- Email: [email protected]
sheath-fibre bundles
Journal of
ELSEVIER
Journal o f Materials Processing Technology 48 (1995) 317-324
Materials Processing Technology
Processing of Thermoplastic Composites from Powder/Sheath-Fibre Bundles
L. Ye a and K. Friedrich b
aCentre for Advanced Materials Technology, Department of Mechanical and Mechatronic Engineering The University of Sydney, NSW 2006, Australia blnstitute for Composite Materials Ltd, The University of Kaiserslautem, 67663 Kaiserslautem, Germany
The relationship between impregnation process, consolidation quality and resulting mechanical properties of GF/PP and CF/PEEK thermoplastic composites manufactured from powder/sheath-fibre bundles have been investigated. A small processing mould was used to simulate the different processing conditions (i.e. applied pressure, holding time and processing temperature). The consolidation quality of finished samples were characterized mainly by microscopic studies of material structure, density measurements and characterisation of mechanical properties through a small transverse flexure testing facility. A model for qualitatively describing the consolidation process of powder/sheath-fibre bundles was developed, which predicts variations of void content during consolidation as well as the time and pressure required to reach full consolidation. Good correlations between predictions and the experimental data indicate the success of the approach. Based on a desired, minimum level of void content (e.g. Xv=5% for GF/PP and Xv=2% for CF/PEEK composites), optimum processing windows for manufacturing of composite parts from powder/sheath-fibre preforms are suggested.
1 INTRODUCTION One of the potential concerns in the manufacturing of thermoplastic composites is impregnation of matrix resin into the network of reinforcing fibres. Great efforts have been made to overcome these difficulties by using the innovative/cost-effective material preforms, such as commingled yarns and powder/sheath-fibre bundles [1-2] (Fig. 1). However, if examining the structures of these material preforms, e.g. powder powder/sheathfibre bundles, up to 60 vol. % of resin may exist in the sheath around the fibre tows. During the consolidation process, sufficient pressure and time must be applied to force the resin to impregnate the fibre tows in order to achieve a fully c o n s o l i d a t e d c o m p o s i t e part [3]. Poor impregnation and consolidation will result in an undesirable shift and significant reduction in the
mechanical properties of the final composites [4]. Although some steps have been made to understand the mechanisms of impregnation in thermoplastic composites [5-7], one is at the moment far away from a comprehensive understanding of what the best processing conditions are to obtain sufficient properties for final parts made out of these new material preforms. In the present study, the fundamental mechanisms which govern impregnation and consolidation processes and thus the material qualities as well as the resulting mechanical properties of composite parts made out of powder/sheath-fibre bundles are investigated. The materials used are GF/PP (320 tex) and CF/PEEK (3K/6K) preforms. An impregnation model was developed to describe the consolidation process. Both approaches help to predict under which conditions of pressure, time
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 6 4 - M
318
L. }re, K. Friedrich /Journal of Materials Processing Technology 48 (1995) 317-324
Commingled Fibers
Film Stacking
i~ Reinforcing Fibers Fiber Woven Polymer F i b e r s / '
ui u i 0 i u
Powder/Sheath-Fibre Bundles Pre-lmpregnated Tape I ]|ul.~ illll hll| i I i IN|1 li~.._ n-u-
nu - u u n u u -
1-
uu - n - u u n
Fiber Polymer
ber Bundle
Polymer Powder olymer Sheath
Fig. 1 Innovative/cost-effective material preforms for thermoplastic composites
and temperature the material preforms result in perfect composite micro/macrostructures and good mechanical performance of the parts made out of them. 2 MATERIALS PROCESSING Powder/sheath-fibre bundles (GF/PP 320 tex and CF/PEEK 3K/6K) were supplied by Enichem, Italy, in which a roving of fibres was mingled with very fine matrix powder and then bound together by a thin sheath of matrix. Consolidation of the unidirectional composites was performed by using a small steel mould with a square cavity and a laboratory heat press. Once the mould reached the desired temperature (185°C-200°C for GF/PP, 360°C-390°C for CF/PEEK), pressure was applied. Different impregnation pressures (0,5-10 MPa) and holding times (5-20 min) were selected to examine the impregnation mechanisms as a function of processing conditions. A typical
processing cycle is illustrated in Fig. 2 The impregnation mechanisms was examined by reflected light microscopy on the polished crosssection of consolidated composites. It was found that the consolidation of the materials can be described by two different procedures, i.e. compaction and impregnation (Fig. 3). Micrographs in Fig. 4 illustrate some typical timedependent impregnation steps in CF/PEEK-6K composites made out of the powder/sheath-fibre preform. During the compaction stage, the initially separated fibre bundles were moved towards one another when pressure was applied. With temperature increasing above Tm, the sheaths around fibre tows melt first. The coalescence between melting sheaths then occurred, so that the interstitial spaces between the fibre bundles were filled by the matrix pool, surrounding the dry fibre tow (Fig. 4a). In the second step, with increased holding time, the applied pressure drove the matrix
L. Ye, K. Friedrich / Journal o f Materials Processing Technology 48 (1995) 317-324
TemperatureCycle
F
•
I.-
--~.m
-
~
paction
rain
AppliedPress /
319
L~.~nation
=1
__ p = 0.5 MPa f--~t (Holding Time)
Time)
t :t Fig. 2 The typical processing cycle consolidation of CF/PEEK composites
for
0
20
40
60
t~o, [mill] Fig. 3 Consolidation procedure of CF/PEEK-6K composites at T=370°C
- .
~~: : ~
•
~
I
~
~
~ •
.
, :
,
..~-~.~.~
Fig. 4 Micrographs for time-dependent impregnation steps in CF/PEEK-6K composites into the reinforcing fibre tows and autohesion between the resin front and the melting powder took place, thus reducing the voids between them (Figs• 4b and 4c). This process continued until the yarn was completely impregnated and full consolidation was achieved (Fig. 4d).
3 IMPREGNATION MODEL By assuming that all fibre bundles undergo impregnation simultaneously in a composite and that all of them are identical in geometry, the consolidation of the entire composite can be described by the inward impregnation of a representative single bundle only. The
320
L. Ye, If. Friedrich I Journal of Materials Processing Technology 48 (1995) 317-324
investigations of Van West, Pipes and Advani [8] indicated that there exists a geometric relationship between a fibre bundle with a circular crosssection and one with an elliptical cross section such that the two bundle geometries have the same time for full matrix impregnation. This allows the use of a one-dimensional impregnation analysis instead of a two-dimensional calculation necessary to track the flow front of matrix impregnating a bundle with a moving external front. The radius of a circular bundle with the same fill time as an elliptical one, is described as:
not changed during impregnation process, and it is equal to req, i.e. fibres are not moved towards each other and the fibre volume fraction is a constant and independent of the compaction. In this case, the compaction of whole bundle and consolidation are simply governed by the impregnation process of the molten sheath matrix into the free space of a fibre tow under a particular holding pressure (Fig. 5). The radial velocity of the matrix passing through the aligned fibres can be expressed as [3]:
- -g
r q = v"2"
(2)
ao b o
(1)
"4 ao+Do where a9r is the radial
velocity
of matrix
where a o and b o are the major and minor half-
impregnation, Kp the permeability of the fibre
lengths of the ellipse, respectively [8]. This equation was used to determine the circular bundle radius in the impregnation model.
tow, m the matrix viscosity, and p the pressure, r designates the radial position. At a desired impregnation step, r = rf, the remnant free space in
As a first order approach, it is further assumed here that the outer border of the fibre tow, ro, is
the fibre tow can be expressed as [11]:
Dimp = degree of impregnation rf = radius of resin front ro = radius of fibre-tow border rb -- current radius of whole fibre bundle
¸
~ ~!~
~i,~,:¸
~ :~y
3
i~¸
Dimp= 0
0 < Dimp< 1
rb tfr
>r°
rb>ro I'f < r_
rb -__-0 r
Dirnp= 1
==r°0
t >0 ~
tt
=tf
Fig. 5 Schematics of impregnation and compaction processes in a representative fibre bundle
L. Ye, IC Friedrich / Journal o f MateKals Processing Technology 48 (1995) 317-324
321
4 CONSOLIDATION QUALITY
Vv -(1 - Vf- Vmp)~ r2 2
(3)
- ~¢ro (1 - D-wap)(1 - Vf- Vine)
where Vf and Vmp are the volume fractions of the
Density measurements were carried out in order to correlate consolidation states with apparent void contents in relation to the processing conditions. The composite density, Pc' for different
fibres and the matrix powder between them. Hence, at a certain impregnation step, the void content, Xv, in a representative bundle is as fol-
processing conditions was determined according to ASTM-792. The theoretical density, Pt' of a
lows:
fully consolidated composite could be estimated by the following equation: Vv
X, - - - ~ nr b
Pf Pra Pt" Wf pm + Wrapt
(4)
(6)
where pf and Pm are the densities and Wf and W m
When the molten sheath matrix is all transferred to the free space in the fibre tow (Dimp=l), full impregnation is achieved, i.e. a
the weight fractions of fibres and matrix, respectively. The apparent void content, X v, was
void
then determined by:
free
consolidated
part
(Xv=Vv=0)
is
obtained. The model proposed by Gutowski et al [9] was used to describe the relationship between the applied pressure and the volume fraction of fibre tow. Once the fibre volume fraction is known, the permeability can be estimated by the Carman-Kozeny equation [10]:
Kp 20
d~
(l - Vf)3
16 k o
Vf2
(5)
~ r q P-n0' .5 MPa o
15
Xv =
Pt - P©
(7)
Pt
Detailed values for the geometry dimensions of the GF/PP and CF/PEEK powder/sheath-fibre bundles and variables used in the evaluations are given in separate reports [11, 12]. Figs. 6-8 illustrate the void contents in the consolidated 20
"=',:2;:
~10
o Pa = 0.5 MPa
~10
A
0 0 ~
5
10 t [min]
15
20
Fig. 6 Void contents versus holding time at different applied pressure (T=185°C) for GF/PP composites
, 0
A
7"-<.
^
10
20
t [mini
Fig. 7 Void content of CF/PEEK-6K versus holding time at two different levels of applied pressure (T=390°C)
L, Ye, K. Friedrich / Journal o f Materials Processing Technology 48 (1995) 317-324
322
& 20
10
o T = 390°C & T = 360°C
&
© CF/PEEK-3K
• CF/PEEK-6K
~ x'lo
=4
o 0(~
6
10 t [rain]
9 20
L
O0
10
20
30
x~ [%1
Fig. 8 Void content of CF/PEEK-3K versus holding time at two different processing temperatures (Pa =1 MPa)
Fig. 9 Transverse flexure strength of CF/PEEK composites versus void content
GP/PP and CF/PEEK composites as a function of holding time at different processing conditions. The symbols indicate the experimentally measured void contents, each one represents a mean value of four measurements. The solid lines indicate the predictions from the model. It can be seen that both applied pressure and holding time greatly affected the quality of consolidation. An increase in either holding time or applied pressure obviously reduced the void content in the composites, and gave better consolidated materials. For example, Fig. 7 illustrates the void contents in the consolidated CF/PEEK-6K composites as a function of holding time at two different levels of applied pressure; the processing temperature was held constant at T=663 K (=390°C), i.e. ~t=380 Pa.s. The data indicates the material reaches fully consolidated status when holding time exceeds 20 min at Pa = 1 MPa.
composites when an inappropriate processing condition was selected.
However, it seems that the CF/PEEK materials do not achieve high quality of consolidation at 360°C (i.e. Ix=1490 Pa.s). From these points, it can even be concluded that (1) the material preforms have brought fibres and matrix together in the nonmolten state and (2) although the reinforcing fibres and the matrix resin may have a good distribution already before processing, the high value of matrix viscosity at low processing temperature still obstructs the impregnation and consolidation processes in achieving void-free
5 CONSOLIDATION QUALITY MECHANICAL PROPERTIES
AND
Characterisations of mechanical properties as a function of impregnation conditions were carried out by using a small transverse flexure (three point bending) testing facility. The length of the span amounted to 40 ram; the width and thickness of the specimens were about 10 mm and 2 mm, respectively. The cross-head speed was set to 2 mm/min. Transverse elastic constants and flexure strength were determined according to ASTM standard D-790. Fig. 9 illustrates effects of void content on the transverse flexure strength of CF/PEEK composites. The ultimate strength, Omax, is obviously reduced as the void content is increased. For a change of the apparent void content from 23% to almost zero, increases in ultimate strength by, on the average, six-fold were identified. Fig. 10 presents the transverse elastic constant E22 as a function of void content. The same trend with an even more significant effect can be measured. The value at 23% void content was only about one ninth of that at a void content just below 5%. Similarly, for GF/PP composites, both ultimate strength and the transverse elastic constant are
L. Ye, K~ Friedrich / Journal of Materials Processing Technology 48 (1995) 317-324
T=220°C
60
323
T = 185°C
4 %" 40
=:2
~.1 20
0
10
20
30
xv 1%1 Fig. l0 Transverse elastic constant of CF/PEEK composites versus void content
obviously reduced as the void content is increased [11]. From these results, it can be stated that both transverse strength and elastic constant highly depend on the consolidation quality of the composites, which is therefore an essential factor which should be considered during manufacture of thermoplastic composites. If the void content is set to be an indicator of consolidation quality in the composite materials, the optimum processing window for manufacturing thermoplastic composites from these powder/sheath-fibre bundles based on a critical level of void content (5% for GF/PP and 2% for CF/PEEK) can be evaluated in Figs. 11-13. For example, assume that Xvc=2% is a desired minimum level of void content for consolidation of the CF/PEEK (3K and 6K) powder/sheath-fibre preforms at two processing temperatures (T=380°C and 400°C). Possible combinations of appropriate processing variables that will lead to composite materials in which the void content is less than this critical value can be selected from the windows. Hence, the mechanical properties of the consolidated composites will not be lower than those corresponding to this critical level of void content. From these processing windows, one can see that if the processing temperature is about
O0
'
1'0
'
2'0 ' 3'0 t [mlnl Fig. 11 Optimum processing window manufacturing GF/PP 320tex composites
40 for
380°C, holding time for high quality consolidation with a void content of less than 2% will be over 20 min (indicated by the point A in Figs. 12 and 13 ), at a processing pressure of 1.0 MPa. 6 CONCLUSIONS The impregnation and consolidation mechanisms in GF/PP and CF/PEEK thermoplastic composites made out of powder/sheath-fibre preforms were investigated. A model has been developed to qualitatively describe the impregnation process during consolidation. This model can predict the current void content as a function of bundle geometry and processing variables (temperature, applied pressure and holding time). Good correlations with the experimental data indicate the success of this approach. Based on a desired, minimum level of void content, optimum processing windows for manufacturing of composite parts from these materials is suggested. In practice, the present results would be of benefit to applications of these preforms for manufacturing engineering structures, especially for fast consolidation/or in-situ consolidation in filament winding and pultrusion manufacturing techniques etc.
L. Ye, K. Friedrich / Journal of Materials Processing Technology 48 (1995) 317-324
324
6--
65 %' 4
/~/T=400°C /
T=400oc
5-
\ J T=380°C
%`
I
4
~T--~380°C
2o/0
~21 ---
Oq
1'0
t [mini
2'0
30
0
0
> 2% m~',-~r_._ , ~n 10 t [min l 20
30
Fig. 12 Optimum processing window for manufacturing of CF/PEEK-3K composites
Fig. 13 Optimum processing window for manufacturing of CF/PEEK-6K composites
ACKNOWLEDGEMENT
4 Ye, L., Klinkmiiller, V. and Friedrich, K., in Proc. ECCM-V, Bordeaux, pp. 423-428, 1992
Thanks are due to Instituto Guido Donegani, Enichem, Milan, Italy, who kindly supplied the testing materials. Dr L. Ye would express his appreciation to the University of Sydney for the support to this study under the University Research Grant Scheme (URG). Prof K. Friedrich addresses his thanks to the Deutsche Forschungsgemeinschaft(DFG) for the support of the research (DFG-FR-675-11-1 ). Further thanks are due to the Fonds der Chemischen Industrie Germany, for his personal research research activities in 1994.
Reference
1. Bigg, D. M., Hiscock, D. F., Preston, J. R. and Bradbury, E. J., J. Thermoplastic Composite Materials, 1 (1988) 146-160 2. Chang, I. Y. and J. K. Lees, J. Thermoplastic Composite Materials, 1 (1988) 277-295 3. Kim, T.-W. and Jun, E. -J., Proc. 34th International SAMPE Symposium, May 8-11, 1989, pp.323-328
5. Lee, W. I. and Springer G. S., J. Composite Materials, 21 (1987) 1017-1055 6. Kim, W. T., Jtm E. J., Um, M. K. and Lee, W. I., Advances in Polymer Technology, 9 (1989) 275-279 7. Set, J. W. and Lee, W. I., J. Composite Materials, 25 (1991) 1127-1142 8. Van West, B. P. Pipes, R. B. and Advani, S. G., Polymer Composites, 12:417-427 (1991) 9. Gutowski, T. G., Cai. Z., Bauer, S., Boucher, D., Kingery, J. and Wineman, S., J. Composite Materials, 21 (1987) 650-669 10. Greenkorn, R. A., Flow phenomena in porous media, Dekker, New York, 1983 11. Ye, L., Klinkmiiller, V. and Friedrich, K., J. Thermoplastic Composite Materials, 5:32-48, (1992) 12. Ye, L., Friedrich, K., Cutolo, D., and Savadori, A., Composites Manufacturing, 5:41-50 (1994)
ELSEVIER
Journal o f Materials Processing Technology 48 (1995) 317-324
Materials Processing Technology
Processing of Thermoplastic Composites from Powder/Sheath-Fibre Bundles
L. Ye a and K. Friedrich b
aCentre for Advanced Materials Technology, Department of Mechanical and Mechatronic Engineering The University of Sydney, NSW 2006, Australia blnstitute for Composite Materials Ltd, The University of Kaiserslautem, 67663 Kaiserslautem, Germany
The relationship between impregnation process, consolidation quality and resulting mechanical properties of GF/PP and CF/PEEK thermoplastic composites manufactured from powder/sheath-fibre bundles have been investigated. A small processing mould was used to simulate the different processing conditions (i.e. applied pressure, holding time and processing temperature). The consolidation quality of finished samples were characterized mainly by microscopic studies of material structure, density measurements and characterisation of mechanical properties through a small transverse flexure testing facility. A model for qualitatively describing the consolidation process of powder/sheath-fibre bundles was developed, which predicts variations of void content during consolidation as well as the time and pressure required to reach full consolidation. Good correlations between predictions and the experimental data indicate the success of the approach. Based on a desired, minimum level of void content (e.g. Xv=5% for GF/PP and Xv=2% for CF/PEEK composites), optimum processing windows for manufacturing of composite parts from powder/sheath-fibre preforms are suggested.
1 INTRODUCTION One of the potential concerns in the manufacturing of thermoplastic composites is impregnation of matrix resin into the network of reinforcing fibres. Great efforts have been made to overcome these difficulties by using the innovative/cost-effective material preforms, such as commingled yarns and powder/sheath-fibre bundles [1-2] (Fig. 1). However, if examining the structures of these material preforms, e.g. powder powder/sheathfibre bundles, up to 60 vol. % of resin may exist in the sheath around the fibre tows. During the consolidation process, sufficient pressure and time must be applied to force the resin to impregnate the fibre tows in order to achieve a fully c o n s o l i d a t e d c o m p o s i t e part [3]. Poor impregnation and consolidation will result in an undesirable shift and significant reduction in the
mechanical properties of the final composites [4]. Although some steps have been made to understand the mechanisms of impregnation in thermoplastic composites [5-7], one is at the moment far away from a comprehensive understanding of what the best processing conditions are to obtain sufficient properties for final parts made out of these new material preforms. In the present study, the fundamental mechanisms which govern impregnation and consolidation processes and thus the material qualities as well as the resulting mechanical properties of composite parts made out of powder/sheath-fibre bundles are investigated. The materials used are GF/PP (320 tex) and CF/PEEK (3K/6K) preforms. An impregnation model was developed to describe the consolidation process. Both approaches help to predict under which conditions of pressure, time
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 6 4 - M
318
L. }re, K. Friedrich /Journal of Materials Processing Technology 48 (1995) 317-324
Commingled Fibers
Film Stacking
i~ Reinforcing Fibers Fiber Woven Polymer F i b e r s / '
ui u i 0 i u
Powder/Sheath-Fibre Bundles Pre-lmpregnated Tape I ]|ul.~ illll hll| i I i IN|1 li~.._ n-u-
nu - u u n u u -
1-
uu - n - u u n
Fiber Polymer
ber Bundle
Polymer Powder olymer Sheath
Fig. 1 Innovative/cost-effective material preforms for thermoplastic composites
and temperature the material preforms result in perfect composite micro/macrostructures and good mechanical performance of the parts made out of them. 2 MATERIALS PROCESSING Powder/sheath-fibre bundles (GF/PP 320 tex and CF/PEEK 3K/6K) were supplied by Enichem, Italy, in which a roving of fibres was mingled with very fine matrix powder and then bound together by a thin sheath of matrix. Consolidation of the unidirectional composites was performed by using a small steel mould with a square cavity and a laboratory heat press. Once the mould reached the desired temperature (185°C-200°C for GF/PP, 360°C-390°C for CF/PEEK), pressure was applied. Different impregnation pressures (0,5-10 MPa) and holding times (5-20 min) were selected to examine the impregnation mechanisms as a function of processing conditions. A typical
processing cycle is illustrated in Fig. 2 The impregnation mechanisms was examined by reflected light microscopy on the polished crosssection of consolidated composites. It was found that the consolidation of the materials can be described by two different procedures, i.e. compaction and impregnation (Fig. 3). Micrographs in Fig. 4 illustrate some typical timedependent impregnation steps in CF/PEEK-6K composites made out of the powder/sheath-fibre preform. During the compaction stage, the initially separated fibre bundles were moved towards one another when pressure was applied. With temperature increasing above Tm, the sheaths around fibre tows melt first. The coalescence between melting sheaths then occurred, so that the interstitial spaces between the fibre bundles were filled by the matrix pool, surrounding the dry fibre tow (Fig. 4a). In the second step, with increased holding time, the applied pressure drove the matrix
L. Ye, K. Friedrich / Journal o f Materials Processing Technology 48 (1995) 317-324
TemperatureCycle
F
•
I.-
--~.m
-
~
paction
rain
AppliedPress /
319
L~.~nation
=1
__ p = 0.5 MPa f--~t (Holding Time)
Time)
t :t Fig. 2 The typical processing cycle consolidation of CF/PEEK composites
for
0
20
40
60
t~o, [mill] Fig. 3 Consolidation procedure of CF/PEEK-6K composites at T=370°C
- .
~~: : ~
•
~
I
~
~
~ •
.
, :
,
..~-~.~.~
Fig. 4 Micrographs for time-dependent impregnation steps in CF/PEEK-6K composites into the reinforcing fibre tows and autohesion between the resin front and the melting powder took place, thus reducing the voids between them (Figs• 4b and 4c). This process continued until the yarn was completely impregnated and full consolidation was achieved (Fig. 4d).
3 IMPREGNATION MODEL By assuming that all fibre bundles undergo impregnation simultaneously in a composite and that all of them are identical in geometry, the consolidation of the entire composite can be described by the inward impregnation of a representative single bundle only. The
320
L. Ye, If. Friedrich I Journal of Materials Processing Technology 48 (1995) 317-324
investigations of Van West, Pipes and Advani [8] indicated that there exists a geometric relationship between a fibre bundle with a circular crosssection and one with an elliptical cross section such that the two bundle geometries have the same time for full matrix impregnation. This allows the use of a one-dimensional impregnation analysis instead of a two-dimensional calculation necessary to track the flow front of matrix impregnating a bundle with a moving external front. The radius of a circular bundle with the same fill time as an elliptical one, is described as:
not changed during impregnation process, and it is equal to req, i.e. fibres are not moved towards each other and the fibre volume fraction is a constant and independent of the compaction. In this case, the compaction of whole bundle and consolidation are simply governed by the impregnation process of the molten sheath matrix into the free space of a fibre tow under a particular holding pressure (Fig. 5). The radial velocity of the matrix passing through the aligned fibres can be expressed as [3]:
- -g
r q = v"2"
(2)
ao b o
(1)
"4 ao+Do where a9r is the radial
velocity
of matrix
where a o and b o are the major and minor half-
impregnation, Kp the permeability of the fibre
lengths of the ellipse, respectively [8]. This equation was used to determine the circular bundle radius in the impregnation model.
tow, m the matrix viscosity, and p the pressure, r designates the radial position. At a desired impregnation step, r = rf, the remnant free space in
As a first order approach, it is further assumed here that the outer border of the fibre tow, ro, is
the fibre tow can be expressed as [11]:
Dimp = degree of impregnation rf = radius of resin front ro = radius of fibre-tow border rb -- current radius of whole fibre bundle
¸
~ ~!~
~i,~,:¸
~ :~y
3
i~¸
Dimp= 0
0 < Dimp< 1
rb tfr
>r°
rb>ro I'f < r_
rb -__-0 r
Dirnp= 1
==r°0
t >0 ~
tt
=tf
Fig. 5 Schematics of impregnation and compaction processes in a representative fibre bundle
L. Ye, IC Friedrich / Journal o f MateKals Processing Technology 48 (1995) 317-324
321
4 CONSOLIDATION QUALITY
Vv -(1 - Vf- Vmp)~ r2 2
(3)
- ~¢ro (1 - D-wap)(1 - Vf- Vine)
where Vf and Vmp are the volume fractions of the
Density measurements were carried out in order to correlate consolidation states with apparent void contents in relation to the processing conditions. The composite density, Pc' for different
fibres and the matrix powder between them. Hence, at a certain impregnation step, the void content, Xv, in a representative bundle is as fol-
processing conditions was determined according to ASTM-792. The theoretical density, Pt' of a
lows:
fully consolidated composite could be estimated by the following equation: Vv
X, - - - ~ nr b
Pf Pra Pt" Wf pm + Wrapt
(4)
(6)
where pf and Pm are the densities and Wf and W m
When the molten sheath matrix is all transferred to the free space in the fibre tow (Dimp=l), full impregnation is achieved, i.e. a
the weight fractions of fibres and matrix, respectively. The apparent void content, X v, was
void
then determined by:
free
consolidated
part
(Xv=Vv=0)
is
obtained. The model proposed by Gutowski et al [9] was used to describe the relationship between the applied pressure and the volume fraction of fibre tow. Once the fibre volume fraction is known, the permeability can be estimated by the Carman-Kozeny equation [10]:
Kp 20
d~
(l - Vf)3
16 k o
Vf2
(5)
~ r q P-n0' .5 MPa o
15
Xv =
Pt - P©
(7)
Pt
Detailed values for the geometry dimensions of the GF/PP and CF/PEEK powder/sheath-fibre bundles and variables used in the evaluations are given in separate reports [11, 12]. Figs. 6-8 illustrate the void contents in the consolidated 20
"=',:2;:
~10
o Pa = 0.5 MPa
~10
A
0 0 ~
5
10 t [min]
15
20
Fig. 6 Void contents versus holding time at different applied pressure (T=185°C) for GF/PP composites
, 0
A
7"-<.
^
10
20
t [mini
Fig. 7 Void content of CF/PEEK-6K versus holding time at two different levels of applied pressure (T=390°C)
L, Ye, K. Friedrich / Journal o f Materials Processing Technology 48 (1995) 317-324
322
& 20
10
o T = 390°C & T = 360°C
&
© CF/PEEK-3K
• CF/PEEK-6K
~ x'lo
=4
o 0(~
6
10 t [rain]
9 20
L
O0
10
20
30
x~ [%1
Fig. 8 Void content of CF/PEEK-3K versus holding time at two different processing temperatures (Pa =1 MPa)
Fig. 9 Transverse flexure strength of CF/PEEK composites versus void content
GP/PP and CF/PEEK composites as a function of holding time at different processing conditions. The symbols indicate the experimentally measured void contents, each one represents a mean value of four measurements. The solid lines indicate the predictions from the model. It can be seen that both applied pressure and holding time greatly affected the quality of consolidation. An increase in either holding time or applied pressure obviously reduced the void content in the composites, and gave better consolidated materials. For example, Fig. 7 illustrates the void contents in the consolidated CF/PEEK-6K composites as a function of holding time at two different levels of applied pressure; the processing temperature was held constant at T=663 K (=390°C), i.e. ~t=380 Pa.s. The data indicates the material reaches fully consolidated status when holding time exceeds 20 min at Pa = 1 MPa.
composites when an inappropriate processing condition was selected.
However, it seems that the CF/PEEK materials do not achieve high quality of consolidation at 360°C (i.e. Ix=1490 Pa.s). From these points, it can even be concluded that (1) the material preforms have brought fibres and matrix together in the nonmolten state and (2) although the reinforcing fibres and the matrix resin may have a good distribution already before processing, the high value of matrix viscosity at low processing temperature still obstructs the impregnation and consolidation processes in achieving void-free
5 CONSOLIDATION QUALITY MECHANICAL PROPERTIES
AND
Characterisations of mechanical properties as a function of impregnation conditions were carried out by using a small transverse flexure (three point bending) testing facility. The length of the span amounted to 40 ram; the width and thickness of the specimens were about 10 mm and 2 mm, respectively. The cross-head speed was set to 2 mm/min. Transverse elastic constants and flexure strength were determined according to ASTM standard D-790. Fig. 9 illustrates effects of void content on the transverse flexure strength of CF/PEEK composites. The ultimate strength, Omax, is obviously reduced as the void content is increased. For a change of the apparent void content from 23% to almost zero, increases in ultimate strength by, on the average, six-fold were identified. Fig. 10 presents the transverse elastic constant E22 as a function of void content. The same trend with an even more significant effect can be measured. The value at 23% void content was only about one ninth of that at a void content just below 5%. Similarly, for GF/PP composites, both ultimate strength and the transverse elastic constant are
L. Ye, K~ Friedrich / Journal of Materials Processing Technology 48 (1995) 317-324
T=220°C
60
323
T = 185°C
4 %" 40
=:2
~.1 20
0
10
20
30
xv 1%1 Fig. l0 Transverse elastic constant of CF/PEEK composites versus void content
obviously reduced as the void content is increased [11]. From these results, it can be stated that both transverse strength and elastic constant highly depend on the consolidation quality of the composites, which is therefore an essential factor which should be considered during manufacture of thermoplastic composites. If the void content is set to be an indicator of consolidation quality in the composite materials, the optimum processing window for manufacturing thermoplastic composites from these powder/sheath-fibre bundles based on a critical level of void content (5% for GF/PP and 2% for CF/PEEK) can be evaluated in Figs. 11-13. For example, assume that Xvc=2% is a desired minimum level of void content for consolidation of the CF/PEEK (3K and 6K) powder/sheath-fibre preforms at two processing temperatures (T=380°C and 400°C). Possible combinations of appropriate processing variables that will lead to composite materials in which the void content is less than this critical value can be selected from the windows. Hence, the mechanical properties of the consolidated composites will not be lower than those corresponding to this critical level of void content. From these processing windows, one can see that if the processing temperature is about
O0
'
1'0
'
2'0 ' 3'0 t [mlnl Fig. 11 Optimum processing window manufacturing GF/PP 320tex composites
40 for
380°C, holding time for high quality consolidation with a void content of less than 2% will be over 20 min (indicated by the point A in Figs. 12 and 13 ), at a processing pressure of 1.0 MPa. 6 CONCLUSIONS The impregnation and consolidation mechanisms in GF/PP and CF/PEEK thermoplastic composites made out of powder/sheath-fibre preforms were investigated. A model has been developed to qualitatively describe the impregnation process during consolidation. This model can predict the current void content as a function of bundle geometry and processing variables (temperature, applied pressure and holding time). Good correlations with the experimental data indicate the success of this approach. Based on a desired, minimum level of void content, optimum processing windows for manufacturing of composite parts from these materials is suggested. In practice, the present results would be of benefit to applications of these preforms for manufacturing engineering structures, especially for fast consolidation/or in-situ consolidation in filament winding and pultrusion manufacturing techniques etc.
L. Ye, K. Friedrich / Journal of Materials Processing Technology 48 (1995) 317-324
324
6--
65 %' 4
/~/T=400°C /
T=400oc
5-
\ J T=380°C
%`
I
4
~T--~380°C
2o/0
~21 ---
Oq
1'0
t [mini
2'0
30
0
0
> 2% m~',-~r_._ , ~n 10 t [min l 20
30
Fig. 12 Optimum processing window for manufacturing of CF/PEEK-3K composites
Fig. 13 Optimum processing window for manufacturing of CF/PEEK-6K composites
ACKNOWLEDGEMENT
4 Ye, L., Klinkmiiller, V. and Friedrich, K., in Proc. ECCM-V, Bordeaux, pp. 423-428, 1992
Thanks are due to Instituto Guido Donegani, Enichem, Milan, Italy, who kindly supplied the testing materials. Dr L. Ye would express his appreciation to the University of Sydney for the support to this study under the University Research Grant Scheme (URG). Prof K. Friedrich addresses his thanks to the Deutsche Forschungsgemeinschaft(DFG) for the support of the research (DFG-FR-675-11-1 ). Further thanks are due to the Fonds der Chemischen Industrie Germany, for his personal research research activities in 1994.
Reference
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