| As a renewable and widely distributed material with high strength to weight ratio and other merits,wood is extensively used in the manufacture of ships,architectures and furniture in the long course of human history,and has become one of the most indispensable material around the world.With the development of productivity,wood has also gradually been used to produce various advanced functional materials,while the manufacture of wood-based products is still the most essential pathway for wood applications,which realizes the most crucial relationship between human beings and wood.However,wood inevitably retains a lot of natural properties which are not beneficial to their utilization.For instance,there is still much potential for the improvement of the dimensional stability and fire resistance for wood.Based on this background,wood modification is still a key direction for the scientific researches in wood science.Nowadays,wood modifications cost is sometimes high for part of traditional methods,while the mechanism for some innovative methods is still not clear,both of which bring difficulties to the large-scale industrial applications.Therefore,exploring new wood modifiers is of great importance,which is still a vital approach for promoting the development of wood modification,in both scientific researches and industrial practice.Thermally expandable microspheres(TEMs)is a kind of polymeric material with core-shell structure.Under heating conditions,the shell softens at a temperature higher than its glass transition temperature(Tg),while the core is gasified at its boiling temperature,both of which are required for the expansion of TEMs.If the TEMs could be impregnated into wood and expand in wood cell cavities,the shortcomings of wood might be improved with low weight percent gain,by virtue of the low density of expanded TEMs.Nevertheless,there are still deficiencies as follows:the high-pressure preparation of TEMs is unsafe;the particle diameters of TEMs does not match the size of wood cell cavities;the preparation time is relatively long.To solve these problems,this study will focus on the preparation of TEMs with small particle diameters,during a relatively short time under atmospheric pressure.Afterwards,this study will explore the utilization of TEMs concerning the water repellency and fire resistance of wood,for providing useful theoretical supports and practical experiences to the applications of TEMs in wood modifications.The conclusions of this study can be summarized as follows:(1)The regulation of the particle diameters of TEMs can be realized by adjusting the revolving speed during homogenization of water phase and oil phase.As the revolving speed during homogenization increased from 800 to 3000 r·min-1,the particle diameters of TEMs firstly decreased to 4?m,and then remained stable.The optimized revolving speed during homogenization was 1900?2100 r·min-1,in which the TEMs with small particle diameters of 3.82?4.06?m could be prepared.As for the TEMs prepared under the optimized conditions,the contents of alkane were 16?18%,the expansion ratios were 3.4?4.3and the expansion temperature was 112?.The TEMs obtained under 2100 r·min-1,with evident core-shell structure,possessed relatively higher potential to be applied in wood,which had been impregnated into wood and expanded in wood cell cavities.(2)Compared with the pyrolytic initiating agents,the redox initiating systems performed higher efficiency in initiating polymerization during the preparation of TEMs.The optimized conditions for initiating and reaction were as follows:the dosage of AIBN and triethylamine were 0.20 g and 270?L,respectively,while the temperature of reaction was 55?,in which the preparation time could decline by43%.Under this condition,the particle diameters of TEMs could be further decreased by increasing the amount of water phase,while the expansion ratios also reduced at the same time.As for the TEMs in Group T54 and T57,the average particle diameters were 4.56?m and 3.84?m,with the expansion ratios of 7.08 and 3.78,respectively,which had relatively higher potential to be applied in wood modification.(3)There were two components in the samples of TEMs prepared with redox initiating systems,namely the easy-to-expand TEMs(TEMs-E)and the hard-to-expand TEMs(TEMs-H),among which the TEMs-E played a decisive role in the expansion property of the TEMs samples.Under heating conditions,the total heat absorption of the TEMs samples performed a positive correlation with the content of alkane.The endothermic processes of TEMs-E and TEMs-H,however,were located in the scale of lower(110?120?)and higher(120?150?)temperature range,respectively,which was decided by the properties of the shell in TEMs.If the shell had lower Tg and degradation temperature,the TEMs could expand more easily,with a endotermic process in the scale of low temperatures.After expansion,the motility of the segments in the shells decreased,which was helpful to the TEMs to retain long-term expanded condition in wood cell cavities.Meanwhile,the-CN on the surface of TEMs could be hydrolyzed,while the-COO-could break,which would influence the effects of wood modifications.(4)Once filling in wood cell cavities,the TEMs were not only waterproof on its own,but also performed synergistic water repellency with the dispersion systems.Under the dispersion systems composed of Mg Cl2·6H2O,Na OH and Si O2,the water absorption of wood treated by T54 and T57 TEMs declined by 6.5%and 15.9%,with a reduction of 22.0%and 20.9%in volume swelling rate,respectively.If Si O2 was replaced by the modified Mg(OH)2,the water absorption fallen by 9.3%and 13.0%,with a volume swelling rate decreased by 16.6%and 7.6%,respectively.The wood treated with TEMs had good leaching resistance.After the experiments of water absorption,the rough structures of TEMs still remained on the surfaces of wood,leading to the hydrophobicity.The TEMs in Group T57 could increase the instant contact angle of wood by 20.3%.(5)Physical filling is the determining factor for the effects of wood modifications for TEMs,while the changes of chemical environment,as a secondary factor,contribute to the differences of wood modifications among various TEMs.In the wood treated by the TEMs of Group T54 or T57,the depths of impregnation were both higher than 1 mm,with similar situations of filling.The TEMs in Group T57,however,had less-CN and-CO-on their surface,thus accompanied with weaker hydrophilicity,which makes the treated wood perform better water repellency and hydrophobicity.(6)Since the addition of tributyl phosphate,the expansion properties of P-TEMs became weaker,while the content of alkane still remained up to19.3.When wood was treated by P-TEMs alone with the presence of surfactants,the P-TEMs mainly stacked on the surface of wood,causing longer self-extinguishing time.When wood was treated with P-TEMs and Mg(OH)2/Si O2 dispersion systems,the treated wood could achieve self-extinguishing during relatively short time and form a char layer with an evident boundary,thus having better fire resistance. |
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