Polypropylene is a thermoplastic resin made by propylene polymerization. Its disadvantages are poor low-temperature impact resistance and easy aging, but these can be overcome by modification. Copolymerization modification refers to the modification of propylene monomer in the polymerization stage using a specific catalyst. Copolymerization of propylene monomer with other olefin monomers can improve the low-temperature toughness, impact resistance, transparency and processing fluidity of polypropylene.

"Three highs and two lows" polypropylene (high melt index, high modulus, high impact resistance, low odor, low VOC) is an important variety of copolymer polypropylene. It has excellent mechanical properties, chemical stability, creep resistance and other advantages, and fast processing and molding. It can prepare large thin-walled products and save raw materials. It is widely used in automobiles, home appliances and other fields. In recent years, its demand has continued to increase. Taking high melt index impact-resistant polypropylene washing machine special materials as an example, the domestic market capacity in 2019 has exceeded 400,000 tons.

Background of the development of "three high materials" polypropylene

With the rapid development of the automotive industry, home appliances, logistics industry, electronic and telecommunications industry, etc., combined with the development trend of new polypropylene products of multinational petrochemical companies in recent years, the market of "three highs and two lows" polypropylene will become larger and larger.

In the traditional sense, high crystalline polypropylene refers to a polypropylene resin with high crystallinity, high crystallization temperature, high rigidity and high heat resistance prepared by adding an efficient nucleating agent on the basis of high stereoregular homopolymer polypropylene. It has a wide range of applications in electrical components, home furnishings and other occasions, and is a high-end product among polypropylene.

In recent years, major foreign polyolefin companies, such as SK and LyondellBasell, have been developing high crystalline impact polypropylene technology. Once the products they sell, such as BX3900, BX3920, and 540V, have been widely recognized by users, and the selling price is more than 1,000 yuan/ton higher than ordinary impact polypropylene. It can be foreseen that high crystalline impact polypropylene will replace the existing high crystalline polypropylene and impact polypropylene to a large extent, and become the leading brand of injection molding polypropylene resin in the future.

Characteristics of "three high materials" polypropylene

1. High melt index

In recent years, with the improvement of polypropylene production technology, especially the improvement of new high-efficiency catalysts and polymerization processes, the development and application of polypropylene products with higher melt flow rate (MFR) (referred to as "high flow polypropylene") have ushered in rapid development. The improvement of the fluidity of high-flow polypropylene (melt index is generally higher than 25g/10min, generally 30, 40, 50, 60, 100, 110, 120, etc.) can reduce the processing temperature, injection pressure, clamping force, etc. during the product processing and production process, thereby reducing energy consumption, shortening the molding cycle of products, and greatly increasing product output, which can meet the needs of large products and high-speed injection molding. In addition, the use of high-flow polypropylene can be used for injection molding of thin-walled products, reducing the amount of raw materials and reducing costs.

2. High modulus

In order to maintain the static stiffness of the original wall thickness product, the thin-walled material is required to have a higher bending modulus.

In particular, the new generation of polypropylene automotive materials need to withstand greater forces during use and must maintain a good shape under external forces, so they must have sufficient rigidity and generally require a bending modulus greater than 1700Mpa. Domestic products generally have a low modulus, less than 1700Mpa. High modulus can meet the needs of thin-walling;

3. High impact resistance

Automotive and home appliance polypropylene materials must have good energy absorption and transfer capabilities, which requires the raw materials to have appropriate impact strength and a wide molecular weight distribution. At room temperature (usually 23°C), the simply supported beam notch impact requirement is greater than 5KJ/m2.

Modulus 1700 corresponds to an impact strength of 9-10kJ/m2 (23°C, notched simply supported beam);

Modulus 1800 corresponds to an impact strength of 8-9kJ/m2 (23°C, notched simply supported beam);

Modulus 1900 corresponds to an impact strength of 6-7kJ/m2 (23°C, notched simply supported beam).

4. Low odor

Mainly used in modified polypropylene for automotive interiors, it is an ultra-low emission plastic particle developed to address the problem of air pollution in cars. The material must have balanced mechanical properties, excellent processing performance, high heat resistance, and low odor (polypropylene material odor ≤ level 3). Low-odor polypropylene that solves the odor problem will become the preferred modified raw material for major automotive modification factories. According to Volkswagen's interior odor test standard PV3900, the odor level is ≤3.5. The TVOC content is less than 50. Especially after the new national standard "Guidelines for Passenger Car Interior Air Quality Evaluation" was enforced on January 1, 2017, solving the HCPP odor problem has become one of the key indicators for petrochemical companies to compete;

5. Low VOC

The sources of volatile components in polypropylene materials are mainly from two aspects: one is the solvents added to polypropylene itself during the synthesis process, including hexane, ethanol and heptane; the other is that small molecules such as catalysts will also remain in the material during the synthesis of polypropylene. These low molecular substances will make polypropylene materials have certain volatile organic compounds (VOC). In the field of automotive interiors, the volatility of organic matter in materials needs to be strictly controlled to ensure human safety. When the toxic volatile gases in automotive interior materials reach a certain concentration, people will experience headaches, nausea, and even convulsions in a short period of time, and will damage the liver, kidneys, brain and nervous system, posing a serious threat to human health. After the new low-VOC national standard "Guidelines for Passenger Car Interior Air Quality Evaluation" was enforced on January 1, 2017, the polypropylene VOC problem has become a key indicator

"Three Highs" Polypropylene Market and Application

At present, the domestic demand for HCPP is 250,000-300,000 tons/year, and it is growing at an annual rate of 10-15%, with broad market prospects.

Domestic HCPP demand is mainly imported products, accounting for about 70% of the total demand. The main ones are SK, Hanwha Total, Korea Petrochemical, LyondellBasell, Exxon, etc. SK of South Korea alone exports about 60,000 to 80,000 tons of HCPP to the Chinese market every year, and the price is 2,000 to 3,000 yuan/ton higher than ordinary impact-resistant PP.

HCPP is mainly used for automotive parts, such as pillars, door panels, bumpers, etc., modified home appliance products, large injection molding products, etc. With the development of the domestic automobile industry and the trend of lightweight automobiles, the demand for HCPP is also increasing.

In recent years, with the development of science and technology, plastic products are increasingly inclined to develop in the direction of large-scale thin-walled, lightweight and comfortable. Some large parts of automobiles can reduce the weight of the body by using polypropylene injection molding, which not only reduces fuel consumption, but also reduces carbon dioxide emissions, which is of great significance to environmental protection. At present, the application ratio of non-metallic materials for automobiles has reached more than 50%.

The demand for modified plastic materials in the automotive field in my country has increased from 3.52 million tons in 2015 to 4.66 million tons in 2019. With the steady increase in automobile production and sales and the trend of automobiles towards lightweight, it is expected that the demand for modified materials in the automotive field will maintain a good upward momentum. The annual demand for high-flow, high-crystallization, and high-impact polypropylene injection molding materials in my country is about 150,000 to 250,000 tons, and the annual growth rate is more than 10%. Lightweight, low-cost, and high-flow materials have good processing performance and short production cycles. At the same time, high crystallinity can improve the rigidity of the material, reduce the thickness of the finished product, and reduce the quality of the material. In addition, the high content of EPDM can increase the strength of the finished product. Studies have shown that in terms of fuel consumption, for every 10% decrease in automobile mass, fuel consumption decreases by 6-8%, and emissions decrease by 4%; in terms of power economy, for every 100kg weight reduction, the acceleration power of 0 to 100km/h increases by 7%; in terms of conventional braking performance, for every 100kg weight reduction, the braking distance is shortened by 2 to 7m at low speed, and for every 100kg weight reduction, the braking distance is shortened by 1 to 2m at high speed; in terms of electric vehicle endurance, for every 100kg weight reduction, the endurance of electric vehicles increases by 15 to 20km.

Manufacturers and Typical Brands

At present, the best HCPP is made by SK of South Korea, whose BX3800/3900/3920 stands out, whether in modulus/impact, processing performance, or low odor, all of which are stronger than similar products on the market. The second is the HCPP products of Korea Petrochemical, Hanwha Total, LyondellBasell, Exxon and other companies.

The earliest company to develop HCPP in China is Yanshan Petrochemical, and it has a commercial series of products, with representative brands K6730/K6760/K6100. Maoming Petrochemical, Zhenhai Refining and Chemical, Yangzi Petrochemical, and Sinopec Tianjin also have products with a melt index of ≤60g/10min, but the modulus is generally low. Lanzhou Petrochemical is also developing a series of HCPP products. Products with a melt index of 30 have been put on the market, and products with a melt index of >30 have completed pilot testing and are waiting for commercialization.

Key points of production technology

1. Catalyst

a. High isotacticity/wide molecular weight distribution

The higher the isotacticity, the higher the crystallinity of polypropylene, and the higher its bending modulus and heat deformation temperature. The wider the molecular weight distribution, the higher the bending modulus and the better the processability, especially for large injection molded parts, which require a wider molecular weight distribution.

b. Good hydrogen adjustment sensitivity

Good hydrogen adjustment sensitivity can easily produce high melt index polypropylene to avoid too high hydrogen concentration in the reactor, which makes it difficult to control pressure and temperature.

c. High activity/long life

High activity means high reaction yield and long catalyst life, so that the impact reactor has sufficient activity to generate multiphase copolymers (rubber phase) with appropriate content and uniform distribution.

d. Porosity

The porosity should not be too high to avoid too much gas entrained in the polypropylene powder, resulting in high volatile content and high odor level in the final product.

2. External electron donor

An external electron donor catalyst with high isotacticity, good hydrogen adjustment sensitivity and the ability to produce polypropylene with relatively wide molecular weight distribution should be selected for use together. At the same time, the content of aromatic components such as benzene, toluene, ethylbenzene, and xylene in the external electron donor should be as small as possible or not contained to prevent the odor level from being too high.

3. Auxiliary agent formula

Select a suitable auxiliary agent system, generally including:

Antioxidant: a primary antioxidant + auxiliary antioxidant of appropriate type and content to minimize the degradation of the product during extrusion granulation, and the content of aromatic volatiles should be as small as possible to minimize the odor level;

Halogen absorber: calcium stearate or synthetic hydrotalcite, synthetic hydrotalcite has a synergistic effect with rigid nucleating agent, which can increase some additional bending modulus;

Rigid nucleating agent: a high rigid nucleating agent should be selected to greatly increase the product bending modulus, and has little effect on the product impact and shrinkage warpage.

4. Ethylene content

HCPP room temperature impact requirement is greater than 5kj/m2, therefore, the final product ethylene content is about 7%, and the multiphase copolymer morphology in the second reactor and the dispersion in the homopolymer matrix resin should be as uniform as possible.

5. VOC removal

The powder processing unit needs to strengthen the ability to remove oligomers and volatiles, and extrusion granulation requires vacuum degassing.

Difficulties in the production of "three high" polypropylene

1. Due to the high melt index of the product, the hydrogen concentration in the reactor is very high. When the catalyst is added, the initial reaction is intense, and the later reaction activity is insufficient. This causes reaction fluctuations and is difficult to control;

2. The hydrogen concentration in the reactor is too high, and it is difficult to control the reaction pressure, especially in the gas phase process;

3. In the production of high melt index products, a large amount of oligomers will be produced due to a large amount of hydrogen, and the powder processing unit has high requirements for powder deactivation and degassing;

4. Due to the high melt index of the powder, granulation is difficult, and it is necessary to optimize the extrusion granulation process conditions, modify the extruder template and pelletizing system, and add a vacuum degassing system.