Contributed by Douglas Fox, Department of Chemistry, American University, Washington, DC
Our research group helps to improve the sustainability of buildings and cities by focusing on the development of affordable and effective flame retardants that are synthesized from natural products. These flame retardants are blended with plastics or formulated into a coating for use in flexible foams and wood based products. Some have been used in bio-derived plastics to reduce their flammability without the typical degradation of desirable polymer properties, thereby increasing the number of commercial applications for these types of plastics.
Plastics, polymers, and polymer composites have become ubiquitous over the past 60 years. In 2013, nearly 300 million metric tons of plastic were produced, making it one of the most used materials worldwide on a volume basis. Their increased use can be traced to their inherent properties: strong, lightweight, flexible, moldable, and inexpensive. In addition, it is relatively easy to incorporate additives to tune their properties. These composites can be tailored to achieve desired properties on opposite ends of the spectrum by simply mixing molten polymer and filler using an extruder.
For example, addition of glass, carbon, or natural fibers increase strength and stiffness. Addition of a plasticizer decreases tensile strength and softens the composite. Addition of clay can reduce diffusion of gases or solvents, whereas incorporation of water soluble salts during extrusion can increase voids in the processed plastic. Or, the addition of benzophenones aids used to protect the polymer from photodegradation, while the addition of titanium dioxide can be used as a photoinitiator to accelerate UV degradation of the polymer. But plastics also come with a range of disadvantages, presenting major challenges. They are inherently flammable, increasing the risk and costs of fire. Most of the monomers, or building blocks of the polymer, and many of the fillers used to tailor the properties, are toxic, bioaccumulative, or persistent chemicals, posing many health concerns, both real and perceived. And, they are generally durable. Combined with their low densities (lightweight), this generates a major waste management issue. Recycling has alleviated some of the waste burden, but plastics still represent over 20% of the waste in landfills, and over 5 million tons of plastic finds its way in oceans every year.
One of the more difficult disadvantages to address is the high flammability of plastics. In the United States, fires account for over 3,000 deaths, 16,000 injuries, and $11 billion in property loss every year. This represents a major decline over the past few decades. Part of this reduction can be attributed to the use of flame retardants in residential upholstered furniture, which reduce the toxicity of combustion products and allow for longer egress times during fires. Fires can be started by either open flame sources, such as candles or matches, or by smoldering sources, such as cigarettes or incense. There are many methods that can be used to address these problems, such as use of smolder resistant fabrics, back-coating fabrics, or the addition of flame retardants. Recently, there has been increasing concern over the migration, persistence, potential toxicity, and perceived lack of efficacy of flame retardants typically used in furniture and flexible foam. This has led to public debate, regulation changes, and increased pressure to find alternatives. The alternatives include both approaches to reducing fire risks as well as development of safer, less transient, more efficient flame retardants.
The largest concern of the increased use of plastics is probably the problems associated with their disposal. There is an increased public awareness of the growing risks associated with waste, pollution and the need for more sustainable products and practices. The most widely used plastics are synthesized from petroleum distillates, generating a resource and energy sustainability issue. These plastics do not biodegrade, but will photodegrade into smaller fragments. This leads to the leaching of the starting materials and fillers used in the production of the composites, many of which are toxic to plants and animals alike. In addition, the small photodegraded plastic fragments are consumed by animals in the wild, which are also toxic or disruptive to their life cycle. This persistence, bio-accumulation, and toxicity of leached chemicals leads to a loss in biodiversity, which can affect the sustainability of entire ecosystems. Resolving this issue is quite complex due to technical, societal, economic, and logistical issues. Attempts have been made to produce biodegradable plastics and/or plastics from bio-derived sources, such as starch based plastics, soybean based epoxies, and corn based poly(lactic acid). Unfortunately, these plastics possess properties that are inferior to their petroleum based counterparts and often cost much more to produce. There are a large number of scientists and engineers working on addressing these issues, and much progress has been made. With the addition of select additives and cheaper production practices, these bio-based and biodegradable plastics have slowly been making their way into the marketplace. Much of their use today has been limited to plastic utensils and packaging applications, but their blends with more traditional petroleum based plastics have also been explored for use in textiles and electrical equipment.
We are addressing these issues by developing more sustainable polymer nanocomposites with improved flame resistance and better compatibility between the filler and polymer. These composites would be used primarily in the home or businesses, helping to develop more sustainable cities. We have two primary goals in our work:
- The use of fillers and crosslinkers to improve the water resistance and mechanical properties of bio-derived polymers.
- The modification of natural materials to improve both interfacial adhesion and flame retardant properties of the polymer.
In many of our systems, we have addressed issues in both of these areas simultaneously.
Our primary focus is currently on the development of flame retardants using natural materials. We began with using cellulose as a replacement for one of the components in an intumescing flame retardant. An intumescing flame retardant produces a foaming char barrier through the use of a dehydration source (typically an acid), a carbon source (such as pentaerythritol), and a gas forming source (typically a polyamine). The use of cellulose fibers is more sustainable than traditional carbon sources used in intumescing flame retardants and can improve mechanical strength and stiffness if there is good interfacial adhesion with the polymer. To increase the amount and mechanical durability of the char during combustion, we utilized silicon containing materials. The modification of cellulose using a reactive POSS molecule (POSS is a hybrid between silica (SiO2) and silicone (R2SiO) that can be attached directly to polymer chains) was effective for flame retarding poly(lactic acid) while preventing the hydrolytic decomposition of the polymer during processing and preventing the loss in mechanical properties that typically accompanies this hydrolytic decomposition. The addition of organically modified clay was only partially effective due to the inability to fully separate the clay layers during processing.
Intumescing flame retardants suffer from the need for multiple components with variable surface energies and morphologies as well as moisture sensitivity, leading to the leaching of flame retardant during weathering and poor interfacial adhesion with some polymers. We are examining methods to utilize cellulose fibers for an all-fiber intumescent. We phosphorylated the POSS modified cellulose to add the acid source needed to promote char. The level of phosphorylation was insufficient to replace typical acids, but the improvements observed were encouraging for future studies. We recognized that starting with cellulose nanocrystals (CNCs), which have greater reinforcing potential, may be more advantageous than using cellulose fibers. CNCs that are produced from oxyanionic acids (like sulfuric acid) participate in side esterification reactions with the oxyanion. Use of sulfuric or phosphoric acid would produce CNCs that contain acidic sulfur or phosphorus groups, both of which are typically good flame retardants. We have experimentally determined that sulfuric acid produced CNCs have a lower thermal stability but better flammability characteristics than unmodified cellulose fibrils. We are currently developing modification methods to simultaneously increase the thermal stability of the CNCs and modify the surface energy to improve interfacial adhesion with hydrophobic polymers.
It is difficult to homogenously disperse cellulose fibers or crystals in epoxy thermosets. Most epoxies are quite hydrophobic (poor interfacial adhesion), have high viscosities (mixing difficulties), and take hours to cure (time for suspensions to settle). For these systems, we examined the use of lignosulfonate and tartaric acid as an intumescing flame retardant. Lignosulfonate, which is a by-product of the sulfite pulping process, was very effective as a flame retardant for epoxy composites, but it had a tendency to migrate to the surface during curing. As a result, the protective layer that forms can crack and peel off the surface when exposed to a radiant heat sources for too long. We are currently investigating methods to prevent this migration during epoxy curing.
The largest fuel source in a home is upholstered furniture. To reduce the fire risks associated with them, the foam had been required to pass both smoldering and open flame fire tests. Manufacturers typically used brominated flame retardants to pass the open flame tests. Due to increasing concern over the persistence, potential toxicity, and perceived ineffectiveness of these flame retardants, industry has renewed interest in flame retardant alternatives. The newest regulations for upholstered furniture does not require an open-flame test, but that does not eliminate the risks associated with these type of fires nor guarantee that they will not be re-established. As a result, there is still an urgent need to develop more environmentally benign flame retardants for upholstered furniture or flexible foam. In light of these developments, we have been developing flame retardant coatings using natural materials. We use borate salts (which have been used extensively in wood as anti-fungal agents), phytic acid (which is a phosphate storage molecule in plants), or taurine as the primary flame retardant. To keep the flame retardant from washing away, we use carbohydrates, polydopamine, or food proteins as binding agents. Other natural additives to add char strength, anti-fungal properties, smoldering resistance, or binder crosslinking are currently under investigation.
As we design our approaches, we remind ourselves of the need to maintain processability. This includes both process design as well as operating costs. It is my belief that great studies in sustainable materials and chemistry will not actually improve sustainability unless industry and commercial enterprises are able to technically and economically adopt the process. We welcome additional input from engineers, managers, and industry, especially on optimizing processing techniques, to help us achieve useful, ecologically sustainable products commonly found in homes and cities.
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