MATERIALS | ENGINEERING THERMOPLASTICS


system standard covers diameters up to 400mm. PVDF pipes can be joined using either mechani-


cal fittings or welding. As regards welding, four proven techniques are generally used: n Electrofusion using electro-weldable sleeves, which are available for pipe diameters of 20-63mm. The technique is almost identical to that used for PE;


n ‘Socket fusion’ using a male-female type sleeve, available for pipe diameters of 16-110mm;


n Butt-welding using heating mirrors, for pipe diameters of 20-630mm; and,


n Contactless, infrared, butt-welding, for pipe diameters of 20-450mm. Other techniques, such as high frequency


welding, hot gas welding or friction welding can also be used. However, they appear more suited to the laboratory environment due to the operational constraints existing in the field.


Coiling ability PVDF has a greater stiffness than most other poly- mers. This is related to its higher density, which is about 1.8 times that of HDPE, for instance. However, this stiffness is about a quarter that of steel. This stiffness factor can constitute an obstacle to its ability to be coiled or wound onto a spool, as is usually the practice with PE pipes. Consequently, PVDF pipes are usually only available in straight lengths. The need for new pipes that can be used for sliplining of existing corroded steel pipes – over lengths exceeding those currently available (5m as standard) with a limited number of welded joints – meant that the feasibility of rolling/unrolling such pipes had to be demonstrated. To this end, a joint study was launched between GRTgaz’s Research and Innovation Center for Energy (RICE), Solvay Specialty Polymers and RYB-Groupe Elydan. The study, which began by using digital simulation of the coiling process, helped the researchers to select – from several possible available grades – the


Solvay used finite element simulations to evaluate the effort needed to coil pipe onto a spool


material with the best compromise in terms of ‘coilability’ and long-term mechanical performance. Preliminary digital simulations were needed in


order to evaluate the effort required to coil the pipe onto a spool, as well as the damage caused to the wall of the PVDF pipe as a function of the coiling radius. These simulations were carried out by Solvay using a finite element model implement- ed on the Abaqus platform. The material input data was that of the Solef 1010 grade homopolymer. These simulations used the stiffness modulus at 23°C and the complete Solef 1010 ‘stress-strain’ curve in order to evaluate the occurrence of the plasticity phenomenon during coiling. The simulations showed that – as expected – the maximum principal deformation is greater for the pipe with the smallest wall thickness (SDR17). For both simulated SDRs, the maximum principal deformation is 4-5%. This value is close to the irreversible plastic deformation for this grade of PVDF. In light of this, it was decided to increase the coiling diameter (by using a larger spool) and to use a PVDF grade with a lower stiffness for the physical trials. Of the two suitable grades – Solef 11010 and Solef 60512 – it was decided to use the second. The homopolymer and copolymer grades selected can be differentiated based on a number of factors, including: n A flexural modulus that is two times higher for the homopolymer;


n A stress at elastic limit that is about two times higher for the homopolymer;


n An ultimate tensile strength that is about 10 times higher for the copolymer;


n A crystallisation temperature range that is very close to the operating temperatures; and,


n A thermal expansion coefficient that is higher for the copolymer – and very close to that of HDPE. Based on the digital simulations, a programme of experimental extrusion was carried out at plastic


Source: Solvay 16 PIPE & PROFILE EXTRUSION | January/February 2019 www.pipeandprofile.com


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