Research On The Preparation And Application Of Polyacrylate Redispersible Powder

Compared with the polymeric products of emulsion state, redispersible polymer powderdoes not contain water as dispersion medium and makes it possible to save the packing,storage and transportation cost of products during the distribution process. Also because of itspowder state, redispersible polymer powder could be blended with cement, filler and pigmentpowders to prepare one-pack dry-mixed concrete or architectural coatings, which areconvenient and healthy for use. However there generally exist two disadvantages for currentpolyacrylate redispersible powder products: comparatively poor water resistance and binderperformance. Meantime the preparation technologies have made little progress from thetraditional―soft-core/hard-shell‖and macromolecular protective colloids technologies since1990s. On the theoretical side, the film formation process of dry-mixed coatings and thereason for unstability of PVA-MMA polymerization system are still indistinct, which limit thetechnical innovation and application performance of polyacrylate redispersible powder.In this thesis we attempted to apply some new technologies in the manufacture ofpolyacrylate redispersible powder and did research on several key theoretical questions, inorder to get a deeper understanding of the preparation process and find out some innovativemethods to improve product performance, which are of great significance in both academicand industrial aspects. The main research topics and relevant conclusions are as follows:The preparation process of redispersible powder was studied step by step at first, in orderto find out a basic recipe and relevant spray drying parameters to get redispersible powderwith qualified properties. The research results were shown as below: Tgof "soft core"-10oC,Tgof "hard shell"70oC, mass ratio of core/shell components3:1, MAA amount16wt%, inlettemperature120oC, rotating speed24,000rpm and feed rate20~40ml/min. Then theresultant redispersible powder from above formula was applied in dry-mixed coatings and thecoatings properties were tested, from which the optimal amount of redispersible powder wasdetermined as10.0g with PVC of36.58vol%. Through these studies the whole process ofredispersible powder preparation and application was made clear and the recipe andparameters would act as the groundwork for the following studies on innovative technologiesand fundamental theories. The corsslinking modification technique was applied in the preparation of polyacrylateredispersible powder by adding crosslinking monomer N-hydroxymethyl acrylamide (NMA)into the polymerization of original emulsions, and the influences of NMA amount onemulsion polymerization, spray drying and properties of resultant powders were studied. Inpolymerization aspect it was found that the particle size, particle distribution index andviscosity of original emulsions increased with the increasing of NMA amount, while thecolloidal stability was lowered. This was determined by the high reactivity and watersolubility of NMA, which easily led to homopolymerization of NMA rather copolymerizationwith other monomers. So the NMA amount was selected as3.0wt%to guarantee the stabilityof original emulsion synthesis. In spray drying and property aspect, the increasing of NMAamount was proved to be beneficial for the improvement of flowing, stacking and bondproperties of the redispersible powders. These results confirmed that the crosslinkingmodification worked in the performance enhancement of redispersible polymer powder.The film-formation mechanisms of dry-mixed coatings (based on the redispersiblepolymer powder) and latex coatings (based on the original polymer emulsion) were studiedand compared according to the structure and properties of the coatings films. It was found thatthere were two structural differences in the films of both products, which directly influencedtheir mechanical properties. One was homogeneous degree: There existed stratificationphenomenon in the film of latex coatings, which contained an upper polymer layer near thefilm/air interface and a bottom polymer-filler composite layer near the film/substrate interface,while the film of dry-mixed coatings only had uniform polymer-filler composite structure.This was mainly due to their different settlement conditions. In both coatings products fillerparticles tended to subside, but the settlement behaviors of polymer particles were different.In latex coatings the polymer particles were more stable in the coatings dispersions, so theycould remain in the upper part of the dispersion while filler particles subsided towards bottom,and finally dried to form the upper polymer layer of film. In dry-mixed coatings, unstablepolymer particles tended to subside together with filler particles and finally formed film ofuniform structure. The other difference was the compact degree: It was found that the innerstructure of latex coatings film was much more compact than the dry-mixed coatings film,and this was because that the particle size of re-dispersed polymer particles were much larger than the original ones due to the insufficient re-dispersion of redispersible powder, whichweakened the capillary forces of film formation. With the reasons of structural differencesabove, we found that both the instability and insufficient re-dispersion of dry-mixed coatingswere attributed to the irreversible coalescence of polymer particle in spray drying, and thiswas just the key point to improve dry-mixed coatings performance.Lateral comparison was conducted between PVA-MMA emulsion polymerization andstable PVA-VAc emulsion polymerization, to find out the reason for the instability ofemulsion polymerization of acrylate monomers with PVA as protective colloid. Through theanalysis of the rate of elementary reactions, the proportion of grafted PVA molecules and thegrafting extent of the final copolymers, we could get the conclusion that the grafted amount ofPVA was even larger in the PVA-MMA copolymers than PVA/VAc ones, so the grafted modeof PVA was considered. Based on the reaction kinetics, it was found that the slower initiationand propagation rates of sulfate radical towards MMA was responsible, for they allowed theparticles to grow up all through the reaction process and resulted in―layer-by-layer‖graftingstructure of PVA. As the surface-grafting density of PVA was lowered, the colloidal stabilityof PVA-MMA emulsion was reduced consequently.As inorganic components have higher melting point and better thermo-tolerance thanpolymer components, polymer-SiO2core/shell composite particles were utilized to prepareredispersible powder instead of―soft-core/hard-shell‖polymer particles. First a one-pot routetowards polymer-SiO2core/shell composite particles was developed through the templatesol–gel processing of alkoxysilane in emulsifier-involved aqueous emulsions, and it wasfound that the core/shell structure could be obtained on the basis of cationic templateemulsions for the electrostatic attraction between template particles and siloxane oligomers.Afterwards the composite emulsions were used to prepare redispersible powder and thefactors such as the Tgof core polymer component, the thickness of SiO2shell were studiedbased in the morphology and re-dispersion ability of redispersible powder. It was found thatwith the help of SiO2shell the Tgof inner polymer core could be reduced to10oC, and thiswas of great significance to further lower the MFFT of redispersible powder. The flowingre-dispersion properties of powder were improved with the increasing of TEOS amount, i.e.the thickness of SiO2shell, then declined when the TEOS amount exceeded55wt%. This was because that more free siloxane oligomers were generated with TEOS amount increasing andaggravated the coalescence of particles in spray drying process.