Wood composite prepared with a B-stageable resin

Details Wood composite prepared with a B-stageable resin Wood composite prepared with a B-stageable resin

1997-11-26

2002-08-13

Gallagher, John J. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S335000, C156S381000, C528S147000

Reexamination Certificate

active

06432254

ABSTRACT:

FIELD OF INVENTION
The invention is directed to a wood composite formed using a B-stageable phenol-formaldehyde resin system capable of rapidly curing in a saturated or superheated steam atmosphere.
BACKGROUND OF THE INVENTION
Wood-resin composite products, such as plywood, laminated veneer lumber, oriented strand lumber, oriented strand board, particleboard, medium density fiber board, hardboard and the like, are traditionally made by combining resin with wood components to form a stack or mat which is then consolidated in a hot platen press to cure the resin. The hot pressing process generally relies on a combination of conductive and convective heat flow to provide the necessary heat to cure the resin. The transfer of heat to the wood-resin composite by conductive and convective heat flow becomes increasing less efficient as the thickness of the wood-resin composite increases.
To produce thicker wood-resin composite products such as laminated veneer lumber and oriented strand lumber efficiently, manufacturer's have begun to use hot presses that use saturated steam as the primary heat transfer medium. When saturated steam is introduced into the wood-resin composite, the steam condenses on the composite components giving up its heat of condensation and rapidly heating the composite. Since the condensation takes place throughout the composite, there is almost no delay in the curing time due to the thickness of the composite. The injection of steam into the composite during hot pressing greatly improves the rate of heating, but the increased moisture from the condensing steam can interfere with the cure of condensation resins such as phenol-formaldehyde resins. Since phenol-formaldehyde resins are widely used in the wood industry and are known to provide cost-effective, strong, highly durable bonds with wood components, it is desirable to have a phenol-formaldehyde resin system suitable for use with steam injection pressing.
SUMMARY OF THE INVENTION
The present invention is directed to a method for making a consolidated wood product comprising coating wood components with a B-stageable, phenol-formaldehyde resole resin; heating the coated wood components to about 80 to 140° C. for a time sufficient to advance the resin to a B-stage; forming a mat or stack of the wood components coated with the B-stage resin; exposing the mat or stack of coated wood components to a saturated or superheated steam atmosphere in a hot press; and compressing the layup to form the consolidated wood product. The phenol-formaldehyde resole resin used in the method has a number average molecular weight of between about 200 and 600, has a F:P mole ratio of about 1.3:1 to 2.0:1, and has been modified with 0 to about 5.5 wt % of caustic based on resin solids.
DETAILED DESCRIPTION OF THE INVENTION
In order to prepare the resin-coated wood components for pressing and to ensure that the resin uniformly coats the wood pieces, the resin must be in a highly fluid state. Thus, dilute resin solutions having a significantly high water content have been used. However, using dilute resin solutions, in turn, creates a greater demand for a drying step (B-staging) prior to pressing to remove most of the water.
Typical phenolic resins used in adhesive formulations for wood composite products have relatively high molecular weights. These resins, when sufficiently diluted to coat the wood components uniformly, become too advanced during the step of B-staging, i.e., they do not properly B-stage. For instance, because they are designed to react (cure) faster, they tend to cure prematurely during the drying stage (B-stage) or they cure too soon during the steam injection pressing cycle.
Simply synthesizing a phenolic resin at a lower molecular weight, however, does not solve the problem because lower molecular weight resins tend to over-penetrate the wood components, leaving insufficient resin on the surface of the wood to act as an adhesive binder. Further, some lower molecular weight phenol-formaldehyde resins emit undesirable levels of formaldehyde during product processing and often yielded reduced physical and mechanical properties in the cured wood-resin composite.
In the present invention, thermosettable, B-stageable, phenol-formaldehyde resole resins were discovered having a degree of molecular weight advancement and caustic levels that are well-suited for the steam injection pressing processes. Suitable phenol-formaldehyde resins of the present invention have low molecular weight advancement which allows the resins to survive the drying stage and cure under a variety of steam pressing conditions.
The B-stageable resins used in the present invention have several advantages such as tolerating a wide range of assembly times. Typical phenol-formaldehyde resins used to prepare wood-resin composite materials are prone to glue-line inactivation when subjected to relatively long assembly times or hot mill conditions. The resin of the present invention can remain in an inert state for at least 30 days following the B-stage process, whereby there is no decrease in the resin's bonding ability during activation in a saturated or superheated steam environment. This long term stability allows the B-staged adhesive to survive lengthy product assembly line stops, permits accumulation and storage of dried (inventory) coated material prior to pressing, and also allows shipping of uncured materials to alternate locations for pressing. The stability of the resin of the present invention has been found to be much less dependent on temperature, up to and exceeding those found in a typical manufacturing setting (i.e., >50° C.).
The low molecular weight B-stageable phenol-formaldehyde resin, used in accordance with the present invention, is synthesized at a F:P mole ratio of about 1.3:1 to 2.0:1, preferably about 1.4:1 to 1.8.0:1, and more preferably about 1.5:1 to 1.7:1. The number average molecular weight of the resin is about 200 to 600, preferably 200 to 500, and more preferably about 240 to 340, determined by gel permeation chromatography (GPC) using polystyrene calibration standards.
Suitable resins may be produced by batch style (one pot) cooks or by “programmed” style (staged addition) cooks. Preferably a batch style cook is used to synthesize the resin since this cook style often provides resins that yield composites having higher internal bond strengths (IB), a higher modulus of elasticity (MOE) and a higher modulus of rupture (MOR) compared to composites made using other resins. In a preferred embodiment, the phenol-formaldehyde resin is prepared by single-stage alkaline condensation ;under a vacuum reflux at a temperature between about 60 and 90° C., preferably above about 80° C., and more preferably about 85° C. The resin is then modified by adding caustic.
The B-stageable resin is preferably modified by adding caustic in an amount to aid the dilution of the resin. It is important not to add too much caustic as it will decrease the properties of the consolidated product by softening the wood and causing over penetration of the resin at the glueline and increased thickness swell properties. Suitable amounts are 0 to 5.5 wt % based on resin solids, preferably 0 to 4 wt %, and more preferably 0.5 to 4 wt %. Suitable caustics include, but are not limited to, sodium hydroxide and potassium hydroxide.
The B-stageable resin may be prepared using reactants that are commercially available in many forms. Formaldehyde is available as paraform (a solid, polymerized formaldehyde) and formalin solutions (aqueous solutions of formaldehyde, sometimes with methanol, in 37%, 44%, or 50% formaldehyde concentrations). Formaldehyde also is available as a gas. Any of these forms is suitable for use in the practice of the invention. Further, the formaldehyde may be partially or totally replaced with any suitable aldehyde as known in the art. Typically, formalin solutions low in methanol are preferred as the formaldehyde source.
The phenol component of the resole resin may include any phenol typically used in preparing phenolic resole re

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