Enhancement of thermal,mechanical and barrier properties of EVA solar cell encapsulating films by reinforcing with esterified cellulose nanofibres

Solar cell encapsulating film based on ethylene vinyl acetate copolymer (EVA) was modified by using bacterial cellulose (BC) nanofibres. Bacterial cellulose was chemically modified with propionic anhydride prior to compounding with EVA in a twin screw extruder. The effects of fibre content on the mechanical, thermal, optical and barrier properties of the EVA composite films were investigated. Better mechanical and barrier properties of the EVA films were obtained when the modified BC nanofibres were used. The results were ascribed to the different chemical functional groups on the fibre surface, as verified by FTIR spectra. Deacetylation of the EVA was delayed and visible light transparency of the EVA films above 75% was retained. Overall, our study showed that it was possible to improve the barrier properties of EVA film without sacrificing much transparency by using a suitable type and content of cellulose nanofibres.     

以离子液体为介质的纤维素加工与功能化

纤维素作为自然界中储量最大的天然高分子,被认为是未来世界能源与化工的主要原料.但由于分子链间存在丰富氢键网络以及高度结晶的聚集态结构特点,天然纤维素不熔化、难溶解,造成纤维素的加工极其困难,纤维素材料的传统生产工艺复杂且污染严重,极大限制了纤维素材料的广泛应用.近年来,人们发现一些特定结构的离子液体能够高效溶解纤维素,为纤维素的加工和功能化提供了新的多用途平台.本文从"溶解纤维素的离子液体、纤维素溶解机理与溶液性质、以离子液体制备再生纤维素材料和以离子液体为介质合成纤维素衍生物"4个方面详细介绍了本课题组在此领域的研究进展.     

Effects of polymer concentration and coagulation temperature on the properties of regenerated cellulose films prepared from LiOH/urea solution

Aqueous 5 wt% LiOH/12 wt% urea solution pre-cooled to −12 °C has a more powerful ability to dissolve cellulose compared to that of NaOH/urea and NaOH/thiourea solution system. The influences of the cellulose concentration and coagulation temperature on the structure, pore size and mechanical properties of the cellulose films prepared from LiOH/urea system were investigated. The cellulose films exhibited good mechanical properties either at wet or dry state and their pore size and water permeability at wet state can be controlled by changing the cellulose concentration or coagulation temperature. With a decrease of the coagulation temperature, the mechanical properties and optical transmittance of the cellulose films enhanced, as a result of the formation of relative smaller pore size and denser structures. This work provided a promising way to prepare cellulose films with different pore sizes at wet state and good physical properties at dry state.     

Comparison of nano- and microfibrillated cellulose films

Nanocellulose is an interesting building block for functional materials and has gained considerable interest due to its mechanical robustness, large surface area and biodegradability. It can be formed into various structures such as solids, films and gels such as hydrogels and aerogels and combined with polymers or other materials to form composites. Mechanical, optical and barrier properties of nanofibrillated cellulose (NFC) and microfibrillated cellulose (MFC) films were studied in order to understand their potential for packaging and functional printing applications. Impact of raw material choice and nanocellulose production process on these properties was evaluated. MFC and NFC were produced following two different routes. NFC was produced using a chemical pretreatment followed by a high pressure homogenization, whereas MFC was produced using a mechanical treatment only. TEMPO-mediated oxidation followed by one step of high pressure (2,000 bar) homogenization seems to produce a similar type of NFC from both hardwood and softwood. NFC films showed superior mechanical and optical properties compared with MFC films; however, MFC films demonstrated better barrier properties against oxygen and water vapor. Both the MFC and NFC films were excellent barriers against mineral oil used in ordinary printing inks and dichlorobenzene, a common solvent used in functional printing inks. Barrier properties against vegetable oil were also found to be exceptionally good for both the NFC and MFC films.     

Oxygen and water vapor permeability of fully substituted long chain cellulose esters (LCCE)

Fully-substituted cellulose esters with acyl substituents ranging in size from C2 to C18 were synthesized using the acyl chloride method. Films were prepared from the purified esters by either solvent-casting or compression-molding at elevated temperatures. Oxygen and water vapor permeability was determined under different conditions of pressure and moisture. The relationship between cellulose ester structure and barrier properties was examined. The results revealed linear relationships between water vapor and oxygen permeabilities and molar ester substituent volume as well as several structural factors relating to polymer polarity and hydrophobicity, such as aliphatic (methylene) content, solubility parameter, and contact angle. Films from long chain cellulose esters (LCCE) with acyl substituents in the size range between C8 and C18 were found to represent effective barriers to water vapor transport while their obstruction to the transfer of oxygen remained low. It was concluded that the hydrophobic nature of LCCEs is responsible for the control of water vapor transport, and that spatial factors dominate the transfer of oxygen.     

Influence of the PVOH molar mass on the morphology and functional properties of EVOH/PVOH films prepared by melt blending

The influences of the molar mass (low, medium, and high) and content of poly(vinyl alcohol) (PVOH) dispersed by melt-blending in an ethylene vinyl alcohol (EVOH) copolymer on the morphology, microstructure, thermal, mechanical, and oxygen barrier properties were investigated. Multilayer films with external low-density polyethylene layers and inner EVOH/PVOH blend layer and respective monolayer films were elaborated and characterized. EVOH/PVOH blends exhibited a good compatibility because of the initial presence of PVOH segments in EVOH. The detailed quantitative analysis of the morphology performed for all blends showed that the finest dispersion was obtained with the PVOH with the lowest molar mass. The properties of the films as a function of the PVOH content and its molar mass were determined herein. Significant improvement of barrier properties was obtained at moderated water activities (up to a_w = 0.6) by using the PVOH with the lowest molar mass. Compared to the neat EVOH material, the oxygen permeability coefficients decreased by a factor 2 by adding 15 vol% PVOH while the thermal and mechanical properties remained similar.     

Synthesis,characterization and properties of novel cellulose derivatives containing phosphorus: cellulose diphenyl phosphate and its mixed esters

Using ionic liquid 1-allyl-3-methylimidazolium chloride as reaction medium, a series of novel cellulose esters containing phosphorus including cellulose diphenyl phosphate (C-Dp) and cellulose acetate (CA)–diphenyl phosphate mixed esters was synthesized homogeneously. The degree of substitution was well controlled by altering reaction conditions, such as the molar ratio of the acylating reagents/anhydroglucose unit and reaction time. The structure and thermal properties of cellulose esters were characterized by FTIR, NMR, wide-angle X-ray powder diffraction and differential scanning calorimetry. All the products possessed excellent solubility in some common organic solvents, and transparent films of cellulose esters were obtained by solution casting. In contrast to C-Dp, CA–diphenyl phosphate mixed esters showed clear glass transitions. More interestingly, these cellulose mixed esters exhibited thermoplastic behavior and could be processed by traditional melt processing methods.     

Mechanical,thermal and barrier properties of pectin/cellulose nanocrystal nanocomposite films and their effect on the storability of strawberries (Fragaria ananassa)

In this work, two formulations of pectin/cellulose nanocrystals/glycerol nanocomposites were employed as packaging to extend storage life of strawberries. The effects of incorporating cellulose nanocrystals extracted from bleached Kraft wood pulp on the mechanical, thermal, and barrier properties of pectin‐based nanocomposites were evaluated. Nanocomposite films with different filler levels of cellulose nanocrystals (1, 2, 4 and 8% w/w) were prepared by casting. Compared with the neat film of pectin, improvements in the mechanical properties of the nanocomposites were observed, but these films became fragile. To improve the film flexibility, glycerol was added as a plasticizer and then new variations in the mechanical, thermal, and barrier properties of these nanocomposites were evaluated. The effects of nanocomposite films on storability of strawberries were compared with Poly vinyl chloride packaging films. The Poly vinyl chloride film and the nanocomposites showed similar behavior regarding weight loss by the strawberries, especially in the initial days of storage. The results show that pectin/cellulose nanocrystals/glycerol nanocomposites could be considered as a viable packaging alternative for replaced the Poly vinyl cloride film. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation and Characterization of Modified Cellulose Fiber-Reinforced Polyvinyl Alcohol/Polypyrrolidone Hybrid Film Composites

In this work, cellulose was modified by using 2-(trifluromethyl)benzoylchloride by base catalyzed reaction. Modification of cellulose was confirmed by IR studies. The biodegradable composite films were developed by a film casting method using modified cellulose with poly(vinyl alcohol) and polypyrrolidone in different compositions. Film composites showed good biodegradability. Better barrier and mechanical properties showed by film composites as the percentage of modified cellulose increased. This indicates the importance of modified cellulose as a reinforcing agent. After analyzing these properties of film composites we came to the conclusion that, these biocomposites can be used for membrane and packaging applications.     

纤维素/明胶复合膜的超分子结构与性能

通过一种绿色的方法在NaOH/尿素水体系中制备出纤维素和明胶组成的复合膜(C/G),并且证明这两种大分子间存在强的氢键作用,导致明胶耐水性明显改善.同时,用戊二醛作为交联剂对复合膜化学交联,进一步提高其抗水性.通过红外光谱(FTIR)、紫外光谱(UV-Vis)、13C固体核磁共振谱、扫描电镜(SEM),力学和溶胀测试对...     

Synthesis of degradable crosslinked polymers based on 1,5-dioxepan-2-one and crosslinker of bis-ϵ-caprolactone type

Poly(lactones) may be crosslinked by ring-opening polymerization of the corresponding cyclic esters in the presence of tetrafunctional bis(ϵ-caprolactone). The homopolymer of 1.5-dioxepan-2-one (DXO) has poor mechanical properties but also some very good properties, such as biocompatibility and degradability. Crosslinking of degradable polymer based on the poly(ether-ester) DXO was performed with crosslinkers having the same reactivity as the monomer. 2,2-Bis(ϵ-caprolactone-4-yl)propane (BCP) and bis(ϵ-caprolactone-4-yl) (BCY) with tetrafunctionalities were synthesized from the corresponding diols and then used as comonomers during the polymerization of DXO. The comonomers showed the same reactivity to the initiator, stanneous 2-ethylhexanoic acid, as DXO and perfectly random crosslinked films were obtained. The crosslinked films showed a high degree of swelling already at 2–3 mol % BCP or BCY. The BCP crosslinker was somewhat less soluble in DXO at lower temperatures, but all BCP was soluble at 180°C. These polymeric films were elastic with no crystallinity and the T_g values increased from −39°C for pure DXO to −35°C for BCP crosslinked films and −21°C for BCY crosslinked ones. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1635–1649, 1997     

Solvent impact on esterification and film formation ability of nanofibrillated cellulose

In this study we have manufactured nanofibrillar cellulose and modified the fibre surface with ester groups in order to hydrophobise the surface. Nanofibrillated cellulose was chosen to demonstrate the phenomena, since due to its high surface area the effects at issue are pronounced. The prepared NFC ester derivatives were butyrate, hexanoate, benzoate, naphtoate, diphenyl acetate, stearate and palmitate. X-ray photoelectron spectroscopy, solid state NMR and contact angle measurements were used to demonstrate the chemical changes taking place on the cellulose surface. NFC ester derivatives can be prepared after a careful solvent exchange to a water-free solvent medium has been carried out. Butyl and palmitoyl esters were chosen for film forming tests due to the difference in their carbon chain lengths, and their contact angles and water vapour and oxygen permeation rates were studied. The prepared nanocellulose esters show increased hydrophobicity even at very low levels of substitution and readily form films when the films are prepared from acetone dispersions. The permeation rates suggest a potential use as barrier materials.     

Cationic wood cellulose films with high strength and bacterial anti-adhesive properties

In this work, periodate oxidized birch wood pulp and microfibrillated cellulose (MFC) were cationized using Girard’s reagent T or aminoguanidine. Cationic celluloses were used to obtain films via solvent-casting method, and the effects of the cationization route and the cellulose fiber source on the properties of the films were studied. Thermal and optical properties of the films were measured using differential scanning calorimetry and UV–Vis spectrometry, and the morphology of the films was examined using an optical microscope and a field emission scanning electron microscope. Bacterial anti-adhesive properties of the films were also studied using a modified leaf print method and against Staphylococcus aureus and Escherichia coli. Both cationizing agents exhibited similar reactivity with periodate oxidized celluloses, however, MFC had significantly higher reactivity compared to birch pulp. The films with high tensile strength (39.1–45.3 MPa) and modulus (3.5–7.3 GPa) were obtained from cationized birch pulp, aminoguanidine modification producing a film with slightly better mechanical properties. Modulus of the films was significantly increased (up to 14.0 GPa) when MFC was used as a cellulose fiber source. Compared to the unmodified MFC films, the cationic MFC films were less porous and significantly more transparent; however, they had slightly lower tensile strength values. It was found that aminoguanidine modified celluloses had no culturable bacteria on its surface and also exhibited resistance to microbial degradation, whereas there were culturable bacteria on the surface of Girard’s reagent modified films and they were partially degraded by the bacteria.     

Novel cellulose esters derived from the levopimaric acid-maleic anhydride adduct: synthesis,characterization, and properties

Synthesis of maleated pimaric acid (MPA) cellulose esters is first reported in this work. Cellulose esterification was performed by reacting microcrystalline cellulose with monoacid chloride of MPA (MPA-Cl) in presence of pyridine as catalyst and reaction medium. The syntheses were started in a heterogeneous solid–liquid reaction medium, but as the reaction advanced, the reaction mass turned into a homogeneous solution. The effects of MPA-Cl/anhydroglucose unit molar ratio, reaction temperature, and reaction duration on the yield and degree of substitution (DS) of cellulose esters (CEs) were investigated. CEs with DS ranging from 2.6 to 2.8 were achieved at molar ratios of 5.5–6.0 after 12–16 h at 118 °C. The purified products were characterized by elemental analysis, IR and ¹³C-NMR spectroscopy, and thermogravimetric analysis. CEs are soluble or partially soluble in usual organic solvents, depending on DS. Transparent films were prepared using CE-cyclohexanone solutions.     

Mechanically ground softwood fines as a raw material for cellulosic applications

Utilization of mechanically manufactured lignocellulosic fines (LCNFs) was investigated in making filaments and films. The LCNFs particles were prepared by using a mechanical grinding method with a w-profile grinding stone that produces mostly fines with dimensions in the micrometer scale. The chemical and elemental composition of the w-stone ground LCNFs particles was investigated. It was found that the mechanically manufactured material exhibited the chemical structure of native wood. The LCNFs particles had an anionic surface charge making them colloidally semi-stable in water. The short length of the fines particles prevents their effective mechanical entanglement, which sets some limitations on preparation of filaments and films. Filament manufacturing required the use of a composite approach with carboxymethyl cellulose (CMC) as a binder polymer. The filament was manufactured by using dry-jet wet spinning with aluminium sulfate crosslinking. The chemical composition, crosslinking mechanism, and mechanical properties of the composite filaments were investigated. The composite approach with CMC was also used to prepare composite films with good mechanical performance. The investigated LCNFs material could be utilized in all-lignocomposite applications with cellulose derivatives, where biodegradability and biobased characteristics are desired properties.     

Properties and Applications of Cellulose Acetate

Cellulose acetate is one of the most important esters of cellulose. Depending on the way it has been processed cellulose acetate can be used for great varies of applications (e.g. for films, membranes or fibers). The properties of the applied cellulose acetates are very important for these applications. A special field for using cellulose acetate is the synthesis of porous, spherical particles, so called cellulose beads. Different types of technical cellulose acetates were used and their ability to form such cellulose beads was characterized. First the different types of cellulose acetates were characterized by means of solubility; turbidity and degree of substitution. In addition the molar mass and the distribution of substituents along the polymeric chain were analyzed. Next, the cellulose beads were synthesized within an emulsion process using these different cellulose acetates. Then the properties (particle size, porosity, morphology) of the cellulose beads were determined. Finally, the relationship between the characteristic of cellulose acetates and properties of cellulose beads was investigated.     

甲基丙烯酸异丁酯与甲基丙烯酸-β-羟基酯的乳液共聚合及其应用于显像管的成膜材料

探讨了甲基丙烯酸异丁酯与亲水性甲基丙烯酸酯类的乳液共聚合,所得乳液颗粒很细(0.1μ以下),并具有非常好的放置稳定性,至少10个月未见有明显的凝聚现象。适用于彩色显像管荧光屏的临时涂膜材料。 测定了 PHEMA、PHPMA 及P(IBMA-HPMA)、P(IBMA—HEMA)的热失重曲线。表明,均属于典型的热解聚型高聚物。HPMA、HEMA的含量在20％以下的共聚物适用于有机载膜材料。     

Synthesis of Cellulose Long-Chain Esters in 1-Butyl-3-methylimidazolium Acetate: Structure-Property Relations

In this study, we prepared cellulose long-chain esters homogeneously in 1-butyl-3-methylimidazolium acetate, using cotton linter as the raw material, long-chain fatty acid as the esterification agent and paratoluensulfonyl chloride as the co-reactant. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction were used to characterize the product. The maximum degree of substitution was found to be 1.53 when the molar ratio of cellulose, lauric acid and paratoluensulfonyl chloride was 1 : 6 : 6, provided that the reaction temperature was 60°C and the reaction time was 24 h. The mechanical property of the free-film made of cellulose laurate was also tested. It was found that the toughness of cellulose laurate was much better than that of cellulose acetate.     

Novel cellulose derivatives. IV. Preparation and thermal analysis of waxy esters of cellulose

Cellulose esters with linear aliphatic acyl substituents ranging in size from C₁₂ (lauric acid) to C₂₀ (eicosanoic acid) were prepared in homogeneous solution (DMAc/LiCl) using a novel synthetic method based on the use of a mixed p-toluenesulfonic/carboxylic acid anhydride. The resulting waxy cellulose esters had a high degree of substitution (DS), between 2.8 and 2.9, and showed little degradation. Thermal analysis of these cellulose derivatives by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) revealed a series of transitions that represented motion by both ester substituents and cellulosic main chain. Broad crystallization and melting transitions attributed to side-chain crystallinity were observed in the range between −19 and +55°C; these side-chain T_m and T_c transition temperatures increased by 10°C per carbon atom of the ester substituent. The T_g of these derivatives increased linearly with increasing substituent size from 94°C for C₁₂ (cellulose laurate) to 134°C for C₂₀ (cellulose eicosanoate). Evidence of “main-chain” crystallization was not observed for these samples, except in the case of peracetylated C₁₂ and C₁₄ esters, which had T_m values of 96°C and 107°C, respectively. © 1996 John Wiley & Sons, Inc.     

A novel transesterification system to rapidly synthesize cellulose aliphatic esters

Cellulose aliphatic esters (CEs) are important cellulose derivatives that have been widely used in many fields such as plastics, textiles, membranes, etc. However, in traditional methods, long pretreatment and reaction times limit the manufacture of CEs and their widespread application. Herein, a very efficient method for the preparation of CEs in a heterogeneous system was developed. This method involved the transesterification of cellulose with vinyl esters (from C4 to C14) in dimethylsulfoxide under the catalysis of aqueous NaOH. For better understanding of this new reaction system, factors such as the water content, amount of catalyst, reaction temperature and molar ratio of vinyl acetate to the anhydroglucose unit were explored. Results obtained from FT-IR, ¹H and ¹³C NMR spectroscopies confirmed that CEs could be synthesized at 100 °C within 5 min. High water content or excessive amounts of NaOH were detrimental to the synthesis of CEs. Results from small-angle X-ray diffraction showed that the interplanar spacings of these CEs showed an increasing trend with the length of the aliphatic chain. Thermogravimetric analysis and derivative thermogravimetric analysis showed that CEs had higher thermal stability than cellulose. This work provides a new and highly efficient method to synthesize various CEs.