Kalyan Maity. 91 / OLT / 091013

The most important thermoset material used for the preparation of molding compounds , adhesives , coatings.

The most common type of resin for composites and marine services.

The least expensive composite resin. The easiest-to-cure composite resin.

Polyesters are made from two types of monomers: Di-acids Glycols

+ acid + glycol + acid + glycol +... => ...acidglycolacidglyc ol... + n water

one stage process. The conventional technique results both high and low

molecular weight fractions.

The high-molecular-weight resin fractions raise the viscosity .

The low-molecular-weight fraction are detrimental to both coating performance and enamel solids or volatile organic content(VOC) due to volatisation during cure

Condensation time : 8-30 hours Water removal is enhanced by vacuum or nitrogen

stripping .

Monomers

Glycols G

(di-alcohols)

Acids A(di-acids)

G

G

G

G

A

A

A

A

A

Polyester polymer

Di-acids have active -OH groups on both ends. Glycols have active -H groups on both ends. One end of the di-acid (the OH group) reacts with

one end of the glycol (the H group) to form water (H−OH) .

The water separates from the polymer and condenses out as a liquid.

This type of polymerization reactions are called condensation reactions.

OH C A C OH

O O

1st Step

O C A C OH

O O

OH C A C OH

O O

Many Steps

( )

Ester

Di-acid

Ester Ester Ester

Di-acid

Di-acid

Di-acid

Ester (new bond)

O ...O C A C O

O O

... O C A C

O O

O

HO G OHGlycol

HO-G

HO G OHGlycol

G G

GlycolGlycol

Di-Acids/Anhydrides Attributes

Maleic/Fumaric Unsaturation (crosslink sites)

Orthophthalic Low cost, styrene compatibility

Isophthalic Toughness,

water/chemical resistance

Terephthalic High HDT

Adipic Flexibility, toughness

Brominated Flammability resistance

Glycols Attributes

Ethylene Low cost, rigidity

Propylene Excellent styrene compatibility

Dipropylene Flexibility, toughness

Diethylene Flexibility

Neopentyl UV stability, water/chemical resistance

Bis-phenol A Water/chemical resistance, strength

Solvents Attributes

Styrene Cost

Vinyl toluene Strength, stiffness

Acrylic (PMMA) Low flammability, flexibility

The relatively easy accessible raw materials - PTA or DMT and MEG.

The very well understood and described simple chemical process of polyester synthesis.

The low toxicity level of all raw materials and side products during polyester production and processing.

The possibility to produce PET in a closed loop at low emissions to the environment.

The outstanding mechanical and chemical properties of polyester.

The recyclability.

The wide variety of intermediate and final products made of polyester.

Modifications of unsaturated polyester resins have been done in order to-

achieve lower styrene emission. better styrene solubility. lower processing viscosity. Two concepts are used:- Introduction of liquid crystalline segments into the

unsaturated polyester. End-capping the unsaturated polyester with

poly(ethylene glycol)ols of various molar mass.

C C CC

Unsaturated portion

Polyesters must have unsaturated portions to crosslink

Initiators are sometimes called catalysts.

The crosslinking reaction is began when an initiator reacts with the double bond.

The most common initiators are peroxides.

The peroxides are effective initiators because they split into free radicals (i.e. they have unshared electrons) which react easily with the double bonds.

C C CC

IInitiator ●

C C CCI

●●●

C C CCI

●

Bond (2 electrons)

Unshared electron

C C CCI

*

Free radical (unshared (unbonded) electron)

Free radicals react readily with anyCarbon-carbon double bond they encounter

To react and form a crosslink, the free radical on the polymer needs to encounter (collide with) a double bond on another polymer.

The polymers are long and entangled (highly viscous), thus they don’t move very quickly.

The polymers are bulky and it is hard to get the free radical into the area of the double bond.

Dissolve (dilute) the polymer with a solvent so that the polymers can move around freely.◦ Ideally, the solvent will react during the cross-linking

reaction so that it does not need to be removed from the solid.

◦ These types of solvents are called “reactive solvents” or “reactive diluents” or “co-reactants”.

Added benefit:◦ The solvent will also reduce the viscosity so that the

polymer will wet the fibres more easily.

Styrene is the most common solvent for polyesters. The styrene reacts (is consumed) during the crosslinking

reaction because the styrene contains a double bond and reacts with the free radical.

The styrene serves as a bridge molecule between the polymer chains (as part of the crosslink)

There may be as many as 8 styrene molecules in a bridge.

C

C

C

C

C

C

C C

C C CCI

The styrene is a bridge molecule between the polyester polymers

Styrene

C C CC

New free radical*

New bonds(crosslink)

The new free radical is available to react with another styrene

Called addition or free radical cross-linking reaction.

Proceeds as a chain reaction.

◦ Once started, it will keep going unless specifically terminated.

◦ Doesn’t need more initiator .

◦ Makes its own reactive sites.

Inhibitors are added, usually by the resin manufacturer, to slow down the cross-linking reaction.

◦ Inhibitors typically absorb free radicals.

◦ Inhibitors protect the polymer during storage because sunlight, heat, contaminants, etc. can start the curing reaction.

◦ Molders must add sufficient initiator to overcome the inhibitors and to cause the cross-linking to occur.

Styrene produces the following complaints:

A.Considerable de-fatting of the skin.

B.Irritation of the mucous membrane of the eyes, nose, throat, and trachea .

C.Narcosis ("styrene sickness").

precaution must be taken during polyester resin handling.

Flame Retardant UP-Formulations.

VOC Emissions (volatile organic compounds).

Odourless UP-resins and UP-Networks.

Further Activities related to Human Health and Well Being.

A number of factors are creating opportunities to develop new high performance polyesters at lower cost.

Health concerns over bisphenol-A are also opening the door for the use of polyesters in traditional epoxy applications, such as interior can coatings.

Strong growth in powder coatings, composites and coil are increasing the market need for polyesters.

KALYAN MAITY.