JAIC 1998, Volume 37, Number 1, Article 6 (pp. 69 to 87)
JAIC online
Journal of the American Institute for Conservation
JAIC 1998, Volume 37, Number 1, Article 6 (pp. 69 to 87)

FILLS FOR WHITE MARBLE: PROPERTIES OF SEVEN FILLERS AND TWO THERMOSETTING RESINS

ELEONORA E. NAGY



3 RESULTS


3.1 WORKING PROPERTIES OF THE MATERIALS TESTED

Both adhesives exhibited color change during mixing and curing. The “clear amber” pure Sikadur resin turned pink immediately upon mixing, pure Akemi turned greenish. As the curing proceeded the pure Sikadur gradually changed to a clear amber and the Akemi resin to a cool clear white hue, indicating that they were close to setting.

None of the fillers appeared to affect the setting time of the resins. Noticeable shrinkage of about 1 mm in length (longitudinal direction) occurred with all three pure Akemi samples.

Mixtures of approximately 40% concentration (40% resin content) or greater had properties of very viscous fluids for all samples. At about 30% concentration or less, two characteristic types of behavior were noted:

  1. Calcium carbonate, mica, and Cab-o-sil composites provided a barely kneadable paste and resulted in heterogeneous mixtures. These fillers appeared to “drink up” the resin and the lowest concentration that could be achieved with them was around 20% (20% resin content). At lower concentrations kneading became impossible and the mixtures did not set. Perhaps one component of the two-part resin mixture was preferentially absorbed by these three fillers, making the particles extremely hard and conglomerate. Also, the remains of the absorbed component may have induced an accelerated setting of the resin before the particles could be completely mixed in. Mixtures of these materials yielded significantly smaller volumes than expected from the volumes of resin and filler added.
  2. Conversely, Globe-o-sil, Microspheres, and Eccospheres fillers were easy to mix and yielded composites as low as 1.0% (1.0% resin content) concentration, although, at approximately 10–15% concentration the composites started to fall apart like bread crumbs and appeared too dry to mix. However, with continued mixing and kneading by hand, this crumbling texture was overcome, and further amounts of filler could even be added to the resins. They remained easily kneadable and cured properly afterward. Mixing large volumes of these fillers into the resin was found to be easiest using Zip-loc plastic bags. The bag was filled with the premeasured filler, the thoroughly premixed resin was poured in, and the bag was sealed. Kneading the putty in this manner helped to contain the fine airborne particles and provided better distribution of the resin.

Marble dust did not strictly conform to either type of behavior, but it seemed to behave more similarly to the second group of fillers.

The large difference in the resin-to-hardener ratio (100:3) of the Akemi polyester resin required more precision in mixing and resulted in increased preparation error as compared to the Sikadur resin. The 7–10 minutes pot life of the Akemi resin was found to be too short for uniform distribution of high filler content in the resin. These mixing difficulties were more relevant when used with the “difficult fillers,” calcium carbonate, mica, and Cab-o-sil. Stratification of resin concentration in the mixtures during or after preparation was not noticed in this study. This fact may be due to several factors, such as the use of relatively filler-rich mixtures, a resin viscosity not low enough to easily flow in response to gravity, the relatively fast pot life of the resins, the specific attention paid to proper mixing, or a combination of all these factors. Although there was an empirically noticeable difference between the viscosities of the two resins used in the experiment, the viscosity of the pure resin did not seem to influence the working properties of the mixtures.


3.2 VISUAL OBSERVATIONS

The generally white or off-white fillers yielded a range of colors in the composites, from brilliant white to yellow, gray, and brown. Composites of Microspheres and Eccospheres were the whitest samples after curing. Some of these samples looked whiter than the natural stone controls. Cab-o-sil yielded bluish green composites in Akemi and yellow in Sikadur resin. Marble dust and Globe-o-sil appeared slightly gray in both resins, the latter being more distinct. Calcium carbonate was beige and mica distinctly brown in both resins. None of the calcium carbonate, Cab-o-sil, or mica samples exhibited any resemblance to white marble. The color of the composite was determined by the color and concentration of the filler. The color of the resin had a negligible influence. Only in cases of approximately 30–40% or higher concentration did the color of the resin become significant. In these resin-rich mixtures the amber color of the epoxy resin added a slight warm hue, while the clear transparent Akemi resin tended to modify the color of the composites to a cool blue-green tint. This was especially obvious for the samples made with Cab-o-sil and marble dust.

Fillers with only one refractive index, which matches the RI of the resin, were the most transparent. As glasses and isotropic crystals satisfied this condition, composites of Microspheres and Eccospheres resembled the most translucent white marble. The rest of the fillers were not uniform crystals, so the result was generally one of graying when the average refractive index was close to that of the resin. The greater the RI difference between the filler and the resin, the greater the opacity of the composite became. Samples with an RI difference of only 0.05 between an isotropic filler and the resin exhibited increased transparency, when samples were observed side-by-side with the naked eye. Around this value, the isotropic filler/resin composites compared very favorably to the translucency of the natural marble specimens. This was the case with all glass microballoon (Microspheres and Eccospheres) samples. Anisotropic fillers, however, were very opaque—even well below this 0.05 RI difference. For example, the mean RI of mica filler had only an 0.003 RI difference in combination with Sikadur resin but was distinctly opaque due to the filler's birefringent nature. Consequently, although natural marble and limestones do contain mica and calcium carbonate, composites of these birefringent constituents do not produce translucent fills for stone.

With filler-rich samples (below 30% resin concentration) the texture of the composite was most similar to marble when the particle size and shape of the filler was closest to the grain of the stone. The filler-rich composites of Microspheres and Eccospheres most resembled white marble; these were followed by the samples made with marble dust. The Globe-o-sil composites resembled the decayed “sugary” marble surface. The calcium carbonate, mica, and Cab-o-sil samples, all materials with small particle sizes, had an artificially smooth, densely packed, and dull opaque appearance resembling a plastic substance rather than natural stone.


3.3 DENSITY

Values obtained for density are shown in table 1. The influence of the fillers on the density of the composites was the same in both resins. Calcium carbonate, Cab-o-sil, mica, and marble dust fillers increased the density of the samples. Globe-o-sil, Microspheres, and Eccospheres radically decreased the density of the samples. (Some composites of these fillers had densities so low that they floated on water.)

The density of the filler did not seem to be strictly related to the density of the resulting composite. For example, the extremely low-density Cab-o-sil filler increased the density of the samples, and they sank immediately when immersed in water. Ball-shaped or rounded filler with air content in the particles decreased the density of the composite.


3.4 WATER ABSORPTION

As seen in table 2, a large number of Akemi resin-based composites disintegrated or were damaged by water during the absorption test. Akemi/Microspheres (6%, 20%, 26%) and Eccospheres (6%, 35%) samples swelled, melted like ice cream, and disintegrated immediately after immersion. Five to 10 minutes after immersion, other Akemi/Microspheres and Eccospheres samples of slightly greater concentration produced less severe but similar reactions. The rate of disintegration slowed with increasing amount of resin in the sample. The 23% Akemi with mica sample was also damaged by water. In particular, the corners of the sample became lighter in color and crumbly. Some of the Akemi/marble dust samples became slightly spotty after drying. It should be noted that pure Akemi resin did not show signs of damage by water immersion, and the manufacturer did not indicate the resin's instability in water. Pure Sikadur resin, or its composites comparable to those of the disintegrated Akemi-based samples, did not show any signs of damage by water immersion.

The percentage weight gain of the samples, as compared to their original dry weight, is shown in table 2. As expected, the composites showed the greatest absorption of water, followed by the decayed marble, fresh marble specimens, and the pure resin samples.

The drying rate of the samples is indicated in columns 4–10 of table 2. Dramatic moisture loss for all samples occurred within the first 6 hours of drying. Pure Sikadur epoxy samples dried the fastest (6 hours). The two types of natural marble specimens returned to their original dry weight in 6–12 hours. These were followed by the pure Akemi polyester samples, which dried in 26 hours. All composites took longer to dry than the stone samples, and some retained a small amount of moisture for 72 hours.

In terms of the drying rate of the composites, two groups could be established.

  1. Samples containing calcium carbonate, Cab-o-sil, and mica dried within 26 hours. Marble dust composites returned to dry state shortly thereafter (within 36 hours).
  2. Globe-o-sil-, Microspheres-, and Eccospheres-filled composites belonged to the slower drying group. After the first 6 hours of dramatic moisture loss, which followed the pattern of natural stones, these composites retained a small amount of water until about 72 hours after immersion.


3.5 COLORIMETRY

Table 3 illustrates the mean of the readings displayed on the colorimeter. The L∗ readings show that the two pure resins were the darkest of the samples; the difference between Akemi and Sikadur was subtle though visually perceptible. The Akemi resin was somewhat on the coolish, blue-green side, and the Sikadur resin was on the warmer side. The pure resin samples were significantly darker than both the fresh and decayed natural stone samples. The fresh marble sample was slightly lighter and less yellow than the decayed marble sample.

Increased concentrations of fillers increased the lightness (L∗) of the composites. The degree of this increase in lightness could be divided into two groups.

  1. The addition of calcium carbonate, Cab-o-sil, Globe-o-sil, and mica did not lighten the mixtures significantly even when added in large volumes. Calcium carbonate yielded beige and mica brown color in their composites. Cab-o-sil was consistently bluish with Akemi and yellowish with Sikadur.
  2. The addition of marble dust, Microspheres, and Eccospheres fillers significantly lightened the composites. For example, 67% concentration of Sikadur/Microspheres composite (very small amount of fillers) showed variations of a∗ and b∗ values.

As shown in the last column of table 3, all wet natural marble controls darkened about 10% compared to their dry state. The wet composites decreased in lightness only 0–5%. Varying the concentrations of a filler did not seem to have any decreasing effect on the L∗ values of a wet sample. This decrease appeared to be filler-specific and not concentration-specific for a composite.


3.6 COMPRESSIVE STRENGTH

As seen in table 4, the range of values obtained for compressive strength was approximately 100 to 17,000 pounds per square inch (psi).

Both pure resin samples had significantly greater compressive strengths (approximately 11,000–17,000 psi) than any of the marble controls tested (4,000 and 10,000 psi).

Fillers either increased or decreased the compressive strengths of the pure resins and could be categorized in two basic types:

  1. Calcium carbonate, Cab-o-sil, and mica fillers increased the compressive strength and the brittleness of the composite. For example, the 10,900 psi of 52% Sikadur/calcium carbonate composite compared to the 13,300 psi of the 28% Sikadur/calcium carbonate composite. Under ultimate load these specimens broke with a loud sound into sharp, shell-like fragments.
  2. Fillers of hollow spherical microstructure, such as Microspheres, Eccospheres, and Globe-o-sil fillers, enormously decreased the compressive strength of the pure resin (e.g., pure Sikadur resin at 10,800 psi, as compared to the 120 psi of 1% Sikadur/Microspheres samples). Filler-rich, hollow microstructure composites had a tendency to distort and thicken under compression (load) and partially recovered their original shape after the load was released. These samples, when pressed beyond ultimate load, crumbled rather than broke.


Copyright 1998 American Institute of Historic and Artistic Works