Process-Induced Morphology Distribution in Injection Molded

Article pubs.acs.org/IECR

Process-Induced Morphology Distribution in Injection Molded Syndiotactic Polystyrene Samples Roberto Pantani,† Andrea Sorrentino,*,‡ Vito Speranza,† and Giuseppe Titomanlio† †

Department of Industrial Engineering, University of Salerno, via Ponte don Melillo, I-84084 Fisciano (Salerno), Italy Institute for Composite and Biomedical Materials (IMCB), National Research Council (CNR), Piazzale Enrico Fermi 1, I-80055 Portici (NA), Italy

‡

ABSTRACT: In this work, syndiotactic polystyrene was injection molded with different molding conditions. The morphology distribution inside the molded parts was analyzed by several techniques, including optical microscopy, IR spectroscopy, and X-ray diffraction. The final morphology distribution was found to be strongly affected by mold temperature and holding pressures. A peculiar morphology distribution, an alternation of several amorphous and crystalline layers, was found along the samples thickness. The experimental results were analyzed and discussed by means of a software code developed at the University of Salerno. The software implements a model for crystallinity evolution that is able to consider the effects of high cooling rates and high pressures. Furthermore, the software is able to predict the orientation evolution during the injection molding. On commenting on the differences between the experimental data and the model predictions for crystallinity and orientation, it was possible to gain some insights on the causes of the peculiar morphology distributions inside the molded samples.

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INTRODUCTION Injection molding is one of the most important manufacturing processes for mass production of complex plastic parts.1 The quality and performance of injection-molded parts depend not only on the material and part design but also on how the material is processed during molding.2 For this reason, a continuous effort toward the understanding of the effect of the molding conditions is necessary to optimize the performance of the molded products. When processed by injection molding, slowly crystallizing polymers like polyether-polyketone (PEK), poly(p-phenylene sulfide) (PPS), and syndiotactic polystyrene (sPS) present a peculiar morphology inside the molded parts, due to the complex interplay among the thermo-mechanical history, polymer rheology, and crystallization kinetics. It makes this class of polymers particularly interesting from a scientific point of view.3,4 Syndiotactic polystyrene, in particular, was the object of many studies because of its impressive material properties and unusual polymorphism.5 Despite the numerous papers dealing with crystallization of sPS, the combined effect of flow, pressure, and high cooling rate on the solidification process of sPS is still not fully clarified.6,7 In particular, the investigations on the crystallization kinetics of this polymer were carried out under quiescent, quasiisothermal conditions very far from the conditions experienced by polymers in common industrial processes.8,9 This implies that the information reported in the literature cannot be used to describe sPS solidification taking place in conventional manufacturing processes like injection molding. This work is aimed at shedding some light on the development of morphology and crystallinity inside injection moldings of sPS samples by discussing final morphology findings on the basis of simulation results based on a process simulation code developed at UNISA,10−14 adopting results of © 2012 American Chemical Society

detailed studies of rheological behavior and crystallization kinetics under high pressure and high cooling rates of the sPS adopted.15,16

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EXPERIMENTAL SECTION The material used for all tests was a syndiotactic polystyrene (Dow - QA101) with an average molecular weight of 320 000 g/mol and a polydispersity index Mw/Mn = 3.9. A 65-ton Penta reciprocating screw injection molding machine was adopted for the injection molding experiments. The material was injected into a line gated rectangular cavity of dimensions 120 × 30 × 2 mm with a gate having the same width of the cavity, a length of 6 mm, and a thickness of 1.5 mm (Figure 1). Five Kistler piezoelectric pressure transducers were mounted along the flow path. In particular, one transducer was mounted in the injection chamber, one in the runner (just upstream from the gate), and the others in the cavity 15, 60,

Figure 1. Sample geometry with schematics of the cutting procedures. Received: Revised: Accepted: Published: 10840

May 14, 2012 July 20, 2012 August 1, 2012 August 1, 2012 dx.doi.org/10.1021/ie301253v | Ind. Eng. Chem. Res. 2012, 51, 10840−10847

Industrial & Engineering Chemistry Research

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and 105 mm downstream to the gate. These positions will be referred to as P0, P1, P2, P3, and P4, respectively. The transducer signals were read by a fast data acquisition system and stored in a desktop computer. Injection temperature was 305 °C and holding time was 10 s (larger than gate sealing time) for all experiments. Four series of experiments were performed, tailored to analyze the effect of mold temperature and holding pressure. The list of molding conditions is reported in Table 1: tests are Table 1. Molding Conditions test code

mold temp [°C]

injection flow rate [cm3/s]

holding pressure [bar]

cycle time [s]

P18M70 P45M70 P70M70 P45M25

70 70 70 25

35 35 35 35

180 450 700 450

60 60 60 60

Figure 2. Pressure evolution during test P18M70. Symbols, experimental data; lines, simulation results.

coded on the basis of holding pressure and mold temperature. The morphology distribution was analyzed at gate position. At that location, the section is thinner than inside the cavity (and thus the deformation rates are higher), and, due to the considerable volume downstream, the material undergoes to shear for a long time during the packing phase.17 Three cutting procedures were used to section the injection molding samples (Figure 1). The first procedure (Pr1) consisted of cutting sections parallel to the flow direction in the flow-thickness plane (F−N plane in Figure 1) and centered on the sample width direction. These sections were analyzed by means of optical microscopy. The second procedure (Pr2) consisted of cutting, with a microtome, slices about 100 μm at different depths in the flowtransverse plane (F−T plane in Figure 1): skin (0−250 μm from wall), intermediate (250−500 μm from wall), and core (500−750 μm from wall). The slices were analyzed by X-ray analysis. The third procedure (Pr3) was used to obtain samples for the birefringence distribution measurements (“wedge method”) along the thickness of the moldings.17,18 A schematic view of the positions of the wedges is reported in Figure 1. The wedge angle was kept at about 30° to maintain a distance between interference fringes of the order of 0.1 mm. X-ray diffraction spectra were recorded on the slices cut according to procedure Pr2 by a “Philips PW 1830” X-ray generator and a flat camera with a sample-to-film distance of 220 mm (Ni-filtered Cu Kα radiation) and 1 h exposure time. A “Fujifilm MS 2025” imaging plate (0.1 mm/pitch) and a “Fuji Bio-imaging Analyzer System” were used to gather and digitalize the diffraction patterns. The degree of crystallinity χc was evaluated by analyzing the WAXD spectra.19


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RESULTS AND DISCUSSION Pressure Curves. Pressure recordings during molding tests are reported as symbols in Figures 2−5. The differences in molding conditions are clearly evidenced by the pressure profiles: the increase of holding pressure from 180 to 700 bar causes an increase of the pressure levels everywhere along the flow path (Figures 2−5). Also, the times at which pressure goes to zero inside the cavity increase on increasing holding pressure, so that for the test carried out with the highest holding pressure (Figure 4) a residual pressure is present in the cavity at positions P2 and P3.


The effect of a lower mold temperature can be evidenced on comparing Figures 3 and 5: on decreasing mold temperature, 10841

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opaque when it crystallizes, layers appearing transparent at a crossed polarizers observation must be amorphous with a not negligible degree of orientation. As shown in Figure 6, a transparent layer (white in the figure) is found close to the skin for all of the molding conditions. This is due to the high cooling rate experienced by the layers close to the skin, which prevents the crystallization. The thickness of this layer seems to increase with either an increase of holding pressure or a decrease of mold temperature: indeed, this is consistent with the fact that final crystallinity decreases with increasing cooling rate from the melt and for sPS on increasing solidification pressure.15 The intermediate layers appear opaque (black in the figures) due to a non negligible crystalline content. Indeed, the cooling rate decreases on getting deeper inside the sample, and this increases the final crystallinity degree. Interestingly, a transparent layer (white in the figure) is found also at the midplane, for all of the molding conditions except for the lowest applied holding pressure (Figure 6A). The alternation of crystalline and amorphous layers can be ascribed to a complex interplay between effects of orientation and of cooling rate and pressure on crystallization kinetics and final crystallinity:20−22 the flow enhances the crystallization kinetics of the polymer such that crystalline layers are obtained even if the cooling rate and pressure are high enough to obtain an amorphous material in quiescent conditions; at the midplane, where deformation rate tends to zero, the effect of flow is much lower and the material solidifies as an amorphous. Also, transparency at the midplane (white in the figure) increases on decreasing mold temperature and on increasing holding pressure. Molded Samples’ Crystallinity. Data of crystallinity at the gate position are reported in Figure 7. The opaque layers are shadowed in the figures. Data clearly show that at gate position the crystallinity degree is much lower than the maximum possible for sPS (about 50%). Furthermore, the crystallinity decreases on increasing holding pressure and on decreasing mold temperature. This can be ascribed to the high solidification pressures and the high cooling rates. It can be noticed that in the transparent layers the crystallinity is always lower than about 10%. This crystallinity can be considered as a threshold in the change from transparent and opaque layers, even if the shape and dimensions of crystals are quite relevant in optical phenomena.23 Orientation along Thickness Direction. The orientation of the samples was obtained by measuring birefringence by means of optical analysis performed on wedge cut according to procedure Pr3 (Figure 1). Obviously, the adopted technique cannot be applied in the opaque (crystalline) regions. Molecular orientation is just proportional to birefringence in the amorphous polystyrene. The results of birefringence at gate position are reported in Figure 8. The opaque layers, where according to the procedure it was not possible to measure the orientation, are shadowed in the figures. It is normally expected that the orientation presents the maximum values close to the skin. This is due to the fact that shear rates are larger and temperatures are lower close to the mold wall. In some cases, for the coldest mold and for the highest holding pressure, even a second maximum of orientation was detected at intermediate positions along thickness. This maximum is associated with the effect of packing flow, which despite being less intense than filling flow,


the pressure drop across the transducer positions (the differences between the pressure recordings) increases. Furthermore, if a lower mold temperature is adopted, the peak in position P0 (inside the injection chamber), after about 16 s, due to the counterpressure applied by the screw to prepare the batching for the following shot does not cause any increase in the pressure profile inside the channels (position P1). Such a feature appears when the polymer is already completely solidified in the channels; thus, for the tests carried out at 45 MPa after 16 s, solidification was not complete in the channel for all of the tests carried out with the higher mold temperature and was complete for the test carried out with the lower mold temperature. Morphological Characterization. According to procedure Pr1 (Figure 1), 1 mm thick slices were cut from the samples along the midplane parallel to the flow-thickness, F−N plane, at the gate position. The slices were cold lapped and observed through crossed polarizers: samples showed a distribution along thickness direction of transparent layers and white/opaque layers (which appear as black in transmitted light). Some of the micrographs are reported in Figure 6. Because sPS is white and

Figure 6. Micrographs of sPS molded samples at gate position. The vertical line identifies the location where crystallinity and orientation were assessed. 10842



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Figure 7. Crystallinity distribution in gate position. Symbols, experimental data; lines, simulation results. The shadowed zones in the figures represent the optical opaque layers in the sample.

Figure 8. Comparison between experimental birefringence (symbols) and calculated molecular orientation (continuous lines) in gate position. The shadowed zones in the figures represent the optical opaque layers in the sample.

acts on colder molecules and thus is more effective in orienting the material.17 Simulations of the Injection Tests. Injection molding tests were simulated by means of a software code developed at University of Salerno.10−14 This software is based on the classical creeping-flow lubrication approach. In addition to neglecting the fluid inertia, this formulation also omits calculation of the velocity component and thermal convection in the gapwise direction, and transverse flow at the melt-front region (the fountain flow behavior). The rheological and the thermal data are taken from the data presented in previous work.24−26 In particular, the viscosity η is assumed to be a function of temperature and pressure according to the following Arrhenius equation (eq 1): ⎛ Tb ⎞ η0(T , P) = ηr exp⎜ ⎟ ⎝ T − A1P ⎠

η(γ )̇ =

η0(T , P) ⎡ η (T , P)·γ ̇ ⎤1 − n 1 + ⎢⎣ 0 C ⎦⎥

(2)

A list of the constants for the Cross−Arrhenius model used for describing the viscosity of the melt is reported in Table 2. The values of the thermal property constants for sPS as used in the computer simulation program are summarized in Table 3. In the absence of specific data, thermal conductivity and specific heat were assumed to be independent of temperature and crystallinity degree. The latent heat of melting was Table 2. Cross−Arrhenius Model Parameters

(1)

The dependence from the shear rate was described by the following Cross equation (eq 2): 10843

parameter

value

ηr Tb A1 n C

4.66 × 10−8 13 022 1.26 × 10−5 0.15 65 751

Pa s K K/Pa Pa



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dξi dE 0 = (1 − ξ) i dt dt

Table 3. Thermal Properties of sPS parameter

value k Cp λα λβ

thermal conductivity heat capacity heat of melting

14.73 2.09 48 58

ref J/(cm s °C) J/(°C g) J/g J/g

ξi = χi/χeq, with χeq being the equilibrium volume fraction of the crystal, which is taken to be same for both phases, that is, χeq = 60%, and Ei0 is the expectancy of volume fraction of each ith phase if no impingement would occur.

16 16 16 16

considered to be different for the two crystalline phases that develop in sPS on cooling from the melt, α and β. Material specific volume was described as a weighted average of the pure amorphous (subscript “A”), the α crystalline (subscript “α”) volumes, and the β crystalline (subscript “β”) volumes according to: v = vαχα + vβχβ + vA(1 − χα − χβ )

∫0

Ei0(t ) = ln 2[

vα =

(4)

vβ = vβo[1 − Aβ (T − T0) + Bβ (P − P0)]

(5)

Pressure effects were taken into account by introducing the pressure dependence of the glass transition temperature (Tg) and the melting temperature (Tmi) by means of the following equations:

Ai, and Bi are, for each ith considered crystalline phase, the specific volume at T = T0 and P = P0, thermal expansion coefficient, and compressibility. The volume of the amorphous phase was described as

2

TmP i = Tmi·e(A iP + Bi P ) TgP = Tg ·SgP 2

(6)

is the specific volume at T = Tg and P = P0, αm and αg are the thermal expansion coefficients of melt and glass, respectively, βm and βg are the compressibility of melt and glass, respectively, and Tg is a function of pressure according to

(7)

The parameters of PVT model are reported in Table 4. Table 4. Parameters of sPS PVT Model Equations 3−7 parameter αm [1/K] Tg0 [K] βm [1/bar] Aα [1/K] Bα [1/bar] αg [1/K]

value 7.0 373 9.86 2.67 4.29 9.0

× 10−4 × × × ×

10−5 10−4 10−5 10−5

parameter

value

v0A [kg/m3] v0α [kg/m3] v0β [kg/m3] Aβ [1/K] Bβ [1/bar] βg [1/bar]

1041 1066 1066 2.67 × 10−4 4.29 × 10−5 4.0 × 10−5

(11)

where Ai and Bi are material parameters related to each crystalline phase, and Sg is the pressure dependency of the glass transition temperature. Parameters of kinetic equations, listed in Table 5, were tuned on the basis of several techniques. In particular, DSC data (both under isothermal and cooling mode),16 PVT data,15 and also data of polymer solidification under pressure, at high cooling rates (up to a maximum cooling rate at 200 °C of about 50 °C/s), obtained during experiments performed by an homemade apparatus,27 were adopted for the identification of the value of the parameters of kinetic eqs 8− 11. The values of parameters are listed in Table 5. The model described by eqs 8−11 is able to describe the effect of cooling rate and pressure in the whole range of interest for injection molding. The effect of flow, however, is not described. The differences between crystallinity and orientation experimental data and model predictions can be helpful to shed light on the effect of flow on the final crystallinity distribution inside the moldings. This will be done in the following. Modeling of Molecular Orientation. A viscoelastic equation was adopted by Pantani et al.8 to describe the evolution of molecular orientation in injection molded samples. They described the fractional “deformation” of the population of dumbbell subchains with respect to the equilibrium conformation as:

v0A

⎤ ⎡ βm − βg Tg = Tg0⎢1 + (P − P0)⎥ αm − αg ⎢⎣ ⎦⎥

(9)

(10)

v0i,

⎧ v = v 0 [1 − α (T − T ) + β (P − P ) A m g 0 m ⎪A ⎪ when T > Tg ] ⎪ ⎨ ⎪ vA = vA0[1 − αg(T − Tg) + β (P − P0) g ⎪ ⎪ when ] T ≤ T g ⎩

ki ds]ni

⎡ K gi·(Tmi + Tc) ⎤ ⎡ ⎤ Ui ⎥ ki = Vi ·k 0i·exp⎢ − ⎥ ·exp⎢ − ⎢⎣ 2·Tc2·(Tmi + Tc) ⎥⎦ ⎣ R ·(Tc − T∞) ⎦

(3)

− Aα (T − T0) + Bα (P − P0)]

t

where ni is the Avrami index and ki(T,P) is a kinetic constant. Dependency of ki upon temperature was described by the Hoffman−Lauritzen equation:

The volume of a crystalline phase was taken as a linear function of temperature and pressure: vαo[1

(8)

A=

3 [⟨R̲ · R̲ ⟩ − ⟨R̲ · R̲ ⟩0 ] ⟨R 02⟩

(12)

where ⟨R·R⟩ is the second-order conformation tensor, and ⟨R·R⟩0 is the value of ⟨R·R⟩ under quiescent conditions, when the end-to-end distance of the molecular chain is ⟨R20⟩ = tr[⟨R·R⟩0]. According to this definition, the constitutive equation for the subchain population can be written as:

The solidification criterion was adopted according to the model proposed by Titomalnio et al.10 The level of critical crystallinity was taken to be 1%. Crystallization Kinetics. Modeling of crystallization behavior of sPS was achieved by assuming a parallel of two noninteracting kinetic processes (α and β) competing for the available amorphous volume. The evolution of each phase was described by eq 8:

D 1 A − ∇νT · A − A ·∇ν = − A + ∇ν + ∇νT Dt λ 10844

(13)



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Table 5. Parameters of sPS Crystallization Kinetic Model parameter

value

parameter

value

K0α Uα Kgα Tmα nα Vα Aα Bα χeq,α Tg Sg

194 775 89 900 545 1.5 0.64 −2.07 × 10−5 −1.83 × 10−9 0.60 373 1.03 × 10−5

K0β Uβ Kgβ Tmβ nβ Vβ Aβ Bβ χeq,β

29 775 100 000 555 3.5 0.96 5.86 × 10−5 1.43 × 10−8 0.48

the material is still molten inside the channels. The pressure spike due to counterpressure (after about 16 s) gives rise to an increase of pressure in position P1 for all conditions having higher mold temperature, while, for the test carried out with the lowest mold temperature, the solidification inside the channels does not allow pressure transmission. The simulations nicely capture this. The pressure difference between position P1 and P2 during the packing stage (for times shorter than about 5 s) provides indication concerning the pressure drop across the gate: the comparison between data and simulation is satisfactory for all of the conditions. Eventually, after gate sealing, the pressure evolution inside the cavity is mainly related to volumetric and thermal properties of the polymer and to mold elasticity: apart from minor deviations, the evolution is correctly predicted. Prediction of Samples Morphology. Prediction of overall crystallinity degree distributions at gate position, obtained with the UNISA software, is reported in Figure 7.13,14 The comparison between predicted and measured crystallinity seems satisfactory at intermediate and core positions, where the model correctly predicts the values of crystallinity. The effect of a higher holding pressure and a lower mold temperature toward a reduction of final crystallinity degree is also correctly captured. In layers close to sample skin, the predictions underestimate the experimental values for crystallinity: this can be ascribed to the neglection of the effect of flow on crystallization kinetics. The analysis of molecular orientation can help to understand the reasons of the complex morphology observed (Figure 6). In Figure 8, a comparison is reported between calculated and measured molecular orientation. The values of predicted molecular orientation are obtained in terms of an orientation index, the difference between the two main eigenvalues of the tensor A (which for injection moldings nearly corresponds to the maximum eigenvalue).17,28 Assuming the validity of the stress-optical rule, this index is proportional to the birefringence, and, in this work, a proportionality constant equal to 5 × 10−5 was kept between the orientation index and the birefringence. This value should be equal to the product of the stress-optical coefficient and the modulus of the material, and it is in line with what is found in the literature for polystyrene.28 In the layers where a direct comparison can be performed (in the transparent, nearly amorphous layers), it can be noticed that the model describes many features of the birefringence curves, although very close to skin it gives rise to an early increase of orientation. The orientation is low at the sample

λ 0 (T , P ) ⎡ λ (T , P)·γ ̇ ⎤1 − m 1+⎣ 0 k ⎦

(14)

where λ0(T) can be described by the Arrhenius equation: ⎛ Tb ⎞ λ 0(T , P) = λr exp⎜ ⎟ ⎝ T − A1P ⎠

(15)

A list of the constants of the Cross−Arrhenius model used to describe the relaxation time is reported in Table 6. Table 6. Cross−Arrhenius Model Parameters for the Relaxation Time parameter

value

λr Tb A1 m k

4.66 × 10−12 13 022 1.26 × 10−5 0.44 0.155

[bar] [bar−2] [K] [K/bar−2]

where ∇ν is the velocity gradient, and λ is the relaxation time. The polymer contribution to the stress tensor is obtained from the fractional “deformation” tensor as τ = Gs·A, where GS is the modulus of the polymer. The relaxation time, λ, is considered a function of temperature, pressure, and shear rate. In particular, the following Cross-type equation was adopted: λ(T , P , γ )̇ =

[1/s] [cal/mol] [K2] [K]

s K K/Pa

Results and Discussion. In Figures 2−5, comparisons between experimental and simulated pressure are shown. The general agreement between data and simulation results is quite good. The experimental data of P0 are used as a pressure boundary condition in the postfilling stage, and thus data and simulations overlap for P0. The pressure levels at position P1 provide indications concerning the pressure drops across nozzle and channels. If the comparison among the measured and simulated pressures in position P1 is analyzed with regard to the tests conducted with the higher (P70M70, reported in Figure 4) and the lower (P18M70, reported in Figure 2) holding pressures, it is possible to notice that the effect of pressure on viscosity could be better described: indeed, the viscosity at higher pressures seems to be underestimated (the predicted pressures in Figure 4 are higher), whereas the viscosity at lower pressure seems to be overestimated (the predicted pressures in Figure 2 are lower). It can also be noticed that both the experimental data and the simulations, in all conditions, show that at the release of the packing pressure, the pressure at position P1 immediately drops, indicating that 10845



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predictions underestimate the experimental values for crystallinity: this can be ascribed to the neglection of the effect of flow on crystallization kinetics. The calculated molecular orientation profiles describe many features of the birefringence curves, although very close to skin an early increase of orientation is predicted. The analysis of predicted orientation profiles revealed that the sample is opaque and crystalline in the layers where orientation presents a local maximum, thus confirming that the enhancing effect of orientation on crystallization can overcome the inhibitory effect of cooling rate and pressure.

skin and rapidly increases on increasing the distance from the mold wall due to the effect of shear. A maximum of orientation is found at intermediate layers, almost precisely where the peculiar crystalline layer at about 0.1−0.2 mm from the skin is present. This confirms the fact that these layers are crystalline because of the enhancing effect of flow (of orientation) on crystallization kinetics. This is also the reason why the (quiescent) crystallization kinetic model is not able to describe the correct crystallinity distribution in those layers. On getting deeper inside the sample, the orientation decreases because the cavity is filled and the machine switches from filling to packing phase. In the sample molded with the lowest holding pressure, the orientation remains at a very low level in the core region. Despite this, no intermediate amorphous layer is present because when the pressure is low the crystallization kinetics is fast enough to reach a significant crystalline content. This is correctly predicted by the crystallization kinetics model (Figure 7A). In all of the other samples, just in the layers where orientation becomes low, a transparent layer is present. In those layers, the orientation model nicely captures the evolution of experimental data. On increasing the distance from the mold wall, the orientation level starts to increase again because of the packing flow. In the layers where orientation presents a local maximum, the sample is again opaque and crystalline. Again, the enhancing effect of orientation on crystallization can overcome the inhibitory effect of cooling rate and pressure. Close to the midplane, orientation fades to nearly zero, and an amorphous layer is found again. It is interesting to notice that crystallinity is very well described (Figure 7) in all of the layers where orientation is low.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Rosato, D. V.; Rosato, D. V.; Rosato, M. G. Injection Molding Handbook; Kluwer Academic Publishers: Norwell, MA, 2001; ISBN: 0792386191. (2) Jansen, K. M. B.; Van Dijk, D. J.; Husselman, M. H. Effect of processing conditions on shrinkage in injection molding. Polym. Eng. Sci. 1998, 38, 838−846. (3) Han, S.; Wang, K. K. Shrinkage prediction for slowly-crystallizing thermoplastic polymers in injection molding. Int. Polym. Process. 1997, 12, 228−237. (4) Sorrentino, A.; De Santis, F.; Titomanlio, G. Polymer crystallization under high cooling rate and pressure: a step towards polymer processing conditions. In Lecture Notes in Physics: Progress in Understanding of Polymer Crystallization; Reiter, G., Strobl, G., Eds.; Springer: Berlin, 2007; Vol. 714, Chapter 16, pp 329−344. (5) Sorrentino, A.; Vittoria, V. In Syndiotactic Polystyrene: Synthesis, Characterization, Processing, and Applications; Schellenberg, J., Ed.; John Wiley & Sons: New York, 2010; Chapter 9, pp 155−193. (6) Guerra, G.; Vitagliano, V.; De Rosa, C.; Petraccone, V.; Corradini, P. Polymorphism in melt crystallized syndiotactic polystyrene samples. Macromolecules 1990, 23, 1539−1544. (7) Ruiz de Ballesteros, O.; Di Gennaro, M.; Auriemma, F. Crystallization from the melt of α and β forms of syndiotactic polystyrene. Polymer 2003, 44, 1861−1870. (8) Pantani, R.; Speranza, V.; Sorrentino, A.; Titomanlio, G. Molecular orientation and strain in injection moulding of thermoplastics. Macromol. Symp. 2002, 185, 293−307. (9) La Carrubba, V.; Piccarolo, S.; Brucato, V. Solidification of syndiotactic polystyrene by a continuous cooling transformation approach. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 2688−2699. (10) Titomanlio, G.; Speranza, V.; Brucato, V. On the simulation of thermoplastic injection-molding process. Int. Polym. Process. 1995, 10, 55−61. (11) Titomanlio, G.; Speranza, V.; Brucato, V. On the simulation of thermoplastic injection-molding process 2. Relevance of interaction between flow and crystallization. Int. Polym. Process. 1997, 12, 45−53. (12) Pantani, R.; Speranza, V.; Titomanlio, G. Relevance of mouldinduced thermal boundary conditions and cavity deformation in the simulation of injection moulding. Polym. Eng. Sci. 2001, 41, 2022− 2035. (13) De Santis, F.; Pantani, R.; Speranza, V.; Titomanlio, G. Analysis of Shrinkage Development of a Semicrystalline Polymer during Injection Molding. Ind. Eng. Chem. Res. 2010, 49, 2469−2476. (14) Pantani, R.; Coccorullo, I.; Speranza, V.; Titomanlio, G. Modeling of morphology evolution in the injection molding process of thermoplastic polymers. Prog. Polym. Sci. 2005, 30, 1185−1222. (15) Sorrentino, A.; Pantani, R.; Titomanlio, G. Crystallization of syndiotactic polystyrene under high pressure and cooling rate. Macromol. Res. 2010, 18, 1045−1052.

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CONCLUSIONS Syndiotactic polystyrene samples were obtained by injection molding in different molding conditions. The morphology distribution inside the moldings was analyzed by several techniques. Morphology of the samples was very complex, and, in most of the cases, an alternation of amorphous of crystalline layers was observed from the sample skin toward the center. Such a morphology, analyzed also on the basis orientation distribution, was ascribed to a complex interplay between effects of orientation and of cooling rate on crystallization kinetics and final crystallinity: as the flow enhances the crystallization kinetics, crystalline layers can be obtained even if the cooling rate is high enough to obtain an amorphous material in quiescent conditions; where the effect of flow is much lower, the material may solidify as an amorphous even if cooling rate is lower. The effect of pressure and mold temperature superpose to the competition between high cooling rate and high orientation; high holding pressure favors low crystallization, and low mold temperature, by increasing cooling rate, favors low crystallinity. Injection molding tests were simulated by means of a software code developed at the University of Salerno, which implements models for crystallinity and molecular orientation evolution. The general agreement between data of pressure histories and simulation results was quite good. The comparison between predicted and measured crystallinity was satisfactory at intermediate and core positions: the effect of a higher holding pressure and a lower mold temperature toward a reduction of final crystallinity degree was correctly captured. In layers close to sample skin, the 10846



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Process-Induced Morphology Distribution in Injection Molded

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