Microcellular of Glass Fibre Reinforced PC/ABS: Effect of the Processing Condition on the Morphology and Mechanical Properties

Transcription

1 Microcellular of Glass Fibre Reinforced PC/ABS: Effect of the Processing Condition on the Morphology and Mechanical Properties Andrzej K. Bledzki and Joanna Kühn-Gajdzik Institut für Werkstofftechnik, University of Kassel, Mönchebergstraße 3, 3419 Kassel, Germany Received: 15 October 29, Accepted: 17 December 29 INTRODUCTION Engineering polymers such as fibre-reinforced thermoplastics play a major role in the automobile industry because of the very good strength and stiffness of the final product. The other challenge is to create new lightweight parts with properties comparable to solid materials. That is why the injection moulding of microcellular polymer is expected to be an increasable promise for engineering applications. Microcellular foam of thermoplastics provides many benefits compared to other conventional materials. One of the most important advantages of the foam is its higher specific flexural modulus (1). The microscopic cell size and a large number of cells in the microcellular material can reduce material consumption as well as improve thermodynamics (2). The experiments with 45 wt.% glass filled PP produced with MuCell technology showed that with increased injection speed the cell nucleation density increases and with increased injection time, the flexural and tensile properties could be improved (3). It is possible to achieve the desired cell size and weight reduction of the foamed part by combining the classical injection moulding process with the so called precision mould opening. During this process, the cavity of the mould is volumetrically filled and, directly after injection, enlarged to the desired part thickness by reducing or even removing the clamping pressure. This process can also be performed with chemical blowing agents. Wall thicknesses achieved with this process can be three or four times the thickness of the initial Smithers Rapra Technology, 21 Cellular Polymers, Vol. 29, No. 1, 21 27

2 Andrzej K. Bledzki and Joanna Kühn-Gajdzik wall. Polypropylene foam parts with a wall thickness from 1 to 12 mm have already been produced (4,5). It was demonstrated that the uniform microcellular structures with a cell diameter less then 1 μm could be reached using a combination of precision mould opening and the gas counter pressure process (4,6). PC/ABS blends are well-known commercial products. Their success on the market is due to the complementary properties of the components. Polycarbonate (PC) contributes to good mechanical and thermal properties whereas acrylonitrile-butadiene-styrene (ABS) provides ease of processability and a reliable notched impact resistance (7). The original purpose of adding glass fibres to polymers was an improvement in stiffness, strength and heat distortion temperature compared to unfilled thermoplastics. In particular, the reinforcement of thermoplastic compounds by short fibres has received special attention because of their use in a variety of engineering applications in both the chemical and automotive industries. The addition of glass fibres enhanced the ultimate tensile strength and modulus and reduced elongation (both to yield and to break) (8). Cell size distribution and cell density of foamed unfilled and glass fiber reinforced ABS samples compared at various pressure drops and rates in the batch process were presented by Mahmoodi and Behravesh in his studies. It could be proved that the cell size of GF-ABS samples is much smaller than that of the foamed unfilled-abs ones. The results indicated that the pressure drop rate does not have any noticeable effect on the cell structure of the microcellular foams in the batch process (produced at room temperature) (9). Many research works have been conducted to study the effect of processing parameters on the microstructure of the foamed materials in various processes (9). However, very few investigations have been performed into the injection moulding process to study the influence of glass fibres on the microstructure of microcellular foams in the solid state of PC/ABS. All mechanisms of cell formation take place mostly in the amorphous regions of the polymer which is an after-effect of diffusion and absorption processes (1). Microcellular foam bubbles may be nucleated homogeneously or heterogeneously (11). It is well known that the cell density is strongly affected by using fillers during the foaming process. The filler size affects nucleation during the foaming process, largely due to the fact that the heterogeneous nucleation itself has not been well studied (12). 28 Cellular Polymers, Vol. 29, No. 1, 21

3 The number of density and cell size distribution in foam is the result of coupled nucleation and growth, and possibly of coalescence. Ideal nucleants have a uniform size, shape, and surface properties (13). Nucleation in heterogeneous polymers may occur both heterogeneously at the interface between the polymer and the second phase and homogeneously in the free volume of the single matrix phase. Heterogeneous nucleation occurs when a bubble forms at an interface between two phases such as a polymer and an additive (11). The gas absorption behaviour of filled and unfilled polymers was investigated to explain heterogeneous nucleation in filled polymers. It was found that filled polymers absorb more gas compared to unfilled ones. The gas accumulates between polymer and filler and helps to create nucleation during the foaming process (14). Lin has found an important relationship between cell distribution and mechanical properties of the high impact polystyrene. Smaller cells are able to distribute the tensile force better than larger ones because smaller cells are usually associated with lower gas volume fractions (15). Impact strength is closely related to cell morphology, in addition to weight reduction percentage and skin thickness. Fine cells and uniform distribution are critical factors for a high impact strength, and very helpful for increasing the tensile and flexural strength as well (16). The investigation with natural fibre reinforced polyurethane foam indicated that the specific data was only slightly dependent on the cell (microvoid) content (17). The mechanical properties of rigid high density foams can be significantly modified by fibre reinforcement. Foam reinforcement will change properties such as stiffness, strength, impact, coefficient of thermal expansion, and property-temperature sensitivity (18). Desai et al. concluded that the compressive properties of the composite foams do not depend on the fibre length but instead on other parameters, such as the amount of the blowing agent and the fibre weight fraction (19,2). Bledzki et al. found out that the dynamic mechanical properties of natural fibre reinforced epoxy foams were also affected by the fibre type and orientation, as well as the fibre and cell content of the laminates tested (21). It was also observed that the loss energy increased with increasing cell content in impact tests of the reinforced epoxy foams (22). Cellular Polymers, Vol. 29, No. 1, 21 29

4 Andrzej K. Bledzki and Joanna Kühn-Gajdzik In this paper, the influence of microcellular foaming with chemical blowing agents (CBA) on the structure and mechanical properties of compact and glass fibre reinforced polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) is investigated. The main purpose of this study was to define the influence of processing parameters on microcellular foam morphology and also the relationship between cell structures, parameter and mechanical behaviour of a given PC/ABS microcellular foam combination produced in injection moulding process using precision mould opening. The other major objective of this research work was to study the effect of glass fibres on the structure of compact and glass-filled PC/ABS microcellular foams with a desired density reduction and homogeneity distribution of cells. MATERIALS AND PROCESSING All processing conditions were kept constant except the enlargement of the mould cavity (defined density). The analyzed material was supplied by Bayer s Bayblend PC/ABS: one unfilled, T85 (density 1.15 g/cm³), and T884N with 2 wt.% of glass fibres (density 1.25 g/cm³). The material was preheated at 11 C for 4-6 h to remove any moisture prior to injection moulding. To get the microcellular foam, Hydrocerol HP4P was used as chemical blowing agent in this study. It was obtained from Clariant Masterbacht and used as a weight percentage of the polymer. The test bars ( mm³) were prepared from dried granulates (at 8 C for 24 h before processing) with an injection moulding machine (Arburg) equipped with precision mould opening (PMO) according to DIN EN ISO 294 under the following process condition (Table 1): Table 1. Parameters of the microcellular foam injection moulding Melt temperature T ME 26 [ C] Mould temperature T MO 8 [ C] Injection velocity V I 4 [cm³/s] Opening delay time T O 2 [s] Amount of CBA 3 wt. [%] In this study, the standard injection moulding process was combined with precision mould opening with the following enlargement of the mould cavity: 3 Cellular Polymers, Vol. 29, No. 1, 21

5 5% PMO (initial thickness of the part 3.8 mm final thickness of the part 4 mm), 1% PMO (3.6 mm 4 mm), 2% PMO (3.2 mm 4 mm). The fracture mechanism and morphology of PC/ABS microcellular foam was determined by using a scanning electron microscope type Cam Scan MV 23 LV (SEM). The surface of the specimens was fractured in liquid nitrogen and sputter coated with a thin layer of gold to avoid electrostatic charging during examination. The structure performance was accomplished with the optical microscope at the cross section of the sanded surfaces. This investigation determines the average cell diameter, the thickness of the compact skin layer cell distribution and the cell anisotropy factor. The anisotropy factor describes the uniformity and roundness of the cell size and is defined as the ratio of the relationship between maximal and minimal cell diameter. If the anisotropy factor is determined with 1, this means that all cells possess geometries of an ideal circle. The tensile and flexural tests were realized according to EN ISO 527 and EN ISO 178 on the Zwick testing machine, UPM The Charpy notched impact test was performed with an instrumented impact hammer according to DIN EN ISO 179. The notch angle was always 45 ± 1 and the radius of notch was varied: type A:.25 mm ±.5 mm. All test specimens were prepared from the injection moulding test bars. A sketch of the bars is shown in Figure 1. Figure 1. Sketch of the sample and the bars for tensile, flexural and Charpy measurement Cellular Polymers, Vol. 29, No. 1, 21 31

6 Andrzej K. Bledzki and Joanna Kühn-Gajdzik RESULTS AND DISCUSSION Morphology The morphology of microcellular foam can be reasonably characterized as a three-layer sandwich structure: compact skin layer foam core centre compact skin layer. The enlargement of the cavity leads to higher nucleation and significantly increases the cell size and the thickness of the skin region (Figure 2). The reason for the uniform cell distribution of PC/ABS microcellular foam is the pressure relief of the melt during mould opening. Figure 3 represents the progress of the cell structure in the center area of PC/ABS microcellular foam produced with 3 wt.% of blowing agents with various enlargements of the mould cavity (PMO). In all cases, a microcellular foam structure with a uniform distribution of cells in the center area could be created. At first glance, the cell sizes of unfilled PC/ABS microcellular foam as well as glass fiber reinforced PC/ABS microcellular foam produced with higher density reduction ( ) with 2% PMO are evidently greater than those of other samples. Comparing the microcellular process conditions of both materials produced with 5% PMO, there are no significant differences between the cell sizes; and the trend is similar also with regard to other enlargements of the mould cavity, irrespective of the material. However, it can be noted that with the reinforced PC/ABS microcellular foam, a higher density reduction could be reached. This phenomenon could be explained in the way that the fibre might act as nucleation centres in the matrix promoting a higher cell population during injection moulding. Figure 2. Light micrographs of T85 microcellular foam a) 5% PMO, b) 2% PMO 32 Cellular Polymers, Vol. 29, No. 1, 21

7 Figure 3. SEM micrograph of the centre area of, T85 microcellular foam a) 5% PMO, b) 1% PMO, c) 2% PMO; microcellular foam glass fibre reinforced T884N d) 5% PMO, e) 1% PMO, f) 2% PMO; a-c) without glass fibre, d-e) with glass fibre; (density reduction) One of the disadvantages of glass fibres is their insufficient adhesion to the polymer matrix. This phenomenon can be seen in Figures 3d-e where the glass fibre appear to be intercellular (between cells) and demonstrate a weakened interfacial character indicating a failure at the interface. Figure 4 indicates the effect of density reduction on the thickness of the compact skin layer, the average cell size in the center area and the anisotropy factor with the standard deviation of the microcellular foam samples. The results of the morphology analysis of the microcellular foams are shown in Table 2. The cells of the sample produced with higher enlargement of the cavity (18% ), independent of the polymer matrix, tend toward larger microcells with an average size around 85 μm. By smaller enlargement of the cavity (7% ), the cell size can be reduced up to 5%. At the same time, the thickness of the skin layer decreased from 123 μm to 785 μm in the PC/ ABS microcellular foam and from 863 μm to 763 μm in the reinforced PC/ ABS microcellular foam with increasing density reduction. It is also important to note that the higher cell anisotropy factor occurred in the microcellular foams with glass fibres (Figure 4). These microcellular foams Cellular Polymers, Vol. 29, No. 1, 21 33

8 Andrzej K. Bledzki and Joanna Kühn-Gajdzik Compact skin layer (μm) T85 Anisotropy factor % 8% 18% Average cell diameter (μm) Density reduction (%) Compact skin layer (μm) Anisotropy factor T884N with 2 wt.% GF % 14% 25% Average cell diameter (μm) Density reduction (%) Figure 4. Effect of the density reduction of the compact skin layer and cell diameter left PC/ABS microcellular foam; right reinforced PC/ABS microcellular foam Table 2. Results of the morphology analysis of all microcellular foams Materials Average cell SD Compact SD Anisotropy Density diameter skin region factor [g/cm³] centre area [μm] [μm] T85+5%PMO ,7 1,5 T85+1%PMO ,24 1,3 T85+2%PMO ,38,92 T884N+5%PMO ,23 1,11 T884N+1%PMO ,25 1,7 T884N+2%PMO ,5,94 SD- Standard deviation 34 Cellular Polymers, Vol. 29, No. 1, 21

9 exhibit an irregular cell structure. The cell shape seems to digress from the unfilled microcellular foams of PC/ABS which show an accurate spherical cell geometry. Due to the foaming of unfilled PC/ABS and glass fibre reinforced PC/ABS, a density level in the range of PP could be reached (Table 3). Owing to the increased mechanical characteristics and significantly reduced weight, it offers the possibility to replace some materials for special application. Table 3. Results of the density of the microcellular foams Materials Density [g/cm³] T85+5%PMO 1,5 T85+1%PMO 1,3 T85+2%PMO,92 T884N+5%PMO 1,11 T884N+1%PMO 1,7 T884N+2%PMO,94 The volume fraction and fibre length of glass fibres in the moulded specimens was also determined using ahs test. The length of the glass fibres was not significantly changed during the injection moulding and foaming process and varied between 4-5 μm. Mechanical Properties The mechanical properties of microcellular foams are always related to their morphology, at all levels of detail, and to the properties of the polymer. Figure 5 presents the effect of the compact skin layer on the specific tensile modulus and specific tensile strength of PC/ABS microcellular foam and glass fibre reinforced PC/ABS microcellular foam. Specific properties of foams are the properties divided by density of foams. Due to the lower tensile properties of compact PC/ABS in comparison to reinforced PC/ABS, the tensile modulus of the microcellular foam PC/ABS was observed to be significantly (up to 5%) lower than that of the reinforced PC/ABS microcellular foam (Figure 5). However, the density reduction of both PC/ABS microcellular foams had a considerable effect on the tensile modulus. The specific tensile modulus as well as the specific tensile strength of the PC/ABS (T85) microcellular foam produced with 7% and 8% density reduction was observed to indicate the same value of the tensile modulus irrespective of the different average cell diameter Cellular Polymers, Vol. 29, No. 1, 21 35

10 Andrzej K. Bledzki and Joanna Kühn-Gajdzik Specific tensile modulus (MPa/(g/cm 3 )) T85 Specific tensile modulus Specific tensile strength T Thickness of compact skin layer (μm) Specific tensile strength (xxx) Specific tensile modulus (MPa/(g/cm 3 )) T884N with 2 wt.% GF Specific tensile modulus Specific tensile strength T884N Thickness of compact skin layer (μm) Specific tensile strength (xxx) Figure 5. Effect of the compact skin layer on the specific tensile modulus and specific tensile strength of (left) PC/ABS microcellular foam and (right) glass fibre reinforced PC/ABS microcellular foam and thickness of the compact skin region (Figure 4). The tensile properties of PC/ABS microcellular foam with a thinner compact skin layer tend to drop more rapidly. In case of reinforced PC/ABS (T884N) microcellular foam, a significant linear decrease of the tensile properties with decreasing thickness of the compact skin layer was observed. The level of decrease of the properties of microcellular foam is higher in samples of higher apparent density. The values of the deformation behaviour of microcellular foam of unfilled und reinforced PC/ABS are plotted in Figure 6 as a function of density reduction. The tensile stress at break of both materials had the same decreasing tendency with increasing density reduction. All compact T85 samples broke at an elongation 36 Cellular Polymers, Vol. 29, No. 1, 21

11 Tensile stress at break (MPa) % T85 Tensile stress at break (MPa) Tensile strain at break (%) Tensile strain at break (%) T85 7% 8% 18% Density reduction (%) Tensile stress at break (MPa) T884N with 2 wt.% GF Tensile stress at break (MPa) Tensile strain at break (%) Tensile strain at break (%) T884N 11% 14% 25% Density reduction (%) Figure 6. Tensile stress and tensile strain of microcellular foam PC/ABS and glass fibre reinforced PC/ABS at break vs. density reduction higher than 5% under testing conditions (Figure 6 top). In the microcellular T85 foam, a small increase the percentage of elongation at break was observed with increasing density reduction as well as with decreasing skin thickness. The major role for this behaviour plays the thickness of the compact skin region by defined cell diameter. The improvement in tensile strain at break of T85 microcellular foam from 11% to 18% was associated with a decrease in thickness of the compact skin layer from 123 μm to 785 μm respectively as well as with increase in average cell diameter from 37 μm to 85 μm respectively. The results of this investigation showed that there is a special morphology Cellular Polymers, Vol. 29, No. 1, 21 37

12 Andrzej K. Bledzki and Joanna Kühn-Gajdzik structure with defined cell sizes and a thin compact surface layer to increase the tensile strain at break of microcellular T85 foam. As expected, the introduction of glass fibres into PC/ABS lowers elongation at yielding and at breakage. Evidently, reinforced T884N microcellular foam broke at a strain value which is lower than the strain of the compact polymer (2.6%) and remained quite affected with increasing density reduction (Figure 6 bottom). All of those microcellular foam broke in the rage of 1,5% elongation. Big changes in the density of the samples did not alter the main breaking behaviour. Figure 7 shows the specific flexural modulus of the compact and microcellular foam in bending modes. The specific flexural modulus of compact PC/ABS and PC/ABS with glass fibres is 1.84 [GPa/(g/cm³)] and 3.84 [GPa/(g/cm³)] respectively. As the density reduction of the microcellular foam glass fibre reinforced PC/ABS increased, the specific flexural modulus increased linearly. There is an improvement of up to 1% in the case of higher density reduction combined with greater cell size. Result shows that sandwich structure reinforced with glass fiber have a higher specific flexural stiffness comparing with compact material as thickness of the microcellular foam increase. Due to the increase of compact skin layer thickness in the sandwich structure the higher stress can be concentrated. In the core of the microcellular foam, the stress at the horizontal plane of the neutral is zero. The specific flexural Specific Flexural E Modul (MPa/(g/cm 3 ) T88N with 2wt.% GF T85 Compact 5% PMO 1% PMO 2% PMO Figure 7. Specific flexural modulus of microcellular foam PC/ABS and glass fibre reinforced PC/ABS vs. various rate of enlargement of mould cavity 38 Cellular Polymers, Vol. 29, No. 1, 21

13 modulus of microcellular foam PC/ABS has been noted as not being dependent on the changed compact skin layer, the average cell diameter as well as the processing conditions and more closely resembling compact material. The PC/ABS compact and the microcellular foam without glass fibres did not break under the given flexural testing conditions. In contrast, all of the compact and microcellular foam reinforced PC/ABS samples appeared to break at the same elongation between 2.5% - 2.9%. Figure 8 shows the effect of the compact skin layer on the Charpy impact strength of PC/ABS microcellular foam and reinforced PC/ABS microcellular foam. Charpy notched impact strength (kj/m 2 ) Density reduction 7% T85 8% 18% Compact T μm 9 μm 78 μm Thickness of compact skin layer (μm) RT -3 C Charpy notched impact strength (kj/m 2 ) Density reduction T884N with 2wt.% GF 11% 14% 25% Compact T88N 86 μm 84 μm 76 μm Thickness of compact skin layer (μm) RT -3 C Figure 8. Effect of the compact skin layer on Charpy impact strength vs. (top) PC/ABS microcellular foam; (bottom) reinforced PC/ABS microcellular foam Cellular Polymers, Vol. 29, No. 1, 21 39

14 Andrzej K. Bledzki and Joanna Kühn-Gajdzik When the reduced skin thickness of glass fibre reinforced PC/ABS microcellular foam remains nearly constant (Figure 4), the values of Charpy notched impact strength observed in Figure 8 also remain constant at room temperature (RT) and also at -3 C irrespective of density reduction. Compared to compact PC/ABS samples, PC/ABS microcellular foam as well as reinforced PC/ABS microcellular foam samples showed lower Charpy impact testing values. A higher density reduction of reinforced PC/ABS did not cause any property changes at both testing conditions with respect to the samples at higher density which is similar to what was observed for the PC/ ABS microcellular foam samples. Despite the uniform distribution of cells and the thicker compact skin region of all these systems, the reduction of the notched impact strength did not significantly improve with higher density. Figure 9 shows the fracture surface of compact and microcellular foam T85 and T884N at room temperature. Compact T85 (Figure 9a) exhibits a ductile behaviour with a clearly deformed zone and distinct flow lines on the fracture surface at room temperature as well as at lower temperature. After foaming, the PC/ABS appeared to be brittle which the fracture surface of the microcellular foam PC/ABS indicates (Figure 9b). The present of cell induced low stress concentration in the surrounding PC/ABS matrix, giving significantly drop to a local shear yielding mechanism with a decrease of up to 2% in Charpy notched impact strength. The brittle behaviour, including the fracture surface, of reinforced PC/ABS can be observed in Figures 9c and d. Figure 9. Effect of the cell structure on the fracture behaviour of compact and microcellular foam T85 and T884N 4 Cellular Polymers, Vol. 29, No. 1, 21

15 CONCLUSION Using several process techniques of foaming as well as different rates of enlargement of the mould cavity, a suitable uniform morphology with desired cell diameter and compact skin region can be reached. The cell diameters of PC/ABS microcellular foam as well as reinforced PC/ABS microcellular foam were significantly reduced from 85 μm to 5 μm by using a small rate of enlargement of the mould cavity (small density reduction). This caused additional pressure relief and improved the uniform cell nucleation over the complete cross section of the sample. The addition of glass fibres to PC/ ABS microcellular foam promoted a higher number of cell nucleation and produced a uniform microcellular structure. The higher density reduction of the reinforced PC/ABS microcellular foam could be reached due to the heterogeneous nucleation of the glass fibres with a large number of cells. The glass fibres served as an effective nucleating agent, which tended to activate the instantaneous nucleation and uniform growth and cell distribution. The relationship of the processing parameters with the mechanical properties of PC/ABS microcellular as well as reinforced PC/ABS microcellular foam depends on the cell diameter, the cell distribution and the thickness of the compact skin region. It should be considered that the microcellular foam density and skin thickness presented a major factor in determining the mechanical properties. It has been proved that the most important advantage of reinforced PC/ABS microcellular foam is its higher specific flexural modulus. The breaking mechanism in the Charpy test of PC/ABS microcellular foam and reinforced PC/ABS microcellular foam was not affected by the density reduction and thickness of the compact skin region. One of the main aims of foaming is reducing the weight of the product. Due to a continuous requirement of improvement of the specific mechanical behaviour of foamed materials, it is possible to replace compact materials with foamed ones which at the same time possess better mechanical properties. By foaming unfilled PC/ABS and glass fibre reinforced PC/ABS, a density level in the range of PP could be reached (.92 g/cm³). It is also necessary to take into account the cost factor and the material application aimed at. It has been proved that the most important advantages of glass fibre reinforced PC/ABS microcellular foam are its constant properties like Charpy notched impact strength at RT and -3 C, tensile strain at breakage, flexural strain at breakage during higher density reduction with a larger cell diameter and thin compact skin layer. In case of PC/ABS microcellular foam without glass fibres, the optimum for tensile strain at breakage appears to be larger microcells and a thinner compact skin layer as well. Cellular Polymers, Vol. 29, No. 1, 21 41

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