Rubber is a type of organic polymer material with high elasticity and low modulus. The difference in modulus between different rubber materials is not very big. But there are big differences in adhesion between different rubbers. Some rubbers have good adhesion and are easy to bond, such as NR, CR, PU, ??etc.; while some rubbers have poor adhesion and are difficult to bond, such as EPDM, IIR, Q, etc. This is related to the molecular structure of rubber.
Rubber-to-rubber bonding is common in the manufacturing process of rubber products, such as the self-adhesion of mixed rubber, the mutual adhesion of different unvulcanized rubbers, and the processing of multi-layer or multi-component products. In order to ensure the smooth progress of the molding process, there needs to be good adhesion between layers or components. In addition, when vulcanized rubber products are damaged during use, corresponding bonding technology must be used to repair them and when preparing composite products from vulcanized rubber.
Adhesion between unvulcanized rubber
Unvulcanized rubber refers to rubber that has not been cross-linked, including raw rubber and mixed rubber. Since there is no cross-linking, the rubber molecular chain in unvulcanized rubber still has a linear structure, which can be dissolved in a good solvent of rubber and has high fluidity and molecular mobility. Therefore, unvulcanized rubber bonding is easier to achieve. However, for unvulcanized rubber with poor adhesion such as EPDM, IIR, and Q, it is necessary to strive for certain bonding technology to improve its bonding effect. For NR, CR, PU and other easy-to-adhesion rubber materials, good adhesion can be achieved without special treatment.
1. Surface properties of unvulcanized rubber
Because unvulcanized rubber has good fluidity, is easy to deform, and has surface tension, but the surface tension of rubber, especially non-polar rubber, Smaller, the surface free energy is not high, so the adsorption on the surface of the unvulcanized rubber is not very strong. The surface of polar rubber has polarity, large surface tension, high cohesive energy density, and stronger adsorption than non-polar rubber. Between non-polar or polar unvulcanized rubbers, due to the small surface tension and interfacial tension, the two surfaces can easily reach a wet state and achieve full contact. Moreover, due to good compatibility, the molecules on the two surfaces It is easy to diffuse and penetrate between them, forming a thicker transition layer, or even the transition layer disappears. Therefore, the surface properties of unvulcanized rubber are conducive to adhesion.
2. The self-adhesiveness of unvulcanized rubber and its influencing factors
The self-adhesiveness of unvulcanized rubber refers to the viscosity of the rubber material. It is the effect of two homogeneous films under small load and short time. After lamination, the required force or work is required to peel it off. The greater the peeling force, the greater the viscosity and good self-adhesiveness. The films can be well bonded without increasing the viscosity.
The self-adhesiveness of unvulcanized rubber depends on the nature of the rubber and process conditions. Different types of rubber have different flexibility, mobility, and self-adhesive properties of their molecular chains. Generally speaking, the self-adhesiveness of unvulcanized rubber is affected by the flexibility, saturation, molecular weight of the rubber molecular chain, the activity of free radicals after chain scission, crystallinity, polarity, and the type and amount of fillers in the rubber. Generally, rubber molecular chains with good flexibility have strong mobility and are easy to diffuse and penetrate after surface contact, forming a thicker interface layer and better self-adhesiveness. The saturation and polarity of rubber molecular chains have a great influence on self-adhesiveness. Saturated rubber has poor self-adhesiveness, especially non-polar saturated rubbers such as EPDM, EPM, IIR, Q, etc., which have poor self-adhesiveness. Polar unsaturated rubber (such as CR) has good self-adhesiveness. For the same type of glue, the average molecular weight is small, the fluidity is good, and it is easier to diffuse, penetrate and spread, so it has good self-adhesiveness. One of the purposes of raw rubber plastic is to improve the self-adhesiveness of the rubber material. The crystallinity of rubber molecular chains also has an important impact on self-adhesiveness. Rubber that crystallizes at room temperature has poor self-adhesiveness because the crystal particles limit the movement of molecular chain segments, resulting in poor diffusion and permeability, such as TPI. However, the rubber molecular chain induces crystallization due to orientation during the stretching process. At this time, the crystallized particles will limit the separation of molecules in the two surface layers and increase its Green's strength. Therefore, rubber with self-reinforcing properties has better self-adhesiveness, such as NR, CR, and IR. Although IIR is self-reinforcing, due to the dense side methyl groups on both sides of the IIR molecular chain, the molecular chain has poor mobility and is difficult to diffuse and penetrate after surface contact. The interface layer is very thin, so the self-adhesion is not good. EPDM, BR, and SBR have poor self-adhesiveness, which is related to their non-self-reinforcement.
In addition, fillers are added to the rubber compound. Since the fillers can adsorb to the rubber molecular chains, on the one hand, it can increase the Green's strength of the unvulcanized rubber, and on the other hand, it can also limit the mobility of the molecular chains.
Because different fillers have different particle sizes, structural levels, surface activities, and different adsorption effects on rubber, they have different effects on self-adhesiveness. When the amount of filler is small (not enough to seriously affect the mobility of the molecular chain), the particle size is small, the surface activity is high, and the filler with high structure has a strong interaction with the rubber. The Green's strength of the rubber is high, so the self-adhesiveness is good. However, when the amount of filler is too large, the viscosity of the rubber is too high, the fluidity is poor, the diffusion and penetration capabilities of the molecular chains at the interface become poor, and the self-adhesiveness decreases.
The self-adhesiveness of unvulcanized rubber is also related to process conditions. Under high temperature and high pressure, the two surfaces can fully contact, the diffusion and penetration capabilities of the rubber molecular chains are enhanced, and the self-adhesiveness is good.
Methods to improve the viscosity of unvulcanized adhesive
① Improve the plasticity of the rubber and reduce the molecular weight through the plasticizing and mixing process.
② Add a plasticizer with good compatibility to the rubber compound.
③ Brush gasoline and other good solvents on the adhesive surface.
④ Apply glue on the adhesive surface.
⑤ Use tackifiers in the rubber compound.
⑥ Increase the temperature of the bonding surface.
⑦ Increase the pressure on the bonding part.
⑧Keep the bonding surface clean and cover the bonding surface with plastic film before bonding.
⑨Prevent the rubber from blooming. (Improvements in formula and process)
⑩ Use as little or no lubricating processing aids as possible, such as wax, Vaseline, etc.
3. Mutual adhesion of unvulcanized rubber
If there are two different unvulcanized rubbers, the viscosity of the bonding surfaces becomes mutual adhesion. The mutual adhesion of unvulcanized rubber is more complicated than self-adhesiveness, such as the wettability and compatibility of the adhesive surface, which also have an impact on the mutual adhesion. If the interfacial tension on the surfaces of the two unvulcanized rubbers is small, the compatibility between the rubber molecular chains is good, and the two surfaces are easy to wet, diffuse and penetrate, so the bonding effect is good and the mutual adhesion is good. If the rubber molecular chains in two unvulcanized rubbers are the same (only the type and amount of compounding agents are different), the mutual adhesion between the two rubber materials depends on the self-adhesiveness of the rubber type. If the glue types in the two glue materials are different, the mutual adhesion is not only related to the self-adhesiveness of the fish, but also the compatibility of the two glue types. The compatibility is good and it is easy to spread and penetrate, so the mutual adhesion is good.
Section 2: Adhesion of vulcanized rubber
1. Surface properties of vulcanized rubber
Vulcanized rubber is closer to a solid, because linear macromolecules are cross-linked into a network Like structure, the mobility of molecular chains and segments is greatly reduced, the surface is not easy to bend and shrink, so it has a high surface free energy, and under the action of vulcanization pressure, the surface of vulcanized rubber is relatively smooth. Therefore, when the surfaces of two vulcanized rubbers come into contact, due to the network of molecular chains, diffusion and penetration are difficult to proceed. Strong adhesion force cannot be directly generated between the two vulcanized rubbers, and bonding is relatively difficult.
2. Adhesion of vulcanized rubber
The adhesive method is usually used to bond vulcanized rubber, that is, apply adhesive on the surfaces of two vulcanized rubbers and then allow the liquid adhesive to solidify. The bonding effect between vulcanized rubbers depends on the cross-linking density of the vulcanized rubber, the type and composition of the adhesive, and the bonding process. The deeper the degree of cross-linking of vulcanized rubber, the poorer the mobility of the molecular chains, and the more difficult it is to bond.
1. Adhesive formula and manufacturing process
Rubber adhesives are divided into single-component and two-component, and are divided into two categories: solvent-based and water-based. One-component adhesive is usually a viscous liquid made from rubber (dry rubber or latex) and resin as the main raw materials, and is mixed with a thickening compound or coupling agent, curing agent, accelerator, filler or solvent. Mixtures, in which polar resins are often added during the manufacturing process to improve the bonding effect. The medium adhesive material of two-component adhesive is composed of two polymer materials, such as different rubbers, different resins, rubber and resin, etc.
Solvent-based adhesive manufacturing process:
① Raw rubber plastic Refining (multi-stage refining), the plasticity requirement is above 0.5.
② Preparation of rubber mix: Mix the rubber material in an open mill or internal mixer.
③Mixed rubber granulation: The mixed rubber is cut into strips and then granulated.
④Add good solvent in proportion and stir with a stirrer to dissolve.
2. Adhesive bonding process
(1) Polishing of the adhesive surface. (Grinding wheel or sandpaper)
(2) Clean the bonding surface. (Clean with a brush and scrub with solvent)
(3) Apply adhesive on the bonding surface, which needs to be thin and even.
(4) Dry and remove the solvent.
(Solvent evaporates completely)
(5) Adhesion requires pressure.
(6) Park it for a period of time before using it.
3. Bonding method between vulcanized rubbers
(1) Adhesive room temperature vulcanization method: use room temperature curing adhesive.
(2) Adhesive thermal vulcanization method: use thermal vulcanization adhesive.
(3) Semi-vulcanization method: The rubber is semi-vulcanized, and adhesive rubber or adhesive is added between the semi-vulcanized rubber sheets, and then vulcanized under pressure and heat.
(4) Mixed rubber sheet lamination and then vulcanization method: two homogeneous or heterogeneous mixed rubber sheets are first bonded and then pressurized and vulcanized.