Polyurea technology is a type of elastomer used across many industries worldwide as a superior coating system for waterproofing and corrosion protection. In the wastewater industry, existing facilities can be quickly and cost-effectively refurbished with polyurea and new treatment facilities permanently protected against hydrogen sulphide attacks.
Some of the first product formulations of polyurea were associated with high-profile coating failures, which resulted in some scepticism about the product. One famous failure, the San Mateo Bridge in the San Francisco Bay Area, has been the subject of many articles over the years. Key problems in this famous failure included improperly specified surface preparation, lack of product knowledge and inadequate application equipment.
The Polyurea Development Association (PDA) was set up in 1999 followed by the Polyurea Development Association Europe (PDA Europe) to further the sustainable growth of the polyurea industry and establish protocol and standards for polyurea applications. With improvements in the application technology, acknowledgement of the product chemistry and an appreciation for the importance of correct application by trained specialists, it may now be time to think again about polyurea technology.
Definition and chemistry
PDA Europe defines polyurea as an elastomer obtained chemically by the polyaddition reaction of aliphatic or aromatic isocyanate or of an isocyanate prepolymer with a polyfunctional amine mixture of amines, generally in a mixing ratio by volume 1-to-1.
Polyurea technology is based on the chemical reaction of the two-component systems applied by spraying, through the use of two-component pumps.
In aromatic systems, the isocyanate (NCO) component is made from prepolymers based on methylene diphenyl diisocyanate (MDI); in aliphatic systems, from hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) or H12MDI methylene dicyclohexyl diisocyanate, and it constitutes the hard segment of the chain. The isocyanate prepolymer selection influences most of the polymer’s properties.
MDI-based prepolymers, with an NCO content between 15 and 16 percent, are typically used in standard polyurea formulation.
This range of NCO features a good compromise between the viscosity of the material and the reactivity of the system. With lower values of NCO, prepolymers have higher viscosities but give the system a greater elasticity and a lower reactivity.
The amine component of the polyurea is generally much more complex than the isocyanate component, and consists of:
- High molecular polyetheramine, which constitutes the soft segment of the chain because of its flexible nature
- Low molecular weight polyetheramine as chain extenders
- Pigments and additives
The choice of the amines is crucial for the processing and subsequent performance of the polyurea.
The polyether amines are propylene oxide/ethylene oxide-based polyethers that are amine terminated, generally with a molecular weight between 200 and 5,000 grams per mole. The primary amino group of these molecules reacts rapidly with the isocyanate, which eliminates the need for a catalyst.
The polyether amines can be bi/trifunctional, aromatic or aliphatic; the latter are used in applications where the stability of color exposed to light is obviously a priority since they are expensive.
The chain extenders are key for polyurea’s properties and reactivity. The diethyltoluenediamine, mostly used in aromatic polyurea formulation, contributes to the hard segment and improves the heat resistance. In recent years, chain extenders such as secondary and/or sterically hindered amines have been specifically designed to slow down polyurea reactivity for particular types of application.
Pigments and additives must be used in limited quantities since the viscosity of the two components must be kept under control during the application. Substantial amounts of fillers or reinforcing additives can be added as a third component.
No uniformity exists with regard to the denomination of components A and B of the polyurea. In Europe, the isocyanate component is commonly component B (resulting from polyurethane chemistry), while other countries have an inversion of the denomination.
The term ‘polyurea’ describes the technology. Many different possible formulations can be used to achieve the desired properties, and therefore the selection of the appropriate raw materials is important.
The polyurea term has been improperly used in the past, creating confusion between pure polyurea and hybrid polyurea. Pure polyurea should not contain hydroxyl groups in its formula, unlike hybrid systems which are characterized by the presence of OH groups and catalysts.
A hybrid system’s composition is a combination of the aforementioned systems (polyurethane and polyurea). The isocyanate component may be the same one used for pure polyurea. The mixture of resins is instead a combination of terminated amine and terminated polymeric hydroxyl resins and/or chain extenders.
The addition of one or more catalysts is required to obtain the same reactivity. For this reason, hybrid systems, despite having a wide scope of applications, are more sensitive to moisture than pure polyurea.
Since the catalyzed reaction between the polyol and isocyanate is affected by changes in temperature at the application phase, unlike the reaction of amine and isocyanate, the system performs more poorly.
Polyurea is formed when the amine reacts with the isocyanate. The reaction is fast and autocatalytic (therefore it does not require a catalyst even at low temperatures, unlike the polyurethane and hybrid systems) and it acquires many specific properties that distinguish it from other types of polymers.
Preparation of substrate
The preparation of surfaces to which the system is applied is fundamentally important for its final success. Preparation will depend on several factors: substrate type and state, coating cycle and total loads.
In its most recent Code of Good Practice (2014), the PDA describes preparation methods including:
- Grinding — Mechanical action performed with abrasive wheels or abrasive paper (sanding) to remove laitance, dirt or other material from the crust of the surface
- Scarification — Mechanical action carried out by a rotating or non-rotating scarifier aimed at removing the surface crust from 3 to 5 meters
- Milling — Mechanical action of a rotary cutter to achieve homogeneous and total removal to a constant thickness, regardless of the resistance of the substrate
- Shotblasting — Mechanical action of metallic granules propelled by special machines with a complete recirculation, separation and recovery of sandy and other materials, all dust-free.
It is imperative to undertake preparatory treatment, including:
- Polishing — For new substrates without special hardening surface treatments
- Brush hammering or scarifying — For old surfaces with friable parts, not spread over the entire surface
- Milling — For old substrates that are particularly degraded or contaminated where it is necessary to remove a continuous and homogeneous layer
- Shot preening — For concrete, stone, brick, metal and tile substrates
Polyurea is a two-component product for spray applications that requires particular mixing conditions to allow a proper chemical reaction during application. The machine for spraying (and mixing) is the heart of the system. It heats the two fluids, pressurizes them and keeps them at a constant during the spraying. To use the equipment, a general understanding of how each part functions is necessary.
The PDA sets the following minimum requirements for correct application in its most recent Code of Good Practice (2014):
- Two-component management system
- Spray pressure between 150 and 240 Bars
- Product temperature between 50°C and 80°C
- Product flow rate between 2 and 10 liters per minute
- Measuring and mixing the products in the right ratio
- Ability to produce and maintain the desired operating pressure
- Separate adjustment of the temperatures (A heater, B heater, heated hoses) to thin application products until the correct viscosity is reached
- Deliver the desired flow rate at the desired pressure
The application of polyurea requires a fine-tuning of the complete system to meet the conditions of the specific channels:
- Supply system
- Dosage and heating units
- Heated hoses
- Spray gun
An air compressor, essential for the supply pumps and purging of the gun
The PDA stipulates that all components must be properly selected to meet the requirements of the desired applications regarding flow rate, temperature and pressure.
Training is another key element of a successful application of polyurea. The PDA sets the following criteria as a minimum. Operators should know:
- How to use the equipment for successful application.
- How to use the equipment provided by the manufacturer of dosage units.
- How to intervene in case of problems.
- How to perform on-site maintenance.
Hugo Herault, current PDA Europe president who assisted in providing detail for this article, said: "As these chemicals are converted into a polymer only when finally processed and applied, there has been an important evolution in the marketplace with regard to the training of installers around substrate preparation, priming and application. Polyurea is a very interesting tool which satisfies many needs but requires respect to the key parameters (health and safety, substrate preparation, priming with the right product, spraying properly and with an adequate unit and protecting the coatings from [ultraviolet] or application of the right anti-skid coating). Successful installers recognize this and have a very clear idea on how to manage projects before, during and after polyurea has been applied."
In 1990, less than 5 tonnes (t) of polyurea was produced worldwide. By 2006 the quantity (polyurea and polyurethane-polyurea hybrids) had reached 35,000 t (PDA market study 2007). It is estimated that the market will continue to grow as the following benefits of pure polyurea as an optimum coating are realized worldwide:
- Rapid reactivity
- High chemical and mechanical resistance
- Resistance to high temperatures Excellent elastic properties and crack bridging
- Resistance to abrasions and impacts
- High resistance to tearing
- Water resistance
- Absence of solvents (100 percent solids)
- Thick application even on vertical surfaces
- Application possible on most substrates
Author’s note: This article was produced with support from PDA Europe.
Mark Lemon is managing director of CSC Services. He oversees the strategic direction of the company, implementing innovative procedures, systems and products, adhering to the latest health and safety practices and policies. CSC Services provides high-performance cleaning, concrete repair, specialist coatings and leak sealing solutions. Visit csc-services.co.uk for more information.