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Hybrid clay functionalized biofibres for composite applications
Composites: Part B 47 (2013) 260–266
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Hybrid clay functionalized biofibres for composite applications Carl Lange a,1, Farid Touaiti b, Pedro Fardim a,⇑ a b
Åbo Akademi University, Laboratory of Fibre and Cellulose Technology, Porthansgatan 3-5, FI-20500 Åbo, Finland Åbo Akademi University, Laboratory of Paper Coating and Converting, Porthansgatan 3-5, FI-20500 Åbo, Finland
a r t i c l e
i n f o
Article history: Received 20 July 2012 Received in revised form 19 September 2012 Accepted 12 October 2012 Available online 21 November 2012 Keywords: A. Hybrid A. Wood B. Fibre/matrix bond E. Injection moulding Layered double hydroxide (LDH)
a b s t r a c t Micro-injection moulded composites of softwood fibres from two different pulping processes in thermoplastic polypropylene matrix have been studied. Surface modification of hydrogen peroxide bleached thermo mechanical pulp fibres (BTMP) from spruce (Picea abies L.) and chemical sulphate pulp fibres (BKraft) from pine (Pinus sylvestris L.) was achieved by co-precipitation of layered double hydroxide (LDH) particles that were further functionalized with sodium dodecyl sulphate (SDS) surfactant in conditions similar to wet end in paper production. Micro-injection moulded test specimens were subjected to uniaxial tension tests, water absorption, micromechanical deformation and microscopic studies. Optical micrographs show that functional pulp was homogeneously dispersed into polypropylene matrix where as the untreated fibres agglomerated during moulding. When composites were prepared with BTMP fibres the electrostatically bound SDS on fibre surface increased elongation at break by 70%. LDH particles intensified dissipation of shear energy to BKraft fibres degrading the average fibre length by 50% during moulding. Functionalization with SDS surfactant, however, increased average fibre length up to 125% in comparison to LDH modification. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Justification for utilizing renewable fibres as reinforcement in composites is often deducted from predicated scarcity of oil reserves and public awareness of environmental problems. These concerns work as an engine to drive the industry towards biobased products and simultaneously create a market for natural fibre based composites. In contrast to glass fibres, natural fibres provide less abrasive, lower density, recyclable, renewable and cost efficient fillers for composites [1]. On the other hand considerable effort is needed to obtain good adhesion and compatibility between a thermoplastic polymer and bio-based fibres [2]. Also, a disadvantage of wood fibres in particular, is their relatively small aspect ratio that affects tensile and impact strength of the final product [3]. Therefore, substantial amount of hemp, jute, flax, kenaf and sisal is used instead as fillers in synthetic polymers [4,5]. Aside from laminates, majority of composites consisting of wood particles, fibres or flour derive from industrial by-products that are processed and extruded with thermoplastic matrix to flooring, furniture and deck board [6]. From economical and environmental point of view the injection moulded automotive parts have become an important sector as well [7]. Although the market for wood par-
⇑ Corresponding author. 1
E-mail addresses: clange@abo.fi (C. Lange), pfardim@abo.fi (P. Fardim). Principal corresponding author.
1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.10.032
ticle composites (WPC) has been steadily rising since 1990s, pulp and paper industry as a producer of WPCs is a rather new concept. The most common coupling agents for natural fibres in thermoplastic applications are maleated co-polymers and silanes [8]. A typical silanization process employs either a vinyl or aminopropyl functionalized silane monomer, especially in polypropylene (PP) and polyethylene (PE) composites, with three methoxy or ethoxy groups able to covalently link with the hydroxyl groups of the fibrous material [9]. Silanes and siloxanes may undergo various hydrolysis and condensation reactions in aqueous environment depending on the pH, temperature, ionic strength of the solution and concentrations of the monomer species. Trace amounts of water may induce condensation reactions between the siloxane groups which is preferred if crosslinks are needed within the matrix and matrix–fibre interface. Crosslinking vinyl alkoxy silanes with PP require peroxide initiators and small amounts of catalyst [10]. It should be noted however, that there is usually a maximum in silane-fibre adsorption isotherm as the condensation can overcompensate the rate of adsorption [11]. Maleated co-polymers are often applied during the extrusion. Another route is to treat fibres with a maleated co-polymer in toluene prior to moulding [12]. The anhydride couples with hydroxyl functional group of fibrous material through esterification. Other surface modification techniques involve mercerization [11], isocyanate functionalities for urethane bond formation [13] as well as physical modification techniques via corona or plasma treatments [14,15]. To the best of our knowledge, research of biofibres involving functionalization
C. Lange et al. / Composites: Part B 47 (2013) 260–266
with inorganic–organic layered double hydroxide (LDH) particles for composite applications is absent in the literature, although nanocomposites from LDH alone can be found [16]. In this article we would like to present results obtained with hydrogen peroxide bleached thermo mechanical pulp (BTMP) from spruce (Picea abies L.) and bleached chemical sulphate pulp (BKraft) from pine (Pinus sylvestris L.) that were functionalized with sodium dodecyl sulphate (SDS) modified LDH particles in process conditions similar to so called wet end in paper line and used as fillers in thermoplastic PP matrix, which is one of the most important industrial plastics [17]. According to ICIS Chemical Business reports (September 5th, 2011) the packing industry holds the highest annual consumption of PP while an increasing trend lays in automotive parts and specialty products. It has relatively low price and offers an advantage gained in reduction of article weight when compared to other plastics. PP has a low intrinsic Gibbs surface free energy and is inert against chemical attacks, which have important consequences when the end products as well as the utilization of various fillers and reinforcing agents within the polymer matrix are in consideration. The applied inorganic–organic LDHs are mixed metal hybrid clays with layered structure that can incorporate organic functional units with anionic charge in between the stacked metal hydroxyl sheets [18,19]. A general formula of a LDH is:
h
3þ M2þ 1x Mx ðOHÞ2
ixþ
Az x=z nH2 O
The formula suggests that a LDH carries two OH groups for each excess positive charge (x) and it can be synthesised in water phase. This is an important factor when the pulp and paper production is considered where implementation of organic solvents is unwanted and reactions are often carried out in alkaline conditions. Pulp fibres contains carboxylic groups that carry anionic charge in alka line suspensions. Also, the divalent M2þ and trivalent M3þ x x metals can be chosen quite freely as long as the ionic radii does not differ too much from each other. The charge balancing anion that carries the function can be a monomer or a polymer. Az x=z The first co-precipitation synthesis of LDH was apparently conducted by Feitknecht in 1942 [20] and by the early 1980s its properties as a heterogeneous catalyst and as a molecular sieve were already reported [21]. Other research areas of LDHs cover controlled drug delivery [22–24], thermal degradation of fibres [25], flame retardant paper and composites [26,27] modified electrodes [28], adsorption [29,30] as well as photo- and biocatalysis and gas-phase conversion [31–33] to mention but a few. It is also possible to substantially decrease wood fibres surface energy via functionalization with surfactant modified LDHs [34]. These fibres should in principle perform better in composite applications. Because the LDHs are known to provide fire retardant properties for composites, it is important to study whether these particles can be applied onto fibres surface to carry both the fire protection as well as the functional organic moiety into composites. If succeeded, the properties of the filler could be tuned separately for each composite application with a simple method. Therefore, besides being a novel approach for surface modification of wood fibres, the aim of our research in this article is also to address the boundary interactions of atactic polypropylene (a-PP) copolymer matrix with hybrid functionalized pulp fibre fillers in micro-injection moulded structures. 2. Materials and methods 2.1. Materials Aluminium nitrate (Al(NO3)39H2O, 98%, Germany) was purchased from Fluka. Magnesium nitrate (Mg(NO3)26H2O, 98–
261
102%, UK), Sodium Dodecyl Sulphate (C12H25NaSO4, 99%, Japan), NaOH (97%, Europe) and isomeric mixture of Xylenes (98.5%, Germany) were purchased from Sigma Aldrich. Bleached thermo mechanical pulp (BTMP) from spruce (P. abies L.) was produced and kindly supplied by UPM-Kymmene (Rauma Mill, Finland) and chemical sulphate pulp from pine (P. sylvestris L.) by Metsä Fibre (Rauma Mill, Finland). All received pulp samples were stored in a freezer and used after defrosting in a cooler at 10 °C. An atactic polypropylene (a-PP) (PP RD204CF, Borealis, Denmark), with density around 900 kg m3 and melt flow rate of 0.8 g min1 (230 °C, 2.16 kg), random copolymer was chosen to serve as the matrix. Small amounts of ethylene and antioxidants that were not specified in the suppliers specifications were present in the matrix. All reagents were used without further purification. 2.2. Methods Fractionation of pulps were performed with a Bauer–McNett fractionator (Lorentzen and Wettres, Kista, Sweden). Fraction from mesh with nominal opening of 1.19 mm was used in composite preparation. Prior to fractionation a Valley Beater (Lorentzen and Wettres, Stockholm, Sweden) was applied to beat the BKraft fibres to Schoppler–Riegler value of 30. The modification of pulp fibres via co-precipitation is explained in elsewhere [34]. In short: the particle precipitation system was set up with two automatic titrators. NaOH solution (2.0 M) was used to keep the pH in the reaction medium close to 10 while the metal salt solution (0.3 M) was dispensed from another titrator. Titration rate was set to 1.0 ml min1. Metal salt concentrations in the reaction medium were calculated from the final volume (1500 ml) and kept at 0.1 M. After modification, pulp fibres were treated in hydrodynamic conditions at 135 °C for 16 h and thoroughly washed at 1% consistency in büchner funnel under reduced pressure. Fibres were assigned according to functionalization: LDH containing fibres (100-0) and LDH containing fibres with 6% (w/w) of SDS surfactant (100-6). Reference fibres (ref) did not contain LDH or SDS but were treated 2 h in alkaline conditions at pH 10. Fibres size distribution and physical parameters were investigated with Kajaani FiberLab analyzer (Metso Automation, Kajaani, Finland) with applicable standard (ISO 16065-2) utilizing FiberLab software version 3.5.3. To remove the polymer matrix the samples were boiled twice in xylenes for 4 h, hot filtrated through sintered glass büchner funnel with pore size number 2, rinsed with xylenes and subsequently dispersed with magnetic stirrer (24 h) into acidic ethanol/water (1:100) solution prior to analysis. DSM Xplore 5.5 ml Injection Moulding Machine (DSM Xplore, Netherlands) was applied to compound the fibres and a-PP to match an ISO standard (ISO 527-2 1BA) specimen with composition of 20% (w/w) of pulp fibres. Temperature in mixing chamber was kept at 190 °C and screw speed at 200 rpm while applied torque was recorded. The mould temperature was set to 40 °C while injection and holding pressures were 4 bars and the processing time 1.0 s. Resulted composite samples were conditioned at 52 ± 2% of relative humidity in 23 ± 1 °C for 60 days. For water absorption studies, three moulded specimens were immersed into distilled water (23 ± 1 °C) in 250 ml. Maximum immersion depth was 10 cm. Sample weights were recorded after removing the surface bound water by drying the composites gently with paper towel and conditioning them in oven at 105 °C for 12 ± 1 s. Also, water desorption kinetics was studied under dynamic air flow at 60 °C. The investigation of the composite mechanical properties was performed with Instron 8872 machine (Instron, England) at ambient temperature. Tensile tests were conducted on ISO standard (ISO 527-2 1BA) samples with constant uniaxial strain rate of
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5.0 mm min1 and 50 mm min1 until rupture. Stress (r) versus strain () curves were used for comparing composite properties like Youngs and resilience moduli, yield and ultimate strengths and elongation at break. Micromechanical deformations were examined from ruptured surfaces with a Leo Gemini 1530 field emission scanning electron microscope equipped with In-Lens detector (LEO Electron Microscopy Ltd., Oberkochen, Germany). All samples were air dried and coated with carbon in Temcarb TB500 sputter coater (Emscope Laboratories, Ashford, UK). Optimum accelerating voltage was 2.70 kV and the working distance 4 mm. Optical microscopy was applied to investigate fibres orientation in moulded specimen. 3. Results and discussions 3.1. Fibre length distribution Results for fibre length distribution after fractionation and modification are presented in Table 1. Column A shows the physical parameters after fractionation while columns B and C presents the results before injection moulding and after their retrieval from polymer matrix respectively. The weight fraction of fibres (W) in composites obtained from extraction with xylenes is also shown. Fractionated BTMP fibres were 500 lm shorter in average length (lav) but 6 lm greater in average diameter (dav) than bleached Kraft pulp fibres (column A). Difference in lav was in major part, due to higher fines content, determined as particulates smaller than 100 lm in size. Average aspect ratio (L) for BKraft pulp fibres was 74 and for BTMP it was 47. Typically, the tracheid cells in coniferous trees have diameters ranging from 20 to 40 lm and the maximum length is approximately 3 mm; thus, the aspect ratio can be close to 100. Fibre dimensions are greatly affected by the pulping process and other treatments such as a washing sequence. Therefore synergic effects on lav values are difficult to address and more stress should be put on the change in aspect ratio (L) after subsequent preparation steps. The steps were divided into modification/functionalization and injection moulding. Reference pulp fibres (column B) were treated in alkaline water for 2 h to simulate conditions of pulp that was modified with LDH particles. For some reason, modification of BTMP with LDH was effectively removing fines from pulp during the washing. It is reasonable to assume that the stacking of LDH particles onto fibre surface did not contribute to dav to any significant degree in Kajaani FiberLab analyzer. Reduction in dav was addressed to peeling and fragmentation of primary (P) and secondary (S1) fibre cell walls. After modification/functionalization step L was approximately 65–70 for BKraft and around 60 for BTMP.
Injection moulding degraded lav effectively (column C). Values for reference pulp were 16 for BKraft (76% reduction) and approximately 10 (84% reduction) for BTMP. The smaller diameter observed in xylene retrieved fibres of both pulps was attributed to complete lumen collapse as well as to partial fragmentation and fibrillation during the moulding process. Interestingly, LDH particles on surface of BKraft induced greater degradation of fibres (L = 8), thus dissipating shear energy. SDS functionalization showed significant improvement in the aspect ratio (L = 19) in comparison to bare LDH. BTMP fibres, on the other hand, were affected by the functionalization with SDS to a lesser degree. The lav increased only 20% in comparison to modified fibres leaving L practically unchanged. Observed differences can be explained by the differences in fibre cell wall structure. Lignin that remains on fibre cell wall of mechanical pulps affects to fibres overall performance by providing rigidity and toughness while making the fibre more brittle. When lignin is removed, fibres flexibility increases. The physical beating that is often applied in chemical pulping process increases flexibility even more through micro-deformations that plasticises the fibre cell wall. When LDH is precipitated on fibre surface the toughness of BTMP might be less affected than it is in flexible BKraft fibres. Agglomeration of fibres in polymer melt causes shear to be dissipated into fibre bundles where the flexibility of individual fibres is inefficient to sustain cell wall structure. This in turn could explain the high reduction in lav of BKraft. After functionalization with surfactant the flexibility of BKraft retains lav as fibres are better dispersed into the polymer matrix.
3.2. Injection moulding The differences in between BTMP and BKraft pulps fibres can be attributed to surface lignin content, the shape and amount in fines as well as fibres cell wall structure. Lignin is highly cross-linked polymer and when associated with fibres it has glass transition temperatures (Tg) ranging from 95 °C to 235 °C depending on the water content [35]. In our work, the water content of fibres prior to injection moulding was less than 2%. As the temperature in the mixing chamber was kept below 200 °C it is possible for the lignin not to reach its Tg. Shear forces may, however, locally increase the fibres surface temperature above the limit of Tg. Softening of lignin should allow fibres to become more durable against shear forces. However, the micro-injection moulding process created a large amount of fines with BTMP fibres. Even if lignin was softened it was not able to afford fibres with plasticity enough to withstand shear similar to flexible BKraft fibres, which practically contains only cellulose.
Table 1 The length weighted averages of fibre lengths (lav), widths (dav), fines content (fines) and aspect ratios (L) after fractionation (A), functionalization (B) and compounding (C) are presented. Fillers used in composites were the reference (ref), the LDH modified (100-0) and the LDH modified fibres with 6% (w/w) of SDS surfactant (100-6). The weight fractions (W) of fibres in composites retrieved by xylene extraction are also presented. Fibres
Parameter
A
B
C
Ref
100-0
100-6
Ref
100-0
100-6
BKraft
lav (mm) dav (lm) fines (%) L W (%)
2.28 ± 0.03 30.8 ± 0.2 0.4 74
2.10 ± 0.03 31.3 ± 0.2 1.0 67
2.06 ± 0.06 30.9 ± 0.6 1.0 67
1.95 ± 0.04 31.0 ± 0.4 1.1 63
0.37 ± 0.01 22. 6 ± 0.3 76.3 16 15
0.19 ± 0.01 23.0 ± 0.5 93.8 8 12
0.43 ± 0.02 22.9 ± 0.3 69.6 19 14
BTMP
lav (mm) dav (lm) fines (%) L W (%)
1.74 ± 0.03 36.8 ± 0.3 4.0 47
1.63 ± 0.07 28.7 ± 0.7 2.0 57
2.06 ± 0.01 32.9 ± 0.2 0.3 63
1.69 ± 0.02 29.4 ± 0.3 1.9 57
0.25 ± 0.01 27. 7 ± 0.4 91.0 9 17
0.22 ± 0.01 24.8 ± 0.3 93.1 9 17
0.27 ± 0.01 32.5 ± 0.2 87.6 8 12
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3.3. Water adsorption Water adsorption was followed until reasonable level of quasistatic kinetics (1600 h) and evaluated with generalized Langmuir’s adsorption isotherm (Eq. (1)) where S is the adsorbed amount, Smax is the maximum adsorption, b is the binding energy term and x the function against which the adsorption is measured. Polypropylene matrix adsorbed negligible amount of water (<0.1%).
S ¼ Smax
ðbxÞn 1 þ ðbxÞn
m=n ð1Þ
14 12 10
Ψweight (%)
For density measurements, the composites were weighted individually after equilibration at 40 °C for 10 days. The volume was determined by choosing 10–12 samples to be immersed in water (23 ± 1 °C). BKraft, BTMP and a-PP were tested separately. Density of PP can be as low as 0.850 g cm3, but it increases rapidly as a function of the molecular weight [36]. The density of injection moulded a-PP matrix polymer was determined to be (0.91 ± 0.01) g cm3, while, regardless of the sample treatment or applied fibres, for composite specimen it was (1.01 ± 0.01) g cm3. According to Whiting et al. the average density for lignin is approximately 1.4 g cm3 and for polysaccharides it is 1.5 g cm3 [37]. Sixta uses similar values except for carbohydrates (1.58 g cm3) [38]. Bleached chemical pulp contains only fractions of lignin, while high yield pulps are similar to native wood. When lumen collapse occur the density of fibres should be equal to the cell wall structure. Therefore, BKraft pulp should have a density close to 1.5–1.58 g cm3 and BTMP from spruce, containing 28% of lignin, approximately 1.47–1.53 g cm3 depending on the average density used for carbohydrates. By using standard specimen volume (1.0 cm3 for ISO 527-2 1BA), the apparent fraction of the applied fibres was calculated to be 0.15–0.17 for BKraft and 0.16–0.18 for BTMP. These values deviate from the aimed 0.20 but correlated reasonably well with the xylene extracted fractions (Table 1). It is believed that air, which was introduced into the mixing chamber with pulp and carried into the sample specimen, precluded complete filling of mould with composite melt. The applied torque was followed at 200 rpm screw speed during the moulding process (Table 2). Required force increased 15% when native pulp or LDH modified pulp was introduced into the chamber with a-PP. Surprisingly torque increased up to 30% when a-PP was mixed with functional BKraft fibres. The surfactant was expected to decrease the viscosity as observed with functionalized BTMP fibres. Therefore, as functionalization of BKraft fibres was able to increase lav, the dynamic viscosity of the polymer melt with longer fibres and LDH particles increased in comparison to melt flow of shorter fibres.
8 6 4 2 0 Reference
LDH-100
LDH-100-6
Fig. 1. The relative mass increase (Wweight) of the composites with different filler compositions at dynamic equilibrium in water according to Langmuir adsorption isotherm are presented. Filler compositions were the same as in Table 1. The results with thermomechanical pulp (BTMP) are shown with lighter shade and the chemical pulp (BKraft) with darker shade.
Eq. (1) reduces to Langmuir’s adsorption isotherm when m = n = 1. In our experiment the hydrostatic pressure, temperature and activity of adsorbing species were kept constant while the change in total mass (Dw) of adsorbent was followed with respect to time (t). Correlation between the data and fitted Langmuir’s isotherms was greater than 99.9% in all samples. Fickian diffusion predicts that an average mass flux of molecules at time t (nspec) is linearly correlated with respect to the square of time according to Eq. (2). This is the case when initial diffusion occurs in a plane [39]. If a = 1 then the diffusion is said to follow Ficks law. Deviation from nspec / t0.5 was observed after approximately 200 h and hence implied to pseudo-Fickian diffusion kinetics after this point. Anomalous adsorption phenomenon is known to occur in porous media [40,41].
nspec nspec;1
2 ¼
a 16 D L2
p
ta
ð2Þ
Calculated average initial diffusion coefficients (D) are presented in Table 2. It should be noted here that the fitted isotherms, in terms of maximum adsorption (nspec,1), were approximate at best because the equilibrium state of adsorption was not reached within a given time scale. The nspec,1 values for BTMP were 11.5%, 7.5% and 11.5% and for BKraft 8.5%, 6.5% and 7.5% in order of reference, LDH-100 and LDH 100-6 respectively (Fig. 1). Thickness of the sample was 2 mm and plane diffusion occurred from both sides (L = 1 mm). Functionalization was observed to increase adsorption kinetics while slowest adsorption was achieved with large amount of LDH (Fig. 2A and C). This was counter intuitive
Table 2 Test parameters of the uniaxial stretch deformation included yield strength (ryield), Young’s modulus (Ymod), modulus of resilience (Rmod), ultimate strength (rultimate), elongation at ultimate strength (ultimate), strength at break (rbreak) and elongation at break (break). The applied average torque (T) during moulding and the diffusion coefficients (D) from the water immersion tests are also presented. Filler contents were the same as in Table 1. Parameter
BTMP
PP
50 mm/min
ryield (MPa) Ymod (GPa) Rmod (kJ m3) rultimate (MPa) ultimate (%) rbreak (MPa) break (%) T (N) D (m2/s 1012)
BTMP
BKraft
5 mm/min
Ref
100-0
100-6
22 ± 1 1.08 ± 0.08 220 ± 30 41 ± 1 9±1 40 ± 1 9±1
21 ± 2 1.14 ± 0.06 200 ± 30 35 ± 3 7±2 31 ± 3 14 ± 2
13 ± 1 1.07 ± 0.08 80 ± 10 31 ± 3 10 ± 1 29 ± 4 15 ± 2
8±1 0.50 ± 0.04 61 ± 9 27 ± 2 22 ± 1 21 ± 2 39 ± 3 1.15 ± 0.03 0
Ref
100-0
100-6
Ref
100-0
100-6
19 ± 2 0.98 ± 0.08 180 ± 40 36 ± 2 11 ± 1 34 ± 2 14 ± 3 1.33 ± 0.03 4.06
18 ± 1 1.05 ± 0.04 160 ± 20 35 ± 1 11 ± 1 32 ± 1 16 ± 1 1.33 ± 0.03 7.68
12 ± 1 0.94 ± 0.03 70 ± 9 29 ± 1 12 ± 1 24 ± 2 24 ± 3 1.28 ± 0. 03 4.01
17 ± 1 1.08 ± 0.05 130 ± 20 39 ± 3 14 ± 1 37 ± 3 16 ± 1 1.30 ± 0.03 6.25
14 ± 2 0. 96 ± 0.08 110 ± 20 31 ± 4 13 ± 1 27 ± 4 21 ± 1 1.35 ± 0.03 6.94
14 ± 2 0.92 ± 0.04 100 ± 20 34 ± 3 14 ± 1 31 ± 3 176 ± 1 1.50 ± 0.03 10.2
C. Lange et al. / Composites: Part B 47 (2013) 260–266
B
A
1.0 0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
1.0
1.0 Ψw
0.8
D
C
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 0
10
20
30
40
0
5
10
40
σUltimate
32
σBreak
24
σYield
16 8
Rmod
εYield
0
15
m od
-1
nspec (mmol g )
0.8
Y
1.0
σ (MPa)
264
0
3
0.5
εUltimate
6
9
12
εBreak
15
ε (%)
t (h ) Fig. 2. The water adsorption kinetics (A) and (C) in specific amount (nspec(mmol g1)) and desorption kinetics (B) and (D) in weight fraction (Ww) are presented as a function of time (t (h0.5)). Results obtained with chemical pulp (BKraft) are shown in figures A and B while results with mechanical pulp fibres (BTMP) are shown in figures C and D. Symbols corresponds to the reference pulp (j), the LDH modified pulp () and to the LDH modified pulp with 6% (w/w) of SDS (+).
as the surface energy of the functionalized fibres was lower than the one in reference or LDH modified pulps [34]. The diffusion parameter (D) is to be taken as an average composite mass transport term which includes the adsorption phenomenon. Initial desorption rate was slightly higher for BTMP (13.2 h1/2) composite samples than for BKraft (9.3 h1/2). However, a larger relative amount of water was lost from BKraft composite samples after approximately 16 h. Also, dispersion of particles into polymer phase and degradation of fibre length increases the filler surface area providing defects in polymer matrix and channels for water to penetrate into fibres via fibre ends thus promoting greater adsorption. At this point however, it is impossible to distinguish in between the fraction of water adsorbed by the fibres from the adsorption provided by percolation and tortuosity. Based on the observations from SEM analysis (vide infra) and our previous results the percolation and tortuosity affects significantly to water adsorption.
Fig. 3. The figure shows an example of a stress–strain curve from the reference BKraft and the derivation of the parameters listed in Table 2.
Table 3 A comparison of the ultimate tensile strength (rultimate) and elongation at break (break) with specific mass fraction (W) of fibres in reinforced polypropylene composites under 5 mm min1 uniaxial strain rate is presented. Fibres Hardwood sawdust Sisal Kenaf Pine Eucalyptus Softwood TMP
W
rultimate
break
(%)
(MPa)
(%)
Compression
20
22
4
[47]
Compression Compression Injection Injection Injection
40 30 50 20 16
35 45 35 33 29
– – 1.5 14 24
[48] [48] [49] [50] This work
Moulding
Reference
3.4. Tension tests Results from uniaxial deformation experiments are presented in Table 2. Yield strength (ryield) was evaluated from a linear region at 0.1% offset. Elastic modulus (Ymod) was defined from the linear portion at 0.1–1.1% strain. Modulus of resilience (Rmod) was calculated according to Eq. (3) from the secant with 0.1% offset. Elongation and strength at break (break and rbreak) were defined as the point where the rate of change in stress exceeded 5 MPa. For comparison, the maximum rate of change in stress at the necking region of the matrix polymer was taken as its breaking point. Ultimate strength (rultimate) is the peak value of the stress–strain curve. An example of BKraft reference and derived parameters are shown in Fig. 3. The BTMP fibre composites were tested at two strain rates (5 and 50 mm min1). The results reflect the viscoelastic nature of these composites as at higher strain rate the ryield and Ymod increased while break decreased as expected. For comparison results obtained with maleated or isocyanate coupled reinforcing fibres from other authors are presented in Table 3.
Rmod ¼
r2yield 2Y mod
ð3Þ
Mechanical pulp fibres are known to lose its hemicelluloses and extractives in aqueous solution at elevated pH and temperature [42] such as those used in our experiment. These are known to af-
Fig. 4. A SEM image of a uniaxially deformed composite showing air pockets (arrows) is presented. The scale bar equals to 500 lm in length.
fect fibres physical properties. It is also known that specific surface charge of fines is governed by the amount of uronic acids [43,44]. According to polyelectrolyte titration, peroxide bleached thermo mechanical pulp fibres from spruce have a surface charge of approximately 175 leq g1 where as for fibrils and flakes these values are 190 leq g1 and 250 leq g1 respectively [45]. The specific surface area of fines is obviously higher than bulk fibres. We therefore propose that electrostatically bound LDH was more prone to precipitate on surface of fibrils, flakes and ribbon-like fragments. The reference pulp fibres enhanced the load bearing capacity of a-PP composites. A 100% increase in ryield and Ymod was recorded.
C. Lange et al. / Composites: Part B 47 (2013) 260–266
265
Fig. 5. The optical micrographs from the skin layer of the chemical pulp (BKraft: A–C) and mechanical pulp (BTMP: D–F) fibre filled composites are presented. The arrows are pointing to the melt flow direction and the length scale of each of the arrows is 250 lm. Composites were filled either with the reference pulp (A) and (D), LDH modified pulp (B) and (E) or with the LDH modified pulp fibres containing 6% (w/w) of SDS (C) and (F). Insets show the break region of the samples tested with 5.0 mm min1 uniaxial strain rate. Darker shades observed in images A, B, D and E are identified as agglomerated fibres.
Fig. 6. The SEM images of the BTMP fibre–matrix failure interface after 5.0 mm min1 uniaxial deformation are presented. Composition mode of back scattered electron imaging was applied in images D–F to enhance the contrast in between the fibres and matrix polymer. Fillers that were used in composites were the reference pulp (A) and (D), LDH modified pulp (B) and (E) and the LDH modified pulp with 6% (w/w) of SDS (C) and (F). Scale bars denotes a length of 1 lm in A–C and 100 lm in D–F. Images A–C shows a single boundary interface of the filler and the matrix polymer.
Both LDH and SDS, on the other hand, enhanced plasticity. LDH particles improved the resistance of BTMP composite to deformation by 5% and the break strain by 10%. The impact of LDH on the BTMP composites Ymod and break increased with strain rate. The recorded values were 5% and 50% respectively. Electrostatically bound SDS on particle surface provided substantial increase in ductility of BTMP composites as break increased by 70% at both strain rates. Based on the decrease in Rmod the functionalization tends to reduce the overall elasticity of the composite. Applied torque increased when functionalized BKraft fibres were used in composite preparation (see above). Comparing the LDH modified and SDS functionalized fibre reinforced composites the Ymod was left practically unaffected. However, a small increase in rultimate and rbreak were observed. Simultaneous 20% decrease in break was evident. Higher lav could partially explain this behaviour. The total work needed for inducing plastic deformation was not affected by SDS while LDH particles decreased it by 25% in comparison to reference. Tentatively, the layer of LDH particles on the fibre surface induced irreversible slippage on polymer–particle interface
under tension. After applying SDS, LDH particles acted as a physical bearing in between the filler and matrix polymer. 3.5. Scanning electron and optical microscope analysis Fig. 4 confirms the assumption that air was precluding complete filling of the mould, which explains the deviation in the mass fraction of fibres. In Fig. 5A–F the optical micrographs from the skin layer and the breaking region are presented. BKraft fibres (A–C) suffered length reduction upon modification with LDH particles whereas functionalization was able to retain lav much better. This confirms the assumption made from increased torque (see above). Fibres are also seen to orient randomly, even perpendicularly against the melt flow in Fig. 5B. Longer fibres appear to orient more parallel to the flow (Fig. 5C). With BTMP fibres this was less obvious. However, evolution of microcracks and necking induced by modification and functionalization are clearly observable in the break region. In short fibre reinforced composites (lav is below critical fibre length) it is less likely for fibres to break. The parallel oriented fibres tend to be pulled out and perpendicularly oriented
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fibres debonded [46]. Notably, functionalization with SDS surfactant provided homogenous dispersion of both BTMP and BKraft fibres in the hydrophobic polymer melt. However, even though the SDS reduced drastically the fibre aggregation problem in these composites, the mechanical properties were not enhanced due to the lack of fibre–matrix adhesion which is illustrated in Fig. 6A–F. The electron diffraction spectrum revealed that majority of LDH particles remained on the fibre surface after moulding. Dispersion of particles into polymer melt was also observed. Functionalization was more effective with this respect as the polymer surface was covered by particles at the break failure interface (Fig. 6A–C). Also, the fibre pull out effect was more pronounced with functionalized pulp (Fig. 6D–F). While matrix polymer seemed to be unable to wet the LDH modified fibre surface, the stress failure in composites prepared from functionalized pulp fibres seemed to occur at the particle–particle interface.
4. Conclusions We have elucidated interactions of the atactic polypropylene matrix and softwood pulp fibres in micro-injection moulded composites. Applied fibres were functionalized with LDH particles in a system readily implementable to industrial pulp and paper processes. The mass fraction of LDH particles on pulp fibres was 0.045, while fibres to matrix mass ratio was approximately 0.16. The provided functionalization method with SDS surfactant enhanced plasticity of natural fibre reinforced composites and allowed homogeneous dispersion of pulp fibres in a-PP matrix. After functionalization the BKraft fibres from chemical pulping process endured the applied shear force better during compounding than the BTMP fibres from mechanical pulping process. Primary reason was addressed to both, the structural differences of fibre cell wall layers and the surface modification with SDS. High shear forces dispersed some of the functionalized particles into polymer phase. The particle cohesion forces were therefore identified as the bottle-neck in this process while addition of the SDS surfactant seemed to promote particle matrix adhesion. Since the electrostatically bound non-functionalized LDH remained, regardless of the relatively high shear, largely on fibre surface, the optimization of the pulp fibres performance in composite applications requires the control of the LDH particle layer thickness on fibre surface. Although an injection moulding of small parts was not suitable for the proposed system, we are confident that the method provides a new functionalization platform for natural fibres in composite applications that can be exploited in industrial scale.
with Instron and Professor Pedro Fardim was responsible for overall supervision. References [1] [2] [3] [4] [5] [6] [7]
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Acknowledgments We would like to thank TEKES (Finnish Funding Agency for Technology and Innovation) for their financial support. We would also like to thank UPM-Kymmene, Stora Enso and Metsä Fiber for their financial support and both the UPM-Kymmene Rauma mill and Metsä Fibre Rauma mill for providing the pulp for the experiments. Finally we acknowledge all the laboratory personnel at Åbo Akademi University who assisted with the instrumentation. Carl Lange carried out the study, as part of his Ph.D. thesis, and was responsible for all parts of the research project, including writing the paper. Farid Touaiti was responsible of tensile experiments
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Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Hybrid clay functionalized biofibres for composite applications Carl Lange a,1, Farid Touaiti b, Pedro Fardim a,⇑ a b
Åbo Akademi University, Laboratory of Fibre and Cellulose Technology, Porthansgatan 3-5, FI-20500 Åbo, Finland Åbo Akademi University, Laboratory of Paper Coating and Converting, Porthansgatan 3-5, FI-20500 Åbo, Finland
a r t i c l e
i n f o
Article history: Received 20 July 2012 Received in revised form 19 September 2012 Accepted 12 October 2012 Available online 21 November 2012 Keywords: A. Hybrid A. Wood B. Fibre/matrix bond E. Injection moulding Layered double hydroxide (LDH)
a b s t r a c t Micro-injection moulded composites of softwood fibres from two different pulping processes in thermoplastic polypropylene matrix have been studied. Surface modification of hydrogen peroxide bleached thermo mechanical pulp fibres (BTMP) from spruce (Picea abies L.) and chemical sulphate pulp fibres (BKraft) from pine (Pinus sylvestris L.) was achieved by co-precipitation of layered double hydroxide (LDH) particles that were further functionalized with sodium dodecyl sulphate (SDS) surfactant in conditions similar to wet end in paper production. Micro-injection moulded test specimens were subjected to uniaxial tension tests, water absorption, micromechanical deformation and microscopic studies. Optical micrographs show that functional pulp was homogeneously dispersed into polypropylene matrix where as the untreated fibres agglomerated during moulding. When composites were prepared with BTMP fibres the electrostatically bound SDS on fibre surface increased elongation at break by 70%. LDH particles intensified dissipation of shear energy to BKraft fibres degrading the average fibre length by 50% during moulding. Functionalization with SDS surfactant, however, increased average fibre length up to 125% in comparison to LDH modification. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Justification for utilizing renewable fibres as reinforcement in composites is often deducted from predicated scarcity of oil reserves and public awareness of environmental problems. These concerns work as an engine to drive the industry towards biobased products and simultaneously create a market for natural fibre based composites. In contrast to glass fibres, natural fibres provide less abrasive, lower density, recyclable, renewable and cost efficient fillers for composites [1]. On the other hand considerable effort is needed to obtain good adhesion and compatibility between a thermoplastic polymer and bio-based fibres [2]. Also, a disadvantage of wood fibres in particular, is their relatively small aspect ratio that affects tensile and impact strength of the final product [3]. Therefore, substantial amount of hemp, jute, flax, kenaf and sisal is used instead as fillers in synthetic polymers [4,5]. Aside from laminates, majority of composites consisting of wood particles, fibres or flour derive from industrial by-products that are processed and extruded with thermoplastic matrix to flooring, furniture and deck board [6]. From economical and environmental point of view the injection moulded automotive parts have become an important sector as well [7]. Although the market for wood par-
⇑ Corresponding author. 1
E-mail addresses: clange@abo.fi (C. Lange), pfardim@abo.fi (P. Fardim). Principal corresponding author.
1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.10.032
ticle composites (WPC) has been steadily rising since 1990s, pulp and paper industry as a producer of WPCs is a rather new concept. The most common coupling agents for natural fibres in thermoplastic applications are maleated co-polymers and silanes [8]. A typical silanization process employs either a vinyl or aminopropyl functionalized silane monomer, especially in polypropylene (PP) and polyethylene (PE) composites, with three methoxy or ethoxy groups able to covalently link with the hydroxyl groups of the fibrous material [9]. Silanes and siloxanes may undergo various hydrolysis and condensation reactions in aqueous environment depending on the pH, temperature, ionic strength of the solution and concentrations of the monomer species. Trace amounts of water may induce condensation reactions between the siloxane groups which is preferred if crosslinks are needed within the matrix and matrix–fibre interface. Crosslinking vinyl alkoxy silanes with PP require peroxide initiators and small amounts of catalyst [10]. It should be noted however, that there is usually a maximum in silane-fibre adsorption isotherm as the condensation can overcompensate the rate of adsorption [11]. Maleated co-polymers are often applied during the extrusion. Another route is to treat fibres with a maleated co-polymer in toluene prior to moulding [12]. The anhydride couples with hydroxyl functional group of fibrous material through esterification. Other surface modification techniques involve mercerization [11], isocyanate functionalities for urethane bond formation [13] as well as physical modification techniques via corona or plasma treatments [14,15]. To the best of our knowledge, research of biofibres involving functionalization
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with inorganic–organic layered double hydroxide (LDH) particles for composite applications is absent in the literature, although nanocomposites from LDH alone can be found [16]. In this article we would like to present results obtained with hydrogen peroxide bleached thermo mechanical pulp (BTMP) from spruce (Picea abies L.) and bleached chemical sulphate pulp (BKraft) from pine (Pinus sylvestris L.) that were functionalized with sodium dodecyl sulphate (SDS) modified LDH particles in process conditions similar to so called wet end in paper line and used as fillers in thermoplastic PP matrix, which is one of the most important industrial plastics [17]. According to ICIS Chemical Business reports (September 5th, 2011) the packing industry holds the highest annual consumption of PP while an increasing trend lays in automotive parts and specialty products. It has relatively low price and offers an advantage gained in reduction of article weight when compared to other plastics. PP has a low intrinsic Gibbs surface free energy and is inert against chemical attacks, which have important consequences when the end products as well as the utilization of various fillers and reinforcing agents within the polymer matrix are in consideration. The applied inorganic–organic LDHs are mixed metal hybrid clays with layered structure that can incorporate organic functional units with anionic charge in between the stacked metal hydroxyl sheets [18,19]. A general formula of a LDH is:
h
3þ M2þ 1x Mx ðOHÞ2
ixþ
Az x=z nH2 O
The formula suggests that a LDH carries two OH groups for each excess positive charge (x) and it can be synthesised in water phase. This is an important factor when the pulp and paper production is considered where implementation of organic solvents is unwanted and reactions are often carried out in alkaline conditions. Pulp fibres contains carboxylic groups that carry anionic charge in alka line suspensions. Also, the divalent M2þ and trivalent M3þ x x metals can be chosen quite freely as long as the ionic radii does not differ too much from each other. The charge balancing anion that carries the function can be a monomer or a polymer. Az x=z The first co-precipitation synthesis of LDH was apparently conducted by Feitknecht in 1942 [20] and by the early 1980s its properties as a heterogeneous catalyst and as a molecular sieve were already reported [21]. Other research areas of LDHs cover controlled drug delivery [22–24], thermal degradation of fibres [25], flame retardant paper and composites [26,27] modified electrodes [28], adsorption [29,30] as well as photo- and biocatalysis and gas-phase conversion [31–33] to mention but a few. It is also possible to substantially decrease wood fibres surface energy via functionalization with surfactant modified LDHs [34]. These fibres should in principle perform better in composite applications. Because the LDHs are known to provide fire retardant properties for composites, it is important to study whether these particles can be applied onto fibres surface to carry both the fire protection as well as the functional organic moiety into composites. If succeeded, the properties of the filler could be tuned separately for each composite application with a simple method. Therefore, besides being a novel approach for surface modification of wood fibres, the aim of our research in this article is also to address the boundary interactions of atactic polypropylene (a-PP) copolymer matrix with hybrid functionalized pulp fibre fillers in micro-injection moulded structures. 2. Materials and methods 2.1. Materials Aluminium nitrate (Al(NO3)39H2O, 98%, Germany) was purchased from Fluka. Magnesium nitrate (Mg(NO3)26H2O, 98–
261
102%, UK), Sodium Dodecyl Sulphate (C12H25NaSO4, 99%, Japan), NaOH (97%, Europe) and isomeric mixture of Xylenes (98.5%, Germany) were purchased from Sigma Aldrich. Bleached thermo mechanical pulp (BTMP) from spruce (P. abies L.) was produced and kindly supplied by UPM-Kymmene (Rauma Mill, Finland) and chemical sulphate pulp from pine (P. sylvestris L.) by Metsä Fibre (Rauma Mill, Finland). All received pulp samples were stored in a freezer and used after defrosting in a cooler at 10 °C. An atactic polypropylene (a-PP) (PP RD204CF, Borealis, Denmark), with density around 900 kg m3 and melt flow rate of 0.8 g min1 (230 °C, 2.16 kg), random copolymer was chosen to serve as the matrix. Small amounts of ethylene and antioxidants that were not specified in the suppliers specifications were present in the matrix. All reagents were used without further purification. 2.2. Methods Fractionation of pulps were performed with a Bauer–McNett fractionator (Lorentzen and Wettres, Kista, Sweden). Fraction from mesh with nominal opening of 1.19 mm was used in composite preparation. Prior to fractionation a Valley Beater (Lorentzen and Wettres, Stockholm, Sweden) was applied to beat the BKraft fibres to Schoppler–Riegler value of 30. The modification of pulp fibres via co-precipitation is explained in elsewhere [34]. In short: the particle precipitation system was set up with two automatic titrators. NaOH solution (2.0 M) was used to keep the pH in the reaction medium close to 10 while the metal salt solution (0.3 M) was dispensed from another titrator. Titration rate was set to 1.0 ml min1. Metal salt concentrations in the reaction medium were calculated from the final volume (1500 ml) and kept at 0.1 M. After modification, pulp fibres were treated in hydrodynamic conditions at 135 °C for 16 h and thoroughly washed at 1% consistency in büchner funnel under reduced pressure. Fibres were assigned according to functionalization: LDH containing fibres (100-0) and LDH containing fibres with 6% (w/w) of SDS surfactant (100-6). Reference fibres (ref) did not contain LDH or SDS but were treated 2 h in alkaline conditions at pH 10. Fibres size distribution and physical parameters were investigated with Kajaani FiberLab analyzer (Metso Automation, Kajaani, Finland) with applicable standard (ISO 16065-2) utilizing FiberLab software version 3.5.3. To remove the polymer matrix the samples were boiled twice in xylenes for 4 h, hot filtrated through sintered glass büchner funnel with pore size number 2, rinsed with xylenes and subsequently dispersed with magnetic stirrer (24 h) into acidic ethanol/water (1:100) solution prior to analysis. DSM Xplore 5.5 ml Injection Moulding Machine (DSM Xplore, Netherlands) was applied to compound the fibres and a-PP to match an ISO standard (ISO 527-2 1BA) specimen with composition of 20% (w/w) of pulp fibres. Temperature in mixing chamber was kept at 190 °C and screw speed at 200 rpm while applied torque was recorded. The mould temperature was set to 40 °C while injection and holding pressures were 4 bars and the processing time 1.0 s. Resulted composite samples were conditioned at 52 ± 2% of relative humidity in 23 ± 1 °C for 60 days. For water absorption studies, three moulded specimens were immersed into distilled water (23 ± 1 °C) in 250 ml. Maximum immersion depth was 10 cm. Sample weights were recorded after removing the surface bound water by drying the composites gently with paper towel and conditioning them in oven at 105 °C for 12 ± 1 s. Also, water desorption kinetics was studied under dynamic air flow at 60 °C. The investigation of the composite mechanical properties was performed with Instron 8872 machine (Instron, England) at ambient temperature. Tensile tests were conducted on ISO standard (ISO 527-2 1BA) samples with constant uniaxial strain rate of
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5.0 mm min1 and 50 mm min1 until rupture. Stress (r) versus strain () curves were used for comparing composite properties like Youngs and resilience moduli, yield and ultimate strengths and elongation at break. Micromechanical deformations were examined from ruptured surfaces with a Leo Gemini 1530 field emission scanning electron microscope equipped with In-Lens detector (LEO Electron Microscopy Ltd., Oberkochen, Germany). All samples were air dried and coated with carbon in Temcarb TB500 sputter coater (Emscope Laboratories, Ashford, UK). Optimum accelerating voltage was 2.70 kV and the working distance 4 mm. Optical microscopy was applied to investigate fibres orientation in moulded specimen. 3. Results and discussions 3.1. Fibre length distribution Results for fibre length distribution after fractionation and modification are presented in Table 1. Column A shows the physical parameters after fractionation while columns B and C presents the results before injection moulding and after their retrieval from polymer matrix respectively. The weight fraction of fibres (W) in composites obtained from extraction with xylenes is also shown. Fractionated BTMP fibres were 500 lm shorter in average length (lav) but 6 lm greater in average diameter (dav) than bleached Kraft pulp fibres (column A). Difference in lav was in major part, due to higher fines content, determined as particulates smaller than 100 lm in size. Average aspect ratio (L) for BKraft pulp fibres was 74 and for BTMP it was 47. Typically, the tracheid cells in coniferous trees have diameters ranging from 20 to 40 lm and the maximum length is approximately 3 mm; thus, the aspect ratio can be close to 100. Fibre dimensions are greatly affected by the pulping process and other treatments such as a washing sequence. Therefore synergic effects on lav values are difficult to address and more stress should be put on the change in aspect ratio (L) after subsequent preparation steps. The steps were divided into modification/functionalization and injection moulding. Reference pulp fibres (column B) were treated in alkaline water for 2 h to simulate conditions of pulp that was modified with LDH particles. For some reason, modification of BTMP with LDH was effectively removing fines from pulp during the washing. It is reasonable to assume that the stacking of LDH particles onto fibre surface did not contribute to dav to any significant degree in Kajaani FiberLab analyzer. Reduction in dav was addressed to peeling and fragmentation of primary (P) and secondary (S1) fibre cell walls. After modification/functionalization step L was approximately 65–70 for BKraft and around 60 for BTMP.
Injection moulding degraded lav effectively (column C). Values for reference pulp were 16 for BKraft (76% reduction) and approximately 10 (84% reduction) for BTMP. The smaller diameter observed in xylene retrieved fibres of both pulps was attributed to complete lumen collapse as well as to partial fragmentation and fibrillation during the moulding process. Interestingly, LDH particles on surface of BKraft induced greater degradation of fibres (L = 8), thus dissipating shear energy. SDS functionalization showed significant improvement in the aspect ratio (L = 19) in comparison to bare LDH. BTMP fibres, on the other hand, were affected by the functionalization with SDS to a lesser degree. The lav increased only 20% in comparison to modified fibres leaving L practically unchanged. Observed differences can be explained by the differences in fibre cell wall structure. Lignin that remains on fibre cell wall of mechanical pulps affects to fibres overall performance by providing rigidity and toughness while making the fibre more brittle. When lignin is removed, fibres flexibility increases. The physical beating that is often applied in chemical pulping process increases flexibility even more through micro-deformations that plasticises the fibre cell wall. When LDH is precipitated on fibre surface the toughness of BTMP might be less affected than it is in flexible BKraft fibres. Agglomeration of fibres in polymer melt causes shear to be dissipated into fibre bundles where the flexibility of individual fibres is inefficient to sustain cell wall structure. This in turn could explain the high reduction in lav of BKraft. After functionalization with surfactant the flexibility of BKraft retains lav as fibres are better dispersed into the polymer matrix.
3.2. Injection moulding The differences in between BTMP and BKraft pulps fibres can be attributed to surface lignin content, the shape and amount in fines as well as fibres cell wall structure. Lignin is highly cross-linked polymer and when associated with fibres it has glass transition temperatures (Tg) ranging from 95 °C to 235 °C depending on the water content [35]. In our work, the water content of fibres prior to injection moulding was less than 2%. As the temperature in the mixing chamber was kept below 200 °C it is possible for the lignin not to reach its Tg. Shear forces may, however, locally increase the fibres surface temperature above the limit of Tg. Softening of lignin should allow fibres to become more durable against shear forces. However, the micro-injection moulding process created a large amount of fines with BTMP fibres. Even if lignin was softened it was not able to afford fibres with plasticity enough to withstand shear similar to flexible BKraft fibres, which practically contains only cellulose.
Table 1 The length weighted averages of fibre lengths (lav), widths (dav), fines content (fines) and aspect ratios (L) after fractionation (A), functionalization (B) and compounding (C) are presented. Fillers used in composites were the reference (ref), the LDH modified (100-0) and the LDH modified fibres with 6% (w/w) of SDS surfactant (100-6). The weight fractions (W) of fibres in composites retrieved by xylene extraction are also presented. Fibres
Parameter
A
B
C
Ref
100-0
100-6
Ref
100-0
100-6
BKraft
lav (mm) dav (lm) fines (%) L W (%)
2.28 ± 0.03 30.8 ± 0.2 0.4 74
2.10 ± 0.03 31.3 ± 0.2 1.0 67
2.06 ± 0.06 30.9 ± 0.6 1.0 67
1.95 ± 0.04 31.0 ± 0.4 1.1 63
0.37 ± 0.01 22. 6 ± 0.3 76.3 16 15
0.19 ± 0.01 23.0 ± 0.5 93.8 8 12
0.43 ± 0.02 22.9 ± 0.3 69.6 19 14
BTMP
lav (mm) dav (lm) fines (%) L W (%)
1.74 ± 0.03 36.8 ± 0.3 4.0 47
1.63 ± 0.07 28.7 ± 0.7 2.0 57
2.06 ± 0.01 32.9 ± 0.2 0.3 63
1.69 ± 0.02 29.4 ± 0.3 1.9 57
0.25 ± 0.01 27. 7 ± 0.4 91.0 9 17
0.22 ± 0.01 24.8 ± 0.3 93.1 9 17
0.27 ± 0.01 32.5 ± 0.2 87.6 8 12
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3.3. Water adsorption Water adsorption was followed until reasonable level of quasistatic kinetics (1600 h) and evaluated with generalized Langmuir’s adsorption isotherm (Eq. (1)) where S is the adsorbed amount, Smax is the maximum adsorption, b is the binding energy term and x the function against which the adsorption is measured. Polypropylene matrix adsorbed negligible amount of water (<0.1%).
S ¼ Smax
ðbxÞn 1 þ ðbxÞn
m=n ð1Þ
14 12 10
Ψweight (%)
For density measurements, the composites were weighted individually after equilibration at 40 °C for 10 days. The volume was determined by choosing 10–12 samples to be immersed in water (23 ± 1 °C). BKraft, BTMP and a-PP were tested separately. Density of PP can be as low as 0.850 g cm3, but it increases rapidly as a function of the molecular weight [36]. The density of injection moulded a-PP matrix polymer was determined to be (0.91 ± 0.01) g cm3, while, regardless of the sample treatment or applied fibres, for composite specimen it was (1.01 ± 0.01) g cm3. According to Whiting et al. the average density for lignin is approximately 1.4 g cm3 and for polysaccharides it is 1.5 g cm3 [37]. Sixta uses similar values except for carbohydrates (1.58 g cm3) [38]. Bleached chemical pulp contains only fractions of lignin, while high yield pulps are similar to native wood. When lumen collapse occur the density of fibres should be equal to the cell wall structure. Therefore, BKraft pulp should have a density close to 1.5–1.58 g cm3 and BTMP from spruce, containing 28% of lignin, approximately 1.47–1.53 g cm3 depending on the average density used for carbohydrates. By using standard specimen volume (1.0 cm3 for ISO 527-2 1BA), the apparent fraction of the applied fibres was calculated to be 0.15–0.17 for BKraft and 0.16–0.18 for BTMP. These values deviate from the aimed 0.20 but correlated reasonably well with the xylene extracted fractions (Table 1). It is believed that air, which was introduced into the mixing chamber with pulp and carried into the sample specimen, precluded complete filling of mould with composite melt. The applied torque was followed at 200 rpm screw speed during the moulding process (Table 2). Required force increased 15% when native pulp or LDH modified pulp was introduced into the chamber with a-PP. Surprisingly torque increased up to 30% when a-PP was mixed with functional BKraft fibres. The surfactant was expected to decrease the viscosity as observed with functionalized BTMP fibres. Therefore, as functionalization of BKraft fibres was able to increase lav, the dynamic viscosity of the polymer melt with longer fibres and LDH particles increased in comparison to melt flow of shorter fibres.
8 6 4 2 0 Reference
LDH-100
LDH-100-6
Fig. 1. The relative mass increase (Wweight) of the composites with different filler compositions at dynamic equilibrium in water according to Langmuir adsorption isotherm are presented. Filler compositions were the same as in Table 1. The results with thermomechanical pulp (BTMP) are shown with lighter shade and the chemical pulp (BKraft) with darker shade.
Eq. (1) reduces to Langmuir’s adsorption isotherm when m = n = 1. In our experiment the hydrostatic pressure, temperature and activity of adsorbing species were kept constant while the change in total mass (Dw) of adsorbent was followed with respect to time (t). Correlation between the data and fitted Langmuir’s isotherms was greater than 99.9% in all samples. Fickian diffusion predicts that an average mass flux of molecules at time t (nspec) is linearly correlated with respect to the square of time according to Eq. (2). This is the case when initial diffusion occurs in a plane [39]. If a = 1 then the diffusion is said to follow Ficks law. Deviation from nspec / t0.5 was observed after approximately 200 h and hence implied to pseudo-Fickian diffusion kinetics after this point. Anomalous adsorption phenomenon is known to occur in porous media [40,41].
nspec nspec;1
2 ¼
a 16 D L2
p
ta
ð2Þ
Calculated average initial diffusion coefficients (D) are presented in Table 2. It should be noted here that the fitted isotherms, in terms of maximum adsorption (nspec,1), were approximate at best because the equilibrium state of adsorption was not reached within a given time scale. The nspec,1 values for BTMP were 11.5%, 7.5% and 11.5% and for BKraft 8.5%, 6.5% and 7.5% in order of reference, LDH-100 and LDH 100-6 respectively (Fig. 1). Thickness of the sample was 2 mm and plane diffusion occurred from both sides (L = 1 mm). Functionalization was observed to increase adsorption kinetics while slowest adsorption was achieved with large amount of LDH (Fig. 2A and C). This was counter intuitive
Table 2 Test parameters of the uniaxial stretch deformation included yield strength (ryield), Young’s modulus (Ymod), modulus of resilience (Rmod), ultimate strength (rultimate), elongation at ultimate strength (ultimate), strength at break (rbreak) and elongation at break (break). The applied average torque (T) during moulding and the diffusion coefficients (D) from the water immersion tests are also presented. Filler contents were the same as in Table 1. Parameter
BTMP
PP
50 mm/min
ryield (MPa) Ymod (GPa) Rmod (kJ m3) rultimate (MPa) ultimate (%) rbreak (MPa) break (%) T (N) D (m2/s 1012)
BTMP
BKraft
5 mm/min
Ref
100-0
100-6
22 ± 1 1.08 ± 0.08 220 ± 30 41 ± 1 9±1 40 ± 1 9±1
21 ± 2 1.14 ± 0.06 200 ± 30 35 ± 3 7±2 31 ± 3 14 ± 2
13 ± 1 1.07 ± 0.08 80 ± 10 31 ± 3 10 ± 1 29 ± 4 15 ± 2
8±1 0.50 ± 0.04 61 ± 9 27 ± 2 22 ± 1 21 ± 2 39 ± 3 1.15 ± 0.03 0
Ref
100-0
100-6
Ref
100-0
100-6
19 ± 2 0.98 ± 0.08 180 ± 40 36 ± 2 11 ± 1 34 ± 2 14 ± 3 1.33 ± 0.03 4.06
18 ± 1 1.05 ± 0.04 160 ± 20 35 ± 1 11 ± 1 32 ± 1 16 ± 1 1.33 ± 0.03 7.68
12 ± 1 0.94 ± 0.03 70 ± 9 29 ± 1 12 ± 1 24 ± 2 24 ± 3 1.28 ± 0. 03 4.01
17 ± 1 1.08 ± 0.05 130 ± 20 39 ± 3 14 ± 1 37 ± 3 16 ± 1 1.30 ± 0.03 6.25
14 ± 2 0. 96 ± 0.08 110 ± 20 31 ± 4 13 ± 1 27 ± 4 21 ± 1 1.35 ± 0.03 6.94
14 ± 2 0.92 ± 0.04 100 ± 20 34 ± 3 14 ± 1 31 ± 3 176 ± 1 1.50 ± 0.03 10.2
C. Lange et al. / Composites: Part B 47 (2013) 260–266
B
A
1.0 0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
1.0
1.0 Ψw
0.8
D
C
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 0
10
20
30
40
0
5
10
40
σUltimate
32
σBreak
24
σYield
16 8
Rmod
εYield
0
15
m od
-1
nspec (mmol g )
0.8
Y
1.0
σ (MPa)
264
0
3
0.5
εUltimate
6
9
12
εBreak
15
ε (%)
t (h ) Fig. 2. The water adsorption kinetics (A) and (C) in specific amount (nspec(mmol g1)) and desorption kinetics (B) and (D) in weight fraction (Ww) are presented as a function of time (t (h0.5)). Results obtained with chemical pulp (BKraft) are shown in figures A and B while results with mechanical pulp fibres (BTMP) are shown in figures C and D. Symbols corresponds to the reference pulp (j), the LDH modified pulp () and to the LDH modified pulp with 6% (w/w) of SDS (+).
as the surface energy of the functionalized fibres was lower than the one in reference or LDH modified pulps [34]. The diffusion parameter (D) is to be taken as an average composite mass transport term which includes the adsorption phenomenon. Initial desorption rate was slightly higher for BTMP (13.2 h1/2) composite samples than for BKraft (9.3 h1/2). However, a larger relative amount of water was lost from BKraft composite samples after approximately 16 h. Also, dispersion of particles into polymer phase and degradation of fibre length increases the filler surface area providing defects in polymer matrix and channels for water to penetrate into fibres via fibre ends thus promoting greater adsorption. At this point however, it is impossible to distinguish in between the fraction of water adsorbed by the fibres from the adsorption provided by percolation and tortuosity. Based on the observations from SEM analysis (vide infra) and our previous results the percolation and tortuosity affects significantly to water adsorption.
Fig. 3. The figure shows an example of a stress–strain curve from the reference BKraft and the derivation of the parameters listed in Table 2.
Table 3 A comparison of the ultimate tensile strength (rultimate) and elongation at break (break) with specific mass fraction (W) of fibres in reinforced polypropylene composites under 5 mm min1 uniaxial strain rate is presented. Fibres Hardwood sawdust Sisal Kenaf Pine Eucalyptus Softwood TMP
W
rultimate
break
(%)
(MPa)
(%)
Compression
20
22
4
[47]
Compression Compression Injection Injection Injection
40 30 50 20 16
35 45 35 33 29
– – 1.5 14 24
[48] [48] [49] [50] This work
Moulding
Reference
3.4. Tension tests Results from uniaxial deformation experiments are presented in Table 2. Yield strength (ryield) was evaluated from a linear region at 0.1% offset. Elastic modulus (Ymod) was defined from the linear portion at 0.1–1.1% strain. Modulus of resilience (Rmod) was calculated according to Eq. (3) from the secant with 0.1% offset. Elongation and strength at break (break and rbreak) were defined as the point where the rate of change in stress exceeded 5 MPa. For comparison, the maximum rate of change in stress at the necking region of the matrix polymer was taken as its breaking point. Ultimate strength (rultimate) is the peak value of the stress–strain curve. An example of BKraft reference and derived parameters are shown in Fig. 3. The BTMP fibre composites were tested at two strain rates (5 and 50 mm min1). The results reflect the viscoelastic nature of these composites as at higher strain rate the ryield and Ymod increased while break decreased as expected. For comparison results obtained with maleated or isocyanate coupled reinforcing fibres from other authors are presented in Table 3.
Rmod ¼
r2yield 2Y mod
ð3Þ
Mechanical pulp fibres are known to lose its hemicelluloses and extractives in aqueous solution at elevated pH and temperature [42] such as those used in our experiment. These are known to af-
Fig. 4. A SEM image of a uniaxially deformed composite showing air pockets (arrows) is presented. The scale bar equals to 500 lm in length.
fect fibres physical properties. It is also known that specific surface charge of fines is governed by the amount of uronic acids [43,44]. According to polyelectrolyte titration, peroxide bleached thermo mechanical pulp fibres from spruce have a surface charge of approximately 175 leq g1 where as for fibrils and flakes these values are 190 leq g1 and 250 leq g1 respectively [45]. The specific surface area of fines is obviously higher than bulk fibres. We therefore propose that electrostatically bound LDH was more prone to precipitate on surface of fibrils, flakes and ribbon-like fragments. The reference pulp fibres enhanced the load bearing capacity of a-PP composites. A 100% increase in ryield and Ymod was recorded.
C. Lange et al. / Composites: Part B 47 (2013) 260–266
265
Fig. 5. The optical micrographs from the skin layer of the chemical pulp (BKraft: A–C) and mechanical pulp (BTMP: D–F) fibre filled composites are presented. The arrows are pointing to the melt flow direction and the length scale of each of the arrows is 250 lm. Composites were filled either with the reference pulp (A) and (D), LDH modified pulp (B) and (E) or with the LDH modified pulp fibres containing 6% (w/w) of SDS (C) and (F). Insets show the break region of the samples tested with 5.0 mm min1 uniaxial strain rate. Darker shades observed in images A, B, D and E are identified as agglomerated fibres.
Fig. 6. The SEM images of the BTMP fibre–matrix failure interface after 5.0 mm min1 uniaxial deformation are presented. Composition mode of back scattered electron imaging was applied in images D–F to enhance the contrast in between the fibres and matrix polymer. Fillers that were used in composites were the reference pulp (A) and (D), LDH modified pulp (B) and (E) and the LDH modified pulp with 6% (w/w) of SDS (C) and (F). Scale bars denotes a length of 1 lm in A–C and 100 lm in D–F. Images A–C shows a single boundary interface of the filler and the matrix polymer.
Both LDH and SDS, on the other hand, enhanced plasticity. LDH particles improved the resistance of BTMP composite to deformation by 5% and the break strain by 10%. The impact of LDH on the BTMP composites Ymod and break increased with strain rate. The recorded values were 5% and 50% respectively. Electrostatically bound SDS on particle surface provided substantial increase in ductility of BTMP composites as break increased by 70% at both strain rates. Based on the decrease in Rmod the functionalization tends to reduce the overall elasticity of the composite. Applied torque increased when functionalized BKraft fibres were used in composite preparation (see above). Comparing the LDH modified and SDS functionalized fibre reinforced composites the Ymod was left practically unaffected. However, a small increase in rultimate and rbreak were observed. Simultaneous 20% decrease in break was evident. Higher lav could partially explain this behaviour. The total work needed for inducing plastic deformation was not affected by SDS while LDH particles decreased it by 25% in comparison to reference. Tentatively, the layer of LDH particles on the fibre surface induced irreversible slippage on polymer–particle interface
under tension. After applying SDS, LDH particles acted as a physical bearing in between the filler and matrix polymer. 3.5. Scanning electron and optical microscope analysis Fig. 4 confirms the assumption that air was precluding complete filling of the mould, which explains the deviation in the mass fraction of fibres. In Fig. 5A–F the optical micrographs from the skin layer and the breaking region are presented. BKraft fibres (A–C) suffered length reduction upon modification with LDH particles whereas functionalization was able to retain lav much better. This confirms the assumption made from increased torque (see above). Fibres are also seen to orient randomly, even perpendicularly against the melt flow in Fig. 5B. Longer fibres appear to orient more parallel to the flow (Fig. 5C). With BTMP fibres this was less obvious. However, evolution of microcracks and necking induced by modification and functionalization are clearly observable in the break region. In short fibre reinforced composites (lav is below critical fibre length) it is less likely for fibres to break. The parallel oriented fibres tend to be pulled out and perpendicularly oriented
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fibres debonded [46]. Notably, functionalization with SDS surfactant provided homogenous dispersion of both BTMP and BKraft fibres in the hydrophobic polymer melt. However, even though the SDS reduced drastically the fibre aggregation problem in these composites, the mechanical properties were not enhanced due to the lack of fibre–matrix adhesion which is illustrated in Fig. 6A–F. The electron diffraction spectrum revealed that majority of LDH particles remained on the fibre surface after moulding. Dispersion of particles into polymer melt was also observed. Functionalization was more effective with this respect as the polymer surface was covered by particles at the break failure interface (Fig. 6A–C). Also, the fibre pull out effect was more pronounced with functionalized pulp (Fig. 6D–F). While matrix polymer seemed to be unable to wet the LDH modified fibre surface, the stress failure in composites prepared from functionalized pulp fibres seemed to occur at the particle–particle interface.
4. Conclusions We have elucidated interactions of the atactic polypropylene matrix and softwood pulp fibres in micro-injection moulded composites. Applied fibres were functionalized with LDH particles in a system readily implementable to industrial pulp and paper processes. The mass fraction of LDH particles on pulp fibres was 0.045, while fibres to matrix mass ratio was approximately 0.16. The provided functionalization method with SDS surfactant enhanced plasticity of natural fibre reinforced composites and allowed homogeneous dispersion of pulp fibres in a-PP matrix. After functionalization the BKraft fibres from chemical pulping process endured the applied shear force better during compounding than the BTMP fibres from mechanical pulping process. Primary reason was addressed to both, the structural differences of fibre cell wall layers and the surface modification with SDS. High shear forces dispersed some of the functionalized particles into polymer phase. The particle cohesion forces were therefore identified as the bottle-neck in this process while addition of the SDS surfactant seemed to promote particle matrix adhesion. Since the electrostatically bound non-functionalized LDH remained, regardless of the relatively high shear, largely on fibre surface, the optimization of the pulp fibres performance in composite applications requires the control of the LDH particle layer thickness on fibre surface. Although an injection moulding of small parts was not suitable for the proposed system, we are confident that the method provides a new functionalization platform for natural fibres in composite applications that can be exploited in industrial scale.
with Instron and Professor Pedro Fardim was responsible for overall supervision. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
Acknowledgments We would like to thank TEKES (Finnish Funding Agency for Technology and Innovation) for their financial support. We would also like to thank UPM-Kymmene, Stora Enso and Metsä Fiber for their financial support and both the UPM-Kymmene Rauma mill and Metsä Fibre Rauma mill for providing the pulp for the experiments. Finally we acknowledge all the laboratory personnel at Åbo Akademi University who assisted with the instrumentation. Carl Lange carried out the study, as part of his Ph.D. thesis, and was responsible for all parts of the research project, including writing the paper. Farid Touaiti was responsible of tensile experiments
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