Polystyrene Used In Lighting Fixtures

Polystyrene Used in Lighting Fixtures

INTRODUCTION Several years ago, the use of plastic in the lighting is beyond the imagination. But now a days it is quite possible. There are number of plastics both thermoplastic and thermosets used in lighting fixtures. Although this field is ruled by the acrylics, polystyrene is steadily gaining ground as an excellent means of light control, and through its excellent electrical and optical properties. In 1930, IG Farben, installed a plant for producing 100 tonnes of polystyrene per annum.

The consumption of polystyrene in lighting fixtures in

USA was 31 million pounds which shows that the growing trend in using polystyrene for light control. In India, where there is now a growing emphasis on scientific lighting, polystyrene offers additional advantages of indigenous manufacture and hence low cost over the imported acrylics. The purpose of delivering this seminar to show the way towards effective use of polystyrene and its advantages in lighting fixtures.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

STRUCTURE OF PS Chemically PS is a homopolymer of styrene, with a predominantly unbranched chain.

~ CH – CH2 – CH – CH2 ~

|

|

PREPARATION METHODS OF PS : i) Mass polymerisation.(German tower method) ii) Solution polymerisation. (Dow process) iii) Suspension polymerisation. iv) Emulsion polymerisation.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

PROPERTIES OF PS i) Good electrical insulation properties. ii) Good optical properties. iii) Reasonable chemical resistance. iv) Good mouldability. v) Low cost. vi) Hard, rigid transparent thermoplastic which Emits a characteristic metallic ring when dropped. vii) Low moisture absorption. viii) Colourability.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

GRADES OF PS USED IN ILLUMINATION The recommendations for light characteristics serve to identify and to characterize thermoplastic polystyrene compounds for use, particularity in the manufacture of fluorescent light diffusing luminaries.

Factors of fabricated

luminaire design and performance and considered beyond the scope of the recommendation. These recommendation covers the following three types of polystyrene compounds : Class A :

Unmodified polystyrene with or without addition of diffusing the pigments and small amount of lubricants.

Class B :

Essentially, class A material characterized by improved heat distortion properties.

Class C :

Essentially, polystyrene with or without the addition of diffusing pigments and incorporating additives to inhibit discoloration resulting from fluorescent light radiation.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

YELLOWING OF POLYSTYRENE & IT’S CAUSES The yellowing or discoloration is the disadvantages of polystyrene when it exposed to UV rays and fluorescent lamp radiation are mainly influenced by the following. i) Presence of impurities, such as short chain molecules, sulphur, lubricants etc. ii) Oxygen iii) Exposure to UV radiation. i) Presence of impurities : The short chain molecules are produced due to the thermal degradation of polystyrene during fabrication process. These molecules or duration products are distinctly yellow in colour and their polystyrene product. Sulphur is used as a distillation inhibitor in the manufacture of styrene monomer and it is non present to extent of 0.0015 %. Experiments carried out on polymerisation of styrene containing known amount of sulphur have indicated that there is a linear relationship between the degree of yellowness and the sulphur content. By improved distillation techniques, it is now possible to manufacture styrene not containing free sulphur. Through combined sulphur may be present in the range of 0.003 – 0.006 %, it is only half harmful as free sulphur in causing discolouration.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

Colourless internal lubricant do not contribute to any yellowness, but the external lubricant degrade to yellow rapidly when exposed to high temperatures or to ultra-violet radiation. ii) Oxygen : Oxygen is one of the most active influences in causing the yellowing of polystyrene. Oxygen is just as important to yellowing of polystyrene as it is to rushing of iron. The effect of oxygen on polystyrene is catalysed by light radiation. The precise role of oxygen and light radiation have been determined in a simple but effective test. Identical samples of polystyrene were sealed in pyrex glass tubes containing different gaseous atmospheres and exposed to sunlight. Samples in test tubes containing nitrogen and other inert gases. Showed negligible discolouration even after 6 year’s exposure. In light, with oxygen or air in the tubes, pronounced yellowing occure. Samples stored in the dark did not yellow even though oxygen was present. Since only samples exposed to oxygen and light yellowed, it is easy to see the contribution of light and oxygen to the yellowing process. iii) Exposure to ultra-violet radiation : This exposure causes the most severe yellowing of polystyrene when exposed to sunlight and to the fluorescent lamps. In sunlight, as it reaches the earth on a clear bright day there is a continuous spectrum of wave lengths ranging from 300-500 mµ.. This spectrum can be divided into three regions. C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

i) Visible light rang : Located in the central part of the shown spectrum and includes that portion of sunlight visible to eye as white light, extends form 400-600 m ii) Infra-red radiation range: Located to the right of the above range and includes the wavelengths longer than 700 m . iii) Ultra-violet radiation range: Located to the left of the visible light range and covers the shorter wavelengths. Although only 5% of the total sunlight at the earth's surface falls in to the category of the U.V. radiation, it is the only portion subject because this is the range of radiation that is emitted by standard fluorescent lamps and is capable of severe yellowing of polystyrene on exposure to it.

Although the fluorescent lamps emit

relatively small amount of U.V. radiation as compared with sunlight it is sufficient high to cause yellowing of polystyrene on prolonged exposure. Light is a radiant energy and the intensity of its energy varies inversely with its wavelength. The shorter wavelengths, i.e. the ultraviolet, have the highest energy to the extent of 82 K calsmole at 350 m which is more than the energy required to break a bond between atoms in a molecule which generally ranges from 50 – 100 K cal/mole. Thus, when polystyrene is exposed to this radiation, as in case of outdoor and fluorescent lamps exposure, it is partly or completely absorbed by the atoms. This absorption of energy activates them to a stage sufficient to cause rupture of bonds holding these atoms. The result is degradation of polystyrene and the subsequent yellowing.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

LIGHT - STABILISED POLYSTYRENE Knowing that the polystyrene yellows on exposure to sunlight and fluorescent lamps and also the causes of yellowing, it will be worthwhile to know how polystyrene is modified to withstand the degradation action of the ultra-violet radiation and oxygen. This special formulation of polystyrene is called the "Light stabilised polystyrene". It is a special formulation of the general purpose polystyrene so modified in composition that it can resist the action of oxygen and light in causing yellowing of polystyrene. As briefed earlier, oxygen-attack catalyst by light is the most critical influence in causing degradation and subsequent yellowing of polystyrene. Obviously, to keep polystyrene from yellowing in the presence of light, something has to be done to keep oxygen away from the polystyrene molecules. To do this, antioxidants and ultra-violet absorbers are incorporated in polystyrene formulation. This formulation is known as the "Light-stabilised polystyrene". How does this formulation resist yellowing? The antioxidants combine with free oxygen and then oxygen is no longer available to degrade polystyrene. That eliminates oxygen.

The ultra-violet radiation causes degradation through

photochemical reaction. Hence to prevent radiation, this photochemical reaction should be postponed effectively or even prevented entirely. This is precisely what U.V. absorbers do. These absorbers prevent the reaction and protect polystyrene by C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

preferentially absorbing the destructive high-energy U.V. radiation and re-emitting this energy at non-destructive wavelengths before the polystyrene atoms can absorb these radiation and re-emitting this energy at non-destructive wavelengths before the polystyrene atoms can absorb these radiation.

PREPARATION OF LIGHT STABILISED POLYSTYRENE Light - stabilised PS is prepare by using additives such as antioxidants, U.V.absorbers, flame retardants, antistats by means of compounding. The process incorporation of ingradiants such as plasticisers, vulcanising agents, stabilisers, fillers, colouring agents, flame retardants and lubricants in to the virgin resin is known as " compounding ". Compounding ingradients in the form of fine powder are blended with fine powder of PS using V-blenders, ribbon blenders etc. Blending can be done at ambient temperatures which should, however, normally be much below the softing temperature of the polystyrene. ANTIOXIDANTS :Antioxidants function by blocking chain reactive oxidative degradation mechanisms. Polymers not protected by antioxidants are subjected to oxidative attack that may shorten the polymer's life due to discoloration, cracking, brittleness and loss of mechanical properties. The scavenging free radical mechanism is widely accepted as the means of whereby polymeric materials are protected by antioxidants. The proposed free radical mechanism involving the antioxidant Irganox 1076. Hydrocarbon molecule break down in to the two radicals. C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

R-R

2 R*

Where, Hydrocarbon radicals (R*) rapidly combines with oxygen to form a peroxide radical. R*+ O2

Fast

R*O2

The propagation reaction. OH | R* O2+ C(CH3)3

C(CH3)3 O

| || CH2CH2 COC18C37 Irganox 1076 O* | RO2H + C(CH3)3 C(CH3)3 O

C(CH3)3

| || CH2CH2 COC18C37 O || C(CH3)3 O

| || CH2CH2 COC18C37 Where, R* Polymer chain. Other antioxidants are, Trade name

- AO 68

Chemical name - Tris ( 2, 4 - di - tert - butylphenyl ) phosphite C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

Chemical structure C( CH3)3 | O

( CH3)3 C

P 3

Antioxidants are effective in concentration of 0.01 to 0.05%

ULTRA-VIOLET ABSORBERS : Every plastics degrades in sunlight in number of ways, the most common being discoloration & loss of physical properties. When a molecule (A) absorbs a quantum of light it is activated to an electronically excited state A*, after which a number of process may occur. These may be summarised as follows : 1) Photophysical process a) Emission of energy

( e.g. fluorescence ) A*

Ao + Energy emitted.

A*

Ao + Heat

b) Generation of heat. c) Energy transfer A* + B

Ao + B*

2) Photochemical process : d) consequent on energy transfer. A* + B

Ao + B* product.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

e) Reaction of excited molecule equivalent to effect occurring with thermally excited molecules. Reaction d) and c) occurs much less frequently than reaction a) and c) but do so at a sufficient rate to cause changes in most polymers. Examples : Trade Name : Tinuvin P Chemical Name : 2 – ( 2' – Hydroxy – 5' methylphenyl ) benzotriazole Structure : OH N N N CH3 2) Trade name Chemical name

- Tinuvin 770 - bis ( 2, 2, 6, 6 - tetramethyl - 4 - piperidyl ) Sebacate

Chemical structure – HN

O O || || – O – C – (CH2)8 C – O –

NH

U.V. absorbers are effective in concentration of 0.05 to 1%.

ANTISTATIS : Antistats function to help bleed of static electricity that is inherent in the polymer. The low amount of water on the surface of the resin results in static electricity. The antistat's function involves increasing the hydrophillic and

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

hydroscopic nature of the surface of polymer's surface. This facilitates a leak off path. Internal & external antistat agents may be used. An internal electrostatic dissipation is often remedied by using a conductive filler such as carbon black. External antistat agents are commonly quaternary amines and ammonium salts. Examples : 1) Trade name - Hoststat Chemical name - Sodium alkyl Sulfonate Chemical structure – O || Na – O – S – O – alkane || O 2) Trade name – Larostat Chemical name - Quaternary Ammonium Compound Modified Fatty Diethyl Methyl Ammonium Sulfate. Chemical Structure :O C2H3 || | HO – S – O – CH2 – CH2 – N – CH3 || | O C2H5

FLAME RETARDANTS :

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

Flame retardants function by forming a layer on the surface of the polymer that serves a barrier to oxygen penetration and serves to protect the polymer from heat. Flame retardants cool the combustion by diluting the combustion gases and then react with free radicals framed during the decomposition. Examples : 1)

Br

Br

Br – Br

Br

Br

– O –

– Br

Br

Br

Br

Great Lakes 83 Brominated diphenyl ether.

2)

Br

CH3

HO– Br

| – C – | CH3

Br – OH Br

Great Lakes 59 Halogenated bisphenol A

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

TEST FOR LIGHT STABILISED POLYSTYRENE The marked improvement in the lights ability of polystyrene brought about by incorporation of antioxidants and ultra-violet absorbers can be illustrated from the following test results. In test, four samples of same stabilised material were exposed continuously to standard cool, white 40 W fluorescent lamps at lamp-to-specimen distances ranging from ½” to 3”. The results are shown in table. The change in yellowness after one year continuous exposure is practically negligible as is the change after 2 years. Even in the most severe case of ½”, discoloration is negligible. Table Lamp to specimen Yellowing factor distance Inches ½ 1 1½ 3 A

comparison

12 months 0.9 0.3 0.0 0.0 of

24 months 1.4 0.9 0.3 0.0

un-stabilised

and

36 months 4.5 2.0 1.7 0.6

light-stabilised

polystyrene

(Polystron600’) exposed in the fadeometer for 80, 160,320,400 and 500 hours in the laboratory of polychem. Table Time in fadeometer Yellowing factor C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

Hrs. 0 80 160 320 400 500

Unstabilised 0.55 0.80 1.96 3.43 6.25 10.71

Stabilised 1.11 1.53 1.73 1.62 1.47 2.80

The un-stabilised sample shows a gradual increase in yellowness up to 160 hours, with a rapid, almost autocatalytic range to yellowing factor of 10-71 at 500 hours. In sharp contrast, the light stabilized sample remains practically unchanged through 500 hours exposure.

Here the fadeometer is predicting a marked

improvement in the stabilised material over the un-stabilised material.

The

fadeometer has continuous output of radiation in the area of the most harmful radiation given off by a cool white fluorescent lamp. Particularly noteworthy is the fact that fadeometer, in contrast to other testing devices, has minimum radiation in the lower U.V. region. Basically, a fadeometer consists of a carbon arc around which plastic samples are rotated in the central part of the equipment. The light level is very high, roughly comparable to that of sunlight on a bright, clear day. The testing is carried out at controlled temperature and humidity. In actual operation, results are expressed as hours to reach a predetermined colour or degree of colour change, but more preferably as the degree of yellowness after a given number of exposure for a specific period, the percentage transmittance of the sample is determined with

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

reference to a specific wavelength. The degree of yellowing is expressed in the form of ‘Yellowing Number’ calculated from the equation.

Yellowing Number =

100 × (T 420 − T '420) − (T 680 − T '680) T 560

Where T420, T560, T680

= % transmittance of the unexposed sample at wavelengths 420, 560 and 680 m respectively.

T’420, T’680

= % transmittance of the exposed sample at wavelengths 420 and 680 respectively. The number gives a fair measure of the light stability of polystyrene.

Larger the number, greater is the yellowing and hence lower is the light stability.

ADVANTAGES & DISADVANTAGES OF LIGHT STABILISED PS Advantages : 1) Good weather resistance 2) Much longer service life 3-5 times long compared with unmodified PS.

Disadvantages : 1) U. v. absorber gives PS a faint yellow tint. 2) Strength and elongation at break slightly reduced compared with unmodified PS. C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

PROCESSING METHODS FOR LIGHT STABILIZED PS The techniques of injection moulding, extrusion and vacuum forming can be most suitably employed in moulding polystyrene lighting fixtures. It has been found that the fabrication conditions maintained during moulding play important part in the light stability of the moldings.

A) INJECTION MOULDING : 1) The cylinder temperature maintained as low as possible.

This is best

accomplished by operating within 80% of machine capacity.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

2) The maximum injection pressure utilised for best overall part strength. 3) A constant cycle with as few delays as possible maintained to avoid overheating the material. 4) If a delay is necessary, the material from the cylinder thoroughly purged out before starting the production.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

Specification of injection molding machine : Compression ratio

=

1.6 to 2.2

Length/Diameter ratio =

About 20

Melt temperature

=

Varies from 200 to 2500 C

Injection pressure

=

Varies between 8 & 13 Mpa

Mould shrinkage

=

0.5 %

Mould temperature

=

20 - 500c.

B) EXTRUSION :1) The stock temperature be maintained in the range of 360 - 4000 F. 2) The extrusion inventory time should be the minimum possible.

C) VACUUM FORMING : This method is commonly used for moulding of luminous ceilings due to its inherent design flexibility. Since abrupt changes in sheet thickness will show up more vividly in lighting fixtures than in other commercial products, the forming job requires particular care and attention.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

COMPARING ACRYLICS , UNMODIFIED PS , RIGID PVC WITH LIGH-STABILISED PS Property Weatherability Maximum light transmission % Fluorescent life year Cost

C.O.E.& T., Akola.

Unmodified

Rigid

Light-stabilised

Good

PS Poor fair

PVC Fair good

PS Fair good

92

90

80-88

90

10 +

2.3

5+

5+

Very High

Less

Fair high

Less

Acrylics

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Polystyrene Used in Lighting Fixtures

1) Injection Molded Louvers for diffused lighting either singly or in continuous lengths for luminous ceilings , 2) Injection Molder Snap-on Diffusers for Fluorescent Tube Fixtures.

Typical Fluorescent Downlight Fixture

PROBLEM OF STATIC-DUST COLLECTION A common problem, characteristics of thermoplastics and especially those used in light control is the static-dust collection or dust pickup from the atmosphere. This problem requires our much attention , as PS is the worst in this respect. The reason for this is that polystyrene is an excellent electrical insulator and has very high surface resistivity. This leads to accumulation of static electric charge on the surface of the moulding , which then attracts dust. The useful result is an unsightly appearance and reduced efficiency of the lighting unit. Although, attempts to manufacture permanent anti-static polystyrene so far been unsuccessful the problem is not without a solution. For ordinary purpose , washing with a mild 10% solution of any commercial detergent does good clearing

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

job without affecting polystyrene. But it is necessary that the solution be allowed to dry on the fitting by itself which leaves a film imparting antistaticity to polystyrene.

TYPICAL APPLICATIONS OF POLYSTYRENE A few of the typical-end-uses of light-stabilised polystyrene are listed below : 1) Lamp shades, injection moulded and thermoformed. 2) Louvers 3) Extruded fluorescent diffusers. 4) Formed luminous ceilings. 5) Diffused lighting. 6) Illuminated advertisement displays. 7) Reflectors for indicator - lights on equipments. 8) Reflectors for cars , motor cycles , etc. 9) Dial lenses of equipments. C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

CONCLUSION Light - stabilised polystyrene satisfactorily fulfills the requirements of "Light - Industry " for long term service life and encourages new concepts in interior lighting decoration. Many attractive and functional lighting fixtures are made up of moulded and thermoformed or fabricated light stabilised formulations. Today, there is growing emphasis on proper lighting and the idea of light control is steadily gaining ground. This has created a vast potential market for plastics in the form of light - control media and has posed a challenge to the imagination of architects , contractors building engineers and the designers for economical , attractive and scientific lighting both for homes and industries.

C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

BIBLIOGRAPHY 1) "Encyclopedia of polymer science and Engg". Volume - 16 , 2nd edition. By Mark , Bikales , overberger , Menges. (Page No : 106, 111 , 112 , 139 , 148 , 161 ) 2) "Plastic Materials" Sixth edition By J. A. Brydson. ( Page No : 130 , 139 , 410 , 414 , 418 , 419 ) 3) "Polymer science", New Age publication C.O.E.& T., Akola.

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Polystyrene Used in Lighting Fixtures

By V. R. Gowarikar , N. V. Viswanathan , Jayadev Sreedhar . ( Page No : 218 , 219 , 327 ). 4) "Styrene - based plastics and their modification" Ellis Horwood Ltd. By Peter Svec, Ladislav Rosik, Zdenek Horak, Frantisek Vecerka. ( Page No : 44 , 54 , 55 , 56 , 64 , 66 , 66 - 73 ) 5) "Recommended Light Characteristics of polystyrene used in illumination" Modern plastics , sept. 1959 ( Page No : 134-204 , 142 - 144 )

Wed Sites : www.ormecon.com

C.O.E.& T., Akola.

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