From Customer Requirements to Focused Coating Development -
The Design of A Unique Waterborne, UV Curable
Hardcoat
Van Technologies, Inc.
NOTE: Tables and
Figures are presented at the end of this document.
Needed: Waterborne hardcoat composition for application to flat sheet plastic or metal that is post formed into various 3 dimensional shapes. Must be cured at ambient temperatures, highly abrasion resistant, clear, and capable of low to high gloss appearance.
On first impression, this seems to be a reasonable request and then the performance implications are considered. Hardcoats typically are compositions that exhibit very tough, abrasion resistant surface properties. Being hard, they do not lend themselves well to flexing which is a requirement when the coated flat stock is post formed. Also, since the sheets are post formed, it is implied that the sheets are handled prior to forming and should be tack free. Lastly, waterborne compositions have come a long way, but to provide such a balance of characteristics is quite a task. All of a sudden a reasonable request represents a sizable challenge. This is considered a good opportunity for Van Technologies, Inc.
Van Technologies is frequently involved in engineering unique coating compositions that provide solutions to very demanding client problems. As a specialty manufacturer and designer of industrial coatings, Van Technologies places great emphasis on environmental compliance and, therefore, places emphasis on Waterborne, High Solids, and Radiation Curable chemistries. Recently, a research and development project was conducted to create an environmentally compliant hardcoat composition for the plastic and polished metal markets. Specifically, the hardcoat composition needed to exhibit excellent scratch and abrasion resistance, solvent and water resistance, adhesion, low VOC content, capability of low application thicknesses, and had to be formable after application to the substrate to produce a 3 dimensional article. This discussion will describe the process of creatively reviewing customer or client requirements and/or specifications to determine the best formulation options available to satisfy them. The result in this particular case resulted in a very unique waterborne UV/EB curable coating composition.
The Review of
Customer/Client Requirements and/or Specifications
The actual requirements desired by the client are listed in Figure 1. As any formulator would recognize, this list is very demanding on current technology. For example, the hardness and flexibility requirements present complications with an elongation requirement. Consideration of the performance requirements, and the manner in which they must interact, enabled the formulation design team to focus on certain coating technologies.
1. Adhesion:
The requirement stated cross hatch adhesion to vinyls, acrylics, polycarbonate, and aluminum flat sheet substrate. The standard method for testing uses a cutter with eleven teeth spaced at 1 mm intervals. Two perpendicular cuts are made on the coated surface and 3M Scotch 610 tape is applied to the cut pattern and rapidly pulled off at an angle perpendicular to the plane of the coated substrate. Adhesion is then evaluated according to the method of ASTM D3359, method B.
Adhesion to various substrates can pose difficulties since the respective surface tension or energy associated with each may vary significantly. To promote adhesion to any particular substrate, the surface energy to the coating should match that of the substrate as closely as possible thus minimizing interfacial tension. Equation 1. identifies the parameters for adhesional wetting between the coating. 1
Eq. 1
WA = gSA
+ gLA - gSL
Where, gSA denotes the surface tension of the substrate under air, gLA denotes the surface tension of the liquid coating under air, and gSL denotes the interfacial tension or free energy of the substrate/liquid coating interface. If at all possible, it is desired to elevate the surface tension of the substrate to further enhance adhesion. This is typically done using primers, tie-coats, adhesion promoters, corona or plasma treatment, and by surface roughening. The desired hardcoat composition for this project, however, was to be applied via direct, one coat application to the substrate.
Of course, the design team recognized that the coating composition also had to exhibit smooth application and excellent uniformity when dried and cured. Although the desired performance properties do not identify the nature of the composition to be waterborne, it became evident for reasons discussed below that water was the best option. The high surface tension of water required the use of surfactants to minimize coating fluid/substrate interfacial tension. Equation 2. illustrates the spreading action of a coating composition on a surface.
Eq. 2
SL/S = gSA
- (gLA + gSL)
When SL/S is:
a. positive, coating fluid spreading is spontaneous,
b. zero, coating fluid spreading is spontaneous,
c. negative, coating fluid spreading is not complete.
Keep in mind that surface tension may change upon evaporation of volatile components and with the use of surfactants, usually causes further depression of the surface tension of the coating. Surfactants are used very carefully and at a minimum concentration so as to avoid conflict with adhesional performance.
Examining other performance requirements implied that the adhesion of the hardcoat composition had to be maintained throughout the forming process as illustrated in Figure 2. The forming process was anticipated to exert a minimum of 200% elongation on certain areas (Figure 3.) of the substrate. Cracking and delamination were serious concerns during the flexing and drawing of the substrate. Therefore, the hardcoat composition must be designed to yield appropriately to the stress and strain exerted.
To the design team assigned to this project, it was quite apparent that the hardcoat composition would have to be cured after the forming process. This also implied that the hardcoat composition would have to be dry and tack free prior to the forming process. By achieving this blend of characteristics, flat sheets could be handled without damage to the coated surface. Essentially the design team was charged with the task of creating a coating composition of low to moderate modulus that, upon cure after post forming, would result in a hardcoat composition of high modulus. Figure 4. illustrates stress-strain behavior patterns of materials of different modulus. The final hardcoat composition required conformance to the characteristics of system C, prior to forming and cure, and of system A, after cure.
Therefore, the consideration of adhesion also required attention to substrate and coating surface tension and to the modulus of the coating composition prior to and after cure.
2.
Water, Chemical, and Abrasion Resistance:
Water resistance tests consisted of 24 soaking in water at 90o F. Chemical resistance tests were spot tests for 1 hour exposure while covered with a watch glass. The desired abrasion resistance requirement identified the use of a Taber Abraser test instrument fitted with CS-10 wheels at a weight of 1000 gm. The number of cycles required was 500.
When considering the coating chemistry to be used, the design team began to focus on highly crosslinked systems. It is common to generate highly crosslinked systems using hydroxyl, carboxyl, epoxy, and amino functionality. Typically, with these functional groups, crosslinking is promoted by either acid or base catalysis and the application of heat. Additionally, free radical initiated chemistries lead to highly crosslinked systems via applied thermal or radiation energy. Therefore, the design team’s preliminary discussions were directed toward epoxies and epoxy acrylates, 2 component urethanes and urethane acrylates, and acrylics, acrylates, and acrylic/melamine systems. Review of the following requirements further narrowed the choice of chemistry.
3.
Hardness, Flexibility, & Elongation:
As discussed above, two other properties needed to be considered with the necessity for hardness: flexibility and elongation. Typical hardcoat compositions are far too brittle for the degree of flex and elongation that would occur in the targeted process. Therefore, it was proposed to develop a coating composition that, when dry, would exhibit tack free properties in the pre-cured state. This composition would also be capable of being flexed and stretched sufficiently to survive the forming process.
Hardness, therefore, needed to be addressed in both the pre-cured and post-cured state. It would also be beneficial, in the dried, pre-cured state, for the hardcoat to resist blocking or sticking between individual stacked sheets or between layers of coated substrate stored in roll form. This established a considerable challenge in the development of initial pre-cured hardness and, as many other formulators experience, trade-offs had to be debated. (The resulting hardcoat composition does succeed in permitting limited stacking for short duration and work continues to further the initial hardness without compromising other properties.)
In the screening of materials and in the testing of various hardcoat compositions, flexibility was evaluated by the “T” Bend test as illustrated in Figure 5. This method is extremely simple and effective where a 0 T was required without cracking, delamination, or loss of adhesion. Elongation was evaluated by using a forming press to deform the flat sheet into a rectangular open box shape. For metallic substrates, a ruled printed grid pattern on the surface of the sheet allowed measurement of elongation after forming. Plastic materials were subjected to a heated membrane pressing into a similar shape. Corners and edges were carefully inspected for defects.
In general , flexibility and elongation are characteristics that can be designed using many types of materials provided that the degree of crosslinking is relatively low. Through numerous other development projects, highly crosslinked systems have used low molecular weight multi-functionalized monomers and oligomers in combination with functionalized polymers to create unique physical properties in the final cured state. This project presented an interesting twist since it was now necessary to consider the pre-cured state properties.
In the uncured, dry state the system should be, on the average, relatively high in the molecular weight to exhibit sufficient hardness and exhibit high flexibility and elongation. Materials too low in molecular weight will typically exhibit physical properties that are a.) too soft, b.) potentially a liquid, or c.) a crystalline solid. However, these materials may be blended with other, higher molecular weight materials to balance desired properties. Therefore, one of the greatest challenges to the design team was the selection of coating composition ingredients to arrive at the best compromise in physical and chemical properties.
4.
Non-Yellowing and Gloss:
The non-yellowing performance requirement significantly narrowed the considered coating technologies to that of the aliphatic materials. Furthermore, the aliphatic urethane and acrylic materials having stability to sunlight ultraviolet radiation were selected for initial screening. Gloss values of coatings from either of these systems, from prior experience, was felt to be highly controllable. The principle factors effecting gloss that were monitored throughout the design process were the solid content, flatting pigment/agent particle size, and application thickness.
5.
Application Method and Dry Coating Thickness:
Roll coating was selected by the client for hardcoat application to the flat substrate in either sheet or roll form. The roll coating equipment usually dictates the range in coating fluid viscosity and, the clients’ range was between 900 cps and 1500 cps (Brookfield RVT @ 21o C, 20 rpm). With the viscosity range targeted, the design team had to enable the roll coating process to establish a dry coating thickness between 0.5 mil to 1.0 mil. A quick approximation of dry coating thickness was made knowing the percent non-volatile content of the composition and the wet thickness applied to the substrate according to Equation 3.
Eq. 3
Dry Coating Thickness = Wet Coating Thickness X (Percent Non-Volatile/100)
This equation is a good tool for the formulator to permit the estimation of wet thicknesses, or percent non-volatiles required of a coating to achieve a specific dry thickness. It is essential to understand that this equation is only a rough approximation since actual values depend on the nature of mixing (always deviating from ideal) and the density of the matter being mixed. Let us say that we have found this equation useful to place the formulator in the correct vicinity of thicknesses and percent non-volatiles required. Follow-up lab evaluation defines the true interaction of the variables involved.
As an example of how Equation 3 is useful, consider the current circumstance and how it leads the design team to certain formulation characteristics:
The client, in the present case, desired a hardcoat dry thickness between 0.50 mil and 1.00 mil and in process, a nominal 0.75 mil thickness would be targeted ± 0.25 mil. By Equation 3, coating compositions that are 100 percent non-volatile will require an estimated 0.75 mil wet thickness ± 0.25 mil. This would represent a very tight control parameter for the particular client’s roll coater, since their ability to control wet applied thickness is approximately ± 0.50 mil (as determined through initial process screening). Based on Equation 3, it is possible to create a plot of wet coating thickness vs. percent non-volatiles vs. wet applied thickness tolerance given the target dry thickness and the allowable dry thickness range. Figure 6 shows these specific relationships to achieve a dry thickness of 0.75 ± 0.25 mil and directs the design team to focus on compositions that are 40 percent or less in non-volatile content.
6.
Maximum Process Temperature:
The client required a maximum process temperature of less than 80o C and the through speed, or line speed, is not necessarily vital (of course, faster is better). The design team now had to reconsider the technical options for achieving the final degree of hardness required via appropriate cured chemistry. The following systems were discussed with respect to their potential:
a. Two component compositions
b. Moisture cured one component systems
c. Highly catalyzed, low temperature cured compositions
d. UVEB curable compositions
Discussions with the client revealed further factors. First, two component systems typically indicate a pot life that needs to be monitored and the client did not want to manipulate this type of variable in their manufacturing process. Second, the clients’ experience in moisture cured systems showed a range of cure response with humidity, a factor they had difficulty in controlling. Third, highly catalyzed, low temperature cured compositions also seemed to be problematic in the past relative to their stability. Fourth, ultraviolet (UV) cured systems have been and were in current use but not electron beam (EB) cured systems. The design team immediately focused on UV curable chemistries.
7.
Low V. O. C. and Zero H. A. P. Content:
In keeping with recent environmental concerns, the client required that the volatile organic compound (V. O. C.) and hazardous air pollutant (H. A. P.) content be consistent with aggressive regulations. Therefore, the requirement of < 2.3 lb/gallon V. O. C. (via EPA method) and < 0.8 lb/(lb non-volatile) H. A. P. was established. The EPA method of V. O. C. calculation used Equation 4.
Eq. 4
V.
O. C. (lb/gallon) = lb per gallon volatile organic compound
(1 gallon minus the gallons of exempt solvent)
The design team considered the V. O. C. and H. A. P. requirements with specific attention to the roll coating process and the resulting dry thickness to be obtained. A solvent or volatile content would be necessary to effectively control viscosity and the non-volatile content. Water was selected as the primary agent for this purpose and, by Equation 4., little or no additional organic solvent could be tolerated.
Summary
The conclusion of the initial requirement/specification review process allowed the design team to effectively narrow the technical focus to consider a waterborne, aliphatic urethane acrylate UV curable composition. The key parameters discussed above permitted a clear view of other specific composition characteristics. For example it is understandable why the non-volatile content target was established between 30 and 40 percent and the viscosity between 900 and 1500 cps. Therefore, much can be established, prior to actual laboratory work, by a complete review of coating composition requirements and/or specifications, including effective clarifying discussions with the customer or client. Too often an incomplete review process occurs that severely limits the effective focus of the design group and often results in poor, inconsistent coating performance and possible failure. The best advice to design teams and formulators is to take ample time to ask the right questions about the target process. Remember, also, that the customer or client may be yourself or others in your organization.
References
1. M. J. Rosen, Surfactants and Interfacial Phenomenon, 2nd Ed., 240-255 (1989) John Wiley & Sons, Inc.
About the Author
Lawrence C. Van Iseghem is the president and founder of Van Technologies, Inc. Van Technologies designs, formulates, and manufactures specialty industrial coatings with emphasis on environmentally compliant technology.
Figure 1. Desired Hardcoat
Performance Requirements 1. Cross
hatch adhesion to vinyls, acrylics,
polycarbonate, and aluminum flat sheet substrate (0.5
mil to 40.0 mil thickness) 2. Water
resistance 3.
Chemical resistance (MEK, alcohols, glycols, gasoline, detergents,
ammonia, acetic acid, oils, and greases) 4.
Abrasion resistance to pass Taber 1000g load, 500 cycles, CS-10
wheels) 5.
Flexible to pass 0 T (direct fold without cracking on edge of
crease) 6. ³
200% elongation to permit formability to various 3D shapes 7. ³
3H pencil hardness 8.
Non-yellowing upon exterior exposure 9.
Variable gloss from 2 to 90 units @ 60o 10. Roll
coat applied to dry thickness between 0.5 mil and
1.0 mil 11.
Maximum process temperature < 80o C. 12. VOC <
2.3 lb/gal via US EPA method, HAP <
0.8 lb/lb non-volatile
Figure 6.
