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Hydrolytic stability of unsaturated polyester networks with controlled chain ends
Polymer Degradndon and Stability 56 (1997) 93-102 0 1997 Elsevier Science Limited
ELSEVIER
PII:
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
SO141-3910(96)00185-l
Hydrolytic stability of unsaturated polyester networks with controlled chain ends F. B&n/
V. Bellengeti & B. Mortaigne’*
“Cray Valley/Total, ‘DGA/Centre
Centre de Recherches de I’Oise, Pare ALATA, BP 22, 60550 Verneuil en Halatte, France bENSAM, 151 Boulevard de I’Hcpital, 75013 Paris, France de Recherches et d’Etudes d’Arcuei1, 16 bis Avenue Prieur de la CGte d’Or, 94114 Arcueil, France
(Received 18 July 1996; accepted 23 August 1996)
Four unsaturated polyester networks with chain ends modified by isocyanate or dicyclopentadiene (DCPD) were immersed in distilled water at 100°C or exposed in humid atmosphere at 70°C and 50°C. The hydrolytic stabilities of these materials were determined by gravimetric analyses and infrared spectrometry. The results obtained were compared with those obtained on standard polyesters under similar conditions. For a given hydrolysis rate, the modification of chain ends by an isocyanate very clearly induces a decreasing rate of weight loss within the framework of degradations involving osmotic phenomena. The controlling of chain ends by DCPD does not reduce mass loss efficiency, but it does limit the hydrolysis rate. 0 1997 Elsevier Science Limited. All rights reserved.
1 INTRODUCTION
Subcutaneous crack propagation parallel to the surface is mechanically favoured and leads to blistering. Cracks ending at the surface can also be observed in aged parts. More or less empirical studies have shown the following features of osmotic cracking:
Unsaturated polyester resins are increasingly used as composite matrices in the building of large ships, pressurised water pipes etc. as they are non-expensive materials and easy to process.’ Unfortunately, these resins contain ester groups in their chains, especially maleate and phthalate characterized by a relatively high groups, sensitivity to hydrolysis2y3 compared, for instance, to vinylesters containing methacrylic esters.4 In such initially brittle materials, chain scission is not expected to have strong direct consequences on mechanical properties. However, chain scissions occurring on dangling chains (initially present or created by hydrolysis events) lead to the formation of small organic molecules capable of inducing cavitational damage by an osmotic process. The corresponding crack propagation mechanism is now well understood5y6 but the initiation step remains to be elucidated, although interesting hypotheses have been proposed.7 Osmotic cracking is very important from the practical point of view since it leads to macroscopic cracks capable of seriously limiting the durability of the whole structure in laminates. * To whom correspondence
-It occurs in the polyester matrix and depends sharply on the ester nature (reactivity towards water) and concentration.7 -Its rate is an increasing function of the polyester hydrophilicity which depends also on the above cited factors. -It is accelerated in the presence of impurities or structural irregularities2~5 capable of displaying a catalytic effect on hydrolysis. It has been shown that the concentration of dangling chains is a very important facto?? for at least two reasons: 1. They are terminated by groups (acids and alcohols) making an especially strong contribution to hydrophilicity.‘*” Furthermore, acids could have a catalytic influence on hydrolysis, although this hypothesis has been questioned.
should be addressed. 93
94
F. Btlan, R. Bellenger,
2. As previously reported, they are potential sources of formation of small molecules, which play a key role in osmotic cracking. The initial concentration of dangling chains is a hyperbolic function of the molar weight of the starting polyester, but this latter cannot be greatly increased because highly viscous prepolymers would not be suitable for processing. The only ways of improving the material’s durability in wet environments are thus: -Optimization of the choice of diacids and diols in order to decrease the reactivity towards hydrolysis. It is now well known, for instance, that isophthalate must be preferred to orthophthalate* and that neopentyl glycol is better than propylene glyco1.7,11,12However, this approach is limited by the fact that fumaric esters are especially sensitive to hydrolysis. -Complete change of the crosslinking species, for instance replacement of the fumaric ester copolymers by dimethacrylate capable also of copolymerising with styrene. Durability is then strongly improved, but the new matrix is considerably more expensive than common polyesters. -Modification of the structure of chain ends. For instance, the replacement of acids by non-catalytic, non-reactive species could have a stabilizing effect. Difunctional molecules could even suppress chain ends, thus having a double effect: acidic suppression and decrease of the yield of small molecules per chain scission. The aim of the present work is to investigate the above third way of improving durability. Two eventual solutions will be studied. In the first one, the initial dangling chains will be terminated by dicyclopentadiene-0-maleic acid groups (DCPD-0-MAA). An advantage of this approach is to significantly limit styrene emission In the second solution, during processing. diisocyanates are used to establish bridges between chain ends and thus to suppress chain ends in the final processing step. The corresponding prepolymers are characterized by an especially low molecular weight, allowing their use in processes such as RIM or RTM. These new systems will be compared with classical ones in order to assess quantitatively the chain-end effects of stabilising eventual modification.
B. Mortaigne
2 EXPERIMENTAL 2.1 Materials Six unsaturated polyester networks were studied. These unsaturated polyester prepolymers were prepared by polycondensation reactions between isophthalic acid (IPA) or orthophthalic acid (OPA) and maleic anhydride (MAA) with 1,2-propyleneglycol neopentylglycol (PG), (NPG), ethyleneglycol (EG) and diethyleneglycol (DEG). In the course of a hybrid polyester network process, prepolymers were polycondensed with 4, 4’-methylene bis (phenyl isocyanate) (MDI). The theoretical structure of the networks based on the initial monomers is shown in Scheme 1. The chemical composition of the prepolymers and the styrene level in the resins are shown in Table 1. The acid value (ZJ and hydroxyl number (IOH), determined by standard volumetric titration,13 and the molecular mass of the prepolymer chains calculated from the relationship M,, = 2/(Z, + Z,,), are also listed. Polyester prepolymers A, B and V were copolymerised with styrene at ambient temperature, via a route which gives rise to free radicals, by successively adding 0.5% of a cobalt octate solution (AKZO Chimie-6% cobalt concentration) and 1.5% of methyl-ethyl-ether-ketone peroxide (Butanox MSO-AKZO Chimie). The mixture was degassed under a 15 mm Hg vacuum, then poured into a vertical metal mould at ambient temperature (hand lay-up process). After 2 h crosslinking, the 300 mm X 200 mm X 1 mm plates were removed from the mould and cut into sections. To achieve maximum crosslinkage, the networks were post-cured for 10 h at 80°C and 2 h at 120°C. Polyester prepolymers H, G and M were with 4.4’-methylene diphenyl copolymerised di-isocyanate (MDI). The mixture was agitated then quickly poured (approx. 40s) between two metal plates, enabling a reaction to take place between the OH chain ends of the polyester prepolymer and the isocyanate groups.14 The rise in temperature due to the reaction enables the crosslinking of the copolymers obtained with styrene to form a standard bi-phase unsaturated polyester network,15 with two interpenetrating (IPN) networks.‘6*17 After being kept at 70°C for 1 min, the material plates, measuring 200 mm X 200 mm x 5 mm, were removed from the mould and cut into sections. Crosslinking was completed by post-curing for 4 h at 120°C.
95
Hydrolytic stabilityof unsaturatedpolyester networks
Network A
0 -
-
C
S = 0.45
j = 3.147
CH3
CH3
NetworkB
II
C-0-_-Hz-C-_-Hz-
CH-C-0-CHz-CH-O-C
-00C-CH II 0
I
S = 0.38
Network G
C -
-O-C-CH-CHII 0
0 -
(PG)r,.r, -
-
0 -
C -
CH -
II 0
l
CH
-
C-
-
0 -(PG)o.st
-
(DEG)o.a-
0 -(PG),,
73 -
(NPGjo.36 -
(MDUo.7
(NPG)o.n
UWo.43
j = 2.713
(DEG)o.97-
-
0
S = 0.35
DCPDaS -
j = 2.718
II
I
0
Network V
(MW-
-
II 0
-00C-CH-CH-C II
(NPGh
II 0
I
S = 0.35
Network M
CH3
j = 2.308
(DEG)o.~ -
S = 0.35
Network H
I
II 0
II 0
0
j = 3.22
0 -C-CH-CH-C-O-D-(OPA-D),,,z-OII 0
i
II 0
S = 0.4
scheme l. Theoretical
j = 2.009
structure of the polyester networks studied.
-
(MDh
F. Btlan, R. Bellenger,
96
B. Mortaigne
Table 1. Prepolymer characteristicsand glass transition temperaturesof networks measured by DSC Material
A B H G M V
Glycol
Diacid
MD1
Styrene
(mol)
W)
Chemical titration
DSC network
PG (mol)
DEG (mol)
EG (mol)
IPA
OPA
MAA
I, x 10”
(mol)
(mol)
(mol)
(mol/g)
I,, x 10” (mol/g)
% (g/moI)
Ts (“C)
(mol)
1 0 0.51 0.6 0.51 0.55
II 0.5 0.51 0.6 0.51 0
0 0 0.68 0.8 0.68 0.56
0 0 0 0 0 1.71
0.54 0.54 0 0 0.3 0
0 0 0 0 0 1.12
0.46 0.46 1 1 0.7 2
428 44s 80 71 86
500 695 5526 6595 4866
2150 1750 314 300 404
104 100 11s 118 98 87
NPG
PG = 1,2-ProPYleneglycol, NPG = neopentylglycol, DEG = diethyleneglycol, maleic anhydride, MD1 = 4,4’-methylene his (phenyl isocyanate)
For all materials, the conditions of use were defined so as to achieve a crosslink density of the materials very close to the asymptotic value. The glass transition temperatures (T,) of networks determined by differential scanning calorimetry (DSC) at the inflection point of the AH = f(T) curve, with a heating rate of lO”C/min, are listed in Table 1. 2.2 Ageing Ageing tests were carried out by immersing samples measuring 60 mm x 10 mm in water at lOO”C, 70°C and 50°C. The immersion tests at 100°C were carried out in spherical flasks with a cooling agent above them and the temperature controlled by boiling water; the tests at 70°C and 50°C were performed in thermostatic baths set at *l”C. 2.3 Gravimetry The relative weight variation of the samples was monitored as ageing took place. The samples were weighed before ageing, when they came out of the immersion bath and after reconditioning at ambient temperature and humidity. The balance used was a Mettler laboratory balance, AE 163, with an accuracy of 10p4. For each material, three types of curve of weight variation as a function of time were plotted: -Curve Pl = f(t), showing the relative weight variation of the samples immediately after being removed from the ageing baths. -Curve P2 = f(t), showing the relative weight variation of the samples after drying at 50°C under vacuum and reconditioning at ambient temperature and humidity. After reconditioning, the sample weight was stabilised. -Curve (Pl - P2) = f(t), which displays the
0 0 0.7 0.7 0.7 0
EG = ethyleneglycol,
45 38 3s 3s 3s 40
IPA = isophthalic
acid, GPA =
change in the material’s during ageing.
orthophthalicacid,MAA =
absorption
of water
2.4 FTIR The samples immersed in water at 100°C were dried, reconditioned at ambient temperature and humidity, then ground with potassium bromide (KBr). They were analysed by infrared spectrometry using a Perkin-Elmer FTIR spectrometer, model 1725X, between 4000 and 400 cm-‘, with a resolution of 4 cm-‘.
3 RESULTS 3.1 Gravimetry The gravimetric curve characteristics of the A networks aged by immersion at 100°C are listed in Fig. 1. The curves obtained for the B networks aged by immersion at 100°C are similar. These gravimetric curves display four phases: -Phase 1: weight increase in network due to water absorption, visible on curve Pl during the first hours of exposure. -Phase 2: extraction of free molecules, such as initiator or catalyst, solvents, free styrene molecules etc. by water; this phase is shown on curve P2 at the very beginning of exposure. -Phase 3: weight loss related to the extraction of small molecules resulting from rapid hydrolysis of the material, mainly at chain ends,’ leading to the formation of cracks. -Phase 4: accelerated weight loss related to hydrolysis after the cracks formed by osmosis have pierced the surface, which enables very rapid extraction of hydrolytic products and even of heavy fragments which still contain ester properties. This phase, detected on curves Pl and I?!, is shown by an interruption in the gradient on these curves during ageing.
97
Hydrolytic stability of unsaturated polyester networks
gravimetric curves display three phases: -Phases 1 and 2, corresponding to the same phenomena as those noted in the case of networks A and B (phases 1 and 2), and displaying identical curve shapes. -A third phase, characterised by an exponential decay in weight loss of the samples. This phase can be likened to phase 4 of the ageing detected, in the case of networks A and B, by immersion at lOO“C, with a balance between the hydrolysis rate and the rate of extraction of hydrolysed molecules. This is also confirmed by the fact that, for these materials, the hydrophilicity determined by the Pl - P2 curve remains constant after reaching a peak at the very beginning of ageing.
Time (ham)
b)
4.00
‘.
-6.00
‘.
a. 8 l
-7.00 .'
4
1
-8.00
Time (hours)
s
4.00
F
I
”
4.00
E -12.00 -16.00
’ a a ! #i
a. '*
t
8
-20.w Time (hours)
0.00) 0
I
: 600
100200300400500
700
b)
800
Time (ham)
1.Variation
in mass of networks A on immersion at 100°C: (a) Pl curve of mass variation immediately after immersion; (b) P2 curve of mass variation after drying and reconditioning at ambient temperature and humidity; (c) Pl - P2 hydrophilic variation curve of the network. Fig.
4.00
s f tJ
!*a
a.00 ..
2000 jooQ 4w0
‘000
‘4
e
-12.w ..
NO 8
-16.00 .-
"8
'0
'8,
‘
30.00 1
The Pl - P2 curve of these A and B networks displays a constant increase in hydrophilicity during phase 3, with stabilisation in phase 4. The transition from phase 3 to phase 4, which is also accompanied by an increase in network hydrophilicity, corresponds to the appearance of cracks or blisters as a result of osmotic mechanisms5y6 on the surface of the samples. The gravimetric curves characteristic of the H networks aged by immersion at 100°C are listed in Fig. 2. The curves obtained for the G and M hybrid polyester networks are similar to those obtained for the H networks. Those obtained for the V networks, whose prepolymer chain ends are of DCPD type, are also similar. In the case of H, G, M and V materials, the
Time (hcuts)
cl
6.00
1 .
0
:
1000
2000
.
3000
Time (hours) Fig. 2. Variation in mass of H networks (UP/PU hybrid material) on immersion at 100°C: (a) Pl curve of mass variation immediately after immersion; (b) P2 curve of mass variation after drying and reconditioning at ambient temperature and humidity; (c) Pl - P2 hydrophilic variation curve of the network.
98
F. B&an, R. Bellenger,
In the case of ageing by immersion at 70°C and at SO”C, the gravimetric curves obtained for the networks A, B and V (1 mm thick) display three phases, similar to those described in Fig. 2. For the 5 mm thick H, G and M hybrid networks, after water absorption corresponding to phase 1, the variations in recorded mass corresponding to ageing are too low. Even taking into account the thickness of the samples, this makes the distribution of hydrolytic products incomplete and the curves difficult to explain. The relative weight variation remains constant throughout ageing, as the extraction of hydrolysed molecules is offset by the additional mass gained through hydrolysis (an increase of 18 g/mol). To simplify matters, in the rest of this paper we shall call the hydrolysis rate constant in phase 3 of ageing for A and B networks by immersion at 100°C ‘k3’, and the hydrolysis rate constant in phase 4 of ageing for A and B networks by immersion at lOo”C, and the hydrolysis rate constants of materials in phase 3 for ageing at temperatures below 100°C ‘keq’. 3.2 Fl’IR A comparison of the sample spectra in their initial state and during ageing shows a decrease of the absorption band at 1295 cm-’ representative of the CO group of the aliphatic ester. Calculating the decrease rate of this absorption band allows the determination of the hydrolysis rate of fumarate esters. To quantify the ageing phenomenon, we determine the indices of the absorption band at 1295 cm-’ defined by the relationship: I (t)1295
=
A(r)l29bL
(1)
where A(,, = absorbance of the infrared band at time t, and Aref= absorbance of the reference infrared band. The reference band chosen, located at 700 cm-‘, is representative of the styrene benzene rings (?CH aromatic ring) and does not change during ageing, as styrene cannot be hydrolysed. To quantify ageing, we can say: EC,,= Z&o&
(2)
in the where EC,, is the ester concentration material at the end of ageing time t, E, corresponds to the concentration of ester in the unaged material, calculated from the composi-
B. Mortaigne
tion, and I,,, and Z0are respectively the indices of the absorption band at 1295 cm-’ of the material spectrum at the end of time t and initially. Figure 3 shows the change in the E/E, ratio in the case of ageing by immersion at 100°C of the six types of network studied.
4 DISCUSSION 4.1 Gravimetry In the case of modified networks aged by immersion at lOO”C, phase 3 of ageing is not detectable. This confirms the theory according to which this phase of hydrolysis corresponds to the hydrolysis of free ends of the network,7*l’ and the fact that networks synthesised from modified prepolymers have little or no free chains. This is consistent with the theoretical structure of classical unsaturated polyester networks? at 100°C hydrolytic damage is quite fast because one chain scission in a branch segment is enough to produce extractible molecules responsible for the well-known osmotic damage. In the case of modified networks, two successive scissions on the same macromolecular segment are necessary to produce extractible molecules. As the overall hydrolysis rate is slower, the hydrolytic products formed have time to be extracted from the polyester network, which limits their involvement in the development of osmotic pressure. Within the context of this paper, we are interested only in phases 3 and 4, i.e. the k, and k,, constants corresponding to the hydrolysis phenomenon. These constants are calculated for ageing times between 500 and 2000 h, by linear regression of the experimental curve. k = -l/m,.
dmldt
(3) are
The values calculated for these constants listed in Table 2. On the basis of these hydrolysis rate constants calculated for ageing by immersion at lOO”C, we find that the rates of weight loss of the A and B networks are very close to one another and well above those of the modified networks. We also find that the order of hydrolytic stability established for prepolymersl’ is also valid for classifying the hydrolytic stability of networks. In the group of modified networks, we can see that the effect of adding isophthalic acid when passing from network H to network M is spectacular. The k,, rate constant is divided by a
99
Hydrolytic stability of unsaturated polyester networks b)
a> I
I
1.1 l
14,
1.1
e
l
le
0
l
l
0.9 --
l
0.9
l
l
l l
0.6
0.8 ..
I 0.7 .. 0.6 .. 0.5 +
-i
0
250
600
750
WOO
I
Time (hours)
Time (hours)
I
4
c) 1.1 1
1
lle e
0.9 -.
0.9
l
l
0.8 --
l
l
0.8
I o.7
1000
2000
1000
2000
Time(hours)
Time (hours)
1.1
1.1 le
1
I l
0.9I l
0.8
%e
0.9 l
e
0.8
0.5:
l l
l
l
t
‘0.61 0
1000
2000
3000
Time (hours)
Fig. 3.
l
0
1000
2000
3000
Time (hours)
Monitoring by FTIR of the change in aliphatic ester concentration in networks as a function of ageing by immersion water at loo”: (a) material A: (b) material B; (C) material H; (d) material G: (e) material M; (f) material V.
factor of 3. The hierarchy usually observed, where PG is more stable than NPG and IPA is more stable than MAA,‘,‘*” is also checked for these modified polyester networks. Similarly, controlling chain ends by DCPD greatly reduces the rate of weight loss. Chemical modifications
in
which enable chain end crosslinking, as in the case of vinylester materials, enable hydrolytic stability to be increased. The esterification of acid chain ends by DCPD not only reduces the concentration of chain ends but also limits the effects of autocatalysis noted in the case of PET
F, B&lan, R. Bellenger,
100
B. Mortaigne
Table 2. Hydrolytic constants of networks determined from gavimetric tests and activation energies, with pre-exponential hydrolytic factors calculated from an Arrhenius law Materials
100°C water immersion k,
x lo5 (h-l) 4.2 4.3 -
A B H G M V
k,,
x lo5(h-l)
70°C water immersion k,, x 10’ (h-‘)
50°C water immersion k,, x 10’ (h-‘)
Es/R (kJ/mol)
1nA
0.73 0.40 1.5
o-10 0.05 0.20
8.9 10.7 10.8
13.9 18.6 20.4
2.5
0.14
15.9
35.9
13.9 15.6 17.9 9.7 5.7 7.2
by some authors,2*,22 although these effects have not yet been demonstrated in the case of unsaturated polyesters.23 Using these gravimetric data, we can try to estimate the activation energy of hydrolytic degradation from the hydrolytic rate constants determined at the various test temperatures, using an Arrhenius law: k,,
= A
exp(-Ea’RT)
(4)
Figure 4 is a graph of In k as a function of l/T, the hydrolytic rate constants determined for the various materials, and the activation energies of the hydrolytic process calculated by linear regression. The activation energy values and pre-exponential factors are given in Table 2. Note that the activation energies of the H and V networks are slightly higher than those of the A and B networks, and the pre-exponential factor calculated for the hybrid network H is higher than that of the other networks. 4.2 FI-IR The curves showing the variation in ester group concentration as a function of ageing time were modelled by a simple exponential law: E = E, expcek’)
(5)
-7 t
:.:Iiii -1.5 0.0026
I 0.0027
(a) When miscibility is high, esterification starts to compete with hydrolysis: I 0.0028
I 0.0029
I o.OQ30
4. Arrhenius
plot obtained from hydrolytic determined by gravimetry.
ester + water $ alcohol + acid
0.003 1
k’
l/T (K-l) Fig.
The hydrolysis first-order rate constants k calculated for each network and for each test temperature are given in Table 3. Figure 5 shows the hydrolysis rate constants for hybrid unsaturated polyester materials H, G and M in a graph of In k plotted as a function of l/T. Linear extrapolation is possible to a first approximation and the activation energy of the hydrolytic process may be calculated. The activation energy values and pre-exponential factors are given in Table 3. We find that the activation energies of the hydrolytic process calculated by FTIR are of the same magnitude as those calculated by gravimetry, which indicates a good correlation between the disappearance of ester groups and the concentration of extracted product evaluated by gravimetry. It also shows that in the case of hybrid network G, the activation energy of hydrolysis and the pre-exponential factor are higher than those of the other materials, whereas in the case of hybrid network M they are much lower. In the case of network V, where T, = SYC, a kinetic study of hydrolysis has been made in both states: vitreous and rubbery. Figure 6 shows an Arrhenius plot where In k =f(l/T) for this material. The glass transition produces a major discontinuity in the curve: in the glassy state, k is an increasing (but not Arrhenius) function of temperature, as one might expect. In contrast, in the rubbery state, k tends to be a decreasing function of temperature. We can put forward several theories to explain such paradoxical behaviour:
constants
k’
would appear where T > Tg.
to increase
rapidly
with T
Hydrolytic stabilityof unsaturatedpolyester networks
101
Table 3. Hydrolytic rate constants of networks obtained by IRTF and activation energies, with pre-exponential hydrolytic factors cnkolated from an Arrhenius law Material
k x 10s (h-l) at 100°C
k x lti (h-l) at 70°C
k x 10s (h-l) at 50°C
A
14.5
-
-
B
21.4
-
-
H G M V
136.1 47.3 16.1 11.5
20.5 3.6 8.2 3.8
k x l@ (h-l) at 80°C
k x lo5 (h-l) at 90°C
EaIR (kJ/mol)
1nA
17.5
12.1
9.6 14.6 2.6 7.3
19.6 31.9 -1.7 11.1
2.4 0.1 5.4 1
(b) This is a physical effect, for instance a decrease of hydrophilicity in the rubbery state. For lifetime prediction at low temperature, the apparent activation energy may be derived from the gradient of the In k = f(l/T) curve around 50°C. This Ea value is given in Table 3. As osmotic degradation is mainly controlled by the concentration of small molecules in the medium, even if their size means they cannot be easily extracted, in Fig. 7 we have shown the k,, values determined by gravimetry at 100°C as a function of the k values determined by infrared spectrometry at 100°C. We find two families: family A, B and V, where k,,/k = 1; and family H, G and M, where k,,/k CC0.35. Therefore, at a given hydrolytic rate, we see that the modification of chain ends by an isocyanate very clearly induces a decreasing rate of weight loss. The modification of chain ends by DCPD does not reduce the weight loss efficiency, but it does help in limiting the hydrolysis rate.
5 CONCLUSIONS The hydrolytic stability of polyester networks with chain ends modified either by reaction with
isocyanate groups to form polyester/polyurethane networks, or by reaction with dicyclopentadiene, was studied. Ageing by means of immersion in water was monitored by gravimetric measurements and infrared analyses, and the hydrolytic rate constants were calculated. Controlling the concentration of chain ends by either isocyanate or dicyclopentadiene addition enables hydrolytic stability to be improved significantly. For a given hydrolysis rate, the modification of chain ends by isocyanate decreases the weight loss rate very clearly, which is an important factor within the framework of degradation involving phenomena of osmosis, as these phenomena are controlled by the concentration of dissolved molecules in absorbed water. The modification of chain ends by dicyclopentadiene does not reduce weight loss efficiency but it does help to limit the hydrolysis rate. The hydrolytic constants determined enabled us to calculate, using the Arrhenius law, the activation energies of the hydrolytic process. We were able to establish that the hybrid network G has a relatively high activation energy, whereas that of network M is much lower; the pre-exponential factor of the latter is also much lower.
-5
-7
-7
-7 . .
_ c 5
-9 .
lG
-II
;.i
-13 :
-151 0.0026
5.
:
.
:-:;:;=--:--
I 0.0027
I 0.0028
I 0.0029
I 0.0030
I 0.0031
0.0026
Arrhenius plot obtained from hydrolytic constants determined by FTIR for networks H, G and M.
0.0027
0.0028
0.0029
0.0030
0.003 1
l/T (K-l)
l/T (K- 1) Rg.
*y-y-y
AH .M
Fig.
6. Arrhenius plot obtained from hydrolytic determined by FTIR for network V.
constants
102
F. Btlan,
R. Bellenger,
. H
B. Mortaigne
Swanpillai, G. J., Polymer, 23 (1982) 1785. 6. Castaing, P., Lemoine, L. & Gourdenne, A., Compos. Struct.. 30 (1995) 223.
7. Mortaigne,‘B.,
Bellenger, V. & Verdu, J., Polym. Netw.
Blends, 24 (1992) 187.
8. Bellenger, V., Ganem, M., Mortaigne,
B. & Verdu, J.,
Polym. Degrad. Stabil., 49 (1995) 91.
9. B&n,
F., Bellenger,
V., Mortaigne,
B. & Verdu, J.,
Macromol. Svmv.. 93 (1995) 81
10. Bellenger, -15
20
40
60
80
100
120
140
k.105 (h-l)
7. Hydrolytic constant determined by gravimetry in phase 4 of ageing as a function of the hydrolytic constant determined by FIIR.
Fig.
ACKNOWLEDGEMENT
The authors would like to thank the Direction de la Recherche et de la Technologie for financially supporting this study.
V., Mortaigne,
‘B-‘&
Verdu,
J., .I. Appl.
Polym. Sci., 41 (1990) 1225.
11. Scott, K. A. and Paul, K. T., Composites, 1974, 201. 12. Misaki, T. & Iwatsu, T., .I. Appl. Polym. Sci., 30 (1985) 1083.
Method for analysis of alkyd resins, IUPAC Div. Appl. Chem. Org. Coating Sect., 1974, p. 470.
13. IUPAC,
14. Hsu, T. J. and Lee, L. J., Composites Institute 42nd Annual Conference, SPI Inc., 2-6 February 1987, Session 18C. 15. Yang, Y. S. & Lee, L. J., Polymer, 29 (1988) 1793. 16. Chou, Y. C. and Lee, L. J., in Interpenetrating Polymer Networks, D. Kempner, L. H. Sperling and L. A. Utracki (Eds.), Advances in Chemistry Series N” 239, ACS edition, 1993, p. 305. 17. Chou, Y. C. & Lee, L. J., Polym. Eng. Sci., 3416 (1994) 1239.
1. Updegraff, I. H., Unsaturated polyester, in Handbook of Composites, G. Lubin (Ed.), Van Nostrand Reinhold, 1982, pp. 19-37. 2. Ashbee, K. H. G., Franck, F. C. & Wyatt, R. C., Proc. R. Sot., A300(1967) 495. 3. Ashbee, K. H. G. & Wyatt, R. C., Proc. R. Sot., A312 (1969) 553. 4. Ganem, M., Mortaigne, B., Bellenger, V. & Verdu, J., Polym. Netw. Blends, 43-4 (1994) 115. 5. Abeysinge, H. P., Edwards, W., Pritchard, G. &
18. Belan, F., Amelioration de la resistance a l’hydrolyse de resines polyester, PhD thesis, ENSAM, Paris, 18 December 1995. 19. Belan, F., Bellenger, V., Mortaigne, B., Verdu, J. and Yang, Y. S., Composites Science and Technology, 56 (1996) 733.
20. Denis, V., PhD thesis, Bordeaux 1, 1991. 21. Ravens, D. A. S. & Ward, I. M., Trans. Farad. Sot., 57 (1961) 150.
22. Zimmerman,
H. & N’Guyen, T. K., Polym. Eng. Sci., 20
(1980) 680. 23. McMahon,
W., Birdsall, H. A., Johnson, C. A. & Camilli, C. J., J. Chem. Eng. Data, 4 (1959) 57.
ELSEVIER
PII:
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
SO141-3910(96)00185-l
Hydrolytic stability of unsaturated polyester networks with controlled chain ends F. B&n/
V. Bellengeti & B. Mortaigne’*
“Cray Valley/Total, ‘DGA/Centre
Centre de Recherches de I’Oise, Pare ALATA, BP 22, 60550 Verneuil en Halatte, France bENSAM, 151 Boulevard de I’Hcpital, 75013 Paris, France de Recherches et d’Etudes d’Arcuei1, 16 bis Avenue Prieur de la CGte d’Or, 94114 Arcueil, France
(Received 18 July 1996; accepted 23 August 1996)
Four unsaturated polyester networks with chain ends modified by isocyanate or dicyclopentadiene (DCPD) were immersed in distilled water at 100°C or exposed in humid atmosphere at 70°C and 50°C. The hydrolytic stabilities of these materials were determined by gravimetric analyses and infrared spectrometry. The results obtained were compared with those obtained on standard polyesters under similar conditions. For a given hydrolysis rate, the modification of chain ends by an isocyanate very clearly induces a decreasing rate of weight loss within the framework of degradations involving osmotic phenomena. The controlling of chain ends by DCPD does not reduce mass loss efficiency, but it does limit the hydrolysis rate. 0 1997 Elsevier Science Limited. All rights reserved.
1 INTRODUCTION
Subcutaneous crack propagation parallel to the surface is mechanically favoured and leads to blistering. Cracks ending at the surface can also be observed in aged parts. More or less empirical studies have shown the following features of osmotic cracking:
Unsaturated polyester resins are increasingly used as composite matrices in the building of large ships, pressurised water pipes etc. as they are non-expensive materials and easy to process.’ Unfortunately, these resins contain ester groups in their chains, especially maleate and phthalate characterized by a relatively high groups, sensitivity to hydrolysis2y3 compared, for instance, to vinylesters containing methacrylic esters.4 In such initially brittle materials, chain scission is not expected to have strong direct consequences on mechanical properties. However, chain scissions occurring on dangling chains (initially present or created by hydrolysis events) lead to the formation of small organic molecules capable of inducing cavitational damage by an osmotic process. The corresponding crack propagation mechanism is now well understood5y6 but the initiation step remains to be elucidated, although interesting hypotheses have been proposed.7 Osmotic cracking is very important from the practical point of view since it leads to macroscopic cracks capable of seriously limiting the durability of the whole structure in laminates. * To whom correspondence
-It occurs in the polyester matrix and depends sharply on the ester nature (reactivity towards water) and concentration.7 -Its rate is an increasing function of the polyester hydrophilicity which depends also on the above cited factors. -It is accelerated in the presence of impurities or structural irregularities2~5 capable of displaying a catalytic effect on hydrolysis. It has been shown that the concentration of dangling chains is a very important facto?? for at least two reasons: 1. They are terminated by groups (acids and alcohols) making an especially strong contribution to hydrophilicity.‘*” Furthermore, acids could have a catalytic influence on hydrolysis, although this hypothesis has been questioned.
should be addressed. 93
94
F. Btlan, R. Bellenger,
2. As previously reported, they are potential sources of formation of small molecules, which play a key role in osmotic cracking. The initial concentration of dangling chains is a hyperbolic function of the molar weight of the starting polyester, but this latter cannot be greatly increased because highly viscous prepolymers would not be suitable for processing. The only ways of improving the material’s durability in wet environments are thus: -Optimization of the choice of diacids and diols in order to decrease the reactivity towards hydrolysis. It is now well known, for instance, that isophthalate must be preferred to orthophthalate* and that neopentyl glycol is better than propylene glyco1.7,11,12However, this approach is limited by the fact that fumaric esters are especially sensitive to hydrolysis. -Complete change of the crosslinking species, for instance replacement of the fumaric ester copolymers by dimethacrylate capable also of copolymerising with styrene. Durability is then strongly improved, but the new matrix is considerably more expensive than common polyesters. -Modification of the structure of chain ends. For instance, the replacement of acids by non-catalytic, non-reactive species could have a stabilizing effect. Difunctional molecules could even suppress chain ends, thus having a double effect: acidic suppression and decrease of the yield of small molecules per chain scission. The aim of the present work is to investigate the above third way of improving durability. Two eventual solutions will be studied. In the first one, the initial dangling chains will be terminated by dicyclopentadiene-0-maleic acid groups (DCPD-0-MAA). An advantage of this approach is to significantly limit styrene emission In the second solution, during processing. diisocyanates are used to establish bridges between chain ends and thus to suppress chain ends in the final processing step. The corresponding prepolymers are characterized by an especially low molecular weight, allowing their use in processes such as RIM or RTM. These new systems will be compared with classical ones in order to assess quantitatively the chain-end effects of stabilising eventual modification.
B. Mortaigne
2 EXPERIMENTAL 2.1 Materials Six unsaturated polyester networks were studied. These unsaturated polyester prepolymers were prepared by polycondensation reactions between isophthalic acid (IPA) or orthophthalic acid (OPA) and maleic anhydride (MAA) with 1,2-propyleneglycol neopentylglycol (PG), (NPG), ethyleneglycol (EG) and diethyleneglycol (DEG). In the course of a hybrid polyester network process, prepolymers were polycondensed with 4, 4’-methylene bis (phenyl isocyanate) (MDI). The theoretical structure of the networks based on the initial monomers is shown in Scheme 1. The chemical composition of the prepolymers and the styrene level in the resins are shown in Table 1. The acid value (ZJ and hydroxyl number (IOH), determined by standard volumetric titration,13 and the molecular mass of the prepolymer chains calculated from the relationship M,, = 2/(Z, + Z,,), are also listed. Polyester prepolymers A, B and V were copolymerised with styrene at ambient temperature, via a route which gives rise to free radicals, by successively adding 0.5% of a cobalt octate solution (AKZO Chimie-6% cobalt concentration) and 1.5% of methyl-ethyl-ether-ketone peroxide (Butanox MSO-AKZO Chimie). The mixture was degassed under a 15 mm Hg vacuum, then poured into a vertical metal mould at ambient temperature (hand lay-up process). After 2 h crosslinking, the 300 mm X 200 mm X 1 mm plates were removed from the mould and cut into sections. To achieve maximum crosslinkage, the networks were post-cured for 10 h at 80°C and 2 h at 120°C. Polyester prepolymers H, G and M were with 4.4’-methylene diphenyl copolymerised di-isocyanate (MDI). The mixture was agitated then quickly poured (approx. 40s) between two metal plates, enabling a reaction to take place between the OH chain ends of the polyester prepolymer and the isocyanate groups.14 The rise in temperature due to the reaction enables the crosslinking of the copolymers obtained with styrene to form a standard bi-phase unsaturated polyester network,15 with two interpenetrating (IPN) networks.‘6*17 After being kept at 70°C for 1 min, the material plates, measuring 200 mm X 200 mm x 5 mm, were removed from the mould and cut into sections. Crosslinking was completed by post-curing for 4 h at 120°C.
95
Hydrolytic stabilityof unsaturatedpolyester networks
Network A
0 -
-
C
S = 0.45
j = 3.147
CH3
CH3
NetworkB
II
C-0-_-Hz-C-_-Hz-
CH-C-0-CHz-CH-O-C
-00C-CH II 0
I
S = 0.38
Network G
C -
-O-C-CH-CHII 0
0 -
(PG)r,.r, -
-
0 -
C -
CH -
II 0
l
CH
-
C-
-
0 -(PG)o.st
-
(DEG)o.a-
0 -(PG),,
73 -
(NPGjo.36 -
(MDUo.7
(NPG)o.n
UWo.43
j = 2.713
(DEG)o.97-
-
0
S = 0.35
DCPDaS -
j = 2.718
II
I
0
Network V
(MW-
-
II 0
-00C-CH-CH-C II
(NPGh
II 0
I
S = 0.35
Network M
CH3
j = 2.308
(DEG)o.~ -
S = 0.35
Network H
I
II 0
II 0
0
j = 3.22
0 -C-CH-CH-C-O-D-(OPA-D),,,z-OII 0
i
II 0
S = 0.4
scheme l. Theoretical
j = 2.009
structure of the polyester networks studied.
-
(MDh
F. Btlan, R. Bellenger,
96
B. Mortaigne
Table 1. Prepolymer characteristicsand glass transition temperaturesof networks measured by DSC Material
A B H G M V
Glycol
Diacid
MD1
Styrene
(mol)
W)
Chemical titration
DSC network
PG (mol)
DEG (mol)
EG (mol)
IPA
OPA
MAA
I, x 10”
(mol)
(mol)
(mol)
(mol/g)
I,, x 10” (mol/g)
% (g/moI)
Ts (“C)
(mol)
1 0 0.51 0.6 0.51 0.55
II 0.5 0.51 0.6 0.51 0
0 0 0.68 0.8 0.68 0.56
0 0 0 0 0 1.71
0.54 0.54 0 0 0.3 0
0 0 0 0 0 1.12
0.46 0.46 1 1 0.7 2
428 44s 80 71 86
500 695 5526 6595 4866
2150 1750 314 300 404
104 100 11s 118 98 87
NPG
PG = 1,2-ProPYleneglycol, NPG = neopentylglycol, DEG = diethyleneglycol, maleic anhydride, MD1 = 4,4’-methylene his (phenyl isocyanate)
For all materials, the conditions of use were defined so as to achieve a crosslink density of the materials very close to the asymptotic value. The glass transition temperatures (T,) of networks determined by differential scanning calorimetry (DSC) at the inflection point of the AH = f(T) curve, with a heating rate of lO”C/min, are listed in Table 1. 2.2 Ageing Ageing tests were carried out by immersing samples measuring 60 mm x 10 mm in water at lOO”C, 70°C and 50°C. The immersion tests at 100°C were carried out in spherical flasks with a cooling agent above them and the temperature controlled by boiling water; the tests at 70°C and 50°C were performed in thermostatic baths set at *l”C. 2.3 Gravimetry The relative weight variation of the samples was monitored as ageing took place. The samples were weighed before ageing, when they came out of the immersion bath and after reconditioning at ambient temperature and humidity. The balance used was a Mettler laboratory balance, AE 163, with an accuracy of 10p4. For each material, three types of curve of weight variation as a function of time were plotted: -Curve Pl = f(t), showing the relative weight variation of the samples immediately after being removed from the ageing baths. -Curve P2 = f(t), showing the relative weight variation of the samples after drying at 50°C under vacuum and reconditioning at ambient temperature and humidity. After reconditioning, the sample weight was stabilised. -Curve (Pl - P2) = f(t), which displays the
0 0 0.7 0.7 0.7 0
EG = ethyleneglycol,
45 38 3s 3s 3s 40
IPA = isophthalic
acid, GPA =
change in the material’s during ageing.
orthophthalicacid,MAA =
absorption
of water
2.4 FTIR The samples immersed in water at 100°C were dried, reconditioned at ambient temperature and humidity, then ground with potassium bromide (KBr). They were analysed by infrared spectrometry using a Perkin-Elmer FTIR spectrometer, model 1725X, between 4000 and 400 cm-‘, with a resolution of 4 cm-‘.
3 RESULTS 3.1 Gravimetry The gravimetric curve characteristics of the A networks aged by immersion at 100°C are listed in Fig. 1. The curves obtained for the B networks aged by immersion at 100°C are similar. These gravimetric curves display four phases: -Phase 1: weight increase in network due to water absorption, visible on curve Pl during the first hours of exposure. -Phase 2: extraction of free molecules, such as initiator or catalyst, solvents, free styrene molecules etc. by water; this phase is shown on curve P2 at the very beginning of exposure. -Phase 3: weight loss related to the extraction of small molecules resulting from rapid hydrolysis of the material, mainly at chain ends,’ leading to the formation of cracks. -Phase 4: accelerated weight loss related to hydrolysis after the cracks formed by osmosis have pierced the surface, which enables very rapid extraction of hydrolytic products and even of heavy fragments which still contain ester properties. This phase, detected on curves Pl and I?!, is shown by an interruption in the gradient on these curves during ageing.
97
Hydrolytic stability of unsaturated polyester networks
gravimetric curves display three phases: -Phases 1 and 2, corresponding to the same phenomena as those noted in the case of networks A and B (phases 1 and 2), and displaying identical curve shapes. -A third phase, characterised by an exponential decay in weight loss of the samples. This phase can be likened to phase 4 of the ageing detected, in the case of networks A and B, by immersion at lOO“C, with a balance between the hydrolysis rate and the rate of extraction of hydrolysed molecules. This is also confirmed by the fact that, for these materials, the hydrophilicity determined by the Pl - P2 curve remains constant after reaching a peak at the very beginning of ageing.
Time (ham)
b)
4.00
‘.
-6.00
‘.
a. 8 l
-7.00 .'
4
1
-8.00
Time (hours)
s
4.00
F
I
”
4.00
E -12.00 -16.00
’ a a ! #i
a. '*
t
8
-20.w Time (hours)
0.00) 0
I
: 600
100200300400500
700
b)
800
Time (ham)
1.Variation
in mass of networks A on immersion at 100°C: (a) Pl curve of mass variation immediately after immersion; (b) P2 curve of mass variation after drying and reconditioning at ambient temperature and humidity; (c) Pl - P2 hydrophilic variation curve of the network. Fig.
4.00
s f tJ
!*a
a.00 ..
2000 jooQ 4w0
‘000
‘4
e
-12.w ..
NO 8
-16.00 .-
"8
'0
'8,
‘
30.00 1
The Pl - P2 curve of these A and B networks displays a constant increase in hydrophilicity during phase 3, with stabilisation in phase 4. The transition from phase 3 to phase 4, which is also accompanied by an increase in network hydrophilicity, corresponds to the appearance of cracks or blisters as a result of osmotic mechanisms5y6 on the surface of the samples. The gravimetric curves characteristic of the H networks aged by immersion at 100°C are listed in Fig. 2. The curves obtained for the G and M hybrid polyester networks are similar to those obtained for the H networks. Those obtained for the V networks, whose prepolymer chain ends are of DCPD type, are also similar. In the case of H, G, M and V materials, the
Time (hcuts)
cl
6.00
1 .
0
:
1000
2000
.
3000
Time (hours) Fig. 2. Variation in mass of H networks (UP/PU hybrid material) on immersion at 100°C: (a) Pl curve of mass variation immediately after immersion; (b) P2 curve of mass variation after drying and reconditioning at ambient temperature and humidity; (c) Pl - P2 hydrophilic variation curve of the network.
98
F. B&an, R. Bellenger,
In the case of ageing by immersion at 70°C and at SO”C, the gravimetric curves obtained for the networks A, B and V (1 mm thick) display three phases, similar to those described in Fig. 2. For the 5 mm thick H, G and M hybrid networks, after water absorption corresponding to phase 1, the variations in recorded mass corresponding to ageing are too low. Even taking into account the thickness of the samples, this makes the distribution of hydrolytic products incomplete and the curves difficult to explain. The relative weight variation remains constant throughout ageing, as the extraction of hydrolysed molecules is offset by the additional mass gained through hydrolysis (an increase of 18 g/mol). To simplify matters, in the rest of this paper we shall call the hydrolysis rate constant in phase 3 of ageing for A and B networks by immersion at 100°C ‘k3’, and the hydrolysis rate constant in phase 4 of ageing for A and B networks by immersion at lOo”C, and the hydrolysis rate constants of materials in phase 3 for ageing at temperatures below 100°C ‘keq’. 3.2 Fl’IR A comparison of the sample spectra in their initial state and during ageing shows a decrease of the absorption band at 1295 cm-’ representative of the CO group of the aliphatic ester. Calculating the decrease rate of this absorption band allows the determination of the hydrolysis rate of fumarate esters. To quantify the ageing phenomenon, we determine the indices of the absorption band at 1295 cm-’ defined by the relationship: I (t)1295
=
A(r)l29bL
(1)
where A(,, = absorbance of the infrared band at time t, and Aref= absorbance of the reference infrared band. The reference band chosen, located at 700 cm-‘, is representative of the styrene benzene rings (?CH aromatic ring) and does not change during ageing, as styrene cannot be hydrolysed. To quantify ageing, we can say: EC,,= Z&o&
(2)
in the where EC,, is the ester concentration material at the end of ageing time t, E, corresponds to the concentration of ester in the unaged material, calculated from the composi-
B. Mortaigne
tion, and I,,, and Z0are respectively the indices of the absorption band at 1295 cm-’ of the material spectrum at the end of time t and initially. Figure 3 shows the change in the E/E, ratio in the case of ageing by immersion at 100°C of the six types of network studied.
4 DISCUSSION 4.1 Gravimetry In the case of modified networks aged by immersion at lOO”C, phase 3 of ageing is not detectable. This confirms the theory according to which this phase of hydrolysis corresponds to the hydrolysis of free ends of the network,7*l’ and the fact that networks synthesised from modified prepolymers have little or no free chains. This is consistent with the theoretical structure of classical unsaturated polyester networks? at 100°C hydrolytic damage is quite fast because one chain scission in a branch segment is enough to produce extractible molecules responsible for the well-known osmotic damage. In the case of modified networks, two successive scissions on the same macromolecular segment are necessary to produce extractible molecules. As the overall hydrolysis rate is slower, the hydrolytic products formed have time to be extracted from the polyester network, which limits their involvement in the development of osmotic pressure. Within the context of this paper, we are interested only in phases 3 and 4, i.e. the k, and k,, constants corresponding to the hydrolysis phenomenon. These constants are calculated for ageing times between 500 and 2000 h, by linear regression of the experimental curve. k = -l/m,.
dmldt
(3) are
The values calculated for these constants listed in Table 2. On the basis of these hydrolysis rate constants calculated for ageing by immersion at lOO”C, we find that the rates of weight loss of the A and B networks are very close to one another and well above those of the modified networks. We also find that the order of hydrolytic stability established for prepolymersl’ is also valid for classifying the hydrolytic stability of networks. In the group of modified networks, we can see that the effect of adding isophthalic acid when passing from network H to network M is spectacular. The k,, rate constant is divided by a
99
Hydrolytic stability of unsaturated polyester networks b)
a> I
I
1.1 l
14,
1.1
e
l
le
0
l
l
0.9 --
l
0.9
l
l
l l
0.6
0.8 ..
I 0.7 .. 0.6 .. 0.5 +
-i
0
250
600
750
WOO
I
Time (hours)
Time (hours)
I
4
c) 1.1 1
1
lle e
0.9 -.
0.9
l
l
0.8 --
l
l
0.8
I o.7
1000
2000
1000
2000
Time(hours)
Time (hours)
1.1
1.1 le
1
I l
0.9I l
0.8
%e
0.9 l
e
0.8
0.5:
l l
l
l
t
‘0.61 0
1000
2000
3000
Time (hours)
Fig. 3.
l
0
1000
2000
3000
Time (hours)
Monitoring by FTIR of the change in aliphatic ester concentration in networks as a function of ageing by immersion water at loo”: (a) material A: (b) material B; (C) material H; (d) material G: (e) material M; (f) material V.
factor of 3. The hierarchy usually observed, where PG is more stable than NPG and IPA is more stable than MAA,‘,‘*” is also checked for these modified polyester networks. Similarly, controlling chain ends by DCPD greatly reduces the rate of weight loss. Chemical modifications
in
which enable chain end crosslinking, as in the case of vinylester materials, enable hydrolytic stability to be increased. The esterification of acid chain ends by DCPD not only reduces the concentration of chain ends but also limits the effects of autocatalysis noted in the case of PET
F, B&lan, R. Bellenger,
100
B. Mortaigne
Table 2. Hydrolytic constants of networks determined from gavimetric tests and activation energies, with pre-exponential hydrolytic factors calculated from an Arrhenius law Materials
100°C water immersion k,
x lo5 (h-l) 4.2 4.3 -
A B H G M V
k,,
x lo5(h-l)
70°C water immersion k,, x 10’ (h-‘)
50°C water immersion k,, x 10’ (h-‘)
Es/R (kJ/mol)
1nA
0.73 0.40 1.5
o-10 0.05 0.20
8.9 10.7 10.8
13.9 18.6 20.4
2.5
0.14
15.9
35.9
13.9 15.6 17.9 9.7 5.7 7.2
by some authors,2*,22 although these effects have not yet been demonstrated in the case of unsaturated polyesters.23 Using these gravimetric data, we can try to estimate the activation energy of hydrolytic degradation from the hydrolytic rate constants determined at the various test temperatures, using an Arrhenius law: k,,
= A
exp(-Ea’RT)
(4)
Figure 4 is a graph of In k as a function of l/T, the hydrolytic rate constants determined for the various materials, and the activation energies of the hydrolytic process calculated by linear regression. The activation energy values and pre-exponential factors are given in Table 2. Note that the activation energies of the H and V networks are slightly higher than those of the A and B networks, and the pre-exponential factor calculated for the hybrid network H is higher than that of the other networks. 4.2 FI-IR The curves showing the variation in ester group concentration as a function of ageing time were modelled by a simple exponential law: E = E, expcek’)
(5)
-7 t
:.:Iiii -1.5 0.0026
I 0.0027
(a) When miscibility is high, esterification starts to compete with hydrolysis: I 0.0028
I 0.0029
I o.OQ30
4. Arrhenius
plot obtained from hydrolytic determined by gravimetry.
ester + water $ alcohol + acid
0.003 1
k’
l/T (K-l) Fig.
The hydrolysis first-order rate constants k calculated for each network and for each test temperature are given in Table 3. Figure 5 shows the hydrolysis rate constants for hybrid unsaturated polyester materials H, G and M in a graph of In k plotted as a function of l/T. Linear extrapolation is possible to a first approximation and the activation energy of the hydrolytic process may be calculated. The activation energy values and pre-exponential factors are given in Table 3. We find that the activation energies of the hydrolytic process calculated by FTIR are of the same magnitude as those calculated by gravimetry, which indicates a good correlation between the disappearance of ester groups and the concentration of extracted product evaluated by gravimetry. It also shows that in the case of hybrid network G, the activation energy of hydrolysis and the pre-exponential factor are higher than those of the other materials, whereas in the case of hybrid network M they are much lower. In the case of network V, where T, = SYC, a kinetic study of hydrolysis has been made in both states: vitreous and rubbery. Figure 6 shows an Arrhenius plot where In k =f(l/T) for this material. The glass transition produces a major discontinuity in the curve: in the glassy state, k is an increasing (but not Arrhenius) function of temperature, as one might expect. In contrast, in the rubbery state, k tends to be a decreasing function of temperature. We can put forward several theories to explain such paradoxical behaviour:
constants
k’
would appear where T > Tg.
to increase
rapidly
with T
Hydrolytic stabilityof unsaturatedpolyester networks
101
Table 3. Hydrolytic rate constants of networks obtained by IRTF and activation energies, with pre-exponential hydrolytic factors cnkolated from an Arrhenius law Material
k x 10s (h-l) at 100°C
k x lti (h-l) at 70°C
k x 10s (h-l) at 50°C
A
14.5
-
-
B
21.4
-
-
H G M V
136.1 47.3 16.1 11.5
20.5 3.6 8.2 3.8
k x l@ (h-l) at 80°C
k x lo5 (h-l) at 90°C
EaIR (kJ/mol)
1nA
17.5
12.1
9.6 14.6 2.6 7.3
19.6 31.9 -1.7 11.1
2.4 0.1 5.4 1
(b) This is a physical effect, for instance a decrease of hydrophilicity in the rubbery state. For lifetime prediction at low temperature, the apparent activation energy may be derived from the gradient of the In k = f(l/T) curve around 50°C. This Ea value is given in Table 3. As osmotic degradation is mainly controlled by the concentration of small molecules in the medium, even if their size means they cannot be easily extracted, in Fig. 7 we have shown the k,, values determined by gravimetry at 100°C as a function of the k values determined by infrared spectrometry at 100°C. We find two families: family A, B and V, where k,,/k = 1; and family H, G and M, where k,,/k CC0.35. Therefore, at a given hydrolytic rate, we see that the modification of chain ends by an isocyanate very clearly induces a decreasing rate of weight loss. The modification of chain ends by DCPD does not reduce the weight loss efficiency, but it does help in limiting the hydrolysis rate.
5 CONCLUSIONS The hydrolytic stability of polyester networks with chain ends modified either by reaction with
isocyanate groups to form polyester/polyurethane networks, or by reaction with dicyclopentadiene, was studied. Ageing by means of immersion in water was monitored by gravimetric measurements and infrared analyses, and the hydrolytic rate constants were calculated. Controlling the concentration of chain ends by either isocyanate or dicyclopentadiene addition enables hydrolytic stability to be improved significantly. For a given hydrolysis rate, the modification of chain ends by isocyanate decreases the weight loss rate very clearly, which is an important factor within the framework of degradation involving phenomena of osmosis, as these phenomena are controlled by the concentration of dissolved molecules in absorbed water. The modification of chain ends by dicyclopentadiene does not reduce weight loss efficiency but it does help to limit the hydrolysis rate. The hydrolytic constants determined enabled us to calculate, using the Arrhenius law, the activation energies of the hydrolytic process. We were able to establish that the hybrid network G has a relatively high activation energy, whereas that of network M is much lower; the pre-exponential factor of the latter is also much lower.
-5
-7
-7
-7 . .
_ c 5
-9 .
lG
-II
;.i
-13 :
-151 0.0026
5.
:
.
:-:;:;=--:--
I 0.0027
I 0.0028
I 0.0029
I 0.0030
I 0.0031
0.0026
Arrhenius plot obtained from hydrolytic constants determined by FTIR for networks H, G and M.
0.0027
0.0028
0.0029
0.0030
0.003 1
l/T (K-l)
l/T (K- 1) Rg.
*y-y-y
AH .M
Fig.
6. Arrhenius plot obtained from hydrolytic determined by FTIR for network V.
constants
102
F. Btlan,
R. Bellenger,
. H
B. Mortaigne
Swanpillai, G. J., Polymer, 23 (1982) 1785. 6. Castaing, P., Lemoine, L. & Gourdenne, A., Compos. Struct.. 30 (1995) 223.
7. Mortaigne,‘B.,
Bellenger, V. & Verdu, J., Polym. Netw.
Blends, 24 (1992) 187.
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7. Hydrolytic constant determined by gravimetry in phase 4 of ageing as a function of the hydrolytic constant determined by FIIR.
Fig.
ACKNOWLEDGEMENT
The authors would like to thank the Direction de la Recherche et de la Technologie for financially supporting this study.
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