Polychloroprene Mixture

To produce high-quality rubber articles of complicated shape, it is necessary to use chloroprene polymers which have exceptionally good processing properties. A material suitable for this purpose must have low shrinkage and low swelling upon extrusion and must be able to fill complicated moulds completely. An unvulcanised moulding must retain its shape exactly and must have a smooth surface; the material itself must be capable of rapid and easy moulding, e.g. extrusion.

Chloroprene polymers which have these properties can be obtained, for example, by mixing a benzene soluble polychloroprene with a benzene insoluble chloroprene copolymer, i.a. a pre-crosslinked product. Copolymers of this kind are, for example, the copolymers of chloroprene with alkanediol dicarylates or dimethacrylates, e.g. ethanediol dimethacrylate. Copolymers of this kind and mixtures thereof with benzene soluble polychloroprenes have been described e.g. in U.K. Pat. No. 1,158,970. The described products have the desired properties, prior to vulcanisation but their vulcanisates are unsatisfactory.

U.S. Pat. No. 3,042,652 discloses a mixture of a benzene soluble polychloroprene and a polychloroprene which has been rendered insoluble in benzene by exposure to .gamma. or .beta. radiation. This mixture, however, does not have the desired properties in the unvulcanised state.

The present invention provides a chloroprene polymer mixture of

A. 90 TO 20 PARTS BY WEIGHT, PREFERABLY 85 TO 50 PARTS BY WEIGHT, OF A BENZENE SOLUBLE CHLOROPRENE POLYMER WHICH HAS A Mooney viscosity ML-4' at 100.degree.C of 30 to 100, and

B. 10 TO 80 PARTS BY WEIGHT, PREFERABLY 15 TO 50 PARTS BY WEIGHT, OF A PRE-CROSSLINKED, BENZENE-INSOLUBLE COPOLYMER OF 90 TO 99 % BY WEIGHT, PREFERABLY 95 TO 98 % BY WEIGHT OF CHLOROPRENE AND 1 TO 10 % BY WEIGHT, PREFERABLY 2 TO 5 % BY WEIGHT OF A COPOLYMERISABLE MONOMER WHICH CONTAINS TWO ETHYLENICALLY UNSATURATED CARBON-CARBON DOUBLE BONDS AND WHICH HAS BEEN TREATED WITH HIGH ENERGY RADIATION AND HAS A Mooney viscosity ML-4' at 100.degree.C of 30 to 75, preferably 40 to 55.

This mixture has the required properties of easy processing and yields vulcanisates with the desired properties.

Component (a) of this mixture, according to the present invention, is a chloroprene homo- or co- polymer containing up to 10% by weight, based on the chloroprene, of an ethylenically unsaturated comonomer. Examples of suitable comomoners are: 2,3-dichlorobutadiene-(1,3), 1-chlorobutadiene and styrene. These polymers and copolymers are known. They can be obtained by known methods, e.g. polymerising the monomers in aqueous emulsion with activators in the presence of molecular weight regulators and emulsifiers. Chloroprene polymers of this kind and their preparation have been described e.g. in U.S.-Pat. No. 2,567,117 and U.K. Pat. No. 512,458.

The copolymers of component (b) in the above mixtures are also known. It is preferred to use copolymers of 90 to 99% by weight, preferably 95 to 98% by weight, of chloroprene and 1 to 10% by weight, preferably 2 to 5% by weight, of diesters of aliphatic dialcohols and ethylenically unsaturated monocarboxylic acids of the formula ##EQU1## in which R denotes hydrogen or an alkyl radical containing from 1 to 6 carbon atoms and n denotes an integer of from 2 to 6. R is preferably hydrogen or methyl and n is preferably 2. The corresponding copolymers of chloroprene with aromatic divinyl compounds such as divinylbenzene or diisopropenylbenzene are also suitable. The preparation of such copolymers is known. The conventional polymerisation process in aqueous emulsion, as used for the preparation of the compounds of component (a), is also used in this case, but here polymerisation is advantageously continued to a higher rate of conversion, namely 80 to 95%.

The thus obtained copolymer latices are subsequently treated with ionising radiation, preferably .gamma. rays or high speed electrons from an accelerator. In principle, any high energy electromagnetic radiation may be used as ionising radiation, e.g. X-rays or .gamma. rays, also corpuscular radiation, e.g. irradiation with .alpha. particles, deuterons, protons or electrons.

For irradiation with high speed electrons, e.g. from a Van de Graaff accelerator, a layer of latex 1 to 30 mm in thickness, depending on the energy of radiation, is exposed to a dobe of from 0.1 to 10.0 Mrad, preferably from 1 to 5 Mrad, in an inert gas atmosphere, (e.g. nitrogen), or air at from 0.degree. to 100.degree.C, preferably at approximately 20.degree.C. The energy of radiation may be from 0.1 to 10 MeV and preferably from 0.3 to 3.0 MeV. The intensity of radiation may be from 0.01 to 2000 Mrad/h and preferably from 1 to 500 Mrad/h.

The method of preparing benzene soluble chloroprene polymers, Component (a), and benzene insoluble precross-linked chloroprene polymers, Component (b) are known. These products are generally prepared by polymerisation in aqueous emulsion. Any of the conventional emulsifiers may be used, e.g. water-soluble salts, in particular alkali metal salts of long chain fatty acids, resinic acids, disproportionated resinic acids, condensation products of formaldehyde and naphthalene sulphonic acid, alkyl and aryl sulphonates or sulphates and alkoxylated alcohols and phenols.

Polymerisation may be initiated by means of the usual free-radical catalysts, for example cumene hydro peroxide, paramenthane hydroperoxide or water-soluble salts of peroxydisulphuric acid, usually in combination with .beta.-anthraquinone sulphonic acid, azo-bis-isobutyronitrile and salts of formamidine sulphinic acid. Suitable molecular weight regulators are the aliphatic mercaptans known for this purpose, such as N-dodecylmercaptan and dialkyl or dialkoxyalkyl xanthogen disulphides, for example of the formula ##EQU2## in which R denotes an alkyl group containing from 1 to 6 carbon atoms or an alkoxyalkyl group containing from 2 to 10 carbon atoms and from 1 to 3 oxygen atoms.

According to the present invention, the polymer mixture is preferably prepared by mixing the components in the form of their latices and precipitating the latex mixture. The polymers are usually precipitated by addition of electrolytes; the solid rubbers are filtered off and dried in a screw extruder. Alternatively, the polymers are precipitated by freezing the latex mixture, the solid polymers being dried in a drying chamber.

The chloroprene polymer mixtures obtained according to the present invention can easily be processed to form rubber compositions containing the usual fillers, plasticisers and other auxiliary substances.

The mixtures themselves can easily be formed into sheets and can readily be shaped by extrusion; they have little tendency to swell upon extrusion and have high green strength and also a smooth surface. Furthermore their vulcanisates have exceptionally high tensile strengths and structural stability, e.g. when compared with the products described in U.K. Pat. No. 1,158,978, in some cases as much as a 30 % increase in strength.

a. Benzene soluble polychloroprene

The mixture, activated with 1% aqueous formamidine sulphinic acid solution, is allowed to polymerise at 43.degree.C. Polymerisation is terminated at approximately 65% conversion and the unreacted monomer is removed by steam distillation at reduced pressure.

Formulation:

As in Example 1, except that 0.40 parts by weight of diethylxanthogen disulphide is used instead of 0.28 n-dodecylmercaptan as molecular weight regulator.

b. Benzene insoluble, precross-linked polychloroprene

The mixture activated with 1% of aqueous formamidine sulphinic acid solution, is allowed to polymerise at 43.degree.C. Polymerisation is terminated at a monomer conversion of approximately 80% and unreacted chloroprene is removed by steam distillation at reduced pressure. The latices are flushed with nitrogen for 10 minutes and then passed, as a continuous 4 mm thick layer, under the beam of a Van de Graaff accelerator (Beam voltage = 2 MeV, Beam current intensity = 200 .mu. A, Dose 1 Mrad).

The mixture activated with 1% aqueous formamidine sulphinic acid solution, is allowed to polymerise at 43.degree.C. Polymerisation is terminated at approximately 80% monomer conversion and unreacted chloroprene is removed by steam distillation at reduced pressure.

The latices prepared according to Examples 1 to 4 were mixed in various combinations to produce mixtures containing the quantities of polymer shown in Table 1. The polymers were isolated by effecting precipitation by freezing and then drying in a drying cupboard.

Vulcanizable mixtures were prepared from the polymer mixtures of Table 1 in accordance with the following formulation:

These mixtures were vulcanized by heating them to 150.degree.C for from 20 to 30 minutes.

The physical properties of the polymer mixtures and vulcanized materials are summarized in Table 2: Mixtures I, III and V are for comparison purposes, Mixtures II, IV and VI are examples of the present invention. On comparing the pairs I-II, III-IV and V-VI, it is found that the stability of the raw materials and their working properties are substantially similar in both cases, but that the vulcanized materials obtained from the mixtures according to the present invention are substantially stronger.