The manufacturer of automatic sealing machine reminds us that the chemical factors affecting the bonding strength of adhesives mainly refer to the polarity, molecular weight, molecular shape (side group number and size), molecular weight dispersion, molecular crystallinity, molecular stability to the environment (transition temperature and degradation), and the properties of adhesives and other components in the adhesives, such as PH value.
1. Polarity. The polarity of adhesives and adhesives affects the bonding strength, but it does not mean that the increase of the polarity of these molecules will certainly improve the bonding strength. In order to improve the bonding strength, it is better to change the exterior polarity of the interface area than to change the polarity of the adhesive and the global part of the adhesives. For example, polyethylene, polypropylene and polytetrafluoroethylene (PTFE) have been treated by plasma. Many polar groups, such as hydroxyl, carbonyl or carboxyl groups, have been produced on the surface, which greatly improves the bondability.
2. Molecular weight. Molecular weight (or degree of polymerization) of polymers directly affects the intermolecular forces of polymers, and the magnitude of intermolecular forces determines the melting point and boiling point of materials, and the glass transition temperature (Tg) and melting point (Tm) of polymers. Whether polymer is used as adhesives or as adhesives, its molecular weight affects the bonding strength.
3. Side chain. The side groups on long chain molecules are important factors to determine the properties of polymers. Considering the intermolecular forces, the effect of polymer branched chains is to increase the length of branched chains and reduce the intermolecular forces when the branched chains are small. When the branched chain reaches a certain length, it begins to crystallize, increases the length of the branched chain, and improves the intermolecular force. This should be the reason for reducing or improving the bonding strength.
4, pH value. As for some adhesives, their PH value has a close relationship with the application life of the adhesives, which affects the bonding strength and bonding life. Ordinary strong acid and alkali, especially when acid and alkali have a great impact on bonding data, are often harmful to bonding, especially porous wood, paper and other fibers are more vulnerable.
Because the curing process of phenolic and urea-formaldehyde resins, such as thermosetting resins, is greatly affected by the PH value, the acidity is often requested. For example, p-toluene sulfonic acid or phosphoric acid is added to phenolic resin during curing, and ammonium chloride or hydrochloric acid is added to urea-formaldehyde resin. Therefore, it is suitable to use neutral m-phenol formaldehyde resin in places where acidity is not expected to be high and adhesion is required.
The wood appearance can be pre-treated with alkali, and generally a strong joint can be obtained. However, it is necessary to pay attention to the PH value of the adhesive layer, which has more influence on the surface of the adhesive layer than on the bonded surface.
5. Filling. The fillers in the adhesives have the following functions: (1) increasing the cohesive strength of the adhesives; (2) adjusting the viscosity or technological properties (such as thixotropy); (3) improving the heat resistance; (4) adjusting the thermal shrinkage coefficient or shrinkage; (5) increasing the fillability of the gap; (6) giving conductivity; (7) reducing the price; (8) improving other properties.
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