Technology
& Innovation

The discolouring of polyethylene is a phenomenon that is well-known in the polymer industry. It is also described as yellowing, pinking or gas fading. The colour change of the material is generally a result of the auto-oxidation of the phenolic antioxidants in the basic polymer used for a product. These phenolic antioxidants are added to the resin by the producer to protect and stabilise the polymer during processing and use. The change in colour of the polymer is strictly cosmetic and does not affect the physical properties of a product.

The sources of discolouration in polyethylene are diverse and can vary from packaging to inappropriate material composition. The most common source though comes from excessive levels of atmospheric pollutants, such as nitrogen oxides (NOx). The relatively high levels of NOx are usually caused by exhaust gasses, for example from fork lift trucks or from heaters in storage warehouses.

The chemical reaction between atmospheric NOx, even in low concentrations, and phenolic antioxidants in a polymer triggers discolouration ranging from yellow to red depending on the polymer composition. The severity of the discolouration increases with increasing exposure to NOx and from an increasing concentration of phenolic antioxidants in the polymer. Other parameters influencing the severity of discolouration are the use of titanium dioxide (TiO2, used as white pigment) and the use of basic (high pH) additives. These reactions can occur in polymers in any form, including pellets, moulded parts, film and yarn. Pinking is especially visible in white products, because of the colour and the use of TiO2 as the pigment.

The chemical reaction is reversible and can be eliminated with changes in the environmental conditions, by exposing the material to UV-light (sunlight). This means that, in most instances, the pinking of white yarn will disappear after installation of an outside pitch. However, when using artificial grass indoors, exposure to sunlight before installation will remove the pinking and the exposure to exhaust gasses should be minimised after installation anyway.

It is well known that plastics in general and polyolefins in respect of synthetic turf are sensitive to degradation when exposed to sunlight. This degradation is called photo-degradation. Photo-degradation is a process that generally results in the polymer chains breaking down within the polymer/product. Unless stabilisers are added to the polymer, this circular chain reaction will accelerate. Ultimately photo-degradation causes cracking, chalking, colour changes and loss in physical properties.

In particular, the non-visible UV radiation in sunlight is responsible for the photo-degradation of polymers. The UV radiation is characterised by its short wavelengths and makes up less than 5% of sunlight. The solar UV radiation spectrum is divided into three categories UVA, UVB and UVC. The UV radiation that reaches the earth’s surface consists of more than 98% UVA, because of absorption in the atmosphere's ozone layer of UVB and UVC. Although UVC is completely absorbed, a small part of UVB reaches the earth’s surface. This small portion of UVB radiation is very aggressive with respect to photo-degradation compared to UVA, because of its very short wavelengths.

In order to test the UV stability of polymers, outdoor exposure to natural weathering is a very realistic but very time consuming method. Several weathering test methods have been developed using artificial light sources to provide accelerated test procedures. These test methods are used to predict the UV stability of a product over a number of years. In general there are three types of artificial weathering light sources available for accelerated weathering tests: UVA, UVB and Xenon. The relationship between the spectrum of these light sources and sunlight can be found in the next table.

One of the main advantages of synthetic turf is that it can be used as an all-weather surface. Whilst this is true in many respects, there are extremes of weather conditions that may adversely affect the playing characteristics and safety of an artificial grass surface.

Low temperature resistance

The temperature resistance of synthetic turf surfaces depends on the base material used for the synthetic turf fibres. Polypropylene synthetic turf fibres should not be used in weather conditions below 0°C (32°F). In general, rubber infilled pitches are made of polyethylene synthetic turf fibres and this base material provides the opportunity to use the pitch down to -20°C (-4°F) in frosty weather. The playing surface will however be harder than usual under sub-zero conditions.

Snow

In principle, snow and ice are not harmful to an artificial grass surface and can be permitted to melt naturally. There are however some special rules that apply to using the field in snowy conditions. Fields with rubber infill can still be played on when they are covered with snow, but play will compact the snow to form ice. This means that the snow will take longer to melt and once the snow has compacted it is no longer possible to clear it off the field. The snow forms a layer of ice with the grass fibres frozen in. Trying to remove this layer would damage the fibres. It is possible to clear the snow before playing on the field provided this is done in the correct manner. Removing snow with a brush or snow shovel could damage the turf. If a snow shovel has to be used, it should be made of wood or plastic, or fitted with a rubber strip to avoid damage as far as possible. Using a snow-blower to clear snow does not damage the fibres, so this is recommended. Remember that when snow is cleared off the field, some infill material will come away with it. This should be put back into the field as soon as the weather permits.

Black ice and thaw

A synthetic turf field cannot be used in icy conditions or during thaw. Ice and sometimes also thaw make the field slippery, increasing risk of injury to the players. Playing on black ice can also damage the fibres. During a thaw, some sections of the field may have thawed out while other parts are still icy. When the base is still frozen, there may be standing water on the field. Playing in these conditions can result in deformation of the field and could also lead to nasty injuries. The playing surface is not consistent, when some parts are thawed and others are not. A player cannot anticipate this, so it is dangerous to play in these weather conditions. The field should only be used again once the ground has thawed out completely.

Within a synthetic turf system, the backing of the tufted carpet provides strength and dimensional stability and plays a major part in the tuft-lock of the synthetic turf fibres. The choice for the right backing is therefore essential for long term performance and durability.

A well-known problem in synthetic turf systems is "line dancing." Under the influence of constant stress the backing slowly starts to permanently deform and the carpet starts to stretch. This effect is caused by viscoelastic creep in the backing.

Creep is a phenomena present in viscoelastic materials like polypropylene and polyethylene. It is defined as the tendency of a solid material to slowly deform permanently under the influence of stress.

Circumstances that can increase the chance of creep in a synthetic turf system are:

• Low friction coefficient between carpet and subbase
• Significant difference in height between middle and sides of the pitch

In order to minimise creep in a synthetic turf system, it is essential to choose a backing that matches the needs of the application. An intensively used pitch with strong player movements and high impact, for example rugby, needs a backing with very high dimensional stability and thus creep resistance. A very good option for this application is a backing with a glass scrim. The glass scrim provides an enormous increase in dimensional stability, as it is not sensitive to the effects of creep. Other backings with increased dimensional stability compared to plain woven fabrics are multilayer backings and backings using polyester.

Again the choice of a suitable backing depends mainly on the application and the combination with other system components. It is all about balancing needs.