The Processing of Fiber Reinforced Thermoplastics Using Co-Rotating Twin Screw Extruders

DAVID WALL

Baker Perkins Chemical Machinery Stoke-on-Trent, STl 4DW, England

For short fiber reinforced thermoplastics the effect of fiber length and the fiber/matrix bond on the ultimate properties of the material are well understood. But, how the process conditions under which the composite is made affect the fiber length and the fiber matrix bond has not been so thoroughly reported in the literature. A study of this relationship has been made on a co-rotating twin screw extruder incorporating 30 percent by weight of glass fiber into nylon 66 to make the composite.

Experimental results are presented to show how machine variables such as mixing configuration and screw speed affect the material properties and process efficiency. The properties of the composite were assessed by measurement of the fiber length distribution and tensile strength.

From the results in the text it is possible to select machine variables to give desired production conditions. Although most of the work concerns the twin screw extruder, some of the consequences of subsequent injection molding are also shown.

INTRODUCTION

he use of strong brittle fibers to reinforce ther- T moplastics has become increasingly popular since the 1960’s. However, the full potential rein- forcement offered by such fibers is never reached in commercial products because of the methods used to incorporate the fibers into the thermoplastic. The most commonly used method is continuous com- pounding, but this causes severe degradation of the fiber length, reducing its effectiveness as a reinforc- ing agent. This has been shown in many pieces of work (1, 2, 5, 6). Bowyer and Bader (3) have verified the use of the equation below for relating the strength of the composite to its composition and the physical properties of its components:

This equation may be written schematically as:

oc = cx + CY + z C is an orientation factor. X, Y, and 2 are the contri- butions to reinforcement made by the sub-critical fibers, super-critical fibers, and the polymer respec- tively.

The importance of fiber length and the fiber matrix bond is clearly illustrated by this equation. It is pos- sible to preserve the length of the fibers passing through the compounder by reducing the amount of

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mixing, but this may lead to poor distribution and wetting-out of the fibers. Poor distribution of fibers may leave fiber-fiber bonds in the finished article, these are weak and break easily under load. When the fiber surface is not well wetted-out with polymer the average value of T will be low. Depending on the magnitude of these effects, the benefits of preserving the fiber length may therefore be more than offset from the reduction in the average fiber matrix bond strength. For any process then, it is important to find the magnitude of these effects and a n operating win- dow in which the correct balance is struck between fiber length and strength of the fiber matrix bond.

EXPERIMENTAL APPARATUS AND PROCEDURE

The study was concerned with how the process affected the properties of the composite, so only the process parameters were varied; the material used for the study remained the same throughout. This was polyamide 6,6 reinforced with 30 percent by weight of 4.5 mm chopped glass fiber strands. A layout of the experimental apparatus is shown in Fig. 1 . It shows polymer entering the co-rotating twin screw extruder, and glass fiber being injected by a side feeder at its mid-point. At the discharge of the twin screw it is coupled to a single screw crosshead extruder to pump the melt up to the die. The aspect ratio of both machines was 1O:l L/D. The overall format of the screw configuration in the twin screw extruder remained similar throughout the experi-

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T h e Processing of Fiber Reinforced Thermoplas t ics

Experimental apparatus

,

Fig. 1 . The experimental apparatus.

ment. It consisted of a feed section to take polymer into the extruder and convey it to the first mixing section. The screws used in this section were two start square pitch screws of a self-wiping design. The first mixing section terminated immediately prior to the side feeder near the mid-point of the extruder. It consisted of eight mixing elements each of D/4 in length. The elements were designed to have two tips and had a self-wiping geometry. A detailed descrip- tion of such geometry is given by Booy (4). The ele- ments were chosen as they have been found to be very effective for mixing in co-rotating geometry and are the most commonly used elements in practice. The first six elements were offset at 60" along a forward conveying helix. The last two mixing ele- ments were offset at 90" to one another. The final element used a restricting disc to hold material in the mixing section. Another melt conveying/feed section followed the first mixing stage; here the glass was injected into the melt with the aid of a single screw side feeder.

Up to this point the screw configuration remained the same for all experiments. The following mixing stage used to mix the glass into the polymer was arranged and consisted of elements similar to the first with the offset angle between elements being 60". In this section the number of elements and hence the length of the mixing section were varied to study what effect it had on the composite and the process. The remaining screw configuration was identical to screws used in the feed section; their function was to transport material to the single screw crosshead extruder. Alterations to the length of the second mixing stage were achieved by substitution of conveying screws.

The general procedure used for experiments was to feed the polymer into the main feedport of the extruder, and obtain a full melt in the first mixing stage so that the glass strands entering via a side feeder were injected into a homogeneous melt. A second mixing stage was then used to incorporate the glass into the polymer. A single screw crosshead extruder was used as a low shear melt pump to push the material through the strand die, minimizing fiber breakage in the pressure development region. The

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strands were quenched in a water bath and pelletized in the usual manner. Fiber length distribution was measured in these pellets before injection molding tensile bars. The bars were molded to comply with the ASTM D790 standard. Tensile strength and fiber length distribution measurements were taken from the tensile bars. All bars were dried for 12 h at 80°C. and then left for 5 h in the test room to acclimatize.

Although it was the compounding stage that was under consideration, it was necessary to involve the injection molding process to examine tensile bars. The injection molding machine will have had some influence on the parameters measured, but all proc- ess conditions were kept constant except for the one being examined. It is therefore reasonable to assume that the differences seen in the measured parameters were due to the varied condition.

The amount of mixing experienced by the com- pound in the extruder may be expected to affect the fiber/matrix bond and the degree of fiber length deg- radation. The amount of mixing is greatly affected by the number of mixing elements contributing to a mixing length and the screw speed. To investigate how these two parameters affect the material prop- erties a series of experiments was conducted. The first series varied only the length of the second mix- ing stage. Then, choosing the configuration from the above which gave the longest average fiber length, the screw speed only was varied between 200 and 500 rpm.

The results were analyzed in terms of tensile strength of the product, fiber length, and energy utility of the process. The former two parameters were chosen for analysis, since one of the objectives of the work was to produce compounded composites with maximum fiber length while ensuring a wetted- out fiber surface. Moreover, there is a clear theoreti- cal relationship between fiber length and tensile strength.

RESULTS AND DISCUSSION

As the length of the second mixing stage (the stage responsible for mixing the glass fiber into the poly- mer) was increased, there would be an increased reorientation of flow and higher probability of fibers passing through a sufficiently high stress region to cause fiber breakage: so it might be expected that the fiber length would decrease as the mixing length increases. Figure 2 confirms this hypothesis and shows that, over the range considered, there was a linear fall-off in fiber length.

As a consequence of injection molding, the fiber lengths in the tensile bars were all very similar and hence should not have contributed significantly to the differences in tensile strengths of the bar. Figure 3 reveals that there is a very noticeable difference in the strength of the tensile bars. When a 2D mixing section was used the bars were 1 . 1 8 times stronger than when a Y2D section was used. This then would appear to be due to the longer mixing section improv- ing the degree to which the fiber surface has been

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David Wall

P O 5 1 lNJtCTlDN UOUL"iN0

F I B R E LENGW v s MIXING LENGTH

TENSILE STRENGTH vs MIXING LENGTH

As the mixing length increased from I/2D to 2%D length, the distribution of fiber lengths in the com- pounded product made a gradual transformation (see Fig. 4), from distribution (a) to distribution (b). The use of the 2%D mixing length resulted in a narrow fiber length distribution with a standard deviation of 142 pm.

There are very few of the longer fibers present and none beyond 775 pm. In contrast, the short mixing section has given a broad fiber length distribution with a standard deviation of 2 13 pm; 12.5 percent of the total fibers counted were above 775 pm and 4.7 percent above 1030 pm. The differences in fiber lengths are a reflection of the history of the fibers on their passage through the compounder with regard to the shear stress and rate of reorientation they experienced. The longer mixing lengths increase the changes of a fiber passing through a high stress region and rapid reorientation. From the shape of the graphs, the greater uniformity of fiber length illus- trated by distribution (b) appears to be predominantly due to the break down of the longer fiber length. This may suggest that the mechanism of fiber reduction, Czarnecki and White (5), favors the breakdown of long fibers rather than the continued reduction of short fibers. The shear stress generated in the melt will normally be much less than the tensile strength of the fiber, so if applied along the direction of the fiber, they will not break it. Fibers are easily bent if the stress is acting at an angle to the axis of the fiber and buckled if the flow is decelerating. The former is likely to occur after reorientation of flow and intui- tively, it might be expected that long fibers are more susceptible to breakage this way. Reorientation does however have an important role in the separation of fibers and their even distribution in the matrix.

High speed co-rotating twin screw extruders offer a great range of screw speeds at which they may be operated, typically 0 to 500 rpm, as was the case in the apparatus used in these experiments. Such a range of screw speeds will considerably affect the rate of shear, reorientation, and the number of times reorientation takes place. Figure 5 shows that vary-

Fig. 3. The effect of mixing length on product strength. EFFECT ff M I X I N G L E N j T H ON FIBRE L E W T H DISTRIBUTION

la.

Izo-

110-

wetted out, thus increasing the average value of 'T

over the fiber surface. Figure 3 shows that initially considerable improvement in fiber wet out is Lrn-

achieved as the mixing length is increased. However, the rate of improvement declined sharply to the point where the addition of more mixing elements did noth- ing to improve the tensile strength measured. Yet at the same time the fiber length was still decreasing linearly in the compounded product over this range of mixing lengths. The inference here is that com-

m e m-

f m-

b 70-

d m-

? Y)-

- 10-

x)-

pounded products from the 2D and 2Y2D mixing sec- tion have a similar overall fiber matrix bond strength, but because the product from the 2D section has a

the extruders, it will be stronger than product made on the 2Y2D mixing section prior to injection molding.

0 l W Z O U X a 4 w 5 a , m 7 w a w g x , I D greater average fiber length after passing through Fibre Length f p l

Fig. 4. The effect of mixing length on fiber length distri- bution.

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T h e Processing of Fiber Reinforced Thermoplas t ics

ing the screw speed has only a small effect on fiber length in the extrudate for the screw configuration considered. The graph indicates a slight drop in fiber length as the screw speed was increased. This was probably due to an additional number of reorienta- tions, as well as the higher shear rates.

The tensile bars produced from samples taken at each screw speed indicate that, for most of the range of screw speeds tested, there was an improvement in tensile strength with increasing screw speed (Fig. 6). Again, the magnitude of this effect is not as marked as that caused by varying the mixing length. How- ever, the improvement in fiber/matrix bonding can be explained in a similar manner: as the screw speed was increased, the number of flow divisions (required to distribute fibers) would also have increased and the additional shear stress at the fiber/matrix inter- face would help to wet out the fiber surface.

As well as producing good quality product, it is desirable to do so as efficiently as possible. One criterion of efficiency is the amount of energy used to produce a unit mass of good quality product. In

EFFECT OF SCREW SPEED ON FIBRE LENGTH

zoo a0 +ca Mo

Screw Speed l c p m l

7

Fig. 5. The relationship between screw speed and fiber length.

EFFECT ff SCREW SPEED ON TENSILE STRENGTH

Zoo 3y1 403 yx)

screw Speed Ir-PrnI

Fig. 6. The relationship between screw speed and prod- uct strength.

POLYMER COMPOITES, APRIL 1989, Vol. lo, No. 2

this case the specific energy is a convenient way of examining energy efficiency. Figure 7 shows that specific energy consumption does increase as the mixing length increases. There appears to be an al- most linear relationship between the two parameters over the range considered. Comparing the results in Fig. 3 with those in Fig. 7 shows that there was no further benefit to matrix properties once the mixing length had reached 2D. If a 2Y2D mixing length is used, the energy consumed will be higher but with no additional material property benefits, so the en- ergy efficiency will be reduced.

The production capacity for this and many other thermoplastic composites is limited by the power available for processing, which for a D.C. drive (com- monly used on these machines) is directly propor- tional to the screw speed (see Fig. 8). The results so far obtained showed that the detrimental effects of

EFFECT ff M I X l f f i LENGTH ON SPECIFIC EKff iY CONSUPTION

3

Fig. 7. The relationship between screw configuration and specijXc energy.

TH1WGHPUT vs S C R W SPEED L

70

=% S u e r %mod lrPnl m 100

Fig. 8. The relationship between screw speed and throughput.

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David Wall

operating at high screw speeds, in terms of fiber breakage, were very small and there were, up to a point, some benefits in terms of fiber wet out. But the gains in production from operating at higher screw speeds are considerable.

It is apparent from Fig. 2 that the average fiber length in the injection molded bars was virtually independent of the average fiber length in the prod- uct from the twin screw extruder; a similar observa- tion was made by S.R. Schweizer (6) in his work. The interference from this is that the design of the injec- tion molding system has to be carefully considered if advantage is to be taken of long fiber length in the compounded product. However, with many current designs of mold, if a compound is chosen where very long fiber length has been preserved at the cost of fiber matrix bonding, then the injection molded item will not be as strong as that produced from a com- pound where the average fiber length may be shorter but the fiber/matrix bond is strong. This is due to the further reduction in fiber length during injection molding.

CONCLUSIONS

In conclusion this work has shown that the average fiber length and the fiber matrix bond are affected by the conditions under which the fibers are mixed into the polymer. When the mixing of the glass into the polymer is performed in a co-rotating twin screw extruder, it is the length of the mixing section that has the most significant effect on both these param- eters. As the mixing length increases, the fiber length will decrease, while the fiber matrix bond will im- prove, rapidly at first, then leveling off.

Over the wide range of screw speeds tested, 200 to 500 rpm, the effects upon fiber length and tensile strength were relatively small when compared to that of the mixing length. Although screw speed had a limited influence on product quality (considering only fiber breakage and the fiber/matrix bond) it greatly affects production rate, as this was controlled by the power available, which in turn is directly propor- tional to screw speed for a D.C. drive.

For a given set of operating conditions, the specific energy consumption was proportional to mixing length. So for energy efficiency it is undesirable to operate with mixing length beyond that required to achieve the maximum fiber/matrix bond strength.

If the injection molding cycle causes considerable fiber length degradation, then a compound where the fiber/matrix bond is strong, rather than one where

the fiber length has been maintained at the cost of this bond, will give a stronger product, as injection molding will reduce their average fiber lengths to similar values.

To approach the efficiency of reinforcement ob- tained from continuous fibers, long fibers that are well bonded to the polymer must be obtained in the product. A fiber length of 5 Lc should give approxi- mately a 90 percent reinforcement efficiency. To achieve this, considerable improvements are re- quired to current compounding and injection molding techniques. But even with commercially available technology this paper has shown that the correct processing conditions will make significant differ- ences to product quality and line efficiency.

NOMENCLATURE

C = Orientation factor. E = Young’sModulus. L = Fiberlength. r = Fiberradius. V = Volume fraction. X = Contribution to reinforcement from sub-criti-

cal fibers. Y = Contribution to reinforcement from super-crit-

ical fibers. e = Strain. T = Stress at fiber/matrix interface. Subscripts c = Critical. f = Fiber. i = Sub-critical fractions. j = Super-critical fractions.

REFERENCES

1 . M. G. Bader and W. H. Bowyer, J. Appl. Phys. . 5,2215 (1972).

2. M. G. B i g , S.P.E. ANTEC Tech. Papers , 30,668 (1984). 3. W. H. Bowyer and M. G. Bader, J . Muter. Sci., 7, 1315

(1972). 4. M. L. Booy, Polyrn. Eng. Sci., 18, 973 (1978). 5. L. Czarnecki and J. L. White, J. Appl. Polyrn. Sci.,25,

1217 (1980). 6. R. A. Schweizer, J . Polyrn-Plust. Tech. Eng., 18(1), 81

(1982). 7. K. Eise, J. Curry, and J. Farrell, S.P.E. RETEC, paper

19, (1984). 8. J. L. White, L. Czarnecki, and H . Tanaka, Rubb. Chern.

Technol., 53. 823 (1980). 9. Y. Chan, J. L. White, andY. Oyangi, J. Rheology, 22(5),

507 (1978). 10. L. Mascia, “Thermoplastics: Materials Engineering,” Ch

4, Applied Science, New York (1982).

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