Richard Gendron

Foaming Polystyrene with a Mixture of CO2 and Ethanol

Use of mixtures of blowing agents in thermoplastic foam extrusion has been an industrial practice for a long time. However, it has gained renewed interest in the past few years due to the introduction of difficult-to-process alternative gases, targeted as potential replacement for the banned ozone-depleting blowing agents. Reasons for blending physical foaming agents (PFA) are numerous. The incentives may be economical, environmental, or technical. With respect to that latter factor, blending suitable PFAs is often regarded as providing a better control of processing conditions. For example, a specific PFA could be selected for its inflation performance and blended with other co-blowing agents chosen for their stabilizing role. Although a considerable amount of work has been done in that area, very little information has been disclosed in open literature. Carbon dioxide (CO2) has been reported as an interesting candidate for low-density polystyrene (PS) foaming, although the required concentrations are associated with high processing pressures due to the low solubility of the gas. Thus, stable processing conditions are difficult to achieve. This work studies the effect of blending CO2 with ethanol (EtOH) as a co-blowing agent for PS foaming. Extrusion foaming performance of this mixture is discussed, with respect to its solubility (i.e., degassing conditions) and rheological behavior. The function of each blowing agent during the process is analyzed with respect to the plasticization, nucleation, expansion, and stabilization phases. Attention is also paid to the interaction involving the two PFA components.

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.

Autoclave Foaming of Poly(ε-Caprolactone) Using Carbon Dioxide: Impact of Crystallization on Cell Structure

The purpose of this study is to develop a better understanding of the main mechanisms controlling the foaming of poly(ε-caprolactone) (PCL), a semi-crystalline biodegradable polymer, using batch processing and CO 2 as the physical blowing agent. A detailed study of the dissolution of CO2 is conducted in order to establish its impact on the transition temperatures of PCL. In a following step, particular attention is paid to the effects of crystallization on cell structure. This foam structure is subsequently linked to the processing window, expressed in terms of the processing temperature with respect to the shifted crystallization temperature and modified crystallization kinetics of PCL. The effects of talc addition on PCL crystallization and foam structure are also investigated, as talc is expected to modify the nucleation rate of crystallites and cells.

Extrusion Foaming of Poly(Lactic acid) Blown with CO2: Toward 100% Green Material

This paper presents a thorough investigation of the continuous extrusion foaming of amorphous poly(lactic acid) (PLA) using carbon dioxide (CO2) as the blowing agent. Detailed results describing the plasticization induced by CO2 dissolution as measured from two different methods, on-line rheometry and in-line ultrasonic technique, are given. Characteristics of the foams obtained from extrusion performed under various processing conditions are also reported. Extrusion of PLA foams in the 20–25 kg/m3 density range was achieved. However, the associated processing window was very narrow: CO2 content lower than ca. 7 wt% did not lead to significant foam expansion while foams blown at CO2 contents larger than 8.3 wt% showed severe shrinkage upon ageing.

Residence time distribution in extruders determined by in-line ultrasonic measurements

The knowledge of residence time distribution (RTD) in industrial extruders is critical, notably when dealing with easily degradable polymers or when using extruders as chemical reactors. Many methods have been proposed for RTD determination but there are some drawbacks associated with each; they are expensive, hazardous, time consuming, or lacking sensitivity. A novel ultrasonic technique, sensitive to the filler concentration of polymer suspensions, is proposed. Ultrasonic properties (ultrasonic velocity and attenuation) were evaluated with regard to parameters such as linearity of the response, resolution of the measurement, and especially robustness to pressure and temperature variations. The attenuation, chosen for RTD evaluation along with a specific grade of calcium carbonate filler as the tracer, was then monitored to yield the RTD of the material in a twin-screw extruder, for different experimental conditions where quantity of filler utilized, as well as the method used to feed it, were changed in order to optimize the technique.

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.

Blends of CO2 and 2-ethyl hexanol as replacement foaming agents for extruded polystyrene

By the year 2010, HCFC 142b will be banned for use as a foaming agent for extruded polystyrene (PS) foam in North America. Many blends of foaming agents have been patented as replacements to expand PS. In this study the optimal concentration of a previously unexplored blend of CO2 and 2-ethyl hexanol (2-EH) is shown to allow the production of PS foam of 30 kg/m3 density. The glass transition temperature reduction of the PS, due to the incorporation of 2-EH, is believed to be an important contributor to the success of this foaming agent blend. In long-term use of the foams, the 2-EH does not measurably diffuse out of the PS, whereas the half-life of CO2 diffusion out of the foam is measured in weeks.

Mechanical-Morphology Relationship of PS Foams:

The relationship between the morphology and the mechanical behavior of commercial PS foams has been investigated. The foams studied had a closed cell morphology with densities between 25 and74 kg/m3 and number-average cell sizes between 75 and 230 mm, and a normal cell size distribution (dv/dn 1.20). Mechanical results showed that the compressive strength and modulus could be expressed as a function of the foam morphology, using a unique morphological parameter taking into account the cell size and foam density. Flat sheet impact tests showed that three stages, i.e. damage initiation, damage propagation and damage coalescence leading to foam collapse, could be identified in the impact behavior of the foams. The maximum stress and the energy absorbed during impact could also be related to the morphological parameter proposed. A transition from a brittle to a ductile behavior could be rationalized using the proposed parameter.

Rheological behavior of mixtures of polystyrene with HCFC 142b and HFC 134a

The processes for the production of thermoplastic foams, often in the shape of extruded sheets or boards, are largely dictated by the rheology of the mixture of a polymeric matrix with a blowing agent. This paper focuses on the theological behavior of mixtures of polystyrene (PS) and physical blowing agents (HCFC 142b and HFC 134a), examined using a commercial on-line rheometer mounted on an intermeshing co-rotating twin-screw extruder. The PS resin was extruded while the blowing agent was injected at high pressure and various compositions. The extent of plasticization was found to be dependent on the blowing agent concentration and its type. The effects of type of blowing agent, composition, pressure, shear rate and temperature on the theological response were measured. These variables were incorporated into a generalized mathematical model, which described the viscosity of the PS/blowing agent mixture over the studied range.

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.