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Fiberglass

4 min read

Fiberglass is made by laying glass-fibre mat or cloth into catalysed polyester or epoxy resin, rolling out all air, and curing it, which gives a hand-laid laminate of 100-200 MPa tensile strength at 1.5-2.0 g/cm³, against 7.8 for steel. A resin share of 50-60% by weight is the usual hand-layup target. Strength is lost to dry spots, trapped air and uncured resin, and the resin, not the glass, is what limits heat and sunlight resistance.

Part of Fiber-Reinforced Composites

Fiberglass, or glass-reinforced plastic, is a composite of glass fibre in a thermoset resin matrix, used for hulls, tanks, panels and pipe.

Making The Fibre Is The Hard Part

The Advanced Materials article’s route is to melt soda-lime glass at 1,100-1,200 °C, draw fibre through a small hole with a rotating drum, aim for 10-20 micrometres, and coat the fibre with a sizing, offering starch solution. Glass melting and drawing are covered in the corpus’s glass articles, such as Glass Types for Different Uses. Two cautions from standard practice:

  • Soda-lime glass fibre is weakened by water and alkali. The glass industry uses low-alkali compositions for this reason. Fibre from ordinary bottle glass loses strength in a wet environment, and a damp structure will deteriorate. Treat it as a short-life material unless protected by resin.
  • A starch size protects the fibre in handling but can stop the resin from wetting it. Commercial practice removes handling sizes or replaces them with ones that bond to resin. Test a small panel.

For most settlements the fibre cloth will be salvaged (boat repair kits, building insulation fabric, old hulls) rather than drawn.

Layup, Step By Step

From the Advanced Materials article:

  1. Make or find a mould in wood, plaster or an existing part.
  2. Apply mould release, wax or PVA film.
  3. Brush on a layer of catalysed resin.
  4. Lay mat or cloth onto the wet resin.
  5. Roll with a ribbed roller to remove air and saturate the fibre.
  6. Repeat until the thickness is reached. Each layer adds about 1 mm.
  7. Cure 24 hours at room temperature, or heat to 60 °C for 4 hours.
  8. Demould, trim, sand.

Resin share is the variable that matters. The Advanced Materials article targets 50-60% resin by weight for hand layup. Too little leaves dry spots and delamination, and too much gives a weak, heavy part. Mat takes more resin than woven cloth for the same wetting, so adjust by looking: the fibre should turn clear and no white dry fibre should remain.

What The Properties Mean

The Advanced Materials article’s typical hand-laid values are tensile strength 100-200 MPa, density 1.5-2.0 g/cm³, excellent corrosion resistance and good repairability by patching with more glass and resin.

Worked example, strength to density: 100 / 1.5 = 67 MPa per g/cm³ to 200 / 2.0 = 100 for fiberglass, against 345 / 7.8 = 44 to 483 / 7.8 = 62 for mild steel using the converted figures in Mechanical Properties. So fiberglass is comparable to or better than mild steel per unit weight, but it is much less stiff, so thin panels flex and need ribs or a curved shape.

Polyester and epoxy are thermosets (see Thermosets and Epoxy Systems). They soften or degrade with heat and with long sunlight exposure, and polyester styrene fumes are harmful. Making polyester resin from scratch is outside this article. Salvaged resin kits have a shelf life, so test a small mix before committing to a large layup.

Catalyst, fumes and fibre

The peroxide catalyst used with polyester resin is a strong oxidiser. Never mix it directly with the cobalt accelerator, and measure it exactly as the supplier states. Too much catalyst in a thick mass can overheat, smoke or ignite. Styrene fumes irritate and affect the nervous system, so work outdoors or with strong ventilation. Glass fibres and sanding dust irritate skin, eyes and lungs, so wear gloves, long sleeves, goggles and a mask. Keep resin waste away from flame.

Common Mistakes

  • Layering without rolling — trapped air makes weak voids.
  • Overloading with resin — it adds weight and not strength.
  • Skipping the test panel — resin, catalyst and fibre combinations vary.

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