The present invention concerns wood products, in particular a modified wood product in which the wood has been reacted with acetic anhydride to produce acetylated wood.

Acetylated wood is a known product. The wood is treated by reaction with acetic anhydride so that at least some of the available hydroxyl groups on and in the wood structure become acetylated. Acetylation renders the wood less susceptible to damage by biological agents such as bacteria or fungi and more hydrolytically stable. Acetylation also changes the wood's physical properties, including tensile strength and density. Acetylated wood is a premium product for use in many commercial applications, including in the construction of buildings. Since acetylated wood is sold on the basis of its durability and stability over a long period, it is desirable for the manufacturer of such products to have a means of identifying products it has made after the products have been sold and possibly many years after a wood product has been used for e.g. building purposes. When a wood product fails, a purchaser of the wood product may wish to claim recompense from the manufacturer and in such circumstances the manufacturer requires a means by which he can positively identify his own product and distinguish his products from those made by another manufacturer. It is an object of the invention to provide such a means.

According to the invention, we provide a method of modifying wood comprising the step of reacting wood with a composition comprising acetic anhydride to produce acetylated wood, characterised in that said composition contains at least one marker compound mixed with said acetic anhydride. The marker compound is an organic compound selected from the group consisting of heterocyclic compounds in which the heteroatom is N or O, and compounds containing carbon, hydrogen and a functional group selected from the group consisting of nitro groups, chloro-, fluoro-, bromo-, nitrile, ether, thio-ether, amide, urea, phosphine oxide and sulfonyl.

We further provide, according to the invention, a modified wood product comprising acetylated wood, characterised in that said modified wood product contains at least one marker compound and in that said marker compound is present and identifiable in the wood at least 5mm below the surface of said wood product.

It is preferred that the marker compound does not occur naturally in wood, paint, varnish, glue or any other typical wood coating / product. In this way the identification of the marker compound in a sample of a wood product may be uniquely associated with the production of an acetylated wood product by a particular manufacturer who has added the marker compound to the wood product.

By "identifiable" we mean that the marker compound is found at a level above the detection limit when extracted and analysed by any appropriate method.

The marker compound is present and identifiable at least 5mm, and preferably at least 10mm below the surface of said wood product. The permeation of the marker compound into the structure of the wood provides the benefit that, after the surface of the wood product has been removed, for example by abrasion, or covered by a coating such as paint, the marker may be detected in a sample taken from beneath the surface of the wood product. Such samples may be taken by means of a core sample or otherwise. It is desirable for the marker compound to permeate the entirety of the wood product. Although the penetration depends on the size of the wood product and the method of adding the marker to the wood product, it is believed that when the marker is added to the acetic anhydride treatment during the process of forming acetylated wood it may be carried into the bulk structure of the wood with the acetylation mixture. In order to maximise penetration and distribution of the marker within the wood structure, the marker compound preferably has a molecular weight < 600 and more preferably < 500 Daltons. Preferred marker compounds are hydrophobic. Preferred compounds have a log Pow value between 2 and 7, where log Pow is the partition coefficient of the marker compound between octanol and water. Preferred markers cannot be completely removed from the wood by submersion in water.

Suitable marker compounds are stable at temperatures up to 180°C, preferably at least 200°C, and pressures of at least 10 bar, preferably at least 15 bar, in contact with acetic acid and/or acetic anhydride and/or water. By stable we mean that the marker compound does not react or decompose to form other compounds under such conditions. Suitable marker compounds are unreactive towards acetic anhydride and acetic acid. It is preferred that the marker compound has a boiling point which is greater than the boiling point of acetic acid. More preferably the marker compound has a boiling point which is in the range from 130 - 500 °C. It is advantageous for the marker compound to be capable of remaining in the acetic anhydride fraction when a mixture of marked acetic anhydride and acetic acid is separated by distillation. This is because the process of forming acetylated wood forms acetic acid as a by-product and sale or re-use of the acetic acid by-product is important in reducing waste and for economic reasons. In order for the acetic acid to be re-used or sold, it is usually separated from the post acetylation mixture of acetic anhydride and acetic acid (and other wood or acetic anhydride derived by-products), typically by distillation. Removal of excess of the marker compound from the by-product acetic acid is therefore facilitated by selection of a marker compound with boiling properties which enable the marker to be separated from such by-product mixtures. We have found that suitable marker compounds may be selected from heterocyclic compounds in which the heteroatom is N or O, and compounds containing carbon, hydrogen and a functional group selected from the group consisting of nitro groups, halo (especially chloro, fluoro and bromo), nitrile, ether, thio-ether, amides, ureas, phosphine oxides and sulfonyls. Marker compounds containing aromatic structures are particularly suitable. For example, nitro- or dinitrobenzenes containing additional alkyl, halogen or haloalkyl groups are useful marker compounds. Compounds containing amine, hydroxyl, ester, ketone or aldehyde groups are not considered suitable for the present invention because of their potential to react with the acetylation mixture. Preferred marker compounds may be selected from the following: pyridines or naphthyls optionally substituted with alkyl, phenyl, ether, halogen or haloalkyi substituents; nitro- or dinitro- benzenes, pyridines or naphthyls optionally substituted with alkyl, phenyl, ether, halogen or haloalkyi substituents; nitrile- or dinitrile-benzenes, pyridines or napthyls, optionally substituted with alkyl, phenyl, ether, halogen or haloalkyi substituents. The marker compound may interact with the wood through ionic interactions, covalent bonds, dipole interactions, hydrogen bonding, London dispersion forces, ion-pi bonds or pi-pi interactions.

The following are non-limiting examples of compounds considered as suitable marker compounds for the present invention:- 1 ,3,5-tri-tert-butyl-2,4-dinitro-benzene, 1 ,4-diisopropyl-benzene, 1 ,3- diphenyl benzene, , 4,4'-dimethyl-biphenyl, 2,3,2',3'-tetramethyl-biphenyl, 1 ,5-di-tert-butyl-3-heptyl- 2-nitro-benzene, 1 -tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene, diphenyl ether, perfluorodecalin, benzonitrile, triphenylphosphine oxide, 1 ,3-dichloro-5-nitro-benzene, benzyl ether, octahydro- pentalene, 1 ,3,5-triethoxy-benzene, 2,2,1 1 ,1 1 -tetramethyl-dodecane, octahydro-3a,6-methano- indene, stilbene, diphenyl thioether, diphenyl sulphone, diphenylacetylene, N,N-dimethyl- benzamide, tetramethyl-urea, 1 -(4,4-dimethyl-pentyloxy)-4,4-dimethyl-pentane, 1 -methyl-1 H- indole, 1 -methoxy-2-[2-(2-methoxy-ethoxy)-ethoxy]-ethane, tert-butyl-cycloheptane, 2,2- diphenylpropane, 1 -methoxy-6-methyl-octane.

The marker compound may be detected in the wood product by means of sampling and analysis. Typically a sample would be taken from a wood product, for example by drilling, scraping, cutting etc and then powdered, typically by grinding. The powdered sample may then be contacted with an organic solvent to extract any marker compound which is present. The solvent extract containing the marker compound (if any) may then be analysed to determine whether the marker compound is present. The analysis may be quantitative or qualitative. A suitable method of analysis is chromatography coupled to a mass-spectrometer. Typically a gas chromatography method is used (GC-MS). Detection could also be undertaken using liquid chromatography coupled to a mass spectrometer (LC-MS), gas chromatography with electron capture detector (GC- ECD), Inductively-coupled plasma- mass spectrometry (ICP-MS), X-Ray fluorescence (XRF), Infrared spectroscopy (IR), nuclear magnetic resonance (NMR), surface-enhanced Raman spectroscopy (SERS), surface-enhanced resonance Raman spectroscopy (SERRS), fluorescence spectroscopy, phosphorescence spectroscopy, HPLC or other variations of these chromatographic or spectroscopic techniques.

The results of the analysis may be compared with analysis of a reference sample containing a sample of a known marker compound in an organic solvent at a known concentration.

Comparisons may be carried out by comparing a spectrum or analytical response from a sample of an "unknown" wood with a reference spectrum or response. Alternatively, a spectrum or response derived from analysis of the unknown sample may be compared with a calibration. A calibration may be carried out using known samples of marker compounds. An analytical instrument may be programmed with a calibration against which a marker compound may be identified. Analysis may be carried out using a programmed instrument which is adapted to compare an analytical result with a stored value. The instrument may be programmed to indicate whether the analysis of an unknown sample is statistically likely to indicate the presence of a particular marker compound. The indication may be visual or aural. The instrument may provide a report in electronic or hardcopy format.

More than one marker compound may be present in a single wood product. Normally the number of marker compounds present is from 1 to 20, preferably from 1 to 10, for example 1 - 5 marker compounds may be used.

The invention will be demonstrated in the examples below. Example 1 : Marking wood with triphenylphosphine oxide

A sample of untreated pine, approximately 40 x 40 x 20 mm, was placed in a Parr reactor vessel. Acetic anhydride containing triphenylphosphine oxide (1 mg per litre of acetic anhydride) was added to the vessel and the system was then sealed and heated to 150°C for 5.5 hours. The reaction vessel was allowed to cool to room temperature and the wood sample was recovered. Excess acetic anhydride was washed from the surface of the wood using toluene and the sample was dried in an oven at 80°C for 1 hour. The surface layer (2 mm) was removed and a core sample in the form of a 1 cm diameter x 2 cm length cylinder of the wood (1 .43 g of shavings) was collected. The wood shavings were placed in a cellulose thimble and a 6-hour soxhlet extraction was performed with toluene (300 ml_). The resulting toluene-extraction mixture was concentrated to ca. 2 mL prior to analysis by GC-MS (Agilent® 6890 GC with HP5-MS 30m* 0.25mm* 0.25μηι GC Column and Agilent 5973N MS detector). A clear signal with mass 277 Da and retention time 5.57 min was observed, identical to that expected for triphenylphosphine oxide. As a comparison, the sampling, extraction and analysis described above was repeated for wood that had been acetylated in the absence of triphenylphosphine oxide. No signal with mass 277 Da and retention time 5.57 min was observed in the comparison.

Example 2: Marking wood with 1 -tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene

A sample of untreated pine was placed in a Parr reactor vessel. Acetic anhydride containing 1 - tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene (1 mg per litre of acetic anhydride) was added and the system was sealed and heated to 150°C for 5.5 hours. The reaction vessel was allowed to cool to room temperature and the wood sample was recovered. Excess acetic anhydride was washed from the surface of the wood using toluene and the sample was dried in an oven at 80°C for 1 hour. The surface layer (2 mm) was removed and a core sample of the wood (0.98 g of shavings) was collected using a drill. These shavings were placed in a vial with toluene (20 mL) and left to stand for 5 hours. The resulting toluene-extraction mixture was analysed by GC-MS as described in Example 1. A clear signal with mass 251 Da and retention time 4.42 min was observed, identical to that expected for 1 -tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene. No similar signal was observed in a comparison sample of acetylated wood which had been acetylated in the absence of 1 -tert-butyl-3,4,5-trimethyl-2,6-dinitrobenzene.

Example 3: Resistance to leaching in water

A sample of acetylated wood (20 x 20 x 40 mm), marked with triphenylphosphine oxide (as described in Example 1) was submerged in water (50 ml_) in a glass jar. The sample was left at room temperature for 1 week, after which time the wood was removed from the water and dried in an oven at 80°C for 1 hour. The surface layer (2 mm) was removed and a core sample of the wood (1 .20 g of shavings) was collected using a drill. These shavings were placed in a cellulose thimble and a 6-hour soxhlet extraction was performed with toluene (300 ml_). The resulting toluene- extraction mixture was concentrated to ca. 2 ml_ prior to analysis by GC-MS as described above. GC-MS analysis of the extract from the leached sample showed a > 95% response for the marker relative to a reference sample of triphenylphosphine oxide- marked acetylated wood which had not been subjected to 1 week in water.