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Plasma impregnation of wood with fire retardants
Nuclear Instruments and Methods in Physics Research B 272 (2012) 365–369
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Plasma impregnation of wood with fire retardants Karel G. Pabeliña, Carmencita O. Lumban, Henry J. Ramos ⇑ Plasma Physics Laboratory, National Institute of Physics, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Dielectric barrier discharge Wood Fire retardant Fourier transform infrared spectroscopy Thermo gravimetric analysis
a b s t r a c t The efficacy of chemical and plasma treatments with phosphate and boric compounds, and nitrogen as flame retardants on wood are compared in this study. The chemical treatment involved the conventional method of spraying the solution over the wood surface at atmospheric condition and chemical vapor deposition in a vacuum chamber. The plasma treatment utilized a dielectric barrier discharge ionizing and decomposing the flame retardants into innocuous simple compounds. Wood samples are immersed in either phosphoric acid, boric acid, hydrogen or nitrogen plasmas or a plasma admixture of two or three compounds at various concentrations and impregnated by the ionized chemical reactants. Chemical changes on the wood samples were analyzed by Fourier transform infrared spectroscopy (FTIR) while the thermal changes through thermo gravimetric analysis (TGA). Plasma-treated samples exhibit superior thermal stability and fire retardant properties in terms of highest onset temperature, temperature of maximum pyrolysis, highest residual char percentage and comparably low total percentage weight loss. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For years, man has searched for ways to enhance the resistance of wood to burning and flame spread. The chemical structure of wood, made up mainly of carbon and hydrogen, make it highly combustible. There are three elements involved in a combustion reaction: fuel, an oxidizer and a source of heat. When these three elements are combined in the appropriate environment, combustion will occur. The combustion of wood is mainly the pyrolysis of the cellulose and its reaction to oxygen. When temperature is increased, pyrolysis occurs. If any of the elements is removed, combustion stops [1]. Flame retardants are substances added or a treatment applied to a material in order to suppress, significantly reduce or delay the combustion of a material [2]. Many fire-retarding techniques have already been established, namely: surface treatment with fire-retardant chemicals such as fire resisting coatings [3], pressure impregnations of chemical solutions into wood [4] and even cutting-edge technologies of adding nanocomposites during product-manufacturing processes [5]. Nonetheless, these methods are capital-intensive and complicated. The use of direct chemical application is increasingly being discouraged due to the volatile by-products endemic in them, which pollute the atmosphere. Low-temperature plasmas have been given much attention in recent years because of its great commercial and technological impact particularly in manufacturing and material processing [6,7]. Leading applications of cold plasma technology include semicon⇑ Corresponding author. Tel.: +632 920 9749; fax: +632 928 0296. E-mail address: [email protected] (H.J. Ramos). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.102
ductor processing, plasma polymerization and coating, flat panel displays, ion implantation, and plasma surface treatment of carbon fibers, textiles and other polymeric materials [6]. This extensive use of plasma in modern technology is a result of its ability to excite gas atoms and molecules into transient and non-equilibrium conditions with very high gas densities thereby altering the normal pathways through which chemical systems evolve from one stable state to another enabling production of novel materials [7]. Plasma treatment of wood provides a sterile procedural environment in that hazardous chemical components are ionized and decomposed into innocuous simple compounds. Thus the technique is more ecosystem friendly compared to direct chemical treatment of wood. The method is advantageous in that it is straightforward, relatively inexpensive and does not involve heating and high vacuum requirements. The main purpose of the study was to obtain an improved fire performance of wood products. Known flame retardants were impregnated on the samples and compared to direct application and vacuum/pressure application of flame retardants. Thermo gravimetric analysis (TGA) was performed on the treated samples and the effects of plasma treatment on the surface of the wood sample and its surface structure were investigated using Fourier transform infrared (FTIR) spectroscopy. 2. Methodology Plywood samples of dimensions 3.75 3.75 1 cm served as test samples. The surface of the plywood was cleaned with sandpaper to insure uniform surface and remove residues. After this, samples were placed inside the plasma enhanced chemical vapor
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deposition (PECVD) facility for vacuum treatment. The plasma device was originally for chemical vapor deposition (CVD) of diamond and diamond-like carbon thin films. The device is a DC glow discharge source which operates under reduced-pressure conditions. The PECVD facility was used as a dielectric barrier discharge (DBD) source since the wood samples act as a dielectric between the two electrodes. Shown in Fig. 1 is the schematic diagram of the low pressure glow discharge facility utilized. Vacuum treatment of wood reduces the moisture content in the pores of the wood and can physically remove organic materials. It has no effect on the chemical structure of the sample. After treatment, the wood samples were placed in a zip lock and stored in a desiccator with silica gels to insure that the moisture content is maintained. Boric acid (BA) of 10% concentration, phosphoric acid (PA) with 85% concentration and Palmer-Asia Flame Guard were the flame retardant (FR) solutions used. Boric acid and phosphoric acid are both non-flammable and have been known for their insecticidal properties. The Palmer-Asia Flame Guard is a commercially-available flame retardant (CFR) solution. For the first chemical treatment, the acid was directly painted over the surface of the untreated wood sample. For the vacuum impregnation and plasma treatment by PECVD, the solutions were placed in a stainless steel cylinder which is connected to the main chamber. The chamber was preliminarily evacuated down to 0.01 Torr before feeding. The amount of vapor flowing through the connecting tube is controlled by a needle valve. To determine the effects of the solutions (PA, BA and CFR) on the wood surface, two methods were performed. First is the conventional chemical treatment wherein the solutions were painted on the surface of the plywood and allowed to dry for 24 h in ambient temperature. Then the samples were placed inside a zip lock and stored in a container with silica gels. The second method is through pressure impregnation. To plasma impregnate with flame retardants, the wood samples were exposed to known flame retardants such as boric acid (BA), phosphoric acid (PA) and commercially-available flame retardant (CFR) solution and/or reactive gases, such as high purity hydrogen and high purity nitrogen for 30 min with varying discharge current (15, 20 and 25 mA). Using the appropriate discharge parameters (i.e., gas feed pressure and plasma current) the charged particles in the plasma induced the impregnation of the retardant chemicals into the wood.
Fig. 1. Schematic of the PECVD facility.
Hydrogen was also tested since it was said to inhibit flammability [8]. Also nitrogen was investigated since it is said to increase the effectiveness of flame retardants, especially when combined with other chemicals such as boron and phosphorus compounds [9]. Exposure time started just after the current was set and the discharge plasma was produced. The operating pressure and gas flow rate were chosen such that the resulting glow discharge is uniform and of relatively high intensity. To insure that the test samples were not subjected to heating, the temperature was monitored throughout the experiment. Thermo gravimetric analysis (TGA) was used to measure weight changes in a material as a function of temperature (or time) under a controlled atmosphere. This was useful in investigating the thermal stability and properties such as the onset temperature (Tonset), maximum pyrolysis temperature (Tmax) and characteristic values like the residual char% (RC%) and total weight loss% (DW%). TGA measurements were carried out at a heating rate of 10 °C/min under N2 atmosphere over the range 34–500 °C using 2960 SDT V3.0F. The thermograms were analyzed using TA Universal Analysis 2000 (Version 4.7A, Build 4.7.0.2) software and Origin 8 (v8.0724). To determine and compare the effects of the chemical and plasma treatment on the wood surface chemistry, FTIR analysis was performed. FTIR tests on these samples were done using a Shimadzu Prestige 21 FTIR spectrophotometer with 4 cm 1 resolution and Pike Technologies Miracle ATR (single reflectance).
3. Results and discussion The effects of chemical and plasma treatment on the flame retardancy of wood samples were investigated. For the chemical treatment, direct application and exposure to vapor of FR were performed. For plasma treatment, the wood samples were treated with different FR plasma for 30 min with varying discharge current (15, 20 and 25 mA). To investigate the effects of the chemical and plasma treatment, we performed thermo gravimetric analysis. Thermally stable material exhibits minimum weight loss even at high temperatures. The TGA thermograms of the different plasma flame retardant treated wood at 25 mA discharge current are shown in Fig. 2. Based from the TGA plot, specific temperature such as the onset temperature (Tonset), maximum pyrolysis temperature (Tmax) and
Fig. 2. Comparison of the TGA thermograms of the different flame retardant plasma-treated wood samples at discharge current 25 mA.
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characteristic values like the residual char% (RC%) and total weight loss% (DW%) of the test samples were determined [10–13]. Shown in Table 1 is a summary of the onset temperature, maximum pyrolysis, residual char (RC%) and the total weight change of temperature of the wood samples treated using different plasma flame retardants. The onset temperature (Tonset) represents the temperature at the start of the thermal decomposition of the sample. This was determined by getting the intersection of the first and second tangents of the thermogram. A low Tonset means that the test sample was easily degraded while a high Tonset indicates that the material does not easily degrade. Therefore, a high Tonset possessed high thermal stability as it can resist change in mass. The Tonset for the different plasma treatments at 25 mA discharge current was obtained to determine which plasma treatment would result to better flame retardant property. The maximum pyrolysis temperature (Tmax) is the temperature where the maximum rate of pyrolysis occurs wherein the material undergoes rapid thermal decomposition. This was determined through getting the maximum of the first derivative of the TGA curve. Based on Table 1, phosphoric acid plasma has the highest Tonset and Tmax. This meant that among the different flame retardant plasma treatment, phosphoric acid plasma is the most thermally stable. It does not degrade easily and it needs a much higher temperature for it to undergo rapid thermal decomposition. The residual char% (RC%) gives the difference between the weight% of the treated wood sample with respect to the untreated sample at a reference temperature. For this case, the reference temperatures used were 375 and 400 °C. It provided the char yield of the sample at the reference temperatures. A high RC% translates to more char formed. The greater the char yield the better flame retardant the material becomes. This may be due to the effect of char as a protective layer and barrier against the heat of the flame. Total weight loss% (DW%) is the difference of the initial and final weight. It presents the total weight change of the sample after the thermo gravimetric tests. The samples which exhibit the low DW% can be considered thermally stable therefore it is more flame retardant as it has retained most of its weight and consequently its mass. Based from Table 1, phosphoric acid plasma-treated wood sample was the most thermally stable since it has retained most of its weight as shown by having the lowest change in weight, DW (%), which is 1.013 and the highest residual char, RC (%), of 1.617 and 1.569 at 375 and 400 °C, respectively. This agrees with Rowell [9] which showed that phosphoric acid has the least percent weight loss as compared to the other flame retardant additives. The high residual char RC% yield of the phosphoric acid plasma-treated wood sample signified that more char was formed at the surface of the treated samples. Therefore there is more protective layer against further thermal decomposition due to heat as observed in its reduced weight loss%. To compare the effects of the chemical and plasma treatment we use the TGA plot of a phosphoric acid-treated wood sample shown in Fig. 3. Using the same method of analysis as before, the
Fig. 3. TGA thermograms of the untreated and treated wood samples.
specific temperature such as the Tonset and Tmax and the values like the RC% and DW% of the test samples were determined. The ‘painted’ and ‘vapor’ denotes the chemically treated wood samples while 15–25 mA indicates the plasma treated ones. Table 2 shows a summary of the results computed from Fig. 3. We first compared the plasma-treated samples with varying discharge current. Based on the results, wood samples treated with 25 mA discharge current have the highest Tonset and Tmax as well as the highest RC% and lowest DW%. Comparing now the results for the 25 mA plasma treated sample with the painted and vapor impregnated wood samples. For the onset temperature, Stevens [14] established that the effect of the direct application of phosphorous compound in the form of alkyland phenyl-chlorophosphorous FR solution applied on wood in ambient atmosphere is to lower Tonset but increase the residual char% at increased temperature. It can be seen that this is also true with our results. However, for the vapor impregnated and plasmatreated wood samples, there had been an increase in the Tonset although the residual char% was still increased. This increase may have been caused by the vacuum treatment on the wood sample. For combustion to occur it needs air but since the sample have been evacuated there is not much air present on the sample. Therefore the Tonset would have to be increased for thermal decomposition to commence. For the Tmax, Stevens [14] also reported that phosphoric compound treatment lower the Tmax but increase the residual char% at elevated temperature. The painted sample yield the lowest Tmax, while the plasma treated yield the highest which was found to occur at 384.54 °C. This simply suggests that with a 25 mA plasma treatment, sample will not undergo rapid thermal decomposition until it reaches the 384.54 °C temperature. It implies that the thermal stability was improved.
Table 1 Summary of the Tonset, Tmax, DW% and residual char% of the different plasma flame retardants.
Untreated CFR H PA + N PA BA
Tonset (°C)
Tmax (°C)
DW (%)
281.12 282.28 290.78 291.28 321.26 290.48
312.32 320.10 335.34 332.42 384.54 315.90
2.60 3.73 1.31 1.59 1.01 1.38
At 375 °C
At 400 °C
Weight (%)
RC (%)
Weight (%)
RC (%)
97.73 96.61 98.79 98.52 99.31 98.69
–
97.54 96.55 98.76 98.49 99.07 98.68
–
1.15 1.09 0.81 1.62 0.98
1.02 1.25 0.97 1.57 1.17
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Table 2 Summary of the Tonset, Tmax, DW% and residual char% comparing the effects of chemical and plasma treatments.
Untreated Painted Vapor 15 20 25
Tonset (°C)
Tmax (°C)
DW (%)
281.12 194.54 282.64 288.06 295.79 321.26
312.32 215.36 367.08 327.08 365.70 384.54
2.60 1.84 0.97 1.14 1.85 1.01
FTIR tests were done on untreated, vacuum treated and plasmatreated samples. The FTIR spectra are illustrated as a plot of the %transmittance versus the wavenumber (cm 1). The bands and its peak in the spectra belong to the chemical bond which absorbs unique frequencies in the IR region yielding information on the molecular structure of the chemical species present in the sample surface. An increase in the %transmittance (%T) of a peak indicates a decrease in the concentration of its assigned compounds, whereas, a decrease in the %transmittance indicates an increase in the concentration of their corresponding compounds. Fig. 4 shows the spectra of an untreated wood sample. The wood surface is mainly composed of the derivatives of wood components such as cellulose, hemicellulose and lignin which are basically made up of OAH, CAH, C@O and CAO functional groups or structural fragments. Table 3 summarizes the prominent peaks of an untreated wood sample. The comparison of the FTIR transmittance spectra of untreated and phosphoric acid-treated wood samples is shown in Fig. 5 and is summarized in Table 4. Based on the spectra, the chemical treatment of the wood surface with phosphoric acid by exposure to vapor leads to the increase in the transmittance of the OAH, CAH and CAO but a decrease in the C@O. Also that of phosphoric acid painted on the surface leads to an increase in the transmittance of the OAH but a decrease in the C@O, CAH and CAO. Between the two chemical treatments, the sample exposed to vapor exhibited the greater increase in the transmittance and the higher reduction in the concentration of OAH, CAH, C@O and CAO functional groups. This suggests that between the two, exposure to vapor lead to a more effective chemical modification of the wood surface. The spectra of the plasma-treated wood samples are also shown in Fig. 5. Similar with the wood sample exposed to vapor, the 25
At 375 °C
At 400 °C
Weight (%)
RC (%)
Weight (%)
RC (%)
97.7 98.36 99.18 98.93 98.44 99.19
– 0.68 1.52 1.26 0.76 1.53
97.54 98.33 99.06 98.9 98.26 99.07
– 0.81 1.56 1.39 0.74 1.57
Table 3 Peaks of the FTIR transmittance spectra of an untreated wood samples. Position (cm
1
)
Functional groups
3500–3200 3000–2850 1850–1600 1260–1000
OAH: broad, strong absorption band CAH: stretching absorption C@O: weak polar carbonyl CAO: stretching region
Fig. 5. FTIR spectra of treated and untreated wood samples.
Table 4 Peaks of the FTIR transmittance spectra of a phosphoric acidtreated wood. Position (cm 2440–2275 1250–1299 1260–855
Fig. 4. FTIR spectrum of an untreated wood sample.
1
)
Functional groups PAH P@O PAO
and 20 mA plasma-treated samples exhibited considerable increase in the transmittance of the OAH, CAH, C@O and CAO as compared with the untreated sample although the 15 mA exhibited a decrease in the transmittance. This implies that the concentration of such functional groups decreases with the increase in the discharge current of the plasma. Moreover, comparison of the FTIR results between the chemical treatment and the plasma treatment, reveals that the plasma-treated samples with discharge current at 25 mA exhibited the highest increase in the OAH, CAH, C@O and CAO transmittance while the painted sample has the least. The results for 25 and 20 mA samples suggest that the plasma species of the phosphoric acid were more energetically reactive at such high
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discharge current allowing these particles to penetrate more throughout the wood. In addition, these results also showed that more chemical reaction occurred at the surface of the plasma-treated wood at high current as compared to the chemical treatment through painting and vapor exposure. Phosphoric acid (H3PO4) when activated by plasma can dissociate into H+, various phosphate ions (i.e. H2 PO4 , HPO24 , PO34 ) and other charged and neutral particles. Since H+ has been found to yield increase in the transmittance CAO bonds, the FTIR results for the plasma-treated wood samples and chemically treated suggests that the phosphate ions would have been the one responsible for the decrease in the number of in the CAO bonds. 4. Conclusions and recommendations Flame endurance of wood samples was increased using chemical and plasma treatment. The chemical treatment involved the conventional method of spraying the solution over the wood surface at atmospheric condition and chemical vapor deposition in a vacuum chamber. In the plasma treatment, wood samples were immersed in reactive plasmas comprised of various ratios of flame retardant solutions and/or reactive gases. Based on the TGA results, chemical treatment by exposure to vapor was found to be more effective compared to the direct application. However, comparing the chemical treatment and plasma treatment, the 25 mA phosphoric acid plasma treatment was found to yield superior thermal stability and flame retardant properties; in terms of the highest onset temperature and temperature of maximum pyrolysis, highest residual char% and comparably low total weight%. It is recommended therefore to: 1. Perform additional thermal analysis and flame performance tests such as limiting oxygen index (LOI) test, cone calorimetry, and standard vertical and horizontal flame tests.
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2. Test the residues for presence of toxic materials. 3. Investigate more on the plasma parameters and the combination of the flame retardant solutions and reactive gases to be used. 4. Apply biasing as a focusing mechanism.
Acknowledgements The authors wish to acknowledge the support of a research grant from the Philippine Council for Industry, Energy and Emerging Technology Research and Development (PCIEERD). References [1] http://virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/burning/ burning.html. [2] http://www.inchem.org/documents/ehc/ehc/ehc192.htm#PartNumber:1. [3] H.W. Eickner, J. Fire Flamm. 6 (1975) 16. [4] R.H. White, M.A. Dietenberger, Wood Handbook – Wood as an Engineering Material (General Technical Report FPL-GTR-113), US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1999. pp. 17–1. [5] D. Porter, E. Metcalfe, M.J.K. Thomas, Fire Mater. 24 (2000) 45. [6] F.F. Chen, Phys. Plasmas 2 (6) (1995) 1084. [7] T.C. Chang, Journal of Industrial Technology 15 (1) (1998–1999) 1–7. [8] G.Q. Blantocas, P.E.R. Manteum, R.W. Orille, R.J.M. Ramos, J.L.C. Monasterial, H.J. Ramos, L.M.T. Bo-ot, Nucl. Instrum. Methods Phys. Res. B 259 (2007) 875– 883. [9] R.M. Rowell, S.L. LeVan-Green, in: R.M. Rowell (Ed.), Handbook of Wood Chemistry and Wood Composites, Chapter 6, CRC Press, 2005, p. 16. [10] M.E. Brown, Handbook of Thermal Analysis and Calorimetry, vol. 1, Elvesier Science B.V., Netherlands, 1998. [11] R.F. Speyer, Thermal Analysis of Materials, Marcel Dekker Inc., New York, 1993. [12] S. Liu, H. Ye, Y. Zhou, J. He, Z. Jiang, J. Zhao, X. Huang, Polym. Degrad. Stab. 91 (2006) 1808–1814. [13] Y. Soudais, L. Moga, J. Blazek, F. Lemort, J. Anal. Appl. Pyrolysis 78 (2007) 46– 57. [14] R. Stevens, D.S. van Es, R. Bezemer, A. Kranenbarg, Polym. Degrad. Stab. 91 (2006) 832–841.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Plasma impregnation of wood with fire retardants Karel G. Pabeliña, Carmencita O. Lumban, Henry J. Ramos ⇑ Plasma Physics Laboratory, National Institute of Physics, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Dielectric barrier discharge Wood Fire retardant Fourier transform infrared spectroscopy Thermo gravimetric analysis
a b s t r a c t The efficacy of chemical and plasma treatments with phosphate and boric compounds, and nitrogen as flame retardants on wood are compared in this study. The chemical treatment involved the conventional method of spraying the solution over the wood surface at atmospheric condition and chemical vapor deposition in a vacuum chamber. The plasma treatment utilized a dielectric barrier discharge ionizing and decomposing the flame retardants into innocuous simple compounds. Wood samples are immersed in either phosphoric acid, boric acid, hydrogen or nitrogen plasmas or a plasma admixture of two or three compounds at various concentrations and impregnated by the ionized chemical reactants. Chemical changes on the wood samples were analyzed by Fourier transform infrared spectroscopy (FTIR) while the thermal changes through thermo gravimetric analysis (TGA). Plasma-treated samples exhibit superior thermal stability and fire retardant properties in terms of highest onset temperature, temperature of maximum pyrolysis, highest residual char percentage and comparably low total percentage weight loss. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For years, man has searched for ways to enhance the resistance of wood to burning and flame spread. The chemical structure of wood, made up mainly of carbon and hydrogen, make it highly combustible. There are three elements involved in a combustion reaction: fuel, an oxidizer and a source of heat. When these three elements are combined in the appropriate environment, combustion will occur. The combustion of wood is mainly the pyrolysis of the cellulose and its reaction to oxygen. When temperature is increased, pyrolysis occurs. If any of the elements is removed, combustion stops [1]. Flame retardants are substances added or a treatment applied to a material in order to suppress, significantly reduce or delay the combustion of a material [2]. Many fire-retarding techniques have already been established, namely: surface treatment with fire-retardant chemicals such as fire resisting coatings [3], pressure impregnations of chemical solutions into wood [4] and even cutting-edge technologies of adding nanocomposites during product-manufacturing processes [5]. Nonetheless, these methods are capital-intensive and complicated. The use of direct chemical application is increasingly being discouraged due to the volatile by-products endemic in them, which pollute the atmosphere. Low-temperature plasmas have been given much attention in recent years because of its great commercial and technological impact particularly in manufacturing and material processing [6,7]. Leading applications of cold plasma technology include semicon⇑ Corresponding author. Tel.: +632 920 9749; fax: +632 928 0296. E-mail address: [email protected] (H.J. Ramos). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.102
ductor processing, plasma polymerization and coating, flat panel displays, ion implantation, and plasma surface treatment of carbon fibers, textiles and other polymeric materials [6]. This extensive use of plasma in modern technology is a result of its ability to excite gas atoms and molecules into transient and non-equilibrium conditions with very high gas densities thereby altering the normal pathways through which chemical systems evolve from one stable state to another enabling production of novel materials [7]. Plasma treatment of wood provides a sterile procedural environment in that hazardous chemical components are ionized and decomposed into innocuous simple compounds. Thus the technique is more ecosystem friendly compared to direct chemical treatment of wood. The method is advantageous in that it is straightforward, relatively inexpensive and does not involve heating and high vacuum requirements. The main purpose of the study was to obtain an improved fire performance of wood products. Known flame retardants were impregnated on the samples and compared to direct application and vacuum/pressure application of flame retardants. Thermo gravimetric analysis (TGA) was performed on the treated samples and the effects of plasma treatment on the surface of the wood sample and its surface structure were investigated using Fourier transform infrared (FTIR) spectroscopy. 2. Methodology Plywood samples of dimensions 3.75 3.75 1 cm served as test samples. The surface of the plywood was cleaned with sandpaper to insure uniform surface and remove residues. After this, samples were placed inside the plasma enhanced chemical vapor
366
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deposition (PECVD) facility for vacuum treatment. The plasma device was originally for chemical vapor deposition (CVD) of diamond and diamond-like carbon thin films. The device is a DC glow discharge source which operates under reduced-pressure conditions. The PECVD facility was used as a dielectric barrier discharge (DBD) source since the wood samples act as a dielectric between the two electrodes. Shown in Fig. 1 is the schematic diagram of the low pressure glow discharge facility utilized. Vacuum treatment of wood reduces the moisture content in the pores of the wood and can physically remove organic materials. It has no effect on the chemical structure of the sample. After treatment, the wood samples were placed in a zip lock and stored in a desiccator with silica gels to insure that the moisture content is maintained. Boric acid (BA) of 10% concentration, phosphoric acid (PA) with 85% concentration and Palmer-Asia Flame Guard were the flame retardant (FR) solutions used. Boric acid and phosphoric acid are both non-flammable and have been known for their insecticidal properties. The Palmer-Asia Flame Guard is a commercially-available flame retardant (CFR) solution. For the first chemical treatment, the acid was directly painted over the surface of the untreated wood sample. For the vacuum impregnation and plasma treatment by PECVD, the solutions were placed in a stainless steel cylinder which is connected to the main chamber. The chamber was preliminarily evacuated down to 0.01 Torr before feeding. The amount of vapor flowing through the connecting tube is controlled by a needle valve. To determine the effects of the solutions (PA, BA and CFR) on the wood surface, two methods were performed. First is the conventional chemical treatment wherein the solutions were painted on the surface of the plywood and allowed to dry for 24 h in ambient temperature. Then the samples were placed inside a zip lock and stored in a container with silica gels. The second method is through pressure impregnation. To plasma impregnate with flame retardants, the wood samples were exposed to known flame retardants such as boric acid (BA), phosphoric acid (PA) and commercially-available flame retardant (CFR) solution and/or reactive gases, such as high purity hydrogen and high purity nitrogen for 30 min with varying discharge current (15, 20 and 25 mA). Using the appropriate discharge parameters (i.e., gas feed pressure and plasma current) the charged particles in the plasma induced the impregnation of the retardant chemicals into the wood.
Fig. 1. Schematic of the PECVD facility.
Hydrogen was also tested since it was said to inhibit flammability [8]. Also nitrogen was investigated since it is said to increase the effectiveness of flame retardants, especially when combined with other chemicals such as boron and phosphorus compounds [9]. Exposure time started just after the current was set and the discharge plasma was produced. The operating pressure and gas flow rate were chosen such that the resulting glow discharge is uniform and of relatively high intensity. To insure that the test samples were not subjected to heating, the temperature was monitored throughout the experiment. Thermo gravimetric analysis (TGA) was used to measure weight changes in a material as a function of temperature (or time) under a controlled atmosphere. This was useful in investigating the thermal stability and properties such as the onset temperature (Tonset), maximum pyrolysis temperature (Tmax) and characteristic values like the residual char% (RC%) and total weight loss% (DW%). TGA measurements were carried out at a heating rate of 10 °C/min under N2 atmosphere over the range 34–500 °C using 2960 SDT V3.0F. The thermograms were analyzed using TA Universal Analysis 2000 (Version 4.7A, Build 4.7.0.2) software and Origin 8 (v8.0724). To determine and compare the effects of the chemical and plasma treatment on the wood surface chemistry, FTIR analysis was performed. FTIR tests on these samples were done using a Shimadzu Prestige 21 FTIR spectrophotometer with 4 cm 1 resolution and Pike Technologies Miracle ATR (single reflectance).
3. Results and discussion The effects of chemical and plasma treatment on the flame retardancy of wood samples were investigated. For the chemical treatment, direct application and exposure to vapor of FR were performed. For plasma treatment, the wood samples were treated with different FR plasma for 30 min with varying discharge current (15, 20 and 25 mA). To investigate the effects of the chemical and plasma treatment, we performed thermo gravimetric analysis. Thermally stable material exhibits minimum weight loss even at high temperatures. The TGA thermograms of the different plasma flame retardant treated wood at 25 mA discharge current are shown in Fig. 2. Based from the TGA plot, specific temperature such as the onset temperature (Tonset), maximum pyrolysis temperature (Tmax) and
Fig. 2. Comparison of the TGA thermograms of the different flame retardant plasma-treated wood samples at discharge current 25 mA.
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characteristic values like the residual char% (RC%) and total weight loss% (DW%) of the test samples were determined [10–13]. Shown in Table 1 is a summary of the onset temperature, maximum pyrolysis, residual char (RC%) and the total weight change of temperature of the wood samples treated using different plasma flame retardants. The onset temperature (Tonset) represents the temperature at the start of the thermal decomposition of the sample. This was determined by getting the intersection of the first and second tangents of the thermogram. A low Tonset means that the test sample was easily degraded while a high Tonset indicates that the material does not easily degrade. Therefore, a high Tonset possessed high thermal stability as it can resist change in mass. The Tonset for the different plasma treatments at 25 mA discharge current was obtained to determine which plasma treatment would result to better flame retardant property. The maximum pyrolysis temperature (Tmax) is the temperature where the maximum rate of pyrolysis occurs wherein the material undergoes rapid thermal decomposition. This was determined through getting the maximum of the first derivative of the TGA curve. Based on Table 1, phosphoric acid plasma has the highest Tonset and Tmax. This meant that among the different flame retardant plasma treatment, phosphoric acid plasma is the most thermally stable. It does not degrade easily and it needs a much higher temperature for it to undergo rapid thermal decomposition. The residual char% (RC%) gives the difference between the weight% of the treated wood sample with respect to the untreated sample at a reference temperature. For this case, the reference temperatures used were 375 and 400 °C. It provided the char yield of the sample at the reference temperatures. A high RC% translates to more char formed. The greater the char yield the better flame retardant the material becomes. This may be due to the effect of char as a protective layer and barrier against the heat of the flame. Total weight loss% (DW%) is the difference of the initial and final weight. It presents the total weight change of the sample after the thermo gravimetric tests. The samples which exhibit the low DW% can be considered thermally stable therefore it is more flame retardant as it has retained most of its weight and consequently its mass. Based from Table 1, phosphoric acid plasma-treated wood sample was the most thermally stable since it has retained most of its weight as shown by having the lowest change in weight, DW (%), which is 1.013 and the highest residual char, RC (%), of 1.617 and 1.569 at 375 and 400 °C, respectively. This agrees with Rowell [9] which showed that phosphoric acid has the least percent weight loss as compared to the other flame retardant additives. The high residual char RC% yield of the phosphoric acid plasma-treated wood sample signified that more char was formed at the surface of the treated samples. Therefore there is more protective layer against further thermal decomposition due to heat as observed in its reduced weight loss%. To compare the effects of the chemical and plasma treatment we use the TGA plot of a phosphoric acid-treated wood sample shown in Fig. 3. Using the same method of analysis as before, the
Fig. 3. TGA thermograms of the untreated and treated wood samples.
specific temperature such as the Tonset and Tmax and the values like the RC% and DW% of the test samples were determined. The ‘painted’ and ‘vapor’ denotes the chemically treated wood samples while 15–25 mA indicates the plasma treated ones. Table 2 shows a summary of the results computed from Fig. 3. We first compared the plasma-treated samples with varying discharge current. Based on the results, wood samples treated with 25 mA discharge current have the highest Tonset and Tmax as well as the highest RC% and lowest DW%. Comparing now the results for the 25 mA plasma treated sample with the painted and vapor impregnated wood samples. For the onset temperature, Stevens [14] established that the effect of the direct application of phosphorous compound in the form of alkyland phenyl-chlorophosphorous FR solution applied on wood in ambient atmosphere is to lower Tonset but increase the residual char% at increased temperature. It can be seen that this is also true with our results. However, for the vapor impregnated and plasmatreated wood samples, there had been an increase in the Tonset although the residual char% was still increased. This increase may have been caused by the vacuum treatment on the wood sample. For combustion to occur it needs air but since the sample have been evacuated there is not much air present on the sample. Therefore the Tonset would have to be increased for thermal decomposition to commence. For the Tmax, Stevens [14] also reported that phosphoric compound treatment lower the Tmax but increase the residual char% at elevated temperature. The painted sample yield the lowest Tmax, while the plasma treated yield the highest which was found to occur at 384.54 °C. This simply suggests that with a 25 mA plasma treatment, sample will not undergo rapid thermal decomposition until it reaches the 384.54 °C temperature. It implies that the thermal stability was improved.
Table 1 Summary of the Tonset, Tmax, DW% and residual char% of the different plasma flame retardants.
Untreated CFR H PA + N PA BA
Tonset (°C)
Tmax (°C)
DW (%)
281.12 282.28 290.78 291.28 321.26 290.48
312.32 320.10 335.34 332.42 384.54 315.90
2.60 3.73 1.31 1.59 1.01 1.38
At 375 °C
At 400 °C
Weight (%)
RC (%)
Weight (%)
RC (%)
97.73 96.61 98.79 98.52 99.31 98.69
–
97.54 96.55 98.76 98.49 99.07 98.68
–
1.15 1.09 0.81 1.62 0.98
1.02 1.25 0.97 1.57 1.17
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Table 2 Summary of the Tonset, Tmax, DW% and residual char% comparing the effects of chemical and plasma treatments.
Untreated Painted Vapor 15 20 25
Tonset (°C)
Tmax (°C)
DW (%)
281.12 194.54 282.64 288.06 295.79 321.26
312.32 215.36 367.08 327.08 365.70 384.54
2.60 1.84 0.97 1.14 1.85 1.01
FTIR tests were done on untreated, vacuum treated and plasmatreated samples. The FTIR spectra are illustrated as a plot of the %transmittance versus the wavenumber (cm 1). The bands and its peak in the spectra belong to the chemical bond which absorbs unique frequencies in the IR region yielding information on the molecular structure of the chemical species present in the sample surface. An increase in the %transmittance (%T) of a peak indicates a decrease in the concentration of its assigned compounds, whereas, a decrease in the %transmittance indicates an increase in the concentration of their corresponding compounds. Fig. 4 shows the spectra of an untreated wood sample. The wood surface is mainly composed of the derivatives of wood components such as cellulose, hemicellulose and lignin which are basically made up of OAH, CAH, C@O and CAO functional groups or structural fragments. Table 3 summarizes the prominent peaks of an untreated wood sample. The comparison of the FTIR transmittance spectra of untreated and phosphoric acid-treated wood samples is shown in Fig. 5 and is summarized in Table 4. Based on the spectra, the chemical treatment of the wood surface with phosphoric acid by exposure to vapor leads to the increase in the transmittance of the OAH, CAH and CAO but a decrease in the C@O. Also that of phosphoric acid painted on the surface leads to an increase in the transmittance of the OAH but a decrease in the C@O, CAH and CAO. Between the two chemical treatments, the sample exposed to vapor exhibited the greater increase in the transmittance and the higher reduction in the concentration of OAH, CAH, C@O and CAO functional groups. This suggests that between the two, exposure to vapor lead to a more effective chemical modification of the wood surface. The spectra of the plasma-treated wood samples are also shown in Fig. 5. Similar with the wood sample exposed to vapor, the 25
At 375 °C
At 400 °C
Weight (%)
RC (%)
Weight (%)
RC (%)
97.7 98.36 99.18 98.93 98.44 99.19
– 0.68 1.52 1.26 0.76 1.53
97.54 98.33 99.06 98.9 98.26 99.07
– 0.81 1.56 1.39 0.74 1.57
Table 3 Peaks of the FTIR transmittance spectra of an untreated wood samples. Position (cm
1
)
Functional groups
3500–3200 3000–2850 1850–1600 1260–1000
OAH: broad, strong absorption band CAH: stretching absorption C@O: weak polar carbonyl CAO: stretching region
Fig. 5. FTIR spectra of treated and untreated wood samples.
Table 4 Peaks of the FTIR transmittance spectra of a phosphoric acidtreated wood. Position (cm 2440–2275 1250–1299 1260–855
Fig. 4. FTIR spectrum of an untreated wood sample.
1
)
Functional groups PAH P@O PAO
and 20 mA plasma-treated samples exhibited considerable increase in the transmittance of the OAH, CAH, C@O and CAO as compared with the untreated sample although the 15 mA exhibited a decrease in the transmittance. This implies that the concentration of such functional groups decreases with the increase in the discharge current of the plasma. Moreover, comparison of the FTIR results between the chemical treatment and the plasma treatment, reveals that the plasma-treated samples with discharge current at 25 mA exhibited the highest increase in the OAH, CAH, C@O and CAO transmittance while the painted sample has the least. The results for 25 and 20 mA samples suggest that the plasma species of the phosphoric acid were more energetically reactive at such high
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discharge current allowing these particles to penetrate more throughout the wood. In addition, these results also showed that more chemical reaction occurred at the surface of the plasma-treated wood at high current as compared to the chemical treatment through painting and vapor exposure. Phosphoric acid (H3PO4) when activated by plasma can dissociate into H+, various phosphate ions (i.e. H2 PO4 , HPO24 , PO34 ) and other charged and neutral particles. Since H+ has been found to yield increase in the transmittance CAO bonds, the FTIR results for the plasma-treated wood samples and chemically treated suggests that the phosphate ions would have been the one responsible for the decrease in the number of in the CAO bonds. 4. Conclusions and recommendations Flame endurance of wood samples was increased using chemical and plasma treatment. The chemical treatment involved the conventional method of spraying the solution over the wood surface at atmospheric condition and chemical vapor deposition in a vacuum chamber. In the plasma treatment, wood samples were immersed in reactive plasmas comprised of various ratios of flame retardant solutions and/or reactive gases. Based on the TGA results, chemical treatment by exposure to vapor was found to be more effective compared to the direct application. However, comparing the chemical treatment and plasma treatment, the 25 mA phosphoric acid plasma treatment was found to yield superior thermal stability and flame retardant properties; in terms of the highest onset temperature and temperature of maximum pyrolysis, highest residual char% and comparably low total weight%. It is recommended therefore to: 1. Perform additional thermal analysis and flame performance tests such as limiting oxygen index (LOI) test, cone calorimetry, and standard vertical and horizontal flame tests.
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2. Test the residues for presence of toxic materials. 3. Investigate more on the plasma parameters and the combination of the flame retardant solutions and reactive gases to be used. 4. Apply biasing as a focusing mechanism.
Acknowledgements The authors wish to acknowledge the support of a research grant from the Philippine Council for Industry, Energy and Emerging Technology Research and Development (PCIEERD). References [1] http://virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/burning/ burning.html. [2] http://www.inchem.org/documents/ehc/ehc/ehc192.htm#PartNumber:1. [3] H.W. Eickner, J. Fire Flamm. 6 (1975) 16. [4] R.H. White, M.A. Dietenberger, Wood Handbook – Wood as an Engineering Material (General Technical Report FPL-GTR-113), US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1999. pp. 17–1. [5] D. Porter, E. Metcalfe, M.J.K. Thomas, Fire Mater. 24 (2000) 45. [6] F.F. Chen, Phys. Plasmas 2 (6) (1995) 1084. [7] T.C. Chang, Journal of Industrial Technology 15 (1) (1998–1999) 1–7. [8] G.Q. Blantocas, P.E.R. Manteum, R.W. Orille, R.J.M. Ramos, J.L.C. Monasterial, H.J. Ramos, L.M.T. Bo-ot, Nucl. Instrum. Methods Phys. Res. B 259 (2007) 875– 883. [9] R.M. Rowell, S.L. LeVan-Green, in: R.M. Rowell (Ed.), Handbook of Wood Chemistry and Wood Composites, Chapter 6, CRC Press, 2005, p. 16. [10] M.E. Brown, Handbook of Thermal Analysis and Calorimetry, vol. 1, Elvesier Science B.V., Netherlands, 1998. [11] R.F. Speyer, Thermal Analysis of Materials, Marcel Dekker Inc., New York, 1993. [12] S. Liu, H. Ye, Y. Zhou, J. He, Z. Jiang, J. Zhao, X. Huang, Polym. Degrad. Stab. 91 (2006) 1808–1814. [13] Y. Soudais, L. Moga, J. Blazek, F. Lemort, J. Anal. Appl. Pyrolysis 78 (2007) 46– 57. [14] R. Stevens, D.S. van Es, R. Bezemer, A. Kranenbarg, Polym. Degrad. Stab. 91 (2006) 832–841.