Abstract:
A method of producing a multilayer plastic film from resin starting material includes irradiating a batch of plastic resin material and using irradiated resin from the batch to produce at least one layer of a multilayer plastic film.

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/ 924488 filed May 17, 2007. 
    
    
     FIELD OF INVENTION 
     This invention relates to multilayer plastic film. 
     BACKGROUND OF THE INVENTION 
     Multilayer plastic films are used for many purposes, for example in the food packaging industry. Such films are described for example in U.S. Pat. No. 6,159,616 (Planeta et al) issued Dec. 12, 2000 and U.S. Pat. No. 6,218,024 (Tamber et al) issued Apr. 17, 2001, the contents of which are hereby incorporated herein by reference. 
     It is known to irradiate such multi-layer films to improve their properties, for example mechanical properties such as puncture resistance, tensile strength and abuse resistance. However, the problem with irradiating a multi-layer film is that the radiation which improves the properties of one or more layers may adversely affect the properties of another layer or layers. It has been proposed to irradiate a layer before it is bonded with other layers to produce a multi-layer film, but this technique has presented production difficulties. 
     SUMMARY OF THE INVENTION 
     According to the invention, at least one layer of a multilayer plastic film is formed from a plastic resin, i.e. the starting material, which has been irradiated before it is formed into a layer. The invention thus enables the resin or resins from which a layer or layers are subsequently formed to be irradiated to an extent which optimizes the properties of the layer or layers concerned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will flow be described, by way of example, with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic view showing irradiation of plastic resin pellets which will be formed into a layer of a multilayer plastic film, 
         FIG. 2  is a schematic view showing a method of production of a multilayer plastic film by means of a double bubble process, 
         FIG. 3  is a schematic view of a five layer film of which at least one layer was produced from a plastic resin which had previously been irradiated in the manner shown in  FIG. 1  and  FIG. 4  is a similar view of a seven layer film. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings,  FIG. 1  shows plastic resin pellets  10  being irradiated to produce irradiated plastic pellets  11  prior to being formed into a layer of a multilayer plastic film. Depending on the composition of the plastic resin pellets, the pellets are irradiated with radiation of at least 2MR by means of x-rays or gamma rays. 
     A multilayer plastic film in accordance with the invention may be produced by a process including the known double bubble method. Referring to  FIG. 2 , a multilayer plastic film is produced by using an appropriate number of extruders  12  (only one of which is shown). At least one extruder  12  is supplied with plastic resin which has been irradiated in accordance with the invention. The resins from extruders  12  are fed to an annular die  14  and a multilayer tubular films is extruded downwardly therefrom in bubble form. The films is cooled in a cold water tank  16  located under the die  14  and containing water at a temperature of about 25° C. or lower. The bubble formed by the multilayer films is terminated by nip rollers  20  in the cold water tank  16  which collapse the films from bubble to flat form. The cold water in tank  16  quenches the films to maintain the amorphous state of the plastic material and to lower the temperature thereof to facilitate subsequent biaxial orientation at a later stage in the process. 
     The collapsed and quenched film  21  from the cold water tank  16  is passed over idler rollers  24  and then fed into a hot water tank  28  where the film  21  passes through nip rollers  25 ,  26  and then upwardly through an air ring  32  beyond which the film is blown to form a second bubble  30 , which is subsequently collapsed by a collapsing frame  33 . The collapsed film passes through nip rollers  34  at a speed which is from about  3  to about  5  times the speed with which the film passes through nip rollers  26 , with the air in the bubble  30  being trapped therein by the rollers  26 ,  34 . This results in biaxial orientation of the film lengthwise and breadth wise. The collapsed film  31  then passes an optional annealing station  38  which stabilizes the film to prevent subsequent shrinkage when in a roll. The film  31  then passes over further idler rollers  36  and is then wound into a roll  39 . Examples of a multilayer plastic film made in accordance with the present invention will now be described. 
     Referring first to  FIG. 3 , a five layer film  40  is made in accordance with the method described with reference to  FIGS. 1 and 2 . The film  40  comprises an outer layer  42 , a bonding layer  44 , a PVDC layer  46 , a further bonding layer  48  and a second outer layer in the form of a sealant layer  50 . 
     The central layer  46 , namely the third layer, is a PVDC barrier layer with a thickness in the range of from about 2 to about 15 microns and comprising a co-polymer of vinylitine chloride and methyl acrylate. The PVDC layer  46  provides good oxygen and moisture barrier properties as well as oil resisting properties. Neither the PVDC layer  46  nor the starting material therefor was subjected to irradiation. 
     The PVDC layer  46  is bonded by a bonding layer  44  to an outside layer  42  which comprises low density polyethylene to provide abuse resistance during handling and transportation, cold resistance and heat resistance to around 90-98° C. which is required for cook-in uses. The resin used to produce the outside layer  42  was subjected to irradiation in the manner indicated in  FIG. 1  in the range from about 2 to about 20MR, preferably from about 2 to about 10MR, and still more preferably from about 2 to about 6MR. The outer layer  42  may have a thickness in the range from about 10 about 40 microns, preferably from about 20 to about 40 microns. 
     The outer layer  50 , namely the sealant layer, may have a thickness in the range of about 10 to about 40 microns, preferably from about 10 to about 30 microns and comprises low density polyethylene. The sealant layer  50  provides heat sealing ability, good seal strength in the presence of fats or oils, seal strength for cook-in uses and during shelf life of the film, and puncture resistance for bone-in-meat usage. The resin used to produce the sealant layer  50  was irradiated to a level in the range of from about 2 to about 20MR, preferably from about 2 to about 10MR, and more preferably from about 2 to about 6MR. 
     The bonding layers  44 ,  48  may comprise ethylene vinyl-acetate copolymer, with each layer having a thickness in the range from about 3 to about 10 microns. The resin from which the bonding layers  44 ,  48  is produced is irradiated at a level in the range from about 2 to about 10MR, preferably from about 2 to about 6MR and more preferably from about 2 to about 4MR. 
       FIG. 4  shows a seven layer film  60  which is similar to the five layer film described with reference to  FIG. 3 , except that a shrink layer  64  is provided between sealant layer  62  and bonding layer  66 , and a further shrink layer  70  is provided between bonding layer  70  and sealant (outer) layer  74 , with the central layer  68  being a PVDC barrier layer as before. 
     The shrink layers  64 ,  72  may each have a thickness in the range of from about 5 to 20 microns, with the resin from which these layers are formed being irradiated in the range from about 2 to about 6MR, preferably about 2MR. 
     Both films  40  and  60  can be produced with good physical and mechanical properties, with the film  60  having a total thickness of about 60 microns and the film  70  having a total thickness of about 65 microns. 
     The advantages and other embodiments of the invention will now be readily apparent to a person skilled in the art from the foregoing description, the scope of the invention being defined in the appended claims.