Graft Polymerization of Vinyl Compounds on Ethylene-Vinyl Acetate

31 Graft Polymerization of Vinyl Compounds Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 31, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch031

on Ethylene-Vinyl Acetate Copolymers HERBERT BARTL and DIETRICH HARDT Polymerization Department, Central Research Laboratory, Farbenfabriken Bayer AG, 5090 Leverkusen, West Germany

Ethylene-vinyl acetate copolymers represent a good backbone for the radical graft polymerization of vinyl compounds. The grafting tendency of the vinyl compounds used is proportional to the activity of the monomer radicals, according to Mayo and Walling, in the following order: vinyl chloride, vinyl acetate, ethyl acrylate, methyl methacrylate, and styrene. The dependence of the degree of grafting on monomer type suggests that under the conditions of suspension polymerization, grafting points in the backbone are formed predominantly by transfer reactions of monomeric or primary radicals. In solution polymerization in tert-butyl alcohol, acrylonitrile shows a greater tendency to graft than vinyl chloride or vinyl acetate. This behavior does not follow the activity series of the monomer radical.

"ethylene and vinyl acetate can be copolymerized in any desired proportion. Depending on their vinyl acetate content, the copolymers, hereafter called E V A copolymers, exhibit very different properties. With a low vinyl acetate content, they are similar to high pressure polyethylene but possess an improved transparency and elasticity. With about 40-50% vinyl acetate, the copolymers show pronounced elastomer properties. With higher contents of vinyl acetate, the copolymers represent soft compounds, and finally, with a very high content of vinyl acetate, their properties approach those of poly (vinyl acetate). Since E V A copolymers, owing to their saturated character, possess excellent resistance to weath ering, it seemed interesting to try to combine the elastomer type with approximatly 45% vinyl acetate by graft polymerization with monomers forming plastics, such as vinyl chloride or styrene-acrylonitrile to impart to the plastics improved impact resistance. 477 In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

478

ADDITION A N D C O N D E N S A T I O N

PROCESSES

\ l / H H H H C H H

H

POLYMERIZATION

H

-c—c4-c-)-c—C4-C4-C—

I IT ι ι τ I

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 31, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch031

H

H Ο H H H H CO

H Preferred Braching Points of an EVA Copolymer Because of their content of hydrogen atoms, which can be separated by a radical reaction, ethylene-vinyl acetate copolymers have a good basis for grafting. In earlier papers on the radical crosslinking of E V A copolymers with peroxides, we showed that polyunsaturated compounds, such as triallyl cyanurate, can be incorporated completely in the copoly mers by graft reactions (2). To survey as completely as possible the grafting behavior of E V A copolymers toward various vinyl compounds, our investigations covered the grafting of vinyl acetate, vinylidene chloride, and acrylic and methacrylic esters. As polymerization processes, at first we preferred suspen sion polymerization to exclude the influence of solvents by terminating or transfer reactions during polymerization. Grafting by emulsion poly merization, in which the E V A copolymer was dissolved in the monomer before polymerization, was difficult because coagulate was formed as polymerization proceeded. The various monomers were compared as to the degree of grafting and the reaction of the backbone under identical reaction conditions— i.e., only Porofor Ν ( AZBN = a,c/-azodiisobutyronitrile ) was used as the initiator in the same concentration, and the polymerization took place at 60°C. at about equal monomer conversion rate. The backbone used was Levapren 450, an ethylene-vinyl acetate copolymer with a vinyl acetate content of 45% and an average molecular weight of about 100,000. When the starting ratio was two parts of backbone to three parts of monomer and when the conversion was about 75% complete, the result ing polymers contained about 45% of the original backbone and 55% of the polymerized monomers. The degree of grafting was determined by fractionation or extraction, on the basis of the different solution be havior of the E V A copolymers and the grafted products or pure polymers,

In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

BARTL AND HARDT

31.

479

Graft Polymerization

respectively. [In determining the degree of grafting by extraction, rela tively broad scattering may occur owing to too low solubility differences.]


Table I.

Product

a

Grafting of Various Monomers on Levapren 450°

Monomer

1

Styrene

2

Grafting of Grafting of the Monomer, the Backbone, Instructions % % Processing 5

8

A

Methyl methacrylate

28

29

A

4

Ethyl acrylate

88

38

A

6

Vinylidene chloride

—

67

Β

95 —100

45 82

A Β

3 Vinyl acetate 5 Vinyl chloride Farbenfabriken Bayer AG.

Table I gives the composition of the grafting products investigate. The monomers are listed in the order of their increasing tendency to graft— i.e., in the order of an increase in the degree of grafting of the monomer and of the grafted backbone portion. A similar sequence was determined by Hayes for the grafting of vinyl chloride, vinyl acetate, and styrene by emulsion polymerization on poly (vinyl chloride), polyacrylonitrile, or poly (vinyl acetate) (7). Obviously, the sequence in Table I corre sponds to the order of the relative activities of the monomer radicals according to Mayo and Walling. The dependence of the degree of grafting on monomer type leads to the conclusion that under the polymerization conditions used here the grafting points in the backbone are formed predominantly by transfer reactions of monomer radicals or primary radicals, respectively, obtained by a reaction of initiator and monomer and not by the radicals of the initiator α,α'-azodiisobutyronitrile. The reaction of the polymer radicals with a hydrogen atom of the backbone, which leads to a homopolymer by chain termination, can also be of minor importance only because in the case of vinyl chloride, for example, when larger proportions of the EVA copolymer are used, no homo-PVC is obtained during grafting (see Tables I and II ). Thus, it is the highly active vinyl chloride radical that is responsible for the extensive grafting of vinyl chloride, while the inert resonance-stabilized styrene radical imparts only slight grafting. After investigating the grafting behavior of styrene on polyethylene, Czvikovsky and Dobo emphasized the importance of certain primary radicals for the formation of grafting points (4). The effectiveness of AZBN in forming grafting points has already been shown (1, 3, 8, 9). An argument in support of this is that it is impossible by the action of



480

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AND CONDENSATION

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AZBN, in contrast to various peroxides, to crosslink E V A copolymers radically, which means that the formation of radicals at the macromolecules does not take place to a sufficient extent. For vinyl acetate there seems to be a contradiction regarding the extent of grafting of the backbone because it is essentially lower than for vinyl chloride, although the vinyl acetate radical (according to Mayo and Walling) is approximately as active as the vinyl chloride radical. This observation might be accounted for by the poly (vinyl acetate) chains which, during polymerization have been formed and grafted, and compete as a new graftable backbone with the E V A copolymer, in which, according to a well-known reaction, poly (vinyl acetate) branchings are formed. Consequently, a lower proportion of E V A copolymer is grafted than expected. On the basis of a series of experiments with vinyl chloride, which, owing to its high tendency to graft with the backbone, offers favorable conditions for investigation, let us now consider some influences on the grafting reaction such as proportion, composition, and differences in the molecular weight of the backbone. Table II shows the dependence of Table II.

Product 9 10 11 12

Grafting of Vinyl Chloride on Levapren 450 with Increasing Quantities of Levapren

Content of Backbone, % 6 24 42 63

Grafting of Vinyl Chloride, % 10-15 65-70 -400 —100

Grafting of the Backbone, Production % Instructions 90 81-85 80 60

Β Β Β Β

the extent of grafting on the proportion of backbone. The extent of vinyl chloride grafting rises with an increasing quantity of backbone, so that from about 40% backbone content homo-PVC is no longer de tectable in the products by fractionation from tetrahydrofuran solution with petroleum ether. For instance, a graft copolymer containing about 50% E V A and 50% PVC has the values of fractionational precipitation shown in Table III. Such graft polymers, which virtually contain all the poly (vinyl chloride ) in a grafted condition, can be crosslinked easily with peroxides via the E V A copolymer chains. This corresponds to an indirect state ment on the degree of grafting of the products. The degree of reaction of the backbone occurs in the reverse direc tion: when the quantities are lower than 10%, the backbone is grafted largely down to small low molecular weight proportions extractable from


31.

B A R T L

Table III.

481


A N D H A R D T

Fractional Precipitation of Graft Copolymer of 50% E V A (45%~VAc) and 50% P V C a

Amount Precipitated


Fraction

a

35.5 34.2 33.5 31.1 26.2 5.3 0.8 0.2

1.85 1.68 1.65 1.54 1.38 1.00 0.72 0.46

37.6 17.6 11.5 12.5 4.6 3.3 2.2 8.4 2.3

1 2 3 4 5 6 7 R Loss

Chlorine Content

(v)

—

Solvent, tetrahydrofuran; precipitating agent, petroleum ether. Table IV.

Product 14 7 5 8 15

—

Grafting of Vinyl Chloride on Ethylene-Vinyl Acetate and Poly (vinyl acetate)

Proportion of Backbone, % 40 42 41 46 45

Vinyl Acetate in the Backbone, % 17 30 45 66 100

Degree of Grafting of the Backbone, Production % Instructions 30 50 81 82 —20

C Β Β Β D

the product. The dependence of the degree of grafting of the backbone upon the content of vinyl acetate is shown in Table IV. The tendency to graft rises with increasing content of vinyl acetate and reaches its maximum at 45-70% vinyl acetate. As expected, the degree of reaction of the backbone is greater the higher the concentration of the activator. More interesting is the observation that the grafted quantity of backbone depends on the molecular weight (Table V ) . Table V . Grafting of Vinyl Chloride on Ethylene-Vinyl Acetate Copolymers with Non-uniform Molecular Weight (50% V A C in the Backbone)

Product 16 17

Content of Backbone in the Polymer, % 26 27

Molecular Weight —3 3

Χ Χ

10 10

4 5

Degree of Grafting of the Backbone, Production % Instructions —20 73


Ε Ε


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Two graft polymers of the same composition, which have been pre pared under identical conditions and differ only in the molecular weight of the backbone, are compared in Table V. The data show that the high-molecular weight E V A is subject to grafting, as far as quantities are concerned, to a much higher degree than the low molecular weight product. If one intends to graft on very little monomer, the E V A polymer granules may be swelled slightly with the monomers containing the initia tor, dispersed in water, and polmerized (Figure 1). The upper part of Figure 1 shows the E V A copolymer used as a starting product (A) (vinyl acetate content = 45% ). The granulated particles have a diameter of about 1 cm. Swelling with vinyl acetate and subsequent polymeriza tion lead to the homogeneous and transparent graft product (B), the particle shape being maintained.

Figure 1. Grafting of a small amount of monomer onto an EVA polymer A: EVA starting product B: Swollen and polymerized

Polymerization thus takes place at a high backbone concentration so that the backbone becomes much more effective owing to its transfer and terminating reaction. Consequently, with different monomers not only high degrees of grafting are achieved but the grafting reactions can proceed so far that with low quantities of some monomers, partly crosslinked products are obtained (Table VI). This method is also suitable for grafting monomers which are not able to dissolve the E V A copolymer but only swell it more or less. One of these monomers is acrylonitrile, which we could not graft by the method of suspension polymerization because of its poor solvent properties.


31.

BARTL AND HARDT

Table VI.

483


Grafting in a Swollen Condition (Instruction F) Polymerized Monomers in the Polymer, %


Vinyl acetate

a

Solubility

a

5 12 30

+ ±

Vinyl chloride

3 6.5 15

+ + +

Acrylonitrile

12 20 25

±

Styrene

8 20 41

+ + +

±

±

+ = soluble, ± = partly soluble, — = insoluble.

When grafting acrylonitrile in a swollen condition of the E V A copolymer, only partly soluble or insoluble products are obtained (Table VI). Therefore, we investigated the graft polymerization of acrylonitrile in solution and compared its grafting behavior with that of vinyl chloride and vinyl acetate. The solvent was terf-butyl alcohol, which is known to show a very low transfer tendency (Table VII). Table VII.

Graft Polymerization in Solution (Instruction G) Vinyl Chloride

Vinyl Acetate

190

110

174

47

9

43

0

0

33

Acrylonitrile Yield, parts by weight Content of the polymerized monomer in the total polymer, % Homopolymer in the total polymer, %

When the polymers were analyzed for their content of homopolymer, it was found that the acrylonitrile polymer is soluble in chlorobenzene and the vinyl chloride polymer in toluene. Since chlorobenzene is unable to dissolve polyacrylonitrile and toluene cannot dissolve poly (vinyl chloride), it must be assumed that no homopolymer has been formed. This has been verified by fractionations. It is possible to extract with methanol from the vinyl acetate polymer 33% of a substantially pure polyvinyl acetate with a vinyl acetate content of 95%. Hence, acrylo-



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nitrile grafts better in tert-butyl alcohol than vinyl acetate, although in the activity series of the monomer radicals it falls after vinyl acetate. It is also remarkable that vinyl chloride, under the same reaction conditions, polymerizes only slightly. If higher conversion rates of vinyl chloride are desired, it is necessary to use a considerable excess of vinyl chloride over the E V A copolymer or to reduce the quantity of tert-butyl alcohol. It seems that during the polymerization in the presence of tertbutyl alcohol the solvent inhibits the polymerization of the vinyl chloride. This means that by using a further component—i.e., a solvent—the poly merization conditions in graft polymerization are changed to such an extent that the grafting behavior of the various monomers is completely different. The physical properties of the graft polymers depend largely on whether the end products—namely the graft polymer, the backbone, and Table VIII.

Graft Copolymer Containing 8 % Ethylene-Vinyl Acetate Copolymer (45% VAc) and 92% P V C a

Physical Properties of the Raw Material Test Method

Property

—

EVAc content Chlorine content Κ value (according to Fikentscher) Density Bulk density Ash content Water content Average grain size

DIN 53.474 DIN 53.726 DIN 53.479 DIN 53.468

—

K. Fischer

—

Unit

Value ca. 8 ca. 52.5 ca. 68

% %

— grams /ml. grams/ml. % %

—

ca. 1.35 ca. 0.5

Graft Polymerization of Vinyl Compounds on Ethylene-Vinyl Acetate

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