Mechanical properties of phenol/formaldehyde resin composites reinforced by cellulose microcrystal with different aspect ratio extracted from sisal fiber

Sisal cellulose fiber (SCF) and sisal fiber cellulose microcrystal (SFCM), produced with sulfate pulping method and ball‐milling approach separately, were in‐situ polymerized and dispersed into phenol/formaldehyde (PF) resin, to manufacture SCF/PF and SFCM/PF composites via rolling and molding method and investigate the effect of SCF and SFCM on the impact, flexural, and dynamic mechanical properties of the SCF/PF and SFCM/PF composites. As a result, under the condition of same content, SFCM could preferably enhance these properties maybe resulting from the better dispersion in resin matrix than SCF. In particular, when SFCM content was 7%, the impact strength and equilibrium relaxation modulus of the SFCM/PF composite were increased by 26.5% and 37.7%, while the creep deformation was decreased by 26.5%. In addition, when SFCM content was 5%, the flexural strength, initial storage modulus and glass transition temperature of SFCM/PF composite were increased by 8.5%, 22.6%, and 13°C. Copyright © 2016 John Wiley & Sons, Ltd.     

Resin impregnation of cellulose nanofibril films facilitated by water swelling

Flexible composite films were produced by impregnating aqueous phenol formaldehyde (PF) resin into water-swollen cellulose nanofibril (CNF) films. CNF films were prepared using a pressurized filtration method in combination with freeze drying. The freeze-dried films were swollen with water then impregnated with PF resin by soaking in aqueous resin solutions of varying concentrations. Small amounts of PF slightly enhanced the tensile properties of CNF films. The formulation with the best mechanical properties was CNF/PF films with 8 wt % resin exhibiting tensile stress and toughness of 248 MPa and 26 MJ/m³, respectively. Resin concentrations higher than about 8 % resulted in composites with decreased tensile properties as compared to neat CNF films. The wet strength of the composite films was significantly higher than that of the neat CNF films. The resulting composites showed greater resistance to moisture absorption accompanied by reduced thickness swelling when soaked in water as compared to neat CNF films. The composites also showed decreased oxygen permeability at low humidity compared to neat films, but the composites did not show improved barrier properties at high humidity.     

Press forming of short natural fiber-reinforced biodegradable resin: Effects of fiber volume and length on flexural properties

The effects of the volume fraction and length of natural fibers on flexural properties of biodegradable composites were investigated. Kenaf and bagasse were mixed with corn-starch biodegradable resin, and composite flexural specimens were fabricated by press forming. Cox's model that incorporates the effect of fiber compression in the cross section was applied to calculate the flexural modulus. Bagasse was found to be more compressible than kenaf due to the difference structure in the cross section. For up to 60% volume fraction kenaf and 66% bagasse composites, the flexural modulus increased with increase in fiber volume fraction, and these results were in good agreement with the calculated values. However, above 60% fiber volume fraction kenaf and 66% bagasse, the flexural modulus decreased due to insufficient resin. Moreover, the decrease of the flexural modulus occurred at fiber lengths 2.8 mm kenaf and 3.2 mm bagasse. Above these fiber lengths, there was no change in the flexural modulus of the composite specimens. This trend was also identified by the calculation.     

Effect of Lignin Structure Characteristics on the Performance of Lignin Based Phenol Formaldehyde Adhesives

As the second most abundant biopolymer, lignin remains underutilized in various industrial applications. Various forms of lignin generated from different methods affect its physical and chemical properties to a certain extent. To promote the broader commercial utilization of currently available industrial lignins, lignin sulfonate (SL), kraft lignin (KL), and organosolv lignin (OL) are utilized to partially replace phenol in the synthesis of phenol formaldehyde (PF) adhesives. The impact of lignin production process on the effectiveness of lignin-based phenolic (LPF) adhesives is examined based on the structural analysis of the selected industrial lignin. The results show that OL has more phenolic hydroxyl groups, lower molecular weight, and greater number of reactive sites than the other two types of lignins. The maximum replacement rate of phenol by OL reaches 70% w/w, resulting in organosolv lignin phenolic (OLPF) adhesives with a viscosity of 960 mPa·s, a minimal free formaldehyde content of 0.157%, and a shear strength of 1.84 MPa. It exhibits better performance compared with the other two types of lignin-based adhesives and meets the requirements of national standards.     

Properties of cellulose pulps from acidic and basic processes

Research has intensified in recent years on organic solvent pulping processes to supplement or replace conventional pulping processes. One of the main problems with organosolv pulps is the inferior tear strength compared to kraft pulps. An investigation of the properties of two acidic (acetic acid organosolv and acid sulfite) and one basic white spruce pulp (kraft) was carried out to determine factors affecting differences in tear strength. Properties evaluated were lignin and sugar content, mineral composition, ESCA oxygen-to-carbon ratios, acid-base characteristics, water wettabilities, degree of polymerization and crystallinity of cellulose, fiber length and coarseness, and physical properties of the various pulps. Differences in tear strength have been attributed to degradation and changes in the cellulose structure, the hemicellulose-lignin matrix in which the degree of polymerization of hemicelluloses plays the most important role in low yield pulps, and finally, the bonding capacity of the fiber surfaces.     

Comparison of hydrothermal,hydrotropic and organosolv pretreatment for improving the enzymatic digestibility of bamboo

Pretreatment is the crucial step to disrupt the recalcitrant structure of lignocellulosic biomass for improving the enzymatic hydrolysis efficiency. Typically, hydrothermal, organosolv and hydrotropic pretreatments are environmentally benign and effective methods. In this work, effects of hydrothermal, organosolv and hydrotropic pretreatments on improving enzymatic hydrolysis of bamboo were comprehensively compared. Hydrotropic pretreatment was more effective in removal lignin and xylose from bamboo fiber cell wall. However, the surface coverage by lignin and extractives were dramatically displaced during organosolv pretreatment as investigation by X-ray photoelectron spectroscopy. After pretreatments, the crystallinity of cellulose in pretreated substrates has a significant reduction, and pores were exposed on fiber surface. The residual content of acetyl and phenolic groups in hydrotropic pretreated substrates is lower than organosolv pretreated substrates. In order to deeply assess the delignification of pretreatments, the isolated lignins obtaining from pretreatments process were characterized by Fourier transform infrared spectroscopy also. It was revealed that hydrotropic lignin contained more phenolic hydroxyl group and syringyl units than organosolv lignin. Compared to hydrothermal and organosolv pretreatment, cellulase adsorption capacity of pretreated substrates was notably improved by hydrotropic pretreatment, which indicating the better enzyme accessibility of cellulose. Eventually, the maximum glucose yield was obtained from hydrotropic pretreated substrates.     

The dynamic mechanical behavior of random-in-plane short fiber-reinforced epoxy resin composites

In the present paper, the dynamic mechanical properties of random-in-plane short fiber-reinforced epoxy resin composites were studied by using a rheometrics solids analyzer. The three-point bend testing of the four composites (glass fiber/913 epoxy resin, glass fiber/924 epoxy resin, carbon fiber/913 epoxy resin and carbon fiber/924 epoxy resin) was carried out over temperatures from −100°C to 200°C at a frequency of 10 Hz and strain 0.05%. The composites based on 924 epoxy resin, which has been designed specially for high temperature applications, have less energy loss than the 913 epoxy resinbased composites. For the same resin, the carbon fiber-reinforced composites have less energy loss than the glass fiber-reinforced composites. All the composites have less energy loss than their corresponding matrices; the greater the fiber content, the lower the energy loss. The beta transition of 913 epoxy resin has been shifted to a higher temperature after being reinforced. It was shifted from −50°C to −30°C after being reinforced with glass fiber and made a diffuse shoulder-like peak commencing at −30°C after being reinforced with carbon fiber. The 924 epoxy resin has undergone the same change in beta transition as the 913 resin, though to a smaller extent. The phenomenon suggested that interactions between the macromolecules of the epoxy resins and the molecules along the fiber's surface.     

Effect of carbon fiber surface functionality on the moisture absorption behavior of carbon fiber/epoxy resin composites

Polyacrylonitrile (PAN)‐based carbon fibers were electrochemically oxidized in aqueous ammonium bicarbonate with increasing current density. The electrochemical treatment led to significant changes of surface physical properties and chemical structures. The oxidized fibers showed much cleaner surfaces and increased levels of oxygen functionalities. However, it was found that there was no correlation between surface roughness and the fiber/resin bond strength, i.e. mechanical interlocking did not play a major role in fiber/resin adhesion. Increases in surface chemical functionality resulted in improved fiber/resin bonding and increased interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composites. The relationship between fiber surface functionality and the hydrothermal aging behavior of carbon fiber/epoxy composites was investigated. The existence of free volume resulted from poor wetting of carbon fibers by the epoxy matrix and the interfacial chemical structure were the governing factors in the moisture absorption process of carbon fiber/epoxy composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of gamma irradiation and fiber surface treatment on the properties of bagasse fiber-reinforced waste polypropylene composites

Biocomposites of waste polypropylene (WPP) with bagasse fiber as reinforcing component can be readily prepared based on waste management application. Bagasse was subjected to chemical treatments using sodium hydroxide (NaOH) and vinyl triethoxysilane to modify the fiber properties. Scanning electron microscope and Fourier-transform infrared spectroscopy were used to elucidate fiber modification after treatments. Compounding of WPP at various ratios of bagasse was produced by melt mixing. Treated and untreated fiber composites were investigated under conditions of gamma irradiation at 20?kGy. In general behavior, at different fiber loadings, treated biocomposites have better properties than untreated one and fiber treated with silane is the best. Furthermore, treated biocomposites represented more biodegradability under soil than untreated one. The results of mechanical properties showed that the as-prepared fiber composites have superior irradiation-resistant properties.     

The influence of polypyrrole coatings on the mechanical and friction and wear properties of bamboo fiber filled PA6 composites

The effects of polypyrrole coatings on the tensile and tribological properties of bamboo fiber reinforced polyamide 6 (PA6) composites were studied. Tribological tests were conducted using a block‐on‐ring arrangement. It was observed that the polypyrrole coatings played a main role in the tensile‐resistant and wear‐resistant properties of the PA6 composites. The tensile properties were ruled by the fiber‐matrix adhesion. And the excellent tribological performance of the fillers improved the tribological properties of PA6 composites. The optimum content of polypyrrole coating concentration is 7vol%.     

Lignocellulosic composite

Banana pseudostem fiber which is a lignocellulosic material, relatively inexpensive, and abundantly available was assessed in terms of its fiber‐matrix adhesion and dispersion in composites. Different types of adhesives were used. The mechanical and water absorption properties were investigated. Overall, for the produced composites, the incorporation of sawdust‐urea‐formaldehyde resin into prehydrolyzed banana fiber resulted in the best mechanical properties. Good adhesion‐fiber interaction is believed to be responsible for the good ultimate performance. The superior reinforcing characteristics of sawdust resin were shown by scanning electron microscopy (SEM), which revealed better fiber‐matrix adhesion. Water absorption tests revealed that the presence of the adhesives affected the amount of water absorbed. Copyright © 2004 John Wiley & Sons, Ltd.     

Production of 5-Hydroxymethylfurfural and Furfural from Lignocellulosic Biomass in Water-Tetrahydrofuran Media with Sodium Bisulfate

5-Hydroxymethylfurfural (HMF) and furfural (FF), two bio-based platform chemicals, were produced from various raw lignocellulosic materials (corncob, corn stover, wheat straw, rice straw and sugarcane bagasse) in a water-tetrahydrofuran media by using NaHSO₄ as catalyst. The in fluences of reaction temperature (160-200 oC), reaction time (30-120 min), solvent volume ratio, feedstock concentration (2.4wt%-11.1wt%) and catalyst dosage were studied. The highest HMF and FF yields obtained from corncob were 47mol% and 56mol% under condition of 190 oC, 90 min, 10/1 of THF/H₂O. Besides, the lignin in the raw biomass wasalso depolymerized into organosolv lignin.     

Effect of internal mold release agent on flexural and inter laminar shear properties of carbon and glass fabric reinforced thermoset composites

The study is focused on thermoset composites reinforced with carbon and glass woven fabrics. Two types of thermoset resins, for example, epoxy and vinyl ester were used as the matrix. Varying concentrations of internal mold releasing (IMR) agent was used in the resin. The composites were cured both at room temperature and at 80°C. The flexural properties were studied using 3‐point bending test method. Further theinter‐laminar shear strength (ILSS) was investigated using the short beam shear strength test based on 3‐point bending. The flexural modulus of room temperature cured epoxy resin is higher than that of high temperature cured epoxy resin and cured vinyl ester resin. The flexural modulus is lowest for 1% IMR sample in epoxy system and the modulus for 0% and 2% epoxy are not significantly different. Lowest flexural strength and modulus can be observed for the combination of reinforcement and curing conditions for samples containing 1% IMR for the epoxy systems. Carbon fiber is found to be less compatible with the vinyl ester resin system and the addition of IMR to the resin degraded the properties further. Inter‐laminar shear strength for epoxy‐based composites is not much affected by presence of IMR, but in case of vinyl ester based composites there is a decrease in ILSS on addition of IMR agent. The study explains variation in flexural properties on addition of IMR and change of curing conditions. These results can be used for ascertaining variation in mechanical properties in real use.     

Effect of pre‐treatment of sugarcane bagasse on the cellulose solution and application for the cellulose hydrogel films

Sugarcane bagasse was used as a cellulose resource, and the transparent cellulose hydrogel films were obtained from the purified cellulose by phase inversion process without chemical cross‐linking, when the dissolved cellulose in lithium chloride/N,N‐dimethyl acetamide was transformed into the solid film. On these processes, bagasse was pre‐treated by 10 wt% sodium hydroxide in the absence and presence of bleaching of 10 vol% sodium hypochlorite (NaOCl) solution in order to obtain cellulose fibers. Here, the bleaching temperature was varied from 40 to 50°C. The effect of pre‐treatment conditions on the resultant cellulose solution and hydrogel films was investigated. It was seen that strong bleaching removed most of lignin component from the bagasse. However, viscosity and size exclusion chromatogram of the cellulose indicated that this operation decreased average molecular weight of the cellulose fibers from 2.1 × 10⁶ to 4.8 × 10⁵. These property changes of fibers also caused increase of water content and weakening of mechanical strength of the resultant hydrogels. In addition, scanning probe microscopy in wet state revealed that the porous fiber network structure in the hydrogel was greatly influenced by bleaching with NaOCl. The average pore size of fiber network was decreased from 8.1 to 5.9 nm as the NaOCl treatment was at 50°C, because of expanded fibers in the swollen hydrogel. Copyright © 2016 John Wiley & Sons, Ltd.     

Novel preparation of glass fiber reinforced polytetrafluoroethylene composites for application as structural materials

A novel method was developed to fabricate continuous glass fiber reinforced polytetrafluoroethylene (PTFE/GF) composites which includes the use of conventional sintering and vacuum assisted resin transfer molding (VARTM), successively. The RTM resin (coded as M4506‐1) “fills” the porosity and defects of original PTFE/GF composites prepared by traditional sintering processing, improves the overall interface bonding between the matrix and fibers, and thus significantly improves the mechanical properties such as the flexural and interlaminar shear strength of fiber reinforced PTFE composites. The present work suggests a new way to produce fiber (especially continuous fiber) reinforced PTFE composites with high mechanical properties, and thus make it potentially possible to use PTFE‐based composites as structural materials. Copyright © 2008 John Wiley & Sons, Ltd.     

Cross‐linked waterborne alkyd hybrid resin coatings modified by fluorinated acrylate‐siloxane with high waterproof and anticorrosive performance

Waterborne alkyd resin coatings are ideal for use as corrosion protection coatings because of its high cost‐effective and environmental advantages. However, their uses are restricted to general applications due to their poor acid, water, and alkali resistance. In this work, waterborne alkyd hybrid resins modified with fluorinated acrylate‐siloxane were synthesized via a surfactant‐free miniemulsion polymerization process using maleic anhydride and silicon modified alkyd resin, dodecafluoroheptyl methacrylate, methyl methacrylate, and butyl acrylate as monomers. And then, crosslinking alkyd resin films were prepared at room temperature using trimethylolpropane‐tris‐(β‐N‐azir‐idinyl) propionate (XR‐100) as the crosslinking agent. The acquired films had lower water absorption and higher water contact angles and had better mechanical/thermal properties, as well as good waterproof property. Most importantly, the electrochemical corrosion studies revealed that the cross‐linked coating exhibited superior corrosion resistance performance with an inhibition efficiency of 99.95% and a corrosion rate of 6.95 × 10^?3 mm per year.     

Spruce Bark-Extracted Lignin and Tannin-Based Bioresin-Adhesives: Effect of Curing Temperatures on the Thermal Properties of the Resins

In this study, formaldehyde-free bioresin adhesives were synthesised from lignin and tannin, which were obtained from softwood bark. The extraction was done via organosolv treatment and hot water extraction, respectively. A non-volatile, non-toxic aldehyde, glyoxal, was used as a substitute for formaldehyde in order to modify the chemical structure of both the lignin and tannin. The glyoxal modification reaction was confirmed by ATR–FTIR spectroscopy. Three different resin formulations were prepared using modified lignin along with the modified tannin. The thermal properties of the modified lignin, tannin, and the bioresins were assessed by DSC and TGA. When the bioresins were cured at a high temperature (200 °C) by compression moulding, they exhibited higher thermal stability as well as an enhanced degree of cross-linking compared to the low temperature-cured bioresins. The thermal properties of the resins were strongly affected by the compositions of the resins as well as the curing temperatures.     

Thermal degradation of lignin–phenol–formaldehyde and phenol–formaldehyde resol resins

Resol resins are used in many industrial applications as adhesives and coatings, but few studies have examined their thermal degradation. In this work, the thermal stability and thermal degradation kinetics of phenol–formaldehyde (PF) and lignin–phenol–formaldehyde (LPF) resol resins were studied using thermogravimetric analysis (TG) in air and nitrogen atmospheres in order to understand the steps of degradation and to improve their stabilities in industrial applications. The thermal stability of samples was estimated by measuring the degradation temperature (T _d), which was calculated according to the maximum reaction rate criterion. In addition, the ash content was determined at 800 °C in order to compare the thermal stability of the resol resin samples. The results indicate that 30 wt% ammonium lignin sulfonate (lignin derivative) as filler in the formulation of LPF resin improves the thermal stability in comparison with PF commercial resin. The activation energies of degradation of two resol resins show a difference in dependence on mass loss, which allows these resins to be distinguished. In addition, the structural changes of both resins during thermal degradation were studied by Fourier transform infrared spectroscopy (FTIR), with the results indicating that PF resin collapses at 300 °C whereas the LPF resin collapses at 500 °C.     

Nanoclay modified silica phenolic composites: mechanical properties and thermal response under simulated atmospheric re‐entry conditions

Ablative nanocomposites based on nanoclay‐dispersed addition curable propargylated phenolic novolac (ACPR) resin, reinforced with chopped silica fiber, were investigated for their thermal response behavior under simulated heat flux conditions corresponding to typical atmospheric re‐entry conditions. Organically modified nanoclay (Cloisite 30B) was incorporated to different extents (1–10%) in the ACPR resin matrix containing silica fiber to form the composite. The composites displayed optimum mechanical properties at around 3 wt% of nanoclay loading. The resultant composites were evaluated for their ablative characteristics as well as mechanical, thermal and thermo‐physical properties. The reinforcing effect of nanoclay was established and correlated to the composition. The mechanical properties of the composites and its pyrolysed product improved at moderate nanoclay incorporation. Plasma arc jet studies revealed that front wall temperature is lowered by 20°C and that at backwall by 10–13°C for the 3 wt% nanoclay‐incorporated composites due to impedance by nanoclay for the heat conduction. Nanoclay diminished the coefficient of thermal expansion by almost 50% and also reduced the flammability of the composites. The trend in mechanical properties was correlated to the microstructural morphology of the composites. The nanomodification conferred better strength to the pyrolysed composites. Copyright © 2014 John Wiley & Sons, Ltd.     

热致液晶聚酯共混物与复合材料研究

评述了热致液晶聚酯的共混物与复合材料的研究新进展,揭示了热行为与相容性、流变性与加工性以及形态结构与多种性能之间的内在联系,展望了应用于高强高模纤维与塑料、低热膨光纤涂层及低透气包装膜等高性能材料的前景.