Adaptables2006, TU/e, International Conference On Adaptable Building Structures 8-187 Eindhoven [The Netherlands] 03-05 July 2006

Photochromic Glassfibre Reinforced Plastic

Hui ZHANG School of Architecture Southeast University Si Pai Lou 2 Hao 210096, Nanjing P. R. China [email protected]

KEYWORDS photochromic materials, glassfibre reinforced plastic (GRP), photochromic GRP, adaptive building skin

Paper

Sunshine in summer is not always desirable. The summer sunrays that reach inside buildings with large glazed envelopes and windows can make computer work difficult, with strong glaring on computer screens seriously affecting working conditions. The heat gained through glazed envelopes and windows, and from the buildings’ skin, also burdens the building with increased cooling loads in summer.

Among various solar control methods, the applications of photochromism are gaining increasing interest in architecture. Energy-absorptive photochromic systems and materials change their optical properties, such as color and light transmission, when exposed to the UV rays in sunlight and revert back to their original properties in diffused light. This material behavior can be used to control the effects of sunlight on the interior light environment and regulate lighting and heating levels for energy load management. At the present time, this photochromic effect is being applied in energy-efficient glazing, which can be either automatically or electrically switched from colorless (or weakly colored) to colored, or the inverse, in accordance with light conditions. These types of windows normally use glass panes as substrates which either have a photochromic coating or are laminated with photochromic films, or they are supplied with complicated device structures. Windows using the photochromic effect are just the beginning. Actually, photochromic materials have many more potential applications in architecture that can be further exploited.

An adaptable composite - photochromic glassfibre reinforced plastic - might contribute to the development of smart building skin which can adapt itself to the changing conditions of sunlight.

1 Glassfibre Reinforced Plastic (GRP) Glassfibre Reinforced Plastic (GRP), one of the Polymer Matrix Composites (PMC’s), consists of a polymer-based resin as the matrix and glassfibres as the reinforcement.

In general, a composite material is composed of at least two components integrated together to produce material properties that are different from the properties of the components on their own. The material properties of the composite can be, to a certain extent, regulated by virtue of adjusting recipes. In PMC, the polymer-based resin matrix carries the load applied to the composite between Adaptables2006, TU/e, International Conference On Adaptable Building Structures 8-188 Eindhoven The Netherlands 03-05 July 2006 each of the individual fibres (which have high tensile and compressive strength) and also protects the fibres from damage caused by abrasion and impact. Therefore, PMC’s are characterized by advantageous properties like high strength and stiffness, ease of moulding complex shapes, and high environmental resistance, all coupled with low densities. This makes PMC’s superior for many applications in the fields of aerospace, shipbuilding, plant construction, motor vehicles and so on.

As a PMC, GRP provides all these remarkable properties. It is also a comparatively economic composite thanks to the low cost and light weight of glassfibre. Concerning energy efficiency, GRP exhibits good thermal features, e.g. low heat conductivity and a small thermal expansion coefficient. [see Table 1] Additionally, this composite is also characterized by many aesthetic merits like ease of free-forming, a large range of color choices, various degrees of transparency, and good surface qualities. All of these properties endow GRP with a wide range of applications in the building industry. With continual improvements, GRP can no longer be regarded as a secondary, low-grade industrial material. It is now known as a promising building material which is being broadly applied in civil architecture for building structure, roofing, and flooring, as well as both interior and exterior cladding, window frames, and finishing.

GRP GRP Steel Timber Glass Al Pultrusion laminate S 235 JR S 10 Kalknatron tensile strength [N/mm 2] 240 ~ 60 360 14 30-90 150-230 E modulus [N/mm 2] 23.000 ~ 6800 210.000 11.000 70.000 72.000 elongation at break [%] 1-3 ~ 1,0 26 ~0,8 0,1 2-8 density [g/cm 2] 1,8 ~ 1,4 7,85 0,6 2,5 2,7 thermal expansion 9 ~ 25 12 ~4,5 8-9 23 coefficient [10 -6/K] heat conductivity [W/mK] 0,25 ~0,25 50 0,13 0,8 160 Table 1. Comparison of properties (Data from ITKE, Germany).

In the 1960s and 70s, GRP gained a certain prominence in the rush to building with plastic, which was demonstrated with successful projects such as Monsanto House, Futuro, Rondo, and fg2000. Nowadays, some experimental buildings like the Eyecatcher in Basel, Switzerland, the D-Tower in Doetinchem, Holland, and a gas transfer station in Dachau, Germany represent the potential of novel GRP applications.

The research on photochromic GRP is aimed at an adaptable composite that integrates the merits of both GRP and photochromism. It is intended to exploit GRP’s potential in architecture further, such as smart glazing or adaptable facades that can also contribute to solar control and energy efficiency.

2 Possible photochromic GRP – Dyeing GRP with photochromic pigments Photochromism can be simply defined as a light-induced reversible change of color. One of its well- known applications is photochromic spectacles that darken in the sun and recover their transparency in diffuse light. Basically, this phenomenon is the reversible transformation of a single chemical species between two energy states having different absorption spectra, i.e. colors. This change in states is activated by electromagnetic radiation (usually UV light) and generally accompanied with energy absorption. Although photochromism is not limited to colored compounds, photochromic materials with simple manipulation techniques are more appealing to use in practice.

One large class of photochromic materials are the organics. Certain organic photochromic dyes can be applied to polymers. This ability to be combined with polymers has attracted much attention in actual applications. Sunglasses made of photochromic plastics are more popular in recent years. The most suitable materials for photochromic glazing are announced to be the derivatives of spiro-oxazine applied to plastics.

Photochromic GRP, Hui ZHANG

Adaptables2006, TU/e, International Conference On Adaptable Building Structures 8-189 Eindhoven The Netherlands 03-05 July 2006 Inspired by photochromic plastics, it is possible to produce photochromic GRP by combining polymer-based GRP resin dyed with photochromic colorants. Applied in this way, photochromic GRP could be manufactured in similar processes like normal GRP and therefore require no further investment.

3 Experiments and Outcomes The primary object of the first phase of experiments is to prove the practical feasibility of photochromic GRP. Correspondingly, the trade-off between cost and benefit should also be taken into consideration.

On these terms, the earliest experiments were conducted under the following conditions: ·Samples were simply made by hand lay-up at room temperature in the laboratory of Lange+Ritter GmbH in Germany. ·Polyesters and epoxy resin systems were selected for laminating, since both of them are easily cured at room temperature by the addition of a suitable hardener or accelerator. Additionally, they are the two main types of resin used in the manufacturing of GRP products. Polyesters are easy to use and have the lowest cost of resins available. Epoxies, with increased adhesive properties, outperform most than other resin types in terms of high mechanical and thermal properties, and resistance to environmental degradation. It should be mentioned that most epoxy systems offered in the market are equipped with certain UV-resistant additive which might alter the photochromic performance. However, this potential defect might be overcome by the adjustment of pigment dosage. ·Certain photochromic dyes from England were applied due to their acceptable cost and comparative stability as well as simple manipulation properties. These highly concentrated pigments are offered in paste form and can be used as normal colorants. Two color options are available: one shows green in the activated state; the other violet. To make the color change effect of photochromic GRP more distinct, a colorless or slightly colored original state of the samples was intended and no other pigments were added. ·Three alternatives were conceived of for the incorporation of photochromic dyes in GRP. Thanks to GRP’s laminate structure, dyes can be applied either in the whole laminating resin or in the resin of interlayers. Otherwise, a dyed gel coat can be applied to the surface of GRP. ·In view of the potential applications for photochromic GRP as an adaptable anti-glare glazing for roofs and facades, a high translucency GRP-panel was also intended. The selection of resin and glassfibre types, as well as additives, was based on our experience in previous research.

The outcomes of the first experiments are satisfying and convincing. Some samples do react quickly to lighting conditions. Upon exposure to sunlight, photochromic GRP-panels either switch from colorless to an even-colored state, or are further darkened. [Fig. 1] Once the irradiation is blocked, these panels revert rapidly to their original appearance. Both the coloration and the bleaching process take place in only a few seconds.

Figure 1. The photochromic GRP samples before (left) and after (right) illumination with sunlight.

Photochromic GRP, Hui ZHANG

Adaptables2006, TU/e, International Conference On Adaptable Building Structures 8-190 Eindhoven The Netherlands 03-05 July 2006 Among the seven samples showed in Fig. 1, Panels 1 and 7 (numbered from bottom to top) display a relatively large shift in color. Panels 2 and 3 darken perceptibly in the sunlight. It is proved that both epoxy (used in Panels 2 and 7) and polyester (used in Panels 1 and 3) are capable of producing photochromic GRP. Photochromic dyes can be incorporated in the interlayer (as in Panels 1 and 2), into a gel coat (as in Panel 3) or in the laminating resin (as in Panel 7).

Not only the colors but also the light transmission of the first samples changed with ultraviolet irradiation. For example, the transmission ratio of Panel 1 dropped from approximately 56% to 50% after illumination with UV-Light. Panel 7 experienced a transmission drop of over 4%. This reduction can be increased with a higher dose of photochromic pigments, or by using other manufacturing processes such as resin transfer moulding.

The failed samples indicate that hardeners or accelerators needed for curing must be used very carefully. The curing of resin is an exothermic reaction, and certain additives can increase the heat release further within a very short time causing the dyes to lose their photochromic performance. Furthermore, photochromic dyes kept in unsealed storage containers could not be dissolved in the resins, supposedly because of oxidation.

4 Conclusions and Perspectives The feasibility of photochromic GRP is clearly confirmed with the first phase experiments via hand lay-up. A GRP-panel with photochromic properties has also been manufactured using an infusion process, but the coloration is uneven. This might be overcome by pre-heating the laminating resin before the input of dyes. In terms of photochromic performance, the first samples exhibit a good sensitivity to activation. As the next step, other manufacturing processes are to be undertaken. The photochromic characteristics of GRP should be enhanced with optimized recipes and optimal dyes. The integration of normal colorants and functional pigments in GRP is also planned.

Parameters like stability and durability are yet to be investigated through accelerated ageing tests. Whereas most organic photochromic compounds demonstrate poor fatigue resistance, stability might be another major obstacle for the application of photochromic GRP. Nevertheless, it has been verified that the photochromic properties may be different in polymers and vary according to the nature and to the oxygen content of the matrix. Moreover, GRP with a laminate structure might maintain photochromism longer, supposing that outer layers could act as protection for the middle layers. Some UV-resistant gel coat might help stability improvement as well. Clearly, the successful development of photochromic GRP requires a close cooperation between academics, professionals, and manufacturers.

Novel photochromic GRP applications would prospectively gain a large range of applications in architecture, such as adaptable building envelopes that, inter alia, contribute to energy efficiency.

Photochromic GRP glazing could adapt light transmittance to the intensity of sunlight and thus also offer protection against glare and interior space heating. With a high translucency, such glazing would be used at places where natural lighting is required but a see-through view is neither allowable nor desirable. Photochromic GRP could also be applied as a panel blind for windows or combined with transparent glass, which helps with solar control in a similar way. Comparatively, such applications are neither costly nor complicated: ·In comparison to coated photochromic glass, which demands high dye concentrations necessary for a darkening coating but with a thickness of only a few micrometers, photochromic GRP embodies the pigments in the major component and thus is much simpler to manufacture at a low-cost level. ·In contrast to electrochromic systems, photochromic GRP can automatically react to the change of lighting conditions, since sunlight provides the energy for the coloring process. In this way, it does not require an external power supply or control. The need for expensive transparent electrodes is also

Photochromic GRP, Hui ZHANG

Adaptables2006, TU/e, International Conference On Adaptable Building Structures 8-191 Eindhoven The Netherlands 03-05 July 2006 avoided. Furthermore, photochromic GRP remarkably reduces the needs for maintenance and thus is more stable.

Facade cladding is one common application of GRP. Manufacturing techniques and material quality have been greatly improved in recent years. On this basis, the integration of photochromism could further enhance the applicability of this composite with some added values. With sunshine in the daytime and through “black light” at night, photochromic GRP facades could turn on an attractive, dramatic appearance changing according to lighting conditions. Furthermore, smaller building cooling loads in summer are achievable thanks to the low thermal conductivity of GRP and the absorption of solar heat via photochromism.

On the supposition that photochromic GRP can be used as chameleon facade cladding, translucent sun blinds, a colorful shelter from glare, or median facades, photochromic GRP additionally allows architecture to turn on a variable expression and to bestow more freedom on architects in their design.

Figure 2. Simulations of application performance of photochromic GRP (up in diffuse light and below in the sunshine).

Note: The bottom left photo in Fig. 2 is figured in Bauwelt , vol. 95 (2004), no.21, p.2; the other were photographed by the author and simulation is done with computer.

5 Acknowledgments This research is supervised by Prof. Dr. Jan Knippers from the Institute of Building Structures and Structural Design (ITKE) at the University of Stuttgart, Germany and supported by Mr. Sven Raskob from Lange+Ritter GmbH, Stuttgart, Germany.

6 References 1. Larnpert , C.M. 1995 , ‘Chromogenic Switchable Glazing: Towards the Development of the Smart Window’, Window Innovations ’95, Toronto, Canada, June 5th-6th, 1995. 2. Bouas-Laurent, H., Duerr H. 2001, ‘Organic Photochromism (IUPAC Technical Report)’, © IUPAC, in Pure and Applied Chemistry , vol. 73, no. 4, pp. 639–665. 3. Kaltenbach, F. 2002, ‘Kuenstliche Transparenz’, in Detail , December 2002, pp. 1608-1615. 4. Einhaeuser S., Stelzer K. 2004, ‘Faserverstaerkte Kunststoffe’, in archplus , December 2004, vol. 172, pp. 34-37. 5. Schneider K. 2005, ‘Photochromic Systems on their way towards Architectural Applications’, © Fraunhofer ISE, Press Release on 8 March 2005. 6. Knippers, J., Zhang, H. 2005, ‘Moeglichkeiten der Gestaltung mit Glasfaserverstaerkten Kunststoffen: Transparenz, Licht und Farbe’, Seminarbericht WS 04/05, Stuttgart: Universitaet Stuttgart, Institut fuer Tragkonstruktionen und Konstruktives Entwerfen. Available from: www.itke.uni-stuttgart.de/de/forschung/transluzenz/Gestaltungsmoeglichkeiten_GFK.pdf .

Photochromic GRP, Hui ZHANG