RUBBER TO METAL ADHESION

Introduction

The processes of adhering rubber to metal components are widespread techniques in the rubber industry. Generally the same considerations and procedures are applicable for rubber to rigid non-metallic components, but the adhesion values may be lower. Only the broad aspects of rubber-to-metal molding are covered here, and more precise information can be provided by the rubber manufacture involved.

GENERAL INFORMATION

Application

Various adhesion levels can be obtained. For instance, to obtain adhesion on critical products, such as engine mounts, very close controls are required, both on metal and rubber preparation. Some products may require only enough adhesion for assembly.

The adhesion level (tear/tensile strength) is directly affected by type of metal, surface preparation, non-metallic inserts, compound composition, curing conditions, and type of adhesive.

Drawings should clearly state adhesion requirements and any other factors which can explain the degree of adhesion required and the method of testing. A clear understanding between customer and rubber manufacturer is essential.

Methods of Obtaining Adhesion

The method most commonly used to obtain adhesion between rubber and metallic or non-metallic components is the use of adhesive cements. Prior to the use of these special adhesives, the surface of the insert must be clean and free of contamination.

The inserts may be prepared by suitable methods such as degreasing, blasting, and/or a suitable chemical treatment. When any one of these preparatory processes is objectionable, it should be noted on the drawing. The rubber compound is then vulcanized to the prepared inserts to obtain the desired adhesion.

Design Factors and Limitations

1. Avoid localized stress raising irregularities.
2. Minimize edge effects. Break, coin or otherwise eliminate sharp edges of all metallic members covered by the rubber.
3. Minimize surface roughness of metallic members in area adjacent to adhered rubber.
4. Avoid welding a molded rubber component to a machine or structure to prevent unnecessary heat deterioration. When welding is mandatory, design metallic member as a heat sink and provide for assembly techniques which will keep the adhered rubber area of the metallic member below 150°C (302°F).

Test Methods for Determining Adhesion Values

Adhesion testing is done in several ways, depending upon the application and the product design. The methods recognized for this testing are treated in full detail in ASTM Test Method D429. These methods are:

Method A. Rubber adhered between two parallel metal plates.

Method B. Ninety degree stripping test, rubber adhered to one metal plate.

The above methods are used primarily for laboratory development and testing production parts. These methods may be modified and applied as described under RMA Production Test Methods section as follows.

RMA PRODUCTION TEST METHODS

Method A. Used where two metal surfaces, not necessarily parallel, can be separated until the specified adhesion value is obtained using the projected adhered area. The area to be considered should be the projected active adhered working area of the smallest metallic member, excluding fillets, overedge, and radii. Very irregular areas are to be given special consideration.

Method B. Used where the rubber can be stripped from the entire width of the part to obtain a specified adhesion value or where the rubber can be cut in 25.4mm (1.0in) wide strips. Specimen rubber thickness shall not exceed 9.5mm (.375 in). In rubber sections over 9.5mm (.375in), values should be negotiated between customer and supplier.

Acceptance Criteria Looseness contiguous to the adhered areas at corners, fillets, mold parting lines, and back-rinding will ordinarily be acceptable.

The adhesion strength is usually considered to be satisfactory if the failure causes permanent distortion of a metallic member.

If the deformation of the rubber section under test far exceeds the functional service requirements, this factor should be taken into consideration when establishing a reasonable adhesion value.

It is recognized the conditions for adhesion will exist where a quantitative value cannot be obtained. In these instances, it is customary to pull the rubber from the metallic member and examine the nature of the failure. The acceptable degree of adhesion must be agreed upon between the customer and the rubber manufacturer. Customer's test methods and fixtures should be identical with those of the rubber manufacturer and correlation procedures established.

METHODS OF DESIGNATING ADHESION VALUES

The design engineer when writing specifications should use a designation to obtain suitable adhesion for the purpose intended.

Methods of testing, such as tension pull or shear pull (RMA Production method "A") or 90 degree stripping (RMA Production Method "B") and the minimum destruction values, as well as the design of special testing fixtures should be specified on the drawings. ASTM D2000-SAE J200 has two types of adhesion designations for adhesion of vulcanized rubber to metal:

1. Adhesion by vulcanization, designated by K11 or K21.
2. Adhesion by the use of cements or adhesives after vulcanization, designated by K31.

This section is concerned only with K11 and K21.

Drawing Designation
B1 (specify method and grade from Table 9)	Production 100% tested to 70% of the minimum destruction values as noted in Table 9, Method A only. In addition, sample parts tested to destruction must exceed the minimum destruction values as noted in Table 9. (Specify Method A or Method B and Grade.)
B2 (specify method and grade from Table 9)	Sample parts tested to destruction must exceed the minimum destruction values as noted in Table 9.
B3	Rubber must be adhered to metal. This designation would ordinarily be used on products where adhesion is not critical to product function.
B4	Mechanical attachment only. Rubber is not adhered to metal.
B5	Products requiring special consideration.

Drawing Designation	S.I. Metric Units		USA Customary Units
Grade 1	2.8 MPa		400 psi
Grade 2	1.75 MPa	For rubber compounds over 10.5 MPa (1500psi) tensile strength and 50 or greater hardness (SHORE "A")	250 psi
	1.4 MPa	For rubber compounds under 10.5 MPa (1500psi) tensile strength or under 50 hardness (SHORE "A")	200 psi
Grade 3	0.35 MPa		50 psi

Drawing Designation	S.I. Metric Units		USA Customary Units
Grade 1	16 KN/m width		90 lbs/in width
Grade 2	9 KN/m width	For rubber compounds over 10.5 MPa (1500psi) tensile strength or greater 50 hardness (SHORE "A")	50 lbs/in width
	7 KN/m width	For rubber compounds under 10.5 MPa (1500psi) tensile strength or under 50 hardness (SHORE "A")	40 lbs/in width
Grade 3	2.7 KN/m width		15 lbs/in width