By Ron Dubs, Senior Formulations Scientist, Fielco Adhesives
One-component epoxy based materials, long relegated to a minor role in the adhesives marketplace, have finally come into the limelight. These materials were once confined to thin bond line applications between similar substrates. Manufacturers endured excruciatingly long cure schedules to get true glass transition temperatures over 100ºC, and the ever-present danger of runaway exotherm always remained a possibility. Thankfully, those days have past. During the past few years one-part technology has changed dramatically, due to a wide variety of new, advanced raw material resins, flexiblizers, latent curing agents, and accelerators. Applications now include uses as diverse as large castings, optically clear coatings, and semi-flexible adhesives for dissimilar substrates. Cure times can now be measured in minutes instead of hours, and the danger of runaway exotherm has been greatly reduced.
From a manufacturing standpoint the advantages of one-component materials over two-component is enormous. One component materials eliminate the metering and mixing errors which are the cause of most epoxy curing problems. These problems, if undetected, can cause significant headaches for a production line manufacturing facility. One-component materials can be run indefinitely through dispensing devices, and be left unpurged in lines and equipment for a night, a weekend, or even longer.
Contrast this with conventional two-component materials, and the benefits are overwhelming. Virtually all two-component materials must have lines of mixed material purged and equipment cleaned daily. Two-component mixing tubes must be changed regularly, and all unused mixed material must be scrapped. Increased contact with epoxy resins, solvents, and especially amine curing agents during equipment cleaning can lead to employee sensitivity problems and the associated medical costs. Two component materials require twice the amount of inventory control, paperwork and environmental compliance. Variations in mixing equipment volumes, differing specific gravities of materials, and alterations in shipped materials often lead to excess of one side of a formulation–and that increases disposal expenses! Considering all these issues, it is small wonder that one component materials are rapidly becoming the choice of savvy production engineers.
The electronics industry has long been one of the prime users of one-component epoxy materials. This has been in response to both the demanding end-use parameters, as well as the extremely rapid line speeds that have become industry standard. The electronics business is famed for both creating new products and continually miniaturizing existing products. Both of these paths lead to the same point: high technology one-part adhesives. The latest one-part electronics adhesives can effectively bond numerous electronic-grade substrates (including conductors, semiconductors, insulators, glass, metals, thermoplastics, and reinforced composites). Resulting materials can stand up to ever increasing operating temperatures and long term thermal cycling, culminating in a finished product that will withstand harsh environmental conditions without losing conductive or dielectric properties.
Adhesives for SMDs (Surface Mounted Devices) positioned on PC boards, and dies positioned on chips, are both assembled by pick and place equipment operating at speeds undetectable by the human eye. These materials must maintain their rheology throughout various seasons, temperature changes, and plant conditions. They must have good green strength, be able to cure rapidly in tunnel ovens, and then withstand the heat of component soldering without degredation.
In addition to SMD and die bonding, the electronics industry is now making extensive use of one-component conductive adhesives. These generally silver-filled materials are designed to replace solder without the use of excessive heat. They can be print screened on to substrates such as Mylar™ to form complicated circuitry, then overlaid with one-component dielectric compounds, with additional circuitry printed on that surface, creating multi-layer flexible circuitry. These are state-of-the-art assembly techniques. Increasingly, it is a one-component, high thermal conductive epoxy compound that is chosen for heat sinks, for materials as diverse as hybrid chips to rectifiers and resistors. One-component epoxy encapsulants are being used in dam and fill techniques, and as glob top coverings for naked integrated circuit and hybrid chips. Clear one-component epoxies cured by Boron Trifluoride are being used as lenses over LEDs and as sight glasses for monitoring equipment. The standard one-component solder masks, typically cured onto circuit boards for directing the flow of liquid solder, are constantly being challenged to provide even finer resolutions for more densely-packed circuitry. The electronics possibilities of one-component epoxies are limited only by engineering vision and formulating realities.
Structural adhesives are also trending to one-component materials. Structural adhesives can loosely be defined as materials that are applied as an adhesive, and after curing become integrated into the structure of the end product. Cured structural adhesives are invariably critical to a product’s successful operation. The use of these materials is limited to products that can be either oven cured or spot cured. One such application is grinding wheel adhesives. In some examples, solid grinding wheels have been made by loading one-component epoxy materials with fillers and abrasives, then casting and curing the mixture. This process can produce wheels that are rigid or flexible. Other grinding wheels use a series of coated abrasive pads which are bonded around a phenolic hub. These are known as flap disks. Both these applications are extremely demanding, due to the combination of high heat build up and centrifugal force. With any application like this comes the associated concerns for liability to the grinding wheel manufacturer. Not every epoxy is ready for this critically important job.
The Automotive Industry is another large user of one-component epoxy structural adhesives. Automotive one-component adhesive applications are certain to increase, as every model year brings more composite plastic parts, and with them the need for proper bonding. Auto manufacturers continue to reduce the number of welded metal parts, turning to structural adhesives as a preferred joining technology. Automotive one-component applications include the manufacture and attachment of trim, the filling and strengthening of hem flanges at doors, hoods, fenders and stress points, as well as the filling of seams in the body, frame, and other places where metals are joined. In addition to direct auto manufacture, one-component materials are becoming a preferred choice among automotive component manufacturers, as well as among aftermarket parts suppliers.
You will find one-component materials used in high-end loudspeaker manufacture. They are typically selected for bonding the speaker cone to the voice coil in highly advanced systems, where performance standards are evaluated critically. Here, as speaker assemblies are indexed into place and rotated, a dispensing unit steadily places a bead of adhesive on the critical junction. Some manufacturers may oven cure at this stage, other extremely advanced operations may induction cure the material almost instantly by employing electromagnetic coils at microwave frequencies. These types of materials must be able to snap cure without offgassing, have good bond strength and toughness, and be well packaged without trapping unwanted air.
We foresee that one-part materials will soon rival two-component materials in other applications. As an example, you will find one-component materials used for large castings. We have been casting large masses of one-component epoxies of up to three pounds and 44 cu. in. volume without significant exotherm (don’t try this with Dicyandiamide). We believe these materials will be prevalent in the marketplace, with modifications for flame retardancy, thermal conductivity, low shrinkage, low thermal expansion and high dielectric properties. Look for them in the electronic encapsulation of sensors, coils, voltage regulators, transformers, telecommunications equipment and similar products. Another promising one-component application involves developing flexible materials with good adhesion properties. This has been an elusive goal, long sought after by manufacturers. Without divulging any trade information, we can share that the key lies in a successful marriage of resins and flexibilizers before addition of the curing agent. We do not know where the applications for these flexible one-components will lead, but the possibilities are intriguing.
In short, the use of one-component epoxy systems is increasing rapidly due to three major factors:
- Current applications in electronic, automotive, and structural adhesive bonding will increase their use, from industry volume growth and the introduction of new products.
- The economics of one-part materials. While two-component systems have long been industry standard, and are easily understood by manufacturers, it’s also apparent that there are significant savings with one-part material usage. Savings cover a broad range of manufacturing, including manpower, materials, and elimination of mixing errors. Savings more than offset any additional expense of a one-part system.
- New technology applications, such as large volume casting, encapsulation, and flexible adhesive bonding, with many more yet to come.