Polymer Degradation and Performance - American Chemical Society

Chapter 19

Degradative Transformation of Poly(vinyl chloride) under Mild Oxidative Conditions

Downloaded by CORNELL UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch019

Györgyi Szarka and Béla Iván* Department of Polymer Chemistry and Material Science, Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, P.O. Box 17, H-1525 Budapest, Hungary

Nowadays there are more and more demands to reuse waste materials, especially in the case o f polymers. O u r research seeks to reveal the oxidative degradation processes o f poly(vinyl chloride) ( P V C ) , which may provide new methods for treating and recycling of PVC waste materials.

Introduction Polyvinyl chloride) (PVC) is produced and used in the third largest amounts among polymers in the world nowadays. This material is one o f the most versatile polymers. It can be processed with a variety o f conventional and economic techniques to products with an enormously broad spectrum o f properties, that is from soft rubbery to very hard materials. However, it is well-known that PVC severely degrades at elevated temperatures, i.e. at the usual processing temperatures (see e.g. Refs. 7-5 and references therein). Therefore, the use o f stabilizers is inevitable for obtaining PVC products. The large amounts o f PVC production and use result in large quantities o f waste materials as well. In relation to o i l , energy and environmental concerns recycling and reuse o f plastic wastes have become an important issue nowadays. This is even more significant in the case o f non-biodegradable materials, such as the major commercial polymers including polyethylene, polypropylene, polystyrene, PVC etc.

© 2009 American Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

219

220


Considering the importance o f P V C recycling, we have carried out investigations aiming at utilizing the inherent l o w stability o f P V C in order to find new ways for broadening the reuse o f this polymer. Recently, we have published (4-6) that epoxidation o f degraded P V C and chemical modification o f vinyl chloride copolymers with functional comonomers may be utilized to obtain new properties which P V C itself does not possess. Epoxidation o f P V C has already been performed successfully under usual processing conditions (7). In this study, we report on our recent findings on an environmentally advantageous m i l d thermooxidative degradative process for oxidative transformation and/or decomposition o f P V C containing conventional stabilizers.

Effect of Lead Stearate on the Thermo-oxidative Degradation of PVC Production and use o f P V C occur in the presence o f air, i . e. in the presence o f oxygen. Therefore, it is surprising that the mechanistic details o f thermo oxidative degradation o f P V C are still not fully revealed. The major reactions o f this process are shown in Scheme 1. A s indicated in this Scheme, thermal dehydrochlorination yields HC1 and simultaneously sequences o f conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation o f radicals. Subsequent chain reactions result in additional initiation o f HC1 loss and further oxidative processes (7, 8). It has been shown that conventional P V C stabilizers act according to the reversible blocking mechanism (9, 10) even under oxidative conditions. However, there is no information on the fate o f the polymer chains in this process in the presence o f additives. It has been reported earlier that thermo oxidative degradation o f solid P V C leads to simultaneous cross-linking and chain scission (7, 77). In order to avoid the problems arising in the course o f investigating degradation o f solid P V C s , such as mixing, mechanical effects, inhomogeneous distribution o f additives etc., we have carried out thermo oxidative treatment o f P V C in solution, using dioctyl phthalate ( D O P ) as solvent, that is the most widely applied plasticizer o f P V C . Figures 1 and 2 show the molecular weight distributions ( M W D s ) o f P V C s treated with oxygen in the absence and presence o f lead stearate (PbSt ) stabilizer for different times. A s shown in these Figures, the M W D s in both cases are significantly shifted towards lower molecular weights with increasing degradation times. This clearly indicates that chain scission occurs, the extent o f which increases with increasing degradation times. The M W D s are monomodal in all cases, that is, there are neither low or high molecular weight tailings in these M W D curves. This means that the chain scission o f P V C chains is a random process, on the one hand. The absence o f high molecular weight 2



221

other products Scheme 1. The major reactions of thermo-oxidative

degradation

of PVC.

2.5-

IgM Figure

1. Molecular (DOP,

weight distributions

of PVCs in the absence of

02, 200 °C) at different degradation

stabilizer

times.


222


bimodality proves that cross-linking o f P V C does not take place under the investigated conditions, on the other hand. The absence o f cross-linking in D O P solutions is quite surprising on the basis o f results obtained with solid samples; in which simultaneous chain scission and cross-linking occur during thermo oxidative degradation o f P V C (7, 77). It is important to note that the oxidative chain scission o f P V C is a relatively rapid process indicated by the considerable shift in the M W D curves even after one hour degradation time. A t longer times, the molecular weight o f most o f the polymer chains fall in the range o f 1000 - 10000, that is in the oligomeric region. The M W D s o f the low molecular weight polymers appear narrower than that o f the rest o f the polymers which might be due to loss o f the even lower molecular weight polymers, i . e. below few thousand, formed by the scission process.

-u,* -i

,

3,0

,

,

3,5

•

,

4,0

,

,

4,5

•

,

5,0

.

1

.

,

5,5

6,0

igM Figure 2. Molecular weight distributions of PVCs in the presence of lead stearate stabilizer (DOP, 02, 200 °C) at different degradation times.

The oxidative chain scission o f P V C is better illustrated in Figure 3 depicting the decrease o f the number average molecular weight ( M ) o f P V C samples as a function o f reaction time. A s shown in this Figure, significant decrease o f M takes place in a short period o f time followed by slower M n

n


n

223 decrease. Surprisingly, there is no difference between the stabilized and unstabilized cases indicating that P b S t does not act as stabilizer for oxidative chain scission o f P V C .


2

This is also confirmed by the data in Figure 4 exhibiting the number o f chain scission per monomer units (S) as a function o f time. This was calculated according to the following equation: S = M V C ( 1 / M n - 1/Mn,0) where M V C , M n and M n , 0 are the molecular weights o f the v i n y l chloride monomer unit, the number average molecular weights o f the oxidized and starting P V C samples, respectively. It is interesting to note that chain scission proceeds with constant rate independent o f the extent o f degradation for a relatively long period o f time. The structural change during oxidation o f P V C was followed by F T I R spectroscopy. Figure 5 shows a representative F T I R spectrum. A s shown in this Figure, strong carbonyl signals (-1600-1800 cm" ) and a broad hydroxyl signal (-3100-3600 cm" ) appear indicating the formation o f carbonyl and hydroxyl containing new functional groups in the P V C chain formed during the oxidative process. This structural transformation may offer new application possibilities for the oxidized P V C s , such as subsequent functionalizations, blending with other polar polymers which cannot be mixed with untreated P V C etc. 1

1

Effect of 2,6-di-tert-butyl-4-methyIphenol Antioxidant on the Thermo-oxidative Degradation of PVC In another set o f experiments, we have investigated the effect o f 2,6-di-tertbutyl-4-methylphenol ( B H T ) antioxidant on the thermo-oxidative degradation o f PVC. A s data in Table 1 indicate, the presence o f this antioxidant leads surprisingly to more chain scission than that observed in the absence o f this compound. The molecular weight data in Table 1 show the large differences. After 3 hours o f treatment, the molecular weight decreases under 1000 which means extremely large numbers o f chain scissions. C o m p a r i n g the M values o f P V C samples with and without B H T antioxidant interestingly indicates that the B H T antioxidant (the additive given to avoid degradation) makes the chain breaking faster. After 4 hours degradation, oily products were obtained. Thus, it can be concluded that even the presence o f B H T does not prevent the oxidative chain scission o f P V C and this additive even accelerates the chain breaking decomposition process. n

Conclusions Systematic experiments were conducted with poly(vinyl chloride) ( P V C ) in order to introduce functional groups in the polymer chain by m i l d thermooxidative conditions in dioctyl phthalate ( D O P ) , a widely used plasticizer in this


224 60000

• •

with PbSt without stabilizer 2

50000

40000-

30000


20000-

10000-

t/h Figure 3. Number average molecular weight (Mn) of PVC as a function of degradation time during thermo-oxidative degradation of PVC in the absence and presence of lead stearate stabilizer (DOP, 02, 200 °C). 0.8 0.7 0.6 0.5 H

8 0.4-I 0.30.2-

with PbSt without stabilizer 2

0.1 0.0

H

5

'

r~ 6

t/h Figure 4. Number of chain scission (S) as a function of degradation time during thermo-oxidative degradation of PVC in the absence and presence of stabilizer (DOP, 0 , 200 °C). 2



225

Table 1. Number average molecular weights ( M ) of P V C as a function of reaction time in the absence and presence of B H T antioxidant during thermo-oxidative degradation (DOP, 0 , 2 0 0 ° C ) . n

2

M

n

BHT

(mmol/VC)

(g/mol)

Oh

3h

6h

0

42300

13800

8550

2

42300

750

-

10

42300

930

-

polymer. The effects o f lead stearate (PbSt ) stabilizers and 2,6-di-tert-butyl-4methylphenol were also investigated. Analysis o f the resulting products by gel permeation chromatography showed that chain scission o f P V C occurs under the investigated conditions in both the presence and absence o f P b S t and B H T antioxidant. Thus it can be concluded that neither the P b S t stabilizer nor the B H T antioxidant are able to prevent chain scission in thermooxidation o f P V C in dilute solutions. Surprisingly, the B H T antioxidant even accelerated the chain scission in dioctyl phthalate. The formation o f polar oxo groups in the polymer 2

2

2


226 was confirmed by I R spectroscopy. The appearance o f oxygen containing groups i n P V C is expected to result in easier blending o f such degradation products with a large number o f other polymers, organic and inorganic materials. Thus, the m i l d thermooxidative degradation and simultaneous functionalization o f P V C may lead to new environmentally advantageous, profitable recycling processes o f P V C waste materials.

Acknowledgements


G P C measurements by D r . M . Szesztay and M r s . E . Tyroler are gratefully acknowledged.

References 1.

Vol. 2.

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B i c a k , N.; Serkal, B. F . ; G a r i , M. Polym. Bull. 2003, 51, 231-236. N a g y , T . T . ; T u r c s á n y i , B.; K e l e n , T . ; T ü d ő s , F . React. Kin. Catal. 1978, 8, 7-11.

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9.

Lett.

Iván, B.; T u r c s á n y i , B.; K e l e n , T . ; T ü d ő s , F . J. Vinyl Technol. 1990, 12, 126-135. 10. Iván, B.; T u r c s á n y i , B.; K e l e n , T . ; T ü d ő s , F . Angew. Macromol. Chem. 1991, 189, 35-49. 11. Iván, B . ; Nagy, T . T.; T u r c s á n y i , B.; K e l e n , T . ; T ü d ő s , F . Polym. Bull. 1980, 2, 461-467.


Polymer Degradation and Performance - American Chemical Society

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