Pressure and Non Pressure Preservation Methods for Rubber (Hevea brasiliensis) Wood Treatment by Boron Preservatives

Abstract

Wood is susceptible for insect and microorganism attack, though heartwood of some species is naturally durable. Rubber is cultivating as a plantation species in Sri Lanka, and it is the second largest plantation crop in the country (covers 124,000 ha). Rubber wood is less durable. Preservative application 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 1964. Vacuum pressure impregnation method for timber treatment is not very common for rubber wood treatment in Sri Lanka. Boron is a better preservative compared to other preservative chemicals as it does not cause 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 cm * 10.16 cm * 1.60 cm (16 inches * 4 inches * 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 two boron solution strengths were used (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 80 ºC hot boron solution and temperature was maintained for 4 hours and allows them to cool within the same solution for 12 hours. In steam cold quench method, timber samples were steamed for 4 hours and immediately transferred them in to cold boron solution and allow them 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 difference before and after the treatment, chemical penetration was measured using Atomic Absorption Spectrometer and effectiveness against termite test was measured by the sandwich method. Cost of treatment of one cubic meter of rubber wood was also calculated for diffusion, hot and cold open tank method and vacuum pressure impregnation 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 in green condition. Hot and cold open tank method was 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. Cost of treatment of one cubic meter of rubber wood by vacuum pressure impregnation was the highest then hot and cold open tank method and diffusion method gave the lowest treatment cost. If high retention and penetration are needed vacuum pressure impregnation method should be use for the dry rubber wood treatment. Although retention and penetration is lower in Hot and cold open tank method and steam cold quench method compared with vacuum pressure impregnation method, those methods can be recommended for low hazardous situation of timber use such as indoor applications.