Introduction to PP raw materials for spunbond and meltblown nonwoven production

Polypropylene is currently the most commonly used raw material and the most important polymer fiber raw material. Nearly 94% of the raw materials used in spunbond fabrics and meltblown fabrics in my country are polypropylene. According to U.S. statistics, polypropylene fiber is increasingly used in nonwovens and has largely replaced polyester and viscose fibers. In the fields of medical, sanitation, and health care, the application volume of polypropylene nonwovens accounts for 50% of the total nonwoven production.

Polypropylene fiber is different from other synthetic fibers in two major ways:

1. The first is that its hygroscopicity is very low, which gives polypropylene fiber excellent stain resistance and almost the same dry and wet properties;

2. The second is that it has the smallest density of all fibers, which gives it better coverage at the same weight.

The basic properties of polypropylene fiber are as follows:

1. Very low density (0.91g/cm3);

2. Low moisture content (less than 0.1%,);

3. Good chemical resistance (acid, alkali, solvent resistance);

4. Anti-mildew, anti-corrosion, anti-moth, anti-bacterial;

5. Good heat insulation, thermal insulation and insulation properties;

6. High stiffness, good tensile strength (4.5~7.5 cN/dtex);

7. Good fatigue resistance and wear resistance;

8. Good hydrolytic stability;

9. The original solution has good dyeability (can dye a wide spectrum);

10. Temperature resistance is 120℃, melting point is 165℃;

11. Easy to recycle.

The hydrocarbon structure of polypropylene fibers (i.e., the lack of any polarity) makes them highly water-repellent, thus preventing staining from polar materials. For nonwovens, this characteristic can also be changed through appropriate post-finishing to make PP nonwovens hydrophilic or have other special properties.

Polypropylene has a very low melting point (165°C), so the fiber web is suitable for hot-rolling bonding and consolidation processing. The product has been widely used as a covering material for sanitary napkins and diapers.

Due to the non-ionic chemical properties of polypropylene fiber, it is difficult to dye using ordinary processes. Therefore, PP nonwoven fabrics all adopt the melt dyeing process, that is, the color masterbatch is mixed into the polymer slice raw material, and the melt blend spinning and dyeing is performed. In addition to product dyeing, functional masterbatch can also be added to the melt to give the product specific functions. Under normal circumstances, the degradation phenomenon induced by light (ultraviolet UV) will cause PP nonwovens to fade, become brittle, and reduce mechanical strength. Now you can add anti-aging agents to reduce the impact of ultraviolet rays on products and extend the effective use time of products.

However, it should be noted that PP nonwovens will break down when exposed to gamma rays (a sterilization method used in the production of hygienic materials), release an odor, change color, and become brittle. This is an issue that should be paid attention to when PP nonwovens are used in the medical and health field.

1: Rheological properties of polypropylene

In polypropylene forming processing, melt flow performance is an important indicator, which is generally characterized by the melt flow index (MFI). The larger the MFI value, the better the fluidity of the melt. It refers to the weight of the melt flowing through a standard (diameter 2.095mm, length 8mm) capillary tube under a standard load (2160g) within 10 minutes at a molten state of 230°C. The unit is g/10min. This is a test method commonly used by enterprises at present, and it is usually used to characterize the flow characteristics of raw material slices. Melt flow index is also called melt index.

The size of the melt index (MFI) is related to the relative molecular mass of polypropylene. Generally, the average relative molecular mass of isotactic polypropylene is between 180,000 and 300,000. The larger the relative molecular mass, the smaller the MFI value. Due to different resin types and molding methods, the melt index of the selected resins varies greatly.

In the production of spunbond nonwovens, since spinning, drafting, and laying are required continuously, and high-speed spinning is used to produce fine fibers, the required melt index is generally 30 or 40g/10min.

The MFI value of resins for other purposes or processes is slightly different from the resins used for spunbond spinning. For example, the MFI value of resins for film-splitting fibers is generally 4~7g/10min; the MFI values of resins for long and short fibers are generally 4-7g/10min. At 14~20g/min; the MFI value of fine denier filament, high-speed spinning filament, and resin for cigarette filters is generally 30~40g/10min.

The production of melt-blown nonwovens requires resin with better flow characteristics, and the melt index is generally between 400 and 1500g/10min. Generally, the higher the melt flow rate and the lower the viscosity of the melt, the easier it is to draw into finer fibers, and the strength of the single fiber will be smaller. If stronger fibers are required, a resin with a lower melt flow rate should be used.

The main factors that affect the strength of nonwoven fabrics are fiber thickness, fiber web uniformity and consolidation conditions. Therefore, the flow rate of resin has a certain impact on the strength of nonwoven fabrics, but the impact is not too great.

The normal stress difference generated when the melt flows in the capillary pores causes the oriented molecules to expand at the outlet. This swelling effect depends on the size of the capillary pores, especially the length-to-diameter ratio (L/D) of the pores, which decreases as the (L/D) value increases and the average shear rate gradient of the capillary pores decreases; As the melt temperature decreases, the relaxation effect slows down and the viscous damping increases, causing the extrusion expansion effect to become significant. The soft molecular chain structure of polypropylene makes it impossible to ignore this outlet expansion behavior during the processing and forming process. This is a factor to be considered when designing the distance between the spinneret holes and the aspect ratio of the spinneret holes.

2: Spinnability of polypropylene

Spinnability is an important application characteristic that measures the ease of spinning polymer raw materials into fine fibers. It refers to the fact that after the melt is ejected from the spinneret, a thin stream of the melt forms a continuous fiber with certain physical and mechanical properties. characteristics. The properties of polymer raw materials related to spinnability are mainly divided into two categories: physical properties and chemical properties.

The physical properties of raw materials mainly refer to melting point, softening point, glass transition temperature, shear viscosity, extensional viscosity, crystallinity, moisture content, impurity content, etc.

The chemical characteristics of raw materials mainly refer to molecular structure, molecular weight, molecular weight distribution, stability, etc. The physical and chemical properties of polymer raw materials have an important impact on the spinnability of the melt. Taking PP as an example, the molecular structure is required to be isotactic, the molecular weight is small, and the melt index is appropriate (spunbonding and melt-blown Methods vary), the molecular weight distribution width is less than 4, and the relative molecular weight distribution of the polymer depends on the type of catalyst and polymerization process conditions. The moisture content is less than 500mkg, the impurity content is less than 250m/kg, etc.

When the polymer's spinnability is poor, its symptoms include: fluctuations in spinning melt temperature, high melt pressure, melt dripping, inability to withstand high-speed drafting, prone to broken filaments, easy degradation, excessive monomers, The smoke is large, the service life of the filter and the spinneret is short, and it is easy to cause shutdown failure.

The glass transition temperature of polypropylene is -35~10°C, which varies according to different sample purity, testing methods and conditions. The melting point of polypropylene is related to isotacticity. Generally, the melting point of polypropylene is 164~170°C, and the melting point of pure isotactic polypropylene is 176°C. The spinning temperature needs to be controlled above the melting point. The specific melt temperature is related to the raw material MFI, spinning process and machine model. If the same raw material is used on different machine models, the melt temperature can differ by dozens of degrees. During the spinning process, as the MFI value of polypropylene increases, the relative molecular mass decreases, and the spinning temperature should decrease accordingly.

Due to the high melt viscosity of polypropylene, if it is spun at a lower spinning temperature, it is easy to cause orientation and crystallization to occur simultaneously and form a highly ordered monoclinic crystal structure; on the contrary, at a higher spinning temperature Under the condition, due to the greater fluidity before crystallization occurs, the pre-orientation degree of the primary fiber is low, and an unstable butterfly-like liquid structure is formed, which can achieve higher multiples of stretching, thereby obtaining high-strength fibers.

In recent years, due to the development of metallocene catalysts, the production and application of metallocene-catalyzed homogeneous polymerization of isotactic polypropylene can prepare polymers with narrower relative molecular weight distribution and improved homogenization. Metallocene-catalyzed polypropylene has excellent rheological properties and can have a larger flow rate at normal spinning temperatures. It can reduce spinning pressure and extrude thinner filaments, which is comparable to spinning monofilaments using ordinary raw materials. Fibers with smaller fineness and nonwovens with lower basis weight values. Metallocene catalyzed polymer raw materials have good effects in improving product uniformity and coverage, improving hand feel, and saving energy. They are currently important raw materials for the production of fine denier fibers. The table below shows a comparison of the properties of metallocene polypropylene and conventional polypropylene.

Since the metallocene complex catalyst can adjust the melting point of polypropylene between 130 and 170°C, which is about 15°C lower than conventional polypropylene, the polymer can be easily mixed with a matrix with a lower melting point such as polypropylene during the polymerization process. Copolymers, polyethylene, etc. are layered. At the same time, it can reduce the residue rate and improve productivity, showing excellent resistance to oxidation fracture during repeated extrusion.

3: Raw material requirements for spunbond and meltblown polypropylene chips

In the polypropylene spinning process, different brands of raw materials are often used, and their spinnability will be greatly different, such as the length of the spinneret assembly cycle, the occurrence rate of broken filaments, and the uniformity of the fibers. . This is often caused by impurities contained in the resin.

Impurities in resin can be divided into inorganic impurities and organic impurities. Inorganic impurities include external impurities and impurities in the resin. The former comes from the impurities brought in during the resin chip production environment, storage, transportation and use; the latter mainly comes from catalysts and impurities. Various additives, such as color masterbatch, flame retardant masterbatch, etc. Inorganic impurities include titanium, aluminum, silicon, iron, sodium, etc. Some people think that sodium is the main component that affects filtration performance.

Organic impurities may be some extremely high molecular weight (more than 1 million) and branched polymeric high melting point foreign matter, which is related to crystal points, fish eyes or gel particles on the resin index. During the melt spinning process, a small part of these impurities with larger particle sizes are filtered out by the filter medium, while a part of the impurities with smaller particle sizes can pass through the gaps of the filter medium and remain in the primary fibers.

Excessively high impurity content can easily cause the filter in the spinning assembly to become clogged and the melt pressure to rise too quickly, especially when the ash content is high and the gel particles are large and numerous, this situation is more serious. Therefore, it is often necessary to replace the filter screen of the melt filter frequently, otherwise it will easily cause melt pressure fluctuations, melt leakage, breakdown of the filter screen, and shorten the service life of the components. Therefore, the impurity content in polypropylene chips should be limited to 0.025 to ensure the continuous spinning process.

Since polypropylene does not contain hydrophilic groups in its molecules and has a slow hydrolysis rate, it does not require high moisture content in the slices. It only requires that it does not affect spinnability and does not produce bubbles during forming. However, due to the fast spinning speed of the spunbond process, the moisture content in the slices is required to be lower, generally within 0.05%.

The high-end PPH Y1500 meltblown material produced by Qingdao primetech Plastic Co., Ltd. uses high-quality raw materials. The PP resin base material is: Hanwha Total Petrochemical's food-grade PP, which has FDA certification; the peroxide is selected from: United States AkzoNobel's Trigonox101; the peroxide Trigonox101 is the earliest peroxide used in the field of degrading polypropylene. Due to its good performance and safety risks, it has the US FDA license and the European BFR license. It can be used in the modification of polypropylene; high-quality raw materials lay a solid foundation for high-quality products.