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.
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.
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 email@example.com, by telephone at 215-674-8700, or through the Fielco Web site at www.fielco.com.
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.