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Novel foaming method to fabricate microcellular injection molded polycarbonate parts using sodium chloride and active carbon as nucleating agents.

INTRODUCTION

The commercially available microcellular injection molding process blends supercritical fluids (SCF; usually nitrogen or carbon dioxide) with polymer melts in the injection molding machine barrel to create a single-phase polymer/gas solution that subsequently foams during the injection molding stage to produce lightweight, microcellular injection molded parts [1-5]. In the supercritical fluid state, SCF nitrogen or carbon dioxide can be accurately metered into the machine barrel like a fluid and yet quickly dissolved into the polymer melt like a gas to form a single-phase polymer/gas solution due to the high shear generated by the screw. Upon injection, the sudden change in pressure and temperature as the polymer/gas solution enters the cavity triggers thermodynamic instability. As a result, cells start emerging from the polymer/gas solution through homogeneous and/or heterogeneous cell nucleation, which is followed by cell growth that helps to fill the entire cavity during the mold cavity filling stage and continues to pack out the mold during the postfilling stage. Because cell expansion can compensate for material shrinkage, no packing pressure or pack/hold stage is needed; in fact, the normal holding pressure could prevent cell growth. Furthermore, the melt viscosity and transition temperature of the polymer also decrease as the SCF serves as a plasticizer. Accordingly, this technology enables an injection molding process with lower processing temperatures and pressures, leading to a reduction in the clamp tonnage requirement, cycle time, and energy consumption [6, 7]. In addition, the uniform packing resulting from cell growth throughout the entire part leads to reduced residual stress and improved dimensional stability [8, 9].

However, the benefits of this technology also come with some limitations, notably higher equipment costs as well as parts with rougher surfaces and lower mechanical properties compared with those of normal solid injection molded parts. Microcellular injection foamed parts have surface finishes with swirl marks and silver streaks resulting from the stretching and collapsing of bubbles on the part surface. For applications with stringent part appearance requirements, the surface swirl marks on the molded part limit the adoption of microcellular injection molding unless the surface quality can be improved with other techniques. To eliminate the surface defects of microcellular injection molded components, several approaches, some of which were previously used for structural foam molding, have been used. These approaches include the following: hot/cold mold cycling [10], expanding mold, insulating film coating [II. 12], mold surface treatment [13], co-injection molding [14], gas counter-pressure [15, 16], and blowing agent application of expandable thermoplastic microspheres [17, 18].

The relatively rough surface characteristics of microcellular injection molded parts are mainly due to gas emerging from the polymer melt as well as cells nucleating and forming at the advancing melt front during the filling stage. Those nucleating cells at the melt front are first pushed and elongated toward the mold surface resulting from the "fountain flow" behavior (an outward velocity trajectory as material at the hot core turns toward the mold surface and subsequently solidifies) and subsequently trapped, collapsed, and stretched at the part-mold interface by the incoming melt. Combined with escaped gas from the polymer/gas solution near the part surface, silver streaks and swirling patterns become visible at the part-mold interface. To improve the surface quality of microcellular injection molded parts, a strategy is needed to prevent gas from coming out of the polymer melt or to delay cell nucleation during the mold filling process. Once a solid, unfoamed skin layer is established due to solidification after the polymer melt contacts the cold mold walls, subsequent cell nucleation and growth in the hot core will not affect the external surface of the microcellular injection molded part. To reduce trapped gas on the part-mold interface and develop a relatively simple microcellular injection molding process without dedicated SCF generation and injection systems, attempts were made to use water as the physical blowing agent assisted by nucleating agents to injection mold foamed polycarbonate (PC) parts. Furthermore, functional fillers serving as nucleating agents are able to act as reinforcements to strengthen foamed parts.

Water can be used as either a chemical blowing agent or a physical blowing agent. Water has been widely used as a chemical blowing agent to produce rigid polyurethane foams, in which water reacts with diisocyanate, thus resulting in the reaction products of gaseous carbon dioxide and polyurea [19, 20]. Conversely, water is also capable of acting as a physical blowing agent for rigid polyurethane foams that contain a small amount of water in bamboo residues [21, 22]. In polystyrene foam production, water expandable polystyrene (WEPS) beads containing small water droplets as the blowing agent expand when heated above the glass transition temperature. Those tiny water droplets are encapsulated in beads via suspension polymerization of styrene monomers in an emulsion [23, 24]. To trap more water in the polystyrene beads, nanoclay and carbon particles have been used to increase the water content before bead expansion [25, 26]. Unlike SCF nitrogen or carbon dioxide, water does not dissolve into or emerge from the polymer melt easily or quickly leading to a slower cell nucleation rate and thus reduced cells at the melt front, which is advantageous for improving the surface quality of foamed parts.

In this study, an alternative and cost-effective microcellular injection molding technique is investigated that uses water vapor as the physical blowing agent. This has the potential benefit of reducing equipment costs by eliminating the need for dedicated SCF generation and injection systems. Two kinds of nucleating agents (i.e., cubic NaCl and non-uniform AC) were used to produce vapor-foamed PC parts. In addition to the known benefit of acting as reinforcing fillers, the effects of these two kinds of nucleating agents on the surface roughness, mechanical properties, and microstructure of solid and foamed parts were characterized and compared.

EXPERIMENTAL PROCEDURE

Melt Processing

Polycarbonate (PC, Lexan 141R), which had a melt flow rate of 14.4 g/10 min (300[degrees]C/1.2 kg), was obtained in pellet form from GE plastics. Hi-grade evaporated salt (NaCl) was obtained from Cargill Salt and had an original size range of 200-600 pm. Active carbon (AC) powder of non-uniform shape and an average diameter less than 25 [micro]m (Hardwood Powdered Activated Charcoal of Food Grade; cf. Fig. 1) was obtained from the Multavita Company. Tensile test bars (ASTM D638-03, Type I) of solid and foamed PC were injection molded via an Arburg Allrounder 320S (Lossburg, Germany) molding machine with commercial Mucell SCF injection units. The barrel temperatures from hopper to nozzle were set at 38, 290, 310, 315, 315, and 300[degrees]C, and the mold temperature was 85[degrees]C. The circumference speed of the injection screw was 15 m/min and the injection speed was 60 [cm.sup.3]/s, leading to a very short melt mixing time in the machine barrel (less than 1 min).

A commercially available microcellular injection molding process (MuCell) was used to fabricate microcellular injection molded PC-N tensile test bars as control parts, with N denoting SCF nitrogen. For the foamed molding processes using nitrogen and water vapor as blowing agents, the back pressure of the screw was maintained at 5.5 MPa, as well as 1.0 MPa for the solid parts. Since the expansion of the cells compensated for the volumetric shrinkage of the solidifying melt, no packing/holding pressure was needed for the microcellular injection molding processes.

Two procedures were used to fabricate vapor-foamed microcellular injection molded parts, denoted as PC-NaCl-W and PC-AC-W for using recrystallized NaCl and AC as nucleating agents, respectively. Water was used as a physical blowing agent to fabricate vapor-foamed PC-NaCl-W parts based on a typical injection molding machine with the addition of a valve and meter mounted on top of the barrel hopper to dispense the water (or 2 wt% water/salt solution) and control its feed rate at 0.5 ml/min [27]. At relatively high temperatures (315[degrees]C) and pressures (5.5 MPa) in the machine barrel, the liquid salt solution quickly transformed into pressurized vapor and recrystallized salt crystals that uniformly dispersed in the PC melt matrix. The original salt sizes of 200-600 pm were reduced to ~20 [micro]m after the recrystallization process transition.

PC-AC (0.5 wt%) solid batch pellets were compounded via twin-screw extruder as AC could not be dissolved in water as the salt. To enhance the ability of PC-AC pellets to absorb water, solid pellets were immersed in water in an ultrasonic tank for 6 h, which is different from directly adding a salt solution to the PC melt through the machine barrel. Prior to the microcellular injection molding process, the wet pellets were dried at ambient temperature and the water absorption content was measured every 15 min until there was no obvious water on the surface of the pellet. The average water absorption content of PC-AC pellets for injection molding was controlled at 0.38 wt%. The wet batch pellets were microcellular injection molded into tensile bars with the same processing parameters as previously foamed parts. The foamed parts had an average weight reduction of approximately 6-16% in comparisons with their solid counterparts, as shown in Table 1, together with the calculated nitrogen and water contents in terms of weight percentage. No burns or charring were found on the surface of the vapor foamed PC parts with NaCl as the nucleating agent and water vapor as the blowing agent. Figure 2 shows the overall appearance of a typical microcellular injection molded part, the PC-NaCl-W part, and the PC-AC-W part.

Testing Techniques

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 FTIR spectrometer at a resolution of 1 [cm.sup.-1] in the frequency range of 4000-600 [cm.sup.-1]. The analysis was performed on the surface of the solid and foamed bars at ambient temperature.

Gel permeation chromatography (GPC) was used to measure the molecular weight distribution of polycarbonate (PC) solid and foamed parts. The solvent DMF was used as the mobile phase with a flow rate of 1 ml/min, and the samples were dissolved in DMF at a concentration of 2 mg/ml. Weight-averaged molecular weights and polydispersity indices were calculated from a calibration curve using linear PMMA.

Two-dimensional surface roughness (2D-SR) measurements were taken via a surface roughness analyzer (Surfanalyzer 4000). This instrument amplifies the deflection of the stylus and calculates the surface roughness parameter, Ra, from the reading. Ra is the arithmetic mean of the departures of the profile from the mean and is used here to quantify the roughness of the samples. The stylus traversed 2.5 mm in the melt flow direction (cf. Fig. 3) on the sample surface at a fixed rate of 0.25 mm/s. Fifteen samples were analyzed for each molding trial condition. An optical microscope (Olympus) with a magnification of 100 was used to examine the three-dimensional surface roughness (3D-SR).

The mechanical tensile properties (modulus, strength, and strain-at-break) were measured at room temperature with a 50 kN load cell on an Instron Model 3369 tensile tester. The crosshead speed was set at 50 mm/min for solid and foamed PC samples. All tests were performed according to ASTM D638-03 guidelines. Ten samples from each molding trial were tested and the average results were reported. The cell morphologies (porous microstructures) were observed using a scanning electron microscope (SEM; Model LEO 1530, JEOL, Japan) operated at an accelerating voltage of 3 kV. The as-molded samples were then cryogenically fractured at three locations on the tensile bar with liquid nitrogen, as shown in Fig. 3. After the cross sections perpendicular to the flow direction were sputter coated with gold, the SEM images from those cross sections were taken to reveal the cell morphology. The average cell size and cell density were analyzed using an image analysis tool (UTHSCSA Image Tool).

RESULTS AND DISCUSSION

Analysis of Polycarbonate Degradation

High temperature and moisture are two potential causes of PC thermal degradation or hydrolytic degradation. To check the degradation caused by melt processing and the moisture vapor foaming process, Fourier transform infrared spectrometry (FTIR) was tried in an attempt to locate peaks which might be associated with degradation, and gel permeation chromatography (GPC) was used to determine the average molecular weight and polydispersity due to degradation.

Figure 4 shows the FTIR spectra of solid PC-AC-S and vapor-foamed PC-AC-W parts. The neat PC had major vibrational frequencies at 2800-3200 [cm.sup.-1] corresponding to CH stretching. Absorption peaks at 1771 and 1503 [cm.sup.-1] corresponded to carbonate carbonyl and aromatic C=C stretching vibrations, respectively. After the water vapor foamed microcellular injection molding procedure, the degradation of PC was characterized by a decrease in the IR absorption band at 1100-1250 [cm.sup.-1], with the largest shifts involving the broad C-O stretching at 1220 [cm.sup.-1], the carbonate C-O stretching at 1188 [cm.sup.-1], and the carbonate C-O stretching at 1160 [cm.sup.-1]. The thermal melting procedure was expected to change the rotation of various PC single bands, but the shift in the stretching bands of C-O and C-[H.sub.3], resulted from PC degradation instead. A broad peak at 3400 [cm.sup.-1] was observed on PC-AC-W as a result of the formation of hydroxyl groups, thereby suggesting the hydrolysis of the carbonate bonds.

The molecular weight and molecular weight distribution are critical parameters in evaluating the degradation of a polymer (cf. Table 2). Small reductions in the average-weight molecular weight and average-number molecular weight, as well as a broadened molecular weight distribution, were found for vaporfoamed samples. However, the reductions of PC-AC-W seen here (7.2% reduction compared with solid injection molded PCS parts) were not significant.

Surface Roughness

The two-dimensional surface roughness (2D-SR) was characterized at the middle location of the solid and foamed injection molded tensile test bars along the melt flow direction. The arithmetic 2D-SR averaged across 15 solid and foamed samples is plotted in Fig. 5, while the 3D optical micrographs are shown in Fig. 6. It is evident that solid PC-S exhibited the lowest surface roughness, and PC-NaCl-W presented the most desirable surface roughness, which was comparable to that of PC-S. PCAC-S and PC-AC-W samples showed relatively rough surfaces due to the appearance of AC powders on the parts' surfaces. In comparison with typical solid or vapor-foamed counterparts (i.e., PC-NaCl-W), PC-N parts foamed using SCF nitrogen as the blowing agent possessed a rougher surface due to their characteristic swirling patterns. This suggests that using water vapor as a blowing agent had a positive effect on the surface finish of the foamed parts.

The rougher surface characteristics of microcellular injection molded PC-N parts are mainly attributed to gas escaping from the PC/nitrogen solution and nucleated cells (bubbles) at the melt front and being trapped, collapsed, and stretched at the interface between the part and the mold. Meanwhile, the cells in the center of the tensile test bars will continue to expand before the material solidifies. Thus, a typical multi-layered structure is formed with skin layers of smaller or no visible cells sandwiching a center layer with more and larger cells. Conceivably, some small bubbles in the skin layer of the tensile test bar ruptured at the part-mold interface because of the friction between the PC melt and the mold wall. Thus, gas could diffuse out of the PC/nitrogen solution and be captured at the interface, causing swirls marks on the surface of the part.

In comparisons with PC-N manufactured with a commercialized SCF injection unit, PC-NaCl-W has a desirable surface quality. The water vapor foaming mechanism is different than that of the SCF foaming process. Because of water's low solubility and long absorption time in PC, the water and PC melt cannot quickly form a single-phase solution like PC/nitrogen in the machine barrel. Distilled water, or a water/salt solution, is dispensed into the machine barrel through the hopper, unlike SCF nitrogen which must be injected into the mixing zone of the machine barrel. Recall that the melt temperature and the back pressure within the machine barrel are 300[degrees]C and 5.5 MPa, respectively, whereas the supercritical state for water are at 374[degrees]C and 22.8 MPa, respectively, according to the water phase diagram. Hence, the initial water temperature and pressure in the machine barrel are not high enough for water to convert to the supercritical state. When a PC/water mixture is injected into the mold cavity, the sudden pressure drop will allow the small water vapor droplets to turn into expanding vapor bubbles in the PC matrix. In the meantime, tiny salt particles recrystallized from the water/salt solution will act as nucleating agents, thus greatly reducing the cell size and increasing the cell density in the microcellular PC-NaCl-W parts.

Unlike SCF nitrogen or carbon dioxide and without the nucleating agent, water does not emerge from the polymer melt quickly, which results in a slower cell nucleation rate and thus reduced cells at the melt front. Even when vapor cells are formed at the advancing melt front and subsequently contact the mold wall (85[degrees]C), it will condense at the mold interface, forming lubricating layer that helps to reduce the surface roughness. Such a phase change also induces a large volume contraction in the cells, compared to cells filled with nitrogen that stay in gaseous form, thereby further minimizing any visible surface defects.

Active carbon (AC) powder is another kind of nucleating agent. It has a lower density, smaller size, and higher surface area-per-volume ratio (cf. Fig. 1) in comparison with recrystallized cubic salt crystals, even though they have the same weight content of 0.5 wt%. The surface quality of both solid PC-AC-S and foamed PC-AC-W parts are unsmooth, and the AC powder particles appear on the surface of the parts as shown in Fig. 5b and e. The AC particles on the surface reduce the PC-AC-S surface quality and simultaneously increase the friction of the part-mold interface during mold filling, thus leading to more bubble (or vapor cells) rupture. Although the phase transition from vapor to liquid is effective at reducing the gas volume, the surface quality of foamed PC-AC-W is rougher than that of PC-NaCl-W. Further studies will be needed to investigate the effects of AC ratios and water absorption content on the surface roughness to help determine the optimal compositions and processing conditions.

Tensile Properties

The representative stress-strain curves of solid and foamed parts (PC-N, PC-NaCl-W, and PC-AC-W) are featured in Fig. 7. Table 3 tabulates the values of the average mechanical properties such as Young's modulus, ultimate strength, and strain-at-break of tensile bars according to the ASTM D638-03 standard. Table 3 also lists the specific Young's modulus and ultimate strength taking into consideration the lighter part weight and lesser material used in the foamed parts. As expected, solid injection molded PC-S parts have a higher Young's modulus and ultimate strength than foamed tensile test bars. Apparently, the cells within the foamed part negatively affect the ultimate strength and strain-at-break as they reduce the effective cross sectional area or serve as stress concentration points. Broadly speaking, foamed parts of PC-NaCl-N and PC-AC-W have similar values to that of PC-N, with some specimens outperforming PC-N due to the nucleation agents that also act as reinforcements.

In comparison with solid and other foamed parts, foamed PC-AC-W parts have a higher specific Young's modulus and a specific strength due to the addition of AC powder (size is much less than 25 [micro]m). In the foamed components, AC and NaCl serve not only as nucleating agents, but also as rigid reinforcing fillers. Hence, AC powder can enhance the mechanical properties (i.e., Young's modulus and ultimate strength) of solid and foamed PC composites. At the same weight concentration, AC has more particles due to its low density and small size in comparison with recrystallized NaCl crystals (10-20 [micro]m). Hydrophobic AC powder is capable of achieving a uniform dispersion in the PC matrix via twin-screw extruder compounding, and this uninform dispersion improves the mechanical properties of PC-AC-W. Furthermore, even though the tiny AC particles are not good for the surface quality of foamed parts, they do effectively enhance mechanical properties. The next section will present the cellular microstructure and multi-layered structure of the tensile test bars, which dictate the mechanical properties of the foamed parts.

It should be pointed out that PC is a hygroscopic material. For the retention of mechanical properties, the moisture content should not be more than 0.01%. Microcellular processing of PC using water as a physical blowing agent may potentially cause hydrolytic degradation of the polymer, thereby leading to inferior material properties. Surprisingly, PC-NaCl-W and PC-AC-W foamed parts had similar tensile properties, even better than PC-N foamed parts, as shown in Table 3. In fact, the main reason for the deterioration of the material properties resulted from the cells (voids) generated by the foaming agents (either nitrogen or water vapor).

Morphology of the Fractured Surface and Microcells

To investigate the effects of blowing agents (SCF nitrogen and water vapor) and nucleating agents (AC and NaCl) on the microstructure of foamed PC samples, cryogenically fractured cross-section surfaces perpendicular to the flow direction were examined using scanning electron microscopy (SEM) at three different locations along the length of the tensile test bar (cf. Fig. 3). Figures 8-10 show the SEM microstructure images of PC-N, PC-NaCl-W, and PC-AC-W samples. All foamed parts exhibited a multi-layer structure; namely, a foamed core region sandwiched between barely foamed skin layers. The skin layers possessed a higher density resulting from fewer cells, whereas the core layer contained most of the cells. Figure 8 presents the microstructure of cells of PC-N parts foamed using SCF nitrogen as the blowing agent. At the end of the tensile test bar there is a thin layer of concentrated tiny cells near the part surface (cf. Fig. 8c). The exact cause for these tiny cells remains unclear although they are thought to have resulted from a high degree of deformation that triggered cell nucleation followed by rapid cooling that froze the cell structure. The average cell size and cell density of microcellular injection molded parts in the core region at the three test locations are plotted in Fig. 11.

Figure 9a-c shows the evolution of cell size along the flow direction of the foamed PC-NaCl-W tensile test bar. It can be seen that the cell size increased from near the gate (Fig. 9a) to the middle of the part (Fig. 9b), and stayed at approximately the same size toward the end of the part as shown in Fig. 9c. The smaller cell size near the gate was due to the high melt pressure. In addition, recrystallized salt crystals were found on the inside cell walls (cf. Fig. 9d), suggesting that they served as nucleating agents. The sizes of the salt crystals shown in Fig. 9d ranged from 10 to 20 [micro]m and were almost 30 times smaller than their original size (200-600 [micro]m). It should be noted that nucleating agents are critical for the water foaming process to generate fine and dense cells in the PC matrix.

Figure 10 presents the microstructure of microcellular injection molded PC-AC-W parts. Vapor-foamed PC-AC-W parts possessed smaller cells than that of PC-N and PC-NaCl-W parts at various counter locations along flow direction. The smaller cell sizes were attributed to high amounts of AC powder serving as nucleating agents. When cell nucleation and cell growth were triggered by water vapor expanding after PC/AC or PC/NaCl was melt injected into the mold cavities, the higher density of the nucleating agents resulted in smaller cell sizes and higher cell densities. Taking the tensile bar profile (cf. Fig. 3) into consideration, location 2 exhibited a tapered crosssection area that signified that the expanding water vapor had a relatively small space in which to expand compared to counter locations 1 and 3. At the mold cavity filling stage, where the advancing melt front experienced the lowest melt pressure (atmospheric pressure for a well-vented cavity), a relatively large number of cells formed and were subsequently pushed toward the mold surface by the "fountain-flow" behavior. As a result, the cells at location 3 were the largest, while those at location 2 were much smaller. The tiny, uniform cell structures at location 2 helped to maintain the foamed parts' mechanical properties.

Moreover, the cell morphology affected the part weight and part weight reduction as well as the surface quality and mechanical properties. Table 1 lists the values of average part weight and weight reduction for solid and foamed tensile test bars not including the runner and sprue. Based on the solid bars, the weight reductions of the foamed parts strongly depended on the processing parameters (i.e., shot volume and injection speed), blowing agents, and nucleating agents. Based on comparisons of foamed parts with the same processing conditions and filler con tent, PC-AC-W achieved the highest average weight reduction at a value of - 16.4%.

CONCLUSIONS

This article introduces a novel vapor-foamed microcellular injection molding process to fabricate PC foamed parts using water as the blowing agent and investigates the effects of various nucleating agents on the surface roughness, mechanical properties, and microstructures of foamed parts. The setup of the vapor-foaming process based on conventional injection molding is much simpler and more cost-effective than commercial microcellular injection molding systems. PC-N with SCF nitrogen as the blowing agent uses thermal instability triggered by a pressure and temperature drop, while PC-NaCl-W and PC-AC-W make use of the expansion of water vapor at low pressure in the mold cavity. PC-NaCl-W foamed parts with recrystallized salt crystals as nucleating agents yielded surface qualities comparable to that of solid PC parts. Conversely, PCAC-W possessed the highest specific mechanical properties (i.e., specific Young's modulus and ultimate strength), as AC particles serve as nucleating agents as well as reinforcing fillers. Foamed PC-NaCl-W has a large cell size of about 250 [micro]m, while foamed PC-AC-W has a much smaller cell size of about 50 [micro]m at a central location on the tensile test bar. PC-AC-W foamed parts also had higher cell densities than their PC-NaCl-W counterparts owing to AC's low density and larger surface area-per-volume ratio. A weight reduction of ~15% was easily achieved by adjusting the shot volume, vapor content, and nucleating agent concentration. High temperature and moisture likely caused some PC degradation, although the average molecular weight and polydispersity showed minor material degradation, likely due to short contact time between PC and water in the machine barrel. Furthermore, PC-AC-W and PC-NaCl-W vapor-foamed parts had better mechanical properties than PC-N parts foamed with SCF nitrogen, thanks to the reinforcing effects of the nucleating NaCl or AC particles.

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Jun Peng, (1,2,3) Xiaofei Sun, (1,2) Haoyang Mi, (1,2,3) Xin Jing, (1,2) Xiang-Fang Peng, (3) Lih-Sheng Turng (1,2)

(1) Polymer Engineering Center, University of Wisconsin-Madison, Madison, Wisconsin 53706

(2) Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, Wisconsin 53715

(3) National Engineering Research Center of Novel Equipment for Polymer Processing, South China University of Technology, Guangzhou 510640, China

Correspondence to: Lih-Sheng Turng; e-mail: turng@engr.wisc.edu Contract grant sponsors: The Wisconsin Institute for Discovery and the Graduate School at the University of Wisconsin-Madison and the Wisconsin Alumni Research Foundation (WARF) Accelerator Program.

DOI 10.1002/pen.24001

TABLE 1. Injection molding parameters of solid and foamed PC
parts using sodium chloride and active carbon as nucleating
agents, respectively.

            Shot volume         Back        Packing    NaCl or
Samples     ([cm.sup.3])   pressure (MPa)   time (s)   AC (wt%)

PC-S            20.5            1.0           4.3        N/A
PC-N            19.5            5.5            0         N/A
PC-NaCl-W       19.5            5.5            0         0.5
PC-AC-W         19.5            5.5            0         0.5

            Nitrogen or      Average part      Average weight
Samples     water (wt%)       weight (g)       reduction (%)

PC-S            N/A       9.40 [+ or -] 0.01        N/A
PC-N           0.22       8.54 [+ or -] 0.11       -9.1%
PC-NaCl-W      1.98       8.81 [+ or -] 0.21       -6.3%
PC-AC-W        0.76       7.86 [+ or -] 0.16       -16.4%

S, N, and W denote solid, nitrogen, and water, respectively.

TABLE 2. Molecular weight of solid and vapor-foamed PC parts.

Samples     [M.sub.w] (g/mol)   [M.sub.n] (g/mol)    PD

PC-S             32,947              19,611         1.68
PC-N             32,183              18,182         1.77
PC-NaCl-W        29,481              15,598         1.89
PC-AC-S          31,594              19,265         1.64
PC-AC-W          30,577              17.472         1.75

TABLE 3. Average mechanical tensile properties of solid
and foamed PC parts with NaCI and AC as nucleating agents.

                                 Specific
               Young's       Young's modulus        Ultimate
Samples     modulus (MPa)   (MPa [m.sup.3]/kg)   strength (MPa)

PC-S          8.12E+02           7.21E-01           5.55E+01
PC-N          6.37E+02           6.23E-01           4.71E+01
PC-NaCl-W     6.95E+02           6.59E-01           4.90E+01
PC-AC-W       7.53E+02           8.00E-01           4.79E+01

                 Specific
            ultimate strength     Strain-at-
Samples     (MPa [m.sup.3]/kg)   break (mm/mm)

PC-S             4.93E-02          9.20E-01
PC-N            4.61 E-02          2.40E-01
PC-NaCl-W        4.65E-02          1.60E-01
PC-AC-W          5.09E-02          9.00E-02
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Author:Peng, Jun; Sun, Xiaofei; Mi, Haoyang; Jing, Xin; Peng, Xiang-Fang; Turng, Lih-Sheng
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jul 1, 2015
Words:5313
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