Ionic Liquids as Alternative Plasticizers for Poly(vinyl chloride

Chapter 7

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 16, 2014 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0913.ch007

Ionic Liquids as Alternative Plasticizers for Poly(vinyl chloride): Flexibility and Stability in Thermal, Leaching, and UV Environments Mustafizur Rahman, Hugh W. Shoff, and Christopher S. Brazel Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, A L 35487-0203

Ionic liquids (ILs) based on ammonium, imidazolium and phosphonium cations were proven to be good plasticizers for polyvinyl chloride), P V C , offering similar or better properties compared to traditional plasticizers. Formulations containing as much as 50 wt % plasticizer were made, and tested for flexibility in comparison to traditional plasticizers based on phthalate, trimellitate, and citrate esters. The flexibilities of all plasticized samples were nearly equal, with slight variances in glass transition temperatures, depending primarily on the plasticizer content, but to a smaller extent on the chemical structure o f the plasticizer. Because ILs are known for their wide liquid range and non-volatility, they can be used in plastics at higher temperatures than traditional plasticizers. The thermal stability o f plasticized P V C was evaluated at moderate and high temperatures, as was each plasticizer's ability to reduce brittleness caused by ultraviolet ( U V ) light exposure. Solid-solid migration and solid-liquid leaching experiments were carried out to determine the suitability of I L plasticizers for the automotive and medical industries.

© 2005 American Chemical Society In Ionic Liquids in Polymer Systems; Brazel, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Background Plasticizers are added to polymer formulations to reduce glass transition temperatures and impart flexibility to materials under conditions where they are normally used. Many traditional plasticizers are based on alkyl esters that often surround a ring structure and include phthalates, trimellitates, adipates, and, more recently, citrates. Di(2-ethyl hexyl) phthalate (DEHP), commonly known in industry as dioctyl phthalate, is the most widely-used plasticizer, as it is highly compatible with P V C and other polymers. D E H P accounts for nearly 50 % of plasticizers used in industry [1], and is suitable for a broad range of applications: consumer products (garden hoses, shower curtains), automotive parts, home construction materials, toys, and medical plastics. However, there remain a number of challenges in the plasticizer industry to develop new plasticized materials that have better performance in harsh environments, longer lifetimes, and reduced toxicity. A recent review paper [2] discusses some o f the major approaches to developing these improved materials. Among the approaches to develop novel flexible materials, ILs have many advantages for use as plasticizers. The low volatility and high temperature stability of many ILs make them useful for applications at elevated temperatures with minimal loss in flexibility and extended material lifetime. The cations and anions in ILs also have a strong affinity for each other, making plasticizer loss by liquid leaching, solid-solid migration, or evaporation much less likely compared to molecular plasticizers. Some o f the potential advantages o f using ILs as plasticizers are listed in Table 1. The use o f ILs in polymer systems has been explored on a number o f fronts, but the majority of early papers in this field have focused on polymerization using ILs as solvents, with emphasis on the environmental aspects and ability to form new structures [3-5]. Attributes o f these reactions, such as long free radical lifetimes, high molecular weight products, rapid polymerization, and the ability to form block copolymers without having to use tedious anionic or cationic polymerization schemes, have made a significant impact in the polymer field. However, industrial polymers made using ILs as environmentally-sound solvents have had considerably less success, as the separation step after the reaction has proven difficult and uneconomical. On the other hand, ILs can be formulated as plasticizers by at least three methods: direct compounding (using a high shear mixer, as most plasticizers are added), solvent casting, and polymerization using the plasticizer as the reaction solvent. Imidazolium-based ILs have been used effectively to plasticize P M M A , and with a higher degree o f compatibility compared to D E H P [6]. After the experiments reported earlier, we sought to expand the range o f ILs studied, focus on a more widely-used flexible plastic, P V C , and probe industrially-relevant properties including UV-stability and leaching. Results reflecting these new areas are presented here.

In Ionic Liquids in Polymer Systems; Brazel, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Table 1. Potential Advantages of I L s as Plasticizers Good Low Temperature Lubricity High Temperature Stability Reduced Leaching in Aqueous and Lipophilic Solutions Enhanced Stability to Ultraviolet Light Reduced Flammability Reduced Solid-Solid Migration Reduced Evaporative Loss High Degree of Compatibility in Polymer Formulations Reduced Human Exposure

Objectives The primary objective o f this research was to determine the feasibility of using ILs to replace traditional plasticizers in P V C and extend the application range of flexible materials. More specifically, the experimental objectives were to: - determine compatibility of phosphonium, ammonium and imidazolium ILs with P V C , - measure thermal stability of plasticized P V C at high (> 200 °C) and moderate (~ 100 °C) temperatures, - determine how the plasticizer molecular structure affects the flexibility of plasticized samples, - measure any loss (or gain) in flexibility after exposure to ultraviolet light, and - determine mass loss due to solid-solid migration and solid-liquid leaching.

Experimental Polymers were plasticized with ILs and traditional plasticizers (see Figure 1). Traditional plasticizers included di(2-ethylhexyl phthalate (DEHP, Sigma Chemical Company, St. Louis, M O ) , trioctyl trimellitate ( T O T M , Aldrich Chemical Company, Milwaukee, WI), and diisodecyl phthalate (DIDP, Fluka Chemie GmbH, Switzerland); all were used as received. Two citrate esters (Morflex, Inc., Greensboro, N C ) that were developed for use in medical plastics were also studied. ILs studied included three that contained phosphonium cations ([thtdPh ][deca], [thtdPh^[Tf N], and [thtdPh ][Cf]), one with an ammonium cation [tbam ][doss~], which is commercially sold by Sachem, Inc. as Terrasail™ and two with an imidazolium cation. The phosphonium ILs were supplied by +

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Ionic Liquids as Alternative Plasticizers for Poly(vinyl chloride

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