A promising alternative for today’s construction and applied coatings

By Morten Sorensen, Director MC Technology Group

The coating of a new construction structures or their maintenance is a time consuming and costly undertaking. The coating of projects is often the last criteria or concern, however one that most often directly leads to delays and cost over-runs, usually big costs. Time and money is everyone’s concern puts enormous pressure on engineers and contractors. Coating systems should ideally be chosen for, suitability, the widest tolerances and with the longest life.

Typical Traditional Coatings Issues Facing Contractors

1. Surface preparation, many coatings require an SA2.5 or SA3, which is difficult to achieve and maintain. Structures offshore or close to the sea flash rust due to airborne salt content, requiring reblasting, 2 or more times. Coatings applied on a poor surface preparation leads to a premature corrosion/adhesion failure.
2. Coatings have an overcoat time limitation and subsequent coats often beyond these limitations, causes premature adhesion failure.
3. Adverse climatic conditions can affect cure and ultimately the longevity of coatings.
4. Touch up and maintenance project and the coating's compatibility with previous coatings.

The owners and their contractors are often faced with these situations frequently causing delays for the completion of a project. Delays are usually unacceptable and the applications are often carried out in conditions beyond the manufacturer's recommendations, potentially causing again pre-mature paint failures.

The MCU Technology

This unique technology coating invention, MCU – Moisture Cure Urethane (or Moisture Cured Urea is a single component pure urethane/urea product, that goes through a rather complicated production procedure. The final cure is by exposure to trace of moisture forming a polyurea. This is quite a different product to the widely known two component polyurethane coatings. The MCU is much safer product for the applicator in primarily due to the lack of isocyanate (carcinogenic) free monomer. The polymer technology for this technology is used for many common products today including artificial heart valves, tennis shoes, automobile parts and caulking compounds for example.

MCU Coatings are extremely tough, pore free and have superior chemical resistance. It offers long-term adhesion and very high abrasive resistance while remaining flexible during its long life. Independent third party testing has proven this to be more effective for corrosion resistance and long term monitoring of projects have demonstrated the MCU's to be more durable and have a longer lifespan, when compared to plural component technology.

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Potential Problems Facing Contractors

Owners and engineers need to be familiar with critical problems a contractor faces in order to make the right specification.

The surface preparation of new steel requires the removal of milscale (the top layers of ferrous oxide of steel plate). This can be done by wheel abrading, dry abrasive blasting or wet abrasive blasting. Once this is done a profile will exist for the life of that plate.

Traditional epoxy coatings require a 50-55 minimum micron angular profile and a cleanliness of the steel to be SA 2 to SA 2.5.

Today, the environment and the safety of workers is of a high importance. Therefore many are looking for alternative methods of surface preparation as dry blasting has harsh environmental and health problems. For over two decades WJ (water jetting) and WAB (wet abrasive blasting) has been implemented with great track records in many parts. These are dust and spark free and create 90-99% less waste. However projects within 50-100 km from the coast will have airborne salts accelerating surface flash rust. If this preparation technology is used with a non-tolerant coating system (and most are), then one or more reblast's will be required. This adds costs and if the contractor does not have a good margin to cover this, the result will be a sub-standard preparation and coating and a potential failure of the system prior to the expected life span.

The MCU Coatings are surface tolerant and can be directly applied onto a "medium" flash rust surface. This generally means within 12-24 hours of the surface preparation.

MCU Miozinc is the world's first and still only surface tolerant zinc rich primer. Miozinc can be applied on a minimum profile of 30-35 microns and also to ST 2 and ST 3 surface preparation.

Figure 1. The upper photo indicates flash rust after UHP surface prep and acceptable for MCU Miozinc. The lower photo Miozinc displayed no cracking or blistering even when applied over 300% above the recommended dft.

The over-coat limits are the minimum and maximum time between one coating to the next. This is critical to the progress of the job. Uncured coatings can cause delays and the wrong mix of catalyst may require the coating to be removed and the job re-started (this could be weeks of delay). However the main concern is the maximum. In the case of zinc rich epoxy or inorganic zinc as well as many epoxy coatings it can be within 6-60 hours depending upon temperature and humidity. If this variable limited time period has been surpassed then the coated area will have to be abraded and re-applied or a tie coat applied causing extra costs.

Another key area is maintenance of structures. Overcoating traditional epoxy and polyurethane requires costly abrading and tie coats. This often means full removal. If the coating is mostly intact then a full removal is an unnecessary budget cost. In case of a sweep more areas will become exposed which will require priming and an additional tie coat may be required, again incurring additional costs and a greater environmental hazard impact. Overcoating alkyds with epoxy just will not work.

Figure 2. Upper macro photo of a proper UHP blasted corrosion spot, small omega pittings visible. Lower – Good filling properties of the MCU Miozinc into the fine cavities.

The MCU MIO system Coatings have no maximum recoat limit and can be primed in the fabrication shop and completed at site even months later with excellent adhesion on a clean surface. The situation could also arise from the field during maintenance; such as all coats could not be completed in a limited shut down, a wind driven sand storm or rain or fog could limit the window.

Figure 3. Left is new tank primed w Miozinc. Right old tank prepared by WJ and ST3, spot primed and overcoated.

Regarding maintenance, MCU Coatings really have no equal. Their superior adhesion will marry to aged epoxy, polyurethane, alkyds as well as metalized, galvanized or aluminium thermal sprayed surfaces, without the need for abrading! This can save considerable time and costs.

Being a moisture cured product MCU Coatings can safely be utilized to maintain online gas and water pipes with assured success, without requiring to shut them down.

Figure 4. Pipelines, specially water and gas can be coated in operation even while sweating.

Adverse climate conditions also cause numerous failures, as most traditional coatings also require them to be applied and have a cure period before subjecting to dew point or moisture. Otherwise the cross-linking causing the cure may not be realized, whereas the system will quickly fail months after application is completed. This means that in the GCC or any tropical region the coating application should not start prior to mid morning and should end in the mid afternoon. This is a very short period to complete a major project and not realistic to have good production, so no attention is placed on this and another of many factors to the failure of coatings.

Fig 5. The BW Offshore Group has chosen for MCU coatings for the rehabilitation for their FPSO's Enterprise, Endeavor/Peace and Pioneer projects. At Sembawang and Keppel yards, Singapore. All coating applied with only 4 products.

In addition the MCU Coatings have no restriction to the dew point or relative humidity. This means that coating jobs can be carried out 24 hours a day with no hold ups due to the climatic conditions as long as the surface is visibly dry. Productivity can be increased 2 X- 3X and schedules maintained or even advanced! 

MCU Product Highlights

These MCU Coatings offer many applications as well as performance advantages.

1. Single component – no mixing errors & less product handling
2. No humidity (up to 99% RH) & no dew point restrictions & temperatures from -15°C to +50°C
3. Surface tolerant to flash rust (even zinc rich)
4. Excellent for use over WJ or ST 3 preparation
5. Excellent long-term adhesion & flexibility
6. Excellent abrasion resistance
7. Excellent atmospheric chemical resistance
8. Best for over-coating all coatings and metalized surfaces
9. Long-term tests show longest performing coating life
10. 3 coat shop applications in 1 day, with QuickCure™

There is a full range of MCU coatings available, and 4-5 coatings can complete most projects.

Figure 6. All New York City bridges have been maintained with MCU Coatings working 24-7 year round and has been the major bridge coating supplier in North America for many years.

 

Why MCU's Outperform Typical Epoxies

During the past 25-years of the varied applications of these products and numerous third-party [3] testing have shown them to be a superior coating in many ways offering:

• Out-performance of most coatings in corrosion testing, passing, salt spray 20,000+ hrsExcellent adhesion requiring less blast profile (20-35µm), primers are surface tolerant to flash rust and magnetite
• Better wetting out properties, necessary to penetrate into deep pittings
• Excellent adhesion to and often rejuvenating old coatings
• Better edge retention
• Application without dew point restrictions and in humidity's to 99%
• Remain flexible, inert after full cure, no change chemically or physically
• Better resistance over salt contamination
• Better UV stability
• Better abrasion resistance – superior to glass flake and ceramic pigmented epoxy
• 2-3 times longevity over typical epoxy – urethane system

Fig 7. Table compares costs for systems: MCU (blue) & Epoxy (mauve), based on US Hwys & ODOT for severe marine environment. Maintenance on MCU structures expected after 40+ years.