Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-08-15
2004-01-27
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C422S122000, C422S901000
Reexamination Certificate
active
06683153
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to polymer recycling, and in particular, to a method for separating technical and biological polymers by means of liquid-liquid separation.
2. Background of the Invention
Polymer recycling involves separating polymers. This separation could mean: (1) the separation of polymers of one type according to molecular weight; or (2) the separation of different types of polymers (for example polystyrene and polyester) regardless of molecular weight.
Polymers, in this case technical polymers, are large-scale products in the chemical industry with production reaching approximately 6.4 million tons in 1990. By far the majority of the produced polymers are thermoplastics, in other words, polymers with finite molecular weight. Thermoplastics represent approximately 90% of the produced amount and can be subdivided into approximately 11 different chemical types such as, for example, polyvinyl chloride, polystyrene, polyolefin, polyester, copolymers and so forth. Recycling depends on whether the different types that form part of mixed waste, can be separated into pure substances since mixed polymers usually do not result in a high quality product.
The recycling can be performed in accordance with the following methods:
Thermal recycling: In other words, combustion of the mixed waste for the extraction of the combustion heat. This method converts the polymers into the substances CO2 and H2O, which are basically not suitable for further synthesis.
Recycling according to type: This type of recycling is usually reserved for the processor, since commercial polymers usually are mixed.
Mechanical separation: This method takes advantage of the different polymer densities or the different wetting characteristics of the polymers. It is not suitable for hollow or mechanically connected parts consisting of different polymers nor for polymers that contain additives (softeners, flame-retardants and others).
Type recognition mechanical separation: This method relies on fast analysis methods that recognize the polymer type and sort the recognized parts. This method is not suitable for mechanically connected parts consisting of different polymers.
A common disadvantage of all the above-mentioned separation methods is the fact that they cannot respond to the additives and auxiliary agents nor decomposed polymers that are often present (for example as a result of the use of UV radiation).
Thermal separation methods should also be considered for polymer separation. Due to the very low vapor pressure of polymers, distillation cannot be considered, leaving extractive and adsorptive methods as possibilities.
SUMMARY OF THE INVENTION
When examining the solubility of a polymer in a solvent it becomes apparent that it is very dependent on both the temperature and the molecular weight of the polymer.
FIG. 1
illustrates this correlation.
In
FIG. 1
, the polymer concentration in a solvent (0 to 100%) is plotted against temperature. The marked two-phase boundaries (
1
) and (
2
) separate a region of total miscibility (
4
) from the regions (
3
) or (
5
) of separation into two liquid phases, one of which is rich in solvent and the other is rich in polymers. It is generally accepted that two separation areas exist, one at lower temperatures (
3
) and one at higher temperatures (
5
). An increase in polymer molecular weight leads to an expansion of the two-phased separation and, in extreme cases, leads to the formation of a single connected two-phase area (watch-glass diagram).
For technical separation tasks, separation at higher temperatures is particularly interesting, since at lower temperatures and with high polymer content (> 20 wt. % polymer), the viscosity of the solution is very high, which leads to difficulties with material transfer and heat transfer and hence leads to higher costs (high-viscosity technique). The two-phase area (
5
) can be shifted to lower temperatures (see, for example, B. Bungert, G. Sadowski, W. Arlt,
Fluid Phase Equilibria
139 (1997) 349-359) by compressing a gas, which is particularly useful for thermally unstable polymers. In this manner, the advantages of low viscosity can be combined with the advantage of lower temperature requirements. An additional advantage when using a compressed gas is that, as opposed to liquid precipitants (through the use of which a separation also could be achieved), it can be quasi-quantitatively removed from the system when the pressure is lowered, eliminating the need for additional processing steps such as, for example, distillation.
If two polymers in one common solvent are examined, a segregation into two liquid phases usually occurs as soon as the total polymer content reaches a value of 5-10 wt. % even when both the pure polymers are completely soluble in the solvent being examined (see, for example, S. Krause,
J. Macromol. Sci.—Revs. Macromol. Chem
., C7(2), (1972) 251-314).
FIG. 2
illustrates these interrelations using a triangle diagram. The triangle diagrams of
FIG. 2 and 3
are in accordance with J. Gmehling, B. Kolbe,
Thermodynamics
, 2
nd
edition, Weinheim 1992.
Referring to
FIG. 2
, while the corners of the triangle represent the pure substances polymer P
1
, polymer P
2
and the solvent LM, the sides of the triangle represent the corresponding binary subsystems. All possible concentrations of the ternary system polymer P
1
/polymer P
2
/solvent LM are located within the triangle surface. Line
8
separates the area of complete miscibility (
9
) from the area (
10
) of separation into liquid phases. In area
10
, two phases are formed that contain mainly one of the respective polymers as well as additional detectable amounts of the other polymer. A polymer separation with a high degree of purity (>99%) taking advantage of this segregation is theoretically possible when working with total polymer content of between 30-50 wt. %. In practice, however, it is not feasible since when doing this, two (or more) phases are obtained, each with a high polymer content. These phases are so viscous that a mechanical separation of the phases is not possible.
WO091/03515 to E. B. Nauman describes a method for the selective dissolving of a polymer mixture using a specifically chosen solvent. The disadvantages of this method are the extensive use of solvent, low selectivity and over sensitivity with regard to the molecular weight of the polymer. Additionally, the dissolving and separation of the polymers is only partially successful due to the high viscosity in the solution. Nauman himself points out that the method is not suitable for composite materials consisting of different polymers.
An additional thermal separation method is liquid chromatography that can be controlled in such a way that a separation of polymers is achieved regardless of molecular weight (see, for example, W. Arlt, A. Lawisch, German Patent Application, File No. 197 14063.7-41). The separation of one polymer type is achieved using distribution between a mobile and a stationary phase. Such a chromatography installation consists of the following parts: machine used to maintain the flow of the mobile phase; feeder device for the substances that are to be separated; column with the stationary phase; detector for the polymer types that are to be separated. The described installation configuration is employed for analytical purposes and under certain circumstances also for technical separation. In this connection, the column material, solvent and elution agent must be chosen in such a manner that the retention times for all types are finite, in other words, so that none of the types irreversibly adhere to the column (again, see, for example, W. Arlt, A. Lawisch, German Patent Application, File No. 197 14063.7-41). The limitations of this method are that it is necessary to work with very dilute solutions because of the modest simultaneous solubility of several of the polymers in the common solvent at simultaneous high viscosity, which affects costs.
The method according to the invention offers a significant improv
Arlt Wolfgang
Behme Stefan
Bungert Bernd
Sadowski Gabriele
Baghdadi Aslan
Boykin Terressa M.
Der Grune Pukt - Duales System Deutschland AG
Shaw Pittman LLP
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