OXYGEN-SCAVENGING COMPOSITIONS

Abstract: For the storage of foodstuffs which are susceptible to oxidative degradation it is proposed to use powdered metal - and especially aluminium - in wet-corrosion compositions using strong corrosion agents, and specifically materials (such as strong acids and strong alkalis) that will reduce the pH to well below 4 or increase it to well above 9. More particularly, it is proposed that the metal and a large-surface-area, reaction-site material be utilised in a very finely powdered form, and especially one in which a significant proportion of the particles are deformed, possibly with fracturing. The compositions can be dispersed within a water- and oxygen-permeable sheet that can be disposed in actual, direct contact with the oxygen-sensitive substance to be protected.

Description: OXYGEN-SCAVENGING COMPOSITIONS This invention relates to oxygen-scavenging compositions, and concerns more particularly such scavenger compositions that employ a metal as the primary oxygen- removing ingredient . There are many fields of human endeavour where it is very much preferred, even essential, that in some defined volume there be very little free oxygen present. Such fields include the storage of cosmetics, pharmaceuticals, chemical compounds, and other materials which are susceptible to oxidative degradation. Perhaps the most common field is that relating to the storage of food; not only is free oxygen itself sufficiently chemically active such that, even in tiny quantities, it can react with components of the food to produce unpleasant tastes or odours, but, in addition, as the main "life" chemical, oxygen is an essential factor for the growth of many of the types of micro-organism that commonly infect food and whose unchecked growth can spoil the food or even render it toxic. Now, while such chemical or biological reactions can be minimised (or even wholly prevented) by the addition of other chemicals to the food - typically anti-oxidants and biocidal or biostatic agents - nevertheless the very-much preferred answer, especially in the light of the present world view that everything should be "natural", ecologically sound and chemical-free, is the apparently simple one of effectively removing all free oxygen both from the food itself and from the container within which the food is being stored. Much work has therefore been carried out on oxygen removal. The obvious starting point is to evacuate all the air from the container once the food has been placed therein, and in many cases this works well. However, in some circumstances such physical oxygen removal is not sufficient, and there is left behind enough free oxygen to give deleterious results. For example, oxygen is easily trapped in - absorbed on or into - the food itself, and with some foodstuffs (such as liquids like fruit juices) that oxygen is difficult physically to remove in an acceptably short time without "damaging" the food itself. Again, many foods are packaged in containers that exhibit some oxygen permeability (despite efforts to prevent this) , and so even if starting oxygen-free they can end up not being so. As a result of these problems, there has also (and additionally) been employed a quite different oxygen-removal technique, namely that of using materials that react chemically with the last traces of the free oxygen to bind it away, safely. This technique is known as "oxygen scavenging", and the chemicals are "oxygen scavengers" or "oxygen-scavenging compositions" . Of course, using chemicals like this has its own problems, and it has been necessary to select substances which - either as themselves or as the products that result from their use - have no, or no significant, deleterious effect either on the food or on the person subsequently consuming that food. Moreover, much effort has been devoted to the manner in which the chemical is physically associated with the foodstuff, so as to ensure that it (and its products) does not end up being consumed at all. The invention concerns both these areas; first, it relates to novel chemical compositions usable as oxygen scavengers for stored oxygen-sensitive substances, especially packaged foodstuffs, and second it relates to the manner in which such compositions can be physically associated with the food. In many applications it is desirable to remove the oxygen rapidly not only from the oxygen-sensitive substance itself but also from the gaseous environment which may surround the substance in its storage container. Earlier oxygen scavengers were chemicals which were added directly to the oxygen-sensitive substance. These are typified by butoxy hydroxy anisole (BHA) ; they themselves react with free oxygen to remove it from the environment and hence prevent whatever else that oxygen might have done or facilitated. However, such scavengers are not themselves entirely without unacceptable side effects, and in order to avoid these a second approach was developed in which a different type of oxygen-scavenging material - an oxidisable metal such as iron in powdered form - was placed in a physically separate compartment, such as a sachet, within the container in which the oxygen-sensitive substance was stored. This approach was successful at removing oxygen from the ambient atmosphere in the container, and thus indirectly from the substance itself; placing the sachet (say) in direct contact with the substance was also a possibility, but generally this was not satisfactory - especially when the substance was a liquid food product - due to undesirable migration of the contents of the sachet into the substance. Incorporation of the oxygen-scavenging material typically iron powder - into a thin plastics film which can be placed in direct contact with the oxygen-sensitive substance is another approach which has been tried, particularly in the case where the substance is a liquid foodstuff, in an attempt rapidly to remove oxygen from the substance itself. However, migration is still a problem - the corrosion products of such scavengers have an undesirable tendency to migrate out of the film into the substance - and in order to mitigate this, or avoid it entirely, elaborate multi-layer film structures have been required. An alternative to powdered iron has been powdered aluminium (and its alloys) , but the use of aluminium - which works as an oxygen scavenger in a manner rather different from that of iron - is beset by a number of what hitherto appeared to be practical difficulties, principally the evolution of excessive amounts of hydrogen. The proposals for the use of aluminium as an oxygen scavenger have generally suggested that the compositions should be of neutral pH (indeed, in certain of the disclosures relating to this there has been the clear suggestion that neither acid nor alkaline compositions - none of which seem actually to be described - should be utilised because they result in excessive production of molecular hydrogen) . These neutral compositions work, but there are problems with their use which can now be seen to stem from the relatively low rate at which they produce atomic hydrogen (the form required for the major oxygen-scavenging reaction) . Once this is understood it becomes possible to see how better, faster-acting compositions can be formulated using compositions of the type apparently previously avoided - that is, wet-corrosion compositions using strong corrosion agents, and specifically materials (such as strong acids and strong alkalis) that will reduce the pH to well below 4 or increase it to well above 9. Firstly, then, the present invention proposes - contrary to all suggestions in the Art - that a wet-corrosion oxygen scavenger should be formulated as a metal-based - and most preferably as an aluminium-based - composition which is, or becomes in use, not approximately neutral but instead either highly acid or, and preferably, highly alkaline. Under these conditions some metals, and particularly aluminium, decompose water with the evolution of hydrogen. For aluminium, the overall equation involved is believed to be: 2A1 + 6H 2 0 -> 2A1(0H) 3 + 3H 2 (1) the molecular hydrogen being a consequence of the reaction 2H + + 2e _ -> H 2 (2) In the presence of oxygen a proportion of the atomic hydrogen (H + ) is thought to combine with the oxygen according to the reaction 0 2 + 4H + + 4e " -> 2H 2 0 (3) The efficiency of oxygen removal is increased if there is included a catalyst which promotes the following reaction between oxygen and molecular hydrogen (H 2 ) , 0, + 2H 2 -> 2H 2 0 (4) One of the purposes of the invention is to provide compositions and processes in which there is increased the efficiency of Reactions 3 and 4, thereby significantly decreasing the amount of molecular hydrogen evolved and concomitantly increasing the rate of oxygen removal . In an environment including a strong corrosion enhancer - a strongly acid or strongly alkaline environment - a metal such as aluminium (even "pure" aluminium, without any alloying ingredients save for the normal impurities present after its production) , corrodes sufficiently well to be of value as an oxygen scavenger. However, as previously noted in such an environment there may be a significantly increased evolution of hydrogen, leading to the formation of molecular hydrogen at levels which may become seriously disadvantageous. The present invention proposes a number of ways in which this potential problem can be resolved. For the first, it is suggested - and especially in the preferred embodiments of the invention - that at least one active constituent of the composition, especially the metal, be utilised in a very finely powdered form. More specifically in the case of aluminium, the form is one wherein not only are the particles of the order of 1 to 20 micrometres in size, but also a significant proportion of the particles are deformed, possibly with fracturing, so that their effective crystal size is significantly reduced and their surface-area- to-volume ratio is significantly increased. The proportion of particles reduced in effective crystal size is preferably at least 20%, very preferably as much as a half, and most preferably all (95% or more) . As is explained further hereinafter, oxygen-scavenging compositions including a metal powder which is in the deformed/fractured state can provide excellent control of molecular hydrogen production. Such a deformed, possibly fractured, form can be prepared by a high- energy milling of a more conventional fine powder, this milling causing deformation, perhaps with fracturing, of the particles into the required state; hereinafter it is referred to as the "deformed/fractured" state. As will be discussed hereinafter, this milling is advantageously effected in the presence of a large- surface- area, reaction-site material, because this results in a composition which provides good control of molecular hydrogen production. This material is preferably a carbonaceous material, particularly carbon. A composition comprised of powdered aluminium and very finely powdered carbon provides some control of molecular hydrogen production. A higher degree of control is obtained with very finely powdered aluminium mixed with very finely powdered carbon. Oxygen- scavenging compositions made up using a metal powder which has been milled with a suitable reaction-site-providing filler, so that both become of a very finely powdered form and that the particles of the composition become of the deformed/fractured state provide excellent control of molecular hydrogen production. For a second way of dealing with the possible problem of hydrogen evolution, the invention suggests a way of physically associating its oxygen scavenger metal powders with the site where the oxygen is expected such that the formation of molecular hydrogen is significantly hindered - namely, dispersed within a water- and oxygen-permeable film (within which term is included a layer that is not self- supporting) that can be disposed in actual, direct contact with the oxygen-sensitive substance to be protected. Again, and as is explained further hereinafter, such film- incorporated, oxygen-scavenging powder compositions provide excellent molecular-hydrogen-production control. More specifically, then, the invention suggests the use of an oxygen scavenger which is a powder-like composition of finely-particulate metal, for example aluminium, together with: a filler, that acts both as a porous dispersant and carrier and also as a site for the reaction of any produced atomic hydrogen with oxygen to produce water (and is typified by an activated carbon material) ; and a strong corrosion enhancer, especially a strongly alkaline corrosion enhancer (e.g. an alkali metal hydroxide, or a combination of an alkali metal hydroxide with an alkali metal halide) to increase the wet corrosion of the metal. Optionally, the composition will also contain a catalyst for the reaction between molecular hydrogen and oxygen to produce water (and typified by a platinum metals group element, such as palladium) . A third way of controlling the amount of hydrogen evolved is by calibrating the amount of metal which is available for reaction against the total amount of oxygen required to be consumed by the oxygen scavenger. Thus, to control the amount of hydrogen evolved following the end of the oxygen scavenging reaction by the presence of excess metal, the amount of metal present in the composition (or oxygen-scavenging polymer formulation) may be adjusted in relation to the total amount of oxygen required to be consumed by the oxygen scavenger. For example, in ideal circumstances, O.lβlg aluminium in a composition used as a powder is sufficient to remove 100ml of oxygen. However, it is necessary to bear in mind that, in less than ideal circumstances, in which not all of the aluminium is available to be reacted, for example in which part of the aluminium is inaccessible to reaction processes, it would be necessary to increase to above the ideal level the amount of aluminium initially incorporated, but not up to a level at which a significant quantity of hydrogen is evolved following the end of the oxygen scavenging reaction. In a first aspect, therefore, this invention provides a method for the scavenging of oxygen from a site where it is present, in which method there is placed in communication with the site an oxygen-scavenging composition comprising a finely-particulate metal, itself capable of corroding in the presence of water with the evolution of hydrogen, and a strong corrosion enhancer appropriate to the metal. In a second aspect the invention provides an oxygen scavenger composition which is a powder-like mixture of finely-particulate metal together with a strong corrosion enhancer . In a sub-set of the compositions of this second aspect, the invention provides an oxygen scavenger composition which is a powder-like mixture of finely-particulate metal together with a filler and a strong corrosion enhancer, and wherein the metal itself is capable of corroding in the presence of water with the evolution of hydrogen. In a third aspect, the invention provides an oxygen scavenger composition comprising finely-particulate metal with particles that are of the order of 1 to 20 micrometres in size and are deformed/fractured in the manner discussed hereinbefore. In a strongly preferred sub- set of the compositions of this third aspect, the invention provides an oxygen scavenger composition which is a powder-like mixture of the finely- particulate metal together with a large-surface-area, reaction-site filler, and wherein the metal is itself capable of corroding in the presence of water with the evolution of hydrogen . Use of the materials and methods of the invention can result in the achievement of very low residual oxygen concentration, such as 0.01 to 0.04% (100 to 400 ppm) or better. Those materials that function by a corrosion process require humid conditions . These latter can be furnished by the packed product, for example, certain packed foods (relative humidity above 60%) . In very dry conditions (less than 22% relative humidity) , the oxygen absorption is or approaches zero, for purposes of storage before use. The physical nature of the composition The present oxygen scavenger is a finely-particulate composition of metal together with various other materials. The particles making up the composition are generally very small, conveniently of the order of 1 to 20 micrometres and less. Indeed, very preferably the composition's particles are no greater that 5 micrometres in diameter. The metal The metal utilised in the compositions of the invention is most preferably one that is itself capable of corroding in the presence of water with the evolution of hydrogen. Suitable examples of such a metal are magnesium and, especially, aluminium. The metal is conveniently "pure" - that is, without any alloying ingredients save for the normal impurities present after its production, and thus with a typical purity of ^ 97% or better. However, both metal alloys and metal mixtures can be employed, and typical such materials are aluminium alloys containing calcium, magnesium, zinc or iron, and aluminium mixtures with iron, zinc or copper. The term "metal" used herein means any of these forms, as appropriate in the context . When the composition is to be utilised as a free powder, the metal is in a very finely powdered form in which the particles - and very preferably all or a large proportion of them - are deformed/fractured as described above. In this form the metal seems to have an enhanced corrodibility . What is surprising is that if at least a significant proportion of the particles are of such a deformed/fractured nature then the evolution of molecular hydrogen from the composition appears to be much reduced. The proportion may be as little as 20%, but for substantial hydrogen control it is preferably at least 50% and most preferably 95% or higher. In principle, the proportion of deformed/fractured particles may be as high as desired. The deformation and possible fracturing of the metal particles will involve a reduction in the effective crystal size of the particulate material. This reduction should normally be at least 10%, preferably at least 20%, but greater reductions - by 40% or 50%, say - are both feasible and beneficial. When the oxygen scavenger was used in a free powder form, a reduction of around 40% in the effective crystal size was associated with a maximum hydrogen concentration of less than 5% throughout scavenging. The amount of metal in a suitable amount of composition should be selected carefully such that sufficient metal is present to absorb the required amount of oxygen but that excess metal is not present which would result in excessive hydrogen evolution after the oxygen has been scavenged. When the metal is aluminium, the aluminium content of the composition is preferably in the range of 10 to 50% by weight, but may be as high or as low as is practical. Fillers The oxygen scavenger powder composition comprises a metal in a particular form, and preferably together with a number of other materials, one or more of which acts as a filler. The nature of the filler will depend upon its purpose (s) . The primary purpose of the filler is to act as a site for the reaction of atomic hydrogen with oxygen to produce water. For this purpose the filler should include a large-surface-area particulate material, particularly activated carbon, through which both hydrogen and oxygen can readily diffuse and be adsorbed. It is advantageous if this carbon be processed - ie, milled - to increase even further its effective surface area. Related carbonaceous materials - graphite and carbon black, for instance - may also be useful in this respect. The secondary purpose of the filler is as a carrier and as a dispersant for the other components. In this respect the filler may be or include an inert particulate solid. The filler may also meet a third purpose, which is to be a milling agent. In this respect the filler may include inert abrasive particulate solids such as sand. Many other substances exist which could be used as fillers or filler components, and examples include gypsum, talc, alumina, active alumina, zeolite, silica gel, active clay, diatomaceous earth, bentonite, synthetic aluminium silicate, and aluminium hydroxide. However, for the required abrasive properties, materials such as talc and ordinary sand (Si0 2 ) are, in fact, sufficiently abrasive and so it is not necessary, although it is possible, to use harder materials such as alumina, zirconia and silicon carbide. A preferred material is ordinary sand. The amount of filler may vary widely depending to some extent on the amounts of the other components of the composition and the functional form in which the composition is used (for example, as a free powder in a sachet or as a powder dispersed within a polymer matrix) . When the composition is used as a free powder, then, based upon the metal, quantities of from 10 to 1000 parts (for example 50 or 100 parts) by weight filler per 100 parts metal are acceptable. It may be convenient to formulate the filler as a 50:50 mixture of reaction site material and carrier material, such as sand. The corrosion enhancer In the present oxygen scavenger compositions there is a strong corrosion enhancer - basically a material which provides the extreme pH conditions for the required increase in the rate of the wet corrosion of the metal. The corrosion enhancer may be strongly acid or strongly alkaline,- when the metal is aluminium it is very preferably strongly alkaline, particularly an hydroxide, and is most conveniently an alkali-metal hydroxide, typically sodium or potassium hydroxide, optionally together with a metal halide, most conveniently an alkali-metal halide such as sodium chloride. A suitable hydroxide and a suitable metal halide have a synergistic corrosion-enhancing action. The amount of corrosion enhancer may also vary depending to some extent on the amounts of the other components of the composition and the functional form in which the composition is to be used. However, based upon using aluminium as the metal, quantities of from 1 to 50 parts by weight corrosion enhancer per 100 parts aluminium have proven satisfactory. The catalyst As noted hereinabove, the present metal -based oxygen scavenger compositions may include a number of additional components, and specifically a catalyst for the reaction between molecular hydrogen and oxygen to produce water. The catalyst may be a substance such as carbonyl iron, but is typically one of the platinum metals group; it may for instance be palladium, palladium oxide, or platinum itself. Conveniently, it is employed in an amount of from 0.001 to 1% by weight based on the amount of the metal . Preferred compositions The more preferred compositions are those which include aluminium powder, an alkali-metal hydroxide/alkali -metal halide corrosion enhancer mixture, and a catalyst, and, when the composition is to be utilised as a free powder (in a sachet, say), a filler (which provides reaction-sites), and the aluminium powder is in the deformed/fractured state. Thus, very preferably, the present oxygen scavenger powder compositions are mixtures of deformed/fractured aluminium together with: a) an alkaline corrosion enhancer and b) a catalyst for the reaction between molecular hydrogen and oxygen,- c) a large-surface-area reaction-site material, and possibly d) an inert particulate solid. Using an alkaline corrosion enhancer - typically an alkali-metal hydroxide such as sodium hydroxide - can provide considerably- improved oxygen absorption, although this may also result in increased hydrogen evolution. However, where the metal is aluminium in the deformed/fractured state, and properly dispersed in a suitable particulate filler and so has increased contactability therewith, the evolution of hydrogen is significantly reduced even though there is still the improved oxygen uptake . The most preferred corrosion enhancer advantageously includes an alkali-metal halide such as sodium chloride. While such compositions give only a small improvement when the metal - aluminium, say - is in conventional powdered form - that is, is not in a deformed/fractured state - they provide significantly better results, both in terms of increased, and quicker, oxygen uptake and as regards reduced hydrogen evolution, if the aluminium is utilised in the preferred deformed/fractured state and properly dispersed in a suitable particulate filler and so has increased contactability therewith. Preparative method The present powder-like oxygen scavenger compositions may be prepared in any convenient manner - thus, by simply blending all the ingredients together. However, it may be advantageous to make them in a simple multi-stage mixing and milling/grinding operation, and to use the filler as a milling agent for the metal (most metals likely to be useful are generally available primarily as micrometre-sized fine powder with no significant initial deformed/fractured particle content) . In such an operation the components are ground (conveniently in a high power ball mill) for long enough to decrease the average effective crystal size of the metal by at least 10% if not at least 20%, and desirably by up to 50%. Although the metal and the filler are always ground from the beginning, the other components may be added in stages during the grinding process. It is particularly advantageous to mill the metal together with a large-surface area, reaction-site filler, so that their surface-to-volume ratio is significantly increased and the contact between them increased. As will be apparent from what has been said above, the main objectives of the milling are to: a) increase all the particles' surface-area-to-volume ratio (of the aluminium and active carbon, say) ; and b) improve the metal's dispersion, and its contactability (and in particular to increase the contact area between the aluminium and the carbon) . The activity of the compositions By selecting the amount of composition provided in a container in relation to the amount of oxygen expected to have to be scavenged, the resulting compositions scavenge oxygen efficiently over a range of temperatures from 30°C down to 6°C. Typically, the oxygen content in a closed 500 ml container can rapidly be reduced at room temperature by a mere 0.57g of scavenger powder composition (containing 31wt%Al) from 20.8% (as in ambient air) down to less than 0.1% in a period of 12 to 24 hours. Those compositions when they contain a proper particulate filler and have been well milled scavenge oxygen efficiently without causing excessive hydrogen evolution. Use of the compositions One of the significant advantages of the present oxygen scavenger compositions is that they can quite readily be incorporated into water- and oxygen-permeable polymer (plastic) films. Such a film can be produced by extrusion of a compounded mixture of the composition and the polymer. Indeed, such a film can be extrusion-coated onto the surface of a substrate to constitute a laminate whereof that film is directly in contact with a product - e.g., a food or pharmaceutical product to be packaged, such as wrapped or cartoned, therein. A self-supporting film can be laminated onto the surface of such substrate to constitute such laminate or be used alone to wrap the product. Thus, the film can provide the inside surface of a foodstuff container. Provided that the film is not exposed to moisture the scavenger composition will stay inert - and fresh - and so the empty container will have a long "shelf life". However, as soon as the container is actually filled with the (wet) foodstuff and some of its water content seeps (as water vapour) through the film into the composition, then the composition will be activated to start scavenging oxygen. A particular advantage of such films is that migration of components of the oxygen-scavenging composition into foods in direct contact with the polymer is extremely low owing to the insolubility of the aluminium and its reaction products. The need for expensive and complex ulti- laminate structures is therefore eliminated. Therefore there can be provided oxygen- scavenging polymer formulations suitable for making protective, oxygen- scavenging films on the inside surface of containers for foodstuffs, which formulations comprise the present oxygen- scavenging powder compositions dispersed within suitable polymers that are permeable to both water vapour and oxygen. In a fourth aspect the invention provides an oxygen- scavenging polymer formulation suitable for making a protective, oxygen- scavenging film in communication with an oxygen- sensitive substance inside a container, which formulation comprises an oxygen-scavenging powder composition of the invention dispersed within a suitable polymer that is permeable to both water vapour and to gaseous oxygen. The oxygen- scavenging composition and the polymer In this aspect the invention provides an oxygen- scavenging polymer formulation which comprises an oxygen- scavenging powder composition of the invention dispersed within a suitable film-forming polymer that is permeable to both water vapour and oxygen. The oxygen-scavenging composition used with the polymer may in essence be any of the present compositions described herein. The film- forming polymer within which the oxygen- scavenging composition is dispersed may be almost any of those film- forming polymer materials already proposed for use as the inner protective coating for a container. However, it has to meet certain special conditions, and these constrain what sort of polymer can be employed. Firstly, it has to be permeable to water vapour (a minimum permeability of around 1 gram per square metre per day) , so that some of the water within the foodstuff in the container can permeate through the polymer to the oxygen-scavenging composition therewithin and then activate that composition. Secondly, it must be permeable to oxygen (a minimum permeability of around 1 litre per square metre per day) . Now, unfortunately it is the case that these tend to be mutually exclusive,- polymers with high water vapour permeability seem to have low oxygen permeability, and vice versa . However, the requirements are for the most part met by various forms of ethylene vinyl acetate copolymer (EVA) , typically that sold as OPTENE by Borealis A/S, which have quite a high oxygen transmissibility (6 litres/square metre per day) and are reasonably permeable to water (18 grams/square metre/day. Another well-known film- forming polymer material is low density polyethylene (LDPE) ; this typically has a high oxygen permeability (4 l/m 2 /day) but rather a low water permeability (3 g/m 2 /day) . Of course, mixtures of materials can be utilised, as can laminates; EVA and LDPE can be employed like that . Other suitable film- forming polymers are the various ethylene butene-l copolymers, and blends of two or more of modified polyethylene oxide, vinyl alcohol polymer, sodium acrylate polymer, nylon, and acrylic acid/vinyl alcohol copolymer with olefin resins. The amount of oxygen- scavenger composition to be incorporated within the film- forming polymer can vary quite widely, typically from 1 to 20wt%, but it should be noted that, when actually dispersed within the polymer rather than being directly exposed to the ambient conditions, the oxygen- scavenging compositions may not be so effective at scavenging oxygen . The resulting films are effective at removing oxygen from headspace air as well as oxygen actually dissolved in liquids (such as foodstuffs) . Typically, at room temperature 400cm 2 of a 50 micrometre film containing 9wt% oxygen- scavenging composition will remove 15ml of oxygen over a 7 day period. The same film will typically reduce the dissolved oxygen content of 200ml of water from 6mg/l to less than lmg/1 in the same time. Evolution of hydrogen by the films is typically less than 2%, and this is not considered to be significant in practice. Moreover, the films do not permit any noteworthy migration of aluminium (the amount migrating is very much lower than even the natural content of aluminium in many foods) . Alternative uses of the polymer formulations If the oxygen- scavenger composition is to be incorporated in an extruded film, then the maximum amount of composition cannot be much more than 20%. However, if it is incorporated into a polymeric sheet material thicker than a film, then the amount of composition could be as high as desired, and even up to 80wt.%, except where the composition is to be in substantially direct contact with a product, such as food, where migration of a consitituent of the composition into the product is unallowable, in which case the amount of composition is more typically from 1 to 20 wt% . For packaging of solid foods with high moisture content, it is possible that an oxygen absorber in the form of a disc or strip inside the packaging material and consisting of the metal-containing scavenging composition incorporated in a polymer could prove the simplest solution. This is due to the fact that direct contact with the food is allowable. In a very simple way, the scavenging formulation can thus be applied as a supplement to vacuum packaging and Modified Atmosphere Packaging systems . Still, a number of liquid and solid foods are foreseen as being packaged in specialised polymer laminate packages having as much transparency as possible. This may mean that the scavenging formulation is applied in spots or patterns in the laminate to allow viewing of the packaged food as well as showing that the package contains an oxygen scavenging composition to protect the food. When foods such as beer, juices etc. are packaged in glass bottles or jars, the scavenging formulation would possibly be placed in the closure of the container, preferably as part of a polymer-based liner in the closure. This liner would allow direct contact with the food over prolonged periods without adverse effects in the form of migration into the food etc. A number of paints and printing inks are today water based, and susceptible to microbial and oxygen-based degradation. The present scavenging formulations have great potential for packaging of such products. When the metal in the powder composition is aluminium, the aluminium content in the formulation is preferably in the range of 0.25 to 15% by weight but may be as high or as low as is practical . The packaging material The present oxygen- scavenging polymer formulations are suitable for making a protective, oxygen- scavenging film providing the inside surface of a packaging material . The packaging material is preferably in the form of a laminate, and may be used to wrap solid products, for example solid foodstuffs, or to form containers, such as cartons, to contain "fluid" products, for example powders and liquid foodstuffs. The film is "protective" in the sense that it protects the foodstuff - the contents of the package - from the ambient conditions, and is oxygen-scavenging in the sense that it removes - scavenges - the dissolved or free oxygen from the contents and inner space of the package. Since the oxygen-scavenging composition relies upon the presence of water or water vapour to function efficiently, such water or water vapour has to be obtained from either or both of the ambient atmosphere and the contents of the package. Thus, where the packaging material is relatively impervious to ambient moisture external of the package, the packaging material may be for any sort of product provided that the product does include sufficient moisture to permeate through the film into the scavenging composition therein and thus initiate the oxygen-scavenging reactions. Typical products that can be packaged in this way are liquids - e.g., dairy products such as cream, and fruit juices (both natural and pasteurised, dilute and concentrated) such as orange or apple juice, as well as a range of pharmaceuticals and cosmetics - e.g., moistening creams, lotions and ointments, preparations of antibiotics, and diagnostic kits - as well as chemicals such as paints and detergents. The substrate itself may be of any suitable material - and, moreover, when the packaging material takes the form of a container the physical nature and form of the container are not really relevant - but in fact the present oxygen- scavenging formulations are particularly suited to use with containers of the sort made from paperboard (such containers find much employment for packaging fruit juices) . Various Examples and Test Results are now given, though by way of illustration only, to show details of the oxygen- scavenging compositions of the invention, of their preparation and of their use, together with some Test Results showing how effective they are. General Procedures I. Preparation of oxygen-scavenging composition A. Small-scale preparation using a Fritsch P7 mill The oxygen scavenger was prepared by placing 1.9 g aluminium (15 micrometres particle size, 99.7 % purity supplied by The Aluminium Powder Company Limited, West Midlands, UK) together with 1.9 g sea sand (supplied by Riedal de Haen, Germany) and 1.78 g activated carbon (supplied by Riedal de Haen, Germany) in a 45 ml cemented carbide bowl containing 10 cemented carbide balls of 10 mm diameter. Two such cemented carbide bowls were then sealed each with a cemented carbide lid, and placed in a Fritsch P7 mill (supplied by Fritsch GmbH, Germany) . The mill was operated in three milling phases, each of 15 minutes duration. For low energy milling, which was always used for the second and third phases, and sometimes for the first phase, the ball acceleration was 6 g (six times that of gravity) , but for high energy milling, which was used only in the first phase, the acceleration was 23 g . The aluminium, the sea sand and the activated carbon were present through all three phases. Between the first and second phases there were added to each bowl either 0.114 g of activated carbon or 0.12 g of activated carbon containing 5 % palladium (supplied by Aldrich Chemical Company, UK) . Between the second and third phases the corrosion enhancer was added to each bowl (when, for the Comparison Compositions, no enhancer was to be used, the second and third phases were combined into a single 30 minute phase) . B. Larger- cale preparation using a Fritsch P5 mill The oxygen scavenger was prepared in a Fritsch P5 mill with four cemented carbide grinding bowls of 250 ml capacity, each containing 50 cemented carbide balls of 10 mm diameter. 19 g aluminium (15 micrometres particle size), 19 g sand and 8.33 g activated carbon were placed in each bowl, and milled for 60 min at 15.2 g ball acceleration. 12 g of activated carbon containing 5% palladium was then added to each bowl, and the composition milled for 15 min at 6 j ball acceleration. Finally, 2 g NaCl and 2 g NaOH were added to each bowl, and the composition milled for 15 min at 6 g ball acceleration. To avoid excessive heating, for each one minute of milling three minutes of cooling was necessary. II. Measurement of deformation content of aluminium particles The effective crystallite size and the strain of the aluminium particles were measured using the method of integral breadths (H P Klug & L E Alexander, X-ray diffraction procedures for polycrystalline and amorphous materials, John Wiley & Sons, 1974, p 661) . The effective crystallite size and the microstrain were calculated, and the deformation content was determined as the ratio of the effective crystallite size after milling to the effective crystallite size prior to milling. III. Compounding of Compositions in polymers and preparation of EVA or LDPE films Compositions were compounded at 9 % by weight in^ either 14-18% vinyl acetate content EVA (as typically supplied by Borealis or Exon) or in Low Density Polyethylene (LDPE: Novex LDPE as supplied by BP Chemicals) . This was done using a single screw extruder (Leistriz AG) . For the compounding the screw melt temperature was maintained at 140°C with a rotation speed of 200-220 rpm. The compounded material was air cooled at room temperature on a conveyer belt at a rate of 15 ft/min and cut into small granules . The compounded material was blown into film with a thickness of 50-100 micrometres using a Queens Film Blowing machine (Taiwan) with the four heating zones of the blower set at 180, 190, 200 and 200°C. IV. Experimental setup for analysis of oxygen uptake by powders in the gas phase Unless stated otherwise, experiments were performed in 500 ml glass bottles with hermetically sealing lids. The lids were punctured, and silicone rubber septa were glued on both sides of each lid to cover the holes using silicone glue. 20 ml of distilled water was placed in the bottom of each bottle following which a quantity of formulation containing 0.177g aluminium was placed in a glass dish and this was positioned on the bottom of the glass bottle keeping the powder out of direct contact with the water. The lids were placed on the bottles and the bottles incubated at 22°C unless otherwise stated. Gas samples were removed at frequent intervals and analysed by gas chromatography . V. Experimental setup for analysis of oxygen uptake by films in the gas phase Unless stated otherwise, experiments were performed in 200 ml glass bottles with hermetically sealing lids. The lids were punctured, and silicone rubber septa were glued on both sides of each lid to cover the holes using silicone glue. 400 square centimetres of film were loosely rolled and placed in the bottle followed by 5 ml of distilled water. The lids were placed on the bottles and the bottles incubated at 20°C unless otherwise stated. Gas samples were removed at frequent intervals and analysed by gas chromatography as described in General Procedures Section IV Measurement of oxygen uptake and hydrogen evolution. VI. Experimental setup for analysis of oxygen uptake by films in the liquid phase Unless stated otherwise, experiments were performed in 200 ml glass bottles with hermetically sealing lids. 400 square centimetres of film were loosely rolled and placed in the bottle which was filled to the top with distilled water equilibrated with oxygen. The lids were placed on the bottles and the bottles incubated at 20°C unless otherwise stated. Dissolved oxygen levels in individual bottles were measured using a Microprocessor One Channel Analyser (MOCA) for oxygen measurement Series 3600 Indicating Instrument produced by Orbisphere Laboratories (Sheffield, UK) with a flow rate of between 45 and 60 ml per minute. Example 1: Preparation and testing of aluminium- containing compositions Using the small scale preparative and test techniques described hereinbefore, there were prepared and tested (for oxygen uptake and hydrogen evolution) a number of different aluminium-containing compositions. These incorporated 100 parts (1.9g) Al, 100 parts (1.9g) sand, 99.7 parts (1.89g) active carbon (with or without 0.3 parts [0.006 g] Pd, and none, either, or both of 10.5 parts (0.2g) NaCl and 10.5 parts (0.2g) NaOH. Some of these were low-energy-milled powders, while others included some high-energy-milled powders,- some employed no corrosion enhancer, while others included one or both of NaCl and NaOH. All the test samples included the same weight of aluminium, and each was tested at 22°C. The results wi thout Pd and with either NaOH only or NaCl and NaOH are shown in Table 1 below (in which the bracketed figures give the maximum percentage molecular hydrogen concentration reached during the oxygen-absorption process and the time in hours when that was reached) . Table 1 Energy of Steady state percentage of oxygen milling remaining after oxygen absorption process commencing with 20.8% NaΩH NaCl+NaOH Low 9.8@64 8.7@24 (hydrogen) (27.7@64) (24.8@24) High 0.48@27 0.03@26 (8.6@27) (7.0@48) It will be noted that the oxygen absorption process giving the Table 1 figures resulted in an end oxygen concentration considerably higher than 0.1%. The results with Pd, and with none, either or both of NaOH and NaCl are shown in Table 2 below. Table 2 Energy of Time (hours) for the 0 2 concentration milling to fall to <0.1% Corrosion enhancer None NaCl NaOH NaCl+NaOH Low 168 * 50.3 45 17.6 (hydrogen) (1.55@3) (16.2@21) (16.6@10) High not 54 11 10 noted (1.3@2.5) (4.15@10) (2.31@6) * In 168 hours the 0 2 concentration had dropped from 20.8% to 20.28%, and then stabilised, indicating that no further uptake could be expected. The bracketed figures represent the maximum molecular hydrogen concentration which resulted during the oxygen- absorption process, and the time (in hours) when that was reached. The Tests show that with low energy milled aluminium- containing compositions NaOH provided approximately the same oxygen absorption as did NaCl, but with a much greater hydrogen evolution, while this hydrogen evolution could be very substantially reduced by using high energy milling. More particularly, the Tests showed the following: - 1) In the absence of any corrosion enhancer (NaOH or NaCl), there was no significant oxygen uptake with low energy milling, and we assume from that that there would not be any significant oxygen uptake with high energy milling. 2) Using a weak corrosion enhancer (NaCl) provided a substantial improvement in the oxygen- absorbing activity of the compositions compared with omission of corrosion enhancer, but it was still not particularly impressive. 3) Using a strong corrosion enhancer (NaOH) with a low energy milled aluminium- containing composition produced better oxygen- absorbing activity, but with a significant upturn in hydrogen evolution. 4) However, using a strong corrosion enhancer (NaOH) with high energy milling produced not only very much better oxygen- absorbing activity but also no significant hydrogen evolution during the oxygen-absorption process. 5) The use of both the two weak and strong corrosion enhancers (NaOH and NaCl) and also low energy milling gave results surprisingly better than was expected from the pair of results for each corrosion enhancer used on its own. 6) The use of both the two weak and strong corrosion enhancers (NaCl and NaOH) and also of high energy milling gave results no better than was expected from the pair of results with one or other on its own. Example 2 : Testing of activity of EVA polymer film containing the oxygen- scavenging composition Using the large-scale preparative and test techniques hereinbefore described there was prepared a high-energy milled composition containing aluminium, sand and activated carbon as well as palladium, NaOH and NaCl. The composition contained 100 parts Al , 100 parts sand, 50 parts active carbon (with 0.3 parts Pd, 10.5 parts NaCl and 10.5 parts NaOH) . The formed composition was then compounded into EVA (16% vinyl acetate content), and films blown as described hereinbefore in the General Procedures. The ability of the resulting films to remove both headspace and dissolved oxygen was tested as described in the General Procedures. The results are shown in Tables 3, 4 and 5 below. Table 3 Headspace oxygen uptake by EVA polymer films containing the composition Values in ml Elapsed time (days) 0 7 14 21 28 6°C 0.00 8.86 10.3 10.3 11.0 20°C 0.00 14.8 18.6 19.68 23.52 Table 4 Headspace hydrogen present using EVA polymer films containing the composition Values in ml Elapsed time (days) 0 7 14 21 28 6°C 0.00 3.01 1.44 1.48 1.50 20°C 0.00 4.32 3.84 3.08 1.86 Tabl e 5 Dissolved oxygen uptake from water by EVA polymer films containing the composition Values in mg/1 Initial Storage time (days) concentration 3 7 6°C 6.00 3.46 0.45 20°C 6.00 2.07 0.035 These data demonstrated that the composition when incorporated into an EVA polymer was particularly effective in removing oxygen from both the gas phases and the liquid phases. Furthermore the resulting polymeric material showed good activity in absorbing oxygen at refrigerated temperatures. Example 3 : Testing of EVA polymer film containing the composition against oxygen in orange juice The ability of polymer film containing the composition described in Example 2 (but prepared according to the larger scale preparative procedure described previously) to remove dissolved oxygen and headspace oxygen in orange juice was determined as described in the General Procedures (except that 800 sq cm areas of film were placed in 1 litre glass bottles into which 900 ml of single strain orange juice was placed) . Control bottles containing no film were also set up. The effect of the film in maintaining the vitamin C content of the orange juice was determined by taking samples of orange juice at intervals and measuring the vitamin C content by titration. The effect of the film in preventing oxidative browning of the orange juice was analysed by taking samples of the orange juice at intervals and measuring the absorbance at 420 nanometres in a spectrophotometer . The results are shown in Tables 6, 7, 8 and 9 below. Table 6 Dissolved oxygen uptake from orange juice at 20°C. Values in microgram/1. Initial Storage time (days) concentration 14 28 42 Control 1600 457 310 21 Composition 1600 177 150 95 Table 7 Effect on headspace oxygen at 20°C above orange juice Values in % Initial Storage time (days) concentration 14 28 42 Control 20.9 18. .8 17.1 15.1 Composition 20.9 16, .5 3.3 2.8 Table 8 Effect on ascorbic acid content of orange juice at 20°C Values in mg/lOO ml juice Initial Storage time (days) concentration 14 28 42 Control 58.7 52. .9 48. .4 43 Composition 58.7 55 .9 55 .9 55.7 Table 9 Effect on browning index of orange juice at 20°C Values are Absorbance at 420 nm Initial Storage time (days) Absorbance 14 28 42 Control 0.14 0.13 0.15 0.17 Composition 0.14 0.13 0.13 0.13 These data demonstrated that the incorporation of the composition into EVA provided a film which was effective at removing oxygen from a liquid food product such as orange juice. The rate of removal of oxygen by the film was sufficient to maintain the dissolved oxygen levels and the headspace oxygen levels below those seen in the controls. The reduced oxygen levels were reflected in a prevention of vitamin C oxidation and inhibition of oxidative browning reactions in the juice exposed to the film in comparison with the controls. Example 4: Analysis of migration from polymer film containing the composition The EVA polymer film containing the composition used in Example 2 was also used to determine the amount of migration of aluminium from the film at 6°C into orange juice and (separately) into 3 % acetic acid (which is a recognised orange juice simulant) in the ratio of 20 ml liquid per 20 sq cm of film in a glass bottle. The amount of aluminium in the orange juice and 3 % acetic acid was determined at various times after exposure to the film by inductively-coupled plasma atomic emission spectrometry . The results are shown in Table 10. Table 10 Migration of aluminium from the composition incorporated in EVA polymer film Values are in mg/1. Elapsed time (days) 0 1 4 7 14 21 28 3 % acetic acid 0.00 0.05 0.05 0.06 0.37 0.90 1.35 Orange juice 0.036 0.037 These data show that the migration from the film into the aggressive acetic acid simulant was less than 1.5 mg per litre which represents less than 1.5 % of the total aluminium present. Thus, incorporation of the composition into an EVA film was effective in reducing the amount of aluminium which was released into the acetic acid. In the case of a typical liquid food product such as orange juice the amount of aluminium released into the food product was barely detectable (less than 5 microgram per litre) . Thus, owing to these very low levels of migration, the present oxygen- scavenging compositions when incorporated into suitable polymers are particularly good for food applications where direct contact with the food may take place. Example 5 : Testing of activity of LDPE polymer film containing the oxygen- scavenging composition A high-energy-milled composition containing aluminium, sand, activated carbon and palladium was prepared as described in Example 2. The formed composition was then compounded into Novex LDPE, and films blown as described hereinbefore in the General Procedures. The ability of the resulting films to remove both headspace and dissolved oxygen was tested as described in the General Procedures. The results are shown in Tables 11, 12 and 13 below. Table 11 Headspace oxygen uptake by LDPE polymer films containing the composition Values in ml Elapsed time (days) 0 7 14 21 28 6°C 0.00 3.46 4.06 4.55 5.57 0°C 0.00 5.43 9.59 12.15 13.13 Table 12 Headspace hydrogen present using LDPE polymer films containing the composition Values in ml Elapsed time (days) 0 7 14 21 28 6°C 0.00 1.88 0.06 0.04 0.16 0°C 0.00 2.48 0.10 0.08 0.03 Table 13 Dissolved oxygen uptake by LDPE polymer films containing the composition from water Values in mg/1 Initial Storage time (hours) concentration 1 2 5 29 47 6°C 7.55 3.15 3.05 2.49 0.57 0.09 Initial Storage time (hours) concentration 1 2 5 23 53 0°C 6.00 3.26 2.20 0.45 0.15 0.07 These data demonstrated that the composition when incorporated into an LDPE polymer was also effective in removing oxygen from both the gas phases and the liquid phases. Furthermore the resulting polymeric material showed good activity in absorbing oxygen at refrigerated temperatures. It is therefore concluded that evolution of hydrogen by the oxygen absorbing compositions can be controlled by incorporation of the composition into a suitable polymer as well as by the energy of the milling process.

Claims: Claims 1. An oxygen scavenger composition which is a powder-like mixture of finely-particulate metal together with a strong corrosion enhancer. 2. An oxygen scavenger composition as claimed in Claim 1, wherein said finely-particulate metal has particles of the order of 1 to 20 micrometres in diameter. 3. An oxygen scavenger composition comprising finely- particulate metal with particles that are of the order of 1 to 20 micrometres in size and are deformed/fractured. 4. An oxygen scavenger composition as claimed in Claim 3, and further comprising a finely-particulate, strong corrosion enhancer. 5. An oxygen scavenger composition as claimed in any preceding Claim, wherein the metal itself is capable of corroding in the presence of water with the evolution of hydrogen . 6. An oxygen scavenger composition as claimed in any one of Claims 2 to 4, or Claim 5 as appended to Claim 2 or 3 , wherein the particles are no greater that 5 micrometres in size. 7. An oxygen scavenger composition as claimed in any of the preceding Claims, wherein the metal is aluminium. 8. An oxygen scavenger composition as claimed in Claim 7, wherein the aluminium content of the composition is in the range of 10 to 50% by weight. 9. An oxygen scavenger composition as claimed in Claim 3, or any one of Claims 4 to 8 as appended to Claim 3, wherein at least 50% of the metal is deformed/fractured. 10. An oxygen scavenger composition as claimed in Claim 9, wherein at least 95% of the metal is deformed/fractured. 11. An oxygen scavenger composition as claimed in any " preceding Claim, and further including at least one filler. 12. An oxygen scavenger composition as claimed in Claim 11, wherein the filler or one of the fillers comprises a large-surface-area, reaction-site material. 13. An oxygen scavenger composition as claimed in Claim 12, wherein the large-surface-area, reaction-site material is carbonaceous. 14. An oxygen scavenger composition as claimed in any one of Claims 11 to 13, wherein the filler (s) comprise (s) talc and/or sand and/or alumina and/or zirconia and/or silicon carbide . 15. An oxygen scavenger composition as claimed in in any one of Claims 11 to 14, wherein the fillers are an activated carbon material and sand. 16. An oxygen scavenger composition as claimed in any one of Claims 11 to 15, wherein the composition is a free powder, and, based upon the metal, contains from 10 to 1000 parts by weight filler per 100 parts metal. 17. An oxygen scavenger composition as claimed in Claim 1, 2, or 4 , or any one of Claims 5 to 16 as appended to Claim 1 or 4, wherein said strong corrosion enhancer is an hydroxide. 18. An oxygen scavenger composition as claimed in Claim 17, wherein said hydroxide is sodium or potassium hydroxide . 19. An oxygen scavenger composition as claimed in Claim 17 or 18, and further comprising another corrosion enhancer which is a metal halide. 20. An oxygen scavenger composition as claimed in Claim 19, wherein said metal halide is an alkali-metal halide. 21. An oxygen scavenger composition as claimed in Claim 20, wherein the alkali-metal halide is sodium chloride . 22. An oxygen scavenger composition as claimed in Claim 7 or 8 as appended to Claim 1 or 4, or any one of Claims 9 to 21 as appended to Claim 7 as appended to Claim 1 or 4 , wherein the amount of the corrosion enhancer is from 1 to 50 parts by weight corrosion enhancer per 100 parts aluminium. 23. An oxygen scavenger composition as claimed in any preceding Claim, and further including a catalyst for the reaction between molecular hydrogen and oxygen to produce water . 24. An oxygen scavenger composition as claimed in Claim 23, wherein the catalyst is one of the platinum metals group. 25. An oxygen scavenger composition as claimed in Claim 23 or 24, wherein the catalyst is employed in an amount of from 0.001 to 1% by weight based on the amount of the finely-particulate metal. 26. A method of preparing an oxygen scavenger composition as claimed in any preceding Claim, in which all the ingredients are blended together in a mixing and milling operation, using a filler as a milling agent for the metal. 27. A method as claimed in Claim 26, in which the components are ground for long enough to decrease the average effective crystal size of the metal by at least 10%. 28. A method as claimed in Claim 27, in which the grinding is for long enough to decrease the average effective crystal size by approaching 50%. 29. A method of preparing an oxygen scavenger composition as claimed in any preceding Claim, in which the amount of the metal which is available for reaction is calibrated to match the total amount of oxygen required to be consumed by a specific amount of the oxygen scavenger composition. 30. A method for the scavenging of oxygen from a site where it is present, in which method there is placed in communication with the site an oxygen-scavenger composition comprising a finely-particulate metal, itself capable of corroding in the presence of water with the evolution of hydrogen, and a strong corrosion enhancer appropriate to the metal . 31. A method as claimed in Claim 30, in which the oxygen- scavenger composition is a composition as claimed in Claim 2, 3, 4, or any one of Claims 6 to 25. 32. An oxygen-scavenging polymer formulation suitable for making a protective, oxygen-scavenging sheet in communication with an oxygen-sensitive substance inside a container, which formulation comprises an oxygen-scavenger composition as claimed in any one of Claims 1 to 25 dispersed within a polymer that is permeable to both water vapour and to gaseous oxygen. 33. A polymer formulation as claimed in Claim 32, wherein the polymer is an ethylene vinyl acetate copolymer (EVA) or a low density polyethylene (LDPE) , or a mixture or laminate thereof .

Inventors: JOHNSTONE KEITH, MEHLMANN ALEXANDER, JOHNSTONE, KEITH, MEHLMANN, ALEXANDER
Applicants: ALUTECH LTD, JOHNSTONE KEITH, MEHLMANN ALEXANDER, ALUTECH LTD., JOHNSTONE, KEITH, MEHLMANN, ALEXANDER
Application year: 1998
Main classification: C09K15
IPC codes: A23L3, C01B3, C09K15