Study On Reinforcing Flame Retardant Cellulose Nanofibril Based Aerogels

With the increasing exhaustion of non-renewable resources and the increasing environmental pollution,the high value utilization of natural renewable resources has become the social consensus and research hotspot.Cellulose nanofibril(CNF)obtained from wood has the high aspect ratio and net-like winding structure,and can be easily converted into a kind of lightweight,high porosity,and low thermal conductivity of three-dimensional porous solid materials-aerogels;compared with other synthetic polymer aerogels and inorganic aerogels,it also has the advantages of high-flexibility,biocompatibility,and degradability.However,the inherent flammability and weak strength of pure CNF aerogels,which greatly reduces its practical effect and use safety.Therefore,to carry out the CNF aerogels enhancement and flame retardant research is very important to expand the application field of the material.The main contents and conclusions of this thesis are as follows:(1)A high-flexibility chemically crosslinked CNF flame-retardant aerogel was successfully prepared by using the butanetetracarboxylic acid(BTCA)which can esterify the hydroxyl groups of both CNF and N-methylol dimethylphosphonopropionamide(MDPA)to form covalent bridges,via an eco-friendly vacuum freeze-drying and post cross-linking method.The results showed that BTCA was successfully esterified with CNF and MDPA,and the MDPA and CNF were covalently linked together;BTCA/MDPA system·Kept the aerogel with low density(8.98~12.35 Kg/m~3),high porosity(>99.17%),and low thermal conductivity(<32.6 mW/(m·k)).By adjusting the mass ratio between BTCA,MDPA,and CNF,the high flexibility of CNF aerogel could be preserved while improving its compression strength,resilience and flame retardancy.When m(CNF)/m(BTCA)/m(MDPA)was 10/1/4,the LOI value reached 26.1%,the mass residual ratio(600°C)increased to 35.56 wt.%;and the heat release rate(HRR),total heat release(THR),and maximum heat release rate(PHRR1)were62.5%,63%,and 61%lower than those of pure CNF aerogels,respectively.The flame retardant mechanism of BTCA/MDPA system is dominated by the catalytic action of MDPA in the condensed phase,making the CNF aerogel exhibit good flame-retardant properties and self-extinguishing properties;its mechanical enhancement mechanism is that BTCA plays an effective bridging role between CNF and MDPA by covalent bond,and a new crosslinked network is formed in the aerogel structure,thereby giving the CNF aerogel flame retardant properties at the same time improved its mechanical strength.(2)A highly flame-retardant CNFs aerogels were successfully prepared by using the synergistic flame retardant effect of boric acid(BA)and nitrogen-phosphorus compound(FR)via the vacuum freeze-drying method.The results showed that BA/FR can also maintain the CNFs flame retardant aerogels with the low density(7.78~9.36 Kg/m~3),high porosity(99.32~99.45%)and low thermal conductivity(27.07~29.03 mW/(m·k)),and to some extent improve its compressive strength,which was mainly attributed to the strong hydrogen bonding force between BA,FR and CNFs,the aerogel presents a three-dimensional network structure composed of filaments and bulky skeletons with higher yield behavior,thus showing better compression strength and resilience performance.Compared to the BTCA crosslinked CNF flame retardant aerogels,the CNFs flame retardant aerogels prepared from BA/FR systems exhibit higher thermal stability and flame retardancy.When m(CNFs)/m(BA)/m(FR)was10/2/4,the LOI value and mass residual rate(at 600°C)of the aerogel reached 51.3%and 51.2wt.%,respectively;and its heat release mode changes from single peak to double peak,the HRR,THR,and PHRR1 were 82%,76%and 77.8%lower than those of pure CNFs aerogels,respectively.But their compressive resilience and flexibility were significantly lower than those of BTCA crosslinked CNF flame retardant aerogels.The excellent flame-retardant effect and high temperature stability of the BA/FR system was mainly achieved through the synergistic effect of the catalytic char formation of FR and the physical coverage of the BA in the condensed phase.(3)TOCNF/MUF composite flame retardant aerogel with high mechanical strength was prepared with TOCNF as a raw material,which prepared by Tempo oxidation method,with melamine-urea-formaldehyde(MUF)pre polymers as strengthening agent and with pentaerythritol phosphate ester melamine salt(PPMS)as flame retardant,by in situ polymerization and vacuum freeze drying merhod.The results showed that the TOCNF/MUF composite flame-retardant aerogel still has the characteristics of low density,high porosity,and low thermal conductivity.When m(TOCNF)/m(MUF)/m(PPMS)is 10/4/5,the compression modulus and specific modulus are 73.1 KPa and 7.45 MPa/Kg·m~3,respectively,which are 25times and 16 times of the pure TOCNF aerogel.And they are significantly higher than those of the two kinds of CNF flame retardant aerogels in previous.This is because MUF and PPMS with TOCNF are combined by electrostatic adsorption or charge neutralization,respectively,which improves the mechanical strength of the TOCNF aerogel.In addition,the LOI value and mass residual rate(at 600°C)of the aerogel reaches 30.2%and 32.3 wt.%,respectively;the UL-94 grade is V-0,and the heat release rate(HRR),total heat release(THR),and maximum heat release rate(PHRR)were 857.4%?35.7%and 40.7%lower than those of pure TOCNF aerogels,respectively.However,compared with BA/FR system,its flame retardant performance is poor.The MUF/PPMS system works through the condensed phase flame-retardant mechanism of PPMS and the gas-phase flame-retardant mechanism of MUF simultaneously,so that the composite aerogel exhibits good flame-retardant properties and self-extinguishing properties.(4)Combining the flame-retardant,mechanical and thermal properties of three types of nano-cellulose flame-retardant aerogels,their potential applications and suitable applications were predicted respectively.The results showed that:a.BTCA cross-linked CNF flame-retardant aerogel was expected to be applied to the thermal insulation layer for thermal protective clothing and special-shaped components of precision instruments used in medium and low temperature;b.CNFs/BA/FR flame retardant aerogel was expected to be used in public places with high requirements for flame retardant grade,such as insulating lining materials on mobile devices such as high-speed rail;c.TOCNF/MUF/PPMS composite flame retardant aerogel was expected to be used as thermal insulation materials with relatively large demand.