Preparation And Properties Of Low Thermal Expansion Epoxy Polymer

Thermosetting epoxy resin（EP）has many excellent properties,such as excellent bonding performance,good insulation performance,high tensile strength and modulus,and chemical corrosion resistance.It can be used as adhesives and packaging materials for key weapon components such as integrated circuits and rocket engines,and is widely used in aerospace,defense,military industry,electronic instruments and other fields.With the continuous development of high-tech represented by aerospace and electronics and the increasing complexity of the environment faced by weapon systems,more stringent requirements are put forward for the dimensional stability of epoxy resin.However,due to the non-harmonic vibration of the internal atoms of the epoxy resin,the epoxy curing material exhibits a thermal expansion and contraction behavior,resulting in the change of material size with ambient temperature,which not only seriously affects the design accuracy and function of the instrument.Long-term reciprocating temperature cycles can also lead to micro stress concentration and shorten the service life of materials and devices.Therefore,effectively reducing the thermal expansion coefficient（CTE）of epoxy resin,thereby inhibiting the positive thermal expansion of epoxy resin materials and even realizing zero thermal expansion is an important way to improve the dimensional stability of materials,which is of great significance to broaden the application range of epoxy resin.In order to reduce the thermal expansion coefficient of epoxy resin,low thermal expansion LaFe_10.5Co_1.0Si_1.5/epoxy resin composites were prepared based on the negative thermal expansion filler LaFe_10.5Co_1.0Si_1.5.The effects of LaFe_10.5Co_1.0Si_1.5filler content on the mechanical properties and thermal expansion properties of the composites were studied.The gradient distribution of filler particles in the resin matrix was induced by magnetic field driving method,and the functional materials with thermal expansion gradient change were prepared.The results show that LaFe_10.5Co_1.0Si_1.5can effectively reduce the linear expansion coefficient of epoxy resin matrix composites between 228 K and 323 K.When 70 wt.%LaFe_10.5Co_1.0Si_1.5filler is added,the linear expansion coefficient of composites can be reduced by 58%.In addition,the variation laws of TGA,thermal conductivity,tensile strength and other properties prove that LaFe_10.5Co_1.0Si_1.5particles a gradient distribution along the magnetic field direction driven by an external magnetic field.At the same time,the thermal expansion properties of composites also show significant gradient changes.This proves that the magnetic field driving method is an effective and simple method for preparing low thermal expansion epoxy gradient materials.Subsequently,in order to control the thermal expansion of epoxy resin to greater extent,and on the basis of not reducing the mechanical strength of epoxy polymer,the stability of the material is improved by organic modification of epoxy resin.Based on the huge negative thermal expansion of the dibenzocyclooctadiene（DBCOD）molecule due to the conformation transition,DBCOD was introduced into the epoxy cross-linked network to prepare the thermal shrinkage epoxy polymer containing special eight-membered carbon ring molecules.The structure of the modified epoxy polymer was characterized,and the influence of the polymer content containing DBCOD on the thermal expansion behavior and mechanical properties of the modified epoxy was studied.The results show that the conformational inversion and conformational recovery of DBCOD molecules can still be realized in the thermosetting epoxy crosslinking network.The polymer containing DBCOD effectively reduces the thermal expansion coefficient of epoxy polymer.When the polymer content containing DBCOD is 15%,the average thermal expansion coefficient of the modified epoxy polymer in the temperature range of 50-120℃decreases to-49.4 ppm/K,showing a negative thermal expansion material.In addition,the mechanical properties show that the compressive strength of the polymer modified epoxy containing DBCOD did not decrease compared with the pure epoxy resin,maintaining good mechanical properties and increasing the tensile strength of the material（up to 29.8 MPa）,indicating that the modified epoxy has excellent mechanical properties.Finally,in order to reduce the difficulty of preparing low thermal expansion modified epoxy polymers and realize their large-scale production in the industrial field,a series of modified polyamides（PEG-PA）with different PEG ratios were synthesized by covalently linking polyethylene glycol（PEG）segments to the main chain of polyamides.Based on this modified polyamide,polyamide modified epoxy polymers containing flexible segments were prepared.The effect of PEG content on the thermal properties of modified polyamide was studied by TGA and DSC,and the curing conditions of modified polyamide/epoxy resin composites were determined by non-isothermal curing kinetics.The results show that the more PEG content in the main chain of the modified polyamide,the lower the glass transition temperature（Tg）of the modified polyamide.Moreover,due to the flexibility of the PEG segment,the solubility of polyamide in epoxy resin is significantly improved without affecting the conformational inversion and conformational recovery process of DBCOD.