NON-CONTACT MEASUREMENTS OF COATINGS

Transcription

1 APPLICATION NOTE NON-CONTACT MEASUREMENTS OF COATINGS USING QCL SPECTROSCOPY Abstract Many companies need to characterize or monitor coatings on surfaces in real time during a manufacturing process or in difficult environments. Mid infrared (IR) spectroscopy, specifically using Fourier transform infrared (FTIR) instruments, have demonstrated the ability to do this, but FTIR instruments require contact with the surface which is totally impractical in many manufacturing processes, time consuming in other processes and potentially results in contamination or damage of the surface. What is needed is a way to characterize or monitor coatings in a fast efficient manner without contacting the surface. This Note describes a new non-contact technique, using Quantum Cascade Laser (QCL) spectroscopy that in real time can characterize or monitor coatings. Introduction The characterization of coatings on surfaces is a major industrial application. Coatings range from chemically modified surfaces to physical layers of both thin and thick organic films. Coatings serve multiple purposes which may be protective, functional, and decorative, including: Protective coatings to prevent corrosion or chemical attack, such as rust and corrosion inhibitors Coatings to prevent surface adhesion and to reduce friction, such as lubrication films and release agents Coatings to promote adhesion for the bonding of surfaces, such as adhesive layer or intermediate layers Decorative coatings to provide protection and to promote an attractive finish, such as a painted surface Functional coatings applied in a hazardous environment, such as an insulating surface in a high voltage application Mid-IR FTIR spectrometers have long been used to identify, characterize and analyze these and other types of coatings but FTIRs require contact with the surface. This is time consuming, often impractical, and potentially results in contamination. Furthermore, most FTIRs are laboratory devices, which are difficult to use under field conditions. This document describes a new optical technique using QCL IR spectroscopy that provides real-time, noncontact, non-destructive characterization of coatings in a rugged, handheld configuration. The latest embodiment of this technology from Block Engineering is called the LaserScan, and is shown in Figure 1. Figure 1. LaserScan QCL reflection spectrometer for real time, non-contact, non-destructive analysis of coatings and other surfaces.

2 Coatings, including those used in an assembled product, can be measured conveniently with no contact and without any risk of damage to the quality or finish of the surface. Likewise, the non-contact measurement can enable safe measurements in areas that can be hazardous, such as protective painted surfaces on high voltage equipment like a transformer or on a high temperature surfaces such as an engine exhaust pipe. For these areas of application, the LaserScan can be considered a service tool to evaluate the quality of a surface in situations where contact measurements are not practical or even possible. Quantum Cascade Laser Technology The LaserScan is a revolutionary portable instrument from that can be an ideal tool for characterizing coatings on surfaces. The LaserScan is the result of many years of development at Block Engineering and utilizes QCL infrared spectroscopy. IR technology has been used for decades under laboratory conditions, most commonly by FTIR as stated above. Because the LaserScan uses a low power, collimated QCL beam, it is eye safe but can measure coatings on surfaces from distances ranging from six inches to several feet depending away on the application. The fundamental principle behind that technology is that when infrared light reflects off coatings, it gets absorbed or reflected at rates that are absolutely unique and characteristic of the coating s chemistry. Therefore, when the reflected light is collected by the LaserScan s built-in detector, a fingerprint pattern emerges, which contains all the necessary information to characterize or identify the coatings. Block s proprietary algorithms are then used to provide real-time identification and characterization of the coating. Furthermore, LaserScan is a lightweight portable unit that provides readings in seconds. Coatings Applications facilitated by LaserScan Analysis There are many coating applications that could benefit from non-contact real time measurements with the LaserScan. For example: 1. Analyzing Metal Conversion Coatings 2. Monitoring the Curing or Drying of Decorative/Protective Paints 3. Monitoring Adhesive Curing 4. Non-contact Real Time Characterization of Protective Safety Coatings Each of these is described in more detail below with actual data taken with Block s LaserScan. 1. Analyzing Metal Conversion Coatings A conversion coating adds a protective surface layer on a metal that is created by a chemical reaction between the metal and a chemical coating solution. One of the most common techniques is anodization of aluminum using electrolysis. Use of the LaserScan for this example is described here. However, there are many other ways of creating metal conversion coating, including anodization of other metals (such as titanium, zinc, magnesium, niobium, zirconium), reactions with various phosphate compounds, and electroceramic and nanoceramic techniques to name a few. Because all of these techniques involve a reaction with the metal and/or use harsh chemicals, a non-contact method of analyzing the coating is preferred and often necessary. For example, aluminum anodization forms a chemically-formed hard layer of oxide that protects the surface from further corrosion and (dependent on the method used) provides what can become a AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 2 OF 10

3 decorative surface. With traditional anodizing, the coating is visibly black but not optically opaque in the mid-ir. The color is imparted by the chemical composition of the electrolytic bath, often based on a chromate conversion process. The chemicals in these electrolytic baths can be very toxic. Therefore, the ability to analyze the parts to insure adequate coatings without contacting them is very important when they are removed from the bath. The coating can be colorless or colored, and in the latter case chemical agents or dyes are added to provide a color that is decorative as well as protective. All of these surfaces (whether black, colored or colorless) can be clearly identified as indicated in Figure 2 by the distinctive characteristics of their LaserScan IR spectra collected from a standoff distance of 12 in. Figure 2. LaserScan QCL measurements of three different anodized aluminum surface: traditional black anodized (black), blue-colored anodized (blue) and hard anodized aluminum (red). Decorative surfaces are often known by various commercial names such as Alodine and Iridite conversion coatings. They range from colorless (natural color of polished or brushed aluminum) to simple colors like red, green, blue, yellow, purple, etc. The color density and clarity are dependent on exposure time (hence coating thickness), which can be measured and controlled remotely using the LaserScan as samples are removed from the bath, as shown in Figure 3. For this application the LaserScan serves as a process tool and a quality control tool for post production inspection, where an objective method of measurement of coating thickness is provided. This can also be implemented above a moving belt where coated parts are inspected. Figure 3. LaserScan QCL measurements of different thicknesses (as a result of exposure time) of gold anodizing (a.k.a., gold Iridite or Alodine) based on a chromate conversion process. AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 3 OF 10

4 An extended version of the anodizing process known as an Anodized Hard Coat is formed under more extreme conditions than traditional anodizing and it forms a dense, thin film similar to sapphire. These surfaces can be colored or transparent, and it is common to use them for engineering applications where the parts can take on the appearance of the base metal and that require an extremely hard and corrosion resistant layer. Hard anodized parts are used for chemical process and automotive applications where chemically reactive or corrosive fluids (to aluminum) are involved. These coatings are easily characterized by IR measurements as shown in Figure 4, and in stand-off mode can be identified on fully manufactured and fabricated products. Figure 4. LaserScan QCL measurements of the surface of a complex machined part that is hard anodized to provide resistance to a chemically aggressive liquid. The external appearance of the hard anodized part used for monitoring a corrosive fluid is identical to a traditional anodized part. It is essential that the correct coating is used, for the accidental use of the incorrect coating could cause the part to catastrophically fail. Such parts are used on aircraft, and the LaserScan can be used as an inspection and a service tool. The QCL spectra shown in Figure 4 provide the responses from different locations on the part, including areas that are not easily reached. Distinct spectral features can be seen. It is especially important that the coating is applied to these hard to reach surfaces as well as exposed external surfaces. So the key advantage of the LaserScan is to avoid contact with corrosive fluids as well as interrogate hard to reach surfaces. 2. Monitoring the Curing or Drying of Decorative/Protective Paints Probably the most common forms of coatings are painted or laminated surfaces where a resin (uncured, partially cured or cured) is deposited on to the surface from a solvent or carrier fluid. As discussed below the ability to monitor the drying and/or curing process is critical to the successful coating process and this monitoring is greatly facilitated with a non-contact measurement as provided by the LaserScan. In addition, the portability of the LaserScan makes it a unique tool for monitoring coatings in the production of transportation vehicles where the monitoring needs to be done either at-line or in-line during the production process. Like the previous application, painted surfaces are applied for different reasons, from a simple decorative application to a protective surface. Protective surfaces include corrosion and safety protection, such as where the coating serves as an insulating barrier as in electrical applications. Also, as in the previous application it is important to characterize the surface to ensure the correct paint is being AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 4 OF 10

5 used, especially for critical applications. Figure 5 shows spectra that illustrate how painted surfaces may be differentiated. The Clear-Coat surface provides a characteristic spectrum of the base acrylic resin. The white pigmented acrylic has some spectral features that are in common with the clear acrylic and other bands associated with the pigment base which contains inorganic compounds. Characterizing pigments is important as these materials can define key properties of the finish, including color fastness and weathering resistance. The third example is a reinforced resin paint used as an under vehicle protective coating for water, rust and abrasion protection. The paint is a urethane resin that includes an isocyanate pre-polymer as well a carbon based filler material. Figure 5. LaserScan QCL measurements of three different painted surfaces: a transparent, Clear-Coat acrylic paint (blue), a pigmented acrylic automotive paint (red) and a carbon pigmented urethane-based paint (black) One of the most important applications of the LaserScan measurement of a painted surface is the ability to monitor the drying and/or curing process. Most paints have a solvent carrier which may be aqueous or organic that provides the mobility for the application of the coating via manual brushing or spraying. For the successful application of paint it is important to determine when the paint is dry. Touching a painted surface prior to it being completely dry can mar or permanently destroy the appearance. Knowledge of complete drying is also important for multi-layer painting as in the case of the painting of an automobile on an assembly line. The non-contact measurement of paint drying is illustrated in Figure 6. In this example the natural drying is monitored for Clear-Coat over approximately a 1 hour period. Curing Drying Figure 6. Non-contact LaserScan QCL monitoring of paint drying and curing (Clear-Coat). AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 5 OF 10

6 As indicated, the drying of the paint can take place in two or more stages: either simple evaporation of the solvent to leave a residual coating of a cured resin, or a coating of the uncured or partially cured resin such as an acrylic or a urethane pre-polymer that undergoes a second curing stage to leave hardened surface. These can be defined as the Drying phase (wet to tacky surface) and the Curing phase (tacky to hard dry surface). The Drying phase can be quick and may only take minutes, whereas the Curing phase might take up to an hour (or even days) to complete. For paint resins such as acrylics, the Curing may be air initiated (by oxygen), and the process can be accelerated by heating the painted surface. The ability to monitor these two phases with the LaserScan is shown in Figure 7, where the spectra from time-sequential measurements are referenced to the first sample at time = 0. The phases can be clearly discriminated as the spectra change from the Drying to the Curing shape in the figure. Curing Drying Figure 7. Non-contact LaserScan QCL monitoring of paint drying and curing (Clear-Coat) with a differential output (sample spectrum referenced to first sample at time = 0). The LaserScan provides a convenient, safe and objective method for monitoring the complete drying and curing of a painted surface without the risk of blemish or damage. As a painted surface builds up, underlying layers may not be completely dry or cured. If atmospheric contact is important in the curing stage, there is a risk of incomplete or prolonged curing if a second coat is applied too early. Figure 8 is the output of just the curing phase of the water vapor initiated cure of an under-vehicle chassis paint. This paint acts as a protective barrier for the underside of the vehicle and a full cure of this finish is essential to get the full strength of the carbon reinforced urethane resin. Curing Figure 8. Non-contact LaserScan QCL cure monitoring of a carbon reinforced urethane-based paint. AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 6 OF 10

7 The convergence of the last few spectra in Figure 8 at a final time of approx. 1.5 hours from the original application of the paint indicates the completion of the curing process. Note that this application exemplifies several of the key benefits of the LaserScan technology, such as portability that permits measurements of the underside of a vehicle and high (yet eye safe) optical output that provides measurements of the highly IR absorbing carbon reinforced urethane paint resin surfaces. The monitoring of the paint drying/curing process is important in many industries, in particular the automotive industry, where an unblemished final finish is essential. Automobiles are painted with many layers, starting from a basic primer and corrosion inhibitors, through various colored and pigmented gloss surfaces, to the final protective Clear-Coat surface. It is essential that the paint is dry and cured at each of these steps, a process that may be accelerated by heat lamps. Monitoring the process at all stages is critical for a good final finish. The LaserScan works very well for this application, as illustrated in Figure 6 Figure 8, and, due to its portability and flexible non-contact configuration, it can be integrated into a vehicle assembly line for use as a quality and process control tool. 3. Monitoring Adhesive Curing Specialized resins are also used as adhesive coatings, and such materials are used to bond surfaces together. Precise non-contact monitoring of the curing of these adhesives in real time at the product site is a key requirement met by the LaserScan. For this application, the use of at-line witness surfaces may be required for resins such as epoxies that are used for aircraft construction. Red: Blue: 0 min 17 min Brown: 48 min Figure 9. Non-contact LaserScan QCL cure monitoring of an epoxy resin adhesive. The strength of an adhesive bond is typically controlled by the curing process, the degree of curing (and cross-linking), and the nature of the chemical bonds formed. Common materials used for these applications are urethanes (and urethane pre-polymers), moisture cured cyanoacryates ( Super Glue ), epoxy resins, and silicones. In all cases a range of formulations exists that defines the strength and the adhesive properties from the coating. Often heat and pressure are applied to form the final bond. An example where both are applied is in the construction of advanced aircraft wing and fuselage surfaces that are used on modern high performance aircraft. In such applications both strength and high adhesion are essential. Note that the curing of an adhesive can take many hours, and even if the resin appears externally to have hardened after a designated time, it can take 3 or 4 times longer for a full AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 7 OF 10

8 cure. Figure 9 provides an example of the cure of a 30 minute epoxy, which still shows spectral indicators of cure after 150 minutes. The use of the LaserScan provides non-contact, real-time monitoring of the curing process. In the example shown there is some convergence of the spectra, but it is likely that more time is required for the adhesive bond to obtain full strength. 4. Non-contact Real Time Characterization of Protective Safety Coatings The ability to identify and differentiate coatings used for packaging food products is essential for food safety reasons. These coating are applied during the production of the base metal material, during can fabrication and the final sealing process. Monitoring the coating process and confirming that the proper coating is applied to each part must be done on moving production lines so a fast non-contact method is needed. LaserScan can meet such a requirement. A classic example is the coatings applied to steel and aluminum surfaces that are used to make food canning products. Ranging from soda beverages to canned tomatoes, the construction of the can and the coatings applied are essential to prevent corrosion. Soda products and fruits like tomatoes are acidic and extremely corrosive to both steel and aluminum. However, these metals are traditionally used for canning applications and the internal surfaces are coated to protect both the metal and the food or beverage products. The choice of coating can differ widely and is dictated by the product (to be canned) and the level of protection needed. Figure 10 shows non-contact LaserScan measurements of the coatings on a metal can surface used for meat products. The figure shows the measurements on the top and bottom of the cans on their interior and exterior surfaces. Bottom Top Figure 10. Non-contact LaserScan QCL measurements of coatings on iron surfaces used for a meat product. Materials used for bottom and top of cans, interior (blue) and exterior (red). In the example illustrated, the top features a reinforced coating to support a pull-tag style top (note the higher contrast spectrum for the top coatings). If wrongly applied, or if the incorrect material is used, food/beverage spoilage eventually can occur. If the correct material is not used for the top, then the pull-tag may not be sufficiently strong and leakage from the top can occur. The application of the LaserScan can be considered in a non-contact mode in a production line for preparation of the coated steel (or aluminum), the can fabrication, and the final canning process. This can confirm that the correct coating is being used. The LaserScan can be applied as a monitoring quality tool on a moving line in real time. Therefore this is an attractive application for both metal companies who make cans, and for food and packaging companies making the final canned products. AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 8 OF 10

9 Summary of the Value Proposition of LaserScan The examples described above are just a few of the many applications for LaserScan in the coatings industry. Coatings are very important in most products. They are important for protection of surfaces, for maintaining the appearance of decorative surfaces, for preventing corrosion, for making surfaces safe (for food, beverage and medical applications), and for holding things together. The LaserScan is an important tool for monitoring the application of coatings, confirming that the correct coating is being used and for confirming the quality of an applied coating. The key advantages of using LaserScan in all of these applications are summarized below. Monitor Drying/Curing Processes: The LaserScan is very useful for non-contact monitoring of the curing process for paints and other liquid-based coating materials. The value is realized in paint application processes, such as in the automotive industry, where multiple layers of paint are applied and where the appearance of the final finish is critical for the sales value of the manufactured product. The noncontact measurement of paint drying/curing can be implemented in real-time on the manufacturing line. The monitoring process can help improve quality and speed up the manufacturing process by precisely determining when drying/curing is complete. Similar monitoring can be applied to resin curing for adhesives, where bond adhesion is critical. For this application, the use of at-line witness surfaces may be required for resins such as epoxies that are used for aircraft construction. The value proposition is improved production quality and confirmation that the strong mechanical boding of adhesives consistently meets product specifications. Material Characterization: The ability to identify a material prior to or during the application of a coating in a manufacturing process is critical to the quality of a final product. Real time validation that the correct coating is being applied in the correct order is imperative for quality and performance of the final product. LaserScan provides such real time characterization on-line or in-line without contact during the manufacturing process. This is related to the first application where multi-layer coatings (paints) are applied. The same applies to the application of food safety coatings. In the simple example application discussed, it was shown that the top and bottom of food cans are different and it is important to get the correct coatings in the right place. Failure to do this can result in a bad batch of product, and more seriously it can result in a batch of product that might give way to bacterial growth because of a failure to completely seal the product. There are also medical device applications where product sterility requires that the correct surface coatings have been applied to all surfaces that will eventually interface with human tissue. The value proposition is real-time monitoring for product quality and the assurance that correct materials are being used for critical manufacturing where product safety is on the line. Surfaces in Hazardous Locations: Examples of hazardous locations include (1) high temperature zones as encountered with catalytic surfaces in industrial processes or for exhaust systems on engines/vehicles, (2) the top of a high voltage electrical transformer where a paint or resin surface may be required to provide electrical insulation, or (3) coatings used in radioactive or chemical hazard locations that need to be monitored remotely and safely. In these cases the value proposition of using LaserScan is its ability to measure the integrity of the coating with a remote and noncontact method. In equipment used outdoors, the integrity may be compromised by weathering of the coating surface. If the integrity of the coating is critical to the performance of a piece of equipment, its continued operation, or worker safety, the LaserScan becomes a safety and system integrity service tool. AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 9 OF 10

10 Moving Surfaces: Several examples have been cited in this document that implies the measurement of coatings on moving surfaces. This ranges from the ability to monitor the coating of metal surfaces used in food canning applications to the measurement of paint drying on a moving automobile assembly line. LaserScan is the only infrared instrument that can make these measurements without contact. Also important is its ability, due to the high output of the laser, to make these measurements rapidly in real time consistent with process parameters. The speed and energy of the system will be particularly important for applications that involve coatings on papers or fabrics such as water proofing or fireproofing agents. The value proposition here is the ability to monitor a coating or a coating process on moving surface by a noncontact method that also provides for high-speed spectrum acquisition with a high energy light source. AN-N03-01: MEASUREMENTS OF COATINGS BY QCL SPECTROSCOPY PAGE 10 OF 10