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Fluoropolymer Coating Processing Technology
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���� ������������ This chapter discusses baking and curing in detail. A large percentage of all coating performance problems reported to coating manufacturers by applicators are due to curing problems. The first part of the discussion focuses on what physically or chemically happens during baking of fluorinated coatings. Important baking scenarios are discussed in this chapter. This is often called ������, which is somewhat of a misnomer for reasons explained later in this chapter. A basic understanding of the baking/curing process will lead to a better understanding of the potential problems that are caused during this crucial step in the coating process. Next, a discussion is presented on the measurement or monitoring of baking temperature. While manufacturers and formulators try their best to provide wide operating windows for cure, these hightechnology coatings frequently have narrow windows, or precise bake schedules. For example, if the specification states that the bake is five minutes at 740°F760°F (393°C404°C) metal temperature, anything significantly different from that may lead to performance problems in the end use. There are three basic ways to heat a coating: convection, infrared, and induction heating. Each of these heating methods is reviewed in terms of theory of operation. Advantages and disadvantages are also discussed.
���� ����������������������������� ��������� The term������� is often used to describe the baking process of fluoropolymer coating systems. According to the free online encyclopedia, Wikipedia (http://en.wikipedia.org), curing in polymer chemistry and process engineering refers to the toughening or hardening of a polymer material by the crosslinking of polymer chains, brought about by chemical additives, ultraviolet radiation, or heat. The key point here is crosslinking, which is a chemical reaction. Strictly speaking, the common fluoropoly-
mers undergo no crosslinking or significant chemical change during baking. It is a melting process. Most of the fluoropolymer coatings undergo no curing reaction by this definition unless they are blended with thermosetting resins, which by definition become insoluble and infusible by a chemical reaction. However, the term������� is used so prevalently, it will continue to be used here to describe the physical process of taking the liquid or powder coating to its final film state. If one looks at a simple fluoropolymer coating, such as a dispersion of FEP, as it is baked at different temperatures, the curing process for fluoropolymers becomes clear. Figure 12.1 shows a series of micrographs taken of a thin coating of an aqueous FEP dispersion. FEP has a melting point of 525°F (274°C). The micrograph in Fig. 12.1(a) shows the applied coating at 500°F (260°C), below the melting point. Severe mud-cracks have formed. As the temperature is raised to 550°F (288°C), 25°F (14°C) above the melting point, the FEP starts to melt and flow, but just barely. See Fig. 12.1(b). As the temperature is raised to 75°F (242°C) above the melting point, the FEP melts and flows, starting to heal cracks, shown in Fig. 12.1(c). Figure 12.1(d) shows that as the temperature is raised to 650°F (343°C), well above the melting point, the FEP melts and flows well. This temperature is often the recommended temperature for FEP liquid and powder coats. At 700°F (371°C), well above the melting point, the FEP melts and flows very well. Even at this high temperature, though, the mud-cracks did not completely heal, as shown in Fig. 12.1(e). It is possible if the coating was held at this temperature for double or triple the time, the mud-crack defects may have disappeared.
164
(a)
(b)
FLUORINATED COATINGS AND FINISHES HANDBOOK
The FEP has a melting point of 525°F (274°C). This micrograph shows the applied coating at 500°F (260°C), below the melt point. Severe mud-cracks have formed as the water and much of the surfactant have become volatile. This dispersion would need a filmforming additive that would minimize or eliminate the cracks if this were to be sold.
As the temperature is raised to 550°F (288°C), 25°F (14°C) above the melt point, the FEP starts to melt and flow, but just barely. The melt viscosity of the FEP at this temperature is still very high.
(c)
As the temperature is raised to 600°F (316°C), 75°F (242°C) above the melt point, the FEP now melts and flows well. However, the mud-cracks were very severe and they are just starting to fill in or heal.
(d)
As the temperature is raised to 650°F (343°C), well above the melt point, the FEP now melts and flows well. This temperature is often the recommended temperature for FEP liquid and powder coats. The mud-cracks were very severe but show significant healing.
(e)
At 700°F (371°C), well above the melt point, the FEP now melts and flows very well. Even at this high temperature, the mud-cracks did not completely heal. It is possible that if the coating was held at this temperature for double or triple the time, the mudcrack defects might have disappeared.
Figure 12.1 Micrographs�������� of aqueous FEP dispersion-based fluoropolymer coating baked under different conditions.����������������������������������
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY The process for coating and curing fry pans with fluoropolymers is basically the same for almost all coatings being used. These coatings are complex and generally consist of two-, three-, or four-coat systems. 1. There is usually a primer layer made of a blend of a fluoropolymer and a thermosetting resin like polyamide-imide (PAI). The polyamide-imide chemistry is discussed in Sec. 4.3.1 of this book. 2. The primer is dried with heat lamps, but the temperature remains below the boiling point of water. 3. �Additional coating layers are applied over the primer. 4. The pans enter an oven on a chain link belt and pass through three separate heating zones. This coating undergoes several stages while it is in the oven. These stages are described in Fig. 12.2. A typical bake of a fry pan takes fifteen minutes. The pan passes through three zones in the oven, each one about five minutes long. The first zone is typically set around 300°F (149°C). The water and volatile solvents are removed from the coating by slowly raising the temperature of the coating to 300°F (149°C) ± 50°F (28°C). This step must de done slowly so that the solvents and water do not boil off rapidly. Boiling solvent would physically disrupt the film.
Figure 12.2 Typical fry pan bake. �
165 The next zone is designed to raise the coating temperature up to near the final baking temperature. Once the solvents have been removed, the temperature is ramped up to about 700°F (371°C). As the coating reaches 400°F (204°C), the polyamideimide in the primer starts to imidize by liberating a water molecule. The primer stratifies (see Sec. 4.2) with the fluoropolymer concentrating at the primer interface with the midcoat or topcoat and the PAI concentrating at the substrate interface. Stratification and imidization continue as the temperature rises but are complete by the time the coating has reached 700°F (371°C). The primer has been cured. Other volatile materials diffuse or decompose, and diffuse out of the film at above 700°F (371°C). These volatiles are primarily surfactants, high boiling solvents, and possibly the film-forming aid. The final bake zone raises the coating to a temperature between 700°F820°F (371°C438°C) and holds it there for three to five minutes. That temperature depends on the particular coating system being used and the substrate. At this temperature, PTFE sinters and other fluoropolymers, if present in the formulation, melt and flow. The coating usually needs two to five minutes at this temperature to allow PTFE to sinter or fluoropolymer(s) to melt and develop the final film properties. The remaining volatile components, surfactants, and film foaming aides diffuse out of the film. Some of these must decompose to lower molecular weight materials in
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FLUORINATED COATINGS AND FINISHES HANDBOOK
order to become volatile. This is why the presence of oxygen in the oven air is important for many coating products. If carbon is used as a pigment, it starts to degrade if the bake is above 750°F (399°C). Most fluoropolymer-based coatings have a specified baking schedule in terms of times at different metal temperatures. These are usually specified by the coating manufacturer and found in their fact sheets. Many coatings are processed in a batch oven. A batch oven set-point temperature can be changed through the bake. Multiple coat systems can have complicated bake schedules, particularly if many coats are applied with bakes between the coats. This is common in high-build systems. Often, the bake temperature is lowered slightly with each additional coat. This is to minimize the chance of bubbling due to decomposition of the fluoropolymers in the underlayers. One must follow the fact sheets for the products being used. It is never advisable, unless specifically instructed, to bake at a higher temperature than the previous coat when using highbuild systems.
12.3 Monitoring Bake The key point of the baking specification mentioned above is metal temperature. This does not mean the oven temperature setting. One must measure the temperature of the part being cured and monitor the temperature during the bake to verify that the proper temperature is reached. When an applicator is asked why he has not actually measured
the substrate temperature, the usual response is, but I know my oven. This presumption is almost always proven wrong. There are a number of ways to measure part temperature, but only two are common in high-temperature coating processes: thermocouples and noncontact IR thermometers. Two devices that will not be discussed are the Resistance Temperature Detector (RTD) and the Thermistor (Bulk Semiconductor Sensor).
12.3.1 Thermocouples A thermocouple is a thermoelectric temperature sensor; it is the most common temperature measuring method. It consists of two dissimilar metallic wires. One is referred to as +, one as -. The two combinations of wires most commonly used are called J-type and K-type. These are described in Table 12.1. The wire pair is connected at the end, preferably twisted and welded. The temperature is detected by measuring the change in voltage between the two wires, which varies with the temperature at that junction. Thermocouples are ideally attached directly to the part being baked. The best way is by welding directly to the substrate. However, this is impractical because it damages the part and that is unacceptable to the eventual user of that coated part. If there is a threaded fastener hole on the substrate, the end of the thermocouple can be held in place by a screw or bolt that fits that hole, thus avoiding damage.
Table 12.1 The Properties of Two Common Thermocouple Wires � ����
�����������������������
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J
Iron & Constantan (Cu-Ni alloy)
-350°F ~ 2200°F (-210°C ~ 1200°C)
K
Chromel (Ni-Cr alloy) & Alumel (Ni-Al alloy)
-450°F ~ 2500°F (-270°C ~ 1350°C)
* Constantan, Alumel, and Chromel are trade names of alloys.
����� Max of ±0.75% or ±2.2°C (4°F) Below 0°C (32°F): max of ±2% or ±2.2°C (4°F) Above 0°C (32°F): max of ±0.75% or ±2.2°C (4°F)
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY The thermocouple can be taped to the part using a high-temperature tape. The tape of choice for this approach is 3M Glass Cloth Tape #361. This tape is made of fiberglass with a silicone adhesive and is rated to about 550°F (288°C), but it can be used at higher temperatures. Care must be taken that the thermocouple wire is held as tightly as possible against the part, preferably with several layers of tape. The tape can still come off; the user must verify that this has not happened during the bake. The adhesive may also stick to the part and may need to be cleaned off. When attachment to the part must be avoided, the thermocouple can be attached to a similar scrap or uncoated part that is baked as closely as possible to the coated part. When such a part is not available, any part of similar metal and mass can be used. The thermocouple wires are usually double insulated. Each wire of the thermocouple is wrapped with a fiberglass insulation that is color coded for its particular type. An additional layer of fiberglass insulation wraps the pair of wires again. The condition of these wires must be checked before every use after the wires have been exposed to a high temperature. The sizing in the fiberglass decomposes, and the insulation becomes very delicate and brittle. (The wires should be handled with gloves and inhalation of fibers of insulation should be avoided after exposure to the first high-temperature bake.) If the wires short-circuit, then that creates a new thermocouple junction and the temperature being measured will be the temperature at that junction, not necessarily the one at the ends of the wires. The baking of a part should be monitored and documented with a recording device such as a chart recorder, computer, data logger, or by hand.
12.3.2 Non-Contact Temperature Measurement Non-contact temperature measurement theoretically would be ideal, avoiding any part damage by the measuring device. There are a number of similar devices that work by measuring the IR radiation or heat coming off the part being baked. These are called IR thermometers or optical pyrometers. Pyrometers take advantage of the fact that all objects radiate energy. The intensity and wavelength of that radiated energy can be used to calculate the
167 temperature. One of two theories is used by the pyrometers: Plancks law or the Stefan-Boltzmann law. Plancks law is used in narrow-band pyrometers, where only one or a few specific wavelengths are targeted. The Stefan-Boltzmann law is used in broadband pyrometers, where a wide range of wavelengths is measured. The principle is the same, differing only in the mathematics. It is important to understand the principle since that leads to an understanding of the limits of this type of temperature measurement. A black body is a theoretical object that absorbs 100% of the incident radiation. Therefore, it reflects no radiation and appears perfectly black. It also is a perfect radiator of energy. At a given temperature, the black body emits the maximum amount of energy possible for that temperature. This value is known as the black-body radiation. It would emit at every wavelength of light, because it absorbs at every wavelength of incoming radiation. It also emits a predictable amount of energy at each wavelength for a given temperature. The radiation intensity and wavelength from a black body at a given temperature T is governed by Plancks law, one mathematical form of which is given in Eq. (12.1):
Eq. (12.1)
I (ν) =
2hν 3 c2
1 hν exp −1 kT
where: I(ν) = �The amount of energy emitted at per unit time per unit surface area per unit solid angle per unit frequency ν ν = �frequency h = �Plancks constant, 6.625 × 10-34 joule·sec c = �Speed of light, 2.998 × 108 m/sec k = �Boltzmanns constant, 1.380 × 10 -23 joule/K T = �Black-body absolute temperature (K) The data at various temperatures and wavelengths calculated from Planks law are shown in Fig. 12.3.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Figure 12.3 Black body radiation output as a function of temperature and wavelength.
The peak wavelengths are all in the infrared part of the spectrum. The graph shows that as temperature increases, the peak wavelength emitted by the black body decreases. It moves from the infrared towards the visible part of the spectrum. Some visible light is emitted at any temperature, but at intensities too low for the eye to see except at the highest temperatures. Hot metal appearing red is therefore cooler than metal appearing yellow or white. The graph also shows that as temperature increases, the total energy emitted increases, because the total area under the curve increases. The remote temperature-sensing devices measure the energy in the infrared or visible parts of the spectrum and calculate what the temperature is assuming the light is coming from a black body. However, real world objects are not perfect black bodies. Some are better than others. A correction for the temperature calculation is called emissivity. Emissivity is defined by the following formula: Eq. (12.2) Emissivity =
Radiant energy of an object Radiant energy of a black body with the same temperature as the object
Emissivity ranges between 0 and 1 depending on the dielectric constant of the object, surface
roughness, temperature, wavelength, and other factors. What does this mean for temperature measurement? If the object one is trying to measure has an emissivity of 0.5, but the device assumes it is a perfect black body with an emissivity of 1.0, then the calculated temperature will be lower than it really is, perhaps significantly underestimated. If emissivity is set too low on the non-contact measuring device, then the temperature is overestimated. The measuring device needs to have the correct emissivity set to get a correct measure of the temperature. Table 12.3 shows the emissivity of many common substrates or materials. To further complicate the matter, the emissivity is sometimes temperature dependent, apparent in some of the table entries. Ideally, one would measure the emissivity of the coated object using a thermocouple on the part and just adjusting the emissivity on the remote sensing device until the temperatures are the same. Additionally, many times these non-contact devices are used on batch ovens, where the door must be opened to make the measurement, which should be avoided.
12.4 Types of Ovens There are several types of ovens or baking methods. Each has its advantages and disadvantages, which are discussed in the following sections.
12.4.1 Convection Heating Convection ovens are the simplest and most common method to heat up a coating. They are often called conventional ovens. With convection processes, energy is transferred to the product by first heating the air. The heated air must contact the coating and substrate to raise their temperatures. A thin boundary layer exists around the coated part that repels the hot air, making it difficult to raise the coating temperature. Most convection ovens have circulating fans in them to move the air around the oven to improve heat transfer and to make the temperature throughout the oven chamber as uniform as possible. In forced convection, the heated air is directed at the surface to break up the boundary layer. This offers the potential for damage to coated surfaces, particularly powder-coated surfaces.
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY
169
Table 12.2 Emissivity of Various Metal Substrates[1] ��������
���������������
���������������
����������
Unoxidized
77
25
0.02
Unoxidized
212
100
0.03
Unoxidized
932
500
0.06
Oxidized
390
199
0.11
Oxidized at 599°C
390
199
0.20
Heavily Oxidized
200
93
0.09
Heavily Oxidized
940
504
0.18
Highly Polished
212
100
0.09
Roughly Polished
212
100
0.04
Commercial Sheet
212
100
0.06
Highly Polished Plate
440
227
0.04
Bright Rolled Plate
338
170
0.04
Bright Rolled Plate
932
500
0.05
Alloy A3003 Oxidized
600
316
0.40
Alloy A3003 Oxidized
900
482
0.40
200800
93427
0.05
73% Cu 27% Zn, Polished
674
357
0.03
62% Cu 37% Zn, Polished
710
377
0.04
83% Cu 17% Zn, Polished
530
277
0.03
Lampblack
77
25
0.95
Unoxidized
77
25
0.81
Candle Soot
250
121
0.95
Black Oxidized
100
38
0.78
Etched
100
38
0.09
Matte
100
38
0.22
Roughly Polished
100
38
0.07
Polished
100
38
0.03
Highly Polished
100
38
0.02
Rolled
100
38
0.64
Rough
100
38
0.74
ALUMINUM
Alloy 1100-0 BRASS
CARBON
COPPER
���������
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Table 12.2���������� ��������
���������������
���������������
����������
Oxidized
212
100
0.74
Unoxidized
212
100
0.05
Red Rust
77
25
0.70
Rusted
77
25
0.65
Oxidized
390
199
0.64
Strong Oxidation
482
250
0.95
Cold Rolled
200
93
Polished Sheet
500
260
0.10
Mild Steel, Polished
75
24
0.10
Mild Steel Smooth
75
24
0.12
212
100
0.08
77
25
0.80
Type 301, Polished
450
232
0.57
Type 316, Polished
450
232
0.57
Type 321
200800
93427
0.270.32
Type 350
200800
3161093
0.180.27
Type 350 Polished
3001800
149982
0.110.35
Type 446
3001500
149815
0.150.37
Bright Galvanized
100
38
0.23
Commercial 99.1%
500
260
0.05
Galvanized
100
38
0.28
Oxidized
5001000
260538
0.11
Polished
100
38
0.02
Polished
500
260
0.03
IRON
CAST IRON
STEEL
Steel Unoxidized Steel Oxidized
0.750.85
STEEL ALLOYS
ZINC
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY Sources of heat are either gas or electricity. Electric systems can use open coils, metal-sheath heaters, or fin-type heaters to heat the air. Direct-fired gas ovens use a combustion flame to heat the air directly, and the products of combustion become part of the oven process air. Indirect-fired gas systems use a heat exchanger to separate the process air from the combustion air. That reduces the transfer efficiency but protects the product from contamination by the combustion products. In a convection oven, the product spends a significant portion of the total dwell time in the oven just reaching the process temperature. This is the major energy consumption portion of the process. The amount of heat transferred to the coating and substrate is determined by: 1. Thermal conductivity of the substrate 2. The surface area and mass of the substrate 3. Temperature gradient 4. The air velocity in the oven Ovens of this type are batch ovens or continuous open-ended tunnel ovens.
12.4.2 Infrared Baking (IR) High tech heat-lamps heat by radiating infrared light and can be used even for high bake-temperature coatings.[2] These are not the kind of heat lamps one sees in restaurants that keep the food warm until it is served. Those types of lamps can be used to warm up parts before coating. They can also dry the paint layers between applications. The infrared sources for curing and melting have significantly more power output. IR is the most efficient of all radiation for transfer of heat where one material emits the IR radiation and the other absorbs it. There are several types of infrared sources including gas-fired and electric emitters. The method of IR source heating and the material of construction have no effect on the wavelength characteristics of the different sources. The IR radiation maximum is determined only by temperature, and is described by the Wien Displacement law: Eq. (12.3)
λ max =
2.9 ×10 −3 T
171 Plancks law describes the intensity versus the wavelength or frequency (Fig. 12.3). The IR sources are generally divided into three regions depending on the most-generated wavelength of IR energy: 1. �Short wave. IR peak output is in the range of 0.762 µm. Short infrared emitters have an evacuated tube, or inert atmosphere, containing tungsten filament heated to 3632°F (2000°C) to 4532°F (2500°C). Up to five percent of the radiative energy output is in visible light, so it looks bright yellow. Power output can approach 200 W/in². Short-wavelength IR has the tendency to penetrate through thin organic coatings and heat the substrate directly. 2. �Medium wave. IR peak output is in the range of 24 µm. Medium infrared emitters have a nickel-chrome filament, at a temperature of 1292°F (700°C) to 2372°F (1300°C). Up to one percent of radiative energy output is in visible light, giving the operating lamps a dull red color. The tubes are not evacuated nor in an inert atmosphere. Power output can approach 15 60 W/in2. Medium-wavelength heaters are available in many configurations including lamps, tubular quartz, flat panel heaters, ceramic, bulbs, metal-sheath heaters, and more. Medium-wavelength infrared has the tendency to be absorbed by organic coatings directly. The peak absorption of water falls within this regime, making it suitable for efficiently heating moisture-rich products or waterbased coatings. 3. �Long wave. IR peak output is in the range of 4 µm to 1 mm. Long infrared emitters are glass radiating panels that are electroconductive on the surface, or are ceramic panels that operate at 572°F (300°C) to 1112°F (600°C). Power output can approach 15 W/in2. The choice of heater for a particular process is determined by the actual process and product demands. This relates back to the electromagnetic absorption spectra of the product being heated and how much energy transmission is required by the process, that is, how hot it needs to get and how fast it needs to reach this temperature.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
The amount of heat transferred from the IR source to the substrate (called���������) receiving the heat depends on several things:
3. �Heating homogeneity due to radiation penetration of the coating (less skinning over) 4. Can do some operations not possible by other methods
Temperature of the IR source Temperature of the heat receiver
5. �Ease of installation as complement to other heating processes (booster ovens)
Absorption and emission coefficients Physical dimensions and shape of the source and receiver
These lead to: 1. �Good heat transfer accuracy and control, and higher product temperatures are possible
The distance between the source and the receiver The heat transfer is described by Eq. (12.4). 4
Eq. (12.4) ��� =��V ×��S ×��T ×�� × (�S ��T
2. High productivityshorter cure times 4)
where: ��� = Infrared heat transfer (W/cm²) �V = Geometric view factor (01) �S = Emissivity of the source (01) �T = Absorptivity of the target (01) � = Stefan-Boltzmann Constant �S � = Source temperature (K) � T = Target temperature (K) This equation shows what parameters are important in infrared heating. The view factor (�V) relates to how well the heating target ���� the IR energy source and takes into account the geometry of the substrate (target). Ideal IR substrates are flat or cylindrical without nooks and crannies. The target absorptivity factor (�T) is a measure of how well the surface absorbs the IR energy. It is related to the emissivity factors discussed earlier in this chapter. The source temperature (�S) controls infrared energy output, the power put into the IR source. The amount of radiant heat flux emitted from the source is proportional to the fourth power of temperature. At high emitter temperatures, >1800°F (~1000°C), IR produces significantly higher heat flux than convection heating. In considering the characteristics of conventional versus IR curing, advantages of IR in industrial applications include: 1. Direct transfer of thermal heat without intermediate (air) 2. Rapid heat up
3. Oven size reductionsless floor space 4. �Quality improvementscleaner ovens (less dirt due to lower air flows) 5. Somewhat lower capital cost, lower operational cost 6. Instantaneous start-up and shutdown 7. �Simplified construction 8. Reduced pollution by ovenenergy efficiency 9. More comfortable working conditions, less wasted heat 10. �Improved safety 11. Maintenance requirements are reduced 12. �Modular and flexible designs with many zones are practicaloffers versatility
������ ���������������� Heating by induction[3] is another approach to direct heating of the substrate. It is a common misconception that the substrate must be magnetic to be a candidate for induction heating. To be heated by induction, the substrate must conduct electricity. Technically, it must also resist the flow of electricity or have resistance, but that is true of all materials except superconductors. The principle of induction heating depends on understanding that, when electricity flows, a magnetic field is generated, and the reverse is also true. Where there is a magnetic field and a conductor, electricity will flow.
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY Induction heaters (Fig. 12.4) make use of this principle. The heater uses alternating electricity in a coil to generate a magnetic field. When a piece of metal is placed close to (not touching) this coil, the magnetic field generated by the coil interacts with the metal, generating electric current. That current is called an ������������. The resistance to current flow in the metal leads to loss of electric power as described by the basic electrical formula in Eq. (12.5). Eq. (12.5)
�����2�
In this equation, � is the amount of current, � is the resistance of the metal, and�� is the power loss or the heat gained. The equation also indicates that doubling the current quadruples the heat generated. Because the coil uses alternating current, the magnetic field averages out to zero over time. The strength of the magnetic field drops off with distance from the induction coil. Because the eddy currents are related to the strength of the magnetic field, the heating is strongest at the surface. The process seems simple, and in a way it is. It is complicated to control however, but it can be controlled. The heat-up rate of the metal underneath the coating being cured with induction heating depends on several properties of the substrate metal: 1. Specific heat 2. Magnetic permeability 3. �Resistivity
����������� Commercial induction heating equipment showing an array of induction coils.���������������� �����������������������
173 All of these properties of the substrate vary with temperature. The weight and shape of the substrate metal will affect the heat-up rate. Since most of the heat is generated at the surface closest to the coil, the thermal conductivity of the substrate will also affect the peak temperatures at the surface as heat moves towards the cooler areas of the substrate. The control parameters of the induction coil include: 1. Power 2. Frequency There is a relationship between the frequency of the alternating current and the depth to which it penetrates the substrate. The induced current flow within the part is most intense on the surface. The current decays rapidly below the surface. The metal closest to the surface will heat more quickly than the inside. The skin depth of the part is described as the depth within which 80% of the heat in the part is produced. The skin depth decreases when resistivity decreases, permeability increases, or frequency increases. High frequencies of 100 to 400 kHz provide shallow penetration, which is usually ideal for curing surface coatings. Low frequencies of 5 to 30 kHz are effective for thicker materials requiring deep heat penetration such as those coated items with complex shapes. Magnetic materials such as steel are easier to heat than non-magnetic materials such as aluminum. This is due to a secondary heating mechanism called ����������. Magnetic materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces its own additional heathysteresis heatingin addition to eddy current heating. A visual explanation is given in Fig. 12.5. A metal that offers high resistance is said to have high magnetic permeability. Permeability can vary on a scale of 100 to 500 for magnetic materials; non-magnetics have a permeability of 1. The advantages of induction heating over conventional convection heating include: 1. ����������������� Heat can be developed directly and nearly instantly inside the substrate, allowing a much quicker startup than conventional convection heating. Bake cycle times can be dramatically reduced.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Figure 12.5 Hysteresis in magnetic materials.[3] Energy is required to turn the small internal magnets around. The resistance to this is like friction; the material increases in temperature.
2. �Controlled directional heating. Very small areas of the substrate can be heated without affecting other surrounding areas or the fixturing that holds the part. With precise power input control, one can achieve the exact temperature required either slowly or quickly. 3. �Repeatability. With modern induction heating systems, the heating pattern is always the same for a given set-up, cycle after cycle and day after day.
4. �Non-contact heat. Nothing touches the coated part when it is placed in the induction coil, the process induces heat within the part without actually touching it. 5. Energy efficiency. In summary, one can buy or formulate the finest quality fluorinated coating, but if it is not applied correctly and baked correctly, it may fail miserably in use.
REFERENCES 1. �Lide, D. R., CRC Handbook of Chemistry and Physics, CRC Press (2003) 2. �Technology Guidebook for Infrared Heating, Electric Power Research Institute: CMF Report No. 93-2, Palo Alto, CA (1993) 3. �Haimbaugh, R. E., Practical Induction Heat Treating, ASM International, Materials Park, OH (2001)
���� ������������ This chapter discusses baking and curing in detail. A large percentage of all coating performance problems reported to coating manufacturers by applicators are due to curing problems. The first part of the discussion focuses on what physically or chemically happens during baking of fluorinated coatings. Important baking scenarios are discussed in this chapter. This is often called ������, which is somewhat of a misnomer for reasons explained later in this chapter. A basic understanding of the baking/curing process will lead to a better understanding of the potential problems that are caused during this crucial step in the coating process. Next, a discussion is presented on the measurement or monitoring of baking temperature. While manufacturers and formulators try their best to provide wide operating windows for cure, these hightechnology coatings frequently have narrow windows, or precise bake schedules. For example, if the specification states that the bake is five minutes at 740°F760°F (393°C404°C) metal temperature, anything significantly different from that may lead to performance problems in the end use. There are three basic ways to heat a coating: convection, infrared, and induction heating. Each of these heating methods is reviewed in terms of theory of operation. Advantages and disadvantages are also discussed.
���� ����������������������������� ��������� The term������� is often used to describe the baking process of fluoropolymer coating systems. According to the free online encyclopedia, Wikipedia (http://en.wikipedia.org), curing in polymer chemistry and process engineering refers to the toughening or hardening of a polymer material by the crosslinking of polymer chains, brought about by chemical additives, ultraviolet radiation, or heat. The key point here is crosslinking, which is a chemical reaction. Strictly speaking, the common fluoropoly-
mers undergo no crosslinking or significant chemical change during baking. It is a melting process. Most of the fluoropolymer coatings undergo no curing reaction by this definition unless they are blended with thermosetting resins, which by definition become insoluble and infusible by a chemical reaction. However, the term������� is used so prevalently, it will continue to be used here to describe the physical process of taking the liquid or powder coating to its final film state. If one looks at a simple fluoropolymer coating, such as a dispersion of FEP, as it is baked at different temperatures, the curing process for fluoropolymers becomes clear. Figure 12.1 shows a series of micrographs taken of a thin coating of an aqueous FEP dispersion. FEP has a melting point of 525°F (274°C). The micrograph in Fig. 12.1(a) shows the applied coating at 500°F (260°C), below the melting point. Severe mud-cracks have formed. As the temperature is raised to 550°F (288°C), 25°F (14°C) above the melting point, the FEP starts to melt and flow, but just barely. See Fig. 12.1(b). As the temperature is raised to 75°F (242°C) above the melting point, the FEP melts and flows, starting to heal cracks, shown in Fig. 12.1(c). Figure 12.1(d) shows that as the temperature is raised to 650°F (343°C), well above the melting point, the FEP melts and flows well. This temperature is often the recommended temperature for FEP liquid and powder coats. At 700°F (371°C), well above the melting point, the FEP melts and flows very well. Even at this high temperature, though, the mud-cracks did not completely heal, as shown in Fig. 12.1(e). It is possible if the coating was held at this temperature for double or triple the time, the mud-crack defects may have disappeared.
164
(a)
(b)
FLUORINATED COATINGS AND FINISHES HANDBOOK
The FEP has a melting point of 525°F (274°C). This micrograph shows the applied coating at 500°F (260°C), below the melt point. Severe mud-cracks have formed as the water and much of the surfactant have become volatile. This dispersion would need a filmforming additive that would minimize or eliminate the cracks if this were to be sold.
As the temperature is raised to 550°F (288°C), 25°F (14°C) above the melt point, the FEP starts to melt and flow, but just barely. The melt viscosity of the FEP at this temperature is still very high.
(c)
As the temperature is raised to 600°F (316°C), 75°F (242°C) above the melt point, the FEP now melts and flows well. However, the mud-cracks were very severe and they are just starting to fill in or heal.
(d)
As the temperature is raised to 650°F (343°C), well above the melt point, the FEP now melts and flows well. This temperature is often the recommended temperature for FEP liquid and powder coats. The mud-cracks were very severe but show significant healing.
(e)
At 700°F (371°C), well above the melt point, the FEP now melts and flows very well. Even at this high temperature, the mud-cracks did not completely heal. It is possible that if the coating was held at this temperature for double or triple the time, the mudcrack defects might have disappeared.
Figure 12.1 Micrographs�������� of aqueous FEP dispersion-based fluoropolymer coating baked under different conditions.����������������������������������
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY The process for coating and curing fry pans with fluoropolymers is basically the same for almost all coatings being used. These coatings are complex and generally consist of two-, three-, or four-coat systems. 1. There is usually a primer layer made of a blend of a fluoropolymer and a thermosetting resin like polyamide-imide (PAI). The polyamide-imide chemistry is discussed in Sec. 4.3.1 of this book. 2. The primer is dried with heat lamps, but the temperature remains below the boiling point of water. 3. �Additional coating layers are applied over the primer. 4. The pans enter an oven on a chain link belt and pass through three separate heating zones. This coating undergoes several stages while it is in the oven. These stages are described in Fig. 12.2. A typical bake of a fry pan takes fifteen minutes. The pan passes through three zones in the oven, each one about five minutes long. The first zone is typically set around 300°F (149°C). The water and volatile solvents are removed from the coating by slowly raising the temperature of the coating to 300°F (149°C) ± 50°F (28°C). This step must de done slowly so that the solvents and water do not boil off rapidly. Boiling solvent would physically disrupt the film.
Figure 12.2 Typical fry pan bake. �
165 The next zone is designed to raise the coating temperature up to near the final baking temperature. Once the solvents have been removed, the temperature is ramped up to about 700°F (371°C). As the coating reaches 400°F (204°C), the polyamideimide in the primer starts to imidize by liberating a water molecule. The primer stratifies (see Sec. 4.2) with the fluoropolymer concentrating at the primer interface with the midcoat or topcoat and the PAI concentrating at the substrate interface. Stratification and imidization continue as the temperature rises but are complete by the time the coating has reached 700°F (371°C). The primer has been cured. Other volatile materials diffuse or decompose, and diffuse out of the film at above 700°F (371°C). These volatiles are primarily surfactants, high boiling solvents, and possibly the film-forming aid. The final bake zone raises the coating to a temperature between 700°F820°F (371°C438°C) and holds it there for three to five minutes. That temperature depends on the particular coating system being used and the substrate. At this temperature, PTFE sinters and other fluoropolymers, if present in the formulation, melt and flow. The coating usually needs two to five minutes at this temperature to allow PTFE to sinter or fluoropolymer(s) to melt and develop the final film properties. The remaining volatile components, surfactants, and film foaming aides diffuse out of the film. Some of these must decompose to lower molecular weight materials in
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FLUORINATED COATINGS AND FINISHES HANDBOOK
order to become volatile. This is why the presence of oxygen in the oven air is important for many coating products. If carbon is used as a pigment, it starts to degrade if the bake is above 750°F (399°C). Most fluoropolymer-based coatings have a specified baking schedule in terms of times at different metal temperatures. These are usually specified by the coating manufacturer and found in their fact sheets. Many coatings are processed in a batch oven. A batch oven set-point temperature can be changed through the bake. Multiple coat systems can have complicated bake schedules, particularly if many coats are applied with bakes between the coats. This is common in high-build systems. Often, the bake temperature is lowered slightly with each additional coat. This is to minimize the chance of bubbling due to decomposition of the fluoropolymers in the underlayers. One must follow the fact sheets for the products being used. It is never advisable, unless specifically instructed, to bake at a higher temperature than the previous coat when using highbuild systems.
12.3 Monitoring Bake The key point of the baking specification mentioned above is metal temperature. This does not mean the oven temperature setting. One must measure the temperature of the part being cured and monitor the temperature during the bake to verify that the proper temperature is reached. When an applicator is asked why he has not actually measured
the substrate temperature, the usual response is, but I know my oven. This presumption is almost always proven wrong. There are a number of ways to measure part temperature, but only two are common in high-temperature coating processes: thermocouples and noncontact IR thermometers. Two devices that will not be discussed are the Resistance Temperature Detector (RTD) and the Thermistor (Bulk Semiconductor Sensor).
12.3.1 Thermocouples A thermocouple is a thermoelectric temperature sensor; it is the most common temperature measuring method. It consists of two dissimilar metallic wires. One is referred to as +, one as -. The two combinations of wires most commonly used are called J-type and K-type. These are described in Table 12.1. The wire pair is connected at the end, preferably twisted and welded. The temperature is detected by measuring the change in voltage between the two wires, which varies with the temperature at that junction. Thermocouples are ideally attached directly to the part being baked. The best way is by welding directly to the substrate. However, this is impractical because it damages the part and that is unacceptable to the eventual user of that coated part. If there is a threaded fastener hole on the substrate, the end of the thermocouple can be held in place by a screw or bolt that fits that hole, thus avoiding damage.
Table 12.1 The Properties of Two Common Thermocouple Wires � ����
�����������������������
�����������������
J
Iron & Constantan (Cu-Ni alloy)
-350°F ~ 2200°F (-210°C ~ 1200°C)
K
Chromel (Ni-Cr alloy) & Alumel (Ni-Al alloy)
-450°F ~ 2500°F (-270°C ~ 1350°C)
* Constantan, Alumel, and Chromel are trade names of alloys.
����� Max of ±0.75% or ±2.2°C (4°F) Below 0°C (32°F): max of ±2% or ±2.2°C (4°F) Above 0°C (32°F): max of ±0.75% or ±2.2°C (4°F)
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY The thermocouple can be taped to the part using a high-temperature tape. The tape of choice for this approach is 3M Glass Cloth Tape #361. This tape is made of fiberglass with a silicone adhesive and is rated to about 550°F (288°C), but it can be used at higher temperatures. Care must be taken that the thermocouple wire is held as tightly as possible against the part, preferably with several layers of tape. The tape can still come off; the user must verify that this has not happened during the bake. The adhesive may also stick to the part and may need to be cleaned off. When attachment to the part must be avoided, the thermocouple can be attached to a similar scrap or uncoated part that is baked as closely as possible to the coated part. When such a part is not available, any part of similar metal and mass can be used. The thermocouple wires are usually double insulated. Each wire of the thermocouple is wrapped with a fiberglass insulation that is color coded for its particular type. An additional layer of fiberglass insulation wraps the pair of wires again. The condition of these wires must be checked before every use after the wires have been exposed to a high temperature. The sizing in the fiberglass decomposes, and the insulation becomes very delicate and brittle. (The wires should be handled with gloves and inhalation of fibers of insulation should be avoided after exposure to the first high-temperature bake.) If the wires short-circuit, then that creates a new thermocouple junction and the temperature being measured will be the temperature at that junction, not necessarily the one at the ends of the wires. The baking of a part should be monitored and documented with a recording device such as a chart recorder, computer, data logger, or by hand.
12.3.2 Non-Contact Temperature Measurement Non-contact temperature measurement theoretically would be ideal, avoiding any part damage by the measuring device. There are a number of similar devices that work by measuring the IR radiation or heat coming off the part being baked. These are called IR thermometers or optical pyrometers. Pyrometers take advantage of the fact that all objects radiate energy. The intensity and wavelength of that radiated energy can be used to calculate the
167 temperature. One of two theories is used by the pyrometers: Plancks law or the Stefan-Boltzmann law. Plancks law is used in narrow-band pyrometers, where only one or a few specific wavelengths are targeted. The Stefan-Boltzmann law is used in broadband pyrometers, where a wide range of wavelengths is measured. The principle is the same, differing only in the mathematics. It is important to understand the principle since that leads to an understanding of the limits of this type of temperature measurement. A black body is a theoretical object that absorbs 100% of the incident radiation. Therefore, it reflects no radiation and appears perfectly black. It also is a perfect radiator of energy. At a given temperature, the black body emits the maximum amount of energy possible for that temperature. This value is known as the black-body radiation. It would emit at every wavelength of light, because it absorbs at every wavelength of incoming radiation. It also emits a predictable amount of energy at each wavelength for a given temperature. The radiation intensity and wavelength from a black body at a given temperature T is governed by Plancks law, one mathematical form of which is given in Eq. (12.1):
Eq. (12.1)
I (ν) =
2hν 3 c2
1 hν exp −1 kT
where: I(ν) = �The amount of energy emitted at per unit time per unit surface area per unit solid angle per unit frequency ν ν = �frequency h = �Plancks constant, 6.625 × 10-34 joule·sec c = �Speed of light, 2.998 × 108 m/sec k = �Boltzmanns constant, 1.380 × 10 -23 joule/K T = �Black-body absolute temperature (K) The data at various temperatures and wavelengths calculated from Planks law are shown in Fig. 12.3.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Figure 12.3 Black body radiation output as a function of temperature and wavelength.
The peak wavelengths are all in the infrared part of the spectrum. The graph shows that as temperature increases, the peak wavelength emitted by the black body decreases. It moves from the infrared towards the visible part of the spectrum. Some visible light is emitted at any temperature, but at intensities too low for the eye to see except at the highest temperatures. Hot metal appearing red is therefore cooler than metal appearing yellow or white. The graph also shows that as temperature increases, the total energy emitted increases, because the total area under the curve increases. The remote temperature-sensing devices measure the energy in the infrared or visible parts of the spectrum and calculate what the temperature is assuming the light is coming from a black body. However, real world objects are not perfect black bodies. Some are better than others. A correction for the temperature calculation is called emissivity. Emissivity is defined by the following formula: Eq. (12.2) Emissivity =
Radiant energy of an object Radiant energy of a black body with the same temperature as the object
Emissivity ranges between 0 and 1 depending on the dielectric constant of the object, surface
roughness, temperature, wavelength, and other factors. What does this mean for temperature measurement? If the object one is trying to measure has an emissivity of 0.5, but the device assumes it is a perfect black body with an emissivity of 1.0, then the calculated temperature will be lower than it really is, perhaps significantly underestimated. If emissivity is set too low on the non-contact measuring device, then the temperature is overestimated. The measuring device needs to have the correct emissivity set to get a correct measure of the temperature. Table 12.3 shows the emissivity of many common substrates or materials. To further complicate the matter, the emissivity is sometimes temperature dependent, apparent in some of the table entries. Ideally, one would measure the emissivity of the coated object using a thermocouple on the part and just adjusting the emissivity on the remote sensing device until the temperatures are the same. Additionally, many times these non-contact devices are used on batch ovens, where the door must be opened to make the measurement, which should be avoided.
12.4 Types of Ovens There are several types of ovens or baking methods. Each has its advantages and disadvantages, which are discussed in the following sections.
12.4.1 Convection Heating Convection ovens are the simplest and most common method to heat up a coating. They are often called conventional ovens. With convection processes, energy is transferred to the product by first heating the air. The heated air must contact the coating and substrate to raise their temperatures. A thin boundary layer exists around the coated part that repels the hot air, making it difficult to raise the coating temperature. Most convection ovens have circulating fans in them to move the air around the oven to improve heat transfer and to make the temperature throughout the oven chamber as uniform as possible. In forced convection, the heated air is directed at the surface to break up the boundary layer. This offers the potential for damage to coated surfaces, particularly powder-coated surfaces.
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY
169
Table 12.2 Emissivity of Various Metal Substrates[1] ��������
���������������
���������������
����������
Unoxidized
77
25
0.02
Unoxidized
212
100
0.03
Unoxidized
932
500
0.06
Oxidized
390
199
0.11
Oxidized at 599°C
390
199
0.20
Heavily Oxidized
200
93
0.09
Heavily Oxidized
940
504
0.18
Highly Polished
212
100
0.09
Roughly Polished
212
100
0.04
Commercial Sheet
212
100
0.06
Highly Polished Plate
440
227
0.04
Bright Rolled Plate
338
170
0.04
Bright Rolled Plate
932
500
0.05
Alloy A3003 Oxidized
600
316
0.40
Alloy A3003 Oxidized
900
482
0.40
200800
93427
0.05
73% Cu 27% Zn, Polished
674
357
0.03
62% Cu 37% Zn, Polished
710
377
0.04
83% Cu 17% Zn, Polished
530
277
0.03
Lampblack
77
25
0.95
Unoxidized
77
25
0.81
Candle Soot
250
121
0.95
Black Oxidized
100
38
0.78
Etched
100
38
0.09
Matte
100
38
0.22
Roughly Polished
100
38
0.07
Polished
100
38
0.03
Highly Polished
100
38
0.02
Rolled
100
38
0.64
Rough
100
38
0.74
ALUMINUM
Alloy 1100-0 BRASS
CARBON
COPPER
���������
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Table 12.2���������� ��������
���������������
���������������
����������
Oxidized
212
100
0.74
Unoxidized
212
100
0.05
Red Rust
77
25
0.70
Rusted
77
25
0.65
Oxidized
390
199
0.64
Strong Oxidation
482
250
0.95
Cold Rolled
200
93
Polished Sheet
500
260
0.10
Mild Steel, Polished
75
24
0.10
Mild Steel Smooth
75
24
0.12
212
100
0.08
77
25
0.80
Type 301, Polished
450
232
0.57
Type 316, Polished
450
232
0.57
Type 321
200800
93427
0.270.32
Type 350
200800
3161093
0.180.27
Type 350 Polished
3001800
149982
0.110.35
Type 446
3001500
149815
0.150.37
Bright Galvanized
100
38
0.23
Commercial 99.1%
500
260
0.05
Galvanized
100
38
0.28
Oxidized
5001000
260538
0.11
Polished
100
38
0.02
Polished
500
260
0.03
IRON
CAST IRON
STEEL
Steel Unoxidized Steel Oxidized
0.750.85
STEEL ALLOYS
ZINC
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY Sources of heat are either gas or electricity. Electric systems can use open coils, metal-sheath heaters, or fin-type heaters to heat the air. Direct-fired gas ovens use a combustion flame to heat the air directly, and the products of combustion become part of the oven process air. Indirect-fired gas systems use a heat exchanger to separate the process air from the combustion air. That reduces the transfer efficiency but protects the product from contamination by the combustion products. In a convection oven, the product spends a significant portion of the total dwell time in the oven just reaching the process temperature. This is the major energy consumption portion of the process. The amount of heat transferred to the coating and substrate is determined by: 1. Thermal conductivity of the substrate 2. The surface area and mass of the substrate 3. Temperature gradient 4. The air velocity in the oven Ovens of this type are batch ovens or continuous open-ended tunnel ovens.
12.4.2 Infrared Baking (IR) High tech heat-lamps heat by radiating infrared light and can be used even for high bake-temperature coatings.[2] These are not the kind of heat lamps one sees in restaurants that keep the food warm until it is served. Those types of lamps can be used to warm up parts before coating. They can also dry the paint layers between applications. The infrared sources for curing and melting have significantly more power output. IR is the most efficient of all radiation for transfer of heat where one material emits the IR radiation and the other absorbs it. There are several types of infrared sources including gas-fired and electric emitters. The method of IR source heating and the material of construction have no effect on the wavelength characteristics of the different sources. The IR radiation maximum is determined only by temperature, and is described by the Wien Displacement law: Eq. (12.3)
λ max =
2.9 ×10 −3 T
171 Plancks law describes the intensity versus the wavelength or frequency (Fig. 12.3). The IR sources are generally divided into three regions depending on the most-generated wavelength of IR energy: 1. �Short wave. IR peak output is in the range of 0.762 µm. Short infrared emitters have an evacuated tube, or inert atmosphere, containing tungsten filament heated to 3632°F (2000°C) to 4532°F (2500°C). Up to five percent of the radiative energy output is in visible light, so it looks bright yellow. Power output can approach 200 W/in². Short-wavelength IR has the tendency to penetrate through thin organic coatings and heat the substrate directly. 2. �Medium wave. IR peak output is in the range of 24 µm. Medium infrared emitters have a nickel-chrome filament, at a temperature of 1292°F (700°C) to 2372°F (1300°C). Up to one percent of radiative energy output is in visible light, giving the operating lamps a dull red color. The tubes are not evacuated nor in an inert atmosphere. Power output can approach 15 60 W/in2. Medium-wavelength heaters are available in many configurations including lamps, tubular quartz, flat panel heaters, ceramic, bulbs, metal-sheath heaters, and more. Medium-wavelength infrared has the tendency to be absorbed by organic coatings directly. The peak absorption of water falls within this regime, making it suitable for efficiently heating moisture-rich products or waterbased coatings. 3. �Long wave. IR peak output is in the range of 4 µm to 1 mm. Long infrared emitters are glass radiating panels that are electroconductive on the surface, or are ceramic panels that operate at 572°F (300°C) to 1112°F (600°C). Power output can approach 15 W/in2. The choice of heater for a particular process is determined by the actual process and product demands. This relates back to the electromagnetic absorption spectra of the product being heated and how much energy transmission is required by the process, that is, how hot it needs to get and how fast it needs to reach this temperature.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
The amount of heat transferred from the IR source to the substrate (called���������) receiving the heat depends on several things:
3. �Heating homogeneity due to radiation penetration of the coating (less skinning over) 4. Can do some operations not possible by other methods
Temperature of the IR source Temperature of the heat receiver
5. �Ease of installation as complement to other heating processes (booster ovens)
Absorption and emission coefficients Physical dimensions and shape of the source and receiver
These lead to: 1. �Good heat transfer accuracy and control, and higher product temperatures are possible
The distance between the source and the receiver The heat transfer is described by Eq. (12.4). 4
Eq. (12.4) ��� =��V ×��S ×��T ×�� × (�S ��T
2. High productivityshorter cure times 4)
where: ��� = Infrared heat transfer (W/cm²) �V = Geometric view factor (01) �S = Emissivity of the source (01) �T = Absorptivity of the target (01) � = Stefan-Boltzmann Constant �S � = Source temperature (K) � T = Target temperature (K) This equation shows what parameters are important in infrared heating. The view factor (�V) relates to how well the heating target ���� the IR energy source and takes into account the geometry of the substrate (target). Ideal IR substrates are flat or cylindrical without nooks and crannies. The target absorptivity factor (�T) is a measure of how well the surface absorbs the IR energy. It is related to the emissivity factors discussed earlier in this chapter. The source temperature (�S) controls infrared energy output, the power put into the IR source. The amount of radiant heat flux emitted from the source is proportional to the fourth power of temperature. At high emitter temperatures, >1800°F (~1000°C), IR produces significantly higher heat flux than convection heating. In considering the characteristics of conventional versus IR curing, advantages of IR in industrial applications include: 1. Direct transfer of thermal heat without intermediate (air) 2. Rapid heat up
3. Oven size reductionsless floor space 4. �Quality improvementscleaner ovens (less dirt due to lower air flows) 5. Somewhat lower capital cost, lower operational cost 6. Instantaneous start-up and shutdown 7. �Simplified construction 8. Reduced pollution by ovenenergy efficiency 9. More comfortable working conditions, less wasted heat 10. �Improved safety 11. Maintenance requirements are reduced 12. �Modular and flexible designs with many zones are practicaloffers versatility
������ ���������������� Heating by induction[3] is another approach to direct heating of the substrate. It is a common misconception that the substrate must be magnetic to be a candidate for induction heating. To be heated by induction, the substrate must conduct electricity. Technically, it must also resist the flow of electricity or have resistance, but that is true of all materials except superconductors. The principle of induction heating depends on understanding that, when electricity flows, a magnetic field is generated, and the reverse is also true. Where there is a magnetic field and a conductor, electricity will flow.
12 FLUOROPOLYMER COATING PROCESSING TECHNOLOGY Induction heaters (Fig. 12.4) make use of this principle. The heater uses alternating electricity in a coil to generate a magnetic field. When a piece of metal is placed close to (not touching) this coil, the magnetic field generated by the coil interacts with the metal, generating electric current. That current is called an ������������. The resistance to current flow in the metal leads to loss of electric power as described by the basic electrical formula in Eq. (12.5). Eq. (12.5)
�����2�
In this equation, � is the amount of current, � is the resistance of the metal, and�� is the power loss or the heat gained. The equation also indicates that doubling the current quadruples the heat generated. Because the coil uses alternating current, the magnetic field averages out to zero over time. The strength of the magnetic field drops off with distance from the induction coil. Because the eddy currents are related to the strength of the magnetic field, the heating is strongest at the surface. The process seems simple, and in a way it is. It is complicated to control however, but it can be controlled. The heat-up rate of the metal underneath the coating being cured with induction heating depends on several properties of the substrate metal: 1. Specific heat 2. Magnetic permeability 3. �Resistivity
����������� Commercial induction heating equipment showing an array of induction coils.���������������� �����������������������
173 All of these properties of the substrate vary with temperature. The weight and shape of the substrate metal will affect the heat-up rate. Since most of the heat is generated at the surface closest to the coil, the thermal conductivity of the substrate will also affect the peak temperatures at the surface as heat moves towards the cooler areas of the substrate. The control parameters of the induction coil include: 1. Power 2. Frequency There is a relationship between the frequency of the alternating current and the depth to which it penetrates the substrate. The induced current flow within the part is most intense on the surface. The current decays rapidly below the surface. The metal closest to the surface will heat more quickly than the inside. The skin depth of the part is described as the depth within which 80% of the heat in the part is produced. The skin depth decreases when resistivity decreases, permeability increases, or frequency increases. High frequencies of 100 to 400 kHz provide shallow penetration, which is usually ideal for curing surface coatings. Low frequencies of 5 to 30 kHz are effective for thicker materials requiring deep heat penetration such as those coated items with complex shapes. Magnetic materials such as steel are easier to heat than non-magnetic materials such as aluminum. This is due to a secondary heating mechanism called ����������. Magnetic materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces its own additional heathysteresis heatingin addition to eddy current heating. A visual explanation is given in Fig. 12.5. A metal that offers high resistance is said to have high magnetic permeability. Permeability can vary on a scale of 100 to 500 for magnetic materials; non-magnetics have a permeability of 1. The advantages of induction heating over conventional convection heating include: 1. ����������������� Heat can be developed directly and nearly instantly inside the substrate, allowing a much quicker startup than conventional convection heating. Bake cycle times can be dramatically reduced.
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FLUORINATED COATINGS AND FINISHES HANDBOOK
Figure 12.5 Hysteresis in magnetic materials.[3] Energy is required to turn the small internal magnets around. The resistance to this is like friction; the material increases in temperature.
2. �Controlled directional heating. Very small areas of the substrate can be heated without affecting other surrounding areas or the fixturing that holds the part. With precise power input control, one can achieve the exact temperature required either slowly or quickly. 3. �Repeatability. With modern induction heating systems, the heating pattern is always the same for a given set-up, cycle after cycle and day after day.
4. �Non-contact heat. Nothing touches the coated part when it is placed in the induction coil, the process induces heat within the part without actually touching it. 5. Energy efficiency. In summary, one can buy or formulate the finest quality fluorinated coating, but if it is not applied correctly and baked correctly, it may fail miserably in use.
REFERENCES 1. �Lide, D. R., CRC Handbook of Chemistry and Physics, CRC Press (2003) 2. �Technology Guidebook for Infrared Heating, Electric Power Research Institute: CMF Report No. 93-2, Palo Alto, CA (1993) 3. �Haimbaugh, R. E., Practical Induction Heat Treating, ASM International, Materials Park, OH (2001)