Star Hydroxyl Polymers And High-solid Content Two-component Polyurethane Coatings

Traditional solvent-based two-component polyurethane coatings?2K-PU?contained more than 50%volatile organic compounds?VOC?,which released to pollute environment and endanger human health.However,solvent-based 2K-PU coatings cannot be completely substituted due to the excellent mechanical properties and the outstanding adaptability to construction environment,the main developing trend is the high-solid content?HSC?2K-PU coatings.2K-PU coatings were composed of hydroxyl resins and polyisocyanate curing agents.The low viscosity and high-solid content polyisocyanate curing agents have been popularized for many years.Therefore,hydroxy resins have become the key factors of the HSC 2K-PU coatings.The viscosities of hydroxy resins used in the traditional solvent-based2K-PU coatings were as high as 20000 mPa s at 80%solid content,and the VOC contents of2K-PU coatings were above 550 g/L,does not meet the existing 420 g/L of the regulatory requirements.In this paper,four new technologies were developed to synthesize star hydroxyl polymers?SHP?with low viscosity and high solid contents,and the effects of 2K-PU coating properties were studied.?1?A series of intermediate products were synthesized via the“one-pot”molten polycondensation by using small molecular polyols,2,2-dimethylolpropionic acid?DMPA?and?-caprolactone??-CL?as raw materials,then these intermediate products were reacted with epichlorohydrin?ECH?to offer a series of star hydroxyl polyesters?SHP-2?.The molecular structures of SHP-2 were determined by FTIR and ¹H NMR.The molecular weights of SHP-2 were about 800-2100 with 1.62-1.84 of the PDI index measured with GPC,and the molecular weights of SHP-2 were increased with increasing the hydroxyl functionality of polyols.The pot life of 2K-PU coatings of SHP-2 were tested through NK-2 Viscosity Cup.It was found that converting some of the primary hydroxyl groups into secondary hydroxyl groups could effectively decrease the reactivity between SHP-2 with polyisocyanate curing agents,and lengthen the pot life of 2K-PU coatings.The solid content of HSC 2K-PU prepared from SHP-2-5 was high up to 70%,and the VOC content was as low as 305 g/L,in the meanwhile,the cured film exhibited excellent adhesion,flexibility,impact resistance,high hardness,as well as high thermal stability,e.g.,its 5%mass loss temperature was as high as 274.1?.?2?A series of star hydroxyl polyesters?SHP-3?were synthesized through the ring-opening reactions between the epoxy groups of core molecules?trimethylolpropane triglycidyl ether,resorcinol diglycidyl ether,1,4-cyclohexyl diglycidyl ether?with carboxyl groups of DMPA and mono-carboxylic acids.The molecular structures of SHP-3 were determined by FTIR and ¹H NMR.The molecular weights of SHP-3 were measured by GPC.It was found that the measured molecular weights of SHP-3 were 540-1390,which were approached to their theoretical value,and the PDI index were in the range of 1.09 and 1.29,and the introduction of?-CL enlarged PDI index.The viscosities of SHP-3 exhibited decreasing trend with increasing the carbon-chain length of mono-carboxylic acids,such as the viscosity of SHP-3-5 obtained from olic acid was low to 7560 mPa s,while SHP-3-1 prepared with n-hexanoic acid possessed the higher viscosity up to 25750 mPa s.In the meanwhile,reducing the amount of mono-carboxylic acid and increasing the content of DMPA could increase the viscosities of SHP-3rapidly,and the films hardness of 2K-PU also gradually increased.The spraying-applied HSC2K-PU coatings were prepared using SHP-3-13 as the preferrable star hydroxyl polyesters,and the solid content were up to 72%,while the VOC content as low as 290 g/L.The cured films possessed good adhesion,flexibility,high hardness,and good thermal stability.?3?The cationic ring-opening reaction of mono-epoxides initiated by the primary hydroxyl groups of small molecular polyols were conducted using the“one-pot”process,and then prepared a series of star hydroxyl polymers?SHP-4?.The molecular structures of SHP-4 were characterized by FTIR and ¹H NMR,and the molecular weights of SHP-4 were tested by GPC.It was found that the measured molecular weights of SHP-4 were 720-960,which were approached to their theoretical value,and the PDI index were in the range of 1.1 and 1.2.The viscosity of SHP-4-2 prepared with phenyl glycidyl ether?PGE?was up to 57500 mPa s,and the cured film hardness obtained from G21 curing agent was 0.82.Meanwhile,SHP-4-6prepared with dodecyl glycidyl ether(C₁₂GE)displayed the lower viscosity only 9800 mPa s,and its 2K-PU film hardness was only 0.78.The spraying-applied HSC 2K-PU coatings was prepared with SHP-4-4,and the solid content of 2K-PU formulations was up to 75%,the VOC contents was as low as 250 g/L.The cured films possessed excellent adhesion,flexibility,impact resistance,film hardness and good thermal stability.?4?Through the reaction between small molecule polyols with different anhydrides to synthesize intermediate products containing carboxyl groups,and then a series of star hydroxyl polyesters?SHP-5?were prepared via the ring-opening reaction between the carboxyl groups with different mono-epoxides.The molecular structures of SHP-5 were characterized by FTIR and ¹H NMR,their molecular weights were also measured by GPC.It was found that the measured molecular weight of SHP-5 were 630-1450,which were close to their theoretical value,and their PDI were ranged from 1.1 to 1.3.It was found that the viscosity of SHP-5increased rapidly with increase the content of anhydrides,while the film hardness of SHP-5showed the maximum value at SHP-5-5.The viscosity of SHP-5-2 prepared with phthalic anhydride?PA?at the 80%solid content was 2910 mPa s,and while SHP-5-1 prepared with succinic anhydride?SA?exhibited the lowest viscosity and film hardness.For these products obtained from PMDA and different monoepoxides,SHP-5-15 prepared with C₁₂GE showed lower viscosity and film hardness due to the longest carbon chain in the branchend sturcture.SHP-5-11 and SHP-5-13 prepared with PGE and BzGE exhibited higher viscosities due to the introduction of rigid benzene ring structures on the branched structures,while the film hardness were not significantly enhance.HSC 2K-PU coatings were prepared with SHP-5-3 and N3900,and the solid content was above 66.5%,with 345 g/L of the VOC contents.The cured film of SHP-5 has excellent heat resistance and its 5%mass loss temperature is as high as307.9?.