Assembly Planbook - June 2008 - (Page 8)

Bonding LINES The Cure for the Common Adhesive M An adhesive must be cured correctly and completely—the required cure cycle must be followed for the adhesive to work. ■ By George W. Ritter, Ph.D. Technology Leader for Adhesives and Plastics Edison Welding Institute Columbus, OH EWI is the largest research and applications facility in North America dedicated to the art and science of materials joining. Contact Dr. Ritter at: george_ritter@ewi.org any structural adhesives undergo some form of cure to achieve their full performance. Curing involves a chemical reaction that causes cross-linking or chain growth to form complete polymers. This builds the internal strength of the adhesive and also increases temperature and environmental resistance. Chemical reactions require activation to begin. Once the reaction begins, heat is evolved as the cross-linking proceeds. Adhesive components that generate enough heat from this exothermic reaction to cure on their own are said to display ambient or room temperature cure. Mixed components that are dormant at room temperature require added heat to initiate curing. When curing begins, the internal temperature of the mix may climb rapidly, depending on the reaction chemistry. This happens for two reasons. First, a rapid exothermic reaction causes heat to build up quickly. Second, most polymers are poor thermal conductors. If heat is being generated rapidly, but cannot escape the mass of low conductivity material, the internal temperature will increase rapidly. This makes the reaction go faster, producing even more heat. This snowball effect can build to the point of smoke or fire. The amount of some adhesive that can be safely mixed in one batch is limited because of concern over an uncontrolled exothermic reaction. Normally the exothermic reaction is quite controlled. The progression of cure begins with initiation, which may require added heat. As the reaction proceeds, the adhesive viscosity drops with the rising temperature. When gellation begins viscosity increases and then strength develops as the adhesive cross-links and hardens. Epoxies and urethanes cure by mixing two reactants. These adhesives cure to a given state, but the cure rate will slow considerably as the viscosity builds and it becomes more difficult for the reactive components to find each other and continue reacting. An initial cure that takes place in minutes may require weeks or months to reach full cure. Epoxies and urethanes are often tested a week after sample assembly to allow for adequate cure time. Acrylic adhesives cure by the addition of free radicals or ions. Free radical reactions are rapid and move quickly to completion. This reaction depends on a catalytic initiator, rather than mixed co-reactants. The catalyst causes the monomers or prepolymers to react with themselves. Adhesives based on free radical or ionic cures include acrylics, cyanoacrylates and acrylic-based ultraviolet curables. The curing reaction of epoxies and urethanes combines two or more prepolymers in a stepwise fashion into a polymer network or chain having performance representing the additive characteristics of both. On the other hand, in a free radical or ionic reaction, the initiator starts a chain but does not contribute to the properties of the material. Because of the high reaction rate, the curing adhesive contains only monomer and high polymer; there are no intermediate, lowstrength oligomers. Many adhesives require added heat to begin curing. Ovens are the most common source of added heat. Curing at 180 to 350 F (80 to 180 C) is common. The bond line must reach the final cure temperature within the oven. Large pieces of metal or composite can require several hours for the bond line to reach the actual cure temperature. Certain one-component epoxy adhesives will not cure much below 250 F (120 C). These one-part systems contain an insoluble co-reactant that dissolves into the heated epoxy resin allowing the reaction to begin. Once the parts reach reaction temperature, curing occurs in minutes. An adhesive must experience its maximumuse temperature during curing to perform at that temperature. This may be achieved by an exothermic spike on top of the oven temperature. The chemical and environmental resistance of many room temperature curing adhesives will increase with postcuring to a modestly elevated temperature. Postcuring ensures that the adhesive is cured to the desired state each time. After all, “room temperature” depends on the room and possibly the time of year. Other curing methods include ultraviolet light, RF induction heating, microwave heating and ultrasonic heating. In each case, energy is added to begin a reaction sequence. Once curing starts, the resulting exothermic reaction takes it to completion. However the adhesive is cured, it is important that it be cured correctly and completely—the required cure cycle must be followed for the adhesive to work. A good curing process really is the cure for the common A adhesive. 8 ASSEMBLY / June 2008 www.assemblymag.com http://www.assemblymag.com

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Assembly Planbook - June 2008 Contents Editorial Bonding Lines Quality in Assembly Assembly Lines Assembly in Action Fastening or Bonding? Loose Lips Sink Assembly Systems DFMA Cuts Downstream Costs Robots Lend a Hand Products Advertisers Index Classified Leading Lean

Assembly Planbook - June 2008

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