Epoxy resins, epoxy glues, epoxy adhesives (whatever name you prefer) are excellent potting compounds for filters.
The picture shows cross-section of an oil filter potted with epoxy.
When considering using a potting compound for filter end caps, there are several basic factors to take into consideration:
This is not all you have to think about, but it is a good start when you are in the early throes of considering epoxy resins for potting filter end caps or casting end caps.
Filter media varies greatly – what material is it made of? Is it pleated? How dense are the pleats? What is the porosity like? Will the epoxy resin flow around the media sufficiently to fill all the gaps before it starts to cure?
Epoxies are renowned for their excellent chemical resistance, making them the ideal choice for filter potting for oil, fuel, and chemical filters. They can also be used for air filters – often low cost polyurethane compounds are used for disposable air filters but for filters with more rigorous requirements, epoxies are selected due to their high strength adhesion, low shrinkage, and good environmental durability.
Cure speed is a juggle. For an efficient production line, a fast cure can be desirable. However, if large or deep filters are being filled or cast, a slow setting two-part epoxy is a more sensible option because a) you need plenty of time to mix bulk and b) you don’t want the material to exotherm. When epoxies are mixed they are exothermic – giving off heat when they react. Slow setting epoxies aren’t as reactive, minimizing the heat given off during the curing process.
Other important considerations include:
-Adhesion strength to the end cap material and whether any surface preparation is required
-Adhesion strength to the filter media
-Method of application – guns, bulk mixing, manual, or automatic
-Whether any approvals are required – medical filters may require an ISO10993 or USP Class VI approved adhesive. Food and beverage filters may require an FDA approved epoxy.
-Colour – occasionally, there is some preference. Normally, two-part epoxies have different colored resins and hardeners so that when you mix them, they can be cross-pigmented, and you can ensure proper mixing (no streaks or color variations are visible).
-Shore Hardness – epoxies are normally measured on the Shore D scale, although there are some softer products available these days which are on the Shore A scale. Shore Hardness is measured with a durometer on a disc of cured material.
-Price (of course, is a major factor). If you’re tempted to go for the cheapest option, you need to consider the consequences of a filter failing – a faulty oil filter can stuff an engine. A dodgy vacuum cleaner air filter might be less important. If a sewage filter failed at a treatment plant, I wouldn’t want to be the maintenance person having to remove and replace it!
There are so many different types of filters out there with different technical requirements that it is rare to be able to supply a one-product-does-all type epoxy. Generally, it is necessary to tweak things like viscosity and cure speed to suit a particular manufacturer’s production line.
It can be hard to mix bulk epoxy without entrapping air. Many filter manufacturers opt for cartridge packaging with mixing nozzles or automatic measuring / mixing / dispensing equipment. However, some – particularly for casting large filter end caps, measure and mix bulk epoxy in pails. This can cause air entrapment, but if you have an epoxy that has low enough viscosity and a slow enough cure, any bubbles can rise up to the surface. Other methods to remove air bubbles are to place the filter in a degas chamber or on a vibration table. The bubbles soon work their way to the top. If bubbles are sitting on the top but not bursting (which is often the case because of the surface tension), this can easily be remedied by quickly passing a hot air gun over the surface.
If using epoxy as a casting resin, check that it is fully compatible with your silicone mould or any release agents you might be using.
Speeding up epoxy cure – you may decide you want to chivvy things along a bit. Increasing the curing temperature by 8 degrees C will approximately halve the cure time. BUT you do run the risk of triggering an exotherm if you have a large volume of mixed adhesive. It is important to note variations in cure speed depending on the working environment. Working in a cold factory during winter will cause the adhesive to cure more slowly.
For further help and information about Permabond adhesives for filter potting, please contact Permabond. We offer an excellent custom formulating service to match your adhesive requirement.
