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The Properties of Resins Used for Varnishing and Retouching of Paintings Part3

Feb 20, 2018

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Jose E Batista
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    Synthetic Resins: Polymers

    Poly (Vinyl Acetate) 1930s Relatively stable but low Tg causes embedding of dust and dirt

    Poly (Alkylmethacrylates) 1950s Paraloid B-72 (ethyl methacrylate / methyl acrylate copolymer)

    R1=C2H5; R2=CH3 / R1=CH3; R2=H)

    Very stable but requires aromatic solvent Poly(butyl methacrylate)

    R1=C4H9; R2=CH3 Soluble in mineral spirits but crosslinks upon aging

    Polymeric varnishes generally produce a different appearancethan natural resin varnishes.

    C

    H2

    C

    CO

    R1

    *

    n

    O

    R2

    *

    CH

    2

    CH

    O

    C

    CH3

    O

    **

    n

    The Properties of Resins Used for Varnishing and Retouching of PaintingsApril 2011 25

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    Synthetic Low Molecular Weight Resins

    Oligomers rather than polymers Molecular weights typically below a few thousand

    Can produce an appearance similar to that achieved with natural resins

    Only those soluble in hydrocarbon solvents of interest

    Ketone (Polycyclohexanone) Resins - 1950s From cyclohexanone, methylcyclohexanones & formaldehyde

    MS2 (Howards, Laporte)

    AW2, Ketone Resin N, Laropal K 80 (BASF)

    Not very stable and all discontinued

    Chemically Reduced Ketone Resins - 1960s

    MS2A, MS2B (Howards, Laporte; currently Linden)

    More stable but brittle

    CH2 CH2

    O O O

    CH2OH

    n

    CH3

    CH2 CH2

    OH OH OH

    CH2OH

    n

    CH3

    The Properties of Resins Used for Varnishing and Retouching of Paintings 26April 2011

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    Urea-Aldehyde Resins (BASF) - 1990s

    From aldehydes & urea:

    Experimental Aldehyde Resin (not available)

    Laropal A 81 requires 30-40% arom.

    Laropal A 101 requires ca. 50% arom.

    Not to be confused with Laropal K 80!

    Considerably more stable than ketone resins

    Hydrogenated Hydrocarbon Resins -1990s

    Pure, saturated hydrocarbons: many available

    Arkon P Resins (Arakawa) e.g. Arkon P 90

    Regalrez Resins (Eastman) e.g. Regalrez 1094

    Soluble in aliphatic hydrocarbon solvents (0% aromatics)

    Insoluble in acetone and the lower alcohols

    Arkon resins not recommended; Regalrez resins most stable

    April 2011 The Properties of Resins Used for Varnishing and Retouching of Paintings 27

    CH

    2CH

    3

    CH3

    N N

    O

    CH3

    CH3

    C

    CH3

    CH3

    CH

    2

    C O

    O

    CH

    2

    C

    CH3

    CH3

    CH

    2

    N

    CH3

    CH3

    C

    CH3

    CH

    2

    O

    N

    CH3

    O

    O

    CH3

    CH3

    O

    O

    CH3

    CH3

    C

    MeO MeO OMeOMe

    CH

    CH2

    C CH2

    CH3

    n

    m

    +

    Styrene -Methylstyrene Hydrogenated Hydrocarbon Resin

    Synthetic Low Molecular Weight Resins

    Regalrez Resins

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    Molecular Weight Distribution (SEC)

    of Low Molecular Weight Resins

    14

    19

    24

    29

    34

    39

    44

    600 700 800 900 1000 1100

    Retention Time (sec)

    Intensity(V)

    Dammar

    Resin

    Regalrez 1094

    Arkon P 90

    Laropal K 80

    Experimental Aldehyde Resin

    Laropal A 101

    Laropal A 81

    MS2A

    100000 30000 10000 3000 1000 300 100

    Approximate Molecular Weight

    Resin Mw Mn Mw/Mn

    Dammar 1616 528 3.06

    Laropal K 80 725 459 1.58

    MS2A 1561 586 2.66

    Arkon P 90 872 531 1.64

    Regalrez 1094 930 573 1.62

    Exp. Ald. Resin 1147 644 1.78

    Laropal A 81 3640 1266 2.88Laropal A 101 2979 1179 2.53

    Average Molecular Weights:

    The Properties of Resins Used for Varnishing and Retouching of PaintingsApril 2011 28

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    Stability of

    Synthetic Low Molecular Weight Resins

    Light Aging in

    Xenon Arc Weather-ometer

    UV component Included

    300 350 400 450 500 550 600 650 700 750

    Wavelength (nm)

    Xenon BS/SL

    Arizona Daylight

    UV Visible

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    The Properties of Resins Used for Varnishing and Retouching of Paintings

    Solvent Removability

    April 2011 30

    Aging Time (Hours)

    0 1000 2000 3000 4000 5000 60000

    Reduced Ketone Resin (MS2A)

    Ketone) Resin

    (Laropal K80)

    Arkon P 90

    Aldehyde Resin

    Regalrez 1094

    Dammar

    200

    100

    SolventStrength

    Unstabilized

    0 1000 2000 3000 4000 5000 6000

    Aging Time (Hours)

    MS2A - 1%Ketone Resin - 3%

    Arkon P 90 - 1%

    Aldehyde Resin - 0.5 %

    Regalrez 1094 - 0.5%

    Dammar - 3%

    Arkon P 90 - 2%

    200

    0

    SolventStrength

    100

    Stabilized with Tinuvin 292

    0

    5

    10

    15

    20

    25

    0 1000 2000 3000

    Solvent

    Strength

    Aging Time (Hours)

    Lar. A 101

    A 81 / 2% Tin 292Lar. A 81

    A 101 / 2% Tin 292

    Urea Aldehyde Resins

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    SEC: Laropal A81

    23

    24

    25

    26

    27

    28

    29

    30

    750 800 850 900 950 1000 1050

    Time (sec)

    Intensity

    (V)

    T=0

    T=1500

    T=3000

    The Properties of Resins Used for Varnishing and Retouching of PaintingsApril 2011 32

    23

    24

    25

    26

    27

    28

    29

    30

    750 800 850 900 950 1000 1050

    Time (sec)

    Intensity

    (V)

    T=0

    T=1500

    T=3000

    Unstabilized 2% Tinuvin 292

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    Resin Composition nd

    Regalrez 1094 Hydrogenated Hydrocarbon Resin 1.52 815

    MS2A Chem. Reduced Polycyclohexanone Resin 1.52 1773

    Dammar Triterpenoid Resin 1.54 1463

    Laropal A81 Urea Aldehyde Resin 1.50 3566

    Mastic Triterpenoid Resin 1.54 6537

    Paraloid B-67 Poly (Isobutylmethacrylate) 1.48 24876

    Paraloid B-72 Ethylmethacrylate/ Methylacrylate Copol. 1.49 75455

    AYAT Polyvinylacetate 1.47 209596

    Polymers used for varnishing have somewhat

    lower RIs than low molecular weight resins.But differences are small - much smaller thandifferences in molecular weight. Whenrounded off all are 1.5.

    Varnish RI should be close to that of bindingmedium (~1.5) to avoid reflection at themedium-varnish interface, which generallyspeaking is the case for all resins.

    Appearance Differences

    Roles of Refractive Index and Molecular Weight

    nd= refractive index = weight-average molecular weight

    The Properties of Resins Used for Varnishing and Retouching of PaintingsApril 2011 33

    When pigment particles are exposed at the paint surface

    varnishing causes darkening because nvarnish > nair.However greatest optical effect is due to leveling ofvarnish over microscopically rough surface, whichreduces light scattering. Molecular weight differencesare very large from ~1000 to ~100,000. Lowmolecular weight varnishes level more. This is the mainreason for the appearance differences.

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    Drying of Solvent-Based Varnish over

    Microscopically Rough SurfaceNo-Flow-Point (NFP) Concept

    The viscosity of the resin solution increasesduring drying due to solvent evaporation.

    The varnish levels until the No-Flow-Point(NFP) is reached, after which viscosity is toohigh for leveling to occur.

    Subsequent shrinkage due to solvent loss causes

    replication at the varnish surface of the peaksand valleys of the paint surface, but with loweramplitude.

    The amount of shrinkage and the roughness ofthe dry varnish depends on the amount ofsolvent present at the NFP.

    Polymer solutions have much higher viscositiesand reach their NFP with much more solvent

    present than solutions of low molecular weightresins.

    Polymeric varnishes are therefore drying withrougher surfaces

    0

    2

    4

    6

    8

    10

    12

    14

    HeightA

    boveLowestValley(m)

    No-Flow-Point Dry Varnish Surface

    Paint Surface

    The Properties of Resins Used for Varnishing and Retouching of Paintings 34April 2011

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    Quantitative Study of Varnished Surfaces:

    Roughness and Light Scattering

    Study in collaboration with NIST &Universit Pierre et Marie Curie, Paris

    Glass plates, ground using abrasivecompounds of different particle size

    (5-19 m) Plates look frosted white and have low

    gloss

    Varnishes applied from solutions intoluene using drawdown bar

    After varnishing plates look clear,smooth and glossy

    AYAT and Regalrez 1094 used asmolecular weight extremes

    The Properties of Resins Used for Varnishing and Retouching of PaintingsApril 2011 36