Preparation And Properties Of Polyvinyl Alcohol Soluble Support Materials For Fused Deposition Molding

Fused deposition modeling(FDM)is an additive manufacturing technology to create plastic production from concept model to reality in a short time and low quantity.To build complex models with cantilever structure or dangling structure appeared on layer by layer manufacturing process,support materials that support the floating parts will help build the final parts or prototypes.Soluble support material could be soluble in water or other solvents,which provides the possibility of making small features in the depths or the embedded area of the parts and provide a solution for constructing assembly integrated parts in one print.Therefore,it is important for the development of FDM process to study the preparation and properties of soluble support material.Polyvinyl alcohol(PVA)was considered as a potential ideal candidate used in FDM process,while its properties and process-ability need to be improved for the wide application.This article attempts to use urea/caprolatam(UC)as modification additive for tuning the thermal and mechanical properties of PVA.PVA-based films with different content of additives were prepared by solution casting method.Differential scanning calorimetry(DSC),thermogravimetric analysis(TGA),X-ray diffraction(XRD),scanning electron microscopy(SEM)and tensile test were conducted.The results showed that with the increase of UC,PVA films showed decreased crystallinity and melting temperature,while the initial degradation temperature and melt index were increased.The microstructure examination via scanning electron microscopy showed that when the content of UC was less than 40%in the composites,homogeneous phase could be observed,indicating good compatibility between these two components.When the amount of UC was 30 wt%,the corporation of UC decreased the melting temperature to 173.0? and PVA composite with 4.5%crystallinity was fabricated.Due to the formation of hydrogen bonding between UC and the hydroxyl group of PVA,the tensile strength and modulus of PVA were slightly decreased,while strain-at-break was significantly enhanced,as high as 470.2%,indicating UC behaved as good plasticizing effect.The plasticizer achieved its plasticizing effect by destroying the intermolecular interactions of PVA,which inevitably lead to the decrease of elastic modulus.The interaction mechanism in PVA composites were investigated though elastic modulus,intermolecular interaction energy and radial distribution function by the molecular dynamics(MD)simulation.The results showed that the elastic modulus of PVA composites decreased with the increasing plasticizer content.The intermolecular interaction energy of PVA decreased with increase in the plasticizer content,which was the reason of the decrease of the elastic modulus.The pair correlation functions analysis showed that the interaction between polar functional groups and surrounding hydrogen atoms in PVA composites is mainly hydrogen bonds.Compound plasticizer disrupted the intramolecular and intermolecular hydrogen bonding of PVA,which was the fundamental reason of the decrease of the elastic modulus.Diffusion behavior of water molecules in PVA was investigated by MD simulation.The effects of polymerization degree,temperature,density,and pressure on the diffusion coefficient were conducted to gain insights into their diffusion mechanisms under different processing conditions.The results showed that the simulation data of diffusion coefficient was consistent with experimental values.With the increase of polymerization degree of PVA,the diffusion coefficient began with a sharply decrease and then became stable,while the diffusion coefficient increased with increasing temperature.It was also found that PVA matrix with a smaller cell density had a larger free volume,and then the diffusion coefficient was larger.The H2O sorption isotherm could be described by the Langmuir sorption isotherm and the diffusion coefficient decreased with the increase of pressure.The diffusion behavior and dynamics of water in PVA composites were studied by the MD simulation.The diffusion coefficients of water in PVA composites,interaction energy,radial distribution function and rotational time correlation function were investigated.Results showed that the diffusion coefficient of water in PVA composites increased with increasing UC content.The diffusion resistance of water in PVA decreased with the increase of UC.The analysis of the radial distribution function showed that the main interaction between the polar functional groups of UC and PVA was hydrogen bond.With the increase of UC content,the hydrogen bonding interaction of PVA and water decreased,and more hydrogen bonds formed between PVA and UC.Rotational time correlation time showed that the rotation of the PVA chain and the small molecules became faster with the increase of the UC,and the relaxation time became shorter.Extrusion process of the wire was given according to the physical and mechanical properties of PUC30 composite.The water solubility,melt flowability,shrinkage rate,melting temperature,mechanical properties,and thermal stability of PUC30 composites were investigated.Results showed that the melt flow rate of PUC30 composites was 3.74 g/10 min(190?,2.16 Kg),the melting temperature and decomposition temperature were 173.0? and 210.0?,respectively.The longitudinal shrinkage was 0.49,and the transverse shrinkage was 0.62.Bending strength and tensile strength at room temperature were 31.03 MPa and 24.17 MPa,respectively..The PUC30 composite material could meet the requirements of FDM process for soluble support materials,and its dissolution rate was superior to the same type of foreign products.It could be found though the application of in FDM process that extruded PUC30 wire could be hot-melt deposited and meet the requirements of FDM soluble support material.The post-processing of the prototype indicated that the support material could be successfully removed by its water solubility.In this dissertation,urea/caprolactam was used as modification additive for tuning the thermal and mechanical properties of PVA.The plasticization mechanisms were studied by experimental methods and molecular dynamics simulation method.It was of great theoretical and practical value to promote the research and development of the soluble supporting material.