A Study on the Synthesis - Structure - Property - Performance Relationship of Bulk Functionalized Polyurethane Foams for Water Filtration Applications
Date of Award
Doctor of Philosophy
Nidal H. Abu-Zahra
Benjamin C. Church, Changsoo Kim, Rani El-Hajjar, Stanislav D. Dobrav
Adsorption, Bes Chain Extender, Bulk Functionalization, Ion Exchnage, Lead Ions, Polyurethane Foams
Polymers, macromolecules made of repeat units, are one of the major building blocks of life. They exist naturally in the form of DNA, proteins, sugars, cellulose, natural rubber etc. However, it was only in the mid twentieth century man began to understand its true nature and developed a new class of materials called ‘Plastics’. In recent years, the concept of a truly tailor made polymer has become a reality as a result of better understanding of the polymer structure-property relationships, newer polymerization techniques and due to the availability of new low cost monomers. Polymers from different elements with any desired property can be produced today. These polymers may be in the form of solid plastics, fibers, elastomers and foams. The endless possibilities of the structure – property – processing relationships, has led to fascinating applications of these materials.
The pliable nature of polymer materials render them lucrative for many applications. They are becoming prominent in the water industry as the need for water and waste water treatment products increase to solve the global water woes. Contamination of water sources by lead has been a cause of concern for several decades. Stringent regulations to treat water or waste water from lead are in place throughout the world due to the harmful health effects of lead toxicity.
Polymer based, surface functionalized ion exchange resins can be fabricated to selectively prefer multiple heavy metal ions such as lead, arsenic etc. This advantage of tailor ability among various others outweigh the limitations of polymer resins, such as, fouling and high operational costs. It has been found that bulk functionalization of polymers compared to surface functionalization can improve the efficiency and lower the operation costs of heavy metal ion filtration systems. Hence research in this area to develop and analyze other ion exchange systems is vital for practical purposes.
This research is thus aimed at developing and understanding the merits and limitations of a bulk functionalized filtration system to overcome the above said limitations. Foams are usually preferred over monolithic forms for their increased surface area. Polyurethane (PU) foams have been considered by researchers for their adsorptive and ion exchange nature. The ease of processing PU foams has kindled the interest of many researchers to develop PU foam based, novel heavy metal ion filtration systems.
This research work focuses on synthesizing bulk functionalized PU foams using a suitable chain extender. A chain extender is usually multifunctional: one, as a low-molecular weight species it links linear chains or chain segments to obtain the final polymer; two, it provides functional groups capable of reacting with other chemical species. N, N- bis (2-hydorxyethyl)-2-aminoethane-sulfonic acid (BES), is a chain extender containing sulfonic acid groups which are capable of exchanging lead ions from aqueous solutions. PU foam synthesized using BES will thus be bulk functionalized with selectivity to lead ions.
Bulk functionalized PU foams, have been synthesized by earlier researchers as a cation exchange media for continuous electro deionization (CEDI) process with an ion exchange capacity of 2.5 meq/L, though no understanding of the process – structure – properties – performance relations were investigated. The cation exchange nature of the aforesaid system is exploited in this research to selectively treat lead ions from water. The structure-property-process-performance relationship of this foam system are investigated thoroughly to determine the effect of the process and application variables on the lead ion removal performance of the foam at parts per billion levels. In the due course of the study, the optimum composition, foam structure and morphology, and the mechanism of lead removal were determined to identify the rate limiting factors for lead ion removal.
Results show that poly propylene glycol (PPG) and toluene diisocyanate (TDI) used in a molar ratio of 1:2 are best suited to produce open cell PU foams and the choice of this polyol and isocyanate system is suitable to incorporate the chain extender, BES by dissolving it in a solvent, dimethyl sulfoxide (DMSO). However, DMSO is the rate limiting factor in the foam formulation, as it is a strong solvent capable of dissolving PU foam. Higher amounts of this solvent in the foam formulation will affect the foam structure at a molecular level. Batch adsorption isotherms and column tests indicate that the functionalized PU foam is capable of removing lead ions by both adsorption and ion exchange mechanisms. The lead ion removal efficiency of the foam depends on several parameters such as the initial concentration of lead ions in the solution, pre-conditioning of the foam, pore size, pore distribution and the foam formulation itself which determines the structure of the foam. The pH of the solution has little effect on the lead removal efficiency of the foam unlike other ion exchange resins. Batch and column tests show 87-88% lead removal efficiency of the functionalized PU foam.
The performance of the functionalized PU foams was benchmarked against commercial activated carbon and ion exchange resins. The results show that the efficiencies of the functionalized PU foam are comparable to that of the commercial media, until equilibrium is reached and soon after, it starts to drop as the foam surface is saturated by lead ions due to adsorption which limits the ion exchange ability of the functionalized PU foam. A case study on product development, suggests that the functionalized PU foam has the potential to be developed as a pre-filter to complement the existing reverse osmosis (RO), ultraviolet (UV), granulated activated carbon (GAC) and ion exchange systems by extending their life while serving as a multipurpose pre-filter, capable of removing lead ions and filtering sediment and silt from the influent.
Gunashekar, Subhashini, "A Study on the Synthesis - Structure - Property - Performance Relationship of Bulk Functionalized Polyurethane Foams for Water Filtration Applications" (2015). Theses and Dissertations. 874.