ES2279704A1 - Polymer foam processing system using carbon dioxide as foaming agent, comprises modified extruder with heat exchanger, static mixer, viscometer and water injection nozzle - Google Patents

Abstract

System for processing thermoplastic polymeric foams, using carbon dioxide (CO{sub,2}) as foaming agent. The production of thermoplastic foams by extrusion using fluorocarbon nucleating agents (HCFC type), produces a high environmental impact and poor properties of the final product. To counteract these disadvantages, in recent years extrusion processes have been developed to obtain thermoplastic foams using CO{sub,2} as a foaming agent. In the production of foams with CO{sub,2} it is necessary to control the processing variables to obtain foams with satisfactory and homogeneous properties. Due to the high cost of the extruders specially designed for this purpose, it was decided to modify a single-screw extruder, designed to work with HCFC as a foaming agent, adding a heat exchanger/static mixer, a viscometer and a foaming needle to the production line. production; which allows controlling the processing conditions and obtaining a good quality final product.

Description

System for processing polymeric foams thermoplastics, using carbon dioxide (CO2) as foaming agent.

Object of the invention

The present invention refers to Single spindle extruder modifications to process controlled thermoplastic polymeric foams using carbon dioxide carbon (CO2) as a foaming agent.

Background of the invention

Foaming of polymeric materials is produced by dissolving in the molten polymer a substance, the agent foaming, which produces through physical or chemical processes the melt expansion. There are two types of agents foaming agents: chemical and physical agents.

Chemical foaming agents, such as HCFC, were the first to be used. They produce the dilation of the molten polymer mass when generating gases by chemical reaction within the mass of the polymer. It must have special care in the amount of chemical foaming agent that introduce since an excess of it remains unreacted Contaminating the foam.

On the other hand, physical agents (such as the CO 2) are inert gases injected into the extruder in the state of supercritical fluid and, once dissolved in the polymer mass, it depressurizes them in order to produce the melt expansion. Being gases, the foaming agents leave the polymeric mass once depressurized, avoiding contaminating the final product; this being its main advantage over foaming agents Chemicals

It is very interesting to produce foams thermoplastics through the extrusion process since this method It combines low cost with a good surface finish. Yes during The dosing stage controls the temperature and pressure at in order to create the optimal conditions to inject so controlled a supercritical fluid, such as CO2, this is dissolve in molten plastic forming a homogeneous solution which, after being depressurized, will produce the foaming of the polymer.

Polymer Foaming Technology thermoplastics by extrusion using supercritical fluids (FSC) It is currently in full development. There are several patents where compositions and processing parameters are specified in order to  obtain polymeric foams using carbon dioxide as foaming agent (for example: Document JP2005088328, Document JP2002028963, among others). However, there is no document where specify the modifications to be made in a machine, originally designed to produce foams using agents foaming HCFC type, so that it can produce foams polymeric and control the processing parameters using carbon dioxide as a foaming agent.

In view of the above, to be able to produce foams by extrusion it is necessary to understand and control the variables that affect the behavior of the polymer solution and CO 2, inside the extruder. That is, control both the solubilization of the gas in the polymer as the evolution of the process, then analyze the effect of the process parameters on the extrusion of the material and the effects of these factors on the quality of the final product

Thus, to produce polymeric foams using CO2 as a foaming agent, it is necessary to control adequately the three steps in which the process consists:

2.1)

Solution Formation polymer / gas

2.2)

Cell nucleation

2.3)

Growth of cells and final form from the same

Description of the invention

The present invention integrates the following elements:

Mixers

In order to improve the formation of the solution, proposes the design of a dynamic mixer attached to the spindle of the thread extruder and a static mixer that acts as heat exchanger placed next to the extruder (figure 2).

The dynamic mixer, which is schematized in the Figure 3, consists of an extension of the extruder spindle, which in addition to having the profile of spindle fillets has longitudinal stretch marks, parallel to the direction of the axis of the spindle Fixing the dynamic mixer to the spindle of the Extruder is made using a thread. This design causes a cross mixing to the polymer flow channel thanks to the longitudinal stretch marks, thus improving the formation of the solution.

The static mixer that also acts as heat exchanger, consists of 5 copper tubes longitudinal and 2 streamers, also copper, concentric to rotation axis of the extruder spindle and placed in a housing that is fixed to the end of the heated extruder electrically with resistors and whose temperature is controlled by thermocouples. A liquid circulates inside the tubes coolant against current and in turbulent regime, while that the polymer solution circulates in the state in the housing molten. This arrangement allows to precisely control the temperature of the circulating polymer bed and thus regulate homogeneously both the viscosity of the solution and the solubility of both components with each other. On the other hand, the tubes they impede the creeping flow of the polymer, acting from this way as deflectors and thus improving the mixing of both components. Taking into account the transfer equations of heat in exchangers and the increase in pressure generated by the Mixer aggregate at maximum flow rate that is capable of delivering extruder, the exchanger was sized to be capable of varying the temperature of the polymer in 30 ° C. The exit of refrigerant communicates with an open pit tank what large enough to ensure that its temperature does not increase and maintain the steady state of the mixer static and its normal operation.

An outline of the static mixer

Viscometer

In order to control the rheology of the solution and thus find the optimal processing conditions for a given solution flow rate, a capillary viscometer was developed placed next to the static mixer so that it can measure the viscosity of the molten product that circulates through its inside. The tube is electrically heated with resistors and the temperature inside can be controlled with thermocouples. The Pressure drop along the tube is measured at its ends, being able in this way to determine the apparent viscosity of the solution to the flow and operating temperature, by means of Haggen equation - Poiseville.

The device is schematized in the figure 5.

Injection needles

To control the pressure variation and the speed of pressure variation at the time the foaming, which affect the nucleation of cells, is due incorporate an injection needle at the end of the viscometer, which is coupled to the mold where the final foaming of the polymeric material

The needle consists of three sections: a reduction conical adjusting the radius of the viscometer to the radius of the needle, to then a cylindrical part that produces an increase in pressure and finally a conical expansion where foaming occurs. Both the radius of the needle, and the length of the conical expansion,  determine the pressure profiles throughout the machine and, therefore Therefore, they directly affect product quality final.

As a priori the processing conditions are unknown, several needles with different geometry were designed in order to generate different pressure profiles.

Figures 6 and 7 detail the main characteristics that these needles must present.

Description of the figures

To complete the description that is being performing and in order to help a better understanding of the characteristics of the invention, is accompanied herein descriptive, as an integral part of it, a set of figures in where, for illustrative and non-limiting purposes, the next:

Figure 1: Shows a general scheme of the mono spindle extruder machine to modify. In it, the polymer or mixture of them, along with the processing additives, are introduced in the form of pellets by the feed hopper (1). Once the material is melted, carbon dioxide is injected (CO 2) using a positive displacement pump (no included in the figure) in order to produce the mixture of components (2). When the molten material leaves the extruder, it is circulated through a dynamic mixer (not shown in the figure) and by a static mixer (3) in order to homogenize the mix and get a uniform temperature distribution throughout the melt Subsequently, the polymer circulates through the inside a capillary viscometer (4) for time measurement actual viscosity of the molten material to finally produce the foaming and forming of the piece to be processed in the injection needle holder head (5).

Figure 2: Shows a scheme of the mixers proposed. In it, molten polymer flows first between the Extruder barrel (6) and spindle (7) and then flow between the dynamic mixer first (8) and the static mixer after (3) and the mixer barrel (9). The material temperature is maintains constant thanks to the heat input generated by the heating belts (10), which are controlled by measuring thermocouples placed in the material flow (no shown in the figure), and thanks to the cooling fluid that circulates inside the tubes that make up the static mixer from the input (11) to the output (12) in countercurrent with regarding molten polymer.

Figure 3: Show the mixer details dynamic. The dynamic mixer (8) has a left thread (14) that fixes it to the spindle of the extruder (7), allowing it turn solidarity with it. In order to cause mixing transverse to the flow direction of the molten polymer, the Static mixer features longitudinal stretch marks (15).

Figure 4: Shows the main Features of the static mixer. Molten polymer from the dynamic mixer, it flows between the interstices present between the mixer barrel (9), the streamers (16) and the tubes (17) through which a cooling fluid circulates from its input (11) to the exit (12). In order to maintain the mass of melted at the desired temperature while circulating through the mixer static, the measuring thermocouples (18), activate or deactivate the heating tapes (10). Finally, molten polymer leaves the static mixer towards the capillary viscometer.

Figure 5: Shows details of the viscometer capillary. The molten polymer from the static mixer, enter the capillary barrel of the viscometer (19), where records the inlet and outlet pressure thereof by pressure gauges (20). The temperature of the molten material is measured by thermocouples (18), which activate or deactivate tapes heaters (10) in order to maintain the same temperature at capillary viscometer length. After crossing the viscometer, the flow of the polymer is directed towards the needle holder head of injection.

Figure 6: Shows details of the head holder injection needle The molten polymer from the barrel capillary viscometer capillary (19), flows through the needle injection (21) fixed to the barrel with a thread (22) where produces an abrupt pressure drop due to the sudden change in diameter, which causes the molten polymer / CO2 mixture become insoluble and the gas expands the material, which is molded to the final shape in the injection needle holder head with  preform (5).

Figure 7: Shows the details of the needle injection. It can be seen that the injection needle (21), fixed threaded to the barrel of the capillary viscometer (19), it has three clearly distinguishable parts: a conical region of decreased size (a), a cylindrical region (b) and finally a region Conical size increase leading to the preform (c). The lengths and characteristic diameters of each of the regions are fundamental variables in the pressure profiles that they develop in the needle and therefore affect the expansion of the material.

Preferred Embodiment of the Invention

As an example of application of the modifications We can mention the extrusion foaming process of polystyrene (PS) for the manufacture of parts that are used as thermal insulators. In this case, the pieces produced using carbon dioxide (CO2) as a foaming agent they have better properties, lower cost and pollute less than produced using carbonated fluoro foaming agents (type HCFC). Thus, an extrusion production line originally designed to produce foaming by an agent sparkling HCFC type, thanks to the proposed modifications you can produce parts using CO2, so that the factory in issue avoids investing in a specially designed extruder for this process, obtaining a lower cost product, less pollutant and of better quality.

Thus, the PS, along with the processing additives (talc, dye, flame retardant agent, etc.), are introduced into pellet shape by the extruder feed hopper (Figure 1). Once the material has melted, the dioxide of carbon (CO2) by a positive displacement pump to produce the mixture of the components. Once the material molten leaves the extruder, it is circulated through a mixer dynamic that, thanks to the longitudinal stretch marks, produces a bidirectional mixing that helps homogenize the mixture. Subsequently the polymer / gas mixture circulates through the interstices present between the housing and the tubes and streamers that make up the static mixer, which helps to further homogenize the mixing and, at the same time, thanks to the cooling fluid that circulates inside the tubes and streamers allows to obtain a uniform temperature distribution throughout the melt. TO then, to determine the optimal processing conditions (which will strongly influence the final quality of the piece), the polymer circulates inside a viscometer capillary that allows to measure in real time the viscosity of the material cast to finally produce foaming and forming the part to be processed in the injection needle holder head where regulates the pressure profile along the machine by varying the needle geometry

Claims (5)

1. System for processing polymeric foams thermoplastics, using carbon dioxide (CO2) as foaming agent, which is composed of a dynamic mixer, a static mixer, a capillary viscometer and an injection needle for the manufacture of polymeric foams, using carbon dioxide carbon (CO2) as a foaming agent in extruders originally designed to produce polymeric foams, using HCFC type foaming agents, which in turn controls the processing variables that affect the final quality of the product (profiles of pressure, temperature, viscosity, etc.). 2. System for processing thermoplastic polymeric foams, using carbon dioxide (CO2) as a foaming agent, according to claim 1, characterized in that the dynamic mixer has longitudinal grooves, and is fixed to the spindle of an extruder by a left thread, allowing you to rotate solidarity with it. 3. System for processing thermoplastic polymeric foams, using carbon dioxide (CO2) as a foaming agent, according to claims 1 and 2, characterized in that the static mixer is arranged inside a heated housing, coupled next to the dynamic mixer. The static mixer is composed of tubes and streamers through which a refrigerant fluid circulates, whose design allows to control, by means of thermocouples, temperature variations in a range of 30ºC and homogenize the mixture and the distribution of temperatures throughout the polymer mass. 4. System for processing thermoplastic polymeric foams, using carbon dioxide (CO2) as a foaming agent, according to claims 1, 2 and 3, characterized in that the capillary viscometer that is placed next to the static mixer records the inlet pressure and exit it by means of pressure gauges. It also controls the temperature of the molten material by means of thermocouples, which activate or deactivate heating tapes placed on the outer surface of the capillary viscometer, so that the same temperature is maintained throughout the entire capillary viscometer. 5. System for processing thermoplastic polymeric foams, using carbon dioxide (CO2) as a foaming agent, according to claims 1, 2, 3 and 4, characterized in that the injection needle is threaded to the barrel of the viscometer, and has three clearly distinguishable parts: a conical region of decrease in size, a cylindrical region and, finally, a conical region of increase in size that leads to the mold preform. The lengths and the characteristic diameters of each one of the regions are fundamental variables in the pressure profiles that develop in the needle and, therefore, affect the expansion of the material.