One-Part Epoxies are Versatile and Convenient

Process Efficiencies

In a one-part epoxy (assuming it isn’t premixed and frozen or UV curable), the resin and curing agent are both present in the initial mixture. Typically, they polymerize when heat is added. Most one-part epoxies require temperatures  of 125° to 150°C for curing, although specialty grades now exist that cure at lower temperatures (80° to 100°C). These are particularly useful for bonding heat-sensitive substrates. One-part epoxies tend to be more exothermic than two-part versions, and in most situations, are limited by curing depths of 0.25 inches or less.

Curing times for one-part epoxies range from a few minutes (so-called snap cures) to several hours. Generally, there can be trade-offs between curing time and performance, specifically in terms of temperature resistance. Epoxies post cured at higher temperatures for longer times are usually stronger with better electrical insulation properties.

One-part epoxies eliminate the need for mixing and, in a sense, have “unlimited working life” at room temperature. That is to say, the open time is comparable to the shelf life, in that the epoxy is applied first and will not fully cure until the appropriate amount of heat is added.

Shelf life and working life are essentially synonymous, so particular attention should be given to storage temperatures. One-part epoxies should be kept at 40°F to 75°F so that the inhibitor does not lose potency, and refrigeration is often even better. Some epoxies degrade when stored at temperatures above 75°F.

Tailoring Characteristics

Most epoxies can be modified in terms of processing or final characteristics by choosing the right resins and hardeners and by adding fillers and diluents. Although two-part epoxies have the broadest array of properties and applicability among adhesives, one-part epoxies can also serve several functions.

There are a number of resins for one-part epoxies, among them high temperature, flexible, and low-ionic variants. The selection of a suitable curing agent, however, is more restricted, unlike two-part versions which have a huge array of options for curing agents.

Curing agents for one part epoxies require heat activation and continued heating. If heat is withdrawn before the material fully cures, the process ceases and the epoxy remains incompletely cured.

Many cured one-part epoxies provide good bond strength to a wide variety of substrates, outstanding mechanical and physical strength, and resistance to high temperatures and chemicals. They can be formulated for almost any viscosity imaginable. However, many users prefer a paste consistency for bonding applications to avoid material flow when heat is applied for curing.

They can also be modified for resistance to chemicals and high temperatures, electrical properties, optical clarity, thermal conductivity, and cure speed. For example, there are epoxy formulations that withstand cryogenic conditions, meet NASA’s low outgassing specifications,  or comply with USP Class VI biocompatibility standards.

One-Part Epoxy Applications

One part epoxies are used for bonding, sealing, coating, potting and encapsulation, and impregnation. In electronics and opto-electronics, one-part systems can be used as an underfill in which the epoxy protects and supports delicate flip chips. Important considerations in this application would be good flow and dimensional stability upon curing.

Ultra-low-viscosity one-part epoxies are widely used for various vacuum impregnation applications, ranging from transformer coils to carbon fiber composites. On the other side of the spectrum, thixotropic epoxies are used to attach dies, and as encapsulants and glob tops. Glob tops are applied to circuits, wire bonds, and even delicate electronic components to provide mechanical support and keep out contaminants.

In general, one-part epoxies protect electronic assemblies from environmental factors, as well as more extreme thermal and mechanical stresses. In electro-optical devices, one part epoxies bond LED and display assemblies, as well as bonding, sealing, and coating fiber optics.

One-part epoxies are employed by OEMs as adhesives in applications as diverse as securing motor magnets and joining iron and glass in automobile headlights. Some firms seal and isolate halogen lamps from moisture and other contaminants with these epoxies. They are also applied as protective coatings on armature coils and motors of all sizes.

Some epoxies withstand temperatures as low as 4°K and are suitable for use in cryogenic seals in centrifugal cryogenic pumps, infrared telescopes and detectors, satellite tracking systems, drones, and missile warning systems.

One-part epoxy films are also becoming increasingly popular where uniform bond-line thicknesses are needed. This includes creating a seal on lids, bonding electronic components to heat sinks, and attaching substrates in microelectronic packages. They are even being used somewhat unconventionally in the aerospace industry to bond large substrates on cryogenic tanks.

In all these industries and applications, selecting an appropriate one-part epoxy depends mainly on its cured properties—especially bonding and physical strength, chemical and temperature resistance, electrical and thermal properties, and color. Handling and processing characteristics such as viscosity, the amount of heat they generate while curing, curing conditions, and temperature play roles in the selection process.

Additionally, there are applications where standards such as biocompatibility, UL, mil specifications, and low outgassing influence the selection process.  

One-part epoxies offer a range of properties and perform well in a variety of applications. If heat curing is possible and practical, they are an excellent option based on their performance and relatively easy and convenient processing.