Polymers in Preservation and Restoration: Two Examples

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CH 2

CH 3 CH 3 CH 3 I I I =CH-Si -0-(-3Si -0-)--Si-CH=CH 2 1 1 fn 1 2 CH 3 CH 3 CH 3 iv 353

Mat. Res. Soc. Symp. Proc. Vol. 462 ©1997 Materials Research Society

H -cSi-O.. Ij-j=3, orj=4 V

The shorter the chain of iv (the smaller the value of n), the higher is the modulus of elasticity. Very dense glass-like networks were created in situ by mixing a 44 functional hydrogenosiloxane (vi) with divinyl tetramethylsiloxane (vii). Both components were obtained from Petrach Systems (now United Chemicals, Inc.). Upon addition of chloroplatinic acid, the very fluid initial mixture became a transparent glass when warmed to 50'C, owing to the high density of cross-links via the ethane bridge (iii). H H H CH 3 - Si -O-( Si -O-) 42 -Si -CH I I CH 3 CH 3 CH 3

3

vi

CH

CH3

CH 2 =CH-Si -0- Si -CH=CH 2 eH3 &3 vii THEORY AND EXPERIMENT: ACRYLIC TECHNOLOGY RELATED TO "BONE CEMENT" The technology behind "bone cement" as used by the orthopedic surgeon to fix a part of the artificial hip in a patient is reconsidered in the context of restoration/ consolidation. The principal acrylic monomers, out of which acrylic resins are polymerized, are named in Table I, along with the structural unit which is formed in the polymer chain from the monomer. The glass transition temperature Tg is shown for each of the homopolymers. (The glass transition temperature is such that 10*C below it the polymer is hard and glassy, and 10*C above it the polymer is somewhat flexible, leathery.) The synthesis of polymer from monomer at or near room temperature, as relevant to architectural restoration, is accomplished by free radical initiation by a so-called redox initiator pair. [7] Two classical, widely used pairs are: dibenzoyl peroxide DBP and dimethyl aniline (DMA) dibenzoyl peroxide DBP and dimethyl paratoluidine (DMPT)

354

TABLE I H X General Formula of Principal Acrylics as Monomers C=C H Y H X and as polymer units - C - C H Y Tg of Polvmer °C

Y

Structural Unit

Abbreviation

X

methyl methacrylate

MMA

-CH 3

ethyl methacrylate

EMA

-CH 3

n-propyl methacrylate

nPMA

-CH 3

methyl acrylate

MA

-H

ethyl acrylate

EA

-H

-100

0II

OCH3 -1C 0 II -COCH 2 CH 3 0 1

_______-COCR

2 2

65 35

CHR 2 CH 3

0 II -C OCH 3 0 iI

10 -24

-COCH 2 CH 3 acrylic acid

AA

0

-H

-COH

355

NA

At room temperature, the mixing of DMA or DMPT with DBP produces free radicals at a conveniently rapid rate to transform the acrylic monomer(s) into the corresponding polymers. DBP by itself will cause polymerization at a reasonable rate, but only if the temperature is 60°C or more. Furthermore, mixtures of two or more of the acrylic monomers may be polymerized into copolymers and terpolymers having properties unobtainable from homopolymers (i.e., polymers having only one kind of unit). The so-called reactivity ratios of the acrylics toward each other are usually favorable in the sense that the composition of the initial monomer mixture will determine to a reasonable degree the compositions of the resulting polymer. Consider what would be appropriate to consoli