14 November 2010

Will transparent polymers kill glass?


A silent revolution is taking place these days. Due to a number of reasons, the glass position as the one and only transparent filling for curtain walls is being threatened. Who is the new kid on the block? Well, it has been around for a while, but it has grown bigger now: transparent recyclable polymers, commonly called thermoplastics.

Tokyo glass and acrylic cantilevered structure, Dewhurst Macfarlane


The attack of the polymers has already started, in the way barbarians entered the Roman empire: as an alliance. If you need a good bullet resistant glass you will end up in a laminate called glass-clad polycarbonate. Beware: the higher the bullet resistance requirements, the less glass there will be in the laminate. If you are after a blast-resistant curtain wall, the options are heavy PVB laminated glass (1.52mm or more of polyvinyl butiral layers) or glass combined with an ionoplastic interlayer as SetryGlas, in widths of 2.28mm or more. If you want to have overhead glazing or horizontal glass with live loads, there you will find the plastic companions again. Structural glass is in fashion, and you may want to achieve all-glass, non metal-supported transparent structures. When you do that, most of the time it's due to the help of polycarbonate sheets glued to the glass with transparent polyurethane interlayers.

Tokyo International Forum, glass canopy details. PMMA was not required structurally but it was added as a safety measure against typhoons and earthquakes.


This was the situation up to now. Glass is still in complete command if we want to clad a facade with a transparent, low U-value, durable and non combustible material. A curtain wall is still synonym for a glass curtain wall. But things are starting to change, and the big polymers suppliers have focused their attention onto the new frontier: to introduce Transparent Composite Facades (TCF) in lieu of Glass Curtain Walls (GCW).

I have taken the TCF name from a PhD dissertation in the University of Michigan, submitted by Kyoung-Hee Kim in 2009, and advised by Professor Harry Giles. The title is Structural evaluation and life cycle assessment of a Transparent Composite Facade system. I have also got ideas for this post from a presentation at the last Glasstec conference in Sept 2010: New materials for transparent constructions, by Eckhardt and Stahl.

Prism Cultural Centre in West Hollywood, California by PATTERN Architects. Translucent facade in resin-based composite polycarbonate. 3Form, an advanced material fabrication company specializing on resin-based composites, has collaborated on the facade solution.

The contenders to take the place of glass in curtain walls are four thermoplastics: polycarbonate (PC), polymethylmethacrylate (PMMA or acrylic), polyethylene terephthalate (PET or nylon) and polypropylene (PP). PET and PP are still lagging behind in the race, mostly because of their low stiffness and low ultimate strength. PET and PP have a Young's modulus 1/7 and 1/2 that of polycarbonate and acrylic, and their ultimate strenght is 1/3 that of polycarbonate and acrylic. The main advantages of polycarbonate and acrylic, on the other hand, is that they are 20 times less brittle than glass and their ultimate streght can be two times higher than glass (see data on the table below). So, we will focus on polycarbonate and PMMA / acrylic from now on.

From 'Materials and design' by Ashby and Johnson, plus www.matweb.com


Some may say these two materials have been around for too long to worry about them now. That is true, but doesn't tell the whole story. Let's call them by their best-known brands: polycarbonate is better known as Lexan, Makrolon or Danpalon. PMMA / acrylic is sold under the brands Plexiglas, Lucite or Perspex. Polycarbonate, either in solid or in multilayered sheets, has found a safe place in architecture as a translucent wall cladding, not transparent. A great example is the Laban Dance Centre in Southern London, a proyect by the Swiss architects Herzog & de Meuron with PC sheets supplied by Rodeca in Germany.

Laban Contemporary Dance Centre, London. Herzog & de Meuron architects


The cladding on the Laban Centre has four layers with a U-value of 1.45 W/m2°K, better than a low-e double glass unit. But present multiwall polycarbonate sheets, only 60mm thick, have already reduced the U-value to 0.85W/m2ºK, in the range of a triple glass with argon and low-e coatings. Polycarbonate can also reduce reliance on secondary solar control; the panels used at the Laban Centre feature dimpled inner skins that diffuse the light. Concerns over the durability and UV resistance of polycarbonate have now been reduced thanks to new film protection technology. Manufacturers now guarantee that the polycarbonate will lose no more than 1% of its transparency over the first 10 years. All this is fine, but this is still a translucent wall, a cladding material for gyms, dance centres, swimming pools or industrial buildings.

V-profile by Rodeca, a mullion in extruded polycarbonate
The next step for solid polycarbonate and acrylic is to become the transparent filling of a curtain wall, both in the fix elements and in the windows. A transparent composite facade (TCF) is well described by Kyoung-Hee Kim's dissertation as:

A composite construction consisting of a polymer double skin with an inner composite core, configured to provide a stiffer, safer, energy efficient and lightweight alterative to a glass façade system. 
This new 'glazing' system has spurred studies that evaluate the material performance of polymer and composites as a cladding material. The polymer skin has a sustainable characteristic due to its recyclability, which can help to reduce the environmental impact associated with raw material depletion and disposal.

Let's use an existing example to visualize the new TCF coming. Kalwall is a well known US translucent facade and skylight system, whose filling material by the way is a thermo-set polymer reinforced with fiberglass, with modified properties regarding UV-resistance and reaction to fire. The basic panel comes in standard dimensions and is installed as a very simple curtain wall unit sistem.

City & Islington College, London. Van Heyningen and Haward architects. Kalwall and glass facade.

The best version of Kalwall, pushed by Stoakes (the UK distributor) to comply with EU directives, has really low U-values by using thermally broken profiles combined with aerogel insulation. Even without aerogels you can have a 100mm panel, filled with polycarbonate fibres, with a U-value of 0.83W/m2ºK and a solar factor of 0.15. But, alas, it's still translucent. Now, imagine you replace the fiberglass reinforced thermoset at both sides of the panel with a high resistant solid policarbonate sheet, and add some intermediate transparent sheets to improve its U-value and acoustic performance. The outcome would be something similar to Kalwall - with the same Japanese paper-like rectangular pattern - but wholly transparent, no glass required whatsoever. Stop imagining, this concept is already being developed somewhere in Europe.

If we move to PMMA / acrylic, a similar story is being written these days. The material can be easily molded to achieve fuzzy shapes at a cost which is only a fraction of glass. In solid state, acrylic sheets can be cut and mechanized using laser cut CNC machines to provide extrusion-like profiles. A great example is the facade of the Reiss HQ building in London by Squire and Partners architects, with a machined PMMA external skin, and lit with an LED system at the bottom of each floor level.












Reiss London. Double skin facade with acrylic and glass. 
Reiss London. Acrylic milling process and finished facade panel.































Evonik Röhm, the company that owns the Plexiglas brand, was founded by Mr Röhm, the inventor of PMMA in 1933. After almost 40 years, the Olympic stadium in Munich by Frei Otto is still a forward-looking piece of architecture. The complex grid of steel cables was clad with a solid, tinted Plexiglas sheet to provide shelter against the summer sun rays. Need versatility? The monolithic thickness of PMMA is 200mm in a dimension of 3 x 8m - larger than glass, and you can even weld acrylic with hidden joints for bigger units. In the extrusion process you can obtain 25mm thickness, a width of 2m and no limit with length.

Olympic Stadium, Munich 1972. Frei Otto and Günther Behnish.


Curving a polymer is easy, but equally so is molding. Transparent facades as the one at the Liquid Wall in Berlin, the flagship store of Raab Karcher, would be a nightmare in glass, but are feasible in acrylic. Home Couture Berlin is a showroom for tiles and spa accessories. The store provides an ideal presentation platform for Raab Karcher and its joint-venture partners from the premium tile and bathroom fittings sector. The store functions as an elongated shop window for passers-by.

Raab Karcher flagship store, Berlin
The ‘liquid wall’ installation of milled Plexiglas appears as a vertical wall of water and serves as an eye-catcher in the Ku'Damm facade. The distorting lens makes the illuminated back wall oscillate as you wander past it. The shop window has been made after a 50mm Plexiglas sheet milled, formed and polished to get convex and concave surfaces. This form creates an effect of a moving room while walking by the window, as if the inside were a swimming pool.

If you need more inspiration, have a look at these four polymer materials suppliers: 3form, Panelite, Lightblocks and Krystaclear. These are all US-based companies at the verge of another revolution: they are not just suppliers, but also fabricators, engineers, materials researchers and high-end designers. They are in the front of the 'design to manufacture' concept that is changing the way materials are used. And they are all based in polymers.

When you can't win your enemies, join them. Another interesting product is Gewe-composite by the German glass supplier Schollglas. This laminated safety glass made with two sheets of glass and an intermediate 2mm polymeric membrane, can replace thicker glass-PVB laminates, is very easy to cold bend and does not stop UV radiation, thus making it very appropriate for winter gardens. The Amazonienhaus botanical greenhouse in Stuttgart is a good example of high UV-transmission, low-e coating composite transparent cladding. Another striking use of this composite in a bent application - non heat mold required - is the Mobile Formula 1 Event Centre for McLaren in the UK. The cold bent composite here integrates metal sheets and comes with a highly selective coating.

Amazonienhaus Stuttgart. Glass laminated panels with high UV transmission from Schollglas











Gewe-composite from Schollglas


There are still issues to solve and improve with thermoplastics before they can replace glass in curtain walls. Even if their mechanical properties are OK in general (impact resistance being great in particular) long term creep deformation is a clear disadvantage. The elastic modulus of extruded polycarbonate, for example, can be reduced to 40% after 1000 hours of constant loading. Regarding durability, polymers and acrylics offer a lower durability and weatherability under outside exposure compared to glass. Coated protections against UV have improved this, but there is still a way to go. The yelowness index (YI) measures discoloration levels under UV exposure, and values above YI-8 are not recommended for external use.

Another issue that must be integrated in the design is the thermal movement of plastics. The coefficient of thermal expansion of both polycarbonate and PMMA is 6 to 7 times greater than that of glass. Beware of the expansion pockets and frame movements! Abrasion resistance of plastics has improved with external coatings, but its value is still 2 to 4 times lower than that of glass. The biggest issue with plastics as external facade elements is probably their flammability or fire reaction. On one side, PC, PMMA and glass all conform to the flammability requirements of the ASTM codes. However, full compliance with the International Building Code (IBC) has to be checked case by case. The IBC limits the installation of plastic glazing to a maximum area of 50% of a building facade.



It is interesting to note that the U-value of a single layer of 6mm of transparent glass, polycarbonate or acrylic is practically the same: between 5,2 and 5,8W/m2ºK (glass being the highest). We get a similar result with g-value and light transmittance: uncoated glass and polycarbonate transmit the same amount of solar and visible radiation, while acrylic is slightly more transparent in both cases. But when we introduce selective coatings, glass performs much better than plastics in energy and visible light terms. Up to now, though.

Nobody knows the end of this story. Will new recyclable polymers completely replace glass as a transparent filling for curtain walls? It seems uncertain, but at least I would vote for a future co-habitation of both materials. If I had money to invest in the stock market, I would buy plastic shares rather than Saint Gobain ones. Well, don't follow my advice too quickly: Saint Gobain is investing in plastics right now, so think twice...

31 comments:

Anonymous said...

Your blog is awesome! I really enjoy reading it. Keep up the great work!

Ignacio Fernández Solla said...

Thanks a lot. That's really encouraging. I will!

butt weld fittings supplier said...

Thanks for sharing. Such very useful post for me.........

Anonymous said...

I was looking for a cladding material that was transparent, colourable and withstood considerable wind stress for a project i have...your page is really helpful!!
thank you for posting the info...dow does the material withstand uv?

claire

Ignacio Fernández Solla said...

Claire,

That's a weak point in general. Polymers offer relatively low durability and weatherability under exposure to natural environment compared to glass.

UV radiation causes the deterioration of mechanical toughness and optical clarity − its effects are measured with two parametres: the Yellowness Index (YI) and % Haze. In order to prevent UV degradation, UV-resistant additives and fillers as well as UV-protective coatings and films are applied to plastic materials. The weathering performance of an uncoated PMMA and a coated PC undergoes significantly less change in color (YI) and optical properties (% Haze) after 10 years of UV exposure, as opposed to uncoated plastics.

ASTM D 1925 Standard Test Method for Yellowness Index of Plastics is used in the plastics
industry to measure discoloration levels under UV exposure. The yellowness becomes visibly detectable when the YI is greater than YI-8. The light transmitting properties of plastics are measured in accordance to ASTM D 1003 Haze and Luminous Transmittance of Transparent Plastics. Materials with greater than 30% haze are considered diffusing materials.

Hope it helps,
Ignacio

Anonymous said...

Great site you have here Inacio... mui bueeno!
Very informative and helpful

Leads Facade said...

fantastic,informative,innovative...over all grt piece of info.

Anonymous said...

Great read, informative. I intend to see more of these articles in the near future..thanks!

Kirinrin said...

Wow, very interesting blog. Thank you for sharing your knowledge. I'm also working at the facade industry.

Anonymous said...

Ignacio,
beautiful article, great illustrations, thank you so much for posting this!
x x
archizu

Cadisch MDA said...

Great post, thanks for sharing it.

Glassware melbourne said...

I guess there would be a possibility since polycarbonate glasses are less prone to breakage than ordinary glasses.

Sri Ramana said...

Thanks for this post. Very Useful.. Lexan is basically the brand name generally used for polycarbonate and resin in a varied range of products.

Lexan Polycarbonate sheet

Window Maintenance said...

I think yes! Transparent polymers will kill glass.

Laminated Glass Walls

Dermaskin said...

12. You blog post is just completely quality and informative. Many new facts and information which I have not heard about before. Keep sharing more blog posts.

Manufacturing Of Technical Glasses

Anonymous said...

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manufacturing of technical glasses

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Keller Products Jan said...

polycarbonate plastic extrusions are one of their specialties that are very beneficial to customers. This type of material is best for projects that are often exposed to spills, dirt and scratches. The structure of this plastic material is very strong. They can resist pressure and abrasions imposed by the busy and stressful environment. They can withstand high temperature that is why they are commonly used for automotive and other housing equipment. Being transparent makes it perfect for outside lighting and is a great replacement for glass. Its flexibility makes it perfect for products that requires angle or curve forms. They are always available in large quantity.

natasha5kextension said...

Thank you for the excellent informative article. 2 questions:
1. how can you further insulate polycarbonate sheeting if it is the exterior cladding?
2. Can the use of a petrochemical generated material be considered sustainable by the virtue of the fact that it is recycled

Keller Products Jan said...

Plastics have been a part of our lives. This is because they are the most convenient yet durable materials that we can have aside from the fact they are way cheaper than those materials made of glass or steel. Polycarbonate is the material often selected for plastic extrusion due to its superb properties in terms of stiffness, resistance to impact, robustness, resistance to heat, insulation to electricity, and dimension stability. It is commonly clear but it is available to be colored according to your preference and it makes a glossy finish that makes it classy. polycarbonate plastic extrusions are perfect for various tubing projects because of the properties that Polycarbonate executes. Because of the many useful properties that it has, Polycarbonate plastic extrusions are applicable to electrical constituents, auto and trim parts, corner guards, housing, frames, fairings, etc.

John Benny said...

Fantastic blog post, I am totally pleased by your excellent work. It contains really very useful information. Thanks for sharing.
Manufacturing of Technical Glasses

The Digital Illusion said...
This comment has been removed by the author.
Anonymous said...

Keller Products offers you the best of both worlds when it comes to your Plastic Profiles. We make the safest of their kind because we use only biodegradable materials. Aside from that, our products have this stain resistant property, that ensures a long lasting color preservation. It will take you a couple of years before you can replace them. Our products will show you what quality and durability means. Our plastic profiles do not catch fire. This property made them known to be “self extinguishers”. Our products are the toughest and highly resistant to impacts. They are glossy and you can choose the color that you want for them upon purchase. They will definitely meet any expectations that you may have.

Anonymous said...

Ignacio, Excellent Post. Do you know if Fiberglass Reinforced Plastics (Kalwall), or Thermoset Plastic (Polycarbonate such as danpalon) has the greater tendency to yellow more over time? I'm having trouble finding yellowing indices for the two, and the manufacturer reps are useless.

Stefano Malnati said...

Hello
just to pont out:
PC isn't so expensive
Flame resistance for the most odf the product is Bs1d0 and also complying ASTM E84
U.V. resistance is very good and warranty is from 10 to 20 years
Cold bending radius is 150 times the ticknes and thermobending is easy
For any question contact s.malnati@ansatermoplastici.it

Stefano Malnati said...

Hello
just to point out:
PC isn't so expensive
Flame resistance for the most odf the product is Bs1d0 and also complying ASTM E84
U.V. resistance is very good and warranty is from 10 to 20 years
Cold bending radius is 150 times the ticknes and thermobending is easy
For any question contact s.malnati@ansatermoplastici.it

Glass Supplier said...

Thanks for the info about polymers

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Anamika Singh said...

Thank you for posting such great information.
Secondary Glazing

Glass Supplier said...

I think the glass is much better

Red Tab said...

i think Using Tempered reflective Glass and Back Painted Glass is a perfect combination for wall cladding....