Plastic Recycling

Flashback: Plastic is rather an undefined term as it includes any organic material (usually derived from coal or petroleum) with "plastic" properties; i.e. the ability to flow into a desired shape when heat and pressure are applied to it and to retain the shape when they are withdrawn. A more precise definition groups them into two basic types: Thermoset (duroplast), which cannot be resoftened after being subjected to heat and pressures; and Thermoplast, which can be repeatedly softened and reshaped by heat and pressure. At the turn of the last century the very first important plastic product emerged in the form of celluloid. Since then, it has been replaced by a wide variety of plastics known by such trade names as Plexiglas, Lucite, Polaroid, and cellophane. New uses continue to be found and include contact lenses, machine gears, non-moving engine parts, artificial body parts in clinical applications, and many more. Modern industrial production facilities generate a huge selection of different varieties that originate from different prime materials with quite incompatible physico-chemical properties. These incompatibilities do pose sever restrictions for an efficient recycling loop on both the consumer's and producer's side.

For the average consumer, the most common plastic materials encountered are those for packaging and wrapping. In fact this is its most widespread application and hence has been the focus of much public debate in recent years. Heavy use of this material has led to serious environmental problems. Since plastic products do not readily decay, large amounts accumulate as waste. Careless disposal introduces these substances into the most unimaginable places (trekking trash left behind by lazy mountaineers, to floating material dumped by short-minded shipping operators). It is not only from an esthetical point that this material does not belong there - marine turtles for example mistake plastic bags for jelly fish, try to eat them, suffocate and utlimatley die.

It is recognized that packaging has an important function in protecting goods and enabling their handling and transportation, however, there needs to be a dramatic change in how this is conceptualised, used and re-used. There are a few simple rules customers can follow: where possible, buy minimal packaged goods or bulk commodities that have a good weight/quantity relationship (particularly foodstuffs and hardware items - customers should not refrain from complaints to the manufacturer when dealing with over-packaging). Other alternatives regard innovative materials such as biodegradable wrapping materials based on starch; e.g. the edible potato waver cup. In the meantime recycling should be the order of the day. Recycling is economically sustainable if the material is of a single type (PE, HDE, HD, etc.) and available in large quantities; e.g. the 1.5L (PET-) bottles used for soft drinks and spring water. Efficient use of plastics can be achieved in the soft drink market when manufactures take back empty bottles; and after an intense cleansing program, refilled them with the same or similar contents.

Key to plastic categories

Nonetheless, the whole area of environmentally low impact packaging has a long way ahead. What is not acceptable anymore, is the continuation of non-biodegradable and non-recyclable materials. Disposal of these substances is difficult because thermoset plastics melt when burned, clogging incinerators and often emitting harmful fumes, e.g., the hydrogen chloride gas given off by polyvinyl chloride (PVC, that ultimately leads to the formation of dioxins and furanes). Nonetheless, less problematic, heterogeneously mixed plastic material are currently destined for large scale incineration in the production of heat and electricity. Plastic bags, cups, jars, and any other unsealed containments most likely are processed in this way.
Þ For further details, refer to the residual waste (incinerator) section.

The non-combustible fraction, is usually quite bulky wasting precious space as these do not compact as readily in landfills. In addition, to lower the total organic carbon load (TOC) that would otherwise would add to the pile of residual solid waste, plastics with such a high carbon content need to be collected and disposed off separately - particularly in communities with limited landfill capacity.
Þ For further details, refer to the landfill section.
In this regard, the ultimate aim has to be the elimination of this kind of packaging from the waste stream.

Collection: In central Europe, collection points are frequently found throughout cities. In Austria, plastic recycling containers are not yet available for every household - and still an average of 300 inhabitants share one large recycling container. Once the (dominating) packaging industry has completed the shift towards biodegradable and recyclable materials, only then it is feasible to provide every major block with a recycle bin for plastic collection. Currently, it is a major challenge for the average consumer to differentiate between the various forms and types of plastic materials. This simple but disturbing fact makes plastic processing a challenge for any recycling and processing station. Packaging material is scattered loosely amongst the huge amount of light weighted but spacious stockpiles of collected plastics in its manifold forms.

Pile of recycled plastics

Separation: Currently, due to the vast diversity of this material, trained staff is still used to manually differentiate between reusable (thermoplast), unusable plastics (thermoset) - besides the other stuff that does not at all fit into these categories. Even though manual screening is very monotonous, separation has to be done with care to obtain a constant output of reusable substances. Extra caution is required, to avoid injuries due to irresponsible disposal of hazardous waste (syringes, glass, and other bio-hazardous contaminates), which may harm a workers health.

Manual preselection

Processing of Reusable Plastics: The reusable and more homogenous material is finally packed and piled before being forwarded to appropriate processing plants.
Before being introduced back into the production line for the assemblage of new packaging material, it is shredded into tiny pellets and according to its colour automatically separated. Based on the qualitative requirements, a distinct fraction is mixed with prime material to maintain the qualitative properties of the final product; i.e. in general not more than 50% of recycled plastics are mixed to the virgin plastic; doing so avoids major changes in the chemo-physical properties of the product.

Screened plastics for further processing

There are two essential prerequisites for high-quality recycling of plastics: according to the plastic type,
1. the material has to be sorted properly and
2. it has to be free of contaminants.
Irrespective of their colour, the "Slidespec" separator is able to identify plastics within a fraction of second. A sparking pistol device is pressed onto the plastic pellets, vaporising a small part of the plastic's surface. This process results in an atomised vapour (atomically brought in an excited state) that emits small amounts of light radiation upon relaxation of the excited atoms back to ground level. The type of plastic is then identified on the basis of a programmed pattern match with the emission lines in the optical spectrum. According to the slide-spectrometric results, the plastic materials are sorted, pulverised and, if necessary cleaned. Finally, they are melted in an extruder and processed into granulates or prefabricated compounds, which are then manipulated in subsequent production steps.

Plastic granulate

Plastics are made up chiefly of a binder consisting of long chainlike molecules called polymers. Such binders can be natural materials, e.g., cellulose, or (more commonly) synthetic resins, e.g., bakelite. The permanence of thermoset plastics are due to the heat- and pressure-induced cross-linking reactions the polymers undergo. Thermoplastic can be reshaped because their linear or branched polymers can slide past one another when heat and pressure are applied. Adding plasticizers (controversially, some manufacturers still use heavy metals for that purpose), stabilizers, and fillers improve the properties of the plastic compound, e.g. hardness, elasticity, and resistance to heat, cold, or chemicals. Adding pigments imparts colour. Plastic products are commonly made from plastic powders. In compression moulding, heat and pressure are applied directly to the powder in the mould cavity to obtain the final shape of the product.

Alternatively, the powder can be plasticised by outside heating and then poured into moulds to harden (transfer moulding). It can be dissolved in a heating chamber and then forced by a plunger into cold moulds to set (injection moulding); or be extruded through a sieve, which results in a continuous rod to be cut into lengths or coiled (extrusion moulding).
Extruders: Plastic profile extrusion starts with a raw material; in this case thermoplastic pellets. The material is placed in a hopper chamber mounted on top of the extruder and gravity fed into the rear end of the extruder.

Extruder

The extruder itself is made up of a number of key components: The barrel with heaters acts as an oven; the screw powered by the motor feeds the pellets towards the flange while creating pressure that liquefies the plastic. The downstream end houses several post forming units (cooling, sizing, cut off section, etc.).

Processing of non-Reusable Plastic Components: Roughly half of the scanned plastic material (usually of thermoset properties) is not reusable. This fraction is collected separately and delivered to incinerating stations. The Hich carbon content of these materials raise the caloric value of the otherwise poorly combustible solid waste fraction.
Þ For further processing of plastics, refer to the residual waste (incinerator) section.

In many cases though, this fraction is often shredded and added to the garbage that is destined for a landfill or other approved dumping site.
Þ For further processing of plastics, refer to the landfill section.

Non-reusable plastics

References:

Fry T. (1992); Green Desires; EcoDesign Foundation Inc.; University of Sydney, NSW - AUS

For additional information visit one of the following web-sites:

http://www.world-plastic.com/extrusion.html
http://www.plastex.co.uk/
http://www.agr.de/show/show_eng.asp
http://www.recycle.net/recycle/Plastic/
http:// Injection Mold-Driven Funding

Intro / Paper / Glass / Plastic / Metal / Compost / Toxic / Residual / Sewage / Landfill