Polyvinyl chloride (PVC) is a kind of general plastics with excellent comprehensive properties, but PVC as hard products can not be widely used as structural materials, mainly due to poor impact performance of PVC. Therefore, it is an important research topic to toughen PVC and develop high-strength and high toughness PVC to promote the engineering of this kind of plastics.

Significance

Wollastonite is a new type of natural short fiber mineral, which has the advantages of small hygroscopicity, good thermal stability and not easy to scratch on the surface. It is filled with polymer, with rapid dispersion and low viscosity. Needle wollastonite is better than other inorganic fillers in improving tensile strength and flexural strength, and widely used in automobile and construction industry. China is rich in wollastonite mineral resources. It is of practical significance to study the application of wollastonite in plastics.

In this study, two kinds of wollastonite rigid particles were selected as fillers, and the wollastonite powder was chemically coated with methyl methacrylate (MMA) to improve the compatibility. The mechanical properties of the blends were investigated and the toughening mechanism of wollastonite powder was discussed.

Wollastonite coated with methyl methacrylate and its characterization

Under the polarized light microscope, the shape of wollastonite rigid particles is fiber, sheet column and cube. The particle size distribution of primary crushing is wide, and the particle size distribution of secondary crushing is relatively uniform. Surface modification of wollastonite: weigh a certain amount of wollastonite powder and put it into a three port flask, add water and MMA according to the proportion, stir vigorously, slowly raise the temperature to 70-75 ℃, add 0.6% - 0.7% water-soluble initiator according to the single volume, and filter after reaction for a period of time. Dry to constant weight in an oven at 120 ℃ for standby. Infrared spectrum analysis: take the appropriate amount of wollastonite powder filtered by the above reaction, filter it three times by anhydrous ethanol and 85 ℃ distilled water respectively, dry it to constant weight in an oven at 120 ℃ and dry the unmodified wollastonite powder for infrared spectrum analysis. It can be seen that the treated wollastonite powder has obvious carbonyl stretching vibration absorption peak at the wave number of 1729 cm-1, which proves that the wollastonite surface is coated with PMMA with tight binding

injection molding

Front temperature: 180 ℃; middle temperature: 175 ℃; rear temperature: 165 ℃; injection pressure: 60 MPa ～ 70 MPa; cooling time: 15 ～ 60 s; holding time: 15 ～ 40 s.

Performance Testing

① Determine the tensile strength of the wollastonite / PVC spline according to the GB1040—92 plastic tensile test method. The experimental speed is 10 mm / min.

② Determine the impact strength of simply supported beams of wollastonite / PVC splines according to the GB1048-93 plastic simply supported beam impact test method.

③ Determine the flexural strength of the wollastonite / PVC spline according to the GB1042-79 plastic bending test method. The experimental speed is 25 mm / min.

④ Observe the phase structure of the sample on a transmission electron microscope (TEM). The sample is taken from the injection spline along the direction of melt flow.

⑤ Observe the internal morphology of the sample on a scanning electron microscope (SEM). The sample is an injection spline cross-section at normal temperature, and the surface is sprayed with gold.

Results and discussion

Both the modified and unmodified wollastonite rigid particles increased the impact strength of the composite system within the scope of the investigation, but the modified particles increased significantly more than the unmodified particles. Studies by Darilyn HRobertsh and He Changcheng also show that wollastonite can improve the toughness of the composite. Under this system, the maximum impact strength is 8.3 kJ / m2 when the primary modified particles are filled at 20 parts, and the maximum impact strength is 9.1 when the secondary modified particles are filled at 50 parts. kJ / m2, which is 108% and 128% higher than the unfilled particle system, respectively. It also shows that primary crushed unmodified wollastonite rigid particles are generally tougher than secondary unmodified particles, because the primary crushed particles have a larger particle size and a broader distribution. It is easier to orient in the melt flow, which helps to penetrate the matrix, transfer stress, and stop crack growth; but at the same time, the particle size is large, and there are many surface defects, and large cracks that cannot be effectively terminated are easily formed, and it is difficult to achieve high filling; secondary crushing of silicon Graystone rigid particles have a small and relatively uniform particle size.If the surface is not modified, loose aggregates are more likely to form in the PVC matrix resin, and the mechanical properties are worse than using coarse-grained particles of the same size as the aggregates. After the particles are surface-modified and coated with PMMA, the particles are uniformly dispersed in the PVC matrix resin, and the particles and the matrix have good compatibility. The matrix and the particles are integrated into a whole, and the internal stress of the interface is adjusted to reduce the stress concentration. , The impact toughness gradually increases, reaching a maximum when the filling amount is higher.

Wollastonite rigid particles improve the tensile strength of the system, which is mainly due to the contribution of the orientation of the wollastonite fiber crystals in the melt flow. Unmodified particles have many weak interfaces in the matrix. When the tensile force is applied, the particles can easily slip between the matrix and the particles, so the tensile strength is lower. Modified wollastonite particles with a small and uniform particle size can improve the tensile strength more, because the modified particles have a good interface bonding in the matrix, which is caused by large internal friction when subjected to tensile stress; during tensile mechanical property testing At 30, the previous composite splines had much greater elongation at break than pure PVC, with (ε) greater than 200%. Tensile strength

The increase first increases and then decreases gradually.

 The secondary crushing of unmodified wollastonite rigid particles results in a reduction in flexural strength, while the modified particles can increase the flexural strength. This is because PMMA on the surface of the modified wollastonite particles fills holes, grooves and other defects on the surface of the wollastonite. As the amount of modified filler increases, the flexural strength increases first and then gradually decreases. The above results show that the secondary crushed modified wollastonite rigid particles have more balanced mechanical properties than the primary crushed modified wollastonite particles, and it is easy to achieve high filling. Therefore, the mechanism of the secondary crushed wollastonite rigid particle filling system is discussed, and the phase and morphology are observed and analyzed.