Adhesion Promotions

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C TECHNOLOGY OVER STORY

Adhesion Promotion by Surface Modification of Polymer

Introduction Polyolefin’s constitute the most often surface treated materials. The good chemical resistance and the non-polar nature of the polyolefin’s surface prevent a good adhesion of the printing ink as well as labels. Amongst polyolefin’s polyethylene has the lowest surface free energy of 31 dynes/cm @20°C. Materials with surface energy below 33 dynes/ cm require pre-treatment and above 36-38 dynes/cm may usually be directly printed. Use of corona, flame and other methods increases the surface energy level in excess of 42 dynes/ cm. Ideally, the surface energy of the plastic should be 7 to 10 dynes/cm higher than the surface tension of the solvent or liquid. For example, a printing ink having a surface tension of 30 dynes/cm would not wet to a material having a surface energy less than 37-39 dynes/cm. Hence polyolefin’s are frequently subjected to surface treatment to improve their bonding characteristics.

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One important reason is to produce a greater range of multicolor decoration where single color moulding is more economic. Printing of plastics offers greater scope to the designer in selective decoration, as

Product Manager Bloom Packaging Pvt. Ltd., Mumbai

The most often used methods are solvent cleaning and etching, corona discharge for films, flame treatments for moulded articles, plasma treatment and UV treatment method.

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Need for Printing on Plastics

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Bhupendra Singh

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more then a single color may be used. Printing can be used to alter the surface appearance giving a less plastic image by reducing the gloss. Certain decorative effects can be conferred which are difficult to produce by moulding in colours, such as polychromatic, wood grained and pearl finish. Light colored finishes are obtained from dark colored resin such as phenolics. Plastics are painted for covering the defects such as flow lines, glass fibers in reinforced plastics. Self-coloured mouldings are more expensive if a range of colors is necessary. Maintaining the continuity in the color and low gloss is difficult. It also permits the use of mixed residues of regenerated plastics of various colors. It adds eye appeal to the articles. Printing reduces the electrostatic charge and hence dusts attraction on the surface. THE ECONOMIC TIMES POLYMERS

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Ultra violet resistance of the plastics can be improved for exterior applications.

Methods for Surface Modifications Various methods are used for the surface modification such as • Solvent cleaning and etching • Mechanical abrasion • Chemical etching • Additives • Flame treatment • Corona discharge • Plasma treatment • Ultra violet irradiation The most often used methods are solvent cleaning and etching, corona discharge for films, flame treatments for moulded articles, plasma treatment and UV treatment method.

Solvent Cleaning and Etching Polyolefin's constitute the most often surface treated materials. The good chemical resistance and the non-polar nature of the polyolefin's surface prevent a good adhesion of the printing ink as well as labels.

Solvent cleaning and etching is one of the most common techniques of surface treatment. The surface of plastic is normally contaminated with grease and dust which comes from various sources such as mold release agent, rust preventives, lubricants, dust etc. It is because of these foreign matters, adhesion of paint, ink or adhesive to the plastic surface becomes very difficult. To improve the adhesion of the paint, ink or adhesive, the surface of the substrate is cleaned using solvents such as isopropyl alcohol (IPA), acetone, methanol, hexane, ethyl acetate, toluene and other organic solvents. During solvent etching some surface change also takes place along with removal of contaminants. Solvent swells the amorphous and low molecular weight impurities and portions of polyolefin surfaces, making it susceptible to the penetration of ink or coating. LDPE, HDPE and PP surfaces can be etched by immersing it in hot solvent at 80°C. Limitations of Solvent Cleaning Method • Many organic solvents are toxic • Most of the solvents are flammable • Some organic solvents can dissolve the plastic

groups at the polymer surface. A corona discharge system consists of a generator, transformer and treater. The generator takes in low voltage at lower frequency of 50 Hz and amplifies the frequency to higher value of 25-30 kHz. Transformer increases the voltage to the required voltage in the range of 15000-30000V. The treater consists of electrode, dielectric cover and grounded metal body. The electrode in the form of a solid bar or segmented electrode is used. Segmented electrode minimises the sparking. A schematic diagram of a segmented ceramic electrode corona treater is shown in Fig.1. Mechanism Since the voltage used is A.C., the electrodes become alternatively positive and negative with respect to each other. The electrons and protons start moving and changing directions. Electrons, protons, excited atoms and ions in the form of corona discharge break C-C and C-H bonds on the plastic surface forming radicals. These radicals react with oxygen and nitrogen in air and forms polar groups on the plastic surface. Many factors play an important role in effective corona discharge treatment such as power supply, frequency, air gap, dielectric material & discharge electrode configuration. Corona discharge treatment lowers the heat sealing temperature of many polymers. Polyethylene treated by corona discharge method can be heat-sealed at temperatures as low as 75°C; while treated polyethylene terphthalate film can be heat sealed at 140°C. Problems Encountered with Corona Discharge Treatment • Greater level of treatment increases the blocking tendency of the film. • Contact of Corona discharge treated film and metal should be avoided as it erases the treatment on the surface. • Additives such as slip additives and other processing aids reduces the treatment over time as these additives migrate to

Surface Treatment by Corona Discharge Corona discharge treatment is the most widely used method for improving the adhesion properties of the plastic films such as polyolefin films prior to lamination. This treatment is used for various polymers such as polyolefins, polyfluorocarbons, polyesters, polyvinylchloride, silicone, nylon and others. Corona discharge treatment incorporates various functional groups such as carbonyl 38

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Fig. 1: A schematic diagram of a segmented ceramic electrode corona discharge treater

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mer such as polyethylene, unless the branches are sufficiently bulky to sterically hinder oxidation. If the polymer contains both aliphatic and aromatic moieties; only the aliphatic portion is oxidised.

Fig. 2: Corona treatment system for film blowing plant

the surface and partially mask the polar groups formed during the treatment. Hence the corona discharge treatment should be carried out just before the printing operations. The corona treatment system is introduced into the film blowing equipment usually at the top of the tower as shown in the Fig.2. Some systems incorporate the treatment midway up the tower or at the base.

Flame Treatment of Moulded Article Light surface oxidation by flame treatment of moulded article with a non-luminous flame. The surface is first thoroughly cleaned to remove dirt and grease. The apparatus consists of the one or more flames, which are held at a fixed distance from the sample and scanned over it at controlled speed. To treat the large areas or complicated shapes special multiple or robotized systems have been in use. Flame Treatment Apparatus for Polymer Films A schematic representation for the flame treatment of polymer film is as shown in the Fig.3

Flame Treatment Flame treatment is widely used for surface modification of polyolefin surfaces mainly to improve printability or paintability. Basically, the process consists of applying heat to the outer surface of polyolefin’s. The surface should be hot enough to suffer modification while the body remains at much lower temperature. Sheeting’s below a thickness of 0.6 mm is usually treated by corona discharge while flame treatment is used for heavier sheets. Heat can be applied by means of: • Hot air • Infra red radiation • Flaming The flaming techniques are very popular and used extensively for treating blow moldings prior to printing, labeling, or the application of other decorative matter. Mechanism Flame contains excited species of O, NO, OH, and NH, which can remove hydrogen from the surface. The oxidation that follows is propagated by a free radical mechanism. Equations for the reactions, which can occur during simple thermal oxidation of hydrocarbon takes, place in three steps: Initiation, Propagation and Termination. Surface of the polymer exposed to simple thermal oxidation greatly affects ease of oxidation. Highly branched polymers oxidise at a much more rapid rate than a linear poly-

Fig.3: Diagram of a flame treater for the polymer films

This equipment resembles the corona treatment apparatus in that the central drum is used as the base for the plastic sheeting. The plastic goes over the drum and under a series of burners similar to those found in a gas furnace. Factors Affecting the Correct Flaming Treatment • A good non-luminous fishtail flame should be used. The use of oxygen instead of air tends to make the treatment more effective. • The surface to be treated should be located on the side of the blue part of the flame so that the flame impinges on the surface well above the blue portion. Inner core length should be around 0.25” to 0.75” while the distance from inner core tip to the surface about 0.25”. • Flame contact time: Flame contact time is generally a fraction of second but depends on: • Flame intensity THE ECONOMIC TIMES POLYMERS

Plasma treatment is a very efficient method for modifying plastic surfaces. Polyethylene surfaces can be effectively treated by inert gas plasma generated by a radio frequency field.

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Air to gas ratio Wall thickness The design of the part The type of flame used and The distance of the burner from the article for flame treatment The gas is burned using 10-15% excess air over the stoichiometric ratio, in order to obtain an oxidizing flame with a temperature of 1090-2760ºC.

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Corona discharge treatment is the most widely used method for improving the adhesion properties of

Benefits of the Flame Treatment Higher treatment levels Low treatment decay rates Suitable for various product shapes and sizes

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polyfluorocarbons, polyesters,

Limitations of the Flame Treatment Flame treatment is not suitable for moulded three dimensional objects where the distance between the flame and the substrate can vary. • Sheets having thickness less than 0.6mm are not suitable for flame treatment. Also treatment cannot be used for films due to high shrinkage in films. • Melting occurs if the flame is too close. • Open flame, particularly in or near a painting operation is usually forbidden for safety reasons.

polyvinylchloride, silicone, nylon and

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the plastic films such as polyolefin films prior to lamination. This treatment is used for various polymers such as polyolefins,

others.

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Applications Flame treatment is most often used to improve ink adhesion to molded PE and PP surfaces. • Flame treatment is also used for acetal, acetal copolymers and for polyethylene terephthalate. • Round containers can be treated on all sides by rotating the container by dropping it through a ring burner.

species which are used to activate many chemical reactions. A typical example of UV action on polymer surfaces is their degradation by the sun exposure. In most cases, modification is carried out by • Introducing functional groups to the material surface by • Applying UV light to oxidize the material surface or • By allowing the material to contact a gas or sensitizer to cause a photochemical reaction. • Allowing UV irradiation graft polymerization to occur at the material surface. UV lamps are widely used for the treatment of polymer surfaces and the apparatus involves essentially a lamp and sample illumination device, which makes selective irradiation of substrate possible. UV irradiation system consists of a UV source, the Coating and the Substrate as shown in Fig.4.

Fig.4: Schematic of UV treatment

Lamps are operated between 250-370 nm wavelengths. Effectiveness of UV treatment The effectiveness of UV treatment depends on: • The intensity of UV source • The distance between the UV source and sample • The coating of benzophenone • The time of exposure

Modification by Ultraviolet Irradiation (UV) UV irradiation produces surface modification, which improves the wettability and bondability of the plastics. Photons, usually those with low wavelength, are energetic

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Advantages UV irradiation treatment has various advantages • Reaction occurs at ordinary temperature and pressure. • Selective reaction is possible. • Light energy can be focused on the surface of the material. UV treatment shows improvement in adhesion but the time taken for the treatment is high hence for commercial purpose this treatment is not feasible.

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Plasma Treatment Plasma treatment is a very efficient method for modifying plastic surfaces. Polyethylene surfaces can be effectively treated by inert gas plasma generated by a radio frequency field. The utilization of plasma is a typical dry method. Application of Plasma Treatment Plasma treatment has developed into a commercial important method for difficult to treat surfaces. It is used in the industry to improve the painting performance of plastic motor vehicle bumpers and the printability of films. Difference between Plasma Treatment,Corona Discharge Treatment and Flame Treatment Plasma differs from corona and flame treatment in that plasma treaters are operated at less than atmospheric pressure. This difference is the primary reason for it’s seldom use in industry for adhesion promotions. In addition a system under a partial vacuum inherently requires a batch process which also lowers cost effectiveness. Instrumentation A simplified diagram of a plasma treater is as shown in Fig.5. The material to be treated is placed in a vessel that is evacuated. The pressure in the

tion. Only a small number of electrons present in the gas have high energies. The energy of electrons present in low temperature plasma is in the range of 1 to 10 eV which causes the ionization and excitation in the molecule of gas. The activated particles react with the polymeric material so that polymeric radicals are produced on the surface layer of the material. The polymeric radicals eventually cause the surface layer to be oxidised, crosslinked or decomposed. It should be noted that the effects of the plasma reactor depends to a great extent on the type of plasma reactor, the oscillation frequency, the high frequency output, the gas type the flow rate and pressure of the gases, the treatment time, the position of the sample and so on. • Crosslinking: It has been reported that when a polymeric material is plasma treated in an inert gas such as helium or argon; cross linkage are introduced into the surface layer of the material. • Oxidation and Decomposition of Polymer Surfaces: When polymers are plasma treated under relatively mild conditions, oxidation reactions generally occur. As a result oxygen atoms can be introduced into polymeric surfaces. Studies indicate that enhancing wettability by plasma treatment results in high bonding strengths.

Bibliography

Fig.5: Plasma reactor

vessel is increased by the addition of a gas in which the plasma is to be struck. The most commonly used gases in plasma treatment for adhesion promotion is noble gases such as argon and xenon. Alternately nitrogen, oxygen and fluorinated gases can also be used. After the addition of active gas, a radio or microwave frequency signal is placed on the coil surrounding the vessel. The coil induces plasma inside the vessel. Plasma Reaction of Polymers In low temperature plasma for polymer treatment, relatively few electrons and ions are present in the gas. This is different from the high temperature plasma of the sun and in nuclear fusion. Hence the temperature of the whole low temperature plasma gas is so low that the degree of heat resistance of the material being treated requires little atten-

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UV irradiation produces surface modification, which improves the wettability and bondability of the plastics. Photons, usually those with low wavelength, are energetic species which are used to activate many chemical reactions.

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