M. Gajo, G. Gajo
Elsy Research s.n.c., V. Mons. Zoppas, 20 - 31020 Scomigo di Conegliano (Treviso).
A. Zingales, G. Quartarone, T.Bellomi
Università di Venezia - Dipartimento di Chimica
Università di Padova - Dipartimento di Ingegneria Meccanica
This article is a new edition of a publication made in 1994 in which already the problem of nickel-free was examined. The old publication had a technical character and was based on working experience. This new article adds to working experience a more scientific cut, thanks to researches carried out in collaboration with the University of Padova and Venice, and faces the problem coming from the new European law regarding the release of nickel.
THE NEW LAW
With directive 94/27/CE, emanated in 1994, the European Parliament has established that the release of nickel from products intended to come into direct and prolonged contact with the skin does not have to be advanced to 0.5 µg/cm2/week, but does not establish the criteria with which testing the nickel release.
The new law have been published the 20 July 1999 on the European official gazette and establishes the parameters to measure the release of nickel, that is through norms EN1810, EN1811 and EN12472.
Norm EN1811 establishes the criteria with which the test to the artificial sweat must be carried out on the objects: the pre-treatment, the formula of synthetic, duration and temperature of the test.
Norm EN1810 indicates in that way must be carried out the analysis of the nickel dissolved in the solution of synthetic sweat, that is through atomic absorption.
The true new is represented by the norm EN12472: it contains the method for the simulation of wear and corrosion to carry out before the artificial sweat test. The law establishes that the objects to test must endure a test of wear and corrosion of 4 hours in corundum. It has been calculated that such test is removing more or less 7 µ of material, including coatings of noble metal.
As said before, the new law has been published on the European official gazette the 20 July 1999 and has been in force the 20 January 2000. Manufacturers have 6 months from the coming in force of the law to produce spectacle frames according to law, that is until the 20 July 2000. Detailers have time until July 2001 to end old supplies.
There are strong oppositions to this new law, as actually more or less all spectacle frames production would be out of law. Moreover, that law is considered too strong for spectacle frames.
It is not so important which directive will be definitively approved, what it is sure is that in the meantime it is convenient to examine the possible solutions.
The first solution is very simple, that is to protect spectacle frames with a double layer of lacquer and to maintain to unchanged all the other parameters of production.
This solution, ventilated from more parts and for this reason cited here, is ineffective. In fact, the coating of varnish it does not cover the material in homogenous way, but it will coat the spectacle frames as indicated in Fig. 1 because of a famous physical phenomenon that is “capillarity”, that it consists in the tendency of liquids to aggregate in the lower energy level. That means the alive angles will be less protected and therefore easy attachable during the wear and corrosion test. Therefore such solution does not give guarantees against disagreeable surprises.
Fig. 1: comparison between homogenous coating (on the left) and that one obtained with a coating lacquer
As showed before, it is not suitable to operate only modifying the thickness of lacquer. That means it is necessary also to deposit a galvanic coating at least 10 µ totally nickel-free. 10 µ of coating are necessary to create an effective barrier layer against nickel contained in the base material.
Now the problem becomes what to deposit instead of nickel to have a surface levelled, bright and keeping the costs to acceptable levels. That excludes noble metals, which, for cost reasons, cannot be deposited at the thickness required.
The only metal in position to replace nickel is copper, as its cost is comparable to nickel, but copper has the following problems:
• It is an optimal levelling but not brightener
• If it is not blocked adequately, it tends to " migrate " towards the surface making oxidation.
Therefore copper must be protected with a coating that makes the surface bright and that forms an excellent layer barrier.
One of the solutions provided was a coating of pure palladium, but it is easy polluted and now it is too expensive to use because of the cost of the metal.
After eight years of working experience and researches in laboratory in collaboration with the University of Venice and Padova, it is possible to assert that an ideal protecting layer for copper is constituted from a coating of “white bronze” which it has the following advantages:
- Same aspect than a coating of palladium.
- It stops totally the migration of copper towards the surface.
- Good resistance to corrosion. For example, in jewellery market it is used like final layer without protective lacquer (experience made on objects carried on persons).
- It gives to the base material the typical brightness of nickel.
- Easy to strip in case of mistakes (contrary to palladium).
- It is a germicidal.
- Low cost.
More informations about white bronze coatings can be found in the publication made in the N° 5 of 1999 of Vedere Tech). 10 µ of copper plus 2 µ of white bronze give all guarantees to have spectacle frames according to law.
Moreover, it is important to observe that unlike noticed in the case of lacquer coating (first solution), where on alive angles the thickness was lower, in the case of electrochemical deposition there is the opposite effect: the thickness is higher on the alive angles because of the higher current density (“tips effect”, Fig. 2). Therefore, the galvanic coating goes to compensate the imbalance produced during the lacquer deposition.
Fig. 2: homogenous coating (on the left) and typical deposition of a galvanic bath
It is important to remember that the target of this galvanic treatment is to replace the nickel. Therefore, even if the final decorative effect is of all similar to a palladium coating, it cannot be expected that an alloy copper-tin-zinc has the characteristics of a noble metal regarding resistance to wear and corrosion. That treatment demands a protective coating, considering the excellent characteristics of the alloy, can be also one simple lacquer coating.
It is the most radical solution: to produce spectacle frames with a nickel-free raw material. A valid example is represented from the Valbruna company that produces steel alloys without nickel. In this case, the thickness of galvanic coating is not important, provided that it is nickel-free.
Independently from the norm that will be confirmed, it is becoming more and more frequent the demand for spectacle frames free at low release nickel or nickel-free. The tendency in the coming years will be to eliminate such metal from the production, so that a study to orientate the production in this direction is not only a good commercial target, but is becoming necessary.