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Understanding Polyurethane: History, Chemistry & Uses

Polyurethane coatings are used as decorative coatings, product finishes, automotive coatings, and industrial maintenance. They are among the most flexible coating types that are available. A corrosion prevention system introduced topcoats made of polyurethane to the marine and industrial maintenance market around the mid-1970s. 

The topcoat has outstanding weathering properties, i.e., high-quality retention and non-yellowing qualities in the presence of sunlight. They can also offer excellent abrasion resistance and hydrolytic stability, and resistance to a range of chemicals and chemicals. Polyurethane topcoats have evolved over the years into a coating technology that can be formulated to give low-VOC coatings, productivity-enhancing coatings, and long-lasting, cost-effective coatings.

There have been reports of polyurethane topcoats suffering a decline in performance in the last few years, especially weathering. Market forces that continue to push the cost of painting jobs certainly contributed to the issue. The coating formulators responded by making formulation adjustments that reduced the cost of raw materials, which ultimately led to less efficient polyurethane paints. Although they're still polyurethanes, they may be less effective in performance than the original standard set in the 1970s.

The first response to this drift in performance was to develop an appropriate weathering performance standard for polyurethane topcoats. This is useful to the end-user/specifier and the coating formulator as it clarifies what is expected of them.

There is a need for the coating to offer an aesthetic appearance and protection against corrosion. However, there are calls for high-quality weathering in certain market areas for industrial maintenance, such as sports facilities and bridges, and water tanks. Specifications and end-users are considering alternative coating kinds, e.g., polysiloxanes and fluoropolymers, to attain more high performance in weathering than is described by Paint Spec No. 36. However, these coating kinds have an intrinsically higher cost of raw materials than aliphatic polyurethanes.

Polyurethane Coating Chemistry

The chemistry of the Isocyanate group (-N=C=O) forms the basic foundation of polyurethane coatings chemical chemistry. Its superior chemical Reactivity and its ability to interact with a wide range of chemical partners make the isocyanate group ideal for the market for coatings. The isocyanate group reacts with any substance that contains oxidizing hydrogen, such as alcohol (R-OH) as well as an amine (R-NH2) or also water (H2O). There are many kinds of amine and hydroxyl-containing raw materials (often known as polyols or co-reactants) that can react with isocyanate-containing raw materials. The three chemical reactions (shown in Schemes 3 and 4) are particularly important.

OCN-R'-NCO + HO-R-OH -(R-O-C(O)-NH-R')n1.

Polyurethane H2N-R-NH2 + NCO-OCN-R'-N -(R-NH-C(O)-NH-R')n2

Polyurea R-NCO + H2O [RC(O)OH R-NH2 + CO2 3

Two-component polyurethane coatings, also known as two-package coatings, are the most widely known polyurethane coatings. Two-component is a term used to describe a process where two different resin packages (often known as Part A and part B) are blended immediately before application. One package has resins that react with chemical groups (e.g., alcohols, hydroxyl, or amines), and the second has a polyisocyanate resin capable of reactivity with chemical groups. The main benefit that comes with the double-component coating is its theoretically unlimited storage stability coupled with the speedy curing reactions once they are combined.

Polyisocyanates

The polyurethane coating is typically defined as the kind of polyisocyanate used in the coating (i.e., aromatic or an aliphatic). The major distinctions are that polyisocyanates with aromatic properties (e.g., the toluene diisocyanate [TDI] and the MDI-derived polyisocyanates) are generally less expensive. Polyisocyanates are apathetic (e.g., Hexamethylene Diisocyanate [HDIIPDI], isophorone diisocyanate, and H12MDI-derived Polyisocyanates) are superior outdoor weathering properties, i.e., excellent retention of gloss and color; however, polyurethane coatings based on aromatic diisocyanates can yellow in the sun. Because of this crucial distinction in light stability, aromatic polyisocyanates are commonly used as primers and intermediate layer coatings. They are commonly employed for outdoor topcoat applications where high gloss and color retention are required.

The potential health risks associated with isocyanates have prompted many discussions throughout time. While there are risks to health arising from exposure to isocyanates as part of applying paint, the dangers are fully understood. Forty years of industrial use has resulted in documented safe handling procedures and information about health dangers. (10-12) Like is the norm when working with anything chemical product or coating, good hygiene practices and limiting exposure to isocyanates are essential in handling polyurethane coatings. The industrial hygiene monitor programs can be particularly helpful in confirming the safety of polyurethane coatings.

Co-Reactants

Co-reactants form the second layer of the 2-component polyurethane coating. They should have a minimum of two hydrogens that react for polymerization. If either the co-reactant or the polyisocyanate can have more than two reaction sites and is crosslinked, a thermoset polymer is created. More crosslinking leads to an incredibly tough and resilient chemically-resistant polymer. Co-reactants are usually identified by their backbone chemistry and can include polyacrylate, polyester (acrylic) or polyether vinyl, fluoro or polyaspartic, and even epoxy. The most popular ones utilized in the marine and industrial maintenance industry are polyacrylic, polyester, and polyaspartic polyurethane topcoats.

It is the composition and composition that plays a crucial role in determining the performance of the polyurethane coating. Figure 1 shows how the use of different polyacrylate polyols produces different weathering outcomes when it comes to polyurethane topcoats. (For an overview of the properties, including performance properties for various acrylic polyols, check out reference 13.) In general molecular weight (higher MW leads to quicker advancement of physical properties) and the content of hydroxyl (higher OH content results in more weathering resistance as well as chemical resistance) and Tg (higher Tg results in more rapid drying of the physical properties and more durable films) as well as the chemical makeup of backbone of polyacrylate (more styrene reduces costs and produces less weather resistance) all play an important role in the performance and application of polyurethane of the final polyurethane topcoat.

A typical corrosion-proof polyurethane topcoat formulation will be described in Table 1. The fundamental components of the formulation comprise binder (resin) as well as solvents, pigments, and additives. There are various factors in a typical corrosion protection formulation that impact the effectiveness, performance, properties of the application, and the cost structures for the coating. This article will examine the type of resin and polyol/isocyanate index (NCO/OH).

Results of a Comparative Weathering Study

A variety of polyurethane topcoats were evaluated using an SSPC Paint Spec. No. 36 criteria to be used as a basis for comparing weathering performance. The degree of gloss retention and the amount of yellowing in the polyester urethane topcoat and two other polyacrylic polyurethane topcoats following QUVA exposure are listed in Table 2. The three polyurethane topcoats listed in Table 2 can meet the performance level 3 standards - the top quality - of the SSPC Paint Specific. No. 36.

But topcoats like these can achieve an even higher level of performance. Beyond the 2000 hours of QUV, A exposure prescribed in SSPC Paint Spec. No. 36 provides some information about the performance capabilities that are possible with polyurethane topcoats and how they could be compared to other coating kinds. Exposure information, 2000 and 5000 hours for acrylic urethanes A and B are shown in Table 3. In addition to the extended exposure time, Table 3 shows the improved weathering that can be achieved by over-indexing formulation, i.e., the NCO/OH ratio is higher than the normal 1.05 ratio. The practice of over-indexing the polyurethane formulation is regarded as an inverse curing process that isocyanate/polyol cure (Scheme 1) combined with an encapsulation process that cures extra the polyisocyanate (Scheme 3.).

By applying the standard that 25% reduction in the initial gloss as specified by SSPC Paint Spec. No. 36. It's fascinating to present the data from Table 3 in the same manner illustrated in Figure 2. The duration of duration at which the different topcoats reach the 25 % gloss loss threshold is as below:

* Polysiloxane C 2300 hrs

* Urethane A 3800 hours;

* polysiloxane A = 5100 hrs

* urethane A over-indexed up to 6000 hours;

* polysiloxane B = 6200 hours;

*Urethane B = 6600 hrs;

* Fluoropolymer - Urethane crosslinked = more than 10,000 hours.

The urethane-crosslinked fluoropolymer is superior to, in a significant way, other topcoats in this weathering test, never exceeding the 25% loss in gloss threshold.

There are many polysiloxane coating types that are the same. Three topcoats are capable of lasting 10,000 hours of QUV exposure without losing all gloss. One of them does not meet SSPC Paint Spec. No. 36, performance level 3 in relation to yellowing of delta E (Figure 3.). Applying Paint Spec. No. 36 type of criteria The polyurethane topcoat formulations in this study have QUV A weathering properties that generally are similar to the three polysiloxanes that are commercially available during this investigation.

But, this study poses the following questions.

* What duration of exposure is suitable for an accelerated test of weathering?

* What degree of gloss retention would be appropriate for the marine and industrial market?

* How will these topcoats perform with weathering that occurs naturally?

* Paint Spec. No. 36 could be altered to incorporate more levels of performances? Maybe an additional level 4.

Conclusions/Putting it in Perspective

The QUV A performance in weathering of polyurethane topcoats varies based on the type of the raw materials. The high-quality acrylic co-reactants that are employed for polyurethane topcoat formulations are able to dramatically outperform acrylic polyurethanes that are low-cost.

Formulation variation can influence the results, for example, the isocyanate index. In particular, over-indexing isocyanate can result in significant improvements in the weathering QUV properties that are present in the polyurethane topcoats in this study. It is anticipated that this will be the case for polyaspartic and polyester urethane topcoats.

The comparison of high-quality polyurethane topcoat formulations with other coating types revealed that they perform better than commercially available polysiloxanes with respect to QUV A weathering properties. This study suggests that polysiloxane topcoats aren't all alike in QUV A weathering capabilities. Three of the topcoats are capable of providing 10,000 hours of QUV A exposure with no loss of gloss. However one of them doesn't meet the delta E requirements set by Paint Spec. 36.

The urethane crosslinked fluoropolymer performs better than the majority of other topcoats used in this weathering test, never exceeding the gloss loss of the 25% threshold that is set in the SSPC Paint Spec. No. 36. Delta E's performance of this coating was also exceptional.

The polyurethane topcoats in this study were of the most stringent QUV A performance standard that is required by SSPC Paint Spec. No. 36 relating to yellowing and loss of gloss.

How can one arrive at a conclusion on which topcoat has the best performance? It is dependent on the definition of the term "better. We would encourage caution when looking into one test method. This topcoat weathering research focuses on only one aspect that is part of the coating selection procedure for a corrosion-proof coating application. You must also take into consideration

1)  The characteristics of the paint e.g. pot life

2) Dry time and VOC levels, corrosion performance when it is part of a multi-coat system as well as

3) The cost of a multi-coat system.

Much attention has been paid to rapid weathering outcomes in recent years in comparison to the other aspects. Also, one must be aware of the importance of accelerated weathering in comparison to natural weathering. The results of this study were also submitted to South Florida weathering and will be released in the near future when the results are accessible.