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High modulus polypropylene: effect of polymer and processing variables on morphology and properties.

INTRODUCTION

Advances in the polymerization chemistry of polypropylene (PP) result in materials with improved stereoregularity, molecular weight, and poly-dispersity control (1). These advances have led to materials that give exceptional stiffness. It has long been recognized that processing variables can also have a significant effect on the material response of injection molded PP. Many studies have addressed the influence of processing conditions on the morphology and properties of injection molded PP (2-10). The influence of melt temperature, mold temperature, injection speed, and part thickness are all well established. In addition to material parameter influences, orientation and crystallinity in these moldings are governed by a number of competitive, interdependent dynamic processes. These are i) temperature dependent melt orientation and subsequent relaxation, ii) heat transfer between the hot melt and the cold mold walls, and iii) crystallization kinetics.

The present study develops quantitative characterization/correlation of material parameters, morphology, and mechanical properties in injection molded propylene homopolymer. It was of interest to quantitatively separate the contributions of crystallinity and orientation to flexural modulus and to provide a quantitative correlation of structure and properties. The ability to provide a quantitative link between morphological structure and properties provides a necessary framework for prediction of properties for resin development and specific molding applications.

EXPERIMENTAL

The material set used in this work is summarized in Table 1. The materials, propylene homopolymer, were characterized for melt flow rate (MFR) (11), molecular weight ([M.sub.w]), and polydispersity ([M.sub.w]/[M.sub.n]) (PI). Tacticity was characterized from both the solution 13CNMR pentad tacticities (MMMM) and the room temperature xylene soluble percentage (XSRT). All materials were compounded and pelletized on a Leistritz twin-screw extruder prior to molding, with addition of anti-oxidant and thermal stabilizer additive packages. Nucleators were not present.

Compression moldings were prepared on a platen press operated at 10 tons of pressure and at 260 [degrees] C (500 [degrees] F), with 12.7-cm (5-inch) square templates having a thickness of 0.3 cm (0.12 inch). The molding time was 5 min, after which the templates were TABULAR DATA OMITTED rapidly transferred to a cooling press maintained at [approximately] 50 [degrees] C (120 [degrees] F). Injection molded plaques were formed that were 7.62 cm (3 inches) wide, 25.4 cm (10 inches) long, and 0.3 cm (0.12 inch) thick. The gate cross section was [approximately] 0.5 cm (0.2 inch) square. Molds were prepared with melt temperatures of 204.4 [degrees] C (400 [degrees] F) and 260 [degrees] C (500 [degrees] F), with injection times of 2 s and an injection speed of 2.2 cm/s (0.85 inch/s). The mold temperature was held constant at 50 [degrees] C (120 [degrees] F). Test specimens for mechanical testing with dimensions 1.27 cm x 6.03 cm (0.5 inch x 2.375 inches) were cut from the plaques in locations both near and away from the gate. Figure 1 shows a schematic of the mold geometry and locations used for the test specimens. Flexural modulus measurements were performed on an automated Instron Model 4202 with a 5.08-cm (2-inch) span at 1.27 cm/min (0.05 inch/min). One percent secant modulus values are reported as an average of three replicates.

Density measurements were performed on void-free samples ([approximately] 50 mg) using the density gradient column technique (12). The column was prepared with a mixture of 2-propanol and water over a density range of 0.850 to 0.925 g/[cm.sup.3]. Calibration of the column was made with glass floats and allowed for an absolute uncertainty of approximately [+ or -] 0.0005 g/[cm.sup.3]. Volume fraction crystallinities, [[Phi].sub.c] were calculated from [[Phi].sub.c] = ([Rho] - [[Rho].sub.a])/([[Rho].sub.c] - [[Rho].sub.a]), where p is the measured density and [[Rho].sub.c] and [[Rho].sub.a] are the crystalline and amorphous densities. These values were chosen as 0.936 and 0.855 g/[cm.sub.3], respectively (13). The values chosen for [[Rho].sub.c] and [[Rho].sub.a] produce high-end values for [[Phi].sub.c] given the published spread for [[Rho].sub.a] and [[Rho].sub.c] (13). Small corrections to the calculated crystallinities were made to account for the presence of the [Beta]-crystalline phase (14) as evaluated by wide-angle X-ray diffraction. These corrections assumed a density of 0.922 g/[cm.sup.3] (13).

DSC scans were performed at heating and cooling rates of 20 [degrees] C/min. Samples of 5 to 6 mg, which were cut through the entire thickness of the compression molded part, were evaluated using a Perkin-Elmer DSC7 equipped with a subambient intercooler and purged with nitrogen. Calibration was performed with an indium standard at the same rates as the sample testing. Visual observations of the skin-core morphology were performed using cross-polarized light with a Leitz Aristomet optical microscope. Cross sections were cut to a thickness of approximately 10[[micro]meter], and the view was through the length of the part.

Wide-angle X-ray (WAXS) measurements were performed using a Scintag theta-theta goniometer modified for transmission measurements. A 0.2 [degrees]-divergence slit and 0.3 [degrees]-receiving slit were used, with lateral divergence of the beam limited by Soller slits. The beam was further collimated by a 1-mm pinhole at the sample position. Data were acquired as a function of the 2[Theta] angle using continuous scans at 1 [degree]/min and a 0.025 [degrees]-integration increment with the sample flow direction both parallel and perpendicular to the goniometer radius. Azimuthal intensity measurements were performed at fixed 2[Theta] angles for the 110, 040, and 130 planes of the [Alpha]-phase crystal modification of polypropylene (15), as well as the 300 plane of the [Beta]-phase modification (14). Azimuthal data collection was performed over the range of 0 to 360 [degrees] at 3 [degrees]-increments and 20-s counting times. Empty beam scattering was subtracted from the 2 [Theta] and azimuthal data, and normalizations for sample thickness, transmittance, and angle dependent absorption were applied. Amorphous scattering was subtracted from the aximuthal data by deconvolution of the 2 [Theta] scans, with amorphous halo orientation effects (16) taken into account empirically by assuming a linear change in amorphous scattering intensity within a quadrant. Relative estimates of the [Beta]-crystalline phase were made on the quadrant-averaged peak intensities integrated over the azimuthal angles and calibrated to an integral Turner-Jones index for unoriented systems (14). [K.sub.[Beta]] values ranged from 0 to 0.14. As with previous studies on injection molded PP (8-10), approximations to the c-axis orientation functions were calculated by Wilchinsky's method (17), assuming uniaxial symmetry as outlined by Samuels (18). Good agreement was obtained by analysis of either the 110/040 or 130/040 pairs, and the reported values are averages of the two analyses.

RESULTS

Macrostructure

Previous work on injection molded PP has indicated that the moldings are characterized by a skin-core macrostructure (4-6,9) and frozen-in orientations, which are sensitive to the molding conditions (8-10). Skin layers are generally associated with gradations of frozen-in orientation, which result from the coupling of the melt orientation and subsequent chain relaxation processes during cooling of the melt in the mold (8, 9). The orientation is effectively frozen-in when the temperature at any given location in the mold falls below the crystallization temperature.

Figures 2 through 4 show optical micrographs for representative materials studied in this work. These results illustrate the influence of gate location, melt temperature, and resin type on the observed macrostructure. All of the micrographs show a skin-core structure that is sensitive both to the molding conditions and material parameters. The area fraction of skin, [A.sub.s], is defined schematically in Fig. 5. Figure 2 shows the skin-core structure of sample H near and away from the gate at melt temperatures of both 204 [degrees] C and 260 [degrees] C. From Fig. 2, it is seen that [A.sub.s] decreases both with increasing melt temperature and with increasing distance from the gate.

Figure 3 shows the optical micrographs from Samples C and D. As noted in Table 1, these samples have similar molecular weight and distribution parameters, but differ in tacticity. Both samples show a decrease in [A.sub.s] with increasing distance from the gate; however, the skin-core morphologies at the same location are quite similar for the two materials. Figure 4 shows the skin-core morphologies of Samples I and B near and away from the gate at a melt temperature of 204 [degrees] C. Both samples exhibit high MFR but differ in the breadth of the molecular weight distributions (MWD). Sample I, with a broad MWD, exhibits much larger values of [A.sub.s], irrespective of the location from the gate.

Orientation

Figure 6 shows a representative X-ray azimuthal intensity distribution. The 110, 040, and 130 planes are associated with the [Alpha]-phase crystal modification (15), and the 300 plane is associated with the minority [Beta]-phase modification (14). Generally, the orientation of the crystallographic axes in injection molded PP is characterized by mixed a*- and c-axis orientations (8, 9, 18). This is indicated by the pronounced 110 intensity maxima in the meridian and equatorial directions in Fig. 6.

Figure 6 shows that the 300 [Beta]-reflection is also highly oriented. The c-axis orientation could not be determined directly for chain sequences participating in [Beta]-crystallites owing to a limited number of useful planes for characterization. However, Fig. 7 shows the negative orientation of the 300 plane of the [Beta]-crystallites qualitatively tracks the orientation of the 040 plane of the [Alpha]-crystallites. The presence of the [Beta]-form in these injection moldings is also indicated by the highly birefringent spherulitic regions seen in the optical micrographs. The internal spherulitic microstructure that gives rise to the high birefringence of the [Beta]-form has previously been detailed (19).

WAXS can give direct information on the orientation functions of the three crystallographic axes [with qualifications noted in the literature (17, 18, 20)]. Figure 8 shows the orientation balance of the crystallographic a*-, b-, and c-axis orientations by the orientation triangle formalism discussed in detail by Samuels (18) and Stein (20). The upper dotted line in Fig. 8 describes the limit of uniaxial c-axis orientation, while the lower dotted line describes the limit of uniaxial negative b-axis orientation. The data in Fig. 8 were taken at locations near and away from the gate at two different melt temperatures. These results give a composite representation of the orientation behaviors and indicates that the absolute value of the c-axis orientation is generally observed to decrease with increasing melt temperature and at locations away from the gate at both melt temperatures. These results, at the molecular level, are in agreement with the macroscopic skin-core results discussed earlier and are consistent with observations in the literature (8-10).

It has been commonplace to associate the skin thickness with the degree of orientation (4-6, 8, 9). With the understanding that the macrostructure actually contains a hierarchy of structure (4, 5), the relationship of the area fraction of skin ([A.sub.s]), as defined in Fig. 5, with the c-axis orientation function is shown in Fig. 9. Figure 9 shows that, although highly scattered, there is a rough correspondence between the measured area fraction of skin ([A.sub.s]) and the bulk averaged c-axis orientation.

Representative effects of stereoregularity on the volume fraction crystallinity ([[Phi].sub.c]) and the X-ray determined c-axis orientation function ([f.sub.c]) are shown in Fig. 10 and 11. Figure 10 shows that, at similar MFR and PI values (samples A and B), the effect of increased XSRT (decreased tacticity) is to reduce the volume fraction crystallinity of the molding, irrespective of the melt temperature and location from the gate. The XSRT/tacticity differences in these samples do not seem to have a strong effect on the frozen-in orientation, as indicated in Fig. 11. Similar conclusions were drawn from the comparison of skin-core morphologies of Samples C and D in Fig. 3. In contrast with the XSRT/tacticity dependence, MFR and PI changes have a dominant influence on the frozen-in orientation. This is illustrated by the representative data in Fig. 12 and 13. The frozen-in orientation is observed to increase both with decreasing MFR and increasing PI.

Structure Property Correlations

Figure 14 shows the flexural modulus of the compression moldings vs. the volume fraction crystallinity, [[Phi].sub.c]. In these compression moldings, which are relatively free of orientation, the modulus increases strongly with [[Phi].sub.c]. The solid line in Fig. 14 is a fit of the modulus vs. [[Phi].sub.c] relationship to a logarithmic dependence (21) for the quench-cooled compression moldings.

log(E) = K1 + (K2 * [[Phi].sub.c]) (1)

Although lacking in theoretical justification, this general functional dependence is useful for correlative purposes and bounded by limiting volume filling mixing rules as discussed shortly.

As an empirical means of investigating the role of orientation on flexural modulus, Fig. 15 shows the relation of [Log(E) - K1]/[[Phi].sub.c] to c-axis orientation for the injection moldings. This plot contains five data points per sample: one for the compression molded result ([f.sub.c] = 0) and four others for the mold locations near and away from the gate at both melt temperatures. According to the compression molding results discussed above, this type of plot should give a constant (K2) as a function of orientation if crystallinity is the only factor governing flexural modulus. The results from Fig. 15 show that orientation strongly influences the observed modulus. The empirical crystallinity normalization gives rise to a representation for flexural modulus in terms of the crystalline volume fraction and the frozen-in orientation. Figure 16 shows the analogous mechanical property correlation in terms of the area fraction skin, [A.sub.s], for selected samples.

Molecular orientation in injection molded parts will also invariably lead to mechanical anisotropy relative to the flow direction (4). When the crystallinity normalization is performed in an analogous fashion as in Figs. 15 and 16, the following parameter is defined:

CNDA = [(log([E.sub.in-flow]) - K1/log([E.sub.cross-flow]) - K1) -1] * 100 (2)

This parameter is intended to allow for an empirical comparison of the mechanical anisotropy, independent of crystallinity. The empirical crystallinity-normalized differential anisotropy (CNDA) describes the anisotropy in the crystallinity-normalized moduli less the anisotropy expected for an unoriented system (value of unity) multiplied by a scaling factor. The results are summarized in Fig. 17 and show the expected increase in the anisotropy at higher levels of orientation.

DISCUSSION

The characterization of the skin-core macrostructure (Figs. 2 through 4) is generally consistent with numerous investigations of injection molded PP (4-6, 9). Lower melt temperatures and locations near the gate give the largest skin-layer thickness. The quantitative relationship in Fig. 9 of the area fraction skin ([A.sub.s]) and the bulk-averaged c-axis orientation function ([f.sub.c]) shows a reasonable correspondence. This is somewhat surprising, given the expectation that for equivalent skin thickness, material dependencies should be expected to give rise to differences in orientation within the skin, and correspondingly lead to shifts in the [A.sub.s] vs. [f.sub.c] relationship. Similar arguments could be made regarding the variation in melt temperature. No systematic tendencies could be seen at the fixed orientation level (or alternatively fixed [A.sub.s]). However, differences in orientation within the skin layers may be one source of the scatter in Fig. 9. At low orientation levels, the [A.sub.s] vs. [f.sub.c] relationship in Fig. 9 falls below the line of [A.sub.s] = [f.sub.c]. This is expected, given that the actual orientation extends to depths below the observed skin layer (8, 9). At higher orientations, the [A.sub.s] vs. [f.sub.c] relationship must rise above the line of [A.sub.s] = [f.sub.c] because the orientation within the skin layers is less than 1. That is, even for 100% skin ([A.sub.s] = 1), the orientation is much less than unity (6, 8, 9).

The results of the skin-core measurements (Fig. 3) and WAXS orientation measurements (Fig. 11) indicate that changing the stereoregularity does not seem to have a strong effect on the frozen-in orientation. Changes in stereoregularity do have a large effect on the volume fraction crystallinity of the molding (Figs. 10 and 14). In addition to XSRT/tacticity, the melt orientation may also have an effect on the volume fraction crystallinity. This is implied from analogies with the strain-induced crystallization mechanism in stretched rubbers (22) and theoretical relationships between melt strain rate and crystallization kinetics (23, 24). To estimate the magnitude of this effect, the difference in the volume fraction crystallinity near and away from the gate has been plotted against the corresponding difference in the c-axis orientation functions in Fig. 18. Although Fig. 18 indicates that there is a weak flow-induced crystallization effect, it appears to be a second-order effect when compared with the influence of the intrinsic chain regularity.

The results discussed above show that the XSRT/tacticity has a relatively weak influence on the frozen-in orientation but a large effect on the volume fraction crystallinity of the molded part. This is not unexpected, provided i) the overall crystallinity is sufficiently high, and ii) the crystallization temperatures/rates are similar. The DSC data from Table 2 show similar crystallization temperatures for the resins investigated. For chain parameters and crystallization characteristics outside the bounds investigated here, increased sensitivity of the frozen-in orientation to XSRT/tacticity might be anticipated with modification of the crystallization temperature. A similar situation is expected with the addition of nucleators.

In contrast with the XSRT/tacticity dependence, MFR and PI changes have a dominant influence on the frozen-in orientation. This is illustrated by the representative data in Figs. 4, 12, and 13, where frozen-in orientation is observed to increase with both decreasing MFR and increasing PI. This is expected because of the slow relaxation of higher molecular weight chains and is consistent with observations noted in the literature (8).

The balance of orientations has been used as an explanation of flex-like behavior in molded-in hinges (25) owing to its role of creating interfibrillar contacts. This feature of the injection molded morphology is also a relevant consideration for the cross-flow mechanical TABULAR DATA OMITTED integrity in injection moldings. The experimentally observed orientation balance shown in Fig. 8 is generally consistent with the presence of a bi-modal orientation of the a*- and c-axis. Fujiyama et al (8) have proposed a morphological model of the skin structure of injection molded PP to explain similar orientation behavior. The primary components of the model were chain-extended crystals with a secondary growth of mixed orientation crystallites.

Previous work on the solid-state drawing of PP films has described the path of a point in Fig. 8 in terms of the morphological state with increasing draw ratio (18, 26). During solid state drawing, the orientation balance of the crystallographic axes is governed by the initial morphology and the drawing conditions. The change of this balance with increasing draw ratio is described by the path on the orientation function triangle. An analogy can be drawn with solid-state deformation processes for the injection molded samples studied here, where the "melt orientability" acts like a melt draw ratio. This orientability is determined by the resin and molding conditions. The results shown in Fig. 8 indicate that the orientability, as opposed to other morphological factors, appears to be a dominant factor governing the orientation balance of the crystallographic axes. This analogy is illustrated schematically in Fig. 19. The conclusion summarized in Figs. 8 and 19 is significant in terms of correlating orientation behavior to mechanical response because it indicates that the "mixed-mode" orientation is correlatable in terms of the average c-axis orientation over the range of material parameters and molding conditions investigated. The gradation of orientations shown in Fig. 8 at fixed melt temperatures and gate locations is achieved through variation of the resin type. As discussed above, the extent of orientation is primarily adjusted by variation of the resin molecular weight and polydispersity.

The results associated with Fig. 14 indicate, as expected, that a major factor contributing to property enhancement in highly stereoregular materials is the resultant increase in crystallinity. In the absence of orientation, the flexural modulus appears to be described as a single function of crystallinity. Crystallinity is likely not the only factor governing flexural modulus in the absence of orientation. This is due to the fact that in long chain molecules the crystallites act not only as volume filling particles (27, 28) but also as an interconnected temporary network (29). Numerous theories and empiricisms have been proposed and applied to account for the volume filling nature of semicrystalline materials (21, 27, 28, 30-34). Most of the volume-falling equations are bounded by either the parallel loading model (E [varies as] [[Phi].sub.c]) or the series loading model (1/E [varies as] [[Phi].sub.c]). In this sense, the logarithmic "mixing" rule given by Eq 1 represents an intermediate behavior (31). Descriptions of the modulus of semicrystalline materials have also invoked a crystallinity-dependent lamellar aspect ratio, in the context of the Tsai-Halpin equation (35), as well as crystallinity-dependent intrinsic moduli (31, 33). While it is likely inappropriate to suggest a "universal" crystallinity/modulus relationship in unoriented polypropylene, because of morphological dependencies, crystallinity is a dominant factor.

The type of normalization in Figs. 15 through 17 is empirical in nature and has many approximations. Among these, as discussed above, is the presumption that the flexural modulus is governed by only one parameter in the absence of orientation, the bulk volume fraction crystallinity, [[Phi].sub.c]. It is also recognized that the crystallinity in an injection molded part can exhibit gradients across the thickness (8, 36), which will contribute differently to the flexural strength. An additional primary assumption is that the crystallinity and orientation contributions to the flexural modulus can be "decoupled." In the three-parameter theory of Seferis and Samuels applied to the tensile modulus of PP film (32), this type of decoupling is suggested if the amorphous orientation component is near zero. The quantitative effects of these complications require further study. However, over the range of crystallinities and orientations investigated, the approach summarized in Fig. 15 through 17 seems to be reasonably effective at correlating the morphology and properties. Still, for reasons outlined above, extrapolation of these results is expected to be limited.

Despite the many quantitative difficulties in developing a generalized formalism for the flexural modulus of injection molded PP, the present results demonstrate the strong influence of crystallinity and orientation in governing the properties of injection moldings. The quantitative correlations appear to have a wide range of applicability for different intrinsic resin parameters. The resultant property/morphology balance is summarized by the three-dimensional crystallinity/orientation surface depicted in Fig. 20. These results were previously depicted in normalized form in Fig. 15. In general, the injection moldings studied here (nonzero values of [f.sub.c]) exhibit reduced crystallinity relative to the compression moldings because of a higher effective quench rate, but higher moduli due to the orientation effect. In addition, the sensitivity of the flexural modulus, E, to [[Phi].sub.c] and [f.sub.c] can be estimated by taking the appropriate partial derivatives of the crystallinity/orientation surface. The calculated derivative surfaces for crystallinity and orientation are shown in Figs. 21 and 22, respectively. Both figures imply that materials with either high crystallizabiltty or high orientability will be more sensitive to perturbations in [[Phi].sub.c] and [f.sub.c]. This indicates that high modulus resins, which generally lie on the property/morphology surface corresponding to high [[Phi].sub.c] and/or high [f.sub.c], can exhibit increased process sensitivity relative to resins with lower crystallizability and/or orientability.

Figure 20 shows the wide range of properties and morphologies resulting from the variation of the resin type, molding conditions, and mold location. Despite this wide rage, the stiffness characteristics of injection molded PP can be collapsed into a relatively simple description (with qualifications noted). The resin characteristics and molding conditions influence the resultant properties by movement along the crystallinity/orientation surface. This surface is able to describe adequately the modulus data for all of the resins, molding conditions, and locations from the gate that were examined in this work.

CONCLUSIONS

Structure/property relationships in compression molded and injection molded PP have been quantitatively evaluated. Flexural modulus in quench-cooled compression moldings can be correlated to the volume fraction crystallinity, [[Phi].sub.c], although there are complicating morphological factors. A primary factor influencing [[Phi].sub.c] is the resin tacticity. The injection moldings are characterized by a skin-core morphology and mixed a*- and c-axis orientations. The resulting orientation balance of the crystallographic axes is governed largely by the total chain axis orientation because of the melt orientability of the resin type and the molding conditions. The resulting net orientation balance lies between the limiting regimes of uniaxial c-axis orientation and uniaxial negative b-axis orientation. Generally, the chain axis orientation is observed to decrease with increased distance from the gate and with increasing melt temperature at a fixed location. Chain regularity/XSRT is the dominant factor influencing the crystallinity, whereas the molecular weight and polydispersity are the dominant factors influencing the orientation. All of the flexural modulus results, irrespective of the resin type, molding conditions, and gate location, can be described by a single empirical correlation in terms of the bulk volume fraction crystallinity and the c-axis orientation. Similar expressions can be developed using alternative measures of orientation, including the skin-layer area fraction from optical microscopy. Limitations of the approach include the assumption of decoupled orientation and crystallinity contributions, the effects of crystallinity and orientation gradients through the thickness of the part, in addition to an independent accounting of complications in the crystallinity normalization that result from intercrystalline connectivity and other morphological effects. The results demonstrate, however, that the effective control of resin crystallizability and resin orientability through variation of stereoregularity, molecular weight, and polydispersity can give rise to unique and predictable properties.

ACKNOWLEDGMENTS

The authors would like to acknowledge with appreciation the contributions of Deborah Morgan, Dean Spencer, Vicki Allen, and Patrick Faline for help in data acquistion, and the assistance of Dr. Dinshong Done, Ken Klinger, Dr. Bill Long, and Helena Rychlicki.

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Author:Phillips, R.; Herbert, G.; News, J.; Wolkowicz, M.
Publication:Polymer Engineering and Science
Date:Dec 15, 1994
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