LATEX COMPOUNDING

Natural latex is a unique natural substance which finds numerous uses in our daily life. We assume that the reader has the understanding about how latex is colleted, preserved and concentrated; and also about the usable forms of natural rubbers. This article is limited to compounding of natural latex and pre vulcanized latex.

The basic and conventional ingredients for latex compounding are Sulphur, Zinc Oxide (ZnO), and ZDC (Zinc Diethyldithiocarbamate). Apart from these materials, we may require stabilizers, antioxidants, plasticizers, extenders, fillers, modifiers etc.

The compound design is different for each application. It may appear almost similar, and a common man may wonder whether certain small variation in percentage of some chemicals is going bring in any change in properties. The fact is, as in any polymerization reaction, in rubber also we can bring about variations in properties, very critical for some applications by suitably adjusting the chemicals used for compounding. This is not very much thought about because; at present the critical application of latex products is limited to a very small number of products like surgical gloves, condoms, catheter and medical tubing. The aim of this article is to point out some vital points in determining the type and quantity of ingredients for compounding.

Before going further into compounding, we wish to make clear that we use phr (per hundred parts of rubber) values in this article, which is the universal approach.

Unlike dry rubber compounding, latex compounding requires to satisfy one essential condition. That is, the stability of latex at the time of compounding and after. The mechanism of stabilization is beyond the scope of this article. Generally, Potassium Hydroxide is used as a static chemical stabilizer and Fatty acid soaps as mechanical stabilizer. The quantity requirement of stabilizers depends on various factors like age, MST, temperature, load (quantity of powders added, duration of agitation, mechanical load applied for transfers and in production), viscosity of compound to be maintained, duration of storage etc. and it ranges from 0.0 phr to 3.0 phr.

The addition of fatty acid soaps brings about some undesirable properties also to the latex; froth is one among them. The others are reduction of tack and surface tension and viscosity reduction. Stabilizers are found to have some impact on cure also. Though, some of these are requirements in certain applications like foam and in spraying, they are undesirable in most applications. Thus, excess stabilizer is detrimental in most applications.

Some stabilizers are added for preservation of latex, details of which most producers do not disclose. Therefore, initial MST of latex should be taken as a base for determining the quantum of additional stabilization. The MST and drying time taken to form a film can be used as an indicator. The quantity requirement of stabilizer can vary from 0 phr to 3 phr in a latex compound and is solely depended on production requirements.

Another consideration in compound design is the viscosity. Viscosity of raw latex depends on its concentration. (Sometimes aged latex may show more viscosity due to thickening which is a different scenario). Commercially available latex has 50% - 60% rubber content. De-sludged PFL (preserved field latex) which has about 38% rubber content is also available. Thus, selection of latex becomes very important in compound design. Modifiers of various types are being used to increase viscosity of compound where it becomes essential.

Addition of compounding ingredients increases viscosity. Therefore, TSC (total solid content) of the compound has to be decided before compounding depending on the required viscosity. For example, a lean compound may be suitable for spraying, but will be unsuitable for dipping. All ingredients into latex compound are added as their dispersions, emulsions, solutions, or as water miscible liquids. Therefore, concentration of dispersions and emulsion is also predetermined in compound design.

Having designed the physical characteristics of the compound, now it is time to decide on the characteristics of product. Thus, quantity and type of vulcanizing ingredients, protective system, reinforcing materials and extenders have to be designed according to required vulcanization properties of the product.

Sulphur is inevitable in vulcanization of rubber except for a sulphurless cure system. The quantity of sulphur varies from 0.5 phr to 3.5 phr for most latex applications. Hardness increases with increase of sulphur. Unlike in dry rubber where with high sulphur we get ebonite, latex gives no similar product. Zinc oxide plays a multiple role. Apart from activation, some excess zinc oxide forms reinforcing filler. But, it tends to form a complex with ammonia and gel, which in turn reduce stability. Quantities from 0.1 phr to 5 phr is used depending on the process requirement, which again is dependent on production requirements.

Latex requires ultra fast accelerators like dithiocarbamates as their Zinc or Sodium salts. Zinc mercapto benzothiozole is sometimes used as a secondary accelerator for scorch safety and it also is said to improve tensile properties. About 0.7 to 1.0 phr antioxidants have been found sufficient for most requirements. Phenolic or trimethyl dihydroquinoline antioxidants are preferred in latex compounds for they are non-staining or mild staining. But, if you ask us there is no limit of choices.

Synthetic latex and some polymeric materials generally called modifiers are added to latex to impart certain properties. Materials like oil, paraffin, metallic stearates etc, which we generally call plasticizers can increase flexibility. But, consideration in selection of these modifiers or plasticizers should be based on need and suitability. Many materials that are compatible with dry rubber cannot be used with latex.

Fillers are considered as cheap materials which add weight. But it is not so. They impart many properties to the product if wisely used. The common fillers used in latex are china clay, precipitated silica, and whiting (calcium carbonate). Among these, china clay and precipitated silica can be considered as reinforcing fillers. The others are extenders used either to increase volume or weight. There is a common problem with fillers and extenders. Their particle size if not reduced properly and dispersed well, settle to the bottom carrying rubber particles with them destroying the homogeneity of compound or even coagulating it. Some of the fillers have acidic character, their impact go unnoticed because of the high pH of latex compound due to ammonia. They give rise to a curd like compound, not tacky, not coagulated, but unstable.

Natural latex has a unique property, that it is possible to prevulcanize it. Prevulcanized latex (PVL) finds application in many products which require low temperature curing. But, it is found that freshly compounded latex gives more tack, tensile strength and higher modulus than PVL. However, prevulcanized latex also can be suitably modified to give equal performance.

Finer details of compound design depend on the product. Any reader with a specific requirement may contact us. We are happy to serve them always.