35.1 I am here too – a faultless protective: Silo 111.


We are always attracted by innovations for two main reasons: firstly we wish to be continuously updated, then we are looking for something that let us 'forget' the solutions of the past. If we take a glance at CTS old catalogs we may come across with CTS111, a classical product abandoned in the mid '90s, later revived and renamed as Silo111. That is a poly-dimethyl-siloxane. As a siloxane based product, it belongs to chemicals with a long history of tests and certifications. Earliest applications were recorded at the beginning of the '60s.

Let's have a look at the strengths and weaknesses of these materials.

First we need to be clear about the terminology; a siloxane is a functional group in organosilicon chemistry with the Si–O–Si linkage(According to IUPAC definition: “Saturated silicon-oxygen hydrides with unbranched or branched chains”). Siloxanes are also called polyorganic siloxanes and, depending of the 'grafted' organic groups, they could be classified as alkyl-siloxanes or aryl-siloxanes. Examples of combinations with identical or different alkyl functional groups are depicted below [poly-methyl-siloxane and poly-methyl-phenyl-siloxane respectively]. Sometimes siloxanes are miscalled 'silanes' (= compounds of the general formula SinH2n+2).

Inside the simple structure of the poly methyl siloxane, the small -CH3 groups do not represent a barrier for rotation so these chains are flexible at room temperature and even at very low temperatures; in fact their transition glass temperature is Tg = -123°C. Properties depend not only on substituents but also on chain length: if the chains contains less than ten -Si- atoms, we are dealing with an oligomer; that is certainly liquid at room temperature. For longer chains we have waxy consistence. 
According to all those definitions we may label Silo 111 as a 
reactive oligomer methyl siloxane.

High molecular weight poly-methyl siloxanes after evaporating, make just the surface hydrophobic (they cannot penetrate the porous substrates because of their size).

Silo 111, on the contrary, because of the short chains, offers some advantages:
1- Slow evaporating rate solvent helps the oligomer to go deep
 inside the substrate.
2- Reaction of ethoxyl groups linking to the stone and producing ethyl alcohol.

In other words, the product can deeply penetrate the substrate and it shows high level of resistance to the aging. Despite their specific average molecular weight, all the siloxanes (poly- and oligo-mers) have good reputation as hydrophobic agents. Here are some related characteristics:
  • They confer high level of hydrophobicity and transpiration at the same time
  • siloxanes are colorless, therefore they do not modify the tonality of the stone
  • they are resistant to sunlight and weathering; physical and chemical properties on surfaces affected by different phenomena evolve the same.
  • Their are classified as non-toxic products.

Siloxanes show some weaknesses too. For example they are sensitive to sulfur dioxide. Scientific research [1] reported several cases of protective films damaged by the attack of SO2.

If the film is too thin it may be crumbled and turn into powder. This issue may arise in heavily polluted environment, such as industrial areas.

Anyway, despite that sensitiveness, we know th
at a protective treatment on a surface (even with consolidation agents) by using Silicon based products prevents from SO2 deposits (in comparison to a not treated stone)[2].

One study [3] was conducted on marble samples coming from Mount Pentelicus, Greece, and compared the protective action of Paraloid B-72 and Silo 111 against the attack of SO2 (after aging the treated samples). As reported on the table below, you can see the absorption of SO2 depends on temperatures: at 40°C surfaces treated with Paraloid absorb the same amount of SO2 the natural marble does, while samples treated with Silo 111 absorb only 1/5 of that. More relevant differences were detected at 80°C (simulating the outdoor temperatures in the Summer); Silo 111 absorb just 1/50 of the amount got from the natural surfaces and 1/5 of that inside the samples treated with Paraloid B-72.

Temperature (°C)                                                          Amount of absorbed SO2 (μmol/g)
                                                                        not treated                Silo 111                  Paraloid b-72
40                                                                         1.401                       0.603                          1.383
80                                                                        23.593                      0.429                          2.183 

Beside that study case, Silo 111 has been studied for a long time in order to get a better understanding of the efficacy through the years; here are some of them:

Application on tufaceous stones [4]_ Silo 111 gives the better performances in terms of change of porosity; reduction of permeability; abrasion resistance and color change.

Protection of the faces and decorations of the Palacio de San Esteban, Murcia
 [5]_the restorers compared Silo 111 and Estel 1100 (ethyl silicate +siloxanes) and a third product based on hydrophobic polysiloxane (not specified). They registered similar results. With regard to the changing of color, Silo 111 and Estel were both reported to be very good agents.

Combination of Silo 111 with biocides [6]_When attempting to improve the resistance to biodegradation they evaluated the effect of four agents for consolidation and protection (plus biocides) on two types of stone and intonaco. It is well known that siloxanes are very sensitive to the attack of microorganisms, especially when humidity is high. For more details, please refer to Bollettino CTS 32.1 “Polimeri slow food”.

Treatments on Baroque churches in Lecce
 [7]_In the '90s some scientists of CNR conducted a research on the surfaces of churches being restored. Their faces are made of Lecce stone, particularly porous and absorbent. The surfaces were treated with ethyl silicate based consolidants (Estel, Wacker OH, RC70) and with poly siloxanes (Silo 111 or Silirain 50) and with combined treatments of Estel 1100 or RC80/90.

Following the outcomes, the authors realized these hydrophobic agents are still effective after ten years. For example, the front side of San Mathew's Church was treated in 1993 and checked ten years later. Water absorption of treated surfaces was still low in 2003-2004 (27-28 mg/cm2) compared to the absorption of the natural Lecce stone (100 mg/ cm2).

In conclusion, Silo 111 is very stable over time, gentle on colors, versatile and user friendly, fitting to different kinds of substrates. These properties explain why this product became so popular and appreciated among the 'insiders'.

1. Mavrov G.; "Aging of silicone resins”, Studies in Conservation 28 (1983), 171-178. 
2. Elfving P., Johansson L.G., Lindqvist O.; "A study of the sulphatation of silane treated sandstone and limestone in a sulphur dioxide atmosphere” Studies in Conservation 39, (1994), 199-209. 
3. Kapolos J.,Bakaoukas N.,Koliadima A.,Karaiskakis G.;"Evaluation of acrylic polymeric resin and small siloxane molecule for protecting cultural heritage monuments against sulfur dioxide corrosion”. Progress in Organic Coatings 59, (2007), 115-159. 
4. Dell’Agli G., Ferone C., Mascolo G.; "Durability of tufaceous stones treated with protection and consolidation products”, 9
th International Congress on Deterioration and Conservation of Stone, Venice, (June, 2000) 
5.Arana,R.; Mancheno, M.A.;Hernandez J.M.; "Estudio de la proprietades fisicas de unas muestras de roca de construccion del conjunto Palacio de San Esteban, Murcia”, in Estudio del estado de deterioro del conjunto Arquitectonico de San Esteban, (2000). 
6. Pinna D., Salvadori B., Galeotti M.; "Monitoring the performance of innovative and traditional biocides mixed with consolidants and water-repellents for the prevention of biological growth on stone” Science of The Total Environment, Volume 423, 15th April, 2012, Pages132–141. 
7.Calia A, Laurenzi Tabasso M., Lettieri M.T., Mecchi A.M., Quarta G.; "Una metodologia per il monitoraggio sostenibile dei trattamenti effettuati sui monumenti in pietra” Arkos 13 (2006).