AU713360B2

AU713360B2 - Process for enhancing wood preservation

Info

Publication number: AU713360B2
Application number: AU30551/95A
Authority: AU
Inventors: Peter Raynor Soundy Cobham
Original assignee: CHEMICCA Ltd; University of Melbourne
Current assignee: University of Melbourne; Osmose Australia Pty Ltd
Priority date: 1994-09-12
Filing date: 1995-09-11
Publication date: 1999-12-02
Anticipated expiration: 2015-09-11
Also published as: AU3055195A

Description

Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

Name of Applicants: The University of Melbourne and Chemicca Limited Actual Inventor: Address for Service: Invention Title: COBHAM, Peter Raynor Soundy DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

"Process for Enhancing Wood Preservation" Details of Associated Provisional Application No.: PM8064/94 The following statement is a full description of this invention, including the best method of performing it known to us: -1la- PROCESS FOR ENHANCING WOOD PRESERVATION The present invention relates generally to a process for enhancing the penetration of preservatives and/or other additives into wood. In particular, the present invention relates to a process which is performed at low pressures.

Existing processes used for treating wood with preservatives include the Bethell, Lowry, Reuping and Mississippi State University (hereinafter referred to as "MSU") processes.

The Bethell process involves using an initial vacuum to remove air from the wood cells and then flooding a cylinder loaded with the wood under vacuum. Positive pressure is then applied, the preservative solution is removed and a final vacuum is drawn.

In the Lowry process, no initial vacuum is applied and the cylinder is flooded under atmospheric pressure. Positive pressure is then applied for a predetermined period, the cylinder is then drained and a final vacuum drawn. The preservative uptake 2 is lower because the air is not removed from the cells.

The Reuping process involves applying positive air pressure and then flooding the cylinder holding this initial air pressure. Positive pressure is then applied and after a predetermined time, the pressure is released and the cylinder drained. A final vacuum is then drawn. This process has a lower uptake than both the Bethell and Lowry processes.

The MSU process is a modification of the Reuping process. The Reuping process is carried out and the cylinder is then drained maintaining a positive pressure slightly above the initial air pressure. Heat is then applied to fix the copper-chromearsenic (hereinafter referred to as "CCA") preservative. After the fixation period, kickback is allowed to occur and a final vacuum is drawn.

95091 1,p:\oper\dab,WOOD.SPE, 1 -2- Pulsation or processes which cycle pressure have also been used to improve the treatment of relatively impermeable wood. Specialised treatment schedules have been developed involving oscillating, alternating or pulsation pressures to improve penetration and hence treatment of impermeable wood. Some of these processes involve higher pressures than is used in conventional treatment plants.

These processes involve rapid changes in pressure and it is believed that this causes a greater pressure difference through obstacles within the wood, while the total pressure within the wood increases slowly allowing the preservative to enter small pores.

Care must be taken using high pressure treatments as the wood cells are likely to collapse.

The oscillating pressure method (hereinafter referred to as "OPM") is suitable for treating wood species such as spruce which are difficult to treat once dry. The process is carried out with an oscillating change of pressure between vacuum and pressure. The pressure range is -93 kPa (gauge) to 600-1500 kPa (gauge). During the pressure phases of the process, preservative solution is forced into the wood where it mixes with the wood sap. During the vacuum cycles, air entrapped in the wood expands, forcing a mixture of wood-sap preservative and air out of the wood. As the cycles continue their is a gradual replacement of wood sap in the wood with preservative solution.

The wood to be treated must be sap fresh (green) meaning the moisture content **must be above fibre saturation in all parts of the sapwood. Air must be present to expand during the vacuum phase and escape from the wood so that the sap can be sucked out of the wood and the impregnating solution pressed into it.

The OPM can be carried out on easy to treat species, such as pine in semi-dry or fully dry condition. The time to treat air dry poles by the OPM is two to four hours i compared to 14 to 18 hours for sapfresh pine poles. For dry wood, the OPM gives approximately the same results as the Bethell process. Considerably improved impregnation is obtained on unseasoned wood.

95091 1 p:\oper\dab,WOOD.SPE,2

I

-3- In New Zealand, the OPM has been successfully used to treat pine species after steam conditioning. It had been found that freshly cut pine was too saturated to be treated green by the OPM.

The OPM process was modified in New Zealand to exclude the vacuum phase.

The resultant process which is known as the Alternating Pressure Method (hereinafter referred to as "APM") is a number of cycles at pressure from 0 to 1400 kPa (gauge).

This is equivalent to a series of Lowry empty cell treatments.

The APM is possible because of the action of steam preconditioning. Species used in New Zealand with the APM are P.radiata and P.nigra.

Initial APM schedules required one hour cycling for every inch on sapwood depth. Later research showed that 15 cycles was sufficient for complete sapwood penetration. The heartwood of sawn timber is treated partially by cycling and further by maintaining the final cycle on pressure for an extended time. The cylinder is flooded without an initial vacuum and then the APM cycles are 1 to 2 minutes on pressure at 1400 kPa (gauge) and 1 minute off pressure.

20 The pulsation process is a further modification of the OPM. It was developed to increase the penetration in refractory species like white spruce. Pulsation trials using both creosote and water-borne CCA have been conducted with white spruce roundwood and sawn timber.

The pulsation process alternates between high and low pressures of 300 kPa (gauge) to 2100 kPa (gauge). The 2100 kPa (gauge) is well above the normal pressures used for treating wood. The aim of pulsation is to treat refractory species. These species may also be prone to collapse.

Pulsation is based on the Reuping process with a sequence as follows: Initial air pressure of 350 kPa (gauge).

Cycling between 350 kPa (gauge) and 2100 kPa (gauge). Some of the 95091 1,p:\oper\dab,WOOD.SPE,3 -4schedules involve increasing the pressure to 2100 kPa (gauge) over several cycles i.e. first to 1000 kPa (gauge), second to 1200 etc. up to 2100. This slow rise is to minimise collapse caused by the high pressure.

The cylinder is then drained and a final vacuum drawn.

Total treatment time varies between 7 and 20 hours depending on the number and duration of the cycles. Improvement in the treatment of refractory spruce has been achieved.

The Fast process was developed in New Zealand to increase productivity in treatment plants. The process involves the use of 5 cycles of pressure from 0 to 1400 kPa (gauge), i.e. a short APM. However, instead of using steam preconditioned timber, air dried or kiln dried timber is treated.

Less time is taken in treating the timber because there is no initial vacuum. The process was validated by carrying out trials with matched samples treated by a Bethell process. It was found that there was no significant difference in penetration or retention between the Fast and Bethell processes. The fast process is now used by a number of 2 plants in New Zealand.

S• The problems with the existing processes is that there is a high potential for °"*kickback after treatment which results in considerable drip and sludge formation. Drip or kickback is when contaminants, such as, water soluble wood sugars come out of the wood with the preservative when pressure is removed. The presence of drip means that the time for fixation is longer and causes significant environmental problems.

S. *We have now found that drip is due to high residual pressure built up in the wood due to the use of high pressures in the existing processes. A requirement accordingly exists for a process in which drip is eliminated or substantially minimised.

According to the present invention there is provided a process for enhancing the penetration of preservatives and/or other additives into wood which comprises the steps 950911 ,p:\oper\dab,WOOD. SPE,4

I

P:\OPER\AXD\1773447RS2 6/10199 of: steam drying the wood; and immersing the wood in a treatment solution and subjecting the wood to multiple pressure cycles, wherein each of said pressure cycles comprises raising the pressure from a minimum pressure of about 0 kPa (gauge) to a maximum pressure of about 700 kPa (gauge) or less and decreasing the pressure to about 0 kPa (gauge).

The present invention also provides wood whenever treated by the process described in the immediately preceding paragraph.

The preservatives may be waterborne or oilborne preservatives. Preferably the preservatives are waterborne and may be selected from chromium, copper and/or arsenic containing preservatives, for example, copper-chrome-arsenic (hereinafter referred to as "CCA") or oxides or salts thereof, acid copper chromate or chromated zinc chloride; ammoniacal 15 preservatives, for example, ammoniacal copper arsenate, ammoniacal copper zinc arsenate, ammoniacal copper carboxylates, ammoniacal copper dithiocarbamates or ammoniacal copper citrate; boron compounds, for example disodiumoctaborate tetrahydrate or zinc borates; or alkylammonium compounds or "quats", for example, ammoniacal copper quats.

20 The wood may also be treated with other additives either before, after or simultaneously with the preservatives. These other additives may include water repellents, such as, waxes, resins or polymers, for example, polyethylene glycol; fire retardants, such as, phosphates; mildewicides; insecticides; mouldicides; dyes or pigments.

The wood may be any timber or wood based product, such as, refractory timber, softwoods or hardwoods. The softwood may include pine species, such as, P. Radiata, for example, heartwood or sapwood. Heartwood is the most difficult part of P. Radiata to treat with preservatives. The hardwoods may include eucalypts or spruce species.

1 P:\OPER\AXD\1773447.RS2 6/10/99 -6- The wood is pre-conditioned by steaming. This drying technique is advantageously more economical than other drying techniques, such as air or kiln drying, as it can generally be achieved in much less time than air or kiln drying. The steaming may be carried out on green or partially dry wood, preferably at a temperature above about 100 C.

The pre-conditioned wood is subjected in step to multiple pressure cycles each comprising raising the pressure from a minimum pressure of about 0 kPa (gauge) to a maximum pressure of about 700 kPa (gauge) or less, preferably about 550 kPa (gauge), more preferably to about 350 kPa (gauge). The pressure cycles are preferably applied over a time period of about 1 to about 5 minutes on pressure and about 30 seconds to about 2 minutes off pressure.

Preferably, up to 15 pressure cycles are used.

o After the pressure step a vacuum of about -85 kPa (gauge) is preferably drawn and held for about 15 to about 40 minutes so as to remove any residual preservative.

The process can be used to enhance the penetration of preservatives and/or other additives in various types of fixation processes, such as, for example a two stage fixation process in which fixation follows treatment. In the two stage fixation process, the fixation step may be achieved by allowing the wood to sit at ambient temperatures for a prolonged period of 20 time or the fixation may be accelerated. An accelerated fixation step generally involves the use of heat from various sources including steam, hot water, kiln or hot oil.

The use of low pressures in the process of the invention means that substantially no residual pressure is built up in the wood which reduces the possibility of kickback out of the treated wood thereby eliminating or substantially reducing drip. This will in turn minimise contamination of the preservative during treatment and reduce the potential of sludge formation resulting from contaminated preservatives and/or other additives. In summary, the use of low pressures confers the following advantages: lower residual pressure within the wood which means less kickback leading to less drip from the wood; -7reduced potential for sludge production because of the smaller volume of kickback after each pressure cycle; and reduction in weight of the treated product after treatment.

When the process of the invention is used to enhance the penetration of preservatives and/or other additives in a two stage fixation process, the use of low pressures confers the following additional advantages: shorter fixation time due to a lower residual pressure in the wood; fixation can be carried out both in blockstack and fillet form as kickback will not accumulate between the layers of wood; lower residual pressure within the wood which means less drip from the wood when heat is used to accelerate fixation; better visual appearance; improved heartwood penetration due to heating of the preservative; :ooo incomplete saturation of the wood; and increasing the concentration of preservative did not result in higher leaching from the wood.

The economical advantages of the process of the present invention are also substantial. A reduction in the weight of the treated wood of up to about 10% can be .9 9• achieved thereby reducing the cost of transport. As the wood treated by the present *process is substantially drip free, the fixation time is not as long and the benefits to the environment are substantial. Furthermore, the heat transfer is faster when an accelerated fixation step is used in a two stage fixation process as the wood is not fully saturated with the preservative. This process also minimises the costs involved in disposing of preservative contaminated sludge.

The process may be conducted using any suitable apparatus, such as, for example, apparatus used in the existing APM.

A preferred plant operation of the process of the invention will now be described 95091 1,p:\oper\dabWOOD.SPE,7 8 by way of example only and with reference to the accompanying drawing in which: Figure 1 shows a conceptual design of a plant for operating the process of the invention.

The waterborne preservatives are agitated with valve 7 open and the agitation pump 13 on. Heating may be achieved either by an in-tank heater or a heat pump in the agitation line. Agitation of the storage tank 8 is continuous.

A pressure cylinder 9 is loaded with wood and the door 12 closed and sealed. The pressure cylinder 9 is then flooded with preservatives. Once the cylinder 9 is flooded, pressures such as 0 to 700 kPa (gauge) are applied using a high volume pressure pump 11 with valve 5 open. A pressure control valve 3 maintains the required pressure. The presence of the high volume pump 11 means that there is constantly fresh solution passing through the pressure cylinder 9 treating and heating the wood. Pressure is released via the vacuum control valve 1 to ramp down the pressure to 0 kPa (gauge).

Once treatment has been completed, there are two alternatives for draining the 20 pressure cylinder 9 as follows: closing valve 5, opening valves 3, 4 and 6 and using the high volume pressure pump 11 to pump the cylinder dry; or using the vacuum pump 10 as an air compressor so that the liquid can be blown out of the pressure cylinder 9 via a line using valve 6 and by-passing the 25 pump.

-9- The advantage of using alternative is that the pressure cylinder 9 can be emptied at the same pressure as it was treated or at a positive pressure meaning that any kickback can be segregated. The kickback can then be collected after final vacuum and cleaned up prior to returning to the storage tank 8.

After draining, all the valves are closed apart from valve 2 and a vacuum such as -80 to -98 kPa (gauge) drawn on the pressure cylinder 9. After a predetermined time, the vacuum is vented through valve 1 and any residual liquid is then cleaned and/or recycled.

The door 12 is then opened and the treated timber removed for storage under cover until it is despatched. A short holding period may be required before the wood leaves the treatment containment area. If a two stage fixation process is involved, then the wood is transferred to a secondary fixation system.

The invention will now be described with reference to the following Examples.

These Examples are not to be construed as limiting the invention in any way.

Example 1 0 Six Pinus radiata Don.) posts were steamed at 1271C (137 kPa (gauge)).

The posts were 150 mm in diameter with a heartwood diameter of 50 mm, therefore the steam time was 2 hours 40 minutes. After steaming, the posts were allowed to cool for 24 hours. They were then cut into 3 pieces and end sealed with epoxy resin.

The posts were then divided into three treatment charges with matching samples in each. Two charges were carried out with the following schedule: Flood the cylinder under atmospheric pressure.

95091 1,p:\oper\dab,WOOD.SPE,9 Apply 15 cycles of pressure from 0 to 350 kPa (gauge). The time on pressure was 1 minute, 10 seconds was allowed for pressure release, then 1 minute off pressure.

At the end of the cycling, the preservative was pumped out of the treatment vessel.

A vacuum was then drawn of -85 kPa (gauge) and held for 15 minutes.

The third charge was subjected to a traditional APM as follows: Flood the cylinder under atmospheric pressure.

Apply 15 cycles of pressure from 0 to 1400 kPa (gauge). The time on pressure was 1 minute, 10 seconds was allowed for pressure release, then 1 minute off pressure.

A vacuum was then drawn of -85 kPa (gauge) and held for 15 minutes.

The preservative used was CCA salt.

After treatment the posts were assessed for weight gain, percentage sapwood :penetration and drip.

Each charge was left on a collection pan to collect drip. For the two low pressure charges there was no drip or solution released from the samples.

The APM showed considerable drip from kickback of the preservative.

Spot testing with 5% ammonia followed by a solution of rubeanic acid (copper spot test) showed complete sapwood penetration for all three charges.

Weight gain, on average the low pressure process showed a 7% lower weight gain compared to the high pressure APM.

95091 l,p:\oper\dab,WOOD.SPE, 1O 11 Treatment time for the low pressure process was 24% less than the traditional

APM.

Example 2 Twelve matched Pinus radiata (D.Don.) sawn (100 x 50) samples were steamed at 127 0 C (137 kPa (gauge)), high temperature kiln dried (130/70 0 C 18 hours), conventional kiln dried (70/50°C 60 hours) and air dried.

The samples were mixed heartwood and sapwood.

Steaming time was 1 hour and twenty minutes at temperature. After steaming, the samples were allowed to stand for 14 days. They were then cut into 4 pieces and end sealed with epoxy resin.

:o The samples were then divided into four treatment charges with matching samples in each. Two charges were carried out with the following schedule.

The treatment process used was: Initial vacuum of -85 kPa (gauge) held for 5 minutes.

S Cylinder flooded maintaining -85 kPa (gauge).

Pressure of 350 kPa (gauge) applied over 5 minutes.

Pressure of 350 kPa (gauge) maintained for 40 minutes.

A vacuum was then drawn of -85 kPa (gauge) and held for 15 minutes.

The third charge followed a low pressure cycling process.

Flood the cylinder under atmospheric pressure.

Apply 15 cycles of pressure from 0 to 350 kPa (gauge). The time on pressure was 1 minute, 10 seconds was allowed for pressure release, then 1 minute off pressure.

95091 ,p:\oper\dab,WOOD.SPE, 1 M M 12- At the end of cycling the preservative was pumped out of the treatment vessel.

A vacuum was then drawn of -85 kPa (gauge) and held for 15 minutes.

The fourth treatment was a Bethell process at 1400 kPa (gauge).

Initial vacuum of -85 kPa (gauge) held for 5 minutes.

Cylinder flooded maintaining -85 kPa (gauge).

Pressure of 1400 kPa (gauge) applied over 5 minutes.

Pressure of 1400 kPa (gauge) maintained for 40 minutes.

At the end of the cycling the preservative was pumped out of the treatment vessel.

A vacuum was then drawn of -85 kPa (gauge) and held for 15 minutes.

The preservative used was CCA salt.

After treatment the samples were assessed for weight gain, percentage sapwood penetration and drip.

Each charge was left on a collecting pan to collect drip. For the three low pressure charges there was no drip or solution released from the samples. The fourth 1400 kPa (gauge) charge showed considerable drip particularly from the exposed heartwood.

Spot testing with 5% ammonia followed by a solution of rubeanic acid (copper spot test) showed complete sapwood penetration for all three charges. Heartwood penetration was sufficient in the kiln dried samples but poor in the air dried samples.

There was an average 8% weight reduction between the 350 kPa (gauge) treatment and the 1400 kPa (gauge) treatment. The cycling process showed a 12% weight reduction.

A hot water fixation was carried out after treatment (within 30 minutes of 95091 1,p:\opcrdab,WOOD.SPE,12 -13 treatment). The fixation schedule was 2.5 hours at 85 0 C. The higher pressure treatment showed surface sludge due to the kickout of preservative.

By comparison the low pressure treatments showed good surface colour. Even after this time at temperature the higher pressure treated samples were showing further kickback drip from the heartwood surface.

Example 3 0 Four charges of 3.5% CCA oxide treated Pinus radiata steamed round wood were carried out at ambient temperature. The pressure used for each cycle was 550 kpa. The results are shown in Table 1 below.

TABLE 1 Charge No. Process Time Uptake Usage Appearance (min) (1/m 3 (Kg) 1 10 cycles 99 106 107 Slight drip min vac 2 16 cycles 115 128 116 Slight drip min vac 3 10 cycles 110 92 89 Drip free 40 min vac 4 16 cycles 107 145 121 Slight drip min vac Boring samples were taken from these charges mixed, ground together and analysed for Copper, Chromium and Arsenic using Atomic Absorption Spectroscopy.

The results are shown in Table 2 below.

S

95091 l,p:\oper\dab,WOOD.SPE,13

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14- Mass/Mass results TABLE 2 Cu w/w 0.21 0.21 Cr w/w 0.36 0.36 As w/w 0.24 0.24 Total Active Elements w/w 0.81 0.81 Sample Duplicate Sample The total active elements complies with the Australian Standard 1604 (1993) and New Zealand Stand and MP 3640 for wood preservation of 0.63% w/w.

Throughout the specification and the claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

e 95091 l,p:\oper\dab,WOOD.SPE,14

Claims

1. A process for enhancing the penetration of preservatives and/or other additives into wood which comprises the steps of: steam drying the wood; and immersing the wood in a treatment solution and subjecting the wood to multiple pressure cycles, wherein each of said pressure cycles comprises raising said pressure from a minimum pressure of about 0 kPa (gauge) to a maximum pressure of about 700 kPa (gauge) or less and decreasing said pressure to about 0 kPa (gauge).

2. A process according to Claim 1, wherein the pressure cycles from a minimum pressure of about 0 kPa (gauge) to a maximum pressure of about 550 kPa (gauge).

3. A process according to any one of the preceding claims, wherein the pressure cycles from a minimum pressure of about 0 kPa (gauge) to a maximum pressure of about 350 kPa (gauge). S. 15 4. A process according to any one of the preceding claims, wherein the pressure cycles are applied over a time period of about 1 to about 5 minutes on pressure and about seconds to about 2 minutes off pressure.

5. A process according to any one of the preceding claims, wherein up to pressure cycles are used. 20 6. A process according to any one of the preceding claims, wherein the drying is carried out on green or partially dry wood.

7. A process according to any one of the preceding claims, wherein the steam drying is carried out at a temperature above about 100'C.

8. A process according to any one of the preceding claims, wherein the preservatives are waterborne or oilborne preservatives. 16

9. A process according to Claim 8, wherein the waterborne preservatives are selected from chromium, copper and/or arsenic containing preservatives, ammoniacal preservatives, boron compounds and alkylammonium compounds (quats). A process according to Claim 9, wherein the chromium, copper and/or arsenic containing preservatives are copper-chrome-arsenic (CCA) or oxides or salts thereof, acid copper chromate or chromated zinc chloride.

11. A process according to Claim 9, wherein the ammoniacal preservatives are ammoniacal copper arsenate, ammoniacal copper zinc arsenate, ammoniacal copper carboxylates, ammoniacal copper dithiocarbamates or ammoniacal copper citrate.

12. A process according to Claim 9, wherein the boron compounds are disodiumoctaborate tetrahydrate or zinc borates.

13. A process according to Claim 9, wherein the alkylammonium compounds are 20 ammoniacal copper quats. S

14. A process according to any one of the preceding claims, wherein the wood is treated with other additives either before, after or simultaneously with the preservatives. 25 15. A process according to Claim 14, wherein the other additives are water repellents, fire retardants, mildewicides, insecticides, mouldicides, dyes or pigments.

16. A process according to Claim 15, wherein the water repellent is a wax, resin or polymer.

17. A process according to Claim 16, wherein the water repellent is polyethylene glycol.

18. A process according to any one of Claims 15 to 17, wherein the fire retardant is a 17 phosphate.

19. A process according to anyone of the preceding claims, wherein the wood is a timber or wood based product. A process according to any one of the preceding claims, wherein the wood is softwood or hardwood.

21. A process according to any one of the preceding claims, wherein the wood is a pine species.

22. A process according to Claim 21 wherein the pine species is P. Radiata. S

23. A process according to any one of Claims 1 to 20, wherein the wood is a eucalypt or spruce species. **oo

24. A process according to any one of the preceding claims, further comprising (c) 20 applying a vacuum of about -85kPa (gauge) to the treated wood for a period of about to about 40 minutes. o A process for enhancing the penetration of preservatives and/or other additives into wood substantially as hereinbefore described with reference to the Examples and/or drawing.

26. Wood whenever treated by the process according to any one of the preceding claims. Dated this 27th day of AUGUST, 1999 THE UNIVERSITY OF MELBOURNE and CHEMICCA LIMITED By Its Patent Attorneys DAVIES COLLISON CAVE