Injection Molding of Thermotropic Liquid Crystal Polymers

ZOHAR OPHIR* and YOSHIAKI IDE

Celanese Research Company 86 Morris Avenue

Summit, New Jersey 07901

Thermotropic liquid crystal polymers consist of rod-like mole- cules and are often called “self reinforcing thermoplastics.” Their rheological behaviors as well as orientation development during processing are often very similar to those of short fiber-filled com- posites. Without reinforcement, the polymer shows superior me- chanical properties to conventional glass fiber-reinforced engineering resins. The orientation distribution in the cross- section as well as flow patterns in the molded thermotropic poly- mers are clearly visible to the naked eye due to color differences. This makes it particularly convenient to study the orientation dis- tribution as well as the flow patterns of packing, back flow, jet- ting, flow instabilities, and weld line formation in injection molding. This paper discusses physical properties of a typical ther- motropic polymer and their relationship to mold filling process in the injection molding.

INTRODUCTION n recent years, a new class of polymeric materials I has been developed through molecular engineering.

These materials consist of rigid backbone molecules and thus, even in the quiescent condition, they take extended chain conformation to form optically aniso- tropic melts (thermotropic liquid crystal polymers). All aromatic rigid backbone homopolymers have very high melting points (1). In order to reduce the melting points to the range where these materials can be injec- tion molded or melt spun, molecules were modified in several ways (2) but still retain extended chain confor- mation. Specifically these rigid backbone moieties are randomly copolymerized (3) or copolymerized with a controlled amount of a moiety with flexible linkages (4), bulky side groups (5) , or kinks (6). These poly- meric materials produce a state that lies between solid and liquid. More specifically, these materials form a nematic phase characterized by polymer chains ar- ranged in an offset parallel order (7).

Unlike lyotropic liquid crystal polymers such as KevlaP, thermotropic liquid crystal polymers can be processed with conventional thermoplastic processing techniques. For example, when these materials are ex- truded and drawn down in the melt, the extrudates show extraordinary molecular orientation (8). When these materials are injection molded, the molded parts show very high mechanical properties without fibrous reinforcements. The molded articles also show high anisotropy of physical properties between the flow di- rection and across the flow direction (9-ll), but the anisotropy is reduced as the thickness of the parts in-

*Present address: Rafael, Israel Ministry of Detense, Department 63, P.O. Box 2250, Haifa. Israel.

creases (9). Rheological behaviors (12) , specifically unbounded viscosity at low shear rate as well as signif- icantly reduced extrudate swell, high die entrance pressure drop, etc., are very similar to those of fiber filled resins (13). These thermotropic liquid crystal polymers are therefore often called “self reinforcing polymers.” Due to the many similarities, studies of liq- uid crystal polymers can complement those of compos- ites, particularly short fiber reinforced composites. This paper describes physical properties of a typ- ical thermotropic polymer and their relationship to the mold filling process in the injection molding operation.

ORIENTATION DISTRIBUTION IN THE MOLDED PARTS

Analysis of injection molding process is highly complex since polymer melt flow into a complex mold cavity is further complicated by highly non-isothermal condi- tions. Thus the orientation distribution is extremely complex because every fluid element of the molded part has a different temperature and deformation history.

Several investigators have characterized the orien- tation distribution in simple molded parts using bi- refringence (14), shrinkage measurements (15) , or op- tical microscopy and x-ray diffraction (16).

It has been found that in the direction of flow the orientation is maximum at the surface and decreases to zero at the center but has secondary maximum near the surface. The orientation at the surface skin is caused by elongational flow in the advancing frqnt, whereas orientation in the core is related to shear flow (17). The elongational flow in the advancing front, of-

792 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1983, Vol. 23, NO. 14

Injection Molding of Themnotropic Liquid C y s t a l Polymers

ten called "fountain flow" can give rise to uniaxial or biaxial orientation depending upon flow cross section.

Short fiber orientation distribution in the injection molded parts is highly complex and shows many dis- tinct layers. For example, layer structures up to 6 dis- tinct layers is reported in glass fiber filled nylon (18). Generally the glass fiber orientation is found highly sensitive to elongational flow. Specifically, the fiber orientation is mostly determined by the change of the flow cross-section (19,20). For example, when the cross-section contracts (converging flow), the fibers are oriented to the flow direction, but when the cross- section expands (diverging flow), the fibers are ori- ented to the transverse direction. Thus molded parts often show transversely oriented core surrounded by a longitudinally oriented skin. And further, the core ori- entation injected through fan gates is often consid- erably more transverse than that through edge gates (19).

MATERIAL The material studied was an all aromatic copolyester that was polymerized in the melt from p-aceto- xybenzoic acid (60 mole percent), terephthalic acid (20 mole percent) and naphthalene diacetate (20 mole percent). This polymer was invented by Calundann and the details of polymerization conditions are de- scribed in the patent (21). The DSC curve did not show much calorimetric activity until the very small melting point peak. This implies that the three dimen- sional crystallinity of the copolyester is not very high and/or the melting enthalpy is quite low. The melt vis- cosity was found to be very sensitive to temperature and shear rate and the zero shear viscosity was not evi- dent.

INJECTION MOLDING APPARATUS Standard ASTM 8.5" (21.6cm) tensile and 5" (12.7cm) flex bars were molded with a 2.5 oz. (71g) Stubbe in- jection molding machine. The machine was equipped with a TransTech linear velocity transducer to mea- sure the linear speed of the screw, and with a Dynisco melt pressure transducer mounted on the nozzle. The mold was equipped with Vanzzeti infra-red melt tem- perature transducers that were fitted inside two of the knockout pins, and with pressure transducers that were mounted under the temperature probes. The infra-red probes were selected because of their instant response and because they did not impose any obstacle on the flow field. It should be noted that such probes detect only the skin temperature. However, because of the fountain flow field at the advancing melt front, the reading of the probe at the moment of contact with the melt is closely related to the bulk melt temperature at that point. The readings of the transducers were re- corded on a six channel, fast response Gould recorder. An example of such readout during a molding cycle is shown in Fig. 1. The molding condition was melt tem- perature 340"C, mold temperature 100°C, filling time 0.6 seconds, injection time 10 seconds, and cool- ing time 20 seconds.

Fig. 1 . Injection speed, temperature and pressure readings during a molding cycle (filling time 0.6 sec., injection time 10 sec., and cool- ing time 20 sec.).

MECHANICAL PROPERTIES Mechanical properties of molded bars as a function of several molding parameters are given in Table 1. It is seen that most of the physical properties are above the range of engineering thermoplastics. Particularly high are the tensile and flex moduli, and the notched im- pact toughness is in the high end of tough thermoplas- tics. The properties of the molded parts are quite sensitive to variations in the molding conditions. It is also seen that slow injection speed tends to give higher tensile and flexural properties but reduces the notched impact strength.

Table 1. Injection Molded Properties of the Liquid Crystal Polymer

Melt Temperature ec, 320 340 340 340 340 340

Mold Temperature 03 40 40 40 100 100 100

Packing Pressure ( M W 28 21 39 39 28 0

Injection Speed Fast Fast Slow Slow Fast Fast Tensile Strength

( M - 4 192 173 189 208 176 153 Tensile Elongation

(Yo) 1.8 1.3 1.5 1.6 2.5 1.5 Tensile Modulus

(GP4 17.2 20.0 19.3 18.6 15.2 16.5 Flex Strength

W a ) 178 174 175 175 170 176 Flex Modulus

(G Pa) 15.2 15.2 15.2 14.5 13.1 13.1 Notched Impact

(Jim) 395 427 230 283 283 347

POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1983, Vol. 23, NO. 14 793

Zohar Ophir und Yoshiuki Ide

TEMPERATURE AND OTHER ENVIRONMENTAL RESISTANCE

Tensile properties of the molded bar at widely differ- ent temperatures were measured. It was found that the modulus is significantly higher at very low temper- atures. Fracture tests of the specimen in liquid nitro- gen indicated that the material retained substantial amount of ductility even at this temperature. Shear modulus behavior measured by the Dynamic Mechan- ical Spectrometer in a 2" by0.5" by 0.125" (5.08cm by 1.27cm by 0.318cm) bar at 1 Hz is shown in Fig. 2. Several transitions are seen in the figure but particu- larly noticeable is one around 100°C which resembles a glass transition in conventional thermoplastics but is much smaller in magnitude. Above this temperature, the modulus remains quite high until temperature ap- proaches the melting point.

Room temperature tensile properties after aging at 250°C of air are shown in Fig. 3. It is seen that after

DYNAMIC MECHANICAL SPECTRUM

lo'll---T1o'

i Fig. 2. Dynamic shear modulus spectrum measured at 1 Hz fie- quency.

TENSILE PROPERTIES

AGED IN AIR AT 2 5 0 ' C

MEASURED AT ROOM TEMPERATURE

- - 8 '1

300

-250 i E

$150

,200 + "

+ v)

2100 m z c" 50

2

40-

30-

20 -

50 TIME ( Days ) 100

Fig. 3. Room temperature tensile properties after aging at 250°C.

such extreme conditions, the tensile properties did not decrease substantially. The material is also quite flame resistant and vertical burn test of UL-94 rates V-0 without any flame retardant. The oxygen index is also as high as 38 percent without any additives.

Electrical properties are also quite good. For exam- ple, dielectric strength was 600 v/mils (23.6 kv/mm) for 0.125'' (3.18mm) and 1560 vlmils (61.4kvlmm) for 0.038" (0.97mm).

ANISOTROPY Qualitative data about the distribution of properties can be gained from measuring the flex modulus of a disc (4" by 1/16") (10.2cm by 1.59mm) in different di- rections (F ig . 4 ) . The liquid crystal polymer (LCP) was compared to polybutylene terephthalate (PBT) and to glass-filled PBT. It is obvious that the liquid crystal polymer exhibits much higher anisotropy of properties than conventional thermoplastics. This ani- sotropy is also reflected in a variety of other physical properties including thermal expansion coefficient, electrical arc resistance, and surface abrasion. The extent of the anisotropy can be reduced by raising the mold's temperature and/or increasing the part's thickness.

LAYER STRUCTURE Cross-sections of the molded tensile bars were cut in the longitudinal direction and the cut surface was fur- ther polished after embedding the samples in epoxy matrix. Black-and-white photomicrographs were taken with the aid of polarized light to enhance the contrasts. Figure 5 is the cross-section of a tensile bar that was molded at 340°C melt and 100°C mold tem- peratures at fast injection speed. Although the details of the structure could not be fully reproduced, one can observe four basic layers: a light skin layer (A) (the

FLEX MODULUS DISTRIBUTION

1.5 I b LCP

M D TO

0 30 6 0 90 I I I I I 0

-90 -60 -30 TEST DIRECTION ANGLE. Degrees

Fig. 4 . Flexural modulus distribution of the liquid crystal polymer (LCP), polybutylene terephthelute (PBT), and glass filled PBT.

794 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1983, Vol. 23, NO. 14

Injection Molding of Thennotropic Liquid Crystal Polymers

Fig. 5. Cross-sectional view of a tensile bar (sample was takenfiom the center of the bar, amow indicates primayflow direction).

edges of the bars are marked on the picture), a dark layer (B) under it (actual color is brown), a third layer (C) in which flow traces are observed as arcs, and a dark core layer (D) at the center. It can be easily ob- served on the part that the skin is highly oriented in the flow direction.

Wide angle x-ray diffraction patterns of layers A and B are shown in Fig. 6. The x-ray photographs were taken with the incident beam normal to the flow direction on the samples of 0.2mm thick. The scatter- ing angles (full width at half maximum) of the equato- rial (110) reflection was 45” for A and 110” for B. It is apparent that the skin layer is highly oriented in the flow direction while the sub-skin layer is almost un- oriented.

Origin of these layers can be interpreted as the same mechanism proposed for conventional resins as dis- cussed in a previous section. The highly oriented skin layer originates from the fountain flow at the melt front and deposited on the wall with the flow direction orientation. The elongational flow field can induce high molecular orientation (8) and the orientation is

immediately frozen in upon contact with the molds surface. Indeed the rapid cooling rate of the skin layer is apparent from Fig. 1, where we observe that the cavity melt temperature (actually skin temperature) drops below the melting point in a fraction of a sec- ond. The sub-skin layer B experienced mainly spread- ing radial flow and at the same time had more time to relax and, therefore, its orientation is minimal. Layers C and D show shearing and converging flows in the core of frozen layers A and B. Layer C shows some flow direction orientation caused by such flows into the center of dumbbell shaped tensile bar and layer D represents plug flow.

The thickness of the layers is affected by the mold- ing conditions; however, quantitative analysis of them is not a simple task because the morphology of the bars varies also with the position along it. We observed that along the bar not only the thickness of the layers varies but also the whole nature of the flow field. Near the dead end, in particular, the flow field sometimes be- comes unstable.

MOLD DESIGN It has been pointed out that the extrudate swell of the thermotropic liquid crystal polymers is extremely low (8,12). In designing the gate, it is important to make sure the melt touches the mold wall in order to avoid the “jetting” phenomena (22). Side gating or fan gates were found to reduce such phenomena.

It is very important to avoid or place the weld line in non-critical area since the weld line strength is signifi- cantly lower than the remainder as with conventional glass filled resins. This is the result of two fountain flows meeting each other. The orientation induced by such local flow fields is perpendicular to the main flow direction. Weld line photographs clearly have shown that the flow pattern and the orientation were pre- served at the weld line. Improvements of the weld line strength are possible by the conventional methods of fast injection and high melt and mold temperatures.

CONCLUSIONS It has been shown that the thermotropic liquid crystal polymers (LCPs) exhibit unusually high mechanical properties when injection molded. Without reinforce- ment, the polymer shows superior mechanical proper- ties to conventional glass fiber reinforced engineering resins. This is because LCPs consist of extended chain molecules (“self reinforcing thermoplastics”) and these molecules can be easily oriented. In addition to the high orientability, the polymer exhibited ductile be- haviors at very low temperatures and did not age sub- stantially at very high temperatures.

Their rheological behaviors as well as orientation development during processing are often very similar to those of glass fiber filled thermoplastics. The aniso- tropy as well as layer structures also show the similar- ity. Orientation distribution in the cross-section as well as flow patterns in the molded LCP are clearly visible to the naked eye due to color differences. This makes it particularly convenient to study the orientation distri-

Fig. 6 . Wide angle x-ray scattering patterns of the skin layer A and sub-skin layer B and their orientation angles.

POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1983, Vol. 23, NO. 14 795

Zohar Ophir and Yoshiaki Ide

bution as well as the flow patterns of packing, back flow, jetting, flow instabilities, and weld line forma- tion in injection molding.

ACKNOWLEDGEMENTS The authors would like to acknowledge Mrs. L. Saw- yer for providing micrographs and Dr. J. Stamatoff for the x-ray photograph. Helpful discussions with Drs. S. Kenig, K. Wissbrun, G. Calundann, M. Jaffe, H. Yoon, and A. Buckley are also greatly appreciated. Thanks are also due to Celanese Research Company for permission to publish this work.

5. 6. 7.

8. 9.

10. 11.

12. 13. 14. 15. 16.

17. 18.

20.

REFERENCES J. Economy, B. E. Nowak, and S. G. Cottis, Am. Chem. SOC. Polym. Prepr., 2, l(1970). 19. W. J . Jackson, Brit. Polym., 1. 154 (1980).

3,637,595 (1972) (to Corborundum). 21. H. E Kuhfuss and W. J. Jackson, U.S. Pat. No. 3,778,410 (1973)

S. G. Cottis, J. Economy, and B. E. Nowak, U.S. Pat. No.

22. (to Eastman Kodak).

T. C. Pletcher, U.S. Pat. No. 3,991,013 (1976) (to DuPont). G. Calundann, U.S. Pat. No. 4,067,852 (1978) (to Celanese). G. W. Gray, “Molecular Structure and Properties of Liquid Crystals”, Academic Press, N.Y., (1962). Y. Ide and Z. Ophir, Polym. Eng. Sci., 23, 261 (1983). W. J. Jackson, H. E Kuhfuss, and T. E Grey, SOC. Plastics Ind. Tech. Conf. 17-D, l(1975). W. J. Jackson and H. E Kuhfuss, I . Polym. Sci. 14,2043 (1976). E E. McFarlane, V. A. Nicely, and T. G. Davis, Cont. Topics in Polym. Sci. 2, 109 (1977). K. F. Wissbrun, Brit. Polym. I., 163 (1980). Y. Chan, J. L. White, and Y. Oyanagi, 1. Rheol. 22,507 (1978). G. B. Jackson and R. L. Ballman, SPE I., 16, 1147 (1960). G. Menges and G . Wubken, SPE ANTEC Prepr., P519 (1973). M. R. Kantz, N. D. Newman, Jr., and F. H. Stigale, 1. Appl. Polym. Sci. 16, 1249 (1972). Z. Tadmor, I . A w l . Polym. Sci. 18, 1753 (1974). D. McNally, Polym. Plast. Technol. Eng., 8, 101 (1977). L. A. Goettler, Mod. Plast., 48, 140 (April 1970). L. A. Goettler, MonsantolWashington Univ. ARPA Report HPC 72-151 (1974). G. Calundann, U.S. Pat. No. 4, 184,996 (1980) (to Celanese). K. Oda, J. L. White, and E. S. Clark, Polym. Eng. Sci. , 16,585 (1976).

796 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1983, Vol. 23, No. 14