Analysis and Countermeasures for Failure of Rubber/Metal Vulcanized Bonding

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The bonding between rubber and metal has a long history. The direct bonding method, hard rubber method, brass plating method and adhesive bonding method are usually used. Among them, the adhesive bonding method is currently one of the most widely used and most effective methods. This article will start from the bonding principle and bonding process, analyze various causes that may cause rubber/metal vulcanization bonding failure, and propose corresponding solutions. 1 Bonding Principle Rubber and metal are two different materials with huge differences in chemical structure, physical and mechanical properties. The adhesives used for hot vulcanization bonding between the two are mostly composed of base rubber, curing agent and other ingredients dissolved, suspended and dispersed in solvents or polymer emulsions. phase system, so the thermal vulcanization bonding between rubber and metal involves the interaction between multiple component systems. It is a complex phenomenon involving surface physics, surface chemistry, polymer chemistry, inorganic chemistry, mechanics, electricity and other disciplines, and the influencing factors are intricate. Regarding the bonding mechanism of rubber/metal, the current main bonding theories include adsorption theory, electromagnetic theory, co-crosslinking theory, etc. For thermal vulcanization bonding of rubber/metal, the bonding mechanisms of single-coat adhesive and double-coat adhesive are shown in Figure 1 and Figure 2 respectively. The bonding between adhesive or primer adhesive and metal is mainly through the adhesive infiltrating the metal surface and then penetrating into the metal surface. In the gaps and concave holes, and eliminate the adsorbed air on the interface, it fully contacts the metal surface, and then produces adsorption and various forms of mechanical engagement (some adhesive molecules will chemically react with metal surface molecules to form chemical bonds) to produce sufficient bonding strength. ； Adhesive and rubber are bonded through mutual diffusion, penetration and co-crosslinking of molecules or chain segments. ； At the same time, a series of physical and chemical reactions occur inside the adhesive and rubber, thereby forming a strong connection between rubber and metal. Figure 1 Schematic diagram of single-coated rubber/metal bonding principle Figure 2 Schematic diagram of double-coated rubber/metal bonding principle 2 Rubber/metal thermal vulcanization bonding process The typical process flow of rubber and metal thermal vulcanization bonding is as follows: Metal surface treatment → Apply adhesive → Laminate compound → Pressurized heating vulcanization Vulcanized adhesives mainly include three categories: phenolic resin, polyisocyanate and halogenated polymer. Currently, the commonly used ones are Chemlok series and Thixon series from the United States, Chemsil series and Megum series from Germany. 3 Failure types of rubber/metal thermal vulcanization bonding Common failure types of rubber/metal thermal vulcanization bonding mainly include the following six categories. The schematic diagrams are shown in Figures 1 and 2. (1) Destruction between primer adhesive and metal (MC type) ； (2) Internal damage to the adhesive (type C) ； (3) Destruction between topcoat adhesive and primer adhesive (CC type) ； (4) Destruction between rubber and surface-coated adhesive (RC type) ； (5) Internal damage of rubber (R type) ； (6) Mixed damage, that is, two or more of the above situations occur at the same time. 4 Failure cause analysis and countermeasures 4.1 Damage between primer adhesive and metal 4.1.1 Improper metal surface treatment (1) Cause analysis ① Insufficient metal surface treatment ； The main function of metal surface treatment is to remove the rust layer, grease, dirt, etc. on the metal surface to obtain a clean, dry and active surface with sufficient roughness to facilitate the infiltration and adsorption of the adhesive. If the metal surface treatment is not enough, the loose oxide layer remains or the surface roughness is too small, then under the same painting area, the effective specific surface area of ​​the adhesive surface is less, the density of contact points between metal and adhesive is small, and the bonding strength is also small. ②Metal surface is not clean: If the metal surface is not cleaned cleanly or is contaminated again after cleaning, resulting in oil stains, impurities, residual cleaning agents, etc. on the surface, it is actually equivalent to creating an interface layer on the metal surface. The interface layer will not only * * Lowering the metal surface creates an interface layer. The interface layer will not only * * Reducing the surface free energy of the metal material significantly increases the contact angle between the adhesive and the metal surface, thereby reducing the wettability of the adhesive to the metal surface. It may also open up gaps on the metal surface, reducing the actual contact area between the metal and the adhesive, thereby reducing the bonding strength. (2) Solutions ① Treat the metal surface to remove rust and oil on the metal surface and ensure that the metal bonding surface has sufficient roughness. Commonly used methods for metal surface treatment include mechanical methods (such as sandblasting, mechanical grinding, etc.) and chemical methods (such as pickling, alkali washing, phosphating treatment, surface coating, high-temperature degreasing, etc.). In addition, attention should be paid to the metal surface not being too rough. If the metal surface is too rough, its irregularity will affect the wettability of the adhesive, and it will easily absorb gas, causing discontinuity in the bonding boundary, forming defects and stress concentration, thereby reducing the bonding strength. Surface roughness is determined based on the fluidity and wettability of different adhesives. ②Before applying glue, clean the metal surface with chemical solvents to remove oil stains, impurities, etc., and pay attention to drying and avoiding re-contamination. Some data show that the cleaner the adhered metal surface, the smaller the contact angle between the adhesive and the metal surface, and the higher the bonding strength. 4.1.2 Improper selection of adhesive (1) Cause analysis ① The viscosity of the adhesive or primer adhesive is too high and cannot effectively wet the metal surface or generate bubbles at the metal/adhesive interface and cause stress concentration around the bubbles. ②Although the adhesive can effectively wet the metal surface, its interaction with the metal after curing is too low. (2) Solution: Choose an appropriate adhesive to ensure that the adhesive has good wettability on the metal surface, and that the physical, mechanical or chemical interaction with the metal after curing meets the bonding strength requirements. 4.1.3 Improper gluing process (1) Cause analysis ① The adhesive is too thick or the solvent evaporates too quickly: Solvent, diluent or dispersed phase liquid is an effective carrier for the adhesive to infiltrate and penetrate into the metal surface. If there is too little or it evaporates too quickly after painting, it will lead to insufficient fluidity or insufficient flow time of the adhesive, resulting in incomplete infiltration of dynamics, and the ideal bonding strength and durability will not be obtained. ②The adhesive is not mixed evenly: The adhesive is not stirred evenly, and active components such as base material or curing agent are unevenly dispersed. The adhesive formed at a lower concentration has low bonding strength, which may cause bonding failure between the metal and the adhesive. ③Improper thickness of adhesive application: The adhesive layer is too thin, with few adhesive molecules per unit surface area and low strength. ； If the adhesive layer is too thick, it will easily produce bubbles, defects and early fractures, and the expansion stress will be large after being heated, which will easily lead to joint damage, leading to bonding failure. (2) Solutions ① Choose a suitable painting process, and pay attention to diluting the adhesive to ensure that the adhesive has a suitable infiltration speed and infiltration time. ②The adhesive must be stirred thoroughly and evenly before coating to prevent the effective solid material from settling. ③The adhesive should be applied at a moderate thickness. 4.2 Internal damage of the adhesive, damage between the top-coat adhesive and the primer-coat adhesive (1) Cause analysis ① The adhesive is not left open for enough time, and the solvent is not completely evaporated, causing defects ； ②The cohesive strength of the adhesive is not low after curing ； ③The bonding surface after applying the primer adhesive is contaminated with oil stains, dust, impurities, etc. After the adhesive is applied to the painted surface, an isolation boundary layer is formed between the two adhesives, resulting in stress concentration, resulting in bonding failure. (2) Solutions ① Dry the adhesive completely after painting to prevent small solvent molecules from remaining ； ②Choose an adhesive with higher cohesive strength ； ③After applying the adhesive, try to prevent hands, dust, debris, etc. from coming into contact with the adhesive surface during storage and transportation to prevent the adhesive surface from being contaminated again. 4.3 Destruction between rubber and surface-coated adhesive During the thermal vulcanization bonding process, the rubber molecules and the adhesive molecules first undergo a physical reaction of mutual penetration and diffusion between the two-phase molecules, and then a cross-linking chemical reaction occurs between the two-phase molecules and the internal molecules of each phase, thereby combining the two phases into a solid body. 4.3.1 Unsuitable rubber compound (1) Reason analysis If the compounding agent in the compound blooms or is extracted and migrates to the surface of the compound, a layer of isolation surface will be generated between the rubber surface and the adhesive, which will affect the molecular diffusion and co-crosslinking reaction between the rubber and the adhesive, making it difficult to form an effective bond. (2) The solution is to design the rubber compound formula and try to follow the following principles on the basis of meeting the product performance requirements.: ①When selecting raw rubber types, try to choose rubber with high polarity and high unsaturation and good bonding properties. ； ②For general rubber, especially diene rubber, the sulfur vulcanization system has better bonding effects. ； ③Softeners, paraffin, processing aids and other additives that are not conducive to bonding should be used as little or as little as possible, especially ester plasticizers. ； ④The dosage of antioxidant D, sulfur and other ingredients that are easy to bloom should not be too much. 4.3.2 Adhesive factors (1) Cause analysis ① The adhesive does not match the adhered rubber ； ②The adhesive is not stirred evenly, the drying time after painting is not enough, or the adhesive surface is contaminated. (2) Solutions ① Choose the appropriate adhesive according to the type of rubber to be adhered. For example, polyisocyanate and halogenated polymer adhesives are better for bonding non-polar rubber, while phenolic ester resin adhesives are less effective. ； ②The curing system of the adhesive must match the vulcanization characteristics of the rubber. For example, polyurethane rubber using a peroxide vulcanization system has a better cross-linking matching effect with phenolic resin and isocyanate adhesives. ； General-purpose rubbers such as sulfur-vulcanized NR and NBR have better compatibility with maleimide and quinoxime cross-linking systems. 4.3.3 Improper vulcanization process During the rubber/metal hot vulcanization bonding process, any improper selection of vulcanization pressure, temperature, and time will cause bonding failure. The main measures to avoid bonding failure caused by vulcanization are:: ①The vulcanization temperature must be able to overcome the chemical reaction barrier and simultaneously trigger the curing reaction of the adhesive and the vulcanization reaction of the rubber compound. ； On the other hand, on the premise of meeting the above conditions, the vulcanization temperature needs to be appropriately lowered, especially for exothermic reactions or excessive bonding expansion stress that will damage the bonding interface. ②Regarding the vulcanization pressure, as long as the other properties of the product and the equipment and process are allowed, the higher the pressure, the better. Especially for adhesives with a large content of low molecular polymers or small molecules produced during reaction, surface wetting, diffusion and discharge of small molecular products must be provided. ③If the reactivity of the adhesive is lower than the vulcanization activity of the rubber or the metal parts are large in size, in order to ensure the synchronous reaction of rubber and adhesive, measures such as preheating the metal parts can be considered to prevent the cross-linking reaction of the adhesive from following the vulcanization reaction of the rubber. 4.4 Internal rubber damage In terms of bonding damage form, the ideal damage form required by the general rubber/metal bonding system is 100% rubber body failure. The bonding strength at this time mainly depends on the physical and mechanical properties of the vulcanized rubber. If the bonding strength at this time has not reached the bonding strength, it mainly depends on the physical and mechanical properties of the vulcanized rubber. If the bonding strength at this time still does not meet the bonding requirements, it may be that the strength of the rubber itself is too low, or the adhesive modifies the rubber at the interface when it diffuses, migrates and undergoes physical and chemical reactions into the rubber phase, reducing the strength of the rubber there. At this time, replacement and improvement of the adhesive or rubber formula should be considered. 5 Conclusion With the development of society and the progress of industry, rubber/metal bonding composite systems are increasingly used in various fields such as automobiles, aerospace, ships, and construction. The requirements for bonding performance and bonding processes are also getting higher and higher. Mastering a series of physical and chemical changes that occur during the bonding process of rubber/metal bonding composite systems and various causal relationships that lead to the success or failure of bonding plays an important role in successfully realizing rubber/metal bonding and improving the bonding performance of the composite system.