The chemistry of polyurethanes

Polyurethanes are incredibly versatile (Figure 1); they are flexible, have high impact and abrasion resistance, strong bonding properties, are electrically insulating and are relatively low cost compared to other thermoplastics. 

Figure 1. Polyurethanes are versatile materials and can be used to make hard and rigid materials through to soft flexible foams.  Common applications for polyurethane include automotive seats, shoes, floor coatings and furniture.

Furniture foams are the dominant application (Figure 2) however uses of polyurethane also include:

• rigid foam (construction and insulation)
• moulded foam (furniture and automotive)
• flexible foam (slabstock for furniture, mattresses and cushions)
• elastomers (footwear, synthetic leather)
• adhesives and sealants (construction, packaging, textiles)
• coatings (automotive refinish and OEM, flooring)

Figure 2. Polyurethane consumption worldwide (2016).  Flexible foams for furniture and automotive account for the largest share of polyurethane usage followed by rigid foams for construction and insulation applications. 

The polyurethane reaction

Polyurethane and its related chemistries were first discovered in 1937 by Otto Bayer however it wasn’t until the 1950’s that they became commercially available.  The basic synthesis involves the exothermic condensation reaction of an isocyanate (R’-(N=C=O)_n) and a hydroxyl-containing compound, typically a polyol (R-(OH)_n) (Figure 3).  

The reaction proceeds readily at room temperature, regardless of a catalyst, and is typically completed in a few seconds to several minutes depending on the formulation, in particular the choice of isocyanate.  Therefore compared to other polymers such as polyethene or polypropene which are produced then heated and moulded at a later stage, polyurethanes are made directly into the final product via reaction injection moulding (RIM), or applied onto the substrate in the case of adhesives and coatings.

Figure 3. The condensation polymerisation of an isocyanate (R’-(N=C=O)n) and a polyol (R-(OH)n) to form polyurethane.

An important side reaction involves the isocyanate component and water.  If moisture is present in the mixture (Figure 4), then the isocyanate will react with this water to form an unstable carbamic acid which then decomposes to form urea and carbon dioxide gas thus resulting in foaming.  The selection of an appropriate catalyst can either suppress this reaction or can promote this reaction if foam formation is desired.

Figure 4. Isocyanates are highly reactive with hydroxyl (-OH) groups.  When in contact with water, isocyanates react to form carbamic acid which then decays to form an amine and carbon dioxide gas.  This gas is responsible for foaming and is often used in the production of PU foams for furniture or construction applications. 

Polyurethanes are typically supplied as two-component formulations; a part A containing the polyol, catalyst, and any additives, and a part B compromising of the isocyanate.

Raw materials used to produce polyurethane

Part A: Polyol

The majority of polyols used in polyurethane production are hydroxyl-terminated polyethers though hydroxyl-terminated polyesters are also used.  The choice of polyol ultimately controls the degree of cross-linking and therefore the flexibility so formulators must consider not only the size of the molecule, the degree of branching but also the number of reactive hydroxyl groups present.

If a polyol containing two hydroxyl groups (a diol) is reacted with TDI or MDI, then a linear polymer is produced.  Polyols with a greater number of reactive hydroxyls result in a higher level of crosslinking and a more rigid final product.

Part B: Isocyanate

The most commonly used isocyanates for polyurethane production are the aromatic diisocyanates toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) which form the basis for >90% of all polyurethanes (Figure 5).

TDI is a mixture of two isomers and is primarily used in the production of low-density flexible foams whereas MDI is a more complex mixture of three isomers and is used to make rigid foams and adhesives.

Figure 5. Chemical structures of the aromatic isocyanates toluene diisocyanate (TDI) and methylene diphenyl isocyanate (MDI).  TDI and MDI account for 90% of all isocyanate usage globally and are mostly used to produce flexible and rigid foams.

Less reactive are the aliphatic isocyanates (Figure 6) however these are important for coatings applications due to their excellent UV and colour stability.  Aliphatic isocyanates account for <5% of isocyanate usage worldwide and include hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI).

Figure 6. Chemical structures of the aliphatic isocyanates hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI).  HDI and IPDI mostly find use in coatings applications and account for <5% of isocyanate usage.

Blocked isocyanates

Blocked isocyanates are a relatively new development whereby the reactive NCO- groups are further reacted with groups such as dimethyl malonate (DEM), dimethyl pyrazole (DMP) or methylethyl ketoxime (MEKO) to produce inert and non-hazardous materials.  These materials can be selectively unblocked at elevated temperatures (+100°C) thus opening up a greater variety of applications such as usage in 1K or waterbased formulations, or for lower free isocyanate levels.

Catalysts

Catalysts play an important role in the production of polyurethane as not only do they increase the reaction rate and control gelling time, they also assist with balancing the side reactions including the water reaction and therefore control gas-formation and foaming. 

Broadly speaking, the catalysts used for polyurethane manufacture fall into two categories: amines or organometallic catalysts including organotin, bismuth and zinc.

Amine catalysts

Amine catalysts are derived from ammonia (NH₃) by substituting one (primary) or two (secondary) or three (tertiary) of the hydrogen atoms with an alkyl group.  Their catalytic activity is determined by both the structure and the bascity with increased steric hinderance of the nitrogen atom resulting in decreased activity and increased bascity increasing activity.  Tertiary amines are predominantly used in the manufacture of foam as whilst they drive urethane formation, they also promote the water reaction leading to CO₂ gas generation.

Mercury catalysts

Mercury catalysts such as phenylmercuric acetate, propionate, and neodecanoate are highly efficient at driving urethane formation and characteristically result in a long pot life in combination with rapid back-end cure. However despite their excellent performance, mercury catalysts are less common due to their poor toxicological status.

Tin catalysts

Outside of amine catalysts, organotin catalysts are the most widely used in polyurethane production with grades such as TIB KAT® 218 (dibutyltin dilaurate DBTL), TIB KAT® 216 (dioctyltin dilaurate DOTL), and TIB KAT® 318 (dioctyltin carboxylate) widely used in CASE applications (coatings, adhesives, sealants, and elastomers).

TIB KAT® 218 (DBTL) is the workhorse grade (Figure 7) and strongly drives the urethane reaction however in some instances longer ligand dioctyltins such as TIB KAT® 216 (DOTL) or TIB KAT® 318 are preferred due to more favourable labelling. 

Other grades such as TIB KAT® 223 or TIB KAT® 214 can provide varying curing profiles such as a rapid cure in the case of TIB KAT® 223 or a “mercury-like” curing profile with TIB KAT® 214.

Figure 7.  Mechanism of polyurethane catalysis using TIB KAT® 218 (dibutyltin dilaurate DBTL).  DBTL acts as a Lewis acid and accepts the non-bonding electrons from the oxygen on the isocyanate molecule to initiate the reaction. 

Bismuth and zinc catalysts

Bismuth and zinc catalysts are growing in popularity due to their low toxicity and both TIB KAT® 716 (bismuth) and TIB KAT® 616 (zinc) are used in CASE applications as they are strongly selective towards the urethane reaction. 

Bismuth, in particular, can mimic DBTL performance and in some instances offers a shorter pot life than organotins. However, bismuth typically requires higher dosage levels than organotins and is sensitive to hydrolysis; even low moisture levels can have a detrimental effect on activity. 

Zinc on the other hand results in increased pot life with a good through cure and is especially useful when curing at elevated temperatures (>60 °C).

Other metallic catalysts

Other catalysts such as aluminium, titanium and zirconium complexes are being used in some instances though are not widespread as have lower activity and can require much higher dosages.  They can also be more selective towards primary alcohols in a polyol mixture leading to poorer and breakable polyurethane material.

Table 1: Advantages and disadvantages of amine and metallic catalysts for polyurethane production.

Other components

Depending on the final application, polyurethane formulators will also include other additives in the formulation including, but not limited to:

• Pigments to create coloured polyurethanes
• Fillers for mechanical reinforcement and to reduce overall costs such as those from Quarzwerke, from KaMin or from 3M
• Plasticisers to increase flexibility
• Cross-linkers and chain-extenders to improve physical properties
• Blowing agents and foam stabilisers to control bubble formation and cell structure in foams
• Moisture scavengers and zeolites to remove moisture and suppress foam formation such as those from Zeochem
• Flame retardants and smoke suppressants to reduce flammability and reduce smoke generation if burnt
• UV absorbers and antioxidants to minimize degradation such as those from Chitec

Summary

Polyurethanes can be used in a wide variety of applications and their properties can tailored through the correct selection of isocyanate and polyol components, catalysts and other additives in the formulation.  Lawrence Industries are supplying a number of materials into PU applications including organometallic catalysts, functional fillers and additives.  For further insight and for recommendations for your formulation, contact your account manager or call us to discuss your requirements.