WO2015095900A2

WO2015095900A2 - Timber treatment process

Info

Publication number: WO2015095900A2
Application number: PCT/ZA2014/000077
Authority: WO
Inventors: Stuart PRIOR; Scott SARGENT
Original assignee: Rhino Modified Wood (Pty) Limited
Priority date: 2013-12-17
Filing date: 2014-12-15
Publication date: 2015-06-25
Also published as: ZA201604486B; WO2015095900A3

Abstract

A method of producing a modified wood product which includes the steps of sourcing a quantity of timber which has a moisture content in the range of 12% to 20%: subjecting the timber to a first stage in which the timber is heat treated; and subjecting the timber to a second stage in which the timber is impregnated with a protective wax-based composition which has a melting point in excess of 100°C.

Description

TIMBER TREATMENT PROCESS C G^j O OEJilE INVENTION

[0001] This invention relates generally to the modification of timber and more specifically to a method of producing a modified wood product with a numbe of enhanced -characteristics.

[0002] Timber Is used In many applications worldwide. Generally, hardwood timbers have a greater durability and aesthetic appeal than softwoods and, consequently, have a higher value.

[0003] Softwood timber Is -used as a renewable resource for construction thereof is regarded as sustainable. Also, to some extent defects in softwood timber, compared to hardwood, can be addressed, by processes which alter the molecular composition of the softwood timber or which change the way the timber reacts to exposed conditions.

[8004| Timber is anisotropic and thus is too weak for certain applications. Timber is also hygroscopic and Is susceptible to decay attributable- to natural rot and Insects. Timbe is flammable and may be deformed by varying temperatures and humidity levels. Chemical treatment through Impregnation and heat treatment are forms of timber -modification that attempt to address these shortcomings.

[OO0S Some forms of timber treatment require chemical Impregnation with heavy metals and toxic components such as copper, chrome and arsenic (CCA timber). These techniques are however frowned upon due to environmental and adverse side effects. At least for this feasors attention has been focused on the use of paraffin wax to impregnate timber. The impregnation^' of timber with a wax-like substance causes moisture which is expelled from the timber, to be replaced with the substance. This restricts the likelihood that the moisture level in the timber will rise. This method is known as hyd aphobafion.

[0008] DEI 0160424 discloses a method for impregnating wood, which is still damp from being freshly cut, in a pressure-resistant container at an elevated temperature using paraffin wax and a super-plasticiser. A modification of this concept is described in D £202004021063 which specifies that the moisture content of the timber on its surface must be at least sixty percent. After impregnation wit a suitable agent th moisture content is reduced to twenty percent.

|0007] EP1B62278 describes a technique of timber treatment in which the moisture content of the timber Is reduced to about twenty percent

[0808] The applicant has identified problems associated with impregnation. The current technology has been developed to modify softwood timber that is suitable for use in mild and temperate climates in which there are relatively High humidify levels and fewer rapidly fluctuating changes in atmospheric moisture levels. The modified timber is however not necessaril suitable for use in dryer or warmer climates.

[0008] The wax which is used for impregnation has a melting point below 65°C. In a hot climate the surface temperature of timber may well exceed this level and, If this occurs, the wax may melt producing unwanted side effects. [0010] Of particular concern is the fact that the timber which is to be treated must have a specific moisture content prior to treatment and must be harvested a a specific time of year. The cost of transporting wet off-saw limber is high because of Its moisture content. The timber must be constantly conditioned so that its moisture content remains within a predetermined range. This adds to energy requirements. Additionally, as the timber may- only be cut at a specific time In the year, problems can arise in respect of cash flow, stock reserves and production limits.

10011] Nutrients such as glucose, citric acid and resin, which are principal agents of wood decay, remain in the timber core ar d the impregnation treatment may not be effective for timber which Is exposed in use to insects or water,

[0012] Timber which is impregnated with wax does not change in colour, A consumer may therefore perceive that timber, which has its natural colour, does not have enhanced qualifies. fOQISj Thermal techniques have also been used to modify timber properties. US5687324 describes a process in which timber, in a chamber, is subjected In a controlled manner to various levels of heat and steam. Specific benefits are Identified in this patent.

$0143 Negative factors associated with thermal treatment include the following: the timber can lose its strength and hence its load-bearing capability; the timber may become brittle - this can result in a requirement for expensive and intricate machining techniques and the brittleness may render the limber more susceptible to attack by adverse weather conditions. There is also little evidence to show that thermally treated timber has an enhanced resistance to termite and Insect attack, particularly In subterranean conditions. fOOi S] An object of the present invention is to provide a timber modification process which addresses a number of the aforementioned negative factors. The timber treatment process of the invention is intended to create a product with, low- VOC (volatile organic compound) emission. The invention can be used to improve the qualities of softwoods so that these qualities rival or even surpass the corresponding qualities of hardwoods. More specifically the invention seeks to provide a modified wood product which has enhanced durability and dimensional stability, which exhibits an improved colour, resistance to insect attack, surface hardness and strength, and which possesses, substantially zero toxicity.

SUMMARY OF THE INVENTION

[Q0i$J In general terms the invention provides a method of producing a modified wood product with a number of enhanced characteristics which includes the steps of sourcing a quantity of timber; with selected dimensions, which has a moisture content in the range of 12% to 20 and then - ubjecting the timber to a first stage in which the timber is heat treated and a second stage in which the timber is impregnated with a protective composition.

[6017] During the first stage, under controlled conditions, the timber is thermally processed to reduce its moisture content to 5% to 8%, and to produce a desired colour in the timber, in the second stage the thermally treated timber is cyclically subjected to impregnation treatment, using a suitable protective composition such as a wax based composition which has a meting point in excess of 100°C.

[00181 More specifically,_, the first stage may- include the steps of: (1) subjecting the timber to a temperature in the range of f00°C to 130°C for a period the duration of w ich is dependent at feast on the dimensions of the timber, to reduce the moisture content of the timber to a level in the range of 3% to 8%, typically of the order of

(2) subjecting the timber, in a controlled presence of steam, to a temperature in the range of 1-8GX to 230^*0 for a period the duration of which is dependent, at ieast, on the dimensions of the timber and

(3) gradually cooling the timber, in a controlled presence of steam, during a period the duration of which is. dependent at least on the dimensions of the timber to reduce a core temperature of t e timber to less than 50°C and, preferably, a core temperature of 4Q°C or lower.

[0019] In th first step of the of the first stage, by way of example, for timber which has dimensions of 114 X 26mm, .. the duration of the period may be of the order of 120 to 180 minutes. For timber which has dimensions of 228 X 78m ra the period may range from 240 to 360 minutes. At the end of this step the core temperature of the timber may be of^" the order of 100^;:C.

|002δ| During the second step of the .first stage, by way of example,, timber with dimensions of 1 14 X 25mm is kept in the temperature range of 180Χ· to 230°C for a period which endures from 60 to 180 minutes. An additional factor which is taken into account in determining the duration of the period is the required shade (colour) of the timber. The longer the period the darker the timber will become. O

|0021] The timber may be exposed to steam either continuously or at controiled intervals in order to coo! outer sides of the timber. It has been observed thai significant differences between the surface temperature and the core temperature of the timber may produce surface cracking. By using the steam an outer surface of the timber is cooled and the temperature differentia! between the core and surface is reduced.

[0022] In the third step of stage one the timber may be cooled in a controlled manner for a period which can endure for up to 300 minutes depending on the dimensions of the timber. Steam and water may be used in a continuous or controi ed manner so ^'that at the end of the process the moisture content of the timber is in the range of 5% to 8%.

[0023] in all steps during the first stage air may be circulated past and through the timber in different directions in an attempt to expose all element of the timber to equal amounts of heat and steam,

[0024] n the second stage the timber is subjected to an impregnation process. The second stage may include the following steps:

(1) placing the timber produced by the first stage in a sealed pressure vessel,

(2) creating a negative pressure in the pressure vessel,

(3) raising the temperature in the pressur vessel to vaporise moisture remaining in the timber,

(4) introducing the protective composition into the pressure vessel to cover the timber,

(5) allowing moisture in the vessel to escape to atmosphere, and

(6) sealing the pressure vessel and subjecting the timber and the protective composition to one or more pressure cycles wherein, in each cycle ?

(a) the pressure n the pressure vessel is raised to a value in the range of 7 to 12 bar for a predetermined time period, and

(b) the pressure in the pressure vessel Is reduced to atmospheric level and an interior of the pressu re vessel Is then exposed to atmosphere to allow moisture inside the vessel to escape to atmosphere.

[002S] The number of cycles employed to impregnate the timber with the wax compound depends, at least, on the dimensions of the timber. By way of example timber with dimensions of 114 X 25mm is subjected to from 3 to 8 cycles while timber with dimensions of 228 X 76mm Is subjected to from 10 to 20 cycles, [0026] After step. 6 in the second stage the^' ax compound in the pressure vessel -may be drained to a level below the lowest- level of the timber and, thereafter, the vessel may be sealed, A reduced pressure environment may then be established inside the vessel for a period of from 2 to 5 minutes in order to draw excess wax compound from -the timber.

BRJEFj^OHiFTfO OF THE D WI GS [0027] The invention is- further described -by way of example with reference to the accompanying drawings irt which :

Figure 1 schematically depicts an oven used for heat treatment of timber in accordance with a first stage of a method according to the Invention, and

Figure 2 depicts apparatus which is used in a second stage of the method of the invention to impregnate timber which is produced during the first stage. DESCRIPTION OF PREFERRED EMBODIMENT

[0028] Figure 1 of the accompanying drawings illustrates a thermal treatment oven 10 which includes- a stainless steel housing 12, a plurality o? high speed fans 14, a number of heating elements 16 and temperature sensors 18 located at strategic positions inside the oven.

[0029] The oven is sized to accommodate a stack of timber 20, Steam 22, produced in a remote source, not shown, can be introduced in a controlled manner into the oven.

[0Q30J The oven is built specifically to deal with high temperatures. The housing 12 is made from stainless steel so that it is resistant to highly corrosive resins thai are exuded from timber during heat treatment. The elements 18 are placed in front of the fans 14 to ensure maximum possible radiation and heat efficiency. The fans 14 are designed to circulat heated air at a high speed throughout the oven. This is important in achieving even treatment of the timbe and to ensure that some parts of the stack of timber 20 are not subjected to_: irregular peaks, in temperature. Output signals from the temperature sensors 18 are directed to a proprietary control system which is designed to ensure that all locations inside the oven are subjected to the same degree of heat. Steam and water are provided to help with the cooling of the timber.

10031] Figure ^:2 shows wax Impregnation equipment 3.0- which is used I a second stage of the invention. The equipment includes a pressure vessel 32 which can withstand a working pressure of the order of up to 12 bar and which is designed to accommodate a stack 34 of a required quantify of timber. A plurality of steam coils 36 underlie the stack of timber 34, [0032] A feeder tank 37 which is. used to supply a molten wax-based protective compound. 33 to the pressure vessel Is coupled to the pressure vessel via a positive displacement pump 40. The feeder tank 37 s higher than the pressure vessel so that a gravity feed to the pressure vessel Is possible.

[S033] A monitor 42 is provided to measure the level 38A of the molten wax compound 38 In the pressure vessel.

[0034] The equipment 30 includes two mufflers 44 and 46 respectively and a cooling towe 48 to ensure that air is cooled sufficiently to allow a vacuum pump 50 to work efficiently.

I0O3S1 The equipment 30 also includes a number of safety and regulating valves, not shown, to ensure thai operating conditions: of the equipment are kept within design capabilities.

[0038] In the Implementation of the method of the Invention timber 20 is sourced directly from a saw mil! at which the timber has undergone a kiln-drying process so that the moisture content of -the timber l es lo the range of 12% to 20%, South African grown pine has been found to be suitable for use in the method of the invention. Generally the more dense and mature the timber the better Is the outcome of the method of the Invention.

[0037| It Is known that the dimensions of the timber can impact on the treatment process but optimal treatment Is readily effected on timber with dimensions ranging from 114 X 25mm to 228 X 76mm. 0038] In a first heat treatment step of a first stage, carried out in the oven 10, the timber 20 Is placed in a stack which is fastened with straps to restrict timber movement during treatment. The timber is then fully dried at a temperature of 100^*0 to 130°C for a period of time that is determined by the timber dimensions. For example (this is non-limiting), timber with dimensions of 1 14 X .25mm needs to be conditioned for from 120 to 180 minutes, while timber with dimensions of 228 X 76mm is conditioned for a period of from 240 to 360 minutes^, The conditioning of the timber requires that the moisture content must be reduced to 5% or below, and that the internal core temperature of the timber reaches a value of about 100°C.

[003.9] In a second step of the first stage, the timber is subjected to a temperature of from 180°C to 230°C for a period which is determined by the dimensions of the timber. Generally timber of 114 X 25mm Is subjected to this elevated temperature for a period of from 60 to 180 minutes, depending on the degree of shading to be imparted to the timber. The longer the timber is kept at an elevated temperature the darker the timber becomes. Timber with dimensions of 22S X 78mm is held at the elevated temperature referred to for a period of from 180 to 240 minutes, again depending on the colour that Is required.

[O0 OJ During the second step, steam Is introduced into the oven at various Intervals. This is done in order to saturate cooler outer sides of the timber slightly, it has: been: observed that differences between the surface temperature and the core temperature of the timber can cause surface cracking. The steam effectively cools, the outer sides of the timber and helps to reduce the temperature differential

[0041] In a third step of the first stage, the timber is cooled in a controlled manner. Steam and water are used to reduce the core temperatur of the timber gradually. The third step can endure for up t 300 minutes- depending on the dimensions of the timber. The third step is regarded as having been completed when the core temperature of the timber reaches a value of 40°C or lower. The steam and water wet the timber and, at the end of the third step, the timber has a moisture content of from 5% to 8%.

[00421 As noted the direction of air How induced by the fans 14 Is varied at intervals to ensure even heating and dispersion of steam throughout the timber stack.

[C104SJ The equipment shown in Figure 2 is used, in a second stage of the method of the Invention, to Impregnate the timber.

£0044] The heat treated timber 20_» produced in the first stage, is placed in the form of the stack 34 in the pressure vessel 32 which is. then sealed, A pressure regulating valve Is closed and a negative pressure environment (vacuum) is established in the pressure vessel The steam coils 38 are used to increase the temperature inside th pressure vessel to about i00°C. This temperature is high enough to vaporise most of the moisture remaining in the timber.

PCN-S] At this, point a sufficient quantity of the molten wax compound 38 Is introduced into the pressure vessel, up to a level 38A, so that the timber stack 3 is completely covered. Typically the temperature of the molten wax compound, is of the order of 130^oC. nitially the pressure vessel is exposed to atmosphere so that excess water in the pressure vessel, arising from the heated timber, Is driven off,

[0046J In the formulation of the wax-based compound use is made of a blend of saturated straight chain hydrocarbons with almost no branches. This has a linear structure as opposed to the branch structure of paraffin waxes that the applicant has noted are used in prio art processes. The blended wax-based compound is inherently stable and ca be exposed to heat for long periods of time without any deterioration in its properties. The wax-based compound includes at least four waxes and, optionally, a synthetic resin, an anti-oxidant and an anil-foaming agent. The waxes employed include a first synthetic wax with a high melting point and a low acidic value, a second synthetic wax with a medium acidic value which, it has been found, Imparts shine to the end product, a third synthetic wax with a high acidic value which gives the product tack and reduces slipping, and a microcrysialllne wax which binds the blended wax compound to the timber and reduces blooming and leaching, particularly during use under high temperatures.

[004?] Use is optionally made of a synthetic resin which bonds with the timber, acting as an adhesive, to reduce blooming and leaching. [00 83 Irs order to impregnate the timber with the wax compound the timber is subjected to a series of pressure cycles, In each cycle the pressure in the vessel is raised to a value which lies in the range of 7 to 12 bar. Each cycle endures for a period of up to 30 minutes. Thereafter the pressure is released and the pressure vessel is opened to atmosphere to allow moisture to escape. Th cycle Is. then repeated. The number of cycles required to impregnate the timber completely depends on the timber dimensions. By way of non - limiting example timber that has dimensions of 1 14 X 25mm requires from 3 to 8 cycles, while timber that is 228 X ?6mm requires from 10 to 20 cycles.

£004$] After the cyclical impregnation process has been completed the molten wax compound In the pressure vessel Is drained to below the level of the stack 34. The vessel is then sealed and a negative pressure environment is established for a period of from 2 to 5 minutes. This process draws excess wax compound from the timber,

£0050] The method of th invention, applied for example to softwood, produces a product which Is as durable as many forest hardwoods. The product can be used as a durable timber in outdoor applications. The product can be given the appearance of a dark hardwood - this opens up the application of th product of the invention to situations which were only previously available for hardwoods.

[0051] The end product has a greatly increased density which reduces its femrnability. The unique combination of waxes and resins ensures that the end product is non-slippery. Also, components in the wax compound are FDA approved and have zero toxicity. Thus the end product is huma and pet friendly and, when necessary, disposal of the product can be done in the same way as ordinary untreated timber,

[00521 When the timber is removed from the pressure vessel: its modulus of elasticity is initially relatively iow. The timber can thus be bent, or shaped to be as straight as possible. As the wax compound hardens inside the timber, the shape of the timber is preserved.

[00S3! The timber also has improved strength and load bearing capacity. It is also unattractive to insects, as they cannot digest the long chained linear molecules which are present in the compound.

Claims

CLAI S

1. A method of producing a modified wood product which includes the steps of SQurcing a quantity of timber, with selected dimensions, which has a moisture content, in the range of 12% to 20%; subjecting the timber to a first stage in which the timber is heat treated; and subjecting the timber to a second stage in which the timber is impregnated with a protective composition.

2. A method of producing a modified wood product according to claim 1 wherein the protective composition is In the form of a wax-based composition which has a melting point in excess of 1QD°C.

3. A method of producing a modifie ^'wood product according to claim 1 or 2 wherein during the first stage the timber Is thermally processed to reduce its moisture ..content to between 5% to 8%, and to produce a desired colour in the timber.

4. A method of producing a modified wood product according to any one of claims 1 to_.

3 wherein during the second stage the thermally treated^' timber is cyclically subjected to impregnation treatment, using the protective^' composition.

5. A method of producing a modified wood product according to any one of claims 1 to

4 wherein the first stage includes:

1} a first step of subjecting the timbe to a temperature in the range of 100°C to 13CTC for a period, the duration of which is dependent at least on the dimensions of the timber, to reduce the moisture content of the timber to a ievel in the range of 3% to 8%, is) a second step of subjecting the timber, in a controlled presence of steam, to a temperafyre in the range of 180°C to 230°C for a period the duration of which is dependent, at least, on the dimensions of the timber: and

lis) a third step of gradually cooling the timber, in a controlled presence of steam, during a period the duration of which is dependent at least or? the dimensions of the timber to reduce a core temperature of the timber to less than 50^eC.

A method of producing a modified wood product according to claim 5 wherein during the first step the moisture content of the timber is reduced to a levei in the order of 6%.

A method of producing a modified wood product according to claim 5 or 8 wherein the duration of the period during the first step Is determined by th required shad of the timber.

A method of producing a modified wood product according to claim 7 wherein during the first step, timber with dimensions of 114 X 25mm is kept in the temperature range of 100 to 130 for a period which ranges from 120 to I SO minutes.

A method of producing a- modified wood product according to any one of claims 5 to. 8 wherein during the second step, timber with dimensions of 114 X 25mm Is Kept In the temperature range of 18CTC to 230*G for a period which ranges from 80 to 180 minutes,

A method of producing a modified wood product according to claim 7 wherein during the first step, timber with dimensions of 228 X 76mm is kept in the 18

temperature range of 100^eC to 130°C for a period which ranges from 240 to 360 minutes,

11. A method of producing a modified wood product according to any one of claims 5 to

10 wherein at the end of the first step, the core temperature of the timber is in the order of 10Q .

12. A method of producing a mod fied wood product according to any one of claims..5 to

11 wherein during the second step, the timber is exposed to steam in order to cooi outer sides of the timber.

13. A method of producing a modified wood product according to any one of claims 5 to

12 wherein during the third step, the timber is cooled in a controlled manner for a period of up to 300 minutes depending o the dimensions of the timber.

14. A method of producing a modified wood: product according to claim 13 wherein during the third step, the core temperature of the timbe is reduced to 4D°C or lower.

15. A method of producing a modified wood product according to claim 14 wherein steam and water are used so that th moisture content of the timber is in the range of 5% to ,8% at the end of the third step,

16. A method of producing a modified wood product according to any of claims 1 to 15 wherein during the first stage ai is circulated past and through the timber in different directions in a attempt to equally expose all the timber to heat and steam. A method of producing a niodsfied wood product according to any one of claims 1 to 16 wherein the second stage includes the steps of:

i) placing the timber produced by the first stage in a sealed pressure vessel;

ii) creating a negative pressure the pressure vessel;

m) raising the temperature in the pressure vessel to vaporise moisture remaining in the t mber;

Iv) Introducing the protective composition into the pressure vessel to cover the timber;

v) allowing moisture in the vessel to escape to atmosphere; and

vi) a) sealing the pressure vessel and subjecting the timber and the protective composition to one or more pressure cycles wherein, in each cycle a pressure In the pressure vessel is raised to a value in the range of 7 to 12 ba for a predetermined time period; and

b) the pressure in the pressure vessel is reduced to atmospheric level and an interior of the pressure vessel is then exposed to atmosphere to allow moisture inside the vessel to escape to atmosphere.

A method of producing a modified wood product according to claim: 17 wherein during step (vl)(a) the number of cycles to impregnate the timber with the protective compound is determined by taking into account at least the dimensions of the timber.

A. method of producing a modified wood product according to claim 18 wherein timber with dimensions of 114 X 25mm Is subjected to between 3 to 8 cycles of impregnation. A method of producing a modified wood product according to claim 18 wherein timber with dimensions of 228 X 7Smm is subjected to between 10 to 20 cycles of impregnation.

A method of producing a modified wood product according to claim 19 or 20 wherein after step yi)(a) and (v1)(b) the protective composition in the pressure vessel is- drained to a level below the lowest level of the timber and, thereafter, the vessel is sealed.

A method of producing a modified wood product according to claim 21 which is followed by a step of establishing reduced pressure environment inside the vessel for a period of from 2 to 5 minutes in order to draw excess protective composition from ^'the limber.