Method and apparatus for microcellular polymer extrusion

Abstract

Continuous polymeric extrusion nucleation systems and methods useful for making polymeric microcellular foamed materials, including crystalline and semi-crystalline polymeric microcellular materials, are provided. Pressure drop rate is an important feature in some embodiments, and the invention provides systems for controlling these and other parameters. One aspect involves a multiple-pathway nucleator that is separated from a shaping die by a residence chamber. Another aspect involves a die for making advantageously thick articles, including a multiple-pathway nucleation section. Microcellular material can be continuously extruded onto wire, resulting in a very thin, essentially closed-cell microcellular insulating coating secured to a wire. Other very thin microcellular products can be fabricated as well.

Description

RELATED APPLICATIONS [0001] This application commonly-owned, co-pending is a continuation of U.S. application Ser. No. 09 / 626,808, filed Jul. 27, 2000, which is a divisional of U.S. application Ser. No. 09 / 258,625, filed Feb. 26, 1999, which is a continuation of Int'l patent application serial no. PCT / US97 / 15088, filed Aug. 26, 1997, which claims priority to commonly-owned, co-pending U.S. provisional patent application Ser. No. 60 / 024,623 entitled “Method and Apparatus for Microcellular Extrusion”, filed Aug. 27, 1996 by Burnham, et al., to commonly-owned, co-pending U.S. provisional patent application Ser. No. 60 / 026,889, entitled “Method and Apparatus for Microcellular Extrusion”, filed Sep. 23, 1996 by Kim, et al, and to commonly-owned, co-pending U.S. patent application Ser. No. 08 / 777,709, entitled “Method and Apparatus for Microcellular Extrusion”, filed Dec. 20, 1996 by Kim, et al, each of which is incorporated herein by reference.FIELD OF THE INVENTION [0002] The present in...

AI Technical Summary

Benefits of technology

The present invention provides methods for producing microcellular material and microcellular articles by creating a first stream of fluid polymeric material and a second stream of a gas under ambient conditions, and then nucleating each stream separately by diverting them. The separate streams can be recombined into a single stream of nucleated material, which can then be shaped into a desired shape while lowering pressure. The methods involve introducing the gas into the polymeric material solution in a way that allows for a relatively constant pressure profile in the material. The invention also provides methods for creating very high pressure drop rates in material to be foamed, and for extruding microcellular material onto a wire substrate or with cells of uniform size.

Problems solved by technology

Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results.

The number of voids or cells per unit volume of material typically is relatively low according to that technique and often the material exhibits a non-uniform distribution of cells throughout the material.

Therefore, thin sheets and sheets having very smooth finishes typically cannot be made by the technique, and materials produced typically have relatively low mechanical strengths and toughness.

The technique is designed to form a high-density skin layer on the foamed material since, according to Hayashi, et al., with single-hole dies the extrudate is deformed by foaming after the material leaves the die and it is not possible to form a skin layer uniformly.

The stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells.

While conventional foam processing can operate at very high output rates, typical known continuous microcellular extrusion production rates do not approach the rates achievable with conventional processes.

However, extruding very thin material or very thick microcellular material can be difficult.

With respect to thick sheets, it has been difficult or impossible to create the necessary solubility change uniformly throughout a thick product produced by extrusion to produce a thick microcellular article continuously.

Additional control problems exist in many known thin foam sheet extrusion techniques.

Accordingly, it has been a challenge to extrude thin coatings of conventional foam cellular material onto substrates such as wire.

Therefore, it has been difficult or impossible to extrude thin, closed-cell polymeric material onto wire to form a coating having acceptable electrical insulation properties under various conditions.

In part due to environmental problems associated with these agents, however, effort has been directed towards the use of low environmental impact atmospheric gases such as carbon dioxide, nitrogen, and air as blowing agents, and success has been met in some cases (see, e.g., U.S. Pat. No. 5,158,986 (Cha), above).

But successful control during foaming with atmospheric gases has been more difficult to achieve than with conventional agents.

Higher processing and melt temperatures can produce reduced polymer melt strength as compared to similar conditions using conventional blowing agents, resulting, in many cases, in explosive cell expansion upon release of the melt to atmosphere.

Due to the process and material limitations described above, and in particular temperature limitations, those of ordinary skill in the art would not expect to achieve highly-controlled, high volume microcellular processing of crystalline and semi-crystalline polymers, especially when using atmospheric gases.

If cooled to Tm, these materials will abruptly solidify, making further processing impossible.

That is, crystalline and semi-crystalline polymers must be processed at temperatures well above (relative to Tg) ceiling temperature for amorphous polymers, driving cell expansion and making it extremely difficult to maintain small cell sizes.

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