03 polyesters

7/29/2019 03 Polyesters

1/34

Polyesters

Brent Strong


2/34

Polyesters (Unsaturated)

The most common type of resin for

composites

The least expensive composite resin

The easiest-to-cure composite resin

Polyesters are made from two types of

monomers: Di-acids

Glycols


3/34

Polyester polymerization

Monomers

Glycols G

(di-alcohols)

Acids A

(di-acids)

G

G

G

G

A

A

A

A

A

Polyester polymer


4/34

Polyester polymerizationn

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 formwater (HOH)

The water separates from the polymer and

condenses out as a liquid

These type of polymerization reactions are called

condensation reactions


5/34

Polyesters polymerization

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 OH

Glycol

G G

GlycolGlycol


6/34

Customizing the Polyester Acids

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


7/34

Customizing the Polyester Glycols

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


8/34

Customizing the Polyester Solvents

Solvents Attributes

Styrene Cost

Vinyl toluene Strength, stiffness

Acrylic (PMMA) Low flammability, flexibility


9/34

Customizing the Polyester

Adding Other Monomers or Resins

Resin Purpose

Dicyclopentadiene(DCPD)

Lower cost, improvestiffness

Styrene butadiene

rubber (SBR)

Toughness

Thermoplastics Surface quality


10/34

Polyesters specific molecules

COCCOCCCCOH

O O O

COCCCCOCCOC

O O O

C OH

Iso (meta)

Isophthalic Polyester

unsaturationunsaturation

CCOCCOOC

O

CCC

OH

O

C

C

C C

C

C

O

O

C

C C C O

O

C O

C

C

C

O C C

OH

C

Bisphenol A Fumaric Acid Polyester

RepeatingUnit

The number of repeating units is usually shown by an n

n


11/34

Polyesters crosslinking (curing)

C CC

C

Unsaturated portion

Polyesters must have unsaturated portions to crosslink


12/34

Initiators (catalysts)

Initiators are sometimes called catalysts.

The crosslinking reaction is begun when aninitiatorreacts with the double bond.

The most common initiators are peroxides.

The peroxides are effective initiatorsbecause they split into free radicals (that

is, they have unshared electrons) whichreact easily with the double bonds.

Free radicals have unshared electrons.


13/34


C CC

CIInitiator


14/34


C CC

CI


15/34


C CC

CI

Bond (2 electrons)

Unshared electron


16/34


C CC

CI

*

Free radical (unshared (unbonded) electron)

Free radicals react readily with any

Carbon-carbon double bond they encounter


17/34

Polyesters Reaction Problem

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 (highlyviscous), thus they dont move very quickly

The polymers are bulky and it is hard to get the

free radical into the area of the double bond

The chances oflining up just right are not good


18/34

Polyesters Reaction Solution

Dissolve (dilute) the polymer with a solvent so

that the polymers can move around freely

Ideally, the solvent will react during the crosslinking

reaction so that it does not need to be removed fromthe 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 fibers more easily


19/34

Styrene

Styrene is the most common solvent for polyesters

The styrene reacts (is consumed) during thecrosslinking reaction because the styrene contains adouble bond and reacts with the free radical

The styrene serves as a bridge molecule between thepolymer 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


20/34

Polyester forming the crosslink

C CC

CI

The styrene is a bridge molecule between the polyester polymers

Styrene

C CC

C

New free radical*

New bonds

(crosslink)

The new free radical is available to react with another styrene


21/34

Crosslinking Reaction

Called addition orfree radical

crosslinking reaction

Proceeds as a chain reaction

Once started, it will keep going unless

specifically terminated

Doesnt need more initiator

Makes its own reactive sites


22/34

Continuing and Terminating

Once started, each free radical-based reactionwill continue until the new free radical site stopscolliding with double bonds Runs out of reactive diluent

Stops encountering other polymers with double bonds Perhaps because the polymers get so long they dont move

much (the polymer becomes a rigid solid)

Post-curing can induce some movement in solids andincrease the amount of crosslinking

The free radical site might cease to exist React with another growing chains free radical site

React with another free radical in the solution From an initiator (danger of adding too much initiator)

From ozone (using wax to help exclude air)


23/34

Inhibitors

Inhibitors are added, usually by the resinmanufacturer, to slow down thecrosslinking reaction

Inhibitors typically absorb free radicals Inhibitors protect the polymerduring storage

because sunlight, heat, contaminants, etc.can start the curing reaction

Molders must add sufficient initiatortoovercome the inhibitors and to cause thecrosslinking to occur


24/34

Promotors (accelerators)

Added to the polymer to make the initiator workmore efficiently or at a lower temperature Each type of peroxide has a temperature at which it

will break apart into free radicals

These temperatures are usually above roomtemperature

For room temperature curing, a chemical method forbreaking apart peroxides is needed

The most common promoters (accelerators) arecobalt compounds and analines (DMA)

Neveradd a promoter directly into an initiator


25/34

Additives

Additives are components (usually minor) that

have various functions that are not related to the

curing reaction

The most common types of additives are: Fillers (to lower cost and/or give stiffness)

Thixotropes (to control viscosity)

Pigments

Fire retardants

Surfactants (to promote surface wetting)

UV inhibitors/Anti-oxidants


26/34

Factors influencing cure

Mix ratios

Resin, initiator, inhibitor, accelerator, solvent

Fillers, pigments, other additives

Storage time after activation

Thickness of the part

Cure time Humidity

Temperature


27/34

Thermal effects

The rate (speed) of chemical reactionsincreases as the temperature is increased Arrhenius equation: exponential relationship between

rate and temperature

Rate roughly doubles for each 10C rise

Molecularcollisions are required

Heat increases molecular movement

Highly reactive entities (like free radicals) havesuccessful reactions with almost every collision

Some reactions create heat (exothermic)


28/34

Time-Temperature Curve in the sample

at constant applied temperature

100

200

300

0 5 10 15

Time, min

Temperature oF oC

38

93

149


29/34

Temperature-Viscosity Curve

Viscosity

Time/Temperature

Liquid-Solid Line

Solids

Liquids

Region A Region B

Thinning due to

temperature

Crosslinking

Combination

(What is seen)

Gel Point


30/34

Thickness-Exotherm Curve

Thickness

Heat

Buildup

Heat builds up because of the poor thermal conductivity of polymers


31/34

Peroxide content

Temperature and Time Curves

Peak Exotherm,oC

% methyl ethyl ketone peroxide (MEKP)

0.5% cobalt

naphthenate

0 0.5 1.0 1.5 2.0100

110

120

130

140

150

F

210

230

250

270

290

310


32/34

Accelerator Content Time Curve

Gel Time,

Min

% Cobalt Naphthenate

50

100

150200

250

300

.004 .008 .012 .016 .020

At 0.25% MEKP

At 0.50% MEKP


33/34

Polyesters


34/34

Thank you

A. Brent Strong

03 polyesters

Documents