There are many quality index items of polypropylene resin. The more important and generally detectable and practical quality index items are: isotacticity, melt index, molecular weight, ash content, chlorine content, titanium content, volatile content, density, apparent Density, tensile yield strength, tensile strength at break, elongation at break, impact strength (notched and unnotched impact strength), heat distortion temperature, Vicat softening point temperature, embrittlement temperature, hardness, electrical properties such as electrical resistance coefficient, breakdown voltage strength, dielectric loss tangent).
For the fiber-grade polypropylene resin used for spinning high-strength polypropylene yarn and non-woven fabrics, there are also gel particle (or "fish eye") content indexes and molecular weight distribution indexes. The concept and significance of some important quality indicators of polypropylene are briefly introduced below.
Polypropylene is a long-chain polymer, which contains three different main structures, namely isotactic polypropylene, syndiotactic polypropylene and atactic polypropylene. Isotacticity of polypropylene is the content of isotactic polypropylene in the whole polymer, expressed in mass percentage, and isotacticity is also called isotactic index.
Isotacticity affects the crystallinity of polypropylene, and the higher the isotacticity, the higher the crystallinity. Crystallinity affects many properties of polypropylene. Within a certain range, the crystallinity is high, the tensile yield strength of the resin is high, and the hardness is high. The low isotacticity, low crystallinity, and good impact strength, especially low-temperature impact performance, are the main reasons why atactic polypropylene can be used as an impact modifier for polypropylene or other resins.
Polypropylene isotacticity has a significant effect on the spinning properties and fiber quality of polypropylene fibers. It has high isotacticity and good crystallinity, which is beneficial to the filament formation performance and the quality of the filament.
In addition to affecting the properties of the product, the isotacticity of polypropylene also affects the processing properties of the resin. The isotacticity is low, the product is sticky, and the fluidity is poor. It is easy to form lumps and agglomerates during packaging and storage, and it is difficult to add materials during processing, or even impossible to process.
Polypropylene isotacticity is too low will also affect the normal production and operation of polypropylene. Generally, when the isotacticity is lower than 90%, the product will appear sticky. When the isotacticity is lower than 85%, the product is very sticky, which will cause serious sticking phenomenon on the inner wall of the loop reactor, the discharge port, the flash tank and the pipeline, which will affect the operation and normal production.
The isotacticity of polypropylene products is generally determined by boiling n-heptane extraction method or infrared spectroscopy, nuclear magnetic resonance method, because isotactic polypropylene is insoluble in n-heptane, and polypropylene with other structures can be dissolved in boiling n-heptane, so it can be It is measured and expressed by the percentage of insoluble in boiling n-heptane in the whole sample, which is also the most simple and feasible method for the determination of polypropylene isotacticity. In addition, the content of isotactic propylene in polypropylene can also be represented by xylene insoluble matter, which is slightly different from the isotacticity measured by n-heptane extraction method.
Melt Fluid Rate
Melt flow rate, also known as melt index (MI for short), refers to the mass of the thermoplastic resin passing through a standard die every 10min under a certain load and temperature on a melt flow rate measuring instrument, expressed in g/ 10min is expressed in units. For polypropylene resin, the conditions for measuring the melt flow rate are: temperature 230°C, load 2160g, and sample pressure 0.2982MPa.
Melt index is an index to measure the flow properties of polypropylene resin in the molten state. The larger the MI, the better the melt flow properties of the polypropylene resin, whereas the smaller the MI, the worse the melt flow properties of the polypropylene resin. Since polypropylene is a thermoplastic resin, it is processed into various products in a molten state, so MI is an important indicator that affects the processing performance of polypropylene, and it is also one of the most important indicators of polypropylene product quality.
In polypropylene homopolymer products, various grades are mainly distinguished by their MI. Different uses, product grades are also different, correspondingly require different specifications of MI. The same grade of polypropylene has a certain MI range. Different MIs require different processing temperatures.
If batches of products with very different MIs are mixed together and processed together, "fish eyes" will occur during processing, that is, the resin with high MI has melted, while the resin with low MI has not melted, resulting in white spots in the product, which will cause The quality of the products is greatly affected, especially when spinning polypropylene fibers, the existence of fisheyes has a greater impact on the spinning performance and the quality of polypropylene fibers. Therefore, the MI of PP resin is required to be stable and controllable to ensure good processing performance and product quality of the product.
Polypropylene resin is a polymer compound, which is composed of many macromolecules, and the molecular weight of each macromolecule is not the same, so there is no uniform molecular weight, but can only be expressed by the average molecular weight. Generally, the molecular weight of polypropylene is a statistical average molecular weight. Due to the different statistical methods used in practice, there are generally four methods for expressing the average molecular weight, namely number average molecular weight, weight average molecular weight, Z average molecular weight, and viscosity average molecular weight. In the production of polypropylene, generally only the viscosity average molecular weight is measured, and other average molecular weights are measured when necessary. The so-called viscosity-average molecular weight is the molecular weight measured by the viscosity (intrinsic viscosity) method. The molecular weight of polypropylene directly affects its melt flow rate. The larger the molecular weight, the smaller the MI, and the poorer melt fluidity; conversely, the smaller the molecular weight, the larger the MI, and the better the melt fluidity.
The molecular weight distribution
The molecular weight distribution represents the degree of dispersion of molecular weights in a polymer compound. The molecular weight distribution can be described by the molecular weight distribution index. The molecular weight distribution index can be expressed as the ratio of weight-average molecular weight to number-average molecular weight, and the range of its value indicates broad and narrow molecular weight distribution.
The molecular weight distribution of polypropylene has an important influence on its physical and mechanical properties and processing properties. For general extruded and injection molded products, when the molecular weight distribution is wide, the processing performance is good, but the tensile strength is poor and the elongation becomes small. For fiber-grade polypropylene resins, a narrower molecular weight distribution is required, which is beneficial to fiber-forming properties and fiber quality. The molecular weight distribution index of fiber grade polypropylene resin is required to be between 3 and 5.
Ash, Titanium, Chloride, Volatile Content
Ash, titanium, chloride ion, and volatile content represent the impurity content in polypropylene resin. They are all important indicators of the quality of polypropylene products and have an important impact on polypropylene processing and application.
Polypropylene resin is an organic compound composed of carbon and hydrogen elements. It will decompose at high temperature, some of which will become low molecular organic compounds and volatilize, and some will become CO2, CO and H2O and volatilize. Completely pure polypropylene should be at high temperature. All volatilized, there should be no ash, but in fact, there will always be more or less non-volatile substances in the polypropylene tree, such as catalysts, activators remaining in the resin and various impurities mixed in the production process. . The content of impurities contained in polypropylene that cannot be volatilized after high temperature burning and remains in the whole sample is called the ash content of polypropylene, generally expressed in mass percentage or mass ppm. Fiber grade polypropylene resin requires the ash content to be controlled below 100ppm. If the ash content is high, it means that there are many infusible substances during processing, which will easily cause equipment blockage and affect the strength and other properties of the product. However, ash has little effect on Tengda extrusion and injection molding.
(2) Titanium content
The content of titanium in the catalyst remaining in the polypropylene product in the entire sample is called the titanium content and is generally expressed in mass ppm. The presence of titanium affects the stability of polypropylene resin. With high titanium content, the resin is easy to age and deteriorate. Since the current PP production process has generally adopted high-efficiency supported catalysts, the content of titanium has been reduced to below a few ppm.
(3) Chloride ion content
The content of chloride ions in the catalyst remaining in the polypropylene product in the whole sample, called chloride ion content, is usually expressed in mass ppm. The presence of chloride ions in polypropylene resin mainly has a corrosive effect on post-processing equipment. At present, due to the use of high-efficiency carrier catalysts, the chlorine content in the product has been reduced to a very low level. After the steaming process, the chlorine content can also be greatly reduced.
(4) Volatile component
The content of volatile matter (such as organic solvents, propylene, moisture, etc.) contained in polypropylene products below the decomposition temperature of polypropylene is called volatile matter content, which is usually expressed in mass percentage. Different from the concept of ash content, when measuring volatile content, the temperature must be controlled below the decomposition temperature of polypropylene, generally 105℃, and the polypropylene resin itself is not allowed to decompose and volatilize.
Volatile component content also affects processability. The high content of volatile components will cause bubbles and pores in the product during processing, thereby affecting its mechanical properties. During spinning, spinnability is also affected.