Patent Publication Number: US-2007122584-A1

Title: Multilayer material

Description:
The present invention relates to multilayer material comprising foamed starch, in particular biodegradable multilayer material, for example for use in thermal insulation, and to a method for the production thereof.  
      Biodegradable foamed starch products are known for use as packaging materials and provide an environmentally friendly alternative to traditional synthetic polymer foams such as expanded polystyrene (EPS) or foamed polyethylene, which are not readily degraded after use and are made from non-renewable petroleum resources. The manufacture of a biodegradable packaging comprising an ordered structure of lengths of foamed starch rod is disclosed in GB-A-2 352 230. This document discloses the adhesion bonding of lengths of foamed starch (such as wheat) rods formed by high temperature short time (HTST) extrusion cooking. Biodegradable packaging in the form of loose-fill chips can also be made from extruded starch (such as wheat). These foamed starch packaging materials are biodegradable and water soluble, making them readily disposable, and have comparable cushioning properties to other types of non degradable packaging, such as EPS chips.  
      It has been desired to expand the applications of foamed starch.  
      According to the present invention there is provided a multilayer material comprising: a foam core comprising compressed foamed starch; and a facing layer bonded to a surface of the foam core.  
      Preferably, the foamed starch is in the form of pieces of foamed starch, such as foamed starch loose-fill, and particularly preferably, the pieces are at least partially adhered together.  
      Foamed starch is biodegradable and the multilayer material is therefore substantially biodegradable, even if the facing material is non-biodegradable. Preferably, however, the facing material is biodegradable. The material of the present invention therefore provides an environmentally friendly alternative to non-biodegradable conventional EPS foams.  
      Foamed starch comprises starch, and optionally, additives such as plasticisers, for example polyvinyl alcohol, or nucleating agents, for example talc or bran. The starch may be unmodified starch. A currently preferred starch source for the preparation of foamed starch is wheat flour containing around 9% protein.  
      The multilayer material may comprise two facing layers bonded to opposing surfaces of the foam core, which may be of the same material as each other or of different materials. The combination of the foam layer with one or more facing layers can significantly enhance the mechanical and thermal insulation properties of the multilayer material. The facing layers may also provide additional desirable properties, such as moisture resistance, fire retardancy and a desired surface finish.  
      The facing material may be paper, (the term paper includes card, cardboard and other cellulose fibre based sheet material), or a biodegradable polymeric material such as starch film, polyvinyl alcohol, polylactic acid, polycaprolactone, polyester or materials containing any of these. The facing material may be a biodegradable material, such as those noted above, or a non-biodegradable material, such as polyethylene. Preferably, the facing material is corrugated card.  
      Preferably, the facing layer or layers are water resistant or comprise a water-resistant coating.  
      Also according to the present invention there is provided a method for making a multilayer material comprising: 
      a) forming a core of foamed starch;     b) compressing the foamed starch core; and     c) bonding a facing layer to a surface of the foamed starch core.    

      Preferably, the foamed starch core comprises foamed starch pieces, more preferably, the foamed starch pieces comprise foamed starch loose-fill.  
      Preferably, a bonding agent is applied to the surfaces of the foamed starch pieces.  
      Preferably the bonding agent is a liquid, more preferably water. Starch adhesives or latex based materials are also suitable.  
      The method may also comprise the step of bonding facing layers to opposing surfaces of the foam core and/or the step of applying a water-resistant coating to the facing layer or layers.  
      Also according to the present invention there is provided apparatus for making a multilayer material comprising a transport surface, a first station for supplying foamed starch pieces to said transport surface, a second station, downstream of said first station, for applying a bonding agent to surfaces of said foamed starch pieces, a compressor for compressing said foamed starch pieces downstream of said second station and a bonder for bonding a facing layer to at least one surface of said compressed foamed starch.  
      Preferably said second station comprises a mist chamber.  
      Preferably said compressor comprises at least one roller or a hydraulic plate press. 
    
    
      An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:  
       FIG. 1  is a perspective view of a panel of biodegradable multilayer material according to the invention;  
       FIG. 2  is a cross section through the panel of  FIG. 1 ;  
       FIG. 3  is a schematic drawing of apparatus according to the invention, for the production of a multilayer material; and  
      FIGS.  4  to  7  show the results of a comparative experiment comparing the insulation properties of EPS foam with a multilayer panel according to the invention. 
    
    
      The biodegradable multilayer insulation panel  2  shown in  FIGS. 1 and 2  comprises a foam core layer  4  of compressed foamed starch loose-fill  6 , a first facing layer  8  bonded to the upper surface of the foam layer  4  and a second, identical facing layer  10  bonded to the opposing lower surface of the foam layer  4 . The total thickness of the multilayer panel is around 100 mm. Both facing layers  8 , 10  are corrugated cardboard and are coated with high hydrolysis polyvinyl alcohol  12 , to provide a water-resistant external surface.  
      As shown in  FIG. 2 , the foam core layer  4  has three components: the compressed loose-fill chips  6 ; the three dimensional network of interfaces  14  formed by fusion of the loose-fill chips  6  with each other at the contact areas; and the air pockets  16  between the loose-fill chips  6 . At the interface regions the foamed starch is of a higher density and rigidity than elsewhere, reinforcing the foam core layer  4 . The air pockets  16  contribute to the performance of the board as a thermal insulator.  
      To make the multilayer board  2 , the foamed starch loose-fill  6  is formed by conventional HTST extrusion. The loose-fill chips  6  are transferred to a hopper  40 , which releases metered amounts of the chips  6  onto a moving conveyor belt  42  having side walls  44 . The thickness of the layer of loose-fill  6  and therefore of the foam core layer  4  is determined by the height of the conveyor belt side walls  44  and the speed of the belt  42  relative to the hopper  40 . The foam core layer  4  is then formed by moistening the surfaces of the chips  6  with water in a mist chamber  46  and subjecting a layer of the moistened chips to compression. The water causes the surfaces of the chips  6  to become adhesive and this causes the surfaces to fuse together where they are in contact with each other. Compression may be applied by continuous rolling using rollers  48  on the chips  6  on the moving conveyor belt  42 . Alternatively, (not shown), compression may be applied to the chips within a flat-bed mould using a hydraulic press comprising heavy duty plates. In the latter case, the chips are transferred from the moving conveyor belt into the mould before compression and the belt and the mould may be moveable in relation to each other, to ensure a uniform distribution of the chips in the mould. In order to achieve the desired density of the foam core, a force of up to 14,000N is applied during the compression step. The compression step can take place with or without heating.  
      The first  10  and second  12  facing layers are bonded to the foam core layer  4  by applying moisture to the external surfaces of the foam core layer  4 , making them adhesive. This avoids the need for the application of a separate adhesive between the foam  4  and facing layers  10 , 12  which can be employed if necessary. The desired facing layer or layers  10 , 12  may be positioned beneath and on top of the loose-fill prior to compression. In the apparatus shown in  FIG. 3 , the first facing layer  8  from roller  50  is positioned on the surface of the conveyor belt  42  prior to deposition of the loose-fill from the hopper  40  and the second facing layer  10  from roller  52  is positioned on top of the deposited layer of loose-fill on the conveyor belt  42  prior to compression of the loose-fill layer by the rollers  48 . The facing layer or layers may be coated with a water-resistant layer before use, or the final product may be coated with a water-resistant layer.  
      The process described may be easily adapted to produce a shaped body of the multilayer material instead of a planar board, by carrying out the compression step inside a mould cavity of the shape required.  
      The density of the bulk foam layer  4  can be varied by altering the degree of moistening, that is, the weight of water per unit surface area of the loose-fill chips and/or the compression pressure. The loose-fill in its uncompressed form has a density of between 6 kgm −3  and 12 kgm −3  and it has been found that optimal thermal insulating properties are obtained when the loose-fill is compressed to a density of between 15 kgm −3  and 50 kgm −3 . The process described is, however, capable of producing a foam core layer with a density greater than 50 kgm −3  if required.  
      Foamed starch loose-fill is particularly suitable for use in an insulation material. When the layer of loose-fill is compressed, air pockets are created between the chips of loose-fill, as shown in  FIG. 2 . The distribution of air pockets throughout the foam core layer is irregular but relatively even throughout the material. The air pockets enhance the insulating performance of the material due to the poor heat conductivity of air. The compression of the core layer during manufacture tends to lead to the air pockets being closed, so that there are no, or only relatively few, open channels through the board which would allow for the creation of convection currents. This means that the transfer of heat through the board by means of conduction or convection is low. The thermal conductivity of the foam core layer  4  described is typically less than 0.035 W/mK. This is comparable to conventional EPS insulation materials, which usually have a thermal conductivity in the range of 0.033 W/mK to 0.036 W/mK. The thermal conductivity of the corrugated card facing layers is also low, in the region of 0.045 W/mK, depending on the density of the card.  
     COMPARATIVE EXAMPLE  
      FIGS.  4  to  7  show the results of a comparison of the insulating properties of multilayer panel according to the present invention with conventional EPS insulation panel. For each material, an empty pallet box, with dimensions 1180 mm×780 mm×655 mm, was lined on all internal surfaces with 65 mm thick panels of the material, having a density of around 20 kgm −3 , and the temperature at the centre of the pallet box was monitored at ten minute intervals throughout the following test sequence: 
      1) Holding the pallet in an environment at 23° C. at 50% relative humidity for 24 hours; then     2) Holding the pallet in an environment at 3° C. for 96 hours; then     3) Holding the pallet in an environment at 43° C. for 96 hours.    

      As can be seen from the graphs of FIGS.  4  to  7 , the pallet box containing multilayer board according to the present invention shows a time lag in each of the temperature transitions compared to the EPS lined pallet box. In particular,  FIG. 4  shows the change in temperature over time inside each of the pallet boxes for the complete cycle,  FIG. 5  shows the ambient to cold transition,  FIG. 6  shows the complete cold to hot transition and  FIG. 7  shows the initial cold to hot transition. The results indicate that the multilayer panel provides improved thermal insulation over the conventional EPS insulation.  
      While in the embodiment described the foamed starch loose-fill chips  6  are fused with each other through the application of water to the surfaces of the chips, it will be appreciated that the chips could be adhered together in other ways, for example by the addition of a different bonding agent such as polyvinyl alcohol, latex, or a starch adhesive. Liquid bonding agents are preferred for ease of handling and control and may be non-aqueous or aqueous.  
      While in the embodiment described, the multilayer board comprises two identical facing layers of corrugated card, it will be appreciated that the two facing layers can be of different materials to each other and that in some cases a single facing layer may be sufficient. It will also be appreciated that the material of the facing layer can be chosen to suit the particular application of the panel. The facing layer or layers may be a paper material other than corrugated card, (with or without additives such as plasticisers), film or sheet materials based on biodegradable polymeric materials such as starch film, polyvinyl alcohol, polycaprolactone or polyester, or a non-biodegradable material such as polyethylene. Alternatively, the facing layer may be provided by encapsulating the foam layer with a thin film so that the foam layer is sealed inside the facing layer. It will also be appreciated that, while in the embodiment described, the water-resistant coating is high hydrolysis polyvinyl alcohol, other water-resistant materials may provide suitable alternatives.  
      This invention provides an at least substantially biodegradable multilayer material, for example for use as in thermal insulation. The material finds particular application in the transport or storage of chilled objects, as a result of the combination of the thermal insulation properties and the suitability of foamed starch products as packaging materials.