A principal quality control concern in a food manufacturing and processing plant is the contamination of food products by foreign bodies from the surrounding environment, such as: general atmospheric dust and dirt; particles from the food processing machinery; debris emanating from human operators e. g. skin, hair etc.; insects or small animals; cleaning and protective materials; and particles of food packaging. The invention is concerned with the detection of particles of packaging, cleaning or protective materials.

It is known to detect metallic foreign bodies, such as particles originating from the manufacturing or processing machinery by passing the prepared food through a metal detector that is suitably positioned on the food production line. The most common method of detection used in the food industry is to recognise a disturbance in the magnetic flux of a coil by the proximity of the metallic foreign body. However, this method is not particularly effective in the detection of non-ferrous materials unless they are sufficiently large.

Metal detectors can be operated selectively to ignore either electrical conductivity or magnetic reactance signals; one set of signals being detected at the expense of the other. One known disadvantage of such detectors is that they are prone to"blind spots"whereby the particle is not detected owing to its particular orientation relative to the detector. Non-metallic foreign bodies may be detected by means of X- ray equipment or, alternatively, by visual inspection. Whilst X-ray equipment is suitable for detecting certain foreign bodies it is expensive and cannot readily detect the presence of certain paper, fabric or plastics fragments that emanate from sources such as food packaging, cleaning cloths, dust sheets, protective clothing etc.

Fragments of such material can be inadvertently released into the food production line by, for example, cutting or tearing open food ingredient packaging to release its contents. Such material is often coloured blue to render it easily visible so that torn or severed fragments can be detected by visual inspection. However, such fragments are often not detected as they are hidden within the food or are so small that they are not easily visible to the naked eye.

One solution to the problem of contamination by packaging, cleaning and protective materials etc is to design them so that they are sufficiently strong so as to reduce the risk of tearing or severing. However, this can often add to the cost of the materials.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.

According to the present invention there is provided a laminar plastics film for use in food packaging and processing, the film comprising outer layers of a polyolefin material separated by an intermediate layer including a metallic material so as to render the film electrically conductive and magnetically reactive such that it is detectable by a metal detector.

The film thus has protective outer layers that are appropriate for coming into contact with food but has an intermediate layer that has an increased chance of being detected in a metal detector irrespective of how the detector is tuned. Such a film can therefore be used in food packaging or as a protective dust sheet for food or food processing machinery without risk of contaminating the food with an undetectable material. It can be produced in a thin, flexible and lightweight form.

It is to be appreciated that the film may comprise more than one intermediate layer, each intermediate layer being sandwiched between polyolefin layers.

The intermediate layer may comprise a layer of extruded polyolefin material containing metallic particles, the layer being extruded and then secured between the outer layers of polyolefin by means of an adhesive or, alternatively, coextruded with the outer layers.

In an alternative embodiment the intermediate layer comprises a printed ink containing magnetically reactive metallic particles and may additionally comprise an electrically conductive layer of metallic material formed by vapour deposition.

Preferably the printed ink or electrically conductive layers are disposed at an interface between the inner and outer layers.

The metallic material may comprise an ink containing magnetically reactive metallic particles printed on a surface of at least one of the outer or inner layers. In addition it may comprise vapour deposited metal on a surface of at least one of the outer or inner layers.

The ink may be printed in a predetermined pattern comprising a plurality of square or rectangular spirals. The spirals are preferably arranged into rows and columns across the surface of which they are printed, adjacent spirals in each row being offset relative to one another. Adjacent spirals in a column are preferably also offset relative to one another.

In a preferred embodiment there are at least two layers of substantially identical printed ink patterns, one being rotationally offset relative to the other.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a laminar plastics film in accordance with the present invention; Figure 2 is a vector diagram showing the signals obtained from a metal detector when using certain metals ; and Figure 3 is a first embodiment of a film in accordance with the present invention; Figure 4 is a second embodiment of a film in accordance with the present invention; Figure 5 is a third embodiment of a film in accordance with the present invention; Figure 6 is a fourth embodiment of a film of the present invention; Figure 7 is a close-up view of the principal component of a printed ink pattern forming part of the film of the present invention; Figure 8 is a diagrammatic view illustrating the initial steps in the construction of a pattern for printing the ink on a film of the present invention; Figure 9 is a diagrammatic view of the pattern of figure 8 shown rotated through 22.5° in the form it is printed on to a layer of film; and Figure 10 is a diagrammatic plan view through a film of the present invention having two layers of printed ink, the figure illustrating the resulting combined ink pattern.

Referring now to the drawings, the exemplary laminar plastics film, shown in figure 1, in diagrammatic form comprises outer film sheets of a polyolefin material such as, for example, polyethylene between which is sandwiched a core layer that contains or carries a metallic material. The laminar film is intended for use in the packaging of foodstuff ingredients that are to be used in the manufacture of prepared foods, as a protective dust sheet for covering food processing machinery or for covering prepared or partially prepared food. However. there may be other applications.

The outer sheets are hygienic and therefore suitable for contact with food without risk of contamination, serve as a barrier to prevent the metallic material from contacting the food and are heat sealable. The core layer is designed to be detectable by a metal detector so that stray fragments of the film that enter the prepared food may be identified and removed. The core layer is both electrically conductive and magnetically reactive so that it can be detected by the metal detector regardless of its mode of operation (i. e. whether it is tuned to detect electrical conductivity or magnetic reactance).

Figure 2 illustrates the operation of a typical metal detector in the form of a vector diagram and in particular the vector signals that are generated by passing different common materials through the detector. As described above the metal detector is able to identify the presence of objects on the basis of their electrical conductivity or their magnetic attraction. In the diagram shown magnetic attraction of the detected object is represented on the x-axis (zero degrees) and the electrical conductivity on the y-axis (90°). The metals aluminium (AL). stainless steel (SS) and food products (PR) are principally conductive and have little magnetic content and so are closely aligned to the (conductive) y-axis. In contrast iron (FE) has significant magnetic attraction and lesser conductivity and so is closely aligned to the (reactive) x-axis.

In practice a metal detector is tuned to ignore background electrical or magnetic noise and so there is an elongated narrow envelope representing a dead zone (indicated in dotted line in figure 2). The zone in which signals are not considered can be selectively aligned to be of any orientation but is generally used in one of two modes of operation i. e. with the dead zone aligned to either the x or y axis. Thus by making the core layer both electrically conductive and magnetically attractive it will always be detected regardless of the mode of operation of the detector.

The core layer can be applied to the polyolefin sheets in several ways as will now be described.

In a first exemplary embodiment of the invention (shown in figure 3) the film comprises outer layers 1 of polyethylene or polypropylene that may be clear or coloured and an intermediate layer 2 of a similar material. It is to be understood that other polyolefin materials may be used for any of the layers. At the interfaces between the intermediate and each outer layer there is disposed a metallic layer comprising an electrically conductive metallic coating 3 and a printed metal-enriched ink 4. In the embodiment shown in figure 3 a conductive coating 3 is disposed on both major surfaces of the intermediate layer 2 and the ink 4 is printed an inwardly facing surface of each of the outer sheets 1. However, in an alternative embodiment (not shown) the relative positions of the ink and the conductive coating may be reversed. Each layer of the film is typically 10 to 18 microns thick.

The ink 4 comprises a carrier resin such as PVB in which metallic particles such as iron flakes are disposed. The iron flakes are magnetically reactive and thus impart the desired magnetically reactive property to the film.

The carrier may be any form of appropriate resin that serves to bond the metallic particles to the surface of the sheets. It may be solvent (including water) based, the solvent evaporating after the ink has been applied. It is applied to the sheets in any appropriate configuration with the resin providing a bond to the surface of the polyolefin. The ink is applied, for example, in a pattern so that the area between ink dots may be applied with an adhesive material so as to combine the sheets with maximum lamination strength. The advantages of a PVB carrier are that it is suitable for contact with food and allows relatively dense packing of the iron flakes such that the ink is not only magnetically reactive but also has electrically conductive properties. The dry ink weight is typically 1.8g/m2 of film and comprises 7 parts PVB to 38 parts of iron flake although can be more or less if desired.

The outer layers 1 of polyoleiin may be produced by blowing them as a sheet of film in the conventional way. printing them with ink and then laminating them on to the conductively coated intermediate layer in a gluing process. Alternatively, the outer layers may be applied to the intermediate layer by extrusion coating whereby the intermediate layer is pre-coated with the electrically conductive metal, printed and then coated one surface at a time by an extruded outer layer 1.

The electrically conductive coating 3 is applied, for example, by means of vapour deposition, the technique for which is well known and not discussed here. The thickness of the deposited layer of metal should be sufficient to offer relatively high electrical conductivity (the actual conductivity being dependent on the metal used and the thickness of application) but thin enough for the laminate to have film-like qualities. As an example, using aluminium with a thickness of 225 angstrom and an optical density of 2.8 the resistivity is approximately 1.5Q rri 2. Suitable metals contemplated for use in this technique are silver, gold, aluminium, copper, iron and cobalt. In the embodiment of figure 3 where there are two deposited layers of metal, the film is a more efficient gas barrier than a film laminate with only one such layer.

Examples of such techniques are described in European Patent Nos. 0154428 and 0209362.

In an alternative example embodiment shown in figure 4 the metallic layer 3,4 is provided between the intermediate polyethylene/polyester layer 2 and only one of the outer layers 1 and in the alternative embodiment shown in figure 5 the metallic layer 3,4 is sandwiched directly between the outer layers 1 without the need for an intermediate layer.

In the embodiment shown in figure 6, the film comprises outer layers 1 of polyethylene or polypropylene and an intermediate layer 5 comprising metallic particles that are incorporated into a polyolefin carrier (such as polyethylene and/or polypropylene) during extrusion. The metallic particles are flakes of metal such as, for example, iron that are milled to a size of, typically, 8 to 15 microns before being incorporated into pellets of polyolefin. The pellets are then used in conventional extrusion apparatus to form a sheet of plastics material that is loaded with metallic particles. The ratio of metallic particles to polyolefin is such that extrusion may be performed without risk of the plastics sheet being severed. It has been found during tests that using 70% iron particles by weight and 22% by volume can be achieved and renders the film readily detectable by a metal detector.

The intermediate layer 5 of the embodiment of figure 6 may be co-extruded with the outer layers 1 of polyolefin or may be laminated separately with an appropriate adhesive. The completed film is primarily detectable by virtue of the magnetic properties of the iron particles, but if they are packed with sufficient density they have a limited amount of electrical conductivity which can also be detected.

Alternatively, fragments of electrically conductive materials are also incorporated in the core layer. In the case where the detector is tuned to detect the magnetic reactance and the main metallic constituent of the core layer is iron, only a small amount of electrically conductive material is necessary to ensure that the vector signal can be detected.

It will be understood that other metallic elements may be used in the extruded intermediate layers. For example copper, silver, conductive carbon and aluminium can be used to provide increased electrical conductivity but reduced magnetic attraction.

A specific example of an ink pattern for use in the embodiments of figures 3 to 5 will now be described with reference to figures 7 to 10.

The ink is printed on the film as a repeated pattern of square or rectangular block spirals, one of which is shown in figure 7. Tests have established that an arcuate spiral shape is the most effective in terms of disturbing the magnetic flux of a metal detector and therefore is the design that is most readily detectable. The square or rectangular form of the spiral has been adopted as a compromise to facilitate tessellation of the design across the full extent of the film surface on which the design is printed. The spiral design, which may have any convenient number of turns, is arranged in a repeated pattern from left to right to form a row one of which is shown, for illustrative purposes, at the top of figure 8. Each spiral (following the first) in the row is disposed in a position vertically offset downwards from the previous spiral.

The next row is then started immediately underneath the preceding row but offset to the left slightly. Figure 8 illustrates the manner in which the pattern is constructed.

The bi-planar offsetting and tessellation of the spirals ensures that no continuous straight lines of significant length of printed or unprinted film are present when the pattern is viewed from any orientation.

The final ink pattern when printed on the film is shown in figure 9. The pattern is printed in a rotated orientation of 22.5 degrees anticlockwise as compared to that shown in figure 8.

In the embodiment of the film shown in figure 3 where there are two layers of printed ink 4, the second layer of ink is printed in the same manner as the first but with the pattern disposed on the respective film layer in a rotated orientation of 45° relative to the pattern applied to the first layer. The resulting superimposed pattern of ink is shown in figure 10. The print coverage area is 75% of the area of the composite film and is arranged such that printed or unprinted areas do not combine to form high or low density localised areas of reactive ink. Such a design is tolerant to misalignment of a printed layer of film relative to the other printed layer in the horizontal plane. That is, during assembly of the final film by way of lamination or extrusion coating etc, misalignment of the respective printed layers in longitudinal, lateral or rotation directions in the plane of the final film does not adversely affect the superimposed pattern on the final composite film in that it does not suffer from moire fringe patterns.

Tests have shown that for a spiral of the dimensions shown in figure 7 a piece of film having a surface area of 0. 25cm2 can be detected by an average metal detector.

The provision of a metallised core in any one of the methods referred to above has several additional benefits to the finished film. First, the presence of metal which is light reflective, particularly in the patterned ink form improves the susceptibility of the film to be detected by visual inspection. Secondly, since at least one of the polyolefin layers of the film (usually the intermediate layer) is to be coated with metal it is made from a more durable material so as to withstand the high temperature associated with the vapour deposition process thus rendering the final film stronger per unit thickness than conventional films.

It is to be understood that the core layer may be formed by one or a combination of the above processes to achieve a film with a core that is both conductive and reactive. For example, one of the outer sheets may have an aluminium vapour deposit to give conductivity and the other may be printed with an iron- enriched ink to provide the magnetic content.

It will be appreciated that the film of the present invention may also be coloured to aid visual inspection.

By providing a core layer that is both magnetic and electrically conductive the chances of detecting a fragment of the film irrespective of its orientation relative to the metal detector is significantly increased.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, the plastics film may be constructed from more than three layers as shown in figure 1. For example separate magnetic and electrically conductive core layers may be provided and each may be sandwiched by polyolefin sheets.