Rapid Adhesive Bonding: Kinetics or Process?
Adhesive bonding provides solutions to bonding problems that are otherwise insolvable. Bonding of dissimilar materials or materials that are too thin to weld are just two of the applications where use of adhesives is highly recommended. When compared with welding or even mechanical attachment, adhesives provide superior bonding performance by improved stress distribution and inherent sealing capabilities.
A major drawback to adhesive bonding is the speed of strength formation during the bonding process. Welding and mechanical attachments provide self-fixturing strength build within seconds of use, while strength development with many adhesives systems can take between minutes and days. In adhesive bonding, fixturing must be used for long periods of time to hold the joint regions in rigid fashion until the adhesive develops sufficient strength to maintain a bond and the required rigidity for the joint.
To overcome the inherent slowness of adhesive strength development, many process enhancements are used. The most common is the curing oven, which is relatively inexpensive to operate. Unfortunately, however, this equipment heats not only the joint region but also the entire construct and the accompanying fixturing--and the excess heat must be removed. It may be recycled in some manner but is typically dumped as waste energy. In many adhesives curing operations, upwards of 95% of the input energy is wasted by heating an entire structure to curing temperature rather than heating only the bond region.
Several other methods can be used in place of inefficient bake oven cures. Room temperature curing systems are one solution, but in most cases they are not particularly fast. Rapid curing systems, which use acrylic and some epoxy chemistries, are another option. For temperature resistance in excess of about 170[degrees]C epoxies are a typical choice, but even rapid curing epoxies have limited formulation range and often produce brittle bond lines.
Two other methods of rapid adhesive bonding are induction curing, which is in widespread use, and microwave curing, which is predominantly a laboratory procedure. In most induction-cured systems, the entire joint region is heated from the outside. This process heats the adhesive along with the joint region but can result in uneven heating, top to bottom, or can cause distortion of parts because of excessive heat buildup in the joint region. Metals typically use induction curing because of their susceptibility to the magnetic field. Fixturing is critical, and even small variations in the spacing between the parts and the field coil result in cure variations.
Microwave curing sees limited use because of the need to ground metals at microwave wavelength distances and because of the difficulty of concentrating the microwave radiation in the bond region. The relatively large size of the bond region in comparison with the microwave energy wavelength results in standing wave patterns and heating pattern disruptions. While these problems can be overcome by use of multimode cavities, multiple horns, and stirring paddles, field uniformity is still a major concern. Similarly, use of co-joined conductors and nonconductors presents problems because of heating nonuniformity and poor cure at the "cold" surface of the nonsusceptor.
Both induction and microwave curing present the problem of how to transfer heat into nonsusceptible joint adherends. If plastics are used with either induction or microwave curing, it is likely the heating patterns will be poor unless the plastic can support eddy currents (induction) or can act as a lossy dielectric (microwave). Instances of inherent susceptibility are better with microwave systems because of poor dielectric behavior. Ferromagnetic susceptibility needed for induction curing is uncommon in plastics.
An obvious solution to these problems is to make the adhesive into a susceptor by the addition of suitable materials that can interact with the impinging field energy. The analogous approach has met with good success in plastics welding applications, but the widespread use of modified adhesives for curing applications has not occurred. In our work, we have undertaken an examination of some of the factors that affect the practical application of susceptor fillers for microwave or induction curing of adhesives.
SUSCEPTOR FILLERS FOR ADHESIVES
Susceptor fillers for induction and microwave heating operate by different mechanisms but produce the same effect: heat. It is the heat from the adhesive modification that causes the adhesive to cure. Usually, the included susceptor material is the only real source of heat. It interacts with the radiation, acting as a lossy interface, and generates heat by absorbing radiation energy. At some point, it tries to dissipate that energy into its surroundings. If the surroundings also happen to be made up of susceptor material, the temperature rise can be rapid and dramatic, e.g., when heating a metal in an induction or microwave field. If the surroundings are not a susceptor, as in the case of a polymeric material, the thermal conductivity of the surrounding material determines how fast the energy can be transferred through the matrix. If the thermal conductivity is poor, the susceptor material will continue to increase in temperature but the heating of the surroundings will be poor. The susceptor will continue to rise in temperature (and may even melt), but the temperature of the area close to the susceptor will remain relatively low. If one depends on the susceptor particle heat to cure an adhesive, regions intimately in contact with the susceptor will cure (or even char), while regions between particles may never reach cure temperature.
One solution to this problem is to add more particles, along with thermally conductive fillers, to help distribute the heat more evenly and produce a better cure. Unfortunately, in poorly conducting media, the amount of filler necessary may become so great that the actual physical performance of the adhesive will be affected. For rapid microwave curing, susceptor particles may be needed at levels of 20 to 25 vol% to achieve acceptable cures. This level of material will clearly affect viscosity and flow properties of the adhesive and may well affect ultimate strength.
Inclusion levels for induction curing are similar. Ferrous materials result in extremely high weight-percent loadings. Even if one allows for the needed high filler levels, the next concern is the particle size distribution of the filler. This is especially an issue with microwave curing. Susceptor fillers in microwave heating become more efficient as the size approaches an appreciable fraction of a wavelength, typically one-half wavelength. Heating a 1-cm toroid core in a microwave field produces rapid, visible results, while pulverizing that same mass of material into -600 mesh powder and incorporating it into a polymer mass may result in little effective heating. Again, the response is to add more filler. Large filler particles improve heating rates, but the inclusion of more filler and larger filler ultimately affects the rheological properties of the adhesive. Particle size is not so important as particle quantities with induction heating. With induction heating, the generation of eddy current heating w ith magnetic susceptibility losses is not size dependent.
A final problem in using susceptor fillers is the effect of available adhesive mass on the heating rate and interfacial temperature development. When susceptible adherends are being heated and bonded, the joint region is amply heated and heat is transferred through the joint into the adhesive. Thus, the adhering surface is heated before the adhesive itself and the adhesive is able to cure at the surface at a preheated curing temperature. When the adhesive is used as the susceptor, the adhesive must heat the joint. This means the curing surface is the last area to receive heat, and in many cases, the adhesive is already admitted to be a poor conductor. Portions of the adhesive can cure before the bond line region ever reaches curing temperature.
It can be readily demonstrated that larger masses of adhesive will heat much more quickly in an impinging field than will smaller amounts. Thus, a mass of perhaps 10 grams of adhesive may cure in less than 10 seconds, while a standard joint containing perhaps 300 mg of adhesive may require minutes to cure or may never cure quickly enough to be considered a rapid cure. The "heating" adhesive must struggle to heat both itself and the interfacial bond line to curing temperature. Meanwhile, the adherends are sinking whatever heat they receive to areas away from the bond line. The result is improper cure or poor adhesion.
The largest obstacle to good adhesive curing with internal susceptors appears to be related to mass. Microwave or induction heating for modified plastics works well for plastics welding because the mass of material is high. In plastics welding, success is dictated by good interfacial flow under applied pressure. The modified welding material provides sufficient mass to heat itself to forging temperature. Any heat that is sunk into surroundings effectively helps to heat the broader joint region and becomes participatory. In adhesive bonding, the participating heat mass is too low and the surroundings are detrimental rather than being participatory. Success is measured by effective cure.
HOW TO MAXIMIZE PERFORMANCE
Beyond the need to heat the bond line surfaces to curing temperature, the critical component in the curing of adhesives is the adhesive itself. For rapid bonding, there is not only the need to heat the adhesive (and the bond line) to a curing temperature but also the need for the adhesive to have sufficient kinetic activity at the curing temperature in order to cure quickly. Thus, despite the foregoing concerns about heating patterns, the most significant factor in rapid curing is the speed of cure at temperature rather than the speed of reaching that temperature. Therefore, the emphasis should not be on the particular heating method, but rather on how that heat will be used once it is supplied.
The above conclusion has led to the development of several guidelines for general bonding practice. First, the lowest practical curing temperature should be used. This means that the adhesive curing package should be tailored to the curing method being used, while being consistent with the performance requirements of the application. This also means that the curing process best able to reach the desired temperature should be used.
Second, the fastest practical curing package should be chosen. This is not necessarily the same as the lowest temperature curing package, but the two often coincide. In curing a structural epoxy, one might select either a polyamide or a boron trifluoride cure. Either may cure well at 70[degrees]C, but the boron trifluoride cure will often be faster. If the end use can tolerate the presence of residual catalyst, the boron trifluoride cure is preferable. Because the trifluoride is a catalytic cure, the adhesive formulation itself will need to be changed to provide the necessary toughness that might otherwise be imparted by a polyamide cure. Nonparticipatory filler inclusion may also be changed. For example, alumina or alumina trihydrate fillers are not compatible with many trifluoride cures, whereas they can be used with polyamide cures. Instead, a silica filler may need to be used with a trifluoride cure. Fillers that improve thermal conductivity of the adhesive are beneficial.
Third, one should choose the curing method that best suits the curing adhesive, a requirement that will include considerations of substrate type as well. Rapid curing is a means to an end. If induction will work better than microwave, or vice versa, then one should choose the process to fit the application, rather than make the adhesive fit the process.
Fourth, one must be realistic. The ratio of mass of available internally heated adhesive to the mass of the joint region must be kept relatively high for good cures. In microwave curing, the cross section of the joint area must be an appreciable fraction of a wavelength for good absorption. Small amounts of material in relatively large joints can be handled better with induction curing.
The adhesives under study were prepared using Epon 828 DGEBA epoxy resin from Shell Chemical and a polyamide curing agent. Susceptor materials were added as needed.
Two-part epoxy adhesives, based on commercial materials or custom formulations developed at the Edison Welding Institute, were used throughout for induction curing studies. When added, susceptors included red iron oxide ([Fe.sub.2][O.sub.3]), black iron oxide ([Fe.sub.3][O.sub.4]), and tungsten-coated microballoons (3M NPE 3049). Induction-cured samples were prepared using standard ASTM D1002 lapshear constructions with either steel, aluminum, or aluminum and polyester SMC. The induction frequency was about 50 KHz. Power levels used ranged from 1 to 5 kW with coil-to-work spacing of approximately 0 to 2.5 cm.
The same basic epoxy-polyamide resin system was used for studies with microwave curing. All the work for microwave curing was done on SMC-SMC samples. Susceptor materials included carbon black and tungsten-coated microballoons added at up to 25 vol%. Lapshear samples were produced at either 1.3 cm or 2.5 cm overlap. Cure times were about 2.5 min at 750 watts in a 2450 MHz chamber. Cure times as low as 1.5 min were obtained using very rapid curing agents, such as boron trifluoride catalysts, instead of the common polyamide curing agents.
Is rapid adhesive bonding a matter of process or kinetics? It is both, but kinetics is more important than process. The adhesive must cure by internally generated heat. How the heat is supplied is not so significant as the speed with which that heat can be used, once supplied. Anything that removes heat from the process is detrimental and will slow the cure. It is better first to tailor the adhesive for rapid kinetic cure using the available supplied heat and then increase the heating rate by use of added susceptors. Also, whenever possible, one should choose a process method that includes participatory heating of the adherends.
This work was supported under the Cooperative Research Program of Edison Welding Institute and represents ongoing sponsored research. Information presented has been released with the permission of the Institute
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|Comment:||Rapid Adhesive Bonding: Kinetics or Process?|
|Author:||Ritter, George W.|
|Date:||Dec 1, 1999|
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