Studies on Gum from Grevillea robusta as a Wood Adhesive

J. G. Mwangi - Associate Professor, Wood Science and Technology Department, Moi University, P.O. Box 3900, Eldoret, Kenya

Abstract:

A natural gum from Grevillea robusta (silk oak) has been studied. Various tests for physical and woodworking properties were carried out.

Wood samples were glued together using the gum and then tested for shear stress

along the band. The results obtained are compared to a commercial, synthetic wood adhesive.

The comparative analysis indicated that with suitable modifications gum from G. robusta can be used as a wood adhesive where cost and availability are decisive factors.

Keywords: Natural gum, physical and working properties, shear and tensile strength, banding, solubility.

Introduction

Plant extracts such as gum and resin have been used for many years to bind things together. The use of natural gum as an adhesive has, however, been reduced due to discovery of synthetic adhesives mainly produced from petroleum products. Recently, due to the escalated prices of petroleum and preference of non-chemical products by consumers and environmentalists, investigations have been initiated to substitute or supplement synthetic wood adhesives with natural gums, one being from G. robusta.

G. robusta is a native, deciduous tree of sub-tropical Eastern Australia which has been widely introduced in various warm temperature sub-tropical and highland areas of the world where it is grown primarily for shade or timber. In East Africa, it is used as an agroforestry tree in either outlining boundaries or among the crops. It produces firewood, poles, fodder and sawn timber.

On injury of a mature tree’s (6 - 10 years) bark, epithelial cells produce a gum exudated at the point of injury. The gum is a water-soluble, brown coloured substance. It is a mixture of polyuronides consisting of calcium and magnesium salts of D-glucoronic acid attached to D-galactose and L-avibinose (%).

This study investigates the physical and working properties including shear stress along the bond lines. The results are compared to those of commercial glue standard qualities. In developing countries, this information is viable as the synthetic glues are expensive or not available at all; furthermore, the raw material sources are abundant especially in the tropics.

Materials and Methods

Tapping

Trees of 6-10 years of age were selected. A V-cut was made on the tree trunk at a height of 1.5m above the ground (diameter breast height - Dbh). A container was placed and tied on to the tree just beneath the V-cut and this assembly was left for one week. The tapping operation took one month. The collected gum was transferred into a beaker and water added 10mls at a time till all the gum was dissolved into a thick paste. The process was accelerated by slight heating (45^Os C). The thick paste was allowed to cool and was stored in a sealed container ready for the tests.

Basic tests

Colour, PH, ash content, solubility in water, ethanol and Benzene tests were carried out using ASTM standard procedures.

Shear stress along the Bandline

Pieces of air-dried Cupressus lusitanica timber measuring 20x10x20 mm each were used for this test. Two pieces were glued to get dimensions of 20x20x20 mm and these were clamped using F-clamps and allowed to set for 6 hours. The glued samples prepared included cold and hot applications as well as a cold application commercial glue (polyvinyl alcohol/acetate). A T - 4283 machine was used to determine the shear stresses along the bandlines.

Water resistance of bandline

The test for water resistance was done by immersing the banded samples in water and observations were made and recorded every hour. The time taken before complete separation of glued pieces was noted.

Survey on availability of gum source

Information regarding the availability of G. robusta establishments was obtained from Rural Afforestation Extension Station (RAES) Nakuru from the 1993 records.

Results and Discussion

Table 1. Physical properties

Table 2. Comparison of physical and working properties of G. robusta gum and casein.

The data above indicates comparability of G. robusta gum to casein which is still widely used for everyday simple glueing purposes. This makes the natural gum just as competitive in the international adhesives market as is casein, though with modification to improve its colour and reduce solubility in water.

Table 3. Results of shear stress along the bandline.

Table 4. Number of G. robusta seedlings planted in Nakuru District in 1993.

The number of trees (6-10 years) in Nakuru District area (Kenya) as per 1993 records were approximately 4 x 10⁶.

Conclusion

The average production per tree was estimated at 1 kg per annum and tapping could be carried out for about 10 months in a year. This is enough gum to sustain cottage industries at a fairly lower production cost of about 25% that of synthetic commercial wood adhesives. Though not superior to synthetic wood adhesives in shear strength of bandline, water and heat resistance, G. robusta gum can still be used for local internal applications under low (22^O C) temperatures where high performance is not required. It is water soluble and thus does not require elaborate processing and storage methods.

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