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The bonding of rubber to metals has a long history, with methods such as direct bonding, hard‑rubber bonding, brass‑plating, and adhesive bonding commonly employed. Among these, adhesive bonding is currently one of the most widely used and effective approaches. This paper examines the underlying bonding mechanisms and processing techniques, analyzes the various factors that may lead to failure in vulcanized rubber–metal adhesion, and proposes corresponding mitigation strategies.

1. Adhesion Principle:
Rubber and metal are two fundamentally distinct materials, differing markedly in their chemical structure as well as their physical and mechanical properties. The adhesives used for their thermosulfurized bonding typically consist of a multiphase system formed by dissolving or dispersing resins, curing agents, and various additives in solvents or polymer emulsions. Consequently, the thermosulfurized adhesion between rubber and metal involves complex interactions among multiple constituent systems, representing a multifaceted phenomenon that spans surface physics, surface chemistry, polymer chemistry, inorganic chemistry, mechanics, electrical science, and other disciplines, with an intricate web of influencing factors. Regarding the adhesion mechanisms of rubber–metal joints, the prevailing theories include adsorption theory, electromagnetic theory, and co‑crosslinking theory. For thermosulfurized bonding between rubber and metal, the adhesive mechanisms associated with single‑coat and double‑coat systems are illustrated in Figures 1a and 1b, respectively. In these cases, adhesion between the adhesive—often a primer—and the metal primarily occurs through the adhesive’s wetting of the metal surface, allowing it to penetrate surface pores and micro‑voids while displacing adsorbed air at the interface, thereby achieving intimate contact. This facilitates both adsorption forces and various forms of mechanical interlocking (with some adhesive molecules even undergoing chemical reactions with surface atoms to form covalent bonds), thus generating sufficient bond strength. Meanwhile, adhesion between the adhesive and the rubber is achieved via molecular or chain‑segment diffusion, penetration, and co‑crosslinking. Simultaneously, a series of physicochemical reactions take place within both the adhesive and the rubber, ultimately resulting in a robust, integrated bond between the two materials.
 
2. Rubber/Metal Hot Vulcanization Bonding Process:
The typical process flow for hot vulcanization bonding of rubber to metal is as follows:
Metal surface treatment → Application of adhesive → Bonding with compounded rubber → Pressurized and heated vulcanization
Sulfur-crosslinked adhesives mainly comprise three major categories: phenolic resins, polyisocyanates, and halogenated polymers. Commonly used brands today include the Chemlok series from the United States, the Thixon series from Rohm and Haas, as well as the Chemsil and Megum series from Germany.

3. Failure modes of rubber-to-metal heat‑vulcanized adhesion:
The common failure modes of rubber-to-metal hot‑vulcanized adhesion can be broadly categorized into the following six types, as illustrated in Figures 1 and 2.
(1) Adhesion failure between the primer-type adhesive and the metal (M–C type);
(2) Internal failure of the adhesive (Type C);
(3) Failure between the topcoat adhesive and the primer adhesive (C–C type);
(4) Failure between the rubber and the surface-coating adhesive (R–C type);
(5) Internal rubber degradation (Type R);
(6) Mixed failure, which occurs when two or more of the aforementioned failure modes are present simultaneously.

4. Failure Cause Analysis and Countermeasures:
4.1 Cohesion Failure Between Primer-Type Adhesives and Metals
4.1.1 Improper metal surface treatment

(1) Cause Analysis:
① Insufficient metal surface treatment: The primary purpose of metal surface treatment is to remove rust, oils, and contaminants, thereby producing a clean, dry, and sufficiently rough active surface that facilitates adhesive wetting and adhesion. If the surface treatment is inadequate—leaving behind loose oxide layers or resulting in insufficient surface roughness—the effective specific surface area of the bonding interface will be reduced for a given coating area, leading to a lower density of contact points between the metal and the adhesive and, consequently, lower bond strength.
② Unclean metal surface: If the metal surface is not thoroughly cleaned or becomes recontaminated after cleaning, resulting in oil stains, impurities, residual cleaning agents, and other contaminants, an interfacial layer effectively forms on the metal surface. This interfacial layer significantly reduces the surface free energy of the metal, markedly increasing the contact angle between the adhesive and the metal surface, thereby diminishing the adhesive’s wettability. Moreover, it may fill the microscopic voids on the metal surface, reducing the actual contact area between the metal and the adhesive and consequently lowering the bond strength.

(2) Mitigation measures:
① Treat the metal surface to remove rust and oil, ensuring that the bonding interface has sufficient roughness. Common methods for metal surface treatment include mechanical techniques (such as sandblasting and mechanical grinding) and chemical processes (such as acid pickling, alkaline cleaning, phosphating, surface coating, and high‑temperature degreasing).
In addition, care should be taken to ensure that the metal surface is not excessively rough. If the surface roughness is too high, its irregularities will impair the adhesive’s wettability and promote gas adsorption, leading to discontinuities at the bonding interface, the formation of defects and stress concentrations, and ultimately a reduction in bond strength. The appropriate level of surface roughness should be determined based on the flow characteristics and wettability of the specific adhesive being used.
② Before applying the adhesive, clean the metal surface with a chemical solvent to remove oil, contaminants, and other impurities, ensuring it is thoroughly dried and protected from recontamination. According to some reports, the cleaner the surface of the metal to be bonded, the smaller the contact angle between the adhesive and the metal surface, resulting in higher bond strength.

4.1.2 Improper selection of adhesives:
(1) Cause Analysis:
① The adhesive or primer has excessively high viscosity, preventing effective wetting of the metal surface or leading to bubble formation at the metal/adhesive interface, with stress concentrations developing around these bubbles.
② Although the adhesive can effectively wet the metal surface, its adhesion to the metal after curing is too low.
(2) Mitigation measures:
Select an appropriate adhesive to ensure good wetting on the metal surface, and verify that the physical–mechanical or chemical interactions between the adhesive and the metal after curing meet the required bond strength.

4.1.3 Improper adhesive application process:
(1) Cause Analysis:
① Adhesive is too viscous or the solvent evaporates too rapidly: Solvents, thinners, or dispersing-phase liquids serve as effective carriers that enable the adhesive to wet and penetrate the metal surface. If their quantity is insufficient or if they evaporate too quickly after application, the adhesive may exhibit inadequate fluidity or an insufficient open time, leading to kinetic‑driven incomplete wetting and, consequently, suboptimal bond strength and durability.
② Inadequate mixing of the adhesive: If the adhesive is not thoroughly mixed, the active components—such as the base resin or curing agent—will be unevenly dispersed. Areas with lower concentrations will exhibit reduced adhesive strength, potentially leading to bond failure between the metal and the adhesive.
③ Improper adhesive coating thickness: If the adhesive layer is too thin, there are fewer adhesive molecules per unit surface area, resulting in reduced bonding strength; if the adhesive layer is too thick, it is prone to bubble formation, defects, and early failure, and, upon heating, generates significant thermal expansion stresses that can lead to joint failure and ultimately adhesive bond failure.
(2) Mitigation measures:
① Select an appropriate coating process and ensure that the adhesive is properly diluted to achieve an adequate wetting rate and wetting time.
② Before applying the adhesive, ensure it is thoroughly and evenly mixed to prevent the effective solid components from settling and aggregating.
③ The adhesive should be applied at an appropriate thickness.

4.2 Internal failure of the adhesive, and failure between surface‑coated adhesives and primer‑coated adhesives:
(1) Cause Analysis:
① Insufficient curing time for the adhesive results in incomplete solvent evaporation, leading to defects.
② The cohesive strength of the adhesive after curing is not low;
③ After the primer adhesive has been applied, if the bonding surface becomes contaminated with oil stains, dust, and other impurities, a separating boundary layer will form between the two adhesives upon subsequent application of the adhesive. This leads to stress concentration and ultimately results in bond failure.
(2) Mitigation measures:
① After applying the adhesive, allow it to dry completely to prevent residual solvent molecules.
② Select an adhesive with high cohesive strength;
③ After applying the adhesive, during storage and handling, minimize contact between the coated surface and hands, dust, debris, or other contaminants to prevent recontamination.

4.3 Failure between the rubber and the surface-coating-type adhesive:
During the hot vulcanization bonding process, rubber molecules and adhesive molecules first undergo a physical interaction characterized by mutual penetration and diffusion between the two phases. Subsequently, crosslinking chemical reactions occur between the molecules of the two phases and within each phase, thereby firmly integrating the two materials into a cohesive whole.

4.3.1 Inappropriate rubber compound:
(1) Cause Analysis:
If compounding agents in the rubber compound migrate to the surface through blooming or extraction, they will form an isolating layer between the rubber surface and the adhesive, hindering molecular diffusion and co‑crosslinking between the rubber and the adhesive, thereby making effective bonding difficult.
(2) Mitigation measures:
In the design of rubber compound formulations, while meeting the performance requirements of the final product, the following principles should be followed as much as possible:
① When selecting raw rubber grades, prioritize those with high polarity, high unsaturation, and excellent adhesion performance.
② For general-purpose rubbers, particularly diene rubbers, a sulfur vulcanization system yields superior bonding performance.
③ Plasticizers, paraffin wax, processing aids, and other compounding agents that are detrimental to adhesion—especially ester-based plasticizers—should be used sparingly or avoided altogether.
④ The dosage of anti‑aging agent D, sulfur, and other additives prone to blooming should not be excessive.

4.3.2 Adhesive Factors:
(1) Cause Analysis:
① The adhesive is incompatible with the rubber to be bonded;
② The adhesive was not mixed uniformly, the surface was allowed to dry for an insufficient time after application, or the glued surface became contaminated.
(2) Mitigation measures:
① Select an appropriate adhesive based on the type of rubber to be bonded; for example, polyisocyanates and halogenated polymer adhesives deliver superior bonding performance when adhering nonpolar rubbers, whereas phenol‑ester resin adhesives exhibit relatively poor performance.
② The curing system of the adhesive must be compatible with the vulcanization characteristics of the rubber. For example, polyurethane rubbers cured with peroxide systems exhibit good crosslinking compatibility with phenolic resins and isocyanate-based adhesives, whereas general-purpose rubbers such as NR and NBR vulcanized with sulfur show better compatibility with maleimide- and quinone‑oxime–based crosslinking systems.

4.3.3 Inappropriate vulcanization process:
During the hot vulcanization bonding process of rubber to metal, improper selection of any one of the vulcanization parameters—pressure, temperature, or time—can lead to bond failure. The primary measures for preventing bond failure caused by vulcanization are:
① The vulcanization temperature must be high enough to overcome the activation energy barrier of the chemical reaction, while simultaneously initiating both the adhesive’s curing reaction and the rubber compound’s vulcanization reaction. On the other hand, provided these conditions are met, it is necessary to appropriately reduce the vulcanization temperature—particularly in cases involving exothermic reactions or excessive interfacial tensile stresses that could compromise the bonded interface.
② With regard to vulcanization pressure, provided that the product’s other properties and the equipment and process constraints are satisfied, higher pressure is generally preferable—particularly for adhesives with a high content of low‑molecular‑weight polymers or those that generate small molecules during curing—since such conditions are essential for ensuring surface wetting, diffusion, and the removal of these low‑molecular‑weight byproducts.
③ If the adhesive’s reactivity is lower than that of the rubber compound during vulcanization, or if the metal component is relatively large in size, measures such as preheating the metal part may be adopted to ensure synchronous curing between the rubber and the adhesive, thereby preventing the adhesive’s crosslinking reaction from lagging behind the rubber compound’s vulcanization.

4.4 Internal Damage to Rubber:
In terms of the mode of adhesive failure, the ideal failure mode for typical rubber–metal bonding systems is 100% cohesive failure within the rubber itself; in this case, the bond strength is primarily governed by the physical and mechanical properties of the vulcanized rubber. If, under these conditions, the measured bond strength still fails to meet the required level—again, because the bond strength is dominated by the vulcanized rubber’s properties—the issue may stem from either insufficient intrinsic strength of the rubber compound or from the adhesive diffusing into and migrating within the rubber phase, where it induces physicochemical reactions that modify the rubber at the interface and reduce its local strength. In such instances, it becomes necessary to consider revising or improving either the adhesive formulation or the rubber compound composition.

5. Conclusion:
With the development of society and advances in industry, rubber–metal adhesive composite systems are finding increasingly broad applications across sectors such as automotive, aerospace, shipbuilding, and construction. Consequently, the demands on bonding performance and bonding processes are steadily rising. Gaining a thorough understanding of the physicochemical transformations that occur during the bonding of rubber–metal composites, as well as the causal relationships underlying bonding success or failure, is crucial for achieving reliable adhesion and enhancing the overall bonding performance of these composite systems.