Fabrication of High Expansion Microcellular Injection-Molded

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Fabrication of High Expansion Microcellular Injection-Molded Polypropylene Foams by Adding Long-Chain Branches Long Wang, Shota Ishihara, Megumi Ando, Atsushi Minato, Yuta Hikima, and Masahiro Ohshima Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03641 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 5, 2016

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Industrial & Engineering Chemistry Research

Fabrication of High Expansion Microcellular InjectionInjection-Molded Polypropylene Foams by Adding LongLong-Chain Branches Long Wang, Shota Ishihara, Megumi Ando, Atsushi Minato, Yuta Hikima, Masahiro Ohshima∗ Department of Chemical Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan

ABSTRACT Herein, 10-fold expansion ratio foams were successfully fabricated for the first time by a core-back foam injection molding (FIM) technique using a newly developed long-chain branching polypropylene (LCBPP). Different rheological curves were investigated to distinguish the strain hardening behavior and melt strength of the LCBPP from isotactic polypropylene (iPP). Fast scanning chip calorimetry (FSC) results revealed that the long-chain branches could increase the crystallization temperature even at fast cooling rates. The FIM of LCBPP could notably not only reduce the cell sizes but also increase the cell densities through the capability of LCBPP to stabilize cells and prevent cells from coalescence. Furthermore, the foam processing window of manufacturing 10-fold expansion foams was significantly broadened for LCBPP foams. This study revealed that the introduction of long-chain branches is a good approach to fabricate high expansion microcellular foams for different potential applications such as construction and transportation.

Keywords: Long-Chain Branches, High Expansion Foams, Melt Strength, Crystallization, Foam Injection Molding

1. INTRODUCTION ∗

Correspondence to: Masahiro Ohshima; [email protected] 1

ACS Paragon Plus Environment


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Thermoplastic foams exhibit several merits as engineering materials, such as high impact strength, low specific weight, and strong energy absorption as well as low thermal conductivity and dielectric constants.1 These outstanding characteristics make thermoplastic foams ideal in many industrial areas such as cushioning, automotive parts, food packaging, sporting equipment, insulation, and low dielectric insulators.1 Polypropylene (PP) is a fast growing commercial polymer because of its prominent functional properties, such as good thermal stability, excellent chemical resistance, good mechanical properties, and easy recycling.2 For instance, PP exhibits higher rigidity than other polyolefins and it gives higher strength than polyethylene (PE) as well as better impact strength relative to polystyrene (PS). In addition, PP offers a higher servicing temperature and thermal stability than PS and PE. These superior properties have made PP foams more attractive than other thermoplastics. However, linear PP has some difficulty in foaming because of the weak melt strength; cell coalescence and cell collapse occur easily in PP foaming because the cell walls cannot withstand any extensional force exerted during the bubble growth stage.1,3 Consequently, linear PP foams have been inevitably characterized with a high content of coalesced and open cells as well as non-uniform bubble distribution,4−6 which has constricted their application in many areas. To conquer the ultra-low melt strength and improve PP’s foamability (especially isotactic polypropylene, iPP), several methods have been employed, such as blending,7−9 cross-linking,6,10,11 compounding,12−15 and introducing long-chain branches.16−22 Blending of PP with other polymers, such as PE, has been considered one of the most practical schemes for improving the foaming behavior of PP.23−25 However, the desire to control the phase morphology of polymer blends has been challenging. Thus, the resultant improvement in foamability is still limited. Compounding with various fibers and particles, e.g., talc, nanoclay, and carbon nanotubes (CNTs), has also been applied to enhance the melt strength and crystallization properties of PP with variable 2


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success.12−15,26,27 Okamoto et al.13 revealed nanoclay to be greatly effective in enhancing the cell nucleation and cell density for PP. Their study showed that nanoclay causes biaxial flow-induced alignment at the cell boundary, and such alignment behavior helps cells resist the extensional stress from breaking the thin cell wall.13,14 Compounding the polymer with nanoparticles could also modify the melt strength of PP or even induce a strain hardening property.14,28,29 However, the incorporation of nanosized particles produces several challenges such as poor dispersion and weak interfacial adhesion, and it also presents significant environmental and health issues associated with manufacturing, application, and disposal of nanocomposites, which further decreases the attractiveness of nanocomposites. Currently, introducing long-chain branching in PP’s backbone is considered the most effective and practical approach to improve its melt strength and foamability.4,16−22 It was found that polymer with just a few long-chain branching could increase the melt strength and display strain hardening behavior.30−33 Strain hardening signifies that the transient elongational viscosity suddenly increased higher than the linear viscoelastic viscosity plot at a constant strain rate.30 Many studies have shown that long-chain branched polypropylene (LCBPP) or blending of LCBPP with iPP could result in the production of PP foams with higher expansion ratios, less cell coalescence and more uniform cell structure than that of the linear PP foams.4,16−22,34 Liao et al. 35 studied the batch foam behaviors of iPP and LCBPP employing carbon dioxide (CO2) as a physical blowing agent (PBA). They showed that LCBPP could be foamed with an increased cell density, smaller cell sizes, and improved homogeneous cell structures. Foaming of blends of iPP with LCBPP was also investigated in extrusion foaming processes using azodicarbonamide as the chemical blowing agent.35 They found an optimum content of LCBPP existed in the blend ratio, which gave the highest expansion ratio and finest cell structures. Park and coworkers4,18,36 performed several studies on the extrusion foaming of LCBPP using a PBA and produced PP foams with a very low density and fine cell 3



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structure. Similar results on the extrusion foaming of LCBPP or linear PP/LCBPP blends using a PBA have also been reported by other researchers.16,17,22,35 Although many attempts have been conducted to identify the influence of long-chain branched structures on the batch and extrusion foaming of PP, very few studies have been conducted on foaming LCBPP using a foam injection molding technique.37 Microcellular foam injection molding (FIM) is an extension of injection molding with foaming and it is regarded as a cost-effective and practical method for fabricating foams with complex, three-dimensional geometries.38 It offers many advantages such as an absence of the sink mark on the part surface, fewer residual marks, better geometric accuracy, and less product shrinkage.38,39 Additionally, foam injection-molded products have also exhibited higher acoustic and thermal insulation39,40 and improved mechanical performance40−42 in contrast with their solid counterparts. It should be mentioned that compared with the extrusion and batch foaming process, manipulation of FIM appears more challenging due to the presence of many additional manipulated variables such as shot size, injection speed, injection pressure, mold temperature and back pressure.38 Recently, substantial studies have been carried out on many different thermoplastics such as PE,40,43 PP,37,40,44 polylactide (PLA),39,40 and polyamide 6 (PA6)45,46 prepared by the FIM technique. However, there has been very little study in these areas for preparing LCBPP foams. Guo et al.37 investigated various experimental conditions on the cell morphologies and dynamic mechanical properties of linear and branched PP foams. A chemical blowing agent was used in this study. However, compared with the batch and extrusion foaming products, one main shortage of regular FIM is its low weight reduction (expansion ratio), which is restricted to the fixed mold in the FIM machine. For example, the weight reduction in the high-pressure FIM is only approximately 5−15 wt.%.41,46 In contrast, low pressure FIM with partial cavity filling is another technique, called short-shot scheme, which can increase the void fractions as high as 30 wt.%.39,41,46 For

4


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low-pressure FIM, however, it is very difficult to control the cell nucleation and growth and achieve uniform cell structure. To achieve higher expansion foams with uniform cell structures, FIM techniques with core-back or mold-opening operation have been proposed, which can greatly increase the expansion ratio (mold cavity volume).44,47−49 In this process, the mold cavity is first completely filled out with the polymer, and then a portion of the mold is suddenly moved along the thickness direction after a period of dwelling time. By opening the cavity, the dissolved gas in the polymer experiences a significant pressure drop and the foaming process starts uniformly in the entire injection-molded product.44,47−51 Ishikawa et al.50,51 performed some fundamental work on the cell nucleation and growth processes through in-situ foaming visualization of the FIM process with mold opening. Fixed at a 2-fold expansion ratio, Stumpf et al.52 investigated the effect of a new class of organic supramolecular nucleating agents on the cell structures of iPP prepared by core-back FIM. In addition to conventional thermoplastic materials, Ameli and coworkers39,53 also fabricated biopolymer PLA composite foams with an expansion ratio of about three using FIM with mold opening and gas counter-pressure. Recently, Chu et al.54 simulated the foaming temperature of the mold-opening foam injection process at a fixed expansion ratio of two. Spӧrrer and coworkers49 foamed branched PP with different expansion ratios (the maximum expansion ratio was three) using a high content of N2 (2 %) as the PBA. However, no effort has yet been dedicated to fabricating high expansion ratio (as high as five) LCBPP foams using FIM. In our previous work,44 foam injection-molded iPP foams with crystal nucleating agents using core-back operation were developed to achieve expansion ratios as high as five, and microcellular PP foams with highly open cells were obtained. In the present work, the newly developed LCBPP polymer was employed to prepare high expansion foams by a core-back FIM technique. For comparison, an iPP resin with the same melt flow index and similar weight-averaged molecular weight was foamed. 5



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The dynamic rheological behaviors and uniaxial elongational properties of LCBPP and iPP were firstly analyzed. Then, the crystallization behaviors of these polymers under an application relevant cooling rate were further characterized. In foaming experiments, the effects of various expansion ratios and foaming temperatures on the cell structures of iPP and LCBPP foams were investigated. Finally, the ability of long-chain branches to stabilize cell structure was discussed based upon the results of open cell content and the foam processing window.

2. EXPERIMENTAL SECTION 2.1. Materials A newly developed metallocene-based LCBPP (MFX6, Japan Polypropylene Corporation, Tokyo, Japan) with 3.0 g/10 min (230 °C/2.16 kg) of melt flow rate (MFR) and 345 kg/mol of weight-averaged molecular weight (Mw) was kindly provided and used as is. iPP product F133A (97 % tacticity, Prime polymer, Tokyo, Japan) with the same MFR and a similar Mw (379 kg/mol) was employed as a reference PP polymer. Nitrogen (N2) (Izumi Sangyo, Tokyo, Japan) was used as the PBA with a purity above 99 %.

2.2. Sample preparation Foam injection molding experiments were conducted using the injection molding machine supplied from Japan Steel Work (J35EL III-F, Tokyo, Japan) with a 35-ton clamping force and an additional gas dosing unit (SCF device SII TRJ-10-A-MPD, Trexel Inc., Wilmington, MA, USA). N2 was used as a PBA. By pressurizing the gas to 24 MPa, it was conveyed to the polymer melt in the molding machine through an injector valve to set the PBA concentration in the polymer at a given set point. The injected N2 was mixed with the polymer melt at high pressures and high temperatures by a specially designed screw, which could promote the dissolution of N2 and stably create a 6


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homogeneous polymer/N2 mixture. The temperature profile in the FIM’s zones was 195, 195, 195, 210, 230, 200, and 180 °C from feed section to the nozzle zone, respectively. The temperature of the mold was remained at 40 °C by an automatic temperature control unit. To minimize the degree of pre-foaming during the injection-filling process as well as to minimize the temperature difference of filled polymer at the positions between cavity end and gate, a high injection speed of 200 mm/s was set. The other processing parameter details are summarized in Table 1. The mold used in this study was the box-type cavity with a gate of 1.5 mm in diameter. The mold dimension was 70 mm × 50 mm × 1 mm. Unlike conventional FIM, the core-back foam injection molding technique can achieve a high expansion foam by expanding the mold cavity in the following way: after filling up the cavity with the polymer/gas solution, the mold can be shifted at a designated speed and a given core-back timing (the end of the dwelling time), which can significantly enhance the cavity volume and suddenly decrease the melt pressure (Figure 1). This large pressure difference starts foaming process in the entire polymer in the cavity, enhancing the cell nucleation and growth processes. Foams with small cell size and uniform cell structure can thereby be produced. The expansion ratio of foams and the pressure drop rate can be controlled through the changes of the core-back rate as well as the core-back distance. Herein, the core-back rate is fixed at 20 mm/s, while the mold-opening distance was changed to four various values of 1, 4, 6, and 9 mm. Further detailed information on the “FIM with core-back operation” can be found in our previous work.44 The temperature and pressure in the mold cavity were recorded by a data collecting device (Mold Marshaling system EPD-001, Futaba, Chiba-ken, Japan) at 20 ms sampling time. Typical temperature and pressure profiles of polymer in the mold cavity are illustrated in Figure 1. As shown, the temperature decreased as a function of time. Thus, crystallization occurs during the cooling process. Control of crystallization behavior is 7



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the key factor to succeed in foaming semi-crystalline polymers.44,55,56 The foaming temperature, crystallinity and melt viscosity at the initiation of the foaming process can be controlled through the manipulation of dwelling time. Dwelling time denotes the time difference between the finish of polymer filling and the beginning of the mold-opening operation. 2.3. Rheological characterization in shear flow Shear rheological experiments were conducted on a strain-controlled rheometer (ARES, TA Instruments Inc., New Castle, DE, USA), using a 25 mm parallel plate geometry. Prior to measurements, the PP pellets were compression-molded into disk-shaped sheets with a diameter of 25 mm and in thickness of 2 mm at 190 °C and 10 MPa in a hot press machine. The dynamic frequency sweep tests were conducted at a fixed strain of 1 % in the frequencies ranged from 0.01 to 100 rad/s. Thermal stability of iPP and LBPP samples was guaranteed by the time sweep.

2.4. Rheological measurements in uniaxial elongational flow Uniaxial elongational flow using an extensional viscosity fixture measurement was conducted on an ARES rheometer at 180 °C under several elongational rates between 0.05 and 0.5 s−1. The rectangular specimens with dimensions of 17 mm × 10 mm × 0.9 mm were fabricated by a molded-compression at 190 °C under 10 MPa in the hot press machine. A pre-stretch of 10 s was conducted to make sure that no slide happens between the specimen and the fixture.

2.5. Fast scanning chip calorimetry measurements Fast scanning chip calorimetry (FSC) tests of non-isothermal crystallization kinetics of PP and LCBPP were carried out with a Mettler-Toledo Flash DSC1 (Mettler-Toledo, LLC, Columbus, OH, USA) with UFS1 sensor. To allow rapid cooling, a Huber intracooler TC45 was attached to the Flash DSC1. Experiments were performed by 8


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purging the calorimeters with dry N2 at a 30 mL/min flow rate. To increase thermal contact with the specimen, the heating portion of the sensor was coated with the silicone oil in advance of loading the sample. Specimens with a typical thickness of 10−20 µm and a lateral dimension in the range of 50−100 µm were prepared using a microtome. The specimens were firstly heated to 200 °C and maintained at 200 °C for 5 s to completely melt the crystals. The efficiency of this melt annealing procedure in erasing any thermal history on subsequent crystallization was carefully verified. Analysis of the non-isothermal crystallization behaviors were carried out by cooling the polymer melt to −50 °C at cooling rates in a range from −1 to −1000 °C/s.

2.6. Foam morphology characterization To observe the cell morphology, a small portion of specimen was cut from the middle part of the foamed products and then cryogenically fractured after immersion in liquid nitrogen for 30 minutes. The fractured cross-sections were covered with gold employing a VPS-020 Quick Coater (Ulvac Kiko, Ltd., Kanagawa, Japan) and the cell morphologies were then studied using a tiny scanning electron microscopy (Tiny-SEM Mighty-8, Technex, Tokyo, Japan), at an accelerating voltage of 17 kV. The obtained images were analyzed with an ImageJ Software (National Institutes Health, Bethesda, Maryland, USA). The cell density, Nd, was calculated as the number of bubbles per cubic centimeter with respect to the non-foamed polymer using the following equation: Nd = (

N 3/ 2 ρ p ) A ρf

(1)

where N is the number of bubbles in a defined area of the image (A), and ρp and ρf are the density of the solid specimen and the foamed sample, respectively. The ρp and ρf values were obtained according to ASTM D792-00 with the water displacement method.

2.7. Open cell content 9



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Open cell content (OCC) of the iPP and LCBPP foams was measured using a gas pycnometer (AccuPycII, Shimadzu, Kyoto, Japan). The volume obtained from the gas pycnometer excluded the open cell volume of the specimen. OCC then could be obtained using the following expression, OCC (%) =

Vapparent − Vmeasure Vapparent

× 100

(2)

3. RESULTS AND DISCUSSION 3.1. Oscillatory shear flow properties The polymer melt strength can be evaluated using elastic properties under shear flow.57 To explore the influence of long-chain branched structures on changes in the melt strength, e.g., the storage modulus, loss tangent, and complex viscosity, small amplitude oscillatory shear rheological measurements were conducted at 180 °C. Figures 2a shows the changes of the storage modulus (G′) as a function of frequency, ω, for iPP and LCBPP, respectively. It revealed that the G′ of iPP were higher than those of LCBPP at high frequencies. The G′ decreased as the frequency was decreased, but iPP showed a more rapid decrease with the frequency decrease. In contrast, at low frequency, the G′ of LCBPP clearly showed higher values compared with the linear PP. It is known that neat linear polymer chains relax fully and follow the typical terminal flow, which was characterized by G′ ~ ω2 and G″ ~ ω1 in the low frequency region.57 As shown in Figures 2a, iPP exhibited a typical terminal behavior, which indicated that it is a linear PP with a fast relaxation process.57 On the other hand, LCBPP showed a deviation from the terminal behavior: with the presence of a long-chain branch structure, G′ notably became higher than that of iPP in the low frequencies and the gradient of the terminal slope of storage modulus reduced to 0.70 for LCBPP while at 1.52 for iPP. This behavior of LCBPP revealed that a much slower relaxation process existed which could be attributed to an increase in chain entanglements caused by long-chain branches.30−32 This slow 10


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relaxation behavior is helpful for the foaming process since long relaxation process benefits the formation of larger local stress around the crystals in cell nucleation.55,56 The higher melt elasticity of LCBPP relative to iPP was also demonstrated by the changes of loss tangent (tan δ) against frequency (Figure 2b). For iPP, tan δ obviously increased with a decrease of frequency, and this was a characterized terminal property for a viscoelastic liquid.32,57 In comparison with iPP, tan δ of LCBPP did not increase with the frequency decrease, but rather leveled out at low frequency range. The data indicated that the elastic response became more prominent with the presence of long-chain branched structures, as LCBPP showed an enhanced viscoelastic behavior, which was analogous to the findings for other LCB polymers.32,58 The introduction of long-chain branches could also modify the melt viscosity.33,57 The complex viscosity, η∗, of iPP and LCBPP is shown as functions of frequency in Figure 2c. Compared with iPP, the complex viscosity of LCBPP was higher at low frequency but lower at high frequency. In other words, a typical Newtonian plateau region existed at the low frequency of iPP melt, while in LCBPP this Newtonian region did not exist, but a strong frequency-dependency of η∗ did exist. As the strong frequency dependency of η∗ for LCBPP at low frequency indicated, a significant shear thinning behavior can be observed in LCBPP at low frequency regions as reported by others,29−31 and at high frequency region, the shear thinning behavior of LCBPP was more pronounced than that of iPP. These findings strongly reveal that the presence of long-chain branching in PP could significantly change its rheological properties. Those changes in rheological properties would be beneficial for the following foaming process. Thermo-rheological behavior of molten polymer in its linear viscoelastic regime can be qualitatively analyzed on plots of the phase angle, δ, as a function of the complex modulus, |G∗|, at different testing temperatures, the so-called van Gurp-Palmen plot.59 For a thermo-rheologically simple fluid, the δ-|G∗| curve has no temperature-dependence, while thermo-rheological complexity is characterized by a temperature-dependence of the 11



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curves.60−62 The van Gurp-Palmen plots for both iPP and LCBPP measured at different temperatures are displayed in Figure 3. For iPP, all the measured data seems to superimpose on “one single curve” which indicates thermo-rheological simplicity.60−62 Behaving differently, the δ-|G*| curves of LCBPP demonstrated a clear split among the measured results at various melting temperatures, and they could not be superimposed on one single curve. A clear trend against temperature was observed in the presence of LCBPP as illustrated in Figure 3; the higher the temperature, the larger the phase angle became at the same |G∗|. This tendency was produced by the fact that only some of the relaxation times of polymer were influenced by the change in temperature.59−61 This behavior is termed thermo-rheological complexity. Thus, our LCBPP polymer is thermo-rheologically proven to possess long-chain branches.

3.2. Uniaxial elongational flow behaviors A measurement of the uniaxial elongational flow characteristics was carried out employing the ARES rheometer. When the transient elongational viscosity was illustrated as the changes of elongational time under certain Hencky strain rates, the sudden increase in elongational viscosity above the linear viscoelastic predictions is named the strain hardening property, which were demonstrated for different polymer with LCB.32,57,58 It should be mentioned that this strain hardening property is extremely significant for actual processing process such as foaming, high-speed fiber spinning, and blowing molding, where the high melt strength is needed.16−18,32,63 The uniaxial elongational flow experiments were conducted at 180 °C with the Hencky strain rate, ε& , in the range of 0.05−0.5 s-1. Figure 4 illustrates the elongational viscosity, η e+ (t ) , versus elongational time curves for iPP and LCBPP obtained with different Hencky strain rates. The maximum achieved Hencky strains for iPP and LBPP samples were ranged from 2.5 to 3.5. The range depended on the Hencky strain rate as well as the content of long-chain branches. A curve representing the estimated linear 12


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viscoelastic range of deformation was also plotted, which is equal to 3 times the time-dependent shear viscosity, η 0+ (t ) . The value was calculated using the discrete relaxation spectrum according to Eq. (3), as follows:32,57 N

η + (t ) = 3∑ λi Gi (1 − e − t / λ ) i

0

(3)

i =1

where Gi and λi are the generalized Maxwell model parameters. Gi and λi is the modulus and relaxation time, respectively. According to the standard nonlinear regression approach provided in the Trios Software (TA Instruments Inc., New Castle, DE, USA), the values of Gi and λi were determined through the original data of G′ and G″.32 Figure 4 shows good agreement between the 3 times of the time-dependent shear viscosity, 3 η 0+ (t ) , and the transient elongational viscosity, η e+ (t ) , at small strains and times. This agreement indicates the accuracy of the measurement is sufficiently high. For the iPP, no strain hardening behavior was observed at any applied strain rates, and the specimen was prematurely fractured before it reached the maximum Hencky strain of 3.5. As shown in Figure 4a, the higher plateau viscosity equal to the 3η 0+ (t ) for iPP provided additional evidence of the linearity of the molecules.30−32 On the contrary, LCBPP could bear the extensional force above the maximum Hencky strain of 3.5 without breaking up and showed a prominent hardening property as shown in Figure 4b, which manifested as a large deviation of η e+ (t ) from the predicted 3η 0+ (t ) . Furthermore, the strain hardening behavior became more prominent with the increase in Hencky strain rate, which was further evidence of the existence of long-chain branches in this newly developed LCBPP. For a further analysis of the strain hardening property, the strain hardening factor, Xe, is calculated by Eq. (4), as follows:31,32

13



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Xe =

η e+ (t , ε& ) 3η + (t )

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(4)

0

where η0+(t) is the shear viscosity as a function of time in the linear viscoelastic regime. Figure 5 shows the changes of strain hardening factor, Xe, at different Hencky strain rate, ε& , for iPP and LCBPP under a fixed strain of 2.7. The reported strain hardening factor values for LCBPLA, LCBPP and LDPE were about 2−7 at a Hencky strain of 2.7, and the value of Xe increased with an increase content of long-chain branches.31,32,64 However, an extremely high value of strain hardening factor was observed for the LCBPP used in this study. This unusual characteristic could be attributed to a comb-like chain structure of the LCBPP made by the metallocene catalyst-based polymerization technique. Therefore, it is anticipated that this novel LCBPP with such a high melt strength could have high foamability with uniform cell structures.

3.3. Crystallization kinetics at a fast cooling rate In general, the crystallization kinetics can be analyzed using the differential scanning calorimetry (DSC). The conventional DSC technique can cover only a limited temperature range at a rather low degree of undercooling. However, the cooling rate of polymers in industrial processes may be much faster. Since the fastest cooling rate in conventional DSC is around a few hundred °C/min, crystallization behavior below several hundred °C/s cannot be directly obtained using the regular DSC, but it can be measured using FSC. The FSC can permit the measurement of crystallization at cooling rates up to 1000000 °C/s by significantly reducing the specimen mass as well as the heat capacity of the furnace.66−68 While there have been many studies on the crystallization kinetics of polypropylene at low-undercooling temperature ranges, the influence of LCB on the crystallization kinetics of PP at application-relevant cooling conditions is not yet known. It is worth mentioning that variation of the crystallization process is connected

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with variation of the crystalline morphology, which results in dramatic alteration of the final cell morphology and consequent application-relevant performance.7,20,44,55 Figure 6 shows the selected FSC crystallization curves for iPP and LCBPP specimens at cooling rates in the range from −2 to −20 °C/s, which was equivalent to the range of cooling rates observed in our foam injection molding process. It was found that the crystallization peak temperature, Tp, of LCBPP samples was larger than iPP sample. For example, the Tp of an LCBPP specimen at −2, −5, −10, and −20 °C/s was 106.5, 102.6, 99.1, and 95.4 °C, respectively, while the corresponding Tp of an iPP specimen at −2, −5, −10, and −20 °C/s was 105.3, 100.6, 96.1, and 90.4 °C, respectively. These increases in

crystallization temperature could be due to an increase in heterogeneous nucleation sites provided by the long-chain branched points, which was a commonly reported fact on the crystallization process of LCB polymers at low undercooling rates.19,32,69 The effects were further validated with the crystallization behaviors observed at faster cooling rates. Figure 7 illustrates the changes of the cooling rate as a function of the peak temperatures for iPP and LCBPP. The peak temperatures of both polymers decreased as the cooling rate increased. The observed cooling rate dependency of the peak temperature of iPP is a typical characteristic of linear PP:66−68 a single crystallization peak was observed in the low cooling range from −1 to −90 °C/s, whereas at cooling rates higher than −100 °C/s, double peak temperatures was observed. The high peak temperature and low peak temperature were attributed to the α-phase crystallization and mesophase crystal formation, respectively.66−68 Behaving differently, LCBPP showed only a single peak temperature at all cooling rates in the investigated range. There was no mesophase formation as observed in iPP.67,68 This indicates that the long-chain branches could provide the heterogeneous crystal nucleation sites. The observation was similar to the results reported by Rhoades et al.66 They found that two peak temperatures were obtained for neat iPP and iPP with low amounts of nucleating agents, but only one peak temperature was observed for iPP with a 15



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very high nucleating agent content at cooling rates to −2000 °C/s. This signified that the long-chain branched structure could act as an effective crystal nucleating agent for PP, and a critical cooling rate much faster than −1000 °C/s should be needed to suppress the crystallization of LCBPP. Thereby, in comparison with iPP, an increase in peak temperature at the industrial level of cooling rate was observed for the LCBPP. This characteristic would be beneficial for preparing PP foams with a uniform and fine cellular structure and higher cell density

3.4. Influence of expansion ratio on cell structures The cell morphology was studied along the core-back direction. In other words, foam prepared by FIM has a multilayered cellular structure comprised of a skin and core layer (foamed layer). Figure 8 displays the cell structure at the center of the core layer of the foams with various volume expansion ratios. The FIM was conducted with a dwelling time of 3 s and a core-back rate of 20 mm/s. To prepare foams with different expansion ratios, the mold-opening distance, i.e., the distance of shifting the movable mold part was changed from 1 to 2, 5, 7, and 10 mm in accordance with the expansion ratios of 2, 5, 7, and 10, respectively. The cell morphology in the core layer was observed from views parallel and perpendicular to the mold-opening direction as was conducted in our previous work.44 As shown in Figures 8a and 8a′, 2-fold expansion foams maintained a spherical cell shape. Meantime, elliptical cells were observed when the expansion ratio increased beyond 5 as shown in Figures 8b, 8c, 8d, 8b′, 8c′ and 8d′. At expansion ratios higher than 5, cells were highly stretched in the core-back direction because highly extensional stress was induced during the core-back manipulation. As clearly presented in Figures 8d and 8d′, microcellular foams with a 10 expansion ratio were successfully fabricated from LCBPP, i.e., the highest reduction of weight, almost 90 wt.% reduction, was achieved by the FIM using LCBPP polymer. As far as we know, this is the largest reported degree of weight reduction of foam. To the best of 16


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authors’ knowledge, most of the reported values of weight reduction in foam injection molding have been much lower than 80 wt.%, which might be due to the confinement of the mold as well as the complexity of precisely controlling the foaming process.37,39,41,43,52,54 The cell shapes in the micrographs taken from the cross-section perpendicular to the mold-opening direction looks spherical as shown in Figures 8b−d even though they were elliptic with the elongation in the melt flow direction as displayed in Figures 8b′−d′. To quantitatively evaluate the cell morphology, the cell diameter and the number density of cells were used. They were calculated from the SEM micrographs at the center of the core layers taken from the views perpendicular to the molding-opening direction assuming that the bubble was spherical through the entire foam. Therefore, it is somewhat inappropriate to apply Eq. (1)−(2) to calculate the number density of cells from the SEM image of the center of the core-layer; Eq. (1) is not valid unless the cell morphology is uniform. As exhibited in the SEM micrographs, the cell morphology of the high expansion foams shows elongated cell structure. Thus, in the following section, the number density of cells calculated by Eq. (1) from SEM images is only used as a reference value. The changes of cell densities as a function of expansion ratio of iPP and LCBPP foams are presented in Figure 9a. It is shown that the cell density of iPP was 106-7 cells/cm3, which is not exceedingly high and is similar to values reported in the literature.37,44,52 Using LCBPP, the cell density of the foams was significantly raised to 108 cells/cm3. This is due to the strain-hardening behavior, which reduces cell coalescence.17−20 The cell densities of iPP and LCBPP foams with the 2-fold expansion ratio were 1.11 × 107 and 1.96 × 108 cells/cm3, respectively. This means that the cell density of LCBPP foam increased nearly 18 times compared with the linear iPP foam. These FIM results revealed that the long-chain branched structure in PP significantly strengthened the melt strength, effectively suppressed the cell breakage and coalescence 17



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and consequently increased cell densities of the foams. The cell sizes of the LCBPP foams were clearly smaller than those of iPP foams, as shown in Figure 9b, which was also an effect of higher melt strength produced by the long-chain branched structure. Interestingly, the cell density did not monotonically decrease with the increase of expansion ratio. Taking LCBPP foams for an example, the cell density decreased when the expansion ratio increased from 2 to 5, but increased with the increase of the expansion ratio from 5 to 7 and 10. It is speculated that flow-induced cell nucleation (void formation) might be induced during core-back operation and it might be enhanced when the core-back distance becomes higher than 5. During the core-back operation, cell coalescence and cell nucleation (void formation) occur simultaneously and competitively; it is easily imagined that the degree of cell coalescence would be increased when the cell wall thickness is reduced with the increase of core-back distance (expansion ratio). Meantime, during the core-back operation, the stretching force exerted on cell walls and elongational flow is induced on cell walls. The elongational flow promotes void formation as observed in PP and PP-talc composites.56 The polymer chains are oriented in the elongational flow. Because the relaxation time of oriented polymer chains is longer than the time scale of cell nucleation and growth, residual stress may exist on the cell and enhance cell nucleation and the final cell density.56 In addition, a strain-induced crystallization may occur in cell walls when applying extensional stress (stretching force) at higher degrees. The resulting crystals might increase the exclusion of physical blowing agent from growing crystalline domains into the boundary of crystal or the amorphous region. With these phenomena, the amorphous region surrounding crystals would become more supersaturated and cell nucleation would be enhanced at the region.55,56 Generally speaking, at the low expansion ratio (low strain) from 2 to 5, cell coalescence was dominant and the resultant cell density was thus reduced; however, by further increasing the expansion ratio to 7 or 10, the effect of extensional stress-induced cell nucleation and/or strain-induced crystallization were 18


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dominant and the cell density increased and the cell size was reduced by increasing expansion ratios. However, it was difficult to differentiate one from the other, and further investigation is therefore needed.

3.5. Effects of foaming temperatures and long-chain branches on cell structures Figure 10 illustrates the relationship between various foaming temperature and cell structures in the core layer for the five-fold expansion ratio of iPP and LCBPP foams. The foaming temperature is the polymer temperature in the mold cavity at which core-back operation is applied. The polymer injected into the mold cavity was cooled and the temperature of the polymer was decreased. Thus, by changing the dwelling time, the foaming temperature was manipulated. For iPP foams, the cell sizes were reduced by prolonging the dwelling time and decreasing the foaming temperature, which increases viscosity and induces crystallization.44 At all foaming temperatures, the cell sizes of the five-fold expansion LCBPP foams became smaller than those of the iPP foams, which is attributed to the high melt strength of LCBPP. As shown in Figure 10, microcellular foams with five-fold expansion could be prepared from both iPP or LCBPP by carefully adjusting the foaming temperature. In our previous work,44 iPP foams were fabricated using core-back FIM, by which a core-back operation was conducted to shift the movable part from 2 mm initial thickness to 10 mm in thickness. The cell sizes of iPP foams prepared in this study were much smaller than those reported in the previous work. This might come from the higher injection speed and the larger dwelling pressure adopted in this research. Figure 11 illustrates the cell morphology of ten-fold expansion iPP and LCBPP foams prepared at different foaming temperatures. As shown, only the iPP foam prepared at 107 °C showed a fibrillary morphology, which was a consequence of the large degree of elongational deformation of cell wall inducing cell coalescence and void formation. Due to the poor melt strength and low elasticity of iPP, the cell walls were not strong 19



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enough to suppress cell rupture and fibril breakage. In contrast, the cell morphology of ten-fold expansion LCBPP foams could maintain spherical shapes in the cross-section perpendicular to the mold-opening direction. The long-chain branches provide the strain hardening property and suppress the complete cell rupture and breakage of the fibrils. Furthermore, LCBPP polymer widens the operating windows for producing 10-fold expansion foams. Figure 12 illustrates the relationship between the cell density and the foaming temperature of iPP and LCBPP foams prepared with various expansion ratios. Because of the fibrillary structure of ten-fold expansion iPP foam, the cell density could not be calculated from SEM images, and only the cell densities of ten-fold expansion LCBPP foams are presented in Figure 12d. In both foams, the cell densities were enhanced with decreasing foaming temperature, which might be due to the increasing melt strength as well as the proceeding crystallization with reducing melt temperature.44 Cell density of the LCBPP foams was noticeably higher than that of the iPP foams even at the very high expansion ratio of 10. The average cell diameter of the iPP and LCBPP foams is illustrated in Figure 13. At all foaming temperatures and expansion ratios, the cell sizes of LCBPP foams was smaller than iPP foams. With the presence of LCB, the average cell diameters of LCBPP foams at an expansion ratio of five-fold were significantly reduced to 20 ~ 30 µm. It should also be mentioned that the average cell diameter of ten-fold expansion LCBPP foams was also small (approximately 30 µm). It is clearly shown that the long-chain branches produce the strain hardening property, which could suppress cell growth during the foaming process.17−20 In addition, the cell size distribution of iPP foams was much wider than that of LCBPP foams, which experimentally demonstrates that the long-chain branches could stabilize the cell structures and give a more uniform cell structure. Herein, these results demonstrated that

20


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introducing long-chain branch in PP was a viable approach to manufacture microcellular PP foams with a high expansion ratio for different potential applications.

3.6. Analysis of open cell content Figure 14 illustrates the changes of open cell content, OCC, of iPP and LCBPP foams at different foaming temperatures. The data of the two-fold expansion foams is not shown because the sample size was too small to measure the OCC precisely by the pycnometer. To measure the OCC, the skin layer was peeled from the foamed sample. As shown in Figure 14, the OCC of iPP and LCBPP foams obviously increased with the increase of the expansion ratio. During the core-back operation, the cells were highly stretched in the mold-opening direction, with an increase of expansion ratio, the cell wall tended to become thinner at a high expansion and thus resulted in a higher OCC. As shown in Figure 14b, the OCC of iPP foams, especially the seven-fold expansion iPP foams, prominently increased with the increase in foaming temperature. As the foaming temperature was increased, the polymer viscosity was reduced. The cell opening occurs more easily with the reduction of viscosity. Comparing the OCC of iPP foam with that of LCBPP foams, the effect of foaming temperature on the OCC was fairly small for LCBPP foams, which could be ascribed to the strain hardening property. The inclusion of long-chain branches in the LCBPP sample obviously reduced OCC. It is clearly demonstrated that long-chain branches could stabilize the cell morphology and keep the cell structure even at a high expansion ratio.

3.7. Foam processing window Figure 15 exhibits the influence of long-chain branches on the foam processing window at which foams with the specific expansion ratio could be produced. The “foam processing window” was calculated from the difference between the maximum and the minimum foaming temperatures. When the foaming temperature exceeds the maximum 21



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temperature of the windows, the foam has large holes or a hollow structure inside the foam. When the foaming temperature was lower than the minimum foaming temperature of the window, the designated expansion ratio could not be achieved because the melt viscosity of PP was too high to be foamed. It shown that the foam processing window of LCBPP foams at low expansion was widened to 1.5 times of that of iPP foams. Moreover, broadening of foam processing windows was more remarkable at the highest expansion ratio of ten-fold, where the 2 °C of window of iPP foams was expanded to 10 °C for the LCBPP foams. The results firmly demonstrated that introduction of long-chain branches is a good method of fabricating high volume expansion ratios of microcellular PP foams and has great potential capacity for preparing microcellular foams with possibly greater than 10 times the expansion ratio.

4. CONCLUSION In this study, ten-fold expansion microcellular foams were successfully fabricated for the first time using a FIM technique with core-back operation. The high expansion foams can overcome several issues that conventional FIM cannot; the lower volume expansion was one of the most serious drawbacks for applying microcellular FIM to plastic foam production. The presence of long-chain branches in polymer structures can greatly change rheological and crystallization properties of polymers so that foamability can be increased. The linear viscoelasticity properties of LCBPP were changed to be much different from iPP, such as a larger storage modulus, a higher complex viscosity, and the existence of plateau in the tan δ−ω curve. The van Gurp and Palmen plots illustrated that LCBPP exhibited the typical thermo-rheological complexity. Additionally, a strain hardening property with various extensional rate dependence was caused by long-chain branches. Flash DSC measurements demonstrated that the long-chain branches could increase the crystallization temperature even at high cooling rates. Moreover, the long chain branches produce the strong heterogeneity that could enhance crystal nucleation. 22


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Only one crystallization process was observed in LCBPP samples even at the cooling rate of 1000 °C/s, whereas iPP showed two different crystallization processes when the cooling rate reached 100 °C/s. Furthermore, a reduced cell size, uniform cell structures, and higher cell density were achieved for LCBPP foams, which was ascribed to the strain hardening property. The foaming results obtained in this study suggested that the long-chain branches could stabilize the cell growth and clearly reduce cell coalescence as well as cell rupture, which could also be identified from the OCC data. Introducing of long-chain branched structures widens the foam processing window, especially at high volume expansion ratios. This study demonstrates that use of long-chain branches for foaming could be an ideal approach to fabricate high expansion microcellular foams and open a promising route to industrially manufacturing injection-molded foams with high weight reduction.

AUTHOR INFORMATION Corresponding Author *Tel.: +81 075-383-2666. Fax: +81 075-383-2646. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was conducted in associated with Grants-in-Aid for Scientific Research (B), Number: 26289289 of Japan Society for Promotion of Science (JSPS).

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(46) Yuan, M.; Turng, L. S.; Gong, S.; Caulfield, D.; Hunt, C.; Spindler, R. Study of Injection Molded Microcellular Polyamide-6 Nanocomposites. Polym. Eng. Sci. 2004, 44, 673–686. (47) Djoumaliisky, S.; Touleshkov, N.; Kotzev, G. Structure of PP Structural Foam Moldings Made by the Gas-Counterpressure Process. Polym. Plast. Technol. Eng. 1997, 36, 257–271. (48) Egger, P.; Fischer, M.; Kirschling, H.; Bledzki, A. K. Kunstst. Versatility Mass Production in MeCell Injection Moulding. Plast. Eur. 2005, 95, 66–70. (49) Spӧrrer, A. N. J.; Altstӓdt, V. Controlling Morphology of Injection Molded Structural Foams by Mold Design and Processing Parameters. J. Cell. Plast. 2007, 43, 313–330. (50) Ishikawa, T.; Ohshima, M. Visual Observation and Numerical Studies of Polymer Foaming Behavior of Polypropylene/Carbon Dioxide System in a Core‐Back Injection Molding Process. Polym. Eng. Sci. 2011, 51, 1617–1625. (51) Ishikawa, T.; Taki, K.; Ohshima, M. Visual Observation and Numerical Studies of N2 vs. CO2 Foaming Behavior in Core‐Back Foam Injection Molding. Polym. Eng. Sci. 2012, 52, 875–883. (52) Stumpf, M.; Spӧrrer, A.; Schmidt, H. W.; Altstӓdt, V. Influence of Supramolecular Additives on Foam Morphology of Injection-Molded i-PP. J. Cell. Plast. 2011, 47, 519−534. (53) Ameli, A.; Nofar, M.; Jahani, D.; Rizvi, G.; Park, C. B. Development of High Void Fraction Polylactide Composite Foams Using Injection Molding: Crystallization and Foaming Behaviors. Chem. Eng. J. 2015, 262, 78−87. (54) Chu, R. K. M.; Mark, L. H.; Jahani, D.; Park, C. B. Estimation of the Foaming Temperature of Mold-Opening Foam Injection Molding Process. J. Cell. Plast. doi: 10.1177/0021955X15592069. (55) Taki, K.; Kitano, D.; Ohshima, M. Effect of Growing Crystalline Phase on Bubble Nucleation in Poly(L-Lactide)/CO2 Batch Foaming. Ind. Eng. Chem. Res. 2011, 50, 3247–3252. (56) Wong A. S.; Guo, Y. T.; Park, C. B. Fundamental Mechanisms of Cell Nucleation in Polypropylene Foaming with Supercritical Carbon Dioxide—Effects of Extensional Stresses and Crystals. J. Supercrit. Fluids. 2013, 79, 142–151. (57) Dealy, J. M.; Larson, R. G. Structure and Rheology of Molten Polymers, Hanser, Cincinnati, OH, 2006.

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Table Captions

Table 1. Processing conditions for foam injection molding experiments.

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Table 1 Parameters

Values

Barrel temperature (°C)

195, 195, 195, 210, 230, 200, 180

Mold temperature (°C)

40

Cooling water (°C)

20

Injection speed (mm/s)

200

Injection pressure (MPa)

160

Dwelling time (s)

2.6~5.0

Shot size (mm)

35

Packing pressure (MPa)

60

Core-back rate (mm/s)

20

Core-back distance (mm)

1, 4, 6, 9

N2 content (wt.%)

0.2

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Figure Captions

Figure 1. Temperature and pressure profile of the polymer melt after injection into the mold cavity. Figure 2. Change of (a) storage modulus, G′, (b) loss modulus, G″, (c) loss tangent, tan δ, and (d) complex viscosity, η∗, as a function of frequency for iPP and LCBPP. Figure 3. van Gurp-Plamen plots for iPP and LCBPP at various measuring temperatures. Figure 4. Elongational viscosity as a function of elongational time at 180 °C for (a) iPP and (b) LCBPP under various Hencky strain rates.

Figure 5. Strain hardening factor, Xe, as a function of Hencky strain rate, ε& , of iPP and LCBPP at a fixed Hencky strain of 2.7. Figure 6. Flash DSC crystallization curves of iPP and LCBPP under cooling rates of −2, −5, −10, and −20 °C/s. Figure 7. Temperature of the crystallization peaks as functions of cooling rates for iPP and LCBPP. Figure 8. SEM micrographs of the cross-section of LCBPP foams in the core layer at expansion ratios of: (a, a′) 2-fold, (b, b′) 5-fold, (c, c′) 7-fold, and (d, d′) 10-fold at a fixed dwelling time of 3 s (foaming temperature = 109 °C). (a−d), and (a′−d′) are taken from the views parallel and perpendicular to the core-back direction, respectively. Figure 9. Change of (a) Cell density, and (b) Cell diameter as a function of expansion ratio for iPP and LCBPP prepared at a foaming temperature of 107 °C and 109 °C, respectively. Figure 10 SEM micrographs of iPP (a−d) and LCBPP (e−h) 5-fold expansion foams fabricated at foaming temperatures of (a) 100 °C, (b) 95 °C, (c) 92 °C, (d) 89 °C, (e) 109 °C, (f) 105 °C, (g) 101 °C, and (h) 98 °C, respectively. Figure 11 SEM micrographs of iPP 10-fold expansion foams fabricated under the foaming temperature of (A) 107 °C, and the corresponding LCBPP 10-fold expansion foams at foaming temperatures of (B) 109 °C, (C) 113 °C, (D) 119 °C. Figure 12. Change of cell density as a function of foaming temperature for iPP and LCBPP foams at expansion ratios of (a) 2-fold, (b) 5-fold, (c) 7-fold, and (d) 10-fold. 32


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Figure 13. Change of average cell diameter as a function of foaming temperature for iPP and LCBPP foams at expansion ratios of (a) 2-fold, (b) 5-fold, (c) 7-fold, and (d) 10-fold. Figure 14. Change of open cell content as a function of foaming temperature for iPP and LCBPP foams at the corresponding expansion ratios of (a) 5-fold, (b) 7-fold, and (c) 10-fold. Figure 15. The foam processing window (∆T) for iPP and LCBPP foams prepared at various expansion ratios.

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Fabrication of High Expansion Microcellular Injection-Molded

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