Hybrid Sol-Gels - Novel Functionality in Hybrid Sol-Gel Coatings Set to Offer Greater Corrosion Resistance by MERI

Topics Covered

Background
Introduction
Breakthrough in the Development of Hybrid Sol-Gels
Evaluating Properties and Performance of Hybrid Sol-Gel Coatings
Summary

Background

MERI (formerly MRI) has been a centre for research excellence in materials since 1990. This excellence was recognised in 2001 when the Research Assessment Exercise (RAE) awarded a 5 rating to the research carried out at MRI. This makes MERI the highest rated department of its type in the new university sector and rated alongside the materials departments of universities such as Liverpool and Queen Mary's.

MERI's research and consultancy activities are supported by a large advanced equipment base ranging from the latest electron microscopes to high performance computing hardware.

Introduction

Corrosion plagues the construction industry and costs the UK economy some 3-4% of GDP. A common strategy used to prevent corrosion is to apply coatings. Historically some of the most effective coating systems for metals (including pre-treatments, primers and top-coats) contain the carcinogenic substance Chromium VI (Cr6+). Current environmental legislation is now reducing or imposing a total ban on the use of Cr (VI) based systems. This has led to the search for alternative systems offering equal corrosion resistant properties.

A potential alternative pre-treatment/primer, one which is gaining interest and popularity, is that of 'sol-gel', where the 'sol' is particles (between 0.0001 and 0.001mm in size) dispersed and suspended in a liquid, and where the 'gel' can be thought of as a liquid and solid dispersed 'within each other'.

To date the vast majority of applications of sol-gels have been based upon purely inorganic formulations; typically silica- or aluminium-based which, respectively, produce silica/oxygen and aluminium/oxygen molecular networks. These systems require long cure times or a high cure temperature (often above 300°C), which has thus far limited the range of applications for this type of coating. More importantly, they have a limited coating thickness (less than 0.001mm for a single-coat system) and are inflexible, so often crack.

Breakthrough in the Development of Hybrid Sol-Gels

Recently researchers at the Materials & Engineering Research Institute at Sheffield Hallam University have made a breakthrough in the development of hybrid sol-gels, which contain both inorganic and organic components. The organic component is typically carbon-based - and the resulting silica-oxygen and silicon-carbon bonds allow a range of properties to be developed, including varying degrees of hydrophilicity/hydrophobicity (the ability to absorb or reject water respectively), flexibility and the ability to functionalise the coating by encapsulating materials with specific properties.

The range of hybrid coatings currently developed are flexible, thicker (up to 0.015mm) and, depending on the inorganic/organic ratio, can be cured at relatively low temperatures at times down to tens of seconds. This has created the possibility of using sol-gel coatings for high volume in-line applications.

Alternatively, if more time is available curing is possible at room temperature, which has made possible 'biologically active' coatings; and here, the biologically active component prevents any naturally occurring microbes in the environment colonising the surface and producing a 'bio-film', i.e., biofouling, which can lead to highly localised 'microbial-induced corrosion'.

In addition, varying the inorganic/organic components and ratios introduces other interesting possibilities. For example, the addition of certain rare earth corrosion inhibitors (such as cerium nitrate) provides improved corrosion resistance and 'self-healing' qualities. In other words, a scratched surface may see a small amount of localised corrosion followed by passivation of the exposed metal at the scratch site but would not mark the start of more widespread corrosion.

Evaluating Properties and Performance of Hybrid Sol-Gel Coatings

A range of laboratory tests have been conducted to evaluate the properties and performance of these coatings, including neutral salt spray and prohesion tests, simple long-term immersion, electrochemical impedance spectroscopy, adhesion and flexibility bend tests and cut-edge corrosion tests.

In addition, a number of field tests have been conducted - the most notable of which so far has been at the Thames Barrier in London.

A new variant of sol-gel coating is now being produced. This system is solvent-free (100% water-based) and has the added environmental benefit that no volatile organic compounds (VOCs) are released during the curing process.

Summary

In summary, sol-gel systems have potential applications in numerous areas including pre-treatment and primers. In addition, sol-gel may be used in a wide range of architectural and construction applications, either as a protective top-coat to prevent surface corrosion impairing the aesthetics or as a primary coating to support a range of final top coats.

Source: MERI - Materials and Engineering Research Institute

For more information on this source please visit MERI - Materials and Engineering Research Institute.

Date Added: Oct 12, 2008 | Updated: Jun 11, 2013
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