How to Monitor Curing of Adhesives and Sealants? A partially cured or incompletely cured adhesive may contain unreacted compounds. This can result in reduced bond strength and poor environmental resistance and durability. These unreacted compounds can also be a source of contamination as they can migrate out of the "cured" adhesive over time. Such contamination is especially detrimental to electrical, electronic, and optical applications. Cure monitoring of adhesives is the efficiency of processing, for quality assurance, and to study the curing process. The use of cure monitoring has become common because it can provide several important functions including: Prevent waste due to short working life or incorrect processing procedures Control aging the use of an incorrect mixing ratio Determine improper mixing Determine cure point to apply pressure for optimum properties Measure the effect of external environments such as humidity, substrates, etc. on the cure properties of the adhesive Assure batch-to-batch uniformity Knowledge of the degree of cure is essential concerning the final properties the adhesive joint is expected to have. The cure rate is an important factor when the expense of jigs and fixturing equipment is high or fast production rates are critical. It is also used as a quality control test to determine if the curing mechanism within the adhesive has changed from lot to lot or if it may have been spoiled by storage, moisture contamination, etc. There are many different test methods that are used to measure the reactivity or cure rate of the epoxy adhesive. Some of these, such as working life, are very practical and are used to plan the production process. Others, such as exotherm, are used to determine reaction kinetics. Yet others such as thermal analysis, rheology, and dielectrometry are used to characterize the epoxy network as it cures for the purposes of determining the degree of crosslinking and the rheological properties of the curing adhesive. These cure monitoring techniques can generally be separated into two broad categories: indirect methods and direct methods. They can also be performed either on an adhesive test joint, on the actual production joint, or on the bulk adhesive material. Let's review the indirect and direct methods of cure monitoring of adhesives in detail. Indirect Methods of Assessing Degree of Cure Structural adhesives usually require curing in three ways. First is by the application of heat, second by the addition of a catalyst, and third by the addition of pressure. A combination of the three is sometimes required. Indirect methods of monitoring the cure of an adhesive include: Tests on prepared specimens Mechanical and durability tests are used to determine the joint strength after various periods of cure. The measured bond strength values are compared with that of a joint that is known to be fully cured. Mechanical tests relate to the inherent cohesive strength of the adhesive system. Durability tests relate more to the chemical resistance and toughness of the adhesive system. ASTM D-1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and cantilever tests can also be used effectively. These tests are commonly used to determine when an adhesive is fully cured or when the system reaches a "handling" strength so that the assembled product can be moved with moderate care. The measured bond strength values of partially cured test specimens are compared with that of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive and the effect of the joint design and substrates may override the effect of cure development. Working life The working life or pot life of an adhesive is characterized by: The time from when the adhesive is ready for use (i.e., mixed and ready to apply to a substrate). The time the system is no longer usable because the setting mechanism has progressed to such an extent that the adhesive is no longer workable. ASTM D-1338 establishes two procedures for determining working life. One method uses viscosity change, and the other uses shear strength tests as the criteria for determining when the effective working life has expired. Working life is usually determined on a volume of adhesive or sealant materials that is practical and normally used in production. The mass of the tested material must be defined in the test report because many adhesives and sealants have a working life that is dependent on sample mass. Cure rate is very important for sealants as well as adhesives. Often the sealant will be required to function as a barrier or resist the movement of substrates very soon after it is applied. With construction sealants, for example, it may not be possible to delay the environmental conditions until after the adhesive cures. Thus, curing time becomes a critical parameter in selection of the sealant. ASTM C 679 is a method for determining the time that a mechanic can work the sealant into the joint before the sealant starts to skin or solidify. Simple chemical tests The simplest test method to determine the extent of cure is to rub a cotton swab that has been soaked in a suitable solvent (e.g., methyl ethyl ketone) against the surface of the cured adhesive. If the adhesive softens, it is very far from a fully cured condition, and the degree of softness is a very gross indication of the degree of cure. The cotton swab will remove any uncured material. The contents of the swab can be analyzed for traces of unreacted material. Solvent extraction can be used to chemically remove unreacted components from the swab for analytical measurement. Hardness The hardness of the adhesive or sealant itself may be used as an indication of a cure. It may also be used as a quality control check on certain substrates. Hardness may be determined in several ways: Resistance to indentations Rebound efficiency, and Resistance to scratching or abrasion The first method is the most commonly used technique. There are several ways of measuring indentation, but they only differ based on the type of equipment used. Basically, they all measure the size of the indentation produced by a hardened steel or diamond tool under a defined pressure. A durometer is an instrument for measuring hardness by pressing a needle-like instrument into the specimen. Durometers are available in several scales for measuring relatively hard, brittle materials to soft elastomers. The two types appropriate for most cured adhesives and sealants are: Shore Type A and Shore Type D. ASTM C 661 offers a method for measuring the indentation hardness of elastomeric-type sealants. Lower hardness readings than expected may be an indication of under-cure or a formulation change in the adhesive. It may also be an indication of entrapped air in the adhesive or sealant or an unwanted chemical reaction with the environment. Higher hardness readings than expected may be an indication of over-cure. A simple, but not very quantitative, hardness test has been used for hundreds of years - the fingernail indentation test. The indentation that a fingernail makes on the edge of an adhesive bond or the body of a sealant can often be used as an approximate indication of the hardness of the material. https://adhesives.specialchem.com/tech-library/article/cure-monitoring-of-adhesives