Styrene-butadiene rubbery adhesives

Figure 6.9 Cohesive and adhesive joint tensile fracture stresses plotted against effective rate of extension, kaj, at 23 °C (styrene-butadiene-rubbery adhesive bonding poly-(ethylene terephthalate substrates) [43]. A Cohesive tensile strength of adhesive. Cohesive-in-adhesive failure of butt joints. O Interfacial failure of butt joints.

For example. Fig. 7.9 shows the results from studying crack growth in a simple-extension joint (Fig. 7.7) consisting of a crosslinked styrene-butadiene rubbery adhesive (a non-linear-elastic material) adhering to a rigidly supported poly(ethylene terephthalate) substrate [103]. From Equation 7.44, with ks = 7T, the data is plotted as t/dc versus and the linear relationship which is... 

This concept is illustrated in Fig. 8.11 for a poly(ethylene terephthalate) substrate and a mild steel (ferric oxide) substrate with, in both cases, water as the hostile environment. Values of y and yl of the various adhesives may be measured, as described in Chapter 2, or extracted from the literature (see Table 2.3) for example, considering a styrene-butadiene rubbery adhesive the values are 27.8 and 1.3 mJ/m, respectively, and for a typical epoxy adhesive they are 41.2 and 5.0 mJ/m, respectively. Hence, it is evident that these (and most other) adhesives will form an environmentally water-stable interface with the poly(ethylene terephthalate) substrate but an unstable interface with mild steel. Indeed, the data confirm that if only secondary molecular forces are acting across the interface then water will virtually always desorb organic adhesives, which typically have low surface free energies of less than about 60 mJ/m, from a metal oxide surface. Hence, for such interfaces, stronger intrinsic adhesion forces must be forged which are more resistant to rupture by water. 

All grades of regular butyl rubber are tacky, rubbery and contain less unsaturation than natural rubber or styrene-butadiene rubber. On the other hand, low molecular weight grades of polyisobutylene are permanently tacky and are clear white semi-liquids, so they can be used as permanent tackifiers for cements, PSAs, hot-melt adhesives and sealants. Low molecular weight polyisobutylenes also provide softness and flexibility, and act as an adhesion promoter for difficult to adhere surfaces (e.g. polyolefins). 

This type of adhesive is generally useful in the temperature range where the material is either leathery or rubbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mostly uncompounded state. 

Only types (l)-(4) fall within the scope of this chapter. No further reference will be made to emulsion-polymerized prolybutadiene rubbers, because they are now of little industrial significance relative to the styrene-butadiene rubbers. Poly(vinyl chloride) is discussed elsewhere in this book. Brief reference will also be made in this chapter (Section 15.5) to the production and properties of carboxylated variants of styrene-butadiene rubber latexes. It may also be noted that latexes of rubbery terpolymers of styrene, vinyl pyridine and butadiene, produced by emulsion polymerization, have long been of considerable industrial importance for the specialized application of treating textile fibres (e.g., tyre cords) in order to improve adhesion between the fibres and a matrix of vulcanized rubber in which they are subsequently to be embedded. 

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. 

Thermoplastic rubber is a relatively new class of polymer. It has the solubility and thermoplasticity of polystyrene, while at ambient temperatures it has the toughness and resilience of vulcanized natural rubber or polybutadiene. These rubbers are actually block copolymers. The simplest form consists of a rubbery mid-block with two plastic end blocks (A-B-A), as shown in Figure 5.7. Examples of commercial products are Kraton and Solprene . These materials are often compounded with plasticizers to decrease hardness and modulus, eliminate drawing, enhance pressure-sensitive tack, improve low-temperature flexibility, reduce melt and solution viscosity, decrease cohesive strength or increase plasticity if desired, and substantially lower material costs. Low levels of thermoplastic rubbers are sometimes added to other rubber adhesives. These materials are used as components in the following applications PSAs, hot-melt adhesives, heat-activated-assembly adhesives, contact adhesives, reactive contact adhesives, building construction adhesives, sealants, and binders. Two common varieties of thermoplastic rubber adhesives are styrene-butadiene-styrene (S-B-S) and styrene-isoprene-styrene (S-I-S). ... 

Manufacture and compounding The majority of organic solvent-based adhesives are based on rubbery polymers, the main ones being natural rubber, polychloroprene, butadiene-acrylonitrile, styrene-butadiene and polyisobutylene. Traditionally, the rubber was placed in a heavy-duty mixer and solvent was added slowly till a smooth solution was formed. In some cases, the rubber was milled beforehand to reduce viscosity and produce smoother solutions. Nowadays, it is possible to obtain some grades of material that only require stirring in a comparatively simple chums. 

It appears that carboxyl-modification is necessary for bonding polymer latexes through amine-functional silanes. A series of experimental styrene-butadiene-acid terpolymers were compared as film formers and primers with 5% added Z-6020 (Table V). Difunctional acids gave the most stable mixes, but poorest water resistance. Polymers with acrylic acid modification were best primers for plaster. Alkylmaleate modified polymers had poor stability with silane, and deposited rubbery films with poor wet adhesion to glass. 

Impact modifiers are added to many formulations. As the name implies, they impart toughness to the polymer article or film. Many of them are butadiene copolymers that disperse in the polymer matrix. One type - the so-called core shell modifiers - has a rubbery core surrounded by a harder acrylate layer. They have been compared to an egg soft on the inside and hard on the outside. The outer shell also has some adhesion to the matrix so that the modifier can be dispersed. Other impact modifiers include methacrylate-butadiene-styrene copolymers or EPDM, ethylene-propylene-diene monomer copolymers. Acrylonitrile-butadiene-styrene (ABS) and ethylene-vinyl acetate (EVA) are also used. 

Block copolymers with incompatible blocks which are able to microphase separate are good candidates for PSA properties. Indeed, blends of ABA triblocks and AB diblocks, where the rubbery midblock of the ABA is the majority phase and the glassy endblocks self organize in hard spherical domains and form physical crosslinks, are widely used as base polymers for PSA. The actual adhesives are always compounded with a low molecular weight tackifier resin able to swell the rubbery phase and dilute the entanglement network. Linear styrene-rubber-styrene copolymers, with rubber being isoprene, butadiene, ethylene/propylene or ethylene/butylene, are the most widely used block copolymers in this category. 

Block copolymers of styrene with either butadiene or isoprene are used in pressure-sensitive adhesives. These are three-block copolymers of type ABA where A are polystyrene blocks and B are polydienes. The two phases are incompatible, each phase having a separate glass transition temperature. The rubbery phase is continuous and the styrene phase consists of small particles 20-30pm diameter. The styrene content is generally above 25%. The Tg of the polybutadiene phase is about —80°C, and it is about — 50°C for polyisoprene. The Tg of polystyrene is about 100°C, and this temperature must be exceeded in thermal processing. They are also used as thermoplastic rubbers. 

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