Preparation Of High Performance Transparent Polyurethane Coatings And The Relationship Between Structure And Properties

The unique chain and aggregation structure enables polyurethanes (PU) with excellentphysical and processing properties, such as high strength, flexibility, excellent wear andchemical resistance, and they have been applied widely in industry, agriculture and daily life.Transparent polyurethanes not only have excellent properties of PU, but also have hightransparency, they are an important research direction in recent years.According to the theory of polymer, crystalline polymers are always opaque because ofthe different refractive index between the crystalline and amorphous region, and thetransparent materials are always amorphous polymers. Polyester polyurethanes possess highpolarity structure in the form of–(C=O)-, causing polyurethanes synthesized from thesematerials to crystallize easily. Polyether macrodiols are the most commonly used materials inpreparing transparent PU as weak force between molecular chains in these materials reducescrystallinity in the PU so synthesized. However, there are some disadvantages for polyetherpolyurethanes, such as: lower mechanical strength, ultraviolet and solvent resistance are poor.On the basis of previous research, we synthesized transparent and high performancepolyurethane coatings with polyester polyols and isocyanate trimers according to therelationship between structure and properties of polymers, and we researched the relationshipbetween structure and property in PU. At the same time, polyurethanes were modified to havespecial properties and make them suitable for coating with special requirements. The specificresearch and achievements are as follows:(1) We synthesized a series of linear and branched transparent polyester polyols withcontrollable molecular weight and distribution. The structure of monomer had a significanteffect on the glass temperature (Tg) and viscosity of polyester polyols. The cyclic monomerincreased Tg and viscosity of polyester polyols, while the long linear monomer decreased theTg and viscosity, small amounts of three functional monomer also increased the Tg andviscosity of polyester polyols.(2) Transparent polyurethane coatings were synthesised with homemade polyesterpolyols and aliphatic isocyanates. The physicochemical, mechanical, optical properties andthe structure in PU were characterized. Results indicated that the linear monomers inpolyester polyols enabled PU with excellent impact resistance and flexibility, the cyclicmonomers could increase the hardness and shear strength of PU, and the impact resistanceincreased first and then declined with the increased amount of cyclic monomers. The cyclic structure in polyisocyanate endowed PU with toughness and hardness, the linearpolyisocyanate enabled PU with good flexibility. The results indicated that high transparentpolyurethane coatings with excellent mechanical properties could be synthesized bycontrolling the microphase separation in PU to a certain degree.(3) Transparent fluorinated polyurethane coatings were synthesized with fluorinatedisocyanates and polyester polyol. The length of fluorocarbon chain on the properties of PUwas discussed. Results indicated that the introduction of fluorocarbon chain increased theinternal short-range ordered structure in PU, resulting in a decline in transparency, and thedegree of the structure increased is inversely proportional to the length of fluorocarbon chain.The mechanical properties were declined as the fluorocarbon chains were introduced topolyurethanes, and the moderate micro-phase separation in fluorinated polyurethanes loweredthe degree of mechanical performance degradation. The introduction of fluorine improved thethermal resistance of polyurethanes, and the short the fluorocarbon chain, the better theperformance of heat-resistant properties. Fluorine would migrate to the surface of PU andreduced the surface energy, and the degree of migration was related to the degree ofcrystallization in PU.(4) A series of nano silica sols with different particle size were prepared by sol-gelmethod, and introduced them to polyester polyol by in situ polymerization. Transparentorganic-inorganic hybrid polyurethane coatings were prepared by nano hybrid polyester resinsand polyisocyanates. Results indicated that the viscosity of hybrid polyester resins wasimproved with increasing silica particle size. The mechanical properties, abrasion resistanceand Tg were obviously increased as nanosilica particles were introduced to PU, especiallywhen the diameter was35nm. However, the effect on UV shielding was not obvious.(5) One-dimensional nanostructure material, silica nanofibers were prepared by chemicaldispersing and acid leaching route using chrysotile as raw material, and introduced them toPU by blending method. Results showed that: the transmittance of PU decreased withincreased amount of silica nanowires, and the appearance changed from colorless to black.However, silica nanofibers improved the UV shielding to a certain extent. At the same time,silica nanofibers could be used as reinforcing filler to improve the hardness, shear strength,wear resistance and thermal stability of PU, but they would decrease the flexibility of PU.