Nanocomposite-based lignocellulosic fibers 2: Layer-by-layer modification of wood fibers for reinforcement in thermoplastic composites

Composites: Part A 42 (2011) 84–91

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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa

Nanocomposite-based lignocellulosic fibers 2: Layer-by-layer modification of wood fibers for reinforcement in thermoplastic composites Zhiyuan Lin a, Scott Renneckar b,⇑ a b

Sustainable Engineered Materials Institute, Virginia Tech, Blacksburg, VA 24061, United States Department of Wood Science and Forest Products, Virginia Tech, Blacksburg, VA 24061, United States

a r t i c l e

i n f o

Article history: Received 24 June 2010 Received in revised form 21 September 2010 Accepted 9 October 2010

Keywords: E. Layer-by-layer A. Clay thin films A. Nano-composites A. Wood thermoplastic composites

a b s t r a c t The present study investigated the layer-by-layer (LbL) modification of wood fibers for reinforcement in thermoplastic composites. The modification process involves the manipulation of surfaces with highly controlled chemistries using polyelectrolytes and nanoparticles. Fibers were modified with layers of montmorillonite clay and poly(diallyldimethylammonium) chloride (PDDA) using sequential adsorption steps to create the LbL modified fibers. The composites were prepared by melt compounding polypropylene (PP) and unmodified/LbL modified fibers, followed by compression molding. The thermal properties of the composites were characterized by differential scanning calorimetry and dynamic mechanical analysis, while mechanical properties were tested in tensile and flexural test modes. LbL modified fiber composites have similar modulus values but lower strength values than those of unmodified fiber composites. However, composites composed of LbL modified fibers display enhanced elongation at break, increasing by more than 50%, to those of unmodified samples. DSC results indicated that crystallization behavior of PP is promoted in the presence of wood fibers. Both unmodified and LbL modified wood fibers are able to act as nucleation agents, which caused an increase of the crystallinity of PP by the presence of wood fiber. Moreover, results from tensile and flexural strength, dynamic mechanical analysis and water absorption tests reveal that the material (PDDA or clay) at the terminal (outer) layer of LbL modified fiber influences the performance of the composites. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Over the past two decades, the use of lignocellulosic fibers as reinforcing elements for thermoplastic composites has experienced a dramatic growth in North America in applications for automobiles, building materials, and packaging industries [1–3]. The lignocellulosic fibers offer a combination of attractive properties such as low density, high specific strength and modulus, renewability, biodegradability, wide availability, and low cost, which make them alternatives to traditional synthetic fibers in many applications. However, some constraints do exist for the use of lignocellulosic fibers in thermoplastic composites. The low thermal stability of lignocellulosic fibers restricts the allowable processing temperature, which is generally lower than 200 °C. The hydrophilic nature of lignocellulosic fibers makes them difficult to process in many hydrophobic thermoplastics, such as polyethylene and polypropylene. Many efforts have been made to overcome these limitations including a variety of surface modification methods for lignocellulosic fiber such as maleic anhydride polypropylene

⇑ Corresponding author. Tel.: +1 540 231 7100; fax: +1 540 231 8176. E-mail address: [email protected] (S. Renneckar). 1359-835X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2010.10.011

(MAPP), and other coupling agents including polyisocyanates and silanes [4–7]. These surface modification methods either reduce the number of polar groups of the lignocellulosic fibers or introduce cross-linking and entanglements by physical and chemical bonds between the fibers and the matrix resin [8]. In recent years, the layer-by-layer (LbL) self-assembly technique has gained attention because it can modify substrate surface chemistry with a simple process of adsorption [9]. This technique involves the sequential adsorption of oppositely charged polyelectrolytes to the fiber surfaces. A great variety of organic or inorganic components have been incorporated into the LbL system to create advanced materials with nanoscale architectures with desired properties [10–18]. The LbL modification method has been investigated in the field of paper science and successfully applied to cellulosic pulp fibers to enhance the properties of paper [19– 23]. Limited research has been carried out on LbL modification of wood fibers for reinforcement in thermoplastic composites. Due to the ability to tailor the surface chemistry of substrates, the LbL fiber modification method, if properly exploited, may be able to alleviate the above-mentioned issues encountered for wood thermoplastic composites. This technique may be significant in creating specific interactions, rather simply, amongst the reinforcing agents and matrix.

Z. Lin, S. Renneckar / Composites: Part A 42 (2011) 84–91

Specific surface area of the substrates impacts the loading of the LbL films on the wood fibers. Steam-explosion processing of biomass can create a high surface area fiber as compared with the traditional ‘‘disc refiner” approach [24–26]. Steam-explosion processing is a thermo-hydrolytic-mechanical process, where wood biomass is pressurized with high steam pressure and elevated temperature, followed by a sudden decompression. This biomass treatment process is very similar to the industrial ‘‘Masonite Process”, which was developed for fiberboard production [27]. The steam explosion affects all the constitutive polymers of wood [28]. Available surface area of wood fibers increases after steam explosion treatment, which favors the subsequent LbL polyelectrolyte adsorption since it is a surface modification technique. In this study, we investigated the layer-by-layer technique as a modification method for lignocellulosic wood fibers as reinforcement in thermoplastic composites. The inorganic montmorillonite clay nanoparticles were incorporated in the LbL system as the negatively charged components because of its unique physical and mechanical properties: adding clays to organic polymers has shown to be a productive method to improve the stiffness, barrier, and thermal properties of thermoplastics and elastomers [29–32]. Additionally, poly(diallyldimethylammonium) chloride (PDDA) was used as the positively charged component, which quaternary ammonium groups have been shown to have anti-microbrial properties. The mechanical, thermal, and sorption properties of composites were characterized by static and dynamic mechanical tests, thermal analysis, and water absorption test to determine the effect of the fiber coating as a function of the number of layers deposited and terminal surface chemistry. 2. Experimental 2.1. Materials Wood fibers were obtained through steam explosion of yellowpoplar (Liriodendron tulipifera) veneers of dimensions 25  25  4 mm. The wood chips were steam-exploded at 240 °C for 3 min, resulting in a steam explosion severity factor (log R0) of 4.6 [33]. Positively charged poly(diallyldimethylammonium) chloride (PDDA) used in the study was an aqueous solution (Mw 200,000– 350,000) obtained from Sigma–Aldrich Inc., USA. Montmorillonite clay used was a high purity sodium montmorillonite donated by Kunimine Industries, Japan. Isotactic polypropylene (PP) pellets (average Mw 340,000, density 0.9 g/mL, melting index 4 g/10 min, Sigma Aldrich) was the thermoplastic matrix used in this study. 2.2. Fiber treatment and layer-by-layer modification Both the extraction and layer-by-layer adsorption process were performed using a stirred and jacketed Pfaudler reactor (60 L) with a large anchor-type agitator to prevent significant shearing of the fibers during mixing. Prior to the layer-by-layer modification, the dark brown steam-exploded fibers (SEF) were first extracted with water at 60 °C for 24 h followed by alkali extraction with 20% (based on fiber solids weight) aqueous sodium hydroxide using an 8:1 liquor-to-fiber ratio at 60 °C for 30 min [34]. After water and alkali extraction, the light brown fibers were isolated and washed multiple times with ultrapure Mill-Q water using a centrifuge until the supernatant pH became neutral. Positively charged PDDA and negatively charged montmorillonite clay were alternately adsorbed onto the fiber surface using the layer-by-layer assembly process. Before the layer adsorption, ultrasonication (750 W, VC 750 Sonics & Materials, Inc.) was applied to the clay suspension for 6 min to break the clay into nanoscale platelets [17]. The detailed LbL deposition process can be described as fol-

85

lows: 500 g (dry weight basis) of alkali extracted fibers were added to 50 L of a 5 mg/mL cationic PDDA solution. The deposition time was 30 min with stirring. After PDDA adsorption, the samples were recovered using a centrifuge. Next, 50 L of 1 mg/mL montmorillonite clay suspension was added to coat the fibers with the first anionic layer. The procedure was repeated multiple times to achieve the desired number of bi-layers. No salt and pH adjustment was performed during the PDDA adsorption, which was the optimal condition for PDDA and LbL adsorption determined previously [35]. To investigate the potential effects of LbL modification (the number of the bi-layers and the outmost adsorption layer) on the performance of final composites, LbL modified fibers with five different numbers of bi-layers were prepared: 0.5, 1, 2, 3, 4, and 4.5, with 1 bi-layer designates a combination of 1 layer of PDDA and 1 layer of clay adsorbed onto the fiber. Therefore, bilayer number 0.5 refers to fiber that was only coated with a single layer of PDDA. The unmodified alkali extracted SEF (bi-layer #0) were used as control fibers. After fiber treatment and LbL modification, wood fibers were air-dried and ground with a Thomas-Wiley mill to pass through a 10-mesh screen. 2.3. Compounding of PP and wood fibers Prior to compounding, all the fibers were oven dried at 80 °C for 24 h to further remove the moisture. Compounding of wood fibers and polypropylene pellets was performed in a Brabarder compounder (CW Brabarder Prep-Center) with an internal mixer. The mixing was carried out at 180 °C with a rotation speed of 60 rpm and a blending time of 15 min. For each batch, the total starting mass of materials was 45 g. The fiber volume fraction was kept constant at 25% for all the fiber composites. Following compounding, all the blends were cooled to room temperature and ground into 10-mesh powder with a Thomas-Wiley mill. 2.4. Preparation of composites The wood–polypropylene composites were prepared by compression molding process using a square steel mold with dimensions of 152.4  152.4 mm. The wood–polypropylene blends were first added into the cold mold. The platens of a 152.4  152.4 mm Carver hot press were preheated to 185 °C. A minimum pressure (close to zero) was applied during the preheating step to maintain the contact between the platens and mold. The pressure was then increased slowly to 2 MPa in 2 min and held at 2 MPa for an additional 10 min. The mold was then removed from the hot press and cooled to room temperature in a separate cold press under the same pressure (2 MPa). The target thickness of plaques was 3.18 mm. 2.5. Thermogravimetric analysis (TGA) Thermogravimetric analysis was performed on the fiber samples to determine the weight loss as a function of temperature. Thermal stability of unmodified fibers (0 bi-layer), LbL modified (1, 2, 3, and 4 bi-layers) fibers and LbL modified fiber reinforced PP composites were analyzed using a TA Instruments Q-500 with a platinum sample pan. All the samples were vacuum-dried (40 °C, 1.33 mbar) and stored in desiccators prior to tests. Samples of ca.10 mg were ramp heated from room temperature to 800 °C with a constant heating rate of 10 °C/min under air atmospheres. Each sample type was measured in triplicate. 2.6. Mechanical tests The tensile and flexural mechanical properties of the composites were measured in accordance with the ASTM D638-03 and

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ASTM D790-03, respectively. The tensile tests were carried out on a MTS testing machine equipped with a 44,482 N load cell. The length and width of the Type I test specimens (dumbbell-shaped, cut on a router table) were 152.4 and 19.05 mm with a gauge length of 50.8 mm. The width and length of the narrow section was 12.7 and 50.8 mm, respectively. The constant crosshead speed was 0.25 mm/min. An extensometer was used to measure the displacement within the gauge length of test specimens. The dimensions of specimens for flexural tests were 63  12.7  3.18 mm (length  width  depth). The span-to-depth ratio of 16:1 was used to avoid significant shear affects during bending. The three point bending tests were performed at a constant crosshead (midspan deflection) speed of 0.64 mm/min. The midspan deflection was determined by a linear variable differential transformer (LVDT) under the specimen in contact with it at the center of the support span. The specimens were deflected until rupture occurred in the outer surface of the test specimens or until a maximum midspan deflection of 5.08 mm was reached, whichever occurred first. Tensile modulus, tensile strength, flexural modulus, and flexural strength properties of composites were calculated using the equations from ASTM D638-03 and ASTM D790-03. At least ten and twelve replications of each sample type were tested for tensile and flexural tests, respectively. Tukey’s Honestly Significant Difference (HSD) multiple comparison analysis was performed to determine the statistical significance of the means among different treatments (a = 0.05). 2.7. Dynamic mechanical analysis (DMA) The dynamic mechanical properties of unfilled neat polypropylene, unmodified, and LbL (4, and 4.5 bi-layers) modified fiber composites were determined by a TA instruments Q800 dynamic mechanical analyzer using a single cantilever mode. DMA specimens, 35  12.7  3.18 mm, were heated from 70 to 150 °C with a heating rate of 3 °C/min and a frequency of 1 Hz in dry air atmosphere. The constant strain of 0.03% determined by the strain sweep tests was used in DMA. At least eight specimens were tested for each formulation. 2.8. Differential scanning calorimetry analysis (DSC) All DSC experiments were conducted on a Q100 DSC from TA instruments. Sample sizes of ca 10 mg were sealed in aluminum crimped pans. The DSC temperature program consisted of: (1) ramp from 80 to 200 °C at 10 °C/min, (2) cool from 200 to 80 °C at 5 °C/min, and then (3) ramp from 80 to 200 °C at 10 °C/min. The first heating cycle was intended to remove any prior thermal history resulted from the compression molding and cooling process in the composite manufacture. Each sample type was measured in triplicate. The degree of crystallinity (Xc) of the each polymer and composite was calculated using the following equation:

Xc ¼

DHf  100 DH0f W p

ð1Þ

where DHf is the heat of fusion of PP and composites, DH0f is the theoretical heat of fusion of 100% crystalline PP (138 J/g) [36], and Wp is the mass fraction of PP in the composites. 2.9. Water absorption test The sorption properties of unmodified and LbL (0.5, 1, 2, 3, 4, and 4.5 bi-layers) modified fiber composite materials were evaluated in accordance with ASTM D570-98. 24 h water immersion tests were performed to assess the water absorption behavior of

composites. The test specimens were in the form of a bar 76.2 mm by 25.4 mm with a thickness of 3.18 mm. The specimens were entirely immersed in distilled water at 25 °C for 24 h. The weights of the specimens before and after soaking were recorded. Prior to the test, the specimens were dried in an oven for 24 h at 50 ± 3 °C, cooled in desiccators and immediately weighed. Percent weight gain (%) after water adsorption was calculated by the following equation.

Percent weight gain ¼

Ww  Wc  100 Wc

ð2Þ

where Ww represents the wet weight of specimen, Wc is the conditioned weight of specimen before water immersion. Eight replications of each sample type were tested. 3. Results and discussion 3.1. TGA Thermal stability properties of unmodified and LbL modified fibers are summarized as a function of the number of bi-layers (Table 1). Temperature at 5% and 10% weight loss, maximum degradation rate and residue weight at 800 °C were listed. Note that the 5% and 10% thermal degradation is calculated after taking in account the loss of moisture and this value can be considered weight loss of dry fiber mass. LbL modified fibers show enhanced thermal stability relative to the unmodified fibers. As can be seen from Table 1, the temperatures at 5% and 10% thermal degradation increase as the number of bi-layers increases; average temperature at 5% and 10% weight loss after 4 bi-layers modification increases by 24 and 15 °C, respectively. LbL modification also decreases the maximum degradation rate. The value is reduced by 44% (from 3.16% to 1.78%) for 4 bi-layer coating. In addition, significant char residue formed for the LbL modified wood fibers (29.4% for 4 bi-layers) suggests this clay-based coating serves as a barrier creating an insulating layer to prevent further decomposition of the material [17]. It should be noted that improvements in thermal stability of LbL modified fibers in this study are greater than previously reported values [17] because the optimal deposition conditions were used in this study, which resulted in higher PDDA/clay adsorption [35]. Table 2 shows the thermal stability properties of neat PP, unmodified, and LbL modified fiber composites. The maximum degradation rate of composites decreases with the increase of number of bi-layers, reducing from 1.7 (unmodified fibers) to 1.36 %/°C after 4 bi-layer adsorption. As similar to that of fibers, the residue weight of composites also increases with increasing number of bi-layers. The char residue of composites reaches 11.1% for 4 bi-layer modification as compared to only 0.2% for the unmodified fiber composites. However, no significant difference in degradation temperature at 5% and 10% weight loss is de-

Table 1 Thermal stability of unmodified and LbL modified fibers with different number of bilayers. Bi-layer #

Temperature at 5% weight loss (°C)

Temperature at 10% weight loss (°C)

Maximum deg rate (%/°C)

Residue weight after heated to 800 °C (%)

0 1 2 3 4

296 ± 1.7 307 ± 1.1 310 ± 0.02 314 ± 0.06 320 ± 0.03

317 ± 0.9 324 ± 0.9 327 ± 0.2 329 ± 0.07 332 ± 0.4

3.16 ± 0.01 2.57 ± 0.03 2.09 ± 0.06 2.02 ± 0.01 1.78 ± 0.01

0.6 ± 0.08a 6.0 ± 0.9 17.0 ± 0.9 21.0 ± 1.3 29.4 ± 2.8

a Residue weight of unmodified fibers (bi-layer #0) was measured after heated to 600 °C.

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Z. Lin, S. Renneckar / Composites: Part A 42 (2011) 84–91 Table 2 Thermal stability of neat PP, unmodified and LbL modified fiber composites.

26

Temperature at 10% weight loss (°C)

Maximum deg rate (%/°C)

Residue weight after heated to 800 °C (%)

PP 0 1 2 3 4

275 ± 1.4 272 ± 2.1 276 ± 0.8 270 ± 0.6 272 ± 2.4 272 ± 1.2

284 ± 1.2 291 ± 2.3 292 ± 1.2 287 ± 0.6 291 ± 3.1 289 ± 1.5

1.60 ± 0.04 1.70 ± 0.04 1.56 ± 0.09 1.27 ± 0.04 1.35 ± 0.01 1.36 ± 0.06

0.2 ± 0.1 0.2 ± 0.08 2.0 ± 0.6 6.5 ± 0.9 9.3 ± 1.7 11.1 ± 0.7

42

Tensile strength (MPa)

Temperature at 5% weight loss (°C)

tected between unmodified and LbL modified fiber composites, which may due to the relatively low fiber volume fraction (25%) employed in this study.

3 5 E11 þ E22 8 8

36

18

16

34

PP

0

0.5

1

2

3

4

4.5

Fiber treatments

ð3Þ

3.0

2.5

2.0

1.5

1.0 0

0.5

1

2

3

4

4.5

Fiber treatments Fig. 3. Elongation properties of unmodified and LbL modified fiber composites.

where Ec is the modulus of random fiber reinforced thermoplastic composites, which we assumed our composites to be. E11 and E22 are longitudinal and transverse tensile moduli of corresponding unidirectional fiber composites, which can be estimated by the simple ‘‘rule of the mixtures” models (Eqs. (4) and (5)):

E11 ¼ Ef V f þ Em V m

ð4Þ

Ef Em Ef V m þ Em V f

ð5Þ

3.0

E22 ¼ 2.5

2.5

2.0

2.0

1.5

1.5

tensile modulus flexural modulus 1.0

Flexural modulus (GPa)

Tensile modulus (GPa)

38

20

Elongation at break (%)

Tensile (modulus of elasticity [MOE], tensile strength, and percent elongation at break) and flexural (MOE, flexural strength [MOR]) mechanical properties of neat PP, unmodified, and LbL modified fiber composites are shown in Figs. 1–3 and Table 3. Tukey grouping letter (a = 0.05) and density of fiber composites are also presented in Table 3. The error bars in the figures represent the standard deviation of data. The addition of unmodified wood fibers to the neat PP significantly increases both tensile and flexural modulus of composites (Fig. 1 and Table 3). MOE of unmodified fiber composites (sample #0) increases by 87% (1.31– 2.45 GPa from tensile tests) and 139% (0.95–2.27 GPa for flexural tests) as compared with the neat PP. This increase in modulus demonstrates that higher modulus wood fibers can stiffen the thermoplastic PP matrix. The LbL adsorption of PDDA and clay multilayers to the surface of the fibers has marginal influence on the modulus properties of composites (Fig. 1 and Table 3). One reason for this occurrence may be attributed to the comparable MOE values between steam-exploded fiber and adsorbed PDDA/clay bi-layers. The reported MOE of PDDA/clay multilayers is 11 GPa [37], which is very similar to that of unmodified steam-exploded fibers (also 11 GPa); this value was determined using the semi-empirical Tsai-Pagano micromechanical model [38]:

3.0

40 22

Fig. 2. Tensile and flexural strength properties of neat PP, unmodified and LbL modified fiber composites.

3.2. Tensile and flexural mechanical properties of composites

Ec ¼

tensile strength MOR

24

MOR (MPa)

Bi-layer#

1.0

PP

0

0.5

1

2

3

4

4.5

Fiber treatments Fig. 1. Tensile and flexural MOE of neat PP, unmodified and LbL modified fiber composites. X-axis categorical values of 0, 0.5, 1, 2, 3, 4, and 4.5 indicate the number of bi-layers of the fiber reinforced PP composites. Neat PP is listed as control.

where Ef and Em are modulus of fiber and matrix, respectively. Vf and Vm are volume fractions of fiber and matrix, respectively. In this study, fiber volume fraction is 25%; tensile modulus of polypropylene and unmodified fiber composites are 1.31 and 2.45 GPa, respectively, as measured in Table 3. In combination with Eqs. (3)–(5), the tensile modulus of unmodified steam-exploded wood fiber can be back-calculated (11 GPa). The inclusion of unmodified fibers to PP decreases the strength properties of composites (Fig. 2). Tensile strength of composites reduces from 23.99 to 21.92 MPa by the unmodified wood fiber loading. Similar results were also reported that addition of 40 wt.% hardwood fibers to PP decreased tensile strength of composites, from 28.5 to 28.2 MPa as compared to neat PP [39]. The LbL modification of fibers has a statistically significant influence on strength properties of composites (Table 3). Both tensile strength and MOR of all LbL modified fiber composites are lowered than

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Table 3 Tensile, flexural mechanical properties and density of neat PP, unmodified and LbL modified fiber composites. Bi-layer#

PP 0 0.5 1 2 3 4 4.5

Density (g/cm3)

Flexural a

MOE (GPa)

Strength (MPa)

Percent EAB (%)

MOE (GPa)

MOR (MPa)

1.31 ± 0.05(C)b 2.45 ± 0.12(B) 2.54 ± 0.09(AB) 2.64 ± 0.08(A) 2.58 ± 0.10(AB) 2.64 ± 0.10(A) 2.61 ± 0.12(A) 2.40 ± 0.10(B)

23.99 ± 1.03(A) 21.92 ± 1.10(B) 19.04 ± 0.92(C) 19.08 ± 0.39(C) 18.00 ± 0.50(C) 17.95 ± 0.85(C) 18.45 ± 0.12(C) 17.41 ± 0.34(D)

Did not break 1.37 ± 0.10(D) 2.12 ± 0.13(A) 1.57 ± 0.19(CD) 1.72 ± 0.15(C) 1.82 ± 0.21(BC) 2.05 ± 0.19(AB) 2.09 ± 0.19(A)

0.95 ± 0.09(C) 2.27 ± 0.14(A) 2.23 ± 0.09(A) 2.27 ± 0.08(A) 2.18 ± 0.09(AB) 2.19 ± 0.11(A) 2.22 ± 0.08(A) 2.07 ± 0.08(B)

N/A 41.30 ± 1.57(A) 38.45 ± 1.10(B) 38.50 ± 0.82(B) 36.83 ± 0.79(C) 36.26 ± 0.99(CD) 36.94 ± 0.82(C) 35.19 ± 0.70(D)

0.9 1.02 1.02 1.03 1.05 1.06 1.07 1.07

EAB refers to elongation at break. Letters in parentheses refer to Tukey grouping letters.

of multilayered structure of PDDA and clay when extending the specimens because the surface is under considerable shear stress [37]. 3.3. DMA The temperature dependence of storage modulus and tan d of neat PP, unmodified and LbL modified fiber composites are shown in Fig. 4. In Fig. 4, it is observed that the neat polypropylene has the lowest storage modulus compared to all composites throughout the entire temperature range. Storage modulus curves of both polypropylene and composite samples display three different distinct regions with the increase of temperature: a fairly flat glassy region, a glass to rubber transition region, and a slower declining rubbery region. From the tan d data, neat PP exhibits a clear thermal transition in the vicinity of 10 °C. This transition corresponds to b relaxation of unrestricted amorphous portion, which often refers to the glass transition temperature (Tg) of PP. Table 4 summarizes the glass transition temperature of neat PP, unmodified, and LbL modified fiber composites determined from peak of tan d

4000

E' (MPa)

those of unmodified fiber composites (Fig. 2 and Table 3). For short fiber reinforced thermoplastic composites, strength is impacted by stress transfer efficiency from matrix to fibers. Therefore, these reductions in strength after LbL modification may suggest that the interfacial adhesion between LbL modified fiber and matrix is decreased as compared to the unmodified one. More specifically, the strength properties of composites with LbL fiber after only 0.5 bi-layer adsorption (PDDA as the outmost layer) are lower than those of unmodified fiber composites. This observation may suggest that PDDA covered fiber surfaces have reduced interfacial adhesion with PP as compared with untreated fibers. Moreover, one consistent finding from both tensile and flexural tests is that composites with LbL modified fiber after 4.5 bi-layers modification (PDDA as the outmost layer) shows reduced strength properties to that of 4 bi-layers (clay as the outmost layer). The material as the outmost layer (PDDA or clay) plays a role in strength properties of LbL modified composites. The strength properties suggest that LbL fiber with clay as the terminal layer displays a better interfacial interaction with PP than PDDA covered fiber. It is interesting to note that the MOR of the unmodified fiber composites in our study is 41.3 MPa, which is substantially higher than the result from previous work with steam-exploded fiber. Yin et al. reported a MOR of 27.86 MPa for steam-exploded fiber reinforced polypropylene composites in a similar system, in which fiber weight fraction was 50% [40]. The lower strength values obtained from Yin et al.’s study might be attributed to insufficient wetting of the fibers and increased fiber to fiber interactions in the case of high fiber content [41]. Otherwise, the values are within range of reported wood–polypropylene composites as reported in the Wood Handbook accounting for the differences in fiber loading [39]. Percent elongation at break (EAB) of fiber composites increases upon the modification of wood fiber with bi-layers (Fig. 3). After 4 bi-layers of PDDA and clay, EAB of composites increases by 50% (from 1.37 to 2.05%) as compared with the unmodified fiber composites, which indicates that fiber composites become more ductile after LbL modification. Similar to strength properties, elongation properties of modified fiber composites are also influenced by the terminal layer of LbL adsorption, which directly contacts PP matrix. The LbL modified fiber composites exhibit the greatest elongation responses at 0.5 and 4.5 layer modification, where PDDA is the terminal layer on the fiber surface. This finding further confirmed that the interfacial adhesion between fiber and PP is diminished after PDDA adsorption. The reduced interfacial adhesion between PDDA coated fibers and polypropylene leads to decreased restraint of the PP matrix by the fibers, allowing the matrix to behave more like a neat PP under the tensile stress. This phenomenon ultimately contributes to the elevated elongation properties of PDDA modified fiber composites. Percent elongation at break of composites also increases with increasing number of bi-layers, which may be attributed to potential parallel slippage

3000

2000

PP 0

1000

4 4.5

0.10

0.08

tan δ

a b

Tensile

0.06

0.04

0.02 -100

-50

0

50

100

150

Temperature (°C) Fig. 4. Storage modulus and tan d spectra of neat PP, unmodified and LbL modified fiber composites.

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Z. Lin, S. Renneckar / Composites: Part A 42 (2011) 84–91 Table 4 Glass transition temperature of neat PP, unmodified and LbL modified fiber composites.

14.5 ± 0.8 11.8 ± 0.3 12.5 ± 0.4 10.6 ± 0.8

curves. As seen in Table 4, Tg is slightly shifted to lower temperature for unmodified composites as compared to the neat PP. Tg decreases from 14.5 to 11.8 °C as determined from tan d curve. This reduction in Tg implies that the addition of the wood fibers alters the response of the polymer chains. Similar finding has also been reported in other studies [40,42,43]. Nunez et al. concluded that the nucleating effect of the wood fibers accelerated the crystallization process of PP. This crystallization led to an amorphous zone with altered mobility, which resulted in a lower Tg value [42]. The observation of a change in crystallization is confirmed in the DSC data in the subsequent section. No evident change in glass transition temperature of PP is observed between composites reinforced by unmodified fibers and LbL modified fibers with 4 bi-layers of adsorption. However, as shown in Table 4, glass transition temperature shows a slight but statistically significant decrease from modified composites with 4 bi-layers modification to those with 4.5 bi-layers: Tg decrease by 1.9 °C (from 12.5 to 10.6 °C). The main difference between those two fibers is whether PDDA or clay serves as the outmost layer. This finding is consistent with strength properties of LbL modified fiber composites in the previous section. The LbL modified fiber with PDDA as the outmost layer (4.5 bi-layers), compared to fiber with clay as the outmost layer (4 bi-layers), impacts the response of the polymer composites. The data suggests that polypropylene polymer chains are not as restrained when PDDA is the terminal layer for fiber modification, which leads to the drop in Tg for 4.5 bi-layer modified fiber composites. Moreover, as illustrated in Fig. 4, the magnitude of tan d peak drops by the addition of wood fibers in the transition region. Lower tan d value suggests that the composites display more elastic (spring-like) than viscous (dashpot-like) behavior as compared with neat PP, as the volume fraction of PP decreases in the case of composites. In addition, composites with fibers after 4.5 bi-layers modification displayed higher tan d values than those of fibers with 4 bi-layer and unmodified fibers. Again, this suggests a relative weaker interfacial adhesion for the fibers with PDDA as the terminal layer as compared to the fiber with clay as the terminal layer and the unmodified fiber. 3.4. DSC Figs. 5 and 6 show the 1st cooling and 2nd heating curves of neat PP, unmodified and modified fiber composites. The melting temperature (Tm), crystallization temperature, heat of fusion (DHf), and calculated degree of crystallinity (Xc) for the neat PP, unmodified and, LbL modified composites determined from cooling and second heating cycles of DSC experiments are summarized in Table 5. The crystallization onset temperature (Tos) represents the beginning point of the crystallization process and the temperature at the crystallization peak is represented by Tc. Therefore, Tos–Tc can be used as an index to indicate the crystallization rate of materials; a small Tos–Tc value refers to a fast crystallization process. An increase in the crystallinity of PP is observed by adding the wood fibers to the PP matrix (Table 5). The crystallinity of PP increases from 59.9 to 66.9 with the inclusion of unmodified fibers, which is due to the fact that wood fibers are able to act as nucleat-

Endotherm

Tg (°C)

PP 0 4 4.5

60

80

100

120

140

160

180

Temperature (°C) Fig. 5. DSC cooling curves of neat PP, unmodified and LbL modified fiber composites.

Endotherm

Bi-layer#

PP 0 4 4.5

PP 0 4 4.5

100

120

140

160

180

200

Temperature (°C) Fig. 6. DSC 2nd melting curves of neat PP, unmodified and LbL modified fiber composites.

ing agents during crystallization of PP [40,42,44]. The crystallization peak temperature of PP (Tc) also increases in the presence of wood as shown in Fig. 5. For the neat PP, the exothermic peak occurs at Tc = 111.3 °C. These exothermic peaks shift to higher temperature of Tc = 125.7 °C for unmodified wood composites. Moreover, the crystallization rate (Tos–Tc) increases as wood fibers are incorporated, from 5.7 to 3.2 °C (Table 5). This increase in crystallization rate, in combination with the higher crystallization peak temperature (Tc), indicates that the crystallization is favored in the presence of wood fibers [42,45]. This phenomenon further supports the role of wood fibers as nucleating sites, inducing the crystallization of the matrix. The accelerated crystallization rate by wood fiber loading leads to an inhomogeneous amorphous phase with greater mobility in the unmodified wood fiber–PP composTable 5 Cooling and melting parameters of neat PP, unmodified and LbL modified fiber composites. Bi-layer#

PP 0 4 4.5

Cooling

Second heating

TOS (°C)

Tc (°C)

Tos–Tc (°C)

Tm (°C)

DHf (J/g)

Xc (%)

117.0 128.9 131.0 131.0

111.3 125.7 127.1 127.1

5.7 3.2 3.9 3.9

161.8 161.5 163.6 163.3

83.3 59.9 56.4 56.4

59.9 ± 0.7 66.9 ± 1.5 66.2 ± 0.7 66.3 ± 0.7

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ites, which supports the lower Tg value detected by the DMA measurements in the previous section. LbL modified fiber induces the crystallization of PP at higher temperature as compared with the unmodified fiber (Table 5). Both the crystallization onset and peak temperature (Tos and Tc) is moved upwards, ca 1.5–2 °C, by incorporation of PDDA/clay bi-layers. The crystallization rate of LbL modified fiber composites is slower compared with the unmodified one, as indicated by Tos–Tc values (from 3.2 to 3.9 °C). This result implies that larger spherulites may be obtained close to the LbL coated fiber surface than those in the unmodified fiber composites [45]. The crystallinity of composites does not significantly change upon LbL modification (Table 5). However, the melting temperature of LbL modified fiber composite does increase from 161.5 to 163.6 °C as compared to the unmodified one. This increment in melting temperature might be attributed to the formation of different crystalline phases in PP matrix [46,47]. It has been reported that both a and b crystalline phases, which have varying melting temperature, can be formed in PP in the presence of heterogeneous lignocellulosic fibers [46]. The higher melting temperature of LbL modified fiber composites may imply that lower portion of b crystalline phases (with lower melting temperature) is present in these materials. No difference in melting and crystallization behavior (Tos, Tc, Tm, Xc) of modified fiber composites is observed between PDDA and clay as the terminal layer of adsorption (4 vs. 4.5 bi-layers).

3.5. Water absorption tests The percent weight gain values of unmodified and LbL modified fiber composites after 24 h water immersion are presented in Fig. 7. Both unmodified and modified fiber composites show excellent water resistant properties due to the high volume fraction of hydrophobic PP matrix in the composites. As illustrated in Fig. 7, percent weight gain of all the composites are less than 0.5%. The water sorption properties of composites slightly increase with increasing number of bi-layers, which is attributed to increased weight fraction of total LbL modified fiber components. The composites made of modified fiber with PDDA as terminal layer (both 0.5 and 4.5 bi-layers) show significantly higher water absorption properties than those of unmodified fiber and modified fiber with clay as terminal layer. Relatively weaker interfacial adhesion between PDDA coated fiber and PP may provide more access along the interfaces for water absorption, which results in the higher water absorption values for composites reinforced by fibers with PDDA as the terminal layer [48].

A novel route to control the surface chemistry of the reinforcing fiber element was studied in terms of the effect that layered film modification of the fiber on the mechanical properties of the resulting composite materials. Layered adsorption on the surface of wood fibers created films comprised of a positively charged water soluble polymer and negatively charged clay nanoplatelets; the modification of the fibers with each bi-layer treatment was illustrated by the increase in residual weight after heating to 800 °C. However, the bi-layer treatment did not have a large impact on the resulting wood–polypropylene composite. LbL modification of fibers did not have a net effect on the stiffness of the composites with the inclusion of the bi-layer films. The reported stiffness of the PDDA/clay films was similar to the stiffness of the wood component, as estimated from the data for the unmodified fiber composites. Strength properties were marginally, but statistically, affected by the surface treatment with the films. Both tensile strength and MOR of all LbL modified fiber composites were lower than those of unmodified fiber composites. The change in strength values implies a difference in interfacial adhesion between the thermoplastic matrix and the wood fiber, either brought by a difference in the work of adhesion or yielding of the film during deformation. This phenomenon was also observed as the composites with the layered films had increased percent elongation at break values. The terminal surface chemistry impacted the glass transition of the composite, the elongation at break values, and water absorption. However, the differences amongst these samples, although statistically significant and reproducible, did not greatly impact the overall composite performance. Because the terminal surface layer can be tuned dependent on coating sequence, future work should concentrate on terminal components that may serve to enhance adhesion to the matrix. Since the composites still retain adequate mechanical properties with the layered coatings on the reinforcing element, the work provides a novel path to functionalize fibers in composites that contain functional nanoparticles and polymer chemistries.

Acknowledgment This project was supported by the USDA CSREES Special Grant No. 2008-34489-19377 and the Sustainable Engineered Materials Institute, College of Natural Resources, Virginia Tech.

References

0.6

Weight gain (%)

4. Conclusions

0.4

0.2

0

0.5

1

2

3

4

4.5

Fiber treatments Fig. 7. Percent weight gain of unmodified and LbL modified fiber composites after 24 h water immersion.

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