Thermally cross-linked fluoropolymer

Details Thermally cross-linked fluoropolymer Thermally cross-linked fluoropolymer

1999-05-20

2003-05-06

Wilson, D. R. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S381000, C525S382000, C525S374000

Reexamination Certificate

active

06559238

ABSTRACT:

FIELD OF THE INVENTION
This invention is in the field of thermoplastic fluoropolymers, more particularly crosslinked thermoplastic fluoropolymers.
BACKGROUND OF THE INVENTION
Thermoplastic fluoropolymers are well known for outstanding combinations of properties including chemical resistance, unique surface characteristics, high service temperatures, and good dielectric characteristics. As a result, fluoropolymer resins are used in a wide variety of applications including wire insulation, cable jacket, hose, tubing, film, linings for chemical process equipment, articles for fluid handling in laboratory and manufacturing situations and the like. The service temperature in some of these applications can be high. As is common for thermoplastics, some properties of fluoropolymers change as temperature increases. Modulus and tensile strength, for example, typically decrease with increasing temperature.
Efforts have been made to improve the physical characteristics of fluoropolymers at elevated temperatures, largely by cross-linking. Approaches to cross-linking usually involve the incorporation of a cross-linking promoter, also called,a coagent, such as triallyl cyanurate or triallyl isocyanurate (U.S. Pat. No. 4,353,951) or metallic diacrylate (U.S. Pat. No. 5,409,997) into a fluoropolymer such as ETFE copolymer, followed by treatment with ionizing radiation to effect the cross-linking.
Improved ways to cross-link fluoropolymers and thereby achieve improved properties are desired.
SUMMARY OF THE INVENTION
This invention provides a cross-linked fluoropolymer, the fluoropolymer being thermoplastic and melt-fabricable prior to cross-linking, wherein the cross-linking of the fluoropolymer is carried out thermally.
The invention also provides a process for cross-linking a melt-fabricable thermoplastic fluoropolymer having pendant functional groups, comprising combining the fluoropolymer with cross-linking promoter before melt fabrication of the fluoropolymer into a shaped article is completed, and then completing melt fabrication. The fluoropolymer can also be shaped into an article before exposure to the cross-linking promoter, which is then infused into the article. Preferred cross-linking promoters are nucleophiles, and include monofunctional and polyfunctional amines.
DETAILED DESCRIPTION
It has been discovered that functionalized fluoropolymer can be effectively cross-linked by a thermal process that does not require the costly use of ionizing radiation and the expensive equipment necessary for such treatment. Functionalized fluoropolymer that can be so cross-linked has pendant functional groups and is melt-fabricable prior to cross-linking. The cross-linking takes place in the presence of cross-linking promoter. Since the cross-linking occurs thermally, the cross-linking process can be coordinated with the fabrication of the melt-fabricable fluoropolymer into a shaped article.
As used herein, “thermal process” and “occurs thermally” are understood to mean that the cross-linking process of the invention is activated by temperature alone. It suffices that functionalized fluoropolymer and cross-linking promoter are brought together and intermixed, and the temperature is raised to at least a level sufficient to activate cross-linking for a time sufficient to achieve the desired cross-linking. One skilled in the art will recognize that the time required to achieve cross-linking will in general depend on the temperature, with cross-linking occurring more rapidly as temperature increases. Typically, a temperature of at least 75° C. and more often at least 100° C. will be required for cross-linking to occur to any appreciable extent. Thus, cross-linking initiation as might be provided by such means as a catalyst or by ionizing radiation is not necessary. One skilled in the art will recognize the possibility that the thermal cross-linking of the invention could be supplemented by such auxiliary means. Such supplemented cross-linking processes are within the scope of the present invention if cross-linking occurs in the absence of such auxiliary means.
The cross-linking promoter used in the thermal cross-linking of the instant invention is present in minor amount. When the cross-linking promoter is a low molecular weight compound, such as a compound having a molecular weight of 500 or less, the amount of cross-linking promoter will generally be no more than 2 wt % based on the weight of fluoropolymer resin, usually no more than 1 wt %. The amount of such cross-linking promoter present is generally at least 0.01 wt %, more commonly at least 0.05 wt %, based on weight of fluoropolymer. When cross-linking promoter is polymeric, then larger amounts of cross-linking promoter can be used, such as 3-30 wt % based on combined weight of fluoropolymer and cross-linking promoter, more commonly 10-25 wt %. Cross-linking promoter should be stable by itself at processing temperatures.
Cross-linking promoters that can be used include nucleophiles, including monofunctional and polyfunctional nucleophiles. Preferred nucleophiles include amines, including primary and secondary amines. Polyfunctional amines that can be used include, for example, aromatic diamines such as para-phenylenediamine, meta-phenylenediamine and methylene dianiline. aliphatic diamines such as 1,6-diaminohexane, and aliphatic triamines such as bis(hexamethylene)triamine. Monofunctional amines that can be used include, for example, aniline, diethyl amine and ammonia. The use of monofunctional cross-linking promoters is particularly surprising. Polymeric cross-linking promoters that can be used in the present invention include polyamides. One skilled in the art will recognize that more than one cross-linking promoter can be used. Thus, at least one cross-linking promoter is used.
Since the cross-linking of the present invention occurs thermally, it is difficult to provide a cross-linkable composition, containing fluoropolymer having pendant functional groups and cross-linking promoter, in conventional cube form because such cubes are normally prepared by melt extrusion, i.e., a high-temperature process which would promote the thermal cross-linking and render the composition less suitable or unsuitable for subsequent fabrication into desired articles or shapes. Additionally, some care must be exercised so that thermal cross-linking does not take place too rapidly and/or to too great a degree within melt processing equipment, e.g., an extruder or an injection molding machine, and thereby impede or even prevent the flow of resin within or out of the equipment. Cross-linking promoter and fluoropolymer having pendant functional groups can first be combined in desired proportions at room temperature, and the mixture then shaped by the intended melt fabrication technique, in which case the cross-linking temperature will usually be at least 250° C. and more often at least 275° C. Promoter and fluoropolymer can be blended with each other in the dry state, such as by tumbling in a drum, or can be combined by simultaneous or separate metering of the feed of one or more of the components to the melt processing device. As illustrated by examples below, low molecular weight cross-linking promoter can also be deposited on fluoropolymer resin from a solution of promoter in an appropriate solvent. Preliminary combining of promoter and fluoropolymer can be satisfactory if residence time in the melt processing equipment is relatively short, but may not be satisfactory if residence time is relatively long or cross-linking is relatively rapid. Promoter and functionalized fluoropolymer can also be combined during melt processing in a way that reduces residence time of the melt-processible composition of the invention in the melt processing equipment, thereby reducing or avoiding the risk of undesired melt viscosity (MV) increase inside the equipment. This can be accomplished, for example, by injecting promoter, such as in a solution, into the melt of fluoropolymer having pendant functional groups as it (the melt) is transported along the barrel of a melt extrud

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