JPH07278498A - Low-hydroxyl-value modified rosin, method for reducing hydroxyl value of rosin derivative, tackifier for pressure-sensitive adhesive, and pressure-sensitive adhesive composition - Google Patents

Abstract

PURPOSE:To obtain the subject rosin useful as a tackifier for enhancing the adhesion strength and holding power of a pressure-sensitive adhesive by reacting a hydroxylated rosin derivative with a specific compound. CONSTITUTION:This rosin is obtained by reacting a hydroxylated rosin derivative (a) with an isocyanate compound (e.g. tolylene diisocyanate). The derivative (a) is desirably an ester of a rosin with a polyhydric alcohol, a formalin-modified rosin, or a modified rosin alcohol, each preferably having a hydroxyl value of 10-180 (e.g. a glycerol ester of an asymmetric rosin).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rosin-based resin useful as a tackifier for pressure-sensitive adhesives, and in particular, a residual hydroxyl group in a rosin-based resin such as a polyhydric alcohol ester of rosin is replaced with an isocyanate compound. The present invention relates to a low hydroxyl value-modified rosin obtained by the reaction.

[0002]

2. Description of the Related Art Generally, in pressure-sensitive adhesives,
A rosin-based resin is used as a tackifier, and in particular, esters of rosin or various modified products of rosin and polyhydric alcohol are widely used.

Rosin is a mixture of various resin acids such as abietic acid, which has a hydrophenanthrene skeleton and has a double bond and a carboxyl group in one molecule in terms of chemical structure.

Thus, the acid value is lowered by esterifying the carboxyl group with a polyhydric alcohol.

[0005]

However, since the carboxyl group in rosin is tertiary, it has poor reactivity and the esterification reaction requires a long time at high temperature.

When the amount of polyhydric alcohol used is increased to reduce the acid value and more hydroxyl groups than the equivalent of the carboxyl group are reacted, the carboxyl group disappears, the acid value further decreases, and the reaction time is short. However, the resulting rosin ester has a large amount of unreacted hydroxyl groups and a high hydroxyl value.

On the other hand, if the hydroxyl value is lowered, the amount of the polyhydric alcohol used is suppressed and the hydroxyl group corresponding to the equivalent of the carboxyl group is reacted, but the reaction takes a long time and the acid value is increased. It is difficult to lower it sufficiently.

Therefore, when the rosin ester is used as a tackifier for an adhesive, the reactivity of the hydroxyl group hinders the adhesive function, so that the hydroxyl value is preferably low. In particular, when a curing agent is used in combination with the adhesive, the hydroxyl group consumes the curing agent and loses its function, so the hydroxyl value should be lowered.

For example, in an acrylic pressure sensitive adhesive,
When a rosin resin is used as a tackifier in an acrylic polymer, usually a small amount of a polyisocyanate compound is added before coating on the tape to improve the physical properties of the adhesive, and then aged after tape coating. It has been practiced to crosslink a polymer with a polyisocyanate compound and cure the polymer to increase the internal cohesive force.

At this time, if the rosin-based resin added as a tackifier has a high hydroxyl value, the hydroxyl group reacts with the isocyanate group of the polyisocyanate compound and the polyisocyanate compound does not sufficiently crosslink, resulting in a pressure-sensitive adhesive. As the adhesive, and the holding power as an adhesive is reduced.

This tendency is observed in pressure-sensitive adhesives other than acrylic ones, but the effect of the hydroxyl value is strongly observed especially when rosin ester is used as a tackifier in acrylic pressure-sensitive adhesives.

In order not to increase the hydroxyl value, it is preferable to react the polyhydric alcohol in the vicinity of the equivalent of the carboxyl group of rosin, but as mentioned above, the reaction takes a long time and the acid value is lowered sufficiently. I can't.

There is also a method in which the residual hydroxyl group in the rosin ester is reacted with a monobasic or dibasic acid, but this also has poor reactivity and requires a long reaction time, and also sufficiently lowers the hydroxyl value. I can't.

The present invention has been made in view of such circumstances, and is used as a tackifier for a pressure-sensitive adhesive composition by efficiently reducing the hydroxyl value of a rosin-based resin having a residual hydroxyl group such as a rosin ester. In this case, the purpose is not to impair the physical properties of the adhesive.

[0015]

The low hydroxyl value modified rosin of the present invention is characterized by reacting a rosin derivative having a hydroxyl group with a compound having an isocyanate group.

In this low-hydroxyl-modified rosin, the above-mentioned rosin-containing rosin derivative is an ester of rosin and polyhydric alcohol, formalin-modified rosin or modified rosin alcohol, which has a residual hydroxyl group. be able to.

The rosin derivative having a hydroxyl group is
It is suitable to apply the one having a hydroxyl value of 10 to 180.

The method of lowering the hydroxyl value of the rosin derivative of the present invention is characterized by reacting a rosin derivative having a hydroxyl group with a compound having an isocyanate group.

Further, the tackifier of the pressure-sensitive adhesive of the present invention is characterized by containing the above-mentioned low hydroxyl value modified rosin as a main component.

Further, the pressure-sensitive adhesive composition of the present invention may be prepared by adding 100 parts by weight of a polymer containing an ester of (meth) acrylic acid and an alkyl alcohol having 1 to 18 carbon atoms as a main component. Low hydroxyl value modified rosin 3 to 5
It is characterized in that 0 part by weight was added.

In this pressure-sensitive adhesive composition, as the polymer, a copolymer of 50% by weight or more of the (meth) acrylic acid alkyl ester and at least one vinyl monomer copolymerizable therewith Is preferably used.

The rosin derivative having a hydroxyl group in the present invention is one in which various modifications are made to rosin and the hydroxyl group remains as a result, and most commonly, there is an ester of rosin and a polyhydric alcohol. .

The rosin referred to here is gum rosin,
In addition to unmodified rosins such as wood rosin and tall oil rosin, there are disproportionated rosins based on these rosins, hydrogenated rosins, polymerized rosins, and purified products thereof. Further, a reinforced rosin obtained by adding maleic acid, maleic anhydride, fumaric acid, acrylic acid or the like to these rosins, or a modified product thereof can be used.

As the polyhydric alcohol, dihydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol and dipropylene glycol, trihydric alcohols such as glycerin and trimethylolpropane, tetrahydric alcohols such as pentaerythritol and diglycerin, and dihydric alcohols. There can be a hexahydric alcohol such as pentaerythritol.

The rosin ester is usually obtained by heating rosin and alcohol in an inert atmosphere such as nitrogen gas. The reaction conditions at this time are generally 250 to 280 ° C.
It takes 5 to 20 hours.

As other rosin derivatives having a hydroxyl group, formalin-modified rosin, modified rosin alcohol and the like can be applied, and further rosin modified by phenolization and the like can also be applied.

The hydroxyl value of the rosin derivative having a hydroxyl group is preferably about 10 to 180. Hydroxyl value is 10
If it is less than 100, it is difficult to further reduce the hydroxyl value by the present invention. Further, when the hydroxyl value exceeds 180, it is necessary to react a large amount of isocyanate compound accordingly, and the isocyanate group attacks the part other than the hydroxyl group by the side reaction, so that the physical properties of rosin are lowered and the hydroxyl value is sufficient. Hard to fall to.

As the compound having an isocyanate group in the present invention, an isocyanate compound such as tolylene diisocyanate, xylylene diisocyanate or hexamethylene diisocyanate can be used, and polyisocyanate obtained by adding a polyhydric alcohol to these compounds. Can also be used, that is, any compound having at least one isocyanate group in the compound can be widely used.

The low hydroxyl value-modified rosin of the present invention can be widely used as a tackifier for pressure-sensitive adhesives.
As the pressure-sensitive adhesive, in addition to the acrylic pressure-sensitive adhesive,
There are various rubber-based pressure-sensitive adhesives such as natural rubber-based, SBR-based, urethane-based, and CR-based, and these solution-based or water-dispersed pressure-sensitive adhesives can be applied.
It can also be used as a tackifier in a photopolymerization type or radiation polymerization type pressure-sensitive adhesive.

The low-hydroxyl-modified rosin of the present invention is particularly suitable as a tackifier in an acrylic pressure-sensitive adhesive composition, and is excellent in the effect of modifying the adhesive physical properties of the acrylic pressure-sensitive adhesive, especially in its holding power. ing.

The pressure-sensitive adhesive composition of the present invention is of the above-mentioned acrylic type, and is mainly composed of an ester of (meth) acrylic acid and an alkyl alcohol, to which a low hydroxyl value-modified rosin is added as a tackifier. Is added.

In this specification, the term (meth) acrylic acid is used as a generic term for acrylic acid and methacrylic acid. Acrylic acid, methacrylic acid, and these are used together at an arbitrary ratio. Including cases.

In the invention of the pressure-sensitive adhesive composition, the alkyl alcohol preferably has 1 to 18 carbon atoms, and these can be used alone or in combination.

That is, as the above ester, (meth)
Methyl acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n- (meth) acrylate
Butyl, sec-butyl (meth) acrylate, (meth)
Examples thereof include t-butyl acrylate, n-octyl (meth) acrylate, and 2-ethylhexyl acrylate. These are used alone or in combination of two or more.

The pressure-sensitive adhesive composition of the present invention uses a polymer containing the above-mentioned (meth) acrylic acid alkyl ester as a main component. This polymer may be a polymer of the above-mentioned (meth) acrylic acid alkyl ester alone, but is copolymerized with the above-mentioned (meth) acrylic acid alkyl ester as a sub-component for improving the adhesive force and cohesive force. It is also possible to copolymerize possible vinyl monomers.

Examples of the monomer copolymerizable with the (meth) acrylic acid alkyl ester include acidic monomers such as (meth) acrylic acid, maleic anhydride and itaconic acid, and 2-hydroxyethyl (meth) acrylate, (meth ) A monomer component having a functional group such as a hydroxyl group-containing monomer such as hydroxypropyl acrylate can be used alone or in combination of two or more.

Also, a copolymerization component of a monomer containing a nitrogen atom such as (meth) acrylic acid amides and vinylpyrrolidone and styrene, α-methylstyrene, vinyl acetate, glycidyl (meth) acrylate, etc. is the same. For the purpose of, the (meth) acrylic acid alkyl ester can be copolymerized.

(Meth) acrylic acid alkyl ester,
When copolymerizing with the above-mentioned various subcomponents, the main component should contain at least 50% by weight or more of (meth) acrylic acid alkyl ester.

In the pressure-sensitive adhesive composition of the present invention,
3 to 50 parts by weight of the low hydroxyl value modified rosin as a tackifier is added to 100 parts by weight of the polymer containing the (meth) acrylic acid alkyl ester as a main component. If the amount of the low hydroxyl value-modified rosin added is less than 3% by weight, the adhesive strength will be insufficient, and if it exceeds 50% by weight, a series of physical properties as an adhesive will deteriorate.

In the pressure-sensitive adhesive composition of the present invention, it is preferable to add a crosslinking agent or curing agent component in an amount of about 0.1 to 10% by weight in addition to the polymer and tackifier. Examples of the crosslinking agent or curing agent component include isocyanate compounds, epoxy compounds, modified epoxy compounds, metal chelate compounds, amino compounds and modified amino compounds.

Further, depending on the application, an organic or inorganic filler can be added to improve the strength of the pressure-sensitive adhesive, and a softening agent, a plasticizer or the like can be added to impart flexibility. You can also The addition amount of these fillers and softeners should be 10% by weight or less.

[0042]

The low-hydroxyl-modified rosin of the present invention has a much lower hydroxyl value than conventional rosin resins, and when added to a pressure-sensitive adhesive as a tackifier, it has few hydroxyl groups. In addition, the cross-linking agent and the curing agent component hardly react with the hydroxyl group to inhibit the cross-linking reaction, and the adhesive strength is large and the holding property is excellent.

When a rosin containing a hydroxyl group is reacted with an isocyanate compound, an isocyanate group having excellent reactivity is bonded to the hydroxyl group to form a urethane bond, and the hydroxyl group disappears. The hydroxyl value of the rosin can be efficiently reduced.

[0044]

【Example】

 [Production of acrylic polymer solution]

Reference Example 1 80 parts by weight of n-butyl acrylate as a polymer component, 20 parts by weight of 2-ethylhexyl acrylate and 4 parts of acrylic acid were placed in a reaction vessel equipped with a thermometer, a nitrogen introducing tube, a stirrer and a cooling tube. In addition, 60 parts by weight of toluene and 60 parts by weight of ethyl acetate are charged as a solvent, 0.3 part by weight of benzoyl peroxide is further added as a polymerization initiator, and the mixture is stirred for 20 minutes while blowing nitrogen gas. The air in the system was replaced with nitrogen.

After that, the temperature was raised to 75 ° C. by heating, and the polymerization reaction was carried out at the same temperature for 8 hours to obtain a solution of an acrylic copolymer having pressure-sensitive adhesiveness. As a result of measuring the molecular weight of this copolymer by gel permeation chromatography, the weight average molecular weight was about 530,000. The nonvolatile content of the copolymer solution was 45.8%.

Reference Example 2 In the reaction vessel described in Reference Example 1, 60 parts by weight of n-butyl acrylate, 40 parts by weight of 2-ethylhexyl acrylate, 3 parts by weight of acrylic acid and 2 parts of acrylic acid were added as polymer components.
0.5% by weight of hydroxyethyl, 36 parts by weight of toluene and 84 parts by weight of ethyl acetate as a solvent,
Furthermore, as a polymerization initiator, benzoyl peroxide 0.
Add 3 parts by weight and operate in the same manner as in Reference Example 1,
A polymerization reaction was carried out at 75 ° C. for 8 hours to obtain a solution of an acrylic copolymer having pressure-sensitive adhesiveness.

The weight average molecular weight of this copolymer was about 550,000, and the nonvolatile content of the copolymer solution was 45.6%.

Reference Example 3 In the reaction vessel described in Reference Example 1, 50 parts by weight of n-butyl acrylate, 50 parts by weight of 2-ethylhexyl acrylate, 4 parts by weight of acrylic acid, vinyl acetate 1.
5% by weight and 0.5% by weight of 2-hydroxyethyl acrylate, 24 parts by weight of toluene as a solvent and 100 parts by weight of ethyl acetate were charged, and 0.2 part by weight of benzoyl peroxide was further added as a polymerization initiator. By operating in the same procedure as in Example 1, a polymerization reaction was performed at 75 ° C. for 8 hours to obtain a solution of an acrylic copolymer having pressure-sensitive adhesiveness.

The weight average molecular weight of this copolymer was about 500,000, and the nonvolatile content of the copolymer solution was 45.5%.

[Production of Low Hydroxyl Value Modified Rosin] Example 1 A glycerol ester of asymmetric asymmetric rosin (Harima Kasei Co., Ltd.) was placed in a reaction vessel equipped with a thermometer, a nitrogen introducing tube, a stirrer, a cooling tube and a drainage tube. Harrier Star DS-90S: acid value 8, softening point 95 ° C, hydroxyl value 25, weight average molecular weight 6
00) 100 parts by weight and toluene 81% as a solvent.
Charge parts by weight, start stirring while blowing nitrogen gas,
Then, the temperature was raised to 100 ° C., and the mixture was stirred at the same temperature for 60 minutes to completely dissolve it.

Then, the mixture was cooled to 50 ° C., and tolylene diisocyanate (T, manufactured by Mitsui Toatsu Chemicals, Inc.
DI-80) (3.6 parts by weight) was added, the temperature was raised to 100 ° C. over 1 hour, and the reaction was performed at the same temperature for 4 hours.

As a result of infrared spectrum analysis, 23
It was confirmed that the absorption of the isocyanate group at 00 cm ^-1 had disappeared. The nonvolatile content of the obtained resin solution is 5
The resin had a softening point of 102 ° C., a weight average molecular weight of 800, and a hydroxyl value of 9.

Example 2 The reaction vessel described in Example 1 was charged with pentaerythritol ester of asymmetrical rosin (Hariema Kasei Co., Ltd. Hariestar DS-110S: acid value 15, softening point 106 ° C.).
A hydroxyl value of 45, a weight average molecular weight of 800) and 100 parts by weight of toluene and 83 parts by weight of toluene as a solvent are charged, stirring is started while blowing nitrogen gas, the temperature is raised to 100 ° C., and the mixture is stirred for 60 minutes at the same temperature. Completely dissolved.

Then, the mixture was cooled to 50 ° C., and 6.5 parts by weight of tolylene diisocyanate (same as above) was added at the same temperature.
After that, the temperature was raised to 100 ° C over 1 hour, and the temperature was increased to 5 ° C.
Reacted for hours.

As a result of infrared spectrum analysis, the absorption of isocyanate groups had disappeared. The nonvolatile content of the obtained resin solution is 55.5%, the softening point of the resin is 114.5 ° C,
The weight average molecular weight was 1,600 and the hydroxyl value was 15.

Example 3 In the reaction vessel described in Example 1, pentaerythritol ester of asymmetric asymmetric rosin (Hariema DS-90 manufactured by Harima Kasei Co., Ltd .: acid value 18, softening point 90 ° C., hydroxyl value 75, weight) 100 parts by weight of average molecular weight 750), and
87 parts by weight of toluene was charged as a solvent, stirring was started while blowing nitrogen gas, the temperature was raised to 100 ° C., and the mixture was stirred at the same temperature for 60 minutes to completely dissolve it.

Then, the mixture was cooled to 50 ° C., 10.5 parts by weight of tolylene diisocyanate (same as above) was added at the same temperature, the temperature was raised to 100 ° C. over 1 hour, and the reaction was carried out at the same temperature for 8 hours. It was

As a result of infrared spectrum analysis, the absorption of isocyanate groups had disappeared. The nonvolatile content of the obtained resin solution was 55.3%, the softening point of the resin was 113.3 ° C,
The weight average molecular weight was 4,400 and the hydroxyl value was 21.

Example 4 The reaction vessel described in Example 1 was charged with 100 parts by weight of pentaerythritol ester of disproportionated rosin (Hariester DS-90 manufactured by Harima Kasei Co., Ltd.) and 87 parts by weight of toluene as a solvent. Then, stirring was started while blowing nitrogen gas, the temperature was raised to 100 ° C., and the mixture was stirred at the same temperature for 60 minutes to completely dissolve it.

Then, the mixture was cooled to 50 ° C., 10.2 parts by weight of hexamethylene diisocyanate (Takenate 700 manufactured by Takeda Pharmaceutical Co., Ltd.) was added at the same temperature, and then 1
The temperature was raised to 100 ° C over time, and the reaction was carried out at the same temperature for 5 hours.

As a result of infrared spectrum analysis, the absorption of isocyanate groups had disappeared. The nonvolatile content of the obtained resin solution was 55.5%, the softening point of the resin was 105 ° C., the weight average molecular weight was 3,800, and the hydroxyl value was 15.

Example 5 The reaction vessel described in Example 1 was charged with rosin alcohol (Rika Hercules Co., Ltd. avitol: hydroxyl value 14).
100 parts by weight of 8) and 94 parts by weight of toluene as a solvent were charged, and stirring was started while blowing nitrogen gas.
The temperature was raised to 0 ° C., and the mixture was stirred at the same temperature for 60 minutes and completely dissolved.

Next, 21.5 parts by weight of tolylene diisocyanate (same as above) was added at the same temperature, the temperature was raised to 100 ° C. over 1 hour, and the reaction was carried out at the same temperature for 5 hours.

As a result of infrared spectrum analysis, the absorption of isocyanate groups had disappeared. The nonvolatile content of the obtained resin solution was 55.2%, the softening point of the resin was 81.2 ° C, the weight average molecular weight was 560, and the hydroxyl value was 12.

Comparative Example 1 A reaction vessel equipped with a thermometer, a nitrogen inlet tube, a stirrer and a drainage tube was placed in an asymmetric rosin (acid value 155, softening point 80 ° C.).
Charge 100 parts by weight and blow while blowing nitrogen gas to 200
It is heated up to ℃ and melted by heating. After the rosin was completely melted, 9.8 parts by weight of glycerin was added, the temperature was raised to 270 ° C., and the mixture was reacted for 12 hours at the same temperature to obtain a glycerol ester of asymmetric rosin. The rosin ester had a softening point of 95 ° C., an acid value of 6, and a hydroxyl value of 15.

Comparative Example 2 The reaction vessel described in Comparative Example 1 was charged with 100 parts by weight of disproportionated rosin and heated to 200 ° C. while being blown with nitrogen gas to be melted by heating. After the rosin was completely melted, 11.3 parts by weight of pentaerythritol was added, the temperature was raised to 280 ° C., and the reaction was carried out at the same temperature for 15 hours to obtain a pentaerythritol ester of asymmetric rosin. The rosin ester had a softening point of 108 ° C., an acid value of 13, and a hydroxyl value of 28.

Comparative Example 3 100 parts by weight of polymerized rosin (Polypar resin made by Rika Hercules Co., Ltd .: acid value 150, softening point 94 ° C.) was charged into the reaction vessel described in Comparative Example 1 at 180 ° C. while blowing nitrogen gas. It is heated up to and melted. After the rosin was completely melted, 9.5 parts by weight of glycerin was added,
The temperature was raised to 0 ° C., and the reaction was carried out at the same temperature for 13 hours to obtain a glycerin ester of polymerized rosin. The rosin ester has a softening point of 115 ° C., an acid value of 11.3 and a hydroxyl value of 2
It was 4.

Comparative Example 4 Polymerized rosin (same as above) 100 was added to the reaction vessel described in Comparative Example 1.
Part by weight is charged and the temperature is raised to 200 ° C. while blowing nitrogen gas to heat and melt. After the rosin has completely melted,
Add 11.0 parts by weight of pentaerythritol and add 280
The temperature was raised to 0 ° C. and the reaction was carried out at the same temperature for 14 hours to obtain pentaerythritol ester of polymerized rosin. The rosin ester had a softening point of 123 ° C., an acid value of 13.4 and a hydroxyl value of 31.

[Preparation of Pressure Sensitive Adhesive Composition] Reference Examples 1 to 1
Toluene was added to the solution of the acrylic copolymer obtained in 3, and the non-volatile content was diluted to 45%.

Toluene was added to the resin solutions obtained in Examples 1 to 5 and the rosin ester obtained in Comparative Examples 1 to 4 was dissolved in toluene to adjust the nonvolatile content to 45%. It was adjusted.

Next, the above-mentioned acrylic copolymer and the rosin ester solutions of Examples and Comparative Examples were blended as shown in Table 1 and mixed by stirring, and then tolylene diisocyanate and tri-diisocyanate were used as curing agents. Ethyl acetate was added to an ethyl acetate solution of an adduct with methylolpropane (Nippon Polyurethane Industry Co., Ltd .: Coronate L) to obtain 4 active ingredients.
What was adjusted to 5% was added and mixed in the amounts shown in Table 1,
A solution of the pressure sensitive adhesive composition was prepared.

Next, a 25 μm thick polyester film (Lumirror film manufactured by Toray Industries, Inc.) was coated with the pressure-sensitive adhesive composition solution using an applicator so that the coating film thickness after drying was 40 μm. It was dried in a hot air dryer at 80 ° C. for 3 minutes to obtain a pressure-sensitive adhesive film.

After the pressure-sensitive adhesive film was stored in a thermo-hygrostat at a temperature of 20 ° C. and a humidity of 65% for 7 days, the adhesive physical properties were tested.

[Physical property test] (1) Adhesiveness (tack) According to JIS Z-0237, a test piece was fixed on a slope having an inclination angle of 30 degrees, and 1/32 to 32 / on the test piece.
A 32-inch steel ball was rolled and the No. of the maximum steel ball stopped on the adhesive surface was measured.

(2) Adhesive Strength (Peel) According to JIS Z-0237, the peel strength was measured using a stainless steel plate as an adherend. Number unit is g / 25
mm.

(3) Holding power (creep) According to JIS Z-0237, a stainless plate was used as an adherend, and the offset distance of the tape was measured after 24 hours in an atmosphere of 25 ° C and 80 ° C. The unit of the numerical value is mm.

[Results] Table 1 shows the composition and adhesive properties of the pressure-sensitive adhesive composition.

[0079]

[Table 1]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ansho Sanada 67-4 Mizuguchi, Noguchi-cho, Kakogawa-shi, Hyogo 4-4, Harima Kasei Co., Ltd. Central Research Laboratory (72) Takashi Iwasaki 671 Mizusui, Noguchi-cho, Kakogawa-shi, Hyogo Address 4 Harima Kasei Co., Ltd. Central Research Laboratory

Claims (7)

[Claims] 1. A low-hydroxyl-modified rosin, which is obtained by reacting a rosin derivative having a hydroxyl group with a compound having an isocyanate group. 2. The rosin derivative having a hydroxyl group is an ester of rosin and a polyhydric alcohol, a formalin-modified rosin or a modified rosin alcohol, and has a residual hydroxyl group. Low hydroxyl value modified rosin described 3. The low-hydroxyl-modified rosin according to claim 1, wherein the rosin derivative having a hydroxyl group has a hydroxyl value of 10 to 180. 4. A method for reducing the hydroxyl value of a rosin derivative, which comprises reacting a rosin derivative having a hydroxyl group with a compound having an isocyanate group. 5. A tackifier for a pressure-sensitive adhesive, which comprises the low hydroxyl group-modified rosin according to claim 1, 2 or 3 as a main component. 6. A low hydroxyl value-modified rosin 3 according to claim 1, 2 or 3 is added to 100 parts by weight of a polymer containing an ester of (meth) acrylic acid and an alkyl alcohol having 1 to 18 carbon atoms as a main component. To 50 parts by weight are added. 7. The polymer is a copolymer of 50% by weight or more of the (meth) acrylic acid alkyl ester and at least one vinyl monomer copolymerizable therewith. The pressure-sensitive adhesive composition according to claim 6.