There are many kinds of auxiliaries, about hundreds of them, and thousands of varieties. The selection and application of additives must take into account many factors, such as the type of application object, processing method, product characteristics and matching components. Taking additives for polymers as an example, the following issues should be considered when selecting additives.

1. Compatibility of additives and products

The additives should match with the polymer, which is the first consideration when selecting additives. The compatibility of additives and polymers includes their compatibility and the effect on stability.

Generally speaking, additives must be stable and uniform in products for a long time to play their application effectiveness. Therefore, it is generally required that the selected additives and polymers have good compatibility. If the compatibility is not good, the additives are easy to precipitate.

The precipitation of solid additives is commonly referred to as "frost spraying", while the precipitation of liquid additives is referred to as "exudation" or "sweating". The precipitated additive not only loses its function, but also affects the appearance and feel of the product. The compatibility of additives and polymers mainly depends on their structural similarity.

Not all additives must have good compatibility with polymers. For example, inorganic fillers and inorganic pigments are insoluble in polymers. In terms of incompatibility, their dispersion in polymers is heterogeneous and not completely precipitated. For this kind of additives, they are required to be small in fineness and good in dispersion. Not all additives have better compatibility with polymers. For example, if the compatibility of lubricant is too large, it will play the role of plasticizer and cause the softening of polymer.

Another important problem of the compatibility of additives and polymers is their effect on stability. The decomposition products of some polymers (such as PVC) are acidic (releasing HCl), which will make some additives ineffective, such as salt formation with alkaline additives. Some additives will accelerate the degradation of polymer.

2. Durability of additives

The ability of polymer materials to maintain their original properties under service conditions is called durability. To maintain durability is to prevent the loss of additives.

The loss of additives is mainly through three ways: volatilization, extraction and migration.

Volatility depends on the structure of the additive itself. Generally speaking, the smaller the molecular weight, the greater the volatility.

The extractability is directly related to the solubility of additives in different media. We should select the appropriate additives according to the use environment of the products.

Mobility refers to the property that some auxiliary components in the polymer can be transferred to the materials in contact with them. The mobility is related to the solubility of additives in different polymers, and the additives should have the ability of water resistance, oil resistance and solvent resistance.

3. Adaptability of additives to processing conditions

The most important requirement of processing conditions for additives is heat resistance, that is, the additives are not decomposed, volatile and sublimated at processing temperature. No corrosive effect on processing equipment and molds.

For the same polymer, the additives required may be different due to different processing and molding methods.

4. Adaptability of additives to product use

The use of products often restricts the selection of additives. Products with different uses have certain requirements for the appearance, smell, pollution, durability, electrical performance, thermal performance, weather resistance, toxicity, etc. of the additives to be used. For example, light color products cannot use easily polluting additives.

In particular, the toxicity of additives has attracted widespread attention. The controversial toxic additives limit its application in food and drug packaging materials, water pipes, medical devices, toy plastic and rubber products and textile products. Different countries have formulated different health standards.

5. Synergism and antagonism in the coordination of auxiliaries

A polymer often uses multiple additives at the same time. These additives are in a polymer system at the same time, and have influence on each other. Such as synergy, additive effect and antagonistic effect.

Additive effect means that when two or more additives are used together, their total effect is equal to the sum of their respective use efficiency. Synergistic effect refers to that when two or more additives are used together, their total effect exceeds the sum of their respective use efficiency.

On the contrary, the antagonistic effect means that when two or more additives are used together, their total effect is less than the efficiency or sum of their individual use. Therefore, the selection of different additives with synergistic effect must be considered when selecting the additives to prevent the occurrence of antagonistic effect.