Morphological, thermo-mechanical, and thermal

Original article

Morphological, thermo-mechanical, and thermal conductivity properties of halloysite nanotube-filled polypropylene nanocomposite foam

Journal of Cellular Plastics 0(0) 1–17 ß The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021955X16681449 cel.sagepub.com

Renan Demori1, Eveline Bischoff1, Ana P de Azeredo2, Susana A Liberman1, Joao Maia3 and Raquel S Mauler1

Abstract Studies about polypropylene nanocomposite foams are receiving attention because nanoparticles can change physical and mechanical properties, as well as improve foaming behavior in terms of homogeneous cell structure, cell density, and void fraction. In this research, the foaming behavior of polypropylene, polypropylene/long-chain branched polypropylene (LCBPP) 100/20 blend, and polypropylene/LCBPP/halloysite nanocomposites with 0.5 and 3 parts per hundred of resin (phr) is studied. The LCBPP was used to improve the rheological properties of polypropylene/LCBPP blend, namely the degree of strain-hardening. Transmission electron microscopy observation indicated that halloysite nanotube particles are well distributed in the matrix by aggregates. Subsequent foaming experiments were conducted using chemical blowing agent in injection-molding processing. Polypropylene foam exhibited high cell density and cell size as well as a collapsing effect, whereas the polypropylene/LCBPP blend showed a reduction of the void fraction and cell density compared to expanded polypropylene. Also, the blend showed reduction of the collapsing effect and increase of homogeneous cell size distribution. The introduction of a small amount of halloysite nanotube in the 1

Chemistry Institute, Federal University of Rio Grande do Sul (UFRGS), Brazil Braskem S/A, III Po´lo Petroquı´mico, Triunfo, Brazil 3 Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH, USA 2

Corresponding author: Raquel S Mauler, Chemistry Institute, Federal University of Rio Grande do Sul, Av. Bento Gonçalves 9500, Porto Alegre, 91501-970, Brazil. Email: [email protected]

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Journal of Cellular Plastics 0(0)

polypropylene/LCBPP blend improved the foaming behavior of the polypropylene, with a uniform cell structure distribution in the resultant foams. In addition, the cell density of the composite sample was higher than the polypropylene/LCBPP sample, having increased 82% and 136% for 0.5 and 3 phr of loaded halloysite nanotube, respectively. Furthermore, the presence of halloysite nanotube increased crystallization temperature (Tc) and slightly increased dynamic-mechanical properties measured by dynamicmechanical thermal analysis. By increasing halloysite nanotube content to 3 phr, the insulating effect increased by 13% compared to polypropylene/LCBPP blend. For comparative purposes, the effect on foaming behavior of polypropylene/LCBPP was also investigated using talc microparticles. Keywords Polypropylene foams, halloysite nanoparticles, talc microparticles, injection-molding, nanocomposites foams

Introduction Polypropylene (PP) is a commodity polymer that has been considered as a substitute for other thermoplastic polymers due to its balanced physical and mechanical properties, recyclability, and low material costs. PP foams are widely used in multiple fields due to their combination of lightness, reduced thermal conductivity, and high absorption energy.1 Despite these excellent properties, linear versions of PP have not been used much in the foaming industry due to their weaker melt strength.2,3 In the foaming processing, low melt strength leads to the rupture of the cell walls under the elongational forces during cell growth. As a result, final foam morphology presents coalesced and open cells. When cells coalesce, cell density as well as cell size uniformity deteriorates. Also, the volume expansion ratio is greatly decreased due to accelerated gas loss through opened cell walls. Therefore, using a long-chain branched polymeric material is essential for producing low-density fine-cell PP foams. Prior research has shown that to increase the melt strength of the polymer, one of the current solutions is to blend PP with special PP grades known as high melt strength PPs. By using long-chain branching PP (LCBPP), the melt strength of the blend can be increased significantly.4–6 In the particular case of blending linear PP and LCBPPs, it has been demonstrated that the addition of 20–30 wt% of LCBPP showed improvements in foaming capacity of the blend.7,8 Thus, a certain amount of strain-hardening promoted by the fraction of LCBPP is sufficient for an optimized foaming behavior.7,8 Currently, the introduction of nanoclay has attracted attention for improving PP’s foaming behavior. Nanofillers act as nucleating agents that reduce the energy barrier of cell nucleation by inducing a local stress variation in polymer/gas solutions and consequently, enhance cell nucleation as well as increase cell density.9–12 Additionally, the high aspect ratio and large surface area of nanoclay particles can improve foaming process in terms of dimensional, thermal stability, and

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reinforcement effect. Also, nanofillers act as cell nucleating agents that lead to smaller cells. The foams made using nanoparticles are useful in many applications such as acoustic, thermal insulators, and improvements in mechanical properties because of the reinforcement effect of the filler.13–16 Halloysite nanotubes (HNT), a naturally occurring clay mineral, have attracted interest as nanofiller-reinforced polymers. Besides its chemical formula of Al2Si2O5(OH)4 2H2O having a similar composition of kaolin, they have a tubular structure with a high length-to-diameter ratio (L/D).17–20 Typically, the size of HNT nanoparticles varies from 50 to 70 nm in external diameter, 100 to 2000 nm in length, and about a 15 nm in diameter of lumen. The nanorod-like geometry of HNT is not intertwined like carbon nanotube (CNT) nor interlinked like layered silicates, which allows the HNT dispersion in the polymer matrix to be easier than CNT and layered silicates.17,19 With small modifications and physical or chemical blowing agents (CBAs), a conventional injection-molding machine can be used to produce PP-foamed parts. Foaming injection-molding has several advantages such as the absence of sink marks on the final part surface, reduced weight, lower back pressure, faster production cycle, and higher stiffness to weight ratio.21,22 This research aims to improve the foamability of PP through blending with 20 parts per hundred of resin (phr) of the branched PP (LCBPP) and using HNT as cell nucleating agent. The primary purpose of the research was to investigate the processing benefits and improvements in properties as a result of combining nanocomposites by foam injecting-molding process. For comparative purposes, the same technique was used to produce neat PP and PP/LCBPP foams using talc as nucleating agent.

Experimental Materials PP homopolymer and branched PP (LCBPP) with a melt flow index (230 C/ 2.16 kg) of 10 g/10 min and 3.5 g/10 min, respectively, were obtained from Braskem S.A. Brazil. The tubular HNTs were purchased from Sigma Aldrich and used as received. The HNT has specific gravity of 2.53, surface area of 64 m2/g, pore volume of 1.26–1.34 mL/g, cation exchange capacity of 8.0 mEq/g, and Zeta potential of 222.543 mV. Azodicarbonamide (ADC), with an average particle size of 3.9 0.6 mm, was added as CBA. Talc particles were from Magnesita S.A., with density of 0.75 g/cm3, minimum superficial area of 5 m2 g1, and 0.1% of retention of the talc particles in 325 mesh sieve (45 mm).

Sample preparation PP with 20 phr of LCBPP was obtained using a Coperion ZSK18 twin-screw corotating extruder (screw diameter of 18 L/D ¼ 38), operating at 200 r/min with

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Figure 1. Scheme of high shear screw profile used to prepare the formulations before the injection-molding and foaming.

constant feed ratio of 4 kg/h, temperature profile: 165–170–175–175–180– 185–190 C using the high shear screw profile as shown in Figure 1. The expansion process was performed using a Battenfeld injection-molding machine. The mold employed contained a 115 mm 55 mm 7 mm cavity and the gate is located at the center of the sample. A constant injection temperature at 200 C and a shut-off nozzle were employed to produce the foam. ADC (1.5%) was used as CBA and zinc oxide (0.5%) was used as an activator of ADC. The conditions for injection-molding process were kept constant in order to examine the effect of the filler type and its content on the morphological, mechanical, and thermo-mechanical properties. The injection-molding parameters used were: injection pressure, hold and back pressure of 55 bar; volume of polymer to be injected was 85% of capacity of the machine; injection, hold, and cooling time were 2, 5 and 60 s, respectively; and mold temperature of 25 C. The duration of cycle was 1 min and 15 s. All samples for testing were taken by placing it in same position, as shown in Figure 3 indicated by a red circle.

Transmission electron microscopy (TEM) The filler morphology was examined using TEM (JEOL JEM-120 Ex II) operating at an accelerating voltage of 80 kV. Ultrathin sections of the specimen (70 nm) were obtained under cryogenic conditions using a RMC CRX microtome. The microtome was equipped with a glass or diamond knife at 80 C, and the film was retrieved onto 300 mesh Cu grids. The sample used in this analysis was noexpanded formulation.

Foam analysis The apparent density of the foams was determined according to the standard method ASTM D3575. The scanning electron microscopy (SEM) was carried out to characterize the specimens’ cell morphology using a Jeol JSM 6060. The samples were cut after frozen with liquid nitrogen.

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Quantitative measurements such as cell distribution statistics, cell density, and average cell diameter were performed using an image analysis software (Image Tool). The average cell diameter was calculated by using the Feret diameter tool presented in the software database. Also, it was possible to count the number of cells (n) displayed in the micrograph using the software. The cell density (N0) and void fraction (Vf) were determined using the following equations (1) and (2)22: 3 N0 ¼ n=A 2 s f Vf ¼ 1 f =s

ð1Þ ð2Þ

where n is the number of cells in the micrograph, A is the area of the micrograph (in cm2), and s and f are the densities of the solid and the foamed material, respectively. The solid density used was 0.90 g/cm3.

Thermal properties (DSC analysis) Thermal properties were determined using a DSC Thermal Analyst 2100 from TA Instruments. All measurements were performed under a nitrogen atmosphere. The samples were heated from 50 C to 200 C at a heating and cooling rate of 10 C/min. Melting temperature (Tm) and crystallization temperature (Tc) were obtained in the first heating and cooling cycle, respectively. The degree of crystallinity was determined using Hm0 ¼ 190 J/g for PP.23 The differential scanning calorimetry (DSC) instrument was calibrated with indium before use.

Thermal conductivity measurements A transient plane source (TPS) hot disk thermal constants analyzer (Hot disk TPS1500)22 was used to measure the thermal conductivity (k) of the no-expanded PP and foamed samples.24 The sensor was sandwiched between two halves of the sample (55 mm x 55 mm each), and during testing, a constant electric power was supplied to the sensor and the temperature increase was recorded. The power output and test duration ranged between 0.8 W and 20 s, respectively. In each case, two samples were tested with at least two replications for each sample and the average values are reported. The test was performed according to ISO 22007-2.

Dynamic-mechanical thermal analysis The specimens for dynamic-mechanical thermal analysis (DMTA) testing were cut from the injected plaques, and the testing was carried out using a TA Instruments Q800 DMA testing machine operation in a single cantilever for specimens measuring 12 mm 13.75 mm 7 mm. The strain was set at 0.02% with a frequency of 1 Hz and a temperature ramp of 3 C/min, from 30 C to 130 C. From the

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experimental data, the storage modulus (E’), Tg, and Tan values were obtained. For each sample and for comparative purposes, the specific stiffness was calculated by the quotient between storage modulus and apparent density.

Results and discussion Transmission electron microscopy The microstructures of no-expanded PP/LCBPP/HNT composites and PP/LCBPP/ Talc with 0.5 phr of loading (Figure 2) were observed by TEM. Despite the high shear screw profile, the fillers still showed the presence of small aggregation, but were well distributed in the matrix. Besides, HNT or talc can be observed as individual and separate particles.

Figure 2. Transmission electron microscopy micrographs of dispersed 0.5 phr of talc or halloysite nanotube (HNT) in injection-molded PP/LCBPP blend. (a) PP/LCBPP/HNT 100/20/0.5; (b) PP/LCBPP/TALC 100/20/0.5 with 0.5 phr of loading. LCBPP: long-chain branched polypropylene; PP: polypropylene.

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Foaming behavior and cellular structure As a general pattern, all foamed samples showed a frozen layer structure (skin) with 1.4 0.1 mm of width in the right and 1.0 0.1 mm of width in the left side of the sample and the core displayed a cellular structure, as shown in Figure 3. This morphology is attributed to the higher temperatures developed inside the core part of the mold permitting cell expansion in the warm and less viscous section, whereas

Figure 3. Pattern of the frozen layer (skin-core) structure presented in all injected foamed samples. All samples for testing were taken from the same part of the sample, in the position showed within the red circle.

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Table 1. Cellular structure characterization of PP/LCBPP/filler foams.

Sample PP PP/LCBPP 100/20 PP/LCBPP/Talc 100/20/0.5 PP/LCBPP/Talc 100/20/3 PP/LCBPP/HNT 100/20/0.5 PP/LCBPP/HNT 100/20/3

Apparent density g/cm3

Cell density (103 cell/cm3)

Void fraction

Cell size (diameter) (mm)

Thermal conductivity (W/mK)

0.48 0.01 0.64 0.03 0.53 0.01

139 48 134

0.46 0.28 0.41

291 176 157 105 110 75

0.11 0.01 0.11 0.00 0.12 0.01

0.59 0.04

187

0.34

106 62

0.15 0.02

0.62 0.01

89

0.31

140 100

0.12 0.01

0.58 0.03

115

0.35

126 83

0.09 0.00

HNT: halloysite nanotubes; LCBPP: long-chain branched polypropylene; PP: polypropylene.

the thermal gradient difference due to the mold’s cooler wall inhibits cell growth near the frozen layer; this behavior is usual in injection-molded foams.14,22,25,26 The material’s code, foam density, volume fraction, cell diameter, and thermal conductivity are presented in Table 1 as well as SEM pictures of the cell characterized foams are presented in Figure 4. The average cell sizes are similar for all the analyzed foams, due to their same foaming process conditions. The expanded PP showed an average density of 0.48 g/ cm3 and non-homogeneous cell distribution, larger cell size, and collapsed cells. The PP/LCBPP blend showed homogeneous cell distribution and reduction of the cell collapsing effect. It is known that branched polymers have higher expandability than linear ones because of the strain-hardening under elongational forces.2,7,27,28 In the foaming experiments described in this research, a certain amount of strainhardening was sufficient for an optimized foaming behavior. In fact, the cell density of the PP foam was 13 104 cells/cm3 and the volume fraction was 0.28. The blend showed a reduction in cell density of about 65% compared to neat PP. The decrease of the cell density in the sample PP/LCBPP 100/20 can be attributed to the lower nucleation rate of the LCBPP portion in the blend, as previously discussed by Antunes et al.7 and Stange and Mu¨nstedt.8 The effect of the fillers used was particularly evident on cell nucleation, which is related to the cell density results (Table 1). Both talc and halloysite enhanced the nucleation efficiency, acting as heterogeneous sites. The addition of 0.5 or 3 phr of talc increased the cell density up to 175% and 285%, respectively. The HNT also increased cell density, 82% and 136% for 0.5 and 3 phr, respectively, but the results were inferior to the ones obtained with talc. Although the addition of the 3 phr of HNT increased cell density, the better and homogeneous cell distribution was achieved with 0.5 phr of HNT in the formulation.

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Figure 4. Scanning electron microscopy (SEM) micrograph of the foams. (a) Neat PP; (b) PP/ LCBPP 100/20; (c) PP/LCBPP/TALC 100/20/0.5; (d) PP/LCBPP/TALC 100/20/3; (e) PP/LCBPP/ HNT 100/20/0.5; (f) PP/LCBPP/HNT 100/20/3.

Related to the cell size distribution, the results of the image analysis using the image software are shown in the graph of frequency versus cell size distribution (Figure 5). The frequency is defined as the number of cells in a particular size range in relation to the total number of cells in that sample. The statistics showed that cells in the range up to 100 mm of diameter represented 60% of total cells in expanded PP, while PP/LCBPP 100/20 sample showed 45% of cells in the range up to 100 mm but 90% of total cells up to 200 mm. Thus, the modification with LCBPP did not increase the number of cells but increased the homogeneity of the cell distribution.

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Figure 5. Statistics of cell size distribution of the expanded samples.

The addition of filler to the PP/LCBPP sample showed that talc with 0.5 and 3 pcr increased 65% and 31% the number of cells in the range up to 100 mm related to PP/LCBPP 100/20, respectively. The addition of HNT with 0.5 or 3 pcr presented similar statistical effect than talc. Additionally, HNT slightly increased the number of smaller cells, where 80% of the cells are in the range up to 200 mm. The use of filler by comparison with PP/LCBPP increased the number of smaller cells, thus acting as nucleating sites. The effect of the filler in polymeric foams can be explained by the studies about cell nucleating behavior. These studies confirmed that the presence of particles induce a local stress variation in polymer/gas solutions that tend to reduce the critical bubble radius significantly. The majority of these theoretical studies reveal that the fundamental mechanisms of cell nucleation processes were based on extensions from the classical nucleation theory.29–32 The nucleation efficiency of the nanoparticles is dependent on the dispersion, particle geometry, aspect ratio, concentration, and particle surface treatment. As a result and relating to the studies on Classical Nucleating Theory, nanoparticles can generate more nucleating sites for bubble growth.9,14,33 These studies also showed that when nanoparticles are used, compared to microparticles, a maximum amount of 1% of the well-dispersed nanoparticles in the matrix is sufficient to yield a maximum increase of the cell density in the foamed material.34

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As a final consideration, a slight nucleation occurred when HNT was used compared to talc microparticles. Additionally, the use of 3 phr of the HNT was not sufficient to increase nucleating sites because of the aggregates and then compromising the cell density during the expansion when compared to 0.5 phr of HNT or talc addition. However, even with particle aggregation, the resulted sample with 0.5 and 3 phr of HNT still displayed a higher cell density and an average cell size reduction compared to PP/LCBPP foam. Studies using talc as nucleating agent showed that the nucleating capacity is related to its morphology. Talc is composed of very fine flakes with irregular morphology.35 Additionally, these irregularities provide crevices that help to form nucleating sites.31,35 Such crevices retain larger volume of gas derived from expansion agent and then nucleate cells. Probably this is the reason that talc microparticles increased the number of cells during foaming when compared to HNT.

Differential scanning calorimetry Thermal properties of the formulations were evaluated to find their role in the foaming process. In fact, higher crystallization temperature (Tc) helps to freeze as well as stabilize the cellular structure formed during molding and cooling, thus reducing the collapsing effect. The crystallization temperature (Table 2) shows that the onset of PP crystallization was at 130 C (peak crystallization temperature of 122 C), whereas PP/LCBPP crystallization starts at 134 C, with a peak temperature at 126 C. The changes in Tc values are correlated with changes in cell morphology. In the PP/LCBPP sample, the enhancement in the Tc is related to the ramification introduced by LCBPP in the blend that acted as crystalline nucleating agent.7,8 As a result, the

Table 2. Performed DSC analysis of the PP/LCBPP/filler foamed samples. Sample

Tm ( C)

Xc(%)a

Tc ( C)b

Tc onset ( C)

PP PP/LCBPP 100/20 PP/LCBPP/TALCO 100/20/0.5 PP/LCBPP/TALCO 100/20/3 PP/LCBPP/HNT 100/20/0.5 PP/LCBPP/HNT 100/20/3

166 164 163 164 165 163

35 39 32 33 43 31

122 126 130 129 128 129

130 134 138 138 136 137

DSC: differential scanning calorimetry; HNT: halloysite nanotubes; LCBPP: long-chain branched polypropylene; PP: polypropylene. a Standard deviation 10%. b Standard deviation 1 C.

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cell morphology of the respective sample enhanced the homogeneity as well as reduced the average cell size with respect to the PP sample. In the fillers, the addition of 0.5 or 3 phr of the talc or HNT showed similar enhancement in Tc, where the average values for Tc onset were at 138 C (Tc peak 130 C). Adding talc or HNT to the formulations shifted Tc and Tc onset to higher temperature than PP/LCBPP blend. The main effects when filler are used to produce foams are as follows: increase of Tc and its hole in the cell nucleation during bubble growth. These effects during the expansion process can avoid coalescence, as well as help the formation of bubble nuclei and can stabilize a homogenous cellular structure.34,36,37 The better cell morphology obtained for PP/LCBPP expanded sample can be associated to the increase of nucleating sites, attributed to the portion of LCBPP inside the sample.8 When HNT or talc particles are added, the enhancement of morphology can be associated with the increase of the Tc values and also the increase of bubble nucleating sites promoted by these particles as discussed previously by the studies with cell nucleating agent.

Thermo-mechanical properties (DMTA) The results of the DMTA tests presented similar behavior among the samples, as shown in Table 3. In terms of specific stiffness, the apparent density and the width of frozen layer govern the mechanical response. Additionally, the non-uniform cellular structure can decrease the storage modulus. As reported by Go´mezGo´mez et al.,22 when two sections with similar width of frozen layer (skin-core) have the same mechanical response of the skin, a more homogeneous cellular structure and the high apparent density (specific stiffness) will contribute to the mechanical performance of the sample.22 Generally, the use of fillers in the foaming process decreases the cell size and increases cell density, which could increase the

Table 3. DMTA properties of the foamed samples PP/LCBPP/filler. Sample

E0 (23 C) (MPa)

Specific stiffness (MPa/cm3.g)

Tg ( C)

transition ( C)

PP PP/LCBPP 100/20 PP/LCBPP/TALC 100/20/0.5 PP/LCBPP/TALC 100/20/3 PP/LCBPP/HNT 100/20/0.5 PP/LCBPP/HNT 100/20/3

77 113 97 106 114 73

159 176 182 179 183 125

4 2 6 2 2 2

77 79 75 79 80 81

DMTA: dynamic-mechanical thermal analysis; HNT: halloysite nanotubes; LCBPP: long-chain branched polypropylene; PP: polypropylene.

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mechanical performance. In this research, the 0.5 phr of filler showed a slight reinforcement effect in the expanded samples, showing similar specific stiffness values independent of the filler type used. In addition, during bubble growth in the foaming process, the filler inside the polymer will be accommodated in the cell walls. Ameli et al.33 studied this effect in PP foams filled with CNTs and found that after expansion the filler has to be accommodated in the cell walls resulting in increased interconnectivity between the fillers. According to their study, depending on the expansion ratio and the distribution of filler inside the polymer matrix, the final properties can be changed. In the present study, the samples showed the same fraction of frozen layer structure. Since the dispersion of the HNT was by aggregates, when 3 phr was used the interconnectivity among HNTs increase and acted as tension concentration points. Therefore, there was no improvement in reinforcement measured by thermo-mechanical analysis and the mechanical response can be attributed to cellular structure, void fraction, as well as, interconnectivity changes in the evaluated samples. The addition of the 3 phr of talc showed the same level of properties than PP/LCBPP. Thus, using the technique described in this work, the addition of 0.5 phr of filler led to the maximum value of the specific stiffness. Probably the thermomechanical properties could be improved by increasing the filler dispersion inside the matrix. The values of Tg measured by DMTA showed no significant changes.

Thermal conductivity Recent studies about thermal conductivity of polymeric foams have shown that the insulation capacity is related to apparent density, void fraction, cell anisotropy, the nature of the filler, and also the strategy of the molding used to produce the foams.24,38,39 In Antunes et al.’s24 research on PP foams without filler, they achieved improvements in insulation effect by reducing cell size, cell anisotropy, and increase void fraction. As highlighted in prior studies, in the case of talc or nanoclay, the better performance can also be associated with the reduction of the cell aspect ratio and improvements of cell distribution. The filler presenting void fraction can also help to reduce thermal conductivity.14,39–41 The unexpanded PP used in this research showed a thermal conductivity of 0.24 W/mK, measured by the same technique. Overall, as expected, the thermal conductivity decreased according to the apparent density after expansion. Among the expanded samples, the addition of 0.5 phr of the talc or HNT showed no influence on thermal conductivity. In this study, the addition of 3 phr of talc or HNT showed two different results according to Table 1. In this research, 36% of thermal conductivity increased when talc was used and 13% of reduction occurred with HNT compared to the PP/LCBPP foam. As noted before, the use of talc as nucleation agent led to higher cell density. Indeed, the HNT clay has a natural hallow structure and very low thermal conductivity