Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Production of continuous or running length
Reexamination Certificate
1998-05-15
2001-01-30
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Production of continuous or running length
C264S211240, C264S331210
Reexamination Certificate
active
06180053
ABSTRACT:
The present invention relates to a process for manufacturing a biodegradable blown film.
Various industries are increasingly seeking biodegradable polymers. Such polymers are, for example, aliphatic polyesters such as &egr;-caprolactone polymers (referred to more simply as poly-&egr;-caprolactones, or PCL). These polymers constitute well-known biodegradable and biocompatible thermoplastic polymers, whose most promising openings lie, precisely on account of these specific properties, in the field of films, such as films for packaging, for protecting crops or for disposable nappies.
However, in certain applications, these polymers have unsatisfactory Theological properties, in particular as regards the manufacture of blown films. It is known in particular that the manufacture of PCL-based blown films is difficult and often leads to instability of the bubble, resulting in unacceptable fluctuations in the properties of the film obtained. This leads to the flow rates and the swelling ratios (radial) and draw ratios (axial) being greatly limited, thus reducing the advantage of this technique; in particular, this limits the width of the film and thus the yield of the manufacturing line. In addition, the operating conditions as a whole must be the subject of limitations and/or precise regulation, in order to avoid any risk of instability. This situation is regrettable, given that extrusion blow-moulding constitutes a particularly advantageous and economical process for manufacturing films when it is not subjected to such constraints. Moreover, the need to limit the extrusion temperature (for the sake of stability) does not always make it possible to obtain a film with a satisfactory surface state.
To overcome these problems, various solutions have already been proposed, each being carried out starting with granules of a presynthesized polymer. One solution consists, in particular, in cooling the bubble using refrigerated air, which makes it possible to accelerate the crystallization, to lower the neck of the bubble and thus to stabilize the bubble. In a comparable manner, Japanese patent application JP 06/143,412 proposes to use water-cooling of the bubble. This solution is complex and does not make it possible to exceed a swelling ratio of 2. In addition, this abrupt cooling rigidifies internal constraints in the film, which leads to considerable shrinkage if the film is subsequently heated. Document EP 708,148 confirms that the extrusion blow-moulding of aliphatic polyesters is extremely difficult, and alternatively suggests facilitating it by mixing these polyesters with from 5 to 50% of polymers bearing polar groups: the films thus obtained are thus not fully biodegradable. Certain Theological properties of aliphatic polyesters, in particular their behaviour in the molten state, can moreover be improved by increasing their molecular mass, but this leads to an increase in the viscosity, which causes an increase in the mechanical energy consumption and higher self-heating in the extruder. Now, such an increase in temperature slows down the crystallization, such that the sheath runs the risk of rewelding between the pinch rolls in certain cases. An excessive temperature can also give rise to degradation of the polymer. Moreover, the batchwise synthesis of very high molecular mass PCL in an autoclave is impossible to carry out in an economically viable manner, since the viscosity of the polymer excessively extends the time required to empty the autoclave, imposes the use of more powerful mixers and reduces the efficacy of the heat exchange with the walls. It may also be added that an increase in the viscosity causes considerable efforts in radial and axial drawing to be required, which induce constraints and then give rise to considerable shrinkage.
Consequently, the present invention is directed towards providing a process for manufacturing blown films essentially consisting of an aliphatic polyester, which is simple and of high stability. This stability would thus make it possible to achieve high swelling ratios and draw ratios, without requiring a high molecular mass; more generally, this stability would reduce the requirements imposed in the regulation of the operating conditions.
More precisely, the present invention relates to a process for manufacturing a blown film essentially consisting of an aliphatic polyester, according to which:
(1) the polyester is synthesized from at least one cycloaliphatic ester, with the intervention of an initiator comprising at least one metal trialkoxide and/or metal tetraalkoxide, in an extrusion device;
(2) the polyester is extrusion blow-moulded, immediately after it has been synthesized in the said extrusion device, fitted for this purpose with an extrusion blow-moulding die, with a swelling ratio of at least 2.5.
The fact that the synthesis of the polymer and its use are simultaneous offers several advantages, in particular as regards the simplicity (single apparatus) and the cost efficiency (only one heating). As regards the cost efficiency, this process also makes it possible to dispense with a step of granulation, of packaging and of storage. Moreover, it allows any manufacturing error to be detected without delay, in contrast with the conventional process in which it may transpire that an entire batch has to be eliminated when a problem is detected during its use, which is not simultaneous with its synthesis. In addition, it has been observed, surprisingly, that, according to this process, the extrusion blow-moulding takes place in an extremely stable manner, even with high swelling ratios and/or high draw ratios, with polyesters of low molecular mass, and/or at high temperatures.
As indicated above, the aliphatic polyester of which the film is essentially made is synthesized from at least one cycloaliphatic ester, by opening of the ring. This mode of synthesis is preferable, in the perspective of a synthesis in an extruder, to modes of synthesis involving a polycondensation reaction, which releases water. As examples of cycloaliphatic esters which can be used, mention may be made of lactones and lactides (for example 3,5-dimethyl-1,4-dioxane-2,6-dione or 1,4-dioxane-2,6-dione (diglycolic anhydride)). One or more lactones (&egr;-caprolactone, &bgr;-propiolactone, &dgr;-valerolactone, etc.) are advantageously used.
The polyester can either be a homopolymer obtained from a single cycloaliphatic ester or a copolymer obtained from several different cycloaliphatic esters. It is preferred to use homopolymers, and most particularly polylactone homopolymers. In the context of the present invention, copolymers comprising at least one cycloaliphatic ester as well as up to 20% (on a molar basis) of one or more other cyclic comonomers which can be copolymerized by ring opening, for example chosen from cyclic carbonates, cyclic anhydrides and cyclic amides, are also considered as aliphatic polyesters. With a view of obtaining a biodegradable polymer, an aliphatic comonomer will preferably be chosen, for example tri-methylene carbonate, 2,2-dimethyltrimethylene carbonate, propylene carbonate or succinic anhydride.
The copolymers can be random or block copolymers. In order to obtain block copolymers, the process can be performed in particular starting with oligomers, or alternatively the monomers can be introduced at different places in the extruder.
Advantageously, the polyester is a poly-&egr;-caprolactone (PCL), i.e. an &egr;-caprolactone homopolymer. Advantageous results have been obtained when the polyester has a number-average molecular mass (M
n
) of less than 74,000 g/mol; these results are noteworthy when M
n
is less than 60,000 g/mol and in particular less than 50,000 g/mol. These values are particularly low when compared with the molecular masses which have been used hitherto for the purpose of blowing films based on such polyesters.
It will be noted that these preferred values relate to the molecular mass which the polyester has in the film finally obtained, i.e. after optional hydrolysis of any hydrolysable bonds. It may thus be that the
Claeys Ivan
Dehennau Claude
Jones Kenneth M.
Schneller Marina V.
Silbaugh Jan H.
Solvay S.A. (Societe Anonyme)
Venable
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