Abstract:
Two paper webs, saturated with a B-phase phenolic resin and dried, are conveyed through separate low melting point metal alloy baths, one web after being corrugated, to convert the resin to a fully cured phase, whereafter the webs are joined to form a structurally rigid waterproof single face corrugated web.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the manufacture of corrugated paperboard for use in structural applications in which waterproofing is imperative. More particularly, the invention pertains to a method and apparatus for converting corrugated medium and liner webs impregnated with a B-phase phenolic resin to a fully cured phase to produce a waterproof single face corrugated web. 
     U.S. patent application Ser. No. 11/769,879, filed on Jun. 28, 2007, which is incorporated herein by reference, describes a method and apparatus for manufacturing open core elements from paperboard webs for applications which might include exposure to water and high humidity. In such applications, the paperboard web must be treated to prevent damage and loss of strength in the presence of water. The hollowcore elements produced in accordance with the above identified patent lend themselves to many structural applications, including relatively narrow structures such as doors and much wider and deeper structures such as walls, decks, floors and beams. 
     One advantage of the method described in the above identified application, in addition to the broad flexibility of the process, is the high output attainable by the unique method for laying up the open core elements. If a waterproof paperboard web is required, it is important that the waterproofing process is fast and accurate enough to fit into the lay-up process without loss of time and quality. 
     SUMMARY OF THE INVENTION 
     In accordance with the basic method of the present invention, a method for curing a paper web impregnated with a B-phase phenolic resin, includes the steps of (1) providing a bath of a low melting point metal alloy that is hot enough to convert the resin to a fully cured phase, (2) carrying the web through the bath to provide direct contact of a web face with the metal alloy, and (3) maintaining contact of the web with the molten alloy for a time sufficient to fully cure the resin. The conveying step may comprise immersing the web completely in the molten alloy bath. Preferably, the conveying step comprises (1) providing the bath with opposite side walls that define coplanar upper edges, and upstream and downstream end walls that have upper edges below the upper edges of the side walls, and (2) conveying the web on the underside of the conveyor in a path between the side walls and over the end walls. Preferably, the method includes the step of sealing the interface between the lateral edges of the conveyor and the side walls. 
     In another aspect of the invention, the web is corrugated prior to conveying the web through the molten alloy bath. The corrugating step comprises carrying the web between upper and lower fluting conveyors having interengaging fluting bars. In accordance with this aspect of the invention, the corrugated web is maintained on the upper fluting conveyor for travel through the bath. Specifically, the method includes the steps of (1) conveying a paper liner web that is impregnated with a B-phase phenolic resin, through a molten alloy bath to convert the phenolic to a fully cured phase, and (2) joining the converted corrugated web to the converted liner web to form a composite single face web. The joining step preferably comprises (1) applying an adhesive to the flute tips of the corrugated web, and (2) pressing the liner web against the flute tips. 
     The present invention also includes an apparatus for curing a fluted paper web that is impregnated with a B-phase phenolic resin, comprising a heated bath for holding a molten low melting point metal alloy, the bath having a bottom wall, opposite side walls extending vertically upward from the bottom wall and defining upper edges of the bath, an upstream alloy supply header that extends between the side walls and has a horizontal upper edge below the upper edges of the side walls and defines a molten metal distribution reservoir. A downstream weir dam has an upper edge that is coplanar with the upper edge of the supply header and defines a trough for receiving molten metal alloy overflowing the weir. A pump supplies the molten metal alloy to the upstream supply header and returns molten metal to the header from the downstream trough in a closed circuit. A web conveyor including a plurality of interconnected articulated flights that are shaped to form and adapted to carry the fluted web on the underside thereof to run through the molten metal bath between the side walls and over the upper edges of the supply header and the weir dam. 
     The molten metal distribution reservoir preferably comprises an inlet for molten metal alloy that is centered between the side walls of the bath, and a distribution manifold that is adapted to equalize the distribution of the molten alloy returned by the pump laterally across the length of the manifold. The distribution manifold preferably has a symmetric pattern of alloy feed holes that extend laterally in opposite directions from the center inlet. The conveyor flights may be heated to preheat the incoming web. The flights preferably comprise aluminum extrusions. A continuous sealing strip is provided between the side walls and the lateral edges of the conveyor to inhibit leakage of the molten metal alloy. The sealing strips preferably comprise low friction plastic strips that are attached to the side wall. 
     A key feature of the present invention is an apparatus for making a waterproof corrugated single face web from two paper webs that are impregnated with a B-phase phenolic resin. The apparatus comprises a corrugator for one of the webs that has a pair of interengaging upper and lower conveyors, each of which has a plurality of interconnected articulated flights shaped to form a corrugated web from the web carried therebetween. The web  10 , with the phenolic resin in the B-phase, is quite flexible and readily corrugated. A low melting point alloy bath in the path of the upper conveyor provides direct contact of the alloy with the corrugated web on the upper conveyor sufficient to convert the resin to a fully cured phase. Means are also provided for heating the other paper web to a temperature sufficient to convert the resin to the fully cured phase. A single facer is provided to join the converted corrugated web and the other web to form the single face web. The heating means for the other web preferably comprises another low melting point alloy and a separate conveyor to immerse and carry the other web through the second bath. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation schematic of the curing apparatus for a corrugated paper web; 
         FIG. 2  is a side elevation schematic showing the  FIG. 1  curing station and the curing station for the liner web; 
         FIG. 3  is a schematic top plan view of the curing bath for the corrugated web shown in  FIGS. 1 and 2 ; 
         FIG. 4  is an upstream end elevation of the alloy supply header; 
         FIG. 5  is an enlarged schematic sectional view of the support and transfer arrangement for the web fluting conveyor; 
         FIG. 6  is a side elevation detail of the fluting conveyor shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a corrugated medium web  10  made from a paper web impregnated with a B-phase phenolic resin must be heated to a curing temperature sufficient to convert the B-phase resin to a fully cured phase in which the web is fully waterproof. The cured web also becomes substantially more stiff and severe bending of the web is thereafter restricted. However, web stiffness is an important characteristic of the corrugated web and the treated liner web to which it is attached, as will as discussed below, for processing in accordance with the method of open core element manufacturing disclosed in the above identified copending application. 
     In the embodiment shown, the medium web  10  is corrugated between interengaging upper and lower corrugating conveyors  11  and  12 , respectively. Each of the conveyors  11  and  12  comprises a belt of interconnected articulated flights  13  that have flute-forming teeth  14  to provide flutes of a desired depth and pitch. For example, flutes having a pitch of ¾ in. (19 mm) and a depth of ½ in. (13 mm) are satisfactory. The flights, preferably of aluminum, may be heated to minimize heat loss in the treatment bath to be described. 
     A heated bath  16  is positioned to receive the corrugated medium web  10  after it is formed and the lower corrugating conveyor  12  is directed away from the web and downwardly in a return run. The corrugated web  10  is retained on the underside of the upper corrugating conveyor  11  where the web flutes  15  remain in intimate contact with the teeth  14  of the conveyor flights  13 . 
     The bath  16  contains a low melting point metal alloy that is used to heat the web  10  and cure the phenolic resin as it passes through the bath  16  in contact with the molten alloy. One particularly well suited alloy is a 60/40 bismuth-tin alloy which is heated to about 400° F. (about 200° C.). Electric resistance heating may be used to maintain the bath temperature, but other heat sources may also be used. The bath has a generally horizontal bottom wall  17 , enclosed laterally by a pair of side walls  18  defining coplanar upper edges  20 . The upstream end of the bath is defined by an alloy supply header  21  that extends between the side walls  18  and has a horizontal upper edge  22  that is lower than the upper edges  20  of the side walls. The supply header  21  defines a molten metal distribution reservoir  23  for the uniform supply of molten alloy. The downstream end of the bath  16  is defined by a weir dam  24  that has a horizontal upper edge  25  that lies generally coplanar with the upper edge  22  of the upstream supply header  21 . The weir dam  24  defines an open slot  29  for receipt of the molten metal alloy that overflows the weir. 
     The molten metal is circulated through the bath in a closed circuit including a pump  26  receiving molten metal flowing into the slot  29  in the weir dam  24  and returning it to the alloy supply header  21  where it is distributed evenly and uniformly across the upstream end of the bath and downstream of the upstream end wall  19 . 
     In operation, the corrugated medium web  10  is carried by the upper conveyor  11  such that the tips of the flutes  15  slide over the upper edge  22  of the upper end wall and into contact with the molten alloy. The alloy in the bath is forced by pump pressure up into the flutes on the conveyor teeth  14 . Pump pressure is adjusted to provide sufficient head to fill the web flutes, preferably with a slight over-pressure to assure the underside of the fluted web  10  is fully contacted by the molten alloy. Movement of the conveyor causes the flutes to assist in carrying the alloy downstream and over the weir dam  24 . This action assures that the corrugated medium web  10  carried on the conveyor  11  is fully contacted by the molten alloy. This, in turn, assures that the entire web  10  is heated sufficiently to convert the phenolic resin to the fully cured phase. As the upper conveyor  11  and attached corrugated web  10  reach the downstream end of the bath, the flutes  15  engage and slide over the upper edge  25  of the weir dam  24  and the alloy drops into the slot  29  and travels through return passages  28  in the side walls  18  of the bath by operation of the pump  26 . 
     With a medium web  10  saturated with about 15% by weight of B-phase phenolic, the web is fully cured if it is retained in a bath of alloy at the indicated temperature for about 4 seconds. 
     Referring also to  FIG. 2 , a liner web  28 , also impregnated with B-phase phenolic, is directed with a liner conveyor  31  through a second bath  30  of molten metal alloy. The liner web  28  is cured in the same manner whereby the phenolic is converted to the final phase and fully cured. A suitable adhesive is supplied to the tips of the flutes  15  by an adhesive applicator roll  32  while the medium web  10  remains carried on the underside of the upper conveyor  11 . One suitable adhesive is a hot melt polyamide. The glued flute tips are joined to the cured liner web  28  on a contact roll  33  to form a fully cured single face web  34 . 
       FIGS. 3 and 4  show details of the molten alloy bath  16 . The alloy return lines  27  are connected beneath the bath to a center alloy supply tube  35  connected to the alloy supply header  21 . The supply header includes the distribution reservoir  23  which, as shown best in  FIG. 4 , includes an upwardly sloping lower wall  36  and an upper wall  37  that is provided with a pattern of outlet holes  38  that increase in size from the center laterally in both directions. This arrangement assures uniform distribution of the molten metal alloy across the entire width of the bottom wall  17  of the bath. 
     As shown schematically in  FIG. 5 , the conveyor flights  13 , which preferably comprise aluminum extrusions, are carried on a plurality of parallel laterally spaced roller chains  40  to which are attached pairs of oppositely extending upper and lower C-shaped attachments, each having horizontal mounting legs  41  and  42 , respectively. The lower mounting legs  42  are secured to the flights  13  and the upper legs  41  are captured in slots  39  in a low friction plastic bearing rail  43 . The bearing rail is preferably made of PTFE. 
     In order to inhibit leakage of the molten alloy between the conveyor  11  and the side walls  18  of the bath, the inner surface along the upper edge of each side wall is provided with a sealing strip  44  against which the opposite ends of the flights  13  of the upper conveyor  11  bear in operation. The sealing strip may be seen in FIGS.  1  and  3 - 5 . It is preferable to apply a vacuum to the upper corrugating conveyor  11  to aid in holding the corrugated medium web  10  in intimate contact with the conveyor flights  13 . One means of providing vacuum is to support the conveyor  11 , via the bearing rails  43 , on the underside of a vacuum plenum  45 , as shown schematically in  FIG. 2 . The conveyor flights  13  are attached to the carrying roller chains  40  such that the faces of adjacent flights  13  are spaced apart slightly, thereby allowing the vacuum to be applied directly to the corrugated medium web  10 . The sealing strip  44  also assists in sealing against vacuum loss.