How mobility affects additive selection

Three performance characteristics related to diffusion must be considered  

The diffusion process is a function of properties of polymer and additive and therefore the mobility of additive alone cannot be used as an indicator of suitability for these applications. Additives have to be tested in particular system and conditions of performance because temperature, pressure, rate of additive loss are important factors which have to be taken into consideration. 

The additive that performs in certain operation and later has no influence on other stages of processing or material use is desirable. From the examples in the previous section it was seen that at least partial reabsorption of additive is possible as is the accumulation of additives on metal parts. This reduces their presence on a formed part. It is difficult to find additives which perform in this manner. Diffusion coefficient is reduced with temperature decreasing which slows down the rate of additive movement when part is formed. In order to accelerate additive migration from the surface to material bulk, a mechanism is required which reduces the additive concentration in the material bulk to create concentration gradient which promotes such migration. These types of mechanisms require complex properties which are not a common part of additive design. 

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