• 1930: Polysyren discovery by  I.G. Farben, in Germany .
• 1940: The Styrofoam (extruded polystyren) is developped by Dow Chemical in USA. The first use of this material was as a floating element for life raft and boats. As the foam cells of Styrofoam are closed, this allows the material to be very resistant to water. 
• 1950: strength plus insulation properties allows the foams to be used as insulating material in the building trade. 
• En 1963: Introduction in Europe. It is now produced in nearly 23 plants over the world.

The choice of the foam block will depend on the expected use of your wings. You should take into account parameters as weight per meter cube, strength, and over all the acceptable final weight. This insulating plastic foams pannels are available in two categories - extruded (PSX) or expanded (PSE) and four types (I to IV). The type I has the lowest thermal resistance and the highest permeability to water vapor and water absorption capability. Type IV has the best thermal resistance and lowest permeability. About insulating materials, weight (density actually) is usually proportional to thermal resistance.

So we may use many types of foams.

• Extruded polystyren: 

The extruded pannels are made from polystyren, solvents and pressurized gas. It is noted that with time the puffing agents (HCFC which are less agressive than CFC against ozone layer), styren and  ethylen chlorid are slowly freed in ambiant air. The styrène is classified as a potentially cancerogen agent for workers. It is recommended to use an adequat ventilation and protection (eyes,lungs) against fumes whilst cutting polystyren. Exhaust gas when polystyren is heated are very toxic.
An example of extruded polystyren is the Styrofoam or the Roffmat manufactured by Dow Chemical.
This material is easy to procure and its price is relatively high.

• The expanded polystyren: 

The expanded pannels, generally white or green, are made from balls containing a polystyren and penthane mixt, a liquid which expand when reaching gazeous state. The polystyren balls are then expanded with vapor. Compared with other foam types, these pannels have a higher permeability to water and may break up in time, especially when used under ground level. They are less used than before, but in the building facing.
Application methods and health effects are mainly identical to those of extruded polystyren. However, penthan does not destry the ozone layer.

This material is easy to procure and its price is low.
 

• Polyurethan and  polyisocyanurate 

 

 

these insulating materials are available in pannels and as squirted foam forms. The rigid insulator is the result of a reaction between an isocyanate and a resin, together with catalysts and puffing agents (HCFC), and other additives. Gas and HCFC are slowly released by these pannels, which reduce their thermal resistance and require them to be isolated from inhabited areas with a good vapor shield in order to limit gas infiltration.
Their good compression strength allows to use them for exterior and building foundations under concrete slab. They are also used as  roof isolation material. If used inside, as they are inflammable, they should be coated with gypsum which offer half an hour shield against fire. One should also use protection against their dust when cutting these pannels.

Besides, a new type of polyurethan, with low density and an open alveolar structure, squirted with water, without any ozon layer agressive CFC or HCFC, has recently been introduced. Alike traditionnal insulating materials, it is squirted in wall cavities from the inside, and shall be separated from inhabited areas with a gypsum fireguard and a vapor shield.
Very good insulator, very best when you have to fill in air leakage and they do not release any toxic gas or fibers after they have been layed in place.
Unfortunately for modellers, this material is difficult to obtain and its pricing is high.

As you already understood, the styren is the main component. Styren is a petroleum derivativen and is the main component of our foams. It has been introduced by 1930 and has had an important role during second world war because of its use in synthetic rubber manufacturing.   
The styren is a clear liquid, without color, which is a component of materials used to make thousands of every day products, for home, school, work and play. It is used everywhere, from food packaging to computers, car or boat games...
Here is a polystyren chunk picture taken through an electron microscope.
 

 

The polystyren foams actually contains 95% and 5% polystyren.
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To distinguish easily the different types, check for ball size. The bigger the balls, the ligther the foam, but the lower the strength. Take care of foam core weight, as it is a non neglectable part of the final weight. Here is a few hints about possible choices: 

• Type 1 : 

Very big balls (5mm diameter and more). Never use this type. Too much soft and scattered with rubbish (wood, concrete, plaster) which give very bad cut.

• Type 2: 

Medium size balls (3 to 4 mm diameter). The best for most uses. All kinds of airplanes, from 1to 2 meters span, with a minimal 12/13 % relative thickness airfoil section. Also works for compacts gliders (2.5 to 3m) (not for high span/chord ratio machines).
Stabilizer, rudders and other parts may also be made from this foam.
Light polystyren.

• Type 3: 

Small balls(2-3mm diameter). Intended for bigger machines which requires more strength, or equipped with thinner section (8 to 10% relative thickness). It is also possible to consider high aspect ratio wings, with internal reinforcements or spars. Do not use it on small airplanes, the weight is too high.