Why does calcium carbonate make plastics so “tough”?

At present, plastic products are used in all aspects of our lives, such as building materials, transportation, automobile manufacturing and so on. However, in the process of using plastic products, the toughness and strength of the products are often not well combined. It can be said that plastic toughening and strengthening is an important problem to be solved in the application of polymer materials.
From the perspective of materials science, high specific strength, high specific modulus, high toughness, and wear resistance in plastic applications are all related to plastic toughness and strength. Plastic strength and toughness are two particularly important but conflicting mechanical properties in structural materials.

1 Advantages of calcium carbonate toughened plastics
Calcium carbonate is a common inorganic filler that is widely used in the plastics industry. It has abundant raw materials, mature technology and affordable price, and is widely used in five major plastic products. In terms of ultra-fine or nano-calcium carbonate material, it has a very high surface area, which can significantly improve the impact resistance (toughness) of the material in the polymer, and also improve the tensile strength and elongation at break. When composited with polymers, the surface effect, small size effect, quantum effect and synergistic effect of nanoparticles will greatly improve the overall performance of the composite material.

2 Methods of filling modified plastics with calcium carbonate

2.1 Sol-gel method

The method is carried out in a precursor system such as a highly chemically active siloxane metal compound. The precursor undergoes a hydrolysis reaction and a condensation reaction to form a stable and uniform transparent gel system. At the same time, CaCO3 particles are formed, and the particles are highly dispersed in the gel. in the system. With the passage of time, the gel system gradually loses its fluidity, and then the gel is dried or sintered to obtain nanostructured composites. This method makes CaCO3 highly dispersed in the organic matrix, and gives full play to the performance advantages of nanomaterials, and the prepared composite materials have excellent properties. However, due to the shrinkage stress generated in the drying process, it is difficult to obtain large-scale products, and industrial production cannot be carried out.

2.2 In situ polymerization method

The method is to uniformly mix CaCO3 into the plastic monomer in a certain proportion. When the plastic monomer undergoes a polymerization reaction to prepare a polymer plastic, due to the physical or chemical reaction between the CaCO3 particles and the plastic monomer, the CaCO3 can be effectively attached to the plastic monomer. surface, and is uniformly dispersed in the plastic matrix with the polycondensation process of the monomer. Using this method to prepare composite materials, the reaction conditions are mild, and the excellent molding effect can be obtained without changing the properties of the inorganic nanoparticles themselves. However, the current technology is not mature enough to be widely used.

2.3 Blending method

The method utilizes physical blending to mix CaCO3 particles into the polycondensation-molded plastic matrix in the form of emulsion blending, melt blending and mechanical blending. The blending method has the advantages of simple and controllable process, simple equipment, the preparation of calcium carbonate and plastic matrix can be carried out in steps, without mutual interference, and can be mass-produced. It is also one of the most common methods for plastic modification at present. However, how to uniformly disperse calcium carbonate in the plastic matrix during the blending process is also a long-term issue.

In addition, the addition of inorganic minerals with special structures during the blending process to form a benign adjustment in the material structure also contributes to the increase of toughness. For example, minerals such as calcium carbonate, wollastonite, brucite, and sea foam may produce special structures, such as spherical, flake, fibrous, reticulated, layered, and porous.

3 Research progress of calcium carbonate toughened plastics

3.1 ABS toughening

Jiang et al. prepared nano-CaCO3/ABS composites and micron CaCO3/ABS composites respectively by melt blending method. The results show that the nano-CaCO3 is more uniformly dispersed in the composite, and the mechanical properties of the former are obviously better than those of the latter. According to the analysis, the main reason for this situation is the synergistic effect of the increase of the interface area between nano-CaCO3 and ABS and the cavitation shear yield of the ligament.

3.2PP toughening

In order to improve the bonding properties of PP fibers and cement matrix, Feng et al. used nano-CaCO3 to modify the surface of PP fibers. It was found that the surface roughness of PP fibers increased after nano-CaCO3 modification, forming dense hydration products with high degree of hydration. The modified PP fiber was filled with cement, and it was found that the flexural performance of the composite was significantly improved.

3.3 Toughening masterbatch

Patent CN102875869A discloses a nano-calcium carbonate reinforced and toughened plastic masterbatch and a preparation method thereof. The masterbatch is composed of nanometer calcium carbonate, micrometer calcium carbonate, metallocene polyethylene, carrier resin and additives. The blending of nanometer and micrometer calcium carbonate improves the strengthening and toughening effect of the masterbatch, and the metallocene polyethylene has high strength. , Good toughness, can improve the strengthening and toughening effect of masterbatch at the same time.

The masterbatch is prepared in two steps, and the nano-calcium carbonate passes through the co-rotating twin-screw extruder twice, which further strengthens its dispersibility in the masterbatch and fully reflects its strengthening and toughening effect. The advantage of the invention is that the nano-calcium carbonate is uniformly dispersed in the carrier resin, and the prepared masterbatch greatly reflects the unique reinforcing and toughening properties of the nano-calcium carbonate.

4 Prospects of Inorganic Powder Toughened Plastics Technology
From a macro perspective, in the future, plastic toughening and strengthening will mainly develop towards the material composite technology route. For example, by choosing low-cost inorganic materials, through nano powder, one-dimensional structural materials (wollastonite, calcium carbonate whiskers, short glass fibers), two-dimensional structural materials (talc powder, mica powder, graphene) three-dimensional structure ( Barium sulfate, glass microbeads, etc. can improve the comprehensive properties of materials, and can be prepared into masterbatches, which can replace both nano-toughened masterbatches and glass fiber reinforced masterbatches, with wide adaptability.

For example, Zhang Lingyan of Wuhan University of Technology studied the toughening mechanism of the composite system and found that the improvement of the strength and toughness of PP composites is not due to a single factor of mica, wollastonite, calcium carbonate or LDPE, POE, but The result of the synergy of the three in the system. Therefore, the synergy between several materials will become the development trend of plastic toughening and strengthening technology in the future.

Epilogue

For calcium carbonate, price is an advantage, which can effectively ensure the amount of use; morphology and processability is another advantage, such as calcium carbonate whiskers, spherical, porous, cake-shaped, etc.; rich in categories, including light calcium, heavy calcium , Nano-calcium and other homogenous and diversified products, the powder properties have various properties, and can be compounded with a variety of minerals. In the face of ordinary powder materials and plastic products, the wide universality of calcium carbonate is also a big advantage.