Polyethylene comes in various qualities, which has encouraged some to start specifying the quality of the raw material in an attempt to safeguard themselves from investing in an inferior quality synthetic turf product. However, the quality of synthetic turf is achieved through a variety of factors.
The introduction of polyethylene polymer has made synthetic turf viable for use all over the world and for a variety of sports that, until then, were deemed unsuitable to play on synthetic turf. Football and rugby are, perhaps, the biggest examples. Polyethylene has the advantage that it is softer on the skin and remains functional even when the temperatures drop below zero. Other raw materials tend to stiffen up, making the synthetic turf surface less comfortable or even dangerous to play on.
Much of the quality of the polymer is determined by the type of catalyst and co-monomer used in the polymer production process. For many years, a Ziegler-Natta (ZN) catalyst was the reference until the introduction of a metallocene-based catalyst. The latter has the advantage over a ZN catalyst that every active site reacts in an identical way. This results in a polymer of more equal chain length, i.e., having a narrower molecular weight distribution. As polymer chains that are too short cannot contribute to the mechanical properties (of the yarn) and molecules that are too long are more difficult to process, metallocene-based resins have an intrinsic advantage.
A second advantage of a metallocene-based catalyst concerns the insertion of the co-monomer. The co-monomer is inserted to control the density (softness, melting point) of the polymer. It also influences mechanical properties like split resistance.
Different co-monomers can be used, and those commonly used in the industry for synthetic turf include butene (C4), hexene (C6) and octene (C8). Short-chain co-monomers (C4) have, in general, a cost advantage but can seldom achieve the mechanical performance associated with the C6 or C8-co-monomers. This is best reflected in the split resistance and resilience. “Each polymer category is also available in a range of densities, and sometimes different qualities are mixed together to combine properties to fulfil customers demands,” says René Heitkönig, a former manager of TenCate Grass yarns and nowadays a consultant to several companies in the synthetic turf industry.
Both the polymer producer and their clients, the yarn producing companies, add additives to improve their product. “The polymer producer adds anti-oxidants to protect the polymer during the yarn-making process,” Pieter Geeurickx of TotalEnergies points out. The yarn producer adds pigments that determine the final colour of the yarn, as well as UV-stabilisers. These so-called hindered amine light stabilisers (HALS) neutralise the free radicals formed in the polymer by exposure to UV light, and thus prevent the chemical decomposition of the yarn, which affects the colour, flexibility and strength of the yarn. “Saving money by adding less UV-stabiliser will result in the yarn becoming brittle more quickly, and, eventually, falling apart, which will be detrimental to the synthetic turf carpet. Depending on the geographical location and the UV intensity experienced, it can be a matter of years or just months before this becomes visible and for the owner to experience the consequences of this lesser quality synthetic turf.” Generally speaking, a good quality fibre has around 10,000 parts per million (ppm) of the additive in its recipe, while yarn with anything below 7,000 ppm can be considered as “poor” quality yarn.
Differential scanning calorimetry
Buyers wanting to verify claims made about a synthetic turf product can ask testing institutes to verify the chemical composition of the product. “A differential scanning calorimetry (DSC) test is not a standard procedure, but every so often we are being asked by clubs or municipalities to establish whether the synthetic turf carpet sample that is being offered meets the quality they specified in the tender,” says Gert-Jan Kieft of Dutch testing institute Kiwa ISA-Sport. “While we are not able to exactly establish chemical DNA of the product, we can determine from what quality polymer the yarn was produced.” He points out that, in addition to the chemical composition of the synthetic turf, the shape of the fibre as well as the extrusion process used for producing the yarn influence the final quality and performance of the synthetic turf carpet.
Several quality tests
The international sports governing bodies don’t validate the chemical composition of a synthetic turf product. Their quality programs predominantly focus on the performance of the turf. “Nonetheless, results for tests like the UV-test, the test that determines the tensile strength of the yarn and the Lisport XL, all say something about the quality of the raw material as well as the volumes used in the final product,” Kieft points out. Conducting these tests can take weeks or even months, and subject the yarn or carpet to a dose of UV light or usage similar to what the synthetic turf carpet will experience over a five-year period when installed in Europe. Unfortunately, the conditions in Europe are not always representative for areas elsewhere around the world, meaning that buyers in countries in the southern hemisphere particularly, have to be very cautions when buying a synthetic turf product. “Some companies try to outsmart the system by being economical in their use of UV-stabilisers or the number of fibres they place in the carpet,” Heitkönig remarks. Using less UV-stabiliser means less cost for the producer, while a lower Dtex (mass in grams per 10,000 m yarn) will save the tufting company money. A lower Dtex also results in a lower weight for the synthetic turf carpet that is shipped for installation. Combined, these savings can become financially significant, especially at a time when the price for oil or shipping products remains high. While this lesser quality can often go unnoticed for a long time in areas with a moderate or mild climate like in Europe, the consequences in areas with a harsher climate can become visible in a matter of months or even weeks after installation.
Establishing the softness of the yarn, a key selling point for polyethylene yarns, is also not part of the compulsory testing programs. “We don’t test that for the yarn individually. However, long-pile synthetic turf products are subjected to a skin-friction test, as we want to know how sliding-friendly the carpet with infill will be. The outcome gives a good indication of the softness of the yarn,” Kieft says. In his experience, buyers of synthetic turf can establish the softness themselves. “We have done panel tests in the past whereby panel members were asked to rate the softness of various synthetic turf carpet or yarn samples. Almost all the time, their observations and conclusions were right.”
Check for the checks
In Kieft’s experience, companies that failed a test as part of the compulsory testing program from one of the governing authorities often learn very quickly from their mistakes. “In most cases, they also have the capacity to adjust their production process rapidly,” he observes. Heitköning points out that many of the emerging producers still lack the experience or technical knowhow for a giant leap forward to narrow the gap with established synthetic yarn (product) producers in Europe and North America. “You don’t really buy that knowhow off the shelf,” he says. If up to him, he would like to raise the bar and include wear, REACH, lifetime performance and environmental impact as well as a field design to facilitate easy recycling, as part of a quality program. Nevertheless, he views the current test protocols of the testing institutes and quality programs of the international sports governing bodies as solid. It’s a view Geeurickx agrees with. “Ask the installing company to guarantee a certain performance like colour retention, or preservation of the visual appearance related to things like fibre flattening, fibre loss or -splitting for a certain period of time rather than specifying the raw material it should be made of.” Focusing too much on the quality of the raw material will likely not yield the desired result when you are not familiar with the subject. “There is a big chance you will jump to the wrong conclusion.”