Testing the Hardness of an Adhesive

In this interview, AZoM talks to Christine Desplat, R&D Engineer at Master Bond, about how to test the hardness of adhesives.

What is hardness testing and why is it important for an adhesive?

Hardness is typically indicative of an adhesive's overall functionality. Testing hardness also helps to ensure that proper measuring, mixing, and curing was done. Master Bond's hardness testing is based on ASTM D 2240 standards. Commonly used hardness scales are A and D. Shore A is used for softer materials, whereas shore D is used for harder materials. To compare hardness measurements for common items, please note the typical values for the following:

• Rubber band    ~ 30 Shore A
• Car tire tread    ~ 60 Shore A
• Golf ball    ~ 60 Shore D
• Bone        ~ 100 Shore D

How are samples prepared for testing hardness?

Hardness testing requires fully cured test samples that are at least 0.125 inches thick. For a two-part system, parts A and B must be mixed together according to the mix ratio on the technical data sheet (TDS). Next, the mixed materials would be applied as a casting and then cured, based on the particular system's technical instructions on the TDS. This is often done by adding heat in an oven. One part systems are easier to test since they don’t require mixing. They can be either heat-activated or UV cured. Once the cured samples are prepared, they are ready for testing. Note that if the mix ratio proportions were incorrect, or if parts A & B were not mixed thoroughly or properly, the adhesive will cure “tacky” or with blotches. 

It must be noted that in some cases-- especially highly exothermic adhesives, extremely viscous adhesives, or some UV curing compounds--it may be difficult to cure the adhesive in a thickness of at least 0.125 inches. Therefore, measuring the hardness in these cases may not be practical. 

How is the hardness test performed?

Hardness testing is based on ASTM D 2240 standards and requires a durometer, durometer stand, and a fully cured sample, as noted above. To perform the hardness test, place the adhesive sample beneath the indentor point, on the table of the durometer (i.e. the flat surface). Make sure the red ancillary hand on the durometer is lined up with the “0” by turning the center knob to align. In one continuous fluid motion, turn the lever on the right in a clockwise direction until the indentor point is pressed into the sample and stops moving. Record the value. This procedure is repeated a few times on one side by moving the sample piece to a new spot each time. The sample should then be turned over and repeated a few more times. Finally, average the readings to arrive at the hardness figure. To see this process in action, please watch this video.

How is hardness used to determine certain performance properties?

Hardness can be a useful tool to ascertain how well a material might be holding up to various experiments and accelerated aging tests. For example, 85°C + 85% humidity testing is often used to establish the expected operating life for devices stored in damp heat. Master Bond has performed extensive hardness testing on many adhesives and potting compounds by subjecting the cured samples directly to 85°C + 85% humidity, for 1000 hours. If the hardness measure did not shift significantly before and after the exposure, the integrity of the adhesive was deemed as not being impacted by such exposure. 
Another example where hardness testing might come in handy is chemical resistance testing. Cured samples can be immersed in various chemicals for a prolonged time period, and the hardness can be periodically measured. Again, if there is little to no change in the hardness measurements, one might conclude that the adhesive performed well and was not impacted by chemical exposure.

What is the range of hardness values that Master Bond products fall within?

Master Bond formulations are available in a wide range of hardnesses. For example, the hardness scales for epoxy and silicone formulations range from rigid to flexible. Please see chart below for hardness ranges:

To find the right compound for your application, please contact a technical expert.

About Christine Desplat

Christine Desplat is an R&D Engineer at Master Bond where she analyzes application-oriented issues and provides product solutions for companies in the aerospace, electronics, medical, optical, and oil/chemical industries. She received a Bachelor's Degree in Chemical Engineering from Rensselaer Polytechnic Institute.