Study On The Shrinkage Stress And Volume Shrinkage Of Photo-curable Acrylate Materials

Photopolymerization of acrylates-based resin has been widely used in coatings,adhesives,inks,3D printing and electronic devices due to its fast curing rate,non-VOC release formulations and energy-saving processes.However,the volume shrinkage and shrinkage stress developed during the photopolymerization process has long been a nuisance to most end applications.During the rapid photopolymerization process,the resin turns from liquid to solid film and at the same time the intermolecular van der Waals interaction is changed into covalent bond.This leads to volumetric shrinkage and shrinkage stress inside the coating as well as on the interface.However,another problem that limits the shrinkage rate research is the method of characterizing the shrinkage volume.The existing methods cannot accurately evaluate the volume change of the material.Therefore,it is very important to establish a method to accurately monitor the shrinkage rate.Based on the establishment of an accurate method for shrinkage rate,the main contents of this work uses RT-MIR-photo-rheology technology and 3D line laser confocal instrument to study the effect of UV components and curing conditions on the shrinkage of the material.Then we uses Pickering emulsion method to prepare porous elastic microspheres.The microspheres offset the locally concentrated polymerization stress through the porous structure to reduce the shrinkage rate of UV-curable materials while maintaining the overall balance of material properties.The details are as follows:(1)A method for in-situ monitoring the shrinkage stress of the photocurable materials was established by using the RT-MIR-photo-rheology technology,which could simultaneously record the changes of the macroscopic rheological properties and the microscopic degree of polymerization;Next,we used the 3D line scanning laser confocal instrument to monitor the shrinkage volume of material.The equipment and principle of shrinkage testing are described.Among them,the standard deviation of shrinkage stress test is less than 1.7 N,and the standard deviation of shrinkage rate test is less than 0.4%,which is caused by optical scanning blind area.(2)This work employed infrared-rheology technology and 3D line scanning laser confocal instrument to monitor the acrylate free radical photopolymerization process.The effects of curing conditions(curing temperature,radiant intensity and sample thickness)and system compositions(diluent functionality,diluent segment length,ratio of diluent to resin,initiator concentration)on the shrinkage rate,polymerization rate and material modulus were fully investigated.The results show that the shrinkage rate of photosensitive resin is consistent with the variation trend of shrinkage stress.The shrinkage stress develops from the gel point to the glassy state,and increases with the concentration of the double bonds involved in the reaction,which is independent of the rigidity of the crosslinking network.The increase of curing temperature and thickness can significantly increase the shrinkage rate,while the increase of radiation intensity can reduce the shrinkage rate.The reduction of the diluent chain length or functionality will increase the shrinkage rate;the increase in initiator concentration will not improve the shrinkage volume of the material;the increase in the oligomer content will reduce the shrinkage volume and increase the rigidity of the material.(3)The porous elastic microspheres were prepared by Pickering emulsion method combined with UV-curing technology,and the effects of microspheres content on the mechanical properties,thermal properties,coating properties and shrinkage of acrylic photocurable materials were investigated.Next,the effects of particle size and shell mechanical properties on shrinkage stress reduction were investigated.Results: The shrinkage rate of TPGDA/EA system can be reduced to half by adding 1.5wt% of porous microspheres,and can maintain the excellent mechanical properties of the UV-material.The shrinkage volume of the resin decreases with the increase of the particle size of the microspheres.For TPGDA/EA system,it is more suitable for the microsphere system with lower shell rigidity.