Patent Application
20120315389
1. Field of the Invention
The invention relates generally to a method for contactless application of an adhesive to a finger surface of a substrate and more particularly to condensation adhesives for contactless application to a finger surface of a wood panel and subsequently joining the fingerjoints of two wood panels.
2. Description of the Related Art
Finger joints are used both within non-structural and structural applications. Finger joints are for example, rectangular or jagged edges of a substrate, such as wood or laminate, that are to be pieced together. Each jagged piece in the edge of a substrate may be considered a finger joint. There may be a whole row and/or rows and columns forming a matrix of joints at the edge of a substrate. In order to piece together and to secure to substrates at these finger joints, adhesive at these joints is required.
There are several methods for joining finger joints together via adhesives. Conventional methods are a roller or matrix method, where glue is deposited and thereby roller is placed over the substrate. This method works for most types of adhesives, but has drawbacks in that it is messier and less efficient. The equipment must be cleaned often because the roller or matrix becomes contaminated by the substrate, particularly when the substrate is wood.
An alternative is a contactless application method. Contactless means that there is no physical contact between the application head and the wood. As this method does not require the applicator to physically touch the surface of the substrate, thereby increasing efficiency and reducing stop time. For example, EP 1985676 describes a contactless method for joining two components, such as wood structures, through the use of applying adhesive to an application unit, monitoring the adhesive bead through an optical sensor, and combining components by contacting them together. The glue lines are delivered to the top of the fingers via nozzles. No subsequent spreading is done before joining the fingerjoints of the substrates to be adhered. The spreading of the adhesive over the surface of the fingers in the fingerjoint is done when the fingerjoints of the substrates are pressed together. In sum, contactless application generally obviates the need for washing and reduces stop time for the equipment, thereby increasing efficiency. Adhesives used in contactless finger joint application are polyurethane and emulsion polymer isocyanate adhesives.
EP 0949309 A1 discloses a cure promoter composition for phenolic resin based or resorcinol resin based adhesive system comprising a carbonate and a method of joining cellulosic fibre containing surfaces by means of a phenolic resin based or resorcinol resin based adhesive system whereby at least one of the surfaces is treated with such a cure promoter prior to the adhesive system being applied. EP 1862277 A1 discloses a process for adhesive bonding of at least two complementary joining elements to a joining surface by the steps: (a) application of the adhesive to the first and/or second joining element perpendicular to the fingerjoints, (b) direction of an air stream onto the applied adhesive to blow the adhesive into the fingerjoints and (c) pressure bonding together of the two elements. In particular the invention relates to contactless finger joint application of adhesives and especially to preventing loss of adhesive by dripping at the end of a joint by blowing the applied resin between the finger joint. JP 19970280309 describes an adhesive coating apparatus for coating a workpiece with an adhesive coating, which apparatus comprises a nozzle for blowing cold air for cooling the coated workpiece.
The problem of the method of known contactless fingerjoint application is that the adhesives are expensive, not versatile or easy to use. Therefore, there is a desire for an improved method for contactless fingerjoint application that uses inexpensive adhesives that are easy to use, environmentally friendly and result in good and reliable fingerjoint bonding.
The present invention addresses these problems by providing a method applying an adhesive to a finger surface of a substrate, comprising:
In one embodiment, at least one of the rheology of the hardener and the rheology of the adhesive are modified by at least one of the following measures: cooling to a temperature between 5 and 15° C., preferably 5 to 10° C. to achieve a viscosity of at least 5000 mPas and preferably less than 20,000 mPas, including into the adhesive a viscosity increasing substance to increase the viscosity to at least 3000 mPas and preferably less than 20,000 mPas, including a thixotropic increasing substance in an effective amount to prevent sagging of the adhesive on the surface.
In the method according to the invention, the rheology modification of the condensation resin adhesive can provide glue-line strings on the fingers which are easy to detect with a quality control system thereby achieving guaranteed quality of the adhesive bond. The modified condensation adhesives have excellent adhesive properties, are water based, easy to use and environmentally friendly.
In contactless fingerjoint application, the adhesive is applied onto the tips of the fingers of the finger joints. The typical period between the time the glue is applied to the fingers of the substrate and the time they are joined in a press is 3-15 seconds. During this time the substrates are transported and handled. The inventors found that the most important parameter in achieving reliable and good quality adhesive bonding of substrates in contactless applications is to prevent dripping and sagging of the glue on the fingers during the time interval between applying and joining of the substrates. In order to ensure safe and effective gluing, glue strings should stay on the fingers, and not sag significantly during a period of 60 seconds after application.
It is possible to overcome some of the problems with sagging from the glue line by “pushing” the glue more into the fingers from the nozzles by increasing the pressure in the applicator. However this technology makes it more difficult to control and ensure that the surface of the fingers in the finger joint are completely covered with adhesive.
Through the inventive method, condensation resin adhesives can be used in this very advanced but demanding application; something that was not previously considered as possible and brings several advantages as mentioned. It is surprising that, through the use of only small amounts of rheology modifiers that hardly affect adhesive properties (if at all) and/or temperature rheology modification, condensation adhesives that otherwise have unsuitable characteristics for contactless application can be utilized in such contactless finger joints and can reliably provide a very good coverage of the surface of the fingers and reduce sagging.
The hardener can be mixed into the adhesive or can be applied separately to the surface of the substrate. In this context, separate means that the adhesive and the hardener are not mixed. The adhesive includes the hardener in this application unless the hardener is separately stated. Applying the adhesive in mixed form is easier, but sometimes it can be advantageous to apply the adhesive separately in unmixed state to prevent premature curing. The adhesive and hardener can be applied separately and unmixed on the same surface or each separately to the opposing substrate surfaces to be adhered.
According to one embodiment of the invention, the hardener is applied separately to a substrate and wherein also the rheology of the hardener is modified by at least one of the following measures
In the embodiment where the hardener and the resin are applied as a mixture, the hardener to resin dosage is 5 to 50 wt % hardener to 100 wt % resin and preferably 10 to 30 wt % hardener to 100 wt % resin. In the embodiment where the hardener and resin are applied separately, the hardener to resin dosage can be from 10 to 200 wt % hardener to 100 wt % resin, preferably 50 to 150 wt % hardener to 100 wt % resin.
The condensation resin according to the invention may comprise any suitable condensation resin for producing a condensation adhesive preferably selected from the group of urea formaldehyde (UF) resin, melamine formaldehyde resin (MF), modified melamine formaldehyde resin (xMF) and phenol resorcinol formaldehyde resin (PRF) or combinations thereof. The resin preferably hardens at room temperature. One example of xMF resins include melamine modified urea formaldehyde (MUF) resins. The adhesives should have high boiling resistance, making melamine—formaldehyde, modified melamine formaldehyde resins and phenol resorcinol formaldehyde resins particularly suitable. Further, it is desired that the resins harden at room temperature. It is known in the art that these resins are suitable resins for use in condensation adhesives to provide excellent properties in gluing and low formaldehyde emission. For example, PCT/EP2009/060462, which is hereby incorporated by reference in its entirety, relates to an improved two component adhesive composition.
It has been found that the condensation resin, when implemented in the adhesive, works particularly well when the condensation resin is urea formaldehyde resin comprising between 20% and 60% wt %, preferably 30 to 55 wt %, most preferably 35 to 50 wt % urea or when the condensation resin is a melamine formaldehyde resin comprising between 25% and 50%, preferably 30 to 45 wt %, most preferably 35 to 45 wt % melamine or when the condensation resin is liquid melamine modified melamine formaldehyde resin comprising between 3% and 50% wt %, preferably 5 to 40 wt %, most preferably 15 to 30 wt % melamine.
In the embodiment where the resin is a PRF resin, the resorcinol content is between 5% and 50% wt %, preferably 10 to 45 wt %, most preferably 15 to 40 wt % resorcinol (calculated on total resin solids weight). When PRF resin is mixed with the hardener, then the hardener to resin dosage is 10 to 50 wt % hardener to 100 wt % resin and preferably 20-40 wt %, more preferably 10 to 30 wt % hardener to 100 wt % resin.
The rheology modified adhesive can be used to adhere miscellaneous wood products, but the advantages of the rheology modified adhesive are most pronounced when used in contactless fingerjoint application, in particular in substrates like laminated wood articles, in particular laminated panels and laminated beams.
The viscosity and/or thixotropy increasing substance (together referred to as rheology modifying substances or rheology modifiers) is preferably selected from the group of a carboxymethyl cellulose, hydroxyethyl cellulose, cellulose fibers, polyvinyl alcohol, polyurethane thickener, polyurea sag control agents (SCA), polyvinyl pyrrolidone thickener, swellable emulsions, xanthan gum, fumed silica and clay. Rheology modifiers include thickeners or rheology additives, both organic or inorganic compounds. Based on this disclosure the skilled person can easily find similar or other known rheology modifiers that when added to at least one of the resin and the hardener, provide the desired rheology modification effects. Preferably, the amount of rheology modifying substances is from 0.1 to 10 wt %, preferably 0.1 to 5 wt %, more preferably 0.1 to 2 wt %, calculated based on the total weight of at least one of the liquid resin and the hardener.
According to various aspects of the present invention, the composition described above can be used in an adhesive. In another embodiment, it is applied to a substrate through cooled nozzles.
In yet a further embodiment, the invention relates to a contactless finger joint applicator for cooling an adhesive to 10-15° C. and/or a contactless application device for carrying out the cooling step of cooling an adhesive composition to arrive at a desired viscosity.
In yet another embodiment, the invention comprises an apparatus for providing and/or cooling an adhesive composition comprising a means for cooling an adhesive to 10-15° C. and a viscosity of between 5,000 mPas and 20,000 mPas.
In accordance with another aspect of the present invention, an adhesive composition is used in contactless finger joint application method, said composition comprising a modified adhesive comprising: a condensation resin selected from the group of urea formaldehyde (UF) resin, melamine formaldehyde resin (MF), modified melamine formaldehyde resin (xMF) and phenol resorcinol formaldehyde resin (PRF) or combinations thereof, and optionally a hardener, wherein the composition further comprises at least one of a viscosity increasing substance and a thixotropic increasing substance.
Suitable contactless fingerjoint application devices to use with the present invention are commercially available, and include, for example, the Oest system devices, such as Oest Ecopur models. See http://www.oest.de, which is hereby incorporated by reference in its entirety. A typical pressure inside the equipment for contactless application of adhesives is 1.5-3.0 106 Pa (15-30 bar).
The features and advantages of the invention will be appreciated upon reference to the following drawings, in which:
The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.
The control unit may comprise a variety of functions, including regulating the temperature, pressure and other conditions of the adhesive components, the rheology modifier can also be added in the control unit. Further, the control unit ensures that the correct amount of resin to hardener wt % ratios according to the invention are met. This is performed prior to use of the static mixer 120. The application unit 140 may further contain a pressurizing device and pressure regulator, an optical sensor to monitor the mixed adhesive 115 as it is dispensed, and temperature regulation mechanisms, such as a heat exchanger to adjust remove heat from the resin and/or the hardener right before mixing and application as an adhesive. This temperature regulation can modify the rheology of the adhesive, as further discussed below.
The adhesives 115 according to the present invention comprise rheology properties that prevent and/or minimize sagging when applied to a finger 145 in a contactless application 100, as shown in
In this invention resin and hardener can be applied in two ways. Either the glue and hardener are mixed before application (as in
The fingers of the lamella, when pressed together, form the joints. The amount of time that it takes for the fingers to join after application of the adhesive is about 1-3 seconds. The pressure applied in pressing the lamellas together is specified in production control systems that are known and readily available to one of skill in the art. For example, 10 N/mm2 can be a typical pressure when pressing fingers together in a finger joint, but this depends on the density, temperature, finger profile etc.
The rheology modifiers are added to the resin or the hardener during production of the adhesive components. For separate application the modifiers will work in each component without influence of the other component. For mix in systems the viscosity and thixotropy of each component might influenced on each other. The behaviour of the final system will often be dependent on the pH of the final mix. Some rheology modifiers works at low pH while others works in high pH. For the amino systems the resin and hardener differs a lot in pH. The resin is on the alkaline side and the hardener on the acid side. For mix in systems it is therefore important to choose rheology modifiers that will give an effect at the pH of the final glue mixture.
The rheology of the resin may also be modified by applying a temperature to the adhesive just before application. For example, a heat exchanger, situated in the application unit 140 of the contactless applicator 110 may be applied before application of the hardener or the resin or both to reduce the temperature. Suitable temperature ranges include 5-15° C., and preferred 5-10° C.
An EPI adhesive mixture has a viscosity of app. 15,000 mPas (measured with Brookfield RVT viscometer at 20° C. with spindle 4 or 6 and 20 rpm) and this mixture will stay on the fingers as shown in
The following examples illustrate the surprising results when a condensation adhesive according to the present invention is applied in a contactless environment. All viscosities are measured with Brookfield RVT viscometer at the specified temperature with spindle 4 or 6 and 20 rpm. Generally a spindle 4 is used for viscosity measurements up to app. 10 000 mPas, and at higher viscosities spindle 6 is used.
The results are given as a tendency to sag within 60 seconds.
The following adhesives, hardeners and additives were used in Examples 1-4 below:
The viscosity and rheology of the MUF adhesive and the hardener is strongly temperature dependent. Table 1 shows typical viscosities for MUF adhesive and hardener at 25 and 10° C. and the related results in a contactless application equipment expressed as sagging.
A typical MUF adhesive at room temperature, approximately 20° C., has a viscosity of 3-6000 mPas. Decrease in temperature will improve the performance in contactless application equipment as shown in table 1.
A typical MUF glue mixture has a viscosity of app. 3500 mPas at 20° C. Adding a small amount of a swellable emulsion will give a more thixotropic adhesive mixture, reduce the sagging and ensure good performance as strings on the fingers.
Example 1 shows the performance in contactless application for a system without rheology additives. Improvements by adding rheology additives are shown in table 3.
As is demonstrated, in the present invention adhesive rheology is modified by increasing the temperature, changing the viscosity to greater than 5000 mPas per second at a shear rate of less than 10 reciprocal seconds, and/or adding a thixotropic agent.
The examples mentioned so far have been melamine based adhesives. The same approach can be used for a phenol resorcinol adhesive. The viscosity and sagging at 20° C. are too high at room temperature and addition of a rheology modifier or reduce the temperature of the glue mixture are required to obtain a system well suited for contactless application, as seen in Table 4.
Thus, the invention has been described by reference to certain embodiments discussed more than. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art.
Further modifications in addition to those described may be made to the structures and techniques described herein. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1004458.4 | Mar 2010 | GB | national |
This application is a continuation of international application number PCT/EP2011/054075 filed on Mar. 17, 2011 and claims priority from UK application number GB 1004458.4 filed on Mar. 17, 2010, the contents of which are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2011/054075 | Mar 2011 | US |
| Child | 13590249 | US |
