It is an indisputable fact that various synthetic polymer materials such as plastic and rubber are ubiquitous in our life. But among the many polymer materials, I am afraid that none of them can penetrate into every corner of our lives as deeply as polyurethane. Perhaps you are wondering where this strange-sounding material is. Then follow me to find its footprint in every day life.
At breakfast, you wash up and walk to the kitchen, ready to take out today’s breakfast from the refrigerator, think about what is inside the thick box and door of the refrigerator? The answer is the polyurethane foam, the cold air inside can not get out, the hot air outside and can not get in? The answer is the polyurethane foam enclosed in the refrigerator body and door. They do their best to stop the exchange of heat inside and outside the refrigerator, if there is no polyurethane, I am afraid the compressor of the refrigerator will endlessly roar, and you have to worry about the huge power consumption of the refrigerator.
Next, please move your eyes to the plywood cabinets next to the refrigerator. Plywood is a number of layers of thin wood with glue, phenolic resin and urea-formaldehyde resin was once the main force of the bonded wood, but these two types of materials will release a strong carcinogenic effect of formaldehyde, so in recent years has been replaced by polyurethane. Afterwards, you are invited to move your eyes to the corner of the wall, where the polyurethane sealant is faithfully performing its duty to seal the gap firmly.
After breakfast you leave home to come to the unit. The company has just moved into a new office building, and when you walk into the lobby, the solid wood floor is shiny as a mirror, thanks to a layer of polyurethane coating covering the floor, otherwise the wood floor would have been worn out by the foot traffic.
You notice the eye-catching posters in the corridor, proudly announcing to everyone that the company’s new office building is huge and its interior is luxurious, but it is exemplary in terms of energy efficiency. All aspects of reducing the building’s energy consumption are involved, but one of the major contributors is the polyurethane sandwiched inside the walls and ceilings, much like the polyurethane foam used in your refrigerator at home, except here they serve a much larger “refrigerator.
After a hard day’s work, you come to the sports field ready to exercise. Is there a polyurethane footprint here? Of course there is. Let’s start with your workout clothes. Why do they fit snugly without losing comfort? If you look at the label, you may notice the words “10% spandex” and therein lies the answer. Spandex is a fiber of polyurethane, which has excellent elasticity. Although the amount of spandex in clothing fabrics is generally not high, but if it is missing, clothes like swimsuits, sportswear and leggings will feel very bad on your body. And say your feet on the pair of sports shoes.
Many shoes have comfortable, hard-wearing soles made of polyurethane, which often rely on polyurethane adhesives to stick firmly to the upper. After the plastic track under your feet. The basic raw material of the plastic runway is of course the scrap rubber crushed into small particles, but without the polyurethane adhesive to hold them together, these small rubber particles will just fly around like sand.
Night falls and you return home. Finally the day’s work is over, and at this moment you may be lazily leaning on the sofa enjoying a TV show or lying in your own big bed ready to go to sleep. If you remove the outside fabric of the mattress or sofa cushion, you may see a soft and light material in white or cream, which is still polyurethane. Polyurethane is responsible for making mattresses and sofa cushions so comfortable.
After that, when you and your lover enjoy the intimate world of two people, do not forget another important contribution of polyurethane, that is, condoms. Although polyurethane is not the mainstay of condom production – most condoms are made from natural rubber – it offers a great alternative for those who are allergic to natural rubber.
Figure 1 Several important applications of polyurethane: top left: mattresses; top right: shoe soles; bottom left: coatings; bottom right: elastomers
Did you realize from the above that polyurethane is indispensable in almost every aspect of our daily work and life? So what kind of amazing material is this and how do we get it?
As we all know, all kinds of synthetic polymer materials are made of countless small molecules reacting with each other, for example, polyethylene is the product of the reaction of ethylene molecules with each other. Polyurethane is naturally no exception, and to get it, two important types of small molecules are indispensable.
One type of molecule is called an alcohol, which is a compound with hydroxyl groups [1]. This is an all-too-familiar class of substances, and we are drinking ethanol, a type of alcohol, every time we have a drink. However, in order to synthesize polyurethane, each alcohol molecule must have at least two hydroxyl groups, so ethanol is not enough and needs to be replaced by its “close relative”, ethylene glycol.
Another molecule, which is relatively less well known, is called isocyanate, and the one that appears here is a member of the group called diphenylmethane diisocyanate [2]. Although the name is difficult to remember, it is definitely one of the “big names” in the polyurethane industry, with annual production of millions of tons worldwide. As the name suggests, there are two isocyanate structures in each diphenylmethane diisocyanate molecule.
Once a molecule of ethylene glycol and a molecule of diphenylmethane diisocyanate meet, it is like an old friend who has not seen each other for a long time, one stretches out the hydroxyl group and the other stretches out the isocyanate, holding each other’s “hands” tightly and becoming one molecule. But the reaction doesn’t stop there. Ethylene glycol has another “hand” – another hydroxyl group, which can also grasp another diphenylmethane diisocyanate molecule. The two diphenylmethane diisocyanates caught by the glycol also each have another isocyanate structure free to catch another glycol molecule. The result of this constant hand-pulling is the formation of a polymer compound, or polyurethane.
Figure 2 The reaction process of polyurethane from ethylene glycol and diphenylmethane diisocyanate
Why is polyurethane so widely used? The mystery lies in these two raw materials. In the example, we have obtained polyurethane by reacting ethylene glycol with diphenylmethane diisocyanate, but if you replace ethylene glycol with propylene glycol or butylene glycol, we can still obtain polyurethane as long as each molecule contains at least two hydroxyl groups. Likewise, we can replace diphenylmethane diisocyanate with other materials as long as each molecule has at least two isocyanate structures. In other words, polyurethanes actually comprise a large class of materials with similar structures but very different properties, from hard plastics to flexible rubbers and powerful adhesives.
Of course, regardless of the variation in properties, they all share many advantages, such as low production costs, ease of processing, corrosion resistance, and so on. With the exception of polyurethanes, there is hardly any other polymer that allows us to adjust its properties to suit different applications so readily.
Polyurethane is also present in almost every corner of our daily life thanks to another of its masterpieces – foaming. If we add a certain amount of alcohol and isocyanate to a beaker, they will turn into a hard solid after stirring for a while. However, if we then add a little water to the beaker, the mixture in the beaker becomes like a freshly opened bottle of beer, a lot of foam spurts out. It turns out that the isocyanate as soon as it encounters water, it quickly turns into another organic substance – amine, while quickly releasing carbon dioxide gas. Due to the liquid barrier, these gases are too late to escape into the air, so we saw a large number of bubbles.
But over time, these bubbles do not fade away like beer foam or soap bubbles, but remain permanently. It turns out that the amine generated by the reaction will continue to seek out and react with the isocyanate that has not yet reacted with water, turning itself into part of the polyurethane. In other words, it is no longer the fragile water film that forms the outer wall of these foams, but the strong polyurethane, so these foams are retained permanently. And because the interior is filled with a large number of tiny air bubbles, polyurethane also becomes incredibly light and puffy, with only a few tenths of the original density [3].
Figure 3 Basic principle of polyurethane foam preparation
Do not underestimate these polyurethane foams, which contribute to more than half of the output of polyurethane materials worldwide. So where does so much polyurethane foam go? By adjusting the specific composition of the raw material, we can get two different kinds of polyurethane foams. The holes in the kind of polyurethane foam are connected to each other, and the polyurethane that forms the outer wall of the foam is soft and flexible. Such foam can be easily deformed by external forces, but it will not be crushed and will regain its own shape once the external forces are removed, thus bringing great comfort to the user. Many of the aforementioned mattresses and sofa cushions have applied this type of polyurethane foam. In addition, car cushions also need it.
Another type of polyurethane foam is more rigid, and the tiny holes inside are no longer connected, but isolated from each other. We know that if we want to isolate the heat transmission, no material can be as effective as air, and the insulation ability of carbon dioxide in air is much better than other components such as oxygen and nitrogen. If we use polyurethane to insulate carbon dioxide in tiny spaces, preventing them from circulating with each other or escaping to the outside world, such polyurethane foam has a good insulation ability almost comparable to air, which is difficult to be achieved by other materials.
If you use bricks instead, the insulation effect of a brick more than 30 cm thick is only as good as a 1 cm thick polyurethane foam! And because polyurethane foam has good mechanical strength, it is often much better than air alone for insulation, so it is widely popular. Polyurethane foam has not only become necessary for the production of refrigerators, but is also widely used in building energy saving. These polyurethane foams effectively help us to improve energy efficiency and thus save valuable resources.
Figure 4 Operators are using polyurethane foam to improve the insulation capacity of buildings
Since its inception, polyurethane has come a long way in nearly 80 years and has changed our lives dramatically. Looking to the future, the continuous innovation and advancement of polyurethane materials will make our lives even better.
Notes.
[1] To be precise, alcohols are compounds in which the aliphatic carbon chain is linked to a hydroxyl group. Compounds in which the hydroxyl group is directly attached to the benzene ring are called phenols.
[2] Diphenylmethane diisocyanate has three different structures, 2,2′, 2,4′ and 4,4′, of which 4,4′-diphenylmethane diisocyanate is commonly used in the polyurethane industry and is the structure shown in Figure 1.
[3] Many polyurethane foams are also produced with the help of other gases that are introduced during the production process
