Photodegradation of Vinyl ChlorideâVinyl Ketone Copolymer - ACS

19 Photodegradation of Vinyl Chloride-Vinyl Ketone Copolymer

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch019

M. HESKINS EcoPlastics Ltd., 201 Consumers Rd., Willowdale, Ontario, Canada M2J 4G8 W. J. REID,* D. J. PINCHIN,** and J. E. GUILLET DepartmentofChemistry,UniversityofToronto,Toronto,Ontario, Canada M5S 1A1

Recently it has been shown that photodegradable polymers suitable for use in packaging applications can be prepared by copolymerization of ethylene or styrene with vinyl ketone monomers (1). Since PVC is the third important plastic used for packaging, we have investigated the possibility of using the same method to develop photodegradable PVC compositions. Experimental Synthesis. The copolymers were prepared by suspension polymerization using a 1 gal stainless steel reactor operating at the autogenous pressure of vinyl chloride at 50° C. The vinyl chloride (Matheson) was purified by passing the gas over KOH pellets. A 9:1 ratio of water to vinyl chloride was used, the suspending agent being methyl cellulose (Methocel 25 cps, Dow Chemical). Percadox 16 (Noury Chemical Corp.) was the catalyst. Because the calculated reactivity ratios indicated that the vinyl ketone would be used up faster than the vinyl chloride, the methyl vinyl ketone was added at frequent intervals throughout the run to maintain an approximately constant feed composition. Yields of 7085% copolymer were obtained for the copolymers in 5-6 hr. The properties of the resulting copolymers are given in Table I. Film Preparation. The PVC homopolymer and the copolymers were reprecipitated from THF solution with methanol. Experimental compositions were made up containing 100 parts PVC homopolymer or PVC copolymer, 3 parts Ferro 75-001 (Ba-Cd stabilizer), 1 part Mark C (organic phosphite chelator), and 4 parts Drapex 3.2 (Epoxy plasticiPresent address: *Uniroyal Limited, Research Laboratories, Guelph, Ontario, Canada. ** Cavendish Laboratories, Cambridge University, Cambridge, England. 272 In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

19. HESKiNS E T A L .

Vinyl Chloride-Vinyl

Ketone Copolymer

273

T A B L E I. Copolymers of Vinyl Chloride and Methyl Vinyl Ketone

Ketone content mol-%


6 1.6 0

[η] dl/g 0.72 0.95 0.83

Tensile Elongation strength, psi at break, %

M ν

4400 4900 4900

54,000 78,000 65,000

130 130 110

zer). This stabilizer system was found to be more effective for the co polymers than other systems tried, including the tin salts. The above ingredients were dissolved in THF (15 ml/gm) and subsequently cast onto glass plates. The glass plates rested on mercury in order to en sure constant film thickness. Films of thickness 0.08, 0.20 and 0.25 mm were prepared in this manner. The presence of Drapex 3.2 im parts a slight plasticization to the film. Degradation. The films were weathered either on wooden racks placed on the roof of the Chemistry Department (University of Toronto), facing south at 45°, or in an American Ultraviolet Accelerometer (Model NS-1200). The temperature in the weatherometer was 37° C. Molecular Weight. The viscosity average molecular weight of the homopolymer and copolymers was determined from single point deter minations of viscosity of a 0.25% solution in Fisher purified cyclohexane at 30.0+0.01° C. The molecular weight is calculated from the intrinsic viscosity, [η], using the relationship (2) [η] = 1.63x10"* M 0.77 l

v

Results Preliminary experiments showed that when thermally stabilized thin films (80 μ thickness) of vinyl chloride-methyl vinyl ketone copoly mers were subject to UV radiation they became brittle without color for mation. When the ketone concentration in the copolymer was approxi mately 6 mol-%, the time to brittleness in the Accelerometer was less than 20 hr, whereas 40 to 50 hr were required if the ketone concentra tion was only 2%. Samples of PVC homopolymer, identically stabilized, were photolyzed along with the copolymers. No color or chemical change was apparent in these samples, even after 100 hr photolysis. Quantitative information was then obtained on the degradation pro cess by irradiating 0.2 mm thick film samples in the Accelerometer for varying periods of time and subsequently measuring their elongation at

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

UV LIGHT


274

INDUCED

REACTIONS

IN

POLYMERS

break. The results are shown in Fig. 1, while Fig. 2 shows how the molecular weight varied with photolysis time for both the homopolymer and copolymer. The molecular weight results are plotted in the form of [(l/M) - (1/MQ)] which gives the number of breaks per chain, corrected for differing initial molecular weight. As expected, the molecular weight and percent elongation remaining decreased with time for the copolymer, while little change was noted for the homopolymer. Outdoor weathering of the various samples confirmed the high susceptibility to UV degradation of the copolymer relative to the homo polymer. The elongation, tensile strength and molecular weight all de creased rapidly outdoors. The results are shown in Figs. 3-5. Figure 3 shows that the copolymer with 6% ketone degrades rapidly for the first three weeks outdoors. The film rapidly turns a brown color thereafter, possibly because the barium soap has been exhausted and the cadmium chloride is catalyzing the formation of color centers. The same dis coloration was noticed after 30 hr in the Accelerometer. The homopoly mer stabilized in the same way and exposed to the same conditions r e mains colorless for much longer periods of time. The stabilizers appear to be depleted in the copolymers at a higher rate than in the homopolymer. Moreover, the depletion rate increases with ketone con tent as the films with approximately 1. 5% present were still colorless after six weeks outdoor exposure. Infrared spectra of the crude copolymer showed a main carbonyl peak at ~1710 cm"* with a small peak at 1770 cm" . Reprecipitation of the copolymer removed the 1770 cm" peak completely. Upon irradia tion of the purified copolymer, the 1710 cm" peak immediately began to decrease while the 1770 cm" peak together with another peak at 1725 cm" , reappeared. These effects are shown in Fig. 6, which shows the IR spectrum of the irradiated film in the carbonyl region using an un irradiated film as reference. However, when the degraded polymer was reprecipitated at an early stage of photolysis, the 1770 cm" band again disappeared. Continued photolysis leads to a broadening of the 1770 cm" peak and the eventual disappearance of the 1710 cm" peak. The rates of appearance and disappearance of these bands were unaffected by the stabilizer system since a copolymer sample without stabilizer showed similar rates of change. 1

1

1

1

1

1

1

1

1

Discussion Previous studies by Hartley and Guillet (3) and by Amerik and Guillet (4) have shown that polymers containing ketone groups undergo photolysis by the Norrish type I and type II reactions. The relative proportions of type I and Π depend on the polymer concerned and the particular environment of the carbonyl group. These reactions are


HESKINS E T A L .

Vinyl Chloride-Vinyl Ketone Copolymer


Τ

Hours in Figure 1.

Accelerometer

Percent elongation remaining with irradiation in the accel erometer;filmthickness, 0.2 mm

Hours

in

Accelerometer

Figure 2. Normalized chain breaks with irradia tion in accelerometer;filmthickness, 0.2 mm



U V L I G H T INDUCED REACTIONS I N

Outdoor Figure 3.

POLYMERS

exposure, weeks

Normalized chain breaks on irradiation (outdoors)




HESKINS E T A L .

pvc/MVKT^i 6% ketone I Time (weeks) Figure 5.

Unirradiated

2

Percent residual tensile strength, outdoor weather ing; film thickness, 0.2 mm

Λ Ι

I5i/ hr irradiation 2

32 hr irradiation

JL

2000

JL 1700 Wavelength,

cm'

1

Figure 6. Photodegradation of PVC copolymer, differential infrared spec tra


278

U V L I G H T INDUCED REACTIONS IN

POLYMERS

represented in the scheme below : CI

CI

I C H - C H - C H


2

I - C H - C H

2

J c=o I

~CH

wwv

2

Partly because it is known that thermal stabilizers for PVC also inhibit UV degradation, it was thought that the photodegradation of PVC followed a mechanism similar to that established for its thermal degra dation. This mechanism is based on the free radical initiated dehydrochlorination reaction of alkyl chloride which, in PVC, results in conju gated unsaturation. This unsaturation is responsible for the rapid discoloration of PVC on heating. In photooxidation studies it is found, however, that HC1 begins to be produced only at a relatively late stage and that ketonic structures are observed at a relatively early stage. Kwei (5) has shown that these ketones are present as β chloroketones in the chain, i.e., — CH —C(=0)— CHg-CHCl— . A simple model for this type of ketone is 4-chloro-2-butanone which undergoes type I scission. Although this type of ketone can also undergo chain scission by type Π, this mode was not considered because no unsaturation was observed during photolysis. In the MVK/PVC copolymers the type I reaction does not cause chain scission so that, for chain scission to occur, the type II or some secondary mechanism is necessary. In order to minimize the effects of the secondary mechanisms which, in these copolymers also cause crosslinking and discoloration, the polymers were irradiated in the presence of stabilizers. Under these conditions the copolymers did indeed undergo chain scission. Only a very small amount of vinyl double bond absorption was observed in IR or the irradiated films, but 2



19.

HESKiNS E T A L .


279

even in the thermal degradation, where conjugated unsaturation is ob served in the UV spectrum, it is difficult to observe unsaturation in the IR. Photolysis of chloro ketones of structure similar to the copolymers has not been reported so we have examined the photolysis of 5 chloro2-hexanone, the simplest compound of similar structure, to see if it can undergo type II reaction. Irradiation of 5 chloro-2-hexanone in hexadecane at room temperature with 313 nm radiation results in the Norrish type II reaction, producing acetone and 2 chloropropene. Under the conditions of the experiment, the type I product observed would be expected to be 2 chlorobutane, but this was not detected. The study of the photolysis of 5 chloro hexanone is being continued and will be r e ported later. Although no type I product could be observed in this model compound it is probable that the type I would be significantly higher in the copolymer since in this case the carbonyl is attached to a secondary carbon instead of primary carbon and in simple ketones this increases the type I yield. Infrared studies of the carbonyl absorption of the copolymers as photolysis proceeds indicates the appearance of two new peaks and the loss of the original peak. The new absorption at 1725 cm" can be ex plained by several processes. One is the change from a secondary ketone to a primary ketone as the type II reaction proceeds. A second explanation is the oxidation of free radical sites generated by type I. This is similar to the mechanism postulated by Kwei for the production of the β chloro ketones. It is also possible that this absorption is due to the acetyl fragments split off by type I photolysis. We have observed that the carbonyl absorption at 1770 cm" can be removed by reprecipitation from THF solution by methanol. This indicates that the group responsible for this absorption is not chemically attached to the mole cule. The acetyl radical produced by the type I can abstract hydrogen, or chlorine, either from the polymer or the stabilizer to form either acetaldehyde or acetyl chloride which may hydrolyze to the acid. Both acetic acid and acetaldehyde have an absorption in the 3100 cm" region of the infrared and are unlikely since no absorption in this region was observed. When the IR spectrum of homopolymer PVC im pregnated with acetyl chloride was measured, two absorptions in the carbonyl region are observed at 1776 and 1722 cm" . These two absorp tions correspond quite well with those of the degraded copolymer. It does however indicate that the radical has abstracted a chlorine from the polymer, a process generally considered unlikely. 1

1

1

1


UV LIGHT INDUCED REACTIONS IN POLYMERS

280 Summary

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet light and solar radiation. Free radicals formed by the homolytic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involv ing the production of HC1 and conjugated double bonds.


Acknowledgements The authors wish to acknowledge the financial support for this r e search by EcoPlastics Limited and from the National Research Council of Canada under a P.R.A.I, grant number S-22. We also wish to thank Dr. H. G. Troth and members of the staff of Van Leer Research Labora tories, Passfield, England, for helpful discussions.

Literature Cited 1. Guillet, J. E., in "Polymers and Ecological Problems", J. Guillet, ed., Plenum Publishing Corp., New York, 1973. 2. Dailer, K. and Vogler, K., Makromol. Chem.,(1952), 6, 191. 3. Hartley, G. H. and Guillet, J. E., Macromolecules, (1968) 1, 165. 4. Amerik, Y. and Guillet, J. E., Macromolecules, (1971) 4, 375. 5. Kwei, K.-P. S., J. Polym. Sci., Part A-1, (1969) 7, 1975.


Photodegradation of Vinyl ChlorideâVinyl Ketone Copolymer - ACS

Recommend Documents

Photodegradation of Vinyl ChlorideâVinyl Ketone Copolymer - ACS