Microstrcture development during the injection molding of PET/LCP blends

Microstructure Development During the Injection Molding of PET/LCP Blends

E. TURCOTT, K. T. NGUYEN, &A. GARCIA-WON

Industrial Materials Institute National R e s e a r c h Council Canada

75 De Mortagne, Boucherville, Quebec Canada J4B 6Y4

A liquid crystal polymer (LCP) was blended with polyethylene terephthalate (PET) in different concentrations to improve the barrier properties of PET in injection stretch blow molded bottles. The improvement depends on the microstructure developed at various stages of the process. In this work, the emphasis is on the injection molding stage of the preform. The characteristics and number of morphological layers were directly related to the amount and type of LCP in the blend and the location within the preform. It was found that at 10% LCP, three morphological layers were found across the thickness of the part, while at 30% LCP, five morphological layers could be identified. The LCP structure can be classified into four types: droplets, thick rods, thin fibrils and ribbons. Each morphological layer is made up of one or more types of structures. The evolution of one type of structure to another depends on the particular flow regime ongoing at various locations in the mold. This microstructure development, during the flow, was examined in detail.

INTRODUCTION

n recent years, a si@icant amount of interest has I been generated on the study of liquid crystal polymers (LCPs) and their blends with thermoplastics. Several books on the subject have been published (14). and a number of articles and reviews have ap- peared in the literature. For a comprehensive review, the interested reader is referred to (5) and to a recent article by Isayev (6). which includes an extensive list of references.

In addition to the study of their rheological and mechanical properties, special attention has been devoted to the study of the morphology induced by different processing methods. For the case of pure LCPs, the classical works of Sawyer and co-workers (7, 8). as well as those from other groups (9-11), should be considered. With regard to studies done with LCP/thermoplastics blends, the contributions of Kiss (12) and Joseph et al. (13) are noteworthy.

The work related to the morphology of these blends is a rapidly growing subject in the literature. Most of this research has focused on the study of samples processed by extrusion followed by extrudate drawing or melt spinning, which involves a sigmficant extensional flow component, see, for instance (14-18). The predominant morphological feature observed in these samples was the presence of long LCP fibers and fibrils embedded in the thermoplastic matrix. However,

in some cases a core region with fine droplets of LCP was observed when low extensional ratios were used. The LCP concentration in the blend and its temperature dependent viscosity ratio are other factors influ- encing the formation of fibrils [see (4, 19, 20)l. Studies concerned with the morphology of injection molded parts with LCP/thermoplastic blends are less preva- lent. Authors analyzing injection-molded parts have reported distinctive skin-core morphology (2 1-25) with different distributions of LCP fibrils in the skin.

The tendency of LCPs to form fibrous structures and their low melt viscosity are two of the reasons why they act as reinforcement and processing aids when added to a thermoplastic matrix (5). Because of their outstanding properties, thermoplastic/LCP blends can be considered for applications in areas such as electronic (molded components with excellent dimensional stability and dielectric properties), auto- motive (ease of processability and high temperature resistance) and packaging (improved permeability and strength) (26). Nevertheless, to achieve the desired properties, the processing technique has to allow for a controlled orientation of the LCP in the molten state and the subsequent freezing of the desired morphology. In this context, several aspects have to be considered. These include the conditions required for the LCPs to form the fibrillar reinforcing phase and the in- fluence of the fibril orientation on the final properties. For packaging applications, PET is the material of

POLYMER ENGiNEERlNG AND SCIENCE, APRll2001, Vol. 41, No. 4 603

E. Turcott, K. T. Nguyen, and A. Garcia-Rejon

choice and the addition of LCP has shown to improve its barrier properties (27). This has to be achieved during the injection (molten state) and stretching and blowing (rubbery state) of a preform. Therefore, it is important to establish which processing step, injection or stretching and blowing of the preform, has the most important effect on the properties of the final Part-

In this work, the emphasis is on the morphology development of the PET/LCP blends during the injection molding of the preform. Special interest was devoted to the type of LCP and the LCP content. The microstructure of the pellets and of the molded parts was examined using a scanning electron microscope (SEM). The microstructure observations were correlated with the deformation patterns occurring during the flow in the mold.

EXPERIMENTAL

A typical blow molding grade polyethylene terephthalate (PET-Eastman 9921W) was used as the thermoplastic matrix. ’Ifivo PJ3T/LCP blends having 10 and 30% LCP (T’mel,g = 277°C) content were hot blended in a twin screw extruder and subsequently pelletized (28). In addition, a blend having 30% high temperature LCP (Tmelting = 320°C) was also studied. The rheological properties of the PET/LCP blends were mea- sured using both small amplitude oscillatory shear (Rheometrics Mechanical Spectrometer) and capillary flow [Rosand RH-7, capillary rheometer (29)l. Figure 1 shows the complex (q*) and the shear viscosity (q) for the PET and PET/LCP blends. As expected the viscosity of the blends decreases with increasing LCP content. The u3P orients in the flow field, the droplets are stretched, the flow resistance decreases and the viscosity is lowered. The results also show that the LCP blends do not follow the Cox-Merz rule. Furthermore, the temperature dependence of the blends decreases

m. 2. Storage (G’) and loss (G”) moduli: 1 0 & 30% PET/L&P blends T = 270°C.

10000

.- - 3 -0 g 1000

c c! 9 100 v) v)

‘D c m c

0) 0)

10

L? 3 v)

1

1000

u) m e P F v

8 : 100

u,

z v)

b - F X

P

Y

- E, 0

10 0.1 1 10 100 1000 1oM)o

Frequency (0) or Shear Rate (y) (l/s)

Flg. 1 . complex (.I *) and shear (q) viscosity PET: & PET/LcP blends T = 270°C.

with increasing LCP content. This could be also asso- ciated with the fact that at 280°C the LCP is already in the molten state.

m u r e 2 shows the storage (G’) and loss (G”) moduli for the 1Ooh and 30% PET/LCP blends. The results show that, over the entire frequency range, the vis- cous component (tan 6 > 1) governs the material be- havior. However, the storage modulus (G’) of the 30% LCP blend is higher than the 10% blend, particularly at low frequencies.

m u r e 3 shows the Entrance Pressure Drop (Po) and the shear viscosity (q) as a function of Shear Stress (7,) for the PET and the PET/LCP blends. The dual bore capillary rheometry technique (Rosand) (29) was used, together with the converging flow data analysis of Cogswell (30). Notice that Po is higher for the 30% LCP blend than for the 10% blend. Since the entrance pressure drop can be related to the extensional be- havior of polymer melts (30). it can be concluded that

G - 10% LCP

G - 30% LCP

0.1 1 10

Frequency, o (radls)

100

604 POLYMER ENGINEERING AND SCIENCE, APRIL 2001, Vol. 41, No. 4

Microstructure Development

FTg. 3. Viscosity [q) and entrance pressure drop (Pd: PET & PET/ LCP blends T = 270°C.

h

Q e n le+6 g n 2 3 rn rn

le+5

8 c c w e c L 0

-& 10% LCP - q A 10% LCP - P"

the extensional viscosity is highest for the 30% LCP blend. This fact is important since it is widely ac- cepted (21) that in the case of injection molding of LCPs, different composite structures are formed through the thickness of the part. There are highly oriented fibrils in the skin layer and undeformed droplets in the core. These differences are related to the difference in deformation and temperature history experienced by the material during processing. Exten- sional flow is much more effective than shear to in- duce orientation and thus produce LCP fibers. The material near the skin undergoes extensional flow due to the fountain flow effect at the melt front. This effect is compounded with the strong shear present near the mold wall. The increase in extensional viscosity is most probably due to the orientation of the LCP fibers.

The preforms were molded using an Aoki (SBIII- 100H-15) one step injection stretch blow molding machine. The preform (Rg. 4) has the following dimensions: L = 85 m; D, = 28 mm; and havg = 3.7 mm. The material is injected at the bottom of the preform through a sprue gate (Dg = 1 mm]. The operating conditions were as follows: Tmelt = 275°C: Tmold = 22°C; and tfiu = 1.65 s. These injection conditions cor- respond to those that produced a good bottle after stretching and blowing. It should also be noted that the high temperature LCP blend was injected at 300°C. During the injection, the shear rate varied from approximately 500 l/s to an average value of 80 1 / s on the preform's body.

1 e+4 le+5 le+6

Shear Stress (Pa)

MICROSTIIUCTURAC EXAMINATIONS

The microstructure investigation of the preform was done using a Jeol JSM-T220 scanning electron microscope (SEM). All samples were cryo-fractured in liquid nitrogen to produce the observation surfaces and sputter-coated with a thin layer of Au:Pd (-15 nm). Figure 5 depicts three regions identified for the study

of the microstructure: the gate region at the preform's bottom, the curve region between the bottom and the vertical wall of the preform, and the preform's vertical wall. Cross sections of the wall and curve regions were produced in order to coincide with the parallel (11) and perpendicular (I) to the direction of flow within the preform (Fig. 41.

Pallet

The microstructure of the original pellets was used as reference. m u r e 6 shows a typical cross section image of the 30% LCP content pellet with an average drop size of - 6 pm. The 10% LCP blend average drop size is - 4 pm. Both, the 10% and 30% LCP pellets presented a slightly finer fraction of spherical drops at the edge.

Gate Region

The 30% blend preform was used for the study of the microstructure in the neighborhood of the injection gate located at the bottom of the preform. After

FTg. 4. S E M cross sectional uiews fie., I] and I to the direction ofmw indicated by the m w ) and preform dimensions.

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E. Wcott , K. T. Nguyen, and A. Garcia-Rejon

Flg. 5. Preform cross section: Gate, transition d f u u y developed regions.

mold ejection, this preform was immediately quenched in liquid nitrogen to preserve the internal morphology. A general cross section view of the gate region at the preform bottom is shown in Fig. 7, where the injection point is located at the lower right comer. Several

distinctive layers could be identified throughout the thickness of the preform bottom: outer skin, outer transition, core, inner transition and inner skin layers. The morphology of each of these layers can be correlated to the flow in this region as described below.

606

Fig. 6. Morphology within the original pellets: 30% PET/LCP blend

POLYMER ENGINEERING AND SCIENCE, APRIL 2001, Vol. 41, No. 4


Ffg. 7. General view of gate region [injection point at lower right comer): 3096 PET/LCP blend.

As the material goes through the gate, it undergoes a dominantly elongational flow. The LCP droplets are stretched resulting in long, thick fibers as shown in Fig. 8. Some of these fibers turn the comer quickly and remain near the outer surface. These undergo shearing and elongation, retaining their shapes or evolving into finer fibrils and thus forming the outer skin layer.

Since the point on the internal wall of the mold op- posite to the gate is a stagnation point, the remaining fibers slow down as they move on toward this point. The flow character in this region is changing from uniaxial elongation to biaxial stretching. Some of the fibers turn toward the curve region before reaching the wall. In this layer, the shearing is weaker and therefore not sufficient to maintain the fiber shape. As they slow down, two phenomena may occur. First, hydrodynamic instability sets in giving rise to wavy in- terfaces; as the disturbance grows, the fiber breaks up into drops: this is the capillary wave mechanism. Second, capillary pressure causes the leading end of the fiber to become spherical; this spherical end will then be pinched off forming a drop. This second mechanism of breakup, called "end-pinching," under transient condition has been demonstrated by Stone et al. (31). The end-pinching mechanism was shown to occur on a shorter time scale than capillary wave mechanism and thus was more likely in this case. The result is the type of morphology shown in Fig. 9,' de- noted as the outer transition layer, which is estimated to be of the order of 20% of the thickness and consists

of wavy rods and interconnecting drops. In the core layer, the shearing is essentially absent and the fiber breakup is complete. This layer is characterized by the presence of spherical inclusions of LCP as shown in Fig. 10 and covered only lo?? to 20% of the thickness. This layer's morphology is similar to that of the initial pellet (see Flg. 6) but with a finer average drop size (- 2 km).

The rest of the fibers will move on toward the inner surface before turning toward the curve region to form another transition layer and the inner skin layer. The inner transition layer, Fig. 11, shows the presence of rods, joint droplets and ribbons. The joint droplets are probably from the final stage of the breakup of the fibers. It should be noted that the flow here is biaxial stretching in nature. Therefore, it is possible that the ribbons come from the biaxial deformation of the droplets. Another possible explanation for the formation of ribbons will be discussed later.

The inner skin layer consisting of fine fibrils and flat ribbons was identified close to the edge of the internal surface of the preform bottom. However, some fluctu- ations and narrow inclusions of transition layer morphology (i.e. rods and interconnected drops) within this skin layer were also detected. The presence of fine fibrils, somewhat away from the edge, is probably due to the stretching of droplets from the fountain flow as well as from the high shearing near the mold wall. The above mentioned complex morphology is shown in Flg. 12, where the edge of the inner skin layer may be seen at the left of the micrograph. Since the flow in


608


Flg. 8. Gate region (morphology close to the injedion point): 30% PET/LcP blend

Ffg. 9. outer transition layer of gate region: 30% PET/LCP blend.



Rg. 10. Core layer of gate regiorx 30% PE7'/L.CP blend

Rg. 1 I. Inner transition layer of gate region: 30% PE"/L.CP blend



F~LJ. 12. Inner skin layer of gate region (edge hated at the &?I: 30% PET/L.CP blend.

the preform bottom (gate region) is &symmetric, a view in any plane of symmetry should be somewhat similar. Moreover, as the directionality of flow is not well defined relative to the coordinate of the picture, features corresponding to the 1 1 and I directions may be found in the same cross section.

Curve Region This is the region where the flow changes its nature

from a predominantly biaxial stretching to predominantly shear. Figure 13 depicts a general view of this region. It was found that a uniform flow regime is rapidly established and the five distinctive layers discussed above were better defined. In addition, the relative proportions of each of these layers across the thickness were completely different. It was observed

that both the skin and the transition layers were of the same order of magnitude as those at the bottom of the preform (i.e. 200-300 pm each]. However, in this case, each of these layers represented only 10% of the total thickness. Furthermore, the inner and outer skin layers of the preform became alike and the same thing happened for both transition layers. The outer skin layer (Fig. 14) is characterized by large ribbons interspersed with h e fibrils. While the outer transition layer consists of smaller ribbons and larger fibers or interconnected drops (see Fg. 15). Finally, the most significant difference was detected in the core layer (Rg. 16) which covered between 60% and '70% of the cross section. The drop size in the core layer (- 4 pm) is somewhat between that in the pellets (- 6 pm) and the one in the gate region core (- 2 pm), indicating

61 0

Fig. 13. Overall view of the m e region. 30% PET/U=P blend.

POLYMER ENGINEERING AND SCIENCE, APRIL 2001, Yo/. 41, No. 4


Flg. 14. Curue region (outer skin layer): 30% PET/LCP blend.

Flg. 15. Curve region (outer transition layer): 30% PET/LCP blend

POLYMER ENGINEERING AND SCIENCE, APRIL 2001, Vol. 41, No. 4 61 1

E. Turcott, K . T. Nguyen, and A. Garcia-Rejon

Flg. 16. Curve region [core layer): 30% PET/LCP blend-

that some coalescence has occurred. It should be noted that the thickness of the preform bottom is approximately 1.8 mm whereas the vertical walls thickness is 3.7 mm (Fig. 4). Therefore, although the ab- solute sizes of the skin and transition layers are similar in the gate, the curve and the fully developed regions, their relative dimensions are smaller.

Fully Developed Region The 30% LCP preform in the fully developed region

also presents a non-uniform morphology across the wall thickness. The five different morphological layers could be detected as well. In this case, the inner and outer skin layers of the preform consist of very fine fibrils and ribbon like morphology. This morphology is shown in Fig. 17 corresponding to the 11 view. The exterior edge is located at the left of this image. w e 18 shows the I view of the 30% LCP blend where the exterior edge can be seen at the top right comer of the micrograph. In this image, the formation of ribbon like elongated sheets may be clearly identified.

Two different transition layers could be detected between both skin layers and the core layer of the preform. The morphology of these transition layers consists of rods and interconnected drops as in the previously mentioned regions. Similarly, the predominant morphology in the core layer is also formed by spherical LCP inclusions in the PET matrix. This process-induced morphology in the skin and transition layers will tend to enhance, among others, the mechanical and barrier properties of the preform, and consequently, the properties of the bottle.

Upon examination of the 10% LCP blend preform, only three layers could be identifled, two skin layers of - 50-70 Fm each and one core layer. The outer skin layer is illustrated in Fig. 19 (flow direction or )I view). This morphology consists of clearly defined rods and a few interconnected drops, similar to the so-called transition layer for the 30% LCP blend. Figure 20 shows the I view of the same sample where the exter- nal surface edge can be seen in the top left comer of the micrograph. The core layer has a similar morphology to the one found in the original pellet, i.e. spherical drops of LCP dispersed in the PET matrix. A difference between the size distribution (- 2 pm) in the preform and the original pellet (- 4 km) was also detected for the 10% LCP blend.

Finally, with regard to the 30% high temperature LCP ("me,,g = 320°C) a uniform morphology across the preform wall in both the ( 1 and I views was found. The SEM observations showed that the LCP phase could not melt at the processing temperature (300°C) and its original pellet morphology (spherical drops of LCP inclusions) did not evolve durhg the injection of the preform. This may explain the difficulties encoun- tered during the stretch-blow molding attempts done with this type of blend.

F o d o n of Ribbons

One of the distinct morphological features identified in this study is the formation of ribbons. A possible explanation for this microstructure is the biaxial stretching of the droplets. If the ribbons are formed by biaxial stretching of droplets, they should be present

61 2 POLYMER ENGINEERING AND SCIENCE, APRIL 2001, Vol. 41, No. 4


Fig. 17. M y developed region (outer skin layer, view 11 to the direction ofjbw): 3096 PET/LCP blend.

Hg. 18. Fury developed region (outer skin layer, View I to the direction ofmw): 3096 PET/LCP blend


614

E. Ttcrcott, K. T. Nguyen, and A. Garcia-Rejon

Q. 19. Fuuy developed region [outer skin layer, uiew )I to the direction offiw): 10% PET/LCP blend

Q. 20. Fuuy developed region (outer skin lager, uiew i to the direction offiw]: 1096 PET/LCP blend


Microstnrcture Development

Fig. 21. Detail image of ribbons (outer skin layer, view i to the direction ofmw): 30% PET/LCP blend.

in blends of all compositions. However, these ribbons are absent in the ZO?! LCP blends. On the other hand, the presence of ribbons is clearly demonstrated in Rg. 21 for the 30% LCP blend. It is therefore speculated here that the ribbons are actually formed by the coalescence of fibrils in the skin layers. Figure 22 illus- trates what appears to be an ongoing process of coalescence of the fibrils in the outer skin layer of the preform (30% LCP blend). It is conceivable that in the 30% LCP blends, the fibril concentration is much higher and therefore close enough for the coalescence to take place. The formation of the ribbons favors an improvement in the barrier properties as depicted in Rg. 23.

CONCLUSIONS

The characteristics and number of morphological layers were directly related to the amount and type of LCP in the blend and the location within the preform. The relative size of each layer thickness throughout the cross section was also related to preform geometry and the location with respect to the injection gate. The injection process generated a five layers (30% LCP) or three layers (1 0% LCP) structure across the wall of the preform when the low melting point LCP was used. In contrast, the high temperature LCP blend resulted in a d o r m morphology similar to the one in the original pellet.

The original LCP droplets in the pellet formed long thick fibers under elongational flow through the gate. These fibers maintained its form or deformed slightly resulting into thicker rods, wavy rods from hydrodynamic instability or interconnected drops present in the transition layers. In the core layer, these rods broke up completely into drops of different sizes under hydrodynamic instability and thermal effects. The drop size distribution depends on the position along the flow length as coalescence may occur. The most complicated structure occurs in the skin layers where fine fibrils interspersed with ribbon-like structures can be identified. The fine fibrils were probably formed by the stretching of droplets due to the fountain flow effect. It is speculated that the ribbon-like structure was formed by the coalescence of elongated fibrils since they existed only in high concentration LCP blends. The presence of ribbon like morphology in the injection-molded preform, is an important component responsible for the enhancement of its barrier properties.

ACKNOWLEDGMENTS

The authors would like to thank: R. Lusignea, Superex Polymer Inc., Waltham, MA, for supplying the PJZT/LCP blends and for many interesting discus- sions; C. DeGrandpre and MA. Rainville for their in- sight during the molding trials.


E. lbcot t , K. T. Nguyen, &A. Garcia-Rejon

Flg. 22. Ribbon formation (outer skin layer, view I I to the direction offlow): 3096 PET/LCP blend.

0.01

0.001

61 6

- 9 - 0

,

I I I

0 5 10 15 20 25 30 35

LCP Content (YO) Flg. 23. Oxygen permeability of preforms molded with 1096 and 3096 PJCT/LCP blends and pure PET.

POLYMER ENGlNEERiNG AND SCIENCE, APRiL 2001, Voi. 41, No. 4


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Microstrcture development during the injection molding of PET/LCP blends