Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
2000-05-23
2002-05-21
Hess, Bruce H. (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S409000, C428S500000
Reexamination Certificate
active
06391432
ABSTRACT:
The present invention relates to a process for processing a thermoplastic, enabling highly transparent articles, in particular films, to be manufactured. It also relates to special films which can be obtained by this process.
Transparent films constitute a very extensive application of thermoplastics, especially in the field of packaging. In particular, by way of example, among the polymers used for manufacturing transparent films, propylene polymers occupy an increasing place.
Improving the transparency of films, which remains a constant objective of the industries in question, may not only be achieved by optimizing the chemical nature of the polymers but also by the choice of particular processing conditions. As a general rule, the transparency of semicrystalline polymers may be increased by reducing their crystallinity and/or the size of their crystallites.
With regard to the chemical nature of the compositions used, techniques known for this purpose are in particular the addition of a comonomer so as to obtain a less crystalline random copolymer, or the addition of a nucleating agent (talc, etc.) which reduces the size of the crystallites and increases their number. With regard to the processing conditions, conventional techniques consist in particular in drawing the film in the solid state so as to break up the crystallites (for example, in the case of biaxially oriented films), or else by very rapidly cooling the films (quenching) immediately after their manufacture, so as to reduce the size of the crystallites and simultaneously to reduce the crystal-linity of the polymer. However, in the latter case, the crystallinity will tend to increase over time, and will do so all the more rapidly if the film is reheated.
Several methods have already been proposed for the purpose of improving certain mechanical and optical properties of thermoplastic-based films. Thus, in the document FR 1,211,161, such a film, manufactured beforehand, is reheated above its melting point (T
f
), then cooled between T
f
−12° C. and T
f
, then drawn at least 150% at this temperature, and finally cooled to at least 75° C. under tension.
A drawback of this process is that it requires a separate step to manufacture the film, followed by a reheating step, this being highly disadvantageous within the framework of industrial exploitation.
The present invention therefore aims to provide a process which is simple to employ and which makes it particularly possible to manufacture flat thermoplastic-based articles having improved optical properties, in particular transparency.
More specifically, the invention relates to a process for processing a semicrystalline thermoplastic, which includes, in succession, the following steps:
(1) a molten semicrystalllne thermoplastic extrudate is cooled to a temperature T
1
which is greater than the crystallization temperature (T
c
) of the thermoplastic and less than T
f
+5° C., T
f
denoting the melting point of the thermoplastic;
(2) next, the extrudate is drawn in the molten state;
(3) next, the extrudate is relaxed, at a temperature T
3
which is between T
c
and T
f
, for a duration of at least equal to the average relaxation time (&tgr;
3
) of the thermoplastic at the relaxation temperature and after the drawing;
(4) next, the extrudate is quenched to a temperature T
4
which is less than T
c
.
This process makes it possible to manufacture flat articles such as films (having a typical thickness of the order of a hundred &mgr;m) or sheets (having a typical thickness of the order of a millimetre). For reasons of simplicity, the specific details of the process of the invention will essentially be described within the context of the manufacture of such flat articles, although this process also makes it possible to manufacture any other article of constant cross-section, such as a bar, profile, pipe, etc., by means of a few modifications obvious to those skilled in the art.
Conventionally, the prior step of melting the thermoplastic takes place at a temperature well above its melting point (T
f
). Such prior melting may be carried out by means of any known device, such as an extruder or mixer. The mass of thermoplastic produced, generally continuously, by this device will be termed the extrudate, even if an extruder is not involved.
Compared with the prior known process mentioned above, the process thus proposed is simpler given that it is carried out starting from a molten thermoplastic and does not require a prior step of forming the article and a subsequent step of reheating the latter. The proposed process is furthermore distinguished by the presence of a high-temperature relaxation step (3) followed by a short and vigorous cooling (4), whereas, according to the previously known process, the film is simply left to cool naturally, and under tension.
It will be recalled that the melting point (T
f
) and crystallization temperature (T
c
) correspond to the endothermic and exothermic peaks measured by differential scanning calorimetry (DSC), respectively during heating and cooling, of the thermoplastic in question, in the absence of any external stress.
The thermoplastic is semicrystalline, that is to say that it comprises at least 50% by weight of one or more semicrystalline thermoplastic polymers, such as, for example, polyolefins or polyvinylidene fluoride. It may furthermore include a minor proportion of one or more other thermoplastic polymers, as well as, optionally, modest amounts of additives such as antioxidants, stabilizers, pigments, etc. Preferably, the thermoplastic includes at least one polyolefin. More particularly preferably, the thermoplastic material includes at least 50% by mass of one or more polyolefins. Ideally, the thermoplastic essentially consists of one or more polyolefins. A propylene polymer may advantageously be used as the polyolefin. Excellent results have been obtained with ethylene-propylene copolymers, as well as with propylene homopolymers.
The cooling (1) of the extrudate preferably takes place at a temperature T
1
less than T
f
this cooling may be carried out in any known manner, for instance by contact with at least one chill roll placed in the immediate vicinity of the device from which the extrudate leaves. This roll may, for example, be cooled by the internal circulation of a fluid maintained at the appropriate temperature by a thermal conditioning device. In order to reduce the cooling time, it is advantageous to cool both sides of the extrudate, for example on two consecutive conditioned rolls, the extrudate describing an S-shaped path. Instead of a single roll, it is also possible to use a pair of rolls pinching the extrudate, when this is in the form of a strip. The drawing (2) may be carried out in any known manner, for instance by exerting tension on the extrudate by means of a roll or a pair of rolls, this or these being placed downstream and driven by a motor whose speed is judiciously controlled. Good results are obtained if the draw ratio during step (2) is at least 200% and preferably at least 300%. When at least one roll is used for each of the cooling (1) and drawing (2) steps, the draw ratio is approximately the ratio of the peripheral velocities of the roll or rolls used for the cooling (1) and of the roll or rolls used for the drawing (2).
The drawing conditions are advantageously such that the velocity gradient is at least 1/(20&tgr;
0
) where &tgr;
0
denotes the average relaxation time of the thermoplastic at the extrusion temperature (T
0
) in the absence of any drawing stress. Preferably, this velocity gradient is at least 1/(10&tgr;
0
). The relaxation time &tgr;
0
is defined by the
τ
0
=
2
η
0
M
c
ρ
RT
0
in which &eegr;
0
denotes the dynamic viscosity of the thermoplastic for velocity gradients tending towards 0, &rgr; is its density, M
c
is the critical (weight-average) molecular mass above which &rgr;
0
is proportional to the power 3.4 of the molecular mass, and R and T
0
denote the ideal gas constant and the absolute extrusion temperat
Dehennau Claude
Gilliard Pierre
Karsten Petrus J.A.
Hess Bruce H.
Schneller Marina V.
Shewareged Beth
Solvay ( Societe Anonyme)
Venable
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