1. Introduction
Particleboard has been defined as generic term for a panel manufactured from linger cellulosic materials, usually wood, primarily in the form of discrete pieces or particles, as distinguished from fibers, combined with a synthetic resin or other suitable binder and bonded together under heat and pressure in a hot press by a process in which the entire interparticle bond is created by the added binder, and to which other materials have been added during manufacture to improve certain properties.
Historically, the products from the light wood technology were very expensive and exclusive. They were used in the aeronautic field or in the automotive field. Over the time, the light wood products could be produced cheap, but with a better quality through increased efficiency in production processes, research, and development. This trend is very strong in the furniture industry.
Particleboard is cheaper, denser and more uniform than conventional wood and plywood and is substituted for them when appearance and strength are less important than cost. However, particleboard can be made more attractive by painting or the use of wood veneers that are glued onto surfaces that will be visible. There are over a hundred particle board plants in operation today worldwide and particle board is one of the strongest reconstituted panel products and is considered as an ideal substitute to wood and plywood.
Urea-formaldehyde is a non-transparent thermosetting resin, made from urea and formaldehyde heated in the presence of a mild base such as ammonia or pyridine. It is characterized by high tensile strength, flexural modulus, and heat distortion temperature, low water absorption, mould shrinkage, high surface hardness, elongation at break, and volume resistance. It is used in many manufacturing.
Processes. Examples include decorative laminates, textiles, paper, foundry sand moulds, wrinkle-resistant fabrics, cotton blends, rayon, corduroy, etc. It is also used to glue wood together. Urea formaldehyde was commonly used when producing electrical appliances casing (e.g. desk lamps).
Urea-formaldehyde (UF) resin is extensively used as a binder adhesive for the production of wood-based panels such as medium density fiberboard, particleboard (PB) and hardwood plywood for interior uses. UF resin is an excellent adhesive with features such as virtually colorless glue lines, low price, mold and fungi resistance, and ease of handling (e.g.; mixing, applying and cleaning). UF resin can be successfully bonded with most species of wood in many combinations. However, formaldehyde emission is the main disadvantage property of the UF resin. The lower formaldehyde/urea (F/U) molar ratio, the lower the content of free formaldehyde in the UF resins. The formaldehyde emission of UF resin has been declining as a result of new resins and resin technologies.
The use of Urea-formaldehyde for the production of particle board and as an adhesive resin has been widely reported due to its high reactivity, good performance, and low price. The use of Urea-Formaldehyde for the production of particle board using saw dust has also been reported. Studies on the use of buckwheat stalk in particleboards bonded with urea-formaldehyde resin adhesive have been reported. It has reported that one of the major difficulties in the processing of wood-based particle boards with urea-formaldehyde is that urea-formaldehyde is a volatile gas with strong odor.
Production of particleboards, as stated earlier, involves the use of a binder (resin). A large amount of binder is being used in particleboard industry for the production of highquality products. The binder accounts for up to 32% of manufacturing cost in the glue-wood composite industry. Various types of binders have been used in the manufacture of particleboards and they are classified as satisfying interior or exterior use requirements primarily on the basis of their response to moisture and/or temperature. At present, the principal ones are formaldehyde condensation polymers, such as Urea-formaldehyde (UF), Phenol-formaldehyde (PF), Phenol-resorcinol formaldehyde (PRF), Melamineformaldehyde (MF) and their derivatives (i.e. modified formaldehyde condensation polymers). Others include; isocyanate adhesive, gypsum, nitrogenous modified amylaceous binder, magnesia, etc. The relatively low cost and proven performance of phenol-formaldehyde and ureaformaldehyde resins have made them the most important adhesive systems for composite wood products. These two resins are formed by step-growth (condensation) polymerization reaction of formaldehyde (CH2O) with phenol (C6H5OH) and urea (H2NCONH2) respectively.
Urea is a colorless and odorless crystalline compound, CO(NH2)2, with a melting point of 132.7°C, also known as carbamide. It is found abundantly in the urine of humans and other mammals. In lesser quantities, it is present in the blood, liver, lymph, and serous fluids and is found in the excrement of fish and many other lower animals. Urea is produced mostly in the liver as the end product of protein metabolism.
The nitrogen in urea, which constitutes most of the nitrogen in the urine, is produced mainly from food protein, but part comes from the breakdown of body cells. Urea is also present in various fungus molds as well as in the leaves and seeds of numerous legumes and cereals. The compound is soluble in both water and alcohol and is slightly soluble in ether. Urea is prepared synthetically by the Wohler synthesis, which was devised in 1828 by the German Chemist Friedrich Wohler.
CO2 + 2NH3 → NH3CONH3 + H2O ↔ H2NCOONH4 (1)
Due to its high nitrogen content, commercially prepared urea is used in the manufacture of agricultural fertilizers. Urea is also employed as a stabilizer in nitrocellulose explosives and is a basic constituent of synthetically prepared resins. Urea-formaldehyde (UF) resins are composed of molecules that cross-link into clear, hard plastics. Properties of UF resins are similar to the properties of phenolic resins. As the names imply, these resins are formed by condensation reactions between urea (H2NCONH2) and formaldehyde (CH2O). Two main steps are recognized in the reaction of formaldehyde with amino compounds to form useful resins.
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