PEETT, Fraser (14 Kings Road, Henley-on-Thames, Oxfordshire RG9 2DG, GB)
1 . A method for improving fire retardant properties of an item of wood, the method comprising:
a) providing an item of wood to be treated;
b) if the wood has a moisture content of greater than 20%, drying the wood so that it has a moisture content no greater than 20%;
c) coating a first major surface of the item of wood with a first coating formulation comprising an aqueous solution containing boric acid or a salt thereof, an amine or ammonium salt, and phosphate;
d) repeating steps b) and c) and b); and then
e) coating the first major surface with a second coating formulation comprising a film-forming polymer material in an aqueous carrier. 2. A method according to claim 1 , wherein the first coating formulation further comprises an adjuvant to promote wetting and aid penetration of the wood by the aqueous solution, the adjuvant preferably comprising one or more of: a glycol, a hydrotrope, and a surfactant. 3. A method according to claim 2, wherein the surfactant is a non-ionic surfactant, preferably selected from alcohol ethyoxylate, alkyl polyglucoside, ethylene oxide-propylene oxide block copolymer, alkyl ethoxylate propoxylate, ethoxylated sorbitan monooleate, alkyl aryl alcohol ethoxylate. 4. A method according to claim 2, wherein the hydrotrope is an aryl ethoxylate phosphate ester salt or an alkyl ethoxylate phosphate ester salt.
5. A method according to any one of the preceding claims, wherein the film- forming polymer material is a styrene butadiene copolymer.
6. A method according to any one of the preceding claims, wherein the second coating formulation further comprises an agent to promote keying of the polymer material into the surface of the wood; the agent preferably being a glycol, particularly preferably butyl glycol.
7. A method according to any one of the preceding claims, wherein the second coating formulation further comprises a crosslinking agent, preferably a zinc salt, to promote crosslinking of the polymer material.
8. A method according to any one of the preceding claims, wherein the second coating formulation further comprises a UV-protecting agent, preferably titanium dioxide, to reduce UV damage of the polymer material.
9. A method according to any one of the preceding claims, wherein the second coating formulation further comprises a material for increasing surface hydrophobicity, said material preferably being silicon dioxide. 10. A method according to any one of the preceding claims, wherein the first coating formulation contains boric acid or borate ions in the range 48-72 g/L, ammonium or amine ions in the range 88-132 g/L and phosphate ions in the range 236-354 g/L. 1 1 . A method according to any one of the preceding claims, wherein the first coating formulation contains boric acid in the range 48-72 g/L and ammonium phosphate in the range 324-486 g/L.
12. A method according to any one of the preceding claims, wherein step c) comprises applying the first coating formulation at a rate of 260±10 g/m2.
13. A method according to any one of the preceding claims, wherein the film- forming polymer material makes up about 20-32%, preferably about 26%, of the second coating formulation by weight.
14. A method according to any one of the preceding claims, wherein step e) comprises applying the second coating formulation at a rate of 100±10 g/m2.
15. A method according to any one of the preceding claims, wherein the item of wood is a piece of timber which further comprises a second major surface opposed to the first major surface, the method further comprising:
f) if the wood has a moisture content of greater than 20%, drying the wood so that it has a moisture content no greater than 20%;
g) coating the second major surface with a first coating formulation comprising an aqueous solution containing boric acid or a salt thereof, an amine or ammonium salt, and phosphate;
h) repeating steps f) and g) and f); and then
i) coating the second major surface with a second coating formulation comprising a film-forming polymer material in an aqueous carrier.
BACKGROUND a. Field of the Invention
The invention relates to a method for improving fire retardant properties of an item of wood, and to an item of wood having at least a major surface with fire-retardant properties. Various standards are recognised in the timber and building industries for fire retardant properties of timber for use in building. It is particularly desirable that wood meet the Class B fire retardancy standard, as tested under BS EN
13823:2010. This Single Burning Item (SBI) test simulates the conditions experienced by a construction product in the corner of a room, when exposed to the thermal attack of a single burning item positioned in that corner. BS EN
13823:2010 is included within the testing protocol EN 13501 -1 :2007+A1 :2009, Fire Classification of Construction Products and Building Elements.
European legislation around the Construction Products Regulation, using
EN 13986 has mandated that for a product to carry a CE mark it must be supported by a Declaration of Performance (DoP) Certificate which lists the relevant performance of that product in terms usable by the
builder/engineer/specifier/architect. The DoP is created and owned by the manufacturer and passed on to any subsequent owners so long as the product itself has not been changed in any way.
EN 13986:2004 is the harmonised European standard for wood -based panels. It defines the performance characteristic and test methods for wood-based panels (including plywood, oriented strand board, and laminated veneer lumber) enabling their use in construction applications. A series of tables lists the relevant performance characteristics for load-bearing (structural) and non-structural applications in the three service environments: dry, humid and exterior. With pressure treatment to improve fire retardant (FR) properties, the wood owner who has made the change to the FR of the product can improve the EN13501 -1 class on the DoP from usually Class D s2 dO to whatever they have achieved though independently verified testing, for instance Class B or C, s1 or 2, dO. As it is not then known how the product has been changed in terms of strength, bonding quality etc, the wood owner must remove all other data on the DoP and replace it with NPD (No Performance Declared). This removes any claim of structural use. If the panel was CE marked as suitable for structural use with a AVCP Class 2+ (CE2+) before hand, it will be dropped down to Class 1 which is non-structural and they must legally declare as such to the next owner.
The only way that the owner can return the product to being defined as CE2+ structural is to prove to the original manufacturer that they have not changed the properties of the boards, which given the aggressive physics involved, is highly improbable. It may well be that the manufacturer can introduce FR into their own production process and therefore do their own testing with the improved fire resistance already present, but it is not possible for a post-production pressurised treatment to do this. b. Related Art
Various treatments are known for improving fire retardant properties of wood, including pressure treatments, often in conjunction with improvements to other properties such as mould resistance or insect resistance. US 2005/0217537 discloses a composition for rendering a material flame retardant and resistant to moulds and insects. The composition comprises a metallic borate, a methyl donor, an acid, an alcohol, and an ammonium base. SUMMARY OF THE INVENTION
According to the invention there is provided a method for improving fire retardant properties of an item of wood, as specified in claim 1 . Preferred features are specified in the dependent claims.
We have surprisingly found that by using a double coating of borate, amine or ammonium ions, and phosphate ions, in combination with a top coat of a film- forming polymer material, it is possible to improve flame retardant properties of wood, and to achieve Class B fire retardancy standard, as tested under BS EN 13823:2010. The use of an adjuvant is particularly beneficial because this promotes homogenous spreading and penetration of the fire retardant treatment into the wood. The boric acid or borate may be provided in an ammoniated salt, which is particularly water soluble and also provides an ammonium source.
The adjuvant may comprise one or more of: a glycol, a hydrotrope, and a surfactant. The term 'hydrotrope' is used herein to refer to a compound that solubilises hydrophobic compounds in aqueous solutions. Typically, hydrotropes consist of a hydrophilic part and a hydrophobic part (like surfactants) but the hydrophobic part is generally too small to cause spontaneous self-aggregation
The second coating formulation provides a film which we refer to as a MicroBarrier. Without wishing to be bound by theory, we believe that application of the MicroBarrier seals salts therein by creating a hydrophobic and moisture- resistant film, aiding retention of salts within a matrix in the wood.
The first formulation and the second formulation may be coated onto the wood surface by any suitable application techniques, notably by spray application or by roller application. DETAILED DESCRIPTION
Solution 1 , containing boric acid and diammoniunn phosphate was prepared follows.
Solution 1 Manufacturing Method
1 . To a clean tank charge with required amount warm (ca. 30°C) water;
2. Begin stirring;
3. Add required amount of Boric Acid and stir; 4. Add Diammonium Phosphate and stir;
5. Continue stirring until product fully dissolved, clear and free from insoluble material;
6. When filling, filter with a 100 pm filter.
Solution 2, containing adjuvant materials to promote wetting and aid penetration of wood, was made up as follows. Formulation
1 . To a clean tank charge with Butyl Glycol and Phenol Ethoxylate Phosphate Ester Salt in that order and stir;
2. Stir until homogenous and add remaining ingredients;
3. Stir a final time for 20 minutes. First Coating Formulation
A first coating formulation was prepared by mixing Solution 1 and Solution 2 in a 95:5 volume ratio (1 16.7:4.85 weight ratio). Moderate agitation was maintained throughout the process to prevent phase separation.
Second Coating Formulation
A second coating formulation was prepared as follows. Chemical name EC /CAS Number Content
Styrene Butadiene Copolymer
CAS: 9003-55-8 500 grams Solution, Grade CLI 3008
Butyl Glycol 100 grams
Phenol ethoxylate (4 Mol)
Phosphate Ester, Potassium Salt CAS: 72283-31-9 25 grams Solution 65% Solution
Zinc Oxide CAS: 1314-13-2 1 gram
Titanium Oxide CAS: 236-675-5 1 gram
Silicon Oxide CAS: 231-545-4 1 gram
Water 375 grams
Second Coating Formulation
1 . To a clean tank charge with required amount cool (ca. 20°C) water;
2. Begin stirring;
3. Add CLI 3008, Butyl Glycol and Phenol Ethoxylate Phosphate Ester Salt in that order and stir;
4. Stir until homogenous and add remaining ingredients;
5. Stir a final time for 20 minutes.
Example 1 The first coating formulation was spray coated onto a substrate of Wisa Finnish Spruce Plywood 2440 mm x 1220 mm (8' x 4') 18 mm thick, 7 veneer, under the conditions summarised in Table 1 .
Ambient temperature and relative humidity may require the specifications to be changed. Increase / modify the combination dryer as necessary to yield a dry wood at the end.
Calibrate machine to apply 260 g/m 2 ± 5 g/m 2 of the first coating
formulation described above. This equates to 32.5 grams on an 1/8 th m 2 tester board
Verify scale-up on first board passed, record weight.
Weight increase of a 2440 mm x 1220 mm (8' x 4') board is 772.95 g / board ± 14.9 grams
Allow the machine to warm up to the specifications above
Once verified, load the wood into the machine and run through to apply the fire retardant. Surface 1 , Pass 1
Before proceeding, the operator checks that the wood is dry. Randomised wood moisture content must be no more than 20% measured using a wood moisture meter. If necessary, the wood is dried to a moisture content of no more than 20%, preferably about 15%.
Surface 1 , Pass 2
After the first coating pass, the same process is repeated for the second pass, with measured moisture content of the wood being no more than 20%, preferably about 15%, prior to the second coating pass.
Typically after drying in a rack for at least 24 hours (or longer depending on ambient temperature and humidity) the panel is rotated, and Surface 2 is treated with the same two-pass coating process as was done for Surface 1 .
In this embodiment, each coating pass applies the first coating formulation at a rate of about 260 g/m 2 . After further drying, until the measured wood moisture content is no greater than 20%, preferably about 15%, the second coating formulation is applied to each major surface of the wood, in this example using the process conditions summarised in Table 2. It is important to control the moisture content, because we have found that high surface moisture hinders the MicroBarrier from adhering to the surface. With too high a moisture content prior to coating, the MicroBarrier may flake off after drying. Machine specifications:
1 . Calibrate machine to apply 100 g/m 2 ± 5 g/m 2 of the combined solution described above. This equates to 12.5 grams on an 1/8 th m 2 tester board
2. Verify scale-up on first board passed, record weight.
Weight increase of 2440 mm x 1220 mm (8' x 4') board is 297.3 g / board ± 14.9 grams;
3. Once verified, load the treated wood into the machine and run through to apply the MicroBarrier 4. Flip 180°;
5. Return to the start for treatment;
6. Load the panel and run through to apply the MicroBarrier on the second side;
7. Stack with spacers and allow 2 days for final drying/curing.
Samples of the treated wood were tested at an independent laboratory as detailed in Table 3. Results are summarised in Table 4 and Table 5. Name of Name of Test Reports / Test Method Laboratory Sponsor Extended
PFS Coatings BMT/RFP/
BM TRADA BS EN ISO 1 1925-2:2010
Ltd F141 16/01
PFS Coatings BMT/RFP/
BM TRADA BS EN 13823:2010
Ltd F141 16/02
Two tests were performed on each layer. The worst performing layer was identified and four further tests were performed on that layer in accordance with EGOLF recommendation ER 07:2003. BS EN 13823:2010
FIGRA o.2 MJ 1 10.5 compliant
BS EN 3 LFS
13823:2010 None compliant
(to edge of specimen)
F141 16/02 THR 600s 7.5 compliant
SMOGRA 7.5 compliant
TSP 600s 48.0 compliant
None compliant particles
Wood treated in accordance with the method of the invention was compliant with BS EN 13823:2010 and also with BS EN ISO 1 1925-2:2010, which describes methods of test for evaluating linear joint (gap) seals. It will be appreciated that the invention is not limited to the use of only two applications of the first coating formulation. If desired, further coatings of the first coating formulation may be applied, with drying between the further applications if necessary, to ensure that the moisture content of the wood is no more than 20%, prior to application of the second coating formulation.
It will also be appreciated that more than one application of the second coating formulation may be made, with drying before the second application if necessary, to reduce the moisture content of the wood to no more than 20%. The preferred first coating formulation is a blend of about 0.9 % B, about 7.0 % N and about 7.73 % P, though the invention is not limited to these ratios. The relative ratios make a significant impact on the fire retardant performance.
We have found that styrene-butadiene copolymers as the MicroBarrier provide an optimum performance compared to all other options. Polyacrylate adhesives for example can impart the binding and durability properties required; however the wood panels will be glued together and prevent use. Polyurethanes could also be used; however this was not a viable option for the methods of use of the present examples.
Vinyl chloride was also tested; however this was grossly incompatible with all chemistry tested. It was also on a list of restricted substances. Additives could have been added to impart fire retardance by making use of the halogen; however this would have increased smoke output and made the product not suitable for indoor use.
Styrene-Butadiene copolymers can have a range of properties depending on the ratio of styrene to butadiene monomers, as well as whether the product is post- modified. The grade chosen provides a non-tacky finish, hydrophobic finish and good adhesion into the wood, without increasing smoke output. It also did not cause flaming droplets, something that foaming MicroBarriers such as
polyurethane could have caused.
It is desirable to incorporate a zinc salt, notably zinc oxide in the second coating formulation as it promotes the crosslinking of the butadiene component of the styrene-butadiene copolymers upon drying, and reduces tackiness.
Silicon dioxide has reported uses in providing surface hydrophobicity, and further improving the durability of the fire retardant.
Titanium dioxide, albeit in small quantities, provides UV protection and prevents the long term photolysis of the styrene-butadiene copolymer. Styrene-butadiene is a very large chromophore, conjugated or not depending on the molecular construct, and some protection against UV damage is desirable. Mechanisms of action
Without wishing to be bound by theory, we believe that the following proposed mechanisms are involved in the fire retardant performance of the invention.
Boron will react with hydroxyl (-OH) functional groups of cellulose in wood. The chemical reaction (endothermic) will form a glassy ester that coats the surface and reduces degradation into flammable gases whilst simultaneously releasing water (H 2 O) which provides a cooling effect. Boron (as boric or borate species) will also catalyse the dehydration of wood, catalysing the char formation if synergistic substances (e.g phosphate, PO 4 3" ) are present.
Phosphate species (e.g. ammonium phosphate) will thermally decompose to release phosphoric acid. Ammonium phosphate will also release ammonia gas (see below). The phosphoric acid will extract water from the pyrolysing substrate, causing char build-up. This mechanism is a combination of dehydration of cellulose (-OH groups) by forming ester bonds, similar to boron, decomposition and release of char (i.e. carbon material). Phosphate-based fire retardants act directly in the pyrolysing stage of fire, reducing the available flammable gas to continue propagation and forming a protective char layer.
The char layer provides a non-combustible thermally insulating layer.
Nitrogen is present in the formulation in its ammoniacal (NH + ) form. Unlike the other components, ammonia has a diluent property only. Exposure to high temperatures will cause degradation of the ammonia salts (endothermic) and release of ammonia gas which dilutes the flammable gases. The nitrogen could be present as an amine group, such as those found in Melamine, which can then decompose to yield ammonia. Structural Integrity
With the present invention, we have created a product that is balanced in terms of FR efficacy, lifetime durability of the coating against weathering/humidity AND retains the structural integrity as CE2+ that is lost to pressure treatment. Due to the chemistry and low/no pressure factory production application method, the boards are not physically damaged by the ingress and we have had BM TRADA as a UKAS body perform extensive testing to ensure that the key numbers have not been changed significantly. The chemistry and its application method do not affect the structural properties of the structural engineered wood-based panels; something which is unique in the industry.
Non-factory production FR 'systems' such as intumescent paints are able to claim FR efficacy but these coatings are applied after the boards have been used within the build environment over which there is far less control in the application and the actual classification offered is usually flawed by this.
Durability Durability is determined by TS 15912:2012 - Durability of reaction to fire performance - Classes of fire-retardant wood-based product in interior and exterior end use applications. This standard is due to be adopted as EN16755 after a public consultation period has been completed. Wood treated according to the present invention has been tested and demonstrated INT2 Durability: for permanent use in interior applications and certain protected exterior applications, service class 2 (eg wall and ceiling products).