Pultruded Glass Reinforced Polyester
Sustainability Considerations when this Composite Material is used in the Manufacture Of Startlink Building Componenents
by John Anthony Hutchinson
Introduction
I am John Anthony Hutchinson BSc (Hons) Dip Arch RIBA MCIArb, an architect qualified and registered to practice in the UK since 1973. Until March this year, I was in full time employment with Feilden + Mawson Architects, of 36 Grosvenor Gardens, London SW1W 0EB, as an Associate of the practice. I am now semi-retired, but am still retained on a part-time basis by F+M as a consultant on materials specification and good practice in building construction matters, particularly with regard to historic building conservation.
In addition to my membership of the Royal Institute of British Architects and the Chartered Institute of Arbitrators, I am a member of the Network Group for Composites in Construction in the UK. I was elected on to this body's Steering Group at its annual conference last November. I am also a member of the Association for Environment Conscious Building, the Green Chemistry Network based at York University and the Building Limes Forum.
A Sheltered Housing scheme which I designed in 1972 for a town in the West Midlands of the UK won an RIBA/DoE design award in 1976. A larger housing scheme which I designed in collaboration with a colleague, Martin Starbuck, in the late 1970s won an RIBA/DoE design award in 1980. I consider myself, on the basis of the expertise and experience which I have cited above, to be sufficiently well qualified to comment usefully and with some authority on sustainability issues as they affect the Startlink System of building components which are made of pultruded glass reinforced polyester and which is principally intended for house building. (DoE is an abbreviation for Department of the Environment).
Pultruded Glass Reinforced Polyester
Although composites have been used in building construction for over forty years, little is known within the industry about the superior properties and structural performance of pultruded glass reinforced polyester. These are summarised below:
*Pultruded GRP has almost twice the strength-to-weight ratio of mild steel which, in itself, has excellent strength and stiffness. It has almost five times the strength-to-weight ratio of reinforced concrete. A table of comparative properties of steel, reinforced concrete and pultruded GRP is included as an annexe to this document.
*Pultrusion optimises the fibre-matrix ratio, which has the effect of stiffening the composite. With a polyester matrix, the value for Young's Modulus can be as high as 39GPa (or kN/mm2). The comparable figure for conventional GRP is between 7 and 10GPa. Softwood suitable for structural applications in buildings has a figure of between 7 and 11GPa.
*The values for tensile and compressive strength are impressive, as can be inferred from the table below, because the composite starts to exhibit the mechanical properties of the E glass fibres, which have a tensile strength of up to 2,000MPa (N/mm2).
*The preponderance of glass in the composite that is achieved by pultrusion improves the performance in relation to thermally induced movement, so that it behaves much more like glass than plastic. The linear coefficient of thermal expansion is about 5x10-6/unit length/degree Celcius, which is slightly less than half that of reinforced concrete or mild steel.
*Pultruded GRP is electrically insulating; it is intrinsically good as a thermal insulator; it is acoustically absorbent and attenuates the passage of structure-borne sound; it is entirely impermeable both to liquid water and to vapour; it is resistant to 'freeze thaw'; it is acid resistant and alkali resistant up to a ph of 13 (vinyl ester rather than polyester would have to be used as matrix material for excessively alkaline environments); it is stable and chemically inert, so that it does not release VOCs while it is present in a building and, finally, it has better performance in a fire than either steel or timber, because the surface of the composite tends to 'char', which protects the core of the structural section against further burning.
*Provided that the correct production techniques are adopted, it can be made to be entirely resistant to the destructive effects of the uv components in sunlight.
*It can be confidently asserted that pultruded GRP is a highly competent engineering material, which is eminently capable of being specified for a large number of building construction applications, not least components for house building.
*Despite the fact that the polyester matrix is made from oil based chemicals, pultruded GRP turns out on detailed analysis to have one of the best environmental impact profiles of any strong building material, including structural timber. I will explain this surprising result in detail, below:
The Inherent Sustainability of Factory Based, Modular Building Materials
*Building construction is a complicated but often wasteful process. Waste and off-cuts from materials used on site are seldom recycled (at least in the UK). On site construction tends to be chaotic unless the standard of site management and supervision is of the highest order, which is the exception rather than the rule in jobbing building work. Site confusion and disorderliness gives rise to wasteful practice. If building processes were to be moved away from site to the more controlled conditions of a factory, the standards of supervision and quality control would automatically improve immeasurably, which would have the effect of greatly reducing waste.
*Another means of reducing waste is to construct modular buildings in which all the parts are pre-designed and fabricated to fit neatly together without cutting. Modular construction is far more suited to factory production, because the standard, modular kit of parts can be mass-produced, thus saving time, materials and a great deal of money.
*If an entire building can be made as a kit of parts to fit on to one low-loading truck, there need only be one delivery to site by means of a large vehicle consuming hydrocarbon fossil fuel. Normally, conventional building construction necessitates many vehicle movements to and from site, not only by delivery trucks, but also by the cars and other vehicles carrying the tradesmen to work and home again. If site time were greatly reduced as a result of the recourse to modular, prefabricated building techniques, there would be far fewer journeys made to site by building workmen.
*If the material used for the fabrication of the kit of parts was, in itself, extremely light in weight and easy to handle, there would be a considerable saving in fuel consumed, both in transportation and by the reduction in the need for motorised cranes and platform lift vehicles.
*Modular buildings are the easiest to recycle. The reason is that the component parts can be dismantled at the end of a building's life and taken away for re-use in another modular building. The long life-expectancy of pultruded GRP would enable Startlink components to be re-used in other buildings after a first building performance life of forty or even fifty years.
*The Startlink system is modular and factory based with the minimum need for any site presence. It uses the lightest low cost engineering material currently available, which is pultruded GRP. All of the fuel saving features I have described in this section are evident to the maximum possible degree in the Startlink system.
The Environmental Suitability of Pultruded GRP
*Although the matrix is made from the refined products of crude oil, the pultrusion process means that the preponderant constituent of the composite is glass which is made from silica - one of the most abundant minerals on the planet's surface. There are realistic prospects of matrix resins being made from plant-derived substances in the very near future. For example, there is a company in the UK, Cambridge Biometrics, that is already offering epoxidised vegetable oil for these types of application. Phenolic resins can be obtained from organic sources. Pultrusion can be done with polyurethane as well as polyester. The polyol component of PU can be made from organic material although, at the present time, it is not yet possible to make the diisocyanate cost effectively from anything other than petro-chemical feedstock.
*The embodied energy in pultruded GRP is similar to that of mild steel or reinforced concrete when measured by weight. The figure for mild steel is 60 megajoules per kilogram. For reinforced concrete it is 50 mJ/kg (source: J E Gordon, The New Science of Strong Materials). Pultruded GRP is between these two figures at about 55 mJ/kg. However, the fact that it is so much lighter and less dense than either of the two conventional structural materials has the effect of greatly reducing the embodied energy in any structure made from it.
*Lightness in pultruded GRP structures automatically reduces their dead-loads which, in turn, reduces the quantity of material needed to make the building, for the simple reason that less structure is needed in its lower parts to support the dead-loads imposed by the upper parts. Lightweight buildings consume far less of the earth's resources and non-renewable hydrocarbon fossil fuels than heavy ones. Pultuded GRP is the lowest cost lightweight material currently available for building construction.
*Lightweight buildings are much easier and less energy demanding to heat in winter. The heating plant can be used to heat the occupants. No energy is wasted in heating massive structure.
*Similarly, there is no heavy structure to become heated as a result of summertime solar gain. No mechanical cooling would be required to remove heat from massive building elements. The summertime performance of pultruded GRP buildings can be greatly improved by application of reflective materials to the external surfaces, which could easily be incorporated in the pultrusion process.
*Pultruded GRP buildings can be made to be air tight and vapour tight without the need to install vapour control layers, air sealing mastics in joints or 'breather' membranes behind rain-screen elements. There is no need for rain-screening, because this material is entirely and permanently resistant to the passage of water, either in liquid form or as diffused water vapour. There is minimal need for additional fire protection layers by virtue of the inherently good fire performance of pultruded GRP, which can be improved even more, should the need arise, by switching to a phenolic matrix instead of polyester. This remarkable material is, therefore, a 'one stop shop'. Entire buildings can be made from it with no need for the addition of other membranes, mastic sealants or cladding/siding. The only extra item required would be a layer of insulation such as ‘Rockwool’ (which adds additional fire resistance and an acoustic barrier) to be installed between the two construction leaves, which is a necessity for any well insulated, energy saving building, whatever it is made of. The slim Startlink profiles allow 90% of the wall to be filled with additional insulation and the resulting simplicity of the building envelope saves time in the construction process. It also saves energy, because all of the additional layers needed in other forms of construction consume energy in their manufacture and transportation.
*The dimensional stability of pultruded GRP has the effect of eliminating the possibility of ambient air infiltration at the joints between panels and window/door frames. Differential movement induced in commonplace building materials either thermally or by changes in humidity would impose stress on these joints with an attendant risk of rupture or fatigue failure of sealing tapes or mastic sealants, which would result in copious infiltration. For the best possible results, it would be sensible to specify pultruded GRP window and door frames, thus ensuring materials compatibility between the panels and the frames.
*The greatest potential for energy saving is below ground. Because pultruded GRP is so light, mass concrete foundations would not be needed. Instead, a Startlink building would be best supported on pultruded GRP piles driven into the ground beneath the building to a depth determined by its bearing capability. A Startlink dwelling accommodating five people could be adequately supported on as few as three piles. The quantities of raw materials and energy required to construct the foundation of a Startlink building would be a tiny fraction of those required in a conventional building.
Recycling Pultruded GRP
*The thermo-setting plastic matrix cannot be recycled as would be the case for thermoplastics such as polythene or polypropylene. When Startlink components have finally reached the ends of their useful lives, the disposal method would be to cut up the sections. The cut pieces would be conveyed to a high temperature incinerator dedicated to the safe disposal by burning of time expired GRP.
*The burning temperature in the incinerator would have to be in excess of 1200 degrees Celcius to effect the complete destruction of the time-expired composite. The combustion of the polyester would maintain this temperature once it had been reached, so the only extraneous energy needed would be for the start of the incineration. Recycling trials for composite parts are progressing very satisfactorily in several European programmes, using the material as a source of heat energy and the resultant residue as raw material in cement manufacture.
Conclusion
Pultruded GRP is the most energy efficient low cost, structurally competent building material available for construction use. Startlink is a modular building system of great elegance and simplicity. The combination of these two elements offers the possibility of making the most energy efficient buildings that it would be possible to erect cost effectively, given the range of materials tested as being reliable in use currently available to the construction industry.
If you find this interesting, email me and learn more!
Mark Singleton, e-mail: markATstartlink.co.uk
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