Reactive Polymers Fundamentals and Applications || Acrylic Resins

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  • 9 Acrylic Resins

    Acrylic resins are polymers of acrylic or methacrylicesters. They are sometimes modified with monomerssuch as acrylonitrile and styrene. The most com-mon acrylates are methyl acrylate, ethyl acrylate,n-butyl acrylate, and 2-ethylhexyl acrylate. Methacry-lates include methyl methacrylate, ethyl methacrylate,butyl methacrylate, and higher alcohol esters.

    The resins are used either as molding powders orcasting syrups. Acrylic resins are often used as hybridresins in combination with urethanes, epoxides, andsilicones.

    Since coatings are not the primary goal of thistopic, coating applications will be dealt with onlymarginally, even when acrylic resins contributegreatly to this topic. Acrylic resins are also widelyused for dental applications. We treat this topicbecause of its importance in a special chapter. Herewe focus on non-dental applications of acrylic resins.

    An overview on acrylic and methacrylic ester poly-mers is given in the literature [1,2].

    9.1 HistoryAcrylic acid was obtained through the air oxidation ofacrolein by Redtenbacher in 1843. Methacrylic acidwas first prepared in 1865. Otto Rhm observed thepolymerization of acrylics. The production of acry-lates was begun in 1927 by Rhm and Haas. In 1936poly(methyl methacrylate) (PMMA) was prepared bya casting process.

    9.2 MonomersA large variety of monomers is known, because ofthe possibility of esterifying the acrylic acid andmethacrylic acid with various alcohols. The mostcommon monomers are shown in Table 9.1. Someacrylate-based monomers are shown in Figure 9.1.

    9.2.1 Specialities9.2.1.1 Cyclohexyl MethacrylatesPolymers containing cyclohexyl methacrylateand related compounds such as 4-methylcyclo-hexylmethyl methacrylate exhibit high weather

    resistance. This is due to its low hygroscopicfunctional group. It is used for coating materials.

    9.2.1.2 Methacryloyl Isocyanate andDerivatives

    Alkenoylcarbamates can be readily polymerized bythemselves or with any other vinyl compounds. Thecarbamates formed from alcohols with a small numberof carbon atoms are available in a stable solid formunder atmospheric conditions and can be dissolvedeasily in various solvents.

    The acylurethane structure contributes to anenhancement of cohesion. Therefore, copolymerscontaining alkenoylcarbamate units show variousadvantageous properties such as high elasticity andgood adhesion.

    The introduction of an epoxy or aziridino groupintroduces further reactive moieties. The modifica-tion to a blocked isocyanate structure provides thealkenoylcarbamate compounds with the latent reac-tivity of an isocyanate group, which is produced fromthe blocked isocyanate structure under heating.

    Methyl N-methacryloylcarbamate, phenyl N-methacryloylcarbamate, benzyl N-methacryloyl-carbamate, and a series of other mathacryloyl-carbamates can be synthesized from methacryloylisocyanate by adding the appropriate alcohols tomethacryloyl isocyanate [8]. The synthesis is shownin Figure 9.2. The reaction is conducted at lowtemperatures. Also, an exchange of the alcoholgroup in the carbamate is possible. For example,ethyl N-methacryloylcarbamate can be reacted with2-ethylhexyl alcohol in the presence of a radicalpolymerization inhibitor such as hydroquinone (HQ)at 120 C. The ethoxy moiety is then replaced bythe 2-ethylhexyloxy moiety to result in 2-ethylhexylN-methacryloylcarbamate. This product is a viscousliquid [9].

    9.2.2 Synthesis9.2.2.1 MonomersAcrylic acid is synthesized by the oxidation ofpropene via acrolein. Methyl methacrylate is

    Fink: Reactive Polymers Fundamentals and Applications. http://dx.doi.org/10.1016/B978-1-4557-3149-7.00009-7 2013 Elsevier Inc. All rights reserved. 235

  • 236 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS

    Table 9.1 Monomers for Acrylic Resins

    Monomer Remarks ReferenceAcrylic monomersAcrylic acidMethyl acrylateEthyl acrylaten-Butyl acrylate2-Ethylhexyl acrylateTrimethylol propane triacrylate a [3]Aziridine derivatives a [4]Methacrylic monomersMethyl methacrylate bEthyl methacrylate2-Hydroxyethyl methacrylaten-Butyl methacrylate cEthylene glycol dimethacrylate aPoly(ethylene glycol) dimethacrylate3-Methacryloxypropyl-trimethoxysilane (MPTS) [5]Cyclohexyl methacrylate d [6]4-Methylcyclohexylmethyl methacrylate [7]Methacryloyl isocyanate [7]2-Methacryloyloxyethyl isocyanate [7]Methyl N-methacryloylcarbamate [8]Phenyl N-methacryloylcarbamate [8]2-Ethylhexyl N-methacryloylcarbamate [9]2-Isocyanatoethyl methacrylate [10]2-(Acryloyloxy)ethyl piperidine-1-carboxylate e [11]2-(Acryloyloxy)ethyl morpholone-4-carboxylate e [11]2-(Perfluoro-(1,1-bis-isopropyl)-2-propenyl)oxyethyl methacrylate [12]a Crosslinkerb Standardc Flexibled Improved weatherabilitye Low viscosity

    CH2 CH C

    O

    O CH3

    Methyl acrylate

    CH2 CH C

    O

    O CH2 CH3

    Ethyl acrylate

    2-Ethylhexyl acrylate

    CH2 CH C

    O

    O CH2 CH CH2 CH2 CH2 CH2 CH3

    CH2

    CH3

    Trimethylolpropane triacrylate

    CH2 CH C

    O

    O CH2 C

    CH2

    O

    O

    CH HC C2

    CH2

    O

    O

    HC HC C2

    CH2 CH3

    Figure 9.1 Acrylate-based monomers.

    CH2 CCH3

    C N C OO

    CH3OHCH2 C

    CH3C N C OO

    H

    OCH3

    N-Methacryloylcarbamate

    CH2 CCH3

    C N C OO

    CH2 CCH3

    C N C OO

    H

    O

    Phenyl N-methacryloylcarbamate

    OH

    Figure 9.2 Synthesis of methyl N-methacryloyl-carbamate and phenyl N-methacryloylcarbamate [8].

  • 9: ACRYLIC RESINS 237

    CH2 CH CH3 CH2 CH CO

    H

    O2

    CH2 CH C

    O

    OHCH2 CH CO

    H

    O2

    CH2 C

    CH3C

    O

    OCH3

    CH3

    C

    CH3

    O HCNCH3

    C

    CH3

    NOH C H2SO4CH2

    C

    CH3

    C NH2

    H2SO2

    CH3 OH

    Figure 9.3 Synthesis of acrylic acid and methylmethacrylate.

    synthesized from acetone via the acetone cyanhy-drin (ACH). The reactions are shown in Figure 9.3.The conventional process for the synthesis of methylmethacrylate runs via the acetone cyanhydrin. Othertechnical processes include:

    the ACH-based process (Rhm and Haas, Mit-subishi Gas Chemical),

    the i-butylene oxidation process (Lucky, JapanMethacrylic),

    the tert-butanol oxidation process (Kyodo, Mit-subishi Rayon),

    the propyne carbonylation (Shell, ICI), and the hydrocarbonylation of ethene [13].

    9.2.2.2 EsterificationThe reaction of methacrylic acid with an alcoholresults in the respective ester. Also, an olefin can beadded to the acid in the presence of anhydrous cata-lysts. Ethylene oxide reacts to form the hydroxyalkylesters. Diazomethane reacts to form the methyl esters.The reactions are shown in Figure 9.4.

    9.2.3 ManufactureVarious structural elements, such as rods, sheets,tubes, and molding powders, are produced by bulk

    CH2 C

    CH3

    C

    O

    OH HO R CH2 C

    CH3

    C

    O

    OR+

    CH2 C

    CH3

    C

    O

    OH CH2 CH R

    CH2 C

    CH3

    C

    O

    O CH2 CH2 R

    +

    CH2 C

    CH3

    C

    O

    OH H2 C CH2

    O

    CH2 C

    CH3

    C

    O

    O CH2 CH2 OH

    +

    CH2 C

    CH3

    C

    O

    OH CH2 C

    CH3

    C

    O

    OCH3CH2N2+

    Figure 9.4 Esterification reactions of methacrylicacid.

    polymerization. The most common method for theproduction of sheets is the batch cell method. Thepolymerization process releases a lot of heat and hasto be carried out slowly in order to avoid an adverseeffect on the optical properties. If the polymerizationin bulk quantities proceeds too quickly, even the boil-ing point can be crossed and thus bubbles are formed.Inhomogeneous temperature distribution during poly-merization may cause streaks in the material.

    9.3 Special Additives9.3.1 Ultraviolet AbsorbersExamples of ultraviolet absorbers are shown inTable 9.2.

    9.3.2 Flame RetardantsFlame resistance can be imparted by incorporatingcertain organic phosphoric acid esters into acrylicresins. Some flame retardants are shown in Table 9.3and in Figure 9.5.

    However, these organic phosphoric acid esters usu-ally have a plasticizing effect. They are likely not only

  • 238 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS

    Table 9.2 Ultraviolet Absorbers for Acrylic Resins [14]CompoundBenzotriazole ultraviolet absorbers2-(5-Methyl-2-hydroxyphenyl)benzotriazole2-[2-Hydroxy-3,5-bis(,-dimethylbenzyl)phenyl]-2H-benzotriazole2-(3,5-Di-tert-butyl-2-hydroxyphenyl)benzotriazole2-(3-tert-Butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzothiazole2-(3,5-Di-tert-Butyl-2-hydroxyphenyl)-5-chlorobenzothiazole2-(3,5-Di-tert-amyl-2-hydroxyphenyl)benzotriazole2-(2-Hydroxy-5-tert-octylphenyl)benzotriazole2-Hydroxybenzophenone ultraviolet absorbers2-Hydroxy-4-methoxybenzophenone2-Hydroxy-4-octoxybenzophenone2,4-Dihydroxybenzophenone2-Hydroxy-4-methoxy-4-chlorobenzophenone2,2-Dihydroxy-4-methoxybenzophenone2,2-Dihydroxy-4,4-dimethoxybenzophenoneSalicylic acid phenyl ester ultraviolet absorbersp-tert-Butylphenyl salicylatep-Octylphenyl salicylate

    Table 9.3 Flame Retardants

    Compound Remarks ReferencePhosphoric acid esters [15]Chlorinated polyphosphates [15]Halogenated polyphosphonate [15]Ammonium polyphosphonate, carbon nanotubes [16]Alkyl acid phosphate Synergist [15]Zirconium phosphate Inorganic [17]Tetrabromobisphenol A [18]Tri(acryloyloxyethyl)phosphate Reactive [17]2,2-Bis(4-hydroxy-3,5-dibromophenyl)propane [18]Tricresyl phosphate [18]Tris(2-chloroethyl)phosphate [18]Antimony trioxide Inorganic [18]Zirconium hydroxide Inorganic [18]Barium metaborate Inorganic [18]Tin oxide Inorganic [18]Montmorillonite, butyl acrylate Inorganic [19]

    P

    O

    OOCH2CH2

    CH2

    Cl

    CH

    CH3

    P

    O

    O

    O

    CH2CH2Cl

    CH

    CH3

    P

    O

    O

    O

    CH2CH2

    CH2

    Cl

    CH2 CH2Cl Cl

    Figure 9.5 Chlorinated polyphosponate (phosgardC-22 RTM, Monsanto).

    to substantially lower the heat distortion temperatureof the acrylic resin products, but also to lower theirmechanical strength. Further, the water absorptivity ofthe resin products tends to increase by the incorpora-tion of such flame retardants, and when used outdoorsthe resin products are likely to undergo deformationor crazing upon absorption of water.

    For a copolymer of methyl methacrylate, -methylstyrene, styrene, maleic anhydride, andmethacrylic acid, a synergism has been observed.

  • 9: ACRYLIC RESINS 239

    When two types of flame retardants, i.e., a halogen-containing condensed phosphoric acid ester or ahalogenated polyphosphonate and an alkyl acid phos-phonate, are combined, superior flame resistance andphysical properties will be imparted by the synergisticeffect of the components.

    Here, it is possible to reduce the amount of the mainflame retardant to a level of about 20% even when theflame resistance needs to meet the standard V-0 of theUL Standards [20].

    Therefore, it is possible to avoid the deterioration ofthe physical properties, particularly the deteriorationof the heat resistance, which is a serious problem whena great amount of the flame retardant is added [15].

    UV curable flame retardant resins can be obtainedby blending phosphate acrylate with an epoxy acrylateresin [21]. The flammability has been characterized bylimiting oxygen index, UL 94 flammability rating, anda cone calorimeter. Further, thermogravimetric anal-ysis (TGA) and several IR-based methods were used.The flame retardant efficiency increases as expectedwith the amount of flame retardant. The TGA indi-cated that the blends with flame retardant have lowerinitial decomposition temperatures and higher charresidues than the neat samples. In contrast, the releaserate increases remarkably. IR measurements indicatea lower thermooxidative stability [21].

    9.4 CuringThe polymerization of acrylic resins occurs essen-tially by a radical mechanism.

    A method has been developed to monitor the pho-topolymerization of resins, based on the rigid-bodypendulum rheometer. Data obtained from a differen-tial photocalorimeter and from the rigid-body pendu-lum rheometer were compared. The differential pho-tocalorimeter data obtained with polyester acrylatesfrom a tetrafunctional polyester acrylate and propoxy-lated neopentyl glycol diacrylate showed only mini-mal variations. However, when a rigid-body pendu-lum rheometer was used, differences in curing rate,crosslinking, and hardening processes were readilyobservable [22].

    9.4.1 Initiator SystemsTraditional radical polymerization initiators may beused for the casting polymerization. Common cata-lysts are shown in Table 9.4. Polymerization initiators

    particularly suitable for the continuous sheet-formingprocess are those having a decomposition tempera-ture, at a half-life of 10 h, in the range of 4080 C.

    9.4.2 PromotersSpecial initiator systems for cold curing were devel-oped. An effective initiator promoter system con-sists of a zinc 2-ethylhexanoate solution, a cobalt2-ethylhexanoate, and as peroxide source tert-butylperoxybenzoate [25].

    9.5 PropertiesAcrylic resins are appreciated for their exceptionalclarity and optical properties. Acrylics show a slowburning behavior and can be formulated as self-extinguishing.

    Acrylic resins have excellent transparency, translu-cency, surface gloss, and weather resistance and fur-ther have a high surface hardness and a superior designadaptability.

    Therefore, they find a wide variety of applicationsin interior materials for vehicles, exterior materials forhousehold electrical appliances, and building materi-als (exterior), for example, regardless of whether theyare outdoor or indoor applications.

    However, acrylic resins generally exhibit poor flex-ibility and low impact resistance and, therefore, pose aproblem in that they are prone to fracture when givenan extraneous load or impact.

    9.5.1 Electrical PropertiesAcrylic resins are easily electrically charged by fric-tion because of their high surface resistivity. Thus theydeteriorate in appearance due to adhesion of rubbishor dust, or they bring about an undesirable situationof electrostatic electrification in parts of electronicequipment. Antistatic properties to the acrylic resincan be imparted by [26]: Kneading a surfactant with the acrylic resin, or

    applying a surfactant on the surface of the acrylicresin.

    Kneading a vinyl copolymer having a poly(oxyethylene) chain and a sulfonate, carboxylate orquaternary ammonium salt structure with an acry-late resin.

    Kneading a polyether-ester amide with a methylmethacrylate-butadiene-styrene copolymer.

  • 240 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS

    Table 9.4 Polymerization Initiators for Casting

    Initiator RemarksAzobis-type catalysts [15]2,2-Azobis(isobutyronitrile) Preferred2,2-Azobis(2,4-dimethylvaleronitrile) PreferredDiacylperoxide-type catalysts [15]Lauroyl peroxideDibenzoyl peroxideBis(3,5,5-trimethylhexanoyl)peroxide PreferredPerester-type catalysts [23]tert-Amylperoxy-2-ethylhexanoatetert-Butylperoxy-2-ethylhexanoatePercarbonate-type catalysts [15]Bis(4-tert-butylcyclohexyl)peroxydicarbonate PreferredUV curing catalysts [3]2,2-Dimethoxy-2-phenylacetophenoneBenzophenone and

    methyldiethanolamineAcylphosphine oxide [24]

    Adding a functional polyamide elastomer. Adding a polyamide-imide elastomer having a low

    content of hard segments.

    9.5.2 Hydrolytic andPhotochemical Stability

    Methacrylate-based polymers have a better hydrolyticstability than the corresponding acrylate polymers.They are much more stable than vinyl acetatepolymers.

    Acrylic and methacrylic resins are not very sensitiveto ultraviolet radiation. However, ultraviolet absorbersimprove stability. Adding ultraviolet absorbers, e.g.,to acrylic windows, also protects the interior from UVradiation.

    9.5.3 RecyclingPMMA depolymerizes nearly qualitatively (ca. 96%)on pyrolysis into the...

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