Hitherto, the long life (i. e. shelf life of one year or more) container of choice for perishable goods has been the traditional tin can. Plastics containers for perishable goods are well known and, unlike tin cans, their contents can successfully be heated by irradiation with microwaves.

However, such containers do not provide a long shelf life for perishable contents. This is particularly the case where the contents are both cooked and sterilised; for example, by an aseptic filling (hydrogen peroxide sterilisation) or by a water immersion or a steam retort process.

This invention seeks to provide plastics sheet suitable for fabricating into containers which are microwaveable and which also provides a long life for perishable contents.

According, therefore, to one aspect of this invention, there is provided a multi-layer plastics sheet which comprises i) a gas barrier layer; ii) a tie layer disposed contiguously on either side of layer i); iii) one or more external layers of plastics material adhered contiguously to one of the tie layers ii); and iv) an internal layer of plastics material which includes an oxygen scavenger, the layer being adhered contiguously to the other of the tie layers ii).

Suitably, the sheet may further comprise a layer of plastics material v) contiguously adhered to layer iv) and not including an oxygen scavenger.

In a preferred embodiment of the invention, layer i) may comprise polyvinyl alcohol or an ethylene-vinyl alcohol copolymer, suitably wherein the degree of hydrolysis of the polyvinyl alcohol may be from 70 to 90 %. It is preferred that the vinyl alcohol content of the ethylene- vinyl alcohol copolymer may be from 50 to 75 %, preferably 55 to 70, such as 68 % weight.

The ethylene content of the copolymer may be from 25 to 45 %, preferably 30 to 40% by weight.

Suitably, each tie layer ii) may comprise a graft copolymer of an unsaturated carboxylic acid or unsaturated carboxylic acid anhydride with the polymer of the plastics material contiguous therewith. Typically, the tie layer may comprise a graft polymer of maleic anhydride with propylene, especially isotactic polypropylene. Desirably, the external layer of plastics material (iii) directly adhered to layer ii) may comprise isotactic polypropylene. Furthermore, the internal layer of plastics material iv) may comprise isotactic polypropylene. Suitably, layer v) may comprise isotactic polypropylene.

The oxygen scavenger may be any of the proprietary products available as such and which often incorporate polyhydroxy phenolic derivatives. Alternatively, they may include a transition metal in a lower oxidation state, for example Fe (II) which is oxidised to a higher oxidation state by trace oxygen which is thereby scavenged. The transition metal may be suitably masterbatched; for example with the plastics material, such as polypropylene, used.

SHELFPLUS, especially SHELFPLUS 2500 has been found to be very suitable.

The sheet according to this invention may be preparable by coextrusion. Desirably, it may have a thickness from 400 to 2500, such as 1000 to 1500,, um as prepared and from 200 to 400 pm after fabrication into a container.

In accordance with a preferred feature of this invention layer i) may comprise from 2.5 to 10 %, preferably 3.0 to 8 %, such as 6.5 %, of the thickness of the sheet. Each tie layer ii), which may be the same or different, may comprise from 1 to 3 %, such as 2 % of the thickness of the sheet.

The, or all of the, layer (s) iii) may comprise from 30 to 20% of the thickness of the sheet.

Preferably, layer iii) does not include any added oxygen scavenger. The layer iv) may comprise from 35 to 55 %, such as 43 %, of the thickness of the sheet. Preferably, layer iv) comprises from 2 to 8%, such as 5%, by weight of oxygen scavenger. Below 2% the scavenger is unlikely to be effective while above 8% the scavenger has no additional effect and may adversely effect the mechanical properties of the sheet. The scavenger is suitably first masterbatched with the plastics material from which (iv) is fabricated. Layer iv) may comprise up to 100% by weight of regrind, typically 50% by weight of regrind. Preferably, layer v) comprises from 15 to 25 %, such as 16.5 % of the thickness of the sheet.

In accordance with an especially preferred embodiment of this invention, it is found that particularly beneficial effects are obtained when the gas barrier layer i) comprises from 3.0 to 8 % of the thickness of the sheet while the concentration of oxygen scavenger in layer iv) comprises from 3 to 7 % by weight of layer iv).

In accordance with a further aspect of the invention, there is provided a sheet according to the invention formed as a container. Preferably, the container is thermoformed.

It is highly desirable that the container of the invention may be subject to microwave radiation without loss of dimensional stability. Thus, the components of the layers, as fabricated, must not melt when irradiated with microwaves.

This invention also provides such containers which are filled and hermetically sealed. The sealing is preferably effected with a closure, such as a lid, fabricated from a sheet according to the invention. Suitably, the fill comprises a comestible or a beverage product. Desirably, the filled, sealed container is then sterilised, for example by autoclaving.

The invention is further illustrated, by way of example, with reference to the sole Figure which is a diagrammatic cross-section of the sheet according to this invention.

The production line includes blending equipment (eg. COLORTRONIC OR MAGUIRE blenders) for separately blending five different feedstocks. These feedstocks are then fed to five different extruders (eg. EGAN or KUHNE extruders) in which the material is heated, pressurised and mixed and next the five streams are fed, without mixing, to a layer delivery block which assembles the layers. A typical melt temperature is about 200°C. The molten multilayer structure is then spread through a die assembly which delivers a molten multilayer sheet to a cooled calender roll stack where the sheet is next solidified prior to being wound on to reels.

The sheet comprises an external layer (1) comprising 30 % of the total thickness and comprising isotactic polypropylene and a copolymer of propylene (such as MONTEL T3 1SE and MONTEL EPT30R, respectively); a tie layer (2) comprising 2% of the total thickness and comprising a maleic anhydride-polypropylene graft copolymer (such as ADMER QF55IE) ; a barrier layer (3) comprising 6.5 % of the total thickness and comprising ethylene-vinyl alcohol of 32 % by weight polymerised ethylene content (such as EVAL T101B) ; a tie layer (4) as in (2); an internal layer (5) comprising 43% of the total thickness and comprising regrind including a total of 5% oxygen scavenger (such as SHELFPLUS 2500); a further layer (6) as in (1), but comprising 16.5% of the total thickness.

Containers are formed from the sheet using a thermo forming process in which a combination of air pressure, temperature and a mechanical forming assist or plug are used to fabricate the sheet in a thermoformer (eg ILLIG 70k) into the desired form for the container. Typical forming temperatures are around 180°C to 185°C such as 183°C. The shelf life of the food with which the containers are filled and hermetically sealed can be in excess of 18 months. Significantly inferior results were obtained when the scavenger was included in layer iii) but not in layer iv).