How To: Optimizing the Glass Transition Temperature (Tg)

Tech Tip

When a polymer system such as an epoxy adhesive, potting compound or sealant is heated, significant changes occur in mechanical strength properties and thermal, electrical and chemical resistance which do not necessarily recover upon cooling. While these changes generally take place over a limited temperature range, a single temperature called the glass transition temperature (T_g) is generally selected for convenience.

Polymeric compounds, which are exposed to a temperature below the T_g, exhibit much higher physical strength and stiffness, as well as greater electrical insulation, dimensional stability and chemical resistance than they do above their T_g. Furthermore, performance upon structural loading is greatly enhanced. For convenience, the T_g is often considered the maximum sustainable operating temperature for a polymer system, especially in structural applications.

Epoxies with High Glass Transition Temperatures The T_g is measured by methods such as Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analyzers (DMA) or Differential Thermomechanical Analyzers (DTA). It is greatly affected, not merely by the chemical composition of the epoxy compound, but by cure conditions such as time, temperature, specific response to heating, amount of load applied, degree of orientation and rate of testing. For many applications the Heat Deflation Temperature (HDT) (ASTM D 648-21, 1978) is utilized in place of T_g. This method employs castings with an applied load of 1.8 MPa (264 psi).

The T_g of epoxy adhesives, sealants and potting compounds varies from as low as 50°C to above 250°C. This offers a wide range of properties to best meet specific service conditions. The table below shows the T_g for some of the most widely employed Master Bond epoxy systems:

Glass Transistion Temperatures For Selected Master Bond Epoxy Compounds
Product	T_g(°C)	Compound Description
EP21	50-80	Two component, room temperature curing epoxy for general purpose, high performance bonding, sealing and coating. For service from -60°F to +250°F.
EP30	80-120	Two part, low viscosity epoxy for coating, bonding and potting. Optically clear. Excellent insulator. For service from -60°F to +250°F.
EP42HT-2	120-150	Low viscosity, room temperature curable, two component epoxy system. USP Class VI biocompatible. Withstands repeated autoclaving. Serviceable up to 450°F.
EP62-1	120-160	Two part, heat and chemically resistant, oven curing epoxy. Superior electrical insulation properties. Serviceable from -60°F to +400°F.
Supreme 10HT	130-160	One part, heat cured (250-300°F) epoxy system with very high shear and peel strength and superb temperature resistance. Serviceable from cryogenic temperatures up to 400°F.
EP13	160-180	One part, oven cured system (cures at 300°F) with excellent serviceability at higher temperatures, up to 500°F.
EP121CL	200-250	Two part, electrically insulative epoxy with excellent thermal stability and outstanding dimensional stability. For coating, sealing, potting, encapsulation and impregnation. For service up to 500°F
EP46HT-1	230-260	Two part structural epoxy offering temperature resistance up to 550-600°F. Requires oven curing at 250-300°F. Unsurpassed chemical resistance.

Careful selection of suitable T_g values for a proposed application is important to optimize the performance of an epoxy. Master Bond’s experienced technical staff will be glad to help you select the epoxy compound with the ideal blend of properties for your particular application.

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Featured Products in this Tech Tip

EP21

Moderate viscosity room temperature curing epoxy adhesive, sealant, coating, encapsulant. Convenient one to one mix ratio, weight or volume. Mix ratio can be altered to give a more rigid or flexible cure. High strength system. Withstands exposure to many chemicals. Reliable electrical insulator. Serviceable from -60°F to +250°F.

EP30

Low viscosity two part high performance epoxy resin system. High strength rigid bonds. Low linear shrinkage after cure. Superb optical clarity and light transmission properties. Serviceable from -60°F to +250°F. Cures readily at room temperature. Withstands 1,000 hours 85°C/85% RH.

EP42HT-2

Bonding, sealing, coating, potting compound. Heat, chemical, steam resistant. Superior optical transmission properties. Solvent free. Withstands exposure to acids, alkalis and many solvents. Castable up to 2-3 inches. Cures readily at ambient temperature. Service temperature range -60°F to +450°F.

EP62-1

Cures rapidly at moderate elevated temperatures. Superior resistance to moisture. Two part epoxy has long pot life at ambient temperatures. High bond strength properties. Reliable electrical insulator. Serviceable from -60°F to +450°F. Withstands aggressive chemical exposure. Tg 160-165°C. Shore D hardness >80.

Supreme 10HT

One part structural epoxy with high bond strength and excellent adhesion to similar and dissimilar substrates, especially composite materials. Requires oven curing at 250-300°F. NASA low outgassing approved. Serviceable from 4k to +400°F. Tough and durable. 85°C/85% RH resistance.

EP13

One component, heat curable epoxy adhesive. Serviceable from -60°F to +500°F. Paste viscosity. High tensile shear and compressive strength. Chemical resistant. Machinable.

EP121CL

Low viscosity epoxy with an extraordinary long open time. Superior dielectric strength. Impressive optical clarity. Excellent chemical resistance. Serviceable from -80°F to +500°F.

EP46HT-1

Oven curing two part epoxy system. Resists from -100°F to +600°F. Tg of 230-240°C . Superb choice as matrix resin for fiber reinforced heat resistant composite structures. Exceptional strength properties. Robust chemical resistance. Long working life after mixing.

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Latest Developments

Case Study

EP114: Utilized to produce nanocomposites via vacuum infiltration

Master Bond EP114 is a two-component nano silica filled epoxy system which offers excellent dimensional stability and a very high glass transition temperature (Tg > 200°C upon curing. Its ultra-low initial mixed viscosity (500–1500 cps at room temperature) can be further reduced by preheating to a slightly higher temperature, making EP114 ideal for vacuum infiltration to produce dense nanocomposites, as shown in the case study below.

EP42HT-2ND-2Med Black Two Part Epoxy System

Technical Data Sheet Excerpt

EP42HT-2ND-2Med Black Product Information

Two component epoxy paste adhesive. Resists sterilization exposure. USP Class VI approved. Outstanding thermal stability and electrical insulation properties. Cures at ambient temperatures. Withstands multiple cycles of autoclaving, radiation, EtO and chemical sterilants. Post curing is recommended to optimize physical properties. Serviceable from -60°F to +450°F.

White Paper

Epoxy Compounds Get Even Tougher

With adhesive products, high performance and rigidity are often thought to go hand in hand. And it is true that the very best strength, thermal, chemical and electrical properties tend to be found in rigid compounds, especially epoxies. Yet there is a growing class of adhesives, sealants and coatings that add ductility to the long list of desirable epoxy properties. Master Bond’s white paper examines the use of these toughened epoxies.

White Paper

Spotlight on Silicones

Silicone adhesive systems have several important performance characteristics that distinguish them from other adhesive families. Learn more about their unique combination of flexibility, high temperature resistance, and more specialized engineering properties.

White Paper

Electrically Conductive Adhesives Make Right Connections

As electronic circuits become more complex, engineers are finding new ways to assemble and package them. Electrically conductive adhesives provide durable bonds with conductive paths to suit a variety of electronics applications. They can be engineered to combine bond strength and electrical conductivity with other service-critical properties.

Technical Data Sheet Excerpt

EP37-3FLF Product Information

Highly flexible, low viscosity, optically clear adhesive. Resistant to severe thermal cycling and thermal shock. Bonds well to dissimilar substrates. Low exotherm system. Long working life. Superior electrical insulation properties. Service temperature range from 4K to +250°F. One to one mix ratio by weight or volume.

Lab Report

Testing Adhesives for Resistance to Xylene

Many Master Bond epoxy systems are formulated with superior chemical resistance. In the lab, we continually test our materials by exposing them to specific chemicals over a long period of time. A common way of testing the chemical compatibility of an epoxy is immersing a sample in a chemical and measuring its change in weight over time. A significant loss or gain in weight would indicate a decreased ability of a material to stand up to chemical exposure. These tests allow us to more accurately recommend the right adhesive based on specific application requirements.In this experiment, we focused on testing our epoxies for their resistance to xylene. The compounds Master Bond used for testing are a variety of two component epoxies and one component UV curing systems with good overall chemical resistance. For the first round of testing the products tested were EP41S-5LV, EP42HT-2AO-1, EP46HT-2AO Black, UV16, UV22DC80-1, and UV26. For the second round of testing the products tested were EP125, EP42HT-2 and UV25. For the final round the products tested were EP41S-5, EP41S-1HT, EP62-1HT and EP62-1. These were compared against the resistance of a generic or standard non-chemically resistant epoxy. A few thin castings, roughly 2 inches in diameter and around 0.125 inches thick, were made for each product and cured in accordance to their specifications. Once the cured samples were created and initial weight was recorded, we submerged them in xylene. Then we continued recording weekly weight measurements. We carried on these tests with frequent weight recording for varying lengths of time based on the products performance. Below you will see the results for soaking results for around 10 months in the first batch, 2 years for the next batch, and greater than 2.5 years for the third batch of products. The castings were weighed periodically and the graphs shown below demonstrate the percentage of weight change over time.For the purpose of comparison, please notice that for each testing timeframe, a casting of a standard epoxy was also tested under these same conditions which served as a reference. As can be seen in the graphs, the standard epoxy was not as resistant to xylene as the other epoxies tested. It demonstrated a significantly greater change in weight over time, and the casting ultimately dissolved in xylene after 15 weeks.In general, a swelling of less than 4-5% can be considered excellent, especially since this test may be more rigorous compared to actual service conditions. It is also worth noting that in the context of a bonded joint or a potted assembly the exposure to xylene might not be as severe or direct as in the above test conditions.Please note, when choosing an epoxy for an application where the resistance to xylene is critical, many other factors must be considered in addition to the chemical resistance. Each of the epoxies in the charts above offers a distinct set of performance properties. For example, if thermal conductivity and high temperature resistance are needed, EP46HT-2AO Black may be a good option. If a fast cure is required and there is full access to a light source with no shadows, UV26 can be considered. For longer term exposure to xylene over several months to a year or longer, the likes of UV25, EP125 EP41S-5, and EP62-1 are excellent candidates. They all need to be processed or cured differently and although some of them can be cured at room temperature alone, for enhancing/optimizing chemical resistance especially to xylene, it is critical to add heat.Disclaimer: The findings in this article are not meant to be used for specification purposes.

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