R. de Palo
Development in Plastic Materials # 2004 Milan
One of the most critical points in the design of a material with long term high burst pressure performance is to limit the so called "brittle failure", where slow crack growth propagation mode takes place. Since many years, polypropylene has been produced via cascade processes, where different PP based structures are combined together to optimise the mechanical performances requested. It is well known that, for pipe applications, the burst pressure resistance depends on the matrix, while impact resistance by the addition of a thermoplastic rubber. This bipolymer is generally made by a combination of 2 co-monomers and it gives no contribution to the resistance to creep. To increase burst pressure performance, the most successful method has been to improve the SCG. Generally speaking, this has been obtained by adding some % of C2- to the PP matrix. The big limitations of matrix randomisation with C2- are the following : 1) C2 modification is limited by the high XYLENE solubles in the PP matrix. For C2- > 4% the level of XYLENE solubles increase dramatically, decreasing the burst stress performance (rubber formation) 2) A C2- >4% leads to plugging problems, especially in liquid phase polymerisation plants 3) The use of C2- dramatically decreases the stiffness of the product, with possible issues on pipes collapse when installed, due to external loads. The present paper deals with the results of an investigation about the use of higher alternative co-monomers to C2- (C4 and combinations c2-c3-c4), in order to increase the modification level without increasing the XYLENE soluble content. The data obtained gave promising results in order to increase both SCG and burst pressure resistance curves, as it is shown in the following picture.