Bonding of Rubber to Substrates

Bonding rubber to a solid insert as part of the injection moulding was a process first begun in the 1960s. Since that era the process has become, like rubber processes in general, a science rather than the black art as it was once regarded. The process does need to be closely controlled to ensure 100% compliance with automotive requirements. This chapter will help the reader understand the processes and how components can be produced that will surpass the customers expectations [1,2]. 

The design parameters of the product control the selection of the rubber compound and the insert (substrate) material. These, in turn, determine the means of surface preparation for the insert and the bonding agent to be used. 

It is possible to bond most compounds to a metal or plastic substrate. The ease with which they may be bonded depends principally on two factors the polymer type and the curing system. 

General purpose polymers (natural rubber and SBR) pose no difficulty unless they are of high viscosity or conversely are of very low hardness, i.e., s 40 IRHD (NR) and styrene-butadiene rubber (SBR). 

Saturated polymers such as butyl and ethylene-propylene-diene terpolymer are more difficult to bond and generally exhibit lower bond strengths compared to those found with the general purpose polymers. Once a degree of functionality is introduced into the polymer such as with chloro or preferably bromobutyl rubber and chlorosulfonated polyethylene (Hypalon ) then the ability to bond is enhanced. 

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Although there is some discussion of relevant theory in various sections of text, the emphasis in this volume has been to concentrate on the practicalities of bonding of rubbers, to themselves and substrates. It is considered that this type of information is of immediate interest to the practising technologist dealing with shop floor problems on a daily basis. 

Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates, Method D - Adhesion Test - Post Vulcanisation (PV) Bonding of Rubber to Metal, 2002. 

BS 903 PA21 Physical testing of rubber Part A21 Determination of rubber to metal bond strength Section 21.1 Adhesion to rigid substrates, 90-degree Peel method. 

Chem. Descrip. Disp. of polymers, org. resins, and tillers in org. solv. Uses Cover coat bonding agent which bonds wide variety of rubbers to metal and other substrates during vulcanization Features General purpose exc. environmental resist. 

Uses Cover coat bonding agent tor EPDM, nonpolar elastomer Features General purpose bonds a wide variety of rubbers to metals and other substrates... 

Rubber to substrate bonding is used for three main applications. Antivibration, the chemical or physical properties of the rubber, and a combination of antivibration with the chemical physical properties of the rubber. This is shown in Table 3.1. 

By changing from solvent degreasing to aqueous degreasing systems a 25% reduction in pollution can be achieved. To meet the next target reduction of 67% solvent emission it will be necessary to use only waterborne bonding systems for rubber to substrate bonding. 

This section shows some typical applications where cyanoacrylates have been used to bond all types of rubber to an assortment of other substrates. It is important to note that each application has its own imique features, and that an adhesive which works well in one circumstance, may not necessarily be suitable in another. 

Bonds between rubber and substrates can fail for a number of reasons. Section 12.1 deals with some of the causes of rubber to metal bond failures. Section 12.2 examines the type of failures which are adhesion related, in fabric or cord reinforced power transmission belts. Section 12.3 discusses a phenomenon which causes service failures of rubber components, mainly in sealing applications. This phenomenon arises through a bond which is formed between the rubber (nitrile) and the metal mating surface of a valve or similar, which is of sufficient strength to rupture the rubber surface when the valve is opened. 

The applications of rubber as an engineering material almost invariably involve bonding to a rigid substrate or reinforcement. In some instances these bonds need to be established to a fully cured thermoset rubber or a molded thermoplastic rubber, and a wide variety of adhesives suitable for this purpose are available. In... 

Finally, the solubility parameter of the adhesive and the substrate must be close. Without getting too teehnieal, the solubility parameter is a rough estimate of polarity. The old saying like dissolves like can be extended to like bonds like. More aeeurately, the solubility parameter is the ealeulated potential energy of 1 em of material for eommon solvents. Polymers are assigned solubility parameters of solvents in which they are soluble. Table 19.3 lists solubility parameters for various solvents and polymers. As an example of how to use this table, butadiene-acrylonitrile rubber with 6= 9.5 bonds natural rubber (6= V.9-8.3) to phenolic plastics (6= 11.5). Note that its solubility parameter is between that of the two substrates. 

For several years the bonding agents have consisted of proprietary polymer/solvent solutions, with a primer coat based on phenolic-style resins and a topcoat formed from solutions of polymers and other ingredients. The formulation of these materials is not disclosed, but much patent literature is available. Bond formation appears to be associated with the development of a very high modulus layer in the rubber immediately adjacent to the surface of the substrate. The thickness of the layer is of the order of 15 pm and... 

The ideal surface for contact with human blood is the surface of blood vessels, and the immediate surface contains heparinoid complexes. Heparin, a negatively charged polysaccharide, has been bonded to silicon rubber and other polymers. In one procedure, a quaternary ammonium compound is first adsorbed on die polymer substrate and heparin is 111 turn adsorbed on the positively charged surface. Chemical bonding of heparin has also been achieved. Such surfaces do not cause clotting of contacted blood. 

Zircoaluminate adhesion promoters constitute a novel class of compounds which have proven themselves useful in arresting corrosion on coated metals, enhancing adhesion of adhesives to rubber and metal, and improving bonding of formed rubber articles containing ZA to other substrates. 

An HgCdTe epitaxial layer is patterned to form individual devices 10a, 10b and 10c on a substrate 12 of HgCdTe. The substrate is bonded to a second substrate 14 by a silicone rubber adhesive 16. 

Proprietary bonding agents are used almost exclusively in modern manufacturing processes for metal-rubber bonded products. Isocyanates were proved to be of value in the immediate postwar period, but their moisture sensitivity and tendency to be wiped off the metal during processing have proved as disadvantages. The majority of commercially available adhesives in use today are a complex mixture of undisclosed composition, and in many instances are specifically used for particular rubbers and substrates. 

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar structure, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These rubber-modified products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

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