COMPLETE SPECIFICATION

DEVELOPMENT OF FILLER GRADE PTFE POWDER OBJECTIVE OF THE INVENTION

The main objective of the present invention is to develop a cost-effective mixing technique of non- free flow PTFE powder with free flowing fillers and to transfer the technology to the Indian industries dealing with filler grade PTFE products who at present either import or buy the filler grade PTFE at a higher price from a few proprietary companies. The idea was conceived way back in 1975 in BARC, Trombay which uses the cryogenic (liquid nitrogen) mixing of PTFE with luminescent material which is cost intensive and hence not acceptable to industries.

SUMMARY OF INVENTION

A mixing instrument of proper choice for making filler grade PTFE is invented. It mimics jet mill in action but is cost effective. It produces no heat and hence no coagulation if properly handled.

In order to get products of higher quality, it is necessary to make the PTFE powder more free flowing while mixing if necessary with additional lubricant. Some fillers like graphite need no lubricant. Other fillers like carbon need lubricant. Graphite serves as the best lubricant for carbon filled PTFE. For other fillers suitable lubricants would improve mixing quality.

A low grain size and high thermal stability of the fillers is found necessary to get products of smooth surface. Fillers having a high concentration of volatile impurities which produce ash on burning should not be used. When carbon burns, temperatures as high as 800°C is reached. At 800°C, the C-F bonds break and the strength of PTFE polymer comes down. Other possible polymer degradation mechanisms include exchange of carbon with sulfur at such high temperatures. Though such fillers are to be avoided, this result by itself is an interesting one since the only agent known so far to damage sintered PTFE is ionizing radiation! Present study, however, shows that fillers which contain volatile impurities could very well damage sintered PTFE more easily.

By means of thermography (TG), differential TG (DTG) , and differential thermal analysis (DTA), it has been shown earlier that the source of carbon can affect the decomposition and reaction characteristics of black powder compositions. Carbons with high volatile content lower the activation energy and reduce the ignition temperature of black powder. The removal of these volatiles causes an increase in ignition temperature and activation energy. It also showed that although sulfur has no effect on the ignition temperature, it does affect the pre-ignition reaction. Sulfurless black powder also has larger activation energies than regular black powder. Compositions of black powder containing channel black have lower activation energies than those with charcoal.
Dust control is another problem to be taken proper care with the help of proper air vents.

Several fillers were mixed uniformly with PTFE using the above protocol and tapes skived from sintered billets made from these mixtures passed all the four tests - density, surface smoothness, fiber strength and plastic quality.

The mixing technique developed is ready for technology transfer to the Indian industries dealing with filler grade PTFE products who at present either import or buy the filler grade PTFE at a higher price from a few proprietary companies. Keen interest has been expressed by several Indian companies in this invention.

BRIEF DISCUSSION OF THE DRAWING

Fig. 1 shows the intermolecular attractive force in PTFE polymer structure which leads to its coagulation.

Fig.2. shows the typical interwinded PTFE polymer. Due to strong intermolecular forces, the polymer chains are tangled.

Fig.3. shows the nonfree flow HIFLON-71 (HFL) or 70IN (Dupont) grade PTFE powder used in this study (left). As fluorine atoms distribute evenly around the C-C bond, PTFE is a highly non- polar molecule. It floats in water (right). Hence wet mixing PTFE with fillers in aqueous media is not feasible.

Fig.4 shows a high speed domestic mixer. Although it can be used for deagglomorating the virgin PTFE powder, it cannot be used for mixing PTFE with fillers due to heat production which reagglomorates PTFE, if operated for more than 5 to 10s.

Fig.5. shows two different types of dust collectors during mixing. If the dust collector is made of cloth, the pores in the cloth gets clogged with the fine mixture (blue pigment filled PTFE) powder coming out of the mixing instrument which prevents further air leakage (a). As a result there is a reverse flow and the dust comes out of the feeder inlet which clogs the inlet. If a plastic bag is used instead of cotton cloth, no clogging or backlash occurs as long as the excess pressure at the collector is vented out through an L-shaped PVC pipe (b). This technique was found useful for mixing different fillers with virgin PTFE.

Fig. 6 shows Bronze (40%) filled PTFE billets. The quality of the commercial billet was better than the one made in this work due to oxidation problems of the locally available bronze powder as mentioned in text.

Fig. 7. Blue Pigmented PTFE powder immediately after mixing (b) and after ageing at RT (c). The agglomeration of PTFE can be seen in (b).

Fig.8. Blue pigment, mica and bronze loaded billets and tapes. CaSO4;Dy luminescent phosphor filled PTFE discs used for radiation dosimetry are also shown. No volatile impurities were present in these fillers. Therefore, the tapes and the sliced discs exhibited the strength and plastic property of PTFE.

Fig. 9. Skieved tape from carbon (obtained from a commercial source) mixed PTFE billet. The tape was brittle and could be easily torn as shown in the picture due to the presence of volatile impurities in the carbon filler used. A better quality carbon filler which was thermally stable was used subsequently to make good quality carbon filled PTFE tape.

Fig. 10. Sintered billets of blue pigment, glass and graphite and filled PTFE billets and their skievings. The turnings and the sliced disc of graphite filled PTFE exhibited the plastic property of PTFE. The die & plunger as well as the hydraulic press used to obtain cold pressed billets before sintering is also shown.

DETAILED DISCRIPTION OF THE INVENTON

Fillers hinder the relative movement of the PTFE molecules past one another and in this way reduce creep or deformation of the parts, reduce the wear rate of parts used in dynamic applications, and reduce the coefficient of thermal expansion. Glass fiber, carbon and graphite fillers are used in corrosive services. For electrical applications, non-conductive fillers such as glass fibre, mica, MbS2 have the least effect on the electrical properties of PTFE.

As PTFE is hydrophobic, wet mixing PTFE with fillers in aqueous media is not feasible. Although mixing in nonpolar liquids such as ethanol is feasible, it is not cost-effective. Pollution from organic solvents is another problem to be met with. Cryogenic mixing is yet another method used (widely in the Department of Atomic Energy) but this is also not cost-effective. Industrial mixers such as double cone, V-type, mass mixer and Octagonal blenders are normally used for mixing two or three free flowing materials. They are not suitable for mixing PTFE with fillers since PTFE is a non-free flow material. Frigmaires (Mumbai) make Shear Plough Mixer used by Rollon Bearings Pvt Ltd (Bangalore) for mixing steel powder with PTFE works on the principle of hurling and whirling. In this machine, a shear plough rotates inside a horizontal cylindrical drum. A high speed chopper is additionally installed to disperse lumps. However, in the absence of an air jet mil, it is suited for mixing fillers such as steel powder with only non-free flow type PTFE and hence this machine was not adopted in this study. Mixing was not perfect with this machine even with this type PTFE. White spots could be seen against a blue background in blue pigmented steel mixed PTFE products.

In this invention, two compact jet-mill type mixing machines - one made of stainless steel with fixed blades having a flat tray at the top for sample feeding and the other made of cast iron with free moving heavy duty hammers having a sloped tray for sample feeding were used. The former was found to be more convenient to use since sample feeding was found to be easy and the dust generation was less since the holes in the blades reduced the air flow volume. With the cast iron type machine both the above issues posed problems. Since it is a continuous feed type, large volumes of mixing required by the industries could be easily carried out. Other mixers mostly use batch type mixing which will require refilling after each mixing. This will limit the output capacity.

Industrial dust collectors are designed based on the principle of central vacuum systems. Dust collection systems with fixed motor creates suction, which is distributed through the space via a network of rigid ducting and flexible hoses to the point of use. They are expensive. Cleaning such systems is quite difficult. Therefore different fillers will require different machines. Use and throw systems used in this invention seemed to be the best. Two mixers, one for black and other for white filler are sufficient.
Fillers in powder form (40 μm) tried in this work include synthetic graphite, two different grades of carbon and coke, glass, luminescent grade CaSO4;Dy, mica, blue pigment, and bronze, mostly procured from India. For instance, bronze (80 % Cu and 20% Sn) powder (Rs 800/kg) bought from the Metal powder company Ltd, India turned black on storage at RT in ambient conditions for more than 2 months. Even the freshly bought bronze powder discolored on mixing it with PTFE and during sintering due to oxidation problem. The quality of the bronze filled PTFE billet made from imported bronze powder was, however, found to be better than that of the billet made with freshly bought bronze powder as shown. Billets made from bronze filled PTFE powders stored for 3 yr in ambience but sealed in a polythene bag were totally black.

Specially treated synthetic graphite powder and carbon powders yielded good quality PTFE billets and the discs skieved out of it as well the turnings turned out of it were strong and possessed the plastic nature exhibited by those of unfilled PTFE. Fine grade white glass powder/fibre has to be imported! High temperature withstanding blue pigments such as cobalt aluminate are difficult to procure in India. Locally made mica powder has, however, been tested successfully.

WE CLAIM

1) We have invented a suitable instrument which works on the principle of air jet mill for mixing nonfree flow PTFE powder with various fillers.

2) We have invented a cost-effective technique to deagglomorate the PTFE powder so it is free flowing without producing heat.

3) We have invented a mixing technique which can be used for by Indian industries for manufacturing filler grade PTFE powders. Since it is a continuous feed type, large volumes of mixing required by the industries could be easily carried out.

4) We have invented a special treatment procedure which make certain fillers like carbon and
graphite suitable for mixing with PTFE.

5) We have designed a special dust collector with a proper vent which contains the dust within the collector without producing reverse flow to prevent dust pollution. It is use and throw type and cost effective.

6) We have discovered that fillers such as carbon containing a high concentration of volatile impurities degrade the sintered PTFE very easily. Though such fillers are to be avoided, this result by itself is an interesting one since the only agent known so far to damage sintered PTFE is ionizing radiation!

7) We have invented that in order to get products of higher quality, it is necessary to make the
PTFE powder more free flowing while mixing if necessary with additional lubricant. Some fillers like graphite need no lubricant. Other fillers like carbon need lubricant. Graphite serves as the best lubricant for carbon filled PTFE. For other fillers suitable lubricants would improve mixing quality.

8) The sintered products obtained from filler grade PTFE powders made with the instrument invented by us were of comparable quality as the commercial ones.

9) The technology developed by us is therefore ready for know-how transfer to industries in India. Several companies have expressed a keen interest in buying this technology.