Plastics

Cellulose Acetate (CA)

General information

Description

Cellulose acetate (CA) replaced cellulose nitrate as a less flammable film base, first in the motion picture industry and later also in photography. It can be injection moulded and from the 1920s onwards it was used to imitate objects that hitherto were made of expensive natural materials (ivory and tortoise). After the second world war modern plastics took over. CA is sensitive for degradation which is noticeable by its acetic acid smell, deformation of the material, craquelure, and formation of white crystals on the surface. Degradation depends strongly on the added plasticisers.

History

Cellulose acetate was discovered in 1865. From 1904 onwards products could actually be made of and with it. In 1910 it became commercially available. Until the 1960s large quantities were produced. Subsequently, its use has declined.

Production, Application, Appearance

Cellulose acetate is a semi-synthetic plastic. Its high gloss, transparency and strength make it a good imitation for tortoise shell. It was used extensively for hairpins, cigarette cases, toys (Lego 1949-1963) and as foil. Nowadays it is mostly used for special applications such as eyeglass frames and film substrate.

Properties

Material properties

Thermoplastic
Density: 1.27-1.34 g/cm3
Melting point: 230-260°C
Glass transition temperature: 100-135°C

Identification properties

Cell structure (foam): not applicable
Smell: acetic acid smell upon degradation (vinegar syndrome)
Touch: no characteristic touch
Sound: Rigid = clear. Can sound tinny; film = (?) no characteristic sound
UV-radiation (when clear): fluoresces distinctly yellow
Polarizing filters (for clear CA): does not produce a colour pattern; hardly a difference with or without filters

Degradation

Process

Upon hydrolysis acetic acid is produced which accelerates further degradation (auto-catalysis).

Details

CA is a sensitive plastic owing to its auto-catalytic degradation.

Symptoms

Yellowing; craquelure; acetic acid smell; loss of mechanical properties resulting in tears and fractures; deformation (warping); surface becomes moist (weeping); formation of white crystals on the surface.

Susceptibility

UV-radiation: Medium
Light: Medium
Oxygen/Ozone: Low
Temp: High
RH: Highly sensitive to hydrolysis

Preventive conservation

Recommendations

UV-RADIATION: keep below 75 µW/lm UV filter for daylight and fluorescent light - reduce intensity
LIGHT: 1 just noticeable change in approx. 30 Mlx.h Moderate light dose. Control intensity and exposure time
OXYGEN / OZONE: ambient conditions
TEMP: keep cool (5°C) or cold (-20°C) to slow down auto-catalytic degradation
RH: 20-30% RH fluctuations: keep constant - setpoint ±5%
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Note: the emitted acetic acid corrodes metals and can affect other objects. Store CA as cold as possible. Do not seal packing. Ensure good ventilation. Keep away from susceptible objects. Acid absorbing material such as activated charcoal or buffered paper can be placed close to the object.