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Thermal behavior of liquefied wood polymer composites (LWPC)
Composite Structures 68 (2005) 103–108 www.elsevier.com/locate/compstruct
Thermal behavior of liquefied wood polymer composites (LWPC) Geum-Hyun Doh, Sun-Young Lee *, In-Aeh Kang, Young-To Kong Laboratory of Functional Wooden Materials, Department of Forest Products, Korea Forest Research Institute, Seoul 130-712, South Korea Available online 9 April 2004
Abstract The thermal behavior of liquefied wood polymer composites was characterized by means of thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses. Low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) as polymer matrices were used with liquefied wood (LW). HDPE showed a better thermal stability than PP. Melt index did not show a significant effect on the thermal stability. The thermal stability of LW decreased with the increase of LW content up to 40%. Higher heating rate provided the higher thermal stability, resulting from the decelerated decomposition rate. According to DSC analysis, the melting temperature of HDPE decreased with the increase of LW loading level. The melting temperature of virgin LDPE and HDPE decreased with the addition of 10% LW. The melting temperature of virgin PP also decreased with the addition of LW. Enthalpy of virgin polymers also decreased with the addition of LW. This study proves the thermal stability necessary for the consolidation process of composite materials. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Liquefied wood–polymer composites; Low-density polyethylene; Polypropylene; High-density polyethylene; Melt index; TGA; DSC; Weight loss; Activation energy; Melting temperature; Enthalpy
1. Introduction For effective utilization of biomass, liquefaction of wood resources has been manufactured by a technique that converts lignocellulosic wastes such as sawdust into substances soluble in organic solvent and liquefying agent. Whole lignocellulosics can be completely liquefied without producing any residues. In general, liquefied wood powder is manufactured after removing the liquefying agent and solvent [2,4,5,9,11,13]. The use of liquefied wood materials in the production of thermoplastic composites is highly beneficial at these materials, improving the toughness and strength of virgin polymers. Moreover, these materials are biodegradable, non-toxic, and flexible. In addition, these composites can be easily processed by common techniques such as injection- and compression-moulding [6]. In order to be applicable for a massive and economic production the matrices have to be economic, easily available and well established. For this reason, lowdensity polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) are the most used *
Corresponding author. Tel.: +82-2-961-2573; fax: +82-2-961-2597. E-mail address: [email protected] (S.-Y. Lee).
0263-8223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.03.004
polymers, especially due to economic reason, ease of processing, environmental and working safety, and recyclability. Most of the polymers are generally subjected to a degradation of the mechanical and physical properties under the increase of temperature. Since there is always thermal stress during the manufacturing of filler reinforced composite materials with polymer matrices, it is important to know the effects of the processing temperature associated with the processing duration. Fundamental information regarding the thermal stability of the composites materials to be processes is obtained from the thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses [14]. TGA is one of the thermal analysis techniques used to quantify weight change and thermal decomposition of the sample [7]. It was reported that the chemical composition, heating rate, temperature, and inorganic substances are the major factors that affect the thermal behavior of biomass [12]. The kinetics of exothermic reactions is important in assessing the potential of both materials and systems for thermal exposition. The determination of detailed kinetic parameters such as activation energy from TGA is carried out to analyze their thermal degradation behavior. The activation
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G.-H. Doh et al. / Composite Structures 68 (2005) 103–108
energy at peak temperature (Tp ) from TGA is determined from the slope of linear regression [3,10]. On the other hand, DSC provides the melting temperature Tm at which a transition occurs. It also provides the enthalpy DH associated with the transition. Studies of the dependence of DH on temperature can also be used to obtain the change in heat capacity that occurs with melting, i.e. the DCp . Understanding these three parameters permits a complete characterization of the thermal stability of the a given materials. Recently, the authors have reported on the thermal properties of polymer composites filled with wood flour and sisal fiber [1,8]. However, no work was reported on the thermal behavior of LWPC. In the present paper, the effects of temperature, heating rate, polymer type, melt index, and liquefied wood level on the thermal behavior were investigated. Activation energy of the composites was calculated using Arrhenius expression.
2. Materials and methods 2.1. Sample preparation Polymers were obtained courtesy of Dae-Lim Industrial Co., South Korea. Two kinds of polypropylene (PP) were homopolymer pellets with a melt flow indices of 3.5 g/10 min and 12.5 g/10 min, respectively. Both have a density of 0.9 g/cm3 . Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have a melt index of 3.0 g/10 min with a density of 0.923 g/cm3 and a melt index of 0.28 g/10 min with a density of 0.938 g/cm3 , respectively. 2.2. Manufacture of liquefied wood mill Liquefied wood (LW) mill was sawdust from municipal wood waste in Korea Forest Research Institute, South Korea. LW mill was dried to 1–2% moisture content using an oven-drier at a temperature of 100 °C for 24 h, and then stored in polyethylene bag until needed. The sawdust was mixed with phenol solution at the mixture ratio of 1:3, and then added with catalyst of 2.0 wt.% H2 SO4 based on the weight of phenol in the rotary-digester of 20 l for 90 min. Dissolute and residues were separated by a centrifuge and filtered through a glass filter (25G2) and the LW was recovered. Unreacted phenol was removed with rotary-vacuum evaporator at 170–180 °C and the LW was obtained. The LW was then granulated in a cutting mill (Fritsch Co., Germany). 2.3. Manufacture of pellets from liquefied wood mill and polymer A major type of laboratory size extruder was a single screw which blended polymer with LW mill. The LW
mill (particle size of 100 mesh per 25.4 mm) was mixed with polymer in a Hakke Rheomix for 20 min and at 200 °C at a mixture ratio of 1:9. The mixture of LW mill and polymer was cooled in the air and then granulated in the cutting mill (Fritsch Co., Germany). The granulate was finally dried under vacuum (p ¼ 100 mbar) for 16 h at 60 °C. Extrusion of the injection-moulded granulate was carried out on a Brabenderâ stand-alone extruder from Germany. This extruder had a screw diameter of 19 mm with an L=D ratio of 25. A circular nozzle of 6 mm in diameter was used for extrusion. The extrudate, in the form of strands, was cooled in the air and palletized with Barbenderâ pelletizer (Germany). The resulting pellets were dried at 105 °C for 24 h before being injection moulded into ASTM test specimens. The pallets were extruded in the form of tensile, flexural, and impact test specimens on a Nissei NC-8300PZ injection-moulding machine. Moulding temperatures for LDPE and HDPE and for PP were 170 and 180 °C, respectively. 2.4. Thermogravimetric analysis and differential scanning calorimetry Thermogravimetric analysis (TGA) measurements were carried out 5–10 mg of LWPC and virgin polymers at four heating rates of 5, 10, 20, and 50 °C/min in a nitrogen atmosphere using a Thermogravimetric Analyzer (TA Instrument SDT Q600). LWPC and virgin polymers were subjected to TGA in high purity nitrogen under a constant flow rate of 10 ml/min. Thermal decomposition of each sample occurred in a programmed temperature range of 30–600 °C. The continuous records of weight loss and temperature were determined and analyzed to determine the following TGA indices: thermal degradation rate (% weight loss/ min), initial degradation temperature and residual weight at 600 °C. All differential scanning calorimetry (DSC) measurements were made on a DSC Q10 (TA instrument) thermal system, using a sealed aluminum capsule. Each test specimen was weighed to about 10 ± 0.5 mg, and was held at a single heating rate of 10 °C/min and a scanning temperature from 30 to 300 °C. Each of the data reported represents an average of three runs. 2.5. Activation energy of thermal decomposition In order to calculate activation energy, TGA measurements were performed with 5–10 mg of LWPC and virgin polymers at four heating rates of 5, 10, 20, and 50 °C/min. From a plot of y ¼ lnðb=Tp2 ) versus x ¼ 1=Tp and fitting a straight line, the activation energy can be calculated from the slope: where b ¼ heating rate (°C/ min), Tp ¼ peak temperature at the exothermic peak (K),
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108
Ea ¼ activation energy of thermal decomposition (KJ/ mol), and R ¼ gas constant (8.314 J/mol).
3. Results and discussion 3.1. TGA––induction time Weight loss of a virgin polymers and LWPC as a function of time or temperature is commonly determined by the technique of TGA and is an irreversible process due to thermal degradation. TGA curves of LW, LDPE, HDPE, PP, and LWPC are shown in Figs. 1 and 2. The TGA curves of those materials show a single stage of weight loss. The initial weight loss of LW started approximately until 200 °C, due to the heat of evaporation of moisture in the sample and the initial decomposition of cellulose and hemicellulose. The severe weight loss from 200 to 410 °C is due to the major components of wood, namely cellulose, hemicellulose, and lignin [7,12]. It can be deduced that cellulose and
PE
120
Weight (%)
100 80 60 LW
40
LDPE LDPE-LW
20
HDPE HDPE-LW
0 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 1. TGA thermogram of LW, LDPE, and HDPE with/without LW.
105
hemicellulose components of LW are the major contributors to decomposition between 200 and 400 °C, whereas lignin is mainly responsible for the char formation of the LW over 400 °C. From these results, the chemical modification of major components of LW occurred during manufacturing LW with phenol and sulfuric acid, resulting in the lower decomposition temperature. The weight losses of LDPE (MI ¼ 3.0 g/10 min) and HDPE (MI ¼ 0.28 g/10 min) started at 390 and 425 °C, respectively (Fig. 1). The thermal stability of LDPE is somewhat lower than that of HDPE. On the other hand, the weight losses of PP with different MI of 3.5 and 12.5 g/10 min began at approximately 349 and 389 °C, respectively (Fig. 2). It indicates that a better thermal stability was shown for both LDPE and HDPE. Our results showed that low MI value showed a better thermal stability, but the effect of MI was not significant on the thermal decomposition in the case of PP. The thermal stability of LDPE and HDPE with LW are higher than that of neat LDPE and HDPE. Both PP with different MI values was thermally less stable, compared with LDPE and HDPE. The addition of 10% LW to LDPE, HDPE, and PP showed no significant effect on the thermal decomposition. The deformation temperature of 10% LW-filled composites from LDPE and HDPE provided no shift of TGA curves (Fig. 1). LWPC from PP (MI ¼ 3.5) also showed no difference of deformation temperature from virgin PP (Fig. 2). However, LWPC from PP (MI ¼ 12.5) showed some increase of deformation temperature by adding 10% LW. Fig. 3 represents the thermogram of TGA for HDPE at the different level of LW (0–40%) and a heating rate of 10 °C/min. As the LW level increased, thermal stability of composites decreased. This result indicates that the compatibility and interfacial bonding decreased by mixing both LW and polymer. It shows that the thermal stability of LWPC is lower. Tar and ash content after thermal degradation over 500 °C, however, increased with increase of LW level.
PP
120
HDPE7.5
120 100
Weight (%)
Weight (%)
100 80 60 LW
40
PP3.5 PP3.5-LW
20
80 60 HDPE-0%LW HDPE-10%LW HDPE-20%LW HDPE-30%LW HDPE-40%LW
40
PP12.5 PP12.5-LW
20
0 0
100
200
300
400
500
600
o
Temperature ( C)
0 0
100
200
300
400
500
600
Temperature (oC)
Fig. 2. TGA thermogram of LW, PP3.5 and PP12.5 with/without LW. 3.5 and 12.5 indicate the melt indices of PP.
Fig. 3. TGA results of HDPE with various loading levels of LW.
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Thermal degradation curves for virgin HDPE and PP with LW at various heating rates of 5, 10, 20, 50 °C/min are shown in Figs. 4 and 5. All the experiments were performed under same conditions of non-isothermal method. As shown in Table 1, decomposition temperature (Td ) was determined at the starting point of severe weight loss and increased with increase of heating rate [14]. It indicates that higher heating rate improved the thermal stability, resulting from the decelerated decomposition rate. In other word, the thermal defor-
PP-LW 120
Weight (%)
100 80 60 o
5 C/min o 10 C/min o 20 C/min o 50 C/min
40 20 0 0
HDPE-LW
200
300
400
500
600
o
Fig. 5. TGA results of PP with LW at various heating rates.
100
Weight (%)
100
Temperature ( C)
120
80
mation is influenced by a factor of time at the different heating rates.
60 o
5 C/min
40
3.2. Activation energy of thermal decomposition based on TGA
o
10 C/min o
20 C/min
20
o
50 C/min
0 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. TGA results of HDPE with LW at various heating rates.
As shown in Fig. 6, activation energy of HDPE (223.439 KJ/mol) with low MI value was higher than that of PP (55.262 KJ/mol) with high MI value. As LW was added to polymer, the activation energy of LWPC decreased 8.7–36.4% for HDPE and PP, respectively. It
Table 1 Results of TGA analysis of liquefied wood–polymer composites Polymer
MI (g/10 min)
LW (%)
Heating rate (°C/min)
Tp a (°C)
Degradation (%)
Residue at 600 °C (%)
LDPE
3.0 3.0
0 10
10 10
466.09 469.90
28.25 36.98
)0.703 1.006
HDPE
0.28 0.28
0 10
10 10
471.75 473.68
32.98 41.31
)0.774 3.939
0 10 0 10
10 10 10 10
458.55 451.74 447.20 456.28
26.58 38.59 27.85 39.27
)3.104 2.069 )0.293 5.869
0 0 0 0 10 10 10 10
5 10 20 50 5 10 20 50
458.23 471.75 485.99 500.68 462.22 470.03 489.04 506.05
30.97 32.98 33.49 43.77 30.97 41.31 34.23 39.56
)0.640 )0.774 )0.022 0.302 )0.641 3.939 3.877 3.165
0 0 0 0 10 10 10 10
5 10 20 50 5 10 20 50
426.71 458.55 461.75 485.24 442.35 459.21 469.94 494.04
25.65 27.85 35.81 33.53 33.52 39.27 35.70 37.15
0.307 )0.209 0.921 0.387 0.335 5.883 3.919 4.389
PP
HDPE
PP
a
3.5 3.5 12.5 12.5 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5
Indicates the peak temperature of DSC analysis.
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108 300
0.0
223.43
Heat flow (W/g)
Activation Energy (KJ/mol)
—0.5
240.08
250 200 150 100
107
—1.0 —1.5 —2.0 LDPE LDPE—LW HDPE HDPE—LW LW
—2.5
75.38 —3.0
55.26 50
—3.5 0
0 PP
PP-LW
HDPE
indicates that the dispersion and/or interfacial bonding between polymer and LW is lower than that of virgin HDPE and PP. Therefore, the lower activation energy of LWPC is highly related to the low dispersion and/or interfacial bonding between LW and polymer matrix. It is obvious that the addition of coupling agents or dispersing agents may improve those behaviors. 3.3. DSC––melting temperature and enthalpy Fig. 7 shows a heating thermogram of LW, LDPE, and HDPE with LW used in this study; a single peak was obtained. This is actually a characteristic of semicrystalline polymers and is visible as an endothermic peak. The thermograms of other polymers selected for the current investigation had the same single peak characteristic. The temperature corresponding to the peak represents the melting point (Tm ) of the composite
100
150
250
200
Temperatre (°C)
HDPE-LW
Fig. 6. Activation energy of thermal decomposition for PP and HDPE with/without LW, based on TGA data.
50
Fig. 7. DSC results of LDPE, HDPE with/without LW.
materials concerned (Table 2). As expected, virgin PP with MI of 12.5 gives the highest melting points (167.45 °C) and LDPE has the lowest (107.50 °C). Virgin PP with MI of 3.5 provides little bit lower melting point, compared with that of MI of 12.5. Similarly, HDPE with MI with 7.5 shows the higher melting point than that with MI of 0.28. The addition of LW to virgin polymer showed no significant effect on the melting point of polymer (Table 2). When heating a binary blending (virgin polymer:LW) of a specific blending ratio (90:10% w/w) for LDPE and HDPE, the melting point decreased virtually 0.2–1.3 °C (Fig. 6). As shown in Fig. 8, on the other hand, the positions of the thermal peak for PP with two MI had no shift with a binary blending (90:10% w/w). The melting point of HDPE (MI of 7.5) decreased 4.8 °C up to a binary blending (80:20% w/w), but it reversibly increased up to 60:40% w/w (Fig. 9). Nevertheless, it is worth pointing out that there is some incompatibility between virgin polymer and LW.
Table 2 Results of DSC analysis of liquefied wood–polymer composites Polymer
MI (g/10 min)
LW (%)
Tm a (°C)
Enthalpy (J/g)
LDPE
3.0
0 10
107.50 106.21
74.36 @ 97.6 °C 58.63 @ 96.4 °C
HDPE
0.28
0 10
127.07 126.49
114.2 @ 118.6 °C 99.92 @ 120.3 °C
PP
3.5
0 10
165.47 165.28
77.28 @ 151.2 °C 67.82 @ 150.1 °C
12.5
0 10
167.45 166.73
88.42 @ 157.8 °C 72.18 @ 155.4 °C
7.5
0 10 20 30 40
138.03 136.27 133.23 137.62 139.38
HDPE
a
Indicates the melting temperature of DSC analysis.
165.5 163.2 160.3 134.0 149.6
@ @ @ @ @
125.4 124.6 123.7 124.1 127.3
°C °C °C °C °C
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G.-H. Doh et al. / Composite Structures 68 (2005) 103–108 0.0
Heat flow (W/g)
—0.5 —1.0 —1.5 —2.0 PP3.5 PP3.5—LW PP12.5 PP12.5—LW LW
—2.5 —3.0 —3.5 0
50
150 100 Temperatre (°C)
200
250
Fig. 8. DSC results of PP with/without LW and with different MI.
0.0
200 to 410 °C is due to the major wood components, mainly lignin. HDPE showed a better thermal stability, compared with PP. Melt index showed no significant effect on the thermal stability. The thermal stability of LW decreased with the increase of LW. Higher heating rate provided the better thermal stability, resulting from the decelerated decomposition rate. The melting temperature of HDPE decreased with the increase of LW loading level up to 20%, but increased over 30% LW loading. The melting temperature of LDPE and HDPE decreased little bit with the addition of 10% LW. The melting temperature of PP also decreased with the addition of LW. Enthalpy of polymer decreased with the addition of LW.
References
Heat flow (W/g)
-0.5 -1.0 -1.5 -2.0
HDPE-10% LW HDPE-20% LW HDPE-30% LW HDPE-40% LW HDPE
-2.5 -3.0 -3.5 0
50
100
150
200
250
o
Temperature ( C)
Fig. 9. DSC results of HDPE with different LW loading levels.
An enthalpy of caloric processes by measuring the heat flow between sample and reference with isothermal heating was determined (Table 2). The area under the peak is the enthalpy of the transition. The enthalpy of virgin HDPE at transition temperature was highest (165.5 J/g), especially with MI of 7.5, but the enthalpy of virgin LDPE was the lowest (74.36 J/g). When LW of 10% was added to virgin polymers, overall enthalpy decreased, indicating the decreased thermal stability. In the case of HDPE with MI of 7.5, the enthalpy at a given transition temperature decreased with increase of LW level up to 40%. 4. Conclusions TGA curves of virgin polymer with LW showed a single stage of weight loss. The initial weight loss of LW started at nearby 200 °C, due to the evaporation of moisture in the sample and the initial decomposition of cellulose and hemicellulose. The severe weight loss from
[1] Coutinho FMB, Costa THS, Carvalho DL, Gorelova MM, De Santa Maria LC. Thermal behavior of modified wood fibers. Polym Test 1998;17:299–310. [2] Doh GH, Kong YT. Liquefaction of wood (II)––analysis of liquefied wood components. J Kor Wood Sci Technol 1995; 22(2):19–25. [3] Kissinger HE. Anal Chem 1957;(29):1702. [4] Kurimoto Y, Doi S, Tamura Y. Species effects on woodliquefaction in polyhydric alcohols. Holzforschung 1999;53(6): 617–22. [5] Lin L, Yoshoka M, Yao Y, Shirashi N. Liquefaction of wood in the presence of phenol using phosphoric acid as a catalyst and the flow properties of the liquefied wood. J Appl Polym Sci 1994;52:1629–36. [6] L opez-Manchado MA, Arroyo M. Thermal and dynamic properties of polypropylene and short organic fiber composites. Polymer 2000;41:7761–7. [7] Mansanray KG, Ghaly AE. Thermal degradation of rice husks in nitrogen atmosphere. Bioresour Technol 1998;65:13–20. [8] Nair KCM, Thomas S, Groeninckx G. Thermal and dynamic mechanical analysis of polystyrene composites reinforced with short fibres. Compos Sci Technol 2001;61:2519–29. [9] Ono H, Sudo K. Japanese Pat. Appl. 217070, 1989. [10] Park BD, Riedl B, Hsu EW, Shields J. Differential scanning calorimetry of phenol-formaldehyde resins cure-accelerated by carbonates. Polymer 1999;40:1689–99. [11] Pu S, Shiraishi N. Mokuzai Gakkaishi 1993;39:446. [12] Shafizadeh F, DeGroote WF. Combustion characteristics of cellulosic fuels. In: Shafizadeh F, Sarkanen KV, Tillman DA, editors. Thermal uses and properties of carbohydrates and lignins. New York: Academic Press; 1976. p. 1–8. [13] Shiraishi N, Onodera S, Ohtani M, Masumoto T. Dissolution of etherified or esterified wood into polyhydric alcohol or bisphenol A and their application in preparing wooden polymeric materials. Mokuzai Gakkaishi 1985;31:418–20. [14] Wielage B, Lampke T, Mark G, Nestler K, Starke D. Thermogravimetric and differential scanning calorimetric analysis of natural fibers and polypropylene. Thermochim Acta 1999;(337): 169–77.
Thermal behavior of liquefied wood polymer composites (LWPC) Geum-Hyun Doh, Sun-Young Lee *, In-Aeh Kang, Young-To Kong Laboratory of Functional Wooden Materials, Department of Forest Products, Korea Forest Research Institute, Seoul 130-712, South Korea Available online 9 April 2004
Abstract The thermal behavior of liquefied wood polymer composites was characterized by means of thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses. Low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) as polymer matrices were used with liquefied wood (LW). HDPE showed a better thermal stability than PP. Melt index did not show a significant effect on the thermal stability. The thermal stability of LW decreased with the increase of LW content up to 40%. Higher heating rate provided the higher thermal stability, resulting from the decelerated decomposition rate. According to DSC analysis, the melting temperature of HDPE decreased with the increase of LW loading level. The melting temperature of virgin LDPE and HDPE decreased with the addition of 10% LW. The melting temperature of virgin PP also decreased with the addition of LW. Enthalpy of virgin polymers also decreased with the addition of LW. This study proves the thermal stability necessary for the consolidation process of composite materials. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Liquefied wood–polymer composites; Low-density polyethylene; Polypropylene; High-density polyethylene; Melt index; TGA; DSC; Weight loss; Activation energy; Melting temperature; Enthalpy
1. Introduction For effective utilization of biomass, liquefaction of wood resources has been manufactured by a technique that converts lignocellulosic wastes such as sawdust into substances soluble in organic solvent and liquefying agent. Whole lignocellulosics can be completely liquefied without producing any residues. In general, liquefied wood powder is manufactured after removing the liquefying agent and solvent [2,4,5,9,11,13]. The use of liquefied wood materials in the production of thermoplastic composites is highly beneficial at these materials, improving the toughness and strength of virgin polymers. Moreover, these materials are biodegradable, non-toxic, and flexible. In addition, these composites can be easily processed by common techniques such as injection- and compression-moulding [6]. In order to be applicable for a massive and economic production the matrices have to be economic, easily available and well established. For this reason, lowdensity polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) are the most used *
Corresponding author. Tel.: +82-2-961-2573; fax: +82-2-961-2597. E-mail address: [email protected] (S.-Y. Lee).
0263-8223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.03.004
polymers, especially due to economic reason, ease of processing, environmental and working safety, and recyclability. Most of the polymers are generally subjected to a degradation of the mechanical and physical properties under the increase of temperature. Since there is always thermal stress during the manufacturing of filler reinforced composite materials with polymer matrices, it is important to know the effects of the processing temperature associated with the processing duration. Fundamental information regarding the thermal stability of the composites materials to be processes is obtained from the thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses [14]. TGA is one of the thermal analysis techniques used to quantify weight change and thermal decomposition of the sample [7]. It was reported that the chemical composition, heating rate, temperature, and inorganic substances are the major factors that affect the thermal behavior of biomass [12]. The kinetics of exothermic reactions is important in assessing the potential of both materials and systems for thermal exposition. The determination of detailed kinetic parameters such as activation energy from TGA is carried out to analyze their thermal degradation behavior. The activation
104
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108
energy at peak temperature (Tp ) from TGA is determined from the slope of linear regression [3,10]. On the other hand, DSC provides the melting temperature Tm at which a transition occurs. It also provides the enthalpy DH associated with the transition. Studies of the dependence of DH on temperature can also be used to obtain the change in heat capacity that occurs with melting, i.e. the DCp . Understanding these three parameters permits a complete characterization of the thermal stability of the a given materials. Recently, the authors have reported on the thermal properties of polymer composites filled with wood flour and sisal fiber [1,8]. However, no work was reported on the thermal behavior of LWPC. In the present paper, the effects of temperature, heating rate, polymer type, melt index, and liquefied wood level on the thermal behavior were investigated. Activation energy of the composites was calculated using Arrhenius expression.
2. Materials and methods 2.1. Sample preparation Polymers were obtained courtesy of Dae-Lim Industrial Co., South Korea. Two kinds of polypropylene (PP) were homopolymer pellets with a melt flow indices of 3.5 g/10 min and 12.5 g/10 min, respectively. Both have a density of 0.9 g/cm3 . Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have a melt index of 3.0 g/10 min with a density of 0.923 g/cm3 and a melt index of 0.28 g/10 min with a density of 0.938 g/cm3 , respectively. 2.2. Manufacture of liquefied wood mill Liquefied wood (LW) mill was sawdust from municipal wood waste in Korea Forest Research Institute, South Korea. LW mill was dried to 1–2% moisture content using an oven-drier at a temperature of 100 °C for 24 h, and then stored in polyethylene bag until needed. The sawdust was mixed with phenol solution at the mixture ratio of 1:3, and then added with catalyst of 2.0 wt.% H2 SO4 based on the weight of phenol in the rotary-digester of 20 l for 90 min. Dissolute and residues were separated by a centrifuge and filtered through a glass filter (25G2) and the LW was recovered. Unreacted phenol was removed with rotary-vacuum evaporator at 170–180 °C and the LW was obtained. The LW was then granulated in a cutting mill (Fritsch Co., Germany). 2.3. Manufacture of pellets from liquefied wood mill and polymer A major type of laboratory size extruder was a single screw which blended polymer with LW mill. The LW
mill (particle size of 100 mesh per 25.4 mm) was mixed with polymer in a Hakke Rheomix for 20 min and at 200 °C at a mixture ratio of 1:9. The mixture of LW mill and polymer was cooled in the air and then granulated in the cutting mill (Fritsch Co., Germany). The granulate was finally dried under vacuum (p ¼ 100 mbar) for 16 h at 60 °C. Extrusion of the injection-moulded granulate was carried out on a Brabenderâ stand-alone extruder from Germany. This extruder had a screw diameter of 19 mm with an L=D ratio of 25. A circular nozzle of 6 mm in diameter was used for extrusion. The extrudate, in the form of strands, was cooled in the air and palletized with Barbenderâ pelletizer (Germany). The resulting pellets were dried at 105 °C for 24 h before being injection moulded into ASTM test specimens. The pallets were extruded in the form of tensile, flexural, and impact test specimens on a Nissei NC-8300PZ injection-moulding machine. Moulding temperatures for LDPE and HDPE and for PP were 170 and 180 °C, respectively. 2.4. Thermogravimetric analysis and differential scanning calorimetry Thermogravimetric analysis (TGA) measurements were carried out 5–10 mg of LWPC and virgin polymers at four heating rates of 5, 10, 20, and 50 °C/min in a nitrogen atmosphere using a Thermogravimetric Analyzer (TA Instrument SDT Q600). LWPC and virgin polymers were subjected to TGA in high purity nitrogen under a constant flow rate of 10 ml/min. Thermal decomposition of each sample occurred in a programmed temperature range of 30–600 °C. The continuous records of weight loss and temperature were determined and analyzed to determine the following TGA indices: thermal degradation rate (% weight loss/ min), initial degradation temperature and residual weight at 600 °C. All differential scanning calorimetry (DSC) measurements were made on a DSC Q10 (TA instrument) thermal system, using a sealed aluminum capsule. Each test specimen was weighed to about 10 ± 0.5 mg, and was held at a single heating rate of 10 °C/min and a scanning temperature from 30 to 300 °C. Each of the data reported represents an average of three runs. 2.5. Activation energy of thermal decomposition In order to calculate activation energy, TGA measurements were performed with 5–10 mg of LWPC and virgin polymers at four heating rates of 5, 10, 20, and 50 °C/min. From a plot of y ¼ lnðb=Tp2 ) versus x ¼ 1=Tp and fitting a straight line, the activation energy can be calculated from the slope: where b ¼ heating rate (°C/ min), Tp ¼ peak temperature at the exothermic peak (K),
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108
Ea ¼ activation energy of thermal decomposition (KJ/ mol), and R ¼ gas constant (8.314 J/mol).
3. Results and discussion 3.1. TGA––induction time Weight loss of a virgin polymers and LWPC as a function of time or temperature is commonly determined by the technique of TGA and is an irreversible process due to thermal degradation. TGA curves of LW, LDPE, HDPE, PP, and LWPC are shown in Figs. 1 and 2. The TGA curves of those materials show a single stage of weight loss. The initial weight loss of LW started approximately until 200 °C, due to the heat of evaporation of moisture in the sample and the initial decomposition of cellulose and hemicellulose. The severe weight loss from 200 to 410 °C is due to the major components of wood, namely cellulose, hemicellulose, and lignin [7,12]. It can be deduced that cellulose and
PE
120
Weight (%)
100 80 60 LW
40
LDPE LDPE-LW
20
HDPE HDPE-LW
0 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 1. TGA thermogram of LW, LDPE, and HDPE with/without LW.
105
hemicellulose components of LW are the major contributors to decomposition between 200 and 400 °C, whereas lignin is mainly responsible for the char formation of the LW over 400 °C. From these results, the chemical modification of major components of LW occurred during manufacturing LW with phenol and sulfuric acid, resulting in the lower decomposition temperature. The weight losses of LDPE (MI ¼ 3.0 g/10 min) and HDPE (MI ¼ 0.28 g/10 min) started at 390 and 425 °C, respectively (Fig. 1). The thermal stability of LDPE is somewhat lower than that of HDPE. On the other hand, the weight losses of PP with different MI of 3.5 and 12.5 g/10 min began at approximately 349 and 389 °C, respectively (Fig. 2). It indicates that a better thermal stability was shown for both LDPE and HDPE. Our results showed that low MI value showed a better thermal stability, but the effect of MI was not significant on the thermal decomposition in the case of PP. The thermal stability of LDPE and HDPE with LW are higher than that of neat LDPE and HDPE. Both PP with different MI values was thermally less stable, compared with LDPE and HDPE. The addition of 10% LW to LDPE, HDPE, and PP showed no significant effect on the thermal decomposition. The deformation temperature of 10% LW-filled composites from LDPE and HDPE provided no shift of TGA curves (Fig. 1). LWPC from PP (MI ¼ 3.5) also showed no difference of deformation temperature from virgin PP (Fig. 2). However, LWPC from PP (MI ¼ 12.5) showed some increase of deformation temperature by adding 10% LW. Fig. 3 represents the thermogram of TGA for HDPE at the different level of LW (0–40%) and a heating rate of 10 °C/min. As the LW level increased, thermal stability of composites decreased. This result indicates that the compatibility and interfacial bonding decreased by mixing both LW and polymer. It shows that the thermal stability of LWPC is lower. Tar and ash content after thermal degradation over 500 °C, however, increased with increase of LW level.
PP
120
HDPE7.5
120 100
Weight (%)
Weight (%)
100 80 60 LW
40
PP3.5 PP3.5-LW
20
80 60 HDPE-0%LW HDPE-10%LW HDPE-20%LW HDPE-30%LW HDPE-40%LW
40
PP12.5 PP12.5-LW
20
0 0
100
200
300
400
500
600
o
Temperature ( C)
0 0
100
200
300
400
500
600
Temperature (oC)
Fig. 2. TGA thermogram of LW, PP3.5 and PP12.5 with/without LW. 3.5 and 12.5 indicate the melt indices of PP.
Fig. 3. TGA results of HDPE with various loading levels of LW.
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Thermal degradation curves for virgin HDPE and PP with LW at various heating rates of 5, 10, 20, 50 °C/min are shown in Figs. 4 and 5. All the experiments were performed under same conditions of non-isothermal method. As shown in Table 1, decomposition temperature (Td ) was determined at the starting point of severe weight loss and increased with increase of heating rate [14]. It indicates that higher heating rate improved the thermal stability, resulting from the decelerated decomposition rate. In other word, the thermal defor-
PP-LW 120
Weight (%)
100 80 60 o
5 C/min o 10 C/min o 20 C/min o 50 C/min
40 20 0 0
HDPE-LW
200
300
400
500
600
o
Fig. 5. TGA results of PP with LW at various heating rates.
100
Weight (%)
100
Temperature ( C)
120
80
mation is influenced by a factor of time at the different heating rates.
60 o
5 C/min
40
3.2. Activation energy of thermal decomposition based on TGA
o
10 C/min o
20 C/min
20
o
50 C/min
0 0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. TGA results of HDPE with LW at various heating rates.
As shown in Fig. 6, activation energy of HDPE (223.439 KJ/mol) with low MI value was higher than that of PP (55.262 KJ/mol) with high MI value. As LW was added to polymer, the activation energy of LWPC decreased 8.7–36.4% for HDPE and PP, respectively. It
Table 1 Results of TGA analysis of liquefied wood–polymer composites Polymer
MI (g/10 min)
LW (%)
Heating rate (°C/min)
Tp a (°C)
Degradation (%)
Residue at 600 °C (%)
LDPE
3.0 3.0
0 10
10 10
466.09 469.90
28.25 36.98
)0.703 1.006
HDPE
0.28 0.28
0 10
10 10
471.75 473.68
32.98 41.31
)0.774 3.939
0 10 0 10
10 10 10 10
458.55 451.74 447.20 456.28
26.58 38.59 27.85 39.27
)3.104 2.069 )0.293 5.869
0 0 0 0 10 10 10 10
5 10 20 50 5 10 20 50
458.23 471.75 485.99 500.68 462.22 470.03 489.04 506.05
30.97 32.98 33.49 43.77 30.97 41.31 34.23 39.56
)0.640 )0.774 )0.022 0.302 )0.641 3.939 3.877 3.165
0 0 0 0 10 10 10 10
5 10 20 50 5 10 20 50
426.71 458.55 461.75 485.24 442.35 459.21 469.94 494.04
25.65 27.85 35.81 33.53 33.52 39.27 35.70 37.15
0.307 )0.209 0.921 0.387 0.335 5.883 3.919 4.389
PP
HDPE
PP
a
3.5 3.5 12.5 12.5 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5
Indicates the peak temperature of DSC analysis.
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108 300
0.0
223.43
Heat flow (W/g)
Activation Energy (KJ/mol)
—0.5
240.08
250 200 150 100
107
—1.0 —1.5 —2.0 LDPE LDPE—LW HDPE HDPE—LW LW
—2.5
75.38 —3.0
55.26 50
—3.5 0
0 PP
PP-LW
HDPE
indicates that the dispersion and/or interfacial bonding between polymer and LW is lower than that of virgin HDPE and PP. Therefore, the lower activation energy of LWPC is highly related to the low dispersion and/or interfacial bonding between LW and polymer matrix. It is obvious that the addition of coupling agents or dispersing agents may improve those behaviors. 3.3. DSC––melting temperature and enthalpy Fig. 7 shows a heating thermogram of LW, LDPE, and HDPE with LW used in this study; a single peak was obtained. This is actually a characteristic of semicrystalline polymers and is visible as an endothermic peak. The thermograms of other polymers selected for the current investigation had the same single peak characteristic. The temperature corresponding to the peak represents the melting point (Tm ) of the composite
100
150
250
200
Temperatre (°C)
HDPE-LW
Fig. 6. Activation energy of thermal decomposition for PP and HDPE with/without LW, based on TGA data.
50
Fig. 7. DSC results of LDPE, HDPE with/without LW.
materials concerned (Table 2). As expected, virgin PP with MI of 12.5 gives the highest melting points (167.45 °C) and LDPE has the lowest (107.50 °C). Virgin PP with MI of 3.5 provides little bit lower melting point, compared with that of MI of 12.5. Similarly, HDPE with MI with 7.5 shows the higher melting point than that with MI of 0.28. The addition of LW to virgin polymer showed no significant effect on the melting point of polymer (Table 2). When heating a binary blending (virgin polymer:LW) of a specific blending ratio (90:10% w/w) for LDPE and HDPE, the melting point decreased virtually 0.2–1.3 °C (Fig. 6). As shown in Fig. 8, on the other hand, the positions of the thermal peak for PP with two MI had no shift with a binary blending (90:10% w/w). The melting point of HDPE (MI of 7.5) decreased 4.8 °C up to a binary blending (80:20% w/w), but it reversibly increased up to 60:40% w/w (Fig. 9). Nevertheless, it is worth pointing out that there is some incompatibility between virgin polymer and LW.
Table 2 Results of DSC analysis of liquefied wood–polymer composites Polymer
MI (g/10 min)
LW (%)
Tm a (°C)
Enthalpy (J/g)
LDPE
3.0
0 10
107.50 106.21
74.36 @ 97.6 °C 58.63 @ 96.4 °C
HDPE
0.28
0 10
127.07 126.49
114.2 @ 118.6 °C 99.92 @ 120.3 °C
PP
3.5
0 10
165.47 165.28
77.28 @ 151.2 °C 67.82 @ 150.1 °C
12.5
0 10
167.45 166.73
88.42 @ 157.8 °C 72.18 @ 155.4 °C
7.5
0 10 20 30 40
138.03 136.27 133.23 137.62 139.38
HDPE
a
Indicates the melting temperature of DSC analysis.
165.5 163.2 160.3 134.0 149.6
@ @ @ @ @
125.4 124.6 123.7 124.1 127.3
°C °C °C °C °C
108
G.-H. Doh et al. / Composite Structures 68 (2005) 103–108 0.0
Heat flow (W/g)
—0.5 —1.0 —1.5 —2.0 PP3.5 PP3.5—LW PP12.5 PP12.5—LW LW
—2.5 —3.0 —3.5 0
50
150 100 Temperatre (°C)
200
250
Fig. 8. DSC results of PP with/without LW and with different MI.
0.0
200 to 410 °C is due to the major wood components, mainly lignin. HDPE showed a better thermal stability, compared with PP. Melt index showed no significant effect on the thermal stability. The thermal stability of LW decreased with the increase of LW. Higher heating rate provided the better thermal stability, resulting from the decelerated decomposition rate. The melting temperature of HDPE decreased with the increase of LW loading level up to 20%, but increased over 30% LW loading. The melting temperature of LDPE and HDPE decreased little bit with the addition of 10% LW. The melting temperature of PP also decreased with the addition of LW. Enthalpy of polymer decreased with the addition of LW.
References
Heat flow (W/g)
-0.5 -1.0 -1.5 -2.0
HDPE-10% LW HDPE-20% LW HDPE-30% LW HDPE-40% LW HDPE
-2.5 -3.0 -3.5 0
50
100
150
200
250
o
Temperature ( C)
Fig. 9. DSC results of HDPE with different LW loading levels.
An enthalpy of caloric processes by measuring the heat flow between sample and reference with isothermal heating was determined (Table 2). The area under the peak is the enthalpy of the transition. The enthalpy of virgin HDPE at transition temperature was highest (165.5 J/g), especially with MI of 7.5, but the enthalpy of virgin LDPE was the lowest (74.36 J/g). When LW of 10% was added to virgin polymers, overall enthalpy decreased, indicating the decreased thermal stability. In the case of HDPE with MI of 7.5, the enthalpy at a given transition temperature decreased with increase of LW level up to 40%. 4. Conclusions TGA curves of virgin polymer with LW showed a single stage of weight loss. The initial weight loss of LW started at nearby 200 °C, due to the evaporation of moisture in the sample and the initial decomposition of cellulose and hemicellulose. The severe weight loss from
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