We answer your questions
Solvent separation of aluminium pastes is no quality problem, there can be two reasons: A real settling of the aluminium pigments or a condensation of the solvent (or a combination of both).
The absorption of the solvent from the pigment depends directly on the surface area of the pigment. Coarse pig-ments have a smaller surface than fine ones; silverdollars have a smaller and smoother surface than cornflakes in the same fineness. This means, coarse silverdollars are potentially the product most affected from this issue.
If the pigment is stored or transported with big differences in temperature - very hot during the day, cool in the night – the solvent is evaporated and condensates at the lid, from where it is dropping back to the pigment in the drum.
Vibrations during transport (up to heavy shaking on the truck depending on the road conditions) force a kind of settling. That does not result in a solvent separation in every case, but it makes the paste compact and equalizes the surface of the paste. The condensate solvent drops back to this even surface creating little lakes or, if the amount is higher, a solvent layer is created.
With plastic liners the paste sweats even more and in addition the solvent cannot drop back into the paste, so the "separation", which is just a function of the temperature, is more intensive. As long as the complete content of the drum is used in one, no loss of quality will occur. If not the whole drum content is needed at the same time, a cus-tomer-specific solution is recommended. In this case, please contact our Technical Service.
Gold bronze pigments consist of copper or copper/zinc alloys (brass). If the binders or additives used in the coating contain acids, copper ions may be flushed out, and this can lead to the gelling effect mentioned. This problem can be avoided by using only coating components with a very low acid index (guide value: acid index < 10 mg KOH/g). Encapsulated gold bronze pigments, which are sold under the name Standart gold bronze powder Resist and our new product line Standart Gold bronze Powder DOROLAN, are much more resistant to gelling.
Gold bronze pigments are highly temperature-sensitive. The colour changes at a temperature as low as approx. 80°C. "Normal" gold bronze pigments can thus only be used in air-drying coatings that are not subsequently exposed to high temperatures (e.g. decorative effects). A considerable improvement in colour stability is achieved by using encapsulated gold bronze pigments that are sold under the name Standart Gold bronze Powder Resist.
Gold bronze pigments for aqueous systems are available under the product name, RotoVario aqua. These are pigment concentrates that, in addition to the gold bronze pigments, contain mainly water, together with small quantities of anti-settling agents and additives.
There are three possible causes for the formation of gas in aqueous metallic coatings. These three causes are as follows:
- High shearing forces were applied when dispersing the aluminium pigments into the coating system; this damages the aluminium pigments. This means there are unprotected aluminium surfaces, which lead to the formation of gas. Usual conditions for dispersing aluminium pigments are to produce a pigment slurry consisting of the aluminium pigment, as well as a suitable solvent and possibly a dispersing additive. These components are then stirred carefully for approximately 20 minutes at shearing rates of 500 - 1000 rpm. Subsequent storage at room temperature can also be helpful.
- As an amphoteric metal, aluminium reacts both in the acidic and alkaline range, it is therefore important that, when formulating the coating, the pH value is in the 7 - 8 range. DMEA, TEA or AMP from Angus Chemicals have proven useful as neutralising agents.
- Despite all the described measures, as a result of the coating components used, the stabilisation methods applied to the aluminium may still be insufficient to prevent the formation of gas. In this case, a more stabilised aluminium pigment has to be used. In such cases, chrome-encapsulated qualities (Stapa Hydrolux), as well as chrome-free encapsulation in the form of a silicate coating with subsequent additive treatment (Stapa Hydrolan), have proven successful.
Due to the poor wetting behaviour of water, wetting agents are often used when dispersing aluminium pigments in aqueous coating systems. These lead, to varying degrees, to a deterioration in the humidity resistance. Exchanging certain tensides for others can result in a considerable improvement. Additives are used with some stabilised aluminium pigments in order to suppress the development of gas. These additives also lead to a reduction in the resistance to condensation. In such cases, the use of either encapsulated aluminium pigments (Stapa Hydrolux, Stapa Hydrolan) or aluminium pigments stabilized with a smaller amount of additives is recommended.
The Stapa© Aloxal types are oxidized Aluminium pigments. This oxid layer works indeed in one way as an inhibitor, but the pigments are not additionally stabilized.
In the most less aggressive paint formulations the oxid layer is sufficient to get an acceptable gassing stability, in a few more aggressive formulations it is not. The gassing stability has to be tested in the customers own binder system.
Ideal storage conditions for aluminium pigments are in the original closed drum at a room temperature of 25°C. As a rule, if aluminium pastes that are usually manufactured as pastes are stored incorrectly, the solvent can evaporate. This then leads to changed optical effects or to an agglomeration of the flakes. If the drum is opened, it should be carefully closed after use. It has also proved best to use all the contents of a drum at once. In the case of drums containing aluminium paste residues, increased agglomeration or solvent evaporation has been observed.
Product / Product group - Shelf life
Stapa Aluminium Paste leafing - 12 months
Stapa Aluminium Paste non-leafing - 12 months
Stapa Metallic - Mobilux - Metallux - 12 months
Stapa Hydrolac - 12 months
Stapa Hydroxal - 12 months
Stapa Hydrolux - 6 months
Stapa IL Hydrolan - 12 months
Stapa .. Reflexal (for plastics) - 12 months
Stapa .. Reflexal (for printing inks) - 6 months
Stapa .. PCR - 6 months
Stapa .. Aluminium Powder (pasted) - 6 months
Stapa Aloxal - 12 months
When using leafing aluminium pigments, non-polar solvents and binders should always be used (e.g. white spirit, solvent naphtha, toluene, xylene or hydrocarbon resins). All binders used should have as low as possible an acidic value (guide value: acid index < 10 mg KOH/g). and should not contain any functional groups. Any moisture in drums or in solvents should be avoided; this is particularly important during production.
If not stored under optimum conditions or because of temperature oscillation during transport, lubricant from the manufacturing process may collect on the surface of the paste and subsequently "cristallize" at lower temperatures. In this case, the best thing to do is to use the complete drum.
In general the pigmentation rate is between 5% and 15% aluminium acc. to the complete formulation. However, this depends greatly on the particle size distribution of the product, on the coating parameters and the requested effect. The coarser the product, the greater the pigmentation required in order, for example, to achieve the desired coverage at the same layer thickness.
Yes, however, in this case, special products have to be used that have been especially stabilised for this purpose, such as STAPA Hydroxal, STAPA Hydrolac, Stapa Hydrolux or STAPA Hydrolan. Unstabilised aluminium reacts with water to generate hydrogen according to the following reaction equation:
2 Al + 6 H2O ^ 2 Al(OH)3 + 3H2
This reaction can leads to a build-up of pressure in the drum and to the loss / deterioration of the optical properties.
In both cases we recommend pre-dispersing the pigments in a suitable solvent. With pastes, a weight ratio of 1:1 (paste : solvent) is recommended and, with powders, a weight ratio of approx. 1:2 (powder : solvent). In addition, it should be noted that aluminium pastes / aluminium powders have a limited shelf life. If the product is well past the expiry date, or if the drum has been stored incorrectly, this may lead to agglomeration, which causes the poor dispersion properties.
There are two possible explanations for this effect:
- The choice of pigments may be unsuitable for the required use, i.e. the selected type has too high a proportion of fine particles.
- The fine particles in the aluminium pigment are created as a result of high shear forces during dispersion (pearl mill). In order to prevent this effect, high shear forces should be avoided in general.