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Influence of hemp fibers with modified surface on polypropylene composites
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JIEC-2865; No. of Pages 10 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Influence of hemp fibers with modified surface on polypropylene composites Denis Mihaela Panaitescu a,*, Cristian Andi Nicolae a, Zina Vuluga a, Catalin Vitelaru b, Catalina Gabriela Sanporean c, Catalin Zaharia d, Dorel Florea a, Gabriel Vasilievici a a
Polymer Department, National Institute of Research and Development in Chemistry and Petrochemistry, 202 Splaiul Independentei, 060021 Bucharest, Romania National Institute for Optoelectronics INOE 2000, 409 Atomistilor St., 077125 Magurele, Romania c Department of Mechanical and Manufacturing Engineering, Aalborg University, Fibigerstræde 16, 9220, Aalborg, Denmark d Advanced Polymer Materials Group, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania b
A R T I C L E I N F O
Article history: Received 9 January 2016 Received in revised form 5 March 2016 Accepted 9 March 2016 Available online xxx Keywords: Hemp fibers Silane Surface treatment Nanoindentation Polypropylene
A B S T R A C T
Hemp fibers (HF) were treated with g-Aminopropyltriethoxysilane, g-Glycidoxypropyltrimethoxysilane, g-Methacryloxypropyltrimethoxysilane (MPS) and potassium permanganate (KP), without alkaline pretreatment, to improve the mechanical properties of polypropylene (PP)/HF composites for automotive parts. MPS and KP treatments were the most efficient in splitting HF bundles and separation of elementary fibers and led to better micro- and nano-mechanical properties in composites. PP modulus-of-elasticity increased with 67% in PP/HF–MPS, with 69% in PP/HF–KP and with only 30% in PP/untreated fibers. KP led to mild oxidation of HF with good effect on mechanical properties and was proposed as cheap and effective treatment of HF, easily applicable industrially. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Natural fibers polymer composites (NFPC) are increasingly used as substitutes for glass fiber composites by car makers. A recent study by Nova Institute of Technology from Germany shows robust growth of wood and natural fiber composite market in the last years and encouraging future trends [1]. Compared to glass fibers, natural fibers are obtained from renewable sources and are recyclable, they have lower density and are cheaper, and they are health friendly providing better working conditions [2,3]. NFPC may contribute not only to high specific stiffness, but also to weight and cost savings when used in car industry, also ensuring a reduction of oil-based material consumption [2–9]. Despite these advantages, the large scale application of NFPC is impeded by: (i) the high water absorption of natural fibers and their strong sensitivity to moisture; (ii) the high thermal sensitivity of the fibers which can be degraded during the melt processing when the polymer matrix needs high melting temperature; and (iii) the poor interfacial adhesion between the highly hydrophilic
* Corresponding author. Tel.: +40 021 3163068. E-mail address: [email protected] (D.M. Panaitescu).
fibers and the hydrophobic polymer matrices [11]. Several physical and chemical treatments of the fibers surface were tried to overcome these drawbacks [3,9–13]. Chemical modification of natural fibers, aiming to improve the adhesion to the hydrophobic matrices, was reviewed by several papers [14–16]. Most of the methods involve silane treatment, esterification, graft copolymerization or the use of compatibilizers. Hemp fibers (HF), one of the most inexpensive and available natural fibers, attracted much interest of both scientists and car makers for polymer reinforcement and fabrication of automotive parts [3,5,11–13,17]. Polypropylene (PP) is, usually, the polymer of choice for the matrix, due to its good properties relative to density, easy processing and its low price. The influence of maleated polypropylene (MAPP) on the morphology and mechanical properties of PP/HF composites was intensively studied [3,5,18,19], but fewer works deal with other treatments in the case of PP modified with HF [13,20]. Chemical modification of HF is important because it increases the hydrophobicity of the fibers by adding functional groups to their surface. Likewise, during the chemical treatment, some of the non-cellulosic components from HF surface, such as waxes and pectins, that hinder the access of reagents to the cellulose, are removed, favoring the chemical modification [13,20]. Alkaline treatment of HF is mostly used. An
http://dx.doi.org/10.1016/j.jiec.2016.03.018 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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increase with almost 50% of Young’s modulus was observed when PP was reinforced with 25 wt% alkali treated HF [5] and an important improvement of tensile strength and modulus when both alkali treated HF and MAPP were added to PP [21]. The surface treatment of HF with silanes has not always lead to significant improvement of mechanical properties in PP/HF composites [22,23]. Alkali treated HF were chemically modified with gmethacryloxypropyltrimethoxysilane, g-aminopropyltriethoxysilane (APS) or g-anilinopropyltrimethoxysilane, and used together with MAPP in the fabrication of PP composites. No significant improvement of Young’s modulus or tensile strength was observed in comparison to the composite containing only alkali treated HF and MAPP [22]. A remarkable increase, with 40%, of the tensile strength of PP/HF composite was obtained when two functionalized organosilanes and a reactive compounding process were simultaneously used [11]. However, complicated or costly approaches could reduce the advantage of low cost of fibers and matrix. In search of a cheap and effective treatment of HF, easily applicable at industrial level, we studied the influence of several silane treatments and of permanganate on the surface properties and thermal stability of hemp fibers. Theoretically, the repeating unit in cellulose structure, anhydroglucose, has many OH groups available to react with different functional groups. However, it has been reported that only the hydroxyl groups of hemicellulose, which is amorphous and some of the OH groups of the amorphous cellulose generally react with the functional groups of the coupling agent or other reagents [24]. The hydroxyl groups of crystalline cellulose, closely packed with hydrogen bonds, are hardly penetrated by reagents and can scarcely participate in these reactions. Many studies have shown that the alkaline pretreatment removes completely hemicellulose and, partially, amorphous cellulose from HF surface [20]. Therefore, the silane and permanganate treatments were applied directly on the surface of HF, without alkaline pre-treatment, trying to preserve more reactive hemicellulose and amorphous cellulose on HF surface. The effect of various treated HF on the thermal and mechanical properties of PP/HF composites, prepared by a melt compounding technique, was also investigated, taking PP/HF composite modified with MAPP as a reference. In previous works, HF were directly treated with g-glycidoxypropyltrimethoxysilane (GPS) [23] and triethoxyvinylsilane [25], but without a curing step. Our study, aiming to investigate the influence of different types of silanes,
with different functionality on the properties of HF and their composites may help to select the best treatment for a foreseen application. Experimental part Raw materials Polypropylene copolymer BJ380MO with a MFI of 80.0 g/ 10 min (230 8C/2.16 kg) and a density of 0.906 g/cm3 was purchased from Borealis AG (Austria). The maleic anhydride grafted polypropylene, Polybond 3200 (Crompton, USA), with a density of 0.91 g/cm3 and a melting point of 157 8C, was used as compatibilizer. Short cut hemp fibers with the length of about 2 mm (Fig. 1) were kindly donated by Euro Master (Italy). gAminopropyltriethoxysilane, g-Glycidoxypropyltrimethoxysilane and g-Methacryloxypropyltrimethoxysilane were purchased from Dow Corning (USA) and potassium permanganate from Sigma-Aldrich and used as received. Fiber surface treatments A solution was prepared from 2 wt% silane (APS, MPS or GPS) in 80/20 ethanol/water mixture and gently stirred at room temperature for 1 h. The pH of the solution was adjusted close to 5 by acetic acid. An amount of 100 g HF were added to 250 mL silane solution containing 2 wt% silane, kept at room temperature for 24 h and mixed from time to time and, then, decanted. A curing step at 120 8C for 1 h was applied to all silane treated fibers. The silanetreated hemp fibers were dried in air for 24 h. HF modified with APS, MPS and GPS were denoted as HF–APS, HF–MPS and HF–GPS, respectively. HF was also treated with 0.05% KP solution in acetone for 30 min. Afterwards, the solution was decanted and the treated fibers (HF–KP) were dried in air for 24 h. Preparation of composites and specimens Before using, unmodified (HF) and surface modified hemp fibers were heated at 80 8C for 3 h to remove the absorbed moisture. Composites from PP and 20, 30 or 40 wt% of different treated hemp fibers were prepared by melt mixing in a Brabender LabStation (Germany), equipped with a 50 cm3 cell at 170 8C. The fibers
Fig. 1. Cut hemp fibers used in the composites and the surface treatments applied to HF.
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were added after PP melting and, than, they were mixed for 8 min, with rotors speed of 75 min1. Plates, 100 100 1 mm, were compression molded from these mixtures using an electrically heated press (Dr. Collin, Germany) at 180 8C, 180 s of preheating at 0.5 MPa and 90 s of compression at 15 MPa. Specimens for mechanical characterization were cut from these plates. Samples were denoted as PP/HF for the composite with untreated fibers and PP/HF–APS, PP/HF–MPS, PP/HF–GPS and PP/HF–KP for the composites containing HF modified with APS, MPS, GPS and KP, respectively. Composites from PP, MAPP and HF were prepared in the same conditions and served as reference. Morphology Olympus BX41 light microscope (Japan) equipped with live view E330 7.5MP Digital SLR Camera and Quick Photo Micro 2.3 software was used for microscopic investigation of fibers. Images were collected in transmission mode. The morphology of PP/HF composites fractured in liquid nitrogen was analyzed by scanning electron microscopy (SEM) using a tabletop SEM TM 3030 (Hitachi, Japan) at an accelerating voltage of 15 kV. Surface morphology of untreated and treated HF was investigated with an AFM MultiMode- 8 atomic force microscope (Bruker— USA) operating in peak force (PF) QNM (Quantitative nanomechanical analysis) mode. This method uses the peak force (maximum interaction force between the tip and the sample) as the feedback signal and can provide quantitative information on mechanical properties of sample surface at nano-scale. AFM measurements were performed at room temperature, with a scan rate of 1 Hz and a scan angle of 90o using a silicon tip with the nominal radius of 8 nm, cantilever length of 225 mm and resonant frequency of 90 kHz. NanoScope version 1.20 software was used for data analysis. Thermal characterization Thermogravimetric analysis (TGA) of fibers and composites was performed on a TA-Q5000 V3.13 (TA Instruments, USA). Duplicate samples weighing between 8 and 10 mg were heated from room temperature to 700 8C at a constant heating rate of 10 8C/min. The purge gas was nitrogen with a flow rate of 40 mL/min. Differential scanning calorimetry (DSC) was performed using a DSC Q2000 from TA Instruments under helium flow (100 mL/min). The samples, weighing between 10 and 12 mg were heated from the room temperature to 200 8C, isothermal for 3 min, then cooled to 50 8C and heated again to 190 oC at a constant heating/cooling rate of 10 oC/min. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm (second melt scan) and the degree of crystallinity (Xc) of PP was calculated as: XC ¼
DH 100 DH0 wPP
(1)
where, DH and DH0 are the enthalpy of fusion of the composite and of 100% crystalline PP, respectively, while wPP is the weight fraction of PP in the composite. DH0 was taken as 207 J/g [26]. The measurement error was 0.5 8C for the temperature and 1% for the heat of fusion. Lamellar thickness was calculated from DSC results using the Gibbs–Thomson equation [26]: T m ¼ T 0 1
2s LDHe
(2)
with the equilibrium melting point of the crystalline lamella of infinite thickness, T0 = 460 K, the equilibrium enthalpy of fusion for the crystalline phase DHe = 184 106 J/m3, and the surface free energy of the basal plane of the crystalline lamella s = 0.0496 J/m2 [27].
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Fourier Transform Infrared Spectroscopy (FTIR) analysis The efficiency of hemp fibers treatment was characterized by attenuated total reflectance (ATR) FTIR using a TENSOR 37 spectrometer from Bruker. The data were collected at room temperature from 4000 to 400 cm1 for 16 scans and a resolution of 4 cm1. The reproducibility was confirmed for each sample by repeating the experiment three times. Mechanical characterization Tensile properties of the composites were determined with a Universal Testing Machine Instron 3382, equipped with Bluehill 2 software. The tests were conducted at room temperature with a crosshead speed of 50 mm/min for the tensile strength measurements and 2 mm/min for Young’s modulus, in accordance with ISO 527. Five specimens were tested for each sample and the mean value was reported. Nanoindentation tests were performed at room temperature using a Hysitron TI Premier Nanoindenter (Hysitron Inc. USA), with Berkovich tip (radius of curvature of 100 nm) and TriboScan software. A loading—unloading function with a maximum peak load of 500 mN was applied, at loading– unloading rate of 100 mN/s. Five indentations were made randomly on each specimen. In all the cases, the penetration depth was 300–400 nm and the distance between indents larger than 7 mm. Hardness and elastic modulus were determined by the Oliver–Pharr method [28].
Results and discussion Characterization of untreated and treated hemp fibers Optical microscopy and AFM imaging The optical microscopy images (Fig. 2) show that the treatments applied to HF modified their size and surface aspect, leading to the fragmentation of the bundles in smaller ones and the separation of elementary fibers on their surface. The intensity of these processes seems to be different for different treatments. Images from Fig. 2 were selected from about 10 images for each type of treatment. Untreated fibers (HF) showed bundles of elementary HF, with the thickness from 100 mm to 200 mm and almost smooth surface, without elementary fibers detached from the bundles. Silane treated fibers showed different degree of fragmentation of bundles in smaller ones. Both the splitting of bundles and the detachment of elementary fibers occurred because of the action of ethanol/ water, higher temperature and mechanical forces, which remove waxes, pectin, some hemicellulose and lignin, that hold cellulose microfibrils together [29,30]. Unlike all these, KP treatment of HF in acetone mostly led to very rough surface, with many elementary short fibers detached from the bundle and less splitting. Both splitting, with subsequent increase of aspect ratio, and higher surface roughness are benefic for fiber–matrix interface, increasing the surface area and the adhesion between PP and HF. AFM topographic and deformation images of treated and untreated HF are shown in Fig. 3. It can be observed that all the treated HF show bundles of nanofibers of different size, most frequently from 150 nm to 300 nm. Nanofibers of less than 50 nm were also observed on the surface of HF–KP and HF–MPS. The average roughness of fibers surface (Ra) was determined from AFM topographic images. Ra is defined as the arithmetic average of the absolute deviations from the mean surface plane. HF–MPS and HF– KP showed higher Ra values, 82 nm and 68 nm, respectively, compared to 53 nm for HF–GPS and 32 nm for HF–APS. The MPS and KP treatments seem to be more efficient in HF fragmentation and release of elementary fibers.
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Fig. 2. Light microscopy images of untreated hemp fibers (HF) and treated hemp fibers: HF–APS, HF–MPS, HF–GPS and HF–KP.
TGA analysis The thermal stability of unmodified and surface modified hemp fibers was studied by TGA (Fig. 4a and b). Thermal decomposition of HF in nitrogen atmosphere is a three-stage process. A first weight loss, due to the removal of adsorbed water, was observed under 120 8C. A broad peak related to pectin and hemicellulose degradation was noticed in the DTG diagram (Fig. 4b) in the range from about 200 8C to 300 8C [30,31]. The bump around 295 8C in the DTG diagram of HF was ascribed to hemicellulose [32]. Shen et al. observed two peaks in the DTG diagram of hardwood hemicellulose, one corresponding to the cracking of xylan chain, close to 295 8C and the other, due to the decomposition of the side units, at about 250 8C [32]. Interestingly, this bump did not appear in the DTG diagram of treated fibers, regardless the treatment, which indicates partial removal of hemicellulose during the treatment. The degradation of cellulose started later than that of hemicellulose, close to 300 8C, since cellulose is mostly crystalline and more energy is necessary for its degradation [32]. The decomposition of lignin is different because it starts from about 150 8C to more than 400 8C and occurs with very low mass loss rate [31]. Considering the small proportion of lignin in HF (below 5%) and its low degradation rate, the detection of a peak corresponding to lignin decomposition in TGA diagrams is unlikely. The most important degradation step of HF, corresponding to the degradation of the main component, cellulose, showed the maximum rate at 335 8C, similar to other observations [30]. The values of Td, the peak in the DTG diagram corresponding to the highest rate of the main degradation process, the temperature at 10% and 20% weight loss (T10% and T20%) and residue at 600 8C (R) were collected in Table 1. The treated fibers showed similar TGA curves with that of untreated fibers in nitrogen atmosphere, but with some differences. The content of water and volatiles (in terms of weight loss at 100 8C) was smaller and the residue was higher for the treated than for untreated fibers (Table 1). Td appeared at different temperature, depending on the applied treatment, slightly higher for APS and MPS treated fibers. It was noted that MPS, APS and GPS treatments have favorable effect on the thermal stability of hemp fibers, increasing T10% and T20% values and KP treatment has small influence, a slight decrease of Td compared to that of unmodified HF being observed in this case.
FTIR analysis The effect of chemical modification of HF surface by silanes and KP was investigated by ATR–FTIR spectroscopy (Fig. 5). Several changes were noted in the FTIR spectra of silanes treated fibers. A new peak was observed at about 1200 cm1 in the spectra of all silanes treated fibers (Fig. 5—inset). This new band, corresponding to ‘‘Si–O–celulose’’ asymmetric stretching mode, was reported for different types of cellulosic fibers modified by silanes [33–35] and this is an indication that silanes have been grafted on HF surface. Another change is related to the broad peak between 3000 and 3700 cm1, coming from inter- and intra-molecular hydrogen bonded OH in cellulose [36]. The decrease of this broad band intensity, observed in all silane treated fibers compared to untreated ones, indicates the decrease of free and bounded OH as a result of the effective binding of silanes to HF. Moreover, the shoulder at 1648 cm1, assigned to absorbed water [37] and visible in the spectra of untreated HF, almost disappeared in silanes treated samples that showed less adsorbed water due to the silane treatment. The most important changes in the FTIR spectra of silane treated HF were observed in the case of HF–MPS: (i) the peak at 1734 cm1, assigned to C5 5O stretching from hemicellulose and lignin, was covered by the higher intensity peak at 1720 cm1, associated to the stretching vibrations of the carbonyl groups of MPS [38]; (ii) new pair peaks at 1452 cm1 and 1321 cm1, assigned to C–O bending and stretching in methacryloxy group, were observed in HF–MPS spectra; (iii) new peak at 1637 cm1, assigned to C5 5C stretching and the new small peaks at 940 cm1 and 816 cm1, characteristic to 5 5CH2 wagging and twisting, respectively, coming from methacryloxy unsaturation, were also observed [38]. These new FTIR bands show that HF–MPS has free methacryloxy groups able to attach to PP chains. It is worth noting that most of the bands characteristic to Si–O–Si and Si–O–C bonds (1030–1200 cm1) were overlapped by C–O and C–O–C stretching vibrations in the fingerprint-region of cellulose [39]. FTIR spectra of HF–KP showed several modifications relative to untreated HF: (i) the decrease of the peak at 1648 cm1, corresponding to the O–H bending of the absorbed water [20]; (ii) the increase of the absorbance at 1737 cm1 (stretching vibration of C5 5O group in ester), which could be related to the oxidation reactions of cellulose due to KP treatment; (iii) the increase of the peak intensity at 1507 cm1, indicating splitting of
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Fig. 3. AFM topographic and deformation images of untreated (a, b) and treated fibers, HF–APS (c, d), HF–MPS (e, f), HF–GPS (g, h) and HF–KP (i, j); scanned area: 5 mm 5 mm.
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Weight loss at 100 8C, %
T10%, 8C
T20%, 8C
Td, 8C
R, %
HF HF–APS HF–MPS HF–GPS HF–KP
6.0 4.4 4.8 4.0 5.5
237.1 241.0 247.6 243.0 237.3
294.5 298.5 300.7 299.5 292.6
335.3 338.0 339.4 334.7 332.0
24.8 27.8 27.7 29.1 27.8
All calculations (including T10% and T20%) were made compared to the initial weight of the sample, before TGA test.
Correlating with TGA results, it can be concluded that KP led to very mild oxidation of HF, evidenced by the slight decrease of the thermal stability of HF–KP. Characterization of PP composites with untreated and treated hemp fibers
Fig. 4. Thermal stability of untreated and treated hemp fibers: TGA curves (a) and DTG curves (b).
the aliphatic side chains in lignin and cross-linking [40]; (iv) the increase of the absorbance at 1241 cm1, ascribed to the C–O–C stretching vibration of the ether groups in lignin [41]. These changes indicate the occurrence of oxidation and/or degradation reactions of cellulose and lignin, following KP treatment.
Micro- and nano-mechanical properties The tensile modulus of elasticity (TM) of PP composites was influenced by both the concentration and the treatment of HF (Fig. 6a and b). PP reinforced with HF–MPS showed more significant increase of modulus compared to the composites containing untreated fibers, when the amount of HF–MPS has grown from 0 wt% to 40 wt%. For example, TM of PP increased with 67% in PP/40 wt% HF–MPS and with only 30% when PP was reinforced with the same amount of untreated fibers (Fig. 6a). Composites with 40 wt% treated fibers showed different increase of TM relative to pure PP, depending on the treatment (Fig. 6b). HF–GPS led to similar increase of PP modulus as untreated fibers while the use of HF–KP and HF–MPS led to a stronger increase of the modulus (with 69% and 67%, respectively). Composites with HF–APS showed intermediate behavior. An increase with almost 90% of TM relative to PP matrix was obtained with MAPP and 40 wt% HF (untreated). The effectiveness of MAPP in improving the adhesion between PP and natural fibers has been reported in many works [6,9]. MAPP can react with the hydroxyl groups on the surface of fibers by its MA moieties and it is compatible with the matrix, thus
Fig. 5. FTIR spectra of untreated (HF) and treated hemp fibers (HF–MPS, HF–GPS, HF–APS and HF–KP); inset—FTIR spectra detail for untreated and treated fibers.
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Fig. 6. (A) Tensile modulus of elasticity of composites with different concentration of untreated fibers (HF) and MPS treated fibers (HF–MPS), (B) Relative modulus of elasticity (tensile modulus of the composite/modulus of PP) for the composites with 40 wt% of different treated HF, (C) Elastic modulus and Hardness (D) resulted from nanoindentation tests.
contributing to good interfacial bonding and enhancement of the mechanical properties [6]. The differences which appeared between silane treatments, i.e., better results for MPS treatment than for APS and GPS ones, could be related to their different adsorption and interaction with HF. After hydrolysis, the reactive silanol monomers or oligomers can be physically adsorbed to hydroxyl groups of HF by hydrogen bonds or they can polymerize to form polysiloxanes [15,42]. The adsorbed silanol is converted into grafted silane under heating, but the new cellulose–O–Si bond is not stable and it can hydrolyze restoring silanol oligomers. Lu et al. obtained lower contact angle for GPS than for APS treated microfibrillated cellulose (648 instead of 908), so lower hydrophobicity indicating poorer interaction between GPS and cellulose [43]. This can explain the low modulus of elasticity of the composite with HF–GPS. Lower modulus can also be obtained when the silane has lubricating effect or the selfcondensation of the silane prevails over grafting. In this last case, silanol oligomers and polysiloxanes, with low melt viscosity and modulus of elasticity [44] could appear on the surface of fibers. The variation of torque values of composites containing the same amount of HF but different applied treatments could emphasize the effect of the treatment on the melt viscosity. Very close values of the torque (after 7 min of processing) were observed for the composites containing untreated HF (with and without MAPP) and HF–MPS, 10.1 Nm, 9.2 Nm and 9.7 Nm, respectively. PP composites with HF–APS, HF–GPS and HF–KP showed lower torque values, 7.8 Nm, 8.0 Nm and 8.2 Nm, respectively, indicating lower melt viscosity. A decrease of the melt viscosity measured by the melt
flow index was also reported for PP composites with silane treated CaCO3 compared to that containing untreated filler [45]. Therefore, the lubricating effect due to the organic moiety or self-condensation products prevailed in the case of composites with HF–GPS and HF–APS, leading to weaker mechanical behavior. Another factor that can influence the mechanical behavior of composites is the increase of the contact area because of the mechanical compounding. Nanoindentation was used to characterize the mechanical properties of composites at nanoscale. The load–depth of penetration curves of composites from Fig. S1, obtained with a force of 500 mN, pointed out the different hardening effect of treated HF, better for KP, MPS and MAPP treatments, intermediate for APS and worse for GPS, similar to macro-level mechanical results. Some difference was observed between the increase of modulus determined by nanoindentation (NM) and TM for the composites with treated fibers compared to that with untreated fibers. For the composites with MAPP and KP treatments, TM increased with 47% and 31%, respectively, and NM with 26% and 37% (Fig. 6C). It should be noted that NM is mostly related to the surface properties of composites, which may differ from the bulk properties. For example, a larger concentration and size of cavities was found closer to the surface than in the bulk for a polycarbonate sample [46]. Likewise, the grafted MA units decrease the regularity of PP structure, leading to lower degree of crystallinity [47], lower strength and modulus compared to PP [48], which can influence the surface properties of the composites compatibilized with MAPP. Moreover, unreacted KP on the fibers surface can be decomposed during the melt processing of
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composites, determining local oxidation of the matrix and releasing manganese dioxide. All these can locally increase the stiffness and hardness of the matrix, since the reinforcing effect of manganesse oxides was already reported [49]. Similar variation was observed for the hardness and NM of composites, as expected (Fig. 6D). Fracture surface characteristics of PP/HF composites Fig. 7 shows the SEM images at the same magnification for better comparison of cryo-fractured surfaces of PP composites
with untreated and treated HF. Holes from pullout fibers and interstices at the fiber/matrix interface were observed in the composite with untreated fibers (Fig. 7a), suggesting poor interfacial adhesion. In the absence of a treatment, only the high melt flow rate of PP used as matrix and the surface roughness of HF may help to cover the fibers with PP. The SEM images of the composites with HF–APS (Fig. 7c) and HF–GPS (Fig. 7e) show fractured fibers but also pullout fibers and gaps at the fiber/matrix interface. This could be caused by imperfect adhesion between APS treated HF and, especially, GPS
Fig. 7. SEM micrographs of fractured surface of PP composites with different treated HF: (a) PP/HF (b) PP/HF/MAPP (c) PP/HF–APS (d) PP/HF–MPS (e) PP/HF–GPS (f) PP/HF–KP.
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treated fibers and PP. When MAPP was used as compatibilizer (Fig. 7b) most of the fibers were broken or fractured, showing the transfer of the stress from matrix to fibers. This suggests better affinity at HF/PP interface due to the compatibilizing action of MAPP. Good adhesion at interface was also observed for the composites with HF–MPS (Fig. 7d) and HF–KP (Fig. 7f). Neither holes nor interstices were detected, only broken fibers, most of them at the base, or fractured fibers, showing that the interface strength was higher than that of the fibers. The SEM observations are in good agreement with the variation of modulus of elasticity. Thermal analysis of composites Weight loss curves from Fig. 8 show slight differences between PP composites with untreated and treated HF (40 wt%), almost no difference being observed up to 270 8C. All the composites have lost less than 10% of their weight up to 230 8C, which is sufficient for their melt processing. It is worth noting that about 6% of this weight loss was released up to 100 8C, representing the adsorbed water. The small differences detected above 270 8C (see detail in Fig. 8) can be correlated with the thermal stability of the fibers (Fig. 8, Table 1): MPS, APS and GPS treatments showed slightly favorable effect on the thermal stability of the composites and KP treatment an opposite effect. No influence was noted for MAPP. DSC cooling curves of PP and its composites are shown in Fig. S2. Both the crystallization temperature and the onset temperature of the composites with MPS and APS silane treated fibers are higher than that of the composite with untreated fibers and similar for the rest of the composites (Table S1). The slight increase of these temperatures for the two composites pointed out the nucleating ability of HF–MPS and HF–APS, leading to faster crystallization of PP. Fig. 9 shows the DSC melting curves (second scan) for the composites with 40 wt% treated HF. An increase of Tm was observed for the composites with silane treated fibers compared to PP with untreated HF (Table 2), higher for PP/HF–MPS, with about 5 8C, intermediate for PP/HF–APS (3 8C) and lower for PP/HF–GPS (1.7 8C), in line with their micro- and nano-mechanical behavior. The increase of Tm means thicker lamellae and this is obvious from the variation of lM (Table 2) for these composites (MPS > APS > GPS). PP/HF–MPS also showed the highest crystallinity between the composites with silane treated fibers, which emphasizes the ability of MPS–HF to improve the orderliness and stability of PP crystallites, in good agreement with the mechanical tests results.
Fig. 9. DSC curves-second scan of PP composites with untreated and treated HF.
Table 2 DSC data-second scan for PP composites with treated and untreated HF. Composites
Tm, 8C
DH, J/g
XC, %
lma, nm
lMa, nm
PP PP/HF PP/HF/MAPP PP/HF–APS PP/HF–GPS PP/HF–MPS PP/HF–KP
163.5 162.0 163.5 165.1 163.7 166.8 162.0
80.3 53.9 49.3 49.0 49.2 50.1 48.1
38.7 43.4 39.7 39.5 39.6 40.8 38.7
6.1 6.3 6.6 6.9 6.4 6.9 6.3
14.4 14.8 16.0 17.5 14.5 19.8 14.8
a lm, lM are the minimum/maximum value of the lamellar thickness calculated using the onset and final melting temperature, respectively.
Although the addition of untreated HF in PP led to a small increase of crystallinity (12%), the fibers treatment, whatever it was, seems to hinder the crystallization process. Similar results were reported for PP reinforced with 40 wt% MAPP treated Opuntia ficus-indica [47] or for a PP/HP composite compatibilized by a PP modified with glycidyl methacrylate [50]. No significant increase of Tm was noted for PP/HF/MAPP compared to PP/HF, which was expected since MAPP decreases both Tm and crystallinity of PP [47]. In the case of PP/HF–KP, the local degradation induced by KP could explain its thermal behavior. Conclusions
Fig. 8. TGA curves of PP composites with 40 wt% untreated and treated fibers (inset—detail of the TGA curves).
In search of a cheap and effective treatment of HF, easily applicable at industrial level, several silane treatments and permanganate were directly applied on HF and their influence on the surface properties and thermal stability of HF was investigated. The chemical treatments led to the splitting of the bundles in smaller ones and separation of elementary fibers, MPS and KP being the most efficient. TGA analysis highlighted the favorable effect of silane treatments on the thermal stability of HF and an opposite effect of KP, which led to the mild oxidation of the fibers, also evidenced by FTIR. The HF treatments influenced the mechanical properties of PP/HF composites: the tensile modulus of elasticity of PP increased with 67% in the composite with 40 wt% HF–MPS, with 69% in that with 40 wt% HF–KP and with only 30% when PP was reinforced with the same amount of untreated fibers. Nanoindentation results show good modulus and hardness for the composites with KP and MPS modified HF, similar to that of the reference containing MAPP as compatibilizer. KP treatment was proposed as a good and cheap treatment of HF leading to better results than some commonly used silanes.
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Acknowledgments This work was supported by the European Community’s Seventh Framework Programme under grant agreement 314744 (EVOLUTION project) and by INOVA-OPTIMA SMIS Project Code 49164 (contract 658/2014). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.jiec.2016.03.018. References [1] Bioplastics MAGAZINE Polymedia Publisher GmbH hhttp://www. bioplasticsmagazine.com/en/news/meldungen/20150621-New-WPC-study-novaInstitute.phpi (accessed 22.06.15). [2] M. Pervaiz, M.M. Sain, Macromol. Mater. Eng. 288 (2003) 553. [3] E. Espino-Perez, S. Domenek, N. Belgacem, C. Sillard, J. Bras, Biomacromolecules 15 (2014) 4551. [4] R.P.O. Santos, B.V.M. Rodrigues, E.C. Ramires, A.C. Ruvolo-Filho, E. Frollini, Ind. Crops Prod. 72 (2015) 69. [5] A. Elkhaoulani, F.Z. Arrakhiz, K. Benmoussa, R. Bouhfid, A. Qaiss, Mater. Des. 49 (2013) 203. [6] K.L. Pickering, M.G. Aruan Efendy, T.M. Le, Composites, Part A 83 (2016) 98–112. [7] D.M. Panaitescu, D.M. Vuluga, H. Paven, M.D. Iorga, M. Ghiurea, I. Matasaru, P. Nechita, Mol. Cryst. Liq. Cryst. 484 (2008) 452. [8] A. Elsabbagh, L. Steuernagel, G. Ziegmann, J. Appl. Polym. Sci. 111 (2009) 2279. [9] D.M. Panaitescu, Z. Vuluga, M. Ghiurea, M. Iorga, C. Nicolae, R. Gabor, Composites: Part B 69 (2015) 286. [10] Z.Q. Yu, S.L. You, H. Baier, Polym. Compos. 33 (2012) 1516. [11] A. Rachini, G. Mougin, S. Delalande, J.Y. Charmeau, C. Barre`s, E. Fleury, Polym. Degrad. Stab. 97 (2012) 1988. [12] M.K. Thakur, R.K. Gupta, V.K. Thakur, Carbohydr. Polym. 111 (2014) 849. [13] M.A. Sawpan, K.L. Pickering, A. Fernyhough, Composites A 42 (2011) 888. [14] V. Kumar Thakur, A. Singh Singha, Surface Modification of Biopolymers, Wiley, New York, 2015. [15] Y. Xie, C.A.S. Hill, Z. Xiao, H. Militz, C. Mai, Composites A 41 (2010) 806. [16] X. Li, L.G. Tabil, S. Panigrahi, J. Polym. Environ. 15 (2007) 25. [17] S. Panthapulakkal, M. Sain, J. Appl. Polym. Sci. 103 (2007) 2432. [18] A. El-Sabbagh, Composites, Part B 57 (2014) 126. [19] M.E. Vallejos, F.X. Espinach, F. Julia´n, L. Torres, F. Vilaseca, P. Mutje´, Compos. Sci. Technol. 72 (2012) 1209.
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JIEC-2865; No. of Pages 10 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Influence of hemp fibers with modified surface on polypropylene composites Denis Mihaela Panaitescu a,*, Cristian Andi Nicolae a, Zina Vuluga a, Catalin Vitelaru b, Catalina Gabriela Sanporean c, Catalin Zaharia d, Dorel Florea a, Gabriel Vasilievici a a
Polymer Department, National Institute of Research and Development in Chemistry and Petrochemistry, 202 Splaiul Independentei, 060021 Bucharest, Romania National Institute for Optoelectronics INOE 2000, 409 Atomistilor St., 077125 Magurele, Romania c Department of Mechanical and Manufacturing Engineering, Aalborg University, Fibigerstræde 16, 9220, Aalborg, Denmark d Advanced Polymer Materials Group, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania b
A R T I C L E I N F O
Article history: Received 9 January 2016 Received in revised form 5 March 2016 Accepted 9 March 2016 Available online xxx Keywords: Hemp fibers Silane Surface treatment Nanoindentation Polypropylene
A B S T R A C T
Hemp fibers (HF) were treated with g-Aminopropyltriethoxysilane, g-Glycidoxypropyltrimethoxysilane, g-Methacryloxypropyltrimethoxysilane (MPS) and potassium permanganate (KP), without alkaline pretreatment, to improve the mechanical properties of polypropylene (PP)/HF composites for automotive parts. MPS and KP treatments were the most efficient in splitting HF bundles and separation of elementary fibers and led to better micro- and nano-mechanical properties in composites. PP modulus-of-elasticity increased with 67% in PP/HF–MPS, with 69% in PP/HF–KP and with only 30% in PP/untreated fibers. KP led to mild oxidation of HF with good effect on mechanical properties and was proposed as cheap and effective treatment of HF, easily applicable industrially. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Natural fibers polymer composites (NFPC) are increasingly used as substitutes for glass fiber composites by car makers. A recent study by Nova Institute of Technology from Germany shows robust growth of wood and natural fiber composite market in the last years and encouraging future trends [1]. Compared to glass fibers, natural fibers are obtained from renewable sources and are recyclable, they have lower density and are cheaper, and they are health friendly providing better working conditions [2,3]. NFPC may contribute not only to high specific stiffness, but also to weight and cost savings when used in car industry, also ensuring a reduction of oil-based material consumption [2–9]. Despite these advantages, the large scale application of NFPC is impeded by: (i) the high water absorption of natural fibers and their strong sensitivity to moisture; (ii) the high thermal sensitivity of the fibers which can be degraded during the melt processing when the polymer matrix needs high melting temperature; and (iii) the poor interfacial adhesion between the highly hydrophilic
* Corresponding author. Tel.: +40 021 3163068. E-mail address: [email protected] (D.M. Panaitescu).
fibers and the hydrophobic polymer matrices [11]. Several physical and chemical treatments of the fibers surface were tried to overcome these drawbacks [3,9–13]. Chemical modification of natural fibers, aiming to improve the adhesion to the hydrophobic matrices, was reviewed by several papers [14–16]. Most of the methods involve silane treatment, esterification, graft copolymerization or the use of compatibilizers. Hemp fibers (HF), one of the most inexpensive and available natural fibers, attracted much interest of both scientists and car makers for polymer reinforcement and fabrication of automotive parts [3,5,11–13,17]. Polypropylene (PP) is, usually, the polymer of choice for the matrix, due to its good properties relative to density, easy processing and its low price. The influence of maleated polypropylene (MAPP) on the morphology and mechanical properties of PP/HF composites was intensively studied [3,5,18,19], but fewer works deal with other treatments in the case of PP modified with HF [13,20]. Chemical modification of HF is important because it increases the hydrophobicity of the fibers by adding functional groups to their surface. Likewise, during the chemical treatment, some of the non-cellulosic components from HF surface, such as waxes and pectins, that hinder the access of reagents to the cellulose, are removed, favoring the chemical modification [13,20]. Alkaline treatment of HF is mostly used. An
http://dx.doi.org/10.1016/j.jiec.2016.03.018 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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increase with almost 50% of Young’s modulus was observed when PP was reinforced with 25 wt% alkali treated HF [5] and an important improvement of tensile strength and modulus when both alkali treated HF and MAPP were added to PP [21]. The surface treatment of HF with silanes has not always lead to significant improvement of mechanical properties in PP/HF composites [22,23]. Alkali treated HF were chemically modified with gmethacryloxypropyltrimethoxysilane, g-aminopropyltriethoxysilane (APS) or g-anilinopropyltrimethoxysilane, and used together with MAPP in the fabrication of PP composites. No significant improvement of Young’s modulus or tensile strength was observed in comparison to the composite containing only alkali treated HF and MAPP [22]. A remarkable increase, with 40%, of the tensile strength of PP/HF composite was obtained when two functionalized organosilanes and a reactive compounding process were simultaneously used [11]. However, complicated or costly approaches could reduce the advantage of low cost of fibers and matrix. In search of a cheap and effective treatment of HF, easily applicable at industrial level, we studied the influence of several silane treatments and of permanganate on the surface properties and thermal stability of hemp fibers. Theoretically, the repeating unit in cellulose structure, anhydroglucose, has many OH groups available to react with different functional groups. However, it has been reported that only the hydroxyl groups of hemicellulose, which is amorphous and some of the OH groups of the amorphous cellulose generally react with the functional groups of the coupling agent or other reagents [24]. The hydroxyl groups of crystalline cellulose, closely packed with hydrogen bonds, are hardly penetrated by reagents and can scarcely participate in these reactions. Many studies have shown that the alkaline pretreatment removes completely hemicellulose and, partially, amorphous cellulose from HF surface [20]. Therefore, the silane and permanganate treatments were applied directly on the surface of HF, without alkaline pre-treatment, trying to preserve more reactive hemicellulose and amorphous cellulose on HF surface. The effect of various treated HF on the thermal and mechanical properties of PP/HF composites, prepared by a melt compounding technique, was also investigated, taking PP/HF composite modified with MAPP as a reference. In previous works, HF were directly treated with g-glycidoxypropyltrimethoxysilane (GPS) [23] and triethoxyvinylsilane [25], but without a curing step. Our study, aiming to investigate the influence of different types of silanes,
with different functionality on the properties of HF and their composites may help to select the best treatment for a foreseen application. Experimental part Raw materials Polypropylene copolymer BJ380MO with a MFI of 80.0 g/ 10 min (230 8C/2.16 kg) and a density of 0.906 g/cm3 was purchased from Borealis AG (Austria). The maleic anhydride grafted polypropylene, Polybond 3200 (Crompton, USA), with a density of 0.91 g/cm3 and a melting point of 157 8C, was used as compatibilizer. Short cut hemp fibers with the length of about 2 mm (Fig. 1) were kindly donated by Euro Master (Italy). gAminopropyltriethoxysilane, g-Glycidoxypropyltrimethoxysilane and g-Methacryloxypropyltrimethoxysilane were purchased from Dow Corning (USA) and potassium permanganate from Sigma-Aldrich and used as received. Fiber surface treatments A solution was prepared from 2 wt% silane (APS, MPS or GPS) in 80/20 ethanol/water mixture and gently stirred at room temperature for 1 h. The pH of the solution was adjusted close to 5 by acetic acid. An amount of 100 g HF were added to 250 mL silane solution containing 2 wt% silane, kept at room temperature for 24 h and mixed from time to time and, then, decanted. A curing step at 120 8C for 1 h was applied to all silane treated fibers. The silanetreated hemp fibers were dried in air for 24 h. HF modified with APS, MPS and GPS were denoted as HF–APS, HF–MPS and HF–GPS, respectively. HF was also treated with 0.05% KP solution in acetone for 30 min. Afterwards, the solution was decanted and the treated fibers (HF–KP) were dried in air for 24 h. Preparation of composites and specimens Before using, unmodified (HF) and surface modified hemp fibers were heated at 80 8C for 3 h to remove the absorbed moisture. Composites from PP and 20, 30 or 40 wt% of different treated hemp fibers were prepared by melt mixing in a Brabender LabStation (Germany), equipped with a 50 cm3 cell at 170 8C. The fibers
Fig. 1. Cut hemp fibers used in the composites and the surface treatments applied to HF.
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were added after PP melting and, than, they were mixed for 8 min, with rotors speed of 75 min1. Plates, 100 100 1 mm, were compression molded from these mixtures using an electrically heated press (Dr. Collin, Germany) at 180 8C, 180 s of preheating at 0.5 MPa and 90 s of compression at 15 MPa. Specimens for mechanical characterization were cut from these plates. Samples were denoted as PP/HF for the composite with untreated fibers and PP/HF–APS, PP/HF–MPS, PP/HF–GPS and PP/HF–KP for the composites containing HF modified with APS, MPS, GPS and KP, respectively. Composites from PP, MAPP and HF were prepared in the same conditions and served as reference. Morphology Olympus BX41 light microscope (Japan) equipped with live view E330 7.5MP Digital SLR Camera and Quick Photo Micro 2.3 software was used for microscopic investigation of fibers. Images were collected in transmission mode. The morphology of PP/HF composites fractured in liquid nitrogen was analyzed by scanning electron microscopy (SEM) using a tabletop SEM TM 3030 (Hitachi, Japan) at an accelerating voltage of 15 kV. Surface morphology of untreated and treated HF was investigated with an AFM MultiMode- 8 atomic force microscope (Bruker— USA) operating in peak force (PF) QNM (Quantitative nanomechanical analysis) mode. This method uses the peak force (maximum interaction force between the tip and the sample) as the feedback signal and can provide quantitative information on mechanical properties of sample surface at nano-scale. AFM measurements were performed at room temperature, with a scan rate of 1 Hz and a scan angle of 90o using a silicon tip with the nominal radius of 8 nm, cantilever length of 225 mm and resonant frequency of 90 kHz. NanoScope version 1.20 software was used for data analysis. Thermal characterization Thermogravimetric analysis (TGA) of fibers and composites was performed on a TA-Q5000 V3.13 (TA Instruments, USA). Duplicate samples weighing between 8 and 10 mg were heated from room temperature to 700 8C at a constant heating rate of 10 8C/min. The purge gas was nitrogen with a flow rate of 40 mL/min. Differential scanning calorimetry (DSC) was performed using a DSC Q2000 from TA Instruments under helium flow (100 mL/min). The samples, weighing between 10 and 12 mg were heated from the room temperature to 200 8C, isothermal for 3 min, then cooled to 50 8C and heated again to 190 oC at a constant heating/cooling rate of 10 oC/min. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm (second melt scan) and the degree of crystallinity (Xc) of PP was calculated as: XC ¼
DH 100 DH0 wPP
(1)
where, DH and DH0 are the enthalpy of fusion of the composite and of 100% crystalline PP, respectively, while wPP is the weight fraction of PP in the composite. DH0 was taken as 207 J/g [26]. The measurement error was 0.5 8C for the temperature and 1% for the heat of fusion. Lamellar thickness was calculated from DSC results using the Gibbs–Thomson equation [26]: T m ¼ T 0 1
2s LDHe
(2)
with the equilibrium melting point of the crystalline lamella of infinite thickness, T0 = 460 K, the equilibrium enthalpy of fusion for the crystalline phase DHe = 184 106 J/m3, and the surface free energy of the basal plane of the crystalline lamella s = 0.0496 J/m2 [27].
3
Fourier Transform Infrared Spectroscopy (FTIR) analysis The efficiency of hemp fibers treatment was characterized by attenuated total reflectance (ATR) FTIR using a TENSOR 37 spectrometer from Bruker. The data were collected at room temperature from 4000 to 400 cm1 for 16 scans and a resolution of 4 cm1. The reproducibility was confirmed for each sample by repeating the experiment three times. Mechanical characterization Tensile properties of the composites were determined with a Universal Testing Machine Instron 3382, equipped with Bluehill 2 software. The tests were conducted at room temperature with a crosshead speed of 50 mm/min for the tensile strength measurements and 2 mm/min for Young’s modulus, in accordance with ISO 527. Five specimens were tested for each sample and the mean value was reported. Nanoindentation tests were performed at room temperature using a Hysitron TI Premier Nanoindenter (Hysitron Inc. USA), with Berkovich tip (radius of curvature of 100 nm) and TriboScan software. A loading—unloading function with a maximum peak load of 500 mN was applied, at loading– unloading rate of 100 mN/s. Five indentations were made randomly on each specimen. In all the cases, the penetration depth was 300–400 nm and the distance between indents larger than 7 mm. Hardness and elastic modulus were determined by the Oliver–Pharr method [28].
Results and discussion Characterization of untreated and treated hemp fibers Optical microscopy and AFM imaging The optical microscopy images (Fig. 2) show that the treatments applied to HF modified their size and surface aspect, leading to the fragmentation of the bundles in smaller ones and the separation of elementary fibers on their surface. The intensity of these processes seems to be different for different treatments. Images from Fig. 2 were selected from about 10 images for each type of treatment. Untreated fibers (HF) showed bundles of elementary HF, with the thickness from 100 mm to 200 mm and almost smooth surface, without elementary fibers detached from the bundles. Silane treated fibers showed different degree of fragmentation of bundles in smaller ones. Both the splitting of bundles and the detachment of elementary fibers occurred because of the action of ethanol/ water, higher temperature and mechanical forces, which remove waxes, pectin, some hemicellulose and lignin, that hold cellulose microfibrils together [29,30]. Unlike all these, KP treatment of HF in acetone mostly led to very rough surface, with many elementary short fibers detached from the bundle and less splitting. Both splitting, with subsequent increase of aspect ratio, and higher surface roughness are benefic for fiber–matrix interface, increasing the surface area and the adhesion between PP and HF. AFM topographic and deformation images of treated and untreated HF are shown in Fig. 3. It can be observed that all the treated HF show bundles of nanofibers of different size, most frequently from 150 nm to 300 nm. Nanofibers of less than 50 nm were also observed on the surface of HF–KP and HF–MPS. The average roughness of fibers surface (Ra) was determined from AFM topographic images. Ra is defined as the arithmetic average of the absolute deviations from the mean surface plane. HF–MPS and HF– KP showed higher Ra values, 82 nm and 68 nm, respectively, compared to 53 nm for HF–GPS and 32 nm for HF–APS. The MPS and KP treatments seem to be more efficient in HF fragmentation and release of elementary fibers.
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Fig. 2. Light microscopy images of untreated hemp fibers (HF) and treated hemp fibers: HF–APS, HF–MPS, HF–GPS and HF–KP.
TGA analysis The thermal stability of unmodified and surface modified hemp fibers was studied by TGA (Fig. 4a and b). Thermal decomposition of HF in nitrogen atmosphere is a three-stage process. A first weight loss, due to the removal of adsorbed water, was observed under 120 8C. A broad peak related to pectin and hemicellulose degradation was noticed in the DTG diagram (Fig. 4b) in the range from about 200 8C to 300 8C [30,31]. The bump around 295 8C in the DTG diagram of HF was ascribed to hemicellulose [32]. Shen et al. observed two peaks in the DTG diagram of hardwood hemicellulose, one corresponding to the cracking of xylan chain, close to 295 8C and the other, due to the decomposition of the side units, at about 250 8C [32]. Interestingly, this bump did not appear in the DTG diagram of treated fibers, regardless the treatment, which indicates partial removal of hemicellulose during the treatment. The degradation of cellulose started later than that of hemicellulose, close to 300 8C, since cellulose is mostly crystalline and more energy is necessary for its degradation [32]. The decomposition of lignin is different because it starts from about 150 8C to more than 400 8C and occurs with very low mass loss rate [31]. Considering the small proportion of lignin in HF (below 5%) and its low degradation rate, the detection of a peak corresponding to lignin decomposition in TGA diagrams is unlikely. The most important degradation step of HF, corresponding to the degradation of the main component, cellulose, showed the maximum rate at 335 8C, similar to other observations [30]. The values of Td, the peak in the DTG diagram corresponding to the highest rate of the main degradation process, the temperature at 10% and 20% weight loss (T10% and T20%) and residue at 600 8C (R) were collected in Table 1. The treated fibers showed similar TGA curves with that of untreated fibers in nitrogen atmosphere, but with some differences. The content of water and volatiles (in terms of weight loss at 100 8C) was smaller and the residue was higher for the treated than for untreated fibers (Table 1). Td appeared at different temperature, depending on the applied treatment, slightly higher for APS and MPS treated fibers. It was noted that MPS, APS and GPS treatments have favorable effect on the thermal stability of hemp fibers, increasing T10% and T20% values and KP treatment has small influence, a slight decrease of Td compared to that of unmodified HF being observed in this case.
FTIR analysis The effect of chemical modification of HF surface by silanes and KP was investigated by ATR–FTIR spectroscopy (Fig. 5). Several changes were noted in the FTIR spectra of silanes treated fibers. A new peak was observed at about 1200 cm1 in the spectra of all silanes treated fibers (Fig. 5—inset). This new band, corresponding to ‘‘Si–O–celulose’’ asymmetric stretching mode, was reported for different types of cellulosic fibers modified by silanes [33–35] and this is an indication that silanes have been grafted on HF surface. Another change is related to the broad peak between 3000 and 3700 cm1, coming from inter- and intra-molecular hydrogen bonded OH in cellulose [36]. The decrease of this broad band intensity, observed in all silane treated fibers compared to untreated ones, indicates the decrease of free and bounded OH as a result of the effective binding of silanes to HF. Moreover, the shoulder at 1648 cm1, assigned to absorbed water [37] and visible in the spectra of untreated HF, almost disappeared in silanes treated samples that showed less adsorbed water due to the silane treatment. The most important changes in the FTIR spectra of silane treated HF were observed in the case of HF–MPS: (i) the peak at 1734 cm1, assigned to C5 5O stretching from hemicellulose and lignin, was covered by the higher intensity peak at 1720 cm1, associated to the stretching vibrations of the carbonyl groups of MPS [38]; (ii) new pair peaks at 1452 cm1 and 1321 cm1, assigned to C–O bending and stretching in methacryloxy group, were observed in HF–MPS spectra; (iii) new peak at 1637 cm1, assigned to C5 5C stretching and the new small peaks at 940 cm1 and 816 cm1, characteristic to 5 5CH2 wagging and twisting, respectively, coming from methacryloxy unsaturation, were also observed [38]. These new FTIR bands show that HF–MPS has free methacryloxy groups able to attach to PP chains. It is worth noting that most of the bands characteristic to Si–O–Si and Si–O–C bonds (1030–1200 cm1) were overlapped by C–O and C–O–C stretching vibrations in the fingerprint-region of cellulose [39]. FTIR spectra of HF–KP showed several modifications relative to untreated HF: (i) the decrease of the peak at 1648 cm1, corresponding to the O–H bending of the absorbed water [20]; (ii) the increase of the absorbance at 1737 cm1 (stretching vibration of C5 5O group in ester), which could be related to the oxidation reactions of cellulose due to KP treatment; (iii) the increase of the peak intensity at 1507 cm1, indicating splitting of
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Fig. 3. AFM topographic and deformation images of untreated (a, b) and treated fibers, HF–APS (c, d), HF–MPS (e, f), HF–GPS (g, h) and HF–KP (i, j); scanned area: 5 mm 5 mm.
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Weight loss at 100 8C, %
T10%, 8C
T20%, 8C
Td, 8C
R, %
HF HF–APS HF–MPS HF–GPS HF–KP
6.0 4.4 4.8 4.0 5.5
237.1 241.0 247.6 243.0 237.3
294.5 298.5 300.7 299.5 292.6
335.3 338.0 339.4 334.7 332.0
24.8 27.8 27.7 29.1 27.8
All calculations (including T10% and T20%) were made compared to the initial weight of the sample, before TGA test.
Correlating with TGA results, it can be concluded that KP led to very mild oxidation of HF, evidenced by the slight decrease of the thermal stability of HF–KP. Characterization of PP composites with untreated and treated hemp fibers
Fig. 4. Thermal stability of untreated and treated hemp fibers: TGA curves (a) and DTG curves (b).
the aliphatic side chains in lignin and cross-linking [40]; (iv) the increase of the absorbance at 1241 cm1, ascribed to the C–O–C stretching vibration of the ether groups in lignin [41]. These changes indicate the occurrence of oxidation and/or degradation reactions of cellulose and lignin, following KP treatment.
Micro- and nano-mechanical properties The tensile modulus of elasticity (TM) of PP composites was influenced by both the concentration and the treatment of HF (Fig. 6a and b). PP reinforced with HF–MPS showed more significant increase of modulus compared to the composites containing untreated fibers, when the amount of HF–MPS has grown from 0 wt% to 40 wt%. For example, TM of PP increased with 67% in PP/40 wt% HF–MPS and with only 30% when PP was reinforced with the same amount of untreated fibers (Fig. 6a). Composites with 40 wt% treated fibers showed different increase of TM relative to pure PP, depending on the treatment (Fig. 6b). HF–GPS led to similar increase of PP modulus as untreated fibers while the use of HF–KP and HF–MPS led to a stronger increase of the modulus (with 69% and 67%, respectively). Composites with HF–APS showed intermediate behavior. An increase with almost 90% of TM relative to PP matrix was obtained with MAPP and 40 wt% HF (untreated). The effectiveness of MAPP in improving the adhesion between PP and natural fibers has been reported in many works [6,9]. MAPP can react with the hydroxyl groups on the surface of fibers by its MA moieties and it is compatible with the matrix, thus
Fig. 5. FTIR spectra of untreated (HF) and treated hemp fibers (HF–MPS, HF–GPS, HF–APS and HF–KP); inset—FTIR spectra detail for untreated and treated fibers.
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Fig. 6. (A) Tensile modulus of elasticity of composites with different concentration of untreated fibers (HF) and MPS treated fibers (HF–MPS), (B) Relative modulus of elasticity (tensile modulus of the composite/modulus of PP) for the composites with 40 wt% of different treated HF, (C) Elastic modulus and Hardness (D) resulted from nanoindentation tests.
contributing to good interfacial bonding and enhancement of the mechanical properties [6]. The differences which appeared between silane treatments, i.e., better results for MPS treatment than for APS and GPS ones, could be related to their different adsorption and interaction with HF. After hydrolysis, the reactive silanol monomers or oligomers can be physically adsorbed to hydroxyl groups of HF by hydrogen bonds or they can polymerize to form polysiloxanes [15,42]. The adsorbed silanol is converted into grafted silane under heating, but the new cellulose–O–Si bond is not stable and it can hydrolyze restoring silanol oligomers. Lu et al. obtained lower contact angle for GPS than for APS treated microfibrillated cellulose (648 instead of 908), so lower hydrophobicity indicating poorer interaction between GPS and cellulose [43]. This can explain the low modulus of elasticity of the composite with HF–GPS. Lower modulus can also be obtained when the silane has lubricating effect or the selfcondensation of the silane prevails over grafting. In this last case, silanol oligomers and polysiloxanes, with low melt viscosity and modulus of elasticity [44] could appear on the surface of fibers. The variation of torque values of composites containing the same amount of HF but different applied treatments could emphasize the effect of the treatment on the melt viscosity. Very close values of the torque (after 7 min of processing) were observed for the composites containing untreated HF (with and without MAPP) and HF–MPS, 10.1 Nm, 9.2 Nm and 9.7 Nm, respectively. PP composites with HF–APS, HF–GPS and HF–KP showed lower torque values, 7.8 Nm, 8.0 Nm and 8.2 Nm, respectively, indicating lower melt viscosity. A decrease of the melt viscosity measured by the melt
flow index was also reported for PP composites with silane treated CaCO3 compared to that containing untreated filler [45]. Therefore, the lubricating effect due to the organic moiety or self-condensation products prevailed in the case of composites with HF–GPS and HF–APS, leading to weaker mechanical behavior. Another factor that can influence the mechanical behavior of composites is the increase of the contact area because of the mechanical compounding. Nanoindentation was used to characterize the mechanical properties of composites at nanoscale. The load–depth of penetration curves of composites from Fig. S1, obtained with a force of 500 mN, pointed out the different hardening effect of treated HF, better for KP, MPS and MAPP treatments, intermediate for APS and worse for GPS, similar to macro-level mechanical results. Some difference was observed between the increase of modulus determined by nanoindentation (NM) and TM for the composites with treated fibers compared to that with untreated fibers. For the composites with MAPP and KP treatments, TM increased with 47% and 31%, respectively, and NM with 26% and 37% (Fig. 6C). It should be noted that NM is mostly related to the surface properties of composites, which may differ from the bulk properties. For example, a larger concentration and size of cavities was found closer to the surface than in the bulk for a polycarbonate sample [46]. Likewise, the grafted MA units decrease the regularity of PP structure, leading to lower degree of crystallinity [47], lower strength and modulus compared to PP [48], which can influence the surface properties of the composites compatibilized with MAPP. Moreover, unreacted KP on the fibers surface can be decomposed during the melt processing of
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composites, determining local oxidation of the matrix and releasing manganese dioxide. All these can locally increase the stiffness and hardness of the matrix, since the reinforcing effect of manganesse oxides was already reported [49]. Similar variation was observed for the hardness and NM of composites, as expected (Fig. 6D). Fracture surface characteristics of PP/HF composites Fig. 7 shows the SEM images at the same magnification for better comparison of cryo-fractured surfaces of PP composites
with untreated and treated HF. Holes from pullout fibers and interstices at the fiber/matrix interface were observed in the composite with untreated fibers (Fig. 7a), suggesting poor interfacial adhesion. In the absence of a treatment, only the high melt flow rate of PP used as matrix and the surface roughness of HF may help to cover the fibers with PP. The SEM images of the composites with HF–APS (Fig. 7c) and HF–GPS (Fig. 7e) show fractured fibers but also pullout fibers and gaps at the fiber/matrix interface. This could be caused by imperfect adhesion between APS treated HF and, especially, GPS
Fig. 7. SEM micrographs of fractured surface of PP composites with different treated HF: (a) PP/HF (b) PP/HF/MAPP (c) PP/HF–APS (d) PP/HF–MPS (e) PP/HF–GPS (f) PP/HF–KP.
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treated fibers and PP. When MAPP was used as compatibilizer (Fig. 7b) most of the fibers were broken or fractured, showing the transfer of the stress from matrix to fibers. This suggests better affinity at HF/PP interface due to the compatibilizing action of MAPP. Good adhesion at interface was also observed for the composites with HF–MPS (Fig. 7d) and HF–KP (Fig. 7f). Neither holes nor interstices were detected, only broken fibers, most of them at the base, or fractured fibers, showing that the interface strength was higher than that of the fibers. The SEM observations are in good agreement with the variation of modulus of elasticity. Thermal analysis of composites Weight loss curves from Fig. 8 show slight differences between PP composites with untreated and treated HF (40 wt%), almost no difference being observed up to 270 8C. All the composites have lost less than 10% of their weight up to 230 8C, which is sufficient for their melt processing. It is worth noting that about 6% of this weight loss was released up to 100 8C, representing the adsorbed water. The small differences detected above 270 8C (see detail in Fig. 8) can be correlated with the thermal stability of the fibers (Fig. 8, Table 1): MPS, APS and GPS treatments showed slightly favorable effect on the thermal stability of the composites and KP treatment an opposite effect. No influence was noted for MAPP. DSC cooling curves of PP and its composites are shown in Fig. S2. Both the crystallization temperature and the onset temperature of the composites with MPS and APS silane treated fibers are higher than that of the composite with untreated fibers and similar for the rest of the composites (Table S1). The slight increase of these temperatures for the two composites pointed out the nucleating ability of HF–MPS and HF–APS, leading to faster crystallization of PP. Fig. 9 shows the DSC melting curves (second scan) for the composites with 40 wt% treated HF. An increase of Tm was observed for the composites with silane treated fibers compared to PP with untreated HF (Table 2), higher for PP/HF–MPS, with about 5 8C, intermediate for PP/HF–APS (3 8C) and lower for PP/HF–GPS (1.7 8C), in line with their micro- and nano-mechanical behavior. The increase of Tm means thicker lamellae and this is obvious from the variation of lM (Table 2) for these composites (MPS > APS > GPS). PP/HF–MPS also showed the highest crystallinity between the composites with silane treated fibers, which emphasizes the ability of MPS–HF to improve the orderliness and stability of PP crystallites, in good agreement with the mechanical tests results.
Fig. 9. DSC curves-second scan of PP composites with untreated and treated HF.
Table 2 DSC data-second scan for PP composites with treated and untreated HF. Composites
Tm, 8C
DH, J/g
XC, %
lma, nm
lMa, nm
PP PP/HF PP/HF/MAPP PP/HF–APS PP/HF–GPS PP/HF–MPS PP/HF–KP
163.5 162.0 163.5 165.1 163.7 166.8 162.0
80.3 53.9 49.3 49.0 49.2 50.1 48.1
38.7 43.4 39.7 39.5 39.6 40.8 38.7
6.1 6.3 6.6 6.9 6.4 6.9 6.3
14.4 14.8 16.0 17.5 14.5 19.8 14.8
a lm, lM are the minimum/maximum value of the lamellar thickness calculated using the onset and final melting temperature, respectively.
Although the addition of untreated HF in PP led to a small increase of crystallinity (12%), the fibers treatment, whatever it was, seems to hinder the crystallization process. Similar results were reported for PP reinforced with 40 wt% MAPP treated Opuntia ficus-indica [47] or for a PP/HP composite compatibilized by a PP modified with glycidyl methacrylate [50]. No significant increase of Tm was noted for PP/HF/MAPP compared to PP/HF, which was expected since MAPP decreases both Tm and crystallinity of PP [47]. In the case of PP/HF–KP, the local degradation induced by KP could explain its thermal behavior. Conclusions
Fig. 8. TGA curves of PP composites with 40 wt% untreated and treated fibers (inset—detail of the TGA curves).
In search of a cheap and effective treatment of HF, easily applicable at industrial level, several silane treatments and permanganate were directly applied on HF and their influence on the surface properties and thermal stability of HF was investigated. The chemical treatments led to the splitting of the bundles in smaller ones and separation of elementary fibers, MPS and KP being the most efficient. TGA analysis highlighted the favorable effect of silane treatments on the thermal stability of HF and an opposite effect of KP, which led to the mild oxidation of the fibers, also evidenced by FTIR. The HF treatments influenced the mechanical properties of PP/HF composites: the tensile modulus of elasticity of PP increased with 67% in the composite with 40 wt% HF–MPS, with 69% in that with 40 wt% HF–KP and with only 30% when PP was reinforced with the same amount of untreated fibers. Nanoindentation results show good modulus and hardness for the composites with KP and MPS modified HF, similar to that of the reference containing MAPP as compatibilizer. KP treatment was proposed as a good and cheap treatment of HF leading to better results than some commonly used silanes.
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