date：2020-03-06Source: Yongjiang group
（1） High melting refers to the market demand of PP
The melt flow properties of polypropylene are closely related to its molecular weight. The weight average molecular weight of commercial polypropylene resin prepared by Ziegler Natta catalyst system is generally between 3 × 105 and 7 × 105. The melting index of these conventional polypropylene resins is generally low, which limits their application.
With the rapid development of chemical fiber industry and textile machinery industry, nonwovens industry is rising rapidly. A series of advantages of polypropylene make it the first choice for nonwovens. With the development of society, the application of nonwovens tends to be broad: in medical and health fields, nonwovens can be used to make isolation clothing, masks, surgical clothing, women's sanitary napkins, baby diapers, etc.; as building and geotechnical materials, nonwovens can be used in roof waterproof, road construction, water conservancy engineering, or the production of advanced roof by using spunbond and acupuncture composite technology The service life of felt is 5-10 times longer than that of traditional asphalt felt. Filter material is also one of the fastest-growing products of nonwovens, which can be used for gas and liquid filtration in chemical industry, medicine, food and other industries, with great market potential. In addition, nonwovens can be used for the manufacture of synthetic leather, bags, clothing linings, decorative cloth and wiping cloth in household use.
It is because of the continuous development of nonwovens that the requirements for their production and application are also increasing, such as melt blown, high-speed production, thin products, etc. Therefore, the processing performance of polypropylene resin, the main raw material of nonwovens, is also required to be improved; in addition, the production of high-speed spinning or fine denier polypropylene fiber also requires polypropylene resin to have good melt flow performance; some pigments that can not withstand high temperature need to process polypropylene with relatively low temperature as the carrier. All of these require ultra-high melt index polypropylene resin which can be processed at low temperature as raw material.
The special material for melt blown cloth is polypropylene with high melt index. Melt index refers to the mass of melt passing through the capillary of standard die every 10 minutes. The larger the value is, the better the processing fluidity of the material is. The higher the melt index of polypropylene is, the finer the fiber will be and the better the filtration of the melt blown cloth will be.
（2） Preparation of high melt index polypropylene resin
One is to control the molecular weight and molecular weight distribution of polypropylene by controlling the polymerization process. For example, to reduce the molecular weight of polymer by increasing the concentration of polymerization inhibitor such as hydrogen, so as to improve the melt index. This method is limited by catalytic system, reaction conditions and other factors, so the stability of melt index is difficult to control and implement.
The metallocene catalyst used in Yanshan Petrochemical industry has been directly polymerized with melt blown materials of more than 1000 in the past few years. Due to the difficulty in controlling the stability, there has been no large-scale polymerization. Since the outbreak of this year, Yanshan Petrochemical Co., Ltd. has adopted the controllable degradation polypropylene melt blown material production technology developed in 2010 to produce the polypropylene melt blown non-woven fabric special material on February 12. At the same time, industrial tests have been carried out on the device with metallocene catalyst. At present, the product has been produced and is being sent to downstream users for trial use.
Another method is to control the degradation of polypropylene, reduce its molecular weight and improve the melt index.
In the past, high temperature degradation method was often used to reduce the molecular weight of polypropylene, but there are many disadvantages in this high temperature mechanical degradation method, such as additive loss and thermal decomposition, process instability and other adverse factors. In addition, there are also ultrasonic degradation methods, but these methods often need the presence of solvents, which increases the difficulty and cost of the process. In recent years, the chemical degradation of polypropylene has been widely used.
（3） Production of high melting point PP by chemical degradation
The chemical degradation method is to make polypropylene react with chemical degradation agents such as organic peroxides in screw extruder, so as to break the molecular chain of polypropylene and reduce its molecular weight. Compared with other degradation methods, it has the advantages of complete degradation, good melt fluidity, simple and easy preparation process, and is easy to carry out large-scale industrial production. This is also the most popular method used by modified plastics manufacturers.
1、 Equipment requirements
The high melting point is totally different from the ordinary PP modification equipment. The equipment used for spray melting material needs a longer length diameter ratio, and the head needs to be vertical, or underwater granulation (Wuxi Huachen has similar underwater cutting); the material is very thin, and it needs to contact water immediately after coming out of the head, so it is cold;
In the production of conventional polypropylene, the cutting speed of extruder is 70 meters per minute, while that of high melting point polypropylene is more than 120 meters per minute. In addition, due to the high flow rate of HDPP, the cooling distance should be increased from 4m to 12m.
The machine for melting and spraying material requires continuous wire mesh changing, and generally adopts double station mesh changing device. Motor power requirements are much larger, screw components inside the shear block will use more;
According to the introduction of Keya Pangong, the twin screw line for melt blown special material has considerable particularity
1. Feed (PP, DCP, etc.) stably;
2. According to the half-life of the compound formula, the appropriate length diameter ratio and the axial position of the opening should be determined (it has evolved to the third generation to ensure the smooth operation of cr-pp reactive extrusion);
3. It is necessary to ensure that the fusion finger has a higher output within the tolerance range (more than 30 finished bars have a higher cost performance and blending basis than only a dozen);
4. It must be equipped with special water mould head. Melt and heat should be even, and waste material should be less;
5. It is better to equip the melt blown material mature cold pelletizer (there is a reputation in the industry), so as to ensure that the