Introduction to Organic Silicon Adhesives

Cited from: https://pixabay.com/zh/photos/paper-block-write-note-pen-3406867/

Cover Photo cited from: https://pixabay.com/zh/photos/wall-carton-background-image-2720952/

In recent years, composited adhesives are widely applied in different departments, it is since the exploration of the universe space, and the rapid developments of missiles, rockets and universal navigations, the requirement on heat resistance is getting higher. Organic silicon adhesive is the main heat resisting adhesive.

Organic silicon adhesive is the adhesive that use linear or chain organic silicon macromolecular compounds as base materials. It could be divided to the ones that use silicone resin as the base material like coatings and the ones that use silicone rubber as base material like sealing materials. They have differences in structures and crosslinking densities, so the final properties are different. They also have difference in chemical structures: organic silicone resin’s main chain structure is formed by Si-O key, it is a three-dimensional structure which could be condensate in the further step to become highly crosslinking resian which is hard and crispy. Organic silicon elastomer which is also called silicone rubber, is a main chain structure macromolecular elasticity matter that formed by linear Si-O keys that contain molecular quantity from tens of thousands to hundreds of thousands. It could only be condensate to crosslinked elastomers in vulcanizing agent or catalyst. It is due to the differences in crosslinking densities, therefore the final physical forms and properties are different.

At present among all composited adhesives, the productivity and consuming amount of organic silicon adhesives take a smaller percentage. Even though it belongs to structural adhesive, but it is always been used as non-structural adhesive and it is applied widely. This feature is non-alternative by other adhesives. It is because organic silicon adhesive has inorganic structural Si-O key, thus it has both of organic and inorganic features and could main its physical and chemical properties in a large temperature range. Especially in high temperature, it has excellent thermal stability therefore it is always used as the high temperature protective layer. In the meantime, it also has excellent waterproof, weatherproof, radiation resistance, corrosion resistance and electrical isolation. The pure organic silicone resin adhesive could be used for a long time even in extreme temperatures. However, it is crispy and has lower strength, thus it is affected by these weaknesses in applications. Now it is in order to get better high temperature physical and chemical properties, sometimes to use other chemicals like epoxy to modify it. It could be stick to metals, plastics, rubber, glasses and ceramics. It is widely used in universal navigation, aircraft manufacturing, electrical gears, mechanical processing, automotive and other industries.

Silicone Resin Adhesive

Organic silicone resin is the organic polysiloxane macromolecular compound. Before solidification it is the side chain structure with multiple functional groups. After solidification it becomes body structure. The most outstanding property of silicone resin adhesive is its heat resistance. Also, as the phenyl percentage is increasing in its side chains, the heat resistance will be better. It also has cold resistance, electrical isolation and weather resistance.

As the sticking adhesive of silicone resin, it is due to the helix structure of polysiloxane has curbed the polarity of Si-O key, and the side functional group R block the Si-O key to make the whole molecule non-polar, thus its inner force is small and the adhesion of silicone resin is bad and the adhesion strength is low. There are some ways to solve these:

1.      To add filler like metal oxidation and glass fiber to the silicone resin to increase its inner force;

2.      To lead some polar alternative functional group like –OH, -COOH, -CN or to use organic polymer to modify polysiloxane to improve its adhesion and elasticity.

3.      To use other processing agent on the surface of the stick matter to increase its adhesion with polysiloxane.

Other resins could also be modified like epoxy resins, phenolic resins and polyester resins. Even if their heat resistances may be weaker, but their adhesion strengths are higher and they could have solidification in lower temperatures, thus they are more useful in practice.

Pure organic silicone resin adhesive is to use silicone resin as the base material and to add moulding inorganic filler, curing agent and organic solvents. It has good heat resistance and it is mainly used to stick the metals, alloys, glass steel, ceramics, micas and composited materials.

PSA

PSA is a special type of adhesive which is in the form of “half dry” for a long time. It could be just to be pressed by finger lightly to stick to the matter tightly. From today’s development of PSA, there are already many design systems. To divide it according to the main body material, there will be natural rubber PSA, composite and recycled rubber PSA, thermal elastomer PSA, acrylic vinegar PSA and organic silicon PSA. Although the former four types have different properties, but they could be only used in a lower temperature and they are widely used in pressure sensitive tags, tapes, slims and other similar products. They are convenient and reliable, and they could be granted other types of functions and have a rapid developing trend.

PSA is developed from 70s of 20^th century and now there are many big companies have patented about it like Dow Corning, ShinEtsu and 3M. They also produce different types of PSA products.

Organic silicon PSA is one excellent and high-end special PSA. It has chemical inertia and it has small stimulating and it still has excellent using properties in high and low temperature. It also has excellent weather resistance, waterproof, electrical isolation, acid and alkaline corrosion resistance and aging resistance. It not only has the necessary adhesion strength required by PSA but contain outstanding heat resistance cutting strength. Therefore, it could be widely used in sticking to metals, non-metals and other types of materials that have low surface energy.

Summary

As another type of important application of organic silicon, its adhesive is also important in some specific industries and it has good commercial potential since it is now being researched and developed by big companies and governments.