Coatings Technology Handbook Episode 2 Part 2 potx

Plasma Surface Treatment 39 -5 the excited species recombine to their original stable and nonreactive form. In most cases, treatment of the exhaust effluent is not required. Gases that contain oxygen are generally more effective at increasing the surface energy. For example, plasma oxidation of polypropylene increases the initial surface energy of 29 dynes/cm to well over 73 dynes/cm in just a few seconds. At 73 dynes/cm, the polypropylene surface is completely water wettable. Increased surface energy results in a plasma that yields polar groups, such as carboxyl, hydroxyl, hydro- peroxyl, and amino. A higher energy (hydrophilic) surface translates to better wetting and greater chemical reactivity of the modified surface to adhesives, paints, inks, and deposited metallic films, providing for improved adhesion and permanency. The enhanced surface reactivity is characterized in the laboratory by studying water wettability. Wet- tability describes the ability to spread over and penetrate a surface; it is measured by the contact angle between the liquid and the surface. The relationship between contact angle and surface energy is inverse — the contact angle decreases with increasing surface energy. Wettability can easily be induced on normally nonwettable materials such as polyolefins, engineering thermoplastics, fluoropolymers, ther- mosets, rubbers, and fluoroelastomers. Noble gases (argon, helium, etc.) generate surface free radicals that react either with other radicals on the surface, yielding molecular weight changes, or with the air, when the part is removed from the chamber, thus increasing the surface energy. Process gases such as fluorocarbons will generally provide a lower energy, or hydrophobic, surface by substitution of abstracted hydrogen with either fluorine or trifluoromethyl radicals to form a fluorocar- bon surface. Fluorination is favored in some medical applications, where it undesirable to have catheters be wetted by blood. The nonwettable barrier layer also inhibits chemical penetration, a consideration that is important for specialty packaging. 39.7 Adhesion Bonding in manufacturing processes is a specialized field, but generally, cleanliness and wettability are necessary for good adhesion. High surface energy alone does not guarantee better adhesion; however, the versatility of the process enables tailoring of the surface chemistry for optimal adhesion or superior product performance. It is not uncommon to move the focus of failure from the bond line into the adherent or into the adhesive with a many-fold increase in the adhesion. Examples of typical plasma improvement on a range of materials for epoxy bonding 3,4 5 TA BLE 39.2 Typical Bond Improvement a after Surface Treatment: Solvent-Washed Plasma Material Shear Strength (pai) Failure Mode b Valox (polyester) 522 Adh 1644 Coh Noryl (polyphenylene oxide) 617 Adh 1799 Coh Durel (polyarylate) 250 Adh 2161 Coh Vec tr a (LCP) 939 Adh 1598 Coh Lexan (polycarbonate) 1705 Adh 2242 Coh Delrin (polyacetal) 165 Adh 857 Adh a Lap shear strength 3M Weldbond #2256. b Adh = adhesive failure, Coh = cohesive failure of adherent. DK4036_book.fm Page 5 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC and for coating are shown in Table 29.2 and Table 39.3. 40 -1 40 Surface Pretreatment of Polymer Webs by Fluorine 40.1 Introduction 40- 1 40.2 The Fluorination Process 40- 1 40.3 Pretreatment with Fluorine: Application Examples 40- 4 40.4 Advantages of Surface Pretreatment with Fluorine 40- 6 40.5 Summary 40- 6 References 40- 6 40.1 Introduction Bonding, coating, laminating, painting, and printing require good substrate adherence. This requires, above all, surface polarity, which permits mechanical and, in particular, chemical bonds. For this reason, polymeric materials are treated by means of oxidation processes entailing three main groups: corona discharge, flame treatment, and chemical. All processes are more or less disadvantageous. The chemical methods have been proved only in narrow, limited fields of application (e.g., as liquid pickling agents), or they require high supervision and a substantial maintenance effort (e.g., ozone treatment). State-of-the-art fluorination is troublesome, because it is a discontinuous process and is not feasible in many cases, especially for web-shaped substances. Corona pretreatment requires high investment and is strongly liable to interference. In the area of the dielectric material, fires occur frequently, causing short circuits in the pretreatment station. Additionally, only one side of the web material can be activated by the corona discharge. This chapter describes an attractive new pretreatment method implemented by Lohmann GmbH, featuring the continuos fluorination of web materials. 1 40.2 The Fluorination Process The surface of web-shaped, polymeric substrates is subjected continuously, for a short time, in a suitable reaction chamber, to elemental fluorine — attenuated with an inert gas. Thus, the surface energy of the polymeric material is increased to such an extent that excellent adherence to other polymers (e.g., lacquers and adhesive agents) is attained. 2 R. Milker Lohmann GmbH Artur Koch Ahlbrandt System GmbH DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC Fluorine • Plant for Continuous Surface Fluorination • Safety Precautions Sheeting • Air Cushion Sheeting • Terpolymer Rubber Polyethylene–Vinyl Acetate Copolymer Foam • Plastic 41 -1 41 Calendering of Magnetic Media 41.1 Introduction 41- 1 41.2 Calendering Magnetic Media Products 41- 1 41.3 Calender Design 41- 2 References 41- 3 41.1 Introduction Calendering is a continuous web process: two or more cylindrical rolls are nipped together under mechanical load and driven via a system motor. This chapter discusses the calendering of coatings in the manufacture of magnetic media products. 41.2 Calendering Magnetic Media Products In the magnetic tape industry, the calendering process relates to improving performance of the manu- factured product. Improvements to the surface of the applied coatings on the substrate, usually polyester film, can be varied depending on the end use of the magnetic media product; they include the following: •Improved surface finish •Densified coating •Improved aesthetics •Reduced product thickness •Improved product electrical output •Reduced recorder head wear •Promotion of coating durability Some of the product improvements listed are interrelated and can be optimized. As an example, improvement of the surface finish will be achieved by the calendering process, and the coating will be densified. Iron oxide in the coating binder system must be maintained at the coating surface and not caused to migrate toward the base film. The surface can become very smooth if a percentage of binder system is caused to migrate to the coating surface, displacing the iron oxide particles away from the eventual recording head. The condition of “too smooth” can also be achieved, and the lack of surface boundary lubrication air will allow contact between the tape and recording head, resulting in excessive drag on the tape. The surface conditions required on the magnetic media products are critical. The hydrodynamic lift clearances are 5 to 10 microinches ( µ in.) (0.12 to 2.5 µ m), as compared to a coated surface roughness (after calendering) of 0.6 to 1 µ in. (0.014 to 0.025 µ m) for the magnetic media products. To achieve this smooth finish, the coating to be calendered is placed in contact with the polished surface of a steel hot John A. McClenathan IMD Corporation DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC 42 -1 42 Embossing 42.1 General 42- 1 42.2 Thermoplastic Webs 42- 1 Embossing Machines for Thermoplastic Webs 42.3 Nonthermoplastic Embossing 42- 5 42.1 General Embossing is a method by which a web is textured by the use of a pattern roll pressing against a backup roll under controlled conditions. One can emboss both thermoplastic and nonthermoplastic webs by choosing the proper roll arrangement to deform the web. To emboss thermoplastic materials, the web is deformed by preheating and pressing it with a cooled embossing (pattern) roll to set the pattern and cool the web to retain that pattern. The degree of preheating to soften the web must be carefully controlled so that no melting or degrading of the web will take place. To make the heat removal process as efficient as possible, no more heat should be applied than is needed to satisfactorily emboss the product. To emboss a nonthermoplastic web, such as paper, textile, or foil, one must apply pressure that exceeds the elastic limit of the substrate and imparts the pattern. This type of embossing involves the use of male and female rolls, either two rolls with matched patterns made of steel or other metal, or a steel pattern roll that comes in contact with a filled backup roll, which takes a permanent deformation for a given pattern by running the steel embossing roll in contact with the backup roll during a “running in” period. In some cases, special rubber-covered rolls can be used; their behavior eliminates the need for “running in.” 42.2 Thermoplastic Webs Embossing thermoplastic webs is achieved by using an engraved metal roll pressing against a rubber- covered backup roll. The metal roll is cooled with a refrigerated solution to remove heat from the product and to set in the embossed pattern. The backup roll is internally cooled, mainly to increase the life of the rubber covering. The roll may also be cooled externally by a water bath and squeeze roll system, especially if the web is an unsupported thermoplastic film having a tendency to adhere to a hot rubber surface. Embossing of a web depends on many variables, such as the following: •Degree of preheating and rheological properties of the product • Sheet thickness •Hardness of the rubber backup roll • Embossing roll pattern and its cooling capacity •Postemboss cooling A fine balance exits between the preheating and the removal of heat to set the pattern. Applying the appropriate amount of preheat but insufficient cooling results in the inability to deform the web, which John A. Pasquale III Liberty Machine Company DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC 42 -2 Coatings Technology Handbook, Third Edition has not been softened enough. The best embossing system is one that optimizes heat input and removal for a given thickness and speed. The hardness of the backup roll plays a role in the finished product’s texture. If a roll is too hard, a good definition of fine surface patterns might be achieved, however, displacement of material within the product for deep embossings may not be possible. If the roll is too soft, it will allow deep embossings to show through the back of the product, an effect that is objectionable in some applications. The need to cool the product after it has left the embossing roll is another important factor. Appropriate postcooling facilities, usually cooling rolls, are used to bring the sheet temperature as close to ambient conditions as possible before the product is rewound into a roll. When cooling a thin sheet, the problem of retained heat is minor, because a thin sheet releases heat easily. In heavy sheet embossing, although the surface of the web might feel cool, the heat is retained in the body of the sheet. This heat, if it remains, will cause a loss of embossed grain when the product is later rewound for further processing. Embossing units can be placed in various geometric positions. They are usually either vertical, where the web path enters in a horizontal manner, as shown in Figure 42.1, or they can be placed in a horizontal fashion, as shown in Figure 42.2. Under special conditions and for certain applications, it might also be The preheated web should enter the embossing nip perpendicular to the line of action of the embossing and backup rolls, to ensure that the web is not prematurely cooled by striking either the embossing roll or the backup roll first. It is acceptable and sometimes desirable, however, that the backup roll be contacted first. Many webs are unstable in their preheated condition and will be easily creased if they enter the nip unsupported. A short arc of contact before the embossing action of the nip allows the web to be flattened. It is also preferable to contact the backup roll first, because the surface temperature of the backup roll is higher than that of the cool embossing roll; thus, the amount of heat removed from FIGURE 42.1 Ve rtical embossing unit. FIGURE 42.2 Horizontal embossing unit. Embossing Roll Web Path Backup Roll Embossing Roll Web Path Backup Roll DK4036_book.fm Page 2 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC advantageous to find them disposed at a particular angle, as shown in Figure 42.3. 42 -4 Coatings Technology Handbook, Third Edition The drum is heated by steam or hot oil and is usually 1 to 1.5 m (36 to 60 in.) in diameter. Steam heat is preferred, as it responds to temperature changes more readily than does hot oil. For this thickness of web, the arc-shaped radiant heat unit around the periphery of the drum is not required. The steam-heated drum has a double-shell construction as shown in Figure 42.5. Saturated steam enters from one end and passes through the center shaft of the drum; then it is fed through passages to the annular section formed by the inner and outer shells of the drum. As the steam fills the annular chamber and performs its work, condensation takes place and is removed through similar passages at the opposite end of the drum by a siphon tube and exits through the shaft opposite the steam inlet. Both the steam and the condensate enter and leave the drum through rotary joints furnished with bronze hoses to withstand the temperature of the steam. The rotary joints are pipe connections, which allow the drum to rotate freely while they remain stationary to provide a solid connection to the steam pipe and condensate return system. The embossing section consists of a 250 to 300 mm (10 to 12 in.) diameter, double-shelled embossing roll, which is designed to allow a cooling solution to pass through it in an efficient manner to remove heat as rapidly as possible, thereby setting in the embossing. Embossing rolls typically have a double-shell design with a spiral wrap for the most efficient passage of the cooling medium. For drums heated by hot oil, the construction is similar, but spiral windings forming passageways or channels are provided in the annular space between the inner and outer shells to allow the oil to flow in a prescribed path and in the most efficient manner, to promote good heat transfer. The hot oil is pumped through the center shaft of the drum and enters through conduits similar to the steam-heated drum design; after it has done its work, it leaves by similar passages at the opposite end of the drum. It is appropriate to pump the hot oil at rates that will cause it to flow in a turbulent manner through the drum annulus passageways, to provide the best heat transfer coefficient for optimal results. The preheated web is stripped away from the main heating drum by use of the heated stripper rolls. These rolls help to maintain the temperature of the web and allow it to be passed into the embossing section. The stripper rolls are of single-shell construction and vary in size from 100 to 150 mm (4 to 6 in.) for steam-heated systems. The infrared heater shown on the drop into the embossing section is used to maintain the ambient temperature around the web and to assure that the surface to be embossed will be entering the nip at approximately 160 ° C (320 ° F). FIGURE 42.5 Steam-heated drum. +− Flexible Hose Center Pipe Inner Shell Condensate Return Hose with Syphon Tube Rotary Joint Condensate Return Annular Steam Chamber Outer Shell Spoked End Plates Bearing Journal Steam Supply DK4036_book.fm Page 4 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC 43 -1 43 In-Mold Finishing 43.1 Introduction 43- 1 43.2 Process 43- 2 43.3 Conclusion 43- 4 43.1 Introduction In the past few years, a lot of attention has been focused on the utilization of in-mold finishing of thermoplastic parts. In-mold foiling, available since the early 1970s, has recently developed into an important market because of refinements in tool design and improvements in foils, and through the expansion of in-mold capabilities. Insert molding has also been available for a number of years and has continued to develop as manufacturers’ search for technologies that will provide them with superior part quality. In-mold foiling offers the customer a product that benefits from the economics of having the part finished simultaneously with the injection molding process. The part is also more durable than a component finished with one of the conventional techniques because of foil quality and the inherent benefits of transferring the finish at the melt temperature of the thermoplastic. In-mold foiling also provides the designer with new opportunities because this application of finishes is not restricted to flat surfaces. Developing technologies include the application of specialized films such as wood, vinyls, and leathers, and the expansion of design capabilities through interfacing in-mold components with other techniques like electroplating. With the opportunity to look at designs in their earliest stages, instead of trying to fit the process to an awkward design, the processors began to fully utilize the benefits afforded by in-mold applications. The obvious benefits lie in the reductions in labor and burden, because the part is partially or completely finished in the molding cycle and, in many cases, without a drastic change in the cycle time required. Another positive aspect of the in-mold processes is the ability to conform to geometries that would be impractical or impossible to finish by more conventional techniques. A third benefit is the variety of materials that may be utilized in the process, which allows for efficiencies of scale when a single mold is utilized to produce a variety of versions with different colors or patterns; moreover, the materials selected can be customized without regard to physical or chemical properties. In-mold decoration has a long history. Inserts, preformed and trimmed, have been utilized for deco- rative components with deep draws for at least two decades, and commercial applications of in-mold foiling began in the early 1970s. Robert W. Carpenter Windsor Plastics, Inc. DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC Laminates • Foiling • New Materials 44 -1 44 HVLP: The Science of High-Volume, Low-Pressure Finishing 44.1 The Principles behind HVLP 44- 1 44.2 The Benefits of HVLP 44- 1 44.3 HVLP versus Conventional Air Spray 44- 3 44.4 HVLP versus Airless and Air-Assist/Airless 44- 3 44.5 HVLP versus LVLP 44- 4 44.6 Compliant Technologies: HVLP and Electrostatic 44- 4 44.7 Components of an HVLP System 44- 4 44.8 Differences between HVLP Systems 44- 5 44.9 Operating an HVLP System 44- 7 44.10 The Use of Air Cap Test Kits 44- 7 44.1 The Principles behind HVLP High-volume, low-pressure (HVLP) atomization utilizes a high volume of air delivered at 10 psi or less to atomize fluid material into a soft, low-velocity pattern. This reduction in the velocity of the airstream over the 40 to 70 psi typically delivered by conventional spray methods results in a more controlled spray pattern, less bounceback, and enhanced transfer efficiency. Transfer efficiency can be defined as the amount of paint sprayed that goes onto the part as compared to the amount lost due to overspray and bounceback. In general, HVLP can be used with most low-to-medium solids materials including two-component paints, urethanes, acrylics, epoxies, enamels, lacquers, stains, and primers. Some HVLP application 44.2 The Benefits of HVLP High transfer efficiency enhances both productivity and finish quality. Less overspray improves visibility, which limits operator error. It also reduces deposits on adjacent surfaces, which typically results in a dry, sandy finish. Reducing overspray will reduce spray booth maintenance, filter replacement, waste disposal, Depending on the application, two-thirds or more of every gallon of material sprayed by conventional methods can be lost to overspray. But with HVLP, typically one-third or less is lost to overspray. Pro- ductivity does not suffer either because more paint is applied per pass, and fewer passes are required. But while finish quality and materials savings are important benefits, perhaps the most compelling reason to consider HVLP is the current trend toward legislated transfer efficiency requirements. The Steve Stalker ITW Industrial Finishing DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC and materials costs (Figure 44.1 and Figure 44.2). equipment can atomize higher viscosity materials and/or higher fluid flow rates. 44 -6 Coatings Technology Handbook, Third Edition the air from a turbine is not always controllable; pressure is not always sufficient to provide effective atomization with higher viscosity materials; and turbines generally require a higher level of maintenance. The second design (Figure 44.8) diverts shop air through an air conversion unit, which reduces atomization air pressure to 10 psi or less before the air reaches the spray gun. When fitted with air heaters, the heat can be adjusted or eliminated. They can also be regulated to deliver consistent pressure. Plus, they are more reliable than turbine generators. However, larger internal diameter (ID) air hose and a separate air conversion are required. The final design also utilizes shop air (Figure 44.9). However, it reduces the air pressure to the required 10 psi or less within the gun. This design eliminates the need for a separate air conversion unit while delivering the same degree of control over air pressure. In addition, it offers added convenience because it can be connected with a standard air line (1/4 ″ fitting and 3/8 ″ or 5/16 ″ air hose). FIGURE 44.7 Turbine generator air supply configuration. FIGURE 44.8 Shop air supply with air conversion unit configuration. FIGURE 44.9 Shop air supply with gun air conversion configuration. Turbine Unit Material Supply 50–100 PSI Air Supply Optional Heater Material Supply Air Conversion Unit 50–100 PSI Air Supply Material Supply DK4036_book.fm Page 6 Monday, April 25, 2005 12:18 PM © 2006 by Taylor & Francis Group, LLC [...]...DK4036_book.fm Page 1 Monday, April 25 , 20 05 12: 18 PM 45 A Practical Guide to High-Speed Dispersion 45.1 What Is a Disperser? 45-1 45 .2 How Does It Work? 45-1 45.3 What Is the Difference between a Disperser and an Agitator? 45 -2 45.4 When Do I Need to Use a Disperser rather than an Agitator? 45-3 45.5 What Are the... rapidly break apart lumps of powdery material, uniformly distributing and wetting them in a liquid It is also used to dissolve soluble solids in a liquid 45 .2 How Does It Work? A disperser works on the principle of energy transfer A disc-type blade is mounted at the bottom end of the mixing shaft and rotated at relatively high tip speed (Figure 45.1) (Tip speed is the speed at the 45-1 © 20 06 by Taylor... Herman Hockmeyer Hockmeyer Equipment Corporation 45.10 What Other Factors Affect the Performance of My Disperser? 45-6 45.11 How Do I Operate My Disperser for Optimum Performance? .45-7 45. 12 What Safety Measures Must I Follow and Why? 45-7 Every year, new people join the various businesses that use mixing machines in their laboratories and manufacturing facilities What seems obvious to . DK4036_book.fm Page 1 Monday, April 25 , 20 05 12: 18 PM © 20 06 by Taylor & Francis Group, LLC 42 -1 42 Embossing 42. 1 General 42- 1 42. 2 Thermoplastic Webs 42- 1 Embossing Machines. Machine Company DK4036_book.fm Page 1 Monday, April 25 , 20 05 12: 18 PM © 20 06 by Taylor & Francis Group, LLC 42 -2 Coatings Technology Handbook, Third Edition has not been softened enough April 25 , 20 05 12: 18 PM © 20 06 by Taylor & Francis Group, LLC advantageous to find them disposed at a particular angle, as shown in Figure 42. 3. 42 -4 Coatings Technology Handbook, Third