Polyurethane

Introduction

Polyurethane are a discovery and development of O.Bayer and his chemist in Germany beginning about 1937.

The reaction of an isocyanate and an alcohol results in the formation of a urethane. By the same reaction polyhydroxy materials will react with poly isocyanate   and give polyurethane.

The raw materials used for the polyurethane elastomers are
1.polyols
2.Diisocyanates
3.Chain extenders

1.Polyols

The polyol components of polyurethane elastomers is usually ba macro glycol of molecular weight about 500 to 4000.

2.Diisocyanate

The diisocyante components may be any of a multitude of relatively small molecules.

3.Chain extenders

Chain extenders or crosslinkersvof polyurethane elastomers is relatively small usually di functional molecule chain extenders or glycols or diamines.

Polyurethane can be processed by
a. Castable PU
b. Millable PU
c. TPE based PU

Simple reactions between the glycol/diamines and diisocyante give the product with urethane.

a.Castable PU

The castable PU is through the formation of a pre polymer obtained by reacting polyol and diisocyante.

The castable PU are of 4 types.
1. Unstable prepolymer.
2. Stable pre polymer
3. Quasi prepolymer
4. One shot system

Unstable prepolymer:

Here the pre polymer  contains free isocyanate groups which are highly reactive they are mixed with chain. Extender and poured into moulds where it is cured at room temperature for one day or in presence of catalyst and at high temperature is about an hour or more.
This system remains important because of excellent load bearing and for excellent heat resisting characteristics.

Stable pre polymer:

  In this the isocyanate in the pre polymer will be end capped and this after mixing the chain extenders.Heating will liberate the free isocyanate which then reacts to give the product the advantage is better safety.
These grades are important for harder grades of printer , rollers and wear resistance application such as pipes, pumps and impellers used in mining industries.

Quasi prepolymer

The prepolymer is prepared with n excess of isocyanate to give an isocyanate terminated molecule. This is then reacted with unreacted glycol to other ingredients.
The system is used mainly for the manufacturer if microcellular products

One shot system

In this system three components bare mixed in the mould and cured
The disadvantage is difficult vin controlling the structure of the resulting product.
It is used in foams manufacturer of soft rollers for the printing industry.

ii) Millable gums.

Urepen 600 in this material the isocyanate is reacted with a slight excess if polyester so the prepolymer is produced. The prepolymer are rubber like gums which may be compounded on a 2 roll mill.
They may be cured by addition of di isocyanate which changes into a active isocyanate during moulding operation.
Adiprene-c it may be vulcanized with sulfur.

iii) Thermoplastic polyurethane

The manufacturing process is a prepolymer. it is first produced by reacting a linear polyester with terminal hydroxyl group with an excess of di isocyanate to give an isocyanate terminated polyol prepolymer plus free isocyanate.This blend is then reacted with a chain extenders usually a glycol such as 1,4- butane glycol to give a polymer with hard urethane segments. The block length depends on the excess of isocyanate.

Properties:

1.they have higher tensile strength
2.Excellent rear and Abrasion resistance
3.Outstanding resistance to ozone and oxygen
4.They have low resilience.
5.Some grades can withstand Up to 110 degree Celsius.



Millable Polyurethane Compound- Starting Point formulation

Millathane® E40 Cure: S	100
Zinc Stearate	0.5
N550 Black	20
Cumar P-10 Color:	10
B DBEEA (TP-95)	5
AC617A Polyethylene	1
Struktol WB-222	0.5
MBTS	4
MBT	2
Thanecure® ZM	1
Sulfur	1.5