Study On Preparation Of Flexible Protective Composites Based On STG And Its Low-speed Impact Properties

With the development of economic globalization,the threat of terrorism has gradually spread all over the world,and people’s awareness of self-protection is increasing.Due to its light weight and good flexibility,flexible protective composites have gradually become a research hotspot in the field of protective materials.So far,it has been widely used at home and abroad to combine the shear thickening liquid（STF）with high-performance fiber fabric to prepare flexible protective materials.However,there are some shortcomings in the application of STF:under normal conditions,the liquid is difficult to carry;it can be easily layered for a long time;the nanoparticles sticking on the surface of fabric easily fall off and so on.To solve these problems,a new type of composite material―shear thickening gel（STG）based on molecular colloid has been developed.In this paper,the preparation of STG was preliminarily explored,and STG was compounded with polyurethane（PU）foam and ultra high molecular weight polyethylene（UHMWPE）separately,in order to prepare composite materials with good low impact resistance.In this work,shear thickening gels were prepared with boric acid and silicone oil as the main raw materials,and the FT-IR,TG and rheological properties of the prepared STG were analyzed.The shear thickening mechanism of STG was briefly described.The PU foam and STG with different dilution ratio were used to compound.And the changes of the weight and thickness of the foam before and after impregnation were studied experimentally.The effects of STG on creep,modulus and low speed impact resistance of foam materials were also studied.UHMWPE fabrics were compounded with STG diluted with isopropanol,and the weight,thickness,flexibility and low impact resistance of the fabrics before and after impregnation were tested and analyzed.The molecular structure and thermal stability of the prepared STG were analyzed by infrared and thermal gravimetric tests,and the rheological properties were also tested.The experimental results show that STG has an absorption peak at 1335 cm^-1 due to stretching vibrations of the B-O bond,and has the characteristic absorption peaks of Si-O-B bonds at 892cm^-1 and 853 cm^-1,and all these show that B atoms are introduced into the main chain structure of-Si-O-.About TG,the temperature T_i at the start of the reaction is 234.96℃,the temperature T_p at which the reaction rate was the fastest is 442.61℃,and the temperature T_f at the end of the reaction is 562.89℃.The weight loss between 400 and 500℃is about 40.51%,and at this time,the main molecular chain of siloxane was decomposed into oligomers.When applying a pressure of 25 Pa to the STG,the strain of the STG is about 150%,and the strain is incr eased to 350%at 50 Pa,and the strain reaches 750%when the stress reaches 75 Pa.For the dynamic rheological properties,the storage modulus of STG is from 55 Pa(G’_min)to 6.29×10⁵ Pa(G’_max),and the difference is four orders of magnitude,with excellent shear hardening effect.While the energy loss modulus shows a tendency to increase first and then decrease.The required STG/PU composites were prepared by dipping and drying.The experiments not only studied the basic properties such as weight and thickness of STG/PU,but also analyzed the rheological properties and low-velocity impact resistance of the composites.The results show that:with the increase of addition amount,the weight of STG/PU gradually increases,but the thickness does not change significantly.Compared with STG,the creep of STG/PU decreases greatly,and PU can provide good skeleton support for STG;compared with neat PU,the impact resistance of STG/PU at low speed is obviously improved.When v=6 m/s,the residual impact load per unit mass of pure PU foam is approximately 2208.76 N/g,while that of PU（1:1）is only 197.71 N/g.The load is reduced by about 91.13%.The STG/UHMWPE composites were prepared by impregnation-drying method.The weight,thickness,flexibility and low-velocity impact resistance of the composites were tested and analyzed.The results show that:the addition of STG increases the weight of the material,but it decreases the thickness;the fabric’s flexibility also decreases,and the more the amount of STG,the poorer the fabric’s flexibility.The protective effects of the UH（1:1）,UH（1:2）,and UH（1:3）are better than pure fabrics in terms of residual impact load per unit mass and peak duration.However,from the aspect of the energy absorption coefficient,since the fabric after being compounded with the colloid becomes hard and the cushioning performance is reduced,the values of the four materials are not much different.