Eco Compliant and Low Emission PUD Concept with enhanced flooring properties

Marc Roelands, R. Satguru and R. Swaans, DSM NeoResins+, Waalwijk, NetherlandsContinuation of FAPU March.

LEPUD - Dry Film Properties - Combination of excellent film formation and a high performance level
The ratio of urethane bonds and urea bonds is highly important for the phase separation of hard and soft domains of the resin, as well as for the cohesive strength of the hard domains (5,6). Urea segments provide much more dense hydrogen bonding interactions (especially in the case of chain extension with hydrazine), compared to the urethane bonds in the backbone.

Grafik-2-deFigure 2: Hydrogen bonding of urea (left) vs urethane bonds (right).

In the LEPUD system, more than 70% of all isocyanate groups are reacted to form a urethane bond and less than 30% is reacted to urea bonds. This results in a large reduction of the minimal filmforming temperature. The concomitant loss of intrinsic hardness is compensated by crosslinking.

The excellent film formation of the LEPUD system in the absence of coalescing agent or additives, is illustrated by Atomic Force Microscopy in figure 3. The phase image shown is clearly indicative of the coherency of the film with high degree of coalescence.

The crosslink mechanism is based on reaction between carbonyl and hydrazide groups(7). The crosslinking groups are incorporated in the flexible part of the urethane backbone and the degree of crosslinking is demonstrated to be of high importance in obtaining the right balance in properties.
Grafik-3-deFigure 3: AFM phase image of the coherent film, obtained from unformulated LEPUD.

Grafik-4-enFigure 4: Effect of crosslink density on mechanical properties of LEPUD system.
Crosslink density is expressed as mmol crosslinks per gram of solid resin.
Mc is expressed in kDa. BHMR: 0 = very poor, 5 = excellent

This obvious increase in mechanical properties is depicted in Figure 4a and b. König Hardness and BHMR are plotted against crosslink density. The molecular weight between crosslinks (Mc), was used to assess the different crosslink densities, relative to each other.

Upon increasing the amount of crosslinker in the LEPUD system, the coating hardness and BHMR is improved up to levels that are accepted for parquet flooring applications. Important here is that the film forming temperature is however unaffected. The LEPUD sample with the highest crosslink density shows a minimum film forming temperature below 0°C and therefore requires no cosolvent to form a coherent film.
Grafik-5-enFigure 5: Dirt pick up resistances upon variation of crosslink density
of the LEPUD system, expressed as delta E values and depicted via
photographs of the cleaned films.

Dirt pick up resistance of LEPUD

Dirt pick up resistance (DPUR) is a much sought after feature for flooring and decorative coatings. It reflects the resistance against the adherence of dust and dirt in the environment and the appearance of the film after cleaning. Often it is measured as a parameter during durability testing, where the colour change compared to the initial white lacquer is expressed as a delta E value.

A semi quantitative test which gives relatively good correlation with outdoor tests, was employed for this purpose. In this test, a solution of a black pigment in ethanol/water is cast on a white pigmented coating and dried for 30 minutes at 60°C. Subsequently, the surface is cleaned with a soap solution and the discolouration was assessed via the measurement of delta E value. Low delta E value indicates good dirt pick up.

Grafik-6-enFigure 6: König hardness and minimum filmforming temperature of blends
between a hard particulate emulsion and the LEPUD system, at various ratios.

In Figure 5, delta E value of LEPUD system as a function of crosslink density is shown. Also shown in Figure 5, are the test panels after the cleaning procedure. It is clearly evident that the resistance against the adherence to dirt improves significantly with the increase in crosslink density.

LEPUD - Wet film properties
Water based resins for parquet lacquer applications are of course always designed with careful consideration to deliver a maximum performance profile of the final dried coating. In the final usage of the floor, the lacquer should be able to withstand high level of mechanical and chemical resistances, as floor coatings are often exposed to high traffic environments.

However, equally important, are the aesthetic properties of parquet lacquers. "Look and feel" of a coated floor is the first impression that the applied parquet lacquer leaves in the mind of the final end-user.

Traditional fatty acid modified (oxidative cure) polyurethane dispersions tend to exhibit significant colour differences in the case of varying layer thicknesses. These layer thickness variations are often the result of the practical mode of application of the parquet lacquer onto the floor. Such lapping issues are unwanted as they leave disturbed and irregular appearances in the final dried coated floor.
This disadvantage is overcome in the LEPUD concept where the cross linking mechanism employed is independent of layer thickness. Moreover, the design of the LEPUD system takes into account of minimizing lapping characteristics.

Yet another wet film property of crucial importance is the open time of the applied parquet lacquer. As open time is often steered by the addition and selection of co solvents, this property is getting more challenging when low emission demands are requested. The LEPUD concept described in this paper provides the required good open time characteristics. This is effected by (i) judicial choice of polyurethane backbone polymer in providing an excellent basis for keeping the drying film open longer and (ii) LEPUD concept is designed in that it hardly needs any co solvent to provide its maximum dry film properties (i.e. mechanical and chemical resistances) thus enabling one to consider addition of selective co solvents which contribute to further open time optimizations, without influencing emission measurements in a negative way.


Combinations with other resins
As illustrated above LEPUD concept provides excellent set of flooring properties without the need for cosolvent. Additionally, it has also the virtue of combining with other resins to achieve certain advantageous effects. For example, LEPUD tolerates the addition of a significant amount of a hard, non film-forming resin, without compromising film formation. For example, up to 25 wt% of precrosslinked, hard particulate dispersion can be combined with LEPUD, without requiring cosolvent for coherent film formation. It is important to note however, that at this blend ratio, a significant increase in König Hardness of LEPUD is evident. This effect is illustrated in Figure 6.

The LEPUD Concept has been shown to exhibit the required flooring properties, which are not only eco-compliant but also satisfying low emission criteria whilst maintaining the high performances. The importance of achieving optimum dry and wet property balances with excellent chemical resistances together with mechanical properties at near zero coalescent demand have been highlighted.

Authors wish to thank Paul de Kok, Lex Donders and Michel van Hoof for their contributions in preparing this paper.

Drs. Marc Roelands holds degrees Physical Organic Chemistry from the University of Utrecht, Holland. He started his career with DSM NeoResins+ in 1997. His Research and Development programs have focused in the technology fields of acrylic and polyurethane dispersions and hybrid concepts, as well as water based UV dispersions. Currently he holds the position of Industry Manager, being responsible for the market segments of coatings for decorative, flooring and construction applications.
Dr. Guru Satguru obtained a PhD in polymer chemistry at University of Lancaster UK and post doctoral research in polymer colloids at University of Bristol UK. At DSM NeoResins+, he holds the position of Business Research Associate with research interests in waterborne surface coatings.
Drs. Roel Swaans studied chemistry at the University of Nijmegen, The Netherlands. After his graduation in 2000, he joined DSM NeoResins as a research scientist.

(1) J. C. Padget, J. Coatings Tec., 66, (839) 89, (1994)
(2) B. K. Kim, Colloid Polm Sci., 274, 559, (1966)
(3) M. Hirose, J. Zhou, K. Nagel, Prog Org Coatings, 38, 27, (2000)
(4) R. Satguru, J. Mcmahon, J. C. Padget, R. G. Coogan, J. Coatings Tech., 66, (830) 47, (1994)
(5) Wang, Macromolecules, 16, 775-786, (1983)
(6) Delpech, Coutinho, Polymer Testing 19, 939-952, (2000)
(7) F. Buckman, A. Overbeek, T. Nabuurs, Eur. Coating J., 6, 53, (2001)