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OCR for page 312
The Suitability of
Polymer Composites as
Protective Matenals
ANTHONY T. DIBENEDETTO
One method of minimizing the erosion of stone surfaces is to deposit into the
surface pores organic monomers or prepolymers, which are then polymerized
into a protective layer. This forms a stone-polymer composite near the surface
that is meant to protect the stone from further erosion by water and reactive
gases. The effectiveness of such a composite as a protective material depends
on the mechanical properties of the composite, the stability of the interface
between the stone and the polymer, and the permeation characteristics of the
polymeric coating. This paper discusses the properties of composite systems
relevant to the preservation of stone.
There are no materials of construction that are totally immune to
environmental degradation. Even stone, undoubtedly the most durable
of all traditional materials, is subject to physical and chemical erosion.
The rate of degradation depends on both the type of stone and the
nature of the environment. The realization that many historic struc-
tures are slowly deteriorating under centuries of exposure to atmo-
spheric conditions, a deterioration often accelerated by the gaseous and
particulate pollutants so common in today's urban environment, has
led to the development of many novel techniques of preservation.
Among those techniques is the application of synthetic organic mono-
Anthony T. DiBenedetto is Professor of Chemical Engineering, University of Connect-
icut, Storrs.
312
OCR for page 313
Polymer Composites as Protective Materials
313
mere and polymers for the purpose of repairing, consolidating, and
protecting the stone structures.
Many properties of polymers and polymer composites make them
attractive for these purposes. For example, patching materials may be
developed using polyester or epoxy resins filled with both stone par-
ticles and pigments, forming composite materials- of strength, dura-
bility, and compatibility with the stone substrate. Consolidation and
surface protection can be attained by impregnating the stone with low-
viscosity monomers and subsequently polymerizing the monomers in
the surface pores. This process forms coatings that resist water and
gaseous-vapor penetration, in addition to strengthening the material.
The suitability of such treatment is strongly dependent on the prop-
erties of the composite materials formed. A few of the characteristics
that determine whether the treatment will be helpful or harmful are
the solids content of the composite, the adhesion between the resin
matrix and the filler, the adhesion between the polymeric composite
and the stone substrate, the matching of physical properties between
the polymer composite and the stone, and especially the choice of resin
matrix.
POLYMER COMPOSITES AS PATCHING MATERIALS
A low-viscosity polyester or epoxy prepolymer can be filled with crushed
and graded stone particles and a pigment to produce an adhesive patch-
ing compound. Since it is essential to bond the resin to both the filler
particles and the stone substrate, it is usually necessary to incorporate
a bonding agent into the compound formulation. There are a number
of such treatments, the most popular for polymer/silicate-type com-
posites being the addition of a few percent of an organosilane. An
organosilane coupling agent is a compound of the general form R-Si-
{XJ3, where R is a resinophilic group and the X's are organic groups
capable of interacting with silanols. Some examples of commercial
organosilane coupling agents are listed in Table 1. For example, the
three methoxy groups {OCH3) of y-glycidoxy-propyltrimethoxysilane
(the seventh compound in Table 1) are capable of reacting with the
hydroxyl groups of a silicate surface to form a chemically bonded layer
of epoxy groups (CH2CH-) on the inorganic surface (see Figure 11.
The other methoxy groups are also capable of reaction with the
silicate or can interpolymerize in the presence of water to form a
protective organosilane polymeric coating. The pendant epoxy group
can react with an epoxy resin, so that the matrix is chemically bounded
to the silicate substrate.
OCR for page 314
314
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OCR for page 315
Polymer Composites as Protective Materials
Glass
Glass
R'
NH
Si—o \ cl H2
Si—O SiR—CH—O—
—si-o/
>SIR—CH—
- si-o I ,
o CH2
NH
R'
—Si—OH
—Si—OH
—Si—OH +X3 SiR—CH - CH2 ~ Glass
—Si—OH \O/
—Si—OH Coupling Agent, e.g.,
X =—OCH3
R =—CH2 O(CH2 )3
Resin
Matrix
FIGURE 1 Coupling reaction at a glass/silane/epoxy interface.
315
- si—0
- Si—O ~ SiR—CH—CH2
- si-o 'Cal
- - Si_O > Si—R—CH—CH2
Ol
+ Resin Matrix
+ R NH2
Proper choice of a coupling agent is essential to the development of
maximum physical properties and, perhaps more importantly, to long-
term stability in the presence of moisture or soluble gases, such as
sulfur dioxide. In the absence of good adhesion between the phases,
thermal stresses will lead to debonding of the imbedded. particles and
a microcavitation of the materials. Water and gases can then collect
at the interfacial void spaces, causing an accelerated degradation of the
inorganic phase. This is illustrated schematically in Figure 2.
Even with the proper choice of components, the wettability of a
surface must be considered. When a fluid polymer is placed-on a silicate
surface, it will form a contact angle, as shown schematically in Figure
3. A large contact angle, 0, represents poor wetting, while a contact
angle of zero represents spontaneous wetting. Wetting is favored when
the substrate is free of contamination, when the polymer has an affinity
for the substrate, and when the surface tension of the polymer is low.
Surface roughness affects the wetting characteristics, since the fluid
must move up and over asperities. Most important is the possibility
of air being trapped under a spreading fluid, thereby creating many
voids at the adhesive interface. The inevitable thermal cycling of the
material could then more easily lead to adhesive failure at the joint
interface.
When patching a porous surface, a strong mechanical bond can be
OCR for page 316
316
CONSERVATION OF HISTORIC STONE BUILDINGS
H2O or Gas Penetration
Collection of H2O
at Interfaces
m_
1. In`. ~
Composite
Patch
~o: ' ~ ° ° '-°' ~ d° ~
~ _~ - o
O o ~
. Q
Substrate
FIGURE 2 With poor adhesion between phases in a silicate-resin patching material,
thermal stresses will lead to debonding of imbedded particles and microcavitation of
matenals. Water and gases can then collect at interracial voids and cause accelerated
degradation of inorganic phase.
cleveloped by forcing the fluid into the capillary passages leading to
the interior. First, the fluid must wet the capillary passages in order
to displace the air in the pores (see Figure 41. Second, enough time must
be allowed for penetration to occur. In a cylindrical, open pore of
diameter, d, the depth of penetration is equal to:
l
l
/COS ~ ~LV d t
V 4~ '
11)
where ~ is the contact angle, Kiev is the surface tension of the fluid, ~
is the viscosity of the fluid, and t is the time of penetration. Most
patching compounds will be highly filled, so that the viscosity is ex-
tremely high, causing penetration to be very slow and in many cases
negligible.
Even when the mechanical joint is perfectly made, one still has to
accept the fact that there is a mismatch of physical properties between
the patch and the substrate. Perhaps the most important is the differ-
ence in thermal expansion coefficients. The polymer composite will
always have a higher thermal expansion coefficient, and thus normal
thermal cycling will lead to internal stresses at all particle/polymer
interfaces and at the composite/substrate boundaries. Under adverse
conditions this could lead to microcracking of the patch material and
even accelerated damage to the substrate. Thus, a great deal of care
OCR for page 317
Polymer Composites as Protective Materials
317
Moderately Good Wetting
Poor Wetting
FIGURE 3 Contact angle is a measure of wettability. Fluid polymer forms contact
angle (~) at a surface. The smaller the contact angle, the better the wetting; a contact
angle of zero represents spontaneous wetting. In this schematic, LV is the liquid-vapor
interface, SL is the solid-liquid interface, and SV is the solid-vapor interface.
must be taken in choosing the proper patching component for a given
. .
app 1catlon.
A critical variable is the choice of polymer matrix. Polyester and
epoxy are generic names for a multitude of different resin formulations
with vast differences in both physical properties and environmental
stability. Some polyesters, for example, have relatively Tow resistance
to atmospheric humidity, while others are highly resistant.
Filled Pore
1
Polymer Coating
Stone
Substrate
Coated Pore
FIGURE 4 Bonding strengthened by forcing polymer into pores. Patching material can
form strong mechanical bond with porous substrate when forced into capillary passages.
Depth of penetration depends on contact angle, surface tension of fluid, viscosity of
fluid, and time of penetration {see equation 1 and Figure 31.
OCR for page 318
318
CONSERVATION OF HISTORIC STONE BUILDINGS
TABLE 2 Illustrative Sorption and Transmission Rates of Water in
Polymers at 40° C
Water transmission
% H2O rate
24-hour immersion 90-95% RH
Polymer 1/8-inch-thick sample {g/m2/24 hours/mil)
Polyethylene {0.92 glrnl) 0.01 28
Polyethylene 10.96 g/ml) 0.01 4
Polylvinyl chloride) 0.03 32
Polylvinyl choride) Plasticized) 0.4 88
Poly~methyl methacrylate) 0.2-0.4 550
Polytethylene terephthalate) Mylar) 0.03-2.5 30
Silicone rubber 0.1-0.15
Epoxy resin 0.01-0.5
The five varieties of polyester resins commercially available are (11
general purpose, {2) isophthalic polyester, (31 bisphenol-A-based poly-
ester, (41 chIorine-bearing polyesters, and (51 viny} ester resins.
The general purpose types, composed of phthalic anhydride, maleic
anhydride, and propylene glycol, are the lowest-cost resins, but they
generally offer the least corrosion resistance. The isophthalic types use
a phthalic acid monomer and exhibit better resistance in both acidic
environments and saltwater. The other types have higher temperature
resistance and can be formulated for chemical resistance under very
severe conditions. The bisphenol-A-based polyesters are commonly
used and are highly resistant to a wide variety of harsh environmental
conditions.
TABLE 3 Some Gas Permeation Values Through Polymer Films
at 30° C
P x 10~° (cc (STp)/mm/cm2/sec/cm Hg)
02 H2S CO2 H2O
Film
Poly(vinylidene chloride) (Saran) 0.05 0.29 0.31 15-100
Polyester (Mylar A) 0.22 0.71 1.50 1300
Polychlorotrifluoroethylene (Kel-F) 5.60 12.50 2.9
Polyethylene (0.92 g/ml) 55.0 448.0 350.0 800
Natural rubber 230.0 1200.0 1330.0
OCR for page 319
Polymer Composites as Protective Materials
319
The epoxy resins are formed by the reaction of epichIorohydrin with
a hydroxyI-containing compound, such as bisphenoT A, and then cured
to a thermoses material with anhydride or amine curing agents. They
have somewhat better thermal properties than the polyesters end: are
generally more resistant to corrosion, except in the presence of strong
oxidizing agents. Once again, however, the specific properties are highly
dependent on the monomer fo~ulation. The epoxy chain can be ar-
omatic-based (starting with bispheno} Al or cycloaliphatic or aliphatic
(starting. with glycerol!. These prepolymers can be cured with long-
chain or short-chain amines or a variety of acid or anhydride catalysts.
Other commercially available formulations are simply too numerous
to mention. It is possible to formulate epoxies with softening points
ranging from 50° C to 200° C, with water;solubilities ranging from less
than 0.1 percent to more than 5 percent, with gas-transmission rates
varying by orders of magnitude, and with mechanical properties rang-
ing from high ductility and impact resistance to extreme brittleness.
Water and gas permeability depend on both the solubility and the
_
diffusivity of the penetrant in. the polymer matenat. Corn ot. anise
properties can be varied over wide ranges by choosing the prepolymer
components appropriately. Tables 2 and 3 give some idea of the range
of gas and water vapor transmission rates possible with different types
of polymer films. In any particular application..these properties would
have to be measured for the compounds being considered as treating
agents. Epoxy formulations, for example, could be highly aromatic or
highly aliphatic and have properties at either end of the spectrum
illustrated by Tables 2 and 3.
CONSOLIDATION OF STONE USING MONOMERS AND
PREPOLYMERS
An increasingly popular method of protecting and strengthening the
surface of a stone structure is to impregnate the surface with an organic
monomer or prepolymer and then to polymerize the material in the.
surface pores of the stone. If this is done properly, the monomer wiB
coat deeply the internal surface pores, adhere to them, and then po-
lymerize into a tough polymer film~with low permeability to~water
and corrosive gases. Many polymers have been tried, including epoxies,
silicones, fluorocarbons and poly~methy} methacrylatel. The. consoli-
dated surface is a composite structure the properties of which depend
on the constituent phases as well as Ol1 the adhesion between the two
phases.
OCR for page 320
320
CONSERVATION OF HISTORIC STONE BUILDINGS
The first step of the impregnation process requires that the monomer
penetrate the stone pores. As previously expressed by equation 1, the
depth of penetration is a function of contact angle, surface tension,
viscosity, pore size, and time. Very often, some of the properties de-
sirable in the consolidated structure cause difficulties in the processing.
For example, a silicone might be desirable because of its water resist-
ance, while that very characteristic may mean it is relatively non-
wetting (high contact angle d. Thus, it may be difficult to make a silicone
penetrate the surface pores, which would~ result in shadow impreg-
nation, clogging of pores, and a surface fiLn that could be subject to
peeling. Some of the most satisfactory epoxy prepolymers also have
very high viscosities, which decrease the depth of penetration. Under
certain conditions it might be necessary to dilute the epoxy with either
a reactive or nonreactive solvent to permit penetration. Upon curing,
however, the presence of a solvent wiD result in greater shrinkage,
leading to higher internal stresses and, perhaps, cracking of both the
polymer coating and the stone. Thermoplastic materials, such as
poly~methy] methacrylateJ, have a combination of good wetting char-
acteristics, mechanical strength, and impermeability and can be ap-
plied in a low-viscosity monomeric form. Upon long-term exposure to
wet, polluted air, and under the internal stresses created at the pore
surfaces, however, there can be a tendency for the polymeric coating
to craze, thus resulting in loss of protection.
The above-mentioned rlifficulties should not discourage the use of
polymers in stone consolidation. It is possible to choose a resin with
proper characteristics for both processing and long-term stability. The
environmental stability, so strongly dependent on the permeability
characteristics, is determined by choosing the proper resin for the
known environmental conditions.
When choosing a material for consolidation, one should seek expert
advice. Also, regardless of the degree of expertise, one should elect
to obtain and analyze accelerated test data on the composite system.
The improper choice of consolidation matenals can lead to a bigger
problem than was there onginally. Our historic structures are too
important for us to make hasty decisions on the means of saving them.
BIBLIO GRAPHY
Anonymous. Chemistry arid Physics of Interfaces, Americal Chemical Society: Wash-
ington, D.C., 1965.
DePuy, G.W., L.E. Kukacka, A. Aushem, W.C. Cowan, P. Colombo, W.T. Lockman,
A.J. Romano, W. G. Smoak, M. Steinberg, F.E. Causey. Concrete-Polymer Materials,
Fifth Topical Report, BNL 50390 and USER REC-ERC-73-12, 1973.
OCR for page 321
Polymer Composites as Protective Materials
Gauri, K. Lal. The preservation of stone. Scientific American, June 1978.
Manson, J.A., and L.H. Sperling. Polymer Blends and Composites, pp. 335 371 in Filled
Porous Systems. Plenum Press: New York, 1976.
Sternman, S., and J.G. Marsden. Bonding Organic Polymers to Glass by Silane Coupling
Agents. 1h Fundamental Aspects of Fiber Reinforced Plastic Composites, R.T. Schwartz
and H.S. Schwartz, eds. Interscience: New York, 1968.
321
Representative terms from entire chapter:
polymer composites
