Page 110 - Handbook of Adhesives and Sealants

78 Chapter Two

Figure 2.15 Competition between an adhesive and other chemicals for surface sites
leading to displacement of the adhesive from the surface. (a) Adhesive adsorbed at
surface sites. (b) Adhesive displaced from surface sites. 21

vided into three periods: application of the adhesive, setting, and ser-
vice. The behavior of each of the five layers during the three periods
indicates that there may be up to 15 different mechanisms occurring
during the life of a single joint. All of these could affect the perform-
ance characteristics of the joint.
The possible combinations of adhesives, adherends, stresses, and
environments are so great that reliable adhesive strength and aging
data are seldom available to the design engineer. Where time and
funds permit, the candidate adhesive joints should always be evalu-
ated under actual or simulated operating loads in the intended envi-
ronment.
Environmental aging generally causes: additional internal stresses
on the adhesive bond; degradation of the adhesive, adherend, or in-
terphase area; or creation of new interphase regions. The service en-
vironment may produce additional stress on the adhesive bond that
could contribute positively or negatively to the stresses that are al-
ready inherent in the joint.
Consider, for example, aging of an adhesive or sealant joint at mod-
erately elevated temperatures. The adhesive or sealant, depending on
the actual chemical composition, could shrink due to loss of plasticizer
or increased crosslinking. It also could increase in modulus of elastic-
ity for the same reasons. However, if the glass transition temperature