Solvent-Free Polymerizations and Processes - ACS Publications

Chapter 1

Solvent-Free Polymerizations and Processes: Recent Trends in the Minimization of Conventional Organic Solvents 1

Timothy E . Long and Michael O. Hunt

Downloaded by 198.245.14.11 on February 14, 2016 | http://pubs.acs.org Publication Date: January 28, 1999 | doi: 10.1021/bk-1998-0713.ch001

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Polymer Research Division, Eastman Chemical Company, Kingsport, T N 37662 Milliken Chemicals, P.O. Box 1927, Spartanburg, S C 29304 2

Solvent-free polymerization processes and polymer melt processing have received significant attention during the past decade i n both industrial and academic laboratories. Environmental concerns and federally mandated emission regulations have primarily driven this intense interest, and w i l l continue to fuel scientific developments i n the future. Research efforts in such diverse scientific areas such as controlled bulk free radical polymerization, polyester melt polycondensation, and the development of suitable polymerization reactors has focused on the reduction and often elimination of traditional organic solvents. This chapter w i l l highlight the importance o f polymer synthesis and processing i n the absence of organic solvents, and focus on remaining technical obstacles to be addressed. In addition, the following chapters w i l l overview current research activities in solvent-free processes and w i l l illustrate the broad-based approach to the reduction of organic solvents in future polymer-based technologies.

The reduction or elimination of traditional organic solvents is a goal that crosses all boundaries i n polymer science. The purpose o f this symposium series book is to consolidate state-of the-art synthetic strategies in melt/solid state condensation polymerization, bulk free radical polymerization, synthesis and processing i n high pressure carbon dioxide, gas phase polymerization, reactive melt processing, and network formation. Although a single reader w i l l not find all of these topics relevant to their individual research interests, the combination o f these dissimilar approaches to the minimization of organic solvents may yield some synergistic breakthroughs toward waste reduction. The commercial products o f these solvent-free processes are used in such diverse applications as textile fibers, packaging resins, high performance films, medical devices, ink components, aerospace coatings, and microelectronic

© 1 9 9 8 American Chemical Society In Solvent-Free Polymerizations and Processes; Long, Timothy E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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circuit board constituents. A viable scientific goal is the development of new polymer products that are prepared in the absence of conventional organic solvents and water, processed into useful forms i n the molten state, and depolymerized efficiently to yield high purity starting materials. Growing concerns regarding the environmental fate of industrial chemical waste streams have prompted significant research efforts to reduce or eliminate traditional organic solvents from industrial processes. Figure 1 depicts the decline of various types of air emissions and surface discharges during the past decade. Although outstanding strides i n pollution reduction and awareness have been made in the last twenty-five years, significant opportunities remain. Recent advances in polymer science have included the development of bulk/solid state polymerization techniques, modification of existing polymers in traditional thermal processing equipment, applications of "pre-polymers" in network formation, and the exploration of environmentally benign solvents such as supercritical carbon dioxide. The added constraints imposed by using "non-optimum" reaction conditions require efficient reactor engineering, and strong multi-disciplinary efforts between research chemists and engineers to develop industrially feasible, solvent-free systems.

Figure 1: Distribution of Toxic Releases from 1988 to 1995 (Reproduced from Reference 1)

In Solvent-Free Polymerizations and Processes; Long, Timothy E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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Due to the advent of environmental concerns and the associated regulatory and product stewardship issues, the re-examination of conventional chemical processes using organic solvents suggests that opportunities exist for improved processes. These processes not only lead to waste reduction, but also i n favorable cases may lead to lower process costs through improved efficiency, higher purity products, and safer working environments. The Chemical Manufacturers Association ( C M A ) has advocated a formal program entitled Responsible Care that establishes business protocols to preserve the environment in a responsible manner (2). In addition, this industry-focused initiative attempts to limit the risks of the chemical business on workers and local communities, and promotes safe chemical manufacturing and transportation. Figure 2 depicts the quantities and corresponding percentages of Toxics Release Inventory (TRI) chemicals managed i n 1995. The data illustrates that greater than 70% of T R I chemicals were either recycled or treated on-site. Although industrial programs such as Responsible Care have raised awareness of environmental responsibility from the management level, a fundamental scientific focus on environmentally conscious synthetic methodologies w i l l contribute to these important initiatives. Issues including energy required to manufacture a new product, consumption of natural resources, definition of the waste stream, process or product impact on the environment, and recycling/disposal methodologies w i l l become more important as polymer research continues into the next millennium (3). The first section of this volume deals with the utility of melt phase and solid state polymerization strategies. The favorable economics of the elimination of a solvent as a raw material and elimination of cost incurred to dispose or recover the solvent in accordance with environmental regulations often justifies the expense of initial research efforts and subsequent process engineering work. In addition, melt phase and solid state polymerization provide a final product that does not typically

Figure 2: 1995 Toxics Release Inventory Public Data Release (1)



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require further purification. The product is in a suitable form for the final consumer, and deleterious impurities derived from organic solvents are not present. For example, polymerization of polymeric amide esters at lower temperatures in the solid state reduces the propensity for deleterious side reactions and associated color body formation. In fact, poly(ethylene terephthalate) (PET) is prepared commercially in the absence of solvent and in the solid state. In addition, polyesters are viewed as ideal candidates for biodégradation and recycling due to the susceptibility of the polymeric ester functionality to subsequent hydrolysis or glycolysis. One can imagine the complexity of polymerization kinetics in the solid state including such complicating factors as simultaneous morphological changes, restricted diffusion of condensation products, and gradients in molecular weight. Many of these factors are explored in the contribution from General Electric dealing with the melt phase polymerization of polycarbonate in the absence of solvent. Despite these apparent complexities, sufficient macromolecular mobility exists to prepare high molecular weight materials at significantly lower temperatures and in the absence of organic solvents. B u l k free radical polymerization and the utility of living free radical processes, e.g. T E M P O mediated polymerization, have resulted in a resurgence of interest in radical polymerization chemistry. The second section of this volume describes various synthetic avenues that employ living free radical methodologies. Although the discovery of nitroxide mediated free radical polymerization was first disclosed by industrial scientists at Xerox, the recent literature has exploded with novel variations and extensions. For example, bulk free radical polymerization in a living manner offers the potential to prepare conventional thermoplastics with controlled molecular architecture and molecular weight. Several chapters describe the utility of living free radical polymerization for the preparation of well-defined macromolecules. This technology offers the potential for well-defined macromolecules to impact commercial products in an economically feasible manner. The development of future polymerization catalysts and initiators that function efficiently in melt, solid, and gas phases w i l l continue to receive significant attention. Although this symposium was directed towards the elimination of low molar mass solvents and water, the utility of supercritical fluids represent an attractive alternative to many halogenated hydrocarbons. One can envision that the supercritical fluid, e.g. carbon dioxide, can be safely transferred and recycled. T w o chapters in this section describe the use of supercritical fluids in polycondensation and free radical polymerization processes. In addition, the gas can be utilized to generate well-defined microcellular polymer products as described in relation to biodegradable polymers. Both homogeneous and heterogeneous processes are described in the presence of supercritical carbon dioxide. In addition, supercritical carbon dioxide is a viable polymer plasticizer that facilitates crystallization and impurity extractions. Crosslinking and photopolymerization in the solid state are receiving increased attention, and two sections describe recent research efforts in these areas. For example, photopolymerization is a vital component of the fabrication of polymer dispersed liquid crystal displays, and other complex electronic circuitry. Pearce outlines a novel approach for the chemical crosslinking in a miscible polymer blend,


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and crosslinkable sites are selectively introduced to polymer chain ends for subsequent chain extension and crosslinking. Polymer modification during melt processing is becoming increasingly important, and the final section deals with this very interdisciplinary research area. Peroxides are often used for the modification of hydrocarbon polymers via reactive extrusion. In fact, commercial processes involving the maleation of polyolefins lead to novel intermediates for polymer blending. A subsequent chapter elucidates the criteria for peroxide choice and processing conditions in reactive extrusion. In addition, the design of reactive extrusion profiles that are capable of handling higher melt viscosities w i l l provide a unique reaction environment for polymer modification reactions in the absence of solvent. The new chemical processes that are developed to meet these solvent-free constraints w i l l require very sensitive engineering controls due to the required dissipation of exothermic heat and reaction equilibria involved in both chain and step growth polymerization respectively. Thus, significant advances in process control w i l l be an essential element of future efforts. A n important issue in all cases is the ability to transport viscous polymeric melts, and efforts to prepare polymers that "flow like water" but have the mechanical properties of high molecular weight analogs w i l l remain as a viable research goal in the future. This edited volume of manuscripts dealing with various aspects of solvent free polymerizations and processes represents only a small fraction of the research efforts currently in progress in both academic and industrial laboratories. A s one can imagine, significant advances in the development of environmentally benign chemical processes w i l l require strong interdisciplinary programs and synergistic combinations of industrial and academic laboratories. This volume is intended to stimulate the scientific creativity of the reader through a broader understanding of some current directions in polymer science and engineering.

References 1.

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Chapter 5: Year-to-Year Comparisons. 1995 Toxics Release Inventory Public Data Release; E P A 745-R-97-005; United States Environmental Protection Agency: Washington, D . C . , 1997; 119 Chemical and Engineering News, M a y 11, 1998, p.13.

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G . G. Bond, Environmental

Protection, February 1998, pp. 29-34.


Solvent-Free Polymerizations and Processes - ACS Publications

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