KR20070020404A - Organic acid resistance improvement in polymer coated metals - Google Patents

Abstract

A method of inhibiting attack of organic acids such as acetic acid in a metal container body, a metal container end, or a thermoplastic polymer coated on the metal container body and the end,

Packaging the organic acid-containing material by heating the polymer on the components above their melting temperature, including flash heat treating the entire metal part of the polymer coated container in contact with the organic acid. It is characterized by producing a container suitable for the.

Description

ORGANIC ACID RESISTANCE IMPROVEMENT IN POLYMER COATED METALS}

The present invention relates to a method for inhibiting attack by organic acids such as acetic acid in a thermoplastic polymer coated on a metal container body and / or a metal container end.

Polymer coated metals have been developed for many applications. One such field is the manufacture of polymer coated metal containers for packaging materials containing organic acids, such as tuna seasoned with white wine.

Aggressiveness of organic acids, such as acetic acid, differs from that of other substances, such as, for example, salt solutions, because of the mechanism of interaction. The organic acid solution may directly attack the bond between the metal substrate and the polymer coating layer while the salt-solution first causes the corrosion process.

Although the packaging of organic acid-containing materials is very aggressive and likely to attack the contact surfaces, there are no or very weak problems of lamination separation without heat treatment processes such as pasteurization and sterilization.

Known polymer coatings have been specifically designed to ensure good adhesion of the coating even after deformation.

However, according to the present invention, the problem of attack by organic acids such as acetic acid is more serious than the case of heat treatment such as sterilization of a filled container.

Organic acids can diffuse through the coating in their non-dissociated state, and this diffusion rate is very temperature sensitive (see Table 1). Dissociation can occur at the polymer-metal interface and aggressive in the accumulation of acids. These acids will result in a double: corrosion reinforcement and coating separation.

A particular problem involved is the result of thermal process treatments used to package foods to increase shelf life. This heat treatment depends on the content and is performed for 1 hour at 80 ° C. to 120 ° C. or higher in hot fill applications.

For example, many fish products packaged in cans are sterilized at approximately 115 ° C. This heat treatment is an important factor in producing a good product and is also an important factor in the packaging process of this type of product.

There is currently no polymer coated metal container useful for packaging such products. It is therefore an object of the present invention to find a suitable solution for packaging acetic acid and other organic acid containing products to be heat treated, such as sterilization.

According to the present invention, even heating the container body to near the hole is insufficient to prevent problems associated with the packaging of the organic acid-containing product to be heat treated, such as sterilization.

According to the invention the polymer is also made of coated steel and all the parts constituting the container (for example the container body and / or the lid forming the container) which withstand the substantial deformation at the risk of weakening the contact surface between the metal and the polymer. Special heat treatment is proposed. The container then becomes resistant to the adverse effects of thermal processes such as retorting in the presence of organic acids such as acetic acid. Experiments have shown that any heat treatment as well as local heat treatment is not sufficient.

US2003 / 0198537 provides a method of suppressing delamination of a thermoplastic polymer coating extruded from a container body by inductively heating the open end of the container body before attaching the can end to the body to bond the polymer to the container. It is known. The container body is made by forming a cylindrical body having an outer surface, an inner surface and an edge which is a hole. The body inner surface is coated with a polymer liner, and the body outer surface can be optionally decorated. Nearly perforated container body edges are inductively heated and join the ends and the body to form a complete container. According to US2003 / 0198537 the polymer must flow into an incomplete portion of a very small surface of the inner surface of the can body. This phenomenon already occurs above the glass-point of the polymer when the polymer is in an amorphous phase.

However, according to the present invention, only heat treatment above the melting point of the polymer is sufficient to make the coating resistant to organic acids.

It is not necessary to specifically use induction heat to process the container. It has been found that the effects of the present invention can also be obtained with 'normal' oven treatments. The treatment may also protect the container. However, thermal treatment by induction (or any other rapid heating method or “flash heating”) is advantageous in preventing unwanted decomposition, resulting in embrittlement of the polyester chain in the presence of oxygen. .

It was further confirmed by the present invention that it is essential to observe a certain time of heat treatment in order to obtain the optimum effect. The longer time is disadvantageous, and it has been confirmed from the examples that the period of approximately 4 seconds is considered to be optimal for packaging.

In summary, according to the present invention for polymer coated metal containers suitable for heat treatment, for example, retort packaging, for organic acid containing materials, less than 5 seconds is preferred for conventional polymer coated metal containers, but not too long to prevent degradation of the polymer. It is desirable not to, and it is proposed to heat the polymer inside the container above the melting temperature of the polymer for a period of time that should not be too short to have an effect.

The invention will now be described using the drawings and examples.

drawing

FIG. 1 shows two cans exposed to 1.5% by weight acetic acid (HAc) at 121 ° C. for 90 minutes, with one can (left) without induction heat treatment on the entire surface clearly black. It shows severe delamination and corrosion, and the other with induction heat treatment (right) shows no corrosion or delamination.

Figure 2 shows a picture of a cut open non-treated can, the top picture of the can is filled with 1% HAc solution and stored for 4 months, the bottom picture is filled with 1% HAc solution 1 Would have been saved for months.

Figure 3 shows a canned open cans processing photo according to the present invention, the cans of the top picture is stored for 4 months filled with 1% HAc solution, the bottom picture is stored for 1 month filled with 1% HAc solution.

Figure 4 shows the implementation of various heat treatments, in particular the instantaneous heat treatment FH according to the invention.

Example 1

There are several factors that affect the resistance of polymer-coated ECCS-packed steels to organic acids.In other words, the chemical resistance to acids varies among the polymers applied to polymer-coated steels, so that the polymer type used and the thickness of the layer increases. As the chromium layer thickness increases, the increased coating thickness increases the barrier, the coating thickness, the increased crystallinity increases the diffusion inhibition, the crystallinity of the polymer, additives in the polymer layer that can increase the barrier properties, and the acid accumulates from the metal surface. There is an air entrapment because of the presence of air pockets between the substrate and the coating from which the polymer can separate.

For flat, unstrained materials, the chromium layer and polymer coating are the best combination. The modified material significantly lost the positive effects of metal selection during subsequent experiments. During the heat treatment, it was confirmed that the attack with acetic acid caused the portion with the largest strain rate, resulting in a weak contact between the polymer and the steel.

Improving the adhesion of the starting material (flat plate) does not improve the performance of the product. Therefore, it was concluded that the option to implement a complete solution was to reinforce the adhesive side after the container was manufactured and filled and before heat treatment such as retorting.

One option to achieve this is to heat the polymer in an air oven so that the bond groups of the polymer can be directly bonded to the surface. Experiments were conducted in hot-air ovens with PET at various temperatures (range from 90 ° C to 260 ° C, ie slightly above the glass transition temperature to slightly above the melting point of PET) and at various times (5 to 50 minutes). The cans prepared from the prepared ECCS (DRD cans used in this test) are heated. The cans are exposed to 5 wt% acetic acid solution and pasteurized at 100 ° C. for 1 hour. These experiments showed that the only way to fully improve the efficacy was to completely melt the polymer to restore functionality (see Table 2).

A problem encountered in restoring functionality by heating the polymer above the melting point is that the polymer is severely embrittled because it remains relatively long at these high temperatures. Even if the adhesion and corrosion resistance are restored, the coating is too fragile and no durable cans are produced at the end. The solution to this problem is to use a rapid heating method (also referred to herein as flash heating). Induction heating is used here, but other methods may also be used. This heating method can melt the polymer coating of the can within seconds.

The heated DRD-can confirmed that the acetic acid concentration was able to withstand sterilization cycles of up to 90 minutes at 121 ° C. at 5% by weight or less (see Table 3).

Analysis of the can walls and can bottoms confirmed that the coating itself did not change significantly: the crystallinity was the same and the orientation was only slightly lower than in the DRD-can. Subsequent coating crystallizations resulted in much better but lower results than the effects of the melting process.

Diffusion through free film

Diffusion experiments were set up with glass PET coatings at different temperatures in order to evaluate the transport properties of acetic acid through the PET coatings. Table 1 shows the results for the diffusion from one compartment of the diffusion cell containing 3% by weight acetic acid solution to the adjacent compartment containing dematerialized water (permeate) through the PET foil membrane. These results show the importance of temperature in the diffusion of acetic acid and organic acids in general on the diffusion coefficient. It can also be seen why the heat treatment of the acetic acid-containing food is very aggressive to the packaging steel.

Diffusion of acetic acid through PET foil (20 μm). (Volume cell: 4.40 × 10 ⁻⁵ m 3; membrane surface: 4.91 × 10 ⁻⁴ m 2) —Part B (deionized water) in compartment A (3% HAc = acetic acid) Spread for up to 24 hours. Temperature (℃) [H ₃ O ⁺ ] (mol / l) in compartment B after 24 hours [HAc] (mol / l) in compartment B after 24 hours HAc (mol.m ^-2 ) diffused through the membrane for 24 hours HAc calculated through the membrane per ^second (mol.m ² .s ^-1 ) D (m ² .s ^-1 ) 20 7.90E-8 3.94E-10 1.83E-5 2.1E-10 6.9E-4 60 2.40E-7 4.27E-9 1.98E-4 2.3E-9 7.4E-3 90 5.62E-5 1.81E-4 8.38 9.7E-5 3.1E + 2

The diffusion of acetic acid at 20 ° C. is very slow. As temperature increases, diffusion also increases rapidly; The diffusion of HAc through the membrane is a much higher factor 10,000 at 90 ° C than at 60 ° C. This property corresponds to the loss of coating resistance that occurs at temperatures above 60 ° C. during sterilization of the polymer coated steel DRD-can.

Hot air oven heating of polymer

Table 2 shows the heat treatment effect in a standard hot air oven. It can be seen that the main improvement is over the melting temperature of the polymer.

Performance of polymer-coated cans for 60 minutes after the cans were heat treated and exposed to 5% by weight acetic acid at 100 ° C. 5 ℃ 90 minutes bad 125 ℃ 5 minutes bad 170 ° C 5 minutes slightly better Slightly better 5 minutes at 220 ℃ 260 ℃ 5 minutes good, no corrosion 10 ℃ 90 minutes bad 10 minutes at 125 ℃ 170 ° C 10 minutes slightly better Slightly better 10 minutes at 220 ℃ 260 ℃ 10 minutes good, no corrosion 90 ℃ 25 minutes bad 25 ℃ 25 minutes bad 170 ° C 25 minutes slightly better 220 ° C 25 minutes slightly better 260 ℃ 25 minutes good, no corrosion 90 ℃ 50 minutes bad 125 ℃ 50 minutes bad 170 ℃ 50 minutes slightly better Slightly better for 50 min at 220 ℃ 260 ℃ 50 minutes good, no corrosion

As mentioned above, even if the results seem acceptable, embrittlement of the coating material renders the method unusable even when used for a very short time.

Induction heating of polymers

The DRD-can was inductively heated to investigate the melting properties of the polymer coating. Table 3 shows the different treatments for melting the polymer and its success.

Different treatments for melting the polymer Coating material thickness (㎛) Inductor Power (kW) Heat time (seconds) Polymer phase visible as a result 20 10 4 No melting zone 20 10 5 No melting zone 20 10 6 No melting zone 20 10 10 Weak yellowing (decomposition) of coating material 20 20 2 No melting zone 20 20 3 No melting zone 20 20 4 Good, fully melted 20 20 5 Good, fully melted 20 20 6 Weak yellowing (decomposition) of coating material 20 20 10 Coating disassembly 20 40 2 Weak yellowing (decomposition) of coating material 20 40 3 Weak yellowing (decomposition) of coating material 30 20 4 No melting zone 30 20 5 Good, fully melted 30 40 2 No melting zone 30 40 3 Good, fully melted 30 40 4 Weak yellowing (decomposition) of coating material

All completely melted coatings were tested to ensure good resistance to 1.5% by weight acetic acid (90 min test at 121 ° C.) to 5% by weight acetic acid (1 hour test at 100 ° C.). For the unmelted cans the coating was completely separated in the test and the can turned black due to the accumulation of corrosion products.

1 shows two cans exposed to 1.5 wt% acetic acid at 121 ° C. for 90 minutes. Cans without induction heat treatment (left) show corrosion and stacking separation on the entire surface where the black color is apparent. Cans with induction heat treatment (right) showed no corrosion or delamination.

Example 2

A series of pack tests was run to evaluate the results. Two polyester coatings were tested, transparent and white PET coatings. Steel is a deep drawn can made of a material coated on both sides with a polyester layer. The parts of the can were then instant heat treated. Experiments A, B, C and D were subjected to instant heat treatment, and controls 1 and 2 were not treated.

The cans were then charged directly to the test medium or subjected to further heat treatment. The heat treatment used is a crystallization process or print simulation in which the can is heat treated at 170 ° C. for 5 minutes.

The crystallization process yielded PET with maximum crystallization. These experiments are run to assess the effect of crystallization on performance. One of the controls (Control 2) performs this treatment. Simulation of print curing is performed to evaluate the effect of ink curing used to decorate the can. 40 minutes is selected as the ink cure time, which is 20 minutes for commonly performed experiments, 20 minutes for ink cure and 20 minutes for cure to enhance gloss.

Cans are filled with simulants containing chemicals that have a strong impact on the performance of commercial foods or cans. After filling, the cans are sealed and sterilized or pasteurized and stored in a temperature controlled area at 20 ° C. for 6 months.

The results are shown in Table 4 below:

Fillings on the market Simulant 85 ℃ 30 minutes 115 ℃ 45 minutes 121 ℃ 90 minutes 115 ℃ 45 minutes 85 ℃ 30 minutes 115 ℃ 45 minutes 115 ℃ 45 minutes type Pretreatment of cans  Sour pickles  Chicharilos Inn Escabece  1% HAc  1% HAc  1% HAc  1% HAc + 0.4% NaCl  2.5% HAc material Judo crystallization Print simulation Control group 1 Transparency Did not 4 One 4 One Control 2 Transparency Did not 170 ℃ 5 minutes 4 2 4 Example A Transparency Did 5 5 5 4 5 3 Example B Transparency Did 170 ℃ 5 minutes 5 5 5 4 5 4 2 Example C Transparency Did 170 ℃ 40 minutes 5 5 5 5 4 5 Example D White Did 5 5 4 5

Escabece is the result of three months.

115 ° C with 2.5% HAc 45 min is the result of 4 months after just 6 months of drilling.

0 = perforation

1 = severe corrosion

2 = severe corrosion

3 = slightly corrosive

4 = almost no corrosion

5 = no corrosion

It has been evident that instantaneous heat treatment strongly improves the performance of the coating in all cases. This is further illustrated in Figures 2 and 3, in which the untreated and processed cans are shown in accordance with the present invention, respectively, wherein the cans in the upper photo are stored for 4 months and the cans in the lower photo are stored for 1 month.

Commercially available fillers showed that chicharrillos in escabeche had already corroded three months after being placed in an untreated can, whereas the treated can no longer exhibited corrosion.

Crystallization treatment shows some improvement. This effect is most evident at 2.5% HAc.

The effects observed with HAc concentration and sterilization with respect to time and temperature vary. 2.5% HAc cans are good after 4 months, but after 6 months all cans are completely corroded and consequently punctured. Perforations were not observed when a lower acetic acid concentration was used as the filler material.

As a result, a very weak difference can be seen between the white and the transparent coating.

In FIG. 4, heat treatment including instantaneous heat treatment FH and cooling is described. The horizontal axis represents time and the vertical axis represents temperature. It is important to shorten the period of time for which the material is above the melting temperature (Tm). In the examples a heating period of several seconds is selected (including heating but not cooling). The can is cooled after the instantaneous heating FH. The cooling time can vary. Several heat treatment curves are illustrated in FIG. 4, showing 1, 2 and 3. Line 3 can be applied in Example 2. The coated cans cool down quickly. During cooling the coating passes through the crystallization zone. The crystallization temperature Tc for PET is approximately 160 ° C. depending on various conditions and the correct formulation. In FIG. 4 it is evident that the crystallization zone time in line 3 is relatively short so as to lead to a coating with little or no crystallization. But if line 1 is followed, the material will be in the crystallization zone for a very long time. This allows the polyester to crystallize more. By choosing the right treatment it is possible to optimize the effect of the can after sterilization and storage, especially with respect to the selected formulation of the composition of the food and the polymer.

Claims (4)

A method of inhibiting attack of organic acids such as acetic acid in a metal container body, a metal container end, or a thermoplastic polymer coated on the metal container body and the end, Packaging the organic acid-containing material by heating the polymer on the components above their melting temperature, including flash heat treating the entire metal part of the polymer coated container in contact with the organic acid. A method for producing a container suitable for use. The method of claim 1, And holding the polymer above the melting temperature of the polymer for up to 10 seconds, preferably up to 5 seconds. The method according to claim 1 or 2, Heating the polymer above the melting temperature of the polymer by induction heating. The method according to any one of claims 1 to 3, A process for holding the container at or below the melting temperature of the polymer, preferably at a temperature in which the crystallization of the polymer takes place after the instant heat treatment.

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