Pvc Soil Contamination

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Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy

INTRODUCTION

Contamination in recycling is an extremely broad topic encompassing a diversity of forms such as dirt, partially oxidized polymers, printing inks, paper, pesticides, metals, foil, additives (e.g., fire retardants and antioxidants) and their transformation products. Contamination in polymers for recycling is ubiquitous; be it soil in LDPE mulch film or metal fragments from aluminum caps in PET. Furthermore, plastic products are increasingly multicomponent systems comprising pigments, fillers, labels, glues, laminates and coatings. The potential for undesirable/antagonistic interactions and the probability of undetected contamination being present, leading to a reduction of the recycle quality, is high. Even in curbside collection programs solely for PET and HDPE bottles, collected in plastic boxes, it has been found there can be 15 to 20% contamination by foreign bodies.1 Table 1 is a comprehensive list of common contaminants found in recycled polymers.

Table 1: Common contaminants in recycled polymers

Polymer

Recycle source

Contamination

PET

beverage bottles

PVC, green PET, Al, water, glue, oligomers

HDPE

milk/water bottles

PP, milk residue, pigments, paper, EPS, cork

LDPE

greenhouse films

insecticides, soil, Ni, oxidation products

LDPE

shopping bags

paper receipts, printing ink, food scraps

PP

battery cases

Pb, Cu, acid, grease, dirt

HDPE

detergent bottles

paper, glue, surfactant, bleach, white spirit

PET

photographic film

silver halides, gelatin, caustic residues

phenolic

circuit boards

Cu, tetrabromobisphenol A

LDPE

multilayer film

ethylene vinyl alcohol, polyamide, ionomer

PVC

beverage bottles

PET, PE, paper, Al foil, PP

ABS

appliance housings

polybrominated flame retardants

SBR

automobile tires

steel wire, fiber, oil extender

LDPE

mulch film

soil (up to 30%), iron (up to 3% in soil)

SBR - styrene-butadiene rubber; EPS - expanded polystyrene; ABS - acrylonitrile-butadiene-styrene terpolymer

SBR - styrene-butadiene rubber; EPS - expanded polystyrene; ABS - acrylonitrile-butadiene-styrene terpolymer

Recycling of polymers is generally considered to be the most environmentally friendly alternative for dealing with the waste disposal problem. Even if consumers can be persuaded/educated to segregate polymer products prior to disposal in the municipal waste stream, there will always be the problem ofsep-arating inadvertent mixtures, multicomponent products and adventitious contamination. One process, which is economically attractive and widely used to separate impurities from a stream which is predominately one polymer, is by using differences in density. The separation of the HDPE base cup from the PET beverage bottles is one such example. There is, of course, the alternative not to separate the plastic waste and to use it in its commingled form. A greater degree of separation and purification is possible if the polymers are solvated. Multicomponent plastic products can be separated using mixtures of solvents that allow for selective dissolution.2

Finally given that today's polymer recycle is comprised of materials made perhaps 20 years ago, this raises the question over the types of additives that are present and their conversion products after the recycling process. Fire retardants, in particular, are a class of additives that are used in relatively high concentrations (e.g. 10-15%) in many polymeric applications from building products, electrical and electronic systems, transportation vehicles to furniture and appliance housings. Certain halogenated fire retardants, which have been banned due to potential toxic hazards, are nonetheless still present in many of the polymers in use today and may inevitably enter the recycling stream.

CONTAMINATION BY OTHER POLYMERS

PVC in PET

In Europe, the majority of bottles for carbonated beverages and spring water are manufactured from either polyethylene terephthalate, PET, or polyvinyl chloride, PVC. As such, these polymers are the most commonly encountered recycled plastics and provide the main feedstock for the plastics recycling industry. PET and PVC, however, are often found to contaminate each other mutually by virtue of their very similar specific weights. As a consequence, they cannot be separated through ordinary physical processes such as water flotation or air elutriation and, therefore must be separated by more sophisticated means.1 PVC is not stable at the processing temperatures of PET (i.e., approximately 280oC). At these temperatures PVC readily undergoes dehydrochlorination leading to copious evolution of hydrogen chloride. As a result, many purchasers of recycled PET specify that PVC be "undetected". The presence of as little as 100 ppm of PVC as flake in recovered PET can cause serious polymer discoloration of PET during its drying phase and lead to contamination of the polymer with black specks during extrusion.3 Yet there are applications where up to several hundred ppm of PVC is tolerable.

HDPE in PET

Another common polymer used in consumer bottles, especially milk bottles in the US, is high-density polyethylene, HDPE. Since the consumer does not always segregate HDPE and PET bottles and because of the high-volume usage there is a high probability that these two polymers are commingled when arriving at the recycling plant. These two polymers are incompatible in the melt, and upon solidification remain as separate phases. The HDPE domains are visually objectionable, and can cause mechanically weak zones leading to failure in the PET matrix.4

PP in HDPE

Polypropylene, PP, is often used to manufacture closures and caps for HDPE bottles (particularly shampoo, detergent and bleach bottles). Since HDPE and PP have similar specific gravities they are virtually inseparable by common physical separation techniques. Sophisticated IR spectrophotometric separation methods are, however, capable of discriminating between these two polyolefins. A problem arises during melt processing of HDPE and PP due to the incompatibility of these polymers. Typical defects that arise due to low level PP contamination are unfused 'lumps' of PP in profiles, poorly-formed parisons in blow-molding and melt-fracture in extruded films. Furthermore, the addition of a few percent of PP to HDPE can lower its low temperature impact strength. This contamination has been thoroughly investigated by FTIR microscope techniques. In contrast, to these findings, work by engineers at Quantum Chemical Co. (US) claims that up to 10% PP contamination can be tolerated in recycled HDPE.5

PET in HDPE

Since PET is a solid at the normal processing temperatures of HDPE, unmelted PET can quickly plug melt delivery channels and injection nozzles in melt processing machinery.4

CELLULOSE in HDPE

Many HDPE and PET bottles (such as milk and beverage containers) have paper labels which are usually removed during the washing stage in the recycling process. However, residual paper in the form of cellulose fibers have been found to cause major problems (creation of holes and surface defects) during blow molding of the reprocessed resin.

MULTILAYER POLYMERS

The advent of coextruded and multilayer films for barrier applications has complicated the recycling of polymer films due to the presence of polyamides and EVOH which lack compatibility with the PE or PP recycle.

Table 2: General effect of non-polymeric contamination on polymer recycling

Contaminant

Effect on recycling metals paper, fibers soil, dirt pigments, dyes water lubricating oil milk terephthalic esters hydroperoxides herbicides flame retardants caustic residues plugs injection nozzles; catalyzes polymer oxidation

'blow-outs' in molded bottles; 'bubble' collapse in film extrusion lowers aesthetic quality of recycle; causes gels and stress concentrations unwanted color variation; catalysis of polymer oxidation hydrolytic degradation in PET; surface defects in PE

undesirable odors; processing fumes recycle plasticization, lowering impact strength; rancid odor of butyric acid discoloration of PET

initiates thermal and photooxidative reactions toxic fumes present hazard to operators may produce supertoxic compounds fogging of photographic film emulsion on PET

CONTAMINATION BY PACKAGING CONTENTS

Plastic packaging, such as HDPE bottles, are often contaminated by their contents that have migrated into the polymer or by residues which are difficult to remove by standard cleaning methods. Often such contamination of the polymer can also arise from non-intended secondary use ofplastic containers by the public for the storage of chemicals. Table 2 summarizes the effects of common non-polymeric contamination on the recycling of polymers.

Hope et al.6 studied the effect of various contaminants (types of household industrial chemicals) on the recycling of HDPE bottles. Detergent, bleach, lubricating oil and white spirit bottles were recycled. They found that the contaminants generally were absorbed into the walls of the container and were not removed by thorough washing. The main contaminants in the recycled pellets were 1-2 wt% of white spirit, 0.7 wt% of hydrocarbon oil, and between 100-500 ppm chlorine from the bleach.6 Another common contaminant in post-consumer recycle, PCR, HDPE bottles is by residues of motor oil. As a consequence of reprocessing, the oil can impart objectionable odors to the recycled product. Such oil contamination can be a problem even at levels below the threshold of detection ofsensitive analytical instruments such as gas chromatography-mass spec-troscopy, GC-MS.

The effect of milk residues on the quality of recycled HDPE from used milk bottles has been studied.7 It was found that the major component of the rancid milk was butyric acid which can diffuse into the walls of HDPE bottles. Other studies have shown that butyric acid contamination in recycled HDPE can lower its tensile strength and mechanical properties due to an internal plasticization/lubrication effect.8

Some organochlorine compounds, especially chlorophenols (e.g. dichlorophenol, DCP, and trichlorophenol, TCP) have the ability to taint HDPE at very low levels. In fact, dichlorophenol can produce objectionable odors in HDPE at levels in the parts per billion range. DCP and TCP are notorious in the dairy industry for tainting milk bottles where levels as low as 5 ppb can be readily detected in milk due to their low threshold oftaste detection. The typical scenario by which DCP enters the recycle stream is where post consumer beverage containers are used to store herbicides. The residue in one such container which enters the reuse stream has the ability to taint thousands of unaffected containers.

Recently there has been effort focused on the recycling ofHDPE containers which contain pesticide residues.910 This area poses special problems which are difficult to overcome since the products formed by pyrolysis or incineration recycling of these containers have not been adequately characterized to date and may involve the production of highly toxic species.

Contamination of PET by foreign substances has been investigated11 using health-endangering chemicals to evaluate leaching and migration effects of these substances in PET. It was concluded that, even with highly toxic substances such as organochlorine pesticides, the migration into PET is extremely low and moreover, the leaching of these compounds from the PET is so low (only a fraction of the migration rate) that the final values are well below the average daily intake specified by the FDA.11 The incidental contamination of PET has also been studied using model compounds and diffusion modelling which determined that the cleaning process associated with conventional PET reprocessing is sufficient to eliminate harmful migratory contaminants.12

The studies to date thus show that while HDPE is easily contaminated by liquid contents, PET is less affected. This can be attributed, in part, to the oleophilic nature of HDPE and its susceptibility to environmental stress cracking. While PET, when biaxially oriented (as is the case in the stretch-blown beverage bottles), is highly crystalline and generally impervious to the ingress of migratory compounds. In addition, the relatively high processing temperature of PET means that any sorbed contamination is likely to break down during reprocessing. HDPE only otherhand, is reprocessed at considerably lower temperatures and many toxic compounds could be expected to survive this heat treatment.

CONTAMINATION BY THE ENVIRONMENT

SOIL IN MULCH FILM

The recycling of mulch film has been actively investigated by Dow Chemicals because the normal practice of disposal by burning it on the fields is environmental undesirable. The contamination levels in mulch film make its recycling particularly challenging. For instance, soil contamination can be as high as 30-40%. Furthermore, the soil can contain up to 3% of iron which is a polyethylene prodegradant.13 In addition, it was found that vegetable matter comin1g3 from harvested plants could not be removed during the washing operations. Other contaminants are fumigants (e.g. methyl bromide) and the oxidized fractions of LDPE due to photodegradation of the mulch film.

MOISTURE CONTAMINATION IN PET

Water is a contaminant which induces hydrolytic chain cleavage of PET and as such the polymer should be rigorously dried before melt reprocessing.14 The drying of recovered PET flakes is performed at temperatures of 160-180oC in order to lower the moisture content to below 50 ppm which is essential for proper blow-molding of PET in order to avoid MW reduction.11 Any moisture not devolatized before the PET becomes molten, rapidly reacts and a surprisingly small amount of moisture can reduce the viscosity of the polymer to such a level that acceptable bottles cannot be be blown.4

CONTAMINATION BY ADDITIVES AND THEIR CONVERSION PRODUCTS

The polybrominated diphenylethers (deca-, octa-, and penta-bromo-diphenylether, PBDE) are excellent flame retardants which act in the gas-phase by "flame poisoning". However, there is concern that these materials form supertoxic compounds such as polybrominated dibenzofurans, PBDF, and polybrominated dibenzodioxins, PBDD, during high temperature decomposition as shown in Figure 1. Nevertheless, because of the outstanding efficiency of PBDE's they are universally used as flame retardants in most plastics. Such is their effectiveness that there is currently no equivalent substitute for imparting flame retardance to upholstered furniture. Moreover, though the use of these systems may be curtailed in the near future, the immediate predicament is that PBDE-containing polymers which were fabricated 10-20 years ago are now reaching the end of their useful life and entering the recycle stream.

Deca Pbde Degradation

Brx Bry pbdd

Figure 1. Schematic of reaction pathways to the production of polybrominated dibenzofurans and polybrominated dibenzodioxins from polybrominated diphenylethers (a common family of flame retardants for polymers). Note: x+y can equal 5, 8, or 10 implying penta-, octa- or deca-substitu-tion.

Brx Bry pbdd

Figure 1. Schematic of reaction pathways to the production of polybrominated dibenzofurans and polybrominated dibenzodioxins from polybrominated diphenylethers (a common family of flame retardants for polymers). Note: x+y can equal 5, 8, or 10 implying penta-, octa- or deca-substitu-tion.

A recent paper by Meyer et al.16 indicates that it is the flame retardants that determine the recyclability of a polymer. Engineering plastics such as acrylonitrile-butadiene-styrene which contain PBDE are unsuitable for recycling because of the generation of PBDF and PBDD's. Moldings containing PBDE must therefore be separated before recycling.16 The recycling of printed circuit boards has been investigated by Lorenz et al.17 and it was found that the combination of the presence of flame retardants (such as, tetrabromobisphenol A), elevated temperatures and the catalytic effects of copper represent typical formation conditions for PBDD. Levels of PBDD up to 4.5 ng/g were detected after heating the printed circuit boards in an oven at 300oC.17

Another important fire retardant is polybrominated biphenyl, PBB, which is both intrinsically toxic and can form PBDF and/or PBDD in pyrolysis or combustion.18 Today, however, its use is highly limited, but it has been widely used in the past.

Flame retardant additives can also modify the types of products that are released during incineration of waste polymers and in this way the flame retardants are contaminants which also affect the quaternary recycling of polymers (that is, the incineration of polymers to recover heat). For example, the thermal degradation of polyurethane, PU, can produce a complex mixture of products. However, in the presence of a common phosphorus-based flame retardant (such as ammonium polyphosphate), aniline (which is quite toxic) becomes a relevant volatile combustion product.19

Another example of additive modification leading to contamination of the recycle stream involves UV stabilizer based on metal complexes. Low-density PE greenhouse films often contain UV stabilizers based on nickel complexes which quench excited states (the so-called nickel quenchers). Whilst, nickel complexes are stable at polyolefin processing temperatures (i.e., 200-250oC), they degrade at higher temperatures such as those encountered in the processing of PET (i.e., >280oC). Therefore, during reprocessing of mixed plastic scrap, nickel ions can be produced and these, by virtue of their polyvalent nature, can act as prodegradants and catalyze oxidative decomposition of the polymer.

The pigments used in PE moldings and bags are often based on inexpensive metal oxides. For instance, the common brown, gray and orange pigments are based on various iron oxides and hydrates which act as prooxidants/prodegradants at the high temperatures (i.e., 220oC) encountered during reprocessing of HDPE. Moreover, green pigments are usually based on chromium(III) oxide which can readily catalyze the thermooxidative degradation of HDPE even in trace quantities. Virgin PE is usually adequately stabilized so that these catalytic compounds do not cause in-service degradation of the polymer, however, with reprocessing the antioxidants are usually consumed and these pigments may then be able to exert their prodegradant effects.

The recycled polymer can also be contaminated by pigmented constituents in the feedstock. In the recycling of RDPE bottles by melt reprocessing, a major effort has been directed towards producing a naturally-colored recycle stream.20 The major barrier to overcome to reach this end is the removal of the colored bottle caps. Typically blue and red caps are used to differentiate the fat contents of the milk. Manual removal is too labor intensive and is thus not economical. Since the weight of the cap is typically 10% of the weight of the bottle, some recyclers have opted to include the caps into the recycle stream. The effect of this is that the recycled product has an olive or "dirty" green color due to the presence of red and blue caps.21 As the caps are generally also PE they cannot be removed by the float/sink process. Some companies have had limited success removing the caps by air classification and elutriation techniques, whereby the caps which are more thicker-walled than the bottles are separated out in a column of air. There is now a move to produce the caps from PP but this causes other problems due to its higher melting point. A number of manufacturers have installed color sorters which rely on electronic optical scanners in combination with high-pressure air jets to selectively remove colored fragments from the recycled material. Companies such as Simco, Pulsar, ESM, Satake, and Sortex have commercially available models of such color sorters that are capable of sorting up to 6 tones of material per hour. Although colored recycle is acceptable for most applications it is of concern for blow molding of lightly colored bottles.

Color contamination can also occur in PET. Clear PET bottle flake can sometimes contain some green chips. Perfect manual separation of green from clear bottles is possible but expensive. Although melt extrusion homogenizes the polymers and the colors, it takes only 1000 ppm of green PET chips to create an observable color shift in clear PET.

LDPE film is extensively used for packaging and for the production of shopping bags. These films often contain a fatty-acid amide lubricant (usually cis-docosenamide) which can be oxidized during thermal reprocessing of LDPE film. The lubricant readily undergoes cleavage at the unsaturated site to give a homologous series of aldehydes which have very low odor thresholds. Such contamination imparts to the recycle, a rancid odor which may restrict its application potential.

Recycled PET from used x-ray film must be carefully analyzed for contamination if it is to be used for the production of new photographic base film. This is because caustic residues from the silver recovery process can cause fogging of the photographic emulsion.22

CHEMICAL MODIFICATION OF THE POLYMER

Environmental aging and oxidation of polymers can induce chemical modification of the polymer structure and the production of oxygenated species.

These oxygenated species are in fact "impurities" which can cause sensitization of the polymer towards further thermal and photooxidation. The most destructive self generated impurity in many polymers is the hydroperoxide group. Since hydroperoxides are thermally unstable and photolabile they can readily break down to initiate a chain of radical reactions. Other impurities are carbonyl groups which are products of oxidation and can act as chromophores in photooxidation. For example, greenhouse films, usually based on LDPE, are frequently recycled. These films are usually quite degraded by UV light and are used to manufacture films for low-grade application. Such films usually have low mechanical strength and low intrinsic stability.23

A number of recent papers in the literature have discussed the recycling of

fluoropolymers. Although, the reuse of fluoropolymers is relatively scarce compared with commodity thermoplastics, their high cost provides incentive for recovery and recycling. During the reprocessing and pyrolysis of fluoropolymers, the possibility exists for the production of perfluoroisobutene, PFIB,25 which is a cause for concern, even in small amounts, because of its extremely high toxicity (the reported 2 hour lethal dose for PFIB in rats is 1 ppm). It has been found that PTFE begins to form PFIB at 525oC, while other fluoropolymers such as poly(tetrafluoroethylene-hexafluoropropylene) copolymers form PFIB at temperatures as low as 380oC.25 The toxic nature of gaseous products of fluoropolymers emphasizes the need for good exhaust ventilation during the reprocessing of such polymers.

This problem is exacerbated where PTFE is used as a flame retardant additive in engineering plastics such as polyphenylene ether, polycarbonate and ABS26 where it imparts non-dripping characteristics during burning of the polymer. In the temperatures encountered in polymer incineration, PTFE would almost certainly produce dangerous levels of PFIB.

CONTAMINATION DURING PROCESSING

METAL CONTAMINATION

Metal contamination (also referred to as "tramp" metal) can processing equipment itself(such as wear fragments from extruder gates, granulators, aluminum adapters, etc.). Small fragments of metal can cause blockage to injection nozzles of injection molding machines resulting in defective moldings and nozzle damage. In addition, ferrous/ferric ions due to their multivalent nature, act as oxidation catalysts through redox reactions. Processing of PVC waste can also promote acid-induced corrosion of the extruder components resulting in rust particle contamination.

GEL CONTAMINATION

Recycling of PCR HDPE (usually milk bottles) by melt extrusion can lead to crosslinking during the thermal reprocessing stage since the antioxidant added initially (during manufacture of the polymer) is consumed.27 Loosely crosslinked regions (known as 'gels') can acts as stress concentrations in film and cause 'blow-outs' in bottles made from recycled HDPE.

DEGRADED POLYMER CONTAMINATION

A common source of contamination in recycled HDPE (as well as virgin HDPE) is 'black specks'. These are small areas of highly degraded polymer that have been carbonized due to excessive residence time in an extruder. These 'black specks' typically occur in low flow regions in the extruder where 'hang-ups' form. Often, such contamination may also appear yellow, brown, or amber depending on the extent of degradation. In fact, under microscopic examination, a color gradient may be evident indicating that the darkest side has been in direct contact with the wall of the extruder barrel. Black specks cause a major problem in the blow molding of natural or white bottles where they are aesthetically undesirable.

In PET, degraded resin impurities can also arise as a result of reprocessing. Such impurities are often self-generated during the melt reprocessing step. During extrusion of PET, oligomers (both linear and cyclic) can form as a result of polymer break-down. Such oligomeric impurities diffuse towards the surface of the recycled PET fibers or film and greatly affect their surface properties and hence adhesion. Problems with printability, dyeability, and treatment can often

occur. Oligomeric products of PET include terephthalate esters (e.g., monohydroxyethyl-terephthalate, monomethyl-terephthalate and bis-hydroxyethyl-terephthalate). These oligomeric contaminants can also lead to discoloration (i.e., yellowing) of the recycle.22

DETECTION OF CONTAMINATION

A number of patents have been issued in which the nature of impurities and their detection in recycled plastic is achieved by measuring their IR emis-

sion and where the post-consumer plastic itself is categorized by IR imaging. A novel method for detecting contamination in mixed plastics uses a pulsed laser beam to form a plasma which is then spectrally analyzed. A rapid technique for determining the level of PVC in mixed plastic waste has been described where the commingled polymer powder is thermally degraded using a linear temperature ramp from 260-360oC and the vinyl chloride polymer content is calculated from the HCl concentration in the evolved gas.29 PVC contamination is of particular concern in polymer recycling since it can adversely impact upon the recycling process (see Table 3).

Table 3: The effect of PVC contamination on various methods of recycling

Classification

Method

Effect of contamination

primary

in-house scrap recovery

polymer discoloration due to conjugated sequences

secondary

melt reprocessing

evolution of HCl at 170oC; machinery corrosion; discoloration and black specs in PET

tertiary

pyrolysis liquefaction

formation of CaCl2 which blocks reactor tubes poisoning of cracking catalyst

quaternary

incineration

HCl and possibly chlorinated dioxins

An automated system developed by Magnetic Separation Systems, Inc. can segregate commingled plastic containers and remove contamination through the use of special sensors. First, a mechanical device debales the bottles and screens out small size and non-plastic contamination. The containers are then presented to an optical sensor which can identify PET, PP, natural and colored HDPE. Another sensor is used to further identify green and amber PET from clear PET containers, while a surface sensing x-ray based sensor identifies PVC

containers.

REMOVAL OF CONTAMINATION

REMOVAL OF CONTAMINATION IN POLYOLEFINS

Equipment has been developed for the recycling of HDPE containers, waste oil and detergent bottles into saleable flakes at rates of 1000 to 5000 lbs per hour. The most used system is based on hydrocyclone separation, a process which is effective in separating various types of plastics while virtually eliminating paper and aluminum contaminants. This represents a major improvement in quality and cost saving over the "float sink" process.31

Schimpf32 describes a process for separating polyolefins from household and industrial plastic waste which relies on a two step process comprising firstly a sink-and-float separator and then a hydrocyclone. The hydrocyclone separation allows separation of polystyrene and PVC.32

A process to remove polyester and cellulose contamination from PP bale wrapper for cotton and polyester fibers involves treating the PP with aqueous sodium hydroxide solution followed by an oxidizing agent such as aqueous sodium hypochlorite.33

REMOVAL OF CONTAMINATION IN PET

A method has been devised for the microseparation of PVC from PET in which the PVC is subjected to a process of selective bulking which causes it to float.34 This method is ideally suited for the removal of small quantities of PVC from large quantities of PET bottle recycle as is generally the case in the USA. However, in Europe, PVC can be a major component in the recycle stream and thus must also be recycled. Froth flotation of PVC contaminants provides an efficient way for PVC removal in the recycling of PET bottles.35

A process for separating PET and PVC has been recently patented in which the commingled stream is treated with NaOH and a nonionic surfactant (e.g., Neodol 91-6) in order to reduce the contact angle ofPET flakes to less than 25o whilst maintaining the contact angle of PVC greater than 45o. Then, the flake mixture is added to water and agitated to allow PVC flakes to come in contact with gas bubbles causing the PVC flakes to float and the PET flakes to sink. The system can separate a 50:50 mixture of PET and PVC to yield 97% pure PVC.36

Another method for separating PVC from PET relies on air flotation whereby the PVC is separated through a combination ofvibration and flotation, to ensure separation on the basis of specific weight. This method is especially suited to separating PVC labels from PET bottles.1

A recent patent describes a procedure to remove foreign material from post-consumer PET. The process involves the melt depolymerization of PET by feeding the melt from an extruder into a special separation vessel where low-density contamination floats to the surface of the melt and high-density con tamination sinks to the bottom of the melt and purified oligomeric PET is removed from the intermediate region.37

A method for removing the contaminants in the recycling of PET waste has also been devised.38 In this process, PET is transesterified by heating with ethylene glycol to give a solution containing short chain PET and bis(2-hydroxyethyl) terephthalate. By hydrolysis of this mixture at elevated pressure and temperature, ethylene glycol and terephthalic acid crystals are produced. Activated carbon and clay are used to absorb contaminants for separation by filtration.38

REMOVING CONTAMINATION IN PVC

Contamination of PET in PVC can be successfully removed by melt filtration through continuous screen changing equipment at a processing temperature of 204oC which is below the melting point of PET and thereby allows PET and other solid contaminants to be filtered out.3

Recycled PVC from post-consumer bottles can be purified from contaminants other than PET (e.g., paper, aluminum foil, HDPE, PP, polycarbonate, crosslinked PVC, and fluoropolymers) by melt filtration specially designed to maintain low back pressure. '

REMOVING CONTAMINATION IN PC

Polycarbonate can be purified during the recycling operation by dissolving it in an organic solvent. Thus insoluble contamination (e.g., glass fibers, paints, or metals) can be separated by filtration via special filter beds. Also, unwanted colorants can be absorbed in this step. A solvent process for recycling PC scrap is currently used.41

CONCLUSIONS

Due to the myriad of sources of contamination in polymer recycle, the effect of contamination is difficult to predict and even more difficult to quantify. The plastics recycler may not be aware ofthe presence ofsome contamination since it is present in such low amounts. It should be bourn in mind that, despite the possible low levels, some contamination, for example, transformation products of flame retardants are extremely toxic and thus can pose a health risk to operators of the recycling process and end-users of the recycled product. Thus in all forms of recycling, especially pyrolysis and incineration, a careful approach is necessary in order to avoid the production of supertoxic compounds.

In other areas, the presence of contamination may not pose any health risk but simply lowers the quality of the recycle and devalues the economics of the recycling process.

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  • bobby
    Why does pcr hdpe smell?
    10 years ago
  • riikka
    Do pp catalyst residues affect cyanoacrylate glues?
    9 years ago
  • prisca
    Can guanidine hydrochloride cause stress cracking in hdpe bottles?
    6 years ago
  • rosa
    Why does hdpe and pet have to be separated from pvc?
    3 years ago
  • SAMI
    Can pvc has metal contamination?
    12 months ago

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