Research Article pubs.acs.org/journal/ascecg
Green Biocomposites from Nanoengineered Hybrid Natural Fiber and Biopolymer Muhammad Arshad, Manpreet Kaur, and Aman Ullah* Department of Agricultural, Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, University of Alberta, Edmonton, Alberta T6G 2P5, Canada S Supporting Information *
ABSTRACT: The surface grafting of polyhedral oligomeric silsesquioxanes (POSS) nanocages onto keratin biofiber and development of hybrid keratin fiber by dissolution of feather keratin, exfoliation/intercalation of nanoclay in the keratin matrix and regeneration into fiber are reported, respectively. The graft polymerization of POSS on to the surface of keratin fibers was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and confirmed with X-ray photoelectron spectroscopy (XPS). The presence and dispersion of nanoclay, in in situ reinforced and regenerated fiber, was investigated and confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and TEM. The nanomodifications resulted in substantial improvements in the properties of all modified fibers including enhanced thermal stability and reduced moisture uptake compared to unmodified native fibers. The native and modified fibers were further blended with copolymer matrix of 30% styrene with 2-(acryloyloxy) ethyl stearate to prepare the biocomposite films. The properties of the resultant biocomposites were investigated using dynamic mechanical analysis (DMA), flame tests, and SEM. The investigations demonstrated improvements in storage moduli, fiber−matrix adhesion, and reductions in flammability of modified fiber reinforced biocomposites as compared to the neat fiber reinforced biocomposites. KEYWORDS: Keratin fiber, POSS, Nanoclay, Hybrid fibers, Flammability, Biocomposites
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their hydrophilic (moisture absorbing) nature,4 low thermal stability,6 seasonal quality variations, high flammability, high UV degradation,7 and lack of sufficient adhesion between fiber and the polymer matrix,8 resulting in poor properties of composite products. There have been several efforts to reduce moisture sensitivity, increase fiber−matrix compatibility, and thermal stability of plant-based fibers, such as sisal, flax, and hemp, by surface treatments.9−11 Most of the chemical treatments have resulted in individual property improvements. However, collective improvements in thermal stability, moisture resistance, UV radiation stability, and fire-retardancy are extremely important for wider applications of natural fibers. Furthermore, almost all the recent research has been focused on plant-based fibers and limited attention has been paid to the modification and utilization of protein fibers. Currently, the keratin fiber from chicken feathers is recognized as an almost infinite source of natural keratin which can be used to develop high performance fiber materials. Poultry processing plants only in United States generate more than 4 billion pounds of waste feathers each year.12 Feathers are light and tough with over 90%
INTRODUCTION In the last few decades, research interest has been shifting from traditional materials to fiber reinforced polymer-based materials due to their unique advantages of high strength to weight ratio, noncorrosive property, and high fracture toughness.1 Polymerbased composites are important commercial materials with diverse applications including the aerospace, leisure, automotive, construction, and sporting industries. Most of the conventional polymer composites are produced by reinforcing synthetic polymers with synthetic fibers, such as glass, carbon or aramid as fillers.2 Unfortunately, the synthetic fibers used in composite development have numerous serious drawbacks, as they are nonrenewable, nonrecyclable, require high energy consumption in the manufacturing process, pose a health risk when inhaled, and are nonbiodegradable.1 In recent years, several studies on the use of natural fibers as substitute to manmade fibers in the fiber-reinforced composites3−5 have been carried out to reduce our dependency on fossil-fuel resources and alleviate some of the environmental concerns associated with the use of synthetic fibers in composites. Natural fibers offer some advantages over synthetic fibers in terms of low density, high toughness, biodegradability, reduced dermal and reduced respiratory irritation, and low cost.2 However, their potential use, as a reinforcement, is greatly reduced because of © XXXX American Chemical Society
Received: December 28, 2015 Revised: February 2, 2016
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DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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protein.13,14 However, poultry feathers are generally considered as a waste byproduct because their existing uses are economically marginal and they contribute to environmental pollution due to the disposal problems. Keratin fibers from feathers are nonabrasive, eco-friendly, biodegradable, insoluble in organic solvents, of consistent quality, exhibit good mechanical properties, and have the lowest density value compared to the all other natural and synthetic fibers.15 These characteristics make keratin fiber a suitable material to be used as high performance fiber reinforcement in polymer composites. In addition to a few applications as biosorbents,16,17 some composites using native keratin fibers as reinforcement in synthetic polymers have been prepared and characterized. Barone et al.18 prepared and characterized native keratin fiber reinforced polyethylene composites. Authors observed lack of adherence of the polymer to the fibers. Uzun et al.,19 used native chicken fibers and quills to reinforce vinylester and polyester resins. The authors observed decrease in mechanical properties by increase in fiber loading, which was attributed to poor fiber−matrix interactions. Wool and co-workers developed fiber reinforced epoxy composites for printed circuit board application using feather fibers and compared with conventional glass fibers, they observed enhanced flammability of keratin fiber reinforced composites.20 Although feather keratin is great renewable resource, their weaknesses such as hygroscopic nature,21 high flammability,20 lack of adhesion between fiber and matrix,1 and odor release during thermal processing need to be addressed to demonstrate a basis for their use in innovative technologies and high performance composite applications. An emerging approach, to improve properties, can be the nanoengineering of fibers via incorporation of nanostructures. POSS nanocages and nanoclays are emerging as a new feedstock for the preparation of high performance nanostructured materials.22−24 An emerging approach, to improve such properties, is the incorporation of nanostructures. A complete transformation of the material’s properties can be achieved by the proper addition of nanoparticles. POSS molecules, with sizes from 1 to 3 nm in diameter, are odorless, thermally very stable and environmentally friendly.25 Incorporation of POSS can dramatically improve material properties including thermal stability, oxidation resistance, improved mechanical properties, increased moisture resistance, and reductions in flammability.26 Nanoclays are one of the most affordable nanofillers and are wellknown to enhance properties of polymeric materials. Both POSS and nanoclays are well-known in improving barrier properties, flame resistance, thermal stability, reductions in UV transmission and scratch resistance in synthetic polymers provided they are well dispersed in the matrix.27 Herein we report the surface and in situ modifications of chicken feather keratin to develop nanoreinforced hybrid biofiber using nano-POSS and nanoclay, respectively. The nanoreinforced fiber was then used in the preparation of biocomposites. The properties of the developed biofiber and composite materials were investigated in detail. To the best of our knowledge, no report on surface grafting of POSS molecules on keratin fibers is available in literature nor has in situ dispersion of nanoclay and regeneration into a hybrid keratin fiber been described. In addition, these modified fibers were blended with copolymer (30% styrene with 2(acryloyloxy)ethyl stearate) matrix for composite formation to evaluate the effect of POSS molecules and nanoclay on their properties.
Research Article
EXPERIMENTAL SECTION
Materials. Chicken feathers obtained from poultry research center (University of Alberta) were first washed with water, dried, and then the fiber part was cut down with scissor, separated, and further ground using a Fritsch cutting mill (Pulverisette 15, Laval Lab. Inc., Laval, Canada) at a sieve insert size of 0.25 mm. The ground fiber was further washed with hexane solvent several times by using Soxhlet apparatus. Finally it was dried in an oven at 80 °C and used for modifications. Styrene (≥99%, passed through a column of neutral alumina to remove inhibitors before use), 4-(dimethylamino)pyridine (DMAP, ≥99%), dicyclohexylcarbodiimide (DCC, 99%), 2-hydroxyethyl acrylate (96%), sodium chloride (99.9%), anhydrous sodium sulfate (≥99%), dichloromethane (DCM, 99.9%), azobis(isobutyronitrile) (AIBN, 98%, recrystallized in methanol), thin layer chromatographic plates (TLC, silica gel matrix), ethylenediaminetetraacetic acid (≥99%), 2-mercaptoethanol (≥99%), sodium thiosulfate (≥99%), nanoclay modified with 35−45 wt % dimethyl dialkyl (C14−C18) amine (Aldrich-682624), nanoclay-hydrophilic bentonite (Aldrich682659), and n-hexane (≥95%) were purchased from Sigma-Aldrich. Tris(hydroxymethyl)aminomethane (THAM) (≥99.8%), urea (99%) and potassium persulfate (99.6%), stearic acid (reagent grade), and sodium bicarbonate were purchased from Fisher Scientific. Methacryl POSS (MA0735) and acryloisobutyl POSS (MA0701) were obtained from Hybrid Plastics, and copper sulfate pentahydrate was purchased from Acros Organic. Unless otherwise specified, all chemicals were used as received. In Situ Modification of Feather Keratin with Nanoclay. Chicken feathers (4 g) were taken in a round-bottomed flask containing distilled water (100 mL). In this mixture, EDTA (117 mg), THAM (3.229 g), urea (31.8 g), and 2-mercaptoethanol (1.168 mL) were added and stirred at a temperature of 70 °C for 48 h. After the proteins became soluble, the solution was divided into two parts (60 mL each). In each part, 100 mg of modified clay (MC) and 100 mg of unmodified clay (UC) were added separately. These keratin−clay mixtures were separately stirred for 10 min and then sonicated for 20 min. The stirring and sonication process was repeated three times to enhance dispersion of nanoclay and then dialyzed against water for 48 h. The solutions became viscous during dialysis; these viscous solutions were used for the fiber preparation. Fiber Preparation by Wet Spinning. For the fiber preparation, the viscous solution of keratin/nanoclay was taken in a syringe fitted with a needle of 0.7 mm diameter. A gentle pressure was applied on the piston of syringe to spin the solution into fibers in a bath of hexane solvent (details in ESI, page S3). The obtained fibers were given a certain time for aging and dried in the fume hood at room temperature and then in an oven at 100 °C overnight. These fibers were cut and ground into powder form and used for composite formation and characterization. Surface Modification of Native Fibers with Acrylic POSS. The washed and ground fibers (5 g), 100 mL of deionized water, and 5 mL of 1 M HCl solution (to maintain pH ∼ 5−6) were placed in a three neck round-bottomed flask. The mixture was stirred and purged with nitrogen gas for 30 min. Then initiator potassium persulfate (27 mg) and sodium thiosulfate (15 mg) were added into the reaction mixture followed by the addition of POSS (1 g of methacryl POSS and 1 g of acryloisobutyl POSS separately) under inert conditions. The reaction mixture was stirred at a temperature of 80 °C for 6 h. The grafting reaction was quenched by exposing the reaction mixture to the air and by keeping the reaction flask in the cold water. The reaction mixture was filtered and fibers were thoroughly washed with distilled water to remove salts and extracted with hexane to remove the unreacted POSS. The modified fibers were dried in the oven at 80 °C overnight and then characterized. Water Uptake. To investigate the moisture uptake behavior of surface and in situ modified fibers, the samples were first dried in an oven at a temperature of 100 °C until constant weight was achieved. A weighed quantity of all fibers including neat fiber was conditioned at room temperature and placed in a desiccator with 98% relative humidity (maintained by placing a saturated solution of CuSO4·5H2O B
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 1. SEM images of (A) neat fiber, (B) and fiber modified with AIB-POSS and (C) with MA-POSS. overnight). The samples were taken out at specific intervals and weighed on five digit balance to measure the gradual uptake of moisture. The following formula was used to calculate the water uptake (WU) of all samples:
WU (%) = (Wt − Wo)/Wo × 100
storage modulus (MPa) was measured in the temperature range of −40 to +50 °C at a heating rate of 2 °C/min. Flammability Test. The ignitability and flame spread behavior of all biocomposites was evaluated by using the Underwriter Laboratories 94 vertical burning test (UL94 V) with some modifications.27 A laboratory type burner having a supply of methane gas (99.9%) was used. The bar specimens of approximately 125 × 13 × 0.65 mm (length, width, thickness) were hung vertically with the help of clamp. The flame was applied to the bottom of the specimen while the distance between the top of the burner and the bottom edge of the specimen was 10 mm. The flame was applied for 1 s and removed; the time (t) required to burn the whole strips was recorded. Attenuated Total Reflection Fourier Transform Infrared (ATRFTIR) Spectroscopy. ATR-FTIR analysis of the neat and modified fiber samples was done by using Nicolet 8700 spectrometer (Madison, WI, USA). The sample pellets were prepared by mixing the ground fiber with the KBr powder. Spectra were recorded using 64 scans within the frequency range of 4000−500 cm−1. All sample spectra were recorded at 32 scans and 4 cm−1 resolution. X-ray Photoelectron Spectroscopy (XPS). The chemical composition of the keratin fiber surface was studied by XPS. The XPS measurements were conducted on ULTRA spectrometer (Kratos Analytical). The base pressure in the analytical chamber was lower than 3 × 10−8 Pa. Monochromatic Al Kα source (hν = 1486.6 eV) was used at a power of 140 W. The analysis spot was 400 × 700 μm. The resolution of the instrument is 0.55 eV for Ag 3d and 0.70 eV for Au 4f peaks. The survey scans were collected for binding energy spanning from 1000 eV to 0 with analyzer pass energy of 160 eV and a step of 0.4 eV. For the high-resolution spectra the pass-energy was 20 eV with a step of 0.1 eV. Electron flood gun was used to compensate the sample charging. Vision-2 instrument software was applied to process the data. The spectra were calibrated for C 1s binding energy position at 284.8 eV. Compositions were calculated from the high resolution spectra using linear background and sensitivity factors provided by the instrument database. Wide-Angle X-ray Diffraction (WAXD). WAXD measurements were carried out using Rigaku Ultima IV unit operating at 38 mA (current) and 38 kV (voltage), having a D/Tex detector containing iron filter and cobalt tube with wavelength (λ) 1.790 260 Å. Samples were run from 5−45° at 1° 2θ per minute with a step size of 0.02. Finally, the results were displayed in the range of 5−35°, after converting into copper wavelength (λ = 1.540 59 Å).
(1)
Where Wo is the initial weight of the sample and Wt is the weight of sample calculated at specific intervals after exposure to 98% relative humidity. Composite Film Preparation. For the preparation of biocomposite films, 30% fiber was mixed with copolymer matrix of 30% styrene and 2-(acryloyloxy)ethyl stearate (methodology and results are given in ESI, page S2). The copolymer matrix was heated to melt and components were blended efficiently. The mixture was cooled to room temperature and hot pressed in a Carver Press at a temperature of 55 °C and pressure between 1000 and 1500 psi for 15 min. The compression molded films were cut into the required size for flammability tests and mechanical analysis. Instrumentation. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). DSC analysis of nanomodified and neat fiber was performed on calorimetric apparatus (2920 Modulated DSC, TA Instrument, USA) under the stream of nitrogen gas. Pure indium sample was used to calibrate the heat flow and temperature of the instrument. All samples were scanned in a temperature range of 25−270 °C at a heating rate of 5 °C per minute. Thermogravimetric analysis of all composites was performed on TGA Q50 (TA Instrument, USA) under the flow of nitrogen stream. The measurements were carried out by heating the samples in the temperature range of 25−600 °C with heating rate of 10 °C per minute. Transmission Electron Microscopy (TEM). TEM images of nanoclay containing fiber samples were taken on the CM20 FEG TEM/STEM (Philips) instrument operated at 200 kV. An FEI Morgagni 268 instrument operated at 80 kV, equipped with Gatan Orius CCD camera and EDX capability, was used to investigate the surface modified fiber samples. For analysis, the suspension of powdered samples was directly loaded on the copper grid and dried at room temperature before scanning. Scanning Electron Microscopy (SEM). The morphology of the modified fibers and fractured surfaces of biocomposites was observed using scanning electron microscopy (SEM, FEI XL30, USA) operating at 20 kV. Before analysis, the samples were coated with Au/Pd with the help of hummer 6.2 sputter coater by Anatech Ltd. Dynamic Mechanical Analysis (DMA). To investigate the mechanical properties of all biocomposites, DMA Q800 (TA Instrument) was used under the flow of nitrogen gas at a constant frequency of 1 Hz. The rectangular specimens with specific dimensions of length, width, and thickness (13 × 10.1 × 0.75 mm) were used. The
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RESULTS AND DISCUSSION Many functional groups (−SH, −COOH, −OH, and −NH2) are present inside/on the surface of keratin fiber, which is largely composed of amino acids. These functional groups can C
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
fibers show some dark lines, which are actually nano silicate layers. The dark lines are the sections of the silicate layers showing both well dispersed and poorly dispersed regions. TEM micrograph of UCF (Figure 2C) and MCF (Figure 2D) represent mixed intercalated and exfoliated regions. This distribution/dispersion can be attributed to insertion of protein chains into the inter galleries of nanoclay, which on regeneration results to high exfoliation/intercalation of silicate layers. X-ray Photoelectron Spectroscopy (XPS). The elemental composition and functional groups of the neat and POSS grafted fibers were observed by using XPS. The peaks observed in the spectrum at about 532, 399, 282, 154, and 101 eV are due to O 1s, N 1s, C 1s, Si 2s, and Si 2p, respectively.28 Figure 3X showed that the neat fiber is largely composed of carbon,
be activated via radical formation resulting in the grafting of POSS molecules. The nanoparticles can also be dispersed inside the keratin chains by in situ breaking and reformation of different interaction within the keratin chains through wet spinning leading to reinforced hybrid fiber. The modified fibers were characterized by various techniques to access the effect of nanomodifications on material properties. These modified fibers were blended with copolymer matrix and compression molded. The effects of nanomodifications on thermal, mechanical, flame retardancy, and fiber−matrix adhesion were investigated. Scanning Electron Microscopy (SEM). SEM was performed to observe the morphology of neat and POSS grafted fibers. The SEM images of untreated, AIB-POSS and MA-POSS grafted fibers are shown in Figure 1. Remarkable differences of the surface topography between the untreated and modified fibers can be observed. It is quite obvious from Figure 1 that surface of the neat fiber seems to be relatively neat and smooth, whereas the POSS grafted fibers have rough surface with spots ranging from few nanometers to a few hundred nanometers in size. These spots are actually the POSS molecules, which are chemically linked with different functional groups present on the surface of the fiber. To confirm further the grafting and presence of POSS on the surface, the samples were analyzed by TEM and XPS. Transmission Electron Microscopy (TEM). TEM analysis of the prepared samples was carried out to confirm further the POSS grafting on the surface of fiber and also the dispersion of nanoclay during in situ modification of keratin and regeneration into fibers. Interestingly, some dark spots were observed in the TEM images of modified fibers. These darker zones in TEM micrographs actually correspond to the POSS-rich zones due to higher electron density of the silicon atoms present in the POSS molecules.31 The presence of silicon in the dark regions on the surface was further confirmed by EDX (Supporting Information page S5, Figure S3). The TEM images (Figure 2A,B) clearly show the presence of POSS molecules, which have been successfully grafted on the surface of fiber. Whereas the other two images (Figure 2C,D) of the clay-reinforced
Figure 3. XPS survey spectra (X) and high resolution C 1s spectra (Y) of neat and POSS grafted fibers.
oxygen and nitrogen. Whereas the POSS grafted fiber contained additional peaks of silicon with increased ratio of carbon and oxygen contents due to insertion of POSS molecule. This confirms the successful grafting of POSS onto the surface of fiber. The presence of different functional groups onto the surface of neat and POSS grafted fibers were estimated by using high resolution C 1s spectra given in Figure 3Y. The neat fiber
Figure 2. TEM micrographs of POSS grafted fibers (A) MA-POSS, (B) AIB-POSS and in situ nanoclay dispersed and regenerated hybrid fibers, (C) unmodified clay fiber (UCF), and (D) modified clay fiber (MCF). D
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering consists of mainly four peaks with binding energies 284.9, 286.6, 287.9, and 289.0 eV, which are usually assigned to C C/CH, C−N/CO, CO, and N−CO functionalities, respectively.29 On the other hand, with these four peaks, POSS grafted fiber has one additional peak of OCO with binding energy of 289.6 eV, which has been assigned to an ester linkage,30 confirming the grafting of acrylic POSS on the surface of fibers. Differential Scanning Calorimetry (DSC). DSC measurements of all of the modified and neat fibers were carried out by heating the samples from 20 to 260 °C as presented in the Figure 4. The first broad curve of all the fibers in the DSC
Figure 5. TGA thermograms of neat fiber, POSS grafted fibers (MA and AIB POSS), and in situ clay modified fibers (MCF and UCF).
Figure 4. DSC thermograms of neat fiber and fibers reinforced with POSS (MA and AIB POSS) and nanoclay (MCF and UCF).
temperature range of 40−200 °C, of neat fiber is 7.5%, which is lower compared to in situ clay reinforced fibers MCF and UCF with weight loss of 9.3%. This weight loss is attributed to the moisture loss and is high in case of MCF and UCF, which may be due to more interaction of water molecules on the surface as well as inside the fiber during their in situ modification with clay. This weight loss was not observed in case of surface modified fibers with POSS, which implies that the grafting of POSS molecules on the surface of the fiber reduces number of polar group on surface and makes surface more hydrophobic. The starting thermal degradation temperature for neat fiber is 205 °C, whereas for POSS modified fibers (MA-POSS and AIB-POSS) is almost 227 °C as can be observed from the TG and DTG results. The DTG curves of all modified fibers showed 15 to 30 °C higher maximum weight loss temperature as compared to neat fiber. Interestingly, the grafting of POSS nanocages led to much enhanced thermooxidative stability: the maximum weight loss temperature of POSS grafted fiber was delayed by approximately 31 °C with respect to the pure keratin fiber. In addition, the delay in 70% weight loss temperature from 441 °C (neat fiber) to 573 °C (POSS modified fiber) was also observed (Table S1, Supporting Information). It has been reported in literature that the incorporation of POSS generally improves char yield and enhances the properties of oxidation resistance and the flame retardancy of the materials.31 Further details on thermal stability and corresponding weight loss with the increase of temperature are given in the Supporting Information (page S3, Table S1). Moisture Uptake (MU). The water molecules can be adsorbed directly by the polar groups on the surface of the fiber. The moisture uptake behavior of modified and neat fibers
spectrum was assigned to the loss of water molecules attached to polar groups of the fiber surface through hydrogen bonding. The figure shows a relatively sharp peak assigned to the loss of water molecules at lower temperature (68 °C) for neat fiber, whereas broad smaller peaks at higher temperatures of 132 and 151 °C were observed for surface grafted fibers with AIB-POSS and MA-POSS, respectively. The presence of less moisture and delay in moisture loss in the case of surface modified fibers is due to the presence of POSS molecules on the surface of the fiber. The grafting of POSS molecule with the polar groups on the surface of fiber decreased the number of polar groups available for moisture uptake through hydrogen bonding. A sharp transition in the melting range of modified fibers especially clay modified fibers was observed in comparison to neat fiber, which could be attributed to the presence of nanoclay in the keratin biopolymer galleries. In addition, some of the hydrophilic groups of clay interact with hydrophilic groups of keratin and remaining hydrophilic groups may be available for moisture uptake. In the case of keratin modified with POSS molecules, the majority of the hydrophilic groups present on the surface are modified with POSS through grafting, which reduces moisture uptake capacity of surface group and also subsequently shows significant delay in moisture loss, which was originally present in the interior of keratin fiber. This behavior in fact is due to surface covering of fiber with POSS nanocages, which makes loss of internal moisture difficult and, therefore, a broad and delayed peak is observed. Thermogravimetric Analysis (TGA). Degradation behavior of neat and modified fibers was investigated by TGA and DTG (derivative thermogravity) in the temperature range of 30−600 °C as shown in Figure 5. The weight loss, in the E
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering was carried out by placing the samples in a relative humidity (RH) of 98%. The moisture uptake behavior of neat and modified fibers is shown in Figure 6. It can be observed from
Figure 7. X-ray diffraction pattern of original modified (MC) and unmodified (UC) clay and in situ reinforced fiber with modified (MCF) and unmodified (UCF) clay.
fiber (MCF) whereas the peaks at 2θ of 26.6° and 19.8° have been greatly reduced. Similarly in UCF, all of the major crystallinity peaks of unmodified nanoclay have been greatly reduced and/or disappeared. The disappearance or reduction of crystallinity peaks in the modified fibers may be attributed to the homogeneous dispersion of nano silicate layers into keratin biopolymer chains, through exfoliation/intercalation, during in situ modification and regeneration of fibers. FTIR-ATR. Figure 8, shows the ATR (FTIR) spectra of neat fiber and in situ reinforced and regenerated fibers with modified
Figure 6. Plots showing the moisture uptake behavior of neat fiber and fibers grafted with POSS (MA-POSS, AIB POSS) and reinforced with clays (MCF, UCF) during conditioning at RH of 98%.
Figure 6 that the rate of moisture uptake is high in the first few hours. The surface and in situ modified fibers sorbed less moisture compared to neat fiber. The surface grafted fibers with POSS molecules (MA-POSS and AIB-POSS) displayed about 70% less moisture uptake compared to neat fiber, which may be attributed to reduction in the number of hydrophilic groups on the surface due to grafting of POSS molecules, which prevents adsorption/bonding of water molecules on the fiber surface. On the other hand, the presence of nanoclay (nonpolar in nature) in the in situ modified fibers (MCF and UCF) results in the ∼40% less sorption of water molecules than the neat fiber. In addition, the slopes of the sorption curves for clay and POSS treated fibers were lower than the neat fibers even in the initial few hours of the sorption experiment suggesting that sorption rate was also reduced for the treated fibers. Nevertheless, both nanoclay and POSS modified fibers still had about 15 and 30% moisture uptake close to the plateau region suggesting that not all the surface group were fully grafted/covered by the nanoparticles. Altogether all of the modifications lead to moisture resistance and showed less absorption of moisture by modified fibers compared to native fibers. Nevertheless, POSS modified fibers had least moisture absorption compared to both neat and clay modified fibers. The enhanced moisture resistance of POSS modified fibers is due to dual effect of POSS grafting. First, POSS is chemically grafted on the surface through hydrophilic functional groups (−SH, −COOH, −OH, and −NH2) of fibers, which reduces the overall numbers of hydrophilic functional groups on the surface of keratin fiber leading to lower moisture uptake. Second, each POSS molecule has multiple hydrophobic groups on the surface, which further reduces moisture uptake. X-ray Diffraction (XRD) Spectroscopy. X-ray diffraction patterns of native nanoclays, both modified (MC) and unmodified (UC), and in situ nanoclay reinforced fibers are shown in Figure 7. XRD diffractograms of MCF and UCF show lower % crystallinity in comparison to pure nanoclay samples. It can be observed from Figure 7 that the peaks assigned to modified nanoclay32,33 (MC) at 2θ of 27.7°, 24.2°, and 6.8° have been completely disappeared in the nanoclay reinforced
Figure 8. FTIR (ATR) absorbance spectra of neat fiber and in situ modified fibers containing modified clay (MCF) and unmodified clay (UCF).
clay (MCF) and unmodified clay (UCF). The native fiber mainly consists of keratin proteins made up of amino acids. In the FTIR spectrum, the characteristic peaks of secondary structure of proteins can be observed including amide I (CO stretching) and amide II (CN stretching and NH bending modes) in the frequency range of 1620−1680 and 1495−1560 cm−1 respectively.34,35 Nevertheless, the spectra of clay reinforced fibers MCF and UCF contain some additional absorption peaks at 1044, 915, and 521 cm−1 assigned to SiO stretching, AlOH, and SiO bending, respectively. The presence of these characteristics peaks confirms the presence of nanoclay in the fiber. At the higher frequency region, the disappearance of peak at about 3410 cm−1 in the neat fiber and increase in peak intensity at 3293 cm−1 was also observed. This F
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering peak at 3400 cm−1 is generally assigned to stretching vibrations of OH group and NH groups. The disappearance of peak at 3410 cm−1 and increase in peak intensity at 3293 cm−1 indicated that there might be new H-bonding/interaction between OH and NH groups of fibers with nanoclay. Flammability Tests of Hybrid Composites. Figure 9 shows the flame retardancy behavior of biocomposites. It was
leaving behind more char than that of neat fiber composite. Similarly, the flammability behavior of biocomposites prepared by using surface modified fiber with both POSS molecules was also investigated. Both these composites ceased fire after first exposure and these composites only caught fire on longer exposure to flame. Both of these strips burnt completely within ∼85 s and a significant amount of char was observed at the end. It can be concluded that the modification of fibers with POSS/ nanoclay substantially improved their resistance against fire. SEM of Composites. SEM images of the fractured surface of biocomposites prepared by mixing 30% fiber in the polymer matrix are shown in Figure 10. It can be observed from the image of neat fiber composite (Figure 10A) that a significant number of fibers have been pulled out, indicating weaker interaction of fiber with the polymer matrix. Whereas modified clay reinforced fiber (MCF) composite (Figure 10B) and unmodified clay reinforced fiber (UCF) composite (Figure 10C) show strong adhesion with the polymer matrix. Similarly, the composite prepared by using surface modified fibers with MA-POSS and AIB-POSS also displayed a very good bonding with polymer matrix as can be observed in the micrograph shown in Figure 10D,E, respectively. The strong bonding of modified fibers with the polymer matrix can be assumed due to stronger interaction of nonpolar POSS groups with the nonpolar polymer matrix. In case of clay modified hybrid fibers, the addition of clay leads to enhanced interaction with keratin hydrophilic groups and therefore more hydrophobic groups become available to interact with polymer matrix. It means either POSS grafting on the surface of the fiber or its in situ modification with nanoclay is accountable in the improvement of fiber and fiber−matrix composite properties. Overall, both surface and in situ modifications of keratin biofiber improved dispersion of biofiber in the resin matrix and
Figure 9. Flame test images of biocomposites reinforced with 30% of different fibers: neat fiber (Fiber), modified clay reinforced fiber (MCF), unmodified clay reinforced fiber (UCF), methacryl-POSS reinforced fiber (MA-POSS), and acryloisobutyl-POSS reinforced fiber (AIB-POSS).
observed in the flame test that the neat fiber composite quickly caught fire when exposed to flame for 1 s. The flame propagated and burnt the whole specimen within 50 s, leaving small quantity of char. Whereas in situ clay modified fiber-based composites initially caught fire when exposed to flame, but both of these composites ceased fire within a few seconds. However, when modified fiber composites were exposed to fire for longer time, they caught fire and took 80 and 73 s, respectively for modified fiber reinforced composites to burn completely
Figure 10. SEM images of fractured surfaces of biocomposites containing (A) neat fiber, (B) modified clay reinforced fiber (MCF), (C) unmodified clay reinforced fiber (UCF), (D) surface modified fiber with methacryl-POSS (MA-POSS), and (E) acrylisobutyl POSS (AIB-POSS). G
DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering enhanced fiber−matrix interaction leading to improvements in the composite properties compared to neat fibers.
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CONCLUSIONS Through this study, we have demonstrated that keratin fibers can be nanoreinforced by grafting of polyhedral oligomeric silsesquioxanes (POSS) nanocages by exploiting the reactive functional groups (−SH, −NH2, −COOH, −OH) present on the fiber surface. The grafting of POSS molecules on the surface of fiber was observed and investigated with the help of SEM and TEM. The grafting was further confirmed with the help of X-ray photoelectron spectroscopy (XPS). The keratin− clay hybrid fibers were also prepared by complete dissolution, in situ dispersion of nanoclay, and regeneration of reinforced keratin into hybrid fiber. The presence and dispersion of nanoclay in regenerated fiber was confirmed by FTIR, XRD, and TEM analysis. A significant increase in thermal stability and substantial decrease in moisture uptake (MU) was observed in all modified fibers compared to unmodified neat fibers. These surface tailored and regenerated modified keratin fibers were used to make films by blending them with a copolymer matrix to evaluate their storage moduli, fiber matrix adhesion, and effects on flame retardancy. SEM of fractured surfaces of modified fiber-based composites displayed improved adhesion with the copolymer matrix compared to neat fiber. The storage moduli of the biocomposites reinforced with modified fibers are substantially different to the ones obtained by using unmodified fibers as reinforcement. Furthermore, substantial reduction in flammability of modified fiber reinforced composites compared to native fiber reinforced composites was also observed. These improvements in various properties suggest that keratin fibers, being renewable and environment friendly, have great potential to be used as a reinforcement material in the composites to supplement and/or substitute the nonrenewable and nonbiodegradable synthetic fibers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01772. Additional results of monomer (AOES) and copolymer synthesis, EDX spectrum of surface modified fiber, and DMA study of biocomposites and their thermogravimetric analysis (PDF).
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AUTHOR INFORMATION
Corresponding Author
*A. Ullah. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support for current work by The Natural Sciences and Engineering Research Council of Canada (NSERC).
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DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acssuschemeng.5b01772 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
