| Polyvinyl chloride(PVC)as one of the five general-plastics,with good plasticity,chemical corrosion resistance,permeability resistance and low cost has been widely used in daily production and life.Because the thermal stability and impact strength of pure PVC cannot meet the requirements of processing and use,it is necessary to add additives in the preparation and processing stage to improve the thermal stability,impact strength and plasticization performance of PVC.With the development of nanotechnology,the application of inorganic nanoparticles with high surface activity,high strength and good thermal stability in polymers has attracted extensive attention.In this paper,surface modified nano-silica was prepared by liquid phase in-situ surface modification technology and PVC was modified by in-situ and melt blending methods with nano-silica.The effects of the structure and surface properties of nano-silica on the structure and properties of PVC were studied.The main contents and results are as follows:(1)PVC was modified with nano-silica by melt blending method.Nano-silica was dispersed into PVC matrix by mechanical mixing and melt blending.The influences of the structure and surface properties of nano-silica on the plasticizing properties of PVC resin and the mechanical properties and thermal stability of the products were studied.After adding 3 phr of HB120F nano-silica,the plasticizing time of PVC resins was significantly reduced from 84s to 39 s and the impact strength of PVC was increased from 5.36 kJ/m~2 to 10.19 kJ/m~2,which is equal to commercial impact resistance modifier CPE.While the bending strength of PVC modified by CPE decreased by 8.24%.Nano-silica has high surface activity and can be adsorbed on the surface of resin particles to improve the friction coefficient,increase torque and promote plasticization.Nano-silica can also play a stress concentration role in PVC matrix,inducing the energy absorption by silver streaks and preventing them from developing into destructive cracks.Adding 3 phr of nano-silica with double bond on the surface can slow down the HCl release rate generated by PVC under thermal decomposition.This is mainly because the double bond can combine with HCl,which weakens the catalytic effect of HCl on PVC decomposition and improves the thermal stability of PVC.(2)PVC was modified with nano-silica by in-situ polymerization.Before the suspension polymerization reaction begins,nano-silica was added into the polymerization reaction system to change the surface morphology and internal pore structure of PVC resins by affecting the micro,sub-micro and macro granulation process of PVC resins.The effects of addition ways,hydrophilicity/hydrophobicity and dosage of nano-silica on oil absorption rate,apparent density and other conventional indexes of PVC resin were studied.The results show that nano-silica is easy to enter monomer droplets through water phase after wetting,which has little influence on polymerization,and thus resin particles in normal state can be obtained.However,hydrophobic nano-silica directly added in powder form will be blocked by water phase(continuous phase),which makes it difficult to enter monomer droplets.Due to the strong interaction between nano-silica and polar groups on the dispersant,mixing nano-silica powder with organic dispersant in advance will make it difficult for the dispersant to spread in the oil-water interface,affecting the dispersing and stabilizing effect of the dispersant on monomer droplets,resulting in unqualified coarse particles.Adding 0.1%HB120F nano-silica with water contact angle about 150°makes oil absorption rate of PVC resins increase by 28.2%and thermal stability increase too.This is because nano-silica can prevent the primary particles from coalescing in the polymerization process,increase the internal porosity and improve the oil absorption rate.Meanwhile,nano-silica itself can adsorb and stabilize the plasticizer,which is beneficial to increase oil absorption rate.Comparing with the methods of changing the production process and adding organic solvent as the pore-forming agent to improve the oil absorption rate,the method of adding nano-silica into the reaction system before the reaction has the advantages of no introduction of organic solvent,environmentally friendly,simple operation and easy scale production.(3)Nano-silica stabilized reaction monomer in the suspension dispersion system.The effects of nano-silica on suspension dispersion system,oil absorption rate and apparent density of PVC resins were studied.With the addition of 0.03%secondary dispersant TCR-4040,the oil absorption rate of PVC resins was increased slightly from 19.0%to 20.2%,while the apparent density decreased from 0.49g/mL to 0.48g/mL.The oil absorption rate of PVC resins were increased from 19.0%to 22.4%by adding 0.025%HB120F nano-silica and the apparent density remain stable.High oil absorption rate is beneficial to the absorption of additives and plasticization of PVC and high apparent density is good to the high-speed extrusion,reducing the bubbles on the surface of products.HB120F nano-silica with water contact angel about 150°can replace the secondary dispersant TCR-4040.Nano-silica itself is non-toxic and harmless,storable,stable and biocompatible.The use of nano-silica instead of organic dispersant is good to reducing chemical oxygen demand(COD)in wastewater and is environmentally friendly.Nano-silica in HB120 emulsion has more silica hydroxyl groups on its surface and better hydrophilicity.HB120 emulsion is used to disperse simulated liquid EDC/water system,which is more conducive to reducing the oil-water interfacial tension and playing the role of dispersing and stabilizing colloid(glue retention).As the amount of HB120 emulsion increased,the simulated solution was gradually dispersed into small droplets.It is expected to replace organic dispersants of polyvinyl alcohol or hydrophilic cellulose ether and finally realize the complete substitution of organic dispersants and the improvement of PVC resin properties. |
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