GB2579813A - Improvements in relating to the bonding of solid wood, mdf, particle board or such like - Google Patents

Abstract

A method of industrially bonding wood-based materials comprises: applying a layer of slow curing adhesive 10, such as polyvinyl acetate, to a first piece 4; applying droplets of a fast curing adhesive 6, such as cyanoacrylate, to a second piece 2; and pressing the two pieces together in an industrial press 16, 18. The PVA glue or similar slow drying adhesive may be applied via a roller applicator 12, while the CA glue or similar fast drying adhesive may be dispensed via a pinch tube or other suitable dispensing system 8. The fast setting adhesive may be heated, for example by an infra-red heater, prior to the step of pressing. Having been pressed until the fast curing adhesive has cured, the assembled piece may then be stored for a period of time sufficient for the slow curing adhesive to cure. There is also disclosed a sheet of wood-based material formed via this method.

Description

A METHOD OF INDUSTRIALLY BONDING WOOD-BASED MATERIALS

Field of the Application

The present application relates to techniques for the industrial production of wood-based products and more particularly to bonding pieces of wood-based products together during such processes.

Background

The present application is directed at bonding thinner sheets together to form thicker sheets of wood-based products. Such techniques are commonly employed in furniture and joinery making, for example in making doors or frames for fitted kitchens, windows or wardrobes. The pieces employed in such manufacture are generally wood based. Such wood based products may include but are not limited to solid hard or soft woods, plywood, fibreboard e.g. medium density fibreboard (MDF), particle board also known as chipboard and oriented strand board.

For decades, the bonding of wooden strips together to create a larger panel or wood section has been carried out. The requirement to do so this is because of the limited supply of suitable wood in the market with the result that smaller size of planks or strips dominate in the market place. Accordingly, to obtain a larger size, a conventional approach is to bind smaller pieces together.

From the early days when wood strips were first bonded with animal glues, to the present day where synthetic and many other forms of glues are available, the general process remains the same.

The first step is in the process is to apply a layer of adhesive to a first piece and more particularly to the surface of the first piece to be bonded.

The surface of the first piece with the adhesive is then brought into contact with the surface of the second piece. The two pieces are then held together under pressure to allow the adhesive to cure. Traditionally and as is the process still used today, the two pieces are clamped together and left to cure overnight.

Increasingly, manual clamps are replaced by larger presses in a technique which is referred to generally as pressing.

Traditionally, this pressing process involved pressing the two pieces together in a press under pressure and leaving overnight to allow the adhesive to sufficiently cure. This technique is commonly referred to as cold pressing. Whilst several panels may be pressed at the same time, the press is required until the adhesive has cured.

Alternatively, hot pressing is a technique in which heat is applied during the pressing process.

The advantage of hot pressing is that the time required for curing is significantly reduced and in this context a figure of approximately 1.5min per 1mm of panel thickness is exemplary of the time required, meaning that bonding two 10mm panels is achieved in a time of 15 minutes as opposed to being required to be left overnight, i.e. a period of at least 12 hours.

Regardless of the process, a period of time is required to allow the curing to occur. This time is referred to as the cure dwell time.

In the context of factory production, the cure dwell time adds significantly to the manufacturing time and occupies additional factory space to allow for the panels to be stored whilst curing.

Whilst hot pressing reduces the cure dwell time and thus improves the throughput, it has caused other problems. Such problems include that the heat from the process can give rise to panel distortion, which means that panels coming from hot presses may be required to be stacked on top of each other so as to cool and flatten out, which in turn disrupts the production process.

There are a variety of adhesives which may be employed in bonding wood together. These include animal glue, polyurethane glue, polyvinyl acetate (PVA), Urea Formaldehyde (UF) Animal glue, especially hide glue, was the primary adhesive of choice for woodworking, including furniture, for many centuries. It is manufactured from rendered collagen from the skins (hides) of animals. Hide glue is still used today in specialized applications, such as making musical instruments but is otherwise not employed.

Polyurethane glue (trade names include Gorilla Glue and Excel) is becoming increasingly popular for use in DIY, but is not generally employed in factory assemblies.

PVA glue is an aliphatic rubbery synthetic polymer. It belongs to the polyvinyl esters family and is a type of thermoplastic.

UF glue, also known as Plastic Resin Glue, is a range of synthetic resins produced by the chemical combination of formaldehyde (a gas produced from methane) and urea (a solid crystal produced from ammonia).

Aliphatic resin emulsions, commonly referred to as "yellow glue" or "carpenter's glue", has a similar use profile and relative ultimate strength as PVA. The two glues differ in grip characteristics before initial set, with PVAs exhibiting more slip during assembly and yellow glue having more initial grip.

Cyanoacrylates are a family of strong fast-acting adhesives, commonly referred to by the brand name SUPERGLUE. They have application in bonding small pieces together but are not employed for bonding larger pieces together since by the time the adhesive has finished been applied to one of the surfaces to be bonded, the adhesive applied first has already cured. Additionally, the cost of these glues is of an order of magnitude higher than that of PVA or UF glues.

In the context of factory assembly of panels for making furniture and similar applications, PVA and UF are the primary adhesives employed used where large areas of panel have to be glued and are used in both hot and cold pressing.

The present application aims to overcome the problems associated with the heat of hot pressing whilst seeking to reduce cure dwell times in the factory environment so as to improve line production to avoid the need for storage on the factory floor. The present invention at the same time aims to reduce the requirement for extra expensive bonding equipment.

SUMMARY

The present application provides an economic method of industrially bonding wood-based materials eliminating the use of heated presses and labour requirements. The method comprises the steps of applying a layer of first adhesive to a first surface of a first piece of wood-based material and applying droplets of a second adhesive to a first surface of a second piece of wood-based material. The first surface of the first piece is then pressed against the first surface of the second piece. This pressing is suitably performed in an industrial press. The first adhesive is selected to have a fast cure time relative to the cure time of the second adhesive.

In this context, the first adhesive may be a cyanoacrylate or similar fast cure adhesive. 30 The second adhesive may be a polyvinyl acetate or similar slow cure adhesive.

The second adhesive may be heated on the first surface of the second piece prior to the step of pressing. This heating may be performed by an electric heater, for example an infra-red heater.

The assembled piece of wood-based material may be removed from the press after the first adhesive has cured. After removal, the assembled piece may be stored for a period of time sufficient to allow for the second adhesive to cure.

The industrial press may be selected to be capable of applying a pressure of at least 20psi. More suitably, the applied pressure is in a range between 65psi and 150psi. Desirably, the pressure is applied uniformly across the area of the surfaces.

The droplets of first adhesive are generally spaced apart from one another by a distance of between 3 and 10 cm. The droplet spacing selected may depend on the rigidity of the first and second pieces and the droplet volume. The individual volume of each droplet is suitably in the range of.000023m1 to.25m1.

The method is particularly suited to joining sheets of wood based material. These sheets may have a surface dimensions of at least.5m x.5m and may be greater than 1mx 1m.

The wood-based material of at least one of the first and second pieces may be hard wood, soft wood, plywood, fibreboard (including MDF), particle board and oriented strand board.

The application also extends to a sheet of material produced by such a method.

Such a sheet of material will comprise two sheets of wood based material bonded together by an adhesive layer. The adhesive layer will have a first set of regions, in which the adhesive layer comprises a mixture of a first adhesive and a second adhesive. Separating the individual regions will be areas of the adhesive layer where only a first adhesive is present. In the adhesive layer, the first adhesive will be a fast set adhesive relative to a second adhesive being a slow set adhesive.

Description of Drawings

The present application will now be described with reference to the accompanying drawings in which: Figure 1 illustrates the steps of assembly of a thick panel from two thin panels; and Figure 2 is a flowchart corresponding to the steps of assembly of Figure 1.

Detailed Description of Drawings

The present application is directed at assembling wood-based sheet materials and in particular for making a thicker sheet from two or more thinner sheets in a factory for furniture and joinery fabrication. In particular, the application is directed at assembling wood-based sheets using two or more thinner sheets which are laminated together with an adhesive under pressure in a press. These sheets may be employed for making door panels and the like.

The present application overcomes the problem of cold pressing, i.e. the need to leave the panels in a press for an extended duration of time, typically more than 8 hours, 10 commonly overnight.

The process of assembly 20 will now be described with reference to figures 1 and 2. The process commences with a first step 22 (STEP A) of selecting of two sheets 2, 4 of wood-based product, i.e. two wood-based sheets. The sheets will be of the order of several mm thick. Accordingly, individual sheets may have a thickness in the range of 4mm to 20mm. More typically, the individual sheets would have a thickness in the range of 6mm to 16mm. However, it will be appreciated that the overall thickness of the materials to be bonded will generally only be limited by the height of the press opening.

The sheets are suitably sized to fit into the press used in a subsequent step in the process. Accordingly, the sheets may be of a standard size for wood-based products, and thus may be greater than lm x 2m in surface size. Conventional sizes would include 1220 x 2440 mm, 1250 x 2500 mm, 1500 x 2500, 1500 x 3000 mm, 1525 x 3050 mm and 1525 x 3660 mm.

At the same time, it will be appreciated that door panels and the like practically tend to have a minimum size and so if an individual door, for example, for the front of a kitchen drawer was being produced from two sheets bonded together, then the surface area being bonded would correspond to the dimensions of the drawer panel. In this context, it is likely that for sheet materials, the sheets will have a minimum dimension along one axis of 10cm and along another of 30cm corresponding to the likely practical smallest panel size. More practically, the dimensions of the first surface area are at least.2m x 30.2m. As the panel sizes are increased, i.e. to an area.5m x.5m or even 1 m x 1m significant advantages arise as it would be entirely impractical for cost or manufacturing reasons to employ fast set adhesives for such large areas.

Equally, it will be appreciated that the process may be employed for joining non-sheet like materials, e.g. for forming an adhesive joint between two pieces, such as for example to join rails and stiles in a panel or to join wood sections so as form a desired wood profile with a minimum of machine time and waste.

The sheets may be cleaned or otherwise prepared to remove any dust or similar material which may interfere with a proper bond.

In a second step 24 (STEP B), a fast set adhesive, suitably a cyanoacrylate adhesive is selectively applied to a first sheet 2. In particular, droplets 6 of adhesive are applied at intervals onto a first surface of the first sheet using a suitable dispensing system 8.

Various types of dispensing systems are known for dispensing cyanoacrylate adhesive in manufacturing applications, including pinch tubes, diaphragm valves and pneumatic-actuated syringe dispensers and air-free syringe dispensers.

In the case of the latter, the syringe and piston are made of polypropylene and polyethylene respectively, which minimizes air and moisture infiltration. The piston and tip cap are coated with a noncontaminating release agent to inhibit bonding between the adhesive and the plastic surfaces. The piston is advanced in the syringe by a linear actuator driven by a stepper motor. The linear actuator ensures that a specific volume of adhesive material is displaced during each dispensing cycle. Such linear actuators allow for a droplet size as small as 0.00023 ml. At the same time, an end-of-cycle "draw-back" prevents dripping of the adhesive. An X-Y positioning system may be used in combination with the dispensing system to ensure that the droplets of adhesive are applied at regular intervals across the sheet. Suitably, the droplets are less than.25m1 in volume.

Alternatively, a series of dispensers could be arranged at intervals in a line, with the length of the line corresponding to the width of a sheet, with the sheet being placed on a conveyor which transports the sheet in a direction transverse to the line allowing for droplets to be deposited at regularly spaced intervals across the width and length of the sheet.

A third step 26 (STEP C), applies a layer 10 of slow set adhesive to the second sheet 4. 30 The slow set adhesive may for example be a PVA type adhesive or similar.

With reference to the terms cure or set, which are used herein interchangeably, fast and slow are relative with respect to one and other. A fast set may be taken to indicate a cure time which is of the order of seconds or minutes, for example less than six minutes, and a slow set time may be taken to indicate a cure time that is of the order of hours, for example at least one hour.

Generally, it may be taken that a slow set has a cure time at least ten times that of fast set. More typically, this ratio will be greater than fifty.

The third step may use a roller applicator 12 to apply the slow set adhesive to the second sheet. More particularly, the second sheet may be transported along a conveyor passing over the roller applicator 12. Other techniques that could be employed to dispense the slow set adhesive may include spraying. Similarly, one or more dispensers may be employed to dispense the slow set adhesive with a squeegee or similar tool employed to spread the slow set adhesive evenly on the second surface.

In an optional step, the surface of the second sheet may be heated. This heating may be achieved by passing over a heater such as an infrared heater so as to partially dry the slow set adhesive. This partial drying can reduce the overall cure time for the slow set adhesive.

It will be appreciated that the second and third steps may be performed simultaneously, or the third step may be performed in advance of the second step.

is A fourth step 28 (STEP D) in the process brings the sheets together. This step aligns the sheets and then brings them together.

In a fifth step (STEP E), the two sheets are pressed between the plates 16 and 18 of a press, suitably a cold press. The pressure applied by the cold press, which is suitably at least 20psi, causes the individual droplets of fast set adhesive to spread out by a factor of ten to twenty times in surface area. A more general pressure range for a press to obtain reliable results is between 65psi and 150psi.

At the same time, the pressure causes the slow set adhesive to mix with fast set adhesive. This starts a chain reaction polymerizing the fast setting adhesive and which then achieves solid state and relative speaking an almost instant cure (measured in seconds). The moisture present in the slow set adhesive increases the bond speed of the fast set adhesive. At the same time, the presence of the slow set adhesive appears to improve the bond strength of the fast set adhesive.

The two sheets are left under pressure in the cold press for a period of time sufficient to allow the fast set adhesive to cure. This time will be reasonably short in the range of 30 to 90 seconds. It will be appreciated that the sheets may be left in the press longer and so this time represents a minimum time rather than a maximum time.

Once the fast set adhesive has cured, the sheets may be removed from the press and placed in a suitable storage area to allow a period of time for the slow set adhesive to cure. This for example may be overnight. The bonding achieved by the fast set adhesive effectively acts like a clamp keeping the two sheets tight together allowing for them to be handled. At the same time, the pseudo clamping performed by the cured fast set adhesive allows for the slow set adhesive to cure and form a strong bond over the entire panel.

Equally, it will be appreciated that the bonded sheets may be processed further in advance of the slow set adhesive curing, provided the process does not introduce excessive force. Thus as an example, the surfaces may be machined in a CNC or milling machine, sanded or coated with a surface finish (e.g. paint or varnish).

Thus the previous interruption to the production flow necessitated by the slow cure time of the adhesive may be substantially eliminated.

It will be appreciated that once the sheets have been removed, the press may then be re-used for subsequent sheets. The effect of this is that the cycle time for a (cold) press is reduced from the conventional time frame of hours (overnight) to minutes.

An advantage of the present approach is that as only a relatively small amount of fast set adhesive is used, it may readily be applied to a sheet in the time available before the adhesive cures. In contrast, were fast set adhesive to be applied to cover the entire surface of a sheet, the adhesive might already have cured by the time the entire sheet is covered. Equally, an advantage of the present approach is that the cost of the fast set adhesive is relatively small since a reasonably small amount is employed.

The spacing and size of the droplets of fast adhesive will be dependent on a variety of factors, including the nature of the fast adhesive, the materials being joined and the thickness of the sheets.

Generally speaking, the size and spacing of the droplets will be selected to result in a bond which is able to withstand handling of the sheets when removing them from the press. In this context, it will be appreciated that such a determination can be readily be made by experimentation. It has however been determined that typically the required inter droplet spacing may be between 3 and 10 cm. A consequence of the inter drop spacing and size of the droplets is that when the droplets are spread out under pressure, the result will be that there will be areas between the two sheets where there is a mix of the slow and fast set adhesives (i.e. centred on where the droplets are placed) and areas where there is only slow set adhesive present.

A further advantage of the described method is that heating requirements for areas in which materials are stored may be reduced as the need for keeping moisture content low is not as important.

Thicker sheets will typically require larger droplets spaced at greater intervals with thinner sheets requiring smaller droplets at closer intervals. This is because thinner sheets are more flexible necessitating the distance between points of bonding to be closer. It is for this reason, that a lower limit of 4mm was selected for the thickness of the sheets An observed advantage of the process is that a sheet produced by the present process appears to be stiffer than a sheet produced using a slowing set adhesive.

The present technique is intended for industrial manufacture where the press used would be an industrial press and where the forces would be applied by relatively large plates, of at least 1m2 in cross sectional area. In such industrial presses, the pressure is applied by an actuator rather than manually. Typically, the actuator is a pneumatic, hydraulic or electric actuator.

In this context, an advantage of the present approach is that a cold press may be used in place of a hot press since once adequate pressure is applied, a sufficient bond will be created by the fast set adhesive which will allow handling of the sheets.