natural fiber composites

Composites: Part A 38 (2007) 1664–1674 www.elsevier.com/locate/compositesa

Preparation and properties of recycled HDPE/natural fiber composites Yong Lei, Qinglin Wu *, Fei Yao, Yanjun Xu School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA Received 11 October 2006; received in revised form 29 January 2007; accepted 13 February 2007

Abstract Composites based on recycled high density polyethylene (RHDPE) and natural fibers were made through melt blending and compression molding. The effects of the fibers (wood and bagasse) and coupling agent type/concentration on the composite properties were studied. The use of maleated polyethylene (MAPE), carboxylated polyethylene (CAPE), and titanium-derived mixture (TDM) improved the compatibility between the bagasse fiber and RHDPE, and mechanical properties of the resultant composites compared well with those of virgin HDPE composites. The modulus and impact strength of the composites had maxima with MAPE content increase. The composites had lower crystallization peak temperatures and wider crystalline temperature range than neat RHDPE, and their thermal stability was lower than RHDPE.  2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymer-matrix composites; A. Recycling; A. Fibres; A. Thermoplastic resin

1. Introduction Natural organic fibers from renewable natural resources offer the potential to act as a biodegradable reinforcing materials alternative for the use of glass or carbon fiber and inorganic fillers. These fibers offer several advantages including high specific strength and modulus, low cost, low density, renewable nature, biodegradability, absence of associated health hazards, easy fiber surface modification, wide availability, and relative nonabrasiveness [1–4]. Much work has been done on virgin thermoplastic and natural fiber composites, which have successfully proven their applicability to various fields of technical applications, especially for load-bearing application. Thermoplastics such as polyethylene (PE) [5–8], polypropylene (PP) [9–11], poly(vinyl chloride) (PVC) [12,13], polystyrene (PS) [14], and poly(lactic acid) (PLA) [15,16] have been compounded with natural fibers (such as wood, kenaf, flax, hemp, cotton, Kraft pulp, coconut husk, areca fruit, pine-

*

Corresponding author. Tel.: +1 225 578 8369; fax: +1 225 578 4251. E-mail address: [email protected] (Q. Wu).

1359-835X/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.02.001

apple leaf, oil palm, sisal, jute, henequen leaf, ovine leather, banana, abaca, and straw) to prepare composites. However, work done on recycled plastic/natural fiber systems is still limited. Plastics account for an increasing fraction of municipal solid waste around the world. In the United States, plastic wastes (mainly consisting of PE (above 40 wt%), PVC, PP, and poly(ethylene terephthalate)) had a total volume of 19.2 million tons in 2001, accounting for about 8.4% of total municipal solid wastes [17]. Thus, used plastics are becoming a potential worldwide source of raw materials. Some studies on the reinforcement of recycled PE with wood fiber have been done [18–20]. Oksman and Lindberg reported that tensile strength of recycled PE/wood particle composites was improved with the addition of maleated styrene-ethylene/butylenes-styrene (SEBS-MA) triblock copolymer and reached its maximum level with 4 wt% SEBS-MA [18]. Paraffin reduced the agglomeration of wood fiber and increased the tensile strength and modulus, but lowered the impact strength of recycled PE/sawdust composites [19]. Use of coupling agents (CAs) to improve adhesion between the fibers and matrix can significantly enhance the composite performance [20].

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Bagasse is the fibrous residual material of sugarcane stems left after sugar extraction process from sugar mills, and contains cellulose (46.0%), hemicellulose (24.5%), lignin (19.95%), fat and waxes (3.5%), ash (2.4%), silica (2.0%) and other elements (1.7%) [21]. It is estimated that the output of bagasse fiber is 75 million metric tons worldwide per year [22]. Most of bagasse is currently used lowvalue applications. Composites based on bagasse and matrices of thermosets such as phenolic [23,24], melamine [25], and thermoplastics such as polyamide 6 [26], PP [27], PE [28–30], and poly(vinyl alcohol) [31] have been reported. It was observed that the tensile strength and the elongation at break of the polypropylene matrix composite decrease with the incorporation of bagasse fiber without treatment. However, isocyanate and mercerization treatments enhance the tensile properties of the composite [27]. Manzur et al. [28] studied the effect of the growth of phanerochaete chrysosporium in a blend of low density polyethylene and sugar cane bagasse, and Raj and Kokta [29] used PE as a binding material to improve the mechanical properties and dimensional stabilities of steamexploded bagasse composites. The fiber characteristics (i.e., the fiber type, morphology, and dimension) and polymer melt flow index (MFI) significantly affected mechanical properties of sugarcane fiber/HDPE composites, and HDPE resins with a low MFI value presented high tensile and impact strengths [30]. In this work, composites based on recycled high density polyethylene (RHDPE) and natural fibers (i.e., pine wood flour and bagasse) were prepared by melt compounding, and the effect of fibers and coupling agents on the properties of the resultant composites were investigated. 2. Experimental 2.1. Materials Recycled high density polyethylene (RHDPE) pellets were obtained from Avangard Industries, Ltd. (Houston, TX, USA). The material has a melt flow index of 0.7 g/ 10 min at 190 C, a density of 939.9 kg/m3, and melting temperature range from 110 to 140 C with a peak at 131.5 C with a heating rate of 10 C/min. Pine (Pinus sp.) flour with 20-mesh particle size from American Wood Fibers Company (Schofield, WI, USA) was used in the experiment. The raw bagasse fiber was obtained from a local sugar mill in Louisiana. Before grinding, bagasse fiber was oven-dried at 95 C for 24 h. The moisture content of oven-dried fiber was lower than 2%. The oven-dried fiber was ground with a Thomas-Wiley miller (Model 3383L10, Swedesboro, NJ) to pass through a 20-mesh screen, and then was stored in sealed plastic bags prior to compounding. Pine flour was also oven-dried at 95 C before use, and its moisture content was controlled at lower than 2%. Three active coupling agents were used: (1) Epolene G-2608, a maleated polyethylene (MAPE), and (2) Epolene

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C-16, a carboxylated polyethylene (CAPE), both from Eastman Chemical Company (Kingsport, TN, USA), and (3) KEN-REACT CAPS L38/L, a titanium-derived mixture (TDM) from Kenrich Petrochemicals, Inc. (Bayonne, NJ, USA). The melt flow index and acid number are 6– 10 g/10 min and 6.5–11 mg-KOH for the MAPE, and 1700 g/10 min and 2 mg-KOH for the CAPE. The initiator used was dicumyl peroxide (DCP) from Aldrich Chemical Company (Saint Louis, MO, USA). 2.2. Preparation of RHDPE/fiber composites All blends were made with a fiber to plastic weight ratio of 30/70. The levels of the CAs used varied from 0.6% to 4.5% for MAPE, from 0.15% to 0.9% for TDM, and from 0.9% to 2.1% for CAPE, based on the total weight of RHDPE and fiber (Table 1). For TDM, 3% DCP based on the weight of titanate was added together with fiber. The compounding process used included two steps. First all RHDPE was kneaded with and without coupling agent (depending on formulations) in a Haake rotor mixer (Model Rheomix 600, Dreieich, Germany) for 5 min. The fiber was then added and mixed for an additional 10 min. The compounding temperature was 165 C and the rotation speed was 60 rpm. All compounded blends were collected and stored in zipper bags. For sample manufacturing, each blend with a target weight was placed in a three-piece stainless molding set and compression-molded in a Wabash V200 hot press (Wabash, ID, USA) at 180 C for 5–10 min depending on sample thickness and then cooled to room temperature under pressure. The pressure for heating and cooling was controlled at 30 ton. For tensile testing and dynamic mechanical analysis, the nominal sample thickness was 1 mm, while the nominal thickness for the impact strength specimens was 4 mm. 2.3. Measurements Wide-angle X-ray diffraction (XRD) analysis was carried out with a MiniFlex diffractometer (Rigaku, Tokyo, Japan) to investigate the change of crystalline thickness of RHDPE in the composites. XRD samples were taken from compression-molded specimens and were mounted to the XRD platform for analysis. To study the effect of wood and bagasse fibers on the XRD results of the composites, both loose fiber (20 mesh) and fiber mat compressed at 30-ton pressure to form a plate shape were analyzed. A 2h range from 5 to 35 in reflection mode was scanned at 5/min. A computer-controlled wide angle goniometer coupled to a sealed-tube source of Cu Ka radi˚ ) was used. The Cu Ka line was filation (k = 1.54056 A tered electronically with a thin Ni filter. The crystalline thickness perpendicular to the reflection plane was calculated using Scherrer’s equation with the instrument width of 0.16.

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Table 1 Effect of CA content on balance torque and temperature of RHDPE/fiber compositesa System

CA

Content (wt%)

Stage 2 Torque (N m/kg)

Temperature (C)

Torque (N m/kg)

Temperature (C)

RHDPE/bagasse

MAPE

0 1.5 3.0 4.5 0.9 1.5 2.1 0.3 0.6 0.9

339.3 356.6 383.6 372.5 342.0 353.1 339.8 359.2 347.0 346.1

176.6 178.2 177.6 178.5 182.8 181.1 182.3 177.0 175.8 177.4

385.0 426.1 436.9 440.2 413.8 413.8 399.1 408.8 402.6 398.9

190.2 195.5 192.5 192.6 193.4 191.1 193.8 187.1 187.3 187.6

0 0.6 1.2 1.8 2.4 0.15 0.3 0.6 0.9

342.9 354.1 347.6 354.1 359.1 352.8 361.3 347.6 345.0

176.7 176.1 176.7 175.8 176.8 176.4 175.9 176.4 178.6

382.5 405.1 402.7 405.2 424.8 374.4 378.9 372.8 359.3

183.4 185.8 185.7 185.3 191.7 184.2 183.4 183.7 184.6

CAPE

TDM

RHDPE/pine

MAPE

TDM

a

Stage 4

The weight ratio of the fiber to HDPE was fixed at 30:70.

The crystallizing behavior of RHDPE in the composites was measured using a differential scanning calorimeter (DSC Q100, TA Instruments, New Castle, DE, USA). Samples of 8–10 mg were taken and placed in aluminum capsules, and were heated from 40 to 160 C at the rate of 10 C/min to eliminate the heat history before cooling at 10 C/min. Thermogravimetric analysis was used to study the thermal stability characteristics of resultant composites with a thermogravimetric Analyzer (TGA Q50, TA Instruments, New Castle, DE, USA), under nitrogen at a scan rate of 10 C/min from room temperature to 650 C. Dynamic mechanical properties of the composites were measured with a Dynamic Mechanical Analyzer (DMA Q800, TA Instruments, New Castle, DE, USA). The tests were done in a dual cantilever mode at a frequency of 1 Hz under room temperature. Prior to each test, test samples (60 · 10 · 1 mm) were conditioned for 72 h at a temperature of 23 C and a relative humidity of 50%. After the DMA test, all the specimens were measured for tensile strength according to the ASTM standard ASTM D638, using an INSTRON machine (Model 1125, Boston, MA, US). For each treatment level, six replications were conducted. A TINIUS 92T impact tester (Testing Machine Company, Horsham, PA) was used for the Izod impact test. All the samples were notched on the center of one longitudinal side according to the ASTM Standard ASTM D256. For each treatment level, five replications were conducted. The morphology of the composites was studied by a Hitachi VP-SEM S-3600N (Hitachi Ltd, Tokyo, Japan) scanning electron microscope. The fracture surfaces of the specimens after impact test were sputter-coated with gold before analysis.

2.4. Data analysis Duncan’s multiple range tests were done to test difference of mechanical properties at different treatment levels at the 5% significance level. 3. Results and discussion 3.1. Compounding characteristics Fig. 1 shows the relationship between mixing torque or temperature with mixing time. The RHDPE/bagasse blends underwent four stages during the compounding. The measured torque was corrected for the loading. During stage 1, the mixing torque sharply increased as RHDPE or mixed RHDPE/CA was added. The chamber temperature quickly decreased at the same time due to feeding of cold materials. The melting process was endothermic. After the materials was melted, the torque decreased and gradually leveled off at stage 2, where the temperature was also stabilized after it increased to a temperature even higher than the set temperature because of the dispersion friction. After the feeding of bagasse fiber during stage 3, there was a rapid increase in the mixing torque mainly due to the surface friction between the fiber and the melt. The melt torque of the RHDPE/ bagasse fiber decreased with continuous compounding and became stable at stage 4. The melt temperature dropped when the bagasse fiber was fed, and then quickly increased to about 185 C. With continuous blending, the blend temperature increased very slowly and was finally stabilized (stage 4). The same phenomenon happened to the RHDPE/pine system during the compounding. For the RHDPE/bagasse system containing 3% MAPE, the compounding showed the similar process, but there

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700

1

600 Torque (N.m/kg)

3 500 400

4

2

300 200

RHDPE/Bagasse

100

RHDPE/Bagasse+3%MAPE

0 0

3

6

9

12

15

Time (min) 200

4 Temperature (ºC )

188 2 176 3 164 RHDPE/Bagasse

152

RHDPE/Bagasse+3%MAPE

1

140 0

3

6

9

12

15

Time (min) Fig. 1. Mixing (a) torque and (b) temperature for RHDPE/bagasse (70/30 w/w) blends compounded at 165 C and 60 rpm.

appeared a wide peak for the melt temperature after the bagasse fiber was added, as shown in Fig. 1. The appearance of this wide temperature peak was perhaps related to the coupling reactions between the fiber and the CAs [32]. The same thing happened to other RHDPE/fiber systems containing active CAs. The balance values of torque and temperature at stages 2 and 4 are listed in Table 1. The acid groups of MAPE and CAPE can react with OH groups of cellulose to form ester linkages, and the grafting of vinyl monomers onto cellulose is mostly initiated by free radicals [33]. According to the chemical structure of titanate shown in Fig. 2, one titanate molecule has two allyl groups which can react with cellulose molecules through

Fig. 2. Chemical structure of titanate in TDM.

radical additions when a free radical initiator is used. Thus, the compatibility between the matrix and fibers may be improved through coupling reactions between MAPE or CAPE or TDM with cellulose during compounding and shaping. For the RHDPE/bagasse system, MAPE, CAPE, and TDM increased the balance torques at stages 2 and 4, especially MAPE, as shown in Table 1. An incompatible blend, characterized by no interaction between phases, frequently exhibits interlayer slip that promotes a reduction of the torque of the blend [34]. The increased torques in the stage 2 were probably derived from the improved compatibility between components in RHDPE because the flowability of these coupling agents was better than that of RHDPE, whose melt flow index was as low as 0.7 g/10 min. For a given CA at the same loading level, the torque increase caused by the CA at stage 4 was more than that at stage 2, suggesting that the affinity between RHDPE and bagasse fiber could be improved by these CAs. For the RHDPE/ pine system, MAPE could increase the balance torques at

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stages 2 and 4, but TDM lowered the balance torques at stage 4 (Table 1). The balance torque of RHDPE/pine at stage 4 was lowed from 382.5 Nm/kg to 359.3 when 0.9% TDM was added. TDM acted as a lubricant for the RHDPE/pine system. The balance torque increase for RHDPE/bagasse by the addition of 1.5% was about 20 N m/kg higher than that for RHDPE/pine by the addition of 1.8%, indicating MAPE acted better for the RHDPE/bagasse system.

3.2. Mechanical properties of the resultant composites The effects of the CAs on the mechanical properties of the composites are listed in Table 4. It was noticed that the impact strength of the RHDPE was much higher than those of virgin HDPE [30], possibly resulting from impact modifiers added during previous processing. When 30% bagasse or wood fibers were added, the modulus of the RHDPE was increased by about 50%, but the tensile

Fig. 3. SEM micrographs of the impact fracture surface of RHDPE/bagasse (70/30 w/w) composites containing: (a) no coupling agents; (b) 1.5% MAPE; (c) 1.5% CAPE; (d) 0.9% TDM and RHDPE/pine (70/30) composites containing (e) no coupling agents; (f) 1.2% MAPE; (g) 0.9% TDM.

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the RHDPE/pine system when the MAPE content increased up to 2.4%. The tensile strength increased as the MAPE content increased. The tensile strength, impact strength, and storage modulus of the RHDPE/pine composite were increased by about 49%, 9%, and 11%, respectively, by 1.8% MAPE. When CAPE was added up to 2.1%, little change happened to the mechanical properties of RHDPE/bagasse composite with the exception of tensile strength, which increased slowly with CAPE content increase. TDM could improve the mechanical properties of the RHDPE/bagasse system, and the maxima of the impact strength seemed to appear at 0.3% TDM when the TDM content increased up to 0.9%. When 0.3% TDM was added, the tensile strength, impact strength, and storage modulus of RHDPE/bagasse composite increased about 5%, 11%, and 9%, respectively. But for RHDPE/pine composite, the addition of TDM lowered the impact strength, and had little influence on tensile strength and storage modulus.

strength and impact strength obviously decreased. The loss of tensile strength could be compensated with the aid of the coupling agent of MAPE. Without coupling agents, the RHDPE/bagasse composites had a little higher modulus than the RHDPE/pine system, but the tensile strength and impact strength of RHDPE/bagasse composites were lower than those of the RHDPE/pine system, especially the impact strength. The impact strength of RHDPE/pine composite was 6.1 kJ/m2, and was about 24% higher than that of the RHDPE/bagasse composite. Based on RHDPE/bagasse system, the storage modulus, loss modulus, tensile strength, and impact strength had maxima with the MAPE content increase from 0% to 4.5%. The best properties seemed to appear at 1.5–3.0% MAPE. When 1.5% MAPE was added, the tensile strength, impact strength, and storage modulus of RHDPE/bagasse composite was increased, especially the tensile strength increased by over 40%. Except for tensile strength, there also existed maxima for other mechanical properties of

16000 (110)

RHDPE RHDPE/bagasse RHDPE/bagasse+1.5%MAPE

Intensity (Counts)

12000

RHDPE/bagasse+1.5% CAPE RHDPE/bagasse+0.9% TDM Bagasse

8000 (200)

4000

0 15

19

23

27

31

35

2-Theta (º) 16000

(110)

RHDPE RHDPE/pine RHDPE/pine+1.2%MAPE

Intensity (Counts)

12000

RHDPE/pine+0.9% TDM Pine flour

8000

(200)

4000

0 15

19

23

27 2-Theta (º)

31

35

Fig. 4. XRD patterns of RHDPE and RHDPE composites reinforced with: (a) bagasse and (b) pine at 2/min for 2h angles of 15–35.

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The tensile strength and impact strength of HD 9856B/ bagasse (70/30) were 18–27 MPa and 3.0–6.1 kJ/m2, respectively [30]. Thus, the mechanical properties of the fiber reinforced recycled HDPE compared well with those of virgin

HDPE/fiber composites. It needs to be pointed that there are many grades of virgin and recycled HDPE products. Thus, a direct comparison of the composite properties from virgin and recycled HDPE is not always possible. 3.3. Morphology of the composites

Intensity (Counts)

2000 (002)

Compact pine flour

1500 Loose pine flour

1000

Compact bagasse

500

Loose bagasse

0 5

10

15

20

25

30

2-Theta (º) Fig. 5. XRD patterns of pine and bagasse fiber at 2/min for 2h angles of 5–30.

The fractured surface of the RHDPE/bagasse and RHDPE/pine composites are presented in Fig. 3. Without coupling agents, there were obvious separation between RHDPE and the fibers, as shown in Figs. 3a and e, because of the incompatibility between hydrophobic matrix and hydrophilic fibers. For RHDPE/bagasse composites, the gaps between the matrix and fibers were reduced when the coupling agents were added (Fig. 3b–d). The MAPE significantly improved the compatibility between the matrix and bagasse fibers, and the matrix was bonded to the fibers well. Thus, the MAPE improved the compatibility between RHDPE and bagasse fibers more than the CAPE and TDM.

16 RHDPE

Heat Flow (W/g)

RHDPE/bagasse RHDPE/bagasse+1.5%MAPE

12

RHDPE/bagasse+1.5%CAPE RHDPE/bagasse+0.9%TDM Bagasse

8

4

0 55

75

95 Temperature (ºC)

115

135

115

135

16 RHDPE

Heat Flow (W/g)

RHDPE/pine RHDPE/pine+1.2%MAPE

12

RHDPE/pine+0.9%TDM Pine flour

8

4

0 55

75

95 Temperature (ºC)

Fig. 6. DSC curves of RHDPE and its composites with: (a) bagasse and (b) pine for a cooling rate of 10 C/min in N2.

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The MAPE also could greatly improve the interfacial adhesion of the RHDPE/pine composites (Fig. 3f), but the gaps between the matrix and pine fibers still apparently existed when 0.9% TDM was added (Fig. 3g). Possibly that was why the TDM could not improve the mechanical properties of the RHDPE/pine composites. 3.4. Effect of the fiber and CAs on RHDPE crystallization X-ray diffractograms of RHDPE and its composites are presented in Fig. 4, and Fig. 5 shows X-ray diffractograms of pure pine and bagasse fibers. The cell walls of most of the natural fibers mainly consist of cellulose, hemicellulose, and lignin. Cellulose has both amorphous and crystalline regions, although hemicellulose and lignin are amorphous. As shown in Fig. 5, peaks existed at about 22 for both pine and bagasse fibers, which correspond to the (0 0 2) lattice plane of cellulose [35]. Reasonably, the peaks for the pine flour increased with density. The diffraction peaks for the (1 1 0) and (2 0 0) planes of RHDPE shifted little when the fiber and CA were added, suggesting that the dimensions of the polyethylene cell did not change. However, the intensity of the peaks did change, suggesting differences in crystallinity. This was further explored by DSC, and the DSC cooling curves of RHDPE and RHDPE/fiber composites are shown in Fig. 6.

Table 2 Crystalline peaks and thickness of RHDPE and its composites by XRDa System

Peak position h () (1 1 0)

(2 0 0)

L1 1 0

L2 0 0

RHDPE RHDPE/bagasse RHDPE/bagasse + 1.5% MAPE RHDPE/bagasse + 1.5% CAPE RHDPE/bagasse + 0.9% TDM RHDPE/pine RHDPE/pine + 1.2% MAPE RHDPE/pine + 0.9% TDM

10.8 10.7 10.8 10.7 10.8 10.8 10.8 10.8

12.0 11.9 12.0 11.9 11.9 12.0 12.0 12.0

18.9 16.7 18.9 20.4 15.9 19.6 20.0 21.4

15.9 16.5 17.0 16.6 16.2 18.7 19.6 20.1

a

Crystalline thickness (nm)

The weight ratio of the pine fiber to HDPE was fixed at 30:70.

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Based on Scherrer’s equation, the crystalline thickness perpendicular to the reflection plane (Lhkl), the long period, can be calculated as [36,37] Lhkl ¼

Kk b0 cos h

ð1Þ

b20 ¼ b2M  b2I

ð2Þ

where b0 is the width of the diffraction beam (rad); bM is the measured width of the diffraction beam (rad); bI is the instrument width (rad); K is the shape factor of crystalline thickness, related to b0 and Lhkl. When b0 is defined as the half-height width of diffraction peaks, K = 0.9. The crystallization peaks were treated using a multipeak separation program MDI Jade 5.0 to separate the amorphous peaks, which covered the peak of the cellulose (0 0 2) lattice plane. The calculated results are summarized in Table 2. The crystallinity level (vc) of the RHDPE matrix was evaluated from the following relationship: vc ¼

DH exp 1   100% Wf DH

ð3Þ

where DHexp is the experimental heat of fusion or crystallization determined from DSC, DH is the assumed heat of fusion or crystallization of fully crystalline HDPE (293 J/g) [37], and Wf is the weight fraction of RHDPE in the composites. The corresponding results are listed in Table 3. The values of Lhkl of RHDPE were 18.9 nm for the (1 1 0) plane and 15.9 nm for the (2 0 0) plane. As shown in Tables 2 and 3, the introduction of 30% bagasse reduced the Lhkl for the (1 1 0) plane, and almost kept the same crystallinity level. When the 30% pine fiber was added to RHDPE, both of the Lhkl values were increased, as well as the crystallinity level. The increased values of Lhkl and crystallization degree might result from the nucleating ability of wood fiber [38] and crystal imperfection. Compared with neat RHDPE, RHDPE/fiber composites had higher melting peak temperatures, lower crystallization peak temperatures, much lower crystallization rates at crystallization peak temperatures, and wider crystalline temperature range, as shown in Fig. 4. The addition of 30% fiber certainly increased the matrix viscosity at the crystallizing

Table 3 DSC analysis results for RHDPE and its compositesa Systems

Melting Peak temperature (C)

Crystallinity level (%)

Peak temperature (C)

Crystallinity level (%)

RHDPE RHDPE/bagasse RHDPE/bagasse + 1.5% MAPE RHDPE/bagasse + 1.5% CAPE RHDPE/bagasse + 0.9% TDM RHDPE/pine RHDPE/pine + 1.2% MAPE RHDPE/pine + 0.9% TDM

130.8 133.4 133.8 132.9 132.2 133.2 132.8 134.1

55.1 56.8 52.0 51.7 55.2 60.9 61.2 53.7

118.3 115.2 115.1 115.1 115.3 115.2 115.4 115.1

56.9 57.0 52.0 52.0 55.5 60.9 62.7 54.0

a

The weight ratio of the pine fiber to HDPE was fixed at 30:70.

Crystallizing

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temperatures because of the poor flowability of neat RHDPE (melt flow index 0.7 g/10 min). The diffusion rate of PE chains was reduced. As a result, the Tc shifted to a lower temperature, and the crystallization rate was obviously lowered. For RHDPE/bagasse composites, the addition of 1.5% MAPE or CAPE increased the L1 1 0, but lowered the crystallinity level. Both the Lhkl and crystallinity level was lowered by 0.9% TDM. The lowered crystallinity level suggested that the compatibility between the RHDPE matrix and fiber was improved by these coupling agents, and the perfection of RHDPE crystals was reduced. For the RHDPE/pine fiber system, the addition of 1.2% MAPE did not obviously influence the Lhkl and crystallinity level, while 0.9% TDM increased the Lhkl and lowered the crystallinity level.

3.5. Thermal stability of RHDPE and its composites The thermogravimetric curves are plotted in Fig. 7 and the corresponding data are listed in Table 5. The degradation of neat RHDPE began at about 440.5 C, and the maximum decomposition rate appeared at about 470 C. The onset degradation temperature (To) of the pine flour was 256.5 C, and the fastest decomposition temperature (Td) appeared at 351.7 C. Because of the carbonization of pine fiber, the residue weight was 15.4%. For bagasse fiber, To and Td were 247.6 and 327.2 C, respectively. Thus, the pine fiber had better heat resistance than the bagasse fiber, but the bagasse fiber had a higher residue weight. With the fiber addition, the To of RHDPE was lowered due to the fiber degradation, being about 255 C for the RHDPE/bagasse system and about 260 C for the

100

Weight (%)

80

60

40

RHDPE RHDPE/bagasse

20

RHDPE/bagasse+1.5%MAPE RHDPE/bagasse+1.5%CAPE RHDPE/bagasse+0.9%TDM Bagasse

0 150

230

310 390 Temperature (ºC)

470

550

470

550

100

Weight (%)

80

60

40 RHDPE/pine RHDPE/pine+1.2%MAPE

20

RHDPE/pine+0.9% TDM pine

0 150

230

310 390 Temperature (ºC)

Fig. 7. Temperature dependences of weight loss for: (a) RHDPE, bagasse fiber, and RHDPE/bagasse composites and (b) pine fiber and RHDPE/pine composites at 10 C/min in N2.

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Table 4 Effect of CA content on mechanical properties of compositesa System

CA

Content (wt%)

Storage modulus (GPa)

Loss modulus (GPa)

Tensile strength (MPa)

Impact strength (kJ/m2)

RHDPE

–

–

1.55 (0.04)

0.15 (0.01)

32.6 (1.1)

12.30 (0.26)

RHDPE/bagasse

– MAPE

– 1.5 3.0 4.5 0.9 1.5 2.1 0.3 0.6 0.9

2.29(b) (0.14) 2.50(a) (0.09) 2.50(a) (0.14) 2.19(b) (0.09) 2.26(a) (0.10) 2.22(a) (0.06) 2.25(a) (0.11) 2.49(a) (0.11) 2.45(ab) (0.08) 2.45(ab) (0.08)

0.19(b) (0.01) 0.20(a) (0.01) 0.20(a) (0.01) 0.17(c) (0.01) 0.18(ab) (0.01) 0.18(b) (0.01) 0.18(b) (0.01) 0.20(a) (0.01) 0.19(a) (0.01) 0.19(a) (0.01)

21.4(c) (0.5) 30.6(a) (0.9) 29.8(a) (1.0) 27.2(b) (1.0) 21.6(b) (0.5) 22.5(a) (1.3) 25.1(a) (1.0) 22.5(ab) (0.9) 22.9(ab) (1.2) 23.6(a) (1.8)

4.92(b) (0.27) 5.70(a) (0.26) 5.90(a) (0.27) 5.82(a) (0.25) 4.96(a) (0.30) 4.92(a) (0.16) 4.95(a) (0.35) 5.49(a) (0.24) 5.03(b) (0.29) 4.76(b) (0.24)

– 0.6 1.2 1.8 2.4 0.15 0.3 0.6 0.9

2.23(b) (0.13) 2.30(b) (0.13) 2.34(ab) (0.14) 2.49(a) (0.11) 2.26(b) (0.11) 2.20(c) (0.13) 2.19(c) (0.08) 2.45(a) (0.12) 2.39(ab) (0.08)

0.18(b) (0.01) 0.19(ab) (0.01) 0.19(ab) (0.01) 0.20(a) (0.01) 0.19(ab) (0.01) 0.18(a) (0.01) 0.18(a) (0.01) 0.19(a) (0.01) 0.19(a) (0.01)

23.5(c) (1.5) 32.6(b) (1.3) 32.6(b) (1.9) 34.9(a) (1.2) 36.1(a) (1.5) 22.3(b) (1.2) 23.1(b) (1.2) 23.7(a) (0.4) 23.1(b) (1.0)

6.10(b) (0.20) 5.87(b) (0.21) 6.74(a) (0.22) 6.67(a) (0.26) 6.09(b) (0.29) 5.67(b) (0.31) 5.58(b) (0.20) 5.57(b) (0.19) 5.09(c) (0.23)

CAPE

TDM

RHDPE/pine

– MAPE

TDM

a

The weight ratio of fiber to the matrix was fixed at 30:70. For the same system containing the same kind of coupling agent, means with the same letter for each property are not significantly different at the 5% significance level. The values in the parentheses are standard deviations.

Table 5 Thermal degradation temperatures and residue weight of RHDPE and its compositesa System

Tob (C)

Tdc (C) Stage I

Stage II

RHDPE Bagasse Pine RHDPE/bagasse RHDPE/bagasse + 1.5% MAPE RHDPE/bagasse + 1.5% CAPE RHDPE/bagasse + 0.9% TDM RHDPE/pine RHDPE/pine + 1.2% MAPE RHDPE/pine + 0.9% TDM

440.5 247.6 256.5 255.7 255.4

– 327.2 351.7 342.1 341.3

469.7 – – 469.1 469.7

0 26.4 15.4 7.0 7.1

254.8 254.2 261.8 262.9 259.8

341.7 342.4 352.5 353.3 348.5

468.7 469.1 469.2 469.0 468.2

7.8 8.5 5.5 5.4 5.8

a b c

Residue (%)

The weight ratio of the pine fiber to HDPE was fixed at 30:70. Onset thermal degradation temperature. The fastest decomposition temperature.

RHDPE/pine system. Compared with the neat fiber, the increased To was derived from the RHDPE coating around the fiber. There were two thermal degradation stages for the systems containing fiber. One degradation stage was for the fiber added, and the other was for the RHDPE. The Td for the first stage appeared at 341.3–342.4 C and 348.5–353.3 C for the RHDPE/bagasse and RHDPE/pine systems, respectively. The Td of the composites for the second stage appeared at almost the same level as the neat RHDPE. The residue weight of the composites was mainly derived from the fiber degradation. The coupling agents seemed to have little influence on the thermal degradation of the composites.

4. Conclusions In this study, RHDPE/fiber (i.e., bagasse and pine) composites were made by melt compounding and compression molding. The influence of coupling agent type and contents on the compounding rheology, RHDPE crystallization behavior, and properties of RHDPE/fiber composites were investigated. The coupling agents, MAPE, CAPE, and TDM improved the compatibility between bagasse fiber and RHDPE. For the RHDPE/pine system, MAPE acted as an effective coupling agent, but TDM played a lubricant role. Without coupling agents, the bagasse fiber reduced the RHDPE Lhkl for the (1 1 0) plane, but did not change crystallinity level. The pine fiber increased the Lhkl values and crystallinity level. Compared with the neat RHDPE, the composites had lower crystallization peak temperatures and crystallization rates, and wider crystalline temperature range. For the RHDPE/bagasse composite, the Lhkl was increased by 1.5% MAPE or CAPE, and the crystallinity level was lowered by coupling agents. For the RHDPE/ pine composites, 1.2% MAPE increased crystallinity level, while 0.9% TDM increased the Lhkl but lowered the crystallinity level. Without coupling agents, the RHDPE/bagasse composites had a little higher modulus, but had lower tensile strength and impact strength than the RHDPE/pine system. The modulus, tensile strength, and impact strength of RHDPE/bagasse composites had maxima with MAPE content increase from 0% to 4.5%. Except for tensile strength, there also existed maxima for other mechanical properties of the RHDPE/pine system when the MAPE

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