In order to improve performance, increase profitability and gain environmental acceptance, many companies are switching to high performance, solventless coatings cured by ultraviolet (UV) light or electron beams (EB). The advantages gained by using UV/EB curing include the use of an environmentally sound technology, increased production speed, process optimization, improved product performance and the ability to develop new value-added products.
Table I is taken from a successful conversion from waterborne materials to UV coatings. It illustrates the difference obtained on one line using three products. UV cured material has a large advantage in physical properties, which not only increases value in the market but decreases waste by offering a more durable product. The higher crosslinked density available in energy-cured systems allows for improved physical properties. The dramatic increase in solvent resistance and scrub resistance of the UV cured coating is a good example of this point.
|TABLE I-Physical Property Comparison|
|Scrub Test||40% Failure||100% Failure||0-10% Failure|
|100 MEK Rubs||NR||100% Failure||Pass|
|Nail Polish Remover||2||FD||0|
|Rating systems are different due to different individuals performing tests. However, conclusion can be drawn if an equivalency can be made between test rating systems.|
|0=NE/No Effect • 1-3=SE/Slight Effect • 4-6=ME/Moderate Effect 7-8=FD/Permanent Effect • 9-10=Failure|
UV/EB coatings contain monomers and oligomers that react together under UV or EB energy to cure rapidly and completely. This eliminates the need for sophisticated and lengthy ovens that would be necessary with solvent- or waterborne materials.
UV/EB provides extremely fast cures, partly because there are no solvents to evaporate. They can also give high-gloss coatings. High depth of image (DOI) is achievable without a rub and buff operation. Scratch and abrasion resistance is often times superior to that achieved using other coating systems.
A cursory comparison of just cost per gallon of materials may seem to indicate a high cost for UV/EB, but this is misleading. A more realistic approach is to look at actual applied cost per dry mil. The coatings previously used in the physical property comparison (Table I) can prove this point. To see the cost comparison, refer to the article "UV Coating Myths" and Myth No. 3.
Implementing UV Curing
The rapid growth and advancement of the UV industry (with many chemical companies offering new raw material products) and resulting advances in UV/EB chemistry have given the formulator a number of chemical classes from which to choose. With this flexibility, the industry is able to easily meet customer requirements and develop products that best fit customer applications. The formulator has a number of chemical classes from which to choose in meeting customer requirements. The basic oligomers used in UV/EB coatings are based on the same resins that conventional coatings use. Each class of materials possesses different attributes. A formulator can therefore choose a resin class that best fits the customer's end property requirements.
Urethane acrylates can exhibit minimal yellowing upon cure and have good outdoor weathering capabilities, although aromatic urethanes are poorer performers than their aliphatic cousins. Both types of urethanes give tough, resilient coatings that are abrasion resistant. The tough nature of the films produced allows high film weights to be applied to dimensionally unstable substrates without cold check cracking. This same high film build yields a coating with good DOI. Urethane acrylates are expensive compared to other classes of acrylates. They cure at slower speeds than the other materials.
Epoxy acrylates are frequently used resins. Their low cost and fast cure speeds combined with very hard cured films and excellent chemical resistance makes them an easy choice for many applications. Structural modifications with amines or by ethoxylation further enhance their utility.
Epoxy acrylates are prone to yellowing. The aromatic structures of most epoxy acrylates contribute significantly to their resulting color. Yellowing can be reduced through judicious choice of a photoinitiator. However, cost constraints of some applications that demand the use of epoxies at times preclude the option. The high tensile strength and low elongation that give epoxy acrylates their brittleness can also be a detriment when higher film builds or superior adhesion at high film builds is required.
Polyester acrylates' physical properties fall between epoxy acrylates and urethane acrylates. Polyesters exhibit better non-yellowing properties than epoxies but not as good as aliphatic urethanes. Polyesters are less brittle than epoxies but not as flexible or tough as urethanes. Cure speeds are faster than urethanes but slower than epoxies. The cost of polyester acrylates also falls between epoxy acrylates and urethane acrylates. Polyester acrylates are good candidates for coatings in their own right but find use as modifiers to promote specific coating properties in the epoxy acrylate and urethane acrylate classes.
Cationic Cure Epoxides
Cationic cure epoxide systems occupy a small percentage of the UV coating market. These materials have advantages that are exploited in coatings for metal. The thermal post cure reaction of cationic epoxides can be useful in a beverage can line. Because of the typically lower shrinkage in UV cationic cure coatings, adhesion can be enhanced.
Vinyl ethers can cure by both cationic and free radical mechanisms. Vinyl ethers in combination with cationic epoxides cure extremely fast, producing very hard, high-gloss coatings. Blending vinyl ethers with unsaturated polyesters can eliminate major obstacles in each class of materials. Using low cost unsaturated polyester with vinyl ether lowers the overall system cost of pure vinyl ether coating. Replacing styrene with vinyl ethers in unsaturated polyesters eliminates the VOC and toxicity concerns associated with styrene. This also dramatically increases the cure speed.
Rules of Thumb
|TABLE II-Formulating Rules of Thumb|
|1. Epoxy||1. Epoxy||1. Epoxy||1. Urethane|
|2. Polyester||2. Unsaturated Polyester||2. Polyester||2. Polyester|
|3. Urethane||3. Polyester||3. Unsaturated Polyester||3. Epoxy|
|4. Unsaturated Polyester||4. Urethane||4. Urethane||4. Unsaturated Polyester|
A summary of formulating rules of thumb for the chemical classes of UV coatings can be seen in Table II. With these rules of thumb and an understanding of the market segment served, one can see why a given chemical class dominates certain market segments. Manufacturers of hardcoats for plastics typically use urethane acrylates (of the aliphatic variety if good outdoor weathering is required) with a high crosslinked density. This can be obtained by the use of multifunctional oligomers or multifunctional reactive diluents (also known as monomers). If the coating is pigmented, polyester acrylates are often employed to enhance pigment wetting and improve the flow of the coating. Similarly, high-build dome coatings are often formulated with aliphatic urethanes but with a more flexible backbone. These type of coatings can also be formulated with acrylated acrylics depending upon the performance that is required.
Most metal coatings contain slightly acid-modified UV curable materials that serve as adhesion promoters. Depending upon the weather resistance required of the coating, steel pipe coatings can be formulated with either acrylated urethanes or epoxies. Protective coatings for brass are typically formulated with aliphatic urethane acrylates, while primers for tin are often formulated with acrylated acrylics or specialty polyester acrylates to ensure good adhesion to the metal surface and good intercoat adhesion to the topcoat.
Just as there is a "best fit" for each chemical class to a market segment, there is a best fit of application technique for the article to be coated. Each application method has its advantages and disadvantages. A look at the advantages and disadvantages for each application method-spray, roll coat, curtain coat and vacuum coat-suggests a market segment that has a "best fit." Each of these application methods is only a general category. Spray may be by conventional air spray, HVLP, air-assisted airless or electrostatic bell or disc. Rollcoat may be differential, direct or reverse with various roll types. Curtain coat can be pressure head, flow head or roller curtain coater. Vacuum may be slot or edge. Each specific variation in application technique represents a refinement. Not all variations dramatically change the application rules. If we attempt to match each application method with an area of use, assuming no change in current production operations, a logical association can be seen.
Paneling and plywood lines run at relatively high speed and apply low coating weights of 5-10 g/m2. This is an excellent fit for roll coating. Achieving a richer look often means applying more material. A high-gloss coating can achieve a high DOI with more material. Curtain coating fits well with higher film builds and finishing flat stock. Vacuum coating is an efficient method of coating long, linear pieces (such as wood or plastic molding or pipe) and edges of panels. Transfer efficiencies can approach 100% with UV curable coatings. Line speeds are relatively fast, making this method particularly UV friendly.
It All Adds Up
How can one put this all together to produce an aesthetically pleasing product that meets all the physical property requirements? A partnership must be formed with all the parties concerned. The formulator knows the chemical class needed to meet the physical property requirements and can pick a formulation, or develop one, that specifically matches the capabilities of the application equipment. At times, the coating raw material supplier is asked to develop new resins (oligomers or monomers) when there are gaps in the performance characteristics of the existing products. The equipment supplier knows the equipment necessary to meet the manufacturer's production needs and methods of operation.
Through a series of tests specifically designed to test both the equipment and the coating, all parties learn the operations needed to produce an acceptable product. Only through the information exchange that results during these tests can the equipment supplier and the supplier of the coating meet the needs of the person finishing the product. The manufacturer learns the strengths and limitations of all pieces in this coating operation needed to produce his product. Each finished product has a unique solution and best-fit operation.
UV curable coatings and application methods are not magic. Only through sound matching of required physical properties, coating formulation, application technique and substrate configuration can a coatings system succeed with the minimum of frustration, labor and capital expenditure. Through a partnership in which all parties educate and support each other, a finishing system can be devised that produces the maximum profit for all involved.