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

7/24/2019 The Properties of Resins Used for Varnishing and Retouching of Paintings Part3

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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


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:



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Stability of


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


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


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 Paintings Part3

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