The development of a new family of PE materials with significantly improved processability and long-term strength at high temperatures is discussed. Polyethylene of Raised Temperature resistance (PE-RT) is a PE-based compound with unique molecular architectures that provides outstanding hydrostatic strength at elevated temperatures. Polyethylene of raised temperature resistance (PE-RT) is a copolymer of ethylene and 𝛼-olefin, and the commonly used 𝛼-olefins are C3 (propylene), C4 (1-butene), C6 (1-hexene) and C8 (1-octene). The PE-RT pipes show excellent internal pressure resistance under ambient and elevated temperature conditions, flexibility and high chemical and mechanical properties. The PE-RT products can be used for hot and cold-water pressure pipes, underfloor heating and cooling, radiator connection and drinking water installation. PERT compound could be used in both 3 layer and 5-layer pipes.
Keywords: Polyethylene, PE-RTThe PE-RT pipes are made from polyethylene, which can resist high water temperatures. They come with improved processability and long-term power at higher temperatures. Industrial system designers and installers are responding to the mechanical and processing benefits of PE-RT resins in larger dimension pipe applications, where regular Polyethylene cannot be used, or is restricted by temperature limitations. The versatility of PE-RT and the ability to be used at higher temperatures without the need for cross linking, makes it a preferred choice for a wide range of applications where temperature profiles can range from sub-ambient to beyond what is considered normal for a traditional PE system. The PE-RT be specially for cold and hot water supply with a kind of ethene and octene, hexene or butylene copolymer that tubing designs, be the brand-new polyvinyl piping materials that occur in the middle and high warm water transportation art. In recent years, PE-RT relies on the characteristics such as its excellent physicals, erosion resistance, snappiness, processability and construction and installation, can replace atactic copolymerized polypropylene (PPR), polybutene (PB), chlorinated polyvinyl chloride (PVC-C) and crosslinked polyethylene (PEX) to be used for the Application Areas such as floor heating tube, heat exchanger, cold and hot water conveying.
The new PE-RT materials are designed with bimodal resin technology. Graphic 1 illustrates bimodal (two) molecular weight peaks to produce pipe resins that achieve excellent raised temperature performance – without the need for cross-linking.The lamellar crystal structure is connected through amorphous polymer segment: the tie chain. The probability of tie chain formation increases with the polymer chain length. Tie chain molecules are known to increase toughness and ESCR or long-term creep properties, by “tie-ing” multiple crystal together. Tie chains show extensibility and mobility can as such absorb and dissipate energy. This is presented in Fig. 2.
The new PE-RT material contains a greater number of high molecular weight chains than ordinary PE materials. Higher molecular weight determines the durability of the material. Long-term strength, toughness, ductility, and fatigue resistance all improve as molecular weight increases. However, the higher molecular weight makes resins more difficult to process. The bimodal technology provides the ability for an increased number of high molecular weight chains while at the same time improving extrusion processability. In Fig. 3 is shown that the breaking strength and the impact toughness were enhanced with increase in the length of the short-chain branch for polyethylene materials with different types of comonomer but similar contents. 1-octen comonomer performance is better than shorter comonomers.
According to the established international standard ISO22391-2 for PE-RT pipes, PE-RT materials are divided into two classes, types I and II, in which type II PE-RT has a better high-temperature resistance and pressure resistance. There are two main methods to produce PE-RT, namely solution polymerization developed by Dow Chemical Company and slurry polymerization developed by Basell Company. Although qualified PE-RT materials can be produced by different manufacturing processes, the differences in the comonomer type, comonomer content, comonomer distribution, and the molecular weight and molecular weight distribution of the copolymer can cause differences in the molecular structure, the super-molecular structure and the performance of the material. However, there are few reports on PE-RT materials from this aspect. In the research, influence of comonomer types, contents and distributions, and molecular weights and molecular weight distributions were assessed the molecular structure, super-molecular structure and properties of three PE-RT materials manufactured by different processes.
For example:
1) PE-RT (type II), made from ethylene and 1-butene with a melt index (MI) of 0.14g (190∘C, 2.16 kg)
2) PE-RT (type I), made from ethylene and 1-octene with an MI of 0.7 g (190 ∘C, 2.16 kg)
3) PE-RT (type II), made from ethylene and 1-octene with an MI of 0.55 g (190 ∘C, 2.16 kg)
Sample | Yield strength (MPa) | Strain hardening modulus | Flexural modulus (MPa) |
PE-RT-1 | 16.2 | 0.0525 | 652 |
PE-RT-2 | 10.2 | 0.0463 | 426 |
PE-RT-3 | 15.2 | 0.0575 | 554 |
The comonomer distribution over molecular chains was investigated by SSA. The SSA is a temperature-dependent separation process in which crystallizable segments of different lengths are recrystallized in the melt. The longer the crystallizable chain is, the thicker the lamellae that can be formed at a higher temperature. Eight main melting peaks were fitted according to temperature, which were respectively recorded as peak 1 to peak 8 from high to low. As shown in Fig. 4, a series of mutually separated melting peaks exist in each curve, indicating that the comonomer was not uniformly distributed over the molecular chain. The SSA curves of 2-PE-RT and 3-PE-RT are similar in shape, but the melting peak at high temperature of 3-PE-RT shifts slightly to higher temperature. Compared with the others, 1-PE-RT has one more high-temperature peak at 133 ∘C. The peak fitting result shows that 1-PE-RT has more regular molecular chains and thicker lamellae than 3-PE-RT. 3-PE-RT has thicker lamellae and a lower content of thinner lamellae than 2-PE-RT. The mechanical properties and crystallization kinetics of three different PE-RT materials were researched. 1-PE-RT and 3-PE-RT, which belong to the same class but with different comonomers, differ in many aspects of performance mainly because of the different comonomer distribution. 1-PE-RT has better crystallization ability and higher flexural modulus and yield strength than those of 3-PE-RT. Most of the comonomer of 1-PE-RT is relatively uniformly distributed over the long-chain molecules resulting in a higher content of the tie chain, while a small part of the comonomer is not uniformly distributed in the short-chain molecules, which gives it a better crystallization ability. As a result, 1-PE-RT has a higher modulus and better long-term performance. Although the ESCR of 3-PE-RT is slightly better than that of 1-PE-RT, the thick lamellae at low molecular weight do not contribute as much as those of 1-PE-RT. So, 3-PE-RT cannot balance stiffness and long-term performance as well as 1-PE-RT. Compared with 3-PE-RT with the same comonomer, 2-PE-RT has weaker crystallization ability, lower flexibility modulus and lower yield strength showing that the ESCR of 2-PE-RT is better than that of 3-PE-RT, but its rigidity is insufficient.
1. The PE-RT pipe has a dual effect of the UV-light resistance and oxygen barrier. 2. The PE-RT pipe has an excellent thermal stability and can withstand long term pressure. Under a working temperature of 70℃ and a pressure of 0.4Mpa, the pipe can be safely used for more than 50 years. 3. The PE-RT pipe has an excellent flexibility, which makes it convenient and economical. The bending radius of the PE-RT pipe is small (Rn=5D) and doesn’t rebound after bending. This makes it ideal for construction usage, and stress on bending parts is quickly relaxed, thus avoiding damage to the sinuosity caused by the stress concentration while servicing. 4. The PE-RT pipe has an excellent impact resistance and high security, and its low temperature brittleness temperature is -70 °C, so that the PE-RT pipe can be transported and constructed in low temperature environment. Its ability to resist external force is much higher than that of other pipes, and does not need to be heated in order to bend. Also, Graphic 5 shows where PE-RT provides greater than 20 times the slow crack growth resistance compared to PE4710 requirements.
5. A heat fusion joining is available for the PE-RT pipe to make the product easy to install and repair. In the underfloor heating plumbing system, if the system is damaged due to an external force, the PE-RT pipe can be repaired using a heat fusion, which is more convenient, faster and safer. 6. The PE-RT pipe can be produced without crosslinking process and controlling crosslinking degree and evenness. The PE-RT pipe has less production processes and an overall, even wall thickness.
Among many advantages, PE-RT pipes can transport water of high temperatures between different places. The pipes are also: • Light-wight and easy to transport • Flexible and require fewer fittings • Easy to operate • The flow capacity is 30% greater than the metal pipe • Eco-friendly and can be recycled • Provide no toxic auxiliaries in the production of the pipe • The high ability to resist slow crack growth or environmental stress cracking P E-RT is flexible enough to bend where the angles are needed in the heating and cooling systems. When used in a larger diameter pipe, a PE-RT pipe is an ideal option for the ultimate processability of water and liquids.
PE-RT is a copolymer of ethylene with an α-olefin (ethene, octene, hexane, or butene) that is resistant to impact, light, temperature, and pressure in cold and hot water piping systems by adjusting comonomer distribution, molecular weight, and molecular weight distribution.
1. H. W. Long, Advanced Applications for HDPE Pipe with New PE-RT Material, 2017, 1-5. 2. D. Schramm, M. Jeruzal, PE-RT, A New Class of Polyethylen for Industrial Pipes, The Dow Chemical Company. 3. Z. Chao, B. Zhao, L. Ding, D. Zhang, F. Yang, M. Xiang. Influence of comonomer distribution on crystallization kinetics and performance of polyethylene of raised temperature resistance. Polymer International 68(10) 2019, 1748-1758.