Compositions: coating or plastic – Coating or plastic compositions – Wax containing
Patent
1997-03-21
1998-09-01
Brunsman, David
Compositions: coating or plastic
Coating or plastic compositions
Wax containing
106271, 10628735, 106499, 106502, C09D19106, C09D19108, C09D 503, C09D 546
Patent
active
058006052
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This a 371 of PCT/US95/01372 filed 13 Jun. 1995.
The present invention relates to a process for the preparation of electrostatically charged particles and, in particular, to the preparation of electrostatically pre-charged powders of high electrical resistivity such as powder paint, household cleaning particles and charged particles for biomedical applications.
BACKGROUND OF THE INVENTION
Electrostatic coating techniques are increasingly Used in commercial applications where durable, high quality, finishes are required. Electrostatic coating techniques involve the application, usually by spraying, of electrostatically charged particles, which preferably are pigmented, onto a substrate, generally but not exclusively a metal substrate, followed by baking to form a uniform and adherent coating. The electrical charge of the pigment-containing paint particles causes the attraction of the particles onto the substrate and holds them in place until the heat treatment is applied. Electrostatic coating techniques are used almost exclusively in the domestic white-goods market, and increasingly in the automobile industry. Less well known areas of application include the furnishing industry and bottle coating applications.
There are two known processes for electro-statically charging paint particles which are by corona charging or by tribo or frictional effects. Both of these techniques are known to be inefficient and extremely unpredictable. The important process of charging the powder is the responsibility of the end user and is usually accomplished by means of commercially available equipment.
The corona charging process relies on the creation of a high charge density unipolar ionic cloud, the ions of which subsequently attach onto the outer surface of discreet particles which are dispersed into an airborne cloud. Typically, only about 0.5% of all the charged ions become attached to the airborne particles, the remaining 95.5% remaining as free ions which can be extremely detrimental in coating applications to the quality of the coating.
Tribo or frictional charging of particles is unpredictable and the charging process is sensitive to a number of parameters including the nature of the materials used, relative humidity, duration of contact, relative permittivity, surface conditions, the type of contact and possibly electrical resistivity.
Accordingly, the prior art processes suffer from the disadvantage that the charging of the particles relies upon surface mechanism and the charging process is generally under the control of the end user who will have very little control over the level of charge deposited onto individual particles.
We have now developed a more reproducible process for charging powder particles which overcomes the disadvantages of the prior art.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for the preparation of electrostatically charged particles of a high resistivity material which process comprises incorporating a unipolar charge into the material at a temperature at or above the glass transition temperature thereof or above the melting point thereof, the said unipolar charge being incorporated into the bulk of the material and the charged material being subsequently comminuted, or the said charge being incorporated into the material whilst forming particles thereof.
DETAILED DISCLOSURE
As is known by those skilled in the art the term glass transition temperature is used to define the temperature at which an amorphous material, such as a polymer or glass, changes from a brittle state to a plastic state.
The process of the present invention may be effected either by the incorporation of the unipolar charge into the material by the injection of unipolar charge or by the induction of unipolar charge whilst the material is at or above its glass transition temperature or above its melting point. The unipolar charge may be incorporated into the bulk of the material and particles thereof subsequently formed by comminution, for example by g
Brunsman David
University of Southampton
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