Gerald Scott

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Aston University, Birmingham B4 7ET, UK

PVC presents particular difficulties in disposal due to the potential environmental hazards associated with the chlorine content of the polymer. In principle materials recycling provides an ecologically acceptable way of re-utilising the energy content of the polymer but the processing operation is damaging to the durability of recycled artifacts unless the provenance of the waste is known. The principles involved in the protection of PVC against the mechanochemical damage that occurs during processing are discussed for both rigid PVC and its modified forms. The importance of knowing the previous history of PVC waste is emphasised, with particular reference to the stabilisers used. It is suggested that the most effective method of recycling PVC is in a "closed loop", so that the previous history and particularly the stabiliser formulation of the recovered polymer is known.

1 POLYMERS IN WASTE MANAGEMENT

There is no unique solution to the problem of plastics waste and litter. A hierarchy of complementary conservation and disposal techniques are required to solve the escalating problem of waste management in industrial societies.

It is generally agreed that the "systems" approach to waste management should include the procedures outlined below.1 There is as yet no universal agreement as to the order of importance in which they should be placed. It is the opinion ofthe present author that the preferred disposal technique must depend on the type of waste and where it is located. For example, products that appear mainly as litter must be considered quite separately from those which appear predominantly in the sewage system and these should again be distinguished from waste that appears in a controlled waste collection systems. Indeed, some engineering polymers (e.g. from the automotive industry), should be considered

not as a waste but as a resource. In this case, closed loop recycling to the original application with additional stabilisers is a realistic possibility.3

1.1 PHOTO-BIODEGRADATION OF PLASTICS LITTER

The cost and energy usage in the collection and disposal of plastics which appear as litter is unacceptable. Much of this arises from agriculture in the form of mulching film, binder twine and increasingly irrigation tubing, silage bags, and fertiliser packaging. Non-degradable plastics in agricultural land have an adverse affect on the fertility ofthe soil and on the economics ofautomated agricul-ture.4 Controlled photo-biodegradability is the only viable solution to this kind of waste which is almost always heavily contaminated and thus unsuitable for recycling even if it could be economically collected and segregated. By the same token, the only way to reduce plastics packaging litter in the sea and on the seashore5 is to ensure that it photodegrades and/or biodegrades rapidly. Some polymers, notably those containing chlorine, present problems in photo-biodegradation due to the potentially dangerous nature of the ultimate low molecular weight degradation products, almost certainly containing chlorine, that may be released into the environment. It has to be said, then that PVC would not be the first choice for agricultural applications and this must be taken into consideration at the materials design stage.

1.2 COMPOSTING OF PLASTICS IN MIXED HOUSEHOLD AND GARDEN WASTE

The technology of composting to give added value to domestic waste is likely to increase rapidly during the next few years. It is claimed that by the end of the century, every household in Germany will be connected to a municipal composting system6 and other European countries are expected to follow. Ironically, in rural China and in other parts of the Far East, composting has always been the most important way of using waste and with the advent of plastics, severe problems are now being experienced with non-degradable plastics packaging and mulching film. Many manufacturers of de6gradable plastics are targeting the composting environment for their products6 and several commercial photo-biodegradable 7polyolefins are readily oxidised and bioassimilated in aerobic composters. For the reasons given above, biodegradable PVC would not be acceptable in such an application.

1.3 MATERIALS RECYCLING OF SEGREGATED, CLEAN INDUSTRIAL WASTE

Clean, segregated plastics arising from industrial operations (including the ultimate disposal of motor car components) should be recycled in a closed loop system into the primary application.3 Some packaging, notably film wrap, that can be easily collected in bulk from industrial waste may be economically resourced in the same way.8 Clean PVC waste can be effectively reprocessed in this way provided the nature of the plasticiser/stabiliser formulation is known. This will be discussed in detail in the following sections.

Mixed plastics packaging is best dealt with in one of the following ways.

1.4 CHEMICAL RECOVERY FROM MIXED PLASTICS WASTE

Some polymers, (e.g. polymethylmethacrylate, polystyrene) lead to good yields of monomer on pyrolysis.9 Similarly, poly(ethylene terephthalate), PET results in good recovery of monomers on hydrolysis. There is also a case for pyrolysing contaminated mixed plastics packaging to obtain a mixture of hydrocarbons. It can be seen from Table 110 that at moderate temperatures in a fluidised bed pyrolysis unit, ethene and propene are major products with methane and ethane in almost equal amounts. The latter and the scores of other compounds that have been detected in small amount have fuel value.

Table 1: Composition of gaseous products from the pyrolysis of mixed plastics, wt%10

Product

680*

735*

790*

Hydrogen

0.667

0.683

1.868

Carbon monoxide

8.376

14.155

6.316

Carbon dioxide

20.418

20.807

3.38

Methane

16.734

22.661

46.491

Ethene

18.383

20.69

25.994

Ethane

10.118

7.189

7.765

Propene

13.758

7.797

3.311

C3-C9 hydrocarbons

11.546

6.504

4.875

  • temperature, oC
  • temperature, oC

1.5 INCINERATION WITH HEAT RECOVERY

Many plastics have a higher calorific value than coal and in principle it would seem logical to use waste plastics instead of fossil fuels. However, in Europe (although not in Japan) public opinion is turning away from burning waste because of the perceived (although not necessarily real) dangers of toxic effluent. PVC and the other chlorinated polymers present a particular problem in incineration because of the massive evolution of hydrogen chloride. Nevertheless, it can be readily accommodated in many modern incinerators by the use of calcium oxide to absorb the hydrogen chloride if PVC is only a minor component of the plastic waste. However, incineration is not seen by the public to be a safe process in urban environments and as PVC is considered to be a major source of toxic effluent, PVC-containing wastes should be directed into recycling or landfill.

1.6 SANITARY LANDFILL

Disposing of plastics by burial in sanitary landfill, contrary to popular belief, is by far the safest method of dealing with waste plastics, since, due to the absence of oxygen, they do not oxidise or biodegrade under these conditions.1'7 Properly stabilised PVC is stable almost indefinitely in anaerobic landfill and the chlorine will remain locked away without harm to the environment almost indefinitely. The main problem here is the shortage of landfill sites in many of the developed countries. However, in contrast to the behaviour of putrescible materials, plastics do not cause subsequent subsidence and the land can be re-used.

2 POLYMER PROCESSING: A DAMAGING OPERATION

Organic polymers, unlike metals and glass, undergo irreversible changes in chemical structure during manufacture and use. Thus, the metal content of a can may be recovered in its entirety by a recycling process and, assuming it has not been contaminated by other metals, can be refabricated to an identical can. This is not possible with the commodity plastics, which undergo environment-induced oxidation during service which adversely affects their subsequent performance.

Chemical changes begin in the extruder itself and unless steps are taken to minimise damage by the use of additives, the process of screw extrusion has a profoundly deleterious effect on the polymer structure and leads to the introduction of chemical impurities which affect the subsequent durability of the product.11 Furthermore chemical and mechanical changes are accentuated during recycling since the chemical defects introduced during the first processing operation and during subsequent use sensitise the polymer to further degradation.8'12'13 Stabilising systems have been developed to minimise degradative effects during the first processing operation (processing stabilisers) and during use (heat and light stabilisers). Multicomponent stabiliser "package" are formulated not only for individual polymers and their blends but more particularly for specific applications. The constituent chemicals of most commercial antioxi-dant/stabiliser packages are generally not disclosed and are almost always empirical in origin.

However, the mechanochemistry involved in polymer processing is now well understood and this provides a rational basis for design ofantioxidants and stabilisers for products which are to be recycled.2

3 MECHANODEGRADATION OF PVC

Repeated recycling through the manufacturing process and exposure to the environment inflicts incremental chemical damage on the macromolecular structure of polymers and reduces the durability of fabricated products. Screw extrusion involves the mechanical scission of the polymer chain in its viscous environment in the polymer melt. This produces highly reactive macroradicals at the ends of the chain which, in the presence of the small amounts of oxygen dissolved in the polymer' cannot recombine but form peroxyl radicals and hydroperoxides.11-18 This process, which is common to all polymers, is illustrated for PVC in Scheme 1, reactions (a) and (b).16-18 However, in this case, other reactions occur in addition to oxidation, ofwhich the most important is loss ofhydro-gen chloride (Scheme 1, reactions (c) and (d)) to give unsaturation. This is the major cause of discolouration in PVC due to "unzipping" of hydrogen chloride from the polymer backbone. (Scheme 1). The two major products of mechanodegradation, namely HCl and hydroperoxides (ROOH) react together in a redox reaction (reaction 1) to give new and highly reactive oxygen radicals and chlorine atoms which in turn attack the polymer chain to initiate further elimination of HCI or initiate further oxidation in the PVC backbone (see Scheme 2).18

Scheme 1: Mechanooxidation of PVC

Shea

  • 0) / yl '
  • HCHjCHOO i
  • mchjÍMOOH

Scheme 2: Sensitisation of PVC degradation by hydroperoxides

Degradation Polymethylmethacrylat Co2

Alkoxyl radicals and chlorine atoms are highly reactive in hydrogen abstraction leading to the formation of macroalkyl radicals (P ), which in turn react with almost zero activation energy with ground state oxygen which is itself a diradical (reaction 2);

Figure 1. Relation between the initial rate of photooxidation of PVC and the concentration of functional groups after processing. (Reproduced with permission from Developments in Polymer Stabilisation-2, Ed. G. Scott, App. Sci. Pub. p. 61, 1980.)

Photooxidation of PVC is initiated primarily by hydroperoxides.18 There is a direct relationship between the hydroperoxide content of PVC after processing and its rate of photooxidation as measured by carbonyl formation (see Figure 1). Photolysis of in-chain hydroperoxides leads to rapid reduction in molecular weight, with associated impairment of mechanical properties, notably elongation at break and impact resistance.

4 THE EFFECTS OF PLASTICISERS, IMPACT MODIFYERS AND "COMPATIBILISERS" ON PVC PERFORMANCE

Modifying agents are added to PVC, sometimes in appreciable quantities in order to improve performance in specific applications. Many of these have an adverse effect on the stability of the polymer both during processing and on its durability during subsequent exposure to the environment.

4.1 PLASTICISERS

Plasticisers have been used for very many years in order to reduce the glass transition temperature of the polymer and give it rubbery properties at normal temperatures.19 Plasticisers may have both positive and negative effects on PVC during the processing operation. The advantage is that the temperature of the processing operation may be substantially less than in the case of rigid PVC. The disadvantage is that many plasticisers are readily oxidisable materials forming hydroperoxides, which, as discussed in Section 3 lead to further oxidation of the polymer substrate and reduction of mechanical properties. This is particularly true when plasticised PVC is exposed to light and in this context, the branched chain alkyl phthalate and phosphate esters are much more readily photooxidised than the straight chain or aromatic esters.19'20 The use of conventional chain-breaking antioxidant (e.g. hindered and semi-hindered phenols) are essential for the protection of plasticisers from oxidation during mixing and calendering processes. These operations are particularly damaging to the polymer since the latter is readily accessible to oxygen of the atmosphere at the relatively high temperatures involved. Recycling of plasticised PVC must therefore be undertaken with considerable circumspection. It is not enough to simply add new antioxidants and/or stabilisers to collected PVC waste. It is necessary to know whether the mechanical properties and durability have been impaired during first use. If they have it must be concluded that the stabilisation system has been depleted or was not adequate in the first place. The cost involved in this type of analytical investigation may be much more than the value of the product made from the PVC waste.

4.2 IMPACT MODIFIERS

A major deficiency of PVC for many applications (for example in the building industry) is its poor resistance to impact.21 Consequently it became a common practice during the 1970s to incorporate rubber-based modifiers such as ABS, MRS, and MARS to improve the toughness of PVC. In this particular respect, they were very successful and much higher levels of initial impact resistance were achieved. However, PVC impact modifiers all contain butadiene segments which are oxidatively very unstable, particularly at high temperatures and in sunlight,22-27 so that their effect on the durability of PVC is catastrophic.2829 Figure 2 shows that the incorporation of 10% of ABS into PVC causes a rapid deca^y of impact strength during the first few hours of exposure in a weatherometer. Degradation is initiated during the processing operation due to the formation of hydroperoxides24 25 and although it can be retarded by the use of effective peroxide decomposing antioxidants, it cannot be eliminated.23 Recent research has concentrated on eliminating the unsaturated components of the impact modifiers, but it seems unlikely that rubber modified PVC will ever achieve the thermooxidative and photooxidative stability of rigid PVC. For this reason, it is not a good candidate for recycling to high quality products.

IRRADIATION TIME (h|

Figure 2. Effect of photooxidation on the falling weight impact resistance of PVC and impact modified PVC (10% ABS). □ unmodified PVC, O, A modified PVC (duplicate results). (Reproduced with permission from European Polymer Journal, 13, 997(1977)).

IRRADIATION TIME (h|

Figure 2. Effect of photooxidation on the falling weight impact resistance of PVC and impact modified PVC (10% ABS). □ unmodified PVC, O, A modified PVC (duplicate results). (Reproduced with permission from European Polymer Journal, 13, 997(1977)).

4.3. "COMPATIBILISER" (SOLID PHASE DISPERSANTS)

PVC often appears as a relatively minor component in mixed plastics waste in which the major components are the polyolefins, particularly polyethylene. When present in a blend with these polymers, it has a disastrous effect on mechanical performance.30 The addition of some rubbers (notably EPDM but not the impact modifiers discussed in section 4.2) improves the impact strength of PE/PVC blends very considerably,30 but relatively large amounts of these expensive polymers are required and the unsaturation in the polymer again causes problem with subsequent durability. The thermal and photooxidative stability of polymer blends containing EPDM can be improved by the use of preventive antioxidants8 (see Section 5). The reprocessing of blends of waste polymers containing PVC as a substantial component is not at present a very promising approach. Segregation from the less dense polymers by flotation would seem to offer the best prospect of success.

5 STABILISATION OF PVC

In most polymers (e.g. polyolefins) process stabilisation for first use can be achieved with relatively low concentrations (0.05-0.1%) of hindered phenol or phosphite antioxidant. The stabilisation of PVC requires much higher concentrations (generally 2-4% on the PVC content) in order to combat the mechanically initiated HCl elimination referred to in Section 3. Some of them may also stabilise the polymer to the environment, but their ability to do this depends entirely on the amount of stabiliser that remains after the reprocessing operation and how much chemical damage has been done to the polymer during the primary processing operation and in subsequent service.

It follows from the mechanism of mechanodegradation of PVC discussed above that PVC may be stabilised in one or more of the following possible ways during reprocessing.18

5.1 HCl SCAVENGING

Since HCl is such a potent redox initiator for the decomposition of hydroperoxides to radicals, its removal, as it is formed, constitutes an important stabilisation mechanism. Most of the metal salts which form the basis of commercial PVC stabilising systems act in this way. They include the long established but now ecologically unacceptable, lead salts (e.g. lead carbonate), the metal soaps (e.g. Ca, Zn, Cd, and Ba carboxylates).19 However, the most effective class of PVC stabilisers encompasses the alkyl tin compounds and in particular the alkyl tin maleates, I, and mercaptides, II. The main reason for the efficiency of these agents is that, as well as being able to scavenge HCl (reactions 3 and 4), they can also perform other functions as outlined below.

5.2 REMOVAL OF CONJUGATED UNSATURATION

It has long been recognised that maleic anhydride and its derivatives interrupt the conjugated unsaturation in PVC and thus remove the colour due to this mechanodegradation product,20 reaction 5.

By the incorporation of a maleate entity in a tin compound (I) complementary synergism occurs between the HCl scavenging function and the dienophilic function. DBTM is not only a good processing stabiliser, it also protects the polymer during UV exposure.17 However, it is partially or wholly removed during processing and subsequent service. Figure 316 shows that during processing of PVC in the presence of DBTM, the concentration of the latter is decreased comensurately with the severity of the processing operation. Consequently, the subsequent light stability of the product is compromised in the same proportion (see Figure 4). The concentration of the stabiliser is similarly reduced on exposure to the environment and when the polymer is reprocessed there may be little or no stabiliser present. It is therefore essential that the stabiliser is replenished before the polymer is reprocessed if it is to be used in durable applications, particularly out-of-doors.

5.3 REMOVAL OF MONOENIC UNSATURATION

As indicated above, isolated olefin groups are the first to be formed in PVC during processing. The resulting allylic methylene groups are very readily oxidised and their selective removal represents a very powerful preventive stabilisation mechanism. Thiols are known to behave in this, way25,31 and it seems likely that the heat stabilising effect of the dialkyl tin thioglycolate esters is due in part to the liberation of thioglycolic esters, reaction 4, followed by their reaction with monoenic unsaturation, reaction 6,32 and phosphite esters have been reported to fulfill a similar function.33

Plastisol Absorption Spectrum

Figure 3. Decay of tin maleate i.r. carboxylate absorbance (1575 cm- ) during the processing of PVC containing a synergistic mixture of dibutyl tin maleate (2.5 g/100 g) and Wax E (0.65 g/100 g. - total sample; --- soluble phase. Processing temperatures are indicated on the curves. (Reproduced with permission from European Polymer Journal, 14, 913 (1978)).

Figure 3. Decay of tin maleate i.r. carboxylate absorbance (1575 cm- ) during the processing of PVC containing a synergistic mixture of dibutyl tin maleate (2.5 g/100 g) and Wax E (0.65 g/100 g. - total sample; --- soluble phase. Processing temperatures are indicated on the curves. (Reproduced with permission from European Polymer Journal, 14, 913 (1978)).

Irradiation lime. hr x ICT2

Figure 4. Effect of processing time (at 210oC) on the photooxidation of PVC containing a tin maleate stabiliser (numbers on curves indicate processing times in min.) (Reproduced with permission from European Polymer Journal, 15, 51 (1979)).

ROCOCH2SH + CH2=CH- ^ ROCOCH2SCH2CH2- [6]

Reaction 6, where R contains a hindered phenol or a UV absorber function, has also been used to introduce antioxidant and UV stabiliser groups into PVC during processing (see Section 7).

5.4 HYDROPEROXIDE DECOMPOSITION

Compounds containing sulphide groups are widely used as synergistic antioxi-dants in hydrocarbon polymers.34 They are the precursors of sulphur acids and the dialkyl tin thioglycolates (II) have been shown to act in the same way in PVC.35 The tin thioglycolates are much more effective heat stabilisers than the tin maleates, almost certainly for this reason. Like the thiodipropionate esters, they are sensitisers for the photodegradation of PVC due to the sensitivity of the intermediate sulphoxides (e.g. III) to photolysis, giving initiating free radicals34 (see Scheme 3). UV absorbers protect the sulphoxides from photolysis and they show synergism with this class of light stabiliser.

Scheme 3: Photosensitizing action of the tin thioglycollates

5.5 CHAIN-BREAKING ANTIOXIDANTS

By themselves, chain-breaking donor antioxidants such as the hindered phenols, IV, are ineffective processing stabilisers for PVC since alone they are not able to inhibit the HCl unzipping reactions discussed above. However, they do inhibit hydroperoxide formation and are widely used as synergists with the basic stabilisation systems discussed above and are always included in commercial PVC stabiliser packages for plasticised formulations (see above, Section 4.1).

IVa R=CH3, BHT

IVb R=CHjCH2COOC18H37. 1076

5.6 REAGENTS FOR ALLYLIC CHLORINE

For many years it was believed that an important mechanism of PVC stabilisation was the replacement oflabile chlorine atoms adjacent to double bonds in the main chain.35 This conclusion was primarily based on the thermal stability of model chlorine-containing compounds (e.g. V, VI) designed to simulate the olefinic imperfections of the PVC molecule.

v vi

R IV

R IV

The concentration of Vis an order of magnitude higher than that of VI, but the evidence suggests that its destabilising effect on PVC is less than that of VI.36-39 It has also been proposed that a ,|3-unsaturated ketones, VII, are more potent sensitisers for thermal degradation than the a-chloroallyl groups in V and VI. Although the metal carboxylates do displace chlorine from PVC during processing (reaction 7), it is not now clear how important this reaction is in stabilising PVC in practice.

Allylic methylene is much more strongly implicated under the oxidative conditions experienced by PVC in a screw extruder (see 3 above) and since the unsaturated ketone, VII, is formed from the allylic hydroperoxide, VIII, it seems probable that under processing conditions the latter rather than the former is the primary cause of instability by reaction 6. A similar chlorine displacement mechanism has been proposed for a variety of synergistic compounds (IX-XI).40

6 ANTAGONISM AND SYNERGISM

It will be clear from the above that recycled polymers are normally unsuitable for out-door use unless considerable care is taken in devising the correct formulation for such possible applications as piping, bumpers, etc. The provenance of the waste plastic is critically important since many synergistic heat stabilising systems accelerate out-door weathering. Some sulphur antioxidants (e.g. thiodipropionate esters) antagonise with the common hindered phenols under these conditions41 and are generally antagonistic toward the hindered amine class of light stabiliser.41 This makes the design of stabiliser formulations for recycled engineering plastic extremely difficult unless the provenance of the waste is known with some certainty. This is often made more difficult by the fact that polymer manufacturers and fabricators are generally reluctant to disclose the composition of their stabiliser "packages". Blending of two identical polymers containing different stabilising systems could lead to a recyclate with reduced stability due to antagonism between the components of the residual stabilisers. Some stabilisers show antagonism in thermal stabilisation but this is less common than during weathering.

In general, an effective light screening pigment such as carbon black or titanium dioxide will provide UV protection to the bulk of a thick-walled artifact but this stratagem is not effective in thin films unless coupled with an effective

synergist. The dialkyl tin processing stabilisers I and II differ sharply in their light stabilising ability; the maleate (I) being more effective than the thioglycollate (II). However, when combined with a UV absorber, II become a much more effective synergist than I.43 Polymer-bound sulphur synergists behave similarly.42'43

The most satisfactory strategy for successful recycling of engineering polymers to durable end products is to use only one source of plastic waste whose composition and history are known. This then permits the design of a stabiliser system which will synergise effectively with what remains ofthe previous stabiliser package or, at worst, will not antagonise with it.

7 ANTIOXIDANT-MODIFICATION OF POLYMERS DURING PROCESSING

The physico-chemical behaviour of antioxidants and stabilisers in polymers is equally as important to their effectiveness as is their chemical activity. At its simplest; if the stabiliser is lost from the polymer due to volatilisation or leaching then it will not protect the polymer under aggressive technological conditions, however effective it might be in a closed system. It has been shown that at high temperatures, particularly in a moving air stream, many antioxidants are physically lost by volatilisation.44'45 Similarly, at much lower temperatures, all antioxidants and stabilisers are extracted from polymers by solvents, lubricating oils, and greases.45-48 An example of this behaviour in PVC plastisols is the degradation that occurs in leathercloth, particularly in sunlight, due to the depletion by oil leaching, evaporation, etc., initially of stabilisers and antioxidants and subsequently ofplasticiser, leading to cracking and embrittlement. Similar effects occur in impact modified PVC subjected to leaching by the weather.

Diffusion of additives through the polymer bulk and subsequent loss by volatilisation can be reduced in the above examples by incorporating the stabiliser function in a large molecule which is soluble in the substrate. The shape and size of an additive affects its rate of diffusion and rate of volatilisation31 and in the case of the large compact molecules, both of these are minimised compared with lower molecular weight compounds (e.g. IV, BHT, 1076, etc.).

Some attention has been directed to making antioxidants and stabilisers part of the polymer molecule.46-49 Two main processes are used; copolymerisa-

tion during manufacture and grafting to an existing polymer chain. The first is expensive compared with the use of conventional additives and can only be justified when antioxidant permanence cannot be achieved any other way. The second is economically more attractive but generally suffers from the disadvantage that conventional grafting normally gives relatively low yields of adduct due to competition from homopolymerisation49 and homopolymerised vinyl antioxidants are normally incompatible with the main polymer phase.50

However, it has been found possible to use the relatively low levels of monoeneic unsaturation formed in PVC during processing (Scheme 1) to give a very high level of chemical binding of thiol antioxidants (e.g. XII, XIII) by the addition reaction shown in Scheme 4.48 Table 2 illustrates the effective synergism between these polymer-reactive stabilisers and conventional processing stabilisers, DBTM and DOTG in an accelerated weathering test when added during a normal processing operation.42'51 Both thiols were found to be completely bound to the polymer within the first few minutes of processing.51

Table 2: Sulphur-containing antioxidants as photoantioxidants in PVC. Total concentration 5.8x10'3 mol/100g.48

Stabilizer

Embrittlement time, h

DBTM (I)

1000

DOTG (II)

460

DBTM + BHBM (XII)

1400

DOTG + BHBM

400

DBTM + EBHPT (XIII)

1850

DOTG + EBHPT

1000

Control (no antioxidant)

670

An alternative approach to stabiliser substantivity under aggressive conditions involves reactive processing in the presence of a peroxide. Symmetrical derivatives of maleic acid, e.g. XIV, are very reluctant to homopolymerise but do readily graft to PVC (see Scheme 5).52 Under the appropriate conditions, this can be done without any degradation of the PVC.

Pvc Stabilizer Scheme

Scheme 4: Antioxidant adduct formation in PVC during processing

Scheme 5: Maleic ester adduct formation in PVC

8 CONCLUSIONS

8 CONCLUSIONS

  • Due to the high chlorine content of PVC some of the techniques at present being considered in the "Systems" waste disposal options are unfavour able. In particular, degradability and composting are not suitable because of the unknown hazards associated with the oxidative degradation of PVC in the environment. Incineration and pyrolysis are similarly disfavoured because of the large amounts of hydrogen chloride and other toxic products that might be produced.
  • Of the two acceptable disposal technologies, materials recycling and landfill, the former is preferred when the provenance of PVC waste is known. Thus, clean PVC whose composition (particularly with respect to stabilisers) and previous history is known is suitable for "closed loop" recycling in non-foodstuffs applications.
  • The problem of antioxidant and stabiliser loss from consumer products is exacerbated by the recycling process but can be mitigated by designing the stabiliser system to be an integral part of the polymer structure.
  • The discipline of closed-loop recycling will require a new approach to the design ofpolymer stabilisers in order to maximise synergism and minimise antagonism between individual components of the stabiliser package both-during initial processing and use and subsequent recycling. Designing PVC formulations for recycling rather than for a single use after which the artifact is discarded involves a new approach to stabiliser design with careful monitoring of the formulation at each stage whenever recovered material is blended with virgin polymer.

ACKNOWLEDGMENT

I am grateful to colleagues and former students who have contributed to the research that has made possible the conclusions reached in this review and I thank in particular Dr. B. B. Cooray, Dr. X. Xing, and Dr. J. Li for the opportunity to refer to previously unpublished results.

REFERENCES

  1. G. Scott in Degradable Materials, Eds. S. A. Barenberg, J. L. Brash, R. Narayan, and A. E. Redpath, CRC Press, p. 143, 1989.
  2. G. Scott in Recycle'93, Proceedings of 6th Annual Forum on Recycling, Davos, March 22-26, 1993, Maack Business Services.
  3. H. Harata in Recycle'93, Davos, March 22-26, Paper 15/4, 1993.
  4. D. Gilead and G. Scott in Developments in Polymer Stabilisation-5, Ed. G. Scott, App. Sci. Pub., Chapter 4, 1982; D. Gilead, Polym. Deg. and Stab., 29, 63 (1990);
  5. Gilead in Degradable Polymers, Principles and Applications, Eds G. Scott and D. Gilead, Chapman & Hall, in press.
  6. G. Scott in Proc. Second Int. Conf. on Marine Debris, Eds R. S. Shomura and
  7. L. Godfrey, Vol. 1, U.S. Dept of Agriculture, p. 827, 1990; G. Scott, Polym. Deg. and Stab., 29, 135 (1990).
  8. G. Hanna in Recycle'93, Davos, March 22-26, Paper 18/2, 1993.
  9. G. Scott in Biodegradable Plastics and Polymers, Proc. 3rd Int. Conf.,
  10. Y. Doi et al., Elsevier Science Pub., in press; G. Scott in Degradable Polymers, Principles and Applications, Eds. G. Scott and D. Gilead, Chapman & Hall, in press.
  11. C. Sadrmohaghegh, G. Scott, and E. Setudeh, Polym. Plast. Technol. Eng., 24, 149 (1985).
  12. N. Grassie and G. Scott in Polymer Degradation and Stabilisation, Cambridge University Press, p. 105, 1985.
  13. W. Kaminsky in Recycle'93, Davos, March 22-26, Paper 7/4, 1993.
  14. G. Scott in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Vol. II, Chapter 3, Elsevier Science Pub., 1993; G. Scott in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Volume II, Elsevier Sci. Pub., Chapter 8, 1993.
  15. G. Scott, Polym. Plast. Technol. Eng., 11, 1 (1978).
  16. G. Scott, Resource Recovery and Conservation, 1, 381 (1976).
  17. G. Scott, Proc. Int. Conf. on Advances in the Stabilisation and Controlled Degradation of Polymers, Luzerne, May 1990, p. 215.
  18. G. Scott in Developments in Polymer Degradation-1, Ed. N. Grassie, p. 205, 1977.
  19. G. Scott, M. Tahan, and J. Vyvoda, Europ. Polym. J., 14, 913 (1978).
  20. G. Scott, M. Tahan, and J. Vyvoda, Europ. Polym. J., 15, 51 (1979).
  21. B. B. Cooray and G. Scott in Developments in Polymer Stabilisation-2, Ed. G. Scott, App. Sci. Pub., p. 89 et seq., 1980.
  22. F. Chevassus and R. De Broutelles in Stabilisation of Polyvinyl Chloride, Arnold, p. 223, 1963.
  23. G. Scott in Biodegradable Plastics and Polymers, Eds. Y. Doi and K. Fukuda, Elsevier Science B.V., p. 307 et seq., 1965.
  24. J. J. Gormley, Reg. Tech. Conf. SPI, March 24, 1970, p.63.
  25. A. Ghaffar, A. Scott, and G. Scott, Europ. Polym. J., 11, 271 (1975).
  26. A. Ghaffar, A. Scott, and G. Scott, Europ. Polym. J., 13, 83 (1977).
  27. M. Ghaemy and G. Scott, Polym. Deg. and Stab., 3, 233 (1981).
  28. M. Ghaemy and G. Scott, Polym. Deg. and Stab., 3, 253 (1981).
  29. G. Scott and M. Tahan, Europ. Polym. J., 13, 981 (1977).
  30. G. Scott in Developments in Polymer Stabilisation-1, Ed. G. Scott, App. Sci. Pub., p. 309, 1979.
  31. G. Scott and M. Tahan, Europ. Polym. J., 13, 989 (1977).
  32. G. Scott and M. Tahan, Europ. Polym. J., 13, 997 (1977).
  33. A. Ghaffar, C. Sadrmohaghegh, and G. Scott, Europ. Polym. J., 17, 941 (1981).
  34. G. Scott in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Elsevier Sci. Pub., Vol. II, Chapter 5, 1993.
  35. G. Scott in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Elsevier Sci. Pub., Vol II, p. 184 et seq., 1993.
  36. K. S. Minsker, M. I. Abdullin, S. V. Kolesov, and G. E. Zaikov in Developments in Polymer Stabilisation-6, Ed. G. Scott, App. Sci. Pub., Chapter 5, 1983.
  37. S. Al-Malaika, K. B. Chakraborty, and G. Scott in Developments in Polymer Stabilisation-6, Ed. G. Scott, App. Sci. Pub., p. 73, 1983.
  38. B. B. Cooray and G. Scott, Polym. Deg. and Stab., 2, 35 (1980).
  39. A. H. Frye and R. W. Horst, J. Polym. Sci., 40, 419 (1959).
  40. K. S. Minsker, V. V. Listiskii, and G. E. Zaikov, Polym. Sci. USSR, 23, 535 (1981).
  41. V. V. Lisitiskii, S. V. Kolesov, R. F. Grataullin, and K. S. Minsker, Z. Analit. Khimii, 33, 2202 (1978).
  42. E. N. Zilberman, Perepletchikova, Y. N. Getmanenko, V. I. Zegelman, T. Molova, and Y. A. Zvereva, Plast. Massy, 3, 9 (1975).
  43. K. S. Minsker, A. A. Berlin, V. V. Lisitskii, and S. V. Kolesov, Vysokomol. Soed., 19, 32 (1975).
  44. A. Guyot and A. Michel in Developments in Polymer Stabilisation-2, Ed. G. Scott, App. Sci. Pub., p. 89, 1980.
  45. G. Scott in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Elsevier Sci. Pub., Vol. II, Chapter 9, 1993.
  46. B. B. Cooray and G. Scott, Europ. Polym. J., 17, 229 (1981); G. Scott in Developments in Polymer Stabilisation-6, Ed. G. Scott, App. Sci. Pub., p. 67, 1983.
  47. G. Scott, Pure & App. Chem., 30, 267 (1972).
  48. N. C. Billingham in Atmospheric Oxidation and Antioxidants, Ed. G. Scott, Second Edition, Elsevier Sci. Pub., Vol. II, Chapter 4, 1993.
  49. G. Scott in Developments in Polymer Stabilisation-1, Ed. G. Scott, App. Sci. Pub., Chapter 9, 1979.
  50. G. Scott in Developments in Polymer Stabilisation-4, Ed. G. Scott, App. Sci. Pub., Chapter 6, 1981.
  51. G. Scott in Developments in Polymer Stabilisation-8, Ed. G. Scott, Elsevier App. Sci., Chapter 5, 1987.
  52. D. Munteanu in Developments in Polymer Stabilisation-8, Ed. G. Scott, Elsevier App. Sci., Chapter 4, 1987.
  53. B. W. Evans and G. Scott, Europ. Polym. J., 10, 453 (1974).
  54. B. B. Cooray and G. Scott, Europ. Polym. J., 17, 385 (1981).
  55. J. Li, G. Scott, and X. Xing, unpublished work.

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