Selecting resins for effective resistance in end-use environments
31 October 2021
Electrolube IoT smart car park
When embarking on resin selection, there will be critical points that arise for effective resistance in end-use environments. Here, Beth Turner, Senior Technical Manager & Resins Specialist at ELECTROLUBE, part of MacDermid Alpha Electronics Solutions, takes a closer look at the fundamental chemistries of potting & encapsulation resins, examining how each type offers individual properties that can be exploited to maximise performance under a wide range of environmental conditions.
A good starting point is to look at the arch enemy of electrical and electronic devices, the dreaded ‘moisture’. On top of producing short-circuits, moisture also causes corrosion, which leads to premature deterioration of components. So, what resin is best suited to combatting moisture? Polyurethane resins! These are usually supplied as two-part products that achieve the desired cure when mixed, and provide that all-important moisture resistance, as well as excellent electrical insulation, flexibility and good adhesion to most substrates, both metal and plastic.
For more intensive moisture resistance, such as marine applications, there are polyurethane resin systems available that provide exceptional resistance to sea water and temperature extremes, making them an ideal choice for marine applications such as sensor encapsulation. Another good example of a polyurethane resin encapsulated component that requires maximum protection against water ingress outdoors is an LED lighting unit; these resins are also optically clear and UV stable, making them ideal for the task.
There are some epoxy resins that offer good moisture resistance, as well as a fast cure, hardness and high stability, particularly in variable temperature environments. These epoxies also adhere well to most substrates, even difficult substrates with a very low surface tension, such as PTFE, ensuring a water-tight encapsulation.
By way of example, Electrolube was challenged to provide a protection solution for a smart car park, where protection was needed to pot domed shaped sensors within a car park floor. The sensors not only had to withstand cars driving over them and be resistant to temperature extremes and water ingress, but also allow effective RF communication, whereby the signal can travel through the sensor as well as the resin.
Beth Turner MSc (Chemistry), MRSC, Senior Technical Manager & Resins Specialist at ELECTROLUBE
The customer initially approached Electrolube following issues due to the unit’s current material not being tough enough to withstand the impact of cars driving over it. Additionally, the surface energy of the housing material was too low to allow a good wetting of the existing resin and subsequently evaded good adhesion. This caused the unit material to bend and, despite a reasonable adhesion of the resin, they found delamination and water ingress inside the unit.
The desired resin needed to be low viscosity to fill all gaps in the small unit, show good adhesion to the material of the unit and provide protection at low temperatures during the winter and high temperatures during the summer.
Consequently, the unit’s design, hardness and material for the housing was changed. Electrolube provided the UR5118 polyurethane resin to avoid RF interference and provide a dielectric constant of between 3-4 @50Hz. The UR5118 has a viscosity of 2300cP @ 23°C and 860cP @ 60°C ensuring sufficient flow in small gaps. It has a wide temperature range -60 to +125 °C and a low exotherm (<35°C). The reformed unit was tested with the new UR5118 resin and passed all tests.
Electrolube UR5118 PU Resin
You might also need to protect electrical or electronic components from encountering chemicals, including acids, alkalis, solvents and other substances that pose a threat to delicate circuits and components. Chemical resistance is very much the territory of epoxy resins, although some of the stronger polyurethane products, as well as some silicone-based formulations, will provide a degree of protection. Epoxy resin products are effective at protecting electrical/electronic units that undergo frequent or permanent immersion in solvents, such as break fluid, antifreeze, unleaded petrol and cellulose thinners.
If protecting components from mechanical shock and vibration are a concern, then a more flexible cured resin is likely to be the best solution, as it adds a level of compliance that helps to insulate the potted components against adverse mechanical movements. In addition to their moisture resistant properties, polyurethane resins also provide this desired level of flexibility, and so they are often the first choice for shock and vibration protection. Silicone resins also offer excellent flexibility and, in addition, will tolerate high operating temperatures.
Thermal shock can have a devastating effect on electronic components, shortening their operating lives at best and completely destroying them completely at worst. The adverse effects of thermal stress can be vastly improved by using two-part epoxy resins with a low Coefficient of Thermal Expansion (CTE), which are the best solution for those applications that undergo severe temperature cycling or which are likely to suffer thermal shock. Additionally, some epoxy resins also provide the bonus of flame retardant technology.
Lastly, it’s worth citing that, aside from providing all the protections listed above, opaque potting and encapsulation resins also conceal what lies beneath. This could provide an effective foil against counterfeiters or those wishing to copy a circuit layout, helping you to protect your intellectual property.
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