KR102818598B1 - Thermal curable resin composition and phenol resin composition prepared therefrom - Google Patents

Abstract

The present invention relates to a thermosetting resin composition which enables the production of a phenol resin exhibiting improved thermal stability and a phenol resin produced therefrom. The thermosetting resin composition comprises (a) a phthalonitrile compound having a specific chemical structure; and (b) a phenol resin prepolymer.

Description

THERMAL CURABLE RESIN COMPOSITION AND PHENOL RESIN COMPOSITION PREPARED THEREFROM

The present invention relates to a thermosetting resin composition which enables the production of a phenolic resin exhibiting improved thermal stability and a phenolic resin composition produced therefrom.

Phenolic resin is a type of thermosetting resin known to have excellent heat resistance properties. Due to these heat resistance properties, phenol resin is widely used in various mechanical devices/parts that are exposed to high heat, such as brake pads and various automobile and construction parts.

These phenol resins can be, for example, cured with a heat curing agent in a prepolymer state to produce a cured resin having higher heat resistance properties. However, it is known that when the phenol resin is cured with a conventionally known heat curing agent, the phenol resin in the final cured resin form can exhibit heat resistance up to a temperature of 400 to 500°C.

However, with the recent increase in development and demand for various high-performance parts, various machine parts are now exposed to higher temperatures, for example, up to about 700℃. However, since existing phenol resins only exhibit heat resistance up to a maximum temperature of 400 to 500℃, it is true that there were limitations in applying them to machine parts exposed to higher temperatures as mentioned above.

Due to this, there is a continuous demand for the development of technologies that can further improve the heat resistance or thermal stability of phenol resins.

The present invention provides a thermosetting resin composition which enables the production of a phenolic resin exhibiting improved thermal stability.

The present invention also provides a phenol resin composition produced from the thermosetting resin composition, which exhibits improved thermal stability.

The present invention provides a thermosetting resin composition comprising (a) a phthalonitrile compound of the following chemical formula 1; and (b) a phenol resin prepolymer:

[Chemical Formula 1]

In the above chemical formula 1,

L ^P2 is independently -O-, -S-, -C(=O)-, -S(=O)-, -S(=O) ₂ -, -C(CH ₃ ) ₂ -, -C(CF ₃ ) ₂ -, or -C(=O)NH-,

L ^P3 is a directly bonded or unsubstituted C _1-5 alkylene group.

The present invention also provides a phenol resin composition comprising a cured product of the thermosetting resin composition. In this phenol resin, the cured product comprises (b) a phenol resin in which the phenol resin prepolymer is polymerized; and (a) a phthalonitrile resin comprising a phthalonitrile compound or a unit derived therefrom.

The phenol resin may have a structure in which a cyano group of the phthalonitrile compound or resin is cross-linked to a portion of the hydroxyl group of the phenol resin. In addition, the phenol resin composition may further include a phenol resin that is physically mixed with the phthalonitrile compound or resin without forming a cross-link.

Hereinafter, a thermosetting resin composition and a phenol resin composition according to embodiments of the invention will be described in detail.

Prior to that, unless explicitly stated otherwise in this specification, terminology is used only for the purpose of referring to specific embodiments and is not intended to limit the present invention.

As used herein, the singular forms include the plural forms as well, unless the phrases clearly indicate the opposite.

The term "including" as used herein means specifying a particular characteristic, region, integer, step, operation, element, and/or component, but does not exclude the presence or addition of any other particular characteristic, region, integer, step, operation, element, component, and/or group.

The term 'prepolymer' as used herein refers to a polymer component that is not yet in a fully cured state, has a reactive functional group such as a hydroxyl group that can be further cured, and is in a state where a further processing/curing reaction can occur because the degree of polymerization of the repeating unit forming the prepolymer is, for example, 1000 or less.

Meanwhile, according to one embodiment of the invention, a thermosetting resin composition comprising: a) a phthalonitrile compound of the following chemical formula 1; and (b) a phenol resin prepolymer is provided:

[Chemical Formula 1]

In the above chemical formula 1,

L ^P2 is independently -O-, -S-, -C(=O)-, -S(=O)-, -S(=O) ₂ -, -C(CH ₃ ) ₂ -, -C(CF ₃ ) ₂ -, or -C(=O)NH-,

L ^P3 is a directly bonded or unsubstituted C _1-5 alkylene group.

The present inventors continued their research using various heat curing agents, etc., in order to further increase the heat resistance and thermal stability of the phenol resin. As a result of the present inventors' continued research, they confirmed that by using a specific phthalonitrile compound of chemical formula 1, which can be used to form a phthalonitrile resin, as a heat curing agent to cure and polymerize a phenol resin prepolymer, a phenol resin and a composition thereof exhibiting heat resistance and thermal stability even at a temperature of up to 700°C, which is higher than previously known, can be produced, and thus the invention was completed.

However, it was confirmed that this effect of improving thermal stability could only be achieved when the compound in the form of a monomer of Chemical Formula 1 was used together with a phenol resin prepolymer, and could not be achieved when the compound of Chemical Formula 1 was cured or polymerized and used in the state of a phthalonitrile oligomer, prepolymer, or resin, or when another phthalonitrile compound outside the scope of Chemical Formula 1 was used. In addition, the effect of this embodiment could not be achieved by using any component that had been used as a thermal curing agent for a conventional phenol resin.

It is presumed that the effect of improving thermal stability by the compound of chemical formula 1 is because the compound of chemical formula 1 has a similar three-dimensional structure to the phenol resin prepolymer, so that it can easily form cross-linking by heat curing while minimizing steric hindrance, etc., and also because it has the largest number of cyano groups capable of forming cross-linking in the monomer form.

Therefore, by using the thermosetting resin composition of one embodiment, a phenol resin composition exhibiting heat resistance and thermal stability at temperatures of 500 to 700°C, and even up to 700°C, which conventional phenol resins could not achieve, can be produced by curing and polymerization. Such a phenol resin composition can be very preferably applied to high-performance machine parts or automobile parts that require higher heat resistance.

Meanwhile, in the thermosetting resin composition of the above embodiment, the (b) phenol resin prepolymer is not particularly limited in its scope of application, and may be a prepolymer of any phenol resin known in the art, such as a general novolac-type resin or a resol-type resin. More specifically, in one example, the (b) phenol resin prepolymer may be a prepolymer of a phenol resin represented by Chemical Formula 2 or 3:

[Chemical formula 2]

[Chemical Formula 3]

In the above chemical formulas 2 and 3, R ₁ to R ₃ are each independently hydrogen or C _1-5 alkyl, and n is an integer from 1 to 1000.

In order for the above (b) phenol resin prepolymer to be manufactured into a phenol resin composition having superior heat resistance by heat-curing and polymerization reaction with the (a) phthalonitrile compound, the above prepolymer needs to be in a prepolymer state having appropriate viscosity, fluidity, and reactivity. To this end, the (b) phenol resin prepolymer may have a degree of polymerization n of the repeating unit of the above chemical formula 2 or 3 of 1 to 1000, or 2 to 800.

Meanwhile, the thermosetting resin composition of one embodiment comprises the (b) phenol resin prepolymer described above, together with the (a) phthalonitrile compound of chemical formula 1, which acts as a thermosetting agent to improve the heat resistance of the phenol resin by being cured together with the (b) phenol resin prepolymer.

In the structure of the above chemical formula 1, the cyano group can form a plurality of cross-linking bonds with the hydroxyl groups of the phenol resin prepolymer, and together with this, the chemical formula 1 can be cured or polymerized. Therefore, when the thermosetting resin composition of the above embodiment is cured and polymerized to produce a phenol resin and a composition thereof, polymer chains derived from the phenol resin and the like are each included, while a plurality of cross-linking bonds can be formed, and the phenol resin composition including these structures can exhibit more improved high-temperature stability.

In order to enable the above phthalonitrile compound to more effectively form curing and cross-linking with the phenol resin prepolymer, thereby producing a phenol resin and composition having further improved heat resistance, in the above chemical formula 1, L ^P2 is each independently -O-, and L ^P3 can be an unsubstituted C _1-3 alkylene group, for example, a methylene group.

In a more specific example, the phthalonitrile compound of the above formula 1 can be a compound of the following formula 1a:

[Chemical formula 1a]

In addition, the (a) phthalonitrile compound may be included in the composition of one embodiment in an amount of 5 to 100 parts by weight, or 5 to 60 parts by weight, or 7 to 55 parts by weight, or 10 to 50 parts by weight, relative to 100 parts by weight of the (b) phenol resin prepolymer. If the amount of the (a) phthalonitrile compound is too small, the heat resistance, etc. of the phenol resin and the composition manufactured through final curing may not be sufficient. Conversely, if the amount of the (a) phthalonitrile compound is too large, the basic physical properties of the phenol resin and the composition themselves may deteriorate, and it also becomes difficult to obtain an additional improvement in heat resistance.

The above-described (a) phthalonitrile compound can be manufactured according to a method known through the inventors' published patent publications, such as Patent Publication No. 2017-0082993 or Patent Publication No. 2018-0074043.

Meanwhile, the composition of the above-described embodiment may further include general additives in addition to the two components described above. For example, the composition may further include a general thermal curing agent within a range that does not inhibit the curing reaction of the (b) phenol resin prepolymer and (a) phthalonitrile compound. The type of such additional thermal curing agent is not particularly limited, and polyvalent amine curing agents, which have been known to be usable for curing phenol resins, can be used without particular limitation.

In addition, the composition of the above embodiment may further include a thermal curing catalyst such as an acid or base that has been used in the curing process of a phenol resin, and may further include additives according to the application field and purpose.

Non-limiting examples thereof include, but are not limited to, additives for the application or use thereof, including reinforcing fibers such as metal fibers, carbon fibers, glass fibers, aramid fibers, potassium titanate fibers, celluloid fibers, sepiolite fibers, ceramic fibers, and acrylic fibers; inorganic fillers such as barium sulfate, calcium carbonate, zirconia, alumina, zirconium silicate, and silicon carbide; and lubricants such as graphite, polytetrafluoroethylene, tungsten disulfide, molybdenum disulfide, and milled carbon fibers; and various other additives may be included.

The thermosetting resin composition of the above-described embodiment can be easily manufactured by mixing the above-described respective components in a solid state such as a powder, for example. In addition, the thermosetting resin composition can undergo curing and polymerization reaction at a temperature of 150 to 300° C. and normal pressure for 0.5 to 18 hours, according to typical curing and polymerization conditions of a phenol resin. Under these conditions, each component of the thermosetting resin composition can be cured and polymerized, so that a cured phenol resin and a composition thereof having superior heat resistance can be manufactured at a higher yield.

Meanwhile, according to another embodiment of the invention, a phenol resin composition prepared from the thermosetting resin composition of the above-described embodiment is provided. This phenol resin composition may include a cured product of the thermosetting resin composition of the above-described embodiment.

In a more specific example, the cured product may have a structure in which the (b) phenol resin prepolymer is polymerized; and the (a) phthalonitrile resin including a phthalonitrile compound or a unit derived from such a compound, wherein a cyano group of the phthalonitrile compound or the resin is cross-linked to a part of a hydroxyl group of the phenol resin. In addition, the phenol resin composition may further include a remainder of the phenol resin that is physically mixed with the phthalonitrile compound or the resin without forming a cross-link.

In addition, the phenol resin composition of the other embodiment can exhibit heat resistance and heat stability that are significantly improved compared to those previously known, as it has the cross-linking structure described above.

For example, when the phenol resin composition is analyzed by a thermogravimetric analyzer (TGA) at a temperature of 700°C, the composition may exhibit a characteristic in which the relative resin residual ratio calculated by Equation 1 below is 10% or more, or 30 to 500%, or 50 to 400%:

[Formula 1]

Relative resin residual ratio (%) = [(residual ratio of phenol resin composition after TGA analysis) - (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis)] / (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis) * 100

The above formula 1 represents the relative ratio of the residual ratio of the phenol resin composition to the residual ratio of the phenol resin manufactured without using a heat curing agent such as a phthalonitrile compound at a high temperature of 700°C. This represents the degree to which the heat stability of the phenol resin composition under 700°C is improved compared to the phenol resin manufactured without a heat curing agent, and since this relative resin residual ratio exhibits a characteristic of 10% or more, it can be supported that the phenol resin composition of other embodiments exhibits more improved high-temperature heat stability. In contrast, as supported by the test examples described below, when other types of phthalonitrile compounds, etc. were used as heat curing agents, it was confirmed that the heat stability was worse (the above formula 1 had a negative value) than the phenol resin manufactured without a heat curing agent, or the improvement in high-temperature heat stability was minimal.

Due to such excellent thermal stability, the phenol resin composition of other embodiments can be preferably used in various automobile parts or machine parts, etc., which require higher thermal stability.

According to the present invention, by using a specific phthalonitrile compound as a thermal curing agent together with a phenol resin prepolymer, it is possible to produce a cured phenol resin and a composition thereof exhibiting thermal stability significantly improved over that previously known, and to provide a thermally curable resin composition therefor.

By using the composition of the present invention, a phenol resin composition can be produced that exhibits heat resistance and thermal stability even at a temperature of up to 700°C, which is higher than previously known, and therefore, the phenol resin composition can be very preferably applied to high-performance machine parts or automobile parts that require higher heat resistance.

Hereinafter, preferred examples are presented to help understand the present invention. However, the following examples are only intended to illustrate the invention and do not limit the present invention to these examples.

Manufacturing Example 1. Synthesis of phthalonitrile compound (chemical formula 1a) (see Manufacturing Example 1 of Patent Publication No. 2018-0074043, etc.)

[Chemical formula 1a]

30.04 g of bisphenol F95 (a mixture of 95 wt% of 4,4'-dihydroxydiphenylmethane and 5 wt% of 2,4-dihydroxydiphenylmethane and 2,2'-dihydroxydiphenylmethane), 34.63 g of 4-nitrophthalonitrile, and 197.1 g of DMF (Dimethylformamide) were placed in a 3-neck round bottom flask (RBF) and stirred at room temperature to dissolve. After all of the introduced compounds were dissolved, 41.46 g of K ₂ CO ₃ , a base catalyst, was added, the temperature was increased to 85 ℃, and stirred for 12 hours.

After the reaction product was cooled to room temperature, K ₂ CO ₃ was removed using a filter, and the reaction product was collected by precipitating in water. The collected precipitate was extracted with methanol to remove the residual reactants, and dried in a vacuum oven to obtain the compound represented by the chemical formula 1a in a yield of 90 wt%.

Comparative Manufacturing Example 1. Synthesis of Phthalonitrile Compound (Chemical Formula 4)

[Chemical Formula 4]

2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane) 21.98 g, 4-nitrophthalonitrile 20.78 g, and DMF(Dimethylformamide) 124.9 g were added to a 3-neck round bottom flask (RBF) and stirred at room temperature to dissolve. After all of the added compounds were dissolved, 18.24 g _{of K2CO3} , a base catalyst, was added, the temperature was increased to 85 ℃, and stirred for 5 hours.

After the reaction product was cooled to room temperature, K ₂ CO ₃ was removed using a filter, and the reaction product was collected by precipitating in water. The collected precipitate was extracted with methanol to remove the residual reactants, and dried in a vacuum oven to obtain the compound represented by the chemical formula 4a in a yield of 78 wt%.

Comparative Manufacturing Example 2. Synthesis of Phthalonitrile Compound (Chemical Formula 5) (Refer to Manufacturing Example 1 of Patent Publication No. 2017-0064870, etc.)

[Chemical Formula 5]

4,4'-methylenebis[2-[(2-hydroxy-5-methylphenyl)methyl]-6-methyl-phenol (103.09 g) and 4-nitrophthalonitrile (152.39 g) were placed into a three-necked reaction flask, together with 145.95 g of potassium carbonate and 605.9 g of DMF (dimethyl formamide). The reaction flask had a capacity of 1000 mL and was equipped with a mechanical stirrer, a distillation apparatus, and a nitrogen inlet. Then, a nitrogen stream was passed through the reaction flask, and the flask was heated and stirred at about 85°C for about 5 hours. Then, the mixture in the flask was cooled to room temperature (about 20°C to 25°C), and the mixture was precipitated in 4 L of an aqueous hydrochloric acid solution (concentration: 0.2 N), and then filtered to remove the remaining inorganic salts and DMF. The powder obtained after filtration was dispersed again in methanol (1 L), filtered again to remove organic matter, and the reactant was vacuum-dried in an oven at 50°C to obtain the target product.

Comparative Manufacturing Example 3. Synthesis of Curing Agent

A compound represented by the following chemical formula 6 was synthesized by the following method.

[Chemical formula 6]

4,4'-oxydianiline (12.04 g) and DMF (Dimethylformamide) (70 g) were placed in a 3-neck round bottom flask and stirred at room temperature to dissolve. The mixture was cooled using a water bath, and 4,4'-oxydiphthalic anhydride (9.31 g) was slowly added in three portions together with 70 g of DMF. When all of the added compounds were dissolved, 28 g of toluene was added to the reactant for the azeotrope. A Dean-Stark apparatus and a reflux condenser were installed, and toluene was added to the Dean-Stark apparatus and filled. 3.1 mL of pyridine was added as a dehydration condensation catalyst, the temperature was raised to 170 ℃, and stirred for 3 hours.

The water generated by the formation of the imide ring was removed using a Dean-Stark apparatus, and the mixture was further stirred for 2 hours, and residual toluene and pyridine were removed. The reaction product was cooled to room temperature and recovered by precipitation in methanol. The recovered precipitate was extracted with methanol to remove residual reactants, and dried in a vacuum oven to obtain the compound represented by the chemical formula 6 in a yield of 85 wt%.

Examples 1 to 3. Preparation of thermosetting resin composition and phenol resin composition

The thermosetting resin compositions of Examples 1 to 3 were prepared by mixing the phenol resin prepolymer (Kangnam Hwasung Co., Ltd., KC-3002 product) and the phthalonitrile compound of Manufacturing Example 1 in a powdered state, respectively. The phthalonitrile compound was mixed in an amount of 10 parts by weight (Example 1), 30 parts by weight (Example 2), and 50 parts by weight (Example 3) based on 100 parts by weight of the phenol resin prepolymer, respectively, to prepare the thermosetting resin compositions of Examples 1 to 3.

For these thermosetting resin compositions, thermal curing and polymerization were performed at a temperature of 200°C for 2 hours and at a temperature of 250°C for 2 hours, respectively, to produce the phenol resin compositions of Examples 1 to 3.

Comparative Examples 1 to 3. Preparation of thermosetting resin composition and phenol resin

In Examples 1 to 3 above, the thermosetting resin composition and the phenol resin composition of Comparative Examples 1 to 3 were each prepared in the same manner as in Examples 1 to 3, except that the compound of Comparative Preparation Example 1 was used in an amount of 10 parts by weight (Comparative Example 1), 30 parts by weight (Comparative Example 2), and 50 parts by weight (Comparative Example 3) instead of the phthalonitrile compound of Preparation Example 1.

Comparative Examples 4 to 6. Preparation of thermosetting resin composition and phenol resin

In Examples 1 to 3 above, the thermosetting resin composition and phenol resin composition of Comparative Examples 4 to 6 were each prepared in the same manner as in Examples 1 to 3, except that the compound of Comparative Manufacturing Example 2 was used in an amount of 10 parts by weight (Comparative Example 4), 30 parts by weight (Comparative Example 5), and 50 parts by weight (Comparative Example 6), instead of the phthalonitrile compound of Manufacturing Example 1.

Comparative Examples 7 to 9. Preparation of thermosetting resin composition and phenol resin

First, the phthalonitrile compound of Manufacturing Example 1 and the curing agent of Comparative Manufacturing Example 3 were mixed, and then mixed and melted at a temperature of 200°C for 10 minutes, and cured to produce a phthalonitrile prepolymer.

In Examples 1 to 3 above, instead of the phthalonitrile compound of Preparation Example 1, the phthalonitrile prepolymer was used in an amount of 10 parts by weight (Comparative Example 7), 30 parts by weight (Comparative Example 8), and 50 parts by weight (Comparative Example 9), respectively, and the thermosetting resin compositions and phenol resins of Comparative Examples 7 to 9 were each prepared in the same manner as in Examples 1 to 3.

Test Example: Evaluation of physical properties of phenol resin composition after curing

TGA Analysis: TGA analysis was performed using a TGA Q900 instrument from Mettler-Toledo. The measurement was performed in an air atmosphere while heating the phenol resin compositions of the examples or comparative examples from 25°C to 900°C at a heating rate of 10°C/min.

Through this TGA analysis, the temperature-dependent weight loss and residual ratio for the resin composition were measured, and based on the measurement results, the relative resin residual ratio was calculated according to the following Equation 1. In calculating this relative resin residual ratio, the temperature-dependent weight loss and residual ratio measured by TGA analysis of a phenol resin manufactured under the same conditions without using a separate heat curing agent were used as the reference values for calculation:

[Formula 1]

Relative resin residual ratio (%) = [(residual ratio of phenol resin composition after TGA analysis) - (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis)] / (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis) * 100

temperature
(℃) Relative resin survival rate (%) Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 7 Comparative Example 8 Comparative Example 9 300 0.66 0.37 0.89 -0.36 -0.55 -0.70 0.33 0.42 1.01 -0.89 -0.91 -0.56 350 0.93 0.36 1.04 -0.37 -0.78 -1.05 0.33 0.46 1.24 -2.21 -2.18 -1.58 375 0.95 0.40 1.39 -0.60 -1.02 -1.36 0.26 0.38 1.61 -3.15 -3.07 -2.36 400 1.11 0.94 2.38 -0.75 -0.99 -1.20 0.14 0.37 2.32 -4.37 -4.35 -3.44 500 2.24 3.64 7.06 -1.06 -4.82 -2.67 -0.14 1.68 7.38 -11.71 -11.64 -10.01 600 0.81 15.00 28.91 -15.23 -43.40 -26.46 -9.62 0.58 22.96 -22.41 -22.48 -21.38 700 62.42 190.78 375.40 -50.50 -95.45 -80.22 -70.61 -8.29 77.22 -28.70 -29.40 -28.24

Referring to Table 1 above, it was confirmed that the phenol resin composition of the example exhibited excellent thermal stability with a high resin residue ratio compared to a phenol resin manufactured without a heat curing agent, and in particular, exhibited a significant improvement in thermal stability at high temperatures of 500 to 700°C.

In comparison, the phenol resin composition of the comparative example was found to exhibit poor thermal stability or only a minimal improvement in high-temperature thermal stability compared to a phenol resin manufactured without a thermal curing agent.

Claims (8)

(a) a phthalonitrile compound represented by the following chemical formula 1; and (b) a thermosetting resin composition comprising a phenol resin prepolymer represented by the following chemical formula 2 or 3:
[Chemical Formula 1]

In the above chemical formula 1,
L ^P2 is independently -O-, -S-, -C(=O)-, -S(=O)-, -S(=O) ₂ -, -C(CH ₃ ) ₂ -, -C(CF ₃ ) ₂ -, or -C(=O)NH-,
L ^P3 is an unsubstituted C _1-5 alkylene group,
[Chemical formula 2]

[Chemical Formula 3]

In the above chemical formulas 2 and 3, R ₁ to R ₃ are each independently hydrogen or C _1-5 alkyl, and n is an integer from 1 to 1000.
delete A thermosetting resin composition in claim 1, wherein in the chemical formula 1, L ^P2 is each independently -O-, and L ^P3 is an unsubstituted C _1-3 alkylene group.
A thermosetting resin composition in claim 1, wherein the (a) phthalonitrile compound is contained in an amount of 5 to 100 parts by weight relative to 100 parts by weight of the (b) phenol resin prepolymer.
A phenol resin composition comprising a cured product of the thermosetting resin composition according to claim 1.
In the fifth paragraph, the cured product comprises (b) a phenol resin polymerized with a phenol resin prepolymer; and (a) a phthalonitrile resin comprising a phthalonitrile compound or a unit derived therefrom.
A phenol resin composition in which some of the hydroxy groups of the phenol resin are cross-linked with a cyano group of the phthalonitrile compound or resin.
A phenol resin composition further comprising a physically mixed phenol resin without forming a cross-linking bond in claim 6.
In the 7th paragraph, when the phenol resin composition is analyzed by a thermogravimetric analyzer (TGA) at a temperature of 700°C, the phenol resin composition has a relative resin residual ratio of 10% or more calculated by the following formula 1:
[Formula 1]
Relative resin residual ratio (%) = [(residual ratio of phenol resin composition after TGA analysis) - (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis)] / (residual ratio of phenol resin manufactured without using a thermal curing agent after TGA analysis) * 100