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Dynamic thermomechanical analysis of a prepreg— applications to industrial curing
Fibre Science and Technology 18 (1983) 109-118
Dynamic Thermomechanical Analysis of a Prepreg Applications to Industrial Curing J. H o g n a t Aerospatiale Central Laboratory, 12 Rue Pasteur--BPN. 76, 92152 Suresnes Cedex (France)
S UMMA R Y The fabrication of a composite structure with a fibre/resin reinforced system simultaneously requires: (a) (b)
a thermal cycle to cure the matrix, a pressure to compact the system.
The optimisation of composites with high mechanical properties from a curing cycle is a compromise between the effect of the thermal cycle on the resin and that of pressure on the prepreg compacting capabilities. With dynamic thermomechanical methods of tests, it is possible to follow a given cycle for the progress of resin curing and to determine the starting point in time of gellification. This point is the point appropriate to pressure application on the lay-up so as to obtain the best possible composite material.
1.
INTRODUCTION
The fabrication of a composite with organic matrix implies at the same time: (a) a thermal cycle to cure the resin, (b) a compacting pressure which will be applied to the prepreg lay-up at a m o m e n t in time during thermal cycle when the resin viscosity is adequate and will produce a satisfactory inter-layer fibre/resin bond as well as a composite structure without voids and with high mechanical properties. 109 Fibre Science and Technology 0015-0568/83/0018-0109/$03-00 O Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
110
J. Hognat
The various phases of the study were the following: (a) determination of the rheologic characteristics of the resin by means of the following tests: (i) Vanhographe test and (ii) flow test. (b) experimental verification: manufacture of test plates submitted to the compacting pressure precisely at those typical moments in time which had been determined by the previously mentioned methods.
2.
T H E R M O M E C H A N I C A L TEST
2.1. The 'Vanhographe' (Aerospatiale patent) The 'Vanhographe' (Fig. 1) is a device which has been developed to measure directly on the prepreg specimen the curing process of a thermosetting resin during the course of a thermal cycle. A constant dynamic impulse generated by a rod/motor/crank system is applied to a control jaw. This control jaw thus loaded transmits more or less the shear effect to a receiving jaw through the prepreg. This variation in the transmission is a function of the relative rigidity of the prepreg and thence of the state of cure of the resin. The receiving jaw is connected to a gauge which transforms this mechanical impulse into a signal which after having been amplified is transmitted, in turn, continuously to a potentiometer recording unit. The Vanhographe test provides information on the resin change during ventilated heating enclosure
// ,. ,..o,i.,
unit~. /
•
./
rixlo
/ arm
/
/
/
/
,/
movable arm
<_s
/ / ./
laW
p
~
\, ', ,
/
rlceivini flexible metal strips
ee~ilittiq flexible metal strips
Fig. 1. Vanhographe, functional diagram.
Dynamic thermomechanical analysis of a prepreg
Rg LIQUID PHASE
RUBBER PHASE
J ,.,,n, /
111
~ GLASS
T
//
/,/
........... ;;
Stilt
ond
TIME
Fig. 2. Vanhographe recording.
the curing process and more precisely on the point in time when resin gellification starts. The compacting pressure should be applied to the resin 'rubber zone' which corresponds to the initiation of gellification (Fig. 2). 2.2. Flow tests
The resin flow tests consist in determining with the weight change method, the quantity of resin which may flow from a prepreg during the application at a given moment during the thermal cycle of a pressure of 7 bars (100 psi) to the prepreg lay-up. 2.3. Vanhographe and flow tests results
We selected for three different thermal cycles, these points in time which could be appropriate for the application of the compacting pressure (they are referenced from 1 to 9). Cycle at 125°C For the thermal cycle studied, the flow test results corresponding to points 4, 5 and 6 and the curve pattern are shown in Fig. 3. At point 4 as well as points 5 and 6, the compacting pressure is applied in the "rubber zone' which corresponds to the initiation of resin
J. Hognat
112
T
(c) I
100
50
50 Fill. 3.
lOO
TIME
Imn)
Flow and Vanhographe results at 125°C.
gellification; moreover, the resin flow is quite sufficient to obtain a satisfactory interlayer resin compaction. Cycle at 135°C The flow test results corresponding to points 1, 2 and 3 and the Vanhographe curve pattern are shown in Fig. 4. The test results show that the pressure applied at point 1 is within the 'rubber zone' which
• l, l , ow
\
,I
.... 06
Fig. 4.
PLATEAU
-
So
/ G
AT /
o
' ' ' 100 Time Flow and Vanhographe results at 135°C. +
=
(c)
/
~.
<=,
~
Dynamic thermomechanical analysis of a prepreg
113
corresponds to the initiation of resin gellification and that the resin flow is sufficient to obtain a satisfactory compaction. On the contrary, for points 2 and 3, the gellification is almost completed and it is too late to obtain a satisfactory compaction of the composite; likewise resin flow is not sufficient to produce a good resin interlayer compaction. Cycle at 145°C The flow test results corresponding to points 7, 8 and 9 and the Vanhographe curve pattern are shown in Fig. 5.
% ,Low,
T (C) 150
w
~/~
Rg
100
,5O
F:
. . . . . . .
~0
_LG_ I(~)
Fig. 5.
TIME
(ran) ' '
Flow and Vanhographe results at 145°C.
The results demonstrate effectively that the pressure applied at 7 and 8 is within the 'rubber zone' which corresponds to the initiation of resin gellification and that the resin flow is quite sufficient to produce a satisfactory compacting of the resin. On the contrary, when pressure is applied at 9, the gellification is almost completed and it is too late to obtain a satisfactory composite compaction. Resin flow is consequently too low to produce a satisfactory interlayer resin compaction. When all these tests were completed, we fabricated test plates which we loaded successively at points 1 to 9.
J. Hognat
114
3.
DESCRIPTION OF THE C U R I N G CYCLES
Based on the following thermal cycle pattern: (a) (b) (c) (d)
temperature rise from ambient to 0 °C, lhatO°C, temperature rise to 180°C, final cure: 2 h at 180°C,
we fabricated for three different temperatures (125 °C, 135°C, 145 °C), three composite test pieces with a carbon fabric/epoxy resin prepreg on which we applied the compacting pressure at those points in time determined previously by the flow and Vanhographe tests.
At At At At At At At At At
O°C
Application of pressure
Plate
125 °C 125°C 125°C 135 °C 135°C 135 °C 145°C 145°C 145°C
for 20 MN for 40 MN for 60MN for 20 MN for 40MN for 60 MN for 0 M N for 10MN for 20 MN
4 5 6 1 2 3 7 8 9
The mechanical properties obtained are indicated in Tables 1, 2 and 3. TABLE 1
Parameter
Temperature/Time Volumic fibre content (~o) Void content (~o) Flexion (MPa) Interlaminar shear (MPa)
Plate
¢7
E 20c 120c
4
5
6
125/20 58 0 760 50810 67 53
125/40 59 0 830 51840 66 55
125/60 58 0 940 52810 64 56
Dynamic thermomechanical analysis of a prepreg
115
Plates 4, 5 and6. No porosity, good quality material. Both flexural and interlaminar shear strength characteristics are in conformity with this type of material. TABLE 2 Parameter
Plate
I Temperature/Time Volumic fibre content (~o) Void content (~) Flexion (MPa) Interlaminar shear (MPa)
o-
E 20c 120c
135/20 61 0.07 870 55280 72 51
2 135/40 58 4.5 530 48840 46 37
3 135/60 57 4.8 590 50550 48 37
Plate 1. No porosity: good quality material. The flexural and interlaminar shear strength are in conformity with this type of material. Plates 2 and 3. M a n y voids. The flexural and interlaminar shear characteristics are too low. TABLE 3 Parameter
Temperature/Time Volumic fibre content (~o) Void content (Vo) Flexion (MPa) Interlaminar shear (MPa)
Plate
o-
E 20c 120c
7
8
145/0 61 0 810 54400 72 58
145/10 59 0 850 56040 79 61
9
145/20 57 3.0 750 50840 49 45
Plates 7 and 8. No porosity. G o o d flexural and interlaminar shear characteristics. Plate 9. M a n y voids. The flexural and interlaminar shear characteristics are too low.
116
J. Hognat
~
Fig. 6. Plate 4. The structure is homogeneous, without voids. The compacting pressure has been applied in the appropriate part of the resin gellification zone. ( × 30)
Fig. 8.
Plate 6. No voids: good quality material. ( × 30)
Fig. 10. Plate 2. Porous structure. The compacting pressure has been applied too late during the cure cycle when the resin was gellified. ( x 32)
:r
. ~ , ~ i ~ ~L ....
i¸
Fig. 7. Plate 5. No voids. The compacting pressure has been applied in the appropriate zone of the resin gellification. ( × 30)
Fig. 9. Plate I. Homogeneous structure without voids. The state of the resin gellification was appropriate for the application of the compacting pressure. ( x 32)
Fig. 11. Plate 3. Porous structure. The compacting pressure has been applied too late during the cure cycle. ( x 32).
Dynamic thermomechanical analysis of a prepreg
117
The mechanical characteristics obtained on the nine plates show that the compacting pressure is satisfactory in the case of: (i) plates 4, 5 and 6: cycle at 125°C, (ii) plate 1: cycle at 135°C, (iii) plates 7 and 8: cycle at 145°C. On the contrary for plates 2 and 3 (cycle at 135°C) and plate 9 (cycle at 145 °C) the compacting pressure has been applied too late during the cycle at a moment when the resin had reached a state of cure which was inadequate to produce a satisfactory resin interlayer bond. The micrographs made of the nine plates (Figs 6-14) confirm that the interlayer cohesive state of cure of the resin is dependent upon the point in time when pressure is applied during the thermal cycle.
Fig. 12. Plate 7. G o o d quality material. No voids. The compacting pressure has
been applied in the appropriate part of the resin gellification zone favourable to the production of a satisfactory composite.
Fig. 13. Plate 8. No voids; homogeneous structure. The compacting pressure has been applied in an appropriate part of the resin gellification zone. ( × 30)
( x 30)
Fig. 14.
Porous structure. The compacting pressure has been applied too late during the cycle when resin was already gellified. ( × 32)
Plate 9.
118
J. Hognat
4.
CONCLUSIONS
It appears from this study that for the development of industrial cure cycles applicable to composites, thermomechanical methods such as the flow and Vanhographe techniques are perfectly adapted to determine, in a simple manner, the gellification zone of a resin and consequently the point in time when compacting pressure should be applied to a prepreg lay-up, so as to obtain a void-free composite with high mechanical properties.
Dynamic Thermomechanical Analysis of a Prepreg Applications to Industrial Curing J. H o g n a t Aerospatiale Central Laboratory, 12 Rue Pasteur--BPN. 76, 92152 Suresnes Cedex (France)
S UMMA R Y The fabrication of a composite structure with a fibre/resin reinforced system simultaneously requires: (a) (b)
a thermal cycle to cure the matrix, a pressure to compact the system.
The optimisation of composites with high mechanical properties from a curing cycle is a compromise between the effect of the thermal cycle on the resin and that of pressure on the prepreg compacting capabilities. With dynamic thermomechanical methods of tests, it is possible to follow a given cycle for the progress of resin curing and to determine the starting point in time of gellification. This point is the point appropriate to pressure application on the lay-up so as to obtain the best possible composite material.
1.
INTRODUCTION
The fabrication of a composite with organic matrix implies at the same time: (a) a thermal cycle to cure the resin, (b) a compacting pressure which will be applied to the prepreg lay-up at a m o m e n t in time during thermal cycle when the resin viscosity is adequate and will produce a satisfactory inter-layer fibre/resin bond as well as a composite structure without voids and with high mechanical properties. 109 Fibre Science and Technology 0015-0568/83/0018-0109/$03-00 O Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
110
J. Hognat
The various phases of the study were the following: (a) determination of the rheologic characteristics of the resin by means of the following tests: (i) Vanhographe test and (ii) flow test. (b) experimental verification: manufacture of test plates submitted to the compacting pressure precisely at those typical moments in time which had been determined by the previously mentioned methods.
2.
T H E R M O M E C H A N I C A L TEST
2.1. The 'Vanhographe' (Aerospatiale patent) The 'Vanhographe' (Fig. 1) is a device which has been developed to measure directly on the prepreg specimen the curing process of a thermosetting resin during the course of a thermal cycle. A constant dynamic impulse generated by a rod/motor/crank system is applied to a control jaw. This control jaw thus loaded transmits more or less the shear effect to a receiving jaw through the prepreg. This variation in the transmission is a function of the relative rigidity of the prepreg and thence of the state of cure of the resin. The receiving jaw is connected to a gauge which transforms this mechanical impulse into a signal which after having been amplified is transmitted, in turn, continuously to a potentiometer recording unit. The Vanhographe test provides information on the resin change during ventilated heating enclosure
// ,. ,..o,i.,
unit~. /
•
./
rixlo
/ arm
/
/
/
/
,/
movable arm
<_s
/ / ./
laW
p
~
\, ', ,
/
rlceivini flexible metal strips
ee~ilittiq flexible metal strips
Fig. 1. Vanhographe, functional diagram.
Dynamic thermomechanical analysis of a prepreg
Rg LIQUID PHASE
RUBBER PHASE
J ,.,,n, /
111
~ GLASS
T
//
/,/
........... ;;
Stilt
ond
TIME
Fig. 2. Vanhographe recording.
the curing process and more precisely on the point in time when resin gellification starts. The compacting pressure should be applied to the resin 'rubber zone' which corresponds to the initiation of gellification (Fig. 2). 2.2. Flow tests
The resin flow tests consist in determining with the weight change method, the quantity of resin which may flow from a prepreg during the application at a given moment during the thermal cycle of a pressure of 7 bars (100 psi) to the prepreg lay-up. 2.3. Vanhographe and flow tests results
We selected for three different thermal cycles, these points in time which could be appropriate for the application of the compacting pressure (they are referenced from 1 to 9). Cycle at 125°C For the thermal cycle studied, the flow test results corresponding to points 4, 5 and 6 and the curve pattern are shown in Fig. 3. At point 4 as well as points 5 and 6, the compacting pressure is applied in the "rubber zone' which corresponds to the initiation of resin
J. Hognat
112
T
(c) I
100
50
50 Fill. 3.
lOO
TIME
Imn)
Flow and Vanhographe results at 125°C.
gellification; moreover, the resin flow is quite sufficient to obtain a satisfactory interlayer resin compaction. Cycle at 135°C The flow test results corresponding to points 1, 2 and 3 and the Vanhographe curve pattern are shown in Fig. 4. The test results show that the pressure applied at point 1 is within the 'rubber zone' which
• l, l , ow
\
,I
.... 06
Fig. 4.
PLATEAU
-
So
/ G
AT /
o
' ' ' 100 Time Flow and Vanhographe results at 135°C. +
=
(c)
/
~.
<=,
~
Dynamic thermomechanical analysis of a prepreg
113
corresponds to the initiation of resin gellification and that the resin flow is sufficient to obtain a satisfactory compaction. On the contrary, for points 2 and 3, the gellification is almost completed and it is too late to obtain a satisfactory compaction of the composite; likewise resin flow is not sufficient to produce a good resin interlayer compaction. Cycle at 145°C The flow test results corresponding to points 7, 8 and 9 and the Vanhographe curve pattern are shown in Fig. 5.
% ,Low,
T (C) 150
w
~/~
Rg
100
,5O
F:
. . . . . . .
~0
_LG_ I(~)
Fig. 5.
TIME
(ran) ' '
Flow and Vanhographe results at 145°C.
The results demonstrate effectively that the pressure applied at 7 and 8 is within the 'rubber zone' which corresponds to the initiation of resin gellification and that the resin flow is quite sufficient to produce a satisfactory compacting of the resin. On the contrary, when pressure is applied at 9, the gellification is almost completed and it is too late to obtain a satisfactory composite compaction. Resin flow is consequently too low to produce a satisfactory interlayer resin compaction. When all these tests were completed, we fabricated test plates which we loaded successively at points 1 to 9.
J. Hognat
114
3.
DESCRIPTION OF THE C U R I N G CYCLES
Based on the following thermal cycle pattern: (a) (b) (c) (d)
temperature rise from ambient to 0 °C, lhatO°C, temperature rise to 180°C, final cure: 2 h at 180°C,
we fabricated for three different temperatures (125 °C, 135°C, 145 °C), three composite test pieces with a carbon fabric/epoxy resin prepreg on which we applied the compacting pressure at those points in time determined previously by the flow and Vanhographe tests.
At At At At At At At At At
O°C
Application of pressure
Plate
125 °C 125°C 125°C 135 °C 135°C 135 °C 145°C 145°C 145°C
for 20 MN for 40 MN for 60MN for 20 MN for 40MN for 60 MN for 0 M N for 10MN for 20 MN
4 5 6 1 2 3 7 8 9
The mechanical properties obtained are indicated in Tables 1, 2 and 3. TABLE 1
Parameter
Temperature/Time Volumic fibre content (~o) Void content (~o) Flexion (MPa) Interlaminar shear (MPa)
Plate
¢7
E 20c 120c
4
5
6
125/20 58 0 760 50810 67 53
125/40 59 0 830 51840 66 55
125/60 58 0 940 52810 64 56
Dynamic thermomechanical analysis of a prepreg
115
Plates 4, 5 and6. No porosity, good quality material. Both flexural and interlaminar shear strength characteristics are in conformity with this type of material. TABLE 2 Parameter
Plate
I Temperature/Time Volumic fibre content (~o) Void content (~) Flexion (MPa) Interlaminar shear (MPa)
o-
E 20c 120c
135/20 61 0.07 870 55280 72 51
2 135/40 58 4.5 530 48840 46 37
3 135/60 57 4.8 590 50550 48 37
Plate 1. No porosity: good quality material. The flexural and interlaminar shear strength are in conformity with this type of material. Plates 2 and 3. M a n y voids. The flexural and interlaminar shear characteristics are too low. TABLE 3 Parameter
Temperature/Time Volumic fibre content (~o) Void content (Vo) Flexion (MPa) Interlaminar shear (MPa)
Plate
o-
E 20c 120c
7
8
145/0 61 0 810 54400 72 58
145/10 59 0 850 56040 79 61
9
145/20 57 3.0 750 50840 49 45
Plates 7 and 8. No porosity. G o o d flexural and interlaminar shear characteristics. Plate 9. M a n y voids. The flexural and interlaminar shear characteristics are too low.
116
J. Hognat
~
Fig. 6. Plate 4. The structure is homogeneous, without voids. The compacting pressure has been applied in the appropriate part of the resin gellification zone. ( × 30)
Fig. 8.
Plate 6. No voids: good quality material. ( × 30)
Fig. 10. Plate 2. Porous structure. The compacting pressure has been applied too late during the cure cycle when the resin was gellified. ( x 32)
:r
. ~ , ~ i ~ ~L ....
i¸
Fig. 7. Plate 5. No voids. The compacting pressure has been applied in the appropriate zone of the resin gellification. ( × 30)
Fig. 9. Plate I. Homogeneous structure without voids. The state of the resin gellification was appropriate for the application of the compacting pressure. ( x 32)
Fig. 11. Plate 3. Porous structure. The compacting pressure has been applied too late during the cure cycle. ( x 32).
Dynamic thermomechanical analysis of a prepreg
117
The mechanical characteristics obtained on the nine plates show that the compacting pressure is satisfactory in the case of: (i) plates 4, 5 and 6: cycle at 125°C, (ii) plate 1: cycle at 135°C, (iii) plates 7 and 8: cycle at 145°C. On the contrary for plates 2 and 3 (cycle at 135°C) and plate 9 (cycle at 145 °C) the compacting pressure has been applied too late during the cycle at a moment when the resin had reached a state of cure which was inadequate to produce a satisfactory resin interlayer bond. The micrographs made of the nine plates (Figs 6-14) confirm that the interlayer cohesive state of cure of the resin is dependent upon the point in time when pressure is applied during the thermal cycle.
Fig. 12. Plate 7. G o o d quality material. No voids. The compacting pressure has
been applied in the appropriate part of the resin gellification zone favourable to the production of a satisfactory composite.
Fig. 13. Plate 8. No voids; homogeneous structure. The compacting pressure has been applied in an appropriate part of the resin gellification zone. ( × 30)
( x 30)
Fig. 14.
Porous structure. The compacting pressure has been applied too late during the cycle when resin was already gellified. ( × 32)
Plate 9.
118
J. Hognat
4.
CONCLUSIONS
It appears from this study that for the development of industrial cure cycles applicable to composites, thermomechanical methods such as the flow and Vanhographe techniques are perfectly adapted to determine, in a simple manner, the gellification zone of a resin and consequently the point in time when compacting pressure should be applied to a prepreg lay-up, so as to obtain a void-free composite with high mechanical properties.