Jacques Tatibouët

Application of Ultrasonic Sensors in the Study of Physical Foaming Agents for Foam Extrusion

The extrusion foaming process involves several critical steps, in which the physical foaming agent plays a significant role: plasticization, solubility, nucleation and bubble growth. Although these aspects can be studied by different techniques, a novel method based on ultrasonic sensors has proven to provide valuable information with respect to the thermoplastic foaming process. This technique can be either used off-line as a characterization tool to improve our understanding of the foaming agent characteristics, or it can be installed in-line, on the extrusion line, as a control device. Review of the different applications of this technique will be covered in this paper, with numerous examples given to the mixture of PS with CO2. The degree of plasticization of the polymer as a function of the blowing agent concentration will be addressed first, followed by the detection of the conditions prone to induce nucleation, in terms of pressure, temperature, type of nucleating agents and flow conditions. The evaluation of the kinetics of bubble growth will also be explored.

A Study of Strain-Induced Nucleation in Thermoplastic Foam Extrusion

The conditions that induce the phase separation and the bubble nucleation for the thermoplastic foam extrusion process in which physical foaming agents (PFA) are involved are obviously linked to the solubility parameters: temperature, PFA content, and pressure. However, it has been reported that flow or shear can significantly modify these degassing conditions. An in-line detection method based on ultrasonic sensors was used to investigate the influence of the flow on the foaming conditions of polystyrene/HFC134a mixtures, for PS resins of various melt flow rates. An increase of the apparent degassing pressure at low melt temperature was observed for high viscosity resins. Deviations from solubility data have been attributed to the combined effects of elongational and shear stresses.

Foam extrusion of polystyrene blown with HFC-134a

There is much interest in developing industrial processes to manufacture extruded polystyrene foam that do not involve ozone depleting blowing agents. A popular alternate candidate is HFC-134a. It has a zero ozone depletion factor and is nearer in chemical structure to standard blowing agents (CFC-22 and HCFC-142b) than carbon dioxide. Although exhibiting main good features, HFC-134a is not used widely as a blowing agent as low foam density is not readily achieved and extruder operation is difficult. A review of past and on-going works on the use of HFC-134 will be addressed first. Then attention will be paid mainly on some processing aspects, with emphasis on the plasticization behavior of polystyrene (PS) by HFC-134a and the effect of screw design on dynamic dissolution of HFC-134a in PS during foam extrusion. Solubility efficiency during extrusion processing has been assessed for different screw configurations by an in-line ultrasonic technique. These results have also been correlated to off-line solubility and diffusivity properties.

Ultrasonic characterization performed during chemical foaming of cross-linked polyolefins

Abstract Injection molding of cross-linked low density foams made from poly(ethylene-co-octene) resins results from simultaneous reactions occurring during the process. The ultrasonic quasi-static characterization technique (no flow) can mimic adequately the conditions prevailing during the molding process (pressure, temperature, time) and monitor the blowing process through sound velocity, attenuation and specific volume measurements. In this work, compounds prepared from resins with different melt flow indices are investigated, showing the influence of the degree of cross-linking on the chemical blowing agent (CBA) decomposition, as well as the effect of viscosity on the degassing conditions. Experiments demonstrate the complexity of CBA decomposition and of gas molecule diffusion in the polymer matrix.

In-line ultrasonic characterization of shear dispersion processes of polydisperse fillers in polymer melts

Shear break-up processes of polydisperse fractal clusters are investigated by the ultrasound scattering technique. Within the framework of fractal aggregation and the hybrid approach model for polydisperse correlated scatterers, the concept of variance in the local filler concentration is used to derive a new expression for the scattering cross-section for polydisperse fractal aggregates in the Rayleigh scattering regime. Considering the scaling laws for the shear-induced disruption of the clusters, the shear stress dependence of the ultrasound scattered intensity for polydisperse fractal aggregates is also derived. The fractal scattering regime is further discussed for both monodisperse and polydisperse clusters of size larger than the wavelength. In-line ultrasonic measurements for the shear disruption processes of silica fume fillers compounded with polypropylene during extrusion are investigated. A critical disaggregation shear stress is determined and is found to decrease with the filler surface treatment concentration. This stress is representative of the particle adhesiveness and aggregate dispersion in the matrix. This is confirmed by the improvement in impact resistance tests. On the basis of the scaling laws and the self-consistent-field approximation usually used in the microrheological models, the shear-thinning behaviour of silica fume clusters is successfully simulated.

Heterogeneous Nucleation in Foams as Assessed by In-line Ultrasonic Measurements

The conditions prevailing during the nucleation stage of the thermoplastic foam extrusion process in which a physical foaming agent is involved are one of the key steps in the design and optimization of the process. The conditions that induce phase separation and bubble nucleation are obviously linked to the solubility parameters, i.e., temperature and pressure at a given foaming agent content, conditions that can be probed during degassing experiments. It has been reported that adding a nucleating agent can significantly modify these degassing conditions. An in-line detection method based on ultrasonic sensors has proven to be sensitive to the onset of the phase separation. This method was used to investigate the influence of both talc and foaming agent concentrations on the nucleation cell density and degassing conditions for polystyrene-HFC-134a mixtures. A qualitative model based on foaming agent concentration gradient near the nucleating agent particles is proposed.

Foaming Cyclo-Olefin Copolymers With Carbon Dioxide

Cyclo-olefin copolymers (COC) based on ethylene-norbornene (E-NB) structures have been investigated with respect to their foamability. COC present interesting characteristics, such as extremely low water absorption, excellent water vapor barrier properties, high strength and very good electrical insulating properties. Varying the ratio of E versus NB monomers enables to tune the glass transition temperature (Tg) over a wide range. Due to the rigid ring structure of NB, the resin remains amorphous. This study presents basic information related to the foam processing of different grades of COC resins with Tg varying between 80 and 130 °C, adding a special emphasis on process-relevant material characteristics: rheology, solubility and plasticization. CO2-laden COC sheets, where the gas sorption was obtained using high pressure autoclave, were foamed using a biaxial stretcher. The impact of selected processing parameters (temperature, time of pre-heating, speed of deformation, magnitude of stretching) on the microcellular foam morphology and density will be reported throughout this paper.

ON-LINE ULTRASONIC CHARACTERIZATION OF POLYMER FLOWS

Although ultrasonic techniques have proven useful for investigating elasticity of solids and viscosity of fluids and gases1, they have seldom been used for polymer studies. Notwithstanding, all reports2,3,4 point out the distinctive behavior of ultrasound in polymers and suggest numerous prospects for fundamental studies, and industrial applications5,6. Thermomechanical properties of polymers are usually measured at low frequencies between 0.01 and 100 Hz, with deformations e ≈ 10-4, while ultrasonic techniques involve frequencies in the MHz range and strains near e ≈ 10-7. Whilst rheology measures global properties associated to long range diffusion of molecules, ultrasonic waves probe the mobility of short chain segments. In an attempt to relate the different measurements, we described experiments7,8 using an apparatus9 that measures the complex ultrasonic modulus with close control of the thermodynamic history. Although successful, the technique involves no macroscopic flow of molecules.

Ultrasonic diagnoses of melting and degradation of poly(ethylene terephthalate)

The melting and thermo-mechanical degradation of poly(ethylene terephthalate) (PET) samples with different moisture levels were investigated in an internal mixer under different processing conditions. The investigation used an ultrasonic diagnosis system and a torque rheometer. The experiment results showed that: 1) a PET sample with a lower humidity level was more stable than that having higher water content under the same processing conditions; 2) higher temperatures and blade speeds accelerated melting and degradation rates. However, the transition point from a partially melted state of a PET sample to a completely melted state could only be identified by ultrasound and the onset of degradation was evident from the ultrasonic measurement as opposed to the torque measurement.

The Supercritical State Paradigm in Thermoplastic Foaming

“Supercritical” has become automatically and intrinsically associated with “carbon dioxide foaming”, with this combination generally resulting into claimed benefits such as microcellular structure, high cell density and enhanced mechanical properties. However the supercritical state remains accessible to any blowing agents. The newly-introduced hydrofluorocarbons (HFCs), following the ban put on chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) in the polystyrene foam industry, are other examples of foaming agents that are processed under temperature and pressure conditions that belong to the supercritical region. Unfortunately, such unusual conditions have resulted in the past into typical processing difficulties that make the resulting benefits associated to the supercritical state questionable. The supercritical state may not be apparently the panacea usually claimed… and sought for! In this work, density characteristics of supercritical fluids were reviewed and linked to the foam nucleation stage. Foaming experiments using a specific polymercarbon dioxide system were conducted at the vicinity of the critical locus and the sequence leading to phase separation monitored using an ultrasonic technique. The results suggested a phase separation mechanism similar to that of spinodal decomposition. Density fluctuations reported for supercritical fluids close to their critical locus would translate for the foaming process into concentration fluctuations typical of the spinodal decomposition.