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Measuring the development of fibre orientation during the melt extrusion of short glass fibre reinforced polypropylene
Composites Purt A 28A ( 1997) 949-958 0 1997 Elqevier Science Limited Printed
in Circa
PII: S1359-835X(97)00070-5
ELSEVIER
Britain. Ail rights reserved 1359-835)
Measuring the development of fibre orientation during the melt extrusion of short glass fibre reinforced polypropylene
P. J. Hines, S.-W. Tsuib, P. D. Coatesb, I. M. Warda and R. A. Ducketta alRC in Polymer Science and Technology, University blt?C in Polymer Science and Technology, University (Received 30 January 7997; accepted 12 June 1997)
of Leeds, Leeds, UK of Bradford, Bradford,
UK
In this paper we describe an investigation into the development of hbre orientation during melt extrusion through two convergent dies: a conical die and a slit die. The material used was a short glass fibre (aspect ratio = 24) reinforced polypropylene at two fibre weight fractions, 20% and 30%. The development of fibre orientation through the two convergent zones was measured in detail using sophisticated image analysis facilities developed in-house. It was found that the ‘pseudo-affine’ deformation model predicted the development of hbre orientation very well for both die configurations, in terms of the applied macroscopic elongational strains. The addition of a breaker plate placed in the barrel of the extruder, between the extruder screw and the convergent Row zones, was shown to produce the highest degree of fibre alignment in the extrudates, by introducing additional pre-orientation of the fibres. Mechanical measurements on the extrudates showed that for the very high degrees of fibre alignment attained, the bending modulus in the extrusion direction reached 90% of the theoretical maximum for the fibre aspect ratio used, and the crack resistance parallel to the preferred fibre direction remained high. 0 1997 Elsevier Science Limited. (Keywords: composites; short fibres; extrusion; mechanical properties)
INTRODUCTION The use of glass fibre reinforced thermoplastics has increased rapidly over the last few years, offering a combination of low cost, fast cycle times, and good mechanical properties. A number of different processing routes are available for these materials, and the choice of process governs both the type of reinforcement used (continuous, long or short hbres) and the fibre orientation in the final composite and hence the balance of composite mechanical properties. Pultruded continuous glass fibre reinforced thermoplastics (e.g. ref. ‘) have the advantage of high axial stiffness and strength, but the disadvantage of low crack resistance along the fibre direction, a consequence of the high fibre fractions which are used in this process. Injection moulded short fibre composites (e.g. refs 233)offer advantages such as ease of manufacture, low cycle times and lower cost, but generally lead to random or at best partially aligned fibres. This results in improved crack resistance compared to pultruded materials, but lower stiffness and strength. From our previous studies of short fibre composites, it is clear that only a small amount of fibre misalignment is needed for a substantial improvement in the fracture toughness parallel to the preferred fibre direction’,‘. There is therefore a possible window of opportunity for materials that are highly, but not perfectly aligned, which
combine high axial stiffness and strength with good crack resistance. There are a significant number of published papers which describe methods for producing preferred fibre orientation in short libre reinforced composites. The techniques described include converging flow extrusion6mx, diverging flow extrusion”.” and injection moulding’3”, while the paper by Guell and Graham” lists centrifugal force, vibration, vacuum drums and electric fields as being used. Interestingly it appears that only the work of Goettler” on tube extrusion, and the work of Bevis in shear controlled injection moulding”, have so far resulted in industrial exploitation. While it is widely recognised that extensional or convergent flow can orient fibres in the flow direction (e.g. refs ‘-‘), and that divergent flow can orient hbres perpendicular to the floods”‘, little work has been done to accurately measure the development of this fibre orientation. If it can be shown that the rotation of fibres in simple deformation fields (either elongational, shear or a combination of both) can be simply modelled in terms of geometrical apsects of the deformation, then more complicated flow regimes, produced for example during injection moulding, can be predicted with confidence. The majority of published work on the prediction of fibre rotation during flow, particularly with regard to simulations of injection moulding, is based on the work of Jeffrey” who
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derived a relationship to predict the motion of ellipsoidal particles immersed in a moving viscous fluid. We have approached the modelling of fibre rotation during convergent flow melt extrusion from a different standpoint, based on the background in this laboratory for predicting the orientation of polymer crystals during solid state deformation. Recently14 we reported a study of the injection moulding of circular dumbbells of short glass fibre reinforced polypropylene. It was shown that as the fibre filled melt flowed from the larger diameter end section to the smaller diameter centre section, the convergent flow region induced preferred fibre orientation in the centre section. Furthermore it was shown that the development of this fibre orientation could be predicted very well by the ‘pseudo affine’ deformation schemeI used extensively to predict the effects of an imposed deformation on the development of orientation in crystalline polymers’6, and more recently to predict accurately the rotation of fibres during the solid state extrusion of short glass fibre reinforced polyoxymethylene17. The rotation of the fibre is calculated from the imposed affine deformation of the composite, and is therefore dependent only on the applied external strains. This is a similar approach to that of Modlen’* who described the re-orientation of fibres during mechanical working. In this paper we use the pseudo-affine deformation scheme to model the rotation of fibres during convergent flow through: (i) a conical die, where the deformation is uniaxial; and (ii) a slit die, which produces a reduction in thickness at constant width. The equations derived can be shown to be special cases of Jeffrey’s equation. The aims of the research now described were therefore threefold: first to measure the rotation of fibres during convergent flow melt extrusion and compare it with theoretical predictions based on the simple pseudo affine scheme; second to measure the maximum level of preferred fibre orientation that we could produce during convergent melt extrusion; and third to investigate the effects of the attained levels of fibre orientation on the axial Young’s modulus and the crack resistance of the extrudates for cracks propagating in the weakest direction, parallel to the main fibre direction.
EXPERIMENTAL
General details Two commercial grades of glass fibre filled polypropylene were used in these studies; G2N02 and G3NOl manufactured by Hoechst AG. These had fibre weight fractions of 20 and 30% (fibre volume fractions of 8 and 13%) respectively. The average fibre length was measured to be 425 pm in the starting pellets, falling to 330 pm after extrusion: with an average fibre diameter of 13.4 pm, this gave a final aspect ratio for the fibres of 24. The polypropylene matrix was isotactic and the composites had melt flow indices of 5 and 4.5 (as given by the literature) for G2N02 and G3NOl manufacturers’ respectively.
950
Figure
1
A schematic
of the convergent
flow region
Breaker
Plate diameter
Number
of holes
45mm 38mm 3.0mm
Figure
2
Ratio of bole size to tibre length
66 10
Ratio of total hole area to barrel area
0.41
Details of the breaker plate
The extrusion experiments were conducted using a Betol BK38 single screw extruder fitted with a single 38 mm diameter general purpose screw which had a 2:l compression ratio. There were six zones of temperature control, four along the extruder barrel and two around the convergent dies. The barrel zones were set to 180, 190, 200 and 200°C (denoted from the inlet hopper to the end of the screw), while the two die zones were set to 200°C. A pressure transducer was located within the die region to monitor melt pressure. All the tests used a screw speed of 20 rpm. After the material left the die, it was passed though a water bath and then through driven haul-off rollers, producing strip at approximately 1 m/min. Figure 1 shows a schematic diagram of the convergent flow region. A breaker plate is normally present in the extruder at the end of the screw to help translate helical motion to axial motion. However the breaker plate causes complicated fibre orientation structures to be developed downstream, making modelling the effect of the convergent dies more difficult: therefore when modelling the effect of the convergent zone on fibre orientation, the breaker plate was removed. Two die geometries were used in this study: a conical die with a 9 mm diameter exit; and a slit die with an exit cross section of 27 mm by 3 mm. Both dies had a semiangle of 15” and a 25 mm land after the end of the convergence region. Experiments were also carried out using both dies, with the breaker plate in place, to assess its effect on the final extrudate orientation. Figure 2 shows a diagram of the breaker plate, which contained 66 holes, each 3.0 mm in diameter giving a total hole/extruder barrel area ratio of 41%. The ratio of the hole diameter to fibre length was approximately 10: the holes were 9.5 mm long.
Fibre orientation
measurement
The fibre orientation zones was measured
before and after the convergent flow from sections taken from frozen
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Direction
Figure 3
Image analysis details: (a) the definition of the orientation angles 0 and $: and (b) the definition of the sample axec
material in the die zone, produced after the extrudates had been manufactured. Fibre orientation was measured using a transputer controlled image analysis system developed at Leeds University’9.20. Images are produced directly from a polished composite section, where each fibre appears as an ellipse. Measuring the ellipticity and orientation of each fibre image enables the two angles 19and 4, which specify the orientation of the fibres, to be determined. 0 is defined as the angle the fibre makes with the extrusion direction (Z axis), while 4 is the angle between the X axis and the projection of the fibre axis on the XY plane, as shown in Figure 3a. The definition of the sample axes are shown in Figure 3b. Further details of the image analysis system and the measurement of fibre orientation can be found in ref. 19. Fracture
properties
Crack propagation parallel to the main fibre direction (i.e. the extrusion direction) was investigated for both materials. Initial fracture tests, using a standard double cantilever beam geometry, proved unsuccessful in producing crack propagation. The combination of a relatively low axial stiffness, and a high toughness resulted in deformation in the arms of the specimen before crack propagation occurred. The solution to this problem was to use a reinforced double cantilever beam geometry”. The arms of the specimen were reinforced with steel strips, effectively increasing the stiffness and strength of the sample arms allowing a crack to be propagated through the sample. Using this sample geometry, cracks were successfully propagated through the extruded strips. During crack propagation the load and displacement were monitored as the crack extended across a series of lines 2 mm apart on the sample. The sample compliance (C) was determined for each crack length (a), and by fitting a polynomial to these data, a value for the rate of change of compliance with crack length (dC/da) was determined. The strain energy release rate, G,, was then calculated from the Irwin-Kies equation
THEORY In this paper we compare the measured fibre orientation distributions in the extrudate (i.e. after the convergent die region) with those predicted from the measured fibre orientation distributions before the die region (position B in Figure I) using the pseudo-affine deformation scheme”,“. In general, material travelling through a convergent flow zone will experience both elongational and shear deformations. Modlen”, in his paper on the effect of mechanical work on fibre orientation, included both elongational and shear deformations but concluded that for uniaxial deformation in particular, internal shearing becomes of decreasing importance as the extrusion ratio increases above 4. As the extrusion ratios used in this work are greater than 9, we have opted to ignore the shear contribution. In addition, determination of the deformation fields for the die angle (15”) and extrusion ratios used in this study by implementing the relationships of Avitzur”, indicate a low level of shear in the extrudate. Using the pseudo-affine deformation scheme, the orientation 19 and 4 of a fibre after a general elongational deformation defined by the deformation ratios Xx, Xy and hz, along the X, Y and Z axes respectively, is related to the original orientation angles 0’ and 6’ by the following two equations.
xx tan0=-tan0’
(l+($tan’6’)
’
(2a)
(1 + tan’+‘)
AZ I
tan+=
I
XtanO’
(2b)
X
For the conical die the deformation
is uniaxial
such that
..2% 2B da ‘
where P is the fracture load at a particular B is the sample thickness.
crack length, and
assuming constant hx.Xy.Xz = 1).
volume
during
deformation
(i.e.
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Development
Table 1
of fibre orientation:
The nominal deformation XX
Conical die Slit die
P. J. Hine et al.
ratios imposed by the two dies hY
0.323 0.11
AZ
0.323 1.0
eqn (2a) and eqn (2b) then simplify
9.61 9.0
to
3 tan 8 = X 2tan 8’
(3a)
where h = Xz This equation is identical to that determined by a number of authors (e.g. Dinh and Armstrong23) by solving Jeffery’s equation for purely elongational flow in a conical die. For the slit die the deformation occurs at approximately constant width so that Xy = 1 and Xx = l/(hz). Eqn (2a) and eqn (2b) then reduce to
(b)
500
1
45.0
tan$=htan4’
(4b)
The deformation ratios for the two dies, determined entrance and exit sizes, are shown in Table 1.
Theta
from the (c)
200
,
160
-
1
RESULTS
Modelling the effect of the convergence breaker plate
zone-G3NO1
-no
In this section results are presented for measurements of fibre orientation through both the conical die and the slit die with the breaker plate removed from the barrel of the extruder. The results presented are for G3NO1, the high fibre volume fraction material only: very similar results were obtained for the G2N02 grade. Figure 4a shows an image analysis schematic and Figures 4b and 4c the 19and 4 distributions, from solidified material taken from the conical die zone. The section is taken in the XY plane (perpendicular to the extrusion direction) and the analysis position is just before the convergence region of the die (position B shown in Figure I). Similar results were found at different positions between B and the tip of the screw (position A). The image analysis schematic was recreated from the data stored during the measurement. Over this measurement field the fibres lie predominantly in the XY plane with a well defined orientation in that plane: i.e. the fibres are mostly perpendicular to the extrusion direction (maximum 8 = 82”) with I$ in the range (-45 > 4 > +lO). The region shown in Figure 4a (approximately 5 mm square) is just a small section of the overall barrel diameter. In other regions the measured 0 distribution was similar, but the fibres had a different preferred 9 orientation as a result of an overall
-90.0
-45.0
0.0
45.0
90.0
Figure 4 G3NOl with no breaker plate: (a) image analysis schematic of a section of the barrel before the entrance to the convergence region; and (b), (c) the 0 and 6 distributions determined at this position
helicity of the flow imparted by the screw of the extruder. A similar result was found from analysing solidified material taken from the slit die. It is significant that the fibre orientations at this point are not isotropic. Figure 5 shows a comparison of the 0 distributions measured before the convergence region and after the convergence region (i.e. in the extrudate), for (a) the conical die and (b) the slit die. The effect of both convergence zones is clear, with the material after the convergent zones showing a significant degree of preferred fibre alignment in the extrusion direction. Also shown in Figure 5 are the theoretical predictions of the 6 distributions after the two convergent regions, calculated based on the f3 and #J distributions before the die and the pseudo-affine deformation scheme as given by eqns (3a), (3b), (4a) and (4b). The
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I .
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(a) ”
‘7 Before
die
Theory (P.Affine) After
22.5
45.0
67.5
die
90.0
Theta
-)-
Before
0
dvs
Theory (P Affine)
-+--
0.0
22.5
45.0
67.5
After
die
90.0
Theta Figure 5 A comparison of the 0 distributions measured before and after the convergence breaker plate: (a) for the conical die: and (b) for the slit die
fibre orientation distribution in the barrel before the melt reached the convergent dies was built up from a number of image scans (taken in the XY plane) approximately 5 mm square. Although for the best comparison between theory and experiment it would be necessary to scan the complete barrel diameter, the size of this area meant that this was not practical. Instead, four image scans were taken at different positions across the barrel diameter, and therefore different local values of c$, to attempt to produce as representative a sample as possible: area scans were taken where the local average values of 4 were approximately 0,45, -45 and 90”. The agreement between the measured extrudate fibre orientations, and the predictions of the pseudo-affine deformation scheme is excellent for the conical die, and good for the slit die. The reason for the poorer fit between experiment and theory for the slit die is believed to be due to the uncertainties in the way the fibre orientation distribution prior to the die is built up, as described above. Eqns (3a), (3b), (4a) and (4b) show that for the conical die, the final 8 is independent of the original 9, whereas for the slit die the orientation of the fibre with respect to the die (i.e. the original 4) is obviously important. Therefore for the conical die the predicted theoretical 8 distribution is independent of
region. with the theoretical
prediction
for G3NOI without the
the size of area scan taken (as long as it is large enough to be representative), whereas the slit die would require a complete scan of the barrel, rather than choosing four representative areas. Figure 6 shows a comparison of the fibre orientation in the extrudate for the two die geometries. The orientation average (cos* 0,) for these two distributions was measured to be 0.808 for the slit die and 0.880 for the circular die ((cos* 02 = 1 for perfect alignment). Although in general terms the two distributions are similar, it is clear that the conical die is more effective in aligning the fibres along the flow direction, although this will of course depend on the chosen dimensions of the two die geometries. In particular it is seen that for the strip die there are more fibres remaining at angles greater than 40” when compared to the conical die. The reason for this can be seen by considering eqns (3a), (3b), (4a) and (4b) again. Because the deformation for the strip die is at constant width (X, = l), fibres that lie at high values of 4’, i.e. parallel to the Y axis and the die slit, are not as effectively rotated towards the Z axis as fibres that lie perpendicular to the slit (4’ = 0”). Figure 7 shows a comparison of the effectiveness of the two die shapes in rotating fibres towards the extrusion (Z) axis, for a fibre with
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Development
of fibre orientation:
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et al.
2500
-B-
4
0.0
45.0
22.5
67.5
Conical
-
Strip
die
die
90.0
Theta Figure
6
A comparison
100
Iii 0
of the 19distributions
in the extrudates
1
I
~,-.-.-.-.-*-.-.-.-,I
80.
z g
from the conical die and the slit die
-A-
Original
-.-
Strip
-U-
Conical
B
60. die
m
0.0
Original
Figure 7
45.0
22.5
Phi
The effectiveness
(before
67.5
die
90.0
die) Z%
of the two die geometries
in rotating a fibre
towards the extrusion direction
an original value of 8’ = 85” for various values of 9’. For the conical die, the effect of the convergence zone on the final value of 0 is independent of 4’ as shown in eqns (3a) and (3b). For the conical die shape used in this work, a fibre which has a value of 6’ of 85” before the die, will be predicted to have a value of 0 of 20” after the die. In comparison, for the strip die the final value of B depends on both 8’ and 6’. For an original value of 8 of 85” (the most probable value as shown from Figure 3a and b) and 4’ greater than 20” (or less than -20” because the relationship is symmetrical around 4’ = 0, the strip normal), the strip die is less effective than the conical die in aligning the fibre along the flow direction. If 4’ is less than 20” the strip die is more effective than the conical die.
The effect of using the breaker plate-G3NO1 For the initial experiments the breaker plate was placed in its normal position at the end of the extruder screw, which
954
Figure 8
G3NOl with a breaker plate 50 mm from the convergent zone entrance: image analysis schematic of a section of the barrel directly after the breaker plate
left a distance of 50 mm between the breaker plate and the entry to the convergent region (Figure 1). Figure 8 shows an image analysis schematic for an XY section (8.1 X 7.4 mm) taken directly after the breaker plate (position A, Figure I). It is clear from this picture, when compared to Figure 4a, that the breaker plate has a profound effect on the fibre orientation. In general the fibres appear more aligned along the extrusion direction after going through the breaker plate, although there are roughly circular regions where the fibres are still aligned perpendicular to the flow. These regions are related to fibres flowing through the holes in the breaker plate. The flow through these holes (3 mm diameter) will have an elongational component due to the area reduction, and a shear component due to the velocity profile of the molten material in the hole. After the breaker plate there will be diverging flow which, as will be shown shortly, will tend to rotate the fibres away from the extrusion direction due to stretching in the XY plane.
Development
0.0
of fibre orientation:
22.5
45.0
67.5
P. J. Hine et al.
90.0
Theta
Figure 10 A comparison plate and at the convergent
0.0
0.5
1.0
Scan
1.5
2.0
width
2.5
3.0
of the 0 distributions zone entrance
directly after the breaker
3.5
(mm) 00
22.5
45.0
57.5
900
Figure 9
Reduced scan area from Figure 8 to correspond to a ‘hole’ region: (a) image analysis schematic; and (b) strip orientation analysis across this scanned area
Theta
Figure 11 A comparison
of the 0 distributions
before the die with and
without the breaker plate
In order to study the heterogeneous fibre orientation seen after the breaker plate in more detail, a smaller area was chosen and analysed. It is worth emphasising that this section is taken directly after the breaker plate and so does not include the effect of the diverging flow after the breaker plate on fibre orientation. The size and orientation of this smaller area (3.3 X 1.5 mm), is shown by the dotted box on Figure 8. The scan length (in the X axis) was chosen to centre on a hole position, while the scan width (in the Y axis) was chosen to correspond approximately to a breaker plate hole size. The actual area analysed, shown in Figure 9a, was not at the position shown on Figure 8, but for ease of interpretation was chosen at a point in the barrel where the fibres in the centre of the scan were preferentially aligned along the Y axis. After scanning, the area shown in Figure 9a was analysed using a strip analysis, the results of which are shown in Figure 9b. For this analysis the sample area was divided into strips, in this case parallel to the X axis as shown by the dotted box on the image analysis scan [Figure Sal, and then the orientation averages (cos2 ox), (cos’ 0,) and (cos’ 0,) determined for each strip (note that (cos* 19,) f (cos’ 0,) + (cos2 6a = 1). The values of these averages indicate how the fibres are aligned along the three principal axes of the sample. For perfect alignment along one axis, the value of the corresponding orientation average would equal one with the other two equal to zero. Conversely for random fibre orientation in three dimensions all three averages would equal one-third.
At this position in the barrel, in the absence of the breaker plate, the fibres would be expected to lie parallel to the Y axis due to the helical flow, with an average value of (cos2 0,) = 0.07. It is seen from Figure 9 that there is a region, roughly 1 mm in diameter, where the fibres are preferentially aligned parallel to the Y axis and have travelled through the hole relatively unaffected. At the centre of the breaker plate hole the major contribution to fibre rotation is through elongational flow as a consequence of the area reduction of the breaker plate (and a low shear contribution at this position). The average orientation of fibres in this small region with respect to the extrusion direction is (cos2 03 = 0.12 which when compared to the value without the breaker plate, suggests that elongational flow through the holes is causing some fibre rotation. Away from this central region of the hole the fibres are more predominately aligned along the flow direction (Z axis), most likely as result of shear between the moving material and the hole edges. Further work on understanding the effect of the breaker plate holes on fibre orientation, is currently in progress. Figure 10 shows a comparison of the 0 orientation distribution directly after the breaker plate (position A) and just before the convergence region (position B). It is seen that during the travel from position A to position B (50 mm)
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600
(a) 600
*
Breaker Plate at
+
No Breaker Plate
-B-
No Breaker Plate
-e-
Breaker Plate 50mm
-*-
Breaker Plate at
50m
45.0
Theta
0.0
22.6
45.0
67.5
die
90.0
Theta Figure 12 A comparison of the t9distributions in the extrudate for various positions of the breaker plate: (a) with no breaker plate and with the breaker plate 50 mm from the convergent zone entrance; and (b) as above but with the addition of the breaker plate at the entrance to the convergent zone
the fibre alignment caused by the breaker plate has relaxed back. As alluded to above, this is most likely due to diverging flow (elongational flow in the XY plane) after the breaker plate, causing the fibres to rotate away from the extrusion axis. As will be seen shortly, the final fibre alignment seen in the extrudate is determined by the fibre alignment at the entrance to the convergence region, and the shape of the convergent zone. Therefore if maximum fibre alignment is required, it would seem beneficial to place the breaker plate as close to the constriction region as possible. A final indication of the profound effect the breaker plate has on the fibre orientation before the die is shown in Figure 11, which shows a comparison between an XY section taken just before the convergent region (position B, Figure I) with and without the breaker plate at the end of the screw. Figure 12 shows the effect of the breaker plate, and its position relative to the entrance to the convergent zone, on the final fibre orientation in the extrudate. Figure Z2a shows results for samples made using the conical die, without the breaker plate, and with the breaker plate 50 mm from the die. As expected, the preferred orientation resulting from
966
flow through the breaker plate, results in a higher degree of orientation in the extrudate. The measured values of (cos’13J were 0.880 without the breaker plate and 0.944 with the breaker plate. Figure 12b shows a comparison of samples made using the slit die, without the breaker plate, with the breaker plate at 50 mm from the entrance to the convergent region and with the breaker plate at the convergent zone entrance. The trends in the final extrudate orientation were as expected with the highest value of preferred orientation seen for the samples made with the breaker plate located at the entrance to the convergent zone. Values of (cos* L~J were 0.808 without the breaker plate, 0.896 with the breaker plate 50 mm from the die and 0.940 with the breaker plate at the entrance to the convergent zone. A summary of the measured extrudate orientation averages, for the two dies and the various configurations, is shown in Tabk 2. It is noticeable that for the slit die, the emerging fibres are more preferentially aligned in the YZ plane (the plane of the slit) than the conical die which is more balanced in the XY plane as would be expected.
Development
Table 2 ____~~
A summary of the measured
fibre orientation
of fibre orientation:
P. J. Hine
averages
Die shape
(cos? e7)
(co? l9,)
(cos? f7,)
No breaker plate Before the convergent zone (position A) After the convergent zone After the convergent zone
slit slit conical
0.069 0.808 0.X80
0.404 0.16 I 0.086
0.031
Breaker Breaker Breaker Breaker
slit slit conical
0.896 0.940 0.944
0.083 0.048 0.025
0.02 I 0.0 I2 0.03 I
plate: plate plate plate
Mechanical
extrudate orientation 50 mm from the entrance at the entrance 50 mm from the entrance
properties-G2N02
et al.
and G3NOl
Young’s modulus. The aim of developing high degrees of preferred fibre alignment in the extrudates is to maximise the Young’s modulus in the extrusion direction. The Young’s modulus of strips extruded from the slot die was determined using a three-point bend test geometry. For the low volume fraction material (G2N02-Vt = 8.2%) voiding was minimal in the extruded strips. However for the higher volume fraction material (G3NOl-Vr = 13.2%) significant voiding was found in the extrudates. As it was likely that this voiding would affect both the Young’s modulus and the crack resistance, the extruded G3NOl strips were compressed in a hot press below the melting point of the PP matrix (at 160°C) in order to remove the voids. A measurement of fibre orientation both before and after this hot pressing stage, confirmed that the fibre orientation was not affected by this process. Current research is aimed at removing this voidage during processing. For the highest aligned strips the values measured were 6.3 GPa for the G2N02 grade and 8.2 GPa for the G3NOl grade. Theoretical calculation has shown that for both materials this equates to approximately 85% of the theoretical maximum for fibres of aspect ratio 24 and 70% of the theoretical maximum for long fibres. Higher values of modulus could be achieved by reducing fibre breakage. increasing the fibre volume fraction further, or by increasing the fibre orientation, although at the high values of preferred orientation achieved further increase may not be worth pursuing. One aspect that could be worth further investigation would be to increase the degree of orientation before the convergence zone by further understanding of the role of the breaker plate. Increasing the degree of alignment before the convergence zone, would allow the dimensions of the die exit to be increased, thereby allowing larger section material to be produced. Fracture results. Figure 13 shows typical fracture results for extruded strips of the two glass filled PP grades, tested using the reinforced DCB geometry with the crack growth along the extrusion direction. The figure shows that even for these highly aligned strips (for both these samples (co? 6,) > 0.90 i.e. made without the breaker plate) the materials are very tough, with a value of fracture toughness rising from around 10 kJ/m* for crack initiation, to about 20 kJ/m2 after 30 mm of crack propagation. The grade with the higher percentage of polypropylene (G2N02) is seen to show the
0.527 0.034
_~_
30
z-
20
*
E : Y
G3NOl (Vf=
.
13%l
G2N02 Wf=8rn)
6
10
0 10
0
Crack Figure
13
30
20
length
40
50
(mm)
Fracture results for the two materials
highest toughness at all crack lengths, indicating that the major contribution to the toughness is from the polypropylene matrix at the comparatively low fibre loadings used in this study. It is clear from the results for both materials that the extruded products have excellent fracture properties, despite the high degree of fibre alignment.
CONCLUSIONS The development of fibre orientation through the convergent dies has been shown to be predicted well by the ‘pseudo-affine’ deformation theory, for both a conical die (uniaxial deformation) and a slit die (constant width deformation). It has been shown that melt extrusion through convergent dies can produce high degrees of fibre alignment. If a breaker plate is positioned between the extruder screw and the convergent die this increases further the degree of fibre orientation in the extrudate by introducing additional pre-orientation of the fibres due predominantly to shearing flow, with the highest degree of alignment obtained when the breaker plate is positioned at the entrance to the convergence zone. Young’s modulus values in the extrusion direction are found to be of the order of 85% of the theoretical maximum for the aspect ratio of fibres used (i.e. 85% of the expected value for perfect alignment). In addition the crack resistance of the highly aligned extrudates, for cracks propagating parallel to the extrusion direction. is excellent.
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ACKNOWLEDGEMENTS We would like to thank Dr M. Fleissner from Hoechst AG, Frankfurt, for arranging the supply of the glass fibre filled PP feedstock.
11.
12.
13.
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2. 3.
4.
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8.
9. 10.
Brandt Goldsmith, W., In Composites Technology, S. W. Lee (ed.), Chapter 9, Technomic Publishing Company, Pennsylvania, PA, 1989. Folkes, M. J., In Short Fibre Reinforced Thermoplastics, Research Studies Press, Letchworth, UK, 1982. Tucker, C. L. and Advani, S. G., Processing of short-fiber systems, In Flow and Rheology in Polymer Composites, Elsevier, Amsterdam, 1994. Hine, P. J., Brew, B., Duckett, R. A. and Ward, I. M., The fracture behaviour of carbon fibre reinforced poly(ether etherketone). Composite Science and Technology, 1988, 33, 35. Truss, R., Hine, P. J. and Duckett, R. A., Interlaminar and intralaminar fracture toughness of uniaxial continuous and discontinuous carbon libre/epoxy composites, Composites, Part A, 1997, 28, 621. Dospisil, D., Kubat, J., Plesek, M. and Saha, P., Extrusion of short fibre reinforced PP. International Polymer Processes, 1994, IX. Mutel, A. T. and Kamal, M. R., Rheological behaviour of fiber orientation in slit flow of fiber reinforced thermoplastics. Polymer Composites, 1991, 12, 137. Doshi, S. R., Dealy, J. M. and Charrier, J. M., Flow induced fiberorientation in an expanding channel tubing die, Polymer Engineering and Science, 1986, 26, 468. Goettler, L. A., Short fiber reinforced hose-A new concept in production and performance. Polymer Composites, 1984, 1, 60. Ausias, G., Jarrin, J. and Vincent, M., Optimisation of the
15. 16. 17.
18. 19.
20.
21.
22. 23.
tube-extrusion die for short-fiber-filled polymers. Composite Science and Technology, 1996,56,719. Allan, P. S. and Bevis, M. J., Shear controlled orientation in extrusion. Plastics, Rubber and Composites, Processing and Applications, 1991, 16, 133. Guell, D. C. and Graham, A. L., Improved mechanical properties in hydrodynamically aligned, short fiber composite materials. Journal of Composite Materials, 1996, 30, 2. Jeffery, G. B., The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London, 1922, A102, 161. Cook, A., Hine, P. J., Duckett, R. A., Allan, P. S., Kalay, G., Bevis, M. J. and Ward, I. M., A comparison of fibre development in conventional injection moulding and using the SCORIM process, In ICAC95, Nottingham, 1995. Kratky, O., Zum deformationsmechanisms der fasenstoffe I. Kolloidzschr., 1933, 64, 213. Ward, I. M., Optical and mechanical anisotropy in crystalline polymers. Proceedings of the Physical Society, 1962, 80, 1176. Hine, P. J., Davidson, N., Duckett, R. A. and Ward, I. M., Hydrostatically extruded glass-fiber-reinforced polyoxymethylene. I: The development of fiber and matrix orientation, Polymer Composites, 1996, 17, 720. Modlen, G. F., Re-orientation of fibres during mechanical working. Journal of Materials Science, 1969, 4, 283. Clarke, A. R., Davidson, N. and Archenhold, G., A large area high resolution image analyser for polymer research, Proc of Transputing 91, IOS Press. Hine, P. J., Davidson, N., Duckett, R. A. and Ward, I. M., Modelling the elastic properties of fibre reinforced composites. I: Orientation measurement. Composite Science and Technology, 1993,47, 65. Gurney, C., Mai, Y. W. and Owen, R. C., Quasistatic cracking of materials with high fracture toughness and low yield stress. Proc. Roy. Sot. (London), 1974, A340,213. Avitzur, B., Flow characteristics through conical converging dies, Tram of the ASME, 1966, 4 10. Dinh, S. M. and Armstrong, R. C., A rheological equation of state for semiconcentrated fiber suspensions. Journal of Rheology, 1984, 28, 207.
in Circa
PII: S1359-835X(97)00070-5
ELSEVIER
Britain. Ail rights reserved 1359-835)
Measuring the development of fibre orientation during the melt extrusion of short glass fibre reinforced polypropylene
P. J. Hines, S.-W. Tsuib, P. D. Coatesb, I. M. Warda and R. A. Ducketta alRC in Polymer Science and Technology, University blt?C in Polymer Science and Technology, University (Received 30 January 7997; accepted 12 June 1997)
of Leeds, Leeds, UK of Bradford, Bradford,
UK
In this paper we describe an investigation into the development of hbre orientation during melt extrusion through two convergent dies: a conical die and a slit die. The material used was a short glass fibre (aspect ratio = 24) reinforced polypropylene at two fibre weight fractions, 20% and 30%. The development of fibre orientation through the two convergent zones was measured in detail using sophisticated image analysis facilities developed in-house. It was found that the ‘pseudo-affine’ deformation model predicted the development of hbre orientation very well for both die configurations, in terms of the applied macroscopic elongational strains. The addition of a breaker plate placed in the barrel of the extruder, between the extruder screw and the convergent Row zones, was shown to produce the highest degree of fibre alignment in the extrudates, by introducing additional pre-orientation of the fibres. Mechanical measurements on the extrudates showed that for the very high degrees of fibre alignment attained, the bending modulus in the extrusion direction reached 90% of the theoretical maximum for the fibre aspect ratio used, and the crack resistance parallel to the preferred fibre direction remained high. 0 1997 Elsevier Science Limited. (Keywords: composites; short fibres; extrusion; mechanical properties)
INTRODUCTION The use of glass fibre reinforced thermoplastics has increased rapidly over the last few years, offering a combination of low cost, fast cycle times, and good mechanical properties. A number of different processing routes are available for these materials, and the choice of process governs both the type of reinforcement used (continuous, long or short hbres) and the fibre orientation in the final composite and hence the balance of composite mechanical properties. Pultruded continuous glass fibre reinforced thermoplastics (e.g. ref. ‘) have the advantage of high axial stiffness and strength, but the disadvantage of low crack resistance along the fibre direction, a consequence of the high fibre fractions which are used in this process. Injection moulded short fibre composites (e.g. refs 233)offer advantages such as ease of manufacture, low cycle times and lower cost, but generally lead to random or at best partially aligned fibres. This results in improved crack resistance compared to pultruded materials, but lower stiffness and strength. From our previous studies of short fibre composites, it is clear that only a small amount of fibre misalignment is needed for a substantial improvement in the fracture toughness parallel to the preferred fibre direction’,‘. There is therefore a possible window of opportunity for materials that are highly, but not perfectly aligned, which
combine high axial stiffness and strength with good crack resistance. There are a significant number of published papers which describe methods for producing preferred fibre orientation in short libre reinforced composites. The techniques described include converging flow extrusion6mx, diverging flow extrusion”.” and injection moulding’3”, while the paper by Guell and Graham” lists centrifugal force, vibration, vacuum drums and electric fields as being used. Interestingly it appears that only the work of Goettler” on tube extrusion, and the work of Bevis in shear controlled injection moulding”, have so far resulted in industrial exploitation. While it is widely recognised that extensional or convergent flow can orient fibres in the flow direction (e.g. refs ‘-‘), and that divergent flow can orient hbres perpendicular to the floods”‘, little work has been done to accurately measure the development of this fibre orientation. If it can be shown that the rotation of fibres in simple deformation fields (either elongational, shear or a combination of both) can be simply modelled in terms of geometrical apsects of the deformation, then more complicated flow regimes, produced for example during injection moulding, can be predicted with confidence. The majority of published work on the prediction of fibre rotation during flow, particularly with regard to simulations of injection moulding, is based on the work of Jeffrey” who
949
Development
of fibre orientation:
P. J. Hine et al.
derived a relationship to predict the motion of ellipsoidal particles immersed in a moving viscous fluid. We have approached the modelling of fibre rotation during convergent flow melt extrusion from a different standpoint, based on the background in this laboratory for predicting the orientation of polymer crystals during solid state deformation. Recently14 we reported a study of the injection moulding of circular dumbbells of short glass fibre reinforced polypropylene. It was shown that as the fibre filled melt flowed from the larger diameter end section to the smaller diameter centre section, the convergent flow region induced preferred fibre orientation in the centre section. Furthermore it was shown that the development of this fibre orientation could be predicted very well by the ‘pseudo affine’ deformation schemeI used extensively to predict the effects of an imposed deformation on the development of orientation in crystalline polymers’6, and more recently to predict accurately the rotation of fibres during the solid state extrusion of short glass fibre reinforced polyoxymethylene17. The rotation of the fibre is calculated from the imposed affine deformation of the composite, and is therefore dependent only on the applied external strains. This is a similar approach to that of Modlen’* who described the re-orientation of fibres during mechanical working. In this paper we use the pseudo-affine deformation scheme to model the rotation of fibres during convergent flow through: (i) a conical die, where the deformation is uniaxial; and (ii) a slit die, which produces a reduction in thickness at constant width. The equations derived can be shown to be special cases of Jeffrey’s equation. The aims of the research now described were therefore threefold: first to measure the rotation of fibres during convergent flow melt extrusion and compare it with theoretical predictions based on the simple pseudo affine scheme; second to measure the maximum level of preferred fibre orientation that we could produce during convergent melt extrusion; and third to investigate the effects of the attained levels of fibre orientation on the axial Young’s modulus and the crack resistance of the extrudates for cracks propagating in the weakest direction, parallel to the main fibre direction.
EXPERIMENTAL
General details Two commercial grades of glass fibre filled polypropylene were used in these studies; G2N02 and G3NOl manufactured by Hoechst AG. These had fibre weight fractions of 20 and 30% (fibre volume fractions of 8 and 13%) respectively. The average fibre length was measured to be 425 pm in the starting pellets, falling to 330 pm after extrusion: with an average fibre diameter of 13.4 pm, this gave a final aspect ratio for the fibres of 24. The polypropylene matrix was isotactic and the composites had melt flow indices of 5 and 4.5 (as given by the literature) for G2N02 and G3NOl manufacturers’ respectively.
950
Figure
1
A schematic
of the convergent
flow region
Breaker
Plate diameter
Number
of holes
45mm 38mm 3.0mm
Figure
2
Ratio of bole size to tibre length
66 10
Ratio of total hole area to barrel area
0.41
Details of the breaker plate
The extrusion experiments were conducted using a Betol BK38 single screw extruder fitted with a single 38 mm diameter general purpose screw which had a 2:l compression ratio. There were six zones of temperature control, four along the extruder barrel and two around the convergent dies. The barrel zones were set to 180, 190, 200 and 200°C (denoted from the inlet hopper to the end of the screw), while the two die zones were set to 200°C. A pressure transducer was located within the die region to monitor melt pressure. All the tests used a screw speed of 20 rpm. After the material left the die, it was passed though a water bath and then through driven haul-off rollers, producing strip at approximately 1 m/min. Figure 1 shows a schematic diagram of the convergent flow region. A breaker plate is normally present in the extruder at the end of the screw to help translate helical motion to axial motion. However the breaker plate causes complicated fibre orientation structures to be developed downstream, making modelling the effect of the convergent dies more difficult: therefore when modelling the effect of the convergent zone on fibre orientation, the breaker plate was removed. Two die geometries were used in this study: a conical die with a 9 mm diameter exit; and a slit die with an exit cross section of 27 mm by 3 mm. Both dies had a semiangle of 15” and a 25 mm land after the end of the convergence region. Experiments were also carried out using both dies, with the breaker plate in place, to assess its effect on the final extrudate orientation. Figure 2 shows a diagram of the breaker plate, which contained 66 holes, each 3.0 mm in diameter giving a total hole/extruder barrel area ratio of 41%. The ratio of the hole diameter to fibre length was approximately 10: the holes were 9.5 mm long.
Fibre orientation
measurement
The fibre orientation zones was measured
before and after the convergent flow from sections taken from frozen
Development
of fibre orientation:
P. J. Hine et al.
Direction
Figure 3
Image analysis details: (a) the definition of the orientation angles 0 and $: and (b) the definition of the sample axec
material in the die zone, produced after the extrudates had been manufactured. Fibre orientation was measured using a transputer controlled image analysis system developed at Leeds University’9.20. Images are produced directly from a polished composite section, where each fibre appears as an ellipse. Measuring the ellipticity and orientation of each fibre image enables the two angles 19and 4, which specify the orientation of the fibres, to be determined. 0 is defined as the angle the fibre makes with the extrusion direction (Z axis), while 4 is the angle between the X axis and the projection of the fibre axis on the XY plane, as shown in Figure 3a. The definition of the sample axes are shown in Figure 3b. Further details of the image analysis system and the measurement of fibre orientation can be found in ref. 19. Fracture
properties
Crack propagation parallel to the main fibre direction (i.e. the extrusion direction) was investigated for both materials. Initial fracture tests, using a standard double cantilever beam geometry, proved unsuccessful in producing crack propagation. The combination of a relatively low axial stiffness, and a high toughness resulted in deformation in the arms of the specimen before crack propagation occurred. The solution to this problem was to use a reinforced double cantilever beam geometry”. The arms of the specimen were reinforced with steel strips, effectively increasing the stiffness and strength of the sample arms allowing a crack to be propagated through the sample. Using this sample geometry, cracks were successfully propagated through the extruded strips. During crack propagation the load and displacement were monitored as the crack extended across a series of lines 2 mm apart on the sample. The sample compliance (C) was determined for each crack length (a), and by fitting a polynomial to these data, a value for the rate of change of compliance with crack length (dC/da) was determined. The strain energy release rate, G,, was then calculated from the Irwin-Kies equation
THEORY In this paper we compare the measured fibre orientation distributions in the extrudate (i.e. after the convergent die region) with those predicted from the measured fibre orientation distributions before the die region (position B in Figure I) using the pseudo-affine deformation scheme”,“. In general, material travelling through a convergent flow zone will experience both elongational and shear deformations. Modlen”, in his paper on the effect of mechanical work on fibre orientation, included both elongational and shear deformations but concluded that for uniaxial deformation in particular, internal shearing becomes of decreasing importance as the extrusion ratio increases above 4. As the extrusion ratios used in this work are greater than 9, we have opted to ignore the shear contribution. In addition, determination of the deformation fields for the die angle (15”) and extrusion ratios used in this study by implementing the relationships of Avitzur”, indicate a low level of shear in the extrudate. Using the pseudo-affine deformation scheme, the orientation 19 and 4 of a fibre after a general elongational deformation defined by the deformation ratios Xx, Xy and hz, along the X, Y and Z axes respectively, is related to the original orientation angles 0’ and 6’ by the following two equations.
xx tan0=-tan0’
(l+($tan’6’)
’
(2a)
(1 + tan’+‘)
AZ I
tan+=
I
XtanO’
(2b)
X
For the conical die the deformation
is uniaxial
such that
..2% 2B da ‘
where P is the fracture load at a particular B is the sample thickness.
crack length, and
assuming constant hx.Xy.Xz = 1).
volume
during
deformation
(i.e.
951
Development
Table 1
of fibre orientation:
The nominal deformation XX
Conical die Slit die
P. J. Hine et al.
ratios imposed by the two dies hY
0.323 0.11
AZ
0.323 1.0
eqn (2a) and eqn (2b) then simplify
9.61 9.0
to
3 tan 8 = X 2tan 8’
(3a)
where h = Xz This equation is identical to that determined by a number of authors (e.g. Dinh and Armstrong23) by solving Jeffery’s equation for purely elongational flow in a conical die. For the slit die the deformation occurs at approximately constant width so that Xy = 1 and Xx = l/(hz). Eqn (2a) and eqn (2b) then reduce to
(b)
500
1
45.0
tan$=htan4’
(4b)
The deformation ratios for the two dies, determined entrance and exit sizes, are shown in Table 1.
Theta
from the (c)
200
,
160
-
1
RESULTS
Modelling the effect of the convergence breaker plate
zone-G3NO1
-no
In this section results are presented for measurements of fibre orientation through both the conical die and the slit die with the breaker plate removed from the barrel of the extruder. The results presented are for G3NO1, the high fibre volume fraction material only: very similar results were obtained for the G2N02 grade. Figure 4a shows an image analysis schematic and Figures 4b and 4c the 19and 4 distributions, from solidified material taken from the conical die zone. The section is taken in the XY plane (perpendicular to the extrusion direction) and the analysis position is just before the convergence region of the die (position B shown in Figure I). Similar results were found at different positions between B and the tip of the screw (position A). The image analysis schematic was recreated from the data stored during the measurement. Over this measurement field the fibres lie predominantly in the XY plane with a well defined orientation in that plane: i.e. the fibres are mostly perpendicular to the extrusion direction (maximum 8 = 82”) with I$ in the range (-45 > 4 > +lO). The region shown in Figure 4a (approximately 5 mm square) is just a small section of the overall barrel diameter. In other regions the measured 0 distribution was similar, but the fibres had a different preferred 9 orientation as a result of an overall
-90.0
-45.0
0.0
45.0
90.0
Figure 4 G3NOl with no breaker plate: (a) image analysis schematic of a section of the barrel before the entrance to the convergence region; and (b), (c) the 0 and 6 distributions determined at this position
helicity of the flow imparted by the screw of the extruder. A similar result was found from analysing solidified material taken from the slit die. It is significant that the fibre orientations at this point are not isotropic. Figure 5 shows a comparison of the 0 distributions measured before the convergence region and after the convergence region (i.e. in the extrudate), for (a) the conical die and (b) the slit die. The effect of both convergence zones is clear, with the material after the convergent zones showing a significant degree of preferred fibre alignment in the extrusion direction. Also shown in Figure 5 are the theoretical predictions of the 6 distributions after the two convergent regions, calculated based on the f3 and #J distributions before the die and the pseudo-affine deformation scheme as given by eqns (3a), (3b), (4a) and (4b). The
Development
2500
I .
-
of fibre orientation:
P. J. Hine et al.
(a) ”
‘7 Before
die
Theory (P.Affine) After
22.5
45.0
67.5
die
90.0
Theta
-)-
Before
0
dvs
Theory (P Affine)
-+--
0.0
22.5
45.0
67.5
After
die
90.0
Theta Figure 5 A comparison of the 0 distributions measured before and after the convergence breaker plate: (a) for the conical die: and (b) for the slit die
fibre orientation distribution in the barrel before the melt reached the convergent dies was built up from a number of image scans (taken in the XY plane) approximately 5 mm square. Although for the best comparison between theory and experiment it would be necessary to scan the complete barrel diameter, the size of this area meant that this was not practical. Instead, four image scans were taken at different positions across the barrel diameter, and therefore different local values of c$, to attempt to produce as representative a sample as possible: area scans were taken where the local average values of 4 were approximately 0,45, -45 and 90”. The agreement between the measured extrudate fibre orientations, and the predictions of the pseudo-affine deformation scheme is excellent for the conical die, and good for the slit die. The reason for the poorer fit between experiment and theory for the slit die is believed to be due to the uncertainties in the way the fibre orientation distribution prior to the die is built up, as described above. Eqns (3a), (3b), (4a) and (4b) show that for the conical die, the final 8 is independent of the original 9, whereas for the slit die the orientation of the fibre with respect to the die (i.e. the original 4) is obviously important. Therefore for the conical die the predicted theoretical 8 distribution is independent of
region. with the theoretical
prediction
for G3NOI without the
the size of area scan taken (as long as it is large enough to be representative), whereas the slit die would require a complete scan of the barrel, rather than choosing four representative areas. Figure 6 shows a comparison of the fibre orientation in the extrudate for the two die geometries. The orientation average (cos* 0,) for these two distributions was measured to be 0.808 for the slit die and 0.880 for the circular die ((cos* 02 = 1 for perfect alignment). Although in general terms the two distributions are similar, it is clear that the conical die is more effective in aligning the fibres along the flow direction, although this will of course depend on the chosen dimensions of the two die geometries. In particular it is seen that for the strip die there are more fibres remaining at angles greater than 40” when compared to the conical die. The reason for this can be seen by considering eqns (3a), (3b), (4a) and (4b) again. Because the deformation for the strip die is at constant width (X, = l), fibres that lie at high values of 4’, i.e. parallel to the Y axis and the die slit, are not as effectively rotated towards the Z axis as fibres that lie perpendicular to the slit (4’ = 0”). Figure 7 shows a comparison of the effectiveness of the two die shapes in rotating fibres towards the extrusion (Z) axis, for a fibre with
953
Development
of fibre orientation:
P. J. Hine
et al.
2500
-B-
4
0.0
45.0
22.5
67.5
Conical
-
Strip
die
die
90.0
Theta Figure
6
A comparison
100
Iii 0
of the 19distributions
in the extrudates
1
I
~,-.-.-.-.-*-.-.-.-,I
80.
z g
from the conical die and the slit die
-A-
Original
-.-
Strip
-U-
Conical
B
60. die
m
0.0
Original
Figure 7
45.0
22.5
Phi
The effectiveness
(before
67.5
die
90.0
die) Z%
of the two die geometries
in rotating a fibre
towards the extrusion direction
an original value of 8’ = 85” for various values of 9’. For the conical die, the effect of the convergence zone on the final value of 0 is independent of 4’ as shown in eqns (3a) and (3b). For the conical die shape used in this work, a fibre which has a value of 6’ of 85” before the die, will be predicted to have a value of 0 of 20” after the die. In comparison, for the strip die the final value of B depends on both 8’ and 6’. For an original value of 8 of 85” (the most probable value as shown from Figure 3a and b) and 4’ greater than 20” (or less than -20” because the relationship is symmetrical around 4’ = 0, the strip normal), the strip die is less effective than the conical die in aligning the fibre along the flow direction. If 4’ is less than 20” the strip die is more effective than the conical die.
The effect of using the breaker plate-G3NO1 For the initial experiments the breaker plate was placed in its normal position at the end of the extruder screw, which
954
Figure 8
G3NOl with a breaker plate 50 mm from the convergent zone entrance: image analysis schematic of a section of the barrel directly after the breaker plate
left a distance of 50 mm between the breaker plate and the entry to the convergent region (Figure 1). Figure 8 shows an image analysis schematic for an XY section (8.1 X 7.4 mm) taken directly after the breaker plate (position A, Figure I). It is clear from this picture, when compared to Figure 4a, that the breaker plate has a profound effect on the fibre orientation. In general the fibres appear more aligned along the extrusion direction after going through the breaker plate, although there are roughly circular regions where the fibres are still aligned perpendicular to the flow. These regions are related to fibres flowing through the holes in the breaker plate. The flow through these holes (3 mm diameter) will have an elongational component due to the area reduction, and a shear component due to the velocity profile of the molten material in the hole. After the breaker plate there will be diverging flow which, as will be shown shortly, will tend to rotate the fibres away from the extrusion direction due to stretching in the XY plane.
Development
0.0
of fibre orientation:
22.5
45.0
67.5
P. J. Hine et al.
90.0
Theta
Figure 10 A comparison plate and at the convergent
0.0
0.5
1.0
Scan
1.5
2.0
width
2.5
3.0
of the 0 distributions zone entrance
directly after the breaker
3.5
(mm) 00
22.5
45.0
57.5
900
Figure 9
Reduced scan area from Figure 8 to correspond to a ‘hole’ region: (a) image analysis schematic; and (b) strip orientation analysis across this scanned area
Theta
Figure 11 A comparison
of the 0 distributions
before the die with and
without the breaker plate
In order to study the heterogeneous fibre orientation seen after the breaker plate in more detail, a smaller area was chosen and analysed. It is worth emphasising that this section is taken directly after the breaker plate and so does not include the effect of the diverging flow after the breaker plate on fibre orientation. The size and orientation of this smaller area (3.3 X 1.5 mm), is shown by the dotted box on Figure 8. The scan length (in the X axis) was chosen to centre on a hole position, while the scan width (in the Y axis) was chosen to correspond approximately to a breaker plate hole size. The actual area analysed, shown in Figure 9a, was not at the position shown on Figure 8, but for ease of interpretation was chosen at a point in the barrel where the fibres in the centre of the scan were preferentially aligned along the Y axis. After scanning, the area shown in Figure 9a was analysed using a strip analysis, the results of which are shown in Figure 9b. For this analysis the sample area was divided into strips, in this case parallel to the X axis as shown by the dotted box on the image analysis scan [Figure Sal, and then the orientation averages (cos2 ox), (cos’ 0,) and (cos’ 0,) determined for each strip (note that (cos* 19,) f (cos’ 0,) + (cos2 6a = 1). The values of these averages indicate how the fibres are aligned along the three principal axes of the sample. For perfect alignment along one axis, the value of the corresponding orientation average would equal one with the other two equal to zero. Conversely for random fibre orientation in three dimensions all three averages would equal one-third.
At this position in the barrel, in the absence of the breaker plate, the fibres would be expected to lie parallel to the Y axis due to the helical flow, with an average value of (cos2 0,) = 0.07. It is seen from Figure 9 that there is a region, roughly 1 mm in diameter, where the fibres are preferentially aligned parallel to the Y axis and have travelled through the hole relatively unaffected. At the centre of the breaker plate hole the major contribution to fibre rotation is through elongational flow as a consequence of the area reduction of the breaker plate (and a low shear contribution at this position). The average orientation of fibres in this small region with respect to the extrusion direction is (cos2 03 = 0.12 which when compared to the value without the breaker plate, suggests that elongational flow through the holes is causing some fibre rotation. Away from this central region of the hole the fibres are more predominately aligned along the flow direction (Z axis), most likely as result of shear between the moving material and the hole edges. Further work on understanding the effect of the breaker plate holes on fibre orientation, is currently in progress. Figure 10 shows a comparison of the 0 orientation distribution directly after the breaker plate (position A) and just before the convergence region (position B). It is seen that during the travel from position A to position B (50 mm)
955
Development
of fibre orientation:
P. J. Hine et al.
600
(a) 600
*
Breaker Plate at
+
No Breaker Plate
-B-
No Breaker Plate
-e-
Breaker Plate 50mm
-*-
Breaker Plate at
50m
45.0
Theta
0.0
22.6
45.0
67.5
die
90.0
Theta Figure 12 A comparison of the t9distributions in the extrudate for various positions of the breaker plate: (a) with no breaker plate and with the breaker plate 50 mm from the convergent zone entrance; and (b) as above but with the addition of the breaker plate at the entrance to the convergent zone
the fibre alignment caused by the breaker plate has relaxed back. As alluded to above, this is most likely due to diverging flow (elongational flow in the XY plane) after the breaker plate, causing the fibres to rotate away from the extrusion axis. As will be seen shortly, the final fibre alignment seen in the extrudate is determined by the fibre alignment at the entrance to the convergence region, and the shape of the convergent zone. Therefore if maximum fibre alignment is required, it would seem beneficial to place the breaker plate as close to the constriction region as possible. A final indication of the profound effect the breaker plate has on the fibre orientation before the die is shown in Figure 11, which shows a comparison between an XY section taken just before the convergent region (position B, Figure I) with and without the breaker plate at the end of the screw. Figure 12 shows the effect of the breaker plate, and its position relative to the entrance to the convergent zone, on the final fibre orientation in the extrudate. Figure Z2a shows results for samples made using the conical die, without the breaker plate, and with the breaker plate 50 mm from the die. As expected, the preferred orientation resulting from
966
flow through the breaker plate, results in a higher degree of orientation in the extrudate. The measured values of (cos’13J were 0.880 without the breaker plate and 0.944 with the breaker plate. Figure 12b shows a comparison of samples made using the slit die, without the breaker plate, with the breaker plate at 50 mm from the entrance to the convergent region and with the breaker plate at the convergent zone entrance. The trends in the final extrudate orientation were as expected with the highest value of preferred orientation seen for the samples made with the breaker plate located at the entrance to the convergent zone. Values of (cos* L~J were 0.808 without the breaker plate, 0.896 with the breaker plate 50 mm from the die and 0.940 with the breaker plate at the entrance to the convergent zone. A summary of the measured extrudate orientation averages, for the two dies and the various configurations, is shown in Tabk 2. It is noticeable that for the slit die, the emerging fibres are more preferentially aligned in the YZ plane (the plane of the slit) than the conical die which is more balanced in the XY plane as would be expected.
Development
Table 2 ____~~
A summary of the measured
fibre orientation
of fibre orientation:
P. J. Hine
averages
Die shape
(cos? e7)
(co? l9,)
(cos? f7,)
No breaker plate Before the convergent zone (position A) After the convergent zone After the convergent zone
slit slit conical
0.069 0.808 0.X80
0.404 0.16 I 0.086
0.031
Breaker Breaker Breaker Breaker
slit slit conical
0.896 0.940 0.944
0.083 0.048 0.025
0.02 I 0.0 I2 0.03 I
plate: plate plate plate
Mechanical
extrudate orientation 50 mm from the entrance at the entrance 50 mm from the entrance
properties-G2N02
et al.
and G3NOl
Young’s modulus. The aim of developing high degrees of preferred fibre alignment in the extrudates is to maximise the Young’s modulus in the extrusion direction. The Young’s modulus of strips extruded from the slot die was determined using a three-point bend test geometry. For the low volume fraction material (G2N02-Vt = 8.2%) voiding was minimal in the extruded strips. However for the higher volume fraction material (G3NOl-Vr = 13.2%) significant voiding was found in the extrudates. As it was likely that this voiding would affect both the Young’s modulus and the crack resistance, the extruded G3NOl strips were compressed in a hot press below the melting point of the PP matrix (at 160°C) in order to remove the voids. A measurement of fibre orientation both before and after this hot pressing stage, confirmed that the fibre orientation was not affected by this process. Current research is aimed at removing this voidage during processing. For the highest aligned strips the values measured were 6.3 GPa for the G2N02 grade and 8.2 GPa for the G3NOl grade. Theoretical calculation has shown that for both materials this equates to approximately 85% of the theoretical maximum for fibres of aspect ratio 24 and 70% of the theoretical maximum for long fibres. Higher values of modulus could be achieved by reducing fibre breakage. increasing the fibre volume fraction further, or by increasing the fibre orientation, although at the high values of preferred orientation achieved further increase may not be worth pursuing. One aspect that could be worth further investigation would be to increase the degree of orientation before the convergence zone by further understanding of the role of the breaker plate. Increasing the degree of alignment before the convergence zone, would allow the dimensions of the die exit to be increased, thereby allowing larger section material to be produced. Fracture results. Figure 13 shows typical fracture results for extruded strips of the two glass filled PP grades, tested using the reinforced DCB geometry with the crack growth along the extrusion direction. The figure shows that even for these highly aligned strips (for both these samples (co? 6,) > 0.90 i.e. made without the breaker plate) the materials are very tough, with a value of fracture toughness rising from around 10 kJ/m* for crack initiation, to about 20 kJ/m2 after 30 mm of crack propagation. The grade with the higher percentage of polypropylene (G2N02) is seen to show the
0.527 0.034
_~_
30
z-
20
*
E : Y
G3NOl (Vf=
.
13%l
G2N02 Wf=8rn)
6
10
0 10
0
Crack Figure
13
30
20
length
40
50
(mm)
Fracture results for the two materials
highest toughness at all crack lengths, indicating that the major contribution to the toughness is from the polypropylene matrix at the comparatively low fibre loadings used in this study. It is clear from the results for both materials that the extruded products have excellent fracture properties, despite the high degree of fibre alignment.
CONCLUSIONS The development of fibre orientation through the convergent dies has been shown to be predicted well by the ‘pseudo-affine’ deformation theory, for both a conical die (uniaxial deformation) and a slit die (constant width deformation). It has been shown that melt extrusion through convergent dies can produce high degrees of fibre alignment. If a breaker plate is positioned between the extruder screw and the convergent die this increases further the degree of fibre orientation in the extrudate by introducing additional pre-orientation of the fibres due predominantly to shearing flow, with the highest degree of alignment obtained when the breaker plate is positioned at the entrance to the convergence zone. Young’s modulus values in the extrusion direction are found to be of the order of 85% of the theoretical maximum for the aspect ratio of fibres used (i.e. 85% of the expected value for perfect alignment). In addition the crack resistance of the highly aligned extrudates, for cracks propagating parallel to the extrusion direction. is excellent.
957
Development
of fibre orientation:
P. J. Hine et al.
ACKNOWLEDGEMENTS We would like to thank Dr M. Fleissner from Hoechst AG, Frankfurt, for arranging the supply of the glass fibre filled PP feedstock.
11.
12.
13.
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