Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-12-28
2001-06-12
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S444000, C528S481000, C528S50200C, C528S503000
Reexamination Certificate
active
06245863
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to an improved process for the production of polyester resin.
Aromatic polyester resins employable in applications such as molding, extrusion, injection and similar operations require relatively high molecular weights corresponding to intrinsic viscosity (IV) values generally higher than 0.65-0.75 dl/g. The resins for film and fibre, on the contrary, have lower IV values, between about 0.6 and 0.75 dl/g.
The preparation of the resins is typically carried out by polycondensation of an aromatic dicarboxylic acid, normally terephthalic acid or its alkyl diesters with an aliphatic glycol operating under temperature and pressure conditions such as to obtain a resin with IV values as high as possible. It is difficult, however, to reach IV values above 0.65-0.75 dl/g because of the high melt viscosity which prevents an efficient removal of the byproducts of reaction. Therefore, the reaction of polycondensation in a melt state (MPC) is carried out under high vacuum to remove the reaction byproducts.
The MPC polycondensation, however, is an expensive operation which is desirable to avoid.
The resin to be used for molding and similar operations, after the MPC stage, is then subjected to polycondensation treatment in solid state (SSP) with the aim of increasing the IV to the desired values (0.75-1.2 dl/g).
Prior to the SSP treatment, the resin granules are subjected to a crystallization treatment directed at increasing as much as possible the crystallinity of the polymer so as to avoid in the subsequent SSP step packing and sticking of the granules which under severe conditions can lead to plant stoppage.
The SSP step, however, requires a relatively long time (from several to 10 or more hours depending on the final IV value to be obtained).
Working in the MPC step with non high melt viscosity values and therefore with the IV of polymer relatively low, it is possible to more easily remove the reaction byproducts and to lower in this way its duration. The increase in IV to desired values could thereafter be obtained by SSP.
However, there exist limits in decreasing of melt viscosity mainly due to the presence of a large quantity of olygomers which are formed when operating under such conditions. The olygomers, in the next SSP step, cause the formation of cyclic compounds whose presence has a negative influence on the flowability of granules and therefore the regularity of the SSP operation.
With the aim of overcoming the above inconveniences it was proposed in U.S. Pat. No. 5,376,734 to add, with the resins having an IV lower than 0.57 dl/g, a dianhydride of a tetracarboxylic acid, for example pyromellitic dianhydride (PMDA), and to conduct the SSP reaction in the presence of such a dianhydride.
The IV values of the resins with which the dianhydride is added are not in practice lower than 0.4 dl/g (in the examples 0.408 dl/g) . After addition of the dianhydride, the resin is pelletized by conventional systems and then has to be subjected to crystallization with the aim of being able to carry out the SSP treatment.
SUMMARY OF THE INVENTION
It has now been unexpectedly found that starting from resins with IV values lower than 0.4 dl/g and between 0.1 and 0.4 dl/g, preferably 0.2-0.3 dl/g, added in the melt with a dianhydride of a tetracarboxylic acid, preferably an aromatic tetracarboxylic acid, that resins possessing sufficient melt strength to be extruded in the form of a cuttable strand are obtained further, if the strand or granules obtainable from the strand are subjected to crystallization at the extruder exit, operating at a temperature between about 150° and 210° C. for sufficiently long time, crystalline organizations are obtained such that in the DSC curves of the resin after crystallization premelt peaks are not present, or are present only in limited quantity.
The crystallized resin thus obtained can be treated in the next SSP step at higher temperatures than those normally used, with consequently significant reductions in the treatment duration.
The process of the invention presents therefore the present advantage not only of being able to eliminate the crystallization treatments following the pelletization of the resin, but also exert a positive influence on the SSP step even though starting from resins with a low IV.
Furthermore, the addition of dianhydride with the melted resin results in a crystallized resin with an IV higher than that of the starting resin. The resins thus obtained are characterized by an IV higher than 0.4 dl/g. After SSP treatment, the resins show IV values generally higher than 0.8 dl/g. In their DSC curves, there is no presence of premelt peaks, or if present, their melt enthalpy is lower than 5 J/g. These resins are new products.
The process of the present invention when it is realized continuously starting from the preparation step of the resin to the final SSP steps, comprises the following phases:
a) polycondensation of an aromatic dicarboxylic acid, or of its alkyl diester, preferably terephthalic acid, with an aliphatic diol under conditions such as to obtain a resin with an IV between 0.1 and 0.40 dl/g;
b) mixing of the resin in melt state with a dianhydride of a tetracarboxylic acid, preferably pyromellitic dianhydride, in quantity between 0.01 and 2% by weight;
c) extrusion of melted resin in strand form;
d) maintaining the strand at a temperature between 150° and 210° C. for sufficient time for the crystallization of the resin, so that in the DSC curves of the same there are no premelt peaks or, if present, their enthalpy is less than 5 J/g;
e) cutting of the strand to form chips, operating preferably at temperatures near to those of crystallization (the cutting of the strand can be carried also in cold conditions);
f) SSP treatment of the chips operating at temperatures between ca. 160° and 250° C., preferably between 210° and 230° C., until the desired rise in IV (0.8-1.2 dl/g) is obtained.
Steps from a) to e) can be realized separately from step f) The cutting of the strand can precede the crystallization phase, which is then realized on the chips and not on the strand.
The SSP treatment is preferably carried out in a polymer fluid bed reactor in current or counter-current of an inert gas (nitrogen) . The relationship in weight between the hourly rate of gas and that of the polymer withdrawn is preferably between 0.2 and 0.6. Preferably the cooling of the strand at a temperature suitable for the crystallization is carried out utilizing nitrogen coming from the SSP step. Preferably the crystallization temperature of the strand is between 170° and 190° C., with time between about 5 to 30 minutes. The strand can be collected on a metal conveyor belt maintained at the crystallization temperature, operating in an inert gas atmosphere. Normally, after crystallization, the chips obtained can be subjected to heat setting with the aim of obtaining improved homogenization of polymer crystallinity.
The mixing of resin melt with the dianhydride of tetracarboxylic acid of stage b) is realized in conventional type mixers, for example static mixers, formed of a pipe provided with streambrakers.
Residence time in the mixer is selected so as to avoid an excessive increase in resin IV, higher for example than 0.6-0.7 dl/g. The time is generally less than 180 seconds.
The polycondensation of the resin is carried out according to known techniques.
It is preferable to operate under conditions to obtain resins with an IV of 0.2-0.3 dl/g.
The polyester resins utilized in the process of the present invention are obtained by polycondensation according to known methods of an aromatic dicarboxylic acid or its alkyl diester, preferably terephthalic acid or naphthalene dicarboxylic acids, with aliphatic diols with 2-10 C, such as ethylene glycol, butylene glycol, 1,4 cyclohexane dimethylol, 1-3 propylene glycol.
Polyethyleneterephthalate and ethylene terephthalate copolymers, wherein up to about 15% by weight of units deriving from terephthalic acid are substituted by units deriving from isophthalic acid an
Acquah Samuel A.
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Sinco Engineering S.p.A.
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