Rubber base adhesives ingredients

In the next sections, the manufacture, chemistry and properties of the main ingredients of the rubber base adhesives will be considered. 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

This chapter reviews the main aspects of rubber base adhesives. A detailed description of the main ingredients of rubber base adhesive has been given because most of the previous contributions in this area did not consider this important aspect. On the other hand, update of the literature in water-base rubber adhesives has been done. 

The curing of CR adhesives is different from that for most rubber-based adhesives as it involves the labile chlorine atoms. This is reflected in the compounding ingredients. The most common room curing agent is zinc oxide, but isocyanates are also very common. Further discussion is given in Polychloroprene rubber adhesives modifiers and additives. 

Elastomeric adhesives typically contain an elastomer and a tackifying or modifying resin as key components, but in general, other ingredients are also included. The formulation of rubber-based adhesives may contain nine, or even more, different components (Table 1). Typical formulations of various types of adhesives are given in Tables 2-5. 

In solvent-borne rubber adhesives, a variety of solvents can be chosen to control drying rate, adjust viscosity and dissolve important ingredients. Resins can be added to improve tack, wetting properties, heat resistance, bond strength and oxidation resistance. The most common resins nsed in rubber-based adhesives are rosins, rosin esters, and terpene, coumarone-indene, hydrocarbon and phenobc resins. Plasticizers and softeners reduce hardness, enhance tack and decrease cost of rubber adhesive formulations. Paraffinic oils, phthalate esters and polybutenes are typical plasticizers. Fillers are not often added to rubber adhesive formulations because they reduce adhesion. However they are sometimes used because they decrease cost and increase solution viscosity. Carbon black and titanium dioxide are also used to provide colour to the adhesives. Clays, calcium carbonate and silicates are also common fillers in rubber adhesive formulations. For water-borne adhesives, typically protective colloid, preservative, defoamers, wetting agents and emulsifiers are included in the formulations. 

There now follows a more detailed consideration of the main features of ingredients for rubber-based adhesives. 

Several elastomers can be used in rubber-based adhesives. The elastomer provides the backbone of the adhesive, so the main performance of the adhesive is provided by the rubber properties. However, several specific properties for application are imparted by adding other ingredients in the formulations. The most common elastomers used in rubber-based adhesives are natural rubber (NR), butyl rubber (BR) and polyisobutylenes, styrene-butadiene rubber (SBR), nitrile rubber (NBR) and polychloroprene rubber Neoprene) (CR). 

The major ingredients for rubber based adhesives in addition to the elastomer are given below ... 

Abstract The basic concepts, formulations, and test methods of pressure-sensitive adhesives are presented. The importance of interfacial interactions, viscous loss, and extensibility are stressed. The common rheological tests are described and the equivalence of deformation rate and test temperature is emphasized. The much longer time scale for bond formation versus the rate of deformation upon debonding in peel or tack is exploited by the formulator to optimize properties. The formulation principles and common ingredients for preparing acrylic- and rubber-based adhesives are described, and the performance capabilities of these two types of pressure-sensitive adhesives are contrasted. 

Styrene-butadiene-styrene (SBS) rubbers are either pure or oil-modified block copolymers. They are most suitable as performance modifiers in blends with thermoplastics or as a base rubber for adhesive, sealant, or coating formulations. SBS compoimds are formulations containing block copolymer rubber and other suitable ingredients. These compounds have a wide range of properties and provide the benefits of rubberiness and easy processing on standard thermoplastic processing equipment. 

With respect to the use of biobased sources, adhesive products have been ahead of plastics. They are often called natural adhesives. Animal- and plant-based adhesives have been used for thousands of years. Three prominent classes of natural adhesives include proteins (polyamides), carbohydrates (polysaccharides), and natural rubber (mainly cw-polyisoprene). Three specific examples are starch, a carbohydrate gelatin, a protein and rubber cement made from natural rubber. Advantages of biobased adhesives include recyclability and environmental safety. The latter is an important consideration because of presence of solvents and hazardous ingredients in some synthetic adhesives. 

Resorcinol formaldehyde latex (RFL) cord dips have only a limited application within the general rubber goods industry and for adhesion to be achieved with synthetic fibres it is necessary to use the systems developed by Bayer and Degussa. These systems are based upon a combination of resorcinol, a formaldehyde donor and a hydrated silica filler (commonly called the RFK system). This system is incorporated as dry ingredients into the rubber compound and is activated by the application of heat. 

Selected examples have been cited to demonstrate that areas of acrylic, anaerobic and radiation-curable adhesive developments have begun to consider telechelic polybutadiene/acrylonitrile liquids as useful formulating ingredients either in direct admixture or in the preparation of rubber-modified oligomers serving as an adhesive formulating base. These cited adhesive examples cover structural, semi-structural, gap-filling and pressure-sensitive types. 

Rubber to metal adhesives contain polymeric materials that are compatible with the ingredients in the primer, as well as the rubber compound to be bonded. Many are based on halogenated polymers. Halogenated polymers or resins are known to wet metals efficiently and can be used in both the primer and adhesive formulation. They provide effective barriers to chemicals that can undermine the adhesive bond. The adhesive also contains very powerful curatives that react with both the polymers in the rubber and the polymers in the adhesive [19]. Difunctional and polyfimctional chemicals are capable of making the film forming polymer a thermoset as well as reacting across the interface of the film to link into the rubber. 

An area that has seen a lot of interest is in the use of waste rubber to produce sound insulation materials. Zhao and co-workers [68] have investigated the properties of waste tyre rubber/wood composites for sound insulation applications. They used a four-microphone method to measure and compare the sound transmission losses of three different composite panels the waste tyre/wood material, a commercial wooden floorboard, and a commercial wood-based particle board. The waste tyre rubber/wood product was manufactured in the laboratory with commercial urea-formaldehyde and a PU adhesive. The results obtained indicated that the sound insulation properties of the waste rubber/wood material were better than the other two products. Also, its insulation properties were significantly affected by the amount of rubber crumb and the amount of PU adhesive used, the performance improving as the quantity of both ingredients was increased. In the case of the PU adhesive, this was due to the formation of more complete rubber-adhesive interfaces within the material. 

In North America approximately 5-10 % of the dispersions used for labels are based on styrene-butadiene rubber (SBR) [24]. SBR used in pressure sensitive adhesives are produced by emulsion polymerization with butadiene contents typically between 25 and 45 %. With carefijl control of process conditions (temperature, styrene and butadiene feed rates) and ingredient feed levels (chain transfer agent, initiator, monomers), intermediate molecnjlar weight, lightly cross-linked elastomers having an excellent balance of cohesive and adhesive properties can be obtained. 

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