Bamboo and Rattan/Rattan/Course-2 Unit-2
Unit-2: Preservative Treatment: Natural durability, Agents of Degradation,Traditional & Non-Chemical Treatment Methods
Permanent preservative treatment
Adequate loading of preservative chemicals needs to be assured in treated materials. National Standards/code of practice for preservative treatment generally prescribes the required retention either in terms of dry salt retention (DSR, kg/m3) or % a.e. (active ingredient) and the extent of penetration of the preservative in mm. The nature and type of chemical and threshold level of retention is decided based on end-use conditions. Achievement of the required retention depends on various factors like species or anatomical structure, moisture content, method of treatment, concentration of treatment solution, etc. Due to these reasons, it is not practical to suggest one standardized treatment system applicable to all rattans at all moisture conditions. This has to be evolved by trial and error method, by using different concentrations, different treatment methods, different chemicals, different treatment conditions (like duration of dipping time, diffusion storage period in the dip diffusion treatment method; extent and duration of vacuum and pressure in the vacuum – pressure impregnation (VPI) treatment method), etc. Indian Standard (BIS 1961) suggests a DSR of 4 kg/m3 boron compounds for protection against decay fungi in treated rattans for non-structural applications.
If any type of conversion of the rattans (splits, peels, core, etc) is intended, treatment should be done in the converted form as this will be more effective and wastage of chemicals can be avoided. Due to the same reason, if any scraping operation is intended in the round rods, it is advisable to treat rattan poles after this (in case, scraped round rod rattans are marketed commercially); but this may not be applicable or practical to the scraping of individual rods during secondary processing at the product manufacturing units. Treating at final product form is not desirable as this will not give uniform penetration or distribution of preservative due to various reasons (use of sections of different dimensions, shapes or curvatures; different forms such as round rods, splits, peels, cores, etc in the same article, fastened parts, etc). Further, this can lead to undesirable dimensional problems due to wetting.
As boron is more eco-friendly in terms of low mammalian toxicity than the conventionally used heavy metal based preservative chemicals, and as the main end-use of rattans are under cover and not in contact with water and soil, boron formulations are recommended for treating rattans. Prepare the required quantity of 10% BAE boron solution by dissolving boric acid and borax in the ratio 1:1.5 (e.g. 5.0 kg boric acid and 7.5 kg borax in 100 liter water); add 0.5% Na PCP or 0.05% a.i. ’Busan 30’, in case prophylactic treatment (protection against immediate attack by sap staining fungi) is also intended; i. e., when permanent treatment is done at the on-site prophylactic treatment stage itself. The concentration of the treatment solution should be checked after every batch of operation with a standardized hydrometer and adjusted. Use of solutions of lower concentration (than 10% BAE) may be sufficient while employing VPI treatment process. Use of glows is recommended while handling treatment chemicals.
Method of treatment:
Dip diffusion treatment method can be adopted for treating green canes. Dipping can be done either by Butt treatment (dipping round rod rattan poles in the treatment solution in a standing position for prolonged duration) (Fig. 16) or by steeping horizontally in a treatment tank of the type mentioned above (Fig. 15 & 17) (for round rods as wells for converted forms). The same prophylactic treatment method suggested above can be made use of for the permanent treatment also, provided a prolonged duration is employed and boron compounds are also incorporated in the desired concentration in the treatment system.
Fig. 16. Butt treatment. Fig. 17. Dip diffusion treatment – steeping.
Dipping duration and diffusion storage period:
Even though no standardized dipping and diffusion storage period is available from literature, on a safer side, one hour dipping is suggested. Similarly, a diffusion storage period of two weeks under cover is recommended for achieving uniform distribution of chemicals within the dip diffusion treated material.
Many studies are not reported in literature on the effect of different concentration of treatment solution, moisture content, length of poles, species, treatment duration, etc on the achievement of required DSR in rattans (Dhamodaran and Bhat 1992 a). Use of a 10% boric acid equivalent (BAE) solution of boric acid and borax in the ratio 1:1.5 is suggested from the experiences on the dip diffusion treatment of perishable wood species. Other treatment conditions need to be optimized for the desired DSR levels by trial and error method. One of the main constraints in diffusion treatment is that the treated poles need to be stored under cover for prolonged periods (diffusion storage period) for uniform distribution of the preservative chemicals within the treated material. This may affect the process flow in handling bulk quantities.
VPI method (Fig. 18)
Using the same chemicals mentioned above may be more appropriate to treat bulk quantities within short time. But, here the material needs to be transported to a commercial treatment plant. The cost for establishing VPI plants will be high, as vacuum and pressure pumps and electrical power connections are required. Diffusion storage period can be avoided in the boron impregnated material; prolonged treatment periods can also be avoided, better penetration/distribution of chemicals within the treated material can be insured and bulk quantity can be treated within short time. Due to these reasons, VPI system is more suitable to commercial level treatment operations. But, if there is much time lag between felling and treatment by the dip diffusion/VPI system, an initial prophylactic treatment immediately after harvesting should be insisted to prevent the possible attack of sap staining fungi (before it is reaching to the permanent treatment site) and subsequent discoloration of rattan stem (discoloration due to the attack of sap staining fungi will not get removed by the subsequent application any preservative treatment method and by the use of any preservative chemicals). Even though much information on the impregnation schedules for rattans are available, a treatment schedule consisting of initial vacuum of 56 cm Hg for 15 minutes followed by the application of a pressure of 150 psi for another 15 minutes and a final vacuum of 56 cm of Hg for 5 minutes is recommended for the pressure treatment of rattans (Dhamodaran and Bhat 1992 b). In the VPI method, both green partially dry and dry rattans can be effectively treated.
Fig. 18. Pilot plant for VPI treatment
Quality control in preservative treatment
Checking the concentration of treatment solution and determining the boron content in the treated material by chemical analytical methods is the best way to assure quality of the treated material. Among the many methods for quantitative determination of boron the Indian Standard method for determining boron in treated wood, IS: 2753 (BIS 1991), is given below:
Mix about 2.5 to 5.0 g of finally ground wood to a paste with saturated barium hydroxide in a platinum crucible. Dry on a water-bath, ash slowly in a muffle furnace first at low temperature and then gradually raise the temperature to a dull red (about 500 – 600 0 C). Ashing should be completed in about 1.5 hours. When no trace of carbon remains, half fill the crucible with distilled water. Add dilute hydrochloric acid to dissolve the ash. Keep the crucible covered during this operation. Make the solution to a known volume in a graduated flask. This solution should be used for determining boron.
Take 10 ml aliquots (of about 1 to 1.25%) of the above solution in a nickel or platinum evaporating basin, add 2 to 3 ml of 105 (weight/volume) sodium hydroxide solution and evaporate to nearly dryness over a water-bath. Ignite the residue for about five minutes to dull red hot.
Dissolve the residue in about 10 ml of warm distilled water into an Erlenmeyer flask. Add two drops of phenolphthalein and then concentrated hydrochloric acid drop wise till color fades. Add two drops of methyl orange and few drops of concentrated hydrochloric acid till the solution is acidic to this indicator.
By using dilute sodium hydroxide solution, adjust the solution to methyl orange end-point and boil gently for 15 minutes under air reflux condenser, making sure that steam does not issue from he top of the condenser (to remove carbon dioxide from the solution). Cool the solution and re-adjust to methyl orange end-point. Add a measured amount of (approximately equal to 1.5 times the volume of the solution) of glycerol to the solution. Add 0.5 ml phenolphthalein and titrate with 0.1normal carbonate free sodium hydroxide to the phenolphthalein end-point.
Calculate the boron content on the basis that 1 ml of 0.1 normal sodium hydroxide is equal to 0.00619 g of boric acid. Glycerol is often acidic in reactions; hence, correction should be applied by taking measured quantity of glycerol diluted by equal volume of distilled water and titrating with 0.1 normal sodium hydroxide solution.
Concentration of the boron treatment solution can be checked after every batch of operation simply by the use of a standardized hydrometer. However, it is suggested to cross-check sometimes the result with chemical analytical method.
Penetration of boron in the treated material can be tested as per Indian Standard IS: 2753 or IS: 401 - 2001 (BIS 1991; BIS 2001). Extent of preservative penetration needs to be tested in the mid cross-section of the treated poles. Through and through penetration is a simple indicator of the success of treatment. The method is as follows:
Prepare an alcoholic extract of turmeric powder by refluxing (by using a water condenser) 10 g of turmeric powder with 100 ml of 95% ethyl alcohol for an hour, cool, filter and store in a refrigerator. Also, prepare an extract of salicylic acid and hydrochloric acid. This can be prepared by saturating salicylic acid (about 13 g per 100 ml) in a mixture of 80 ml 95% ethyl alcohol and 20 ml of 30% hydrochloric acid.
Apply the alcoholic extract of turmeric powder on a reasonably dry mid-length cross-cut surface of the material to be tested. Allow the surface to dry for few minutes and apply the extract of salicylic acid and hydrochloric acid. Development of red color in the treated surface indicates the presence or extent of penetration of boron.
The treated material needs to dry before further processing operations.
i. Bleaching and Fumigation
Although, bleaching and fumigation are practiced in the rattan rich South-East Asian countries the code of practices varies widely among the different industries and countries depending on their traditional belief. Severe reaction conditions can cause hydrolysis of the wood material resulting reduction in physical and mechanical properties of rattans. No clear distinction of the effects of bleaching and fumigation on the strength properties of canes has been established. As there is no scientific information on the effects of these practices on utilization value, no standards are available with regard to the concentration of reactants or duration of treatment. Thus the existing practices are quite diverse in the field of product manufacture. In an attempt to throw light into the effects of bleaching and fumigation conditions on the utilization value of rattans, Dhamodaran and Bhat (1995) reported the following:
Even though hydrogen peroxide and chemical grade sodium or potassium hypochlorite are used in South-east Asian countries for bleaching rattans, commercial grade bleaching powder is the cheap bleaching agent available in market. A tank of appropriate size may be used for the bleaching operation.
Bleaching the round rod rattans in green condition is found to be more effective than bleaching dry rattans as far as their color improvement is concerned. About the concentration and treatment duration, almost similar color grades are obtained with the use of bleaching solutions of concentration 5% for 24 to 48 hours and 7.5% for 12 to 24 hours and 6 to 12 hours with the use of 10% solution. Strength parameters like modulus of rupture (MOR), modulus of elasticity (MOE) and wood density are not adversely affected due to the bleaching of round rod rattans. Maximum compressive stress (MCS) is found adversely affected due to bleaching. Thus, the use of more concentrated solution for shorter periods of bleaching is suggested as far as mechanical properties are concerned. The use of a 7.5% solution for 12 hours in round rod green rattans produces the optimum beneficial effects as far as color improvement and mechanical properties are concerned. Effect of bleaching conditions on the tensile properties of round rod rattans is also not available.
For bleaching rattan peels, the use of 5% bleaching solution for 24 hours and 7.5% solution for 12 hours and 10% solution for 6 hours produced almost similar grades of color, but will certainly adversely affect the tensile strength (ultimate tensile stress – UTS) of bleached peels. The tensile strength of rattan peels of different species representing small, medium and large diameter categories were found almost reduced to half of its original value due to bleaching. Hence, bleaching of rattan peels is not recommended in view of the reduction in strength.
Fumigation is done by exposing rattans to fumes from burning sulphur. As mentioned in the case of bleaching, for fumigation also no standard practices that without adversely affecting the utilization value were arrived at from systematic scientific studies. The preliminary study reported by Dhamodaran and Bhat (1995) reported the following:
The use of 250 g sulphur for 24 hour fumigation and 500 g sulphur for 12 hours fumigation in a chamber of approximate capacity of 2.3 m3 produced almost similar grades of color to round rod dry rattans. Fumigation in any of these conditions is found not adversely affecting the mechanical properties. Fumigation of green rattans is not recommended due to the possibility of acid formation and subsequent chemical reactions resulting chemical degradation of the wood materials.
In the case of rattan peels, the tensile strength was significantly adversely affected due to fumigation. Hence, it is not advisable to fumigate rattan peels, as far as their utilization value is concerned.
Data on the tensile strength of rattan peels prepared from bleached/fumigated round rod rattans versus directly bleached/fumigated peels is also not available in literature. This makes the situation difficult to comment on the effect of bleaching or fumigation on rattan peels prepared from bleached or fumigated round rods. All the scanty information available indicates the need for further research inputs in this aspect for standardization of the bleaching or fumigation conditions for avoiding adverse effect on their utilization value. Till such information is available, extreme care has to be taken while choosing the reaction conditions.