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Acetal / POM adhesion to dissimilar materials for structural bonding

What is Acetal?

DELRIN® is an acetal homopolymer (polyoxymethylene, acronym POM) resin manufactured by DuPont Engineered Plastics, which is also manufactured by Ticona as an acetal copolymer under the trade names of Hosaform®, Duracon® and Celcon®, by BASF as Ultraform®, by Quadrant EPP as Acetron®. There a many more trade names with more than 800 grades available.

What are some uses of ACETAL?

Acetal is lightweight and has excellent dimensional stability, machinability and high mechanical strength and stiffness due to its high degree of crystallinity. As a strong polymer, POM provides good wear and abrasion resistance, a low coefficient of friction and low moisture absorption, making the product a good choice for use in high moisture environments. POM is an excellent choice for its long term mechanical stability in applications such as valves, washers, gears, bushings, bearings, seals, rollers, timing screws, impellers, electrical, medical and anti-friction components. It is most often used for electronic office equipment, advanced conveyor technology and automotive applications.

It is important to note that although acetal is strong mechanically, and offers excellent resistance to moisture, gasoline, solvents, and many other neutral chemicals, it cannot tolerate strong acidic (<pH 4) or strong basic (>pH 9) conditions long term. Also note the are material performance differences between the homopolymer and copolymer.

One example involves bonding acetal components together, using the RELTEK BONDiT B-46TH as the optical section of scopes used for commercial and military rifles and related armory.

How do you bond DELRIN with an adhesive to dissimilar materials?

One of the properties of POM that makes it difficult to bond to is its low surface energy. Acetal can be bonded to itself as well as dissimilar materials using BONDiTTM brand adhesives manufactured by RELTEK LLC. BONDiTTM products are among the few available that can be used to bond acetal to itself, and to other substrates for harsh environments and long term deployment without debonding.

Industry producers of acetal generally report it cannot be bonded without special surface conditioning, due to the high degree of polymer chain alignment and hydrogen pairing on the surface. However, DELRIN CAN be bonded to itself and to other substrates without significant preparations (light abrasion is usually recommended) using BONDiTTM brand adhesives manufactured by RELTEK. With light abrasion some of the paired hydrogen can be broken up to create tertiary hydrogen (connected to a tertiary carbon and not sterically hindered or paired with another adjacent hydrogen) which are chemically active and increase the surface energy. BONDiT B-4x series epoxy products are designed to take advantage of the tertiary hydrogen to form good bonds, producing with a acetal/steel lap joint a typical lapshear strength of 350 PSI to 450 PSI (pounds-per-square-inch) depending on curing method.

Among the other substrates which acetal can be bonded to using BONDiTTM products are: UHMW, fluoropolymers, rubber, polyethylene, polyolefin, polypropylene, polyurethane, silicone, thermoplastics, thermoset, glass and ceramics, wood, metals, stone, aggregate and concrete.

Corona or flame (plasma) etch is occasionally used to prepare the POM surface which strengthens the bond by reacting surface chains with oxygen to form oxane bonds, significantly increasing the bond strength. BONDiT B-4x series epoxies readily form high strength bonds to the oxide rich substrate of 600 PSI to 700 PSI. Chemical etching with chromic acid at elevated temperatures has also been successfully employed to increase the surface energy of the POM, but is a much more complex process compared to flame etching.

Most application do not need extensive enhancement and work well with just the B-45TH adhesive on a lightly abraded (#100-120 grit) surface. The need to pretreat the acetal by etching occurs in about 15% of the applications. If abrasion is not possible the BONDiT B-4682TH and B-46TH has the capability of adhering to the unabraded acetal surface. They will even bond similarly to untreated Teflon and UHMW.

If you think that your application may require this treatment, please call us at (707) 284-8808 or email us at for technical support as this technique requires some skill and experience. We can provide you with written instructions to guide you through a process for which in our experience there is a high degree of miss information extant within industry.

How do you design with DELRIN for Harsh Environments?

BONDiTTM products are among the few available that as an acetal adhesive will bond to itself or other substrates with differing coefficients of expansion (COE). BONDiTTM products are renowned for their resistance to harsh environments and for withstanding long term deployment without debonding. Bonding acetal to dissimilar materials is a particular problem due to it's very high thermal expansion coefficient--seven times that of steel. As a consequence the adhesive bond joint must be designed to accommodate the expansion/contraction differential between materials to avoid bond failure. The BONDiT B-4X series epoxies are especially designed for that purpose, ranging in elongation (stretch) from 10% to 500%. Adhesive joints designed with these products have been successfully thermal cycled -50°F to+150°F, and even cryogenic temperatures (liquid nitrogen) and as high as 300°F.

Our first product recommendation for bonding DELRIN is BONDiTTM B-45TH or BONDiTTM B-482TH.

Our recommendation would change if you are overmolding or if there are other special adhesive needs.

If overmolding, we would recommend priming the substrate with our
BONDiTTM A-43. An example would be molding either, thermoplastic polyurethane, two-part urethane rubber or rubbers in general such as EPDM to UHMW.

Or, if exceptional chemical resistance is desired, we would recommend the BONDiTTM B-481 or B-4811.

Abundant engineering support is provided at RELTEK for our customers

Please feel free to call or email us with your questions. Our recommendations are founded in compressive experience. We are willing to share that experience with you to ensure your successwhether or not you become a RELTEK customer, we welcome the opportunity to serve you.

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