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10The article has entered in publishing office 13.01.10. Ed. reg. No. 751
Статья поступила в редакцию 13.01.10. Ред. рег. № 751
ISOCYANATES FOR COATINGS APPLICATIONS F. Richter, R. Halpaap, U. Meier-Westhues
Bayer MaterialScience AG, D-51368 Leverkusen, Germany e-mail: frank.richter@bayermaterialscience.com
Referred: 26.01.10 Expertise: 31.01.10 Accepted: 06.02.10
The article describes the chemistry of polyisocyanates for coatings applications starting from the basic principles and reactions in polyurethane (PUR) chemistry. The derivatization of diisocyanates to polyisocyanates with low residual monomer content suitable for coatings is discussed. Curing of the two component systems lead to PUR coatings with the known outstanding performance due to urethane linkages and intermolecular hydrogen bonding resulting in high chemical resistance and durability of the final film. Adjustment of components - polyisocyanates as well as polyols as reaction partners - is the basis of new developments of so called "self healing" PUR systems for coatings applications. Modern ,very high solids' coatings containing more than 90% non volatile components as well as hydrophilically modified hardeners for waterborne systems are the basis of highly efficient coatings and improvement of the ecological profile of the whole technology of surface protection.
Keywords: polyisocyanate, polyurethane, isocyanate, polyol, coatings.
ИСПОЛЬЗОВАНИЕ ИЗОЦИАНАТОВ В ПОКРЫТИЯХ
Ф. Рихтер, Р. Хальпаап, У. Майер-Вестхьюс
Заключение совета рецензентов: 26.01.10 Заключение совета экспертов: 31.01.10 Принято к публикации: 06.02.10
В статье описана химия полиизоцианатов с точки зрения их использовании в покрытиях, начиная с общих принципов и реакций в химии полиуретанов. Рассмотрена дериватизация диизоцианатов в полиизоцианаты с низким содержанием остаточного мономера, пригодные для использования в покрытиях. При отверждении двухкомпонентных систем образуются полиуретановые покрытия, которые известны своими исключительными эксплуатационными характеристиками благодаря уретановым группам и межмолекулярным водородным связям, которые обеспечивают высокую химическую устойчивость и износостойкость получаемых пленок. Подбор компонентов - полиизоцианатов, а также полиолов в качестве реагирующих веществ - лежит в основе новых разработок - так называемых «самовосстанавливающихся» полиуретановых соединений для использования в покрытиях. Современные высокотвердые покрытия с содержанием нелетучих компонентов более 90%, а также модифицированные гидрофильными соединениями отвердители для систем на водной основе - это базис для создания высокоэффективных покрытий и улучшения экологических характеристик технологии защитных покрытий в целом.
Ключевые слова: полиизоцианат, полиуретан, изоцианат, полиол, покрытия.
Dr. Frank Richter, born 1962, studied chemistry at the TU Dresden and Martin-Luther University, Halle-Wittenberg followed by a postdoctoral stay at the Massachusetts Institute of Technology. Since 1994 he is working on isocyanates at Bayer AG, now Bayer MaterialScience, Business Unit Coatings, Adhesives and Specialties.
Frank Richter
International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010
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Dr. Reinhard Halpaap, born 1949, started within Bayer AG after studying chemistry at the TH Darmstadt in April 1981. Since that time he is involved in the development of polyisocyanates for coatings applications with the main focus on aliphatic moieties. Currently he is responsible for "Product and Process Development" of Polyisocyanates within Bayer MaterialScience, Business Unit Coatings, Adhesives and Specialties.
Dr. Ulrich Meier-Westhues, born 1955 in Pirna/Elbe, studied chemistry at the RWTH Aachen. After four years employment at Herberts GmbH in 1989 he changed to Bayer AG. Today at Bayer MaterialScience, Business Unit Coatings, Adhesives and Specialties he is responsible for Business Development Polyisocyanates.
Ulrich Meier-Westhues
Isocyanates - known to the organic chemist for more than a century - experienced a dramatical boost in importance to mankind as whole when Otto Bayer discovered the diisocyanate polyaddition process [1-3] giving birth to a new class of macromolecules: the polyurethanes.
Initially intended to rival the commercially eminently successful nylon fibers [3], polyurethanes have soon shown to give impact to a vast variety of applications, coatings being a rather 'young' example among the latter.
Otto Bayer and co-workers recognized that the properties of paints and varnishes based on alkyd resins could be improved if the starting material was modified with an aromatic diisocyanate.
However, the major breakthrough of polyurethane technology in the coatings area is attributable to the discovery of K. Wagner [4] who developed a process for the manufacture of polyisocyanates derived from aliphatic diisocyanates that were virtually free of monomer, low in viscosity and - most importantly - led to lightfast polyurethane coatings. The abbreviation DD (for the Bayer tradenames Desmodur - for the polyisocyanate - and Desmophen for the reaction partner, typically a polyol) is now a generally accepted synonym for superior quality two component coatings raw materials.
Chemistry of Isocyanates
Polyurethane chemistry for coatings applications is based on the combination of polyisocyanates with polyols. Polyisocyanates - also called ,hardener' - play the most important role with respect to processing, curing and the properties of the final paint.
Polyisocyanates are manufactured from diisocyanates [5]. Depending on the properties required, aliphatic as well as aromatic components may be employed. The technically most important representatives of both classes are given in Fig. 1.
An important attribute of isocyanates is their high reactivity. One makes use of the reaction of isocyanates with one another or with small, NCO-reactive molecules such as water or short-chain primary or polyhydric alcohols, respectively, in order to obtain polyisocyanates suitable as a hardener for coatings applications. While isocyanate reactions with most alcohols and even more with amines are fast in practice, most NCO-NCO-reactions require the presence of a catalyst. The nature of the latter has a significant influence on the type of products formed [5, 6]. Fig. 2 and 3 illustrate the broad variability of isocyanate chemistry as a whole that can be used for the control of the final properties of the hardener obtained.
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Fig. 1. Technically important aromatic (top) and aliphatic diisocyanates (bottom) except for HDI, different isomers and oligomers (in the case of MDI) exist
R
v + r-nco N==N -»
R R R R
I I I I
•YyYY
O O O O
Homopolymer (1- or a-Nylon)
O
R^ JL. ^R
N N
o^Y^O
R
Isocyanurate (,trimer')
R
Carbodiimide
n-r
A
R-N N-R
Y
O
Uretone Imine
R—NCO
*2]
O
%An'r
Iminooxadiazindione (,asymmetrical trimer')
O
N N O^O^O Oxadiazinetrione
O
A
R-N N-R
Ï O
Uretdione (,dimer')
Fig. 2. Cyclopolymerization and polymerization of isocyanates (schematic)
International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010
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2
n
Reaction partner / product Schematic equation
Alcohol / Urethane R 1 R-NCO + HO—R' -H'N^fa4R' O
Urethane /Allophanate R R R R-Nco + hny%' -^ hnyny%' O o o
Water / [unstable] Carbamic Acid / Urea r—nco + hoh - r o rr 1 1 + rnco n n - co2 11 o
Ammonia, primary, or secondary Amine / mono- di- or trisubstituted Urea r' r r' r—nco + ^ —- Hnyn-R" o
Urea / Biuret r r' r r r' Il III r nco + h y r'' h y Y o o o
Carboxylic Acid / Amide R 1 ' R-NCO + H п -H'NNf"'R' O - co2 H п O
Amide /Acylurea R R R 1 1 1 R—NCO + H-NyR' - O O O
Anhydride /Imid O O RNCO + O J -- RN) у-у - co2 OO
Epoxide / Oxazolidone O R' l r-nco + vr ^cat^ r-n^o O 4 R'
Oxime / Oximcarbamate r' r r' ^nco + o_n^r" -- h^y^Ar- h o
Fig. 3. Reactions of isocyanates with various reactants (schematic)
Polyisocyanate Hardeners
Urethanization, allophanatization, biuretization, as well as ,dimerization' and ,trimerization' are of technical importance for the manufacture of polyisocyanate hardeners from the starting monomeric diisocyanates.
A prominent example for a modification reaction conducted by urethanization is the synthesis of the first ever example of a polyisocyanate especially made for coatings applications: Desmodur L (L in this context stands for 'Lack', the German word for paint or lacquer). It is manufactured from an aromatic diisocyanate, TDI, and a triol. In order to ensure acceptable workability of
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the resin the composition is further adjusted. Coatings derived from this polyisocyanate exhibit good mechanical and chemical stability but suffer from low lightfastness. The latter reduces their scope in practice to indoor applications, e.g. as a furniture finish.
Coatings based on aliphatic polyisocyanates do not suffer from that disadvantages and find use in various applications. The first ever representative, Desmodur N 100
(N stands for the German phrase ,nicht vergilbend' - non yellowing), containing the biuret structure in its chemical backbone, is still a benchmark in the polyisocyanate hardener market. Todays most important polyisocyanates for weather resistant, lightfast polyurethane-based coatings are the HDI-biuret (e.g. Desmodur N 75) and the HDI-isocyanurate (e.g. Desmodur N 3390) (Fig. 4).
Fig. 4. HDI-biuret (left) and -isocyanurate (right) - idealised structures
Properties of Polyurethane Coatings [7]
Polyurethane technology is based on a combination of polyisocyanate hardener and hydroxyfunctional polymers such as polyesters, polyacrylates, polyethers and - increasingly important - polycarbonates (Fig. 5).
Polyols are first processed to a masterbatch and the polyisocyanate hardener is added just prior to application due to their high reactivity (two component technology). The final properties of the polymer film such as gloss, reflow, scratch and chemical resistance as well as light-and weather stability can be adjusted by the proper choice of polyol and polyisocyanate. Properties like ,easy-to-clean' or ,soft-feel' may thus be adjusted by skillful formulation of the whole coatings recipe.
Polyurethane coatings owe their outstanding performance to the urethane linkage and their chemical inertness to hydrolysis and saponification. But the ability of the polyurethane network to form intermolecular hydrogen bonds is important too (Fig. 6).
These linkages increase the durability of the polymer film and are the structural reason for the so called reflow or 'self-healing' properties of polyurethane coatings (Fig. 7). Minor damages like scratches or mars of a polyurethane coat may thus, above the glass transition temperature, 'heal' by the aid of the restoring forces of the inherent hydrogen bonds. This phenomenon is made use of e.g. in two component polyurethane clear- or topcoats for motor vehicles.
O
H \ 1
~*NCO + —
H
Polyisocyanate Polyol Polyurethane
Fig. 5. Polyurethane formation
R
N-H—Ov R'
M hO
R' O->- H-N
\
_R
Fig. 6. Hydrogen bonding in polyurethane networks
Fig. 7. ,Self-healing' two component polyurethane clearcoat with high reflow ability
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Applications of Polyurethane Coatings
Polyurethane coatings today are established in a wide variety of applications due to their capability for tailor-made developments. And this will even improve in the future, as less volatile organic compounds (VOC) containing systems will be more and more enforced for ecological and also economical reasons. In the past, coatings producers regulated the viscosity of their coating systems by adding solvent. Today and even more so in the future, statutory environmental protection requirements restrict this practice. The coatings industry's answer is "high solids," often with prefixes such as "very" or "ultra," up to "100%" instead of "high". Waterborne systems are another alternative [8].
Two component polyurethane systems based on aromatic polyisocyanates are, as has already been outlined above, especially applied in the wood coatings sector, for corrosion protection and in flooring applications.
Aliphatic Polyisocyanates may be employed in a much broader range of applications. Aircraft coatings were the first market captured by them as they combined fast hardening even at ambient temperature with an excellent light stability, also against UV-rich radiation and robustness to frequent and fast temperature fluctuations.
Today the application range has broadened significantly and spans from car refinish and OEM coatings (original equipment manufacturing: approximately 25% of all cars produced worldwide are protected by a polyurethane clearcoat) over metal-, wood-, plastics-, glass-, textile coatings, to marine coatings, protection of pipelines against corrosion and even in dental applications one may find aliphatic polyisocyanates as one key ingredient.
Perspectives of Polyisocyanate Developments
New Polyisocyanates will help to increase efficiency while maintaining and even improving the ecological profile of the whole technology of surface protection. One aspect is the economy of the application process itself (reduction of coating layers and/or production steps e.g. by the so called 'wet-in-wet' process) and also the aspect of long term durability of the object coated by increasing its robustness to chemically or mechanically induced degradation.
As polyisocyanates are always higher in viscosity compared to the diisocyanates they are made from, a general requirement for new hardeners is a reduced viscosity. The general trend can be found in the Table.
Viscosity of typical HDI-based polyisocyanates in mPas at 23° C
Structural type Technical product Bayer trade name Desmodur ... Average NCO-functionality
Uretdione 150 +/- 80 N 3400 < 3
O-Alkyl-Allophanate 350 +/- 100 XP 2580 < 3
Iminooxadiazindione 700 +/- 100 N 3900 > 3
Isocyanurate 1200 +/- 300 3000 +/- 750 N 3600 N 3300 > 3
Biuret 10000 +/- 2000 N 100 > 3
Laboratory scale isolated products with the respective lowest molecular weight of "Ideal structures" (see Fig. 2 and 3) have even lower viscosities.
However, viscosity isn't everything; the number of NCO groups in the hardener molecule, the (average) NCO functionality, determines the quality of the final coating film. A trimer, an isocyanurate and particularly an iminooxadiazinedione, is always superior to a dimer -more precisely, a uretdione, or an allophanate - even if the latter two exhibit a significantly lower viscosity (Table).
Modern, ,very high solids'-coatings, containing more than 90% of non volatile matter, may be formulated with the new polyisocyanate hardeners enabling the end user to obtain a high film thickness even in one process step.
Moreover, besides hydrophilically modified hardeners [8], low viscous hardeners may be incorporated into waterborne systems as the dispersibility of a hydrophobic material is always improved by a reduction in viscosity.
References
1. DRP 728981 (1937) I.G. Farben; O. Bayer, Angew. Chem. 1947, 59, 257-288.
2. D. Dieterich // Chemie in unserer Zeit. 1990, 24, 135.
3. J.H. Saunders and K. Frisch. Polyurethanes, Chemistry and Technology, part I. Interscience Publishers, 1962.
4. DE-A 1101394 (1959) Bayer AG; K. Wagner.
5. H.J. Laas, R. Halpaap, J. Pedain // J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200.
6. F. Richter // Chem Eur. J. 2009, 15, 5200-5202.
7. U. Meier-Westhues. Polyurethanes: Coatings, Adhesives and Sealants. Vincentz Network, 2007, ISBN 3-87870-334-1, ISBN 978-3-87870-334-1.
8. Avtomonov E., Ristic V., Vollmer M. Waterborne Technologies for Polyurethanes Coatings // ISJAEE. 2010. No. 4. P. 14-21.
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