One-Pack Epoxy Adhesive Systems: New Technologies, New Applications
Conventional Plural-Component SystemsThe high reactivity of epoxy groups with conventional curing agents such as acids, anhydrides, polyamines, polyamides and amidoamines makes one-pack systems impossible. Products must be supplied in plural-component packages to provide shelf life. The epoxy components are mixed prior to usage either by hand, or in most manufacturing environments, by meter/mix/dispense equipment.
The product components are pumped either directly from shipping containers or transferred into reservoirs. Each component is delivered to a gear or piston metering pump that dispenses the prescribed volume of each component in relation to its mix-ratio requirement. The metered components then pass through either a dynamic or a static mixing element to deliver homogenous adhesive mixture to the assembly process.
Filament-winding applications either manually premix or directly meter the impregnating thermoset material into coating reservoirs. Additional material is added when the reservoir level reaches a minimum volume.
Screen-printing applications typically hand-mix the adhesive or coating material and apply the product to the screen after the squeegee has retracted from the printing cycle.
Pot life or open time of thermoset adhesives and coatings has a dramatic effect on processing conditions. It is a constant challenge to balance long open pot life with the rapid green-strength development and minimized set time of the final adhesive or coating. Changes in product viscosity and consistency can affect a product’s wet-out on surfaces to be coated or assembled, filament impregnation, coating quality, flow, and leveling of the material. Considerable resources are required to maintain and clean production equipment, vats, reservoirs, printing screens and metering equipment.
Adhesives gelling or curing in reservoirs or on production hardware can cause considerable downtime and repair costs. Additional expenses are incurred by the necessity to use cleaning solvents, as well as associated disposal costs (which, due to evermore stringent environmental regulations, are climbing notably).
Many of the curing agents used in epoxy systems have significant safety considerations. Various amines are corrosive by DOT CFR 49 standards and result in contact skin burns if not promptly and properly removed. Some systems have high volatility, resulting in vapor emission during mixing and product assembly, and can often be odiferous. In contrast, one-pack systems have reduced skin-irritation levels and can be formulated with virtually no odor.
One-Pack AdvantagesSeveral advantages and improvements associated with the replacement of two-pack systems with one-pack products are as follows:
- Elimination of plural-component dispensing and metering equipment and associated spare parts.
- Reduction of operator labor needed to maintain meter/mix equipment and components, or to manually weigh and mix the product components.
- Reduction of labor required to oversee, maintain and clean pumps, vats, scales and other associated manufacturing equipment.
- Improved raw-material procurement and flow by replacing two containers associated with two-pack products with one container for one-pack systems.
- Elimination of pot life considerations and line downtime resulting from product advancement exceeding process tolerances.
- Elimination of mix-ratio tolerance concerns, with the ever-present danger of using an epoxy system outside mix-ratio ranges.
- Improved final product quality as a result of more consistent adhesive or coating.
- Opportunities for improvement in adhesive or coating performance where specific sacrifices may have been made to maximize pot life and formulate to required viscosities and mix-ratio requirements.
- Improved worker safety and reduced hazardous-communication requirements with one-pack epoxies resulting from the elimination of volatile and corrosive curing-agent components.
Formulation GuidelinesOne-pack systems are formulated to the required viscosity, flow, activation temperature and specific mechanical, electrical and chemical-resistance properties. This is accomplished through the proper selection of epoxy-resin backbone structure, modifiers, diluents and tougheners.
The curing agent is compounded into the system using proper dispersion technique and temperature control. The curing agents used are typically latent salts delivered in a liquid form or micronized blocked amines. The amines reside heterogeneously in the formulation until the activation temperature is reached, resulting in very rapid cure reaction.
Adhesives are easily formulated with activation temperatures of 120°C and as low as 60°C. The cure process proceeds rapidly above the activation temperature with complete cure occurring in as short as a 20-minute timeframe. Shelf life of these products is superb and ranges from three to six months when stored at 25°C. The excellent latency of these systems makes manufacturing, shipment, storage and end-product utilization simple. It also accommodates normal processing conditions.
This is a marked improvement over the workhorse curative dicyandiamide, which has an activation temperature of 177°C with very good latency – as long as six months in a compounded system. This activation temperature can be decreased through the utilization of various accelerators. However, pot life is reduced to weeks or days. This requires the material to be stored at 0°C or less during shipment and storage.
One-pack systems can be formulated over a wide range of viscosities and mechanical properties. Photographs 1 and 2 demonstrate the full range of viscosities achievable. The first photograph is a low-viscosity impregnating resin used for continuous-fiber reinforcement. The second photograph shows a one-component reactive-epoxy hot melt adhesive designed for structural bonding. This unique product has a melt temperature of 30°C. It is used in abrasive wheels, structural metal bonding, appliances and office-furniture construction, where immediate green strength is needed resulting from resolidification of the adhesive after component assembly combined with rapid cure rates above 120°C.
Historically, dicyandiamide-cured products have resulted in high strength, and chemically resistant adhesives and coatings; however, they are extremely brittle. Through utilization of proper thermoplastic modifiers, plasticizers and tougheners, very resilient, highly flexible, low-modulus adhesives can be formulated. This is demonstrated by the ability of the adhesive pictured in photograph 3 to survive a 180-degree bend without splitting or cracking.
Shore hardnesses as low as 80 have been reached with high shear strength of 4,500 psi combined with exceptional peel strength as high as 30 pounds per linear inch (both tested at 20°C). The combination of these properties and the overall adhesive toughness resulted in bond lines surviving 1,000 hours of thermocycling from 15°C to 100°C at 30-degree ramp steps. By combining these fundamental properties with specific fillers and filler loading, coefficient of thermal expansion and thermal conductivity can be greatly improved, resulting in low shrinkage and shock-resistant systems required for outdoor structural applications and electronic-component assembly.
Formulation TrendsWith the increasing availability of latent epoxy-curing agents, one-pack epoxy adhesives are going to be utilized in the marketplace at an ever-increasing rate. Flexibility, flame retardancy, thermal conductivity, low shrinkage, thermal expansion, low dielectric properties and larger mass castings will open up areas of electronic encapsulation for sensors, coils, voltage regulators, transformers and telecommunication equipment historically utilizing two- component systems.
The improved economics of one-pack systems, including rapid cure, improved worker safety and less material handling will deliver markedly better productivity, efficiency and cost savings to the end user.
For further information, please contact Tony Ring directly at firstname.lastname@example.org, by telephone at 215-674-8700, or through the Fielco Web site at www.fielco.com.
Sidebar: Solder MasksIn the assembly process of electronic circuit boards, solder masks are needed to stop-off certain areas from receiving solder during the border-processing cycle. Solder masks also aid in minimizing contamination of circuit traces and add an additional insulating layer to the top surface of the circuit board. Solder-mask applications are demanding from the aspects of application techniques to the thermal resistance needed to survive temperatures of 245°C and higher during wave-solder and hot-air solder leveling processes, as well as being resistant to cleaning chemicals after solder application. The solder mask must have excellent electrical-insulation properties and not blister, lift out gas or decompose during the manufacturing process. Maskants can be pattern-imprinted onto the circuit board through a screen-printing process and then thermally cured. Our laboratory is currently investigating the viability of rapid-curing, screen-printable solder masks using blocked-amine curatives, which will result in a complete cure of the maskant layer in as short a time as 20 minutes at 115°C. In-line infrared heaters could be integrated into the curing process to accelerate board heat-up rate, which could shorten this cure cycle. Fundamental properties such as adhesion to various surfaces, electrical properties, moisture and chemical resistance will be evaluated and integrated into future formulations for surface-mount device bonding.
Sidebar: ElectronicsOne-pack epoxy adhesives are used extensively for bonding surface-mount devices to printed circuit-board assemblies. They are also used for bonding heat sinks to various electronic components. The heat-sink application shown demonstrates the bonding of a nickel-plated copper power transistor to an anodized-aluminum heat sink. The heat sink is used to pull thermal energy away from the component and dissipate it into the environment. The heat sink also had a secondary function, serving as a handle to transport sub-assemblies throughout the manufacturing process. Originally, the end user was using a two-part acrylic adhesive for the assembly and had periodic batch failures as high as 10,000 components – the result of contamination on either the transistor or the heat sink, which poisoned the cure mechanism of the acrylic adhesive. The two-component system was difficult to use in the production environment because of the inconsistency of activator application. The fundamental objective of this project was to develop an adhesive with heat-conduction characteristics in the range of 0.5 watts per meter per degree C, with coefficient of thermal expansion in the range of 700 ppm per degree C. The material also was required to pass a 1,000-hour thermal cycling from 15°C to 100°C. High tensile strength with specific adhesion to anodized aluminum and nickel plate was of utmost importance. After further evaluating the application, it was determined that rapid set time and filler selection to regulate bond line regulation would be of benefit. Our laboratory’s formulation effort resulted in a metal-filled, thermally conductive, one-pack adhesive with a cure time of 15 minutes at 120°C, having a self-regulating bond line of 0.003”. Bond-line regulation was achieved by adding filler having a narrow (75-micron) particle-size distribution, which would limit the depth the transistor could be pushed onto the heat sink. The table lists the properties of this assembly adhesive.
Sidebar: AbrasivesSeveral abrasive products have been fabricated using one-component epoxy adhesives: Metal-hub mounting onto phenolic grinding wheels. This process historically used two-component epoxy adhesives. The adhesive is meter/mix dispensed onto the hub assembly that rotates on a chuck, with the grinding wheel centered onto the metal hub with subsequent flanging. The wheel is cured for seven days under ambient conditions and then shipped.
Manufacturing problems typically arise when mix ratios vary outside tolerance requirements to maintain high levels of shear strength needed for wheel performance. A one-pack epoxy was successfully used in this application, integrating production consistencies and rheological benefits. Induction curing resulted in very rapid property generation and complete cure in less than 30 seconds. Additional benefits were realized as a result of the high temperatures the metal hub reached. This improved the epoxy’s ability to cut through surface oils and contaminants present from the hub’s manufacturing process, creating more-consistent, higher bond strength of the grinding wheel to the metal job.
Flap-disk abrasive products are manufactured using a hot melt thermosetting, one-pack epoxy adhesive. The first-generation epoxy products required cure temperatures of 180°C for one hour to ensure maximum performance. More-rigorous production requirements, the need to decrease cure time and, most importantly, the cure temperature facilitated the development of a specialty line of flap-wheel adhesives. These systems were specifically developed to be dispensed at the moderately low temperature of 50°C, resulting in rapid green-strength development after flap insertion to ensure concentricity of the flap array. Primary benefits of this formulation are rapid cure of 30 minutes at 120°C while maintaining excellent shelf life of four months at 77°F. Additional rheological properties were incorporated into the system to maintain adhesive-bead configuration with zero sag under curing conditions. Greatly improved flexibility of the adhesive resulted in improved higher rpm speed-rating results. Flap-disk users are driving the development of flexible, conformal products needed for polishing and grinding contoured surfaces. These products will require a flexible plastic or cloth backing and adhesive. Developmental work is ongoing to formulate this highly flexible material with the same adhesion, and inherent strength and durability as the current adhesive series. The photograph demonstrates formulation work to date showing a highly flexible material bonding abrasive flaps to denim backing.
Another abrasive application currently using a one-pack epoxy is fiberglass reinforcement of the exterior of grinding wheels for metal-rail dressing, as used in all major freight and passenger railroads. The grinding wheels are phenolic-based and are inherently brittle. The end user required additional fiber reinforcing on the circumference of the wheel to eliminate catastrophic failure during high-speed use. The application required a low-viscosity epoxy of 1,000 centipoise at room temperature to ensure filament wet-out. High flexural strength, tensile strength, and adhesion to glass and phenolic surface were also required. Conventional systems were either polyester two-component or epoxies with a solvent cut. The polyester system could not achieve the thermal-stability properties required. The solvent-cut epoxy resulted in starved bond lines between the glass and the wheel as a result of the material wicking into the phenolic wheel. We developed a system that solved both of these problems, combined with three month’s shelf life and a rapid-cure schedule.