Temperature effects structural adhesives

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittle. Tims, the utility of unmodified common resins as structural adhesives would be very limited For highly cross-linked resin systems to be useful structural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin flexibility7, and thus increases the fracture resistance of the cured adhesive. Recently, structural adhesives have been modified by elastomers which are soluble within the uncured structural adhesive, but then phase separate during the cure to form a two-phase system The matrix properties are mostly retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved... 

The durability of the properties of epoxy adhesives is often limited by aging effects. Depending on the aging conditions, such as humidity and temperature, various structural properties of the adhesives can be strongly influenced. However, the underlying mechanisms in the early stage of aging have not been understood sufficiently. 

The effects of water and temperature on the adhesive itself are also of utmost importance to the durability of bonded structures. In the presence of moisture, the adhesive can be affected in a number of ways, depending on its chemistry and how rapidly the water permeates through and causes significant property changes [51,86-88]. The potential efficacy of moisture penetration on the locus of failure of bonded joints has been discussed in the previous section. As expected, elevated temperature conditions tend to degrade joint strength at a faster rate. 

In summary, the rehabilitation and repairing techniques for timber structures involving structural adhesives should always take into consideration the effect of service temperature on the adhesive performance, thus care should be taken regarding the structural joint design and the adhesive selection. 

The values of Poisson s ratio for some polymeric materials are shown in Table I. The stress-strain characteristics for several cured commercial structural adhesives were determined by Schjeldrup and Jones. These workers found that the modulus, at room temperature, for epoxy-based commercial adhesives varied in the range 322-375 ksi, while the ultimate stresses varied from 6,050 to 7,853 ksi and the ultimate strains varied from 2.4 to 15.6%. Sancaktar et al. investigated the effect of cure schedule on the tensile properties of a commercial structural adhesive. They found that the ultimate stress and modulus were higher for a longer, lower-temperature cure... 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenolic resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be supported by a braid, a scrim, or paper. This does not influence the kinetic results and can be corrected in the calculations of dynamic mechanical properties (qv). Recent DMA work on phenolic resins has been used to optimize the performance of structural adhesives for engineered wood products and determine the effect of moisture in wood product on cure behavior and bond strength (75-77). 

Cruz, H., et al. (2004a). Efeito da temperatura no desempenho de colas epoxfdicas usadas em refor90 estrutural (Temperature effects on the performance of epoxy adhesives used in structural reinforcements). 2nd National Construction Congress, Rethink Construction (CONSTRUQAO 2004), Porto, Portugal. 

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