Polystyrene - Part 1

Description

    Polystyrene (PS) is a clear, colorless polymer used extensively for low-cost applications. It is available commercially in both pellet and sheet form. The most serious deficiencies are low impact strength, poor weatherability and poor chemical resistance. Numerous modified grades which seek to correct these shortcomings are commercially available.

   Polystyrene (IUPAC Poly(1-phenylethane-1,2-diyl)), abbreviated following ISO Standard PS, is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. Polystyrene is one of the most widely used kinds of plastic.

    Polystyrene is a thermoplastic substance, which is in solid (glassy) state at room temperature, but flows if heated above its glass transition temperature (for molding or extrusion), and becomes solid again when cooled. Pure solid polystyrene is a colorless, hard plastic with limited flexibility. It can be cast into molds with fine detail. Polystyrene can be transparent or can be made to take on various colors.

    Solid polystyrene is used, for example, in disposable cutlery, plastic models, CD and DVD cases, and smoke detector housings. Products made from foamed polystyrene are nearly ubiquitous, for example packing materials, insulation, and foam drink cups.

    Polystyrene can be recycled, and has the number "6" as its recycling symbol. Polystyrene takes a very long time to biodegrade, and is often abundant as a form of pollution in the outdoor environment, particularly along shores and waterways.

History

    Polystyrene was discoveraed in 1839 by Eduard Simon, an apothecary in Berlin. From storax, the resin of the Turkish sweetgum tree (Liquidambar orientalis), he distilled an oily substance, a monomer which he named styrol. Several days later, Simon found that the styrol had thickened, presumably from oxidation, into a jelly he dubbed styrol oxide ("Styroloxyd"). By 1845 English chemist John Blyth and German chemist August Wilhelm von Hofmann showed that the same transformation of styrol took place in the absence of oxygen. They called their substance metastyrol. Analysis later showed that it was chemically identical to Styroloxyd. In 1866 Marcelin Berthelot correctly identified the formation of metastyrol from styrol as a polymerization process. About 80 years went by before it was realized that heating of styrol starts a chain reaction which produces macromolecules, following the thesis of German organic chemist Hermann Staudinger (1881–1965). This eventually led to the substance receiving its present name, polystyrene.

    The company I. G. Farben began manufacturing polystyrene in Ludwigshafen, Germany, about 1931, hoping it would be a suitable replacement for die-cast zinc in many applications. Success was achieved when they developed a reactor vessel that extruded polystyrene through a heated tube and cutter, producing polystyrene in pellet form.

    Before 1949, the chemical engineer Fritz Stastny (1908–1985) developed pre-expanded PS beads by incorporating aliphatic hydrocarbons, such as pentane. These beads are the raw material for moulding parts or extruding sheets. BASF and Stastny applied for a patent which was issued in 1949. The moulding process was demonstrated at the Kunststoff Messe 1952 in Düsseldorf. Products were named Styropor.

     In 1959, the Koppers Company in Pittsburgh, Pennsylvania, developed expanded polystyrene (EPS) foam.

Structure of Polystyrene

    The chemical makeup of polystyrene is a long chain hydrocarbon with every other carbon connected to a phenyl group (the name given to the aromatic ring benzene, when bonded to complex carbon substituents). Polystyrene's chemical formula is (C8H8)n; it contains the chemical elements carbon and hydrogen. Because it is an aromatic hydrocarbon, it burns with an orange-yellow flame, giving off soot, as opposed to non-aromatic hydrocarbon polymers such as polyethylene, which burn with a light yellow flame (often with a blue tinge) and no soot. Complete oxidation of polystyrene produces only carbon dioxide and water vapor. Because of its chemical inertness, polystyrene is used to fabricate containers for chemicals, solvents, and foods.

  This addition polymer of styrene results when vinyl benzene (styrene) monomers (which contain double bonds between carbon atoms) attach to form a polystyrene chain (with each carbon attached with a single bond to two other carbons and a phenyl group). Polystyrene is generally flexible and can come in the form of moldable solids or viscous liquids. The force of attraction in polystyrene is mainly due to short range van der Waals attractions between chains. Since the molecules and long hydrocarbon chains that consist of thousand of atoms, the total attractive force between the molecules is large. However, when the polymer is heated (or, equivalently, deformed at a rapid rate, due to a combination of viscoelastic and thermal insulation properties), the chains are able to take on a higher degree of conformation and slide past each other. This intermolecular weakness (versus the high intramolecular strength due to the hydrocarbon backbone) allows the polystyrene chains to slide along each other, rendering the bulk system flexible and stretchable. The ability of the system to be readily deformed above its glass transition temperature allows polystyrene (and thermoplastic polymers in general) to be readily softened and molded with the addition of heat.

Ball-and-stick model of part of the crystal structure of isotactic polystyrene

    A 3-D model would show that each of the chiral backbone carbons lies at the center of a tetrahedron, with its 4 bonds pointing toward the vertices. Say the -C-C- bonds are rotated so that the backbone chain lies entirely in the plane of the diagram. From this flat schematic, it is not evident which of the phenyl (benzene) groups are apolymerization can produce an ordered syndiotactic polystyrene with the phenyl groups on alternating sides. This form is highly crystalline with a Tm of 270 °C (518 °F).

Extruded polystyrene is about as strong as unalloyed aluminium, but much more flexible and much lighter (1.05 g/cm3 vs. 2.70 g/cm3 for aluminium).

Ordinary atactic polystyrene has these large phenyl groups randomly distributed on both sides of the chain. This random positioning prevents the chains from ever aligning with sufficient regularity to achieve any crystallinity, so the plastic has a very low melting point, Tm<<TRT. But metallocene-catalyzed

Properties:

The unique physical and chemical properties of polystyrene are responsible for its use in a wide range of applications.

A] Processing properties:

    Flow properties may be the most important properties of polystyrene processes. There are two widely accepted industry methods for the measurement of processing properties. These include the melt flow index and the solution viscosity. The melt flow index is measured by ASTM method as a measure of the melt viscosity at 2000 C and a 5kg load. The melt flow index of polystyrene is generally controlled by adjustment of the molecular weight of the material and by the addition of such lubricants as mineral oil. Polystyrenes are commercially produced with melt flow ranges of less than 1 to greater than 50, although the most widely available grades generally have melt flows between 2.0 and 20g per 10min. Solution viscosity is another method for measuring the molecular structure of the polystyrene. Solution viscosity can be measured as an 8% solution in toluene and increases with increasing molecular weight.

B] Rheological properties:

    Polystyrene is a non-Newtonian fluid with viscoelastic properties. The viscosity of polystyrene melts or solutions is defined as the ratio of shear stress to shear rate. Generally, as the molecular weight of the polymer is increased or mineral oil is decreased, melt viscosity increases.

C] Mechanical properties:

    Crystal polystyrenes have very low impact strengths of less than 0.5ft-b.Commercially available impact polystyrene grades can be obtained with values of 1.0 - 4.0 ft-lb. Generally, polystyrenes are not produced with greater than 15% total rubber because of polymerization processing constraints. Nevertheless, impact properties can be increased substantially without additional rubber by the proper control of rubber particle size, percentage of grafting, cross-linking, and percentage of gel. Tensile and flexural properties are also important representation of the strength of polystyrenes. Increasing the rubber modification of polystyrene generally leads to lower tensile strength, crystal grades being stiff and brittle. Tensile strength is also  ecreased by the addition of lubricants, such as mineral oil. Flexural strengths for polystyrenes can be obtained from 5000 to 18000psi and are also decreased by the addition of rubber and other additives to the polystyrene. Elongations can be obtained from 1% for crystal polystyrene to 100% for some impact polystyrene grades.

D] Thermal properties:

    Annealed heat distortion is one popular method for measuring he resistance to deformation under heat for polystyrenes. The heat distortion temperature is decreased by the addition of rubber, mineral oil, or other additives to polystyrene. The glass transition temperature for unmodified polystyrene is 373 K, and the glass transition temperatures for polybutadienes are 161-205 K, subject to the cis, trans, and vinyl content.

 E] Chemical properties:

    Solvent crazing of polystyrene is a commercially important phenomenon.High impact polystyrenes are susceptible to solvent crazing at the interface between the rubber particles and the polystyrene phase. The resistance of polystyrene to this crazing is referred to as environmental stress crack resistance (ESCR). For food-packaging applications, such as butter tubs and deli containers, polystyrenes with high ESCR properties are desirable. Increasing the percentage of gel, percentage grafting, and rubber particle size can increase stress crack resistance. Residual levels of low molecular weight materials are also important to polystyrene performance. Some of the chemical impurities in the polystyrene are styrene monomer and ethyl benzene solvent. Residual levels of styrene below 200 ppm and ethyl benzene levels below 30 ppm are obtainable for very specialized applications.

F] Optical properties:

    Crystal polystyrene is a transparent and colorless polymer; high impact polystyrene is generally opaque as a result of the rubber particles. Developmental grades of translucent impact polystyrenes have been produced but have not gained wide acceptance. The major optical; property for high impact polystyrene is gloss. Gloss is a measure of the percentage of light reflected is generally controlled by the size of the rubber particle. In general, the smaller rubber particle gives higher gloss. Values from 20 to 95% reflectance are commercially available. High impact polystyrene is naturally white and crystal polystyrene is naturally clear, but both can be readily colored.

G] Gas and water permeability of polystyrene:

    When styrene polymers are used in packaging applications, the gas andwater permeability characteristics take on an important aspect. Polystyrene itself has its limitations and in consequence is often used with other polymers so as to achieve different permeability properties. These properties can change dramatically as other monomers are introduced into the molecule.

H] Weatherability and ageing:

    Polystyrene and the copolymers are susceptible to degradation by the action of sunlight; the main effect being due to UV radiation in the wavelength band of 300-400nm. the action of the UV radiation is accompanied by the oxidation so that the overall degradation reaction is one of photo oxidation. The extent of degradation varies from location to location owing to the differences in the intensity of the  adiation. This is of considerable importance in many applications because the degradation is reflected, in the case of transparent compositions, in a yellowing effect and generally in a loss of mechanical properties such as a lower elongation at break and a reduced impact strength.

I] Toxicity:

    Polystyrene is a low toxic product. The FDA for the food contact applications approves almost all commercially available polystyrenes. The polymer itself is not digestible and is not normally biodegradable.

Overview of physical properties of polystyrene :

·         Density - 1.05 g/cc

·         Dielectric constant - 2.4 to 2.7

·         Thermal conductivity - 0.08 W/(m.K)

·         Young's modulus - 3000 to 3600 Mpa

·         Tensile strength - 46 to 60 Mpa

·         Melting point - 240 ºC

·         Water absorption - 0.03 to 0.1

    This was all about polystyrene properties. Copolymers of polystyrene, that have improved properties, are made by adding other polymers with desired properties such as polybutadiene rubber, during the process of polymerization. Some examples of copolymers include high impact polystyrene and acrylonitrile butadiene styrene

Polystyrene Foam Insulation Properties

    The polystyrene foam is not at all heavy and is impermeable, which is the reason why they are perfect for the construction insulation. The closed unit arrangement it has got means that the polystyrene foam attain a very high R value. The R value is the capability of an element to conduct hotness. The greater R value of the element means that it resists more heat.

    Perfect insulation will not permit the hotness to enter or leave the house and because of that the expenses of warming and cooling are considerably decreased. This tells us that thanks to the filling, the temperature of the home will remain comfortable in the hot as well as cold season.

    The arrangement of this kind of insulation shows that there are almost zero vacant gaps in between its units and because of that it is very tough and water resistant. Due to its quality of avoiding water, mildew cannot produce on it and as they are very powerful and strong, they can be utilized again which will in turn reduce the further expenses of restoration and maintenance.

    Polystyrene foam insulation is slashed into panels that are set up in the exterior walls or fixed to the border of the constructions. The foam may be coated with several elements to form a particular appearance like granite or any other mineral. Beam may corrupt the polystyrene foam insulation. In order to solve this trouble, the foam must be set up with a beam obstructing material for the purpose of protection.

    This kind of insulation is also utilized as ground and roofing insulation. Moreover they can be set up in a cottage as well. When it has been placed on roofing element, then other elements may be laid over it. Afterward, grits can be set up on that.

uses

    Polystyrene, commonly known as 'Styrofoam' is one of the most widely used type of plastics. It is an inexpensive and hard plastic that finds use in a number of applications. The outside housing of your computer, housings of most kitchen appliances, model cars and airplanes, toys, molded parts in your car are all made of polystyrene. It is also made in the form of foam that is used for packaging and insulating.

CD case made from general purpose polystyrene (GPPS) and high impact polystyrene (HIPS

Disposable polystyrene razor

Polystyrene is commonly injection molded or extruded, while expanded polystyrene is either extruded or molded in a special process. Polystyrene copolymers are also produced; these contain one or more other monomers in addition to styrene. In recent years the expanded polystyrene composites with cellulose and starch have also been produced. Extruded closed-cell polystyrene foam is sold under the trademark Styrofoam by Dow Chemical. This term is often used informally for other foamed polystyrene products.

Polystyrene is used in some polymer-bonded explosives It is also a component of napalm and a component of most designs of hydrogen bombs.

    Polystyrene Petri dishes and other laboratory containers such as test tubes and microplates play an important role in biomedical research and science. For these uses, articles are almost always made by injection molding, and often sterilized post-molding, either by irradiation or treatment with ethylene oxide. Post-mold surface modification, usually with oxygen-rich plasmas, is often done to introduce polar groups. Much of modern biomedical research relies on the use of such products; they therefore play a critical role in pharmaceutical research

Foams

    Polystyrene foams are good thermal insulators and are therefore often used as building insulation materials, such as in structural insulated panel building systems. They are also used for non-weight-bearing architectural structures (such as ornamental pillars). PS foams exhibit also good damping properties, therefore it is used widely in packaging.

Keep reading Polystyrene - Part 2