Polystyrene (PS) Recycling
Polystyrene is an amorphous, glassy polymer that is generally rigid and relatively inexpensive. Unfilled polystyrene has a sparkle appearance and is often referred to as crystal PS or general purpose polystyrene (GPPS). High impact polystyrene grades (HIPS) are produced by adding rubber or butadiene co-polymer which increases the toughness and impact strength of the polymer. Polystyrene possess good flow properties at temperatures safely below degradation ranges, and can easily be extruded, injection molded, or compression molded. Considerable quantities of polystyrene are produced in the form of heat-expandable beads containing a suitable blowing agent which ultimately results in familiar foamed polystyrene articles.
Polystyrene (PS) has been known for well over one hundred years but its real molecular nature was not clarified until about 1920 when the work of Staudinger elucidated the materials molecular structure in the very early days of polymer science. About 1930 I.G. Farben, in Germany, first produced polystyrene, while at the same time the Dow Chemical Company commenced their ultimately successful development of the material.
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 poly-butadeine rubber. The two such materials can never normally be mixed because of the amplified effect of inter-molecular forces on polymer insolubility, but if poly-butadiene is added during polymerization it can become chemically bonded to the polystyrene, forming a graft co-polymer, which helps to incorporate normal poly-butadiene into the final mix, resulting in high-impact polystyrene or HIPS, often called “high-impact plastic” in advertisements. Common applications of HIPS include toys and product casings. HIPS is usually injection molded in production. Autoclaving polystyrene can compress and harden the material.
High Impact Polystyrene (HIPS) Granules in Black Color Coding
The table provided below is a typical test report ordered on presence of heavy metals and chemical contents in recycled HIPS resins in accordance with European Union’s Restrictions of Hazardous Substances (RoHS) directives. Note that for better quality recycled resins, the factory can control the manufacturing process by having relatively clean homogeneous industrial polystyrene scraps as input material and this can be specified for the production. These test results are inconsistent between replicates, thus test result is based on average result of replicate analysis.
RoHS Directive 2011/65/EU
|Test Item(s)||Unit||Method||MDL||Result No.1||Limit|
|Cadmium (Cd)||mg/kg||Ref. IEC 62321-5:2013 and performed by ICP-AES||2||n.d.||1000|
|Lead (Pb)||mg/kg||Ref. IEC 62321-5:2013 and performed by ICP-AES||2||14.2||1000|
|Mercury (Hg)||mg/kg||Ref. IEC 62321-4:2013 and performed by ICP-AES||2||n.d.||1000|
|Hexavalent Chromium CR(VI)||mg/kg||Ref. IEC 62321:2008 and performed by UV-VIS||2||n.d.||1000|
|Sum of PBBs||mg/kg||–||n.d.||1000|
|Monobromobiphenyl||mg/kg||Ref. IEC 62321-6:2015 and performed by GC/MS||5||n.d.||–|
|Sum of PBDEs||mg/kg||–||396||1000|
|Halogen-Chlorine (CI) (CAS No: 22537-15-1)||mg/kg||Ref. BS EN 14582:2007 Analysis performed by IC||50||357||–|
|Halogen-Bromine (Br) CAS No: 10097-32-2)||mg/kg||Ref. BS EN 14582:2007 Analysis performed by IC||50||1690||–|
|Polybrominated biphenyls (PBB)
Polybrominated diphenyl ether (PBDE)
Plastic Recycling + Semi Manufacturing
Plastic recycling is the process of recovering scrap or waste plastic and reprocessing the material into useful products, sometimes completely different in form from their original state. Since plastic is non-biodegradable, recycling it is a part of global efforts to reduce plastic in the waste stream, especially the approximately eight million metric tons of waste plastic that enter the earth’s ocean every year. This helps to reduce the high rates of plastic pollution.
Plastic recycling includes melting down soft drink bottles and then casting them as plastic chairs and tables. However, this kind of “recycling” is rather a misnomer since plastic beverage bottles (soda, juice, milk) are never truly reformed into new beverage bottles, as this requires virgin plastic. So there is actually no true cycle in the “recycling” of plastic beverage containers, which actually and more precisely should be referred to as ‘down-cycling’. Plastics are also recycled during the manufacturing process of plastic goods such as polyethylene film and bags. A percentage of the recycled pellets are then re-introduced into the main production operation. This closed-loop operation has taken place since the 1970s and has made the production of some plastic products amongst the most efficient operations today.
Compared with lucrative recycling of metal, and similar to the low value of glass, plastic polymers recycling is often more challenging because of low density and low value. There are also numerous technical hurdles to overcome when recycling plastic.
A macro molecule interacts with its environment along its entire length, so total energy involved in mixing it is largely due to the product side stoichiometry. Heating alone is not enough to dissolve such a large molecule, so plastics must often be of nearly identical composition to mix efficiently.
When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications.
Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more often. Yet another barrier to removing large quantities of plastic from the waste stream and landfills is the fact that many common but small plastic items lack the universal triangle recycling symbol and accompanying number. An example is the billions of plastic utensils commonly distributed at fast food restaurants or sold for use at picnics.
The percentage of plastic that can be fully recycled, rather than down-cycled or go to waste can be increased when manufacturers of packaged goods minimize mixing of packaging materials and eliminate contaminants. The Association of Plastics Recyclers has issued a Design Guide for Recyclability.
Some examples of plastic identification codes
Before recycling, most plastics are sorted according to their resin type. In the past, plastic recyclers used the resin identification code (RIC), a method of categorization of polymer types, which was developed by the Society of the Plastics Industry in 1988. Polyethylene terephthalate, commonly referred to as PET, for instance, has a resin code of 1. Most plastic recyclers do not rely on the RIC now; they use automatic sort systems to identify the resin. Ranging from manual sorting and picking of plastic materials; to mechanized automation processes that involve shredding, sieving, separation by rates of density i.e. air, liquid, or magnetic, and complex spectrophotometric distribution technologies e.g. UV/VIS, NIR, Laser, etc. Some plastic products are also separated by colour before they are recycled. The plastic recyclables are then shredded. These shredded fragments then undergo processes to eliminate impurities like paper labels. This material is melted and often extruded into the form of pellets which are then used to manufacture other products.
Main articles: Depolymerisation and Thermal depolymerisation
Another process involves the conversion of assorted polymers into petroleum by a much less precise thermal depolymerisation process. Such a process would be able to accept almost any polymer or mix of polymers, including thermoset materials such as vulcanized rubber tire separation of wastes and the biopolymers in feathers and other agricultural waste. Like natural petroleum, the chemicals produced can be made into fuels as well as polymers. Gasification is a similar process, but is not technically recycling, since polymers are not likely to become the result.
Yet another process that is gaining ground with start-up companies is heat compression. The heat compression process takes all unsorted, cleaned plastic in all forms, from soft plastic bags to hard industrial waste, and mixes the load in tumblers (large rotating drums resembling giant clothes dryers). The most obvious benefit to this method is the fact that all plastic is recyclable, not just matching forms. However, criticism rise from the energy costs of rotating the drums, and heating the post-melt pipes.
For some waste plastics, recent technical devices called recyclebots enable a form of distributed recycling. Preliminary life-cycle analysis indicates that such distributed recycling of HDPE to make filament of 3-D printers in rural regions is energetically favourable to either using virgin resin or conventional recycling processes because of reductions in transportation energy.
A process has also been developed in which many kinds of plastic can be used as a carbon source in the recycling of scrap steel. There is also a possibility of mixed recycling of different plastics, which does not require their separation. It is called Compatibilization and requires use of special chemical bridging agents compatibilizers. It can help to keep the quality of recycled material and to skip often expensive and inefficient preliminary scanning of waste plastics streams and their separation/purification.
Post-consumer polyethylene terephthalate (PET or PETE) containers are sorted into different colour fractions, and baled for onward sale. PET recyclers further sort the baled bottles and they are washed and flaked (or flaked and then washed). Non-PET fractions such as caps and labels are removed during this process. The clean flake is dried. Further treatment can take place e.g. melt filtering and palletising or various treatments to produce food-contact-approved recycled PET (RPET).
RPET has been widely used to produce polyester fibres. This sorted post-consumer PET waste is crushed, chopped into flakes, pressed into bales, and offered for sale.
One use for this recycled PET that has recently started to become popular is to create fabrics to be used in the clothing industry. The fabrics are created by spinning the PET flakes into thread and yarn. This is done just as easily as creating polyester from brand new PET. The recycled PET thread or yarn can be used either alone or together with other fibers to create a very wide variety of fabrics. Traditionally these fabrics are used to create strong, durable, rough products, such as jackets, coat, shoes, bags, hats, and accessories since they are usually too rough for direct skin contact and can cause irritation. However, these types of fabrics have become more popular as a result of the public’s growing awareness of environmental issues. Numerous fabric and clothing manufacturers have capitalized on this trend.
Other major outlets for RPET are new containers (food-contact or non-food-contact) produced either by (injection stretch blow) moulding into bottles and jar or by thermoforming APET sheet to produce clam shells, blister packs and collation trays. Other applications, such as strapping tape, injection-moulded engineering components and even building materials account for some percentage of RPET production.
Plastic # 2, high-density polyethylene (HDPE) is a commonly recycled plastic. It is typically down-cycled into plastic lumber, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery (e.g. rulers) and other durable plastic products and is usually in demand.
Most polystyrene products are currently not recycled due to the lack of incentive to invest in the compactors and logistical systems required. As a result, manufacturers cannot obtain sufficient scrap. Expanded polystyrene (EPS) scrap can easily be added to products such as EPS insulation sheets and other EPS materials for construction applications. 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 moulding from recycled PS.
Recycled EPS is also used in many metal casting operations. Insulated concrete form is made from EPS that is combined with cement to be used as an insulating amendment in the making of concrete foundations and walls. Since early 90s, manufacturers have produced insulating concrete forms made with approximately 80% recycled EPS.
This plastic recycling code indicates that the type of plastic in question is made of a resin other than the six listed above, or is made of more than one resin listed above.
The white plastic polystyrene foam peanuts used as packing material are often accepted by shipping stores for reuse. Plastic films recovered from mixed municipal waste streams can be recycled into useful household products such as buckets
Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled into much larger products for industrial applications such as plastic composite railroad ties. Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.
Low national plastic recycling rates have been due to the complexity of sorting and processing, unfavourable economics, and consumer confusion about which plastics can actually be recycled. Part of the confusion has been due to the use of the resin identification code which is not on all plastic parts but just a subset that includes the recycling symbol as part of its design. The resin identification code is stamped or printed on the bottom of containers and surrounded by a triangle of arrows.The intent of these symbols was to make it easier to identify the type of plastics used to make a particular container and to indicate that the plastic is potentially recyclable. The question that remains is which types of plastics can be recycled by your local recycling centre. In many communities, not all types of plastics are accepted for sidewalk recycling collection programs due to the high processing costs and complexity of the equipment required to recycle certain materials. There is also sometimes a seemingly low demand for the recycled product depending on a recycling centre’s proximity to entities seeking recycled materials. Another major barrier is that the cost to recycle certain materials and the corresponding market price for those materials sometimes does not present any opportunity for profit. The best example of this is polystyrene (commonly called Styrofoam), although some communities are moving toward banning the distribution of polystyrene containers by local food and coffee businesses.
Plastic identification code
Five groups of plastic polymers, each with specific properties, are used worldwide for packaging applications (see table below). Each group of plastic polymer can be identified by its Plastic Identification code (PIC), usually a number or a letter abbreviation. For instance, Low-Density Polyethylene can be identified by the number “4” or the letters “LDPE”. The PIC appears inside a three-chasing-arrow recycling symbol. The symbol is used to indicate whether the plastic can be recycled into new products.
The PIC was introduced by the Society of the Plastics Industry, Inc., to provide a uniform system for the identification of various polymer types and to help recycling companies separate various plastics for reprocessing. Manufacturers of plastic products are required to use PIC labels in some countries/regions and can voluntarily mark their products with the PIC where there are no requirements. Consumers can identify the plastic types based on the codes usually found at the base or at the side of the plastic products, including food/chemical packaging and containers. The PIC is usually not present on packaging films, since it is not practical to collect and recycle most of this type of waste.
|Plastic Identification Code||Type of plastic polymer||Properties||Common Packaging Applications||Melting (°C) and Glass Transition Temperatures||Young’s Modulus (GPa)|
|Clarity, strength, toughness, barrier to gas and moisture.||Soft drink, water and salad dressing bottles; peanut butter and jam jars; small consumer electronics.||Tm = 250;
Tg = 76
|Stiffness, strength, toughness, resistance to moisture, permeability to gas.||Water pipes, hula hoop rings,
five gallon buckets, milk, juice and water bottles; grocery bags, some shampoo/toiletry bottles.
|Tm = 130;
Tg = -125
|Versatility, ease of blending, strength, toughness.||Blister packaging for non-food items; cling films for non-food use. May be used for food packaging with the addition of the plasticisers needed to make natively rigid PVC flexible . Non-packaging uses are electrical cable insulation; rigid piping; vinyl records.||Tm = 240;
Tg = 85
|Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture.||Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids.||Tm = 120;
Tg = -125
|Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture.||Reusable microwaveable ware; kitchenware; yogurt containers; margarine tubs; microwaveable disposable take-away containers; disposable cups; soft drink bottle caps; plates.||Tm = 173;
Tg = -10
|Versatility, clarity, easily formed||Egg cartons; packing peanuts; disposable cups, plates, trays and cutlery; disposable take-away containers.||Tm = 240 (only isotactic);
Tg = 100 (atactic and isotactic)
|Other (often polycarbonate or ABS-acrylonitrile butadiene styrene)||Dependent on polymers or combination of polymers||Beverage bottles; baby milk bottles. Non-packaging uses for polycarbonate: compact discs; “unbreakable” glazing; electronic apparatus housings; lenses including sunglasses, prescription glasses, automotive headlamps, riot shields, instrument panels.||Polycarbonate: Tg = 145;
Tm = 225
|Polycarbonate: 2.6; ABS plastics: 2.3|
REDUCE, REUSE, RECYCLE