SHEERGARD™ Hi-Performance Composites
SHEERGARD™ Hi-Performance Composite Materials
Saint-Gobain Performance Plastics high performance radomes and electromagnetic windows shield antenna systems while maintaining optimum RF performance. The key to high performance is Saint-Gobain's SHEERGARD™ family of microwave transmissive PTFE (polytetrafluoroethylene) Teflon® composites. They combine PTFE with specially woven structural fabrics utilizing fiberglass, aramid (Kevlar®), and similar high strength, high temperature resistant yarns.
Saint-Gobain Performance Plastics maintains complete product control; from twisting and weaving the structural fibers to coating and laminating the woven reinforcements. Even the multilayer PTFE film used as a surface laminate is produced by Saint-Gobain.
- Low dielectric constant and loss tangent with minimal thickness as required to meet tensile strength design requirements
- Wide band RF performance
- Permanent hydrophobicity with unexcelled performance in rain
- Long service life (20 years+) virtually maintenance free
- High tensile strength, shear modulus, and tear resistance
- Heat weldable, permitting lap joints having strengths equal to or greater than the base material
- Excellent solar rejection properties
- Capacity to include integral permanent colors
- Manufacturing begins with the selection of a fabric construction best meeting the projected requirements.
- Special yarns are twisted and plied as required. These individual yarns are tested for strength and elongation.
- The special yarns are combined on large warp "beams" in preparation for weaving.
- Weaving takes place on industrial looms capable of weaving up to 166" wide.
- After inspection, the greige goods are coated with PTFE Teflon® in special towers. Multiple passes are fused to the base fabric under high temperatures.
The PTFE dispersion may contain colorants or other formulations dependent upon project requirements.
- In some cases, such as SHEERGARD™ SX-12, the coating processes is limited to provide a basis for heat laminating a Saint-Gobain Performance Plastics multilayer PTFE cast film. The film is then an integral part of the composite.
- In other cases, such as SHEERGARD™ M26OS, only one side of the base fabric is coated. This permits adhesives to adhere to the non-Teflon side.
- Final acceptance testing insures that the resultant roll goods meet specification requirements.
Numerous physical tests of every item included in the specification are conducted on every roll produced. Supplemental measurements, such as biaxial elongation, are required to properly pattern the radome or structure.
The RF microwave properties of each SHEERGARD™ composite are measured in an independent testing facility. In many cases, extended RF measurements are conducted to verify compliance with specific project requirements.
The SHEERGARD™ series is continually being expanded to both meet new applications and to obtain enhanced performance. Much of this effort is presently directed at increasing the shear modulus, useable tensile strength, and tear resistance of composites applicable to large radomes.
SX-12 combines PTFE film surfaces with a Kevlar® fabric, principally for use in mobile applications and where chemical/biological decontamination is required.
M series is available in wide widths and is principally for use in EM window applications and for planar radomes.
R series provides higher tear strengths and greater flex fold capacity for use in spherical radomes.
K series is a high strength, Kevlar Aramide fabric with a Teflon coating and an outer PTFE film. The coating and film provide both UV shielding of the Aramide and a highly hydrophobic surface.
Q series is based upon a multiple ply construction providing high shear modulus and maximum tear resistance.
You may select any of the Data Sheets to obtain detailed data in .pdf format.
Normally, SHEERGARD™ composites are produced in a natural, off-white color. In addition, the following standard colors are available for the products listed:
SHEERGARD™ M26 Standard Colors
FED STD 595A Color No.*
SHEERGARD™ M26 O.S. Standard Color
FED STD 595A Color No.*
* The Standard color is similar to the indicated FED STD 595A identification number. It is not manufactured to specifically match FED STD 595A.
Three areas important to the electromagnetic performance of a radome or EM window membrane are:
- Material electrical properties (dielectric constant and loss tangent) combined with thickness
- Water absorption
The reflective and absorptive loss, as well as insertion phase delay, are all complex functions of the electrical properties and thickness of the membrane. SHEERGARD™ composites are specifically compounded to obtain the optimum electrical properties consistent with strength and environmental resistance requirements. The SHEERGARD™ composite consists of a PTFE (Teflon®) coating which is fused or sintered to the woven structural substrates.
The dielectric constant of Teflon ®shows less change over a wide range of frequencies than any other solid material. (Source: Dupont Technical Data Sheet for Teflon®)
Each SHEERGARD™ series of composites are similar in composition with increasing thickness corresponding to heavier structural base fabric for higher strength. This allows the engineer to balance the required tensile strength for any given design against the desired electrical performance.
Water absorption in any radome material can be extremely detrimental (a frequent problem in the core of rigid sandwich panels). The manufacture of SHEERGARD™ microwave composites includes special processing resulting in a material which is essentially nonabsorbent.
Water absorption of the radome membrane with unsealed edges is less than 0.5% by weight after 48 hours total immersion in water.
The hydrophobic properties of SHEERGARD™ are an inherent characteristic of the PTFE, due to its low critical surface tension (16.5 dynes/cm). Low surface-free energy, which is analogous to low surface tension, is an inherent property of the PTFE polymer. It is this low surface-free energy that inhibits the formation of a water film. Unlike simple coatings, laminates, and painted finishes, SHEERGARD™ PTFE cannot peel, delaminate, or wear off.
Since PTFE is virtually inert chemically, its properties do not change even after years of exposure to high ozone or strong ultra-violet light.
Saint-Gobain has funded the development of a computer algorithm that was used at Georgia Tech to quantify the relative effects of water droplet and water film conditions. At all frequencies between 1 GHz and 40 GHz, the droplet condition resulted in a much lower transmission loss than a water film condition, as shown in the graph. The results of this study were published in the June 1986 issue of Microwave Journal. The article also indicates relative performance of various membrane materials in wet conditions and their retention of hydrophobic qualities over time.
It is of interest to note that the formation of a water film on any radome surface significantly increases the loss, which can result in noise temperature increases approaching 100° K. In this regard, the selection of radome envelope material with hydrophobic characteristics which prevent such a water film is an obvious major advantage.
For example, tests conducted on a 93-foot diameter radome at a scientific site (where an older non-hydrophobic radome was replaced with a SHEERGARD™ radome) gave the following results at X-band:
Old Radome - Dry: 60-65° K
Rain: 160° K
New SHEERGARD™ Radome - Dry: 58-63° K
Light Rain: No change
Very heavy down pour: 70° K
(These are noise temperatures of the entire system. Original site computations indicated that the old radome contributed about 5-7°K to this total. Their tests indicate the new radome is 2° K lower.)
On going testing of SHEERGARD™ composites are conducted by Damaskos Inc., Chadds Ford, PA using both free space and resonant cavity techniques. Horn to horn free space measurements at various incidence angles are made in an anechoic chamber with a HP 8510 Vector network analyzer using gating techniques to eliminate spurious reflective paths.
In addition, samples may be furnished to the customer for testing at their facility.
Typical signal magnitude and phase delay measurements are shown in the graphs. The graphs are from measurements of orange SHEERGARD™ M26 as might be used in Planar Radomes. There is a slight roll off in the equipment bandwidth at either end of the frequency range.
This data is then reduced to effective real and imaginary dielectric permittivity, which may also be expressed as a material Dielectric Constant and Loss Tan. Ignoring the initial equipment roll-off, note the relatively consistent dielectric properties throughout the range. These values are used to compute the RF performance of a specific radome or EM window.
Teflon, Kevlar and Nomex are registered trademarks of E.I. DuPont de Nemours & Co.
SHEERGARD is a pending trademark, and CHALLENGE is a registered trademark of Saint-Gobain.