Preparation Of Styrcne-acrylic Emulsion Modified By CaCO₃Nanoparticles And Its Application In Paper Coatings

Styrene-acrylic emulsion was generally known to be synthesized via emulsion polymerizationprocess in the presence of styrene and butyl acrylate monomer. As an important non-cross-linkedemulsion, it has been widely used in various application fields such as paint, ink, leather andparticularly paper coatings. However, there are still some limitations encountered in developingthe conventional styrene-acrylic emulsion, involving low water resistance, poor thermal stability,low weather resistance and limited membrane forming ability, all of which essentially exerted anegative impact on the enhancement in the optical and printing performances of coated paper.With the development of nano-science and nanotechnology, the preparation and potentialapplication of novel emulsion modified by inorganic nanoparticles have received great attentionin recent years. In the present work, surface modification of CaCO3nanoparticles with silanecoupling agent KH570was investigated to improve the compatibility between inorganicnanoparticles and organic monomers. Then styrene-acrylic emulsion modified by CaCO3nanoparticles was synthesized via in-situ emulsion polymerization process. Eventually, incomparison with the conventional styrene-acrylic emulsion, the potential application of modifiedstyrene-acrylic emulsion in paper coatings was investigated, which might be considered as aparticular reference for the development of styrene-acrylic emulsion.Owing to their extremely high hydrophilicity, CaCO3nanoparticles were required toundergo complicated treatment to satisfy the demands of good dispersion state in organic system.A well-known approach to overcome the disadvantage is to modify the surface of CaCO3nanoparticles. Here, silane coupling agent KH570was employed as a modifier to treat theCaCO3nanoparticles. The effect of KH570amount on the microstructure and surface propertieswas stressed. According to FTIR result, some organic characteristic peaks occurred at2923cm-1,2852cm-1and1641cm-1, suggesting that silane coupling agent was successfully grafted onto thesurface of CaCO3nanoparticles. Furthermore, TG revealed that the maximum grafting ratio and the maximum utilization ratio were1.5%and36.7%, respectively, and the optimum amount ofKH570should be set to5%. Contact angle test showed the reduced polarity and increasedhydrophobicility of CaCO3nanoparticles as a result of surface modification. In addition, TEMimages indicated that raw CaCO3nanoparticles exhibited a serious agglomeration state while themodified one displayed an improved dispersion. It should be pointed out that the carbon-carbondouble bonds on the surface of nanoparticles derived from the surface modification wouldsubsequently have a positive influence on the in-situ emulsion polymerization.Then, the synthesis of styrene-acrylic emulsion modified by CaCO3nanoparticles wasdesigned and performed. The optimum process conditions on modified styrene-acrylic emulsionwere systematically discussed, involving the ratio between hard and soft monomer, the selectionand amount of emulsifier and CaCO3nanoparticles amount. The optimum results were as follows:styrene (St)/butyl acrylate (BA)=1/1, functional monomer methyl methacrylate (MMA) of10%(to the total monomers amount, similarly hereinafter), emulsifier (SDS/OP-10=1/2) of3%,co-emulsifier HD of0.6%and NaHCO3of0.5%. Additionally, miniemulsion polymerizationexperienced a reaction at70oC for5h. Based on the measurements and analysis, it can beconcluded that emulsifier types and amount were the key factors affecting the emulsion particlesize distribution and average particle size, and CaCO3nanoparticles amount was found to play animportant role in the reaction rate and conversion during polymerization. The morphologies ofmodified emulsion were further characterized. TEM images showed that the as-obtainedemulsion exhibited a core-shell structure with a particle size of130.1nm. It was apparent thatCaCO3nanoparticles acted as the core and organic polymer served as the shell, indicating theideal combination between inorganic nanoparticles and organic polymer. In addition, comparedwith conventional styrene-acrylic emulsion, modified styrene-acrylic emulsion possessed goodcalcium ion stability, mechanical stability and storage stability, showing a percentage increase of48.2%in water resistance. TG analysis further confirmed that modified styrene-acrylic emulsionshowed an improvement in thermal stability properties, with a thermal decompositiontemperature higher than that of conventional styrene-acrylic emulsion.Finally, the potential application of styrene-acrylic emulsion modified with CaCO3nanoparticles in paper coatings was preliminarily investigated. It was shown that the brightness and scattering coefficient increased with the increased amount of modified styrene-acrylicemulsion. When increasing the amount to15%, all parameters achieved an optimum level.Meanwhile, upon coating process, the hydrophobicility and surface strength of coated paperwere significantly enhanced. In comparison with conventional emulsion, styrene-acrylicemulsion modified with CaCO3nanoparticles showed visible improvement in brightness,scattering coefficient and printing performance. In particular, compared to the conventionalstyrene-acrylic emulsion, modified styrene-acrylic emulsion imparted a percentage increase of20%in surface strength and10.9%in surface hydrophobicility to the coated paper, whichfurther verified the enhancement in water resistance and film-forming properties ofstyrene-acrylic emulsion modified by CaCO3nanoparticles. SEM images gave an indication thatpaper coating layer behaved three-dimensional microporous network structure, whichsubsequently governed the functional and physical feature of coated paper. The coating layercontaining modified styrene-acrylic emulsion showed a fairly less microcellular structure, whichfacilitated the bonding strength between pigment particles and base paper. In addition, the SEMimages of cross section of coated paper provided direct evidence that the cavities and sittingfibers of base paper were fully covered by a coating layer with a thickness of about25μm.