Strength adhesion

Adhesives. High concentration (>10%) solutions of poly(ethylene oxide) exhibit wet tack properties that are used in several adhesive appHcations. The tackiness disappears when the polymer dries and this property can be successfully utilized in appHcations that require adhesion only in moist conditions. PEO is also known to form solution complexes with several phenoHc and phenoxy resins. Solution blends of PEO and phenoxy resins are known to exhibit synergistic effects, leading to high adhesion strength on aluminum surfaces. Adhesive formulations are available from the manufacturers. 

A fully automated microscale indentor known as the Nano Indentor is available from Nano Instmments (257—259). Used with the Berkovich diamond indentor, this system has load and displacement resolutions of 0.3 N and 0.16 nm, respectively. Multiple indentations can be made on one specimen with spatial accuracy of better than 200 nm using a computer controlled sample manipulation table. This allows spatial mapping of mechanical properties. Hardness and elastic modulus are typically measured (259,260) but time-dependent phenomena such as creep and adhesive strength can also be monitored. 

A simpler method would be using seratehing knives and making small squares in the surfaee (minimum 100) around I mm for primer and 2 mm for the final surface and applying adhesive cellophane tape 25.4 mm wide, with an adhesion strength of 40 2.8 g/mm then pulling it off suddenly. Not more than 5% of the squares must peel off. 

Fig. 5. Examples of ihe correlation between measured adhesive strength and (l+cos6). (a) Plot of data from Raraty and Tabor [171J for adhesion of ice to various solids, (b) Plot of data of Barbaris [172] for adhesion of a mixture of epoxy and polyamide resin to low density poly(ethylene) treated in various ways. Both figures from ref. [31], by permission.

Fig. 9. The effect of voids due to poor wetting on adhesive strength, (a) The zippering effect of voids aligned in the plane of shear, (b) Macro-voids in the resin formed during the manufacture of a carbon fiber reinforced prepregs. (c) Micro-voids caused by axial crenulations along carbon fiber surfaces.

Mangipudi et al. [63,88] reported some initial measurements of adhesion strength between semicrystalline PE surfaces. These measurements were done using the SFA as a function of contact time. Interestingly, these data (see Fig. 22) show that the normalized pull-off energy, a measure of intrinsic adhesion strength is increased with time of contact. They suggested the amorphous domains in PE could interdiffuse across the interface and thereby increase the adhesion of the interface. Falsafi et al. [37] also used the JKR technique to study the effect of composition on the adhesion of elastomeric acrylic pressure-sensitive adhesives. The model PSA they used was a crosslinked network of random copolymers of acrylates and acrylic acid, with an acrylic acid content between 2 and 10%. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

Table 2 indicates that only the acrylic foam tapes comjjete in adhesive strength comparison in any realistic sense with the conventional adhesives. 

To produce a suitable rubber base adhesive, three key aspects are required (I) tack and wetting properties (2) adhesive strength (3) cohesive strength. 

Adhesive strength refers to the bond produced by contact of an adhesive to a surface. It used to be measured by peeling tests. This ultimate strength depends on temperature, applied pressure and time of contact. 

Cohesive strength is the internal strength of an adhesive or the ability of the adhesive to resist splitting. Unlike tack and adhesion strength, cohesive strength is not influenced by the substrate. 

The properties of the solvent-bome CR adhesives depend on the molecular weight, degree of branching and rate of crystallization of the polymer. The ability of polychloroprene adhesives to crystallize is unique as compared to other elastomers. The higher the crystallization rate, the faster the adhesive strength development. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

Crosslinking has been claimed to improve thermal resistance of the cyanoacrylate adhesive [18]. However, in other reports [6], little or no improvement in thermal resistance of the adhesive was demonstrated by the addition of a difunctional monomer. As seen in Fig. 2, the addition of varying amounts of crosslinker 7 provided no improvement in the tensile adhesive strength of ethyl cyanoacrylate on steel lapshears after thermal exposure at 121 °C for up to 48 h. 

Even low concentrations of DEMM in ECA significantly reduce lapshear adhesive strength, probably because of the reduction in molecular weight of the adhesive polymer. However, even though the initial adhesive strength is lower. 

An example of this improvement in toughness can be demonstrated by the addition of Vamac B-124, an ethylene/methyl acrylate copolymer from DuPont, to ethyl cyanoacrylate [24-26]. Three model instant adhesive formulations, a control without any polymeric additive (A), a formulation with poly(methyl methacrylate) (PMMA) (B), and a formulation with Vamac B-124 (C), are shown in Table 4. The formulation with PMMA, a thermoplastic which is added to modify viscosity, was included to determine if the addition of any polymer, not only rubbers, could improve the toughness properties of an alkyl cyanoacrylate instant adhesive. To demonstrate an improvement in toughness, the three formulations were tested for impact strength, 180° peel strength, and lapshear adhesive strength on steel specimens, before and after thermal exposure at 121°C. 

This increase in polymer thermal stability translates to improved thermal stability of the adhesive, as shown in Fig. 10 for the steel lapshear adhesive strength after thermal aging at 121 °C for 48 h. 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

This difference in reactivity between the different classes of amines explains the difference in the primer performance on polyolefin substrates with ethyl cyanoacrylate-based adhesives [37J. Since primary and secondary amines form low molecular weight species, a weak boundary layer would form first, instead of high molecular weight polymer. Also, the polymer, which does ultimately form, has a lower molecular weight, which would lower adhesives strength [8,9]. 

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