Abstract:
A coextruded multilayer film laminate featuring a polyolefin based formulation which closely matches the mechanical performance criteria of a plasticized PVC film is disclosed. This film is particularly suitable for use as a replacement film for plasticized PVC in a variety of medical and non-medical applications. The multilayer films of the present invention offer a particular set of mechanical properties, normally associated with plasticized PVC, including easy stretch, high degree of recovery, low fatigue and minimal permanent set. Moreover, these polyolefin based formulations do not contain any known or suspected carcinogenic compounds and may be produced at costs that are highly competitive with the costs associated with a conventional, plasticized PVC film.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to a multilayer film and a method of manufacture therefor. Specifically, the multilayer film of the present invention features a polyolefin based material which closely replicates the mechanical properties normally associated with a film of plasticized polyvinylchloride (PVC). This material is particularly suitable for use as a substrate for bandages and surgical dressings, but the film of the present invention is not limited to only medical applications and could be used as a substitute material in other PVC based articles. 
     BACKGROUND OF THE INVENTION 
     Solution cast, plasticized polyvinylchloride (PVC) films are frequently used as substrates for bandages and similar disposable articles. Plasticized PVC films are used in these applications primarily because they offer a particular set of mechanical properties. Plasticized PVC films possess desirable properties including easy stretch, high degree of recovery, low fatigue and minimal permanent set. However, plasticized PVC film has become less desirable because of known or suspected carcinogens associated with both the PVC monomer and the various plasticizers used in its production. Obviously, given the widespread use of these films in bandages and other medical applications where the polymer substrate may come into direct contact with open skin, blood, and other bodily fluids, it would be highly desirable to produce a new polymeric film which behaves mechanically like a plasticized PVC film, but is formed of materials that are free of suspected or known carcinogenic components. 
     Such a substitute or replacement film for plasticized PVC film has been desired for some time. One category of polymer films, polyolefins, are quite common and are used in a wide variety of applications. However, polyolefins in general do not recover from stretching as well as plasticized PVC films do. Ideally, a PVC replacement film will stretch easily, but recover completely. An ideal film would not fatigue or retain a permanent set. Additionally, if a substitute material could also provide improved breathability (i.e. higher MVTR) as compared to plasticized PVC film, this would also be a plus. 
     In short, there is a need for polymeric films which can replace plasticized PVC films in a variety of medical and non-medical applications. Specifically, there is a need for polyolefin based materials which have similar hysteresis (stress/strain) characteristics to plasticized PVC film as well as having a similar folding and conforming nature to plasticized PVC films. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a coextruded multilayer film particularly adapted for use as a replacement film for plasticized PVC in a variety of medical and non-medical applications. It has been discovered that polyolefin based formulations exist that closely match the mechanical performance criteria of a control plasticized PVC material. These polyolefin based formulations do not contain any known or suspected carcinogenic compounds and may be produced at costs that are highly competitive with the costs associated with a conventional plasticized PVC film. 
     In one preferred embodiment, the multilayer film of the present invention comprises a core layer which is coextruded and disposed between two outer skin layers. The core layer is generally about 65% to about 95% of the multilayer film thickness, and the two exterior skin layers are each about 2.5% to about 17.5% of the multilayer film thickness. One preferred material for the core layer is a blend of metallocene ultra low density polyethylene (ULDPE) polyolefin plastomer and an ethylene methyl acrylate (EMA) copolymer. Each coextruded skin layer may be made of another polyolefin blend such as linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: 
     FIG. 1 is a cutaway view of a multilayer film, according to the present invention. 
     FIG. 2 is a side elevational view of a conventional matte embossing arrangement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to FIG. 1, a multilayer film  100  according to the present invention is depicted in a cutaway side view. In one preferred embodiment the multilayer film  100  is a three layer film having a core layer  10  which is disposed between two exterior skin layers  15 ,  20 . 
     The core layer  10  is normally about 65% to about 95% of the total multilayer film  100  thickness T, and in one preferred embodiment the core layer  10  is about 75% to about 85% of the total thickness T. Consequently, the two exterior skin layers  15 ,  20  are each about 2.5% to about 17.5% of the multilayer film  100  thickness T, and in one preferred embodiment the two exterior skins layers  15 ,  20  are each about 7.5% to about 12.5% of the total thickness T. 
     It has been discovered that one polyolefin based formulation which is suitable for use as a core  10  material is a blend of metallocene ultra low density polyethylene (ULDPE) polyolefin plastomer, such as Dow PL1280, and an ethylene methyl acrylate (EMA) copolymer, such as Exxon TC120. Typically, the core layer  10  will be a blend of about 55% to about 75% metallocene ULDPE and about 25% to about 45% EMA copolymer. In a more preferred embodiment, the core layer  10  will be a blend of about 55% to about 60% metallocene ULDPE and about 40% to about 45% EMA copolymer. Please note that, unless specified otherwise, the blend percentages provided herein are understood to be on a weight percent (wt%) basis. 
     One desirable blend which may be used in each of the exterior skin layers  15 ,  20  is made up of linear low density polyethylene (LLDPE), such as Dow 2517, and a low density polyethylene (LDPE), such as Chevron 1017. Each exterior skin layer  15 ,  20  will normally be a blend of about 45% to about 55% LLDPE and about 45% to about 55% LDPE. In one preferred embodiment each exterior skin layer  15 ,  20  will be a blend of about 50% LLDPE and about 50% LDPE. 
     Several alternative embodiments of the multilayer film  100  according to the present invention have also been discovered. One alternative embodiment involves the substitution of an ethylene vinyl acetate (EVA) copolymer in place of the EMA copolymer. It has been observed that the EVA material may be substituted on a one-for-one weight percent (wt%) basis with the EMA material in the core layer  10  with very little change in the overall mechanical properties of the multilayer film  100 . 
     Another alternative material which has been tested for use in the core layer  10  is a family of atactic polypropylene (PP) materials which possess the hysteresis or stress/strain characteristics necessary to be considered for a replacement of plasticized PVC. Atactic polypropylene materials are created using specialized catalysts and are also commonly referred to as flexible polyolefins (FPO), and are commercially available from companies such as Huntsman (WL201) and Montell (Catalloy). Additionally, syndiotactic polypropylenes from Fina were also considered and have been thought to give similar results. Note that in cases where specialty polypropylene materials are used, they comprise 100% of the core layer  10  in the multilayer film  100 . 
     It is also desirable in producing a multilayer film  100  to provide the film  100  with an embossed exterior surface on a first side  25 , a second side  30 , or both sides of the multilayer film  100 . It was noted that of the various materials which were suited for use in the core layer  10 , many of these materials tended to adhere aggressively to the embossing roll. It was at least in part to overcome this propensity that the blended LLDPE/LDPE skin layers  20  were added to multilayer film  100 . It was also discovered that by using a “fine” embossed pattern such as matte, FS II, or JMAC I, the resulting coextruded film  100  can be deglossed to give the film  100  an attractive dull finish while maintaining the desired hysteresis characteristics. In all of the embodiments disclosed, the coextruded film may be direct cast embossed using an engraved pattern of choice. In one preferred embodiment, either an FS II (a regular, repeating, square cell pattern with about 145 cells/inch) or JMAC I (an offset, repeating, circular cell pattern with about 22 cells/inch) is desirable as it is believed that these patterns do not alter the base hysteresis characteristics of the film and the patterns give the film an attractive dull finish (e.g. 45 degree gloss of about 3.0 to about 7.0) that is often desired in the marketplace. 
     Additionally, it should be noted that the multilayer films described herein may also be corona treated on one exterior side to satisfy the printing needs of consumers. It should also be noted that the core and skin polymers may also have select additives incorporated into the blend in very low concentrations (about 0.10% to about 2.0%) of titanium dioxide or other colorants or pigmenting materials to again provide the multilayer film  100  of the present invention with a desired appearance. 
     An additional disclosure applicable to each of the embodiments above includes the addition of a siloxane polymer into at least one skin layer  15 ,  20  to improve the refastenability characteristics of a bandage or other finished article. This refastenability characteristic is often referred to as a differential release property. 
     In short, differential release refers to a material such as a multilayer film  100  of the present invention in which a first side  25  and a second side  30  possess different affinities for an adhesive coating or glue. For example, in a bandage, it may be desirable to have a first side  25  to which an adhesive coating, not shown, may be applied and should remain attached thereto, and a second side  30 , which is generally the exterior side of the bandage but which may come into contact with the adhesive on the first side  25  as the bandage is applied and wrapped around a finger or other part of the body. In this example, it is desirable that the first side  25  of the multilayer film  100  has a greater affinity for the adhesive than the second side  30  does. This allows the adhesive layer, not shown, to remain attached to the first side  25  when applied to the skin or other surfaces, and also allows the adhesive to be peeled away from the second side  30  if the bandage is overlapped. 
     One way to achieve a differential release for a multilayer film  100  is to incorporate a low surface energy material such as a siloxane polymer into one skin layer  20  to make its exterior surface  30  more resistant to an adhesive or glue than the exterior surface  25  of the other skin layer  15 . This has been done successfully by incorporating a small amount of ultra-high-molecular-weight functionalized siloxane polymer, such as master batch MB50-313 available from Dow Corning, into one of the skin layers  20 . Master batch MB50-313 is a 50/50 wt % blend of ultra high molecular weight siloxane polymer and LLDPE resin. Several experimental samples have been made incorporating between about 1.0% to about 10.0% additions of MB50-313 added to a single skin layer of  20 . In one preferred embodiment, between about 2.5% to about 3.5% MB50-313 is incorporated into a single skin layer  20 . This skin layer  20  would thus comprise about 50% LLDPE (Dow 2517), about 46.5% to about 47.5% LDPE (Chevron 1017), and about 2.5% to about 3.5% master batch polymer blend (MB50-313). Thus, in this particular embodiment, the ultra-high molecular weight functionalized siloxane polymer content of the resulting skin layer  20  would be about 1.25% to about 1.75%. 
     Additional additives which may also be used to develop a differential release characteristic in a multilayer film  100  include: synthetic silica such as Grace Siloblock 45, Behenamide organic antiblock, or Fuji Sylesia at 6 and 12 micron particle size; cross-linked silicone spherical particles such as Toshiba GE Tospearl; hollow glass spheres such as Zeospheres; and treated talc. These materials may be blended with the LLDPE/LDPE skin layer  20  at appropriate levels to develop the controlled release desired. 
     A method of manufacturing a multilayer film  100  according to the present invention will now be disclosed. As best seen in FIG. 2, a simplified manufacturing line  200  for making the multilayer film  100  is illustrated. Prior to forming the multilayer film  100 , it is necessary to blend a core composition and at least one skin composition in separate extruders or mixers, not shown, as known in the art. The core composition  10 ′ and the at least one skin composition  20 ′ are fed simultaneously into a slotted film casting die  50  and coextruded to form a multilayer film  100 ′ with a core layer  10  and at least one skin layer  20 . The multilayer film  100 ′ is then embossed using a nip roll apparatus  260  which has a metal embossing roll  265  and a rubber roll  270 . As the multilayer film  100 ′ is pressed between the metal embossing roll  265  and the rubber roll  270 , it is possible to impart an embossed finish onto one or both sides of the multilayer film  100 ′. The embossed multilayer film, now referred by the numeral  100 , is then allowed to cool and taken up on rolls  290 , as known in the art. Optionally, the line may further include a corona discharge bar  280  for corona treatment of at least one side of the film for later printing. It should be further noted that the multilayer film  100  may subsequently be printed, apertured, coated with an adhesive and a backing sheet, and cut into various shapes and sizes to form finished articles such as bandages. 
     By way of example only, test data for several PVC replacement films according to the present invention is collected in Table 1 below. The four example films are three layer coextruded films having a core layer of about 58% metallocene ULDPE polyolefin plastomer and about 42% EMA copolymer; a first exterior skin layer of about 50% LLDPE and about 50% LDPE; and a second exterior skin layer of about 50% LLDPE, about 47% LDPE, and about 3% master batch siloxane polymer blend. The core layer in each film is about 80% of the overall thickness and the skins are each about 10% of the overall thickness. The films were also fine embossed with an FS II or JMAG I pattern on one side, although this does not appear to measurably affect mechanical properties. The plasticized PVC film data shown in Table 1 is provided for comparison purposes only, and it should serve to give an indication of how closely the polyolefin based films of the present invention replicate the mechanical properties of a typical plasticized PVC film in use today. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 PVC 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Basis Weight 
                   
                 102.7 
                 49.0 
                 51.5 
                 54.3 
                 60.2 
               
               
                 (g/m 2 ) 
               
               
                 Thickness by 
                   
                 98.3 
                 53.5 
                 56.2 
                 59.1 
                 65.8 
               
               
                 Weight (μ) 
               
               
                 Specific Gravity 
                   
                 1.045 
                 0.916 
                 0.917 
                 0.918 
                 0.916 
               
               
                 (g/cc) 
               
               
                 Tensile Strength 
                 MD 
                 1096 
                 1591 
                 1148 
                 1280 
                 1532 
               
               
                 at Break (g/cm) 
                 TD 
                 1017 
                 1210 
                 883 
                 1019 
                 1966 
               
               
                 Elongation 
                 MD 
                 182 
                 647 
                 286 
                 284 
                 710 
               
               
                 at Break(%) 
                 TD 
                 188 
                 682 
                 497 
                 502 
                 676 
               
               
                  5% Stress 
                 MD 
                 100 
                 70 
                 109 
                 125 
                 157 
               
               
                 (g/cm) 
                 TD 
                 91 
                 123 
                 123 
                 139 
                 145 
               
               
                 10% Stress 
                 MD 
                 191 
                 164 
                 200 
                 225 
                 238 
               
               
                 (g/cm) 
                 TD 
                 175 
                 190 
                 197 
                 218 
                 234 
               
               
                 Elmendorf Tear 
                 MD 
                 1.03 
                 5.18 
                 2.65 
                 3.06 
                 6.08 
               
               
                 (g/μ) 
                 TD 
                 1.03 
                 12.87 
                 14.73 
                 15.96 
                 12.48 
               
               
                 Gurley Flexural 
                 MD 
                 26.64 
                 18.87 
                 17.39 
                 14.43 
                 10.73 
               
               
                 Stiffness (mg) 
                 TD 
                 14.98 
                 15.91 
                 11.10 
                 15.17 
                 10.73 
               
               
                 Coefficient of 
                 F 
                 0.80 
                 0.24 
                 0.29 
                 0.31 
                 0.35 
               
               
                 Friction 
               
               
                 Static (film/steel) 
                 M 
                 1.01 
                 0.39 
                 0.35 
                 0.39 
                 0.39 
               
               
                 Coefficient of 
                 F 
                 0.52 
                 0.26 
                 0.33 
                 0.34 
                 0.37 
               
               
                 Friction 
               
               
                 Kinematic (film/ 
                 M 
                 0.65 
                 0.46 
                 0.37 
                 0.41 
                 0.43 
               
               
                 steel) 
               
               
                 Haze (%) 
                   
                 89.4 
                 93.2 
                 91.3 
                 91.9 
                 94.3 
               
               
                 Low Load Thick- 
                   
                 111 
                 75 
                 129 
                 128 
                 78.3 
               
               
                 ness (μ) 
               
               
                 TD Force at 25% 
                   
                 373 
                 237 
                 242 
                 263 
                 295 
               
               
                 Strain (g/cm) 
               
               
                 TD Force Relaxa- 
                   
                 49 
                 18 
                 18 
                 23 
                 18 
               
               
                 tion at 25% Strain 
               
               
                 (%) 
               
               
                 TD Permanent 
                   
                 1.8 
                 2.3 
                 2.1 
                 2.2 
                 2.1 
               
               
                 Set at 25% Strain 
               
               
                 (%) 
               
               
                 TD Force at 50% 
                   
                 581 
                 272 
                 270 
                 289 
                 329 
               
               
                 Strain (g/cm) 
               
               
                 TD Force Relaxa- 
                   
                 54 
                 19 
                 22 
                 23 
                 19 
               
               
                 tion at 50% Strain 
               
               
                 (%) 
               
               
                 TD Permanent 
                   
                 5.5 
                 4.9 
                 6.7 
                 6.1 
                 5.3 
               
               
                 Set at 50% Strain 
               
               
                 (%) 
               
               
                   
               
             
          
         
       
     
     In Table 1, the abbreviations MD and TD are understood to refer to the machine direction (MD) and the transverse direction (TD) of the film. The machine direction of a film may be defined as the direction in which the film is pulled during its production or the direction in which the film is taken up onto rolls. The transverse direction (TD) may be defined as being perpendicular to the MD within the plane of the film. Mechanical properties are measured in this manner because long chain molecules within polymer films tend to become oriented in the direction of strain, usually the machine direction in cast films. Also, please note that the abbreviations F and M are understood to refer to the female (i.e. embossed or steel roll) side of the film and the male (i.e. rubber roll) side of the film. Note that the data provided on the plasticized PVC film is intended to be representative of a typical commercial film of this type, but that properties may be somewhat higher or lower depending on the manufacturer and batch tested. 
     Hysteresis properties, namely force relaxation and permanent set, are often measured in accordance with a laboratory test procedure utilizing a test instrument which applies a load to a specimen through a constant rate of motion. By way of example only, one such test instrument is an Instron Tensile Tester—Model #1130. The test procedure is run in two parts on each specimen. The first cycle applies a load to the specimen and places the sample in tension to achieve the desired strain (% elongation), holds at that strain for a designated time, and then returns to an unloaded condition. The curve which is generated during this cycle is used to calculate force relaxation. The second cycle applies a load and places the sample in tension to obtain the desired strain (% elongation) as in the first cycle, holds that strain for a designated time, and then returns to an unloaded condition. The tensile set or permanent set is calculated from this second curve. 
     For the hysteresis data of Table 1, specimens are taken from various areas across the film and are cut 1.0 inch wide by about 7.0 inches long. The polymer test samples should be free of surface damage, wrinkles, and blemishes which might have a detrimental effect on the test results. Testing is carried out at about 73±2° F. and a humidity of about 50%±2%. After the testing machine is calibrated, the desired % elongation is set using an upper limit stop. A test specimen is placed in the jaws of the tensile testing machine which are set 3.0 inches apart (original gage length), the jaws are moved apart at a rate of 20 inches/minute to reach the desired % elongation and the force (f 1 ) is noted. The sample is held for 30 seconds at the desired % elongation and the force (f 2 ) is noted again. The sample is then returned to a no load condition. After a rest period of 30 seconds, the test sample is again cycled to the desired % elongation, held for 30 seconds, and returned to zero load. During this second cycle, the take-up distance or elongation (a) of the film before the film resists deformation and a load is applied by the testing machine is noted. 
     After the test data is collected, it is possible to compute the force relaxation and the permanent set for each sample. Force relaxation is defined as the loss in force (f 1 −f 2 ) during the hold phase of the first test cycle. The loss may be expressed as a force relaxation %=(f 1 −f 2 )/f 1 *100%. Permanent set, also known as tensile set, is a measure of permanent deformation of the sample as a result of the initial elongation, hold, and relax procedure. The permanent set is the ratio of elongation (a) of the sample before a load is applied, as measured in the second test cycle, divided by the original gage length of the sample. This may also be expressed as a permanent set %=a/gage length*100%. 
     It is also notable that in many cases the replacement films not only reproduce the hysteresis characteristics of the plasticized PVC film, but are actually physically superior in other mechanical properties. For example, the replacement films have significantly lower specific gravity and may be made thinner than conventional plasticized PVC films, which means that it will require less weight of raw polymer to manufacture the same area of film. Moreover, properties including Elmendorf tear and elongation at break are also greatly improved by the additional stretch before failure which is provided by the polyolefin films. Also, the coefficient of friction is significantly reduced which allows the replacement films to be handled at higher line speeds and with less mechanical resistance. 
     Although preferred embodiments of the invention have been described in the Examples and foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements and modifications of parts and elements without departing from the spirit of the invention as defined in the following claims. Therefore, the spirit and the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.