Study On The Structure And Performance Regulation Of Thermosetting Polyurethane Bonding Agent For Polishing Abrasive Tools

Resin abrasive tools such as grinding wheels and belts are the key tools for ultra-precision machining of modern difficult-to-machine materials.With the improvement of the technology of the industrial system,the development of high-precision machinery requires increasing surface quality of workpieces.The application of ultra-precision machining also challenges the grinding and polishing performance and durability of traditional grinding tools.However,at present,most of the domestic precision and ultra-precision processing abrasives are mainly imported,which results in a huge impact on the domestic abrasives market and restricts the development of the core technology of abrasives.The grinding and polishing performance and durability of resin abrasive tools are related to the performance of resin bond and abrasives and the structure of tools.Therefore,it is of great engineering significance to research the improvement of resin bond performance and the design of the new abrasive composite structure to improve the performance of traditional grinding tools.Polyurethane（PU）resin bonded abrasive tools were studied in this work.Polyurethane foam（PUF）grinding wheels and PU/EP diamond dome pyramidal array structural composites were prepared based on the study of the effects of chain extension coefficient,inorganic fillers and epoxy（EP）on the organization and properties of thermosetting PU matrix by using DSC,FTIR,SEM,super depth-of-field microscope and white light interferometer.The grinding mechanism of PU resin abrasives in the polishing process and the influencing factors of workpiece surface quality were studied respectively.In the PU system,the PU prepolymer is cured by the chain extender of Dimethyl Thio-Toluene Diamine（DMTDA）and 3,3′-dichloro-4,4′-diaminodiphenyl-methane（MOCA）.When the chain extension coefficient is between 1.2 and 1.25,both chain extenders can significantly improve the tensile properties of the PU matrix.MOCA has better flowability and is more suitable for the preparation of PU abrasive tools.The addition of inorganic fillers decreases the tensile properties of the PU composites.When 3 vol.%graphite is added,the tensile strength of the PU composites decreases from 27.5 MPa to 17.5 MPa and the elongation at break decreases from 441%to 289%,which is helpful to improve the self-sharpening of abrasive tools.The addition of foaming agents,diluent,and inorganic filler had a significant effect on the structure and properties of PUF.The porosity of PUF increased with the addition of foaming agents,and the reaction of the foaming agent increased the viscosity of the PU system,which instead inhibited the growth of pores.When the foaming agent is increased to 5 wt.%,the porosity of PUF is only 16%.The diluent reduces the viscosity of the PU foaming system and increases the porosity of PUF.Compared with one-component dimethylformamide（DMF）and cyclohexane（CYH）,a two-component（DMF,CYH）diluent can effectively regulate the formation and growth of pores of PUF in the foaming process and obtain the best foaming effect,when the compound diluent（40%DMF and60%CYH）was 10 wt.%and the foaming agent was 3 wt.%.The addition of inorganic fillers such as graphite,Cr₂O₃ and Ce O₂ can further promote the formation of pores in PUF.When graphite,Cr₂O₃ and Ce O₂ are added at 3 vol%,1 vol%and 1 vol%,respectively,PUF achieved complete pore structure and uniform performance.The effect of white corundum abrasive addition on the structure and performance of PUF abrasives was investigated.When the content of white corundum was 40 wt.%,the pores and abrasive distribution of PUF abrasives were uniform.Two PUF white corundum polishing wheels with 3 vol.%graphite and 1 vol.%Cr₂O₃ were prepared.In the PU/EP system,the increase of chain extender can provide more crosslinking points for the copolymer reaction and help to form the polymer interpenetration network.The 1:1 addition ratio of PU/EP increased the tensile strength from 27.6 MPa to 53.6 MPa by 94.2%.FTIR results show that the PU/EP system can complete the curing reaction within2 hours.A lower curing temperature or shorter time would cause the pyramid structure to collapse,while a higher semi-curing temperature or longer time would lead to difficulty in embossing and molding.Finally,the PU/EP diamond dome pyramidal array structural composite was prepared by semi-curing at 120℃for 50 min.Polishing tests were conducted on 304 stainless steel using white corundum PUF grinding wheels and PU/EP diamond pyramidal pads,respectively,and the effects of processing parameters and abrasive grit on the surface quality and grinding mechanism of the workpiece were analyzed.The PUF grinding wheels with 3 vol.%graphite achieved a lower surface roughness of stainless steel workpiece.Graphite accelerates the fracture of the PU matrix during friction by reducing the tensile properties of the PU composites,which increases the self-sharpening of PUF grinding wheels during machining.The PU/EP diamond domed pyramid structure provides a strong cutting ability during the 0-60 s contact with the rough workpiece surface.As the wear test proceeds,debris from the workpiece tends to adhere to the rough platform of the pyramid structure and causes adhesive wear.At this time,the grinding behavior changes from cutting to slip abrasion due to the generated adhesive wear,which in turn affects the removal efficiency of the abrasive on the workpiece material,but facilitates the improvement of the workpiece surface,promoting the integration of processing grinding and polishing.