Research On The Preparation And Properties Of Waterborne Polyurethane Ink Binder Used For The Plastic Film Printing

Up to now, the plastic film printing inks for food package are still mainly solvent-based polyurethane inks. With the increasing environmental protection requirements, the transformation of solvent-based inks to water-based inks is an inevitable trend in the development of ink industry. The preparation of waterborne polyurethane emulsion with excellent property is the key to prepare the waterborne polyurethane(WPU) inks. However, WPUs exhibits some shortcomings in the water resistance, heat resistance and adhesion performance, especially for the adhesion of waterborne polyurethanes on the non-polar plastic film. This paper aims to prepare waterborne polyurethane emulsions with excellent property for potential application in waterborne inks used for food package. To this end, we have carried out the following several aspects of research:1) Waterborne polyurethane emulsions with 2,2-dimethylolpropionic acid(DMPA) as the hydrophilic monomer were synthesized, the effects of DMPA content on the emulsion property and adhesion of WPUs were investigated. The results showed that as the DMPA content increased, a decrease in the mean particle size of the WPUs was produced and the increase in zeta potential and viscosity of the WPUs was obtained, leading to the improved storage stability of the WPUs. The adhesion strength of the WPUs increased initially and then decreased with the increase of DMPA content. When the content of DMPA was 4.0~4.5 wt%, the synthesized WPU emulsions had good storage stability and adhesion property.2) Several polyfunctional amine chain extenders were added for the post-chain extension after the dispersion of polyurethane prepolymers to improve the performance of WPUs and reduce the amount of dilution solvent. The waterborne polyurethane with low organic solvent content and excellent property was obtained. The post-chain extension process and the influences of these chain extenders on the properties of WPU dispersions were investigated. The results indicated that the post-chain extension mainly occurred on the particles surface, and the actual maximum chain extension degree was approximately 60%. XRD, DSC, TG and T-peel strength revealed that trifunctional chain extender diethylene triamine(DETA) was an effective chain extender and crosslinker that improved the properties of WPU dispersions. The chain-extended WPU dispersion by EDA and DETA with a mole ratio of 2:3 had excellent properties. The effects of the NCO/OH molar ratio on the storage stability and solid content of WPUs were also investigated. The results showed that the NCO/OH molar ratio of 1.6 was suitable for preparing stable WPU dispersion with solid content of about 40 wt% and organic solvent content of 5 wt%. The synthesized WPU dispersion with low organic solvent content could be used as waterborne polyurethane ink binder and avoided the cost of organic solvent removal. The aqueous ink with the prepared WPU dispersion as a binder showed the excellent steaming and boiling resistance and could be used for the food package PET film printing.3) In order to improve the adhesion of WPUs on the non-polar plastic film, trimethylol propane monooleate(TMPM) with long aliphatic chains as diol chain extender was incorporated into polyurethane molecular chains during the synthesis of polyurethane prepolymers to synthesize the WPUs containing long branched aliphatic chains. The influences of TMPM content on the properties of WPU dispersions were investigated. FT-IR and 1H NMR spectra revealed that TMPM was successfully incorporated into polyurethanes. As the TMPM content increased, the mean particle size of WPUs increased and the viscosity of WPUs decreased. The increase in TMPM content improved the wetting property of WPUs on PET and OPP films. The adhesion strength and thermal stability of WPUs initially increased and then decreased with the increase of TMPM content. The WPU with 6 wt% TMPM had the highest adhesion strength and thermal stability.4) In order to further improve the adhesion of WPUs on the non-polar film, on the base of the WPU with 6.0 wt% TMPM,(3-(2-aminoethyl)aminopropyl)trimethoxysilane(AEAPTMS), containing two amine groups and three methoxysilane groups, was used as a diamine chain extender to prepare a series of WPUs with siloxane groups. The trimethoxysilane group was designed in the side chain of PU. The effects of the AEAPTMS content on the properties of WPUs were investigated. FT-IR and 1H NMR spectra revealed that AEAPTMS was successfully incorporated into polyurethanes. The increase in AEAPTMS content improved the wetting property of WPUs on OPP films. The methoxy of AEAPTMS hydrolyzed to generate Si-OH, which would react with the hydroxyl of polyurethane molecular chains by the condensation reaction, enhancing the cohesive strength of WPU films, and could also react with the hydroxyl and carboxyl groups on the surface of OPP films treated by corona treatment to form chemical bonds, improving the interfacial adhesion strength of WPU films and OPP films, and therefore the peel strength of WPUs on OPP films increased. However, when the AEAPTMS content was too large, the crosslinking density and cohesive strength of WPUs were too high, which would restrain the motion of the polyurethane chain segments, resulting in the decreased peel strength. The WPU with 2.0 wt% AEAPTMS had the highest adhesion strength, and had excellent water and heat resistance. The waterborne inks with this WPU exhibited excellent adhesion on non-polar OPP film and could be used for non-polar plastic film printing.5) The internal crosslinked waterborne polyurethanes were synthesized by introducing trifunctional trimethylolpropane(TMP) into polyurethane to investigate the relationship between crosslinking density and adhesion property of WPUs. The crosslinking density was changed by varying the TMP content. The fitting curve of the relationship between crosslinking density and adhesion property was obtained by mathematical analytical methods, and empirical equation was also obtained. The results showed that as the TMP content increased, the crosslinking density and adhesive strength of WPUs increased, and thereby the peel strength of WPUs increased. However, when the content of TMP was too high, the high crosslinking density limited the motion of PU molecular chains, decreasing the interfacial adhesion strength of WPUs on PET films. Therefore, the peel strength decreased, and the type of peeling failure changed from cohesive failure to interface failure. When the content of TMP was 2.0 wt%, the peel strength was the maximum, and the type of peeling failure was the combined failure of cohesive failure and interface failure. An empirical equation, T=1.94+0.81x-0.22x2, revealing the relation between peel strength(T) and TMP content(x), was obtained via binomial fitting. The relation between peel strength and crosslinking density(υe) was also obtained and could be showed by the equation T=1.59+750.44υe-128313.62υe2. The equation υe=0.00045+0.0013 x or υe=0.00050+0.0013 x could be used to illustrate the relaltionship between crosslinking density and TMP content. The obtained equations showed that a suitable crosslinking density was one of the key factors for synthesizing a kind of WPU with excellent adhesion strength. In this study, in order to obtain the maximum peel strength, the optimal υe was 0.00292 mol/cm3.6) Graphene was used to modify waterborne polyurethaneto improve its heat resistance and mechanical properties, etc. Firstly, we prepared graphene oxide(GO) by the modified Hummers method and functionalized the GO with 3-aminopropyltriethoxysilane(APTES). Then diethanolamine(DEA) was used to reduce the resulting functionalized GO with amine groups, and the reduced graphene oxide(A-r GO) containing amine groups was obtained. At last, the obtained A-r GO as a reinforcing agent was covalently incorporated into the WPU matrix to prepare the A-r GO/WPU composites by in situ polymerization. The structure, morphology and property of raw graphite, GO and A-r GO were investigated, and the influences of the A-r GO content on the properties of A-r GO/WPU composites were also investigated. SEM and TEM images showed A-r GO sheets with low content(≤0.5 wt%) could homogeneously disperse in the WPU matrix. With the increase of A-r GO content, the interaction between A-r GO sheets and WPU matrix gradually increased, which increased the mechanical property, storage modulus, and thermal stability of A-r GO/WPU films. However, when the A-r GO content was too high, the dispersibility of A-r GO sheets decreased and thereby the mechanical property of A-r GO/WPU films decreased. The A-r GO/WPU films with 0.5 wt% A-r GO sheets exhibited good mechanical property and thermal stability. The water-based ink with the A-r GO/WPU hybrid emulsion as a binder had excellent adhesion on PET film as well as steaming and boiling resistance, and could be used for food package PET film.