TEM phase and SEM morphology observation and Mechanism Discussion

The theory and practice show that the surface of micro powder can produce van der Waals force when it is filled with ultra-fine or micro filler, so that the primary particles can recombine, form agglomerates, and then form agglomerates, which are difficult to disperse in the polymer. The macro effect is equivalent to the increase of particle size. After the surface treatment of filler, the apparent particles are obviously refined, and the dispersion situation is greatly improved. Therefore, the surface It is of special significance for fine particle size fillings to form acicular wollastonite. The results of mechanical properties test show that the mechanical properties of the blends are good, and the best impact strength of the modified wollastonite particles is 9.1 kJ / m when the filling amount is up to 50 phr

Discussion on mechanism

The tensile strength is 31.8 MPa, which is 128% and 9% higher than that of the matrix resin. This is because:

① the wollastonite rigid particle needle wollastonite used in this study is a combination of fiber, sheet column and cube. Therefore, in fact, wollastonite rigid particles of different shapes disperse needle like wollastonite powder in PVC matrix resin. The composite modification of multi shape rigid particles and PVC needle like wollastonite powder resin is beneficial to exert the synergistic effect of various shape particles, overcome the shortage of single shape particles and improve the overall advantage;

② the rigid particles of modified wollastonite are uniformly dispersed in PVC matrix and have good interface structure, because the PMMA on the particle surface not only has the appropriate interface adhesion strength with PVC, but also has obvious low-temperature β - relaxation ester base movement, γ - relaxation methyl rotation and so on, which makes it have certain deformation ability. When subjected to external force, the PMMA interface layer itself can deform and relax the stress Stress and induced deformation of PVC matrix to improve fracture toughness of composite system

③ PMMA on the surface of wollastonite particles forms a solidified non shrinking interface layer, which greatly eliminates the residual stress at the interface caused by the large difference of thermal expansion coefficient between rigid particles and matrix during the curing of PVC resin. With the decrease of residual stress in the interface area, the number of microcracks decreases, and the material will absorb more impact energy under impact load. See the TEM photo of Fig. 3 (a), unmodified wollastonite Rigid particles have good dispersion in PVC matrix, which may be the reason why unmodified wollastonite particles can still improve the mechanical properties of PVC composite system, but compared with modified wollastonite particles, there are more particle aggregates in the dispersion phase, the interface between wollastonite particles and PVC matrix is clear, and the binding force between the two phases is obviously not strong. From the TEM photos in Fig. 3 (b), it can be seen that the modified wollastonite rigid particles coated with PMMA are evenly dispersed in PVC matrix, the apparent particles are obviously refined, and the single particles are greatly dispersed in PVC matrix, and the phase interface between them is fuzzy. This is due to the formation of hydrogen bond between the carbonyl group of PMMA on the surface of wollastonite particles and α - hydrogen in PVC, which has a real coupling effect and good compatibility. Therefore, before the optimal filling amount, the volume of modified wollastonite particles is greatly reduced under the same filling amount, while the number of particles dispersed in PVC matrix is correspondingly increased, and the fine particles are more easily distributed among the large particles, which is not only conducive to the increase of rigid particle filling amount, but also conducive to the increase of impact energy absorption units, so as to improve the toughness and reduce the cost Ben. The TEM images also show that the wollastonite fiber crystal is oriented along the melt flow direction, which is helpful to improve the tensile strength.

Due to the unmodified wollastonite particles and the small binding force with the matrix, it is easy to form a weak interface, the gap and crack on the impact section are obvious, the edge angle of wollastonite particles is clear, and holes are formed after falling off. Therefore, the poor compatibility leads to the poor dispersion of Wollastonite particles in the system, which can not transfer energy well. The composite system debonds before the crack growth and breaks at the interface, which is not conducive to toughening.

The cross section is undulating, and the shedding particles are mainly small-sized particles with smooth surface, forming holes after falling off, while some large-sized fiber crystals and sheet columnar particles have been pulled out at one end, but the other end is still well combined with the matrix, so it can be seen that the dissipation of impact energy in the modified wollastonite / PVC composite material is through the interface debonding between the wollastonite rigid particles and the matrix, and the fiber pulling out, Friction movement between rigid particles and matrix and plastic deformation of interface layer are realized. The results show that the impact fracture is ductile failure, and its meso damage mechanism is strong shear yield failure. Craze and shear band are two ways of energy consumption in the process of impact, but they show different forms due to the differences of materials and conditions. Among them, shear yield is an effective way of energy consumption. Only when the shear yield mechanism exists, the toughness of materials will be greatly improved. Under the condition of uniform distribution of dispersed phases in the composite system, Liu zh et al. Proposed the critical particle spacing Criterion Formula for brittle ductile transition of polymer composite

Tc =dc[ (π/6 )1/ 3exp(1 .5ln2 б)-ex p(0 .5ln2 б)]

When t > TC, the system is brittle, when t = TC, the system has brittle ductile transition, when T < TC, = "system is ductile, =" б increases, = "effect is equivalent to the increase of particle size d =," system toughness decreases. Rigid particles with a certain particle size and particle size distribution are dispersed in the matrix resin, = "and brittle ductile transition occurs at the point when the particle concentration reaches the critical value =" "C =" ", which is related to the matrix resin, particle shape, particle size, particle size distribution and interface condition, =" when the filling amount of rigid particles does not reach the optimal value, = "the notched impact strength of composite increases with the increase of =" ", and when it reaches the optimal value When it is over the optimum value, the notched impact strength of the system decreases with the increase of. When the content of wollastonite particles is 50 phr, the matrix thickness between particles is smaller than the critical matrix thickness of brittle ductile transition, and the stress concentration caused by Wollastonite particles and interface debonding promote the shear yield of PVC matrix The mechanism changes from craze and microcrack &amp; mdash; cavitation to shear yield, = "" can dissipate a lot of impact energy, = "" can lead to brittle ductile transition of materials, = "" can greatly improve the impact toughness of blends.

Among them, fiber crystal and sheet columnar particles can not only induce a large number of matrix to yield and absorb impact energy, hinder and passivate the crack growth, but also have a large contact area with PVC matrix, and the coupling is good and not easy to fall off. However, the thickness and size of the matrix in the local area of the small particles between the large particles are small. Before the body stress reaches the yield stress of the matrix, the matrix in the local area near the small particles will yield first, and then debonding will generate holes, which will cause the surface of the large particles and the surrounding resin matrix to yield, and then the impact toughness will be greatly improved.

The impact section is relatively neat, obviously the toughness of the system becomes poor, which is caused by the excessive content of wollastonite particles. Because of the excessive wollastonite particles, the volume of the matrix that can produce shear yield is too small, and the amount of wollastonite particles is too high, it is difficult to evenly disperse in the PVC matrix resin, and it is easy to form aggregates, resulting in large cracks, but it can not effectively terminate it, resulting in the system toughness Sexual deterioration.

conclusion

(1) PMA can be coated on wollastonite by MMA chemical coating, and the compatibility between PMMA and PVC is good.

(2) the mechanical properties of PVC filled with wollastonite rigid particles are improved, and the modified particles are easier to improve the mechanical properties.

(3) wollastonite particles with different shapes have synergistic effect, which is conducive to the improvement of mechanical properties of the composite system.

(4) when the amount of filling is 50 phr, the best impact strength is 9.1kj/m2, and the tensile strength is 31.8mpa, which is 128% and 9% higher than that of unfilled.