Yarn is the building block of woven & coated textiles. It is woven to form the base fabrics which make up the foundation of grain storage materials.
Yarn can be created from almost any type of polymer. Base fabrics used in the grain industry are generally made from polypropylene, polyester or polyethylene. Yarn matters because it is the key element in giving coated fabrics their strength. Without a high quality yarn, tear strength, dimensional stability and burst strength can all suffer, leading to premature fabric failure in the field.
Two distinct yarn types are used in grain storage fabrics: multifilament spun yarn and tape yarn. Each has different physical characteristics and is suited to different applications within the bunker storage system.
Multifilament Spun Yarn
Multifilament spun yarn is formed by drawing and twisting together many fine continuous filaments.
A close up image of multifilament spun yarn.
The performance of a spun yarn is measured by four principal properties:
- Fibre tenacity, the breaking strength of the fibre relative to its linear density, expressed in centinewtons per decitex (cN/dtex). Higher tenacity means greater load-bearing capacity for a given yarn weight.
- Fibre length, for staple-spun constructions, longer fibres produce a more uniform, cohesive yarn with better tensile consistency.
- Twists per metre (tpm), the number of times the filaments are twisted along a given length. Twist consolidates the yarn structure, balancing tensile strength against flexibility. Too few twists and the yarn is weak; too many and it becomes stiff and prone to fatigue.
- Yarn weight, expressed as denier (grams per 9,000 metres) or decitex (grams per 10,000 metres). Heavier yarns contribute more mass and generally more strength to the finished fabric.
In grain storage applications, multifilament spun yarn is typically polyester (PET) or polypropylene (PP) and is used for grain (top) covers. It forms the base cloth of the cover before coating is applied.
Tape Yarn
A close up image of tape yarn.
Tape yarn is produced by slitting a flat polymer film into narrow strips and stretching them under heat, in a process called drawing, to align the polymer chains. This substantially increases the tensile strength of the tape. Drawn tape is then annealed: held at a slightly lower temperature without further stretching, which relieves the internal stress created by drawing and stabilises the tape against later shrinkage. Tenacity, length, twists per metre and linear weight are all also properties measured in tape yarns. However, twists per metre (tpm) of the tape is not measured, as the yarn relies on the film extrusion and stretching process for it’s tensile strength, rather than any twisting.
A further process, fibrillation, can be applied to tape yarn. Controlled fibrillation splits the tape’s surface into a network of fine fibres, giving the yarn an appearance similar to multifilament spun yarn. More importantly, fibrillation increases yarn density and contact area within the weave, which raises the fabric’s overall strength without requiring a heavier yarn.
In grain storage applications, tape yarn is predominantly polyethylene (PE) and is used in ground sheets and wall liner rather than in the grain cover base cloth.
The importance of yarn UV stabilisation
Regardless of type, yarns used in grain storage fabrics require ultraviolet stabilisation. Ground sheets are exposed to reflected and direct solar radiation during deployment and storage. Without UV treatment the polymer chains degrade, leading to premature embrittlement and loss of mechanical strength. This requirement is essential across all yarn types and all applications in the bunker system.
Once yarn is woven into a base fabric, the weave construction itself further determines the material’s properties, an important subject that will be covered in a future article.
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