PSI - Issue 30

A.A. Chirikov et al. / Procedia Structural Integrity 30 (2020) 23–27 Chirikov A.A., Sokolova M.D., Gogoleva O.V. / Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Welding is a method of monolithic fastening of parts by connecting their molten sections. During welding the interdiffusion of molecular chains or their parts into the surface layers of the fused parts occurs. Many welding methods for various polymeric materials are described in the works of Kryjanovskiy et al. (2004), Vlasov et al. (2004), Zaytcev et al. (1988), Polkop and Chernyak (1987), Zaytcev and Matsyuk (1978), Komarov (1979) and Schwartz et al. (2005). It is known that products made of ultra-high molecular weight polyethylene cannot be welded by the same methods that are used for welding traditional thermoplastics and standard high density polyethylene (HDPE), since UHMWPE does not transform into a viscous flow state when heated (Andreeva et al. (1982)). During welding, standard HDPE undergoes a viscous flow when heated, and the viscosity of the movable melt is 10 2 -10 3 Pa·s. In the area of the surfaces to be welded, interdiffusion of macromolecules occurs, which ensures physical contact with a small clamp and, ultimately, a durable weld. Physical contact occurs only when the macromolecules come closer to distances of the order of the size of the crystal lattices (0.4-0.5 nm). In this case, when the polymer is cooled below the melting temperature, it becomes possible to form a continuous crystal lattice over the entire contact surface area. The situation is different with UHMWPE. The viscosity of the UHMWPE melt in a highly elastic state is of the order of 10 5 -10 6 Pa·s, therefore, the mutual diffusion of macromolecules occurs at a very low speed. To create a strong joint, it is necessary to ensure very close contact of the molten polymer layers. This is achieved by increasing the pressure above the yield strength. Thus, UHMWPE parts can be welded when heated under pressure. HDPE molecules, including UHMWPE, practically do not contain polar groups. Associated with this are the low adhesive properties of HDPE. Some methods of activating its surface are used, to increase the adhesive properties of HDPE, (Volkov (2001)), which are also suitable for UHMWPE. The methods of polyethylene surface treating with gaseous chlorine, ozone, a chromium mixture and other oxidizing agents by Kardashov (1976) as well as with gas burner flame by Baloshin et al. (2013) are known. Chemical methods of surface activation are laborious, they are associated with the use of harmful substances and therefore are not widely used. More often, physical methods to activate the surface of polyethylene are used. Thus, the activation of polyethylene films by corona discharge or radiation treatment on a cobalt installation RC- γ -30 with various radiation doses is widely used. Known works where UHMWPE was welded by friction by Patell et al. (2015) and using a heating device by Ershov et al. (1991). Thus, the development of an innovative method for welding ultra-high molecular weight polyethylene is an actual task of polymer materials science.

Nomenclature T

temperature of the welding, °C. high density polyethylene

HDPE

PTFE polytetrafluoroethylene UHMWPE ultra-high molecular weight polyethylene σ p

tensile strength of welded UHMWPE samples, MPa. elongation at break of welded UHMWPE samples, %.

ε p

2. Objects and research methods The object of research is UHMWPE brand Gur-4150 with a molecular weight of 9.2 million. Welding was carried out on UHMWPE samples in the form of pressed parts, bar size 11x8.5x50 mm. The tensile strength and elongation at break was determine on the testing machine “UTS-20K” at a speed of 50 mm/min. We have developed a welding method by installing a ferromagnet in the welding zone, which is heating to a temperature sufficient to melt ultrahigh-molecular polyethylene by exposing of magnetic induction. Welding pressure is supplied using a butt welding machine "ROWELD P160B". A distinctive feature of our welding method