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

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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