A Little Guide to Resins

Electrolube’s Alistair Little starts a whole new series of Encapsulation Resin Guides starting this month with a ‘back to basics’ piece that questions the core rationale for potting – why and how do we do it, what are we trying to achieve or protect? In future pieces, Alistair will be delving deeper into the subject, examining the resin chemistries in detail, their properties, applications and the processes you should follow in order to achieve a successful outcome.

Alistair Little joined Electrolube as Global Business Technical Director for the company’s Resins Division in January 2016. He currently heads up a team of chemists developing formulations that encompass epoxy, polyurethane and silicone resin systems for encapsulating or potting electrical devices or electronic components in order to protect them against a wide range of environments.

In his previous appointment at Gurit (UK) he held the position of Senior Product Development Chemist, responsible for the development of new epoxy and vinyl ester resin systems for aerospace, automotive, engineering and marine composite systems. Prior to this, he worked for 2K Polymer Systems, where he developed civil engineering and structural adhesives, and at SI Group, where he worked on UV curing and high performance resins. As a chemist for Schenectady-Beck, he was responsible for the development of primary and secondary electrical insulation resins.

Alistair completed his BSc. (Hons) in Chemistry, at the University of East Anglia in 1995. He then subsequently undertook an MSc. in Polymer Science and Engineering, at London Metropolitan University in 2003. His technical competences and academic achievements cover a broad range of related chemistries, including benzoxazine and phenolic resins for high-temperature applications; epoxy, epoxy meth/acrylate, urethane meth/acrylate, silane terminated polymer, polyamide-imide, polyester-imide, polyurethane and unsaturated polyester systems. Alistair has wide experience of working within ISO 9000, 14001 and 18001 accredited systems, and is an expert in vinyl ester resin systems (for which he is in the process of completing a PhD in Polymer Chemistry at Loughborough University). He is a Chartered Chemist, an active member of the Royal Society of Chemistry and a member of the Society of Adhesion and Adhesives.

Resins: why, where and how?
I would like to start this series of encapsulation resin pieces by going back to basics, questioning the core rationale for potting and encapsulation with resins, their fundamental chemistries and how each resin type differs one from the other – indeed, how their individual properties can be exploited to maximise performance under a wide range of environmental conditions.

Let’s start with moisture – the arch enemy of electrical and electronic devices that, quite apart from causing short-circuits, can also lead to premature deterioration of components due to corrosion. Polyurethane resins – usually supplied as two-part products, which, when mixed achieve the desired cure – provide that all-important moisture resistance, as well as excellent electrical insulation, flexibility and good adhesion to most substrates, both metal and plastic. Some polyurethane resin systems provide exceptional resistance to sea water, as well as temperature extremes, making them an ideal choice for marine applications such as sensor encapsulation. A good outdoor example of a polyurethane resin encapsulated component that requires maximum protection against water ingress is an LED lighting unit; these resins are also optically clear and UV stable, making them ideal for the task.

There are one-part moisture curing polyurethane resins available, but these are a no-no if you want to achieve moisture resistance, as moisture penetration is necessary in order to obtain full cure on potting or encapsulation. Some epoxy resins, too, offer good moisture resistance, as well as a fast cure, hardness and high stability, particularly in variable temperature environments. These also adhere well to most substrates, even difficult substrates with a very low surface tension, such as PTFE, ensuring a water-tight encapsulation.

You might also need to protect electrical or electronic components that are likely to come into contact with chemicals, including acids, alkalis, solvents and a whole raft of other substances that pose a threat to delicate circuits and components. Chemical resistance is very much the province of epoxy resins, though some of the tougher polyurethane products – as well as some silicone based formulations – will provide a degree of protection. Epoxy resin products are available to protect electrical/electronic units that undergo frequent or permanent immersion in solvents such as diesel fuel, leaded and unleaded petrol and cellulose thinners.

Protecting components from mechanical shock and vibration may also be on your must-do list. Here, a more flexible cured product is likely to provide the best solution as it adds a level of compliance which will help 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, too, offer excellent flexibility and, in addition, will tolerate high operating temperatures.

One more ‘shock’ that I should mention is thermal shock, which can have a devastating effect on electronic components, shortening their operating lives at best – destroying them completely at worst. The adverse effects of thermal stress can be ameliorated by using two-part epoxy resins, which are best for those applications that undergo severe temperature cycling or which are likely to suffer thermal shock. Some also provide the added bonus of flame retardant technology.

Finally, it’s worth mentioning that, quite apart 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.

When it comes to the choice and applications of resins, there’s a great deal more to discuss, and over the following months I hope to provide some useful tips and design advice that will help you in your quest for reliable circuit protection.

Alistair Little