08_chapter1

Dibqufs!.!1!

INTRODUCTION Natural rubber latex (NRL) is a natural commodity that has tremendous economic and strategic importance. NRL is used for the production of dipped goods, extruded threads, adhesives, carpet backing and moulded foams. This is primarily because of the unique characteristics of NRL such as high strength, excellent elasticity and flexibility, good formability and antivirus protection. The additional factors which gave considerable interest in NRL products are their low cost and biodegradability.1-5 NRL is obtained from Hevea brasiliensis tree and contains 30-40% rubber dispersed as rubber latex particles in water.6-12 The rubber particles have an average diameter of 1 µm and surrounded by a layer of the non-rubber components such as proteins, lipids, carbohydrates, sugars and metal ions.13-28 Structural studies of NR showed that the rubber molecule is composed of one ωterminal, two trans-1, 4- isoprene units, a long chain of cis-1, 4- isoprene repeating units ending in an α-terminal.29-34 Some of the proteins or phospholipids are thought to co-exist at the terminal ends of the rubber molecules and present as a charged layer covering the rubber latex particle, thereby stabilizing the latex particles against aggregation.35-37

Figure 1.1: Surface nanostructure of NRL particle obtained using AFM imaging 1 

Proteins ω - Terminal

Hydrogen bond

Figure 1.2: Structure of proteins present in NRL

H3C

H

n Figure 1.3: Chemical structure of NRL (cis-1, 4-polyisoprene)

The unique mechanical properties of NR results from both its highly stereoregular microstructure and the rotational freedom of the α-methylenic C-C bonds and from the entanglements resulting from the high molecular weight which contributes to its high elasticity.38 NRL proteins are organic substances that contain carbon, hydrogen, nitrogen, oxygen, and sulphur. They are naturally occurring polymers that may contain hundreds of individual amino acid residues linked together by peptide bonds. The smaller degraded NR proteins of 10 amino acids or less are typically called peptides. The proteins of NRL constitute about 2% by weight of the field latex.12 These proteins may be expected to be attached on the surface of the rubber particles by excluded volume effect of the lipids. Lipids associated with NR are comprised mainly of neutral lipids and phospholipids. The principle phospholipids of the rubber particles are α-lecithin.6 2 

Figure 1.4: NRL obtained from Hevea brasiliensis tree by tapping NRL is obtained from rubber tree by the process of tapping. Ammonia is usually added to the latex as a preservative to increase the alkalinity (pH) and retard microbial growth. The additional benefit from adding ammonia is the increase in stability of the NRL due to the increase in negative surface charge of the rubber particles. NRL undergoes several changes during ammonia addition and storage. Typically some of the proteins are partially degraded by the alkaline conditions imparted by the ammonia. The changes in the composition of proteins in NRL affect the physical properties of the latex. Some of the proteins lose their secondary structure and are partially hydrolyzed to small peptides. The resulting field NRL is further processed to a concentrate by creaming or centrifugation. These processes concentrate the latex to about 60% rubber content. Significant quantities of non-rubber material such as proteins are removed during this processing step. This concentrated form of ammoniated latex is used for the production of dipped rubber products such as medical gloves.12 Natural rubber is sticky and nonelastic by nature. The vulcanization is a process that transforms the predominantly thermoplastic or raw rubber into an elastic or hard ebonite-like state. This process is also known as “crosslinking” 3 

or “curing” and involves the association of macromolecules through their reactive sites. The crosslinking imparts various properties to rubber. It improves its tensile strength, becomes more resistant to chemical attack and is no longer thermoplastic. It also makes the surface of the material smoother, prevents it from sticking to metal or plastic, chemical catalysts and renders it impervious to moderate heat and cold. In addition, it is a good insulator against electricity and heat. These attractive physical and chemical properties of vulcanized rubber have revolutionized its applications. This heavily crosslinked polymer has strong covalent bonds, with strong forces between the chains, and is therefore an insoluble and infusible thermosetting polymer. The most important vulcanizing agent for rubber is sulphur because of its low cost, ease of availability and minimal

interference

with

other

compounding

ingredients.

Various

concentrations of sulphur are used to manufacture different kind of rubber compounds. During vulcanization, sulphide bridges are formed between adjacent rubber chains.39

Vulcanisation S

Figure 1.5:

Vulcanisation (Formation of sulphide bridges between adjacent rubber chains.)

Thus in order to enhance the physical properties of the rubber products, NRL must be prevulcanised before use.40 1.1 Prevulcanisation Prevulcanisation is an important term used in latex technology, which is the process of crosslinking the rubber particles in latex stage without affecting the colloidal stability of the latex. Thus prevulcanised latex, in effect, is a latex of vulcanized rubber. The appearance of prevulcanised latex is very similar to unvulcanized latex and the original fluidity of latex is retained during 4 

prevulcanisation. During prevulcanisation, crosslinking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex without affecting their state of dispersion appreciably. The particles in the prevulcanised latex exhibit similar Brownian movement as in unvulcanised latex and after prevulcanisation particles have same shape, size and size distribution as those in the initial unvulcanised latex.41-44 Using prevulcanised latex effective control of the physical properties can be exercised before the articles are manufactured from it. Prevulcanised latex is used nowadays for the development of products, since initial crosslinking of the rubber particle is possible during prevulcanisation, and complete vulcanization is achieved by simply drying the product.45,46 This enables the manufacturer to decrease the time required for an optimum cure in the circulating hot air oven. An additional benefit is that less time at elevated temperatures means less opportunity for oxidative degradation. Prevulcanised latex is widely used for the manufacture of various dipped goods such as gloves, toy balloons, condoms, catheters, adhesives, latex foam, latex thread, textile combining, latex composites and blends.47-50 There are different techniques used for the prevulcanisation of NRL. They are reaction of rubber particles with sulphur or peroxide and irradiation of latex with UV and γ-rays. The rate of prevulcanisation reaction varies with different vulcanizing systems and the extent of prevulcanisation has a profound effect on the final vulcanisate properties. 1.2 Radiation Prevulcanisation Radiation

vulcanization

of

NRL

involves

radiation

induced

crosslinking of macroscopic particles of NR dispersed in the aqueous medium.

5 

1.2.1 γ-radiation/Electron Beam Prevulcanisation

Prevulcanised NRL prepared by using gamma radiation or high energy electron beams in place of sulphur in conventional process is known as Radiation Vulcanized Natural Rubber Latex (RVNRL).51-71 The main components of NRL are NR and water. Upon irradiation, both NR and water molecules absorb radiation energy independently. The radiolysis products of water are diffused into NR particles and react with NR molecules. The OH radicals, H radicals and hydrated electrons formed are involved in the radiation induced crosslinking of NRL.39

. Radiolysis of NR

Radiolysis of water

Hydrogen abstraction

R

RH

.

OH

H2O

RH

.

+ OH

.

+

H

. +

H

. R

+

H2O

Figure 1.6: Radiation prevulcanisation of NRL Radiation vulcanization needs more than 200 kGy radiation dose to achieve maximum tensile strength. The use of very high dose of radiation for vulcanization of NRL deteriorates properties of the rubber due to the main chain degradation. In practice, radiation vulcanization (RV) accelerators or sensitizers are used to reduce the radiation dose at which maximum tensile strength could be obtained (Dv). Polyfunctional monomers which contain more than two polymerizable C=C double bonds in a molecule, monofunctional acrylic monomers and halogenated hydrocarbons were used as vulcanization accelerators.53-57 6 

The commonly used sensitizers are n-butyl acrylate (n-BA), neopentyl glycol diacrylate, dimethacrylate, 2-ethyl hexyl acrylate (2-EHA), phenoxy ethyl acrylate (PEA), trimethylol propane trimethacrylate (TMPTMA), trimethylol propane triacrylate (TMPTA), chloroform (CHCl3) and carbon tetrachloride (CCl4).54-57 Using these vulcanization accelerators radiation dose could be reduced from 300 kGy to 15 kGy. Among the monofunctional monomers, n-BA is the most effective vulcanization accelerator because it not only imparts much better physical properties at lower Dv but also unreacted n-BA is not retained in the final product.53,58 PEA is more effective accelerator for RVNRL since it imparts approximately equivalent physical properties compared to that of n-BA. However, when compared to n-BA, the sensitizing effect of PEA is comparatively lower, and therefore, similar physical properties could be obtained at a slightly higher dose. Even though TMPTMA is a polyfunctional monomer, it is less capable of imparting better physical properties at similar dose.57 The physical and mechanical properties of irradiated NRL depend on many factors such as irradiation dose, sensitizer content, dry rubber content and storage time.53-59 Radiation vulcanization improves the properties of rubber and has a number of advantages over the conventional method of vulcanization. In γ-ray vulcanization, the crosslinking occurs by elimination of hydrogen and direct carbon-carbon bonds are formed, which are more stable than sulphur bonds. RVNRL was claimed to be more user friendly than NRL prevulcanized by sulphur curing system. This process uses no curatives like sulphur, ZnO and dithiocarbamates. The absence of Type IV allergy inducing chemicals in RVNRL make it a suitable material for manufacturing of many kinds of latex products, especially those come into direct contact with users. The products prepared using RVNRL possess low cytotoxicity and are more transparent and soft.60

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The radiation vulcanization process is a much simpler energy saving process compared to the conventional thermal process which has two vulcanization steps before and after dipping. But the tensile strength and 100% tensile modulus values of rubber films made from this type of latex is not as high as found in many traditionally prevulcanised latex films which incorporate a post vulcanization step. Also, there are a lot of health hazards associated with this process and the expense of the equipment needed is very high. During irradiation of NRL for vulcanization, the latex proteins undergo disintegration which leaves a high soluble protein content in the latex products. The soluble protein content in the cream phase decreased where as that in the serum phase increased with radiation dose. Thus the amount of EPs in RVNRL is greater than in conventional sulphur vulcanisate. NRL can be radiation vulcanized quite effectively and easily in presence of styrene butadiene rubber latex (SBRL).60 One important application stipulated for RVNRL is for the manufacture of dipped products like surgical gloves, examination gloves, condoms, catheters, and so on.70 Various steps involved in the development of RVNRL products are shown in Figure 1.7. Antioxida n t N R la te x M ixin g

Irrad iation

Pre vu lca n ise d N R la te x

n -BA

D ip pin g

L ate x p ro du cts

Figure 1.7: Various steps involved in the development of RVNRL products

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1.2.2 UV-radiation Prevulcanisation Ultraviolet (UV) light is widely applied for modifying the properties of polymer bulk materials and polymer surfaces.72-76 The prevulcanisation of NRL based upon an UV induced pre-curing of the latex emulsion has been described recently by S.Schlogl et al.77-80 This new technology makes the manufacture of examination and surgical gloves feasible without using any sensitizing or allergenic processing agents. In the photochemical process, a selected photoinitiator and a polyfunctional thiol are added to the NRL. On UV irradiation the bond cleavage of the photoinitiators occurs, generating free radicals and crosslinking of the latex particles is then achieved by a thiol-ene addition reaction. The thiol-ene reaction involve the reaction of unsaturated C=C bonds with thiol derivatives such as alkyl thiols, thiol glycolate esters, or thiol propionate esters.81,82 Thiols based on propionate esters and glycolate esters comprise a higher reactivity compared with alkyl thiols due to a weakening of the thiol moiety by hydrogen bonding of the thiol hydrogen groups with the ester carbonyl. Due to their commercial availability and widespread use, trimethylol propane tris (3mercaptopropionate) (TriThiol) and pentaerythritol tetra (3-mercaptopropionate) (Tetra Thiol) were used as crosslinking agents in the photochemical prevulcanisation. The initiation mechanism involves the excitation and cleavage of a photoinitiator. In the prevulcanisation process, Type I photo fragmenting photoinitiators, which undergo homolytic bond cleavage under UV irradiation to yield free radicals, has been used. Regarding the efficiency of the photoinitiator system, not only the reactivity of the compound but also the fit of the light source to the absorption spectrum of the photoinitiator, the transmission properties of the reaction mixture and photophysical (e.g. quantum yield) have to be considered. For photochemical crosslinking to proceed efficiently, the absorption bands of the photoinitiator must overlap with the emission spectrum of the light source. Regarding the prevulcanisation of the latex, an undoped 9 

medium pressure mercury lamp was used. Besides the photoinitiator concentration, the incident light intensity controls the rate of initiation in photochemical crosslink reactions. The prevulcanisation process was carried out by emulsifying the processing chemicals such as photoinitiator and thiol crosslinker in deionized water. The emulsion of the chemicals was then added to high ammonia NRL comprising a dry rubber content of 40 wt%. The UV induced reaction involves a radical addition of thiols to nonactivated C=C bonds. Using thiol derivatives bearing more than two thiol moieties crosslinking of diene rubbers can be achieved efficiently. The photoreactions have been investigated using real time FTIR and Raman spectroscopy. To provide a continuous irradiation of the liquid latex the prevulcanisation is carried out in a falling film photo reactor. Falling film reactors are well known and are used in various industrial technologies including water waste treatment and preparative organic chemistry.83 Due to the low light transmissivity of NRL, the concept of the falling film process allows a homogeneous irradiation of the reaction mixtures in thin films with thicknesses in the range of millimeters. Compared with thin film reactors there is no contact between the reaction medium and the UV lamp. This new technology operates at room temperature and crosslinking is accomplished within minutes, allows enormous cost saving options compared with conventional sulphur vulcanization processes which are carried out at elevated temperatures for hours in batch processes.79

10 

Figure 1.8: Schematic representation of a falling film photo reactor used for the production of UV prevulcanised surgical gloves UV prevulcanised NRL was used for the production of latex gloves by a conventional dipping process.78,79The dipped latex articles display excellent mechanical properties as well as good stability against ageing and gamma sterilization. Skin sensitization, irritation and cytotoxic tests prove the good skin compatibility of UV crosslinked NRL gloves. Various steps involved in the manufacture of UV prevulcanised surgical gloves are illustrated in Figure 1.9.

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Falling Film Photoreactor

Coagulant Bath

UV pre-cured Latex

Oven

Cleaning Bath

Chain Machine

Stripping

Powdering

Quality Control

Packaging Unit

Beading Unit

Oven

Leaching Bath

Gamma

UV pre-cured

Sterilization

Surgical Gloves

Figure 1.9: Processing steps involved in the manufacture of UV prevulcanised surgical gloves 1.3 Peroxide Prevulcanisation Prevulcanised latices prepared by using organic peroxide and/or hydroperoxide involving free radical crosslinking is referred to as Peroxide Vulcanized Natural Rubber Latex (PVNRL).84-88 The process involves the heating of NRL in the presence of organic peroxide. After cooling, films are casted and dried, yielding vulcanized rubber films with a tensile strength as high as 251 kg/cm2. The organic peroxides undergo homolytic cleavage at elevated temperature. The resulting free radicals induce abstraction of hydrogen atoms from the polymer backbone. These in turn create free radicals on the polymer backbone, which allow the chains to form carbon-carbon crosslinks with one another. These carbon-carbon bonds are stronger and more stable than those 12 

formed by sulphur crosslinking. Also, the bond is very predictable one, unlike the many different types carbon-sulphur bonds.89-94 It is also possible to add coagents during compounding to add a slightly different characteristic to some of the carbon-carbon bonds. The additions of certain coagents help to improve certain specific physical properties, such as tear strength. The mechanism of peroxide prevulcanisation of NRL is given in Figure 1.10. H3C RO

+

CH2

C

CH

CH2

H3C ROH

+

CH2

C

CH

CH

H3C

2

CH2

CH

C

CH

H3C CH2

C

CH

CH2

C

CH

HC

CH

H3C

Figure 1.10: Mechanism of peroxide prevulcanisation of NRL Organic peroxides break apart (homolytically cleave) in a very predictable manner. At any given temperature, organic peroxides have a half 13 

life. This is the time it takes for one half of the currently present peroxide to homolytically cleaves. In order to make sure that only trace amounts of organic peroxides remain after curing, it is important to keep the latex film at its predetermined curing temperature for the proper number of half-lives. For instance, after six half lifes, approximately 1.6% of the peroxide is left intact, and after eight half-lives only about 0.4% of the peroxide is left intact. A presumed safe target is eight or more half-lives. There are a very large number of organic peroxides, combinations of organic peroxides and coagents to choose from. Some peroxides decompose at very low temperatures, such as dibenzyl peroxide, while others decompose at very high temperatures, such as 2, 5Dimethyl-2, 5-di (t-butyl-peroxy) hexane. It is important to use an organic peroxide which does not homolytically cleave at too low of a temperature, since it would then not be possible to dry the water out from the latex prior to vulcanization subjecting the film to severe degradation. It is preferable to choose a peroxide that can homolytically cleave rapidly at a temperature lower than the degradation temperature of the base polymer. By choosing the right peroxide and temperature combination, cure times of about two minutes to about nine minutes are conveniently used, while allowing adequate time for latex particles to fuse, and sufficient safety in the water drying process. In peroxide prevulcanisation, along with hydrogen peroxide (H2O2) an activator is used which does not encourage the decomposition of the H2O2 to produce molecular oxygen under the condition of treatment. The activator is suitably added to the unsaturated polymer as an aqueous solution or dispersion. The amount of such an activator employed may be as little as one millimole per mole of H2O2. Usually inorganic and organic compounds which yield peracids or persalts by reaction with H2O2 in aqueous medium were commonly used as activator. Examples for inorganic activators are sodium and potassium salts such as the molybdates, tungstates, stannates, borates, pervandates, aluminates, bicarbonates, lithium chloride and boric acid. Examples of organic activators include formic acid, formaldehyde and fluoroacetic acid. Such an activator may be conveniently formed in situ but may alternatively performed if desired. For 14 

example performic acid may be formed by the reaction of formaldehyde with H2O2 preferably in the mole ratio 1:2. The peracid so formed is present as one component of an equilibrium system comprising water, H2O2, organic acid and organic peracid. In some instances no activator needed to be added since the unvulcanized polymer latex may contain such an activator owing to its method of preparation. This process requires large and expensive heated pressure vessels. To avoid the use of pressure vessels, some PVNRLs are made with the use of catalysts at low temperatures. Such methods generally produce lower tensile strength films and leave chemical residuals in the film. The physical properties and ageing resistance of the peroxide vulcanisate are poor. 1.4 Sulphur Prevulcanisation The possibility of vulcanizing the dispersed phase of NRL without any concomitant colloidal destabilization was first investigated by Schidrowitz in the early years of this century.95-97 His interest was in making a cellular rubber directly from NRL. Although he succeeded in prevulcanising NRL, he did not achieve his principal objective because prevulcanised NRL is not a suitable material for the production of latex foam rubber due to the inherently poor gel strength. The method of prevulcanising NRL disclosed in his first patent was to heat the latex with sodium polysulphide, S and ZnO at 145ºC for 30-45 minutes.96,97 At the conclusion of the process it was necessary to reduce the steam pressure very slowly to prevent the latex from boiling over. Relatively high vulcanization temperatures were necessary because very active organic accelerators of sulphur vulcanization were not available at first. Subsequently, water soluble accelerators of high activity became available. By their use, it was possible to reduce the intensity of vulcanization conditions.

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1.4.1 Preparation of Sulphur Prevulcanised NRL (SVNRL) It is done by allowing the latex particle to react with sulphur and one or more organic vulcanization accelerators and also with an inorganic vulcanization activator. Sulphur prevulcanisation process involves the following steps (1) De-ammoniation by aeration The purpose of reducing the ammonia content and replacing it with a fixed amount of alkali is to minimize the risk of ZnO thickening when the latex is subsequently heated for vulcanization. (2)

Latex is colloidaly stabilized by the addition of small amounts of a caustic alkali and a hydrocolloid stabilizer like sodium carboxy methyl cellulose.

(3)

Compounding of the latex with appropriate amounts of sulphur, accelerator and ZnO. It is desirable to warm the compounded latex for 30 minutes at 300C prior to the addition of the vulcanizing ingredients. This will help to reduce the variability of the latex to the vulcanization reaction by accelerating certain chemical changes which occur slowly at normal ambient temperatures.

(4)

Heating the latex slowly to a temperature in the range 55-80ºC and then maintaining the temperature at this level until the desired degree of vulcanization has been achieved. Care must be taken to (a) maintain constant stirring keeping the vulcanizing ingredients in

suspension (b) minimize the skin formation

(c) control temperature. (5)

Cooling When the desired degree of vulcanization has been attained, the latex is cooled, run off into containers, strained and then bulked. 16 

(6)

Removal of residual vulcanizing ingredients Vulcanizing ingredients sediment during this time and so can be eliminated at this stage.

(7)

Maturation Prevulcanised latex is allowed to mature for sufficient time before product manufacture. During maturation, crosslinking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex.43 Maturation will impart good technical properties and will remove the air bubbles introduced into the latex compound while compounding.

The variables which affect sulphur vulcanization are 1.

The level of sulphur Vulcanization systems used (efficient and conventional vulcanization system) have a profound effect on the extent of crosslink formation during prevulcanisation. The conventional vulcanization (CV) system is characterized by a low accelerator and high sulphur combination, while efficient vulcanization (EV) system has a high accelerator and low sulphur combination.41

2.

Particle size of sulphur dispersion.

3.

The nature and levels of the accelerators.

4.

Whether the vulcanization accelerators are to be water soluble or water insoluble or a combination.

5.

The particle size of dispersions of any water insoluble accelerator which are used.

6.

Whether or not to use an inorganic activator and if so the type and amount. 17 

7.

The temperature-time profile which is imposed for the reaction.

8.

The extent to which the reaction is allowed to continue. Polysulphidic content is high as the extent of prevulcanisation of the latex compounds increases. It was reported that the extent of prevulcanisation affects the aged and unaged physical properties of latex threads.48 The extent of prevulcanization has been analyzed by crosslink density measurements.41

1.4.2 Mechanism of Sulphur Prevulcanisation of NRL The chemical mechanism of the vulcanization is not well understood and is still contoverisal.43 Early studies suggested, and others just assumed that vulcanization takes place upon direct contact between the latex particles and the vulcanizing reagents, since the latter (sulphur, ZnO and ZDC) are insoluble in water. Yet others preferred the premise that the reactants must first dissolve in the aqueous phase before diffusing into the latex particles.98-100 The presence of water in the latex is essential for the occurrence of the sulphur prevulcanisation of NRL at unexpectedly low temperatures. The enhanced solubility of sulphur and accelerator in the latex aqueous phase facilitate sulphur prevulcanisation. The first important step of the reaction is the formation of a sulphur-accelerator species in the aqueous phase of the latex. This species or some derivative of it then transfers to the rubber phase and crosslink the rubber molecules therein. Many hypotheses have been proposed on the transport of the vulcanizing reagents through the aqueous phase. If this species is surface active, the most obvious mode of transfer is adsorption from the aqueous phase on to the surface of the rubber particle. Van Gils, Porter, Rawi and Rahim showed that both the accelerator and sulphur dissolve in the aqueous phase of ammonia preserved NRL before migrating to the rubber phase.98-100 The Zn atoms of the accelerator form complex with hydroxide ions from the water to provide the sulphur-accelerator species with water solubility for transfer to the rubber phase. 18 

The sequence of events following the arrival of the vulcanizing reagents at the surface of the latex particles is of paramount importance in influencing the morphology of the latex particles.43 The microscopy techniques, such as AFM and transmission electron microscopy (TEM), have been used in the elucidation of chemical mechanism of latex prevulcanisation.43,45,46 The surface morphology of prevulcanised NRL films monitored using AFM provides new insight into the prevulcanisation mechanism. Based on AFM studies, C.C.Ho and M.C.Khew proposed the following mechanism for the prevulcanisation of NRL.43 Basically the prevulcanisation mechanism is controlled by the relative rates of the diffusion of vulcanizing reagents and the crosslinking reaction within the latex particles. The rate of crosslinking reaction is much faster than the diffusion rate of the reagents in the rubber phase during prevulcanisation. Hence crosslinking takes place on the surface of the latex particles before the reactants can diffuse into the core. This leads to an unvulcanised core surrounded by a highly crosslinked shell of rubber molecules, resulting in inhomogeneous latex particles. This crosslinked shell acts as the recipient surface for the arrival of more crosslinking reagents and becomes a hindrance to further diffusion of these materials into the core. The difference in diffusion rates of the two phases during the gradual coalescence of the film results in the collapse of the denser shell of the particle. The hardened shell of the partially vulcanized particles also retards the diffusion of the rubber molecules and thus hinders further gradual coalescence of the latex particles in the film. Hence the film surface remains rough and uneven for the highly crosslinked latex films. A homogeneously vulcanized particle is not achieved by prevulcanisation. Thus complete vulcanization could not be achieved even after postvulcanisation.43

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Figure 1.11: Proposed mechanism of latex prevulcanisation and film formation The phase transfer/bulk polymerization/TEM technique has been successfully applied to elucidate the internal morphology of prevulcanised NR and skim latex particles.45,46,101-105 The technique involves the titration of negatively charged latex particle with an aqueous solution of cationic surfactant such as benzyldimethyl hexadecylammonium chloride (BHAC) in the presence of a non-water miscible monomer e.g., styrene, until reaching the end point. Due to the neutralization, the latex particles completely transfer from the aqueous phase into the organic phase. The monomer containing swollen rubber particles was then polymerized. After sectioning the specimen and staining with OsO4, the rubber particles embedded in polystyrene (PS) was studied under TEM. By using this process, the freeze-drying of latex normally required for specimen preparation is omitted and, hence, the disturbance of rubber particle structure is minimized. The internal morphology of prevulcanised NRL particles under TEM showed that the sulphur and γ- radiation systems caused homogeneous crosslink inside each particle while a non-uniform network, a dense network near the surface of the particle was noticed in the sample employing peroxide.

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Figure 1.12: TEM micrographs of crosslinked NR embedded in a PS matrix: (a, b) sulphur prevulcanised and (c, d) peroxide prevulcanised latex particles 1.4.3 Methods for Assessing the Degree of Vulcanization of Prevulcanised NRL (1) The chloroform-coagulation test A sample of prevulcanised latex is coagulated by mixing with an equal volume of chloroform and the degree of vulcanization is evaluated from the appearance of the coagulum. 106

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(2) The equilibrium swelling test In this method, the degree of vulcanization is judged from the extent of equilibrium swelling of films dried down from the latex, the swelling being carried out under controlled conditions using a suitable solvent.107-110 (3) The relaxed –modulus In this method, the degree of vulcanization is judged from the relaxed modulus at 100% extension of films dried down from the latex.111-114 (4) The prevulcanisate relaxed-modulus (PRM) test. This method assesses the degree of latex prevulcanisation by way of measurement of the elastic modulus of a film dried down from the latex. Only the chloroform coagulation test is peculiar to prevulcanised NRL which is applied to the latex itself, and not to films dried down from the latex. The other three tests are applied to films from the latex and are in principle applicable to any elastomeric films including post vulcanized films derived from rubber latices. 1.4.4 Structure of Films Derived from SVNRL Particles in prevulcanised latex would dry down to coherent, well integrated films because the crosslinks between the rubber macromolecules would restrict the ability of the molecular segments in one particle to interdiffuse with those of contiguous particles. Three theories have been proposed to explain the strength and coherence of films derived from prevulcanised NRL.44 They are (1) The primary valence bond theory (2) The bond adhesive theory (3) The secondary valence bond theory 22 

(1) The Primary Valence Bond Theory Here strength is attributed to the formation of interparticle covalent chemical bonds between the rubber macromolecules in contiguous particles, such bonds being formed as the film dries. These bonds immobilize the rubber macromolecules within the individual particles. Two types of network can be distinguished in deposits from sulphur prevulcanised latices. One type is formed during the prevulcanisation reaction and the other type is formed as the film dries. There is a ready mechanism by which such interparticle bonds can form. Rearrangements of polysulphidic links among themselves at elevated temperature together with formation of further sulphur crosslinks, if residual vulcanizing ingredients are present, which lead to the formation of interparticle crosslinks. But the formation of interparticle crosslinks during drying is not essential for the formation of strong coherent deposits from prevulcanised latices. Due to this reason, the primary valence bond theory has been rejected. (2) The Bond Adhesive Theory Here the strength and coherence of deposits from SVNRL, is due to a close packed assemblage of small particles of vulcanized rubber bonded together with non-rubber substances situated in the interfaces between the particles. If all the non-rubber substances are removed from the film and the theory is correct, then it would be expected that the film would disintegrate. But results of Humphreys, Wake and Ghazaly revealed that the strength and coherence of the films is not primarily depending upon the presence of appropriate non-rubber substances in the latex. (3) The Secondary Valence Bond theory Strength is attributed to secondary valence bonds formed between segments of the rubber macromolecules in the surface of contiguous particles. These interactions are developed as the latex dries and the surface of

23 

contiguous particles come into increasingly close contact and as the region of overlap between the surfaces of the two particles increases in volume. Although individual bonds are very weak, the total force required to separate particles is sufficient to account for the strength and coherence of the deposit. Thus in order to separate two contiguous particles, it is necessary to break the entire secondary valence bonds which have been formed in the region of overlap between the surfaces of the two particles. The tensile strength of prevulcanised latex films and behaviour of prevulcanised latex films towards rubber solvents can be satisfactorily explained using this theory. Lebedev et al. provide evidence for the correctness of this theory. 1.4.5 Factors Affecting Sulphur Prevulcanisation of NRL (1) Temperature Gorton studied the effect of temperature upon the rate of sulphur prevulcanisation of NRL using ZDC as accelerator and a small amount of ZnO as activator.111,112 Over the temperature range 40-70ºC, the rate of crosslinking increased with increasing prevulcanisation temperature as is evidenced by the modulus of the vulcanisate. Tensile strength and elongation at break were better when prevulcanisation was conducted at lower temperatures and at each temperature with a shorter heating time. (2) Level of vulcanizing ingredients Gorton studied the effect of levels of sulphur and accelerator upon the sulphur prevulcanisation of ammonia preserved NRL using a small amount of ZnO keeping the accelerator/sulphur ratio constant.111 Although the level of sulphur and accelerator was increased by a factor of 10, the effects upon film properties after any given time of prevulcanisation were found to be relatively slight. This is because the amount of vulcanizing ingredients which become available for reaction with rubber under these conditions were probably

24 

determined by factors other than merely the amount of ingredients present in the system. (3) Presence/absence of a sparingly soluble inorganic zinc compound It is desirable to minimize the level of sparingly soluble Zn compounds in the compounding ingredients for the following reasons. (1) in order to minimize the colloidal destabilization arising from the formation of Znammine ions in the aqueous phase of the latex. The higher the temperature more, serious is the problem likely to be. (2) to achieve maximum optical clarity in the eventual rubber film. Merril studied the effect of ZnO upon sulphur prevulcanisation reaction. In the absence of ZnO, the level of free accelerator decrease sharply as prevulcanisation proceeds and eventually falls to zero. The rate of decrease of the level of free accelerator is greatly reduced if a small amount of ZnO is present because ZnO regenerates the accelerator. (4) Nature of dithiocarbamate accelerator Gorton studied the effect of nature of dithiocarbamate accelerator upon sulphur prevulcanisation using ZDC, zinc di-n-butyl dithiocarbamate and their corresponding sodium salts as accelerator and ZnO as activator.111,112 Zinc di-n-butyl dithiocarbamate is more active in promoting crosslinking at lower prevulcanisation temperature and also give highest tensile strength than ZDC. (5) Degree of dispersion of insoluble vulcanizing ingredients Results of Gorton and Pendle showed that the particle size of the sulphur and accelerator have no significant effect upon the tensile properties of the films obtained from the latex.112 It indicates that the amount of vulcanizing ingredients which become available for reaction with rubber under these conditions is not significantly affected by the area of interface between the sulphur or accelerator particles and the aqueous phase.

25 

(6) Effect of addition of surfactant It was reported that addition of small amounts of carboxylate, sulphonate and sulphate surfactants enhanced the mechanical and chemical stabilities of NRL by rearranging the indigenous soaps, derived from proteinlipid, and making them more effective as stabilizers. This ability depended largely upon the chain length of their alkyl group and the optimum enhancement was observed when the alkyl chain of the surfactants consisted of approximately 11 carbon atoms.

45

The sulphur prevulcanisation of NR particles was enhanced

by the presence of sodium dodecyl sulphate (SDS) which causes an increase in the amount of negative charge on the surface particles. 1.4.6 Mechanical Properties of SVNRL (1)Tensile strength Blackley and Merril reported that the tensile strength of films from prevulcanised NRL compounds depends on the ability of the particles to coalesce and integrate when the film dries and on the concentration of the crosslinks in the rubber.44 The tensile strength of vulcanized film depends upon the conditions under which the film was dried but is always considerably greater than that of unvulcanised films. The effect of crosslink concentration upon the tensile strength of pre and post vulcanized NRL films has been investigated by Porter.99 The ratio of interparticle to intraparticle crosslinks was found to be higher in a sulphur post-vulcanized film as compared with a sulphur prevulcanized film. 113,114 When the film is prepared from a highly crosslinked latex, coalescence of the rubber particle becomes more and more difficult resulting in lower tensile strength. Justin Che et al. studied the structural development and morphology in unvulcanized and vulcanized (both pre- and postvulcanized) NRL both in a relaxed state and under deformation by multiple-quantum (MQ) NMR and in situ wide-angle X-ray diffraction (WAXD), respectively. Vulcanization was carried out using both sulfur and peroxide, showing important differences on the 26 

spatial distribution of cross-links according to the source of vulcanizing agents. Sulfur prevulcanization promotes the formation of highly homogeneous networks in the dispersed rubber particles, whereas peroxide vulcanization makes broader spatial crosslink distributions. The latter is compatible with the formation of core–shell network structures. Molecular orientation and straininduced crystallization were analyzed by both stress–strain relations and WAXD. An increase in the vulcanizing agent concentration led to an increase in modulus and crystalline fractions. For sulfur vulcanization, the additional heat treatment (postvulcanization) increased the interactions between rubber particles and unreacted vulcanizing agents. For peroxide vulcanization, the additional heat treatment led to chain scission reactions and degradation of network points.114

Figure 1.13: Comparison of pre and post vulcanisation (2)Behaviour towards rubber solvents When immersed in rubber solvents such as toluene or chloroform, it is found that films from SVNRL swell very considerably to give gels which are mechanically rather weak. Humphrey and Wake observed that if significant post vulcanization is avoided, films from SVNRL tend to disintegrate into a mass of very small separate particles of vulcanized rubber swollen with solvent. Prevulcanised film can readily rupture if a drop of solvent is placed upon it whilst it is held in an extended condition. It is for this reason that latex 27 

dipped gloves are more readily damaged by oil and grease than are those which have been vulcanized after the glove film has been formed. However this disadvantage of prevulcanised films relative to post vulcanized films can be mitigated to some extent by subjecting the prevulcanised film to post vulcanization. 1.4.7 Comparison of Sulphur Prevulcanisation of NRL and Sulphur Vulcanization of Dry NR Sulphur prevulcanisation of NRL occurs at much lower temperatures and is more facile than sulphur vulcanization of dry NR using an identical vulcanizing system. 40,42 The results obtained by Loh illustrate that sulphur prevulcanisation occurs much more rapidly than does vulcanization of solid NR at the same temperature using the same vulcanizing ingredients. Analogous results are obtained for artificial latex of synthetic cis-1, 4-polyisoprene and the corresponding dry solid polymer. The unexpected facility of sulphur prevulcanisation of synthetic polyisoprene latex compared to ammonia preserved NRL is due to the absence of non-rubber substances in them. The speed of the prevulcanisation reaction seems to be associated primarily with the presence of water.41 Prevulcanisation of NRL has two particular advantages over vulcanized dry rubber. (i) Prevulcanised latex tends to be very much stronger and elastic as the elastomer chains have not been degraded by the mechanical work needed to incorporate curatives into the dry material. (ii) Since even post vulcanization takes place at a relatively low temperature (generally below 1200C) possible to incorporate colouring chemicals which could not survive the high temperature used for cost effective conventional vulcanization (typically 100-2000C).

28 

1.4.8 SVNRL-Industrial Grades Three grades of high ammonia and one grade of low ammonia prevulcanised latex are available. The high ammonia grades are distinguished as low modulus (LR), medium modulus (MR) and high modulus (HR) according to the modulus of films which dry from the latex. The low ammonia grade used for heat sensitized dipping of films having medium modulus and high optical clarity. Other grades of SVNRLs available are (1) grades having reduced concentration of non-rubber substances by double centrifugation of latex. (2) grades having low levels of extractable N-nitrosoamines and N-nitrosatables in films dried from the latex. (3) grades which are currently available from Revertex under the trade name ‘Revoltex’. There is no need for further vulcanization of films from these latices with additional vulcanizing ingredients. P r e v u lc a n i s a t i o n o f N R l a t e x

R a d ia tio n

S u lp h u r

P e r o x id e

γ − r a y / e l e c tr o n b e a m

U V - ra ys

H ig h a m o n ia te d

H i g h a m o n i a te d

H ig h a m o n i a t e d

H ig h a m o n ia te d

N R la te x

N R la te x

N R la t e x

N R la te x

R e d u c tio n o f N H 3

A d d i t i o n a n d m i x in g o f s t a b il i s e r s , S, ZD C, ZM BT, ZnO and a n t io x id e n t d i s p e r s i o n s

A d d i t i o n a n d m i x in g o f a t-b u ty l h y d ro p e ro xid e

A d d it i o n a n d m i x i n g o f a n - b u ty l a c ry la te s e n s itiz e r

P r e v u lc a n i s a t i o n

γ − r a y i r r a d ia t i o n

A d d it i o n a n d m i x in g o f e m u l s i f i e d p h o t o i n i t ia t o r a n d th i o l c r o s s li n k e r

U V - ir ra d ia tio n

a t 5 0 -7 0 0C

C o m p o u n d m a tu ra tio n a n d A d d itio n o f a n tio x id a n t

p r e v u lc a n i s a t i o n a t 3 5 - 8 0 0 C

P r o d u c t f a b r ic a t i o n , d r y i n g

P r o d u c t fa b r ic a tio n

a n d p o s t v u l c a n i s a ti o n

a n d d ry in g

Figure 1.14: Various methods for prevulcanisation of NRL

29 

1.5 Film Formation Process Polymer latex films are formed by spreading water based latex dispersion onto a substrate and allowing the water to evaporate until the particles come into contact and fuse together.14 The fundamental process involves the transformation of the particles in a stable latex dispersion into a continuous film.115-118 Early work on the mechanism of latex film formation was carried out by Dillon et al., Brown, Voyutskii, Bradford and Vanderhoff, and Mason.119-121 Over the years, various improvements to these early models have appeared, and the general picture that emerged recognizes, conceptually for an idealized model, the film formation process as being divided into three distinct stages. The first stage involves the linear cumulative water loss with time of the concentrated latex dispersion, with increasing restricted Brownian motion of the particles until they come into contact. The packing of the latex particles with interstices water depends on the polydispersity of the particle size and the ionic strength of the original latex dispersion. Further, slower evaporation of water leads to deformation and coalescence of the soft deformable particles in the second stage. Deformation is driven primarily by capillary and osmotic forces and resisted by electrostatic/steric repulsion and viscous and elastic deformation of the polymeric particles. At the end of this stage, the film is dry but particle contours are still discernible, the particles having deformed into a polyhedral structure. At the final stage, interdiffusion of polymer chains across the particleparticle interface occurs (termed further gradual coalescence or autoadhesion), if the film is at a temperature above the glass transition temperature (Tg) of the latex particles, resulting in a mechanically continuous homogeneous film. Any residual water left in the film would escape by diffusion through capillary channels between the deformed particles or through the polymer itself.

30 

Figure 1.15: A pictorial representation of the stages of latex film formation from soft polymer particles (a) the latex dispersion (b) the solvent evaporates leaving the particles in close contact (c) deformation and packing of the particles (d) further coalescence produces a mechanically rigid film. The nascent film is normally weak. Its mechanical properties, such as tensile strength, improve gradually, often requiring days or weeks to reach their final properties. It is commonly believed that the enhancement of tensile strength is a consequence of polymer diffusion across the particle-particle interface in the film. Important factors responsible for interdiffusion capability of polymers in lattices are the molecular weight of the polymer, film formation 31 

temperature, the spatial distribution of chain ends near the interface, and the steric and electrostatic stabilization of the latex.122-127 Many studies of the latex filming process, utilizing a variety of techniques, have been published. TEM of latex films has been the traditional method for examining the film morphology. Roulstone et al. and Wang et al. used freeze-fracture transmission electron microscopy (FFTEM) to investigate the deformation, coalescence and fusion process in poly (butyl methacrylate) (PBMA) particles in the latex film with a particle diameter of 114-400 nm. The molecular interdiffusion phenomena in the third step have also been extensively studied by techniques such as non-radiative energy transfer (NRET) method and small-angle neutron scattering (SANS). 116 Latex film formation studies of synthetic latexes by AFM have been actively pursued recently.128-152 The time and temperature dependence of particle deformation/flattening at the surface of PBMA latex has been measured by Goh et al. and Lin et al. respectively. Besides the influence of time and temperature on the film formation process in the second and third steps, attention was also paid to the effect of particle size on the sintering temperature of latex filming. However, the Tg of almost all the polymers investigated are close to or above room temperature, such as PBMA and polystyrene. Lin et al. monitored the flattening rates of PBMA film from the corrugation heights of the latex particles as a function of time at different annealing temperatures. They found that the kinetics of film formation obey the time/temperature superposition principles and demonstrated a direct relationship between film formation kinetics and rheological properties. In contrast, there is very little work done on the film formation of NRL, even though NRL is used extensively in the manufacture of gloves, prophylactics, and other dipped-goods that also involve film formation of the latex particles. In fact, the necessary condition of such applications is the formation of a continuous film with the appropriate mechanical strength. Apart from the much larger particle size and wider size distribution, one obvious 32 

difference between synthetic and NRL is that NRL also contains a host of nonrubber materials in small amounts. These non-rubbers are either soluble in the aqueous phase or adsorbed on the latex particle surface. The latex particles in commercial latex concentrate are stabilized by an adsorbed layer of mainly longchain fatty acid soaps, polypeptides, and proteins. NRL forms a continuous, almost transparent film when allowed to dry as a thin spread on a substrate. Once initial contact between the latex particles is achieved by gelation following the uniform destabilization of the latex particles, dehydration of the aqueous phase retained in the interstices of the 3-D gel network of latex aggregates follows.

aqueous phase

Figure 1.16: Latex particles in colloidal suspension (Particles dispersed in aqueous phase are separate, uncrosslinked, and homogeneous throughout.)

Figure 1.17: Particles with prevulcanisation (Dispersed particles, still separate but lightly crosslinked.)

33 

Figure 1.18:Particles with procure (Particles with crosslinked surface and very slightly crosslinked interior.)

[Upon drying, precured particles agglomerate and form only a weak film because the thick particle skin interferes with homogeneous vulcanization.]

[Prevulcanised particles agglomerate and form a good film with homogeneous crosslinking.] Figure1.19: Film formation contrasts The extent of prevulcanisation has a profound effect on the formation of the latex film. A coherent film would not be achieved if the particles were highly crosslinked near their surface owing to the restricted mobility of the rubber chains at the particle surface. On the other hand, homogeneously crosslinked particles fuse well and form a film with optimum physical properties.45

34 

1.6 Uses of Prevulcanised NRL Prevulcanised NRL is a very convenient raw material mainly for the manufacture of thin film-products.45 Partially prevulcanised NRL has been found to be capable of yielding thin films which have very satisfactory mechanical properties, especially if those films have been further vulcanized. The principal mechanical properties of interest of thin rubber films are tensile strength, puncture strength and extension at break. Prevulcanised latex widely used for the production of various dipped goods the bulk of which is in the form of examination and surgical gloves.43 In the context of dipping, the use of partially prevulcanised NRL offers a considerable economic advantage relative to post vulcanisable NRL in that its use enables a large quantity of rubber to be partially vulcanized as bulk latex instead of having to be entirely vulcanized when spread out as a thin film over the surface of innumerable formers. Prevulcanised latex when used for making gloves need not undergo a thorough vulcanization process since this has already taken place during the mixing and heating up of the chemical mixtures with latex. Technically the dipping process using NRL involves the following basic steps. (i) latex compounding by mixing the latex with dispersions of vulcanizing reagents (sulphur, ZnO, and organic accelerator such as ZDC and additives such as surfactant, antioxidant, pigment, filler and so forth); (iii) agitating the mixture during a period of maturation at 20-700C; (iii) forming the articles by dipping so as to deposit a thin layer of the compounded latex on a former. A former is a solid surface on to which a thin layer of latex can be coated by dipping it in the latex and then withdrawing it. The thin latex film formed after drying can be stripped off. The former, made of ceramic, takes the shape of a hand in the case of glove manufacturing; (iv) leaching followed by heating to dry and vulcanise the wet gel and (v) stripping the film from the former.43

35 

The principal latex dipping processes are simple or straight dipping, coagulant dipping, heat sensitized dipping, and electrodeposition. In straight dipping, the clean and dry former is immersed into the latex, slowly withdrawn, inverted, rotated, and dried. In coagulant dipping, the former is dipped into a coagulant solution (e.g., calcium nitrate, calcium chloride), withdrawn, and allowed to dry partially. It is then lowered into the latex compound and, after a suitable dwell time, slowly withdrawn, inverted, rotated, and dried. Straight dipping gives a very thin deposit of latex film, whereas coagulant dipping gives a higher thickness of latex film deposited on the former. The thickness of the latex film deposited on the former depends on several factors, namely, the properties of the latex compound, the type and temperature of former, the concentration and nature of the coagulant, the rate of withdrawal, and the dwell time of the former.47

Figure 1.20: Production of latex gloves and toy balloons by dipping process Prevulcanised latex is ideal for small scale manufacturers of balloons and gloves where it does not require compounding the latex to produce superior products.42 This will bring down the cost of production of small scale units. It has long shelf storage life, online process life, consistency and stability.

36 

From the point of view of a manufacturer, the use of prevulcanised latex simplifies a great deal the whole manufacturing process. Depending on what products are being made, the prevulcanised latex could be used as it is (as in the case of toy balloons after the addition of pigments) or after some dilution with water to achieve a final latex solid content of as low as 30% (as in the case of examination gloves). What one needs to do next is to mix for about 30 minutes and allow enough time, usually 16-24 hours for deaeration before the latex is ready for dipping, casting, extrusion, spraying, painting, coating etc. Another important advantage is the low residual chemicals, particularly the accelerators. This results in a cleaner latex compound with low toxicity level which of utmost importance for the manufacturing of medical devices such as gloves, baby teats, condoms, catheters and medical tubings. Testing of these articles has been carried out by medical device manufactures tacking their destination and service conditions. This includes chemical analysis of extracts, skin irritation, skin sensitization, muscle implantation pyrogeneity, cell cytotoxicity etc. Latex medical articles made from prevulcanised latex are in most cases biologically less active than those made from typical post vulcanisable latex compounds. Biological activity is usually caused by ingredients that have not fully reacted in the process of vulcanization. These could migrate into human bodily fluid such as fluid from mucous membranes, blood, saliva and other physiological fluids. Prevulcanisation can be used to control the degree of crosslinking.43 Using prevulcanised latex, a higher degree of cross-linking can be achieved compared with that of a purely post vulcanized latex film. No maturation period is required as in the case of post vulcanisable latex compound where a maturation stage is almost always a prerequisite for making reasonably good quality latex products. Maturation is a stage when sufficient time must be allowed for both the naturally occurring and added surfactant and fatty acid soaps to reach equilibrium. Maturation allowed time for the maximum 37 

dissolution of sulphur-accelerator species to migrate into the rubber particles. Also, a controlled degree of vulcanization must take place during this stage before the latex compound is ready to be used. To use the latex compound too early or too late would result in under curing and over curing respectively. Generally speaking, unlike prevulcanised latex, post vulcanizable latex compound would have a marching curve immediately after compounding in terms of degree of vulcanization. Hence, in the case of post-vulcanisable latex compound, it is more difficult to prevent situations of over-curing when cracking and tearing of, for instance, gloves and condoms are frequently encountered. This is attributed to the fact that the tensile strength reaches a peak before reclining as the crosslink density increases. In short, post- vulcanisable latex compound has short shelf life of usually from 2 days to 2-3 weeks depending on the curative formulation. On the other hand prevulcanised latex generally has a very much longer shelf-life of 6 to 9 months. Therefore less stringent process controls are required for prevulcanised latex. The viscosity of post-vulcanisable latex compound increases with time, unlike prevulcanised latex. This is basically a result of zinc-ammine thickening. This involves the dissolution of ZnO by ammonia in the presence of ammonium salts releasing zinc-ammine complex ions which in turn would react with stabilizers on the latex particles namely the fatty acid soaps and proteins forming insoluble zinc soaps and proteinates. The end result is the loss of stability accompanied with increasing viscosity. Although the vulcanization stage is not required for prevulcanised latex, in practice, an oven is still required to accelerate the drying. Sulphur prevulcanisation can also be extended to epoxidised natural rubber latex (ENRL). ENRL is a chemically modified form of NRL. ENR was produced by attaching the epoxy group to NR molecule. The epoxidation has been mainly carried out in latex stage using freshly prepared peracetic acid, performic acid or the mixture of H2O2 and organic acid, i.e. formic acid or acetic acid.153, 154 38 

Epoxidation

O

H2O2/ HC(O)H n

n-m

Latex medium

Natural Rubber

m

Epoxidised Natural Rubber

Figure 1.21: Synthesis of epoxidised natural rubber ENR has been shown to exhibit beneficial properties, particularly its oil and chemical resistance, air permeability and adhesive properties. It would be useful if these desirable properties of ENR could be exploited in latex dipped product applications. However, uncompounded ENRL has been shown to be incapable of being satisfactorily processed by coagulant dipping due to its inability to gel on the former and because of the formation of a very thin latex deposit. The main reason for this observation is the stabilization of ENRL by high level of non-ionic surfactant. Whilst the non-ionic surfactant is necessary to keep the latex stable during epoxidation reaction with acetic acid and H2O2, it however, keeps the latex chemically too stable against the calcium salt coagulant such as Ca(NO3)2.

Figure 1.22: Schematic representation of γ-radiation prevulcanisation of ENRL particle Skim latex, a byproduct of field NRL concentrated by centrifugation can be prevulcanised using sulphur and peroxide systems. 102,104,155 By applying centrifugation process, most of the non-rubbers solids, about two thirds of the 39 

water-soluble non-rubbers and small NR particles or skim rubber are removed. While the surface-active species or indigenous surfactants remain in the serum and/or on the rubber particles in the concentrate fraction. However, the skim rubber has always been discarded and has not received much attention due to the high ratio of aqueous phase in the latex. Previous study reported that the skim latex contained about 7% TSC and 5% DRC. Although skim rubber was studied for its use as an urea encapsulant in the controlled release application, a better understanding of the properties of skim particle is still needed in order to facilitate its recovery and to explore other potential use.155 Owing to its small particle size, skim rubber would be a good choice for use as agglomerating latex in the heterocoagulation process which involves agglomeration of small particles onto a large core particle. This technique generally offers the possibility of a better control for certain composite latex particle morphology, especially the core–shell type. An interpolymer complex principle was also successfully applied to prepare the heterocoagulated NR/polychloroprene (CR) with core–shell structure. Due to the low glass transition temperature (Tg), the NR/CR core–shell particle was obtained without annealing the aggregate at high temperature. Better oil resistance of film casted from heterocoagulated latex than that of NR film also confirmed the role of high polar CR, the agglomerating latex or shell layer. The study of replacing CR by skim NRL, naturally abundant and low cost products, in the heterocoagulation technique was, therefore, of great interest. To improve oil resistant property, prevulcanisation was the minimum requirement for skim rubber modification.104 Sulphur prevulcanised skim latex has higher degree of crosslinking compared to that of peroxide prevulcanised skim latex. The presence of hydrolysis product of skim latex, protein, could facilitate the rate of diffusion of vulcanising reagents into the rubber phase and act as sulphur-cured activator. Previous works reported that small amounts of proteinaceous substances in NRL, being surface active, resulted in an increase in curative concentrations in rubber particles. Since the rate of diffusion of alkoxy radicals derived from 40 

t-BuHP/fructose system into the rubber phase was lower than that of the vulcanisation, the radicals generated in the aqueous phase reacted first with the rubber molecules on the surface of PVNRL particles followed by abstraction of hydrogen atoms to produce rubber radicals which readily combined to form crosslinks. The high dissolution of alkoxy radicals in the initial stage resulted, therefore, in the high crosslink density and, hence, low swelling ratio of the PVNRL film. However, with longer prevulcanisation time, the constant swelling ratio of PVNRL film was obtained and its value was higher than that of SVNRL film. This might result from the effect of non-rubber substances which acted as an inhibitor in the free radical polymerisation reaction. The role of protein–lipid surrounding rubber particles on relatively low conversion in NR was reported when the in situ polymerisation of styrene in NR and DPNR latex particles was studied. It was believed that the indigeneous proteins in NR acted as free radical scavengers which retard polymerisation and termine the grafting reaction altogether. The small size of skim particle or large particle surface area should also be responsible for the high degree of crosslinking. A mesh structure of all sulphur-crosslinked skim particles containing PS, prepared by using the phase transfer/bulk polymerisation process, irrespective of size, was uniform. In the peroxide-cured latex, the network structure was noticed mostly in the form of film.104 The prevulcanised skim latex having high degree of crosslinking and homogeneous crosslink structure was used for the preparation of composite latex particle of NR/ prevulcanised skim rubber by heterocoagulation technique. Data from zeta potential measurement and the better resistance to toluene of films casted from the heterocoagulated lattices when compared with that of the NR film indicated the existence of SVNR on the outer layer of the composite particles. The use of crosslinked skim as agglomerating latex in the encapsulation of disinfectant agent for the preparation of NR medical glove is also the ongoing research.104

41 

Prevulcanised latex can be used for the preparation of various nano composites and blends.41,49,50 Polymer nano composites represent a new alternative to conventionally (macroscopically)

filled polymers.

Nano

composites are materials that have a nanometer scale dispersion of reinforcing agents (at least one dimension).

Because of their nanometer size filler

dispersion nano composites exhibit markedly improved properties when compared to the pure polymers or their traditional composites. These include increased modulus and strength, outstanding barrier properties, improved solvent and heat resistance and decreased flammability. Clay and clay minerals such as sodium montmorillonite, saponite, hectorite, bentonite etc have been widely used as natural fillers in making the nano composites. The main reasons for adding clay fillers to rubber are to enhance the mechanical properties and to make the final products less expensive. 156-166 Commercial clay has been used as filler for rubber. The reinforcing capacity of clay is poor because of its large particle size and low surface activity. On the other hand, minerals have a variety of shapes suitable for reinforcement, such as fibrils and platelets. Layered clays are comprised of silicate layers having a planar structure of 1 nm thickness and up to 500 nm length. The layers cannot be separated from each other through general rubber processing. Since inorganic ions absorbed by clay can be exchanged by organic ions, research succeeded in intercalating many kinds of polymers and to prepare clay/polymer nano composites. Rubber–clay nanocomposites were prepared from latex by a coagulation method and an improvement in mechanical properties was reported.156-160 The conventional compounding technique was used to prepare nanocomposites from latex. Some layered silicates are suitable additives for latex, provided that they can form dispersions adequate for latex compounding. NRL was compounded with dispersions of layered silicates and other vulcanizing ingredients in order to produce vulcanized NR–clay nano composites. In aqueous dispersions, the clay ‘swells’ and makes its good dispersion in the rubber possible. Recently, Varghese et al. studied the properties 42 

of natural rubber and synthetic rubber lattices reinforced with layered silicates.163,165 Presence of precipitated silica, china clay and whiting decreased tensile strength and elongation at break but increased the modulus of the prevulcanised latex film. Poor tear resistance is a common problem encountered in medical gloves and condoms. Fillers such as carbon black, ultra-fine calcium carbonate, silica and starch are normally added to the NRL products for reinforcement. These fillers however, are not good enough. Besides strength, the NRL products should also be biodegradable. To produce totally biodegradable, NRL products will require the complete change of the raw material (latex). However to produce partial biodegradable NRL products is easier and can be achieved by adding biodegradable material as one of the components in the NRL formulation. A suitable biodegradable material is natural fibers because natural fibers are abundant, cheap and due to their intrinsic properties could also reinforce the NRL products.2 Natural fibers such as flax, hemp, banana and oil palm are known to reinforce polymers. The reinforcement of rubber with natural fibres is possible by combining the elastic behavior of the rubber matrix with the strength and stiffness of the natural fibres. The degree of reinforcement of rubber by natural fibers depends on the fiber concentration, fiber dispersion within the rubber matrix. Poor fiber dispersion is the biggest problem encountered when mixing rubber with natural fibers and this always results in the reduction of the overall strength of the composites. Poor dispersion of the natural fibers with rubber could be improved by modification of fiber surface by the use of bonding agent and chemical modification of the fiber surfaces. Recently A.S.Siri Nuraya et al. reported the use of banana stem powder as reinforcing filler for NRL. The prevulcanised NRL films containing banana stem powder have properties comparable to those films containing the same amount of calcium carbonate and colloidal silica.2

43 

Several investigations have been made in the past few years in the field of blending of polymers. Blending is used extensively to improve the processing characteristics as well as the properties of the end products. Excellent reports regarding latex blends exist in the literature. Okikura conducted a series of studies on the recent trends in the practical blending of various kinds of latices. Shundo, Imoto, and Minoura found out a relation between the properties of unvulcanized and vulcanized blends of NR and SBR prepared by means of solution blending, latex blending, roll blending, and Banbury mixer blending. In most cases, latex blending results in a good degree of dispersion, which cannot be achieved by other blending techniques. Additionally, NR/SBR blends showed a direct relation to their blend ratio, regardless of blending method used. The blends of SBR and NR latices have many potential advantages. For example, blends of 38% solid NRL and 22% solid SBR latex in the 70/30 ratio have good processability. These blends combine better crack resistance, wet grip, and weather resistance of SBR and the superior strength properties and low temperature characteristics of NR.41 The effects of various factors like vulcanizing systems, prevulcanization time, accelerator system, and shear rate on the rheological behavior of NR/SBR latex blends have been analyzed. The rate of prevulcanization reaction in homopolymer latices and their blends has been investigated by the variation of viscosity in each time. NR and NR-rich blends show the highest increase in viscosity with prevulcanization time compared with SBR and SBR-rich blends. This is due to the lower unsaturation and low solid content of SBR latex. In most cases pseudoplastic behavior is observed except in the case of SBR and SBRrich blends. The dilatant behavior of SBR and SBR-rich blends is attributed to the high temperature sensitivity of SBR latex. 41 Prevulcanised NRL can also be used for the modification of asphalt emulsion applicable for high way construction application. Asphalt emulsion is manufactured by emulsification of asphalt, and it is an energy saving, ecologically safe material because it does not need any heating processes which 44 

can emit gas and fire hazard in its use. Using prevulcanised NRL as an admixture of asphalt emulsion, thermal stability and useful mechanical properties of the asphalt emulsion were improved. The mechanical and physical properties including softening point and penetration of asphalt emulsion were also very well modified by the vulcanized rubber phase. The increase in vulcanizing agent could also increase softening point. At low polymer contents, the samples reveal the existence of dispersed polymer particles in a continuous bitumen phase, whereas at high polymer contents a continuous polymer phase was observed.167 Recently it has been reported that prevulcanisation of NRL does not hinder the formation of conjugated sequences, which is a precondition for developing intrinsic electrical conductivity in polymers. So prevulcanised NRL can be dopped by iodine to become electrically conducting and the dopping process is not affecting the flexibility or processability of the films. The incorporation of a steric stabilizer such as starch enhances the solution dopping of the prevulcanised NRL resulting in flexible semiconducting thin films with an optimum enhancement in the electrical conductivity behavior and a decrease in optical band gap. Doped vulcanized NRL promises to be a cost effective and highly efficient material for use in solar cells and light emitting diode materials.168-171 Polymethyl methacrylate (PMMA) particles were deposited onto the sulphur prevulcanised NRL film by using the layer-by-layer technique in order to increase the roughness and hence, decrease the friction on the rubber surface. Prior adsorption of PMMA particles, the sulphur prevulcanised NR sheet was pretreated with argon plasma followed by UV induced graft co-polymerisation of polyacrylamide to generate charge on the NR film. This modified sulphur prevulcanised NRL can be used for the development of gloves designed for the hypersensitive person because here the direct contact between the rubber and skin is decreased. 172-174

45 

Recently it was reported that maleated suphur prevulcanised NRL (MSVNRL) can be prepared by grafting maleic anhydride on to SVNRL particles by using benzoyl peroxide as an initiator. The tensile strength, modulus, hardness and elongation at break of SVNRL film dramatically increased after grafting with maleic anhydride. Due to the reduction of double bond, the thermal stability of M-SVNRL film was better than that of SVNRL. The environmental friendly M-SVNRL would be further applied as a compatibilizer between NR and biopolymer.175,176 However, there are some disadvantages for prevulcanised latex. These are the inherently darker colour and the higher tackiness. Another disadvantage of prevulcanized NRL is that the wet gel strength of the dipped films may be lower than that of nonprevulcanized latex films, thus requiring some changes in the manufacturing process. 1.7 Latex Allergy Allergy to latex was first recognized in the late 1970s and since then it becomes a major health concern as more individuals continue to be impacted. Allergic reactions can affect the skin, eyes, mouth, nose, throat, lungs and heart. Symptoms range from skin rashes, redness, itching to dizziness and abdominal pain. Up to 6% of the general populations are allergic to NRL.177-225 Latex allergies are of two types. They are (1) Type IV allergy Type IV (delayed type) hyper sensitivity is usually caused by chemicals such as accelerator, antioxidants, emulsifiers etc. added to the latex.215 Type 1V reactions are generally not life threatening and tend to be localized near the area of contact. The cytotoxicity and tissue irritancies of NRL materials were correlated to the residual amount of zinc dithiocarbamate accelerators. MBT-type accelerators are known as contact allergens. DTB-type accelerators have been reported to show strong cytotoxicity.199 46 

(2) Type I allergy It is caused by proteins present in the latex and these type reactions tend to be general and can be life threatening.106,203 The proteins help to maintain the latex colloidal stability during collection and transport prior to manufacture. But the presence of proteins in NR has been assumed to exert influence on some undesirable properties such as poor creep and stress relaxation, increasing storage hardening and moisture adsorption. Recently, the extractable proteins in the latex products are found to be responsible for the allergic reactions and it is recommended these proteins should be removed. Consequently, when measures are taken to remove or degrade these proteins, other problems can be introduced, such as destabilization of the latex and changes in its coagulation properties. The proteins can be removed either in latex stage or by treatment of the finished product. 1.8 Protein Removal in Latex Stage Deproteinised natural rubber (DPNR) latex is a purified form of NRL with very low nitrogen and ash content. Either mechanical or chemical means are common for reducing protein content of latex. The mechanical process involves multiple centrifugation or membrane filtration of diluted latex. The chemical treatment involves a combination of digestion with a proteolytic enzyme or displacement of adsorbed proteins using a surfactant and the subsequent purification of the treated latex by centrifugation or creaming.

47 

1.8.1 Mechanical Processes Used for Deproteinisation •

Centrifugation Most of the protein (75%) in field NRL is freely dissolved in the serum

fraction while a minor fraction (25%) is bound to the surface of the rubber particles. NRL is typically concentrated to reduce the water content. When latex is centrifuged, the field latex is concentrated from about 33% rubber solids to 60% solids. About one-half of the soluble proteins are removed from the NRL by this process. Since most of the reduction is from the natural rubber serum fraction, the actual percentage of protein bound to rubber particles is increased from 25% to about 50% of the total.12 If the NRL is processed further by another round of centrifugation, the latex protein content can be reduced even further. This is usually achieved by diluting the centrifuged NRL concentrate with water back to about 30% solids, and then recentrifuging it back to 60% solids. This additional processing can further reduce the serum proteins by another 50% or more. Therefore, double centrifuging of latex can reduce the overall protein content by 63% or greater. Although this approach is a simple way of reducing the protein content of NRL, it may not be the most cost effective. Each time the latex is concentrated by centrifugation, about 10% of the rubber is lost to skim latex, and fresh field latex cannot be processed since the centrifuges are being used. In addition, the double centrifuged latex may have to be supplemented with chemical stabilizers to maintain the NRL stability and to prevent any unacceptable coagulation from occurring. The EP content attributable to serum proteins in finished unleached gloves made from double- centrifuged NRL can be reduced by 60% or more when compared with gloves made from single-centrifuged latex. However, the EP attributable to bound proteins is not affected by this process and therefore the reduction in total antigenic protein content in finished gloves may be somewhat less than 60%. Another 50% reduction in serum-EP might be possible by using triple-centrifuged NRL (recentrifugation of double-centrifuged NRL).

48 

•

Creaming Creamed NRL is produced by modifying the buoyant density of the

latex and thereby accelerating the natural creaming of NRL. Creaming is a process of concentrating NRL so that the rubber particles in the latex rise slowly to the top to form a creamed rubber layer on the liquid. This process is similar to the creaming of milk. Creaming is usually carried out in large vessels with the addition of creaming agents that increase the buoyant density of latex, such as alginate and methylcellulose type polymers. As the rubber gradually rises to the top of the liquid and is concentrated, the aqueous serum phase is removed from the bottom of the vessel, leaving the creamed latex concentrate in the tank. This process of creaming NRL is sometimes more effective but slower than centrifugation. NRL concentrations up to 68% are typical of creamed latex; thus more of the serum phase is removed than in centrifuged latex. Following the creaming of rubber, much of the soluble protein is removed when the aqueous phase is drained from the bottom of the latex tank. The creaming of NRL may be more effective in reducing smaller latex proteins than the centrifugation process. This may be due, in part, to the addition to NRL of colloidal creaming agents that may release some of the proteins that are bound to rubber in the serum phase, thus further reducing the overall total protein levels in the concentrate.12 1.8.2 Chemical Processes Used for Deproteinisation The proteins are efficiently removed from NR in latex stage by using proteolytic enzyme, surfactant and denaturing agents like urea, guanidine hydrochloride etc. 10-13, 204-214 The use of proteolytic enzymes to digest NRL proteins has been known for many years and found broad application in rubber industry.12 Proteases are a form of hydrolytic enzyme that cleaves peptide bonds and are available commercially in large supply. Typically these commercial enzymes are produced from the fermentation of select nonpathogenic strains of bacteria. 49 

Proteolytic enzymes are used in many applications such as in the manufacture of leather, in food processing including meat tenderizing, and in the manufacture of cheese, beer, and baked goods. The use of proteases in washing detergents for home use is fairly common. However, it is only recently that the use of these enzymes has been shown to reduce allergenic proteins in medical gloves. These enzymes break up the proteins into smaller pieces, which facilitate their removal. Concentrated enzyme powders or solutions should be handled carefully to avoid inhalation of aerosols, as they are potential allergens. Care must be taken to remove residual proteolytic enzymes, which may also give rise to an allergic response.106 Moreover, a long incubation time is needed for enzymatic deproteinisation. Stabilized liquid Papain obtained from papaya plant (Carica papaya) can be used for deproteinisation of NRL in a single centrifuging process. This low protein latex is used for the production of surgical gloves with low levels of EP content.211 The use of nonionic surfactant is a comparatively better method for deproteinisation and it will not affect the mechanical properties to a greater extent. Polyethylene glycol (PEG) is an important non-ionic surfactant used for the purpose. EP content of NRL was found to decrease with PEG treatment and reduction increased with increase in the molecular weight of PEG. With PEG treatment there is 35% reduction in the EP content without any compromise on the mechanical properties of the latex; however, thermal stability of low-protein latex was found to be reduced. 106 Tanaka and coworkers have effectively eliminated proteins from NRL employing a proteolytic enzyme/ surfactant combination. The former breaks down the proteins linkages selectively and the latter washes out resulting oligopeptides. Results of nitrogen contents and FTIR of DPNR in the form of dried film indicated the presence of remaining proteins bound to the DPNR particles.103

50 

The enzyme treatment is very effective in reducing antigenic proteins in NRL. While this technology adds marginally to the production cost of standard grades of NRL, it is still quite cost-effective when compared with postwashing NRL products or the use of synthetic latex. Moreover, enzyme-treated NRL maintains the excellent physical properties and performance of NRL. The enzyme deproteinization was found to be less efficient when the rubber particles had been previously irradiated. This might be due to the change of proteins structure when subjected to γ-ray which resulted in the decrease of enzyme activity.The evidences obtained from the morphology study of DPNR latex particles using TEM support this. Here also phase transfer/bulk polymerization/TEM technique was used for morphology study. The enzyme alcalase/SDS system was used for deproteinisation. Two types of crosslinked DPNR latex particles, i.e., γ-radiation vulcanized-deproteinized NR (RVDPNR), obtained from enzymatic deproteinization of the γ-radiation vulcanized natural rubber (RVNR) latex, and deproteinized-γ-radiation vulcanized NR (DPRVNR), prepared by irradiation of the DPNR latex with γ-ray, are titrated with an aqueous solution of cationic surfactant (BHAC) in the presence of styrene. At the critical transfer concentration (CTC), a hydrophobic layer around the latex particles forms and the rubber particles transfer into the styrene monomer. The styrene containing transferred rubber particles is then polymerized in bulk and the rubber particles embedding in polystyrene (PS) matrix can be sectioned for the TEM study. By using this method, the air, freeze or chemical drying step of rubber latex normally required before the embedding step for specimen preparation is omitted and, hence, the disturbance of actual DPNR particle structure is minimized.103 The CTC values of the deproteinized latices, i.e., RV-DPNR and DPRVNR were approximately twice the value of RVNR latex. This could be due to an increase in the amount of negative charges on the surface of the DPNR particles resulted from the added anionic surfactant (SDS) during the deproteinization process. Consequently, the high quantity of the added cationic 51 

surfactant (BHAC) for neutralization of the rubber particles was required. Although, the CTC values of both deproteinized latices were similar, it was observed that the CTC of RV-DPNR latex was slightly higher than that of DPRVNR. Therefore, it was reasonable to assume that the phase transfer technique could be employed to indicate the presence of negative charges, partly derived from residual proteins, on the surface of DPNR latices. Deproteinization of the latex before irradiation (DP-RVNR) removed a large proportion of the proteins and therefore the membrane might be partly destroyed during deproteinization. The degree of crosslinking of γ-radiation vulcanized NRL was found to decrease when the proteins had been previously removed by using proteolytic enzyme.103 The nitrogen content of both deproteinized NR lattices (RV-DPNR and DP-RVNR) was lower than that of concentrated NRL because some of proteins in the former latices were eliminated by using the enzyme alcalase/SDS system. This deproteinization technique was very effective in removal of more than half of the original proteins. It was reported that the nitrogen content of rubber was reduced from 0.3% to 0.01% by using this technique. The proteins bound to NR particles were not completely removed. It is of interest to note that the nitrogen content of the NRL which was vulcanized before deproteinization; RV-DPNR, were greater than those of DP-RVNR.103 DPNR latex contains very low nonrubbers, especially protein (<0.02 wt% nitrogen content). The faster rate of flattening of DPNR films observed in AFM study is attributed to the very low non-rubber content of these films, which poses minimal hindrance to deformation of the articles. The loss of naturally occurring antioxidants during deproteinization reduces the oxidative resistance of the DPNR latex films. 14 The treatment of latex with urea and a polar organic solvent in the presence of surfactant also used to produce protein free NRL. Acetone and anionic surfactant were found to be effective. The combination of urea and acetone in the presence of an anionic surfactant such as SDS is found to be effective for the removal of proteins. 52 

Urea is well-known to change only

conformation of the proteins but not cleave any chemical linkages, the removal of almost all proteins from NRL with urea may suggest that most proteins present in NR are attached just on the surface of the particles with physical interaction.219,220 Acetone was the most effective solvent for the removal of the proteins compared with ethanol and 2-propanol i.e., the total nitrogen content decreased dramatically from 0.053-0.026 w/w% when 2.5 w/w% of acetone is added into the latex. This may be explained to be due to the effect of acetone on the precipitation of the proteins in the aqueous media.13

Figure 1.23: A schematic representation of the proposed mechanism by (a) enzymatic treatment and (b) surfactant washing or physical treatment. Recently, it was reported that the addition of calcium chloride solution and sodium dodecyl sulphate into concentrated NRL effectively removes the proteins from the NRL. The CaCl2 was obtained indirectly from egg shells. The pyrolysis of egg shells from chickens, ducks and birds at 9000C for 2 hours transforms the crystal structure of CaCO3 into CaO with a high purity, creates a 53 

small and uniform particle size, a suitable specific surface area and an average pore diameter that is suitable for reacting with hydrochloric acid to obtain CaCl2.206 1.9 Protein Removal by Product Treatment Certain allergic reactions are caused by proteins leached from the surface of latex products. Recently there have been reports that NRL gloves and other surgical aids can cause hyper sensitivity reactions. Gloves made from NRL contain mainly proteins tightly bound to the rubber particles, having molecular weight of 14 and 24 kDa and the serum derived proteins of molecular weights 14, 24, 29, 36, 45 and 100 kDa. Even after various processing operations during the manufacture, these proteins are found to remain with the glove. The proteins strongly adhering to the rubber particles may not get removed easily. The serum proteins give rise to the bulk of EP.201 1.9.1 Leaching of Gloves Leaching is the process of removal of hydrophilic materials from the latex dipped products by washing them in water.14 The removal of excess water soluble non-rubbers such as proteins, Ca(NO3)2 and other added compounding ingredients results in the improvement of physical properties such as tensile strength, film clarity, prevention of surface blooms and reduction in water absorption

and electrical conductivity of latex dipped products. The

effectiveness of the leaching process is critical in the determination of the overall quality of gloves produced. 12, 200,201 There are two types of leaching processes: wet gel leaching and dry film leaching. The wet gel leaching involves the washing of the wet gel i.e., gelled deposit on former, prior to drying and vulcanization. It is usually carried out online. Dry film leaching consists of washing the dried vulcanized latex product after removal from the former and is an off line process. When complete removal of hydrophilic material is required dry film leaching for an extended period of 16-48 hours depending on the type of product is made. A substantial 54 

amount of water soluble protein is generated upon drying and vulcanization of dipped products and those proteins are drawn towards the surface away from the former during this stage giving rise to asymmetry of EP distribution. Any form of leaching or washing including the slurry dip after drying is therefore expected for further removal of the EP. The most efficient way of reducing water-soluble components in NRL gloves is to perform both wet-gel and dry-film leaching. In either leaching process, it is essential to maintain the cleanliness of the leach water. This is usually accomplished by adding and removing some of the leach water continuously (water turnover of at least 1–2 gal per minute). In addition to water turnover, effective leaching is improved when the ratio of leach water volume to total weight of rubber gloves increases. Poor water turnover and/or low volume of unclean leach water can increase the amount of EP in gloves produced. Hot and cold water leaching is now used extensively within the dipping industry.

12

The efficiency of leaching may be determined by several parameters. These include the leaching time, the leach water temperature, and the rate of leach water turnover.12 Although water leaching is an effective means of reducing the EP content of gloves, the results can vary widely depending on how the latex was compounded. The influence of glove thickness and the processing conditions have been studied by researchers. Their findings indicate the following: • There is a need for the protein to migrate to the glove surface to facilitate extraction. • The thinner the glove, the more effective is protein removal. • Gloves produced from latex with lower rubber content (40%) had a higher level of EPs than gloves generated from latex with higher rubber content (60%). For NRL gloves, wet gel leaching, i.e. leaching before curing the rubber, was found to be less effective than dry film leaching. This was thought to be due 55 

to the protein having insufficient time to migrate to the film surface during the manufacturing process. Wet gel leaching is important for removing salts, peptides, soaps, and surfactants from the rubber so that a continuous rubber film with good barrier properties is formed, but it is not effective in leaching out larger proteins that are sequestered within the hydrated coagulated film. The effect of brief wet gel leaching on EPs is strongly influenced by the thickness of the latex film; the effect could be either positive or negative depending on the film thickness.200 Irrespective of film thickness or latex DRC dry film leaching was found to be far more effective than wet gel leaching for the removal of soluble proteins. Only proteins that had migrated to the surface of the film could be extracted with water rapidly. Migration of soluble proteins to the surface had not yet taken place to an appreciable extent when the latex film was in the wet-gel state. When the wet-gel leached film was completely dried by prolonged heating after the leaching step, more proteins migrated and appeared at the surface from where they are easily extractable. To remove soluble proteins efficiently, therefore the wet–gel leach should be supplemented with a dry film leach since most of the proteins removed are already at the film surface. 200 The migration of soluble proteins to the surface of the latex film as it dries has important implications in the manufacture of dipped latex products. As most of the soluble proteins are concentrated on the surface of the film away from the shaping former, a large proportion of the EPs can be extracted from the product even by washing only one surface of the film on-line (while the film is still on the shaping former). Although the EPs from the surface in contact with the shaping former are left untreated by this approach, the amount of proteins extractable from this surface is very low relatively. Other treatments to reduce EPs (chlorination, dipping with a non-latex coating etc.) can similarly be carried out on-line with one film surface receiving the treatment. The results from an on-line treatment of the latex film might not be as effective in reducing EPs when compared with a thorough off-line operation, but the advantage of the 56 

latter should be gauged by the benefit gained in the context of the allergy response. Once surface proteins have been removed by dry-film leaching, further drying of the washed film before storage or packing does not cause more proteins to migrate to the surface. Indeed, this drying step appears to lock in whatever residual soluble proteins that still remained. 200 The ammonia solution, sodium lauryl sulphate (SLS) solution and water-methanol mixture are reported to be effective in removing proteins from rubber products.201 KOH solution and acid can also be used for the leaching of NRL films. The crosslink density and EP content of the films reduced with increasing pH of the KOH solution.221,222 The better EP removal during the treatment of glove samples with ammonia solution could be attributed to the hydrolysis of proteins into fragments which are removed during leaching. SLS is used for the solubilisation of major proteins in various membranes. Proteins can be denatured to individual polypeptides by treatment with this detergent by breaking down the S-S bond to S-H bonds. SLS gets attached to the polypeptides and makes them more soluble. The better leaching effect of SLS solution may be due to the interaction of the SLS micelles with the rubber particle proteins in the latex glove to form a soluble protein-lipid-detergent complex. Thus the rubber particle proteins will also be leached out during the treatment. Methanol being an organic solvent also has some solubilizing effect on the proteins, though not as effective as SLS or ammonia.201 The use of ultrasonics in the leaching system can accelerate the removal of latex protein over leaching on its own. The effect seems to be more pronounced in pre-vulcanised films than in post-vulcanised films and there is apparently no detrimental effect on tensile properties.

57 

1.9.2 Post Washing and Chlorination of Gloves The washing and surface treatment of NRL gloves with chlorine ions is an effective means of reducing the EP content on the glove surface. Chlorination leads to significant changes on the surface structure. Chlorination performed in a low-pH aqueous leach causes the breaking of bonds in the rubber polymer and a hardening of the film surface. As a result of this surface modification, the rubber film becomes more slippery and the coefficient of friction is much reduced. Although this process is commonly used to improve the donnability of gloves, it can result in decreased grip on the outside of the glove for the wearer. Moreover, the chlorination process can be difficult to control, and result in small fissures in the rubber film that can compromise shelf-life and barrier integrity of the glove. In comparison, extensive postwashing in water alone can be almost as effective as chlorination, but without the adverse side effects of chlorine on film physical properties.12 Various reactions are possible during chlorination. In aqueous solution, chlorine released from sodium hypochlorite by conc.HCl reacts with NR via ionic mechanism to form carbonium ion which crosslinks with each other and undergoes cyclisation. Thus NR surface can act as a barrier for migration of proteins. Denaturation of proteins may also be possible by chlorination which makes them insoluble. The after treatments, neutralization with ammonia solution and subsequent washing, will also enhance the removal of EP.201 There could be crack formation on the surface of the gloves at higher doages or longer durations of chlorination. This is reflected in the physical properties. Tensile strength and ageing resistance are reduced in the case of highly chlorinated samples.12 The use of fumed silica, as little as 1-2 pphr may significantly reduce EP from latex gloves without adversely affecting the dipping process.61 Steam autoclaving can also be used for protein reduction of latex gloves but it will affect physical properties unless precautionary measures are taken at the compounding stage.106 58 

1.10 Importance of Low Temperature Vulcanization Vulcanisation temperature is very important in determining the quality of the rubber products. Optimum properties are obtained when curing is done at the lowest possible temperature. Low temperature vulcanisation results in products of good quality and appearance. The modulus developed decreases as the vulcanization temperature is increased. High temperature vulcanization has the following disadvantages (1) high energy consumption (2) leads to the degradation of rubber (3) less safety (4) high insulation costs (5) less flexibility in designing the compound for each component at high temperature (6) inconvenience in the curing room (7) chances of over vulcanization.226-235 Room temperature prevulcanisation of NRL has the following advantages. (1) It does not affect the colloidal stability of the latex. (2) Vulcanisation is the most energy requiring step in latex technology. If it can be done at room temperature, we can save a large amount of energy. Energy saving decreases pollution and fuel consumption, thus enhancing the environment and of course saving money. (3) It need less sophisticated equipments and thus simplify the process. (4) We can use less thermal stable reinforcements and additives. (5) We can eliminate the chance of premature vulcanization and scorching. 1.11 Objective of the Present Work The primary objective of the present work is to develop a novel accelerator system for the prevulcanisation of NRL at temperature lower than that employed conventionally. At present, sulphur prevulcanisation of NRL is done industrially by heating the compounded latex at 600C for 2-3 hours. This will affect the colloidal stability of the latex and also the physical properties of the films dried down from the latex. Lowering the prevulcanisation temperature has a great influence on improving the quality of the dipped rubber products, since rubber undergoes lesser degradation at lower temperature.

59 

The nature of accelerator used for crosslinking is an important factor affecting the temperature of sulphur prevulcanisation. Xanthates and dithiocarbamates are two important ultra fast accelerators used in rubber industry. In latex technology, the use of mixed accelerator systems for prevulcanisation has received much attention since such systems usually exhibit synergism. In this study, different xanthates in combination with zinc diethyl dithiocarbamate (ZDC) were used for the low temperature prevulcanisation of NRL. Xanthates in the form of sodium, potassium, and zinc salts were used for the purpose. Prevulcanisation of NRL can be effectively carried out at room temperature and at 400C using xanthate/ZDC accelerator combination without affecting the colloidal stability of the latex. Due to the world wide spread of epidemic diseases such as AIDS, hepatitis B and influenza (H1N1), it becomes increasingly urgent to develop high performance NRL gloves. The present accelerator system can be used for the production of examination gloves and toy balloons with improved properties at low temperature.2 The study can be outlined under the following heads (1) Preparation of following xanthates (i)

Zinc butyl xanthate, Zn(bxt)2

(ii)

Zinc isopropyl xanthate, Zn(ipxt)2

(iii)

Sodium butyl xanthate, Nabxt

(iv)

Potassium butyl xanthate, Kbxt

(v)

Potassium isoamyl xanthate, Kiaxt

(2) Characterisation of xanthate using different techniques (i)

FTIR spectroscopy

(ii)

1

H-NMR spectroscopy

(iii)

TG/DTA

(3) Detailed study of zinc butyl xanthate ™ Optimization of preparation method ™

Cytotoxicity study 60 

(4) Low temperature prevulcanisation of NRL using Zn(bxt)2/ZDC accelerator system ¾ Optimization of accelerator concentration ¾ Optimization of prevulcanisation time ¾ Effect of storage on colloidal properties of prevulcanised latex ¾ Thermal ageing study ¾ Comparison

with

conventional

high

temperature

prevulcanisation system (5) Low temperature prevulcanisation of NRL using Nabxt/ZDC and Kbxt/ZDC accelerator system

(6) Low

•

Optimization of prevulcanisation time

•

Comparison of the accelerating properties of Nabxt and Kbxt

•

Thermal ageing study

•

Surface morphology study using AFM

temperature

prevulcanisation

of NRL

using

Kiaxt/ZDC

accelerator system ™ Optimization of prevulcanisation time ™ Thermal ageing study (7) Studies on the effective removal of extractable proteins from prevulcanised NRL film by leaching procedure ¾ Use of different media for effective protein removal ¾ Effect of protein removal on mechanical properties ¾ Comparison with postvulcanised NRL film (8) Production of examination gloves and toy balloons at low temperature ™ Optimization of cure time ™ Evaluation of mechanical properties

! 61