In many cases, it is desirable to de-air the material after mixing. The receptacle is placed in a chamber and subjected to a vacuum of 2 millimeters of mercury (29.8˝ hg) for approximately three to five minutes. The vacuum should be held for a minute or two after the collapse of the foam head while minor bubbling continues and then slowly released.
Some materials will need to be blanketed with nitrogen or argon to ensure that moisture doesn’t contaminate the material. (Anhydrides, Urethane Resins and Isocyanates are some of the common materials that need to be nitrogen layered.)
If a material has separated in the container, the material will need to be re-dispersed before using. Using the material in a settled state may cause issues with cure rates and may inhibit cure completely. Most materials can be remixed by simply using a metal spatula to agitate the material until uniform. If the material is heavily filled, then a paint shaker or mixing blade with a drill or drill press may be needed to properly re-mix filler into the system.
Epoxy resin-hardener ratios are proportioned by weight. In most cases, these ratios are stoichiometric and must be strictly adhered to. For high rates of production, automated meter-mix equipment is a necessity. Small batches are weighed out on a balance scale in a clean container with ample room for mixing. Hardness may vary if the recommended mix ratio is not used.
Silicones and urethanes – mix ratio accuracy is necessary to ± 2% to ensure proper cure and de-mold times. Hardness may vary if the recommended mix ratio is not used.
Durometer hardness is used to determine the relative hardness of soft materials, usually plastic or rubber. The test measures the penetration of a specified indentor into the material under specified conditions of force and time. The hardness value is often used to identify or specify a particular hardness of elastomers or as a quality control measure on lots of material.
The CTE is the coefficient of thermal expansion of a cured material. It is expressed as the amount of linear expansion per degree of temperature. Example: If a material has a CTE of 30 ppm/C, what is the expansion of this material for a 10.0 mm long piece that is taken from 25C to 125C? The material will expand by 0.03 mm, which is 10 mm x 100C x 0.000030 mm/mm per degree C. The expansion rate is determined by the chemistry of the product and the filler content. In general, flexible products have a higher expansion rate and higher filled products have lower expansion rates. The CTE of a material will be lower at temperatures below its Tg and will be higher at temperatures above its Tg.
The use of heat will accelerate the cure of most products; epoxies, silicones and urethanes included. You need to be aware of the mass of material that is being cured to make sure the material does not gel too quickly or generate an uncontrolled exotherm. If the material is gelled too quickly or at a high temperature you may run the risk of increasing the stress due to shrinkage. This applies to epoxies and urethanes but usually not to silicones. For example, in automotive and industrial products, LORD 600 Resin and 64 Hardener can be cured for 2 hours at 65C in small devices, as compared to the typical 24 hours at 25C recommended for larger mass encapsulant applications. Always consult the technical data sheet of your materials to see the suggested cure schedule.
In some cases, adding more hardener may speed up the reaction but it is not recommended because the excess of the unreacted hardener will reduce the integrity of the final cured product. Mild warming is recommended to speed up the reaction, not altering the mix ratio.
Depending on the chemistry employed, some epoxy, urethane and silicone products are more tolerant to slight variations in mix ratio than others. Generally speaking, ± 5% by weight accuracy should yield satisfactory results in most cases. Since the mix ratio is determined by product chemistry, based on the desired degree of cross-linking, it is best to be as close as possible to the stated ratio in order to obtain the optimum cured properties. Some formulations are designed to be very forgiving in the mix ratio and can be varied by 10 – 50 %. The final properties of these materials will vary with the mix ratio used. Ask your material supplier about the specific material properties needed.
You can reduce the amount of entrapped air by warming materials prior to application or by vacuum degassing the materials before use. If you are using a mixing method that traps air bubbles during mixing step, you should vacuum degas the materials after mixing. Another way to reduce air bubbles in potted electronic devices is to preheat the device before potting. By having the electronic device preheated, it lowers the viscosity of the potting material during the potting process without shortening its working life.
The shelf life of a product is the time that the material is usable when stored under proper conditions. This will vary per material and per storage conditions. It is usually defined as the time in storage that a material will still meet all of its original specifications. The working life and/or pot life refers to the time a two-component material is usable after the two parts have been mixed. It is also the time that a refrigerated one-component material is usable at 25C. The exact definition may be change from product to product but it is usually determined by an increase in viscosity which makes the material not usable under standard process conditions.
Store material per your supplier’s recommendations. Normally materials should be stored away from outside walls and should not be in direct contact with concrete floors. Both can act as heat sinks and reduce the temperature of the materials, thus making it more difficult to use. Also, do not store materials near heat sources unless recommended by the supplier. Caps and lids for containers should be cleaned before replacement and very tightly fitted or screwed to prevent the infiltration of air. Rotate stock on a first in/first out basis.
The shelf life of some materials may be improved by refrigeration prior to being opened. Many one-component, heat-cured epoxy and silicone materials are refrigerated or frozen to extend their shelf lives. Some systems that are low viscosity mixtures of polymers and inorganic powders can benefit from cold storage to delay the separation of the mixture (settling). The chance of crystallization may also occur with refrigeration. This may not be noticeable during storage but may affect the material performance during use. Please consult the package labeling or technical data sheet for proper storage conditions.
Most organic solvents are effective in removing uncured materials. Acetone and isopropanol are most commonly used. Once a material is cured it is difficult to remove. Soft materials such as urethanes and silicones can be mechanically removed by scrapping. Hard material such as epoxies may require special methods to remove. Contact your supplier for suggestions.
Specific electronic devices may need UL recognized components as encapsulants or adhesives. These are typically devices for indoor consumer use where flame resistance and electrical insulation are needed. A typical part would be a power supply or power converter. The most common UL requirement for electronic devices is flame resistance detailed by the test method UL 94. Other electrical insulation tests are sometimes needed for higher voltage applications.