Thermal Degradation of Automotive Plastics: A Possible Recycling

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4 Thermal Degradation of Automotive Plastics: A Possible Recycling Opportunity M. Day, J. D. Cooney, C. Klein, and J. Fox National Research Council Canada, Institute for Environmental Chemistry, Ottawa, Ontario K1A 0R6, Canada

The

thermal

degradation

motive plastics -vinyl

of a series of 50/50 mixtures of four

[polypropylene,

acrylonitrile-butadiene-styrene,

chloride), and polyurethane]

mogravimetry experimentally polymers

was studied by using dynamic ther-

in nitrogen. Comparison

along with kinetic data obtained

techniques indicates that interactions weight-loss

of the weight-loss data obtained

with data predicted from the behavior of the

that make up a polymer degradation

meric composition in the polymer

by using

individual

isoconversional

can occur between the

mix. Both stabilization

polymers

and destabilization

processes were noted depending

of the mixture. The presence of poly-(vinyl

esses. The results could have important by automobile shredding

of

on the polychloride)

mixtures had significant effects on the weight-loss

of chemicals from mixed-plastic

autopoly-

proc-

implications for the

recovery

waste streams such as those

produced

operations.

T T H E DISPOSAL OF PLASTIC WASTE INTO LANDFILL SITES is well recognized as both a waste of a valuable, nonrenewable resource and a waste of a source of energy. Pyrolysis or tertiary recycling, meanwhile, represents an opportunity to preserve these hydrocarbons to produce valuable petrochemicals. Over the years a number of projects were developed to produce marketable products from plastic wastes, and these projects had varying success (1-4). This type of recycling has been projected to grow 10-fold in the next 10 years (5) be cause of the anticipated increase in the price of oil and the increased costs associated with current conventional disposal options.

0065-2393/96/0249-0047$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


48

POLYMER DURABILITY

The mixed-waste stream produced when junked automobiles are shred ded to recover ferrous and nonferrous metals represents a particularly attrac tive stream for converting organic wastes to hydrocarbons by pyrolysis (6, 7). This material, known as auto-shredder residue (ASR), is produced when old, discarded automobiles are shredded in a hammer mill. The waste is a complex mix of plastics, rubber, textiles, glass, foam, dirt, rust, etc., contaminated with automobile fluids and lubricants. Analysis and characterization of the material in our laboratories (8) has shown the material to be highly variable in com position, as would be expected based on the variability of materials used in the construction of automobiles. Whereas much information exists on the ther mal degradation of single polymer systems, the thermolysis of mixed polymer systems has received little attention. Most of the work on mixed polymer systems has centered on thermal stability studies of polymer blends, usually containing poly(vinyl chloride) (PVC) (9). These studies revealed that the blending of two polymers can either stabilize or destabilize the polymer com ponents present depending on the polymers and their degradation products. In this chapter, studies of the thermal degradation of 50/50 blends of the four major plastics used i n automobiles will be presented. The data will then be analyzed to determined the types of interaction to be expected when mixedpolymer systems such as those found in A S R are subjected to thermal recy cling processes.

Experimental The four automotive plastics used in this study were obtained from major resin suppliers to the automotive industry. They included: • • • •

A clean, flexible polyurethane (PU) foam used in automotive upholstery Himont's PRO-FAX SV-152 impact-resistant polypropylene (PP) B F Goodrich Geon PVC Dow Magnum acrylonitrile-butadiene-styrene resin (ABS)

Each polymer was ground cryogenically to a fine powder (less than 20 mesh) by using a Wuey Laboratory Mill. The 50/50 mixtures were prepared by weighing equal proportions of the polymers into a container, which was then agitated for 1 h to ensure thorough mixing. The thermogravimetry (TG) studies were conducted on a 2100 system (TA Instruments, Inc.) employing a 951 T G balance. A dynamic nitrogen atmosphere (50 mL/min) and programmed heating rates of 0.1-50 °C/min were used. Sample weights ranged from 11 to 13 mg. The kinetic parameters for the thermal degradation processes were deter mined by the isoconversional method of Ozawa (10) and Flynn and Wall (11). In addition, apparent activation energies were estimated by using Kissinger's peak maximum temperature technique (12). To facilitate the determination oï the peak maximum temperature from tne derivative weight-loss curve, Jandel's Scientific Peak Fit software program version 3.1 was employed to deconvolute the curve.

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

4.

DAY ET AL.

49

Thermal Degradation of Automotive Plastics


Results and Discussion PP/ABS. The thermograms obtained from the PP/ABS mix are shown in Figure 1. The four curves shown in this and the following figures corre spond to the weight losses for the two pure polymers as well as those obtained with the experimental mixture and the anticipated behavior based on the in dividual polymers in the absence of any interactions. The rate of weight loss appears to occur less rapidly in the mixture than would be anticipated. For example, at a heating rate of 5 °C/min at a temperature of 430 °C, the mix lost approximately only 41.1%, whereas the anticipated loss was 51.7%. Calculation of the kinetic parameters for the thermal degradation by using the Flynn-Wall approach gave the apparent activation energies (E) presented in Figure 2. Interestingly, there are significant differences between the values determined from the experimental data points and those obtained from the calculated data points based on the principle of additivity. The rationale for these differences are, at the moment, unclear, because the Kissinger method value for ABS of 163 kj/mol is very close to the anticipated value of 159 kj/ mol, whereas the value for PP is increased slightly from an anticipated value of 161 kj/mol to one of 198 kj/mol found experimentally. However, based on the data presented in Figures 1 and 2, it would appear that some interactions are taking place between these two polymers when subjected to thermal deg-

200

300

400

500

TEMPERATURE (°C) Figure 1. TG curves at 5 °C'/min for PP ( ) ABS ( ), experimental 50/ 50 mixture ( ), and calculated for the mixture in the absence of interaction (--)•



50

POLYMER DURABILITY

40 Γ 0.0

0.1

• 0.2

0.3

0.4

, , , , ι , . , • t , • . , ι • • • • ι • , • . 1 0.5 0.6 0.7 0.8 Ο.θ

FRACTIONAL WEIGHT LOSS Figure 2. Activation energies as a function of fractional weight loss for 50/50 mixture of PP/ABS derived from experimental (O) and calculated (V) weight-loss data. radation, especially in terms of the weight loss associated with the degradation of the A B S . PP/PVC. The experimental and predicted T G thermograms for the mix of P P and P V C and those of the pure polymers are shown in Figure 3. Once again, the weight loss with the mix is less pronounced than would be predicted. For example, the onset of dehydrochlorination appears retarded, and the weight loss associated with the process appears much less than antic ipated (i.e., 17.0% as opposed to 28.6% at a temperature of 350 °C). However, when the kinetic apparent Ε values are examined (Figure 4), an interesting observation is noted. For the initial 20% weight loss associated with the de hydrochlorination reaction, the experimentally observed and anticipated Ε val ues are almost identical and very similar to the Kissinger values of 133 and 130 kj/mol determined at the peak maximum. However, when the region associated with the degradation of PP and the polyenes produced from the P V C are considered (fractional weight loss 0.3-0.9), there is a marked dif ference in the Ε values. Therefore, the incorporation of P P with P V C could apparently lead to a certain amount of stabilization of the initial degradation of P V C , whereas the char formation action of P V C could be responsible for a retardation of the weight loss associated with the degradation of PP. PP/PU. The T G curves obtained for the PP/PU mix are presented in Figure 5. These polymers show their main decomposition processes in very



4. DAY ET AL.


51

200

40 • ' ' ' ' • • ' ' 0.0 0.1 0.2 1

1

0.3

0.4

0.5

•

0.6

0.7

•

0.8

ι . • . • I 0.9

FRACTIONAL WEIGHT LOSS

Figure 4. Activation energies as a function of fractional weight loss for 50/ mixtures of PP/PVC derived from expérimental (Ο) and calculated (V) weightloss data.



52

POLYMER DURABILITY

200

300

400

500

TEMPERATURE (°C) Figure 5. TG curves at 5 °C/minfor PP ( ), PU ( ), experimental 50/50 mixture ( ), and calculated for the mixture in the absence of interactions (-). clearly defined temperature regions that have only a slight overlap. In the case of this polymer mix, the weight loss occurs more rapidly in the mix than would be expected. For example, at 300 °C the mix lost approximately 6.2% more than predicted, whereas at 400 °C the loss was approximately 12.1% more. These results suggest the degradation processes in this particular blend are accelerated due to the presence of the other polymer. The kinetic parameters presented in Figure 6 appear to confirm this interaction for the PP/PU sys tems, although the actual Ε values are only slightly smaller than the predicted values. A B S / P V C . The weight-loss behavior of the ABS/PVC mix (Figure 7) in many ways resembles that of the PP/PVC mix. For example, the weight loss associated with the dehydrochlorination reaction is much more pro nounced than expected, and the weight loss associated with the process is much less (i.e., 18.1% in comparison to 29.5% at 350 °C). In a similar manner to that observed with PP/PVC, the apparent Ε for the dehydrochlorination process (Figure 8) was relatively constant and similar for the experimental and simulated data over the first 25% weight loss. However, differences are noted between the Ε values at higher conversion values, and the experimental values are consistently higher than predicted values. Therefore, like PP, ABS is be stowing a certain amount of stability to the initial dehydrochlorination of P V C .



4. DAY ET AL.

0.0

53


0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Figure 6. Activation energies as a function of fractional weight loss for 50/ mixtures of PP/PU denvedfrom experimental (O) and calculated (V) weight-lo data.

200

300

400

500

TEMPERATURE (°C)

Figure 7. TG curves at 5 °C/minfor ABS ( ), PVC ( ), experimental 501 50 mixture ( ), and calculated for the mixture in the absence of interactio (--)•


54

POLYMER DURABILITY


220

0Γ 0.0

ι

ι

ι

ι- I ι

0.1

ι

ι

ι

0.2

I ,,.

,

,

, ,| . ,

0.3

,

,

,

0.4

I „,

,

,

,

0.5

I ,

,

,

, , I ι, . ι

0.6

• I ... i . i

0.7

ιI

0.8

< .,„ . .

1

0.9

FRACTIONAL WEIGHT LOSS Figure 8. Activation energies as a function of fractional weight loss for 50/50 mixtures of ABS/PVC derived from experimental (O) and calculated (V) weightloss data. A B S / P U . The T G curves for the ABS/PU mixture are presented in Figure 9. Once again, as was noted from the PP/PU mixture, three degrada tion regions have been identified. In all cases the peak degradation temper atures of each region were noted to occur at lower temperatures than predicted. For example, in the case of the P U they occurred at 287 °C and 366 °C (predicted values were 295 °C and 374 °C), whereas A B S peaked at 400 °C when it was predicted to peak at 412 °C. In terms of measured Ε values (Figure 10), the experimentally determined values were consistently higher than expected, especially for the second stage of the P U degradation. This kinetic data further suggests that the presence of A B S is retarding the degradation of P U due to diffusion control in addition to chemical interaction processes. PU/PVC. In the case of the PU/PVC mixture, it is clearly evident from the weight-loss curves shown in Figure 11 that interactions are occurring be tween the two polymers. For example, the experimentally determined tem perature of dehydrochlorination is much lower (273 °C) than was expected based on the additivity rule (295 °C). It is possible that the degradation of the P U is influenced by the accumulation of H C l within the polymer mix that may catalyze the weight-loss processes and cause an acceleration in the initial weight-loss processes of P U . However, as degradation proceeds, possible cross-linking reactions involving P V C may retard the diffusion of liberated



4. DAY ET AL.

55


TEMPERATURE (°C)

Figure 9. TG curves at 5 °C/min for ABS ( ), PU ( ), experimental 50/ 50 mixture ( ), and calculated for the mixture in the absence of interactio (...).

180

Q

I ,

0.0

, , ,

ι

0.1

ι

0.2

0.3

t ,

0.4

0.5

, ,

, ι , , , ,

0.6

ι ,

0.7

t . ,

0.8

I

0.9


Figure 10. Activation energies as a function of fractional weight loss for 50/ mixtures of ABS/PU derived from expenmental (O) and calculated (V) weigh loss data.


POLYMER DURABILITY


56

200

300

400

500

TEMPERATURE (°C) Figure 11. TG curves at 5 °C/min for PVC ( ), PU ( ), experimental 50/ 50 mixture ( ), and calculated for the mixture in the absence of interactions (--)• products at temperatures above 380 °C, where a reduction in rate of weight loss is noted. Interestingly, examination of the kinetic parameters for the ther mal degradation processes as shown in Figure 12 reveal that the Ε values determined from the experimental data very closely resemble those predicted for the behavior of the individual polymers.

Conclusion The thermal degradation of mixed polymer systems cannot be predicted on the basis of the behavior of individual polymers. Both diffusion and chemical interactions appear possible when 50/50 polymer mixtures are being consid ered. The effects may either stabilize or destabilize the component polymers present in a polymer mix and influence the kinetics and mechanism of the degradation processes. In the case of PVC-containing mixtures, the liberation of H C l or chlorine radicals is of particular concern, because they could have a significant effect on pyrolytic degradation processes and subsequent product yields. The data provided in this study identify some of the concerns that must be addressed if pyrolysis is to play a key role in the recycling of automotive plastics produced by automobile shredding operations.


4.

DAY ET AL.

51



260

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

FRACTIONAL WEIGHT LOSS Figure 12, Activation energies as a function of fractional weight loss for 50/50 mixtures of PVC/PU derived from expérimental (Ο) and calculated (V) weightloss data.

References 1. Kaminsky, W. J. Anal. Appl. Pyrolysis 1985, 8, 439-448. 2. Scott, D. S.; Czernik, S. R.; Piskorz, J. Radlein, D. St. A. G. Energy Fuels 1990, 4, 407-411. 3. Bertolini, G. Ε.; Fontaine, J. Conserv. Recycl. 1987,10, 331-343. 4. Kaminsky, W. Makromol. Chem., Macromol. Symp. 1992, 57, 145-160. 5. Anon. Reuse Recycle Wastes 1992, 22(9), 68. 6. Braslaw, J. Melotik, D. J.; Gealer, R. L. Wingfield, R. C. Thermochim. Acta 1991, 186, 1-18. 7. Roy, C.; deCaumia, B.; Mallette, P. In Proceedings of the 16th Institute of Gas Technology Conference on Energy from Biomass and Wastes; Institute of Gas Technology: Orlando, FL, 1992;p1. 8. Day, M.; Graham, J.; Lachmansingh, R.; Chen, E. Resources Conserv. Recycl. 1993, 9, 255-279. 9. McNeill, I.C.;Gorman, J. C. Polym. Degrad. Stab. 1991, 22, 263-276. 10. Ozawa, T. Bull. Chem. Soc. Jpn. 1965, 38, 1881-1886. 11. Flynn, J. H. Wall, L. A. Polym. Lett. 1966, 4, 323-528. 12. Kissinger, Η. E. Anal. Chem. 1967, 21, 1702-1706. ;

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RECEIVED for review December 6, 1993. ACCEPTED revised manuscript January 9, 1995.


Thermal Degradation of Automotive Plastics: A Possible Recycling

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