| Plant fibers are the most abundant renewable resources in the world,among which corncobs(CC)have the advantages of large output and low cost.At present,the mainly use of them have low industrial-added-value and are easy to cause environmental pollution.In this paper,CC was used as the raw material,the relatively clean and environmentally friendly steam-explosion pretreatments were adopted to eliminate the recalcitrant properties of CC,reduce the difficulties of the subsequent treatments,and further to prepare cellulose nanofibers(CNFs)and achieve microfibrillation.These practices added high value to CC and possessed great industrial significance and economic value.CC was first pretreated by intermittent steam-explosion(SE)and continuous steam-explosion(CSE).It was found that pretreated CC showed high thermal stabilities;hemicellulose in SE pretreated CC(SECC)was partially degraded,whereas CSE pretreated CC(CSECC)was considered to reserve all components.SE pretreatment can tear the plant cell wall structure of CC,expose microfibers,and reduce the dificulities of the subsequent chemical treatments.CSE pretreatment can microfibrillate CC,increase its specific surface area,and also had higher processing efficiency and producing capacity.Therefore,based on their own advantages,SE and CSE pretreatments can be applied to the preparation of CNFs and direct filling of polymers,respectively.SECC was used as the starting material for alkali treatment and bleaching in low concentration to remove non-cellulosic components under room temperature.For comparison,CC was also used as the starting material for the same procedures;however,the same result cannot be achieved.High-speed blending can reduce the entanglements among cellulose fibers and homogenize the size of the cellulose microfibers.The cavitaion effect of ultrasonication can fibrillate the cellulose microfibers into nano-scale.As a result,CNFs with an average diameter of 8 nm were prepared following this step-wise method.Under the same routine of preparing CNFs,the dispersion medium was replaced with polyethylene glycol(PEG)solution to investigate the effect of PEG concentration on the size of CNFs and the stabilities of CNFs suspensions.It was found that PEG molecules consumed a portion of the cavitation energy and also increased the distance between cellulose fibers at the same time.These two antagonistic effects determined the final size of CNFs.When PEG concentration was lower than 5 wt%,it was found that the increase of PEG concentration increased the stability of CNFs suspension.When PEG concentration was higher than 5 wt%,the stability of CNFs suspension decreased as the PEG concentration increased due to an increase of the size of CNFs under this condition.CNFs suspensions with different PEG concentrations were freeze-dried and the resulting CNFs/PEG masterbatches(CP masterbatch)had higher thermal stabilities with the increase of PEG content.The CP masterbatch obtained after freeze-drying of CNFs suspension with PEG concentration of 5 wt%processed high redispersibility and high thermal stability,and therefore can be used for the thermal processing to prepare polyvinyl alcohol(PVA)composites.In order to prepare PVA composites using thermal processing,PVA1788 was used as the matrix;different contents of CP masterbatch and PEG were used as fillers for comparison;water was used as the plasticizer.It was found that CNFs and PEG in CP masterbatch can destroy the original hydrogen bonds of PVA.As results,the melting points were lowered,the melting ranges were widened and crystallinities were decreased.In the mean time,the increase of the content of CP masterbatch also improved the thermal stabilities of PVA-CP composites.With the increase of the content of CP masterbatch or PEG,the impact strength of PVA-CP and PVA-PEG composites increased accordingly,however,the imact strength of PVA-CP composites were slightly higher than those of PVA-PEG composites when the content of filling material was the same.It was because CNFs can absorb some of the energy,transfer the energy and delay the propagation the material cracks when composites were impacted by external forces.With the increase of the content of CP masterbatch,the tensile strength and elongation at break of PVA-CP composites increased accordingly,while the tensile modulus increased first and then decreased.The maximum value 108.2 MPa of tensile modulus was reached when the content of CP masterbatch was 20 phr.This was due to the fact that CNFs were easy to have aggregation in the matrix when the content of CP masterbatch exceeded 20 phr.PVA1799 was used as the matrix,different contents of microfibrillated CSECC and CC were used as fillers for comparison and water and glycerin were used as the plasticizers for preparing PVA composites.The results showed that the thermal stabilities of PVA composites increased,the melting points decreased and therefore the thermal processing windows expanded with the increase of CSECC or CC contents.CSECC had a relatively high specific surface area and microfibrillation structure and thus had better dispersion and bonding strength in PVA matrix than those of CC,which had a significant effect on the mechanical properties of PVA composites.When the content of CSECC was 1-5 phr,CSECC can enhance and toughen the PVA at the same time.Compared to the PVA/g/w sample,the impact strength was increased from 27.3 kJ/m~2 to 39.7 kJ/m~2,while tensile strength,tensile modulus,and elongation at break were increased from 28.5 MPa,129.2 MPa,and 402.2%to 36.0 MPa,170.3 MPa,and 482.1%,respectively.When the content of CSECC exceeded 5 phr,tensile and elongation at break of PVA-CSECC composites decreased accordingly.This was due to mechanical defects that caused by CSECC aggregation and uneven dispersion in the matrix.The biodegradabilities of PVA-CSECC composites were slightly better than that of PVA-CC composites.When the content of CSECC or CC was 10 phr,the weight loss was 9.2%or7.4%,respectively,after buried in soil for one month. |
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