41.1 New Products - Nano Estel: research in progress

30/07/15

Defining Nano Estel as a "new product" is a little bit forced, since that product has been added to our catalog in 2011. However, it is hardly surprising that restoration sector is rather "conservative" toward novelties. On the other hand, being cautious is reasonable; as a matter of fact we have noticed some innovations did not live up to what they promised; in addition to that, the value of the artifact subjected to the treatment sets limits to any kind of experimentation. As a consequence, we should rely on products that have already successfully passed the largest number of tests.
For the following two main reasons we promised ourselves to monitor one of the most interesting products, i.e. Nano Estel and keep us updated on a regular basis:
- as a consolidating agent that generates silica during its action, Nano Estel is considered an irreversible agent, and because of that, it is similar to the ethyl-silicate;
- in recent years the market has been flooded with dozens of nano-structured products, which profit by the terminology and claim high performances without the proper support of scientific research.

For this reason let's take up CTS Bulletin no. 29.1 (Nano Estel: early research findings), and let's e
nrich those pieces of information through the latest scientific outcomes, obtained by some Portuguese scientists.

1- Determination of the kinetics of drying

A study by Musacchi and Diaz Gonçalves [1] has compared two consolidating systems based on nano-particles (nano-lime Calosil E25 and nano-silica Nano Estel), applied on three different kind of stones, with the intended purpose to evaluate any variation of the rate of drying. As you know, some treatments make this process more difficult, with the result that water stays longer inside the treated stone, producing negative consequences in terms of exploitation of the chromatic aspect (it turns dark) and microbiological attack.
The study was conducted as a part of the 
Drymass project (Ref. PTDC /ECM/100553/2008), with the purpose to verify other results obtained within the same project [2], according to which an increase of the kinetics of drying is correlated to the application of lime coating. In addition, other authors had already experienced the same acceleration in drying by means of nano-structured limes coating (Nanorestore and Calosil).
The tests were carried out on different specimens as follow: a mortar based lime (capillary porosity = 20.8 % 
±0.4); a soft and porous stone from Coimbra, Portugal(Ançã, c.porosity = 22.9 % ±0.6); and a porous quartz-rich stone from Germany (Bentheimer, c.porosity =17.7 % ±0.7). Photo [1] shows the specimens at the time of the aging process. The drying kinetic profiles were determined according to the RILEM procedure (RILEM 1980). Despite the initial mistake of the scientists when considering Nano Estel as a ready-to use product, it seems the two products do not affect the kinetics of drying. Actually, as reported by the technical data sheet, you are supposed to dilute the product. The use of the pure form is limited only to extremely ruined and absorbent substrates.
We can conclude that the Nano Estel does NOT produce unwanted side-effects, even in the case of excessive treatment.

2- Comparison between nano-silica and a modified ethyl silicate
Approaching the study by Borsoi, Da Veiga and Santos Silva [3], we should keep in mind the properties of nano-silica were compared with those of the ethyl silicate (TEOS) that had been modified by adding nano structured limes. In that case the substrates for the tests were made of mortars poor in binder (binder: inert = 1:4), prepared in such a way as to simulate a degraded and porous mortar. Then they prepared three series of samples:

- not treated (NT)
- treated with a mixture of nano-strucured limes (Nanorestore, 95 %) and TEOS (Estel 1000, 5 %), with a final dry weight of 24 g/l (NLSE).
- Treated with nano-silica (Nano Estel diluted in water (1:8), with a final dry weight of 57 g/l (NS).

The products were sprayed through a sequence of 10 applications, with a pause in between to let them to be absorbed. After being seasoned for 90 days the samples were subjected to a series of measurements as described below. The mechanical resistance measurements revealed that both treatments increased the numerical values considerably. In addition, there was a moderate improvement of the hardness of the surfaces. That moderate increase is preferred because the consolidation agents can go deeper and interact with the inner layers of the substrates; that means a better distribution of the product inside the specimen.


Treatment flexural strength (N/mm2) compression strength (N/mm2) surface hardness (Shore A)
NT 0.28 ±0.03 0.43 ±0.02 73
NLSE 0.38 ±0.04 0.66 ±0.04 79 (8 %)
NS 0.43 ±0.05 0.73 ±0.04 82.5 (13 %)

Even if the consolidants under observation are not hydrophobic, the rate of absorption of water was determined by the Karsten tube. Here are the results:

Treatment Rate of absorption in seconds plus standard deviation
NT 36.50 ± 4.92
NLSE 40.16 ± 7.43
NS 62.33 ±13.04

As we know, the ethyl silicate loses his hydrophobic nature after a few weeks; so, after 90 days from the treatment, the value for the specimens of the NLSE series is almost back to the initial levels. It could be simply the consequence of a partial reduction of the pore volume, also occurred in the case of nano-silica, another product that does not possess an intrinsic hydrophobic effect. Moreover, it seems the mixture NLSE have perfectly adhered to the carbonate based substrate, as if the nano-structured lime had worked as an adhesion promoter, giving birth to calcium silicates. SEM-EDS analysis revealed that also the generated silica assume a different aspect, no longer as in form of flat plates but filaments instead, an evidence of the linear development of silica tetrahedrons.

In conclusion, both products proved to be effective on consolidation; on the other hand the main problem seems to be related to a surplus of the product left on the surface, which in the case of the NLSE leads to whitening, while in the case of nano-silica leads to glossy effect. Even if both the chromatic effects are not detectable by the naked eye, they represent the evidence of a not complete absorption.


Finally, there is also some concern that the researchers adopted "the heavy hand". We think the 10 spray shots sequence (diluted the product may be) seems to be excessive; a restorer in the team could have limited the negative effects by dabbing the surfaces at the end of the application, for removing the excess of both products.
Despite the limits to penetration previously highlighted, the efficacy and compatibility of Nano Estel with all the examined substrates is certainly confirmed by the same studies.
The crucial point is related to the procedures. It is important to avoid any accumulation on the surface in order to prevent from preservation failures. Extreme care and attention are always required when dealing with the consolidation agents.


References

1. J. Musacchi, T. Diaz Gonçalves; “Influence of nano-lime and nano-silica consolidants in the drying kinetics of three porous building materials” Relatório 168/2014. 
2.V. Brito, T. Diaz Gonçalves; “Artisanal lime coatings and their influence on moisture transport during drying. 3rd Historic Mortars Conference, 11-14 September, Glasgow, Scotland, 2013. 
3.G. Borsoi, R. Veiga, A. Santos Silva; “
Effect of nano-structured lime-based and silica-based products on the consolidation of historical renders”, 3rd Historic Mortars Conference 11-14 September 2013, Glasgow, Scotland
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