Adhesives for Assembly

by Edward A.Y. Fisher, engineering manager, Henkel Corp.

Question: When choosing an assembly process for a manufactured part, why should adhesives be considered over other processes, such as welding?

Adhesives offer several major benefits over other assembly methods. They distribute stress load evenly over a broad area, eliminating stress concentration that occurs when mechanical fasteners are utilized, and increase the overall strength of the assembly. Since adhesives are applied inside the joint, they are invisible within the assembly. Adhesives resist flex and vibration stresses, and form a seal as well as a bond, protecting the internal components from harsh environments.

They join irregularly shaped surfaces more easily than mechanical or thermal fastening, negligibly increase the weight of an assembly, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates. Adhesives are one-size-fits-all and assembly can be easily automated.

Question: When selecting adhesives during the design process, what should manufacturers take into consideration to avoid production issues?

To select an appropriate adhesive for an application, a designer should consider how the component will be assembled and what substrates will be bonded. It also is critical that the adhesive specified during the design phase is appropriate for the production process. At a minimum, the following questions should be asked when specifying an adhesive:

• What kind of joint stress will the assembly see? Tensile? Compressive? Peel?
• Does the design include difficult-to-bond substrates like polypropylene or nylon?
• Are there dissimilar metals that may cause thermal expansion problems when heated/cooled?
• Are any of the parts UV absorbing, making a UV curable adhesive inappropriate?
• Are there shadowed areas that will not see UV light?
• Will surface treatments (plasma, corona treatment) enhance bonding?
• Will the substrates and adhesive perform properly in the end use environment?
• Are there temperature-sensitive substrates that can’t tolerate heat, making the selection of a heat cure or even a UV cure adhesive inappropriate?

Question: How can I be sure I’m using the correct adhesive for the manufacturing situation?

Currently there are a multitude of adhesives available, which fall into six families that are most commonly used in manufacturing environments. Each of these families offers a unique combination of performance and processing benefits. Manufacturers that dedicate significant up-front time to research and select the proper adhesive for an application will save significant time and expense later in manufacturing and reliability.

Epoxy Adhesives. Epoxies are one- or two-part structural adhesives that bond very well to a wide variety of substrates, have excellent toughness and offer superior environmental resistance. The major disadvantage of epoxies is that they tend to cure much slower than other adhesive families, with typical fixture times between five minutes and two hours. Available in a variety of cartridge sizes, two-component epoxies are extremely popular as they minimize adhesive waste and accommodate small production runs. One-part, pre-mixed heat cure epoxies are popular since they are already “mixed”; however, they have a short work life once the adhesive is removed from cold storage.

Dual cure (UV and/or heat) epoxies are an option that increases processing speeds. The most recent technological advance in epoxy technology is the introduction of pre-activated epoxies. These single component epoxies are irradiated with a controlled dose of UV energy which “activates” the product. Part assembly takes place immediately after UV exposure and part fixturing takes place seconds after assembly. This Loctite® patented technology allows manufactures to realize the benefits of light cure adhesives on opaque parts discussed on the next page.

UV Cure Acrylic Adhesives. One-part, solvent-free UV curable acrylics offer performance benefits comparable to epoxies. While early UV curable acrylic adhesives relied on high doses of ultraviolet energy, advances in the technology allow for dual cure (UV and visible light) or only visible light energy to cure the adhesive. Because cured acrylic adhesives are thermoset plastics, they offer superior thermal, chemical and environmental resistance.

Light cure acrylic formulations are widely available with a secondary cure mechanism (such as exposure to heat or chemical activators) that allows adhesive in shadowed areas to cure completely. As cure is on demand, light cure acrylics offer extended open times for positioning and repositioning parts. All this, coupled with cure times of only two to 60 seconds, makes UV curable acrylics an attractive manufacturing option.

“Elastomeric” Adhesives. For bonding dissimilar substrates like glass to metal, the best option to ensure a robust assembly is silicone technology. Silicones are flexible, rubber-like materials that cure at room temperature, exhibit excellent resistance to heat and moisture, and bond a wide variety of substrates. The pliability of silicones over a broad temperature range (-40 to 600°F) makes them an ideal stress absorber. Today there are UV/visible light cure, dual UV/moisture cure, heat cure and ultra fast two-part silicone technologies to complement the older RTV chemistry.

Also in the elastomeric adhesive family are modified silane polymers (MSP). MSPs are isocyanate, solvent-free adhesives that offer superior primerless adhesion to a variety of substrates. They are UV stable, non-yellowing, paintable and have high elongation and excellent low temperature performance. MSPs typically are one-component moisture cure products, with two-component versions available when faster cure times are required. They are ideal for applications with high movement/high impact and can be used in processes where the sealant must be applied prior to painting. Primers also are available for difficult-to-bond substrate combinations.

Cyanoacrylates. High strength, one-part cyanoacrylates or instant adhesives are versatile adhesives, which are commonly used to bond elastomeric substrates to metal or plastic and for bonding/sealing plastic components together. These adhesives achieve fixture strength in just seconds and full strength within 24 hours, making them ideally suited for high-speed production.

One major limitation of cyanoacrylates is their temperature resistance. Standard “superglue” fails as temperatures exceed 180°F. While there are high temperature (250°F) products available, the fixture time of these increases significantly with increasing temperature resistance.

Anaerobic Adhesives. Traditional anaerobic adhesives, also known as threadlockers, are single-component adhesives that remain liquid when exposed to oxygen. Once confined between metal substrates, anaerobic adhesives cure or harden into tough thermoset plastics that provide excellent environmental and temperature resistance. These adhesives are ideal candidates for application where vibrational loosening is an issue.

Methyl Methacrylate Adhesives. Methyl Methacrylate (MMA) adhesives are two-component products consisting of a resin and an activator/hardener combination. These products also are available in one-component “no-mix” form where the activator is brushed, dipped or sprayed onto one bonding surface before the adhesive is applied and an external mix form. MMA systems can develop strength in as little as two minutes and have outstanding environmental and impact resistance. The main drawback with this product is the characteristic odor that many users find offensive.

For more information, Ed Fisher can be contacted by phone at 860.571.5359 or via email at [email protected]. Henkel operates worldwide with leading brands and technologies in three business areas: Laundry & Home Care, Beauty Care and Adhesive Technologies. Founded in 1876, Henkel holds globally leading market positions both in the consumer and industrial businesses with well-known brands such as Persil, Schwarzkopf and Loctite. Henkel employs about 47,000 people and reported sales of $21.7 billion and adjusted operating profit of $2.8 billion in fiscal 2011. Henkel’s preferred shares are listed in the German stock index DAX. Learn more at www.henkelna.com/industrial-adhesive-14972.htm.