Polystyrene- Plastjoo

Polystyrene is a type of polymer with thermoplastic properties produced from the petroleum-derived monomer, styrene. The polymerization of styrene has been known since 1839, when German pharmacist Eduard Simon reported its conversion into a solid later named metastyrol. As late as 1930 little commercial use had been found for the polymer because of brittleness and crazing (minute cracking), which were caused by impurities that brought about the cross-linking of the polymer chains. By 1937 American chemist Robert Dreisbach and others at the Dow Chemical Company’s physics laboratory had obtained purified styrene monomer through the dehydrogenation of ethylbenzene and developed a pilot polymerization process. By 1938 polystyrene was being produced commercially. It quickly became one of the most important modern plastics, owing to the low cost of producing large volumes of styrene monomer, the ease of shaping the melted polymer in injection-molding operations, and the optical and physical properties of the material. It is used to produce many products for industrial and consumer use. In fact, its presence as a plastic in everyday life is second only to polyethylene

 

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The chemical structure of this material allows it to be classified as a liquid hydrocarbon, meaning that it is composed exclusively of hydrogen and carbon. Like its precursor, it's an aromatic hydrocarbon that participates in covalent bonding with every other carbon atom being attached to a phenol group. It is produced via free radical polymerization, which means that the reaction involves breaking the bonds between electrons and leaving them “free” to form new bonds. Styrene is obtained by reacting ethylene with benzene in the presence of aluminum chloride to yield ethylbenzene. The benzene group in this compound is then dehydrogenated to yield phenylethylene, or styrene, a clear liquid hydrocarbon with the chemical structure CH2=CHC6H5. Styrene is polymerized by using free-radical initiators primarily in bulk and suspension processes, although solution and emulsion methods are also employed. The structure of the polymer repeating unit can be represented as

 

Polystyrene  


Polystyrene is a not biodegradable plastic and resistant to photolysis. When burned, this material yields black carbon particles, or soot. When completely oxidized, only carbon dioxide and water vapor remain


Properties
Advantages of  Polystyrene
The presence of the pendant phenyl (C6H5) groups is key to the properties of polystyrene. Solid polystyrene is transparent, owing to these large, ring-shaped molecular groups, which prevent the polymer chains from packing into close, crystalline arrangements. In addition, the phenyl rings restrict rotation of the chains around the carbon-carbon bonds, lending the polymer its noted rigidity.Polystyrene (PS) is a


Clear-
Amorphous-
nonpolar-
easy to process and that can be easily converted into a large number of semi-finished products like foams, films, and sheets-
good electrical insulator-
excellent optical clarity due to the lack of crystallinity-
good chemical resistance to diluted acids and bases-


Disadvantages of Polystyrene
poor oxygen and UV resistance-
rather brittle-
flammable-
poor impact strength due to the stiffness of the polymer backbone-
yellows upon prolonged exposure to sunlight (without the addition of chemical stabilizers)-
upper temperature limit for continual use is rather low due to the lack of crystallinity and its low glass transition temperature of about ( Tg = 373 K (100°C)). (softens in boiling water)
medium to high tensile strength (35 - 55 MPa)- 
low impact strength (15 - 20 J/m)-


Some of its weaknesses can be overcome by copolymerization with other monomers. For example 


more than half of all polystyrene produced is blended with 5 to 10 percent butadiene rubber. This blend, suitable for toys and  1. appliance parts, is marketed as high-impact polystyrene (HIPS


polystyrene can be copolymerized with methyl methacrylate. The copolymer poly(styrene-co-methyl methacrylate) (PSMMA) has 2. higher clarity and improved chemical and UV stability


3. One of the most important styrene copolymers is poly(styrene-co-acrylonitrile) (PSAN). It has much improved chemical resistance, better heat stability, and improved mechanical properties. However, these copolymers often yield yellow products


Probably of equal importance are poly(styrene-co-butadiene) (SBR, SBS) and poly(styrene-co-acrylonitrile-co-butadiene) (ABS). 4. Both copolymers have very high stress and impact resistance and ABS has higher tensile strength than pure PS


5. To increase the heat resistance, styrene is sometimes copolymerized with small amounts of maleic anhydride or it is copolymerized with this monomer to an alternating structure. The copolymer poly (styrene-co-maleic anhydride) (PSMA) has a higher Tg than pure polystyrene (400 - 430 K), improved heat resistance and high dimensional stability

 

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Commercial Polystyrenes
:The three most important grades of styrene are
GPPS: General purpose polystyrene, also known as crystal-clear polystyrene, is a fully transparent, rigid and rather brittle low cost thermoplastic made from styrene monomer
HIPS: High impact polystyrene contains usually 5 to 10% rubber (butadiene) and is used for parts which require high(er) impact resistance
EPS: Expandable polystyrene consists of micro-pellets or beads containing a blowing agent (usually pentane). The expanded or foamed polystyrene is thermally insulating, has high impact resistance and good processability


APPLICATIONS
:Polystyrene is a polymer that is cheap and easy to process. It is the material of choice for many applications including
food-packaging-
disposable consumer plastic goods-
parts for optical, electronic/electrical-
medical applications (pipettes, Petri dishes and medicine containers)-
dining utensils-
disposable plastic cups-
housewares-
toys-
CD cases-
trays and fast-food containers-
cushioned packaging-
thermal insulation-

 

Compiler: Nasim Rashidi Nezhad
 

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