Preservation Series - Polyurethane

14 February 2024

Hello all,

Welcome back to the final instalment of the preservation series where we discuss how to identify and care for sensitive plastics that you may have in your collection. In previous posts we have reviewed cellulose nitrate, cellulose acetate, and polyvinyl chloride (PVC) – you can find the posts associated with those plastics here. For our last segment of this series, we will be discussing polyurethane, a tricky plastic to identify though one that is used in a wide range of objects.

Polyurethane is an extraordinarily diverse synthetic plastic which can be manufactured as a foam with thermoset properties as well as fibres and surface coatings that have thermoplastic properties. All forms of polyurethane are created by combining diisocyanates, specifically aromatic or aliphatic diisocyanates with polyols or glycols. Additional components are then added to produce the different variations of polyurethane.   

The discovery of polyurethane is attributed to German chemist, Fredrich Bayer, who developed the earliest known type of polyurethane fibres, which he branded as Perlon U in 1937. A few years later in 1940, the first polyurethane elastomers were developed which became a cheaper alternative to rubber and protective coatings used on wartime airplanes. In 1953, the combination of nylon and synthesized polyurethane threads resulted in the discovery of a new lightweight and stretchable material and the first commercial production of polyurethane foam began.

The numerous forms of polyurethane are used in a variety of products including building and general insulation (refrigerators and freezers), padding for furniture and bedding, footwear and clothing, automotive accessories, and coatings or adhesives. The broad applications of polyurethane make it a common and potentially hidden element in many museum collections; therefore, identification and observation are crucial to providing the best care possible.

Identification:

  • Manufacturing: The diverse nature of polyurethane means that objects can be manufactured through any method. Some manufacturing methods will leave an identifiable mark, though in the case of polyurethane, these marks are not strong enough evidence to rule out other plastics.
  • Smell: Polyurethane does not give off a significant or recognizable scent.
  • Design: As discussed earlier, polyurethane is used for a wide range of objects and takes many forms. However, some objects and components of objects have a higher likelihood to contain polyurethane such as faux leather and furniture padding. Plastic coatings on objects will either be PVC or polyurethane both of which require similar care.
  • Inscription: Documentation on your object in the form of inscriptions will be your best method of identifying polyurethane. Trade names for polyurethane include Lycra, Spandex, Adiprene, and Pellethane among many others. Always research any inscriptions on your object for more information.

Degradation:

Polyurethane is particularly sensitive to environmental factors such as light, temperature, and humidity, especially when in the form of foam or coatings. Listed below are the recommended environmental levels for storing objects containing polyurethane and indications of degradation to be on the lookout for.

Ideal Environment:

Polyurethane objects should be stored following the standard practice guidelines.

  • Stored in a space with a temperature between 18-20 °C and a relative humidity between 45-65% RH (fluctuations of 5% are tolerable in 24-hour period).
  • The storage space should be well ventilated and dark with light levels around 50 to 150 lux – the object should be kept within an archival box to prevent introduction of dust and foreign material.

Signs of Degradation:

  • Discoloration and brittleness leading to peeling are signs that polyurethane coating has begun to degrade.
Image
Polyurethane coating of a jacket that has begun to degrade; turning matte, brittle and cracking.
  • In some cases, plasticizers used to make the plastic more flexible will bloom or seep to the surface of the object resulting in sticky patches or areas with a white residue.
Image
The front of a platform show made of polyurethane. The plasticiser has seeped out of the plastic leaving a residue on the tip of the shoe.
Image
Heel of a platform shoe made of polyurethane in which the plasticiser has migrated to the surface leaving behind a white bloom.
  • Polyurethane foam is the most sensitive to environmental factors due to its increased surface area. Over time, exposure to oxygen and subsequent oxidation causes the foam to harden, crack and crumble.
Image
The sole of a high-heeled shoe made of polyurethane foam. The foam has oxidized and become brittle and cracked.

Degradation of plastics is inevitable; however, the proper care should be taken to preserve these objects for as long as possible. When it is no longer possible to maintain care for plastics objects, appropriate disposal practices should be implemented.

We hope you have enjoyed and found this preservation blog series useful. As always feel free to reach out to us with any questions and check this space for more content.

Shannon Carr, MoDiP Collections Officer

References:

https://www.britannica.com/science/polyurethane

https://custompolyurethane.co.uk/blog/the-history-and-uses-of-polyurethanes/

https://www.l-i.co.uk/knowledge-centre/the-chemistry-of-polyurethanes/

https://www.modip.ac.uk/

https://plasticseurope.org/plastics-explained/a-large-family/polyurethane/