# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 2 Materials and Methods  
The tested PP baling twine is of type T-800. Its structure is shown in Figure 1. As guaranteed by the manufacturer, the mass of the twine package is 1 kg, the twine length is 800 \(\pm\) 64 m, and the linear mass density is 1250 tex (approximately 11,250 den). The twine is made of 100% polypropylene. The essential physical characteristics of the used material are the melting range of 140-165 \({}^{\circ}\)C, white colour, closed cup flash point of about 400 \({}^{\circ}\)C, ignition temperature of about 450 \({}^{\circ}\)C, and density of 0.89-0.91 g/cm\({}^{3}\). Thermal decomposition starts at approximately 300 \({}^{\circ}\)C.  
Figure 1 indicates that the tested PP baling twine has a complex structure. The fibres of non-standard thickness extend in different directions and form a complex geometrical shape (net). During manufacturing, the net must be twisted to form a twine. The twisting changes the planar chaotic arrangement of the fibres, forcing them to assume a more complex, spatial, and chaotic orientation. As hard as it would be to predict and define the specific nature of the contact between the fibres or the size and shape of the contact surfaces, generating a mathematical model for theoretical and numerical analyses of the described material would be even more challenging. Therefore, laboratory testing is necessary for determining and analysing the relevant mechanical characteristics of the described material. The twine was tested on a small laboratory device developed for testing the characteristics of metallic and non-metallic materials by tension (Figure 2). The drive system applies a tensioning force to the mobile grips, which move along the tension axis as the test sample elongates. When the steel samples are tested on that particular device, the mean value of the uncertainty in the elastic modulus measurement is 1.56% [44; 45]. At approximately equal tension force, the strain of PP baling twines is greater than the strain of steel samples, so the expected measurement uncertainty is less than 1.56%. Figure 2: Device for testing metallic and non-metallic materials by tension. Tensile tests were executed on non-prestressed twine samples (series 1) and samples prestressed by twisting around the longitudinal axis (series 2). Free ends of the twine were fixed in the clamping grips by tying the so-called Hangman's Knot. Considering the type and structure of the material, the diameter of the sample, and the deformation speed, the aforementioned particular tying technique [33; 34; 35; 36] provides the best results regarding the compliance and stress concentration in the knot and ensures that the rupture occurs within the gauge length. The mean initial gauge length of the twine samples was 145.3 mm in the first series of tests and 146.5 mm in the second. Due to the different initial lengths of the samples, the resulting elongation is considered through relative deformation. The baling twines are not being lubricated in exploitation. However, lubricated twine samples, i.e., samples immersed in oil, were also tested, as the research aimed to deepen the analysis of the effects of friction between the fibres. Those tests were carried out on samples not subjected to prestress (series 3) and on prestressed samples (series 4) that exhibited the largest increase in tensile strength during the second series of tests. The samples from series 3 were immersed in oil before the tensile testing, while the samples from series 4 were immersed in oil, twisted along their longitudinal axes and then subjected to tension load. The mean length of test samples was 145.8 mm in series 3 and 146.3 mm in series 4. Each test within the four series was repeated 30 times. Figure 3 shows the sample from series 4, prepared for the test, i.e., immersed in oil, prestressed by twisting and fixed. PP is a hydrophobic material with low oil-sorption capacity [46]. After brief contact with oil, the fibre of PP baler twine does not change its volume and chemical composition [6; 7; 8; 9; 10; 11; 47].