Conventional propylene-based resin compositions possess excellent characteristics such as rigidity, heat and chemical resistance, while propylene-based resin pipe members formed using the same are extensively employed in factories, medical field, and architecture field, etc. In particular, owing to acid-alkali resistance in a high-temperature region as well as low cost, a propylene-based resin pipe is suitable for high-temperature chemical and hot water supply pipe systems in the industrial field, and its widespread use in the future is anticipated.
However, in the case where a propylene-based resin pipe is employed in a pipe line that flows high-temperature fluid, as the typical linear expansion coefficient of the propylene-based resin falls within the range of 12×10−5 to 15×10−5/° C., the considerable longitudinal elongation of the pipe associated with thermal expansion poses a problem. Consequently, as a high-temperature fluid of 60° C. and above flows through a fixed pipe, a meandering phenomenon results from the longitudinal elongation of the pipe associated with thermal expansion thereby causing a major distortion stress to arise in the pipe which does not only impair the long-term lifespan of the pipe, fluid leakage caused by distortion that takes place at the connection part to the joint and valve may also occur. As a countermeasure to this problem, a U-shaped passageway (flexible bent pipe) can be set up at regular intervals of the pipe to serve as a buffer to the expansion of the pipe, or by employing a flexible pipe. However, these methods, both present a disadvantage in the cost spent on pipe where a vast amount of space is required for setting up. Thus, reduction of thermal expansion of a propylene-based resin pipe itself is desirable.
Regarding the method to inhibit thermal expansion of a propylene-based resin composition, a method in which a propylene-based resin is mixed with an inorganic filling material has been conventionally employed, wherein a rubber-modified polypropylene resin material is composed of an inorganic filling material comprised of 100 parts of polypropylene and 20 to 50 parts of ethylene-propylene rubber (Japanese Laid-Open Patent Application 63-57653). This resin material is used mainly for bumpers and other automobile parts by virtue of its excellent impact resistance and low linear expansion coefficient.
In order to improve rigidity and heat resistance of conventional propylene-based resin compositions, a polypropylene-based resin composition consisting of 100 parts by weight of polypropylene resin and 30 to 400 parts by weight of surface treated talc is employed as an inorganic filling material that contains the surface treated talc. This polypropylene-based resin composition is surface treated by 0.1 to 5 parts by weight of silicone oil and 0.1 to 5 parts by weight of a higher fatty acid metal salt based on 100 parts by weight of the surface treated talc (Japanese Laid-Open Patent Application 2000-256519). This resin composition is preferably used in various industrial products such as automotive parts by virtue of excellent dispersibility due to the minimal generation of gum.
It could be reasoned that it is possible to inhibit the thermal expansion if the conventional propylene-based resin composition were mixed with an inorganic filling material. However, for a propylene-based resin composition added with an inorganic filling material, technology that takes into consideration the essential characteristics to function as a pipe member has not yet been established. Even if conventional technology is employed in the pipe member by simply adding the inorganic filling material, it is not possible to fulfill the characteristics as a pipe member and hence, such technology, without modification, cannot be applied in the pipe member. Also, one of the crucial properties of the pipe member, creep characteristic, has also not been given adequate consideration. Furthermore, as it is not possible to observe creep characteristic via short-term tests such as tests for tensile strength and Izod impact strength, a propylene-based resin composition with poor long-term creep characteristic cannot be applied as a pipe member.
Conventional propylene-based resin pipe members contain a polypropylene pipe comprised of a polypropylene whose melt flow rate is in the range of 0.005 to 5 g/10 minutes measured at 230° C. under a load of 2.16 kg, the maximum peak region of the endothermic curve measured by a differential scanning calorimeter falls in the temperature range from 128 to 172° C., density is from 898 to 917 kg/m3, the content of units consisting of α-olefin having 4 to 20 carbon atoms is in the range from 0 to 6 mol %, the flexural modulus of a press sheet specimen formed at 200° C. is in the range of 800 to 2,600 MPa (Japanese Laid-Open Patent Application 10-195264). The pipe, free from inorganic filling material or any additive that inhibits the linear expansion coefficient, is formed from a specific polypropylene and hence possesses superior mechanical strength.