Development of a Bead Foam from an Engineering Polymer with

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Development of a Bead Foam from an Engineering Polymer with Addition of Chain Extender: Expanded Polybutylene Terephthalate Tobias Standau,† Bianca Hädelt,† Peter Schreier,‡ and Volker Altstädt*,†,‡ †

Lehrstuhl für Polymere Werkstoffe, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany Neue Materialien Bayreuth GmbH, Gottlieb-Keim-Straße 60, 95448 Bayreuth, Germany

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‡

ABSTRACT: In this work, the development of a bead foam from the engineering polymer polybutylene terephthalate (PBT) in a continuous process is described. The polymer was chemically modified by means of multifunctional epoxy chain extender (CE) to improve the rheological properties, the foaming behavior, and the fusion of the beads. Shear experiments indicated the formation of branched structures as a result of the chemical modification. Viscosity and melt strength were increased due to the chain extender. Also, strain hardening was observed for PBT modified with CE. It was shown that modified PBT has enhanced foaming behavior; namely, a higher expansion and more homogeneous and finer cell morphology were achieved. For the first time, expanded polybutylene terephthalate bead foams (E-PBT) were produced and welded in a steam chest molding process. The parts from steam chest molded beads showed a density down to 170 kg/m3. An optimum concentration of chain extender in terms of expansion and welding behavior was identified as 1 wt %. a patent3 describes the production of expandable bead foams from PET. Detailed knowledge about processing and properties is limited as no scientific publication is available on this topic as yet. It is well-known for polyesters (e.g., PBT, PET, or PLA) that their low melt strength is disadvantageous when it comes to foaming.4 Therefore, often reactive extrusion with countless chemical modifiers such as peroxides,5,6 combinations of pyromellitic anhydrides (PMDA) and bis-oxazolines (PBOZ)7 or triglycidyl isocyanurate (TGIC),2 and many others are used to improve the rheological behavior and consequently to enhance the expansion behavior. Also, numerous publications deal with the commercial multifunctional-epoxy chain extender Joncryl of BASF SE.8−10 This modifier can lead to increased molecular weight (branching and/or cross-linking, higher polydispersity), reduced crystallinity, and changed rheological properties which are favorable for foaming (i.e., increased shear viscosity and melt strength).4 In our previous work bead foam production from PBT was shown11 and processing parameters for continuous bead foam production with foam extruder coupled to an underwater granulator (UWG) were investigated. A high viscosity was identified as one of the key parameters for optimization of foam morphology and expansion. However, it was not possible to

1. INTRODUCTION The consumption of bead foams has increased steadily over the past decades, although only a few polymers are available as bead foams on the market yet. The bead foams with the biggest market share are (i) expandable polystyrene (EPS), which mainly can be found as building insulation and packaging material, and (ii) expanded polypropylene (EPP), which is commonly used in the automotive sector (e.g., in sun visors or bumper cores). The main disadvantage of these established materials is their low thermal resistance, which is 80 °C for EPS and approximately 110 °C for EPP. For certain applications (e.g., close to an engine, so-called under-the-hood-applications) and some processes (e.g., sandwich consolidation by pressing at elevated temperatures which depend on the face sheets properties, cathodic dip painting and drying under heat up to 200 °C), bead foams that possess a higher heat stability are required. Thus, the attention for engineering polymers such as polybutylene terephthalate (PBT) as bead foam material is growing. However, only a few publications deal with the foaming of PBT at all. Jeong et al. published two studies about foam extrusion of PBT. With a chemical blowing agent, a rather low expansion was achieved resulting in foams with intermediate and rather high densities between 600 and 1200 kg/m3.1 The addition of triglycidyl isocyanurate (TGIC) as reactive modifier led to an increase of complex viscosity, and together with isobutane as blowing agent the density was decreased to 330 kg/ m3.2 A current approach with regard to achieving bead foams with higher thermal resistance was done by the company Armacell. They offer a bead foam from polyethylene terephthalate (PET) under the trade name ArmaShape. Only © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 1, 2018 November 17, 2018 November 26, 2018 November 26, 2018 DOI: 10.1021/acs.iecr.8b04799 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research weld the beads at that time, among other reasons because of the high steam pressures which are necessary for fusion. The fusion of semicrystalline bead foams such as EPP usually requires the formation of a double melting peak during the foaming. As stated by Harrison,12 most semicrystalline polymers are able to form multiple melting peaks. This can be caused by either different crystal sizes,13 different crystal structures,14−16 or rearrangement to higher orders (so-called crystal perfection)17 during heating or isothermal phases, respectively. Hingmann et al.18 stated that the creation of the double melting peak for polypropylene depends sensitively on the temperature. Furthermore, in the publication of Nofar et al.19 it was shown for EPP that the isothermal saturation under CO2 atmosphere in an autoclave led to crystal perfection and thereby to formation of a double peak. Also, commercially available EPP is commonly produced in autoclave processes and possesses the characteristic double melting peak.20 The principle to create double melting peak structures due to isothermal saturation is transferable also to other polymers, as it was shown in further works of Nofar et al. for PLA.21,22 These works report that the perfection of crystal structures (i.e., thicker lamellas) takes place at elevated temperatures below the melting point over 10−90 min. Hence, this phenomenon cannot be expected to happen in foam extrusion processes as crystallization and expansion takes place directly at the die exit23 and long isothermal phases cannot be achieved. However, as described by Raps et al.,20 the double melting peak is essential for the fusion of the sole beads to form a molded part in the steam chest molding process. The applied steam temperature range is set between both melting peaks. Hence, the lower melting crystals melt and allow polymer chains to diffuse between the bead interfaces leading to linkage of them, while the unmolten higher melting peak crystals stabilize and retain the foam structure. Also, favorable for the later processing in steam chest molding would be the formation of broader melting peaks, as reported for thermoplastic polyurethane (TPU) foams.24−26 Alternatively, amorphous bead foams such as EPS can be easily molded by heating them above their glass transition temperature Tg, leading to sufficient interbead diffusion of polymer chains and consequently to bead fusion.27 The main issues to make bead foams from polyesters like PBT can be summarized as (i) the need to improve their poor rheological properties (including the low melt strength), (ii) to fit in the narrow processing window, and (iii) to ensure the fusion between the beads. In this study, the chemical modification of PBT with chain extender (CE) to overcome these issues is described.

Figure 1. Sketch of the foam extrusion line coupled with underwater granulator for continuous production of bead foams. Parameter setup during the trials is given in the tables.

Table 1. CE Concentrations Used during the Trials and Sample Notations CE concentration [wt %]

sample notation

0 0.5 1 2 4

neat PBT + 0.5CE PBT + 1CE PBT + 2CE PBT + 4CE

one-phase gas-melt mixture is transferred to the following single screw extruder (D = 45 mm, L/D = 30D) where the gas-loaded melt is successively cooled to 220 °C. Here, the screw speed was varied between 15 and 18 rpm to ensure a stable process (i.e., adjusting screw speed accordingly to rising or falling melt pressure). In the attached underwater granulator, the gas-loaded melt is pressed through a perforated plate at a temperature of 295 °C. Here, the pressure drops and the foaming takes place. Simultaneously a rotating knife (2000 rpm) is cutting the expanding polymer strand to foamed beads which are taken by a continuous water stream (80 °C), get dried, and then are ejected from the machine. Throughput was set as 7.0 kg/h. Welding of the foamed beads was carried out on a steam chest molding machine, Teubert TVZ 162/100PP (Blumberg, Germany), equipped with a specialized steam generator, HaeCo II from Unibell (Hwaseong, South Korea), which is able to provide unconventionally high pressures of up to 25 bar. A custom-made aluminum mold (20 × 20 × 6 cm3) was filled with foamed beads by hand. An empirically optimized steaming process wherein the pressure was applied in two consecutive steps was carried out to weld the beads. Conditions varied depending on the CE concentration. In the first step steam pressures of 7−10 bar were applied for 45−60 s followed by the second steaming with 13−17 bar for 30−45 s. Thermal Characterization. The thermal behavior was determined by the use of a differential scanning calorimeter (DSC 1) from Mettler Toledo (Columbus, OH, USA). Measurements were carried out in nitrogen atmosphere in a temperature range from 25 to 280 °C with a temperature ramp of 10 K/min. The first heating curve was analyzed to determine the influence of the processing. The degree of crystallinity was

2. EXPERIMENTAL SECTION Materials. PBT Pocan B1300 was kindly provided by Lanxess AG (Cologne, Germany). Before processing the material was dried for 4 h at 80 °C. For chemical modification, the multifunctional epoxy chain extender Joncryl ADR 4468 from BASF SE (Ludwigshafen, Germany) was used. Processing. Bead foams were produced using a Dr. Collin tandem foam extrusion line (Ebersberg, Germany) coupled with an underwater granulator LPU from Gala Kunststoff- and Kautschukmaschinen GmbH (Xanten, Germany) as can be seen schematically in Figure 1. The tandem line consists of a twin-screw extruder (D = 25 mm, L/D = 42D) in which the polymer or dry blends of the polymer with different concentrations of chain extender (see Table 1) and the blowing agent CO2 (2 wt %) were homogenized at 245 °C and a screw speed of 150 rpm. The B

DOI: 10.1021/acs.iecr.8b04799 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 2. Foam morphologies of PBT foamed beads with and without CE. Densities and cell sizes are noted.

Figure 3. Frequency sweeps (left) and time sweeps (right) of processed PBT with and without chain extender at 250 °C.

GmbH, Buchen, Germany) at a temperature of 240 °C. The temperature was chosen to avoid die swell or sagging at the die to prevent internal stresses in the strand. The strand is drawn down with a constant acceleration of 12 mm/s2 by two counterrotating wheels of the Rheotens 71.97 device (Göttfert Werkstoff-Prüfmaschinen GmbH, Buchen, Germany) which is located below the capillary rheometer. Therefore, the strand is subjected to uniaxial deformation. The wheel pairs are connected to a force transducer in order to measure the tensile force applied to the stretched strand. The tensile force is then plotted as a function of the draw-down velocity. The maximum force at rupture of the strand is defined as melt strength.29 Measurement was repeated at least five times to ensure reproducibility. Analyses of Foam Morphology and Density. Scanning electron microscopy (SEM) images were made with a JEOL JSM-6510 (Akishima, Japan). By the use of ImageJ software, cell sizes were statistically calculated as an average of all cells of the whole cross section of a bead. Two beads per sample were analyzed; i.e., over 100 cells were considered. Foam density was determined with the Archimedes principle using a Mettler Toledo AG245 balance (Columbus, OH, USA) with density kit. Dynamic Mechanical Analysis (DMA). To first evaluate the temperature potential of the expanded polybutylene terephthalate (E-PBT) bead foams, DMA was carried out on a Gabo Eplexor 500N (Ahlden, Germany) in compression mode. Therefore, cylindrical samples with dimensions of 25 mm

determined with the theoretical value for the heat of fusion of 100% crystal PBT (ΔH0m = 140 J/g) from the literature.28 Rheology. All samples for rheological investigations were prepared from the foamed beads which were cryoground with a mill from Retsch GmbH (Haan, Germany) and then melt pressed into the desired shapes (shear, round plates with 25 mm diameter and 2 mm thickness; elongational, rectangular plates 10 × 14 × 0.6 mm3) on a hot press from P/O/Weber (Remshalden, Germany). Shear rheology experiments were carried out in oscillatory mode on a RDA III from TA Instruments (New Castle, DE, USA) at 250 °C. Strain sweeps from 0.1 to 100% at a constant frequency of 1 rad/s at 250 °C were performed prior to the frequency sweep to determine the linear viscoelastic regime for each material. Frequency sweeps were performed with strains of 10% from 500 to 0.5 rad/s. Additionally, time sweeps were conducted for 60 min at 1 rad/s within the linear viscoelastic regime. Measurements to determine the extensional rheology were carried out on an Anton Paar MCR 702 (Graz, Austria) with a universal extensional fixture (UXF). The measurement temperature was set as 230 °C, which ensured no sagging of the sample. Strain rates were varied (0.1, 0.5, 1.0, 5.0, and 10.0 s−1). Additionally, Rheotens measurements were carried out to determine the melt strength of the materials. Therefore, the cryoground and strongly compacted bead foams were pressed vertically through the die (l = 30 mm, d = 2 mm) of a Rheograph 6000 capillary rheometer (Göttfert Werkstoff-Prüfmaschinen C


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Figure 4. Results of elongational viscosity measurements at 230 °C with different strain rates. Please note: Samples with 0.5 and 2 wt % are not shown here, but behave similarly to PBT with 1 wt %.

From this, it can be seen that two contrary effectschain extension/branching due to chain extender reaction (resulting in a higher viscosity) and degradation due to increasing shear forces (resulting in a lower viscosity and shorter chain fragments)are facing each other. In the case of high CE concentrations (i.e., 4 wt %), the balance is shifted more to degradation. Nevertheless, the viscosity is still higher than for the neat material. Furthermore, time sweep measurements (cf. Figure 3) reveal for all samples with CE a significant increase in viscosity over time while the neat material shows constant viscosity. Conclusively, it can be assumed that the chain extender did not fully react during the extrusion process even though the residence time of the compound in the extrusion line was several minutes. Extensional Viscosity. Extensional viscosity measurements were carried out on melt pressed samples of the cryoground foamed beads of the neat and modified PBT. Representative curves at different strain rates are shown in Figure 4 for the neat PBT and the modified PBT at CE concentrations of 1 and 4 wt %, respectively. According to Spitael et al.34 strain rates that usually occur during foaming (over a short period) are 1−5 s−1. The neat PBT was difficult to measure at all, because of its low viscosity. Especially at high strain rates no signal was detectable, as the samples ripped suddenly. Compared to the samples with 0.5, 1.0, and 2.0 wt % CE no strain hardening was observed. This corresponds to the bad foam morphology visible in Figure 2 (cell rupture and coalescence). Strain hardening is favorable for foaming processes and can be induced by entanglements of polymer chains while stretching the sample. In the measurement of PBT with 1 wt % CE, it is clearly visible as a drastic increase of viscosity (steeper slope) over time. The sample with 4 wt % shows divergent behavior. No clear strain hardening can be noted, most likely due to the before-mentioned shear-induced degradation happening at processing with the highest concentrations of CE. Rheotens. From the Rheotens curves in Figure 5 it can be seen clearly that the neat PBT has almost unmeasurable low melt strength, leading to less expansion and cell coalescence as shown in Figure 2. With increasing CE concentration (up to 2 wt %), the melt strength increases significantly. The mentioned shearinduced degradation at higher CE content of 4 wt % also leads to lower melt strength compared to the samples with less CE (i.e., 0.5, 1.0, and 2.0 wt %) which can be attributed to reduced network structures.33 It can be further summarized that the conclusions from the Rheotens measurements are congruent with the shear and elongational viscosity results.

diameter and 20 mm height were prepared out of the welded blocks. Measurement of the complex modulus was carried out with a force of 150 N at 1 Hz with a heating rate of 1 K/min from 25 to 230 °C. As reference, a commercial EPP Neopolen P 92 HD 180 (220 kg/m3) from BASF SE was used.

3. RESULTS AND DISCUSSION Foaming of the Beads. Round, foamed beads were obtained for neat PBT and at all concentrations of CE (0, 0.5, 1.0, 2.0, and 4 wt % CE). As can be seen from Figure 2, the single beads showed varying morphologies. Also, they exhibit different densities. A rather coarse cell structure with coalescing cells was observed for the neat PBT foamed bead which has a density of 224 kg/m3. Cell size could not be determined. However, some cells exceed a diameter of several millimeters. The addition of CE leads to a finer cell structure and an increased expansion resulting in a lower density. An optimum regarding maximum possible density reduction and homogeneity of cell structure was found for the foamed beads with 1 wt % CE. Here, the density was 179 kg/m3 and cell size was 155 μm. An increased CE concentration of 2 wt % or even 4 wt %, respectively, leads to higher densities and more heterogeneous foam morphologies. Shear Viscosity. In Figure 3 the frequency sweeps can be seen for the cryoground foamed beads of PBT with different CE concentrations. The neat PBT shows a linear curve with a zeroshear viscosity of around 100 Pa·s. All modified samples show an increased zero-shear viscosity, which can be attributed to chain extension and formation of branched structures according to Najafi et al.30 The viscosity measured for the processed PBT beads rises with the CE concentration up to the concentration of 2 wt %. Unexpectedly, the sample with 4 wt % CE shows lower viscosity after extrusion than the samples with 1 and 2 wt % CE. This could be explained as with the reaction of CE and the consequently increased melt viscosity a higher torque in the extruder is coming along. This was also stated by Yang et al.31 for PET modified with similar CE. Hence, high shear forces can be expected to occur. This effect would be most pronounced in the case of the highest CE concentration of 4 wt %. Thus, in the case of CE addition the increased shear during the extrusion process will cause degradation of PBT resulting in shorter chains as a side effect.32 Consequently, the samples with 4 wt % CE prepared out of the cryoground beads after the extrusion process exhibit a lower viscosity. Cailloux et al.33 described comparable effects for PLA mixed with similar CE. Within their study, they reported a rising torque at the beginning of the mixing assigned to the reaction of the CE followed by a drop which was attributed to shear induced degradation resulting from the previously developed network structure. D


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processing, the sample possesses less long-chain branches but therefore most likely short-chain fragments which is reasonable as the viscosity curve is lower compared to other modified samples (cf. Figure 3). Camacho et al. addressed shorter polymer chains that were formed due to chain scission after multiple extrusion steps as the reason for an increase in crystallinity as they can rearrange easily.41 Lower molecular weight, respectively shorter chains, can enhance the crystallinity.42 Since shorter chain fragments are assumed within the sample, the increased crystallinity is plausible. The neat PBT beads could not be welded, but all modified materials could be molded in the steam chest molding process as can be seen in Figure 7. For the CE concentrations of 1 and 2 wt Figure 5. Rheotens measurements of processed PBT with and without chain extender at 240 °C.

Thermal Characterization (DSC) and Welding of the Beads. In Figure 6 the DSC curves of the nonwelded beads are

Figure 7. Foam morphology of welded PBT bead foams (E-PBT).

%, the densities of the molded parts were almost similar to that of the foamed beads. Furthermore, fusion was optimal for the CE concentrations of 1 and 2 wt %, judged based on the appearance, as the lower and higher concentrations showed rather large voids. Also, for the samples with 0.5 and 4 wt % densities are higher for the welded parts corresponding to the densities of the nonwelded foamed beads. The increase in density for E-PBT with 0.5 and 4 wt % CE can be explained by the fact that the cavity was filled with a larger bead volume to induce additional compression during steam chest molding, which was necessary to ensure fusion. More interesting is the fact that the steam chest molding process can be carried out with pressures between 13 and 17 bar. These pressures of saturated steams convert approximately to temperatures between 192 and 205 °C, which are clearly lower than the melting point of the E-PBT beads of approximately 225 °C. Also, no double melting peak is detectable which is a result of the processing by extrusion. The fact that E-PBT can be welded anyhow contradicts the current knowledge about fusion of semicrystalline bead foams by applying steam pressures between a double melting peak, e.g., as it is typically done for expanded polypropylene (EPP). Possible explanations for the fusion of the beads from chemically modified PBT are summarized in two hypotheses: (I) During the underwater granulation, the expanding PBT melt (approximately 220 °C) gets in contact with the much colder water (80 °C). Consequently, at least the bead surface could get quenched. It was already shown for other polyesters, such as PLA,43 that water quenching led to quite amorphous behavior. Possibly, even though the DSC data show a semicrystalline behavior for the beads, their surfaces could be amorphous. Unfortunately, in this study it was not possible with DSC (and even wide-angle X-ray scattering (WAXS)) to clearly

Figure 6. DSC curves of nonwelded E-PBT beads with and without chain extender (10 K/min). The gray area shows the steam temperature (first heating curve is shown).

shown. It is obvious that the addition of chain extender does not change the melting temperature (Tm) but the crystallinity is decreased up to the concentration of 2 wt % CE. From the structure of the chain extender8 it is visible that it (i) contains bulky benzene rings4 and (ii) has multiple epoxy groups that explain the high efficiency to create branched structures35,36 which were reported in the literature before30 and are indicated by the shear rheology curves (cf. Figure 3). Bothsteric hindrances and branched structures at a certain extentcould result in reduced chain mobility.37,38 Furthermore, with increasing branching lower degrees of crystallinity are observable due to disruption of the chain symmetry.39 The phenomenon of a decrease in the achievable crystallinity after addition of chain extender was also observed for foams from branched PLA by Mihai et al.40 They report that, even though the branched structure, which is formed due to the reaction of the chain extender, could accelerate the nucleation, the total crystallinity is found to be decreased as a consequence of the interrupted chain linearity and the creation of steric hindrances. Nevertheless, the sample with 4 wt % CE is an exception as it shows a similar degree (32.4%) of crystallinity to the neat PBT beads (33.9%). As a result of the before-mentioned shearinduced degradation at too-high CE concentration while melt E


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4. CONCLUSIONS In our study, we could show that it is possible to process bead foams from PBT and weld them if they are modified with a multifunctional epoxy chain extender. By the addition of the CE the thermal behavior of the foamed beads changes and lower crystallinities were observed. Rheology experiments revealed that due to the incorporation of the CE a branched topology can be expected and strain hardening was induced at concentrations of 0.5−2 wt %. Furthermore, the melt strength was obviously increased leading to increased expansion and improved foam morphology. The concentration of 1 wt % chain extender was identified as optimum in terms of foam properties (density and morphology). It was possible to create foamed parts with an overall density down to 170 kg/m3 with a homogeneous cell morphology and fewer voids. Additionally, it was shown that a too-high CE concentration of 4 wt % goes along with unfavorable rheological behavior leading to no further improvement of foam morphology or expansion behavior (i.e., density reduction). Furthermore, it was found that fusion is only possible if PBT was modified with the chain extender. However, the thermal behavior, which is also the key factor for the welding process, changes with the addition of this modification and needs more investigations in the future. This is necessary to enhance the understanding of the fusion mechanism, which is obviously different from those of the established bead foams (EPS and EPP). Two possible explanations were hypothesized as (i) due to quenching of the expanding melt in the underwater granulation an amorphous surface appears, allowing inter bead chain diffusion by heating above the Tg while steam chest molding and/or (ii) unreacted CE remains in the beads and is activated by the hot steam in the steam chest molding resulting in covalent bonding at the bead surface. DMA results showed that a higher heat stability under compression can be expected for E-PBT (failure at approximately 200 °C) compared to EPP (failure at approximately 120 °C).

identify differences between surface and core regions of the beads as the cell walls are very thin. However, assuming an amorphous bead surface is created during the expansion in the underwater granulator, chain interdiffusion between the bead interfaces would be possible during steam chest molding. (II) The time sweep measurements revealed that the foamed beads still contain unreacted chain extender. It was reported in the literature before that, after processing, unreacted chain extender can remain in the polymer visible in an increase of viscosity over time.44 Still, the number of functional groups of the chain extender (epoxy groups) are low and could not be tracked in Fourier transform infrared (FTIR) spectroscopic measurements. Yet, it would be plausible that during the steam chest molding again temperatures of around 200 °C are applied to the beads inducing further reactions at the bead interfaces and leading to covalent bonding. Although it is remarkable that only the CE-modified PBT could be welded, it must be emphasized that the amount of unreacted epoxy groups on the bead surface that might contribute to this bonding formation would be rather low. Even a combination of both hypotheses could be possible. However, the surface condition of the beads should be investigated with advanced methods in regard to completing the explanation for bead fusion. Among others X-ray photoelectron spectroscopy (XPS) and NMR would be suitable to evaluate the surface regarding its chemical composition and atomic force microscopy (AFM) would be suitable to reveal and characterize potential crystal structures at the welding line.45 DMA. In Figure 8 a representative curve of E-PBT (1 wt % CE) and a curve of EPP as reference are shown. For EPP the

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 0049 (0)921 55 7471. Fax: 0049 (0)921 55 7473. E-mail: [email protected]. ORCID

Volker Altstädt: 0000-0003-0312-6226 Notes

The authors declare no competing financial interest.

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Figure 8. DMA curve of E-PBT (1 wt % CE) with a density of 170 kg/ m3 and EPP (commercial reference) with a density of 220 kg/m3.

ACKNOWLEDGMENTS We would like to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this project (AL-474/28-1). We also acknowledge the Bavarian Polymer Institute and all students, technicians, and scientific members who were involved in the trials and thank them for their support and fruitful discussion. Special thanks to Sebastian Gröschel for carrying out the extrusion trials and to Michael Fafara and Max Löhner for steam chest molding.

values are higher, which can be at least partially addressed to the higher density of the sample. Also, polypropylene exhibits a higher modulus inherently. The maximum at 49.2 °C of the EPBT curve can be attributed to the glass transition of PBT. Even though the complex modulus decreases with increasing temperature, it clearly can be seen that E-PBT can withstand elevated temperatures of >150 °C better than EPP. Therfore, EPBT shows a good thermal stability over a wide temperature range. However, above 200 °C the complex modulus decreases significantly. Based on this analysis, the thermal resistance of EPP is nearly 125 °C, while for E-PBT it is around 200 °C.

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REFERENCES

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Development of a Bead Foam from an Engineering Polymer with

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