Manufacturing is full of technologies that have quietly faded into the background. They are no longer fashionable, rarely feature in new product launches, and have been eclipsed by newer materials in many applications. Yet they continue to solve a handful of problems better than almost anything else available. Polysulphide sealants are a good example.
First developed in the 1940s, polysulphides (or polysulfides) are among the oldest families of synthetic sealants still in widespread industrial use. Most polysulphides are two-part materials that cure at room temperature over several hours or even days, depending on the formulation and conditions. Over the decades they have been displaced in many sealing applications by silicones, polyurethanes (PUs), silyl modified polymers (SMPs) and other modern chemistries. These alternatives often cure more quickly, are available as convenient one-part products, produce less odour during application and generally offer a simpler processing experience.
Despite this, polysulphides remain the material of choice wherever long-term resistance to fuels, oils and aggressive chemicals is the overriding design requirement. Strong oxidising acids, certain solvents and other aggressive chemicals may require a different sealant chemistry.
Their superpower is fuel resistance
Most flexible sealants perform well in normal environmental conditions. Continuous exposure to fuels and hydrocarbons is a much tougher challenge. This is where polysulphides excel. They combine long-term flexibility with exceptional resistance to aviation fuel, diesel, petrol and hydraulic fluids, maintaining a durable seal where many other elastomeric sealants would degrade. That is why they continue to be specified for aircraft fuel tanks, fuel storage facilities and other demanding chemical containment applications.
Why aerospace still uses them
Aircraft fuel tank sealing remains the defining application for polysulphides. Few sealants have such a long and successful service history under continuous fuel exposure. In aerospace, proven performance matters. When a material has demonstrated reliability over decades in a safety-critical application, there is little incentive to replace it unless a clear technical advantage exists.
Why other technologies have taken over elsewhere
Outside specialist applications, the picture is rather different. Many modern sealing jobs simply do not involve prolonged exposure to fuels or aggressive hydrocarbons. Instead, manufacturers may place greater importance on processing speed, ease of use, appearance, environmental considerations or production efficiency. In these situations, newer chemistries frequently offer practical advantages.
- Silicones provide excellent weatherability and high-temperature performance
- Polyurethanes offer strong adhesion and good mechanical properties across a wide range of construction and industrial applications
- SMPs combine good adhesion with low VOC emissions and are often easier to handle than older chemistries
For many manufacturers these advantages outweigh the exceptional chemical resistance offered by polysulphides because that particular property is simply not required.
Choosing materials for the application, not the fashion
It is tempting to assume that newer materials are automatically better. Materials science rarely works that way. Every adhesive and sealant technology exists because it solves a particular engineering problem particularly well. When that problem changes, the preferred material often changes with it. Polysulphide sealants demonstrate this perfectly. They may no longer be the default choice for general-purpose sealing, but where long-term resistance to fuels and hydrocarbons is essential, they continue to occupy a position that few other flexible sealants can match. Sometimes mature technologies survive for decades not because industry is reluctant to change, but because they remain exceptionally good at the job they were originally developed to do.
For most sealing applications today, polysulphides are no longer the obvious starting point. Unless your application demands long-term resistance to fuels or aggressive hydrocarbons, modern technologies such as silicones, polyurethanes or SMPs may offer a simpler and more practical solution. The right sealant is rarely the oldest or the newest. It is the one whose strengths best match the demands of the application.
Thanks to Kevin for help on this one.
