Adhesion promoter
Pure rubber rubber products are often difficult to meet actual use needs. Most rubber products require composite manufacturing using skeleton materials as the main stress-bearing part. The skeleton materials also play an important role in the stability of the shape of rubber products during use. The strong combination of rubber and frame material can not only protect the frame material, but also the reinforcing effect of the frame material can be fully exerted.
Rubber products have different requirements for skeleton materials. In terms of material, they mainly include metal, natural fiber and synthetic fiber. In terms of structure, they mainly include canvas, rope, cord, etc. Different adhesives should be selected for different composite products. The types of adhesives are shown in Table 1.8.1-26.
Note a: “Catalogue of Substitutes for Toxic and Hazardous Raw Materials (Products) Encouraged by the State (2016 Edition)” (Ministry of Industry and Information Technology [2016] No. 398) will be used for impregnation and treatment of tire cord fabric, rubber conveyor belt canvas, etc. An alternative to phenolic resin (RFL) impregnating agent for impregnating various types of cords. Curtain canvas NF impregnating agent (condensation of the main components hexamethylenetetramine complex (RH) and hexamethoxymethylmelamine substance), solvent-free fiber cord impregnating agent (main components polymethylene polyphenyl polyisocyanate (polymer MDI), polyurethane, liquid rubber (HTPB)) are included in the R&D catalog, among which solvent-free fiber cord impregnating agent is currently The most effective ones are polyurethane aqueous dispersions or polyurethane emulsions. Hydroxymethylation modification and amination modification of lignin are also possible important directions.
The surface treatment of the skeleton material and its adhesion to the rubber matrix are very important issues. In the past few decades, many studies have been conducted on the rubber bonding mechanism, but a unified understanding has not yet been reached. Research on the bonding mechanism of rubber and skeleton materials mainly includes the following types:
①Adsorption theory
Adsorption theory is the most popular theory of adhesion. This theory believes that the adhesive and the adherend are bonded together through adsorption. The adhesion force is mainly caused by the mutual adsorption of molecules or atoms of the adhesion system near the adhesion interface to generate van der Waals forces and bond them together. The bonding process is mainly divided into two aspects. First, the adhesive molecules migrate to the molecular surface of the adherend through molecular motion. Pressure and high temperature are conducive to the process; secondly, when the molecules move to the surface of the adherend, When a small enough distance is reached, van der Waals forces come into play and gradually increase as the distance decreases. Adsorption theory regards adhesion as a surface process based on intermolecular forces. This theory believes that intermolecular forces are one of the main forms of adhesion. However, the adsorption theory is not universally applicable and cannot explain the bonding of the direct bonding system between rubber and copper-plated steel wire.
②Mechanical theory
Mechanical theory believes that adhesion is the penetration of the adhesive into the rough surface of the adherend, and mechanical forces such as hooking and anchoring are generated on the surface of the adherend to combine the adhesive and the adherend. Adhesives bond materials with surface treatments much better than materials with smooth surfaces. However, mechanical theory cannot explain the bonding of materials with smooth surfaces, such as glass and metal.
③Chemical bond theory
Chemical bond theory is currently the most systematic and oldest theory. Chemical bonding theory refers to the strong adhesion between two phase materials obtained by forming chemical bonds at the bonding interface. The chemical bond force is much greater than the intermolecular force and can produce good bonding strength. The chemical bond theory has been confirmed by various experimental facts, such as the bonding of rubber to copper-plated steel wire.
④Diffusion theory
Diffusion theory, also known as molecular penetration theory, means that the mutual bonding of two-phase materials is accomplished through molecular diffusion. Diffusion creates a dense adhesive layer at the interface between the two phases, thereby combining the two-phase materials. This diffusion occurs through interpenetration at the bonding interface. Diffusion results in no obvious bonding interface between the two-phase materials, only the existence of a transition zone, and the bonding system can obtain good bonding performance through diffusion. This theory can well explain the adhesion between polymers with good compatibility, but it cannot explain the adhesion between rubber and metal.
⑤Electrostatic theory
Electrostatic theory, also known as electric double layer theory, refers to the phenomenon of discharge and emission of two-phase materials at the interface in a dry environment. But many scientists believe this theory doesn’t address the nature of bonding. Moreover, the adhesion force generated by static electricity only accounts for a small part of the total adhesion force, and its effect on adhesion is negligible. In addition, electrostatic theory cannot explain the bonding between two phase materials with the same or similar properties.
The bonding of rubber to metal can be traced back to 185Cobalt chloride, cobalt sulfate, cobalt stearate, cobalt naphthenate, etc. In the viscosity-increasing process of organic cobalt salts, cobalt ions mainly play a binding role.
As for the mechanism by which organic cobalt salts improve adhesion, a common view is that the addition of organic cobalt salts can promote the generation of the active product CuxS and adjust the generation rate of CuxS. Different organic cobalt salts have different regulating abilities. The reactivity of various organic cobalt salts is: cobalt borate > cobalt neodecanoate > cobalt naphthenate > cobalt stearate [12]. Among cobalt salt system adhesives, cobalt borate and cobalt neodecanoate are widely used due to their relatively high mass fraction of cobalt and high activity. In particular, cobalt borate also has good anti-aging properties. effect[13]. Generally speaking, in 100 parts of rubber, the content of metallic cobalt should be about 0.3 parts [14]. If the amount of cobalt ions is too large, it will accelerate the formation of a large amount of inactive copper sulfide, reduce the bonding strength, and accelerate rubber aging. If the amount of cobalt ions added is too small, it will be difficult to form a copper sulfide layer at the bonding interface, resulting in a decrease in bonding performance [15~17].
The proportion of copper and zinc in the coating is also an important factor in determining the bonding effect. Metal copper is a relatively active metal. If pure metal copper is used, the reaction will be very violent and cuprous sulfide will be produced quickly, which is difficult to match the vulcanization rate of rubber. The zinc in the coating can effectively inhibit the activity of copper, reducing the rate of generating cuprous sulfide; at the same time, zinc can react with sulfur to form zinc sulfide, which also enhances the bonding effect [18]; finally, zinc can react with sulfur to form zinc sulfide. The steel wire forms the form of a primary battery, which effectively protects the steel wire from corrosion [19].
The cobalt salt bonding system is best for natural rubber, followed by isoprene rubber and butadiene rubber, and butyl rubber, nitrile rubber and chloroprene rubber are worse.
C. Lignin binding system
