Corrosion protection with Epoxy-Polysiloxane

by Abel De la Cruz*

After approximately a decade of the introduction of polysiloxane finishes to the market of protective coatings, and despite having demonstrated where they have been applied, excellent value characteristics, such as their low environmental impact, with low VOC level, free of isocyanates, with high volume of solids, and above all with excellent properties of resistance to degradation by exposure to the weather, providing durability to its brightness and color characteristics compared to those "top coat" coatings also used in industrial and marine environments, such as those of the aliphatic acrylic polyurethane type, we can affirm that in a balance of market share in Peru it is still of low coverage in terms of application and use in the treatment of surfaces of the sector of new construction works and even less in industrial maintenance.

Therefore, it is important to disseminate and support the design and formulation characteristics of this corrosion protection system called high durability especially to work in atmospheric environments with corrosivity category (ISO 12944) classification: C5-I (Very high Industrial corrosivity) and C5-M (Very high Marine corrosivity).

In our midst, few companies can demonstrate in real time a successful performance of polysiloxane-finished systems as an alternative to the two-layer epoxy/polyurethane system. This paint scheme recommended in new constructions with minimum maintenance needs and high durability is constituted by an Inorganic Primer Rich in Zinc and a Polysiloxane-Epoxy Finish, this two-layer system equals or in the best case exceeds the traditional three-layer systems to obtain optimal corrosion resistance, such as the Base: Inorganic Primer Rich in Zinc, epoxy intermediate layer and a layer of aliphatic acrylic polyurethane finish to ensure the retention of color and shine over time.

In 1998, the Peruvian paint company CPPQ S.A., licensee of the Amercoat Brand, applied for the first time in Peru the system: Epoxy- Polysiloxane PSX 700 for the external protection of sulfuric acid storage tanks of the southern peru Cooper Corporation company exposed to an environment of corrosive category C5M. The results of anticorrosive and aesthetic performance have been monitored by periodic annual inspections in the last 11 years and it is estimated that the first major maintenance for corrosion protection reasons should be performed when the system reaches level Ri 3 as defined in ISO 4628-3, which could occur in 2013. Subsequently, applications of this technology have been made in the new construction project of the Zinc Thickener Bridge and the painting of Structures of the Molybdenum-Bismuth Plant of the Antamina Mining Company in Huaraz, among others.

The results of the performance tests of the Polysiloxane-Epoxy PSX 700 coating, which is most widely used in this country, are described below:

It is important to consider that with the polysiloxane finish, a polyurethane finishing layer is not needed to obtain superior weather resistance and better color and gloss retention. This has its explanation in that inorganic polymers based on the chemistry of silicon, have chemical bonds in their molecular structure of the type -Si-O- (-silicon-oxygen-) of high resistance, which are those that hold the atoms together and whose value of bond energy exceeds by approximately 100 Kjoules / mol to the bond force - C-C- (-carbon-carbon-) of organic polymers such as epoxies and polyurethanes.

While it is true that inorganic polysiloxane polymer provides high levels of durability to the coating film, however some degree of chemical modification is required using small amounts of organic polymer (epoxy or acrylic hybrids) to achieve a balance of film-specific practical characteristics, such as flexibility, hardness, gloss, adhesion, edge covering, sprinkler characteristics, and overall appearance, and even an optimization in costs.

The necessary degree of organic modification will depend on the specific design of the painting and needs to be determined by experimental tests. Too low a level can lead to rigid films with risk of weakening, cracking and low adhesion capacity, failing due to delamination. In contrast, too high a degree of incorporation of organic polymer will reduce the durability characteristics in brightness and color, as well as compatibility problems between both polymers.

Through field benchmarking, polysiloxane has been shown to retain brightness and color for much longer than the best aliphatic polyurethane. This prolongs maintenance periods by appearance or aesthetics very common in organic coatings. Likewise, the low attraction properties of the film surface reduces the possibility of trapping organic and inorganic scales making it easier to clean the surface.

Its features for maintenance and ease of application
The wetting capacity of polysiloxane-epoxy is such that it can embed well-adhered oxide residues and bind tightly on a surface prepared with mechanical manual cleaning, making easy application where paint could not be applied before due to cleaning limitations.

Its low viscosity and high thickness retention mean that dry film thicknesses up to 7 mils can be applied in a single brush layer or with conventional application systems.

All these advantages reduce maintenance times and costs without risking the durability of the paint system. In Peru, given the increase in the cost of application (due to the fact that the safety requirements for both the environment and personnel are being implemented with adequate rigor), it is becoming more functional to apply coatings with this new technology, since in addition to the benefits of the product, applying one layer less as a protection system makes the choice of this painting system more attractive in terms of cost reduction.

Its ecological characteristics
Polysiloxane-epoxy has "ecological" characteristics, since it can be formulated containing a high percentage of solids, being suitable with the current regulations of emission of Volatile Organic Components (VOC), likewise, it is compatible with methods of surface preparation of low level of contamination such as the "water jetting" SSPC-SP12 or the manual-mechanical cleaning SSPC-SP11/SP15.

Good chemical resistance
Being this technology of high performance, its effective anticorrosive and aesthetic protection has been proven, testing among other cases, resistance to spills and splashes of sulfuric acid and leachable material, maintaining its performance and color.

Typical uses
Zinc Base (inorganic or organic) or First Epoxy and Polysiloxane-Epoxy Finish systems are commonly recommended for high durability protection (ISO 12944) in environments of high corrosivity (C5I/C5M) and can be applied on typical areas such as:
* Tanks, structures, pipes and equipment.
* Power, gas and oil plants.
* Water treatment plants.
* Offshore Industry: superstructures.
* Mining Plants.
* Chemical and petrochemical industrial plants.
* Dead works and superstructure of boats, Marine Platforms, etc.

References:
* Amercoat PSX 700 Performance Evaluation Study. CPPQ S.A.
* Polysiloxane Coatings Innovations, Norman Mowrer, Ameron International.
* Polysiloxane finishes, a step too far?, Alejandro Lagler.Akzo Nobel.

*Manager- General- Principal Consultant. AmericanConsultPeru. [email protected] http:// www.americanconsultperu.com