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Proper Considerations When Selecting An Epoxy

Thu, 12/10/2009 - 9:22am
Yitzy Feman, Master Bond Inc.

Industries worldwide employ epoxies for their highly reliable properties, especially those that are hard to come by in alternative solutions. Given the range of considerations involved with epoxy selection, manufacturers today offer a wide variety of solutions, each with optimized, application-specific properties. Consequently, the factors contributing to epoxy choice are vast and complex, and proper choice is implicit of first fundamentally understanding the need, and then moving on to researching the specifications for the appropriate solution. On a broader level, there are several practical considerations that influence selection, each as pivotal as its predecessor when it comes to the pursuit of a suitable system.

Processing vs Performance

These considerations may be segmented into two general categories, those properties needed for processing requirements, and those needed for performance requirements. Curing technique is perhaps the largest factor when it comes to an epoxy's processing requirements. The current industry offers a wide variety of methods, ranging from two component systems that cure upon mixture, to one component heat curing systems, to UV cure systems. Barring certain exceptions, two component systems would generally be preferred for large parts, eliminating the need for energy to heat substrates, and serving as a more cost-effective solution. This benefit of two component systems is not without compromise however, as the use of these compounds may limit production times, so manufacturers concerned about speed and efficiency may find the heat or UV cured systems more suitable. An additional trade off which calls for attention is the fact that systems curing without heat will possess a lower cross link density (essentially the strength of the systems polymeric bands), an attribute which inevitably affects the strength of their physical characteristics. This comes in contrast to heat curing systems which possess very strong bonds and consequently strong properties. However, in line with a strong cross link density which heat cured systems possess comes a tendency for the subject to be more rigid and brittle, with poor peel and impact properties. It should be noted that much technological research has provided innovative methods of overcoming these deficiencies, most notably so with the addition of heat to a room curing epoxy for optimized properties, or the use of additives to strengthen the peel and impact properties of a heat cured system.

The benefits of UV cure systems, as opposed to heat cure systems, lay solely in their superior cure rate. Their ability to cure in seconds has established UV cure systems as the ideal choice for situations requiring a very fast processing rate. However, the limitations of UV cures lay in their inability to affect any cure through opaque substrates, making them an effective solution for transparent substrates only. To overcome this deficiency, manufacturers have developed dual heat/UV cure systems which boast the superior cure speed of standard UV systems without any substrate limitations.

Real Compatibility

An additional processing concern worthy of mention is the epoxies’ viscosity which needs to be compatible with its respective dispensing method and application. When choosing epoxy for bonding applications, the determining factor is usually the level of clearance between the bonded substrates. Applications with a larger gap between substrates will require a thicker formula for an effective bond, as opposed to applications with small clearance between substrates which will require a more flowing system. An added factor is the handling time or working life of the respective epoxy after it has been mixed. This generally is dependent on the amount of time needed to work with and manipulate the components before the epoxy cures completely.

An epoxy's performance requirements are direct reflections of its specific service conditions. To this end, prudent manufacturers take steps to anticipate all relevant variables as a means of ensuring optimal system performance. Any component exposed to a modicum of potentially damaging stresses needs an epoxy with a low modulus of elasticity. Commonly referred to as flexible epoxies, these systems protect components from damaging environments by absorbing any stresses imparted on substrates while retaining high strength bonds. These grades are available for applications exposed to a variety of elements, including low or high temperatures, CTE mismatch in tandem with temperature variations, impact, vibration, shock, and other similar entities.

A contrasting formula yields a rigid epoxy which provides engineers with some properties that flexible epoxies fail to address.  These epoxies are extremely tenacious with superior shear strength, and offer outstanding mechanical support and environmental resistance. This and more has made rigid epoxies the system of choice for critical potting and encapsulation applications. Rigid epoxies have found employment in a wide range of industries, most notably in electrical, medical, electronic, optical, and chemical processing applications.
Epoxies with corrosion resistant properties provide highly reliable protection and have been successfully utilized in a wide range of applications. Depending on the particular chemical concentration as well as the degree of exposure—be it an intermittent splash, prolonged immersion, or vapor—manufacturers will seek systems with varying degrees of resistance.

Corrosion resistant systems are available in a wide range of formulations encompassing all relevant processing and performance requirements. In addition, applications to be used in high vacuum environments, with an emphasis on critical assembly processes in the optical, aerospace, or electronics industry, will most likely need a low outgassing system. Low outgassing epoxies preempt any effects of released contaminants or air bubbles, providing consumers with highly reliable service and performance.

Much research has allowed manufacturers to present engineers with special grades possessing augmented properties. Where conditions necessitate the use of such products, these hybrid epoxies incorporate the use of additives, yielding individualized and application-specific systems by enhancing the strength of a formula's properties. A wide range of additives are available for impressive application compatibility ranging from phenolic additions for high temp resistance, rubber based compounds for high resiliency and peel strength, and selected metals for chemical resistance.

True Understanding

The above points are a very general and selected representation of epoxy, and in no way do they represent the extent of epoxy capabilities, which far surpass the boundaries of this composition. Manufacturers must be cognizant of this reality and invest the necessary efforts in thoroughly researching and understanding the needs of their applications. Such efforts will undoubtedly prove worthwhile, and yield confident employment of epoxy regardless of application.

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