# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 1 Introduction  
Polypropylene (PP) is a widely used linear hydrocarbon polymer with good physical and chemical properties that can be adjusted using different additives. It is characterised by moderate elasticity, good hardness and fatigue resistance, and resistance to chemicals and heat. In the production of all commercial types of PP, additives are used to protect the polymer during the production process and improve the final product's properties. Additives can also be utilised to solve the problem of degradation and destruction of macromolecules when PP is exposed to UV radiation. PP is a recyclable and reusable material, so its life cycle does not end with producing a final product [1, 2]. When subjected to tension, recycled PP exhibits similar mechanical characteristics (elastic limit and elastic modulus) as transparent virgin PP, and therefore, it can be used as widely as any non-recycled material, and even has certain advantages such as lower cost and a contribution to environmental protection [3, 4, 5]. Polypropylene has high chemical resistance and excellent resistance (no attack) to most acid solutions, alcohols, bases and mineral oils [6, 7, 8, 9, 10, 11]. The exceptional adaptability and various applications of PP, as well as the previously mentioned advantages, indicate a continuous broadening of the product spectrum where PP could be used. This growing trend started in 1957 when the commercial application ofPP was recorded for the first time. PP is one of the most commonly used materials in the production of plastic items. It is widely used in the packaging and automotive industry as well as in the production of consumer products and products for the industry. Synthetic PP fibres are cheaper than fibres made of other materials, which is why they are often used in the textile industry. A significant number of papers investigate factors affecting the mechanical characteristics of PP. [PERSON] and [PERSON] [12] showed that the mechanical characteristics of both talc-filled and unfilled PP samples depend on temperature and strain rate. The stress-strain curves were nonlinear in both cases, even at relatively small strains. Both samples' elastic modulus and yield strength values decreased with a temperature rise and increased with higher deformation rates [12]. Similar results were obtained by [PERSON] and [PERSON] [13]. During tensioning, the mechanical properties of PP are pretty sensitive to the deformation rate. Properties such as yield stress, elastic modulus, and yield strain increase with strain rate [13]. Applying strain rates ranging from 0.8 to 200 s\({}^{-1}\), [PERSON] et al. [14] determined that tensile strength and elastic modulus increase with strain rate. The behaviour of materials depending on the strain rate can be described using the phenomenological Hyperelasto-Visco-Hysteresis model (HVH model). Observing the fracture surface under a microscope showed that the roughness of the surface increases with an increase in the strain rate [8]. PP's mechanical characteristics are tested on conventional test samples and in compliance with established standards [15; 16; 17; 18]. PP is a component of many nano- to macro-composites due to its convenient mechanical and thermal properties. Polymer composites are also popular because of their simple production and low prices (PP belongs to the family of relatively cheap high-volume thermoplastics). The PP matrix can be combined with reinforcement and used for various raw materials, semi-finished products, or textile fabrics. [19; 20; 21; 22; 23; 24; 25]. [PERSON] et al. [26] investigated the tensile behaviour of a 3D polypropylene/glass fabric at different test speeds. [PERSON] and [PERSON] [27] studied the mechanical and morphological properties of polypropylene (PP)/polyoxmethylene (POM) blends, where the content of PP ranged from 10% to 90%. A decrease in the elastic modulus and tensile strength of POM/PP blends was associated with a PP content rise from 10% to 30%, but their values increased again as the content of PP continued to rise.