Rheology and Surface Chemistry for Screen Printing

Transcription

1 Rheology and Surface Chemistry for Screen Printing Ken Gilleo, PhD Shetdahl, Inc. To the average screen. printer, rheology and surface chemistry can appear to be enigmas rather than useful tools of the trade. These fundamental and critical sciences of material behavior can be used to describe ink flow and surface wetting characteristics that are basic to screen printing. Comprehending these disciplines will help us understand and predict the behavior of ink flow during processing and the subsequent wetting interaction with the substrate. Rheology is the science of deformation and subsequent flow in materials when force is applied to them. Viscosity, the resistance to (low, is the most important Theological characteristic of all liquids, including screen-printing inks. Surface chemistry comes into play after the ink stops flowing: It describes wetting (arid dewetting) phenomena resulting from the mutual attraction between ink molecules and the intramolecular attraction between the ink and the substrate surface. The relative strength of these molecular interactions determines a number of inkperformance parameters. Good print definition, adhesion, and a smooth ink surface all require proper surface chemistry. Bubble formation and ccilated tilmformation defects can also be traced to surface chemistry. ;Theology of Inks Four general categories of rheology are 1 28.SCRFEt4PCItti t (NG recognized: elasticity. plasticity, rigidity. and viscosity. For liquids, the scope of Theology encompasses the changes in the liquid shape as physical force is applied, and viscosity is the main focus. Viscosity is the ratio of shear stress to shear rate (viscosity = shear stress/shear rate). Shear stress is typically expressed in dynes/cm 2 and shear rate is measured in reciprocal seconds (sec -1 ). Applying the equation above, the viscosity unit be - comes dyne-seciculz or poise. For lowviscosity fluids like water (0.01 poise), the poise unit is rather large so the more common centipoise (I/ 100 poise) is used. Therefore, water has a viscosity of about centipoise (cp). Screen-printing inks are much more viscous and range from ,000 cp for graphics and as high as 50,000 cp for some highly loaded polymer-thick film (PTF) inks and adhesives. Throughout Ihe screen-printing process, various types and quantities of mechanical fame are exerted on the ink. The amount of shear force directly affects the viscosity value for the so-called non- Newtonian fluids. [Editor's note: Newtonian, or ideal, fluids display a constant viscosity under varying shear-stress/ shear rate conditions, such as water. The viscosity of non-newtonian tiuids varies disproportionately witft shear rate and shear rate.] Most inks undergo a "shearthinning" phenomenon when "worked" by mixing or running on a press. The ink viscosity drops as shear rate is increased. which seems simple enough except for two additional characteristics. One is called the yield point. This is the amount of shear stress required to cause the initial flow. Catsup, for example. often refuses to flow until a little extra force is applied then it often flows too freely. Once the yield point is exceeded, the solid-like behavior vanishes and the loose network structure of the liquid is broken up. Inks also display this property, but to a lesser degree. Yield point is one of the mast important ink properties. The second factor is time dependency The viscosity of some inks changes over time even when a constant shear rate is applied. This means that viscosity can depend on the amount and duration of mechanical force applied. When shearing forces are removed, the ink will return to its 4481 viscosity. That rate of return, another important ink property, can vary from seconds to hours. Rheology goes far beyond the familiar "snapshot" view of viscosity at a single shear rate that is often reported by ink vendors. It deals with the changes in viscosity at different levels of force, at varying temperatures, and as different solvents and additives are used. Brookfield viscometerreadings, altnot ugh valuable, do not show the full picture for non-newtonian liquids Types of flow behavior Plasticity Rheolcgically speaking, Os-

2 \ \ ti c fluids behave more like plastic solids until a specific minimum force is applied to overcome the yield point. Gels, sots, and catsup are extreme examples. Once the yield point is reached, the liquid begins to approach Newtonian behavior as she'd' rate is increased Figure 1 shows the shear-stress/shear-rate curve and the yield point. Although plastic behavior is of questionable value to catsup, it does have relevancy in inks and paints. Actually, the yield-point phenomenon is of more practical value, No-drip paints are an excellent example of the usefulness of yield point. After the brush-stroke force is removed, the paint's viscosity builds quickly until flow stops. Dripping is prevented because the yield point exceeds the force of gravity The "bleed" of a screen-priming ink, the tendency to flow beyond the printed boundaries. is controlled by yield point. Inks with a high yield point will not bleed, but their flow out may be poor. A very low yield point will provide excellent flow out, but bleed may be excessive. The proper yield point provides the needed flow out and leveling without excessive bleed, Both polymer binders and fillers can account for the yield-point phenomenon. At rest, polymer chains arc randomly oriented and offer more resistance to flow. When shear force is applied, the chains straighten in the direction of [low. reducing resistance. Solid fillers can form loosemolecular-attraction structures that break down quickly under shear. Pseudoplasticity Pseudoplastic liquids, li ke the plastic-behaving materials discussed earlier, drop in viscosity as force is applied. They have no yield point, however. The more energy applied, the greater the thinning. When shear rate is reduced by a given amount, the viscosity increases proportionately. There is no hysto.resis. [Editor's note: In this case, hysteresis refers to liquids that display inconsistent shear-stress/shearlate relationships. resulting in a looped shape when plotted on a graph. Materials without hysteresis exhibit a straight-line correlation between shear stress and shear rate.] The shear-stress/shear-rate curve is the same in both directions, as seen in Figure 1. Figure 2 compares pseudoplastic behavior rising viscositylshear rale nerves. Many screen-printing inks exhibit this kind of behavior, but with time depen dency. They have a pronounced delay in viscosity increase after the force is removed. This form of pseudoplasticity with a hysteresis loop is called thixotropy Figure 3 shows the hysteresis curve produced when shear rate is first increased and then decreased. Thixotropy This form of pseudoplasticity describes the behavior of most screenprinting inks. Mixing and other high-shear forces rapidly reduce viscosity. However, thixotropic. inks continue to thin during shearing, even if the shear stress is constant. This can be seen during mixing when the Brookfield-viscometer readings continue to drop even though the spindle turns at a constant rpm. When the ink is left motionless, viscosity builds back to the initial value. This can Occur slowly or rapidly. Various-shaped curves are possible, but they will all display a hysteresis loop. In fact, this hysteresis curve is used to detect thixotropy ( Figure 3). Thixo tropy is very important to proper ink behavior and we can factually state that the changing-viscosity attribute makes screen printing possible. Di/aterrcy Liquids that display an increase in viscosity as shear force is applied are called dilatent. This is a rather rare property that is of limited value in ink technology. A dilatent ink would not be readily printable since viscosity would increase sharply as the ink was forced through the screen. Ditatency should not he confused with viscosity increases that result from such causes as solvent evaporation and polymerization. plastic Newtonian ti \\N\SS.. \,,,\N :.\%. nn nn pseudoplastic C - 0 a. dilatant Figure 1 Stress/rate curves Rheopexy Sounding more like a disease than a property, rheopexy is the timedependent farm of dilatency where mix ing causes shear thickening --the exact opposite of thixotropy (*Figure 3). Rhearate (sec )

3 Figure 2 Viscosity/rate curves high-viscosity Newtonian liquid w 0 0. dilatent Ta 0 pseudoplastic low-viscosity Newtonian liquid shear rate (sec r ) Figure 3 Viscosity/rate curves with time dependency rheopexic... * '".' P - e tli " Ali' thixotropic rate (sec- 1 1 M sc wi he an es. A r ant Cor 130 SCREENPRINTING

4 - pexy is fortunately rare and is, in any case, a totally useless characteristic for screen-printing inks. Other factors Two important factors that can dramatically alter viscosity are temperature and solvent content. Rheological additives, especially thickeners, are also important. Temperature: Increases in temperature will reduce the viscosity of virtually all inks. Cold inks will often refuse to print and refrigerated inks must be brought to ambient temperature before printing. The ink temperature must also be maintained during printing. The apparent thinning of inks during a printing run can be caused by temperature increases. Increases in thermal energy result in more molecular movement and less resistance to flow. Solvents: All true solvents reduce viscosity. Latent or partial solvents have a lesser effect. Nonsolvents, on the other hand, will generally increase viscosity, a principle used by some ink formulators. However, too much 1 -, Jnsolvent will "kick" the binder out of solution. Viscosity change, therefore, depends on the type and amount of solvent added. Although many inks are formulated to allow the customer to add solvent, this is a questionable practice. An ink should always be mixed before use so that shear thinning will reduce the viscosity (thixotropic). Many inks, especially vinyl, will increase substantially in viscosity and will partially gel during storage. High-shear mixing can often bring them back to a usable range. Solvents should not be added until shear thinning has been attempted. "Recovery" of older ink batches by solvent thinning will never bring back the original ink properties. Thickeners: A variety of silicas and other tillers are used to increase viscosity and to "build" inks. This usually increases the thixotropic behavior of the inks, which can be of value. Inks with excessive bleed can be improved with thickeners and the yield point will frequently increase. the use of thickeners should be left to experienced ink formulators. Many ink vendors are beginning to provide thickeners and formulating guidelines, however Screen-printing process Mixing As mentioned earlier, most screen-printing inks are thixotropic and will undergo shear thinning, Inks should be mixed before use to stabilize viscosity and to provide consistency. Most fillers, especially metals, will eventually stratify. A medium-speed mixer can be adequate and many vendors recommend a specific mixer type, rate, and duration. Shear rate corresponding to squeegee forces, about see- 1, will suffice. The mixing procedure should not add air bubbles or produce excessive shear farce. Squeegee motion A squeegee traversing at normal speed can produce a shear rate from sec -1. This shear rate provides enough energy to overcome the thixotropic forces and keep the ink reasonably thin. A more pronounced squeegee angle will increase shear. Once the squeegee has stopped, viscosity will Table 1 Surface-tension values Surface Tension Material (dyne/cm) Fluorineirf 1 FC-77' 15 Teflon 18 Heptane 20 Silicone Oil 24 Polyethylene 31 Castor Oil 35 Acrylic 39 PVC 40 Polyester 43 Water 72 ' A product of the 3M Co. begin to build. Therefore, several impressions may be required to get an ink "working." Some presses move the squeegee back and forth in a flood mode while waiting for a print cycle. Through-the-screen forces Extremely high shear rates are produced when ink is forced through the screen-mesh openings (typical estimates 1 are ,000 sec -1 ). The ink is reduced to its lowest viscosity as it passes through the inesh 2. the relatively thin ink is deposited onto the substrate in.a pattern corresponding to the screen-mesh openings. While still in a low-viscosity state, the ink "islands" will flow together into a continuous film as the screen is removed from the substrate. Flow out All mechanical shear forces stop the moment the ink is applied to the substrate and the screen is removed. Vis cosity begins to increase immediately. The only forces now acting on the ink are gravity, surface tension, and possibly tern perature. The ink must flow together be foie the viscosity becomes too high. If the viscosity increase is too rapid, a mesh pattern will result, even with a properly made screen. The yield point will also increase with time. The ink must flow together and level out before it becomes immobile. Leveling stops when the yield point is equal to the combined forces of gravity and surface tension. However, if viscosity builds too slowly or the yield point remains too low, the ink will continue to flow past the intended defi ned pattern (bleed). In some instances, especially with UV inks, monomers and oligomers (higher-weight reactants) may bleed out, leaving the pigment and filler behind. Once the thickeners separate, thixotropic behavior is significantly reduced. A surface-chemistry primer: wetting and leveling As mentioned earlier, surface chemistry deals with molecular attractions. Liquid molecules attract one another, resulting in a surface energy. The liquid attempts to reduce its surface area to a minimum by forming a sphere. This is why highsurface-tension materials like water and mercury invariably form spherical droplets. Table 1 lists the surface tension of common liquids and solids. We need not dwell on the units (dyne/cm) except to note that the higher the value, the greater molecular attraction. Wetting The higher the surface tension of a liquid, the greater the liquid's attraction for itself. Therefore, high-surfacetension liquids do not readily wet nonporous substrates, especially plastics (an exception is clean metal). In order for a liquid to wet a solid surface, its surface tension must be lower than the solid's surface tension (usually called surface energy). This means that a high-surface-. energy substrate is easier to wet, while low-surface-energy materials are difficult to wet, Because only a small intramolecular interaction exists between the liquid and solid, the liquid molecules have a higher attraction for one another and a rionwetting spherical shape results. Screen-printing inks must have surfacetension values of less than 40 dynes/cm to properly wet certain materials. As seen in Table 1, common substrates like polyester, vinyl, and acrylic have surfaceenergy values around 40 dynes/cm. Oils, especially silicones, have much lower surface-energy values than inks. Oil contamination, therefore, can cause the familiar dewetting on substrates, sometimes referred to as "fisheyes." Not only will aesthetic imperfections result from poor wetting, but adhesion will also suffer Good adhesion requires good wetting. Increasing an ink's viscosity to reduce fisheyes is the wrong approach. Even though a thickened ink is less prone to apparent dewetting, the low attraction of the ink for the substrate will give poor adhesion. The only ways to overcome a wetting problem are to increase the surface energy of the substrate or decrease the surface tension of the ink, Both approaches are practical and widely practiced. The surface energy of a plastic substrate can FEBRUARY

5 \ \ \\ \\\\ \\\ be increased by first removing any contamination and then treating. Any heatment that activates (produces polar chemical groups) the surface, such as corona discharge, wit increase the surface energy. Various chemical treatments are also useful. The other approach involves the use of wetting agents called surfactants. Fluorochemicals, silicones, and some hydrocarbons will reduce the surface tension of an ink. Fluorosurfectants are very potent and can allow an ink to wet ever) an oiled tinplate stock. Note that the same types of materials that can act as contaminants on substrates can be wetting enhancers in the ink. The surfactant approach, however, is a two-edged sword. We must be careful that the surfactant does not eventually become a substrate contaminant. Some of the early silicones were notorious for this. A silicone wetting agent could become airborne during an ink-baking cycle and contaminate substrates in an entire plant. Today, both nonvolatile and reactive silicone additives are available that are safe to use, but the old notoriety is unfortunately still around. Surfactants must be compatible with the ink, an oftenoverlooked requirement. An incompatible surfactant with an affinity for the substrate can cause dewetting. The additive attaches itself to the substrate and lowers the substrate's surface energy, functioning as a contaminant. Use all surfactants with caution. Too much of a good thing can reduce the quality of the printed image and alter the cured ink's properties. Although ink bleed is usually a rheological problem, high levels of surfactant can promote excessive wetting beyond the print boundaries. At Sheldahl, we have found that reducing the amount of wetting agent also reduces bleed. Another screen defect that we call "shadowing" can also be improved by reducing wetting agents. Shadowing appears to result from an overly efficient wetting of the screen mesh and print-side emulsion. The ink flows onto the emulsion and produces a light shadow or halo in the next impression. We have obtained the best printing results by keeping the ink's surface tension just a little lower than the substrate's. Leveling Leveling is much more complicated than it appears. As mentioned before, the screen-printing process deposits rectangles of ink that are joined together as the screen is removed and then flow out smoothly. At the moment that the ink islands blend together, the ink-film surface is very irregular. A number of forces combine to cause leveling. Empirical relationships for leveling have been discovered, but mainly in the paint and coaling industry'. However, whether surface irregularities are produced by a paint brush or a screen mesh, the qualitative relationships should hold. The only ways to overcome a wetting problem are to increase the surface energy of the substrate or decrease the surface tension of the ink. Most of these model equations indicate that leveling is influenced by the surface tension of the liquid, amplitude of the irregularities, liquid viscosity, leveling time, distance between irregularities, and yield point, In general, the following factors should improve ink leveling: longer flow-out time higher surface tension for the ink lower viscosity lower yield paint (or one that develops more slowly) a thixotropic ink with a slower viscosity increase greater ink thickness smaller distances between ridges (finer mesh) Most of these conclusions are self-evident to the practiced screen-printing operator. Perhaps one that is not so obvious is the effect of surface tension. Higher inksurface tension improves leveling. We are often tempted to add more arid more sur factant to cure an apparent flow problem. If the problem is really poor leveling, then reducing the ink's surface tension only aggravates the problem. Relative surface tensions can be estimated by placing a small drop of ink on the substrate and observing the contact angle between the two. A receding angle where the ink attempts to form a sphere indicates that the ink's surface tension is too high. A rapidly advancing droplet sug gests that the value is too low. A 90 0 contact angle, or one that advances slightly, is ideal. Summary The remarkable process of screen printing is made possible by the special rheology of inks. The process-altered viscosity of the screen-printing ink provides the righl characteristics just when they are needed. The ability of the ink to dramatically change in viscosity and to go from a flowing to an immobile state makes the screen-printing process a reality. The surface cher iistry of the ink is equally important. Wetting, flow out. leveling, and good adhesion depends on the right surface tension. Viewing (neology and surface chemistry together provides a more comprehensive understanding of screen print - ing and its interrelationships. n If you enjoyed this subject and want to read more about it in future issues of Screen Printing, circle n Reader Card No. 290 References Tlheology PRO.thes, Haake Duchler instruments, Inc, Vol. 12, Nu 1 (updated). zfrecska, T '' Rheology Primer," SITE. Nov. 1965, p. 54-5R. 'Orchard, S.E. Applied Science Research, p. 451 (1962). Bibliog raphy Bikales, N M, Adhesion and Bonding, New York, Wiley-Interscience, Martens, C R., Technology of Paint, Varnish, and Lacquers, New York, Krieger Pub. Co Nyle.n, P. and Sutherland, S.. Modem Surface Coatings, John Wiley & Sons. New York Patton, TC.. Paint Flow and Pigment Dispersion, Wiley-Interscience. New York, 2nd ed SCREENPRINVING

6 TABLE 5 SURFACE TENSION TEST KIT Surface Tension Castor Oil Toluene Heptane FC48/FC77 15 dyne/cm 0/ / / (100 glycerol) 72.8 (100 water) Mixtures are in weight percent References: various sources and tests by author

7 VISCOSITY (PO1SEj D 0= r a 8N_ Z o 5. om ri m r -6 ara

8 VISCOSITY (POISE) - '.. r 44- r r * rr I pi 0 I 4 I rn"