Method of making heat-sterilised product package

The invention relates to a polymer-coated Polymer-coated heat-sealable packaging material, heat-sterilized package formed from the material, and use of the packaging material. The material comprises a fiber base 4 (e.g., packaging board made of bleached pulp) with polymer heat-seal layers 1,9 located on both sides thereof. The package comprises, outside the fiber base, first polymer coating layer 2, which contains white pigment for covering the fiber base from sight, and second coating layer 8, which contains light-absorbing pigment. A coating layer pigmented grey inside the fiber base may provide a light shield for the packaged product. Absorbent black pigment may be located outside the fiber base underneath the white layer to stabilize the brightness of the material. A heat-sterilized package is made by providing the packaging material, forming a package containing, e.g., food, and heat-sterilizing the package in an autoclave. Yellowing of the fiber-based material is avoided by means of a white pigment.

FIELD OF THE INVENTION

The invention relates to a heat-sealed, heat-sterilised product package made of a polymer-coated, fibre-based packaging material. The invention also relates to a packaging material suitable for such a package and to the use of the material.

BACKGROUND OF THE INVENTION

There are numerous known applications of polymer-coated fibre-based packaging materials, in which one or both sides of the fibre base are provided with one or more successive coating layers and which are intended for various packaging purposes. The coating can optionally achieve a moisture, oxygen and/or light barrier, which is intended to improve the storage life of the packaged product and thus to extend its effective storage and/or “best before date”. The coating also provides a heat-sealable material, enabling a tight seal of the product package. Food packages constitute one principal field of application for such packaging materials, which are used for instance for cartons, casings and boxes formed of coated packaging board and pouch packages formed of coated paper.

Heat-sterilised packages form a notable group of sealed food packages, the packaging material of these being required to withstand sterilising treatment conditions. Sterilisation may be performed by irradiation or by hot pressurised steam in an autoclave Closed metal cans have typically been treated in autoclaves, however, nowadays also fibre-based packaging materials are available for autoclave applications. WO 02/28637 thus describes a packaging material for autoclave packages, in which the board substrate is equipped with an oxygen barrier layer, polymer heat-sealing layers and binder layers required between these. In this publication, the oxygen barrier layer is preferably formed of an aluminium foil, even though EVOH is also mentioned as an optional material. The proposed material for the binder and heat-sealing layers is polypropene, which resists, without melting, the temperature above 130° C. prevailing in an autoclave.

In tests of heat-sterilised fibre-based food packages, the applicant has found the problem of bleached packaging board coated merely with polymer oxygen-barrier and heat-sealing layers tending to yellow due to heating. This is especially true for packages treated in autoclaves, which are required to resist the high autoclave temperature and pressure over the required treatment period, about 30 minutes in general. In other words, a package made of polymer-coated board gets another appearance during autoclave treatment, and this is a drawback in the market, especially in demanding applications.

SUMMARY OF THE INVENTION

The purpose of the present invention is to eliminate the problem mentioned above and to provide a sterile package made of a fibre-based material, whose material in addition to the fibre base comprises only polymer coating layers, which achieve the sealability, heat resistance and light shield required for the product, and in which also yellowing of the material as a result of heating treatment has been substantially avoided. The product package of the invention is characterised by the fact that the polymer coating layer outside the fibre base of the package contains a white pigment and in that the second polymer coating layer included in the packaging material contains a light-absorbing pigment.

In the package of the invention, white pigment is used to hide the fibre base so that yellowing caused by heating does not significantly affect the aspect of the package, more precisely its ISO brightness. The light absorbent pigment, in turn, which has been placed in a coating layer different from that of the white pigment imparting the package its brightness, shields the packaged product from visible light. In addition, in accordance with the invention, light-absorbing black pigment can also be used as an agent for stabilising the light appearance of the package as described in the following.

In accordance with the invention, without the use of an aluminium foil, a fibre-based packaging material has thus been achieved, which resists heat sterilization, especially autoclave treatment at high temperature and under vapour pressure, the polymer coating layers in the material forming the light shield required for the storage life of the product and which also meets the requirements posed on the appearance of the finished package to be marketed. The light shield can be formed with polymer layer toned grey with a black and a white pigment, e.g. as in WO 01/76976, the polymer layer forming an inner layer that replaces the conventional aluminium foil of the package, whereas on the opposite side of the package, i.e. on its outer surface, merely white pigment can be used, which affects the package appearance substantially by covering the discoloration of the fibre base caused by heating treatment.

An optional solution of the invention is to provide two successive pigmented polymer layers outside the fibre base in the package, with the outer polymer layer containing 5 to 25% of white pigment and the inner polymer layer closer to the fibre base containing black pigment in a small amount. These successive pigmented layers allow the light tone of the packaging material to be effectively stabilised despite yellowing of the subjacent fibre base. The overall colour of the material is then white or light grey, slightly toned grey under the effect of the black pigment underneath the white pigment. The brightness degree depends on the pigment amounts used and their mutual ratios, however, the idea is to adjust the material tone as close to white as possible and to maintain this tone during the heating treatment of the package.

A small amount of black pigment provided under a white-pigmented layer has a slightly light-absorbing effect. This effect may yet be inadequate in shielding the packaged product from light penetrating the material. If desired, more effective light-shielding pigmentation can be achieved by providing the polymer coating inside the package with a sufficient amount of light-absorbing pigment, e.g. by toning at least one inner coating layer of the fibre base grey, as disclosed in the WO patent specification 01/76976 mentioned above.

The desired material toning can be achieved with successive pigmented layers, whose total weight is 25 to 60 g/m2, the black pigment concentration of the inner layer being in the range 0.05 to 0.5%, preferably approx. 0.06 to 0.15%, and the white pigment concentration in the outer layer being in the range 5 to 25%, preferably approx. 7 to 12%. Optionally, the inner layer containing black pigment can also be admixed with white pigment so as to get an overall light grey tone of the layer. The proportion of white pigment in the inner layer could thus be 5 to 25%, preferably 7 to 15%. Carbon black, for instance, is suitable as a black pigment, and titanium dioxide as a white pigment.

In a preferred embodiment of the invention, the package material comprises on both sides of the fibre base an outer transparent heat-sealing layer, with the pigmented polymer layers located inside these. This avoids the pigmented layer from getting into direct contact with the product to be packaged, which is prohibited by foodstuff legislation or regulations in a number of countries. However, the inner pigmented polymer layer may contain the same heat-sealable polymer as the outer layer and it may be located immediately underneath this, so that these layers during heat sealing act as a single uniform material layer forming the seam.

In an autoclave package, the fusion point of the polymer in the heat-sealing layer should be at least approx. 130° C., thus avoiding fusion of the polymer in the autoclave. Polypropene is an example of such heat-sealable polymers.

Depending on the packaged product, the package material may further comprise one or more polymer oxygen-barrier layers, which most advantageously are located inside the package, between the fibre base and the pigmented light-shield layer. Especially ethylene vinyl alcohol copolymer (EVOH) and polyamide (PA) are suitable materials of the oxygen-barrier layer.

In the package of the invention, the fibre base of the material may consist particularly of a packaging board made of bleached pulp, having a weight in the range 130 to 500 g/m2, or a packaging paper having a weight in the range 20 to 120 g/m2. These are suitable for heat-sealable casing or carton packages or poach packages to be treated in an autoclave.

The heat-sealable packaging material of the invention, which is suitable for sterile packages as described above, comprising a fibre base and polymer heat-seal layers on its both sides, is characterised by the fact that pigment has been incorporated in the polymer layers on both sides of the fibre base, with the fibre base being hidden with white pigment on one side of the fibre base, and with a light-absorbing pigment forming a light shield for the packaged product on the opposite side of the fibre base. Such a material can be used for forming a light-shielding package for the product, which can be sterilised without any yellowing caused by heating substantially altering the aspect of the package.

The invention further comprises the use of the fibre-based, polymer-coated packaging material tinted by pigmenting on both sides in packages to be sterilised by heating. The invention relates in particular to the use of one of the packaging materials described in further detail above in food packages that have been sealed by heat sealing and are intended for treatment in an autoclave.

DETAILED DESCRIPTION

The polymer-coated packaging board ofFIG. 1comprises in the following order: an outer heat-sealing layer1made of polypropene (PP), an inner polypropene heat-sealing layer2pigmented white with titanium dioxide, a fibre base4made of bleached board, a polyamide (PA) oxygen-barrier layer5adhered directly to the fibre base, an EVOH oxygen-barrier layer6, a polymer binder layer7, a heat-sealable polypropene layer8, which has been toned grey by means of carbon black and titanium dioxide, and a heat-sealing, layer9made of polypropene. The successive coating layers can be produced on different sides of the board4by means of coextrusion. The material can be bent and heat-sealed to form a closed product package preferably with the polypropene layer2tinted white with titanium dioxide placed on the outer surface of the package, i.e. outside the fibre base4, and with the polypropene layer8tinted grey by means of carbon black and titanium dioxide placed on the inner surface of the package, i.e. inside the fibre base4. Owing to the layer2containing white pigment, the material generally preserves its brightness in a heat-sterilising treatment of the package made from it, such as autoclave treatment, despite any yellowing of the fibrous substrate4. The grey layer8has the function of shielding the packaged product from light.

The packaging material shown inFIG. 2differs from the one ofFIG. 1only in that the polypropene heat-sealing layer placed on the outer surface of the package has been omitted and in that the mutual order of PA and EVOH oxygen-barrier layers5,6in the material has been changed. Thus the polypropene layer2tinted white with litanium dioxide is on the outer surface of the material, where it serves alone as a heat-sealable layer.

The packaging material ofFIG. 3, again, differs from the one shown inFIG. 1only in that the material of the pigmented polymer layers2,8on both sides of the fibre base4is cyclo-olefin copolymer (COC) instead of polypropene. COC is a polymer obtained by copolymerisation of ethene and norbornene, which is poorly vapour-permeable and whose application in layered packaging materials is known per se.

The package material ofFIG. 4consists of a polypropene layer1, a polypropene layer2pigmented white with titanium dioxide, a fibre base4, a polypropene layer9′, a binder layer7′, a PA oxygen barrier layer5, an EVOH oxygen barrier layer6, a second PA oxygen-barrier layer5′, a second binder layer7, a polypropene layer8pigmented grey with carbon black and titanium dioxide, and a polypropene layer9. In this multi-layer structure, the adhesion of the oxygen-barrier layers5,6to the fibre base4has been ensured by means of intermediate polypropene and binder layers9′,7′, which may be necessary in demanding applications, such as packages treated in autoclaves. This application has i.a. the special feature of a symmetrical five-layer combination of oxygen-barrier and binder layers7′,5,6,5′,7, which is advantageous when successive polymer layers are formed by coextrusion. Optionally the five-layer structure of the material consisting of polypropene, binder, EVOH and PA oxygen-barrier layers9′,7′,5,6,5′ can be applied onto the board4as a separately manufactured film laminate, which are commercially available in the market. The laminate is then sealed on the board4at the same time as the outer polypropene layers1,2,8,9are incorporated in the final material by co-extrusion.

The packaging material ofFIG. 5differs from the one shown inFIG. 4in that the grey pigment composition is admixed in the two binder layers8,8′ of the structure, so that the number of coating layers is reduced by one. The material comprises a co-extruded seven-layer structure9,8′,5,6,5′,8,9, which is symmetrical relative to the central EVOH layer6.FIG. 6further shows a variant of the material ofFIG. 5, in which only the innermost binder layer8contains grey pigment. This has the advantage of the separately manufactured five-layer film laminate5,6,5′,7,9mentioned above, which can be sealed to the final layered packaging material as an integral assembly.

In the packaging materials ofFIGS. 1 to 6, the fibre base4may consist of a packaging board containing bleached sulphate pulp, having a weight in the range 130 to 500 g/m2, preferably 170 to 300 g/m2. If the fibre base optionally consists of bleached paper, it may have a weight in the range 20 to 120 g/m2. The weight of the PA and EVOH oxygen-barrier layers5,6may be 3 to 15 g/m2, preferably 50 to 10 g/m2. The weight of the transparent polypropene layers1,9placed outermost in the material can be 5 to 30 g/m2, preferably 7 to 20 g/m2, and the inner pigmented polypropene layers2,8adjacent these outer polypropene layers may have a weight in the range 10 to 50 g/m2, preferably 25 to 40 g/m2. The titanium dioxide concentration of the white-pigmented polypropene layer2may be 5 to 25%, preferably approx. 7 to 12%. The carbon black concentration in the grey-pigmented polypropene layer8may be 0.05 to 0.5%, preferably approx. 0.12 to 0.15% and the titanium dioxide concentration accordingly 5 to 25%, preferably approx. 7 to 12%. The binder layers7, whose material is appropriately i.a. the polymer marketed by the company Mitsui Chemicals under the trade name Admer, may have a weight in the range 3 to 15 g/m2, preferably 5 to 10 g/m2. If the polymer material of the pigmented layers2,8is COC or Admer instead of polypropene, the layer weights and pigment concentrations of the pigmented layers mentioned above apply to these as well.

The polymer-coated packaging board ofFIG. 7comprises in the following order: a coating layer2pigmented white with titanium dioxide, a coating layer3pigmented with carbon black, a fibre base4made of bleached board, a light-shield layer8toned grey with titanium dioxide and carbon black so as to resemble an aluminium foil, and a transparent heat-sealing layer9. The polymer material of all of the coating layers2,3,8,9is polypropene. The successive coating layers can be formed on different sides of the fibre base4by co-extrusion. The material can be bent and heat-sealed to form a closed product package so that the successive, pigmented polymer layers2,3are place don the outer surface of the package, i.e. outside the fibre base4, and the grey light-shield layer8and the transparent heat-sealing layer9are placed on the inner surface of the package, i.e. inside the fibre base4. The outer successive pigmented layers2,3of the package impart the package a light, almost white shades which does not change in a heat-sterilising treatment of the package.

FIG. 8shows a packaging board suitable especially for food packages treated in autoclaves, in which, compared withFIG. 7, there are the following added layers: a transparent polypropene heat-sealing layer1placed on the outer surface of the package, polyamide and EVOH oxygen-barrier layers5,6inside the fibre base4, and a binder layer7adhering to the light-shield layer8of the EVOH layer6, whose material is e.g. the polymer marketed the company Mitsui Chemicals under the trade name Admer. Consequently, the package formed of the material is protected with pigmented layers2,3against any yellowing of the fibre base4occurring in autoclave treatment, and the oxygen-barrier layers5,6and the light-shield layer8inside the package protect the packaged product and extend its storage life and best before date.

FIG. 9shows a modification of the packaging board ofFIG. 8, to which a second polyamide layer5′ and polypropene and binder layers9′,7′ have been added between the fibre base4and the polyamide layer5. The arrangement enhances the adhesion between the oxygen-barrier layers and the fibre base. Additional features consist of the symmetrical five-layer combination of the binder7′, polyamide5, EVOH6, polyamide5′ and binder7, which is an advantage when coating layers are formed by co-extrusion, and the provision for incorporating the five-layer structure of polypropene9′, binder7′, polyamide5, EVOH6and polyamide5′ as a film laminate in the material, such film laminates being commercially available in the market. Such a finished laminate can be sealed to the board at the same time as the other coating layers of the material are produced by co-extrusion.

FIG. 10shows a second modification of the packaging material, which differs from the one shown inFIG. 9only in that a second grey-pigmented light-shield layer8′ has been added inside the fibre base4, so that the layer combination inside the fibre base has been made symmetrical in its totality, which is and advantage in terms of co-extrusion.

In the packaging materials ofFIGS. 7 to 10, the white-pigmented layer2outside the fibre base4may be 20 to 50 g/m2, and the layer3containing black pigment may have a weight in the range 5 to 10 g/m2. The grey-pigmented light-shield layer3inside the fibre base4may have a weight in the range 10 to 50 g/m2, preferably 25 to 40 g/m2. The titanium dioxide concentration in the white-pigmented coating layer2may be in the range 5 to 25%, preferably 7 to 12%. The carbon black concentration in the inner coating layer3may be 0.05 to 0.5%, preferably 0.06 to 0.15%. If the layer3contains also titanium dioxide, it may have a concentration in the range 5 to 25%, preferably 7 to 15%. In other respects, the material layer weights are equal to those defined in conjunction withFIGS. 1 to 6.

The packaging boards intended for use in autoclaves illustrated inFIGS. 1 to 10can be varied in different ways within the scope of the invention. Adequate oxygen barrier can be provided by means of one single EVOH layer6, for instance, and then the polyamide layers can be omitted. If the pigmented layers on the outer surface of the package are used as heat-sealing layers, the outermost transparent heat-sealing layer1may also be superfluous. The innermost layer3containing black pigment, which is possibly provided, may be optionally admixed with white pigment, e.g. titanium dioxide, and then the layer gets a light grey overall shade.

FIG. 11shows a closed package product10shaped as a rectangular prism, which has been manufactured by bending and heat sealing blanks of the packaging material ofFIG. 1. The figure includes a partial enlargement of the package wall, showing that the polymer layer2tinted white by pigmenting is placed outside the substrate board4in the package, and that the grey-pigmented polymer-layer8is placed inside this. Thus the outer surface of the package is white and its inner surface is grey, resembling an aluminium foil. At the seam points11of the package, the blank edges are overlapping and the polypropene layers1,2,8,9of the opposite board sides are tightly heat sealed together. In addition to the outermost transparent layers1,9, the pigmented polypropene layers2,8also contribute to the heat sealing. The package is intended for sterilisation at the autoclave temperature of approx. 130° C., at which the white layer2prevents the package from yellowing.

The material of package10ofFIG. 11can be varied, for instance by using any of the packaging materials illustrated inFIGS. 2 to 10. In accordance with the invention, it is further possible to exclude the light-shield layer8inside the package from the materials ofFIGS. 7 to 10, provided that the outer layer3containing black pigment gives adequate light shield for the product. It is further possible, within the scope of the invention, to provide sterilised heat-sealed poach packages having a fibre base of paper and a weight in the range 20 to 120 g/m2.

The following examples comprise experiments, which have primarily tested the effect achieved with pigments added to the polymer coating in covering yellowing of the fibre base of the packaging material during autoclave treatment. The examples have also measured the protection against humidification of the fibre base provided by polymer coatings. The autoclave treatment does not affect the light shield achieved with pigments on the opposite side of the fibre base, and this is why this effect has not been tested.

Pieces of fibre-based packaging material were tested in an autoclave, in which the saturated vapour had a temperature of 130° C. The fibre base of the material consisted of a packaging board made of bleached pulp having a weight of 250 g/m2. The test material1consisted of this material as such. Test material2consisted of said board, with an aluminium foil laminated on its other side by means of a low-density polyethene (LDPE) layer having a weight of 20 g/m2, which acted as a binder. Test materials3to11were layered packaging materials, whose substrate consisted of said test material2and in which the board was equipped, on the side opposite to the aluminium foil, with one single polymer coating layer. The polymer of this layer was polyethene (test material3), polypropene (test materials4and5), a mixture of 70% polypropene and 30% polyethene (test materials6and7), a pigmented mixture of 92% of the mixture of polypropene and polyethene above and 8% titanium dioxide (test materials8and9), and also cyclo-olefin copolymer (COC), which was the product Topas 8007D, producer Ticona GmbH.

The test material pieces were heated in an autoclave for 20 minutes at room temperature to the treatment temperature of 130° C., at which the pieces were maintained for 0, 20, 40 or 60 minutes. After the treatment, the autoclave was cooled within 20 minutes, resuming room temperature, at which the test pieces were removed from the autoclave. For each test material, the brightness (ISO %) of the polymer-coated side of the material, i.e. opposite to the aluminium foil, was measured before the autoclave treatment (initial brightness) and after autoclave treatments of different durations. Accordingly, the moisture content of the materials was measured before the autoclave treatment (initial moisture) and after autoclave treatments of different durations. The results are presented in the following table 1, which also indicates the weight of the polymer-coating layer in each of the test materials3to11.

The results show that regarding polymer-coated test materials4to11, the fibre base had 14% maximum moisture content after autoclave treatment, which is still an acceptable value in view of the autoclave treatment of a package made of this material. In test material3, the LDPE coating layer melted at autoclave temperature, and this is why the moisture content of the fibre base was not measured. Measurements of test material2, i.e. uncoated aluminium/LDPE/board laminate, indicate strong water soaking of the unprotected fibre base in the autoclave.

A study of the measured brightness values before the autoclave treatment and after various treatment durations shows a progressive decrease in brightness as the treatment period increases, which is due to yellowing of the fibre base, which is not at all covered by the colourless polymer surface. With test materials8and9, in which the polymer coating layer was pigmented, the brightness reduction was however substantially less than with materials equipped with an unpigmented colourless polymer coating. Consequently, white TiO2pigment allows the yellowing of a fibre base occurring in autoclave treatment to be covered to a considerable extent.

A series of tests in conformity with example 1 was made with materials, in which the fibre base consisted of packaging board made of bleached pulp, having a weight of 210 g/m2(test material1). On one side of this board, an aluminium foil (test material2) had been laminated by means of an LDPE binder layer having a weight of 20 g/m2, and the opposite side of the board was additionally equipped with a two-layered polymer coating, whose inner layer contained a mixture of 70% of polypropene and 30% of polyethene, to which had been added titanium dioxide in various amounts, and whose outer layer was pure polypropene (test materials3to12). The measured moisture contents of the fibre base and the brightness on the polymer coating side in the material before and after the autoclave treatment are shown in table 2. The table also contains the layer weights of the inner pigmented and the outer unpigmented polymer layers, respectively.

The results indicate a decrease in the brightness of the material as the autoclave treatment period increases, however, a comparison of the results with the brightness of the materials equipped with an unpigmented polymer coating of table 1 shows the considerable effect of the titanium dioxide pigment in covering the yellowing of the fibre base, the effect increasing with an increase of the pigment concentration and the weight of the pigmented layer, i.e. the layer thickness. A still better covering effect can obviously be achieved by further increasing the layer thickness of the pigmented polymer layer.

In this example, the test series was performed with test materials free from aluminium, whose fibre base consisted of a packaging board made of bleached pulp, having a weight of 210 g/m2(test material1). On one side, the board was coated with a polymer layer, in which polypropene was admixed with 7.5% of titanium dioxide and having a layer weight of 58 g/m2(test material2). The opposite side of the board was coated with an unpigmented polymer coating layer (test material3to6), or with a layered structure formed of two or three successive unpigmented polymer layers (test materials7to11). The materials of the coating layers was polypropene (test materials3and4), a mixture of 70% of polypropene and 30% of polyethene (test materials5and6), a two-layered structure, whose inner layer was COC (Topas 8007D) and outer layer said mixture of polypropene and polyethene (test material7), a three-layered structure, whose innermost layer was polyamide, following layer a binder (Admer NF 912E) and outermost layer said mixture of polypropene and polyethene (test materials8and9), or a three-layered structure, whose innermost layer was ethyl vinyl alcohol copolymer, intermediate layer a binder (Admer NF 912E) and outermost layer polypropene. The weights of the polymer layer(s) on the unpigmented side of the fibre base are indicated in the following table 3.

The autoclave treatments of the test materials were performed in the same manner as in examples 1 and 2, except that the duration of the autoclave treatment at a temperature of 130° C. was 0, 30 or 60 minutes, respectively, and the initial brightness of the material and brightness after a 60 minute autoclave treatment were measured on both sides of the material.

The results of table 3 show the crucial difference in the change of the brightness between the fibre base sides coated with pigmented and unpigmented coating. On the pigmented side, there was a slight decrease in the brightness of the material, and in all the cases, the brightness value after one hour of autoclave treatment was above 80% ISO. On the unpigmented side of the material, the brightness decreased notably as a result of yellowing of the fibre base, which the colourless coating layers are unable to cover.

A test series was performed with a packaging material, whose fibre base consisted of a packaging board made of bleached pulp having a weight of 210 m/m2. The board was coated on one side with a polymer layer covering against yellowing, in which polypropene was admixed with 7.5% of titanium dioxide and whose layer weight was 58 g/m2. The opposite board side was coated with an unpigmented polymer coating layer (test materials1to4) or a layer structure formed of two or three successive unpigmented polymer layers (test materials5to9). The coating layer materials were polypropene (test materials1and2), a mixture of 70% of polypropene and 30% of polyethene (test materials3and4), a two-layered structure, whose inner layer was COC (Topas 8007D) and outer layer said mixture of polypropene and polyethene (test material5), a three-layered structure, whose innermost layer was polyamide, following layer a binder (Admer NF 912E) and outermost layer said mixture of polypropene and polyethene (test materials6and7), or a three-layered structure, whose innermost layer was ethyl vinyl alcohol polymer, intermediate layer a binder (Admer NF 912E) and outermost layer polypropene (test materials8and9).

The test material pieces were heated in an autoclave for 20 minutes at room temperature to a treatment temperature of 130° C., at which the pieces were kept for 60 minutes. After the treatment, the autoclave was cooled in 20 minutes, resuming room temperature, at which the test pieces were removed from the autoclave. The brightness (ISO %) on each side of each test material was measured before the autoclave treatment (initial brightness) and after the autoclave treatment. The results are shown in table 4, which also indicates the weights of the unpigmented polymer coating layers in each test material1to9.

The test results of the table show the problem to be solved with the invention, i.e. the notable decrease of brightness due to yellowing of the fibre base. which the colourless coating layers on the unpigmented side of the fibre base have been unable to cover. Owing to the pigment, the initial brightness on the opposite side of the fibre base, pigmented with titanium dioxide as in FI patent application 20030260, was high and a slight decrease of brightness has occurred in the autoclave treatment, however, with a final brightness above 80% ISO in each case.

A test series in conformity with example 4 was carried out with materials of the invention, in which the fibre base consisted of a packaging board made of bleached pulp, having a weight of 250 g/m2. One side (the outer side) of this board, which is intended to be the outer surface of the autoclave package, was equipped with a two-layered polypropene coating, in the innermost layer of which polypropene was admixed with carbon black and titanium dioxide (test materials1to7) or merely with carbon black (test materials8to22), and whose outermost layer comprised polpypropene admixed with titanium dioxide alone. In part of the materials (test materials20to22), the opposite board side, which will be the inner surface in the autoclave package, was equipped with an inner light-shield layer based on polypropene and pigmented grey with carbon black (0.15%) and titanium dioxide (12.5%), having a weight of 27 g/m2, and with a transparent polypropene layer covering this, having a weight of 23 gm2. The measured brightness of said outer material side before and after the autoclave treatment is shown in the following table 5. The table also comprises the pigment concentrations of the pigmented polymer layers on the outside of the material and the layer weights.

The results show that the packaging material of the invention can be toned to an optimum brightness of above 80% ISO, which undergoes a slight, practically negligible change during autoclave treatment. The final brightness after the treatment is almost the same as in FI patent application 20030260, however, with a clearly improved colour stability of the light shade during the treatment.