Acetylation of plant cellulose fiber in supercritical carbon dioxide

Natural cellulosic ramie fiber was acetylated using supercritical carbon dioxide (sc-CO₂) as a reaction medium. The structure and properties of the acetylated fibers were investigated using infrared spectroscopy, scanning electron microscopy, X-ray diffraction (including synchrotron microbeam X-ray diffraction), nano-Raman scattering, and a tensile test. The acetylation reaction proceeded without using an organic solvent, and it reached to the core part of the fiber within a short period while maintaining the fiber morphology. The crystallites of cellulose triacetate II and cellulose coexist in the fiber. The acetylated fiber with an average degree of substitution of 1.9 showed high modulus (34.5 GPa) and high strength (763 MPa), which are the highest values for cellulose diacetate so far reported to date.     

Characterization of ramie yarn treated with sodium hydroxide and crosslinked by 1,2,3,4‐butanetetracarboxylic acid

Ramie yarns were treated with various concentrations of NaOH at room temperature and subsequently crosslinked with 1,2,3,4‐butanetetracarboxylic acid (BTCA). The microstructure and tensile properties of the treated yarns were characterized. X‐ray diffraction (XRD) and FTIR were used to study the crystalline structure of the resultant ramie yarns. The results showed that the maximum change in the structure of the alkali‐modified ramie took place at 16% NaOH, which would completely transform cellulose I to cellulose II. At the same time, the crystallinity index and fiber orientation decreased to the minimum value while the absorption properties were enhanced. The average degree of polymerization (DP ) of the treated ramie yarns slightly decreased after NaOH treatment. Tensile properties including tenacity, breaking elongation, and modulus of the treated yarns were also investigated. Scanning electron microscopy (SEM) was used to investigate the breakage of the treated yarns. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1857–1864, 2004     

天然竹纤维的固态核磁共振谱表征

利用固态核磁共振碳谱分析研究了天然竹纤维的结晶结构,并与常规的天然纤维素纤维苎麻、亚麻和棉纤维进行了比较。研究表明竹纤维的结晶度、晶粒尺寸与苎麻相似,大于棉纤维和亚麻纤维。利用13C NMR分析计算出了竹纤维中纤维素的晶型含量,结果显示,竹纤维的晶型与棉纤维、苎麻和亚麻相似,纤维素Ⅰβ占优势。利用核磁共振谱计算的几种纤维素纤维的结晶度和晶粒尺寸小于利用X射线衍射计算的结果,只有对于高结晶度的纤维如竹纤维和苎麻偏差才比较小,主要原因在于NMR分析对小范围的有序更为敏感,只有晶区内的材料才能看作结晶区。     

Radiation effect on the thermal conductivity and diffusivity of ramie fibers in a range of low temperatures by γ rays

To understand the influence on the thermal conductivity by the length of the molecular chain in the polymer fiber, the thermal conductivity and thermal diffusivity of ramie fibers and those irradiated by γ rays, which induced molecular chain scission of cellulose, were investigated in a range of low temperatures. The degrees of polymerization, crystallinities, and orientation angles of ramie fibers and those irradiated by γ rays (γ‐ray treatment) were measured by the solution viscosity method, solid‐state NMR, and X‐ray diffraction. Only the degree of polymerization decreased with the γ‐ray treatment, and the crystallinities and orientation angles were almost independent of the γ‐ray treatment. The thermal conductivities of the ramie fibers with and without γ‐ray treatments decreased with decreasing temperature. The thermal diffusivities of the ramie fibers and those irradiated by γ rays were almost constant from 250 to 100 K, increased slightly with the temperature decreasing from 100 to 50 K, and increased rapidly with the temperature decreasing below 50 K. The thermal conductivity and thermal diffusivity of the ramie fibers decreased with the γ‐ray treatment. The mean free path of the phonon in the ramie fibers was reduced by the γ‐ray treatment. This decrease of the thermal diffusivity and thermal conductivity was explained by the reduction of the mean free path of the phonon by molecular chain scission with γ rays. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 5007–5018, 2006     

Wet spinning of cellulose from ionic liquid solutions–viscometry and mechanical performance

A series of regenerated cellulose fibers was produced from dopes prepared by mixing and dissolving cellulose of two different degrees of polymerization in different ratios in the ionic liquid 1‐ethyl‐3‐methyl‐imidazolium acetate. Viscoelastic properties of the spin dopes were characterized by controlled stress rheometry. The cellulose solutions were solidified in pure water by the traditional wet spinning technique. The resulting fibers were characterized by means of wet and dry tensile testing and scanning electron microscopy. The characterization revealed a compact and homogeneous fiber. A nonlinear relationship between degree of polymerization and fiber properties was observed with a moderate difference in mechanical properties in a broad interval of fibers while fibers composed of polymers with the highest degree of polymerization stood out as stronger and stiffer. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal conductivity of ramie fiber drawn in water in low temperature

To understand the effect of extension of molecular chain in amorphous region in polymer fibers to thermal conductivity, the thermal conductivity, tensile modulus and crystal orientation angle of ramie fibers and those drawn by the stress of 17.4 kg/mm² (water treatment) in the water were investigated. The tensile modulus of ramie fiber in fiber direction increased from 61 to 130 GPa by drawing in the water. The crystal orientation angles of ramie fiber with and without water treatment were measured by X‐ray diffraction. The orientation degrees of ramie fibers without and with water treatment were estimated as 92.9 and 93.6%, respectively. Therefore, the tensile modulus increases two times as that of blank ramie by water treatment although crystal orientation angle does not change distinctly. The increasing of tensile modulus of ramie fiber by water treatment was explained by extension of the molecular chains in the amorphous region. Thermal conductivities of ramie fibers with and without water treatment were measured in the fiber direction in the temperature range from 10 to 150 K. Thermal conductivity of ramie fiber in the fiber direction increased by water treatment. The increasing ratio of thermal conductivity by water treatment agreed to that of sound velocity induced by increasing tensile modulus. Those results suggest that thermal conductivity of polymer fiber increase by the extension of molecular chains in the amorphous region. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2196–2202, 2006     

Temperature dependence of the brittleness of cellulose fibers and of chemically modified cellulose fibers

The temperature dependence of the brittleness of cellulose fibers was studied by the breaking twist angle method. The selected fibers were purified ramie cells, the same cells after fibrous acetylation to form the triacetate, annealed acetylated ramie cells, and mercerized ramie cells. Water acts as a plasticizer to reduce the brittleness of ramie and of mercerized ramie cells but does not appear to affect the brittleness of ramie cells after acetylation. The brittleness of ramie cells is reduced by similar amounts by fibrous acetylation and by mercerization. The brittleness of acetylated ramie cells is increased by annealing, which induces additional crystallization in the cells. The brittleness of ramie and acetylated ramie before and after annealing varied as the test temperature was progressively increased in the range 100–200°C. The effect of temperature on the brittleness of the fibers is interpreted in terms of the relaxation transitions previously determined on the same types of fibers, and in terms of the supramolecular structure of the fibers.     

苎麻落麻的表面处理及其复合材料的性能研究

采用甲基丙烯酸甲酯、丙烯腈接枝,硅烷偶联剂(A-151)偶联,聚氯乙烯(PVC)包覆等方法对苎麻落麻纤维进行表面处理;测试了处理前后落麻纤维的吸水率、单丝强度及其与环氧树脂(EP)、酚醛树脂(PF)和不饱和聚酯(UP)等的接触角;观察了偶联和包覆后落麻纤维的表面状况;选取偶联和包覆后的落麻纤维制备了UP/落麻毡和PP/落麻纤维复合材料,测试了其拉伸和弯曲性能并观察了处理前后复合材料的拉伸断面形貌。结果表明,接枝、偶联和包覆不仅降低了落麻纤维的吸水速率。而且也降低了平衡吸水量;接枝对落麻纤维单丝强度影响最大,偶联次之,包覆最小;接枝、偶联和包覆均能大幅度改善落麻纤维与EP、PF和UP的浸润性;偶联和包覆后的落麻纤维表面都比处理前粗糙;偶联使UP/落麻毡复合材料的拉伸强度、拉伸弹性模量提高了21％,弯曲强度提高了34％,弯曲弹性模量提高了40％,包覆使PP/落麻纤维复合材料的拉伸、弯曲强度提高了20％左右。     

Enhancing mechanical properties of gel‐spun polyvinyl alcohol fibers by iodine doping

High strength polyvinyl alcohol (PVA) fibers with a conventional degree of polymerization of 1500 were prepared by doping iodine with PVA spinning solution. The iodine‐doped PVA (I‐PVA) aqueous solution was extruded into cold methanol that provides dark purple PVA‐iodine complex gel fibers. Only a small amount of iodine was required to enhance drawability and molecular orientation by reducing the interaction between PVA chains. An increase of ca. 10% in the maximum draw ratio of the doped fibers compared with that of undoped PVA translated into values for the tensile strength, 2.2 Giga‐Pascal (GPa), and initial modulus (47 GPa) that were more than 30% higher than those of the neat PVA fiber. Easier chain slippage of molecules in the amorphous segments of the I‐PVA fiber during drawing leads to increased orientation in these segments, which is believed to be the source of the improvements in mechanical properties. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Molecular modeling of cellulose in amorphous state part II: effects of rigid and flexible crosslinks on cellulose

It is essential to understand the molecular level response of crosslinked cellulose chain segments upon deformation, in order to develop new agents which convey high durable-press (DP) rating to cellulose fibers with minimal strength loss. In this work models of amorphous cellulose crosslinked with both rigid and flexible crosslinks were constructed computationally for this purpose. Rigid crosslinks bound cellulose molecular segments together and blocked the chain slippage, providing cellulose models with a higher initial modulus and better elastic response. However, the loss of the chain slippage led to stress being distributed unevenly among cellulose chains. Chains in some regions were subjected higher stress and these regions were opened up much more than the rest of the cellulose, which presumably caused models to fail. When conformationally flexible crosslinks were used, breaking strain of cellulose was not significantly reduced but deformation recovery was not improved either, in comparison with the models of untreated cellulose. Conformational transitions were observed in the flexible crosslinks during extension. These results help to explain how and why rigid crosslinks work to provide wrinkle resistant properties and why they also lower tensile strength, and that just using a conformationally flexible crosslinking will not provide any recovery.     

几种天然纤维素纤维的物理结构研究

对竹纤维等几种天然纤维素纤维的物理结构进行了系统的比较研究。实验结果表明:竹纤维聚合度远高于粘胶、稍低于苎麻;结晶度高、取向度高,属于大分子排列整齐、紧密型超分子结构,接近于苎麻纤维;在形态结构上,横截面为腰圆形,有中腔,壁上有裂纹,截面有孔隙,TEM结果表明,次生层为单层次结构,与苎麻相仿。总之,竹纤维从微观的大分子链结构到聚集态结构,到宏观的纤维形态结构,各种物理指标特征均表明其与苎麻纤维有着非常的相似性。     

Hydrophobic surface modification of ramie fibers by plasma-induced addition polymerization of propylene

The low affinity between hydrophilic cellulose fibers and hydrophobic matrices leads to poor interfacial bonding, reducing the mechanical performances of natural cellulose fiber-reinforced composites. This study illustrates plasma-induced addition polymerization of propylene to create a hydrophobic surface on ramie fibers for enhancing their bonding with polypropylene (PP). Plasma treatment with propane is applied for comparison. The advancing contact angles of the plasma-treated ramie fibers are raised from 66.3° to 106.1° and the interfacial shear strengths with PP are enhanced up to 36.4%, likely resulted from the increase in fiber surface roughness observed under a scanning electron microscope and the introduction of plasma-grafted PP and alkyl groups on fibers surfaces proven by X-ray photoelectron spectroscopy. It is also revealed that plasma treatment with propylene is highly effective in increasing surface carbon content (from 68.3% to 82.4% in 0.5 min) and more efficient than the treatment with propane, though both plasma treatments show substantial modification efficacies to the fiber surfaces. The treatment duration affects surface roughness more than surface chemical composition, and the optimized treatment time is around 1 min. The modification method developed in this research has the potential to be used for surface modification of fibers for many applications.     

废弃天然纤维增强高密度聚乙烯复合材料性能研究

选用纺织工业废弃苎麻落麻纤维和造纸工业废弃的竹屑纤维为增强体,采用双螺杆熔融共混挤出工艺,制备天然纤维增强高密度聚乙烯（PE-HD）复合材料。考察纤维种类、含量变化对天然纤维增强复合材料熔体流动速率、微观断面形貌、拉伸性能、弯曲性能的影响。结果表明,2种废弃纤维都能有效提高PE-HD的拉伸性能和弯曲性能,其中苎麻落麻纤维的增强效果优于竹屑纤维,加入20% 苎麻落纤维复合材料拉伸强度比纯PE-HD提高21%,弯曲强度提高了41.9%。     

Treatment of ramie fiber with different techniques: the influence of diammonium phosphate on interfacial adhesion properties of ramie fiber-reinforced polylactic acid composite

Ramie fiber-reinforced polylactic acid (PLA) composites were successfully prepared by hot compression molding. Different treatment techniques were used to modify the surface of ramie fiber. The influence of diammonium phosphate (DAP) on the interfacial adhesion between ramie fiber and PLA composites was investigated by the contact angle measurements, FTIR and SEM analyses. The contact angle measurement results showed that alkali treatment combined with DAP was very efficient in decreasing the hydrophilicity of fibers. After treatment, the hydrophilicity of untreated ramie fiber from 5.9 ± 1.3 decreased to 2.0 ± 0.8 mJ/m². The wettability of alkali/silane/DAP-treated ramie fiber/PLA composite was higher (95.4° ± 1.3°) than that of pure ramie fiber/PLA composite (87.3° ± 1.9°). The FTIR results were consistent with the wetting measurements as the increment of hydrophilicity. Thermal analysis indicated that DAP-modified ramie fiber/PLA composites exhibited a lower thermal decomposition temperature, unique decomposition behavior and more residual char formation at decomposition temperature. The tensile, flexural and impact properties of DAP-modified ramie fiber composites were comparable to those of untreated ramie fiber composite. Moreover, proper alignment and uniform distribution of ramie fibers within the PLA matrix were found to be excellent. The morphological structures observed by SEM showed that well-modified ramie fibers enhanced the failure of the PLA composites in tensile, flexural and impact tests.     

Grafting modification of ramie fibers with poly(2,2,2-trifluoroethyl methacrylate) via reversible addition–fragmentation chain transfer (RAFT) polymerization in supercritical carbon dioxide

Reversible addition–fragmentation chain transfer (RAFT) polymerization was used to control the grafting of 2,2,2-trifluoroethyl methacrylate (TFEMA) with ramie fibers in supercritical carbon dioxide (scCO₂). The hydroxyl groups of the ramie fibers were converted to 2-dithiobenzoyl isobutyrate as the RAFT chain transfer agent (cellulose-CTA). Then, the subsequent grafting with TFEMA was mediated by RAFT polymerization in the presence of 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate (ECPDB) as the free RAFT chain transfer agent (free CTA). The modified ramie fibers were highly hydrophobic with water contact angles of up to 149°. Size exclusion chromatography showed narrow polydispersity (PDI = 1.28) of the grafted poly(TFEMA) chains. This grafting polymerization process is a novel and environmentally friendly approach for the preparation of functional grafted copolymers utilizing ramie fiber biomass.     

Effects of a pretreatment with N‐methylmorpholine‐N‐oxide on the structures and properties of ramie

To improve the dyeing properties of ramie, the ecofriendly organic solvent N‐methylmorpholine‐N‐oxide (NMMO) was used to substitute sodium hydroxide as a ramie‐fiber swelling solvent. Through padding and baking pretreatment, ramie fabric was modified by an NMMO aqueous solution. Ultraviolet–visible spectrophotometry, Fourier transform infrared spectroscopy, X‐ray diffraction, and differential scanning calorimetry were used to investigate the effects of NMMO pretreatment on the structure of the ramie, whereas the color strength (K/S, where K is the light absorption coefficient and S is the scattering coefficient), adsorption isotherm, and dye uptake rate curve were measured to investigate the effects of NMMO pretreatment on the dyeing properties of the ramie. The results show that the ramie fiber experienced a limited and irreversible swelling because of the partial breakage of interhydrogen and intrahydrogen bonds of cellulose molecules in the amorphous area, but the crystal and chemical structure of the ramie fiber did not change obviously under the experimental conditions. The K/S value of the NMMO‐modified ramie fabrics dyed with reactive dyes increased by about 100%, and the dye uptake increased by 27.88% compared to that of the raw sample, whereas the standard affinity and diffusion coefficient value of the reactive dyes on the NMMO‐modified ramie fabric were higher than those of the raw ramie fabric. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Disorder in cellulosic fibers

Ruland's concept of an isotropic disorder function is applied to estimate the disorder parameter and the degree of crystallinity in a few cellulosic fibers: two cottons, native ramie, and a high-tenacity rayon. The results indicate an increase in disorder without any change in crystallinity on mercerization of native celluloses. On hydrolysis, with or without a pretreatment of mercerization, the samples exhibit a higher crystallinity, disorder remaining the same as for native celluloses. A ball-milled sample of “amorphous” cellulose is still found to be fairly crystalline with the lowest disorder. On being wetted in water and oven-dried, a distorted form of cellulose II with higher crystallinity and disorder was obtained. The polynosic fiber, Tufcel, has low values for the degree of crystallinity, disorder parameter, as well as crystallite dimension. A strong dependence of the degree of crystallinity on the crystallite size, particularly the lateral, is observed.     

Chemical surface modification of wheat straw fibers for polypropylene reinforcement

The use of natural materials has grown in the last years in the plastics industry. Natural lignocellulose fibers derived from agricultural waste present potential to be used as a replacement for glass fibers for polymer reinforcement, leading to lower CO₂ footprint products. However, cellulose fibers are hydrophilic and polar and as a result of that, incompatible with hydrophobic polymers such as polypropylene. For this reason, a surface modification on the cellulose fiber is required. This work focuses on the modification of the cellulose fibers to improve the compatibility with polypropylene. Wheat straw fibers derived from agricultural waste were scoured with the purpose to remove lignin, hemicellulose and pectin to facilitate the defibrillation. The fibers were then esterified using acetic anhydride. Thermal gravimetric tests have shown an increase in the thermal stability of the scoured and esterified cellulose fibers, from 246°C for untreated fibers to 292°C and 316°C, respectively. From mechanical tests results it could be seen that the tensile modulus of the composites with esterified cellulose fibers increased 57% compared with the neat PP. Flexural strength increased by 31% and flexural modulus by 70%. The use of esterified fibers led to an improvement of 79% in the impact strength compared with the neat PP. A better compatibilization between fibers and matrix could be seen using maleic anhydride modified polypropylene copolymer as compatibilizer, even with esterified fibers, probably due to residual hydroxyl groups still available on modified cellulose. POLYM. COMPOS., 37:2133–2141, 2016. © 2015 Society of Plastics Engineers     

聚乳酸/酯化纤维素复合材料的制备与表征

通过气固反应利用马来酸酐（MA）对纤维素进行酯化改性,采用熔融共混工艺制备了聚乳酸（PLA）/酯化纤维素复合材料。红外分析表明纤维素与MA发生了酯化反应。力学性能测试、热重分析、差示扫描量热仪（DSC）、扫描电镜（SEM）等分析表明,PLA/酯化纤维素复合材料的拉伸模量和弯曲模量随酯化纤维素含量的增加而升高,拉伸强度、弯曲强度和热稳定性随酯化纤维素含量的增加而降低;复合材料的Tc相对纯PLA较高,说明酯化纤维素的加入起到了异相成核作用,使结晶速率提高。酯化纤维素在复合材料中分散充分,但两者的界面黏结力较弱。     

Influence of plasma treatment on properties of ramie fiber and the reinforced composites

In this research, 9 series of ramie fibers were treated under low-temperature plasma with diverse output powers and treatment times. By analysis of the surface energy and adhesion power with epoxy resin, 3 groups as well as control group were chosen as reinforced fibers of composites. The influences of these parameters on the ramie fiber and its composites such as topography and mechanical properties were tested by scanning electron microscopy (SEM), atomic force microscopy (AFM), tensile property and fragmentation test of single-fiber composites. Contact angle and surface free energy results indicated that with the increased treatment times and output powers, surface energy and adhesion work with epoxy resin improved. Compared with the untreated fibers, surface energy and adhesion work with epoxy resin grew 124.5 and 59.1% after 3 min-200 w treatment. SEM and AFM showed low temperature plasma treatment etched the surface of ramie fiber to enhance the coherence between fiber and resin, consequently fiber was not easy to pull-out. After 3 min-200 w treatment, tensile strength of ramie fiber was 253.8 MPa, it had about 30.5% more than that of untreated fiber reinforced composite. Interface shear stress was complicated which was affected by properties of fiber, resin and interface. Fragmentation test showed biggest interface shear stress achieved 17.2 MPa, which represented a 54.0% increase over untreated fiber reinforced composites.