What is the filler of adhesive?
In the components of adhesives, some matters change the properties without reaction with materials. These solid matters are called fillers. Fillers can reduce the cost and curing shrinkage rate during curing process. In addition, fillers will change adhesive properties according to the practical demands. Some filler could enhance the impact resistance of adhesive layer and other mechanicalstrength.
The frequently used filler is inorganic compounds, such as metal powder, metal oxide and minerals. Adding some fillers, especially polar fillers, can reduce curing shrinkage rate and coefficient of thermal expansion, improve adhesive cohesive force, bonding strength and heat resistance. For example, adding asbestos fiber or glass fiber as filler can improve impact resistance, while adding Quartz powder, porcelain powder, or iron powder can enhance rigidity and pressure resistance. Moreover, graphite powder and french chalk as filler could improve wearability. Alumina and titanium dioxide as filler can enhance bonding strength.
①Fillers, especially fiber fillers, have good thickening properties to avoid lacking adhesives. Because the adhesive might flow during curing process. In addition, fillers can improve the thixotropic to control flowable of adhesive.
② The interaction forces between polymer molecule is weak with little cohesiveenergy and low mechanicalproperties. So appropriate fillers can reinforce materials. Most macromolecular chains are bonded with the surface of filler. When a molecular chain is subjected to stress, joint can separate the stress into other molecules. If one molecular chain has sheared off, the other chains will keep working to improve the mechanicalproperties of adhesives.
③ Adhesives cure in the curing process with the decreasing of volume and the increasing of density. And the volume shrinkage is caused by chemical reaction. Besides the shrinkage in curing process, adhesives will shrink because of their own thermal expansion coefficient. Both may produce the inner stress and cause the cracks of adhesive layer. During the curing process, fillers can adjust the rate of shrinkage and bridge the gap of thermal expansion coefficient between adhesives and substrates to prevent the cracks spreading. Fillers are applied to improve the bonding strength and shear strength. But excessive fillers may decrease the bonding strength because the joint needs to stand more inner stress.
④ Adhesives can be made into conductive adhesive after added metal powder with good conductivity, because metal powder can improve conductivity and thermal conductivity. If the filler is special conductive carbon black, silver powder or magnetic powders, users can choose conductive adhesive for special occasions. Some filler can reduce water absorption capacity of resin. And some could improve the aging resistance and salt spray. All fillers must be stored in dry place to guarantee the effect of adhesion.