Wood foam – The most promising new thermal insulation material

Wood foam is a new wood-based sustainable thermal insulation material that is recently developed by researchers at Germany’s Fraunhofer Institute for Wood Research WKI. The researchers have developed a both efficient and environmental friendly insulation material which aims at replacing petrochemical plastic insulation materials in the future. As more and more attention is paid to environmental effect, and with a better overall performance than conventional thermal insulation materials, wood foam is the most promising thermal insulation material.

Good performance

Wood foam offers the same performance as petrochemical plastic insulation materials such as Polyurethane and Expanded polystyrene, while petrochemicals are not environmentally friendly and not renewable. Typically used petrochemical thermal insulation materials are Polyurethane (PU) foam, Expanded Polystyrene (EPS) foam. The thermal conductivity of polyurethane foam and EPS foam is about 0.03 W/m*K which can be said to be good performance compared with mineral wool insulation whose thermal conductivity is 0.04 W/m*K. Although this performance is not as good as silica aerogel 0.02 W/m*K (EngineeringToolBox, 2014) and Vacuum Insulation Panel (VIP) 0.008 W/m*K (EnergySavingTrust.org, 2010), but aerogel is very brittle and fragile, VIP is too expensive and difficult to produce.

Wood foam can be made into rigid foam panel and flexible foam mats, which is also the advantage of PU. The advantage of PU foams is its wide range of characteristics which offers flexibility for application. There are mainly three kinds of polyurethane foams divided by their characteristic and application: 1. Flexible foam used in bedding, furniture and automotive seating. 2. Rigid foam used for thermal insulation panels. 3. Spray foam insulation for building insulation. (Mixture of isocyanate and polyol resin forms expanding foam and are sprayed onto roofs, concrete slabs and into wall cavities.) And wood foam can fulfill all these application requirements.  “Our wood foam can be used in exactly the same way as conventional plastic spray foams, but is an entirely natural product made from sustainable raw materials,” said Professor Volker Thole (2014) of Frauhofer WKI.

Comparing with EPS which is easily broken off, wood foam has high flexural stiffness.

There are some other wood-based insulation materials already existed, like mats made from wood fibers and wood wool. But they tend to shed fibers and are less stable in shape than plastic insulation materials. “Over time, the currently used insulation mats made of wood fibers tend to sink in the middle due to temperature fluctuations and damp. This to some extend adversely affects its insulating properties,” said Professor Thole. But the wood foam is as good as conventional plastic foams in this aspect.

People may worry about the risk of fire because wood is a typical inflammable material in people’s mind. This can be solved by adding flame retardant. Many types of insulation are retardant because they contain flame retardant and they are also fire accelerant if not treated. Or it can be treated in the way of EPS fireproofing. EPS foam is coated in a fireproofing chemical called Hexabromocyclododecane (HBCD).

Easy production

The foam is produced by grinding the wood particles very finely to a slimy mass, then gas is added to expanded it into a frothy foam and then hardened. The substance aiding the hardening process is contained in the wood itself.  “It’s a bit like baking, when the dough rises and becomes firm in the oven,” Explained Professor Thole (2014). Finally, the base material is made into rigid foam panels and flexible foam mats.

This process is similar with the production method of EPS foam which is normally produced by two mechanisms: 1. Polystyrene beads are expanded to little balls (pre-expanded polystyrene) with blowing agent like pentane. 2. The little balls are glued together with hot steam.

Comparing with VIP which is the very difficult to produce, the advantage of wood foam production is obvious.

Comparing with fiberglass which is the most commonly used thermal insulation material because of its easy production which is by effectively weaving fine strands of glass into an insulation material. But the main disadvantage of fiberglass is danger of handling because of the glass powder and tiny shards of glass which can harm eyes, lungs and even skin if proper safe protections are not equipped.

Low cost

Wood foam is claimed to be made from sustainable raw materials by Professor Volker Thole of Frauhofer WKI. So the price won’t keep increasing because of less and less reservations as petroleum.

Figure 1 Brent spot prices of petroleum in dollar as a function of time from 1987 – 2013 monthly average.  Sources: Energy Information Administration and Bureau of Labor Statistics.

The price for petroleum keeps increasing since the year 2003 till now, and it has been quadrupled from 40$ to 120$. This is caused by the decreasing amount of petroleum, and the price will be higher and higher. The price of petroleum based thermal insulation materials keeps increasing as the price of petroleum keeps increase. It’s urgent to replace EPS, PU with renewable resources based material – wood foam.

Comparing with VIP which is of the most expensive thermal insulations, VIPS use membrane walls to prevent the air from entering the panel. The membrane is supported by a rigid, highly-porous material, such as aerogel and fumed silica against atmosphere pressure and chemicals are needed to collect gas leaked through the membrane. All the components above: membrane wall, aerogel, fumed silica and gas collection chemicals are all expensive subject, besides the vacuum forming is another costing part.

Environmentally friendly

Wood foam is claimed to be an entirely natural product, which means it doesn’t include any synthetic chemicals, even the substance aiding the hardening process is contained in the wood itself. Also the raw material wood is claimed to be sustainable and the wood is biodegradable, which means the whole production chain from raw material to final product is green.

Cellulose may be the most eco-friendly conventional thermal insulation material, because it is made from recycled cardboard, paper, and other similar materials.  But skilled individuals are needed to install it and the newspaper dust can cause allergies that some people may have. (2011)

Comparing with petrochemical based materials, wood foam is much more eco-friendly. Firstly, the petrochemical based thermal insulation materials are mostly non-biodegradable, and chemical additives are added into the product and aid the production process. Besides, there are also specific influences of specific kinds. PU foam is said to have health risk such as skin and respiratory harm present in spray polyurethane foams. But fully reacted polyurethane polymer is chemically inert. Extruded polystyrene (XPS) uses hydro-fluorocarbons (HFC) as blowing agent which has very high global warming potential. EPS is non-biodegradable and there will be rough panel edges and dust when cut.

Applications

The main application of wood foam is aimed at building insulations, because buildings are major demand for thermal insulation to keep indoor climate stable. Conventional construction insulation material are petrochemical based rigid boards or expandable foams, although they have good insulation performance, but they are not eco-friendly. Nowadays, countries and unions are setting goals to become green and sustainable society, wood foam offers a good choice. Spray foam (PU) insulation is a commonly used building insulation. Mixture of isocyanate and polyol resin forms expanding foam and are sprayed onto roofs, concrete slabs and into wall cavities. But it is said to have health risk like skin and respiratory harm present in spray polyurethane foams when installed incorrectly. But by correct installation, fully reacted polyurethane polymer is chemically inert. One interesting thing is PU foams made using aromatic isocyanates discolor exposed to visible light. It turns from off-white to yellow to reddish brown. But visible light has little effect on foam properties. And these drawbacks of spray foam don’t exist in wood foam insulation. Wood foam can also be produced in sheets as the expandable polystyrene sheet used in building insulation. The only drawback is that it needs additional chemicals to make the foam rise and set, but it is still more sustainable and green than petrochemical foams, as explained by Andrew Liszewski (2014).

Wood foam is also aimed at packaging industry. EPS is often used as packaging because it can prevent damage and its thermal insulation required for food and medical transportation. Although EPS is rigid and tough but it has a low sheer strength which makes it easily break off. Wood foam not only can offer all the advantages of damping, light and thermal insulation, but also can prevent the disadvantage of low shear strength, so wood foam is a promising alternative to replace EPS packaging.

Ongoing

The researchers of Fraunhofer is now trying to find the best choice of tree species to be the raw material. And they are also developing industrial scale production process for the wood foam.

Conclusion

As wood foam has good performance, easy production, low cost, eco-friendliness and flexible applications, it can be forecasted that wood foam is the most promising new thermal insulation material especially in replacing conventional petrochemical based foams.

 

References:

Andrew Liszewski. (2014, March). Insulating Foam Made From Wood Makes Your Log Cabin Warm and Authentic. Retrieved from http://gizmodo.com/insulating-foam-made-from-wood-makes-your-log-cabin-war-1548168076

EnergySavingTrust.org. (2010). Insulation materials chart: Thermal properties and environmental ratings. Retrieved from http://www.energysavingtrust.org.uk/content/download/2640/62057/version/1/file/CE71+-+Insulation+materials+chart%E2%80%93+thermal+properties+and+environmental+ratings.pdf

EngineeringToolBox. (2014). Thermal conductivity of some common materials and gases. Retrieved from http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Thermaxxjackets.com. (2011, June). 5 most common thermal insulation materials. Retieved from http://www.thermaxxjackets.com/5-most-common-thermal-insulation-materials/

Volker Thole. (2014a, March). Effective thermal insulation with wood foam. Retrieved from http://www.fraunhofer.de/en/press/research-news/2014/march/effective-thermal-insulation.html

Volker Thole. (2014b, March). Effective thermal insulation with wood foam. Retrieved from http://www.fraunhofer.de/en/press/research-news/2014/march/effective-thermal-insulation.html

Wood foam — advanced green thermal insulation material

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Wood foam is a new sustainable thermal insulation material that is recently developed by researchers at Germany’s Fraunhofer Institute for Wood Research.

Wood foam offers the same performance as petrochemical plastic insulation materials such as Polyurethane and Expanded polystyrene, but petrochemicals is not environmenal friendly, and wood foam is made from sustainable raw materials.

The foam is produced by grinding the wood particles very finely to a slimy mass, then gas is added to expanded it into a frothy foam and then hardened. The hardening process is aided with substances contained in the wood itself. Finally, the lightweight wood foam as base material is made into rigid foam boards and flexible foam mats.

There are some other wood-based insulation materials already existed, like mats made from wood fibers and wood wool. But they tend to shed fibers and are less stable in shape than plastic insulation materials.

The researchers of Fraunhofer is now trying to find the best chioce of tree species to be the raw material. And they are also developing industrial scale production process for the foam wood.

As more and more attention is paid to enviromental effect. Wood foam is an promising alternative in replacing spray foam insulation in builing insulation and also plyurethane boards and expanded polystyrene in packaging.

Here is the reference link:

http://www.fraunhofer.de/en/press/research-news/2014/march/effective-thermal-insulation.html

Polyurethane foam

Polyurethane foam is another commonly used thermal insulation foam like EPS, XPS foam.

Its thermal conductivity is 0.03W/(m*k) which is almost the same with EPS foam. The advantage of PU foams is its wide range of characteristics which offers flexibility for application.

There are mainly three kinds of polyrethane foams devided by their characteristic and application:

1. Flexible foam used in bedding, furnature and automotive seating. Low density, open-cell-structure.

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2. Rigid foam used for thermal insulation panels. Low density, closed-cell-structure.

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3. Spray foam insulation for building insulation. Mixture of isocyanate and polyol resin forms expanding foam and are sprayed onto roofs, concrete slabs and into wall cavities.

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One interesting thing is PU foams made using aromatic isocyanates discolor exposed to visible light. It turns from  off-white to yellow to reddish brown. But visible light has little effect on foam properties.

One disadvantage of PU foam is its health risk like skin and respiratory harm present in spray polyurethane foams. But fully reacted polyurethane polymer is chemically inert.

Compared to EPS foam which is preffered in my thesis, rigid PU foam offers the almost the same thermal conductivity, but with a high bending strength and stiffness. So PU foam sheet can be considered to be put on top of the panel core to improve thermal inulation.

Here is the reference link:

http://www.recticel.com/index.php/company/what-is-polyurethane

http://www.wisegeek.com/what-is-polyurethane-foam.htm

The best high temperature insulation

ImageRecently, I have found an thermal insulation product called Microtherm, which is said to be the best high temperature (up to 1000 °C ) insulation.

The meganism of Microtherm is microporous insulation, which is 90% air but the air is contained in minute cell that are smaller than the average free path of an air molecule. Thus the convection is reduced. It’s kind of nanotechnology in insulation. The main constituent is usually pyrogenic silica. The other more important ingredient is the opacifier that block the infrared radiation which is the main heat transfer at very high temperature.

It not only perfoms the best at high temperatures, but is also the best general thermal insulation at all temperatures from very high to cryogenic temperatures, which can be seen from the figure below.

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As we can see from the figure microtherm offers a very low thermal conductivity at ambient temperature 0.021 W/m*k which is even lower than still air 0.025 W/m*k, while the EPS offers only 0.033 W/m*k. What is more important is that the thermal conductivity only increases a little when temperature increases while the other insulation materials increases very fast at high temperatures.

It’s used  in cookings, refrigerators, transportations and buildings.

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Comparing to my thesis of Econcore panel, microtherm offers more professional and concentrated performance at thermal insulation especially at high temperatures. But it has some reference value for my thesis because one ot the improvement we applied, which is pouring sillica into the cells of the panel, is very similar to the meganism of microporous insulation.

Here is the reference link: http://www.microtherm.uk.com/low/EXEN/site/hightemperature-insulation.aspx

EPS foam — Common packaging insulation

下载EPS(expanded polystyrene) foam is among the largest commodity polymer. It’s also one of the main choice in my thesis.

EPS is consisted of closed cells which are glued together. It is rigid and tough and has good thermal insulation and damping property.

It is often used as packagings because of it can prevent damage and its thermal insulation required for food and medical transportation. It’s also used in building insulation.

The thermal conductivity of EPS varies from 0.032 to 0.038 W/(m·K) depending on its density. Recently the thermal conductivity has reached 0.029 to 0.034 W/(m·K) by adding graphite, aluminum and carbons as fillers and such EPS has grey or black color different from the normal white color.

images (3)It is normally produced by two meganism: 1. Polystyrene beads are expanded to little balls(pre-expanded polystyrene) with blowing agent like pentane. 2. The little balls are glued together with hot steam. In my thesis, we choose to use ready made pre-expanded polysyrene, because it’s easy to pour the little balls into the little cells of the panel.

There is another polystyrene foam called XPS(Extruded polystyrene). It offers higher surface roughness and stiffness. It has a reduced thermal conductivity between 0.029 and 0.039 W/(m·K). It has a higher water vapour diffusion resistance, which makes it more suitable in wet environment than EPS. One disadvantage of XPS is the use of hydrofluorocarbons(HFC) as blowing agent which has very high global warming potential.

There are two famous trade names of EPS. Styrofoam is a brand of XPS but is refered to EPS packaging in United States and Canada. Thermacol is another tradename originated from BASF.

There are a few disadvantages. It is non-biodegradable, but it’s recyclable although the recylcling program can be limited. It is flammable like other organic compounds. There will be rough panel edges and dust when cut, when XPS is cut clean and straight. It’s not water proof as XPS.  It has a very low flexual strength while XPS has exellent flexual breaking strength.

Here is the reference link: http://www.wisegeek.com/what-is-a-polystyrene-foam.htm

Aluminum foil

ImageAluminum foil is not only used in my thesis, but has a lot of applications in daily life. I have found some useful facts about aluminum foil.

Aluminum foil is thin leaves less than 0.2mm thick.  It is pliable and can be wrapped around objects. It is often laminated on other materials like plastics to be more useful as in our situation.

It replaces tin foils in 1950s. It is some times mistaken with metallised films which is plastic films coated with a thin layer of aluminum. And metallised films seems to be a better choice for my thesis.

Aluminum foil thicker than 0.025mm is impermeable to oxygen and water. Aluminum has a shiny side and a matte side due to production process. It’s said that the heat is kept out when shiny side is facing out and heat is kepts in when shiny face is facing in, but the difference is negaligibe without instrumentation.

Aluminum foil is mostly used as packaging. It is used in long-life packaging for food, drinks and pharmatheuticals as it is a total barrier for light and oxygen. It is also used to wrap food to preserve it like left food in fridge, take-away food and fast food as it can prevent ordor exchange. Aluminum foil is also ued in thermal insulation due to its high reflectivity, electromeganetic shielding and cooking like barbecue.

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Some aluminum foil can be recycled but many aluminum laminates are not recycled due to difficulty in separating the components and low yield.

Aluminum foil is commonly used as foil containers and flexible packaging. In my thesis, aluminum foil is used to reduce radiation heat transfer as an important insulation method..

Here is the reference link: http://en.wikipedia.org/wiki/Aluminium_foil

http://www.uspackagingandwrapping.com/blog/Thickness-of-Aluminum-Foil.html

 

Analogy between thermal and electrical

Equivalient_thermal_circuit

In thermal domain, there are basic definitions like thermal conductivity k(W/m*k), thermal resistance R(W/m2*k) whose meanings are confusing to beginners. Recently, I read an article about thermal electrical analogy, which make them easy to understand.

 

1. Definitions analogy

Voltage                             U        <————>     ΔT (k)             Temperature difference

Current                             I          <————>      q(w)              Heat flow rate

Electrical Resistance      R         <————>       Rth(K/W)      Absolute thermal resistance

Electrical Resistance unit area   <———–>     R (K*m2/W)   Thermal resistance

Electrical conductivity        σ       <———–>      k  (W/m*k)      Thermal conductivity

Electrical resistivity            ρ       <———–>       Rλ (K·m)/W         Specific thermal resistivity

Among the four definitions about thermal resistance, absolute thermal resistance Rth is a property of a specific component just like a resistor and the other three are constant property of material or a kind of component. Thermal resistance R is per unit area, thermal conductivity k is per unit area and unit length, and specific thermal resistivity Rλ is the reciprocal of k.

2. Equations analogy

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Rt is the absolute thermal resistance of the component, q is heat flow rate and ΔT is temperature difference.

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K is the thermal conductivity which is reciprocal of thermal resistivity, thus in the oposite position with  electrical resistivity ρ.

These two equations are the main equations used to calculate the thermal conducivity of the material. The first one is used to calculate the performance of the specific sample and the second one is used to calculate the performance of material per unit area and length.

Here is the reference link:

http://www.egr.msu.edu/~raguin/ME812/FinalProjects/Lindberg_FinalProject.htm