Technical Data Sheets : Tensile Strength for Coatings
In the last blog, we were squashed, but today we are going to be stretched — “we” being samples of various coatings and protectants, such as cementitious urethane. Tensile strength is the maximum pull a material can withstand before breaking or otherwise failing. The pulling literally rips intermolecular bonds apart. For the type of materials we use at PennCoat, the test of choice is ASTM International, (formerly known as the American Society for Testing and Material) D638, Standard Test Method for Tensile Properties of Plastics.
D638 was originally set up for plastics and must be slightly modified to make it applicable to coatings. The trick is to apply the coating to a mold, let it cure under controlled conditions and then stretch it in a testing machine. In a procedure described in The Journal of The Southern African Institute of Mining and Metallurgy, molds made of Perspex — a thermoplastic alternative to glass –take the shape of flat dog bones. The mold dimensions differ depending on the type of material tested, but all have end tabs that the pulling machine grips and a slender central portion where failure will occur. The molds are filled with the test material and are allowed to cure for a set of predetermined times, up to 28 days. Care must be taken to assure the molds are uniformly shaped and that the material is packed correctly so that its central axis aligns with that of the mold.
“I’ll Talk! I’ll Talk!”
The heroic but doomed sample is then placed into a tensile load frame in which each dog bone grip is fitted into an inescapable vise. The rack-like device then starts pulling using a pre-determined load and speed while instruments record stress and strain. The stress is the applied load divided by the original cross-sectional area, and strain is the deformation of the material. Testers chart stress vs. strain to find the slope of the resulting “Young’s Modulus” The maximum value of the modulus — where the slope is horizontal — is the tensile strength. It is at this point the material will start to crack, fracture or otherwise break. Continuing to stretch the material beyond the breaking point leads to “necking” — a rapid thinning of the dog bone’s central portion — and finally, sadly, causes the sample to rupture. Gruesome but effective.
By way of reference, we’ve been using Poly-Crete MD, a cementitious urethane from Dur-A-Flex, Inc., as our benchmark for this series on physical properties of coating materials. The product’s data sheet reveals a tensile strength of 2,175 psi, which is only about 24 percent of its compressive strength. That makes sense, because we are much more concerned with a floor coating’s ability to withstand foot and vehicle traffic — compressive forces — than we are with its tensile strength. Nonetheless, we look to materials with sufficient tensile strength to indicate that the product is durable over time.