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The mysterious scent of adhesives

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Researchers succeeded in identifying odourants in acrylic adhesives that can be hazardous for humans

2018-01-05

It is a known fact that adhesives may smell unpleasant. However, as Fraunhofer researchers have recently discovered, this doesn't need to be the case. Through extensive research on acrylic adhesives they were able to identify the substances responsible for the offensive odours. Manufacturers have the opportunity to optimize their production process.

 - The researchers rely on a variety of analysis methods, including gas chromatography, to detect the causes of unpleasant odors in acrylic adhesives.
© Fraunhofer IVV
The researchers rely on a variety of analysis methods, including gas chromatography, to detect the causes of unpleasant odors in acrylic adhesives.

Nowadays almost all products feature adhesives – for example, they are found in windshields, cell phone displays, shoes or carpets. Some adhesives contain solvents that smell unpleasant. However, solvent-free products or those with a low concentration of solvents can also emit a pungent odour. There is as yet very little known about which substances cause the stench and how they affect the human body. The Department of Sensory Analytics at the Fraunhofer Institute for Process Engineering and Packaging IVV is committed to researching this issue. Department head Professor Andrea Büttner and her team have tested four solvent-free acrylic adhesives to find out which components trigger the offensive odours.

Hazardous substances

In their analyses, the researchers were able to identify 27 odourants that were causing the unpleasant smells. These include methyl methacrylate, acetophenone, 1-butanol, 4-methylphenol, phenylacetic acid methyl ester and acetic acid. It was the first time that 20 substances, including the chemical compound borneol, were identified as odour-active compounds in adhesives. This variety of substances was matched by the breadth of odour impressions, which ranged from pungent, fruity and leathery to smoky and moldy. “If a product emits a particularly strong odour, this can indicate the presence of hazardous substances,” says Professor Büttner. Some of the samples contained phenolic compounds suspected of being mutagenic. Professor Büttner sees a clear need for action to optimize the product development of adhesives. “Our analysis shows that a series of substances we found ought to be eliminated, and not just in terms of odour emissions. Strong odours can most definitely cause headaches and dizziness. We should be asking why adhesives smell. The mindset of both the user and the manufacturer needs to change in this regard.”

Different methods

Scientists at Fraunhofer offer manufacturers solutions and targeted strategies to improve adhesives and other everyday non-food products – including paints, binders, furniture, softeners, detergents and plastics – from the point of view of the odourants they contain. “We’ve set ourselves the task of supporting manufacturers in product development, as the methods they employ mean they are often not in a position to know which of the components are causing the odours. This requires special analysis as well as trained test subjects to detect triggers, possible impurities and byproducts arising during the manufacturing process,” emphasizes the food chemist. The results of the research conducted by the scientist and her colleagues provide the basis for psychological and toxicological evaluations of hazardous substances.

For their analyses, the researchers rely on different methods and devices, such as gas chromatography and mass spectrometry; these enable them to carry out simulated exposure studies in real-life situations to detect and quantify the substances being emitted. In other words, the researchers determine how much is being emitted in normal use. In short: the analytical requirements are high. The detection methods must cover as broad a spectrum of chemical substances as possible, since odourants are not a chemically uniform group. Their only commonality is that they are volatile. In addition, the analysis must differentiate between odour-active and -inactive substances. Machine detectors are only partly capable of achieving this, the human nose is still irreplaceable.

Follow your nose

Weekly sensory training sessions are held at Fraunhofer IVV to train test subjects on becoming odour experts. The test subjects are given samples in odourless glass containers. The sensor panel sets characteristic odour attributes for each sample; in a second sensory session, it evaluates them against reference compounds on a predetermined scale. The overall intensity and the personal preference or dislike of an odour impression is evaluated, with the mean values of the evaluations being used to summarize an odour profile.

Philipp Denk researches all types of adhesive components. His current focus is the acrylic adhesive group, following which he will analyze physio tapes, some of which also contain odor-intensive compounds. “A globalized market and an ever-growing e-commerce sector is a major challenge for the official testing authorities – regarding the wide range of products they must test for hazardous components. That’s why we’re developing new technologies in order to support quality control and official entities and allow decentralization of the testing of products for hazardous substances.”