Polyurethane elastic fibre: from the 1930s to the chemistry of modern stretch clothing

If we want to give a definition of what polyurethane fiber is, we can say that it is a synthetic chemical substance characterized by behavior similar to rubber. This fiber is made up of a molecular chain composed of soft segments, called glycols, interspersed with rigid segments called isocyanates.

Polyurethane fiber was born around 1937 when political-military tension in Europe made the trade of raw materials more difficult, in fact until then elastomers were predominantly natural, imported from South America and South East Asia.

As you can read in the article in the NEWS on the history of natural rubber , this was an element known since the times of the Maya and used throughout the world in various sectors.

The real turning point in the field of fabrics occurred in 1823 when Charles Macintosh patented a compound made of natural rubber and oils, suitable for waterproofing fabrics and, subsequently in 1830, Thomas Hancock, subjected the rubbery compound to mechanical actions, mixing additives oily substances, fillers and pigments, so as to make the compound industrially workable in the machine.

It was such a success that exports of natural rubber from Brazil increased exponentially, going from a few hundred tons in 1846 to more than 10,000 in 1880.

This is how the English got wind of the business and in 1876 obtained , from some seeds imported from Brazil, two thousand seedlings of Hevea Brasilienis, which were then sent to what is now Sri Lanka to be replanted.

This English botanical intervention gave rise to a flourishing production, still active today, in Malaysia, Indonesia and Thailand, an area in which 80% of natural rubber is produced today.

In the 1930s, a period in which chemical research was making enormous strides, the first studies began to create a synthetic rubber that could be replicated in any country in the world, without depending on the importation of natural raw materials.

The most interesting studies of the period were carried out by the German company Bayer and so it was that in 1939, Paul Schlack synthesized a polymer with high elastic properties, but we had to wait until the end of the Second World War to see the production, in 1951, of the first fiber polyurethane through the wet spinning process.

Also in the United States, research led the DuPont company, following important investments made in polyurethane elastic fibre, in 1959, to produce elastic polyurethane fibre, through the dry spinning process, which it put on the market in 1962.

The real explosion in the production of these yarns occurred at the end of the 60s of the last century, when the fashion of the miniskirt and the related use of women's stockings spread.

How is synthetic polyurethane fiber produced and processed?

The synthetic elastomeric fiber is produced by extruding the polyurethane polymer in solution or melt, using a spinneret of a mechanical spinning plant.

There are normally four methodologies for fiber production:

• Wet spinning consists of the extrusion of the polymer in a hot water bath, forming the thread by coagulation, and subsequent washing, drying, lubrication and winding into a bobbin.

• Dry spinning is undoubtedly the most used system in the world and consists of the extrusion of the polymer in a vertical cylindrical cell inside which there is a hot gas, which is normally nitrogen. The wire passes from the cell and is subsequently lubricated, with silicone oil or magnesium stearate and then rolled onto a spool placed at the end of it.

• Melt spinning consists of the plasticization of granules in an extruder creating a fluid mass, which is passed through a vertical die which meets a flow of cold air which leads to the solidification of the raw material. The outgoing wire is then lubricated and wound onto spools. Among the four processes presented, fusion spinning is certainly the one with the lowest environmental impact as it does not require solvents and requires less energy.

• Reactive spinning consists of the extrusion of the pre-polymer into a bath of solution containing polyfunctional amines. The isocyanate parts that constitute the raw material react with the amines forming a polyurethane with a higher molecular weight. It is a technology that is little used today due to the low elastic characteristics of the thread compared to other production processes.

What are the main applications of polyurethane fiber?

The uses of this fiber are many, so we only collect some production indications for the articles:

– Tablecloths

– Sofa covers

– Stockings for surgical use

– Elastic bandages

– Graduated compression stockings

– Diapers

– Tracksuits for sports activities

– Diving suits

– Ski trousers and leggings

– Jeans and other stretch fabrics

– Corsetry

– Socks and tights

– Elastic tapes

– And many other articles

Category: news - plastic - circular economy - PU - elastic fibre