Polyurethane meets specific performance and processing requirements
03/03/2012 (The Star Online) - POLYURETHANES have a broader spectrum of properties and applications than any other natural or man-made material as polyurethane products range from solid to cellular, elastomeric to plastic, rigid to soft and hydrophilic to hydrophobic.

Troy Polymers Inc president Vahid Sendijarevic says polyurethanes can be formulated to meet very special performance requirements such as oxygen permeation, electrical resistivity and ion resistance to ultraviolet radiation, optical performance and energy-absorbing performance.

“The key to a broad spectrum of polyurethane application is versatile chemistry and a broad range of starting raw materials,” he says at a presentation titled Natural Oil Polyols in Polyurethanes Industry: Design for Performance during the Palm Oil International Congress 2011.

He says that in contrast to most high-volume polymers, polyurethanes are always formulated for specific performance and processing requirements.

“Polyurethanes can be found in applications in transportation, construction, furniture, appliances, marine, packaging, mining, space, agriculture, electronics and medicine. Polyurethanes are used as foams, elastomers, coatings, adhesives, binders, sealants, films, fibers and composites,” he says.

He adds that because of their versatile chemistry, there is even a confusion of the definition.

Materials that will meet strict chemical definition of polymer with 100% urethane as repeating group (linkage) are produced from monomer diisocyanates and low molecular weight glycols.

Product definition
“However, these strait polyurethanes are very hard, brittle and characterised with high glass transition temperatures,” he says, adding that polyurethanes had been defined as chemically complex polymeric materials formed by the reaction of isocyanates with compounds containing active hydrogen.

He elaborates during an interview with StarBiz (after his speech) that this definition includes urethane groups formed in the reaction of isocyanate with polyols and also urea groups generated in reaction of isocyanates with water as a blowing agent in rigid and flexible polyurethane foams.

“This definition also includes spray and reaction injection molding, polyurea elastomers which are products of isocyanetes with polyetheramines,” he says.

He adds that the polymer matrix of flexible polyurethane foams, which are the most familiar polyurethane products are composed of a hundred times more ether groups originating from high molecular weight polyether polyols and several times more urea group originating from reaction of isocyanates with water and even more polyether than polyurethane.

“There is no single isocyanate or polyol that has been used in all polyurethane formulations. There are dozens of different types of isocyanates and numerous isocyane derivatives available on the market for production of polyurethanes,” Sendijarevic says.

There are numerous polyols and other raw materials reactive with isocyanates that has been used in the manufacturing of polyurethane products.

He says the key factor in selection of raw materials for polyurethane formulations is the understanding of structure-properties relationship as different raw materials impart different performance characteristics of polyurethanes.

“There are three guiding principles for the selection of raw materials for specific polyurethane products one is performance requirements of the final products, number two is the processing requirements for the manufacturing of the product and lastly, the cost limitations,” he says.

It is well understood that some raw materials are not suitable for certain application. Aromatic polyester and aromatic isocyanates, for example, would not be materials of choice to prepare light stable coatings as aromatic groups cause yellowing when exposed to ultraviolet light.

“To produce water-clear, non-yellowing coatings, chemists select raw materials with aliphatic rather than aromatic backbone. Another example is if the polyurethane product requirement is water repellency, chemists will use a hydrocarbon polyol or natural oil based polyol, rather than polyether polyol with high content of hydrophilic oxyethylene groups,” he says, adding that if processing requires fast reactivity, such as rigid spray foams, then amine initiated polyols are selected.

In conventional high resiliency (HR) seating foam formulations, a combination of two or more polyol is frequently required to achieve the load bearing properties, dynamic mechanical properties and reactive requirements.

Combining strength
“Polymer (graft) polyols are used at high levels to enhance the load bearing properties of the cured foams. Polyols with high level of cell opening polyols are used to control timing of cell opening of the foams,” he says.

The combination of polyols to be used in HR molded seating foam formulations also depends on the type of isocyanate used for their manufacturing.

“Once again, there is no single polyols that can be used to formulate different types of HR molded seating foams,” he says.

Sendijarevic says rigid spray foams are another example where a combination of several polyols has been used to meet reactivity requirements, green strength properties and flame resistance of the final product, which is required by the building codes.

“In order to meet stringent flammability requirements, aromatic polyester polyols are almost always a part of rigid spray foam formulations. In order to meet the reactivity requirements, polyols with self-catalytic properties, such as Mannich polyols (Jeffol R-350X, R-425X and R-470X) from Hunstman are used. In addition, spray foam formulations also use high functionality, high hydroxyl value polyether polyols to provide green strength properties,” he says.

He says visco-elastic foams are also an example of polyurethanes produced using combinations of two or more polyols. Selection of polyols depends on the target application of the foam, type of isocyanate used in the formulations and type of process.

“Some of the polyols used in these formulations are the same polyether polyols used in HR foams and other low equivalent weight polyether polyols usually used in semi-regid foam applications and also short chain polyols usually used as chain extenders. The main objective of using a mixture of these polyols is to break symmetry of polyurethane matrix in order to get low resiliency,” he says.

At the time of early development of polyurethanes, castor oil and castor-oil based polyester polyols have been important raw materials for manufacturing flexible and rigid foams, elastomers and other type of polyurethanes.

Dimer acid-based polyester polyol has also been used from the early years of polyurethanes.

“When petroleum-based polyester polyols were developed, they replaced castor oil-based polyols as raw materials for manufacturing of flexible and rigid foams and other applications. However, castor oil-based polyols remain as important raw materials in adhesives, coatings and speciality elastomers for electrical encapsulations,” he says.

With the recent push for environmentally friendly and sustainable products, there has been renewed interest in development and utilisation of natural oil-based polyols.

However, the notion quickly developed that natural oil-based polyols are inferior to petroleum-based polyols.

This has been explained by the fact that these polyols cannot replace 100% petroleum-based polyols in typical flexible and rigid foam formulations.

“However, this expectation is unrealistic as it is well known that in most cases, a combination of various polyether polyols and reagents is required to meet the performance and processing requirements of these polyurethane products,” he says, adding that there is no single petroleum-based polyols which can be used as sole polyol in most molded HR flexible foams, nor is there a polyol that can be used as sole polyol in rigid spray foams, or in cisco-elastic flexible foams.

In all these products natural oil-based polyols have been successfully utilised in a combination with petroleum-based polyols at levels that vary from 5% to 25% depending on the product.

“To emphasise the significance of the natural oil-based polyols levels in some of these formulations, it is important to understand that cell opening polyether polyols are used in molded HR flexible foams at 10% and much higher levels. Woodbridge Foam Corp has reported molded flexible foams prepared with up to 40% of natural oil-based polyols based on total polyols. In the case of visco-elastic flexible foams, natural oil-based polyols can be used at levels higher than 40%,” he says.

Specific application
Any suggestion that natural oil-based polyols are inferior to petroleum-based polyols in polyurethanes without specifying the application will be similar to a claim that automobiles are inferior to Formula One racing cars without specifying in which applications.

“Formula One racing cars are technologically advanced. However, they will not meet the minimum requirements to be registered as a taxi for public transportation. The least expensive automobile, therefore, is better suited than a Formula One car when the application is taxi service for transportation of passengers,” he explains.

Natural oil-based polyols has been marketed as renewable, bio-based raw materials and low cost replacements for petroleum-based polyols. Natural oil-based polyols has been successfully used in automotive seating foams and automotive interior components, rigid spray polyurethane foams and flexible HR and visco-elastic slab-stock polyurethane foams, where they are not viewed as performance products.

“It is well understood that natural oil-based polyols affect resilience and dynamic mechanical properties of HR flexible foams. In rigid spray foams, natural oil-based polyols can reduce reactivity and negatively impact the flammable characteristics of the foam,” he says, adding that there are also some issues with compatibility of natural oil-based polyols in polyol components.

For these reasons, limited amount of petroleum-based polyols can be replaced with natural oil-based polyols in particular applications.

However, he says: “We should view natural oil-based polyols developed so far as the first generation of polyols.”