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Controlled Polymer Recycling and Degradation a Tutorial

Gyrgy BnhegyiDepartment of Advanced Materials and Processes

Bay Zoltn Nonprofit Ltd. for Applied Research

gyorgy.banhegyi@bayzoltan.huABSTRACT

Polymer materials became an indispensable part of our life in the 20th century and they will surely be with us in the 21st century as well. They are, and will be, indispensable because of their advantageous properties, such as low density, relatively low specific energy consumption during production, easy and large scale processability, but they also require keen attention because of their environmental effects, such as permanence, slow degradability, potentially harmful additives etc. In this tutorial only a few relevant aspects of this complicated problem can be touched, and even those very briefly: the origin and distribution of plastic waste; main directions of utilizing/recycling/disposing this waste; recycling of plastic packaging waste; electrical and electronic waste; automotive waste; commingled plastic waste; thermosets and elastomers; the inter-relation between recycling and design; pros and cons of controlled degradation; controlled photo-oxidative degradation; bio-degradation and finally the possibilities of a sustainable polymer technology.Keywords

Keyword 1, recycling 2, plastics 3, environment, 4 thermoplastic, 5 thermoset, 6 rubber1. Introduction

Nowadays it is fashionable and not only in the green circles to blame plastics for almost all environmental problems. Philippics against PVC can be read and heard everywhere, although the simple question: what would the environmentalists do with the huge amounts of chlorine obtained as a byproduct of caustic soda production is rarely answered (in fact, the problem is usually not even recognized). Some of the hardliners would answer: you should not produce caustic soda either. I wonder how they could tolerate each others smell without using soap which requires caustic soda. But let us not be sarcastic: plastic waste is a huge problem that should be handled carefully [1]. Waste should be considered as a potentially valuable resource instead of simply being a burden [2]. Before seriously considering the elimination of plastics, the economic, social and other costs of replacement should also be considered. Simple negation of a technology may sound attractive but a real answer should always discuss the consequences. If we remain at the level of political propaganda, the solutions will not survive the next political cycle.Due to the complexity of the problem, which is not only technical, but also economic and societal in nature, and to demonstrate the actuality of the discussions related to this topic, only a minor part of the references used in compiling this tutorial were taken directly from the technical literature, most of them come from various surveys made by authorities, websites etc. No comprehensive review can be attempted at this length, therefore only brief comments on selected examples can be offered. Nevertheless I hope that at least the versatility of the various approaches and the inter-relatedness of the emerging problems can be presented.2. Basic notionsBefore starting the discussion of plastic waste, some very basic notions should be clarified. Plastics got their name from the fact that they can be shaped relatively easily in the molten state by highly efficient methods (such as injection molding or extrusion) and maintain their shape after cooling.This basic property is closely related to their molecular structure, i.e. to the fact that they are composed of long, relatively flexible, mostly linear molecules - in contrast to low molecular materials. Upon melting low molecular materials (including metals and ionic solids) form low viscosity liquids, mostly described by Newtonian viscosity, while polymers tend to exhibit visco-elastic properties both in molten and in solid states.

Another important difference from low molecular solids and liquids is that in plastics the molecules are rarely uniform. These polymer chains (repetition units are called monomers) are of different length, therefore the molar mass exhibits a distribution characteristic of the polymerization method. This molecular mass distribution has far-reaching consequences on the viscosity of the polymer melt (essentially determined by the average molar mass), while the shape and breadth of the distribution curve influences such phenomena as the frequency or shear rate dependence of the viscosity or the melt strength. Not only the molar mass values of the various polymer molecules are not uniform: the molecules may exhibit various polymerization defects. One typical defect is branching, which may be short chain branching (if the length of the branch is negligible with respect to the longest linear chain) or long chain branching (if they are comparable). Branching means that at least some monomers are trifunctional and not bifunctional. Trifunctional units may be added to the monomers consciously or they may be formed by chance as a polymerization defect. If the branches are long and if the concentration of the branching units reaches a critical concentration the polymers becomes a gel, i.e. a non-flowing network is formed. On this basis so-called thermoplastic and thermoset materials are distinguished: thermoplastics contain linear (or slightly branched) macromolecules where the branching density does not reach the critical value necessary for gelling. Thermoplastics can be re-melted, which makes their recycling easier. Even linear macromolecules can be rendered thermoset by crosslinking reactions (e.g. vulcanization, radiation crosslinking), typically used in rubbers. Typical thermoset resins are, however, relatively low molecular, easy flowing liquids which become multifunctional during the so-called curing reaction. Once the network is formed, it cannot melt (unless if it is based on physical crosslinking or reversible chemical bonding), only degrade at high temperature (hence the name: thermoset).Plastics are typically amorphous (lacking long-range order), exhibiting one typical transition temperature: the glass-rubber transition (Tg). Unlike melting, it is not a thermodynamic transition (although some believe that a so-called second order transition exists close to Tg), but a relaxation transition, where the quasi-solid amorphous material becomes a visco-elastic melt. (At even higher temperatures the melt tends to become more purely viscous). Some other plastics exhibit two, intimately mixed phases: crystalline and amorphous. The crystallites are typically very small and the degree of crystallinity may strongly depend on the thermal and shear history. Such, so-called semi-crystalline polymers exhibit two transitions: the glass-rubber transition (Tg) and melting (Tm). Generally semi-crystalline polymers exhibit higher thermal resistance, although their processing may become more complicated because of secondary crystallization.It should be noted that industrially available plastics are much more than simple polymers, they are rather compounds, usually intricately formulated multi-component materials. Even plastics containing only one kind of polymer need additives that render them well processable and stable under e.g. outdoor conditions. Other plastics are blends, i.e. mixtures of more types of polymers, still others contain organic or inorganic reinforcements or functional fillers. This complexity makes the recycling activity more complicated as an additive designed for a certain plastic compound may be detrimental for another purpose. This emphasizes the necessity of selective collection and/or separation of various plastics from the waste stream.

A last, practically important classification of plastics is the distinction of so-called commodity plastics (polyolefins, vinyls and styrenics nowadays also PET is added here), engineering plastics and high performance polymers (the latter are sometimes divided into high temperature and very high temperature grades). Therefore the commercially available polymers are frequently represented by a pyramid, consisting of two halves: amorphous and crystalline polymers. The broad base of the pyramid is the family of commodity resins. The next stage is the group of engineering plastics (such as polyesters, polyamides, acrylics, polyacetal etc.), while the top of the pyramid include high performance engineering plastics. The higher we climb on the pyramid, the higher are the temperature resistance values and the price and the lower the production volume. The decrease of production volume is faster than linear: almost 75% of the EU plastic consumption in 2011 belonged to the commodity plastics. Almost 40% of the total demand was used by the packaging industry, about 20% by building and construction, 8% by the automotive industry and 5% by the electrical and electronic industry [3] (see Figures 1 and 2).

Figure 1 Plastic consumption in the EU in 2011 by plastic types (Source Plastics Europe)

Figure 2 Plastics consumption in the EU in 2011 by applications (Source Plastics Europe)

3. Plastic waste its origin and distributionNo wonder then that the majority of plastic waste also comes from the commodity plastics. Some important sources plastic wastes are listed below [4]: Plastics conversion, processing waste, scrap, (in-house)

Packaging waste (selectively collected)

Post-consumer waste (selectively collected)

Municipal solid waste

Agricultural waste

Construction and demolition waste

Automotive waste (end-of-life vehicle, tires)

Electrical and electronic waste

While about 45% of the industrial waste is recycled, and only 10% ends up in the landfill, about 70-80% of the domestic plastic waste is incinerated. The composition of municipal solid waste (MSW) may vary from country to country, even from town to town, the definition of plastic waste is also somewhat ambiguous in various statistics (whether rubber, man-made fiber or paint is included or not), but on the average 10-15 wt% of the MSW is plastic [5] (see e.g. Figure 3). The distribution of plastics in the waste closely resembles the production rates [6]: about 81% of them belong to the commodity plastics (see Figure 4).

Figure 3 Distribution of the municipal solid waste in the USA in 2010. Source: www.eschooltoday.com/waste-recycling/sources-of-waste.html

Figure 4 Distribution of plastics in the municipal solid waste. Source: www.naturalstateresearch.com/ProductsPage.html 4. What can be done with plastic waste?There are more levels and possibilities of recycling [7]. The ideal is mechanical or material recycling where the waste is mechanically ground (reclaim, regranulation, scrap reuse). It is feasible with production waste or with selectively collected plastic waste (e.g. packaging waste). It is less efficient with commingled plastic waste (fractionation is necessary), even less with municipal solid waste. The main limitation of this technology is the immiscibility of most thermoplastics with each other and the contamination (partial degradation, soil, foreign particles etc.). Upgrading of plastic waste, where the original properties are reached or even exceeded is very rare downgrading is much more frequent. It means that in the majority of mechanical recycling technologies the original application is not feasible, some less demanding products should be sought. Chemical recycling means that waste is converted into other, valuable raw materials by depolymerisation [8], cracking, hydrogenation, hydrolysis (in the case of polycondensation polymers), solvolysis (PUR), peptization (rubbers). Biological recycling by bacteria, enzymes means conversion of plastics to lower molecular products such as biogas, biomass and so on. This possibility is very much limited with fossil based plastics, it can be used, however with so-called degradable plastics. Energy recovery, i.e. direct burning or co-firing in cement kilns, blast furnace, in combination with other fuels is an obvious option but one should take care of the potentially dangerous byproducts. (This is true for municipal waste incineration as well). One cannot emphasize enough the responsibility of the industrialized states vs. the developing countries: illegal waste trade, pollution, exporting the problems should be prevented by all means!Figure 5 shows the flow chart of new and waste plastic streams in the EU from a survey made in 2010 [9].The European Union issued several directives related to the waste problem, e.g. the EU Directive 94/62/EC of 20 December 1994 on packaging and packaging waste, which prescribed 22.5 wt% recycling by 2008, or Directive 2008/98/EC which demand 50% recycling by 2020. Other relevant directives are 2012/19/EU on the electronic and electrical waste or the end of life vehicle directive 2000/53/EC, which will be discussed later.

Figure 5 Import, consumption, recycling and export of new and waste plastic in the EU based on a 2010 study. [9]The question, whether material recycling or energy recovery is more economic or useful, has been investigated for a long time [10]. From the viewpoint of energy utilization alone, incineration should be preferred. Such questions are, however, more complicated than that and the whole complexity of problems, including environmental pollution and social costs should also be included in the balance. The answer always depends on the technology used therefore decision should be supported by constantly updated calculations. This will be discussed in some length in relation to LCA (Lifecycle Analysis) methods.4.1. Plastic packaging wasteIn view of the large amount of plastics used for packaging, the importance of this kind of waste in environmental pollution and the relatively good chance for collecting at least a substantial part of this kind of waste before getting into the MSW stream plastic packaging waste is a good target of recycling [11,12]. Recycling of agricultural plastic waste (at least that of agricultural films) overlaps partly with the technologies used for recycling plastic packaging waste (films), although it has specific features due to the different nature of contamination and to the differences in the additive package. (We will return to this latter problem when treating the controlled degradation problem).The majority of packaging scrap is in the form of films: bags, sacks, various kinds of food packaging (food contact and non-contact grades as well), shrink films, stretch films, cling films and so on. There are problems even with selectively collected films: one major problem is that most up-to-date films are multilayer structures, where, in addition to polyolefins (LDPE, LLDPE, HDPPE, olefin plastomers, PP) typically there are other layers as well (such as polyamides, tie-layer resins, which are typically olefin copolymers with polar co-monomers or grafts). Another possible problem is contamination [13,14] by greases, other organic components, soil etc. A third problem is that even various types of polyolefins are not necessarily compatible [15] (they are, in fact, normally incompatible) which means severe downgrading in properties (especially in elongation at break) if simple melt mixing is applied. The use of compatibilizers (mostly block copolymers) may be of some help. A further complication may come from composite laminates, such as plastic/paper, plastic/aluminum or paper/plastic/aluminum multilayer packaging. These are very modern and highly functional, but the components cannot be easily separated from each other, thus hampering recycling.In addition to films other plastic packaging materials are available in the form of injection molded or vacuum or thermoformed boxes, cups, blow molded containers or bottles. Boxes and cups are usually made of PE, PP or PS, while bottles and containers from HDPE [16] or PET (PVC is much less used nowadays for this purpose).Collection of selective packaging waste should begin at the retail stores where e.g. a large portion of shrink and stretch films can be recollected after product delivery. Other parts of packaging waste can be collected selectively by the households. Nowadays this selective collection is mostly limited to PET bottles. In less developed countries the majority of plastic waste can be found in MSW. If it goes to landfill, it causes problems; if it is incinerated, at least the energy content is recovered.A typical recycling line [17] which can be used not only for plastic packaging waste, but also for other plastic scraps is shown in Figure 6. The milled waste is first washed (cleaned, decontaminated), the various additives are added and finally the mixture is compounded and granulated. Regranulation lines usually contain easily changeable melt-filtering units, as fine solid particles clog the filters relatively frequently in spite of the initial washing step. In post-consumer waste the polymer molecules are somewhat degraded (oxidized and/or the average molecular weight is reduced). Oxidation may be accompanied by discoloration and the antioxidants and heat stabilizers are typically depleted. Therefore additional stabilization is necessary. Contamination level should be kept low if the recycled polymer is to be used again for food packaging. Leachate level should be very low. The hazards can be significantly reduced if the recycled plastic is used in multilayer films again where the recyclate is in the central layer and the outer layers are made of virgin resin. Co-injection and multilayer blow molding are similar strategies in other applications.Molecular weight reduction in post-consumer PET bottle recyclate may be quite serious. This may be one reason why the majority of PET recyclate is used for fiber purposes and not for bottle production (see Fig. 7.). Fiber spinning requires much more fluid plastic grades than bottle production.

Figure 6 Flow chart of a typical plastic waste regranulation line after Ref. 8.

If PET recyclate is to be used for the original purpose, chain-extension and recrystallization processes are made [18]. In recycled PET bottle manufacturing the multi-layer approach with central recyclate layers is also widely adopted.

Figure 7 Consumption of PET recyclate for various purposes. Source: University of Cambridge, ImpEE project, Topics/Recycling Statistics4.2. Electrical and electronic wasteIn our consumer society the lifecycle of up-to-date electronic equipment became very short, new communication and data storage technologies become completely obsolete within 5 years together with the hardware. This is accompanied by an ever growing amount of electrical and electronic waste, where not only the amount is alarming but the hazardousness of some components. Presently at least two directives deal with this problem is the EU: the WEEE Directive 2002/96/EC and the RoHS Directive 2002/95/EC. Most industrial countries try to regulate the selective collection of electronic waste by the manufacturers and the vendors. This is important because of both the valuable and hazardous materials contents. The electronic waste contains 49% metals, 33% plastic, 12% comes from cathode ray tubes (which may diminish with LCD and other displays), the rest contains different materials [19]. According to the same source in 2005 in the US about 56% of the WEEE plastics fraction was HIPS, 20% was ABS, 11% was PPE (poly-phenylene-ether, which, together with HIPS constituted a popular blend, called Noryl), while the residue (13%) contained other polymers. Nowadays probably the PC content is higher due to the widespread use of PC-ABS alloys, but essentially E-waste plastic fraction is a strongly mixed powder of engineering plastics. The most important fraction of electronic waste is that of metals: some precious metal (Au, Pd, Cu, Ni, Sn, Pb) components are present there in significantly higher concentration than in natural ores [20], and can be recovered after mechanical separation followed by classical or hydro-metallurgical and electrochemical processes. It is quite possible that it will be less expensive to recover some metals from the E-waste than from natural resources. The plastic fraction is less valuable (mainly because of the necessity of component separation and contamination), nevertheless the constituents a relatively expensive engineering plastics. The E-waste recycling rate increased from 10% in 2000 to 24.9% in 2011 [21]. The E-waste problem is not limited to the USA or the EU countries, Asian countries face the same problem [22]. Separation and reprocessing of the E-waste plastic fraction are somewhat similar to those technologies used in the processing of the automotive shredder plastics residue. Material recycling is very much limited by the technical difficulties of component separation, controlled decomposition into low molecular organic compounds, incineration or co-firing are better suited to this kind of waste. Plastics of different chemical composition may be separated from each other by various techniques, based on density, wetting or triboelectric behavior (see the next section). E-wastes are problematic because of the presence of potentially hazardous components: most importantly halogenated flame retardants, halogenated hydrocarbons (coolants in refrigerators and air conditioners), and various kinds of batteries (e.g. lithium ion batteries). This latter problem is dealt with by the RoHS directive, which limits the use of halogenated flame retardants, Cr(VI) based compounds, lead based solder materials etc. The presence of printed circuit boards based on thermosets (mostly epoxies) complicates recycling further which will be discussed in a later section. Prescribing the duty of product collection by the manufacturers and vendors can be easily circumvented by non-EU resident companies. This is a general problem of the consumer society: legislation always lags behind the problems. Alleviation of the E-waste problem is also possible by careful design, which will also be touched briefly in a later section.4.3. Automotive waste

In spite of the slowdown caused by the 2008 economic crisis, production and replacement of older automobiles by the companies and by the population results in a heavy environmental burden. In order to reduce fuel consumption and CO2 emission the portion of plastics used in new car models increased continuously (see Fig. 8).Similarly to the E-waste, again plastics are the least valuable components of the waste. Therefore great efforts are made to recover steel and other non-ferrous metals and the big problem is the economic and useful reprocessing of automotive shredder residue (ASR). As the so-called End-of life vehicle (ELV) 2000/53/EC prescribes 85% reuse and recovery of ELV by 2006 and 95% by 2015 plastic fractions must be dealt with. The average composition of the ASR is shown in Figure 9, taken from a case study in Denmark [24]. Although the margins vary widely, the organic fraction is substantial.

Figure 8 Average material consumption for a domestic light vehicle, model years 1995, 2000, and 2009 (Source: Wards Communications, Wards Motor Vehicle Facts and Figures, 2010Detroit, MI, 2010, pp. 65). The picture is taken from a study made for the National Highway Traffic Safety Administration, USA [23]

Figure 9 Average composition of automotive car shredder residue, taken form Ref. 24The ferrous part is separated by magnetic field, while the rest is mostly sorted based on their density differences [25,26]. Non-magnetic separation technologies include trommel (size) separation, vibration sieving, air classification (cyclons), sink and float, manual sorting, on-line spectroscopy assisted separation, eddy current separation and triboelectric separation. The term sink and float covers methods based on both wetting (froth flotation) and density differences. If using organic liquids (which are less tolerated nowadays for environmental reasons) selective partial swelling of various plastic grades can be utilized to increase density differences. Due to the difficulties involved in plastic separation one may think of burning the organic residue, which has its own problems: high humidity content (2-25%), relatively high inert content (5-40% - resulting in slag formation), widely varying and relatively low heating value (13-25 MJ/kg comparable with that of wood but much less than that of pure polymers, coal or oil). Co-incineration with municipal solid waste or sewage sludge is also possible, but the presence of acidic gaseous products and heavy metal contamination in the dust requires attention. Pyrolysis in reductive atmosphere (CO, CO2, H2 mixture formation) is also possible.Instead of describing a wide variety of technologies, here we briefly refer to a PhD Thesis written by Bodzay, parts of which are published internationally [27-29] in which a density based fractionation method is described for ELV-ASR fraction assisted by on-line spectroscopic methods and various possibilities for upgrading (e.g. flame retardant addition, mechanical upgrading by layered composite formation or transformation to carbon nanotubes by laser pyrolysis in the presence of silicate additives) are suggested. Essentially the following groups of fractions can be treated as independent raw material sources: