Page 517 - Handbook of Adhesives and Sealants

Selection of Adhesives 447

organic adhesives lose some of their strength, and some types soften
or decompose to the extent that they become useless. Up to 160 F,
several types of thermosetting and thermoplastic adhesives may be
used. However, at 250 F, while only a few thermoplastic adhesives will
withstand intermittent exposures at low stress rates, most thermo-
setting adhesives will function satisfactorily under continuous expo-
sure. Above 250 F, only the more thermally resistant thermosetting
adhesives, such as phenolic-nitriles and epoxy-novolacs, will perform
adequately for continuous duty. Some very special and expensive ad-
hesives can withstand 400–500 F for continuous exposure and 600 F
for short periods. Some adhesives have been noted to retain shear
strengths exceeding 500 psi after 1 h exposure at 900 F. The adhesive
engineer must realize that as the temperature service requirement
becomes greater and greater, the number of candidate adhesives will
become less and less. The high temperature resistant adhesives have
onerous curing and production requirements, lack resistance to peel
and cleavage, and are very expensive systems.
Low temperatures cause embrittlement and internal stresses on
many adhesives so that premature cohesive failure of the bond may
occur below 100 F. For subzero temperature conditions, it is prefer-
able to choose flexible thermoplastic or elastomeric adhesive types
that retain some resilience on exposure to cold. Flexible adhesives,
such as urethanes and epoxy nylons, provide shear strengths in excess
of 8,700 psi at cryogenic temperatures as low as 330 F.
Although performance varies considerably, most organic adhesives
are adversely affected by moisture to some extent, especially when
under conditions of stress. Natural adhesives are more vulnerable
than the synthetic adhesives. Adhesives can fail in moist service en-
vironments either cohesively by hydrolysis of the adhesive resin or
adhesively by preferential adsorption of moisture at the interface.
Considerations must also be given to such environmental influences
as chemical reagents, hot and cold water, oils, solvents, hydraulic flu-
ids, chemical atmospheres (e.g., ozone, acidic gases, salt-spray), exte-
rior weathering and aging, and the effects of biological agents (mold
growth, fungi, insect and rodent attack). Also to be considered are the
effects of radiation (sunlight, x-rays, radioactivity, infrared), hard vac-
uum, etc. These environments are especially severe on natural adhe-
sives. Such environments affect the thermoplastic and elastomeric ad-
hesives and have lesser affect on the thermosetting and hybrid
adhesives. However, all potential adhesive systems must be screened
against the harmful effects of these environmental conditions. No ad-
hesive system is resistant to all possible environments. Generally, ad-
hesive systems are resistant to certain environments and less resis-
tant or catastrophically damaged by other environments. So once
again, the adhesive engineer must understand the environmental in-