The major components have been outlined. This is a more detailed look at those items and a description of materials secondary but integral to FRP construction. Figure 4 shows the manufacturing process of various fibreglass reinforcements.
Glass is normally encountered as flat sheets such as windows or formed into containers such as drinking glasses and test tubes. If the chemical composition of this same material is altered when it is in a molten state and it is processed into filaments of 8–14 microns, it can gain an ultimate strength higher than that of steel. A material of this tensile stress obviously has structural potential. For the boatbuilder the specification of the glass is denoted by the type designation. Type A, E or S are generally offered by manufacturers and for tropical marine use Type E only should be used. Figure 5 shows some glass reinforcements.
It can be seen from Figure 4 that the continuous filament forms the basis of most reinforcements. With no further processing other than cutting to lengths of about 50 mm, these short pieces are deposited by machine on a moving conveyor belt and held and held together with a gluing compound (Powder or Liquid Binder) to form a continuous sheet of chopped strand mat of variable thickness. This material is specified by weight: 300, 450, 600 and 900 g/m2 are popular weights of CSM. The boatbuilder purchase it in rolls of 30–35 kg which are about 1 m in width. Note that as total weight and width of rolls is similar, the length of mat will decrease as the weight per square metre increases. The side of the material is slightly smoother than the other which reflects the smooth side of the conveyor belt on which the mat was made. It is the rougher side which should be placed down when laminating.
Figure 4 Reinforcements manufacturing process
Figure 5 Glass reinforcements
The alternative step in the processing of filaments is their formation into strands which are loosely twisted into rovings. The normal range is 60–120 strands per roving. These rovings resemble loose glass rope which can be coiled up into reels or further processed into woven roving. The reels of roving may be used to provide short lengths of glass reinforcement to strengthen areas where access is difficult or used with a spray chopper machine as a magazine supply of glass reinforcement for spray lay up. This is an automated process combining a chopper gun which reduces the roving to short lengths and sprays it with catalysed resin onto a mould (Figure 18). This achieves the same result as a layer of CSM but faster. For a uniform rate of deposit, the operator of this expensive machine should possess similar skills to a paint sprayer. It is useful in mass production situations.
This is the other popular reinforcement. It is purchased in a similar form to CSM and is again specified by weight. Standard specifications are 18 oz per square yard (600 g/m2) and 24 oz (800 g/m2). As in the case of CSM, other weights are available but boat designers will normally specify a laminate which is composed of commonly available materials as it passes on a cost saving to the builder.
During manufacture, the roving is woven into a cloth such that roving in the “warp” direction (length of the cloth) is continuous for the whole length of the roll which results in a high tensile strength. WR also gives a higher glass per unit volume ratio than CSM which reduces the amount of resin needed. Approximate resin to glass ratio for CSM is 2.5:1 by weight (30% glass) and for WR is 1.25:1 (45% glass). So for a large vessel whose hull shell weight is measured in tons, inaccurate resin/glass ratios or a laminate with too much CSM and not enough WR may waste substantial amounts of materials and money.
However, it is rate to find any WR in vessels of less than 6 m and equally rare to find vessels built wholly of WR. CSM laminates are normally adequate for smaller boats while wholly WR laminates do not provide a good inter-laminar bond (adhesion of successive layers) at any size. For these reasons experience has shown that normal hull laminates are best made of alternate layers of CSM and WR with extra CSM near the outside.
This is available in standard widths as previously described. It is characterized by continuous roving in the warp direction only with no transverse roving except a light glass thread at widely spaced intervals to prevent the cloth from falling apart when handled. It is rarely found in workboats as it is difficult to keep in shape, expensive and needed only where high strength and light weight are requirements.
This has a similar appearance to woven roving but on a finer scale. It is available in various widths from rolls down to 25 mm. These smaller sizes, which are known as glass tape, give an indication of its uses which are, for the narrow sizes, bonding of joints and small repairs or in full sizes for giving high strength with a smooth finish and where good draping qualities are required in areas of compound curvature. It is more expensive than WR and normal weight specifications are in the range of 110–400 g/m2.
This is very thin and can be compared to a very fine, smooth CSM but is made from blown glass staple. It is rarely used except to support a gelcoat of above average thickness or to produce a smooth cosmetic finish on the innermost layer of a laminate. It is non-structural and unnecessary on workboats.
Polyester resin is the main type used in the boatbuilding industry worldwide. “Unsaturated” polyester resin is a more correct term for the liquid state in which it is supplied. When cured to the solid state during the laminating process it becomes “saturated”. Terylene is another example of a saturated polyester resin and is clearly a plastic and non-organic material.
Resin is derived from coal and oil. The industrial base for the production of resins is an oil refinery and petro-chemical works which are rarely found in developing countries. The characteristic smell of polyester resin is given by styrene which is added to the polyester base at a late stage in production.
Figure 6 Manufacture of polyester resin
After the basic polyester resin has been produced various changes may be made by the manufacturer to alter the properties so that the resin gains characteristics required for a particular application. For example, improved weather resistance where exposure to a harsh environment is foreseen or resistance to chemical attack if the final product is to be used for a fuel tank.
Properties to be considered for boatbuilding lay-up resin are:
When direct contact with the supplier is not possible, a “Marine General Purpose” resin should be ordered which has been previously approved by a Classification Society such as Bureau Veritas, Lloyds Register of Shipping, Nippon Kaiji Kyokai, Det Norske Veritas or American Bureau of Shipping. An Iso (Isophthalic) rather than Ortho (Orthophthalic) should be requested, better again is an Iso - NPG (Neopentyl Glycol). These qualifications should ensure a suitable material. Lay-up resin for boatyards in developing countries will be supplied in 200-litre drums and to achieve cure (hardening) it requires a catalyst and an accelerator and can be ordered with or without the latter pre-mixed.
The addition of the catalyst is always the absolute final step immediately before applying the resin onto the mould and is ordered in separate containers. It is recommended that pre-accelerated resin be ordered for two reasons:
This is the other type of resin commonly referred to in the boatbuilding industry. When cured it forms the shiny, smooth outer surface of the hull and as such is the first layer to be applied to the female mould during the laminating sequence. The name refers to use rather than any fundamental chemical difference. It also is usually a polyester resin but is more viscous as it must not drain off vertical surfaces when applied to the polished inner face of the mould. When cured it is usually harder than laminating resin and has greater weather and chemical resistance as it forms a protective barrier between the environment and the reinforced laminate of the hull itself.
This material is commonly used for car body repairs and on boats is used for similar purposes. It possesses little strength as it is composed mostly of a filler powder such as chalk dust which is water absorbent and should not be used on underwater surfaces unless based on an epoxy resin. In new construction it can be used for bedding deck fittings or radiusing internal corners on joints which require bonding.
The three items described above can all be supplied pre-accelerated but to cure, the addition of a catalyst is needed. It is the catalyst which changes the monomeric unsaturated polyester resin to a polymeric saturated resin that is, the hardened state, by the production of an exothermic reaction (heat expiration).
This begins IMMEDIATELY on adding the catalyst and once again it is emphasized that the addition of the catalyst to the resin MUST BE THE VERY LAST ACTION before it is applied to the mould.
The accelerator governs the speed of the reaction and without the catalyst the accelerator has no effect on resin. This is why it can be pre-mixed months before use. The catalyst however, cannot, as it will cure the resin in a matter of hours in a hot climate without any accelerator being present. The addition of a specified amount of catalyst to a set quantity of pre-accelerated resin should, if following manufacturer's instructions and in line with worldwide industrial experience, allow working times of 20–40 minutes.
If accelerator must be purchased separately, a purple liquid (Cobalt Naphthanate) is usually offered for use with Methyl Ethyl Ketone Peroxide (MEKP) catalyst. When using unaccelerated resin the accelerator should be thoroughly mixed in first. Accelerator and catalyst must never be mixed directly together as they can cause an explosion. The flashpoint for both can be as low as 20°C. Other combinations are available for special circumstances.
When using materials from separate suppliers or when mixing batches, caution should be taken to ensure the concentration strength of the catalyst purchased agrees with the percentage volume required for catalyzing the resin, 40% MEKP is standard and should have been checked as correct by the supplier/manufacturer if ordered at the same time as other materials from his product range.