SCIENTIFIC PROCEEDINGS X INTERNATIONAL CONGRESS "MACHINES, TECHNOLОGIES, MATERIALS" 2013 ISSN 1310-3946 PLASMA-AIDED FLAME RETARDATION OF CAOBA ( MACROPHYLLA)

Assoc. Prof. M.Sc. Gospodinova D. Ph.D. 1, M.Sc. Ivanov I. 1, Prof. M.Sc. Dineff P. Ph.D. 1, Prof. M.Sc. Veleva L. Ph.D. 2 Faculty of Electrical Engineering - Technical University of Sofia, Bulgaria 1 CINVESTAV - Merida, Ycatan, Mexico 2

Abstract: The plasma aided flame retardation of wood, wooden products and cellulosic fibrous materials has been conceived and devel- oped as a result of plasma aided process of capillary impregnation. The plasma-chemical surface pre-treatment substantially alters its elec- trical, chemical and capillary activity, thus improving some impregnation process basic characteristics, such as penetration depth, solution spreading and adsorption speed, adsorbed solution capacity. Thermal analysis (TGA, DTA and DSC) has been used to the impact of a new phosphor and nitrogen containing flame retardant and plasma-aided capillary impregnation on plasma-aided flame retardation of Caoba or Mahogany (Swietenia macrophylla) wood. This study has been developed as part of a large research on plasma-chemically acti- vated wood surface and flame retardant treated tropical wood. Keywords: DIELECTRIC BARRIER DISCHARGE SURFACE TREATMENT, PLASMA-AIDED CAPILLARY IMPREGNATION, FLAME RETARDATION OF WOOD AND WOODEN PRODUCTS, THERMAL ANALYSIS.

1. Introduction Some experimental results of the investigation are presented here: alteration of the wood surface chemical composition or the This study has been developed as part of a large research on surface functionalization as a result of cold plasma pre-treatment plasma-chemically activated wood surface and flame retardant at atmospheric pressure and room temperature monitored by treated tropical wood. some methods of commonly used thermal analysis (thermogra- The plasma aided flame retardation of wood, and wood vimetric analysis, TGA; differential thermal analysis, DTA; and products has been developed as a result of a new plasma-aided differential scanning calorimetry, DSC); applying solution im- process of capillary impregnation that comprises a surface pregnation on bare or plasma pre-treated Tzalam wood with or plasma pre-treatment for alteration of the chemical activity of without addition of surfactants to the impregnating solution and wood surface as well as its electrical (ionic) and capillary activi- the resulting change of wood thermal degradation depending on ties, and in general for improvement the technological character- the flame retardation under different conditions monitored by istics of the capillary impregnation process. A technological TGA, DTA, and DSC. system of plasma device and applicators has been created to produce cold technological plasma through dielectric barrier 2. Experimental Investigation discharge (DBD) at atmospheric pressure and room temperature. Mahogany Caoba soft wood (Swietenia macrophylla, Yuca- The cold plasma pre-treatment by non-equilibrium DBD of wood, tan, Mexico) with average dried weight: 420 kg/m3; Janka hard- like White (Bulgaria), Tzalam (Mexico) and Douglas , ness: 3.3 kN; maximum stress in compression parallel: 44.5 MPa; (Canada), improves technological characteristics such as a solu- stress in compression perpendicular: 6.4 MPa; maximum shear tion spreading and adsorption speed, as well as a specific amount stress in parallel: 9.7 MPa; modulus of rupture in bending: 83.3 of the adsorbed flame retardant. In this way, the plasma pre- MPa; radial or R - shrinkage: 4.1 %; tangential or T - shrinkage: treatment of wood and wooden products improves its flame 7.2 %, and volumetric shrinkage: 9.5 %; T/R ratio: 1.75; and retardation [1, 4]. [1, 2]. moisture content of 7.8 % was used in this investigation, Fig. 1.

Mahogany Caoba tree grows up in the Atlantic Coast from 2 45 Mexico through Panama and the Amazon region of Peru, Bolivia Parameter: , W/m

a 40 and Brazil. It was used by the Maya to make dugout canoes. In Voltage URMS, kV p the Spanish territories this tree was called caoba. Tickness of glass barrier: 35 Mahogany has a straight, fine and even grain, and is 3 mm B relatively free of voids and pockets. Its reddish-brown color 30 darkens over time, and displays a reddish sheen when polished. 18 This soft wood has excellent workability, and is very durable. 25 17 16 a) High Historically, the tree's girth allowed for wide boards from Voltage 20 15 traditional mahogany species. These properties make it a HV favorable wood for crafting cabinets and furniture. Mahogany is 15 14 A DB still widely used for fine furniture, veneer, furring, ceilings of 13 b) houses and crafts. It also resists wood rot, making it attractive in Power Real of Density Surface 10 12 GE Wood Sample boat construction. It is also often used for musical instruments. 11 Earth 5 10 But due to its fine-texture and surface inactivation it is difficult to 9 apply flame retardants through capillary impregnation. The 8 18 kV 0 plasma-chemical surface pre-treatment by dielectric barrier air 0 4 6 8 12 12 kV 16 discharge at atmospheric pressure (DBD) was specify as new Air (or operating) Gap d, mm good way to Mahogany wood surface functionalization and activation [5]. Fig. 1. Plasma-chemical surface pre-treatment of wood sample by non-equilibrium dielectric barrier air discharge at atmospheric pressure The objective of this paper was to study the effect of plasma in asymmetric coplanar system with one glass barrier (a), technological pre-treatment on the wood surface functionalization as well as the regime of cathode directed streamers (b). effect of different surfactants on the ion activity of the impregna- tion solution, both aiming to improve the Mahogany wood flame On the basis of prior art, as well as on our own former expe- retardation. rience in plasma aided impregnation of wood and wooden mate- rials, [6, 7], an oxidative (nitrogen oxides, NOx) surface plasma pre-treatment has been applied on the test samples for 60 sec in a

64 YEAR XXI, VOLUME 1, P.P. 64-67 (2013) SCIENTIFIC PROCEEDINGS X INTERNATIONAL CONGRESS "MACHINES, TECHNOLОGIES, MATERIALS" 2013 ISSN 1310-3946 non-equilibrium cold plasma of dielectric barrier air discharge Germany) in quantity of 0.1 vol. %, as well as their combina- (DBD) at atmospheric pressure, industrial frequency (50 Hz) and tions, have been used to control the ion activity of the flame 18 kV (RMS) or 25 kV (PV), Fig. 1. retardant impregnation aqueous solution. The capillary impregna- A halogen-free, phosphorus and nitrogen containing flame tion has been applied on bare (for comparison) or plasma pre- retardant as a 30 wt. % aqueous solution has been used in this treated Mahogany wood samples at atmospheric pressure by investigation. The impregnating flame retardant aqueous solution spraying the corresponding flame retardant solution (390 ml/m2). (PhFRIS, dry substance of 30 wt. %; phosphorus content of about 13 wt. %, pH = 7÷8 and density of 1.15 g/cm3) was based on a Thermal analysis – thermogravimetric analysis (TGA), dif- halogen free phosphorus and nitrogen containing flame retardant. ferential scanning calorimetry (DSC), and differential thermal The replacement of the halogen containing flame retardants by analysis (DTA), have been performed in air at a heating rate of halogen-free ones has been imposed by the toxicity of the halo- 10 0C/min within the temperature range of 25÷1 0000C using gens [8]. A new flame retardant product based on ortho- Perkins-Elmer equipment. phosphorous acid, urea and ammonia has been produced and studied. 3. Results and Discussion

The studied plasma-aided capillary impregnation of Mahog- Mahogany Wood Temperature, 0C any wood was based on both: plasma-chemical pre-treatment of % 0 100 200 300 400 500 600 700 , 0 the wood surface, Fig. 1, and flame-retardant impregnation 251 a) treatment by a new aqueous solution with ion activity optimiza-

tion. It was expected that increased wood capillary activity and -20 increased solution adsorption speed and capacity would allow

Mass Losses Mass 291 good enough flame retardant performance of porous wood sur- -40 face [1, 2, 7].

Relative The DBD surface plasma-chemical activation effects, the ex- -60 pected surface reorganization and alternation of the chemical

Glowing Combustion Glowing composition as a result of the plasma pre-treatment as well as of the flame retardant solution impregnation, have been directly glowing, Char Degradation Smoldering, Ignition -80 - monitored by the thermal behaviour of bare and flame protected Desorbtion, Dehydratation Desorbtion, 472

After Mahogany wood – by TGA, DTA, and DSC-analyzes. Flaming Combustion Combustion Flaming -100 0 Mahogany Wood Temperature, C

200 250 300 350 400 450 500 414 % 0 ,

8,1 %

b) 251 , mJ/sec 160 IP a 335 20,1 % 308 -20 Ignition

291

120 Losses Mass Heat Flow

316 -40 b 33,7 %

80 Relative Smoldering, 312

-60 Combustion Glowing

glowing, Char Degradation c 62,5 %

40 IP - -80 Glowing Combustion Glowing After 0 472 0 100 200 300 400 500 600 700 Combustion Flaming 96,2 % -20 -100 Temperature, 0C Fig. 3. Temperature regimes of mass losses after obtained kinetic statistic models (linear regression) of bare Mahogany wood pyrolysis: a - V 40 410 2

µ ML = - 0.5007 T + 117,612; R =0.9961; b - ML = - , 308 330 2 c) 1.2352 T + 331.603; R =0.9970; c - ML = - 0.2655 T + 29.124; R2=0.9947. Signal 30 - ML - relative mass losses in %; T - temperature in degree of Celsius;

2

R - coefficient of determination equation. DTA 20 It is well known that hemicellulose, cellulose and lignin are the three main components of wood biomass which in general

÷ ÷ ÷

Smoldering, Ignition IP cover respectively 20 40, 40 60, and 10 25 wt. % for lignocel- 10 lulosic biomass. Its pyrolysis can be divided into four stages: glowing, Char Degradation - moisture evolution, hemicellulose decomposition, cellulose Flaming Combustion Combustion Flaming Glowing Combustion Glowing

After decomposition and lignin decomposition. Chemical bonds begin 0 to break at temperatures higher than 100 0C and are manifested as 0 100 200 300 400 500 600 700 -5 carbohydrate and lignin weight losses of various types (wood Temperature, 0C pyrolysis) that increase with the temperature according to the pyrolysis models of F. Browne, 1958; R. Miller and J. Bellan, Fig. 2. Thermal analysis - TGA (a), DSC (b), and DTA (c) spectra of 1996, and K. Bryden, 1998). bare Mahogany (Swietenia macrophylla, Yucatan, Mexico) wood in air (heating rate - 10 0C per minute): Mahogany wood pyrolysis stages and The results on thermal degradation of bare Mahogany wood characteristic peaks identification. above 100 0C shown Fig. 2, 3, and 4 allow the following conclu- sions: The non-equilibrium air plasma treatment gives good results A. Mahogany Caoba wood reveals the following characteris- increasing the chemical, anionic and capillary surface activities. tics (TGA, DTA and DSC analysis): Anionic surfactants (AS, “Aniticrystallin A“, Chimatech, Ltd., Bulgaria) in quantity of 5 vol. %, or silicone super spreader (SSP, i) self-sustaining exothermic pyrolysis process and transition Y-17113, Momentive Performance Materials GmbH & Co. KG, to flaming was observed at temperatures over 250 0C;

65 YEAR XXI, VOLUME 1, P.P. 64-67 (2013) SCIENTIFIC PROCEEDINGS X INTERNATIONAL CONGRESS "MACHINES, TECHNOLОGIES, MATERIALS" 2013 ISSN 1310-3946 ii) self-sustaining exothermic pyrolysis process and transition ii) some feasible mechanisms for fire retardency include: to glowing was observed at temperatures over 290 0С. The acid-catalyzed (phosphoric acid) hydrolysis of cellulose and identification of the individual stages of development of pyroly- hemi-cellulose as reactions above 106 0С - lignin does not seem sis is accomplished by a new method based on the representation to be much affected by this FR-chemical; dehydration of Mahog- of the process from a kinetic point of view by a mathematical any wood by FR-chemical; condensation enlargement and cross- model representing a polygon of straight broken lines (linear linking of the carbon skeleton; insulating layer formation, in- regression), Fig. 3. creased dissipation via heat conduction away from the heat The various modes of pyrolysis are successfully presented by source, limited access of oxygen deep in it, and limited release of linear regression of the type, Fig. 3: combustible gases out of it, endothermic reactions to absorb heat 0 ML = B T + A = k B t + A = D t + A (1) (between 100 and 251 C, Fig. 2b); 0 iii) the effective suppression of flaming and glowing combus- where ML is mass losses, mg; T - temperature, C; t - time, 0 0 0 tion within the temperature range from 65 to 472 С, Fig. 5; sek, T = k t, k = 10 C/min = 1/6 C/sek. iv) the change of ionic and capillary activity resulting from iii) a characteristic peak, associated with hemi-cellulose pyro- the anionic surfactant and silicon spreader application in the lysis occurs at a temperature of 280÷290 0С (DTA), Fig. 2b and c, 0 impregnation solution (PhFRIS) accelerates, increases the but it is almost swallowed by the next peak at 308 С, which can amount of the applied flame retardant and the depth of the capil- be associated mainly with the cellulose pyrolysis. A single peak 0 lary impregnation, but does not alter substantially the efficiency, was observed at 308 С (DSC), Fig. 2b and c, corresponding to neither in terms of heat release and heat flow, nor in terms of the hemi-cellulose and cellulose pyrolysis. These experimental mass losses, Fig. 7, 8 and 6. facts comply with the general notion, placing the hemi-cellulose pyrolysis peak in the temperature range from 220 to 315 0С, and Caoba Wood

kg a) 0 J/ that of the cellulose - in the range from 315 to 400 С; 14 M

iv) characteristic peak, associated with lignin degradation , 0 12 exothermic reaction, was represented by peaks at 414 С (DSC), 0 Heat Fig. 3, and at 423 С (DTA), Fig. 2b and c. Generally, the peaks 10 0 of lignin degradation reaction occur between 225 and 450 C; 8 v) during the transition to glowing combustion (pyrolysis), Specific ing, Ignition 72.8 wt. % of the Mahogany wood was solid; 6 vi) remaining wood residue after glowing degradation or after

0

472 C was char which undergoes further degradation by being 4 Combustion Flaming Smolder glowing, Char Degradation - oxidized to CO2, CO, and water during the afterglow stage. The Dehydratation Desorbtion, remaining wood residue above 472 0C was 3.8 wt. %, Fig. 2a 2 After Glowing Combustion Glowing and 3. 0 0 100 200 300 400 500 600 700 B. The examined phosphor and nitrogen containing flame re- 0

Temperature, C

tardant (FR-chemical), applied as an impregnation solution kg b) J/

(PhFRIS), displays the following characteristic features during 14 0.501 (3.6 %) M , the Mahogany wood flame retardency: 12 472

i) this flame-retardant chemicals shift the Mahogany wood Heat ) % thermal degradation to the low temperature pathway of non- 10 .2

combustible gases and greater proportion of remaining wood as 034 72 .

8 ( char residue - the transition to glowing combustion is already Specific 10 taking place at 418 0С (against 291 0С for bare Mahogany) and

6 Combustion Glowing

Flaming Combustion Combustion Flaming 1.134 the remaining char residue is 54 wt. % (against 43 wt. % for bare 1.141 (8.1 %) 4 312 Mahogany), Fig. 2a and 4; 291 (8.2 %) 0 251 Temperature, C 2

% 0 100 200 300 400 500 600 700 800 900 1.103 (7.9 %) , 0 0 Caoba (Control Sample) 250 300 350 400 450 500 0 A + Temperature, C -20 (127) Phosphor and Nitrogen c) 419

kg

Containing Flame Retardant J/ Mass Losses Mass

0.4 M

, 341 -40 373 B Heat Relative (308) 0.3

-60

Specific Specific 0.20

322 Combustion Glowing - Combustion Flaming -80 Flame Retarded Caoba ion, Dehydratation 308 (Cao-18-FR-A5-S) Smoldering, Ignition

0.1 glowing, Char Degradation C 271 - -100 Desorbt After Fig. 4. Thermal analysis - TGA spectra of bare and flame retarded 0 samples of Mahogany (Swietenia macrophylla, Yucatan, Mexico) wood in 0 100 200 300 400 500 600 700 -0.05 0 air: flame retardency effect obtained by plasma aided capillary impreg- Temperature, C nation treatment with flame retardant (phosphor and nitrogen) contain- ing aqueous solution (30 mass %). Fig. 5. Differential Scanning Analysis in air: integral (a, b) and dif- AB - protective char layer production after flame retardant activation ferential (c) specific heat spectra of bare Mahogany (Swietenia macro- at about 125 0C; BC - flame retardency effect of relative mass losses phylla, Yucatan, Mexico) wood - Mahogany wood pyrolysis stages and decrease after obtaining a protective char layer at about 300 0C. characteristic peaks identification.

66 YEAR XXI, VOLUME 1, P.P. 64-67 (2013) SCIENTIFIC PROCEEDINGS X INTERNATIONAL CONGRESS "MACHINES, TECHNOLОGIES, MATERIALS" 2013 ISSN 1310-3946

Temperature, 0C temperature areas; ii - the integral distribution of the heat

% 250 300 350 400 450 500 generated (exothermic), Fig. 7, clearly reveals the delaying effect , 0 of the flame retardant chemicals used in relation to the heat

1 generated up to the temperature range 482÷540 0C; iii - after this -20 3 interval of suppression, however, follows a release of much larger amounts of heat, which is determined by the thermal Mass Losses Mass 4

degradation of the protective char layer; iv - the release of heat -40 2 from bare Mahogany wood sample ends at about 500 0C, i.e. the

Relative entire heat release goes for growth of the fire before reaching the B 0 -60 (308) Combustion Glowing 4 flash point (600 C) and full fire development; v - flame-retarded 3 Mahogany wood releases significantly less amount of heat - up to 2 about 4 MJ/kg, which is one of the effects of flame retardency; vi -80 - plasma-aided flame retardency (curve 4) shifts to the right the point of alignment (540 0С) and reduces substantially the heat, in 1 Flaming Combustion Combustion Flaming relation to the cases where surfactants and spreaders are not used, -100 Fig. 7 (curve 2 and 3). Fig. 6. Thermal analysis - TGA spectra of bare and flame retarded samples of Mahogany (Swietenia macrophylla, Yucatan, Mexico) wood in Conclusion air: 1 - bare Mahogany wood sample: Cao-K; 2 - flame retarded Mahog- any wood sample: Cao-K-FR; 3- plasma aided flame retarded Mahogany The application of thermal analysis (TGA, DTA, and DSC) wood sample: Cao-18-FR; 4 - plasma aided flame retarded Mahogany allows evaluating the Mahogany wood pyrolysis under heat wood sample: Cao-18-FR-A5-S. influence by setting pyrolysis stage temperature ranges and hemi- cellulose, cellulose and lignin characteristic temperature peaks. 30 Thermal analysis helps define and illustrate the impact of the kg Caoba Wood

J/ newly produced phosphor and nitrogen containing flame retar- M 2 3 , 25 dant on Mahogany wood behaviour. It significantly reduces the 540 heat release within the range of 65÷540 0С by suppressing the Heat 4 20 518 flaming and glowing combustion of wood and decreases consid- 482 erably the mass loss.

Specific Specific 15 1 Thermal analysis enables studying the plasma aided capillary EP impregnation technology using aqueous solution containing 10 phosphorus flame retardant and its impact on Mahogany wood plasma-aided flame retardency. The created new approaches for 5 processing of the data from TGA and DSC-analysis allow 0 revealing more clearly the effect of flame-retardant chemicals 0 100 200 300 400 500 600 700 800 900 and of plasma-aided flame retardant capillary impregnation Temperature, 0C technology.

Fig. 7. DSC - thermal analysis: integral specific heat spectra of bare Acknowledgments (1) and flame retarded (2, 3, and 4) samples of Mahogany (Swietenia macrophylla, Yucatan, Mexico) wood: 1 - bare Mahogany wood sample: The authors gratefully acknowledge the financial support of Cao-K; 2 - flame retarded Mahogany wood sample: Cao-K-FR; 3, 4 - Technical University - Sofia, for the Research Project plasma aided flame retarded Mahogany wood sample: Cao-18-FR; Cao- 132ПД0051-01. 18-FR-A5-S. References

200 [1] Dineff, P., D. Gospodinova, L. Kostova, T. Vladkova, and E.

2 Chen., Plasma aided surface technology for modification of materials , mJ/sec 160 3 referred to fire protection, Problems of Atomic Science and Technology, 4 Series Plasma Physics (14), 2008, 6, pp. 198÷200.

120 [2] P. Dineff, L. Kostova. Method for Plasma Chemical Surface Heat Flow 445 Modification. H05H 1/24, Bulgarian Patent Publication No.: BG 66022 1 B1; International Patent Publication No.: WO Patent 2006/133524 A2; 80 459 Priority Date: 14.06.2005 (BG No. 109189). Flaming Combustion Combustion Flaming

[3] Dineff, P., D. Gospodinova, I. Avramova. Energy Characteristics 40 and Parameters of Non-Uniform (Pin-to-Plate) Dielectric Barrier Dis- charge. XVI-th International Symposium on Electrical Apparatus and

Glowing Combustion Glowing Technologies “SIELA‘09”, June 04÷06, 2009, Bourgas, Bulgaria. Pro- 0 ceedings of Papers, 2009, Vol.: 2, pp. 30÷39. 250 300 350 400 450 500 0 Temperature, C [4] Динев, П., Д. Господинова, И. Аврамова. Електродни конфи- гурации на бариерен разряд и ефект на повърхностно химично Fig. 8. DSC - thermal analysis: heat flow effect of plasma aided активиране. ХІХ. Международна конференция по авангардни мате- flame retardency (3 and 4) of Mahogany (Swietenia macrophylla, Yuca- риали и обработки „АМО’09”, 25÷27 юни, 2009, Кранево, България. tan, Mexico) wood: 1 - bare Mahogany wood sample: Cao-K; 2 - flame Сборник с доклади, ДМТ-Продукт, 2009, стр. 191÷196. retarded Mahogany wood sample: Cao-K-FR; 3- plasma aided flame retarded Mahogany wood sample: Cao-18-FR; 4 - plasma aided flame [5] Господинова, Д., И. Иванов, П. Динев, Л. Велева, А. Гутие- retarded Mahogany wood sample: Cao-18-FR-A5-S. рес, Плазмено подпомогната огнезащита на дървесина от ТЦАЛАМ (Lysiloma Bahamensis) - I и ІІ. Годишник на Техническия универси- A new interesting information is obtained from DSC - ther- тет - София, София, България, Том: 62, Кн.: 4, 2012, стр. 103÷122. mal analysis, after switching from the data on heat flow to energy (heat) and its differential and integral distribution, Fig. 5: i - the differential distribution of the heat generated repeats the characteristic peaks of the distribution of heat flow and DTA- signal, Fig.2 b and c, but shifted to the right toward higher

67 YEAR XXI, VOLUME 1, P.P. 64-67 (2013)