Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
2000-03-07
2003-04-08
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C528S083000, C525S440030
Reexamination Certificate
active
06545119
ABSTRACT:
FILED OF INVENTION
The present invention relates to magnetic recording media and more particularly it relates to magnetic recording media such as magnetic tapes and magnetic disks, having high durability, high abrasion resistance and high heat resistance, and exhibiting excellent dispersibility and filling ratio of magnetic particles. The present invention further relates to thermoplastic polyurethane resins suitable for use in the magnetic recording media, which are obtained by reaction in molten state of starting materials.
BACKGROUND OF THE INVENTION
The production of general-purpose magnetic recording media such as magnetic tapes and floppy disks has been carried out by application onto a polyethylene terephthalate film as a non-magnetic support, of a magnetic coating containing magnetic particles in needle shape having a lengthwise size of 1 &mgr;m or smaller, which are dispersed, together with additives such as dispersants, lubricants, and antistatic agents, in the solution of a binder, to form a magnetic layer.
The binder in the magnetic layer should have various characteristics including high dispersibility, high filling ratio, and high degree of orientation of the magnetic particles, high durability, high abrasion resistance, and high heat resistance of the magnetic layer, and good adhesion of the magnetic layer to the non-magnetic support. The binder plays a very important role.
As the conventional binder, there have been used vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinylidene chloride copolymers, polyurethane resins, polyester resins, acrylonitrile-butadiene copolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins, and acrylic resins.
In particular, the polyurethane resins have excellent toughness and excellent abrasion resistance because of their intermolecular hydrogen bonds through urethane bonds as compared with the other resins. It has been well known that the introduction of sulfonic acid metal salt groups into the polyurethane molecules is an effective means of improving the dispersibility of magnetic particles (see JP-A 54-159603).
In recent years, there have been great demands for lowering the price of materials for the production of magnetic tapes or magnetic disks to reduce the cost of the production of general-purpose video tapes or general-purpose floppy disks for public use. In particular, the general-purpose video tapes for public use have a tendency to have thinner magnetic layers to reduce the cost of production. The magnetic tapes having thinner magnetic layers may have their decreased blackness, making it difficult to recognize and detect the leading tape portions at both ends of these magnetic tapes by photosensors provided in tape recorders, and thereby making it impossible to operate auto-stop and other functions.
For this reason, to improve the blackness of magnetic layers made thinner, the amount of carbon black particles to be added has been increased. As disclosed in JP-A 63-103418 and JP-A 59-84339, carbon black particles have been blended so far at ratios of about 3% by weight or lower, based on the weight of magnetic particles. In recent years, however, because of the above problems, it is usual to blend carbon black particles at ratios of at least 5% by weight.
In the system where the amount of carbon black particles blended in the magnetic coating is increased, the viscosity of the magnetic coating may often be increased, making it difficult to disperse magnetic particles. When such a magnetic coating is used to produce magnetic tapes, various problems will occur. For example, the resultant magnetic tapes cannot have satisfactory magnetic characteristics because of the insufficient dispersibility of magnetic particles; the magnetic coating, if it has a remarkably increased viscosity, cannot be applied; the application of the magnetic coating requires prolonged time because of its increased viscosity; and the speed on the application line cannot be raised, thereby lowering the productivity of magnetic tapes.
Even when the amount of carbon black particles to be blended in the magnetic coating is increased, there have been demands for binder resins capable of keeping the sufficient dispersibility of magnetic particles and the low viscosity of magnetic coatings. The conventional binder resins cannot satisfactorily meet these requirements. An object of the present invention is to provide magnetic recording media exhibiting excellent magnetic characteristics by the satisfactory dispersibility of magnetic particles and the low viscosity of magnetic coatings, both of which can be kept even when carbon black particles are blended at higher amounts, ie., at least 5% by weight, based on the weight of magnetic particles.
To meet great demands for lower priced materials of magnetic tapes and magnetic disks, the main type of binder resin as one of the materials is turning from the conventional solution type to the solid type, which is more inexpensive because of its low costs of production and transportation, and typical examples are disclosed in JP-B 58-8053, JP-B 61-231050, and JP-B 3-13648. For further lowering costs and improving performance, magnetic layers have become thinner than ever, so that there arises a need to increase the amount of carbon black particles to be blended in the magnetic layer for decreasing light transmittance, which causes some problems including the lowering of dispersibility and the increased viscosity of magnetic coatings.
The thermoplastic polyurethane resins obtained by reaction in molten state of starting materials are prepared by feeding polyester diols in molten state at a constant flow rate to an extruder and reacting them with the corresponding amounts of isocyanate compounds in molten state. For the quantitative transfer of polyester diols in molten state, they are therefore preferred to have lower melt viscosity. The use of polyester diols having high melt viscosity makes it difficult to feed them at a constant flow rate, so that polyurethane resins of stable quality cannot be obtained.
The polyester diols composed mainly of aromatic dibasic acids are quite useful materials for the design of binder resins having high glass transition temperatures, which are essential to conferring durability on magnetic recording media. It has, however, been difficult to use these polyester diols as the starting materials of solid-type polyurethane resins because of their high melt viscosity.
For increasing the dispersibility of magnetic particles, it is preferred to introduce into polyester diols, polar groups such as sulfonic acid groups or phosphonic acid groups. These polar group-containing polyester diols may, however, have further high melt viscosity.
This results in, for example, broader molecular weight distribution, making it difficult to obtain polyurethane resins of stable quality. Almost all of the polyester diols having low melt viscosity as single components are polyester diols composed mainly of aliphatic dibasic acids.
Therefore, the conventional design of solid-type polyurethane resins having high glass temperatures involves the use of polyester diols composed mainly of aliphatic dibases taking a means of increasing the concentration of urethane bonding groups with aromatic diisocyanate compounds.
However, an increase in the urethane bonding group concentration lowers the solubility of polyurethane resins in general-purpose solvents, and when the polyurethane resins are used in the binders for magnetic recording media, the dispersibility of magnetic particles becomes deteriorated. Thus, there have been demands for solid-type polyurethane resins having high glass transition temperatures and exhibiting high dispersing performance on magnetic particles.
There is another method in which polyester diols containing aromatic dicarboxylic acids and therefore having high melt viscosity are blended with low molecular weight diols or other materials to decrease the melt viscosity; however, when sublimable low molecular diols in solid for
Doi Kuniyuki
Kizumoto Hirotoshi
Sugitawa Chikara
Yatsuka Takeshi
Gorr Rachel
Morrison & Foerster / LLP
Toyo Boseki Kabushiki Kaisha
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