A Review of High-Temperature Glass Epoxy Laminate Materials

Materials technology continues to evolve, offering higher performance materials for more demanding customer applications. Often, manufacturers fail to review their specifications to take advantage of these newer high-temperature glass epoxy laminate materials offer improved performance at a lower cost.

The generator stator may use high-temperature glass epoxy laminate insulation wedges as shown below to secure the conducting bars in the slot.

Unfortunately, there is a tremendous amount of confusion in the marketplace regarding insulation materials’ thermal performance. A related document available from The Gund Company, “Thermal Performance of Insulation Materials – A Review of Terminology & Methods,” addresses this topic specifically.

Similarly, there is confusion in the electrical insulation marketplace regarding the names, grades, and industry standards given to these materials. Many companies assign their own names to materials, and it is not clear whether their material meets or exceeds the industry standards. A related document available from The Gund Company, “What’s in a Name – A Review of Material Data Properties,” addresses this topic specifically.

The purpose of this document is to review the common international standards for high temperature, high-pressure glass epoxy laminates. The paper will review the increasing thermal performance characteristics of the grades available today. Lastly, the document will present side-by-side test data for several high-temperature glass epoxy laminate grades illustrating typical high-temperature laminates.


Large steam generators are one example of electrical equipment that relies on relatively high-temperature electrical insulation materials. Most electrical equipment depends on the electrical insulation components’ thermal performance and insulation systems for long service life

Manufacturers and designers of electrical equipment have the challenging assignment of understanding electrical insulation materials’ performance using the data that electrical insulation suppliers provide. This challenge is daunting when presented with global sourcing options from suppliers that may reference questionable data with no referenced standard or without directly comparable results.

The most widely accepted means of validating the performance of insulation materials from a new source is side by side comparative testing using the most commonly accepted international standards- NEMA and IEC. Unfortunately, this type of testing requires time and money, so it is a step that many suppliers often skip. The Gund Company has offered our test lab services to perform comparative side by side testing of materials to validate materials’ performance. Particularly when considering high-temperature glass fabric epoxy laminate options, the world offers many confusing alternatives. The two most commonly accepted international standards for high-pressure laminate materials are the NEMA standard LI 1 and the IEC standard 60893. The table below shows the most common standard glass fabric epoxy grades described in these two standards.

Region  Standard  Epoxy  Epoxy FR  155 °C Epoxy  155 °C Epoxy FR  180 °C Epoxy
Europe IEC 60893  EPGC 201  EPGC 202  EPGC 203  EPGC 204  EPGC 308
North America NEMA Ll1  G-10 FR4 G-11 FR5

The most common glass epoxy laminate specifications are G-10, FR4, G-11, and FR5 provided in NEMA LI-1. There is no “performance requirement” for the temperature index of these materials in the NEMA standard. There is a “general guide” for design regarding a temperature index of 130 °C provided as “authorized engineering information.” Thus, the NEMA specification does not require a temperature index of 130 °C, but it sets the expectation that the material should have that property. This means that a grade of material could be certified to NEMA properties without achieving that temperature index. G-10 and FR4 have a typical temperature index of 130 °C. G-11 and FR5 have a regular temperature index of 155 °C and are thus considered “high-temperature glass epoxy laminates.” FR4 and FR5 both have a flammability rating of V-1 according to the UL 94 flammability test, but both grades commonly have halogenated (common bromine) resin systems to achieve those flammability ratings.

In IEC 60893, the most common glass epoxy laminate specifications are EPGC 201 (G-10), EPGC 202 (FR4), EPGC 203 (G-11), and EPGC 204 (FR5). The IEC standard also includes a Grade EPGC 308, which references a temperature index of 180 °C though the NEMA standard has no equivalent grade. This grade is also widespread, and in North America, it is commonly referred to as “Class H” or “180 °C” G-11. NEMA G-11 was originally developed as a high-temperature version of the glass epoxy laminate, G-10. Essentially, the requirements of G-10 are the same as G-11, with the addition of a 150 °C flexural strength requirement. Similarly, the IEC Grade EPGC 201 (G-10) was given a need for higher flexural strength at 150 °C resulting in the new IEC Grade EPGC 203 (G-11).

Table 2 – Comparison of IEC and NEMA glass epoxy requirements for 3mm (1/8”) thick sheet material

Region  Standard  Epoxy  Epoxy FR  155 °C Epoxy  155 °C Epoxy FR
Min Breakdown
45 kV 45 kV 35 kV 35 kV 35 kV
Max Permittivity/
Dissipation Factor
5.20/.025 5.20/.025 N/A N/A N/A
Min Izod Impact
7.0 ft-lb/in 7.0 ft-lb/in 34 kJ/m2 34 kJ/m2 34 kJ/m2
Min Flex Strength
23 °C (length)
55 ksi
(380 MPa)
55 ksi
(380 MPa)
49.3 ksi
(340 MPa)
49.3 ksi
(340 MPa)
49.3 ksi
(340 MPa)
Min Flex Strength
150 °C (length)
N/A 30 ksi
(207 MPa)
N/A N/A 170 MPa
(24.6 ksi)
Max Water
0.15% 0.15% 22 mg 22 mg 22 mg
Min Thermal
130 °C 155 °C N/A N/A 180 °C

1-All properties are at room temperature unless otherwise noted. Requirements of properties at different temperatures may not be shown. Note that ISO and ASTM test methods may vary slightly. Hence the difference in some properties which cannot be converted for comparison.
2-Note that NEMA does not require a thermal class or thermal endurance. It is only shown as “Authorized Engineering Data” or a typical value.


Initially, NEMA G11 and IEC EPGC 203 manufacturers provided grades with temperature induces of 155 °C. As technology has advanced in the last 10 years, leading North American and European manufacturers began to provide G-11 and EPGC 203 grades with relative temperature indices (RTI’s) very near or beyond 180 °C. These grades no longer carry a premium cost and offer a premium thermal performance level at a lovely price. However, many electrical equipment manufacturers’ have been slow to update their specifications or approve these new materials that offer premium performance at no additional cost.

Some insulation suppliers try to keep up with the higher temperature performance material trend by claiming performance that they have not achieved. This lack of discipline has resulted in confusion regarding the temperature performance of glass epoxy laminates. Lab testing has proven that not all grades of glass epoxy perform equally at elevated temperatures.

For this reason, The Gund Company recommends performance verification through side by side lab testing evaluation. The Gund Company materials lab has tested products claiming to be G-11 from over a dozen different international suppliers. This testing has provided us insight into the variability found when sourcing materials such as G-11 from laminate suppliers worldwide. A high-pressure laminate producer can quite quickly meet the specifications of “G-11” or “EPGC 203” by adding some higher Tg (glass transition temperature) epoxy resin to a standard G-10 formula. This practice can certainly increase the high-temperature flexural strength enough to pass the NEMA and IEC requirements. However, this blending practice in higher Tg resin doesn’t create a thermally stable epoxy thermoset matrix that can withstand high temperatures over long periods of time. The Gund Company lab studies have shown that while most of the “G-11 products” are passing the 150 °C flexural requirements, they do not perform well under thermal endurance testing at elevated temperatures beyond 150 °C
(See table 3).

Table 3 – Comparison Data showing different G-11 or EPGC 203/308 materials using 150 °C flex strength (G-11 min 30 ksi).

Sample G-11 / EPGC 308 

#1 North
#2 North
#1 Europe  #2 Europe  #2 Asia  #3 Asia  #4 Asia   South Am.
ksi  MPa  G-11  G-11  EPGC 308   EPGC 308  EPGC 308 EPGC 203  EPGC 308  G-11
150°C Flex
ksi (MPa)
30 min 170 min 42.9 (296)
32.4 (223)
46.3 (319)
49.6 (342)
38.1 (263)
8.5 (58)
33.8 (233)
26.2 (181)

Many insulation users today are switching to class H (180 °C) G-11 or IEC Grade EPGC 308 for today’s high-performance electrical machines. The Gund Company uses the thermal endurance testing standards ASTM 2304 and UL746E as a guide to qualifying a material as class H (180 °C RTI). According to these standards, the life of insulating material can be defined when a selected property reaches 50% of its original unconditioned value. The standard requires testing at 50 degrees above the RTI rating of the material. The Gund Company tested dielectric and flexural strength thermal endurance for three G-11 materials in the example shown below. The control in the study is a G-11 grade with a known RTI of >180 °C.





The G-11 produced by an Asian laminate supplier (Asia #4) was tested under the same thermal endurance conditions and only had 34% retention of its original dielectric strength after six days (144 hours). The flexural strength retention was even worse at 12% of its original strength. The picture below illustrates the delamination of this product due to resin degradation.

The control samples show no signs of delamination or warping after 34 days of exposure to 230 °C.

Figure 3 – Thermal endurance sample of Asian produced G-11 after only six days of testing at 230 °C (single piece).

The Gund Company seeks out and qualifies the best global suppliers of electrical insulation
materials to offer those materials to our customers. Our latest G-11 offering provides the highest
mechanical, electrical, and thermal performance in the industry at an economical price. The Gund Company’s Grade G-11 can often replace polyimide and BMI (Bismaleimide) resin thermoset laminates at a significant cost savings. The Gund Company’s G-11 can provide higher performance and can be substituted for EPGM203, EPGC 203, EPGC 308, and standard G-11 at no additional cost in most cases.

Figure 4 – Control material after 34 days of treatment at 230 °C (Four pieces stacked).

Although G-11 is produced to a standard, the performance between one G-11 and another can often be significantly different. By asking for Class H (180 °C) G-11 or EPGC 308, the user can ensure they are getting the best quality G-11, which will have the highest thermal performance. This performance can be achieved at a reasonable cost compared to historical high-temperature electrical insulation materials.

  G-11 Polyimide
EPGM 203  EPGC 203  EPGC 308
Flex Strength (at 23 °C) 486 MPa

400 MPa 400 MPa 380 MPa 350 MPa 400 MPa
Flex Strength (at 150 °C) 276 MPa

200 MPa 200 MPa1 190 MPa 207 MPa 200 MPa
Breakdown Voltage >50 kV

45 kV 49 kV 50 kV 50 kV 50 kV
Dielectric Strength 22 kV/m 20 kV/m 15 kV/m 20 kV/m 15 kV/m 13 kV/m
Permittivity @ 1 MHz 4.5 4.8  4.66  <5  <5.5  4.6
Relative Temperature Index 180 °C  200 °C  180 °C  155 °C  155 °C  180 °C

1– tested at 200 °C


Click here to view the NEMA Grade G-11/G-11H Materials Datasheet

Click here to view the paper as a PDF


The Gund Company is a vertically integrated manufacturer and fabricator of engineered material solutions. Since 1951, we have listened to our customers and learned about their industries’ demanding operating environments. We are AS9100D Certified and ITAR Compliant. Our custom fabricated parts are manufactured according to ISO 9001:2015 certified quality systems.

We understand the challenges of material selection and the demanding operating environment of your application. Our Application Engineering Team takes a consultative approach to understand your requirements. By relying on our material specialists, our customers gain valuable insight into improving component designs to increase efficiency and functionality while reducing cost. In addition to helping with material selection, we challenge ourselves to optimize production for material yield or fabrication efficiency. As a lean enterprise, we focus on continuous improvement and finding the most cost-effective, efficient solutions for our customers.

Please contact us today if we can help answer material property questions or provide an application-specific quote. Thank you for the opportunity to earn your business.



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