The Ravages of the Environment

SpecialChem - Dec 18, 2002

In this issue of, there is an article on the resistance of adhesives and sealants to ultraviolet radiation. So, this may be a good time to discuss the effect of the environment on adhesive / sealant joints in general and what the formulators should keep in mind in the development process. One of the hardest questions that I ever had to answer is "how long will that adhesive or sealant last in service?"
In today's editorial, I will attempt to explain why predicting life is so difficult.

For an adhesive or sealant bond to be useful, it not only must withstand the mechanical forces that are acting on it, but it must also resist the service environment or the chemical forces that are also applied. Thus, one of the most important characteristics of an adhesive joint or sealant is its endurance to the operating environment. Strength and permanence are influenced by many common environmental elements. These include high and low temperatures, moisture or relative humidity, chemical fluids, and outdoor weathering.

The effect of simultaneous exposure to both mechanical stress and a chemical environment is often more severe than the sum of each factor taken separately. Mechanical stress, elevated temperatures, and high relative humidity can be a fatal combination for certain adhesives and sealants if all occur at the same time. Add to this the possible cyclic effects of each factor, and one can easily see why it is important to understand the effects of environment on the joint.

Environmental consequences are so severe that it is usually necessary to test preproduction joints, both in the laboratory and in the field, under conditions as close to the actual service environment as possible. The parameters that will likely affect the durability of a given joint are:

  • maximum stress level
  • average constant stress level
  • nature and type of environment
  • cyclic effects of stress and environment (rate and period)
  • time of exposure

For the development of adhesives that can be used in applications where possible degrading elements exist, the formulated adhesives must be tested under simulated service conditions. Standard lap shear tests, which use a single rate of loading and a standard laboratory environment, do not yield optimal information on the service life of the joint. Important information such as the maximum load that the adhesive joint will withstand for extended periods of time and the degrading effects of various chemical environments are addressed by several test methods.

For the formulators, time and economics generally allow only short-term tests to verify the selection of the adhesive system relative to the environment. It is tempting to try to accelerate service life in the laboratory by increasing temperature or humidity, for example, and then to extrapolate the results to actual conditions. However, often too many interdependent variables and modes of potential adhesive failure are in operation, and a reliable estimate of life using simple extrapolation techniques cannot be achieved.

For example, elevated temperature exposure could cause oxidation or pyrolysis and change the rheological characteristics of the adhesive. Thus, not only is the cohesive strength of the adhesive weakened but its ability to absorb stresses due to thermal expansion or impacts are also degraded. Chemical environments may affect the physical properties of the adhesive and also cause corrosion at the interface; however, the adhesive may become more flexible and be better able to withstand cyclic stress. Exposure to a chemical environment may also result in unexpected elements replacing the adhesive at the interface and resulting in a weak boundary layer. These effects are dependent not only on the type and degree of environment but also on the time the bond is in service.

If there is only one parameter that changes due to environmental exposure, then the application of accelerated test techniques and analysis may yield useful information as to service life. For adhesives used in the electrical insulation industry, Arrhenius plots are often used to predict end of life of insulating materials by simple extrapolation. This can be accomplished because insulation life is dependent on temperature and other factors are relatively minor. However, there are a multiplicity of consequences that can occur within the adhesive / interface / adherend and with each having a possible affect on the other.

The formidable task of determining the end-of-life is one of the most difficult challenges in adhesives science and possibly the single item that most inhibits the consideration of adhesives and sealants in structural applications. It is even more difficult for the formulators to create an appropriate adhesive for a specific application without testing it in the exact environment.

Should you have any comments or feedback, please contact me.

Edward Petrie.

To get expert advice on additives for UV resistance, contact us via TechDirect

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