WO1988004601A1

WO1988004601A1 - Wood preservatives

Info

Publication number: WO1988004601A1
Application number: PCT/GB1987/000914
Authority: WO
Inventors: Gerald Howell
Original assignee: Laporte Industries Limited
Priority date: 1986-12-19
Filing date: 1987-12-21
Publication date: 1988-06-30
Also published as: AU1059888A; GB8630352D0

Abstract

Organic boron compounds having an ability to hydrolyse in use, particularly orthoboric acid esters such as methoxy or ethoxy boranes or the mono or biboranes of glycols such as ethylene or hexylene glycol are used as wood preservatives, the alcohol liberated on hydrolysis or present as a result of manufacture being present as an absorption aid. The technique may particularly be applied to the preservation of wooden railway sleepers.

Description

Wood Preservatives

The present invention relates to the use of borates as preservatives, especially wood preservatives.

The preservation of bio-degradable construction materials has always presented a problem. Of such materials, wood presents the greatest problem, but even brick and concrete are also subject to attack by mosses, lichens, ivy and even fungi.

Various techniques have been devised to hinder such attack and these frequently take the form of some sort of coating such as tar or creosote. Such methods work reasonably well, but do not protect the interior of the material. As construction materials are generally exposed to some degree of stress, cracking is a common occurrence. When this happens, water can penetrate and infection ensues sooner or later.

A particular problem is wooden railway sleepers. These are constantly exposed to the elements and a high degree of stress. The average life of a wooden sleeper is between δ and 40 years, usually around 25 years. When they rot, they must be replaced at enormous expense in terms of man hours, replacement costs and inconvenience to traffic. With more ' than 1.5 billion timber sleepers in use around the world, an effective preservation technique is urgently required. i It has been discovered that fused boric oxide rods, when inserted into holes in sleepers, are capable of effectively inhibiting fungal attack for at least 5 years (UK Patent No. 2008640). However, such compounds are difficult to manufacture commercially, requiring temperatures in excess of 1000°C for fusion to enable shaping to be performed and are extremely corrosive of equipment at such high temperatures. The rods so formed are relatively poorly soluble, even at moisture contents of 25% and above.

Attempts to increase the solubility of rods containing boric oxide and/or other inorganic boron compounds, such as sodium tetraborate penta- or deca- hydrate, by tabletting such compounds in solid powdered form, or by extrusion- forming a paste containing an inorganic boron compound, such as disodium octaborate tetrahydrate, capable of binding water present in the paste as water of crystallisation so as to solidify the extrudates, have resulted in rods which dissolve excessively quickly, in use, so that the boron compound may be washed away before it has been fully absorbed into the sleeper.

The boron rods, tablets and extrudates described above also suffer from the problem that they are reluctant to dissolve in the moisture present in the interior of sleepers when that moisture is below about 25% by weight of the sleeper. Decay can readily occur, however, at lower moisture contents and cannot be co batted satisfactorily by the aforementioned means.

There is therefore a need for a boron-based rod, tablet, extrudate or other artefact suitable for insertion into timber which can be used effectively to treat wood having moisture contents below about 25% -by weight. It would also be of benefit to produce a boron-based composition in any form, for example in liquid form, which is effective in treating wood having the relatively low water contents last envisaged above.

According to the present invention there is provided a wood preservative composition comprising one or more boron compounds the composition being characterised by a content of one or more organic boron compounds preferably of one or more boric acid esters.

The term "organic borate" is used to indicate derivatives of a boric acid or of a boric oxide containing one or more organic groups and includes functional derivatives thereof. While metaboric acid derivatives are not excluded the present invention is primarily concerned with orthoboric acid derivatives. The term 'ester' is used to indicate the compound which would be obtained by reactio between the boric acid and an organic hydroxy functional compound whether obtained by that reaction or by other routes, e.g. by reaction between boric oxide and the organi compound. The last mentioned compounds are generally characterised by the binding of an organic group to the boron atom via a linking oxygen atom.

A wide range of organic borates may be prepared by a reaction between the boric acid and one or more mono- or poly- hydroxy organic compounds. According to the inventio such compounds may be particularly suited to the treatment of wood, particularly wood having a relatively low water content. On hydrolysis by contact with the moisture presen in the wood the active boric acid species is liberated and the mono- or poly- hydroxy compound regenerated. According to one aspect of the present invention the presence of the said hydroxy compound by reason of its water solubility and therefore its solubility in the ambient wood moisture helps to transport the said active species into the wood thereby producing an effective wood treatment system.

The effect of this is not the same as would be obtaine by the treatment of the wood with a mixture of the alcohol and boric acid since both of these are water soluble and would be susceptible to be washed from the point of treatment at high moisture contents and the alcohol could be lost by evaporation at low moisture contents. The hydrolysis mechanism, the speed of which is controllable by suitable selection of the one or more borates, releases the active species and the alcohol transport agent. progressively.

The rate of hydrolysis of organic borates is subject to a number of parameters amongst which are steric parameters, according to which the presence of groups of large steric requirement above and below the BO3 plane hinders hydrolysis, and electronic parameters, according to which substituents which can exert an inductive effect to decrease the electron density on the boron atom, for example halogen substituents, encourages hydrolysis. The matter is discussed in Organoboron Chemistry by Howard Steinberg published by Interscience Volume 1, Chapter 21, and the selection of readily hydrolysable borates is preferably conducted bearing in mind the above. Preferably a borate is selected having an uncatalysed rate of hydrolysis at ambient temperature at least equal to, and preferably at least twice, that of tri 3-heptyl borate. The last mentioned compound may be produced by.reaction between orthoboric acid and heptanol in the presence of an azeotroping agent for the water produced, such as benzene, xylene, toluene, 1-octene or diisobutylene, with removal of the azeotrope from the reactants by distillation. The rate of hydrolysis may be determined by comparison of the time taken for a given proportion of the ester to hydrolyse in an aqueous or organoaqueous medium, a suitable proportion, for example, being from 25% to 50%. Other boric acid esters considered herein may be produced by parallel techniques to that outlined above and known to those versed in the art of organo-boron chemistry.

The physical state of the borate is also of importance. If it is to be utilised in the form of a solid it clearly should be a solid at normal temperatures, and if it is to be used in the form of a liquid it is preferably a liquid at normal temperatures to avoid undue dilution of the active species because of the need to use a solvent. Borates produced using low molecular weight mono- alcohols such as, for example methanol or ethanol or other monohydric alcohols containing up to 10 carbon atoms tend to be liquid at normal ambient temperatures. The above borates are preferably tri substituted with the mono-alcohol residue.

While borates based on simple monohydric alcohols, are envisaged above, parallel methods and considerations apply to the selection for the purposes of this invention of aryl borates, arylalkyl. borates and borates based on glycols an polyhydric alcohols. Useful borates may particularly be produced by the use of glycols to form either an anhydride type of borate containing a single boron molecule, or, particularly preferably, biborates containing two boron atoms and three glycol residues which generally aresolids a ambient temperature. Alternatively, aromatic dihydroxy compounds may be employed. Preferably the glycols contain to 10 carbon atoms for example hexylene glycol although the lower glycols for example ethylene glycol may be preferred on cost considerations. Suitable polyhydric alcohols which may be employed in the production of borates for use according to the present invention are the trihydric alcohols such as glycerol, trimethylol ethane, tetrahydric alcohols such as pentaerythritol or higher polyhydric alcohols such as the sugar alcohols for example sorbitol or mannitol.

While, in the main, unsubstituted alcohols have been envisaged above other hydroxyl compounds notably the alkanolamines such as for example ethanola ine, may be employed. Halogen substituted alcohols, especially chloro substituted alcohols may also be used. The borates envisaged herein generally have boron contents, calculated as B2O3 of from about 10 to 35% weight.

For inclusion in solid wood treatment compositions a borate preferably has a melting point of from 40°C to 200°C particularly preferably of from 50°C to 150°C. Such borates may be formed into rods or other solid artefacts by fusing or extruding although if decomposition is found to occur they may alternatively be moulded or tabletted using a binder, if necessary, without the need to heat to or near the melting point. Preferably the final compositions of this invention cont at least 5% of boron (calculated as the element) to give a reasonably effective boric acid potential in use.

In situ, the organic borates dissolve with hydrolysis at moisture contents as low as 10%, unlike the known inorganic borates which tend to dissolve only when the moisture content reaches 20-25%. This property allows effective pretreatment of the wood or other construction material instead of having the preservative only permeate the article with the influx of water. The fact that organic borates can dissolve at such low moisture contents also means that infections developing elsewhere in the material can effectively be countered at a very early stage.

In further features of the present invention, there are thus also provided use of an organic borate as defined above, wherein the said borate has been fused prior to use, and the use of an organic borate as described, wherein the said borate is further associated with an alcohol.

Preferably, the alcohol used is that from which the organic moiety, or moieties, is or are derived and is present as a result of hydrolysis of the borate 'in situ' in the wood, and/or as a result of manufacture of the borate fro the alcohol.

Particularly preferred compounds for use in accordance with the present invention are trihexylene glycol biborate and trimethoxyborane. It will thus be appreciated that the said organic borate compound need not comprise only one borate moiety.

A particularly preferred embodiment of the present invention concerns railway sleepers. An organic borate, tri(ethylene glycol) biborate (m pt circa 160°C), is fused and extruded to form a rod which, when solidified, is snapped or cut into short lengths of approximately 10 cm, the rod being about 2.5 cm in diameter. These rods are then placed in snug-fitting holes bored in a sleeper, the holes being sufficiently deep to allow insertion of a tight-fitting cap to reduce exposure to excessive quantities of water. Several rods, for example 8 to 12 rods per sleeper, may be used and are spread evenly along the length of the sleeper conveniently in a zig-zag fashion to maximise the volume perfused. More rods may be required in areas of the sleep particularly prone to stress and/or absorption of water, f example near the plates under the rails.

Under such conditions, boric acid distributes up to 50 mm from the point of insertion in wood containing only 10%- 12% by weight of moisture. In wood containing a higher moisture content the penetration is greater but this confirms the operability of the invention at minimum wood water content.

A further feature of this invention are that it is als possible to selectively control the rate of hydrolysis by a combination of various borate esters which will dissolve at predetermined rates.

A further advantage is. that inorganic borates can be combined ith organic borates by melting the latter, and blending in the inorganic borates to form a dispersion whic can, on solidification, be extruded to form solid wood preservative rods. This will provide a solid rod with a fast dissolving fraction (organic borate) with a residual, more slowly dissolving content(inorganic borate) which will be extremely advantageous in the secondary treatment of wooden structures where a rapid distribution of preservativ is often required.

The present invention is useful particularly where there is a danger of attack by insects, fungi and other infestations generally susceptible to the well-known toxic effects of borates.

Claims

1. A wood preservative composition comprising or consisting of at least one organic borate.

2. A composition as claimed in claim 1 wherein the organic boron compound comprises at least one orthoboric acid organic ester.

3. A composition as claimed in claim 2 wherein the orthoboric acid ester has an uncatalysed ambient temperature hydrolysis at least equal to that of 3- heptyl borate.

4. A composition as claimed in claim 2 wherein the ester is a biborate of a dihydric alcohol containing from 2 to 10 carbon atoms.

5. A composition as claimed in claim 2 wherein the ester is a trialkoxyborane wherein the alkoxy group contains from 1 to 10 carbon atoms.

6. A composition as claimed in claim 2 in the form of a rod tablet extrudate, or other solid artefact wherein the ester has a melting point of from 50°C to 150°C.

7. A composition as claimed in claim 2 also containing an alcohol.

8. A composition as claimed in claim 7 wherein the alcohol corresponds to that of the particular ester used.

9. A process for treating wood containing at least 10% by weight of moisture comprising bringing the wood into contact with at least one orthoboric acid ester in the form of a solid or liquid composition. ^W°<»<"«» PCT/GB87/00914

10. A process as claimed in claim 9 wherein the compositio additionally contains the alcohol corresponding to tha of the particular ester used.

11. A process as claimed in claim 10 wherein the alcohol i a polyalkylene glycol and the ester is an ester of sai glycol with one or more orthoboric acid molecules.

12. A process for treating wood to increase the resistance thereof to decay comprising hydrolysing an organic ester of orthoboric acid in contact therewith and absorbing the said hydrolysis products into the wood.