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Weldline Morphology of Injection Molded Modified Poly(phenylene-oxide)/Polyamide-6 Blends.

KOOKHEON CHAR [*]

The weldline morphology of modified-poly(phenylene-oxide)/polyamide-6 blends has been investigated. A distinct contacting layer consisting of fine spherical particles was observed from the V-notch at the surface to the center of the molded part for the low viscosity ratio blend. In contrast, such small particles were not found and a slight deformation was observed near the part's surface for the high viscosity ratio blend. The weldline morphology was found to be dominated by the deformation, the breakup, and the relaxation of the dispersed modffied-poly(phenylene oxide) phase. The effect of injection conditions on the weldline morphology has also been investigated. The morphology of the meldline was quite complicated. Distorted multi-layered structures were observed. It was found that those structures arise from the subsequent flow after merging of the two flow fronts. Weldlines and meldlines have been studied separately, and their formation mechanisms were found to be basically similar.

INTRODUCTION

Among the many defects found in injection molded parts, weldlines are the most prevalent, occurring in most injection molded parts except those of very simple geometry [1,2]. Weldlines are formed by the merging of two melt fronts originally divided either in the runner system or in the mold cavity. The formation of a weldline is regarded as one of the most undesirable phenomena in injection molding processes, since it results in poor appearance as well as poor mechanical properties. It is well known that raising both the melt temperature and the mold temperature can minimize such troubles. High temperatures increase the diffusion at the interface between the two melt fronts and the diffusion time during which the melt fronts penetrate across the interface, thereby making it possible to form a strong bond between the melt fronts [3-7]. In polymer blends, morphology is another critical factor affecting weldline characteristics. Therefore, in order to understand the weldline characteristics of polymer blends, it is necessary to have a clear knowledge of weld zone morphology and its formation mechanism during molding processes.

There have been several studies on weldlines in injection molded polymer blends, but only a few have focused on weldline morphology. Brahimi et al. [8] investigated the weldline strength of injection molded HDPE/SBS/PS blends. They observed that weldline strength decreases to a larger extent in the case of polymer blends compared with the corresponding homopolymers, and that the addition of a small amount of block copolymer significantly improves weldline strength. Mekhilef et al. [9] calculated the weldline strength of PC/PE blends using both the diffusion and the Flory-Huggins theories. The result predicted the effect of injection temperature on the weldline strength of the blends. However, they did not take into account the effect of morphology in the weld zone. Fellahi et al. [10-12] investigated weldline morphology and weldline strength of injection molded HDPE/PA6 blends. They found that the morphology in the weld zone is quite different from that in the bulk, but they could not directly observe the sh ape and size of the dispersed phase in the weld zone. Using SEM (scanning electron microscopy) for morphology observation, they could not find evidence for the existence of a dispersed domain in the weld zone. With the aid of other techniques such as TEM, DSC and XPS, they indeed found a very fine dispersed phase in the weld zone. They also observed that the thickness of the weld zone is approximately twice that of the skin layer, from which they concluded that the weldline is generated by the merging of two flow fronts.

In the present work, we investigated the detailed morphology in the weld zone of injection molded modified-poly(phenylene oxide) /polyamide-6 blends. The effects of injection conditions have also been investigated, and the mechanism of morphology formation in the weld zone is discussed.

EXPERIMENTAL

Materials

The blend components used in this study are polystyrene (PS), poly(phenylene oxide) (PPO), and polyarnide-6 (PA). PS is Starex HF2660 from Cheil Industries Inc. (Mn = 12 X [10.sup.4] g/mol), PPO from Asahi Chemicals (high flow grade, P402), and PA from Kolon Chemicals (Mn = 3 X [10.sup.4] g/mol). Although each blend consists of three components, it has a pseudo binary phase since PS and PPO are quite miscible. PS and PPO form a single dispersed phase, while PA forms a matrix phase. Two blends were prepared, each consisting of 20 wt% of the dispersed phase and 80 wt% of the matrix phase. The two blends differ in the nature of the dispersed phase. One consists of 90 wt% of PS and 10 wt% of PPO, while the other consists of 10 wt% of PS and 90 wt% of PPO. The viscosity ratio of the former is on the order of one, and that of the latter is on the order of ten at high shear rates, which is expected to be close to the condition of the melt flow in a mold cavity (Fig. 1). The former is classified as a low viscosity ra tio blend (denoted by MPPOL/PA) and the latter as a high viscosity ratio blend (denoted by MPPO-H /PA).

Compounding

Prior to the mixing operation, the sample mixtures were dried overnight at 90[degrees]C in an air circulating oven to minimize the hydrolytic degradation of PA during processing. Blending was carried out in a Berstoff intermeshing, co-rotating twin screw extruder with a screw diameter of 40 mm and a length-to-diameter ratio of 34.5. Barrel temperature was 260[degrees]C, screw speed was 250 rpm, and the feeding rate was 50 Kg/hr.

Injection Molding

Prepared pellets were dried in an air circulating oven at 110[degrees]C for 24 hours to remove residual moisture. Dried pellets were then fed into a Gold-Star[TM] injection molding machine with a clamping force of 100 tons. The geometry of the mold cavity is shown in Fig. 2, together with the location of morphology observation. In this study, we investigated the morphology of a weldline as well as a meldline. Type I is for the weldline, and is generated by merging of two flow fronts at an angle of nearly 0[degrees]. That is, the flow fronts advance In opposite directions before merging. Type II in Fig. 2 is for the meldline. The meldline is generated by the merging of flow fronts separated by inserts or obstacles in a cavity. The flow fronts before merging are generally flowing in the same direction. Injection temperature and speed were varied, while all other conditions were kept constant. The mold temperature was 80[degrees]C, injection time and cooling time were set for 6 and 25 seconds, respectively, scre w rotation speed was 100 rpm, and hold pressure was not applied.

Morphology and Image Analysis

Flat surfaces at locations marked in Fig. 2 were made for morphology analysis. A Reichert Jung Super Nova microtome equipped with a diamond knife was used to obtain cross sections of the molded parts. The microtomed samples were then immersed in a chloroform solution to remove the dispersed phase, and then gold coated with a BIO-RAD coater prior to morphology examination with a JEOL 840A scanning electron microscope operated at 20 kV. A fully automatic image analyzer (Image Pro+) was used to measure the aspect ratio of the dispersed phase. About 100 to 300 particles were averaged for each SEM micrograph, and the aspect ratio were treated as a number average. Detailed experimental conditions are available elsewhere [13, 14].

RESULTS AND DISCUSSION

Weldline

Weldline morphology of the low viscosity ratio blend (MPPO-L/PA) is schematically shown in Fig. 3(a). SEM pictures corresponding to each position marked in Fig. 3(a) are shown in Fig. 4 and Fig. 5. A V-notch is clearly seen close to the part surface, known to be generated by entrapment of air in the mold cavity or by post-shrinkage of the molded part [3]. A distinct layer is observed along the center of the weld zone from the V-notch at the surface to the center of the molded part. As the symmetric phase morphology is observed across this layer, it seems that this is the contacting surface of the two flow fronts generated at the time of weldline formation. In the contacting layer, the size of the dispersed phase is much smaller than that observed away from the layer, and the shape is almost spherical even quite close to the part surface. The contacting layer is thought to be generated by the breakup of the dispersed phase that was deformed by elongational flow at the flow front in addition to the relaxation during the cooling process, which will be discussed later. Right next to the contacting layer close to the V-notch, the dispersed phase deforms to the thickness direction as shown in S of Fig. 5 (it should also be kept in mind that the dispersed phase near the contacting surface is not deformed and is spherical; see S in Fig. 5). The dispersed phase close to the surface deforms to the flow direction as it moves away from the V-notch.

The weldline morphology of the high viscosity ratio blend (MPPO20/PA80-H) is shown in Fig. 3(b), and SEM pictures corresponding to each position marked in Fig. 3(b) are shown in Fig. 6. A V-notch is also observed, and it is clear that the degree of deformation of the dispersed phase is much less than that observed in the low viscosity ratio blend. Only domains near the part surface are deformed slightly, and particles elsewhere are rarely deformed. This is because the highly viscous particles are more resistant to deformation under a given flow field.

Figure 7 shows the effect of injection conditions on the weldline morphology for low viscosity ratio blends. All pictures were taken at 0.3 mm beneath the part surface. Clearly visible is a contacting layer in which the dispersed phase is small and spherical. The contacting layer becomes more pronounced at low injection temperatures and high injection flow rates. With a higher injection flow rate and at a lower injection temperature, the force acting on the dispersed phase becomes larger, and the particles become more deformed and are more easily broken up. The size of the dispersed phase, in this case, becomes smaller while the band of the contacting layer increases.

Next to the contacting layer, the particles are deformed in the thickness direction and the size of the dispersed phase is much larger than that in the contacting layer. The degree of deformation near the contacting layer becomes more pronounced as the injection flow rate increases and the injection temperature decreases. For high viscosity ratio blends, no contacting layers are present, since deformation during a filling process is insufficient for breakup of hard particles to take place.

Figure 8 shows the aspect ratio of deformed particles as a function of distance perpendicular to the contacting surface for various injection conditions. With a higher injection flow rate at lower injection temperatures, the aspect ratio generally increases. At injection flow rates of both 123 [cm.sup.3]/s and 62 [cm.sup.3]/s, the degree of deformation looks similar. This may be because more heat is generated owing to viscous dissipation as well as a higher melt temperature at an injection rate of 123 [cm.sup.3]/s. Comparing Fig. 8(a) and Fig. 8(b), it is clearly seen that particles closer to the part surface are more deformed than those away from the surface. This may be due to relaxation during cooling. The deformed dispersed phase relaxes during the cooling process. Near the part surface, the melt solidifies much faster and has less time for relaxation, keeping a high degree of deformation. However, as the melt moves away from the surface, it can have more time to relax, and a lower degree of deformation is possible. It is observed that the dispersed phase deforms to a larger extent at lower injection temperatures in Fig. 8(c) and Fig. 8(d). The viscosity of the continuous phase is higher at lower temperatures, causing the dispersed phase to experience a greater force and larger deform. In addition, there is less time for relaxation of the dispersed phase to occur. The results for a high viscosity ratio blend are given in Fig. 8(e). The dispersed phase is less deformed than that of the low viscosity ratio blends, as mentioned previously. In contrast to the low viscosity ratio blend, the dispersed phase closer to the weldline is deformed to a greater extent. The effect of injection flow rate on the deformation of dispersed phase is the same as in the case of the low viscosity ratio blend.

A schematic diagram demonstrating the mechanism of weldline formation for a low viscosity ratio blend is given in Fig. 9. During the filling stage, two flow fronts advance toward each other. The dispersed phase near the flow front is highly deformed owing to the elongational flow. Since the elongational rate is highest at the free surface of the flow front, particles at the free surface are deformed most. After the conjunction of the two flow fronts, a V-notch is formed by air entrapment. Once the V-notch is formed, there is no flow and the deformed dispersed phase begins to relax. At this stage, the highly deformed dispersed phase breaks up by the capillary instability mechanism, and forms a contacting layer. Closer to the surface, relaxation is not as pronounced and the dispersed phase maintains the deformed state. At the center of the weldline in the thickness direction, the dispersed phase relaxes easily and has a spherical shape. In the case of a high viscosity ratio blend, the dispersed phase near the flow front slightly deforms during the filling stage, and breakup of the dispersed phase is not possible during the cooling stage. Only relaxation is possible for the high viscosity ratio blend during the cooling stage.

Meldline

A schematic illustration of meldline morphology for a low viscosity ratio blend is shown in Fig. 10. There are, however, some differences between meldlines and weldlines. In the case of a meldline, there also exists a contacting layer along the center of the weld zone; however, it is severely distorted and the resulting morphology is much more complicated. It is also worth noting that the contacting layer is surrounded by multiple layers of different morphologies. Aside from the contacting layer, there exists another layer in which the size of dispersed phase is larger than that found in the contacting layer. This layer appears to be connected to the subskin layer away from the weld zone. This layer is again surrounded by another layer with smaller particles, in which the size of the dispersed phase is slightly larger than the one in the contacting layer. These layers are finally surrounded by the core region in which the dispersed phase is quite large and spherical.

In order to understand the formation mechanism of these multiple layers, the morphology has been investigated for two cases as described in Fig. 11. In case I, morphology was taken from a flat surface at a position 1 mm from the insert, unlike the former case in which morphology was taken at a position 3 mm from the insert. Since the location of observation is very close to the insert in case I, the melt at this position solidifies as soon as the two flow fronts contact each other, and there is little effect on subsequent flow. In case II, the flow was stopped artificially when the melt passed approximately 3 mm from the insert after the merging of the two flow fronts. The flat surface for morphology analysis was taken at a position 3 mm from the insert. There is no subsequent flow in both cases as in the case of the weldline. The morphology in both cases was found to be similar to that of the weldline: a straight contacting layer and no multiple layered structures. From this observation, it is believed that the distorted multi-layered structure in the meldline is formed by the subsequent flow after the merging of the two flow fronts. Consequently, at the instant of merging of two flow fronts, the morphology of the meldline is thought to be quite similar to that of a weldline. After merging of the flow fronts, subsequent flow takes place to fill out the mold, and this is believed to cause the distorted multi-layered structure. The effect of injection molding conditions on the morphology of a meldline was found to be similar to that of a weldline. That is, domains near the V-notch deformed more at higher injection speeds and lower injection temperatures.

CONCLUSIONS

Weldline morphology of modified-poly(phenylene oxide)/polyamide-6 blends has been investigated. For a low viscosity ratio blend, a distinct contacting layer was clearly located from the V-notch at the part surface to the center of a molded part. In the contacting layer, finely dispersed spherical particles were observed and are believed to be formed by breakup of the dispersed phase strongly deformed by an elongational flow at flow front. The particles close to the V-notch deformed in the thickness direction. The contacting layer was found to be more pronounced with a high injection flow rate and a low injection temperature. For a high viscosity ratio blend, the finely dispersed spherical particles observed in the case of the low viscosity ratio blend were not found, and a slight deformation was observed only near the part surface, and little deformation was observed elsewhere. In the case of the meldline, the morphology was found to be quite complicated. A contacting layer composed of fine spherical particle s was observed, but it was surrounded by multiple layers of different morphology. By careful investigation into the formation mechanism of this complex morphology, it was found that the distorted multi-layered structure arises mainly from the subsequent flow after merging of two flow fronts. Meldline morphology in controlled experiments, in which subsequent flow effects are virtually eliminated, was found to be quite close to the weldline morphology. We have noted that the formation mechanism is basically identical regardless of whether it is meldline or weldline. The weldline morphology is believed to be dominated by deformation, breakup, and relaxation of the dispersed phase.

ACKNOWLEDGMENT

Financial support of this work by CRM-KOSEF (2000), the Brain Korea 21 Project, is gratefully acknowledged.

(*.) Corresponding author.

REFERENCES

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(13.) Y. Son. K. H. Ahn and K. Char, Polym. Eng. Sci., 40. 1376 (2000).

(14.) Y. Son, K. H. Ahn and K. Char, Polym. Eng. Sci., 40, 1385 (2000).

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Article Details
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Author:SON, YOUNGGON; AHN, KYUNG HYUN; CHAR, KOOKHEON
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:Mar 1, 2001
Words:3276
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