T E C H S P O T L I G H T

One Component Epoxies:Versatility and Convenience

Master Bond Inc. 154 Hobart Street, Hackensack, NJ 07601 USA

Phone +1.201.343.8983 | Fax +1.201.343.2132 | WhitePaper@masterbond.com

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One Component Epoxies: Versatility and ConvenienceOne and two part epoxy based systems are used in countless applications including some of the most sophisticated in the aerospace, electronics, optical, medical, and OEM industries. They have unmatched versatility and are excellent for bonding, sealing, coating and potting/encapsulation type applications. In order to understand the etiology of a one part system, it is important to first understand two part systems.

Two part systems, the most basic of epoxies, are formed through the polymerization of two starting compounds: a resin and a curing agent. The curing process takes place when the reactive constituents of the resin and curing agent combine. As this reaction proceeds, an exotherm develops, enhancing the crosslinking of the two components. The speed of the reaction as well as the ultimate cured properties depend on the nature of the resin and the curing agent.

The majority of industrial grade epoxies are two part systems, which offer a comprehensive range of properties. Two part systems require mixing and have a corresponding open time, which can be defined as the maximum time between mixing and the time when the product can no longer be applied. The time it takes to fully cure a two part system can range from as little as a few hours, to up to two weeks. Often, heat can be added thereby accelerating curing, enhancing crosslinking, and optimizing performance properties.

With two part systems, there are a number of processing related issues that must be considered. First and foremost, all two part systems have a mix ratio. Some may be simple, for example, one to one by weight or volume. Others might be very specific, e.g. an unforgiving 100:7 by weight. Also, the process of mixing requires a certain level of skill, especially when trying to ensure complete mixing of the two components (which is absolutely critical) and avoiding air entrapment. Another issue with two part systems is that the amount of material mixed usually exceeds the amount used, resulting in wastage.

One part systems increase process efficienciesOne part epoxies are highly desirable, especially from a processing point of view. In a one part epoxy, the resin and curing agent are both present within the system. The reaction only occurs upon the addition of heat as the curing agent is not active due to the presence of inhibitors. Adding heat eliminates these inhibitors causing the reaction to take place. Up until then, the stability and shelf life are quite good, and one part systems can be applied at any time prior to the addition of heat.

It should be noted that the heat required must be added for the time period indicated. Adding heat to initiate a cure will never result in a full cure; the heating cycle must be thorough and complete. One part systems eliminate the need for mixing and in a sense have “unlimited working life”. That is to say, the open time is comparable to the shelf life, in that the system will not fully cure until the appropriate amount of heat is added.

• Most one part systems require a temperature of 125°C to 150°C for curing, although specialty grades now exist for an 80°C cure. These are particularly beneficial for bonding heat sensitive substrates. The curing times of one part epoxies range from a couple of minutes (snap cure) to many hours. Generally, there is a trade-off between curing time and performance, with a longer curing system offering more advantageous product properties, for example, electrical insulation properties or physical strength values, etc.

Since shelf life and working life are essentially synonymous, particular attention should be paid to the storage temperature. Normally, with a one-part epoxy, storage

temperatures from 40°F to 75°F are desirable so that the deactivator does not lose its potency and often, refrigeration is desirable. Higher storage temperatures

(above 75°F) can adversely affect the system.

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Chemical constituents determine epoxy characteristicsMost epoxies can be modified through appropriate selection of resins and hardeners and through the addition of fillers, diluents, and other components to achieve a variety of processing conditions and performance properties. While two part epoxy systems offer the broadest array of properties and applicability among all adhesive families, one part epoxies are also highly multifunctional.

There are a number of resins that can be used in one part epoxies including conventional, high temperature, flexible, as well as low ionic resins. The selection of a suitable curing agent is more restricted. Unlike two part systems which have a huge array of curing agent options, the number of compounds available to function as a curing agent in a one part epoxy is markedly limited. All curing agents for one part epoxies require heat activation and continuation of heating. If the heat is withdrawn prior to full cure, the curing process ceases and the epoxy remains incompletely cured. In contrast, for a two part room temperature curing system, one can initiate curing by adding heat. This is primarily done to fixture parts together quickly. If heat is withdrawn the system will continue to cure at ambient conditions. This will not occur with a one part system.

Fully cured one part epoxies offer many desirable properties including excellent bond strength to a wide variety of substrates, outstanding mechanical and physical strength properties, robust temperature resistance and the ability to withstand chemicals. One part epoxies can be formulated in almost any viscosity imaginable. However, a paste consistency is often preferred for bonding applications in order to avoid material flow when heat is applied for curing. Chemical resistance, the ability

to withstand high temperatures, electrical properties, optical clarity, thermal conductivity, and cure speed can also be modified to meet specific applications. For example, formulations are available to withstand cryogenic conditions, or to meet NASA low outgassing specifications or to pass USP Class VI biocompatibility standards. One significant application issue regarding one part epoxies is that they are more exothermic than two part systems, and in most situations, are limited by curing depths 1/4 inch or less.

One part epoxies streamline processing in industrial applicationsOne part epoxy systems are used in electronics, aerospace, optical, opto-electronic, medical, and specialty OEM, among others. These systems can be used for bonding, sealing, coating, potting and encapsulation, and impregnation applications.

Unconventional “One Part” Epoxy SystemsInnovations in polymer engineering and packaging have led to the availability of unique epoxy systems that offer the convenience of conventional single component systems coupled with the high performance of two-part systems.

B-staged systems consist of epoxy resins that have been partially cured during formulation, and are thus said to be “staged” for later processing. These semicured systems can be packaged in several different ways. Some are poured while hot into cans or molds where they solidify, and are then sold in solid form in cans or as “cookies.” Others are formed into sheets of solid films or cut to create precision preforms. B-staged systems offer storage stability at ambient temperatures and extended storage life with refrigeration. After application, solid B-staged epoxies can be cured to completion in an hour or less through the application of heat at 250-300°F. The properties of fully cured B-staged epoxies are comparable to those of liquid adhesives.

Premixed and frozen systems are two part epoxy systems that are thoroughly blended and degassed by the formulator, then frozen and packed in dry ice for shipping. When stored at -40°C, they have a shelf life of six months, during which they retain their physical and handling properties. Premixed and frozen systems require no mixing and cure at room temperature — offering the performance benefits of a two part system along with the ease of use of a single component system.

One part epoxy systems simplify processing and reduce waste.

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In electronics and opto-electronics, one part systems are utilized in many different applications. One instance would be as an underfill system, where the epoxy is designed to protect and support delicate flip chips. Important considerations would be good flow along with superior dimensional stability upon curing. Ultra low viscosity one part epoxies are well suited for various vacuum impregnation applications, ranging from transformer coils to carbon fiber composites. On the other side of the spectrum, thixotropic one part epoxies can be used as glob tops and encapsulants. Glob tops can be applied to circuitry, wire bonds, and even to delicate electronic components. They can also function as die attaches. In general, one part epoxies can be applied to protect electronic assemblies from a host of conditions from environmental to the more extreme thermal and mechanical stresses. In electro-optical devices, one part epoxies are well suited for bonding LED and display assemblies, as well as fiber optics bonding, sealing, and coating.

Versatility and usefulness of one part epoxies extend to specialty OEM applications as well. They can be used as adhesives in applications as diverse as securing motor magnets and joining iron and glass in automobile headlights, as sealants to isolate halogen lamps from moisture and other contaminants, and as coatings to protect armature coils and various motors such as starter, power window, and other electric types. One part systems are available to withstand temperatures as low as 4K and are suitable for use in a number of situations. These would include cryogenic seals in centrifugal cryogenic pumps, infrared telescopes and detectors, satellite tracking systems, unmanned aerial vehicle systems, and missile warning systems.

Many medical products can also benefit from one component epoxies. Typically, these are specially formulated to resist chemicals and various methods of sterilization— including multiple cycles of autoclaving—

while meeting rigorous biocompatibility standards. They adhere well to metals, plastics, glass and other substrates used in medical devices. These medical grade epoxies are commonly used in disposable and reusable devices such as endoscopes, infusion pumps, catheters, and surgical instruments. They are also used in many orthopedic and prosthetic devices as well as diagnostic equipment such as MRI machines and ultrasound devices.

One unusual iteration of these one part systems is epoxy films. Films are becoming increasingly popular, especially where uniform bond line thicknesses are needed. There are a host of applications where they are being used— lid sealing, the bonding of electronic components to heat sinks, and attaching substrates in microelectronic packages among others. In fact, films are now being used somewhat unconventionally, for example, to bond large substrates on cryogenic tanks in the aerospace industry.

In all these industries and varied applications, selection of an appropriate system depends on several factors. Most important are the cured properties, especially bonding and physical strength, chemical resistance, thermal and electrical properties, temperature resistance, thermal cycling capabilities and color. The handling and processing characteristics such as viscosity, exotherm, curing conditions and temperature among others also play a role in the selection process. Additionally, there are applications where standards such as biocompatibility, UL, mil specifications, and low outgassing, may influence the selection process.

Versatile one part epoxies are a great optionOne part epoxy compounds offer a range of properties and perform well in a wide variety of applications. These versatile systems can be formulated to achieve dozens of combinations of physical, thermal, and electrical properties in order to address application requirements. If heat curing is possible and practical, one part epoxies are an excellent option because of their outstanding performance profile and relatively easy and convenient processing.

For further information on this article, for answers to any adhesives applications questions, or for information on any Master Bond products, please contact our technical experts at Tel: +1 (201) 343-8983. USP Class VI approved one part no mix epoxy

adhesives feature high bond strength and resistance to sterilization.