GB1583864A - Treatment of rubber latices to modify the particle size and/or size distribution - Google Patents

Description

(54) TREATMENT OF RUBBER LATICES TO MODIFY THE PARTICLE SIZE AND/OR SIZE DISTRIBUTION (71) We, THE INTERNATIONAL SYNTHETIC RUBBER COMPANY LIMI TED, a British Company, of Brunswick House, Brunswick Place, Southampton, Hampshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to rubber latices and, in particular, relates to the treatment of rubber latices to modify the particle size and/or the particle size distribution.

In British Patent Specifications Nos.

976,212, 976,213, 976,214, 1,039,727 and 1,379,909 we have described processes for the treatment of rubber latices to modify the particle size and/or the particle size distribution of the latex, in particular, to enable concentration of the latex to a high solids content. The processes described comprise flowing the latex through a constriction, such as the orifice of a homogeniser valve, under pressure.

In such processes the pH of the latex is one factor which affects the extent of the change in particle size, known in the art as the "degree of agglomeration". Other factors are the temperature of the latex, the solids content and the pressure which is used in the treatment. pH is relatively simple to control.

It is, therefore, normal practice to control the degree of agglomeration by adjustment of the pH of the latex and by alteration of the pressure used in the treatment. It is desirable that the pH is in the range 7 to 11, and preferably in the range 7 to 10. Since many emulsion polymerisations yield a latex having a pH above 10, and natural latices are stabilised to a pH above 10, the addition of an acid or an acid forming substance is required.

On a commercial scale, sodium silico fluoride is added in controlled quantities.

According to the present invention a noncarboxylated rubber latex having a pH of at least 9 is admixed with a carboxylated latex to reduce the pH to not less than 7 and the latex blend is passed under pressure through a constriction, which is preferably the orifice of a homogeniser valve.

The non-carboxylated rubber latex may be a natural or synthetic rubber latex. The process is, however, particularly applicable to synthetic rubber latices, and especially styrenebutadiene rubber (SBR) latices. The styrene content of the styrene-butadiene copolymer in such latices may be up to 70 /0 by weight of the total monomers, but normally is in the range 10% to 30% by weight.

Carboxylated latices, which are latices of copolymers of at least 20% by weight of at least one conjugated diene with at least one alpha-beta unsaturated carboxylic acid and at least one non-carboxylic copolymerisable monomer, e.g., a vinyl aromatic monomer and/or an unsaturated nitrile, are well known and are produced in substantial quantities for the paper and textile industries. The conjugated diene in the carboxylated latex is generally one having 4 to 8 carbon atoms, typically butadiene. Other examples are isoprene, dimethyl butadiene, 2,4 hexadiene and chloroprene. Mixtures of conjugated dienes may be used if desired. Butadiene is particularly preferred.

The alpha-beta unsaturated carboxylic acid may contain one or more carboxyl groups.

Examples are acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid and alpha-chloroacrylic acid. Mixtures of carboxylic acids may be used if desired.

Examples of the copolymerisable monomers which may be included in the copolymer are vinyl aromatic monomers such as styrene, alpha methyl styrene, vinyl toluene, vinyl naphthalene; alpha-beta unsaturated nitriles, e.g. acrylonitrile and methacrylonitrile; esters of acrylic or methacrylic acid, e.g. ethyl acrylate, methyl methacrylate, hydroxy propyl methacrylate, hydroxy ethyl methacrylate, hydroxy ethyl acrylate and unsaturated acid amides such as acrylamide. Mixtures of copolymerisable monomers may be used.

The conjugated diene is present in the carboxylated copolymer in an amount of at least 20% by weight of the total and may be present combined with other monomers in a wide variety of proportions. Generally, the unsaturated carboxylic acid is present in an amount of 1 to 10% by weight, e.g. 2 or 3 percent of the total. The unsaturated nitrile, if used, may be present in an amount of, for example, 5 to 15% by weight of the total The copolymerisable monomers may be present in an amount of up to 70% by weight of the total. Copolymers of, e.g. 30 to 60% by weight of a conjugated diene, 65 to 30% by weight of a vinyl aromatic comonomer, one or more unsaturated acids and optionally one or more unsaturated acid esters are preferred. Particularly preferred copolymers are butadiene / styrene / unsaturated carboxylic acid copolymers, optionally containing an ester of acrylic or methacrylic acid.The carboxylated latex in the present invention is used in the acid form.

An example of the practice of the present invention will now be described, taking the case of a non-carboxylated synthetic rubber latex. Non-carboxylated synthetic rubber latices to which the process of the invention may be applied may be those of homopolymers of a conjugated diene or a substituted conjugated diene, such as chloroprene, or copolymers of a conjugated diene and another copolymerisable monomer, such as acrylonitrile or, especially, a vinyl aromatic comonomer. After polymerisation the latex temperature is generally 35"C to 500C. At this stage the latex is preferably cooled to below 30"C and, if desired, the solids content of the latex may be adjusted. For optimum agglomeration, the solids content is 35% to 50%, but latices of lower solids content, e.g.

230% may be used if desired.

The carboxylated latex is then admixed with the non-carboxylated rubber latex in controlled amounts to reduce the pH to the preferred range for agglomeration. The preferred pH is 7.8 to 8.5. Using a carboxylated latex of pH about 6, the amount of this latex normally required is in the range 10 to 50 parts dry weight per hundred parts dry weight of the non-carboxylated rubber latex to be agglomerated. However, in one embodiment of the invention the amount of carboxylated latex added does not exceed 10 parts by weight per hundred parts by weight of the non-carboxylated rubber latex.In any event it is preferred that the pH of the non-carboxylated rubber latex and the amount of carboxylated latex added are such that the physical properties of foams prepared from the treated latex are substantially the same as those prepared from the non-carboxylated rubber latex alone otherwise treated in the same way.

The latex blend is passed under pressure through a constriction to effect agglomeration.

Normally this constriction is the orifice of a homogeniser. A suitable homogeniser is one manufactured by the Manton Gaulin Manufacturing Co. Inc. of Everett, Mass. U.S.A., such as the Type K 24 x 8 RAX. The pressure used is preferably at least 1,000 p.s.i.g.

(70 Kg/cm2) although by careful adjustment of the conditions of treatment (for example optimum solids content, optimum pH and/or optimum temperature) pressures as low as 250 p.s.i.g. (about 18 Kg/cm2) may be used.

Desirably the pressure of treatment is in the range 3,000 to 5,000 p.s.i.g. (210 to 350 Kg/cm2). With appropriate equipment pressures as high as 10,000 p.s.i.g. (700 Kg/ cm2) may be used if desired.

After treatment of the latex blend by the process of the invention the surface tension of the latex is lower than before. The surface tension may be reduced from, for example, 60 yo 80 dynes/cm to 30 to 45 dynes/cm, both measured at 25"C. A slight increase in pH is generally also observed.

By the process of the invention noncarboxylated rubber latices may be treated to enable evaporation of the latices to a solids content of at least 60% without the viscosity rising to an excessive level. This may be achieved without the need for the addition of chemicals, other than the carboxylated latex, which in an integrated rubber plant is readily available.

The latices treated by the process of the invention may be used for the same purposes as the rubber latex agglomerated, after pH adjustment, using, e.g. sodium silico fluoride.

Such latices are widely used in the carpet and moulded foam industries.

A particular advantage is the formation of a diagglomerate of an alkaline rubber latex and a carboxylated latex.

The following examples illustrate the invention: Examples 1 to 4.

100 parts dry weight of a styrene-butadiene rubber latex of bound styrene content 20% by weight having a pH of 9.56, a surface tension of 67.2 dynes/cm and a solids content of 31% by weight was blended with varying amounts of INTEX 166 (INTEX is a registered Trade Mark). INTEX 166 is a carboxylated styrene-butadiene latex of solids content 50.1% by weight. The sample had a surface tension of 35.8 dynes/cm and a pH of 6.02 (before stabilisation). The amount of INTEX 166 used in each case is given in the table below. The latex blend was treated in a Manton Gaulin homogeniser at the pressure indicated and at a temperature of 65 C. The pH and surface tension of each sample before and after treatment was noted and is also given in the table.

Initial Fin al Ex. Parts of CEX Surface Surface No. latex (dry) pH Tension Pressure pH Tension 1 10 8.48 67.9 I 3600 9.67 39.8 2 20 8.25 67.2 3300 9.30 39.2 3 30 8.05 66.8 3100 8.84 41.0 4 40 7.85 65.4 3000 8.5 7

The blend containing 30 parts of INTEX 166 was evaporated to give a stable latex of 62.4% total solids content at a viscosity of 275 centipoise. Viscosity and surface tension were measured at 250C.

Similar changes in pH and surface tension are observed with blends containing 2 and 5 parts dry weight of the carboxylated latex.

WHAT WE CLAIM IS: 1. A process for the treatment of an initial non-carboxylated rubber latex having a pH of at least 9 to modify the size and/or the size distribution of the latex particles by passage under pressure through a constriction at a pH which is lower than the initial pH but not less than 7, the process comprising lowering the pH of the latex by addition of a carboxylated latex of a pH which is lower than that of the initial latex.

2. A process according to Claim 1 wherein the pH of the latex is lowered to 7 to 10.

3. A process according to Claim 1 or Claim 2 wherein the amount of carboxylated latex added does not exceed 10 parts dry weight per hundred parts by weight of the noncarboxylated rubber latex.

4. A process according to any preceding claim wherein the non-carboxylated rubber latex is an SBR or polybutadiene latex.

5. A process as claimed in Claim 1 substantially as hereinbefore described with reference to any one of the Examples.

6. A non-carboxylated rubber latex when treated by the process according to any

Claims (1)

1. preceding claim.