Pressure and non-pressure preservation methods for rubber wood treatment by Boron preservatives

Abstract

Wood is susceptible for insect and microorganism attack, though heartwood of some species is naturally durable. Rubber is cultivated as a plantation species in Sri Lanka, and it is the second largest plantation crop in the country (covers 122,000 ha). Rubber wood is less durable. Preservative applications can make rubber wood more durable. Several timber preservation methods are used in Sri Lanka, but rubber wood treatment is mainly done using traditional boron diffusion treatment, which was invented in Sri Lanka in 1969. Vacuum pressure impregnation method which is a more effective method in timber treatment is not very common for rubber wood treatment in Sri Lanka, but a few private sectors industries have recently started using this method using boron preservative chemical. Boron is a better preservative compared to other preservative chemicals as it does not cause for discoloration of treated wood. Aim of this study is to investigate effectiveness of rubber wood treatment by boron preservatives by different treatment methods.

40.63*10.16*1.60 cm (16*4*0.63 inches) size rubber wood samples were treated by boron preservative by four different timber treatment methods. For each method two moisture contents of wood were tested (dry and green) and using two boron solution strengths (8% and 12%). Timber treatment methods used were dip diffusion, hot and cold open tank method, steam cold quench method and vacuum pressure impregnation method.

In dip diffusion method, timber samples were dipped in boron solution for 24 hours and thereafter these were block stacked for 1 hour. In hot and cold open tank method, timber samples were submerged in a 80 ºC hot boron solution and temperature was maintained for 4 hours and allowed it to cool within the same solution for 12 hours. In steam cold quench method, timber samples were steamed for 4 hours and immediately transferred into cold boron solution and allowed to cool for 12 hours. Timber samples were treated by vacuum pressure impregnation method as the 4th treatment method.

Effectiveness of each method was measured by chemical retention, chemical penetration and termite test. Chemical retention was measured by the weight different before and after the treatment, chemical penetration was measured using Atomic Absorption Spectrophotometer and effectiveness against termite test was measured by the sandwich method. Cost benefit analysis was also done for each method.

Vacuum pressure impregnation method was found to be the best method for preservation of dry (12% Moisture content) rubber wood, but it is less effective for green condition. Hot and cold open tank method is the best method for preservation of green rubber wood. All treatment methods are more effective in dry condition than green condition except dip diffusion method. Effectiveness increased with the solution strength. Termite resistance was highest in vacuum pressure impregnated dry timber.