Ask Dr. Dave
Question: Bonding aluminum alloy to itself is a common practice in my company. We also have an application where an aluminum groove is filled with adhesive. My coworkers and I have debated how the aluminum surface should be prepared when applying/bonding adhesive to it. The industry standard is phosphoric anodize with an epoxy primer applied over the anodize. However, when this is unfeasible (due to lead time and cost) the surfaces are typically chem filmed (chromic acid treated). We proved last year through tensile testing that bare aluminum creates a much better bond than a chem-filmed surface, but the aluminum will break down over time because it is not protected, this despite the fact that adhesive covers it. Do you have any thoughts/experience on this?
Answer: Surface pre-treatments are used on aluminum not only to increase adhesive strength but to maximize durability of the joints. In the aerospace industry in particular, corrosion in assembled joints has always been a major concern. Recently, there has been a shift away from chemical-intensive and toxic processes. Several air forces have developed abrasion processes, which include the application of silane coupling agents, to prepare aluminum for adhesive bonding. These organosilanes are widely accepted as primers in several adhesive systems to prevent degradation of bonded joints due to the ingress of water, but you will have to experiment with different organosilanes to find the one best suited for your system.
Question: What is the difference between HMPUR adhesives and TPU hot-melt adhesives, particularly for bookbinding applications?
Answer: The abbreviation HMPUR stands for hot-melt polyurethane reactive adhesive, while TPU stands for thermoplastic urethane. HMPUR are polyurethane adhesives that are applied just like a conventional hot-melt adhesive but subsequently crosslink through reaction with atmospheric moisture. TPU adhesives are linear polyurethanes with no reactive groups.
HMPUR that give instant fixture and then cure to give crosslinked adhesives are considered the most flexible and durable adhesives for bookbinding. They yield products that lie flatter and require less backbone preparation than other adhesives. However, they are more expensive than adhesives like hot-melt ethylene-vinyl acetate (EVA), and their full curing time ranges between 24 and 48 hours - a definite disadvantage if cost and productivity are issues. The unique property that gives HMPUR adhesives their performance advantage over thermoplastic hot melts is that they cure to a thermoset material that resists melting. TPUs are sometimes used as additives to HMPUR to modify properties like rate of crystallization and, thus, set time.
Any views of opinions expressed in this column are those of the author and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media.