Technical Tips

Kohesi bond’s technical tip library offers answers to most common questions related to bonding, sealing, coating, potting and encapsulation. In addition, they provide guidance on achieving better strength, higher production efficiency and tips on ease of application. Get the most out of first-rate adhesive systems, by referring to Kohesi Bond’s technical tips.

AEROSPACE OPTICAL ELECTRICAL

Enhancing Electrical Insulation Properties for Epoxies
Epoxies are inherently excellent electrical insulators. Adhesives, sealants, coatings, potting, encapsulation and impregnating compounds are widely employed in electronic and electrical applications. Choosing the right epoxy not only depends on its dielectric properties, but also on a number of properties such as temperature resistance, mechanical strength, chemical resistance and resistance to thermal and mechanical shocks and cycles. These factors are greatly affected by the choice of the curing agent. Amine type curing agents are commonly used for bonding, sealing, coating and impregnation applications. Anhydride type curing agents are commonly used for potting and encapsulation applications, owing to their low exotherm and long pot life. Before selecting an epoxy for your application, it is first important to understand the various dielectric properties of insulation materials:
Volume and Surface Resistivity: Volume resistivity quantifies how effectively a material can resist the flow of electric current along its thickness. Surface resistivity measures the material’s ability to resist the passage of electric current along its surface. Higher the resistivity, better is its insulative properties. The volume resistivity of an epoxy is measured using the ASTM D257 test method and typically it measures > 1011 ohm-cm at 25°C for an unfilled system.
Dielectric Strength and Dissipation Factor: Dielectric strength measures a material’s ability to resist flow of electric current along its thickness without breaking down. It is typically measured in volts/mil. Dielectric constant is the measurement of the rate of loss of energy under an alternating field. Dielectric strength for an epoxy is measured using the ASTM D149 test method and dissipation factor is measured using the ASTM D150 test method. For a 0.125 inch thick specimen, an epoxy’s dielectric strength typically measures in the range of 420 – 480 volts/mil.
Arc Resistance: Arc resistance measures the time durability of a dielectric material against a high voltage arc discharge. It is measured using the ASTM D495 test method. Typically, epoxies can hold up to the arc for 90-300 seconds.

Kohesi Bond offers a vast selection of epoxy compounds with varying dielectric properties. Specific grades meet various industrial certifications such as NASA standards for low outgassing (ASTM E-595). Our products can withstand extremely high and low temperatures. Notably, KB 1040-2 is an optically clear, cryogenically serviceable product that offers resistivity of > 1015 ohm-cm. Additionally, it is capable of passing NASA’s low outgassing specifications. Kohesi Bond’s technical specialists can help you find the best solution for your electronic application.

Adhesive Selection for Space Applications
Epoxies make an excellent choice of material for space applications. However, selecting the right adhesive is extremely critical for the performance of various optical and electronic components in space. One of the key concerns while selecting an adhesive for space applications is its serviceability in extremely high vacuum environment. Systems operate under very high vacuum (~ 10-8torr) in space. Any outgassing from the adhesive could lead to failure of crucial components of the system.

To avoid such failure during operation, NASA developed the ASTM E-595 low outgassing test standard. This test involves collecting volatiles from the test specimen and weighing the condensate to determine the following:

CVCM: The amount of collected volatile condensable materials (CVCM) on the collector plate.
TML: The amount of total mass loss (TML) by the sample
WVR: The amount of water vapor regained (WVR) by the sample

The material is said to pass or fail the test based on the following conditions:

CVCM < 0.1% and TML < 1% - Pass
CVCM < 0.1% and TML > 1% – Pass if TML – WVR <1%
CVCM > 0.1% or TML – WVR > 1% – Fail

In addition to low outgassing, epoxies need to withstand extreme temperature conditions in space; particularly at cryogenic temperatures.

Kohesi Bond has specially formulated epoxy adhesives, sealants, coatings, potting and encapsulation compounds that are capable of providing excellent performance at high temperatures as well as down to 4K (-269.15°C). They are also capable of passing NASA standards for low outgassing. Furthermore, these epoxy systems can provide optical clarity, thermal and electrical conductivity/insulation, chemical resistance, superior mechanical strength and toughness.

How to Optimize the Glass Transition Temperature (Tg) of an Epoxy
Unlike thermoplastic polymers, when a thermosetting polymer is heated it does not begin to melt. Instead, they turn into a viscous rubbery phase over a short temperature range. This onset of change in the phase of the material is indicated by the glass transition temperature. Heating above this temperature could affect various properties of the polymer, such as mechanical strength, electrical properties and chemical resistance, among others.

The glass transition temperature (Tg) of an epoxy can be measure using various techniques, such as Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analyzers (DMA) or Differential Thermomechanical Analyzers (DTA). Additionally, measuring the Heat Deflection Temperature (HDT) can also indicate the glass transition temperature (Tg) of an epoxy.

The Tg of an epoxy is affected not only by the choice of epoxy, curing agent and filler used, but also by the curing conditions. Epoxies demonstrate a wide range of Tg, from as low as 50°C to upwards of 250°C. In addition to improving its performance properties, adding heat while curing the epoxy helps improve its glass transition temperature (Tg).

Careful selection of the epoxy helps achieve the optimal performance over a wide temperature range in your application. Kohesi Bond’s technical experts can help you select the best epoxy suited for your application. Given below are the glass transition temperatures (Tg) of various Kohesi Bond epoxy systems.

Product Name Tg (°C) Product Description
KB 1427 HT 220-230 One component, heat curing epoxy system suitable for bonding, sealing, coating, potting and encapsulation. It can be serviced at temperatures as high as 340°C.
KB 1427 HT-3 120 One component, snap curing epoxy system suitable for bonding, sealing, coating, potting and encapsulation. Additionally, it offers superior chemical resistance properties.
KB 1372-LP 128 Two component, heat curing epoxy system offering very long open time and chemical resistance.
KB 1372-AO >150 Two component, heat curing epoxy system that offers superior thermal conductivity and high temperature resistance.
KB 1372-LO >172 Two component epoxy system that offers phenomenal high temperature resistance. Additionally, it is capable of passing NASA standards for low outgassing (ASTM E-595).