Understanding Adhesives and Their Chemical Resistance
By Austen Williams, Application Engineer
Henkel Corporation
Introduction
With the advancements made in modern manufacturing, adhesives are becoming
not only a key product for industrial assembly but also a key product for chemical and
corrosion resistance. Adhesives are capable of mating dissimilar materials, providing full
stress distribution along the bond line, and sealing the bonded assembly from chemical
exposure. While an adhesive is capable of fulfilling all three of these requirements, the
chemical resistances of Henkel’s industrial adhesive line vary by application and
chemistry. Before choosing a product that will be exposed to a chemical it is important
to first identify the chemical and the parameters that the adhesive will be exposed to.
Polar vs. Non-Polar Solvents
A solvent is typically a liquid that is capable of dissolving another substance,
water being the most common solvent. While the term solvent is used broadly, it is best
to understand there are two major types of solvents: polar and non-polar. Typically
miscibility, dipole moment, and dielectric constants are used to classify solvents as
polar or non-polar but there is no clear boundary between the two. The polarity of a
solvent is based on the balance between a polar component, like an Oxygen-Hydrogen
bond, and a non-polar component.
Polar solvents have unevenly distributed electric charges and are miscible with
water or hydrophilic meaning they mix homogeneously. A high dielectric constant is
usually a clear sign that a solvent is polar. Water, for example, has a dielectric constant
of 80. This means that electrical energy will pass 80 times as much through water as it
will through a vacuum which has a dielectric constant of 1. A high dielectric constant is
usually a clear sign that a solvent is polar. Other examples of polar solvents are ethanol,
isopropanol, methanol, acetone, methyl ethyl ketone, and acetic acid.
Non-polar solvents have equally distributed electric charges and are lipophilic
meaning they combine with or dissolve in lipids or fats. Lower dielectric constants are a
good indication of a non-polar solvent. For example, hexane, a non-polar solvent has a
dielectric constant of only 1.9 and has a solubility of 0.014 grams per 100 grams of
water. Other common non-polar solvents are toluene, motor oil (10W-30), Gasoline,
Heptane, and tetrachloroethylene.
1
Dielectric Constants of Common Solvents
Polar / NonPolar
Polar
Non-Polar
Solvent
Dielectric Constant
Solubility
(g/100g
Water)
Water
Methanol
Ethanol
Acetone
Methyl Ethyl Ketone
Isopropanol
Acetic Acid
Tetrachloroethylene
Toluene
Motor Oil (10W-30)
Gasoline
Hexane
Heptane
80
33.1
24.3
21.4
18.4
18.3
6.2
2.5
2.4
2.2
2.1
1.9
1.9
--Miscible
Miscible
Miscible
25.6
Miscible
Miscible
0.015
0.05
Immiscible
0.015
0.014
0.01
Figure 1: Common solvents, their dielectric constants, and solubility in water.
In general, it is best to think of oil and water when mixed. The two liquids, or
solvents in this case, do not mix homogenously and the oil sits on top of the water. The
water is polar and the oil is non-polar. Therefore, polar reactants will dissolve in polar
solvents and non-polar reactants will dissolve in non-polar solvents. For example, a
polar solvent like isopropanol can be diluted with water when mixed, forming a
homogenous solution. When speaking about adhesives, the adhesive can be labeled as
the reactant and whatever chemical it is subjected to is the solvent.
Henkel Chemical Resistance Testing and Reporting
Henkel’s large testing capabilities can be used in conjunction with
solvent/chemical resistance testing using exposure to no chemical/solvent as a control.
Typically a bonded assembly is placed inside an ammo bag, placed inside a pressure
reservoir, also known as a bomb, and filled with the customer’s desired solvent. These
bombs are then subjected to the parameters desired for the testing. After the
designated time period for submersion, the specimens are tested according to the
parameters previously established.
For example, an anaerobic thread sealant can be used to assemble nuts and
bolts. After the product is fully cured, the assemblies are placed into an ammo bag, the
2
bag of specimens is placed in a bomb, a solvent, like 10W-30 motor oil, is added until
the parts are fully submerged, and then the bomb is left at either room temperature or
can be heated in an oven for an extended period of time. (See Figures 2 & 3)
Figure 2: A collection of various assemblies bonded with an anaerobic pipe
sealant are left to condition in their properly labeled solvent solutions.
Figure 3: Previously tested specimens show the effects a solvent can have on a
bonded assembly.
3
Loctite Technical Data Sheets typically report Chemical/Solvent Resistance as a
percentage of strength related to the initial strength tested without chemical/solvent
exposure or the test’s control. (See Figure 2)
Figure 2: The Chemical/Solvent Resistance data for E-30CL shows the
environment the product was exposed to, the temperature, and time of exposure. Data
reported is a percentage of the initial cure prior to exposure. For some products, the
strength may increase after exposure. This could be due to the increased heat or a
chemical interaction between the adhesive and chemical causing the adhesive to form a
stronger bond.
Questions to Ask When Choosing an Adhesive




What solvent will the assembly be
subjected to?
For how long will the assembly be in
contact with the solvent?
What adhesive properties must
survive the chemical exposure?
What temperature is the assembly
exposed to and is this a constant
temperature or intermittent?




What is the bond line gap?
Is the solvent in liquid or vapor form?
Is the assembly fully or partially
submerged in the solvent?
Will the part remain static in the
chemical or is it in motion?
4
Conclusion
Before selecting a product to use in an application exposed to various chemicals,
verify the parameters that the adhesive will be exposed to and check the Henkel
Technical Data Sheet for solvent resistance. If further information or testing is required,
Henkel is capable of performing chemical resistance testing. It is always suggest that a
customer, however, verifies all performance criteria with their own validation testing.
Should more information be required, a local Henkel Sales Representative may be
contacted at 1-800-LOCTITE to coordinate further testing.
References
http://www.chemicalland21.com/info/solvents.htm
http://deltacnt.com/99-00032.htm
5