| In recent years,the problem of waste and environmental pollution caused by crop straw incineration has become more and more seriously.The conundrum of straw utilization has been paid more attention.In addition to being used as feed and fuel,crop straw can also be used for the preparation of biomass composites.It is of great significance to study on the composition and properties of straw,because the properties of straw directly affect the properties of the composites.In this paper,five kinds of straws were selected as the research object,such as wheat straw,rice straw,corn stalk,reed stalks and soybean stalks.And this five kinds of straw were pretreated by microwave treatment,water bath heating,dilute H2SO4 and NaOH treatment,this four kinds of different pretreatment methods.The mechanical properties,physical and chemical properties,major chemical functional groups,thermal stability,microstructure of the inner and outer surfaces,and crystallinity degree were analyzed.And the contents of cellulose,hemicellulose and lignin were analyzed.The main conclusions are as follows:(1)Compared with the analysis of the mechanical properties of straw fiber,the results showed that the tensile strength of reed straw was the largest,followed by soybean straw,the tensile strength of rice straw was the smallest,which was 185.46MPa of reed straw,97.35MPa of soybean straw,corn stalk 54.32MPa,wheat straw 49.39MPa,rice straw 37.28MPa;tensile modulus size order and tensile strength were consistent,followed by reed straw 6.18GPa,soybean straw 5.55GPa,wheat straw 3.73GPa,rice straw 3.05GPa,corn straw 2.71 GPa;the maximum shear hardness was corn stalks,followed by soybean stalks,the smallest is rice straw,and their size were:corn stalks 49.60N,soybean straw 40.10N,reed straw 35.27N,wheat straw 5.93N,rice straw 3.57N;the maximum hardness of the displacement of the order of corn stalks 2.31mm,soybean straw 2.25mm,reed straw 1.76mm,wheat straw 1.39mm and rice straw 1.24mm.Compared with the analysis of straw fiber composition and performance,the results showed that the content of cellulose and hemicellulose in the reed straw was the highest,the soybean starch had high lignin content and the ash content of the corn stalk was higher.The cellulose content was as follows:reed straw was 45.85%,wheat straw was 45.32%,rice straw was 42.94%,corn straw was 37.24%andsoybean straw was 30.83%.The content of hemicellulose was 21.15%of reed straw,20.95%of rice straw,19.69%of wheat straw,17.38%of corn stalk,the content of lignin was 14.64%,the content of lignin was 32.8%of soybean straw,23.13%of corn stover,19.21%of reed straw,12.49%of wheat straw and 10.92%of rice straw.The ash content was 7.22%of wheat straw,wheat straw 5.03%,3.16%of rice straw,2.90%of soybean straw and 1.34%of reed straw.(2)There are four stages in the thermal decomposition of five kinds of straw:drying and evaporating stage 30 ??00 ?;heating stage 100 ??250?;thermal decomposition stage 250??400 ?;residual material heating stage 400 ??.The trend of different straw weight loss was basically the same,but the residual mass ratio was different,the decomposition of soybean straw was more thorough;the DSC curve of different varieties of straw was different,and the stability of soybean straw was better.The results showed that the residual mass of different straw fibers was different,and the remaining mass of reed straw was the largest,followed by rice straw and soybean straw residual mass,which were 22.71%of reed straw and 22.57%of rice straw,21.47%for corn stalks,20.18%for wheat straw and 1.88%for soybean stalks.(3)Comparative analysis of five kinds of straw fiber crystallinity we can found:the X-ray diffraction spectra of five kinds of untreated plant straw fibers were similar,and the diffraction peaks of soybean straw,reed straw,rice straw,wheat straw and corn stalk were typical.The reed stalks washighest(CrI=55.90%),wheat straw followed(CrI=54.95%),followed by corn stalks(CrI=54.39%),straw(CrI=48.73%),soybean straw was the lowest(CrI=42.5%).(4)The infrared absorption peaks of five kinds of straw fibers are similar in shape,but the intensity of the absorption peaks was different The intramolecular hydroxyl-OH stretching bands of cellulose,hemicellulose,polysaccharide and monosaccharide in the five kinds of straw were located in the wave number of 3300-3500cm-1,both-CH3-and CH2-anti-symmetric stretching vibration peak is located at the wave number of 2900?2935cm-1,all of which have the-NH2 stretching vibration peak in the wave segment 2230?2400cm-1,and there are carboxylic acid lipid compounds related to lignin or hemicellulose and the absorption peaks of C-O-C asymmetric stretching vibration in the aliphatic compounds were in the wave number of 1150-1160cm-1,all of which are in the carbohydrate.-CO-stretching vibration absorption peak located in the wavenumber segment 970?1100cm-1,both 0-D glucosyl-based properties and molecular hydrogen bond absorption peak in the wavenumber segment 890?900cm-1.The vibration of wheat straw at the wave number 3300?3500cm-1 was the strongest,which indicated that the water absorption of wheat straw fiber was the strongest.The difference between this five kinds of straw in the wave number 2900?2990cm-1 was small,the vibration of wheat straw fiber material is slightly strong.The five kinds of straw in wavenumber 1700?1735cm-1 strong vibration absorption peak from large to small by successive is soybean straw,corn straw,reed straw,wheat straw,rice straw and straw fiber materials,consistent with the order of the lignin's content in straw fiber.(5)Comparison of straw fiber absorbent and hygroscopicity founded:24h water absorption of the wheat straw was the best that it can up to 456.62%,followed was rice straw by 421.85%,corn stover was 352.18%,straw reed was 244.1%,wherein the lowest 24h water absorption was soybean straw with 235.67%;The soybean straw had the highest contact angle,followed by the reed straw,and their size was soybean stalk,reed straw,corn stalk,corn stover,rice straw and wheat straw,and five kinds of straw material contact angle are greater than 90°.(6)Comparative analysis of the microstructure with straw fiber can be found:the outer surface of soybean stalks was relatively rough,with a clear gully;reed straw outer surface of the organization was smooth,dense,bundled fiber longitudinal arrangement,the surface layer of waxy film;the outer surface of corn stalks was smooth and compact,with a small amount of stomata and scratches are not evenly distributed;theoutside surface of rice strawwith different sizes of protruding waxy crystals and siliceous cells,small radius of waxy crystals and silicon the outer surface of wheat straw was similar to reed straw,and the outer surface was dense and had no damage,and there were some single pattern holes.(7)The physical and chemical properties of five straw fiberhave changed after microwave treatment,the crystallinity of the material and untreated had a small increasethan before,The highest crystallinity was reed stalks(CrI=57.87%),followed by corn stalks(Crl=56.67%),soybean straw(Crl=45.75%).The contact angle of the five kinds of straw materials after microwave treatment decreased slightly,from large to small followed by reed straw,soybean stalks,corn stalks,rice straw and wheat straw.The water absorption rate of straw after 24 hours was decreased,and the maximum water absorption rate of wheat straw was 484.6%,followed by Rice straw straw 465.9%,corn stalks 253.96%,soybean stalks 193.64%and reed stalks 131.61%;the moisture absorption rate did not change as much as they were:wheat straw,rice straw,corn stalk,soybean straw and reed straw.(8)The physical and chemical properties of five straw fiberhad a certain change in the material crystallinity on a small increaseafter the water heated treatment,The highest crystallinity was reed straw(CrI=57.54%),followed by wheat straw(CrI=53.29%),rice straw(CrI=48.22%),the lowest crystallinity was soybean straw(CrI=44.45%),the results showed that the contact angle of five kinds of straw fibers was reduced by the treatment of water heated,and they were soybean straw,reed straw,corn stalk,wheat straw and rice straw.The water absorption rate was 555.86%And the rest were reduced by 400.54%of wheat straw,301.48%of corn stalks,230.25%of reed straw and 200.49%of soybean straw.The moisture absorption rate of rice straw was significantly lower than that of untreated,and the rest did not change from large to small in order:rice straw,wheat straw,corn stalk,reed straw and soybean straw.(9)The physical and chemical properties of straw fibers were significantly changed by dilute H2SO4 treatment:their crystallinity was significantly improved,maize straw was the highest(CrI=60.64%),wheat straw followed(CrI=55.54%),reed straw(CrI=54.21%),rice straw(CrI=54.06%),and the soybean straw was the lowest(CrI=51.32%).The contact angle of straw fibers treated with dilute H2SO4 decreased significantly,which was 85.4° of reed straw,78.67° of soybean straw,77.6° of com straw,77.4° of rice straw and 56.83°of wheat straw,respectively.The 24h water absorption rate of the straw reduced,from large to small followed by 397.29%of wheat straw,163.43%of rice straw,95.18%of corn straw,94.16%of soybean straw and 59.59%of reed straw.The hygroscopic rate decreased slightly,and they order from large to small is wheat straw,rice straw,corn straw,soybean straw and straw of reeds.(10)Thestraw fiber treated by NaOH had the biggest change of physical and chemical properties:their crystallinity increased greatly,corn stalks was the highest(Crl=70.11%),the reed straw(CrI=63.64%),rice straw(CrI=62.30%),and wheat straw(CrI=57.80%),the lowest of soybean straw(Crl=53.51%);The contact angle of straw fibers treated with NaOH was also greatly reduced,the order from lager to small was:reed Straw 65.2° corn straw 62.2°,soybean straw 60.2°,rice straw 52.9° and wheat straw 48.9°;In addition to wheat straw 438.97%,the 24h water absorption of rest were significantly reduced;the overall change of moisture absorption rate was small,the moisture absorption rate of reed stalk decrease from large to small was as follows:wheat straw,rice straw,soybean straw,corn straw and reed straw. |
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