Barbie's PVC Body Gets
Dibutyl Phthalate Migrates
Department The National Museum of Denmark 19apr99
Some of the world's most cherished, plastic dolls, manufactured in the 1950s and '60s, have fallen victim to a deadly, highly contagious disease that has baffled experts. Their skin develops green spots and they weep tears. It can't be stopped or cured but now we have the technology to slow its progress and prolong the life of Barbie for a few more years by controlling the indoor climate. Why use costly energy to preserve such inexpensive toys? Old dolls form a record of fashion, history and of the plastics technology available earlier this century. Museums in every country in the world have such objects in their collections and want to preserve them for future generations to study and enjoy. Current research is investigating practical techniques to make this possible.
Polyvinyl chloride behaving badly
Barbie dolls, together with many other toys, clothes and electrical insulation found in museums and private houses are made from polyvinyl chloride (PVC). Mini-skirts, plastic furniture and modern sculpture feature in such collections. PVC is an inexpensive plastic which has been available commercially since 1942. Pure PVC is extremely brittle. In order to make it flexible enough to shape, up to 70% of its weight in plasticiser is added to the pure polymer. Plasticisers physically separate the long polymer chains to help them flex and slide over one another without cracking.
The plasticiser traditionally added is dibutyl phthalate, a pale liquid with a boiling point of 340°C. Dibutyl phthalate does not vaporise at room temperature, but starts to migrate slowly out of the PVC soon after manufacture. Migration causes the formation of sticky 'tears' on the surface of objects, making the damaged PVC unsafe to touch. This usually takes place within ten years of manufacture. Dibutyl phthalate is toxic to humans through contact with skin and by ingestion.
Losing plasticiser leaves the PVC itself highly vulnerable to deterioration. Hydrogen chloride, a corrosive, acidic gas, is produced by deteriorating PVC. If not rapidly removed from the surface of the plastic, degradation progresses three times faster than before. The acid corrodes any metals it contacts before dispersing.
Barbie often wears earrings, and, as hydrogen chloride is produced by her cheeks it reacts to form green copper chloride which stains her skin. Deterioration of PVC is manifested by its change in colour from white to yellow to pink to brown and, eventually, black.
What can be done to save Barbie and her PVC companions? Deterioration of plastics cannot be stopped under the usual indoor climate conditions comfortable for people. Active preservation treatments such as applying protective coatings and paints are complex for technical and ethical reasons. Any coating which adheres successfully to a plastics surface must also soften the surface, thereby increasing the damage and altering the original appearance. This is unacceptable for museum objects.
Taking control of the environment
A more satisfactory approach is to design and control the environment surrounding the object so that the opportunity for deterioration of PVC is significantly reduced. This can be achieved either by limiting the factors causing deterioration or by ensuring that, if it occurs, the concentration of acidic degradation products is as low as possible. The major causes of deterioration of PVC are heat and light. In general the rate of a chemical reaction doubles if the temperature is raised by 15°C. By keeping PVC as cool as possible, and in the dark, deterioration would be slowed dramatically.
A domestic freezer operating around -20°C is a possibility currently being researched by the author. Transparent windows could be incorporated into the freezer design to facilitate inspection of objects, although this would increase its cost. One drawback to this solution is that freezers require considerable space, a commodity lacked by most museums. This option is also impractical if Barbie needs to be displayed or made available for frequent study. Not surprisingly, museum visitors have a rather short attention span if exposed to room temperatures below zero or if required to handle frozen objects!
Once the deterioration process has begun, the concentration of hydrogen chloride produced can be minimised by removing it from the environment surrounding the PVC object. One simple, clean technique is to trap the hydrogen chloride molecules so that they are no longer sufficiently mobile to make contact with the PVC surface and attack ''healthy'' objects. Zeolites are crystalline silicates of calcium and aluminium which are strongly heated to remove water. The heating process opens up spaces in the silicate structure into which small molecules such as hydrogen chloride can flow and be trapped firmly. Once trapped they cannot be released without strongly heating the zeolite, a process which is unlikely to occur in a normal museum environment.
Zeolites are readily available as small pellets, approximately 4mm x 2mm, which are inexpensive, occupy little space, and offer no hazard to the health of people or that of other materials. They are commercially available as Molecular sieve, Type 4A. The author has shown that they are effective at slowing the rate of deterioration of PVC even when simply introduced into a museum showcase, contained in a small glass dish or wrapped in pure cellulose filter paper, and placed close to the object. For more sophisticated systems, zeolites could be incorporated into a filter for the outgoing gases where the air was recirculated in a storage room or showcase.
Future for plastics
Improving Barbie's environment could extend and improve the quality of her life. The cost and energy requirement to install a freezer or to introduce zeolite adsorbants could be minimised. It is less expensive to inhibit deterioration of museum objects than to actively treat damaged material.
The principle of manipulating the environment in museum storage areas and inside showcases so that the rates of chemical reactions are slowed, is being investigated with the purpose of preserving other types of early plastics. Its low cost and simple technology should result in a large number of 20th century objects being saved for future generations. This will make a lot of museum professionals and grown-up children very happy. Not to mention Barbie's boyfriend, Ken.
PhD Student, The National Museum and the Department of Chemical Engineering,
The Technical University of Denmark
Thanks to the National Museum of Scotland for the picture of Barbie's ear.
source: http://www.natmus.dk/cons/lab/barbie/bb.htm 8jul01