Synthesis, Structure And Performance Of Casein-based Emulsifier-free Composite Emulsion With Core-shell Structure

With the deficiency of global oil resources and the increasingly seriousenvironmental pollutions caused by non-degradable synthetic polymer materials,there is a good preference for developing and utilizing renewable resources andenvironmentally friendly materials, which has been listed as one of theinternational frontier disciplines. As a bio-based protein, casein is one of therenewable resources and has unique film-forming features, such as discontinuousproperty, high temperature resistant and so on. In addition, casein containsvarious active groups on its chains, so it can be easily modified. Consequently,casein-based film-forming material is becoming one of the most widely usedchemical materials in leather, paper making, paint and packaging fields. Inleather industry, casein has been chosen as one of the most dominant water-basedfinishing materials since it was used. With the increasing requirement for thewearing comfort of leather product, casein is attracting more and more attentionbecause of the unmatchable hygienic properties of its coatings. However, caseincoatings have serious defects such as poor water resistance and brittleness, whichlimits its further application. The aim of this study is to improve the defects ofcasein films in the soap-free emulsion polymerization system by a series ofmodification.In detail, the main works in this study include the following parts:（1） Caprolactam was firstly employed to modify casein throughpolycondensation reaction to prepare caprolactam modified casein （CA-CPL）.Emulsion properties, film properties and finishing application results were takenas the main indexes to systematically optimize the reaction conditions, includingthe type of pH regulators, caprolactam usage, reaction temperature and reactiontime. Chemical structure, microstructure, particle size and particle distribution ofthe as-prepared CA-CPL were characterized by fourier transform infrared（FT-IR）, solid-state13C-NMR, transmission electron microscopy （TEM）, scanning electron microscopey （SEM）, dynamic light scattering （DLS）, atomicforce microscope （AFM） and contact angle measurement. Based on the aboveresults, the formation mechanism of the CA-CPL latexes and the film wereestablished. When casein was dissolved in triethanolamine solution, caprolactamusage was35%, modification temperature was75℃, modification time was3.0h, CA-CPL emulsion property and its film-forming performance were optimal.The introduction of CPL chains onto casein molecules gave relatively strongerhydrophobic and flexibility to the modified coatings. Characterization resultsverified the successful occurrence of polycondensation between caprolactam andcasein. In addition, the particle size of the CA-CPL decreased dramatically andthe size distribution became more even compared with those of neat casein.（2） Waterborne polyurethane （WPU） was introduced into CA-CPL bymeans of physical blending and caprolactam modified casein/waterbornepolyurethane （CA-CPL/WPU） composite emulsion was prepared. Effects ofWPU usage on CA-CPL/WPU micro-structure, particle size and performancewere mainly investigated. Meanwhile, its application performances in leatherfinishing agent were measured. FT-IR, TEM and DLS characterizations wereconducted to investigate the structure, particle morphology and size distributionof CA-CPL/WPU. In order to find out the relation between the morphology andproperties of the composite, the interaction force between the components in thecomposite latex as well as the film forming mechanism were both discussed, andthe relevant models were established. The results showed that stronger hydrogenbonding interactions existed between CA-CPL and WPU components in thecomposite CA-CPL/WPU. This interaction affected the particle diameter sizeand its distribution to a large extent. SEM and contact angle results showed thatwhen the appropriate usage of WPU was introduced into the composite,dispersibility of WPU was good. Application results showed that when the usageof WPU was proper, the composite materials could endow coatings withexcellent flexibility and hydrophobicity. However, the strength and the watervapor permeability of the coatings decreased.（3） CA-CPL was used as the self-emulsifier and acrylate monomers wereintroduced into casein system to obtain core-shell type caprolactam-acrylateco-modified casein latexes via soap-free emulsion polymerization process. Effects of reaction conditions including the usage and proportion of acrylicmonomers, reaction temperature, and reaction time on the grafting degree,microstructure and performance of the resultant emulsion were discussed. Theoptimal caprolactam-acrylate co-modified casein emulsion was applied in leatherfinishing, and the application performance was studied compared with that of asimilar commercial product. The mechanism models for core-shell latex particlesformation and its film-forming were established. FT-IR results showed that thecaprolactam-acrylate co-modified casein was successfully obtained via soap-freeemulsion polymerization. TEM and DLS results indicated that the as-preparedlatex particles were nanoscale in size with uniform distribution, bearing obviouscore-shell structures. SEM and AFM results exhibited the homogeneous anduniform structure of modified casein-based latex films. Contact angleaccompanying with TGA data showed that hydrophobicity and thermal stabilityimproved significantly after modification compared with those of neat casein.（4） Single-in-situ method was adopted by introducing commerciallyavailable nano-silica （SiO₂） particles into caprolactam-acrylate co-modifiedcasein system to obtain core-shell structural casein-based SiO₂nanocompositelatexes. Effects of the variety of nano-SiO₂, the usage of nano-SiO₂, and usage ofthe initiator on the composite emulsion performance and film-forming propertieswere mainly discussed. The formation mechanism of the as-preparedcasein-based composite latexes and the composite films were also explored thusthe relevant models were established. When RNS-D was chosen as thenano-SiO₂, RNS-D usage was0.3%and initiator usage was3%, performancesof casein-based SiO₂composite emulsion prepared through single-in-situ methodreached best. TEM and DLS results showed that the modified casein emulsionparticles were spherical in shape, and their average particle size was59.34nmwith uniform size distribution. Contact angle and color developing data showedthat casein-based nano SiO₂composite had superior hydrophobicity andmatchable colors developing property. Application results showed that theintroduction of SiO₂could give improved tensile strength and sanitationproperties to the finished leather samples, however, the flexibility declined to acertain extent.（5） To further improve the stability and comprehensive properties of the casein-based SiO₂composite, a method called double-in-situ was employed toprepare the casein-based SiO₂composite latexes. In the double-in-situ method,nano-SiO₂was introduced by adding the precursor ethylorthosilicate （TEOS）instead of directly using the nano-SiO₂powder, and silane coupling agentγ-Methacryloxypropyl trimethoxy silane （KH570） was used to enhance theinterfacial force between inorganic and organic phases, thus obtaining stablecore-shell structural casein-based SiO₂composite latexes. Effects of TEOS usage,KH570usage and their feeding style, reaction temperature and reaction time onthe composite performance were investigated. The mechanism models for latexformation and film formation were also established. To verify the performanceadvantages of casein-based silica composite prepared via double-in-situ method,the control emulsion of casein-based silica composite via physical blending wasprepared and its performance was tested. Contrastive analysis was conducted onthe emulsion prepared via double-in-situ and that prepared via physical blending.TEM and DLS results verified that the resultant double-in-situ casein-based SiO₂nano-composite latexes exhibited evident core-shell structure, and their particlesize was around80nm with uniform particle size distribution. From the SEMand XPS results, SiO₂particles were proved uniformly coated on the shell layerof the core-shell composite latexes. Compared with the composite latexesprepared via single-in-situ method, particles obtained via double-in-situ methodshowed better stability. Through comparison and contrast, it’s noted thatdouble-in-situ method was favor for obtaining more homogeneous particles withhigher stability, heat stability, water resistance and mechanical properties.（6） The film formed from the as-prepared casein-based SiO₂composite viadouble-in-situ method was applied in ibuprofen delivery system. Beforemeasuring its drug-releasing behavior, swelling degree of the composite filmunder different pH conditions was tested. Effects of SiO₂content on the loadingand releasing properties was studied. The morphology and structure of the drug,drug-loaded film and drug-released film were also investigated through SEM,NMR and FT-IR. Based on the above results, slow-release mechanism model forcasein-based SiO₂nano composite film was propsed. The casein-basedcomposite films were proved pH responsive and the swelling degree of the filmunder different pH conditions followed the order in alkaline medium, neutral medium and acid medium. The loading and releasing behaviors results showedthat the existence of SiO₂layer largely contributed to the film’s loading andreleasing performance due to its excellent adsorption ability with drug. In acidmedium, the casein-based film showed superiror controlled release behavior,which suggested that it has a potential application in the stomach sustainedrelease system.