Ocean Insight’s Liquid Transmission Measurement System (LTMS) is a spectroscopic-based instrument designed for real-time, automated concentration measurements in anodization dyes and other liquids in industrial environments.
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Can Spectroscopy Assess Paint Samples Quickly and Easily?
In this application to analyze paint formulations, an Ocean Optics spectrometer setup measures the reflectance of painted surfaces.
Paint and coating materials comprise components including pigments, dyes, fillers and binders, with careful mixing of each element contributing to product quality and performance. The ability to evaluate different paint formulations and UV-protective coatings within minutes versus hours can make a big difference in development time and cost.
Experimental Setup
For fast, convenient assessment of paint samples on flat surfaces and substrates, we used an extended-range (UV-Visible-NIR) spectrometer with a broadband light source, integrating sphere for reflectance, and diffuse reflectance standard. This modular setup allowed us to quickly characterize the diffuse, specular and total reflectance characteristics of each sample.
Equipment Used
- Ocean HDX-XR extended range spectrometer (200-1100 nm)
- INTSMA-025 customer-changeable entrance slit
- DH-2000-BAL deuterium-tungsten halogen light source (200-2500 nm)
- ISP-50-8-R-GT integrating sphere with gloss trap
- (2) QP600-1-XSR 600 µm extreme solarization-resistance optical fibers
- WS-1-SL Spectralon® diffuse reflectance standard
- STAN-HOLDER reflectance standard holder
Because integrating spheres attenuate light levels significantly during reflectance measurements, we selected the Ocean HDX-XR as our spectrometer, for its great sensitivity from 200-1100 nm. The additional slit provides versatility.
The ISP-50-8-R-GT integrating sphere has a 50 mm sphere diameter and gloss trap to enable diffuse and total reflectance measurements. The 600 um core diameter of the fibers matches the optical needs of the integrating sphere, while the fibers’ extreme solarization-resistant properties mitigate degradation that can occur in fibers in the UV region. The STAN-HOLDER is a convenient accessory that can act as both a reflectance standard holder and a sample stage.
Measurements
We analyzed five samples applied to flat surfaces: three different white paints, a highly reflective UV aluminum strip, and a highly absorptive UV strip.
Samples were measured with an integrating sphere containing a gloss trap, so both total reflectance (gloss trap closed) and diffuse reflectance (gloss trap open) could be measured for samples applied to the middle strip (area) of the flat surfaces. Also, measurements were made for high and low UV reflectivity samples and for the top, middle and bottom strips (no paint applied) of the surfaces. Each sample was measured at three different locations and the spectral data averaged.
All samples were referenced to a Spectralon reflectance standard, which was held in a reflectance standard holder. Solarization-resistant fibers were used to connect the light source and spectrometer to the applicable ports on the integrating sphere. The integrating sphere was then inverted so the port of the sphere could be placed on the sample supported by the reflectance standard holder.
Reflectance was measured for three different locations on each sample with the gloss trap open and then closed (Figure 1). To avoid setup variability due to fiber movement during the measurements, the sphere was lifted slightly to change samples instead of moving the sphere between samples.
Figure 1. The total reflectance spectra here are the average of spectra measured at three locations in the middle strip of a flat surface. Data was also acquired for the paint applied to the areas above and below the middle strip. There may be some error in the reflectance data associated with a missing gloss trap piece for the integrating sphere, which we addressed by slightly modifying the accessory.
Why Paint Quality Matters
There’s a lot more to paint than how fast it dries or what a certain color will look like in your living room. For example, paint has additives designed to absorb UV wavelengths, helping to prevent fading and degradation, and even providing disinfection properties. With a broadband spectral system helping paint researchers to test new formulations more efficiently, manufacturers can produce higher quality, longer-lasting and safer paints.
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Are These Sneakers Real or Fake?
Counterfeit goods cost businesses and consumers billions of dollars annually, with knock-offs increasingly difficult to identify. Spectroscopy can authenticate products by identifying embedded markers and substandard materials.
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Example Reflectance Measurement Setup
In this tech tip, we review the components you’ll need – spectrometer, light source, sampling optics — to make reflectance and color measurements. Configure components to optimize your setup for myriad applications.
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The Importance of Dynamic Range and Signal to Noise Ratio in Spectrometers
Spectrometer performance criteria can be challenging to interpret, although a common vocabulary can help. In this tech tip, we consider two important yet often misunderstood terms: dynamic range and signal to noise ratio.
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Raman Spectroscopy for Reaction Advancement Monitoring
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Note: The author of this application note is Amy J. Ray Bauer, Ph.D., Principal Applications Scientist.
Epoxy is a general term referring to epoxy resins, both the individual components and the cured results. Epoxy resins are a family of reactive polymer precursors and polymers that include epoxide groups. These resins are used in an extensive array of consumer and engineering applications because of their durability, strong adhesion, chemical resistance, and other specialized properties.
Epoxy resins are used in the manufacture of adhesives, plastics, paints, coatings, primers and sealers, flooring and other products and materials that are used in building and construction applications. Most materials known as “structural” or “engineering” adhesives are epoxies. These high-performance adhesives are used to make laminated woods for decks, walls, roofing and other building applications, as well as in other products that require strong bonds to a variety of substrates including concrete and wood. Epoxies can stick to metal, glass, stone, wood and some plastics, and are more heat- and chemical-resistant than most glues.
However, all these properties are contingent on proper proportions and mixing. In this application note, we use a readily available two-part epoxy as a model system for monitoring the kinetics and reaction completion of industrial processes, generally, and adhesive curing, specifically.
Experimental Setup
The two-part epoxy used in this demonstration, Devcon® 15 Minute Epoxy, is composed of bisphenol A diglycidyl ether resin and a polymercaptan/polyamine mixture (2,4,6-tri-(dimethylaminomethyl)-phenol and mercaptan amine blend). Mercaptan is an organo-sulfur compound that donates a thiol group to the curing process. Thiols react readily with the epoxide group and are typically used where heat cures are not possible, or a fast cure is needed.
To monitor the curing process, we used an Ocean Insight QE Pro-Raman+ with a 300 mW, 785 nm laser (LASER-785-LAB-ADJ-FC) and a Raman probe (RIP-RPB-785-FC-SMA) suspended at a fixed distance over the curing epoxy. Ambient light was excluded from the laser-sample interaction region with a black cloth. Raman signals were acquired with 3 s acquisition time and 10x averaging. Signals were acquired on the epoxy samples immediately upon mixing and then in 2-3 minute intervals after that. In some cases, signals were acquired the following day, but in no instance did these signals vary appreciably from those collected at 40-45 minutes after mixing.
Measurements
Two samples of different proportions of resin and hardener were mixed, at the recommended ratio of 1:1 and at a 2:1 hardener-to-resin mixture. Following the mixing of the two components, the curing process was monitored with Raman spectroscopy. The analytical focus was principally on the epoxide and thiol features, which diminish upon curing.
Raman bands corresponding to epoxide vibrational frequencies have been reported previously [1], with 1254 cm-1 assigned to be the breathing mode of the epoxide ring (Figure 1). The intensity of this peak is linearly dependent on the concentration of epoxide groups in the resin mixture [1 and references therein]. The peak at 921 cm-1 is likely the epoxide ring deformation and is much weaker. The Raman peaks at 1114 cm-1, 1186 cm-1 and 1610 cm-1 have been assigned to resin backbone vibrations and do not change in intensity during the curing reaction.
The change of the 1254 cm-1 feature with time can be interpreted as the consumption of the free epoxide groups during the vitrification of the epoxy. The reduction of the intensity of the Raman feature at 2575 cm-1 shows the consumption of the thiol in the hardener as the material reacts. The rate of the initial loss of the S-H feature appears to be a function of the initial concentration of the hardener.
The S-H stretch, related to the presence of thiol, is readily observed at 2575 cm-1. In the case of the measurement of 1:1 hardener-to-resin, the S-H feature is still present when the curing process stops. Note that this sample was still slightly sticky to the touch the next day, far after the 15 minute cure time assured on the package. This result could be due to either the incorrect proportions of the two substances or perhaps inadequate mixing of the two.
In the 2:1 hardener-to-resin sample, the initial peak of the thiol feature is slightly higher than in the 1:1 case (Figure 2). This was not evaluated for quantitative behavior, but the ratios of peak heights seem reasonable given that in the 2:1 case, the hardener accounts for 67% of the total material present (two-thirds) and in the 1:1 case, only 50% (one-half). In this second case, there is not enough resin to react stoichiometrically with the available resin, and there is an appreciable amount of thiol remaining in the product epoxy. It is not surprising that the epoxy did not cure well in this case, either.
The S-H feature (analyzed using peak area from 2487-2639 cm-1) in the case of the 2:1 mix comprises about 1.36x more integrated area than that associated with the 1:1 mix. This agrees with the fact that in the 1:1 mix, there is 50% hardener. In the 2:1, there is 67% hardener (a 1.34x increase).
The 2:1 mixture has an appreciable amount of S-H remaining; 1:1 has less S-H remaining at the end of the cure.
Figure 1. Devcon® 15 Minute Epoxy (1:1 resin to hardener), as a function of cure times (minutes).
Figure 2. Devcon® 15 Minute Epoxy (1:2 resin to hardener), as a function of cure times (minutes).
Kinetics Analysis
A kinetic analysis of the S-H loss is readily performed on the data produced with the Ocean Insight Raman system.
Behavior as a function of time of the S-H feature is shown in Figure 3. A visual comparison of the two data traces (markers) shows that first, there is more S-H at the beginning of the process in the case of the 2:1 hardener-to-resin mix than in the 1:1 case.
Additionally, the data markers show that onset of the reaction is slightly accelerated when excess hardener is present. Fitting the data in the region where S-H reactivity is the fastest (excluding the first three points in the 1:1 case) to a simple exponential with an offset (eq. 1), it is apparent that the kinetics of the bulk of the reaction are the same in both mixtures with τ (1/e half-life) of 4.5 minutes. Finally, it is also clear from the tail of both sets of data (markers) that more unreacted S-H remains in the case where more hardener has been added, which is reasonable because the hardener is the source of the thiol.
Equation 1. y = y0 + A * exp [-(x-x0)]/τ
Figure 3. Integrated areas under the S-H feature in both the described cases; also, the exponential fits to the data.
Summary
This brief application note details the use of Raman spectroscopy to elucidate details of the epoxy curing process in real time. This utility can be extended easily to other types of reactive processes, even those that are faster than this relatively slow epoxy vitrification. Ocean Insight supports a line of Raman spectrometers and lasers that are ideal to perform this type of analysis, and the applications expertise to help you solve similar problems.
Download a PDF version of this application note.
1. Křesálek, V. and H. Vašková, “Raman spectroscopy of epoxy resin crosslinking,” Recent Researches in Automatic Control, Conference Paper, May 2011.
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NIR Spectroscopy for Photovoltaics Materials Analysis
Compact, modular spectrometers are attractive tools for photovoltaic materials research and production. Applications include evaluation of solar cell materials and quality control in solar cell module production.
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Understanding Color Measurements
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Laser Induced Breakdown Spectroscopy (LIBS) Podcast: Listen to Expert Insights
The Applied Spectral Knowledge Podcast is a forum for tips and insights on spectral sensing challenges. In the latest episode of our series, Ocean Insight experts offer some valuable advice on LIBS technology.
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