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LimitedoxygenindexlevelsofimpregnatedScotspinewood

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Thermochimica Acta 573 (2013) 181– 185

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hort Communication

imited oxygen index levels of impregnated Scots pine wood

ylem Dizman Tomaka,∗, Ayfer Donmez Cavdarb

Forest Industry Engineering Department, Faculty of Forestry, Bursa Technical University, 16200 Bursa, TurkeyInterior Architecture Department, Faculty of Architecture, Karadeniz Technical University, 61080 Trabzon, Turkey

r t i c l e i n f o

rticle history:eceived 29 July 2013eceived in revised form6 September 2013ccepted 17 September 2013

a b s t r a c t

In this study, effect of various concentrations of boron powder, mixture of boric acid and borax, fire-proof agent based on liquid blend of limestone, and silicon oil on limited oxygen index levels (LOI) ofS. pine wood was investigated. Wood samples were first vacuum treated with the preservatives, andthen were subjected to leaching procedure. Samples treated with fireproof agent showed the best resultsfor improving the fire retardancy of wood, furthermore, samples treated with 25%, 50% and 100% of the

vailable online 30 September 2013

eywords:cots pineoronire retardancy

solution did not burn. Leaching did not considerably change the LOI of wood samples treated with boronpowder and silicon oil; however, LOI levels of samples treated with the mixture of boric acid and boraxand fireproof agent were affected by leaching procedure probably arising those preservatives did notchemically bond to main wood components. All treatments improved fire retardancy of samples despitesome amount of preservatives leached out from wood.

xygen index

. Introduction

Building codes include area and height of the rooms, firestops,oors and exits, automatic sprinklers, fire detectors and typef construction, and fire codes include materials combustibility,ame spread and fire endurance specify fire safety standards

or structures [1]. Fire retardants are usually required in publiconstructions [1]. Burning properties of wood and wood basedomposite materials limit their wide applications in constructionsince they easily burn when exposed to heat [2]. The compoundsontaining bromine, chlorine, or phosphorous, or two or more ofhese elements, antimony, boron, nitrogen, silicon, and zinc haveeen found to be most effective in providing fire retardance [3].any researchers investigated the effect of fire retardant formula-

ions on combustion and thermal degradation of wood and woodased composite/plastic products [3–18]. Depending on the naturef chemicals in the formulations, they can be classified into sixlasses: (1) chemicals that promote the char formation, (2) chem-cals which act as free-radical traps in the flame, (3) chemicals

hich form a coating on the surface, (4) chemicals that increasehe thermal conductivity of wood, (5) chemicals that dilute theombustible gases coming from the wood with non-combustibleasses, and (6) chemicals that reduce the heat content of the volatile

ases [1].

Limiting oxygen index (LOI) measurement is one of the testethods for evaluating the effectiveness of fire retardants, and

∗ Corresponding author. Tel.: +90 2243141756; fax: +90 2243141725.E-mail address: eylemdizman@yahoo.com (E.D. Tomak).

040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.tca.2013.09.022

© 2013 Elsevier B.V. All rights reserved.

has been widely used for determining the fire retardant propertyof wood and wood based/plastic composites [3–6,14,15,17,19–25].Highly flammable materials have a low oxygen index level [19].Inorganic additives increase LOI levels in reducing the yield of lev-oglucosan [1], and thus increase fire retardancy.

There is an increasing attention in searching of environmentallyfriendly and healthy safe fire retardant formulations in nowa-days due to the new regulations and environmental concerns. Forthis purpose, fireproof agent (Firetex) which was developed andpatented by Kale Company in the recent years seemed to be one ofthese formulations. Fireproof agent is water based liquid blend ofa mineral formula of limestone. It increases fire resistance of cellu-losic materials without any harmful adverse effect on humans andanimals. Its’ insecticide and fungicide properties were also reported[26]. Boron powder and silicon oil may also be categorized in thisgroup. It is generally accepted that boron compounds are environ-mental friendly wood preservatives that increases fire retardancyand biological resistance against decay organisms. In many cases,fire retardancy property of wood preservatives recently found orknown for years was reported immediately after impregnationof wood samples. However, leaching of chemicals from wood isvery important parameter for permanent performance of preser-vatives inside wood against degradation factors. In this study,Scots pine samples were treated with fireproof agent, boron pow-der and silicon oil with various concentrations, and subsequentlyexposed to leaching procedure. Leaching procedure was carried out

to determine the permanent performance of those preservatives inwood. LOI levels were determined both for leached and un-leachedsamples according to ASTM D2863 standard. Samples treatedwith boric acid and borax mixture known as having more fire

1 ochimica Acta 573 (2013) 181– 185

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Table 1Retention (kg/m3) of wood samples.

Treatments Retention (kg/m3)

BP 0.25% 1.46 (0.05)a

0.5% 2.97 (0.13)1% 5.95 (0.16)

2% 11.93 (0.48)4% 23.61 (0.70)

BA + BX 0.25% 1.48 (0.06)0.5% 3.02 (0.08)1% 5.60 (0.19)2% 11.47 (0.90)

4% 25.41 (4.82)

FA 5% 30.88 (0.79)10% 64.05 (1.61)

25% 162.38 (5.73)50% 333.67 (15.93)

100% 773.89 (43.84)

SO 100% 166.67 (43.78)

82 E.D. Tomak, A.D. Cavdar / Therm

etardant efficiency than individual boric acid or borax were alsosed as comparison purposes.

. Materials and methods

.1. Materials

Boron powder (BP) and silicon oil (SO) were purchased fromerck Chemicals. Borax decahydrate (BX) and boric acid (BA) were

btained from Eti Mine Works General Management in Turkey.ireproof agent (Firetex) (FA) was provided by Kale Company inurkey. Boron powder with dark brown color has a particle size of

�m and ignition temperature of 700 ◦C. Borax decahydrate com-oses 36.47% boric oxide and 16.24% sodium oxide. Boric oxidemount of boric acid is 56%. Silicon oil has ignition temperaturef 460 ◦C.

Sapwood of Scots pine (Pinus sylvestris L.) timber free of knots,xcessive cross-grain and other obvious defects was obtainedrom Trabzon located in the Northeastern region of Turkey.ine timber was machined into samples with dimensions of0 mm × 4 mm × 150 mm (R, T, L). Samples were carefully chosenor having the same annual ring. Before impregnation, samplesere conditioned at 65% relative humidity and 20 ◦C for two weeks.arious concentrations of boron powder (0.25%, 0.5%, 1%, 2% and%), BA + BX (0.25%, 0.5%, 1%, 2% and 4%) and fireproof agent (5%,0%, 25%, 50% and 100%) were prepared using distilled water. Sil-

con oil was used without any dilution. In the solution of BA + BXixture, the ratio of BA to BX was 7/3 (w/w). Ten replicate samplesere used for each group.

.2. Impregnation with the wood preservatives

Wood samples were first vacuum impregnated with the solu-ions at 600 mm/Hg for 30 min and then they were immersed inhe solutions for 60 min at atmospheric pressure. Retention ofamples (R in kg/m3) was calculated based on the initial weightefore impregnation and the weight after impregnation. Samplesere then conditioned at 20 ◦C and 65% relative humidity for threeeeks.

.3. Leaching procedure

Leaching test was conducted according to AWPA E11-97 [27]tandard method without stirring process. Samples were impreg-ated with distilled water by pre-vacuum for 20 min. After 6, 24,8 and thereafter at 48-h intervals, the leachate was removed andeplaced it with an equal amount of fresh distilled water. Leachingest was continued for a total of 14 days. After leaching procedure,amples were conditioned again at 20 ◦C and 65% relative humidityor four weeks.

.4. Limited oxygen index measurements

LOI measurements were carried out using a Dynisco Limitingxygen Index Chamber according to the principles of ASTM D2863

28] standard. Wood samples were placed vertically in the centerf the glass column using a sample holder and then were burned in

precisely controlled atmosphere of nitrogen and oxygen. LOI wasefined as the lowest oxygen concentration in the carrier gas flowt which full flaming combustion of the samples was observed. Fiveamples for each group were tested.

.5. Statistical analysis

The effect of preservative treatments on LOI of samples in com-arison with controls was analyzed by one-way ANOVA test using

Control – –

a Values in parenthesis are standard deviations.

with SPSS 18.0 program for un-leached and leached groups. Thesignificance (P < 0.05) between the treatments was compared withDuncan homogeneity groups. Statistical analyze was not performedfor fireproof agent treated samples (un-leached) because LOI val-ues were not determined for high concentrations (25%, 50% and100%) despite the highest oxygen content of the test was applied.Mann–Whitney U test was performed when comparing the LOI ofSO treated samples with the LOI of controls.

3. Results and discussion

Table 1 shows the retention values of S. pine samples along withstandard deviations.

Retention values depending on concentration were found to be1.5–24 kg/m3, 1.5–25 kg/m3, 31–774 kg/m3 and 167 kg/m3 for BP,BA + BX, FA and SO treated samples, respectively. Samples treatedwith BP got dark brown color, and at high concentrations (2% and4%) BP cumulated on the surface of samples and totally covered theall surfaces. White crystals of BA + BX mixture on the wood surfacecould be also observed for high loadings of compounds; however,the compounds did not considerably change the original color ofwood as BP did. It was possible to see the light yellowish colorchange in SO treated samples with naked eyes. FA treatment didnot cause observable color change on the wood surface.

Figs. 1 and 2 show LOI levels of un-leached and leached samples,respectively. LOI levels were found to be 25 and 25.5 for un-leachedand leached control samples, respectively. Rapid burning withflames was observed in controls. All treated samples showed higherLOI levels than controls despite leaching procedure was applied.LOI levels increased in treated samples as the uptake of chemicalsinside wood increased. Similar trend was also observed for leachedsamples at high chemical loadings. These results were found sta-tistically significant in one-way ANOVA test (P < 0.05) and Duncanhomogeneity groups. The LOI levels suggested that the combustibil-ity of treated wood decreased with the preservative treatments.

LOI levels of BP treatment were in the range of 26.75–28.5and 26.5–28.75 for un-leached and leached samples, respectively.Char formation occurred in the samples while smoke genera-tion was not observed both for leached and un-leached samples.Char formation during thermal degradation of wood is related

to the degree of flame retardancy exhibited by samples [14,29],and correlates very well with LOI levels [14]. Fire retardants leadto form more char that provides additional surface insulationand also inhibits volatilization of wood [21], and therefore, they

E.D. Tomak, A.D. Cavdar / Thermochimica Acta 573 (2013) 181– 185 183

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Fig. 1. Limited oxygen index (LO

uppress the combustibility and increase the LOI value [14]. Leach-ng did not cause remarkable changes on LOI levels. It was alsoupposed that boron powder still remained in the wood samplesfter leaching procedure although boron compounds could easilyeach out from wood when exposed to water. During the leach-ng procedure, each cycle of leachate got brown color showed thateaching occurred in the samples, however, leached samples stillad dark brown color especially for samples treated with high load-

ngs of boron powder. Boron powder could also be clearly seenith naked eyes on the wood surface after leaching procedure.ne-way ANOVA test results indicated that there were some sta-

istically significant differences between the LOI values of samplesreated with BP and controls (P < 0.05). According to the Duncanomogeneity groups in ANOVA test, LOI of BP treated samplesccurred in the following order: 4%BP = 2%BP ≥ 1%BP ≥ 0.5%BP >.25%BP > control for un-leached group, and 4%BP = 2%BP > 1%BP =.5%BP > 0.25%BP > control for leached group.

Mixture of BA and BX increased LOI levels of wood sam-les, and LOI levels were found to be 28–43.5 and 26–28 forn-leached samples and leached samples, respectively. High

oadings of BA + BX mixture showed better fire retardant propertyhan low loadings did as also found with the Duncan homo-

eneity groups in the following order: 4%BA + BX > 2%BA + BX >%BA + BX > 0.5%BA + BX > 0.25%BA + BX > control. Similar LOI levels,uch as 32.9, 34.6 and 42.1 were also observed for BA + BX treated

Fig. 2. Limited oxygen index (LOI) le

ls of un-leached wood samples.

samples with the loading rates of 0.25%, 1% and 4.7%, respec-tively [21]. Char formation without any smoke was observed forun-leached samples. At the 4% concentration, green flames wereobserved for un-leached samples during the test. Boric acid andborax have a char forming catalytic effect on wood [30]. Wanget al. [31] reported that a physical mechanism was achieved bythe formation of a coating or protective layer on the boron treatedwood surface at high temperatures. It was stated that boric acidand borax mixture have a synergistic effect in retarding flamespread on wood surfaces [30] and char formation [18]. Boraxtends to reduce flame spread but cannot suppress smoldering orglowing. On the contrary, boric acid suppresses smoldering buthas little effect on reducing the flame spread [30]. Leaching proce-dure caused considerably decrease on LOI levels of samples thatsuggested some BA + BX leached from wood under leaching con-ditions. According to the Duncan homogeneity groups in ANOVAtest, LOI of leached samples occurred in the following order:4%BA + BX > 2%BA + BX = 1%BA + BX ≥ 0.5%BA + BX ≥ 0.25%BA + BX ≥control. BA + BX mixture showed better fire retardancy than BPfor un-leached samples. Temiz et al. [12] also found that BA + BXmixture (especially 5%) (7:3, as weight) was more effective thanindividual boric acid or borax. Fire resistance of BA + BX treatment

on wood is also proven by other researchers [21,30,32].

Samples showed LOI levels of 34 and 38 after 5% and 10% FAtreatment. Samples treated with upper concentrations did not burn

vels of leached wood samples.

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espite the highest oxygen content of the test was applied. No obvi-us changes were occurred on those samples. At 10% concentrationf FA treatment, yellow flames were observed without any smoke,nd at 100% concentration colorless flames were observed at thenitial of the test and afterwards reddish flames were observedor un-leached samples. The fire retardancy property of FA onood and cellulosic products was also proven according to ASTM-76 and D-3218 standards [26]. However after leaching proce-ure, LOI of samples decreased to a level of 28 and 29.5. Thesendings showed that FA might not chemically bond with theood components and leached out from the wood and wood

urface. LOI of treated samples were statistically significant fromhe controls (P < 0.05) for leached group. LOI of leached samplesccurred in the following order according to Duncan homogeneityroups: 10%FA = 5%FA > 100% FA = 50%FA = 25%FA > control. As beeneported above, FA is water based liquid blend of a mineral formulaf limestone which consists calcium carbonate. Some of mineralllers decompose and absorb energy during combustion. Similarly,alcium carbonate widely used mineral filler decomposes at tem-eratures between 700 and 900 ◦C and absorbs 1800 kJ/kg, andulfur emissions are reduced since it absorbs oxides of sulfur fromhe combustion of the coal [33]. The decomposition of carbonate

inerals is endothermic and causes a decrease in flame tempera-ure [34]. Apart from cooling effect and quenching of the flames,re retardancy of carbonate minerals is also enhanced by a kind oferamic layer being formed on the surface of forest fuels that pro-ects the ignitable materials from further attacks of flames and heat34,35].

Silicon oil treated samples showed 28.5 LOI levels before andfter leaching procedure. During the test, treatment of wood withO behaved different with aforementioned wood preservatives thatesulted in burning with flames and black smokes. Late ignitioniming but rapid burning occurred on the samples accompany-ng with foam formation on the wood surface. Silicon has longeen used as fire retardant. Water glass ensures fire retardantfficiency by melting and forming an insulating foam layer onhe surface of wood [36]. The mechanism of silicates may bexplained by a barrier theory [37]. A barrier can retard both smol-ering combustion and flaming combustion by preventing theammable products and oxygen from reaching the wood [37].ire resistance of wood treated with cationic silica sol (CSS) [36],ilicate–acetylated wood and silicate–propionylated wood com-osites [4], and SiO2 wood–inorganic composites prepared fromEOS and methyltrimethoxysilane (MTMOS) systems [38] was pre-iously reported. Unfortunately, the water glass coating is notong-term stable since it neutralizes in contact with air [36], andi O C-bonds are susceptible to hydrolysis [39]. In this study,eaching procedure did not change LOI level of silicon oil treatedamples and foam production probable due to water repellent sur-aces were obtained with the SO treatment that prevented somehemical leaching from wood. Mann–Whitney U test showed sig-ificant differences on LOI values between the treated and controlamples both for leached and un-leached groups (P < 0.05). LOI levelf SO treated samples was lower than that of BA + BX treated sam-les. Qu et al. [29] found the flame retardancy of the sodium silicatereated wood was the worst between the borax and potassiumarbonate treatments of wood.

. Conclusions

Treatments with the preservatives increased LOI levels of wood

amples in spite of leaching procedure. The greatest efficiencybtained with fireproof agent treatment and then mixture ofA + BA in un-leached samples. Boron powder and silicon oilhowed similar performance on LOI. Boron compounds showed

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char formation during the LOI test. Fire retardant performance ofsamples increased with the high loadings of preservatives. Leach-ing procedure greatly decreased LOI levels of samples treated withfireproof agent and mixture of BA + BX; however, it did not have aneffect on boron powder and silicon oil treated ones. Treatment withfireproof agent and mixture of BA + BX may be used some applica-tions where a construction material having fire retardant efficiencyis necessary such as wall, floor or roof. However, their leachingeffect should be taken into consideration for the end-use applica-tions. Elemental analyses are needed in leached samples for betterunderstanding of permanent performance of those preservativesafter leaching procedure.

Acknowledgments

The authors would like to thank Prof. Dr. Hulya Kalaycioglufor providing the opportunity to work with the Dynisco LimitingOxygen Index Chamber at the Wood Based Composite MaterialsLaboratory, Karadeniz Technical University, Turkey.

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