JP7714901B2 - Adhesive composition and adhesive composition for forming flat cables - Google Patents

Description

The present invention relates to an adhesive composition containing a polyester resin and an adhesive composition for forming flat cables. More specifically, the present invention relates to an adhesive composition that has a low dielectric tangent, excellent heat resistance even when used as a one-component, and excellent solubility in organic solvents.

Polyester resins have been used in a wide range of applications, such as films, PET bottles, fibers, toner, electrical components, adhesives, and pressure-sensitive adhesives, due to their excellent heat resistance, chemical resistance, durability, and mechanical strength. In addition, polyester resins have high polarity due to their polymer structure, and are therefore known to exhibit excellent adhesion to polar polymers such as polyester, polyvinyl chloride, polyimide, and epoxy resin, as well as to metal materials such as copper and aluminum.
Taking advantage of such excellent adhesive properties, it is used as an adhesive for various applications, and recently, its application as an adhesive for flat cables used in wiring within electrical equipment has been attracting attention.

In recent years, in response to the increasing complexity of internal wiring in computers, liquid crystal displays, mobile phones, printers, automobiles, home appliances, copiers, and various other electrical devices, flat cables have come into use to reduce the labor required for wiring and prevent incorrect wiring.

A flat cable is generally manufactured by laminating an adhesive layer onto a film (which includes the concepts of tape and sheet; the same applies hereinafter) substrate (insulating layer) to create an insulating film, arranging multiple conductors in parallel between the adhesive layers of two insulating films, overlapping the adhesive layers to sandwich the conductors, and heat-sealing them. In other words, a flat cable has a structure in which the conductors are contained within the adhesive layer and insulating layers are provided on both outer surfaces of the cable.
As an adhesive used in forming flat cables, a solvent-soluble crystalline polyester resin is used because of its advantages of high adhesiveness and high heat resistance even when used in a one-component form (see, for example, Patent Document 1).

JP 2009-249472 A

However, in recent years, with technological developments in flat-screen TVs and game consoles that handle high-definition images and video data, there has been a growing need for flexible flat cables that can transmit signals at higher speeds and with lower loss. In addition to the conventional adhesive properties and heat resistance, the adhesive layers used in these cables are now strongly required to have low dielectric properties such as low dielectric constant and low dielectric dissipation factor, especially low dielectric dissipation factor.

The technology disclosed in Patent Document 1 above describes the use of a specific crystalline polyester resin as an adhesive, which provides high adhesiveness and heat resistance, but does not take into consideration low dielectric properties, and does not meet recent demands.

Against this background, the present invention aims to provide an adhesive composition containing a polyester-based resin that has low dielectric properties, particularly a low dielectric dissipation factor, and also has excellent adhesion, heat resistance, and solvent solubility.

However, in view of the above circumstances, the present inventors have conducted extensive research and have found that the adhesive composition described below meets the object of the present invention, thereby completing the present invention.
That is, the present invention relates to an adhesive composition containing a polyester-based resin including a structural unit derived from a polycarboxylic acid and a structural unit derived from a polyhydric alcohol, wherein the polyester-based resin satisfies all of the following requirements (A) to (D):
(A): The structural units derived from polycarboxylic acids contain 90 mol% or more of structural units derived from aromatic polycarboxylic acids relative to the total structural units derived from polycarboxylic acid components. (B): The structural units derived from polyhydric alcohols contain structural units derived from aliphatic diols having side chains. (C): The resin is a crystalline polyester-based resin. (D): The dielectric loss tangent (α) at 10 GHz is 0.01 or less (temperature 23°C, relative humidity 50% RH).

As described in Patent Document 1, crystalline polyester resins typically have excellent adhesive properties and heat resistance, but generally have high dielectric constants and dielectric loss tangents, making them difficult to use as adhesives having the low dielectric properties that are required in recent years.
The present inventors have investigated the composition of the monomers that make up polyester-based resins and found a structure that is effective in reducing dielectric loss tangent, thereby finding that an adhesive composition can be obtained that has an even lower dielectric loss tangent than conventional crystalline polyester-based resins and is also excellent in adhesion, heat resistance, and solvent solubility, and have completed the present invention.

The adhesive composition of the present invention forms an adhesive that has a low dielectric tangent and excellent adhesion, heat resistance, and solvent solubility, and is particularly effective as an adhesive used in the production of flat cables, etc.

In the present invention, it is believed that the low dielectric tangent can be achieved by including structural units derived from aromatic polycarboxylic acids and structural units derived from diols with side chains in the structure of the polyester resin used in the adhesive composition.

The configuration of the present invention will be described in detail below, but these are merely examples of preferred embodiments.
In the present invention, the term "class" added after the name of a compound is a concept that encompasses not only the compound but also derivatives of the compound. For example, the term "carboxylic acids" includes not only carboxylic acids but also carboxylic acid derivatives such as carboxylic acid salts, carboxylic acid anhydrides, carboxylic acid halides, and carboxylic acid esters.

<Polyester-based resin>
First, the polyester resin used in the present invention will be described.
The polyester resin used in the present invention contains, in its molecule, a structural unit derived from a polycarboxylic acid and a structural unit derived from a polyhydric alcohol.
Also preferred is a polyester resin obtained by reacting a polycarboxylic acid with a polyhydric alcohol to form an ester bond.

[Structural units derived from polycarboxylic acids]
The content of structural units derived from aromatic polycarboxylic acids in the polyester resin used in the present invention is 90 mol% or more of the total structural units derived from polycarboxylic acids (requirement (A)). Such a content is preferably 92 mol% or more, more preferably 95 mol% or more, particularly preferably 98 mol% or more, and most preferably 100 mol%. If the content of aromatic polycarboxylic acids is too low, the long-term durability in a humid and hot environment tends to deteriorate and the dielectric loss tangent tends to increase.

The content (molar ratio) of the structural units derived from aromatic polycarboxylic acids relative to the total polycarboxylic acids can be calculated from the following formula.
Content (mol %) of structural units derived from aromatic acids=(structural units (mol) derived from aromatic polycarboxylic acids/structural units (mol) derived from polycarboxylic acids)×100

The content of structural units derived from aromatic polycarboxylic acids in the entire polyester resin is preferably 15 to 70% by weight, more preferably 20 to 65% by weight, even more preferably 25 to 60% by weight, and particularly preferably 30 to 55% by weight. If the content of structural units derived from aromatic polycarboxylic acids is too low, the dielectric loss tangent tends to be high, while if it is too high, the adhesiveness tends to be insufficient.

Examples of structural units derived from polycarboxylic acids include structural units derived from aromatic polycarboxylic acids, which will be described later; structural units derived from alicyclic polycarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,2-cyclohexanedicarboxylic acid and their acid anhydrides; and structural units derived from aliphatic polycarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, and dodecanedioic acid. One or more types of polycarboxylic acids can be used.

Examples of structural units derived from the above aromatic polycarboxylic acids include structural units derived from aromatic dicarboxylic acids and their derivatives (aromatic dicarboxylic acids), such as terephthalic acid, isophthalic acid, dimethyl isophthalate, orthophthalic acid, naphthalenedicarboxylic acid, dimethyl naphthalenedicarboxylate, and biphenyldicarboxylic acid. Other examples include structural units derived from aromatic oxycarboxylic acids, such as p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. Furthermore, structural units derived from trifunctional or higher aromatic polycarboxylic acids introduced into polyester resins for the purpose of imparting a branched skeleton or acid value are also included in the structural units derived from the above aromatic polycarboxylic acids. Examples of structural units derived from trifunctional or higher aromatic polycarboxylic acids include structural units derived from trimellitic acid, trimesic acid, ethylene glycol bis(anhydrotrimellitate), glycerol tris(anhydrotrimellitate), trimellitic anhydride, pyromellitic dianhydride, oxydiphthalic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride, and 2,2'-bis[(dicarboxyphenoxy)phenyl]propane dianhydride.

Among these, structural units derived from aromatic dicarboxylic acids are preferred, and structural units derived from terephthalic acid, isophthalic acid, dimethyl terephthalate, dimethyl isophthalate, and dimethyl naphthalenedicarboxylate are more preferred. From the viewpoint of low dielectric dissipation factor, structural units derived from dimethyl naphthalenedicarboxylate are particularly preferred. Furthermore, structural units derived from terephthalic acid are preferred, and the content is more preferably 35 to 100 mol %, particularly preferably 40 to 90 mol %, and even more preferably 45 to 80 mol % of the total structural units derived from the polycarboxylic acid component. If the content of structural units derived from terephthalic acid is too low, the polyester resin tends to become amorphous and have insufficient heat resistance. If the content is too high, the polyester resin tends to become too crystalline and have insufficient solvent solubility.

In addition, from the viewpoint of low dielectric tangent characteristics of the polyester resin, the content of structural units derived from aromatic dicarboxylic acids having sulfonic acid groups, such as sulfoterephthalic acid, 5-sulfoisophthalic acid, 4-sulfophthalic acid, 4-sulfonaphthalene-2,7-dicarboxylic acid, and 5(4-sulfophenoxy)isophthalic acid, and structural units derived from aromatic dicarboxylic acid salts having sulfonate groups, such as their metal salts and ammonium salts, relative to the total polycarboxylic acids is preferably 10 mol % or less, more preferably 5 mol % or less, particularly preferably 3 mol % or less, even more preferably 1 mol % or less, and most preferably 0 mol %.

[Structural units derived from polyhydric alcohols]
The structural unit derived from polyhydric alcohols includes a structural unit derived from aliphatic diols having a side chain in order to reduce the dielectric loss tangent (requirement (B)). Note that the side chain here refers to an organic group bonded to the main chain, and the main chain refers to an organic group connecting two hydroxyl groups of the diol.

Structural units derived from aliphatic diols having side chains include structural units derived from 1,2-propylene glycol, 2-methyl-1,3-propanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-ethyl-1,3-hexanediol, 2,4-diethyl-1,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, dimer diol, etc. Among these, from the viewpoint of achieving a low dielectric loss tangent, it is preferable for the carbon number of the side chain to be 3 or more, more preferably 5 or more, and even more preferably 7 or more. In particular, structural units derived from dimer diol are preferred, as they can increase the side chain content and achieve a low dielectric loss tangent without significantly impairing the crystallinity of the polyester resin.

Examples of the dimer diol-derived structural units include structural units derived from dimer diols, which are reduced forms of dimer acids (mainly those having 36 to 44 carbon atoms) derived from oleic acid, linoleic acid, linolenic acid, erucic acid, etc., and structural units derived from their hydrogenated products. Among these, structural units derived from hydrogenated products are preferred in terms of suppressing gelation during the production of polyester-based resins.

Furthermore, structural units derived from aliphatic diols having side chains preferably have a primary hydroxyl group, as this makes it easier to increase the molecular weight of the polyester resin, and more preferably have only primary hydroxyl groups.

The content of structural units derived from aliphatic diols having side chains relative to the total amount of polyhydric alcohols is preferably 2 to 50 mol%, more preferably 3 to 40 mol%, particularly preferably 5 to 30 mol%, and even more preferably 7 to 20 mol%. If the content of structural units derived from aliphatic diols having side chains is too low, the low dielectric tangent characteristics tend to be inferior, while if it is too high, the polyester resin tends to become amorphous and have insufficient heat resistance.

Furthermore, the content of structural units derived from aliphatic diols having side chains relative to the total polyester resin is preferably 5 to 60% by weight, more preferably 10 to 50% by weight, particularly preferably 12 to 40% by weight, and even more preferably 15 to 30% by weight. If the content of diols having side chains is too low, the low dielectric tangent characteristics tend to be inferior, and if it is too high, the polyester resin tends to become amorphous and have insufficient heat resistance.

Other structural units derived from polyhydric alcohols include structural units derived from bisphenol skeleton-containing monomers, aliphatic polyhydric alcohols without side chains, alicyclic polyhydric alcohols, and aromatic polyhydric alcohols. One or more types of structural units derived from polyhydric alcohols may be contained.

Examples of structural units derived from bisphenol skeleton-containing monomers include structural units derived from bisphenol A, bisphenol B, bisphenol E, bisphenol F, bisphenol AP, bisphenol BP, bisphenol P, bisphenol PH, bisphenol S, bisphenol Z, 4,4'-dihydroxybenzophenone, bisphenol fluorene, bisphenylphenol fluorene, and hydrogenated products thereof, as well as glycols such as ethylene oxide adducts and propylene oxide adducts obtained by adding one to several moles of ethylene oxide or propylene oxide to the hydroxyl groups of bisphenols.

Examples of structural units derived from aliphatic polyhydric alcohols without side chains include structural units derived from ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, and 1,10-decanediol. Among these, structural units derived from 1,4-butanediol are preferred because they facilitate crystallization of the polyester resin. The content is preferably 30 mol% or more, particularly preferably 40 mol% or more, and even more preferably 50 mol% or more.

Examples of structural units derived from alicyclic polyhydric alcohols include structural units derived from 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, tricyclodecanediol, tricyclodecanedimethanol, spiroglycol, etc.

Examples of structural units derived from aromatic polyhydric alcohols include structural units derived from paraxylene glycol, metaxylene glycol, orthoxylene glycol, 1,4-phenylene glycol, and ethylene oxide adducts of 1,4-phenylene glycol.

In order to achieve a low dielectric tangent, the content of structural units derived from ether bond-containing glycols such as diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol in the entire polyester resin is preferably 20% by weight or less, more preferably 15% by weight or less, particularly preferably 10% by weight or less, even more preferably 5% by weight or less, and most preferably 0% by weight.

In the present invention, it is preferable to increase the content of aromatic rings in the structure of the polyester resin in order to achieve a low dielectric loss tangent.
The aromatic ring content of the polyester resin as a whole is preferably 5% by weight or more, more preferably 10% by weight or more, particularly preferably 15% by weight or more, and even more preferably 20% by weight or more, with the upper limit usually being 50% by weight.

Here, the definition and calculation method of the aromatic ring content in the present invention are as follows.
The aromatic ring content is the weight ratio of atoms constituting aromatic rings in a polyester resin. Note that the two aromatic ring portions derived from the bisphenol skeleton do not contribute to low dielectric properties, and are therefore not included in the aromatic ring content in the present invention. The reason why the two aromatic ring portions derived from the bisphenol skeleton do not contribute to low dielectric tangent is unclear, but it is presumed that, for example, due to steric factors, the two aromatic rings derived from the bisphenol skeleton cannot participate in stacking of aromatic rings with each other.

The aromatic ring content can be calculated from the composition of the polyester resin as follows.
Aromatic ring content = A1×(a11×m11+a12×m12+a13×m13...)/(x1-y1)+A2×(a21×m21+a22×m 22+a23×m23...)/(x2-y2)+A3×(a31×m31+a32×m32+a33×m33...)/(x3-y3)...
A: Content (wt%) of structural units derived from each monomer in the polyester resin
a: atomic weight of an atom constituting an aromatic ring in each monomer (for example, 12 for carbon, 14 for nitrogen, etc. In addition, when two or more types of atoms are present, it corresponds to a11, a12, a13, ... in the above formula, for example, a11: carbon, a12: nitrogen, a13: oxygen).
m: number of atoms constituting the aromatic ring in each monomer; x: molecular weight of each monomer; y: sum of formula weights of leaving groups in each monomer. Note that the two aromatic ring moieties derived from the bisphenol skeleton are not included as atoms constituting the aromatic ring (treated as m = 0) for the reasons described above.

In addition, hydroxycarboxylic acid compounds containing hydroxyl and carboxyl groups in their molecular structure can also be used as raw materials for polyester resins. Examples of such hydroxycarboxylic acid compounds include 5-hydroxyisophthalic acid, p-hydroxybenzoic acid, p-hydroxyphenylpropionic acid, p-hydroxyphenylacetic acid, 6-hydroxy-2-naphthoic acid, and 4,4-bis(p-hydroxyphenyl)valeric acid.

In the present invention, it is preferable to increase the side chain content in order to achieve a low dielectric loss tangent.
The side chain content of the polyester resin as a whole is preferably 2% by weight or more, more preferably 3% by weight or more, particularly preferably 5% by weight or more, and even more preferably 7% by weight or more, from the viewpoint of a low dielectric loss tangent. The upper limit is usually 50% by weight.
.

Here, the definition and calculation method of the side chain content in the present invention are as follows.
The side chain content is the weight ratio of carbon atoms in side chains derived from monomers having side chains in the polyester resin.

The side chain content can be calculated from the composition of the polyester resin as follows.
Side chain content = A1×12×m1/(x1-y1)+A2×12×m2/(x2-y2)+A3×12×m3/(x3-y3)...
A: Content (wt%) of structural units derived from each monomer in the polyester resin
m: number of carbon atoms constituting the side chain in each monomer; x: molecular weight of each monomer; y: sum of formula weights of leaving groups in each monomer.

The polyester resin used in the present invention may be copolymerized with at least one selected from the group consisting of a structural unit containing an aromatic ring of a tri- or higher functional polycarboxylic acid and a structural unit containing an aromatic ring of a tri- or higher functional polyhydric alcohol, for the purpose of introducing a branched skeleton.

In this case, the content of structural units derived from trifunctional or higher polyvalent carboxylic acids includes, for example, trimellitic acid, trimesic acid, ethylene glycol bis(anhydrotrimellitate), glycerol tris(anhydrotrimellitate), trimellitic anhydride, pyromellitic dianhydride, oxydiphthalic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride, 2,2'-bis[(dicarboxyphenoxy)phenyl]propane dianhydride and the like.
Examples of structural units derived from tri- or higher functional polyhydric alcohols include structural units derived from glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and the like.
The structural units derived from tri- or higher functional polycarboxylic acids and the structural units derived from tri- or higher functional polyhydric alcohols may each be used alone or in combination of two or more kinds.

When at least one selected from the group consisting of structural units derived from trifunctional or higher polycarboxylic acids and structural units derived from trifunctional or higher polyhydric alcohols is used for the purpose of introducing a branched skeleton, the content of structural units derived from trifunctional or higher polycarboxylic acids relative to all structural units derived from polycarboxylic acids, or the content of structural units derived from trifunctional or higher polyhydric alcohols relative to all structural units derived from polyhydric alcohols, is preferably in the range of 0.1 to 5 mol%, more preferably 0.3 to 3 mol%, and even more preferably 0.5 to 2 mol%, respectively. If the content of either or both is too high, the resulting polyester resin tends to become amorphous and have insufficient heat resistance, and gelation may also occur during polymerization.

[Production of polyester resin]
The polyester resin used in the present invention can be produced by a known method, for example, by subjecting a polycarboxylic acid and a polyhydric alcohol to an esterification reaction, optionally in the presence of a catalyst, to obtain a prepolymer, followed by polycondensation under reduced pressure while distilling off excess glycol.

The temperature for the esterification reaction between polycarboxylic acids and polyhydric alcohols is typically 180 to 280°C, and the reaction time is typically 60 minutes to 8 hours.

The temperature for polycondensation is typically 220 to 280°C, and the reaction time is typically 20 minutes to 4 hours. It is also preferable to carry out polycondensation under reduced pressure.

The polyester resin used in the present invention is crystalline. Crystallinity, as used here, can be confirmed using a differential scanning calorimeter. For example, crystallinity refers to the observation of an endothermic peak due to crystalline melting when measured at a temperature range of -70 to 260°C and a temperature rise rate of 10°C/min. The measurement temperature range can be changed as appropriate depending on the sample.

[Glass transition temperature (Tg) of polyester resin]
The glass transition temperature (Tg) of the polyester resin used in the present invention is preferably −20° C. or higher, more preferably −10 to 50° C., particularly preferably −5 to 40° C., even more preferably 0 to 350° C., particularly preferably 5 to 30° C., and most preferably 10 to 25° C.
If the glass transition temperature (Tg) is too low, the tack-free property tends to be insufficient, whereas if the glass transition temperature (Tg) is too high, the adhesiveness and flexibility tend to be insufficient.

The glass transition temperature (Tg) was measured as follows.
The glass transition temperature (Tg) can be determined by measurement using a differential scanning calorimeter under the following measurement conditions: a temperature range of −70 to 260° C., and a temperature rise rate of 10° C./min.

[Melting point of polyester resin]
The melting point of the polyester resin used in the present invention is preferably 60°C or higher, more preferably 70 to 180°C, particularly preferably 80 to 170°C, even more preferably 90 to 160°C, and particularly preferably 100 to 150°C.
If the melting point is too low, the heat resistance tends to be insufficient, and if it is too high, the solvent solubility tends to be insufficient.

The melting point was measured as follows.
The melting point can be determined by measuring using a differential scanning calorimeter under the following measurement conditions: a temperature range of −70 to 260° C., and a temperature rise rate of 10° C./min.

[Flow Start Temperature of Polyester Resin]
The flow initiation temperature of the polyester resin used in the present invention is preferably 70°C or higher, more preferably 80 to 180°C, particularly preferably 90 to 170°C, even more preferably 100 to 160°C, and particularly preferably 110 to 150°C.
If the flow initiation temperature is too low, the heat resistance tends to be insufficient, whereas if it is too high, the solvent solubility tends to be insufficient.

The flow starting temperature is measured as follows.
The flow initiation temperature can be determined by measuring by a temperature rising method using a flow tester under the following measurement conditions: test force: 30 kg, temperature rising rate: 3°C/min, measurement temperature range: 40 to 250°C, die hole diameter: 1 mm.

From the viewpoints of adhesiveness, heat resistance, and solvent solubility, it is preferable that the difference between the flow initiation temperature and Tg of the polyester resin used in the present invention be within a specified range. The difference between the flow initiation temperature and Tg (flow initiation temperature - Tg) is preferably 70°C to 160°C, more preferably 80°C to 150°C, particularly preferably 90°C to 140°C, and even more preferably 100°C to 130°C. If the difference between the flow initiation temperature and Tg is too small, the balance between adhesiveness and heat resistance will be insufficient, and if the difference is too large, solvent solubility will tend to be insufficient.

[Acid value of polyester resin]
The acid value of the polyester resin used in the present invention is preferably 5 mgKOH/g or less, more preferably 0.2 to 4 mgKOH/g, and particularly preferably 0.5 to 3 mgKOH/g.
If the acid value is too low, the adhesiveness will be insufficient, whereas if it is too high, the long-term durability in a humid and hot environment will tend to decrease.

The definition and measurement method of the acid value are as follows.
The acid value (mgKOH/g) can be determined by dissolving 1 g of polyester resin in 30 g of a mixed solvent of toluene/methanol (for example, toluene/methanol=9/1 by volume) and performing neutralization titration according to JIS K0070.
In the present invention, the acid value of the polyester resin is determined by the content of carboxy groups in the resin.

[Ester bond concentration of polyester resin]
The ester bond concentration of the polyester resin used in the present invention is preferably 9 mmol/g or less, more preferably 4 to 8.5 mmol/g, even more preferably 5 to 8 mmol/g, and particularly preferably 6 to 7.7 mmol/g.
If the ester bond concentration is too high, the low dielectric loss tangent characteristics tend to be insufficient, and the hygroscopicity tends to increase, which tends to lead to an increase in the dielectric loss tangent due to moisture absorption.If the ester bond concentration is too low, the adhesiveness tends to be insufficient.

The definition and measurement method of the ester bond concentration are as follows.
The ester bond concentration (mmol/g) refers to the number of moles of ester bonds per gram of polyester resin, and can be calculated, for example, from the amounts charged. The calculation method is to divide the number of moles of the smaller of the polycarboxylic acids and polyhydric alcohols charged by the total weight of the resin, and an example of the calculation formula is shown below.
When the amounts of the polycarboxylic acids and the polyhydric alcohols charged are equal in molar amount, either of the following calculation formulas may be used.
Furthermore, when using a monomer having both a carboxyl group and a hydroxyl group, or when preparing polyester from caprolactone or the like, the calculation method will need to be changed appropriately.

(When the amount of polycarboxylic acids is less than the amount of polyhydric alcohols)
Ester group concentration (mmol/g)=[(A1/a1×m1+A2/a2×m2+A3/a3×m3...)/Z]×1000
A: Amount of polycarboxylic acids charged (g)
a: molecular weight of polycarboxylic acid; m: number of carboxylic acid groups per molecule of polycarboxylic acid; Z: weight of finished product (g)

(When the amount of polyhydric alcohols is less than the amount of polycarboxylic acids)
Ester group concentration (mmol/g)=[(B1/b1×n1+B2/b2×n2+B3/b3×n3...)/Z]×1000
B: Amount of polyhydric alcohol (g)
b: molecular weight of polyhydric alcohol n: number of hydroxyl groups per molecule of polyhydric alcohol Z: finished weight (g)

The above ester bond concentration can also be measured by known methods such as NMR.

Furthermore, the concentration of polar groups other than ester bonds and reactive functional groups is preferably low from the viewpoint of low moisture absorption and long-term durability in a humid and hot environment.
Other examples of the polar group include an amide group, an imide group, a urethane group, a urea group, an ether group, and a carbonate group.

The total concentration of amide groups, imide groups, urethane groups, and urea groups is preferably 3 mmol/g or less, more preferably 2 mmol/g or less, particularly preferably 1 mmol/g or less, even more preferably 0.5 mmol/g or less, and most preferably 0.2 mmol/g or less.
Examples of the ether group include an alkyl ether group and a phenyl ether group, and it is particularly preferable to reduce the concentration of the alkyl ether group from the viewpoint of low moisture absorption and long-term durability in a humid and hot environment. The alkyl ether group concentration is preferably 3 mmol/g or less, more preferably 2 mmol/g or less, and particularly preferably 1.5 mmol/g or less.
The phenyl ether group concentration is preferably 5 mmol/g or less, more preferably 4 mmol/g or less, particularly preferably 3 mmol/g or less, and even more preferably 2.5 mmol/g or less.
The carbonate group concentration is preferably 3 mmol/g or less, more preferably 2 mmol/g or less, particularly preferably 1 mmol/g or less, even more preferably 0.5 mmol/g or less, and most preferably 0.2 mmol/g or less.

[Peak top molecular weight (Mp) and weight average molecular weight (Mw) of polyester resin]
The peak top molecular weight (Mp) of the polyester resin used in the present invention is preferably 5,000 to 150,000, more preferably 10,000 to 120,000, particularly preferably 20,000 to 100,000, and further preferably 30,000 to 80,000.
If the peak top molecular weight (Mp) is too low, the heat resistance and long-term durability in a humid and hot environment tend to be insufficient, and the low dielectric loss tangent characteristics tend to be insufficient due to the influence of many hydroxyl groups remaining at the terminals.On the other hand, if the peak top molecular weight (Mp) is too high, the adhesiveness and solvent solubility tend to be insufficient.

The weight average molecular weight (Mw) of the polyester resin used in the present invention is preferably 5,000 to 150,000, more preferably 10,000 to 120,000, particularly preferably 20,000 to 100,000, and even more preferably 30,000 to 80,000.
If the weight-average molecular weight (Mw) is too low, the heat resistance and long-term durability in a humid and hot environment tend to be insufficient, and the low dielectric tangent property tends to be insufficient due to the influence of many hydroxyl groups remaining at the terminals.On the other hand, if the weight-average molecular weight (Mw) is too high, the adhesiveness and solvent solubility tend to be insufficient.

The peak top molecular weight (Mp) and the weight average molecular weight (Mw) were measured as follows.
The peak top molecular weight (Mp) and weight average molecular weight (Mw) can be measured by high performance liquid chromatography ("HLC-8320GPC" manufactured by Tosoh Corporation) using two columns (TSKgel SuperMultipore HZ-M (exclusion limit molecular weight: 2 × 10 ⁶ , number of theoretical plates: 16,000 plates/column, packing material: styrene-divinylbenzene copolymer, packing particle size: 4 μm)) in series, and calculated in terms of standard polystyrene molecular weight.

[Water absorption rate of polyester resin (wt%)]
The water absorption of the polyester resin used in the present invention is preferably 2% by weight or less, more preferably 1% by weight or less, particularly preferably 0.8% by weight or less, and even more preferably 0.6% by weight or less.
If the water absorption rate is too high, the durability against humidity and heat and the insulation reliability tend to decrease, and the low dielectric tangent characteristics tend to be poor.

The water absorption rate was measured as follows.
The polyester resin solution (before the curing agent was added) was applied to a release film with an applicator and dried at 120 ° C for 10 minutes to produce a sheet with a dry polyester resin layer thickness of 65 μm. This sheet was cut into a size of 7.5 cm × 11 cm, and the polyester resin layer surface of the sheet was laminated on a glass plate, after which the release film was peeled off. This process was repeated six times to obtain a test plate having a polyester resin layer with a thickness of 390 μm on the glass plate.
The test plate thus obtained was immersed in purified water at 23° C. for 24 hours, then removed, the surface water was wiped off, and dried for 2 hours at 70° C. The necessary weight was measured in each of these steps, and the water absorption rate (wt%) was calculated from the weight change according to the following formula.
(c-d)×100/(b-a)
a: Weight of the glass plate alone b: Initial weight of the test plate c: Weight of the test plate immediately after removing it from the purified water and wiping off the water d: Weight of the test plate after drying it at 70°C for 2 hours

[Dielectric properties of polyester resin]
(Dielectric constant (Dk))
The dielectric constant of the polyester resin used in the present invention at a frequency of 10 GHz under an environment of a temperature of 23° C. and a relative humidity of 50% RH is preferably 3.0 or less, more preferably 2.9 or less, particularly preferably 2.8 or less, and even more preferably 2.7 or less. If the dielectric constant is too high, when the resin is made into a flat cable, the transmission speed tends to be poor and the transmission loss tends to be large.

(Dielectric loss tangent (Df))
The polyester resin used in the present invention has a dielectric loss tangent at a frequency of 10 GHz under an environment of a temperature of 23° C. and a relative humidity of 50% RH of 0.01 or less, preferably 0.008 or less, more preferably 0.007 or less, and particularly preferably 0.0065 or less. If the dielectric loss tangent is too high, the transmission loss will increase when the resin is made into a flat cable.

The dielectric constant and dielectric loss tangent can be measured using a cavity resonator perturbation method with a network analyzer. If the polyester resin is highly adhesive and it is difficult to prepare a measurement sample on its own, it can be measured sandwiched between film, and the dielectric properties of the polyester resin alone can be calculated by subtracting the film's weight.

Thus, it is possible to obtain a polyester resin used in the present invention that has a very small dielectric loss tangent compared to conventional polyester resins.
Furthermore, polyester resins with extremely small dielectric loss tangents are extremely useful as raw materials for adhesive compositions for flat cables because they can suppress transmission loss in the high frequency range.

In addition, in the present invention, it is preferable that the polyester resin be soluble in organic solvents in order to prepare the adhesive composition described below. If the solubility in such organic solvents is insufficient, it tends to be difficult to prepare the adhesive composition.

Examples of the organic solvent include aromatic solvents such as toluene, xylene, solvent naphtha, and Solvesso; ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohol solvents such as methyl alcohol, ethyl alcohol, isopropyl alcohol, and isobutyl alcohol; ester solvents such as ethyl acetate and n-butyl acetate; acetate solvents such as cellosolve acetate and methoxyacetate; halogenated solvents such as dichloromethane, tetrachloroethylene, and chloroform; and mixtures of two or more of these solvents. Among these, non-halogenated solvents are preferred due to their toxicity to the human body.

<Curing agent>
The adhesive composition of the present invention has excellent heat resistance even when it contains only a polyester resin, but may contain a curing agent as needed. By containing a curing agent, functional groups in the polyester resin react with the curing agent having a functional group reactive with the functional group, resulting in curing, and an adhesive with excellent adhesive strength, heat resistance, and durability can be obtained.
Examples of such curing agents include compounds having a functional group that reacts with at least one of the hydroxyl group and the carboxyl group contained in the polyester resin, such as polyisocyanate compounds and polyepoxy compounds.

Examples of the polyisocyanate compounds include polyisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, xylylene diisocyanate, and hydrogenated xylylene diisocyanate, as well as isocyanate adducts such as the tolylene diisocyanate adduct, hexamethylene diisocyanate adduct, and isophorone diisocyanate adduct of trimethylolpropane. The polyisocyanate compounds may also be used in which the isocyanate moiety is blocked with phenol, lactam, or the like. These isocyanate compounds may be used alone or in combination of two or more.

Examples of the polyepoxy compounds include bifunctional glycidyl ether types such as bisphenol A diglycidyl ether, bisphenol S diglycidyl ether, and brominated bisphenol A diglycidyl ether; multifunctional glycidyl ether types such as phenol novolac glycidyl ether and cresol novolac glycidyl ether; glycidyl ester types such as hexahydrophthalic acid glycidyl ester and dimer acid glycidyl ester; and alicyclic or aliphatic epoxides such as triglycidyl isocyanurate, 3,4-epoxycyclohexylmethylcarboxylate, epoxidized polybutadiene, and epoxidized soybean oil. These polyepoxy compounds can be used singly or in combination.

Furthermore, when the polyepoxy compound contains a nitrogen atom (nitrogen-atom-containing polyepoxy compound), the coating film of the adhesive composition can be brought to a B-stage (semi-cured state) by heating at a relatively low temperature, and the fluidity of the B-stage film tends to be suppressed, improving workability during the bonding operation. It is also expected to have the effect of suppressing foaming of the B-stage film, which is preferable.

Examples of nitrogen-containing polyepoxy compounds include glycidyl amine compounds such as tetraglycidyldiaminodiphenylmethane, triglycidyl para-aminophenol, tetraglycidyl bisaminomethylcyclohexanone, and N,N,N',N'-tetraglycidyl-m-xylenediamine.

When the adhesive composition of the present invention contains a polyepoxy compound, and the polyepoxy compound further contains a nitrogen-containing polyepoxy compound, the content of the nitrogen-containing polyepoxy compound is preferably 30% by weight or less, more preferably 25% by weight or less, and particularly preferably 20% by weight or less, based on the total weight of the polyepoxy compound. The content of the nitrogen-containing polyepoxy compound is preferably 5 parts by weight or less, more preferably 3 parts by weight or less, and particularly preferably 2 parts by weight or less, based on 100 parts by weight of the polyester resin.
If the content of such nitrogen atom-containing polyepoxy compound is too high, the rigidity tends to be excessively high, and the adhesive properties tend to be reduced. In addition, the crosslinking reaction tends to proceed easily during storage of the adhesive sheet, and the sheet life tends to be reduced.

The equivalent weight of the epoxy group to the carboxy group is preferably 0.8 to 5, more preferably 0.9 to 3, particularly preferably 1 to 2.5, and even more preferably 1.2 to 2.
If the equivalent weight is too large, the adhesiveness tends to be insufficient and the low dielectric tangent characteristics tend to be poor, whereas if it is too small, the durability and heat resistance tend to be insufficient.

The equivalent weight of epoxy groups relative to carboxy groups (COOH) can be calculated from the acid value of the polyester resin and the epoxy equivalent weight (g/eq) of the blended polyepoxy compound according to the following formula:
Epoxy equivalent to COOH=(a ÷ WPE)/(AV ÷ 56.1 ÷ 1000 × b)
a: Weight (g) of polyepoxy compound used in the formulation
WPE: epoxy equivalent (g/eq) of polyepoxy compound
AV: Acid value of polyester resin (mgKOH/g)
b: Weight (g) of polyester resin used in the blend

<Adhesive Composition>
The adhesive composition of the present invention contains at least a polyester resin, and if necessary, further contains a curing agent, and exhibits the effects of excellent low dielectric loss tangent characteristics, adhesiveness, and heat resistance.

The adhesive composition of the present invention may contain fillers, flame retardants, etc. In such cases, taking into account the incorporation of fillers, flame retardants, etc., the content of the polyester resin of the present invention in the adhesive composition is preferably 30% by weight or more, more preferably 40 to 95% by weight, particularly preferably 50 to 90% by weight, and even more preferably 60 to 85% by weight, based on the total solids content.

Furthermore, when the adhesive composition of the present invention contains a curing agent, the content of the curing agent is preferably 1 to 30 parts by weight, more preferably 1.5 to 20 parts by weight, particularly preferably 2 to 15 parts by weight, and even more preferably 2.5 to 10 parts by weight, per 100 parts by weight of the polyester resin of the present invention. If the curing agent content is too low, heat resistance and durability tend to be insufficient, while if it is too high, adhesion tends to be insufficient and low dielectric tangent characteristics tend to be inferior.

The adhesive composition of the present invention may contain a solvent to appropriately adjust the viscosity of the adhesive composition and facilitate handling when forming a coating film. The solvent is used to ensure ease of handling and workability when molding the adhesive composition, and there are no particular restrictions on the amount used.

Examples of solvents include ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone, and cyclohexanone; esters such as ethyl acetate; ethers such as ethylene glycol monomethyl ether; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; alcohols such as methanol and ethanol; alkanes such as hexane and cyclohexane; and aromatics such as toluene and xylene. The above-listed solvents may be used alone, or two or more may be mixed in any combination and ratio.

[Other ingredients]
The adhesive composition of the present invention may contain other components in addition to the components listed above to further improve its functionality, such as inorganic fillers, coupling agents such as silane coupling agents, ultraviolet protection agents, antioxidants, plasticizers, fluxes, flame retardants, colorants, dispersants, emulsifiers, elasticity reducing agents, diluents, antifoaming agents, ion trapping agents, leveling agents, and catalysts.
When the adhesive composition of the present invention contains other components, the content of the other components is preferably 70% by weight or less, more preferably 0.05 to 60% by weight, particularly preferably 0.1 to 50% by weight, and even more preferably 0.2 to 40% by weight.

<Adhesive>
When the adhesive composition contains a curing agent, the adhesive of the present invention can be obtained by further carrying out a curing treatment, and exhibits the effects of low dielectric loss tangent characteristics, excellent adhesiveness, and excellent heat resistance.
In the present invention, "curing" means intentionally curing the adhesive composition by heat and/or light, and the degree of curing can be controlled depending on the desired physical properties and application.

When the adhesive composition of the present invention is cured or semi-cured to form an adhesive, the curing method for the adhesive composition varies depending on the components and their amounts in the adhesive composition, but typically involves heating at 80 to 200°C for 1 minute to 10 hours.

When the adhesive composition of the present invention is cured using a curing agent, a catalyst may be used. Examples of such catalysts include imidazole compounds such as 2-methylimidazole, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenyl-4-methylimidazole, and 1-cyanoethyl-2-ethyl-4-methylimidazole; tertiary amines such as triethylamine, triethylenediamine, N'-methyl-N-(2-dimethylaminoethyl)piperazine, 1,8-diazabicyclo(5,4,0)-undecene-7, and 1,5-diazabicyclo(4,3,0)-nonene-5,6-dibutylamino-1,8-diazabicyclo(5,4,0)-undecene-7; and compounds obtained by converting these tertiary amines into amine salts with phenol, octylic acid, quaternized tetraphenylborate salts, or the like; cationic catalysts such as triallylsulfonium hexafluoroantimonate and diaryliodonium hexafluoroantimonate; and triphenylphosphine. Among these, tertiary amines such as 1,8-diazabicyclo(5,4,0)-undecene-7 and 1,5-diazabicyclo(4,3,0)-nonene-5,6-dibutylamino-1,8-diazabicyclo(5,4,0)-undecene-7; and compounds obtained by converting these tertiary amines into amine salts with phenol, octylic acid, quaternized tetraphenylborate salts, or the like are preferred in terms of thermosetting property, heat resistance, adhesion to metals, and storage stability after blending.
The amount of the curing agent added is preferably 0.01 to 1 part by weight per 100 parts by weight of the polyester resin, as this range further enhances the catalytic effect on the reaction between the polyester resin and the curing agent, resulting in strong adhesive properties.

[Application]
The adhesive composition of the present invention has low dielectric loss tangent characteristics, excellent adhesiveness, and heat resistance, and is therefore suitable as an adhesive for flat cables.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Note that "parts" and "%" in the examples are by weight.

<Production of Polyester Resin>
The compositions shown in Table 1 below are the composition ratios of the finished product (resin composition ratios), and are the relative ratios (molar ratios) of the amounts of the constituent monomers of the polyester resin obtained and their weight percentages.

Example 1
[Production of Polyester Resin (A-1)]
A reactor equipped with a thermometer, a stirrer, a rectification column, and a nitrogen inlet tube was charged with 239.0 g (1.4385 mol) of terephthalic acid (TPA) as polycarboxylic acids, 102.4 parts (0.6163 mol) of isophthalic acid (IPA), 237.1 parts (2.6309 mol) of 1,4-butanediol (1,4BG) as polyhydric alcohols, 51.0 parts (0.8217 mol) of ethylene glycol (EG), and 130.5 parts (0.2466 mol) of dimer diol "Pripol 2033" (P2033) (manufactured by Croda), and 0.1 parts of tetrabutyl titanate as a catalyst. The internal temperature was raised to 270°C over 2.5 hours, and an esterification reaction was carried out at 270°C for 1.5 hours.
Thereafter, 0.1 parts of tetrabutyl titanate was added as a catalyst, the pressure inside the system was reduced to 2.5 hPa, and a polymerization reaction was carried out over 2 hours to obtain a polyester resin (A-1).

Aromatic acid content, aromatic ring content, side chain content, ester bond concentration (mmol/g), glass transition temperature (Tg) (°C), acid value (mgKOH/g), melting point (°C), flow onset temperature (°C), peak top molecular weight (Mp), weight average molecular weight (Mw), and dielectric properties were measured according to the procedures described in this specification. Other physical properties were measured as follows:

[Dimer diol content]
The content (wt %) of dimer diol relative to the polyester resin is shown.

[Solvent solubility]
20 g of polyester resin, 64 g of toluene, and 16 g of methyl ethyl ketone were mixed, heated and stirred at the boiling point for 3 hours under reflux, and the mixture was evaluated based on the following criteria.
(Evaluation criteria)
◯: The polyester resin was completely dissolved. ×: The polyester resin was not completely dissolved, and some of it remained as a solid.

[Solution stability]
For those evaluated as "good" in the solvent solubility test, the time until solidification after dissolution was measured when the sample was left to stand at 23°C, and the sample was evaluated based on the following criteria.
(Evaluation criteria)
◎: Did not solidify even when left to stand for 24 hours or more after dissolution. ○: Solidified 1 hour or more but less than 24 hours after dissolution. △: Solidified 0.5 hours or more but less than 1 hour after dissolution. ×: Solidified less than 0.5 hours after dissolution.

<Production of adhesive composition>
Using the polyester resin obtained above, an adhesive composition was produced as follows.

20 g of the polyester resin (A-1) obtained above, 64 g of toluene, and 16 g of methyl ethyl ketone were mixed, and the mixture was heated, stirred, and refluxed at the boiling point for 3 hours to completely dissolve the polyester resin, yielding an adhesive composition.

<Preparation of Laminate>
The adhesive composition prepared above was applied to a 50 μm-thick PET film "Lumirror T60" (manufactured by Toray Industries, Inc.) using an applicator, and then dried at 120°C for 5 minutes to form an adhesive layer with a dry thickness of 25 μm. Next, a 100 μm-thick tin-plated copper foil "SnCu-O" (manufactured by Kawasaki Rolling Co., Ltd.) was laminated to the adhesive layer surface of the adhesive-layered PET film (lamination conditions: 170°C, 0.2 MPa, feed rate 1.5 m/min), and then aged for 3 days in an environment of 23°C and 50% RH to obtain a laminate.
For convenience, a laminate (PET film/adhesive layer/tin-plated copper foil) formed by laminating a PET film and a tin-plated copper foil will be referred to as PET/Sn.

[Adhesive strength]
The laminate obtained above was cut into a 1 cm wide piece to serve as a test piece. The test piece was fixed to a 2 mm thick glass plate using double-sided tape, and the tensile peel strength of the test piece was measured at 80°C using a peel tester (peel speed: 100 mm/min, peel angle: 180°). The evaluation criteria were as follows:
(Evaluation criteria)
◎: 9 N/cm or more ○: 7 N/cm or more and less than 9 N/cm △: 5 N/cm or more and less than 7 N/cm ×: Less than 5 N/cm

(Examples 2 and 3, Comparative Examples 1 and 2)
[Production of Polyester Resins (A-2 to A-3, A'-1 to A'-2)]
Polyester resins (A-2 to A-3, A'-1 to A'-2) were obtained in the same manner as in Example A-1 except that the resin composition was changed as shown in Table 1, and then various evaluations were performed using the same methods as in Example 1.

The resin composition (structural units derived from the components) and various physical properties of the obtained polyester resin are shown in Tables 1 and 2. In Table 1, the abbreviations are as follows:
"TPA": terephthalic acid; "IPA": isophthalic acid; "AdA": adipic acid; "SebA": sebacic acid; "EG": ethylene glycol; "NPG": neopentyl glycol; "CHDM": 1,4-cyclohexanedimethanol; "1.4BG": 1,4-butanediol; "1.6HG": 1,6-hexanediol; "P2033": dimer diol "Pripol 2033" (manufactured by Croda)

The results in Tables 1 and 2 above demonstrate that the adhesive compositions using the polyester resins (A-1) to (A-3) of Examples 1 to 3, which satisfy all of the requirements of the present invention, have low dielectric tangent characteristics, solvent solubility, and excellent adhesion at high temperatures.
On the other hand, the adhesive composition using the polyester resin (A'-1) of Comparative Example 1 was inferior in low dielectric tangent characteristics and was not suitable for use as an adhesive for low-loss flat cables, which is currently required. Also, the adhesive composition using the polyester resin (A'-2) of Comparative Example 2 was inferior in low dielectric tangent characteristics, and in addition, because it was amorphous, it had poor adhesion at high temperatures and poor heat resistance.

The adhesive composition of the present invention is an adhesive composition containing a crystalline polyester resin, and exhibits the effects of a low dielectric loss tangent and excellent adhesion, solvent solubility, and heat resistance. For example, it is effective as an adhesive for use in flat cables, etc.

Claims (4)

An adhesive composition containing a polyester-based resin including a structural unit derived from a polycarboxylic acid and a structural unit derived from a polyhydric alcohol, wherein the polyester-based resin has a glass transition temperature (Tg) of 50°C or less and satisfies all of the following requirements (A) to (D):
(A): The structural units derived from polycarboxylic acids contain 90 mol% or more of structural units derived from aromatic polycarboxylic acids relative to the total structural units derived from polycarboxylic acid components. (B): The structural units derived from polyhydric alcohols contain structural units derived from aliphatic diols having side chains, and in the structural units derived from aliphatic diols having side chains, the aliphatic diols having side chains are dimer diols. (C): The polyester resin is a crystalline polyester resin. (D): The polyester resin has a dielectric loss tangent (α) of 0.01 or less at 10 GHz (temperature 23°C, relative humidity 50% RH), a glass transition temperature (Tg) of -5°C or higher, and a melting point of 60°C or higher. 2. The adhesive composition according to claim 1 , further comprising an organic solvent. 3. The adhesive composition according to claim 2 , wherein the polyester resin is dissolved in an organic solvent. 4. An adhesive composition for forming a flat cable, wherein the adhesive composition according to claim 1 is used as an adhesive interposed between a resin substrate and a conductor in a flat cable.