User Story

On the Trail of Plasticizers

byZEISS Microscopy 8 April 2024 3 min read

Introduction

Christina Elsaesser looks at a box of plastic glasses frames with a smile and a tear in her eye. The frames 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 three employees in the restoration research department at the Deutsches Museum in Munich, is investigating why the frames are in this poor condition and what conclusions can be drawn about the preservation of other 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 | Deutsches Museum

Celluloid – the Dawn of the Plastic Age

One of these endangered plastics is celluloid. Celluloid describes a group of thermoplastics that are made from cellulose nitrate with the addition of camphor as a plasticizer. It is only by adding these plasticizers that plastics become more pliable and three-dimensionally malleable. Regularly structured, long-chain plastics in particular typically tend to form partially crystalline areas that are hard and non-deformable. As a result of mechanical movements, the crystalline areas can grow over time, so that the plastic becomes harder and more brittle as it ages and ultimately breaks. Plasticizers interfere with crystallization to keep the plastic supple. Celluloid was one of the first plastics to be mass-produced on an industrial scale.

A Closer Look

Elsaesser first records the overall condition of the plastic macroscopically and with the help of stereo microscopes such as ZEISS Stemi 508, examining it for microcracks and other abnormalities. The formerly transparent frames show discoloration caused by microcracking and the metal used in the temples.

The microscope image also shows droplet-shaped deposits on individual fragments of the frames. Droplets and plastics are often the result of plasticizers, but in this specific case they are due to nitrous outgassing. Among other causes, this happens when plastics are stored incorrectly together with metallic materials. A valuable insight for the preservation of other exhibits.

Off into the Cold

Among other things, Elsaesser is experimenting with cold storage and investigating what happens to the objects and the chemical processes when they are stored in a refrigerator or freezer. A particular challenge lies in the combination of materials used for the exhibits as well as their different structures and thicknesses. The freezing process can lead to internal stresses, which can also damage the object. “There is still a lot of research to be done,” Elsaesser explains with a wink.

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

Christina Elsaesser | Deutsches Museum

Deusches Museum Munich — Aircraft part from a Messerschmitt Me 262

Restoration Research at the Deutsches Museum Munich

One way or another, the restoration research 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 ZEISS Axio Scope.A1 microscope, a pigment analysis shows how the aircraft was historically painted and which materials were used, thus giving restorers a clue.

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 the Plexiglass and PMMA roof of the Olympic Stadium to learn how to gently clean such objects.

In addition to the conservation and restoration of objects, research into historical materials and constructions as well as their alterations are therefore an important part of the conservation 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.

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