The nature of reinforced plastics:
Reinforced Plastic is the generic term used to describe specific plastic materials reinforced with high strength fibres. Since their development, these materials have been commonly known by names such as “Fibreglass” an GRP (Glass Reinforced Plastic). Though GRP is still the most used term, the development and utilisation of fibres other than glass make FRP (Fibre Reinforced Plastic) a more accurate and comprehensive description. Within the reinforced plastics industry itself, “Composite” is the term felt to best describe this light, durable and astonishingly tough constructional material which can be fabricated into all manner of products. It may be translucent, opaque or coloured, flat or shaped, thin or thick. There is virtually no limit to the size of objects which can be made, and single pieces over 60 metres long have already been fabricated as boat hulls.
FRP is unique amongst materials of construction in that the fabricator actually makes the material. Whether he/she is making roof sheeting, chemical tanks, pipes, silos, buildings, vehicle bodies, or boats he/she is not merely assembling pre-existing components but making the structural material in situ.
What then is FRP?:
It is a composite of a resilient durable resin with an immensely strong fibrous glass. The resin is the main component and is normally a polyester resin. It is supplied in the form of a viscous syrup, which when suitably activated sets to a hard solid. Buttons and castings are made from such a resin, but just as concrete may be reinforced with steel rods, so polyester resin may be reinforced with glass fibres to form FRP. This is what the fabricator does. He/she uses a single surface mould or form on which he/she impregnates layers of glass mat with liquid resin until he/she has built up the required thickness, so forming a laminate or moulding. After removal of this product from the mould he/she can make many more in the same way.
FRP as a Structural Material:
FRP can only be used to the best advantage if the fabricator designs for the material. Able to do this, the designer must take into consideration the mechanical properties of the laminate as well as the methods of fabrication which it is intended to use. The advantage of using FRP over conventional materials is the ability to design and build large structures conceived as a whole and not as an assembly of parts which have to be jointed together. Another advantage of FRP is the possibility of varying the thickness of the material locally and of increasing the strength characteristics at any point in any direction simply by making intelligent use of the glass fibre reinforcement. Each design problem must be approached without thinking of FRP just as a replacement for traditional structural materials.
Building and Construction:
Composites are extremely versatile and have been used in many areas of the building and construction industry for more than thirty years. Modules and cladding are the two most popular ways of using composites in building. Modular composite construction is an extension of long established prefabrication techniques, which utilise to the full the light weight nature of composite mouldings. As they are manufactured in a mould, it is relatively easy to produce large numbers of identical modules in various geometric designs. The ability to be formed into complex shapes, to be textured and to simulate natural materials such as wood, slate, stone etc., are among the reasons for the successful use of composites as external cladding materials. The light weight and excellent strength to weight ratios of composites enable designers to meet specific criteria such as impact resistance, insulation properties and fire resistance. Composite modules and cladding panels are aesthetically pleasing and their strength, durability and weather resistance means they require minimal maintenance compared too many conventional building materials. Resins and gel coats have a proven track record of over thirty years in the building and construction industry. The use of these materials offers Architects, Civil Engineers and other specialists exciting opportunities to provide unique benefits and attractive solutions to building design today and in the future.
The Gel Coat
The durability of a composite moulding is mainly dependent on the quality of its exposed surface. Every possible precaution must be taken to prevent fibres from coming too near this surface where they may be liable to attack by moisture. This is achieved by providing a resin rich area on the working surface of the laminate, and this is the Gel Coat.In many ways the gel coat is the most important part of the laminate. Gel coats in various colours BS or Ral.
In order to produce a moulding or laminate using a polyester resin, the resin must be cured. This is achieved either by the use of a catalyst and heating or at room temperature by using a catalyst and an accelerator. Most Crystic resins are supplied pre-accelerated, incorporating an accelerator system designed to give the most suitable cure characteristics for the fabricator.
Organic peroxides are normally used as catalysts in the composites industry. Since these materials are unstable in the pure form, they are mixed with an inert compound before being supplied commercially. This process is known as phlegmatisation and is carried out during manufacture. Phlegmatisers are usually liquids (e.g. phthalates) or inert fillings (e.g. chalk) but other media are sometimes used.
The types of catalyst most commonly used, particularly in conjunction with polyester resins, are Methyl Ethyl Ketone Peroxide (MEKP), Cyclohexanone Peroxide (CHP), Acetyl Acetone Peroxide (AAP) and Benzoyl Peroxide (BPO).
Glass is an ideal reinforcing fibre for plastics. It is one of the strongest of materials (the ultimate tensile strength of a freshly drawn single filament of 9 – 15 microns diameter is about 3.5GPa). Its constituents are readily available; it is non-combustible and also chemically resistant. Glass Fibre is produced by drawing and rapidly cooling molten glass and is available in a variety of types and formats. Its final format will depend on how the drawn glass is further processed.
In the composites industry today “E” (Electrical) and “C” (Chemical) are the predominant grades of glass used. Developments in glass fibre technology mean that glass reinforcements are now available in a wide variety of styles and formats, suited to diverse applications in many industrial sectors.
Several resin types are employed in the manufacture of composite products. All of these resins are thermosets but they differ in their chemical make up, thus exhibiting diverse properties. This means that manufacturers can choose resin which enable them to tailor their products to meet specific requirements.
FIRE RETARDANT LAMINATES:
Polyester resins are made up of carbon, hydrogen and oxygen atoms and like all organic compounds, they will burn. However, by altering their structure and/or by the use of additives, it is possible to modify their burning behaviour so that laminates made from such resins present a lower hazard under fire conditions. In most applications the use of FRP presents no greater fire hazard than the use of timber. Unfortunately a “standard fire” does not exist and behaviour of FRP in a fire depends on a number of factors amongst which are:
• Ease of ignition.
• Surface Spread of flame.
• Fuel Contribution.
• Fire penetration.
Many tests for fire behaviour exist and almost every country has its own particular methods, often requiring large specimens and special equipment.
Resins are unsaturated polyesters. The raw materials used for the manufacture of unsaturated polyester resins are oil based and to produce a polyester of this type, three basic chemical components are generally required.
A: saturated acid (e.g. phthalic anhydride)
B: unsaturated acid (e.g, maleic anhydride)
C: dihydric alcohol (e.g. propylene glycol)
With the application of heat, these chemicals combine to form a resin which is a viscous liquid when hot, but a brittle solid when cold. The term “polyester” is derived from the link between A or B with C, which is termed an “ester” link. Whilst is is still hot, the resin is dissolved in a monomer which is usually styrene though others can be, and are, used. The monomer performs the vital function of enabling the resin to cure from a liquid to a solid, by crosslinking the molecular chans of the polyester. No by-products are evolved during this process, which means the resin can be moulded without the use of pressure. They are therefore known as contact or low pressure moulding resins.
Once the resin is cured, if will continue to mature, during which time the moulding will acquire its full properties. This process, which can take several weeks to complete at room temperature, can be accelerated by post curing the moulding at elevated temperatures.
The next step in the contact moulding process is the lay-up of the glass fibre reinforcement with polyester resin. Laying up can be started as soon as the gel coat has hardened sufficiently to withstand solvent attack from the laminating resin. Chopped strand glass fibre mat is the most usual reinforcement for contact moulding. The amount of resin required can be calculated by weighing the glass fibre to be used for the moulding. For chopped strand mat the resin: glass ratio should be between 2.5:1 and 2:1 weight (29-33% glass by weight).
Simultaneous depositing of polyester resin and chopped glass fibre by spray moulding equipment. Although much of the manual labour of hand lay-up is eliminated by using a spray process, thorough rolling is still necessary not only to consolidate the deposited glass/resin mixture, but also to ensure that the accelerated and catalysed portions of resins are adequately mixed. Considerable skill is required to control the thickness of the laminate when using a glass/resin depositor and to maintain a consistent glass/resin ratio. The spraying of gel coats can be carried out either by catalyst injection system or the one pot system. Spraying reduces labour costs and when the volume of production is large enough to keep the equipment in constant use, spray techniques are fully justified. Spraying is now widely used throughout the world and in the hands of an experienced operator most types of spray equipment will significantly increase output compared with application by brushing.
Trimming and Finishing:
It will save much time if the laminate can be trimmed while the resin is still in the “green” stage. This can be carried out with a sharpe trimming knife held at right angles to the laminate. Great care should be taken not to disturb or distort the moulding at this stage. Fully Cured FRP is not an easy material to Cut or machine, since it will quickly blunt most ordinary steel tools. Abrasive discs or wheels are recommended for cutting wherever possible. Portable hand tools are often used for awkwardly shaped laminates, and portable reciprocating electric saws have proved useful for trimming and slotting, especially if high grade saw blades are used. It is essential that the resin is fully cured before any finishing operations are undertaken. Even when the pigmented gel coat has been used and subsequent painting is not required. The moulding can then be buffed or polished with any of the normal cutting compounds.
If the moulding is to be painted, extra care must be taken to ensure that all traces of release agent are removed from the moulding. The surface to be clean and dry and it is advisable to first rub the surface with a fine abrasive to obtain efficient keying. Most paint systems can be used on FRP laminates.
The cost of a finished laminate depends not only on the cost of materials but also on the method of fabrication. Speed of production, investment in equipment, amount of waste and labour costs have all to be taken into consideration, and these will be different for each fabrication technique.
One of the most important design considerations is the expected performance of the FRP in the environment in which it is to operate in practice. How well does FRP withstand normal weathering processes? What is the effect of various chemicals, and at varying temperatures? What is the likely loss of strength after constant immersion in water?
The performance of FRP is so dependent on the actual composition of the laminate, the type of polyester resin used, the surface finish and, above all the degree of cure that it is impossible to provide detailed information covering every variable.The weather and water resistance of FRP laminates is largely a function of the gel coat since in most applications it is the gel coat surface which is exposed to attack. For optimum chemical resistance combined with high structural performance a resin-rich surface is essential on the face of the moulding which is exposed to the hostile environment. The resin gives high mechanical strength and excellent strength retention in many chemical environments at temperatures upto about 95oc.
H41 GLASS REINFORCED PLASTICS
To be read with Preliminaries/General Conditions.
TYPE(S) OF CLADDING/FEATURES
* Drawing reference(s): Specialist
Construction: To be Advised.
U/Value: W/sq m degC
Fire rating when tested to BS 476:
Class surface spread of flame.
* Fixing: To manufacture details.
* Joints: To be advised.
* Accessories/Features/Incorporated components:
210 DESIGN: Complete the detailed design of the cladding and associated features shown on the drawings to meet the requirements of this specification.
220 WEATHER RESISTANCE: The cladding and associated features must be weather tight under all conditions with full allowance made for deflections and other movements.
* The cladding and associated features must resist all dead and live loads. Wind loads to be calculated in accordance with CP 3: chapter V: Part 2, making due allowance for any internal pressure. The cladding must accommodate without damage all drying shrinkage, creep, deflections and thermal movements.
260 FIRE RATING: Make specimens of the proposed panel construction (including core if any) and arrange for specified tests to be carried out by an approved independent testing authority. Submit resultsdemonstrating compliance before commencing manufacture.
270 COLOUR FASTNESS OF OPAQUE GRP:
* Before undertaking the verification work required by this clause confirm choice of colour with the CA.
* The manufacturer must report to the CA if he considers that use of the proposed pigments and resins is likely to result in an unacceptable change of colour with time.
* Pigments must have a colour fastness to daylight of not less than standard 6 when measured to BS 1006, Section BO1. Submit evidence of compliance.
* If available obtain samples of GRP, made from the proposed pigments and resins, and naturally weathered for not less than 2 years.
* If naturally weathered samples are not available, prepare samples using the proposed pigments, resins and gel coat thickness, and test for 1500 hours in an accelerated weatherometer which subjects the samples to both moisture and ultraviolet light.
* The weathered samples must not show significant change of colour. Obtain approval of colour fastness before commencing manufacture.
310 DIMENSIONAL CO-ORDINATION:
* The cladding must be detailed to ensure compliance with requirements for accuracy in manufacture and erection, and to accommodate deviations in the building structure.
* Select types and methods of fixing which will give ample adjustability in three dimensions.
* Liaise with the party responsible for construction of the building structure to ensure co-ordination of dimensions and tolerances.
* Submit details of the proposed system of tolerances and adjustments.
320A DESIGN SAMPLE: At an agreed stage in the detailed design work make a sample of the proposed construction profile 1000mm in length showing the proposed colour, texture and incorporating a completed section of a joint. Obtain approval of appearance before proceeding.
410 QUALITY OF WORK:
* Manufacture GRP units carefully to ensure compliance with design and performance requirements, using materials and workmanship appropriate for the purpose.
* All materials must be compatible with each other, and must be stored and used in accordance with the manufacturer's recommendations. Resins must be used as supplied and not adultered: fillers and admixtures may be used only where authorised.
* The standard of finish must be appropriate to the end use and position in the building. Ensure that defects such as wrinkles, spotting, striations, fibre patterning, fish eyes, blisters, crazing, cracking, dry patches and uneven or inconsistent colour do not occur.
420 WORKING CONDITIONS: Workshops must be warm, dry, clean and well ventilated. Cease manufacture if the temperature falls below 10 degC or if dew point is reached.
430 PATTERNS: Inform CA when each master pattern is complete and not less than 7 days before commencing manufacture of moulds.
440 MANUFACTURING ACCURACY: Finished dimensions of completed units to be such that the cladding, when erected, complies with clause 630 and all sizes fall within the following permissible deviations:
|up to 2||2-3||3-4.5||4-5.6|
|Width & height||+0||+0||+0||+0|
Straightness of edges deviation from intended line, any variation to be evenly distributed with no sudden bends or irregularities
Squareness: taking the longer of 2 sides at any corner as a base line, the deviation of shorter side from erpendicular,dimension involved is the shorter side.
Twist: deviation of any corner from the plane containing the other 3 corners: dimension involved is the shorter side.
Flatness – deviation under a 1 m straight edge placed anywhere on a flat panel surface: 3 mm
(Measurements taken at 16-18 degC ambient temperature).
450 GEL COAT(S) TO EXTERNAL SURFACES:
* Mix resins thoroughly. All units of one colour to have gel coats from the same colour batch of resin.
* Apply evenly to give an overall nominal thickness of 500 microns.
* Check the wet film thickness(es) of the gel coat(s) of all units in accordance with BS 3900:Part C5, Method 7, four readings per sq m per coat over the external surface area.
* If single gel coating is used, no reading to be less than 400 microns nor more than 600 microns. Average of readings for each unit to be within the range of 450-550 microns.
* If double gel coating is to be used, submit proposals for coat thickness limits based on the resin manufacturer's recommendations, and obtain approval.
460 LAMINATING: Ensure that:
* Each GRP skin contains not less than 900 g/sq m of glass fibre, in not less than two layers.
* Random reinforcement is distributed uniformly, and non random reinforcement is correctly positioned and aligned.
* Each layer of woven fabric reinforcement has a layer of chopped strand mat on both sides.
* The glass is fully wetted out by the resin, with a resin/glass ratio of not less than 2:1 higher as appropriate.
* There is a good overall bond between all gel coats and all layers of laminate.
* The GRP is well consolidated and free from air voids.
470 CORES, RIBS ETC., Ensure that all core materials, ties, ribs, fixings and accessories are fully bonded to the GRP skin(s) over the full contact surface area.
480 FIXINGS: to be of a suitable type of stainless steel, nonferrous metal or GRP and to be such as to avoid bimetallic corrosion.
490 SEALING OF UNITS:
* Apply a flow coat to all surfaces of the finished units which are not gel coated.
* Thoroughly seal all cut edges, holes etc., to protect the glass fibre from penetration of moisture.
500 CURING: All units must be adequately cured at not less than 50 degC (higher as necessary) for not less than 8 hours (longer as necessary). Ensure that units are not distorted whilst being cured.
510 HARDNESS: After curing, and at time(s) to be agreed with the CA measure hardness of GRP in accordance with BS 2782:Part 10: Method 1001, one test for each 1sq m of external surface area, not less than 2 tests per unit. Reject any unit in which any Barcol hardness measurement is less than 30 at ambient temperature.
530 WEIGHT: The first unit of each type and size produced is to be thoroughly checked for compliance with the design and specification, and the weight recorded. All subsequent units must then be weighed, and must not deviate from the weight of the first identical unit by more than +/- 10%. Inform CA if any units fails to comply.
540 PRODUCTION CONTROL UNIT(S):
* The first unit produced of each of the type(s) listed below is to be inspected by the CA and, if its appearance is approved, clearly marked and kept safely at the shop as a control standard for appearance of subsequently produced units.
1.0m length of cornice.
* Control units to be delivered to site last. H41/4
550 INSPECTION: All completed units must be carefully inspected and checked by the manufacturer for match with approved sample(s) or control unit(s) and compliance with specification before despatch to site. Make arrangements with the CA for him to inspect completed units in the shop.
* Keep completed records for each unit including the following information. Unique identification number.Full details of composition.Date of each stage of manufacture.
Dates and results of all tests, checks and inspections.
Dimensions related to specified levels of accuracy.
Specific location in the finished work.
Details of any damage and making good.
Any other pertinent data, eg. If the unit is an approved production control unit.
* Records to be available for inspection on request.
580 RETENTION OF MOULDS: After manufacturing ceases retain moulds and store in a reusable condition to allow manufacturing to recommence if required. The period of such storage is expected to be until practical completion but do not destroy moulds until authorised by CA.
610 ERECTION, FIXING or JOINTING BY PERSONS OTHER THAN THE MANUFACTURER:
The GRP manufacturer must:
* Provide clear and comprehensive instructions and ensure that they are understood by the site operatives.
* Provide adequate site supervision by a suitably skilled person.
* Prevent mechanical damage and disfigurement. Separate units during transport and storage to prevent chaffing. Pad all slings, ropes, bearers, ladders etc.
* Support units as necessary so they they do not bow, twist or distort.
* Adequately protect units from the weather. Surfaces not having a weathering gel coat must not have prolonged exposure to direct sunlight or water.
* Do not cover units with plastics sheeting or stick adhesive tape on exposed surfaces. * Store fixing and jointing materials indoors.
* Do not deliver to site any units which cannot be erected immediately or unloaded into a suitable well protected storage area.
603A ACCURACY OF ERECTION:
* Survey the structure, including any fixing inserts, before commencing erection. Report to CA immediately if structure will not allow the required accuracy of erection.
* Set out joint centres for a complete elevation at a time unless otherwise agreed with CA. Erect units using temporary spacers to suit the survey results and ensure generally consistent joint widths.
* The widths of joints must be such as to ensure that the joints perform as intended and are within the recommendations of the joint sealant or baffle manufacturer.
* The finished work must have a satisfactory appearance, being square,regular, true to line, level and plane with a satisfactory fit of all junctions,all to approval.
640 FINAL FIXING:
* Obtain approval of appearance of each elevation before tightening fixings or sealing joints.
* Tighten threaded fastenings to torque figures recommended by the manufacturer. Do not over tighten restraint fixings intended to permit lateral movement.
650 SEALING JOINTS:
* Sealant: to CA approval.
* Colour to match GRP Colour.
* Application: As Section Z22.
660 OPEN DRAINED JOINTS:
* Ensure the rear seals are completely airtight.
* Install baffles and flashings securely and accurately to ensure that they function as intended.
670 DAMAGED UNITS: Do not repair without approval: such approval will not be given where the units are badly damaged or where the proposed repair will impair appearance or performance. Obtain approval of appearance of all repaired units. Repairs must be:
* Well keyed onto the surrounding GRP using both abrasion and a suitable chemical primer.
* Carried out using the same materials as used in manufacture of the units.
* Adequately cured, using suitable portable heaters for site curing.
680 CLEANING DOWN: Return to site at Practical Completion or when instructed and thoroughly clean down the entire area of the GRP work. Cleaning agents for the purpose must be approved by the GRP manufacturer.
SOURCE: Scot Bader Co. Ltd