Journal of Wood Chemistry and Technology

ISSN: 0277-3813 (Print) 1532-2319 (Online) Journal homepage: https://www.tandfonline.com/loi/lwct20

Development of Fire Resistant Laminated Wood
Using Concentrated Sodium Polyborate AqueousSolution
Isao Tsuyumoto & Tomokazu Oshio
To cite this article: Isao Tsuyumoto & Tomokazu Oshio (2009) Development of Fire ResistantLaminated Wood Using Concentrated Sodium Polyborate Aqueous Solution, Journal of Wood
Chemistry and Technology, 29:4, 277-285, DOI: 10.1080/02773810903033721
To link to this article: https://doi.org/10.1080/02773810903033721

Published online: 18 Nov 2009.

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Journal of Wood Chemistry and Technology, 29: 277–285, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 0277-3813 print / 1532-2319 online
DOI: 10.1080/02773810903033721

Development of Fire Resistant Laminated Wood
Using Concentrated Sodium
Polyborate Aqueous Solution
Isao Tsuyumoto and Tomokazu Oshio
Kanazawa Institute of Technology, College of Bioscience and Chemistry,
Department of Applied Chemistry, Nonoichi, Ishikawa, Japan

Abstract: A highly concentrated aqueous solution of sodium polyborate (SPB) wasused to treat laminated Japan cedar (Cryptomeria japonica) in order to first develop afire resistant laminated wood. After the pressure-impregnation of a water borne solution
containing 9.78 mol/L of boron, the fire properties were evaluated by cone calorimeter(CCM) and weight loss (WL). The resorcinol formaldehyde resin (RF) laminated wood
did not fracture and the adhesive interfaces kept the pieces together even after heatingat 750◦ C for 20 min or with the CCM test using the heat flux of 50 kW/m2 for 20 min.
Heat emission for 20 min in the CCM test below 8 MJ/m2 was achieved by a WPG of
53% or more.

Keywords: Borate, cedar, cone calorimeter, fire resistant, laminated wood, pressure
impregnation

INTRODUCTION
Boric acid and borates have long been used to make wood more fireresistant.[1–3] However, the solubilities of the conventional boric acid and borates at room temperature are insufficient for meeting fire standards (e.g., boricacid (H3 BO3 ), 3.99; borax (Na2 B4 O5 (OH)4 ·8H2 O), 2.58 at 20◦ C). A highly

The authors are grateful to Messrs. Shingo Sakaguchi and Ryuhei Takehira for
SEM measurements and to Messrs. Ken Ozaki, Kyuo Motoh, and Mitsuteru Senda forhelp in the pressure-impregnation experiments. The wood specimens were supplied by
Kagamokuzai Co. Ltd., Japan.
Address correspondence to Isao Tsuyumoto, Kanazawa Institute of Technology, College of Bioscience and Chemistry, Department of Applied Chemistry, 7-1 Ohgigaoka,Nonoichi, Ishikawa 9218501, Japan. E-mail: tsuyu@neptune.kanazwa–it.ac.jp
277

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I. Tsuyumoto and T. Oshio

concentrated borate solution is desirable to give sufficient fire resistant properties to wood.
We have successfully prepared a highly concentrated aqueous solution ofsodium polyborate with 5.24 mol/kg of boron at 20◦ C by adjusting the Na/B
ratio to 0.22.[4] This is markedly more concentrated than the most soluble
borate solution (Na2 O·B2 O3 ·8H2 O, 2.49 mol/kg at 20◦ C).[5] Evaporation of
the solution at 5.24 mol/kg yielded amorphous solid foam with a density of
0.1–0.3 g/cm3 . This highly concentrated polyborate solution has been applied
to a fire resistant treatment of wood specimens of Japan cedar (Cryptomeria
japonica), and the specimens pressure-impregnated with the solution (14 mol/L
as to boron) at 150◦ C (474 kPa) for 1 h had a weight percent gain of 124% by
uptake of polyborate with a weight loss of 38% when heated at 750◦ C for 20
min in air.[6]In the present study, we applied this concentrated polyborate solution
to laminated wood of Japan cedar (Cryptomeria japonica). In recent years,
laminated wood produced by gluing lumber boards is widely used for both
fixture and structural materials of buildings such as schools, houses, and domes.
It is highly desirable to develop fire resistant laminated wood from the viewpointof disaster prevention. Compared to solid wood, possible poor thermal stabilityof the adhesive interfaces was crucial to the fire resistance. We solved thisproblem by selecting a glue and prepared laminated wood that does not fractureby heating in air at 750◦ C, for 20 min. We present here findings in fire resistanttreatment of laminated wood using the polyborate solution and heat release
tests using the cone calorimeter.

EXPERIMENTAL SECTION
Materials
Specimens of both solid and laminated wood of Japan cedar (Cryptomeria
japonica) were pressure-impregnated to compare permeability. The size was
100 (R) mm × 100 (L) mm × 10–50 mm, and their butt ends were arranged
at the side of the specimens. All the specimens were dried at 105◦ C for morethan 24 h before pressure-impregnation. This drying process did not affectthe adhesives. The adhesive interfaces of the laminated wood were locatedevery 30 mm perpendicularly to the surface and the butt end. The wide surfaceof the laminated wood was quartersawn. To laminate the wood, two kindsof adhesives, aqueous vinyl polymer-isocyanate resin (API) and resorcinolformaldehyde resin (RF), were investigated.
Sodium polyborate (SPB) was prepared according to our previous article[4]and used for fire resistant agent of wood. The pH of the SPB solution (6.68
mol/L as to B) was 6.86 at 20◦ C. In the present study, 150 kg of amorphous
SPB (Na/B ratio 0.22) was dissolved into 100 kg of water at 60◦ C. The boron

Development of Fire Resistant Laminated Wood279

concentration of the obtained SPB aqueous solution was 9.78 mol/ L, and thedensity of the solution was 1.25 g/cm3 . The SPB solution at 9.78 mol/L wasused in all the experiments in the present study, though the boron concentration
can be raised to 14 mol/L. This is because our previous study indicated 9.78
mol/L was sufficient to satisfy the fire resistant properties.

Pressure-Impregnation Treatment
The specimens were pressure-impregnated at 1 MPa, 60◦ C for 1 h using anautoclave. The pressure was directly increased from ambient pressure without
an initial vacuum. The weight percent gain (WPG, %) was calculated fromthe mass increase of the specimen after drying at 105◦ C for 24 h. Most of
the laminated wood boards were accumulated using spacers and sunk in thesolution in the autoclave. Some specimens were accumulated without spacers
and one specimen was floated in the solution to investigate how the arrangementof specimens in the solution affected the WPGs and to establish techniques
to control WPGs using one standard concentration (9.78 mol/L). Bending andcompressive strengths after pressure-impregnation have been found to be almost
the same as those before the impregnation, suggesting that the polyborate has
no or only a minor effect on the strength properties.

Heat Release Test
The cone calorimeter (CCM) test was performed to evaluate the fire resistanceproperties (Fire Testing Technology Ltd). The CCM monitors heat release froma specimen with surface area of 99 mm × 99 mm when it is irradiated withinfrared in air at a heat flux of 50 kW/m2 . The amount of heat release from thespecimen is calculated from O2 consumption in the exhaust gas and recordedfor 20 min. A total amount of heat release below 8 MJ/m2 at a heat flux of 50kW/m2 for 20 min is the necessary criterion for “non-combustible materials”
by the Japanese Building Standards.[7] Weight loss (WL, %) in the CCM testat 20 min was also recorded.

RESULTS AND DISCUSSION
Choice of Adhesives
In order to investigate heat resistance preliminarily, the pressure-impregnatedlaminated wood (WPG, 50%) was heated in an electric furnace at 750◦ C for
20 min. The laminated wood glued with API adhesive was found to separate
from all its adhesive interfaces and to break into its constituent pieces, although

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I. Tsuyumoto and T. Oshio

the pieces were only carbonized and maintained their shapes. In contrast, thelaminated wood with RF adhesive did not break, and the adhesive interfaces
kept gluing the carbonized pieces together even after heating at 750◦ C for 20min. The RF adhesives bore at 750◦ C for 20 min, although the ignition point of
the RF resin is 607◦ C. This is probably because the temperature inside wood
was lower than the wood surface due to the heat-insulating effect of the fire
resistant wood.
Figure 1 shows SEM images of the butt-end surface including the crosssection of the adhesive interface after: (a) impregnation and before heatingand (b) heating at 750◦ C, for 20 min. The adhesive interfaces kept solid after
impregnation (WPG, 50%), that is, the impregnation did not bring about thedestruction of adhesive interfaces (Figure 1(a)). After heating at 750◦ C, theadhesive interfaces still glued the carbonized pieces despite their small partial
transformation as much as 0.1 mm (Figure 1(b)). Laminated wood that did
not separate at 750◦ C in air was first achieved for our developed SPB-treatedcomposite with a RF adhesive. Thus, experiments later were performed using
the RF adhesive.

Permeability of Laminated Wood and Solid Wood
The WPGs of the laminated wood and the solid wood were plotted asa function of thickness in Figure 2. The laminated wood showed higherWPGs than the solid wood, although the impregnation condition was thesame. This is probably because the wide surface of the laminated wood wasquartersawn and quartersawn lumber has better permeability than flatsawn
lumber.
The standard deviations (n = 12) of the WPGs of the laminated wood were
smaller than that of the solid wood, likely because the wood parts comprising
the laminated wood reduced the density variation. The dispersion of the WPGs
is one of the difficulties in quality control of fire resistant wood, so quality
control in fire resistance of laminated wood is easier than solid wood. In thepresent laminated wood study, the adhesive interfaces are arranged every ca.
30 mm and the specimens consist of 3 or 4 parts. The standard deviations of
the laminated wood are statistically predicted to be 0.50–0.58 times of the solidwood, which was similar to that obtained for the specimens thicker than 25mm.

Impregnation and CCM Tests
Heat release (MJ/m2 ) for 10 and 20 min CCM tests are summarized in Table 1
along with the thickness of specimen (mm), dry density (g/cm3 ), WPG (%), andWL (%). Heat releases for specimens with WPGs over ca. 50% were consistent,

Development of Fire Resistant Laminated Wood

281

Figure 1. SEM images of the butt-end surfaces: (a) after impregnation and before heating, and (b) after heating at 750◦ C, for 20 min. The observed surfaces are perpendicular
to the adhesive interfaces, and the cross-sections of the adhesive interface are observed
as plain thick lines traversing the images.

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I. Tsuyumoto and T. Oshio

Figure 2. Comparison of permeability of laminated wood ◦, and solid wood , withdifferent thickness. The error bars denote standard deviation (n = 12).

Table 1. Heat release (MJ/m2 ) for 10 and 20 min in the CCM tests along with the thickness (mm2 ), dry density (g/cm3 ), WPG (%), and WL (%) of the specimens. Specimens
No. 2–4 and 8 were accumulated without spacers, and Specimen No. 6 was floated inthe solution during the pressure-impregnation
Heat emission (MJ/m2 )Thickness Dry density
Specimen no.
(mm)(g/cm3 )WPG (%) 10 min
20 minWL (%)
1234567891011129.510.7
12.1
12.2
12.9
15.8
15.8
16.0
16.0
25.1
35.4
51.0

0.32
0.31
0.32
0.35
0.34
0.38
0.33
0.38
0.35
0.35
0.35
0.35

48.5
38.2
35.6
39.5
44.8
21.1
53.0
43.7
54.0
55.5
53.1
48.0

5.4
0.4
18.0
11.6
1.2
10.0
0.9
0.5
0.0
0.8
1.2
0.2

12.0
8.4
28.4
26.6
13.0
29.2
1.9
1.0
0.9
2.2
2.0
0.3

60.4
61.2
63.6
61.5
58.0
63.2
45.4
43.2
42.2
24.0
13.2
0.0

Development of Fire Resistant Laminated Wood

283

Figure 3. Specimen of fire resistant laminated wood with thickness of 16 mm after
the CCM test. The bottom half remained unchanged, although the upper half wascarbonized.

but for specimens where the WPGs were intentionally decreased were morevariable. All the specimens with thickness over 15.8 mm and WPG over 43.7%
sufficiently satisfied the criterion for “the non-combustible materials” of theJapanese Building Standards, below 8 MJ/m2 for 20 min.
Table 1 shows the relationship between the heat release (MJ/m2 ) in CCM
tests and WPG (%). The heat release decreased with increasing WPG. The
correlation coefficients using only specimens with thickness over 15.8 mm
were R = 0.926 for 20 min and R = 0.924 for 10 min. The specimens withthicknesses of 9.5 and 12.9 mm showed a heat release of 12.0 and 13.0 MJ/m2 ,although the WPGs were 48.5 and 44.8%. This indicates that the thickness ofthe board is an important factor in heat release.
The heat release of the specimens in the WPG range of 35.6–55.5% decreased at greater WPGs at thickness of ca. 15 mm or greater. Figure 3 shows
the side view of the specimen with 16 mm thickness after CCM test. Although the upper half was carbonized, the bottom half remained unchanged.
However, the specimen with 9.5 mm thickness, a little thicker than the carbonized depth, did not satisfy the criterion. Our results suggest that the unchanged bottom half is essential for the suppression of the heat release. The
unchanged bottom half may suppress the development of cracks in the carbonized layer and, subsequently, contributes to the protection of the insidefrom the heat flux. In a word, we developed the technique to satisfy the criterion below 8.0 MJ/m2 for 20 min by using the board with thickness over 15.8mm and by pressure-impregnating up to the WPG 44%.
In Table 1, the relationship between the heat release (MJ/m2 ) and the WL(weight loss, %) in the CCM test showed a good correlation. Thus, measurementof the WL at a heat flux of 50 kW/m2 can be a useful preliminary screening
test before measuring heat release, as an expensive apparatus is needed for theheat release measurement in the CCM test.

284

I. Tsuyumoto and T. OshioFire Resistance Mechanism
As we have reported previously,[6] the fire resistance effect of SPB is due to theinterception of both oxygen gas and heat conduction by the generation of SPBfoam. The SPB impregnated in wood transforms into glass foam barrier uponheating, which enfolds the wood and blocks the contact with oxygen gas. This
brings about the carbonization of wood, and the composite of the glass foam
and the carbonized wood blocks heat conduction.

CONCLUSIONSFire resistant laminated wood of Japan cedar (Cryptomeria japonica) satisfying
the criterion that the total amount of heat release at a heat flux of 50 kW/m2for 20 min is below 8 MJ/m2 was successfully prepared through a process ofpressure-impregnation for 1 h using our developed SPB aqueous solution (9.78mol/L) and the RF adhesive. The heated specimens were carbonized, but theadhesive interfaces glued the carbonized pieces together even after heating at750◦ C for 20 min in an electric furnace or after the CCM test using the 50kW/m2 heat flux for 20 min. In the present study, the criterion was achieved
when the thickness and the WPG were over 15.8 mm and 43.7%, respectively.
There remains a possibility that the specimens thinner than 15.8 mm satisfy thecriterion by increasing the WPG, because the saturated amount of the WPG
with regard to the Japan cedar was ca. 150%. The fire resistant laminated wood
developed in the present study will be useful for fixture and structural materials
in buildings where fire prevention is absolutely essential such as high-risebuildings, stations, theaters, museums, and domes. Fire damage with regard towood materials is expected to be diminished by using the present fire resistancetechnique. Furthermore, our results suggest that the SPB has a fire resistanceeffect on the RF resin, and it is also expected that the SPB will be usefulfor fire retardance of polymers because it does not contain heavy metals orphosphorus.

REFERENCES
1. Grand, A. F.; Wilkie, C. A. Fire Retardancy of Polymeric Materials; Marcel Dekker:
New York, USA, 2000.
2. Ishihara, S. Boron and its compounds for fire retardant of wood. Wood Preservation,
1989, 15(6), 248–230.
3. Ishihara, S. Fire retardants of wood and wood-based materials (I). Wood Industry,1990, 45(1), 2–6.
4. Tsuyumoto, I.; Oshio, T.; Katayama, K. Preparation of highly concentrated aqueous
solution of sodium borate. Inorganic Chem. Communications, 2007, 10(1), 20–22.

Development of Fire Resistant Laminated Wood285

5. Nies, N.P.; Hulbert, R.W. Solubility isotherms in the system sodium oxide-boricoxide-water. Revised solubility-temperature curves of boric acid, borax, sodium
pentaborate, and sodium metaborate. J. Chem. Engineering Data, 1967, 12(3),303–313.
6. Tsuyumoto, I.; Oshio, T. Fireproofing technique of wood using sodium polyborate
solution. J. Society of Mat. Sci., Japan, 2007, 56(5), 472–476.
7. Japan Testing Center for Construction Materials, Fire Preventive and Fireproof
Performance Test and Evaluation Procedure Manual, Japan, 2000, 62–75.