Sealing agent for plastic liquid crystal display panels

A sealing agent for plastic liquid crystal display panels which comprises the following three effective components: PA1 (a) Nylon and/or a linear saturated polyester; PA1 (b) an epoxy resin and/or an isocyanate compound; and PA1 (c) a solvent which can dissolve the components (a) and (b), and a transparent plastic panel substrate. The sealing agent makes it possible to use a transparent plastic film as a substrate for a liquid crystal display panel, thereby enabling inexpensive mass-production of high quality liquid crystal display panels.

TECHNICAL FIELD 
This invention relates to a sealing agent for plastic panels. 
BACKGROUND ART 
A liquid crystal display panel has heretofore been manufactured by applying 
an inorganic or organic sealing agent in the form of a liquid or film to a 
sealing area on a sheet of glass defining a substrate for the panel, 
placing another substrate thereon, and heating the whole to solidify the 
sealing agent. It has, however, been necessary to inject a liquid crystal 
through a hole in the substrate after solidification of the sealing agent 
and close the hole, because the sealing agent in the form of a liquid 
flows into the liquid crystal layer, and the heat applied for 
solidification of the sealing agent causes degradation or decomposition of 
the liquid crystal if the liquid crystal is applied beforehand. Therefore, 
liquid crystal display panels have required a complicated process for 
their manufacture, and have never been expected to lend themselves to 
continuous mass-production. 
Although it appears possible to employ a transparent plastic film for a 
substrate for a liquid crystal panel instead of inorganic glass, there has 
hitherto been no example in which a transparent plastic film is used for 
the substrate of a field-effect display panel employing a nematic liquid 
crystal, because such a substrate is required to have a high level of 
electrical, optical and chemical properties. However, a transparent 
plastic film having properties which make it suitable for the substrate of 
a liquid crystal panel has recently become available. This invention is 
based on the results of the research for the sealing agent which is 
applicable for the manufacture of a liquid crystal panel using such a 
transparent plastic film. 
It is a principal object of this invention to provide a liquid crystal 
panel having no hole for liquid crystal injection inexpensively by way of 
mass production using an organic panel substrate. It is another object of 
this invention to provide liquid crystal panels having a constantly high 
quality inexpensively by way of mass production. Other objects of this 
invention are believed to be easily understood from the following 
description, and not mentioned herein. 
DISCLOSURE OF THE INVENTION 
Thus, this invention resides in a sealing agent for plastic liquid crystal 
panels which comprises the following three effective components (a), (b) 
and (c): 
(a) Nylon, or a linear saturated polyester; 
(b) an epoxy resin and/or an isocyanate compound; and 
(c) a solvent which is capable of dissolving the above-mentioned components 
(a) and (b), and a transparent plastic substrate for the panel which is 
formed from a polyester or polyether sulfone. 
BEST MODE OF CARRYING OUT THE INVENTION 
A plastic film intended for use as a substrate for a liquid crystal display 
panel is required to have excellent physical and chemical properties, such 
as heat resistance, chemical resistance, adhesion of a conductive film, 
electric resistance, visible light transmission, flatness, film strength 
and optical activity. A transparent plastic sheet having nearly all of 
those properties has recently been developed, and efforts are still being 
made for improving it to a further extent. Typical examples thereof 
include a polyester film, such as polyethylene terephthalate (PET) and 
polybutylene terephthalate, a polyether sulfone film, or a polycarbonate 
film, on which a transparent conductive film formed from indium oxide is 
provided. When such a film is used for preparing a panel substrate, it is 
usual to subject the film surface to orientation treatment such as by 
angular deposition of SiO.sub.2 in order to improve the effect of a liquid 
crystal. 
When a liquid crystal display panel is prepared from such material, a known 
adhesive for PET is unsatisfactory because of its low sealing strength, 
gas barrier properties, heat resistance and solvent resistance. A 
thermosetting polyester resin in the form of a powder which has recently 
become known is not acceptable, since its curing temperature is higher 
than the temperature at which PET is deformed. A silicone sealing agent 
has to be solidified before a liquid crystal is applied; therefore, it 
requires a substrate having a liquid crystal injection, and is not 
suitable for low gas barrier and sealing a panel having no such hole. When 
a panel substrate is prepared from the abovementioned plastic film such as 
a PET or polyether sulfone film, there is required a sealing agent: 
(1) having high adhesion strength to the plastic film; 
(2) which is curable at a temperature lower than that at which the plastic 
film is thermally deformed (i.e., below about 150.degree. C.); 
(3) capable of forming a film by preliminary drying, and sealing by hot 
pressing at a temperature equal at least to its melting point, but lower 
than the temperature at which its thermal deformation takes place, without 
flowing into a liquid crystal layer, or causing any change therein (which 
is an essential requisite for sealing a panel having no hole for liquid 
crystal injection); and 
(4) which can form a seal having high chemical and solvent resistances. 
The current status of the art of sealing involves the use of a hot-melt 
sealing agent, or a curable sealing agent in the form of a liquid. A 
hot-melt sealing agent has, however, failed to provide a practically 
acceptable sealing strength for a plastic panel substrate, since the 
substrate surface is relatively inert as opposed to a metal or inorganic 
glass surface. 
A lot of time and labor have been required for forming the sealing agent 
into a film having the shape of a seal, or melting it under heat for 
application, thereby preventing any reduction in the cost of panel 
manufacture, though no hole for liquid crystal injection is required in 
the substrate. 
Although a liquid curable sealing agent has provided a fairly satisfactory 
sealing strength, it has been necessary to employ a substrate having a 
hole for injection of a liquid crystal or the like after solidification of 
the sealing agent in order to prevent any sealing agent from flowing into 
the liquid crystal or the like before solidification, or causing any 
degradation thereof when heated for curing. 
Moreover, no sealing can be achieved between the panels by any known liquid 
curable sealing agent after its solidification, because it loses its 
sealing power upon solidification. 
The sealing agent of this invention, which incorporates the advantages of 
both of the known hot-melt and liquid curable sealing agents, and 
eliminates their disadvantages, overcomes the necessity of making any hole 
for liquid crystal injection, and provides a sufficiently high sealing 
strength. More specifically, the sealing agent of this invention comprises 
thermoplastic linear polymer molecules of the type used in a hot-melt 
sealing agent, reactive polymer molecules of the type mainly used in a 
liquid and/or solid curable sealing agent, and capable of crosslinking 
therewith, and a solvent which can dissolve these two types of polymer 
molecules, and a panel substrate. The sealing agent containing at least 
these three components can be applied to a plastic panel substrate, dried 
for preliminary solidification, and then, heated for penetration into the 
substrate and completing crosslinking. The sealing agent of this invention 
having the aforementioned properties has been derived primarily from the 
results of a lot of experiments, though some theoretical background has 
been required for its recongnition. 
While all of the polyamide resins, such as 6-nylon, 6,6-nylon and 12-nylon, 
can be used for the component (a), it is preferable to use a resin having 
a melting point Tm which is higher than 60.degree. C., but lower than 
150.degree. C. Examples of the nylon resins having such a melting point 
range include a copolymer of 6-, 6,6- and 6,10-nylons, a copolymer of 
6-nylon, 6,6-nylon, bis-(4-aminocyclohexyl)methane-6 and 11-nylon, and 
N-alkoxymethylnylon. 
If a linear saturated polyester is selected for the component (a), it is 
possible to use a saturated polyester synthesized from a saturated 
polycarboxylic acid such as terephthalic acid, isophthalic acid, phthalic 
acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid and 
trimellitic acid anhydride, a saturated polyhydric alcohol such as 
ethylene glycol, propylene glycol and neopentyl glycol, and a monomer such 
as styrene monomer, methyl methacrylate and diallyl phthalate, along with 
various kinds of catalysts and promotors. A saturated polyester resin 
(including a saturated copolymer polyester) synthesized mainly from 
terephthalic acid is particularly superior in heat resistance, impact 
resistance and flexing resistance. It is preferable to use a saturated 
polyester having a melting point Tm which is higher than 60.degree. C., 
but lower than 150.degree. C., and which can be dissolved in an industrial 
organic solvent. The nylon and the linear saturated polyester may be used 
either alone, or together. 
For the component (b) epoxy resin to be mixed with the component (a), it is 
desirable to use a liquid one having a relatively low molecular weight of, 
say, 130 to 1,260, preferably 300 to 400, and which is capable of 
crosslinking with nylon under heat. An epoxy resin having a higher 
molecular weight is undesirable, since it is difficult to mix with nylon, 
and forms a brittle product when cured. The isocyanate compound mentioned 
as an alternative of the component (b) means a resin composed solely of a 
compound containing an isocyanate, or a mixture thereof with a substance 
containing a hydroxyl, amino or carboxyl group, and which is easily 
reactive with an isocyanate. It is characterized by the high polarity and 
reactivity of the isocyanate group. Examples of the isocyanate compounds 
include triphenylmethane triisocyanate (known under the trade name of 
Desmodur R), tris(4-phenylisocyanate)thiophosphate (known under the trade 
name of Desmodur RF), a compound known under the trade name of Coronate L 
or Desmodur L, a TDI dimer (known under the trade name of Desmodur TT), a 
TDI trimer (known under the trade name of Desmodur IL), 
2,4,4'-diphenylether triisocyanate (known under the trade name of Hylen 
DM), and MDI (known under the trade name of Coronate AP). The epoxy resin 
and the isocyanate compound may be used either alone or in combination. 
Further, a curing accelerator, e.g., tetraorthotitanates, chelated 
titanium compounds etc. may be added. 
The solvent (c) which can dissolve the components (a) and (b), and a 
transparent plastic panel substrate formed from a polyester or polyether 
sulfone may be appropriately selected from among individual solvents such 
as haloalcohols, e.g., 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), methyl 
acetate, ethyl acetate, trichloroethylene, methylene chloride, methyl 
ethyl ketone, Cellosolve acetate, dioxane, tetrahydrofuran, chloroform, 
2-nitropropane, nitromethane, phenol, m-cresol etc., or mixed solvents 
such as toluene/ethanol, toluene/acetone, solvent naphtha/acetone, carbon 
tetrachloride/acetone, n-butyl acetate/toluene, and DOP/acetone. 
The suitable ratio in which the components (a) and (b) are mixed is in the 
range of (a) 2.5 to 3/(b) 1. With an increased deviation of the mixing 
ratio from this range, the reactivity of these components is reduced, and 
the quality of the cured product thereof is lowered. It is desirable that 
the proportion of the components (a) and (b) relative to the component (c) 
be less than about 30% by weight of the component (c) which is an upper 
limit to the quantity of the resinous components which can be completely 
dissolved in the component (c) solvent, and more preferably, it is in the 
range of about 10 to 20% by weight in order to provide a sealing agent 
having a suitable viscosity for application to the substrate. If the 
components (a) and (b) have a total concentration which is lower than 10%, 
the low viscosity and easy flowability of the resulting sealing agent 
makes it difficult to apply the agent to a sealing area appropriately. 
When a solution of a mixture of the components (a) and (b) in the component 
(c) is applied to a polyester or polyether sulfone film, the component (c) 
dissolves the film surface, too, and causes the resinous components in the 
sealing agent to penetrate into the molten area in the film. 
According to this invention, this phenomenon is utilized for sealing a 
liquid crystal display panel. The sealing agent is applied to the opposite 
areas in a pair of panel substrates, and after the solvent is volatilized, 
the substrates are put together, and heated for fusion, so that 
crosslinking may take place. When the sealing agent is cured, molecules 
are united with each other in the boundary areas between the sealing agent 
and the surfaces of the panel substrates, and produce a very high adhesive 
power. 
In accordance with this invention, an appropriately prepared sealing agent 
is applied to the opposite sealing areas in upper and lower panels on 
which a predetermined wiring pattern is provided, and which has been 
suitably treated with an orientation agent if required. The application of 
the sealing agent may be performed on an industrial basis by any suitable 
means such as a transfer or screen printing system employing a relief 
plate having a convex configuration which is complementary to the sealing 
areas, while experimentally, a syringe having a needle can be used for 
extrusion application of a solution of the sealing agent. 
The upper and lower panels to which the sealing agent has been applied are, 
then, dried at 40.degree. C. to 80.degree. C. for 30 to 60 minutes. After 
the sealing agent has been dried, a liquid crystal is dropped onto one of 
the panels inwardly of its sealing area, and then, those areas of the 
upper and lower panels to which the sealing agent has been applied are 
placed against each other, and pressed together under heat. For such hot 
pressing, the panels are heated to a temperature of, say, 120.degree. C. 
to 150.degree. C., and a light pressure which may be as low as is required 
merely for holding the sealing areas together is applied to the panels. 
Hot pressing may be completed in two to three seconds. Thus, the upper and 
lower panels form a completely sealed liquid crystal display panel 
enclosing a liquid crystal therein. The resulting panel has a sealing 
strength of 500 to 1,000 g/25 cm. If the sealed portion is further heated 
at 120.degree. C. to 150.degree. C. for 10 to 30 minutes, its sealing 
strength is improved to the extent that any attempt to break the seal will 
result in destruction of the substrate film. 
Alternatively, it is possible to provide a seal for such a panel if a 
solution of the sealing agent is applied only to one of the panel 
substrates, and after the sealing agent has been dried and solidified, the 
other substrate is placed thereon, followed by the hot pressing of the 
whole. While this method advantageously reduces the consumption of the 
sealing agent, the panel thus obtained shows a lower sealing strength 
after hot pressing, and even if the sealed portion is heated again after 
hot pressing. The liquid crystal to be enclosed within the panel may be 
appropriately selected from among, for example, a nematic liquid crystal 
such as a biphenyl, phenyl benzoate, azoxy or Schiff's liquid crystal, a 
cholesteric liquid crystal, and a smectic liquid crystal.

EXAMPLE 1 
An upper and a lower panel is prepared by depositing SiO.sub.2 angularly 
for horizontal orientation on a plastic panel comprising a polyester 
(polyethylene terephthalate) film carrying thereon a transparent 
conductive film of indium oxide containing a small amount of tin. An 
epoxynylon solution of the following composition is applied by a syringe 
to each of the horizontally orientated upper and lower panels in a shape 
defining a sealing area: 
Nylon (having a peak melting point of 123.degree. C.; 
Daisel Chemical Industry Ltd.'s product known under the trade name "Daiamid 
T-250")--15 g 
Epoxy resin (Shell's Epikote 815)--5 g 
HFIP--50 ml 
and the sealing agent is dried at 65.degree. C. for 30 minutes. After the 
sealing agent has been dried (solidified), a biphenyl liquid crystal 
(i.e., a BDH product known under the trade name of E-7) is dropped onto 
one of the panels inwardly of the sealing area, and after the other panel 
has been placed in intimate contact therewith so that no bubble may be 
caught inwardly of the sealing areas of the panels, they are hot pressed 
together at about 130.degree. C. for two to three seconds, whereby the 
liquid crystal is confined within the panels. Then, the panels are 
maintained at 150.degree. C. for ten minutes for curing of the sealed 
portion, whereby a liquid crystal display panel is formed. It has the 
following properties: 
Sealing strength (peeling test): The film is broken, or peeled off the 
conductive film; 
Chemical resistance (as tested for one hour in an atmosphere containing 30% 
of HFIP): The panel surface is partially dissolved or whitened, but the 
sealed portion does not show any change; Water resistance (as tested at 
room temperature and a humidity of 90% for 24 hours): No change is found 
in the value of the current through the panel, or the orientation of the 
liquid crystal. 
The aforementioned properties indicate that the liquid crystal display 
panel is fully satisfactory for practical use. 
EXAMPLE 2 
A liquid crystal display panel was prepared by repeating the procedures of 
the preceding example, except that a sealing agent of the following 
composition was used, and cured at 130.degree. C. for 30 minutes after hot 
pressing: 
Nylon (having a peak melting point of 103.degree. C.; 
"Daiamid.times.1874")--15 g 
Epoxy resin (Shell's Epikote 815)--5 g 
HFIP--50 ml 
It showed the same properties as the product of the preceding example. 
EXAMPLE 3 
A liquid crystal display panel was prepared by using a sealing agent of the 
following composition, and holding the panel substrates between a pair of 
glass plates for curing at 170.degree. C. for 10 minutes: 
Nylon (having a peak melting point of 115.degree. C.; 
"Daiamid.times.2302")--15 g 
Epoxy resin (Shell's Epikote 815)--5 g 
HFIP--50 ml 
It showed the same properties as had been obtained in the preceding two 
examples. 
The glass plates were used for the purpose of preventing thermal 
deformation of the panel substrates. 
EXAMPLE 4 
A liquid crystal display panel was prepared by repeating the procedures of 
Example 1, except that a sealing agent of the following composition was 
used for hot pressing at 130.degree. C. and 3 kg/cm.sup.2 for two seconds, 
and final adhesion at 150.degree. C. for ten minutes: 
Copolymerized nylon (having a peak melting point of 115.degree. C.; 
"Daiamid.times.2302")--15 g 
Saturated polyester (Toyo Spinning's product known under the trade name 
"Vylon 30P")--15 g 
Epoxy resin (having an epoxy equivalent of 190.+-.5)--10 g 
HFIP--80 ml 
It showed the same properties as had been obtained in Example 1. 
EXAMPLE 5 
A liquid crystal display panel was prepared by repeating the procedures of 
Example 1, except that a sealing agent of the following composition was 
used for hot pressing at 130.degree. C. and 3 kg/cm.sup.2 for two seconds, 
and that the panel was left to stand at room temperature for three days, 
instead of being heated for final adhesion: 
Copolymerized nylon (having a peak melting point of 107.degree. C.; 
"Daiamid.times.1874")--15 g 
Saturated polyester (Toyo Spinning's product known under the trade name 
"Vylon 30P")--15 g 
Isocyanate compound (known under the trade name "Coronate L")--8 g 
HFIP--80 ml 
It showed the same properties as had been obtained in Example 1. 
EXAMPLE 6 
The procedures of Example 1 were repeated for preparing a liquid crystal 
display panel, except that a polyether sulfone was horizontally orientated 
as described in Example 1, a sealing agent of the following composition 
was used, and that the procedures of Example 4 were repeated for hot 
pressing and final adhesion: 
Linear saturated polyester (known under the trade name "Vylon 30P")--15 g 
Epoxy resin (Shell's Epikote 828)--5 g 
Mixture of methylene chloride and tetrahydrofuran--80 ml 
It showed the same properties as had been obtained in Example 1. Liquid 
crystal display panels having similar properties were obtained by using a 
sealing agent prepared by employing dioxane instead of the mixture of 
methylene chloride and tetrahydrofuran, and drying it at 110.degree. C. 
for 10 minutes, and a sealing agent prepared by employing instead of the 
aforementioned mixture a mixture of at least two compounds selected from 
among HFIP, methylene chloride, tetrahydrofuran and dioxane, and drying it 
at 100.degree. C. for 30 minutes. 
EXAMPLE 7 
Linear saturated polyester (known under the trade name "Vylon 30P")--15 g 
Triphenylmethane triisocyanate (known under the trade name "Desmodur R")--2 
g 
Mixture of methylene chloride and tetrahydrofuran--80 ml 
A liquid crystal display panel was prepared by repeating the procedures of 
Example 6, except that a sealing agent of the foregoing composition was 
used. It was similar to the panel obtained in Example 1. 
EXAMPLE 8 
A liquid crystal display panel was prepared by repeating the procedures of 
Example 1, except that a sealing agent of the following composition was 
used: 
(a) Nylon (having a peak melting point of 123.degree. C.; "Daiamid 
T-250")--15 g 
(b) Epoxy resin (Shell's product known under the trade name "Epikote 
815")--5 g 
(b') Isocyanate compound (known under the trade name "Desmodur R")--4 g 
(c) HFIP--80 ml 
The results which were similar to those obtained in Example 1 were 
obtained. 
A similar liquid crystal display panel was obtained by using a polyether 
sulfone or polycarbonate film instead of the polyethylene terephthalate 
film. A similar liquid crystal display panel was also obtained by using a 
mixed methylene chloride/tetrahysrofuran or methylene chloride/chloroform 
solvent instead of HFIP. 
EXAMPLE 9 
A liquid crystal panel was prepared by repeating the procedures of the 
immediately preceding example, except that a sealing agent of the 
following composition was used, and that after hot pressing, the panel was 
cured at 150.degree. C. for five minutes, and left to stand at room 
temperature for three days: 
(a) Linear saturated polyester (known under the trade name "Vylon 30P")--15 
g 
(b) Epoxy resin (known under the trade name "Araldite CY 230")--5 g 
(b') Isocyanate compound (known under the trade name "Desmodur R"; 
Ciba-Geigy)--4 g 
(c) HFIP--80 ml 
It showed the same properties as had been obtained in the preceding 
example. Similar results were obtained by using methylene 
chloride/tetrahydrofuran, or methylene chloride/chloroform instead of 
HFIP. 
EXAMPLE 10 
A liquid crystal display panel was prepared by using a sealing agent of the 
following composition, and holding the panels together between a pair of 
glass plates for curing at 130.degree. C. for 10 minutes: 
(a) Nylon (having a peak melting point of 115.degree. C.; 
"Daiamid.times.2302")--20 g 
(a') Linear saturated polyester (known under the trade name "Vylon 
30P")--10 g 
(b) Epoxy resin (known under the trade name "Araldite GY 252")--10 g 
(b') Isocyanate compound (polyisocyanate sold by Konishi Gisuke Shoten 
under the trade name "KU662Y")--8 g 
(c) HFIP--80 ml 
It showed the same properties as the panel obtained in the immediately 
preceding example. 
The glass plates were used for preventing thermal deformation of the panel 
substrates. Similar results were obtained by using methylene 
chloride/chloroform, or methylene chloride/tetrahydrofuran instead of 
HFIP. 
Comparative Example 
A liquid crystal display panel was prepared by using a sealing agent of the 
following composition in accordance with the procedures of Example 1: 
Nylon (having a peak melting point of 123.degree. C.; "Daiamid T-250")--15 
g 
HFIP--50 ml 
Its properties were as follows: 
Sealing strength (peeling test): 300 to 600 g/25 cm; 
Chemical resistance (as tested for one hour in an atmosphere containing 30% 
of HFIP): The panel surface and the sealed portion were whitened or 
dissolved; 
Water resistance (as tested at 110.degree. C. and a humidity of 90% for 24 
hours): The panel showed an increase of 10% in the amount of current 
flowing therethrough, and the liquid crystal showed a disorder in its 
orientation in the vicinity of the sealed portion. The aforementioned 
properties indicate that the liquid crystal display panel is 
unsatisfactory for practical use. 
INDUSTRIAL APPLICABILITY 
The liquid crystal display panels prepared by using the sealing agent of 
this invention do not only simplify the means for incorporating a liquid 
crystal therein because they are not formed with any hole for liquid 
crystal injection, but can always be produced with uniform quality. As a 
transparent plastic film, such as of PET, can be used for panel 
substrates, it is possible to make a liquid crystal display panel by using 
a transparent plastic film in the form of a tape if the sealing agent of 
this invention is employed. The sealing agent is also applicable for 
sealing a plastic display panel employing any other material for display 
than a liquid crystal, or material associated with such display.