Thermal Paper as a Polarity and Acidity Detector

Co-Authored by Dean J. Campbell,* Bozana Lojpur,* and Rebecca Liu*

*Bradley University, Peoria, Illinois

Thermal paper has been used for many years for printed receipts at many points of sale. Instead of applying ink onto paper during “printing” of the receipts, heat is used to change components already in the paper from colorless to colorful and produce visible markings. It is an inexpensive, effective method of rapid printing, which was developed in the late 1960s and became much more popular in the late 1980s and 1990s when it became more cost-effective and versatile.¹ The ability of thermal paper to change color in response to heat has been used to demonstrate heat produced by the reaction of hydrogen in palladium with oxygen in the air.² Thermal paper also changes color in response to interactions with intermediate polarity solvents and acids, described below.   

Thermal paper consists of five layers: the top protective coat, the thermal layer (that contains all of the components for colorization), the pre coat, the base paper itself, and the back (or bottom) coating of the paper. The color-changing thermal layer is composed of many components including dyes, developers, sensitizers and stabilizers.³ Each component of the paper has a purpose in making any dark marks on thermal paper as legible as possible. During the printing process, the printer sends a pulse of heat onto the paper, causing the acidic developer molecules to donate hydrogen ions to leuco dye molecules which transform chemically, changing from their colorless form to their colored form which is usually black.³ Each color-changing dye molecule in the thermal paper typically contains a lactone ring with one of the carbons in that ring near the ester group also connected to three aromatic rings, such as benzene rings. A good example of this type of dye is crystal violet lactone, see Figure 1. When the dye is protonated by the developer in the thermal paper, the lactone ring opens.⁴ The aromatic rings in the dye and their connecting carbon atom become coplanar, and the formation of this larger delocalized structure enables the dye to absorb visible light and change from colorless to a darker color.

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Figure 1. Structures of (LEFT) colorless Crystal Violet lactone and (RIGHT) the darkly-colored protonated form of the dye.

Developers are weak acids needed to donate protons to the leuco dye molecules and produce the dark color of the thermal paper. Common developers used in many commercially available thermal papers are bisphenol A (BPA) and bisphenol S (BPS), Figure 2. BPA is mostly used as a monomer in the production of polymers such as epoxy resins and polycarbonate. BPA can interact with estrogen receptors in biochemical pathways, and it has exhibited toxic effects in animal studies at the systemic, developmental, and reproductive levels. This has raised concerns about it being hazardous to both people and the environment.¹ Thermal paper can be a source of BPA exposure. A thermal paper receipt rubbed five times between two fingers and the thumb was found to transfer approximately 0.030 mg of BPA to the skin.¹ The U.S. Environmental Protection Agency explored 19 possible substitutes for BPA, including BPS, as a developer in thermal paper. Unfortunately, that study did not find any of the alternatives to be significantly safer than BPA for use in thermal paper.¹

Figure 2. Structures of (LEFT) bisphenol A and (RIGHT) bisphenol S.

The sensitizers are organic molecules which are mixed with the other components of the paper. Sensitizers are solids but can be easily melted, allowing the dyes and developers to combine, effectively lowering the temperature at which the dyes change color, and allowing lower energy cost printing.¹ Lastly, stabilizers are organic molecules such as phenols which inhibit recrystallization of the dye and developer, stabilizing the printed image and preventing the color from fading.¹ The multifunctional phenols prevent the new resonance structures from closing their rings and turning colorless.⁴ All these components of the thermal layer on paper make it possible to create a printed image on common register receipts. Knowing a little about the chemistry involved in the color change enables demonstrations to be developed around the thermal paper.

Thermal paper can be darkened by interactions with some solvents of intermediate polarity. Water will usually not darken the paper (some older thermal paper that darkens to blue did turn slight blue). Very nonpolar solvents like aliphatic hydrocarbons will also not darken the paper, but solvents of intermediate polarity will darken thermal paper. It seems likely that these intermediate polarity solvents can help to dissolve the solid sensitizer and enable the dye and developer to come together and react. Figure 3 below lists solvents, their dielectric constants,^5-8 and the typical response of thermal paper when a drop of each solvent is placed on thermal paper placed on paper towels. The thermal layer, or reactive layer, where colorization takes place, is on one side of the paper. In these experiments, each drop of solvent was dropped on both the front and back sides of each paper to observe the differences in colorization. The results varied based on the type of paper that was used and on the type of solvent. For example, all the colors that appeared typically dark on the front of the receipt (such as acetic acid) showed some coloration on the back of the receipt as well, although the color was not as dark. Solvents that created a light pigmentation on the front side of the paper were even more difficult to detect on the back side.

Figure 3. Solvents, their dielectric constants, and their interaction with thermal paper

In addition to placing solvent drops on the thermal paper, the solvents or solutions containing these solvents can be “painted” on the paper (e.g., with the broken end of a toothpick) to make marks appear, as shown in the Abstract Figure above. Figure 4 shows sheets of thermal paper (fax paper) that were sprayed with a 70% 2-propanol in water solution onto a hand placed over those paper sheets. The solution darkens the thermal paper only where they come into contact. Note that spraying onto one of the sides of the sheets leaves the sheet white where the 2-propanol solution initially touched but darkens the opposite side where the 2-propanol solution soaked through.

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Figure 4. Spraying a 70% 2-propanol solution onto a hand over sheets of thermal paper darkens the paper where it touches. Note that spraying onto one of the sides of the sheets leaves the sheet white but darkens the opposite side.

Thermal paper also darkens in response to acids. Drops of aqueous strong acids can darken receipts. Drops of three strong acids (hydrochloric acid, nitric acid, and sulfuric acid) and the weak acid acetic acid at 6 M, 3 M, 1 M and 0.1 M concentrations were placed on various thermal paper samples placed on paper towels. It seemed that for most samples, higher strong acid concentrations produced darker greenish spots at faster rates. For the higher concentrations (e.g., 6 M) the spots also turned reddish in the middle. It is possible that the red color could be due to production of some form of the dye with multiple protons added. For these same paper samples, acetic acid did not change the paper color at the concentrations studied. It seems that the weak acid was insufficient to change the color of these paper samples. However, not all paper samples behaved in this manner. One thermal paper sample, an older sample that darkens to blue instead of black when heated or treated with intermediate polarity solvents, did not change color when treated with higher concentrations of strong acids. For this paper, acetic acid at all concentrations and strong acids at 0.1 M concentration produced a blue color. It is possible that the dye in this paper turns colorless when multiple protons are added. One additional observation from the aqueous acid trials: sometimes when the acid drops were added to the paper, bubbles would appear in the aqueous droplets. This has been observed with other paper samples before, and is attributed to calcium carbonate (added to the paper as a whitening agent) reacting with the acid to produce carbon dioxide gas bubbles.⁹

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Figure 5. White thermal paper exposed to hydrogen chloride vapor darkens and turns reddish. Upon removal from the vapor, the dark color shifts to greenish.

Additionally, many thermal paper samples react with hydrogen chloride vapor. Thermal paper samples were placed in a closed jar and exposed to hydrogen chloride vapor produced by a solution of concentrated hydrochloric acid. The paper darkened over the course of about 30 minutes and turned reddish. When the paper was removed from the HCl vapor exposure, it turned dark gray and then added a slight greenish tint. Figure 5 shows these colors, and Video 1 is a time lapse video (15 minutes condensed to 15 seconds) showing the thermal paper color change from reddish to greenish after removal from hydrogen chloride exposure. The old “blue” thermal paper described above did not appear to change color when exposed to the hydrogen chloride vapor.

Video 1. Time lapse of HCl vapor-treated thermal paper changing colors after removal from vapor exposure. ChemDemos YouTube Channel (accessed 8/2/2021)

A drop of concentrated hydrochloric acid placed on a piece of thermal paper in a fume hood produced a darkened streak on the paper extending away from the drop in the direction of the air flow in the hood. The drop location was initially red, but then dark green. This darkened streak resembles chemical emission plumes at larger scales, e.g., point source gas emissions from smokestacks¹⁰ or volcanoes.¹¹ Figure 6 shows a couple of these streaks on thermal paper, one featuring an outline of the United States. Perhaps the color intensity of the dark marks on the paper could be measured with a cell phone camera and correlated to cumulative hydrogen chloride exposure.

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Figure 6. Darkened streaks on thermal paper produced by hydrogen chloride vapor released in a fume hood from a drop of concentrated hydrochloric placed on the paper. Air flow was from left to right.

The response of thermal paper samples to hydrogen chloride vapor resembles that of solid Crystal Violet dye. Crystal Violet lactone has been used in thermal paper to produce a blue color.¹ Two samples of solid Crystal Violet were placed in a closed jar and exposed to hydrogen chloride vapor produced by a solution of concentrated hydrochloric acid. For both cases, the green powder darkened and turned reddish. This resembled the thermal paper darkening and turning reddish when exposed to hydrogen chloride. Repeating the same process with concentrated aqueous ammonia instead of hydrochloric acid, the powder turned to a blue color from its natural green state, and after a few hours, one sample appeared almost white. A third jar was set up using deionized water, and the samples seemed to darken only a bit, from a dark green to very dark green state. The acid/base influence on thermal paper dyes is also reminiscent of Crystal Violet dye decolorization by base in aqueous solution. Video 2 shows this reaction, where 0.00002 M purple solution of Crystal Violet dye faded much more quickly as it reacted with 2 M NaOH solution than it faded as it reacted with 0.02 M NaOH solution.

Video 2. Crystal Violet fading reaction mediated by basic solution. ChemDemos YouTube Channel (accessed 8/2/2021)

These simple experiments can be interconnected to concepts of solvent polarity, acids and bases, and the relationship between molecular structure and light absorption. Given the controversy associated with the use of BPA in thermal paper, these demonstrations obviously provide a springboard for discussions of concepts of toxicology, environmental chemistry, and Green Chemistry. The EPA study of BPA and possible alternatives provides interesting reading with respect to many of these issues.¹ Among the Twelve Principles of Green Chemistry, two that seem quite relevant are Designing Safer Chemicals and Design for Degradation.¹² It would be desirable to have less toxic alternatives to BPA in thermal paper, and it would also be good to have the components of the thermal paper not persist in the environment when it is discarded. Ultimately, a shift to electronic receipts might cut down on the use of the paper receipts, but there are costs to be considered with this approach, too.¹ A demonstration of thermal paper could help to explain the chemistry behind how it works and could lead to discussions about how to make it better and safer.

Safety All solution containers should be clearly labeled. Precautions, including proper personal protective equipment such as goggles and gloves, should be used when working with solvents, acids, bases, and even the thermal paper itself. Avoid spilling the solutions on clothing. Concentrated solutions of hydrochloric acid and ammonia should be handled in a fume hood to prevent their noxious vapors from being inhaled. When placing liquids on the thermal paper, it is recommended that the paper should be placed on paper towels over a surface so that it will not be damaged by any liquids soaking through the paper. Always wash your hands after completing the experiments and demonstrations.

Acknowledgements This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society, the Illinois Space Grant Consortium, and the Bradley University Building Excellent Scientists for Tomorrow (BEST) Program. We would also like to thank Wayne Bosma, Thomas Kahila, and Darian Bays for ideas and donations of thermal paper.

References

1. Bisphenol A Alternatives in Thermal Paper [Online]; U.S. Environmental Protection Agency; 2015. https://www.epa.gov/sites/default/files/2015-08/documents/bpa_final.pdf (accessed Aug 16, 2021).
2. Klotz, E.; Mattson, B. “Hydrogen and Palladium Foil: Two Classroom Demonstrations.” J. Chem. Educ., 2009, 86, 465-469. 
3. Pergafast™ color developer for THERMAL PAPERS. https://www.solenis.com/en/research-and-development/innovations/pergafas...  (accessed Aug 16, 2021).
4. Koester, Vera. How Does Thermal Paper Work? ChemistryViews, 10.1002/chemv.201900038 (accessed Aug 16, 2021).
5. Drake, F. H, Pierce, G. W, Dow, M. T, Measurement of the Dielectric Constant and Index of Refraction of Water and Aqueous Solutions of KCl at High Frequencies, Ads, 1930, 35 (6), 613-622, DOI: https://ui.adsabs.harvard.edu/abs/1930PhRv...35..613D/abstract (accessed Aug 16, 2021).
6. Dielectric Constant. https://macro.lsu.edu/HowTo/solvents/Dielectric%20Constant%20.htm (accessed Aug 16, 2021).
7. Tert-Butyl alcohol. http://www.microkat.gr/msdspd90-99/tert-Butyl%20alcohol.html (accessed Aug 16, 2021).
8. Octanol. http://www.stenutz.eu/chem/solv6.php?name=octanol  (accessed Aug 16, 2021).
9. Schorr, D. K.; Campbell, D. J. “Demonstration Extensions Based on Color-Changing Goldenrod Paper.” J. Chem. Educ., 2019, 96, 308-312. 
10. Campbell, D. J.; Wright, E. A.; Dayisi, M. O.; Hoehn, M. R.; Kennedy, B. F.; Maxfield, B. M. “Classroom Illustrations of Acidic Air Pollution Using Nylon Fabric.” J. Chem. Educ., 2011, 88, 387-391.
11. Campbell, D. J. “Mt. St. Helens 40 Years Later: Getting More from Soda Volcano Demonstrations.” ChemEd Xchange. May 17, 2020. https://www.chemedx.org/article/mt-st-helens-40-years-later-getting-more... (accessed Aug 16, 2021).
12. Compound Interest. The Twelve Principles of Green Chemistry: What it is, & Why it Matters. https://www.compoundchem.com/2015/09/24/green-chemistry/ (accessed Aug 16, 2021).