Effect of talc on micro-foamed polypropylene (PP) composites

Talc is a common silicate mineral. The flaky structure of talc has a positive effect on the rigidity of plastic materials and the creep resistance at high temperatures, making it a reinforcing filler. The talc powder has a good reinforcing effect on the polypropylene material, and at the same time, can improve the dimensional stability and heat resistance of the polypropylene. In the application of talc-filled PP automotive materials, the sheet-like structure facilitates the optimal balance of stiffness and impact toughness.

Polypropylene (PP)/talc powder is widely used in automotive plastics due to its low cost and excellent mechanical properties. However, the material still has some shortcomings, such as high density, which is not conducive to the weight of the car. The micro-foamed PP/talc powder material not only has excellent properties of the composite material, but also has a reduced density and a reduced quality.

1. Sample preparation (1) PP/talc powder masterbatch is extruded and granulated by co-rotating twin-screw extruder. Process parameters: screw speed 140r/min, feeding rate 1.2-1.5r/min, extrusion heating zone temperature 160-188 ° C.
(2) Micro-foamed PP/talc powder composite
The PP/talc masterbatch and the blowing agent masterbatch are secondarily molded in an injection molding machine to prepare a dumbbell-shaped spline that meets the test standard. The injection temperature was 175 ° C, the injection rate was 95%, and the injection pressure was 500 kPa.

2. Effect of talc on mechanical properties of micro-foamed composites

Figure 2 Effect of mass fraction of talc on specific strength of microcellular foam

Specific strength is an important indicator to measure the light weight and high strength of materials. It can be seen from Fig. 2 that the specific tensile strength of the unfoamed PP material with a mass fraction of 5% of talc powder is lower than that of the pure PP material, and the specific bending strength is increased, and the specific impact strength is basically unchanged; with the talc powder The mass fraction is further increased, and the specific tensile strength, specific bending strength and specific impact strength of the material are both lowered.
Compared with pure PP material, the talc powder has a mass fraction of 5%. After foaming, the specific tensile strength and specific bending strength increase, and the impact strength is slightly lower. Continue to increase the mass fraction of talc, foaming material. The specific tensile strength, specific bending strength and specific impact strength are both reduced. For the same material, the specific strength of the material after foaming is lower than that of the unfoamed material.
Through the analysis of the specific strength of the above materials, after the material is foamed, the specific strength of the talc powder with a mass fraction of 5% is the highest, and the mass fraction of the talc powder is continuously increased, and the specific strength of the material is decreased. Therefore, a small amount of talc filling is beneficial to improve the mechanical properties of the PP material after foaming.

3. Effect of talc powder on cell structure of micro-foamed composites

Figure 3 Optical micrograph of micro-foamed talc-filled PP composite

Compared with the pure PP material, the PP composite with a mass fraction of 5% of talc has a regular cell shape after foaming, uniform and compact, no cell combination and collapse. The talc powder with a mass fraction of 5% is added to the PP matrix to serve as a hot spot nucleation. As the mass fraction of talc powder further increases, agglomeration occurs between the talc powders, and the heterogeneous nucleation is reduced. The latter has a greater influence on the foaming system than the former, so the cell collapse is serious and the cells are sparse. This is mainly because the nucleation of talcum powder is obvious, and the number of cells increases. If the adjacent cells are similar in size, they tend to merge under the action of interfacial tension; if the difference in size is large, due to the difference in gas pressure in the cells, The melt film between the cells is broken and the cells are combined.

4. Effect of talc on the average cell diameter and number of cells in micro-foamed PP composites

Fig. 4 Effect of talc powder on cell diameter and number of cells of foamed material

It can be seen from Fig. 4 that the average diameter of the cells with a mass fraction of talc of 5% is 73 μm, and the number of cells is the largest, reaching 132. Compared with pure PP foaming material, after adding 5% of talc powder, the average cell diameter decreased by 2700 and the number of cells increased by 7800. This is because after the talc powder is added to the PP matrix, the talc particles act as a hot spot nucleation, the number of cells increases, and the cell diameter decreases.
As the mass fraction of talc powder further increases to 10%, agglomeration occurs between the talc powders, the nucleation point decreases, and the heterogeneous nucleation decreases. The latter affects the foaming system more than the former, thus the cells The average diameter increases, and the number of cells decreases. When the mass fraction of talc is continuously increased to 20%, the cell diameter decreases and the number of cells increases. Although up to 20% of the mass fraction of talc will agglomerate, the number of nucleation sites will increase at this time, and the heterogeneous nucleation will have a greater influence on the foaming system. At this time, the cell diameter decreases and the number of cells increases.

5, summary (1) talc powder has a heterogeneous nucleation effect on the foaming of PP material. When the mass fraction of talc powder is 5%, a micro-foamed material with small, uniform and dense cells is obtained. As the mass fraction of talc powder increases, due to the poor dispersion of talc in the matrix, agglomeration occurs, and the cell diameter increases and the number decreases.
(2) The mechanical properties of PP materials decreased after foaming, but when the mass fraction of talc powder was 5%, the mechanical properties of PP materials decreased slightly after micro-foaming, and the specific bending strength was close to that of unfoamed materials.