METHOD FOR TREATMENT OF CREOSOTE IMPREGNATED WOOD TO SUBSTANTIALLY REDUCE OR AVOID BLEEDING

Abstract: The invention relates to a method for treatment of creosote impregnated wood in order to substantially reduce or avoid bleeding, the wood being treated with air at a pressure above atmospheric before impregnation. The method is characterized in that after the creosote impre­ gnated wood has been stored at least until the initial air pressure has been balanced it is subjected to a heat treatment in a heat medium, a vacuum being simultaneously applied.

Description: Method for treatment of creosote impregnated wood to substantiallx reduce or avoid bleeding This invention relates to treatment of creosote impregnated wood and especially to a method of treatment thereof to substantially reduce or avoid bleeding. Impregnation of wood, such as telegraph poles, electric power transmission poles, piles, sleepers etc, with creosote oil is the method used most frequently at present for protecting the wood from attacks by mould fungi and other organisms as well as vermin. In spite of the development of other impregnating agents having a fungicidal effect, which are water-based and contain e.g. compounds of arsenic, copper and chromium, the creosote oil is the impregnating agent mostly used today for wood products in contact with the ground. In addition to the fact that it provides a perfectly satisfactory protection for a very long time the creosote oil has the further advantage that it improves the mechanical resistance of the wood. At pressure impregnation of wood with creosote oil the conventional method according to Roping is used, which means that the wood is given an initial air pressure (of up to 3.4 x 105 Pa) before impregnation. The object of this method is to reduce the consumption of creosote oil and therefore the method is called "Rupings economy method". 13y this method it is achieved at the following impregnation with creosote oil that the walls of the cell cavities are covered while the cavities largely remain unfilled. In this way the amount of creosote oil is reduced. For example it can be mentioned that at impregnation of poles of fir, using the above -mentioned economical method, about 135 kg of creosote oil per m3 of sap are used up while at complete impregnation considerably greater quantities of oil should be required. It is the object of the invention to solve the problem with bleeding of creosote implegnated wood. The expression "bleeding" then refers to the fact tht the creosote oil migrates or spreads from tlie interior of the pole to;Jards its periphery and oozes out on the outside of the pole resultvlg in that this will be sticky and thus ulll)lcas.ant and difficult to handle. Many attempts have been made to solve the bleeding problem. To this end it 1035 been suggested to store creosote impregnated poles in ater basins. With a knowledge of the fact that the storage space of impregnation poles requires an arca of about 60 000 at a big impregnation plant this solution is economically unrealistic. Another way of solving the problem with bleeding of creosote impregnated poles has mcant that the outermost layer of the poles has been peeled off. However, for reasons easy to understand this method cannot be carried out neither industrially nor economically. The method is based on the experience that a very high concentration of creosote oil is found in the outer layers after impregnqtion. In spite of the peeling made the poles will, however, bleed, and therefore the method is little successful merely for this reason. Although the French patent S08 624 does not solve the bleeding problem the contents thereof will be discussed in brief. This patent relates to washing of creosote impregnated ood by treatment thereof with water vapour at 120 - 130 C for 0.5 - 2 h. Obviously the prerequisite of obtaining favourable results by. this method is that all excess of creosote oil has migrated to the outer surface of the wood before washing is carried out. Thus, this patent does not solve the bleeding problem. The only solution that can at present be practically carried out is an enough long storage of the wood after impregnation. However, this solution is not satisfactory mainly for economical reasons as the impregnation industry needs from 1S to 24 months to produce an acceptably dry creosote impregnated pole from wood. According to the invention the problem with bleeding of creosote impregnated wood is solved in a simple and economically favourable way. Thus, the invention relates to a method for treatment of creosote impregnated wood to substantially reduce or avoid bleeding, the wood being treated with air at a pressure above atmospheric before impregnation, and the method has been provided with the cloaracteris features defined 1. in claim 1. The method of the invention for treatment of creosote impregnated wood means in principle that the wood is subjected to a heat treatment under simultancous applying of a vacuum after the creosote impregnated wood has been stored for a certain period, which preferably covers at least 3 months, or until the initial air pressure mentioned above has been balanced. The heat treatment means that the wood is cooked with water as heat medium. In practice the treatment of the creosote impregnated wood is carried out in an autoclave provided with a vacuum pump and internally with steam coils or the like. According to a preferred embodiment 25 1 water per m3 wood are added to the autoclave, after which the temperature in the autoclave is raised to the boiling point of the water at the same time as a vacuum of 90% is applied, resulting in evaporation of the water. The operating conditions in the autoclave are preferably adjusted so that a maximum temperature and vacuum have appeared after about 0.25 to 0.50 h. The negative pressure or the size of the vacuum is not critical and depends on such factors as the size of the autoclave, the wood to be treated etc. However, according to the invention a vaccum of 70 - 90 is used, i.e. a vaccum to the order of 700 - 900 mbar. The vacuum is obtained by means of a conventional vacuum pump optionally connected to an ejector. At the method of the invention the creosote impregnated wood is subjected to said heat and vacuum treatment for up to 4 hours calculated from the start of applying the vacuum. However, the length of the treatment time is not critical to the invention as long as the intended effect is obtained, i.e. bleeding is avioded. To be on the safe side a treatment time of up to 4 hours is used. The invention will be explained more in detail with reference to the following, non-limiting examples. Example 1. About 35 m3 of creosote impregnated wood stored in the open air for about 3 months were introduced into a cylindrical autoclave having a diameter of 2 m and a length of 25 m, which was internally provided with steam coils and to which a vacuum pump working within a range of pressure of about 700 to 900 mbar was connected. Water in an amount of 25 1/m3 wood was introduced into the autoclave and the temperature in the autoclave was raised to the boiling point of water simultaneously as a vacuum of 90% (900 mbar) was applied. This temperature and vaccum state was reached after 20 min. The time of treatment lasted for about 4 hours, after which the vacuum was interrupted and water was cmptied out of the autoclave. During the treatment 7 1 water/m3 wood had been consumed, and it could be established by an analysis of the remaining water that 4 kg creosote oil/m3 wood had been leached out. The treated, creosote impregnated wood was stored on trial for one month and no bleeding could be observed. Example 2. 16 logs designated 12 S, 12 logs designated 12 E and 12 logs designated 11 S, which had together a volume of 27.48 m3, were introduced into a cylindrical autoclave (tube). A vacuum pump and a heat exchanger were connected to the tube. Water in an amount of about 700 1 was introduced at the same time as the wood. The following values of the in and out temperatures of the heat exchanger were noted: Time In Out Remarks 700 990 1120 710 990 1120 720 99 112 730 99 112 Wood was introduced 740 990 1120 o 0 750 99 112 800 990 1120 Started to take in water 810 1000 1120 820 1000 1120 Q30 1000 1120 Water in 840 1000 1120 850 1000 1130 900 1000 1140 910 1010 1140 920 1010 1140 930 1010 1140 940 101 114 Time In Out Remarks 950 101 114 1000 102 115 1010 102 115 1020 1020 1150 1030 1020 1160 1040 1020 1160 1050 102 116 1100 103 116 1120 103 116 1130 103 117 1140 104 118 1150 104 118 1200 1040 1180 1210 1040 1180 1220 1040 1180 1230 1040 1180 Tube was emptied The stop for half an hour between the introduction of the wood and the intake of water was caused by the fact that the intake valve for water did not let through the water but had to be corrected. The water was sucked in by the vacuum existing in the tube, which amounted to about 900 mbar. The temperatures in the above table are measured in the jacket. Exemple 3. The above example was repeated with 30 poles of 12 m length and designated N and 30 poles of 10 m length designated N, these poles totally taking up a volume of 25.5 m3. The amount of water used amounted to about 660 1. For reasons of measurement technique the temperatures indicated in the table below were determined within the cylinder. The vacuum pressure was about 90 %. Time Temp. C Remarks 915 20 Wood in 935 40 Vacuum started 9 45 Started to take in water 1000 50 1010 54 Water in. 905 vacuum Time Temp. C Remarks 1020 56 1030 60 1040 61 1050 64 1100 66 1110 67 1120 69 1130 71 1140 72 1150 74 1200 75 1210 77 1220 78 1230 78 1240 79 1250 79 1300 79 1310 79 1320 79 1330 77 1340 76 1350 76 1400 76 1410 76 The cylinder was empited After the test being finished the volume of remaining water and "excess" of creosote were determined to 250 1.

Claims: PATENT CLAIMS 1. A method for treatment of creosote impregnated wood in order to substantially reduce or avoid bleeding, the wood being treated with air at a pressure above atmospheric before impregnation, characterized in that the creosote impregnated wood, after being stored at least until the initial air pressure has been balanced, is subjected to a heat treatment in a heat medium, a vacuum simultaneously being applied. 2. The method of claim 1, characterized in that the wood is cooked with water in an autoclave and that the vacuum amounts to about 90%. 3. The method of claim 1 or 2, characterized in that maximum temperature or vacuum has appeared after about 0.25 to 0.50 h. 4. The method of claim 1, characterized in that the vacuum is of the order of 700 to 900 mbar. 5. The method of any one of the preceding claims, characterized in that the creosote impregnated wood is stored for at least three months before it is subjected to the heat-vacuum treatment. 6. The method of any one of the preceding claims, characterized in that the time of treatment is at least 4 hours.

Inventors: AKERLUND, EMIL
Applicants: SUECIA IND BYGGNADS AB
Application year: 1984
Main classification: B27K3
IPC codes: B27K5, B27K3