Where Coated Glass Fabric Earns Its Cost
A silicone coated glass fabric sits in places where metal is too heavy and organic textiles burn. Expansion joints on industrial ducting. Conveyor belts in tortilla lines and bakery tunnels. Removable insulation blankets on steam turbines. Welding curtains. Release sheets in composite vacuum bagging. Thermal barriers between a battery pack and a passenger cabin.
Each of those applications loads the material differently. A conveyor belt sees flex fatigue at 200 °C plus abrasion from food. An expansion joint sees 30 mm of movement per cycle plus pressure pulses. A welding curtain sees radiant heat spikes above 1,000 °C for a few seconds. One datasheet cannot describe all of them, and that is where most sourcing mistakes begin.
This guide explains what the coating does, how the fabric and the compound must be matched, which test methods carry real information, and how to write a purchase specification that a coating line can actually meet.
The application list also explains why the market splits into two price tiers. Food belting and release sheets are sold on certification and surface quality, and buyers accept a higher add on weight because a failed belt stops a line. Industrial insulation blankets are sold on price per square metre, and a 120 g/m² single pass coating does the job when the part never flexes.
The Three Layers of a Silicone Coated Fabric
Every coated fabric is a system with three parts.
The base fabric carries the mechanical load. Glass fibre offers high tensile strength, zero flammability, and dimensional stability up to roughly 550 °C. It also has poor flex fatigue, so the construction must follow the flex radius of the part.
The coupling layer is what most buyers never see. Glass is hydrophilic and silanol rich, silicone is hydrophobic and low energy. Without a suitable primer or an adhesion promoter in the compound, the coating sits on the surface and peels after a few heat cycles. Silane based coupling agents at 0.5 to 2.0% in the compound, or a dedicated primer at 3 to 8 g/m², make the difference between a five year part and a five week part.
The silicone coating provides release, weatherability, electrical insulation, and protection of the glass from abrasion and moisture. It also determines the surface friction, which is why the same base fabric can be sold as a conveyor belt or as a release sheet.
Primer or in compound adhesion promoter, the choice has process consequences. A primer adds a coating pass, a drying oven, and solvent handling. An in compound promoter simplifies the process, yet it limits the compound to one substrate type and shortens pot life. Coaters running several fabric types usually keep a primed line and an unprimed line.
Choosing the Base Fabric
Weave and weight
| Base Fabric | Typical Weight | Surface | Best Fit | Watch Out For |
|---|---|---|---|---|
| Plain weave, 200 to 300 g/m² | 0.15 to 0.25 mm | Smooth, balanced | Release sheets, thin thermal barriers | Low tear strength, poor flex life |
| Twill 2x2, 400 to 600 g/m² | 0.35 to 0.55 mm | Dense, good drape | Expansion joints, insulation blankets | Yarn crimp creates coating ridges |
| Satin 8 harness, 600 to 900 g/m² | 0.55 to 0.85 mm | Very smooth face | High pressure release, belt surfaces | Delamination risk at cut edges |
| Bulked or texturised glass | 300 to 500 g/m² | Voluminous, compressible | Gaskets, compression pads | Higher resin demand, weight variation |
Weight tolerance matters. A ±5% band on a 600 g/m² fabric moves tensile strength by a similar margin. Ask for the tolerance and the test method, normally ASTM D3776.
Yarn and finish
ECG 75 1/0 and ECG 150 1/0 are common yarns for coating substrates. Heat cleaned or desized fabric gives better adhesion than greige fabric because the starch size burns off during coating and creates voids. Volatile content of the base fabric, measured by loss on ignition, should stay below 0.5% for consistent adhesion.
Roll width, splices, and handling
Standard coating lines run fabric 1,000 to 1,600 mm wide. Wider finished goods require a spliced roll, and every splice is a stress concentration. For expansion joints and belts, specify a maximum of one splice per 50 m and require a lap splice with a defined overlap, typically 20 to 30 mm, and avoid butt joints.
Edge quality matters as much as width. A frayed edge loses coating coverage within 5 mm of the selvage and becomes the starting point for delamination. Specify a hot knife or ultrasonic cut with sealed edges, and inspect the first metre of every roll at goods inwards.
Coating Chemistry: Addition, Condensation, Peroxide
Three cure systems dominate the market and they behave differently on a coating line.
| Property | Addition Cure (Platinum) | Condensation Cure | Peroxide Cure |
|---|---|---|---|
| Cure temperature | 120 to 150 °C | 140 to 170 °C | 150 to 170 °C |
| Cure speed | Fast, 30 to 90 s at 140 °C | Slow, hours at ambient plus heat | Moderate |
| By products | None | Acetic acid or alcohol | Volatile peroxide fragments |
| Pot life | 4 to 24 h (B stage controlled) | 2 to 8 h | Days |
| Adhesion to glass | Excellent with primer | Good | Moderate, needs primer |
| Heat resistance | Good to 250 °C continuous | Moderate | Good |
| Typical use | Food grade belts, medical, premium release | General industrial, low cost | Legacy lines, thick sections |
| Risk | Platinum poisoning by sulphur, tin, amine | Acid attack on glass, corrosion | Odour, extractables |
For food contact conveyor belts, addition cure is the default. For welding curtains and insulation blankets where cost dominates, condensation cure still holds a large share, provided the acid by product is fully driven off during the cure cycle.
Key Performance Specifications and Test Methods
A coating specification should reference recognised methods, not adjectives.
| Property | Method | Typical Range | Comment |
|---|---|---|---|
| Coating mass per unit area | ASTM D3776 / supplier gravimetric | 80 to 400 g/m² | Determines release life and abrasion resistance |
| Total thickness | ASTM D751 | 0.20 to 1.00 mm | Include tolerance, often ±10% |
| Grab tensile | ASTM D5034 | 500 to 3,000 N | Warp direction usually higher |
| Tear strength, trapezoid | ASTM D5587 | 30 to 250 N | Critical for flex applications |
| Adhesion, coating to fabric | ASTM D751, section on ply adhesion | ≥ 3 N/cm | Below 2 N/cm expect field peeling |
| Heat ageing | ASTM D573, 200 °C, 72 h | Tensile retention ≥ 80% | Compare before and after |
| Continuous service temperature | ISO 188 extrapolation | 200 to 250 °C | Short excursions to 300 °C |
| Flammability, vertical | NFPA 701 | Pass, char length limits | Required for curtains and drapes |
| Surface spread of flame | ASTM E84 / EN 13501-1 | Class A / B-s1,d0 | For architectural membranes |
| Dielectric strength | ASTM D149 | 15 to 25 kV/mm | For electrical insulation blankets |
Two of these matter more than the rest. Adhesion controls service life in dynamic applications. Tear strength controls handling damage during installation, which causes a large share of field failures.
Tolerances deserve the same treatment. Coating weight measured across the width often varies by 8 to 12% on a knife over roll line, and the thin side wears through first. Specify a band with a hard floor, for example 220 g/m² minimum and 280 g/m² maximum, and require a thickness map with each roll.
Thermal performance is a system property. Silicone has a thermal conductivity around 0.2 W/m·K, so a 0.5 mm coating adds little insulation on its own. The insulation value of a blanket comes from the air trapped in the glass structure and the outer reflective layer.
Common Field Failures
Delamination at cut edges. Coating thickness at the slit edge drops and the glass yarns are exposed. Moisture wicks along the yarn and breaks the bond. Specify edge sealing with the same compound.
Thermal cracking. Silicone does not melt. It depolymerises above its service limit and the coating embrittles. At 300 °C continuous, a standard methyl silicone loses elasticity within weeks. For sustained 280 °C, use a phenyl containing compound or add an aluminium foil laminate.
Coating transfer. Soft coatings at 40 Shore A transfer to the counter surface under pressure. In release sheet applications this shows up as ghosting on the moulded part. Raise hardness to 50 to 60 Shore A or add a fluorosilicone topcoat.
Flex fatigue of the glass. Glass breaks at a small strain, roughly 3 to 4%. A belt running over a 50 mm pulley bends the fabric far beyond that on the outer face. Use a thinner fabric, a smaller yarn, or a twill weave with higher crimp interchange.
Edge curl after slitting. The coating shrinks slightly on cooling while the glass does not, which builds a curl into narrow strips. It makes automated feeding difficult and signals unbalanced coating on the two faces. Specify a balanced coating or an annealing step at 100 to 120 °C.
Processing and Conversion Notes
Coating lines run knife over roll, comma bar, or dip and squeeze depending on viscosity. Typical silicone coating viscosity for knife coating sits between 20,000 and 80,000 mPa·s. Lower viscosity penetrates the weave and improves anchorage but reduces coating build on the surface.
Cure profile matters. A two zone oven at 120 °C for 60 s followed by 160 °C for 90 s gives better adhesion than a single high temperature zone, because the primer has time to react before the compound skins over.
Line speed and dwell
Line speed sets dwell time, and dwell time sets adhesion. A line running at 20 m/min through a 6 m oven gives 18 s of exposure. The same oven at 40 m/min gives 9 s, which may skin the coating without completing the bond to the glass. Coaters compensate by raising temperature, which risks blistering because the surface cures before the solvent escapes.
Ask for the cure window in seconds at a stated temperature. A coater who can state the minimum dwell at a given temperature has the process under control.
Solvent and viscosity control
Many coating compounds arrive as 100% solids and need no solvent. Where solvent is used for thin coatings, toluene or xylene at 10 to 30% reduces viscosity and improves flow. The trade off is residual solvent. A coated fabric with more than 0.5% residual solvent will smell in service and can fail a food contact assessment.
If the fabric will be slit, punched, or sewn, tell the coater at quotation. Sewing requires a coating that does not crack at the needle line.
Specifying and Ordering
A workable purchase specification contains six items:
Base fabric: weave, yarn, weight with tolerance, finish type
Coating: chemistry, hardness, add on weight, number of passes
Adhesion: method and minimum value
Service conditions: continuous temperature, peak temperature and duration, flex radius, contact chemistry
Flammability and regulatory requirements: NFPA 701, EN 13501-1, REACH SVHC, ZDHC MRSL if the article touches apparel or footwear supply chains, OEKO-TEX Standard 100 where skin contact is involved
Conversion: roll width, length, edge treatment, splicing method
Buyers comparing quotations often find a 30% price gap between suppliers quoting "the same" fabric. The difference usually sits in coating add on, number of passes, or adhesion promoter. A 120 g/m² single pass coating and a 300 g/m² three pass coating are different products, and both can be described as silicone coated fiberglass.
Order quantities follow line width. A coating run below 500 m is usually uneconomic because setup and purge consume most of the margin. Rolls are normally supplied at 50 m or 100 m on a 76 mm core. Store them horizontally in a dry area, since glass fabric picks up moisture at the selvage.
For compound supply, our /liquid-silicone-rubber/ range covers addition cure grades at 30 to 70 Shore A suitable for direct coating, and our /solid-silicone-rubber/ range covers high consistency grades for calender coating and skim compounds.
If you are developing a coated glass fabric for expansion joints, high temperature belting, thermal barriers, or composite tooling, send the application conditions and the target test list. We will confirm the compound, the recommended add on, and the cure window for your line. Start at /contact-us.