Marc Roelands, R. Satguru and R. Swaans, DSM NeoResins+, Waalwijk, Netherlands
The potential and breadth of the use of waterborne polyurethane dispersions in surface coatings and in particular for flooring industry is widely recognized. The inherent presence of hard - soft segmentation together with the attribute of hydrogen bonding, allows polyurethane dispersions to achieve coherent coatings with excellent mechanical properties.
Concept of designing binders for flooring, which are not only eco-compliant but also satisfying low emission criteria whilst maintaining the high performances, will be the focus of the paper. This will be illustrated by considering the synthesis, morphology, self crosslinking and film formation aspects. To this end, the importance of achieving optimum property balances with excellent chemical resistances together with mechanical properties at near zero coalescent demand will be particularly highlighted.
Waterborne polyurethanes in surface coatings
An essential requirement for a useful surface coating is to be able to (i) provide a defect free and coherent film on the substrate in question and to (ii) develop mechanical and resistance properties. Aqueous polyurethane dispersions are well suited to satisfy the above issues due to their unique dispersion and particle morphology (1-3).
The mode of synthesis of polyurethane dispersions typically results in particles, which are poly dispersed in terms of its size, and swollen in terms of its internal particle morphology. In fact the particles behave as a collection of large macromolecular chains held together by intermolecular interactions. This dispersion morphology is clearly the outcome of the nature of the synthesis, which is based on step growth addition polymerisation followed by emulsification process. Additionally, the internal morphology of an anionic polyurethane particle is known to be swollen via water plasticization. The above two advantageous features (4) allow polyurethane particles to coalesce effectively to achieve excellent film formation. Moreover, the polymer chain make up of a typical polyurethane commonly contains soft and hard segmentation as shown schematically in Figure 1.
Soft segments mainly arise from high molecular weight polyol components while the hard segment contribution arise from diisocyanates, low molecular weight diols and urethane, urea bond sequences. The hard domains provide both physical interaction sites via hydrogen bonding and filler like reinforcement to the soft segment matrix. Dependent on the phase mixing via hydrogen bonding or de-mixing of the soft and hard segments polyurethanes can yield specific film properties, which are uniquely suited for surface coatings (5,6). The above features allow polyurethane dispersions to be employed in a wide variety of surface coating applications, notably in flooring applications where mechanical properties in particular are essential. Superior toughness, elongation, mar and scratch resistance properties are typical virtues of polyurethane systems.
Over and above the useful attributes of polyurethane systems, environmental and legislative pressures have driven resin manufacturers to place critical emphasis on the design of dispersions.
Relevance of ECO compliancy and low emission parquet lacquers
Within the defined EU directives, waterborne parquet lacquers are classified in the group of 1C and 2C performance coatings. The associated maximum level of VOC for this class of coatings is currently set within the European Union at 140 g/l. In addition to this compulsory European Directive, it seems that parquet lacquer manufacturers have the target to formulate at lower VOC levels in order to be able to market their products with eco-compliant certificates, such as Blue Angel, GISCODE classifications and Ecolabel.
Furthermore, one can conclude there is an on-going interest and trend from the European Union as well as certain European countries to extend maximum VOC levels towards maximum allowed measurable VOC emissions over a well defined period of time. The reason for doing so is that for instance, VOC emission and indoor air quality measurements relate much more to the final potential impact of the applied parquet lacquer on the health of the users of the involved building or room.
In Germany the use of building products is subject to the provisions of the building codes of the federal states. These provisions demand that built structures shall be designed, built and maintained in such a way that life, health or natural environment are not endangered. The importance of building products is also accounted for in the European Union by the European Construction Products Directive (CPD). One of the objectives of the building codes of the Federal States and of the European Construction Products Directive is to protect the building users' health.
There is no doubt that the building users' health has to be protected but it was still not clear how this protection can be realized in detail. The Committee of Health-related Evaluation of Building Products, AgBB (Ausschuss für die Gesundheitliche Bewertung von Bauprodukten), considers as one of its main tasks to establish in Germany the fundamentals for a uniform health-related assessment of building products so that the requirements specified in the building codes of the Federal States and the Construction Products Directive are satisfied, and an evaluation procedure will result which is traceable and objective as possible.
The Committee AgBB has constructed a procedural scheme for health-related evaluation of VOC emissions from building products used for indoor applications. Within this scheme, volatile organic compounds include compounds within the retention range of C6 to C16 which are considered both as individual substances and in calculating a sum parameter following the TVOC concept (TVOC = Total Volatile Organic Compounds) and semi volatile organic compounds (SVOC) within the retention range from C16 to C22.
With the above in mind, according to AgBB scheme, the emissions are measured in predefined test chambers, following a European standard; DIN EN ISO 16000 -9 to -11.
Translating this ongoing trend of maximum allowable VOC emissions towards the parquet lacquer industry leads to new lacquer developments needs in order to comply with these new proposed directives. In specific, this has strong implications to the polymers used in the formulation of parquet lacquers.
Traditionally used polymers for formulating parquet lacquers comprise polyurethane dispersions and urethane - acrylic combinations, in hybrid form or by means of physical blending. It is also known that these polymer technologies, to date, require substantial amounts of co solvents in order to deliver the high performance profile often associated with parquet lacquers.
In the last years, the parquet lacquer industry has been facing the challenge of eliminating the raw material (co solvent) NMP. However, being able to develop parquet lacquers which display minimized VOC emissions brings another hurdle to be taken. New polymer concepts need to be designed which require minimum amount of co solvents for proper film formation. Low VOC demanding polymer technologies will provide a basis for minimized VOC emissions over time. However, of crucial importance here is to design the low VOC polymer concept in such a way that it still provides the very high performance profile required for parquet flooring applications.
In this paper, we illustrate the design of a Low Emission Polyurethane Dispersion (LEPUD), aimed at achieving the combination all required facets ideally suited for wet and dry flooring properties.
LEPUD design - flooring property considerations
In general, excellent film formation and coherent coating without the need for cosolvent, can be obtained by using a predominant presence of soft building blocks. However, in such systems, it is often seen that mechanical properties relevant to flooring applications such as hardness development, taber resistance, black heel mark resistance (BHMR) and scratch resistance are not on the level that is required for parquet applications. In the LEPUD system, a delicate balance between hard and soft segmentation was employed, such that film formation in the absence of cosolvent can be attained. Additionally, the soft segments were strengthened by a crosslinking mechanism that is activated during and after filmformation. The crosslinking groups are present in the flexible domains, resulting in high crosslink efficiency. This resulted in LEPUD system exhibiting superior properties in near-zero VOC formulations.
Article to be continued in May.