Polyepoxides as Stabilizers for Poly(vinyl chloride)

5 Polyepoxides as Stabilizers for Poly(vinyl chloride)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch005

HEINRICH HOPFF Swiss Federal Institute of Technology, Zurich 6, Switzerland

The mechanism of the thermal degradation of PVC is not completely understood, but it is generally accepted that the major effect is the splitting off from the polymer of hydrogen chloride, which results in the formation of highly chromophoric conjugated double bonds and coloration. Therefore, HCl-absorbing reagents have proved to be effective stabilizers for chlorine-containing polymers. Epoxy compounds, especially polyepoxides, have found practical application because of the high reactivity of the epoxy group with HCl. A number of new polyepoxides containing aromatic rings have been prepared, and their stabilizing effect has been determined, whereby dibutyltin laurate was used as standard. The epoxidized unsaturated oils can be used as stabilizing plasticizers and show good compatibility with the resinous compounds.

" D o t h heat and light w i l l degrade poly (vinyl chloride) ( P V C ) with elimination of H C l and formation of chromophoric double bonds. These bonds are highly reactive and decrease the mechanical properties of the polymer by oxidation and crosslinking. Although the full mecha nism of all reactions involved in this process is not known, the major effect is the splitting off of hydrogen chloride. For that reason, acceptors for this acid were generally used to stabilize P V C . Wartman (7) has examined a number of metal salts of organic acids, but he could not find a general relationship with the rate of decomposition. Arlman ( I ) has proved that no catalytic effect of H C l exists and that added H C l does not increase the rate of decomposition. In the presence of lead stabilizers the decom position is independent of H C l . Arlman could show that iron salts induce oxidation b y radical formation. F o r that reason, benzoyl peroxide ( 1 % ) 57

In Stabilization of Polymers and Stabilizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

58

STABILIZATION

OF

POLYMERS

A N D STABILIZER

PROCESSES

doubles the rate of decomposition, and diazoaminobenzene increases it 30-fold. For radical formation Arlman gives the following scheme. For the mechanism of the reaction it is supposed that the thermal initiation consists in splitting off the end group:

RCH —CHC1—CH —CHC1—CH.,—CHC1 2

2

i


R- +

ÇH —CHC1—CH —CHC1—CH,—CHC1 2

2

after which propagation might proceed as follows: C H — C H = C H — C H C 1 MM jr CHo=CH—CH—CHC1—CH I ' C H o = C H — C H — C H = C H — C H C l MM 2

2

C H = C H — C H = C H — C H — C H C l — C H — MM 2

2

Marvel and Horning (6) suggested an allyl (zipper) mechanism. More is known about the mechanism of P V C stabilization with tin com pounds as Kenyon (5) has shown. In this case, it could be proved with Cu-labelled dibutyltin diacetate, that only the butyl groups react with the radicals, and the rest adds on the H C l .

S-c

§-c

Η

H

·

Cl H H

H

4 Η β

H

Cl H

|~H H C H 4

°

0 1

Cl H

C

+

\

/

/

\

4H ^ 9

O A c

Q

A

c

^OAc -^oAc

9

There seem to be different kinds of H C l eliminations, and Fuchsman (4) designates them as paradehydrochlorination (double bond forma tion) and diadehydrochlorination, whereby no double bonds are formed (crosslinking) (see top of p. 59). The problem is complex. Commercial types of P V C show great differences in their behavior, depending on the method of production, additives, and impurities. The practical demands for a good P V C stabilizer are manifold. The stabilizer should be efficient in small amounts and not only be effective at high temperatures but also in the presence of light. It should not give any color and should be miscible with P V C to give products with


5.

HOPFF

59

Polyepoxides as Stabilizers

Cl — -HCl H H H H .Cv Cl yC ν Cl C . Cl y c v C l / / H C H C H C H C H H H H

—

y

N

/

N

/

N


H

/

X

H

H

/

H

H

I

H

H

,C V C l y C V C l y C V C l yC ν C l / /

H

N

C H

/

H

X

C / H H

X

C / H H

N

C ^ H

the same refractive indexes. It should have an agreeable odor, be non toxic, not decrease the good electrical properties and working conditions of the polymer. Of course, PVC's good mechanical properties must be maintained, and last but not least, the price of the stabilizer should be low. From this standpoint, the epoxy compounds seemed promising and are practically used on a large scale, very often with other compounds. The mechanism of their effect is certainly caused somewhat by their ability to add H C l and form chlorohydrins, but other effects w i l l play a part too, especially the presence of oxygen. The first type of epoxy product ( phenoxypropene oxide ) from sodium phenolate and epichlorohydrin was used in 1932 by I G Farbenindustrie but showed too high a volatility. Corresponding derivatives of alkylphenols gave excellent results. The condensation product of diisobutylphenol was used under the name, stabilizer D B G . The epoxides do not form insoluble metal chlorides like lead compounds. As the tendency to bind H C l increases with the number of the epoxy groups, many polyepoxides have been studied as stabilizers for P V C . Most of these products were obtained by reaction of polyhydroxy com pounds with epichlorohydrin, for example, diphenylolpropane, 4,4'-dihydroxybenzophenone, hydroquinone. Diglycidyl esters of dicarboxylic acids can be prepared from epichlorohydrin and alkali salts of dicar boxylic acids. A l l these products have the epoxy group in ether linkage. Another category contains the epoxy group in direct bonding with aro matic rings. In our laboratory the following products have been prepared: (1) Diepoxides of aromatic hydrocarbons: 1,4-diepoxyethylbenzene, 4,4'-di(epoxyethyl) diphenyl, 4,4'-(h ( epoxyethyl ) diphenyletber, and 1,35-tri ( epoxyethyl ) benzene.


60

STABILIZATION

OF POLYMERS

A N DSTABILIZER PROCESSES

(2) Diepoxides from Diels-Alder addition products of quinone with two molecules of butadiene, which exist as several isomers (f.p., 179°, 186°, 216°, 260°, and 320°C.


OH-

Η ΟΗ

•"

Η ΟΗ

/λττ.

/rif

ΟΗ775%

dioxyd 260

rrt

H OH

ΗΟ H

ΗΟΗ

ΗΟ Η

ΗΟ Η

Η Ο Η

dioxyd 186

dioxyd 179

τ τ

\

y-v

ττ

ΟΗ-/91%

72% tetrol 320

tetrol 303

The structure of the diepoxides was proved by saponification to the corre sponding tetrahydroxy compounds ( tetrol 303 and 320 ). Many other types with similar structure are known—e.g., CH

2

R = (CH ) -io 2

2

USP 2.750.395

o:

*CH 2

O

\ C H

D R P 931.130

Ο 9

Ο

O-O USP 2.739.161



5.

HOPFF

61

Polyepoxides as Stabilizers

To synthesize the new polyepoxides, two different methods can be considered: (1) epoxidation of the corresponding vinyl-compounds; (2) chlorination of the corresponding acetyl compounds, reduction to the corresponding carbinols, and elimination of H C l by potassium hydroxide. 1,4-Diepoxyethylbenzene was first prepared by Everett and K o n (3) as a liquid which evidently was not pure because analysis showed a dif ference of 4 % in the carbon content. Our product was a white crystalline mass with melting point 79°C. and boiling point 95 °C. at 0.06 mm. Catalytic hydrogénation gave the theoretical absorption. The stabilizing effect in P V C mixtures was about 30% of dibutyltin laurate—i.e., 3.3 times more of the stabilizer is necessary to obtain the same discoloration as dibutyltin laurate in a plasticized mixture of P V C with 30% dioctylphthalate at 200 °C. for 20 minutes. 1,3,5-Triacetylbenzene was prepared by the two methods mentioned above. CO · C H

C H , · CO

CO · CH C1 2

3

CO · CHoCl

CO · C H

C H O H · CH,C1

CHoCl · C H O H

C H O H · CH C1 2

Epoxidation of the trivinylbenzene gave only a poor yield because of polymerization. Chlorination of 1,3,5-triacetylbenzene in chloroform gave the chloroketone in 80% yield and 96% of the chlorohydrin by MeerweinPonndorf reduction. The chlorhydrin could be separated into two iso mers, one of which melts at 146°C. The other racemic product was not obtained in crystalline form. By treatment with potassium hydroxide, both chlorohydrins gave the corresponding epoxides, one melting at 81.5°, the other at 64°C. The triepoxides as well as the trichlorohydrins have three asymmetric centers; four optical isomers exist, which form two pairs of antipodes. Two racemic forms are possible: ddd-lll and dld-ldl. The two isomers mentioned above should correspond to these pairs. The stabilizing effect in P V C is about half that of dibutyltin laurate.


62

STABILIZATION

OF POLYMERS

A N D STABILIZER

PROCESSES


Another interesting triepoxide has been prepared recently from 1,3,4trivinylcyclohexane ( 2 ) . Its stabilizing effect on P V C has not yet been tested. 4,4'-Di(epoxyethyl)diphenyl was obtained from diphenyl by FriedelCrafts reaction with chloracetyl chloride. Meerwein-Ponndorf reduction gave the dichlorohydrin, and treatment with alcoholic potassium hydrox ide gave 9 7 % of the theoretical yield of diepoxide, (f.p., 161 ° C ) . The stabilizing effect was about one-fourth that of dibutyltin laurate. 4,4'-Di ( epoxyethyl ) diphenyl CI

Ç1

Ο

2

2

Ο Cl

C H ^ h Q h ^ - C U H

>

CH -^H-^~~^—^~^6Η-Λ:Η

Ο

OH CI

QH

«·— C H

2

2

^ H - - ^ ^ - - ^ ^ C H - J : H

2

4,4'-Di( epoxyethyl) diphenyl ether was prepared by the same method (f.p., 9 3 ° - 9 4 ° C . ) The stabilizing effect was about one-fourth that of dibutyltin laurate. 4,4'-Di ( epoxyethyl ) diphenylether Ç1 C^"°O

*

C H

Cl C H ^ H - ^ ^ O - ^ ^ C H - i H ,

«.

Ο

™

2

- C - ^ ^ O - ^ - C - ^ C H

OH 2

Ο Cl

OH

2

CI

J H H ^ ^ O - ^ ^ C H - C H

2

1,5-Di(epoxyethyl)naphthalene was prepared according its formula (f.p., 7 4 . 5 ° - 7 5 ° C . ) The stabilizing effect was about one-fourth that of dibutyltin laurate. A l l these polyepoxides form white leaflets. The practical stability tests of these epoxides showed that there is a relationship between the number of epoxide groups and the stabilizing effect, but this behavior does not occur i n all cases. Thus, for example, the epoxides of the Diels-Alder adducts of butadiene are not as effective as the diepoxides of diphenyl. The best effects were obtained with 1,3,5triepoxyethylbenzene (ca. half the effect of dibutyltin laurate). Unfor tunately, the stability of this product on storage is not satisfactory. After four weeks standing at normal temperature a considerable part of the


5.

HOPFF

63

Polyepoxides as Stabilizers 1,5-Di ( epoxyethyl ) naphthalene


Ο Cl II I C—CH

2

product was completely insoluble in ethanol. The diepoxides from d i phenylether were more stable. Storage at lower temperature increases the stability considerably. The importance of polyepoxides is increased by the fact that epoxides of unsaturated oils (linseed or soybean oil) can be used as stabilizing plasticizers. The general structure of such products is shown below. CH —O—CO—(CH, ) — C H 2

1 0

8

ι

CH—O—CO— (CH, ) — C H — C H — (CH ) — C H 7

2

7

3

\κ

ο CH,—Ο—CO— ( C H ) — C H — C H — C H — C H — C H — ( C H ) — C H Ο o 2

7

2

2

4

3

Epoxy-plasticizers of this type are used commercially—e.g., Paraplex resins of Rohm and Haas and the epoxy plasticizer D 81 of Henkel Inter national G m b H , Diisseldorf). The Abrac A (Bush Roake Allen L t d . , London) belong to this class. In addition to their stabilizing properties, all these products have a strong plasticizing action, thus making possible a reduction in the quantity of other plasticizers used. Besides low volatility they show a low tendency


64

STABILIZATION

Table I.

OF POLYMERS

A N DSTABILIZER

Epoxystearate as a Plasticizer for Vinylite V Y R D Exposed for 264 hours


PROCESSES

e

Aged

Plasticizer

Loss of Tensile Strength, %

Loss of Elongation, %

Loss of Tensile Strength, %

Loss of Elongation, %

Butyl epoxystearate Epoxidized diacetomonoolein Epoxidized soybean oil Dioctylphthalate

38 20 4 32

60 35 0 60

22 19 10 24

18 10 0 24

Change in mechanical properties by 264 hours exposure to the atmosphere and 264 hours aging at 140°C. in the presence of air. 0

to migration, good resistance to mineral oil, water, and detergent solu tions. The epoxy content is normally higher than 6%. Aluminum and zinc compounds, when used as the sole stabilizers, have a tendency to form chlorides, which may act as catalysts to decompose P V C . W i t h the epoxidized plasticizers this disadvantage does not exist. Combination of zinc or calcium compounds with epoxidized oils show a synergistic effect, which permits a reduction i n the amount of the expensive tin stabilizers. The increase i n heat stability of epoxy plasticizers is often combined with a better light stability ( Table I ). Literature Cited (1) Arlman, J., J. Polymer Sci. 12, 543 (1954). (2) Derichs, F., Schade, W., Glossauer, O., Franke, W., Ann. Chem. 687, 116 (1965).

(3) Everett, J. L., Kon, G. A. R., J. Chem. Soc. 1950, 3131. (4) Fuchsman, C. H., SPE J. 9, 787 (1959). (5) Kenyon, A. S., Natl. Bur. Std. Circ. S25 (1953). (6) Marvel, S., Horning, E. C., "Organic Chemistry," Wiley, New York, 1943. (7) Wartman, L. H., Ind. Eng. Chem. 47, 1013 (1955). RECEIVED April 19, 1967.


Polyepoxides as Stabilizers for Poly(vinyl chloride)

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