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(1) Thermal degradation can be achieved by using oxidants or peroxidants in the extrusion process, or by mechanical shearing of the extruder or by controlling the working temperature, so as to achieve the purpose of regulating the melt viscosity.
(2) The technology of melt blown nonwoven fabric requires a narrow molecular weight distribution so that uniform ultrafine fibers can be produced. With the new catalytic technologies, such as metallocene catalysts, it is possible to produce polypropylene with very high melting values and very narrow molecular weight distributions.
(3) The heat resistance of polypropylene is sufficient for most product applications, and it has a wide melting range, so it is very beneficial for the thermal bonding process commonly used in nonwovens.
(4) It is beneficial to make ultra-fine fibers. If the melt viscosity of polypropylene is very low and the molecular weight distribution is very narrow, the same energy consumption can be used in the melt blown technology to make very fine fibers under the same drafting conditions. Therefore, the common fiber diameter of polypropylene melt blown nonwoven fabrics is 2 to 5pm, or even smaller.
Since the melt blown technology uses high-pressure hot air for drafting, it is required to use a polymer with a high melt index, which is conducive to increasing production and reducing energy consumption.
At present, the most widely used polypropylene chips for melt blown nonwoven fabrics have a melt index of 400 to 1200g /10min, and a narrow molecular weight distribution so as to produce ultra-fine fibers with the required linear density.
The polypropylene chips used in melt blown production, processing and use should have high and uniform melt index, narrow molecular weight distribution, good melt blown processing characteristics, and uniform and stable chip quality, so as to ensure the technology of melt blown nonwoven materials is stable.
Since the production technology of melt blown nonwoven fabrics is significantly different from that of spunbond nonwoven fabrics, the installation and disassembly methods of spinning modules for spunbond and melt blown nonwoven fabrics are also different due to different spinning technologies.
Spinning modules of melt blown nonwoven fabrics can be divided into two types according to different structural forms: integral type and split type, so the installation and disassembly methods can also be divided into two types according to the structural form.
(1) Replacement method of integral spinning module of melt blown nonwoven fabrics
The integral spinning modules of melt blown nonwoven fabrics refer to the melt distribution plate, filter screen, spinneret and air knife forming a detachable whole. The air gap has been adjusted and tightened before installation. Therefore, the modules can be easily and quickly replaced as a whole.
Using this form of melt blown modules, the installation process is simple and the time used is short, and the efficiency is higher if the preheating technology is used at the same time.
(2) Replacement method of split spinning module of melt blown nonwoven fabrics
When the split spinning module of melt blown nonwoven fabrics is replaced, various parts in the module, such as filters, spinnerets, air knives, etc., are installed on the spinning box one by one in sequence.
During the installation process, the matching relationship between the various parts should be continuously tested, and a comprehensive inspection and adjustment should be carried out after the initial assembly. After reaching the working temperature, heat fastening and fine adjustment should be carried out again or for several times until the technological requirements are met.
After the spinning module is installed on the spinning box, it is necessary to use the drafting airflow and the heat of the box to heat up the module, which requires a long heating and balancing time before it can be put into operation.
If the balancing time is insufficient, the spinneret plate will be turned on before it reaches the working temperature. Due to the large flow resistance of the melt, it is easy to generate a high pressure in the modules and the box, which is very likely to cause the spinneret plate to crack and be damaged.
Each part of the spinning module of melt blown nonwoven fabric is the precise and complex machining part, so great care must be taken when installing the spinning module of melt blown nonwoven fabric.
Although it takes more time to install such a split type module, the size and weight of the parts in this type of spinning module are relatively small, and the operation can generally be completed by using small special tooling or manpower.
Moreover, because the matching between the parts of the spinning module of melt blown nonwoven fabric is not fixed, it can also be optimized and adjusted according to the production technological requirements or the quality of the product, which has a certain flexibility.
