dk nkf - Nordisk Konservatorforbund Danmark

Table 3. Critical Surface Tensions of common plastics and metals
Material Critical surface tension
mN/m at 20ºC
acrylonitrile-butadiene-styrene 35
acrylics 32
aluminium c. 500
cellulose (paper) 45
copper c.1000
cyanoacrylates (superglues) 37
epoxy 47
polyamide 46
polycarbonate 46
polyethylene 31
polyethylene terephthalate 43
polymethylmethacrylate 39
polystyrene 33
polytetrafluoroethylene 18
polyvinyl chloride 39
silicone 24
water 73
such as polyester to accommodate dimensional
changes which occur with variations in relative
humidity and temperature.
An adhesive should have a surface tension lower
than its intended adherend. It is difficult to find
adhesives with lower critical surface tensions than
those of plastics (Table 3). A dilute, water-based
adhesive is unlikely to wet plastic because the
surface tension of water is 73 mN/m compared
with only 30-45 mN/m for most plastics. A surface
contaminated with silicone (18 mN/m) from, say,
waterproofing polish, will also be difficult to wet
and subsequently to adhere. Plastics must be free of
contaminants if adhesion is to succeed.
Selection of an appropriate adhesive for joining
industrial plastics is initially dependant on whether
a structural or a non-structural material is required.
If a structural adhesive is required, thermosetting
adhesives such as epoxies, polyurethanes and
acrylics are those most frequently chosen because
they exhibit toughness and flexibility. Non-structural
materials include pressure-sensitive, contact and
hot-melt adhesives.
Keynote speech
Conclusion
It is now indisputable that both early and modern
plastics have a finite lifetime and will deteriorate.
Because their degradation pathways are complex and
often poorly understood, the preservation of plastics
gives conservators and collectors nightmares.
Once initiated, degradation of plastics cannot be
prevented, reversed or stopped, but only inhibited
or slowed. Slowing the rate of deterioration of most
plastic types is achieved by storing objects in an
appropriate microclimate which removes the main
factors causing degradation including oxygen, acidic
gases and moisture. Broad-spectrum, adsorbents
such as activated carbon and zeolites are currently
used to achieve appropriate microclimates. They
are inefficient and require frequent renewal. Pilot
studies on the effect of cold storage on physical
properties of selected plastics indicate that storage of
plastics in a domestic freezer should be considered
as an alternative.
In addition to prolonging the lifetime of plastics
materials, preservation of their appearance and
significance is a requirement in museums, galleries
and private collections. This area of conservation
has been neglected due to the high sensitivity of
plastics to cleaning agents, solvents, adhesives
and consolidants. However, the risks of causing
irreversible damage to plastics by interventive
treatments can be minimised by adapting established
adhesive and cleaning techniques from the plastics
industry to meet conservation ethics. Applying tools
such as solubility parameters and critical surface
tension values would facilitate intelligent choices of
conservation materials.
Investment in time and resources to collaborate
with the plastics industry who have experience of
production of plastics is required now to develop
more effective and less damaging techniques for
conserving plastics in the future.
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