ES2301388A1 - Method and means for producing moulded pipes with a microcellular cranial structure - Google Patents

Abstract

The invention relates to a method and means for producing moulded pipes with a microcellular cranial structure, of the type made from thermoplastic polymer having at least one foaming agent and one nucleating agent added thereto, which can be introduced into a mould and, with the application of temperature and pressure, produce a product which is suitably cooled with the aid of a second sizing mould in order to obtain an end product having a microcellular cranial structure.

Description

Procedure and means for manufacturing molded pipes with microcellular cranial structure.

Procedure and means for manufacturing molded pipes with microcellular cranial structure (ECM) characterized essentially by being constituted as a procedure suitable for starting from a thermoplastic profile of extrusion, due to the different products added, to the joint of pressure and temperature and controlled cooling under pressure, It becomes a final product with a cranial structure microcellular (ECM).

Object of the invention

The object of the invention is to obtain pipes, sheets and other profiles of thermoplastic products with a microcellular cranial structure (ECM), whose characteristics of finish, resistance and external appearance significantly improve products from which they start.

Field of the Invention

The field of the invention is both for the industry related to the production of improved products derived from the extrusion of thermoplastics like that of the manufacture of auxiliary machinery for extrusion and modification of these products.

Background of the invention

The technique of introducing cellular material as one of the components of these thermoplastic products of Extrusion is a known procedure.

Foamed layers reduce the amount of raw material used and therefore the cost of product.

In recent years there has been an interest growing in these foamed structures, and have been made numerous investigations and patents leading to manufacturing of cellular structures with optimized mechanical properties.

For this, attempts have been made to obtain structures called microcellular that are characterized by presenting a homogeneous cell structure with cell sizes smaller than 100 microns and cell densities above 109 cells per cubic centimeter.

While extrusion technology is fine implanted and studied and its advantages are known there are several Disadvantages of it when used for manufacturing pipes, profiles and sheets, especially when you are structures include cell layers. Among them we can highlight the following:

The extruded piece is stretched in the direction of extrusion, this fact causes a molecular orientation of the polymer in said direction. This molecular orientation improves the mechanical properties of the material in the direction of extrusion, but it decreases the mechanical properties in the directions perpendicular to the extrusion.

For example, in the case of pipes, mechanical properties worsen in the radial direction, property the latter is fundamental in applications where the pipe must withstand an external load, for example in buried pipes.

This orientation effect in the direction of extrusion has a great weight when the pipe is tri-layer with a foamed core, in this case for get a proper tolerance in the thickness of the tube this is stretched in the direction of extrusion (stretch values of up to 60% can be found in commercial pipes), which causes elongation of the cells in the extrusion direction, this elongation reduces the crush resistance of the pipeline.

It is extremely difficult to make pipes Monolayers using extrusion technology. The Product foaming at the nozzle of the extruder is a complex process that is difficult to control and usually leads to cellular structures larger than 150 microns, this fact considerably reduces the surface quality of the product. In addition to the manufacture of pipes by extrusion the The tube is cooled from the outside of the tube so the growth of the cell structure in the inner wall of the tube It is a very complex task.

On the other hand, microcellular structures proposals as a solution to obtain materials with better mechanical properties for a determined amount of mass of product have not achieved the expected success. On the one hand, because they are extremely difficult to manufacture using the technology of extrusion, on the other because the homogeneous structure of these materials is not optimal from a mechanical point of view and it looks  surpassed by that of multistructured structures in which the Material density varies from the surfaces thereof to the interior continuously, presenting these materials a continuous skin and a foamed core.

Within this field there are several patents who have tried to solve part of these problems, but without no success and always in completely different ways than described in this invention, such as:

- Martini-Vvedensky , JE et al . US Patent 4 473 665 ( 1984 )

- Cha , SW et al . US Patent 5 158 986 ( 1993 )

- Colton , JS et al . US Patent 5 160 674 ( 1992 )

- Park , CB et al . US Patent 5 866 053 ( 1999 )

- Suh , NP et al . US Patent 6 005 013

- Park , CB et al . US Patent 6 051 174 ( 2000 )

- Baldwin , DF et al . US Patent

- Cha , SW et al . European Patent 580 777 ( 2002 )

- Anderson , JR et al . US Patent 6 376 059 ( 2002 )

- Xu , J., US Patent 6 322 347 ( 2001 )

- Burnham , TA et al . European Patent EP0923443B1 ( 2002 )

- Blizard , KG et al . US Patent 6 294 115.

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In no case are they directly related to the present invention that develops a manufacturing methodology of structured molded parts with cranial structure designed to measure to manufacture a new product with excellent Mechanical properties for low weight.

This invention solves the difficulties mentioned regarding extrusion technology and microcellular structure manufacturing technology and allows manufacture parts with microcellular structures that resemble those of the human skull.

The control of process parameters allows the custom design of said structure, can be controlled features such as solid skin thickness, density average of the material, the average size of the cell, the Molecular orientation and solid matrix morphology.

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Description of the invention

The generalized method of production of Thermoplastic pipes, sheets and profiles is extrusion. Materials based on PVC, PE, PP, PA, PS among others are manufactured as standard using this manufacturing technology.

For certain applications it is convenient that the final product is constituted by several layers of different materials. For example in the case of pipes there are in the Tri-layer pipe market consisting of one layer central that can be a foamed product or a loaded product and internal and external layers made of a continuous loaded plastic or not. Again the manufacturing technology of this type of Materials are based on extrusion, being in this case specific it would require the co-extrusion of several polymers.

The possibility of introducing cellular material as one of the components of these products is a procedure known. Foamed layers reduce the amount of raw material used and therefore the cost of the product. In the In recent years there has been a growing interest in these structures foamed, and numerous investigations and patents have been carried out conducive to the manufacture of cellular structures with optimized mechanical properties This has been tried get structures called microcellular that are characterized for presenting a homogeneous cell structure with cell sizes less than 100 microns and cell densities above the 109 cells per cubic centimeter.

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Description of the drawings

To complement the description of the invention is being carried out and in order to help better understanding of the characteristics of it, accompanies the present descriptive report, as an integral part thereof two flat sheets, in which identical references indicate identical elements and where for illustrative purposes and not limiting, the following has been represented:

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to.-

\ vtcortauna thermoplastic polymer

b.-

foaming agent

C.-

\ vtcortauna nucleating agent

d.-

other additives

t.-

pipe obtained by extrusion

T.-

final pipe with cellular structure cranial

M.-

metal mold

3.-

outer concentric mold pipe (M)

4.-

inner concentric mold pipe (M).

5.-

ring plunger

6.-

air evacuation valves

m.-

\ vtcortauna second heating mold

7.-

hydraulic pressure system

12.-

mold closure cap (M)

9.-

outer surface of the pipe (T) of oriented microcells

10.-

middle zone of the microcell (T) pipe oriented and dense.

eleven.-

inner zone of the low density pipe (T) without orientation

ms.-

\ vtcortauna supplementary mold

13.-

supplementary mold closing / opening valve (ms)

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Figure 1.- Represents a schematic view of of the pipe (t) inside the mold (M).

Figure 2.- Represents a schematic view of the pipe (t), inside the mold (M) and the assembly inside the mold (m). The supplementary mold (ms) is also represented.

Figure 3.- Represents a schematic view of the microcellular cranial structure (ECM) from a section of a pipe wall (T).

Preferred Embodiment of the Invention

While it is a generalized method for the production of any profile of thermoplastic products, since be curved or circular planes, in this case we are going to refer directly to the preferred application made on pipelines.

Within the manufacturing process applied to this particular case of pipe manufacturing we can distinguish various stages.

Stage 1

The beginning is the manufacture of a pipe (t) of special characteristics, constituted at least by the following elements as raw materials.

to.-

he basic element is a thermoplastic polymer, such as PVC, PE, PP, etc. or also mixtures thereof, in the amounts suitable for obtaining the desired final product.

b.-

a chemical foaming agent. Within the wide variety of these products we can use among others azodicarbonamide, bicarbonate, bicarbonate and citric acid mixtures, compounds hygroscopic that absorb water, etc.) or mixtures of same.

C.-

a nucleating agent, such as talc, calcium carbonate, etc. or mixtures thereof.

d.-

other additives, which could even include loads, reinforcements, process assistants, pigments, crosslinking agents, agent reaction activators sparkling, etc.

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The pipeline with these basic compounds is manufactures at a temperature higher than the polymer processing (a) and less than the decomposition of the foaming agent (b).

The amount of foaming agent (b) entered in the formulation it is far superior to what it would take to Obtain the final density of the product. This fact will allow generate a self-pressure during the process of molded, this self-pressure will be one of the fundamental causes  of the generation of the multistructured structure with leather Foamed and homogeneous cell structure.

The foaming agent (b) is introduced disaggregated in the initial formulation.

Nucleating agents (c) are introduced disaggregated in proportion of 10 9 particles per centimeter cubic.

The dimensions of the pipe (t) will come determined by the dimensions of the final pipe to obtain (T).

The amount of material used will depend on the dimensions of the final pipe and the nominal density objective. The relative density (density of the final pipe divided by the density of the starting pipe) of the pipes that could be manufactured by this process would be between 0.1 and 1.

The pipe thus manufactured is calibrated and cooled by conventional means, as an example by water tanks, until enough temperature so that it can be self-supporting

Standard values of these temperatures would be, 50ºC for PVC and PE and 70ºC for PP.

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Stage 2

The pipe thus manufactured is cut into sections regular and introduced into a metal mold (M).

The dimensions of the manufactured tube will be such that allow its introduction into this mold. Its thickness will be less than the thickness of the pipe to be manufactured and its length will be less than or equal to that of the pipe to be manufactured. These dimensions are conditioned by the dimensions of the mold and by the product density of the final pipe.

The metal mold (M) is constituted by two concentric pipes (3) and (4) and a circular cover (12), which is where the pipe (t) is introduced and where it will be extracted already turned into a pipe (T) with a microcellular cranial structure.

The inside diameter of the pipe (3) matches with the final outer diameter of the pipe (T) that is intended manufacture.

The outer diameter of the inner pipe (4) of the mold matches the final inside diameter of the pipe (T) to be manufactured

The radius difference between the inside of the outer mold (3) and the outer part of the inner mold (4) matches the final thickness of the pipe (T) to be manufactured.

The mold includes one of its ends or n annular embolus (5) capable of generating pressure.

It also includes the mold, usually in the opposite part of the annular plunger (5), a closure cap (12) with a air evacuation valve (6).

Once the pipe (t) has been introduced into the mold, it closes.

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Stage variant 1 one

It may happen that instead of entering the manufactured pipe (t), the polymer of heading in molten and properly dosed state.

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Stage 3

The mold (M), which has inside the pipe (t) manufactured in stage 1 or the molten polymer, is transports up to a second mold (m) that will allow the heating of the first. This second mold in which it is inserted the first is at a temperature higher than decomposition of the foaming agent that has been introduced into the formulation of the pipeline.

Once the mold (M) containing the pipe is has been placed in this second heating mold (m) connect the ring pin to a hydraulic system (7) that will allow moving said plunger along the first mold.

Stage 4

When the pipe temperature (t) is exceeding the softening point of the base polymer (a), the annular plunger (5) is actuated by deforming the molten material and compressing it.

This process continues until everything is extracted the air that has been trapped during the deformation of the tube.

So that air can escape the valves Exhaust (6) open.

Due to the temperature and pressure, the agent Foaming (b) begins to enter the gaseous state. The pressure of Annular embolus (5) is maintained until all the gas that comes of the foaming agent (b) has been generated.

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Stage 5

\hbox{espumante (b).}

At this time the mold (M) containing the pipe (t) is removed from the heating mold and therefore the
cooling of the assembly formed by the thermoplastic polymer (a) with the gas coming from the agent

 \ hbox {sparkling (b).} 

Once the set temperature reaches a value for which the viscosity of the assembly formed by the thermoplastic polymer (a) with the gas coming from the agent sparkling (b) gas is sufficient to retain the gas during the phase of foaming the annular embolus allows a fall of pressure greater than 0.1 GPa / s which causes the nucleation of the cells produced by the gas within the polymer thermoplastic

To obtain polymer foaming thermoplastic (a) to the appropriate extent, the pressure is used produced inside the mold (M) by the annular plunger; as said annular piston (5) moves along the mold (M) and by both controlling the foaming of it, at all times by medium of pressure.

Foaming of thermoplastic polymer it can also be obtained, increased or controlled by means of a third supplementary mold (ms), provided with a shut-off valve (13) and conventional heating devices. In this way the expansion can be faster and also used to obtain Another type of profile.

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Stage 6

Once the material temperature is enough for the material to enter the final solid state proceeds to the expulsion of the tube (T) already configured with the new microcellular cranial structure (ECM), and for this you can apply  pressure to the ring plunger (5) and open the closing cover (12) of the mold (M).

In the case of using a supplementary mold (ms), when the profile made goes to the final solid state, It is extracted by conventional means.

The tube thus manufactured has a structure cranial microcellular (ECM).

Figure 3 shows the model of microcellular cranial structure (ECM) obtained.

It shows the different densities of the product that varies along its thickness, being close to that of the starting polymer on the surfaces and reducing progressively into the tube.

Due to the manufacturing process used, the material presents in the areas near the surface a biaxial orientation of the cellular structure (9) and of the polymer base. Due to the high self-pressures generated during the molding process, which can reach 200 bars, the material has excellent surface quality. Plus towards the inside of the tube structure (T) an area is appreciated dense oriented (10), presenting the central zone of the tube (T) a Low density zone without orientation (11).

It is also important to note that the procedure thus described is applicable not only to products Polymers and polyurethanes, but also to metal products and ceramic

The third supplementary mold (ms), may have different shapes and so depending on your design you can get different profiles; but besides these design changes too they will have to affect the shape of the molds (m) and (M) so that each particular form will involve a set of molds (m); (M) and (ms).

Describe sufficiently the nature of the invention, as well as how to put it into practice, should be noted that the provisions indicated above and represented in the attached drawings are susceptible to modifications of detail as long as they do not alter its principles fundamental, established in the previous paragraphs and summarized in the following claims.

Claims (19)

1. Procedure for the manufacture of molded pipes with microcellular cranial structure characterized essentially by the fact that the procedure begins by the manufacture of a pipe (t), constituted at least by the following elements as raw materials:

to.-

thermoplastic polymer, such as PVC, PE, PP, or also mixtures thereof, in the amounts suitable for obtaining the desired final product.

b.-

a chemical foaming agent, such as azodicarbonamide, bicarbonate, mixtures of bicarbonate and citric acid, hygroscopic compounds that absorb water, or mixtures thereof.

C.-

a nucleating agent, such as talc, calcium carbonate, or mixtures thereof.

d.-

other additives, which could even include loads, reinforcements, process assistants, pigments, crosslinking agents, agent reaction activators sparkling

And also because the pipe (t), composed of the above products is introduced in a mold (M); and said mold (M) is in turn introduced into a second mold (m) that is going to allow the heating of the first (M); being this second mold (m) at a temperature higher than the decomposition of the foaming agent (b). 2. Procedure for the manufacture of molded pipes with microcellular cranial structure according to the 1st claim and characterized in that the pipe constituted by these elements cited as raw materials, is manufactured at a temperature higher than that of polymer processing (a) and lower to the decomposition of the foaming agent (b). 3. Process for the manufacture of molded pipes with microcellular cranial structure according to claim 1 and characterized in that the amount of foaming agent (b) introduced into the formulation is much greater than what would be needed to obtain the final density of the product and It is also introduced disaggregated. 4. Process for the manufacture of molded pipes with a microcellular cranial structure according to claim 1 and characterized in that the nucleating agents (c) are introduced disaggregated in a proportion of 10 9 particles per cubic centimeter. 5. Procedure for the manufacture of molded pipes with microcellular cranial structure according to claim 1 and characterized in that the dimensions of the pipe (t) thus manufactured are determined by the dimensions of the final pipe to be obtained (T), the quantity being of material used in the manufacture of the pipe (t) in direct relation to the dimensions of the final pipe (T) and therefore of the target nominal density; the relative density obtained being between 0.1 and 1. 6. Procedure for the manufacture of molded pipes with microcellular cranial structure according to claim 1 and characterized in that the pipe (t) thus manufactured is calibrated and cooled by conventional means, as an example by means of water tanks, to a sufficient temperature so that it can be self-supporting being standard values of these temperatures 50ºC for PVC and PE and 70ºC for PP. 7. Procedure for the manufacture of molded pipes with a microcellular cranial structure according to claims 1 to 6 and characterized in that the pipe (t) is cut in regular sections and inserted into a metal mold (M), this metal mold being constituted by two concentric pipes (3) and (4) and a circular cover (12), with one or several air evacuation valves (6); including in the opposite cover an annular plunger (5) capable of generating pressure. 8. Procedure for the manufacture of molded pipes with microcellular cranial structure, according to claim 7 and characterized in that the inner diameter of the pipe (3) coincides with the final outer diameter of the pipe (T) to be manufactured and the outer diameter of the inner pipe (4) of the mold coincides with the final inner diameter of the pipe (T) to be manufactured; coinciding the difference in radii between the inner part of the outer mold (3) and the outer part of the inner mold with the final thickness of the pipe (T) to be manufactured. 9. Process for the manufacture of molded pipes with microcellular cranial structure according to claims 1, 7 and 8, and characterized in that the dimensions of the manufactured tube (t) will be such that they allow its introduction into the mold (M), being its thickness less than the thickness of the pipe to be manufactured (T) and its length will be less than or equal to that of the pipe (T) that is intended to be manufactured being these dimensions also conditioned by the dimensions of the mold (M) and by the density of the product of the final pipe. 10. Process for the manufacture of molded pipes with microcellular cranial structure according to claim 1 and characterized in that in certain cases instead of introducing the pipe (t) into the mold (M), the product with which it is manufactured is introduced the pipe in plastic state. 11. Process for the manufacture of molded pipes with microcellular cranial structure according to claims 1, 7, 8, 9 and 10 and characterized in that once the mold (M) containing the pipe (t) has been placed in this second heating mold (m) connects the annular bolus (5) to the hydraulic system (7) that will allow moving said bolus along the first mold (M) and when the temperature of the pipe (t) is greater than the softening point of the base polymer (a), the annular plunger (5) is operated by deforming the molten material and compressing it, continuing this process until all the air that has been trapped during the deformation of the tube is extracted by means of the valves exhaust (6). 12. Process for the manufacture of molded pipes with microcellular cranial structure according to claim 1 and 11 and characterized in that due to the temperature and pressure, the foaming agent (b) begins to enter the gaseous state, maintaining the embolus pressure cancel (5) until all the gas that comes from the foaming agent (b) has been generated. 13. Process for the manufacture of molded pipes with microcellular cranial structure according to claim 11 and characterized in that once the temperature of the assembly reaches a value for which the viscosity of the assembly formed by the thermoplastic polymer (a) with the gas that comes from the foaming agent (b) is sufficient to retain the gas during the foaming phase by means of the annular plunger (5) it is allowed to generate a pressure drop greater than 0.1 GPa / s which causes the nucleation of the cells produced by the gas within the thermoplastic polymer. 14. Method for the manufacture of molded pipes with microcellular cranial structure according to claim 13 and characterized in that the foaming of the thermoplastic polymer can also be obtained, increased or controlled by means of a third supplementary mold (ms), provided with a valve opening / closing (13) and conventional heating devices, connected to the mold (M). 15. Procedure for the manufacture of molded pipes with microcellular cranial structure according to claim 14 and characterized in that the supplementary mold (ms), allows obtaining different profiles depending on their design. 16. Procedure for the manufacture of molded pipes with microcellular cranial structure according to claims 7 to 12 and characterized in that the mold (M) already containing the molded pipe with microcellular cranial structure (ECC) (T) is extracted from the mold of heating (m) and cooling of the assembly formed by the thermoplastic polymer (a) begins with the gas that comes from the foaming agent so that when the temperature of the material is sufficient for the material to enter the definitive solid state it is expelled of the tube (T) already configured with the new microcellular cranial structure (ECM), opening the closure cap (12) and applying pressure to the annular embolus (5). 17. Procedure for the manufacture of molded pipes with microcellular cranial structure according to the preceding claims and characterized in that the tube (T) thus manufactured has a microcellular cranial structure (ECM), in which it can be seen in areas near the surface a biaxial orientation of the cellular structure (9) and of the base polymer, more inward direction of the tube structure (T), a dense oriented area (10) can be seen, the central zone of the tube (T) presenting a low zone density without orientation (11) and due to the high self-pressures generated during the molding process, which can reach 200 bar, the material has an excellent surface finish. 18. Procedure for the manufacture of molded pipes with microcellular cranial structure according to the preceding claims and characterized in that this procedure is applicable to any other thermoplastic extrusion product, including flat or non-cylindrical profiles, simply by modifying the shape of the molds ( m), (M), and (ms). 19. Procedure for the manufacture of molded pipes with microcellular cranial structure according to all the claims and characterized in that the procedure is applicable to all types of polymeric, polyurethane, metallic or ceramic products.