QC and product screening of ink and paint with NIRS

Nov 27, 2023

Article

We encounter ink and paint on a daily basis. These materials are used for writing and printing, protection against the elements, aesthetic reasons, and so much more. Our historical record exists in large part due to these substances. How are paints and inks manufactured and what quality control (QC) measures are taken? This blog article covers a brief history of this industry, the manufacturing process, and how near-infrared spectroscopy (NIRS) can be used as a multi-parameter QC solution.

What is the difference between paint and ink?

Paint is a substance applied as a liquid or paste that dries into a solid coating. This coating protects and/or adds color to the object or surface to which it has been applied.

Ink is a pigment- or dye-based fluid used for writing, printing, etc.

Origins and brief historical overview of ink and paint

Click on the arrows below to learn more about the origins and history of these two materials.

What is paint made of?

Paint typically consists of pigment, resin, solvent, and additives.

How is paint manufactured?

The production process for paint can be divided into four basic steps. First, the paste is made (Figure 1), then the pigments are milled and dispersed for homogeneity purposes (Figure 2). In the third step, the paste is thinned (Figure 3), and finally the end product is packaged (Figure 4).

However, different ingredients require adaptation of the production process, and the displayed example is a generalization. Coatings are made in a batch process and stringent tests are applied throughout the entire production process to ensure quality.

Quality control and screening of ink and paint

Quality control (QC) of ink and paint can be done easily during all stages of production with near-infrared (NIR) spectroscopy. Using NIRS for QC and screening of ink and paint is more efficient and cost-effective than other analytical methods.

The rest of this blog article covers a brief overview of NIR spectroscopy and its applications for the ink and paint industry. Examples are provided of how ink and paint producers can benefit from using NIRS instruments for assurance and quality control to make high quality products.

NIR spectroscopy – how does it work?

NIR spectroscopy is an analytical technique that uses the interaction between light and matter to determine a sample’s chemical and physical parameters. In this situation, light is described by wavelength or wavenumbers rather than the applied energy. The interaction can be measured, e.g., with the Metrohm DS2500 Liquid Analyzer (Figure 5a), which generates NIR spectra (Figure 5b).

As NIRS is quite sensitive to the presence of some molecular functional groups, it is an ideal technique to quantify many chemical parameters. Non-volatile content, volatile organic compounds, dye content, surfactant content, and moisture can be measured simultaneously in paint or ink. Even the detection of physical parameters such as density and viscosity is possible with NIRS.

A single NIR spectrum contains all of this information, making near-infrared spectroscopy suitable for quick multi-parameter analysis.

Choice of NIRS measuring mode

The NIRS measuring mode depends on the type of sample to be analyzed.

When analyzing liquids, the transmission mode is appropriate (Figure 6). During transmission, the NIR light gets absorbed while traveling through the sample, and unabsorbed light goes directly to the detector.

When analyzing pastes, the transflection mode is preferred (Figure 7). Here, a gold stamp is used as a diffuse reflector. In this case, the NIR light is directed through the paste sample while being absorbed and is reflected by the gold stamp. The reflected NIR light is absorbed further by the sample and finally reaches the detector.

Advantages of using NIRS for QC and screening purposes

Near-infrared spectroscopy has several advantages over other analytical techniques, especially when it comes to quality control and screening.

NIRS is fast, providing results in less than one minute. No sample preparation is required, saving even more time. Measurements are non-destructive, so samples can be reused. No reagents are needed when using NIR spectroscopy for analysis. Not only does this lower the cost per sample, it also makes this technique environmentally friendly.

NIRS is also compliant with international standards such as ASTM E1655: Standard Practices for Infrared Multivariate Quantitative Analysis, making it simpler for industries to adopt it. Finally, NIRS is easy to use and can be operated by non-technical personnel, unlike other more complicated analytical techniques.

Quality control and screening parameters for ink and paint production

Ink and paint products are subjected to several standardized test methods to determine their chemical and physical properties. Such laboratory testing is an indispensable part of research and development and quality control. Table 1 lists the most relevant test parameters for the quality control and screening of ink and paint.

Application examples showing the capability of NIRS to analyze several of these parameters simultaneously from the same sample follow in the next sections.

Parameter	Conventional analysis method
Intrinsic, kinematic viscosity	Viscometry
Moisture	Karl Fischer titration
Surfactant content	Titration
Non-volatile content / Solids content	Loss on drying (LOD)
Dye content / Pigment content	Ashing
Volatile organic compounds (VOC)	Multiple wet chemical methods
Additives and wax in packaging paint	HPLC and GC

Application example: Quality control of ink with NIRS

When performing quality control checks during ink production, the typical parameters measured are the content of dye, diethylene glycol (DEG), surfactant, and water. The dye (e.g., triphenylmethane/phenazine or azo dyes) gives the ink its color. Diethylene glycol is used as a solvent and prevents the ink from drying out. Surfactants control the texture and prevent the ink from foaming.

These parameters are usually monitored using different analytical techniques including ashing, titration, and Karl Fischer titration. Sample preparation is time consuming, and the use of multiple determination methods is cumbersome. By using NIRS, several QC parameters for ink can be measured simultaneously, with results given in less than one minute.

NIR spectra from several ink samples are shown in Figure 8 with the respective correlation diagram for the prediction of dye content in Figure 9. The figures of merit for the NIRS prediction of dye, DEG, surfactant, and water in ink are given in Table 2.

Figures of merit	Dye content	DEG content	Water content	Surfactant content
R²	0.996	0.993	0.991	0.977
Standard Error of Calibration (SEC)	0.0835%	0.5037%	0.5571%	0.0368%
Standard Error of Cross Validation (SECV)	0.0949%	0.5888%	0.9614%	0.1316%

Application example: Quality control of paint driers with NIRS

Paint driers decrease the drying time of paints and affect the gloss and clarity of the coating. In this case, the main QC parameters of interest are the metal content, solid content, viscosity, and specific gravity.

All reference test procedures for these parameters are specified in ASTM procedures—ASTM D2373, ASTM D1644, ASTM D5125, and ASTM D2196. Each standard uses different analytical instruments for measurement: balances and ovens, titrators, hydrometers, and viscometers. In contrast, using NIRS to measure all of these parameters simultaneously saves a significant amount of time and reduces the costs per analysis.

Figures of merit	Cobalt content*	Solids content	Specific gravity	Viscosity
R²	0.999	0.999	0.977	0.999
SEC	0.08%	0.24%	0.003%	9.3 MPa
SECV	0.09%	0.29%	0.003%	10.9 MPa

Summary

NIRS is an excellent choice for quality control and screening of ink and paint along the entire production chain, from raw materials to finished products. Its advantages over other analytical techniques cannot be understated. The ability to perform multi-parameter analysis on a single sample with results in less than one minute saves an incredible amount of time and money.

Conventional laboratory techniques usually require sample preconditioning or chemicals and skilled personnel to perform the analyses. Each parameter is measured on a different instrument, and therefore results take much longer to generate.