Research On Poplar Wood Powder/Dynamic Polyurethane Composites Based On Low Transition Temperature Mixtures

Processing and utilization of fast-growing poplar and other wood with intensification,high efficiency,low-carbon,environmental protection,and sustainability have always been the focus of wood science and technology research.It is necessary to make full use of poplar and other wood resources,including small diameter wood,irregular wood and processing residues,etc.The processing process needs to decrease energy consumption and avoid using toxic substances and generating harmful matter.The final products must have no release of small molecular organic matter,have excellent mechanical properties,and can be recycled.This paper develops a new type of green and recyclable poplar wood powder/dynamic polyurethane composites to meet the above requirements.In this paper,poplar wood powder was mixed with dynamic polyurethane based on low transition temperature mixtures(LTTMs)to prepare poplar wood powder/dynamic polyurethane composites with strong mechanical properties,environment friendliness,and recyclability.The LTTMs were prepared by directly stirring the mixture of pyrogallol and polyethylene glycol 600(PEG600)at room temperature.The interaction between pyrogallol and PEG600 was analyzed,and the formation mechanism of the LTTMs was discussed by density functional theory(DFT).A solvent-independent dynamic polyurethane with dynamic phenol carbamate bond was prepared by directly mixing the LTTMs with hexamethylene diisocyanate trimer(HDI-T).Then poplar wood powder was added to the mixture of LTTMs and HDI-T,and they were hot pressed into poplar wood powder/dynamic polyurethane composites.Based on the orientation and deformation behavior of poplar wood powder during the preparation of composite materials and combined with the partial removal of lignin from poplar wood powder to improve its compressibility,the nacre-mimetic poplar wood powder/dynamic polyurethane composites with a"brick and mortar"structure were prepared by simple hot pressing.The main contents and results are as follows:(1)Pyrogallol and PEG600 can be stirred into a clear and transparent mixture at room temperature(22°C).UV and IR spectra show that a strong interaction is present between pyrogallic acid and PEG600 molecules in LTTMs,and the interaction between phenolic hydroxyl groups and ether atoms is significant.Theoretical calculations show that the electrostatic potentials of most surfaces of pyrogallol and PEG600 molecules are negative and positive,respectively.Thus,pyrogallol molecules and PEG600 molecules can form electrostatic attraction over a larger surface area.Five complexes with pyrogallol binding to different regions of PEG600 molecular segments are present in the LTTMs.The interaction energy of the complexes is between-32.07 and 34.79 kcal/mole.The weak interaction analysis shows that the interaction between pyrogallol and PEG600 in LTTMs is mainly through the hydrogen bond formed by the pyrogallol phenol hydroxyl group and the oxygen atom on the ether bond of PEG600 and the intermolecular force.(2)A solvent-free method was used to prepared dynamic polyurethane containing dynamic phenol carbamate bond by direct mixing of LTTMs and HDI-T.The transmittance of the dynamic polyurethanes in the visible light range is about 87%,meaning that they have very good transparency.Its transmittance gradually decreases to 0 in the wavelength less than 400nm,which shows excellent UV shielding ability.The dynamic polyurethane shows remarkable dynamic behaviors when the temperature is above 100°C and the characteristic relaxation time(τ)is less than 10 minutes.The crosslinking density,glass transition temperature,storage modulus,tensile strength and modulus of dynamic polyurethanes increase with the increase of pyrogallol content.The maximum tensile strength and modulus of the dynamic polyurethane measured in the experiment are 53.5±3.25 MPa and 0.59±0.049 GPa,respectively.The dynamic polyurethane has the ability to repair scratches and be recycled;most of the scratches on its surface can be repaired after heating at 120°C for 5 minutes,and the dynamic polyurethane can recover up to 70.11%of the original tensile strength after three repeated recycling processes.(3)Poplar wood powder was added to the mixture of LTTMs and HDI-T,then they were hot pressed into poplar wood powder/dynamic polyurethane composites.Carbamate covalent bonds,hydrogen bonds,and intermolecular forces are present between poplar wood powder and dynamic polyurethanes,which makes poplar wood powder have good phase compatibility and high interfacial bonding strength.The highest tensile and flexural strength of the composites are55.37±4.49 MPa and 88.22±3.67 MPa(poplar wood powder content is 70 wt%),the highest tensile and flexural modulus are 1.83±0.35 GPa and 3.66±1.00 GPa(poplar wood powder content is 80 wt%),and the highest impact strength is 6.80±0.98 k J/m~2(poplar wood powder content is 50 wt%).The storage modulus and the glass transition temperature of the composites were increased,the creep and the thermal expansion of the composites were decreased,and the thermal weight loss in the thermal decomposition stage of poplar wood powder was aggravated as the increase of poplar wood powder content in the composites.The poplar wood powder/dynamic polyurethane composites have good recycling ability;they can recover up to 92.3%and 101.3%of the original tensile strength and modulus,recover up to 90.4%and 87.6%of the original flexural strength and modulus,and recover up to 88.2%of the original impact strength(poplar wood powder content is 50 wt%).(4)The nacre-mimetic poplar wood powder/dynamic polyurethane composites with"brick and mortar"structure were prepared by compounding partially delignified poplar wood powder with the mixture of LTTMs and HDI-T.The nacre-mimetic poplar wood powder/dynamic polyurethane composites have a more obvious"brick and mortar"structure when the content of poplar wood powder is ultra-high(90 wt%and 95 wt%).The highest tensile and flexural strength of the composites are 76.16±2.25 MPa and 134.63±7.02 MPa(poplar wood powder content is90 wt%),the highest tensile and flexural modulus are 2.44±0.19 GPa and 7.59±0.23GPa(poplar wood powder content is 95 wt%),and the highest impact strength is 6.11±0.83 k J/m~2(poplar wood powder content is 80 wt%).After partial removal of poplar powder lignin,the tensile strength and modulus,flexural strength and modulus,and impact strength of the nacre-mimetic poplar wood powder/dynamic polyurethane composites increased by 65%and 27%,165%and 184%,and 72%,respectively(poplar wood powder content is 90 wt%).The storage modulus and the glass transition temperature of the composites were increased,the creep and the thermal expansion of the composites were decreased,and the thermal weight loss in the thermal decomposition stage of poplar wood powder was aggravated as the increase of the nacre-mimetic poplar wood powder content in the composites.The nacre-mimetic poplar wood powder/dynamic polyurethane composites have a certain recycling ability;they can recover up to76.7%and 116.6%of the original tensile strength and modulus,recover up to 77.6%and 84.1%of the original flexural strength and modulus,and recover up to 72.8%of the original impact strength(poplar wood powder content is 80 wt%).