Expanded Polypropylene Bead Research Articles

Compression testing of EPP bead foams in a vacuum chamber: Experimental investigation on the influence of the cell gas

In this study, we systematically investigate the compression behavior of Expanded Polypropylene (EPP) bead foams using precise experiments in a custom-designed vacuum chamber. The material’s response is examined under controlled pressure conditions, including compression tests at both ambient pressure and 4 mbar, at various strain rates. Considering that most foams consist of more than 90% air, our focus is on elucidating the influence of cell gas on mechanical properties, such as compression modulus, plateau stress, plateau slope, and energy absorption. The impact of cell gas on recovery behavior was analyzed by loading–unloading-steps in the vacuum chamber. Notably, the presence of cell gas shows a pronounced effect on the foam’s deformation behavior, particularly in the plateau region, significantly affecting its resistance to deformation and energy absorption. The findings offer valuable insights for the development of foam materials and structures/components made of foams, especially in applications where resilience and durability are paramount.

Comparison of Mechanical Properties of Steam-Free and Steam-Based Specimens Made of Expanded Polypropylene Beads.

Reducing the CO2 emissions of plastic parts is crucial in terms of sustainable product and process designs. Approaches include the use of recycled materials and reducing the energy demands of processes through more efficient technologies. In this context, this study shows the potential of the steam-free processing of particle foam beads into thin-walled moulded parts. Expanded polypropylene (EPP) particle foam beads have been processed in both a steam-free and steam-based process. For this purpose, specimens with different part densities and thicknesses were produced, the mechanical properties were investigated, and the surface quality was discussed. Specimens made of EPP with a part thickness of 5 to 20 mm and part densities of 60 to 185 g/L were produced steam-free. Lower part thicknesses and higher densities increase the mechanical properties. As the density increased, the homogeneity of the surfaces of the steam-free specimens also increased. In comparison, specimens with a thickness of 10 mm and part densities of 35 to 90 g/L were produced on a steam-based process. The results of the mechanical test were compared with those of the steam-free specimens. The steam-based specimens showed higher mechanical properties for the same density.

Analysis of density-dependent bead and cell structure of expanded polypropylene bead foams from X-ray computed tomography of different resolution

Closed-cell bead foams with their hierarchical geometrical structure are a challenge for statistical reconstruction and finite element modelling. For the purpose of providing the fundamental micro- and meso-structural descriptors - wall thickness, cell as well as bead volume and sphericity - of expanded polypropylene bead foams of different density, 3D-images from X-ray computed tomography are analyzed. A detailed description of development and application of an image analysis methodology for the determination of feature distributions from CT-scans of different level of detail is provided. The methods are based on off-the-shelf algorithms provided by the open-source package distribution FIJI. It should be highlighted, that beside essential methods such as thresholding, euclidean distance and watershed transformation here the Trainable WEKA segmentation is applied for separating material phases in the images. Although the methods elaborated are generally very case sensitive, the reader benefits from the validation strategies applied, so that development of individual methods into the direction of reliability, repeatability and automation is supported.

Role of polymorphism in solid-state production of double-melting expanded polypropylene beads

The production of expanded polypropylene (EPP) beads is of great significance for its wide range of industrial applications. However, it requires special grades of polypropylene with high melt strength (HMS-PP) and the capability of forming double-melting expanded beads to enable sintering during post-processing of EPP. In this study, conventional Ziegler-Natta PP grades of heterophasic ethylene-propylene copolymer, random propylene-ethylene-butene-1 terpolymer, and their 50 wt% blends were studied in a wide range of foaming conditions from solid to semi-liquid state. While, all the polypropylenes were capable of producing EPP beads with double-melting behavior when foamed in a semi-liquid state, in solid-state foaming, the double-melting behavior appears only in polymorphic polypropylenes with low temperature and high temperature melting peaks associated with γ and α phase crystals, respectively. The EPP samples foamed in the solid-state show a closed-cell morphology with high cellular density (>1010 number of cells/cm3) and low cell size (0.3–5 μ m) resulting from the high extensional strength of the polymeric matrix when foamed at a temperature prior to the initial melting. Using polymorphism to induce double-melting behavior and performing the expansion in solid-state is promising for producing nano cellular EPP beads.

Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion.

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m3 using CO2 and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

Improved cell morphology and thermal properties of expanded polypropylene beads by the addition of PP with a high melting point

ABSTRACTExpanded polypropylene (EPP) beads have attracted attention in recent years due to their potential use in fabricating complex, three dimensional structures. In this study, a small amount of polypropylene (PP) with a higher melting temperature (HPP) was blended with low melting point PP (LPP) to regulate the cell morphology and thermal behavior peaks of the resulting blends. These materials were then foamed to produce EPP beads in an autoclave with CO2 at different temperatures, producing EPP beads. The rheological, thermal, and foaming properties of the LPP/HPP blends were characterized. The melt strength and elasticity of LPP/HPP were improved because of rapid crystallization and increase in crystal number with increasing HPP content. These results were attributed to the physical crosslinks and nucleation sites provided by the HPP. These enhanced properties were leveraged to produce regular foams without through‐holes, decreased cell size, and increased cell density. Furthermore, the lower melting point decreased with increasing HPP content, producing a wider temperature range over which steam‐chest molding could occur. Thus, the addition of HPP is a facile method for regulating the cell morphology and thermal behavior of EPP. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45121.

A Study on a New 3D Porous Polymer Printing Based on EPP Beads Containing CO2 Gas

We propose a new CO2 gas-based 3D PPP (Porous Polymer Printing) developed based on the Fused Deposition Modeling scheme of Additive Manufacturing. The 3D porous polymer structure was stacked layer by layer, which was a rapid single step manufacturing technique for a multilayer product in single device. A hybrid filament of diameter 1.75 mm was introduced with EPP (Expanded Polypropylene) beads containing CO2 gas. EPP beads, which have several desirable characteristics such as light weight, shock absorption, thermal insulation, heat resistance, weather resistance, oil resistance, and chemical resistance, were also subject to various process conditions such as EPP type, laminate thickness, printing speed and nozzle temperature. In this paper, three types of EPP beads with expansion ratios of 15, 30 and 40% were investigated. We found the experimentally optimal process conditions of a new CO2 gas-based 3D PPP method and finally demonstrated a simple 3D structure. In the future, a faster production and the mechanical shock absorption effect will be studied and tested for application in the automobile industry.

High Melt Strength Polypropylene with Wide Molecular Weight Distribution Used as Basic Resin for Expanded Polypropylene Beads

The high melt strength polypropylene (HMSPP) is fabricated by direct polymerization through controlling the species and ratios of the external electron donors in the Ziegler–Natta catalyst system at different reaction stages. The polypropylene exhibits wide molecular weight distribution and higher melt strength. The expanded polypropylene (EPP) beads are prepared by the above HMSPP through an autoclave-based batch process. The effects of melt strength on the cellular morphology of EPP are investigated by scanning electron microscope and melt strength meter, respectively. Results indicate higher melt strength facilitates the control of the foaming process and the cellular structure compared with a commercial propylene homopolymer such as T03. The bimodal cell structure is obtained with the single foaming agent CO2 in both of the EPP beads from HMSPP and commercial polypropylene.

Effects of saturation temperature/pressure on melting behavior and cell structure of expanded polypropylene bead

The expanded polypropylene beads with microcellular structure were prepared by an autoclave-based batch foaming process using non-supercritical CO2 as the foaming agent. Herein, smaller polypropylene micropellets (∼0.6 mm) were obtained from three kinds of polypropylene resin by underwater micro-pelletizer system. The results from differential scanning calorimetry indicated that the double melting peaks of beads moved to higher temperature with increasing saturation temperature and pressure. The kind of comonomer did not exert noticeable influence on the double melting behaviors. The X-ray diffraction of expanded polypropylene bead revealed that characteristic peaks of α-type crystal did not change compared with that of original polypropylene micropellets. The relationship between cell morphology and saturation temperature/pressure of expanded polypropylene beads are discussed preliminarily by scanning electron microscope.

Characterization of Expanded Polypropylene Bead Foams with Modified Steam-Chest Molding

Steam is commonly used in polymer bead foam sintering due to its high heat transfer capacity. However, the pressure loss unavoidable during flow through the beads causes a temperature decrease and thereby negatively affects the sintering behavior. In order to reduce the sensitivity of the temperature to the pressure variation, hot air was added to the steam line. The effects of the flow rate, the pressure, and the temperature of the hot air on the surface roughness, thermal properties, and mechanical properties of the molded products were studied. The results showed that the introduction of hot air at a flow rate of 120 L/min decreased the total steaming time by about 32%. Further, a decrease of 16 °C in the processing temperature was observed, which decreased the surface roughness by 50%. Hot air improved the heat flow and thereby enhanced the uniformity of the tensile properties across the molded part.

Double Crystal Melting Peak Generation for Expanded Polypropylene Bead Foam Manufacturing

A high-pressure differential scanning calorimeter chamber was used to investigate double crystal melting peak generation in polypropylene. The double crystal melting peak forms during the batch-based, expanded polypropylene (EPP) bead foaming process. This double peak structure is required for good sintering and the desired geometric shape of the final bead foam products. To form the double peak structure, isothermal treatment over a certain period of time is necessary at a high temperature and saturated pressure during the batch foaming process. This study investigated the influences of various saturation temperatures, time, and pressure on double crystal melting peak behavior. It was seen that the temperature was the most sensitive parameter on double peak generation. Also, the longer saturation time increased the amount of perfect crystals with a high melting temperature. Furthermore, as the saturation pressure increased, the required saturation temperature for generating the second peak with perfected...

Autoclave preparation of expanded polypropylene/poly(lactic acid) blend bead foams with a batch foaming process

Due to the well-defined mechanical properties and high service temperature, considerable efforts have been devoted to the development of expanded polypropylene bead foams nowadays. A pilot scale batch foaming equipment with volume of 50 L was carried out to produce polypropylene and polypropylene/poly(lactic acid) bead foams with using n-pentane as the physical blowing agent. The resultant bead foams exhibited high expansion ratio up to 44.4, nice ellipse bead foam shape, well-defined cell structure and quite high cell density. Neat PP exhibited a narrow foaming temperature window around 4—5°C. Poly(lactic acid) possessed high n-pentane solubility, and the presence of 20% poly(lactic acid) was verified to obviously broaden the foaming temperature window to 10—15°C and facilitated the foam expansion. In this study, the fundamental issues such as pressure building in autoclave, n-pentane solubility, and various parameters to control bead foams’ expansion and cell morphology are presented.

Steam-Chest Molding of Expanded Polypropylene Foams. 2. Mechanism of Interbead Bonding

Expanded polypropylene (EPP) bead foams were processed using steam-chest molding. Interbead bonds formed during processing and was characterized by observing both the product surface and the fracture surfaces using scanning electron microscopy (SEM) and by measuring the tensile properties of specimens. It was found that both the degree of interbead bonding and the tensile strength had a direct relationship to the steam pressure/temperature and that high steam temperature enhanced the crystallization behavior of EPP beads during the cooling process. A formation mechanism of interbead bonding during processing was proposed based on the crystal melting behaviors, as discussed in the first article of this series (Zhai et al. Ind. Eng. Chem. Res. 2010, 49, 9822). Possible parameters influencing the formation of interbead bonding, namely, steam temperature/pressure and bead foam expansion ratio, were investigated in this study. The effect of postannealing of the EPP products on the shrinkage and crystallinity of the molded EPP products was also addressed.

Steam-Chest Molding of Expanded Polypropylene Foams. 1. DSC Simulation of Bead Foam Processing

To investigate the evolution of melting behavior during the steam-chest molding process of expanded polypropylene (EPP) bead, a differential scanning calorimetry (DSC) test involving fast heating, isothermal treatment, and fast cooling was used to simulate the bead foam processing. The EPP bead foam had two original melting peaks: a low melting peak, Tmlow, and a high melting peak, Tmhigh. A new melting peak, Tmi, was induced in the DSC curves, resulting from the heating process, when the treatment temperature was 80−150 °C. The Tmi was about 7 °C higher than the treatment temperature and tended to increase linearly with the increased treatment temperature. A new melting area and further a melting peak, Tmc, were observed in DSC curves, resulting from the cooling process, when the treatment temperatures were higher than 135 °C. The high sensitivity of the Tmi and Tmc to treatment temperature provided a clue to the actual temperature inside the mold during the steam-chest molding process. Different steam pressures were applied during EPP bead processing, and the melting behavior of molded EPP samples was measured to check the processing thermal history. A comparison study was done between the DSC simulations and the actual results to understand the melting behavior evolution of EPP bead foam during processing. Some fundamental issues in the steam-chest molding processing, such as the actual steam temperature, the temperature gradient across the mold cavity, and the accurate pressure parameter used to describe the actual processing temperature were also studied based on the DSC simulation.