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The Components of Paint
3 The Components of Paint While there are relatively few commercial fluoropolymers, there are thousands of fluoropolymer paint formulations. Nearly all fluoropolymer coatings come in two forms. They are either dry powders or liquids. This book does not cover exotic approaches like gas or plasma phase reactions and depositions. A basic understanding of paint formulation technology would facilitate understanding fluoropolymers coatings. This chapter provides a general overview and details follow in later chapters. Except for the molecules containing fluorine, fluorocoatings are much like other paints. There are several books that discuss the basics of coating technology and a lot of those works can be consulted to help understand fluorocoatings.1 The components of all paint generally include the following: Binder Solvents (except for dry powder coatings) Pigments and fillers Additives.
3.1 Binder, Vehicle, Film Former The binder is the film-forming component of paint. It is the only component that must be present in a paint formulation. With no other components present, most binders would dry to form a transparent, glossy film; some binders are used without pigments to make clear finishes and varnishes. The other components are included optionally, depending on the desired properties of the cured film. The binder imparts adhesion and strongly influences properties such as gloss, durability, flexibility, and toughness. Binders in fluorocoatings include the fluoropolymers and optionally other organic or inorganic polymers. Binders can be categorized according to the mechanisms for drying or curing. Drying usually refers to evaporation of the solvent or thinner. Curing usually refers to a chemical change to the binders such as cross-linking (see Chapter 4). Cross-linking
is the formation of chemical bonds between polymer chains. Some paints form by solvent evaporation only, but most rely on cross-linking. Those that require melting may or may not form cross-links. Paints that dry by solvent evaporation and contain the solid binder dissolved in a solvent are known as lacquers. A solid film forms when the solvent evaporates, and because the film can redissolve in solvent, lacquers are generally unsuitable for applications where chemical resistance is important. Many household spray paints fall into this category. Paints known as emulsions in the United Kingdom and latexes in the United States are water-borne dispersions of submicrometer polymer particles. These paints cure by a process called coalescence. Water evaporates in the initial step of coalescence. Next the slower evaporating trace (or coalescing) solvents evaporate and draw together to soften the binder polymer particles and fuse them together into irreversibly bound networked structures. As a result the paint cannot redissolve in the solvent/water that originally carried it. House paints cure in this manner and that is why they can be washable. Some paints cure by oxidative cross-linking. When applied, the exposure to oxygen in the air starts a process that cross-links and polymerizes the binder component. Other paints cure by polymerization and are generally one or two package coatings that polymerize by way of a chemical reaction, and cure into a cross-linked film. For UV-curing paints, the solvent is evaporated first. Hardening is then initiated by ultraviolet light, which initiates chemical cross-linking. In powder coatings there is little or no solvent, and flow and cure are produced by heating of the substrate above the melting point after electrostatic application of the dry powder. Thermoplastic coatings typically melt when reheated, whereas thermosets undergo the chemical reaction curing mechanisms mentioned earlier. Curing involving cross-linking prevents remelting. In liquid coatings the binder is dissolved, dispersed, or suspended in the solvent. The solvent is usually a mixture. It liquefies the other paint components
Fluorinated Coatings and Finishes Handbook. http://dx.doi.org/10.1016/B978-0-323-37126-1.00003-5 Copyright © 2016 Elsevier Inc. All rights reserved.
51
F LUORINATED C OATINGS
52
allowing them to spread out over the substrate being coated. Water is considered an important solvent. Nonfluoropolymer binder polymers are discussed in more detail in Chapter 4. Several other terms are common in the paint industry. These include medium, vehicle, and carrier. The generally accepted definitions are as follows: Vehicle is the liquid portion of paint. The vehicle is composed mainly of solvents, resins, and oils Carrier usually refers to the solvent Medium is the continuous phase in which the pigment is dispersed; it is synonymous with vehicle. Pigments and fillers are small particles added to paints to impart color and affect physical properties such as hardness or abrasion resistance or affect corrosion resistance. Pigments can also be used to influence viscosity, cost, adhesion, moisture permeability, gloss, abrasion resistance, electrical and thermal conductivity, and other properties. Alternatively, some paints contain dyes instead of or in combination with pigments to impart color. Fillers are a special type of pigment that serve to thicken the film, support its structure, and increase the volume of the paint. Fillers are usually cheap and inert materials, such as diatomaceous earth, talc, lime, barytes, clay, etc. Not all paints include fillers. On the other hand, some paints contain large proportions of pigment/filler and binder. Pigments and fillers are discussed in more detail in Chapter 5. Besides the three main categories of ingredients discussed above, paint can have a wide variety of miscellaneous additives, which are usually added in small amounts, yet provide a significant effect on the product. Additives are chemicals added to paints to achieve specific effects or solve specific problems. These include: 1. Surfactants, which help stabilize dispersions 2. Viscosity agents 3. Defoamers
AND
F INISHES H ANDBOOK
7. Catalysts 8. Others. Additives are discussed in more detail in Chapter 7.
3.2 Important Properties of Liquid Coatings A number of important properties describe liquid coatings. Sometimes these properties are part of the specifications of the coating. Often they are not, but nonetheless they are important to know, particularly when problems arise.
3.2.1 Rheology/Viscosity Viscosity in its simplest definition is the resistance of a liquid to flow or, as the American Heritage Dictionary puts it, “The degree to which a fluid resists flow under applied force.” The viscosity is usually a specification. It is frequently reported as a single measurement such as: “200e400 cps (measured by Brookfield viscometer at 25 C, #2 spindle at 20 RPM).” At first glance, a specification such as this might imply that the viscosity is a single measurement of a coating. If one looks closely the information in the parentheses, then the implication is that the viscosity depends exactly on how it is measured. The “25 C” in the specification implies that the viscosity is a function of temperature, which it is. The “Brookfield viscometer” is the instrument used to measure the viscosity, and its inclusion in the specification implies that the viscosity also depends on how it is measured. The Brookfield viscosity measurement is described in more detail in Chapter 13.1 “Measurement of Coating Performance.” Finally, the “#2 spindle at 20 RPM” defines two of the variables one can control on the Brookfield viscometer. The spindles of a Brookfield are different designs and each has a different surface area. The more area in contact with the liquid and with the spindle, the more force will be required to turn it. The ratio of that force to the area is called the “shear stress” in Eqn (3.1):
4. Surface modifiers 5. Stabilizers 6. Wetting agents
s¼
F A
(3.1)
3: T HE C OMPONENTS
OF
PAINT
53
where: F ¼ force (dynes) A ¼ area (cm2) s ¼ shear stress (dynes/cm2) The “20 RPM” defines the rotational speed or velocity. The liquids being moved against this surface include not only the liquid that is in direct contact with the spindle but also the liquid near its surface. This creates a velocity gradient, which is called the shear rate in Eqn (3.2): D¼
dv v ¼ dx x
(3.2)
where:
Figure 3.1 Newtonian flow.
D ¼ shear rate (s1) v ¼ shear velocity (cm/s) x ¼ thickness (cm) The viscosity is defined in Eqn (3.3). h¼
s D
(3.3)
where: s ¼ shear stress (dynes/cm2) from Eqn (3.1) D ¼ shear rate (s1) h ¼ viscosity (poise ¼ dyne-s/cm2) The main point here is that the viscosity depends upon the shear rate and stress applied by the measuring device and the temperature at which the measurement is made. This also implies that the viscosity will change depending upon how the coating is applied. Therein lies a key to using and understanding coatings. The viscosity varies with how the coating is used. Actually, viscosity also affects how coatings are manufactured, how they are stored, how they are prepared for use, and how long their shelf life is. The study of viscosity as a function of shear applied to the coating is called rheology. A test instrument called a rotoviscometer can make these measurements quickly. This work will not go into deep detail on the physics and chemistry of rheology.
Many texts develop the theory, measurement, and interpretation of rheology.2 An ideal liquid might have a viscosity that is independent of temperature, shear, and time. Some materials approach this ideal. The ideal is called Newtonian flow. Figure 3.1 shows the viscosity versus shear rate of a Newtonian fluid. Solvents and water are nearly Newtonian. Most coatings exhibit viscosity change with shear change. There are many practical reasons for making coatings behave in this manner. For instance, nearly everyone is familiar with house paint. It has very high viscosity as it sits undisturbed in a can. That is high viscosity at a low shear rate. However, when it is rolled or brushed, the shear rate becomes high. The viscosity drops dramatically. This allows the paint to flow out and level well on a wall or ceiling. Then after it is applied and the shear is removed the viscosity rises dramatically preventing or at least minimizing which keeps the paint from dripping or running. This type of viscosity behavior is called “shear-thinning” or pseudoplastic. Figure 3.2 shows the viscosity versus shear relationships of two coatings. Coating “B” shows a linear relationship while “A” shows a more nonlinear change. Both are considered pseudoplastic. The opposite of pseudoplastic flow is dilatant flow or “shear-thickening.” Figure 3.3 shows dilatant behavior. Dilatant coatings are rare. Thixotropic flow is a special case of shear-thinning behavior. A thixotropic coating thins with shear, but
54
Figure 3.2 Pseudoplastic flow.
F LUORINATED C OATINGS
AND
F INISHES H ANDBOOK
Figure 3.4 Thixotropic flow.
Table 3.1 Approximate Shear Ranges for Common Coating Processes
Figure 3.3 Dilatant flow.
its viscosity does not return to the original value after the shear is removed. There is time dependence. Often with enough time, the viscosity will recover. Figure 3.4 shows a thixotropic coating and what is referred to as a thixotropic loop. The arrows indicate how the experiment was run. Starting from low shear, shear is gradually increased. Then, gradually the shear is removed. This type of behavior is sometimes designed into coatings with additives (discussed in Chapter 7). It can be used to minimize settling in a coating formulation, increasing the time a coating can be stored.
Process
Shear Range, sL1
Sagging
102e101
Leveling
102e101
Dipping
100e101
Flow coating
100e101
Pumping
100e102
Mixing
101e102
Dispersion
102e105
Spraying
103e105
Roller coating
103e105
Brushing
103e104
To give a feeling for the magnitude of the shear forces, several processes and their shear rates are given in Table 3.1. There are many ways to measure or estimate viscosity (discussed in Chapter 13.1). Some are easier than others. Some have more variability than others. Many plants and paint shops use a cup method which times how long it takes for a given volume of coating to drain through a hole of specific size. This work
Table 3.2 Viscosity Conversion Chart3 Seybolt University, SSU
Zahn #1, s
Zahn #2, s
A4
60
30
16
A3
80
34
17
100
37
18
130
41
19
160
44
20
Ford #4, s
Gardner Holdt Bubble
20
5
25
8
Ford #3, s
Krebs Units, KU
Zahn #3, s
Zahn #4, s
Zahn #5, s
Shears, s
Parlin #7, s
Parlin #10, s
Fisher #1, s
0.1
10
27
11
0.15
15
30
12
0.2
20
32
13
30
15
12
10
0.25
25
37
14
35
17
15
12
A2
0.3
30
43
15
39
18
19
14
A1
0.4
40
50
16
50
21
25
18
A
0.5
50
57
17
24
29
22
0.6
60
64
18
29
33
25
0.7
70
20
33
36
28
0.8
80
22
39
41
31
0.9
90
23
44
45
32
1.0
100
25
50
50
34
D
40
530
41
12
10
27
1.2
120
30
62
58
41
E
43
580
49
14
11
31
1.4
140
32
66
45
E
46
690
58
16
12
34
1.6
160
37
50
G
48
790
66
18
13
38
1.8
180
41
54
50
900
74
20
14
40
2.0
200
45
58
52
1000
82
23
16
44
2.2
220
62
I
54
1100
25
17
10
2.4
240
65
J
56
1200
28
18
11
2.6
260
68
58
1280
30
20
12
2.8
280
70
K
59
1380
32
21
13
3.0
300
74
L
60
1475
34
22
14
3.2
320
1530
36
24
15
3.4
340
N
3.6
360
O
3.8
380
4.0
400
4.2
420
4.4
440
Q
4.6
460
R
4.8
480
5.0
500
Poise
Fisher #2, s
Gardner Lithograph
centipoise, cp
B
C
000 H
210
52
22
19
30
260
60
24
20
33
320
68
27
21
35
370
30
23
37
430
34
38
480
37
M
S
1630
39
25
16
41
26
17
1850
43
28
18
1950
46
29
19
2050
48
30
20
2160
50
32
21
66
2270
52
33
22
67
2380
54
34
23
68
2480
57
36
24
64
00
26
1730
62
P
24 10
(Continued )
Table 3.2 Viscosity Conversion Chart3 (Continued )
Poise
centipoise, cp
Parlin #7, s
Parlin #10, s
Fisher #1, s
Fisher #2, s
Ford #3, s
Ford #4, s
Gardner Holdt Bubble
5.5
550
T
6.0
600
U
7.0
700
8.0
800
Gardner Lithograph
0
Krebs Units, KU
Seybolt University, SSU
69
Zahn #1, s
Zahn #2, s
Zahn #3, s
Zahn #4, s
Zahn #5, s
2660
63
37
25
68
71
2900
40
27
74
3375
44
30
77
3880
51
35
9.0
900
V
81
4300
58
40
10.0
1000
W
85
4600
64
45
11.0
1100
88
5200
49
12.0
1200
92
5620
55
13.0
1300
95
6100
59
14.00
1400
96
6480
64
15.0
1500
98
7000
16.00
1600
100
7500
17.0
1700
101
8000
18.0
1800
19.0
1900
20.0
2000
21.0
2100
9850
22.0
2200
10,300
23.0
2300
24.00
2400
25.0
2500
X 1
Y
8500 9000 103
Z
Z-1
2
9400
105
10,750
109
11,200
114
11,600
Shears, s
30.00
3000
35.0
3500
40.0
4000
45.0
4500
50.0
5000
55.0
5500
60.0
6000
65.0
6500
30,000
70.00
7000
32,500
75.0
7500
35,000
80.00
8000
37,000
85.0
8500
39,500
90.0
9000
41,000
95.0
9500
43,000
100.0
10,000
110.0
11,000
51,000
120.0
12,000
55,500
130.0
13,000
60,000
140.00
14,000
150.0
15,000
Z-2
3
Z-3
121
14,500
129
16,500
133
18,300
136
21,000 23,500 26,000
Z-4
Z-5
4
5
28,000
46,500
65,000 Z-6
67,500
160.00
16,000
74,000
170.0
17,000
80,000
180.0
18,000
83,500
190.0
19,000
88,000
200.0
20,000
93,000
300.0
30,000
140,000
F LUORINATED C OATINGS
58
will not deal with all of these tests, but Table 3.2 allows estimation and conversion between some common viscosity measurement devices.
AND
F INISHES H ANDBOOK
where: C ¼ square feet of substrate covered per gallon of paint S ¼ percent volume solids E ¼ transfer efficiency
3.2.2 Weight Solids, Volume Solids
I ¼ dry film thickness of the paint in mils
Users of coatings need to know how much a given volume of coating they need for their particular coating job. This information is important not only in determining how much to buy but also in determining how much it costs if they are a processor, a seller, or a distributor of coated items. Two measures are typically reported by coating manufacturers. Weight solids is frequently a specification and is quite easy to measure. It is simply what is left of the paint on the surface after the volatiles have evaporated during the curing. The American Society for Testing and Materials (ASTM) test for this determination is D1644-01 “Standard Test Methods for Nonvolatile Content of Varnishes.” This measure is not directly useful to a paint user. He needs to know the cured coating density to calculate how much dried paint he has. Volume solids is a more useful measure than weight percent solids. It is the volume of the solid materials left after a gallon of paint’s volatile components are removed. With this number on hand, one can easily calculate how much surface area can be painted with a gallon of a particular coating using Eqn (3.4).
C¼
1604 S E I A
(3.4)
A ¼ part area to be coated in square feet Volume solids is more difficult to measure. The procedure is described by ASTM D2697-03, which is the “Standard Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings.” Typically, weight solids is measured and reported as a specified factor. Volume solids, or coverage, is reported but it is generally not measured. It is usually calculated based on the prescribed mixture of the raw materials using their densities. The underlining assumption is that there are no chemical reactions or unusual interactions. This is sometimes incorrect. The ingredients all can affect the coating properties. The next several chapters look in more detail at each of the components of a fluorinated coating.
References 1. Goldschmidt A. BASF handbook on basics of coating technology. Hanover (Germany): BASF Coatings AG; 2003. 2. Patton TC. Paint flow and pigment dispersion: a rheological approach to coating and ink technology. New York: John Wiley & Sons; April 1979. 3. Viscosity conversion chart based on charts widely available from multiple sources including. online extra to Fine Woodwork March 2004;169.
3.1 Binder, Vehicle, Film Former The binder is the film-forming component of paint. It is the only component that must be present in a paint formulation. With no other components present, most binders would dry to form a transparent, glossy film; some binders are used without pigments to make clear finishes and varnishes. The other components are included optionally, depending on the desired properties of the cured film. The binder imparts adhesion and strongly influences properties such as gloss, durability, flexibility, and toughness. Binders in fluorocoatings include the fluoropolymers and optionally other organic or inorganic polymers. Binders can be categorized according to the mechanisms for drying or curing. Drying usually refers to evaporation of the solvent or thinner. Curing usually refers to a chemical change to the binders such as cross-linking (see Chapter 4). Cross-linking
is the formation of chemical bonds between polymer chains. Some paints form by solvent evaporation only, but most rely on cross-linking. Those that require melting may or may not form cross-links. Paints that dry by solvent evaporation and contain the solid binder dissolved in a solvent are known as lacquers. A solid film forms when the solvent evaporates, and because the film can redissolve in solvent, lacquers are generally unsuitable for applications where chemical resistance is important. Many household spray paints fall into this category. Paints known as emulsions in the United Kingdom and latexes in the United States are water-borne dispersions of submicrometer polymer particles. These paints cure by a process called coalescence. Water evaporates in the initial step of coalescence. Next the slower evaporating trace (or coalescing) solvents evaporate and draw together to soften the binder polymer particles and fuse them together into irreversibly bound networked structures. As a result the paint cannot redissolve in the solvent/water that originally carried it. House paints cure in this manner and that is why they can be washable. Some paints cure by oxidative cross-linking. When applied, the exposure to oxygen in the air starts a process that cross-links and polymerizes the binder component. Other paints cure by polymerization and are generally one or two package coatings that polymerize by way of a chemical reaction, and cure into a cross-linked film. For UV-curing paints, the solvent is evaporated first. Hardening is then initiated by ultraviolet light, which initiates chemical cross-linking. In powder coatings there is little or no solvent, and flow and cure are produced by heating of the substrate above the melting point after electrostatic application of the dry powder. Thermoplastic coatings typically melt when reheated, whereas thermosets undergo the chemical reaction curing mechanisms mentioned earlier. Curing involving cross-linking prevents remelting. In liquid coatings the binder is dissolved, dispersed, or suspended in the solvent. The solvent is usually a mixture. It liquefies the other paint components
Fluorinated Coatings and Finishes Handbook. http://dx.doi.org/10.1016/B978-0-323-37126-1.00003-5 Copyright © 2016 Elsevier Inc. All rights reserved.
51
F LUORINATED C OATINGS
52
allowing them to spread out over the substrate being coated. Water is considered an important solvent. Nonfluoropolymer binder polymers are discussed in more detail in Chapter 4. Several other terms are common in the paint industry. These include medium, vehicle, and carrier. The generally accepted definitions are as follows: Vehicle is the liquid portion of paint. The vehicle is composed mainly of solvents, resins, and oils Carrier usually refers to the solvent Medium is the continuous phase in which the pigment is dispersed; it is synonymous with vehicle. Pigments and fillers are small particles added to paints to impart color and affect physical properties such as hardness or abrasion resistance or affect corrosion resistance. Pigments can also be used to influence viscosity, cost, adhesion, moisture permeability, gloss, abrasion resistance, electrical and thermal conductivity, and other properties. Alternatively, some paints contain dyes instead of or in combination with pigments to impart color. Fillers are a special type of pigment that serve to thicken the film, support its structure, and increase the volume of the paint. Fillers are usually cheap and inert materials, such as diatomaceous earth, talc, lime, barytes, clay, etc. Not all paints include fillers. On the other hand, some paints contain large proportions of pigment/filler and binder. Pigments and fillers are discussed in more detail in Chapter 5. Besides the three main categories of ingredients discussed above, paint can have a wide variety of miscellaneous additives, which are usually added in small amounts, yet provide a significant effect on the product. Additives are chemicals added to paints to achieve specific effects or solve specific problems. These include: 1. Surfactants, which help stabilize dispersions 2. Viscosity agents 3. Defoamers
AND
F INISHES H ANDBOOK
7. Catalysts 8. Others. Additives are discussed in more detail in Chapter 7.
3.2 Important Properties of Liquid Coatings A number of important properties describe liquid coatings. Sometimes these properties are part of the specifications of the coating. Often they are not, but nonetheless they are important to know, particularly when problems arise.
3.2.1 Rheology/Viscosity Viscosity in its simplest definition is the resistance of a liquid to flow or, as the American Heritage Dictionary puts it, “The degree to which a fluid resists flow under applied force.” The viscosity is usually a specification. It is frequently reported as a single measurement such as: “200e400 cps (measured by Brookfield viscometer at 25 C, #2 spindle at 20 RPM).” At first glance, a specification such as this might imply that the viscosity is a single measurement of a coating. If one looks closely the information in the parentheses, then the implication is that the viscosity depends exactly on how it is measured. The “25 C” in the specification implies that the viscosity is a function of temperature, which it is. The “Brookfield viscometer” is the instrument used to measure the viscosity, and its inclusion in the specification implies that the viscosity also depends on how it is measured. The Brookfield viscosity measurement is described in more detail in Chapter 13.1 “Measurement of Coating Performance.” Finally, the “#2 spindle at 20 RPM” defines two of the variables one can control on the Brookfield viscometer. The spindles of a Brookfield are different designs and each has a different surface area. The more area in contact with the liquid and with the spindle, the more force will be required to turn it. The ratio of that force to the area is called the “shear stress” in Eqn (3.1):
4. Surface modifiers 5. Stabilizers 6. Wetting agents
s¼
F A
(3.1)
3: T HE C OMPONENTS
OF
PAINT
53
where: F ¼ force (dynes) A ¼ area (cm2) s ¼ shear stress (dynes/cm2) The “20 RPM” defines the rotational speed or velocity. The liquids being moved against this surface include not only the liquid that is in direct contact with the spindle but also the liquid near its surface. This creates a velocity gradient, which is called the shear rate in Eqn (3.2): D¼
dv v ¼ dx x
(3.2)
where:
Figure 3.1 Newtonian flow.
D ¼ shear rate (s1) v ¼ shear velocity (cm/s) x ¼ thickness (cm) The viscosity is defined in Eqn (3.3). h¼
s D
(3.3)
where: s ¼ shear stress (dynes/cm2) from Eqn (3.1) D ¼ shear rate (s1) h ¼ viscosity (poise ¼ dyne-s/cm2) The main point here is that the viscosity depends upon the shear rate and stress applied by the measuring device and the temperature at which the measurement is made. This also implies that the viscosity will change depending upon how the coating is applied. Therein lies a key to using and understanding coatings. The viscosity varies with how the coating is used. Actually, viscosity also affects how coatings are manufactured, how they are stored, how they are prepared for use, and how long their shelf life is. The study of viscosity as a function of shear applied to the coating is called rheology. A test instrument called a rotoviscometer can make these measurements quickly. This work will not go into deep detail on the physics and chemistry of rheology.
Many texts develop the theory, measurement, and interpretation of rheology.2 An ideal liquid might have a viscosity that is independent of temperature, shear, and time. Some materials approach this ideal. The ideal is called Newtonian flow. Figure 3.1 shows the viscosity versus shear rate of a Newtonian fluid. Solvents and water are nearly Newtonian. Most coatings exhibit viscosity change with shear change. There are many practical reasons for making coatings behave in this manner. For instance, nearly everyone is familiar with house paint. It has very high viscosity as it sits undisturbed in a can. That is high viscosity at a low shear rate. However, when it is rolled or brushed, the shear rate becomes high. The viscosity drops dramatically. This allows the paint to flow out and level well on a wall or ceiling. Then after it is applied and the shear is removed the viscosity rises dramatically preventing or at least minimizing which keeps the paint from dripping or running. This type of viscosity behavior is called “shear-thinning” or pseudoplastic. Figure 3.2 shows the viscosity versus shear relationships of two coatings. Coating “B” shows a linear relationship while “A” shows a more nonlinear change. Both are considered pseudoplastic. The opposite of pseudoplastic flow is dilatant flow or “shear-thickening.” Figure 3.3 shows dilatant behavior. Dilatant coatings are rare. Thixotropic flow is a special case of shear-thinning behavior. A thixotropic coating thins with shear, but
54
Figure 3.2 Pseudoplastic flow.
F LUORINATED C OATINGS
AND
F INISHES H ANDBOOK
Figure 3.4 Thixotropic flow.
Table 3.1 Approximate Shear Ranges for Common Coating Processes
Figure 3.3 Dilatant flow.
its viscosity does not return to the original value after the shear is removed. There is time dependence. Often with enough time, the viscosity will recover. Figure 3.4 shows a thixotropic coating and what is referred to as a thixotropic loop. The arrows indicate how the experiment was run. Starting from low shear, shear is gradually increased. Then, gradually the shear is removed. This type of behavior is sometimes designed into coatings with additives (discussed in Chapter 7). It can be used to minimize settling in a coating formulation, increasing the time a coating can be stored.
Process
Shear Range, sL1
Sagging
102e101
Leveling
102e101
Dipping
100e101
Flow coating
100e101
Pumping
100e102
Mixing
101e102
Dispersion
102e105
Spraying
103e105
Roller coating
103e105
Brushing
103e104
To give a feeling for the magnitude of the shear forces, several processes and their shear rates are given in Table 3.1. There are many ways to measure or estimate viscosity (discussed in Chapter 13.1). Some are easier than others. Some have more variability than others. Many plants and paint shops use a cup method which times how long it takes for a given volume of coating to drain through a hole of specific size. This work
Table 3.2 Viscosity Conversion Chart3 Seybolt University, SSU
Zahn #1, s
Zahn #2, s
A4
60
30
16
A3
80
34
17
100
37
18
130
41
19
160
44
20
Ford #4, s
Gardner Holdt Bubble
20
5
25
8
Ford #3, s
Krebs Units, KU
Zahn #3, s
Zahn #4, s
Zahn #5, s
Shears, s
Parlin #7, s
Parlin #10, s
Fisher #1, s
0.1
10
27
11
0.15
15
30
12
0.2
20
32
13
30
15
12
10
0.25
25
37
14
35
17
15
12
A2
0.3
30
43
15
39
18
19
14
A1
0.4
40
50
16
50
21
25
18
A
0.5
50
57
17
24
29
22
0.6
60
64
18
29
33
25
0.7
70
20
33
36
28
0.8
80
22
39
41
31
0.9
90
23
44
45
32
1.0
100
25
50
50
34
D
40
530
41
12
10
27
1.2
120
30
62
58
41
E
43
580
49
14
11
31
1.4
140
32
66
45
E
46
690
58
16
12
34
1.6
160
37
50
G
48
790
66
18
13
38
1.8
180
41
54
50
900
74
20
14
40
2.0
200
45
58
52
1000
82
23
16
44
2.2
220
62
I
54
1100
25
17
10
2.4
240
65
J
56
1200
28
18
11
2.6
260
68
58
1280
30
20
12
2.8
280
70
K
59
1380
32
21
13
3.0
300
74
L
60
1475
34
22
14
3.2
320
1530
36
24
15
3.4
340
N
3.6
360
O
3.8
380
4.0
400
4.2
420
4.4
440
Q
4.6
460
R
4.8
480
5.0
500
Poise
Fisher #2, s
Gardner Lithograph
centipoise, cp
B
C
000 H
210
52
22
19
30
260
60
24
20
33
320
68
27
21
35
370
30
23
37
430
34
38
480
37
M
S
1630
39
25
16
41
26
17
1850
43
28
18
1950
46
29
19
2050
48
30
20
2160
50
32
21
66
2270
52
33
22
67
2380
54
34
23
68
2480
57
36
24
64
00
26
1730
62
P
24 10
(Continued )
Table 3.2 Viscosity Conversion Chart3 (Continued )
Poise
centipoise, cp
Parlin #7, s
Parlin #10, s
Fisher #1, s
Fisher #2, s
Ford #3, s
Ford #4, s
Gardner Holdt Bubble
5.5
550
T
6.0
600
U
7.0
700
8.0
800
Gardner Lithograph
0
Krebs Units, KU
Seybolt University, SSU
69
Zahn #1, s
Zahn #2, s
Zahn #3, s
Zahn #4, s
Zahn #5, s
2660
63
37
25
68
71
2900
40
27
74
3375
44
30
77
3880
51
35
9.0
900
V
81
4300
58
40
10.0
1000
W
85
4600
64
45
11.0
1100
88
5200
49
12.0
1200
92
5620
55
13.0
1300
95
6100
59
14.00
1400
96
6480
64
15.0
1500
98
7000
16.00
1600
100
7500
17.0
1700
101
8000
18.0
1800
19.0
1900
20.0
2000
21.0
2100
9850
22.0
2200
10,300
23.0
2300
24.00
2400
25.0
2500
X 1
Y
8500 9000 103
Z
Z-1
2
9400
105
10,750
109
11,200
114
11,600
Shears, s
30.00
3000
35.0
3500
40.0
4000
45.0
4500
50.0
5000
55.0
5500
60.0
6000
65.0
6500
30,000
70.00
7000
32,500
75.0
7500
35,000
80.00
8000
37,000
85.0
8500
39,500
90.0
9000
41,000
95.0
9500
43,000
100.0
10,000
110.0
11,000
51,000
120.0
12,000
55,500
130.0
13,000
60,000
140.00
14,000
150.0
15,000
Z-2
3
Z-3
121
14,500
129
16,500
133
18,300
136
21,000 23,500 26,000
Z-4
Z-5
4
5
28,000
46,500
65,000 Z-6
67,500
160.00
16,000
74,000
170.0
17,000
80,000
180.0
18,000
83,500
190.0
19,000
88,000
200.0
20,000
93,000
300.0
30,000
140,000
F LUORINATED C OATINGS
58
will not deal with all of these tests, but Table 3.2 allows estimation and conversion between some common viscosity measurement devices.
AND
F INISHES H ANDBOOK
where: C ¼ square feet of substrate covered per gallon of paint S ¼ percent volume solids E ¼ transfer efficiency
3.2.2 Weight Solids, Volume Solids
I ¼ dry film thickness of the paint in mils
Users of coatings need to know how much a given volume of coating they need for their particular coating job. This information is important not only in determining how much to buy but also in determining how much it costs if they are a processor, a seller, or a distributor of coated items. Two measures are typically reported by coating manufacturers. Weight solids is frequently a specification and is quite easy to measure. It is simply what is left of the paint on the surface after the volatiles have evaporated during the curing. The American Society for Testing and Materials (ASTM) test for this determination is D1644-01 “Standard Test Methods for Nonvolatile Content of Varnishes.” This measure is not directly useful to a paint user. He needs to know the cured coating density to calculate how much dried paint he has. Volume solids is a more useful measure than weight percent solids. It is the volume of the solid materials left after a gallon of paint’s volatile components are removed. With this number on hand, one can easily calculate how much surface area can be painted with a gallon of a particular coating using Eqn (3.4).
C¼
1604 S E I A
(3.4)
A ¼ part area to be coated in square feet Volume solids is more difficult to measure. The procedure is described by ASTM D2697-03, which is the “Standard Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings.” Typically, weight solids is measured and reported as a specified factor. Volume solids, or coverage, is reported but it is generally not measured. It is usually calculated based on the prescribed mixture of the raw materials using their densities. The underlining assumption is that there are no chemical reactions or unusual interactions. This is sometimes incorrect. The ingredients all can affect the coating properties. The next several chapters look in more detail at each of the components of a fluorinated coating.
References 1. Goldschmidt A. BASF handbook on basics of coating technology. Hanover (Germany): BASF Coatings AG; 2003. 2. Patton TC. Paint flow and pigment dispersion: a rheological approach to coating and ink technology. New York: John Wiley & Sons; April 1979. 3. Viscosity conversion chart based on charts widely available from multiple sources including. online extra to Fine Woodwork March 2004;169.