JP5667734B2 - Polyester film for brightness enhancement members - Google Patents

Description

  The present invention relates to a polyester film suitable for a brightness enhancement member.

  As a brightness enhancement film, BEF (Brightness Enhancement Film: manufactured by 3M), a reflective polarizing film, and DBEF (Dual Brightness Enhancement Film: manufactured by 3M) are known as liquid crystal related members that play a role of improving brightness. Widely used from mobile phones to LCD TVs. According to these films, for example, in a liquid crystal TV, the amount of backlight, the number of lights, and the number of LEDs can be reduced, which has an environmental advantage.

  However, these films have concerns such as high price and supply stability. Although these alternative products have been studied, a film having satisfactory characteristics as a brightness enhancement film has not yet been completed.

  There are several ways to achieve brightness enhancement, such as BEF and DBEF. For example, a method that uses reflection of light by imparting the shape of the polymer surface with a mold (BEF), a method that imparts a super-laminated structure to the film (DBEF), and a recycling of light that is incident and not transmitted as it is There are various methods of applying and aligning various nematic liquid crystals, cholesteric liquid crystals, and the like on polymers.

  The method of applying a liquid crystal to a polymer is advantageous in terms of price and supply stability, but the control of the alignment between the polymer surface and the liquid crystal in contact with the polymer surface is the key.

  Usually, polyimide is generally used as a member for applying alignment liquid crystal such as nematic liquid crystal or cholesteric liquid crystal, and the key is surface treatment performed for alignment control, for example, rubbing treatment. When a polyester film is used as an alternative film, the liquid crystal cannot be well aligned due to the difference in orientation with the polyimide after the surface treatment, for example, after the rubbing treatment, or the appearance of uneven or streaky The occurrence of defects is a problem.

JP 2003-43260 A JP 2003-84284 A JP 2004-226945 A JP 2006-113513 A JP 2008-249901 A Special table 2002-502503 gazette Special table 2003-526802

  The present invention has been made in view of the above circumstances, and the problem to be solved is a polyester suitable as a film for a brightness enhancement member that can control the orientation when liquid crystal is applied and does not cause unevenness or streak-like defects. To provide a film.

  As a result of intensive studies in view of the above circumstances, the present inventor has found that the above problem can be easily solved by a polyester film having a specific configuration, and has completed the present invention.

That is, the gist of the present invention is that at least one outermost layer of the polyester film contains 0.075 wt% or more of inorganic particles having a Mohs hardness of 8 or more, and the crystallinity of the polyester film is 35 to 45, A polyester for a brightness enhancement member, wherein at least one surface of the outermost layer is rubbed at a strength of 35 to 50 mm as defined below using a rayon rubbed cloth according to the measurement method described in the specification. Be on film.
Surface treatment strength (mm) = N × l × (1 ± 2Π × r × n / 60 / v)
(In the above formula, N is the number of rubbing treatments, l is the pushing amount of the rubbing roll wound with the rubbing cloth (mm), r is the radius of the rubbing roll (mm), n is the rotational speed of the rubbing roll (rpm: 1/60 s ⁻¹ ), V is the moving speed (mm / s) of the film table, + means the reverse direction with respect to the moving direction of the film table, and-means forward rotation with respect to the moving direction of the film table.

  According to the polyester film of the present invention, it is possible to control the orientation at the time of applying the liquid crystal, and to produce a polyester film that can be suitably used as a film for a brightness enhancement member that does not cause unevenness or streak-like defects at low cost and yield. It can be provided well and its industrial value is high.

Hereinafter, the present invention will be described in more detail.
The polyester film constituting the polyester film in the present invention is excellent in functionality and price, so it is essential to have a laminated structure, as long as it does not exceed the gist of the present invention other than the two-layer or three-layer structure. There may be four or more layers, and there is no particular limitation.

  The polyester used in the present invention is preferably a homopolyester as a result of pursuing reduction of production costs and ease of process work. In the case of a homopolyester, those obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol are preferred. Examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Typical polyester includes polyethylene terephthalate and the like.

It is necessary to contain a specific amount of specific inorganic particles in at least one outermost layer of the film of the present invention. That is, the Mohs hardness of the particles contained in the outermost layer of the polyester film of the present invention is 8 or more. If the Mohs hardness is less than 8, the surface treatment effect cannot be fully exhibited. Specific examples of the particles used in the present invention include aluminum oxide, silicon carbide, vanadium carbide, titanium carbide, boron carbide, tungsten boride, and boron nitrite. Of these, aluminum oxide or silicon carbide, which is easily available industrially, and aluminum oxide are preferable (for example, specific examples are described in JP-A No. 5-128490).
The content of the high hardness particles in the outermost layer is 0.075% by weight or more, preferably 0.10% by weight or more. When the amount of the particles is less than 0.075% by weight, the surface-treated light cannot be sufficiently exhibited.
The reason why the inorganic particles having high hardness are used in the present invention is that the outermost layer of the polyester film to be surface-treated in a later step is made relatively hard so that the surface treatment, in the case of the present invention, specifically, so-called rubbing This is because the processing can be effectively performed even under a certain rubbing condition.
The primary particle size of the particles used in the present invention is usually 0.50 μm or less, preferably in the range of 0.005 to 0.50 μm. As used herein, the primary particle size refers to a so-called average particle size for non-aggregating particles, and refers to the average particle size of fine particles constituting an agglomerate for cohesive particles. When the primary particle size of the particles in the surface layer exceeds 0.50 μm, the size of the irregularities on the particle surface may become prominent, and the proportion of voids between the particles and the polyester increases. Even with a particle type having an approximate refractive index, the scattering of incident light by the gap may not be reduced, and as a result, the film may become opaque. In addition, the granularity of the added particles can be confirmed with human visibility, which may cause a problem in terms of image clarity. The primary particle size of the particles contained in the outermost layer is preferably as fine as possible, and is preferably 0.10 μm or less, more preferably 0.07 μm or less. However, when the primary particle size reaches 0.005 μm, the agglomeration becomes remarkable, and many agglomerates having an average particle size of 0.15 μm or more are formed without fine dispersion even by melt extrusion by a high-shear twin screw extruder. There is a possibility that.
Moreover, it is preferable to use the kneading method rather than the coating method for the inorganic particles in the polyester film in the present invention. In the polyester film for brightness enhancement member, there is a step of applying a liquid crystal in the subsequent step, and in this step, it is common to use a method by lyotropic and thermotropic, and in the lyotropic method, a coating is used because a solvent is used. If the inorganic particles are contained, the coating layer is dissolved and then causes particle aggregation and the like, and there is a high possibility of causing surface defects such as unevenness in the film surface.

  In the polyester film of the present invention, in addition to the above inorganic particles, other types of particles may be used for improving the surface treatment effect by so-called bimodal, and silica particles are preferably used in combination.

  The shape of the silica particles used in combination is not particularly limited, and any of a spherical shape, a block shape, a rod shape, a flat shape, and the like may be used. Moreover, there is no restriction | limiting in particular also about the hardness, specific gravity, a color, etc.

  The average particle diameter of the particles used in combination is usually in the range of 0.01 to 3 μm, preferably 0.1 to 2 μm. When the average particle size is less than 0.01 μm, the slipperiness may not be sufficiently imparted. On the other hand, when the thickness exceeds 3 μm, transparency may be lowered due to the aggregate of the particles when the film is formed, and it tends to cause breakage, which is a problem in terms of productivity. May be.

  The content of the particles used in the outermost layer is usually in the range of 0.001 to 5% by weight, preferably 0.005 to 3% by weight. When the particle content is less than 0.001% by weight, the slipperiness of the film may be insufficient. On the other hand, when the content exceeds 5% by weight, the transparency of the film is insufficient. There is a case.

  The method for adding particles to the polyester layer is not particularly limited, and a conventionally known method can be adopted. For example, it can be added at any stage for producing the polyester constituting each layer, but it is preferably added after completion of esterification or transesterification.

  Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a blending of dried particles and a polyester raw material using a kneading extruder. It is done by methods.

  Although the thickness of the polyester film in this invention will not be specifically limited if it can be formed into a film as a film, Usually, 10-350 micrometers, Preferably it is the range of 50-250 micrometers.

  Next, although the manufacture example of the polyester film in this invention is demonstrated concretely, it is not limited to the following manufacture examples at all. That is, a method of using the polyester raw material described above and cooling and solidifying a molten sheet extruded from a die with a cooling roll to obtain an unstretched sheet is preferable. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum, and an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed. Next, the obtained unstretched sheet is stretched in the biaxial direction. In that case, first, the unstretched sheet is stretched in one direction by a roll or a tenter type stretching machine. The stretching temperature is usually 70 to 120 ° C., preferably 80 to 110 ° C., and the stretching ratio is usually 2.5 to 7 times, preferably 3.0 to 6 times. Next, the film is stretched in the direction perpendicular to the first stretching direction. In that case, the stretching temperature is usually 70 to 170 ° C., and the stretching ratio is usually 3.0 to 7 times, preferably 3.5 to 6 times. is there. Subsequently, heat treatment is performed at a temperature of 180 to 270 ° C. under tension or under relaxation within 30% to obtain a biaxially oriented film. In the above-described stretching, a method in which stretching in one direction is performed in two or more stages can be employed. In that case, it is preferable to carry out so that the draw ratios in the two directions finally fall within the above ranges.

  For the production of the polyester film of the present invention, a simultaneous biaxial stretching method can also be employed. The simultaneous biaxial stretching method is a method in which the above-mentioned unstretched sheet is usually stretched and oriented in the machine direction and the width direction at a temperature controlled normally at 70 to 120 ° C., preferably 80 to 110 ° C. Is 4 to 50 times, preferably 7 to 35 times, and more preferably 10 to 25 times in terms of area magnification. Subsequently, heat treatment is performed at a temperature of 170 to 250 ° C. under tension or under relaxation within 30% to obtain a stretched oriented film. With respect to the simultaneous biaxial stretching apparatus that employs the above-described stretching method, a conventionally known stretching method such as a screw method, a pantograph method, or a linear driving method can be employed.

The crystallinity of the polyester film of the present invention is also an important requirement in the present invention because it creates a special surface state. The crystallinity referred to in the present invention is defined by the heat of fusion method in differential calorimetry (hereinafter abbreviated as DSC) measurement. At this time, the value of Groeninckx et al., 117.6 J / g, was adopted as the heat of fusion (ΔH ⁰ ) of the completely crystalline PET.
The degree of crystallinity of the film of the present invention is 35 to 45, preferably 36 to 40. When the film crystallinity is larger than 45 or smaller than 35, the surface treatment effect is thin, and it is difficult not only to align the liquid crystal alignment film but also difficult to produce.

  The polyester film of the present invention usually indicates that it is processed after being rubbed with a rubbing cloth, and the strength of about 40 is the best as an index of the rubbing density. If the strength value is small, for example, a value smaller than 35, the orientation does not remain, and if the strength value is large, for example, a value larger than 50, the film surface is scratched to the extent that it can be visually confirmed. Will result in defects in appearance.

  In the polyester film of the present invention, measurement of the water droplet contact angle is recommended as one method for evaluating the rubbing effect. The value of the water droplet contact angle on the film surface after the surface treatment by rubbing tends to increase as compared to that before rubbing. This is thought to be due to the formation of nano-sized grooves in one direction. Under optimum conditions, when the water droplet contact angle takes a maximum value and the rubbing is excessive, the nano-sized grooves are attached with anisotropy. As a result, the value drops extremely. In that case, visible scratches enter the surface of the polyester film, causing a problem.

  The value of the water droplet contact angle after the surface treatment of the polyester film of the present invention is preferably about 5 to 10 °, more preferably about 7 to 9 °, compared to that before the surface treatment. If the value of the water droplet contact angle after the rubbing treatment is less than 5%, the orientation does not remain, and if the surface treatment is carried out to a point where the value of the water droplet contact angle drops beyond 10 °, It may cause scratches that can be visually confirmed, resulting in defects in appearance.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. The measurement method and evaluation method used in the present invention are as follows.

(1) Measurement of intrinsic viscosity of polyester 1 g of polyester from which other polymer components and pigments incompatible with polyester have been removed are precisely weighed, and 100 ml of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio) is added. It was dissolved and measured at 30 ° C.

(2) Measurement of average particle size (d50: μm) 50% integrated (weight basis) in equivalent spherical distribution measured using centrifugal sedimentation type particle size distribution measuring device (SA-CP3 type manufactured by Shimadzu Corporation) The value was defined as the average particle size.

(3) Measurement of transmittance of polyester film According to JIS-K7105, the total light transmittance of the polyester film was measured with an integrating sphere turbidimeter NDH-300A manufactured by Nippon Denshoku Industries Co., Ltd. Judgment is based on the following criteria.
○: Transmittance in the range of 88.7 to 88.8% Δ: Transmittance in the range of 88.0 to 88.6% ×: Transmittance lower than 88.0%

(4) Haze (turbidity) measurement of polyester film The total light transmittance of the polyester film was measured with an integrating sphere turbidimeter NDH-300A manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS-K7105. Judgment is based on the following criteria.
○: Lower value than 1.8% △: 1.8-2.0%
X: Value is higher than 2.0%

(5) Calculation of crystallinity by DSC measurement 10 mg of polymer was set in a DSC-1 differential calorimeter (DSC) manufactured by PerkinElmer, and heated at a rate of temperature increase of 20 ° C./min in an N ₂ stream. The endothermic amount (J / g) (measured heat of fusion: ΔHexp) accompanying melting of the polymer was determined in the measurement range of 25 to 300 ° C.
ΔHexp (J / g) = (endotherm associated with polymer melting) / (polymer sample weight)
The degree of crystallinity can be calculated by the following formula.
Χc (%) = ΔHexp / ΔH ⁰ × 100
Actual heat of fusion: ΔHexp (J / g)
Heat of fusion of fully crystalline PET: ΔH ⁰ (J / g)
However, ΔH ⁰ used the value of Groeninckx et al., 117.6 J / g. As a document, “saturated polyester resin handbook” (Kazuo Yuki editor, Nikkan Kogyo Shimbun 1993 edition PP217-) was referred.

(6) Calculation of surface treatment strength The rubbing treatment was performed using a rubbing evaluation apparatus (small rubbing apparatus MRM-100; e-etchy Co., Ltd.). The rubbing cloth used at this time is a rayon cloth (YA-20-R) manufactured by Yoshikawa Processing Co., Ltd. The strength is almost synonymous with the rubbing density which is an important index in the rubbing treatment. The formula of intensity is as follows, and it can be processed with desired intensity by adjusting the following related values.
Surface treatment strength (mm) = N × l × (1 ± 2Π × r × n / 60 / v)
(In the above formula, N is the number of rubbing treatments, l is the pushing amount of the rubbing roll wound with the rubbing cloth (mm), r is the radius of the rubbing roll (mm), n is the rotational speed of the rubbing roll (rpm: 1/60 s ⁻¹ ), V is the moving speed (mm / s) of the film table, + means the reverse direction with respect to the moving direction of the film table, and-means forward rotation with respect to the moving direction of the film table.

(7) Surface water drop contact angle measurement of the surface-treated film At a temperature of 23 ° C. and a humidity of 50% RH, the contact angle between the polyester film before and after the rubbing treatment and distilled water was measured using a contact angle meter CA manufactured by Kyowa Interface Chemical Co., Ltd. -DT-A type was used for each measurement. The contact angle was measured for two points on the right and left sides of the polyester film before and after the rubbing treatment, and a total of 6 points with 3 samples, and the average value was obtained as the contact angle. In addition, the diameter of the water droplet was 1.5 mm, and the numerical value 1 minute after the dropping was read.

About evaluation, it computed with the following formula | equation.
Surface water droplet contact angle increase value (°) = θ1-θ2
θ1: Water droplet contact angle of PET film after rubbing treatment (2θ / °)
θ2: Water droplet contact angle of PET film before rubbing treatment (2θ / °)

The surface water droplet contact angle of the film after rubbing is judged according to the following criteria.
○: Increase in the value of 7 to 9 ° Δ: Increase in the value of 5 to 7 ° or 10 ° ×: The value is lower than 5 ° or decreased beyond 10 ° (the rubbing is strong) Too much, the groove becomes random and the value of the contact angle decreases)

(8) Appearance evaluation after application of liquid crystal to film after surface treatment The film after rubbing treatment was inspected by using general liquid crystal to determine whether or not the alignment liquid crystal can be aligned. Evaluation was performed according to the following criteria.
○: No irregularities or streaks and clean surface shape Δ: Unevenness or stripes visible ×: Liquid crystal alignment film is not aligned or surface shape is terrible

The polyester used in the examples and comparative examples was prepared as follows.
<Method for producing polyester (A)>
Using 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is placed in the reactor, the reaction start temperature is set to 150 ° C., and the methanol is distilled off gradually. The reaction temperature was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After adding 0.04 part by weight of ethyl acid phosphate to this reaction mixture, 0.04 part by weight of antimony trioxide was added, and a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.63 due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure. The intrinsic viscosity of the obtained polyester (A) was 0.63.

<Method for producing polyester (B)>
In the method for producing polyester (A), 0.04 part by weight of ethyl acid phosphate was added, and then 0.2 part by weight of silica particles having a Mohs hardness of 5 dispersed in ethylene glycol having an average particle diameter of 2.0 μm, trioxide Polyester (B) was obtained using the same method as the production method of polyester (A), except that 0.04 part by weight of antimony was added and the polycondensation reaction was stopped at a time corresponding to an intrinsic viscosity of 0.65. . The obtained polyester (B) had an intrinsic viscosity of 0.65.

<Method for producing polyester (C)>
The polyester (A) had an intrinsic viscosity of 0.65, an ester unit, in the same manner as the polyester A except that alumina particles having a primary particle size of 0.05 μm and a Mohs hardness of 8 were added as an ethylene glycol slurry after the transesterification reaction. 99% of the polyester (C) was obtained which was ethylene terephthalate and the rest was diethylene glycol and terephthalic acid condensed ester units. The alumina content was 1.5 parts.

Example 1:
The mixed raw materials in which the polyesters (C) and (B) were mixed in proportions of 92% and 8% were used as the outermost layer (surface layer) raw material, and the polyester (A) was used as the intermediate layer raw material in each of the two extruders. 2 layers and 3 layers (surface layer / intermediate layer / surface layer) on a cooling roll which is extruded from a die and melted at 290 ° C. and then set to a surface temperature of 40 ° C. using an electrostatic application adhesion method. The composition was co-extruded and cooled and solidified to obtain an unstretched sheet. Next, the film was stretched 3.7 times in the machine direction at a film temperature of 85 ° C. using the roll peripheral speed difference, led to a tenter, stretched 4.3 times at 120 ° C. in the transverse direction, and heat-treated at a heat setting temperature of 235 ° C. After that, a polyester film having a thickness of 250 μm (surface layer: 12.5 μm, intermediate layer: 225 μm) having an application layer and an intrinsic viscosity of 0.61 was obtained. About the obtained polyester film, intensity | strength 41 (N = 1, l = 0.5mm, n = 300rpm) using the rubbing apparatus (EH Sea Co., Ltd.) which wound the rubbing cloth made from rayon (Yoshikawa processing Co., Ltd.). , R = 50 mm, v = 10 mm / sec.). The obtained polyester film was so beautiful that the rubbing groove was not visible. Furthermore, the orientation liquid crystal dissolved in chloroform was applied to the polyester film after the rubbing treatment, and the liquid crystal layer was adjusted to 0.03 g / m ² after drying. When the obtained polyester film after rubbing treatment and application of the liquid crystal layer was visually evaluated, no streak or unevenness was observed and the polyester film was clean. In addition, the example of a compound which comprises the said liquid-crystal layer is as follows.

(Example compounds)
・ Nematic liquid crystal (ZLI-2293: Merck)
Chiral dopant (MLC-6248: Merk)
・ Photopolymerizable liquid crystal (RM257: Merck)
・ QtT (Quarter Thiophene)
As a reference, K.K. AMEMIYA et. al. , Applied Physics, Vol. 44, no. 6A, 2005, pp. There are 3748-3750.

Examples 2-7:
A polyester film was obtained in the same manner as in Example 1 except that the crystallinity (heat setting temperature), the content of particles in the outermost layer, and the Mohs hardness were changed. The produced polyester film was as shown in Table 1.

Comparative Examples 1-6:
In Example 1, it manufactured like Example 1 except changing crystallinity, particle content to the outermost layer, and Mohs hardness, and obtained the polyester film. The produced polyester film is as shown in Table 2. However, when the obtained rubbing treatment and the polyester film after application of the liquid crystal layer were visually evaluated, problems such as streaking and unevenness were observed.

  The polyester film of the present invention can be suitably used for a brightness enhancement member.

Claims (1)

At least one outermost layer of the polyester film contains 0.075% by weight or more of inorganic particles having a Mohs hardness of 8 or more, the crystallinity of the polyester film is 35 to 45, and at least one surface of the outermost layer is According to the measurement method described in the specification, a polyester film for a brightness enhancement member, which is rubbed at a strength of 35 to 50 mm as defined below using a rayon rubbing cloth.
Surface treatment strength (mm) = N × l × (1 ± 2Π × r × n / 60 / v)
(In the above formula, N is the number of rubbing treatments, l is the pushing amount of the rubbing roll wound with the rubbing cloth (mm), r is the radius of the rubbing roll (mm), n is the rotational speed of the rubbing roll (rpm: 1/60 s ⁻¹ ), V is the moving speed (mm / s) of the film table, + means the reverse direction with respect to the moving direction of the film table, and-means forward rotation with respect to the moving direction of the film table.