18 European Coatings JOURNAL 06 l 2012 www.european-coatings.com

Technical PaperPolyurethanes

Neil Carter

The popularity of polyurethane chemistry in con-struction sealants is growing. However, moisture ingress and pinholing can be a problem. Oxazolidine latent hardeners can overcome these drawbacks.

Whilst there have been a number of technical ad-vances in the resins used in construction seal-ants, polyurethane (PU) chemistry continues to

offer the best balance between cost and performance. The chemistry is highly versatile, with aromatic and ali-phatic prepolymers offering the formulator a range of properties to meet differing performance requirements. The addition of oxazolidine latent hardeners reduces the negative effects of moisture and speeds up reaction.The main advantages PU offers over other technologies are better mechanical strength the absence of toxic by products of the cure mechanism (in contrast to the pro-

Scavenge moisture and cut pinholes in a triceOxazolidine latent hardeners in polyurethane systems

Contact:Dr. Neil CarterIncorez T +44 1772 201964 ngc@incorez.com

duction of methanol in silane-cured systems),, better substrate adhesion and ease of subsequent coating.The traditional method of curing PU sealants is to use the reaction of the terminal isocyanate groups of the polyurethane binder with moisture. This process requires a high level of isocyanate to give effective cure, which in turn leads to toxicity issues concerning residual isocy-anate monomer. Another problem is the production of carbon dioxide during the reaction. This results in pin-holing, a phenomenon which can then compromise the integrity and aesthetics of the finished coating.An effective way to eliminate such gassing and improve the cure performance of the sealant is to introduce an oxazolidine latent hardener. Such a product allows the formulator to combine the benefits of this type of hard-ener with a prepolymer at a much lower isocyanate lev-el. Typically, this can be reduced by as much as 10-15 %.The benefits of introducing oxazolidine latent hardeners into aromatic PU sealants are:

19www.european-coatings.com 06 l 2012 European Coatings JOURNAL

Technical PaperPolyurethane chemistry

» Elimination of gassing » Excellent through cure in thick film applications (com-pared to moisture cure/aldimine systems) » Retention of good workability of the sealant (skinning control) » Excellent early hardness development, as measured by Shore, through hardener crosslinking » No significant negative impact on mechanical strength or shrinkage through crosslinking at recommended levels of addition of the hardener » No detrimental effect on UV resistance

When handling PU prepolymer mixes containing oxazoli-dine latent hardeners, it is vitally important to keep mois-ture ingress to a minimum. It is good practice to purge of all mixing reactors with nitrogen and to reduce as much as possible exposure to atmospheric moisture during handling.

Gassing eliminatedTraditionally, PU sealants are cured by the reaction of ter-minal isocyanate groups with moisture. A side product of this reaction is the generation of carbon dioxide. This re-mains trapped in the cured sealant system. Figures 1 and 2 illustrate cured aromatic PU prepolymers (MDI & TDI) mixed with two oxazolidine latent hardeners, hardener A (“Incozol LV”) and hardener B (“Incozol EH”), typically at

Results at a glance There are a range of advantages in introducing

oxazolidine latent hardeners into aromatic PU sealant formulations.

The ability of oxazolidine latent hardeners to act as moisture scavengers ensures that the reaction between any moisture present and the isocyanate is prevented, thereby preventing the generation of carbon dioxide which would result in sealant defects.

The addition of an oxazolidine latent hardener into a formulation will promote faster through cure than that achieved by moisture alone. This is due to the ability of the latent hardener to hydrolyse quickly and crosslink the PU prepolymer in preference to the water and isocyanate reaction.

Introducing an oxazolidine latent hardener allows the formulator to use a prepolymer with reduced isocyanate content. Typically, the isocyanate level can be reduced by as much as 10-15 %.

Oxazolidine latent hardeners, such as hardener A and hardener B, bring added benefits to both aro-matic and aliphatic PU sealant formulations. They do this without affecting the physical properties of the system. There is no significant detrimental impact on the workability of the sealant, the tensile strength and the elongation.

a level of 2 % w/w, compared to the same prepolymers cured only with moisture.The addition of an oxazolidine latent hardener not only eliminates sealant defects created by gassing, but also promotes faster through-cure (see Figure 3). This reflects the ability of oxazolidine latent hardeners to hydrolyse quickly and crosslink the PU prepolymer compared to the water-isocyanate reaction.The addition of an oxazolidine latent hardener does not have a significant impact on PU-prepolymer viscosity and, consequently, does not affect the workability of the sealant. The initial viscosities of the oxazolidine/prepoly-mer blends shown in Figure 4, indicate the influence of the latent hardeners on the systems. Hardener A either reduces or retains the viscosity whilst hardener B increas-es it slightly. These differences offer options for formula-tors to retain the desired workability of the system.As regards storage stability, as expected, TDI-based mixes are more stable than MDI systems due to the increased reactivity of MDI prepolymers. For TDI systems, mixing with a latent hardener shows an initial viscosity rise when moisture ingress during mixing and handling occurs. How-ever, stability is excellent, with little rise in viscosity, as long as the containers are sealed from the atmosphere.In the case of MDI mixes, the stability is lower than with TDI. The data in Figure 5 reflects the inclusion of 2 % oxa-zolidine in MDI prepolymer mixes. Dilution of the binder

Figure 1: MDI prepolymer cured without oxazolidine (left), cured with 2 % hard-ener A (centre) and cured with 2 % hardener B (right)

Figure 2: TDI prepolymer cured without oxazolidine (left) and cured with 2 % hardener A (right)

European Coatings Conference

“Polyurethanes for high performance

coatings”25-26 September

2012 Berlin, Germanywww.european-coatings.com/

events

Technical PaperPolyurethanes

20 European Coatings JOURNAL 06 l 2012 www.european-coatings.com

and curing agent in fully formulated systems with dry, inert fillers affords improved stability.The addition of latent hardeners does not significantly alter physical properties such as hardness and tensile strength in the cured PU system. Crosslinking at low lev-els does not detrimentally alter sealant performance. In general, hardener B slightly increases the hardness and tensile strength of PU polymer, while hardener A somewhat reduces the hardness and tensile strength. This is due to the nature of their respective backbones.

Formulation hurdlesToday, formulators using polyurethane also face a number of hurdles when trying to achieve the desired film prop-erties and appearance. Whether the target is a high-gloss finish for a volatile organic component -compliant coating, a fast curing adhesive or a strong, flexible defect-free sealant, the issues facing formulators when attempting to develop new technologies with PU include: » Retaining and improving performance cure and physical properties of PU systems » Meeting legislation demands such as volatile organic component reduction » Reducing toxicity by decreasing isocyanate concentration

One of the biggest issues is the ongoing battle to deal with the presence of moisture often attracted through hydro-scopic polyols and solvents or present in pigments, fillers and plasticisers. The reaction of moisture with isocyanate can seriously compromise performance (for example, prop-erties such as film strength). Furthermore, appearance prop-erties can be affected by the generation of carbon dioxide. The oxazolidine technology offers an alternative low-toxicity solution to formulators of both one and two-component polyurethane systems in overcoming moisture-related for-mulation issues. All oxazolidine products are triggered by the reaction with moisture and it is their preferential reac-tion that helps the formulator limit the moisture-isocyanate reaction problem. Oxazolidine products can be used in aliphatic and TDI-based prepolymers. A number of grades are suitable for use in MDI systems. However, oxazolidine products offer benefits over and above just inhibiting the reaction with moisture. Some of these are described below in more detail.Formulators of sealants, adhesives and high-build elasto-meric coatings are always striving to improve performance properties. Traditionally, polyurethane systems incorporate a high level of isocyanate to speed up the cure rate to an effective level. This often requires the incorporation of toxic catalysts such as lead, mercury and, more recently, variants using tin. Oxazolidine technology however, offers a number of routes forward. For example, high-solid, one-component systems can incorporate oxazolidines in a number of ways. The presence of an oxazolidine not only reduces the prob-lems associated with CO2 generation and subsequent gas-sing, but also enables PU manufacturers to reduce the level of isocyanate present in the formulation. This is achieved by activating the oxazolidine, initially latent in the formula-tion, by mixing it with the formulation components to yield in-can stability. On application, and subsequent exposure to moisture, the oxazolidine-moisture reaction yields ami-no-alcohol functionality which can affect a through cure at much lower isocyanate concentrations, by a so-called

Figure 3: The figures of hardness development indicate faster cure when oxa-zolidines are blended, resulting in increased concentration of reactive groups as well as faster hydrolysis of oxazolidines

Figure 4: Initial viscosities of oxazolidine/MDI systems

Figure 5: Storage stability of oxazolidine/MDI systems at 23 °C

www.european-coatings.com 06 l 2012 European Coatings JOURNAL

Technical PaperPolyurethanes

‘moisture-triggering’ route. This approach allows formula-tors to reduce prepolymer total-isocyanate contents from typically 10-15 % to much lower toxicity levels of 2-5 %. Therefore oxazolidine products can help formulators achieve this aim with their selection based on property requirements. Furthermore, the oxazolidine latent-curing agent imparts an increased level of crosslinking through its four-functionality structure (two secondary-amine and two primary-hydroxyl groups). This helps formulators improve cure rate and impart additional tensile strength to the coat-ing, adhesive or sealant.Other routes are also available to formulators through the use of oxazolidines. The reaction of a hydroxyl-functional oxazolidine enables PU formulators partially to end-cap the isocyanate groups in the prepolymer further to reduce iso-cyanate content. This approach also imparts other benefits such as early tensile-strength development through latent in-situ crosslinking on exposure to moisture.Continued legislative pressure is forcing polyurethane for-mulators to find ways of significantly reducing the amount of solvent used in products to make them more environ-ment friendly. Low-viscosity, oxazolidine-reactive diluents such as hardener A, allow the design of two-component, PU, high-solid systems, with the added benefit of eliminat-ing gassing. Hardener A is a four-functional, bis-oxazolidine curing agent that significantly reduces hydrogen bonding through its carbonato-link and has a low viscosity of 50cps. This allows formulators effectively to reduce the viscosity of the product mixture. Therefore, formulators can continue to use the same application tools. Thus eliminating the need to retrain operators or dispose of the contaminated waste associated with using alternative water-based products.

Better film appearanceThe addition of an oxazolidine to a product can also im-prove coating appearance. The elimination of carbon dioxide pinholes from coatings, which occur as a result of the moisture-isocyanate reaction, by the preferential reaction of a moisture scavenger, provides further signifi-cant benefits for the formulator. Pinhole defects lead to problems such as downglossing or more significantly can compromise film integrity lead-ing to weaknesses such as loss of film strength or ease of attack by a chemical reagent.Oxazolidine technology has been adopted in a wide ar-ray of polyurethane coating, sealant and adhesive ap-plications. The polyurethane-coatings sector has chosen this technology for use in two-component, high-solid systems for applications in the automotive, marine, wind-turbine and aerospace sectors. The use in one-component polyurethane coatings is restricted to the industrial-maintenance sector.Oxazolidines are widely used in one-component aliphatic and aromatic polyurethane cartridge sealants. These high-performance, elastomeric, polyurethane sealants require no mixing and typically no priming to promote adhesion to many substrates, including concrete and ma-sonry.There is a growing interest in the use of oxazolidines in the production of one-component, reactive, hot-melt ad-hesives, where they accelerate the cure rate and improve both green strength and crosslink density. í

www.sili.eu

Milling

Grinding

Dispersing

... better grindingSiLibeads®

The German

spirit of quality

since 1854

More than 155 years:

SIGMUND LINDNER GmbHOberwarmensteinacher Str. 38 · 95485 Warmensteinach / GermanyPhone (+49) 92 77 - 99 40 · Fax (+49) 92 77 - 9 94 99E-Mail: sili@sigmund-lindner.com

Beads were not our invention, but

we make them to perfection.

Please visit us

18th – 22nd June 2012,

ACHEMA Frankfurt/Main,

Hall 6.0, Stand B 42

anz_beads_hoch_105x297mm_06.indd 1 14.05.12 13:15