# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 3 Results  
In the first series, twine samples were taken off the rolls, cut to the appropriate length and tested without prestressing. In the second series, all twine samples were twisted by performing an equal number of turns (from N = 1 to N = 26) around their longitudinal axis. As already mentioned, due to the large amount of data, only characteristic tension diagrams are presented in this paper. Based on those diagrams, it is possible to determine the maximum tensile strength that can be achieved by changing the twist angle. Diagrams  
Figure 4: The characteristic twist angles: (**a**) N = 8; (**b**) N = 14; (**c**) N = 20; (**d**) N = 26.  
also indicate which angle value is related to the maximum increase in tensile strength and how the twine's tensile strength changes with a further increase in the surface helix angle. An analysis of Figures 5 and 6 enables a comparison between the first test series' tension diagrams and the second series' characteristic diagrams (N = 4, N = 8, N = 14, N = 20, and N = 26). The curves in the diagrams represent the mean values of experimentally obtained results for 30 samples from each series. The force-elongation diagram in Figure 5 indicates that the maximum tensile force values in the twine samples that were twisted from 4 to 20 times are higher than those obtained by testing the non-prestressed samples. The changes in the elongation during the test indicate an increase in the deformation properties of the samples that were twisted 8, 14 and 20 times, differing from the results obtained on non-prestressed samples. When N = 26, a decrease in both force and elongation is evident. Compared to the non-prestressed samples, the prestressed samples show a greater increase in tensile strength and relative deformation following the rise in the twist angle  
Figure 5: Force–elongation diagrams for twine samples tested with and without prestressing by twisting. Figure 6: Stress–strain diagrams for twine samples tested with and without prestressing by twisting. up to N = 20 (Figure 6). The greatest changes in tensile strength and strain are at N = 8. A further increase in the helix angle, up to N = 20, is followed by a decrease in tensile strength and relative deformations. When N = 26, there is a noticeable drop in the tensile strength and deformation of the prestressed samples. The diagram of the change in tensile strength as a function of the angle of twist (i.e., the number of twists) is provided in Figure 7a. Figure 7b illustrates the dependence of the relative deformation on the twisting angle (i.e., the number of twists). The breaking points on the diagrams in Figure 7 are at helix angles of 6.8\({}^{\circ}\) (N = 4), 13.9\({}^{\circ}\) (N = 8), 24.4\({}^{\circ}\) (N = 14), 30.5\({}^{\circ}\) (N = 20) and 39.5\({}^{\circ}\) (N = 26). Figure 7a indicates the growth of tensile strength at helix angles from 6.8\({}^{\circ}\) (N = 4) to 13.9\({}^{\circ}\) (N = 8). The maximum tensile strength value is reached at 13.9\({}^{\circ}\) (N = 8), followed by its decline until the angle reaches 39.5\({}^{\circ}\) (N = 26). So, tensile strength decreases as N rises from 8 to 26. In Figure 7b, there is a slight decrease in the strain at the rupture of the prestressed samples up to N = 4, after which the strain at rupture increases and reaches a maximum value at N = 8, indicating the difference in the behaviour between the prestressed and non-prestressed samples. After that, the deformation values decrease with the increase in the angle of twist up to N = 26. Figure 8 shows the results related to all 30 samples from series 1 to 2. The prediction lines obtained using the least square method emphasise a more uniform behaviour of samples from series 2 at N = 8 (i.e., less scattering of experimental data around the arithmetic mean) compared to series 1. The samples from series 1 exhibit a larger dispersion of the experimentally obtained stress and strain values at rupture.