Johannes de Bruin, Dr. Bert Dalmolen, Albo van Hateren
Papers # 2016 Berlin
Normal High Density Polyethylene (HDPE) is commonly used as the fluid contacting element, or the “inner-liner”, in Reinforced Thermoplastic Pipes (RTP) and steel pipes up to 65 ºC. Through a modification of the HDPE polymer matrix, by adding nanoparticles, the E-modulus can be increased. Research shows that Nano-filled HDPE liners can, as a result, be used in RTP and steel pipe up to 85 ºC, a temperature which could previously only be reached by using rather expensive polyamide 11 and polyamide 12 compounds.
Normal High Density Polyethylene (HDPE) is nowadays extensively used as a corrosion resistant liner material in Reinforced Thermoplastic Pipes (RTP) and in Carbon Steel Pipes for temperatures up to 65 ºC. Rather than strength, the retention of a certain stiffness (E-modulus) is the design criterion governing the high temperature limitation for plastics as a liner material. Through a modification of the HDPE polymer matrix by adding nano-particles the E-modulus can be increased.
Hydrocarbon fluids tend to absorb in the HDPE matrix, acting as a plasticizer and lowering the E-modulus. Fluid absorption strongly increases with temperature, and also depends on the composition of the fluid. In general, low molecular weight fluids show higher absorption, and aromatic hydrocarbons show more affinity to HDPE than other olefins. The affinity of hydrocarbons towards HDPE can be expressed as the “Hildebrand solubility parameter”. When this parameter closely matches the Hildebrand parameter of the matrix, hydrocarbon absorption, plasticization, and fluid permeation are at maximum. The Hildebrand solubility parameter can be calculated by the arithmetic average of the Hildebrand parameters of the fluid components. When the composition is known, an accurate estimate of the swelling, lowering of the E-modulus, and the level of fluid permeation can be made.
Tests, to assess the modulus retention at high temperature, were conducted with a worst case hydrocarbon fluid composition (based on the Hildebrand solubility parameter): a synthetic gas condensate consisting of 50 % (m/m) of trimethylbenzene and 50 % (m/m) n-decane. HDPE, PE100 was used as a reference material, which has a modulus of around 300 MPa at 65ºC when saturated with the test fluid. The HDPE based nano-particles filled compound showed a similar modulus as PE100 at a 20 ºC higher temperature.
Results of permeation experiments and slow crack growth tests, support the assumption that HPDE based compound filled with nanoparticles can be used for liner materials in RTP and steel pipe up to 85ºC, a temperature which could previously only be reached by using more expensive polyamide 11 and polyamide 12 compounds.