Thermoplastic adhesives relative strength

Figure 4.21 Relative strengths of thermoplastic and thermosetting adhesives T is the melt temperature of the thermoplastic adhesive Tg is the glass-transition temperature of the thermosetting adhesive).

Most of the polymer s characteristics stem from its molecular stmcture, which like POE, promotes solubiUty in a variety of solvents in addition to water. It exhibits Newtonian rheology and is mechanically stable relative to other thermoplastics. It also forms miscible blends with a variety of other polymers. The water solubiUty and hot meltable characteristics promote adhesion in a number of appHcations. PEOX has been observed to promote adhesion comparable with PVP and PVA on aluminum foil, cellophane, nylon, poly(methyl methacrylate), and poly(ethylene terephthalate), and in composite systems improved tensile strength and Izod impact properties have been noted. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

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). ... 

An example of a plasticizer which destroys cohesive strength is dioctylphthalate (DOP), commonly used in plasticized polyvinyl chloride (PVC) compositions. A relatively small amount of DOP added to an S-B-S thermoplastic rubber reduces the ultimate tensile strength from above 4000 psi to zero. Low molecular weight liquid tackifying resins are also often soluble in both phases and tend to reduce cohesive strength. However, they may still be useful in some adhesive applications. 

An interesting PI which is not a high temperature polymer is shown in structure 7. This material deserved mention because it provided the highest average strength at 25 C for Ti TSS of any known oiganic adhesive. Strengths at 7850 psi at 25 C, 5400 psi at 93 C and 4045 psi at 121 C were reported. The PI has a Tg of 155 C, which permitted the fabrication of TSS from essentially volatile-free adhesive tape at 260 C under 100 psi. The polymer was fabricated as a thermoplastic where the time at temperature was relatively short (< 15 minutes). 

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