Liaoning Xinda Talc Group - Deng Xianghui
Abstract: Talc powder filled polypropylene was mixed, extruded, and injected to produce talc powder/polypropylene composite materials. The shrinkage rate of the composite materials was tested, and the mechanism of the influence of the microstructure of talc powder on the shrinkage rate of the composite materials was explored.
Keywords: Particle size, flake structure, shrinkage rate, composite material
Abstract：Talc-filled polypropylene was processed through mixing, extrusion granulation, and injection molding to produce talc/polypropylene composites. The shrinkage rate of the composites was tested, and the mechanism by which the microstructure and processing methods of talc influence the shrinkage rate of the composites was discussed.
Foreword
Plastic shrinkage rate refers to the proportion of volume or size reduction of plastic after molding and cooling. Usually expressed as a percentage (%).
Inorganic fillers play an important role in plastic modification and have become indispensable modification agents. Thin layered talc powder is an effective reinforcement and stiffening material in plastics, which can endow plastics with high rigidity and creep resistance at both room temperature and high temperature. Adding talcum powder to polyethylene and polypropylene plastics can effectively improve the bending modulus, tensile strength, impact strength, thermal deformation temperature, surface hardness, and shrinkage rate of the products, accelerate the molding speed, and improve the dimensional stability of the products.
Talc powder is an industrial product composed of magnesium silicate mineral talc. It is ground, treated with hydrochloric acid, washed with water, and dried. Its main component is hydrated magnesium silicate. Commonly used as fillers for plastic and paper products, rubber fillers and anti sticking agents for rubber products, advanced paint coatings, etc.
During the PP molding process, fillers such as silicates, calcium carbonate, silica, cellulose, and glass fibers are filled into the polymer to improve the heat resistance, reduce costs, increase rigidity, and decrease molding shrinkage of PP.
This article uses talc powder of different specifications to prepare talc powder/polypropylene composite materials under the same polypropylene resin and processing technology, achieving uniform dispersion. The influence of the microstructure of talc powder on the physical properties of talc powder/polypropylene composites was studied through composite material shrinkage rate testing.
Reasons for Plastic Shrinkage Rate
1. The thermal expansion and contraction effect is the main reason why plastics naturally shrink when they cool from a high-temperature molten state to room temperature.
2. Structural changes of crystalline materials: Some plastics, such as polyethylene and polypropylene, have crystallinity and undergo a transition from amorphous to crystalline structure during cooling, resulting in further volume reduction.
3. Molecular orientation effect: During the injection molding process, the flow direction of the plastic melt will cause the polymer chains to be oriented, and this orientation during cooling will result in shrinkage differences (anisotropy) in different directions.
4. The high pressure applied during pressure release injection fills the mold cavity with molten material. When the holding pressure is completed and the mold is opened, the disappearance of pressure will also cause dimensional changes.
5. The impact of additives. Adding fillers such as talcum powder and glass fiber can alter the shrinkage behavior of the plastic matrix. For example, adding rigid fillers usually reduces the overall shrinkage rate.
The influence mechanism of talc powder on the shrinkage rate of polypropylene
1. Filling effect: Talc powder itself does not shrink, and its addition dilutes the shrinkage behavior of polypropylene in terms of overall proportion, thereby reducing the overall shrinkage rate of the composite material.
2. Changes in Crystallization Behavior: The addition of talc powder affects the crystallinity of polypropylene, reducing the formation of large-sized spherulites. This structural change further suppresses the shrinkage of the material.
3. Heterogeneous nucleation: Talc powder particles can serve as nucleating agents, promoting the formation of finer and more uniform spherulite structures in polypropylene, preventing the formation of larger spherulites, and thereby limiting the shrinkage movement of molecular chains.
Experimental section
1.1 Main raw materials
Polypropylene (PP), K7726H, North Huajin Chemical Co., Ltd; Talc powder, Liaoning Xinda Talc Group; Calcium stearate, BS3818， Huamingtai Chemical Co., Ltd; Aluminum ester coupling agent, JL-0025， Nanjing Jinlaiwang Plastic Technology Co., Ltd.
1.2 Main equipment
Double screw extruder, LNSD-51， Jiangsu Hongyunxiang Rubber Machinery Co., Ltd; Plastic injection molding machine, SA900II/260， Haitian Plastic Machinery Group Co., Ltd; Laser particle distribution measuring instrument, GSL-101BI， Liaoning Instrument Research Institute Co., Ltd; Ma Fulu, SX2， Zhengzhou Xinhan Instrument Equipment Co., Ltd.
1.3 Process Flow and Testing Methods
Grind and dry talc powder, polypropylene, and calcium stearate of different specifications in a ratio of 20:80:1 using a twin-screw extruder, and perform injection molding using an injection molding machine and mold to obtain standard sized test samples. The spline is stored in a constant temperature and humidity chamber for 24 hours before testing. The injection molding process adopts multi-stage injection molding control technology, with main process parameters: plasticizing temperature, 200 ℃; Screw speed, 50r · min-1; Injection pressure, 45%; Injection time, first 30%, middle 50%, and last 30%; Holding pressure, 30%; Holding time, 8s; cooling time, 10s.
Standard for spline production: GBT 17037.1-2019 Preparation of injection molded specimens for thermoplastic materials Part 1: General principles and preparation of multi-purpose specimens and elongated specimens
The testing standard in the experiment is GB/T 17037.4-2003 Preparation of Injection Molding Specimens for Thermoplastic Materials Part 4: Determination of Molding Shrinkage;
Results and discussion
2.1 The effect of fineness of talc powder on the shrinkage rate of talc powder/polypropylene composite materials
Table 1 Shrinkage rate of polypropylene composites filled with talc powder of different particle sizes
Number | DV50（μm） | Silicon content（%） | Shrinkage rate/% |
1 | 2.5 | 50 | 0.85 |
2 | 3.5 | 50 | 0.88 |
3 | 4.5 | 50 | 0.95 |
4 | 5.5 | 50 | 1.03 |
5 | 6.5 | 50 | 1.06 |
The mechanical properties of polypropylene composites filled with talc powder of different particle sizes are shown in Table 1. It can be seen that the smaller the particle size, the lower the shrinkage rate of the composite material.
2.2 Effect of Silicon Content in Talc Powder on the Shrinkage of Talc Powder/Polypropylene Composite Materials
Table 2 Shrinkage rate of polypropylene composites filled with talc powder with different silicon contents
Number | DV50（μm） | Silicon content（%） | Shrinkage rate/% |
1 | 5.5 | 60 | 0.91 |
2 | 5.5 | 58 | 0.94 |
3 | 5.5 | 55 | 0.98 |
4 | 5.5 | 50 | 1.03 |
5 | 5.5 | 45 | 1.12 |
From Table 2, it can be seen that the higher the silicon content of talc powder, the smaller the shrinkage rate of the composite material.
2.3 Effect of the diameter to thickness ratio of talc powder on the shrinkage rate of talc powder/polypropylene composite materials
Table 3 Mechanical Properties of Polypropylene Composites Filled with Talc Powder with Different Diameter to Thickness Ratios
Number | DV50（μm） | Silicon content（%） | Diameter to thickness ratio | Shrinkage rate/% |
1 | 5.5 | 50 | 8.5 | 1.15 |
2 | 5.5 | 50 | 10.5 | 1.11 |
3 | 5.5 | 50 | 12.5 | 1.06 |
4 | 5.5 | 50 | 14 | 0.98 |
5 | 5.5 | 50 | 17 | 0.92 |
From Table 3, it can be concluded that the higher the diameter to thickness ratio of talc powder, the smaller the shrinkage rate.
3. Conclusion
(1) The smaller the particle size of talc powder, the lower the shrinkage rate of its plastic composite material. The reason is that the smaller the particle size, the more particles talc powder has, and the more particles act as nucleating agents, resulting in higher crystallinity and reduced formation of large-sized spherulites, further suppressing material shrinkage. The smaller particle size of talc powder has a larger specific surface area and a larger contact area with the resin matrix, which provides good support and reduces shrinkage better than larger particle size talc powder.
(2) The higher the diameter to thickness ratio of talc powder, the smaller the shrinkage rate. Because talc powder itself does not have shrinkage, the higher the diameter to thickness ratio, the larger the specific surface area of talc powder, and the larger the contact area between talc powder and resin matrix. This will have a stronger suppression of the thermal expansion and contraction of polymer materials, and also have a positive effect on suppressing the orientation of polymer materials.
(3) The higher the silicon content of talc powder, the smaller the shrinkage rate of the composite material. Generally speaking, the higher the silicon content, the smaller the shrinkage rate, which is only a surface phenomenon. The fundamental reason is that the higher the silicon content, the higher the powder diameter to thickness ratio. The higher the purity, the more powder can be used as a nucleating agent, which means that the silicon content factor should be classified into two categories: microstructure reasons and nucleating agent reasons. All of them have a positive impact on reducing shrinkage rate.
