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Hi Guys
A nice web page explain about why exactly PVC figure would become sticky after time. I cannot post the link, here is it summary
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.
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.
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發表於 10-11-2011 22:48:47
發表於 19-11-2011 17:23:08
