Polyester resins are made from di-acids and glycols in a condensation reaction that produces water as a byproduct. They are the most common and inexpensive composite resins, and are used to make products like molding compounds, adhesives, and coatings. Styrene is often added as a reactive diluent to reduce viscosity and allow for easier processing. When cured with an initiator like a peroxide, the styrene participates in free radical chain reactions that crosslink the polyester polymers into a thermoset network. While polyesters have many advantages, styrene emissions during curing can cause health issues, so new formulations aim to reduce or eliminate styrene.

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91 / OLT / 091013

2.  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.

3.  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.

4.  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 .

5. Monomers
Glycols G
(di-alcohols)
Acids A
(di-acids)
G
G
G
G
A
A
A
A
A
Polyester polymer

6.  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.

7. 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 OH
Glycol
HO-G
HO G OH
Glycol
G G
GlycolGlycol

8. 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

9. 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

10. Solvents Attributes
Styrene Cost
Vinyl toluene Strength, stiffness
Acrylic (PMMA) Low flammability, flexibility

11.  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.

12.  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.

13. 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.

14. C C C
C
Unsaturated portion
Polyesters must have unsaturated portions to
crosslink

15.  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.

16. C C C
C
IInitiator ●

17. C C C
CI
●●
●

18. C C C
CI
●
Bond (2 electrons)
Unshared electron

19. C C C
CI
*
Free radical (unshared (unbonded) electron)
Free radicals react readily with any
Carbon-carbon double bond they encounter

20.  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.

21.  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.

22.  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

23. C C C
CI
The styrene is a bridge molecule between the polyester polymers
Styrene
C C C
C
New free radical*
New bonds
(crosslink)
The new free radical is available to react with another styrene

24.  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.

25.  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.

26.  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").

27. precaution must be taken during polyester
resin handling.

28.  Flame Retardant UP-Formulations.
 VOC Emissions (volatile organic compounds).
 Odourless UP-resins and UP-Networks.
 Further Activities related to Human Health and Well Being.

29. 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.

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