Paint and Coatings: How to Sustain Testing

Testing complexity in the paint and coating industry has increased by more strict regulations paired with more complex products. Thus, producers ask for more powerful, sustainable and safe analytical methods. Testing by Vis-NIR spectroscopy is a cost efficient and sustainable alternative to many wet chemical methods. This article describes how Vis-NIR spectroscopy enhances testing procedures for various analyzes during the formulation and production of paint and coatings in an ecological and economic way.

Paint and Coating Industry – A Growing Field

Paints and coatings make life more colorful, easier and safer. The significance of this industry is reflected by the stable growth of its annual production. From 2007 to 2012, the global compound annual growth rate of the coating industry in GDP was 4.3% with a revenue of 110 billion USD for 2012.¹ As a result of a strong demand especially from Asia Pacific, Latin America and Eastern Europe, a worldwide increase in growth to 6% by 2019 is predicted.²

The key reason for the importance of paints and coatings refers to the diversity of application possibilities. Besides being used for beautifying substrates, paints and coatings are also used for protection, as anti-skid surfaces, for insulation or as electrical conductors, to name just a few examples.

To cover this variety, complex formulations are produced, which comprise of four main components: solvents, pigments, binders and additives. The individual amounts of these can differ significantly. For the group of powder coatings, for instance, solvents are not needed (see figure 1).

A product with specific functions according to the user needs is formulated and produced only after the smart combination of these main components. To ensure best results, a fast-performing and reliable quality control is essential. This, however, becomes a lot more challenging as product complexity increases.

Figure 1. Overview of ingredients and possible amounts of sovent-based and powder-based coatings.

Quality Control – Essential at Every Step

To guarantee an optimal end product, quality control has to take place during the entire manufacturing process. The individual production steps have varied requirements regarding analysis. Three main production steps with different analysis requirements can be defined:

High Throughput is Key in the Warehouse

In the warehouse, the incoming raw materials for the production of the inks and paints have to be identified and validated with respect to purity and quality. Analytical methods that help to substitute time-consuming wet chemical procedures should be used preferably in order to meet the demand for high throughput.

Precision and Accuracy for Successful Production

The quantity of several characteristic values is set and the identity of the intermediates is tested during the steps of formulation and production. These control mechanisms are performed atline (next to the process) as well as online (in a process bypass). Accuracy and precision are considered to be vital parameters in these steps. It is necessary for an analytical method to be selected accordingly.

Complex Matrices Demand Powerful Methods

Prior to delivery, the last but nevertheless extremely important step in quality control is the final testing. To guarantee best product performance at the customer site, all relevant properties and the general quality have to be confirmed. As the multiple components of the end products give rise to complex matrices, final testing can turn out to be a majorly ambitious task. Thus, a combination of different analytical instruments is needed in order to obtain reliable results.

Many Parameters – A Challenging Task

Producers have the opportunity to use instrumental as well as wet chemical methods, which are both used during the complete production process. As a reference, the standard operating procedures described in the ASTM methods can be used (see table 1).³ This small section alone reveals that many different analytical instruments and methods are needed, resulting in major training effort for employees.

Wet Chemical Analysis – A Long Tradition

Typical wet chemical analysis methods include, for example, ion chromatography, titration and colorimetry. All of them are highly accurate and precise. They are also well studied and abundantly described in the literature due to their long-standing importance as analytical tools. However, these techniques are generally restricted to specific parameters.

Moreover, additional costs arise for auxiliary chemicals needed for sample preparation and the analysis itself. The regular training of employees and the implementation of the strict safety regulations further add to the bill. Thus, the search for alternative instrumental methods has been trending. Instrumental methods can be performed with only minor training while also being fast and needing less auxiliary chemicals. These benefits are especially true for Vis-NIR spectroscopy.

Table 1. Overview of ASTM Methods for different key parameters and the corresponding methods and instruments.

Vis-NIR spectroscopy – A Simple Solution

The visible-near-infrared (Vis-NIR) technology is considered to be an established analytical method in the pharmaceutical, chemical, and food and feed industries. Its high precision and reliability have resulted in a continuous expansion to several different application fields. The high flexibility of Vis-NIR spectroscopy, which is reflected by its flexible uses in process analysis as an online, inline, offline and atline tool, has also contributed to this expansion.

Functionality of Vis-NIR Spectroscopy

The measurement principle underlying Vis-NIR spectroscopy is based on a nondestructive interaction between matter and light. This interaction is used to determine different chemical and physical parameters within one measurement, which normally takes only a few seconds. The combination of visible and near-infrared light allows determining information related to the color of the sample as well as information linked its chemical side groups.

Economical and Ecological

By detecting multiple parameters in a single measurement, Vis-NIR spectroscopy is able to achieve a high throughput. In addition, it fulfills economical and also ecological criteria. As sample preparation is needed for neither liquid nor solid samples and analyzes are nondestructive, the necessity for expensive auxiliary chemicals is eradicated while, at the same time, costs are reduced and safety is enhanced.

Vis-NIR spectroscopy as a nondestructive, ready-to-use technique shows great promise particularly for the emerging field of powder coatings that are in better compliance with volatile organic compounds (VOC) regulations.

Pigments

The process of beautification is considered to be the most obvious application of paints and coatings. Present in a concentration range of 0–30%, pigments are present among the central ingredients in the majority of paints and coatings. Besides the color effect and intensity, pigments are also responsible for the coating thickness, opacity and the gloss of the coating.

Properties of the pigments are determined by both their chemical structure and their physical structure. These are the crystallinity, the particle distribution and the particle size. DIN 5943 groups pigments according to their chemical structures and their optical effects: The first level of classification groups pigments into organic and inorganic pigments. At the second level, they are further grouped into white pigments, effect pigments, colored pigments and luminous pigments.

In order to obtain the desired properties in the end product, it is important to select the appropriate pigments, and also to get the pigment concentration right. Coating layer thickness and color intensity are both influenced by the total pigment volume concentration (PVC), which relates the volume of pigments to the volume of all solids.³ In addition to the PVC, the relationship between pigment concentration and binder is crucial. This parameter called critical pigment volume concentration (CPVC) is critical to determine, as a shortfall results in a loss of paint stability and strength.³

Thus, determination of the pigment concentration is crucial. The following section presents the analysis by Vis-NIR spectroscopy.

Quantification of Dye Content in Inks

Triphenylmethanphenazin and azo dyes are normally used for ball pen inks. Besides the dye itself, such inks are made up of multiple ingredients which are, for instance, solvents to prevent clogging and surfactants in order to avoid foaming. This complex matrix can make the assessment of the absolute dye content a challenging task.

In application note AN-NIR-026, 20 ink samples were examined with respect to the concentration of blue ink dye in ball pens.⁴ The examined concentration range was between 0.56% and 4.56%. Figure 2a) shows a selection of the spectra. The information extracted from these were linked with the values from the primary method, which resulted in a high correlation as displayed in figure 2b).

Figure 2. a) Exemplary spectra for five different dye concentrations which are typically used for ball pen inks. The visible region 400–800 nm displays a high correlation between the dye quantity and the absorbance of the sample. b) Correlation plot of dye content determined by Vis-NIR spectroscopy and titration. Beside the high correlation of R² = 0.99, the standard error of validation was < 0.1% with two factors used.

Additives

Though the total amount of additives in paint and coating formulations is just minor, additives are the chief means to notably modify properties of paints and coatings for specific applications.

Besides these product-specific additives, which can only be found in small concentrations, three main additives are almost always part of paint and ink formulations: surfactants, paint driers and antioxidants. These are responsible for controlling important product properties such as drying time, processability, durability, brushability and sprayability.

As slight changes in the amount of additives used can bring about considerable change in the behavior of the final product, multiple physical and chemical properties of every additive underlie superior quality standards. Table 2 displays a summary of typical parameters of paint driers and their limits defined by ASTM.

Table 2. Overview of relevant parameters and respective limits for quality control of paint driers according to ASTM.³

Vis-NIR spectroscopy is a valuable tool to support quality control as it is considered to be a method that can be used for determining multiple parameters within one measurement. The use of Vis-NIR spectroscopy for additive analysis is described in the following section. The important additive class of paint driers is employed for this demonstration.

Multiple Parameter Analysis of Paint Driers

Paint driers are an important part of almost every paint or coating product since they are capable of modifying the drying process of paints and coatings.

The drying process can be split into two steps: The first step is the physical process in which the solvent evaporates. The second step is the chemical process were the binder goes through an oxidative crosslinking reaction. Paint driers, which are metal ligand complexes, accelerate significantly the second step and decrease the coating time from multiple hours to just minutes. However, the catalytic function relies strongly on the total metal amount, where even minor shifts to higher or lower concentrations change the behavior to a considerable extent.⁵

Due to this, it is highly important to quantify precisely the concentration during production. Cobalt octoate complexes are the most commonly used paint driers. Formulations made with these additives are studied with respect to the viscosity, metal content and specific gravity, which can be assessed within one measurement with Vis-NIR spectroscopy as demonstrated in AN-NIR-033 (see figure 3).⁶

Figure 3. a) Exemplary spectra for different cobalt octoate paint driers. b) Correlation plot of values for the physical parameter viscosity obtained according to ASTM D1289-85 and measured by Vis-NIR spectroscopy.

Binders

As mentioned in the earlier section, the concentration ratio of binders and pigments (CPVC) can majorly affect the stability of the final coating. Binders are responsible for developing a coating solid and enduring. Binders go through a chemical cross-linking reaction during the drying process. As this fixation of the coating on its substrate is absolutely vital, binders are considered to be the most important ingredient of every paint and coating product and are present in every coating and paint formulation, when compared to pigments and solvents.

The most prominent representatives of binders are polymeric substances such as polyamides, polyester, polyurethanes or epoxy resins (see figure 4). These synthetic resins have completely replaced the previously used natural resins.

The binder affects not only the CPVC, which controls the adhesion and the hardness of the final product, but also the viscosity.⁵ In addition to these direct influences, free amines in qualitatively poor polyurethane binders can negatively affect specific coating procedures. The example in the following section demonstrates how Vis-NIR spectroscopy is capable of supporting the process of quality control by detecting free amines in paints and coatings in a reliable manner.

Figure 4. Chemical structures of two typical binders used in paints and coatings.

Quantification of Amine Value in Dipping Paints

The elctrodeposition process (EPC) for dipping paints is a commonly used method to apply uniform coatings. The key advantage of this process is that the thickness of the coating can be controlled with high precision even for complex structures.

Interfering redox-active molecules can negatively affect the coating result since EPC is an electrochemical process. These interfering molecules can occur as residuals from other coating constituents, for example, free nitrogen as a residual impurity of the used binder.

Application Note AN-NIR-030 describes the use of Vis-NIR spectroscopy for the quantification of the amine number as an indicator for free nitrogen in dipping paints.⁷ In the concentration range of 33.82–48.31 mmol/kg, a correlation of R² = 0.97 with five PLS factors was discovered (see figure 5).

Figure 5. a) Exemplary spectra of the amine number analysis in dipping paints. b) Correlation plot of amine number values determined by Vis-NIR spectroscopy and titration.

Solvents

Solvent-based coatings, whose key constituent is the solvent, are considered to be the most common class of coatings. The two main parameters of the solvent are the solvating power and the evaporation rate, which both affect the wetting behavior and the viscosity of the end product. The solvent additionally influences several aspects of the product-specific behavior. Thus, it needs to be chosen cautiously while taking into account the intended application.

Generally, solvents are grouped according to their chemical properties. Three classes of solvents can be distinguished: oxygenated solvents, hydrocarbon solvents and other solvents, for example, water.³ The latter forms the basis of all water-based coatings, whose significance has majorly increased in the last years since they contain lower levels of volatile organic compounds (VOCs). In some countries and states, the use of VOCs is restricted.

Water-based products are thus becoming more dominant in the market. Vis-NIR spectroscopy as a precise technique is capable of supporting the essential quality control of solvents, specifically for water-based coatings.

Analysis of Cosolvents in Water-Based Paints

As presented in the earlier sections, pigments, binders and additives are vital ingredients of paints. Cosolvents have to be added in order to use these compounds in water-based coatings. Cosolvents such as butyl glycol and propylheptyl alcohol help to solubilize binders, and are also used for rheology control and as plasticizers, respectively. Plasticizers help in determining several coating properties such as dry film appearance or substrate adhesion.⁸

Metrohm Application Note AN-NIR-029 shows the use of Vis-NIR spectroscopy for the quantification of butyl glycol and propylheptyl alcohol content in varied water-based paints.⁹ Prediction models based on spectral data and reference values for each solvent resulted in a high accuracy of prediction by Vis-NIR spectroscopy with a standard error of validation < 1% (see figure 6).

Figure 6. Correlation plots of a) propylheptyl alcohol and b) butyl glycol.

Conclusion

Paints and coatings are used for multiple purposes and need to fulfill high requirements. As a result, these complex products need extensive quality control, which demands the use of refined technologies in order to guarantee accurate and reliable analysis. This need is met by Vis-NIR spectroscopy which fulfills economical and also ecological requirements.

The combined analysis of the visible spectral range and the near-infrared spectral range by Vis-NIR spectroscopy allows determining chemical parameters, for example, the amine number and concentrations of individual constituents, including physical parameters such as the dynamic viscosity and the specific density. Compared to standard methods, Vis-NIR spectroscopy offers the advantage that multiple parameters are assessed simultaneously within one measurement in less than one minute, without causing any damage to the sample.

The applications presented in this article provide but a glimpse of the possibilities offered by Vis-NIR spectroscopy. Because of its key benefits, Vis-NIR spectroscopy can complete and in part replace the standard methods used in the quality control of paints and coatings and is thus expected to gain importance over the next years.

References

[1] Scott Detiveaux, and Charles Bangert, (2016). The State of the Global Coatings Industry : Products Finishing. [online] Pfonline.com.  [Accessed 28^th Sep. 2016].

[2] Ihs.com. (2016). Paint and Coatings Industry Overview - Chemical Economics Handbook (CEH) | IHS Markit. [online] [Accessed 28^th Sep. 2016].

[3] Koleske, J. (2012). Paint and coating testing manual. West Conshohocken, PA: ASTM International.

[4] Partners.metrohm.com. (2016). AN-NIR-026. [online] Available at: https://www.metrohm.com/en_in.html Accessed 28^th Sep. 2016].

[5] Gupta, A. (2013). Determination of optimal quantities of different types of driers for addition in the batches of paint formulation. International Journal of Engineering, Science and Technology, 5(4), pp.1-13.

[6] Partners.metrohm.com. (2016). AN-NIR-033. [online] Available at: https://www.metrohm.com/en_in.html  [Accessed 28^th Sep. 2016].

[7] Partners.metrohm.com. (2016). AN-NIR-030. [online] Available at: https://www.metrohm.com/en_in.html [Accessed 28^th Sep. 2016]

[8] Stoye, D. and Freitag, W. (1998). Paints, coatings, and solvents. Weinheim: Wiley-VCH

[9] Partners.metrohm.com. (2016). AN-NIR-029. [online] Available at: https://www.metrohm.com/en_in.html [Accessed 28^th Sep. 2016].

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.