Abstract:
Preform, serving as a semi-finished product, for a container intended for containing products therein that are sensitive to radiation in particular light sensitive and food and dairy products, consisting of at least one base layer ( 1 ) made of a primary plastic base material, with a certain amount of additives ( 5 ) incorporated in it ( 1 ), characterised in that said preform ( 10, 20 ) is opaque over virtually the whole extent thereof, wherein a relatively low percentage of plastic additives ( 5 ) is incorporated to generate sard opaque appearance ( 22 ), so as to protect the inner space ( 9 ) thereof which is delimitated by it against external radiation (V 1 , V 2 ) particularly electromagnetic radiation, more particularly light, under normal pressure condition.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to containers for containing products that are sensitive to radiation, especially light, essentially of the food industry, more particularly milk and further dairy products, including nutrients and dairy products that are enriched or contain fruit. 
         [0002]    The present invention also relates to a preform, serving as a semi-finished product, for making such containers, consisting of at least one base layer made of is a primary plastic material, with a certain amount of additives incorporated in it. 
       BACKGROUND OF THE INVENTION 
       [0003]    Plastic containers including bottles made of polyesters and notably polyethylene terephthalate (PET) are increasingly employed for packaging food and drinks. PET containers were originally used for carbonated beverages, such as soda water. They have since gained considerable ground in all areas of the food sector, such as drinks, including milk. 
         [0004]    Polyethylene terephthalate is an excellent material for packaging pasteurized milk, which does not keep for long and is distributed and kept cold, with a shelf life of 7-10 days. However, the absence of a built-in light barrier extending across the whole container greatly hampers the use of all-PET plastic formulations for packaging sterilized, long-life ultra-high temperature (UHT) milk, which keeps for 4-6 months at a normal temperature. 
         [0005]    One of the problems with milk and dairy products generally lies in their unstable nature. The fact is that they can be attacked by undesirable external effects forming part of the prevailing conditions of the surroundings. Their keeping properties therefore depend to a great extent on the way they are packed. 
         [0006]    Owing to the absence of protection from light in the existing packaging units, the milk in them undergoes photo-oxidation. This causes undesirable off-flavours associated with the action of light, Riboflavin (vitamin B 2 ) is also readily attacked, and so are some of the other vitamins and nutrients, which similarly undergo photo-degradation in the presence of light. 
         [0007]    It is well known that milk is degraded by exposure to visible but also invisible light, mainly in the wavelength range between 200 and 550 nm. It must therefore be protected at all cost from harmful light of such wavelengths in order to ensure that the quality of milk is retained for the entire shelf life scheduled for it. 
         [0008]    In the case of products containing additional nutrients that are sensitive to oxygen, the penetration of the latter must also be reduced as much as possible in order to stop the deterioration of the quality. Packs have therefore been developed for UHT milk to prevent the penetration both of visible light and of UV radiation. Multilayer carton packs with a full light barrier have thus been introduced, as well as aluminium foil to prevent the penetration of oxygen. However, the keeping qualities of the contents of these packs after opening leave something to be desired, owing to the closure of these packaging units. 
       PRIOR ART 
       [0009]    The Japanese document JP 55 117632 A of MITSUBISHI RAYON describes a plastic container with a transparent neck and an opaque body, so that not all its parts have the same opacity, and the light barrier is not present over the whole container, i.e. it does not extend over the neck section. Furthermore, these containers are only intended for cosmetics. 
         [0010]    The European Patent Application EP 0 273 681 A2 of MOBIL OIL CORP describes a process for making polymer films that become glossy when incorporating high percentages of additives up to 30%, to ensure the required opacity in the end product, but they do not have a definite three-dimensional shape and actually do not even have a shape of their own at all. In addition, the additive concentration in them is quite high. It is also stressed here that the additive must have a higher glass transition temperature T g  and a higher melting temperature T m  than the base polymer used as the primary material, which is a set precondition for being able to keep the mixture in the molten state. This is of course a significant limitation, since the material must inevitably be melted during its processing. Besides, this document does not give any information about the specificity connected with the well-defined three-dimensional shape of the object envisaged here. 
         [0011]    The American patent U.S. Pat. No. 4,410,482 A of SUBRAMANIAN PALLATHERI yet describes extruded and blown bottles made from mixtures of polymers, but again high percentages, up to 40% of additives are used in them, i.e. even more than in the case depicted above. 
         [0012]    The European Patent Application EP 0 974 438 A1 of TEIJIN Ltd yet describes polymer mixtures, but they are intended for transparent containers, whose light-barrier properties appear to be unsatisfactory, or at least call for considerable improvement. 
         [0013]    The European Patent Application EP 0 273 897 A2 of MONSANTO EUROPE S.A. describes aerosol-type pressurized containers made from non-opaque preforms that consist of mixtures of PET and additives of the type of styrene-maleic anhydride (SMA) copolymer, yet with a still high concentration of the latter up to 30%. The purpose of this additive is mainly to make the resulting PET containers more rigid, so that they are able to fairly resist the high pressures used in aerosol-type containers envisaged here. However, this document does not contribute to solve the present problem about the improvement of the walls of the packs for excluding the incident light, which in case of ordinary containers are characterised by a proper shape under normal atmospheric pressure of about 1 atm. Nor does this document describe an opaque preform. 
       AIM OF THE INVENTION 
       [0014]    The aim of the present invention is to solve the problem mentioned above by including additives that are easier to manage and are more suitable, as regards both their nature and amount, in the primary base material under the abovementioned normal conditions of use, mainly pressure but also to some extent temperature, notably under the atmospheric conditions of the surroundings. 
       SUMMARY OF THE INVENTION 
       [0015]    There is thus proposed in the present invention a preform, which is remarkable in that it is opaque and consists of a primary plastic material and a low percentage of additives to ensure a whitish opaque appearance over virtually the whole preform. Thanks to the preform proposed according to the invention, an opaque container such as a bottle can be directly obtained that reliably protects its contents from external radiation, especially electromagnetic radiation and more specifically light, whether natural or artificial and whether visible or ultraviolet. It will be understood that we are dealing here with ordinary containers that have stiff or semi-rigid walls of a predetermined shape and which do not have to meet special requirements such as those needed for high pressure. The containers proposed according to the invention are yet intended for use at normal pressure. Opaque preforms are thus proposed which serve as semi-finished intermediate products that can be easily and directly converted into containers that have efficient light barrier properties. In particular, the refractive index of the primary base material is modified here to such an extent that the incident radiation suffers virtually no refraction. As a result, the drink or food kept in the container is protected from harmful external light under normal operating conditions as regards pressure, especially against photo-oxidation and from the subsequent degradation of products occurring under the influence of photo-catalysis. 
         [0016]    In a preferred embodiment of the present invention, the plastic is PET. This choice of material has several advantages indeed in the applications that are relevant to the invention, including a great flexibility of designing and shaping the container and a more reliable formation of the neck region of it, which makes it possible to drink straight out of the bottle without any problems. 
         [0017]    In a particular embodiment of the present invention, the additives used are polymeric substances. As a result, the containers can be made with a nacreous effect, which ensures that a large part of the incident light is automatically reflected by its surface. In addition, the walls of the container have a large measure of internal refraction. These two phenomena—reflection and refraction—jointly ensure a considerable barrier to the penetration of light, which is desirable in the case of light-sensitive products such as UHT milk. The latter can therefore be kept reliably over long periods even under normal conditions, i.e. at room temperature and in the presence of light, without needing special storage conditions, such as a dark or cool place. A significant improvement is thus achieved over the existing PET structures, because the former are particularly suitable for keeping the products at a normal temperature, which is especially advantageous in the case of containers used for packaging UHT milk, which are kept at room temperature. Another advantage is that the well-known white pigment, which is more expensive, can be replaced by a low percentage of cheaper polymeric additives, which reduces the cost. 
         [0018]    In a specific embodiment of the present invention, the additives are thermoplastic polymers. An excellent opacity may be achieved in the outside wall of the preform in this way, and the base material, generally PET, has a higher T g  and T m  value than the additive admixed to it. 
         [0019]    In a further embodiment of the present invention, the additives are polyolefins. The advantage thereof is that this material is incompatible with the primary base material (PET), their refractive indices being very different from that of PET. When two polymers with different refractive indices are mixed together, they produce a white mixture. 
         [0020]    In a preferred embodiment of the present invention, said additive is polypropylene (PP). Indeed, this material is easy to disperse, especially in PET, which makes it useful when converting the preform into the container. 
         [0021]    The present invention makes it possible to obtain a satisfactory opacity in the outer wall by admixing the above thermoplastic polymeric additives to PET in a ratio of 1:10 in terms of percent by weight. The remarkable thing is that the change to white occurs already with a very little additive of up to only 2%, which is far less than the amounts used in the prior art. On the other hand, when the polymeric additives are present in a fairly high percentage, problems arise with the structure in the form of possible delamination due to incompatibility between the components of the mixture, so that it is preferable to use percentages that do not exceed the critical limit of 10% or even 8%, whereby satisfactory mechanical properties of the mixture are maintained, and a satisfactory barrier effect is ensured at the same time. In a special embodiment of the invention, these additives are introduced into polyethylene terephthalate in an amount of 3-9%, and especially 5-8 wt-percent, which further reinforces the effect mentioned above. A particularly notable advantage here is that it is possible to achieve opaque PET containers whose walls are white and opaque, i.e. have a high colour density without the addition of a white pigment, the colour density being a measure of opacity. 
         [0022]    Another notable special advantage obtained according to the invention by adding polypropylene is that it considerably improves the intrinsic viscosity (IV) of the processed preform material in comparison with that of conventional, mineral-filled PET. The intrinsic viscosity is a measure of the ease with which the preform can be processed in a stretching and blowing device that converts it into the final container. Opaque preforms with quite a large amount of pigment have significantly lower intrinsic viscosity than ordinary preforms, so they lack the required strength in the melt form during the blowing process. This makes it more difficult to stretch and blow the preform into a bottle with the required properties, especially the required wall thickness distribution. 
         [0023]    By contrast, the preforms with added polypropylene instead of added pigments have a high intrinsic viscosity and a high strength in the molten state, so they are much easier to process in conventional stretching and blow-moulding machines. The direct result of this is that containers with a much lower weight can be manufactured with polymeric additives than with large amounts of pigments according to the standard prior art. Since the density of polypropylene is 30% lower than that of PET, the PET-PP mixture is lighter, and the weight of the container is less as well. So both the preforms and the containers obtained in this way are much lighter than the conventional ones. 
         [0024]    A PET structure has recently been introduced that consists of a single layer of an opaque white PET layer but with a fairly large amount of pigment, namely titanium dioxide or zinc sulphate. The disadvantage of this structure is that a relatively large pigment charge of up to 8% is necessary, which is a drawback in Injection moulding. Another undesirable effect occurs in the heating of preforms and their blowing into containers. Furthermore, the protection from light achieved here is unsatisfactory. Finally there is an adverse effect on the cost. 
         [0025]    Some other known polyethylene packaging units have a three-layer structure with a light-barrier insert provided by a black polyethylene layer in between two white polyethylene layers, one on either side of it. A six-layer structure is also known, which is formed by placing the following layers one over the other: a white polyethylene layer, a black polyethylene layer, an adhesive, an ethylene—vinyl alcohol (EVOH) copolymer layer, another adhesive layer, and finally again a black polyethylene layer, the aim being to provide a barrier to both light and oxygen. A three-layer PET structure consisting of a black PET layer between two white PET layers is also known. In an interesting embodiment of the invention, the polymeric additive is incorporated in such a multi-layer structure having a black PET middle layer. Thanks to this measure, virtually all transmitted light can be excluded. So the combination of this polymer addition technique with a central black PET layer in a multi-layer structure has a certain effectiveness. 
         [0026]    However, the disadvantage of especially the first two structures and to some extent of the last of the above structures is that the amount of white pigment incorporated in the outside layer must be quite large in order to prevent the black colour of the middle layer shining through. The fact is that this would cause a colour shift of the bottle surface to grey, which would leave a visible trace at the outer wall which is visible to the consumer. This smudging is most undesirable. To avoid this, the containers must be made with a white outside wall that is thick enough to screen the inner black layer completely in order to make it virtually invisible. However, this makes the bottles relatively heavy and expensive, as well as difficult to blow, since the white pigment must be used in quite a large amount. 
         [0027]    According to an advantageous embodiment of the present invention, a preform with a multi-layer structure is thus proposed with a white PET intermediate layer. 
         [0028]    In another embodiment of the invention the preform contains a certain amount of fragmented metal in the above mixture, especially in powder form and preferably in the form of very small particles having a high dispersibility, so that the metal powder can be homogeneously distributed, the quantity used being especially about 2% and preferably not exceeding 1%. A useful advantage of this is that the resulting containers are considerably more recognizable, due to the presence of metal in them. This makes it easier to sort the containers when they are being recycled. In addition, the containers can also be coded in this way. 
         [0029]    It is also possible here to achieve a particularly remarkable mirror effect on the inside of the wall of the container. This increases the number of possible applications of the containers with a light-barrier effect to include tubes for toothpaste and other cosmetics and for flowing foods such as mayonnaise and ketchup, the containers then having a semi-rigid wall, in addition to the containers with a rigid wall mentioned above. 
         [0030]    According to a further preferred embodiment of the invention, the preform comprises a certain amount of iron-containing metals, especially stainless steel, the magnetism of which is useful when it comes to recycling. 
         [0031]    Alternatively, the preform contains a certain amount of non-ferrous metals in the mixture mentioned above. 
         [0032]    According to a further remarkable embodiment of the invention, the surface of the PET containers can be transformed by changing the nacreous appearance to a metallized one, especially a silvery metallic appearance, by suitably incorporating additives during the blowing of the preforms into containers. The metallized appearance of the surface can be attributed to additional incompatibility between the two polymers, which in turn is due to the stretching of the material in the cold, which makes the nacreous surface additionally turn white, which nacreous effect then makes disappear it or reduces it, creating a mirror-like metallic appearance on the processed product. 
         [0033]    The present invention is also related to a process for making opaque containers, including multi-layer polyester containers, by injection-moulding opaque preforms and by co-injection, followed by blowing the preforms to containers. 
         [0034]    This involves the preparation of an immiscible composition that is naturally white, i.e. white without any pigments. The immiscibility is manifested in the orientation of the preform when it is being blown into a container, since the surface of the material is changed from having a white appearance to having a nacreous one, at least in the regions where the preform is stretched. 
         [0035]    The light transmittance data can be further improved by adding a small amount of colourants to the PET/PP mixture, typically about 2-4 wt-% or about 5-8 wt-%, according to whether the container has a multi-layer or a single-layer structure, respectively. This yields results which are directly visible to the naked eye. 
         [0036]    According to an additional remarkable embodiment of the invention, both the nacreous and the metallized finishes can be coloured by changing the white base either by adding coloured PP pigments to it, or by using a coloured intermediate layer in the case of a multi-layer structure. 
         [0037]    Further features and properties of the preform, the container and the process will emerge from the following description of some embodiments of the invention, which are illustrated with the aid of the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  shows a diagrammatic cross-section of a preform, taken along its longitudinal axis according to a first embodiment of the invention. 
           [0039]      FIG. 2  shows a diagrammatic cross-section of a preform, also taken along its longitudinal axis according to a second embodiment of the invention. 
           [0040]      FIG. 3  represents a front elevation of a first embodiment of a container according to the invention. 
           [0041]      FIG. 4  is a front elevation of a further embodiment of a container according to the invention. 
           [0042]      FIG. 5 to 9  show a first set of graphs based on measurements of the light-barrier properties and some related parameters. 
           [0043]      FIGS. 10 to 21  show a second set of graphs based on measurements of the light barrier properties and some related parameters in the case of single layer preforms represented in  FIG. 2 . 
           [0044]      FIGS. 22 to 24  show a third set of graphs based on measurements of the light barrier properties and some related parameters in the case of multilayer preforms represented in  FIG. 1 . 
       
    
    
     DESCRIPTION 
       [0045]    This invention here is generally involved with preforms and containers which are opaque and intended for containing products that are sensitive to radiation and especially light, such as milk, dairy products, fruit juices and so-called functional drinks with nutrients, which can thus be effectively protected from photo-oxidation and from the degradation of the contents based on photo-oxidation. 
         [0046]      FIG. 1  shows a preform  10  with a wall  7  and a neck  8  in cross-section taken along the longitudinal axis l. This is a three-layer structure consisting of a base material which is composed of a primary plastic, which forms an outer layer  1  and an inner layer  3 , with an intermediate layer  2  between them, consisting of a secondary plastic. The primary plastic is advantageously polyethylene terephthalate, and the secondary plastic may also be polyethylene terephthalate. The primary base layer has a whitish and opaque appearance, so it reflects a large part of the incident radiation, especially light when it impinges on the wall as shown by the arrow y 1 . The outer layer  1  is made opaque by adding a thermoplastic polymeric additive  5  to PET in an amount of even only from 1 wt-% upward, shown here by cross-hatching. The outer layer  1  therefore forms an effective light barrier, the light-blocking effect whereof can be further increased if need be by the intermediate layer  2  which is downstream. 
         [0047]    Said thermoplastic polymeric additive  5  is preferably polypropylene. It can be mixed with PET in an amount of 1-10 wt-%, if required 5-8 wt-%. 
         [0048]    In one of the examples, the intermediate layer  2  containing polypropylene can be completely black, so that any rays that may have traversed the outer layer  1  of the preform are absorbed by the intermediate layer  2 , which has a high radiation-absorbing capacity and acts as a downstream radiation filter having a virtually total radiation blocking function, so that virtually no rays can penetrate past the intermediate layer  2 , as a result of which the content of the container is no longer attacked by external radiation. This is indicated schematically in  FIG. 1  by the arrows y 1  and y 2 , respectively. 
         [0049]    This embodiment is particularly useful when the preform is to be blown into a container and especially into a bottle for UHT milk. In this case, the intermediate layer  2  also acts as a gas barrier, in addition to excluding the light by absorbing it, whereby the oxygen penetrating from the outside is therefore also absorbed by it, in such a way that the milk is not attacked by said outer oxygen particles. This gas barrier effect is therefore combined here with the light barrier action of the outside and inside layers  1  and  3 . 
         [0050]    The general advantage of a multi-layer structure is that undesirable external substances that may penetrate through the outside layer  1  are finally fully blocked by the intermediate layer  2 , acting as an exclusion barrier, which provides extra safety. 
         [0051]    To optimize the structure, the intermediate layer  2  can be changed from black to grey with the aid of polypropylene or to other colours that are supported on grey with the aid of polypropylene, in order to ensure the same maximum light exclusion. 
         [0052]    The amount of additives  5  in the intermediate layer  2  can be increased to very high levels compared with the usual situation, because the intermediate layer, with e.g. only about 10% of the total thickness, does not affect the mechanical characteristics of the container and so it does not influence either the blow moulds used for the preforms or the co-injection thereof. These characteristics mainly come from the inside layer  3  and the outside layer  1 , which jointly make up about 90% of the three-layer structure  10 . 
         [0053]    Furthermore, a plurality of other colouring additives and colourants can be incorporated in the intermediate layer  2  more easily than in the customary situations with PET, because one can use lower injection temperatures for the intermediate layer than for the outside layer  1  and the inside layer  3 . This opens up a very wide range of possibilities for the incorporation of other and/or more additives, particularly in the intermediate layer, which would not be possible with preforms having a single-layer structure. 
         [0054]    With a paler colour for the intermediate layer, a smaller amount of colouring additives is needed in the outside layer, which has a covering function, because a paler colour is easier to hide by a white outside layer. This has a quite favourable effect by reducing the cost and improving the ease of blowing the preform  10 . It is therefore possible to use opaque preforms with a thick wall, which would not be possible otherwise under normal conditions. 
         [0055]    In addition, the colour of the intermediate layer  2  and the colour of the outside layer  1  can be blended and adjusted to each other if the required colour of the outside surface is not white, such as blue, red, gold, yellow or orange, etc. Such situations can mainly arise from the marketing requirements for the recognizability of said containers, in which PET is a good base material because it offers numerous possibilities in this respect, including a great variety of designs and shapes for the containers. The colour combination mentioned above can be utilized to the utmost by making the outside layer  1  transparent but coloured, thereby providing further options by using any possible colour combination required. This also improves the light barrier properties. 
         [0056]    The following examples illustrate the further improvements in the barrier properties of the container wall, not only for light but also for oxygen. An additionally improved oxygen barrier that goes beyond the ordinary PET can be incorporated for the packaging of oxygen-sensitive dairy products that contain basic nutrients such as vitamins, proteins, carbohydrates, starches, essential fatty acids, etc. This can be achieved by incorporating in the intermediate layer  2  materials with improved barrier properties, such as aromatic or aliphatic barrier plastics, nylon and aromatic polyesters such as for example: 
         [0057]    polyethylene 2,6-naphthalate (PEN) 
         [0058]    polyethylene terephthalate ionomer (PETI) 
         [0059]    polyethyleneimine (PEI) 
         [0060]    polytrimethylene naphthalene 2,6-dicarboxylate (PTN) and 
         [0061]    polyethylene terephthalate—polyethylene naphthalate copolymer (PETN). 
         [0062]    Alternatively, the same aim can also be achieved by adding an oxygen scavenger, such as an oxidizable polyester or an oxidizable nylon. 
         [0063]    This may further best be achieved by incorporating both a material with improved barrier properties and an oxygen scavenger, so that the inside of the container is protected not only from light but also from oxygen. 
         [0064]    In this way, the incorporation of polymeric additives in the PET base material in combination with the additional use of colour additives in both multi-layer and single-layer structures can give rise to a great variety of combined colour effects that not only ensure the technically desirable light barrier properties but also offer visual advantages facilitating the identification of the product. 
         [0065]    On the other hand, a single-layer structure  40  is satisfactory for some applications in the dairy sector, especially for products derived from milk, where the degrading action of oxygen is less critical. Said single-layer structure is shown in  FIG. 2 . Any colour can be used in these applications, and a single-layer milk bottle can be made by the addition of the required coloured pigments and colouring materials. 
         [0066]      FIG. 3  shows the front view of a container of the bottle type  20  obtained by stretching and blowing a preform  10  or  40  of the type shown in  FIGS. 1 and 2 . The outer wall  21  is visible and has a special appearance  22  indicated here by light stippling. This remarkable effect is caused by a nacreous appearance  22  that the bottle  20  presents to the consumer, making it not only particularly attractive but also easier to recognize. The nacreous effect is promoted by the biaxial stretching of the preform, i.e. its stretching both in the radial and in the longitudinal direction, and by the blowing of the preform to form the container. This nacreous effect is is achieved from the delamination occurring in the mutually joined but immiscible primary base materials and polymeric additives, wherein their immiscibility is in turn due to their mutual incompatibility. It is therefore the choice in full awareness of incompatible materials as constituents of the plastic mixture which creates surprising nacreous effects. 
         [0067]    This nacreous effect  22  is not only an advantage in the presentation of the product but also serves a technical purpose by making the resulting outer surface  21  quite reflective. The resulting surface therefore already has one of the three fundamental properties characterising a light barrier, which are low transmittivity, high absorptivity and high reflectivity. 
         [0068]    What is ingenious here is that this nacreous effect  22  produces a white gloss if a special polymer is chosen and mixed with PET. Satisfactory barrier properties may be obtained even without the addition of any colouring matter, notably a white one. The whitish pale nacreous appearance  22  can therefore be obtained by stretching the plastic without the use of any colouring matter though. 
         [0069]    The barrier properties can yet be further promoted by the addition of a small amount of colourants, typically about merely 24 wt-%, or about 5-8 wt-%, according to whether the container has a multi-layer structure or a single-layer one. This is a considerable advantage from the technical point of view, since the addition of colourants causes problems when a preform is being blown into a bottle. The more pigment it contains the more difficult is the blowing process. The critical value set above at 8% for coloured pigments is a threshold value beyond which the blowing of preforms into bottles becomes considerably difficult. 
         [0070]    It has been shown experimentally that the wall  21  can reflect up to 92% of incident light even without the use of colourants, but by incorporating polymeric additives alone, which is more than sufficient for a wide range of applications, such as sleeve bottles, where the printed sleeve can be drawn with virtually any pattern on such a container. This is therefore a fundamental characteristic which is proper to the present container. 
         [0071]    An additional advantage lies in the easier blowing of the preform to a container, owing to the possible absence of coloured pigments, which make blowing only difficult. Furthermore, the mechanical properties of the material are not diminished here as they inevitably are when colourants are added. In addition, the thermal stability of the preform is better, so the latter remains stable at considerably higher temperatures. 
         [0072]    In addition, the absence or at least greatly reduced presence of pigments, which are relatively heavier than polymeric additives, means that the container formed is very light, being a reduction up to 20 wt-% lighter, while retaining a reflective index of more than 92, together with the possibility of using the customary blowing equipments. 
         [0073]    However, an improvement in the light barrier properties for a multi-layer structure in comparison with a single-layer one cannot be expected if no colourants are incorporated in it. So the use of a multi-layer structure is only sensible if colourants are present. In the absence of colourants, the cheaper single-layer structure will suffice. For structures of this type, such as that shown in  FIG. 2 , pigments are therefore used in relatively small amounts, yet without exceeding the critical threshold value for blowing. 
         [0074]    Further thermoplastic polymeric additives are formed by polyethylene additives, in particular so-called high-density polyethylene known as HDPE, low-density polyethylene (LDPE), medium density polyethylene (MDPE) and linear low density polyethylenes (LLDPE). Further to be considered are polyolefine acetate co-polymers, such as methyl (EMA), ethyl (EEA), vinyl (EVA) acetate, polyethylene co-polymers of vinyl alcohol (EVOH). 
         [0075]    Polystyrene (PS), polyvinylchloride (PVC), polyethylene-terephthalate (PET), polyethylene-isophthalate (PEI), polybutylene-terephthalate (PBT), polyethylene-naphthalate (PEN), polytrimethylene-naphthalate (PTN), polytrimethylene-isophthalate (PTI), polytrimethylene-terephthalate (PTT), phthalic acid copolymers, polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polyamide 6 (PA6), polyamide 66 (PA 6,6). 
         [0076]      FIG. 4  shows a variant of the bottle  30 , where the darker shaded zones  31  indicate a metallized appearance  32  of the container. 
         [0077]    Said nacreous effect  22 , resp. metallized effect  32 , which are due to the addition of a polymeric additive to the primary base plastic, have the intrinsic advantage for light-sensitive products, such as UHT milk, that the surface  21  or  31  of the container  20  and  30  containing the milk reflects a substantial proportion of the incident light in a natural way. In addition, the wall of the container has a great deal of internal refraction. These two phenomena mutually combine to reduce or even prevent the penetration of light. 
       EXAMPLES 
       [0078]    In a typical comparison, a one-litre multi-layer bottle with the structure white PET -black PET—white PET weighs 26 grams when made with polymeric additives according to the invention and 32 grams when made by the traditional technique using a large amount of pigment, which means an approximately 25% saving of material, i.e. a considerable amount. 
       Experiments 
       [0079]    Said light barrier properties and said associated three parameters—transmission, absorption and reflection—were determined experimentally by means of a spectrophotometer of the “datacolour” type 650™ customarily used for this purpose, and the data obtained were used to construct the graphs shown in  FIGS. 5-9 . 
         [0080]    The graphs in  FIGS. 5 and 6  show the transmission of radiation that is incident on the container as a function of its wavelength λ in the case of a single-layer structure containing 5% of polypropylene in the first case (see  FIG. 5 ) and a structure containing 10% of polypropylene in the second case (see  FIG. 6 ). In the case of light transmission,  FIG. 5  shows that an extremely strong light-blocking effect is observed when polypropylene additives are added to PET as the primary plastic without any colour additives or colourants. In  FIG. 6 , which shows the reflection, the high reflectivity can be observed, which is caused by the nacreous appearance of the wall surface of the container after stretching the original PET/PP preform thereto. 
         [0081]      FIGS. 7 and 8  similarly show the transmission and reflection of multi-layer structures made with the addition of 10% propylene additives and further with the addition in the amount of 2% of a white colourant in the outside layer  1  and with 2% of a black colourant in the intermediate layer  2 . Both  FIGS. 7 and 8  indicate the great effect on the transmission which is generated by the incorporation of a black layer as intermediate layer, ensuring the total exclusion of light. As to the reflectance shown in  FIG. 8 , the reflecting effect of the nacreous outer surface of the wall can be observed, just as indicated in the case of the one-layer structure represented in  FIGS. 5 and 6 , and partly by the internal refraction of light. 
         [0082]    Measurements carried out on single-layer bottles indicated that the transmitted light is reduced to only 5%, which is an excellent result compared with PET, which is not completed with polypropylene additive and without white colourants, as set out hereafter, especially in connection with  FIGS. 10-11 . 
         [0083]    If the container is only made of the primary plastic PET, one could observe that up to about 90% of the light is transmitted. 
         [0084]      FIG. 11  refers to the case when 2% of additives in the form of polypropylene is added to the primary base material. It can be concluded from this graph that even such a modest amount of polypropylene additives causes a significant reduction in the amount of light allowed through. 
         [0085]    It can be observed on  FIG. 12  showing the addition of polypropylene up to 5% that the light rays transmitted through the container wall are further limited to 15%. 
         [0086]    It can be deduced from  FIG. 13  that a light transmission is limited to merely 5% when adding the same additional amount of polypropylene additives of 5% yielding a total amount of 10% PP. It is thus striking that the light exclusion is not linear with the addition of polypropylene additives, but instead decreases relatively faster. For example, one may state when comparing  FIGS. 11 and 13  that five times more additives correspond to ten times less light transmission. A conclusion here is then that the adding of polypropylene additives up to 10% makes the light transmission decrease by 95%, which is thus a quite remarkable result. 
         [0087]    A further group tests shown in  FIG. 14 to 17  is set out hereafter. In this group 5% polypropylene additives are respectively added to the primary base material PET, with a further addition of white colourants in an amount comprised between 2% and 8% respectively, with each time an increase of 2%, i.e. 4 and resp. 6% white. The graphs in  FIG. 14  show that the addition of 2% colourants reduces the transmission of light rays to approximately 2%, while in the addition of colourants is doubled to 4%, the transmission of light is reduced by half to approximately 1% as appears from  FIG. 15 . 
         [0088]    Multiplying colourants by three times up to 6% causes a further reduction of light to merely approximately 0.3% as shown in  FIG. 16 . 
         [0089]      FIG. 17  shows the maximum addition of white according to the present tests in the amount of 8% with a light transmission reduced to approximately merely 0.15% of the incident light. 
         [0090]    It can therefore be deduced from the four preceding test series that the further addition of white colourants by 2% reduces the light transmission from 15% as shown in  FIG. 12  to merely 2% as shown in  FIG. 14 . With regard to this, a moderate addition of white colourants is able to reduce the light transmission to a very low level of only 0.15% light transmission. 
         [0091]    Similarly as in the preceding tests series which are represented In  FIG. 10 to 13 , it can be stated again that the reduction of light transmission is not linear in function of the addition of colourants since multiplying the colourants by four from 2 to 8% generates up to approximately 13 times more light transmission, which can be considered as a remarkable result as well. 
         [0092]    A still further series of four tests represented in  FIG. 18 to 21  is set out hereafter. These tests take place in quite similar conditions, under doubling however of the added percentage of polypropylene additives from 5 to 10%. 
         [0093]      FIG. 18  shows a graph of transmittance in % in function of the wavelength of the incident radiation, wherein it may be observed that adding 2% of colourants with a doubled addition of polypropylene additives to 10%, transmits only approximately 1% of the incident light radiation, i.e. the half of the transmittance under similar conditions, with the addition of the half of polypropylene additives to 5% however, as shown in  FIG. 14 . 
         [0094]    The subsequent  FIG. 19 to 21  are similar representations with each time  2  additional percents of colourants addition. With the first doubling of the colourants to 4% represented in  FIG. 19 , there is still only 0.4% light transmission. When tripling the colourant addition white to 6%, the graph represented in  FIG. 20  shows that the light transmission is still further reduced by half to 0.2% of the incident light radiation. 
         [0095]    Finally when multiplying by four the white colourant addition to 8%, the light transmission is reduced to only 0.1% of the incident radiation as shown in  FIG. 21 . 
         [0096]    A comparison of the test results within this additional group of measures represented by  FIG. 18 to 21  teaches again that the reduction of light transmission is not linear with the increase of colourants, but with a certain acceleration effect with amplifying reduction of the light transmission with respect to the addition of colourant additives. 
         [0097]    Consequently, it can be deduced from the latter series of measurements that the graphs appear two times lower compared to the previous series measurements with the half of polymer additives, i.e. 5% PP, including in the presence of white colourant, when further adding polypropylene as polymer additive up to 10%. 
         [0098]    At last, a last series of measurements is represented in  FIG. 22 to 24  showing analogue graphs, each time with colourant additives in the amount of 8%, the first one whereof in  FIG. 22  in the absence of polymer additives, which means only with colourant additives, whereas the two subsequent figures represent graphs each time with the addition of 5% polymer additives, i.e. 5% polypropylene in  FIG. 23 , resp. 10% polypropylene in  FIG. 24 . 
         [0099]      FIG. 22  lets light radiation through up to approximately 1%, whereas the addition of merely 5% polypropylene transmits light radiation up to merely 0.15% of the incident light radiation. When doubling polypropylene to 10%, the light transmission is limited to approximately 0.1% as shown in  FIG. 24 . 
         [0100]    Both latter  FIGS. 23 and 24  correspond logically with  FIG. 17  and respectively 21 above. It can be deduced from these figures that the addition of white colourants without polymer additives may cause up to 1% light transmission at a wave length of 550 nm, but not less. Only the addition of polymer additive polypropylene may bring back the graphs to a level up to 0.1%, which is extremely low. Lower levels of colourant additions white with polymer additives reproduce the same performances as observed in  FIG. 10 to 21 . 
         [0101]    It is to be noted here that these measurements were carried out by means of a spectrophotometer which is a worldwide recognised device which provides extremely reliable measurement results, so that the tests set out above should be considered as particularly relevant. All abovementioned tests were carried out with each time the same bottle. 
         [0102]    Besides, only the transmitted light radiation getting through the container wall was measured, since only this amount of radiation is detrimental for the product which is to be contained in the container. The results set out above should further be related with respect to admissible radiation transmission values in the intended field. In view thereof, it should be considered that when the product to be contained is milk, the maximum admissible transmission value amounts to 0.3%. In other words, this means that for milk preforms the addition of colourants is suitable in the amount of 6% in case 5% polymer additives are added as represented in  FIG. 13 . In case for instance 10% polymer additives are added, the amount of white colourants may be reduced to a percentage which is comprised between 4 and 6, e.g. approximately 5% of white colourants, as may be assumed by extrapolating the measurement results of  FIG. 19 , resp. 20. This is a remarkable result in the meaning that blowing a preform becomes more difficult as more colourant additives are added. The difficulty of blowing becomes critical, especially as from 4% addition of white colourants and more. It is to be noted here that the performance of the blowing machine may decrease up to 20% and more. In addition, one is also limited in the geometry of the preform because the wall thickness thereof will be smaller than 4 mm, and even up to 3.5 mm. 
         [0103]    When further also considering the costs of white colourants such as titanium dioxide or zinc oxide, the usefulness of a minimum addition of white colourants will be appreciated directly. In this respect, it may be stated that very favourable transmission results may be achieved without the addition of colourants. Example of applications in this respect are a maximum value of 0.7% transmission, which is not enough for the filtering of light for some kinds, in particular UHT milk where 0.3 is the maximum transmission. 
         [0104]    When adding an amount reduced by half of white colourant additives of the UHT type for the same amount of added polymer additives of polypropylene, i.e. 5%, a light transmission of 2% is achieved. 
         [0105]    It can further be observed that the colourants will have a more efficient behaviour regarding light exclusion in the presence of polymer additives of polypropylene. It can therefore be stated that the polymer additives have a synergetic effect on colourant additives. 
         [0106]    It can further be observed on most of the graphs that they present an increasing profile in function of the wavelength, whereby it may be stated that the smaller the wavelength of the incident radiation, the easier the incident radiation may be blocked by the container wall. 
         [0107]    It is particularly worth noting that the multi-layer structure of the container according to the invention can also be used with an intermediate layer  2  that is similarly white instead of being black. The replacement of the latter by the former according to the invention is possible here thanks to said synergistic effect of the polypropylene-type polymeric additives and colouring additives, ensuring an additional intrinsic light-blocking effect for enabling the achievement of this blocking mode of the intermediate layer  2  without the need of a black intermediate layer with its characteristic light-absorbing function. This also has the outstanding advantage that owing to the invention, the black intermediate layer no longer needs to be covered by a white outer layer as in the conventional types of preform. Achieving this quite remarkable effect is only possible by subjecting the initial preform, i.e. the semi-finished product to biaxial stretching in order to obtain the container as the finished product. It is therefore possible to achieve the absorption of the radiation without any pigmentation, i.e. without the addition of colouring additives that are needed for obtaining an absorbing black intermediate layer, but not for a white light-blocking intermediate layer. A similar effect may be obtained without adding colouring additives or pigments, yet by subjecting the initial preform to biaxial stretching in order to form the container. Owing to this method of biaxial stretching, a crystalline structure is achieved in the polyethylene terephthalate, as a result of which the biaxially stretched container becomes white. 
         [0108]    It is therefore possible now to produce a coloured container like a bottle with three layers or more generally a multi-layer structure, by adding a relatively small percentage of colourants or pigments with a suitable incorporation of polymeric additives according to the invention. 
         [0109]    It should further be mentioned that it is rather difficult to load PET. Indeed, incorporating additives like pigments and colourants in PET is relatively difficult because the processing temperature used here is high, i.e. from 250 to 300° C., which is undesirable for pigments and colourants. In addition, the pigmentation of PET is much more expensive than that of other plastics. In this respect, there are pigments allowing higher levels of charges, such as e.g. HCAe used in the tests mentioned above. The same light exclusion effect can therefore be obtained here but at a lower cost. However, a multi-layer structure must be used to reduce the transmission to an absolute minimum, i.e. practically to zero. 
         [0110]    Owing to the invention, light radiation is absorbed instead of being refracted, and this is achieved merely by using polymeric additives, i.e. with very small pigment or colourant charges or even none at all. 
         [0111]    To summarise, multi-layer bottles can be advantageously made with a lower weight and so a lower cost. Another advantage is that the injection moulding and blowing process used here is equivalent as with customary single-layer PET structures, which is not possible with conventional systems. Yet another advantage of the present invention is that the surface of the containers has a nacreous appearance. This is a particularly remarkable effect, which consumers find very attractive. 
         [0112]    Furthermore, none of the existing structures mentioned above can ensure an additional oxygen barrier effect over and above that obtained with conventional PET containers, at least for the packing of products that are sensitive to both light and oxygen. In regard thereof, a still further advantage of the invention is that an oxygen barrier can be incorporated in the walls of the container or preform by replacing polyethylene terephthalate in one or more of the layers by a polyester barrier that absorbs oxygen.