Thermosets

Thermosets are polymers whose molecular chains are joined by covalent bonds into a three-dimensional network during curing. The reaction runs once and cannot be reversed: heat never softens the material again. In electrical insulation, thermosets appear in four forms, as laminates (paper laminate, fabric laminate, glass-epoxy such as FR4), as moulding compounds (SMC, BMC), as casting and potting resins, and as the binder in mica laminates. This article puts limit temperature, thermal class, dielectric strength, CTI and UL 94 side by side for ten materials in one table and derives the most important rule from it: in a laminate, the carrier sets the temperature limit, not the resin. GOBA punches insulation materials from 0.023 to 3.0 mm thick in part sizes up to 1,000 x 2,000 mm into insulation and moulded parts, including paper laminate into the millimetre range, and slits prepregs based on epoxy, phenolic and polyester resin in widths from 5 mm.

Once cured, a thermoset is a single giant molecule

Before curing the resin exists as a liquid or meltable precursor; afterwards every chain is joined into one molecule. Heat cannot undo that network, it eventually destroys it: thermosets do not melt, they char or decompose. The materials standard calls the group duromers, in electrical engineering the German term Duroplast is common, and English datasheets say thermoset. All three words mean the same thing.

Two reaction routes lead to the network. Phenolic, urea and melamine resins crosslink by polycondensation: under pressure and at 140 to 180 °C they release water and join into the thermoset. Epoxy and polyurethane resins crosslink by polyaddition with a hardener, unsaturated polyester resins by radical crosslinking with styrene. That explains why epoxy and polyurethane arrive on the shop floor as two-component systems, while phenolic resin comes as a moulding compound or as pre-impregnated paper.

The finished network determines every property that makes thermosets interesting for insulating parts:

• Dimensional stability under heat. The material neither creeps nor softens until the network breaks down.
• High stiffness and compressive strength, but low elongation at break. A thermoset barely bends, it fractures.
• Chemical resistance to oils, solvents and many acids, because no solvent can swell the network.
• Low thermal expansion and high volume resistivity, both prerequisites for dimensionally stable support parts in switchgear and motors.
• No weldability and no material recycling. Joints are bonded or bolted, end-of-life parts are ground down and used as filler.

Thermoset and thermoplastic: the difference in one paragraph

The difference lies in a single property, crosslinking. Thermoplastics consist of chains held together only by physical forces; they soften under heat and can be welded, reshaped and reused as regranulate. Thermosets are covalently crosslinked and can do none of that. In return they stay stiff and dimensionally stable under heat while a thermoplastic has long since started to creep. Elastomers form the third group, loosely crosslinked and rubber-elastic. In an insulation system all three work together: the slot liner is a thermoplastic film, the impregnating resin on top of it is a thermoset, the cable grommet is an elastomer. The bonding chemistry of the other side and the data from PET to PEEK are covered in the article on thermoplastics, the application view of both groups in electrically insulating plastics.

Which thermosets matter in electrical insulation?

Six resin families cover practically every thermoset insulation material. Each has a typical form in which purchasing encounters it, and a typical limit that decides the selection.

Polyurethane lives in both worlds: crosslinked potting compounds are thermosets, thermoplastic polyurethanes and PUR foams are not. Bakelite is the historical trade name for phenolic moulding compounds and still stands for the whole group. Fibre-reinforced thermosets such as GRP and CFRP formally belong here as well, but they behave as layered composites and are covered in the article on composite materials.

Paper, fabric and glass laminates: the industrial laminates

Most thermoset insulating parts are made from industrial laminates. A carrier of paper, cotton fabric, glass fabric or glass mat is impregnated with resin, dried and pressed under heat and pressure into sheets or tubes. The designations come from three systems that circulate side by side and mean the same materials.

Pertinax is a brand name for paper laminate, held by Dr. Dietrich Müller GmbH since 2014, and in everyday use it stands for any phenolic paper laminate. FR4 comes from the printed circuit board world: the abbreviation means flame retardant, the flame retardancy comes from brominated epoxy resins, and G10 is the same construction without that additive. Anyone who writes HGW 2372.1, EP GC 202 or FR4 in an enquiry is ordering the same material.

Thermoset data: temperature, dielectric strength, CTI and UL 94

The properties of thermosets can only be quantified per material, because resin, carrier and filler shift the values by a wide margin. The following table brings together the figures that decide material selection for insulating parts.

Limit temperature, thermal class to IEC 60085, CTI to IEC 60112 and UL 94 are taken from the material guide values of the laminate manufacturer Mertl (comco catalogue 2021) and from the DuroBest range by AGK, dielectric strength perpendicular to the laminations from the one-minute proof voltage on the 3 mm specimen to DIN 53481 and IEC 60243-1, for FR4 supplemented by the Dotherm datasheet HGW 2372.1 and the printed circuit board values to NEMA LI 1. The BMC row follows the datasheet of the UP moulding compound Kern UP 4385, the mica row the manufacturer data of Dumico and ISO-tech. All figures are guide values; design work follows the supplier datasheet. Thin sheets reach considerably higher values per millimetre than the 3 mm specimen, and the influencing factors are explained in the article on dielectric strength.

Read row by row, the table is a price list; read column by column, it is a selection rule. The temperature column shows that the carrier sets the limit, not the resin: phenolic resin itself withstands more than 200 °C, the phenolic paper laminate ends at 120 °C because the cellulose embrittles first, and the cotton fabric laminate already at 110 °C. Only glass fabric lifts epoxy to 130 to 155 °C, silicone on glass reaches 180 °C, and mica with 10 % silicone binder 500 to 700 °C. Anyone who needs thermal class F or H is therefore looking for the carrier, and the assignment of classes to temperatures is provided in the article on thermal classes.

The CTI column holds the second trap. Phenolic paper and phenolic fabric laminates sit at CTI 100, meaning material group IIIb to IEC 60664-1, the poorest. Melamine glass laminate, polyester glass mat and BMC reach CTI 600 and therefore group I. At the same voltage a part made of GPO3 may be built with a considerably shorter creepage distance than one made of Pertinax. In a polluted or humid environment the inexpensive paper laminate therefore costs installation space, as the article on the comparative tracking index works out. The UL 94 column, finally, shows that V-0 is not a property of the material class: standard paper laminate and G10 carry no rating, FR4 and FR5 earn it only through brominated resin, melamine and UP glass mat bring it from the resin itself. What lies behind the classes is covered in the article on UL 94.

Casting resins and potting compounds: epoxy, polyurethane and silicone

The second major use of thermosets is liquid: as impregnating resin in the winding, as casting resin for bushings and insulators, and as potting compound around electronics, sensors and coils. The three resin families differ less in electrical strength than in temperature range and hardness.

The figures come from the potting compound comparison published by the Swiss processor Silitech and apply to typical two-component systems. For high voltage, epoxy carries the main load: as casting resin, because it stays hard, crack-free and dimensionally stable after curing, and as impregnating resin in the VPI process, because it binds winding, mica tape and slot insulation into a single block that reaches thermal class F or H. Polyurethane is chosen when the potted assembly sees vibration or sensitive electronics cannot tolerate a hard shell. Silicone remains when 150 °C is not enough or the part must stay accessible after potting. How dipping, trickle and VPI work is described in the article on insulating resin, the design of insulating distances in the article on high-voltage insulation. For heat removal from potted modules the filled variant counts, see thermal conductivity.

Mica laminates: the thermoset as binder

Beyond 200 °C the range of pure resin materials ends and the thermoset changes role. In mica sheets and mica tapes a share of around 10 % silicone or epoxy resin holds the mica flakes together, the remaining 90 % is mineral. The result withstands 500 °C continuously as a muscovite sheet and 700 °C as a phlogopite sheet, at around 25 kV/mm. The resin provides shape and workability, the insulating effect comes from the mica. Where a mica tape sits in a high-voltage winding and when a sheet is chosen is explained in the articles on micanite and mica tape. Glass fibre and mica products for high temperatures are part of GOBA's material range for insulating and moulded parts.

How are thermosets manufactured?

Thermosets are always manufactured in two stages: first a meltable or liquid precursor is made, then it is finally crosslinked in the tool or in the component. Four processes cover electrical engineering.

1. Compression moulding of moulding compounds. Free-flowing phenolic or melamine resin compounds with 40 to 65 % filler are compression or transfer moulded in heated tools at 140 to 180 °C. This produces switches, terminal plates and housings.
2. Laminating. Paper, fabric or glass mat runs through a resin bath, is dried and pressed layer by layer under pressure and heat into sheets or wound tubes. This yields paper laminate, fabric laminate and glass laminate as semi-finished stock.
3. SMC and BMC. Unsaturated polyester resin is premixed with glass fibres and mineral filler into a mat (SMC) or a kneadable mass (BMC) and shaped and cured in a single step in the hot tool.
4. Casting and impregnating. Two-component epoxy, PUR or silicone resins are mixed, poured into the mould or drawn under vacuum into the winding, and crosslinked in the oven.

GOBA comes in behind the second process. The semi-finished stock arrives as sheet or roll and leaves as a cut piece or punched part. The systematic classification of these processes is provided by DIN 8580.

What matters when punching and slitting thermosets

A datasheet says nothing about whether a paper laminate sheet turns into a clean series part. With thermosets, five points decide, and all of them follow from the crosslinking.

• Thickness. According to the manufacturer, paper laminate PF CP 201 punches well up to 2.5 mm. Our punching presses handle material thicknesses from 0.023 to 3.0 mm at part sizes up to 1,000 x 2,000 mm, which takes paper laminate into the millimetre range. Anything thicker is milled or waterjet cut at the laminate manufacturer. We confirm the feasible thickness for your material in the quotation.
• Tooling. Glass fabric is abrasive and eats cutting edges. For glass laminates and mica sheets the solid steel tool with its longer service life therefore pays off, while the more economical steel rule die is sufficient for paper laminate in small and medium runs.
• Fracture instead of flow. A thermoset cannot be creased, embossed, bent or hot formed, it breaks at the bending line. Contours need radii instead of sharp inside corners, holes need an edge distance greater than the sheet thickness. Anything that has to be bent belongs in a thermoplastic.
• Moisture. Paper laminate HP 2061 absorbs 600 mg of water on the 4 mm standard specimen, glass laminate HGW 2372 only 28 mg. Paper laminate is therefore stored dry, and dimensions are checked after conditioning, otherwise the hole no longer fits in summer.
• Dust. Punching and machining phenolic laminates produces fine dust that can release phenol and formaldehyde. Extraction at the machine is mandatory, not optional.

Before curing, thermosets are a different material. Prepregs based on epoxy, phenolic and polyester resin, meaning fabrics pre-impregnated with resin, are slit by us in widths from 5 mm at a width tolerance of plus minus 0.1 mm, without heating the material and with temperature-controlled storage so that the resin distribution is preserved. Details are given under composite slitting. Tooling and processes for sheets and roll stock are described under stamping and forming, and if you want to supply a special material yourself, talk to us about custom products.

Which thermoset should you choose?

For support and separating parts in a dry environment at low voltage we recommend paper laminate HP 2061 as the starting point. It is the least expensive thermoset insulation material, punches well and at 120 °C is sufficient for thermal class E. From there we only move away for a specific reason, and the reasons are in the table above:

1. Moisture, oil or medium voltage involved? Then glass laminate HGW 2372 or FR4. Water absorption drops by a factor of 20, dielectric strength at least doubles.
2. Polluted environment or tight creepage distances? Then melamine glass laminate or polyester glass mat GPO3 with CTI 600 instead of phenolic resin with CTI 100.
3. Flammability requirement to UL 94 V-0? Then FR4, GPO3, BMC or the special grade HP 2062.9, not the standard paper laminate.
4. Thermal class H or more? Then silicone glass laminate for 180 °C and mica sheet for everything above.

We advise against cotton fabric laminate as an electrical insulation material. At 2 to 3 kV/mm, CTI 100 and 110 °C it trails paper laminate in every column; its strengths are gears, plain bearings and guides, in other words mechanical engineering. We also advise against listing FR4 as class F by default: depending on the manufacturer, the limit temperature lies between 120 and 155 °C, and that figure belongs on the drawing, not in an assumption. If you are unsure which material carries your temperature, voltage and environment, we clarify it with you under consulting and service before a tool is built.

Are thermosets too brittle for insulating parts?

The objection comes from assembly: a thermoset breaks when pressed in, a thermoplastic yields, so the film must be the safe choice. The objection hits the material property but not the design. Brittleness is the flip side of dimensional stability, and that is exactly what is needed when a part has to hold its dimensions under heat and pressure: as an insulator, terminal plate, partition, spacer or slot wedge. A PET film creeps away at that point, a glass laminate with 340 MPa flexural strength stays where it is.

Fracture risk almost always comes from geometry, not from the material class. Sharp inside corners, holes close to the edge and press fits without clearance are the three causes we see most often in drawings. With radii, adequate edge distance and a glass fabric carrier instead of paper the problem disappears without losing stiffness. Where a part is creased, bent or slid into a slot, however, the thermoset is the wrong choice, and we recommend a moulded part made from film or laminate from our insulation materials. The real question is therefore: which point in the insulation system has to be stiff, and which one may yield?

GOBA Takeaway

Thermosets are the stiff components of the insulation system: paper laminate, glass laminate and moulding compounds as support and separating parts, epoxy as casting and impregnating resin, silicone and epoxy as the binder in mica. Their selection comes down to four figures, limit temperature, dielectric strength, CTI and UL 94, and the temperature comes from the carrier, not from the resin. Anything that has to be bent belongs in the thermoplastic next to it.

We punch paper laminate and other insulation materials to drawing, slit prepregs to width and carry glass fibre and mica products for the high thermal classes. Send us the drawing with temperature, voltage and environment, and we will tell you which thermoset carries the load and whether it can be punched in your thickness.

FAQ on Thermosets

Which polymers are thermosets?

Thermosets include phenolic resin (Bakelite, paper laminate, fabric laminate), epoxy resin (FR4, casting resin, potting compound), melamine resin, unsaturated polyester resin (GRP, SMC, BMC), silicone resin and crosslinked polyurethane potting compounds. In electrical insulation, phenolic paper laminate dominates for low-cost support parts, epoxy glass laminate for moisture and medium voltage, and epoxy as impregnating and casting resin.

What is the difference between thermosets and thermoplastics?

Thermoplastics are not crosslinked, soften under heat and can be reshaped, welded and recycled repeatedly. Thermosets are covalently crosslinked, stay hard until they decompose and can neither be melted nor welded. In the insulation system the thermoset takes on the stiff support parts and the impregnating resin, the thermoplastic the thin, bendable films.

Is FR4 a thermoset, and what does the designation mean?

Yes. FR4 is a glass laminate made of epoxy resin on glass fabric with a brominated flame retardant, listed as HGW 2372.1 in DIN 7735 and as EP GC 202 in EN 60893. FR stands for flame retardant, and the material reaches UL 94 V-0. Depending on the manufacturer it withstands 120 to 155 °C, reaches CTI 200 and around 10 kV/mm on the 3 mm specimen. G10 is the same construction without the flame retardant.

Can thermosets be punched, bent or bonded?

Punched, yes: according to the manufacturer, paper laminate PF CP 201 punches well up to 2.5 mm, and GOBA punches insulation materials from 0.023 to 3.0 mm thick in part sizes up to 1,000 x 2,000 mm. Bent, no: thermosets break at the bending line and cannot be reshaped even under heat. Bonded, yes: since thermosets cannot be welded, they are joined with epoxy or cyanoacrylate adhesives or bolted mechanically.

Are thermosets recyclable?

Not as a material. The covalent crosslinks remain in place on heating, so there is no regranulate as with thermoplastics. Cured thermosets are ground down and used as filler in new moulding compounds or in construction, and fibre composites are partly separated thermally or chemically into fibre and resin fractions.