Rubber and metal bonded products have a wide range of applications in various fields such as automobiles, rail transit, engineering machinery, household appliances, aerospace, etc., such as engine suspensions, shock absorbers, oil seals, brake pads, pipeline joints, etc. However, poor adhesion between rubber and metal has always been a difficulty in product quality control, which can lead to mass scrap, increased after-sales failure rates, and damage to brand reputation in severe cases.

This article will deeply analyze the reasons for poor adhesion between rubber and metal from six dimensions: material selection, formula design, surface treatment, adhesive system, process control, and testing and evaluation, and propose systematic solutions.

1、 Material factors: Control basic conditions from the source

1.1 Rubber substrate selection and polarity matching

The adhesive properties of different types of rubber to metals vary significantly. Polar rubber (such as chloroprene rubber, nitrile rubber, hydrogenated nitrile rubber, polyurethane rubber, fluororubber, etc.) is more compatible with the polarity of metal surfaces and has better adhesion performance. Non polar rubber (such as natural rubber, butadiene rubber, ethylene propylene rubber, etc.) relies more on interfacial bonding systems and surface activation treatments.

1.2 Rubber pollutants and small molecule migration

Residual unreacted monomers, lubricants, plasticizers, waxes, and migration agents in rubber can migrate to the metal surface to form a fouling layer, affecting the wetting and cross-linking of the adhesive. Therefore, it is necessary to strictly control the amount and types of low molecular weight migration components in the formula, such as low molecular weight softeners, aromatic oils, white wax, release agents, etc.

2、 Formula design: Balancing physical and adhesive properties

2.1 Active components and enhanced reactivity

Introducing components that can synergistically react with the adhesive system in rubber formulations, such as resorcinol HMT、 Formaldehyde resin, phosphite coupling agent, isocyanate reactive additives. These components can form chemical crosslinks with metal surfaces and adhesives during the vulcanization process, significantly enhancing interfacial adhesion.

2.2 Sulfurization compatibility of formula system

Adhesive systems usually have certain compatibility requirements for vulcanization systems. For example, traditional RFS bonding systems are often compatible with sulfur-containing systems, while for peroxide vulcanization systems, special grafting monomers or treatment based primers need to be introduced to enhance compatibility. The synergy between the type of vulcanizing agent, sulfur content, promoter type, and adhesive needs to be optimized through experiments.

3、 Metal surface treatment: ensuring the "cleanliness and activation" of the bonding interface

3.1 Degreasing treatment

Metal surface degreasing is the primary step, usually using alkaline degreasing solution or solvent degreasing agent to remove processing oil, hand sweat, and oxide layer. If not thoroughly cleaned, the adhesive is prone to producing "fish eyes" or bubbles. For high demand occasions, it is recommended to adopt a complete treatment process of alkali dehydration water washing acid washing neutralization drying.

3.2 Coarsening treatment

Sandblasting, acid etching, electrochemical treatment, etc. can increase the surface roughness of metals and enhance mechanical biting force. It is recommended to use alumina sand with a particle size of 80-120 mesh for sandblasting, with a pressure controlled between 0.4~0.6 MPa, and to keep the sand particles clean to prevent secondary pollution.

3.3 Coating Primer

The function of the primer is to bridge the interface, with one end reacting with the metal surface to form a strong chemical bond, and the other end synergistically vulcanizing with the adhesive. Common primers such as Chemlok 205, 220, Primer 80, etc. should be evenly applied on clean and dry metal surfaces, with a controlled coating thickness of 5-10 μ m.

4、 Adhesive system: a bridge connecting rubber and metal

4.1 Selection of Adhesive Types

Common adhesives are divided into thermosetting adhesives (such as RFS system, Chemlok series, Thixon series, etc.) and room temperature adhesives (such as epoxy, cyanoacrylic acid, etc.). Rubber and metal bonding products often use a hot vulcanization system, which can co vulcanize with rubber to improve bonding strength.

4.2 Precautions for Adhesive Use

Stir evenly: Multi component adhesives need to be thoroughly stirred before use to prevent resin or filler precipitation.

Control humidity and temperature: Construction in high humidity or low temperature environments can affect the drying rate and crosslinking degree of adhesives.

Validity management: Once the adhesive is opened, it must be used up within the specified time; It is strictly prohibited to use expired materials.

5、 Molding and vulcanization process: process control is the guarantee of quality

5.1 Mold Cleaning and Spraying

If there are residual release agents, oil stains, or carbon black in the mold, it will transfer and contaminate the metal interface, posing a risk of delamination. The mold needs to be cleaned regularly and high-temperature migration resistant release agent should be used to avoid spraying onto the surface of metal parts.

5.2 Compression molding/injection molding parameters

The vulcanization temperature, pressure, and time need to be precisely controlled. If the temperature is insufficient or the heating is slow, the adhesive will peel off before fully crosslinking; If the pressure is insufficient, the rubber cannot flow fully to the metal surface, resulting in areas that are not bonded.

5.3 Preheating and Positioning

Metal inserts need to be fully preheated (such as to 70-100 ° C) to avoid cooling the rubber flowability; At the same time, metal parts need to be fixed by positioning structures or magnetic attraction to prevent displacement during the molding process.

6、 Adhesive Strength Testing and Failure Analysis: Accurately Mastering Quality Fluctuations

6.1 Adhesive Strength Standards

Common testing methods include:

Tensile peel strength (ASTM D429 Method-B)

Peel shear strength (ISO 814)

Shear tensile peeling (GB/T 7762)

Analysis of delamination failure mode: Determine whether it is interface delamination, colloid failure, or adhesive layer delamination. General industrial requirements for adhesive strength

≥ 5 MPa, some key parts require ≥ 8 MPa, and adhesive failure should be the main cause.

6.2 Types and Diagnosis of Adhesive Failure

Interface delamination: usually caused by untreated metal, wet adhesive, or rubber contamination.

Bubble interlayer: Gas is not discharged during the molding stage, or there are too many volatile substances in the rubber material.

Damp heat aging peeling: incomplete cross-linking or insufficient aging stability of the adhesive.

The essence of the adhesion problem between rubber and metal is the synergistic failure of multiple factors. Solving this problem requires interdisciplinary cooperation such as materials engineering, chemical reactions, interface physics, and process control. Any problem in any link may lead to poor adhesion in the end.