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
2000-05-19
2001-11-27
Seidleck, James J. (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...
C526S232500
Reexamination Certificate
active
06323289
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention pertains to a process for enhancing the melt strength of a polypropylene by mixing a peroxydicarbonate with the polypropylene and heating the polypropylene/peroxydicarbonate mixture.
Such a process is known from EP-B-0 384 431. This reference describes a process for preparing polypropylene (PP) with a branching index of less than 1 comprising (1) mixing a low decomposition temperature peroxide, such as a peroxydicarbonate, with the PP, (2) heating or maintaining the resulting mixture at a temperature from room temperature up to 120° C., and then (3) heating the PP at a temperature of 130 to 150° C. to deactivate substantially all the free radicals present in said PP. The obtained PP has a significant amount of long chain branches and has an increased weight average molecular weight giving the polymer a significant strain hardening elongational viscosity. It is mentioned that in step (3) the heating may be done by extrusion or in a fluidized bed. It is further stated that at a temperature above 120° C. an essentially linear polymer with little or no branching is obtained.
Apparently, the significant strain hardening elongational viscosity is related to an improved melt strength of the obtained PP due to long chain branching. It is only mentioned in this publication that step (3) of this process can be performed by extrusion, in the examples the entire process is carried out in a sealed reaction vessel. A disadvantage of this process is that it comprises three steps, which is undesirable in practice.
DE-A-4340194 (U.S. Pat. No. 5,416,169) describes a process for preparing PP having a high melt strength and a chain branching coefficient of 1, by mixing bis(2-ethylhexyl)peroxydicarbonate with a linear, crystalline PP, followed by heating at 70 to 150° C. In a subsequent step, the PP is taken out of the reaction vessel and is melt-kneaded. The peroxydicarbonate may be dissolved in an inert solvent before it is added to the PP. It is stated that other peroxydicarbonates, having a similar decomposition temperature, cannot be used for this purpose.
A disadvantage of this process is that it can only be performed using one specific peroxide, i.e., bis(2-ethylhexyl)peroxydicarbonate. Another drawback is that the process is performed in two steps, which is undesirable and uneconomical in practice.
It is the object of the present invention to provide a process which does not have the above-mentioned drawbacks and which process yields PP having a good melt strength.
SUMMARY OF THE INVENTION
To this end, the present invention provides a process for enhancing the melt strength of polypropylene comprising the steps of:
mixing the polypropylene with at least one peroxydicarbonate;
reacting said polypropylene and peroxydicarbonate at a temperature between 150° C. and 300° C., with the proviso that the peroxydicarbonate is not in the form of an aqueous dispersion in a polar medium, with at least 90% by weight of the initiator particles being smaller than 50 &mgr;m and at least 99% by weight of the initiator particles being smaller than 65 &mgr;m. Preferably, the reaction conditions are chosen such that more than 50% by weight, more preferably more than 70% by weight, and most preferably, more than 80% by weight of the peroxide is still present when the mixture of peroxide and polymer reaches a temperature of 120° C., more preferably 150° C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the non-prepublished patent application, now published as WO 97/49759, a process for enhancing the melt strength of a polypropylene is described, wherein a dispersion of an initiator, such as a peroxydicarbonate, in a polar medium, e.g., water, and with a particular particle size distribution, is contacted with the polypropylene and the resulting mixture is heated. This process may be carried out using an extruder.
The process according to the present invention is a process providing PP having an enhanced melt strength. Since the processing of PP frequently involves extrusion of the PP, either for pelletization in case of storage and transport or for further processing, i.e., the formation of an end product, it is a further advantage that the invention process is an extrusion process, allowing the modification of the PP, to enhance the melt strength, to be combined with said processing step.
The term “polypropylene” (“PP”) refers to polymers or mixtures of polymers containing at least 50% by weight of polymerized propylene. Polymerization catalysts may be Ziegler-Natta, metallocene or other types giving stereospecific polymerization of propylene. Use may be made in this connection of homopolymers of propylene; random, alternating, or block copolymers; or random, alternating, or block terpolymers of propylene and another olefin. Generally, a propylene copolymer or terpolymer will contain one or more other olefins, such as ethylene, butene, pentene, hexene, heptene, or octene, but it may also comprise other olefinically unsaturated monomers or combinations of these, such as acrylates, styrene, styrene derivatives, acrylonitrile, vinyl acetate, vinylidene chloride, and vinyl chloride.
It is preferred here to restrict the content of olefins other than propylene to 30% by weight of the copolymer. Especially suited to be used are homopolymers of propylene, copolymers of propylene and ethylene or mixtures of polypropylene and polyethylene containing not more than 10% by weight of polymerized ethylene.
The melting point of normally solid commercially available PP is about 160-170° C. The melting point of propylene copolymers and tercopolymers in general can be lower. The process of the invention preferably is carried out at a temperature in the range of from 150 to 300° C, more preferably from 160 to 250° C., and most preferably from 170 to 225° C.
The molecular weight of the PP used can be selected from a wide range. Indicative of the molecular weight is the melt flow index (MFI). Use may be made of a PP having a MFI from 0.1 to 1000 g/10 min (230° C., 21.6 N). Preferably, use is made of a PP having a MFI from 0.5 to 250 g/10 min.
The process according to the present invention is suitably carried out in melt mixing equipment known to a person skilled in the art. Preferably, an extruder or a kneader is used. More preferably, use is made of a single or twin screw extruder. An internal mixer such as a Banbury mixer optionally coupled to an extruder may also be used.
The peroxydicarbonate may be mixed first with the PP and then the mixture may be extruded. Alternatively, the peroxydicarbonate may be added to the extruder already containing the PP by injection or spraying, or may be added together with the PP. It is preferred to introduce a solid peroxydicarbonate together with the PP into the extruder, for example, by using a feeder. The temperature setting of the extruder should allow the PP to melt, i.e. above 150° C. The screw speed typically is from about 25 to 500 rpm.
Normal residence time in the extruder is 15 sec.-30 min. The longer residence times can be achieved by using additional static mixers etc.
The extruded strand may be further processed as known to one of ordinary skill in the art. Normally, the extruded strand is fed through a water bath and granulated using a granulator. Alternatively, the extruded modified PP is formed directly into a desired end product.
It is preferred to carry out the process of the present invention in an atmosphere of an inert gas, such as nitrogen or argon. Preferably, nitrogen is used.
Solid as well as liquid peroxydicarbonates may be used in the process according to the present invention. A solution of a peroxydicarbonate in an inert solvent, such as isododecane, or in the form of frozen flakes, may also be used. Suitable inert solvents are known to one skilled in the art. It is preferred to use a solid peroxydicarbonate, in the form of, for example, flakes, finely divided particles (powder), or a liquid peroxydicarbonate, optionally adsorbed on or absorbed in a suitable carrier, such as silica or polypropylene powder or pellets. The use of
Fischer Bart
Hogt Andreas Herman
Spijkerman Geesje Klasina
Akzo Nobel nv
Asinovsky Olga
Fennelly Richard P.
Seidleck James J.
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