Braun and K Krmer

Deutsches Kunststoff-Institut, Schloßgartenstraße 6, 64289 Darmstadt, Germany

INTRODUCTION

With a present world-wide annual production of approximately 19 million tones, 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 forms constitutes the major factor responsible for the versatility of PVC. Because of a low cost and the processability by a wide variety of techniques (e.g. calendering, extrusion, injection molding, and plastisol techniques) combined with good physical, chemical, and weathering properties, PVC has become a universal polymer.2 There are many applications of rigid and plasticized PVC, e.g., 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 arises. Today, the question ofthe disposal ofused plastics 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 about 80% of waste is dumped into landfills. A continuation at the

present rate could exhaust the U.S. landfill capacity in a decade. Also some 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 as a 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 such installations meet with growing difficulties.

The recycling activities can be separated to chemical and material recycling. The chemical recycling is based on the idea of converting polymers back into short-chain chemicals for re-use in polymerization or other petrochemical processes. Four different process technologies are currently considered for chemical recycling: cracking, gasification, hydrogenation and pyrolysis.7 In the meantime, a few commercial-scale plants are working and some interesting

8 10

studies about investigations with these techniques are available. At the end, the economic efficiency will be decisive for the application as any recycling process in the future.

The material recycling is already practiced in plastics industry over many years in regard to post-manufacturing 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 such 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 tones of municipal solid waste in Western Europe in 1990 are plastics materials. Figure 1 illustrates the percentage of different polymer types in the total plastics portion.

The incompatibility of these components is the 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 some practical problem^ some interesting developments for plastics waste separation were found. '

The separation in a hydrocyclone, which works based on the principle of sorting by a centrifugal force field, using density difference of the various polymers is one possible solution.14 Sometimes, prior to separation, it becomes necessary to clean the polymer waste to remove contamination like dirt, food, and paper.

Polymer Waste
Figure 1. Percentage of different polymer types in the total plastics portion of municipal solid waste.

In the future, a clean PVC-stream of the municipal solid wastes and the building sector wastes ready for material recycling can be expected. This evolution will be accelerated by new ordinances made by the government of some countries. In Germany the first item of legislation to be introduced was the "Act on the Avoidance of Packaging Waste".15 The aim of this legislation is to reduce the large amounts of packaging 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.

Generally, one can say that material recycling is a necessary way to reduce the municipal solid waste problem. This paper shows a concept of integrated steps required to make recycling happen. The special problems ofPVC recycling will be presented and illustrated by a number of practical examples.

CHARACTERIZATION OF PVC WASTE

Many different grades and types of PVC are available allowing applications as diverse as flexible sheets, pressure pipes, transparent bottles, and med ical products to be produced. For these articles, a lot of different additives and stabilizer systems are used to get suitable properties for the respective applica-tions.16 Moreover, during a high temperature processing and throughout a service live of the product the polymer might be subjected to degradation.17 Therefore, a 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.

MEASUREMENT OF RESIDUAL STABILITY

The main disadvantage of PVC is the rather limited thermal stability which requires addition of heat stabilizers to prevent dehydrochlorination and discoloration. With respect to a 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.18-21 The elimination of hydrogen chloride, at relatively low temperatures (about 100oC) or under the influence of light is the fundamental aspect of PVC decomposition. In the first stage, this reaction leads to the formation of one double bond followed by a so-called rapid zipper-like splitting off of further HCl molecules to give polyene sequences (Figure 2). These sequences, with a mean length of 6 - 14 conjugated double bonds, cause the polymer to turn yellow, brown, and eventually black.

Figure 2. Schematic formula of the dehydrochlorination of PVC.

Figure 2. Schematic formula of the dehydrochlorination of PVC.

The thermal stability of PVC is considerably lower than that of its low-molecular weight model analogues. 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 it seems that the type ofreaction depends on the conditions (temperature, presence ofoxygen, etc.) during the decomposition.

he 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 replace labile chlorines, 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.

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.21 That makes it necessary to get information about the residual stability of PVC articles before they can be recycled.

For this purpose, the determination of the hydrogen chloride elimination 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. The following picture (Figure 3) shows an apparatus which is very suitable and often used for such studies.23

Thermostat Ether
Figure 3. PVC degradation measuring apparatus. a. rotameter, b. degradation vessel with PVC sample, c. thermostat (180oC), d. conductivity cell (25oC), e. conductivity-meter, f. computer.

The PVC-sample (~ 0.1 g) is introduced into the degradation vessel and then the measurement is carried out under isothermal conditions (e.g., 180oC). A stream of warmed up carrier gas (nitrogen) transports the evolved hydrogen chloride into the conductivity cell filled with distilled water. The HCl determi-

nation is performed by continuous conductometric measurements. As a result the conversion-time-curve is obtained as illustrated in Figure 4.

Degradation Pvc Time
Figure 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, is an important information required to estimate the remaining stability of a PVC specimen and decide whether an additional stabilization is necessary for the material recycling. Also, the rate of HCl split-off, after stabilizer's consumption, can be calculated. In some cases, it can be sufficient to use a simple Congo Red test, e.g., according to DIN 53418, instead of the apparatus for measuring the hydrogen chloride elimination.

ANALYSIS OF THE HEAT STABILIZER AND OTHER ADDITIVES

As mentioned above, a limited thermal stability of PVC requires the addition 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 re-use.

This can be done by some analytic methods based on the experiences ofthe Deutsches Kunststoff-Institut (DKI) with the material recycling of various used

PVC products. As an example, for plasticized PVC, the analysis of PVC roofing sheets is described and shown in Figure 5.25

Figure 5. Analysis of PVC roofing sheets.

The first step consists a Soxhlet-extraction of the powdered PVC sample with diethyl ether to separate plasticizer. After evaporation of the solvent, the amount of plasticizer can be determined. The rest of the material is then dissolved in tetrahydrofurane (THF), and, after filtration, the fibrous materials are obtained. The other components, insoluble in THF, are separated in a centrifuge. The remaining residue can be divided to fillers and crosslinked PVC by burning to ashes. By dropping the THF solution in a surplus of methanol the 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 copolymers and impact modifiers, have 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 was previously discussed.27

Finally, the determination of the heat stabilizer, 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 are the most widely used PVC heat stabilizers 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. Several metal carboxylate soaps are used in combination as PVC stabilizers, e.g., barium-cadmium, barium-zinc, calcium-zinc. Since many years, barium-cadmium systems were used in Europe in white window frames with good weathering properties. But the utilization of cadmium in stabilizers or pigments recently comes under increasing scrutiny. At present, all manufacturers are looking for alternative systems such as calcium-zinc 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. The 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 acid groups present. The toxicity of the dialkyltin decreases rapidly with the chain length of the alkyl group, so that octyl tin compounds are accepted for food contact applications. Also, some sulfur-containing organotin-stabilizers are used because of their excellent heat stability and clarity. The relatively high costs is the main drawback of tin stabilizers.

A simple possibility to obtain a detailed information about the stabilizer system in PVC waste can be seen in the classic analysis methods which are common practice in inorganic chemistry of the separation and determination of cations. The only difficulty is to find an easily practical way to get the metallic cations into water phase. For this purpose, the PVC sample is dissolved in cyclohexanone and the received solution used for a liquid/liquid-extraction with nitric acid containing water. After phase separation, the different cations are found in 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 the remaining methanol/THF solution is used for the identification. Besides, some spectroscopic methods are described for stabilizer analysis.32 The infrared spectroscopy seems to be the preferred method for this purpose because of its easy feasibility combined with a high detection rate.33

DETERMINATION OF MOLECULAR WEIGHT

For 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 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 crosslinked which results in changes in the

molecular weight and molecular weight distribution. There is a correlation between molecular weight, processability, and mechanical properties of PVC, and it is thus important to investigate changes in molecular weight during processing or use.

The simplest method for molecular weight measurement includes the determination of the viscosity of a PVC solution. The PVC is usually dissolved in cyclohexanone and measured at 25oC, e.g., according to DIN 53726. For practical purposes, the obtained K-value gives sufficient information in most cases. Using the Mark-Houwink-equation, the molecular weight can also be calculated from the results of the viscosity measurements.35

The gel permeation chromatography, GPC, is by far the most popular method of molecular weight measurement.21 It not only gives information on the molecular weight but also on the molecular weight distribution. Normally, THF is used as solvent and the columns are calibrated with polystyrene or PVC standards. In some cases, the results might be misleading, either if PVC is not dissolved properly or if the sample contains polymer that is partially insoluble due to former treatment, e.g., crosslinking under the influence of heat or light.

POSSIBILITIES OF ADDITIONAL STABILIZATION OF USED PVC

The recycling of PVC waste offers a lot of problems due to the limited thermal stability which requires, in most cases, the addition of stabilizers. PVC articles are susceptible to degradation at almost all stages of their lifetime: production, storage, processing, transportation, and end-use. Several possible influences, which can be hostile to polymers, are heat, light, oxygen, and mechanical stress. Therefore the stabilizer system is partially consumed during the service life of PVC product. An example is shown in Figure 6 where the conversion-time-curve of a 15 - 20 years old window frame scrap and a post-manufacturing window frame waste are compared.

Degraded Pvc Window Frames
Figure 6. Degradation curve of a post-manufacturing window frame waste and an old window frame scrap (180oC, nitrogen); post-manufacturing waste: —; window frame scrap: ---.

The induction time of the post-manufacturing waste is much longer than the induction period of the old PVC scrap. Also, some other studies about the changes in properties of rigid PVC during weathering show a reduction of the stabilizer's efficiency.36

ADDITION OF HEAT STABILIZERS

As mentioned before the measurement of the residual stability of a PVC article may lead to the result indicating that an additional stabilization is necessary for efficient recycling. The use of a new heat stabilizers for this purpose is connected with some difficulties. First, it is necessary to determine the stabilizer system present in the PVC waste because some stabilizers are not compatible with others. For instance, a sulfur-containing tin stabilizer reacts with a lead stabilizer during processing and the resulting material would have dark spots from lead sulfide formed, besides the reduced efficiency of the system. Moreover, the use of heavy metals in stabilizers might be forbidden in a few years by the governments of some European countries because of their toxicity. If this happens, every processing company will have the problem how to stabilize old PVC products if material recycling is desired.

IMPROVEMENT OF THERMAL STABILITY BY FILLERS

The use of a filler as a co-stabilizer is an alternative possibility to recycle PVC waste without addition of further stabilizers. For this purpose, calcium carbonate is suitable, because it is able to react with hydrogen chloride. Furthermore, chalk has good properties (e.g., wide variety of calcium carbonate materials available, low price, no abrasion of processing equipment, reduced plate-out, increasing mechanical properties, and homogeneous distribution by coating) making it popular as PVC filler.16 In Figure 7, the degradation-curves of two stabilized PVC samples (one with 10 phr (parts per hundred parts resin) calcium carbonate and one without filler) are illustrated.

The addition of chalk increases thermal stability of the filled PVC sample, indicated by a longer induction period of dehydrochlorination. Also, the rate of HCl-elimination is lower in the presence of calcium carbonate. Other investigations with unstabilized PVC samples have shown that the filler acts as a trap for the split-off hydrogen chloride, but it has no influence on the decomposition of PVC. This is confirmed by the UV-spectra of PVC which was heat-treated in the absence and in the presence of chalk.22 Under both conditions, the same unsatu-rated sequences in PVC are formed, as shown in Figure 8 (curve a and b). On the contrary, in the presence of stabilizers (curve c) practically no polyene sequences are formed during the induction period.

Consequently, one can conclude that chalk acts as a co-stabilizer for PVC increasing its thermal stability, but only in connection with a normal heat stabi lizer or the remaining active heat stabilizer in PVC waste. The lower HCl-elimi-nation rate can be explained by the fact that hydrogen chloride, which reacts with the filler^, has no more the well-known catalytic effect on the further PVC

degradation.

Figure 7. Dehydrochlorination of PVC at 180oC under nitrogen. PVC filled with 10 phr chalk —, PVC without filler ---.

RESULTS OF PROCESSING EXPERIMENTS WITH FILLED PVC

Some studies with filled and unfilled rigid PVC were made in our laboratory to describe the effect of chalk as additional stabilizer. A suspension PVC (K-value 70) with an organotin stabilizer (2 phr) and lubricants (1.8 phr) was used as the experimental material. A part of this compound was filled with a stearic acid coated calcium carbonate (10 phr) as an additional component. The PVC powder and the additives were mixed at a high speed in an intensive mixer. The received dry blends were pelletized by extrusion to get a better dispersion of the additives in the polymer material. Finally, the granulates were processed by injection molding to test specimens for measurements of mechanical properties.

Tensile stress-strain data at constant deformation rate and constant temperature are undoubtedly the most valuable mechanical data for the characterization of rigid PVC.38 They are widely used not only for material selection but

Picric Acid Vis Spectra
Figure 8. UV-VIS-spectra of PVC in THF (2 g/l. before (d) and after (a,b,c) thermal degradation (30 min at 180oC under nitrogen).

also to determine the strength of the resulting products and to measure the retention of mechanical strength on outdoor or accelerated exposure. Stress-strain measurements are generally made in tension, e.g., according to DIN 53455. The standard test piece is stretched at a uniform rate until it breaks. From the x-y plot, the tensile strength, the elongation at break, and the modulus of elasticity are obtained. The toughness is a further important mechanical property of a polymer material which can be determined by an impact test.38 The testers are pendulum instruments that break the specimen with a hammer. For this purpose the Charpy-method, according to DIN 53735, with notched standard test pieces can be used.

Table 1: Results of the mechanical measurements of the filled and unfilled PVC, according to DIN 53455 and DIN 53735

Modulus

Tensile

Elongation

Notched

of elasticity

strength

at break

impact strength

N/mm2

N/mm2

%

kJ/m2

PVC without chalk

3190

66.1

10.5

4.3

PVC + 10 phr chalk

3360

62.4

11.0

5.0

The results of the mechanical measurements are given in Table 1. The stress-strain-diagrams of the PVC samples show a ductile behavior with a yield-point. The addition of chalk causes a higher modulus of elasticity, whereas the tensile strength is slightly diminished. The elongation at break is nearly equal, and also the notched impact strengths of the filled and unfilled PVC are on the same level. We can infer from this data that the use of calcium carbonate as filler brings no discredit upon the mechanical properties of rigid PVC if the filler is homogeneously distributed in the polymer material during processing.

Figure 9 illustrates the degradation-curve of the two PVC samples. The induction period of the filled PVC (~ 300 min) is significantly longer than the time of induction of the unfilled PVC (~ 160 min). Also of interest is the fact, that, in the presence ofchalk, the rate ofHCl-elimination after consumption ofthe stabilizer system is lower. A comparison of Figure 9 with Figure 7 shows a corresponding shape of the degradation-curves of the filled and unfilled PVC samples. The additional stabilization of calcium carbonate in PVC compounds could be proved also under realistic processing conditions.

Some more investigations, such as variation of the concentration of filler and change of the particle size, were made to get further details about the influence of chalk in PVC. It could be shown that the stabilizing effect increases with the content of calcium carbonate in the mixture. But there is a maximum at about 30 phr filler because of the increase in shear viscosity in the used processing machines which leads to an increasing thermomechanical treatment of the material. Another limiting factor is the change of the mechanical behavior from ductile to brittle with rising amount of chalk connected with lower values of elongation at break and impact strength.

0 120 240 360 480 600 min

Figure 9. Conversion-time curve of the two processed PVC samples; Stabilized PVC with 10 phr chalk —; Stabilized PVC without filler ---.

PRACTICAL EXAMPLES OF PVC RECYCLING

To confirm the applicability of the suggested recycling concept with real PVC waste, three different used rigid PVC products were investigated as described previously. The obtained results are shown in Table 2.

The three samples were processed according to method used for the virgin PVC as described above. First, the big pieces of PVC waste had to be handled in a mill to get a powdered material. Then, half of the single regrinds were mixed with 10 phr of a stearic acid coated calcium carbonate. After extrusion and injection molding the test specimens were received. Detailed information about the processing conditions of the modification of PVC scrap with chalk, aiming at a good distribution of the filler, are given elsewhere.39

In Table 3, the results of the mechanical measurements of the three processed PVC wastes without addition of filler are compared with the chalk modified samples. The influence of the added amount of chalk corresponds to the effects discussed earlier, except for the notched impact strength. These values are slightly higher for the modified specimens of the materials 1 and 3, whereas the filled sample of material 2 has a lower value.

Table 2: Results of characterization of three different PVC wastes

Article Color

Material 1 sheet grey

Material 2 foil colorless

Material 3 window frame white

PVC (%)

87

93

87

Filler (%)

7

-

8

Other additives (%)

6

7

5

Stabilizer system

Pb

Ca/Zn

Ba/Cd

K-value

61.5

60.2

69.3

Induction time (min)*

260

70

95

  • 180oC, nitrogen
  • 180oC, nitrogen
Table 3: Mechanical measurements of produced specimens, according to DIN 53455 and DIN 53735. Upper value: processed PVC waste without chalk; Lower value: processed PVC with 10 phr chalk

Material 1

Material 2

Material 3

Modulus of elasticity (N/mm2)

2850 2880

2530 2730

2500 2610

Tensile strength (N/mm2)

57.6 53.3

50.3 48.0

50.2 46.5

Elongation at break

(%)

10.1 12.8

15.4 15.1

20.0 20.9

Notched impact strength (kJ/m2)

8.3 12.1

36.6 25.9

15.2 22.9

The following Table 4 shows the induction time of the processed samples and the original PVC waste.

Table 4: Determination of residual stability at 180oC under nitrogen (induction time in min)

Material 1 Material 2 Material 3 Original PVC waste 260 70 95

Processed PVC waste without addition of chalk 120 35 40

Processed PVC waste with addition of10 phr chalk 170 95 65

Plastics Degradation Curve
Figure 10. Degradation curves of material 2; Original PVC waste —; Processed PVC waste without addition of chalk ; Processed PVC waste with additional 10 phr of chalk---.

As expected the period of induction decreases due to the thermomechanical treatment during processing, but a comparison of the unmodified and the modified PVC wastes indicates a considerable stabilizing effect of chalk. Also, the visual evaluation of the produced specimens indicates a better color quality of the modified samples. Especially PVC scrap with a low residual stability (material 2 and material 3) needs an additional stabilization if intended for a re-use. This is also impressively illustrated by the conversion-time-curve of material 2 (Figure 10). The chalk modified sample has a better heat stability than the original product in spite of one more processing cycle.

A feasibility of PVC recycling without additional stabilization is seen in mixing of PVC waste with a new material. Several studies are available about the use of PVC scrap in the manufacturing process of various PVC articles.40-43 They describe mainly the influence of regrind on the properties of virgin PVC. The investigations made with the re-use of roofing sheets show that new PVC roofing sheets can contain up to 10 - 20% of recycled material without any adverse effect on the product quality.25

New processing techniques offer another way to recycle PVC scrap. One example is the production of window frames by coextrusion.44 The regenerated PVC is used in core whereas virgin resin is used as the skin. In this process only 1/3 of new material is necessary to obtain the same properties as a window frame from 100% virgin PVC. The coextrusion process is also feasible for the production of pipes where the inside and the outside layers are made out of a new resin and the old material is used for the thick middle layer.45

CONCLUSIONS

  • The studies on the recycling-ability of used PVC show the importance of a careful characterization, especially if mixed PVC from the municipal solid waste separation should be recycled. The analysis of the composition and a detailed knowledge of the heat history and the molecular weight of a PVC scrap are necessary before reprocessing can be applied.
  • The suggested additional stabilization with calcium carbonate is an effective method for the recycling of PVC waste with a low remaining thermostability. The addition of chalk, up to a content of 10 phr, does not significantly change the mechanical properties, whereas it remarkably increases the heat stability.
  • A number of schemes have been initiated to collect plastics and to reprocess them to useful articles. The most advantageous situation occurs when a source of a single material type can be identified, for example, particular types of packaging, bottles, or window profiles. Such materials can often be simply reformulated and converted into high quality products.
  • Nevertheless, new material recycling concepts for PVC mixtures have to be established because in future the environmental preferences will play a larger role in a material's selection. A practical re-use of PVC requires a continuous stream of suitable scrap and the further development of technologies to reach the specifications for the intended applications. Another important point is that markets for the secondary products must exist to make the material recycling a successful economical enterprise.

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