Huayuan Wollastonite Xinyu Huayuan Wollastonite Co., Ltd. Jiangxi Wollastonite
Polyvinyl chloride (PVC) is a large class of general-purpose plastics with excellent comprehensive properties, but PVC as a hard product cannot be widely used as a structural material, mainly because of its poor impact properties. Therefore, PVC is toughened and modified to develop high strength and toughness PVC is an important research topic to promote the engineering of such plastics.
Wollastonite is a new type of natural short fiber mineral, which has the advantages of low hygroscopicity, good thermal stability, and not easy to be scratched on the surface.It is used to fill polymers with fast dispersion and low viscosity. In terms of flexural strength, acicular wollastonite is superior to other inorganic fillers and is widely used in the automotive and construction industries. China is rich in wollastonite mineral resources, and studying the application of wollastonite in plastics has practical significance for the development of this high-quality mineral.
In this study, two types of wollastonite rigid particles were used as fillers, and the needle-shaped wollastonite powder was chemically coated with methyl methacrylate (MMA) to improve compatibility. The blending system was investigated. The mechanical properties and the toughening mechanism of acicular wollastonite powder were discussed.
Wollastonite coated with methyl methacrylate and its characterization
Observed under a polarizing microscope, the shape of the rigid particles of wollastonite is fibrous, columnar and cubic. The particle size distribution of the primary crushed particles is wide, and the particle size distribution of the secondary crushed particles is relatively uniform. Surface modification of wollastonite: Weigh a certain amount of wollastonite powder in a three-necked flask, add water and MMA in proportion, stir vigorously, slowly warm up to 70 ~ 75 ℃, add 0.6 according to the amount of monomer % ~ 0.7% water-soluble initiator, after a period of reaction, filtered. Dry to constant weight in an oven at 120 ℃ and set aside. Infrared spectroscopy analysis: take the appropriate amount of wollastonite powder after the above reaction filtration, filter with anhydrous ethanol and distilled water at 85 ℃ three times, dry in a 120 ℃ oven to constant weight, and dry the unmodified wollastonite powder for infrared Spectral analysis. It can be seen that the treated wollastonite powder has an obvious carbonyl stretching vibration absorption peak at a wave number of 1 729 cm-1, which confirms that the surface of the wollastonite is coated with tightly bound PMMA
Front temperature: 180 ℃; middle temperature: 175 ℃; rear temperature: 165 ℃; injection pressure: 60 MPa to 70 MPa; cooling time: 15 to 60 s; holding time: 15 to 40 s.
① Determine the tensile strength of wollastonite / PVC splines according to GB1040-92 plastic tensile test method. The experiment speed is 10 mm / min.
②According to the GB1048-93 impact test method for simply supported plastic beams, determine the impact strength of the simply supported beams of wollastonite / PVC spline.
③ Determine the bending strength of wollastonite / PVC splines according to GB1042-79 plastic bending test method. The experiment speed is 25 mm / min.
④ Observe the phase structure of the sample on a transmission electron microscope (TEM). The sample is an injection spline and is sampled along the melt flow direction.
⑤ Observe the internal morphology of the sample on a scanning electron microscope (SEM). The sample is an injection spline impact section at normal temperature, and the surface is sprayed with gold to observe.
Results and discussion
Both modified and unmodified two kinds of wollastonite rigid particles improved the impact strength of the composite system within the scope of the investigation, but the modified particles had a larger increase 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 of primary modified particles is 8.3 kJ / m2 when the filling amount is 20 parts, and the maximum impact strength is 9.1 when the secondary modified particles are 50 parts. The kJ / m2 is 108% and 128% higher than that of the unfilled particle system. At the same time, it also shows that the primary crushed unmodified wollastonite rigid particles are generally easier to toughen than the secondary unmodified particles, because the primary crushed particles have a larger particle size and a wider distribution. It is easier to orient in the melt flow, which helps to penetrate the matrix, transfer stress, and terminate crack propagation; but at the same time, the particle size is large, the surface defects are also many, and it is easy to form large cracks that cannot be effectively terminated, and it is not easy to achieve high filling; secondary crushing of silicon The particle size of the limestone rigid particles is small and relatively uniform. If the surface is not modified, loose aggregates are more likely to form in the PVC matrix resin, and the mechanical properties of the coarse particle size particles that are the same size as the aggregate are worse. After the particles are surface-modified and coated with PMMA, the particles are evenly dispersed in the PVC matrix resin. The particles and the matrix have good compatibility. The matrix and the particles are integrated into one whole, and the internal stress of the interface is adjusted to reduce the stress concentration. , The impact toughness gradually increases and reaches a maximum when the filling amount is high.
The rigid particles of wollastonite improve the tensile strength of the system, which is mainly the contribution of the orientation of wollastonite fiber crystals in the melt flow. Unmodified particles form many weak interfaces in the matrix. When they are under tensile force, the particles and the matrix, and between the particles and particles are easy to slip off, so the tensile strength is lower. Modified wollastonite particles with small and uniform particle size can improve the tensile strength, because the modified particles interface well in the matrix, and the internal friction force is large when subjected to tensile stress; tensile mechanical properties are tested At that time, 30 previous composite splines had a much greater elongation at break than pure PVC, (ε) greater than 200%. The tensile strength increases first with the increase of filler amount and then gradually decreases.
The second crushing of unmodified wollastonite rigid particles leads to a decrease in bending strength, while modified particles can increase the bending strength. This is because the PMMA on the surface of the modified wollastonite particles fills the holes, grooves and other defects on the wollastonite surface. As the amount of modified filler increases, the bending strength first increases and then gradually decreases. The above results show that the second crushed modified wollastonite rigid particles have more balanced mechanical properties than the first crushed modified particles, and it is easy to achieve high filling. Therefore, the mechanism of the second crushed wollastonite rigid particle filling system is discussed and the phase state and morphology are observed and analyzed.