{"id":5126,"date":"2014-09-03T19:05:25","date_gmt":"2014-09-03T23:05:25","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=5126"},"modified":"2017-01-13T14:37:47","modified_gmt":"2017-01-13T19:37:47","slug":"what-do-you-mean-by-polyurethane","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/what-do-you-mean-by-polyurethane\/","title":{"rendered":"What do you mean by \u2018polyurethane?\u2019"},"content":{"rendered":"
\"All<\/a>
All photos courtesy Quest Construction Products<\/em><\/figcaption><\/figure>\n

by Steven Heinje
\n<\/strong>\u2018Polyurethane\u2019 is not a useful specification term on its own. It is akin to saying, \u201cI want a form of transportation,\u201d as opposed to something much more specific, like, \u201cI want a four-cylinder sedan.\u201d Getting to the point where a manufacturer or supplier will only offer products meeting a specific project\u2019s needs will require more precise and non-proprietary terminology that makes performance the outcome.<\/p>\n

The term \u2018polyurethane\u2019 simply means many repeated units of urethane, much in the same vernacular as \u2018polypropylene\u2019 or \u2018polyester.\u2019 Often, these coatings are referred to as \u2018urethanes,\u2019 similar to the way terms like epoxy or acrylic are employed. They are all polymers of the key reactive or function group. The urethane group confers many of the traits of a polyurethane\u2014most notably, adhesion\u2014in much the same way most acrylic paints share ultraviolet (UV) resistance.<\/p>\n

Typically, a Technical Data Sheet (TDS) for a polyurethane coating will include a long list of tests, often with some impressive numbers. A few of these will find their way into various specifications. Perhaps they will include a benchmark product and later a few \u2018equals\u2019 will be added\u2014such a specification is representative of the standard of care for using a polyurethane in today\u2019s construction marketplace.<\/p>\n

This article seeks to add some background and a few key details so specifiers understand what type of product is actually being offered, what are its intrinsic performance attributes, and whether it truly has adequate offsets. (The focus is on coatings; polyurethane foams for insulation are outside this article\u2019s scope.) A little chemistry is required to fully grasp the concepts, but the desired outcome is for design professionals to know what questions to ask and ultimately to be able to write better specifications and avoid failures.<\/p>\n

\"This<\/a>
This 100 percent aliphatic polyurethane has exceptional ultraviolet (UV) resistance<\/span> needed to hold this deep blue color on the very visible roof of Marin County Civic <\/span>Center (San Rafael, California).<\/span><\/figcaption><\/figure>\n
\"The<\/a>
The flexibility and hydrophobicity <\/span>of a specific polyurethane was <\/span>chosen over an epoxy alternative<\/span> for this plant near Shanghai.<\/span><\/figcaption><\/figure>\n

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Why use a polyurethane?<\/strong>
\nPolyurethanes have the broadest range of any product group: they can be soft as a baby\u2019s skin or hard enough to be machined and tooled. Some may yellow and chalk severely, while others are among the most light-stable coatings available. Chemical resistance, including water resistance, is similarly variable.<\/p>\n

Polyurethanes combine toughness and flexibility\u2014a unifying trait that comes from the urethane linkage itself. Urethanes form a molecular spring based on the intense attraction of the urethane groups to each other, so they form hard domains within a softer and more flexible matrix. This link is flexible, and more importantly, it is recoverable; this leads to products that combine high tensile and high elongation, and even maintain high hardness and flexibility at low temperatures (down to ?30C [?22 F]) and up into higher temperatures (75 C [167 F]). This feature allows them to be used as high-performance coatings, as well as durable foams.<\/p>\n

In most cases, another key trait is adhesion. As a rule, urethanes bond to various substrates, often better than styrene-ethylene\/butylene-styrene (SEBS), acrylic, silicone, and even butyl. Adhesion is important to consider because maintenance coatings are often used over unknown, older surfaces, or even multiple surfaces giving the contractor or specifier a good reason to specify a polyurethane above other more-restrictive options.<\/p>\n

Beyond toughness and flexibility, all other traits\u2014gloss retention and resistance against abrasion, water, UV, solvents, and acids\/alkalines\u2014depend on the \u2018backbone,\u2019 as defined later in this article. Application traits are another mixed bag, relying on the nature of the curing chemistry, which is referred to as the \u2018curative.\u2019 With the appropriate formulation, there is no other class of product that has a better balance of adhesion, UV resistance, abrasion resistance, and flexibility. By contrast, epoxies are too hard, acrylics exhibit poor abrasion performance, and silicones show relatively weak physical properties as a product class. There are few properties a urethane cannot be formulated to achieve.<\/p>\n

However, high heat resistance is an overall weakness. Below 75 C is urethane territory: applications include roofing and most industrial and sealant applications. While some specialty urethane products can perform long-term above 100 C (212 F), these high temperature applications typically require silicone and epoxy products.<\/p>\n

\"This<\/a>
This polyurethane system has exceptional fire retardancy for use on sloped roofs.<\/span><\/figcaption><\/figure>\n
\"A<\/a>
A low-toxicity polyurethane coating was used on the structural concrete for this wastewater treatment plant.<\/span><\/figcaption><\/figure>\n

Types of urethanes: Varnish to airplane components<\/strong>
\nASTM D16, Standard Terminology for Paint, Related Coatings, Materials, and Applications<\/em>, defines six types of polyurethanes.<\/p>\n

Type I: One-package (1K) urethane alkyds<\/em>1<\/sup><\/a>
\n<\/a>Like all alkyds, or \u2018oil-based paints,\u2019 these cure by oxidation of a drying oil and solvent evaporation. They are used as varnishes, floor finishes, and abrasion-resistant paints. They do not cure by a urethane reaction. In this category, the oil-based paint becomes a platform for using an aromatic polyurethane to enhance the former\u2019s performance. (This is also done with acrylics and epoxies, but in those cases they are not referred to as \u2018acrylics\u2019 or \u2018epoxies\u2019 in the way varnishes and paints are called \u2018polyurethanes.\u2019) Looking at toxicity, they have no free isocyanate, which means they are no more toxic than other alkyds.<\/p>\n

Type II: 1K moisture-cured paints and industrial coatings
\n<\/em>This important type is often employed in high-performance thin film (i.e.<\/em> 25 to 75-\u00b5m [1 to 3-mil]) floor and industrial paints. This class also includes higher-build (i.e.<\/em> 0.5 to 2-mm [0.02 to 0.08-in.]) elastomerics such as those described in ASTM D6947\/D6947M, Standard Specification for Liquid-applied Moisture-cured Polyurethane Coating Used in Spray Polyurethane Foam Roofing System<\/em>. This class uses atmospheric moisture to act as the curative, allowing them to be reactive 1K products. This can cause problems and limitations along with advantages.<\/p>\n

Unreacted isocyanates, such as those found in Type II urethanes, react with many other chemical species found in wood, metal, epoxies, and acrylics\u2014this gives these resins exceptional adhesion. Moisture cures are often high in solids, relatively low in viscosity, and have moderate volatile organic compound (VOC) content. When they cure, however, they release a lot of carbon dioxide (CO2<\/sub>) gas. Consequently, Type IIs used in flooring and corrosion protection need to be applied thinly, so not too much gas is evolved from curing. Elastomerics have a lower concentration of isocyanate, allowing them to tolerate thicker films before they exhibit the same foaming problems.<\/p>\n

The moisture cures require experienced contractors because bubbles, foaming, and blisters present high risks. Most of these Type II products are aromatic, which means they will yellow. ASTM D6947 moisture-cured coatings used in roofing are typically blended with asphalt. These sealants and coatings are not UV-stable, but when used for roof details and crack-bridging or caulk applications at thicknesses approaching 2 mm, they can weather for more than a decade.<\/p>\n

Type III: 1K heat\/oven-cured elastomers and industrial coatings<\/em>
\nThese elastomerics are generally not used as maintenance coatings. Instead, they are reserved for industrial products, car bumpers, or technical fabrics for sportswear.<\/p>\n

Type IV: Two-package (2K) products<\/em>
\nThese materials are similar to the Type IIs; in fact, some may simply be a Type II product offered with a liquid curative. This eliminates the need to rely on atmospheric moisture to cure the resin, and it avoids the release of CO2<\/sub> gas. They generally have higher solids content and moderate pot-life in the range of an hour. They are still prone to problems with CO2<\/sub> gas when used carelessly.<\/p>\n

As an example of diversity within polyurethanes, there is a Type IV industrial that uses a pure aliphatic resin cut in solvent and is cured as a polyurea\u2014it achieves 34,475 kPa (5000 psi) tensile and 400 percent elongation, is easily sprayed, and has a two-hour pot-life. Further, as a polyurea, it is not affected by moisture at all.<\/p>\n

Another case is a sub-group used in roofing and flooring: moisture-triggered (oxazolidine) cured aliphatic urethanes. They are described in ASTM D7311\/D7311M-10, Standard Specification for Liquid-applied, Single-pack, Moisture-triggered, Aliphatic Polyurethane Roofing Membrane<\/em>. This is another example of a specific urethane chemistry not prone to gassing even in thick films due to the special chemistry of the oxazolidine curative.<\/p>\n

Type V: 2K urethanes<\/em>
\n<\/a>Important commercial products used in industrial applications (and to a lesser extent in roofing), these urethanes are typically used in a 1:1 volume ratio with plural component equipment.2<\/sup><\/a><\/p>\n

Most are 100-percent-solids elastomers or coatings. A sub-group, called ridged polyurethanes, is specifically devised for tank, pipe, and petrochemical use. Some have enough pot-life to be rolled or squeegeed, but most require plural component equipment, in-line heat, and will cure within minutes. <\/a>Also within this group are the so called polyureas, which represent another approach of avoiding the issue of atmospheric moisture and CO2<\/sub> gassing by using curatives that produce an almost instantaneous reaction. These require the use of an impingement mixing plural component spray gun. The reality is most polyureas are hybrids of both polyurea and polyurethane reactions and should be treated as another flavor of the Type V polyurethanes.3<\/sup><\/a><\/p>\n

Type VI: One-package solvent-based urethane lacquers<\/em>
\nVOC restrictions have largely eliminated this type of polyurethane from commerce, but they may still be found in adhesive applications. (Water-based urethanes, or \u2018urethane latex,\u2019 could also be included here.) Often blended with acrylic emulsified resins, these are finding use in floor coatings, primers, and other high-performance and specialty applications.<\/p>\n

There are a few words of caution related to the use of the term \u2018urethane,\u2019 which can be a marketing selling point. There are examples where a urethane material has been used to thicken an acrylic latex product, which is then illegitimately branded as a urethane. In other cases, a low level of a legitimate urethane resin may be added for marketing purposes. A urethane\u2019s key properties\u2014being flexible and tough\u2014does not \u2018play well\u2019 with the high levels of fillers typically included in vinyl, styrene-acrylic, and acrylic paints. If a product has a high density (i.e.<\/em> > 1.4 g\/L [11.7 lb\/gal]), then it is functionally not a urethane.<\/p>\n

\"CS_September_2014.indd\"<\/a><\/p>\n

\"For<\/a>
For this underground concrete storage tank, a\u00a0 polyurethane able to resist alkaline patching materials was required.<\/span><\/figcaption><\/figure>\n

Form follows function: A primer on the chemistry<\/strong>
\nWith so many different types of polyurethanes, a specifier has many factors to consider in selecting the right product for the job. The three key components to understanding what a polyurethane provides in terms of performance are:<\/p>\n