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ULTEM / PEI adhesion to dissimilar materials for structural bonding

What is PEI?

Polyetherimide, acronym PEI, is an amorphous engineering thermoplastic resin manufactured by SABIC Innovative Plastics under the trade name Ultem (developed by GE Plastics). Polyetherimide (PEI) is amber-to-transparent thermoplastic characterized by high heat resistance, high strength and modulus, excellent electrical properties that remain stable over a wide range of temperatures and frequencies, good UV and gama radiation resistance, good hydrolytic stability and excellent processibility. With similar characteristics to PEEK's, it is cheaper, although less temperature-resistant and lower in impact strength.

Ultem 2100, Ultem 2200 and Ultem 2300 glass reinforcement yields a product with an exceptional strength-to-weight ratio and increased tensile strength up to 24,500 psi, making glass reinforced Ultem one of the strongest thermoplastic materials. In addition, the glass reinforcement reduces Ultem's coefficient of thermal expansion close to that of aluminum.

What are some uses of PEI?

It is frequently used in aircraft as a precertified thermoplastic, transportation, electrical appliances particularly for electrical insulation, medical tools, packaging particulalry for sensive electronics that requires low levels of ionic contaminants as in semiconductor processing, mircrowave applications due to its transparency to microwaves, structural applications requiring high strength and rigidity at elevated termperatures, and autoclaving cycles due to its excellent hydrolysis resistance. It performs continuously to 340°F (170°C), making it ideal for high strength/ high heat applications, and those requiring consistent dielectric properties over a wide frequency range.

Polyetherimide resists a wide range of chemicals including most hydrocarbons, alcohol's, and fully halogenated solvents. It resists mineral acids and tolerates short-term exposure to mild bases. Partially halogenated solvents are acceptable to polyetherimide.

Ultem commonly is machined into parts for reusable medical devices, analytical instrumentation, electrical/electronic insulators. Ultem 2100, Ultem 2200 and Ultem 2300 are glass-reinforced versions (10, 20, and 30%, respectively) of Ultem 1000 which provide even greater rigidity and improved dimensional stability while maintaining many of the useful characteristics of basic Ultem. In pharmaceutical process equipment, manifolds machined from Ultem plate offer resistance to hot chemical solutions and daily sanitizing--an application for which acetal has also been used.

Ultem is not designed for use in bearing and wear applications. However, to over come this limitation polyimides are somethimes fill with low friction materials such as graphite, PTFE and molybdenum disulfide. Other suitable products thermoplstics for bearing applications include flouropolymers (e.g. PTFE), nylon, acetal, and UHMW PE. The addition of fillers to the Ultem could interfere with adhesion when present at the interface.

One major military naval contractor successfully employed Ultem for microwave transparant high strength structural components by bonding them to steel hulls using the RELTEK BONDiT B-45TH.

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

Care must be used in selecting adhesives and designing press fit components to avoid stress cracking. In particular watch out for notch-sensitive geometries. Also, the amorphic structure of Ultem can cause relaxation under influence of bondings on the long term. The BONDiT B-45TH has sufficient elasticity to absorb stresses and minimize risk of stress cracking of the Ultem.

Polyimides contain a large variety of chemical functions (e.g. benzene rings, ether linkages (C-O-C), carbonyl groups (C=O) and C-N functions). When the PEI is molded the long hydrocarbon chains align themselves so these reactive components interact to cause cross linking between the chains. This is what give Ultem its properties. Normally undisturbed these create a rather stable structure resistant to temperature and a wide range of chemicals, and correspondingly resistance to adhesion.

PEI can be structurally 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 imide molecular groups in the resin chain can be broken up to create molecular sites which are chemically active, and increase the surface energy.

BONDiT B-4x series epoxy products are designed to take advantage of these available sites to form good bonds, producing with an Ultem/Steel lap joint a typical lapshear strength of up to 500 PSI (pounds-per-square-inch) or 900 PSI with application of the A-43 primer. Glass filled products can produce substantially higher shear and tensile strengths with BONDiT epoxy products, even without the use of primers. Curing method can also make a difference on bonding strength.

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

PEI is resistant to most solvents but is soluable in some chlorinated aliphatic compounds. It is also prone to stress cracking in chlorinated solvents. Hence PEI does not lend itself well to solvent bonding.

Besides light mechanical abrasion method of altering the surface morphology chemical etching (5% sodium hydroxide at elevated temperature), plasma treatments and ultraviolet irradiation of polymer surface, have been shown to improve adhesion of metal thin films to polyimide, measured by a peel test. Surface modification studies of polyimides indicate the formation of carbon
radicals due to carbonyl oxygen losses. These radicals can either react with each other, resulting in crosslink formation among the polyimide chains on the surface and loss of adhesion sites, or with materials deposited onto the surface. It is believe that the different adhesion behaviours observed experimentally might be related to the total number of bonds formed with the radicals rather than the formation of strong bonds. BONDiT B-4x series epoxies readily form good bonds to the surface modified polyimide.

Most application, however, do not need such enhancement and work well with just the B-45TH adhesive on a lightly abraded (#100-120 grit) surface. If you think that your application may require this treatment, please call us at (707) 284-8808 or email us at for technical support.

Other RELTEK adhesives to use with Ultem include B-536 & B-575 and B-755.

How do you design with Ultem for Harsh Environments?

BONDiTTM products are among the few available that as an Ultem 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 Ultem to dissimilar materials is a particular problem due to it's high thermal expansion coefficient--six times that of steel (unfilled PEI). 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 PEI 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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