GB2383052A

GB2383052A - Treating wood with a hydrophilic monomer and polymerising

Info

Publication number: GB2383052A
Application number: GB0120078A
Authority: GB
Inventors: Rachel Lloyd
Original assignee: SMARTWOOD TECHNOLOGY Ltd
Current assignee: SMARTWOOD TECHNOLOGY Ltd
Priority date: 2001-08-17
Filing date: 2001-08-17
Publication date: 2003-06-18
Also published as: GB0120078D0

Abstract

A liquid hydrophilic monomer system is evenly infused into a material comprising natural wood and then polymerised in order to increase the strength of the material and make it less susceptible to fungal and insect attack. The process may be preceded by irradiation of the material.

Description

DESCRIPTION TITLE : HYDROPHILIC POLYMER GRAFTING TO PRE-TREATED WOOD.

BACKGROUND Many types of wood strengthening and preservation have been proposed. All suggest that addition of other agents to increase the material strength and its resistance to fungal and insect attack. These processes increase strength by the simple addition of other materials to the natural wood voidage giving a result, as expected, being the simple sum of the strength of the two materials in proportion to their presence. Additionally chemicals are included to kill or repel insects or fungus. These chemicals all evaporate or degrade over a period of years.

Previous attempts to use non-hydrophilic materials (e. g. methyl methacrylate) have been successful when applied to samples of small cross-section for the manufacture of handles for cutlery where they make the wood resistant to water ingress. These techniques do not alter the chemical structure of the natural wood thereby making it less attractive to insect or fungal attack, nor do they chemically combine the infusion with the wood fibres to give strength much greater than the simple sum of the constituents.

Any wood in which a monomer system is infused and polymerised (whether only with itself or with the cellulose also) will stress and tear any adhesions when the material is subjected to thermal or humidity cycling; unless the polymer system has similar mechanical properties to those of the wood. Since wood is a naturally hydrophilic material a good constitution for the composite is only possible when the polymer system used is similarly hydrophilic.

FIELD OF THE INVENTION The invention relates to a method of processing wood. More particularly, it relates to methods of infusing a hydrophilic polymer into the voids within the structure of the wood and ensuring that this polymer makes best contact with the wood fibres by chemically bonding to the cellulose molecules.

It relates to a novel polymer system, to methods of infusing the monomer or pre-polymer systems into the voids within the structure of the wood and ensuring that it combines with the structure to achieve maximum effect.

This is a preferred objective because it gives the wood a strength greater than the sum of the two individual constituents. It also alters the chemical structure of cellulose molecules making then less susceptible to fungal and insect attack Because natural wood is hydrophilic it also ensures that the new composite has the same dimensional variation, in the presence of humidity changes, as does the original wood.

DETAILED DESCRIPTION OF THE INVENTION The total process comprises of three stages, the first of which is optional but preferred o Pre-irradiation

...

'Infusion 'Polymerisation' PRE-IRRADIATION OF THE WOOD When natural wood is subjected to ionising radiation the molecules of cellulose from which the wood is, in the main, comprised are excited; and some molecular bonds in the long chain carbon structure of the cellulose are broken. These breaks produce chemically active sites at the position in the molecule where the break occurred. Low doses of ionising radiation (lMrad) produce few such breaks whereas large doses (lOOMrad) produce many such fractures, substantially altering the chemistry of the natural wood.

This process must not be performed at high temperatures otherwise the active molecular segments will react with each other due to thermal excitation. Ideally, the process should be performed below-30 C, but certainly not above 80oC.

Ideally this process is performed in a vacuum, or an inert gas, such as argon. This greatly reduced the number of the active sites which react chemically either with each other or the gas filling the voidage, before the second part of the process can take place. Using these two techniques of low temperature and vacuum the maximum number of active sites are produced and retrained for use in the subsequent process.

Therefore because it is desired to retain as many of the active sites, induced by the radiation, as is possible Three steps are preferred to achieve this.

1. The timber is irradiated and retained in a vacuum or inert atmosphere such as argon.

2. The timber is irradiated and maintained at a low temperature 3. The monomer system is introduced to the timber voids as soon as practically possible after irradiation.

The preferred process irradiates the timber in a vacuum at a temperature between-30 C and 0 C and immediately upon delivery of the required dose the monomer system is introduced For bulk timbers, gamma irradiation is needed to access the depth of the timber. For thin sections, suitable for example to make plywood, electron beam irradiation is possible. Pretreatment irradiation doses of between 1 and 100 Mr are preferred. This dose can be applied over periods of between 1 hour and 1 day

POLYMERS FOR USE IN THE PROCESS The preferred embodiment of this invention of using infused polymers with similar hydrophilic properties to that of the natural wood will now be described. This part of the process is an essential step and is performed whether the pre-irradiation step has been used or not.

Most polymers will absorb some water even when the polymers are called hydrophobic polymers for instance most polymers including those which might be considered to be hydrophobic will absorb up to 5% water.

A polymer must be selected to match the hydrophilic nature of the wood been processed, this is usually expected to be in the range 5%-25%, by weight, water uptake.

Suitable polymers are addition polymers or co-polymers of monoethylenically unsaturated monomers and, if the polymer is to be water swellable, the monomers should be hydrophilic or the monomer system should contain a sufficient proportion of hydrophilic monomer that the final product is hydrophilic. Examples of monomers having hydrophilic properties are amino-alkyl acrylates and methacrylates, especially those in which the alkyl group is one containing 1 to 4 carbon atoms, e. g. methyl or ethyl. The amino group may be mono-or di-substituted and any substituent is preferably a CI-4 alkyl group e. g. methyl or ethyl. Specific examples are aminoethylacrylate, dimethylaminoethyl acrylate, methylaminoethyl methacrylate and diethylaminoethyl methacrylate. Hydroalkyl acrylates and methacrylates may also be used as the hydrophilic monomer and in these the alkyl group is preferably of 1 to 4 carbon atoms e. g. methyl or ethyl. A specific example is hydroxymethyl methacrylate.

Preferred hydrophilic monomers include N-vinyl pyrrolidone and other vinyl lactams and acrylamide and methacrylamide and N-substituted derivative thereof Substituted acrylamide and methacrylamid derivatives may be mono-or di-substituted and preferred substituents are alkyl, hydroxylalkyl and aminoalkyl (including mono-and di-substituted aminoalkyl e. g. di-alkyl aminoalkyl groups. Preferably any alkyl group present contains 1 to 4 carbon atoms, methyl and ethyl being especially preferred. Examples of such derivatives are N-methylacrylamide, Nisopropylacrylamide, N, N-dimethylacrylamide, N, N-dimethylaminomethylacryamide, N, N-di- methylaminoethylacrylamide and N-methyl-aminoisopropyl acrylamide.

The polymers are preferable copolymers, two or more hydrophilic monomers, e. g. as described above, being copolymerised together or one or more hydrophilic monomers being copolymerised with one or more other monomers. Preferably a hydrophilic monomer is copolymerised with an alkyl acrylate or methacrylate, especially one in which the alkyl group has from one to four carbon atoms e. g. methyl or ethyl, or acrylonitrile. Styrene may also be used as a comonomer.

Specific examples of suitable copolymers are copolymers of N-vinyl pyrolidone and methyl methacrylate, of N-vinyl pyrolidone and hydroxymethyl methacrylate and of N-vinyl pyrolidone, styrene and acrylonitrile. Co-polymers of terephthalic acid and N-vinyl pyrolidone may also be suitable.

It is greatly preferred that the polymers should be cross-linked and this may be achieved by incorporating di-or poly-functional cross-linking agents in the monomer system. Examples of suitable cross linking agents are compounds containing two or more ethylenically unsaturated groups e. g. allyl methacrylate, divinylbenzene, ethylene glycol dimethacrylate and trimethylol propane trimethacrylate and trimethylol propane trimethacrylate. Usually it is desirable that the polymers should be relatively lightly cross-linked, the cross-linking agent being used, for example, in an amount of about 1% by weight of the monomer system It is much preferred to use a hydrophilic constituent to the monomer mixture, which is ionic such as hydroxyethyimethacrylate, because this combines much more readily with the active sites generated in the cellulose by the pre-irradiation process. Other ionic and highly chemically active monomers such as acrylamido-methyl-propanesulfonic acid can be used to increase the reactivity of the infused monomer system with sites on the cellulose molecules before being polymerised into the polymer system.

High strength polymer systems are also preferred in some cases such as that obtained from using acrylonitrile and vinyl pyrolidone.

INFuSION Natural wood has a voidage between approximately 30-90%. After the initial pre-irradiation the selected monomer system is then infused into the wood as soon as practically possible. This reduces the time available for the active sites induced in the wood by the pre-irradiation to react with each other or other constituents of the wood or any residual atmospheric oxygen. If a natural softwood with a voidage of approximately 50% is infused with a monomer system to reduce this voidage to 30%, it will have a mechanical properties similar to a natural hard wood which typically has a voidage of 30%.

It is desired to evenly distribute the introduced monomer system throughout the wood voidage.

One preferred means of doing this is to introduce the monomer system as a vapour at a temperature between OOC and 200C to the pre-irradiated wood in which the core temperature is very low (between-40 and-20 C) whilst the surface temperature is slightly higher than the vapour temperature. The monomer vapour mixture then permeates the timber voids and deposits as a liquid on the colder core timber fibres. The infusion then gradually deposits evenly as the pressure drops (owing to the infusion of vapour) and the temperature of the whole wood sample rises towards ambient and becomes isothermal.

POLYMERISATION Once the monomer system has been evenly infused into the wood it can then by polymerised.

The polymerisation may be effected by use of a chemical initiator and by heating or by use of radiation in which case neither heating nor an initiator is necessary. A particularly satisfactory chemical initiator is azobispropylpercarbonate or an organic peroxides such as benzoyl peroxide.

Heating at temperatures in the range of 30 to 80 C, preferably 35 to 70 C, is generally suitable and it is often desirable to conduct the heating in a series of stages at increasing temperatures.

After the basic polymerisation has been conducted a post-cure treatment may be effected e. g. by heating the polymer at a temperature of 85 to 95 C, preferably under vacuum. If the polymerisation is to be effected by irradiation various forms of radiation may be used e. g. ionising radiation such as U. V, X or gamma rays or particulate radiation such as electron or photon beams. Preferably the radiation is ionising gamma radiation form a cobalt 60 source or a beam of electrons. After the basic polymerisation has been completed a post-cure treatment may be effected by further irradiation

Commonly the mixture subject to the polymerisation will consist of the monomer or monomers, cross-linking agent and any initiator needed but, if desired, a solvent for one or more of the monomers may be included e. g. water.

For bulk timbers, gamma irradiation is needed to access the depth of the timber. For thin sections, suitable for example to make plywood, electron beam irradiation is possible.

Polymerisation irradiation between 0.1 and 20 Mr can be used, though most hydrophilic polymers are preferably polymerised at doses of between 1 and 4 Mr.

This preferred dose can be applied over periods of between 1 hour and 1 day.

Claims

CLAIMS 1. A material comprising natural wood and into which a suitable liquid hydrophilic monomer system has been evenly infused and then polymerised.
2. A material comprising a natural wood which has been subjected to ionising radiation, with doses between 5 and 100 Mr, into which a hydrophilic monomer system has been evenly infused and then polymerised
3 A material as claimed in claim 1 or claim 2 in which the hydrophilic plastic material is selected from the group containing: hydroxyethylmethacrylate and methyl methacrylate having water uptakes in the range 10% to 30% by wet weight ; and * a cross linked co-polymer of acrylonitrile (or its homologues) with n-vinyl-2-pyrolidone and; 'a cross linked co-polymer of cellulose acetate butyrate (or its homologues) with n-vinyl-2- pyrolidone and;

* a cross linked co-polymer of an aromatic polyamide such as Trogamide T with n-vinyl-2pyrolidone and, 'for cross linked co-polymers involving n-vinyl-2-pyrolidone additional highly reactive ionic monomers such as acrylamido-methyl-propanesulfonic acid.
4. A material as claimed in any preceding claim whereby a fungicide or insecticide is distributed throughout the material in a form suitable to be a permanent deterrent or for gradual release during its working life.
5 A method of forming a material as claimed in claim I or claim 2 or claim 3 in which the preirradiation is performed at low temperatures (preferably-30 C to 0 C) and in a vacuum or inert atmosphere such as argon.
6. A method of forming a material as claimed in claim 1 or claim 2 or claim 3 in which the monomer is infused as a vapour (preferably at a temperature between 0 C and 400C) using a pressure differential.
7. A method of forming a material as claimed in claim I or claim 2 or claim 3 in which the monomer is infused into the pre-irradiated wood immediately after it has received the required radiation dose
8. A material substantially as described herein.
9. A method of producing a material substantially as described herein.