Guide to Engineering Adhesives

A multitude of factors come into play when selecting adhesives. Even market-specific applications can present design engineers with diverse bonding challenges, according to officials at Permabond, Bridgewater, N.J. (www.permabond.com).

For instance, aerospace applications often require high-performance adhesion, while medical-device applications demand advanced dispensing capabilities. Virtually every adhesive has different attributes, so designers must consider characteristics such as viscosity, gap filling, adhesion, cure speed, and bond strength. On the manufacturing side, manual, low-volume processes will differ from high-speed, fully automated lines. Factors such as assembly rates, dispensing requirements, curing time, and even leaching evaluation must be reviewed. Experience is often mandatory to ensure every aspect is properly addressed. Nonetheless, there are some general guidelines to the characteristics of different adhesive families. Here's a quick overview.

UV curable adhesives use advanced acrylic chemistry to accelerate curing under ultraviolet light. New cationic formulations cure quickly when exposed to UV light, and the bond continues to strengthen after the light source is removed. Advantages include:

• Higher production rates. UV curables have the shortest cure times of any adhesive family.
• Suitable for multiple applications, including potting, doming, bonding, sealing, and gasketing; viscosities range from water-thin to pastelike.
• Cures without costly and time-consuming ovens.

Epoxy adhesives generally come in two types. One-part epoxies include a hardener in the adhesive, where it remains inactive until the adhesive is heated. Two-part epoxies have separate resin and hardener components. When mixed, curing results from the reaction between the two.

One-part epoxy advantages include:

• High shear strengths, to 5,000 psi.
• Single-part chemistry speeds dispensing and application.
• Fast curing, within 15 sec using induction heating.

• Cures at room temperature; heat speeds bonding time.
• Joins virtually all surfaces, including metals, composites, ferrite, and cementitious products.
• Shear strength to 3,770 psi.
• Radio-opaque epoxies are specially formulated for shielding electronics.

Acrylic adhesives bond a range of components, including dissimilar materials such as plastic and glass, to metal. Two-part, no-mix adhesives bond when the first part of the formulation is applied to a joint face, and the second part to the opposing face. Curing begins when the two parts react and form the bond. Two-part, premix (or bead-on-bead) formulations combine at the dispensing point, before application to the joint face. They cure at ambient temperatures.

Advantages include:

• Multiple viscosities and versatile gap-fill ability lets them handle close-fit and large-gap applications.
• Cure speeds can be adapted to fit specific production requirements.
• Cures at room temperature; eliminates the need for external heat-cure sources. ast setting speeds help increase component assembly and production rates.

Anaerobic adhesives cure on metal surfaces, including iron, steel, stainless steel, and zinc when air is absent. The liquid adhesive fills surface imperfections and gaps between mated parts. The adhesive rapidly cures to an inert, plasticlike adhesive/sealant creating a solid surface-to-surface contact and physical lock. Uses include retaining applications once reserved for mechanical joining methods such as keyways, interference fits, and shrink fits, as well as threadlocking, pipe sealing, and gasketing.

Advantages include:

• Liquid adhesive provides greater surface-to-surface contact than mechanical fasteners.
• Quick curing without air; speeds assembly
• Resists oils, solvents, and other surface treatments.
• Available in permanent and removable formulations.
• High bond strength; often exceeds that of the substrate.
• Wide temperature range, from 65 to 350F.
• Fills gaps ranging from 0.003 to 0.025 in.
• One product seals, bonds, and locks.

Cyanoacrylate adhesives are one-part products that cure by reacting to minute traces of surface moisture on the bonded material. They cure in seconds at ambient temperatures and can be formulated to bond a range of thermoplastics, from flexible to rigid, as well as rubber and metal.

Advantages include:

• One-part chemistry speeds preparation and application.
• Joins dissimilar materials, such as rubber to metal, without compromising bond strength.
• Cures in seconds at room temperature, which speeds assembly and eliminates the need for costly ovens.
• Fills gaps ranging from 0.006 to 0.020 in.
• Wide temperature range of -76 to 500°F.
• High bond strength often exceeds that of substrate.

For more information

The new 16-page Engineering Adhesive Solutions Guide covers a broad range of adhesive chemistries and applications, and is designed to assist engineers specifying adhesives for product assembly.

The guide features adhesive data, including viscosity, gap fill, set times, and temperature ranges. It also includes process-related information, including handling, dispensing, fixturing, curing, and in-line testing, as well as information on specific markets such as medical devices and electric motors.

For a copy, visit Permabond's Web site or contact the company at 10 Finderne Ave., Bridgewater, N.J. 08807, (800) 653-6523, fax: (908) 575-7203.