聚氯乙烯的回收 毕业论文外文文献翻译.doc

附录a英文翻译（原文）recycling of pvcabstractrecycling of used pvc needs a careful characterization of pvc waste. the analysis of the scrap, especially with respect to the thermal stability and the molecular weight, is useful before reprocessing. additional stabilization of used pvc can be done by up to 10 wt% fillers, e.g. chalk, which does not change the mechanical properties. a literature survey on the proposed concepts and methods for material and chemical recycling of pvc isgiven. q 2002 published by elsevier science ltd.keywords: poly(vinyl chloride); degradation; stabilization; material and chemical recyclingcontents1. introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. characterization of pvc waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32.1. thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. analysis of stabilizers and other additives . . . . . . . . . . . . . . . . . . . . .62.3. molecular weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73. stabilization of used pvc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83.1. addition of heat stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2. improvement of thermal stability by fillers . . . . . . . . . . . . . . . . . . . . 93.3. processing of filled pvc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.4. practical examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124. recent investigations on material recycling. . . . . . . . . . . . . . . . . . . . . . . 155. chemical recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186. conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201. introductionwith a todays worldwide annual production capacity of approximately more than 30 million to poly(vinyl chloride) (pvc) is the second largest volume thermoplastic only to polyethylene as volume leader in the plastics industry 1. its ability to be compounded with many additives to a wide range of flexible and rigid products constitutes the major factor responsible for the versatile application of pvc.because of the low cost and the processability by a wide variety of techniques (e.g. calendering,extrusion, injection moulding, and plastisol techniques) combined with good physical, chemical and weathering properties, pvc has become a universal polymer 2 with many applications, e.g. for pipes,profiles, floor coverings, cable insulation, roofing sheets, packaging foils, bottles, and medical products.at the end of the service-time of these articles, large amounts of scrap arise. therefore, the question of the disposal of used pvc has gained increasing importance in the public discussion because of the environmental problems resulting from the rapid growth of the plastic waste during the last years.landfilling of municipal solid waste is becoming a burden as, for example, in the united states about80% of waste is dumped into landfills. a continuation at the present rate could exhaust the landfill capacity in the near future 3. also the european countries are faced with a similar dilemma because the availability of suitable sites is limited 4. thus, landfilling as a disposal process is increasingly seen asthe last option.the energy recovery by incineration is another way to dispose the municipal solid wastes 5. but environmental argumentation, such as toxic emissions from inadequate equipment or inappropriate incineration conditions, are building up a public resistance against these techniques 6. especially, pvc incineration is connected with some technological problems due to the high chlorine content of this polymer which yields large amounts of hydrogen chloride during thermal decomposition, beside the possibility of formation of toxic dioxines and furans. therefore, plans to expand capacities of suchinstallations meet with growing difficulties.the recycling activities can be distinguished between chemical from material recycling. the chemical recycling is based on the idea of converting polymers back into short-chain chemicals for reuse in polymerization or other chemical processes. four different process technologies are currently considered for chemical recycling: cracking, gasification, hydrogenation and pyrolysis 7. in the meantime, some commercial-scale plants are working and a number of interesting investigations with these techniques are available 810. at the end, the economic efficiency will be decisive for the application of each recycling process in the future.the material recycling is already practiced in plastics industry over many years with postmanufacturing waste. these experiences can be used to develop new concepts for material recycling of post-consumer waste. the major problem in the recycling of used plastics is connected to a great inhomogeneity of the polymers present in the waste 11. a statistical study by the information system on plastics recycling in western europe shows that about 7.4% of the 9 million to of municipal solid waste in western europe are plastic materials. fig. 1 shows the percentage of different polymer types in the total plastics waste.fig. 1. percentage of different polymer types in the municipal solid plastic waste.the incompatibility of these components is the most important reason of the difficult processing and inferior mechanical properties of the resulting products from mixed, chemically different polymers.therefore, it is necessary to separate various polymers to boost their value. although there are many practical problems some interesting developments for plastics waste separation were found 12,13. the separation in a hydrocyclone, which works on the principle of sorting by a centrifugal force field, usingdensity difference of the various polymers is one possible solution 14. also identification by ir analysis in a continuous process can be used for separation of the most common plastics. sometimes,prior to separation, it becomes necessary to clean the polymer waste to remove contaminations like dirt,food, and paper.in the future, a clean pvc-stream of the municipal solid wastes and from the building sector ready for material recycling can be expected. this will be accelerated by new regulations made by the governments in some countries. in germany, the first item of legislation to be introduced was the act on the avoidance of packaging waste 15. its aim is to reduce the large amounts of packaging waste through avoidance and material recycling. another fact is that the manufacturers and traders are made responsible for their used packaging to relieve the local authorities of the burden to dispose the waste. in this respect, material recycling is a necessary way to reduce the municipal solid waste problem.2. characterization of pvc wastemany different grades and types of pvc are available allowing applications as diverse as flexible sheets, pressure pipes, transparent bottles, and medical products to be produced. for these articles, a lot of different additives and stabilizer systems are used to get suitable properties for the respective applications 16. moreover, during the high temperature processing and throughout the service live of the products the polymer might be subjected to degradation 17,18. therefore, characterization of the pvc waste is necessary to obtain information on properties such as the residual stability, molecular weight, and content of additives of the individual pvc species.2.1. thermal stabilitythe main disadvantage of pvc is the rather limited thermal stability which requires addition of heat stabilizers to prevent dehydrochlorination and discolouration during processing and application. with respect to the great practical importance of the polymer, the thermal and photochemical degradation of pvc has been studied for a long time and there is a large number of published surveys 1921. the elimination of hydrogen chloride at relatively low temperatures (about 100 ) or under the influence of light is one of the fundamental aspects of pvc decomposition. in the first stage, this reaction leads to the formation of double bonds followed by a so-called rapid zipper-like splitting off of further hcl molecules to give polyene sequences (fig. 2). these sequences, with an average length of 614 conjugated double bonds, cause the polymer to turn yellow, brown, and eventually black. fig. 2. scheme of dehydrochlorination of pvc.the thermal stability of pvc is considerably lower than that of its low-molecular weight model analogues. therefore, initial sites, such as allylic chlorines adjacent to internal double bonds, tertiary chlorines at branched carbons, head-to-head units, and oxygen-containing structures are believed to be responsible for the instability. the mechanisms, which occur during degradation are not yet fully understood. there are radical or ionic mechanisms suggested, and the type of reaction depends also on the conditions (temperature, presence of oxygen, etc.) during the decomposition.the main function of heat stabilizers is to prevent degradation during processing. they have in common the ability to react with hcl when it is liberated from the polymer. another task is to replacelabile chlorine atoms, which may initiate the dehydrochlorination of more stable groups, and thus to enhance the heat stability. a number of organometallic compounds and inorganic salts are especially effective and since long time in practical use.a part of the stabilizer will be consumed during processing and sometimes during the application period. therefore, the efficiency of the stabilizer system is remarkably reduced after compounding pvc with the necessary additives 22 that makes it useful to get information about the residual stability of pvc articles before they can be recycled.for this purpose, the determination of the thermally induced hydrogen chloride elimination from pvc seems to be the best way. the study of the early stages of the reaction requires a combination of good reproducibility, high accuracy and a low detection limit. fig. 3 shows an apparatus which is very suitable and often used for such studies since many years 23.fig. 3. pvc degradation measuring apparatus. (a) rotameter, (b) degradation vessel with pvc sample, (c) thermostat, (d)conductivity cell, (e) conductivity-meter and (f) computer.the pvc-sample (,0.1 g) is introduced into the degradation vessel and then the measurement iscarried out under isothermal conditions (e.g. 180 ). a stream of warmed up carrier gas (nitrogen or air)transports the evolved hydrogen chloride into the conductivity cell filled with distilled water. the hcl determination is performed by continuous conductometric measurements. as a result the conversiontime curve is obtained as schematically shown in fig. 4.fig. 4. schematic degradation curve of stabilized pvc.the degradation curve of stabilized pvc shows an induction period where no hcl is evolved. during this period, the heat stabilizer is consumed and afterwards the dehydrochlorination begins. the time of induction, ti, gives an important information required to estimate the remaining stability of a pvc specimen and to decide whether an additional stabilization is necessary for the material recycling. in some cases, it can be sufficient to use the simple congo red test, e.g. according to din 53418, instead of the more expensive apparatus for quantitative measuring the hydrogen chloride elimination.2.2. analysis of stabilizers and other additivesas mentioned above, the limited thermal stability of pvc requires the use of heat stabilizers in almost all fields of application. besides, also other additives (e.g. light stabilizers, fillers, lubricants) are used to modify the properties of pvc or to improve its processability. at present, about 1/3 of all used pvc is plasticized by various types of modifiers 24. therefore, it is helpful to get some detailed information about the composition of a special pvc scrap before reuse. as an example, for plasticized pvc, the analysis of pvc roofing sheets is described and show in fig. 5 25.fig. 5. analysis of pvc roofing sheets.the first step is a soxhlet-extraction of the powdered pvc sample with diethyl ether to isolate plasticizers. after evaporation of the solvent, the type and amount of plasticizer can be determined.the rest of the material is then dissolved in tetrahydrofuran (thf), and, after filtration, the fibrous materials are obtained. the other components, insoluble in thf, are separated by a centrifuge. the remaining residue can be divided to fillers and cross-linked pvc by burning to ash. by dropping the thf solution in a surplus of methanol the dissolved pvc is precipitated. the single components are determined gravimetrically and identified by chemical and spectroscopic methods.usually the quantitative analysis of the main parts of a pvc sample (plasticizer, filler, pvc itself) will give enough information about the material. for the qualitative analysis the ir-spectroscopy is particularly suited because the main additives, including co-polymers and impact modifiers, show typical ir-bands 26. also other spectroscopic methods can be used for identification but the expenditures of sample preparation and equipment are higher. a complete qualitative and quantitative analysis of all ingredients of a pvc compound is described in ref. 27.finally, the determination of the heat stabilizers, as an important point in the analysis of pvc waste, is particularly considered. the selection of a stabilizer system for pvc depends on many factors including application, tradition of the market, and local legislation 28. lead stabilizers still are the most widely used pvc heat stabilizers for some technical applications because they provide cost-effective stabilization systems and easy processing. they maintain volume resistivity in plasticized pvc cable insulation and are the principal stabilizers for many general-purpose applications. important are several mixed metal carboxylate soaps as pvc stabilizers, e.g. bariumcadmium, bariumzinc, calciumzinc. since many years, bariumcadmium systems have been used in europe in white window frames with good weathering properties. but the utilization of cadmium in stabilizers or pigments recently became under increasing scrutiny. at present, all manufacturers use alternative systems such as calciumzinc stabilizers 29,30. they are applied for food packaging, water bottles, and medical products. the number of applications is likely to increase with the availability of less toxic additives. organotin compounds form another large group of stabilizer systems where mono- and dialkyltins are the most widely used. their properties depend on the nature of the alkyl and ester groups present. the toxicity of the dialkyltin types decrease rapidly with the chain length of the alkyl group, so that, e.g. octyl tin compounds are accepted for food contact applications. also, some sulfur-containing organotin-stabilizers are used because they offer excellent heat stability and clarity.a simple possibility to obtain a detailed information on the stabilizer system in pvc waste can be seen in the classic analyses methods which are common practice in inorganic chemistry for the separation and determination of cations. the only difficulty is to find an easily practical way to get the metallic cations into the water phase. for this purpose, the pvc sample can be dissolved in cyclohexanone and the received solution is used for a liquid/liquid-extraction with nitric acid containing water. after phase separation, the different cations are found in the water solution.the determination of the metals can also be accomplished by thin layer chromatography, using an organic solution of pvc in thf 31. sometimes, a precipitation of the polymer might be necessary, and he remaining methanol/thf solution is used for the identification. besides, some spectroscopic methods are described for stabilizer analysis 32. the infrared spectroscopy nowadays is the preferred method for this purpose because of its easy feasibility combined with a high detection rate 33.2.3. molecular weightfor different kinds of processing and various applications, industry offers pvc types with k-values between 55 and 80 34. the k-value is a traditional unit of measurement used until now by manufacturers to describe the molecular weight of pvc materials. this information is necessary to decide which processing technique can be used for recycling. also, under the influence of heat, light, and oxygen, pvc chains can be degraded or even cross-linked which results in changes in the molecular weight and molecular weight distribution 22. as there are correlations between molecular weight,processability, and mechanical properties of pvc, it is important to investigate changes in molecular weight during processing or use.the simplest method for molecular weight measurements includes the determination of the viscosity of a pvc solution. the pvc is usually dissolved in cyclohexanone and measured at 25, e.g. according to din 53726. for practical purposes, the obtained k-value gives sufficient information in most cases. using the markhouwink equation, the molecular weight can also be alculated from the results of the viscosity measurements 35.now, the gel permeation chromatography, gpc, is by far the most popular method of molecular weight measurement 21