# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 2 Materials and Methods
However, the twine's complex, net-like structure absorbs a certain amount of oil. Due to prestressing and tensioning, which increases all-around pressure in the material, a considerable amount of oil drains out, so the change in the diameter of the twine samples can be neglected. The universal mineral oil SAE 15W-40 used in experiments has a density of 0.879 g/cm\({}^{3}\) at 15 \({}^{\circ}\)C, kinematic viscosity at 100 \({}^{\circ}\)C of 15 mm\({}^{2}\)/s, index viscosity of 135, total base number of 6 mgKOH/g, and pour point \(-\)24 \({}^{\circ}\)C. The oil was randomly chosen because the aim was not to determine the lubricant properties. The samples were immersed in oil to examine the friction between layers or fibres with different strain rates. The relative motion of adjacent fibres occurs due to the fibres' different stress states across the twine's diameter. As a rule, tensile stresses caused by prestressing are the smallest along the sample's longitudinal axis and increase as the distance from the axis increases. The twine was twisted by fixing one end in the stationary grip and rotating the other end around the axis of tension, whereas the number of twists was predetermined. The number of twists, denoted by N, represents the number of 360\({}^{\circ}\) turns around the longitudinal axis. In the second series of tests, N ranged from 1 to 26, meaning that the tests were performed with 26 different twist angles. The measurement of gauge length, diameter  
Figure 3: The twine sample prepared for testing (previously immersed in oil and prestressed by twisting). and twisting angle is performed under the effect of a tensile force of 10 N, achieved by the kinetic system (and tracked by the measurement system) of the device, which was necessary for reducing the influence of internal stresses caused by twisting (i.e., prestressing) on geometric characteristics of samples. The twist angle was determined by processing the photographs of the samples tensed with a force of 10 N. Due to the tested material's complex structure, fibres have different orientations, which leads to a large dispersion of the twist angle measurement results. Therefore, ten angles of randomly selected strands were measured on each sample. As tests were repeated 30 times in each series, the adopted values of twist angles represent the mean value of 300 measurements. Due to the large amount of data and for the transparency of the results' presentation, only the test results at the characteristic twist angles (N = 4, N = 8, N = 14, N = 20, and N = 26) are further discussed. The twist angles for N = 8, N = 14, N = 20, and N = 26 are provided in Figure 4.  
The cross-sectional area of the samples used in all four series was determined based on the twine's diameter. The diameter was calculated based on Equation (1), using the known deniers and density of the PP twine (0.9 g/cm\({}^{3}\)), where m, \(\rho\) and l are the twine's mass, density and length, respectively. \[\mathrm{d}=\sqrt{\frac{4\cdot m}{\rho\cdot 1\cdot\pi}} \tag{1}\]  
The twist angle depends on the twine's diameter and the number of turns per unit length [43; 48]; therefore, the results can be compared regardless of the device used for tensile testing. The experiments were conducted in controlled microclimatic conditions according to the ISO standard [16] and manufacturer's recommendations (temperature of 23 \(\pm\) 2 \({}^{\circ}\)C and relative humidity of 50 \(\pm\) 10%). The tensioning speed was 2.5 mm/min in all series and test repetitions.