Plastics

Plastics are the newest material in most collections and frequently the first to fail. A design object from the 1960s can be in worse condition than a wooden object from the 1660s, and unlike most materials it can damage its neighbours as it goes.

The four problem polymers

Cellulose nitrate appears as imitation ivory and tortoiseshell, early spectacle frames, combs, handles and film. It degrades to release nitrogen oxides, becomes discoloured, crazed and sticky, and in quantity presents a fire risk. It also corrodes metal fittings on the same object.

Cellulose acetate was the safety replacement for nitrate and has its own failure mode: it releases acetic acid, which catalyses further degradation, and the object warps, shrinks and crystallises. The vinegar smell is diagnostic. Photographs and film covers the film case, where A-D indicator strips give a graded measurement.

Plasticised PVC loses its plasticiser, which migrates to the surface as a sticky or oily film, and the object becomes hard, brittle and discoloured. The migrated plasticiser will mark anything it touches, and acidic degradation products attack neighbouring material.

Polyurethane foam, particularly the polyester-based variety, crumbles to dust within decades. It is common as padding inside objects and, awkwardly, as packing material in older storage, where it disintegrates over whatever it was protecting.

Against these, polyethylene, polypropylene, polyester and acrylic are comparatively stable and are the polymers to choose when a plastic is needed for storage.

Identification

Storage decisions depend on knowing which polymer an object is, and visual identification is unreliable for the very materials that matter most, since nitrate and acetate were used for the same products in overlapping periods.

Non-destructive analytical methods exist and are the right answer where the collection warrants it. Short of that, smell is a genuine indicator: vinegar points to acetate, and a sharp acrid smell to nitrate. Camphor, associated with celluloid, points to nitrate as well. Visible symptoms such as crazing, weeping, stickiness and warping all indicate that degradation is under way whatever the polymer.

Where identification is uncertain and the object is of the right period and type, treating it as suspect is the prudent default.

Isolate, ventilate, cool

The counter-intuitive part is ventilation. For most collection material, sealing is protective. For degrading plastics it is the opposite: the degradation products accumulate and catalyse further breakdown, so a sealed bag accelerates the process.

The usual approach is therefore:

Isolate. Degrading plastics damage neighbours. They come out of mixed storage and go into their own space or their own well-ventilated enclosures.

Ventilate. Perforated or open enclosures rather than sealed bags, so acidic products can disperse, combined with sorbents in the storage space to remove what is released.

Cool. Lower temperature slows the chemistry substantially, and for badly degrading nitrate and acetate cold storage is the main effective intervention.

Keep humidity moderate. Hydrolysis of both cellulose esters is accelerated by moisture, and very dry conditions make some plastics brittle.

Keep light off. Photo-oxidation affects many polymers, and it is entirely avoidable in storage.

Monitoring

Plastics deteriorate slowly and then quickly, so periodic inspection is what catches the transition. A survey of the plastics holdings, with condition photographs and a note of any smell, gives the baseline. Repeating it annually shows the direction of travel.

For acetate film specifically, A-D strips give a numerical grade and are the established method. For general storage areas holding plastics, a VOC sensor is a reasonable change detector, provided its output is treated as a trend rather than a risk figure.

Storage materials, the other way round

The same knowledge applies in reverse when choosing storage materials. PVC sleeves, foam padding of unknown composition and older plastic boxes are all capable of damaging what they hold. Polyethylene, polypropylene and polyester are the safe choices, and acid-free boxes and tissue covers what to ask a supplier.