Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1995-01-03
2001-02-13
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S253000, C526S254000, C526S255000
Reexamination Certificate
active
06187885
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the preparation of vinylidene fluoride polymers, more particularly to the preparation of copolymers of vinylidene fluoride and hexafluoropropylene having improved physical properties.
Abbreviations:
The following materials are referred to in the herein specification by their common abbreviations:
DSC
differential scanning calorimetry
DTBP
di(tert-butyl)peroxide
HFP
hexafluoropropylene
IPP
diisopropyl peroxydicarbonate
TCFM
trichlorofluoromethane
VDF
vinylidene fluoride
PVDF
polyvinylidene fluoride
BACKGROUND OF THE INVENTION
U.S. Pat. Nos. 4,076,929 and 4,569,978 describe an improved emulsion polymerization process for preparing VDF homopolymers and copolymers having improved flexibility. According to the aforesaid process, a comonomer for VDF is added in an emulsion polymerization mixture as part of an initial charge, or the comonomer is introduced continuously in a fixed ratio with respect to VDF. The comonomer may comprise HFP. The invention of U.S. Pat. No. 4,076,929 results in extrusion grade polymer resins having improved melt flow characteristics which may be utilized for the fabrication of objects having greater flexibility and impact resistance. These improvements are diminished as the melt viscosity of the polymer is decreased to 20 kilopoise or lower, measured at 232° C. and 100 sec
−1
(ASTM 3835), unless increasing amounts of HFP comonomer are utilized. However, as the amount of HFP in the polymer is increased, the DSC melting point, and thus the use temperature of the polymer, decreases.
What is needed is a VDF/HFP copolymer having improved physical properties, particularly improved melt flow, which improved properties are obtained without significant reduction in polymer use temperature, as compared to the use temperature of VDF homopolymer. More particularly, there is a need for a VDF-based polymer which has improved melt flow, but which substantially maintains the melting point of VDF homopolymer.
SUMMARY OF THE INVENTION
A process is provided for the production of VDF/HFP copolymer by the emulsion polymerization of VDF and HFP in a stirred aqueous reaction medium. The aqueous reaction medium is formed by charging the following to a reactor: water, VDF, an initiator to start the polymerization, and a water-soluble surfactant capable of emulsifying both the initiator and the reaction mass during polymerization. Additional amounts of VDF and initiator are fed to the reaction medium to continue polymerization of the VDF until from about 50% to about 90% of the total weight of the VDF utilized in the emulsion polymerization process has been added to the reaction medium. There is then added to the reaction medium (i) from about 1% to about 20% HFP by weight, based upon the combined weight of the HFP and the total weight of VDF added to the reaction medium in the emulsion polymerization process, and (ii) the balance of the VDF utilized in the process. VDF/HFP copolymer is then obtained from the reaction medium. The HFP is preferably added to the reaction medium as rapidly as possible. The balance of the VDF monomer is preferably added to the reaction medium at the same or substantially the same rate as added during the VDF homopolymerization phase of the process.
Preferably, from about 65% to about 85%, most preferably from about 70% to about 80%, of the total weight of VDF utilized in the process is added to the reaction medium before the addition of any HFP. From about 5% to about 15% HFP by weight is preferably added to the reaction medium, based upon the combined weight of the HFP and the total weight of VDF added to the reaction mixture during the polymerization process. The aqueous reaction medium for polymerizing the VDF monomer preferably contains a chain transfer agent, e.g. TCFM, for controlling the molecular weight of the resulting polymer.
According to one embodiment of the invention, VDF is added to the reaction mixture, prior to the addition of HFP, as an initial VDF charge sufficient to obtain an operating pressure in the reactor of at least about 450 PSIG, preferably, from about 450 to about 700 PSIG, after which initiator is added in an amount equal to from about 0.25 to about 2.0 grams per kilogram of VDF present in the initial charge. VDF is additionally added to the reaction medium as a continuous or incremental VDF charge to increase the amount of VDF added to the reaction medium to from about 50% to about 90% of the total amount of VDF utilized in the polymerization process. Additional initiator is added to the reaction medium to continue polymerization of the VDF.
Establishment of the reaction medium prior to addition of the initial VDF charge may comprise the steps of initially charging the reactor with a reaction medium comprising water, fluorosurfactant, and paraffin wax; agitating and heating the mixture; ceasing agitation and venting the reaction medium; resuming agitation and adjusting the temperature to an operating polymerization temperature of from about 65° C. to about 150° C.; and optionally adding chain transfer agent in an amount sufficient to obtain the desired molecular weight of polymer.
The invention further relates to VDF/HFP copolymers such as those prepared according to the present invention, having a HFP content of from about 1% to about 20% by weight, preferably from about 1% to about 15%, most preferably from about 5% to about 15%, and a melting point in the range of from about 160° C. to about 170° C., preferably from about 163° C. to about 168° C.
By “VDF/HFP copolymer” or “vinylidene fluoridehexafluoropropylene copolymer” is meant normally solid polymers containing at least 50 mole percent vinylidene fluoride copolymerized with hexafluoropropylene as a comonomer, and optionally containing one or more further comonomers selected from the group consisting of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinylfluoride, pentafluoropropene, and any other monomer that will readily copolymerize with vinylidene fluoride. Most preferably, said further comonomer(s) is/are selected from those monomers which are at least as reactive toward polymerization as vinylidene fluoride, e.g. pentafluoropropene, chlorotrifluoroethylene, and trifluoroethylene.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, VDF/HFP copolymers are prepared which have improved properties. Prior art methods for copolymer preparation by emulsion polymerization wherein all of the HFP monomer is combined with VDF monomer in an initial reaction mixture charge, or introduced continuously in a fixed feed ratio with VDF, generally result in the formation of uniformly random VDF/HFP copolymers. In contrast, VDF/HFP copolymers of the present invention, produced by homopolymerizing VDF monomer, followed by further VDF polymerization with HFP, results in a polymer having discrete VDF homopolymer domains, and a discrete VDF/HFP copolymer domain. Such polymers possess improved properties over the uniformly random VDF/HFP copolymers. Specifically, they display greater melt flow, lower flexural modulus, improved impact resistance, and/or improved chemical resistance, but without sacrifice to the use temperature of the polymer. The aforementioned improvements may be realized in the production of resins according to the practice of the invention useful for various applications within the present spectrum of commercial PVDF-based polymer usage. In particular, the polymers of the invention provide low melt viscosity resins which display excellent flow characteristics under minimal sheer stress, while nevertheless maintaining the integrity of favorable mechanical properties.
While conventional VDF/HFP copolymers display melt temperatures below about 150° C., generally 140°-145° C., the copolymers of the invention are generally characterized by a melting point, as determined by DSC, in the range of about 160° C. to about 170° C., more particularly from about 163° C. to about 168° C., which closely approaches the melting point of VDF homopolymer generally (168°-170° C.). While certai
ATOFINA Chemicals, Inc.
Seidel Gonda Lavorgna & Monaco, PC
Wu David W.
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