radiation and postirradiation crosslinking and structure of two unsaturated polyester resins

Radiation and Postirradiation Crosslinking andStructure of Two Unsaturated Polyester Resins

Irina Pucic, Tanja JurkinDivision for Materials Chemistry, Rudjer Boskovic Institute, Bijenicka 54, Zagreb, Croatia

Radiation and postirradiation crosslinking of two un-saturated polyester (UP) resins were monitored, andsubstantial differences in the reaction course andextents were observed. DSC thermograms of one ofthe resins showed double peaks and significantly lowerresidual reaction heats. Extraction revealed that gela-tion dose of the resin with double peak was twice thegelation dose of the other resin that had single peak inDSC thermograms. Although other components of thepolyesters were identical, NMR spectra of the resinwith a single peak revealed isophthalic units while inthe polyester of the resin having double DSC peaksorthophthalic units were detected. Orthophthalatereduced the compatibility of polyester and styrene andcaused the reaction-induced phase separation, influ-encing gel structure that was visible in scanning elec-tron microscope micrographs. Previously, the doublepeaks in crosslinking thermograms of UP resins wereusually attributed to initiator effects, but here no initia-tor was used, and, in the literature, we found that thedouble peaks are almost exclusively present in thethermograms of UP resins containing orthophthalates,whereas in resins with isophthalates double peaksalmost never appear. Crosslinking extents were signifi-cantly higher in the resin-containing isophthalate andin both cases enhanced by postirradiation reaction thatis often neglected. POLYM. ENG. SCI., 48:1768–1777, 2008.ª 2008 Society of Plastics Engineers

INTRODUCTION

The ionizing radiation produces various changes in

polymers, and it offers fast and clean means of reaction

initiation. No initiator and/or promoter is needed for radi-

ation polymerization, crosslinking, and grafting reactions.

The radiation produces reactive species homogeneously,

which is almost impossible to achieve using chemical ini-

tiators, and reaction rate can be varied independently of

temperature. Depending on polymer structure, degradation

has to be taken into account although it mostly occurs at

relatively high doses. Predominant types of ionizing radia-

tion are fast electrons, and electromagnetic radiation such

as 60Co c-radiation, although fast ions, should not be

omitted. More on the radiation effects on polymers can be

found in a number of newer reviews and books on that

subject [1–5]. One of the aspects of radiation effects in

polymers, which is mostly neglected, are the postirradia-

tion changes; although, more than 40 years ago, Chapiro

[6] described it in solid systems. Ivanov [3] defined the

postirradiation reaction as a continuation of polymeriza-

tion after the irradiation is terminated due to reactive spe-

cies, mostly free radicals, formed under the irradiation,

being trapped, and stabilized in macromolecules or in the

network. The postreaction course largely depends on

polymer under consideration, and, for some polymers, no

postirradiation changes were detected, or they vary

depending on how the samples have been stored. In other

cases, the postreaction can last up to a year or more [7]

and significantly affects the overall radiation reaction con-

version. Postirradiation degradation also occurs. Postpoly-

merization is a base for some types of radiation grafting

that are gaining importance in the production of polymer-

coated nanoparticles [8, 9]. Obviously, determination of

the postirradiation behavior should be a part of any study

on radiation-initiated reaction-concerning polymers. The

first goal of this article is to compare the postirradiation

reaction in two unsaturated polyester (UP) resins irradi-

ated to chosen doses and thus partially crosslinked.

The UP resins are thermosetting resins used mostly as

the polymer matrix for composites and, recently, nano-

composites. The UP resins consist of polyester coils swol-

len to some extent in a vinyl monomer that is at the same

time a solvent and a crosslinking agent (reactive diluent).

The crosslinking reaction of UP resins is usually initiated

by a thermal or redox initiator; promoters are commonly

added. The difficulty of achieving homogeneous mixture

of initiators and/or promoters with UP resin reduces con-

version, especially at room temperature. Incomplete cross-

linking contributes to the release of residual styrene,

which creates problems to the environment and is the

source of odor in many applications. Radiation cross-

linking of UP resins could at least partly solve these

problems.

The course of crosslinking of UP resins is rather com-

plicated, and it is still being studied. The reaction occurs

by free radical mechanism, and its course can be divided

Correspondence to: Irina Pucic; e-mail: [email protected]

DOI 10.1002/pen.21143

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2008

in four major periods: induction period, propagation pe-

riod (not to be confused with radical reaction of similar

name that takes part in all of the mentioned periods), gel

effect, and vitrification. Induction period lasts until the in-

hibitor is being used up, and it is followed by the first

stage of propagation that is driven by the law of mass.

The appearing network leads to diffusion-controlled prop-

agation, and when the network becomes dense enough to

restrict termination of free radicals, propagation rate sig-

nificantly increases, what is called Thormsdorff or gel

effect [10]. The reaction ends on vitrification, when glass

transition temperature of the formed network becomes

equal to the reaction temperature. This is one of the rea-

sons why full conversion is impossible in isothermal con-

ditions [11]. Although it is most often used, unpolar sty-

rene is not a thermodynamically good solvent for rela-

tively polar polyester [10, 12, 13], and so various

saturated diacids or higher dialcohols are added in the

preparation of polyesters to improve compatibility.

Orthophthalic and isophthalic acids are the most often

used diacids, because it is expected that their aromatic

rings will increase compatibility with styrene, and the

reactivity of the resin can be modified in that manner too,

because polyester becomes less flexible and less reactive

[14]. At very beginning of the UP resin, crosslinking

microgels appear. These globular formations of submi-

crometer range [15, 16] appear due to the intramolecular

reaction of polyester unsaturations, and some styrene mol-

ecules present inside the polyester coil. Because styrene is

incompletely solvating polyester, its concentration inside

the coil is lower [16] and reaction-induced phase separa-

tion (RIPS) might occur on crosslinking.

On the appearance of microgels, the two-component

system becomes three-component system, and phase sepa-

ration becomes very likely. De Gennes [17] first suggested

that the network formation and phase separation are collab-

orative events. Although RIPS is mostly studied for UP

resins containing fillers and low-profile additives, it may

occur on the crosslinking of neat resin [18], depending on

styrene–polyester ratio and compatibility. Furthermore, in

the crosslinking reaction, vinyl monomers interconnect

microgels to produce a three-dimensional network, and the

resin system abruptly changes from a viscous liquid into a

hard thermoset solid. Still, a part of polyester double bonds

remain unreacted inside, the microgels what is another rea-

son why full conversion of double bonds cannot be

achieved. Because the differences in radiation reaction

course as well as during postirradiation period were

observed in the two UP resins under consideration, the

other goal of the experiments described in this article was

to investigate whether RIPS contributed to that.

MATERIALS AND METHODS

Two commercial UP resins, Chromoplast A-143, resin

A in further text, and Chromoplast I-175, resin I in further

text, were provided by the producer ‘‘Chromos’’ Zagreb.

The producer stated that the polyester consists mainly of

propylene glycol and maleic anhydride diluted by 30% (by

weight) of styrene and inhibited with hydroquinone to pre-

vent room temperature reaction, and no other details on

composition were available. The UP samples were put into

the glass vials 1 cm in diameter as received, and the cross-

linking was initiated by 60Co c-radiation at a dose rate of 39

kGy/h at 218C, and no initiators or promoters were added.

The irradiation of the samples was terminated at selected

doses, the steps were 1 kGy, and intermediate steps of 0.5

kGy were added if pronounced difference in conversion was

detected. The postirradiation changes in partially cured UP

resins were measured on the first, second, fifth, and fifteenth

day after the termination of the irradiation and compared to

the results of measurements on the day of the irradiation

(zeroth day). During the postirradiation period, the UP sam-

ples were stored at room temperature.

At low doses, the samples remained liquid, whereas those

irradiated to higher doses became semisolid or solid. The

semisolid and solid samples were weighted and transferred

to Soxhlet apparatus, where the continuous extraction was

performed in dichloromethane for a week. The remaining

gel was dried in vacuum for 4 h at 508C and then weighted,

and the procedure was repeated until constant mass was

achieved. To report the postirradiation changes we defined

[19] the fraction of the gel content formed on the nth of post-irradiation period, gn

gn ¼ ðwn � w0Þ=w0 (1)

where wn is the mass of the gel on the nth day after the irra-

diation and w0 the mass of the gel on the day of the

irradiation.

Three UP resin samples of masses between 5 and

10 mg irradiated to each dose and on each chosen day

were placed in aluminum pans and sealed. A PerkinElmer

DSC 7 instrument (calibrated with In and Zn standards

provided by the producer) was operated in a dynamic

mode in the range of 50–3008C, and the heating rate was

108C/min in the extra pure nitrogen environment. Some

samples were reheated, and no heat evolution occurred in

the second heating run, confirming that the residual reac-

tion was completed in the first run. The exothermic reac-

tion heats were determined using the software provided

by the producer of the instrument, and the reaction heats

were calculated as the average heats of three samples.

Unirradiated resins were examined by FTIR and NMR

spectroscopy. FTIR spectra were recorded on Bruker Ten-

sor 27, and the resins were coated over KBr disks. Proton

and 13C NMR spectra of both resins as received and sty-

rene (Merck) were recorded on Bruker Avans NMR

300 MHz spectrometer in deuterated dichloromethane.

The identification of the absorptions was performed using

the data for corresponding monomers obtained at the SDBS

database (http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_

index.cgi?lang¼jp) and compared with literature data on

similar polyesters [20–22].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 1769

For scanning electron microscope (SEM) observations,

the samples of gel grains of both UP resins, which were

partially crosslinked and previously extracted, were

extracted again in dichloromethane (any solvent with

higher boiling point could have produced undesired chem-

ical changes) and dried in high vacuum to remove any re-

sidual monomer. Inspection of the grain surfaces was car-

ried out using thermal field emission SEM (FE SEM),

model SM-7000F, manufactured by JEOL. The sample

surfaces were not coated with electrically conductive

layer, and micrographs were taken with low accelerating

voltage, 1 kV, to avoid electrical discharges.

RESULTS AND DISCUSSION

The samples of commercial UP resins A and I were

crosslinked to various conversions by irradiating them to

selected doses lower than those needed for maximal con-

version and the corresponding courses of postirradiation

curing were compared. It is assumed that the radiation

reaction conditions are isothermal, and because full con-

version of all double bonds in UP resin cannot be

achieved in such conditions despite constant initiation

provided by irradiation, great part of double bonds

remained unreacted, mostly buried inside microgels [18].

Low cure temperature [11] additionally reduced conver-

sion. If a sample of such partially crosslinked resin is lin-

early heated in dynamic DSC experiment, ‘‘residual heat’’

DHR evolves from the reaction of those unreacted double

bonds. The sum of the heat evolved during isothermal

reaction and the heat of the residual reaction is considered

equal to the total heat of crosslinking, and so DHR is

expected to be inversely related to the extent of crosslink-

ing. It is very difficult to directly measure the heat of

radiation crosslinking so monitoring of residual reactivity

is particularly advantageous for the study of radiation-

induced reactions. Previously, we used this method to

investigate the radiation crosslinking [23] and compared

the sensitivities of DSC, extraction analysis, and FTIR to

the postirradiation changes of UP resin [19]. It turned out

that DSC is the most sensitive, quickly providing a lot of

valuable information out of the shapes of the thermo-

grams and from the residual reaction heats.

The thermograms of resin I samples (see Fig. 1) are

simple: there is a single maximum, its area decreases with

dose and during the postirradiation period with a slight

FIG. 1. The DSC traces of the residual crosslinking heat of the resin I irradiated to the selected doses on

the indicated days after the irradiation normalized to the mass of the sample.

1770 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

shift toward lower temperature values. On the other hand,

there are two distinct maxima in almost all thermograms

of resin A (see Fig. 2), exceptions are the samples irradi-

ated to the lowest doses (only the lower-temperature max-

imum appeared) and to the highest dose late in the postir-

radiation period (only the higher-temperature maximum).

The temperature of the single peak in resin I thermo-

grams, about 1458C, is intermediate of the two peaks of

resin A that are at about 105 and 1708C. The appearance

of double peaks in thermograms of UP resins is common,

but there are disputes on its origin. Some authors [24, 25]

assume that double peaks are caused by initiator and/or

promotor, but, in this case, none was added so only reac-

tions of styrene and polyester could have produced two

exothermic processes. Others [26, 27] suggest that double

peaks in UP crosslinking thermograms both arise of poly-

ester and styrene copolymerization, induced by initiator at

lower temperatures and thermally initiated at higher tem-

peratures, but that explanation cannot be applied in this

case. Kubota [28] found that increase of styrene concen-

tration from 25 to 50% increased the heat of lower tem-

perature peak, whereas suppressed the higher temperature

peak. That confirms that the significant portion of polyes-

ter unsaturations may remain unreacted at usual concen-

trations of styrene and that polyester homopolymerization

is likely cause of the higher temperature peak. Hsu and

Lee [18] found that, in the case of RIPS, almost 90%

polyester double bonds remain unreacted inside microgels.

Based on that, we are convinced that in resin A, signifi-

cant RIPS occurred and caused increased concentration of

unreacted polyester double bonds that subsequently pro-

duced higher temperature peaks in thermograms, because

the energy brought in the course of DSC experiment ena-

bles polyester coils to override steric hindrances and

homopolymerize. Lower temperature peak appears at

about 1208C what is according to Lu et al. [29] consistent

with styrene-polyester copolymerization and styrene

homopolymerization that occur in the same temperature

range and cannot be separated. During postirradiation at

room temperature, a small molecule like styrene easily

diffuses and reacts with remaining radicals, and so it was

no surprise that lower temperature maximum decreased

faster and, in some cases, disappeared at the end of the

postirradiation period.

The differences in changes of the residual reaction

heats, DHR, with dose between resins A and I are also

FIG. 2. The DSC traces of the residual crosslinking heat of the resin A irradiated to the selected doses on

the indicated days after the irradiation normalized to the mass of the sample.


significant. In resin I (see Fig. 3), DHR first increases with

dose, has a large maximum at 4 kGy and then decreases

at higher doses, except at 5 kGy because of the gel effect.

During the postirradiation period, the decrease of residual

reactivity in resin I was also the highest at 4 kGy,

because the three-dimensional network has just started to

form and is relatively loose so the reactive species easily

diffuse and react even at the room temperature resulting

in the lowest residual reactivity of all samples on the

fifteenth postirradiation day. At higher doses, the postirra-

diation decrease of DHR becomes smaller, because the

density of the three-dimensional network produced on

irradiation increases and progressively obstructs the con-

tact of the reactive species. The gel effect produced no

rise of postirradiation conversion. The residual reaction

heats of resin A displays different pattern (see Fig. 4),

which was already discussed in the previous work [23];

here, we present additional details and differences from

the behavior of resin I. At doses up to 3 kGy, DHR is in-

significant at 3.5 kGy residual reaction heat sharply

increases, and the higher temperature peak appears at that

dose. At higher doses and higher radiation conversions,

the residual reaction heats on the day of irradiation

decrease and then stabilize. The increase of DHR due to

the gel effect is hardly notable on the day of irradiation.

Alike the resin I, the greatest residual reaction heat on the

day of the irradiation in resin A is again at the same dose,

3.5 kGy, as its greatest decrease during the postirradiation

period. At 6 kGy, the postirradiation decrease of DHR is

the least, because vitrification occurred during the irradia-

tion reaction and prevented contact between reactive spe-

cies at room temperature. The polyester homopolymeriza-

tion remains the sole cause of residual reactivity after the

second postirradiation day, because no diffusion is needed

for it. It should be noted that the greatest residual reactivity

of the resin A is about 2.5 times lower that that of resin I.

The extraction analysis is frequently used to determine

the conversion of radiation-induced polymerization and

crosslinking reactions, but it is limited only to samples

with enough insoluble gel. In resin A, measurable quan-

tity of gel appeared at significantly higher dose than in

resin I, and the gel contents formed during radiation reac-

tion and postirradiation period also significantly differ. At

the dose at which gel just appears in the resin A, the gel

content of the resin I is almost two-third of the maximum;

on higher doses the differences became lower. Because

gel content increases during postirradiation period, styrene

evaporation can be excluded as a cause for diminishing of

reaction heats determined by DSC.

Shyichuk and Tokaryk [30] proposed log–log method

of plotting sol fraction, s, against the dose that unlike

FIG. 3. The heats released during the residual crosslinking of resin I

irradiated to selected doses at the day of irradiation (darker symbols) and

on the fifth day of the postirradiation period (lighter symbols) versus

irradiation dose.

FIG. 4. The heats released during the residual crosslinking of resin A

irradiated to selected doses at the day of irradiation (darker symbols) and

on the fifth day of the postirradiation period (lighter symbols) versus

irradiation dose. Full symbols—the total residual heats DHR, empty sym-

bols—the heats of higher-temperature-peak, DHRHTP; (DHR ¼ DHRLTP þDHRHTP).


most widely used Charlesby-Pinner plot allows the deter-

mination of gel dose without the influence of changes in

molecular weight distribution during the reaction. Such

plot is presented in Fig. 5 for both resins on the day of

irradiation and on the fifth day of postirradiation period.

From these linear plots, gelation doses, Dg, were deter-

mined as doses where the plot intercepts horizontal line at

log s ¼ 0. The gelation dose of the resin A on the day of

irradiation was, DgA0 ¼ 3.07 kGy, twice the gelation dose

of the resin I, DgI0 ¼ 1.52 kGy. Although the slopes of

the plots somewhat changed on the fifth postirradiation

day, the gelation doses of both resins remain almost

unchanged, DgA5 ¼ 3.04 kGy and DgI5 ¼ 1.41 kGy.

Obviously, the extent of postreaction is not high enough

to produce gel if it was not formed on irradiation.

In both resins, the dose at which the residual reaction

heat becomes significant is just little higher than the dose

at which insoluble gel appears, but the curves of gel con-

tent increase do not reveal the details of crosslinking reac-

tion as the changes in residual reaction heat do. Instead,

the plots of gel content fractions formed during nth-daypostirradiation period, gn, shown in Fig. 6, and the resid-

ual reaction heats follow the same pattern. These plots

also demonstrate different reactivities of the resins. The

increase at gel effect dose and the vitrification at greatest

dose are also obvious.

To find out the cause of so-different crosslinking and

postcuring behavior of the two UP resins under considera-

tion, FTIR and NMR spectra of unirradiated resins and

styrene were recorded, and additional styrene spectra were

used to simplify identification of its absorptions. There

are slight differences in the diol region of proton NMR

spectra (3.5–04.5 ppm) as shown in Fig. 7, which can be

explained by somewhat different ratios of 1,2-propanediol

and diethylene glycol used in the preparation of resins A

and I. 13C NMR spectra (see Fig. 8) are almost identical,

and so variations in diol composition are too low to

explain different reactivities. Another possible cause for

various reactivities could be a difference in maleate to fu-

marate ratio. On polyesterification maleate isomerizes to

more reactive fumarate and the reactivity of the resin

towards styrene depends on the extent of this isomerization.

Because, in proton NMR spectra, styrene and fumarate

vinyl protons are almost indistinguishable (chemical shift

of styrene vinyl protons is 6.69 ppm and fumarate 6.65

ppm), 13C NMR spectra were checked and fumarate

absorptions at 134.03 ppm (styrene absorbs at 136.98 ppm)

were seen in both the resins. In 13C NMR, maleate absorp-

tion at 136.76 ppm coincides with styrene absorption but

not in proton NMR, where no maleate absorptions at 6.4

ppm were seen so variations in maleate/fumarate ratio

could not have caused the observed differences in the

course of the radiation and postirradiation crosslinking. No

quinone absorptions are visible in any of the resins, imply-

ing low inhibitor concentration, so that it is safe to assume

FIG. 5. The extraction analysis: plot of soluble part according to Shyi-

chuk for the radiation crosslinking of resins A (lighter symbols) and I

(darker symbols) on the day of the irradiation (zeroth day), full symbols,

and on the fifth day of the postirradiation period, empty symbols.

FIG. 6. The extraction analysis: the fraction of gel formed during pos-

tirradiation crosslinking of UP resin samples at selected doses.


that its effect on the course of crosslinking is the same in

both resins. Even if there was a difference in inhibitor con-

centration, it could only prolong the induction period, but

should not influence the final conversion as is the case

here. FTIR spectra are not shown, because they are almost

identical and do not offer any additional information.

The main difference between resins under the consider-

ation becomes obvious from the proton NMR spectra: the

absorptions at 8.23 and 8.63 ppm in the resin I reveal the

presence of isophthalic group, whereas in resin A there

are orthophthalic group absorptions at 7.63 and 7.75 ppm

(outlined in Fig. 7). The intensity of phthalic NMR

FIG. 7. Proton NMR spectra of resins A and I, the orthophthalic absorptions in resin A, and isophthalic

absorptions in resin I are outlined.

FIG. 8. 13C NMR spectra of resins A and I.


absorptions is rather low, and because the infrared absorp-

tions of these isomers are similar and difficult to separate

from those of styrene and or/polyester, it becomes clear

why FTIR spectra of resins A and I are so alike. The dif-

ference in the type of saturated aromatic diacid seems to

be the only cause for the shift of beginning of radiation

crosslinking to higher doses and lower extents of both

radiation and postirradiation reactions in resin A com-

pared to resin I. Although compounds containing phenyl

rings are usually included into polyester to increase its

compatibility with styrene and reactivity of the resin [12],

from our results, it is obvious that the outcome greatly

depends on the type of the phthalate used. Large carbox-

ylic groups tend to position themselves apart as far as

possible, and so the best conformation of two orthoph-

thalic carboxylic groups is the one in which carboxylic

groups are on the opposite sides of phenyl ring. Thus,

bulky carboxyl groups at least partly shield the phenyl

ring in polyester of resin A from contact with styrene and

might slightly increase some chemical shifts of diol 1H

NMR absorptions in resin A. In such conditions, more

intracoil crosslinking occurs and formation of microgels

with more unreacted polyester double bonds inside

increases. All this lead to RIPS [15, 31, 32], and the num-

ber of polyester bonds available for homopolymerization

becomes great enough to produce the corresponding peak

in DSC thermograms. Higher dose at which the residual

heat becomes measurable and gel appears confirm that

RIPS occurred in the resin A. The effects of RIPS are

also noticeable in SEM micrographs of gel surfaces of

both resins in Fig. 9 (upper micrograph). The inhomoge-

neity of the sample surface is visible in gel of resin A

sample, because, after double extraction, the unreacted

parts were removed leaving spaces whereas microgels

formed clusters [18].

On the other hand, phenyl rings of isophthalic unit in

resin I can improve contact with styrene, because its car-

boxylic groups are in metaposition, and there is a number

of conformations in which carboxylic units are relatively

distant while not shielding the phenyl ring. Because of

that, more polyester double bonds of the resin I become

available to crosslinking, so that the radiation and postir-

radiation crosslinking reactions are faster and have greater

extents resulting in higher overall conversion. At the same

magnification in the SEM micrograph, the surface of the

resin I gel is smooth (Fig. 9, middle), because fine struc-

ture of microgels cannot be seen at that magnification. At

higher magnification (Fig. 9, lower micrograph), typical

granular morphology of densely packed microgels

becomes obvious.

To check whether the relationship between saturated

aromatic unit type in the polyester chain and the shape of

DSC thermogram is generally valid, the literature is ana-

lyzed. The composition of polyester chain, resin composi-

tion, initiators, promoters, conditions of DSC experiments,

number of peaks, and corresponding temperatures (if not

tabulated, values were estimated from the plot) from the

all available articles that provided these data and dynamic

DSC thermograms are listed in Table 1. It can be seen

that in almost all the cases of resin-containing isophthalic

acid unit, single peaks appeared and the exceptions are

FIG. 9. SEM micrographs of gels, upper: resin A, magnification

35500; middle: resin I, magnification 35000; lower: resin I, magnifica-

tion 330,000.


the cases where the mixtures of two or more initiators are

used [32], some extreme initiator–promoter concentration

ratios [40] and at very low temperatures [41] that might

disrupt the compatibility of polyester and styrene. On the

other hand, double peaks are almost always present in the

thermograms of resins containing orthophthalic acid.

However, relatively high concentration of BPO eliminated

the high temperature peak in orthophthalic acid-containing

resin [26]. The same initiators and promoters, in the same

concentrations, did not cause the appearance of double

peaks in resins containing isophthalic acid, but the use of

initiators shifted temperatures of DSC peak to lower val-

ues compared to the our case, where thermally stimulated

reaction of remaining radiation-induced radicals was fol-

lowed. The additional peak(s) due to initiators appeared at

even lower temperatures. No influence of ratio of phtha-

lates to other constituents of the polyester chain or the

concentration of styrene on the number of the peaks was

noted. Our results and literature data confirm that the

chemistry of polyester chain determines whether double

peaks in dynamic thermograms of UP resins appear and

are generally due to the presence of orthophthalic acid.

CONCLUSIONS

Significant postirradiation increase in the crosslinking

extent of two UP resins occurred in the samples kept at

room temperature and lasted for days after the irradiation.

The changes strongly depended on the extent of the radia-

tion reaction and structure of the polyester. These results

emphasize the importance of monitoring of postirradiation

changes in polymers because of the impact on the final

properties, especially if the polymers are used for the

preparation of composites and nanocomposites as is often

the case of polyester resins.

Both radiation and postirradiation crosslinking reac-

tions were highly influenced by the structure of the poly-

ester chain: the presence of orthophthalic units in polyes-

ter chain seems to decrease the compatibility with styrene.

That caused RIPS resulting in double peaks in DSC ther-

mograms, lower extent of radiation, and postirradiation

crosslinking that begun at higher dose and coarser surface

structure on SEM micrographs. Isophthalic units improved

the compatibility with styrene resulting in simple DSC

thermograms, higher reaction extents, and more homoge-

neous surface appearance, that is, signs of macroscopic

RIPS were not observed.

Comparing the literature data, we concluded that the

double peaks in DSC thermograms of UP resins crosslink-

ing appear almost exclusively if orthophthalate is present

in the polyester chain, whereas single peak thermograms

are usual for the resins containing isophthalate. The sty-

rene reactions (with polyester double bonds or its homo-

polymerization) produce the lower-temperature peak,

whereas the higher temperature peak is due to the polyes-

ter homopolymerization. Generally, the appearance of

double peaks in thermograms of UP resins is a strong in-

TABLE1.

CompositionofsomeUPresinsanddataoncorrespondingthermograms.

Polyestercomposition

Styrene(%

)Initiator,promotor

Heatingrate

(8C/m

in)

Peak1(T/8C)

Peak2(T/8C)

Reference

oPA:M

A:PG

¼1:1:2,

34

1%

MEKP,0.2%

CoOct

10

(80)

127.1

Rodriguez

[24]

oPA:M

A:PG

¼1:1:2

34

2%

BPO,0.2%

DMA

10

59.6

/Rodriguez

[24]

oPA:M

A:1,2PG

¼0.54:0.46:1

36

MEKPConaphtenate150ppm

Cuoctoate

585.1

109.7

Penczek

etal.[25]

oPA:M

A:1,2PG

¼0.54:0.46:1

36


Cuoctoate

585.1

112.1

Penczek

etal.[25]

oPA:M

A:1,2PG

¼0.54:0.46:1

36


Cuoctoate

585.1

110.9

Penczek

etal.[25]

iPA:M

A:PG

¼1:3:4

50

Bis(4-t-butylcycloxehyl)peroxydicarbonate

10

60

NgandManas-Zloczower

[33]

oPA:?:?

¼?:?:?

?1%

MEKP,0.1%

CoOct

10

(80)

117.3

Worzakowska[34]

oPA:M

A:PG

¼1:2:2,?

??%

MEKP,?%

Conaphtenate

10

137

(130)

Sim

itziset

al.[35]

140

(130)

iPA:M

A:PG:ST¼

1:1:2:2.5

?%MEKP,0.1%

Cooctoate

?100

Grenet

etal.[36]

oPA:M

A:PG

¼?:?:?

32.6

t-Butyl-peroxybenzoate

10

(125)

(175)

LuandShim

[37]

iPA:M

A:PG

¼1:1:2

38

1.5%

MEKP,0.2%

Cooctoate

10

(100)

Maraiset

al.[38]

iPA:M

A:PG

¼1:2:3

?1.5%

MEKP,0.2%

Cooctoate

10

(135)

YangandLee

[39]

iPA:FA:PG

¼?:?:?

?Percadox,1%

5(90)

Han

andLee

[32]

iPA:FA:PG

¼?:?:?

?Benzoylperoxide,

t-butylperbenzoate,

1%

5(110)

Han

andLee

[32]

iPA:FA:PG

¼?:?:?

?t-Butylperbenzoate,

1%

5(135)

Han

andLee

[32]

iPA:FA:PG

¼?:?:?

?Percadoxþ

t-butylperbenzoate,

(0.1

þ0.9)%

5(100)

(135)

Han

andLee

[32]

MA,maleicacid;FA,fumaric

acid;oPA,orthophthalic

acid;iPA,isophthalic

acid;PG,proyleneglycol;?,

theauthors

did

notprovidedata;

estimated

temperaturesarein

parantheses.


dication of RIPS and subsequent lower crosslinking con-

version that influences the properties of any object pre-

pared from such resin. Double peaks in thermograms of

resins containing isophthalate appear only in extreme

reaction conditions. In rare cases, where the decomposi-

tion of initiator(s) produces an additional peak, it appears

at temperature lower than that of styrene reactions, that

is, below 808C.

ACKNOWLEDGMENT

The authors thank Dr. sc. S. Music for providing SEM

micrographs and his help in their interpretation.

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radiation and postirradiation crosslinking and structure of two unsaturated polyester resins

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