Trexel, Inc. - Technical Description

Background

Foam products made from thermoplastic polymers have been available commercially for many years. Although these products have been successful in a number of markets, their expansion within existing markets or into new markets often has been impeded by certain limiting characteristics of conventional foaming technologies.

The most severe limiting factors in the production and application of conventional thermoplastic foams are the large size of the cells that characterize these materials and the lack of uniformity of those cells: Large, non-uniform cells decrease mechanical properties (e.g., strength, toughness, and fatigue), introduce brittleness, and necessitate relatively thick part cross sections to ensure that the cells are contained within the material. (Thin cross sections are often flawed by breaks and holes.) Conventional foam processes also can be inconsistent due to the inherent difficulty controlling the levels and release of the blowing agent (such as in the use of chemical blowing agents, CBAs). In addition, many foam processes use flammable blowing agents, which require regulatory approvals for their use and release, special handling techniques, and long product-storage times.

Trexel has developed a foaming process for commercial injection molding and extrusion applications that is unaffected by the factors that limit the use of conventionally foamed thermoplastic polymers. Based on the microcellular thermoplastic foam technology, which was invented at the Massachusetts Institute of Technology (MIT), Trexel's innovative process uses high cell nucleation rates within the foaming material to create foams with evenly distributed and uniformly sized microscopic cells (generally between 5-50 microns in size, depending on the material and application). Foam materials produced by this process offer improved consistency and homogeneity of cell structures, which can result in products with superior properties compared to other foaming systems.

The name MuCell® has been registered by Trexel to describe this microcellular process and the materials made by it.

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The Microcellular Foam Technology

Microcellular foam is produced when the cell nucleation rate is both extremely high (orders of magnitude greater than with conventional foaming processes) and much greater than the diffusion rate of the blowing agent into cells (i.e., cell growth). Under these conditions, an extremely large number of cells will be created before any cell growth occurs. Consequently, when the blowing agent diffusion begins to dominate in the foam-creation process, all cell sites will begin to grow at the same time and at approximately the same rate, resulting in a material characterized by a large number of evenly distributed, uniformly sized, microscopic cells.

Atmospheric gases, such as carbon dioxide (C0₂) and nitrogen (N₂) - which are both less expensive than other common blowing agents and unregulated - can be used as physical blowing agents to create this phenomenon. Importantly, the very high nucleation rates needed to produce microcellular foam can be achieved without the use of nucleation agents, such as talc or chalk.

High Nucleation Rates: The Theoretical Solution

The requirement for very high nucleation rates and the resultant creation of a large number of nucleation sites necessitate a fundamental change in the way in which the cells are nucleated. Conventional foaming technologies employ nucleating agents that typically are added to the material in the form of solid particles. These solid nucleating agents induce heterogeneous (not uniform) nucleation in the material at a fixed - and relatively small - number of sites. (The actual number of cell sites is directly related to the quantity of nucleating agents added.) The material produced by this heterogeneous nucleation is characterized by large, non-uniform cells. (Lack of cell-size uniformity results from the relatively slow rate of nucleation.)

To achieve high nucleation rates (and create a large number of nucleation sites), homogeneous nucleation is preferred. In homogeneous nucleation, nucleation sites are formed throughout the mass of the polymer at a molecular level. (The rate of nucleation is dependent upon the concentration of the blowing agent.) The number of sites available for cell growth is several orders of magnitude greater than in the heterogeneous nucleation of conventional foaming processes because homogeneous nucleation is not limited by the uneven distribution and low concentration of nucleation agent particles.

Homogeneous nucleation is driven by a large thermodynamic instability. This large thermodynamic instability is achieved by first dissolving a high concentration of blowing agent into the polymer at a high temperature and under high pressure - creating a single-phase solution - and then lowering the pressure below the saturation pressure. To reach the desired high rate of homogeneous nucleation, both the saturation level of dissolved blowing agent and rate at which the instability is achieved also must be high.

The conditions required for homogeneous nucleation are best illustrated by plotting the solubility of a blowing agent as a function of pressure in a typical polymer system. In the three-dimensional schematic below, the solubility of CO₂ in polypropylene (PP) has been plotted as a function of pressure and temperature. As shown, the solubility of CO₂ increases with increasing pressure (line AB) and decreases with increasing temperature (line AC). Within the plasticating section of injection molding equipment or within an extruder (point A), the pressure and temperature are high and the solubility of the CO₂ blowing agent is high; at this point, the polymer is saturated with the CO₂ blowing agent. In the die (point B), the pressure drops quickly, and the CO₂ blowing agent becomes supersaturated within the polymer; at this point, the blowing agent will begin to precipitate out in the form of gas, foaming the polymer. If the drop in blowing agent solubility from A to B is sufficiently large and sufficiently fast, then conditions will exist for homogeneous nucleation of cells, and a large number of evenly distributed microscopic cells will form and grow uniformly.

Solubility of CO₂ in PP as a function of pressure and temperature.

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Adaptation to MuCell® Injection Molding and Extrusion

Three conditions must occur during the injection molding or the extrusion process for the required rapid homogeneous nucleation to take place:

A larger amount of blowing agent (CO₂ or N₂) than in conventional foaming must be completely dissolved within the polymer - resulting in a single-phase solution - before the pressure is lowered.
The pressure profile must be maintained throughout the system to keep the blowing agent in solution.
A high rate of change of solubility must be established (accomplished by different means in extrusion and injection molding).

To promote the thermodynamic instability that drives the MuCell® process, the blowing-agent delivery system must be capable of transferring the requisite amount of blowing agent at a pressure higher than the pressure in the plasticating section of injection molding equipment or in the extruder. The creation of a single-phase solution (which is accomplished by a diffusion-controlled process), depends on the diffusion coefficient, residence time, and temperature, as well as the distance over which the blowing agent must diffuse. To facilitate the rapid creation of the single-phase solution, Trexel's MuCell process uses a blowing agent (typically CO₂) that is in a supercritical state (i.e., a supercritical fluid, SCF) to speed the process of dissolution. To achieve a uniform mix and dissolution of the required amounts of the blowing agent, the MuCell process employs unique injection-system and screw-configuration designs.

Injection Molding

In the plastication section of the injection molding process, a single-phase solution of blowing agent (typically CO₂) and polymer must be created. (The single-phase solution is formed by applying knowledge about adding and mixing blowing agents in an extruder to the design of the plasticating portion of the injection molding system.) An SCF metering system is required for this process as well as Trexel-designed injectors for the barrel portion of the system.

Screw designs that meet MuCell technology design rules are necessary for the complete development of a single-phase solution of blowing agent and polymer within the plasticating portion of the injection molding system. MuCell-compatible blowing-agent delivery systems and screws can be designed for both screw and accumulator injection molding machines and standard reciprocating-screw injection molding machines, as well as structural foam machines.

Adapting injection molding equipment for the MuCell technology requires the integration of the MuCell process with the standard cyclic function of molding equipment. This integration, usually accomplished through software changes in the molding cycle, ensures that the correct amount of blowing agent for the polymer is delivered at the standard cycle of the equipment and that the single-phase, polymer/SCF solution is held at the conditions required to maintain the single-phase solution before it is delivered to the mold. To expose the single-phase solution to a pressure drop that is sufficiently rapid to cause nucleation, specific molding conditions must be maintained.

To summarize, to adapt the MuCell process to injection molding requires the following changes or additions:

An SCF metering system with sufficient capacity to deliver the blowing agent to the screw at the volume and pressure required for the MuCell process.
A Trexel-designed screw for creating a single-phase solution of the blowing agent and polymer.
Software and system modifications to create and maintain the single-phase solution throughout the injection molding cycle.
MuCell-compatible molding conditions to create the microcellular foam within the molded part.

The MuCell molding technology can be retrofitted easily to installed injection molding and structural foam machines. The MuCell technology also is available as an option on selected new injection molding machines and structural foam machines from leading original equipment manufacturers. All MuCell-capable machines - new and retrofitted - are also capable of conventional, non-foaming operation.

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Extrusion

To make an extrusion system compatible with the MuCell process, an SCF delivery system, Trexel-designed injectors, and modifications to the extruder screw are required. A die with the appropriate flow configuration also is necessary for implementation of the MuCell process, both to maintain the pressure required to keep the blowing agent in solution in the extruder and to create the high rate of change of solubility required for high nucleation rates. (Pressure develops as a viscous fluid is pushed through a restrictive flow path at a known mass flow rate - i.e., in the MuCell process, pressure develops as the single-phase solution is pushed through the die). To determine the proper die configuration, data describing the relationship between polymer viscosity, the melt temperature, and the blowing agent are used to calculate the pressure drop and pressure-drop rate through the die flow path. Using the pressure and pressure-drop-rate requirements for high nucleation rates, the specific die flow geometry needed to produce microcellular foamed products can be designed.

To summarize, to adapt the MuCell process to a conventional extrusion system requires the following changes or additions:

An SCF metering system with sufficient capacity to deliver the blowing agent at the volume and pressure required for the MuCell process
An SCF injector and screw designed according to MuCell technology design rules to create a single-phase polymer solution
An extrusion die with flow paths designed according to MuCell technology design rules to build pressure in the system and provide the appropriate pressure drop rate

The MuCell technology can be retrofitted easily to installed extrusion equipment. All MuCell-capable extruders - new and retrofitted - are also capable of conventional, non-foaming operation.

Summary

Microcellular foams can be made using injection molding or extrusion equipment at commercial production rates with low-cost modifications. The key to producing microcellular foams is the use of rapid nucleation techniques (which require higher levels of blowing agents) and control of the pressure and pressure-drop rate in injection nozzles and extruder dies.

The MuCell® microcellular foam process provides many new application possibilities to molders and extruders. In particular, materials that have been difficult to foam and/or require CBAs with conventional foaming processes can be produced with the MuCell process because of the precise process control inherent in this breakthrough technology. In addition, the microscopic cell sizes inherent in MuCell material permit the successful extrusion and molding of very thin parts.

Trexel, the worldwide leader in the development and commercialization of microcellular foam processing technologies for thermoplastic polymers, licenses the MuCell technology for injection molding and extrusion applications worldwide.

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