What are the factors that affect the strength of adhesives?
In general, the polarity of the adhesive and the bonded molecule affects the bonding strength, but does not mean that the increase in polarity of these molecules will necessarily increase the bonding strength.From the polarity point of view, in order to improve the bonding strength, it is better to change the polarity of the interface area than to change the polarity of the adhesive and all the molecules of the adhesive. For example, polyethylene, polypropylene and polytetrafluoroethylene are treated by plasma surface treatment, and many polar groups, such as hydroxyl groups, carbonyl groups or carboxyl groups, are produced on the surface, thus remarkably improving cohesiveness.
The molecular weight of the polymer (or degree of polymerization) directly affect the force of the polymer molecules, and molecular interactions determines the size of the melting point and boiling point substance level, to determine the polymer glass transition temperature Tg and the melting point of Tm. Therefore, whether the polymer is used as an adhesive or as a binder, its molecular weight affects the bonding strengthIn general, the relationship between molecular weight and bonding strength is limited to unbranched polymers, including two types. In the first type, the cohesive damage of adhesive occurs in the whole range of molecular weight. At this time, the bonding strength increases with the increase of molecular weight, but remains the same when the molecular weight reaches a certain value. The second types differ because of their molecular weight. At this time, occurrence of cohesive failure in small molecular weight range, with the increase of molecular weight increases the bonding strength; when the molecular weight reaches a certain value after cohesion adhesive with adhesion force is equal, while the mixed failure; when the molecular weight further increases, while the cohesion over adhesion force, infiltration is not good is the interface failure. As a result, the adhesive strength is the maximum when the adhesive is a molecular weight.
The side group of long chain molecules is an important factor in determining the properties of polymers. Considering the intermolecular forces, the effect of polymer branch chains is that the branch lengths increase and the intermolecular forces decrease as the branch lengths increase. When the branch chain reaches a certain length, it begins to crystallize, increases the length of the branch chain and improves the intermolecular force. This should be the reason to decrease or increase the bonding strength.