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  Polyurethane, Chemistry tutorial

Introduction

Polyurethane (PUR and PU) is any polymer composed of a chain of organic units linked via carbamate (urethane) links. Polyurethane polymers are shaped through step-growth polymerization, through reacting a monomer (by at least two isocyanate functional groups) through another monomer (by at least 2 hydroxyl or alcohol groups) in the presence of a catalyst.

     n O=C=N-R-N=C=O  +   n HO-R1-OH                  [ C-NH-R-NH-C-O-R1- O ]n

                                                                                       1980_double bond.jpg                1980_double bond.jpg

  diisocyanate                            diol                           O                O

Route to the synthesis of polyurethane from diisocyanates 

Properties of Polyurethane

Polyurethane is a unique substance that presents the elasticity of rubber joined through the toughness and durability of metal. Such polymers have elevated strength, good resistance to gas, oil, and aromatic hydrocarbons, elevated abrasion resistance and excellent resistance to oxygen and ozone, but are susceptible to microbial attack. Since urethane is available in an extremely broad hardness range (eraser-soft to bowling-ball-hard), it permits the engineer to replace rubber, plastic and metal through the ultimate in abrasion resistance and physical properties. 

Advantages of Polyurethane

Abrasion resistant - Parts made of polyurethane will frequently outwear other substances via a margin of 5 to 50/one when severe abrasion is a factor. It has been proven to be greatly superior to rubber plastics and metal in many applications.

Tear resistant - Tear strength ranges between 500-100 Ibs./linear inch that is far superior to rubbers. As a consequence, urethane is frequently utilized as drive belts, diaphragms, roll covers, cutting pads, gaskets and chute liners. Oil and solvent resistant - Polyurethane has excellent resistance to oils, solvents, fats, greases and gasoline.

Excellent noise abatement properties - The solid urethanes are now being utilized as gears in products where engineers wish sound reduction. The soft urethanes are utilized to replace rubbers for progressed sound/vibration dampening. Load bearing capacity - Polyurethane has a higher load-bearing capacity than any conventional rubber. Since of this characteristic, it is an ideal substance for load wheels, heavy duty couplings, metal-forming pads, shock pads, growth joints and machine mounts.  

Weather resistant - Polyurethane has outstanding resistance to oxygen, ozone, sunlight and common weather situations. Flex-Life - Most formulations offer tremendously elevated flex-life and can be expected to outlast other elastomer substances where this feature is a significant requirement. Dust boots, bellows, diaphragms, belts, couplings and similar products are made from urethane for this reason. 

Electrical properties - Polyurethane have excellent electrical insulating properties and are used successfully in many moulded wire and cable harness assemblies.  Heat and cold resistant - Continuous use above 225°F is not recommended nor is urethane recommended in hot water over 175°F. At low temperatures, polyurethane will remain flexible down to -90°F. Gradual stiffening will take place at 0°F, but won't become pronounced until much lower temperatures are attained.   

Applications of Polymers

In metal forming pads, belts, wear strips, bumpers, gears, bellows, machinery mounts, cutting surfaces, sound-dampening pads, chute and hopper liners, prototype machined parts, gaskets, seals, rollers, roller covers, sandblast curtains, diaphragms.

The table below illustrates how polyurethanes are used (US data from 2004)        

Table: Usage per application of polymer

1073_Usage per application of polymer.jpg

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