Effect of acrylonitrile–butadiene–styrene terpolymer on the foaming behavior of polypropylene

Abstract

To improve the cellular foam structure of common polypropylene (PP), acrylonitrile–butadiene–styrene terpolymer (ABS) and compatibilizer were used to blend with PP, and the foaming behavior of PP/ABS blends was investigated. The solid and foamed samples of the PP/ABS blend with different component were first fabricated by melt extrusion followed by conventional injection molding with or without a blowing agent. The mechanical properties, thermal features, and rheological characterizations of these samples were studied using the tensile test, dynamic mechanical analyzer, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and torque rheometry. The results suggest that ABS is a suitable candidate to improve the foamability of PP. The effect of ABS and compatibilizer on the foamability of PP can be attributed to three possible mechanisms, that is, the weak interaction between phases that facilitates cell nucleation, the improved gas-melt viscosity that prevents the escape of gas, and the influence of crystallization behavior that helps to form a fine foaming structure.

Keywords

Foaming compatibility PP/ABS crystallization injection molding

Introduction

Polypropylene (PP) foams are one of the most widely used thermoplastic foaming materials due to their advantages such as lightweight, high melting point, low material cost, excellent chemical resistance, and outstanding mechanical performance.^1,2 However, it is difficult to obtain a fine foamed structure for pure linear PP resin.³ There are two possible reasons, namely, that the low melt strength leads to easy cell rupture or that the semicrystalline characteristic results in difficult gas diffusion in the crystalline region. It has often been reported that blending and/or compounding with other fillers can be successful methods to improve foaming structures,^{4

-19} in addition to improvements in melt strength via self-modifying such as ionic modification²⁰ and grafting.²¹ According to the appearance and structure, the fillers can be roughly classified into three categories: (1) micro- or nanoparticles,^4

-8 (2) fibers,^9

-14 and (3) other polymers.^15

-19 The effect of micro- or nanoparticles on the foaming behavior of PP resins using various types of blowing agents has been widely investigated, and it is believed that the presence of the particles, which includes talc,⁵ carbon nanoparticles,⁶ nanoclay,⁷ and nano-calcium carbonate,⁸ in the PP resins could act as nucleating agents, resulting in a decreased cell radius and a more uniform cell structure. Utilizing fibers as reinforcements is another attractive method to improve the foaming behavior and mechanical properties of PP matrices. Glass fibers,^9

-12 carbon fibers,¹³ and wood fibers ¹⁴ were shown to be effective in improving the foam structure of PP matrix. In addition, it was also found that PP foamability could be significantly enhanced by blending with other polymers such as high-density polyethylene,¹⁵ low-density polyethylene,^16,17 polystyrene,¹⁸ and ethylene–octene copolymer.¹⁹ The two phases of these blend systems are often viewed as immiscible and/or incompatible. However, the differences among phases could offer many opportunities for foaming. For example, it was often reported that the poorly bonded interfaces between the immiscible phases can decrease the gas bubble nucleation energy, which thus facilitates the formation of microcellular structures.²²

Foaming of two-phase blends can be identified as an effective approach to satisfy the steadily growing demand for PP foams with enhanced properties. Acrylonitrile–butadiene–styrene terpolymer (ABS) is an important engineering plastic with excellent mechanical performance, easy processing, and economy. However, there is not yet any published report on the foaming behavior of PP/ABS blends, possibly because the mechanical properties of the PP/ABS blends are even more unsatisfactory than neat PP resin.²³ The weak interfacial interaction between the PP and ABS phases usually cannot prevent the initiation and propagation of cracks along the interface; consequently, PP/ABS blends usually exhibit poor fracture toughness. The poor mechanical properties of the PP/ABS blend may limit its use in various applications and may explain the relatively scarce reports of PP/ABS blends. In fact, it is still possible to blend ABS with PP to combine the advantages of the two resins. For example, montmorillonite and PP-grafted maleic anhydride have been shown to improve the compatibility and elevate the mechanical properties of the blends.²⁴ In addition, the high melt strength of ABS and the incompatibility between PP and ABS appear to benefit the nucleation of PP/ABS foams. Therefore, it is expected that the addition of ABS can modify the foaming behavior of PP. As a result, it would be possible to produce a foamed part of the PP/ABS blend with improved cellular structures. This case arouses our curiosity to explore the possibility of foamability, enhancing the effect of ABS when added to the PP foamed sample.

Combining the sophisticated fields of PP/ABS blends and foams offers the potential for new materials but also poses a significant challenge, as the multiphase characteristics of PP/ABS blends and the complexity of foam processing must be considered. In this study, a method to improve the processing ability of the PP injection-molded foamed component was introduced and the results were carefully compared. A better understanding of the influence of ABS and compatibilizer on the cellular structure, foam morphology, and crystallization behavior of PP/ABS blends was gained from the results and from comparisons with pure PP and ABS specimens.

Experimental work

Materials

The ABS (Qimei, 757) and PP (Huajin, T30S) used in this study were commercial products made in China. Their melt flow indexes were approximately 1.8 g 10 min⁻¹ (200°C, 49.0 N) and 3.2 g 10 min⁻¹ (190°C, 21.6 N), respectively. The ABS content of PP/ABS blends was fixed as 10% and 25% by weight, respectively. Poly(ethylene-co-glycidyl methacrylate) (PE-c-GMA, with a GMA of 8.0 wt%) was used as the compatibilizer. It was purchased from Sigma-Aldrich Co., Ltd (St. Louis, MO, USA), and its melt flow index was approximately 5 g 10 min⁻¹ (190°C, 21.6 N). The above materials were used as received and in pellet form, and were dried in an oven at 80°C for 10 h to remove any moisture. In addition, the azodicarbonamide (AC, Dn8) was used as a blowing agent and the zinc oxide (ZnO) was used as a blowing auxiliary agent. The decomposition temperature of the original AC powder was approximately 200°C, and ZnO could lower the decomposition temperature of AC. All materials used for samples are listed in Table 1. After many experiments, the ratio of AC and ZnO was fixed as 1:0.096 for a suitable decomposition temperature. As a result, the thermal decomposition behavior of the AC/ZnO composite was characterized using a differential scanning calorimetry (DSC) test with a scan rate of 10°C min⁻¹, and the results are shown in Figure 1. As shown in Figure 1, the decomposition temperature of the composite foaming agent was 138–171°C with a peak maximum of approximately 162°C, which was clearly close to the melt temperature (165°C) of neat PP resin. For convenience, a foaming masterbatch including the AC/ZnO composite was fabricated. The content of AC/ZnO composite in the PP/ABS blends was optimized in previous experiments and fixed at 0.8% in this study.

Table 1.

Materials used for processing samples.

Function	Material	Designation	Description
Polymer	PP	Huajin, T30S	MFI: 3.2 g 10 min⁻¹ (190°C, 21.6 N)
Polymer	ABS	Qimei, 757	MFI: 1.8 g 10 min⁻¹ (200°C, 49.0 N)
Compatibilizer	PE-c-GMA	with a GMA of 8.0 wt%	MFI: 5.0 g 10 min⁻¹ (190°C, 21.6 N)
Blowing agent	AC	Dn8	Decomposition temperature of 200°C
Blowing auxiliary agent	ZnO

PP: polypropylene; ABS: acrylonitrile–butadiene–styrene terpolymer; AC: azodicarbonamide; ZnO: zinc oxide; PE-c-GMA: poly(ethylene-co-glycidyl methacrylate); MFI: melt flow index.

Figure 1.

DSC heating flow curve of AC/ZnO, PP, and ABS used at a heating rate of 10°C min⁻¹.

Sample preparation

Sample preparation included two main stages, materials preparation and sample fabrication. In the stage of materials preparation, the dried PP and ABS were firstly blended using a high-speed mixer, and the compositions of the PP/ABS blends by weight percentage were 0/100, 10/90, 25/75, and 100/0. According to the different weight ratios of ABS and PP, the PP/ABS blends were named AP01, AP19, AP13, and AP10, respectively. PE-c-GMA was added and mixed with half of the four blends. The weight ratio of PE-c-GMA in the PP/ABS blends was fixed at 5% after a screening procedure and named as “G” in this study for brevity. There are thus eight PP/ABS blends, that is, AP01, AP01G, AP19, AP19G, AP13, AP13G, AP10, and AP10G.

The foamed samples were fabricated using eight different PP/ABS blends in a conventional injection molding (CIM) machine with the addition of AC/ZnO. The optimal processing parameters were obtained as follows: injection machine melt temperature of 205°C, mold temperature of 60°C, injection pressure of 60 MPa, injection time of 1.5 s, and cooling time of 25 s. The packing time and pressure optimized results were 1.5 s and 15 MPa, respectively. For comparison, solid samples were prepared with the same thermal-mechanical history of the foamed samples but without AC/ZnO; the packing pressure was fixed at 55 MPa, and the packing time was 4 s. A standard tensile test bar mold was used to mold the samples, and its temperature was controlled by circulating oil from a thermal controller. The volume of the cavity of the mold was approximately 9.582 × 10³ mm³, and a detailed description of the molded sample is shown in Figure 2.

Figure 2.

Detailed description of sample size and preparation.

Sample tests

Tensile tests were carried out according to ASTM D638-10 standards, using a screw-driven universal testing instrument (MTS, Sintech 10/GL, Eden Prairie, Minnesota, USA) under ambient conditions. Crosshead speeds of 10 mm min⁻¹ were used to study the stress and strain behavior of the molded tensile samples. Seven tensile bars were tested for each material, and the biggest and the smallest values were excluded. Hence, there were five values selected for analyzing, and the mean and range of ultimate tensile strength, strain at break, and Young’s modulus for each group of samples were calculated and reported. In addition, the tensile strength shown in this study was nominal for convenience. It was obtained by dividing the maximum load by the original cross-sectional area.

The morphologies of the selected molded specimens were examined using a scanning electron microscope (SEM, JEOL JSM-6480, Tokyo, Japan) with an accelerating voltage of 20 kV. The SEM specimens, which are also shown in Figure 3, were taken from the cross-section at the middle of the molded tensile bar that was fractured in liquid nitrogen (N₂). The surfaces of the fractured specimens were sputter coated with gold prior to observation for a period of 60 s. The coating equipment used is an auto sputter coater (Cressington 108, Watford, England). In addition, to better characterize the structural details, the fractured surfaces of the molded samples were chemically etched and then also observed by SEM. Two kinds of etchant were used to reveal the characteristic details and surface topography of the selected samples. Dichloromethane was used to remove ABS from PP/ABS blends, and a solution of sulfuric acid, phosphoric acid, and distilled water was used to expose the crystals morphology of PP. The two kinds of etching lasted for 2 h and 16 h at a temperature of 25°C, respectively. After that, the etched surfaces were washed, dried, and covered with a thin layer of gold for SEM observation.

Figure 3.

SEM images of the eight selective foamed PP/ABS samples: (a) AP01, (b) AP01G, (c) AP19, (d) AP19G, (e) AP13, (f) AP13G, (g) AP10, and (h) AP10G.

Furthermore, the quantitative analysis based on the information of the cell morphology provided by the SEM pictures was performed using an image processing tool of the ImageJ® software (Version v1.64) package (Bethesda, Maryland, USA). The cell diameter could be calculated with the hypothesis of spherical shape cells after the area of each cell in the SEM picture was assessed, and then the mean cell size of the foams could be evaluated. Cell density, the number of cell created per unit volume (cm³), in the foamed samples could be calculated by using the following equation²⁵

N = {(\frac{n}{A})}^{3 / 2} \times \frac{ρ_{s}}{ρ_{f}}

where n is the number of cells in the SEM micrograph, A is the area of the micrograph (cm²), and ρ _s and ρ _f are the density of the solid and foamed materials (g cm⁻³). The density of the solid and the foamed samples was determined by water displacement method according to ISO 1183-1987, respectively, and the results of the eight kinds of PP/ABS blends are listed in Table 2. A 10–12% decrease of the density can be found by the comparison between the solid and the foamed samples.

Table 2.

Density of the eight kinds of PP/ABS solid and foamed molded samples.^a

Samples	Solid	Foamed
AP01	0.908 × 10³	0.819 × 10³
AP01G	0.907 × 10³	0.818 × 10³
AP19	0.922 × 10³	0.815 × 10³
AP19G	0.921 × 10³	0.810 × 10³
AP13	1.009 × 10³	0.896 × 10³
AP13G	1.008 × 10³	0.893 × 10³
AP10	1.043 × 10³	0.928 × 10³
AP10G	1.042 × 10³	0.926 × 10³

PP: polypropylene; ABS: acrylonitrile–butadiene–styrene terpolymer.

^aUnit: kg m⁻³.

A PerkinElmer DSC-8000 (Shanghai, China) apparatus was used to study the thermal behavior of the molded samples. The DSC specimens were also cut from the middle of the selected molded samples, and the detailed locations are also shown in Figure 2. To avoid the possible influence, the DSC specimens were strictly weighed in the range of approximately 4.9–5.1 mg and crimped in aluminum pans loaded at 20°C. The selected specimens were then heated directly from room temperature to 220°C at a rate of 10°C min⁻¹ and then held for 5 min to eliminate any possible thermal and stress history. Then the specimens were cooled to 60°C at a rate of 10°C min⁻¹ to analyze the nonisothermal crystallization kinetics of PP/ABS blends.

X-ray diffraction (XRD) analysis was carried out on a Philips X’Pert X-ray diffractometer (PANALYTICAL, Almelo, Holland) operating at 40 kV and 30 mA with nickel-filtered copper K _α radiation. The XRD patterns were recorded in a scan range of approximately 5–30° with a scan speed of 3° min⁻¹ and a step size of 0.02°.

To determine the interaction on a molecular level between the different components in the polymer blend, dynamic mechanical properties were studied using a dynamic mechanical analyzer (DMA 242E, NETZSCH, Selb, Germany) instrument. As shown in Figure 2, the gauge length of the standard tensile test sample was 80 mm. Hence, the rectangular bars with a 50 mm length, 10 mm width, and 4 mm thickness could be obtained for the DMA test by slicing the gauge section of the tensile bar. The DMA tests were performed using a three-point bending mode at approximately 30–160°C and operated at 1 Hz with the heating rate 3°C min⁻¹ under N₂ atmosphere.

The rheological tests were carried out on a torque rheometer (Haake System 90, Thermo Fisher Scientific, Karlsruhe, Germany) with a measure head (mixing room) of 60 cm³. The rotation speed relation between rollers was 2/3. The different composition of PP/ABS blends with and without the addition of AC/ZnO were tested at 200°C under air atmosphere. The measuring head was generally loaded to 91% of its volume capacity during all the test, in order to avoid the influence of apparent filling degree on torque.²⁶

Results and discussion

Foaming behavior

Typical SEM micrographs of the fractured surfaces of the foamed PP/ABS blend samples are shown in Figure 3. As observed from Figure 3, all foams exhibit a completely closed cell cellular structure. It can also be seen in Figure 3 that the detailed foamed structures are clearly different, owing to the addition of ABS and PE-c-GMA. Compared to the neat PP resin shown in Figure 3(a), the number of cells increases and the size of the cells decreases when the content of ABS is 10%, as shown in Figure 3(c), indicating a uniformly distributed foamed structure. This suggests that the addition of ABS could facilitate the foaming behavior of the PP resins. Furthermore, it can easily be observed that the foamed structure shown in Figure 3(d) is probably the most uniform among all eight pictures shown in Figure 3. This can be attributed to the addition of PE-c-GMA. In addition, from the direct observation and comparison of Figure 3, the following three qualitative conclusions are possible: (1) the addition of PE-c-GMA results in poor foamability, which is evidenced by comparing Figure 3(b) with Figure 3(a), where increased cell size and decreased cell density for PP foams can be observed; (2) there is no obvious difference between Figure 3(e) and Figure 3(f); and (3) PE-c-GMA could improve the cellular structure of neat ABS resin.

Figure 4 shows the mean cell size and cell density of the PP/ABS blend foams, and these data support the morphological qualitative analysis results. As shown in Figure 4, the common PP foamed sample (AP01) has a cell density of 3.4 × 10⁵ cells cm⁻³ and an average cell size of 112 µm. Compared with the AP01 sample, the sample with 10 wt% ABS (AP19) has a more uniform cell size distribution, with a cell density of 9.8 × 10⁵ cells cm⁻³ and an average cell size of 62 µm. Furthermore, the cell distribution of the sample with 10 wt% ABS and 5% PE-c-GMA (AP19G) is most uniform because it has the largest cell density of 1.2 × 10⁶ cells cm⁻³ and the smallest average cell size of 53 µm. Thus, the addition of ABS and the compatibilizer PE-c-GMA could facilitate the foaming behavior of PP resins.

Figure 4.

Statistic results of cell size (a) and cell density (b) from SEM images of Figure 3.

It must be noted that it is not easy to fabricate a foamed sample of PP resin with a nucleation density higher than 1.0 × 10⁶ cells cm⁻³ using the CIM method presented in this study. For example, cell density is generally reported as low as 1.0 × 10⁵ cells cm⁻³, with a cell size larger than 100 µm for the neat PP resin and PP with talc.⁵ This could be attributed to the special requirement for injection molding technology. For example, the melt temperature is generally as high as 200°C in CIM operations for the polymer melt flowing easily into the mold. However, PP has a very narrow foaming temperature window, and 150–160°C is often viewed as its suitable foaming temperature. Hence, the cell density of 10⁶ cells cm⁻³ in a common, conventional PP foamed system can be easily obtained via batch foaming processing, and the maximum cell density can be as high as 10⁸ cells cm⁻³ after optimization by filling or modifying.³ However, it is very difficult to obtain a cell density of 10⁶ cells cm⁻³ for conventional injection-molded PP samples. Furthermore, the uniformity can be easily observed for the neat PP foaming when the cell density is higher enough.²⁷ As a result, the method of fabrication of PP foamed components with the addition of ABS and compatibilizers might be a revolutionary route to conserving materials. It would be optimal if the foamed samples could replace solid samples with 10% material reductions without significantly compromising the material properties. Hence, the mechanical properties must be further investigated.

Mechanical properties

Tensile tests were performed for the PP/ABS blend solid and foamed samples, and the results for the tensile strength, strain at break, and Young’s modulus tests are shown in Figure 5. The tensile strength of the mold solid samples of PP/ABS blends, with or without the addition of PE-c-GMA, is even lower than that of samples fabricated using neat PP resin. The strain at break results also show a similar trend. Hence, it can be concluded that the addition of ABS cannot improve the mechanical properties of PP resin. The poor mechanical properties of the PP/ABS blend can be attributed to the incompatibility between PP and ABS phases. However, compared to the solid samples, the foamed sample of the AP19G blend, with an ABS content of 10% and the addition of PE-c-GMA, shows higher tensile strength than that of the neat PP and PP/PE-c-GMA blend. As seen in Figure 5, the addition of 5 wt% PE-c-GMA to the PP/ABS blend foamed samples causes a substantial improvement in the tensile strength and strain at break of approximately 10.9% and 73.7%, respectively, while the increase is 3.1% and 24.3% for its solid samples. Hence, the incorporation of PE-c-GMA into PP/ABS blend foamed samples can lead to higher strength and strain at break, which could significantly extend its use in various applications where the foamed materials require a small cell size and high cell density. The improvement in tensile strength and strain at break of the PP/ABS blend upon addition of PE-c-GMA can be attributed to improved compatibility. The use of the compatibilizer reduces the interfacial tension between PP and ABS, subsequently allowing ABS to contribute strength much more effectively than in uncompatibilized blends. It must be noted that tensile modulus, which are shown in Figure 5(c), have a similar trend to the tensile strength of the molded solid samples.

Figure 5.

Tensile strength (a), strain at break (b), and Young’s modulus (c) of the molded samples.

The mechanical properties of PP/ABS blends shown in Figure 5 can further be verified by the DMA test results shown in Figure 6. Figure 6(a) shows the storage modulus versus temperature curves of PP/ABS foamed samples. For convenient comparison, the DMA result of the compatibilizer PE-c-GMA was also listed in Figure 6. As shown in Figure 6(a), the storage modulus of the pure ABS and ABS/ PE-c-GMA blend is almost unchanged below the glass transition temperature (T _g, approximately 100°C) and suddenly decreases as the temperature exceeds its T _g. However, the storage modulus of the PP and PP/ABS blends decreases with increased temperature in the experimental temperature range of 30–160°C, higher than the T _g of PP (generally reported as −30°C for the homopolymer PP used in this study²⁸). Furthermore, the storage modulus of the AP10G sample is higher than those of the AP10, AP01, and AP01G samples, indicating that PE-c-GMA improves the storage modulus. This can be attributed to the improvement of the compatibility of PP/ABS due to the addition of PE-c-GMA. The compatibility of PP/ABS was further examined by determining the T _g of ABS, shown as the loss factor versus temperature curves in Figure 6(c) for different PP/ABS blends. The T _g of ABS decreases from 110.5°C to 99.6°C and 98.5°C with the increase in PP. The loss factor peak temperature further shifted to 98.1°C and 96.6°C with the addition of PE-c-GMA for the AP19 and AP13 samples, respectively. These results from the DMA test show that the compatibility of PP/ABS blends increases with the addition of PE-c-GMA.

Figure 6.

DMA curves (storage modulus (a) and loss factor (b) vs. temperature) of the eight selective foamed PP/ABS samples and the compatibilizer PE-c-GMA (simplified as G).

Compatibility

Compatibility is a well-accepted, efficient method for enhancing the interfacial adhesion between blend partners, resulting in improved mechanical properties. To further examine the compatibility of PP and ABS, SEM, one of the most popular techniques for characterizing blend morphologies, was used here. Figure 7 shows the morphological characterization of the fracture surfaces of the eight representative samples. Figure 7(a) and (b) shows the morphology of the pure PP and PP/PE-c-GMA blend, respectively. As seen from the two figures, there is no substantial difference between the two samples, indicating good compatibility of PP and PE-c-GMA. Furthermore, as shown in Figure 7(g) and (h), the pure ABS and PE-c-GMA also have a good compatibility. However, compared to PP and PE-c-GMA and/or ABS and ABS/PE-c-GMA, the compatibility of PP and ABS is obviously poorer, as PP clearly forms a continuous phase, while ABS forms large domains, as shown in Figure 7(c). It can also be observed from Figure 7(c) that most of the ABS particles in the PP/ABS blends are spherical, with a mean diameter of dispersed particles of approximately 10 µm. However, there is an irregularly shaped dark region in the interface, as magnified and shown in Figure 7(i). This is likely due to the nonuniformly distributed ABS droplets that are debonded and removed from the matrix during deformation. Hence, it can be concluded from the coarse and heterogenous phase dispersions at the PP/ABS interface that poor adhesion and high interfacial tension form between the two phases. For this reason, the mechanical properties of the AP10 sample do not sufficiently increase.

Figure 7.

SEM images of the molded samples: (a) AP01, (b) AP01G, (c) AP19, (d) AP19G, (e) AP13, (f) AP13G, (g) AP10, (h) AP10G, (i) AP19 magnification, and (j) AP19G magnification.

When PE-c-GMA is added to the PP/ABS blend (Figure 7(d)), the mean diameter of the dispersed particles decreases to approximately 5 µm, which indicates an increase in compatibility between PP and ABS. Thus, the interfacial adhesion between PP and ABS phases is improved in the presence of PE-c-GMA, subsequently reducing the interfacial tension between the two phases, and leading to good mechanical properties. This can be attributed to the dipole–dipole interactions between the polar interface of the glycidyl methacrylate group of PE-c-GMA and the nitrile group of ABS,²⁴ indicating that PE-c-GMA is an efficient alternative for the reactive compatibilization of the PP/ABS blend. However, the dispersion of ABS particles in the PP matrix appears to not be thoroughly homogeneous, as shown in the magnified photo in Figure 7(j), since PE-c-GMA can react only with one phase and can only be miscible with the other phase, located in the interface between the two phases. Hence, it can be concluded that the compatibility of PP/ABS is partially improved with the addition of PE-c-GMA.

The partial compatibility of PP/ABS/PE-c-GMA can be further verified via the morphological comparison of Figure 7(e) with Figure 7(f). For the AP13 blends, the addition of PE-c-GMA somewhat decreases the dispersed particle size. There is only a slight improvement in the adhesion between the PP and ABS phases that may be attributed to the insufficient content of PE-c-GMA and the insufficient ability to improve the compatibility. Furthermore, the morphology of the etched PP/ABS 90:10 blends was also used to illustrate the partial compatibility of the PP/ABS/PE-c-GMA system, as shown in Figure 8. As shown in Figure 8(b), the removed ABS phase distributes more uniformly in the presence of PE-c-GMA than the result shown in Figure 8(a). However, there is still some room for improvement, as the dark region of unequal size is easily observed in Figure 8(b).

Figure 8.

SEM images of the two representative ABS removed samples: (a) AP19 etched and (b) AP19G etched.

However, the partial compatibility of the PP/ABS/PE-c-GMA system could enable improved foamability for pure PP resin and lead to a fine, denser cell structure. On one hand, improved compatibility generally benefits the mechanical properties, ensuring improved phase adhesion between the blend partners through the reduction of both interfacial tension and coalescence. On the other hand, the number of nucleating sites may be increased with the incorporation of the dispersed phase, since the interfacial boundaries between the two immiscible phases can effectively lower the critical energy barrier for bubble nucleation.

Until now, the incompatibility between PP and ABS has been viewed as a major route to improve the foamability of the PP/ABS system. If so, the foaming effect of AP19 should be better than AP19G. However, the opposite is actually true. Hence, there may be other unknown factors, and further investigation of the forming mechanism of PP/ABS fine foam structure is needed. The rheological test was further used to understand the foaming behavior of the PP/ABS blend.

Relative rheological behavior

Typical foaming injection molding methods are well known. After the processed polymer that contains AC/ZnO is added to the hopper, AC decomposes under high temperature, and the released gas diffuses into the polymer melt under high pressure and shear during the normal plasticizing process. The resulting single-phase gas-melt solution is then injected into the cold mold cavity, and cell nucleation forms due to the sudden pressure drop and sharp decrease in gas solubility in the melt. Foamed injection-molded components are thus created. Hence, foam injection molding generally involves many complex processing conditions, which not only includes processing parameters under CIM such as temperature, pressure, and shear rate but also the interaction between the gas and polymer melt, such as gas release, gas dissolution in the polymer melt, and injection of the gas-melt solution into the mold cavity. The creation of an appropriate mixture of gas-melt solution is a key step in foamed injection molding. Improving the gas-loaded ability allows more gas to be held in the polymer melt, further influencing the foaming processes and improving the foamed effect.

Rheological measurement is generally seen as a conventional and useful method, with important guiding significance on the processing properties of polymers. Such measurements can be classified into absolute and relative methods. To make an absolute rheological measurement, it is necessary to control the deformation (or stress) of the melt and measure the resulting stress (or deformation). A rotational rheometer and capillary viscometer are two common, useful apparatuses to measure the absolute rheological properties of the polymer melt, since they offer a precise, reliable, and thorough analysis of the rheological behavior. However, it is not suitable for testing the gas-melt solution. Typically, it is difficult to prepare the test sample since the gas-melt solution forms during the normal plasticizing stage of injection molding and strongly depends on the processing conditions. Thus, a relative rheological test using a mixing method ensures a more suitable and reliable characterization than the absolute analysis, since the gas-melt solution forms in a similar circumstance between mixing and plasticizing. In addition, the relative results of the torque and rotational speed test can be actually converted into the absolute results of shear stress and shear rate conveniently, after a series of calibrations. There are reports that polymer or polymer composites can successfully characterize the rheological properties using the torque measure during mixing.^29

-32 Hence, a torque rheometer, which can give real-time rheological evolution of the gas-melt solution, was used to study the foaming properties of PP/ABS blends.

Figure 9 shows the torque rheological test results of the selective PP/ABS blends. Notably, the rheological curve of the rich-gas melt in Figure 9 was taken for the first 10 min of the total tests. With increasing time, the rheological curve of the rich-gas melt is close to that of the no-gas sample when the test time exceeded 10 min. As seen from the torque curve shown in Figure 9(a), the balance torques of the rich-gas samples are lower than those of no-gas samples for the three selective PP, AP19, and AP19G systems. It is expected to be attributed to the plasticization effect of the gas. In addition, when the compatibilizer PE-c-GMA is present in PP/ABS blends, there is an increase in the torque, indicating that the reaction between PP and ABS can improve the interfacial tension between the blend components. It can also be observed that the addition of ABS even decreases the torque of pure PP, which means that the balance torque of an immiscible polymer blend did not necessarily increase or decrease linearly with the blending composition,³³ although it is well-accepted that ABS has a significantly higher melt viscosity than pure PP.³⁴ In addition, for the torque rheological test, it can be easily deduced that a higher balance torque is correlated with higher melt viscosity. Hence, AP10G has a lower melt viscosity than pure PP. However, from Figure 9(a), for the gas-melt solution, the AP10G sample has an obviously higher torque value than the pure PP and AP10 rich-gas samples, indicating that AP10G has the highest melt viscosity among the AP01, AP10, and AP10G rich-gas samples. To distinguish this sample, “gas-melt viscosity” was used in this study. One can explain that, for the AP10G sample, the increase of the ability of the PP chains movement caused by the introducing of the part-compatible second phase results in a decrease of the shear viscosity of pure PP resin. However, by introducing gas of the system, the ability of the polymer chains to move freely is also changed. The shear viscosity of rich-gas PP drastically reduces owing to the existence of the gas. The movement of rich-gas PP chains can be slightly influenced by the existence of the incompatible ABS phase. However, the compatibility increases as the addition of PE-c-GMA, the part-compatible ABS phase somewhat limits the free movement of PP chains in a gaseous environment resulting in an increase of the shear viscosity. It can be concluded that AP10G has the highest gas-melt viscosity, which may improve the foamability of PP.

Figure 9.

Torque curves (a) and linear fitting between logarithm of torque and rotational speed (b) of the four selected sample types.

The influence of the presence of gas on the rheological properties can be furtherly clarified by the double logarithmic relation between balance torque and rotational angular velocity, as shown in Figure 9(b). From Figure 9(b), for all rotational velocities, the order of the balance torque from high to low is AP01 solid, AP19G solid, AP19G foamed, and AP01 foamed. Hence, it can be concluded that the addition of ABS and compatibilizer cannot increase the melt viscosity but rather affects the gas-melt viscosity. In addition, the experimental measurements confirm the linearity of the curves logarithm of balance torque (M _T, the recorded result when torque almost remained constant) versus logarithm of rotational speed (S). Hence, the M _T versus S curves follow a power-law model, as suggested by the following equation²⁶

M_{T} = C (n) \times m \times S^{n}

where M _T is the total torque measured by the instrument, S is the angular velocity of the instrument, C and m are model parameters, and n is the flow index which can be obtained from the slope of Ln(M _T) versus Ln(S). In Figure 9(b), the flow index increases in the presence of the gas, indicating that the viscosity of the gas-melt decreases more slightly than the increase in shear rate. This can be attributed to the escape of more gas from the gas-melt under a high shear rate (rotational speed), resulting in a gas-melt closer to the pure melt. It was also observed that the flow index of AP10G foamed sample was smaller than that of the pure PP foamed sample, which indicates that gas escaping from AP10G was less than for pure PP under a high shear rate. Hence, another reason for the improved foamability of PP with the addition of ABS and compatibilizer is the increase in gas-melt viscosity, which results in a better gas loading ability and prevents the escape of gas.

As discussed, it is not easy to characterize the mixture of the gas-melt solution in the foamed injection molding. This is partly due to the high shear rate of the injection stage in the foamed injection molding process. However, the relative rheological test presented in this study using a mixing method can indicate a gas-loaded ability by comparing the variation of the gas-melt with melt viscosity.

However, to date, the influence of ABS on cell growth and distribution throughout the molded PP sample remains completely unclear. It is well known that PP is a typical semicrystalline polymer. Once the gas-melt solution is then injected into the cold mold cavity, the cell growth and distribution occurs with the solidification and the crystallization of the PP gas-melt. Hence, the relationship of crystallization and foaming behavior to the addition of ABS must also be further clarified.

Crystallization behavior

DSC is one of the most popular methods to study the crystallization kinetics of polymers. Figure 10(a) shows the nonisothermal crystallization cures of selective PP/ABS blends at a cooling rate of 10°C min⁻¹. According to the cooling exotherms of Figure 10(a), the evolution of the relative degree of crystallinity (θ) can be obtained³⁵

Figure 10.

DSC curves of the eight foamed PP/ABS samples.

θ (t) = \int_{t_{0}}^{t} \frac{d H}{d t} d t / \int_{t_{0}}^{\infty} \frac{d H}{d t} d t

where t ₀ is the crystallization start time, t is the time at which the measurement was taken, and dH/dt is the heat flow rate during crystallization. The calculated result of θ is shown in Figure 10(b), and all curves show a sigmoid dependence with temperature.

As seen in Figure 10(a), an obvious exothermic peak with different start and end temperatures was observed in the cooling curves of the rich-PP samples. The crystallization starting temperature of the AP19 and AP13 samples are both at least 10°C lower than that of pure PP sample, indicating that the presence of ABS actually hinders the crystallization of PP with decreased temperature. However, the difference in the final crystallization temperature between AP10 or AP13 and pure PP is obviously smaller.

The PP crystallized temperature region also varied due to the addition of ABS and compatibilizer. There was at least a 10°C delay in the crystallization starting temperature of AP19 and AP13 samples, lower than that of pure PP sample. The influence of ABS on the crystallization temperature region of PP can also be attributed to the incompatibility between the PP and ABS phase, and ABS blocks PP crystallization. At the crystallization onset temperature of PP, the presence of ABS can limit the crystallization process of the PP phase since the macromolecules have less freedom to move around and rearrange themselves into crystals. The PP crystallization ability is relatively weak at the high temperature, and due to the blocking effect of ABS, the AP19 and A13 samples cannot form crystals of PP. With the decrease in temperature, the ABS can act as a macromolecule agent and accelerate the crystallization process. Thus, once crystallization arises, the presence of ABS can accelerate the crystallization progress of PP. As can be known from Figures 6 and 10, the T _g of ABS and the crystallization onset temperature of PP are very close. It gives a hint that the different state of incompatible ABS phase has possibly a different influence trend on the crystallization of PP. This needs further study and clarification.

The delay of the crystallization onset temperature may facilitate the foaming behavior of PP. The diffusion of gas in a polymer only occurs in the amorphous phase; the crystalline phase acts as a barrier for the diffusing gas. Hence, it is now well-accepted that cell growth stops when crystallization starts,³⁶ and the gas can be rejected from the crystals. Thus, the nucleated cells can have more time and space to grow due to the delay of crystallization, resulting in higher cell density. Furthermore, the faster crystallization process in the low-temperature region also benefits the foaming. The crystallization process under low temperature leads to greater crystal nucleation and the formation of small crystals.^37
-39 There are more tiny interfaces and amorphous regions in the small “crystallographic group,” which is more suitable for holding gas.

The effect of ABS on crystallization behavior was verified by the XRD test results shown in Figure 11 and the SEM direct observation results shown in Figure 12. As seen in Figure 11, the diffraction peak intensities of PP crystals in the AP19 and AP13 samples are almost same as in the AP01 sample, which indicated that the resisting and accelerating effect of ABS on PP crystallization can cancel each other. More smaller crystals can be found in the AP19 sample than the AP01 sample, as shown in Figure 12.

Figure 11.

XRD curves of the eight foamed PP/ABS samples.

Figure 12.

SEM images of two representative samples: (a) pure PP sample etched and (b) AP19G ABS removed and etched, the crystals are circled in the figures.

Therefore, the presence of ABS increases the cell density, as shown in Figure 3(c) and (e), indicating that a fine foamed structure forms. However, it is more complex with the simultaneous addition of ABS and PE-c-MGA. As seen in Figure 10, the crystallization onset temperature of the AP19G sample was higher than that of AP19 and lower than that of AP01. The temperature region of AP19G is also the intermediate value of AP19 and AP01. The AP13G sample has a similar trend. As discussed, the variation of crystallization behavior of PP is induced by the addition of incompatible ABS phase, and PE-c-GMA can improve the compatibility of the PP and ABS phase. It means that the effect of ABS on PP crystallization is weakened by the addition of PE-c-GMA owing to the partial compatibility.

In conclusion, the influence of ABS on the foaming behavior of PP can be attributed to three aspects of the formation mechanism. First, the interface between the two phases due to the incompatibility increases the chance of cell nucleation. Second, gas escaping during the plasticizing and flow stage decreases due to the increase in gas-melt viscosity. Last, the delay of crystallization and the formation of more small crystals facilitate gas growth and stabilization. The addition of PE-c-GMA makes the influence of ABS on the PP foaming mechanism more complex. The addition of PE-c-GMA benefits the increase in gas-melt viscosity but weakens the beneficial effect of ABS on foamability by changing the crystallization behavior. Although these two aspects cancel each other out, as discussed, the comprehensive effect can significantly facilitate the foaming behavior of PP. As PE-c-GMA can also improve the mechanical properties of PP/ABS blends, the PP/ABS/PE-c-GMA composites with an appropriate ratio of each composition are preferred when the foamed parts of the PP matrix must be fabricated. In addition, while the foaming behavior of PP/ABS and the effects of compatibilization using foaming injection molding processing and AC/ZnO as a blowing agent were analyzed in detail, this method should be adopted to foam additional blends.

Conclusions

In this study, the foaming behavior of PP/ABS and the effects of compatibilization were analyzed in detail, using CIM processing and AC/ZnO as blowing agents. It was found that ABS can improve the cellular foam structure of common PP, and the presence of PE-c-GMA could lead to a more complex interaction. The efficiency of ABS with respect to the foamability of PP can be attributed to three possible mechanisms: (1) the interface between PP and ABS phases is favored as a nucleation site due to the partial compatibility and the weak interaction between the two blend components; (2) the presence of ABS increases the gas-melt viscosity of PP and thus improves the gas loading ability in the foaming process, which can decrease the possibility of gas escaping; and (3) the ABS phase can act as a macromolecular nucleating agent, in a similar manner to that observed for commonly used solid-state fillers, that results in a minimal cell size and maximal cell density. Accordingly, an effective method to improve the foaming behavior of PP foamed injection-molded components was suggested and proposed.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Innovative Entrepreneurship Training Program for Chinese College Students (no. 201710289006), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (nos KYCX17_1833 and SJCX18_0761), and the Guangdong Province Pearl River Scholar Funded Scheme (2012, 2016).

ORCID iD

Ying-Guo Zhou

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