Synthesis And Structural Properties Of Modified Waterborne Polyurethane

Waterborne polyurethane(WPU)is a new type of block copolymer with a molecular backbone containing hydrophilic and urethane groups and using water as the solvent.WPU usually consists of oligomeric polyol(polyether or polyester diol)as the soft chain segments and isocyanate with small molecule chain extender come into being its hard chain segments.The unique alternating soft and hard segment structures give WPU molecule a specific microphase structure which gives it good elasticity,high mechanical strength and wear resistance.However,compared with solvent-based polyurethane resins,waterborne polyurethanes suffer from deficiencies in hydrolysis resistance,mechanical properties,and anti-adhesion,and require the addition of various additives to adjust the surface tension,viscosity,and cross-linking degree during application.In the thesis,firstly,the aqueous polyurethanes prepared from traditional petroleum-based raw materials were modified by dual crosslinking,and then the aqueous polyurethanes prepared from renewable bio-based raw materials were modified by light-curing crosslinking agent and long aliphatic hydrocarbon to systematically investigate the structure and properties of aqueous polyurethanes,and the main research contents are as follows.(1)The introduction of polydimethylsiloxane into aqueous polyurethane can improve the thermal stability and low-temperature flexibility of aqueous polyurethane and reduce its surface energy,but the surface is prone to phase separation during the curing process causing wrinkles on the surface of the adhesive film.A series of polysiloxane-modified waterborne polyurethanes were synthesized by using petroleum-based raw materials polytetrahydrofuran diol as the soft chain and isophorone diisocyanate(IPDI)as the hard chain.In the meantime,a double crosslinking network was constructed by introducing an internal crosslinker trimethylolpropane(TMP)and an external crosslinker polycarbodiimide,which increased the hydrolysis resistance and mechanical properties of waterborne polyurethanes,and meanwhile the surface of the films is smooth and flat,which solves the problem of surface’s wrinkles caused by polysiloxane embedded in molecular chains and will have a broad application prospect in leather industry.(2)Bio-based materials have the advantages of environmental protection,renewable and wide source of raw materials,etc.In order to replace some petroleum-based raw materials,the thesis adopts bio-based castor oil as raw material to synthesize a series of new green waterborne polyurethanes,and focuses on solving the problems of long curing time and slow drying rate of films of aqueous polyurethanes through light-curing crosslinking modification.At first,the effect of castor oil on the performance of bio-based waterborne polyurethanes was investigated.After characterizing the crystallinity and glass transition temperature of waterborne polyurethane films,the tensile strength was 4.46 MPa and elongation at break was 480%at the optimum molar ratio of n(IPDI/Castor oil)equal to 3.In order to improve the curing rate,the tensile strength and elongation at break were measured to be 3.8 MPa and 236.6%,respectively when an additional light-curing crosslinking agent containing double bonds,ethoxylated trimethylolpropane triacrylate(EO3-TMPTA),was used.It is indicated that the direct external addition of light-curing crosslinking agent could not improve the mechanical properties of the adhesive film and further optimization was required.(3)To further improve the tensile strength of the light-cured aqueous polyurethane films,a monomer containing a double bond,pentaerythritol triacrylate(PETA),was introduced into the molecular chain of the bio-based aqueous polyurethane,and light-cured crosslinked aqueous polyurethane films were prepared for performance evaluation.The tensile strength increased to 11.08 MPa when the mass fraction of light-curable monomer PETA in the aqueous polyurethane was 6%.the effect of crosslinking density on the overall properties of the material was analyzed by dynamic thermodynamics.The crosslinking density of the sample PWPU-0 without the addition of monomer PETA is equal to 348.19 mol/m~3.However,when the mass fraction of monomer PETA was 4%,the crosslinking density of the sample PWPU-4 is equivalent to1439.53 mol/m~3,which greatly improved the light-curing strength and solved the problems of long curing time and slow drying rate of the films.(4)Waterborne polyurethane as a surface treatment material or adhesive material also has the problem of poor adhesion of non-polar surface film.The adhesion performance of aqueous polyurethane is related to its wettability on the substrate.By introducing the monomer trimethylolpropane monooleate(TPM)with long aliphatic hydrocarbon in the side chain into the bio-based aqueous polyurethane,it is expected to increase the wettability of aqueous polyurethane on the surface of untreated polypropylene film(OPP),and thus improve the adhesion of aqueous polyurethane.By increasing the mass fraction of monomer TPM,the aqueous polyurethane emulsion gradually enhanced the wettability on the surface of untreated polypropylene.Comparing with the peel force of 1.25 N/25 mm for waterborne polyurethane without TPM addition,the peel force increased to 3.5 N/25 mm when the mass fraction of monomer TPM was 4%,and the side chain aliphatic chain modified waterborne polyurethane would greatly improve the adhesion of non-polar surface film and has important application value.From what has been discussed above,the thesis solves the current problems of waterborne polyurethanes in terms of hydrolysis resistance,mechanical properties and anti-adhesion by dual crosslinking modification,light-curing crosslinking modification and the molecular structure of aliphatic hydrocarbon modification,and lays the theoretical foundation and experimental basis for the modification,synthesis and industrial application of waterborne polyurethanes.