Conductive materials

Details Conductive materials Conductive materials

2001-06-01

2004-11-09

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Elemental carbon containing

Reexamination Certificate

active

06814889

ABSTRACT:

This invention relates to conductive materials and is particularly concerned with conductive materials to serve the purpose of a heating means.
It has long been known that materials that are electrically conductive can generate heat. This phenomenon has spawned a considerable number of heating means for a considerable number of different purposes.
Whilst electrical heaters at large are generally successful, and more than capable of meeting their intended purpose, what has proved to be difficult, is the achieving of a uniform heating effect over a relatively wide area, substantially free from hot and cold spots or areas.
There have been attempts hitherto to create electrical heaters in sheet or web form to serve a wide variety of purposes. However, the most usable form of conductive material is in the form of carbon particles or carbon black embedded in or coated on a carrier or substrate. To attempt to achieve a uniform heating effect, a greater density or concentration of carbon particles is employed, but as a direct result of that, the material becomes less pliable and more brittle, to the detriment of the employment of the material over a wide range of potential uses.
The first object of the present invention is to provide a start or batch material able to be used in a variety of different physical forms to suit a wide variety of electrical conduction heaters.
Second and subsequent objects of the invention are to transform the start or batch material into products that are electrical conductive heaters for a wide variety of different purposes.
According to a first aspect of the present invention, a conductive material comprises finely divided carbon particles uniformly dispersed in an elastomeric carrier there being carbon particle levels of 20% by weight to 75% by dry weight to 80% to 25% by dry weight of carrier levels.
Carbon particle levels of 20% to 40% by dry weight to 80% to 60% by dry weight of carrier levels can be used.
The carbon particles may be so-called carbon black, a finely divided powder, and the elastomeric carrier is an elastomeric polymer, and according to a further feature of the invention, a semi-conductive film or coating comprises a carbon filled elastomeric polymer, preferably with carbon particle levels of 43% to 73% by dry weight to 57% to 27% by dry weight polymer levels, preferably 60% by dry weight to 65% by dry weight. Further preferably, the carbon particle level is 57% by dry weight.
The elastomeric polymer may be an aliphatic polyurethane in solution, and desirably the carbon particles are mixed without milling prior to the addition thereto of the polyurethane solution.
If required in particular applications, flame retardant materials may be provided.
According to a further aspect of the invention a method of forming a compound for an electrically conductive heater comprises stirring fine carbon particles into a polymer base containing an anti-adsorption compound, to achieve carbon particles to polymer levels of 20% by dry weight to 75% by dry weight to 80% to 25% by dry weight of polymer, and subjecting mixture to high speed stirring for a pre-determined period of time, with the maintenance of the mixture below a predetermined level, to grind the carbon particles to a predetermined final fineness. The predetermined level may be at not more than 25° C.
The required carbon particle level may be 20% to 40% by dry weight to polymer level but preferably carbon levels are from 43% to 73% by dry weight to polymer, and more particularly 57% by dry weight.
The resultant mixture exhibits thixotropic rheological properties, and if required, the viscosity of the mixture can be reduced, by the addition of a suitable solvent such as dimethylformamide.
Preferably the carbon black particles have particulate size of approximately 30 En nm. Suitable adsorbants may be selected from the group containing polypropylene glycols, and polyethylene glycols of a required molecular weight. After this process is complete a suitable polymer solution eg an aliphatic polyurethane is added to the master batch such that the ratio of polymer to carbon black is 1:0.57 on a dry basis. The finished compound is then refiltered prior to any coating process.
The duration of the slow stirring-in of carbon black is not critical, but desirably high speed stirring is limited to not more than 30 minutes.
Once the mixture is prepared, it is checked by taking a sample and passing it through a 200 mesh filter, preferably with the aid of a low pressure displacement pump, and checked by using a Heckman gauge, to ensure that there has been no agglomeration of the carbon particles during mixing. If any agglomeration is detected, the mixture should be subjected to further high speed stirring.
Preferably, the prepared start or batch material is subjected to a final filtration step by passing the mixture through a 300 mesh filter cloth, for example by way of a slow, low pressure positive displacement pump, at which stage there should be no residue left on the filter cloth that would signal that there was still a degree of remnant agglomeration of carbon particles.
In the form where the start or batch material employs dimethylformamide as the solvent and polyethylene or propylene glycol as the polymer base, it constitutes an ideal material to serve as a coating or a base or carrier material.
According to a further aspect of the invention a web or sheet is formed by applying a quantity of finished compound as discussed above to a release paper by way of transfer coating, to achieve a uniform coating or film of compound between 90 and 100 grams per square meter dry weight, and subjecting the web or sheer to heat progressively rising from 110° C. to 150° C. to achieve the controlled release of solvents and provide a coating or film free of pinholes.
As the electrical conductivity, and hence the heating effect achievable is a function of coating or film thickness the above process is repeated until a desired thickness of coating or film is created.
At this stage considerable care must be exercised to ensure that reticulation is avoided, and as it is preferred to spread the finished compound on the release paper by employing a doctor blade, equally considerable care exercised to ensure the avoidance of the presence of dirt or grit on the blade edge, to prevent the creation and spread of lines of indentations in the coating or film.
Desirably, the release paper is matt grade and is an unembossed silicone-coated paper.
According to a still further aspect of the invention, a flexible fabric able to serve the purpose of an electrical conductive heater is formed by taking the release paper mentioned above with its coating of finished compound spreading thereon a further quantity of said compound, laying the release paper on a flexible fabric carrier sheet or web, and passing the composite through a fixed gap roller to ensure controlled penetration of said compound into the fabric of the sheet or web, the sheet or web thereafter being subjected to heat progressively rising from 110° C. to 150° C. to achieve controlled release of solvents and provide a coating of film free of pinholes. Also possible is the direct application of a coating of finished compound directly on to a fabric carrier.
Such a sheet or web can be of any desired length, and of any width with the limits of available fixed gap rolls.
By the nature of the coat or film, there is a substantially totally even spread of carbon black at loadings in the polymer material considerably beyond that which has hitherto been believed to be possible whilst at the same time leaving the fabric with its film or coat totally flexible.
The fabric may be a knitted cotton material but can be of any other suitable form, such as a weft knitted polyvinyl alcohol fabric.
The preparation of the said compound and the manner of its application to a carrier is such that a visually smooth coating or film is provided. However, it remains so that the coating will exhibit a microscopic degree of roughness with peaks and troughs formed by carbon particles at the exposed su

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