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Why Treat Wood with Preservatives?

Cross Section of a tree Contains Four Portions as under:

  • Bark - The bark portion is dead and attracts insects for shelter, which later on damage the wood.
  • Sapwood- Sapwood portion of all wood species is a living part of the tree. It conducts sap from root to crown and vice-versa. It may contain starches, which attract fungi & insects. It is lighter in colour. This portion of every wood species is liable to be destroyed by biodegrading agencies like insects & fungi (i.e. non-durable). Therefore protection of this portion of all the wood species against biological degradation is absolutely necessary and treatment with suitable wood preserving chemicals is a prime necessity.
  • Heartwood- This portion consists basically of physiologically dead cells and does not take any active part in the life of the tree except to provide structural rigidity. The time period for the destruction of this portion significantly varies with different wood species. Based upon the service life of this part of the tree, different wood species have been grouped into three classes as under :-
    Class I Durable Life 120 months & above
    Class II Moderately durable Life 60 months and above
    but less than 120 months.
    Class III Non durable Life less than 60 months.
    This portion of all class I timbers like teak, shisham, dark red meranti etc. may not require treatment with wood preserving chemicals. But, timbers classified into class II or class III do require adequate protection against biological degradation and must be given treatment with suitable wood preserving chemicals before being put to use.
  • Pith Portion (Central portion of the log or center heart ) This portion tends to cause mechanical degrade in the form of splitting & cracking and should be avoided as far as possible in the cut wood pieces
 
Treatment Method
There are many methods but a method for treating green timber immediately after felling and the other pressure method Alongwith an alternate pressure method are briefly mentioned below : Impregnation of wood under pressure is the most desirable and best method to get uniform and proper treatment under controlled conditions. This is usually done in a pressure cylinder equipped with vacuumpressure system, storage tanks and mixing tanks for preservative solutions. However, the timber is needed to be dried to below fibresatrution point beforetreatment .
 
Long Term Protection

An important point in impregnation of timber with preservatives is to get adequate penetration coupled with appropriate absorptions. This can be achieved by a number of processes. Among these processes impregnating inorganic & toxic compounds in the timber to make the wood material poisonous to fungus and insects is the most common practice.

Mixtures of various salts which in the presence of wood react and form insoluble complex salts and thus get permanently fixed in the wood treated with them are commonly used. The treated wood is very clean and can be painted, polished or waxed. These are :

  1. Copper-Chrome-Arsenic (CCA)
  2. Copper-Chrome-Boric (CCB)
  3. Acid Copper-Chrome (ACC)