Silicone resin coatings for glass cloth used in fabric structures

Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 601-604

601

Silicone Resin Coatings for Glass Cloth Used in Fabric Structures Beth I. Gutek' and Bernard VanWert Technical Service and Development, Dow Corning Corporation, Midland, Michigan 48640

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A new silicone resin coating with high light transmittance and superior weathering properties has been specifically developed for coating glass cloth used in fabric structures. Glass cloth construction variables and surface treatments were studied. Both are important for determining the light transmittance, strength, and other mechanical properties of fabrics for structures. By selecting a light weight open-weave glass fabric, a high light transmittance material can be produced. Structures built of the coated cloth are suitable for greenhouses, as shown by plant growth experiments as well as by accelerated weathering and mechanical tests on the fabric. By selecting strong, heavyweight glass fabrics as the substrate, composites with high light transmittance can be produced for hrgsspan structures such as tension or air-supported bulldings used either for horticultural or general archfiectural purposes.

Introduction Although fabric structures comprise one of the oldest forms of shelter, they are still being perfected. Today, in addition to portable tents, permanent structures with roofs of flexible, coated fabric are being constructed. Through advances in technology, three basic types of permanent fabric structures have evolved the large-span, air-supported structure, the tension structure, and the double-air-layer greenhouse. Air-supportedstructures have single-membrane roofs supported by a positive pressure differential between the inside and outside air. Tension structures have single-membrane roofs extended between rigid supports. The third type, the double-air-layer greenhouse, is most often made from a flexible film rather than a coated fabric. All three types can benefit by the use of a recently developed silicone resin coated cloth. Because these fabric structures are permanent and are under constant exposure to the elements throughout the year, the coated fabric that forms them undergoes considerable strain compared to the material of portable shelters. Thus, any fabric used in a permanent shelter must maintain its strength, flexibility, and impervious characteristics upon continuous exposure for the life of the building. Glass, the traditional material for greenhouses, has ideal optical properties, but it also has notable disadvantages: it is a poor thermal barrier, it is heavy and requires a strong, expensive support system, and it is subject to thermal expansion and contraction, resulting in gaps between the frame and the glass. These gaps contribute to high infiltration losses; together, the disadvantages add up to high operating expenses. In contrast, a double-air-layer greenhouse covered with flexible glazing eliminates the major disadvantagesof glass. There is an insulating pocket of air between the layer of cloth to prevent conductive heat loss, the glazing is lightweight and requires only a minimal supporting structure, and the greenhouse can be made air tight to reduce infiltration loss. The most common flexible glazing material for this use is inexpensive, yet hazes and loses its strength on exposure to sunlight in a period of about two years. While still less than satisfactory, this problem may be tolerable for the small grower, but for a larger grower the labor costs for replacement become prohibitive. An ideal material for this purpose would offer not only the required optical properties, but also extended weatherability. Silicones are widely known as one of the most resistant types of polymers to harsh environmental conditions. Although they look and feel much like hydrocarbon ma0196-4321/82/1221-0601$01.25/0

terials, silicones are quite different in composition. A chain of alternating silicon and oxygen atoms forms their central structure. This is the same framework that makes up sand, glass, and quartz rock. In silicones, however, the central chain has been modified by organic side groups to make the material handle like hydrocarbon fluids, elastomers, and resins. The result is a hybrid material that is partly organic and partly inorganic; it has properties of both parent materials. Silicone resins are virtually transparent to the UV A&B ultraviolet energy, the UV frequencies which penetrate the atmosphere. Because of this transparency, silicones are not subject to the usual degradation and subsequent failure of conventional coatings. Silicone coatings subjected to exterior exposure for over 12 years are known to show little color change, chalking, loss of gloss, or other sign of failure. Silicone architectural building sealants Faintain their flexibility on exposure to weather and wide temperature variation. Neither time nor weather significantly deteriorates these elastomeric sealants. Although silicone materials maintain their original properties after aging in harsh environments, they do not have sufficient strength to form strong, unsupported T i s , and they certainly do not have enough strength to span a large, air-supported structure. A strong textile reinforcement such as glass cloth is ideal for providing the needed strength, since glass, like the silicone resin, is also resistant to sunlight. Silicone resin coatings for glass cloth are now being commercializedfor use in fabric structures that require durability to sunlight and weather. Development of Silicone Resin Glazing In 1971 H. A. Clark of Dow Corning Corporation first recognized the potential of silicone resin-coated glass cloth. By choosing a resin with a refractive index very near that of glass, he obtained a highly transparent material. DSET Laboratories near Phoenix, Az, conducted accelerated weathering tests on a sample of the coated cloth. Figure 1 is a plot of the translucency (expressed as percent transmittance), showing that the coated glass cloth maintained its original translucency over 10 years of actual weathering, or an accelerated weathering period of approximately 30 years. Resin Selection While Clark was experimenting with glass cloth, a new class of silicone resins known as elastoplastic silicones was being developed by Dow Corning Corporation for the electronic packaging market. These materials are block copolymers of soft diorganosiloxane segments and hard monoorganosiloxane segments. The resulting materials are @ 1982 American Chemical Society

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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 IC

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Figure 2. Torsion braid analysis, conventional silicone resin.

hybrids that show properties of both silicone rubber and silicone resin. These new materials have an extremely low glass transition temperature, like silicone elastomer, but have the stick, hard surface of silicone resin. Figure 2 depicts the torsion braid analysis of a conventional silicone resin. Note the discontinuity at 39 "C in both modulus and damping factor. Figure 3 is a plot of a torsion braid analysis for an elastoplastic silicone. It shows that the major discontinuities are at -124 "C, a very low temperature. This means that at very low temperatures, the material maintains its flexibility and makes it the ideal coating for glass cloth for fabric structures. Glass Cloth Selection Many fiber types were considered as a substrate material for the resin, but glass was selected on the basis of its proven record of sunlight resistance and high strength. Fabric construction variables have an important effect on the properties of the coated cloth. Five of these properties are described in the following subsections. Breaking Strength. Breaking strength is a measure of the tensile strength of the fabric. The value is expressed in pounds per linear inch rather than pounds per square inch. This value is recorded at the point of breakage when the fabric is stressed in tension. As expected, breaking strength is primarily a function of the weight of the cloth. Figure 4 shows that the relationship is nearly linear. Yarn size has a secondary effect; fabrics having coarser yarns, yet of equal weight as compared to fabric with finer yarns, will have slightly higher strength. Solar Energy Transmittance. This value refers to the portion of the solar spectrum transmitted through the

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Figure 4. Breaking strength vs. fabric weight.

coated cloth; it is expressed as a percent. The required transmittance value depends on the ultimate use of the fabric. For greenhouses and other glazing applications, solar energy transmittance must be as high as possible. To maximize transmittance, the yarn count and the fabric weight should be minimized. Figure 5 shows a rough correlation between solar energy transmittance and fabric weight. This relationship, however, has some practical limits. For example, a particular use may require a high-strength fabric with high light transmittance. Minimizing the weight of the fabric also lowers its strength, so some light transmittance may have to be compromised to obtain the desired fabric strength. The most practical solution is to first determine the necessary strength for the intended use of the fabric. Given this required strength, Figure 4 shows corresponding weight ranges of the fabric, and it becomes a simple process to find the lowest count fabric within that weight range. Table I shows that some heavy-strength fabrics can also have a relatively high transmittance value. Trapezoidal Tear. Trapezoidal tear represents resistance of the fabric to tearing at an angle of 30°,breaking each yarn sequentially. A fabric with coarser yarns and a satin weave maximizes this property. The structure of

Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982

Table I. Fabric Descriptions

fabric

fabric wt, oz/yd’

yarn count, warp X fill

A

12

16 X 16

B

25

44

C

19

D

35

14 X 14

12

E

x

finish silicone elastomer silicone elastomer silicone elastomer silicone elastomer silane

1 4 X 16

1.6

20

x

10

weave

coated thick- translucency ness, transmittance, in, %

plain

0.030

45

smaller structure

satin

0.042

30

large span structure

plain

0.038

40

plain

0.030

65

large tropical greenhouse smaller greenhouses

leno

0.016

85

use

B B

B

B solar glazing and double air layer

Table 11. Finishes

fabric


yarn count warp x fill, endsiin.

fabric wt, ozlyd’

Flex-fold, % retained

after 1 0 creases

finish

100 r-

100

-

90 -

-

:

80

E

F

: E

Y

-

P

-

5

-

B

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70 -

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80

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70

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60

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50

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40

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30

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20

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90

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60 -

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5

6

7

8

9

10

11

12

Silane

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l

l

I

10

12

14

16

18

Coating Weight Oz/Ydz

-

D

Fabric

-

0

50

coated thickness, in.

Figure 6. Flex-fold vs. coating weight.

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Ind. Eng. Chem. Rod. Res. Dev.. Vol. 21, No. 4, 1982

604

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Figure 9. Spedral trmmittance of silicone reain-eoated glass cloth, ASTM E424-A integrating sphere.

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Figure 7. Durability of silicone elastoplastic coated glass cloth. Effect of fabric finish Emmaqua. accelerated exposure.

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Figure 10. Double-air-layer greenhouse constructed of silicone elastoplastic.

Figure 8. Dye wicking study.

cloth, the breaking strength retained after creasing is only 18%of original strength, but by applying a thin finish of organosilane A or silicone fluid, strength is increased to approximately 50%. A precoat of silicone elastomer A or B helps to retain about 90% of the original fabric breaking strength. Using this information, we selected a light-weight, leno-weave cloth to be coated in full-size width for greenhouse glazing. This fabric was submitted for a scan of the solar spectrum shown in Figure 9. The scan shows that the material transmits part of the ultraviolet portion of the solar spectrum, most of the visual portion, and only a small part of the infrared portion. Thus, the glazing produces a true greenhouse effect, allowing the shorter wavelengths of the spectrum to pass through the glazing, yet blocking the reradiated infrared wavelengths from inside the greenhouse. An experimental greenhouse constructed at Pennsylvania State University, shown in Figure 10, weathered well, and plant growth was equal to that in glass greenhouses. Energy use was equal to that in the double-air-layer, flexible-film houses.

Architectural Fabrics For large buildings, high breaking strength of fabric is more critical than a high degree of translucency. Fabric B in Table I would be appropriate for a larger tension or air structure, yet it still has fairly high translucency. Pigments can be added to the coating to reduce transluceny if required. Fabric C is a lower yam-count fabric and is of somewhat lower weight. It has a much higher translucency, making it suitable for greenhouses, yet it is much stronger than the coated fabric for glazing, Fabric E in Table I. Seams Strong, durable seams can be produced with silicone adhesives or mechanical fasteners. In laboratory destructive testing, lapped seams were found to be so strong that failure usually occurred in the fabric rather than in the seam. Dirt Pickup On roof-weatheringteats in Midland, MI, an industrial environment where there is some fly-ash in the air, dirt pickup does not reduce translucency by more than 7% in the first six months. After six months, the translucency remains constant. After washing, only 3% translucency is lost. Conclusions Glass cloth coated with silicone resin is a versatile material for fabric structures. The coated cloth is unique in its high translucency and its proteetion of the glass fibers. By selecting appropriate fabric constructions and finishes, the cloth can be adapted to almost any type of fabric structure, from high translucency double-air-layer greenhouses to large-span tension and air structures. Receiued for reuiew February 26, 1982 Accepted June 17, 1982

Silicone resin coatings for glass cloth used in fabric structures

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