Expanded polystyrene
Expanded polystyrene (EPS) is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. Familiar uses include molded sheets for building insulation and packing material ("peanuts") for cushioning fragile items inside boxes. Sheets are commonly packaged as rigid panels (size 4 by 8 or 2 by 8 feet in the United States), which are also known as "bead-board". Thermal resistivity is usually about 28 m·K/W (or R-3 per inch). Some EPS boards have a flame spread of less than 25 and a smoke-developed index of less than 450, which means they can be used without a fire barrier (but require a 15 minute thermal barrier) according to US building codes. A growing use of EPS in construction is insulating concrete forms. The density range is about 16–640 kg/m3. The most common processing method is thermal cutting with hot wires.
Extruded polystyrene foam
Extruded polystyrene foam (XPS) consists of closed cells, offers improved surface roughness and higher stiffness and reduced thermal conductivity. The density range is about 28 – 45 kg/m3.
Extruded polystyrene material is also used in crafts and model building, particularly architectural models. Because of the extrusion manufacturing process, XPS does not require facers to maintain its thermal or physical property performance. Thus, it makes a more uniform substitute for corrugated cardboard. Thermal resistivity is usually about 35 m·K/W (or R-5 per inch in American customary units).
Styrofoam is a trademarked name for XPS; however, it is often also used in the United States as a generic name for all polystyrene foams.
Copolymers
Pure polystyrene is brittle, but hard enough that a fairly high-performance product can be made by giving it some of the properties of a stretchier material, such as polybutadiene rubber. The two such materials can never normally be mixed because of the amplified effect of intermolecular forces on polymer insolubility (see plastic recycling), but if polybutadiene is added during polymerization it can become chemically bonded to the polystyrene, forming a graft copolymer which helps to incorporate normal polybutadiene into the final mix, resulting in high-impact polystyrene or HIPS, often called "high-impact plastic" in advertisements. One commercial name for HIPS is Bextrene. Common applications of HIPS include toys and product casings. HIPS is usually injection molded in production. Autoclaving polystyrene can compress and harden the material.
Several other copolymers are also used with styrene. Acrylonitrile butadiene styrene or ABS plastic is similar to HIPS: a copolymer of acrylonitrile and styrene, toughened with polybutadiene. Most electronics cases are made of this form of polystyrene, as are many sewer pipes. SAN is a copolymer of styrene with acrylonitrile, and SMA one with maleic anhydride. Styrene can be copolymerized with other monomers; for example, divinylbenzene can be used for cross-linking the polystyrene chains to give the polymer used in Solid phase peptide synthesis.
Oriented polystyrene
Oriented polystyrene (OPS) is produced by stretching extruded PS film, improving stiffness.
Disposal and environmental issues
Polystyrene is not easily recycled because of its light weight (especially if foamed) and its low scrap value. It is generally not accepted in curbside collection recycling programs. In Germany, however, polystyrene is collected, as a consequence of the packaging law (Verpackungsverordnung) that requires manufacturers to take responsibility for recycling or disposing of any packaging material they sell.
Environmental impact
Discarded polystyrene does not biodegrade for hundreds of years and is resistant to photolysis. Because of this, very little of the waste discarded in today's modern, highly engineered landfills biodegrades. Because degradation of materials creates potentially harmful liquid and gaseous by-products that could contaminate groundwater and air, today's landfills are designed to minimize contact with air and water required for degradation, thereby practically eliminating the degradation of waste.
Polystyrene foam is a major component of plastic debris in the ocean, where it becomes toxic to marine life. Foamed polystyrene blows in the wind and floats on water, and is abundant in the outdoor environment. Weathering by wind, sun, rain, and wave action degrade polystyrene to known and suspected carcinogens, including styrene monomer (SM), styrene dimer (SD) and styrene trimer (ST). However, styrene is an organic, naturally occurring substance in our environment and to date, no regulatory body anywhere in the world has classified styrene as a known human carcinogen, although several refer to it in various contexts as a possible or potential human carcinogen. Furthermore, styrene is quickly broken down in the air, evaporates quickly in shallow soil and water, and what remains in soil and water can be further broken down by bacteria and microorganisms.
Polystyrene foams are produced using blowing agents that form bubbles and expand the foam. In expanded polystyrene, these are usually hydrocarbons such as pentane, which may pose a flammability hazard in manufacturing or storage of newly manufactured material, but have relatively mild environmental impact. However, extruded polystyrene is usually made with hydrochlorofluorocarbons (HCFC) blowing agents which have effects on ozone depletion and on global warming. Their ozone depletion potential is greatly reduced relative to chlorofluorocarbon (CFC) which were formerly used, but their global warming potential can be on the order of 1000 or more, meaning it has 1000 times greater effect on global warming than does carbon dioxide. That being said, global warming regulations should have minimal direct impact on the PS industry. There are few greenhouse gas emissions generated by the PS industry in comparison to other industries such as oil refineries and automobiles.
Regardless, on September 21, 2007, approximately 200 countries agreed to accelerate the elimination of hydrochlorofluorocarbons entirely by 2020 in a United Nations-sponsored Montreal summit. Developing nations were given until 2030. Ultimately, hydrofluorocarbons (HFCs) will replace HCFCs with essentially no ozone destruction.
While there is less concern about the health effects of EPS itself, the brominated flame retardants used in most EPS foam (decaBDE or hexabromocyclododecane are the most commonly used) could possibly create health and environmental risks that are generating some concern with the EPA. Since the early 1990s, starch-based foam packing peanuts have been used as a replacement for PS packing peanuts. Foamed polylactic acid (PLA), marketed as Biofoam, has also been developed.
In 2007, Eben Bayer, a student at Rensselaer Polytechnic Institute, invented an environmentally-friendly replacement for polystyrene packaging using the mineral perlite bonded with lignin-containing agricultural waste, degraded by fungi, which he calls Ecocradle.
Recycling
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| The resin identification code symbol for polystyrene |
The resin identification code symbol for polystyrene Currently, the majority of polystyrene products are not recycled. Expanded polystyrene scrap can be easily added to products such as EPS insulation sheets and other EPS materials for construction applications. Commonly, manufacturers cannot obtain sufficient scrap because of the aforementioned collection issues. When it is not used to make more EPS, foam scrap can be turned into clothes hangers, park benches, flower pots, toys, rulers, stapler bodies, seedling containers, picture frames, and architectural molding from recycled PS. Recycled EPS is also used in many metal casting operations. Rastra is made from EPS that is combined with cement to be used as an insulating amendment in the making of concrete foundations and walls. American manufacturers have produced insulated concrete forms made with approximately 80% recycled EPS since 1993. However, polystyrene recycling is not a closed loop, producing more polystyrene; polystyrene cups and other packaging materials are instead usually used as fillers in other plastics, or in other items that cannot themselves be recycled and are thrown away.
Incineration
If polystyrene is properly incinerated at high temperatures, the chemicals generated are water, carbon dioxide, a complex mixture of volatile compounds, and carbon soot. According to the American Chemistry Council, when polystyrene is incinerated in modern facilities, the final volume is 1% of the starting volume; most of the polystyrene is converted into carbon dioxide, water vapor, and heat. Because of the amount of heat released, it is sometimes used as a power source for steam or electricity generation.
When polystyrene was burned at temperatures of 800-900 °C (the typical range of a modern incinerator), the products of combustion consisted of "a complex mixture of polycyclic aromatic hydrocarbons (PAHs) from alkyl benzenes to benzoperylene. Over 90 different compounds were identified in combustion effluents from polystyrene."
When burned without enough oxygen or at lower temperatures (as in a campfire or a household fireplace), polystyrene can produce polycyclic aromatic hydrocarbons, carbon black, and carbon monoxide, as well as styrene monomers.
Burial
Foam cups and other polystyrene products can be safely buried in landfills, since it is as stable as concrete or brick. No plastic film is required to protect the air and underground water.
Reducing
Some effort is being made to find alternatives to polystyrene foam, especially in restaurant settings. Restricting the use of foamed polystyrene takeout food packaging is a priority of many solid waste environmental organizations. However, the Plastics Foodservice Packaging Group counters that in US, less than 1% by weight of solid waste disposed is polystyrene. A campaign to achieve the first ban of polystyrene foam from the food & beverage industry in Canada was launched in Toronto as of January 2007, by local non-profit organization NaturoPack. Portland, Ore., and San Francisco are among about one hundred cities in the United States that currently have some sort of ban on polystyrene foam in restaurants. For instance, in 2007 restaurants in Oakland, California were required to switch to disposable food containers that will biodegrade if added to food compost. Although polystyrene can be recycled at recycling facilities, most polystyrene is not recycled. The EPA (United States Environmental Protection Agency) estimates that 25 billion polystyrene cups are tossed every year. Since polystyrene degrades very slowly- more than 500 years for a single cup- the EPA considers this a serious environmental problem. Several green leaders, from the Dutch Ministry of the Environment to Starbucks' Green Team, advise that individuals reduce their environmental impact by using reusable coffee cups.
Finishing
In the United States, environmental protection regulations prohibit the use of solvents on polystyrene (which would dissolve the polystyrene and de-foam most foams anyway).
Some acceptable finishing materials are
· Water-based paint (artists have created paintings on polystyrene with gouache)
· Mortar or acrylic/cement render, often used in the building industry as a weather-hard overcoat that hides the foam completely after finishing the objects.
· Cotton wool or other fabrics used in conjunction with a stapling implement.
Health and fire hazards
There has been concern about the trace presence of polystyrene's production chemicals in the final plastic product, most of which are toxic if not removed. For instance benzene, which is used to produce ethylbenzene for styrene, is a known carcinogen. As well, unpolymerized styrene may pose health risks.
However, Based on scientific tests over five decades, government safety agencies have determined that polystyrene is safe for use in foodservice products. For example, polystyrene meets the stringent standards of the U.S. Food and Drug Administration and the European Commission/European Food Safety Authority for use in packaging to store and serve food. The Hong Kong Food and Environmental Hygiene Department recently reviewed the safety of serving various foods in polystyrene foodservice products and reached the same conclusion as the U.S. FDA. and from 1999 to 2002, a comprehensive review of the potential health risks associated with exposure to styrene was conducted by a 12 member international expert panel selected by the Harvard Center for Risk Assessment. The scientists had expertise in toxicology, epidemiology, medicine, risk analysis, pharmacokinetics, and exposure assessment.
The Harvard study reported that styrene is naturally present in foods such as strawberries, beef, and spices, and is naturally produced in the processing of foods such as wine and cheese. The study also reviewed all the published data on the quantity of styrene contributing to the diet due to migration of food packaging and disposable food contact articles, and concluded there is no cause for concern for the general public from exposure to styrene from foods or styrenic materials used in food-contact applications, such as polystyrene packaging and food service containers.
The LD50 of styrene is 3 mmol/kg as determined by the Registry of Cytotoxicity Data (ZEBET) 7.1, National Institute of Health, Berlin, Germany.
There are, of course, studies concerning polystyrene containers used for food packaging which find that styrene oligomers migrate into the food. For instance, one Japanese study conducted on wild-type and AhR-null mice found that the styrene trimer, which the authors detected in cooked polystyrene container-packed instant foods, may increase thyroid hormone levels. Polystyrene is classified according to DIN4102 as a "B3" product, meaning highly flammable or "easily ignited." Consequently, although it is an efficient insulator at low temperatures, its use is prohibited in any exposed installations in building construction if the material is not flame retardant, e.g., with hexabromocyclododecane. It must be concealed behind drywall, sheet metal or concrete. Foamed polystyrene plastic materials have been accidentally ignited and caused huge fires and losses, for example at the Düsseldorf International Airport, the Channel tunnel (where polystyrene was inside a railcar that caught fire), and the Browns Ferry Nuclear Power Plant (where fire breached a fire retardant and reached the foamed plastic underneath, inside a firestop that had not been tested and certified in accordance with the final installation).
In addition to fire hazard, polystyrene can be dissolved by substances that contain acetone (such as most aerosol paint sprays), and by cyanoacrylate glues.
Alternatives
Besides expanded polystyrene, another material has been developed that has about the same properties and uses as expanded polystyrene. This material is called mycobond and is cradle-to-cradle compliant. It is made from agricultural byproducts, bound together by fungal mycelium.
Read also Polystyrene - Part 1
Read also Polystyrene - Part 1
References:
http://www.polymerprocessing.com/polymers/PS.html
http://www.medical-answers.org/hd/index.php?t=Polystyrene
http://www.pypc.com/epk-ps.htm
http://www.sbioinformatics.com/design_thesis/Polystyrene/polystyrene_Properties%26uses.pdf
http://ezinearticles.com/?Polystyrene-Foam-Insulation---Properties&id=4565977
http://en.wikipedia.org/wiki/Polystyrene
http://www.answers.com/topic/polystyrene
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