Degradation of Polyurethanes
Fashion designers frequently do not consider the lifespan of a material, but they choose material for its properties and design. Clothing is designed to last only for a short time, and then typically gets thrown away or handed down. Since fashion is always changing, people adapt their choice of clothing constantly. Fashion that ends up in museum collections can come from many different sources and its individual history can alter the deterioration behavior. An object can be bought straight from the designer and has never been worn. It can also be a donation and, if this is the case, the object has had a history when it might have been (more) exposed to cold, warmth, rain, tension, cleaning processes, and any other number of conditions. All of these factors will of course accelerate the degradation process of polymers severely .
The most common agents of degradation are:
- light (which causes the shortening of polymer chains and/or cross-linking between chains);
- oxygen;
- moisture;
- additives (such as fillers or plasticizers from within the plastic itself);
- mechanical stress (for example the parts of a jacket that are under pressure when wearing it, or mounted on a bust or mannequin).
The substrate supporting the coating itself also plays a very big role in the route of degradation. For example a cotton substrate is very sensitive to humidity and will likely cause more tension between the substrate and the coating.
Within the class of polymers there are two important families:
- The Ester based polyurethanes are more sensitive to hydrolysis and thermal aging;
- The Ether based polyurethanes are more resistant to hydrolysis, but more sensitive to photo oxidation.
In both cases, degradation will alter the object and result in a loss of its chemical, physical and mechanical properties. An object can become unrecognizable, unstable and eventually fall apart.
The most common forms of degradation that have been established are:
- Blooming: Whitish deposits that can evolve to a more crystalline appearance over time, coming out from the superficial coating layer This white bloom is commonly identified as adipic acid, which is formed by the reaction of the polyester based polyol with water (moisture);
- Peeling: the coating comes loose from the substrate. The most common types of substrate are cotton, silk, polyester and leather;
- Tackiness: Substrate showing an adhering surface due to the migration of additives and/or degradation of constituent materials. Commonly found in aliphatic polyurethane coatings;
- Flaking: Cracking and loss of adhesion of the coating, due to hydrolysis of the polyester polyurethane. The flaking is worsened by the presence of a flexible substrate (e.g. knit) Commonly found in early ester-based and aromatic polyurethane coatings;
- Cracking: Breaks crossing the entire coating due to an increased brittleness of the material. Commonly found in ester-based and aromatic polyurethane coatings;
- Micro cracking: Superficial thin network of breaks at the coating layer;
- Discoloration: a complete change of the original color;
- Powdering: Losses of material due to a disintegration (powdering) of the coating layer. The textile substrate underneath the superficial coating becomes visible;
- Sweating: Liquid onto the surface with a sweat appearance along with a tackiness feeling, possibly caused by the migration of additives or by the degradation of the synthetic coating material.
The type of degradation depends heavily on the period of the object, the kind of substrate and the conditions the object has been exposed to. Understanding the material and knowing its entire composition is fundamental. Anoxic conditions, with low temperature in the dark with an RH of 40-50%, are showing a lot of promise as a preventive measurement for long term storage.
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