Research On Thermal Modification Of Moso Bamboo In Tung Oil

Bamboo is one of the most abundant biomass resources,which has some inherent drawbacks such as its dimensional instability and low resistance to decay.The search for green and sustainable modification method to produce durable bamboo materials remains a challenge in the industry.Synergistic effect of tung oil and heat treatment on bamboo has unique advantages than other heat treatment.According to the application status of raw bamboo in China and the contradiction between supply and demand,the research on tung oil heat treatment of bamboo has a great significance for realizing industrial modification of raw bamboo and expanding the application field of raw bamboo.In this paper,moso bamboo(Phyllostachys edulis(Carr.)J.Houz.)was used as test material,and tung oil was used as a modified reagent.The heat temperature is 100-200 ?,and the duration time is 3 h.This paper offers a systematic study of tung oil heat treatment.Our objective was to explore the synergistic effect of tung oil and heat treatment on the anatomical properties,physical properties,mechanical properties and chemical properties of moso bamboo.The main research results contain:(1)The structure of bamboo was largely preserved during treatment.Tung oil distributed well in the lumen,pits,intercellular space and cell walls of bamboo after oil heat treatment.Moreover,the pits in the vessels started to shrink as treatment temperature up to 140 ?.When the temperature reached 200 ?,the pits almost disappeared and the pits were blocked by tung oil over 95%.(2)Once the bamboo sample was impregnated with tung oil at a certain temperature,most of the oil firstly permeated through the conducting tissue in the longitudinal direction and then gradually diffused into adjacent tissue through pits.In addition,a small portion of tung oil also entered through the parenchyma cells and fibers from the surface.Due to size and connectivity difference,tung oil entered through the conducting tissue had the highest permeability,followed by the parenchyma cells and then the fibers.As the length of the treatment time extended,tung oil gradually achieved full infiltration into the bamboo samples through pits in the tissue.Upon completion of heat treatment,the bamboo samples were removed from the tung oil solution and cooled to room temperature.Subsequently,the oil polymerized to form a protective film over the pore wall inside the bamboo structure and the surface of the samples.(3)Bamboo has the relatively stable content of cellulose and a slight increase in lignin content,after heat treated with tung oil at 100-200 °C.The content of hemicellulose was gradually decreased from 22.92%(untreated)to 20.80%(140 ?)?19.87%(180 ?)and 15.91%(200 ?),respectively.The degradation of hemicelluloses and the rearrangement of cellulose,resulting in the crystallinity index of bamboo was increased from 33.3% to 44.4% by gradually increasing the temperature from 100 °C to 200 °C,which was higher than untreated bamboo(24.5%).The O/C rate of bamboo would decreased as the content of tung oil increased or chemical composition degraded.(4)Improvement in fungi resistance such as resistance to Aspergillus niger is one of the advantages of tung oil heat treated bamboo.The decrease of hemicellulose and moisture content in the bamboo samples after heat treatment in tung oil can efficaciously inhibit the growth of fungi.Besides,the formation of oily films on the surface and inside the bamboo samples also help prevent water and fungi from penetrating into the inside of bamboo,thus considerably improving is fungi resistance.(5)The synergistic effects of tung oil and heat treatment on the surface was able to improve the surface hydrophobic properties and dimensional stability of bamboo.The contact angles could keep > 100 ° after 300 s for the samples that went through heat treatment in tung oil at 200 ?.While,the contact angle in the transverse section of air heat treated sample was disappeared after 300 s.The value of water-saturated swelling gradually decreased with increasing treatment temperature,reducing from 3.17% for the untreated sample to 2.15%(200 ?)in the radial direction.Tung oil heat treatment effectively reduced the content of hydrophilic groups and largely prevented water molecules from accessing the hydroxyl groups,leading to the consequent increase of cellulose crystallinity.The treatment also promoted further crosslink by the polycondensation reactions in lignin,contributing to the improvement of water resistance of the modified bamboo.In addition,after oil heat treatment,the cell walls of bamboo samples were covered with an oily film and most of the pits were filled with tung oil,thus blocking the pathway of water transport.After tung oil heat treatment,tung oil will be oxidized by oxygen and polymerized to form a protective film over the bamboo surface,thus effectively preventing water uptake.(6)Bamboo materials treated in tung oil below 200 ? still maintained good mechanical performance.Bamboo after tung oil heat treatment up to 200 °C did not have reduced modulus of elasticity(MOE),modulus of rupture(MOR)and ultimate stress compared to the untreated bamboo.The ultimate strain of bamboo materials decreased obviously at the treatment temperature over 180 °C.Accordingly,bamboo after tung oil heat treatment below 140 °C had better mechanical performances.Bamboo after tung oil heat treatment over 180 °C was still stiffer and stronger than the untreated bamboo but became more brittle.Heat treatment in tung oil is an effective approach to modify bamboo to produce more durable bamboo materials.After tung oil heat treatment,the bamboo shows that not only was the hydrophobic property of bamboo heat treated in tung oil enhanced,so were its dimensional stability and fungi resistance noticeably improved.It is worth noting that although heat treatment of bamboo in tung oil at high temperature over 180 ? induced an obvious reduction in breaking toughness,but bamboo materials treated in tung oil below 200 ? still maintained good mechanical performance with no obvious drop in MOE and MOR,compared to untreated bamboo.Therefore,tung oil heat treatment can be used as an economical and ecofriendlyapproach to modify bamboo materials,which is highly promising for application in the bamboo industry.