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PRESSURE IMPREGNATION SYSTEM
IN PRESERVATION OF WOOD MATERIAL


Assoc. Prof. Selim ŞEN, University of Düzce
Department of Forest Industry Engineering 

 

Summary
Wood is an economical and renewable building resource. However, untreated wood is subject to attack by insects, micro-organisms, and decay fungi. To ensure long-term structural performance, wood must be protected from its natural predators. Wood is treated with preservatives to protect it from mechanical, physical, and chemical influences. Preserved wood is used primarily in the construction, railroad, and utilities industries to prevent rotting when wood is exposed to damp soil, standing water, or rain, and as protection against termites and marine borers. There are a variety of wood preservatives and application methods used around the world including high pressure impregnation, low pressure and vacuum methods of preservative application, dip treatments and brush or spray-on application methods. The art of wood preservation is matching the best preservative and application method to the wood species and the end use of the finished preserved wood product. One of the treatment methods applied vacuum and pressure in a cyclindir is economic and advantageous approach compared to the others. In addition The preservative chemicals are penetrated into wood quickly in this methods.

1. History of Impregnation Industry 
The significance of preserving wood material in its utilization area was acknowledged many ages ago; to preserve wood people have employed several materials and methods. Of these, the method of carbonization as the first preventive process has come to light by several archeological excavations. Diana Temple at Ephesus was built on wooden columns that were preserved by carbonization. Old civilizations such as China, Egypt, Rome and Greek treated wooden material with vegetable, animal and mineral oils. In these civilizations oils obtained from juniper, cedar and Pine were used in naval vessels in the course of time. In addition to this, in warm and dry climates, for instance in Egypt, wooden materials inside pyramids have survived thousands of years.

There had been no significant development in relation to the impregnation of wooden material till 1000 AD. From 16th century on some merchant vessels had been preserved against decay fungi and marine borers by treating carbonization and sOaking into wood tar. In addition to this, with industrial developments other preservative materials such as oils, glues, resins, rubbers and salts had been used (Bozkurt et. al. 1993). Impregnation industry has advanced for the last 200 years in parallel with the developments in industrial sector.

In the beginning of the 19th century creosote, which was originally derived from destillattion of coal used as fuel in industrial sector, has found wide-ranging utilization as a tar-based preservative for impregnaton of wooden telegraph poles and railroad sleepers until today. 

The first modern impregnation system started when the French J.R. Breant invented a method on the treatment of wood material with chemical preservatives and patented pressure cylinder method. However this method consumed great amount of impregnation material and this made the impregnation treatment expensive. In 1902, M. Rueping developed the more practical method of empty-cell (Berkel 1972).        

2. Development of Impregnation Industry in Turkey
In Turkey the first impregnation establishment was installed in Denizli in 1915 with a capacity of 20.000 m3 per year. The second facility was established in Derince, İzmit in 1931 by Germans with a capacity of 60.000 m3 per year. These establishments that process impregnation with creosote were established for impregnation of railway sleepers of State Railways. In 1956, the third establishment of impregnation was established by British in Bolu with a capacity of 15.000 m3 per year to supply for impregnated telephone poles of PTT by using water-borne salts. Together with several private establishments this number reached to 30 during 2000s (Bozkurt, 1993).  

3. Impregnation Methods
Wood preserving methods to ensure a long life of wood in the place of usage can be roughly divided into two as pressure and non-pressure methods. Non-pressure methods contain simple methods such as brushing and dipping in a bath while pressure methods require installed equipments. To achieve success in wood preservation it is critically important that one chooses the most suitable methods in relation to its place of usage.

3.1. Non-pressure Methods
The most common of these treatments involve the application of the preservative by means of brushing or spraying of chemical preservative onto the surface of wood material. To result in more penetration of chemical material, methods for dipping wood material in preservatives have been developed. The period of dipping into preservative depends upon wood species, the place of utilization and preservative material. There are also methods that involve hot and cold treatment. 

3.2. Pressure Impregnation Methods
Today, amongst the industrial methods in the impregnation of wood material the pressure processes in which the treatment is carried out with applied pressure and vacuum becomes important. In these processes a deeper and more uniform penetration and a higher absorption of preservative, in economically sufficient amount, is achieved in a short period of time. To achieve sufficient impregnation, a special establishment is required where the wood material is run into a long steel cylinder and the sufficient vacuum and pressure can be treated (İlhan, 1983).

In these establishments different wood materials in high volumes and in any sizes could be impregnated and the quantity of impregnation material could be adjusted. A more efficient and secure impregnation is achievable for a deeper and more uniform penetration of preservative into wood. These establishments can either be built fixed or serve mobile on trucks. Pressure impregnation methods are generally divided into two: full-cell and empty-cell methods.      

3.2.1. Full-cell Methods
In full-cell methods, the intent is to keep as much of the liquid absorbed into the wood during the pressure period as possible. The charge of wood material is sealed in impregnation cylinder and a preliminary vacuum (lower pressure) is applied to remove the air from cell gaps and walls. After the impregnation preservative is admitted to the cylinder pressure is applied until it penetrates into all gaps of wood material (Hafızoğlu, 1987).     

Breant method is amongst the first applied methods of full-cell. Early on creosote was used, while over time salts were used as well. In the Burnett method that was invented in 1847 and covered the use of zinc chloride in the beginning, the wood material is treated with steam at 1.5 atmosphere pressure for 30 minutes. However this impregnation material is not commonly used today.

For the first time in 1836 J. Bethell patented, in the literal sense, full-cell method developing the idea of using creosote derived from mineral coal in impregnation industry. In Bethell method the 12-14% moisturized (air dried) wood material was impregnated with creosote before it was treated with a preliminary steam process. Presently this method is amonsgt the most common in the world in which water-borne salt are widely being used (Bozkurt ve ark, 1993). Impregnation of wood material with full-cell method according to Bethell method can be seen in Figure 1.

Figure 1: Treatment according to full-cell method in pressure impregnation system. (Bethell Method)

Bethell Method
1- Vacuum Period: Pressure cylinder is applied 60 cmHg (0.8 bars) vacuum. Following this vacuum process the air in wood material is removed. The vacuum period depends on wood species. It is applied 30 min. for Pine, 60 min. for Beech, Oak and Hybrid and 120 min. for Spruce.      
2- Filling Period: After vacuum period the preservative is admitted to the cylinder. Filling process is applied under pressure (50 cm Hg).   
3- Pressure Period: High pressure application is generally procured between 8-14 atmospheres. Depending on wood species pressure period reaches up to 60 min. for Pine, 120 min. for Beech, Oak and Hybrid and 240 min. for Spruce.
4- Withdraw of cylinder and last vacuum period: When the pressure is completed the preservative is withdrawn from the cylinder. A short final vacuum at 40 cm Hg is applied for 5 minutes to free the charge of wood from dripping preservative, and the vacuum is removed.
Figure 2 shows an impregnation cylinder in which the full-cell method is applied.

Figure 2: A pressure cylinder that impregnates according to full-cell.  I- Wood material loaded in cylinder. II- Unload of impregnated material (Anon 2007).

When creosote used as preservative in full-cell method the consumption turns out to be high. It is reported that there is no need for such consumption to preserve wood material. Today this method is applied for water-borne salts.

Several methods of full-cell have been developed in order to decrease the consumption of creosote and to impregnate those that are resistant to treatment. Of these, the creosote developed by Julius Rutgers aims at decreasing the high level of creosote consumption by adding zinc chloride. For the seasoning and impregnation of green wood the wood is applied a preliminary seasoning with creosote solvent at 105 0C, then full-cell method is applied. Double impregnation method was invented for water-borne salts to penetrate deep and decrease creosote consumption. In 1952 Gilwald-Jonat developed the stepped pressure method to treat the species that are resistant to treatment (Figure 3). Water-bone salts are used effectively for this treatment that turns out satisfactory for species such as Pine and Beech expect Spruce and Abies.           

Figure 3: Treatment of stepped pressure method
The Swedish Sten Henrikson invented the fluctuation pressure method of impregnation that applies variable pressure. Allowing season and green impregnation of the species such as Spruce and Abies that are vital for North European countries and have a wide-range of application in energy and telephone transmission lines this method have been successful. The main rule of the method is to increase deep penetration in resistant woods by applying pressure and vacuum periods with short intervals (Figure 4).   

Figure 4: Diagram for fluctuate impregnation method that applies variable pressure
3.2.2. Empty-cell Methods
Instead of full-cell methods that consumes high amount of preservation material   inexpensive methods of empty-cell have been invented following the increase in consumption of impregnated wood materials for railways, energy transmission lines and communication services. Empty-cell methods differ with the application of preliminary vacuum treatment to wood material. The most common and important of these methods is Rueping method that does not contain a preliminary vacuum, but the remainder of the treatment is the same as described in full-cell methods (Hafızoğlu, 1987).
Rueping Method
Invented in 1902 by Max Rueping this method entails that the creosote amount absorbed by wood walls is adequate for preservation. Yet, creosote absorbed by wood cells was considered unnecessary and this idea was supported by laboratory works. An initial pressure is applied to force the air in order to regain the preservation material forced into cell gaps during the impregnation of wood material (Figure 5).           
1- Preliminary Air Pressure: A preliminary air pressure at 0.5 - 4 atmosphere is forced into the treating cylinder, which contains the charge of wood. For an effective result the wood material should be air dried. The pressure amount is adjusted according to permeability of wood and the amount of preservative material. As the pressured air forces the preservation material out from the treating cylinder the more the preliminary pressure increases the lesser the preservation material absorbed.    
2- Admission of Preservative Material: As the pressure maintained, the preservative material is pumped in the treating cylinder. Creosote temperature is increased until 65-110 0C.
3- Adjustment of Pressure Period: When the treating cylinder was filled and the required temperature was attained, the variable pressure, depending on wood species and sizes, is increased up to 10-14 atmospheres.   
4- Drain of Preservative Material: At the end of the pressure period the preservative is drained from the cylinder to storage tanks.
5- Final Vacuum: The surplus preservative is removed from the wood with a final vacuum injected in treating cylinder for 10 minutes not lesser than 60 cm Hg. Following the treatment, the preservative does not bleeds from wood material and the surface of the wood looks untainted.  

Figure 5: Diagram for treatment according to empty-cell method (Rueping method). 

A number of economical impregnation programs have been developed for the impregnation of resistant woods. For instance, normal impregnation treatment is applied after wood material is maintained in creosote for 2 hours at 100-105 0C temperature (Anon, 2007).

Figure 6: Treating cylinder in an establishment that applies empty-cell method (Temasan)

In many European countries and in Turkey, double Rueping method is applied in impregnation of railway sleepers with creosote. In double Rueping method the same impregnation treatment is applied twice one after another. The absorption amount reaches at 90 kg/m3for Pine, 145 kg/m3 for Beech, 75 kg/m3 for Spruce, 45 kg/m3 for Oak. For the species that are resistant to treatment vacuum and pressure periods vary.
In the four stage impregnation method developed in 1962 for young Spruce sleepers, wood material is impregnated with creosote at 120-130 0C. In this method creosote is filled into cylinder before the preliminary air pressure is applied; 8 A pressure and then 50 cm Hg vacuum is applied, and these treatments are applied four times subsequently (İlhan, 1983).

Table 1: Consumption of Creosote Preservative in Pressure Impregnation Methods

Method

Wood Species

Consumption (kg/m3)

Bethell Method (Full-cell Method)

Pine

270-300

Spruce

325-350

Oak

85-100

Poplar

290-500

Ruping Method (Empty-cell Method)

 

 

Pine

60-90

Spruce

145-150

Oak

45-50

Lowry Method
Different from Rueping method a preliminary high air pressure in the beginning of impregnation treatment is not applied. Preservative is forced into wood material after the creosote is heated up to 85-1000 0C. The air is imprisoned to internal layers with pressure treatment. With the removal of pressure the dilated air pushes some preservatives out, thus making them regained.      

3.2.3. Some Special Methods that Applies Pressure
During the application of some methods wood material is treated in special establishments and vessels instead of treating cylinder.
1- Method of Pushing the Sap Out (Boucherie Method): Consists of attaching a bag or container of preservative solution to a standing or a cut tree with bark, branches and leaves still attached, thereby injecting the liquid into the sap stream. Thus impregnation in species such as Spruce and Abies that are resistant to treatment has been satisfactory with this method. These establishments are generally installed in the forest and they are mobilized (İlhan, 1983).  
2- Pressure Capsule Method: To shorten the impregnation period of 12-15 days in Broucherie method, preservative material is forced into wood material through impregnation capsules. Thus the impregnation period reduced to 2-3 days.    
3- Cellon Method: After the infiltration of impregnation salt, which was dissolved with liquid gases, into wood material the gas is recovered through vaporization. This method is used in both full-cell and empty-cell methods. The preservative material is a mixture of LPG, isopropyl ether and PCP. However following because of several reasons such as the prohibition of PCP, danger of exploitation etc. this method has been recently abandoned. 
4- High Pressure Methods: For resistant woods, 50-60 atmosphere pressure is applied and thus satisfactory absorption and deep penetration is realized. Used in very durable systems this treatment deforms structures of most wood species.    
5- Lower Pressure Methods: Firstly used in 1940 in US, with this method the wood material is impregnated through low pressure. Treated according to full-cell method this method is known as Vac-Vac as it consist two vacuums. Just in the UK there are 250 establishments (Bozkurt et. al., 1993). In Turkey their number increases as well.
6- Vacuum Methods: These are similar to Boucherie methods. It consists speeding the absorption of preservative material through the application of vacuum to the peaks of wood poles that are impregnated. These are reported time consuming and uneconomical methods. 
7- Hydro-jet: Through the aid of jet systems that may apply 1800-3500 atmosphere pressure the preservative solvent is bumped onto wood material in order to obtain deep penetration. In this method, shallow holes made in wood material in such a form and size that would not taint mechanical properties, and the impregnation period shortens significantly. However this method has not promised much development so far.    

4. Wood Preservatives
The preservative materials used in impregnation industry have evolved as another industry field. They are classified with regard to their effect forms, areas they affect and cost. Today the degrees of toxicity with variety of preservatives limit their production and utilization. The use of chemical preservatives in some areas have been restricted, and even prohibited following the protest of environmentalist organizations in recent years. Preservatives are generally of four types:

1- Oilborne preservatives: Such as coal and materials derived from coal and petroleum products. Of these, most significantly the creosote is still being used especially for treating wood material that will be in use outdoor and in the sea for long period. Carbolineum, coal-tar, brown coal-tar and wood-tar are among these types. Besides, creosotes enriched by PCP (pentachlorophenol), copper, arsenic, zinc are also used. Railway sleepers that are treated with creosote endure 35-40 years.     
2- Light Organic Solvent Preservatives (LOSP): Volatile organic solvents derived from distillation of petroleum are obtained by dissolution of active chemicals. Pentachlorophenol, copper naphthenate, copper-8-kinolinat, organic tin glaze components, zinc naphthenate, hydrocorbonate with chloride and liquid gases are amongst these.       
3- Water-borne Metallic Preservative Salts: These preservatives are many; arsenic, zinc, copper, chrome, mercury, boron compounds, chloride and fluoride. Economically expensive ones are not used. Only water has been used as solvent all along.
4- Preservatives Used for Special Purpose: These are the preservatives that are used against blue colored fungi, fungus, also decay and harmful insects. Another class of preservative materials applied is for protection wood against physical and chemical degradation. There a number of chemicals that protect against fire and delay burning.  
5. Conclusions
Amongst the impregnation methods, the favorable, efficient and commercially common treatments are the ones with pressure. Although pressure methods are relatively more costly and applied in complicated impregnation establishments, they have the benefit of keeping the wood material in a cylinder. The advantage of these methods is the control of treatments such as the amount of chemical material required in respect of wood material that is to be impregnated, pressure and vacuum periods, filling and unload of cylinders.      

For the impregnation of wood material with water-borne salts generally full-cell methods are applied, the largest amount of preservation solvent that it can absorb through pressure is infiltrated. Again when a limited amount of penetration desired during the impregnation process, full-cell methods are preferred. Pressure methods are preferred for the impregnation of wood material in high volumes. The utilization area ranges from railway sleepers, telephone, telegraph and electricity poles as well as mine poles, wood construction materials, water construction poles, fence materials and garden fences.    
Sources

  1. Anon, 2003 Wood Protection Association, Industrial Timber Preservation, Osmose, UK.
  2. Anonymous, 2007 http://www.temasanltd.com, Ostim, Ankara
  3. Berkel, A., 1972 Ağaç Malzeme Teknolojisi [Wood Material Technology] II. Cilt, Ağaç Malzemenin Korunması ve Emprenye Tekniği [Preservation of Wood Material and Impregnation Techniques], İstanbul Üniversitesi Orman Fakültesi, İÜ Yayın No:1745, Fak. Yayın No:183, İstanbul, 592 s.
  4. Bozkurt, Y., Göker, Y., Erdin, N., 1993 Emprenye Tekniği [Impreagnation Technique], İstanbul Üniversitesi Orman Fakültesi, İ.Ü. Yayın No:3779, O.F. Yayın No:425, ISBN 975-404-327-2, İstanbul, 429 s.
  5. Hafızoğlu, H., 1987 Ağaç Malzeme Emprenye Tekniği [Wood Material Impregnation Technique], Karadeniz Teknik Üniversitesi, Orman Fakültesi Ders Notları, Trabzon.
  6. İlhan, R., 1983 Ağaç Malzeme Koruma ve Emprenye Tekniği [Preservation of Wood Material and Impregnation Technique], KTÜ Orman Fakültesi, Fak. Yayın No:74, Trabzon, 180 s.
  7. Nicholas, D.D., 1973 Wood Deterioration and Its Prevention by Preservative Treatments, Volume II, Preservatives and Preservative systems, Syracuase University Press., New York.
  8. Morrell, J.J. 1996, Wood Pole Maintenance Manual, Forest Research Laboratory, Oregon State University, Corvals Research Contribution 15. 47 p.

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