Increased Internal Pressure Resistance Polypropylene (MRS 12.5 = PP125): How Polymer Design Has Led to Step-Change Performances and the Implications for PP-R Applications

An innovator and leader in the production of resins used in piping systems, LyondellBasell and its predecessor companies have served customers in the pipe industry for more than 50 years. Customers have succeeded in steadily replacing traditional materials such as cast iron, steel, clay and concrete; plastic pipes have proven a strong, durable and cost-effective solution. Polypropylene-Random (PP-R) has been a material of choice for producing plastic pipes for hot and cold water installations for more than 20 years. PP-R has higher stiffness and higher melting temperature than other polyolefins, giving a feeling to installers more similar to the incumbent metals, but at the same time keeping the typical advantages of plastic pipes such as light weight, fast and reliable jointing by welding and corrosion resistance, among others. Compared to other highly effective materials used in plastic pipe systems, like PEX or PB-1, PP-R has historically required higher wall thickness, due to its pressure resistance characteristics. The customers desire to reduce in wall thickness without compromising the mechanical properties has been a key driver for the R&D team within LyondellBasell. The efforts spent over the last decade have resulted in the innovative next generation of PP-R (PP125). 1-Hexene as a co-monomer is well known in the Polyethylene world, but has never been commercially used for PP because of its low reactivity in conventional polymerization processes. A new polymerization process combined with the catalyst and polymer design capabilities of LyondellBasell R&D has made the use of 1-Hexene as comonomer in Polypropylene a commercial reality. The new polymers, produced with 1-Hexene as comonomer, have been tested and shown much higher tensile creep performance compared to state of the art PP-R polymers. The test results lead to the conclusion that the next generation PP-R have an increased long term pressure resistance, allowing MRS classification 12.5 MPa, hence PP125. Due to the higher material strength compared to standard PP-R, the new Hostalen PP XN series materials have the potential for lower wall thickness in the same application class. The significant potential for reduction in weight provides opportunities for material savings and other possible benefits such as faster processing, easier handling and a higher degree of sustainability. Secondarily, if the outer diameter remains constant, an increase of the inner flow section is achieved. For all application classes it is possible, by simple geometrical calculations of cross-sections as prescribed by EN ISO 15874, to determine the potential weight savings, which according to customer trials may vary between 15% and 50%.Hostalen PP XN Series materials derive their superior strength from a specific and unique microscopic morphology. Apart from the enhanced pressure performance, XN Series materials are compatible with standard PP-R materials and according to customer feedback can be easily welded to standard PP-R fittings. As a further benefit to the XN Series, the resins are available in natural color, allowing maximum flexibility for pipe producers to color as desired to meet customer needs.