Deutsches Museum Munich - On the Trail of Plasticizers

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On the Trail of Plasticizers with Microscopy

byZEISS Microscopy 17 December 2024 3 min read

On the Trail of Plasticizers with Microscopy

Christina Elsaesser looks at a box of plastic eyeglasses with a smile and a tear in her eye. The eyeglasses were given to her by the museum’s optical department. Likely from the 1950s, their sense of fashion is questionable, and some are badly damaged and can no longer be used as exhibits.

Elsaesser, one of two employees in the conservation science department at the Deutsches Museum, is investigating why the eyeglasses are in this poor condition and what conclusions can be drawn for preserving similar plastic objects from this period.

Not all plastics are the same. Compared to materials such as wood, metal, or stone, plastic can be one of the most unstable compounds. In total, there are five types that are considered to be at risk.

Christina Elsaesser Conservation science department at the Deutsches Museum

Celluloid – the Dawn of the Plastic Age

One of these endangered plastics is celluloid. Celluloid is a thermoplastic made of cellulose nitrate with the addition of a plasticizer called camphor. Celluloid was one of the first plastics to be mass-produced on an industrial scale. Celluloid made it possible to faithfully imitate natural materials such as ivory, tortoiseshell, and mother-of-pearl, making it possible to create inexpensive yet luxurious objects for everyday use. This development came at a time when natural resources such as ivory were becoming increasingly scarce in the mid-19th century. As demand for ivory increased, militarily organized groups of hunters began to target African elephants at an alarming rate. This relentless hunt quickly turned these majestic creatures, once the undisputed rulers of the wilderness, into an endangered species in a matter of years.

A Closer Look

Elsaesser first records the overall condition of a celluloid eyeglass frame macroscopically and with the help of stereo microscopes such as ZEISS Stemi 508, examines the extent of microcracks and other deterioration signs. The formerly transparent frame shows discoloration caused by yellowing, microcracking, and metal corrosion. The microscope image also shows droplet-shaped deposits on some areas of the frame. Although the migration of plasticizers to the surface of plastics can be associated with the formation of droplets, in the case of celluloid, the presence of droplets is due to the emission of nitrogen oxides that have reacted with moisture to form acids. Such droplets indicate an advanced stage of celluloid deterioration and require conservation measures.

Off into the Cold

Among other things, Elsaesser is experimenting with cold storage and investigating what happens to celluloid samples when they are stored in a refrigerator or freezer, i.e. how the chemical processes are slowed down. A particular challenge lies in exhibits made of a combination of materials, as well as with different structures and thicknesses. Since the freezing process can lead to internal stresses, which can damage objects, samples with various materials and thicknesses were tested. “There is still a lot of research to be done,” Elsaesser explains with a wink.

Each plastic reacts differently to degradation. For example, early Lego bricks were made of cellulose acetate, which is closely related to cellulose nitrate, the material from which celluloid was made. However, cellulose acetate tends to deform as it ages. Such changes in shape are inconvenient when precision is needed to fit one brick on top of another. So in 1962, the company changed the plastic used in Lego bricks to acrylonitrile butadiene styrene (ABS). Because plastics age differently, they all require individual preservation strategies.

One of our core tasks is to preserve the museum objects. In the case of these eyeglass frames, we can draw conclusions about possible hotspots in our collection and research what can be done preventively to slow down the deterioration.

Christina Elsaesser Conservation science department at the Deutsches Museum

Deusches Museum Munich — Aircraft part from a Messerschmitt Me 262

Conservation Science at the Deutsches Museum Munich

One way or another, conservation science at the Deutsches Museum in Munich reveals the most varied and exciting things. Ski boots catch the eye, as do old yellowed keyboards or an aircraft part from a Messerschmitt Me 262. Between 1943 and 1945, 1,433 of these twin-engine aircraft, which is also known as the Schwalbe or Sturmvogel, were built. Using the Axio Scope.A1 microscope, stratigraphy analysis of cross section samples shows how the aircraft was painted, thus giving historians a clue on the amount of paint layers and specific morphological features of each layer. This information is relevant to learn about past aeronautical paint technologies.

Some of the objects come from the museum’s collection, others from collaborations with universities and other museums. However, projects funded by the German Federal Environmental Foundation (DBU) also play a role. For example, a series of studies on cleaning and protective measures were conducted on samples of the Polymethylmethacrylat (PMMA) roof of the Olympic Stadium to learn how to gently preserve such surfaces.

In addition to the preservation of objects, research into historical materials and constructions as well as their alterations are an important part of the investigation work at the Deutsches Museum in Munich. The research reliably provides valuable information for the long-term preservation and care of objects or even buildings.

Celluloid is a thermoplastic made from cellulose nitrate and camphor, and it was one of the first plastics to be mass-produced. It is prone to instability and deterioration, showing signs such as yellowing, microcracking, and droplet formation due to chemical reactions. These characteristics make it particularly vulnerable compared to other materials.
Elsaesser is experimenting with cold storage techniques, examining how storing celluloid samples in refrigerators or freezers affects their chemical processes and slows down deterioration. She is also conducting tests on various materials and thicknesses to understand the impact of freezing on different plastic compositions.
The department conducts research on the preservation of various objects, including analyzing paint layers on historical aircraft and studying cleaning methods for materials like PMMA. This research not only aids in the conservation of museum artifacts but also provides insights into historical materials and construction techniques, enhancing our understanding of past technologies.