Fibre Reinforced Facade Encyclopedia: Composition
Chopped strand mat is a reinforcement made from glass strands chopped to about 50 mm and laid down in random orientation on a moving belt, then bonded into a handleable sheet with a resin binder. Because the strands point in every in-plane direction, the laminate it produces has the same properties in every direction in the plane of the panel. It is the workhorse reinforcement of hand lay-up laminating and the layer that gives a laminate its conformability and its resistance to print-through.
A 450 g/m² chopped strand mat laminated by hand produces roughly a 1 mm thick ply at about 30 percent glass by weight, or about 18 percent by volume.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Areal weight range | 225 to 900 g/m² | 300, 450 and 600 are the common grades |
| Imperial equivalent | 0.75 to 3.0 oz/ft² | 450 g/m² is the 1.5 oz mat |
| Chopped strand length | 25 to 50 mm | Longer strand drapes less, wets out slower |
| Binder content | 3 to 6% by weight | Emulsion or powder; burns off in ignition loss |
| Fibre orientation | Random in plane | Quasi-isotropic in the plane of the laminate |
| Typical laminate fibre content | 25 to 35% by weight | About 15 to 22% by volume in hand lay-up |
| Cured ply thickness | about 0.9 to 1.1 mm per 450 g/m² | Varies with roller pressure and resin ratio |
| Tensile strength of the ply | 80 to 130 MPa | Same in all in-plane directions |
| Suits | Hand lay-up, spray-up, contact moulding | Not suited to vacuum infusion in emulsion form |
Fibre architecture and what it buys
Every other reinforcement format is directional. Woven roving, non-crimp fabric and unidirectional tape put the fibre where the designer wants it and leave the other directions weak. Chopped strand mat does the opposite: it distributes strength evenly and accepts that the peak value in any one direction will be lower than a woven fabric of the same weight could achieve. In a random in-plane array only about three eighths of the fibre is working in any given direction, which is why a mat laminate reaches perhaps half the tensile strength of a woven roving laminate of comparable areal weight.
That trade is worth making in three situations. Where the load direction is not known or varies, as on a doubly curved architectural panel under wind. Where the geometry is complex and a woven fabric would bridge rather than conform. And where the requirement is a sealed, resin rich, crack resistant skin rather than maximum stiffness.
Binder type, and why it decides the resin
The binder holding the chopped strands together comes in two forms and the choice is not cosmetic. Emulsion binder, typically a polyvinyl acetate, dissolves in styrene. That is exactly what is wanted in a polyester resin or vinyl ester resin laminate: the binder releases as the resin wets in, the mat relaxes, and the strands redistribute to follow the mould. Powder binder, usually a solid epoxy or polyester powder, softens rather than dissolves, holds the mat together more firmly, and is the grade to use with epoxy resin, which contains no styrene to dissolve an emulsion.
Wet-out and laminating practice
Chopped strand mat wets out faster than any woven format because there is no crimp, no tight weave and no dense bundle for resin to push through. Resin is applied, the mat is laid in, and a grooved aluminium or paddle roller is worked through it to displace air and consolidate the ply. The visual test is simple: a properly wet-out mat goes translucent, and any white patch is dry glass or trapped air.
The cost is resin consumption. Because the strands sit loosely and randomly, mat laminates carry a lot of resin, which is why their fibre volume fraction is the lowest of the common formats. Resin is heavier, weaker and more combustible than glass, so a mat-only laminate is heavier and carries more organic content per unit of strength than a woven or infused one. That matters for both panel weight and reaction to fire performance.
Mat is also used as a barrier ply. A layer of 300 or 450 g/m² mat immediately behind a gel coat prevents the coarse pattern of a woven roving from reading through the surface as the resin shrinks on cure, and a mat interleaf between successive woven roving plies raises interlaminar shear strength by giving the weave something to key into.
Related formats
- Continuous filament mat: swirled continuous strand rather than chopped, higher permeability, used in closed moulding and pultrusion
- Surface veil or tissue: a very light mat at 30 to 50 g/m² used to give a resin rich, fibre free surface layer
- Combination mat: a woven roving stitched or bonded to a mat layer, laminated as one ply
- Spray-up: the same random architecture produced in place by a chopper gun rather than from a roll
Spray-up deserves a note. A chopper gun cutting roving at the nozzle produces the same random in-plane array with the same quasi-isotropic properties, faster and with no binder at all, but with less consistent thickness and higher styrene emission than laying mat by hand.
Specifying and verifying
A specification should state areal weight in grams per square metre, strand length, binder type and glass type, and should state the number and sequence of plies rather than a laminate thickness alone. Two laminates of the same thickness with different ply schedules are different materials.
Verification is by ignition loss to ASTM D2584 or ISO 1172 on a cut sample, which burns off the resin and the binder and leaves the glass to be weighed. That gives fibre content by weight, which converts to fibre volume fraction using the densities of glass and cured resin. For an architectural laminate, ask for the ply schedule and one burn-off result per production batch.
Standards and references
- ISO 2559 (textile glass mats, chopped strand: designation and basis for specifications)
- ASTM D578 Standard Specification for Glass Fiber Strands
- ASTM D2584 and ISO 1172 (ignition loss for glass content)
- ASTM D3039 and ASTM D790 (tensile and flexural properties of the laminate)
- ASTM D2734 (void content of reinforced plastics)
- AS 5637.1 and EN 13501-1 (reaction to fire classification of the finished laminate)
Related terms
Questions specifiers ask
What areal weight of chopped strand mat should be specified?
450 g/m² is the general purpose grade and gives about a 1 mm ply. Lighter 225 to 300 g/m² mat is used behind a gel coat and over tight radii; heavier 600 to 900 g/m² mat builds thickness faster on flat areas but conforms poorly.
Can chopped strand mat be used with epoxy?
Only powder bound mat. The common emulsion bound mat relies on styrene to dissolve its binder, and epoxy contains none, so the mat will not release or wet out properly.
Is mat stronger than woven roving?
No. For the same areal weight a woven roving laminate is roughly twice as strong along the weave directions. Mat is chosen for equal properties in all directions, better conformability and a smoother surface, not for peak strength.
Why put mat between layers of woven roving?
Woven roving does not bond well to itself. A mat interleaf raises interlaminar shear strength and stops the weave pattern reading through to the finished surface as the resin shrinks.
How much glass does a mat laminate contain?
Typically 25 to 35 percent by weight in hand lay-up, which is about 15 to 22 percent by volume. That is the lowest of the common reinforcement formats, because random strands trap a lot of resin.
Working out a laminate schedule for a facade panel? Our fibre reinforced polymer system is documented with published mechanical and fire data. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.