Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Reexamination Certificate
2001-04-20
2003-06-03
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
C264S053000, C425S00400R, C425S81700C
Reexamination Certificate
active
06572800
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process and an apparatus for producing polystyrene foam, as well as foam blocks and boards (such as plates and panels) produced therewith.
BACKGROUND OF THE INVENTION
Of particular relevance in this technical field are GB-A-1,220,053 and GB-A-1,230,992 of Imperial Chemical Industries Limited (ICI). Prior to the date of application of said British patents, foaming tests of synthetic resins using substantially inert inorganic gases, especially CO
2
, had already been carried out. These tests did not give the desired results, at least not for foam thicknesses of more than a few mm, owing mainly to the low solubility of CO
2
in the polymeric mixture or melt to be foamed, which made it necessary to operate at high pressures throughout the process (resulting in higher costs), giving rise to high temperatures, with the result that the temperature of the melt at the time of the final extrusion to the atmosphere (below, “final extrusion”) is too high to allow the foam features to be controlled properly; moreover, the polymeric mixture may be subject to thermal degradation. As a result, a foam of very high density is obtained, and foam boards of commercial grade can only be achieved for thicknesses in the order of a few mm, since it is not possible to incorporate in the melt the amount of CO
2
necessary for obtaining greater thicknesses.
Said documents GB-A-1,220,053 and GB-A-1,230,992 established, toward the end of the 1960s, the general basis for producing specific synthetic resin foams, including those of polystyrene. Particularly, GB-A-1,220,053 describes “a process for the production of foamed thermoplastic polymers in which the pressure is released from a pressurised homogeneous mixture of the molten thermoplastic polymer and a blowing agent system, said system comprising a first blowing agent which is completely miscible with the molten thermoplastic polymer at the said pressure and which is a solvent for said thermoplastic polymer under the conditions of temperature and pressure of the homogeneous mixture and a second different blowing agent which has a solubility of at least 0.01% by weight in the thermoplastic polymer and a critical temperature less than the temperature of the homogeneous mixture at the point when said pressure is released, said first blowing agent having a boiling point 10° C. or more below said temperature of the mixture” (claim 1). Moreover, it clearly states that the process can be applied to the foaming of “polystyrene” (page 1, col. 2, line 53), that “the first and the second blowing agents are selected so that they are inert with respect to the thermoplastic polymer under the pressure and temperature conditions of the pressure vessel” (page 2, col. 1, lines 40-44), that “the most suitable substances for use as the first blowing agent are liquids whose boiling points at atmospheric pressure are greater than room temperature” (page 2, col. 2, lines 69-72), that “ethanol” can be used as the first blowing agent (page 2, col. 2, line 83), that it was found that “carbon dioxide” (page 3, col. 1, line 17) is a substance which can be used as the second blowing agent, that it is preferable to use “as much of the second gaseous blowing agent (CO
2
) as possible”, but stating that this amount of CO
2
is usually less than 10% by weight (page 3, col. 1, lines 29-34), and suggesting that, in fact, the foaming produced by the first blowing agent (for example, ethanol) alone may give rise to a small number of very large bubbles or cells, which is detrimental to the commercial value of the product (page 3, column 1, lines 10-14).
In the years following the filing date of said ICI patents, manufacturers tended more and more towards using halocarbon blowing agents (also foreseen in said British patents), either alone or in various mixtures. However, for environmental reasons, since the 1980s, attempts have been made to decrease the use of halocarbon blowing agents and everything points to the increased use of CO
2
as the main blowing agent. From among the patent documents reflecting these trends, the following may be mentioned: WO 93/22371, EP-A-0 318 846, EP-A-0 411 923, EP-A-0 464 581, EP-A-0 597 375, DE 39 43 265 A1, U.S. Pat. No. 5,158,986, U.S. Pat. No. 5,244,927, U.S. Pat. No. 5,250,577 and U.S. Pat. No. 5,269,987, some of them also involving the use of ethanol as “blowing agent”.
Apparatuses of the general kind defined in preamble of present claim 7 are know from EP-A-0 528 536 and U.S. Pat. No. 4,436,679.
As may be gathered from said patent documents of ICI, it would be ideal to work with a single blowing agent, namely CO
2
. However, its low solubility in the polymer and the other drawbacks referred to above make it necessary to use CO
2
in conjunction with other different agents which below that of the polymer melt at the moment of the final extrusion will inevitably have, to a greater or lesser extent, a foaming effect, and in that sense such an agent can qualify as a ?blowing agent?, as is done, for example, in said patent documents of ICI when referring to the “first blowing agent”. However, especially when the agents employed jointly with CO
2
are lower alcohols, the foaming effects (inevitably) produced are in general undesirable, owing to the fact that they tend to give irregular bubbles or cells of which large-sized and/or open ones predominate, rather than cells with the desired features produced by using the proper blowing agent, i.e. CO
2
. For this reason, agents other than CO
2
will not be referred to in the present specification as “blowing agents” but as “control agents” since, rather than being used for foaming, they are used for enabling the foaming produced basically by the proper blowing agent (CO
2
) to proceed adequately.
The present inventors have confirmed through experiments that ethanol acts as a solvent for polystyrene, thereby reducing the viscosity of the melt, making it possible to work at lower pressures along the entire production line, including the final extrusion head, while using the same amount of CO
2
incorporated in the melt. A decrease in the viscosity also results in a decrease in both the internal friction of the melt and its friction with the mechanical components it makes contact with and, as a result of this, in a decrease in the temperature of the melt, which is of special importance near the die of the final extrusion head. The present inventors have also confirmed through experiments the beneficial effects of ethanol, by virtue of absorbing heat from the melt when the foaming occurs.
However, the experiments carried out by the present inventors using ethanol as control agent for the foaming, mainly produced by CO
2
, have also revealed its drawbacks, which chiefly consist of the fact that the foams obtained using ethanol take a considerable amount of time to reach he final value of some of their physical properties which are considered critical, such as their high-temperature dimensional stability, compression strength, and behaviour towards fire or self-extinction capacity, and that (as already stated) ethanol tends to produce foaming with large and open cells.
The aim of the present invention is to overcome these drawbacks.
The inventors have found that where ethanol is used as control agent for the foaming produced by CO
2
the best foam features are obtained if the minimum amount of ethanol is added, just sufficient to give to the melt the proper viscosity for absorbing the highest possible amount of CO
2
, and to keep the foaming produced by ethanol at the minimum possible amount.
Moreover, the inventors have found that this reduction in the amount of ethanol used would by itself have the potential drawback of making it difficult to achieve the required decrease in temperature, since one of the effects of ethanol is to act as coolant of the melt. In order to avoid this drawback, the inventors propose the use of a second control agent consisting of H
2
O
2
which is added to the melt subsequent to the addition of ethanol.
The fact that H
2
O
2
Figueras Nadal Laura
Ladera Sainz Jesús
Jacobson & Holman PLLC
Kuhns Allan R.
Poliglas, S.A.
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