Fire retardancy of sol–gel derived titania wood-inorganiccomposites

Muhammad Shabir Mahr • Thomas Hubert •

Martin Sabel • Bernhard Schartel • Horst Bahr •

Holger Militz

Received: 23 March 2012 / Accepted: 28 May 2012 / Published online: 20 June 2012

� Springer Science+Business Media, LLC 2012

Abstract Sol–gel technology was applied in tailoring

novel wood-made-inorganic composites with improved

thermal and fire properties. In practice, composites materials

were prepared by impregnating pine sapwood wood with

nano-scaled precursor solutions derived from titanium(IV)

isopropoxide followed by a thermal curing process. Thermal

and fire properties were evaluated by thermal analysis and

cone calorimetry, whereas flammability was specified by

oxygen index (LOI) and UL 94 test. Peak heat release rates

were moderately reduced indicating fire retardance potential

in terms of flame spread attributed to the appropriate pro-

tection layer action of the titania-based depositions. LOI

(oxygen index) values of these composites were increased up

to 38 vol.% in comparison to 23 vol.% for untreated wood.

The flame retardancy performance depends on the fire sce-

nario and is strongly influenced by wood loading and crack-

free deposition of the titania layers inside the composite.

Introduction

Wood is one of the most important natural materials due to its

versatile properties and usage in all most in every sphere of

life. Despite of its excellent engineering properties, it has

some serious disadvantages such as low dimensional stability

in moist atmosphere, low resistance against UV-radiation,

heat and fire as well as low biological durability (low resis-

tance against insects, fungi, and other organisms) [1–4]. Due

to these draw backs an additional treatment is required for its

protection. In general, conventional wood preservatives

effectively protect timber from natural diseases, however, due

to their toxic effects some are under strict scrutiny since

decades. Therefore now, to address environmental concerns,

one is seeking environmentally friendly alternatives to wood

preservatives. Due to less toxic impact, cost effective pro-

cessing and easy handling makes sol–gel derived materials as

one of the potential candidate for wood modification and

preservation at industrial scale.

Sol–gel is a well adapted method for producing glass and

ceramic materials at low temperatures [5, 6]. Usually with this

process, metal oxides are formed by the hydrolysis and sub-

sequent condensation reactions initiating from precursors in

wet state (in the presence of acid or base catalyst) [7, 8] and

their desirable features such as chemical composition, struc-

ture can be tuned by varying synthesis parameters as well as

precursors used [9]. Saka et al. [10] applied this approach to

enhance the properties of wood by infiltrating sol–gel-based

solutions into wood to form a new class of novel composite

materials namely ‘‘wood-inorganic composites.’’ In later

trials, they formed a large variety of sol–gel derived wood-

inorganic composites by depositing different sol–gel precur-

sor solutions into untreated wood and studied extensively

their resistive role against moisture, fire and micro-organisms.

Results revealed better fire retardancy, dimensional, and

UV-stabilization [2, 11–17] as well as possessed high decay

resistance against white, brown, and soft rot fungi [18].

Similar studies were reported by other researchers. Donath

et al. [19] used sols derived from different silanes namely,

tetraethoxysilane (TEOS), methyl triethoxysilane (MTES)

and propyl trimethoxysilane (PTEO) by sol–gel process and

M. Shabir Mahr � T. Hubert (&) � M. Sabel � B. Schartel �H. Bahr

BAM Federal Institute for Materials Research and Testing,

Unter den Eichen 44-46, 12203 Berlin, Germany

e-mail: thomas.huebert@bam.de

M. Shabir Mahr � H. Militz

Wood Biology and Wood Products, Burckhardt Institute,

Georg-August-University Gottingen, Busgenweg 4,

37077 Gottingen, Germany

123

J Mater Sci (2012) 47:6849–6861

DOI 10.1007/s10853-012-6628-3

found that wood properties such as wall bulking, anti-swelling

efficiency, moisture uptake, and bio-durability were

improved significantly by treating wood with these sol–gel

silane-based systems. In another study, they used sols derived

from amino-functional silanes [20] and reported a significant

enhancement in fungal decay resistance of the treated wood

with these systems. Bucker et al. [21] developed nano-sized

precursors derived from tetra ethoxysilane (TEOS) by sol–gel

process to get inorganic-wood composites with improved

properties. They stated that these materials showed better

dimensional stability as well as improved resistance against

water, fire, and bio-organisms. Most recently Hubert et al.

[22] used titanium(IV) n-butoxide and titanium(IV) iso-

propoxide as precursors to prepare TiO2 wood composites

by employing sol–gel route. Results indicated that these

wooden products exhibited better dimensional stability and

fire retardancy than their non treated counterparts.

Most of the researchers investigated the fire behavior of

sol–gel derived wood-inorganic composites using thermal

analysis (TGA, DTA) [2, 3, 14, 15, 22]. No doubt, thermal

analysis is very efficient, milligram based and fast method to

qualitatively characterize the thermal behavior of materials

and thus potent to deliver some hints for the fire behavior of a

material such as char yield and pyrolysis temperatures.

However, it should be noted that it is actually not sufficient to

estimate the fire properties of macroscopic specimens or

components in hardly any fire scenario or fire test. Thus,

more comprehensive studies about sol–gel-based wood-

inorganic composites that cover thermal and fire behavior

under different fire scenarios is still missing. The aim of this

contribution was to fill this gap by investigating the thermal

and fire behavior of newly prepared sol–gel derived TiO2

wood-inorganic composites under different fire conditions

and fire scenarios. In addition, it was also the goal of this

study to find out the influence of parameters like composi-

tion, size of the precursor species, and the depositions of

titania sol–gel precursor solutions, as well as dry TiO2 gel

present in resultant products after final curing on the fire

performance under different fire environments. Herein,

thermal properties were evaluated by employing thermo-

gravimetric analysis (TGA) and differential thermal analysis

(DTA) while fire behavior under forced flaming conditions

was characterized using the cone calorimeter. Flammability

in terms of reaction to small flame was investigated using the

limiting oxygen index (LOI) and UL 94 classification.

Experimental

Wood specimens

Wood specimens (pine sapwood, Pinus sylvestris L.) were

prepared for LOI (80 9 10 9 4 mm), UL 94 test

(125 9 13 9 1.6 mm), and cone calorimeter investiga-

tions (100 9 100 9 10 mm). All the samples were oven

dried (18 h, 103 �C) to get their reference weights and

volumes prior to impregnation. The masses of the wood

specimens were measured by means of an electric balance

(TR5003 Mettler Toledo).

Preparation of precursor solutions

Six different precursor solutions (T1 to T6) of different molar

ratios were prepared from titanium(IV) isopropoxide (TIP,

Alfa Aesar, 97 %) by adding in 2-propanol (ISP, Sigma-

Aldrich, 99.5 %) under vigorous stirring (see Table 1). In

each case a catalyst (65 % HNO3) was added to adjust the pH

value 2 of the solutions. TIP is highly reactive with water

therefore; it can readily hydrolyze during the synthesis using

water from the atmosphere. The catalyst contains some water

that can be used to start hydrolyzing the TIP. As a consequence

of TIP–water interaction, particles suspended in the solvent

will be formed. Thus, immediately after synthesis, all the

solutions were tested for evaluating the mean particle sizes of

the titanium containing particles formed and then were stored

at 17 �C in a storage tank until impregnation finished. Usually,

titanium alkoxide precursor solutions are stable for longer

periods (more than 3 months) and can be used for impreg-

nating wood in this period [22]. However, in this study, these

precursors were impregnated within a week after their prep-

aration without further stirring before impregnation.

Wood impregnation and curing treatments

Oven dried samples were impregnated under vacuum

conditions. All the samples were evacuated at 0.1–0.3 kPa

for 1 h before impregnate solutions. Impregnation took

place at 0.1–0.2 kPa subsequently specimens were soaked

at 5–10 kPa for 2 h at 20–23 �C. For each precursor

solution, all the test specimens were treated simulta-

neously. For some selective solutions, samples were doubly

impregnated in the same way. In order to insure complete

hydrolysis and well fixation of the precursor solutions into

Table 1 Characteristic parameters of sol–gel precursor solutions

Precursor

solution

TIP: ISP/

molar

ratio

Equivalent TiO2

solid content/

mass%

Species size

in solution/

nm

T1 1:120 1 0.9

T2 1:15 6.5 1.3

T3 1:10 9 1.6

T4 1:6 12 1.8

T5 1:3 16 4.3

T6 1:1 21 5.7

6850 J Mater Sci (2012) 47:6849–6861

123

the wooden structures, all samples were treated to a special

curing program which included different heating steps like

specimen storage in a desiccators at 20–23 �C and a rela-

tive humidity of 95 % (achieved by supersaturated KNO3

solution) for 1–7 days; open air drying for 1–7 days and a

final curing at 103 �C for 18 h. Described impregnation

and curing procedures are valid for relative small scaled

samples, whose sizes are demanded by standards test pro-

cedures. Until now, samples of (100 9 100 9 10) mm3

dimensions have been treated successfully. Upscaling to

commercial dimensions needs special equipment as well as

larger times for solution infiltration and subsequent soaking

and drying. Even the use of pressure treatment cannot be

ruled out for effective impregnation of sol–gel materials in

bigger wood slices.

Methods

Thermo-oxidative stability of the composites was assessed

by employing TGA/DTA using SETARAM TGA 92-16.

All the measurements were performed in the temperature

range of 23–800 �C at a constant heating rate of 2 K min-1

under air. Prior to all fire tests, the test specimens were

preconditioned at 23 �C with a relative humidity of 50 %

for 88 h. Cone calorimeter (Fire Testing Technology, East

Grinstead, UK) measurements were carried out according

to ISO 5660 [23]. Cone calorimeter works on oxygen

consumption principle to determine the heat release rate

(HRR) during combustion of a material [24]. A set of fire

risks and characteristics like total heat release (THR), time

to ignition, effective heat of combustion, smoke density,

mass loss rate, and CO-yield are measured as well. All

measurements were performed in the horizontal position at

a constant irradiation of 50 kW m-2 in triplicate and the

obtained values were averaged.

The limiting oxygen index (LOI) was determined

according to ISO 4589 [25] (apparatus: Stanton Redcroft,

East Grinstead, UK). LOI is the minimum amount of

oxygen in oxygen–nitrogen mixture required to sustain

candle like combustion in the specified set-up. The higher

the LOI value is, the more flame retardant the material is.

UL 94 horizontal and vertical classifications were deter-

mined using a Plastics HVUL Horizontal Vertical UL

Flame chamber (Fire Testing Technology, East Grinstead,

UK) in compliance with the standard IEC 60695-11-10

[26]. V-2, V-1, and V-0 UL 94 classification are defined by

self-extinguishing occurring within a specified time and

HB classification at least when horizontally burning speed

is below a defined limit.

Morphological information about the sol–gel derived

wood composites was obtained by employing environmental

scanning electron microscopy (ESEM) using a Philips

XL-30 linked to an EDAX energy dispersive X-ray analyz-

ing system (EDX). Typical specimens with cross sectional

cuts in 5 mm thickness were prepared with a Reichert sliding

microtome and sputtered with carbon before recording

ESEM images. EDX-mapping analysis was carried out to

observe the distribution of the impregnated sol–gel-based

inorganic particles present in the cell walls. The particle size

of the solution precursors was measured as hydrodynamic

particle diameter (considering them spherical objects) by

means of dynamic light scattering (DLS) using a particle

sizer (HPPS, Malvern, UK).

Results and discussion

Parameters influencing solid uptakes (WPG)

and composite morphology

All precursor solutions (T1 to T6) were color less liquids at

the time of synthesis but later on turned to light yellowish

upon storage. Characteristic details of titania precursor

solutions parameters like concentration (expressed in molar

ratios and equivalent solid titania content) and hydrody-

namic sizes of the precursor species are shown in Table 1.

Average diameters of titania precursor species present in

precursor solutions having uncertainty of about 0.1 nm

were in the range of 1–6 nm depend heavily on the con-

centration of titanium alkoxide. Precursor solution T1 with

molar ratio of TIP/ISP of 1–120 and an equivalent titania

solid content of 1 mass% contained species of 0.9 nm in

diameters while size was increased to 5.7 nm in T6 solu-

tion (equivalent solid titania 21 mass%). From the results,

it was noted that species size increased with the titania

solid content (titanium alkoxide concentration) and fol-

lowed the order on the basis of titania solid content i.e.,

T1 [ T2 [ T3 and so on (see Table 1).

All TiO2 wood composites were slightly lighter in color

compared to untreated controls due to TiO2 gel solidifica-

tion on the surfaces. However, these color variations were

very small and the original color of wood was not much

affected by the sol–gel treatment. As a result of vacuum

impregnation by the precursor solutions a mass gain of

material was observed due to their penetration into the

cellular wooden structures. Upon final curing infiltrated

material was deposited into wood blocks and termed as

weight percent gains (WPG) in literature [2–4, 19, 22].

Data concerning WPG are placed in Table 2 for all the

composites prepared applying precursor solutions T1 to T6

choosing specimens of various dimensions for performing

different fire tests (LOI, UL 94, and Cone calorimeter). It is

clear from the data that solid uptakes or WPG for all the

samples irrespective to their sizes and dimensions were

increased by increasing molar ratio of TIP/ISP or indirectly

J Mater Sci (2012) 47:6849–6861 6851

123

with titania solid content. The WPG values for composite

(TC1) derived from T1 precursor solution with the smallest

species size (0.9 nm) were about 3 mass% showing ‘WPG/

solid content’’ ratio equal to 3. For all other composites

‘‘WPG/solid content’’ values were not increased in such

extent as the penetration probability of the precursor

solutions tend to decrease due to containing larger size

titania precursor species. Noticeably, precursor solution T5

being more concentrated (solid content of 15 mass%

greater than T1) showed an increase of around 80 % in

WPG in the second deposition cycle for all treated samples

of different dimensions and geometries. T1 apparently

indicated a minor increase in WPG during second

impregnation too, even though the deviation is strictly

speaking covered by the uncertainty. Significant blockage

of cell wall micro voids as a consequence of first impreg-

nation is proposed as the reason for this phenomenon. This

implies higher solid content (titanium alkoxide concentra-

tion) in the precursor does not always lead to higher WPG

values. It may depend on the characteristic properties of the

precursor solution like species size, viscosity, and degree

of hydrolysis etc., as well as influenced by density,

dimensions, and geometry etc., of the wood impregnated.

ESEM investigations of radial and cross sectional parts

of the titania wood-inorganic composites were performed

to extract information about their microstructure. Typical

results are displayed in Fig. 1 a, b, c, d. Sol–gel precursor

solutions with nano-sized titania species (1–6 nm) were

effectively impregnated and fixated in the whole wood

matrix. Being larger cavities most of the material was

absorbed and deposited in the cell lumen in the form of

TiO2 gels upon hydrolysis and subsequent condensation of

the precursor solutions. These titania gels were placed there

in the form of nano-scaled films and coating the internal

surfaces of cell lumen. These depositions were not crack

free; however, very few cracks were found in composite

TC2 formed using T2 precursor with titania solid content

of 6.5 mass% (Fig. 1b). This less cracking may correlate to

low alkoxide concentration of the solution in such a way

that the thickness of the developed films was in the nano

range and thus can more sustain thermal treatment [27–29].

Composite TC5, prepared by relatively more concentrated

precursor solution T5 (titania solid content of 9.5 mass%

greater than T2) contained more cracks (see Fig. 1d). In

this case, the curing thick TiO2 gel films results in cracking

or delaminating due to differences in thermal expansions of

wood matrix and TiO2 deposited films.

Some samples were also subjected to EDX-mapping to

investigate the presence and distribution of titania gels in the

cell walls (see Fig. 2b, d). Figure 2d shows that few TiO2

gels were found in cell wall cavities of T5 (solid content

16 mass%) prepared titania wood composite (TC5);

although the TiO2 gels distribution was not uniform in the

entire cell wall areas. Relatively more gel were apparent in

the cell walls of TC2 composites (with similar random dis-

tribution) prepared with less concentrated solution T2 with

solid content 6.5 mass% as demonstrated in Fig. 2b. In

general, this kind of filling is caused by the penetration of

titanium containing species which are in the dimensions of

the wood cell wall voids (*2–8 nm). The infiltrated titanium

alkoxide solution of low solid content will be subsequently

converted to solid TiO2 upon curing [15, 22]. This further

implies that TiO2 content in the cell walls is dependent on the

concentration of the precursors. Due to limitations of larger

particle size and smaller dimensions of cell wall voids,

higher concentrated solutions were expected to be less

soaked into the cell walls. Furthermore, rapid hydrolysis and

condensation of precursor present in the lumen led to gela-

tion there that consequently interrupted the further diffusion

of liquid into the cell walls [15]. These limitations were well

over come by diluting the precursors. Larger cell wall

sorption can be attributed to small particulate size (1.3 nm)

of T2 solution as well as smaller deposition in cell lumen

because of the low titanium alkoxide concentration (small

solid content) of the solution rendered less hindrance in its

diffusion in cell walls. In contrary, previously reported

results for sol–gel derived TiO2 wood-inorganic composites

prepared from TIP where TiO2 gels were not detected in cell

wall regions [15]. This could be due to the effect of pre-

conditioning before impregnation. Herein, oven dried (0 %

moisture content) samples were used for impregnation while

in the reported work moisture conditioned samples with

20–25 % moisture content were subjected to impregnation

[15]. It is assumed that this huge amount of water in the

lumen speeded up the hydrolysis of TIP precursors followed

by fast condensation to form TiO2 gels in larger amounts into

wood lumen prevented further absorption of the liquid into

cell wall areas. Thus, as a result, the amount of TiO2 gels

present in the cell walls was too low to be detectable with

SEM-EDX.

Table 2 WPG values for titania wood-inorganic composites pre-

pared by single and double impregnation of titania precursor solutions

T1 to T6 and residual mass after TG experiment

Composites Impregnation

cycles

WPG/

mass%

TG residue/

mass%

UN 0 0 \1

TC1 1 3 ± 1 \2

TC1-2 2 4 ± 6 \2

TC2 1 10 ± 1 5

TC3 1 12 ± 2 7

TC4 1 21 ± 1 13

TC5 1 27 ± 3 13

TC5-2 2 49 ± 6 28

TC6 1 39 ± 6 18

6852 J Mater Sci (2012) 47:6849–6861

123

Thermo-oxidative decomposition

To study thermal decomposition, titania wood-inorganic

composites as well as for comparison untreated wood samples

were thermally analyzed with TGA and DTA under air.

Representative TG curves are displayed in Fig. 3a showing

mass loss as a function of temperature. From the TG curve of

untreated wood, three mass loss regimes (1, 2, and 3) were

identified in different temperature ranges. Below 220 �C

(region 1), a slight mass loss was observed. This was related to

moisture content liberated during measurement as well as

partial decomposition of least stable wood component hemi-

celluloses that began to degrade from 200 �C [30, 31]. A

considerable second and third mass loss (region 2 and 3) were

occurred in the ranges of 220–330 �C and 330–460 �C,

respectively. Mass loss in the range of 220–330 �C was

related to oxidative decomposition of wood components [32]

while mass loss in region 3 was attributed to char oxidation

[30]. After 460 �C, no further mass lost implied that untreated

wood was completely thermo-oxidized with very small resi-

due (\1 mass%). For titania wood-inorganic composites,

similar TG curves were obtained, consisting all three mass loss

regions. In region 1 relatively more mass loss corresponding to

release volatile organic residues (un-hydrolyzed alkoxide)

along with water dehydration. In the range of 220–330 �C, a

moss loss of around 40 mass% apparent for both untreated

wood and composites without showing pronounced mass loss

difference in this region. However, in region 3 composites

were decomposed with different final mass loss caused by

remaining titania as stable residues (see Fig. 3a). In this

region, all composites curves were hardly shifted to higher

temperatures indicating any considerable delay in the oxida-

tion of char. For all the titania wood-inorganic composites

yielded residual masses depended on WPG of the tested

composites. In comparison to WPG shown in Table 2, these

values are rather small displaying a disagreement in WPG and

the residual amounts. This implies that WPG contains con-

siderable amount of organic residuals (un-hydrolyzed alkox-

ide as a result of incomplete hydrolysis and condensation).

These organic residuals were thermally instable and released

during measurement leaving only stable titania residues at the

end of TG measurement.

DTA thermograms of untreated wood and all composites

are placed in Fig. 3b. Two distinct exothermic peaks appeared

in temperature ranges of 290–350 �C and 410–460 �C for

untreated wood. First exothermic narrow peak with larger

intensity was mainly ascribed to oxidative decomposition of

wood cell wall components (hemicelluloses and cellulose and

Fig. 1 ESEM micrographs of sol–gel derived titania wood composites: a and b from TC2 while c and d were developed from TC5 composites.

Images a and c are taken from cross sectional while b and d from radial cuts

J Mater Sci (2012) 47:6849–6861 6853

123

lignin) while second was attributed to oxidation of char as

similarly described above for respective TG curve. Appar-

ently, for all cases of composites, the intensity of the first peak

considerably reduced. The intensity of second peak was also

reduced gradually with increasing WPG and was slightly

shifted toward higher temperatures revealing a delay in oxi-

dation of char. These results are similar to previously reported

results for titania wood-inorganic composites [15, 22].

Fire properties

Fire behavior under forced flaming conditions

The investigation of the fire behavior in the cone calorimeter

results in two important characteristics to assess the fire risks.

The HRR related to flame spread and fire growth and the total

heat release (THR) which corresponds to fire load at the end

of the test [24, 33, 34]. The HRR profiles for measured

samples are illustrated in Fig. 4a, b and main cone calo-

rimeter results are summarized in Table 3. In addition, res-

idues of all tested samples collected after end of test are

visualized in Fig. 5. Always, the HRR representative curves

are a two peak profile typical for thick char forming materials

like wood. Out of two peaks, first one is caused by the

increasing amount of volatile fuel oxidizing within the

growing flame due to the rapid increase in mass release rate

before a char layer is formed limiting the pyrolysis. While

second peak at the end of burning corresponds to char layer

cracking and further decomposition [33, 34]. In comparison

to untreated wood, the HRR of titania wood-inorganic

composites was reduced in particular the second peak HRR

(PHRR) toward the end of burning accompanied by pro-

longed burning times (see Fig. 4a, b). These findings showed

similarities to DTA curves presented in Fig. 3b where sim-

ilar effects were observed in term of intensity reduction and

peak shifting in comparison to untreated wood. Of course,

the experiments were performed under different environ-

ments that led to different fire scenarios as well as pyrolysis.

Supported by residual amount obtained for untreated wood:

17.3 mass% in the cone calorimeter whereas\1 mass% in

TG (see Tables 2, 3), pyrolysis was mainly anaerobic in the

cone calorimeter [35] whereas thermo-oxidative decompo-

sition took place in TG under air. Therefore, the shifts and

reduction observed in DTA were not directly related to the

shifts observed for the second PHRR in the cone calorimeter.

Nevertheless, the similarities give a remarkable hint to a

general protection mechanism active in the residues of

homogeneous sol–gel derived titania composites. It is

noticed that HRR considerably decreased (but not tends to

zero) after the second PHRR. This fire response is assigned to

an after glowing of char through thermo-oxidative decom-

position. The reduction in heat release rate is irrelevant for

Fig. 2 Localization of TiO2 gels in the wood matrix of TiO2 wood-inorganic composites as characterizes by ESEM micrographs and EDX

mapping. a and b were taken from TC2 while c and d were developed from TC5 composites

6854 J Mater Sci (2012) 47:6849–6861

123

the first maximum in HRR. Beyond first PHRR, HRR

decreases because of the protection of carbonaceous char. It

is concluded that the protection performance was improved

by the applied titania depositions layers. It was reported

before that degree of reduction, shape, width, and shift

observed in the second PHRR depends on the protection

properties of residues [36–39]. Shift magnitude as well as

decrease in the second peak intensity of HRR was varied for

all the tested samples indicating a change in the residual

characteristics during combustion in the cone calorimeter.

Figure 4c revealed that the composites TC1 (WPG

3 mass%), TC1-2 (WPG 4 mass%) and TC2 (WPG

10 mass%) showed similar order of reduction (19, 26, and

30 %, respectively compared to untreated wood see Table 3)

with increased residual amounts in their second maximum of

HRR indicating similar properties of their residues. TC5 with

WPG 27 mass% and with 28.8 % residual content exhibited

a smaller reduction in the second PHRR indicating a reduced

protection action of the residues. This poorer performance

may be attributed to extensive cracking occurred in the char

formed as well as associated to cracks and imperfections

related to its composite micro structure as confirmed by

ESEM micrographs (Fig. 1d). An improvement in the pro-

tection action was observed when titania gels introduced in

the second impregnation healed most of the cracks formed

during first impregnation consequently reduced the second

PHRR to 25 % for TC5-2. On the basis of these results, it was

concluded that not the residual content (indirectly WPG) as

such controls the second PHRR of the tested composites.

Apparently, protection properties of the titania depositions

are not only dependent on the amount deposited into the

wood (WPG) but on the crack-free effective fixation of

material into the wooden structure that leads to a closed

barrier layer with greater capability to hinder the heat and

mass transportation in the condensed phase.

In contrast to untreated wood, the times to second PHRR

(2.TPHRR) were appreciably altered depending on the WPG

values for tested composites (see Fig. 4c; Table 3). Time

related to second maximum of HRR for TC2 composite (WPG

10 mass%) was delayed about 20 % while that of TC5-2

(49 mass% WPG) about 45 %. Burning times (time of

flameout) for all tested composites were greater than untreated

wood increasing with WPG. What is more, in contrast to the

second PHRR the time to second PHRR increases linearly

with the increasing fire residue. Thus, it is proposed that the

HRR was mainly controlled by the protection properties of

the residue, whereas the time to PHRR goes along also with

the heat capacity of the residue and thus its amount.

In contrary to HRR, total heat release evolved (THE =

THR at the end of the test) of all the investigated samples was

very slightly decreased except for TC5-2 composite. THE for

TC5-2 (WPG 49 mass%) prepared with double impregna-

tion was found to be 19 % greater in magnitude than THE

(57 MJ m-2) for untreated wood. This unexpected THE

value for TC5-2 composite supports the existence of organic

residuals as a large fraction of WPG in the composites as

already predicted by thermal analysis. The effective heat of

combustion deduced from THE/TML remained invariant for

all tested composites (Table 3) ruling out significant flame

retardancy mechanisms in the gas phase such as flame

inhibition or fuel dilution. The CO production for titania-

based composites TC1 and TC2 was minutely increased

while surprisingly, decreased for rest of tested composites

indicating slight changes in the volatile pyrolysis products.

Unlike to untreated wood, in particular total smoke release

was reduced considerably for tested titania composites. A

remarkable reduction in the range of 50–62 % in total smoke

release was achieved for all composites compared to

untreated wood indicating a worth seeing improvement in

reducing fire hazards by titania-based sol–gel treatment to

wood.

Figure 5 shows the residues of untreated wood and the

tested composites after the test in a cone calorimeter.

-1.0

-0.5

0.0

rela

tive

mas

s lo

ss

temperature / °C

TC6TC4TC3UN

(a)

1

2

3

0 200 400 600 800

200 400 600 800

End

o.E

xo.

(b)

TC6

TC5

TC4

TC3

TC2

TC1

untreated (UN)

temperature / °C

Fig. 3 Thermal behavior of sol–gel titania wood composites probed

by TGA (a) and DTA (b) at a heating rate of 2 K min-1

J Mater Sci (2012) 47:6849–6861 6855

123

Untreated wood was combusted to light brownish–black

residues. Cracking of the residue toward the end of burning

and subsequent after glowing finally decomposed the

material to black ash containing larger pieces disintegrate

to each other. Combustion of composites derived from

dilute precursor solutions (WPG up to 10 mass%) yields

white residual masses with more compact and integrated

structures. White appearance of residues attributed to tita-

nia gels network homogeneously distributed to whole wood

matrix. Larger cracks were found in the residual masses of

TC5 and TC5-2 (WPG 27 and 49 mass%, respectively). It

is assumed that thicker TiO2 coatings in the interior of

wood matrix tend to be more frail and hence to crack when

heated. As a result, more cracked and rigid residual mass

were obtained specially for TC5 composite accompanied

by a low reduction in its second maximum of HRR.

A comprehensive assessment of fire risks for untreated

wood and titania-based wood-inorganic composites for all

tested samples in terms of THE, PHRR, and time to igni-

tion (TTI) at an external heat flux of 50 kW m-2 is dis-

played in Fig. 4d. Method is also useful to identify the

system with minimum fire hazards [33]. The ordinate (total

0

50

100

150

200

250 UN TC1 TC2 TC5

HR

R /

kWm

-2

time / s

(a) (b) UN TC1-2 TC5-2

time / s

52

56

60

64

68

0 200 400 600 0 200 400 600

6 8 10 12 1416 20 24 28 32 36160

180

200

220

240

260

280

residue / mass%

2.P

HR

R /

kWm

-2

280

320

360

400

440UN

TC1

TC1-2

TC2

TC5

TC5-2

WPG

2.PHRR/TTI / kWm-2s-1

TH

E /

MJm

-2

UN

TC1

TC1-2

TC2TC5

TC5-22.

TP

HR

R /

s(c) (d)

Fig. 4 Assessing fire risks from

cone calorimeter data obtained

for titania-based wood-

inorganic composites at

50 kW m-2. Heat release rate

(HRR) as a function of time for

a single impregnated, b double

impregnated specimen,

c residue versus second maxima

of HRR and THRR, and d total

heat evolved against peak of

heat release rate per time to

ignition

6856 J Mater Sci (2012) 47:6849–6861

123

heat evolved) serves as a measure for the propensity to

cause long-lasting fire. The abscissa, peak of heat release

rate per time to ignition marking the propensity to cause a

quickly growing fire in the cone calorimeter experiment.

Based on this result, TC1-2 causes minimum fire hazards

among other tested composites because a strong decrease

not only in 2.PHRR/TTI as well as in THE was achieved in

a cone experiment for this system. Least emission of

smoke for TC1-2 strongly supported this finding. On the

other hand, 2.PHRR/TTI for TC5-2 is minimal but due to

unusual increase in THE relative to untreated wood proves

it the worst system in terms of fire hazards.

On the basis of results described above, it was extracted

that sol–gel derived titania wood-inorganic composites

showed slight fire retardancy in terms of reduced flame

spread as well as suppressed fire growth. Heat release rates

and second maximum of HRR for these systems were

decreased and corresponding burning times and time to

second maximum of HRR were increased moderately due

to change in their residual properties under force-flame fire

conditions. No further improvement was realized in terms

of fire load as total heat evolved was unchanged very

similar to untreated wood.

Flammability

Composites materials show better performance in terms of

flame retardancy, what is demonstrated in the following.

LOI value of untreated pine sapwood and their titania

derived composites are summarized in Table 4 along with

UL 94 results. Measured LOI value for untreated pine

wood is 23 vol.%. This value is smaller than the reported

value (29.4 vol.%) for pine wood [40], demonstrating that

this method delivers only relative (LOI) values depending

on origin and dimensions of the specimens. A remarkable

increase in LOI values ranging from 26 to 35 vol.% was

observed for singly impregnated composites. Increase in

LOI was even more promising in case of doubly impreg-

nated composites. Doubly impregnated composite TC1-2

only with 4 mass% WPG achieved LOI value of 29 vol.%

compared to 23 vol.% for pure wood exhibited 27 % rel-

ative improvement in oxygen index. Composites with

higher loadings such as TC5 (WPG 27 mass%) and TC5-2

(WPG 49 mass%) obtained the highest oxygen index val-

ues (33 and 38 vol.%, respectively) from the tested samples

(see Table 4). These results show that titania wood-inor-

ganic composites are excellent flame retardants even with

very low loadings of titania precursor materials.

In order to find the origin of flame resistance as well as

clarify the influence of parameters on flame retardancy,

LOI is plotted against and WPG and fire residue, respec-

tively, as well as second maximum of HRR (2.PHRR)

measured by cone calorimeter for the tested compositesTa

ble

3C

on

eca

lori

met

erd

ata

for

tita

nia

-bas

edw

oo

d-i

no

rgan

icco

mp

osi

tes

atan

irra

dia

tio

no

f5

0k

Wm

-2

Sp

ecim

enT

TI

(s)

TF

O

(s)

Mas

s

(g)

TM

L

(g)

Res

idu

e

(mas

s%)

TH

E

MJ/

m2

TH

E/T

ML

MJ/

m2g

1.P

HR

R

kW

/m2

1.T

PH

RR

(s)

2.P

HR

R2

.TP

HR

R

kW

/m2

TS

R

m2/m

2T

CO

P

(g)

CO

Y

kg

/kg

UN

20

40

94

9.1

40

.61

7.3

57

.01

.42

09

29

26

53

07

29

80

.21

10

.00

52

TC

12

34

71

50

.34

0.2

20

.05

2.9

1.4

22

83

32

14

35

31

42

0.2

25

0.0

06

1

TC

1-2

23

49

45

0.2

38

.12

4.1

51

.01

.32

06

32

19

63

59

11

20

.20

20

.00

52

TC

22

35

15

53

.84

0.4

24

.95

5.6

1.4

19

43

51

86

36

71

36

0.2

20

0.0

05

4

TC

53

35

03

61

.74

3.9

28

.85

6.6

1.3

17

84

12

26

39

31

47

0.1

80

0.0

04

0

TC

5-2

32

57

16

9.4

45

.93

3.9

67

.61

.52

06

60

19

94

46

14

60

.17

70

.00

38

TT

Iti

me

toig

nit

ion

,T

FO

tim

eo

ffl

ameo

ut

(bu

rnin

gti

me)

,T

ML

tota

lm

ass

loss

,T

HE

tota

lh

eat

evo

lved

,P

HR

Rp

eak

of

hea

tre

leas

e(fi

rst

and

seco

nd

),T

PH

RR

tim

eo

fP

HR

R(fi

rst

and

seco

nd

),

TS

Rto

tal

smo

ke

rele

ase,

TC

OP

tota

lC

Op

rod

uct

ion

,an

dC

OY

CO

-yie

ld

J Mater Sci (2012) 47:6849–6861 6857

123

(Fig. 6). Whereas the LOI roughly depends linearly from

the WPG, the linear relationship between LOI and residues

produced in cone calorimeter is rather perfect indicating

that LOI is strongly controlled by the char yield [41]. As

described before, a change in residual properties signifi-

cantly affects the second maxima of HRR in a cone

experiment. It is shown that the correspondence between

the LOI and 2.PHRR is lacking (Fig. 6b) similar to Fig. 4c.

Thus, it was concluded that the treatment of wood with

more concentrated titania precursor solutions as well as

higher titania loadings in the derived composites

remarkably enhance the flame retardancy of these materials

with respect to reaction to a small flame such as LOI.

It is reported before that those composite materials may

pass the UL 94 test with V-0 only when they have achieved

LOI values of larger than 40 vol.% [42]. Beside remarkably

high LOI values of the titania-based composites, their self-

extinguishing performance is still insufficient as depicted

by UL 94 test (see Table 4). Even though all the tested

samples have achieved horizontal burning HB in accor-

dance to UL 94 classification, the composites showed also

in UL 94 clearly reduced fire hazards. Most composites

Untreated wood (UN) TC5

TC5-2 TC2

TC1-2TC1

Fig. 5 Photographs of residue obtained in the cone calorimeter for untreated wood and sol–gel derived titania-based wood-inorganic composites

6858 J Mater Sci (2012) 47:6849–6861

123

specimens show self-extinguishing in HB before reaching

any testing marks in contrast to pure wood. Furthermore,

the flame spread velocity for the few burning specimen was

much slower than for untreated wood. These result well

corresponds to the LOI results and may correspond with

the cone results for the second maximum of HRR implying

that titania-based wood-inorganic composites are slow

burning or flame retardant materials. On the other hand, the

composites did not achieve rating vertical burning in

accordance to UL 94 showing that these systems were still

flammable materials in the more challenging vertical con-

figuration with ignition from the bottom. The difference in

LOI/horizontal burning UL 94 and vertical burning UL 94

testing indicates a strong influence of upward and down-

ward flame configurations on the flammability of the

investigated materials as it is often observed for charring

approaches. It was reported that heat transfer and burning

rates under downward flaming configuration (LOI and HB)

were greatly altered from the upward flaming configuration

(vertical burning UL 94) [43]. Moreover, in both test

geometries, the residual impact was changed that consid-

erably influenced the flame retarding effect [44].

Nevertheless, titania-based sol–gel wood composites are

valuable environmentally friendly materials that show

significant improvements in their most of properties

including fire and flame retardance at low titania loadings

(WPG). In practice, 3–4 mass% critical loading of titania

gels by single impregnation of titanium alkoxide precursors

to wood is sufficient to produce fire retardant wood-

inorganic composites with fewer fire risks such as low CO

and smoke release. An approach to use higher loadings by

infiltrating more concentrated titanium alkoxide solution is

senseless and hardly improves the fire performance of the

derived composites. In comparison with typical boron-

based fire retardants (boric acid, boric acid ? borax mix-

tures etc.) under similar loadings (3–5 mass%) similar fire/

flame retardancy enhancements in terms of second peak

HRR (2.PHRR) reduction as well as LOI increase in the

small scale flammability conditions were realized [40, 45,

46]. However, at higher loadings later performed better

flame retardance (based on their LOI values) than the under

investigated TiO2 wood composites. However, due to

better leach resistance, TiO2-based wooden materials may

have edge over boron-based ones especially under moist

conditions. Wood treated with nitrogen- and phosphorous-

based fire retardants showed almost three times more flame

resistance (increase in LOI) than TiO2 wood composites at

the similar respective loadings of nitrogen and phosphorous

in the tested specimens [47–49]. This comparison reveals

that TiO2 wood composites possess fire resisting character

but there current fire performance needs to improve further

to beat or become competitive with conventional fire

retardants.

In previous reports, on the basis of DTA/DTG results it

was stated that fire properties of sol–gel derived wood

composites were improved [2, 15]. However, the evidence

of the results is limited. Cone calorimeter, LOI and UL 94

results reported herein even provided first deep insight to

clarify the complex description of fire behavior of titania

impregnated wood composites.

21

24

27

30

33

36

39(a)

single impregnated double impregnated

LOI /

vol

.%

WPG / mass%0 6 12 18 24 30 36 42 48

21 24 27 30 33 36 3916

20

24

28

32

36

LOI / vol.%

resi

dues

/ m

ass%

160

180

200

220

240

260

2.P

HR

R /

MJm

-2

(b)UN

TC1

TC2

TC1-2

TC5

TC5-2

Fig. 6 LOI plotted against a weight percentage gains for the

composites and b residue as well as second maximum of HRR

(2.PHRR) obtained from the combustion of titania wood-inorganic

composites in a cone calorimeter at an irradiation of 50 kW m-2

Table 4 Flammability (reaction to small flame) of titania wood-

inorganic composites

Composites Impregnation

cycles

LOI ± 1/

vol.%

UL 94

classification

UN 0 23 HB

TC1 1 26 HB

TC1-2 2 29 HB

TC2 1 28 HB

TC3 1 29 –

TC4 1 30 –

TC5 1 33 HB

TC5-2 2 38 HB

TC6 1 35 –

J Mater Sci (2012) 47:6849–6861 6859

123

Since the main focus of this study was to elucidate the

fire/flame resistive potential through well accepted fire

experiments. However, it is realized that a number of wood

properties alter by applying sol–gel approach. For instance,

beside fire and flame retarding improvements, sol–gel

derived TiO2 wood-inorganic composites displayed excel-

lent anti-fungal characteristics [50]. These materials very

minutely degraded against brown rot fungi C. puteana and

P. placenta compared to untreated pine sapwood that

severely lost its mass due to fungal deterioration. Anti-

fungal efficacy of TiO2 wood composites was attributed to

better shielding by the gel thin layer precipitated on the cell

wood walls as well as the consequence of biocidal action of

the organic residues that acted like fungicides. In addition,

these composites absorbed significantly less moisture in the

moisture sorption test in comparison to untreated wood

exhibiting more dimension stability [22, 50]. Furthermore,

it is expected that these composites have better mechanical

properties like modulus of elasticity, modus of rapture, and

hardness etc., thereby the localization of TiO2 gels into the

cell wall cavities that provide additional strength to the

matrix. Similar mechanical improvements are reported in

literature [51]. Also, composites assume to be leaching

resistant because of the insolubility of TiO2 precipitates in

water as well as due to their better fixation into the matrix.

Conclusions

ESEM/EDX investigations showed that precursor solutions

can infiltrate the whole wood matrix and is fixated mainly

in the lumen forming thin films. It was visualized that these

depositions were not crack free and increased drastically in

composites prepared with precursor solutions of high alk-

oxide concentrations.

Thermo-oxidative decomposition of tested composite

materials was characterized employing thermal analysis. It

was obvious from TGA results that the amount of residues

formed during thermo-oxidative decomposition is lower

than the corresponding WPG values. This indicated that a

fraction of WPG was composed of organic residues, mainly

un-hydrolyzed alkoxide and solvents that released during

the measurement and left only stable inorganic residues.

For the development of titania wood composites with

optimum fire resistance capabilities, it is essential to avoid

possible content of organic residues by improving the

treatment process. DTA thermograms of composites

showed that the intensities were decreased markedly as

well as a shift occurred in the peaks with respect to tem-

perature qualitatively identified change in oxidative

decomposition reaction of these materials.

Cone calorimeter results revealed that heat release rates

after the initial PHRR and in particular, the heat release

rate of the second PHRR in a developing fire were mod-

erately reduced as well as the burning time was increased

significantly indicating some improvements in flame

retardance due to protection properties of the fire residue.

In addition, fire hazards such as CO and smoke production

were remarkably reduced. Beside these improvements, fire

behavior in terms of ignition, initial HRR increase, fire

growth index and fire load remained unchanged for the

tested composites.

An impressive improvement in LOI values up to 64 %

(relative to LOI for untreated wood) was noticed showing

better flame retardant properties of these materials when

reacting to small flame. All the UL 94 tested systems have

achieved HB (horizontal burning) but the composites show

dramatically decreased burning rates in the horizontal

configuration.

It is concluded that fire behavior and the flammability of

these systems readily depended on the fire scenarios. The

observed improvement in fire performance of the sol–gel

derived titania wood composites are due to stabilization of

the deposited thin layers of titania gels as well as fire

response of the depositing material. On the basis of

acquired understanding, it is expected that a considerable

improvement in the fire characteristics of these materials

can be achieved by minimizing the organic fraction of

WPG (by better processing) as well as stabilizing the

deposition layers of titania (crack free) by using more

dilute titanium alkoxide precursors.

Acknowledgements The authors thank Dr. Ina Stephan and Heidi

Lorenz of BAM Federal Institute for Materials Research and Testing

for supplying wood samples and assisting TG measurements.

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