Influence of peroxide cross-linking temperature and time on mechanical, physical and thermal properties of polyethylene

Abstract Polyethylene is a very common liner material for type IV pressure vessels due to its good toughness and easy processing. The property profile of the polymer can be improved by cross-linking thereby changing the nature of the polymer from thermoplastic toward more elastomeric. For this purpose, polyethylene is modified either chemically, using peroxide or silane, or physically by radiation. In the present work, a cross-linkable polyethylene grade that can be processed by rotational molding was peroxide cross-linked under variation of temperature and time. Subsequently, the material was characterized by differential scanning calorimetry, tensile tests, notched bar impact tests and permeation measurements. Two of the altogether six parameter combinations investigated did not lead to successful cross-linking resulting in very poor toughness. Stiffness, strength and permeation barrier properties, however, were much better than those of the other series due to higher crystallinity. Mechanical, physical and chemical properties changed significantly by successful cross-linking. The impact strength could be improved by a factor of more than 10. At the same time, significant losses in stiffness, strength and permeation barrier properties had to be accepted. Peroxide induced randomly distributed formation of cross-links above the melting point interfered with formation of crystalline regions upon cooling.