With Epoxy and Polyurethane compounds, the mix ratio is perhaps the single most important processing parameter in terms of obtaining the desired performance parameters. Since the mix ratio is based on material chemistry, any deviation will result in degraded or different cured properties.
There are "variable ratio" Epoxy systems available but each variation will result in different cured properties.
Parts made to stringent performance criteria, inadvertently encapsulated with off-ratio product, should be discarded. In cases where there is less of an importance placed on the encapsulant in the service environment, post-curing for an extended period of time may improve the properties of an off-ratio mix sufficiently to save the finished components. It must be realized however, that post-curing an off-ratio mix will not restore the desired properties and will only work if the mix ratio variation is relatively minor.
Please contact our technical staff for information regarding the possibilities.
Sticky/tacky surfaces also known as blush on a cured epoxy is due to moisture in the atmosphere reacting with uncured hardener at the surface. The blush is cosmetic and does not affect the performance of the cured epoxy below the surface. Some epoxy hardeners are more prone to blush than others. Castings that are thin and require a long time to gel at room temperature are more prone to blush. This blush may not be apparent when the same system is cast thicker. The thicker casting cures faster and as a result, less time is available for surface blush to develop. The best way to eliminate blush when casting thin sections is to warm the substrate or warm the mixed epoxy prior to application.
Introducing mild warming to the mixed system prior to pouring, or pouring the mixture into a preheated unit or mould will shorten both gel time and development of green strength time.
Some Epoxy and Urethane products require post cure in order to develop full strength. There is usually a temperature and a specific length of time specified by the manufacturer for the post cure. These compounds are usually in a "brittle state" just prior to the post cure process and will crack if subjected to mechanical shock or extreme temperature differences.
The post cure temperatures are usually specified on the technical data sheet covering the epoxy or urethane compound. The parts must reach this temperature to complete the chemical reaction and develop full properties.
Some ovens are not uniform in temperature due to "zoning". This means that some epoxy or urethane parts, in one area of the post cure oven, will be subjected to lower temperatures while in other areas of the oven, the temperature will be higher than specified or what is indicated on the controller. These temperature differences could account for various problems in the cured properties of an epoxy or urethane component.
It is advisable to periodically check the actual temperature of the components being post cured to insure that every part in the oven reaches the required temperature. Temperature controllers are only accurate near the location of the thermocouple so do not assume that the whole oven is at a given temperature just because the controller is set to that temperature.
Epoxy and Polyurethane compounds are widely used to encapsulate electronic circuits. Depending on the type of components contained in the circuit and the performance requirements for the finished components, it is critical to select the appropriate formulation. The type of fillers, pigments, thermal conductivity, the electrical properties, the amount of shrinkage during cure and adhesion to containers can all effect the output of an encapsulated circuit.
The most common causes of change in circuit output or failure are as follows:
Incorrect pigments in the formulation Some pigments are more conductive than others. Changes in output can occur when resins containing more conductive pigments come in contact with high impedance devices. Excessive amounts of impurities in the filler or the liquid components of the formulation Conductive impurities in any of the components of the formulation used can effect the output if the circuit especially those containing high impedance devices. Excessive shrinkage during cure Excessive shrinkage during the curing process will result in pressure on the components. As a general rule, filled products and slower curing materials shrink less. Some pressure sensitive components such as discrete FETs may have to be cushioned with a softer material to eliminate this problem. The dielectric strength of the material is too low Especially with high voltage devices, increased leakage currents can cause a change in the output of the circuit. Dielectric breakdown in certain areas could also cause changes in output. The thermal expansion is too great Operation at elevated elevated temperatures will cause the encapsulant to expand. This expansion can cause internal pressures to be exerted on embedded components. Further to this, dielectric heating can cause expansion resulting in pressure on internal components. Components degraded by excessive exotherm during cure Highly reactive materials, especially when applied in larger masses, can generate heat in excess of 1500C during cure. This heat can degrade the components being encapsulated. The probable solution to the above problems is to select a product with the correct filler content deigned to meet the desired operating criteria. In some cases cushioning extremely pressure sensitive devices may be necessary. Slowly curing systems with maximum filler content are best suited to these applications.When post-curing Epoxy and Polyurethane compounds, the post-cure temperature should be a minimum of 10oC ( 18oF) higher than the intended service temperature of the finished component. For large castings or products designed for high service temperatures, a stepped or staged post-cure (gradual increase in temperature) may be necessary. For best results, the part should remain in the mould until all the curing steps are completed. Smaller components can be removed from the mould and stage cured separately in a suitable oven. It is usual to increase the post-cure oven temperature in increments of 20oC, allowing the part to stabilize at each temperature, until 100C above the intended service temperature is reached. Stage curing allows the material to fully crosslink and, at the same time, will minimize any residual stresses that are conducive to crack formation.
If the finished component contains embedded metal inserts, such as in the case of transformer bushings, the final post cure temperature should be above the HDT (heat deflection temperature) of the encapsulating material. This will minimize any residual stresses that are conducive to cracking.
The parts should not be allowed to cool between the post-cure steps since any cooling would be equivalent to thermal cycling an uncured material.
This is a problem frequently encountered potting or encapsulating various components into containers.
Most often the problem is caused by one of the following:
- Surface contaminants left on the container walls. Frequently there is mould release or greasy residue left on the surfaces from the container manufacturing process causing the epoxy or urethane to adhere to the contaminant covering the walls of the container.
- The container has oxidized and the epoxy or urethane has adhered to the oxide layer which is loosely attached to the sides.
- Excessively high cure temperature. Epoxy and urethane products shrink during cure. As a rule, the faster the cure the higher the shrinkage and the product pulls away from the walls of the container.
- Improper mix ratio causing improper cure.
- Insufficient elongation. Elongation is obtained by properly formulating the epoxy or urethane compound to meet the application requirements.
- A large difference in the coefficient of thermal expansion between the epoxy and the container. The greater the difference the more likely that separation will occur with changes in temperature such as thermal cycling.
- The container is manufactured using materials which are difficult to adhere to. High gloss surfaces an some stainless steel materials are examples of this.
