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Technical Update provide relevant technologicial information, presented in a lively format, and of particular interest to newcomers to the surface coatings industry.
Updates from previous volume 
Updates from next volume
Optimising the graffiti resistance of polyurethane coatings
The importance and market share of polyurethane coatings has steadily increased due to their excellent properties. Typical applications for high-performance polyurethane coatings now include automotive OEM and refinish paints, coatings for commercial vehicles (buses, trucks, rail vehicles, aircraft), industrial metal and plastic coatings, wood coatings and others.
The development of coatings technology for railway coaches and locomotives in most western European countries provides an example of technological changes driven by quality and efficiency. In Germany, for example, drying oils were the dominant resin technology for rail vehicle coatings until the 1930s. These coatings were characterised by high gloss, but gave very slow drying and poor durability, resulting in a need to repaint the vehicles almost every year.
By switching to the fast physically drying nitrocellulose resins (NC), repainting time could be significantly shortened, despite the low solids content of NC paints (~15%, meaning that several layers had to be applied to reach the required dry film thickness). Moreover, repainting intervals could be extended to about two years.
The greatest advance in coating quality, however, was the introduction of solvent-borne 2K polyurethane coatings by Deutsche Bahn in 1967. These coatings showed much better resistance against solvents and chemicals (including the cleaning systems) and dramatically improved durability. Complete repainting of a rail vehicle painted with 2K polyurethane is now only necessary every eight to ten years; after three to six years the vehicles are inspected and if necessary local repairs are made to the coating or a new topcoat layer is applied on top of the existing one.
Excellent film appearance, superior scratch resistance, reduced dirt uptake and a very efficient coating process - due to the high reactivity of the liquid paint - are the main benefits of 2K polyurethane coatings, resulting in widespread use of this technology in rail vehicle coatings through to the present day.
Water-borne coatings in the furniture industry: meeting demands for appearance and performance
The wood and furniture industries are very concerned by the implementation of the European directive relating to the reduction of VOC emissions, published in 1999, and harmonised with the French regulations in May 2000 and May 2002. Companies must use environmentally-friendly technologies to replace solvents which, in 80% of cases, are still used in the wooden furniture industry to produce decorative finishes by using stains, varnishes, lacquers etc.
The alternative coatings must meet the following criteria:
Most companies use many coatings, and must reduce their emissions by 50 to 80%. All options are being considered to reach this target, including VOC destruction equipment as well as technologies with little or no solvents, provided that the new process will comply with the criteria described above, and that the investment costs and plant modifications are not excessive.
Biodegradable nanocomposites and their applications
Synthetic polymers - generally referred to as plastics - have become technologically very significant because they have replaced glass, wood, masonry and other construction materials and even metals in many industrial, domestic, commercial and environmental applications. They have gained ground because of their favourable mechanical and thermal properties, easy processability, stability and durability.
Using hyperdispersants in universal pigment dispersions for UV coatings
UV coatings can be made from 100% non-volatile components, thereby giving essentially zero VOCs. However, due to the viscous nature of the polymers used in UV applications, producing thin films can be an issue, especially if they are also pigmented. Even though low-viscosity materials are available, maintaining a stable Newtonian rheology still remains a challenge for certain UV applications.
Hyperdispersants can help to alleviate the main problems with these systems. In general, UV resins are poor at wetting out the surface of most pigments, which is where hyperdispersants are at their very best. When they are used at the correct dosage, the rheology of a pigmented coating can be dramatically improved.
Naturally, they will not affect the rheological properties of the resin itself, but they will minimise the overall viscosity increase caused by the pigment. This will enable the formulator to achieve higher pigment loadings at a given viscosity, or better flow at a given pigment loading, both of which are essential for certain ink applications such as offset, flexo and digital.
Food contact compliant defoamers for inks and overprint varnishes
Various regulatory bodies around the globe are responsible for ensuring the public is protected against chemicals that may transfer into foodstuffs from packaging. For example, in the USA the Food and Drugs Administration (FDA) regulations specify that if a material will be in direct contact with food, it must be composed only of direct or indirect food additives that are found in its regulation 21 CFR 170-189.
Whilst European Union Regulation (EC) number 1935/2004 does not indicate specific materials that can be used in printing inks and overprint varnishes, Council of Europe Resolution AP (2004) 1 on Coatings and AP (2002) 1 on Paper and Board contain open 'positive' lists of materials that can be used in printing inks and overprint varnishes. In future, these resolutions will become Codes of Practice or legislation.
Details surrounding these various regulations are often confusing to both manufacturers and printers. Often printers request compliance from the printing ink and overprint varnish manufacturers. For example, EC Directive 2002/72/EC sets limits on materials allowed in the manufacture of plastics. Though this is not directly related to printing inks and overprint varnishes, misunderstanding of this legislation often leads to printers requesting compliance with this directive. This in turn leads manufacturers towards using only additives that comply with regulatory bodies' regulations in their printing inks and overprint varnishes.
Changes to volatile organic compound (VOC) regulations in both Europe and the USA are also driving manufacturers and printers to produce water-borne alternatives to current solvent-based technology. For example, European Directive 2004/42/CE sets limits on the allowed emission of VOCs due to organic solvents for both manufacturer and printer. Although it is more environmentally acceptable, water-borne technology brings with it the inherent problem of foam generation.
The use of silicone-polyether copolymers to combat foam in printing inks and overprint varnishes has a number of benefits over traditional polydimethylsiloxane (PDMS) technology, while at the same time they meet the various current regulations for food contact applications. It will also be demonstrated that very low addition levels can be used, leading to low cost in use.
Accuracy of the weathering of paints containing titanium dioxide pigments - A critical retrospective on 20 years of weathering
Decisions we make today depend on our expectations for the future. This also applies to the selling or purchasing of paints for coatings. If we knew that the good properties of the material would only last for a few months, we would never buy it, nor could we sell it. We therefore require a reliable method to assess the durability of coatings.
The worldwide way of doing this is to test the materials in devices for accelerated weathering. There are different kinds of apparatus on the market, which differ mainly as regards the type of radiation source used, namely fluorescent lamps, xenon lamps, carbon arcs or metal halide lamps.
But all results obtained with this equipment must face the same question: how do they correspond to the results of real outdoor weathering? In other words, do they reflect the same ranking regarding the observed properties, is the differentiation of these properties as it would be found outdoors, and will we get the same results if we repeat the test? Ultimately this means - are the weatherometer (WOM) results meaningful for predicting the behaviour of the material in real outdoor exposure?
As we all know, weather is not repeatable and the climate around the world varies considerably. We have to concentrate on the question: how repeatable are the results of accelerated weathering and what is the magnitude of error of the method?
Solvent-free secondary dispersions for water-borne 2K PUR coatings
Two-component polyurethane coatings (2K PUR) have gained a substantial market share since their introduction in the early 1960s. They combine excellent appearance (high gloss, wet look) with high levels of resistance against chemicals, solvents, atmospheric exposure, etc. Another important advantage of 2K PUR systems is their ability to cure under very mild conditions, producing high-grade coatings even without stoving.
Typical applications include automotive OEM and refinishing paints, commercial vehicle coatings (buses, trucks, rail vehicles, aircraft), industrial metal and plastic coatings, wood coatings and others. These are examples of technology changes driven by quality and efficiency. However, coatings and raw material producers are constantly forced to improve their technology in order to remain competitive.
Because of increasing environmental concerns, the volatile organic compound (VOC) emissions of coating systems have become a major issue. The first response from paint companies was the development of medium-, high-, or very high-solids solvent-borne 2K PUR coating systems. However, as can be seen from Figure 1, the VOC level of even the very high solids systems is still substantial. As a result of yet stricter environmental legislation, the VOC level achievable with these systems will no longer be acceptable for certain applications. A significant step in the right direction, though, was the introduction of water-borne 2K PUR coatings.
Nanocomposite dispersions for water-based coatings
Nanotechnology (materials which contain structures or features that have at least one length scale ranging from molecular dimensions to approximately 100nm) has gained much interest over the last decade. Benefits can be achieved from the use of (inorganic) nanoparticles which are not observed when using conventional macroscopic filler materials.
This is due to the very high surface area developed between the phases and/or the confinement effects. It becomes possible to achieve the best of both worlds: the properties of the hard and hydrophilic inorganic material along with the flexibility and film formation of the less polar organic polymer.
The synthesis of water-dispersed composite particles with nanometre-size domains differing in chemical nature has so far mostly been achieved with purely organic structures, such as core-shell particles. These modern, environmentally-friendly binders combine a soft film-forming polymer-phase with a hard, non-film-forming polymer domain.
These binders can be formulated without solvents yet show sufficient film hardness and good elasticity. One disadvantage is their thermoplastic behaviour: at elevated temperatures the polymer softens when insufficiently cross-linked. Here, an inorganic hard phase offers an interesting and new approach to this problem.
Intensive work, especially at universities, has now facilitated the combination of inorganic and organic material on the nanometre scale. In the field of aqueous dispersions, little experience has been gained with inorganic/organic nanocomposite particles in architectural coatings and lacquers, but this area has great value in the future for European architectural coatings businesses.
In this paper, the combination of a film-forming polymer with an inorganic hard phase in a water-based dispersion, and its application potential in the field of coatings, will be highlighted.
Dry film biocide technology: new problems, new solutions
Although the effects of mould growth on the interior of buildings have been recognised for many years, it is only recently that general public demand has led to the widespread availability of specialist fungicidal coatings.
In the last two decades, recognition of the impact of algae in causing defacement and deterioration of exterior surfaces, allied with the increased cost of remedial treatment, has focussed attention on the development and testing of combined fungicidal/algicidal coatings.
This paper reviews the causes and effects of microbial growth on surface coatings, considers changes in regulatory requirements, environmental and consumer pressure and building insulation regulations, and presents a novel approach to preparing biocides to meet these new challenges.
Rosin-modified lac as a binder for road marking paints
Hot-melt thermoplastic (HMTP) road marking paint is very widely used today for road marking. This type of paint dries quickly in less than ten minutes and so is ideal for marking under continuous traffic conditions. The paint is heated to 180 to 200°C in a kettle with constant stirring, released from a closed system and sprayed onto the road surface.
Dual UV/thermal cure systems based on partially acrylated polyols
Traditional UV curing systems based on acrylate monomers and oligomers lack flexibility and adhesion to difficult substrates. Two-pack (2K) isocyanate coatings, on the other hand, lack reactivity and productivity. By combining both chemistries and their intrinsic advantages while bypassing their individual disadvantages, dual cure systems based on two-pack isocyanate coatings and UV formulations open up new horizons for formulators.
As opposed to simple blends of conventional acrylate monomers/oligomers and two-pack isocyanate formulations, the authors present here dual functionality monomers and oligomers bearing hydroxyl and acrylate groups (partially acrylated polyols), allowing cross-linking and the formation of interpenetrating polymer networks (IPNs) between the acrylate and the polyurethane networks. Film forming and cross-linking can take place even in non-UV-exposed areas.
These new high-solids systems are suitable for interior and exterior applications for various end uses in original equipment manufacturing (OEM), car refinishes, plastic coatings or coatings for 3D and complex shapes with shadow areas not exposed to UV.
Optimising binder properties for high-performance automotive coatings
In automotive paint applications, the clearcoat is the outer layer on top of other functional layers. A high-performance clearcoat must serve two important functions. One is to ensure that the coating has excellent appearance, for example with good levelling and high gloss; the other is to protect the coating system against environmental influences such as mechanical stresses and chemical substances.
The most critical mechanical stresses are stone chip and scratching from various sources (car assembly, carwash brushes, bushes, shopping trolleys, keys etc). To improve the resistance to scratches arising from carwash equipment has been a major issue in the development of high-performance clearcoats.
Chemical resistance must also be considered. Humidity, UV light and especially the attack of acidic substances can result in damage to the coating. The latest generation of high-performance clearcoats combine a good resistance against this chemical attack with high scratch resistance.
Ambient plasma processing: A revolution in surface engineering
A continual challenge in coating and printing is matching the surface properties of the substrate to that of the coating or ink to improve adhesion. This may involve changing the surface energy of the substrate to enable better wetting and spreading, or the addition of bonding sites to facilitate grafting.
Plasma techniques can be used to pretreat (ie activate) substrate surfaces to improve the adhesion of the final coating by cleaning and/or oxidising the substrate surface. However, the practical application of these techniques is often limited by traditional plasma processes being batch operations. Dow Corning Plasma Solutions is developing technology and processes using atmospheric pressure plasma that address this limitation, and which can be utilised to enhance coating performance.
Effects of European legislation on the future of the paint industry
CEPE, the European Council of the Paint, Printing Ink and Artists' Colours Industry, was founded in 1951 in order to promote and protect the interests of the industry at European level, and to offer an international forum for the exchange of ideas and information.
CEPE is a non-profit making organisation serving more than 1,100 paint and 75 printing ink companies as well as about 20 artists' colour manufacturers in Europe, representing in total more than 85% of the European market.
The increasing importance of the European Union and its legislation has major impacts on the paint industry. Consequently, the importance and scope of CEPE's work as the representative of the industry's interests at European level is growing.
For CEPE, the most important level with regard to legislation is the European Union. Here, CEPE's main contact is the European Commission. CEPE also maintains close links with the European Parliament, particularly with the Environment Committee dealing with health, safety and environmental matters. To be successful in its advocacy, CEPE needs to achieve a broad consensus between all the stakeholders: the paint manufacturers, the downstream sectors - for example the painters - and upstream sectors - the raw material suppliers.
Photoinitiators old and new: Food for thought
The first patent for a UV-curable ink appeared in 1941 from the Inmont Company, but new technology takes many years to develop into commercial reality and it was not until the 1960s that it could be said that a new industry had taken shape.
At that time there were very few photoinitiators available commercially, and the 1970s and 80s saw a rapid growth of new structures that would bring fast curing, efficient and economic photoinitiators into the market. The range of photoinitiators that became available now covered most wavelengths of the UV spectrum and allowed formulators the opportunity to develop and fine-tune the UV coatings and inks that were being produced.
The rapid growth of UV curing in the 1990s stemmed from the existence of this wide range of commercial photoinitiators. More recently, this green, low-energy technology has expanded into new applications and opportunities, and is applying itself to the problems that occur along the way.
The role of coalescents in architectural coatings
Manufacturers of architectural coatings in Europe are being driven by regulatory directives, retailer requirements and consumer preferences to deliver products with reduced volatile organic compounds (VOCs) and lower odour. This must be accomplished while maintaining optimum paint performance in scrub resistance, block resistance, low-temperature film formation, etc. In many cases, some performance compromises must be accepted to meet existing and future VOC regulations.
There are several methods of achieving regulatory compliance including:
This technical article will review the formulation of architectural paints that comply with European VOC legislation and will also discuss the use of coalescents to produce low-odour coatings. The main objective is to present in detail the results of a study on coalescent versus coalescent-free architectural paints. PRA conducted this independent study for Eastman Chemical Company in 2005.
There is life after NMP
Based on a paper presented at the Water-borne & High-Solids Coatings Conference, 7th to 8th March 2006, Brussels, Belgium
Polyurethanes have been used in the coatings industry in a variety of different guises for many years. Their versatility and excellent physical, chemical and mechanical properties have made them the polymer of choice for a variety of applications, ranging from coatings for wood, where their excellent abrasion/scratch resistance and hardness are paramount, to coatings for fabric where their flexibility, adhesion, and water and chemical resistance are unsurpassed.
Polyurethanes were initially developed as solvent-based systems. However, growing concerns over the impact of solvents, or volatile organic compounds (VOCs), on health and the environment has led to new legislation and a shift towards more eco-friendly water-borne polyurethane dispersions (PUDs).
Although more expensive than solvent-borne systems, water-borne polyurethanes are becoming increasingly attractive as they allow for a significant reduction in VOCs with little or no adverse effect on performance. The purpose of this paper is to provide an overview of the general chemistry behind PUD manufacture, explain current issues associated with their manufacture, what impact recent changes in legislation will have on the industry as a whole, and how recent developments have led to a new breed of PUDs with enhanced performance.
The original chemistry for preparing water-borne polyurethanes was developed in the late 1960s and has remained relatively unchanged over the years,1 although processing techniques have developed to improve performance and long-term stability (see below). The majority of PUD manufacture is still carried out in a batchwise process that involves three key stages, as outlined in Figure 1.
As a first stage, an isocyanate-terminated prepolymer bearing a hydrophilic site is prepared by reaction of an excess of diisocyanate (aliphatic and/or aromatic) with difunctional polyols (ie polyether, polyester or polycarbonate types) and dihydroxy carboxylic acids (or sulphonic acids).
Although not strictly necessary, the incorporation of hydrophilic sites into the prepolymer backbone allows it to become water-soluble by forming an ionic salt when neutralised with a base, such as triethylamine. This then allows for the prepolymer to be readily dispersed into water with a minimum of shear.
Once dispersed, the isocyanate groups are reacted with a diamine to rapidly build molecular weight (via urea formation) that is necessary for film performance. Most of the properties exhibited by ionic polyurethanes can be attributed to the primary polymer structure (ie the nature and sequence of hard and soft domains), which tends to dictate how much hydrogen bonding and other ionic associations occur throughout the polymer lattices. By carefully choosing the polyol or isocyanate type and the conditions under which they are reacted, a whole range of properties may be displayed by the final polymer dispersion.
Smart coatings - Current research and applications
Yesterday's 'science fiction' quickly turns into today's 'scientific fact' thanks to the rapid strides being made in interdisciplinary or hybrid technology, applied materials science and nanotechnology. Research in paint technology has gained an impetus from these developments and is rapidly re-inventing itself to be at the forefront of cutting-edge technology.
One such field of study that has generated considerable interest is that of smart coatings. 'Smart' or 'intelligent' coatings are structured coating systems that exhibit a functional aspect besides the protective and decorative roles of classical paint. The 'intelligence' of these smart coatings depends on their ability to sense and then respond to an external stimulus, which could be physical, chemical or mechanical.
These external stimuli could include outside conditions such as temperature, stress, strain or the environment. Their smart behaviour results from the scientific combination of functional additives, pigments or polymers and not from the incorporation of devices such as microsensors or microprocessors. Thus, the functional aspect is an intrinsic property of the coating.1 This review discusses the current state of research and practical applications of smart coatings, looking initially at different types in terms of their functions in use.
Novel and environmentally-friendly urethane/acrylic hybrid dispersions
Polyurethane dispersions (PUDs) are a unique class of water-borne polymer colloids which offer many advantages to the surface coating industry. They are usually produced through the interaction of a polyfunctional isocyanate with a polyfunctional chemical compound containing an active hydrogen in its structure.