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Secondary Processing of Fluoropolymer Films
6
Secondary Processing of Fluoropolymer Films
Thermoplastics films are often subjected to additional operations to expand their utility as discussed in Section 1.3. Since the fluoropolymer films discussed here are produced from thermoplastics (fluoroplastics), many secondary processing and fabrication methods being used for them are identical or similar. However, some of them differ. The subject is discussed in this chapter.
6.1 Surface Preparation of Fluoropolymer Films Fluoropolymers have inherently lower surface energy than most other polymers and because of that they tend to form poor adhesive bonds without any type of surface treatment. There are many physical and chemical treatments used for that purpose described in Section 1.3.1. Industrial surface treatments for plastics include corona, flame and plasma treatment, and chemical etching. The increase in surface energy of fluoropolymers, surface modification, involves significant dehalogenation, that is, the removal of fluorine and chlorine atoms from the surface of the macromolecules.
6.1.1
Corona Treatment of Fluoropolymer Films
Corona is a stream of charged particles that are accelerated by an electric field. It is generated when a space gap filled with air or other gases is subjected to sufficiently high voltage in order to set up a chain reaction of high-velocity particle collision with neutral molecules, resulting in the generation of more ions. It is reported that corona treatment is used for FEP and PTFE films and polyvinyl fluoride (PVF) (see Section 1.3.1.1) although other fluoroplastic films could be treated by corona when necessary.
6.1.2
Sodium Etching of Fluoropolymer Films
Perfluorinated fluoroplastics are chemically unaffected by nearly all commercial chemicals with the exception of highly oxidizing substances as elemental forms of sodium, potassium, and other alkaline metals. This is the basis for sodium etching of fluoroplastic parts. The original Applications of Fluoropolymer Films. DOI: https://doi.org/10.1016/B978-0-12-816128-9.00006-4 © 2020 Elsevier Inc. All rights reserved.
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method for surface treatment for adhesive bonding was developed for PTFE and was etched by a sodium solution in anhydrous liquid ammonia. An alternative solution of sodium is to prepare a complex with naphthalene followed by dissolution in tetrahydrofuran or dimethyl glycol ether. Newer systems using glycol diethers (referred to as glymes) are much less toxic than tetrahydrofuran [1]. Special precautions must be taken while working with sodium etching solutions. Fluoropolymer products (films, parts), treated by sodium etching solution, should be stored in a cold, dark atmosphere free from oxygen and moisture. The useful shelf life of etched polymer under these conditions at temperature lower than 5 C is 3 4 months. For in-house etching, it is advisable to consider purchasing commercial etching solutions available from a number of sources. Some of them can be used successfully for fluoropolymers other than PTFE. Moreover, there are several companies offering full service contract etching.
6.1.3 Plasma Treatment of Fluoropolymer Films Plasma is considered the fourth state of the matter and is produced by exciting a gas with electrical energy introduced into a vacuum chamber. Since plasma is intensely reactive, it can effectively modify surfaces of plastics. It can be used to treat thermoplastic films to impart hardness, roughness, more or less wettability, and increased adherability to the part surfaces. Plasma treatment oxidizes the surface of the polymer in the presence of oxygen, which is believed to be the reason for roughening of the surface. Atmospheric plasma treatment (APT), also called glow discharge, is operating without the use of vacuum. The plasma creates uniform plasma cloud that completely surrounds small objects or spreads into the boundary layer of the surface. Fluoropolymer films can be treated by APT with PVF responding to it well but PTFE does not as pointed out in Section 1.3.1.2.
6.1.4 Flame Treatment of Fluoropolymer Films Flame treatment is defined as a surface-preparation technique in which the plastic is briefly exposed to a flame. Flame treatment oxidizes the surface through a free radical mechanism, introducing hydroxyl, carbonyl, and amide functional group to the depth of about 4 6 nm, and produces chain scissions and some cross linking. The film is passed over an oxidizing flame formed by an oxygenrich (relative to stoichiometry) mixture of hydrocarbon gases (see Section 1.3.1.3). However, this method is not effective in the
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adhesion treatment of perfluoroplastics, but it is reported that it can be used for PVF and ECTFE [2].
6.2 Lamination of Fluoropolymer Films Lamination is the technique of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of differing materials. Details are given in Section 1.3.2. In extrusion lamination the molten polymer bonds to a solid substrate by melting chemical interaction and in some cases by mechanical interlocking [3]. Fluoropolymer films can be laminated by methods mentioned in Section 1.3.2 and by extrusion lamination. The most widely used lamination is used in the processing of PVF films. PVF films are laminated to metals including aluminum, stainless steel, cold-rolled steel, galvanized steel, copper, and titanium as well as to other substrates, such as cellulosic materials; to films and sheets from other polymers to fiberglass panels; and to vinyl wall coverings. In each case the other substrate has to be cleaned and/or surface treated, and in most of the cases, special adhesives are used. The best current source of information on this subject is Ref. [4].
6.3 Heat Sealing of Fluoropolymer Films Heat sealing is a widely used process in the packaging industry. It involves joining two polymer films by the application of heat and pressure for a specified time. As pointed out in Section 1.3.3, the polymer characteristics that play a critical role in the heat-sealing process involve melting temperature, chain diffusion rate, melt strength, and crystallization kinetics. Thermoplastic films, including fluoroplastic films, can be sealed by any method that heats the contacting surfaces of the film above the melting point of the given polymer and at the same time. Applied pressure assures intimate contact of those surfaces. Even here, the techniques usually used are hot bar heat sealing, impulse heat sealing, and hot air sealing.
6.4 Metallization of Fluoropolymer Films Plastic parts can be coated with metal, in a process called metallization, for both esthetic and mechanical purposes. Metalized plastic
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components are used in similar applications as metal plated parts, but tend to be lower in weight and have higher corrosion resistance, although not in all cases. As pointed out in Section 1.3.4, fluoropolymer films (PTFE, FEP, ETFE, ECTFE, PVDF, and PVF) can be readily metallized by a variety of metals by vacuum and electroless deposition. Aluminum-coated PTFE is used in high temperature resistant capacitors. Good adhesion of fluoropolymers to copper has been achieved by silanization of fluoropolymer film surfaces. Preactivation of PTFE surface improves adhesion of copper applied by electroless deposition of copper [5]. More information on metallization of fluoropolymers is in Ref. [5].
6.5 Orientation of Fluoropolymer Films As pointed out in Section 1.3.5, polymeric films usually emerge from melt simple-processing methods such as extrusion and calendering with the polymer chains arranged in a relatively random order, because of which they exhibit anisotropy. If the film is stretched, the polymer chains tend to line up (or orient) in the direction of the stretch. Consequently, this orientation affects the physical properties of the film in the direction of the stretch, referred to as machine direction orientation and transverse direction orientation that is stretching the film across the process flow. Another possibility is the stretching in both directions, which is referred to as biaxial orientation. Essentially any thermoplastic film can be oriented by these methods. As for fluoropolymer films, mainly PVF, PVDF and PTFE are biaxially oriented on industrial scale. PCTFE alone cannot be oriented because of its high degree of crystallinity but when combined into laminated with other polymeric films, it is possible [6,7]. More details on orientation of fluoropolymer films are in Ref. [5].
6.6 Other Methods for Secondary Processing of Fluoropolymer Films 6.6.1 Laser Marking of Fluoropolymer Films Laser marking is the modification of the surface of a material to create human- and/or machine-readable alphanumeric characters, 1D/2D barcodes, logos, insignia, etc., mainly for the purpose of identification.
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Laser marking of fluoropolymer films can be performed with either a 10.6 or 9.3 µm CO2 laser, with no appreciable difference in process quality. Laser marking of pigmented fluoropolymer substrates is enhanced by using titanium dioxide pigment coated with organosilane [8]. In general, laser marking can be performed on PTFE, perfluoroalkoxy, FEP, and ETFE [9].
6.6.2
Laser Cutting of Fluoropolymer Films
Laser cutting is the complete removal and separation of material from the top to the bottom surface along a designated path. Laser cutting can be performed on a single layer material or multilayer material. Material thickness and density are important factors to consider when laser cutting. Cutting through thin material requires less laser energy than cutting the same material in a thicker form. Lower density material typically requires less laser energy. However, increasing laser power level generally improves laser cutting speed. Laser cutting of fluoropolymer films can also be performed with either a 10.6- or 9.3-µm CO2 laser [8].
6.6.3
Printing on Fluoropolymer Films
Printing on fluoropolymer films requires surface treatment as described in Section 6.1 and often the use of a special primer [10]. The high-speed roll-to-roll techniques used for printing on fluoropolymer films are gravure printing and flexographic printing [11]. The limitation for inks used to mark fluoropolymer substrates is that they have to be able to withstand high curing temperatures, as this is the adhesion mechanism. This limits the selection of the colorants to very heat-stable pigments [12]. But because the melting point of ECTFE, ETFE, and PVDF are lower than those of other commonly used fluoropolymers, a lower curing temperature material can be used. When the material is cured properly, the print will melt into the top layer of the surface of an application. This provides a very good adhesion to the fluoropolymer, and an extremely long-lasting mark. The printing and striping inks used with PTFE and FEP are formulated from the same resins as the substrate. The use of the same matrix material creates a uniformly melted top layer [12].
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6.6.4 Thermoforming of Fluoropolymer Films Single films, sheets, and laminates from fluoropolymer films can be formed into required shapes by a process, referred to as thermoforming. Thermoforming is a manufacturing process in which a plastic film, sheet, or laminate is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a useable product. The film, sheet, or laminate is heated in an oven to a temperature high a
b
(A)
c
(B)
Figure 6.1 Vacuum forming [13]. (A) Preheated sheet prior to forming (B) Formed sheet into female mold a—Preheated, clamped sheet b—Female mold with vacuum holes c—Vacuum
d a b c e (A)
Figure 6.2 Pressure forming [13]. (A) Preheated sheet prior to forming (B) Formed sheet into female mold a—Pressure box b—Preheated, clamped sheet c—Female mold with vacuum/vent holes d—Applied air pressure e—Venting of vacuum
(B)
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d
a
b c (A)
(B) d
Figure 6.3 Matched mold (die) forming [13] (A) Preheated sheet for forming. (B) Sheet formed by simultaneous motion of two mold halves a—Male mold half b—Preheated, clamped sheet c—Female mold half d—Applied force.
enough so that it can be stretched into or onto a mold and cooled to a finished shape [13]. Currently, there are three thermoforming methods, namely, vacuum forming, pressure forming, and match die forming. The principles of these methods are shown in Figs. 6.1 6.3.
References [1] S. Ebnesajjad, Chapter 8 Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013. [2] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 204. [3] B.A. Morris, The Science and Technology of Flexible Packaging, Elsevier, Oxford, UK, 2017, p. 371. [4] S. Ebnesajjad, Chapter 9 Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013. [5] M. Friedman, G. Walsh, Polym. Eng. Sci. 42 (8) (2002) 1768. [6] M.L. Tsai, US Patent 5,874,035 (February 23, 1999) to Allied Signal, Inc. [7] M.L. Tsai, US Patent 5,945,221 (February 23, 1999) to Allied Signal, Inc. [8] Laser Processing, Universal Laser Systems, ,www.ulsinc.com/materials/ fluoropolymers., 2019. [9] Laser Marking, Zeus Industrial Products, ,www.zeusinc.com/products/ value-add/laser-marking., 2019.
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[10] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 214. [11] B.A. Morris, The Science and Technology of Flexible Packaging, Elsevier, Oxford, UK, 2017, p. 46. [12] Considerations When Printing and Marking Fluoropolymers, PolyOne Corporation, ,www.polyone.com., 2015. [13] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 251.
Secondary Processing of Fluoropolymer Films
Thermoplastics films are often subjected to additional operations to expand their utility as discussed in Section 1.3. Since the fluoropolymer films discussed here are produced from thermoplastics (fluoroplastics), many secondary processing and fabrication methods being used for them are identical or similar. However, some of them differ. The subject is discussed in this chapter.
6.1 Surface Preparation of Fluoropolymer Films Fluoropolymers have inherently lower surface energy than most other polymers and because of that they tend to form poor adhesive bonds without any type of surface treatment. There are many physical and chemical treatments used for that purpose described in Section 1.3.1. Industrial surface treatments for plastics include corona, flame and plasma treatment, and chemical etching. The increase in surface energy of fluoropolymers, surface modification, involves significant dehalogenation, that is, the removal of fluorine and chlorine atoms from the surface of the macromolecules.
6.1.1
Corona Treatment of Fluoropolymer Films
Corona is a stream of charged particles that are accelerated by an electric field. It is generated when a space gap filled with air or other gases is subjected to sufficiently high voltage in order to set up a chain reaction of high-velocity particle collision with neutral molecules, resulting in the generation of more ions. It is reported that corona treatment is used for FEP and PTFE films and polyvinyl fluoride (PVF) (see Section 1.3.1.1) although other fluoroplastic films could be treated by corona when necessary.
6.1.2
Sodium Etching of Fluoropolymer Films
Perfluorinated fluoroplastics are chemically unaffected by nearly all commercial chemicals with the exception of highly oxidizing substances as elemental forms of sodium, potassium, and other alkaline metals. This is the basis for sodium etching of fluoroplastic parts. The original Applications of Fluoropolymer Films. DOI: https://doi.org/10.1016/B978-0-12-816128-9.00006-4 © 2020 Elsevier Inc. All rights reserved.
125
126
APPLICATIONS
OF
FLUOROPOLYMER FILMS
method for surface treatment for adhesive bonding was developed for PTFE and was etched by a sodium solution in anhydrous liquid ammonia. An alternative solution of sodium is to prepare a complex with naphthalene followed by dissolution in tetrahydrofuran or dimethyl glycol ether. Newer systems using glycol diethers (referred to as glymes) are much less toxic than tetrahydrofuran [1]. Special precautions must be taken while working with sodium etching solutions. Fluoropolymer products (films, parts), treated by sodium etching solution, should be stored in a cold, dark atmosphere free from oxygen and moisture. The useful shelf life of etched polymer under these conditions at temperature lower than 5 C is 3 4 months. For in-house etching, it is advisable to consider purchasing commercial etching solutions available from a number of sources. Some of them can be used successfully for fluoropolymers other than PTFE. Moreover, there are several companies offering full service contract etching.
6.1.3 Plasma Treatment of Fluoropolymer Films Plasma is considered the fourth state of the matter and is produced by exciting a gas with electrical energy introduced into a vacuum chamber. Since plasma is intensely reactive, it can effectively modify surfaces of plastics. It can be used to treat thermoplastic films to impart hardness, roughness, more or less wettability, and increased adherability to the part surfaces. Plasma treatment oxidizes the surface of the polymer in the presence of oxygen, which is believed to be the reason for roughening of the surface. Atmospheric plasma treatment (APT), also called glow discharge, is operating without the use of vacuum. The plasma creates uniform plasma cloud that completely surrounds small objects or spreads into the boundary layer of the surface. Fluoropolymer films can be treated by APT with PVF responding to it well but PTFE does not as pointed out in Section 1.3.1.2.
6.1.4 Flame Treatment of Fluoropolymer Films Flame treatment is defined as a surface-preparation technique in which the plastic is briefly exposed to a flame. Flame treatment oxidizes the surface through a free radical mechanism, introducing hydroxyl, carbonyl, and amide functional group to the depth of about 4 6 nm, and produces chain scissions and some cross linking. The film is passed over an oxidizing flame formed by an oxygenrich (relative to stoichiometry) mixture of hydrocarbon gases (see Section 1.3.1.3). However, this method is not effective in the
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adhesion treatment of perfluoroplastics, but it is reported that it can be used for PVF and ECTFE [2].
6.2 Lamination of Fluoropolymer Films Lamination is the technique of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of differing materials. Details are given in Section 1.3.2. In extrusion lamination the molten polymer bonds to a solid substrate by melting chemical interaction and in some cases by mechanical interlocking [3]. Fluoropolymer films can be laminated by methods mentioned in Section 1.3.2 and by extrusion lamination. The most widely used lamination is used in the processing of PVF films. PVF films are laminated to metals including aluminum, stainless steel, cold-rolled steel, galvanized steel, copper, and titanium as well as to other substrates, such as cellulosic materials; to films and sheets from other polymers to fiberglass panels; and to vinyl wall coverings. In each case the other substrate has to be cleaned and/or surface treated, and in most of the cases, special adhesives are used. The best current source of information on this subject is Ref. [4].
6.3 Heat Sealing of Fluoropolymer Films Heat sealing is a widely used process in the packaging industry. It involves joining two polymer films by the application of heat and pressure for a specified time. As pointed out in Section 1.3.3, the polymer characteristics that play a critical role in the heat-sealing process involve melting temperature, chain diffusion rate, melt strength, and crystallization kinetics. Thermoplastic films, including fluoroplastic films, can be sealed by any method that heats the contacting surfaces of the film above the melting point of the given polymer and at the same time. Applied pressure assures intimate contact of those surfaces. Even here, the techniques usually used are hot bar heat sealing, impulse heat sealing, and hot air sealing.
6.4 Metallization of Fluoropolymer Films Plastic parts can be coated with metal, in a process called metallization, for both esthetic and mechanical purposes. Metalized plastic
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components are used in similar applications as metal plated parts, but tend to be lower in weight and have higher corrosion resistance, although not in all cases. As pointed out in Section 1.3.4, fluoropolymer films (PTFE, FEP, ETFE, ECTFE, PVDF, and PVF) can be readily metallized by a variety of metals by vacuum and electroless deposition. Aluminum-coated PTFE is used in high temperature resistant capacitors. Good adhesion of fluoropolymers to copper has been achieved by silanization of fluoropolymer film surfaces. Preactivation of PTFE surface improves adhesion of copper applied by electroless deposition of copper [5]. More information on metallization of fluoropolymers is in Ref. [5].
6.5 Orientation of Fluoropolymer Films As pointed out in Section 1.3.5, polymeric films usually emerge from melt simple-processing methods such as extrusion and calendering with the polymer chains arranged in a relatively random order, because of which they exhibit anisotropy. If the film is stretched, the polymer chains tend to line up (or orient) in the direction of the stretch. Consequently, this orientation affects the physical properties of the film in the direction of the stretch, referred to as machine direction orientation and transverse direction orientation that is stretching the film across the process flow. Another possibility is the stretching in both directions, which is referred to as biaxial orientation. Essentially any thermoplastic film can be oriented by these methods. As for fluoropolymer films, mainly PVF, PVDF and PTFE are biaxially oriented on industrial scale. PCTFE alone cannot be oriented because of its high degree of crystallinity but when combined into laminated with other polymeric films, it is possible [6,7]. More details on orientation of fluoropolymer films are in Ref. [5].
6.6 Other Methods for Secondary Processing of Fluoropolymer Films 6.6.1 Laser Marking of Fluoropolymer Films Laser marking is the modification of the surface of a material to create human- and/or machine-readable alphanumeric characters, 1D/2D barcodes, logos, insignia, etc., mainly for the purpose of identification.
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Laser marking of fluoropolymer films can be performed with either a 10.6 or 9.3 µm CO2 laser, with no appreciable difference in process quality. Laser marking of pigmented fluoropolymer substrates is enhanced by using titanium dioxide pigment coated with organosilane [8]. In general, laser marking can be performed on PTFE, perfluoroalkoxy, FEP, and ETFE [9].
6.6.2
Laser Cutting of Fluoropolymer Films
Laser cutting is the complete removal and separation of material from the top to the bottom surface along a designated path. Laser cutting can be performed on a single layer material or multilayer material. Material thickness and density are important factors to consider when laser cutting. Cutting through thin material requires less laser energy than cutting the same material in a thicker form. Lower density material typically requires less laser energy. However, increasing laser power level generally improves laser cutting speed. Laser cutting of fluoropolymer films can also be performed with either a 10.6- or 9.3-µm CO2 laser [8].
6.6.3
Printing on Fluoropolymer Films
Printing on fluoropolymer films requires surface treatment as described in Section 6.1 and often the use of a special primer [10]. The high-speed roll-to-roll techniques used for printing on fluoropolymer films are gravure printing and flexographic printing [11]. The limitation for inks used to mark fluoropolymer substrates is that they have to be able to withstand high curing temperatures, as this is the adhesion mechanism. This limits the selection of the colorants to very heat-stable pigments [12]. But because the melting point of ECTFE, ETFE, and PVDF are lower than those of other commonly used fluoropolymers, a lower curing temperature material can be used. When the material is cured properly, the print will melt into the top layer of the surface of an application. This provides a very good adhesion to the fluoropolymer, and an extremely long-lasting mark. The printing and striping inks used with PTFE and FEP are formulated from the same resins as the substrate. The use of the same matrix material creates a uniformly melted top layer [12].
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6.6.4 Thermoforming of Fluoropolymer Films Single films, sheets, and laminates from fluoropolymer films can be formed into required shapes by a process, referred to as thermoforming. Thermoforming is a manufacturing process in which a plastic film, sheet, or laminate is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a useable product. The film, sheet, or laminate is heated in an oven to a temperature high a
b
(A)
c
(B)
Figure 6.1 Vacuum forming [13]. (A) Preheated sheet prior to forming (B) Formed sheet into female mold a—Preheated, clamped sheet b—Female mold with vacuum holes c—Vacuum
d a b c e (A)
Figure 6.2 Pressure forming [13]. (A) Preheated sheet prior to forming (B) Formed sheet into female mold a—Pressure box b—Preheated, clamped sheet c—Female mold with vacuum/vent holes d—Applied air pressure e—Venting of vacuum
(B)
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d
a
b c (A)
(B) d
Figure 6.3 Matched mold (die) forming [13] (A) Preheated sheet for forming. (B) Sheet formed by simultaneous motion of two mold halves a—Male mold half b—Preheated, clamped sheet c—Female mold half d—Applied force.
enough so that it can be stretched into or onto a mold and cooled to a finished shape [13]. Currently, there are three thermoforming methods, namely, vacuum forming, pressure forming, and match die forming. The principles of these methods are shown in Figs. 6.1 6.3.
References [1] S. Ebnesajjad, Chapter 8 Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013. [2] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 204. [3] B.A. Morris, The Science and Technology of Flexible Packaging, Elsevier, Oxford, UK, 2017, p. 371. [4] S. Ebnesajjad, Chapter 9 Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013. [5] M. Friedman, G. Walsh, Polym. Eng. Sci. 42 (8) (2002) 1768. [6] M.L. Tsai, US Patent 5,874,035 (February 23, 1999) to Allied Signal, Inc. [7] M.L. Tsai, US Patent 5,945,221 (February 23, 1999) to Allied Signal, Inc. [8] Laser Processing, Universal Laser Systems, ,www.ulsinc.com/materials/ fluoropolymers., 2019. [9] Laser Marking, Zeus Industrial Products, ,www.zeusinc.com/products/ value-add/laser-marking., 2019.
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[10] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 214. [11] B.A. Morris, The Science and Technology of Flexible Packaging, Elsevier, Oxford, UK, 2017, p. 46. [12] Considerations When Printing and Marking Fluoropolymers, PolyOne Corporation, ,www.polyone.com., 2015. [13] S. Ebnesajjad, Polyvinyl Fluoride—Technology and Applications of PVF, Elsevier, Oxford, UK, 2013, p. 251.