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- The faster the gel time the higher the exotherm.
- Thick sections have higher exotherm.
- The higher the mass, the higher the exotherm.
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Assuming that the supplier has the products listed is not an adequate defense with the EPA.
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Unlike Polyester materials, adding more or less hardener will not speed up or slow the reaction but will cause degradation in the cured properties.
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Epoxies can be formulated to yield a variety of hardness, down to the Shore A range with elongation up to 100%. However, if a product turns out to be softer than indicated on the technical data sheet, there is a good chance that it was not mixed in accordance with the stated mix ratio.
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The HDT is the temperature where the material begins to lose its mechanical strength however, this is only significant in applications where mechanical strength is required at that temperature. In other cases, such as potting, where mechanical strength may not be necessary at elevated temperatures a material with a lower HDT may be sufficient.
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At this point the linear molecules have become large enough to become solid at room temperature, but the product does not have significant mechanical properties. Depending on the product, the gelled material may have developed enough properties to be handled in production but not enough for its intended end use. In addition, a material in its gelled state is vulnerable to stresses caused by thermal cycling. Inadvertent thermal cycling can occur during processing if the part is allowed to cool prior to post cure.
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Embedded sharp corners represent areas of concentrated stress. Small cracks are initiated during gellation as the material shrinks. Subsequently, these micro cracks will propagate during thermal cycling.
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The viscosity of epoxy resins decrease rapidly with temperature. Lower viscosity will aid in de-airing products while resulting in shorter pot life and faster gellation. As a rule the curing reaction will double (the pot life is halved) for every 100C rise in temperature.
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Thorough and complete mixing of two component materials is extremely important to avoid either resin rich or hardener rich areas in the mix. Under catalyzed or over catalyzed areas contain excess, un-reacted material resulting in soft spots. If the components are mixed by hand or drill it is important that the material from the sides and bottom of the container is properly mixed in. If the materials are mixed with automated equipment, it is important to have the correct mix head (the correct number of elements) and the mix ratio is checked frequently.
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The first shrinkage that occurs is during cross-linking as the molecules attach to each other and take up less space. More significantly, the greater shrinkage occurs due to thermal contraction while cooling down from the exothermic reaction. As a rule, slower reacting or products containing large amounts of fillers shrink less.
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Almost all epoxy and polyurethane compounds pass through a stage, just before the molecules completely align themselves, during which the product is extremely brittle. Depending on the type of material, this stage is passed either with time or the application of additional heat such as post cure. A component or module will be prone to cracking if subjected to undue stress, thermal cycling or thermal shock immediately after the gellation phase.
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It often happens, during material selection, that a product is ruled out because the published dielectric strength is lower. In fact, the published figure is just about meaningless unless the thickness of the test specimen is also stated. A thin test specimen will always yield higher figures because there are less impurities contained within the structure. In order to compare "apples to apples" the two specimens have to be tested in the same thicknesses under the same conditions.
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Without question, unfilled product are most suited to automated equipment. However in most cases they are not suitable for the part under consideration due to their inherently higher shrinkage and lower thermal conductivity. Materials containing fillers can be formulated for reduced wear on automated equipment by employing "non-abrasive" fillers. Crystalline fillers will cause the most severe wear in dispense cylinders while "Amorphous" fillers are the least damaging. It is important for the supplier to know how the material will be dispensed in order to provide the most appropriate material for the process at hand.
