# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 1 Introduction
The strain at the moment of breaking of the samples increased when PP content ranged from 70% to 90%, implying that higher proportions of PP improve the blend's mechanical characteristics [27]. The improvement of materials' characteristics by adding the baling twine fibres was also investigated. The ability of the composite to absorb transient overloads was achieved by using a polypropylene film in a cement matrix [28]. [PERSON] et al. [29] studied six full-scale plastered straw bale wall assemblies. They concluded that wall assemblies with polypropylene reinforcing mesh exhibited better inelastic behaviour than those obtained using other materials and levels of detailing. Another aspect of studying the twines is the influence of knots on the rupturing of twine during baling. [PERSON] et al. [30] summarised the working conditions and requirements of the Chinese knotters and designed an innovative knotter for square balers with high knotting rates, achieving improved knotting performance. [PERSON] et al. [31] investigated the importance of twine strength in maintaining bale integrity and reducing harvesting costs. They focused on the tension of the top strands when baling is performed with a large square baler. Results suggest that tension is highest when the bale exits the chamber, but its value typically remains below 60% of the twine's knot strength. This indicates the potential for design changes to reduce tension and improve knot strength recommendations, ultimately lowering baling costs. [PERSON] et al. [32] introduced a novel rope knotting method and corresponding knotter design. With a high success rate of 98.4% in physical tests, the method demonstrates reliability, meeting the requirements for structure, posture, and coordination during knotting. Also, some detailed studies on the types and purposes of knots describe safe and reliable tying methods that prevent the rupturing of twine [33; 34; 35; 36]. Only a few results regarding tensile tests on 100% PP baling twine can be found in the literature. Baling twine made of 100% PP is not affected by moisture, does not rot or mould, and shows resistance to most chemicals and acids. Until recently, baling twines were not included in the standards, so consumers had to trust the manufacturers \"at their word\" that they would receive a product with the appropriate characteristics. The American Society of Agricultural and Biological Engineers (ASABE) has adopted standards for the quality, testing, labelling, performance and country of origin of agricultural baling twines. The standards include actual length and tensile strength specifications and require a uniform cross-section, stable weatherability, and UV protection characteristics for baling twines. Also, quality control must be carried out in accredited laboratories. Used baling twine is usually disposed of in landfills, becoming a waste instead of a potential resource. For example, it could serve as a low-cost reinforcement in the concrete industry [37], but only after examining its mechanical characteristics. A series of individual fibres, twisted or braided together, form a twine of a certain thickness (diameter). Most twines are made of smaller strands of twisted or braided fibres. In braided twines, there is an outer braided layer and a core of a certain number of parallel fibres. Twisted twine is usually made of three bundles of different numbers of fibres. Thin strands connected in a mesh structure, creating baling twine, are obtained by fibrillation. The PP baling twine's geometry, structure and behaviour under tensile load are complex. Twisting can result in both strengthening and weakening the twine [38]. At small surface helix angles, the radial stress in the twisted twine increases, leading to a faster recovery from the external tensile stress along the fibres, further leading to the twine's overall strengthening. At larger surface helix angles, the sum of all stress components decreases. However, the twine's resistance to external stress decreases as well.