# The Influence of Friction and Twisting Angle on the Tensile Strength of Polypropylene Baling Twine
## 5 Conclusions  
The conducted experimental research has led to the following conclusions:  
* The tensile strength and the appearance of the damages on the ruptured twine change depending on the fibres' twist angle.
* In the investigated case, a surface helix angle of 13.9\({}^{\circ}\) ensures the highest increase in tensile strength of approximately 33%.
* Further increase in surface helix angle causes a decrease in tensile strength and deformation speed.
* An increase in tensile strength is followed by an increase in strain and vice versa.
* An increase in tensile strength occurs due to an increase in the resulting friction force between the fibres of the tested material, which has the opposite direction to the direction of tension. Therefore, the tension force is the sum of the friction force and the resisting force of the material, as proven by additional testing.
* The applied baling twine twisting technique, used to increase the tensile strength, provides results only in exploitation without lubricants.
* The obtained results indicate that an increase in friction force positively affects the tensile strength only up to a certain limit. Exceeding the limit value also changes the type of load, whereby, due to the increase in the surface helix angle, tensile stresses decrease while shear stresses increase, so the tensile strength of the tested twine decreases.  
The data acquired from the presented results can be used to increase the tensile strength of the PP twine mechanically, without changing its chemical composition or complicating the production process, but exclusively by increasing the surface helix angle, thus expanding its field of application. Thus, the PP twine's prestressing level would vary depending on the intended use and the required tensile strength value. The twisting angle can be adjusted during production without modifying the production process or increasing the costs. Future research should focus on modelling and optimising the parameters influencing materials' mechanical properties, such as material type, fineness, structure, and prestressing level. The feasibility of predicting the twisting angle based on mechanical characteristics such as tensile strength and strain should also be investigated.  
Conceptualisation, S.K. and V.K.; methodology, S.K. and V.K.; software, M.D.; validation, S.P.S. and M.D.; formal analysis, V.K. and S.P.S.; investigation, S.K., V.K., J.M. and M.D.; resources, S.K. and D.V.; writing--original draft preparation, S.K., V.K., S.P.S., D.M., J.M. and M.D.; writing--review and editing, S.K., V.K. and D.V.; visualisation, M.D. and J.M.; supervision, D.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding author.  
**Acknowledgments:** This research was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia.  
**Conflicts of Interest:** The authors declare no conflicts of interest.