BG60671B1 - Method for surface treatment of products having vitreous structures - Google Patents

Abstract

The method is used in the glass industry. The resulting continuous coating on the inner walls of the product reliably prevents the migration of lead from the glass into the acidic drinks or food products placed in it. In the method, the vessel is filled with a thixotropic slip in the form of a viscous liquid, then emptied in such a way that its inner walls are coated with a continuous layer of the slip, which forms a coating adhering to their surface. The vessel together with the coating is heated to a temperature in the range of 3000C to the softening temperature of crystal or lead glass for several hours. A surface is obtained on the inner walls, and after the partial removal of lead ions from it, an aluminosilicate diffusion barrier is formed, which prevents the passage of lead in acidic drinks or food products by ion exchange between the glass and the surface containing an aluminosilicate layer. This is followed by cooling of the vessel and removal of the aluminosilicate layer containing lead by washing with water, sandblasting and/or ultrasound.

Description

The invention relates to a method for surface treatment of articles with a vitreous structure in order to prevent the migration of lead from glass into beverages such as cognac, whiskey, wine, fruit juices, as well as acidic food products placed in these articles . The method is used in the glass industry.

A method is known for making a crystal container for storing any acidic product in which, in order to prevent the lead from the crystal from migrating into the products and beverages, the container has a thin, continuous, lead-free film on its inner surface. glass forming a "screen".

This method contains the following steps:

- pre-separation of a certain amount of droplets without droplets;

- coating of unleaded glass with a certain amount of crystalline mass in excess, with the crystal and unleaded glass having almost the same coefficients of expansion and refraction;

- simultaneous blowing of the crystal and unleaded glass in the desired shape of the vessel having a cavity whose inner surface intended for storage of the product will be completely covered with a continuous unleaded glass film / 1 /.

Obtaining a thin film of unleaded glass to completely cover the inner surface of the vessel cannot be controlled, i. the expansion of unleaded glass by blowing does not produce a continuous film. The likelihood of openings in this film to allow lead to penetrate into the liquid is very high.

It is an object of the invention to provide a method for treating the surface of articles with a vitreous structure, according to which the protective coating on the inner walls of the lead-glass article is continuous and therefore the protection against lead migration is complete.

The problem is solved by a method of treating the surface of articles with vitreous structure, in particular lead and crystal glass vessels, the inner surface of which is exposed to acidic media, in order to prevent migration of lead in said acidic beverages and foodstuffs, according to which the following steps are carried out: filling the vessel with a thixotropic slip in the form of a viscous liquid; emptying the container in such a way that its inner walls are coated with a continuous layer of slip, which forms a coating adhering to the surface of these inner walls; heating the vessel together with the coating to a temperature in the range of 300 ° C to the softening temperature of the crystalline or lead glass for several hours, thereby forming a surface on the inner walls and, after partial removal of the lead ions, from it is formed aluminosilicate a diffusion barrier which prevents the passage of lead into acidic beverages or foodstuffs by ion exchange between the glass and said surface containing alumosilicate deposition resulting from heating; cooling the vessel and finally removing the alumosilicate deposition containing lead obtained by heating by washing with water, sandblasting and / or ultrasound.

The slurry is a viscous aqueous suspension of a compound selected from the group comprising phyllosilicates and tectosilicates, such as kaolin or any compound that contains substantially pure hydrated aluminum silicate.

The slurry is preferably an aqueous slurry of kaolin and the coating obtained from this slurry on the inner walls of the vessel is heated at a temperature in the range 400-440 ° C for 4-6 hours.

The amount of water in said aqueous suspension of kaolin is from 40 to 60% by weight.

It is known that any liquid contained in a container made of any material tends to dissolve a certain amount of the chemical constituents of that material. This amount is usually extremely small. For example, water placed in a vessel made of sodium-calcium glass dissolves some Na ₂ O from the glass. The standard test (DIN 1211) allows comparison of the hydrolytic stability of the above-mentioned sodium-calcium glasses / solubility from 30 to 1000 mg of Na ₂ O per g of sodium-calcium glass at 90 ° C for 1 b /.

This phenomenon should be taken into account in analytical chemistry and microchemistry, but it is of little importance in everyday life except in some special cases such as those with heavy metals such as lead, barium and cadmium, which tend to migrate into food, when they are an integral part of materials intended for the manufacture of household or beverage containers. In addition, the crystal contains lead oxide (Afnor standart: NF 30,004) and it has been found that a certain amount of lead up to several ppm can pass into acidic products (pH = 3.5) when these products are stored in crystalline vessels for a long time time.

This phenomenon has been the subject of extensive research and has led to the creation of international norms in order to protect the population from the possible dangers posed by the contact between the food and the surface of glass objects used in the preparation, serving and preserving of food and beverages (International standart ISO 7086 / 1-1982).

In some countries, health authorities have very high requirements for the maximum lead content in alcoholic beverages, which creates problems for both crystal vessel manufacturers and expensive beverage vendors offering their products in such containers.

This method is based on a completely different principle from the known method, which consists solely in isolating the glass from the liquid, aiming to create on the surface of the glass (intended to contact the liquid and after separating the lead ions contained in that surface zone) diffusion barrier preventing the passage of lead ions from the crystal in the middle.

The method used is based on the ion exchange technique, and in this case in particular the exchange of cations between glass containing lead and alkali cations and a substance containing cations H ⁺ and AL ³⁺ brought into contact with the crystal, all of which take place under certain conditions.

It is known that the hydrolytic attack of glass in an acidic medium can be represented as an ion exchange between an H ⁺ ion in excess in solution and the mobile cations of the vitreous network, leaving silica virtually unchanged. Such an exchange is practically restricted by network modifiers (alkali and alkaline earth metals) and some heavy metals such as lead, cadmium and barium (insofar as they are present in the substance).

In fact, in the field of lead glass and crystal compositions, the kinetics of ion exchange between lead from the wall and the cations in the liquid follow Fick's second law, namely the amount of lead passing into the solution is proportional to the square root of the contact time, whereas the diffusion coefficient, in turn, is the Arrhenius function of temperature. On the other hand, the extraction of vessels from acidic solutions can only be carried out at relatively low temperatures, lower than the boiling point of the liquid used under operating conditions, and a contact duration limited by industry requirements. In addition, security requirements create difficulties in dealing with concentrated acids.

The result is that in all cases the amount of effectively extracted lead is very small and the re-contact of the acidic liquid with the vessel so treated is again associated with the diffusion of lead into the liquid, which becomes measurable after a few days: the diffusion rate remains practically the same , as well as before treatment, and in practice the manifestation of the phenomenon slows down slightly, which is completely insufficient from the point of view of storage up to several years.

The method according to the invention is based on these preliminary methods and results preventing the migration of lead contained in lead or crystalline vessels from the walls of the vessel into the liquid stored therein for an extended period, for example, for alcoholic beverages stored in crystalline vessels, and also for a relatively short period, for example, for highly acidic products containing vinegar (sauces, mustard).

To obtain this result, the process of the invention requires, on the one hand, high temperatures and, on the other, compounds capable of exchanging or releasing H ⁺ and AL ³⁺ ions at high temperature.

The first explanation of the phenomenon underlying the invention is the following: all these crystalline bodies are characterized by a structural arrangement of octahedral and tetrahedral types, whose total electrical load is lacking in positive loads due to the replacement of Si ⁴⁺ cations with less loaded cations such as AL ³ *. The excess of negative loads is therefore offset by other cations, mainly monovalent (H ⁺ , alkali metals) and sometimes bivalent (alkaline earth metals). Such cation-compensators are interchangeable with other types. The number of compensated negative cargoes with exchange cations is called "cation exchange capacity" / CCC / or "basic exchange capacity" / CCC /.

It should be noted that since 1945 / 2,3,4 / some authors have shown some possibilities for high temperature ion exchange between the H ⁺ ions of metakaolin and the alkali metal ions of sodium-calcium glasses and have even provided for the application its, namely, the improvement of the strength and some other properties (mechanical and electrical) of these sodium-calcium glasses. The additional manipulations required by this method have led to its abandonment, since the same results are achieved by more convenient methods.

These preliminary studies were limited by the exchange between sodium in the calcium-calcium glasses and the H ⁺ ions of metakaolin, since the difference between such ion exchange at high temperature and extraction at room temperature, as noted above, was found to be found place in the kinetics of the process, on the one hand, and in the state of the surface obtained after processing, on the other.

Thus, it is found that by the method according to the invention at given temperatures between 30 ° C and the temperature of solidification of lead glass or crystal, only a few hours of contact are sufficient to cause diffusion of a certain amount of lead from glass or crystal into the silicon aluminum layer.

In fact, surface analysis of ESCA and SIMS spectrometers show that during this process there is an additional complex exchange between K * and Rb ²⁺ ions of the crystal on the one hand and A1 ³⁺ ions of kaolin on the other. -complicated from the one on which the previous hypothesis is based and generates in the wall of the crystalline vessel a surface silico-aluminum layer about 100 nm thick (1000A) that creates an interphase barrier preventing the internal diffusion of Pb ²⁺ ions from the vessel and H ⁺ ions from the acidic liquid. Kaolin aluminum is involved in the formation of this stable surface layer, which remains after heat treatment at 120 ° C for 17 hours, corresponding to aging at room temperature for several years without contact with any liquid.

The silica layer that remains on the wall at the end of the treatment is removed, for example, by washing with water, ending with sand washing, ultrasonic treatment, etc.

The advantage of the method is the conditions of its application, which allow it to be carried out for any type of lead glass or crystal with conventional technique, as well as obtaining a continuous layer, providing complete protection against the migration of lead through the inner walls of the glass products.

The invention is illustrated by the following exemplary embodiments.

Example 1.

The example illustrates a specific case of application of the method according to the invention for processing crystalline carafes with 30% lead content intended for storage of cognac.

These carafes have a volume of 750 ml. The slurry is made from industrial kaolin, marketed in France by Societe Lambert under the name “Kaolin heavy”. This product is of high purity and its maximum impurities are:

Calcium 250 ppm

Heavy metals 25 ppm

Chlorides 250 ppm

Sulphates 0% by weight

Acid-soluble substances 10 mg

Other physical and chemical characteristics satisfy the 1974 Europian Pharmacopee standards.

After mixing 53% water and 47% kaolin, the slurry is introduced into the casing and stirred to apply it evenly over the entire inner surface. The excess is poured.

Thixotropy of the slip allows to cover the entire surface of the vessel with a uniform layer without subsequently spreading.

The oven is placed in an oven, the temperature is gradually raised to 400-440 ° C, held at this temperature for 4-6 hours and then slowly cooled.

The slurry is first removed by washing with water so as to wash away the larger part and then with a slurry of sand to remove any remaining traces so that the vessel returns to its original appearance.

It should be noted that the treatment temperature is close to the point of solidification of the crystal (about 440 ° C), but lower to avoid surface alteration or additional bonding of the kaolin after cooling.

The amount of water in the slip can vary widely depending on various factors such as:

- the nature of the kaolin used;

- granulometric composition;

- mode of administration (slurry or liquid suspension);

- additives providing thixotropy;

- the geometric shape of the vessel.

An interval of 40 to 60% by weight is optimal.

In order to place the vessel thus treated in a comparative stability test, it was filled with a 4% aqueous solution of acetic acid 5 and the concentration of lead in this solution was measured by atomic absorption at 24 h intervals.

At the same time, the same measurements were carried out on an identical carcase but untreated according to the invention.

After 750 hours of contact / one month / the following results are observed:

Garaf Lead content in solution

Treated Not detected Raw 1,70 mg / 1

This test is proof of the efficiency of the method.

The amount of lead converted to kaolin during the treatment process is 2-3 mg.

Example 2.

This example shows that the efficiency of the same treatment is sufficient to allow the preserved liquids in the treated vessels to meet the requirements of sanitary standards.

To this end, in a series of experiments carried out for 6, 12, 24 and 60 months (five years) using 4% acetic acid (pH = 2.30) as a liquid and a commercial alcoholic beverage (pH = 3) , 5 /, the results presented in Table 1 are obtained and illustrated by the curves grouped in the scheme of FIG. 1:

Table 1.

A 4% acetic acid test according to the method described in Example 1.

B test with alcohol (initial lead content 16.8 g / l).

Duration of trials Lead content in the liquid, μξ, / \ 6 months 12 months 24 months 36 months 5 years A / test with acetic acid 31.0 43,8 62,0 76,0 98 B / alcohol test / 13 18.3 25.9 31.6 40

These results (Table 1 and curves A and B) lead to the following conclusions.

First of all, the five-year test comes with an almost unnecessary guarantee. In fact, the age of an alcoholic beverage such as cognac, as it stands on a crystal vial, does not in any way mean that it has matured throughout the aging period in the vial, which normally takes place in damajani or barrels (usually in oak barrels). The carafe is filled at the moment before being put on sale, regardless of the age of the cognac and the actual duration of contact between the alcohol and the crystal is only during its display in the store. The duration of consumption by the buyer logically rarely exceeds several months and is at most 2-3 years.

Therefore, the above tests were actually performed for up to 12 months, then extended for up to 36 months. and finally extrapolated to 5 years, following Fick's law, officially adopted in the field of ion migration, whose value is calculated as the square root of duration.

Secondly, it will be noted that the amount of lead passing into the solution is significantly less elevated in alcohol than in acetic acid. This is explained by the higher pH and the presence of other ingredients in the alcohol.

Finally, it is established, and this observation is most important, that the highest content of lead in alcohol reaches at the end of the fifth year not more than 40 g / Ι, which is twice lower than the maximum content currently required.

It can be argued that the method according to the invention, applied in the most extreme circumstances, also guarantees the most stringent requirements of the law.

The method according to the invention effectively achieves the desired result, i. prevent the migration of heavy metals from a glassy substance such as a crystal into a liquid that is in contact with the crystal, for example alcohol (cognac).

The invention is based on experimental results of the application of the method, which cannot be limited by theoretical or hypothetical explanations. Only a few hypothetical options can be identified to explain the efficiency of the method:

1. Simple exchange of H ⁺ / Pb ²⁺ between crystal and kaolin. This surface depletion of lead from the crystal will significantly reduce further migration to the liquid.

2. A much more complex exchange between the most important cations of the crystal and kaolin due to the formation of a diffusion silico-aluminum barrier on the surface of the crystal, impeding further migration of lead.

It is quite possible for these two ingredients to exist simultaneously. The former prevails at the beginning of processing, ie. initially, in the absence of the barrier, lead-to-kaolin migration will take place, and this surface modification is desirable. Then, with the development of the method, there is an emergence and development of a barrier preventing the further migration of lead from the crystal to the liquid.

Claims (4)

A method for treating the surface of articles of glassy structure and, in particular, of lead and crystal glass vessels, comprising the steps of: filling the vessel with a thixotropic slurry of pure hydrated aluminosilicate in the form of a viscous liquid emptying the container in such a way that its inner walls are coated with a continuous layer of slurry, which forms a coating adhering to the surface of these inner walls, heating the vessel together with the coating to a temperature in the range of 300 Up to the softening temperature of the crystal or lead glass, which gives a coating on said inner walls and after partial removal of lead ions from it, an alumosilicate diffusion barrier is formed against the passage of lead into acidic beverages or foodstuffs by ion exchange between the glass and said surface containing alumosilicate deposition resulting from heating, cooling the vessel, and finally removal of alumosilicate deposition containing lead resulting from empowered heating, by washing with water, sandblasting and / or ultrasound. A method according to claim 1, characterized in that the slurry is a viscous aqueous suspension of a compound selected from the group comprising phyllosilicates and tectosilicates, such as kaolin or any compound containing pure hydrated aluminum silicate. Method according to claim 2, characterized in that the slurry is an aqueous suspension of kaolin and the coating obtained from this suspension on the inner walls of the vessel is heated at a temperature in the range 400-440'C for 4- 6 h. 4. The method of claim 3, wherein the amount of water in the slurry is 40-60% by weight.