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Adhesives for Assembly
by Edward A.Y. Fisher, engineering manager, Henkel Corp.
Ask the Expert
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
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:
Question: How can I be sure I’m using the correct adhesive for the
- What kind of joint stress will the assembly see? Tensile? Compressive?
- Does the design include difficult-to-bond substrates like polypropylene or
- Are there dissimilar metals that may cause thermal expansion problems when
- Are any of the parts UV absorbing, making a UV curable adhesive
- 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
- Are there temperature-sensitive substrates that can’t tolerate heat, making
the selection of a heat cure or even a UV cure adhesive inappropriate?
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.
||Minutes to Hours
||1 or 2
||1 or 2
||1 or 2
|Temperature Resistance (°F)
||-65° – 300°
||-65° – 300°
||-20° – 500°
||-40° – 600°
||-65° – 300°
||Up to 180°
||Best Temperature Resistance
||Fastest Fixture Time
||Light Must Reach Adhesive
||Long Cure Time
||Needs Metal Substrates
||Poor High Temperature 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
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 firstname.lastname@example.org. 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.