Showing papers in "Cellular Polymers in 2008"

Microcellular Injection Molding of PP and PC/ABS With Precision Mold Opening and Gas Counterpressure

Abstract: The injection molding of microcellular polymers is expected to be increasable promise for engineering applications. The combined effect of precision mold opening and gas counterpressure process pro...

Microcellular Polycarbonate with Improved Notched Impact Strength Produced by Injection Moulding with Physical Blowing Agent

Abstract: Polycarbonate has the reputation of having a tough breaking behaviour, but it is often unknown that this applies only to special conditions. The impact strength of polycarbonate depends on the temp...

The Challenge of Foam Injection-Moulding – Possibilities to Improve Surface Appearance, Foam Morphology and Mechanical Properties:

Abstract: This paper is dealing with structural foam moulding of polypropylene (PP) thermoplastics utilising the MuCell approach to introduce the blowing agent as well as advanced moulding technologies. Till today, structural foam moulding is facing several different challenges, which are limiting a full acceptance in industrial applications. Encountering issues of those parts are the critical surface appearance, the limited density reduction and finally the mechanical properties (stiffness, strength and toughness). Here, two surface improving techniques have been evaluated according to their effectiveness and feasibility. Furthermore, in this experimental study, six different typical automotive PP compounds have been processed to structural foams with a density reduction of 50% and evaluated in regard to their morphologies and mechanical performance. The modulus and strength of structural foams can be correlated to their build-up (skin-layers, foam density), as the toughness performance is heavily dependent on molecular weight, filler-type and filler orientation.

Mechanical Behaviour at Low Strains of LDPE Foams with Cell Sizes in the Microcellular Range: Advantages of Using These Materials in Structural Elements

Abstract: This paper presents the production method and the compressive mechanical response at low strains for a collection of polyethylene foams with high densities and cell sizes in the microcellular range...

Tensile Modulus Prediction of Structural Foams using Density Profiles

Abstract: In this study, the tensile moduli of LDPE structural foams are related to their density profiles measured by X-ray densitometry. A review of the most important mechanical models for cellular materials are presented and adapted to include the complete density profile. Using experimental data, predicted values with this new approach are much closer (4% deviation) when compared with other models for the range of parameters studied.

The Effect of Processing on the Structure and Properties of Crosslinked Closed Cell Polyethylene Foams

Abstract: This paper analyses the relationships between production, structure and properties ofa collection of crosslinked closed cell polyethylene foams. Foams with similar densities produced from a similar base polymer and manufactured using three different technologies (high pressure nitrogen solution process, compression moulding and semicontinuous processing) have been characterised. On the one hand, several foam characteristics such as density, cell diameter, cell wall thickness, cell shape, fraction of mass in the edges, gel content, crystallinity and melting temperature have been measured and related to the way in which the foams were produced. On the other hand, three important physical properties such as thermal conductivity, Young's modulus and thermal expansion have been measured analysing the experimental results in terms of the previously cited foam characteristics. The results have shown that the production route used to manufacture the foam strongly influences the foam structure and as a consequence the main physical properties.

Compression Moulding of Polypropylene Foams and Their Properties

Abstract: Compression moulding was used to produce polypropylene foams using azodicarbonamide as a chemical blowing agent. The morphology of the foams is reported in terms of cell size, cell density, and ski...

Synthesis and Characterization of Porous Polyaniline Conductive Polymers

Abstract: Polyaniline conductive polymers exhibit great potential for linear actuator applications. Many recent studies report methods to develop polyaniline-based materials with increased mechanical properties, electrical conductivity, and faster response time during actuation. In this study, porous blends of poly(methylmethacrylate) and polyaniline are processed using a two phase batch foaming setup. The effect of materials, processing, and system parameters on the physical properties of the resulting cellular structure are investigated. Hence, the effect of density and cell morphology on the electrical conductivity is elucidated.

Pressure Profile in Annular Die using PP/CO2 Solution Viscosity

Abstract: In this paper, a rheological die with a rectangular channel was designed to determine the solution viscosity of supercritical CO 2 in non-crosslinked high-melt-strength (HMS) polypropylene (PP). The shear viscosity of this solution was modeled as a function of temperature, pressure, gas content, and shear rate. With interest in producing highly-expanded foamed wire coatings using an annular die, an analysis of the annular die pressure profile using the measured viscosity data and an analytical method was also conducted.

Extrusion Foam Coating of Coaxial Cables using Chemical Blowing Agent

Abstract: This paper presents an experimental study on manufacture and characterization of an insulating foam coating in the coaxial cables. Coaxial cables with foam insulator are widely used in the communication industries; low loss attenuation coaxial cable is used in the feeders of base stations for mobile phones. The insulating foam plays a principle role in the attenuation characteristics of the cable. In this research work, an extrusion setup was prepared to produce foam insulated coaxial cable where a chemical blowing agent was used as the foaming agent. The effects of blowing agent and die outlet diameter on foam density and loss-attenuation were investigated. The results show that an acceptable loss-attenuation is achievable via adjusting processing parameters.

Crosslinking of Polyolefin Foam. III. Increasing Low-Density Polyethylene Foam Production Efficiency by Incorporation of Polyfunctional Monomers:

Abstract: The effect of introducing polyfunctional monomers into dicumyl peroxide (DCP) crosslinking systems for low-density polyethylene (LDPE) is considered. Effects are characterised as a function of relative concentrations in solid, melt and foamed states. The monomers selected were diallyl phthalate (DALP), trimethylolpropane trimethacrylate (TMPTMA) and triallyl cyanurate (TAC). It is shown that TAC promotes network formation more efficiently than TMPTMA and DALP allowing a considerable reduction in DCP concentration to attain a specific gel content. A TAC concentration of approximately 0.5% was most cost-effective. Crosslinking promotion efficiency was shown to be in the order TAC > DALP > TMPTMA. Polyfunctional monomers containing allyl groups appeared to be significantly more effective than methacrylates and the efficiency improved with increasing functionality. Compression moulding of LDPE foams of similar density showed that the cycle time could be reduced using a DCP/TAC system compared with DCP alone.

Crosslinking of Polyolefin Foam II. Applicability of Parameters to Assess Crosslinking/Foam Density Relationships:

Abstract: The effect of introducing triallyl cyanurate (TAC) monomer, into dicumyl peroxide (DCP) crosslinking systems for low-density polyethylene (LDPE) is considered. In the foam formation, chemical blowing used in a fixed amount of 8.0 phr. Effects are characterised as a function of relative concentrations in solid, melt and foamed states. It was observed that gel content could only be used as a reasonable indicator to predict foaming behaviour only for traditional crosslinking with DCP alone. The results also showed that melt modulus seemed a better indicator when TAC was incorporated in the system but appeared only to be particularly relevant at a specific TAC concentration. It was found that swell ratio better controls expansion prediction whereas foam density determines physical and mechanical properties independent of formulation. Moreover, swell ratio appeared to be able to define expansion characteristics not only of traditional crosslinking systems but also those containing triallyl cyanurate monomer.

On the Nonlinear Creep and Recovery of Open Cell Earplug Foams

Abstract: The compressive creep and recovery of foam earplugs was studied at various applied stresses leading to different fi nal creep strains. Deformation was determined from digital videos of the earplugs via image processing software. Creep could not be modeled by a single exponential in time; creep approximated a power law in time. Nonlinear viscoelasticity was observed: creep compliance depends on stress level. Nevertheless, for strain as high as 24%, recovery follows creep. Recovery does not follow creep at 70% strain. Recovery proceeds to completion given enough time.

Effects of Microstructure of Mesophase Pitch Precursor on the Compression Behaviour of Carbon Foam

Abstract: The compression behaviour ofcarbon foams derived from synthetic naphthalene based and coal tar based mesophase pitch is studied. After carbonization and graphitization, the structure of carbon foam is modified obviously and compression behaviour decreaseddramatically due to the transition of foam from two-dimensional turbostratic structures to graphite structures. The SEM, X-Ray diffraction and compression behaviour are used in the study of modification of mechanical properties. The results show that the compressive strength of carbon foam obtained from synthetic mesophase pitch is only 30% of the foam obtained from coal tar based mesophase pitch. The SEM shows that the length of microcracking of foam derived from synthetic mesophase pitch is as high as 5 μm and that of foam derived from coal tar based mesophase pitch is less than 2 μm. The causes of the differences in properties focused on rod structure of naphthalene based pitch transformed easier to three-dimensional sandwich structure.

Graphitic Foam with High Strength and High Thermal Conductivity Doped by Tungsten

Abstract: A graphitic foam with high strength and high thermal conductivity was prepared through the incorporation of tungsten particle into mesophase pitch precursor. Results show that tungsten acts as catalysts to accelerate the graphitization of carbon, promote more perfect and larger crystallites and enhance the conductive and mechanical properties. Test results reveal that tungsten doped graphitic foam (TDGF) has excellent compressive strength and high thermal conductivity, with highest values reaching 29.6 MPa and 117.8 Wm -1 K -1 for a tungsten concentration of 12 wt% in the precursor materials. More compact struts and cell walls stacked by more uniform pores were observed by scanning electron microscope in graphitic foam. Correlation between the content of dopant and the properties and microstructure of TDGF was discussed.

Cleanroom Insulation Performance of Nitrogen Expanded Low Density Polyvinylidene Fluoride (PVDF) Foams

Abstract: Zotefoams plc is a world leading manufacturer of low density, closed-cell, crosslinked block foams and is recognised globally for producing the highest performance polyolefin foams. In recent years the company has developed a new generation of low density foams based on materials where foaming has traditionally been an insurmountable technical challenge or other limitations have existed to the reduction in density possible. One of these new commercially available foam products, sold under the trademark ZOTEK® F, is based on the fluoropolymer Polyvinylidene Fluoride (PVDF). The properties of the products, not surprisingly, reflect the general properties of the fluoropolymer family however as cellular materials the combination of material attributes is unsurpassed in the industry. This paper will describe these novel foam products covering the key aspects of the current commercial grades of ZOTEK® F and the technology used to manufacture them. The paper will then focus on a specific application area in cleanroom insulation, where the product performance will be reviewed against the combination of technically challenging requirements such as low flammability, low smoke generation, low thermal conductivity, low moisture absorption, chemical inertness and resistance to fungal growth.

Prediction of the Effective Thermal Conductivity of Graphitic Foam Using a Fractal Model

Abstract: Graphitic foams prepared by thermal treating of mesophase pitch under certain temperature and pressure have good thermal conduction after being graphitized. Fractal theory has been applied to study...

A Summary of Work Carried out by EUROPUR into Extractable Materials from Flexible Polyurethane Foam

Abstract: The European polyurethane flexible slabstock manufacturing trade association, EUROPUR, has carried out extensive analytical work over recent years to determine if any extractable or volatile materials can be detected emanating from PU foam. Much of this work concerning aromatic amines and VOCs has already been published. This brief review summarises other work, which has been carried out during this research period.

Effect of As-Mold Rapid Cooling on Shrinkage and Morphology of Core Region of Injection Molded Parts Using Microcellular Foaming Process

Abstract: Shrinkage is a common problem in injection molded parts which considerably affects the dimensional accuracy of the product. In this paper normal cooling (NC) and as-mold rapid cooling (RC) of core region of injection molded samples have been used to study the% shrinkage of Acetal copolymer with increased weight reduction using Microcellular Foaming Process (MCPs). The results show that shrinkage reduction is not only associated with weight reduction (WR) but also directly related with the decrease in crystalline phase and as-mold RC rate.14% decrease in linear shrinkage by NC with 5.5% loss in crystallinity and 24% decrease in linear shrinkage with 9% loss in crystallinity with RC were observed at 10% weight reduction at same mold temperature. Furthermore, RC of core region resulted into decreased cells size and increased cells density. The study can be useful to control the dimensional tolerances and morphology of the injection molded parts by RC of still molten core region.