Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Patent
1994-01-07
1996-01-30
Resan, Stevan A.
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
428694B, 428900, 252 6254, G11B 500
Patent
active
054879515
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to magnetic recording media and in particular to the use of star block copolymers of defined formula as binders for such media.
BACKGROUND ART
Magnetic recording is the preferred method of information storage when data must be capable of being read immediately after writing or where the data is to be processed by a machine. The recording medium usually consists of fine, magnetizable particles which are dispersed in a polymeric binder and coated onto a support. In order to work efficiently, magnetic media must possess certain physical and magnetic properties, e.g., the medium surface must be sufficiently smooth to enable accurate reading of the recorded signal by the magnetic head. It must also be free from asperities and other surface roughness that can lower the signal to noise ratio. In addition, the medium must be durable, so that it is possible to record and reproduce information repeatedly. To be durable, the magnetic particles must be firmly bound to the support and not be worn off by the passing of the magnetic head over the media. This durability must persist under all environmental conditions under which the medium is to be used. In addition, it is important that the media have sufficient lubricity so that the magnetic head passes freely over the surface with the minimum coefficient of friction. Furthermore, the medium should have uniform physical properties over a wide temperature range.
Magnetic recording media are normally produced by passing a non-magnetic support through an apparatus which coats the support with a liquid dispersion of the magnetic medium. This dispersion consists of a binder, in either an uncured or solvated state, having the magnetizable particles homogeneously dispersed therein. After coating, the dispersion dries or cures to give a tough binder film having the magnetizable particles uniformly distributed throughout. The desired physical and magnetic properties of the recording layer are often dictated by certain processing parameters. One important parameter is good pigment wetting in the bulk liquid dispersion. Good pigment wetting is closely related to dispersion stability and is essential to ensure an even distribution of pigment throughout the finished magnetic coating.
Another important processing parameter is the so-called `pot-life` of the dispersion, the time for which the dispersion can be kept at a sufficiently low-viscosity before coating. Most conventional, magnetic media rely on a chemical reaction between a polyfunctional isocyanate crosslinking agent and hydroxy functionality on the binder material to cure the coating and so toughen it. The isocyanate is generally added to the dispersion prior to coating (known as an activation step) and consequently the dispersion has a finite pot-life. if the cure reaction is too fast, the resultant short pot life creates time constraints on the coating process and can make it difficult to obtain sufficiently smooth coatings. If the cure reaction is too slow, the magnetic coating will have poor green strength until the cure reaction has progressed sufficiently. As a result, the magnetic coating will be susceptible to damage during subsequent processing steps unless an inconvenient and expensive time delay is built into the manufacturing process. Furthermore, the need for an activation step (or post-coating cure step as in e-beam crosslinked systems) complicates and thus increases the cost of media production.
The bulk of the binder materials used in the preparation of conventional media are of relatively low molecular weight. Consequently, a cure reaction is essential to produce media having the appropriate mechanical properties. A further problem inherent in this approach arises from the very presence of these low molecular weight species. If the cure reaction, for any reason, is incomplete, then low molecular weight species can remain in the coated media following curing. Such species can plasticize the media leading to poor media durability. The low molecular weight species may
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Evans Christopher M.
Norman Colin F. W.
Griswold Gary L.
Kagan David B.
Kirn Walter N.
Minnesota Mining and Manufacturing Company
Resan Stevan A.
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