New Sulfonic Acid Catalysts for Coil Coatings
March 2, 2009
A new class of blocked sulfonic acid catalysts has been developed that promotes the crosslinking reaction of hydroxyl-functional polymers with amino-formaldehyde crosslinking agents such as hexamethoxymethyl melamine, especially in coil coatings. These catalysts are particularly effective in coil primer formulations containing calcium ion exchange anti-corrosive pigments. In addition, the unique deblocking profile of these catalysts provides the so-called snap cure at the desired peak metal temperature, and within the specified time.
IntroductionCoil coatings are high-performance liquid coatings applied to flat steel and aluminum metal strips before they are fabricated into the final product. Coil coatings are applied in a continuous process at very high line speeds, baked in seconds at very high temperatures and rewound for delivery to the user. The metal strip must then be cut and further fabricated without cracking, chipping or scratching the finish. The process is value added and offers several benefits. It is environmentally friendly, non-polluting and with the ecological advantage of little or no waste. Most lines usually use the exhaust air (containing solvents) from the hoods over the coaters to heat the ovens and hence recycle the residual heat from the oven exhaust. Thus burning the solvent eliminates VOC emissions from coil lines and the fuel value of the solvent is recovered. It offers several technical advantages too, including ease of application and can be applied at speeds of up to 180 meters (600 feet) per minute (fpm) and as fast as 240 meters (800 feet) per minute, at both sides of the metal strip. The faster coating application rate leads to lower labor costs. Film thickness is more uniform as compared to those derived from coating pre-formed parts. The elimination of VOC and HAPs emissions and fire hazards associated with application of these coatings leads to potentially lower insurance cost as well. Thus the manufacturers of the end-use product will not have to deal with VOC concerns, pollution control equipment and paint sludge disposal.
Coil coatings on steel and aluminum currently amount to about 36 MM gallons, valued at over $570 MM, with solventborne polyester/melamine being the most widely used. Use for building products contributes to half of the total volume valued at over $280 MM, with overall growth of about 3%/year.1
Major end-use segments include construction, transportation, appliances, furniture, and HVAC (heating, ventilation and air conditioning). Some coil-coated metal application examples include roofing, original siding of mobile homes, garage doors, Venetian blinds, doors and windows, rain gutters and downspouts, passenger cars, vans and light trucks, buses, kitchen counters, cookware, refrigerator boxes, home and office furniture, cabinets, fixtures and shelving, can ends and bodies, tabs and closures, signs and displays.
One of the controlling factors for coil coating line speed is the oven dwell time necessary to cure the applied coating at the cure oven temperature. A coating composition that can be cured in a shorter time at the oven temperature allows a faster and more economical coil coating process. A number of other properties are important for coil coatings too, such as resistance to degradation on outdoor exposure (weatherability), chemical and water resistance, scratch resistance, gloss, hardness, and resistance to delamination when the substrate is bent. The bending property is important because after being coated the metal is subjected to a forming step. For example, building panels are formed into a three-dimensional shape after coating. It is important that the coating not lose adhesion during the forming step or steps. Weatherability is important for metal that will be used for building panels, gutters, garage doors, sign stock, panels used for vehicle parts, or other such uses where the coated surface is exposed to outdoor weather and sun. While the bending property is generally better with softer, more flexible binders, weatherability and other durability properties are generally better with harder binders.
Typical coatings on aluminum are single coats, but a primer/topcoat system is used on steel. Resin systems used for primers include polyesters, epoxies and polyurethanes. Binders used in topcoats include polyester/melamine for superior exterior durability and corrosion protection, silicone-modified polyesters either alone or with melamine, fluoropolymers like polyvinylidene fluoride for highest exterior durability, and plastisols. Polyester/melamine resins are also used as backer coatings on the reverse side of the coated metal to prevent metal marking on the top surface by rubbing against the bare metal reverse side of the coil.
Environmental concerns about the use of Cr (VI) compounds in anti-corrosion coating technology has resulted in the introduction and use of environmental, health and safety (EHS)-compliant products such as calcium-modified silica gel anti-corrosive pigments, especially in the general industrial and coil coatings segment. This has been a continuous and slow process but has been gaining great momentum in the last few years. The pressure on the European coatings industry towards implementation of VOC-free and EHS-compliant technologies significantly increased with the announcement of the EU VOC legislation that comes into effect in 2007 and the new REACH legislation that is currently planned to become effective from 2008 to 2010. The target of the latter legislation is to ban all chemicals that may be considered as hazardous (approximately 20% of all chemicals used in the coatings industry). In response to this ever-increasing environmental pressure, producers of anti-corrosion primers are currently actively testing alternatives to anti-corrosive materials based on Cr (VI) and Zn compounds.
Calcium-modified silica gel anti-corrosive pigments (Ca/Silica pigments) offer a suitable, environmentally compliant alternative that can effectively address the new legislation changes. These pigments being heavy metal free are essentially non-toxic. They are amorphous micronized particles of controlled particle size distribution, with a density of about 1.8 g/cm3 and very high surface area. They are moderately basic, with the pH of aqueous slurry roughly in the range of 9-10. These pigments are readily available from an ion-exchange reaction at the surface of silica gel between weakly acidic surface silanol groups and calcium hydroxide.
Coil primer formulations typically contain acidic substances in the form, for example, of resins bearing acidic groups or amine-blocked conventional acid catalysts or latent acid catalysts containing a beta-hydroxyl group. In these cases, acid-base interactions can lead to viscosity increase or reduced cure rates. Latent acid catalysts containing beta-hydroxyl groups are more prone to inter-molecular alkylation reactions leading to the liberation of free sulfonic acid upon storage. The slow release of free sulfonic acid is responsible for the observed detrimental interaction with basic components present in the formulation. Such acid catalysts may lose activity due to adsorption onto the basic pigment surface during storage. In many cases, this can be overcome by adjusting paint formulations with special additives, such as viscosity-control additives, use of co-catalysts or use of pigments and inhibitive pigments having acidic surface sites. Choice of pigments having lower calcium content may help minimize the basicity of the Ca/Silica pigments surface, but unfortunately at the cost of the desired anti-corrosive performance of the overall formulation.
Shieldex C-303, a calcium-based anti-corrosive pigment (W.R. Grace), is an excellent non-chrome substitute for strontium and other chromate compounds and was designed to replace strontium chromate in coil primers. Unfortunately this pigment interacts with most known acid catalysts (amine and epoxy blocked) and reduces the cure response. Grace recommends the use of a combination of a silane ester, epoxy-blocked phosphoric acid, epoxy-blocked sulfonic acid and a di-carboxylic acid at up to a 9.76 % loading to prevent the interaction of the basic Shieldex pigment and to provide acceptable cure response.2
We began our research toward a more efficient non-interacting acid catalyst that can provide cure in the presence of these basic anti-corrosive pigments while providing viscosity stability during extended storage periods. Herein we describe the development of a new class of blocked sulfonic acids, derived from aromatic sulfonic acids, for use in coil primers that provide outstanding cure, viscosity stability upon oven aging, and corrosion/salt spray resistance. In addition to resistance to basic pigment deactivation, these catalysts reduce solvent popping defects and provide excellent adhesion/intercoat adhesion while allowing extended storage of formulated coating. In addition, the deblocking profile of these catalysts provides the so-called snap cure at the desired temperatures that are beneficial in reducing solvent popping/blistering, seen in coil applications with epoxy and amine blocked catalysts.
Evaluation in a Coil Primer Containing Basic Anti-Corrosive PigmentA master batch of the white Shieldex C-303 coil primer system was prepared (Table 1) without any catalyst for use in our evaluations. All formulations used in this study were made from this white master batch with the addition of the appropriate catalyst package.
For comparison purposes, control formulations with current state-of-the-art catalysts (W.R. Grace) namely epoxy-blocked dodecylbenzene sulfonic acid (DDBSA), and epoxy-blocked phosphoric acid were prepared as shown in Table 2.
Preparation and Evaluation of Coil Primer CoatingsAll formulations were applied by the drawdown method to B-1000 steel panels, with the average dry film thickness ranging from approximately 0.5 - 0.75 mils. Films were cured in a coil oven for 25 seconds at 325 °C. using a cure schedule that provided a peak metal temperature (PMT) of approximately 230 - 235 °C. The following properties were evaluated with the cured panels: pendulum hardness; gloss (20°/60°); color (yellowness index); adhesion (direct to B-1000 and recoated); salt spray resistance (500 h); humidity resistance (1000 h); and 50 °C oven aging (7, 14, 21, 28 days – viscosity, MEK resistance, and hardness).
Results and DiscussionThe cure response of a typical polyester/melamine-based primer formulation containing the basic anti-corrosive pigment was controlled by a fairly high loading (9.7%) of the catalyst package consisting of two different epoxy-blocked acids (current state-of-the-art).
When this package was replaced by 3.46% of the new blocked acid catalyst Nacure XC-194, better initial cure and superior cure after several weeks of 50 °C oven aging were achieved. Also the true non-interacting behavior of the new catalysts was seen by the superior viscosity stability of these formulations.
Also exceptional humidity resistance and corrosion resistance were seen as compared to the current state-of-the-art formulations.
The results are tabulated in Tables 3-5, and Figures 1-3.
Evaluations in a White Coil CoatingA master batch of the white coil coating system was prepared, as shown in Table 5, without any catalyst for use in our evaluations. All formulations used in this study were made from this white master batch with the addition of the appropriate catalyst package.
Preparation and Evaluation of White PE Coatings
Results and DiscussionThe cure data (Figure 4) indicates that the new acid catalyst, Nacure XC-181, was able to provide rapid cure at a peak metal temperature of 250 °C, (oven 300 °C/35 sec), without any significant cure at lower peak metal temperatures, compared to an amine-blocked and polymer-blocked sulfonic acid catalyst. The snap cure response allows release of volatiles in the formulation, thereby preventing popping seen in coil clear coats with conventional catalysts.
The popping phenomena was evaluated at a peak metal temperature of 240 °C, and the snap cure profile, seen with the new catalyst, resulted in no visible popping compared to the coating cured with the corresponding amine-blocked catalyst (Figure 5).
Summary and ConclusionsIn summary, the new sulfonic acid catalysts are particularly suitable for coil primer formulations containing basic anti-corrosive pigments, where they resist high pH pigment absorption, prevent popping, promote good intercoat adhesion, and provide exceptional storage stability and corrosion resistance. In addition, products from this class of catalysts also are effective in topcoats where their snap cure response allows release of volatiles before cure, thereby preventing popping, while providing exceptional storage stability.
We thank Marvin Blair, Max Gandhi, John Florio and Rui Gloria for experimental support and for evaluations of the new catalysts in various coating systems. We thank King Industries for permission to publish this work.
For further information, visit www.kingindustries.com.