Epoxy resins. III. Application of fourier transform IR to degradation studies of epoxy systems

Epoxy Resins. 111. Application of Fourier Transform IR to Degradation Studies of Epoxy Systems

S. C. LIN, B. J. BULKIN, and E. M. PEARCE, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, New York 11201

Synopsis

Three cured epoxy resins were investigated under various degradation conditions by Fourier transform infrared (FTIR) spectroscopy for measurement. The epoxy resins were the diglyadyl ethers of bisphenol A (DGEBA), phenolphthalein (DGEPP), and 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF). The thermal stability order of functional groups that incurred DGEBA was total methyl group -total benzene ring > methylene > p-phenylene > ether linkage > isopropylidene. The oxidative thermal and photodegradation processes were found to be related to the classical autocatalytical oxidation of aliphatic hydrocarbon segments. The Wieland rearrangement, Nor- rish-type reaction, Claisen rearrangement, and other possible degradation mechanisms were suggested by the data.

INTRODUCTION

Degradation is an irreversible process in which useful polymer properties degenerate when exposed to specific environments. An understanding of the mechanism by which a polymer deteriorates may lead to new approaches to stabilization.

A review of the thermal and oxidative thermal degradation of epoxy resins1,2 includes the literature up to 1966. Since then several workers3-12 have studied the thermal degradation of epoxy resins, but most of the literature has concen- trated on amine-cured systems in which amine was the initiator. Little information that deals with the degradation mechanism derived from the epoxy polymer main chain has been reported.

Another study based on radiothermal analysis of isotopically labeled epoxy systems has been made of the relative thermal stability of heat-labile groups in bisphenol A epoxy resin^.^ The thermal stability of isopropylidene groups in the bisphenol segments of the resin was found to be greater than that of the reacted glycidyl groups under oxidative and nonoxidative conditions with methylene-diamine or phthalic anhydride as the epoxy curing agent.

The thermal stability of a polymer can be evaluated by differential scanning colorimetry (DSC), thermogravimetric analysis (TGA), and thermal mechanical analysis (TMA) techniques, but these results can give only an estimate of stability and do not provide enough information to interpret the deterioration process itself. Several techniques, such as gas chromatography?J3 mass spectro~copy,~~ radiochemical analysis,3,8 or their combinations, have been developed and widely applied in the study of degradation mechanisms. Those proposed from these results were based on degradation product analysis related to the original polymer structure. The degradation of material a t high temperatures may cause rearrangement of the degradation fragments, secondary decomposition of primary

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17,3121-3148 (1979) 0 1979 John Wiley & Sons, Inc. 0360-6376/79/0017-3121$01.00

3122 LIN, BULKIN, AND PEARCE

products, or even more complicated reactions. Therefore product analysis may lead to erroneous degradation mechanisms.

To overcome the difficulties inherent in product analysis studies infrared (IR) spectroscopy has been a t t e m ~ t e d ~ , ~ with dispersive spectrometers. Unfortu- nately it is difficult to achieve sufficiently high signal-to-noise ratios or frequency accuracy in these instruments to detect small changes in the spectra.

Fourier transform infrared (FTIR) spectrometers can produce spectra of high accuracy in both intensity and frequency.15J6 The application to polymeric systems has been discussed by Koenig.17 Of particular relevance to the degradation studies discussed herein is that the high quality of the spectra permits subtraction of absorbance spectra. As discussed in more detail later, absorbance subtraction is a powerful technique for determining relative functional group stability in copolymer.

FTIR has been applied to the study of polymeric materials17J8 which include the oxidation and thermal degradation of p01ybutadiene.l~ By subtracting the spectrum of the pure polymer from the oxidized or degraded sample information regarding the initial stages of oxidation or degradation may be gained.

In this study FTIR is used to determine the thermal and photodegradation of the epoxy polymers in order to explain how the epoxy-TMB systems degrade, what groups are formed or destroyed, and how the functional groups change in concentration under oxidative or nonoxidative conditions.

EXPERIMENTAL

Curing Process

Three diglycidyl ethers of bisphenols were obtained from bisphenol A (DGEBA), phenolphthalein (DGEPP), and 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF) after reaction with epichlorohydrin in the presence of sodium hy- droxide.lg

Epoxy resin mixed with 9.5 g/(equiv epoxy resin) of trimethoxyboroxine was dissolved in acetone which had been dehydrated by K2C03 and distilled at 56°C before use. A smooth, polished aluminum plate (see below) was immersed in the epoxy-TMB solution, taken from solution, and placed in a desiccator to let the epoxy-TMB coating dry on the surface of the aluminum plate. The curing cycle was 135OC for 3 hr, 18OOC for 3 hr, and 220°C for another 3 hr.

Application of FTIR to Curing Reaction and Degradation of Epoxy Resins

The FTIR system used was a Digilab Inc., FTSIR spectrometer, model FTS-20B. In this study all spectra were collected and processed at 2-cm-l resolution, to which 256 interferometer scans were added.

The setup in the sample chamber is shown in Figure 1. The IR beam from the source passes through the sample on the aluminum plate, is reflected, passes back through the sample, and is then directed to the detector. The temperature of the heater was controlled by a Variac (f2OC) and measured by a thermocouple connected to a potentiometer.

3123

I I I I I I I I I I

I I

I I I 1.- - - - - - - - - - .- - - - - - - - - - -- - - - - - - - - - -1

Fig. 1. Diagram of sample compartment of FTIR spectrometer: (a) infrared source; (b), (c), (d), and (el, reflection mirrors; (f) detector; (9) microheater; (h) sample plate; (i) thermocouple.

Thermal and Oxidative Degradation Studies

The cured epoxy coating on aluminum plate was applied to the heater in the sample compartment of the FTIR spectrometer under an inert (N2) or oxidative (air) atmosphere. The degradation at 300°C was followed by collection of the spectra of the sample in several different periods.

Photodegradation Studies

The equipment for photodegradation consisted of a mercury-xenon arc op- erating at 1000-W output power. The ultraviolet (UV) light passed through a lens as a collimator and irradiated the sample in a reaction chamber. The sample was taken out for IR spectral measurement after specific periods of irradiation and put back for further irradiation. Air was pumped into the reaction chamber during the study to control the degradation conditions and the temperature in the chamber (26°C).

RESULTS AND DISCUSSION

The difference spectrum in epoxy resin thermal oxidative degradation studies was first carried out by Park and Blunt20 with a dispersive instrument. They found a decrease in the -OH absorption and an increase in the C=O absorption. Because of the difficulty of preparing uniformly thick films and the low sensitivity of the instrument, much of the detailed information in the degradation difference spectrum apparently was lost. In addition to the high-sensitivity advantage, the FTIR also provided the difference spectrum from the same thin film.


Assignments of Infrared Absorption Bands of Epoxy Polymers

Figure 2 shows the IR spectra of DGEBA, DGEPP, and DGEBF epoxy resins after curing but before degradation. The 3550-cm-l band for the three resins was assigned to stretching the -OH group. The absorptions at 3060 f 8 and 3036 f 2 cm-l are due to C-H stretching of the -CH3 group. C-H stretching of the -CH2- group can be assigned to 2933 f 3 and 2878 f 2 cm-'. The absorption at 1732 cm-l for DGEBA and DGEBF is broad, has a shoulder a t 1765 cm-', and may be assigned to stretching the carbonyl groups in aldehyde (1732 cm-l) and perester (1765 cm-l), which may be formed, respectively, from isomerization of the epoxide group22 and the subsequent oxidation of the aldehyde during the high-temperature curing process.

In DGEPP the strong shoulder absorptions at 1782 and 1750 cm-l are in the range of the C=O stretching mode of the unsaturated five-member lactone ring which produces absorptions of 1772-1787 cm-l in a nonpolar solvent.23 The phthalide derivatives also showed absorptions between 1776 and 1805 cm-l, depending on the s ~ b s t i t u e n t . ~ ~ The doublet from this vibrational mode has been attributed to Fermi

Bands at about 1610 and 1578 cm-l are derived from quadrant stretching of the benzene rings, which are not sensitive to changes in 0-, m-, or p-sub~ti tut ion~~; 0- and p-disubstituted benzene show semicircle stretching at 1510 cm-l in the three spectra. The semicircle stretching of o-disubstituted benzene has another component a t 1465 cm-' (1448 cm-l) for DGEPP (DGEBF). The second component for p-disubstituted benzene semicircle stretching is a t 1412 cm-', observable for DGEPP and as a shoulder for DGEBA and DGEBF. The weak absorption bands between 1400 and 1500 cm-l, other than those already dis-

I.., ._ . I ._ . .I. 36CO 3000 200c 1000

Waver.urrbers (cn-' )

Fig. 2. IR spectra of DGEBA, DGEPP, and DGEBF resins after curing with TMB.

EPOXY RESINS. I11 3125

cussed, are derived from the C-H bending modes.26 The weak absorption at 1290 cm-1 for DGEBA and DGEBF is assigned to the twisting mode of the -CH2- group which appears at 1279 cm-l for polypropylene oxide.26 The strong absorption band at 1288 cm-l observed in the DGEPP spectrum could result from the C-0-C asymmetric stretching of the lactone ring. The 1255-cm-l absorption band of the three resins is the stretching mode for aromatic ether. The 1184-cm-l absorption band seen in the three is not present in phenyl glycidyl ether and can be assigned to C-C stretching of the bridge carbon atom between two p-phenylene groups, linked with another C atom instead of the p-phenylene group; that is,

The absorption at 1120 cm-1 was attributed to C-C stretching of the aliphatic chain derived from the glycidyl group.26 The 1086-cm-l DGEPP and the 1096-cm-l bands of DGEBF may be produced by asymmetric stretching of the ether linkage in the gauche forms. The absorptions at 1035 and 1010 cm-l developed from stretching the trans and gauche forms of the ether linkage, respectively, which showed the same absorption position in polypropylene oxide. The 830- and 750-cm-l absorptions may be assigned to p- and o-phenylene groups, respectively.

The other particular absorptions of DGEPP at 965,940, and 925 cm-l, absent in the other two polymers, are derived from lactone ring modes. The strong, sharp absorption of DGEPP at 692 cm-', also present in phenolphthalein, was most likely due to the out-of-plane sextant ring bending that is considered weak or IR inactive for ortho- and paraphenylenes with two different or two identical sub~tituents.~5 Because of the high polarity of the carbonyl group, which may also be enhanced by mesomerism, this vibration may turn out to be IR active. Table I contains a summary of the results from this discussion.

Application of FTIR for Degradation Studies

Because of the high SIN and frequency accuracy of the FTIR system, absorbance subtraction can be used as a sensitive method for detecting the small changes in a sample during its degradation. The common features in the spectra are canceled and only the change is recorded. Koenig17 applied this technique to study the oxidation of polybutadiene and used the elimination of the interfering absorbance process to isolate the particular absorbance due to a certain component.

Interfering Absorption Treatment l7

For spectrum 1 the following equation may represent the total absorbance,

(1) where A is the absorbance. A similar expression can be written for spectrum 2:

AFI, of all components (x, y, and z ) at frequency v.

A'+, = A:, + Af;, + A:,

TAB

LE I

Te

ntat

ive

Infr

ared

Abs

orpt

ion

Ass

ignm

ents

for t

he T

hree

Cur

ed E

poxy

Res

ins a

nd th

e A

bsor

ptio

n V

aria

tions

Dur

ing

Deg

rada

tion

Vib

ratio

n W

aven

umbe

r D

GEB

A

DG

EPP

D

GEB

F Fu

nctio

nal

(cm

-')

IRa

TDb

TOC

POd

IR

TD

T

O

PO

IR

T

D

TO

PO

gr

oup

mod

e

3570

X

e 35

50

3525

+f

+

3430

3350

33

00

3200

30

68

3060

30

52

X

3038

30

34

X

2970

X

29

35

X

2933

29

30

2880

28

76

X

1808

1790

1784

1782

17

75

- -

+ + +

+ - + +

X

+

- -

X

+ + +

} R-O

H

ArO

H

0 I1

Arc

on

} R-O

OH

Ar-O

OH

Ary

lene

1 Met

hyl

Met

hyle

ne

I 00

RC+-C

R II

II h

0 II RC

OO

H

} Lac

tone

v(0-

H)

v(0-

H)

v(0-

H)

~(0

-H)

v(0-

H)

v(C-

H)

v(C

-H)

v(C-

H)

v(C

=O)

v(C

=O)

v(C

=O)

"E

1765

X

1754

1745

1732

X

+ +

+ +

X

-

- +

X

+ +

+

+ +

+ +

Q I1 RC

OOR

u(C

=O)

Lac

tone

Q

RCQR

II ZH

II Q 0 II

RCO

H

+ +

+ +

+ X

1725

+

+ +

1715

X

1665

16

10

X

+ +

+ +

- -

M

cd

0 5

Phen

oxr

Phen

ylen

e

I Q

uadr

ant

stre

tchi

ng

1578

X

15

10

X

1505

Phen

ylen

e

1480

+

+ +

+ +

-

-

-

X

+ 14

65

Sem

icir

cle

stre

tchi

ng

1448

1440

+

+

+ +

-

-

1425

1412

X


I - -- I + + I + I 1 + 1 I I I +

+ + I I 1 + 1 I I I +

I I 1 + 1 I I I

X X x x x x

I + + + I + I + I I I I I l l +

I I I I I I I I l l +

I + I I I + I I I I I l l

X X X X x x x x x x

+ + I + I I + I I I +

+ + I I I I I +

I I I I I +

X X X X X

840

830

X

825

822

775

754

746

740

736

725

715

692

-

- X

+

- p-

Phen

ylen

e In

-pha

se,

-

out-

of-p

lane

X

X

-

- -

hydr

ogen

w

aggi

ng

o-Ph

enyl

ene

In-p

hase

, 0

out-

of-p

lane

hy

drog

en

wag

ging

$

sext

ant

!2 rin

g 2 z

bend

ing

r/,

i +

+ +

+ -

-

X

+ -

-

- X

+

+ + -

+ +

X

-

-

-

o-Ph

enyl

ene

Out

-of-

plan

e

a IR

Orig

inal

IR sp

ectr

um of

epo

xy re

sin.

TD

: T

herm

al d

egra

datio

n.

TO

: Th

erm

ooxi

dativ

e de

grad

atio

n.

PO:

Phot

ooxi

dativ

e de

grad

atio

n; st

retc

hing

. X

: A

bsor

ptio

n pr

esen

t in

the

orig

inal

spe

ctru

m

+: A

bsor

banc

e in

crea

se d

urin

g de

grad

atio

n.

R -:

Abs

orba

nce

decr

ease

dur

ing

degr

adat

ion.

h:

T

he a

bsor

ptio

n fo

rmed

pos

sibl

y by

dec

reas

ing

peak

, 178

2 cm

-' an

d in

crea

sing

pea

k, 1

784

cm-'.

U

Y

Y


AF2 = A:, + A;2 + Ai2 (2) The absorbance of subtracted spectra (A ,” ) at a frequency u is

A : = A52 - KA51 = (Ai2 - kA:,) + A;2 - kAL;,) + (Ai2 - kAi1) (3) where k is an adjustable scaling parameter. To remove the absorbance due to component x it can be set as

A22 - k A : l = A:, = 0

or

Therefore the difference absorbance will become

A : = (A;2 - kA;J + (Ai2 - kA:,)

and all characteristics of component x can be removed.

Comparison of Functional Group Stability

A polymer molecule normally contains several different functional groups with different stabilities. Sometimes the least stable group may degrade very fast and also initiate the degradation of another functional group. FTIR can be used to rank the stability of each of the functional groups present in a polymeric material.

If two functional groups x and y decrease their IR spectral absorbance at the characteristic frequencies u, and vy, the difference absorbances of each functional group can be expressed as

A:” = A? - krA!x (6)

and

A:Y = A5Y- k‘ATY (7)

By removing functional group x from the difference spectrum the k’ parameter can be obtained:

(8 )

where C is the concentration of the functional group x. Three situations can occur in relation to the absorbance of the functional group y :

(9)

which could indicate that group y is more stable than (>O), as stable as (= 0), or less stable (<O) than group x , respectively, under specific degradation conditions.

k’ = AYi(/AY = C;X;”/c;X

A:Y = A? - krATY = Eyb(C$ - k’C’;) 5 0

Thermal Degradation

Figures 3,4, and 5 show the IR spectra and difference spectra (k‘ = 1) of cured DGEBA, DGEPP, and DGEBF, respectively, before and after thermal degradation at 300°C for 1 hr in a nitrogen atmosphere. I They show that all the absorption bands of the cured resin have decreased in intensity and that new bands


Fig. 3. IR and difference spectra of cured DGEBA before and after thermal degradation a t 3Oo0C for 1 hr.

appear at a few frequencies; their absorption intensities are to be compared with the spectra of the sample before degradation.

To understand the relative stability of the functional groups in the resin the subtracted spectra were obtained by changing the k’ parameters, as shown in Figure 6 for DGEBA. After the band at 830 cm-l (p-phenylene) is canceled = 0) i t can be observed that the difference absorbances and A,:876 change from negative to positive. These changes indicate that the p-phenylene group is not so stable as the -CH3 and -CHz- groups and may rearrange to a more stable form of substituted benzene species. After canceling the -CH3 group absorption = 0) the C-H stretchings of benzene ring at 3052 and 3034 cm-l also almost disappear and those of the -CHz- group at 2935 and 2876 cm-l still keep their negative difference absorbances (Fig. 6). These results indicate that the -CH3 group has similar stability to the total benzene ring content of the polymer and higher stability than the -CH2- group. The ether linkage at 1035 cm-l and the C-C bond of the isopropylidene group, 1184 cm-l, show decreased absorbances with all the canceling processes for the p -phenylene, -CH3, and -CH2- groups. The isopropylidene group seems to be the least stable in the polymer chain. On the basis of these results the order of functional group stability is total methyl group - total benzene ring > methylene > p-phenylene > ether linkage > isopropylidene. The only source of the -CH3 group in DGEBA is the isopropylidene group. The methyl and isopropylidene groups show different stabilities; the former, the most stable and the latter, the least. The only possible reason for this phenomenon is that the isopropylidene group degrades, releases the first -CH3 group, and retains the second methyl group until the latter stages of degradation.

A:”’,


I I I 1

1 I / . 3600 3000 2000 1000

Wavenumbers (car1)

Fig. 4. IR and difference spectra of cured DGEPP before and after thermal degradation a t 3OOOC for 1 hr.

Dissociation of aryl-isopropylidene bond and loss of methyl group have been observed by Paterson-Jones12 for an amine-cured DGEBA system based on pyrolysis product analysis and can account for the high stability of a -CH3 group and the lower stability of the isopropylidene group. No matter which reaction is favored during thermal degradation, the two decompositions retain one or two methyl groups which may not easily be released:

CHI

Both reactions will cause the absorption at 1184 cm-l for the C-CH3 stretching frequency to shift to another position which appears a t 1170 cm-l in the difference spectra (Figs. 3 and 6 ) . Lee27 found phenol (largest component), isopro- pylenylphenol (major component), and isopropylphenol (major component) as the products of the pyrolysis of DGEBA cured with methyl nadic anhydride or methylenedianiline at 475°C in uucuo. This confirmed that the aryl-isopropylidene bond dissociation occurs during the initial decomposition. From the


3 Wavenumbers ( c m - 1 )

Fig. 5. IR and difference spectra of cured DGEBF before and after thermal degradation at 300°C for 1 hr.

dissociation energy viewpoint, the C-CH3 bond in the isopropylidene group has a low bond energy, 60 kcal/mole (Scheme I), which indicates a relative pro-

Scheme I

CHa 83 OH

I O-CH2-CH- CH2-O- CH,-CH-CH,---O - I J

I 0

t I --%* 6o CH, 86 f 86

CH2

I pensity for thermal dissociation. It has been noted that methane is a product of p y r o l y ~ i s . ~ J ~ , ~ ~ Reaction (11) seems to be responsible for the formation of methane at low degradation temperatures. Bowen3 used isopropylidene 2- I4C-labeled and glycidyl-1,3 I4C-labeled epoxy resin to determine the relative thermal stability of heat-labile groups in a bisphenol A epoxy resin. The thermal stability of the isopropylidene groups in the bisphenol segments of the resin was greater than that of the reacted glycidyl groups under both oxidative and nonoxidative conditions with methylene dianiline or phthalic anhydride as the curing agent. These results are consistent with this study, in which the total methyl group is the most stable functional group. Bowen did not recognize the low stability of the isopropylidene group because the carbon atom in the methyl group


0

Fig. 6. Variations in the thermal degradation difference spectrum of cured DGEBA during canceling process.

had not been labeled and thus could not be detected by radiochemical analysis.

The relative stability of the total substituted benzene ring compared with the paraphenylene group may be related to a Claisen rearrangement of the aryl ally1 ether that will change the paraphenylene group to a 1,2,4-trisubstituted benzene with increased thermal stability.

OR

e - C H , C H - O - I - -ROH

(12) Claisen rearrangement

M H , C H = C H - O -

CH-CH=CH,

O- I

The C-C bond formation may not be easily observed because of overlapping of the absorbance of the rapidly decreasing paraphenylene and epoxide ad- sorption as well as its inherent weakness in the infrared. The 1,2,4-trisubstituted benzene can be confirmed by the increase of the 815,1425,1480, and 1600-cm-' bands in the A,:3o = 0 difference spectrum (Fig. 6). Phenol formation can be detected in the range of 1200-1300 em-' if compensation is made for the decrease at 1255 cm-l due to aromatic ether cleavage. Lee27 found that water was the largest fraction in the volatile decomposition products obtained from cured


DGEBA at 350°C and explained it on the basis of alcohol dehydration, another possibility for the formation of unsaturation.

For aryl ether the electron shifts are controlled by the balance of the negative inductive effect of the oxygen atom; the positive mesomeric effect tends to move electrons into the benzene ring7:

The mesomeric effect gives a higher electron density to the-oxygen-phenylene linkage and reinforce this bonding.

The decomposition of aromatic ether is expected to accompany phenol formation. The disappearance of the 1225-cm-l absorption and the appearance of the absorption in the range of 1200-1300 cm-l seem consistent witt this as- sumption:

OR OR

The instability of the ether linkage and the aliphatic chain derived from the glycidyl group is important in relation to the overall weight loss of the cured material during thermal degradation. Lee27 observed 38.9% (the largest component) and 2.52 mole % (one of the major products) of methylcyclopentadiene and cyclopentadiene, respectively, in the volatile products from the pyrolysis of DGEBA resin cured with methyl nadic anhydride. He also noted that methylcyclopentadiene is the major volatile product (next to water) formed during the pyrolysis of methylenedianiline-cured DGEBA at 350°C. On the basis of these results it appeared that the aliphatic ether linkage in the cured resin may undergo some rearrangement before forming and releasing methylcyclopentadiene and cyclopentadiene.

This also gives a possible explanation for the rapid ether and aliphatic chain decompositions. Scheme I1 is assumed to account for the formation of methylcyclopentadiene and cyclopentadiene.

Because of the easier bond cleavage of 0 - C H F compared with o-phenylene, methylcyclopentadiene formation can be expected to form in greater quantity than cyclopentadiene, as indicated in this degradation scheme.

The only difference between DGEBA and DGEBF is the replacement of the isopropylidene group by a fluorene group. The IR and difference spectra (Fig. 3-5) are similar to those of cured DGEBA except for two different groups. In the = 0 difference spectrum for thermally degraded DGEBF at 300°C for 1 hr in nitrogen gas the absorption due to orthophenylene group at 746 cm-l increases sharply. This indicates that the fluorene group is much more stable than the other groups in the DGEBF cured resin. This phenomenon probably accounts for the high char formation of cured DGEBF resin.

The IR and difference spectra of the DGEPP cured resin before and after thermal degradation at 300°C for 1 hr in nitrogen gas (Fig. 4) are much more complicated than the other two cured resins because of the presence of the lactone ring in the phthalide structure. The lactone carbonyl group (1782 and 1754


Q ?


cm-l) decrease is indicative of a lactone ring-opening reaction. Three possible lactone ring opening processes may be occurring:

/- (16)

II 0

The difference spectrum for cured DGEPP (Fig. 4) shows that the 1184-cm-1 band (C-C stretching) decrease is accompanied by an increase in the 1180-cm-l band. This may merely indicate that an absorption frequency shift occurred instead of the destruction of the bond. Furthermore, the in-phase, out-of-plane hydrogen wagging of an o-phenylene group absorption at 754 cm-l shows an increase in intensity. These results imply that reaction (14) is improbable. In the 1184-cm-l band there is a decrease in intensity at 1288 cm-l, a result that may indicate that the bond dissociation of C-0-C in the lactone ring and the formation of ester or carboxylic acid formation happen at about the same time during thermal degradation. In other words, reaction (16) is more likely than reaction (15). Theoretically, the mesomerism between form (i) and form (ii) will increase the charge density of the C-0 bond and this will increase the bond strength.

I 101-

Inversely, this delocalization will weaken the C L O bond, which implies that bond cleavage in reaction (16) is easier than that in reaction (15). Other evidence that supports this view is the strong broad absorption with a maximum at about 3430 cm-' present in the difference IR spectrum of DGEPP only. This, however, could be consistent with reaction (15) or (16).

The absorption intensity decrease a t 692 cm-l may be due to the decomposition of the lactone ring. The slight increase at 750 cm-', orthophenylene, may possibly be due to the increase in the extinction coefficient that results from the lactone ring opening.

Thermooxidatiue Degradation Polymeric materials are used primarily in an oxidative environment. The

thermooxidative degradation of DGEBA, DGEPP, and DGEBF cured with TMB


I I

3600 3000 2000 1000 Wavenumbers (cm-l)

Fig. 7. IR and difference spectra of cured DGEBA before and after thermooxidative degradation a t 300°C for 25 min in air.

were examined by IR difference spectroscopy, as shown in Figures 7,8, and 9, respectively. The three resins were thermooxidatively degraded at 3OOOC for 25 min in air.

The differences between thermal and thermooxidative difference spectra are that the thermooxidative degradation shows a faster deterioration rate, a sharper increase at 3525 cm-', and a strong increase that ranges from 1620 to 1800 cm-l which is indicative of the formation of carbonyl groups and also a specific peroxide absorption band at 885 cm-l.

The increase in hydroxyl group, the increased degradation rate, and the formation of peroxide group (885 cm-1) strongly suggest that the oxidative thermal degradation of the three cured resins is an autocatalytic process proposed for hydrocarbon polymers. This reaction follows the classical scheme that can be written as follows:

Initiation:

P H + P- + H- P. + 0 2 - POO.

Propagation:

POO. + P H -+ POOH + P. POOH + PO. + .OH


Fig. 8. IR and difference spectra of cured DGEPP before and after thermooxidative degradation a t 300°C for 25 min in air.

Chain branching:

PO- + PH 4 POH + P. -OH + PH -+ HOH + P.

Termination:

2 POO. - Inert product

NeimanZ9 also concluded that the thermooxidative degradation of cured DGEBA was an autocatalytic process and radical in type from his study on the relationship of the length of the induction period and the oxygen pressure.

Some other side reactions which can form various carbonyl groups during thermooxidative degradation may also occur. The broadened absorption bands a t 1730 and 1735 cm-l in the initial IR spectrum of DGEBA and DGEBF, respectively, may be due to the formation of aldehyde that may come from the isomerization of the epoxide group as well as the partial oxidative degradation that occurs during the high-temperature curing process. The absorption a t this frequency is absent from the IR spectrum of DGEPP because of overlap with the strong absorption band of the lactone group. The 1765-cm-l absorption (shoulder) in the initial spectra may indicate perester formation30 formed by oxidation during high-temperature curing. Both absorptions show a relatively slower rate of intensity increase with others due to carbonyl groups formed during thermooxidative degradation.


I I I

I 1 3000 2000 1000*

Wavenumbers (cm-1)

Fig. 9. IR and difference spectra of cured DGEBF before and after thermooxidative degradation at :300°C for 25 min in air.

Thermooxidative degradation difference spectra of the three cured epoxy resins (Figs. 7-9) show increases at 1808,1784,1765,1745,1732,1715, and 1665 cm-'. The 1808- and 1745-cm-l bands may be attributed to the formation of acid anhydride which shows double absorptions about 65 cm-' apart in a simple carboxylic acid anhydride.31 In a noncyclic anhydride the high-frequency band is more intense than the low one because of the planar structure of the CO- O-CO group in the cis-c~nfiguration.~~ In this study it was found that the lower wavenumber band has a greater absorbance than the higher-frequency band. Two possibilities are contributory. First, the acid anhydride group is formed from two neighboring segments that are entangled by another polymer chain or have restricted segmental motion at a certain position in the solid glassy material. Hence the CO-0-CO groups formed have more difficulty obtaining the planar cis configuration than the simple acid anhydride. Second, another carbonyl group, such as an ester, with a wavenumber close to 1745 cm-l, is formed during oxidative thermal degradation and overlap with this absorption.

The carbonyl absorptions produced from photooxidative and thermooxidative degradation of p ~ l y e t h y l e n e ~ ~ ~ ~ ~ and polypropylene have been analyzed by comparing them with related simple compounds33 and reacting the carbonyl group with sulfur tetrafl~oride.3~ Seven carbonyl species,

-CH=CH-C. R-C-OH, R-C-R, R-CHO, II 0

II I1 0 0

R-C-OR. R-C---OOR, and R-C-OOH II 0

II 0

I1 0


were proposed as forming during oxidiative degradation and were assigned to the frequencies around 1685,1705,1718,1730,1740,1763, and 1785 crn-', respectively. The degradation difference spectra obtained here show results strikingly similar to those of the polyolefins.

The absorption band at 1665 cm-' may be assigned to semiquinone formation. The quinone is a highly delocalized group that will shift the carbonyl absorption to a lower wavenumber.

In a consecutive reaction, A - B - C, the concentration of B depends on the rate difference between its formation and reaction. Similarly, the IR difference spectrum also shows the total change in the absorbance of intermediate B. The absorptions at 1735 and 1765 cm-l do not show a sharp increase in intensity. This result implies that aldehyde and perester continue to form during oxidative thermal degradation but that the formation rates are less than those of other carbonyls.

On the basis of these and other results, the following possible reaction schemes may be proposed:

OR OR I I

-O--CH$HCH,--O-CH&HCH,-O-

homolysis 1 OR I I -O-CH,CCH-O-CH,CHCH~-O- -O-CH2CHCH2-O-CH2-

I OOH

/-.OH ( i )

RO I I

-O+H,CCH,-O-CH2-

0

I OR

I OR I+.*

RO

k C H 2 - O \ CH,CH-CH2-O-

0-0. \

-O--CH2

rearrangement 1 1 Lssion

OR /OR

80 -OCH,C + *CH,-OCH,- /OR I

b -()--CHIC + C H 2 0 + *O-CH2CHCH,-O-

It is a well-known phenomenon that the a-positions of ethers are easily at- tacked by oxygen. Three a-positions of ethers are present in the aliphatic chain derived from a glycidyl group, among which the one at a secondary carbon is the most active. A similar mechanism should also apply to the other two a-positions with the formation of aldehydes. M a r ~ h a l ~ ~ investigated the oxidative degradation of polymers and organic compounds which included polyethylene oxide and polypropylene oxide (atactic) induced by y radiation and concluded that at room temperature chain scission involves unimolecular decomposition of the secondary and tertiary peroxy radical with a six-membered ring transition state. The increased intensity at 1280 cm-l may also support ester formation:


0 0

(ii) II -.OR II

R - + C O R<OR R-C--O-OR - R 4 - 0 (1745 cm-') (1765 cm") 0

0

I1 + R-C-0-C-R R-COH (1715 cm-') -H,O (1808 and * 745 cm-')

This oxidation scheme may be used to illustrate the formation of five carbonyl species that show increases in the absorbances of the degradation IR ~ p e c t r a ~ ~ , ~ ~ (see iii, p. 3143).

Ovenall:38 investigated the degradation of cured DGEBA resins in air and vacuum with ESR and concluded that when the resins were heated in air both formation and decay of two radical species occurred, whereas in vacuum only decay was observed. The radicals observed from ESR results are of the semiquinone or aryloxy type:

It has been shown that the reaction of

Ph ,-0 - I

follows the Wieland rearrangement":

Ph X'

I L Ph(CH,)kOPh CHI

Hence the semiquinone should be formed from this rearrangement. The thermal degradation routes, of course, should also occur during ther-

mooxidative degradation. Thermal degradation IR spectra show increased absorption bands accountable by thermooxidative degradation that probably come from the oxidation of the polymers by oxygen trapped in them or present as an impurity in the nitrogen gas.


T

1

\ + 0, and then RH

OOH

CH:,

CH -1 Weland rearrangement

CH,


BEFORE DEGRADATION

I I 1 I.

3600 3000 2000 1000 Wavenumber (crn-l)

Fig. 10. IR and difference spectra of cured DGEBA before and after photooxidative degradation a t room temperature for 3 days in air.

Photooxidative Degradation Thermal and thermooxidative degradation of cured epoxy systems have been

studied in detail as previously mentioned but little has been done on photooxidative degradation. FTIR was used to follow the UV-induced oxidative degradation of cured epoxy resins. Figures 10, 11, and 12 show the IR and difference spectra of the three cured epoxy resins before and after photooxidative degradation at room temperature for three days in air. It can be observed that the results are similar to those of thermooxidative degradation. This suggests that the initial steps of photooxidative degradation are comparable to thermooxidative degradation.

The difference between thermooxidative and photooxidative degradation is the kind of energy that induces the degradation. The degradation rates for a certain reaction may be different even though both forms of degradation may have the same mechanism. Thus the -OH stretching at 3350 cm-l during photooxidative degradation increased in intensity much more rapidly than that during thermooxidative degradation. This absorption, combined with the band at 890 cm-', indicates that photooxidative degradation is also an autocatalyzed oxidation-the same as thermooxidative degradation.

The absorption region from 1850 to 1600 cm-' also shows a little difference between thermooxidative and photooxidative degradation. The acid anhydride formation rate was somewhat slower during the photooxidative than during the thermooxidative if this absorption is compared with other carbonyl species.

Norrish-type reactions40 have been noted as the main primary photochemical reactions to be considered in the oxidative photodegradation mechanism.


\ DIFF'ERBNCE SPECTRUN v--- \\f-

-_

I L I I 3600 3000 2000 1000 '

Wavenumber (cm-')

Fig. 11. IR and difference spectra of cured DGEPP before and after photooxidative degradation at room temperature for 3 days in air.

1 I I

I

3600 3000 2000 h'avenumher (cm-' )

1000

Fig. 12. IR and difference spectra of cured DGEBF before and after photooxidative degradation at room temperature for 3 days in air.


A Norrish-type I process is 0 0 II hv II

R-C-CH,R’ --t R-C. + .CHzR

A Norrish-type I1 process is

h RO-CH,-C-CH,-O-CHI-CH-CH,-OR --+

II I 0 0-

0- I 0

II RO-CHL<-CH,, + O=CH-CH-CH,-OR

Both processes create additional carboxylic acid, peracid, and perester during oxidation.

In the carbonyl absorption region the cured DGEPP shows a distinguishable difference spectrum for photooxidative degradation when compared with the difference spectrum for thermooxidative degradation. This may be due to the higher formation rate of peracid and perester which may overlap and cancel the decreasing absorbance at 1754 cm-l for the lactone ring. The lactone ring may follow a Norrish-type I reaction during photodegradation and form peracid:

II 0

II 0

II 0

The absorbance decrease at 746 cm-l and the increase at 3200 cm-1 during the photooxidative degradation of cured DGEBF is indicative that the fluorene ring opens and possibly forms phenol or aryl peroxide:

A0*& && 0 ‘0-0 0-0 0

0 0-OH 0

J

/ HO-0


CONCLUSIONS

In this study three cured epoxy resins were investigated under various degradation conditions with FTIR for measurement. In the thermal degradation study the stability order of the functional groups in DGEBA was total methyl group - total benzene ring > methylene > p-phenylene > ether linkage > isopropylidene. The isopropylidene group degraded, released the first methyl group, and retained the second methyl group until the latter stages of degradation. The p -phenylene group underwent a Claisen rearrangement and formed 1,2,4-trisubstituted benzene. Other possible initial degradation steps were proposed.

The oxidative thermal and photodegradations were found to be related to the autooxidative degradation processes for aliphatic hydrocarbons. The Wieland rearrangement and Norrish-type reactions, as well as other possible oxidation degradation mechanisms, were also suggested.

The authors acknowledge the support in part of the NASA Ames Research Center, Moffett Field, CA, under grant numbers NSG-2182 and NSG-2147, and the helpful discussions with J. A. Parker, S. Riccitiello, and G. Fohlen.

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3148 LIN, BULKIN, -AND PEARCE

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Documents

Epoxy resins. III. Application of fourier transform IR to degradation studies of epoxy systems