Bonding materials

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...

Reexamination Certificate

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C523S210000

Reexamination Certificate

active

06335386

ABSTRACT:

According to the present invention there are provided bonding compositions and methods of bonding.
In the manufacture of panel products from ligno-cellulosic materials, an adhesive composition which is a formaldehyde-based polycondensation resin system is added to the ligno-cellulosic materials before placing in a press which applies heat and pressure to cure the adhesive and bond the materials.
Thus, for example, an adhesive composition will be mixed and applied to the fibres or chips of materials such as wood or other cellulosic materials such as straw in the forming of fibreboards or particleboards or to the surface of a veneer in the forming of plywood.
Such adhesives are usually prepared prior to application in the form of a liquid mass called the glue mix which comprises the adhesive, some other additives and the hardener which catalyses the curing of the resin or bonding agent in the press. Thus one employs a urea- or melamine-formaldehyde or other formaldehyde-based adhesive and a hardening agent which is conventionally an inorganic acid or salt. Blending such a hardening agent into the bonding agent or prepolymer has the disadvantage that, if for any reason there is a delay between the blending of the polymer and hardening agent and application to the ligno-cellulosic material, then cure may proceed prematurely. Particularly this is true when a free acid is employed and for that reason it is preferred to use a salt, particularly an ammonium salt. On the other hand these are less active as hardening agents and therefore require longer in the final heat/pressure stage than the free acids.
A further problem has been the increasing use of ligno-cellulosic materials which may be at a slightly elevated temperature just prior to addition of the bonding agent, which again can result in premature curing of the bonding agent.
A similar problem is found with two other active agents used in such bonded materials namely water absorbents, and flame/fire-retardants.
These agents share the common problem of possibly causing premature curing if incorporated into the bonding material prior to admixture with the ligno-cellulosic material. Flame/fire retardants as additives of the gluing formulation interfere with the curing reaction of the bonding resin and represent therefore a barrier to the control of the timing of the resin curing. Water absorbents interfere with the kinetics of the resin polycondensation reaction due to their water absorbing function, which shifts the reaction towards condensation accompanied by water production and thus towards premature curing. It is also desirable to have them co-acting with active hardeners since the reduction of press cycle can result in blisters in the product because of violent escape of water on press opening.
Various means, such as the use of buffers, have been suggested for delaying the action of such active agents, particularly hardening agents, but, as indicated, these can have the undesirable effect of slowing down the reaction of the hardening agent or other active agent once the product is in the final press stage of manufacture.
Using active agents (eg. hardeners) in a solid, particulate form in which the particles are coated has been one suggestion in epoxy resin formulations but it has been difficult to find a coating which would not adversely affect the reaction of the active agent especially a hardener.
Thus JP 05 310950A, JP07-024303 and JP08-081668 relate to encapsulation of isocyanate binders and are not relevant to the problems of formaldehyde polycondensation products in manufacture of lignocellulosic shaped products.
EP 0 392 170 and EP 0 547 379 relate to encapsulation of one component of a two-component polyurethane adhesive and there is no discussion of condensation polymerisation systems using an active accelerator.
DE-A-2635570, WO-A-9112293, WO-A-960983 refer to various adhesive systems (polysulfides-polythioethers, polyesters, ethylene polymers and epoxy resins respectively) and the encapsulation of the corresponding active agents.
U.S. Pat. No. 4,808,639 relates to a liquid or viscous microcapsules-containing curable adhesive composition composed of a liquid or viscous curable resin, which contains, dispersed therethroughout, a multiplicity of rupturable microcapsules of a resin curing agent. However, the content of these microcapsules can only be released by the application thereto of external pressure, abrasive action, shear stress or other mechanical manipulation.
EP 0 115 307, WO 95/23823 and WO 96/00747 relate to protection of the bonding agent resin as distinct from an activating agent.
EP-A-0 278 582 relates to encapsulation of a water-proofing agent in the paper industry to delay reaction of the hardening agent with a starch adhesive.
U.S. Pat. No. 4,307,206 discloses urea-formaldehyde resin with encapsulated alkaline component to neutralise excess acid after cure and there is no discussion of controlling timing by encapsulating an acidic activator.
EP-A-0 341 569 discloses encapsulating a formaldehyde scavenger eg urea to reduce free formaldehyde emission. There are references to including “catalyst” with the urea but in the context of enhancing the urea formaldehyde reaction and there is no discussion of imposing the shaped article with shorter press times while avoiding premature reaction.
There are various discussions of methods of encapsulating materials to protect those materials against the environment but usually this has been discussed in the context of releasing the content of the capsules as a result of mechanical impact on the capsule wall.
A capsule subject to such mechanical degradation would not provide sufficient protection within the context of the handling of a bonding composition including the application to the ligno-cellulosic material and including the formation of the bonding composition or the precursor of the bonding composition and application material.
According to the present invention there is provided a formaldehyde-based polycondensation system composition for bonding ligno-cellulosic materials to form shaped products which contains an active agent which is at least one of a resin hardening agent, flame/fire retardant and water absorbent characterised in that the agent or agents are incorporated as capsules, the walls of the capsules being capable of degradation to allow access of the agent to the bonding material, such degradation being at a temperature above the temperature prevailing in the system prior to application of the heat in the final compression and the heating stage.
The invention also provides a method of forming a shaped ligno-cellulosic based material with a formaldehyde based polycondensation bonding material under heat and pressure in which the bonding material is blended with an encapsulated active agent which is at least one of a resin hardening agent, a flame/fire retardant agent and water absorbent with the capsule wall resisting direct contact between the bonding material and agent until the product is placed in the final heating stage for manufacture of the shaped product at which time the walls of the capsules are degraded at a temperature above that which the composition has been held before it entered the said stage.
The invention also relates to use of a formaldehyde-based polycondensation resin system which contains an active agent which is at least one of a resin hardening agent a flame/fire retardant and a water absorbent characterised in that the agent is incorporated as capsules, the walls of the capsules being subject to degradation to allow access of the agent to the bonding material, such degradation being at a temperature above the temperature prevailing in the system prior to application of the heat in the final compression and the heating stage.
By degradation is meant collapse or disintegration of the wall material whereby the contents of the capsule are released into or can contact the adhesive. The invention is particularly applicable to hardening agents but can be applied to other two additives of the glue mix namey flame/fire retard

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