Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2001-08-02
2002-06-25
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
C427S487000, C427S521000
Reexamination Certificate
active
06411319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal head that is modified on the surface thereof to be kept low in surface tension without inhibiting thermal conduction, and particularly relates to a thermal head that maintains excellent perforation property for a long period of time when used for perforating heat sensitive stencil sheets.
1. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As one of conventional methods of perforating heat sensitive stencil sheets, known is a stencil making method using a thermal head which is, in general, also called thermal printing head. In this method, the thermoplastic resin film face of a heat sensitive stencil sheet is brought into contact with a thermal head, for melting and perforating the thermoplastic resin film in an area corresponding to an image area of an original, by mean of heat of the thermal head.
However, if this method is used to continuously make stencils, there is a problem that the melt of the film is deposited on the surface of the thermal head to gradually degrade thermal perforation property of the thermal head.
In general, thermal heads can be structurally classified into thin film type, thick film type, semiconductor type, etc. The thin film type thermal head generally has, as shown in
FIG. 1
, a layered structure consisting of an insulating substrate
1
, heat-generating resistor
2
formed on the insulating substrate
1
, an electroconductive layer
3
connected with the heat-generating resistor
2
for supplying electric power to it, and a protective layer
4
covering the heat-generating resistor
2
and the electroconductive layer
3
. The thick film type thermal head generally has, as shown in
FIG. 2
, a similar layered structure consisting of an insulating substrate
1
, a heat-generating resistor
2
and an electroconductive layer
3
formed on the insulating substrate
1
, and a protective layer
4
covering the electroconductive layer
3
and the hear-generating resistor
2
. Therefore, the surface of a thermal head generally means the surface of the protective layer
4
.
As the material of the protective layer
4
, an inorganic material having relatively good thermal conductivity such as Ta
2
O
5
, SiO
2
, SiON or Si
3
N
3
is used. However, since these inorganic materials have high surface free energy, they have high surface tension, and thus have such a nature that the melt of the film is likely to be deposited on the surface of the thermal head.
To solve the above problem, it is proposed to coat the film surface of a heat sensitive stencil sheet with a releasing agent (JP-A-61-170392) or to let a heat sensitive stencil sheet contain a releasing agent in the porous substrate or adhesive layer thereof (JP-A-2-255384). However, since these methods have a releasing agent applied to a heat sensitive stencil sheet, the have such disadvantages that the stencil sheet production process is complicated to raise production cost and that uniform performance is difficult to obtain.
To overcome these disadvantages, it is proposed to further form a water-repellent, oil-repellent and heat-resistant resin layer on the surface of the thermal head, i.e., the protective layer
4
, for preventing the deposition of the melt of the film onto the surface (see JP-Y-4-7967, JP-A-60-2382, JP-A-60-178068, JP-A-62-48569, etc.). The resin layer is typically made of a fluorine resin such as Teflon (trade name of Du Pont: polytetrafluoroethylene). For coating the surface of a thermal ahead with such a fluorine resin, it is usually necessary to prepare a dispersion containing 50 to 60% solid polytetrafluoroethylene, to coat the surface of a thermal head with the dispersion, to preliminary dry and to heat up to about 350° C.
The fluorine resin layer is excellent in making the surface of a thermal head lower in surface tension, but the treatment process (heating process) thermally loads the electronic parts associated with the thermal head. So, the method cannot be said to be a simple and proper treatment method. Furthermore, the fluorine resin has such a problem that bonding strength to vitreous materials such as the protective layer is not sufficient.
Moreover, since the above resin layer is a coating layer of resin, even if thin coating is made, the thickness becomes about 1 &mgr;m, to inhibit the efficient thermal conduction from the heat-generating resistor to the surface. There is also a limit in making the thickness of the resin layer uniform for enhancing the surface smoothness, and the actually obtained thickness and surface roughness are on the order of microns.
Above all, in the case where such a thermal head is used to process heat sensitive stencil sheets into stencils, there is a problem that the roughness of the resin layer formed on the surface of the thermal head inhibits close contact between the thermal head and the heat sensitive stencil sheet, thereby lowering the thermal conductivity. As a result, uniform perforation of the heat sensitive stencil sheet cannot be ensured.
Furthermore, as other methods for making the surface of a thermal head lower in surface tension, proposed are a technique comprising the step of coating the surface of the protective layer with a fluoroalkyl group-containing silane compound for forming a water-repellent, oil-repellent film, and a technique comprising the steps of pre-treating the protective layer using, for example, silicon oxide for forming an undercoating layer and forming said water-repellent, oil-repellent film on the undercoating layer, to make a two-layer structure, in order to improve the bonding strength between the water-repellent, oil-repellent film and the protective layer (Japanese Patent Application No. 2000-30694). The former method is a very simple and advantageous method for making the protective layer lower in surface tension without inhibiting the thermal conductivity since the obtained water-repellent, oil-repellent film is a uniform film of molecular level by virtue of properties of the fluoroalkyl group-containing silane compound. However, the method may be insufficient in performance in applications that require film durability such as scratch resistance. On the other hand, the latter method has a disadvantage that production cost is raised since the work basically consisting of two steps complicates the thermal head production process, though it can be expected that durability will be higher compared with the former method.
The object of this invention is to overcome the problems of the above-mentioned prior art, that is, to lower the surface tension of the protective layer by a simple method for preventing the deposition of the melt on the thermal head for a lone time while maintaining the thermal conductivity from the heat-generating resistor to the surface of the thermal head and the smoothness of the protective layer.
BRIEF SUMMARY OF THE INVENTION
According to this invention, the above object can be achieved by a thermal head which comprises an insulating substrate, a heat-generating resistor formed on the insulating substrate, an electroconductive layer connected with the heat-generating resistor for supplying electric power to it, and a protective layer formed on the heat-generating resistor and the electroconductive layer, wherein said protective layer is treated on the surface thereof with a dry film of a surface-treating agent containing a chlorosilyl group-containing compound and a fluoroalkyl group-containing silane compound.
The surface-treating agent can be produced, for example, by a method of dissolving a chlorosilyl group-containing compound and a fluoroalkyl group-containing silane compound into an organic solvent. Then, the surface-treating agent can be coated on the surface of the protective layer of the thermal head and dried, to form a water-repellent, oil-repellent film on the surface.
Thus, according to another aspect of this invention, there is provided a surface-treating agent containing a chlorosilyl group-containing compound and a fluoroalkyl group-cont
Nakao Terutoshi
Sugaya Kengo
Riso Kagaku Corporation
Tran Huan
LandOfFree
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