Preparation Of Polyimide Based Inks For Direct Writing 3D Printing And Study On Tribological Properties Of Molded Samples

Polyimide（PI）is a kind of polymer material with excellent comprehensive properties.It not only has light weight,low temperature resistance,low dielectric coefficient,but also has excellent mechanical and physical properties and thermal stability.There are many types of PI,among which thermosetting PI has excellent performance,and the main forming method is cold pressing and hot sintering.The porosity and pore distribution of the finished product are mainly controlled by the content and distribution of pore forming agents,making it difficult to optimize pore structure and shape special-shaped structural components.The 3D printing method can be controlled for molding based on the special microstructure and appearance of porous structural components,but thermosetting PI is difficult to shape through general 3D printing methods.The preliminary exploration of the research group has shown that it can achieve additive molding by reasonably configuring printing ink and using direct writing 3D printing.The main method is to modify the rheological properties of PI precursor polyamide acid（PAA）by adding specific reagents,achieving direct writing molding of samples with self-supporting properties,and then preparing porous PI materials with three-dimensional through pore structures through thermal amination.However,there are issues with high shrinkage rate,insufficient guarantee of device forming size and shape accuracy.The key to thermosetting PI direct writing 3D printing technology lies in the allocation of printing ink;Based on the properties of additive materials,the research team explored and found that adding pore forming agents such as sodium chloride（NaCl）,silicon dioxide（SiO₂）,polymethyl methacrylate（PMMA）,polystyrene（PS）,and calcium hydroxide（Ca（OH）₂）to prepare porous PI materials can enhance the performance of printed porous material samples.To determine the ratio of each printing ink and optimize printing parameters,porous PI samples were printed using the aforementioned materials as pore forming agents.The performance of the samples prepared by adding various pore forming agents was explored from the aspects of strength,porosity,and shrinkage performance.It was found that the addition of Ca（OH）₂ not only maintained high strength,but also had a lower shrinkage rate.Further investigate the effect of Ca（OH）₂ content on the performance of PI porous samples.The main work and conclusions are as follows:（1）Build a direct writing 3D printing platform,prepare printing ink,optimize printing process parameters,and prepare porous PI samples.The optimal amount of pore forming agent for printing samples with certain support capacity was obtained:the optimal ink ratio for NaCl was 175%by solution mass,the optimal ink ratio for SiO₂ was 300%by solution mass,the optimal ink ratio for PMMA was 180%by PAA mass,and the optimal ink ratio for PS was 200%by PAA mass.The content of Ca（OH）₂ is related to the storage time,and can be formed between 15%and 45%.The higher the content,the shorter the storage time.（2）Perform performance tests on samples printed from ink prepared with several additives.In terms of mechanical and physical properties,NaCl and SiO₂ additives have little effect on the rheological properties of the ink,and the printed sample has insufficient strength after removing the additives,which is less than 1MPa.The three groups of samples,PMMA,PS,and Ca（OH）₂,showed excellent compressive strength before and after the removal of additives,with values higher than 8MPa before removal and 3MPa after removal.In terms of porosity,the pore structure strength of the two groups of samples,NaCl and SiO_2,is poor,and the porosity exceeds 60%after removing additives;The porosity of the PMMA and PS samples before the removal of additives is less than 20%,and after the removal of additives,the porosity is between 45%and 50%;The porosity of Ca（OH）₂ samples before removing additives is 25%-30%,and after removal,the porosity is 45%-50%.In terms of shrinkage,the shrinkage of NaCl samples is less than 10%,SiO₂ samples are between 25%and 30%,PMMA samples are between 45%and 55%,PS samples are between 40%and 45%,and Ca（OH）₂ samples are between 20%and 30%.The Ca（OH）₂ sample has high strength,low shrinkage,and moderate porosity.Increasing the Ca（OH）₂ content further enhances the shrinkage performance.（3）Further research was conducted on the effect of high content of Ca（OH）₂ on the properties of printed porous PI materials,and the problem of large shrinkage was further improved:the shrinkage of the formed sample was less than 15%;Before removing Ca（OH）₂,the porosity remains in the ideal range,ranging from 25%to 35%;The strength remains at6-10MPa;A large number of pore structures are distributed on the surface and inside,and the spherical pore structure inside is conducive to oil storage;From its thermal decomposition,it can be inferred that this material can work in an environment of 300℃for a long time.（4）Exploring the effect of Ca（OH）₂ content on tribological properties;The oil content is above 22%,and the oil holding rate of PAO#40 lubricating oil is as high as 96%,while the oil holding rate of PAO#10 lubricating oil is also above 93%.It was found that Ca（OH）₂ can increase the friction coefficient,with a dry friction coefficient of up to 0.5,which is higher than the dry friction coefficient of pure PI materials,and the wear rate is less than 1.5%;Under oil lubrication,it can be reduced to 0.2 and the wear amount is less than 1%.From the analysis of its worn surface morphology,with the increase of Ca（OH）₂ content,the wear scratches of the sample are shallow and narrow,which is caused by the increase in hardness due to different proportions of porous PI composite materials.