Adhesion environmental factors

A typical environmental factor which effectively limits the use of possible adhesives for a certain application is the service temperature. In outdoor applications the existence of high humidity may also restrict the number of possible adhesives. Environmental factors such as temperature, humidity and chemicals may have a severe degrading effect on the adhesive properties and, in particular, the creep properties. 

Environmental Tests. It is desirable to know the rate at which an adhesive bond will lose strength due to environmental factors in service. Strength values determined by short-term tests do not give an adequate indication of an adhesive s performance during continuous environmental exposure. Laboratory-controlled aging tests seldom last longer than a few thousand hours. To predict the permanence of an adhesive over a 20-year product life requires accelerated test procedures and extrapolation of data. Such extrapolations are extremely risky because the causes of adhesive bond deterioration are complex (see Sec. 15.2.2). Unfortunately no universal method has yet been established to estimate bond life accurately from short-term aging data. 

Wear is the process of physical loss of material. In sliding contacts this can arise from a number of processes in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. Wear occurs because of local mechanical failure of highly stressed interfacial zones and the mode of failure is influenced by environmental factors. 

Many environmental factors such as moisture and temperature determine the long term adhesion of polysulfide sealants. It is necessary that the substrates be properly prepared surfaces must be clean and structurally sound. To wet the surface and permit the adhesion additives to migrate to the surface, the sealant must have the necessary rheology and cure properties. 

Some accelerated tests are surprisingly simple and intended to give only highly qualitative information, while others have been formulated into standard tests intended to yield more quantitative results. Since heat and moisture, to which adhesive joints are commonly exposed, are environmental factors known to greatly influence adhesive durability, most accelerated tests involve these two agents. 

Besides the environmental factors mentioned above, the materials involved in a structural joint also influence bond strength and durability. The factors in the material category include the adherends the adhesive the design of the joint absence of surface contamination (including contamination with wood extractives) stability of the adherend surface the ability of the adhesive to wet the surface, and entrapment of air/volatiles. Thus, the condition of the adhesive/adherend interface becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint [31]. 

Durability of adhesive materials is affected by environmental factors. The specific environments of concern are (1) extreme high temperatures, (2) extreme low temperatures, (3) extreme high humidity, (4) salt water, (5) fire, (6) corrosive gases or liquids, and (7) external stresses. A detailed discussion of the first five factors is given. Future research needs about these factors are also suggested. 

Environmental factors, such as temperature, moisture and chemicals, may affect adhesive mechanical properties considerably. 

Evidence suggests that the failure behavior of thermoplastics is much the same as for thermoset composites. High joint efficiencies can be obtained with suitable consideration to the joint design, fastener type, and environmental factors. In addition to mechanical and adhesive joining, thermoplastic composites can also be heat welded depending on the concentration and type of fillers within the composite. 

How long an organic coating will be serviceable is dependent upon the durability of the coating itself and its adhesive ability on the base metal. The former is the stability of the coating layer as exposed to various environmental factors, and the latter is determined by the condition of the interface between the organic film and the substrate. 

Taken together, these studies suggest how the ability of PEG-containing PUs to reduce bacterial adhesion can have variable degree of success depending on PEG molecular weight, bacterial species involved, and environmental factors (culture medium). 

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