We present a novel approach that introduces mass as a fundamental metric for characterizing the size and structure of pigments and nanoparticles. This offers the potential to establish a new standard for particle size determination. Leveraging measurement techniques developed in aerosol sciences, we apply them to uniquely analyze pigment particles in an application-oriented manner.

Engineered nanomaterials are produced at a massive scale, ranging from 10³ to 10⁷ tons per year. The physical properties of individual particles, aggregates, fibers, etc., profoundly influence the application performance of these materials. Traditional methods for characterizing physical dimensions are either expensive and time-consuming (e.g., TEM imaging) or limited to equivalent diameters. For instance, laser diffraction size analysis requires the refractive indices for a wide range of color pigments and struggles to accurately measure nonspherical or aggregated particles.

Moreover, various material properties are mass-based metrics, facilitating the calculation of other essential values. For instance, using skeletal density (g/cm³) or specific surface area (m²/g), one can derive absolute particle volume or surface area and key performance indicators like the number of particles per gram or particles per dollar. Additionally, we measure the mobility diameter, enabling the calculation of particle bulk densities, fractality, porosities, and more.

To introduce the potential of this measurement concept, we present the results of three commercially available iron red pigments, three carbon blacks, and a coated iron yellow pigment. First, some background information is provided on the relation between particle mass and diameter, collectively defining the particle structure, and then the experimental setup is briefly discussed.

Background: Mass Plus Diameter Equals Structure

The structure of a particle population is encoded in the relation between mass and diameter of the individual particles. The scaling relation between particle mass (m) and the particle diameter (d) can be expressed by Equation (1):

with a constant (k) and the fractal index (fi). While a fractal index of 3 indicates scaling of a solid object, a fractal index below 3 (and above 2, which would indicate plate-like scaling) signifies the scaling of an increasingly more porous material (see Figure A1). In the context of color pigments, the fractal index describes the branching of the aggregates itself and varies for every material. To visualize that, we examine the effective density, i.e., the density of a particle with mass m and diameter d assuming sphericity. Inserting Equation (1) for the diameter gives Equation (2).

The scaling for three structurally different particle types is illustrated in Figure 1. Solid particles with a fractal index of 3 exhibit a constant effective density (in this special case, equal to the skeletal density). The effective densities of aggregated particles, here, as the example shows, columns and spheres (Figure 1), decreases with increasing mass, indicating a fractal index below 3. The stronger the decrease, the lower the fractal index and the higher the aggregation level of the particles.

FIGURE 1 Particle effective density as a function of particle mass for three structurally different particle types: Solid spheres (top), aggregated columns (middle), and aggregated spheres (bottom). Gray circles indicate a diameter estimate for visual guidance. Note: Same skeletal density is assumed. Particle mass axis is logarithmic.

The electrical mobility analysis is used to measure the geometric dimensions of a particle. Unlike other diameter estimates, the mobility diameter is not affected by the material density and refractive index. The mass is an intrinsic property of a particle and, unlike any equivalent diameter, it does not require any prior assumption and is independent of the measurement method. Therefore, the particle mass is well-suited to serve as a standard for the particle size.

Methods

We utilized a PowMaster system (femtoG AG, Switzerland), a combination of dedicated particle dispersion systems and the newly developed Mass & Mobility Aerosol Spectrometer (M2AS, Cambustion Ltd., UK), which enables the rapid measurement of size distributions for both absolute particle mass and a diameter (electrical mobility) within 5 to 15 minutes.

Powder analysis was initiated by an aerosolization of the powder material. Here we suspended 0.2 g of a pigment in 100 mL ultrapure water. The particles were deagglomerated by a finger-type ultrasound probe until an energy input of 100 J/ml was reached. Subsequently, the suspension was nebulized and the droplets were dried to retrieve the initial pigment particles. By producing small droplets and working with dilute suspensions, re-agglomeration during the drying process could be prevented.

Assuming that paint chemistry, film thickness, and application techniques are matched to the intended service environment, the degree of cure is the single most important characteristic required for a painted system to perform well. During the coating development phase, proper curing parameters are determined through a variety of sophisticated techniques monitoring the glass transition temperature. Thermomechanical Analysis (ASTM E1545) and/or Differential Scanning Calorimetry/Differential Thermal Analysis (E1356) are the usual methods for dissecting the degree of cure on a molecular level to inform the needed curing temperature and time information. Unfortunately, each of these techniques takes a fair amount of time that is inconsequential in a laboratory environment, but impractical for the fastpaced need of a production setting.

Testing has been developed to harness the laboratory curing information by correlating the glass transition temperature with a solvent (resistance) rub test. The standard for this testing is ASTM D5402 Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs.

This is a quick way for a production quality lab to simulate the degree of cure by measuring the number of double solvent rubs it takes to damage the surface. This manual method is quick, surprisingly simple, and exhibits a 95% confidence limit for reproducibility between round robin operators. Coating thickness is also an important variable. Thicker coatings provide higher rub numbers because it takes longer to expose the substrate. Very thin films with pencil hardness of 6H fall a little outside of the 95% confidence limit, at 90%. The film thickness range required number of rubs and type of solvent are listed on the Technical Data Sheet.

Historically, many quick property tests were developed within the coil coating industry where 40,000+ pound coils take only minutes to run through the line, with mere seconds of curing time. This production cycle requires the absolute minimal test completion time. In 1980, the test was codified as National Coil Coating Association No. 11-18. Revised in June 1996, it became NCCA Technical Bulletin 4.2.11 Test Method for Evaluation of Solvent Resistance by Solvent Rub. In 1993, ASTM D5402 was written for use in the laboratory, the field, or the factory.

Coatings that chemically change during the curing process become more resistant to solvents as they cure. They reach specific levels of solvent resistance prior to topcoating or being placed into service. Epoxies, vinyl esters, polyesters, alkyds, and urethanes are typical chemistries that fit this paradigm. In this standard, no solvent is specified. Rather, the producer and user agree on the solvent and the number of double rubs based on formulation chemistry.

ASTM Test Method D4752 Standard Practice for Measuring MEK Resistance of Ethyl Silicate (Inorganics) Zinc-Rich Primers by Solvent Rub is the preferred solvent rub method for ethyl silicate zinc-rich primers as the cure is a reaction with moisture, providing a binder. In this case, MEK is the specified solvent correlating very well with the chemical changes occurring during cure as identified by diffuse reflectance infrared spectroscopy.

The test is remarkably simple. First, the dry film thickness is verified to be within the required range. Cheesecloth is saturated with the solvent of choice. An index finger is placed in the center of the cloth, with the thumb and remaining fingers holding the cloth in place. The index finger is held at an approximately 43° angle as the cloth is rubbed onto the paint, first away from the operator, then back over the same line. The complete forward and back motion takes about one second. The testing continues, adding solvent as needed without lifting the finger off the substrate until either the required double rubs are achieved or the substrate surface is exposed. If the substrate is exposed before the required number of rubs, this is reported as a failure.

In addition to reporting the number of double rubs, sometimes a rating system is used to further describe any damage to the surface, such as burnishing, marring, or depressions in the film, particularly in zinc-rich systems.

In 2013, a mechanical rubbing machine was developed and codified through ASTM D7835/D7835M Standard Test Method for Determining the Solvent Resistance of an Organic Coating using a Mechanical Rubbing Machine. It provides consistent stroke length, rate, pressure, and contact area not subject to variables such as human fatigue and tabulates the number of rubs achieved.

Whether conducted manually or by machine, this simple test is a rapid, effective way to ascertain that the required level of cure is achieved for optimal service life.

Cynthia A. Gosselin, Ph.D.,Ěýis director at The ChemQuest Group,Ěý. Email: cgosselin@chemquest.com.

One of the more discussed paint adhesion and hardness tests involves “scratching” paint off a substrate. Not only does this type of testing depict a qualitative level of adhesion, but in many instances, it also provides a relative idea of degree of cure and hardness of the organic coating.

The genesis of scratch testing is found in the very old (pre-1960) National Coil Coating Testing Manual, Appendix A–Test Method NMCTP–17-92. Scratching could be preceded by environmental testing or conducted immediately after painting. The paint surface was scratched through the paint film with the edge of a nickel—hence also known as the “Nickel Adhesion Test.”

Every paint line superintendent had their favorite nickel in their pocket that, according to their experience, produced consistent results. A photographic standard was used to determine the quality of adhesion as depicted by the thickness of the paint removal line. (Figure 1) The thicker the substrate exposed within the scratch line, the poorer the adhesion. Acceptable adhesion levels were determined between the paint vendor, coated product producer, and the final customer to ensure success in the field.

FIGURE 1 Nickel adhesion photographic standard.

While the Nickel Adhesion Test provided surprisingly consistent relative results, more sophisticated and reliable quick adhesion and hardness testing was needed. In 1974, ASTM published ASTM D3363-74 Standard Test Method for Film Hardness by Pencil Test. This test method was a quick, inexpensive determination of film hardness. Relative adhesion and degree of cure could be inferred during testing. In 1980, the National Coil Coating Association issued an update of the ASTM specification as Technical Bulletin 4.2.5 (NCCA No. 11-12). The next iteration of this specification took place in June 1996 when a revised version was published in a compilation of testing for the coil coating industry. This revision was heavily influenced by the methodology utilized by ASTM.

Currently, ASTM D3363-74 (22) is an updated staple within myriad testing protocols included in ASTM Volume 06.01, routinely undergoing the rigorous five-year review process.

Pencil hardness testing involves holding a variety of flattened pencil leads of differing hardness at a 45° angle. The leads are pushed into the paint, away from the operator, using a 6.5mm (0.25″) stroke.

The process starts with the hardest pencil and continues down the hardness scale to the point where the lead will not cut into the film (hardness) or will not mar the film (scratch hardness). Any defacement of the film other than a gouge (cut) is considered a scratch. Both endpoints are reported to provide a full view of the paint film.

Calibrated pencil drawing leads or equivalent calibrated wood pencils are used. These leads meet the following hardness scale, with the difference between two adjacent leads considered as one unit of hardness.

Sometimes the point at which gouging occurs points to the relative degree of cure, which can affect hardness. High-reflectance white paint is one such product. The system required an H-pencil standard for the paint to adhere without incurring damage during the stamping process. Anything less results in paint peeling in the die, even if volume solids, paint film, color, reflectance, and crosshatch adhesion are perfect. Using the strict H-pencil criteria with mutually calibrated leads allows for the verification of enough cure to withstand the stamping process and a significant decrease in rejected material.

However, critics of this type of human/manual test are always looking for a combination adhesion/hardness test that can be perfectly calibrated and demonstrate excellent precision, repeatability, and reliability. There have been many attempts, but nothing comes close. Often alternative testing has proven too complicated and time-consuming, or it requires expensive, hard-to-calibrate equipment. That may be fine for a research laboratory, but for production labs and field evaluation, it is not practical.

To determine just how good (or bad) this test is, results of precision testing were added to the ASTM standard. A detailed interlaboratory study was conducted, analyzing data both within and between different laboratories. The study found that an interlaboratory standard deviation was 0.52 and 0.61 between laboratories. Based on this study, criteria were developed to judge the acceptability of results at a 95% confidence level. If two operators using the same panels and pencils produce results that differ by more than one pencil unit, the repeatability and reproducibility should be considered suspect.

This surprisingly good result makes pencil hardness one of the quickest and most relevant adhesion/hardness tests within the industry. Yes, it is qualitative and relative (not mathematically absolute), but good enough to provide confidence in the ultimate success of the product in the field.

Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.

By Cynthia A. Gosselin, Ph.D., The ChemQuest Group

This ASTM column is a bit different from previous ones that delved into testing standards and their foibles, histories, how-to instructions, and analytical results. The main theme of this writing is terminology—which, when dissected, leads to exact meaning, precise communication, and a clear understanding of the definitions assigned to various technologies and technical standards.

Each of the more than 140 ASTM Committees has an important standard in the library that is often ignored. This standard governs definitions for each subject category, and yet, it generally goes unnoticed during most Committee Weeks. Often people become aware of this standard when one of their customers questions a specific issue by quoting one of the definitions. Known as the Editorial and Terminology Standards, they are filled with technical, trade, and industry definitions of words found within the technical standards. And, as the Led Zeppelin song states— you must read carefully—“because sometimes words have two meanings.”

For example, ASTM D16 is the primary standard governing the main committee relevant to our industry, ASTM Committee D01 on Paint, Related Coatings, and Materials. This standard is the first one in Volume 06.01. However, due to the diverse and highly technical subject matter associated with paints, coatings, and materials, there are also specialized terminology standards under the jurisdiction of D01. These include:

• D804 Terminology Relating to Pine Chemicals Including Tall Oil and Related Products (Volume 06.03)
• D1695 Terminology of Cellulose and Cellulose Derivatives (Volume 06.03)
• D6440 Terminology Related to Hydrocarbon Resins (Volume 06.01)
• D6488 Terminology Relating to Print Problems (Volume 06.02)
• D7188 Terminology for Printing Inks, Materials and Processes (Volume 06.02)
• E284 Terminology of Appearance (Volume 06.01)

Each of the specialized standards augments the main Committee D01 standards. While the specialized standards may be in separate print volumes, all terms are identified in one place in the online Terminology Dictionary on the ASTM website. The main reason for the specialized standards is to further refine definitions as needed for subject specificity.

The scope description of these critical standards is that of a compilation of terminology relevant to the technical details espoused within the committee or specialized standard subject matter. These definitions identify and precisely define the words used within the context of the science being examined.

For clarity and simplicity, a definition is a prescribed single sentence that can be augmented by discussion notes as needed. However, the goal is to define a term as precisely as possible—defined with as few words as needed for a precise explanation. If a definition gets too wordy, too long, or too detailed, much of the understanding of the term is lost. In fact, a definition with more than one sentence is prohibited by the ASTM Form and Style Manual and is sure to receive a negative vote.

Even so, sometimes even the simplest definitions need a little more context. In that case, a discussion section can be added, which can be as long and as detailed as needed.

A term is included within a terminology standard only after the task group within the responsible subcommittee has reviewed it and reached the consensus that it is either a generally used term (such as in D16) or a descriptive term used in a specific technical area (such as D7188).

The process for publishing a definition begins with the term being voted on within the entire subcommittee. If it passes without any negative votes, it is then balloted again within the main committee. The main committee vote either validates the definition or returns it to the subcommittee for review and revision if there are negative votes. It is important to note that all negatives must be addressed—even those from outside the submitting committee—before final acceptance. Definitions may be identical to those published by other committees or may vary in meaning depending upon the scientific context. The terminology section on the ASTM website provides the location of definitions for every term within all the Editorial and Terminology Standards, highlighting those that have disparate or similar meanings across different technologies.

Some of the ASTM Committees have comparatively fewer Editorial and Terminology Standards than Committee D01, allowing for an easy review of all individual definitions every five years. In Committee A05 on Metallic-Coated Iron and Steel Products, each term also includes the date of original publication. This makes it quite easy to review a term every five years based on its “birthday.”  Because technology changes, manufacturing processes evolve, and steel chemistries are added and subtracted, this makes for a timely modernization of definitions. Note that this committee is also one of the smaller ones with 49 definitions with no additional specialized terminology standards and is considered an ideal case.

Others, such as the D01 Committee Standards on Paint and Coatings, have hundreds of definitions—as represented by more than 600 standards and a total of seven Editorial and Terminology Standards. In this case, practicality demands that definitions tend to be reviewed as technical standards become obsolete, or items within reviewed standards change—as the normal five-year review of hundreds of individual terms is impossible at this point.

New definitions are added as technologies emerge or if new standards contain undefined terms. Sometimes, well-known trade staples such as “pot life” were never defined within the terminology standard (or even within the standard that utilized the term). They were added well after the terms were used for decades, often when someone did a thorough five-year review of one of the associated technical standards or had an argument with a customer. As a result, these huge terminology standards often have legacy terms that have remained even though the definitions have long expired. A thorough review of the D01 Committee standard would take the work of many people with a good depth of understanding of technical standards and practices, along with quite a bit of volunteer time.

To combat the seeming invisibility of Editorial and Terminology Standards, ASTM has taken steps to enhance awareness throughout the organization.

One of those steps is a renewed focus during the prescribed five-year review of all technical standards as they come due to ensure that the Terminology Section within each committee (and thereby each standard) is robust and up to date. This was also codified in the newest ASTM Form and Style Manual. Some older working standards had completely omitted a terminology section in the past and are currently being revised accordingly.

Each standard identifies key terms, which are then incorporated into the main committee standard, lending precision to the wording in each procedure. It is helpful to precisely define the term within the context of the relevant section. More importantly, individual technical standard terminology sections should be mirrored in the Editorial and Terminology Standards. The added advantage is that this focus allows for a practical way to review even the longest list of standing definitions allowing for adjustments, deletions, and additions, hitherto relatively impossible.

Most people are excited to work on technical standards that govern testing, equipment, and scientific analysis. Unfortunately, terminology is usually a stepchild, as wordsmithery, wordplay, and precise definitions are not as “interesting” to those locked into process flow, new material development, or experimental details. Even so, standards are only as good as the words implemented, and testing is based on precisely communicating concepts, making terminology an important part of ASTM.

Full disclosure, I am the chair of both the Committee A05 and D01 Editorial and Terminology Subcommittees (Subcommittees A05.11 and D01.18, respectively) as I have always been interested in describing processes and concepts and precise definitions. We are always looking for volunteer subcommittee participants who are interested in reviewing terms and solidifying and debating the merits of old, new, or revamped definitions. This applies across the board to all 146 committees governing everything from aromatics to zinc.

If you would like to contribute your expertise to a committee, joining is easy. The ASTM website provides a seamless way to connect with the organization and select committees of interest. A nominal fee of $115 per year entitles each member to a free printed or electronic ASTM standards volume of choice, free attendance for the twice-yearly committee meetings, and unlimited networking opportunities in your field or tangential and completely different fields of interest. These are individual memberships, so you can join and represent your company or industry independently.

If you are interested in finding out more information, please feel free to email me with any questions or comments. If you are interested in joining one of the many Main Committees, and hopefully also lend your expertise to one of the Editorial and Terminology subcommittees, please . There you will find all kinds of information about membership and member resources. Looking forward to seeing you at the next Committee Week meeting.

Cynthia A. Gosselin, Ph.D, is director at The ChemQuest Group/ChemQuest Technology Institute/ChemQuest Powder Coating Research;Ěýcgosselin@chemquest.com;Ěý.