Abstract:
A structural panel comprises phenolic skins formed over a honeycomb core. The skins are bonded to the honeycomb under vacuum and heat, providing a panel capable of forming to desired shapes. The panel is 30% lighter than aluminum honeycomb panels of similar thickness, equivalent in strength to aluminum honeycomb panels, and meets the very stringent fire, smoke and toxicity norms of the industry. Additionally the product also reduces the thermal load, has very high heat resistance and is corrosion resistant. The use of this product is not limited to flat profiles, but can also be used to mold double curved or other three dimensional profiles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/846,747 filed Jul. 16, 2013, entitled LIGHTWEIGHT REINFORCED PHENOLIC STRUCTURAL SANDWICH PANEL BASED ON ARAMID HONEYCOMB CORE AND METHOD. 
    
    
     BACKGROUND 
     This disclosure relates to structural panels, for example wall panels used in construction of rolling stock or other vehicles. 
     Wall panels used in the construction of, for example, rolling stock, train cars, airplanes and the like, need to be rigid, light weight and fire resistant. Current practice in the rolling stock industry is to use aluminum honeycomb panels as construction panels. As the rail industry moves to faster trains it also needs lighter products to achieve this target, hence the industry has opted to use aluminum honeycomb panel, wherever light weight nearly flat paneling is needed, such as external paneling. 
     Aluminum honeycomb panels have some further disadvantages in that the shapes to which they can be formed are limited and if the panel is impacted, the aluminum skin &amp; core can permanently dent or deform resulting in a dented appearance. Also being a metal, it has lower corrosion resistance and is a conductor of heat, which are undesirable properties for this kind of application. 
     SUMMARY 
     The present disclosure relates to a processing method, to produce panels with phenolic skins &amp; core, more effectively and securely. 
     In accordance with the disclosure, an alternative material is offered, which is about 30% lighter, equivalent in strength to aluminum honeycomb panels, and meets the very stringent fire, smoke and toxicity (safety) norms of industry. Additionally the product also reduces the thermal load, has very high heat resistance and is corrosion resistant. The use of this product is not limited to flat profiles, but can also be used to mould double curved or other three dimensional profiles. 
     Accordingly, the present disclosure provides an improved structural panel for use in industrial applications. 
     The subject matter of the present technology is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and embodiments thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a portion of an exemplary panel; 
         FIG. 2  is a cross sectional view of a completed panel; and 
         FIG. 3  is a diagram showing a production method. 
     
    
    
     DETAILED DESCRIPTION 
     The system according to a preferred embodiment of the present disclosure comprises a structural panel comprising a honeycomb core with glass reinforced phenolic skin and method of producing the same. 
     Referring to  FIG. 1 , an exploded perspective view of a portion of an exemplary panel  10 , the construction of this panel is typically glass reinforced phenolic skin layers comprising two fabric layers  12 ,  14  bonded via use of bonding medium  16 , comprising a phenolic adhesive, to an aramid honeycomb core  20 . When assembled, the layers  12 ,  14  and bonding medium  16  form a layer  18 , typically 0.5 to 2 mm in thickness. The honeycomb core has uniform cell size of 5-15 mm. The bonding medium is an in-house formulated glue using Catalyzed liquid phenolic resin mixed with a pyrogenic oxide such as fumed silica and other additives, in the range of 0.2% to 10%. This gives the product its thixotropy, low moisture absorption and optimizes its rheological properties. The phenolic skin layers use glass fibers to provide strength and the resin of the phenolic skin provides excellent fire resistance and low smoke and toxicity benefits. This bonding medium has the same chemistry as the resin in the skin layers, and therefore assists in mechanical and chemical bonding between the skins and the core, curing to exhibit similar properties to the phenolic skins. 
     Current established methods for manufacturing of similar panels are typically done using prepegs as layers  12 ,  14  with an optional adhesive film between the core and the prepeg which assists in bonding. The product in accordance with the present disclosure uses a liquid phenolic resin to impregnate the selected glass fibre and specially formulated adhesive in the intermediate layers for the bonding. 
     This process is thereof modified to accommodate glass reinforced fibre skins impregnated with liquid phenolic resin. 
     The skins construction can be varied to include stitched, woven or Aramid based/E glass fibre to enhance the properties as needed. 
     Use of light weight, strong and easily impregnated fibres for layers  12 ,  14  with a certain structure/weave and density of, for example between 30-500 gms/square meter, ensures that the adhesive is retained in the upper skin during application and does not totally seep down to fill the honeycomb cells and increase the weight. Also, since the adhesive is applied in between two layers, this restricts the downward flow of the adhesive, yet allows penetration of the adhesive through the layers in limited amounts towards the core under the effect of vacuum as discussed below. 
     The entire composite panel is then subject to vacuum and high temperature using negative pressure by the process of vacuum bagging. With reference to  FIG. 3 , a diagram showing a production method, a mould  24  is provided which has a face  26  with a desired finished 3 dimensional configuration, whether flat or contoured to form the panel to a desired shape. A bottom skin  28  (which as noted previously may comprise a glassfibre composed of two fabric layers  12 ′,  14 ′ and a bonding medium  16 ′ therebetween) is placed against the mould face  26  and a honeycomb core  30  is then positioned on top of the bottom skin. Top skin  32  (which as noted previously may comprise a glassfibre composed of two fabric layers  12 ,  14  and a bonding medium  16  therebetween) is set on the top face of the honeycomb core, followed by a peel ply  34 , release film  36  and bleeder fabric  38 . Optionally, a thin layer of adhesive coating  29  and  31  may be applied between the skins and the honeycomb core. The entire sandwiched assembly is then placed within a vacuum bag  40  which is sealed to the mould by use of seal  42 . A vacuum connector  44  is provided in the mould to allow connection to a vacuum source, which draws the vacuum bag down to compress the various layers together. 
     The bonding of individual layers is then achieved by exposing the uncured laminate to high temperature while under vacuum. This methodology ensures that the core plus skins develop a mechanical and chemical bonding. Other established processes use high positive pressure for bonding instead of a negative pressure (vacuum) used in the product and process of the current disclosure. 
     The panel is cured completely at temperatures ranging from 60-120 Deg Celsius for 1-8 hours under vacuum, before being taken for the further processing. This produces a product which is very light, has high stiffness and good peel strength in addition to excellent fire smoke and toxicity requirements. 
     The honeycomb core being phenolic based, forms a chemical bond with the skins due to fusing of similar materials used on the skins. A mechanical fixation is ensured by ‘fillet formation’ at the edge of the honeycomb cells. The specially formulated adhesive has a combination of surface tension, surface wetting and controlled flow during the early stages of cure which enable the formation of the fillets. 
     The density and type of the glass used as adjacent skin layers to the core are unique and the fabric assists in the bonding of the skins to the core as it offers the right permeability for the glue to flow through and form localized fillets with the core and improves the peel strength. 
     As a standard industry practice, formation of panels is accomplished with the glue applied immediately between the core and the skin. In contrast, since the product and process of the current disclosure, glue is applied in between two fabric layers, the adhesive is trapped, yet able to penetrate toward the core side as required under vacuum. This prevents the glue from falling into the hollow honeycomb cells and collecting on the lower face, due to gravity. This promotes equal adhesive application on each side, which is more desirable, as unequal glue on either face, leads to an imbalance, improper bonding and lower peel strength on the upper moulded face. 
     The bonding of individual layers, achieved by exposing the uncured laminate to high temperature and negative pressure provided by the vacuum ensures that the core and skins develop a mechanical and chemical bonding. 
     Use of the applicants&#39; bonding methodology reduces the amount of bonding medium that is needed for proper bonding, so therefore reduces weight. 
     Since the core and bonding medium are co-cured along with the skins the resulting product is dimensionally stable and evenly bonded. 
       FIG. 2  is a cross sectional view of a panel after processing. The core, being phenolic based, forms a molecular bond with the layers, and mechanical fastening is ensured by fillets  22  formed at the edge of the honeycomb cells. 
     Table 1 illustrates some properties of an exemplary panel produced in accordance with the disclosure herein. 
                             TABLE 1               10 mm thickness   STD/SPECIFICATIONS   Present product                   Weight kgs/m2   Lightweight   3.2-3.4 kgs/m2       Peel Torque (avg)   ASTM D 1781   24 lbs - in/3 in               (Load - 215 N/3               in)       Flexural   ASTM C 393           Properties   Span ratio of 2:1 -               length:width           Max load       300 N       Facing Bending       48-50 MPA       Stress               Core shear       0.6 MPa       Stress - due to               flexion               Flat wise tensile   ASTM C297   0.8 MPa (failure       strength       mode: core               sheared, bonding               between facings               &amp; core intact       Flat wise   ASTM C365   2.2 MPa       compression               strength               Maximum   Distributed   Elastic       deflection under   vertical load -   deformation -       load   applied at 600   6.8 mm max; No           kgs/m2   permanent               deformation       Maximum   Vertical   Elastic       deflection under   distributed load -   deformation -       load applied   applied at 160   0.7 mm max; No       continuously for   kgs/m2   permanent       48 hrs       deformation       Fire               specification               German   DIN 5510   S4/SR2/ST2       German/European   EN ISO 5659 (as   FED at 30 min =           per DIN 5510 -   0.07           2009)           EU   EN 45545   HL2 for               Interiors       French   NFF 16101   M1 F1       British   BS 6853   Category 1A       American   ASTM E 162   FSI &lt; 10           ASTM E 662   Ds (1.5) &lt; 100;               Ds (4) &lt; 200       Impact resistance   As per EN 438-2/22   No crack,           When ball is   impression of           dropped from a ht   11 mm Φ; &lt;0.2 mm           of 1 m.   in depth       Water absorption   90 hrs continuous   &lt;2.5%           immersion           Performance after   EN ISO 9142 Cycle   No debonding or       Aging: (Thermal &amp;   D3 - for 30 days   failure of       humidity cyclic       construction.       test)               Vibration test   EN 61373   No debonding or           (04/2011) § 10   failure of               tested panel.                    
Case Study
 
     An external panel for a toilet cabin (e.g. for use in a train) was moulded and assembled by replacing an existing aluminum honeycomb panel. Table 2 illustrates the comparison with the prior aluminum honeycomb panel. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Al HC 
                   
                   
               
               
                   
                 existing 
                 Current 
                   
               
               
                   
                 panel 
                 disclosure 
                 Remarks 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Panel weight 
                 18.4 kgs 
                 13.8 kgs 
                 25% lighter 
               
               
                   
                 (as 
                   
                   
                   
               
               
                   
                 assembled) 
                   
                   
                   
               
               
                   
                 Straightness 
                 1.55 mm 
                 2 mm across 
                 Dimensionally 
               
               
                   
                   
                 across span 
                 span length 
                 equivalent 
               
               
                   
                   
                 length 
                   
                   
               
               
                   
                 Deflection 
                 &lt;5 mm 
                 &lt;5 mm 
                 Equivalent in 
               
               
                   
                 at 100 kgf 
                   
                   
                 performance 
               
               
                   
                 in the 
                   
                   
                   
               
               
                   
                 middle of a 
                   
                   
                   
               
               
                   
                 wall over a 
                   
                   
                   
               
               
                   
                 span length 
                   
                   
                   
               
               
                   
                 of 2000 mm 
                   
                   
                   
               
               
                   
                 Assembly 
                 Standard 
                 Exactly the 
                 No change in 
               
               
                   
                 interfaces 
                   
                 same as 
                 fastening 
               
               
                   
                   
                   
                 aluminum 
                 techniques 
               
               
                   
                   
                   
                 honeycomb 
                   
               
               
                   
                   
                   
                 panel 
                   
               
               
                   
                 Curved 
                 Radiuses 
                 Easy to 
                 Not possible 
               
               
                   
                 geometry 
                 lager than 
                 mould, 
                 to bend AL HC 
               
               
                   
                   
                 40 mm only 
                 radius as 
                 to tight 
               
               
                   
                   
                 possible 
                 small as 3 
                 radiuses. 
               
               
                   
                   
                   
                 mm possible 
                 Bends on 
               
               
                   
                   
                   
                   
                 larger 
               
               
                   
                   
                   
                   
                 radiuses are 
               
               
                   
                   
                   
                   
                 at risk of 
               
               
                   
                   
                   
                   
                 deforming 
               
               
                   
               
             
          
         
       
     
     Table 3 shows some exemplary variations of panels constructed in accordance with the present disclosure. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 SPECIALTY 
                   
                   
                   
                   
                   
               
               
                 (10 +/− 1.5 mm 
                   
                   
                   
                   
                   
               
               
                 thickness) 
                 Standard 
                 Panel 1 
                 Panel 2 
                 Panel 3 
                 Panel 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Weight kgs/m2 
                 3.2-3.5 
                 kgs 
                 3.8-4 
                 kgs 
                 5-5.2 
                 kgs 
                 3.3-3.6 
                 kgs 
               
               
                 Skin thickness 
                 0.35-0.45 
                 mm 
                 0.5-0.6 
                 mm 
                 0.8-0.9 
                 mm 
                 0.35-0.45 
                 mm 
               
             
          
           
               
                 Peel strength/ 
                 ASTM D 178 
                 24 lbs - in/3 in (Load 215 N/3 in 
               
               
                 peel torque 
                   
                   
               
             
          
           
               
                 Flexural 
                 ASTM C 393 
                   
                   
                   
                   
               
               
                 Properties 
                   
                   
                   
                   
                   
               
               
                 Max load 
                   
                 300 N 
                 340 N 
                 460 N 
                 390 N 
               
             
          
           
               
                 Facing Bending 
                   
                 45-48 
                 MPa 
                 36 
                 MPa 
                 27 
                 MPa 
                 53 
                 MPa 
               
               
                 stress 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 Core shear 
                   
                 0.55-0.64 MPa 
                 0.72 
                 MPa 
               
               
                 stress due 
                   
                   
                   
                   
               
               
                 to flexion 
                   
                   
                   
                   
               
             
          
           
               
                 Flat wise 
                 ASTM C 365 
                 0.8 
                 MPa 
                 0.8 
                 MPa 
                 1.1 
                 MPa 
                 0.88 
                 MPa 
               
               
                 tensile 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 strength 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Flat wise 
                 ASTM C 365 
                 2.2 
                 MPa 
                 3 
                 MPa 
                 3 
                 MPa 
                 4.4 
                 MPa 
               
               
                 compressive 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 strength 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 Maximum 
                 Vertical 
                 Elastic 
                 Elastic 
                 Elastic 
                 Elastic 
               
               
                 deflection and 
                 distributed 
                 deformation - 
                 deformation - 
                 deformation - 
                 deformation - 
               
               
                 deformation 
                 load applied 
                 6.8 mm max; 
                 5.2 mm max; 
                 3.2 mm max; 
                 6.8 mm max; 
               
               
                 under load 
                 at 600 kgs/m2 
                 No permanent 
                 No permanent 
                 No permanent 
                 No permanent 
               
               
                   
                   
                 deformation 
                 deformation 
                 deformation 
                 deformation 
               
               
                 Impact 
                 EN 438-2/22; 
                 No crack, 
                 No crack, 
                 No crack, 
                 No crack, 
               
               
                 resistance 
                 from a 
                 impression of 
                 impression of 
                 impression of 
                 impression of 
               
               
                   
                 height of 
                 11 mm; &lt;0.2 mm 
                 6 mm; &lt;0.2 mm 
                 5 mm; &lt;0.2 mm 
                 10 mm; &lt;0.2 mm 
               
               
                   
                 1 meter 
                 in depth 
                 in depth 
                 in depth 
                 in depth 
               
               
                   
               
             
          
         
       
     
     Accordingly a panel is provided that is 30% lighter than aluminum honeycomb panels of similar thickness. The panel is formed of two rigid, thin, high strength facings over a thick low density core, with an adhesive attachment which forces the core and facings to act as a continuous structure. The panel has a peel strength, which is a measure of the bonding strength between the core and skin, that is equivalent to aluminum honeycomb panels and which also meets the minimum values set for use in aircraft sandwich panels. No permanent deformation occurs on application of load, due to the elastic nature of the material. Excellent fire performance is provided, meeting the highest level of safety base on standards set across all countries. The panel is extremely lightweight, has excellent stiffness, high strength to weight ration and provides excellent fire resistance, being non combustible and non toxic. The panel is corrosion resistant, has excellent dielectric properties, is thermally insulating and has good thermal stability and acoustic performance. The panels are high temperature resistant being an insulating medium, unlike aluminum which is conductive of heat. High pressure laminates bond well to the phenolic skins on the present panels since they are made of similar materials, and the thermal elongation properties are similar, reducing the risk of debonding when exposed to cyclic cooling and heating. Aluminum in contrast expands/contracts differently from the high pressure laminates, which can cause debonded pockets during the life of a panel. The honeycomb cell size of the current panel is, for example, 5 mm, compared to 12 mm used in aluminum honeycomb. The smaller cell size increases the contact area for bonding and provides better load transfer. Further, cell size in aluminum honeycomb can be non-uniform, as the core is sourced in unexpanded form and is expanded before processing. The non-uniform cell size can cause internal stresses and lead to debonding when the panel is subject to bending forces. The panels in accordance with the present disclosure can be formed to complex acute and oblique curves as well as flat geometries. The panels can be easily repaired, where localized repair is possible using standard glass fibre reinforced panel techniques. In contrast, aluminum panels would dent permanently and could not be brought back into shape. 
     While a preferred embodiment of the technology has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the technology.