Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-03-17
2003-08-19
Yao, Sam Chuan (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S064000, C156S296000, C156S350000, C156S378000, C264S109000
Reexamination Certificate
active
06607619
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a process for bonding lignocellulosic materials such as in the manufacture of plywood, laminated veneer lumber (LVL), hardboard, particleboard, fiberboard, oriented strandboard (OSB), waferboard, and the like. More particularly, the process provides for varying the level of catalyst blended into the resin used for bonding the lignocellulosic material based upon at least one of the temperature and moisture content of the lignocellulosic material (wood substrates).
BACKGROUND OF THE INVENTION
Conventional wood-based composite products are generally made using thermosetting or heat-curing resin or adhesive to bind the lignocellulosic material (wood substrate) together. The resin-binder systems generally include phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde and isocyanate. Phenol-formaldehyde (PF) resins are generally used in the manufacture of products that require durability with respect to exterior exposure, such as plywood, OSB and siding. Such resins require a longer press time and higher press temperature than do products made with urea-formaldehyde resins. Products made using phenol-formaldehyde resin thus have a slower thermal cure, which may be lengthened due to the need to eliminate moisture from the wood substrate during curing. It is known that the curing time may be shortened by the use of additives such as organic or inorganic acids or hexamethylenetetramine, but the additives may cause side effects that are undesirable. The longer press time and higher temperature needed for proper curing result in higher energy consumption and lower line speeds, causing a lower production rate. When phenol-formaldehyde resins are used to make products, the products may have a lowered dimensional stability as a result of a lower moisture content in the finished product. Also, since products manufactured with phenol-formaldehyde resin may tend to be dark in color, such products may be unsuitable for decorative applications such as furniture.
Plywood is a flat panel containing sheets of veneer called plies. Thus, plies are individual sheets of veneer in a panel. The plies are bonded using heat, pressure and a bonding agent to create a panel with an adhesive bond between the plies. Plywood may be made from hardwood, softwood, or a combination of both. Generally, plywood is constructed with an odd number of layers, with each layer having a grain perpendicular to the grain of the previous layer. Layers may be a single ply or a plurality of plies that are laminated such that their grain is parallel. The inner plies are called cores or centers. The outside plies are generally called faces. The inner plies may vary as to number, thickness, species and grade of wood and generally have a panel thickness of 8 mm to 45 mm. Thus, a panel may have an odd or even number of plies, but normally will always have an odd number of layers, i.e., the number of layers is determined by the number of times the grain orientation changes, and the outer layers generally have their grain direction oriented parallel to the length of a panel, while the inner layers generally have their grain direction oriented perpendicular to the length of the panel. Hence, a panel may be described as, for example, four ply, three layer. Alternating grain directions for adjacent plies provides dimensional stability across the width and axial strength in a direction perpendicular to the panel plane. Lamination provides for distribution of defects and minimizes splitting propensity.
Two classes of plywood are generally produced: construction and industrial, and hardwood and decorative. The construction and industrial plywood may include hardwood, and is classified by exposure capability and grade, where exposure capability may be exterior or interior. Exterior capability plywood may not have less than “C” grade veneer, as determined in Product Standard 1, an industrial standard known to those skilled in the art. Interior capability plywood may be “D” grade veneer. Hardwood and decorative plywood may include certain decorative softwood species for non-construction use, but generally includes four types of plywood, listed here in decreasing order of resistance to water: Technical (Exterior), Type I (Exterior), Type II (Interior), and Type III (Interior). The adhesives used in the manufacture of the two classes of plywood are generally different, but are selected to achieve the desired specifications of the end-product wood composite.
After the trees are felled and cut into logs, the logs are graded and sorted to provide the most efficient use of the timber. Lower grade logs may be graded as “peelers”, and higher grade logs may be graded as “sawlogs”. High grade peelers and high grade sawlogs are sent to sawmills. The lower grade peelers and lower grade sawlogs are sent to the veneer mills to make plywood. At the veneer mill, the logs are sorted by grade and species, then debarked and cross cut into peeler blocks. The peeler blocks may be heated, steamed, or immersed in hot water prior to peeling to facilitate peeling and provide better quality veneer material. The peeler blocks are then transferred to a veneer lathe where the blocks are rotated at high speed and are fed against a stationary knife parallel to the length of the block until a predetermined size core remains. The core may be sawed into lumber, sold as fenceposts or timber or chipped to provide wood chips. Thus, veneer is peeled from the block as a continuous, uniform, thin sheet. Veneer typically is 0.8 mm to 4.8 mm thick.
The continuous sheet of veneer is then cut into usable widths; defects are removed; and
the veneer may be dried to a moisture content that is compatible with the adhesive being used to bind the panels. Glue may be applied to a panel by any known method such as spraying, curtain coating, roller coating, extrusion, or foaming. Adhesive may applied to one surface by spraying, curtain coating or foaming, or may be applied to two surfaces by using a roll-coater. Typically, after being coated with adhesive, the core plies are “laid up” to form desired panels and then conveyed from the lay-up area to the pressing area. Generally, the panels are cold pressed to flatten the veneers and transfer the adhesive to uncoated sheets, followed by hot pressing to cure the adhesive. Then panels are solid-piled (hot-stacked) to allow the adhesive to complete cure. Then panels may be sawed to a desired size, sanded where desired, and then graded.
Generally, in part due to their light color, urea-formaldehyde (UF) resins are used for manufacturing interior or decorative products. Products prepared using urea-formaldehyde resins tend to have a smooth surface and uniform dimensions. Curing for such products is generally accomplished at a temperature similar to PF plywood resins, but lower than PF OSB resins, and press times are generally shorter. Due to the cost of raw materials and processing, urea-formaldehyde resins are more economical than phenol-formaldehyde resins to use for manufacturing composite wood products.
Since melamine-formaldehyde (MUF) resins are generally more expensive than phenol-formaldehyde resins, melamine-formaldehyde resins are typically blended with urea-formaldehyde resins and utilized for wood composites used for decorative applications.
Diphenylmethane di-isocyanate often is used to manufacture composite wood products such as OSB. However, the highly toxic nature of the isocyanate requires that special safety precautionary measures be implemented in the manufacturing process.
Clearly, selection of adhesives in the manufacture of composite wood products requires consideration of a number of factors such as, for example, the total costs to be incurred, the materials to be bonded, the moisture content at the time of bonding, and the desired properties and durability of the products to be manufactured. Generally, phenol-formaldehyde and urea-formaldehyde are the most commonly used adhesives for lignocellulosic, i.e., wood-fiber, composite products.
A number of products
Breyer Robert A.
Foucht Millard E.
Banner & Witcoff , Ltd.
Georgia-Pacific Resins Inc.
Yao Sam Chuan
LandOfFree
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