PDA Europe Conference 2017: from blasting to coating

The Polyurea Development Association Europe held its annual conference in London, near Heathrow Airport in November 2017. Simon Robinson paid a visit.

A wide range of papers were presented at the two-day Polyurea Development Association Europe meeting in west London in November 2017, from the importance of operator training, dust exposure regulations and the interaction between European fire regulations and polyurea-based coatings.

The papers are available to members of PDA Europe, and here we concentrate on three: a versatile three-step process to polyurea composites, surface coating polyureas and vapour blasting. The last is a technique to prepare surfaces which, proponents say avoids many of the problems with wet and dry blasting.

Tamas Balogh, technical director of Polinvent, Budapest offered a different, indirect approach to creating polyurea using a 50% water solution of waterglass such as sodium silicate (component ‘A’).

He explained that this way of making ‘indirect’ polyurea has been known in the mining and tunnel construction and rock stabilisation sectors for more than 30 years. Polinvent has tuned the resins to have long and well-controlled pot life. The user can choose among the ‘B’ component types to find pot life and viscosity that fit to the technology and to the expected weather conditions. Waterglass solution is delivered by large manufacturers; Polinvent does not produce it in house.

In this process, the water in which the waterglass is dissolved reacts with isocyanate to produce amines and carbon dioxide; there is then a part-reaction between the sodium hydroxide component of waterglass which produces carbon dioxide and sodium carbonate, and finally a reaction between the amine formed in the first reaction with the rest of isocyanate to produce polyurea.

The carbon dioxide generated in the amine reaction is dissolved into the aqueous medium and reacts with the strongly alkaline waterglass, producing small soda crystals.

In practice, this is done by mixing waterglass and an MDI-based blend together to create a two-phase system. The reaction starts on the surface of the waterglass droplets suspended in the MDI-blend. This gives a mixture with a pot life of between 5 and 150 minutes, and can be used to make composites, Balogh said. Glass fibre, aramid, basalt and carbon fibres have all been used successfully to make composites, he added. The mixtures’ viscosity is between 150 and 3000 mPa/s, depending on the resin, and solids content is typically between 85 and 90%.

Additionally, the resins are stable at 80°C for extended periods, whether dry or wet, and exhibit strongly self-extinguishing behaviour, he added. The materials are widely used in Germany, where more than 100,000 repairs/year are made to wastewater networks in concrete, PVC or vitrified clay with Glass fibre composites using the resins.

In the repair process, an impregnated glass fibre-based fabric-mat combination is wrapped round an inflatable packer with a calculated overlap, and inserted into damaged pipework. When the packer is correctly positioned, it is inflated, the overlap decreases, and the impregnated fabric-mat combination is pushed to the damaged pipe wall. The composite cures exothermically. The packer is deflated and extracted before the pipe is recharged, Balogh added.

Separately, the materials are used to create floors with anti-static properties or electrical conductivity, and in filament-wound objects which are used in the chemical industry, electrolysis plants, the food industry, brewing and the pharmaceutical industry. One customer covered more than 10,000 m² of industrial flooring with 3P resins. Customers prefer to use these resins in the food industry, he said because they resist many organic acids.

Indirect polyureas can also be used to make strengthening components in civil engineering applications for beams and columns, for example with carbon or basalt fibres.

A typical formulation consists of an MDI blend and sodium waterglass in a volumetric mixture of 1 part waterglass to 2 parts MDI blend. This can be mixed manually or by a drill machine at room temperature. Workers like to use it since there is no smell at room temperature and below, Balogh said. The materials are designed to be applied mainly in composites and thick composite coatings, which are reinforced with fibre and can be applied to wet or underwater surfaces and will cure adequately.

Surface coating polyureas

It is often necessary to add a topcoat to polyurea substrates for a number of reasons for mechanical or UV-protection, or for decorative purposes. Hugo Herault and Raul Fernandez of Krypton Chemicals advised the audience on how to avoid some common pitfalls of applying topcoats to polyureas.

A range of materials is available to coat polyureas, said Herault. These include one-component solvent-based aliphatic PU resins; two-component water-based aliphatic PU resins; and two-component polyaspartic resins which may be either solvent based or solvent free.

Each of these has a number of advantages and disadvantages, Herault said.

One-component solvent-based aliphatic PU resins are moisture catalysed fast curing products. Their properties are similar to PU, and their flexibility can be tailored. Their limitations are that the price is usually higher than for two-component systems, and the number of colours available can be limited.

Two-component solvent-free systems are smell-free, are compatible with most substrates, and have similar properties to two-component PUs. However, they cure slowly, he said.

Polyaspartic resins are available as 100% solids formulations which cure very quickly, and have similar properties to two-component polyurethane. However, they are relatively expensive, and some formulations could be too rigid to successfully coat polyureas.

Heralt added that the available top coats are often self-levelling, and that this can often lead to defects because a single coat will lead to thin areas where there are peaks in the polyurea substrate, and that curing can be slow in those parts where there are underlying valleys in the polyurea substrate.

According to the PDA code of good practice, where a topcoat is applied, it must be done between 2 and 72 hours after the polyurea was applied. But, Herault warned, adhesion to polyurea falls dramatically 4-6 hours after application. The substrate should be at least 3°C above the dew point and, ideally, 5°C above.

If an anti-slip finish is needed this can be achieved by sprinkling quartz particles between 0.3 and 0.7 mm diameter onto the surface, and then applying second coat a day later.

Finally, it is important that a flexible topcoat be applied to a flexible substrate. If a rigid topcoat is used then the elasticity of the underlying polyurea main coat can be compromised. This can lead to cracks opening in the polyruea layer, Fernandez warned. However, the top coat’s elongation only needs to be around 50% of the underlying polyurea for the system to work.

Wet abrasive blasting

Peter Bloem of Graco outlined the benefits of surface preparation using wet abrasive blasting. By wrapping abrasive particles in small amounts of water, highly effective surface preparation with lower levels of dust generation is possible.

He explained that the effectiveness of surface preparation media depends to a large extent on its momentum when it hits the surface to be prepared. Dry blasting media particles of a given size are less dense than those coated with a layer of liquid.

Bloem explained the benefits and drawbacks of slurry blasting with a modified sandblaster. This is fairly cheap to do and, according to work carried out by the Center for Construction Research and Training at the University of Iowa, it will suppress between 50 and 85% of the dust which would generated with dry media. However, this uses a large amount of water and a large amount of blasting media, he said.

Instead of using a very wet liquid system, Bloem recommended using a water vapour blasting approach. In this method, water and abrasive are combined in a pot under pressure and then injected into the airflow. This technique can reduce dust by around 92% compared with dry blasting. Additionally, he said, the water/abrasive mixture can be closely controlled which means the approach is effective with a wide range of surfaces, and the amount of water and blasting medium can be managed.

It is very similar to dry blasting, except that there is a small amount of water coating the blast media. The water consumption is typically between 500 and 1000 cm³/min when the system is set up with a high-performance blast media and it can be used with all media denser than water except for steel shot, he added.

The systems are suitable for use in confined areas, where higher speed is needed. The process can be done upside down and vertical, it is more comfortable for operators to use and has a low running cost. Additionally, it is easier to comply with environmental and health and safety regulations with this process than with other methods, he said.

There are concerns that vapour blasting will be a slower process than dry blasting, but this is not the case, said Bloem. ‘These new technologies are comparable to dry abrasive production rates, and in some cases faster,’ he said.

Some fear that the process will be wet and messy, but this is not the case, he added. Vapour blasting uses much less water than conventional wet blasting. While this can be as low as 500 cm³/min for some media, with 80 mesh garnet water use is typically 2-3 litres/min. Additionally, the vapour helps to remove debris and dirt produced in by the process.

Moreover, the process meets standard NACE No2/SSPC-SP 10 Near White Metal Blast Cleaning for joint surface preparation, he said. It can also remove hard coatings and linings as well as mill scale and pitted rust.

It is possible to work with finer grits to achieved a good surface on steel, he added; dry blast systems use 16-20 grit to give deep profiles. 16 Grit averages 1.1 mm, 20 grit averages 0.94 mm on steel sheet. He said that wet abrasive systems can use 80 grit to generate a 0.17 mm profile, and 100 grit for a 0.12mm profile. This also gives surfaces with greater microstructure to help coatings adhere to surfaces better than with other blasting processes. Bloem said that typically it is possible to get around nine times more peaks and valleys in the same surface area than is possible with dry blasting. This can be achieved, he said, because although the technology uses smaller grit, there is additional mass from the water which coats the grit granules.

Vapour blasting can also be used on concrete where the market is increasingly demanding dustless surface preparation, as well as the ability to prepare surfaces which are vertical, horizontal or upside down.

PDA Europe last held its annual conference in France in 2016.