当前位置:首页 >> 机械/仪表 >>

Coatings Technology Handbook, ch1


I
Fundamentals and Testing

I-1
? 2006 by Taylor & Francis Group, LLC

1
Rheology and Surface Chemistry
1.1 1.2 1.3 Introduction ........................................................................1-1 Rheology ..............................................................................1-2
Types of Viscosity Behavior ? Temperature Effects ? Solvent Effects ? Viscosity Measurement ? Yield Value

Surface Chemistry ...............................................................1-8
Surface Tension ? Measuring Surface Tension ? Wetting ? Surfactants ? Leveling

K. B. Gilleo
Sheldahl, Inc.

1.4 Summary............................................................................1-12 References .....................................................................................1-12 Bibliography .................................................................................1-12

1.1 Introduction
A basic understanding of rheology and surface chemistry, two primary sciences of liquid ?ow and solid–liquid interaction, is necessary for understanding coating and printing processes and materials. A generally qualitative treatment of these subjects will suf?ce to provide the insight needed to use and apply coatings and inks and to help solve the problems associated with their use. Rheology, in the broadest sense, is the study of the physical behavior of all materials when placed under stress. Four general categories are recognized: elasticity, plasticity, rigidity, and viscosity. Our concern here is with liquids and pastes. The scope of rheology of ?uids encompasses the changes in the shape of a liquid as physical force is applied and removed. Viscosity is a key rheological property of coatings and inks. Viscosity is simply the resistance of the ink to ?ow — the ratio of shear stress to shear rate. Throughout coating and printing processes, mechanical forces of various types and quantities are exerted. The amount of shear force directly affects the viscosity value for non-Newtonian ?uids. Most coatings undergo some degree of “shear thinning” phenomenon when worked by mixing or running on a coater. Heavy inks are especially prone to shear thinning. As shear rate is increased, the viscosity drops, in some cases, dramatically. This seems simple enough except for two other effects. One is called the yield point. This is the shear rate required to cause ?ow. Ketchup often refuses to ?ow until a little extra shear force is applied. Then it often ?ows too freely. Once the yield point has been exceeded the solidlike behavior vanishes. The loose network structure is broken up. Inks also display this yield point property, but to a lesser degree. Yield point is one of the most important ink properties. Yield value, an important, but often ignored attribute of liquids, will also be discussed. We must examine rheology as a dynamic variable and explore how it changes throughout the coating process. The mutual interaction, in which the coating process alters viscosity and rheology affects the process, will be a key concept in our discussions of coating technology.

1-1
? 2006 by Taylor & Francis Group, LLC

1-2

Coatings Technology Handbook, Third Edition

The second factor is time dependency. Some inks change viscosity over time even though a constant shear rate is being applied. This means that viscosity can be dependent on the amount of mechanical force applied and on the length of time. When shearing forces are removed, the ink will return to the initial viscosity. That rate of return is another important ink property. It can vary from seconds to hours. Rheology goes far beyond the familiar snapshot view of viscosity at a single shear rate, which is often reported by ink vendors. It deals with the changes in viscosity as different levels of force are applied, as temperature is varied, and as solvents and additives come into play. Brook?eld viscometer readings, although valuable, do not show the full picture for non-Newtonian liquids. Surface chemistry describes wetting (and dewetting) phenomena resulting from mutual attractions between ink molecules, as well as intramolecular attractions between ink and the substrate surface. The relative strengths of these molecular interactions determine a number of ink performance parameters. Good print de?nition, adhesion, and a smooth ink surface all require the right surface chemistry. Bubble formation and related ?lm formation defects also have their basis in surface chemistry. Surface chemistry, for our purposes, deals with the attractive forces liquid molecules exhibit for each other and for the substrate. We will focus on the wetting phenomenon and relate it to coating processes and problems. It will be seen that an understanding of wetting and dewetting will help elucidate many of the anomalies seen in coating and printing. The two sciences of rheology and surface tension, taken together, provide the tools required for handling the increasingly complex technology of coating. It is necessary to combine rheology and surface chemistry into a uni?ed topic to better understand inks and the screen printing process. We will cover this uni?cation in a straightforward and semiqualitative manner. One bene?t will be the discovery that printing and coating problems often blamed on rheology have their basis in surface chemistry. We will further ?nd that coating leveling is in?uenced by both rheology and surface chemistry.

1.2 Rheology
Rheology, the science of ?ow and deformation, is critical to the understanding of coating use, application, and quality control. Viscosity, the resistance to ?ow, is the most important rheological characteristic of liquids and therefore of coatings and inks. Even more signi?cant is the way in which viscosity changes during coating and printing. Newtonian ?uids, like solvents, have an absolute viscosity that is unaltered by the application of mechanical shear. However, virtually all coatings show a signi?cant change in viscosity as different forces are applied. We will look at the apparent viscosity of coatings and inks and discover how these force-induced changes during processing are a necessary part of the application process. Viscosity, the resistance of a liquid to ?ow, is a key property describing the behavior of liquids subjected to forces such as mixing. Other important forces are gravity, surface tension, and shear associated with the method of applying the material. Viscosity is simply the ratio of shear stress to shear rate (Equation 1.3). A high viscosity liquid requires considerable force (work) to produce a change in shape. For example, high viscosity coatings are not as easily pumped as are the low viscosity counterparts. High viscosity coatings also take longer to ?ow out when applied. Shear rate, D = velocity (sec?1 ) thickness force (dynes/cm 2 ) area

(1.1)

Shear stress, τ =

(1.2)

Viscosity, η =

shear stress τ = (dynes ? sec/cm 2 ) shear rate D

(1.3)

? 2006 by Taylor & Francis Group, LLC

Rheology and Surface Chemistry

1-3

TABLE 1.1

Viscosities of Common Industrial Liquids
Liquid Viscosity (cP) 0.32 0.58 0.59 1.0000 1.0 1.2 1.5 1.6 12.0 25.4 33.1 84.0 986.0 1490.0 130,000.0

Acetone Chloroform Toulene Water, standard 2(20°C) Cyclohexane Ethyl alcohol Turpentine Mercury, metal Creosote Sulfuric acid Lindseed oil Olive oil Castor oil Glycerine Venice turpentine

Values are for approximately 20°C. Source: From Handbook of Chemistry and Physics, 64th ed., CRC Press, Boca Raton, FL, 1984.1

As indicated above, shear stress, the force per unit area applied to a liquid, is typically in dynes per square centimeter, the force per unit area. Shear rate is in reciprocal seconds (sec–1), the amount of mechanical energy applied to the liquid. Applying Equation 1.3, the viscosity unit becomes dyne-seconds per square centimeter or poise (P). For low viscosity ?uids like water (≈0.01 P), the poise unit is rather small, and the more common centipoise (0.01 P) is used. Since 100 centipoise = 1 poise, water has a viscosity of about 1 centipoise (cP). Screen inks are much more viscous and range from 1000 to 10,000 cP for graphics and as high as 50,000 cP for some highly loaded polymer thick ?lm (PTF) inks and adhesives. Viscosity is expressed in pascal-seconds (Pa?sec) in the international system of units (SI: 1 Pa?sec = 1000 cP). Viscosity values of common industrial liquids are provided in Table 1.1. Viscosity is rather a simple concept. Thin, or low viscosity liquids ?ow easily, while high viscosity ones move with much resistance. The ideal, or Newtonian, case has been assumed. With Newtonian ?uids, viscosity is constant over any region of shear. Very few liquids are truly Newtonian. More typically, liquids drop in viscosity as shear or work is applied. The phenomenon was identi?ed above as shear thinning. It is, therefore, necessary to specify exactly the conditions under which a viscosity value is measured. Time must also be considered in addition to shear stress. A liquid can be affected by the amount of time that force is applied. A shear-thinned liquid will tend to return to its initial viscosity over time. Therefore, time under shearing action and time at rest are necessary quanti?ers if viscosity is to be accurately reported. It should be apparent that we are really dealing with a viscosity curve, not a ?xed point. The necessity of dealing with viscosity curves is even more pronounced in plastic decorating. A particular material will experience a variety of different shear stresses. For example, a coating may be mixed at relatively low shear stress of 10 to 20 cP, pumped through a spray gun line at 1000 cP, sprayed through an airless gun ori?ce at extreme pressure exceeding 106 cP, and ?nally allowed to ?ow out on the substrate under mild forces of gravity (minor) and surface tension. It is very likely that the material will have a different viscosity at each stage. In fact, a good product should change in viscosity under applications processing.

1.2.1 Types of Viscosity Behavior
1.2.1.1 Plasticity Rheologically speaking, plastic ?uids behave more like plastic solids until a speci?c minimum force is applied to overcome the yield point. Gels, sols, and ketchup are extreme examples. Once the yield point has been reached, the liquid begins to approach Newtonian behavior as shear rate is increased. Figure

? 2006 by Taylor & Francis Group, LLC

1-4

Coatings Technology Handbook, Third Edition

Shear Stress

Plastic Pseudoplastic

Newtonian

Yield Point

Dilatant

Rate (sec.?1)

FIGURE 1.1 Shear stress–shear rate curves.

1.1 shows the shear stress–shear rate curve and the yield point. Although plastic behavior is of questionable value to ketchup, it has some bene?t in inks and paints. Actually, it is the yield point phenomenon that is of practical value. No-drip paints are an excellent example of the usefulness of yield point. After the brush stroke force has been removed, the paint’s viscosity builds quickly until ?ow stops. Dripping is prevented because the yield point exceeds the force of gravity. Ink bleed in a printing ink, the tendency to ?ow beyond the printed boundaries, is controlled by yield point. Inks with a high yield point will not bleed, but their ?ow out may be poor. A very low yield point will provide excellent ?ow out, but bleed may be excessive. Just the right yield point provides the needed ?ow out and leveling without excessive bleed. Both polymer binders and ?llers can account for the yield point phenomenon. At rest, polymer chains are randomly oriented and offer more resistance to ?ow. Application of shear force straightens the chains in the direction of ?ow, reducing resistance. Solid ?llers can form loose molecular attraction structures, which break down quickly under shear. 1.2.1.2 Pseudoplasticity Like plastic-behaving materials, pseudoplastic liquids drop in viscosity as force is applied. There is no yield point, however. The more energy applied, the greater the thinning. When shear rate is reduced, the viscosity increases at the same rate by which the force is diminished. There is no hysteresis; the shear stress–shear rate curve is the same in both directions as was seen in Figure 1.1. Figure 1.2 compares pseudoplastic behavior using viscosity–shear rate curves. Many coatings exhibit this kind of behavior, but with time dependency. There is a pronounced delay in viscosity increase after force has been removed. This form of pseudoplasticity with a hysteresis loop is called thixotropy. Pseudoplasticity is generally a useful property for coatings and inks. However, thixotropy is even more useful. 1.2.1.2.1 Thixotropy Thixotropy is a special case of pseudoplasticity. The material undergoes “shear thinning”; but as shear forces are reduced, viscosity increases at a lesser rate to produce a hysteresis loop. Thixotropy is very common and very useful. Dripless house paints owe their driplessness to thixotropy. The paint begins as a moderately viscous material that stays on the brush. It quickly drops in viscosity under the shear stress of brushing for easy, smooth application. A return to higher viscosity, when shearing action stops, prevents dripping and sagging. Screen printing inks also bene?t from thixotropy. The relatively high viscosity screen ink drops abruptly in viscosity under the high shear stress associated with being forced through a ?ne mesh screen. The

? 2006 by Taylor & Francis Group, LLC

Rheology and Surface Chemistry

1-5

High Viscosity Newtonian Liquid

Dilatant Viscosity (poise)

Pseudoplastic Low Viscosity Newtonian Liquid

Shear Rate (sec.?1)

FIGURE 1.2 Viscosity shear rate curves.

momentary low viscosity permits the printed ink dots to merge together into a solid, continuous ?lm. Viscosity returns to a higher range before the ink can “bleed” beyond the intended boundaries. Thixotropic materials yield individual hysteresis loops. Shear stress lowers viscosity to a point at which higher force produces no further change. As energy input to the liquid is reduced, viscosity begins to build again, but more slowly than it initially dropped. It is not necessary to know the shape of the viscosity loop, but merely to realize that such a response is common in decorating inks, paints, and coatings. The presence in decorating vehicles of pigments, ?atting agents, and other solid ?llers usually produces or increases thixotropic behavior. More highly loaded materials, such as inks, are often highly thixotropic. Thixotropic agents, consisting of ?at, platelet structures, can be added to liquids to adjust thixotropy. A loose, interconnecting network forms between the platelets to produce the viscosity increase. Shearing breaks down the network, resulting in the viscosity drop. Mixing and other high shear forces rapidly reduce viscosity. However, thixotropic inks continue to thin down while undergoing shearing, even if the shear stress is constant. This can be seen with a Brook?eld viscometer, where measured viscosity continues to drop while the spindle turns at constant rpms. When the ink is left motionless, viscosity builds back to the initial value. This can occur slowly or rapidly. Curves of various shapes are possible, but they will all display a hysteresis loop. In fact, this hysteresis curve is used to detect thixotropy (see Figure 1.3). The rate of viscosity change is an important characteristic of an ink which is examined later as we take an ink step by step through screen printing. Thixotropy is very important to proper ink behavior, and the changing viscosity attribute makes screen printing possible. 1.2.1.2.2 Dilatancy Liquids that show an increase in viscosity as shear is applied are called dilatants. Very few liquids possess this property. Dilatant behavior should not be confused with the common viscosity build, which occurs when inks and coatings lose solvent. For example, a solvent-borne coating applied by a roll coater will show a viscosity increase as the run progresses. The rotating roller serves as a solvent evaporator, increasing the coating’s solids content and, therefore, the viscosity. True dilatancy occurs independently of solvent loss. 1.2.1.2.3 Rheoplexy Sounding more like a disease than a property, rheoplexy is the exact opposite of thixotropy. It is the timedependent form of dilatancy where mixing causes shear thickening. Figure 1.3 showed the hysteresis loop. Rheoplexy is fortunately rare, because it is totally useless as a characteristic for screen print inks.

? 2006 by Taylor & Francis Group, LLC

1-6

Coatings Technology Handbook, Third Edition

Rheopectic

Viscosity (poise)

Thixotropic

Rate (sec. ?1)

FIGURE 1.3 Shear stress–shear rate curves: hysteresis loop.

1.2.2 Temperature Effects
Viscosity is strongly affected by temperature. Measurements should be taken at the same temperature (typically 23°C). A viscosity value is incomplete without a temperature notation. Although each liquid is affected differently by a temperature change, the change per degree is usually a constant for a particular material. The subject of temperature effects has been covered thoroughly elsewhere.2 It will suf?ce to say that a coating’s viscosity may be reduced by heating, a principle used in many coating application systems. Viscosity reduction by heating may also be used after a material has been applied. Preheating of ultraviolet (UV)-curable coatings just prior to UV exposure is often advantageous for leveling out these sometimes viscous materials.

1.2.3 Solvent Effects
Higher resin solids produce higher solution viscosity, while solvent addition reduces viscosity. It is important to note that viscosity changes are much more pronounced in the case of soluble resins (polymers) than for insoluble pigments or plastic particles. For example, although a coating may be highly viscous at 50% solids, a plastisol suspension (plastic particles in liquid plasticizer) may have medium viscosity at 80% solids. Different solvents will produce various degrees of viscosity reduction depending on whether they are true solvents, latent solvents, or nonsolvents. This subject has been treated extensively elsewhere.2,3

1.2.4 Viscosity Measurement
Many instruments are available. A rheometer is capable of accurately measuring viscosities through a wide range of shear stress. Much simpler equipment is typically used in the plastic decorating industry. As indicated previously, perhaps the most common device is the Brook?eld viscometer, in which an electric motor is coupled to an immersion spindle through a tensiometer. The spindle is rotated in the liquid to be measured. The higher the viscosity (resistance to ?ow), the larger is the reading on the tensiometer. Several spindle diameters are available, and a number of rotational speeds may be selected. Viscosity must be reported along with spindle size and rotational speed and temperature.

? 2006 by Taylor & Francis Group, LLC

Rheology and Surface Chemistry

1-7

TABLE 1.2

Viscosity Conversionsa
Consistency Watery Medium 1.0 100 2.5 250 5.0 500 10 1,000 50 5,000 Heavy 100 10,000 150 15,000

Poise: Centipoise: Viscosity device Fisher #1 Fisher #2 Ford #4 cup Parlin #10 Parlin #15 Saybolt Zahn #1 Zahn #2 Zahn #3 Zahn #4
a

0.1 10 20 5 11 60 30 16

0.5 50

24 22 17 260 60 24

50 34 25 530 37 12 10

67 55 12 1,240 85 29 21

25 2,480

47 4,600

232 23,500

465 46,500

697 69,500

57 37

Liquids are at 25°C. Values are in seconds for liquids with speci?c gravity of approximately 1.0. Source: Binks Inc., ITW Industrial Finishing, 195 International Boulevard, Glendale Heights, IL 60134.

The Brook?eld instrument is a good tool for incoming quality control. Although certainly not a replacement for the rheometer, the viscometer may be used to estimate viscosity change with shear. Viscosity readings are taken at different rpms and then compared. A highly thixotropic material will be easily identi?ed. An even simpler viscosity device is the ?ow cup, a simple container with an opening at the bottom. The Ford cup and the Zahn cup are very common in the plastic painting and coating ?eld. The Ford cup, the more accurate of the two, is supported on a stand. Once ?lled, the bottom ori?ce is unstoppered and the time for the liquid to ?ow out is recorded. Unlike the Brook?eld, which yields a value in centipoise, the cup gives only a ?ow time. Relative ?ow times re?ect different relative viscosities. Interconversion charts permit Ford and other cup values to be converted to centipoise (Table 1.2). The Zahn cup is dipped in a liquid sample by means of its handle and quickly withdrawn, whereupon time to empty is recorded. The Zahn type of device is commonly used on line, primarily as a checking device for familiar materials.

1.2.5 Yield Value
The yield value is the shear stress in a viscosity measurement, but one taken at very low shear. The yield value is the minimum shear stress, applied to a liquid, that produces ?ow. As force is gradually applied, a liquid undergoes deformation without ?owing. In essence, the liquid is behaving as if it were an elastic solid. Below the yield value, viscosity approaches in?nity. At a critical force input (the yield value) ?ow commences. The yield value is important in understanding the behavior of decorating liquids after they have been deposited onto the substrate. Shear stress, acting on a deposited coating or ink, is very low. Although gravity exerts force on the liquid, surface tension is considerably more important. If the yield value is greater than shear stress, ?ow will not occur. The liquid will behave as if it were a solid. In this situation, what you deposit is what you get. Coatings that refuse to level, even though the apparent viscosity is low, probably have a relatively high yield value. As we will see in the next section, surface tension forces, although alterable, cannot be changed enough to overcome a high yield value. Unfortunately, a high yield value may be an intrinsic property of the decorative material. Under these circumstances, changing the material application method may be the only remedy. Although a high yield value can make a coating unusable, the property can be desirable for printing inks. Once an ink has been deposited, it should remain where placed. Too low a yield value can allow an ink to ?ow out, producing poor, irregular edge de?nition. An ink with too high a value may ?ow out

? 2006 by Taylor & Francis Group, LLC

1-8

Coatings Technology Handbook, Third Edition

poorly. As pigments tend to increase yield value, color inks are not a problem. Clear protective inks can be a problem, especially when a thick ?lm is deposited, as in screen printing. When it is not practical to increase yield value, wettability can sometimes be favorably altered through surface tension modi?cation. Increasing surface tension will inhibit ?ow and therefore ink or coating bleed.

1.3 Surface Chemistry
Surface chemistry is the science that deals with the interface of two materials. The interface may exist between any forms of matter, including a gas phase. For the purpose of understanding the interfacial interaction of decorative liquid materials, we need only analyze the liquid–solid interaction. Although there is a surface interaction between a liquid coating and the air surrounding it, the effect is small and may be ignored.

1.3.1 Surface Tension
All liquids are made up of submicroscopic combinations of atoms called molecules (a very few liquids are made up of uncombined atoms). All molecules that are close to one another exert attractive forces. It is these mutual attractions that produce the universal property called surface tension. The units are force per unit length: dynes per centimeter. A drop of liquid suspended in space quickly assumes a spherical shape. As surface molecules are pulled toward those directly beneath them, a minimum surface area (sphere) results. The spherical form is the result of an uneven distribution of force; molecules within the droplet are attracted from all directions, while those at the surface are pulled only toward molecules below them. All liquids attempt to form a minimum surface sphere. A number of counterforces come into play, however. A liquid placed on a solid provides a liquid–solid interface. This type of interface is critically important to the plastic decorator, as liquid molecules are attracted not only to each other (intramolecular attraction) but also to any solid surface (intermolecular attraction) with which they come in contact. We need only concern ourselves with these two interactions; intra- and intermolecular. A fundamental understanding of this interfacial interaction will permit the decorator to optimize materials and processes.

1.3.2 Measuring Surface Tension
Every liquid has a speci?c surface tension value. Liquids with high surface tensions, such as water (73 dynes/cm), demonstrate a high intramolecular attraction and a strong tendency to bead up (form spheres). Liquids with low values have a weak tendency toward sphere formation that is easily overcome by countering forces. A variety of methods are available for measuring liquid surface tension. Table 1.3 gives values for common solvents. Methods are also available for determining the surface tension of solids, which is usually referred to as surface energy. Table 1.4 gives surface energy values for plastics. We need be concerned only with ways of estimating surface tension and with techniques for determining relative differences.

1.3.3 Wetting
A liquid placed on a ?at, horizontal solid surface either will wet and ?ow out, or it will dewet to form a semispherical drop. An in-between state may also occur in which the liquid neither recedes nor advances but remains stationary. The angle that the droplet or edge of the liquid makes with the solid plane is called the contact angle (Figure 1.4). A nonwetting condition exists when the contact angle exceeds 0° — that is, when the angle is measurable. The liquid’s intramolecular attraction is greater than its attraction for the solid surface. The liquid surface tension value is higher than the solid’s surface energy. A wetting condition occurs when the contact angle is 0°. The liquid’s edge continues to advance, even though the rate may be slow for

? 2006 by Taylor & Francis Group, LLC

Rheology and Surface Chemistry

1-9

TABLE 1.3

Surface Tension of Liquids
Surface Tension (dynes/cm) 5.6 15.6 22.1 24.0 26.3 36.8 43.5 48.4 59.1 63.1 70.2 72.8 490.6

Liquid SF6 Tri?uoroacetic acid Heptane Methanol Acetone Dimethylformamide (DMF) Dimethyl sulfoxide (DMSO) Ethylene glycol Formamide Glycerol Diiodomethane Water Mercury, metal

Source: From Dean, J., Ed., Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill, New York, 1985.4

TABLE 1.4

Surface Tension of Polymers
Polymer Surface Tension (dynes/cm) 16 18.5 24 31 34 39 40 43 46

Polyper?uoropropylene Polytetra?uoroethylene (Te?on) Polydimethyliloxane Polyethylene Polystyrene Polymethylmethacrylate (acrylic) Polyvinyl chloride (PVC) Polyethylene terephthalate (polyester) Polyhexamethylene adipate (nylon)

Source: From Bikales, N.M., Adhesion and Bonding, Wiley-Interscience, New York, 1971.5

θ Liquid

FIGURE 1.4 Contact angle.

high viscosity materials. The intermolecular (solid–liquid) attraction is greater in this case. The surface energy of the solid is higher than the liquid’s surface tension. Measuring the contact angle is a simple technique for determining the relative difference between the two surface tensions. A high contact angle signi?es a large departure, while a small angle suggests that the two values are close, but not equal.

? 2006 by Taylor & Francis Group, LLC

1-10

Coatings Technology Handbook, Third Edition

TABLE 1.5
Surface Tension 15 dynes/cm 17 19 22 22.4 24.5 27 30 32.5 35 63 72.8

Surface Tension Test Kit
Castor Oil Toluene Heptane FC48/FC77 0/100 100/100 100/0 12.0 55.2 74.2 0 88.0 100.0 (100 glycerol) (100 water) 49.2 25.0 14.4 100.0 4.5 100 38.8 19.8 11.4 0 3.5

Mixtures are in weight percent. Source: Various sources and tests by author.

One can estimate liquid surface tension by applying drops of the liquid onto smooth surfaces of known values until a wetting just occurs, signifying that the two surface tensions are equal. Conversely, the surface energy of a solid may be estimated by applying drops of standard surface tension liquids until wetting is achieved. A surface tension kit can be made up from simple mixtures for testing surfaces. Table 1.5 provides formulas. Low energy surfaces are dif?cult to wet and can give poor results for coating, painting, and printing. The standard surface tension kit may be used to estimate the surface energy of a plastic to be decorated. If the particular plastic shows a much lower value than that reported in Table 1.4, contamination is suspected. Mold release agents, unless specially made compatible for decorating materials, can greatly lower surface energy of a plastic part, making it uncoatable.

1.3.4 Surfactants
Agents that alter interfacial interactions are called surfactants. The surfactant possesses two different chemical groups, one compatible with the liquid to be modi?ed, and the other having a lower surface tension. For example, the surface tension of an epoxy may be reduced by adding a surfactant with an alcohol group (epoxy-compatible) at one end and a ?uorochemical group at the other. The alcohol group will associate with the epoxy resin, presenting the incompatible ?uorochemical “tail” to the surface. The epoxy coating will behave as if it were a low surface tension ?uorochemical. The addition of a small amount of surfactant will permit the epoxy coating to wet dif?cult, low energy surfaces, even oilcontaminated plastic. Surfactants ef?ciently lower the surface tension of inks, coatings, and paints. Typically, 1% or less is suf?cient. When dewetting occurs because of intrinsically low surface energy of the substrate, use of surfactants, also called wetting agents, is indicated. These materials are not a substitute for good housekeeping and proper parts preparation. Contamination can cause adhesion failure later. Fluorochemicals, silicones, and hydrocarbons are common categories of surfactants. Fluorochemicals have the lowest surface tension of any material and are the most ef?cient wetting agents. Silicones are next in ef?cacy and are lower in cost. Certain types of silicone, however, can become airborne, causing contamination of the substrate. Although it may be desirable to lower the surface tension of a coating, the opposite is true for the substrate. The very agent that helps the decorating material renders the substrate useless. Silicone contamination will produce the notorious dewetting defect called “?sh-eyes.” Coatings, paints, and inks, once modi?ed with surfactants, are usually permanently changed, even after curing. Their low surface energy will make them dif?cult to wet over if, for example, it is necessary to apply a top coat. There are several options for overcoming this problem. The best practice is to use

? 2006 by Taylor & Francis Group, LLC

Rheology and Surface Chemistry

1-11

the smallest amount of the least potent surfactant that will do the job. Start with the hydrocarbon class. Also make sure that the substrate is clean to begin with. Another possibility is to use reactive surfactants. Agents possessing a functional group that can react with coating or binder are rendered less active after curing. Once the surfactant has completed the role of wetting agent, it is no longer needed. One other approach is to add surfactant to the second material to be applied. Often the same surfactant will work, especially at a slightly higher loading.

1.3.5 Leveling
Leveling depends on both rheology and surface chemistry. It is a more complex phenomenon and a more dif?cult one to control. Coatings applied by spraying, dipping, roll coating, and most other methods are often not smooth enough for aesthetic appeal. Splatters, runs, ridges, and other topological defects require that the liquid material level out. It is therefore important to understand the dynamics of leveling. We will ?rst assume that proper wetting has been achieved, by wetting agents if necessary. Important parameters affecting leveling are viscosity, surface tension, yield value, coating thickness, and the degree of wet coating irregularity. Several workers have developed empirical relationships to describe leveling. The leveling equation (Equation 1.4) is quite useful.6 at = a0 where at = amplitude (height) of coating ridge σ = the surface tension of the coating η = coating viscosity h = coating thickness or height t = the time for leveling λ = wavelength or distance between ridges Equation 1.4 shows that leveling is improved by one or more of the following: 1. 2. 3. 4. 5. Longer time (t) Higher surface tension of coating (σ) Lower viscosity (η) Greater coating thickness (h) Small repeating distance between ridges (λ) exp(const σh 3t ) 3λ 4 η

(1.4)

Note that h, the coating thickness, is raised to the third power. Doubling the thickness provides an eightfold (23) improvement in leveling. Also note that λ, wavelength between ridges, is raised to the fourth power. This means that ridges that are very far apart create a very dif?cult leveling situation. Earlier, it was pointed out that a high yield value could prevent leveling. The shear stress on a wet coating must be greater than the yield value for leveling to take place. Equation 1.5 shows the relationship between various parameters and shear stress.7 Tmax = where σ = surface tension of coating a = amplitude of coating ridge h = coating height λ = coating ridge wavelength 4π 3 σ ah τλ 3 or D(coating ridge depth ) = 3 3 λ 4π σh

(1.5)

? 2006 by Taylor & Francis Group, LLC

1-12

Coatings Technology Handbook, Third Edition

Because Equation 1.5 deals with force, the time factor and the viscosity value drop out. It is seen that increasing surface tension and coating thickness produce the maximum shear stress. Coating defect height (a) increases shear, while wavelength (λ) strongly reduces it. If coating ridges cannot be avoided, higher, more closely packed ones are preferable. When the yield value is higher than the maximum shear (Tmax), leveling will not occur. Extending leveling time and reducing viscosity will not help to overcome the yield value barrier, because these terms are not in the shear equation. Increasing surface tension and coating thickness are options, but there are practical limits. As yield value is usually affected by shear (thixotropy), coating application rate and premixing conditions may be important. Higher roller speed (for roll coaters) and higher spray pressure (for spray guns) can drop the yield value temporarily. It should be apparent that best leveling is not achieved by lowest surface tension. Although good wetting may require a reduction in surface tension, higher surface tension promotes leveling. This is one more reason to use the minimum effective level of surfactant.

1.4 Summary
A comprehension of the basic principles that describe and predict liquid ?ow and interfacial interactions is important for the effective formulation and the ef?cient application of coatings and related materials. The theoretical tools for managing the technology of coatings are rheology, the science of ?ow and deformation, considered with surface chemistry, and the science of wetting and dewetting phenomena. Viewing such rheology properties as viscosity in terms of their time dependency adds the necessary dimension for practical application of theory to practice. Such important coating attributes as leveling are affected by both viscosity and surface tension. Knowing the interrelationships allows the coating specialist to make adjustments and take corrective actions with con?dence.

References
1. 2. 3. 4. 5. 6. 7. Handbook of Chemistry and Physics, 64th ed. Boca Raton, FL: CRC Press, 1984. Temple C. Patton, Paint Flow and Pigment Dispersion, 2nd ed. New York: Wiley, 1979. Charles R. Martens, Technology of Paint, Varnish and Lacquers. New York: Krieger Pub. Co., 1974. Dean, J., Ed., Lange’s Handbook of Chemistry, 13th ed. New York: McGraw-Hill, 1985. Norbert M. Bikales, Adhesion and Bonding. New York: Wiley-Interscience, 1971. S. Orchard, Appl. Sci. Res., A11, 451 (1962). N. D. P. Smith, S. E. Orchard, and A. J. Rhind-Tutt, “The physics of brush marks,” JOCCA, 44, 618–633, September (1961).

Bibliography
Bikales, N. M., Adhesion and Bonding. New York: Wiley-Interscience, 1971. Martens, C. R., Technology of Paint, Varnish and Lacquers. New York: Krieger Pub. Co., 1974. Nylen, P. and S. Sunderland, Modern Surface Coatings. New York: Wiley, 1965. Patton, T. C., Paint Flow and Pigment Dispersion, 2nd ed. New York: Wiley-Interscience, 1979.

? 2006 by Taylor & Francis Group, LLC


相关文章:
更多相关标签: