Polyurethane

Introduction

Polymer compounds are one of the most significant compounds as they are vastly used for various purposes. Polyurethane is an organic polymer commonly used for coatings in products. Polyurethane is used to prepare high-quality paints that offer better durability than traditional pains. It does not melt or boil upon heating. These are thermosetting polymers. Polyurethanes are produced with the linkage of urethane molecules bonds.

What is Polyurethane?

Polyurethane, also known as PU or PUR is a type of organic polymer created by the linkage of many organic units via urethane molecules. Most of them do not melt by applying heat and these are known as thermosetting polyurethane. Thermoplastic polyurethanes are specific types of polyurethanes that can be remoulded and melted by applying heat.

Figure 1: Structure of Polyurethane

Polyurethanes are traditionally prepared by chemical reactions between Di-isocyanates or tri-isocyanates and polyols. Polyurethanes are alternating copolymers that feature two types of monomers offering two-step polymerization. The two monomers - isocyanates and polyols are used for polyurethanes production and processing. Every molecule of these monomers usually contains at least two functional groups.

Production of Polyurethane

Diisocyanates and diols react with each other to produce polyurethanes. The production is done in three steps: diol production, isocyanate production and polyurethane production. Polyol is polyester or polyether with terminal hydroxyl groups. It is used for polyurethane production. Methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) is used to produce most Polyurethane. Flexible and soft foams are produced from TDI. Rigid and more versatile polyurethanes are produced from MDI.

The reaction of a diol with MDI or TDI constructs a thermoplastic, linear polymer through the reaction named condensation polymerisation. A cross-linked, rigid and thermosetting molecule is three or more hydroxyl groups present in the alcohol chain-extending agents, surfactants, cross-linking agents, blowing agents, and fillers are some common additives used for feature improvements of Polyurethane.

Polyurethane Foam

A solid polymer in rigid or elastic form is produced by combining these two reactant liquids. A cellular foam material can be formed with the physical or chemical formation of bubbles in this solid substance. Polyol with the addition of water initiates Chemical blowing that leads to the formation of CO2 gas bubbles while reacting with isocyanates Pentane and other low boiling point substances are added for physical blowing. Pentane evaporates in form of bubbles after being heated in the exothermic polymerisation reaction process.

Characteristics of Polyurethane

Some properties of Polyurethane are discussed below.

• Polymerisation reaction of diisocyanates and diols produces Polyurethane.

• Polyurethane molecule is formed by urethane linkage or COONH bonds

• Polyurethane properties have a dependency on production manner.

• The final Polyurethane becomes elastic and soft if the polyol chain is flexible and long.

• The final becomes rigid and tough if the cross-linking quantity is very high.

• 3D networks can form the cross-linked structure of Polyurethane. It can result in polymer featuring very high molecular weights.

• The polymer's thermosetting nature is accounted for by its structure. Polyurethane does not melt or soften typically in heat.

• A linear thermoplastic polymer is produced by the reaction between a diisocyanate and a diol.

Benefits of Polyurethanes

Polyurethanes have foam-making abilities. The foam is produced in the urethane polymerization process with the help of CO2 gas production. Another benefit is polyurethane is its microcellular foams with high density can be formed without the help of blowing agents.

Uses of Polyurethane

Polyurethane is used to produce a wide variety of products. Some of them are as follows.

• Flexible polyurethane foam is mostly used for cushioning purposes in furniture, automotive interiors, bedding, packaging, and carpet underlay.

• Rigid polyurethane foams are known to be the most money and energy-saving insulations. It offers high-quality insulation for walls, rods, doors, and windows. It helps in noise level reduction, and a uniform temperature is maintained. It is used for thermal insulation in freezers and refrigerators.

• The lifespan and appearance of many products can be improved by applying Polyurethane coating.

Figure 2: Uses of Polyurethane

• Polyurethane finish is useful for product shining. It offers better facilities than traditional methods. Polyurethane paint or wipe-on polyurethane is used in concrete or wooden surfaces for durability and colour.

• Water-based polyurethane dries quickly and it is less toxic relating to its increased usage and popularity.

• The adhesives of polyurethane offer strong bonding benefits, especially after its manufacturing. Polyurethane sealants are used to make tighter seals than other traditional procedures.

• Polyurethane elastomers are used for moulding into desired shapes. These are light as compared to metals and can recover increased stress. Polyurethane elastomers are resistant to the environment.

Conclusion

Polyurethane is a type of organic polymers combined by many organic units with the linkage of urethane molecules. Polyurethane can be of two types, thermosetting or polyurethane and thermoplastic polyurethane. One of them can be melted in heat, the other can’t. Both of these polymers have extensive usage. Polyurethane is made by a chemical reaction of isocyanates and polyols. The polymer compound is used for insulation, assembling, coating and many other domestic and industrial applications.

(FAQs)

Q1. What is the difference between monomer and polymer?

Ans. Monomers are small molecules with single atoms having the capability of forming bonds with a minimum of two other monomer molecules. Multiple monomers are combined to form a synthetic substance class called polymers.

Q2. Why Polyurethane foam is useful?

Ans. Polyurethane foam is useful for buildings for waterproofing and air tightening. This is possible due to the capability of high-quality insulation. Polyurethane foam keeps no gap between the substrate and insulation. It is used for exterior insulation and it is a good barrier to air and vapour.

Q3. What is polymerization?

Ans. Polymerization is the process of making polymers from monomers. Monomers are small molecules that are chemically combined to form a chain-like and very large molecules called polymers.