Polypropylene modification technology and its applications in the automotive industry

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I. Polypropylene modification technologies. Polypropylene has two main drawbacks: first, it has a high shrinkage rate, which leads to poor dimensional stability of the products and makes them prone to warping; second, it becomes brittle at low temperatures. Of course, compared to traditional engineering plastics, it also has disadvantages such as low modulus, poor heat resistance, and weak resistance to light and thermal aging. Addressing these shortcomings, scientists pursued two approaches: polymer modification and blending modification, and through relentless efforts, they finally made breakthrough progress in the mid-1980s. From homopropylene to random ethylene-propylene copolymers and ethylene-propylene block copolymers, and with continuous advancements in catalyst technology, impact-resistant polypropylene can be synthesized. Moreover, through Reactor-made and Catalloy technologies, ultra-high-toughness polypropylene with an ethylene-propylene rubber content as high as 35% and a rubber particle size of less than 0.5 microns can be directly produced in reactors, and such production has now reached the commercial stage. In particular, the advent of metallocene catalysts has pushed olefin polymerization technology to a whole new level. The POE thermoplastic elastomer, which was commercialized by the American company Dow Chemical in the late 1980s, is a typical example of this. POE (Polyolefin Elastomer) is a thermoplastic elastomer resulting from the in-situ polymerization of ethylene and octene. Crystalline PE is present in the side chains of the amorphous comonomers, acting as physical cross-linking points under stress. Curled ethylene and octene chains contribute to elasticity. By changing the ratio of ethylene to octene and controlling the polymerization conditions, a range of product series with varying properties and applications can be obtained. A very narrow molecular weight distribution and a certain degree of crystallinity enable it to possess the properties of an elastomer while also being easy to process like a thermoplastic elastomer. Compared to ethylene-propylene rubber, it has lower cohesive energy and higher shear sensitivity; it exhibits good compatibility with polyolefins during processing. Since the dependence of the apparent shear viscosity of POE on shear rate or temperature is similar to that of PP, the compatibility window for processing is wide, which facilitates the achievement of smaller particle sizes for the dispersed phase as well as a narrower particle size distribution. Its toughening effect is often superior to that of EPDM. For polypropylene systems filled with inorganic materials, POE can also improve the dispersion of the fillers. The toughening effect of POE as a toughening agent in ultra-high toughness PP alloy materials and highly filled PP alloy materials is shown in Tables 1 and 2. Table 1 Properties of POE-toughened polypropylene alloy materials: Jinbei Sea Lion, Xiali, ALTO thermoplastic sheets; EPT4045POE, EPT4045POE, EPT4045POE, MI (230°C, 2.16 kg) g/10min: 6.5, 8.3, 54.2, 5.0, 1.9, 1.4; σy MPa: 16.1, 16.1, 18.6, 18.1, 23.7, 21.8; ε%: 510, >600, 540, >600, 525, >600; Is (missing); J/M: 539.47, 49.35, 76.16, 662.7, 650.9, 677.2; σb MPa: 20.0, 23.5, 25.0, 27.0, 26.0, 27.0; Eb MPa: 806, 873, 1125, 1120, 800, 850; Rubber content %: 25, 25, 20, 20, 44; MI – Melt flow rate ； σy—tensile yield strength ； ε–elongation at break ； Is (missing) – Notch impact strength ； σb—ultimate strength ； Eb—Bending modulus. Table 2: Mechanical properties of POE-toughened polypropylene composite alloys
l234EPT4045POEEPT4045POEMI (230°C, 2.16 kg)
g/10 min: 4.2, 4.8, 4.1, 4.5
σy MPa: 22.3, 22.7, 21.2, 21.8
ε%: 55, 160, 70, 200
Is (absent): J/M: 220.5, 247.0, 280.0, 335.0
σb MPa: 35.5, 37.5, 32.3, 36.0
Eb MPa: 2520, 2630, 2400, 2550
Rubber content %: 55, 88

The data presented in Tables 1 and 2 show that, at the same level of toughening agent content, POE provides a better toughening effect compared to EPT4045 produced by Mitsubishi Oil Chemical Company in Japan. Similar results were also observed in polypropylene alloy materials filled with 30–35% talc. Furthermore, POE contains no double bonds and is not sensitive to moisture; its actual weather resistance is superior to that of thermoplastic elastomers such as EPDM, EVA, SBS, and CPE. Its good dispersibility and fluidity result in a tensile strength of the welded seams of its products that is at least 30% higher than that of EPDM blends. It is also cheaper than EPDM. In terms of polymeric modification, significant progress has been made in the copolymerization of propylene and cycloolefins; it is reported that Mitsubishi Oil Chemical in Japan has successfully synthesized propylene/cycloolefin copolymers. This product has good transparency, and its properties such as heat resistance and tensile strength have reached the level of engineering plastics. It has begun to enter the commercialization phase. TPE (Thermoplastic Elastomer) is a polypropylene thermoplastic elastomer developed by the American company AES (Advanced Elastomer System) Co., which uses the reactive extrusion process to create a material containing 100% cross-linked EPDM elastomer dispersions. It is a type of thermoplastic rubber; by varying its components and manufacturing processes, a range of products with different hardness levels, as well as varying degrees of toughness and mechanical properties, can be produced. It can be used not only in the production of injection-molded rubber products but also suitable for extrusion and blister packaging processes. II. Applications of polypropylene and its modified materials in the automotive industry. Cars have become one of the main modes of transportation in modern society. It is a major consumer of energy; in industrially developed countries in the West, approximately 50% of oil consumption is used for vehicles. It is reported that reducing the weight of standard car components by 1% can result in a nearly 1% reduction in fuel consumption, while reducing the weight of moving parts by 1% can lead to a 2% decrease in fuel use. Therefore, lightweighting cars is of great importance for energy conservation and environmental protection. The lightweighting of vehicles relies on the use of plastics, which is why consumption of plastics for automotive use is increasing year by year. In the mid-1980s, the United States had 109 kg per vehicle, accounting for about 9.8% of the vehicle’s weight; Japan had 74.3 kg per vehicle, accounting for about 6% of the weight; in Europe, it was 120 kg per vehicle in 1989. It already accounts for 11% of the vehicle’s weight; recent reports indicate that the amount used per vehicle has reached 200 kg. It accounts for about 15% of the vehicle’s weight. As mentioned earlier, polyolefin plastics and their modified materials still account for the largest share in automotive plastics. In Japan, 61% are regular cars, and 41% are luxury cars. PP modified materials rank first among automotive plastics. In 1990, the consumption of PP for bicycles in Europe was 22.5 kg, and it rose to 38 kg in 1995. It reached 45 kg in 1998. Currently, there are about 60 PP components used in cars, and this number is set to rise to 200 in a few years; the application prospects are very promising. Table 13 shows the distribution of PP plastic components used in Japanese cars. Table 3: Relative proportions of PP plastic components used in Japanese cars – Percentage by component weight%. Controller box: 1.8; Steering wheel: 1.8; Fan: 2.1; Fan guard: 2.1; Battery case: 11.4; Radiator cover: 9.1; Rearview mirror: 3.1; Air filter housing: 1.3; Bumper: 22.1; Dashboard: 14.1; Door interior panel: 12.2; Light box: 10.0; Air conditioner housing: 11.4. China’s automobile industry is experiencing rapid development, and in order to keep up with this growth, efforts to develop supporting industries related to automobiles have been planned since the late 1980s. **The Planning Commission once included polyolefin plastics for use in automobiles and their modified materials in the **“Eighth Five-Year Plan” key scientific and technological research projects. With the exception of plastic fuel tanks and specialized materials for wheel arches in certain models, everything else is a modified material based primarily on polypropylene. Among the polypropylene materials used in automobiles, aside from a few components such as bumpers and wheel covers that are PP/EPDM-toughened or ultra-toughened alloys, talc-filled polypropylene composite alloys hold a very important position. Through more than two decades of unremitting efforts, our country has made tremendous progress in the research, development, and industrialization of polyolefin plastics for use in automobiles as well as their modified materials. The domestic automotive industry is now fully capable of producing polyolefin materials specifically designed for use in automobiles, and the achievements of certain research institutions have reached international advanced levels.