Material selection and development is the most important part of producing a high quality grain cover. A number of specifications govern that decision: weight in grams per square metre, tensile and tear strength in warp and weft, ultraviolet stability and coating adhesion.
Coating adhesion is rarely considered and harder to measure than the rest, and it is a common site of grain cover failure.
What a grain cover is made of
A coated fabric is not one material. It is two bonded together.
- The scrim is the woven base-cloth. It provides the load and determines the tensile and tear strength. Its characteristics and performance are determined by the polymer type, the yarn weight (measured in decitex or denier), the weave pattern (such as ‘plain weave’ or ‘rip-stop’) and the weave density (measured in threads per inch or per centimetre).
- The coating is the polymer layer on each face. It provides the weather-proofing, gas-tightness, the ultraviolet stabilisation and colour. It also provides the surfaces that become welded together along the seams during fabrication.
What is coating adhesion?
Coating adhesion is the strength of the bond holding the coating to the scrim.
Adhesion is measured by peeling the coating back off the scrim and recording the force required, reported as a force per unit of width. The test is run against a published standard, usually ISO 2411, AS 4878.8 or ASTM D751. Results from each are not comparable, because specimen size, pull rate and sample conditioning all differ. A figure quoted without its test method attached does not mean much.
Why it matters for grain cover quality
Grain cover fabricators work with material manufacturers over long periods to develop good coating adhesion in their materials. A material manufacturer has an array of inputs which they manipulate, in a process of trial and error, to improve coating adhesion, including; plasticiser type and level, filler content, coating temperature and coating-line speed.
PP is the harder case, for reasons of chemistry. Polypropylene is ‘non-polar’ and has one of the lowest surface energies of any common polymer, making it a difficult material in which to achieve strong coating adhesion. PP grain cover material is also ‘newer’ to the market, than its PVC counterpart. Therefore, the ‘recipe’ to produce good coating adhesion is not yet widely-held industry knowledge. This means there are a number of low quality PP materials being offered in the Australian grain handling market which pose a significant risk to stored grain – something for Australian grain handlers and cover fabricators to be acutely aware of as the industry moves towards on-farm storage solutions.
Even when materials perform well in the laboratory, it is never certain how they will perform on-stack. Lab conditions cannot perfectly reproduce the real-life demands of a grain bunker. It’s not until a material has been field tested over many years that it be certain that the coating adhesion is fit for purpose.
How it fails, and how to avoid it
Coating adhesion failure presents as:
- Blistering. A bubble where the coating has lifted from the base cloth.
- Peeling off in sheets. In severe cases, coating can begin to peel off the scrim in large sheets.
- Seams opening. Particularly in windy conditions, welds may partly or completely come apart, because the coating it was welded to has let go of the scrim.
- Flex cracking. Poor adhesion increases the risk of cracks forming in the coating, particularly in windy regions and where covers are not fitted tightly. Flex cracking is a significant topic to be covered fully in future articles.
Rough handling and windy conditions are often what expose it. A bunker cover is deployed, walked on, dragged, folded, stored and redeployed. Each is a demand on the interface between coating and base-cloth. Because adhesion degrades with exposure and handling it tends to present after several seasons rather than in the first.
If coating adhesion fails, it cannot be repaired and failure affects not only a whole cover, but a whole batch of material. The covers must be replaced, but at this point it’s often too late for the grain underneath.
Avoiding this requires careful analysis of the grain cover fabric properties, constant monitoring of batch quality, and years of testing before a material is brought to market.
For a grain handler none of this is visible at the point of purchase. Working with a fabricator who has the technical knowledge to procure a quality material, has long-standing relationships with reputable suppliers and whose material has been tested in the field, is essential.
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