Coating processes – Solid particles or fibers applied – Interior or hollow article coating
Reexamination Certificate
2000-02-16
2001-09-11
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
Interior or hollow article coating
C427S195000, C427S231000, C427S234000, C427S240000, C427S242000
Reexamination Certificate
active
06287632
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of rotolining with melt processible fluoropolymers.
BACKGROUND OF THE INVENTION
Fluoropolymers such as tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA) tetrafluoroethylene/hexafluoropropylene (FEP), tetrafluoroethylene/ethylene (ETFE), and the like, exhibit melt flow at a temperature at or above the melting point of the polymer. Such polymers are designated here as “melt processible” and are extensively used as excellent film forming materials that produce coatings with minimal pinholes or voids. Melt processible fluoropolymers are distinguished from polytetrafluoroethylene (PTFE), the homopolymer of tetrafluoroethylene that is processed by other means.
Fluoropolymer coatings are useful as linings for pipes and vessels, providing them with corrosion resistance, non-stickiness, abrasion resistance, and chemical resistance. In addition, being made of fluoropolymers, the linings are effective over a broad temperature range. Traditional means of applying coatings include powder coating, sheet lining, and rotational lining, also known as rotolining. In the case of powder coating, the maximum thickness that can be applied is about 100 &mgr;m. If thicker coatings are attempted, gas bubbles are often entrapped. These bubbles constitute defects in the coating, contributing to surface roughness and to actual or potential thin spots or pinholes. However, for best corrosion resistance, a lining thickness of 500 &mgr;m or greater is desirable. Therefore, it has been necessary to make multiple applications to build up to the desired thickness.
Sheet lining is an alternative method for applying a coating. In sheet lining, a 2 to 3 mm thick film of PFA or PTFE, backed with a glass fabric, is bonded to the substrate with an adhesive, and the joint where the ends of the film meet is sealed or welded. Sheet lining gives coatings of the necessary thickness, but useful temperature range of the coating is limited to that of the adhesive, which is generally less than the useful temperature range of the fluoropolymer.
In the rotolining molding process melt processible polymer in powder form is added to the article to be lined. Then the article is heated as it is rotated around at least two rotational axes. Rotation distributes the melting polymer uniformly over the interior surface of the hollow article resulting in a coating of uniform thickness. Cooling the article causes the polymer to solidify, fixing the lining to the surface of the article.
Rotolining has been applied principally to low melt viscosity resins such as polyethylene, polypropylene, or the like, but the process has begun to be applied to fluoropolymers in order to make use of their excellent properties. There is a tendency however, for substantial bubble formation as the film becomes thicker occurring at 340-380° C. See, for example, European Patent Application 0 778 088 A2, which reports gas bubble formation in the rotolining process as applied to fluoropolymers. This is overcome only by high rotation speeds, that is, high radial acceleration, and operation in a narrow temperature range just above the melting point of the fluoropolymer. Nothing is written about the thickness of the lining attained under these conditions.
A rotolining process is needed that permits the formation, with a single application of fluoropolymer powder, of a fluoropolymer lining at least 500 &mgr;m thick. This lining should be substantially free of defects such as bubbles or voids, and its surface should be smooth, to facilitate flow and prevent fouling by material caught on surface imperfections, such as depressions and asperities.
SUMMARY OF THE INVENTION
A rotolining process which comprises placing a powder having an average particle size of 70-1000 &mgr;m containing a melt processible fluoropolymer, in a cylindrical article to be lined, said powder being present in sufficient amount to make a lining at least 500 &mgr;m thick, rotating said cylindrical article to bring the radial acceleration at the substrate surface to be coated to 100 m/sec
2
or greater, pressing said powder against the article to be lined by means of the centrifugal force generated by that rotation, at the same time heating the melt processible fluoropolymer to a temperature equal to or higher than the melting point of the melt processible fluoropolymer, but not higher than 400° C., thereby adhering the melt processible fluoropolymer to the surface of the article to be lined.
A preferred embodiment of the invention is a rotolining process comprising forming a first layer of a melt processible fluoropolymer powder composition containing a filler on the substrate surface of the article to be lined, and then overlaying a second layer of filler-free melt processible fluoropolymer powder on the surface of said first layer.
DETAILED DESCRIPTION OF THE INVENTION
The melt processible fluoropolymers of this invention include the copolymers tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA) tetrafluoroethylene/hexafluoropropylene (FEP), and tetrafluoroethylene/ethylene (ETFE). Among the melt processible fluoropolymers, PFA, is preferred because of its thermal stability and chemical resistance. The PFA preferably has a specific melt viscosity at 372° C. in the range of 5·10
3
to 1·10
6
poise (of 5·10
2
to 1·10
5
Pa·s). If the specific melt viscosity is lower than 5·10
3
poise (5·10
2
Pa·s), the resin will have inferior thermal stability and resistance to stress cracking, making it an unsatisfactory lining material. If the specific viscosity exceeds 1·10
6
poise (1·10
5
Pa·s) removal of gas bubbles will be retarded, particularly when the fluoropolymer is used with a filler.
The average particle size of the powder used in this invention is 70-1000 &mgr;m, preferably 100-500 &mgr;m. A powder with an average particle size less than 70 &mgr;m will usually cause the powder particles to agglomerate before film formation begins. This results in large secondary particles, which will produce film with a rough surface. A powder with an average particle size greater than 1000 &mgr;m will reduce film forming capability, resulting in a poor surface smoothness.
The rotational rate used in rotolining according to this invention need only be enough to force the fluoropolymer powder against surface to be coated and to prevent its moving while the fluoropolymer is melting and the film is being formed. As shown in the Examples, for lining a tube 81 mm in inner diameter, 500 rpm is adequate. This corresponds to a circumferential speed of about 2 m/sec, or, to state this in terms independent of the diameter of the article to be coated, a radial acceleration of about 100 m/sec
2
. A radial acceleration of 200 m/sec
2
is preferable. As regards the coating, there is no upper limit to the radial acceleration, although mechanical stress on the equipment used and economic considerations impose practical limitations.
It is sometimes desirable to incorporate a filler in the fluoropolymer powder used in this invention so that the coating will have a thermal shrinkage as close to that of the substrate as possible. This will to prevent differential shrinkage when the article is cooled after coating. Therefore, if a filler is compounded with the fluoropolymer for the object of reducing shrinkage, it is preferred to use a heat resistant filler that has at least lower thermal shrinkage than that of the fluoropolymer. A glass fiber filler is particularly effective for reducing the shrinkage.
Adding a small amount of a heat stabilizer such as PPS (polyphenylene sulfide) to prevent the decomposition of the fluoropolymer on heating can give an excellent coating with minimal bubble formation. These additives may include combinations; for example, as proposed in Japanese Patent 2550254, the use of a melt processible fluoropolymer powder composition is preferred in which a small amount of heat stabilizer PPS is added and uniformly incorporated within the melt processible fluoropolymer particles, along with the heat resistant filler.
Despite the benefits of additi
Aida Tsuneo
Nishio Takao
DuPont Mitsui Flurochemical
Parker Fred J.
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