Bonding requirements in coating and laminating of textiles

E. Shim , in Joining Textiles, 2013

10.3.4 Roll coating

Roll coating is a pre-metered coating and uses a series of rollers to meter and apply coating liquid on a substrate. A metered film of coating liquid is first formed on the roller surface before it is applied to the substrate, so the amount of coating material delivered to the substrate is nearly independent of the fabric properties and structures. Mostly determined by the rheology of the fluid and the relative speed of two rotation surfaces, precise control is possible ( Alonso, 2003; Greer, 1995; Abbott et al., 1972). Varying substrate thickness does not result in uneven coating thickness (Scott, 1995; Glawe et al., 2003; 'Laminating and coating: flexible future', 2002).

The simplest roll coating set-up uses a single rotating roller. The bottom half of the roller is immersed in a coating liquid bath and the upper part of the roller is in contact with the fabric substrate (Fig. 10.7). As it rotates, the coating liquid forms a film on the roller surface and part of the liquid film is transferred from the roller surface to a fabric substrate. The amount of coating on the substrate is governed by hydrodynamics. The rotation speed of the roller, the substrate speed, and rheological properties of coating fluid (surface tension, viscosity, and density) are factors determining coating thickness (Wright, 1981). In this set-up, one roller is a metering device as well as an application device. More precise control is achieved by addition of more rollers.

10.7. Roll coating (one roll reverse applications).

Three-roll coating uses a metering roller, an applicator roller and a backup roller. Common three-roll configurations are nip feed coating and L-head coating. In three-roll nip feed coating (Fig. 10.8(a)), the nip formed by a metering roller and an applicator roller is flooded with coating liquid and functions as a reservoir (Greer, 1995; Grant, 1978). The applicator roller picks up the coating liquid from the nip and the amount of coating liquid delivered to the fabric substrate is metered by the metering roller rotating in a reverse direction to the applicator roller. After coating liquid transfers from the metering roller to the applicator roller, any coating liquid remaining on the metering roller surface is cleaned by a doctor blade, otherwise it will result in coating defects, such as streaks or film roughness. Film formed on the applicator roller is deposited onto the substrate surface supported by a back-up roller. This configuration needs only a minimal amount of coating fluid, but a drawback is possible coating fluid leakage. It can be problematic when coating liquid has low viscosity. In three-roll pan feed or L-head coating (Fig. 10.8(b)), a liquid film is formed on the applicator roller rotating through the coating liquid, metered by a metering roller, and deposited on the substrate fabric on a back-up roller. To increase coating speed, one more roller – a pick-up roller running at a reduced speed – can be added, and this system is called a four-roll pan-fed coating system (Greer, 1995; Grant, 1978, 1981).

10.8. Three-roll coatings: (a) nip feed coating and (b) L-head coating.

Configurations shown in Fig. 10.8 are called reverse metering, since applicator and metering rollers rotate in opposite directions. When they rotate in the same direction, it is called forward metering (Hannachi & Mitsoulis, 1990). Reverse metering produces a smoother film with better stability, while forward roll metering is prone to generate unstable, non-uniform films. Therefore reverse roll coating is more commonly used (Greer, 1995; Grant, 1978). Roller coating can use water-based solutions, solvent-based coating materials as well as hot melts (Grant, 1981). In hot melt roller coating, solid pellet is melted between the heated melt rollers, forming a melt film and deposited on a substrate. The substrate fabric is usually preheated before hot melt is applied (Zickler, 1978).

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Coating and laminating processes and techniques for textiles

E. Shim , in Smart Textile Coatings and Laminates (Second Edition), 2019

2.3.7 Gravure or engraved roll coating

Gravure roll coating is the process where engraving on the roller acts as a metering device. The engraved roller is partly submerged in the coating bath and as it rotates, coating liquid fills the engraved pattern and excess coating liquid forms a film on the roller's surface ( Fig. 2.11). A doctor blade removes the excess surface film from the gravure roller surface and it then presses against the substrate to transfer the coating material in the engraved pattern to the substrate. This method can operate in either forward or reverse mode, but, as discussed before, the reverse mode generally provides greater stability (Grant, 1981; Hewson, Kapur, & Gaskell, 2006).

Figure 2.11. Gravure roll coating.

The amount of coating material delivered to the substrate is mainly controlled by the engraved patterns. Various patterns are used, examples being regular dot, irregular, quad, net structured, and rhomboid patterns (Hewson et al., 2006). The most important characteristics of engraved patterns that affect the amount of coating material delivered are the land area, cell openings, cell depths, cell volumes, cell angles, and cell spaces (Robinson & Marrick, 2006). Other parameters include viscosity of the coating material, application pressure on the substrate and type and structure of the substrate. Gravure roll coating can be used for different fluids, including hot melt adhesives (Anon, 2002a). It can also produce patterned coating (Stukenbrock, 2003).

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Current and advanced coating technologies for industrial applications

A.S.H. Makhlouf , in Nanocoatings and Ultra-Thin Films, 2011

1.6.4 Roll-to-roll coating

Roll-to-roll coating is the process of applying a coating to a flat substrate by passing it between two (or more) rollers. In this technique, the coating material is applied by one or more auxiliary rolls onto an application roll after the gap between the upper roller and the second roller has been appropriately adjusted. The coating is wiped off the application roller by the substrate as it passes around the support roller at the bottom. After curing, the coated substrate is then shaped to the final form; this has no effect on the properties of the coating.

Roll-to-roll coating is made up of two different techniques: direct roll coating and reverse roll coating. In the direct roll coating technique, the applicator roll rotates in the same direction as the substrate. In the reverse roll coating technique, the applicator roll rotates in the opposite direction to the substrate. 38

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Adhesive Coating and Lamination of Polyvinyl Fluoride Films

Sina Ebnesajjad PhD , in Polyvinyl Fluoride, 2013

9.6.2.1 Reverse Roll Coating

The reverse roll coating technique is based on the transfer of adhesive material from a trough by means of a pick-up roller partially immersed in it to a contacting transfer roller sheet [10]. Material is continuously coated with adhesive when fed between the transfer roller and a pressure roller, which is adjusted to determine the thickness required. Roll coating is most suitable for applying adhesives to flat sheets and film, and may be used for parts as large as 2 meters. Where feasible, this technique provides the highest production rate and the most uniform coverage.

Figure 9.5 shows a schematic diagram of a reverse roll coater in which a doctor blade is the metering device. The excess adhesive on the metering roll is wiped off by the doctor blade, and the adhesive is then transferred onto the applicator, which in turn deposits it on the metal substrate.

Figure 9.5. Schematic diagram a reverse roll coater.

When multiple coats of an adhesive are required, the most uniform film thickness is achieved by applying the second coat perpendicular to the first. Moreover, the time between successive coats must be carefully regulated. Too short a drying time may result in sagging, bubbling, or blistering, while too long a time may lend to the lifting of earlier coats.

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Other Processing Approaches

Syed Ali Ashter , in Thermoforming of Single and Multilayer Laminates, 2014

10.5.1.5 Reverse-Roll Coaters

A reverse-roll coating process is considered one of the most versatile and important coating methods. This is because it can handle a wide range of viscosities and coating weights and the accuracy is very high. The coating is independent of substrate tension and the variation in substrate thickness and therefore it is able to apply a premetered coating of uniform thickness. Typically, there are two forms of reverse-roll coaters, the three-roll nip and the pan fed. Fig. 10.25 shows the arrangement of a nip-fed reverse-roll coater.

Figure 10.25. Nip-fed reverse-roll coater: (1) applicator roll, (2) metering roll, (3) back-up rubber roll, (4) web, (5) coating pan, (6) doctor blades and (7) drip pan [24].

In this process, a layer of thin coating is metered between the applicator and the metering roll. The coating material is adhered to the applicator roll, which is then carried to the coating nip where the material is transferred to the web moving in the opposite direction. This produces a high level of shearing action. In order to obtain a good coating, the applicator roll is in the opposite direction to the metering roll and to the web. The coating material that does not get transferred from the applicator roll after contact with the web is manually removed, collected in a pan and recycled. The coating thickness is dependent on the gap between the applicator roll and the metering roll, the rotational speed of the applicator roll and the amount of material transferred on the web, which in turn is dependent on the web pressure on the applicator roll adjusted by the backup roll. It is difficult to prevent leaks from the coating reservoir when a low-viscosity material has to be coated. In such a case, a pan-fed coater is recommended. It operates on the same principle as the nip-fed coater but is more suited to low-viscosity materials.

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Thermoplastic Processing

Michel Biron , in Thermoplastics and Thermoplastic Composites (Third Edition), 2018

5.4.2 Roll Dipping, Spreading

These methods process liquid thermoplastics, which are often obtained by dissolving the polymer in solvents. Consequently, it is necessary to obey all health, safety, and environmental precautions and regulations.

Discontinuous coating by dipping in a bath of liquid thermoplastic is very similar to molding by dipping as used for PVC gloves (see Section 5.1.8). Wire articles, for example, can be coated with PVC to protect them and obtain a softer touch.

Continuous coating can be obtained by roll coating or spreading. There are three main versions:

•

direct dipping in a bath of the liquid thermoplastic

•

indirect coating by contact with a spreading roller partially immersed in the dipping bath

•

spreading machine.

Figs. 5.15 and 5.16 display examples of direct and indirect roll dipping.

Figure 5.15. Principle of direct roll dipping.

Figure 5.16. Principle of indirect roll dipping.

Fig. 5.17 shows a diagram of a spreading machine.

Figure 5.17. Diagram of a spreading machine.

The main advantages and drawbacks of coating by dipping are as follows:

•

the section sizes are limited by the machinery size but the length is unlimited

•

thickness can be difficult to control

•

the articles are isotropic with neat resin

•

the aspect can be poor to good

•

a finishing step is often essential

•

the tools are cheap but the machinery cost depends on the output and sophistication

•

the process is difficult to industrialize

•

the output rates are low to medium according to the sophistication of the machinery.

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Pumping

Charles A. Bishop , in Vacuum Deposition Onto Webs, Films and Foils (Third Edition), 2015

4.11 Filtering

Many large roll-to-roll coating systems run without any filtering. The levels of dust or particles are kept low by vacuuming away any debris during the cleaning of the shields during the system downtime. However, there are a number of processes where there is a significant amount of particles that can be generated during the deposition process, which means that the pumps need to be protected from large levels of particles that could abrade the close-fitting pumping surfaces or with oil-based pumps could clog up the oilways. Low melting-point materials or compounds, that may have a high vapor pressure and low sticking coefficient, may get pumped as a vapor or, if the vapor is able to form particulates, the particles may be pumped directly. This type of problem is found in some of the copper indium gallium diselenide photovoltaic deposition processes, where selenium vapor gets everywhere, and in atomic layer deposition processes where the process is designed to coat every surface uniformly. In some cases, it is possible to use a cold trap to condense vapors before they reach the pumps where the liquid or solid can be collected for periodic removal. Alternatively, there are high surface area molecular filters that can do the same job. The particles can be trapped using filters similar to those used as air filters for automobiles and these may be in series with progressively finer filters. For larger particulates, there are also cyclone filters that spiral the particulates to the outermost diameter of the cyclone and then pump from the central axis where only the lightest particles can remain.

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Polymer Thin Films—Processes, Parameters and Property Control

Bertrand Fillon , in Micromanufacturing Engineering and Technology (Second Edition), 2015

Organic coatings deposited in solution

Organic coatings deposited in solution (i.e., using wet methods) are found in very diverse areas. They are commonly produced by coating substrates with thin layers of a liquid or suspension, which are then transformed into solids by gelation, drying, or cross-linking. Such structures are vital elements for an extremely broad range of industrial products. They are developed for paper, steel, aluminum, polymer films, printed materials, selective membranes, photographic film, photosensitive coatings, adhesives, micro-electronics, integrated circuits, etc. However, deposition technologies may differ greatly depending on the final application. For example, the manufacture of microelectronic components uses spin coating, whereas varnish is generally deposited on steel foil using roll or spray coating.

This chapter focuses exclusively on organic coatings deposited with the roll coating technique; this covers a large majority of the polymer thin films produced industrially. Most of the deposition processes based on this technique involve a series of rollers-producing polymer thin films, which can have thicknesses below one micron. These varnishes offer an effective protective barrier for metal substrates (aluminum, steel), paper substrates, or polymer films. These coatings may also be used for decorative or aesthetic purposes (glossy or matte effects, colors, etc.), or act as sealants, etc. They must also offer specific characteristics of durability and resistance to temperature, solvents, or UV radiation depending on the final application of the coated product:

•

Exterior applications (prefinished steel and aluminum for construction);

•

Decorative interior elements, household appliances, etc.

•

Packaging for food products, cosmetics, etc.

The composition of the varnishes or paints, the choice of substrate, and the conditions under which organic coatings are applied, dried, and cured are inextricably linked in determining the properties of the end product [8].

Main polymers used in roll coating

As in extrusion coating, the polymer used to coat the substrate will strongly influence the quality of deposition and the final properties. The most commonly used coatings are indicated in Table 1. Note that the molecular weights of polymers deposited in solution are lower than those of polymers deposited by extrusion.

Table 1. Performance of Main Coatings Used in Solution

Polymer Family Polyester/Amine Polyester/Polyurethane Epoxy PVDF PVC
Flexibility Good Very good Poor Good Very good
Hardness Good Average Very good Average Poor
Adhesion to metal Good Good Very good Poor Poor
Corrosion protection Good Good Very good Average Very good
Weather resistance Good Very good Poor Very good Average
Temperature resistance Good Good Good Good Poor
Recyclability Very good Good Good Poor Poor

In general, six categories of basic polymers (Figure 5 ) are used in roll coating [9]. The different categories are based on:

Figure 5. Morphology of basic resins [13].
•

Solid state (amorphous or crystalline);

•

Solubility in common solvents;

•

Swellability of crystalline or cross-linked polymers.

Amorphous and soluble polymers (a), for example polyesters, epoxies, or acrylics, produce liquid varnishes and paints which are 30–70% nonvolatile. Amorphous and insoluble polymers (b), which are primarily polyesters, can be used in powder coating technology. Semicrystalline polymers (c), which form the basis of the extrusion coating films listed in the previous chapter, can also be used in roll coating after a process of precipitation or grinding to produce fine powders.

Dispersing these fine powders in other basic polymer solutions produces organosols (e), which are 40–60% nonvolatile. Examples of organosols include PVDF films and polyester polyurethane systems modified by polyamide fine powders.

Organosols can also be formulated with cross-linked polymers (d) in the form of micro-gels or ground into fine powders. Certain applications use these substances, specifically cross-linked amine polymers, acrylics, and unsaturated polyesters.

A plastisol (f) is obtained if a fine and crystalline powder polymer is dispersed in a plasticizer that homogeneously dissolves it at temperatures above the crystallite melting point, forming a gel after cooling. Plastisols are more than 90% nonvolatile. Little solvent is needed.

Note that in addition to the specific characteristics for roll application and "flash" forming or curing of the film, the varnishes or paints for the prefinishing of flat steel or aluminum must offer excellent flexibility, e.g., meeting the postforming requirements for drawing food cans.

Principle of organic coating deposition in solution

Applying organic coatings to reels of flat substrate offers major technical and financial advantages due to the following:

•

Increased productivity and yield of finishing systems (with linear speeds of up to 1000   m/min), reducing the cost of varnish and paint application.

•

Flexibility of roll coating; rapid changes can be made without stopping the application process.

•

Various curing temperature levels (80–250   °C), enabling a wide range of binder systems to be used via various film-formation processes (flash cross-linking, gelation, or fusion). Cross-linking increasingly involves the environment-friendly techniques of UV radiation or electron beams.

•

Efficient effluent treatment systems (e.g., incineration of solvent vapors).

Generally, the process of roll coating includes three phases:

•

Surface preparation and treatment: corona surface treatments are widely used on various substrates, e.g., chromate treatment of steel (no-rinse process) which helps to control film weight and eliminate releases.

•

Roll coating of the polymer: can be performed on both sides of the reel simultaneously. Various approaches can be used for this step (Figure 6). More than 35 systems are available on the market to coat a variety of substrates [10]. The varnish properties, surface appearance, and thickness ranges differ according to the process used.

Figure 6. Various systems for varnish deposition [10]. Diagrams from Coatema sales documentation.
•

Solvent evaporation and flash curing: necessary for forming the binding network.

The choice of technology is based on the application, but there are some country-specific trends emerging in the technologies chosen [11]. With the process of varnish deposition, it is possible to influence precision, coating weight, and also surface appearance (Table 2).

Table 2. Three Varnish Deposition Processes most Commonly Encountered

Type of Process Characteristics Main Comments on the Process
Knife coating Thickness range: 5–500   μm
Viscosity: 5000–50,000   mPas
Maximum speed: 100   m/min		 Very dependent on varnish and substrate. A very smooth film can be obtained with 4% precision over the full width of the substrate. For low viscosity varnishes, there may be problems with deposition uniformity. Solvent evaporation during the implementation process may lead to the appearance of agglomerates.
Slot die Thickness range: <1–200   μm
Viscosity: <1000   mPas
Maximum speed: 1000   m/min		 Possible to work with minimum contact. Substrate does not have a big influence. Precision is on the order of 1%. The low viscosity (<200   mPas) often Results in very good precision.
Roll coating Thickness range: 2–100   μm
Viscosity: <2000   mPas
Maximum speed: 1000   m/min		 Very dependent on the surface of the deposition roller (very smooth with very good precision or gravure roller). A very smooth film can be obtained. The precision of deposition depends on the parameters of the application roller and the roller opposite to it. For film thickness, precision is on the order of 2%.

Most often, the coating is applied to the substrate using a series of application rollers. The reasons cited for using such a system are as follows: the ability to transmit shear to a liquid, to smooth the coating before its deposition, to attain an acceleration between the slow movement of the rollers and the speed at which the substrate passes through them (i.e., line speed), and to produce thin films by multiplying the separation phases at each roller. The configurations are selected based on the deposition thickness and precision. In the two-roller configuration, there is only one gap for adjusting and controlling the thickness deposited. In a three-roller configuration, the additional gap allows this thickness to be optimized. Each roller has two functions. First, rollers act as a divider by reducing the thickness and uniformly spreading the varnish between all the rollers. Second, they create the necessary shear to provoke Newtonian behavior in the varnish. There are few publications [12–15] explaining how these deposits form, how a thin film is created, how good the sensitivity is, and how to perfectly control coating weight based on roller speed, gap, and varnish properties.

But obviously there will always be interactions between the varnish type, its viscosity, and the additives (size of charges, chemical nature, form, etc.). Note that for certain production processes, the central chemical treatment and coating section is isolated from the unwinding and winding sections by accumulators, allowing for uninterrupted operation during reel changing.

Process parameters and influences

Example: optimizing optical properties

There is a great deal of demand for coatings with specific visual or optical properties (transparency, gloss, matte finish). By adjusting the parameters of the implementation process, it is possible to influence a coating's surface appearance as well as its thickness and precision.

As the varnish is applied to the substrate, the shear rate changes abruptly, potentially attaining very high values (Figure 7), but this intense shear strain lasts only a short time (<0.01   s). Use of an elastomer roller can be estimated to increase the dwell time and reduce shear by a factor of around 10 compared to a rigid roller. The viscosity of the varnish will thus have a nonnegligible effect on its thickness and visual qualities, and it will interact with the system of application. Eleven types of flow between two rollers have been described. Experimental and theoretical studies have examined some of them [16] and have enabled correlating the quantity deposited with the flow characteristics. Likewise, a strong interaction between the implementation parameters and the optical properties has been demonstrated by the number of capillaries:

Figure 7. Timescale and typical values of process parameters for wet deposition.

C a = η o V σ

where η o   =   viscosity, V =   line speed, and σ  =   surface energy.

Whatever the mode of varnish transfer (corotating or reverse), the optical characteristics of the film vary strongly according to Ca . When Ca is high, an "orange peel" effect or bubbles appear (Figures 8 and 9). This is caused by a resonance phenomenon provoking a wave effect in the liquid solution when it is transferred by the application roller to the substrate. This defect can measure several microns [17] and increases in size when the film is thin and the line speed is high. It is generally agreed that competition exists between the viscosity of the varnish and the surface energy of the surface. This phenomenon is also observed for Newtonian liquids [18].

Figure 8. Typical optical defect with a "bulge" appearance.

Figure 9. Diagram of the two main appearance defects (bulges and bubbles).

Optical properties can be improved by increasing substrate surface energy or by decreasing the viscosity of the varnish as much as possible. However, one of the conditions essential to forming a continuous film of varnish at the surface of the substrate is that the latter has a surface energy higher than the surface tension of the liquid varnish, to ensure good wetting of the substrate. Figure 10 clearly illustrates that a critical surface energy is needed to eliminate these defects.

Figure 10. Changes in appearance defects according to substrate surface energy for an acrylic varnish on a polyethylene film.

Coating quality also plays a very important role in determining its final properties, but several types of defects are possible, depending on the implementation conditions. They can include lines, pores, irregular thickness, etc. Extensive cracking of thin films may be linked to the internal strain in these fine organic coatings. This strain can also develop over time, either due to age or UV radiation. In addition to the loss of optical properties, aging often provokes delamination between the polymer thin film and the substrate [19,20]. There is also consensus that the film's glass transition temperature (T g) plays a role in this mechanism, given that polymer chain mobility may facilitate the penetration of various species in the polymer matrix, leading to a loss of adhesion at the polymer/substrate interface. Figure 11 illustrates changes in glossiness for two families of varnish. Polyester varnish demonstrates poorer durability than PVDF under mercury lamp exposure [9].

Figure 11. Variation in the glossiness of different polymer thin films according to exposure time under a mercury lamp.
Example: barrier properties

Another property in great demand for thin films deposited in liquid form is protection of the substrate by barrier properties or protection against corrosion. In the previous section, it was shown that the substrate's surface energy plays an important role in determining the optical properties of the thin film deposited. Figure 12 illustrates that surface energy is also important for water vapor barrier properties, obtained by depositing a film of polyvinyl alcohol (PVA) on a paper substrate [21,22]. Applying the corona treatment to the paper before PVA deposition increases the substrate's surface energy, while improving deposition quality and therefore, the water barrier properties [21]. This surface energy activation benefits calendered papers as well as noncalendered papers. Note that the substrate's surface roughness also influences the uniform appearance of the film deposited [22].

Figure 12. Changes in water barrier properties (WVTR) according to the type of paper and its surface treatment (with and without corona treatment). Calendered and noncalendered paper, with and without PVA.

A new generation of polymer thin films is currently being developed and involves integrating nano-fillers taken from sheet silicates (clays). Much work has been done on these nano-composites made from platelet nano-fillers, particularly in the packaging and automotive industries. These materials, with nanometric stiffeners that are strongly anisotropic (form factor of 200), offer very attractive properties, in terms of mechanical behavior under high temperatures as well as barrier properties, compared to materials with traditional stiffeners (talc, silica, etc.). Used on polymer, metal, and paper substrates, these thin film materials are opening up new possibilities for food-grade and cosmetic packaging, where additional requirements apply in terms of oxygen and odor barriers and resistance to heat and abrasion [23,24]. The industrial development of such impermeable structures, with their enhanced mechanical properties compared to standard thin films, depends on controlling nano-filler dispersion and exfoliation. Figures 13 and 14 illustrate the difference in nano-filler dispersion depending on the type of polymer matrix and the type of the nano-filler.

Figure 13. Homogeneous dispersion of the filler 2MHBT in a vinyl varnish.

Figure 14. Inhomogeneous dispersion of 2M2HT in a vinyl varnish.

The filler in Figure 14 is not sufficiently polar and the filler/polymer interactions are not sufficiently strong to allow perfect exfoliation and dispersion of the platelets in the polymer matrix.

Example: sealing properties

Certain polymer thin films deposited in solution offer very attractive sealing properties, especially for substrates such as aluminum foil or paper. For example, Table 3 indicates which family of sealing varnishes will be effective based on the substrate, for packaging applications such as seals, pouches, etc.

Table 3. Families of Sealing Varnish According to Type of Substrate

Varnish Family Type of Substrate
Acrylic Film or sheet of polystyrene or bi-oriented polypropylene (BOPP)
Vinyl Aluminum foil, PVC
Polyester Polyester, PVC
Modified acrylic on olefin Film or sheet of polystyrene or bi-oriented polypropylene (BOPP)

Summary on organic coatings

Depositing polymer thin films in solution is a widely used technique for diversifying the surface appearance and functionality of flat substrates. Selecting the binder system for the organic coating is decisive for substrate performance, whether in terms of appearance, moisture or gas barrier properties, corrosion protection, resistance to photochemical aging, or the capacity to undergo forming without cracking.

Prefinishing technologies offer diverse possibilities for new development, thanks primarily to the following:

•

Coatings are constantly being improved;

•

New developments in the area of organic coatings and their implementation.

For exterior applications, the use of new-generation flexible organic coatings with high or ultrahigh durability is enabling the development of wet thin films which are affordable and particularly effective. These new thin films meet the requirements of the final application domain and can ensure aesthetic qualities for over 15   years in some cases. In sectors such as household appliances, new, slightly thicker thin films offer better scratch resistance, for example.

It should be noted that other methods of applying organic coatings without a solvent are expanding rapidly, notably radiation cross-linking techniques (UV, electron beam).

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Life Cycle Tribology

Simon C. Tung , ... Xianghuai Dong , in Tribology and Interface Engineering Series, 2005

2.2.3 Test Solid Lubricants

The solid lubricants used in the present study were applied by spraying or roll coating to a thickness of between 0.00025" and 0.001". All forming lubricants applied are the solid lubricants which have been suspended in a liquid medium for spraying or roll coating. The solid lubricants as shown in Table 2 include: milk of magnesia (Mg(OH)2), graphite, molybdenum (Moly) disulfide, boron nitride (hexagonal boron nitride), and talc. In addition, mixtures of some of these lubricants were also tested. These include a double layer consisting of one graphite and one molybdenum disulfide layer as well as PEKITE, a GM patented mixture of boron nitride and milk of magnesia [9].

Table 2. The Effect of Tooling Materials and Load on Friction and Wear

Test Lubricant Load 200N Tool Materials* 1040 Tool Steel Tool Materials* GM 238 Cast Iron Tool Materials* GM 246 Cast Iron 1040 Tool steel Load 1000 N**
Dry (unlubricated) High friction (μ=1.5-3.0) High Wear (d&gt;8 μm) High friction (μ=1.3-3.2) High Wear (d&gt;8 μm) High friction (μ=1.5-3.0) High Wear (d&gt;8 μm) High friction (μ=1.5-3.0) High Wear (d&gt;8 μm)
Magnesia High friction (μ=1.6-3.1) High Wear (d&gt;8 μm) High friction (μ=1.2-3.0) High Wear (d&gt;8 μm) High friction (μ=1.4-3.1) Moderate Wear (2&gt;d&gt;6 μm) High friction (μ=1.5-2.8) Moderate Wear (2&gt;d&gt;6 μm)
Graphites Low friction (μ=0.3-0.7) Moderate Wear (2&gt;d&gt;6 μm) Low friction (μ=0.3-0.6) Moderate Wear (2&gt;d&gt;6 μm) Low friction (μ=0.3-0.8) Moderate Wear (2&gt;d&gt;6 μm) Low friction (μ=0.3-0.8) Moderate Wear (2&gt;d&gt;6 μm)
Boron Nitride Low friction (μ=0.4-1.0) Low Wear (d&gt;2 μm) Low friction (μ=0.4-0.7) Low Wear (d&gt;2 μm) Low friction (μ=0.4-0.6) Low Wear (d&gt;2 μm) Low friction (μ=0.4-1.1) Low Wear (d&gt;2 μm)
Moly Disulfide Low friction (μ=0.3-0.9) Low Wear (d&gt;2 μm) Low friction (μ=0.3-0.6) Low Wear (d&gt;2 μm) Low friction (μ=0.3-0.7) Low Wear (d&gt;2 μm) Low friction (μ=0.3-1.1) Low Wear (d&gt;2 μm)
Pekite 50 (Mixed 50% BN + 50% Magnesia) Low friction (μ=0.3-0.6) Low Wear (d&gt;2 μm) Low friction (μ=0.4-0.6) Low Wear (d&gt;2 μm) Low friction (μ=0.4-0.7) Low Wear (d&gt;2 μm) Low friction (μ=0.2-0.7) Low Wear (d&gt;2 μm)
*
AII tests in the first three columns were conducted at a constant load of 200N.
*
Tool materials used for tests are different 1040 tool steel, GM 238 and GM 246 cast Iron.
**
Last column indicated that all tests were conducted at a higher load of 1000N.

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