DRYING A COATING OF SOLVENT-BASED ADHESIVE

Abstract: For drying coatings of solvent-based adhesive applied to shoe bottoms a through-flow drying apparatus is provided which utilises air at ambient temperature for the constant rate period (i.e. the period during which the surface of the coating is "wet"), and thereafter applies infra-red heating during the falling rate period (i.e. the period when migration of the solvent through the solid adhesive is to be achieved). In the first stage, jets of air are directed against the shoe bottom at a velocity of not less than 15 metres/second (measured at the inlets (38)), while at the heating station a plate heater (42) of a size which is larger than the area of the shoe bottom is provided. Ideally, when the shoe is removed from the apparatus, an outsole having a reactivated coating of adhesive can be located with the shoe bottom and bonded thereto, without reactivation of the shoe bottom adhesive itself. Also disclosed is a shoe support (46) for supporting shoes of a wide range of sizes with little or no need for adjustment, said support comprising a plurality of pairs of shoe-engaging rolls (60, 62, 64) between which the downwardly depending portions of the shoe last can be accommodated.

Description: This invention is concerned with the drying of a coating of solvent-based adhesive, more particularly with a method of drying such a coating applied to the surface of an article, e.g. a shoe, and with a drying apparatus for use in carrying out such a method. In the drying of a layer of solvent-based adhesive applied to an article surface, the total drying time can be divided into two separate periods: In a first period the rate of evaporation of solvent is determined by the state of the air immediately adjacent the surface, this rate being constant while the surface remains "wet". Thus during this period, which is referred to as the "constant rate period", the rate of drying depends upon the rate of removal of solvent vapour from the layer of air adjacent to the wet surface. In the second period, the rate of removal of solvent depends upon the migration of the solvent through the moist solid to the surface, the resistance to solvent transfer progressively increasing and thus reducing the drying rate. Thus during this period, which is referred to as the "falling rate period", the controlling factor is the rate of vapour diffusion through the solid. There is described in US-A-3,056,984 a method for cementing (i.e. adhesively bonding) soles to shoe uppers, in carrying out which method after adhesive has been applied to the bottom of a shoe to which a sole is then to be attached, firstly the shoe is allowed to remain in open atmosphere for an initial air drying of the adhesive, during which time the solvent at the surface of the adhesive is intended to evaporate off, whereafter the shoe enters a chamber in which the adhesive coating on the shoe bottom is subjected to heating for a further period. At the end of this further period, furthermore, the shoe passes through an air turbulence station at which the solvent-laden air adjacent the surface of the shoe bottom is removed and at the same time the adhesive coating is cooled, whereafter the shoe then passes to a final station within the chamber at which the adhesive coating is reheated and thus activated in readiness for the bonding of the shoe bottom to the shoe sole. It will of course be appreciated that allowing solvent to evaporate in open atmosphere is hazardous both to the health and the safety of the operator. Moreover, as already mentioned the rate of evaporation of solvent from a surface is dependent to some degree upon the proportion of solvent in the layer of air immediately adjacent the surface and, in carrying out the invention described in this US Patent, it is also clear that no steps are taken to remove the solvent-laden air from adjacent the surface of the shoe bottom until after the heating operation has been completed. As a consequence the cycle time for the drying of an adhesive coating applied to a shoe bottom using the method here described is relatively long. It is of course true that in using the process disclosed in this Patent the coating is not only dried but brought to a condition in which it is ready for the bonding of the shoe bottom to the shoe sole. It will, however, be appreciated that in bringing the coating to this condition after drying two further steps, namely firstly cooling and subsequently re-heating, are carried out. This of course leads to an extension of the drying/ preparation period. In US-A-4,336,279 is described an apparatus for drying and curing coating substrates, which apparatus comprises a chamber through which the coated substrate is progressively moved firstly passed a plurality of jets of air at elevated temperature (in the order of 175°C) and thereafter pass a bank of radiant heaters. The jets of air have a relatively high velocity (in the order of 30 to 40 metres/second) and the temperature of the air is in the order of 175°C. In addition, the bank of heaters takes up approximately half the length of the chamber. It will thus be appreciated that in order to obtain such high velocities and also in order to maintain the air at the desired temperature, as well as in order to operate the bank of heaters, a great deal of energy is consumed in this apparatus. On the other hand, the drying time would appear to be in the order of ten seconds. In using this apparatus, therefore, it is generally considered that heat is desirable in both the constant rate period and the subsequent falling rate period, the amount of heat supplied being dependent upon the amount of time which can be allocated to the drying process. It will of course be appreciated that use of heat in this way is expensive in so far as either the energy costs are high if the air is not re-circulated, or, if the air to be re-circulated, then a more complicated apparatus is required, usuall also involving, thermal insulation panels, as well as a system for removing solvent from the now solvent-laden air. It will of course further be appreciated that in using the process and apparatus disclosed in US-A-4,336,279 there is no reference to the further step of bonding, so that it is not clear from this specification that the process there described would readily be adaptable for use in drying solvent-based adhesive for subsequent use in bonding the coated substrate to a further surface. It is thus one of the objects of the present invention to provide an improved method of drying a coating of solvent-based adhesive in which, without significant disadvantage in terms of time requirements, significant savings can be made in energy consumption and overall cost. The invention thus provides, in one of its several aspects, a method of drying a coating of solvent-based adhesive applied to a surface of an article, comprising supporting the article with the coated surface thereof exposed, and passing the article thus supported through a chamber in which for an initial drying period during which the rate of drying of the coated surface is calculated as being constant or substantially constant a plurality of jets of air at ambient temperature is directed through an inlet or inlets towards said coated surface, in a direction extending normally or substantially normally to said surface such that the apical region of each jet impinges on the coated surface, and thereafter for a further drying period during which the rate of drying of the coated surface is calculated as being falling the coated surface is subjected to heating thus to continue the drying of the coating, the condition of the coating at the end of said further drying period being such that the coating is ready, without further treatment thereof, for the bonding of said coated surface to an attachment surface. It will thus be appreciated that, using such a method, firstly a proportion of the drying takes place without the provision of heaters (in general this takes place during the "constant rate" period of evaporation), the requirement for heat being reserved merely for the subsequent ("falling rate") period for which preferably a suitable infra-red heater can be provided. In practice, using a method as set out above it has been found that coatings of solvent-based adhesive applied to a shoe bottom can be dried in a period of less than 2½ minutes, at a cost in terms of energy consumption which is significantly smaller than with current available force-drying apparatus. Moreover, at the end of this drying operation the coating is in a condition ready, without any need for re-activation, for the bonding of the surface to an attachment surface (e.g. where said surface is constituted by the shoe bottom, the bonding of the shoe bottom to a shoe sole). Furthermore, although in a preferred embodiment of apparatus described hereinafter the drying air is recirculated (subject to a proportion being constantly removed for preventing a build-up of solvent in the recirculating air) and further it has been found that, during the course of a working day, the temperature of the re-circulating air may be raised above room temperature merely as a result of the application of heat during said further drying period, nevertheless the need for complicated and expensive thermal insulation to retain the heat (regarded as necessary in previous apparatus) is avoided in carrying the method according to the present invention. In carrying out the method in accordance with the invention the article is preferably moved progressively along a path firstly past said air inlet or plurality of air inlets and thereafter past a heating arrangement. By this progressive system, utilising a continuous conveyor arrangement, the articles can readily be moved from a loading to an unloading position via the air-blowing and heating positions, thus, at the same time enhancing the work flow. The heating arrangement preferably comprises a plate heater having a continuous planar heating surface by which infra-red radiation is emitted, the heater being of greater dimensions, both widthwise and lengthwise, than the shoe bottom so that the radiation is spread substantially uniformly over the surface of the shoe bottom despite variations in the distance of the shoe bottom from the heater by virtue of the heightwise contour of the shoe bottom. Further, by the selection of a suitable band of wavelengths emitted by the heater, it can be ensured that the energy is strongly absorbed by the material of which the shoe upper is made, regardless of its colour, without the need to adjust the output of the heater. The heater may be a metal plate, e.g. an aluminium pate, in which heaters are embedded, or alternatively may be made up of ceramic blocks in which heaters are accommodated. The surface temperature of the plate heater is preferably in the range of 450-500°C. In the initial drying period the velocity of the air jet(s) is preferably not less than 15 metres/second, more preferably at least 17 metres/second, (measured at the air inlet(s)), the arrangement being such that the apical region of the or each jet impinges on the article surface, whereby maximum turbulence is achieved at the article surface, thus ensuring removal of the layer of stagnant air which otherwise would tend to lie adjacent to the article surface and act as a barrier to the solvent evaporation. Where the coated surface is subjected to heating from an infrared radiant heater, preferably an air flow is created between said heater and the coated surface during heating, said air flow being diverted away from the heater. To this end, furthermore, the invention also provides, in another of its several aspects, drying apparatus for use in carrying out a method as set out above comprising a chamber comprising an air-blowing station and a heating station, a support arrangement for supporting an article with the surface thereof to which adhesive has been applied exposed, and drive means for moving the support arrangement to bring an article supported thereby successively from a loading station through the air-blowing and heating stations to an unloading station, wherein the air-blowing station comprises one or more air inlets for directing jets of air towards said surface of an article, in a direction extending normally, or substantially normally, to said surface, so that said jets of air impinge on the surface of the article, together with means for drawing air from the region surrounding the apparatus, and thus at ambient temperature, and wherein the heating station comprises a heating arrangement for directing infra-red radiation towards said surface of an article, a plurality of baffles being arranged at said station between the support arrangement and the heating arrangement and serving to concentrate the heat applied by the heating arrangement to the exposed surface of an article supported by the support arrangement, and further wherein the air-blowing and heating stations are so arranged within the chamber that air is drawn by the air-drawing means from the region surrounding the apparatus and passes through the heating station in a direction towards the air-blowing station, and still further wherein at least one of the baffles at the heating station extends transversely of the flow of air being drawn through said station by the air-drawing means thus to divert such flow away from the heating arrangement and towards the article surface. By drawing air through the heating station as aforesaid, it will be appreciated, the flow of air from the air-blowing station to the heating station is inhibited and the risk of ignition and/or explosion of solvent-laden air passing through the heating station is mitigated. This effect is of course further enhanced by the provision of said at least one of said baffles extending transversely of the air flow. The support arrangement of the apparatus preferably comprises a plurality of article supports, so that articles supported thereby can be moved progressively through the chamber in a continuous flow. One shoe support suitable for use in an apparatus as set out above, furthermore, is adapted to support, bottom uppermost, a shoe comprising a shoe upper on a last and an insole on the last bottom, and comprises a plurality of pairs of support members each providing an arcuate shoe-engaging portion, the members of each pair being arranged spaced apart from one another for receiving part of a shoe therebetween, wherein the members of at least one of said pairs are mounted for movement between first and second pre-set shoe-engaging positions whereby shoes of different sizes can be accommodated in the support. Preferably each support member comprises a roll of resilient material, and the movably mounted members are each mounted on an arm for pivotal movement between the two pre-set positions. As has already been mentioned, the adhesive coating, at the end of the further drying period, is in such a condition that it is ready, without further treatment thereof, for the bonding of the surface to a further or attachment surface. The method thus lends itself readily to a method of bonding a shoe sole to the bottom of a lasted shoe and, in accordance with the invention, in carrying out such a method conveniently a coating of adhesive applied to the shoe bottom is subjected to drying using a method of drying as set out above and is thus ready, without further treatment thereof, for bonding to a shoe sole, while the drying of the adhesive coating on the shoe bottom is taking place, the shoe sole, on the attachment surface of which an adhesive coating is provided, is subjected to a treatment to render the adhesive coating thereon ready for bonding, both adhesive coatings being thus made ready for bonding at the same time, and thereafter the two coated surfaces are brought into a desired location with one another and held pressed together until a bond therebetween is consolidated. There now follows a detailed description to be read with reference to the accompanying drawings, of one method and one apparatus (inlcuding a shoe support) in accordance with the invention. It will of course be appreciated that this method and this apparatus have been selected for description merely by way of exemplification of the invention and not by way of limitation thereof. In the accompanying drawings:- Fig. 1 is a plan view of an apparatus for carrying out the method in accordance with the invention; Fig. 2 is a front elevation of said apparatus; Fig. 3 is a details view in plan of a shoe support forming part of said apparatus; Fig. 4 is a fragmentary view along the line IV-IV of Fig. 3; and Fig. 5 is an underneath plan view of a heater arrangement of the apparatus. The apparatus in accordance with the invention now to be described is suitable for use in drying a coating of solvent-based adhesive applied to the bottom of a shoe for the subsequent attachment of an outsole thereto. Thus, the apparatus comprises a box-like structure 10 within which is provided a chamber 12 which is generally rectangular, but the rear "corners" of which are cut across by inclined side plates 14. Arranged above the chamber 12 is a plenum chamber 16 to which air is supplied by a fan unit 18 through a funnel-shaped conduit 20 arranged at the rear of the chamber. The fan 18 draws air from within the chamber 12 via a port 22 situated to the rear of the chamber, and also through further ports 24, 26 arranged towards the front of the chamber, which is open to atmosphere. Thus, a proportion of the air which is drawn in by the fan unit 18 will be solvent-laden, while a further proportion will be from the area surrounding the apparatus and will be relatively solvent-free. The chamber is also provided with two exhaust ports 28, one arranged in each of the wall portions 14, with each of which also is associated a fan unit 30. Thus, in the operation of the apparatus, solvent-laden air is progressively "bled" off from the chamber and is replaced by the relatively solvent-free air. The chamber 12 comprises an air-blowing station 12A and a heating station 12B. At the air-blowing station 12A, the base of the plenum chamber 16 is constituted by a perforated plate 36, the perforations 38 of which constitute air inlets through which jets of air are directed into the chamber 12 at the air blowing station thereof. At the heating station 12B said base plate is solid and supports, on the under-side thereof, a heater arrangement generally designated 40. The heating station 12B of the apparatus is, as can be seen from Fig.2, arranged adjacent the open front of the chamber 12. Thus, as ambient air is drawn from the area surrounding the apparatus by means of the fans 18, 30, it passes through the heating station 12B in a direction towards the air-blowing station 12A, thereby, militating against any risk of ignition and/or explosion by direct contact of solvent with the heater arrangement 40, and also serving to inhibit the passage of any air which is solvent-laden from the air-blowing station 12A into the heating station 12B. The heater arrangement 40 comprises a plate-type heater 42 which is made up of a plurality of ceramic blocks in which electrical heaters are embedded. The plate heater 42 is thus adapted to emit infra-red radiation and thus heat the shoe bottom presented thereto at a temperature which is substantially uniform over the whole of the surface of the shoe bottom. In the apparatus being described, the surface temperature of the plate heater 42 should be in the order of 450-500°C. Arranged beneath the plate heater 42, furthermore, is a plurality of (in casu nine) baffles 43 extending transversely of the flow path of the air being drawn past the heating station 12B. The baffles 43 are effective not only to act as channels for the heat emitted by the plate heater 42, concentrating it on the coated surfaces of the shoes presented to the heating station, but also act to divert the air passing through the station away from the surface of the plate heater 42, thus still further enhancing the mitigation of risk of ignition and/or explosion of the solvent by contact with the heater surface. The apparatus further comprises a support arrangement generally designated 44 comprising a plurality of shoe supports generally designated 46 for supporting, bottom uppermost, shoes S comprising a shoe upper on a last and an insole on the last bottom. The support arrangement 44 thus comprises a plurality of (in casu twenty two) shoe supports 46, each of which is carried by an inclined arm 48 in the form of two adjacent plates, mounted on a rotating support plate 50. The plate 50 is driven by drive means generally designated 52 and comprising a motor 54 by which a pulley 56 is driven, said pulley being connected by a belt 58 to a further pulley 60 carried by a shaft 62 on which the plate 50 is supported. Thus, when the drive means 52 is operating, and this is continuous in the operation of the apparatus, the support arrangement is driven so as to move the shoe supports 46 supported thereby progressively through the chamber 12. Suitable operator-actuatable control means is also provided for enabling the operation of the drive means to be controlled, whereby the time required for a shoe to pass through the chamber can be set by the operator. Each shoe support 46 supported by the support arrangement 44 comprises a plurality of (in casu three) pairs of support members, in the form of rolls 160,162,164 of a resilient material, e.g. silicon rubber. The first and third pairs of rolls 160,164 are each mounted on rigid lug portions 66,68 carried by the plates forming the support arms 48, the rolls of each pair being so spaced as to receive therebetween a downwardly depending portion of a shoe S for supporting it. Each roll thus provides an arcuate shoe-engaging portion, so that shoes of different widths can readily be accommodated therebetween as aforesaid. The middle pair of rolls 162 are mounted, also on lug portions 70 on said plates, for pivotal movement, about an axis 72, between first and second positions (shown respectively in full and chain line in Fig. 4) whereby to vary the distance between the shoe-engaging portions of the rolls. These two positions of the rolls 162 are determined by engagement of the support pin thereof with abutment surface portions provided on the lug portions 70, the arrangement being such that engagement of a roll by the shoe will not dislodge it from the position in which it has been set by the operator. As shown in Fig. 3, the shoe support in accordance with the invention can accommodate shoes of a wide range of sizes, a men's shoe S1 and also a children's shoe S2 being shown in that Figure. It will also be appreciated from Fig. 3 that, for the men's shoe the middle rolls 162 must be in their outward position (show in full line in Fig. 4) and support the last just forwardly of the break thereof (instep region), while for the children's shoe the middle rolls must be in their inward position (shown in chain-dot in Fig. 4) and support the cone of the heel end of the last (see also Fig. 2). In general, it will not be necessary to adjust the position of the rolls, other than for moving the middle rolls between the two pre-determined positions as aforesaid. If further adjustment becomes necessary, however, a bracket 74 providing the lug portions 66 and 70 is supported by its plate 48 by a pin-and-slot connection 76 such that the bracket can pivot or be moved heightwise according to the particular requirement. In carrying out the method in accordance with the invention shoes to the bottom surface of which solvent-based adhesive has been applied in a coating are supported, bottom uppermost, on the shoe supports 46 and are carried by the support arrangement 44 through the chamber 12 whereby firstly jets of air at ambient temperature are directed through the air inlets provided by the perforations 38 in the base of the plenum chamber 16, towards the shoe bottom surface in a direction extending normally, or substantially normally, to said surface, such that the apical region of each jet impinges on said surface. To this end, the velocity of the jets is not less than 15 metres/second, preferably at least 17 metres/second (measured at the air inlets 38) and the distance of the air inlets from the shoe bottom surface is 115 to 160 mm. (In the apparatus described the distance of the top edges of the plates 48 from the perforated plate 18 is approximately 160mm and the diameter of each air inlet is 20mm.) The speed at which the support arrangement 44 rotates is such that the shoe remains beneath the perforated portion of the plate 18 for a period which is determined according to the constant rate period, ie. the period during which the rate of drying is calculated as being constant, or substantially so, for the particular adhesive coating. This will of course depend upon the thickness of the coating, which in conventional practice in a shoe factory is of the order of 1.0mm or less. At the end of the constant rate period, the shoe passes on its support 46 to the heating station 12B, at which it passes beneath the plate heater 42 for a further period determined according to the falling rate period for the particular adhesive coating. It should be appreciated, however, that this drying period is terminated before completion, viz. when the condition of the adhesive coating is such that bonding of the shoe bottom surface to an attachment surface of an outsole can be effected without re-activation of the coating on the shoe bottom. In practice, it has been found that for a polyurethane-based adhesive of a type conventionally used in the shoe-making industry, an overall period of 2-2½ minutes is adequate for satisfactorily drying a coating with a dry weight of 10.5mg per sq.cm.. By virtue of the construction of the apparatus, furthermore, approximately 85% of the drying time is in the air-blowing station and the remaining 15% at the heating station. If desired, the apparatus in accordance with the invention can advantageously be arranged in a line within the shoe factory, which lines starts with a bottom, roughing operation following by a bottom cementing operation, from which the shoe is loaded into the apparatus. At the end of the drying operation the operator takes an outsole, having a coating of adhesive which has been re-activated at the same time as the shoe is being dried as aforesaid, and locates it accurately with the shoe when removed from the drying apparatus. This assembly of shoe and outsole is then placed in a sole attaching press for a period until a bond is consolidated therebetween.

Claims: Method of drying a coating of solvent-based adhesive applied to a surface of an article, comprising supporting the article with the coated surface thereof exposed, and passing the article thus supported through a chamber (12) in which for an initial drying period during which the rate of drying of the coated surface is calculated as being constant or substantially constant a plurality of jets of air at ambient temperature is directed through an inlet or inlets (38) towards said coated surface, in a direction extending normally or substantially normally to said surface, such that the apical region of each jet impinges on the coated surface, and thereafter for a further drying period during which the rate of drying of the coated surface is calculated as being falling the coated surface is subjected to heating thus to continue the drying of the coating, the condition of the coating at the end of said further drying period being such that the coating is ready, without further treatment thereof, for bonding of said coated surface to an attachment surface. Method according to Claim 1 wherein the velocity of the jets of air is not less than 15 metres/second (measured at air inlets (38) for the jets into the chamber). Method according to Claim 1 or Claim 2 wherein the coated surface is subjected to heating from an infra-red radiant heater, and further wherein an air flow is created between said heater and the coated surface during heating, said air flow being diverted away from the heater. Drying apparatus for use in carrying out a method according to Claim 3 comprising a chamber (12) comprising an air-blowing station (12a) and a heating station (12bB), a support arrangement (44) for supporting an article with the surface thereof to which adhesive has been applied exposed, and drive means (52) for moving the support arrangement (44) to bring an article supported thereby successively from a loading station through the air-blowing and heating stations (12A, 12B) to an unloading station, wherein the air-blowing station (12A) comprises one or more air inlets (38) for directing jets of air towards said surface of an article, in a direction extending normally, or substantially normally, to said surface, so that one or more jets of air impinge on the surface of the article, together with means (18) for drawing air from the region surrounding the apparatus, and thus at ambient temperature, and wherein the heating station (12B) comprises a heating arrangement (40) for directing infra-red radiation towards said surface of an article, a plurality of baffles (43) being arranged at said station (12B) between the support arrangement (44) and the heating arrangement (40) and serving to concentrate the heat applied by the heating arrangement (40) to the exposed surface of an article supported by the support arrangement (44), and further wherein the air-blowing and heating stations (12A, 12B) are so arranged within the chamber (12) that air is drawn by the air-drawing means (18) from the region surrounding the apparatus and passes through the heating station (12B) in a direction towards the air-blowing station (12A), and still further wherein at least one of the baffles (43) at the heating station (12B) extends transversely of the flow of air being drawn through said station (12B) by the air-drawing means (18) thus to divert such flow away from the heating arrangement (40) and toward the article surface. Apparatus according to Claim 4 wherein the support arrangement (44) comprises a plurality of article supports (46), each adapted to support, bottom uppermost, a shoe comprising a shoe upper on a last and an insole on the last bottom, and each comprising a pair of support members (160, 162, 164) each providing an arcuate shoe-engaging portion, the members (160, 162, 164) of each pair being arranged spaced apart from one another for receiving part of a shoe therebetween, and the members (162) of at least one of said pairs being mounted for movement between first and second pre-set shoe-engaging positions, whereby shoes of different sizes can be accommodated in the support (46). Apparatus according to Claim 5 wherein each support member (160, 162, 164) comprises a roll of resilient material, and further wherein said movably mounted members (162) are each mounted on an arm (70) for pivotal movement between the two pre-set positions. Method of bonding a shoe sole to the bottom of a lasted shoe, in carrying out which method a coating of adhesive applied to the shoe bottom is subjected to drying using a method according to any one of Claims 1 to 3 and is thus ready, without further treatment thereof, for bonding to a shoe sole, while the drying of the adhesive coating on the shoe bottom is taking place the shoe sole, on the attachment surface of which an adhesive coating is provided, is subjected to a treatment to render the adhesive coating thereon ready for bonding, both adhesive coatings being thus made ready for bonding at the same time, and thereafter the two coated surfaces are brought into a desired location with one another and held pressed together until a bond therebetween is consolidated.

Inventors: HANSON, RAYMOND, TILLYARD, MALCOLM
Applicants: BRITISH UNITED SHOE MACHINERY LIMITED, USM ESPANA S.A.
Application year: 1989
Main classification: A43D25
IPC codes: F26B15, F26B3, B05D3, A43D43, B05C9, A43D25, C09J5