PROCESS TO JOIN SANDWICH PANELS WITH WATER-BORNE CONTACT CEMENT AND APPARATUS

Abstract: Process for joining sandwich panels is described in which water-borne contact cements are used and the cement-coated panels are dried employing infra-red radiations of wavelength between 2 and 4 µm.

Description: PROCESS TO JOIN SANDWICH PANELS WITH WATER-BORNE CONTACT CEMENT AND APPARATUS BACKGROUND OF THE INVENTION The invention concerns a process to join sandwich panels with waterborne contact cement and the equipment (apparatus) necessary to carry out this process. The realization of sandwich panels used in building industries (sub- roofing panels, sheathing walls, interior partitions), in vehicle body industries, in furniture industries (desk tops, furniture, etc.) necessitates the joining of a panel facing on one or on both surfaces of an insulating material. The insulating materials are, in general, chosen from among the rigid foams: polystyrene foam of variable density, extruded polystyrene with or without surface coating, polyurethane foam, WC foam, phenolic foams, etc. Glass reinforced plastics, flakeboard and plywood materials can also be used. Panel facings are, in general, chosen from among: - metallic sheets, prelacquered or not (aluminum, steel) galvanized or electrogalvanized steel, of variable thickness, generally 4/10 to 10/10 mn; - asbestos cement or cement mixed with asbestos fiber, stabilized or not; - decorative laminates, - plywood of variable thickness, - panels of chip board fibers, - enamelled steel panels, etc. Several gluing processes have already been proposed for the manufacture of sandwich panels. Cited, in the first place, are the techniques of gluing, adhesively bonding, by means of contact cements or adhesives which are polychloroprene based adhesives in solution in organic solvents (ketonic, aliphatic, cycloalkane, aromatic). The term "contact adhesive" comes fran the fact that, to effect a bond, the following process is usually involved: - applying the adhesive through an appropriate method (gun, screen, roller) on each of the two materials to be joined; - drying (evaporating of the solvents), - assembling, and - pressing (2 to 5 bars) for a very short time, under a nip roller, for example. An industrial bonding chain within the "contact adhesive" process comprises the following elements: - a spray apparatus, containing several air spray guns (ordinarily 2 to 6) moving perpendicularly in the direction the conveyor moves; - a drying oven with forced convection hot air, equipped with a hood in which the solvent are vaporized. Depending on the equipment, the time of passage of the glued materials in the oven is 1 to 1.5 minutes; - an indexing table on which the glued materials are assembled upon their exit from the drying oven; - a rotary or nip press in which the assembly is submitted to pressures of 1.5 to 3 bars, critical to obtaining a useful bond. The use of these gluing processes, employing polychloroprene adhesives with an organic solvent base presents certain advantages, among which are: - applying adhesive with spray gun which allows a great seeed; - the versatility of the adhesive, which allows bonding of diverse materials; - the pressing through a rotary press which permits pressing at the speed (of advancement of the chain) of 3 to 7mhninute. These adhesives present, nevertheless, inconveniences, z s, and notably the aggressiveness of most solvents used toward the polystyrene as well as the important presence (approximately 80% in weight of the a adhesive) of very inflammable or noxious solvents: risk of fire, employees' health hazard, environment related hazards, and pollution. Other bonding methods have been proposed to avoid the inconveniences mentioned above. They concern principally the use of polyurethane or epoxy adhesives with two components and 100% dry solids, or polyurethane adhesives with 100% dry solids, monocomponent, moisture curing through the humidity contained in the materials. The advantages of these systems are the total absence of solvent, and the 100% content of dry solids. The application of the adhesive takes place only on one of the materials to be joined. However, the reaction time is long, imposing the immobilization of the bonding to be effected under press during time periods which can vary from 1 to 12 hours. There also exist polyurethane glues, monocemponent, moisture curing through reaction with the humidity of air and the materials, based in organic solvents to lower their viscosity and render them sprayable. Their advantage is the application on only one of the materials to be glued and their drawbacks are: - the presence of organic volatile solvents (40 to 758); - the reaction which results in a workable bond necessitates a relatively long pressing (30 minutes to several hours); - the range of gluable materials is limited to those containing enough water to provoke the moisture curing of the glue line. It follows that it would be beneficial to be able to profit fran the advantages of contact adhesives, without being confronted with the drawbacks mentioned above for this type of adhesive when used in a solvent base. These contact adhesives are available commercially in water base but their use is limited due to existing drying equipnent. The "high density" inf ra-red drying ovens allow to rapidly dry a film of water-based glue (5 to 30 seconds) but only on material insensitive to heat because of the high temperature obtained in the equipment. In hot air drying ovens, the drying time is very long (10 to 30 minutes). The e object of the invention herein is to provide a new process for joining sandwich panels with water-borne contact cement which avoids the drawbacks of the preceding technique and permits the practitioner to take maximum advantage of these adhesives for the joining of sandwich panels, even in the case where certain constituting elements of these panels are sensitive to heat. There is a need to establish a process of drying allowing water (approx. 30 to 50 g/n2) contained in the water-borne contact cement to evaporate in the evaporation time of organic solvents (approximately 60 seconds). This process can be implemented on existing industrial equipment whatever the panel material may be, and notably whether one deals with materials either insensitive to heat or materials very sensitive to heat, such as foamed or extruded polystyrenes which do not withstand a temperature above 70-C. SUMMARY OF TBE INVENTION The present invention provides a process of joining sandwich panels with water-borne contact adhesive cement characterized in that the coated panel is exposed to the action of infra-red radiation of wavelength comprised between 2 and 4 pri, while carrying out the evacuation of the formed steam, and thereafter carrying out in knawn manner, the assembling of gummed and dried backings. The e apparatus necessary for carrying out the rapid drying process according to the invention is characterized in that it requires a drying oven of which the heating device is essentially constituted of a battery of transmitters of infra-red middle-sized rays, of a wavelength comprised between 2 and 4 pmt connected to a battery of transmitters of short infra-red rays of wavelength comprised between 0.76 and 2 jin. The functioning of the latter transmitters is commanded by a device detecting metallic surfaces, the said drying oven being, moreover, equipped with means of extraction of the water steam formed. One skilled in the art can easily conceive the advantages presented by the present invention. These water-borne contact cements present all the interesting properties of solvent-based contact cements without their above-cited drawbacks; notably, replacing volatile and inflammable solvents with water suppresses any fire and pollution risk, while allowing use of products with higher solids. The present invention will be better understood and its advantages will be noted from the following description with reference to the attached drawing, Figure 1, the unique figure represents, in very diagrammatic form, the drying oven constituting the essential element of the apparatus according to the invention. In the drawing (1) designates, in general manner, the drying oven, (2) the battery of average wave length infra-red transmitters, (3) the battery of short wave length infra-red transmitters, (4) the detector of metallic surfaces z s and (5) the means of evacuation of the steam formed from the water in the adhesive. The drying oven (1) represented in the drawing is composed essentially of the following elements: - a battery (2) of nine average IR radiants of 1.8 Kw, of wavelength comprised between 2 and 4 pn; - two batteries (3) of three short IR radiants of 2 Kw, of wavelength comprised between 0.76 and 2 - a device (4) for detecting metallic panels of very weak thickness (less than or equal to 1 mm) and whlCh controls the functioning of batteries of short IR radiants (3); - means (5) of evacuation and ventilation; - a conveyor (6) of adjustable speed, for example, between 0 and 2.7 nRminute which allows the introduction of coated (gummed) panels into the drying stove (1) for drying. A potentiometer (not shown) regulates the power of the average IR radiants (2) from 0 to 100%. Also foreseen, in case of the conveyor (6) stopping, is a device of automatic protection (not represented) avoiding the degradation of the panels present in the drying stove. The process will now be described in detail. The panels (7) which are to be assembled are coated, in conventional fashion, for example, through gun spraying, with a water-borne contact cement which is a mixture of polychloroprene latex (or other type of polymers: acrylics and sBR), of various resins (for example, "tackifying" resins of which the goal is to improve the gluing power armor adherence) and of various adjuvants: tensio active, thickening, vulcanization agents, antioxidants, and the like. Such a water-borne cement contains approximately 30 to 65% dry solids. For example, a polychloroprene cement based on the following is used: - Latex 671 (du Pont de Nbmours) 100 parts - Antioxidant 2 parts - Phenolic terpene resin in emulsion 50 parts - Zinc oxide 10 parts The polychloroprene latexes used can comprise in their molecule reactive groupings, allowing the eventual utilization of a hardener to improve certain performance of the bonds obtained, such as resistance to humidity, and heat resistance. In proceeding with the process, the drying oven is started; - the air exhaust (5) assuring the evacuation of the moist air is started up; and - the battery (2) of average IRs is started up (warm-up time for useful power: 5 to 10 minutes). The glued panels (7) that are to be dried are placed onto the conveyor (6) and the speed is regulated to approx. 2.7 mMminute. In instances where panels of metal sheet (of thickness 5/10mm) are presented on the conveyor under the detector (4), short infra-red batteries (3), all of which-reach sufficient power in from 3 to 5 seconds, will be activated and insure a full drying of the glued metal sheet. Any means of detection can be used, for example, magnetic detection or detection based on the reflection characteristics of metallic surfaces. The battery of short infra-red radiants (3) will not be started up when non-metallic panels (such as polystyrene) pass through; such panels will be submitted only to the action of the average infra-red radiations of the battery (2). A full drying has been obtained in a passing time of 36 seconds under a battery of nine radiants of 1.8 Kw at a distance of 1,650 irni, arranged at 230 lik above the conveyor (6), the power being regulated at 508 of the maximum power. It is advisable to provide, for the case when the process chain unexpectedly stops while a panel is in the oven, an automatic system which by augmenting the supply of air and diminishing the power of the average infra-red radiants lowers the temperature in the oven very rapidly. Other devices such as automatic reversal of the average IR radiants, and autanatic setting up of screens which lower the exposure of the average IRs, can also be used. It has thus been possible to ocxnpletely dry in industrial conditions, in less than a minute, water-based contact cement films deposited by means of automatic air spray guns at the rate of 55 and 80 g/m2 on the following materials: - sheet of metal of pre-lacquered and clad aluminum alloys of 5/10 mm thickness currently designated by the tradename Stuco; - sheet of metal of pre-lacquered aluminum of 5/10 mn thickness; - polystyrene expansed uninterrupted (25 kg/m3) of 25 rtrn thickness; - polystyrene extruded uninterrupted of 25 rrm thickness, such as the one commercialized under the trademark Styrofoam; - plywood of 3 mm thickness; and - fiber panel of thickness 3 mn. After having been glued and dried by passage through the drying oven (1), the following sandwich asemblies have been immediately realized: - aluminum Stuco/plywood/polystyrene/plywood; - aluminum Stuco/foamed polystyrene/wood fibers such as the one commercialized under the trademark Panosom; - smooth alumlinun/Styrofoa/plywood. As soon as the assembly is realized, the thus constituted sandwich panels are pressed between the rollers of a calender (not represented). Panel samples joined by the process herein have been tested in resistance to delamination by pulling the panel apart and canparing the test results to the same samples joined on the same chain employing a solvent-based contact glue. The table below summarizes the test results obtained (average of five observations) of the following tests: - measure of resistance to delamination in mPa, measured at ambient temperature and peel speed of 5 mm/minute, carried out in canparative manner, on panels glued with a contact cement with conventional organic solvent and on panels glued with a water-based contact cement, which panels are dried following the conditions of the process herein. TABLE I WATER-BASED CONTACT CEMENT SOLVENT-BASED CONTACT CEMENT MRTA(1) METHOD OF FAILURE MRTA(1) METHOD OF FAILURE Laminated/ 0.32 In the thickness of the 0.16 Superficial cells Foamed Polystyrene polystyrene of polystyrene Smooth aluminum sheet/ 0.36 Separation of the 0.25 Partial separation extruded polystyrene surface skin of the surface skin (Styrofoam) Panosom/polystyrene 0.18 Separation of 80% of 0.16 Separation of 50% of the superficial fibers the superficial fibers Aluminum sheet 0.16 Separation of the 0.15 Separation of the Stuco/Expansed superficial cells at superficial cells at Polystyrene contact points contact points (1) MRTA = Measure of Resistance to Delamination (expressed in MPa) These results show that use of the present process employing waterbased contact cements, suitably dried on the panels in accordance with the process of the invention, results in sandwich panels having stronger bonds than those obtained with solvent based adhesive.

Claims: WHAT IS CLAIMED IS: 1. A process to join sandwich panels employing water-borne contact cement comprising the steps of exposing the contact cement coated panels to the action of infra-red radiations of wavelength comprised between 2 and 4 pn while carrying out the evacuation of the formed steam, and thereafter assembling the coated dried panels to provide the sandwich panels. 2. The process of Claim 1 wherein the water-borne contact cement is a polychloroprene latex emulsion. 3. The process of Claim 2 wherein the polychloroprene latex emulsion contains a tackifying resin and one or more adjuvants. 4. The process of Claim 1, wherein the water-borne contact cement is an acrylic or SBR emulsion. 5. The e process of Claim 1 wherein at least one of the panels to be joined is a material sensitive to temperatures above 70 CC. 6. The process of Claim 1 wherein at least one of the panels to be joined is polystyrene. 7. The e process of Claim 1 wherein at least one of the panels to be joined is metallic and is additionally suhnitted to the action of short inf ra-red radiations of wave length comprised between 0.76 and 2 pn. 8. Apparatus for carrying out a process to join sandwich panels comprising a drying oven in whiCh the heating device is essentially composed of a battery of transmitters of infra-red rays of average wavelength comprised between 2 and 4 Sm, and is equipped with means for evacuating the formed steam. 9. The apparatus of Claim 8 wherein the battery of transmitters of infra-red rays of average wavelength is associated with a battery of transmitters of infra-red rays of short wavelength comprised between 0.76 and 2 pn, which are activated for drying metallic panels. 10. The apparatus of Claim 8 comprising means for detecting accidental stoppage of the process chain and an automatic system for diminishing the power of the infra-red transmitters or increasing the supply of air.

Inventors: ANCEY, MAURICE, GREPINET, JEAN, KIEFFER, ANNIE, MAGNAUD, GERARD, MARY, ADRIEN, MATHIS, JEAN-FRANCOIS
Applicants: NATIONAL STARCH AND CHEMICAL CORPORATION
Application year: 1986
Main classification: C09J5
IPC codes: B29C65, C09J5