4. Raw materials
A wide variety of thermosetting and thermoplastic resin systems are available for pultrusion. Heading the list are unsaturated vinyl ester-, epoxy-, unsaturated polyester-, and phenol-resins as well as methacrylate resins (due to optimum halogen-free flame retardant properties), Polyurethane; plus various thermoplastics.
UNSATURATED POLYESTER RESIN (UP)
Special types of this resin system are used for pultrusion. Due to their viscosity, they are easy to fill with kaolin, chalk, aluminium trihydrate (ATH) or ammonium polyphosphate (APP).
Four to five different methacrylate resin types are available. They have the following advantages over normal polyester resin types:
- High filler content with ATH up to 200 parts (complies with the most stringent fire safety standards)
- extremely effective flame retardancy if ATH & APP are added
- high reactivity (relatively high pultrusion speed)
- low shrinkage (good-quality surface, no exothermic cracks)
- easy to be pigmented
- Phenol resins
Phenolresins are the oldest known resin systems; they are created by condensation of phenols and formaldehyde. Due to water releasing there is a danger of cracks and pores during the curing process. This may cause considerable processing problems. Very low flammability, but high flue gasemissions. No pigmentability.
Polyurethane (PU) is a two-component thermoset resin which has been successfully used for pultrusion since the early 2000s. The main characteristics of PU in pultrusion are very high impact and fatigue resistance, combined with a low shrinkage & a line speed comparable to that of unsaturated polyesters. PU-based pultruded profiles are used when very high structural performance is required.
Combinations of glass fibres and thermoplastics are another option to improve specific properties such as surface slip characteristics, subsequent deformation during heat exposure, abrasion resistance, and chemical stability.Polyethylenes, polypropylenes, and polyamides are primarily used as matrix materials.
Glass and carbon fibres (in specific cases also aramide fibres) largely determine the strength and the rigidity of the resulting profiles.
GLASS FIBRE REINFORCEMENTS
Glass filaments of the E, C, or S glass types, bundled by melt spinning to form rovings with a weight between 600 and 9600 tex, form a unidirectional reinforcement in pultrusion direction and are mostly an important part of the reinforcing structure. The filaments are surrounded by a silane size considerably enhancing cross-linking with the matrix (adhesion). Three types are used for pultrusion: smooth roving, textured roving, and monofilament roving. With the latter, increased transverse strength may be achieved despite the unidirectional arrangement.
CONTINUOUS FILAMENT MATS
Continuous filament mats (CFM) with a multiaxial arrangement of bonded or needled glass filaments are used primarily for the reinforcement of the surfaces of extensive profiles, with high demands regarding the surface finish (low roughness and porosity). Mat weights of 300, 450, and 600 g/m² are available.
Polyester or glass mats form an overlay on many profiles, affording high-quality closed surfaces and largely determining the weatherability and UV resistance as well as the colourfastness and the chemical resistance, because profiles usually are not painted. Weight categories are between 30 and 100 g/m².
WOVEN FABRICS / NON-WOVEN FABRICS
These reinforcing materials may be used to increase the transverse strength. Fibre combinations in the 0°, 45°, and 90° direction, but also non-woven hybrid fabrics (such as glass / carbon fibre) with different layers are produced, for instance with a roving beneath and a 0°/90° mat on top, or with a mat below and a continuous filament mat on top to attain resin rich and therefore smooth surfaces.
FLAME RETARDANT MATS
Flame retardant mats at the surface, such as graphite mats, make standard polyester resins with a low filler content to comply with the DIN 5510 S4 norm, because they foam in case of fire, thus keeping the oxygen away from the source of the fire (at 160 °C, graphite fibres will expand by a factor of 9).