The pine needle - leaf of the Pinus roxburghii tree - is a renewable natural resource material produced annually in a huge quantity in the western part of the Himalayas (Indian forests: ~ 2.7 million tonne per annum). The needles comprise cellulose (40-43 per cent), hemicelluloses (20-24 per cent), lignin (36-40 per cent), extractives (25-28 per cent) and ash content (2-4 per cent). They are available in a length of 300-380 mm with a density of 0.22 g per cc. Because of their inefficient utilisation, every year, forest fire causes substantial damage to the flora and fauna of the area following the burning of fallen needles on the ground around the trees and liberation of carbon dioxide in the atmosphere.

At present, about 13.34 million cu m of timber is in short supply. The import of industrial round wood is about 3-4 million cu m annually. To meet the requirements of wood scarcity, the utilisation of renewable raw materials such as agro-residues, leaves and stalks of forest plants and natural fibres has been widely recognised for making ligno-cellulosic panel products in the building industry. Because of their ligno-cellulosic similarity with wood, a systematic study was undertaken at CSIR-CBRI, Roorkee, on the use of pine needles to make composite panels, partitions, door inserts, etc. Pine needles in an average size of ~3 mm with desired surface characteristics can be gainfully utilised as supplementary raw materials, either on their own or with secondary wood particles in the manufacture of composite building panels as wood substitutes.

Materials parameter optimisation
Composite panels were produced with pine needles collected from Uttarakhand and isocyanate prepolymer and their physico-mechanical properties were evaluated in terms of adhesive content, flake size, pine needle content, pine needle or wood particle blend and hygro or hydro exposure conditions. Prior to use, wettability of the pine needles treated under various conditions such as alkali, steam and alkali-steam combination was assessed through contact angle measurement. It was found that alkali steam-treated pine needle fibres were more wettable than other treatments. The internal bond strength, modulus of rupture and screw withdrawal load of composites increased with the increase of resin content. The screw withdrawal load of the high resin content panels (~20 per cent) was comparable to natural wood. Fractographic evidences pulled out of needle fibres, fibre fracture and debonding owing to the swelling of fibres in hydrothermally aged samples could be used to explain the loss of strength over the unaged samples. To improve the strength of these composites, the pine needles were blended with eucalyptus wood particles in different ratios. A positive effect in the properties of composites was observed when pine needles were used with the composite adhesive system: Monomeric isocyanate for pretreatment and isocyanate prepolymer for particle-particle bonding. Pine needles and wood particles in a 50:50 ratio in the panels with 5-7 wt per cent resin content gave optimum results. The developed composite panel satisfies the requirements of standard specification: IS: 3087-2005/EN 312-2003 (Table 1).

The performance of pine needle composite panels was assessed for their dimensional stability, flammability, biological attacks under fungi or termite, thermal acoustics and toxicity characteristics. For dimensional stability, the samples were exposed under various levels of humidity (80 per cent RH, 95 per cent RH, 95 per cent RH at 50¦C) and cold water for 60 days. At lower humidity, the composites with varying resin adhesive contents exhibited 2-7 per cent thickness swelling at equilibrium moisture content, whereas at higher humidity, the thickness swelling in the panels ranged between 13 per cent and 23 per cent. Under immersed water, the composite panels swelled two to three times as much as the samples exposed at 98 per cent RH. After ageing, the internal bond strength of panels was reduced by 41-67 per cent in accelerated water and 54-78 per cent in cyclic exposure respectively.

In order to improve fire behaviour, urea phosphate-treated needle furnishes were used in the composites. Cone calorimetry results indicate that the optimum flammability characteristic of needle furnishes was obtained at the retention of 7.48 kg per cu m fire-retardant additive. When the panel was tested as per BS 476-1981 (Part 6), the fire propagation index was only 17.52, indicative of its contribution towards slow fire growth. As per specified criteria of flame spread (limit: 165 ¦ 25 mm), the sample belongs to Class I category according to surface spread of flame test (BS 476-1981, Part 7). During natural decay test, composite boards treated with wood preservatives exhibited 4-8 per cent weight loss after eight weeks of exposure compared to 9-13 per cent for untreated boards, categorising them under the ´highly resistant class´ according to ASTM D 2017-2005. Termites caused ~ 6 per cent less weight loss in the treated samples than the untreated samples after 10 weeks of exposure in microcerotermus bessoni colony showing their moderate resistance behaviour as per ASTM D 3345-2008. Thermal conductivity and sound transmission loss of samples were 0.136 W per mK and 26.51dB respectively, showing their adequate insulation properties. Toxicity index based on gases emitted during the burning of composites was found to be 2.2-3.7 according to the test conducted as per NES-713 standard, which is considered to be safe with respect to other similar materials.

The process
The raw materials used in manufacturing are pine needles, reactive resin adhesive and additives. The composite panels were produced with pine needles and isocyanate prepolymer on a hydraulic press at 140ºC and 10 MPa pressure for 10-15 minutes retention. Various parameters such as resin efficiency, mat moisture content, pine needle flake size and pressing conditions were examined to achieve satisfactory products at minimum cost. The process consists of digestion of pine needles and reduction of their size; blending of processed pine needles and resin adhesive; and mat formation and pressing. After the pressing step, the composite boards are cooled, conditioned, trimmed and surface finished. Various sizes of panels such as 1.2 m + 1 m and 2 m + 1 m were produced. The cost of pine needle boards (Rs 25 per sq feet for 12 mm thickness) is comparable to commercial boards (Rs 26-30 per sq ft).

The salient features of pine needle composite panels include dimensional stability, sufficient internal bond strength, adequate retention of properties under wet or humid conditions, ability to be easily cut and sawn and easily laminated, good screw holding strength, and good sound and thermal insulation. And, of course, the cost is lower than wood like teak and sheesham as well as plastics. The composite panels belong to medium and high density board categories (density 0.07-1.2 g per cc) and meet the requirements of standard specification (IS 3087). The panels can be used as partitions, door inserts, table tops and other wood substitute applications. Compared to commercial resin-bonded wood or ligno-cellulosic particleboard, it was found that pine needle composite board exhibited 40 per cent higher internal bond strength than the specified values of the standard for phenolic or urea-bonded boards.

The screw withdrawal load was comparable with commercial particleboards. The water absorption of pine needle boards was 23.56 per cent and 9.12 per cent less than phenolic or urea-bonded boards after two and 24-hour soaking. In fact, the swelling thickness of the pine needle boards after two-hour water immersion was 26 per cent higher than commercial resin-bonded boards. After adding paraffin wax (1 per cent), the swelling reduced from 12.62 to 7.84 per cent (37.88 per cent), which passes the requirement of the commercial standard. Linear expansion of the pine needle boards is also 80 per cent less than that of phenolic or urea-bonded boards after two-hour water immersion.

Market potential
Existing wood-based ligno-cellulosic panel products have a huge market for doors, windows, partitions and panels. About 25 per cent of total wood production is being used in building construction. The total annual requirement of doors is ~90 lakh. Approximate timber equivalent to 12 lakh trees is utilised for door infill and flush doors. The total market growth rate for prefabricated doors and windows is 10 per cent per annum. Pine needle composite boards or panels can be used as wood substitutes for various building applications. Thus, one can conclude that pine needles in a processed stage can be effectively utilised as an alternative raw material in the manufacturing of composite panels in view of increasing shortage of wood resources. Although the product is ready for commercialisation, it has not been used in any project to date. This work has thus far been an in-house project; the developed panels have been tested as per IS 3087 in a laboratory. In years to come, substituting timber by these composites will certainly present tremendous business opportunities.

About the authors:
Dr B Singh is Chief Scientist and Head of Polymers Plastics and Composites Division, CSIR - Central Building Research Institute, Roorkee.
Dr Hina Tarannum is Assistant Professor, Department of Chemistry, College of Science, Taibah University at Yanbu, Kingdom of Saudi Arabia.

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