Polyurethane

Anna Liu

Arial Foundation Champions Program

Summer 2010

 

A polyurethane is a kind of polymer that contains urethane in its structure. Polyurethanes are produced by reacting polyisocyanate, polyol (a molecule with multiple hydroxyl groups), and a chain extender. They can be elastomeric or thermoplastic. The polyurethane used for wall and roof panels in Arial Homes has a rigid, closed-cell foam structure. It does not absorb water from air, but it can absorb moisture from the building and the ground (and in the form of precipitation). It can withstand a range of 30 to 90 degrees Celsius on a regular basis. It can be heated to a maximum of 250 degrees Celsius for a short period of time. Since it is a thermosetting polymer, it will not melt under heat or in a fire. Thermal conductivity does not increase much upon absorption of water or an increase in temperature (Federation).

 

Polyols are typically derived from petrochemicals. As crude oil is refined, gasoline and other fuels comprise a majority of the petroleum products. However, various petrochemicals are also produced, among them propylene oxide, the substance from which polyols are synthesized. In light of this, polyurethane may be more sustainable than it seems. On the other hand, if the demand for polyurethane increases the demand for crude oil, the opposite conclusion may be valid.

 

Since natural resources are fast being depleted, attention is increasingly directed to using renewable alternatives such as vegetable oils to produce polyols. Soybean, castor, and palm oils are both sustainable and less expensive. Their chemical structures often contain points of unsaturation, which can be altered to yield the desired hydroxyl groups. The subsequent reaction of these polyols with different kinds of polyisocyanates can yield different properties. For example, using triisocyanates rather than diisocyanates results in more crosslinking, which translates into higher density and greater strength (Sharma). It might even be feasible soon to use renewable resources to produce isocyanates (Raquez). Since sustainability is central to the Arial Foundation’s mission, we could investigate what starting products the current supplier uses to produce the polyurethane.

 

In addition to materials selection regarding starting products, nanoparticle additives can enhance the properties of the polyurethane foam. Nanoparticles are small enough to mix well with the foam, although one must ensure that they are dispersed evenly. When 1 wt% of carbon nanotubes (CNFs), Nanoclay, or TiO₂ is added to the liquid polyurethane, the tensile modulus and strength are enhanced significantly, among other mechanical properties (Saha). Also, research has shown that incorporating n-hexadecane and n-octadecane into polyurethane foam improves its ability to insulate (Sarier). 

 

According to the Center for the Polyurethanes Industry, closed cell spray polyurethane foam used in roof and wall panels has an R value of 6.0-6.5 per inch (http://polyurethane.org/s_api/sec.asp?CID=2242&DID=9514). In the Arial Home’s current operations, liquid polyurethane is added between the sheets of metal and allowed to solidify, becoming closed-cell polyurethane foam. A possible alternative to this is to use a vacuum insulated panel (VIP), which is a panel filled with a core material that is then evacuated of air. VIPs can attain very high R values—from 30-50 per inch.  The drawback, however, is that VIPs cost more per unit R. Also, air will seep into the panels over time, decreasing their thermal resistance. VIPs can be made using polyurethane as the core material, but this is typically done with open-cell foam, as that is easier to evacuate (Kwon).

 

At the end of its life, a polymer can be reused, depolymerized, converted to a feedstock, incinerated to produce consumable energy, or sent to a landfill. Unfortunately, depolymerization of polyurethane is hard to accomplish. Its synthesis (via the typical route) is not environmentally sustainable either; Lange states that manufacturing it produces “comparatively large amounts of inorganic wastes.” Given these considerations, the preferred option is to reuse it (Lange). Research in the past few years has found other ways to reuse polyurethane wastes besides conventional methods (e.g. crushing it and producing boards). For example, polyurethane foam wastes can be added to mixtures of cement, producing lightweight concrete (Mounanga). The products from recycling rigid polyurethane foam are unstable, but they can be reheated to release the amine component that causes the instability. This treatment leads to a more environmentally friendly product (Fukaya).

 

For practical purposes, the Arial Foundation may want to identify properties whose enhancement would be particularly beneficial and then research whether the corresponding additives (or polyurethane produced with such additives) are commercially available. Concurrently, we must consider whether any additional costs would be worthwhile. We should also speak with our suppliers to determine the source of their raw materials and whether they are relatively sustainable, especially since recycling options for polyurethane are currently limited.

 

 

Center for the Polyurethanes Industry. “R-value and U-value.” 8 June 2010. <http://polyurethane.org/s_api/sec.asp?CID=2242&DID=9514>

 

Federation of European Rigid Polyurethane Foam Associations. “Thermal insulation materials made of rigid polyurethane foam (PUR/PIR).” Brussels, 2006.

 

Fukaya, Taro, et. al. Reheating decomposition process as chemical recycling for rigid polyurethane foam. Polymer Degradation and Stability 91, 2549-2553 (2006).

 

Kwon, Jae-Sung, et. al. Effective thermal conductivity of various filling materials for vacuum insulation panels. International Journal of Heat and Mass Transfer 52, 5525-5532 (2009).

 

Lange, Jean-Paul. Sustainable development: efficiency and recycling in chemicals manufacturing. Green Chemistry 4, 546-550 (2002).

 

Mounanga, P., et. al. Proportioning and characterization of lightweight concrete mixtures made with rigid polyurethane foam wastes. Cement and Concrete Composites 30, 806-814 (2008).

 

Raquez, J.-M., et. al. Thermosetting (bio)materials derived from renewable resources: A critical review. Progress in Polymer Science 35, 487-509 (2010).

 

Saha, M.C., et. al. Enhancement in thermal and mechanical properties of polyurethane foam infused with nanoparticles. Materials Science nad Engineering A 479, 213-222 (2008).

 

Sarier, Nihal, et. al. Thermal characteristics of polyurethane foams incorporated with phase change materials. Thermochimica Acta 454, 90-98 (2007).

 

Sharma, Vinay, and Kundu, P.P. Condensation polymers from natural oils. Progress in Polymer Science. 33, 1199-1215 (2008).