Polystyrene is a petroleum-based plastic made from the styrene monomer. Polystyrene’s excellent combination of physical properties and ease of processability by injection molding and extrusion make it unique thermoplastic. It is a crystal clear product with excellent thermal and dimensional stabilities, high flexural and tensile
strengths, and good electrical properties.
Commercially, polystyrene was first introduced into the U.S. domestic market by Dow Chemical Company. With the advent of The Second World War in 1941, the U.S. was plunged into a synthetic rubber program requiring tremendous quantities of pure styrene monomer for copolymerization with butadiene to produce styrene. These were constructed to produce styrene.These were operated by Dow, Monsanto, Koppers and Carbide chemical companies.
SYNTHESIS MECHANISMS
Polystyrene which is produced commercially contains up to 2000 styrene units in the polymer chain. These chains are tangled and coiled in a random manner.
Polystyrene is synthesized by a free radical polymerization. An initiator, such as benzoyl peroxide, is used to initiate the free radical polymerization of styrene. Once the radical initiator initiates the polymerization of styrene, propagation occurs which “builds up” the polymer chain. Once the polymer chain has “grown” and at a desirable length or molecular weight, the polymerization is terminated. The polymer is then isolated, possibly purified, characterized, and used for material use.
Although polystyrene is largely commercially produced using free radical polymerization , it can be produced by all four of the major techniques: anionic, cationic, free radical, and coordination-type systems. All of the tactic forms can be formed employing these systems.Cationic catalysts such as boron fluoride, stannic chloride, aluminum chloride and titanium tetrachlordie and anionic catalysts will bring about styrene polymerization.The most important of the tactic forms is syndiotactic PS (sPS) .Metallocene-produced sPS is a semi-crystalline material with a Tm of 270°C.It was initially produced by Dow in 1997 under the trade name of Questra. It has good chemical and solvent resistance in contrast to “regular” PS that generally poor chemical and solvent resistance because of the presence of voids that are exploited by the solvents and chemicals.
Branching occurs especially when the polymer is prepared at high temperatures and high initiators concentrations. PS prepared from Ziegler-type catalysts has no branching is isotactic. Isotactic PS has about 40% crystallinity in the solid state and is less soluble in most solvents than atactic polystyrene.
Copolymerization:Styrene monomer copolymerizes very readily with many other vinyl monomers to form copolymers in which both kind of monomer units are preent in each polymer chain.Monomers frequently used commercially for copolymerization are acrylonitrile, alpha-methyl styrene, butadiene, maleic anhydride, methylmethacrylate.
Graft Polymerization: Graft polymerization of styrene onto synthetic rubber is used quite extensively in the commercial formation of impact resistant resns.Butadiene styrene copolymer is dissolved in styrene monomer and the reaction proceeds with styrene reacting with the pendant vinyl groups in the rubber and by abstraction of hydrogen atom , probably on the allylic hydrogen, from the rubber by a catalyst free radical to give a free radical which can initiate polymerization of styrene.
PHYSICAL PROPERTIES OF POLYSTYRENE
Physical properties of PS are dependent on the molecular weight and the presence of additives.Although higher molecular weight PS offers beter strength and toughness, it also offers poorer processability. Low molecular weight PS allows good processability but poorer strength and toughness. Generally , a balance is sought where intermediate chain lengths are used. Typically employed chain lengths are on the order of 1500 to 3500 with Standard molecular weight distributions of about 2.2 to 3.5. Small amounts of plasticizers are often used to improve processability.Styrene is employed in the formation of a number of co- and terpolymers. The best known is the terpolymer ABS.
AREAS OF USAGE
Major uses of PS are in packaging and containers , toys and recreational equipment, insulation, disposable food containers, electrical and electronics, housewares, and appliance parts. It is also used housings of things like hairdryers and kitchen appliances. Expandable PS is used to package electronic equipment such as televisions, computers, and stereo equipments. Legislation was put in place in some states to ensure the recycling of PS. Interestingly, some of this legislation was written such that all PS had to be recycled within some period of time such as 1 year. This legislation was changed to reflect the real concern of fast-food containers when it was pointed out that less than 10% PS is used in this manner and that well over twice as much was used as house insulation that could not be recycled every year or so.
In 2004 the global styrene demand was reported to be over 24,000 Kt. Although declining in proportion polystyrene is by far the largest segment (46%) primarily used in packaging, disposables, electronics and appliances, followed by expandable polystyrene (16%) and acrylonitrile-butadiene-styrene (ABS) resins (14%). The heat-resistant, tough ABS resins are widely used for appliances and telephone casings, luggage, sporting helmets, pipe fittings and automotive parts. Styrene-Butadiene (SB) latex and SB rubber account for 10% of the global demand. SB latex finds its use as paper coating for glossy magazines, as component of carpet and upholstery backing, for the adhesive production and for latex paints. The majority of SB rubber is consumed in the manufacture of tires, automobile parts and electronic components. Unsaturated polyester resins (UPR) are used over a broad spread of industries, mainly the construction, boat building, automotive and electrical industries (5% of the global styrene polymer derivative demand).
ENVIRONMENTAL EFFECTS OF STYRENE MONOMER AND POLYSTYRENE
The biggest environmental health concern associated with polystyrene is the danger associated with Styrene, the basic building block of polystyrene. Styrene is used extensively in the manufacture of plastics, rubber, and resins. About 90,000 workers, including those who make boats, tubs and showers, are potentially exposed to styrene. Acute health effects are generally irritation of the skin, eyes, and upper respiratory tract, and gastrointestinal effects. Chronic exposure affects the central nervous system showing symptoms such as depression, headache, fatigue, and weakness, and can cause minor effects on kidney function and blood. Styrene is classified as a possible human carcinogen by the EPA and by the International Agency for Research on Cancer (IARC).
A 1986 EPA report on solid waste named the polystyrene manufacturing process as the 5th largest creator of hazardous waste.· The National Bureau of Standards Center for Fire Research identified 57 chemical byproducts released during the combustion of polystyrene foam. The process of making polystyrene pollutes the air and creates large amounts of liquid and solid waste.
Toxic chemicals leach out of these products into the food that they contain (especially when heated in a microwave). These chemicals threaten human health and reproductive systems.
The use of hydrocarbons in polystyrene foam manufacture releases the hydrocarbons into the air at ground level; there, combined with nitrogen oxides in the presence of sunlight, they form tropospheric ozone a serious air pollutant at ground level. Ozone is definitely a dangerous pollutant. The EPA says: "Healthy individuals who are exercising while ozone levels are at or only slightly above the standard can experience reduced functioning of the lungs, leading to chest pain, coughing, wheezing, and pulmonary congestion. In animal studies, long-term exposure to high levels of ozone has produced permanent structural damage to animal lungs while both short and long term exposure has been found to decrease the animal's capability to fight infection.
Polystyrene foam is often dumped into the environment as litter. This material is notorious for breaking up into pieces that choke animals and clog their digestive systems.
Many cities and counties have outlawed polystyrene foam (i.e. Taiwan, Portland, OR, and Orange County, CA).
Kaynaklar:
Manufacture of Plastics Volume 1 ,W.Mayo Smith
-Introduction to Polymer Chemistry , Charles E. Carraher Jr.
-styrenemonomer.org
-environment.gov.am
-courses.chem.psu.edu
-hbogm.meb.gov.tr
-www.marketresearch.com
-www.chemsystem.com
-www.earthsource.org
-www.malzemeci.net
Written by begüm
