Foam Extrusion Research Articles

Development of an extrusion die with rollers for foaming polymers

Ideas concerning the use of a flat slot die with rollers in the technology of extrusion foaming of polymers are developed. Based on the experimental study of low-density polyethylene foaming with isobutane, dependences of the cell density and foam density on the polymer solution viscosity, gap width between rollers, die temperature, and talc concentration have been analyzed. A model is formulated for estimating the intensity of coalescence of supercritical bubbles in an inhomogeneous field of flow velocities. The number density of supercritical nuclei formed in a polymer–gas solution in the limiting case of high pressure drop rate is estimated. The obtained simple estimations allow the results of experiments to be justified on a qualitative level. POLYM. ENG. SCI., 2015. © 2015 Society of Plastics Engineers

Influence of chemical nature, expansion ratio and cellular morphology on the fracture behaviour of flexible polyolefin-based foams assessed by the Essential Work of Fracture (EWF)

Several commercial polyolefin-based flexible foams produced by extrusion foaming were characterized in terms of their cellular morphology and fracture behaviour using the concept of the Essential Work of Fracture (EWF), focusing on the influence of foam's chemical nature, expansion ratio and cellular structure on the values of the fracture parameters. Correction procedures were proposed in order to take into account the complexity of foams in the obtained fracture parameters, particularly a correction procedure based on their expansion ratio, and a second one based on the fraction of polymer present in the foams determined from cellular structure characterization. Although doubts remain about the applicability of the EWF methodology to LDPE foams, the correction procedure based on the expansion ratio seemed to provide more accurate results than that based on polymer fraction, with EWF effectively distinguishing between polyolefin foams having different chemical nature. Comparatively, foams based on a P-E copolymer presented the highest values of the essential work of fracture in the MD direction, while significant differences were only observed in the TD direction for foams having a highly oriented cellular structure. All PP-based foams showed similar non-essential work of fracture values in both MD and TD directions.

Chemical foaming extrusion of poly(lactic acid) with chain-extenders: Physical and morphological characterizations

The effects of an epoxide-based chain-extender (CE) on the properties of poly(lactic acid) (PLA) foams obtained by chemical foaming extrusion using 4wt.% of chemical foaming agent (CFA) were studied. PLA/CE blends with different weight ratios of CE were initially processed using a co-rotating twin-screw extruder. PLA/CE foams were then produced using a single-screw extruder. Various PLA/CE foams characterizations, including solution viscosity, thermal properties (DSC, TGA), cellular structure (void fraction, cell size, open cell content), mechanical and dynamic rheological behaviors, were investigated. Results show that the addition of CE enhances the viscosimetric average molecular weight and rheological properties (viscosity and storage modulus) of PLA, but it has no significant effects on the thermal properties of PLA, except the occurring of a cold crystallization. The CE incorporation also led to a decrease in the void fraction of cellular PLA, in the cell size and in the open cell content and to an increase in the cell density. Furthermore, the tensile mechanical properties such as yield stress and elongation at break of cellular materials increased with the addition of CE.

Influence of graphene on the cell morphology and mechanical properties of extruded polystyrene foam

The incorporation of nanoparticles to polymer foams not only reinforces the cell walls and struts but can also lead to a decrease of cell size and enhanced cell morphology which in turn, yield foams with superior mechanical properties. For this purpose, several studies have focused on identifying close-to-ideal nucleating agents as well as understanding the influence of processing parameters on foam cell morphology. This research provides a systemic approach to low-density polystyrene foams produced with graphene (thermally reduced graphite oxide), talc and carbon nanotubes (MWCNTs) via foam extrusion. Remarkably, the cell morphologies of polystyrene/thermally reduced graphite oxide foams show enhanced cell homogeneity with a tremendous increase of the cell densities by more than one order of magnitude compared to neat polystyrene and its counterparts.

Fabrication of polystyrene/nano‐CaCO3 foams with unimodal or bimodal cell structure from extrusion foaming using supercritical carbon dioxide

The article surveyed the fabrication of polystyrene (PS)/nano‐CaCO3 foams with unimodal or bimodal cellular morphology from extrusion foaming using supercritical carbon dioxide (sc‐CO2). In order to discover the factors influenced the cell structure of PS/nano‐CaCO3 foams, the effects of die temperature, die pressure, and nano‐CaCO3 content on cell size, density, and morphology were investigated detailed. The results showed that the nano‐CaCO3 content affected the cell size and morphology of PS/nano‐CaCO3 foams significantly. When the die temperature and pressure was 150°C and 18 MPa, respectively, the foams with 5 wt% nano‐CaCO3 exhibited the unimodal cellular morphology. As the nano‐CaCO3 content increased to 20 wt%, a bimodal cell structure of the foams could be obtained. Moreover, it was found that the bimodal structure correlated more strongly with the pressure drop than the foaming temperature. The article revealed that unimodal or bimodal cellular morphology of PS/nano‐CaCO3 foams could be achieved by changing the extrusion foaming parameters and nano‐CaCO3 content. POLYM. COMPOS., 37:1864–1873, 2016. © 2015 Society of Plastics Engineers

Extrusion foaming of poly(ethylene terephthalate) with carbon dioxide based on rheology analysis

Although low density poly(ethylene terephthalate) foams take advantage of the combination of good performances and lightness and have a wide range of engineering applications, extrusion foaming of the poly(ethylene terephthalate) is still faced with many challenges, especially the inadequate rheological characteristics. In this work, the melt strength of four types of poly(ethylene terephthalate) was compared on the basis of the analysis on the dynamic complex viscosity, storage and loss modulus, relaxation spectrum, loss factor and extensional viscosity using rheometer. It was found that the two long chain branching poly(ethylene terephthalates) owned overall higher melt viscosity and greater elasticity than the other two linear ones. Furthermore, only one long chain branching poly(ethylene terephthalate), i.e. PET4, succeeded in the extensional viscosity characterization and showed the phenomenon of strain hardening. Thereafter, PET4 was successfully applied for the extrusion foaming with CO2 as the blowing agent. The effects of the foaming temperature, screw revolution speed, gas input and addition of nucleation agent were carefully investigated on the extrusion foaming and the resulted poly(ethylene terephthalate) foams properties. Under the optimization condition, the extruded poly(ethylene terephthalate) foams were produced with the volume expansion ratio of 9.5, average cell size of 265 µm, cell density of 4.58 × 105 cells/cm3 and a narrow cell size distribution.

Fabrication of Porous Poly(ε-caprolactone) Scaffolds Containing Chitosan Nanofibers by Combining Extrusion Foaming, Leaching, and Freeze-Drying Methods

Biomimetic porous poly(ε-caprolactone) (PCL) scaffolds containing chitosan nanofibers (CSNF) were produced by using a hybrid process that combines extrusion foaming, water-soluble phase leaching, and freeze-drying. The high porosity and interconnectivity of the scaffolds were achieved by supercritical fluid foaming and the addition of water-soluble poly(ethylene oxide) (PEO) and sodium chloride (NaCl) as porogens. The porosity of the scaffolds after leaching was over 78%. The structure of the chitosan nanofibers introduced into the micropores of the scaffolds was similar to the collagen bundles in the extracellular matrix (ECM). The chitosan nanofibers enhanced the compressive modulus of the scaffolds slightly and improved the water uptake ability by as much as 35% compared to the scaffolds without chitosan. A 3T3 fibroblast cell culture was performed to investigate the biocompatibility of the scaffolds, which revealed that the cells proliferated better on the PCL/chitosan scaffolds than on the PCL scaffolds. Regarding cell morphology, cells on the PCL scaffolds were rounded in the form of tumoroids, suggesting poor cell adhesion and poor cell–scaffold interactions. In contrast, cells on the PCL/chitosan scaffolds were elongated and spindle-shaped, indicating favorable cell–scaffold interactions. Therefore, the highly porous PCL scaffolds containing chitosan nanofibers fabricated in this study have the potential to be used in tissue engineering applications.

Design and Manufacture of Polymeric Scaffolds

Scaffolds are porous materials designed for organ replecement, which are manufactured with a shape and mechanical properties matched to that of original tissue. Such structures are sieved with cells, genes and proteins, that grow in the scaffold pores and after maturation may be implanted into the body. Scaffolds described in this paper are biodegradable and were fabricated from polylactide by three groups of technology: batch foaming and extrusion foaming, electrospinnig, moreover the microstucture was designed first with CAD technique, then fabricated by means of a rapid prototyping. Microstructure of scaffold was evaluated by the scanning electron microscopy.

Foaming Poly(vinyl alcohol)/Microfibrillated Cellulose Composites with CO2 and Water as Co-blowing Agents

We investigated the continuous extrusion foaming of poly(vinyl alcohol) (PVOH)/microfibrillated cellulose (MFC) composites using supercritical carbon dioxide (scCO2) as the blowing agent. First, the as-received PVOH pellets were compounded with water to decrease their melting point. Then, they were compounded with an MFC solution to prepare the sample. Both scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) showed that the MFC dispersed well in the PVOH/MFC composites. Differential scanning calorimetry (DSC) results showed that adding MFC affected the thermal behavior. This, in turn, affected cell nucleation and cell growth phenomena during foam extrusion. Water also acted as a co-blowing agent, together with scCO2, in creating biodegradable polymer foams with a uniform cell structure and a high cell density. The effects of the MFC content, scCO2 content, and die temperature variations on the cell density and cellular morphology of the PVOH/MFC composite foams were ex...

E-PBT—Bead foaming of poly(butylene terephthalate) by underwater pelletizing

Foams from engineering thermoplastics like poly(butylene terephthalate) (PBT) are a new generation of polymer foams and, probably, the future for lightweight, insulation and damping materials. By means of foam extrusion or bead foaming, it is possible to achieve low-to-medium density foams (< 500 kg/m3). However, foam extrusion of PBT is quite challenging due to its low melt strength and drawability combined with a small temperature-processing window, which is a characteristic of semi-crystalline thermoplastics. This work proves that the problem of cell coalescence and insufficient cell stabilisation can be reduced by choosing the right material and processing parameters in foam extrusion with underwater pelletizing. Therefore, expanded PBT beads could be realised for the first time using CO2 as supercritical blowing agent. To obtain spherical low-density PBT beads with a homogenous foam structure, different process parameters were systematically studied with two different commercial extrusion grades and different blowing agent concentrations. The influence of water pressure and cutting speed of the underwater pelletizer on foam morphology of E-PBT and bead structure was studied. It was shown that using a polymer grade with a sufficiently high-melt viscosity helps to reduce cell coalescence. The lowest achieved density was 230 kg/m3. An increase of the blowing agent concentration did not help in reducing the density. The melting range was investigated by differential scanning calorimetry and yielded reasonable moulding temperatures of 205–215 ℃. This corresponds to steam pressures of 17–21 bar in a steam-moulding machine.

Effect of shear factor on bubble nucleation of polystyrene using supercritical fluid as a foaming agent

Abstract The effect of shear factor on bubble nucleation during the foaming extrusion process with polystyrene (PS) using the supercritical fluid (SCF) carbon dioxide (CO2) as a foaming agent was studied based on the energy transformation in this article. The influence of shear factor (which is caused by the velocity gradient in a conical die) on the dynamics of bubble nucleation was investigated and determined. The critical radius for bubble nucleation is presented through the Taylor deformation theory by using a nuclear embryo to represent the state in the bud of the appearance of the nuclear bubble. Theoretical analysis shows that a spherical nuclear bubble can split into two new nuclear bubbles after the deformation in the shear flow field; the radius of the new nuclear bubble is greater than the critical radius, so it can finally grow into a larger nuclear bubble. The shear factor increased the number of the nuclear bubbles in the polymer melt and so increased the density of the bubbles and decreased the diameters of the bubbles. This article proposes a qualitative judgment which indicates that the shear factor in a conical die has a beneficial effect on bubble nucleation, and helps to produce foamed plastics with fine bubbles.

Extrusion foaming of PHBV

This paper reports work on extrusion foaming of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) with a chemical blowing agent based on sodium bicarbonate and citric acid and calcium carbonate nucleation agent. It includes investigations in the effects of rheological behaviour of the polymer, blowing agent, nucleation agent and processing conditions on the foam density and morphology. The poly(3-hydroxybutyrate- co-3-hydroxyvalerate) is a natural biodegradable polyester with high crystallinity, low melt viscosity and slow crystallisation rate and high sensitivity to the thermal degradation at temperatures above its melting point, making it particularly difficult to control the foaming process. Use of negative gradient temperature profile was found beneficial to minimise the thermal degradation and achieve necessary melt strength to stabilise the cell structure. Solidification of the super-cooled polymer melt occurring at the die was discussed in relation to the selection of the temperature profile and rheological behaviour and solidification of the poly(3-hydroxybutyrate- co-3-hydroxyvalerate) characterised by rotational rheometry. In addition to extrusion foaming conditions, effect of the blowing and nucleation agents on rheology of the polymer, the cell refinement on foam density and morphology were discussed. The poly(3-hydroxybutyrate- co-3-hydroxyvalerate) was extruded with a twin screw extruder fitted with a strand die yielding up to 60% density reduction with uniform fine cell structure.

Prediction of gas solubility in poly(3-hydroxybutyrate-co -3-hydroxyvalerate) melt to inform process design and resulting foam microstructure

Foaming studies on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were performed to evaluate the potential impact of nitrogen gas solubility on foam microstructure and properties. It was found that there was a step change from low to high cell density below and above a specific pressure threshold. This pressure threshold, which is associated with nitrogen gas solubility in PHBV, was calculated in order to inform processing requirements for extrusion foaming of PHBV. In the process, the PVT properties of PHBV were also calculated to account for polymer swelling at high temperature and pressure. To validate the predictive method applied in this work, gas solubility of nitrogen was also determined for polypropylene (PP) and poly(lactic acid) (PLA), which have similar thermal and rheological properties to PHBV. Predicted PVT properties for PP were found to be similar to experimental values in the literature, with the predicted solubility having a 10–14% error for PP and 20–24% error for PLA. Application of the same error range to the solubility predicted for PHBV aligned well with experimental observations. POLYM. ENG. SCI., 54:2683–2695, 2014. © 2013 Society of Plastics Engineers

Rheological Behavior Under Shear and Non-Isothermal Elongational Flow of Biodegradable Polymers for Foam Extrusion

The production of many items, in particular for food packaging applications, is based on foam extrusion and thermoforming. These operations require the use of polymers which can grant some specific rheological properties, both under shear and elongational flow. In this work, the behavior of some biodegradable polymers [Mater-Bi® and poly(lactic acid)] under shear and non-isothermal elongational flow was investigated and compared with a traditional, non-biodegradable polymer, in order to assess their suitability for industrial-scale foam extrusion and thermoforming. The rheological characterization evidenced the differences between the different biodegradable polymers and the reference polystyrene (PS), as well as the effect of humidity on their main rheological properties. This can be of great interest in helping to find an optimum solution in replacing PS for the production of food packaging items.

The FPGA Design and Implementation of Pipeline Image Processing in the GPU System

In the GPU system, pipeline image processing is facing the problem that a large amount of data to be processed, complicated processing procedure, more data transmission channels, and etc. All of these lead to low processing speed and large circuit area. This paper proposed a FPGA design of the pipeline image processing in GPU. The design has been implemented by foam extrusion pipeline architecture and validated on Xilinx Virtex XC6VLX550T FPGA. The results show that the consumption of resources is 390726.09 and the speed is 200MHz.

Extensional Flow Properties of Externally Plasticized Cellulose Acetate: Influence of Plasticizer Content

Elongational flow properties of polymer melts are very important for numerous polymer processing technologies such as blown film extrusion or foam extrusion. Rheotens tests were conducted to investigate the influence of plasticizer content on elongational flow properties of cellulose acetate (CA). Triethyl citrate (TEC) was used as plasticizer. Melt strength decreases whereas melt extensibility increases with increasing plasticizer content. Melt strength was further studied as a function of zero shear viscosity. The typical draw resonance of the Rheotens curve shifts to higher drawdown velocity and the amplitude of the draw resonance decreases with increasing TEC content. With respect to foam extrusion, not only are melt strength and melt extensibility important but the elongational behavior at low strain rates and the area under the Rheotens curve are also significant. Therefore, elongational viscosity as well as specific energy input were calculated and investigated with respect to plasticizer content. Preliminary foam extrusion tests of externally plasticized CA using chemical blowing agents confirm the results from rheological characterization.

Continuous Extrusion Foaming of Lignin Enhanced Thermoplastic Polyurethane (TPU)

Continuous Extrusion Foaming of Lignin Enhanced Thermoplastic Polyurethane (TPU)

In situ fibrillation of CO2-philic polymers: Sustainable route to polymer foams in a continuous process

In this study, in situ polymer–fibrillar blends of polypropylene (PP)/polytetrafluoroethylene (PTFE) are prepared. Dynamic oscillatory shear experiments confirm that the PTFE elongates into fibrils during blending and forms a physical network of entanglements in the melt which results in gel-like properties. Uniaxial extensional flow experiments show strain-induced hardening behaviour. CO2 solvency in the PP/PTFE fibrillar blend is enhanced due to the CO2-philic character of PTFE. Remarkably, adding only 0.3 wt% of PTFE is sufficient to markedly enhance the CO2 sorption capacity of the matrix. Continuous foam extrusion of the in situ fibrillar blend reveals a three orders of magnitude increase in bubble density, a ten-fold increase in volume expansion ratio, and a marked broadening of the foaming window with respect to neat PP. These improvements are attributed to the simultaneous enhancement in CO2 solvency and strain-hardening behaviour of the melt in the in situ fibrillar blend.

Assessment of hydrofluoropropenes as insulating blowing agents for extruded polystyrene foams

Thermally insulating extruded polystyrene foams are currently produced with hydrofluorocarbon blowing agents. Hydrofluorocarbons have zero ozone depletion potential but rather high greenhouse warming potential. Various unsaturated fluoropropenes, with greenhouse warming potential values <15, have been assessed as HFC-134a replacements for styrenic extrusion foaming. The screening is first based on the modeling of solubility and diffusivity properties, followed by foaming experiments with a conventional extrusion process. Some fluoropropenes appear to be excellent blowing agents for extruded polystyrene foams and can be used alone for making very low-density foams with regular and large cell sizes, while some others require the use of a co-blowing agent for processing good quality foam. A few others are not suitable as a blowing agent for extruded polystyrene foams due to their toxicity or their very poor transport properties.

Foam extrusion behavior, morphology, and physical foam properties of organic cellulose ester

Abstract