Rubber latex, agglomerate

Irreversible agglomeration of particles originally dispersed in a rubber latex. 

A rubber latex is a two-phase system consisting of particles of rubber dispersed in water. Protective colloids and other stabilizing agents will be present in a latex to keep the rubber well dispersed and to prevent agglomeration or coagulation of the rubber particles. Natural rubber is collected as a latex from certain species of tropical trees, in particular, Hevea brasilien-sis. The rubber particles exist as globules of 1-3 microns diameter at a solids level of 35-40%. 

The first stage in the emulsion polymoization processes ctmcems the preparation of rubber seed particles of coitrolled diameter (typically 0.1-0.5 im). The rubber prepared most commoily is polybutadiene, diough copolymers of butadiene with (typically <35 wt%) of either acrylonitrile or styrraie sometimes are prepared. In ordo- to achieve hig rates of polym ization, a latex with a smaller particle size (Le. higher particle number omcentration) than required can be prepared first and the particles then agglomerated to attain the necessary seed latex particle diameto. ... 

The SEM micrograph in Figure 3.23 shows good dispersion of carbon black in the rubber matrix, in contrast to that in Figure 3.24, which shows poor dispersion and large agglomerates in a NR latex film. 

There are several NR latices which are prepared as a conduit for obtaining the dry rubber from them later by coagulation. This is because it is easier to incorporate solid rubber chemicals into latex in powder form so as to achieve a uniform dispersion of the material. For example, carbon black is often agglomerated and very difficult to disperse uniformly in dry rubber. A lot of energy is usually needed to mix the rubber and carbon black in an internal mixer to obtain a near homogeneous compound. 

To achieve concentrations above 50% it is necessary that at least a portion of the polymer be present in agglomerates of greater than colloidal size. In a latex, the polymer is present in the form of globules existing as a discrete phase in an aqueous matrix. Natural and synthetic rubbers, vinyl resins, and acrylics are the most important of the adhesive latexes. The polymer is not limited in molecular weight since the viscosity does not depend on what is contained within the discrete particles. The viscosity of the latex depends primarily on solids content and the composition of the aqueous phase. Latexes are commonly available in concentrations of 35 to 55% but it is possible, in some cases, to go considerably higher before reaching an excessive consistency. 

The effect of mean particle size on the rate of dissolution is illustrated in Table 3 for fractions sieved from granulated SMR-5 bale rubber. Particles having a mean diameter of 1 mm are taken up by solvent 5-10 times faster than particles having a mean diameter of 7 mm. A rubber powder prepared from spray dried latex was marketed but proved difficult to produce. A small level of anti-tack agent such as calcium stearate is commonly used to minimize agglomeration during manufacture, and this can usually be ignored. 

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