WO2006017302A1

WO2006017302A1 - Acrylic viscoelastic material

Info

Publication number: WO2006017302A1
Application number: PCT/US2005/024725
Authority: WO
Inventors: Yorinobu Takamatsu; Ken Tokoro
Original assignee: 3M Innovative Properties Company
Priority date: 2004-07-12
Filing date: 2005-07-11
Publication date: 2006-02-16
Also published as: JP2006028224A; JP4606078B2

Abstract

An acrylic viscoelastic material capable of maintaining high vibration damping performance across a wide temperature range. The acrylic viscoelastic material generally comprises a polymer produced by copolymerizing a monomer mixture comprising (1) 80 to 99 wt% of an alkyl acrylate ester whose homopolymer has a glass transition temperature (Tg) of 0°C or lower, and (2) 1 to 20 wt% of methacrylic acid, or a methacrylic acid ester whose homopolymer has a glass transition temperature of 40°C or higher.

Description

Acrylic ¥iscoelastic Material

Field of Invention

The present invention relates to acrylic viscoelastic materials and to vibration damping materials containing such viscoelastic materials.

Background

Vibration damping materials are known throughout a variety of fields. For example, materials used to form scaffolding structures for buildings in the field of architecture are generally capable of absorbing impact displacement and vibration from wind, earthquakes and similar phenomena, while electronic parts such as computer disk drives are protected by materials which absorb vibrations to prevent damage or malfunction caused by the vibrations. Viscoelastic damper materials are generally used for absorption of such displacement and vibrations.

Viscoelastic damping materials generally exhibit stable damping performance across a range of temperatures encountered during use. Published Patent Application WO 96/035458, for example, discloses a tackifying composition comprising (a) a polydiorganosiloxane polyurea segment copolymer obtained by reaction between a polydiorganosiloxanediamine and a polyisocyanate, and (b) a silicate resin, as a vibration damping composition wherein the ratio

between the storage elastic modulus (G') at 0°C and at 40°C, as determined by dynamic viscoelastic measurement in shear mode, is no greater than 10, and the loss tangent (tanδ) based on the measurement at 0- 40°C is at least 0.4. Thus, satisfactorily stable vibration damping performance is said to be exhibited in a relatively wide temperature range. Such silicone-based viscoelastic compositions are useful because they generally exhibit good weather and heat resistance and are reliable with prolonged use. These materials are expensive, and because they exhibit generally poor adhesive properties they do not readily affix to adherends by adhesion. Published Patent Application WO 01/074964 discloses a vibration damping composition having a ratio (G'_0°c/G'_40°c) between the storage elastic modulus (G') at 0⁰C and at 40⁰C, as determined by dynamic viscoelastic measurement in shear mode, of no greater than 15, and a loss tangent (tanδ) based on the measurement at 0-40⁰C of at least 0.4. The constituent component is a block copolymer comprising an aromatic vinyl (for example, styrene) polymer block (a) with a number-average molecular weight of no greater than 10,000, and an isobutylene polymer block (b). Such synthetic rubber-based block copolymers, however, generally have poor weather and heat resistance, and thus exhibit poor reliability with prolonged use. Moreover, the poor adhesive properties impair their attachment to adherends.

Summary of the Invention

In one aspect, the present invention provides an acrylic viscoelastic material capable of maintaining a high degree of vibration damping performance across a wide range of temperatures .

According to one mode of the invention, there is provided an acrylic viscoelastic material composed of a polymer obtained by copolymerizing a monomer mixture comprising: (1) 80-99 wt% of an alkyl acrylate ester whose homopolymer has a glass transition temperature (Tg) of 0⁰C or lower; and (2) 1 to 20 wt% of methacrylic acid, or a methacrylic acid ester, whose homopolymer has a glass transition temperature of 40⁰C or higher, wherein the above wt% is based on the total weight of the monomers in the monomer mixture.

An acrylic viscoelastic material composed of a polymer obtained by copolymerization of the aforementioned monomer mixture generally exhibits low temperature dependency of the storage elastic modulus and loss tangent (tanδ) as determined by dynamic viscoelastic measurement, and therefore is able to maintain high vibration damping performance across a wide temperature range.

In addition, the materials of the invention generally exhibit desirable weather and heat resistance, and higher reliability of performance with prolonged use when compared to conventional synthetic rubber vibration damping materials. Brief Description of the Drawings

Fig. 1 is a graph showing the storage elastic modulus (G') values for the examples. Fig. 2 is a graph showing the loss tangent (tanδ) values for the examples. Fig. 3 is a graph showing the dynamic viscoelastic properties at different temperatures for the examples.

Fig. 4 is a graph showing the dynamic viscoelastic properties at different deformations for the examples.

Detailed Description

A description of preferred, but non-limiting embodiments of the invention follows. The temperature dependency of the acrylic viscoelastic material of the invention will be explained first. During copolymerization of a monomer mixture containing component (1) an acrylic acid ester monomer and component (2) a methacrylic acid or a methacrylic acid ester monomer, the methacrylic acid or methacrylic acid ester (component 2)) reacts rapidly with itself (reaction between component 2)) while the reaction with the acrylic acid ester (reaction between component (2) and component (I)) is slower. Consequently, the monomer components (1) and (2) do not randomly copolymerize, and the resulting polymer has a wide compositional distribution. That is, the resulting polymer comprises a component rich in component (2) monomer units and a component rich in component (1) monomer units. As a result, the glass transition temperature (Tg) of the component rich in component (1) is low since component (1) is the dominant factor, while the glass transition temperature (Tg) of the component rich in component (2) is high since component (2) is the dominant factor. Thus, the loss tangent (tanδ) of the obtained polymer is high in a wide temperature range, while the temperature dependency of the storage elastic modulus G' is low. Throughout the present specification, the "storage elastic modulus" and "loss tangent (tanδ)", unless otherwise indicated, are the values read from a chart obtained upon measurement from -60°C to 160°C with heating at a temperature elevating rate of 3°C/min, by dynamic viscoelastic measurement in shear mode at a frequency of 1.0 Hz and 50% deformation. For the monomer copolymerizability, a correlation exists between the reactivity ratios rl and r2 of the monomers (J. Brandrup, E.H. Immergut, E.A. Gralke, Polymer Handbook 4th Edition, "Free Radical Copolymerization Reactivity Ratios").

-M₁ * + M₁ → -M₁M₁ * kl 1 -M₁* + M₂ → -M₁M₂* kl2

-M₂* + M₂ → -M₂M₂* k22

-M₂* + M₁ → -M₂M₁ * k21

-M*: radical from monomer M in polymer, k: reaction rate, rl = kl l/kl2, r2 = k22/k21

The reactivity ratio between the acrylic acid ester monomer (M₁) and the methacrylic acid or methacrylic acid ester and acrylic acid ester monomer (M₂) is rl « r2. For example, in the case of butyl acrylate (M₁) and methacrylic acid (M₂), rl = 0.31, r2 = 1.25, and the reaction between methacrylic acid itself is rapid, resulting in a wide distribution of the monomer unit composition of the polymer.

The following formulae for the Q and e values according to Alfrey and Price are well known.

Q2 = (Ql/rl) exp{-el(el-e2)}

Ql = (Q2/r2) exp{-e2(e2-el)}

Ql: Q value of monomer (Mi), el: e value of monomer (M₁)

Ql: Q value of monomer (Mi), el: e value of monomer (M₁) rl : Reactivity ratio of monomer (M₁) r2: Reactivity ratio of monomer (M₂)

The Q and e values are described in detail in J. Brandrup, E.H. Immergut, Grulke, Polymer Handbook 4th Edition, "Q and e Value for Free Radical Copolymerizations of Vinyl Monomers and Telogens". The starting monomer component (1) of the viscoelastic material of the invention is a C4-12 alkyl group-containing alkyl acrylate ester. As examples of compounds to be used for component (1) there may be mentioned n-butyl acrylate, 2-ethylhexyl acrylate and isooctyl acrylate. Because the homopolymers of these monomers have Tg values lower than 0°C, they are able to confer vibration damping performance at low temperature, as well as tackiness, to the resulting polymer. This monomer component is generally present at 80-99 wt% based on the total weight of the monomers in the monomer mixture. If the amount of the component is too small, the vibration damping performance at low temperature may be decreased and the resulting polymer may have low tackiness. Also, the material may exhibit low flexibility and poor attachment to adherends. If the amount of the component is too great, the temperature dependency of the performance may be increased or, in other words, the vibration damping performance at high temperature may be lower such that the usable temperature range of the material may be restricted.

The starting monomer component (2) of the viscoelastic material of the invention is methacrylic acid, or a methacrylic acid ester whose homopolymer has a glass transition temperature of 40⁰C or higher, and as examples there may be mentioned methacrylic acid, methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate, benzyl methacrylate, tert-butyl methacrylate, cyclohexyl methacrylate, tetrahydrofurfuryl methacrylate and isobornyl methacrylate. Preferably, component (2) has a Q value of 0.7 or greater. Component (2) preferably has a copolymerizable ratio rl and r2 with respect to component (1) which satisfies the inequality r2/rl > 3. Examples of monomers satisfying this condition include methacrylic acid (Q = 0.98, rl = 0.31, r2 = 1.25 (values with respect to butyl acrylate)), methyl methacrylate (Q = 0.78, rl = 0.13, r2 = 0.92 (values with respect to butyl acrylate)), ethyl methacrylate (Q = 0.76, rl = 0.28, r2 = 2.25 (values with respect to butyl acrylate)), 2-hydroxyethyl methacrylate (Q = 1.78, rl = 0.09, r2 = 4.75 (values with respect to butyl acrylate)), isobutyl methacrylate (Q = 0.82, rl = 0.28, r2 = 2.52 (values with respect to butyl acrylate)) and benzyl methacrylate (Q = 0.88, rl = 0.28, r2 = 2.76 (values with respect to butyl acrylate)). Component (2) is generally present in an amount of 1-20 wt% based on the total weight of the monomers in the monomer mixture. If the amount of component (2) is too great, the vibration damping performance at low temperature may decrease and the resulting polymer may have low tackiness. The material may also exhibit lower flexibility and poorer attachment to adherends. If the amount of this component is too small, the temperature dependency of the performance may be increased or, in other words, the vibration damping performance at high temperature may be lower such that the usable temperature range of the material may be restricted.

The monomer mixture used according to the invention may optionally contain additional monomers. In order to maintain or increase the cohesiveness of the obtained monomer, for example, a polar monomer such as acrylic acid, acrylamide, acrylonitrile, hydroxyethyl acrylate, maleic acid or itaconic acid may also be included in the monomer mixture. Such polar monomers will generally be used in an amount up to about 10 wt% based on the total weight of the monomer mixture.

One or more crosslinking agents may also be used to improve one or more physical properties of the obtained material including shear or other mechanical strength or thermal properties such as heat resistance. It may prove convenient to use crosslinkable monomers such as polyvalent acrylates as crosslinking agents. Polyvalent acrylates suitable as crosslinking agents include 1,6-hexanediol diacrylate. A crosslinking agent will generally be used in an amount of 0.01-5 parts by weight with respect to 100 parts by weight of the monomer mixture. The polymer composing the acrylic viscoelastic material of the invention may be produced by copolymerization of the aforementioned monomer mixtures. For example, copolymerization may be carried out by thermal polymerization or by radiation induced polymerization with an electron beam or ultraviolet rays. In the case of thermal polymerization, a thermal polymerization initiator will generally be used at no greater than 5 parts by weight to 100 parts by weight of the monomer mixture, in the case of ultraviolet polymerization, a photopolymerization initiator will generally be used at no greater than 5 parts by weight to 100 parts by weight of the monomer mixture, and in most cases of electron beam polymerization no initiator is necessary.

One suitalbe production process for the polymer may be illustrated as an example of photopolymerization with ultraviolet rays. When a polymer is obtained by photopolymerization such as ultraviolet polymerization, the photopolymerization initiator will usually be included in the amount described above. Examples of initiators include acetophenone-based initiators such as 4-(2-hydroxyethoxy)phenyl (2-hydroxy-2- propyl)ketone (commercially available under the "DAROCURE 2959" trade name from Merck), α-hydroxy-α,α'-dimethylacetophenone (commercially available under the

"DAROCURE 1173" trade name from Merck) and methoxyacetophenone, 2,2-dimethoxy- 2-phenylacetophenone; benzoin-based ether initiators such as benzoin ethyl ether and benzoin isopropyl ether; ketal-based initiators such as benzyl dimethyl ketal; as well as halogenated ketones, acylphosphine oxides and acylphosphonates. The photopolymerization initiator may be included at 0.01-5.0 parts by weight with respect to 100 parts by weight of the monomer mixture. If the content is less than 0.1 part by weight, the photopolymerization initiator will be consumed at the start of polymerization by the light energy, tending to result in residue of the unreacted monomer and reduced cohesion. Conversely, at greater than 5.0 parts by weight the polymerization reaction rate will be increased but with significant residual odor generated by decomposition of the photopolymerization initiator, and the adhesive power will be reduced due to increasing variation in the molecular weight of the obtained polymer.

Ultraviolet rays will generally be used for the light irradiation. The ultraviolet lamp used may be one having an emission spectrum distribution in a light wavelength range of 300-400 nm, examples of which include chemical lamps, blacklight lamps (product of Toshiba Denzai), low-pressure mercury lamps, high-pressure mercury lamps, extra-high pressure mercury lamps, metal halide lamps, microwave-excited mercury lamps and the like.

The monomer mixture may be loaded into an appropriate reactor, the photopolymerization initiator added to the monomer mixture, and one or more crosslinking agents and other additives may be added as necessary prior to polymerization reaction by a suitable dose of light irradiation such as ultraviolet rays, to obtain a polymer. The reaction is generally conducted in an inert atmosphere of nitrogen or the like. The light irradiation may be carried out in a single shot or across two or more stages. For example, the monomer mixture containing the polymerization initiator may be partially reacted by light irradiation to obtain a polymerizable prepolymer syrup of an appropriate viscosity. Additional monomers may then be added to the syrup, and the mixture introduced between two release-treated bases made of polyethylene terephthalate (PET) and then exposed to light irradiation to obtain the final polymer. The resulting polymer is a cohesive viscoelastic material and can be directly used as a vibration damping material. An alternative structure is one having a rigid sheet sandwiched between multiple polymer sheets. Employing a bonded structure having the aforementioned polymer sheet laminated with one or more rigid sheets can further increase the vibration damping property.

Suitable rigid sheets include metal and polymer resin sheets. Suitable metal sheets include those made of iron materials such as steel sheets, stainless steel, aluminum-plated steel sheets, zinc-plated steel sheets and epoxy-coated steel sheets, and those made of metal materials such as zinc, aluminum and titanium. Suitable polymer resin sheets include sheets made of materials such as fiber-reinforced plastic (FRP), vinyl chloride sheet, polyethylene, polypropylene, polycarbonate and acrylic materials. Such metal sheets and polymer resin sheets may be used in combination with one another.

The polymer for the viscoelastic material of the invention may also optionally include a tackifying resin to impart a high level of adhesion force to the viscoelastic product. Suitable tackifying resins include rosin-based resins, modified rosin-based resins (hydrogenated rosin-based resins, disproportionated rosin resins, polymerized rosin-based resins, etc.), terpene resins, terpene phenol resins, aromatic modified terpene resins, C₅ and Cg-based petroleum resins, coumarone resins and the like.

The polymer for the viscoelastic material of the invention may also optionally include commonly employed additives such as thickening agents, thixotropic agents, bulking agents and fillers. As thickening agents there may be used acrylic rubber, epichlorhydrin rubber, isoprene rubber, butyl rubber and the like. As thixotropic agents there may be used colloidal silica, polyvinylpyrrolidone and the like. As bulking agents there may be used calcium carbonate, titanium oxide, clay and the like. As fillers there may be used inorganic hollow bodies such as glass balloons, alumina balloons and ceramic balloons; organic spheres such as nylon beads, acrylic beads and silicone beads; organic hollow bodies such as vinylidene chloride balloons and acrylic balloons, and filaments of polyester, rayon, nylon and the like.

By employing the monomer mixtures of the type described herein, the viscoelastic material of the invention can exhibit a ratio (G'o/G'₄₀) between the storage elastic modulus GO at 0⁰C and the storage elastic modulus G'₄₀ at 40°C of no greater than 10, and a loss tangent (tanδ) of consistently 0.4 or greater at 0-40°C. Thus, the viscoelastic material of the invention is useful as a vibration damping material with low temperature dependency. The viscoelastic material of the invention may be utilized, for example, as a viscoelastic vibration damping material which absorbs impact displacement and vibration due to wind, earthquakes and similar phenomena in materials used to form scaffolding structures for buildings in the field of architecture, or as a viscoelastic damping material which absorbs vibrations in computer disk drives.

Examples

After 96 parts by weight of 2-ethylhexyl acrylate (2-EHA) (AEH by Nihon Shokubai), 4 parts by weight of acrylic acid (AA) (product of Wako Pure Chemical Industries Co., Ltd.), 5 parts by weight of methacrylic acid (MAA) (product of Wako Pure Chemical Industries Co., Ltd.) and 0.04 part by weight of Irgacure 651 (CIBA-GEIGY, 2,2-dimethoxy-2-phenylacetophenone) as a photoinitiator, were stirred to uniformity in a flask, bubbling was accomplished with nitrogen gas and the mixture was irradiated with ultraviolet rays at a cumulative dose of 90 mJ/cm² using a fluorescent black electric bulb (Sylvania F20T12B) having a light wavelength of 300-400 nm and a maximum emission spectrum of 351 nm, to prepare a polymerizable prepolymer syrup (A) with a reactivity of 10% and a viscosity of 1000 cps.

To 100 parts by weight of the prepared syrup (A) there were further added 0.21 part by weight of 1,6-hexanediol diacrylate (HDDA) (NK Ester A-HD by Shin-Nakamura Kagaku Kogyo Co., Ltd.) and 0.16 part by weight of Irg. 651, and the mixture was stirred to form a uniform solution. The obtained solution was placed between two release-treated polyethylene terephthalate (PET) sheets at a film thickness of 1.0 mm, and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

Dynamic Viscoelastic Measurement

The dynamic viscoelastic properties of the obtained polymer were measured. The dynamic viscoelastic properties measured were the storage elastic modulus G' and loss tangent (tanδ), determined by measurement from -60⁰C to 160⁰C with heating at a temperature elevating rate of 3°C/min, in shear mode at a frequency of 1.0 Hz and a deformation of 50% using an Advanced Rheometric Expansion System (ARES) by Rheometric Scientific.

The results are shown in Table 1: G' (0°C)/G' (40⁰C) = 8.8, tanδ (Min-Max) = 0.46-0.57. Thus, a high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low. Example 2

Ultraviolet rays were irradiated at a cumulative dose of 90 mJ/cm² with a Sylvania F20T12B in the same manner as Example 1, using 96 parts by weight of 2-ethylhexyl acrylate (2-EHA), 4 parts by weight of acrylic acid (AA) and 0.04 part by weight of Irg. 651, to prepare a polymerizable prepolymer syrup (B) with a reactivity of 11% and a viscosity of 10,240 cps. To 100 parts by weight of the prepared syrup (B) there were further added 5 parts by weight of methacrylic acid (MAA), 0.21 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.16 part by weight of Irg. 651, and the mixture was stirred to form a uniform solution. The obtained solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm, and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1: G' (0°C)/G' (40°C) = 6.2, tanδ (Min-Max) = 0.62-0.67. Thus, a high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low. The results are also shown in the graphs of Figs. 1 and 2.

Example 3

To 100 parts by weight of the syrup (B) obtained in Example 2 there were further added 15 parts by weight of 2-hydroxyethyl methacrylate (HEMA) (product of Wako Pure Chemical Industries Co., Ltd.), 0.12 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.17 part by weight of Irg. 651, the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. A high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low. Example 4

Ultraviolet rays were irradiated at a cumulative dose of 90 mJ/cm² with a Sylvania F20T12B in the same manner as Example 2, using 98 parts by weight of 2-ethylhexyl acrylate (2-EHA), 2 parts by weight of acrylic acid (AA) and 0.04 part by weight of Irg. 651, to prepare a polymerizable prepolymer syrup (C) with a reactivity of 11% and a viscosity of 9760 cps.

To 100 parts by weight of the prepared syrup (C) there were further added 5 parts by weight of methacrylic acid (MAA), 0.11 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.16 part by weight of Irg. 651 , the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. A high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low. Example 5

To 100 parts by weight of the syrup (C) obtained in Example 4 there were further added 20 parts by weight of 2-hydroxyethyl methacrylate (HEMA), 0.24 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.18 part by weight of Irg. 651, the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. A high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low. The results are also shown in the graphs of Figs. 1 and 2.

Example 6

Ultraviolet rays were irradiated at a cumulative dose of 90 mJ/cm² with a Sylvania F20T12B in the same manner as Example 2, using 94 parts by weight of 2-ethylhexyl acrylate (2-EHA), 6 parts by weight of acrylic acid (AA) and 0.04 part by weight of Irg. 651, to prepare a polymerizable prepolymer syrup (C) with a reactivity of 11% and a viscosity of 16,250 cps.

To 100 parts by weight of the prepared syrup (D) there were further added 5 parts by weight of methacrylic acid (MAA), 0.11 part by weight of 1 ,6-hexanediol diacrylate (HDDA) and 0.16 part by weight of Irg. 651, the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. A high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low.

Example 7

To 100 parts by weight of the syrup (D) obtained in Example 6 there were further added 10 parts by weight of 2-hydroxyethyl methacrylate (HEMA), 0.11 part by weight of 1 ,6-hexanediol diacrylate (HDDA) and 0.17 part by weight of Irg. 651 , the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing. The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. A high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low.

Comparative Example 1 To the polymerizable prepolymer syrup (B) obtained in Example 2 there were added 0.2 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.15 part by weight of Irg. 651, the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1. The storage elastic modulus G had low temperature dependency, G' (0°C)/G' (40⁰C) = 2.4, but the loss tangent had high temperature dependency and a low value, tanδ (Min-Max) = 0.25-0.69. Comparative Example 2

Ultraviolet rays were irradiated at a cumulative dose of 90 mJ/cm² with a Sylvania F20T12B in the same manner as Example 2, using 87.5 parts by weight of 2-ethylhexyl acrylate (2-EHA), 12.5 parts by weight of acrylic acid (AA) and 0.04 part by weight of Irg. 651, to prepare a polymerizable prepolymer syrup (E) with a reactivity of 11% and a viscosity of 9000 cps.

To 100 parts by weight of the prepared syrup (E) there were further added 0.1 part by weight of 1,6-hexanediol diacrylate (HDDA) and 0.15 part by weight of Irg. 651, the mixture was stirred to form a uniform solution in the same manner as Example 2, and the solution was placed between two release-treated PET sheets at a film thickness of 1.0 mm and then exposed to ultraviolet rays at a cumulative dose of 1500 mJ/cm² using a Sylvania F20T12B for complete reaction curing.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 1: tanδ (Min-Max) = 0.63-1.21, i.e. a large value but with high temperature dependency; G' (0°C)/G' (40°C) = 20.8, with very high temperature dependency.

The results for Examples 1 to 7 indicate that the viscoelastic material of the invention has low temperature dependency of G' and a high tanδ value, and is therefore a satisfactory material exhibiting performance with minimal temperature dependency. On the other hand, the results for Comparative Examples 1 and 2 indicated that lowering the glass transition temperature can reduce the temperature dependency of G but also reduces the loss tangent, such that the glass transition temperature must be increased to increase the loss tangent, thereby increasing the temperature dependency of G'.

Example 8

The use of a viscoelastic material of the invention as a vibration damping device for an architectural structure was simulated, and the dynamic viscoelastic property was measured under conditions similar to a working model.

The viscoelastic sheets obtained in Example 2 were cut to a size of 50 mm x 50 mm x 4 mm. The obtained sheets were laminated between three steel sheets, shear deformation was applied, and the dynamic viscoelastic properties of storage elastic modulus (G') (N/cm²) and loss coefficient η (tanδ) were determined under the following conditions. The test apparatus used was an MTS810 Uniaxial Material Testing System by MTS Co. (maximum displacement: 250 mm, maximum speed: 75 kine, maximum load: 25 kN). The data points for measurement were determined so as to obtain data for at least 300 points per cycle at each frequency.

- Temperature: 10, 20, 30⁰C

- Frequency: 0.33 Hz, 1.0 Hz

- Shear deformation: 10%, 50%, 100%

The results are shown in Figs. 3 and 4 and Table 2.

As seen in Figs. 3 and 4, an elliptical load-deformation curve was obtained for the viscoelastic material and energy was absorbed, thus indicating that the material is useful as a viscoelastic damper with low temperature dependency.

Table 2: Dynamic viscoelastic property (Example 8)

The following examples demonstrate that polymers for the viscoelastic material of the invention can also be produced by thermal polymerization in solution.

Example 9

After dissolving 96 parts by weight of 2-ethylhexyl acrylate (2-EHA), 4 parts by weight of acrylic acid (AA), 5 parts by weight of methacrylic acid (MAA) and 0.21 part by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, by Wako Pure Chemical Industries Co., Ltd.) as a thermal initiator in 157.5 parts by weight of ethyl acetate in a four-necked flask equipped with a cooling tube, stirrer, thermometer and nitrogen inlet tube, solution polymerization was conducted for 20 hours at 50⁰C under a nitrogen atmosphere.

To the obtained solution there was added 4.2 parts by weight of a 5 wt% toluene solution of isophthaloylbis(2-methylaziridine) (0.2 part by weight with respect to 100 parts by weight of the polymer solid portion), and the mixture was coated and dried onto a 50 μm-thick release-treated PET sheet.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 3: G' (0°C)/G' (40⁰C) = 9.6, tanδ (Min-Max) = 0.68-0.89. Thus, a high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low.

Example 10

In the same manner as Example 9, 98 parts by weight of 2-ethylhexyl acrylate (2- EHA), 2 parts by weight of acrylic acid (AA), 5 parts by weight of methacrylic acid (MAA) and 0.21 part by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, by Wako Pure Chemical Industries Co., Ltd.) as a thermal initiator were dissolved in 157.5 parts by weight of ethyl acetate, and solution polymerization was conducted for 20 hours at 50⁰C under a nitrogen atmosphere.

To the obtained solution there was added 4.2 parts by weight of a 5 wt% toluene solution of isophthaloylbis(2-methylaziridine) (0.2 part by weight with respect to 100 parts by weight of the polymer solid portion), and the mixture was coated and dried to obtain a 50 μm-thick layer on a release-treated PET sheet.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 3: G' (0°C)/G (40⁰C) = 6.9, tanδ (Min-Max) = 0.59-0.86. Thus, a high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low.

Example 11

In the same manner as Example 9, 98 parts by weight of 2-ethylhexyl acrylate (2- EHA), 2 parts by weight of acrylic acid (AA), 7.5 parts by weight of methacrylic acid

(MAA) and 0.22 part by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, by Wako Pure Chemical Industries Co., Ltd.) as a thermal initiator were dissolved in 161.25 parts by weight of ethyl acetate, and solution polymerization was conducted for 20 hours at 50⁰C under a nitrogen atmosphere.

To the obtained solution there was added 4.3 parts by weight of a 5 wt% toluene solution of isophthaloylbis(2-methylaziridine) (0.2 part by weight with respect to 100 parts by weight of the polymer solid portion), and the mixture was coated and dried to obtain a 50 μm-thick layer on a release-treated PET sheet.

The dynamic viscoelastic properties of the obtained polymer were measured under the same conditions as in Example 1. The results are shown in Table 3: G' (0°C)/G' (40⁰C) = 8.9, tanδ (Min-Max) = 0.67-0.83. Thus, a high loss tangent was exhibited, and the temperature dependency of the storage elastic modulus G' was low.

Table 3; D namic viseoelastie properties

Claims

What is Claimed is:

1. An acrylic viscoelastic material comprising a polymer produced by copolymerizing a monomer mixture comprising a) 80 to 99 wt% of an alkyl acrylate ester whose homopolymer has a glass transition temperature (Tg) of 0⁰C or lower, and b) 1 to 20 wt% of methacrylic acid or a methacrylic acid ester whose homopolymer has a glass transition temperature of 40⁰C or higher, wherein the above wt% is based on the total weight of the monomers in the monomer mixture.

2. The acrylic viscoelastic material according to claim 1, wherein said alkyl acrylate ester is a C4-12 alkyl acrylate ester.

3. The acrylic viscoelastic material according to claim 1, wherein said methacrylic acid or methacrylic acid ester is selected from the group consisting of methacrylic acid, methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate and benzyl methacrylate.

4. A vibration damping material comprising the acrylic viscoelastic material of claim 1.