ADHESIVE COMPOSITION FOR USE IN METAL-TO-METAL BONDING

Abstract: A thermally conductive, hot melt adhesive composition for use in metal-to-metal bonding is characterised in that it comprises: (i) at least one amorphous, thermoplastic, polyamide resin, the resin having a softening point of no greater than about 150`C, and the resin(s) comprising from 30-90% by weight of the adhesive composition; and (ii) a metal powder filler, the metal being selected from the group consisting of aluminium and copper, and the powder comprising from 10-70% by weight of the composition. A method of bonding metal substrates using this adhesive composition is also disclosed.

Description: "ADHESIVE COMPOSITION FOR USE IN METAL-TO-METAL BONDING" This invention relates to an adhesive composition for use in metal-to-metal bonding and particularly relates to a hot melt adhesive composition for use in bonding of metals where thermal conductivity is the main requirement. It is a particular object of the present invention to provide an adhesive composition which will tolerate both high and low temperatures, which is flexible and heat conductive, and which exhibits high adhesion, particularly when used in metal-to-metal bonding. One particular area in which the bonding of metals using a thermally conductive, hot melt adhesive can be of particular benefit is the bonding of metal tubing to sheet metal in the production of tube-on-sheet type heat exchange units such as evaporator or freezer boxes for refrigerators and freezers and collectors for solar heaters. There is now commercially available a pre-painted sheet steel in which the steel has been provided with a finished coat of a polyester, acrylic, vinyl or al yd paint. One such product is sold in Australia under the trade mark • 'Cclorbond"' . Such a material requires no f * u * rther painting - and thus, when used in fabrication, allows a manufacturer to dispense with a finishing paint line and a drying oven. Since welding, brazing, and soldering techniques- cannot be used in the fixing or bonding of such a coated sheet steel product, the use of metal-to-metal adhesives is of particular interest. The use of such coated sheet steel has been proposed for use in the construction of freezer boxes for domestic refrigerators since the pre-finished nature of the product is of particular value in the . automated assembly techniques which are now used by manufacturers of such products. Such automated assembly 5 techniques may, for example, produce one completed refrigerator or freezer every 15 to 120 seconds so that it is essential that the individual components are _ constructed by efficient and effective techniques. In the manufacture of freezer boxes for refrigerators from coated 0 flat sheet materials, it is proposed that the evaporation, condenser or other metal coil, which is usually copper- coated steel tube, will be formed into a serpentine configuration ready to lay flat onto the sheet. A coil wil then be bonded to the sheet by means of an adhesive and the 5 assembly then formed into the freezer box shape. In adopting such a technique for the manufacture of freezer boxes, the criteria for the adhesive will be thatitwill ha high strength and flexibility, have rapid "cure", be adapte for rapid and simple application, and have good thermal 0 conductivity. It has now been discovered that whilst adhesive .. compositions .based.on.other....types..of--x.esins_v:s.uch.. as-epoxy— " resins cannot meet the above-mentioned criteria, suitable 5 . adhesive, compositions- meeting- these criteria can be - - prepared using polyamide resins. According to one aspect of the present invention there is provided a thermally conductive, hot melt adhesive 0 composition for use in metal-to-metal bonding which - ——compriseS-τ;---_--.- .-" ' - ' " - ' - ■' ■—---::- (i) at least one amorphous, thermoplastic polyamide resin, said resin having a softening point of no greater than about 150°C,. and said resin(s). . 5 comprising from 30-90% by weight of the adhesive composition; and (ii) a metal powder filler, said metal being selected from the group consisting of aluminium and copper, and said powder comprising from 10-70% by weight of the composition. More than one polyamide resin may be used in accordance with the present invention, and a blend of two or more such resins may be used where it is desired to combine the properties of such resins. For example, one polyamide resin may be used to provide flexibility and another polyamide resin to provide strength to the adhesive composition when set. In general, an amorphous thermoplastic polyamide resin of lower softening point, for example up to 150 C, may be used to provide flexibility to the composition, whilst an amorphous thermoplastic polyamide resin of higher softening point, for example above 150 C, may be used to provide strength. In such a blend, the ratio of higher softening point polyamide resin : lower softening point polyamide resin may, for example, fall within the range 1:1 to 1:3. . .. . Suitable amorphous thermoplastic polyamide resins.. for use in the adhesive compositions of the present invention are available commercially and include Macromelt 6240, Macromelt 6239 and Macromelt 6238 (manufactured by General Mills Chemicals Inc.) , and Sunmide HT-150C an Sunmide HT-140E (manufactured by Sanwa Chemical Industry Co, Ltd.) . Whilst the use of certain, ones of these, polyamides.. is■ presently " pre erred, " and " these polyamides are described in detail below, it will be appreciated that other amorphous thermoplastic polyamide resins having an appropriate softening point may be used in. accordance with the present invention. Details of the presently preferred amorphous thermoplastic polyamide resins for use in the composition of this invention are as follows:- Macromelt 6240 Property Specification Method Appearance Amber coloured uniform (lump free) granular form resin with slight 10 amine odor. Softening Point (Ball & Ring) 133-145C ASTM E28 Brookfield Viscosity @ 225C 5500 - 8500 cps Thermosel Color max 8 Gardner 15 Characteristics Tensile Strength @ 75F 450 - 700 psi ASTM D1708 20 Elongation @ 75F 890 - 900% ASTM D1708 Shore A Hardness @ 75F 85 ASTM D1706 Water Absorptio 0.30* ASTM-D57.0- * 2.4 hours 25 Sunmide HT-140E Property Specification Method Appearance Pale yellow clean -3-0 solid..resin . . Softening Point 130 - 150C ASTM E28 (Ball & Ring) Shore A Hardness @ 20C Viscosity @ 200£ 5000 - 10000 cps Brookfield 35 Thermosel Characteristics Brookfield Viscosity BH Type @ 180C 6000-15000 cps @ 200C 4000-11000 cps @ 220C 2000-5000 cps A ine Value <7 Acid Value <12 Tensile Strength § 20C 40.5 kg/cm' Elongation @ 20C 600% Sunmide HT-150C Property Specification Method _ Appearance Pale yellow clean solid resin Softening Point 135 - 150C ASTM E28 (Ball & Ring) Shore A Hardness @ 20C ~80 Viscosity @ 200C 600 1000 cps Brookfield Thermosel/BH type.. .; . _ Characteristics Amine Value < 15 Acid Value < 3 Colour < 12 G/H " Method Specific Gravity Df Tensile Strength Elongation 250% ' Adhesive Shear 101.2 kg/cm 2 Strength T Peel Strength 12.5 kg/cm It will be appreciated from the above that various types of polyamide resins having a softening point of no greater than about 150°C are suitable for use in the therm conductive hot melt adhesive compositions of the present 5 invention. Details of a thermoplastic polyamide resin of higher softening point which may optionally be included in the composition of this invention are as follows:- 10 Macromelt 6300 Property Specification Method 15 Appearance Amber colored uniform bead form resin with slight.amine odor Softening Point 194-205C ASTM E28 Brookfield Viscosity, @ 240C 2500 - 5000 cps Thermosel 20 Color max 8 Gardner Characteristics Tensile Strength 3000 - 3400 psi ASTM .D1708_ 25 ■ &- 75F - Elongation @ 75F 400 - 500 ! ASTM D1708 Water Absorption ASTM D570 24 hours 0.10% 7 days 0.30% 3.0 30 days 0.50%_ " Speci c •" Grav y : :" .s≤ "" ~ :: STM ' D792 Viscosity @ 225C 7000 cps 35 ' Whilst it is essential to the present invention to use an amorphous thermoplastic polyamide resin in the composition, it is also within the ambit of the present invention to include a reactive or thermosetting polyamide resin in the adhesive composition. One suitable thermo- setting resin which is commercially available is Sunmide 300, manufactured by Sanwa Chemical Industry Co. Ltd.. The use of such a thermosetting resin is found to be of assistance in improving the flexibility of certain adhesive compositions of this invention. The filler used in the adhesive composition of the present invention is an aluminium or copper powder. Each of these is a metal which has high thermal conductivity. Aluminium is preferred. The powder preferably has a particle size in the range of 100 mesh to 550 mesh, with particles of sizes 200 mesh and 350 mesh being particularly preferred. Once again, blends of powders having different particle sizes may be used in the composition of the present invention, and typically a blend of aluminium powder 200 mesh with aluminium powder 350 mesh may be used. The adhesive composition of the present invention may also include a plasticizer (for example, in an amount of up to 25% by weight of the composition) , an antioxidant (for example in an amount of up to 5% by weight of the composition) , as well as other usual additives such as colorants and stabilizers. The plasticizers which are optionally included in the adhesive composition of the present invention may be any of the known plasticizing materials for polyamide resins. Such plasticizers are basically of two types, synthetic plasticizers such as N-ethyl-o, p-toluene-sulphonamide (which is available commercially as Santiσizer 8) and Santolite MHP, and naturally occurring types such as gum r~ - 4-τιτ resin and epoxidised soya bean oil. These two types of plasticizers may be used interchangeably. In general, plasticizers which are acceptable for use in the adhesive composition of the present invention are those having low odour and relatively high boiling points, as well as being relatively non-toxic. A gum resin which is used in certai of the preferred adhesive compositions of the present invention is Chinese Gum Resin WW of the following specification: 10 Property Specification Method Appearance Clean medium amber colored, clear hard brittle resin, which should be free of white 15 spots or opaque appearance indicating moisture, dirt or other extraneous matter. Odor Slight, if any- typical abietic acid odor with no turpentine or other object¬ ionable or foreign odor. 20 Softening Point (Ball & Ring) min 70C (70-85C) ASTM E28 Acid Number 160 .+ 5 ASTM D465 " Solubility " / ■' ■ ' '■■ - @.52% TS in thanol,. shall.be complete and show no crystallization after 25 standing overnight. ϋnsaponifiable max 8% ASTM D1065 Matter. Dirt & Foreign max 0.1% ASTM D269 Matter-.. Lovibond Tintometer 20Y 30 Standard 2.1.R ~ 7:_ .... ■' - '■ ]-. " - ■ .-. • :7 ': :- :r - --.-. ■ . -z-.-" * : -.----- ' QB * - Specific Gravity 1.07 @ 15.5C 35 The anti-oxidant which may be incorporated into the adhesive composition of the present invention is selected to act as a heat stabilizer for the composition. Such an anti-oxidant must be non-toxic, have low odour and have a 5 highboiling point. A typical example of an anti-oxidant which may be used is the material available commercially as Irganox 1076. Particularly preferred adhesive compositions in 10. accordance with, the present invention are set out hereunder by way of exemplification of the present invention. EXAMPLE 1 Parts by weight 15 Macromelt 6240 23.81 Macromelt 6300. 7.93 Santicizer 8 4.76 Al powder 350 mesh 63.50 20 The above-mentioned components are formed into the desired hot melt adhesive composition by firstly dry blending the components to obtain thorough mixing, followed by melt extrusion of the dry mixture in a three-zone . extruder in which the temperature ranges from 130°C to 180°C. 25 This final melt extrusion gives a product having the following properties: Viscosity: approx. 50,000 - 100,000 @ 19.5 C Application temp.: 19.5°C - 220°C 30 Specific Gravity:.- -1.63..- - Adhesive Strength: 750 psi (Colorbond/Colorbond) EXAMPLE 2 Parts by weight 35 Sunmide HT-150C 15.87 Sunmide HT-140E 15.87 Chinese Gum Resin WW 4.76 Al powder.350 mesh OR 1:1 blend of Al powder 350 mesh: Al powder 200. mesh 63.50 The components are dry blended and then melt extruded as described in Example 1 to give a product having the following properties: Adhesion : 750 psi CColorbond/Colorbondl M.p CBall & Ring). -141°C + 5°C Specific Gravity: 1.57 + 0.Q5 Tensile Strength: 1000 p.s.i. + 50 p.s.i. Elongation: 40. - 60% If desired, in each of the compositions of Examples 1 and 2 above, an anti-oxidant, for example Irganox 1076, may be added in an amount of up to 0.5% by weight. EXAMPLE 3 Parts by weight Macromelt 6240 15.625 Sunmide HT-150-C. . 15.625 Santi ' cizer " No-8 ""' "_ ■' " . T . " " 6^25 " . ■"' : '"■' " : Al powder 350 mesh 62.50 The components are dry blended and melt extruded as described in Example 1. EXAMPLE 4 Parts., by. weigh -Sunmide HT-Ϊ50-C 15; 625 "' " " "" "' Macromelt 6238 15.625 Santicizer No.8 6.25 Al powder 350 mesh - -6-2.50 ■"• - The components are dry blended and melt extruded as described in Example 1.- EXAMPLE 5 Parts by weight Macromelt 6238 31.25 Santicizer No.8 6.25 Al powder 350 mesh 62.50 The components are dry blended and melt extruded as described in Example 1. It has been found that the above described preferred compositions are particularly suitable in metal-to-metal bonding, especially in the bonding of tube to sheet as required, for example in the manufacture of freezer boxes as previously described. After bonding, the sheet can be bent at an angle of 90 without peeling, and the adhesive has satisfactory cohesive strength at both low (-20 C) and high (95 C) temperatures as well as being resistant to creep when exposed to temperatures of the order of 60 C to 80 C (which are generated during the exothermic foaming reaction used to form in situ insulating materials) . In another aspect of the present invention there is provided a method of bonding metal substrates which comprises the steps of:- (1) applying an adhesive composition as hereinbefore described to a first metal substrate; (2) pre-heating a second metal substrate to a temperature which does not exceed the softening temperature of said adhesive composition; (3) positioning said second metal substrate in contact with said adhesive composition on said first metal substrate and increasing the temperature of said first and said second metal substrates to a temperature in excess of said softening temperature; and (4) cooling said first and said second metal substrates to allow setring of said adhesive compositio . In yet another aspect, the present invention provides a method of bonding a length of metal coil or tube to a sheet metal substrate which comprises the steps of:- (1) applying an adhesive composition as 5 hereinbefore described to said coil or tube; (2) pre-heating said substrate to a temperature which does not exceed the softening temperature of said adhesive composition; (3) positioning said coil or tube in contact with 10 said substrate and increasing the temperature of said substrate and said coil or tube to a temperature in excess of said softening temperature; and (4) cooling said substrate and said coil or tube 15 to allow setting of said adhesive composition. Preferably, the adhesive composition is one of the preferred adhesive compositions specifically described abo The preheat temperature is preferably about 20°C below the 20 softening -temperature of the adhesive composition. Where a metal tube or coil is to be bonded to a. sheet -.7 / " -metal substrate, the substrate " may, if "' desired " -,- * b " e : pre- or into the desired shape prior, to bonding the coil or tube 25 " thereto. " Thus, in the formation of freezer boxes for refrigeration, the sheet metal substrate, which may for example be the pre-painted sheet steel sold under the trad mark "Colorbond", may be pre-formed by bending into the hollow box shape of the final freezer bo_ .. Alternatively, 30 and preferably, however, the coil or tube is . bonded .to the "" - ' " sheet metal substrate " before " the atte ': is formed * into its final shape. It is found that when an adhesive compositio as broadly described herein is used as the bonding agent, the. sheet substrate having the coil or- tube bonded thereto 35 may be bent at an angle of 90 without separation of the coil or tube from the substrate. In addition, the use of such an adhesive composition in either of the above method - U - gives a rapid "cure", so that an entire assembly and bonding operation may be carried on within a period of approximately 60 seconds as required in an automatic assembly line for refrigerators. In a first illustratiye embodiment of this aspect of the invention, a, pre-for ed freezer box for a refrigerator of colorbond material is pre-heated to approximately 120 C. Metal tubing, pre-formed to a serpentine configuration and having adhesive material of Example 2 above applied thereto by means of a hot melt adhesive gun or a heated roller, is placed in position on the box in an assembly jig. Heat is then applied to the assembly by means of hot platens or radiant heaters to raise the metal temperature to approximately 175 C, and the assembly is held at this temperature for approximately 30 seconds. The assembly is then cooled for a further period of approximately 30 seconds by passing cool air over the assembly to complete the assembly. According to a second illustrative embodiment of this aspect of the invention, which uses the adhesive of Example 1 described above, metal tubing to which the adhesive is pre-applied, is bonded to planar Colorbond substrate in a continuous or semi-continuous assembly operation. The substrate is fed onto a belt (such as a Teflon-coated belt) and passed to a pre-heat station at which the substrate is pre-heated to approximately 150 C by means of an electrically heated platen and then the tubing is positioned thereon. The tubing may if desired be held in position on the sheet by a support unit which is detachably secured to the sheet by suction cups or electromagnets. After a period of approximately 15 seconds, the assembly is passed on the same or a separate belt to the heating station at which the metal temperature is raised to a temperature above the softening temperature of the adhesive composition by means of a farther electrically heated platen. After being heated for approximately 15 seconds the assembly is passed on the belt or a further belt to the cooling station where it is cooled by means of a water-cooled platen to a temperature of approximately 60 C or less. After removal of the support unit the assembly is then passed to the next assembly stage in whic it is formed, for example, into a freezer box by bending. Whilst particular embodiments of the present inventio have been described herein by way of illustration, it will be readily apparent to persons skilled in this art that ma modifications and variations may be made to those embodimen without departing from the broad principles of the present invention, and that this invention includes all such modifications and variations.

Claims: CLAIMS : 1. A thermally conductive, hot melt adhesive composition for use in metal-to-metal bonding, characterised in that it comprises: (i) at least one amorphous, thermoplastic, polyamide resin, said resin having a softening point of no greater than about 150°C, and said resin(s) comprising from 30-90% by weight of the adhesive composition; and (ii) a metal powder filler, said metal being selected from the group consisting of aluminium and copper, and said powder comprising from 10-70% by weight of the composition. 2. An adhesive composition according to claim 1, characterised in that it comprises two amorphous, thermoplastic polyamide-resins, each resin ;. having a softening point of no greater than about 150°C. 3. An adhesive composition according to claim 1, characterised in that it further comprises an amorphous, thermoplastic polyamide resin having a softening point greater than about 150 C. 4. An adhesive composition according to claim 3, characterised in that the ratio of higher -.-- softening point resin to lower softening, point resin falls within the range 1:1 to 1:3. 5. An adhesive composition according to claim 1, characterised in that it comprises a reactive or thermosetting polyamide resin. 6. An adhesive composition according to any one of claims 1 to 5, characterised in that said metal . powder has a particle size in the range of 100 mesh to 550 mesh. 7. An adhesive composition according to claim 6, characterised in that said metal powder has a particle size in the range of 200 mesh to 350 mesh. 8. An adhesive composition according to claim 7, characterised in that said metal powder is a blend of 200 mesh particles and 350 mesh particles. 9. An adhesive composition according to any one of ............-.-claims._l...tα._.8,.:characteri.sed:_:in..that_it..further comprises " a plasticizer for said ' polyamide resin. 10. An adhesive composition according to claim 9, characterised in that said plasticizer comprises up to 25% by weight of the composition. 11. A method of bonding metal substrates, -- ■ - ;.. - : -;---chaxacrter±sed ^ (1). applying an adhesive composition according to claim 1 to a first metal substrate; (2.) pre-heating a second metal substrate to a temperature which does not exceed the softening temperature of said adhesive composition; C31 positioning sa,i.d second πjeta,!. substrate in contact with said adhesive composition on said first metal substrate and increasing the temperature of said first and said second metal substrates to a temperature in excess of said softening temperature; and (4) cooling said first and said second metal substrates to allow setting of said adhesive composition. 12. A method of bonding a length of metal coil or tube to a sheet metal substrate, characterised in that it comprises the steps of:- (1) applying an adhesive composition according to claim 1 to said coil or tube; (2) pre-heating said substrate to a temperature which does not exceed the softening temperature of said adhesive composition; (3) positioning said coil or tube in contact with said substrate and increasing the ' . "' . ' ■ ' tem eraϊiαre of ■ said :' substrate '" and " sai " coil or _ '" tube to a temperature in excess of said softening temperature; and (4) cooling said substrate and said coil or tube to allow setting of said adhesive composition. 3 * . A method,according, to claim.11 or. claim 12,. ... characterised in that said- ' pre-heat temperature " " is about 20°C below the softening temperature of the adhesive composition. 14. A method as claimed in claim 11, characterised in that it comprises the steps * of:- (a) passing said second metal substrate to a pre-heat station where it is heated by means of a pre-heat platen; Cb). assembling said first metal substrate to which said adhesive composition has been applied to said second metal substrate at said pre-heat station, and applying support means to hold said assembly of said first and second metal substrates through- subsequent steps; Ccl passing said assembly to a heating station where it is heated by means of a heating platen; Cdi passing said assembly to a cooling station where it is cooled by means of a cooling platen; and Cel removing said support means from said assembly.

Inventors: DOUTHAT M
Applicants: GRACE W R LTD AU, W R GRACE AUSTRALIA LTD
Application year: 1981
Main classification: C09J3
IPC codes: C09J5, C09J177, F28F1