Adhesive joint strength

Bates, R. (1971) Communication from Evode, Ltd, Stafford, England. Benham, P. P. and McCammond, D. (1971) Plastics Polymers, 39(140), 130. Benson, N. K. (1969) Influence of stress distribution on joint strength, Adhesion—Fundamentals and Practice, UK Ministry Technology, MacLa-ren, London, p. 191. 

Simple design rules for single-lap joints were proposed as a function of the main variables that influence joint strength adhesive and adherend properties, adhesive thickness, overlap, and residual stresses. The main rules are ... 

Adhesives and Sealants. Most industrial adhesives contain surface active components and additives, and air entrainment during their mechanical appHcation can significantly reduce joint strength. Defoamers are usually formulated into adhesives to protect users against such difficulties. Additional benefits, such as improved uniformity of products, increased throughput and reduced labor costs can also result from the use of defoamers during adhesive appHcation. The footwear and nonwoven fabric industries are extensive users of defoamers in this way. 

Another major area of use is in the field of adhesives. The main attractions of the material are the absence of a need for mastication, easy solvation of the polymer, which is supplied in a crumb form, the production of low-viscosity solutions and high joint strength. In conjunction with aromatic resins they are used for contact adhesives whilst with aliphatic resin additives they are used for permanently tacky pressure-sensitive adhesives. In addition to being applied from solution they may be applied as a hot melt. 

Fig. I. Comparison of unprimed and eleetroprimed single lap-shear adhesive joint strengths for steel coupons bonded with imidazole-cured epoxy [43].

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

The relative aggressiveness of the environments proved to be consistent for all substrates, with the room temperature control the least hostile (virtually no loss of adhesion), and the cycle tests the most aggressive (up to 100% loss of adhesion within 60 days). Humidity cabinet exposure and 60°C water immersion yielded very similar values. As a result, for reasons of clarity, only water immersion data is actually presented here. Joint strength data obtained from either the Ford APG or Fisher Body Cycle Tests were identical, and were therefore also represented by one set of data points. The relative aggressiveness of the host environments toward... 

CRS which had been phosphated prior to bonding exhibited a significant enhancement of durability and corrosion resistance under the same accelerated conditions (Figure 4). The crystalline barrier layer restricted the exposure of the metal oxide to moisture by reducing the rate of water penetration at the interface. Even samples exposed to the cycle test were able to maintain failure within the adhesive for up to 10 days, after which varying amounts of interfacial failure were noted. Again, room temperature control samples maintained initial joint strength and failure remained cohesive within the adhesive. 

Samples constructed from adherends which had been alkaline cleaned, lubricated or left untreated exhibited similar joint strength values and durability trends (Figure 10). Adhesive joints placed in the room temperature control environment or the 23 C water bath retained lOOZ and 92% of initial joint strength, respectively. Failure remained cohesive within the adhesive for all of the control samples and for the first 20 days of exposure in the 23 C water bath. After 20 days, some failure began to initiate at both the primer/steel and primer/topcoat interfaces. The adhesive/topcoat interface proved to be more durable than those found between the substrate/primer/topcoat layers. Samples exposed to the more severe salt fog, 60 C water bath and cycle tests were able to retain 70% to 50% of their initial strength over a 60-day exposure period. 

Petterson13 has shown that the use of only 50 percent of the stoichiometric amount of hexamethylenediamine imparts to a cured DGEB A adhesive the maximum butt-joint strength, bulk tensile strength, and flexural moduli, whereas higher proportions of the diamine give lower properties. 

Controlling flow or viscosity is an important part of the adhesive formulation process. If the adhesive has a propensity to flow easily before and during cure, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface and getting too thick of a bond line. These factors could result in localized high-stress areas within the joint and reduction of the ultimate joint strength. 

Since real surfaces are not smooth or perfectly flat and most epoxy adhesives are viscoelastic fluids, it is necessary to understand the effects of surface roughness on joint strength. A viscous liquid can appear to spread over a solid surface and yet leave many gas pockets or voids in small surface pores and crevices. Even if the liquid does spread spontaneously over the solid, there is no certainty that it will have sufficient time to fill in all the voids and displace the air. The gap-filling mechanism is generally competing with the setting mechanism of the liquid. 

Solvents are used, however, for special applications. For example, solvents may be added to reduce viscosity and assist penetration on porous substrates. On certain polymeric substrates, solvents may be added to improve adhesion by assisting the diffusion of the adhesive molecules into the substrate. On nonporous substrates, volatile solvents must be evaporated before cure because the solvent could interfere with the degree of crosslinking, and under certain curing conditions, gaseous bubbles could form in the bond line and degrade joint strength. 

Depending on the substrate, the curing temperatures, and the service temperatures that are expected, the adhesive formulator may want to adjust the coefficient of thermal expansion of the adhesive system. This will lessen internal stresses that occur due to differences in thermal expansion between the substrate and the adhesive. These stresses act to degrade the joint strength. 

The application of corrosion-resistant primers has become standard practice for the structural bonding of aluminum in the automotive and aerospace industries. The adhesive-primer combinations are chosen to provide both maximum durability in severe environments and higher initial joint strength. Improved service life is typically achieved by establishing strong and moisture-resistant interfacial bonds and protecting the substrates surface region from hydration and corrosion. 

There also appears to exist a critical water concentration within the adhesive below which water-induced damage of the joint will not occur. This also infers that there is a critical humidity for deterioration. For an epoxy system, it is estimated that the critical water concentration is about 1.35 to 1.45 percent and that the critical humidity is 50 to 65 percent.39,40 Any loss in joint strength by the absorbed water can be restored upon drying if the equilibrium moisture uptake is below the critical water concentration. 

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