Radiant energy – Irradiation of objects or material – Ion or electron beam irradiation
Reexamination Certificate
1999-11-12
2002-11-26
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Ion or electron beam irradiation
C250S435000
Reexamination Certificate
active
06486481
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention involves radiation treatment of polymeric materials, including treatment of polytetrafluoroethylene (“PTFE”) to degrade the material into lower molecular weight forms. That is the electron beam degrades the molecular chain of the polymer thereby reducing the molecular weight. More specifically, the invention has to do with the use a vibratory table to move polymer, such as PTFE, in an even layer, that is at a consistent thickness or depth under an electron beam. The vibratory table apparatus and methods in accordance with the present invention accomplish several desired improvements over apparatus and methods known in the art, and resolves certain deficiencies associated with radiation treatment, particularly radiation treatment of fluoropolymers such as PTFE. The apparatus and methods provide an even layer of material on a continuous basis under an electron beam which allows for more efficient processing without radiation losses and labor requirements of other processes, provides for an enclosed environment which protects against gas evolution and provides for continuous cooling. These and other advantages over radiation process known in the art are achieved with the apparatus and methods of the invention as described herein.
The polymeric material is treated in an enclosed environment which permits treatment to be performed in dedicated environments, including an inert environment and environments rich in particular gases, such as oxygen and nitrogen, or combinations of gases. This feature of the invention allows the apparatus and process to be adapted to permit reaction between materials on the table and chemicals permitted to enter the environment where the radiation treatment occurs. The method can also be adapted to combine at least two materials on the vibratory table to allow for a reaction or reactive coating, such as copolymerization, between the materials using the energy of the electron beam. The apparatus comprises a dual loop cooling water system which provides for the efficient removal of the significant heat generated during the radiation processing, and other cooling systems dedicated to other pieces of apparatus used in the process. The material can be delivered to the vibratory table by any means capable of transporting polymer materials, including a pneumatic flow system, and any means capable of transporting polymer materials, including a pneumatic flow system, can be used to convey treated material from the area of the vibratory table. The radiation processing can embrittle the polymeric material due to molecular wright degradation thus facilitating the physical breaking of the treated polymers in the pneumatic flow system to allow for the reduction in material particle size. Use of a pneumatic flow system with the vibratory table provides for the reduction of the particle size of the polymeric material to about 30 to about 3,000 microns.
2. The Related Art
Radiation treatment has been employed to polymerize organic substances and for treatment of polymers. For example, U.S. Pat. No. 2,921,006 to Schmitz et al. describes polymerization of monomers with an electron beam which is exposed to the monomer in a vacuum chamber partially filled with a cooling medium. An embodiment of the invention in U.S. Pat. No. 2,921,006 involves the continuous treatment of monomer moved through the electron beam on a thin sheet of stainless steel, about 0.002 inches thick, with side flanges of resilient material to prevent material dropping off the sides of the sheet. This patent does not involve the degradation of polymer and does not address process concerns and parameters pertinent to polymeric degradation with an electron beam, and particularly degradation of PTFE.
U.S. Pat. No. 3,081,485 to Steigerwald involves treatment of thermoplastic strands which have softening points below their decomposition points. The strands are moved through an electron beam in a vacuum chamber to soften the thermoplastic. U.S. Pat. No. 5,856,675 to Ivanovich et al. concerns the movement of polymer, on a continuous basis using a conveyor system through an electron beam to cross-link the material.
Radiation can be used to degrade the molecular weight of polymers such as PTFE. For example, U.S. Pat. No. 3,766,031 to Dillon discusses radiation treatment of PTFE with an electron beam at between about 5 and 25 Mrads which renders the PTFE capable of being comminuted to microfineness. This comminuted PTFE is useful as a dry lubricant, for example, in paints and inks. The process employed in U.S. Pat. No. 3,766,031 is not efficient and not appropriate for modern processing because there is no method for cooling the PTFE and the PTFE is unevenly treated. U.S. Pat. No. 4,220,511 to Derbyshire discusses the degradation of unsintered PTFE to be ground into a powder with an average size less than 10 microns. A combination of a radiation at 50-150 Mrads and heat at 150° F. to 600° F. for approximately one half hour is used in the process to degrade the PTFE. U.S. Pat. No. 4,029,870 to Brown, et al. discusses dry lubricant PTFE obtained by subjecting the PTFE to &ggr;-radiation at doses of between 2 and 20 Mrads and subsequently comminuting the PTFE to microfiness. U.S. Pat. No. 5,891,573 to Nueberg et al. addresses subjecting PTFE handled at a temperature below 66° F. to a radiation source to obtain friable PTFE, and the PTFE may be combined with a wetting agent prior to irradiation.
Conventional methods of treating PTFE with electron beams, in use today, including those involving the use of trays are inefficient and results in PTFE products that are unevenly treated. The conventional methods result in uneven radiation treatment, and experiences other deficiencies, including the inability to control the processing environment, process gas evolution and the inability to effectively remove heat.
Several factors contribute to the inefficiency of tray irradiation of PTFE. The most significant occurs as a result of the beam penetration characteristics through the depth of material in the tray. Typically, the dose at the surface is taken as the nominal dose for the material. Beam energy and/or material depth is adjusted so that an equal dose is effected at the opposite surface of the material. Radiation which passes entirely through the product is not utilized. Radiation in excess of the nominal dose is likewise not used. This causes inefficiency, and in some instances may result in undesired properties of the resultant product. This depth-dose characteristic can cause processing inefficiency of up to 50%. Variations in material depth in the tray, gaps between the trays, and overscan of the tray, which is necessary to assure complete and uniform irradiation, can cause additional inefficiencies of approximately 5-15%.
The depth-dose characteristic, overscan and variations in material depth that occurs with tray processing all contribute to uneven treatment of the PTFE. Consequently, some of the PTFE may be overly treated and other portions of the PTFE may not receive adequate treatment. This results in inconsistent properties and, hence, unpredictable product quality of the processed PTFE.
Processing on trays can only be performed in batches requiring time and labor to place material on the tray and move trays into position and from the processing area after treatment. This requires that a human being be so integrally involved in the process that processing is generally performed in ambient conditions. Thus, the conventional tray process is limited in the inability to effectively control the environment where the processing occurs, and the process cannot be readily adapted to allow for reaction between PTFE and other materials, or to react or combine polymers in general.
Hydrogen fluoride gas generation from radiation treatment of fluoropolymers, such as PTFE, is a problem associated with the tray method and other known processes for degradation of fluoropolymers such as PTFE with an electron beam. Presence of hydrogen fluoride gas presents work
Ausimont USA, Inc.
Berman Jack
Norris & McLaughlin & Marcus
Quash Anthony
LandOfFree
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