Polyurethane Foams

8/12/2019 Polyurethane Foams

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INTRODUCTION

Polyurethanes (PU) are developing materials which is used in the form of foams, rubbers

(vulkollan), thermoplastic materials, as surface coating materials and adhesives. Polyurethanes are the

polymeric materials that is widely used in the form of foams which is used in making shoe soles,

furniture cushioning, and crash pads in automobiles, antenna housings and in sandwich structural parts.

POLYURETHANE MANUFACTURING TECHNIQUES

Polyurethanes are polymeric materials which can be manufactured by two routes

i. By the condensation polymerization of diisocyanate with dihydroxy compoundsii. By the reaction of bischloroformates with diamines (less practiced technique).

OCN-R-NCO + HO-R-OH H2N-R-NH2+ Cl-COO-R-COO-Cl

(Less Common Route)

-[-HN-R-NH-COO-R-COO-]n-

PU elastomers are prepared by an excess of an aromatic diisocyanate such as TDI or MDI with a hydroxyl

terminated polyester or polyethers having its molecular weight in the range of 2000-3000. The excess

isocyanate would give an isocyanate terminated prepolymer.

PU FOAMS

PU foams are mainly of three types

Flexible Rigid Semi-rigid

PU foams are prepared out of polyesters, polyethers or natural polyols such as castor oil.

The process for producing PU foams are three, they are

1. One-Shot process2. Pre-polymer3.

Quasi pre-polymer

By varying the raw material and process we can produce 27 basic types of PU, commercially.


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RAW MATERIALS

POLYESTER BASED POLYURETHANES

Flexible PU foams are made only from polyesters and polyethers. All commercial polyesters are

based on diethlene glycol and adipic acid, partially branched by incorporation of small amount of triols.

The reaction scheme is represented as follows

(n+1) HO-CH2-CH2-O-CH2-CH2-OH + n XOOC-(-CH2-)4-COOX

HO-CH2-CH2-CH-[-O-CO-(CH2)4-COO-CH2-CH2-O-CH2-CH2-]n-OH + 2nH2O

The polymerization reaction is carried out in an inert atmosphere in a stainless steel autoclave.

The liberated water is removed by distillation. After the reaction is completed polyester is allowed to

cool and then is filtered. The rate of reaction is noted by rate of increase in viscosity and decrease in free

acid content. The foams produced by polyester are very susceptible to contamination by oil, grease and

silicones.

Polyester foams have higher density, higher tensile strength and elongation at break compared

to polyether foams. They resist actions of solvents than do the polyether foams.

POLYETHER BASED POLYURETHANES

Flexible foams of PU manufactured with polyether such as propylene oxide adducts of

monomeric polyalcohols. There are the following 5 types which are prepared by using an alkaline

polymerization catalyst.

1. POLYOXYPROPYLENE DIOLSPrepared by polymerizing propylene oxide using water and amonomer diol as initiator.

2. POLYOXYPROPYLENE TRIOLSPrepared by using a triol initiator.3. ETHYLENE OXIDE TIPPED TRIOLSProduced by reaction of polyoxy propylene triol chains with

ethylene oxide to increase the proportion of primary hydroxyl end groups.

4. Copolymer triols of both the random and block copolymers of ethylene and propylene oxides5. BLENDED POLYOLSUsually a mixture of diol and triol of both oxy propylene types.

By controlling the proportion of alkaline oxide relative to polyalcohol initiator, the molecular weight of

polyethers are controlled. Most widely used polyether type is Polyoxypropylene triol which has

molecular weight of about 3000 based on glycerol.

ISOCYANATE

The next raw material for the production of PU is isocyanate. The most commonly usedisocyanates are Toluene Diisocyanate(TDI), Methylene diphenyl diisocyanate(MDI).


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METHYLENE DIPHENYL DIISOCYANATE (MDI)

MDI is an aromatic diisocyanate. It exists in three isomers, 2,2'- MDI, 2,4'- MDI, and 4,4'-MDI,

however, the 4,4' isomer is most widely used. This isomer is also known as Pure MDI. MDI reacts with

polyols in the manufacture of polyurethane. It is the most produced diisocyanate, accounting for 61.3%

of the global market. The first step of the production of MDI is the reaction of aniline and formaldehyde,

using hydrochloric acid as a catalyst to produce a diamine precursor under the chemical formulation:

2 C6H5NH2+ CH2O CH2(C6H4NH2)2+ H2O

Then, these diamines are treated with phosgene to form an MDI. MDI is the least hazardous of the

commonly available isocyanates but is not benign. Compared to other organic cyanates, MDI has a

relatively low human toxicity.

TOLUENE DIISOCYANATE (TDI)

Toluene diisocyanate (TDI) is an organic compound. Two of the six possible isomers are

commercially important: 2,4- TDI and 2,6-TDI. 2,4-TDI is produced in the pure state, but TDI is often

marketed as 80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively.

2,4-TDI is prepared in three steps from toluene, which is doubly nitrated with nitric acid to give

dinitrotoluene. This step determines the isomer ratio of the ultimate TDI. Hydrogenation of the

dinitrotoluene produces the corresponding isomers of diaminotoluene (TDA). Finally, the TDA is

subjected to phosgenation, i.e. treatment with phosgene to form TDI. This final step produces HCl as a

byproduct and is a major source of industrial hydrochloric acid.PU FOAM PRODUCTION PROCESS

As seen above PU foams are produced by One-shot process, Pre-polymer process and Quasi pre-

polymer process.

a) ONE SHOT POLYESTER FOAM

Produced by reacting adipic acid and diethylene glycol together with a small quantity of

trifunctional reaction component such as trimethylol propane. The polyester thus produced will have a

molecular weight of about 2000. To the polyester, TDI and water in the ratio of 65:35 is to be added

together with ingredients such as a catalyst (dimethyl benzylamide), an emulsifier (sulphonated castor

oil), a structure modifier and paraffin oil. The last component help in control pore size and prevent

splitting of foams. All the reactants are taken in a mixing head, the nozzle of which is kept closed till the

pressure, due to formation of CO2gas, is sufficiently build up. When the nozzle is opened the foam

gushes out and may be used in coating and such other applications

b) ONE SHOT POLYETHER FOAMOf the flexible PU-foams manufactured today the bulk consists of one-shot polyethers,

particularly because of favorable economics of the process and because polyether foams have better

cushioning properties. A typical recipe consists of a polyol, an isocyanate, a catalyst, emulsifier and

blowing agent.

The one shot polyether foam process become feasible because of most powerful catalyst that

have been developed. Earlier tertiary amines were the catalyst for both the polyesters and polyethers.

In late 1950s, Organotin catalysts were found to be quite powerful for chain extension reactions. It was


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later found that by varying different quantities of a tertiary amine and tin catalyst both the gas evolution

and chain extension could be effectively controlled.

The surfactants help in dispersion of water in the hydrophobic polymer mass by reducing the

surface tension between the phases. When the cross-linking density was required to be increased

independent of isocyanate-water reaction, compounds such as glycerol and several types of amines

have been used. Increase in cross linking density led to increase in rigidity of the foam.

c) POLYETHER PREPOLYMER

Before the invention of the suitable catalyst system it was very difficult to produce PU foams by

one shot process. Hence a prepolymer process was developed. In this process the polyether is reacted

with an excess of diisocyanate to give an isocyanate terminated polymer. This polymer is sufficiently

stable if kept in dry conditions. When needed, the catalyst, the water and other ingredients must be

added to the content and foam would be produced.

d) QUASI-PREPOLYMER

This process is a three-part operation and is considered to be an intermediate between the

other two processes. When the equipment available adapts easily to a two-part system and not to a one

shot process or where the prepolymer is found to be very viscous, this method is adopted.

First part consists of combining all cyanurate with one half of the polyol to obtain a prepolymer

of low molecular weight which would have a large number of terminal isocyanate groups. The second

part consists of bringing the remaining polyol, into initimate contact with water, catalyst, etc., and also

additional hydroxy compounds. Thus a low viscosity system would result. Finally the first part and the

second part are mixed in equal amounts.

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Polyurethane Foams