Abstract:
An improved polymer film material for the packaging of perishable foodstuffs, wherein the surface structure of the polymer is altered by means of an ion bombardment process so as to reduce the rates of transport of oxygen and water vapor through the polymer material wherein the polymer film material is produced by subjecting at least one surface of the film to ion bombardment with ions selected from the group consisting of hydrogen, carbon, nitrogen, helium, and argon at energies of 30-80 KeV and ion doses in the range of 10 12  to 10 18  ions/cm 2 .

Description:
The present invention relates to the treatment of the surface of films made of polymer materials and more particularly to such films for use in the packaging of goods. The treatment may be to reduce the permeability of polymer films, to improve their printability, or both. The invention is concerned, particularly with polymer films used for the packaging of foods and beverages. 
     BACKGROUND OF INVENTION 
     The two most widely-used polymer films for food and beverage packaging are polyethylene terephthalate and biaxially-oriented polypropylene. One of the major factors affecting the shelf life of food packaged in containers including films made of the above two materials is their permeability to oxygen and water vapour. 
     At present, the barrier properties of these materials are improved by vacuum coating them with aluminium to a thickness of a few tens of nanometers and an optical density of about 2.5. Metallization of one side of the film only reduces the oxygen permeation by a factor of about 100 and water vapour permeation by a factor of about 500. Metallization of both sides of the film improves these figures by a further factor of 5. 
     However, the use of aluminium as a coating material has several disadvantages; one is the possible link between the presence of aluminium particulates in food and Alzheimer&#39;s disease; another is that the contents of a package are invisible to a would-be purchaser, a third is that the presence of the aluminium coating means that food which requires cooking cannot be cooked in the package by microwave radiation, and a fourth is that such coated materials cannot be recycled. 
     Other methods which have been suggested for reducing the permeability of polymer films include laminating together two different polymer films, for example, polyvinylidene chloride and polyethylene terephthalate; depositing a film of silica on the polymer film by the thermal evaporation of silica; or the deposition of other oxides of silicon by various vapour deposition techniques. The purpose of the use of silicon oxides other than silica is to attempt to provide barrier layers which do not suffer from the brittleness of silica. 
     It is known that the rate of diffusion of fluorinated hydrocarbons through glassy polystyrene can be reduced by bombarding the polystyrene with H +   ions at an energy of 30 KeV and ion doses between 10 13  and 10 15  ions/cm 2  (see Study of Diffusion of a Fluorinated Hydrocarbon in Ion Beam Irradiated Polystyrene by Nuclear Magnetic Resonance Reaction Analysis. Uwezawa et al Appl. Phys. Lett., Vol. 61(13), Sep. 28, 1992 pp 1603-4). The work was carried out in connection with the development of polymer encapsulants for microelectronic devices. 
     Another problem associated with polymers as packaging materials is that it is difficult to print upon them. The problem is exacerbated by the transition, for environmental reasons to water-based rather than hydrocarbon solvent-based inks. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide an improved polymer film for the packaging of perishable materials, particularly foodstuffs. 
     According to the present invention in one aspect there is provided a method of modifying the surface structure of a film of polymer material for use as a packaging material for perishable materials to reduce the transport of gaseous material therethrough, comprising the operation of subjecting the film of polymer material to bombardment with ions selected from the group consisting of hydrogen, carbon, nitrogen, helium and argon at energies in the range 1 to 80 KeV and ion doses in the range 10 12  to 10 18  ions/cm 2 . 
     According to the invention in a second aspect, there is provided a polymer film for use as a packaging material for perishable foodstuffs, wherein at least one surface of the polymer film material has been modified by being subjected to bombardment with ions for reducing the transport of gaseous material therethrough, the ions being selected from the group consisting of hydrogen, carbon, nitrogen, helium and argon at energies within the range of 1 to 80 KeV and ion doses in the range 10 12  to 10 18  ions/cm 2 . 
     Suitable polymer materials for use in performing the invention are polyethylene terephthalate or biaxially-oriented polypropylene. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The invention will now be described, by way of example with reference to the accompanying drawing which is a schematic representation of an apparatus for carrying out the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing, an apparatus for carrying out the invention comprises a vacuum chamber 1, and an associated pumping system 2. Within the vacuum chamber 1 are an ion source 3 of the gas discharge type known as a bucket source, although other types of ion source such as a twin-anode ion source can be used, a feed spool 4 and a take-up spool 5 for a film 6 of polymer material to be treated according to the invention. There is provided also a power source 7 to energise the ion source 3. 
     In a first example of the invention, samples of polyethylene terephthalate and biaxially-oriented polypropylene were implanted with nitrogen ions at an energy of 50 KeV and ion doses between 10 13  and 10 17  ions/cm 2 . Subsequently the oxygen permeability, water vapour transmission rate and optical density were measured using techniques which are well-known in the film packaging art and are not described herein. The same parameters were measured for untreated samples of the same materials and also for samples of the same materials one surface of which has been metallised. 
     The results are summarised in the following table. 
     
                       TABLE I______________________________________                              O.sub.2WVTR permeability   Ion Dose  gm/m.sup.2 /24 hrs cc/m.sup.2 /24 hrs   (Fluence) Optical @ 30° C. 90% @ 23° C., 50%  Substrate ions/cm.sup.2 Density RH RH______________________________________Poly-   Untreated 0.04    40.0     120.0  ethylene Metallised 3.00 0.04 0.5  tere- 1 × 10.sup.13 0.06 &gt;30.0 140.0  phthalate 1 × 10.sup.14 0.24 &gt;30.0 18.0   1 × 10.sup.15 0.19 &gt;30.0 2.0   1 × 10.sup.16 0.40 0.76 0.17   1 × 10.sup.17 0.55 0.83 0.54  Bi-axially Untreated 0.04 7.0 3000.0  oriented Metallised 2.0 0.5 200.0poly-   1 × 10.sup.13             Not measured (sample damaged)propylene   1 × 10.sup.15             0.16    5.8      36.0   1 × 10.sup.15 0.15 6.1 29.0   1 × 10.sup.15 0.13 6.3 30.0   1 × 10.sup.16 0.40 0.53 73.0______________________________________ 
    
     It can be seen that, for polyethylene terephthalate, ion doses above 1×10 14  ions/cm 2  all gave superior performance in relation to both oxygen permeability and water vapour transmission rate, when compared with untreated material, albeit with an increase in the optical density. 
     An ion dose of 1×10 16  ions/cm 2  gave particularly good results. 
     So far as biaxially oriented polypropylene is concerned, the results are more variable, but they show a marked decrease in the oxygen permeability, particularly for an ion dose of 1×10 16  ions/cm 2 , as before. 
     However, insofar as the oxygen permeability is concerned, it appears that at higher ion doses a correlation between the ion dose and the oxygen permeability may occur. 
     In a second example of the invention, samples of polyethylene terephthalate only were subjected to ion bombardment with ions of hydrogen, nitrogen, and argon at various energies and ion doses. 
     In a first treatment the material was treated with nitrogen ions at energies ranging from 20 KeV to 70 KeV and a fixed ion dose of 10 16  ions/cm 2 . 
     In a second treatment the material was treated with nitrogen ions at a constant energy of 30 KeV but ion doses ranging from 10 15  to 10 17  ions/cm 2 . 
     In a third treatment the material was treated with hydrogen ions at energies ranging from 30 KeV to 70 KeV and ion doses ranging from 10 15  to 10 17  ions/cm 2 , and 
     in a fourth treatment the material was treated with argon ions at energies ranging from 30 KeV to 70 KeV and ion doses of 3×10 15  and 10 16  ions/cm 2 . 
     Again, the results are expressed in the form of a table: 
     
                       TABLE II______________________________________                                   Moisture  O.sub.2 Trans- vapour  Exp  Energy Ion Dose Optical mission rate transmission  No. Ion (KeV) (Ions/cm.sup.2) Density cc/m.sup.2 /day rate g/m.sup.2                                   /day______________________________________1(a) N.sup.+      20      1 × 10.sup.16                     0.21  2.49    8.84  1(b) N.sup.+ 30 1 × 10.sup.16 0.34 1.56 1.67  1(c) N.sup.+ 40 1 × 10.sup.16 0.43 1.71 2.01  1(d) N.sup.+ 50 1 × 10.sup.16 0.53 0.86 1.16  1(e) N.sup.+ 60 1 × 10.sup.16 0.59 0.45 1.55  1(f) N.sup.+ 70 1 × 10.sup.16 0.67 0.29 0.38  2(a) N.sup.+ 30 1 × 10.sup.15 0.23 2.94 12.63  2(b) N.sup.+ 30 3 × 10.sup.15 0.30 0.52 1.27  2(c) N.sup.+ 30 5 × 10.sup.15 0.35 0.73 1.51  2(d) N.sup.+ 30 1 × 10.sup.16 0.39 1.62 1.88  2(e) N.sup.+ 30 3 × 10.sup.16 0.46 3.2 1.72  2(f) N.sup.+ 30 5 × 10.sup.16 0.49 1.95 1.97  2(g) N.sup.+ 30 1 × 10.sup.17 0.52 1.38 1.03  3(a) H.sup.+ 30 1 × 10.sup.16 0.34 0.96 1.80  3(b) H.sup.+ 50 1 × 10.sup.16 0.47 1.09 0.73  3(c) H.sup.+ 70 1 × 10.sup.16 0.53 0.70 2.10  3(d) H.sup.+ 30 3 × 10.sup.15 0.27 0.72 1.91  3(e) H.sup.+ 30 3 × 10.sup.16 0.41 1.37 2.60  3(f) H.sup.+ 30 1 × 10.sup.17 0.57 0.49 1.76  4(a) Ar.sup.+ 30 1 × 10.sup.16 0.34 2.87 2.20  4(b) Ar.sup.+ 50 1 × 10.sup.16 0.50 1.02 1.86  4(c) Ar.sup.+ 70 1 × 10.sup.16 0.61 0.44 0.80  4(d) Ar.sup.+ 50 3 × 10.sup.15 0.35 0.72 2.27______________________________________ 
    
     The above values are to be compared with those for untreated polyethylene terephthalate, which for 12 μm thick material are: 
     a) O 2  transmission rate @23° C. and 50% Relative Humidity; 120 cc/m 2  /24 hours. 
     b) Moisture vapour transmission rate @30° C., 90% RH, 40 grams/m 2  /24 hours. 
     Metallised polyethylene terephthalate has oxygen and moisture vapour transmission rates of 0.30 cc/m 2  /day at 23° C. and 50% RH, and 0.5 gms/m 2  /day at 30° C. and 90% RH respectively. 
     It can be seen that at least some of the treatment shown in Table II, for example, treatments 1(d), 1(f), (b), 2(c), 3(c), 3(d), 3(f), 4(c) and 4(d) give values approaching these. 
     Those treatments (1(f) and 4(c)) which gave the best reductions in oxygen and water vapour transport also produced the highest optical densities. 
     It is believed that the improvements in the barrier properties of the above, and other packaging materials are results of cross-linking between polymer chains in the materials and densification of the surface. Other effects include micro-roughening of the surface, the destruction of waxy surface deposits which often occur on the surfaces of polymer materials and the introduction of new chemical species, particularly oxygen, into the surface of the packaging material. This may occur as a result of the ion implantation treatment or on subsequent exposure to the atmosphere. The above effects improve the ink-retaining properties of polymer materials, particularly for inks which use polar solvents, such as water-based inks. 
     Thus in addition to improving the barrier properties of polymer packaging materials, the present invention also improves their printability.