Unlike high-molecular-weight tetrafluoroethylene, the preparation of low-molecular-weight polytetrafluoroethylene usually requires the use of environmentally harmful organic substances as molecular weight regulators and the use of organic solvents. The conventional low-molecular-weight polytetrafluoroethylene resin adopts the emulsion polymerization method, with deionized water as the polymerization medium, ammonium perfluorooctanoate as the dispersant, methylene chloride as the chain transfer agent, and tetrafluoroethylene monomer as the initiator of persulfate. Polymerization under initiation, the obtained polymerization solution is coagulated and continuously dried to obtain low molecular weight polytetrafluoroethylene resin. This method can prepare low molecular weight PTFE resin with excellent water emulsion stability.
(1) The conventional process uses ammonium perfluorooctanoate as an emulsifier, which is difficult to degrade and may be harmful to the human body, so it needs to be replaced;
(2) The chain transfer agent dichloromethane was used.
This product has anesthetic effect, mainly damages the central nervous system and respiratory system, and is irritating to the skin and mucous membranes.
(3) Use an organic compound molecular weight regulator. The use of this reaction medium will inevitably have adverse effects on the environment and operators, and the molecular weight regulator of organic compounds will also affect the performance of the final product.
Technical research on the neutralization process of low molecular weight polytetrafluoroethylene resin inorganic acid by cylinder-cone filter and washing two-in-one
The new preparation process of low-molecular-weight polytetrafluoroethylene resin does not use perfluorooctanoic acid and its salts, does not add chain transfer agents, and does not require high temperature or radiation methods, so that it will not cause harm to the body of operators during the resin preparation process. The preparation method of the low-molecular-weight polytetrafluoroethylene resin with a molecular weight of 3 to 600,000 comprises the steps:
(1) Preparation of polytetrafluoroethylene dispersion
In the presence of water, 500-5000ppm fluorine-containing surfactant, 200-1000ppm carboxylic acid or its salt, 100-1000ppm anionic non-fluorinated surfactant, polymerize at a temperature of 60-100°C and a pressure of 1.5-2.8MPa Tetrafluoroethylene monomer for 2 to 3 hours to obtain a polytetrafluoroethylene dispersion;
(2) Preparation of low molecular weight polytetrafluoroethylene resin
Add nitric acid to the aqueous dispersion of low-molecular-weight PTFE, and apply severe mechanical shearing force to coagulate the polymer and condense polytetrafluoroethylene resin;
(3) Cylindrical-cone filtration and washing two-in-one inorganic acid neutralization process
In the barrel-cone filter-washing two-in-one, the powder in wet state is neutralized and washed with water, and then the low-molecular-weight polytetrafluoroethylene resin high-purity powder is filtered out.
(4) Low molecular weight polytetrafluoroethylene resin drying process
Dry the filter cake with a special drier for low-molecular-weight polytetrafluoroethylene resin, thereby obtaining low-molecular-weight PTFE powder.
Low-molecular-weight polytetrafluoroethylene resin inorganic acid neutralization special cylinder cone filter washing two-in-one performance advantages
Cylinder-cone filtering and washing two-in-one cone or cylinder is equipped with multi-layer orifice plates and filter media to form a filter device. The inside is equipped with a stirring shaft and a variable-angle variable-lead screw stirring device. Stirring, lifting and scraping operations are performed, and the filter cake is automatically pushed out during discharge. This machine can carry out vacuum filtration or pressure filtration. During the filtration process, the scraper in the machine continuously scrapes the filter cake on the filter screen, so that the material is filtered under the condition of a thin layer of filter cake, and the filtration rate is very high.
Low molecular weight polytetrafluoroethylene resin inorganic acid neutralization process can filter the slurry in the machine first, then add washing liquid, and use the scraper, agitator and extrusion screw in the machine to re-slurry the filter cake. At this time, the re-slurry is filtered into a filter cake by means of pressure or vacuum. In this way, re-slurry filtration is repeated many times to wash the material thoroughly, and finally all the filter cakes are extruded out of the machine.
(1) Fully enclosed system, the operating environment is quite good, especially suitable for filtering and washing toxic, flammable and explosive dangerous products and delicious food biological products, pharmaceuticals and biological products that do not allow pollution.
(2) Continuous operation with extremely low labor intensity. The operation is flexible, and the operation cycle can be adjusted by adjusting the filter pressure difference, rotation speed, screw extrusion lead, etc., so as to adapt to the change of process conditions.
(3) It can not only concentrate, but also press filter.
Especially when it is necessary to wash the material, the material can be concentrated first, washed thoroughly in a suspended state, and then filtered after filtration, which can reduce the handling of the material.
Reduce waste, do not need to re-pulp when washing, and the processed products are of high quality.
(4) The machine can form a very thin filter cake on the surface of the filter medium or filter under the condition of no filter cake. The filtration rate per unit filter area is very high, so the equipment is small and compact.
(5) The materials can be repeatedly filtered and washed conveniently, so that the functions of multiple unit equipment can be completed on one machine, so that the equipment cost, floor area, labor intensity, energy consumption, etc. are greatly reduced.
At the same time, the combination of cylinder-cone filter and wash is especially suitable for high washing requirements, and the filtered materials need to be repeatedly repulped and washed, and can save washing liquid.
(6) Make the material undergo pressure filtration under higher pressure, and vacuum filtration can also be carried out. A jacket is added to the outside of the filter to heat, cool or heat-preserve and filter the material to meet the requirements of various process conditions.
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