BASIC RUBBER COMPOUNDING

Introduction

One of the crucial tasks in rubber industry is to select the basic raw materials for the preparation of a specific commercial product. This requires a personnel with necessary technical background since the processes involve complicated chemical reactions such as vulcanization.  

The technical personnel, say a rubber technologist, may be required to do chemical analysis of the raw materials and of the finished rubber products. Thus he or she should have some background knowledge in order for him or her to make a right decision to select the most appropriate test methods.

As a rubber technologist, one must be capable of describing these processes and the problems involved to those concerned with the production of  a serviceable product at a reasonable cost.  Thus a rubber technologist must have an understanding of various types of raw materials used in making the rubber articles since raw rubber on its own is almost useless. 

In developing a rubber compound, it is  essential to mix the raw dry rubber with various compounding ingredients.  The process of sequentially adding the ingredients into the raw rubber is termed ‘compounding’ and the resulting final homogeneous mix is referred to as the ‘rubber compound’. 

The most common  compounding ingredients  added into the rubber are vulcanizing agents, vulcanization accelerators, activators for the accelerators, fillers, processing aids, softeners, antidegradants and other miscellaneous ingredients for specific purposes.  Each compounding ingredient has specific role in the compound and some give significant effects on the final properties of the resulting products.

Therefore a rubber compound has to be formulated with an expectation to incorporate the desired properties required in the finished rubber product as well as in the production process.  In view of this, the important factors that need to be considered when designing a rubber compound are price, processability and functional properties. 

Compounding Recipes

How much the quantity of each ingredients that is required to be added into the rubber must be specifically prescribed prior to the mixing process. This prescription is a compilation of various ingredients into a “recipe” (formulation).  In the recipe the quantity of each ingredient is quoted as an amount based on a total of 100 parts of the rubber or combinations of rubbers (or masterbatches) used. This notation is generally listed as phr (parts per hundred of rubber). 

Hence if we are to compare the outcome of two or more different recipes with respect to the processing characteristics or the physical properties of the final vulcanizate, the effects of varying any particular ingredient, from one recipe to another, can be easily identified.

Below is an example of a compounding recipe.

INGREDIENTS            phr
SMR 10 (NR)              100
Zinc Oxide                       5
Stearic Acid                     2
MBT                                0.5
Sulphur                           3.0

It is a common practice to list the materials in the general order that they are mixed into the rubber during processing. This will help the mill room workforce in setting up their mixing schedules for processing various compounds and for the preparation of special masterbatches which may be used in many different products.