Retortable packages

DESCRIPTION

This invention relates to the production of retort containers, loaded with a product (in particular a liquid or semi-liquid food product). In the British patent application GB. 2,067,157B, of the applicants herein, the package assembly comprises a container of thermoplastic material, and a diaphragm that is sealed or welded to an edge formed at the loading end or mouth of the container. The diaphragm was constituted by a metalic laminar and it is sealed or welded tightly to the edge of the vessel by means of a thin layer of thermoplastic material carried by the metalic laminar. After the container is closed, it is subjected to a thermal treatment, for example in a retort of water vapor, water / air vapor or water vapor to achieve pasteurization or sterilization.

When using retort packaging, and from a commercial point of view, it is important that after the retort treatment the container is not only intact but does not show visible signs of deformation, so that the consumer is presented with an acceptable appearance . Another requirement is that the package can be kept standing in a stable way, without overturning, on a shelf or similar medium.

In our British patent application GB 2,067,157B, mentioned above, (which is recommended to consult the reader), applicants have described a sealing procedure whereby retort packaging, of a liquid or semi-liquid food product, is hydraulically solids The upper space that was initially present in a thermoplastic container, above the contained product, is removed by extracting the gas contained in the space before sealing the container by an aluminum diaphragm; After sealing, external pressure is applied to the diaphragm to stretch it inelaostically and redistribute the product that is adjacent to the diaphragm. therefore, the diaphragm has a concave configuration outside and fully in contact with the product

Using the procedure of patent application 2,067,157, the applicants have been able to produce containers provided with a high degree of commercially satisfactory dimensional stability, provided that the containers have only undergone a small degree of volume contraction (eg , 3% or less) during the return treatment. However, when trying to use the procedure with containers subject to a greater degree of volume contraction (eg, above 3%), applicants have encountered difficulties due to the substantial deformation of the container produced by the return treatment. This deformation manifests itself in two ways; that is: (a) an outward swelling and / or buckling of the base of the container, and (b) an outward swelling and / or a non-presentable contraction of the metabolic diaphragm in the upper part of the vessel. Defects (a or b) are usually presented to the exclusion of the other, but on some occasions both defects (a and b) are presented and / or the side wall of the container is deformed instead of deforming the base of the container or in addition to this last deformation.

With containers with a large floor area related to their height, in particular shallow trays, the degree of deformation suffered can be visually and mechanically insignificant, and therefore, can be considered commercially acceptable. In containers such as those known as "potitos", cubes and bowls with a relatively small surface area in relation to their height, the deformation is more evident to the consumer and, in the event that deformation occurs at the base, It may result in the container not standing up in a stable manner. Therefore, particularly as regards those containers that undergo a substantial degree of volume contraction during the return treatment, there is a need to control the deformation of the container produced by the contraction of the volume of the container, such that they can be made commercially acceptable.

Following an aspect of the present invention, a retort treatment container is provided, containing a product therein, the package of which comprises a container having a base and a raised side wall projecting to an edge, at least the side wall of a thermoplastic material and being subject to contraction during the retort treatment process, the package also comprising a diaphragm that is sealed or welded to the edge of the container and which is domed over the product contained in the container, so that the package is hydraulically solid, said diaphragm being of thermoplastic and heat shrinkable material so that it contracts in the retort treatment and the concavity of the diaphragm is reduced to virtually compensate for the contraction of the volume of the container produced by the retort treatment.

The packaging could therefore make the effects of the retort treatment not easily visible. In this last case, the cooncave diaphragm is generally flat.

Applicants have obtained satisfactory results using containers made of polypropylene and laminates that incorporate that material, but applicants believe that the invention also has application to containers made of other plastics and single-layer or multilayer (laminated) construction materials. In addition, although having a particular application to thermoformed thermoplastic containers from sheet materials, the invention can be used with containers made by other conformation methods for example, by stretching by inflating a tubular shoulder or a tubular preform that can include containers in the that the base is not an integral part of the side wall, but that it joins it.

Following a second aspect of the present invention, a procedure is provided for

015 058 manufacture a container for retort treatment, comprising the phases of:

(a) forming a container with a base and an adapted side wall projecting to an edge, at least molding the side wall of plastic material and contracting a retort treatment process;

(b) load the container with the product to leave an upper space below the edge;

(c) treat a thermoplastic diaphragm in the perimeter of the edge;

(d) make the diaphragm material co-concave so that the diaphragm, after sealing tightly, occupies the upper space and comes into contact with the product, thereby making the package hydraulically solid; and (e) make the heat shrinkable diaphragm so that, during the retort treatment, the product is contracted to constitute a hydro-solidly solid container and, when the concavity of the diaphragm is reduced, the volume contraction of the container produced by the treatment is virtually condensed in retort.

Other aspects and features of the present invention would be apparent from the description that follows and from the appended claims.

Next, an embodiment of the present invention is described, by way of reference, in relation to the attached drawings, in which:

Figure 1 schematically shows, in a diametral, vertical cross-section, a container after filling and closing and before being subjected to a retort treatment process, the package of which comprises an ermoplastic container made by the thermally forming of a plastics laminate , and a diaphragm seal welded around the container and containing a liquid or semi-liquid food product.

Figures 2 and 3 show two containers of the class of Figure 1, as they appear when they are closed by means of a metalic diaphragm and after having undergone a retort treatment process.

Figures 4 and 5 show, in a similar way, two containers with a plastic cap, following the present invention, as they appear after having been subjected to a retort treatment process; Y

Figure 6 graphically shows the range of base thicknesses measured in 40 individual containers used in a behavior test of the retorted containers with metal diaphragms and plastics caps, according to the present invention.

By way of comparison, the containers, before being subjected to the retort treatment, are represented in the striped lines in Figures 2 a

5.

The tests described below were all carried out in containers formed using conical vessels of circular cross-section, of the class commonly known as 71-mm milk products. The vessels had a diameter ratio of approximately 1: 1 height and were produced by thermoproduction of a coextruded multilayer thermoplastic sheet or sheet. The laminate was made up of two relatively thick outer layers of polypropylene (PP), which were sandwiched between them a thin, oxygen barrier layer of polyvinylidene chloride (PVdC) and thin layers of adhesive on each side of the layer Barrier

For the purposes of the tests performed, the liquid or semi-liquid food product to be contained in the containers was simulated by a starch solution.

The containers were closed after being filled with the product, leaving an upper space, in which it was practiced or after the vacuum. A flat flexible sheet of material was sealed or welded to the edge of the container, to form a diaphragm that would leave the product and the upper space inside the container tight, after which the diaphragm was subjected to an external fluid pressure on the upper space to bring it in contact with the product.

The displacement of the diaphragm towards the interior of the container eliminated or the empty upper space and produced a certain redistribution of the product, resulting in the closed container being essentially hydraulically solid and without any empty part, leaving very little or no permanent gas inside. The stretching of the laamine or sheet was inelastic so that, when the pressure was removed from the fluid, the container was virtually free of remaining stresses.

Said process of sealing or thermal welding has been described in detail in the specification of British patent GB 2,067,157B, which is advised to be read to obtain more detailed information concerning that process. In the resulting package, the diaphragm sealed or welded was a concave outward appearance, shallow, gently curved, and completely in contact with the product contained in the container, as mentioned above. Figure 1 shows a diametral, vertical cross-sectional view of one of the containers produced for testing. In that figure, the sealed container 10 of the package contained a product 24 and comprised a unit container 12 with a side wall 14, a base 16 and an annular edge 18, turned outward, and a closure diaphragm 20 with its perimetric margin 22 tightly sealed to the edge of the container 18.

Test Series 1

For this first series of tests, the containers were sealed with diaphragms consisting of a 40 micron aluminum sheet coated with a 50 micron layer of high density polyethylene, so that the diaphragm could be sealed or welded to the edge of the container.

To produce containers with a wide range of base thicknesses, the containers were formed from two laminate thicknesses, that is: 1.8 mm and 2.5 mm; In addition, the containers formed from the 1.8 mm laminate were made using two different series of thermoforming conditions, which provided them with relative bases.

015 058 tively thin or relatively thick. A four quality thermoforming mold was used for each laminate, and in relation to the 2.5 mm laminate, the cavity of the particular mold used for each measurement was noted.

The test packages underwent 3 different but traditional return treatment procedures; however, it was found that, after the aforementioned processes were completed, all the containers had undergone some substantial and easily visible deformation, so that they have to be considered as commercially unacceptable. Table 1, exposed below, gives the results obtained.

TABLE 1

<td rowspan="2">Type of container</td><td rowspan="2">Weight of container (g)</td><td colspan="2">Deformation (%)</td>

<td>R</td><td>D</td>

<td>Sheet of 1.8 mm - thin base</td><td>6.5 g - 6.8g</td><td> 100</td><td> 0</td>

<td>-based thick Laminated 2.5mm -Cavity of the</td><td></td><td> 86</td><td> 14</td>

<td>mold</td><td></td><td></td><td></td>

<td> (1)</td><td>9.5 g - 10</td><td> 55</td><td> 45</td>

<td> (2)</td><td></td><td> 15</td><td> 85</td>

<td> (3)</td><td></td><td> 28</td><td> 72</td>

<td> (4)</td><td></td><td> 67</td><td> 33</td>

Being:

R = Container D = Diaphragm

The measurements made showed that the recipients had experienced a degree of contraction of volume between the limits of 3% and 8% and it was evident that this contraction had correspondingly reduced the volume available for the product which correspondingly had resulted in a severe deformation, commercially unacceptable, of the container. Normally, the deformation occurred in the base 16 of the container 12 or in the closure diaphragm 20; however, in some cases the container presented or deformed its side wall 14. The deformation of the base of the container and the deformation of the diaphragm are illustrated, respectively, in Figures 2 and 3.

Comparing Figure 2 with Figure 1, it was observed that while in Figure 2 the co-concave shape inside the diaphragm of the container 20 seems unaltered by the retort treatment process, the base 16 of the container 12 has been forced out for the product inside, while in a state softened by heat, bulging down so that the package was mechanically unstable when placed with its base on a horizontal surface, in addition, it gave the container a "swollen" appearance. Therefore, the retort treatment makes this package inappropriate for sale to the consumer. This deformation mode is typical of packages whose containers are formed from thinner laminate (1.8 mm), although some containers formed from thicker laminate (2.5 mm) are similarly affected.

On the other hand, it has been found that, although the base of the container 16 of Figure 3 seems unchanged when compared with the container of Figure 1, the closure diaphragm 20 is pushed up by the contained product, presenting a appearance wrinkled, uneven and bulging, which again is considered unacceptable to the potential consumer. This mode of deformation has been typical of the containers whose containers are formed from the thicker laminate (2.5 mm), but also occur in some containers formed of 1.8 mm laminate not subject to deformation of the container. Therefore, all containers undergoing retort treatment, provided with metal diaphragms, have been considered unacceptable for sale to the consumer if the retort treatment process is used.

From table 1 above, it was found that the containers produced in grades 2 and 3 had a significantly better performance than the vessels of grades 1 and 4 in relation to the deformation of the base of the vessels.

This disparity can be explained by the fact that the vessels of grades 2 and 3 had a medium thickness and walls of uniform bases than the vessels of grades 1 and 4 and therefore could better withstand the stresses generated in the container during retort treatment; nevertheless, a substantial proportion of them experienced a serious deformation of the base. From the Test Series 2, of which a report is now presented, the qualities were combined as groups 1/2 and 3/4, so that the vessels of the two groups had similar ranges of base thicknesses.

Test Series 2

For this series of tests, 40 containers of the same 2.5 mm laminate were molded as used in Test Series 1, using the same 4-cavity thermoforming mold that had been previously used for that laminate. Note was taken of the cavity corresponding to each container. The 20 molded containers in cavities 3 and 4 were then closed using the same lid material and the same closure procedure as was used in Test Series 1; The 20 containers of the cavities 1 and 2 were closed using essentially the same closing procedure as above, but with a plastics (transparent) all-cap material formed of 15 micron polyethylene terephthalate (PET (), laminated by molded polypropylene extrusion 70 microns

The closed containers were subjected to retort treatment immersed in water for 60 minutes at a temperature of 115 ° C and a pressure of 2.11 kg / cm2. Before being subjected to retort treatment, all the packages had the appearance indicated in Figure 1. Afterwards

015 058 of the retort treatment, the containers with a metallic diaphragm had, again, an appearance as illustrated in Figure 2 or Figure 3, and were considered commercially unacceptable; however, the containers with a plastic diaphragm had an appearance normally as illustrated in Figure 4 but, sometimes, as illustrated in Figure 5.

It will be seen from Figures 4 and 5 that the bases 16 of all the plastic-capped containers 12 of this second series of tests had withstood the internal forces produced during the retort treatment; in fact, the containers showed no visible signs of deformation anywhere. Figure 4 depicts a typical vessel after being subjected to retort treatment and shows that the diaphragm still retains its original, gently curved, co-excavated appearance. The concavity of the diaphragm had been reduced, but this change was not evident to a potential customer of this packaged product; In addition, there are no wrinkles, folds, blisters or bulges of the diaphragm, which could give rise to doubts about the condition of the packaged product, or otherwise produce customer rejection.

Figure 6 shows the containers used in the Test Series 2 in relation to the mold cavities in which they were formed and whose data were treated against the thicknesses of the bases. For each container, the respective line represents the range of thicknesses that were measured in a number of points at the base of the container. The larger and more uniform base thicknesses, produced in cavities two and three, could be easily distinguished. The results are presented, in tabular form, in the following table 2:

TABLE 2

<td></td><td></td><td>Thickness</td><td></td><td></td>

<td></td><td></td><td>from the base</td><td colspan="2">Deformation %</td>

<td></td><td>D</td><td>of the container</td><td>R</td><td>D</td>

<td></td><td></td><td>(mm)</td><td></td><td></td>

<td>C1</td><td>T</td><td> 0.74 - 1,57</td><td> 0</td><td> 0</td>

<td>C2</td><td>T</td><td> 1,22 - 1,81</td><td> 0</td><td> 0</td>

<td>C3</td><td>M</td><td> 1,22 - 1,64</td><td> 0</td><td> 100</td>

<td>C4</td><td>M</td><td> 0,63 - 1,28</td><td> 100</td><td> 0</td>

Being:

C = Cavity T = Thermoplastic M = Metal R = Container D = Diaphragm

The reduction in the concavity (or degree of concavity) of the plastic diaphragms, in the Test Series 2, depended on the contraction of the volume of the containers in relation to the volume of the upper space closed by the diaphragms. It was found that the reduction could be adjusted, within wide limits, as desired, by varying the level of filling of the product and, therefore, the volume of upper space, resulting in the maximum reduction of a generally flat diaphragm. as indicated in Figure 5. In this regard, it will be noted that a convex diaphragm, bulging outwards, is considered a commercially unacceptable product from the standpoint of activation capacity, ease of transport and acceptance by the customer.

Applicants believe that the absence of unacceptable deformation of the plastic lid containers, produced by the retort treatment operation, can be attributed to the following reasons:

(1) During the retort treatment, the contraction of the diaphragm acts in the same direction increasing the volume of the container and, therefore, counteracts the loss of volume of the container produced by the contraction of the volume thereof, therefore having the tendency to reduce the pressure inside the container;

(2) Due to the lower thickness of the material and the thermal capacity of the diaphragm material relative to the container material, the diaphragm's thermal response to retort treatment temperatures is faster than that of the vessel and, during retort treatment , the internal pressure of the vessel is not simply essentially more than what it would have been with a non-heat shrinkable diaphragm material (eg, metal), but some in some substantial part of the operation of the retort treatment may in fact be negative in relation to the atmospheric pressure of the retort;

(3) Despite the flexibility due to lack of consistency of the materials of the container and the diaphragm, induced by the operation of the retort treatment, the container can maintain substantial negative pressures in deformation and, therefore, survives the operation of treatment in retort without deformation and with the reduced concavity of the diaphragm to compensate for the contraction of the volume of the vessel;

(4) After the retort treatment, when the container is cooled to normal room temperature, the plastic materials of the container and the diaphragm regain their rigidity and the container is in a virtually stress-free state, as much as the diaphragm material has not fully recovered the original flat state.

Therefore, it is believed that the retrogradation of its canonical and thermoplastic diaphragms towards a virtually planar shape (without concavity), during retort treatment, has made the plastic lidded containers of Test Series 2 commercially acceptable afterwards. of retort treatment.

Test series 3 thermoformed containers in the same mold with four cavities from 2.5 mm laminate. used in the Series 1 and 2 Tests, they indiscriminately underwent the same closure and retort treatment operations as the Series 2 Test vessels. After the retort treatment, the 11 containers that had plastic lids demonstrated be all commercially acceptable and, in particular, showed no apparent deformation5

015 058 visible mind; nevertheless, the 19 containers with metal foil cover all presented deformation of the container or diaphragm and were considered commercially unsatisfactory.

The minimum thickness of the base of the 30 containers of the Series 3 Tests was 0.65 mm. and the applicants believed that this figure responds to the minimum base thickness that could guarantee that a large proportion (eg, 99.9% more) of the particular containers under test would be commercially acceptable after the retort treatment. In this regard, it was observed that the thickness of the mony base of the stisfactorial vessels, with a plastic lid of the Test Series 2, was 0.74 mm.

Series of tests 4 thermoformed containers using 1.8 mm laminin were closed, some with metalic diaphragms and the rest with plastic diaphragms, using the closure procedure of the other Series of Tests. After the retort treatment using the procedure used for Series 2 and 3, it could be verified, as expected, that none of the containers with metal foil lids could be considered commercially satisfactory. However, approximately half of the 28 containers with a plastic lid proved commercially satisfactory after the retort treatment; the failures could be attributed to the deformation of the container produced by an insufficient wall, in particular the base, and its thickness, and in this respect it was observed that the base thicknesses of the containers, it was demonstrated, were within the limits of 0.50 mm and 0.81 mm and, therefore, covered the value of 0.65 mm mentioned in relation to Test Series 3 above. It was therefore created that the results of Test Series 4 support the assertion that the thickness of 0.65 mm is approximately the minimum value of the wall thickness of the container that is likely to be commercially acceptable following the containers used in the tests.

Various plastic materials can be used for the heat shrinkable diaphragm caps of the packages according to the invention. Normally, the cover material will be laminated, even when this factor is not essential. Following a proposal, the lid material can be a five-layer structure, comprising outer surface layers of polypropylene and an intermediate barrier layer of polyvinylidene chloride (PVdC) that is bonded by thin adhesive caps to polypropylene layers on each side.

The heat shrinkage of the diaphragm layers of the packages following the invention can be imparted entirely by an operation that stretches the diaphragm material in contact with the product, as described above in particular in relation to the tests performed by the applicants. . Normally, the diaphragm material will have a degree of contractibility induced therein during its original manufacturing, and that contractibility itself adds to any contractibility created by the stretching operation. However, within the scope of the invention, various methods for its manufacture are included in which the diaphragm is totally or partially concave before its application and thermal welding to the container, for example by a thermoforming operation on a diaphragm material of thermoplastic, relatively thick and normally self-stable; In such circumstances, the heat shrinkage can be induced, again, in the diaphragm by giving it its co-concave shape and possibly also during the original formation of the material.

To give it the required property of thermocontractibility, the diaphragm was normally made of thermoplastic material in its entirety and the package could then be completely subjected to microwave action. In spite of the entire diaphragm, it may be partially metabolic, but any metal content that has the diaphragm material should not be of such magnitude that it destroys the thermocontractive nature of the diaphragm material; therefore, it could normally be in the form of a thin layer, deposited in steam or separate particles added to act as a gas barrier or for attractive reasons.

Although the containers used in the tests set out above presented volume contractions of 3% to 8%, applicants believe that the invention can be used with containers that have volume contractions from 1% upwards. As mentioned above, the containers can be formed in a thermoforming operation on thermoplastic laminin, or by another operation of plastics molding; In addition, the base of the container does not necessarily have to be an integral part of the side wall. In one application, the invention is used to release internal pressures and prevent deformation of the side wall during treatment of the retort vessel, the vessel of which is provided with a generally cylindrical side wall cut into a stretch molded PET (polyethylene terephthalate) tube -inflation. One end of the container, intended to form what can be considered as the base of the container, is closed by a rigid metal lid with double hooking to the side wall, the other end being "the top end" of the container a concave, relatively diaphragm flexible and thermocontroactive, made of plastics material, which is thermally welded to a flange formed at the other end of the side wall and in full contact with the product contained inside, so that the package is hydraulically solid. When undergoing a thermostability operation, the PET side wall may experience a certain volume contraction during retort treatment but any reduction in volume resulting from the vessel during that operation is counteracted by the retrogression of the diaphragm into a flat state, as explained above in relation to the entire plastics container. It was observed that with this particular vessel construction, the re6

015 058 container can be supplied to the food product packager with the diaphragm cap attached but flat (eg, not concave). The packer fills the container with the product at the opposite end, in a vacuum, leaving a thinner upper space; Place a double-hooked metal cap on that end to close the container and then give the diaphragm a conontal shape inwards to eliminate the said empty space, leaving the container hydraulically solid and making the diaphragm thermocontrose.

015 058