Explainer: what is thermal paper?

Thermal paper is widely used for things like receipts and tickets. But to make text appear without using ink, some of these materials rely on toxic chemicals that many countries are starting to phase out. Because of this, scientists are now developing alternatives – for example, a team at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland recently developed a biomass-derived thermal paper that has a much lower toxicity. ¹

So what is thermal paper, why does it sometimes contain harmful bisphenols and what are the alternatives?

What is thermal paper?

‘It’s a type of paper that is really well spread in society where you need something that prints really fast and is easy to handle,’ says Claire Bourmaud, a researcher in the EPFL team. This includes receipts, boarding passes, labels and tickets. Using ink in such scenarios could create problems due to potential stains and smudges, while having to regularly replace ink cartridges would bring additional costs.

Instead, thermal papers contain a heat-activated dye that enables text to appear by changing colour – usually from white to black. The paper consists of several layers: a paper base, a thermal layer containing the dye and colour developer and a protective coating that shields the dye-containing layer against damage from ultraviolet light, moisture and natural oils.

Exposing thermal papers to heat causes the thermal layer to melt, bringing the dye and developer into contact. Most thermal papers use leuco dyes that switch between a colourless and coloured state when exposed to heat or light. Many leuco dyes in thermal papers contain a lactone ring that opens when protonated by the mildly acidic developer. This creates a conjugated π-network that can absorb and transmit visible light.

Why do many thermal papers contain bisphenols?

Bisphenols are used as colour developers in many thermal papers. Bisphenol A (BPA) and bisphenol S (BPS) are the two most prevalent bisphenol compounds currently used in thermal papers.

Because these compounds are derived from crude oil, they are cheap and widely available, explains Manon Rolland, another member of the EPFL team. She adds that they react with dyes efficiently via a well-understood mechanism, and as they are white powders, they don’t leave behind any visible residue on the paper itself.

In 2020, the UK and EU imposed a limit on BPA concentrations in thermal papers of less than 0.02% of the paper’s total weight. However, one study that looked at the impact of the ban in Switzerland, adopted on the back of the EU’s announcement, found that the fraction of thermal papers using BPA dropped from 82% in 2013 to 10% in 2022. However, the use of BPS as an alternative rose from 3% to 19% in the same period, despite it also being banned in thermal papers.²

Gassman explains that customers may also still come into contact with bisphenols if they buy products from countries where bisphenols are not banned or are less regulated. For example, only certain states in the US – Connecticut, Illinois and Washington State – have currently banned the use of BPA in thermal papers, while many countries, including China, Canada and India, still use BPA-containing thermal papers.

Why are bisphenols a problem?

‘The main [issue] is their endocrine disrupting properties,’ says Natalie Gassman, a toxicologist at the University of Alabama at Birmingham in the US. She explains that these compounds bind to oestrogen, testosterone, thyroid and androgen receptors, disrupting regular hormone binding. ‘[This] changes the downstream programming that these hormones are meant to do,’ she adds. Hormone disruption can affect fertility, the immune system, metabolism and cognitive function.

Another issue is that cytochrome P450 enzymes – predominantly found in the liver – oxidise bisphenols to generate quinones. These molecules can directly damage DNA or cause epigenetic changes that affect how much a gene is expressed, says Gassman.

Bisphenols can also generate reactive oxygen species that damage and age mitochondria in cells. Gassman explains that this can either disrupt fatty acid metabolism or encourage the growth of fat cells, leading to these compounds becoming known as ‘obesogens’.

She adds that analogues can act either as agonists – boosting a certain hormone response – or as antagonists. ‘Depending on the [bisphenol] derivative, it’s hard to predict what some of these downstream effects will be,’ explains Gassman. She adds that different companies, industries and countries all use various analogues of BPA, making it hard to study the effects of such compounds.

Who is most affected by the bisphenols in thermal paper?

Most people that come in to contact with bisphenol-containing thermal papers will do so on an irregular basis and at levels that are within daily limits.

However, repeated exposure over long periods can cause such compounds to accumulate in the body to levels above recommended levels of 0.2ng of BPA per kg of body weight per day. As such, cashiers handling receipts, airline workers printing boarding passes or mail workers handling parcels that use such thermal papers are examples of jobs where workers may receive higher doses.

Gassman explains that people in these roles also often use hand lotions, sanitisers and other creams, which can increase the rate of transfer of bisphenols into the skin by up to 100-fold.³

One study in Turkey suggests that on average such workers receive around 10 times the daily limit imposed by the European Food Safety Authority in 2020 , though this is only for a given day rather than a holistic overview of long-term exposure.⁴ The analysis assumed that workers were holding thermal paper receipts containing bisphenols for around 5 seconds at a time, which is roughly the time it takes for such compounds to enter the skin.

What are the alternatives?

While many countries are still using bisphenols, there are alternatives. For example, Pergafast 201 is a now commonly used urea-based alternative that works via the same mechanism as bisphenols, but with fewer risks. Gassman adds that halogenated or highly conjugated derivatives – ideally with a molecular weight over 500 Daltons – are also a potential option, as they won’t absorb through the skin as easily. However, she notes that it will take many years to test and fully understand both the short- and long-term health impacts of such compounds.

Blue4est paper is another option that uses ‘a completely different technology’, says Bourmaud. She explains that small air pockets in a blue top layer of the paper collapse when exposed to heat, revealing a black layer underneath. This eliminates the need for any chemical reactions, instead relying on a purely physical process.

However, she explains that there is no alternative that is non-toxic, bio-based and sustainable. The team at EPFL has tried to overcome these issues by synthesising polymers and bisphenol esters derived from lignin – a structural material found in plant cells.

These compounds still have the necessary phenol group to activate dye molecules but the polymer’s large size and the presence of methoxy substituents next to the phenol groups help reduce the compounds’ toxicities when compared to bisphenols.

‘The main thing that we wanted to do was to find a recipe where we only replaced the toxic bisphenol, but the rest [of the thermal paper] is more or less the same,’ says Bourmaud. ‘We hope that this will facilitate the adoption of our technology because the production facilities won’t have to change the way that they operate.’

‘We have to balance things in our society: we need to adopt more sustainable chemistry to avoid these exposure risks, but then we are going to have to take on the costs of making these substitutions,’ says Gassman.