| Polymer emulsion is a major kind of waterborne resins and has received keen interest because it uses water as the dispersion medium,and is therefore safe and environmentally friendly,and it also has excellent product performance.However,in the preparation of the polymer emulsion,water-soluble initiators,surfactants and the like are introduced,so that the emulsion polymer itself is water-sensitive.When the latex film is in contact with water,small water bladders whose refractive index is different from that of the polymer are generated between the latex particles by water absorption,and the light is scattered at the interface between the two to make it look white,i.e.,water whitening,which affects the appearance and performance of the product.The latex films of most polymer emulsions currently used on the market have poor water whitening resistance.Therefore,it is particularly important to explore the causes of the water-whitening phenomenon of latex film and propose feasible solutions.In this paper,we first prepared polyacrylate emulsions with different glass transition temperatures(Tg).The effects of Tg on the water whitening resistance of latex films were investigated by measuring the changes in the transmittance of latex films.Then the polyacrylate emulsion with a soft to hard monomer ratio of 7:3(Tg=-38℃)was selected as the starting recipe,and polyacrylate latexes with different amount(0.5%-2.5%)of acrylic acid(AA)was prepared and the water whitening resistance of their films were investigatedand compared with that of the latex film formed from emulsion obtained under neutral conditions and film formed under neutral conditions.Finally,based on the study of previous two parts,a polyacrylate emulsion pressure sensitive adhesive with good water whitening resistance was obtained.The specific research contents are as follows:In the first part of the work,we used isooctyl acrylate(2-EHA)as a soft monomer,methyl methacrylate(MMA)as a hard monomer,changing the relative content of the two to make polyacrylates with different Tg Emulsion.The water whitening performance of different Tg polymer latex films was characterized by using a light transmittance meter.It was found that the water whitening resistance of the latex film became better as the Tg of the latex film increased.The effects of emulsifier migration onto the surface of the latex film on its water whitening resistance were characterized by static water contact angle,ATR-FTIR and XPS.When the latex film was subjected to the AFM test,it was clearly seen that the amount of the emulsifier that migrated to the surface increased as the Tg of the latex film increased.In addition,due to the difference in the reactivity ratio of different materials,the composition of the molecular segments before and after the polymerization is inconsistent,which affects the water absorption and swelling rate of the latex particles themselves.The particle size changes of the latex particles were tested at different dilution ratios.It was found that the water swell rate of the latex particles decreased with the increase of Tg,and thus the water whitening resistance of the latex film was affected.In the second part of the work,an emulsion of Tg=-38℃ was selected as the basic formulation,and different contents(0.5%-2.5%)of AA were added as a functional monomer for emulsion polymerization.And compared with the water whitening performance of the latex film prepared under the neutral conditions and under the neutral conditions,the reasons for the difference in water whitening resistance of the latex film were explored.The results showed that with the increase of AA dosage,the water whitening resistance of the latex film deteriorated;under the same AA dosage,the reaction under acidic conditions,film formation under neutral conditions,the latex film was better in water whitening resistance;Under the conditions of the reaction,the latex film has the worst water whitening resistance.The infrared spectrum,emulsion particle size test and emulsion drying kinetics test of the latex film before and after washing were used to characterize the reaction under neutral conditions,the formation of PAA,the enhancement of hydrogen bonding and the increase of the degree of heterogeneity of latex particles.The negative impact of the film’s resistance to water whitening.While reacting under acidic conditions,film formation under neutral conditions effectively avoids the above negative effects.Through TEM,AFM and other tests,it was found that the latex film prepared under such conditions is more dense,the structure of the latex particles is more regular,and the water whitening performance of the latex film is greatly improved.In addition,the reaction under acidic conditions,film formation under neutral conditions,through static water contact angle,XPS and other tests found that there will be more hydrophilic-COOH appeared on the surface of the latex film,thus resulting in the hydrophilic content of the latex film Reduced,thus further improving the water whitening performance of the latex film.In the third part of the work,combined with the research of the first two parts,a pressure-sensitive adhesive with excellent water whitening performance with a certain micro-crosslinked structure was obtained.The effects of silane coupling agent KH-570 and rosin emulsion on the mechanical properties and water whitening resistance of pressure sensitive adhesive were investigated.It was found that with the increase of the amount of KH-570,the crosslink density in the system increased,which led to the weakening of the initial adhesive force of the pressure sensitive adhesive and the increase of the adhesive strength.When the dosage of KH-570 was 1.5%,the number of crosslinking points in the system and its The distribution is in the most ideal state,and the peel strength and water whitening resistance of the pressure sensitive adhesive reach the optimum value at the same time.Adding a rosin emulsion with good compatibility with the polyacrylate emulsion in the system,on the one hand,improves the wettability of the pressure sensitive adhesive on the BOPP film;on the other hand,the addition of the rosin emulsion also reduces the crosslinking density of the system.With the increase of the amount of rosin emulsion,the initial tack and peel strength of the pressure sensitive adhesive showed a parabolic change.The water-resistant whitening performance of the film was 1.5%when KH-570 was used,and the optimum value was reached when the amount of rosin emulsion was 2%.The film began to appear white after 13.5h after soaking. |
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