Short Fiber Reinforced Thermoplastics. II. Interrelation Between Fiber Orientation and Rheological Properties of

Glass Fiber-Reinforced Noryl A. VAXMAN and M. NARKIS

Department of Chemical Engineering Technion-Zsrael Znstitute of Technology

Haifa 32000, Israel

and A. SIEGMANN* and S. KENIG

Department of Materials Engineering Technion-Zsrael Znstitute of Technology

Haifa 32000, Israel

Mechanical properties of short fiber reinforced thermoplastics depend on the fiber orientation distribution produced during the melt flow processing step. An experimental investigation of fiber orientation during shear flow through a cir- cular die is presented. A quantitative determination of the glass fiber orientation distribution was done by analyzing photomicrographs of polished extrudate sec- tions. Planar and axial fiber orientation distributions as affected by shear rate and total shear are discussed. The reliability of a classical metallographic tech- nique for fiber orientation determination was evaluated. Orientation effects are expressed by a single orientation parameter which is used to account for the average fiber orientation contribution to the relative viscosity. The influence of shear rate, total shear, fiber concentration, and temperature on fiber orientation has been investigated. I t is concluded that an orientation parameter should be used to account for fiber concentration effects on the relative viscosity. Calculated values, considering the orientation parameter, show good agreement with the experimental relative viscosity results.

INTRODUCTION distribution first oscillates and then reaches a

hort fiber-reinforced thermoplastic moldings are S characterized by typical microstructures which affect the composite macromechanical properties. Microstructural studies (1-3) have shown that the processing flow regime of the fiber containing melts creates a complex fiber distribution which has at- tracted a growing interest in the relationships be- tween the rheological behavior and the resulting in- ternal structure in such systems. Composite melts are basically described as suspensions of particles of various shapes in viscoelastic fluids. The flow behav- ior of suspended particles in Newtonian and non- Newtonian fluids and of filled polymer melts was recently reviewed by Kamal and Mute1 (4) and Metz- ner (5).

In a flowing dilute suspension, the short fibers undergo continuous rotation; rotating rapidly when aligned perpendicular to the flow direction and slowly when they become aligned in the flow direction. Stud- ies on sheared suspensions of rigid rod and disc shape particles (6) have shown that the orientation To whom correspondence should be addressed.

04

pseudo-equilibrium state which may then continue to slowly change with time. At higher fiber concen- trations, interparticle interactions slow fiber rota- tion, subsequently even preventing further rotational movement, and a stable orientation distribution may develop (9). Theoretical studies of fiber orientation (7-10) are all based on Jeffery’s analysis (1 1) of the motion of an ellipsoidal particle suspended in a New- tonian fluid. Okagawa, et al. (6) have suggested that the relative viscosity 7/qo of dilute suspensions is a function of the particle volume fraction, c , particle geometry and orientation as follows:

9/70 = 1 + kic (1) where 7 and v0 are the viscosities of the suspension and the suspending fluid, respectively, and k l , often referred to as an “intrinsic viscosity”, is given by:

2 3

kl = - C1 (sin4@ sin22$) + 2 (2)

where $ and % are spherical coordinates defined in Fig. 1 . The constant C1 takes into account the parti-

POLYMER COMPOSITES, APRIL 1989, Yo/. 10, No. 2

Short Fiber Reinforced Thermoplas t ics . 11.

U

b Fig. 1. Coordinate system used in determination of fiber spatial orientation (a) and a drawing of a fiber cut parallel to the X2X3 plane (b).

cles' geometry. Blakeney (1 2) suggested that experi- mental viscosity concentration relationships for non- dilute suspensions may be represented by an equa- tion of the form:

7/70 = 1 + klc + k2c2 (3)

where the k2c2 term has been added to include the effect of particles interaction. Frisch and Simha (1 3) expressed the interaction term as k(klc)' and found from theoretical considerations that for purely hy- drodynamic interaction between rigid rods, randomly oriented by Brownian motion, k = 0.73.

a

Several techniques are commonly employed to de- scribe qualitatively fiber orientation including con- tact micro-radiography of thin sections and reflected- light microscopy of polished surfaces (3). McGee and McCullough (1 4) proposed a rapid technique for quan- titative determination of fiber orientation, using a Fraunhoffer diffractometer to obtain diffraction pat- terns characteristic of the average state of orienta- tion in the sampled section. The recent progress in image analysis and its application to polished inner surfaces of composite materials (1 5) allows an effi- cient measurement of the fibers' spatial orientation.

The purpose of the present work is to investigate fiber orientation in shear flow of fiber-filled polymer melts and its relation to the rheological behavior of the composite system. An orientation parameter is proposed to quantify the spatial orientation state which enables an estimate of some of the rheological properties.

EXPERIMENTAL

Commercial glass fiber reinforced Noryl (poly(pheny1ene oxide) polystyrene blend, Tg = 115.5"C, General Electric) was used throughout the present investigation. Materials designated GFN 1, GFN2, and GFN3 denote Noryl with 10, 20, and 30 weight percent glass fibers, respectively. The fiber length distribution and average length are similar in the three systems (16). The glass fibers are 15 pm in diameter and 2 14,230, and 190 pm in average length for the 10, 20, and 30 weight percent compounds, respectively. Fiber length distributions were recently reported (1 6, 17).

The rheological data were obtained using an In- stron capillary rheometer, as described in a previous paper ( 17).

Fiber orientation in extrudates obtained through a capillary die was determined using a classical metal- lographic technique (3). The extrudates were em- bedded in an epoxy resin and then longitudinally polished. The polished extrudates were photo- graphed, using reflected-light microscopy. To obtain a reliable statistical analysis 20 to 30 photographs were taken and about 800 fibers per experiment were analyzed. Two orientation angles were measured from the photographs: the inclination angle, 0, rela- tive to the sectioning plane and the in plane orienta- tion angle, 4, as in Fig. I . The measuring method of the axial orientation is based on the shape, either circular or elliptical, of the fiber cross section within the polished plane (1, 3, 15, 18). The ratio a/b, where 'a' is the fiber diameter and 'b' is the measured ellipse long axis, is related to the angle of fiber incli- nation relative to the sectioning plane (Fig. I ) . The planar fiber orientation in the sectioning plane is equal to the orientation of the long main axis of the ellipse, all related to the extrudate longitudinal axis. The planar, Fp, and axial, Fa, orientation parameters, which define the spatial orientation state of a fiber,

POLYMER COMPOSITES, APRIL 1989, Vol. 70, No. 2 85

A. Vaxrnan , M. Nark is , A. Siegrnann, and S. Kenig

were calculated as follows (20):

F p = 2(c0s2+) -1 where (cos2$)

and Fa = [3(cosz0) -1/2 where (cos20)

entation to the relative viscosity, according to Eq 1 . For is defined by the following expression (16):

1 'I

For = (sin40sin22$) = - 2 ~ i n ~ 0 ~ s i n ~ 2 4 ~ (6)

A graphical presentation of Eq 6 , Fig. 2, shows that For attains maximum values at 0 = 7r/2 and 4 =

n l

7r/4 or 3*/4 and minimum values at 0 = 7r/2 and $ = ~ / 2 , 0 = 0 for all values of 4.

../z

= 1 Na(0)cos20 sin 0 d0 (5)

The terms Np(4) and Na(0) are the orientation distri- bution functions which specify the fraction of fibers within the angular elements d$ and d0, respectively.

F p varies between 1, when the fibers are perpen- dicular to the flow direction (X3) , and -1, when par- allel to the X 3 axis. For a random distribution in the X z X 3 plane, Fp = 0. The orientation parameter Fa quantifies the tendency of fibers to tilt out of the XzX3 plane. Fa varies from -0.5 to 0 as fiber orien- tation changes from normal to the XI axis to a ran- dom distribution, and from 0 to 1 as fiber orientation changes from random to perfect alignment along the X I axis.

The planar and axial orientation angles are com- bined into an orientation function, For, which ac- counts for the contribution of the average fiber ori-

RESULTS AND DISCUSSION

Fiber orientation distributions were measured in extrudates collected from a capillary rheometer , which was operated at different shear rates and tem- peratures. In the previous paper (17) fiber breakage during shear flow is discussed showing that substan- tial breakage was only observed at the highest stud- ied shear rate, 575 s-'. Consequently, reported fiber orientation distributions, in the present paper were only measured in extrudates obtained at shear rates lower than 575 s-'.

Axial and planar fiber orientation distributions in extrudates obtained at 250°C at different shear rates are presented in Fig. 3. From the orientation distri- bution curves it is apparent that both the planar and

3 . 1 4

86 POLYMER COMPOSITES, APRIL 7989, Vol. 70, No. 2

A. Vaxman, M. Narkis, A. Siegmann, and S. Kenig

1

0 30 0 90 120 150 180 PLANAR ORIENTATION ANGLE, deg

0 25

0 15 30 45 60 75 90 AYIAL ORIENTATION ANGLE, dee

Fig. 3. Experimental planar (a) and axial [b) fiber orien- tation distribution in Noryl extrudates containing 20 weight percent glass fiber at 250°C. LID = 20 and dqfer- ent shear rates: 2.87 I---). 28.7 I-----), 1 15 (- - -), 287 s-’ [-- - -1.

axial distributions are affected by the shearing flow and that fiber alignment increases with shear rate. However, in spite of an expected orientation sym- metry in cylindrical extrudates the axial distributions are not symmetric and the two calculated orientation parameters are not identical. Moreover, the shear rate effect on the planar fiber orientation is more pronounced than on the axial one. Calculated values of the orientation parameters, Fp and Fa, as a func- tion of shear rate are listed in Table 1, which shows that Fp is much more shear rate dependent than Fa. This lack of symmetry is apparently due to the me- tallographic polishing technique used for assessment of the fiber orientation distribution; not all the fibers are accounted for. Only fibers intersecting the pol- ished plane are observed, because of the inclination- dependent probability of sectioning a fiber, and be- cause of fiber damage during polishing. The proba- bility for a fiber along the sectioning plane to be cut is much lower than for a vertically oriented one. Moreover, it is possible that these fibers along the sectioning plane (having a high degree of orientation) are pulled out and lost during polishing. Therefore, in spite of the large number of fibers sampled, the orientation peak obtained in the axial distribution, for all investigated samples, was shifted by about 20” in comparison with the planar distribution.

Table 1. Effect of Shear Rate on Orientation Parameters.*

Shear rate

(sec-’) F,, F. F‘ . For F‘or

A. Temperature = 250 OC. 2.87 -0.621 -0.309 -0.353 0.165 0.142

28.7 -0.778 -0.323 -0.394 0.136 0.140 115. -0.826 -0.322 -0.404 0.134 0.136 287. -0.827 -0.323 -0.421 0.112 0.118

2.87 -0.461 -0.265 -0.281 0.255 0.178 28.7 -0.708 -0.321 -0.369 0.155 0.131

115. -0.741 -0.313 -0.383 0.145 0.133 287. -0.791 -0.336 -0.407 0.118 0.109

*The orientation parameters F.‘ and Fw‘ were calculated according to € 9 ~ . 5 and 6, assuming symmetry in the cylindrical extrudates (0 = +).

B. Temperature = 300OC.

To evaluate the orientation measuring technique, the experimental orientation parameters Fa and For were compared with Fa’ and For’ values calculated from Eqs 5 and 6 assuming an orientation symmetry (6’ = 4), characteristic for cylindrical filaments (Table I ). The calculated fiber orientation parameters, Fa’ and F,,’, exhibit reasonable agreement with values based on the experimental measurements, Fa and For. Since Fa’ < Fa within the whole studied shear rate image, it is concluded that a certain fraction of fibers oriented in the flow direction (probably, the most oriented ones) is not detected and obviously unaccounted for. As suggested above, these fibers are either pulled out or are not intersected during the polishing stage. These results demonstrate that the classical metallographic technique usually recom- mended in the literature (3, 15, 18, 19) for measuring the spatial orientation of fibers does not represent a sufficiently accurate quantitative characterization method of the fiber orientation state. Fisher and Eyerer (1 5), describing a computer based measuring strategy to determine spatial orientation of short glass fibers based on cross sections of single fibers, suggested a correction factor to compensate for the fiber inclination error. Sanou, et al. (19) emphasized the necessity of an accurate measurement of fiber orientation and discussed another source of error: the low eccentricity of the ellipse when the fiber orientation angle is less than 45”. On the other hand, it is important to notice that in several recent reports, e.g. (18, 19), a rather low number of fibers was meas- ured to get an “accurate” calculated orientation func- tion.

The present work shows that fiber orientation in the flow direction, as expressed by the orientation parameter, For, increases with increasing shear rate (Fig. 4). Moreover, fiber orientation at low shear rates is shear sensitive while at the higher shear rates, the orientation of the already-oriented fibers is only slightly affected by further increasing shear rates. It should be mentioned that only a few experimental studies, on shear flow-induced fiber orientation, have been reported in the literature. These were basically qualitative descriptions of the orientation state, using either scanning electron microscopy for examining

POLYMER COMPOSITES, APRIL 7989, Vol. 70, No. 2 a7

A. Vaxman, M. Nark i s , A. S i e g m a n n , and S. Kenig

O . l O L - - - - - . - - i 1 10 100 1000

SHEAR RATE, l/s

Fig. 4. Eflect of shear rate on fiber orientation parame- ter, For, in extrudates of Noryl containing 20 weight per- cent glass fiber at 250°C (A] and 300°C (*].

extrudate surface morphology (2 1, 22), or contact microradiography for studying thin sections of extru- dates (23). The data of the present research are in agreement with the observations made by Vincent and Agassant (24). Knutsson and White (21), how- ever, reported that fiber orientation in extruded fila- ments gradually increased with shear rate at low extrusion rates and only beyond 100 s-' orientation rose much faster. Wu (22) has also observed that 'disorder-order' transition occurs in fiber orientation behavior and its distribution during extrusion of fiber filled polymers. The high degree of orientation pres- ently observed, even at the lower shear rate range, is probably a consequence of an additional effect, such as extensional flow-induced fiber orientation occur- ring in the converging capillary entry. Fibers actually begin to align before entering the die since the con- verging flow at the die entrance could result in quite a high level of fiber alignment. Surprisingly, some reports even claim that the subsequent shear flow along the capillary can reduce this initial orientation (23, 25).

Examination of fiber orientation in extrudates of the various filled systems, prepared at different shear rates, reveals an interesting dependence of For on fiber content. The degree of orientation increases with fiber content in extrudates obtained at 287 s-' and decreases in those obtained at 2.87 s-'. The effect of fiber content on fiber orientation (Fig. 5) undergoes a transition in a shear rate range repre- sented by about 1 15 s-'. It is interesting to compare the present results with previous observations (22, 25) in which the degree of orientation decreased as the fiber concentration increased. It has already been shown that an increasing fiber content results in an increase in shear viscosity, an effect diminishing however at the higher shear rates. It is therefore expected that the increasing degree of fiber disorder with fiber content, reported in the literature (22, 25), will decrease at higher shear rates.

Fiber orientation was also found to depend on the capillary L/D ratio. Figure 6 shows the variation of fiber orientation distribution in extrudates obtained

, . , , *--- , . , . . . . . - - - ~ 5 10 15 20

FIBER CONCENTRATION. %vol

0.10

Fig. 5. Orientation parameter For, as a function of fiber concentration at a constant shear rate; 2.87 (01, 1 1 6 (A), 287 s-' (*I.

30 7.

0 30 80 80 120 150 180 PLANAR ORIENTATION ANGLE, deg

25 B

K

0 15 30 45 80 75 I I AXIAL ORIENTATION ANGLE, deg

Fig. 6. Fiber axial angular distribution {a] and planar angular distribution (b) in extrudates of Noryl containing 20 weight percent glass fiber, at 250"C, using capillary dies of various LID ratios (LID: 20 (- ---I, 40 (-- - -), 60 (-----], 80 (--I].

using dies of different L/D ratios. Here again, a dif- ference between Fa and Fp, in spite of the expected fiber symmetry, is observed ( T a b l e 2). Correspond- ingly, F p is practically unaffected by L/D, while Fa decreases with increasing L/D (this asymmetry was already discussed above). The orientation parameter For decreases (Fig. 7 ) with increasing L/D, i.e., the degree of orientation increases with total shear. Pres- ently, since D was kept constant increasing L/D re-

88 POLYMER COMPOSITES, APRIL 7989, Vol. 10, No. 2

Short Fiber Reinforced Thermoplas t ics . ZI.

I

3.0

P i I i 2.5:

2.0 -

I :

sults in higher total shear. These results are in agree- ment with theoretical predictions given by Kenig (7), Dihn and Armstrong (8). and Folgar and Tucker (9). According to their predictions an increasing total shear should enhance fiber alignment in the direc- tion of shear. According to Wu (22), the degree of order in the extrudate first decreases with increasing capillary length (initial L/D = 16, entry angle goo), reaching a minimum at L/D = 33, and then increases in longer dies. In contrast, Crowson, e t al. (23) found that the fibers alignment in the flow direction is more pronounced in extrudates obtained from a die with L/D = 0.15 (entry angle 180") than for L/D = 50. The simultaneous increasing apparent shear viscosity with increasing L/D (Fig. 7) and thus with decreasing F,, is presently unexplicable.

Figure 8 shows the effect of fiber orientation on

g 2.0; 0 .

,- ,- ti'- 1.0 . J

240 250 260 Z I O 280 280 300 310 TEUPERATUREPC

Table 2. Effect of Capillary L/D Ratio on Planar Fp and Axial F. Orientation Parameters.

1 /

20 -0.621 -0.309 40 -0.679 -0.301 60 -0.672 -0.294 80 -0.679 -0.243

0.17 ,. e d

0'16 1 0.15 a z

2 0.14

t O.I3 1.51 . . . . . . . . . . . . . . - - . . / . . . . , . . . . . . . . . . . . . . / 0.12

10 20 30 40 50 60 70 80 90 LENGTH / DIAMETER

Fig. 7. Relative viscosity 0) and orientation parameter, For, (A) as affected by capillary LID ratio.

2.50 1 I

t A

1.75 , .i 0.10 0.12 0.14 0.16 0.16 0.20 0.22 0.24 0.28 0.28

ORIENTATION PARAMETER. For

Fig. 8. Dependence of the relative viscosity on orienta- tion parameter, For, for Noryl containing 20 weight percent glass fiber at 250°C (0) and 300°C (A).

. . . , . , . . . , . . . . , . . , . . . 240 250 280 270 280 280 300 310

TEUPERATLIRE,%

Fig. 9. Temperature effect on relatiue viscosity (a) and orientation parameter, at 287 s-', for Noryl containing 10 weight percent a], 20 weight percent (Aj, and 30 weight percent ("1 glass fibers.

3'5 F-- -7 3.0 j

2 2.51

J 2.0

B 7 1.5 1

FIBER CONCENTRATION. %vol

1.0 1 0

2.5

18

,

4 6 8 10 12 14 16 FIBER CONCENTRATION, Xvol

Fig. 10. Comparison of experimental (0) and calculated relative viscosities using Eq 1 (-----j and E q 3 (- - -1.

POLYMER COMPOSITES, APRIL 7989, Yo/. 70, No. 2 89

A. Vaxman, M . Narkis, A. Siegmann, a n d S. Kenig

the relative viscosity of Noryl containing 20 weight percent glass fiber. The relative viscosity increases with the orientation parameter thus with fiber dis- order. Unexpectedly, the q/qo us. For curve for extru- dates prepared at 300°C is slightly higher than that at 250°C, though it was found that the shear viscosity of the unfilled and filled melts decreases appreciably with an increasing temperature (1 7). As the temper-

of the polymer melt decreases and therefore the rel- ative viscosity of the fiber-filled melt also decreases; however the fibers' presence and their orientation result in a different rate of viscosity decrease with

At this stage, a quantitative comparison of the calculated relative viscosity, according to E q s 1 and 3, using experimental orientation parameters, with experimental relative viscosity values is presented. To calculate the relative viscosity, the coefficient k l was first evaluated according to E q 2, based on For. The constant C1 in Eq 2 was calculated as (6):

(7) < ature is increased from 250 to 300"C, the viscosity c1 = 4(ln2rp - 2.14)

where r, is the fiber aspect ratio. Values of C1 of 42, 47, and 38 were obtained for the 10, 20, and 30

temperature. The study of the effect of temperature on the relative viscosity (Fig. 9a) shows an unex- pected behavior: first the relative viscosity increases as the temperature is increased from 250 to 275°C. followed by a decrease as the temperature is further increased to 300°C. This unexpected behavior is in agreement with the simultaneous changes taking place in the orientation parameter over the same temperature range (Fig. 9b). Thus, the highest vis- cosity is observed at the lowest orientation (high For), as expected. However, the general temperature effect on orientation is presently unexplainable.

weight percent filled Noryl, respectively. As seen in Fig. 10, the experimental values of the relative vis- cosity are higher than the calculated ones according to E q 1 . The relative high fiber concentration (20 and 30 weight percent) used in this work, leads to fiber- fiber interaction, not accounted for in E q 1, which significantly increases the viscosity of the system. A much better fit was obtained using E q 3, where the effects of fiber interaction are included in the k2c2 term. A value of k = 0.78 was determined for all concentrations, temperatures, and shear rates stud- ied for which the relative viscosity and the orienta-

z = RELATIVE VISCOSITY X = ORIENTATION P " E R For Y - FIBER CONCENTRATION, %yo1

,/

/' p.

73

so

2s

0

0 .

- 0 . 3

7 0 . 0 1 .oo

Fig. 1 1 . Graphical representation of the Eq 3 which describes the effect of fiber concentration and fiber orientation distribution on the relative viscosity.

90 POLYMER COMPOSITES, APRIL 1989, Vol. 10, No. 2

Shor t Fiber Reinforced Thermoplas t ics . 11.

tion parameter were measured. This k value is in good agreement with that predicted by Frisch and Simha (1 3). By including the orientation parameter in the third term of Eq 3 it is assumed that fiber orientation has a greater effect on the relative viscos- ity at higher fiber concentration (Fig. 1 1 ) . At low concentrations however, Blakeney (12) found that the 0.73(klc)* term is negligible. These results reflect the structures generated by fiber interaction.

In conclusion, in spite of the complexities presently encountered, the viscosity of fiber containing poly- mer melts can quite accurately be predicted based on fiber and polymer characteristics and fiber orienta- tion. The orientation parameter For should be ac- counted for in the interaction term when the relative viscosity is correlated with filler concentration. This orientation parameter as affected by shear rate, total shear, temperature, and fiber concentration was studied. It was found that F,, decreases with increas- ing shear rate and the effect of fiber concentration on it is shear rate dependent: For increases with fiber content at low shear rates and decreases with fiber content at high shear rates.

Future work will address the effect of fiber length on fiber orientation in shear flow. The fiber orienta- tion distribution will be investigated under elonga- tional flow conditions.

1. 2.

3.

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