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Effects of biopitch on the properties of flexible polyurethane foams
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 1420–1428
www.elsevier.com/locate/europolj
Effects of biopitch on the properties of flexible polyurethane foams Renata Costa Silva Arau´jo, Vaˆnya Ma´rcia Duarte Pasa *, Breno Nonato Melo Departamento de Quı´mica, Universidade Federal de Minas Gerais, Av. Antoˆnio Carlos, 6627 CEP 31270 901 Belo Horizonte, Brazil Received 8 March 2004; received in revised form 13 December 2004; accepted 23 December 2004 Available online 2 February 2005
Abstract Biopitches are industrial residues obtained by the distillation of the tar recovered during Eucalyptus charcoal production and can be used as a renewable polyol source. Flexible polyurethane foams were prepared with different proportions of biopitch and HTPB (hydroxyl-terminated polybutadiene) and using polymeric MDI (4,4 0 diphenyl methane diisocyanate), N,N dimethylcyclohexylamine as a catalyst and water as a blowing agent. Elemental analysis, thermal analysis (TG/DSC), Fourier Transform Infrared Spectroscopy (FTIR), scanning electronic microscopy (SEM), and density results were used aiming to discuss the contribution of biopitch to foams properties. The higher the biopitch content, the higher the thermal stability and the lower the density of the flexible foams (air atmosphere), behaviors similar to those of lignin-based polyurethanes. Biopitch enhanced the oxygen content of the polyurethane foams synthesized, and their reaction with HTPB resulted in stable foams. 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Flexible foam; Biopitch; HTPB
1. Introduction The polyurethane foam market has shown a continuous and fast growth in the last years. This growth can be attributed to the physical and mechanical properties of polyurethane foams, their chemical composition, and density, which render them particularly suitable for many applications, such as upholstering, insulation, and packing [1,2]. These properties make flexible polyurethane foams an important material for the synthetic polymer market, which had a business volume of about
* Corresponding author. Tel.: +55 313 499 6651; fax: +55 313 499 6650/5700. E-mail address: [email protected] (V.M.D. Pasa).
4.5 millions ton/year in 1995, corresponding to 60% of mattress and furniture, 35% of automotive use and 5% of packing [3]. Recently, there has been an increased interest in using renewable resources in the plastic industry. Besides industries, many researchers have focused on polymers based on renewable environmentally friendly resources. Polyurethanes based on castor oil [4], starch [5], lignin [6], and other natural polyols have been used in the preparation of foams and elastomers, but there is nothing about the use of biopitch in the literature besides our work. Another concern of the researchers is the blowing agent required for the production of polyurethane foams. Growing evidences that CFCs damage the ozone layer have led researchers to look for new alternative blowing agents. There are basically three types of blowing agents:
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.021
R.C.S. Arau´jo et al. / European Polymer Journal 41 (2005) 1420–1428
(1) water, which reacts with isocyanate and produces carbon dioxide, (2) liquid chemicals with low boiling point, which evaporate due to the exothermic reaction of polyols and isocyanate, (3) air, which is blown in or whipped into the polyols and isocyanate mixture. The use of water as a blowing agent is preferable for the manufacture of flexible polyurethane foams [7] due to its low cost and environmental impact, but sometimes its expansion power is lower than that of CFCs. Brazil is a broad country with an excellent production index of planted forests, which makes it a large potential producer of biomass-based materials. Many Brazilian iron-making industries use charcoal instead of coke as a thermoreducer. About 75% of all charcoal consumed in Brazil (6 million tons/year) is produced using planted Eucalyptus forests [8]. Besides charcoal, the slow pyrolysis of wood also generates light gases and pyroligneous liquor. After decantation, the pyroligneous liquor separates into two phases: wood vinegar (77%) and wood tar (33%). Wood tar is a complex mixture constituted mainly by phenols, acids, aldehydes, and ketones. Wood tar has been studied as a chemical precursor of fine chemicals and of pharmaceuticals [9,10]. The fractionated distillation of wood tar permits the isolation of phenol, guaiacol, syringol, and the generation of a solid residue (35–60%). This residue, called wood tar pitch or biopitch, has been used as a precursor of polymeric materials in our research works. It is very important to develop applications for biopitch, considering the expressive Brazilian potential production of about 200,000 ton/year, only by integrated companies, which grow their own Eucalyptus forests, and run carbonization units and ironmaking plants. The use of biopitch by polymeric material industries could aggregate value and improve its economical feasibility, generating new revenue for the charcoal making industry and Brazilian poor rural areas. Investigations of biopitch by elemental analysis have demonstrated its aromatic nature, its high oxygen content (24%) [11], and O/C and H/C ratios of about 0.25 and 1.10, respectively. The structure of biopitch is different from that of both coal tar pitch (O/C and H/C ratios of 0.6 · 103 and 8.0 · 103) and petroleum pitch (O/C and H/C ratios of 0.8 · 103 and 4.0 · 103) [12]. The chemical units that constitute the oligomer structure of biopitch are mainly phenyl, guaiacyl, syringyl, and their derivatives. Its structure is similar to that of lignin, presenting almost the same chemical groups, but biopitch shows a thermoplastic behavior, while lignins normally are thermosetting. Lignin is recognized as being a three dimensional, highly branched polymer and has been used as a natural polyol in poly-
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urethane synthesis [13,14]. The glass transition temperature of lignins range from 90 to 180 C, depending on the pretreatment used [15], while that of biopitch has shown results ranging from 25 to 50 C. In Fig. 1, we can observe a simplified model of the main functional groups present in biopitch, considering its complex structure, it is impossible propose a defined formula. Recently, some polymeric materials incorporating biopitch have been synthesized to obtain advanced carbonaceous materials [16–18], phenolic resins [19]. Biopitch has also been used as a polyol in polyurethane elastomers [20,21], composites [22], and coatings [23]. In the different polyurethane systems, biopitch behaves mainly as an aromatic polyol, but its function as a black pigment, flame retardant, and an antioxidant additive have been considered as well. The present work reports results of the synthesis and characterization of flexible polyurethane foams based on biopitch and using water as a blowing agent aiming to evaluate the influence of this renewable polyol on the properties of the flexible PU foams obtained.
Fig. 1. Model structure illustrating the main functional groups of biopitch.
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2. Experimental
3.3. Infrared spectroscopy and elemental analysis
2.1. Materials
The chemical characterization of the biopitch-based foams was made by elemental analysis and infrared spectroscopy. Elemental analyses were carried out in a Perkin Elmer 2400 to assess carbon, hydrogen, and nitrogen contents in the samples, and oxygen by difference. Elemental O/C and H/C ratios were calculated aiming to evaluate the effect of the biopitch oxygen content on foam properties. Fourier Transform Infrared Spectroscopy (FTIR) was carried out in a Perkin Elmer 1000 by ATR—Attenuated Total Reflectance.
The biopitch used as a polyol was obtained from wood tar distillation (T = 200 C and P = 30 mm Hg) in a pilot plant. Flexible foams were obtained using Eucalyptus biopitch, HTPB—Hydroxyl Terminated Polybutadiene (PETROFLEX, Brazil), MDI (VL R 20 BR, Bayer, Brazil), and N,N dimethylcyclohexamine (DMCHA) as a catalyst and distilled water as a blowing agent. 2.2. Foam preparation Biopitch was mixed with HTPB and stirred with distilled water and dimethylcyclohexamine. MDI was added and stirred for 1 min. The mixture was poured into a container and allowed to react completely at room temperature. Different biopitch contents were used in synthesis aiming to evaluate the material properties. Biopitch and HTPB proportions and the biopitch concentration in the formulations are shown in Table 1. The NCO/OH ratio was kept at 1.0.
3. Characterization 3.1. Hydroxyl content Biopitches are characterized as natural polyol sources due to their hydroxyl groups. Hydroxyl content was determined by biopitch acetylation (monitored by infrared spectroscopy) with acetic anhydride catalyzed by pyridine for 100 h. After acetylation, titration was carried out with NaOH (1.69 eq g/L) [21]. 3.2. Nuclear magnetic resonance spectroscopy—solidstate 13C NMR of biopitch Nuclear magnetic resonance spectroscopy—solidstate 13C NMR: spectra were taken on a VARIAN INOVA-300 with probe RT CP/MAS. NMR analysis of the biopitch sample used was carried out.
Table 1 Flexible polyurethane foams using biopitch as a natural polyol in different proportions Sample
HTPB/Biopitch proportion
PU PU PU PU PU
5/0 5/1 5/2 5/3 5/5
HTPB A B C D
3.4. Thermal properties Thermogravimetric Analysis (TGA) was performed with TGA-50 Shimadzu under dynamic nitrogen and air atmospheres and at a temperature range from room temperature to 750 C, with a heating rate of 10 C/min. The samples had an approximate weight of 4 mg. Differential Scanning Calorimetry (DSC) was carried out in a DSC-50 Shimadzu in two scans. First, the samples were scanned up to 150 C, thereafter cooled to 120 C and recorded a second time up to 150 C. Tg was determined from the second scan. Measurements were assessed under helium atmosphere with a flow rate of 50 mL/min, and a heating rate of 20 C/min. The samples had an approximate weight of 16 mg. 3.5. Density Densities were determined by standard method (ASTM) D 792-86. Five samples were tested and the average value is reported. 3.6. Morphological properties Scanning electronic microscopy formed with JSM-840 A scanning to observe foam cellular structure. fractured after immersion in liquid material was coated with a thin observation.
(SEM) was permicroscope–JEOL The samples were nitrogen, and the gold layer before
4. Results and discussion 4.1. Hydroxyl content The hydroxyl content of biopitch was 7% or 153 mg KOH/g, confirming its polyol character. This value is higher than the hydroxyl content of HTPB (47 mg KOH/g), an aliphatic polyol.
R.C.S. Arau´jo et al. / European Polymer Journal 41 (2005) 1420–1428
4.2. Nuclear magnetic resonance spectroscopy—solidstate 13C NMR of biopitch Solid-state 13C NMR analysis (Table 2) showed the different chemical groups that constitute the biopitch macromolecular structure. It is possible to verify that biopitch has about 27.3% of aliphatic carbon, 8.7% of olefinic carbon, 57.2% of aromatic carbon, and 6.8% of carbonyl. It is observed that the hydroxyl group that constitutes polyurethane is basically phenolic. 4.3. Infrared spectroscopy and elemental analysis Table 3 shows the elemental analysis results and O/C and H/C ratios of foams with different biopitch contents. It was observed a high oxygen content (24%) for the biopitch sample used, and consequently, an enhancement of the polyurethane O/C ratio with the increase in biopitch content, mainly due to the contribution of methoxyl, carbonyl, and hydroxyl groups from the biopitch structure. This renewable polyol also contributed to reduce the H/C ratio of the polyurethane foams because of the increase in aromaticity. The biopitch infrared spectrum confirmed the results obtained by solid-state 13C NMR analysis, with phenolic groups (OH 3350 cm1 ; C@C aromatic 1600, 1500 1450 cm1). The synthesis of polyurethane foams was confirmed by FTIR–ATR spectroscopy. It can be seen in Fig. 2 that the urethane bond at 1730 cm1 (carbonyl) is present in all samples. The hydroxyl bond in the polyurethane spectra overlaps with the N–H bonds related to urethanes groups at 3350 cm1. It is possible that residual biopitch OH groups are unreacted because of their low accessibility due to complex biopitch structure. Besides, isocyanate absorption does not appear at 2275 cm1 due to the complete reaction of isocyanate to form urethane bonds and carbon dioxide. The spectrum of PU HTPB presented higher relative intensity of the 2920 cm1 peak than their corresponding values for the others samples due to the aliphatic polyol (HTPB) used. Other absorptions are assigned to polyol and MDI groups. 4.4. Density Flexible polyurethane foam densities were determined and the average values are shown in Fig. 3. It was observed that the incorporation of biopitch into foams caused a decrease in density, which could be attributed to the high aromaticity of the biopitch structure. Almost all hydroxyl groups that formed urethane bonds were phenolic and presented low mobility. As a result, the addition of biopitch caused an increase in cell size, and consequently more expanded materials (higher volumes), and decreased densities. Another important
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aspect is the unreacted biopitch phase, which behaves as an inclusion, and therefore makes chain packing difficult, especially in samples C and D, and decreases density. 4.5. Thermal properties Thermogravimetric analyses were carried out in nitrogen and air atmospheres, as observed in Figs. 4 and 5, respectively. None of the curves showed events at 100 C due to water release, thus, the FTIR absorption at 3350 cm1 cannot be assigned to residual water. In nitrogen atmosphere, the TG curves showed that PU based on HTPB was more stable than other biopitchbased PUs up to 500 C. The TG curve of biopitch showed that it has low thermal stability due to its high oxygen content, and the presence of low molecular weight monomers in its structure. Consequently, the addition of biopitch decreased Tonset temperatures of the foams synthesized in nitrogen atmosphere. Due to the significant biopitch aromaticity, the higher the biopitch content, the higher the foam carbon residue (N2 atmosphere). Samples PU B, PU C, and PU D presented a residual carbon yield higher than 40% at temperatures above 500 C. This value is more significant than the carbon residue observed in the biopitch degradation due to the synergistic contributions of biopitch and MDI used to synthesize foams. We can explain this effect by the difficult heat transfer in cellular materials. The higher the biopitch content, the higher the open cell volume, and consequently, heat transfer is hindered and pyrolysis is less effective. This behavior is not observed in polyurethane elastomers synthesized with HTPB/Biopitch/MDI [21], and with PEG/Biopitch/MDI [20], whose carbon residues are lower and structures are not cellular. In air atmosphere, after a weight loss of about 20%, the presence of biopitch increases the thermal stability of foams and retards degradation reactions. It is important to point out that all biopitch-based foams presented similar maximum degradation temperature, around 520 C, higher than that observed for PU HTPB (420 C). A similar behavior was observed in the thermal study of lignin-based polyurethanes [24]. It was observed that biopitch retards the degradation of the materials in air atmosphere as a consequence of its aromatic structure and the condensation reactions that occur between the sample and the atmosphere oxygen. In Fig. 6, DSC curves show glass transition temperature (Tg) relative to soft segments formed by HTPB. The Tg determined for polyurethane foam based only on HTPB is 80 C, and for that added with biopitch, this value approaches 77 C. The insignificant variation in Tg with the increase in biopitch content indicates that this material is a multiphase system. These results are similar to those obtained for elastomers synthesized with
R.C.S. Arau´jo et al. / European Polymer Journal 41 (2005) 1420–1428
1424 Table 2 Solid-State NMR Region 1
13
C Analysis of the biopitch used to prepare PUs Group functionality
Chemical shift (ppm)
Carbon content (%)
14
7.9
20
4.1
29–50
11.8
50–70
3.5
110–115
8.7
115–140
30.2
O
140–160
27.0
CO OC/H
160–185
6.8
C
190–230
H
H
C
C
H
H
C H3
C H3
H
H
C
C
H
H
CH3
H H H C
C
C
H C/H H C
C/H CH
O
C/H
C/H
2
C
C
C
C/H
C
C
3
C/H
C O
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Table 3 Elemental Analysis of biopitch and polyurethane foams Sample
Carbon (%)
Hydrogen (%)
Nitrogen (%)
Oxygen (%)
O/C
H/C
Biopitch PU HTPB A B C D
69.6 85.0 78.9 79.2 78.4 75.1
6.4 10.4 8.6 8.8 8.5 7.6
0.4 4.3 5.6 5.5 4.8 4.5
23.6 0.3 6.9 6.6 8.3 12.9
0.2543 0.0026 0.0656 0.0625 0.0794 0.1288
1.1034 1.4683 1.3080 1.3333 1.3010 1.2144
Fig. 2. FTIR–ATR of biopitch (a) and polyurethanes foams (b).
HTPB–biopitch–MDI [21], but different from those obtained for elastomers based on PEG–biopitch–MDI [20]. PEG is a polar polyol and affords a greater homogeneity with biopitch, consequently, the Tg of PEG/Biopitch elastomers increases with biopitch content increase. We used HTPB instead of PEG in this flexible foam synthesis because HTPB presents the exceptional property of increasing material flexibility.
4.6. Morphological properties The open cell structure is confirmed by the morphological analysis performed by SEM in Figs. 7 and 8 with enlargements of 40 and 400 times, respectively. In Fig. 7, the photomicrographs show the sizes and the shapes of cell foams containing biopitch in their structure. It can be observed that biopitch content increases cell sizes
R.C.S. Arau´jo et al. / European Polymer Journal 41 (2005) 1420–1428
1426 180
Density (kg/m3)
160 140 120 100 80 60 40 20 0 PU A
PU B
PU C
PU D
Sample Fig. 3. Densities of flexible polyurethane foams based on biopitch.
Fig. 6. DSC curves (second scan) for polyurethanes and biopitch under dynamical helium atmosphere at a heating rate of 20 C/min.
Fig. 4. TG curves of polyurethanes and biopitch under nitrogen dynamical atmosphere at a heating rate of 10 C/min.
Fig. 5. TG curves for polyurethanes and biopitch under air dynamical atmosphere at a heating rate of 10 C/min.
and porosity, which is in accordance with density results. It can be seen in Fig. 8 that there are particles adhered
onto the cell walls in samples A, C, and D. These particles are probably unreacted biopitch that remained in
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Fig. 7. SEM micrographs of foams varying biopitch content. Magnification: 40·.
Fig. 8. SEM micrographs of unreacted biopitch particles in flexible foams. Magnification: 400·.
the flexible foams formed. These results partially confirm FTIR–ATR spectra, which present OH absorption overlapping NH stretching at 3300 cm1. SEM micrographs also confirm the heterogeneity of these PU systems as shown by DSC results. Sample B did not show biopitch particles on the material surface. Possibly, therefore, this sample present better compatibility and thermal properties than others PUs based on biopitch and (Figs. 4 and 5).
5. Conclusion Biopitch, obtained from wood tar distillation, is a renewable polyol precursor for flexible polyurethane foams with phenolic hydroxyls and aromatic carbon content of 57% according to RMN 13C—solid state analysis. It is possible to synthesize flexible foams using a mixture of biopitch and HTPB as a polyol. It was observed by elemental analysis that foam oxygen content
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(O/H) and aromaticity (H/C) increase as density decreases with the addition of biopitch as a consequence of its structural characteristics. Biopitch causes an increase in the cell sizes of polyurethane foams. This fact confirms the decrease in density as biopitch content increases. Calorimetric and thermogravimetric analyses showed the formation of a multiphase material due to negligible variation in Tg with biopitch addition. The lower thermal stability of biopitch causes a decrease in the Tonset of polyurethane foams under nitrogen and air atmospheres, but a significant increase in the carbon residue. This behavior suggests the use of these polyurethane foams as precursors of carbon membranes and carbon foams, which can be used as catalyst supports. Samples containing biopitch are more stable after a weight loss nearly 20% under air atmosphere, and present high maximum degradation temperature (520 C). It is important to point out that no additives were used in these materials, which means that the new flexible foam properties obtained can be improved yet. In addition, biopitch behaves as a polyol, which seems to act as a flame retardant in flexible PU foams.
References [1] Gmitter GT, Fabris HJ, Maxey EM. Plastic foams. New York: Marcel Dekker Inc.; 1972 [Chapter 3]. [2] Bailei FE, editor. Handbook of polymeric foams and foam technology. Munich: Hanser Publishers; 1991. [Chapter 4]. [3] Vilar WD. Quı´mica e tecnologia de poliuretanos. Pronor; 2001. [Chapter 1]. [4] Nayak P, Mishira DK, Parida D, Sahoo KC, Nanda M, Lenka S, Nayak PL. J Appl Polym Sci 1997;63:671–9. [5] Alfani R, Iannace S, Nicolais L. J Appl Polym Sci 1998;68:739–45. [6] Hsu OHH, Glasser WG. Appl Polym Symp 1975;28: 297–307. [7] Dieterich D, Grigat E, Hahn W, Hespe H, Schmelzer HG. Principles of polyurethane chemistry and special applications. In: Oertel G, editor. Polyurethane handbook. Munich: Hanser Publishers; 1993. [8] Statistical Yearbook, Associac¸a˜o brasileira de florestas renova´veis, Belo Horizonte, 1998.
[9] Carazza F, Rezende MEA, Pasa VMD. Fractionation of wood tar. In: Proceedings of developments in thermochemical biomass conversion, vol. 2. 1997. p. 1465–74. [10] Rezende MEA, Pasa VMD, Lessa A. In: Proceedings of developments in thermochemical biomass conversion, vol. 2. 1997. p. 1289–98. [11] Arau´jo RCS, Sı´ntese e caracterizac¸a˜o de elastoˆmeros de poliuretano sistema piche/PBLH e piche/PEG. Belo Horizonte: Instituto de Cihatencias Exatas da UFMG, 1999. (Dissertac¸a˜o: Mestrado em Quı´mica). [12] Bernejo J, Granada M, Mene´dez R, Garcia R, Tasco´n JMD. Fuel 1997;76:179. [13] Mo¨rck R, Reimann A, Kringstad KP. Elastomeric polyurethanes from a kraft lignin–polyethylene glycol–toluene diisocyanate system. In: Proceedings of lignin properties and materials, Toronto, 1988. p. 390–401. [14] Kelley SS, Glasser WG, Ward TC. Effect of soft-segment content on the properties of lignin-based polyurethanes. In: Proceedings of lignin properties and materials, Toronto, 1988. p. 402–13. [15] Glasser WG. Lignin utilization in thermosetting and thermoplastic polymers and materials. In: Proceedings of second international symposium on natural polymers and composites, 1998. p. 33–9. [16] Prauchner MJ, Pasa VMD, Otani C, Otani S. Energy Fuels 2001;15:449–54. [17] Prauchner MJ, Pasa VMD, Otani C, Otani S. Biopitchbased general purpose carbon fibers: processing and properties. Carbon 2005;43(3):591–7. [18] Prauchner MJ, Pasa VMD, Molhallem NDS, Otani C, Otani S, Pardini LC. Biomass Bioenergy 2005;28:53–61. [19] Assis RS, Pasa VMD. In: Proceedings of second international symposium on natural polymers and composites. 1998. p. 277–81. [20] Arau´jo RCS, Pasa VMD. Thermal study of polyurethane elastomers based on biopitch–PEG–MDI system. J Therm Anal Calorim 2002;67:313–9. [21] Arau´jo RCS, Pasa VMD. Mechanical and thermal properties of polyurethane elastomers based on HTPB and biopitch. J Appl Polym Sci 2003;88(3):759–66. [22] Melo BN, Pasa VMD. Composites based on eucalyptus tar pitch/castor oil polyurethane and short sisal fibers. J Appl Polym Sci 2003;89:3797–802. [23] Arau´jo RCS, Pasa VMD. New eucalyptus tar derived polyurethane coatings. Prog Org Coat 2004;51:6–14. [24] Hirose S, Yano S, Hatakeyama T, Hatakeyama H. Heatresistant polyurethanes from solvolysis lignin. In: Proceedings of lignin properties and materials, Toronto, 1988. p. 382–9.
European Polymer Journal 41 (2005) 1420–1428
www.elsevier.com/locate/europolj
Effects of biopitch on the properties of flexible polyurethane foams Renata Costa Silva Arau´jo, Vaˆnya Ma´rcia Duarte Pasa *, Breno Nonato Melo Departamento de Quı´mica, Universidade Federal de Minas Gerais, Av. Antoˆnio Carlos, 6627 CEP 31270 901 Belo Horizonte, Brazil Received 8 March 2004; received in revised form 13 December 2004; accepted 23 December 2004 Available online 2 February 2005
Abstract Biopitches are industrial residues obtained by the distillation of the tar recovered during Eucalyptus charcoal production and can be used as a renewable polyol source. Flexible polyurethane foams were prepared with different proportions of biopitch and HTPB (hydroxyl-terminated polybutadiene) and using polymeric MDI (4,4 0 diphenyl methane diisocyanate), N,N dimethylcyclohexylamine as a catalyst and water as a blowing agent. Elemental analysis, thermal analysis (TG/DSC), Fourier Transform Infrared Spectroscopy (FTIR), scanning electronic microscopy (SEM), and density results were used aiming to discuss the contribution of biopitch to foams properties. The higher the biopitch content, the higher the thermal stability and the lower the density of the flexible foams (air atmosphere), behaviors similar to those of lignin-based polyurethanes. Biopitch enhanced the oxygen content of the polyurethane foams synthesized, and their reaction with HTPB resulted in stable foams. 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Flexible foam; Biopitch; HTPB
1. Introduction The polyurethane foam market has shown a continuous and fast growth in the last years. This growth can be attributed to the physical and mechanical properties of polyurethane foams, their chemical composition, and density, which render them particularly suitable for many applications, such as upholstering, insulation, and packing [1,2]. These properties make flexible polyurethane foams an important material for the synthetic polymer market, which had a business volume of about
* Corresponding author. Tel.: +55 313 499 6651; fax: +55 313 499 6650/5700. E-mail address: [email protected] (V.M.D. Pasa).
4.5 millions ton/year in 1995, corresponding to 60% of mattress and furniture, 35% of automotive use and 5% of packing [3]. Recently, there has been an increased interest in using renewable resources in the plastic industry. Besides industries, many researchers have focused on polymers based on renewable environmentally friendly resources. Polyurethanes based on castor oil [4], starch [5], lignin [6], and other natural polyols have been used in the preparation of foams and elastomers, but there is nothing about the use of biopitch in the literature besides our work. Another concern of the researchers is the blowing agent required for the production of polyurethane foams. Growing evidences that CFCs damage the ozone layer have led researchers to look for new alternative blowing agents. There are basically three types of blowing agents:
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.021
R.C.S. Arau´jo et al. / European Polymer Journal 41 (2005) 1420–1428
(1) water, which reacts with isocyanate and produces carbon dioxide, (2) liquid chemicals with low boiling point, which evaporate due to the exothermic reaction of polyols and isocyanate, (3) air, which is blown in or whipped into the polyols and isocyanate mixture. The use of water as a blowing agent is preferable for the manufacture of flexible polyurethane foams [7] due to its low cost and environmental impact, but sometimes its expansion power is lower than that of CFCs. Brazil is a broad country with an excellent production index of planted forests, which makes it a large potential producer of biomass-based materials. Many Brazilian iron-making industries use charcoal instead of coke as a thermoreducer. About 75% of all charcoal consumed in Brazil (6 million tons/year) is produced using planted Eucalyptus forests [8]. Besides charcoal, the slow pyrolysis of wood also generates light gases and pyroligneous liquor. After decantation, the pyroligneous liquor separates into two phases: wood vinegar (77%) and wood tar (33%). Wood tar is a complex mixture constituted mainly by phenols, acids, aldehydes, and ketones. Wood tar has been studied as a chemical precursor of fine chemicals and of pharmaceuticals [9,10]. The fractionated distillation of wood tar permits the isolation of phenol, guaiacol, syringol, and the generation of a solid residue (35–60%). This residue, called wood tar pitch or biopitch, has been used as a precursor of polymeric materials in our research works. It is very important to develop applications for biopitch, considering the expressive Brazilian potential production of about 200,000 ton/year, only by integrated companies, which grow their own Eucalyptus forests, and run carbonization units and ironmaking plants. The use of biopitch by polymeric material industries could aggregate value and improve its economical feasibility, generating new revenue for the charcoal making industry and Brazilian poor rural areas. Investigations of biopitch by elemental analysis have demonstrated its aromatic nature, its high oxygen content (24%) [11], and O/C and H/C ratios of about 0.25 and 1.10, respectively. The structure of biopitch is different from that of both coal tar pitch (O/C and H/C ratios of 0.6 · 103 and 8.0 · 103) and petroleum pitch (O/C and H/C ratios of 0.8 · 103 and 4.0 · 103) [12]. The chemical units that constitute the oligomer structure of biopitch are mainly phenyl, guaiacyl, syringyl, and their derivatives. Its structure is similar to that of lignin, presenting almost the same chemical groups, but biopitch shows a thermoplastic behavior, while lignins normally are thermosetting. Lignin is recognized as being a three dimensional, highly branched polymer and has been used as a natural polyol in poly-
1421
urethane synthesis [13,14]. The glass transition temperature of lignins range from 90 to 180 C, depending on the pretreatment used [15], while that of biopitch has shown results ranging from 25 to 50 C. In Fig. 1, we can observe a simplified model of the main functional groups present in biopitch, considering its complex structure, it is impossible propose a defined formula. Recently, some polymeric materials incorporating biopitch have been synthesized to obtain advanced carbonaceous materials [16–18], phenolic resins [19]. Biopitch has also been used as a polyol in polyurethane elastomers [20,21], composites [22], and coatings [23]. In the different polyurethane systems, biopitch behaves mainly as an aromatic polyol, but its function as a black pigment, flame retardant, and an antioxidant additive have been considered as well. The present work reports results of the synthesis and characterization of flexible polyurethane foams based on biopitch and using water as a blowing agent aiming to evaluate the influence of this renewable polyol on the properties of the flexible PU foams obtained.
Fig. 1. Model structure illustrating the main functional groups of biopitch.
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2. Experimental
3.3. Infrared spectroscopy and elemental analysis
2.1. Materials
The chemical characterization of the biopitch-based foams was made by elemental analysis and infrared spectroscopy. Elemental analyses were carried out in a Perkin Elmer 2400 to assess carbon, hydrogen, and nitrogen contents in the samples, and oxygen by difference. Elemental O/C and H/C ratios were calculated aiming to evaluate the effect of the biopitch oxygen content on foam properties. Fourier Transform Infrared Spectroscopy (FTIR) was carried out in a Perkin Elmer 1000 by ATR—Attenuated Total Reflectance.
The biopitch used as a polyol was obtained from wood tar distillation (T = 200 C and P = 30 mm Hg) in a pilot plant. Flexible foams were obtained using Eucalyptus biopitch, HTPB—Hydroxyl Terminated Polybutadiene (PETROFLEX, Brazil), MDI (VL R 20 BR, Bayer, Brazil), and N,N dimethylcyclohexamine (DMCHA) as a catalyst and distilled water as a blowing agent. 2.2. Foam preparation Biopitch was mixed with HTPB and stirred with distilled water and dimethylcyclohexamine. MDI was added and stirred for 1 min. The mixture was poured into a container and allowed to react completely at room temperature. Different biopitch contents were used in synthesis aiming to evaluate the material properties. Biopitch and HTPB proportions and the biopitch concentration in the formulations are shown in Table 1. The NCO/OH ratio was kept at 1.0.
3. Characterization 3.1. Hydroxyl content Biopitches are characterized as natural polyol sources due to their hydroxyl groups. Hydroxyl content was determined by biopitch acetylation (monitored by infrared spectroscopy) with acetic anhydride catalyzed by pyridine for 100 h. After acetylation, titration was carried out with NaOH (1.69 eq g/L) [21]. 3.2. Nuclear magnetic resonance spectroscopy—solidstate 13C NMR of biopitch Nuclear magnetic resonance spectroscopy—solidstate 13C NMR: spectra were taken on a VARIAN INOVA-300 with probe RT CP/MAS. NMR analysis of the biopitch sample used was carried out.
Table 1 Flexible polyurethane foams using biopitch as a natural polyol in different proportions Sample
HTPB/Biopitch proportion
PU PU PU PU PU
5/0 5/1 5/2 5/3 5/5
HTPB A B C D
3.4. Thermal properties Thermogravimetric Analysis (TGA) was performed with TGA-50 Shimadzu under dynamic nitrogen and air atmospheres and at a temperature range from room temperature to 750 C, with a heating rate of 10 C/min. The samples had an approximate weight of 4 mg. Differential Scanning Calorimetry (DSC) was carried out in a DSC-50 Shimadzu in two scans. First, the samples were scanned up to 150 C, thereafter cooled to 120 C and recorded a second time up to 150 C. Tg was determined from the second scan. Measurements were assessed under helium atmosphere with a flow rate of 50 mL/min, and a heating rate of 20 C/min. The samples had an approximate weight of 16 mg. 3.5. Density Densities were determined by standard method (ASTM) D 792-86. Five samples were tested and the average value is reported. 3.6. Morphological properties Scanning electronic microscopy formed with JSM-840 A scanning to observe foam cellular structure. fractured after immersion in liquid material was coated with a thin observation.
(SEM) was permicroscope–JEOL The samples were nitrogen, and the gold layer before
4. Results and discussion 4.1. Hydroxyl content The hydroxyl content of biopitch was 7% or 153 mg KOH/g, confirming its polyol character. This value is higher than the hydroxyl content of HTPB (47 mg KOH/g), an aliphatic polyol.
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4.2. Nuclear magnetic resonance spectroscopy—solidstate 13C NMR of biopitch Solid-state 13C NMR analysis (Table 2) showed the different chemical groups that constitute the biopitch macromolecular structure. It is possible to verify that biopitch has about 27.3% of aliphatic carbon, 8.7% of olefinic carbon, 57.2% of aromatic carbon, and 6.8% of carbonyl. It is observed that the hydroxyl group that constitutes polyurethane is basically phenolic. 4.3. Infrared spectroscopy and elemental analysis Table 3 shows the elemental analysis results and O/C and H/C ratios of foams with different biopitch contents. It was observed a high oxygen content (24%) for the biopitch sample used, and consequently, an enhancement of the polyurethane O/C ratio with the increase in biopitch content, mainly due to the contribution of methoxyl, carbonyl, and hydroxyl groups from the biopitch structure. This renewable polyol also contributed to reduce the H/C ratio of the polyurethane foams because of the increase in aromaticity. The biopitch infrared spectrum confirmed the results obtained by solid-state 13C NMR analysis, with phenolic groups (OH 3350 cm1 ; C@C aromatic 1600, 1500 1450 cm1). The synthesis of polyurethane foams was confirmed by FTIR–ATR spectroscopy. It can be seen in Fig. 2 that the urethane bond at 1730 cm1 (carbonyl) is present in all samples. The hydroxyl bond in the polyurethane spectra overlaps with the N–H bonds related to urethanes groups at 3350 cm1. It is possible that residual biopitch OH groups are unreacted because of their low accessibility due to complex biopitch structure. Besides, isocyanate absorption does not appear at 2275 cm1 due to the complete reaction of isocyanate to form urethane bonds and carbon dioxide. The spectrum of PU HTPB presented higher relative intensity of the 2920 cm1 peak than their corresponding values for the others samples due to the aliphatic polyol (HTPB) used. Other absorptions are assigned to polyol and MDI groups. 4.4. Density Flexible polyurethane foam densities were determined and the average values are shown in Fig. 3. It was observed that the incorporation of biopitch into foams caused a decrease in density, which could be attributed to the high aromaticity of the biopitch structure. Almost all hydroxyl groups that formed urethane bonds were phenolic and presented low mobility. As a result, the addition of biopitch caused an increase in cell size, and consequently more expanded materials (higher volumes), and decreased densities. Another important
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aspect is the unreacted biopitch phase, which behaves as an inclusion, and therefore makes chain packing difficult, especially in samples C and D, and decreases density. 4.5. Thermal properties Thermogravimetric analyses were carried out in nitrogen and air atmospheres, as observed in Figs. 4 and 5, respectively. None of the curves showed events at 100 C due to water release, thus, the FTIR absorption at 3350 cm1 cannot be assigned to residual water. In nitrogen atmosphere, the TG curves showed that PU based on HTPB was more stable than other biopitchbased PUs up to 500 C. The TG curve of biopitch showed that it has low thermal stability due to its high oxygen content, and the presence of low molecular weight monomers in its structure. Consequently, the addition of biopitch decreased Tonset temperatures of the foams synthesized in nitrogen atmosphere. Due to the significant biopitch aromaticity, the higher the biopitch content, the higher the foam carbon residue (N2 atmosphere). Samples PU B, PU C, and PU D presented a residual carbon yield higher than 40% at temperatures above 500 C. This value is more significant than the carbon residue observed in the biopitch degradation due to the synergistic contributions of biopitch and MDI used to synthesize foams. We can explain this effect by the difficult heat transfer in cellular materials. The higher the biopitch content, the higher the open cell volume, and consequently, heat transfer is hindered and pyrolysis is less effective. This behavior is not observed in polyurethane elastomers synthesized with HTPB/Biopitch/MDI [21], and with PEG/Biopitch/MDI [20], whose carbon residues are lower and structures are not cellular. In air atmosphere, after a weight loss of about 20%, the presence of biopitch increases the thermal stability of foams and retards degradation reactions. It is important to point out that all biopitch-based foams presented similar maximum degradation temperature, around 520 C, higher than that observed for PU HTPB (420 C). A similar behavior was observed in the thermal study of lignin-based polyurethanes [24]. It was observed that biopitch retards the degradation of the materials in air atmosphere as a consequence of its aromatic structure and the condensation reactions that occur between the sample and the atmosphere oxygen. In Fig. 6, DSC curves show glass transition temperature (Tg) relative to soft segments formed by HTPB. The Tg determined for polyurethane foam based only on HTPB is 80 C, and for that added with biopitch, this value approaches 77 C. The insignificant variation in Tg with the increase in biopitch content indicates that this material is a multiphase system. These results are similar to those obtained for elastomers synthesized with
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1424 Table 2 Solid-State NMR Region 1
13
C Analysis of the biopitch used to prepare PUs Group functionality
Chemical shift (ppm)
Carbon content (%)
14
7.9
20
4.1
29–50
11.8
50–70
3.5
110–115
8.7
115–140
30.2
O
140–160
27.0
CO OC/H
160–185
6.8
C
190–230
H
H
C
C
H
H
C H3
C H3
H
H
C
C
H
H
CH3
H H H C
C
C
H C/H H C
C/H CH
O
C/H
C/H
2
C
C
C
C/H
C
C
3
C/H
C O
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Table 3 Elemental Analysis of biopitch and polyurethane foams Sample
Carbon (%)
Hydrogen (%)
Nitrogen (%)
Oxygen (%)
O/C
H/C
Biopitch PU HTPB A B C D
69.6 85.0 78.9 79.2 78.4 75.1
6.4 10.4 8.6 8.8 8.5 7.6
0.4 4.3 5.6 5.5 4.8 4.5
23.6 0.3 6.9 6.6 8.3 12.9
0.2543 0.0026 0.0656 0.0625 0.0794 0.1288
1.1034 1.4683 1.3080 1.3333 1.3010 1.2144
Fig. 2. FTIR–ATR of biopitch (a) and polyurethanes foams (b).
HTPB–biopitch–MDI [21], but different from those obtained for elastomers based on PEG–biopitch–MDI [20]. PEG is a polar polyol and affords a greater homogeneity with biopitch, consequently, the Tg of PEG/Biopitch elastomers increases with biopitch content increase. We used HTPB instead of PEG in this flexible foam synthesis because HTPB presents the exceptional property of increasing material flexibility.
4.6. Morphological properties The open cell structure is confirmed by the morphological analysis performed by SEM in Figs. 7 and 8 with enlargements of 40 and 400 times, respectively. In Fig. 7, the photomicrographs show the sizes and the shapes of cell foams containing biopitch in their structure. It can be observed that biopitch content increases cell sizes
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1426 180
Density (kg/m3)
160 140 120 100 80 60 40 20 0 PU A
PU B
PU C
PU D
Sample Fig. 3. Densities of flexible polyurethane foams based on biopitch.
Fig. 6. DSC curves (second scan) for polyurethanes and biopitch under dynamical helium atmosphere at a heating rate of 20 C/min.
Fig. 4. TG curves of polyurethanes and biopitch under nitrogen dynamical atmosphere at a heating rate of 10 C/min.
Fig. 5. TG curves for polyurethanes and biopitch under air dynamical atmosphere at a heating rate of 10 C/min.
and porosity, which is in accordance with density results. It can be seen in Fig. 8 that there are particles adhered
onto the cell walls in samples A, C, and D. These particles are probably unreacted biopitch that remained in
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Fig. 7. SEM micrographs of foams varying biopitch content. Magnification: 40·.
Fig. 8. SEM micrographs of unreacted biopitch particles in flexible foams. Magnification: 400·.
the flexible foams formed. These results partially confirm FTIR–ATR spectra, which present OH absorption overlapping NH stretching at 3300 cm1. SEM micrographs also confirm the heterogeneity of these PU systems as shown by DSC results. Sample B did not show biopitch particles on the material surface. Possibly, therefore, this sample present better compatibility and thermal properties than others PUs based on biopitch and (Figs. 4 and 5).
5. Conclusion Biopitch, obtained from wood tar distillation, is a renewable polyol precursor for flexible polyurethane foams with phenolic hydroxyls and aromatic carbon content of 57% according to RMN 13C—solid state analysis. It is possible to synthesize flexible foams using a mixture of biopitch and HTPB as a polyol. It was observed by elemental analysis that foam oxygen content
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(O/H) and aromaticity (H/C) increase as density decreases with the addition of biopitch as a consequence of its structural characteristics. Biopitch causes an increase in the cell sizes of polyurethane foams. This fact confirms the decrease in density as biopitch content increases. Calorimetric and thermogravimetric analyses showed the formation of a multiphase material due to negligible variation in Tg with biopitch addition. The lower thermal stability of biopitch causes a decrease in the Tonset of polyurethane foams under nitrogen and air atmospheres, but a significant increase in the carbon residue. This behavior suggests the use of these polyurethane foams as precursors of carbon membranes and carbon foams, which can be used as catalyst supports. Samples containing biopitch are more stable after a weight loss nearly 20% under air atmosphere, and present high maximum degradation temperature (520 C). It is important to point out that no additives were used in these materials, which means that the new flexible foam properties obtained can be improved yet. In addition, biopitch behaves as a polyol, which seems to act as a flame retardant in flexible PU foams.
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