What Is Polyurea?
Polyurea spray coatings technology is a development in the polyurethane coatings industry in the last 20 years. The polyurethane chemistry is about 60 years old. Since the 1970's elastomeric urethane coatings have been available. The polyurea elastomer technology was introduced some 10 years later. Two main application areas are Reaction Injection Moulding (RIM) and spray-able coatings. Polyurea coatings combine extreme application properties such as rapid cure, even at temperatures well below 0°C, and insensitivity to humidity, to exceptional physical properties such as high hardness, flexibility, tear strength, tensile strength, chemical and water resistance. The result is good weathering and abrasion resistance. The systems are 100 percent-solids, making them compliant with the strictest VOC regulations. Due to its specific curing profile and exceptional film properties, the polyurea spray coating technique developed into various areas, including corrosion protection, containment, membranes, linings and caulks.
The Chemistry
The isocyanate component is often a soft block-prepolymer based on aromatic or aliphatic isocyanates, such as toluenediisocyanate (TDI) and diphenylmethane diisocyanate (MDI) for aromatic systems and isophorone diisocyanate (IPDI) for aliphatic systems. The isocyanates are partially polymerized with high(er) molecular weight amines such as amine-terminated ethylene or propylene oxide based polyethers (Jeffamines®). Common chain extenders are low molecular weight amines such as diethylenetriamine (DETA), triethylene tetraamine (TETA) or etheramines for aliphatic ureas and diethyl-toluenediamine (DETDA), dimethylthio-toluenediamine (DMTDA), or N,N’-di(sec.butyl)- amino-biphenyl methane (DBMDA) for aromatic ureas. The latter two aromatic amines slow down the reaction significantly, whereas most aliphatic amines react rapidly with isocyanates, often within a few seconds. To slow down the reaction, aspartic ester amines are often used which provide secondary amine functionalities that cure noticeably slower than most other aliphatic amines.
Ureas made with aliphatic isocyantes are more UV-stable and are less susceptible to oxidation and degradation than ureas made with aromatic isocyantes. However, they are more expensive and are only used when high UV stability is required such as for exterior coatings. For most other applications, aromatic isocyantes are preferred.
The majority of commercial polyureas are elastomers. The elastic network consists of alternating flexible polyether blocks and short and stiff urea blocks. The urea segments associate to hard domains that are connected via hydrogen bonds. These domains act as physical crosslinks and provide a restorative force when the thermoplastic urea elastomer is stretched. However, urea resins can also be chemically crosslinked, for example by reacting isocyanates with trifunctional amines such as polyoxypropylene triamine.
Polyureas are generally considered moisture insensitivity and exhibit good moisture barrier properties. Polyurethanes, on the other hand, are very sensitive to humidity, both during processing and while curing. The reaction with water produces carbon dioxide gas which leads to foaming and defects which can cause premature coating or adhesive failure. They also have higher heat resistance than comparable formulated polyurethane coatings. Another important advantage of amine/isocyanate reactions is fast and reliable cure even in the absence of a catalyst, whereas the cure of polyurethane coatings typically requires a catalyst and carefully controlled processing parameters such as temperature and humidity.