Adhesion enhancer
Most of the rubber products need to be compounded with skeleton material as the main force-bearing part, and the skeleton material also plays an important role in stabilizing the shape of the rubber products in use. The solid combination of rubber and skeleton material not only protects the skeleton material, but also enhances the role of the skeleton material in order to get full play.
Rubber products have different requirements for skeleton materials, mainly metal, natural and synthetic fibers in terms of material, and canvas, rope and cord in terms of structure. Different composite products should choose different adhesives. Adhesive types as shown in Table 1.8.1-26.
Note a: The “Catalogue of Alternatives to Toxic and Hazardous Raw Materials (Products) Encouraged by the State (2016 Edition)” (Ministry of Industry and Information Lianjie [2016] No. 398) will be used for the impregnation treatment of tire cord fabric, conveyor belt canvas for rubber, etc. with the alternative of phenolic resin (RFL) impregnating agent used for the impregnation treatment of various types of cord canvas NF impregnating agent (the main components of hexamethylenetetramine complex (RH) and hexamethylenetramine (condensation of methoxymethylamine), solvent-free fiber cord impregnating agent (the main components of polymethylene multi-phenyl polyisocyanate (polymeric MDI), polyurethane, liquid rubber (HTPB)) included in the R & D category catalog, where solvent-free fiber cord impregnating agent is currently the most effective for polyurethane water dispersion or polyurethane emulsion, hydroxymethylene-modified, amino-modified lignin is also a possible important direction .
The surface treatment of the skeleton material and the bonding with the rubber matrix are very important issues. In the past decades, a lot of research has been conducted on the rubber bonding mechanism, but no unified understanding has been reached so far. Research on the bonding mechanism of rubber and skeleton material mainly includes the following:
① Adsorption theory
Adsorption theory is the most popular bonding theory. This theory believes that the bonding material and the bonded material are bonded together by adsorption. The bonding force is mainly caused by the adhesion of molecules or atoms of the bonding system near the bonding interface, resulting in van der Waals forces and bonding together. The process of bonding is divided into two aspects: firstly, the molecules of the adhesive migrate to the molecular surface of the bonded material through molecular motion, and the process is facilitated by pressure and high temperature; secondly, when the molecular motion reaches a sufficiently small distance to the surface of the bonded material, the van der Waals force starts to act and gradually increases as the distance decreases. The adsorption theory considers bonding as a surface process based on intermolecular forces, which are considered to be one of the main forms of bonding forces. However, the adsorption theory is not universally applicable and cannot explain the bonding of a direct bonding system of rubber and copper-plated steel wire.
②Mechanical theory
The mechanical theory is that bonding is achieved by penetrating the adhesive into the rough surface of the bonded material and generating mechanical forces such as hooking and anchoring on the surface of the bonded material so that the adhesive is bonded to the bonded material. Adhesive bonding of surface-treated materials is much more effective than that of smooth surface materials. However, mechanical theory cannot explain the bonding of materials with smooth surfaces, such as glass and metal.
③Chemical bonding theory
Chemical bonding theory is the most systematic and oldest theory available. Chemical bonding theory refers to the strong bonding between two phase materials obtained by forming chemical bonds at the bonding interface. The chemical bonding force is much greater than the intermolecular force and can produce a very good bond strength. The chemical bonding 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 by the action of molecular diffusion, which makes a dense bonding layer at the interface of the two phases, thus binding the two phases together. This diffusion is carried out by interpenetration at the bonding interface. Diffusion leads to the absence of a clear bonding interface between the two phases, only a transition zone exists and the bonding system is able to achieve good bonding properties by diffusion. This theory explains well the bonding between polymers with good compatibility, but not the bonding between rubber and metal.
⑤ Electrostatic theory
The electrostatic theory, also known as the double layer theory, refers to the phenomenon that two-phase materials discharge and glow at the interface in a dry environment. However, many scientists believe that this theory does not directly refer to the essence of bonding. Moreover, the bonding force generated by electrostatics is only a small part of the total bonding force, and its effect on bonding is negligible. Moreover, the electrostatic theory cannot explain the bonding between two materials with the same or similar properties.
The bonding of rubber to metal can be traced back to as early as 1850 and has mainly gone through the hard rubber method, phenolic resin method, brass plating or brass method, halogenated rubber method, etc. At present, the method of bonding rubber to metal in rubber products is mainly to bond rubber to metal in the process of vulcanization of rubber. So far, domestic and foreign has developed a variety of adhesives with excellent performance, such as Chemlok, Tylok, Metalok, Thixon, Chemosil (Henkel) series, Megum (Magum) series, etc.. Especially Chemlok series of adhesives are widely used in the field of rubber industry bonding.
①Hard rubber method was discovered around 1860, mainly on the surface of the metal paste a layer of high sulfur amount of hard rubber, and then stick on the surface of the composite material for vulcanization. This method is still widely used in large rubber rollers. Although the products made by this method have a good bonding effect, but the use of temperature generally can not exceed 70 ℃. Moreover, this process takes a long time to vulcanize and does not bond well with copper or copper alloy.
② Brass plating method is a kind of bonding method that can achieve rubber and metal bonding without adhesives, which was gradually developed after the research on rubber and brass plating bonding by Charles and others in 1862 in England. Initially, this method was mainly applied to the shock-absorbing rubber of engines. Now it is also used in the wire cord of tires. The main feature of the brass plating method is that the bonding of rubber to copper-plated steel wire occurs simultaneously with the vulcanization of the rubber at the vulcanization temperature, and there is no need to coat the surface of the wire with adhesive. Its disadvantages are mainly determined by the nature of the surface of the steel wire, and some large products of the surface copper plating difficulties.
③Phenolic resin method was developed after the Second World War. Phenolic resin method of rubber and metal bonding is recognized as occurring through chemical adsorption on the surface of the metal, that is, the bonding between the bonding material and the bonded material occurs, metal bonds or ionic bonds form bonds, a special reaction. This adsorption is generally considered to be a complexation reaction of phenolic organic compounds or a similar reaction.
④The halogenated rubber method was developed by Raymond Varner in 1932 as a result of his research on bonding experiments with brominated rubber. The halogenated rubber bonding system is considered to have good thermoplasticity and does not undergo curing reactions with the vulcanization itself. The most significant advantage is that it can be stored in a liquid state for a long time and used in a wide range of applications.
⑤ Direct bonding of rubber to metal
Direct bonding of rubber and metal is a method of bonding rubber and metal at the interface during the vulcanization process. At present, the commonly used direct bonding systems mainly include inter-methane-white bonding system, organocobalt salt, lignin, organocobalt salt/silica and triazine bonding system.
So far, it is still not clear how the vulcanized layer produced during the bonding process of rubber and copper-plated steel wire enhances the bonding strength between rubber and copper-plated steel wire. The generally accepted view is that during the bonding process of rubber and copper-plated steel wire, a CuxS layer is formed at the bonding interface with an x-value of 1.90 to 1.97.
The bonding process of rubber and copper-plated steel wire undergoes three main processes: formation of the bonding interface, stabilization and bonding [1]. Before the vulcanization of the rubber, there is only physical contact between the rubber and the copper-plated steel wire, forming a monotonic contact interface. As the rubber is vulcanized, the sulfur yellow in the rubber migrates toward the copper-plated steel wire, forming a non-metric coefficient of CuxS at the interface between the rubber and the copper-plated steel wire, and the formed CuxS migrates toward the rubber layer, forming an interlocking structure with the vulcanized rubber and improving the adhesion of the rubber to the copper-plated steel wire.
In the bonding process of rubber and copper-plated steel wire, the reactions of rubber vulcanization and bonding interface formation are synergistic and mutually promoting. Reaction course of rubber and sulfur yellow:
Bonding process of rubber with copper-plated steel wire:
The synergy of the two reactions is determined by the amount of sulfur yellow, the rate of CuxS production and the thickness of the brass layer. It is necessary to increase the amount of sulfur yellow in the rubber formulation to meet the consumption of sulfur yellow in the vulcanization process of the rubber and in the bonding process. It is also necessary to use late accelerators in the rubber formulation to prevent premature vulcanization reactions that can affect the generation of the bonding interface.
Van Ooij [2] pointed out in an early study that the bond between rubber and copper-plated steel wire is mainly established by the CuxS layer and that the bond strength depends on the thickness of the vulcanized layer, i.e. on the copper content in the brass-plated layer. As the rubber vulcanization proceeds the CuxS layer gradually grows towards the rubber layer and forms a strong mechanical interlocking structure with the rubber.Hotaka et al [3] separated the rubber from the bonding interface by placing a filter paper at the bonding interface between the rubber and the brass-plated steel wire during the vulcanization of the rubber. It was found that prior to vulcanization, the surface of the steel wire had
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