Particleboard from Sunflower Stalks with Tannin-Based Adhesives

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Zeinab Abd El Rahim Osman, Hassan
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The availability of tremendous amounts of agricultural residues such as cotton stalks, sorghum, wheat, bagasse, sunflower and usher wood together with the urgent need for construction boards for the reconstruction and development programmes in Sudan, calls for the industrial utilization of these raw materials. However, efficient utilization of local raw materials requires thorough investigation and information especially for low quality wood and such agricultural residues as sunflower stalks. Physical and chemical analysis of whole and wet-depithed sunflower stalks carried out, according to the international standards, showed that with wet-depithing of sunflower stalks the content of ash and extractives were decreased considerably while the content of the major chemical components (lignin and polysaccharides) was increased indicating the positive effect and necessity of wet-depithing of sunflower stalks before use in particleboard preparation . Chemical analysis of Calotropis procera (usher) wood indicated the presence of phenolics other than lignin which were expected to have positive effect on the strength properties of cement-bonded, panels. Acacia nilotica spp. tomentosa ground pods were extracted with water at different temperature (30, 50 and 70 oC) and water-to-pods (2, 4 and 6) ratio in order to find the optimum conditions for large scale extraction as well as the most suitable ground pod fraction by size. Three different pod fraction sizes (2 mm- coarse (C), < 2 mm. medium (M) and < 1mm fine (F)) were used. As control for garad tannin adhesivness commercial mimosa tannin from the bark of Acacia mearnsii was used, solid and liquid extracts from the bark of neem (Azadirachta indica) were also used in the preparation of resins in mixture with garad tannin to find out if adhesiveness pattern close to that of mimosa could be achieved. The tannin content in the different extracts obtained was evaluated by mean of the official hide - powder method. The medium fraction (M) gave the highest percent of tannin (43.7%) with Stiasny number (catechin content) of 27.6 at an extraordinary extraction rate (tannin / nontannin ratio of 11.12) which was much higher than that of the other two fractions (coarse 1.38 and fine 2.5). The most acceptable results from the lab trials were obtained when tannin extraction was carried out at 70°C (initial temperature of extracting water) and water to pods ratio of 6:1 for 24h extraction period. These conditions were further used to extract the tannin on large scale basis using each of the three different fractions (coarse, medium and fine). The results obtained from the evaluation of tannin content in the extracts obtained from the large scale experiments agreed with the best results obtained from the small scale experiments. At the small scale the medium fraction gave the highest percent of tannin (38.9%) compared to that of the other two fractions (coarse 10.2 %, and fine-16.4%). The extracts obtained from the large scale experiments were spray dried and analysed. The highest tannin content (47.9%) was found for the spray dried powder from the medium fraction pods, whereas the tannin content in the other two fractions was 38.9% for the coarse and 46.8% for the fine one. Different types of adhesives were prepared from both liquid and solid (spray dried powder) tannin extracts according to specially developed technology and their adhesion bond strength was tested on veneers specimens. The tannin resin prepared from liquid extract of the medium fraction mixed with urea formaldehyde resin in different ratios showed highest adhesion strength at 3:7 tannin-to-urea formaldehyde ratio Particleboard specimens of varying densities were prepared using different resin mixtures of tannin (T) and urea formaldehyde (UF) and different amounts of particles from both wet-and dry-depithed sunflower stalks according to special expperimental design. The physico-mechanical properties of the particleboard were determined according to the appropriate international standards (DIN 52361 - 73) and statistically analyzed. Particleboard from sunflower stalks with superior properties was prepared with tannin-to-urea formaldehyde resin ratio of 1:9 (or 10%) in the density range of 550 - 650 kgm-3 . Trials to produce cement-bonded panels from whole and dry-depithed sunflower stalks and usher wood particles gave acceptable physico-mechanical properties with the dry-depithed sunflower stalks.
Particleboard from Sunflower Stalks with Tannin-Based Adhesives