Q: Are synthetic turf athletic fields with different hardnesses under-foot, a potential hazard? A: We do not know for sure, but for the athlete’s safety, it’s worth testing because it is an accepted fact that all sections of an athletic field should feel the same under a running athlete’s feet. If a running athlete steps on a hard spot in one part of the field and/or a soft spot in another, it can contribute to injury. Perhaps it’s a reason for unexplained running injuries without contact.
Q: Why does the hardness vary on the same synthetic turf athletic field? A: Because when turf seams are sewn or glued they feel different than the rest of the turf itself. Additionally, two or more different adhesives that have different hardnesses are often used to install on the same field. One adhesive may be used to make a factory prepared turf logo to be installed later in the field; whereas another adhesive or two may be used in the field to bond seams, game lines, numbers, hash marks and other inserts. They have different hardnesses, as does the unbonded turf, especially when tested at different outdoor temperatures, like very cold and/or very hot. There is also a different hardness of the same adhesive in the hot sun vs. a cooler shady area of the same field. Hence, the question then becomes, “Are the hardness differences insignificant or are they something to be concerned about, from a potential running injury standpoint?”
Q: Is there one type of adhesive that is theoretically more likely than other turf adhesives to have wider hardness changes as the temperature varies? A: Yes, we think that thermoplastic hot-melt adhesives are most likely to reveal if there is a problem or not, regarding variable hardness under-foot.
Q: What makes you theorize and arrive at that opinion? A: Thermoplastic hot-melt adhesives become softer, weaker and eventually liquefy as the temperature rises (that’s why they are called hot-melts) and oppositely solidify and become harder as the temperature decreases. They also are often applied in thicker films which make their hardness, or lack of it, become more apparent as compared to thin-film curing adhesives that do not change as much.
Q: If hardness tests are compared on hot-melt adhesive bonded parts of the field versus other sections, at both cold and hot temperatures, and there are insignificant differences, is that enough to overcome your concerns about potential hazards regarding different hardnesses under-foot? A: We think so, because if the problem doesn’t show up with thermoplastic hot-melt adhesives, we doubt that there will be a problem with conventional thin-film curing turf adhesives (they’re not thermoplastic) that are also used to install athletic fields and playground surfaces.
Q: What if a problem with hardness does show up with hot-melts? A: We doubt that there will be a variable hardness problem if thin-film curing adhesives for turf are used on a field, but for safety purposes, it doesn’t hurt to check some installed fields for significant hardness differences in sewn seams or glued sections. The goal is a uniform surface throughout and not one with hard and/or soft spots.