Ionic liquids: New generation stable plasticizers for poly(vinyl chloride)

Polymer Degradation and Stability 91 (2006) 3371e3382www.elsevier.com/locate/polydegstab

Ionic liquids: New generation stable plasticizers for poly(vinyl chloride)

Mustafizur Rahman, Christopher S. Brazel*

Department of Chemical and Biological Engineering, The University of Alabama, 201 7th Avenue, Box 870203, Tuscaloosa, AL 35487-0203, USA

Received 20 January 2006; received in revised form 19 May 2006; accepted 28 May 2006

Available online 20 July 2006

Abstract

Room temperature ionic liquids (ILs), based on ammonium, imidazolium and phosphonium cations, were studied as novel plasticizers forpoly(vinyl chloride), PVC. All the ILs tested were able to produce flexible PVC. Upon 20 wt% plasticization, some of the ILs lowered the glasstransition temperature (Tg) of PVC more than that done by several traditional plasticizers. They showed good thermodynamic compatibility aswell. Several ILs showed better leaching and migration resistance than the traditional plasticizers. This was, in particular, a significant obser-vation considering the ongoing controversy regarding the leaching and migration issues of the commonly-used phthalate plasticizers. High tem-perature and ultraviolet (UV) ray stability of IL-plasticized PVC samples were also studied.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Ionic liquid; Plasticizer; Poly(vinyl chloride); Leaching; High temperature stability; UV stability

1. Introduction

For decades, plasticizers have been extensively used withpolymers for producing flexible plastics to be used in com-modity, engineering and medical applications. According tothe council of International Union of Pure and Applied Chem-istry (IUPAC), as stated in 1951, ‘‘a plasticizer is a substanceor material incorporated in a material (usually a plastic or anelastomer) to increase its flexibility, workability or extensibil-ity. A plasticizer may reduce the melt viscosity, lower the tem-perature of a second order transition, or lower the elasticmodulus of the product’’ [1]. The first time application ofa plasticizer for producing flexible plastics dates back to asearly as 1862 [2]. Since then, plasticizers have come a longway to become an inherent part of current plastic industry.In 2003, worldwide plasticizer market was worth more than10 billion pounds, with approximately 90% consumed byPVC [3]. In North America, the consumption was 2.2 billionpounds. Adipates, azelates, benzoates, phthalates, trimellitatesand phosphates are some of the most frequently used

* Corresponding author. Tel.: þ1 205 348 9738; fax: þ1 205 348 7558.

E-mail address: [email protected] (C.S. Brazel).

0141-3910/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.05.012

plasticizers. Phthalates are the most dominant class of plasti-cizers, composing about 87% of the entire plasticizer industry[4], while di(2-ethylhexyl) phthalate, DEHP, accounts foralmost 50% of total plasticizer consumption [5].

Inside the polymer matrix, plasticizers act by breaking up theprimary bonds holding the polymer chains together and formingsecondary polymereplasticizer bonds, and thus rendering mo-bility to polymer chains or chain segments. Since polymereplasticizer interactions are weak, there exists a dynamic processwhere a plasticizer molecule attached to one site in the polymernetwork may be dislodged and be readily replaced by another[3]. Plasticizing efficiency of different plasticizers is thoughtbe a function of organic/inorganic moetity and functionalgroups present in the plasticizer molecule, structure, chainlength, molecular weight (MW), etc. [6]. Different plasticizers,therefore, yield different plasticization effects because of differ-ences in strength in plasticizerepolymer and plasticizereplasticizer interactions. Commercially available plasticizersoffer a wide range of end-use properties to different polymers.But applications of many of these plasticizers are often associ-ated with a number of potential problems. Limited compatibil-ity, poor stability at high temperatures or when exposed to UVrays, diminished lubricity at low temperatures, and flammabil-ity are some of the common technical challenges in the

mailto:[email protected]

http://www.elsevier.com/locate/polydegstab

3372 M. Rahman, C.S. Brazel / Polymer Degradation and Stability 91 (2006) 3371e3382

plasticizer industry [7]. However, for the past couple of decades,the most debated issue regarding plasticizers has been the leach-ing and migration of plasticizers, especially of the phthalates,from medical and commodity plastics. The shortcomings ofphthalates in outdoor environment as part of engineeringplastics have been well documented [7]. The European Unionhas already banned a number of phthalates from certain applica-tions [8,9] and the US Food and Drug Administration hassuggested that the manufacturers of plastic products considerthe feasibility of replacing DEHP with safer alternatives [10].

Several alternative plasticizers are currently under investi-gation. ILs are one of these few novel alternatives whichhave shown promising results in the early stages of investiga-tion [7]. ILs are actually molten salts that melt below at about100 �C and typically consist of a bulky inorganic cation and ananion [11]. They have attracted much attention and gained rec-ognition as potential environmentally benign solvents due tosome of their unique properties. These properties of ILs resultfrom the composite properties of the wide variety of cationsand anions. Most of the ILs are liquid at room temperatureand usually exhibit negligible vapor pressure, which reducesthe possibility of air pollution and loss of materials at ambientconditions [12]. Many of the ILs are liquid over a wide tem-perature range (often more than 300 �C). They have low melt-ing points (as low as �96 �C has been reported), which can beattributed to the large asymmetric cations having low latticeenergies [13]. As a class of materials, ILs are highly solvatingfor both organic and inorganic materials. Many of them arenonflammable, non-explosive [14] and have high thermal sta-bility. ILs usually have high electric conductivity (10�1e10�2 S m�1) and possess a wide electrochemical window(commonly larger than 3.0 V) [15]. They are also recyclable,which can be helpful in reducing landfill waste.

Even though the use of ILs in an industrial process for organicsynthesis dates back to 1990 [16], most IL-research has been

confined to academic laboratories. They have been studiedas novel solvents for organic synthesis, polymerization, andliquideliquid extraction [17,18], in electrochemical studies[19,20], gas chromatography [21,22], for catalysis [23] and bio-catalysis [24e26], as plasticizers [27e33], storage and transpor-tation media for highly toxic and flammable gases [16] and massspectrometry [34]. One of the unique applications of ILs is asnovel plasticizers, as has been shown in a number of earlier pub-lications [27e33]. Considering the diverse challenges associatedwith different traditional plasticizers [7], ILs hold prospects asalternative plasticizers in the rapidly growing plastic industry.

As plasticizers, ILs were initially found to have better com-patibility with poly(methyl methacrylate), PMMA than DEHP[27]. It was also found that they were capable of lowering theTg of PMMA much more than DEHP does, while improvingthe high temperature stability of PMMA and also providinga wide temperature range for flexible PMMA-based plastics[27e30]. With the breadth of the PVC-plasticizer market inmind, the effectiveness of ILs as plasticizers for PVC wasstudied. Here we focus on the use of ILs as plasticizers fordiverse applications in flexible PVC.

2. Experimental

2.1. Materials

Suspension polymerized PVC (Mn¼ 22,000) was obtainedfrom Aldrich Chemical Company Inc., Milwaukee, WI, andHPLC grade tetrahydrofuran (THF) was obtained from FisherScientific, Fairlawn, NJ. ILs and traditional plasticizers studiedare shown in Table 1 along with their abbreviated names andthe names of the suppliers. Fig. 1 shows the chemical struc-tures of these ILs and the traditional plasticizers.

The effectiveness of these ILs and traditional plasticizerswere studied in terms of flexibility, mechanical properties,

Table 1

List of different ILs and traditional plasticizers, their abbreviated names, and suppliers

Plasticizer Abbreviated name Supplier

Traditional plasticizerDi(2-ethylhexyl) phthalate DEHP Sigma Chemical co., St. Louis, MO

Diisodecyl phthalate DIDP Fluka chemie GmbH, Switzerland

Trioctyl trimellitate TOTM Aldrich Chemical Co., Milwaukee, WI

Acetyl tri-n-hexyl citrate (Citroflex� A6) Citroflex A6 Aldrich Chemical Co., Milwaukee, WI

N-Butyryl tri-n-hexyl citrate (Citroflex� B6) Citroflex B6 Aldrich Chemical Co., Milwaukee, WI

IL

1-Butyl-3-methylimidazolium hexafluorophosphate [bmimþ][PF6�] Dr. William M. Reichert, The Univ. of Alabama

1-Hexyl-3-methylimidazolium dioctylsulfosuccinate [hmimþ][doss�] Dr. Jim Davis Jr., Univ. of South Alabama

1-Hexyl-3-methylimidazolium hexafluoroborate [hmimþ][BF4�] Dr. William M. Reichert, The Univ. of Alabama

1-Hexyl-3-methylimidazolium hexafluorophosphate [hmimþ][PF6�] Dr. William M. Reichert, The Univ. of Alabama

Tetrabutyl ammonium dioctylsulfosuccinate [tbamþ][doss�] Dr. Jim Davis Jr. Univ. of South Alabama

Tetrabutyl phosphonium dioctylsulfosuccinate [tbPhþ][doss�] Sachem, Inc., Austin, TX

Tributyl (tetradecyl) phosphonium dodecylbenzenesulfonate [tbtdPhþ][dbs�] Dr. Rex Ren, IL-Tech Inc.

Tributyl (tetradecyl) phosphonium methanesulfonate [tbtdPhþ][mes�] Dr. Rex Ren, IL-Tech Inc.

Trihexyl (tetradecyl) phosphonium bis(trifluoromethane) sulfonylimide [thtdPhþ][Tf2N�] Cytec Canada, Inc., Ontario, Canada

Trihexyl (tetradecyl) phosphonium chloride [thtdPhþ][Cl�] Cytec Canada, Inc., Ontario, Canada

Trihexyl (tetradecyl) phosphonium decanoate [thtdPhþ][deca�] Cytec Canada, Inc., Ontario, Canada

Trihexyl (tetradecyl) phosphonium dodecylbenzenesulfonate [thtdPhþ][dbs�] Dr. Rex Ren, IL-Tech Inc.

Trihexyl (tetradecyl) phosphonium methanesulfonate [thtdPhþ][mes�] Dr. Rex Ren, IL-Tech Inc.

3373M. Rahman, C.S. Brazel / Polymer Degradation and Stability 91 (2006) 3371e3382

(a) Traditional plasticizers

CH3

1-butyl-3-methylimidazolium hexafluorophosphate

CH3H9C4N N PF6

-

H13C6N N

SO3-

O O O O

1-hexyl-3-methylimidazoliumdioctylsulfosuccinate

(b) Imidazolium-based ILs

N+SO3

-

O O O O

tetrabutyl ammonium dioctylsulfosuccinate

(c) Ammonium-based IL

H3C-C-O-C- C-O-CH2(CH2)4CH3

O

CH2C-O-CH2(CH2)4CH3 O

O

CH2C-O-CH2(CH2)4CH3

O

H3C(CH2)2-C-O-C- C-O-CH2(CH2)4CH3

O

CH2C-O-CH2(CH2)4CH3 O

O

CH2C-O-CH2(CH2)4CH3

Oacetyl tri-n-hexyl citrate n-butyryl tri-n-hexyl citrate

O

C-O-CH2CHC4H9

C-O-CH2CHC4H9

O C2H5

C2H5

di(2-ethylhexyl) phthalate

trioctyl trimellitate

O

O

C-O-(CH2)7CHCH3

C-O-(CH2)7CHCH3

CH3

CH3

C-O-CH2CHC4H9

C-O-CH2CHC4H9

O

O

H9C4HCH2C-O-C

O C2H5

C2H5

C2H5

diisodecyl phthalate

Fig. 1. Structures of ILs and traditional plasticizers.

and high temperature and UV stability they impart to PVC asplasticizers. The leaching and migration tendencies of theseplasticizers were also investigated. Citrate-based Citroflex�

plasticizers offer a wide range of benefits when used as plasti-cizers with aqueous and solvent-based polymers and hencehave a history as low-leaching plasticizers and lubricants formedical plastics and food-contact products. Citroflex plasti-cizers were, therefore, studied only in the leaching experimentsas a reference for the ILs and other traditional plasticizers.

2.2. Sample preparation

Plasticized samples of PVC were prepared using a solventcasting method. For a fair comparison of plasticizing

effectiveness of different plasticizers, all samples were formu-lated using 80 wt% PVC and 20 wt% IL or traditional plasti-cizer. PVCeplasticizer mixtures were dissolved in THF andplaced in a flat aluminum pan. THF was allowed to evaporate,first at atmospheric pressure, and then in a vacuum chamberfor approximately two weeks until a constant weight was ob-served. Unplasticized PVC samples were made in the samemethod for migration testing, as described later.

2.3. Characterization of PVC samples

PVC samples plasticized with different ILs and traditionalplasticizers were tested for flexibility, high temperature and


(d) Phosphonium-based ILs

P+

SO3-

SO3-

SO3-

O O O O

C4H9H9C4

H9C4 C14H29

P + H25C12

P + CH3SO3-

C6H13

C4H9

C6H13

C6H13

C14H29

C6H13

C14H29

H25C12

C14H29

C6H13

C14H29

C14H29

C14H29

H13C6

H9C4

H9C4

H13C6

H13C6

H13C6

H13C6H13C6

H13C6

H13C6

H13C6

H13C6

P +

N

SS

CF3F3C

O O O O

_

P + Cl-P + C9H19COO-

CH3SO3- P + P +

tetrabutyl phosphonium dioctylsulfosuccinate

tributyl (tetradecyl) phosphonium dodecylbenzenesulfonate

tributyl (tetradecyl) phosphonium methanesulfonate

trihexyl (tetradecyl) phosphoniumbis (trifluoromethane) sulfonylimide

trihexyl (tetradecyl) phosphonium chloride

trihexyl (tetradecyl) phosphoniumdodecylbenzenesulfonate

trihexyl (tetradecyl) phosphonium decanoate

trihexyl (tetradecyl) phosphonium methanesulfonate

Fig. 1. (continued)

UV stability, and resistance to leaching and migration ofplasticizers.

2.3.1. FlexibilityGlass transition temperatures (Tgs) of PVC samples were

obtained from the onset of storage modulus in a temperatureramp test using a Dynamic Mechanical Analyzer (Rheomet-rics Solids Analyzer, RSA II, Rheometrics Inc., Piscataway,NJ). Rectangular samples were subjected to a maximum of0.5 N tensile and compressive force at an incremental strainrate of 0.01% and by using a temperature ramp of 10 �C/minfrom �50 to 50 �C. The frequency of vibration of the fixturewas 1 rad/s. Elastic moduli of samples at 25 �C were alsonoted during the temperature ramp. Another mechanical prop-erty that was studied as a measure of flexibility of plasticizedPVC samples was elongation at break. Dog-bone shaped sam-ples were subjected to elongation tests (INSTRON 5581, Ins-tron, Grove City, PA) at 2.54 mm/min strain rate using a 500 Nstatic load cell.

2.3.2. High temperature stabilityShort-term high temperature stability of different plasti-

cizers and PVC samples, measured as mass loss, was testedusing a Thermogravimetric Analyzer (Model 2950 TGA, TAInstruments, Newcastle, DE). For dynamic stability tests, sam-ples weighing w10 mg were subjected to a temperature rampof 10 �C/min starting from room temperature up to 350 �C.Isothermal experiments were performed by heating samplesof w10 mg to 200 �C and keeping the temperature constantfor 30 min. Long-term thermal stability of samples was ana-lyzed by placing w100 mg of rectangular samples (approxi-mately 33� 7 mm2) at 100 �C in a dry oven and measuringtheir weight after successive intervals over a period of twomonths.

2.3.3. UV StabilityThe stability of plasticized PVC samples in the far UV

region was studied. The wavelength that was used (254 nm)belongs to the UV-C range and is dominant in outer space.


This type of irradiation is often applied to induce crosslinkingof polymers. Dog-bone shaped plasticized and unplasticizedPVC samples were exposed to UV rays at 200 wpi for 5 minusing an UV irradiation chamber (Ultraviolet System andEquipment, Houston, TX). The temperature inside the irradia-tion chamber was approximately 90 �C. To eliminate any ther-mal effects, samples were placed on a conveyer belt andpassed through the chamber where the exposure time for onepass was only 10 s. Samples were allowed to cool prior to ad-ditional UV exposure to reach a total exposure time of 5 min.Mechanical properties of the samples, as measured by Instron,were compared before and after irradiation.

2.3.4. LeachingLeaching of plasticizers from plasticized PVC samples was

studied by placing thin rectangular samples (approximately30� 10 mm2) weighing w0.125 g in 100 ml of deionized(DI) water in Erlenmeyer flasks and shaking at 100 cyclesper minute for 10 days. To enhance the effect of leaching, sothat observations could be made in a short-time period, thetemperature of the water bath was elevated to 50 �C. Sampleweights were measured at different stages of the experimentand elastic moduli before and after leaching were observed.Citroflex A6 and Citroflex B6, which are often used as plasti-cizers in medical plastics and several food-contact applica-tions, were studied particularly for the leaching experiments.

2.3.5. MigrationThe migration of plasticizers from plasticized to unplasti-

cized PVC films was studied at 40 �C over a two-week period.Rectangular plasticized PVC sheets of 33� 7 mm2 surfacearea, weighing w0.1 g were sandwiched between two unplas-ticized PVC sheets of the same shape. The sandwiched mate-rials were kept in contact by placing them between glassmicroscope slides secured by binder clips. The amount of plas-ticizer that migrated to the unplasticized sheets was deter-mined by gravimetry and mass balance.

3. Results and discussions

3.1. Flexibility

In an ideal plasticizer, the solvating groups are usually lo-cated internally rather than at the ends of the plasticizer mol-ecule [6]. Thus, the compatibility is controlled by the solvatinggroups along with the significant influence of the non-polargroups creating a barrier between polymer chains. The phtha-late and trimellitate plasticizers studied, as shown in Fig. 1,clearly fall into this category. Notably, the structures of theILs are not much different. The ester groups and the aromaticrings of the traditional plasticizers, which supposedly formsecondary bonds with the polar ends of PVC according to Leu-ch’s model [35], are replaced by distinct charged species inILs. The phthalates, trimellitates and ILs also share commonaliphatic hydrocarbon segments in their structures. Therefore,similar performances were expected with ILs in terms of inte-grating with polymer chains and providing flexibility. All ofthe ILs studied were able to produce flexible PVC films, asconfirmed by the lowered Tg of PVC samples shown inTable 2. The plasticized films produced were transparent forall of the phosphonium and ammonium-based ILs as werethe PVC samples containing the traditional plasticizers. How-ever, the film plasticized by [thtdPhþ][deca�] had a yellowishappearance due to the color of the IL. Upon 20 wt% plastici-zation, some of the ILs lowered the Tg of PVC samples evenbelow those offered by some of the traditional plasticizers,as was shown previously for IL-plasticized PMMA [27e30].The imidazolium-based ILs, except [hmimþ][doss�], showedpartial phase separation as evidenced by somewhat opaquefilms. Despite this, all of the imidazolium-based ILs success-fully lowered the Tg of PVC.

The elastic modulus was also used to show the degree offlexibility imparted to PVC by ILs and traditional plasticizers.All of the plasticizers, upon 20 wt% plasticization in PVC,lowered the elastic modulus of PVC (Table 2). The traditionalplasticizers were found to lower the modulus of PVC to the

Table 2

Flexibility analysis of plasticized PVC samples

Plasticizer content Glass transition temperature, Tg (�C) Elastic modulus, E at 25 �C (GPa) Elongation at break at 25 �C (%)

None (bulk PVC) 80.10� 0.71 1.313� 0.072 1.432� 0.64

20% DEHP 14.20� 3.67 0.279� 0.012 56.91� 2.05

20% DIDP 22.90� 1.94 0.388� 0.026 51.23� 1.02

20% TOTM 10.10� 0.09 0.552� 0.042 59.10� 3.77

20% [bmimþ][PF6�] 34.43� 1.36 a a

20% [hmimþ][doss�] 39.64� 2.40 1.238� 0.053 2.255� 0.21

20% [hmimþ][PF6�] 31.41� 1.69 a a

20% [tbamþ][doss�] 25.20� 0.45 0.755� 0.013 108.6� 3.83

20% [tbPhþ][doss�] 27.85� 0.15 a 71.16� 4.60

20% [tbtdPhþ][dbs�] 22.81� 1.44 0.966� 0.023 58.06� 6.32

20% [tbtdPhþ][mes�] 11.23� 0.93 0.823� 0.058 94.61� 4.76

20% [thtdPhþ][Tf2N�] 36.61� 0.39 0.929� 0.073 4.475� 0.35

20% [thtdPhþ][Cl�] 17.80� 0.65 0.855� 0.009 46.08� 0.93

20% [thtdPhþ][deca�] 21.80� 0.50 0.582� 0.025 32.66� 1.98

20% [thtdPhþ][dbs�] 20.01� 1.67 1.023� 0.086 55.74� 8.64

20% [thtdPhþ][mes�] 16.51� 1.65 1.098� 0.194 84.58� 1.64

a Not tested.


greatest extent. The performance of ILs in lowering the modulivaried, but many of the ILs did impart significant flexibility toPVC by lowering the modulus of unplasticized PVC(1.313 GPa) by 50% or more. Theoretically, solvency of poly-mer by plasticizer, as indicated by polymereplasticizer inter-action, is the key factor in determining how much flexibilitythe plasticizer will impart. However, molecules that interactstrongly with the polymer make the polymer chains stiffer[3]. We theorize that [hmimþ][doss�], which lowered the Tg

of PVC by the least amount, showed similar behavior in low-ering the modulus of PVC due to the carbonyl groups presentin the anion, which caused additional secondary bonding ef-fects and thus restrained free movement of PVC chains, unlikethe case with other plasticizers. Comparing the structure of[hmimþ][doss�] to that of the other dioctylsulfosuccinateILs, [tbamþ][doss�] and [tbPhþ][doss�], the difference inthe structure of the cation can explain the difference in theirlubricating performance. While the protonated [hmimþ] cationhas the aromatic ring with high solvating effect, the [tbamþ]and [tbPhþ] cations have a relatively coordinating charge sur-rounded by four alkyl chains, as expected in the case of idealplasticizers.

Elongation at break is an important parameter for analyzingthe flexibility imparted by different plasticizers to PVC. Asshown in Table 2, unplasticized PVC samples broke at an aver-age of 1.4% elongation, whereas IL-plasticized samples werestretched up to as much as 108% of the original length beforethey failed. Of course, the extent to which different ILs impartedflexibility to PVC varied to some extent, but the overall perfor-mance of ILs in producing flexible PVC films, compared to thetraditional plasticizers, was quite satisfactory. A rudimentaryanalysis of these results allows a comparison of the different an-ions and cations. In addition to lowering the Tg of rigid PVC,[tbamþ][doss�] imparted the highest flexibility in terms of elon-gation at break. [hmimþ][doss�], which was the only imidazo-lium IL tested that was fully compatible with PVC, offered verylittle flexibility as a plasticizer, as did [thtdPhþ][Tf2N�]. Whilethe possible interactions for [hmimþ][doss�] have been dis-cussed earlier, the performance of the latter may have similarlybeen caused by the presence of several double bonded oxygenatoms, forming additional hydrogen bonds with PVC chains.ILs with the same [thtdPhþ] cation, but different anions dis-played a range of performance. While [thtdPhþ][deca�] and[thtdPhþ][Cl�] produced less flexible samples than the tradi-tional plasticizers, the effectiveness of [thtdPhþ][dbs�] wassimilar to that of the traditional ones and [thtdPhþ][mes�] of-fered a significant improvement in elongation at break, proba-bly because of a smaller, more coordinating anion comparedto the aromatic ring of [dbs�]. For the ILs with [dbs�] and[mes�] anions, when butyl groups replaced the hexyl groupsin the phosphonium cations, the ILs became more lubricatingas observed by the decreased elastic modulus and increasedelongation at break of the plasticized PVC samples. This effectwas more pronounced in the [mes�] ILs, may be because thepresence of C12 chains in the [dbs�] anion offsets any effectof alkyl chain length in cations for the [dbs�] ILs. While theeffect of alkyl chain length on plasticizing efficiency has not

been addressed in literature, this may have occurred due tothe decreasing solvency and compatibility of plasticizers withincreasing MW [6].

3.2. High temperature stability

Fig. 2(a) shows the onset temperature for decomposition ofdifferent ILs and traditional plasticizers as observed duringa TGA temperature ramp of 10 �C/min. Plasticizers are ar-ranged in an ascending order of their decomposition tempera-tures in this figure. It is evident from the bar graph that all ofthe three traditional plasticizers are more susceptible to thermaldegradation compared to the ILs studied. Especially the twophthalates were the most vulnerable to high temperature withtheir decomposition temperature being close to 200 �C. Studyof vapor pressure up to 250 �C has shown that the vapor pres-sure of DEHP>DIDP> TOTM [35] and our observation is inagreement to literature. Most of the ILs studied were found tobe fairly stable up to and above 300 �C. This conforms to the

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Ons

et te

mpe

ratu

re fo

r dec

ompo

sitio

n (º

C)

(a)

20

30

40

50

60

70

80

90

100

Wei

ght R

emai

ning

, (M

/Mo)

%

(b)

Fig. 2. (a) Onset temperature for decomposition of different plasticizers when sub-

jected to a temperature ramp of 10 �C/min, (b) percentage of weight remaining for

different plasticizers when subjected to an isothermal condition at 200 �C for

30 min. Plasticizers, as denoted by different numbers, are as follows: 1. DEHP,

2. DIDP, 3. TOTM, 4. [tbPhþ][doss�], 5. [hmimþ][doss�], 6. [tbamþ][doss�],

7. [thtdPhþ][deca�], 8. [hmimþ][BF4�], 9. [thtdPhþ][Cl�], 10. [bmimþ][PF6

�],

11. [thtdPhþ][Tf2N�], 12. [hmimþ][PF6�], 13. [tbtdPhþ][mes�], 14.

[tbtdPhþ][dbs�], 15. [thtdPhþ][mes�], and 16. [thtdPhþ][dbs�].


early studies done by the group of Rogers [12] on several imi-dazolium-based ILs. ILs with [Tf2N�] anion are known to behigh temperature stable [36], but phosphonium ILs with[mes�] and [dbs�] anions were found to be slightly more stablethan their [Tf2N�] analogue in short-term temperature ramptests. The [doss�] ILs, irrespective of their cationic counterpart,were relatively less stable at higher temperature since theystarted to degrade above 250 �C. As a general observation, al-most all of the samples lost some early weight probably due toevaporation of moisture content, impurities or dissolved gases.

When the plasticizers were subjected to an isothermal con-dition at 200 �C for 30 min, a significant difference wasobserved between the ILs and the traditional phthalate plasti-cizers. As shown in Fig. 2(b), all of the ILs displayed good sta-bility, losing w5% or less of their initial weight during thistime, whereas DIDP and DEHP lost around 20% and 65%weight, respectively. TOTM, however, was fairly stable, losingslightly over 3% of its initial weight.

When plasticized samples were subjected to the same tem-perature ramp, dramatic differences were observed comparedto the above results for the plasticizers alone. UnplasticizedPVC was stable up to 212 �C, whereas the addition of plasti-cizers deteriorated its thermal stability to some extent for ev-ery plasticizer studied. Table 3 offers a comparison of the PVCsamples where the temperatures at which samples lost 10% oftheir initial weights have been reported. TOTM produced themost stable plasticized PVC samples of all. Even though thephthalate plasticizers were not high temperature stable bythemselves, they showed significantly better thermostabilizingeffects with PVC, compared to the ILs studied. This effect hasbeen observed earlier by Stepek et al. [37]. This is, however,an interesting contrast to data reported by our group earlierfor the improved high temperature stability of IL-plasticizedPMMA compared to those plasticized with DEHP [27e30].[thtdPhþ][Tf2N�] was the only IL which offered considerablyhigh temperature stability to PVC upon 20 wt% plasticization.PVC samples plasticized with all other ILs lost weight

Table 3

Short-term high temperature stability analysis of plasticized PVC samples

Plasticizer content Temperature (�C) at which

sample losses 10% of initial

weight during a 10 �C/min

temperature ramp

Percentage of weight

remaining after being

subjected to 200 �C

for 30 min

None (bulk PVC) 270.9� 0.4 99.50� 0.40

20% DEHP 244.1� 0.9 78.30� 1.73

20% DIDP 254.6� 4.1 88.19� 0.45

20% TOTM 267.4� 1.2 95.95� 0.20

20% [bmimþ][PF6�] 223.3� 3.1 71.06� 0.73

20% [hmimþ][doss�] 210.6� 3.4 77.39� 0.68

20% [tbamþ][doss�] 205.6� 2.5 56.22� 0.67

20% [tbPhþ][doss�] 203.4� 1.1 56.28� 0.91

20% [tbtdPhþ][dbs�] 201.2� 3.3 58.53� 1.88

20% [tbtdPhþ][mes�] 191.9� 3.5 52.46� 3.76

20% [thtdPhþ][Tf2N�] 242.0� 0.1 86.72� 0.58

20% [thtdPhþ][Cl�] 195.1� 3.4 57.50� 1.73

20% [thtdPhþ][deca�] 194.0� 2.3 52.40� 0.53

20% [thtdPhþ][dbs�] 203.8� 4.3 55.60� 0.12

20% [thtdPhþ][mes�] 195.3� 3.9 56.27� 2.47

drastically above 170 �C and ended up losing 10% of theirinitial weight at around 200 �C. Some of the samples, e.g.the one plasticized with [thtdPhþ][deca�], were found tohave started degrading at as low as w92 �C. When the sam-ples were kept at 200 �C for 30 min (Table 3), the same trendwas observed with only a couple of notable differences. The[thtdPhþ][Tf2N�]-plasticized samples were more stable thanDEHP-plasticized samples and [hmimþ][doss�] offered nostatistical difference in thermal stability compared to DEHP.The remaining IL-plasticized samples lost 40e50% of theirinitial weights during this time.

To analyze a more realistic condition for end-use, plasti-cized PVC samples were stored at 100 �C in aluminum pansand weight loss was observed over a two-month period(Fig. 3). Once again, unplasticized PVC was the most stable,followed by PVC plasticized with TOTM. This time, however,[thtdPhþ][Tf2N�], [thtdPhþ][dbs�] and [hmimþ][doss�]were found to offer better thermal stability to PVC than didDEHP. The stability of [tbtdPhþ][dbs�] and [tbamþ][doss�]-plasticized samples was comparable with that of DEHP. How-ever, DEHP-plasticized samples lost more than 15% of theirinitial weight on an average in two months which is significantconsidering any kind of end-use application at elevated tem-peratures. Thus some of the ILs were found to perform betterin combination with PVC at relatively low heating conditions.The remaining IL-plasticized samples performed poorly, asthey did in short-term studies.

[thtdPhþ][deca�] is the IL that caused the most rapiddegradation of PVC samples at elevated temperatures whilesome other ILs with the same cation were relatively stable.To investigate if it was the anion that was causing the degra-dation of PVC, a homogeneous mixture of 5 wt% PVC in[thtdPhþ][deca�] was subjected to 2 �C/min temperatureramp in a Fisher Johns melting point apparatus and PVC par-ticles were closely observed. This is part of the protocol fol-lowed for determining the solidegel transition temperature ofPVC [38]. It was found that PVC starts to degrade at as lowas w92 �C while in the mixture with the IL. However, whenthe same study was performed using decanoic acid, no degra-dation of PVC was observed up to w200 �C. This leads to theconclusion that rather than due to a particular anion or cation,the poor thermal stability of IL-plasticized PVC samples mayhave been due to the IL taking part in the catalytic degrada-tion of PVC, similar to the well-known autocatalytic degrada-tion of PVC by HCl given off as PVC degrades [39]. Hightemperature thermodynamics of different IL-PVC systemsare currently being investigated.

3.3. UV stability

The permanence of a plasticizer is critically affected by itschemical structure, concentration and conditions to which theend product is exposed. Under most end-use conditions, plas-ticized PVC can remain compatible for fairly long time, andeventually phase separate. PVC is known to show agingphenomena quite often [3]. Therefore, laboratory tests areoften designed to accelerate incompatibility under extreme


500 10 20 30 40 50 60 70 80

60

70

80

90

100

Time, t (days)

Wei

ght R

emai

ning

, (M

/Mo)

%

Bulk PVC

TOTM

DIDP[thtdPh+][Tf2N-]

[thtdPh+][dbs-][hmim+][doss-]DEHP[tbam+][doss-][tbtdPh+][dbs-]

[thtdPh+][mes-]

[tbtdPh+][mes-]

[thtdPh+][Cl-]

[thtdPh+][deca-]

Fig. 3. Long-term thermal stability of 20 wt% plasticized PVC samples at 100 �C.

conditions. UV degradation of PVC in the far UV region hasbeen studied to a lesser extent than in the near UV region. De-composition of PVC by UV light is largely dependent on theadditives present in PVC [39]. For all the samples studied inthis experiment, neither weight loss, nor exudation of any plas-ticizer was observed during or after the high intensity UV ir-radiation period. Elongations at break were measured foreach sample before and after irradiation. In almost all cases,elongation at break increased, indicating an increase in flexi-bility theorized to be caused by extensive scission of the poly-mer chains (Table 4). It has been observed before that chainscission is the dominating effect that takes place at lowerend UV irradiation, resulting in reduced Mn and Mw [40],though there is simultaneous crosslinking taking place at thesurface. But the thickness of the surface layer is limited toapproximately 0.004 cm, where only the crosslinking densityincreases with extended UV exposure [41]. The objectivehere was to compare the relative changes in elongation at

Table 4

Change in elongation at break (at 25 �C) of plasticized PVC samples after

254 nm wavelength UV exposure at 200 wpi for 5 min

Plasticizer content Elongation

at break

before exposure

Elongation

at break

after exposure

Ratio

(after/before)

None (bulk PVC) 1.432� 0.64 34.93� 5.20 24.4� 11.5

20% DEHP 56.91� 2.05 92.39� 2.39 1.62� 0.07

20% DIDP 51.23� 1.02 66.98� 2.62 1.31� 0.06

20% TOTM 59.10� 3.77 66.19� 5.26 1.12� 0.11

20% [tbamþ][doss�] 108.6� 3.83 82.24� 4.33 0.78� 0.05

20% [tbtdPhþ][dbs�] 58.06� 6.32 94.28� 9.22 1.62� 0.24

20% [tbtdPhþ][mes�] 94.61� 4.76 110.0� 10.8 1.16� 0.13

20% [thtdPhþ][Tf2N�] 4.475� 0.35 9.077� 2.03 2.03� 0.48

20% [thtdPhþ][Cl�] 46.08� 0.93 56.72� 1.31 1.23� 0.04

20% [thtdPhþ][deca�] 32.66� 1.98 34.58� 7.58 1.06� 0.24

20% [thtdPhþ][dbs�] 55.74� 8.64 100.5� 1.10 1.80� 0.28

20% [thtdPhþ][mes�] 84.58� 1.64 106.5� 3.80 1.26� 0.05

break for samples plasticized with different plasticizers. Theinitial effect of UV exposure is usually the formation of freeradicals in the PVC structure owing to chain scissions [42].The chain ends or the free radicals, however, often reunite lateron, forming Y-linked or H-linked crosslinked structures unlessthe plasticizer molecules hinder the crosslinking of chains byscavenging at the open ends. The scavenging of free radicalsby different plasticizers occurs to different extents and can becompared to chain transfer to solvent constants used in evalu-ating free radical polymerization kinetics [43].

The ratios of elongation at break after and before irra-diation showed no particular pattern for any of the ILs, orfor the traditional plasticizers. The unplasticized PVC, how-ever, had much higher chain scission as indicated by the in-creased elongation after irradiation. Another significantobservation was the partial phase separation that occurredin the [thtdPhþ][Tf2N�]-plasticized PVC samples after UVexposure, resulting in opaque regions along the sides of thesamples. These particular samples also showed the highestdegree of increase in elongation at break upon UV exposureamongst all the plasticized samples (Table 4). While the sam-ples with other plasticizers remained unchanged in theirappearances, the phase separation in the [thtdPhþ][Tf2N�]-plasticized PVC samples was visually observable. The edgesof the otherwise transparent samples turned white, clearlyindicating presence of multiple phases and prompting furthermicroscopic analysis. Scanning electron microscopic (SEM)images of the cross section of the UV-exposed sampleshowed distinct phase separation and layering of the pla-sticizer molecules along the sides, compared to the typicalrough surface of the unexposed sample which only hadscratches due to the cutting procedure (Fig. 4). UV-inducedphase separation of ILs, of course, has more to do with thesolubility parameter and compatibility of the IL and PVCat the molecular level.


Fig. 4. SEM micrograph of cross sectional view [150�] of 20 wt%

[thtdPhþ][Tf2N�]-plasticized PVC sample (a) before UV irradiation and (b)

after UV irradiation at 254 nm for 5 min.

3.4. Leaching

In a polymereplasticizer system, the polymer and plasti-cizer molecules are continuously undergoing association andsegregation. Hence, the tendency of the plasticizer to be re-moved from the system is very real. Usually removal of plasti-cizers can be affected by two primary factors: (i) the rate atwhich plasticizer molecules diffuse through the polymermatrix, and (ii) the rate at which plasticizer molecules are re-moved from the outer surface [6]. At lower plasticizer concen-trations, the diffusion of plasticizers is thought to be the ratelimiting step. But in the case with this particular experiment,where PVC samples contained 20 wt% plasticizers and the ex-tractant is a small molecule (water), it can be presumed that thewater molecules can easily diffuse through the wide enoughpores to reach and solubilize the plasticizer molecules. Leach-ing will therefore be a function of the strength of the existingsecondary bonding effect in the polymereplasticizer systemand miscibility of the respective plasticizers with water.

The trend of plasticizer loss due to leaching over a period of10 days is shown in Fig. 5. For all the samples, most of theweight loss took place on the first day and somewhat stabilizedafter a week. This happened because once the plasticizers inthe outer layers were removed, secondary bonding effectsbetween PVC chains became dominant in the absence of theshielding effect and reduced access to the pores, restrictingfurther leaching of plasticizer molecules [6]. Of all the sam-ples, [thtdPhþ][Tf2N�]-plasticized samples lost the minimumweight, understandably because of its hydrophobicity andalso because of its bulky structure and additional secondarybonding effects which restrained its movement along the poly-mer network. Citroflex A6 and Citroflex B6-plasticized sam-ples lost less weight than all the remaining samples,indicating good resistance to leaching of respective plasti-cizers in aqueous media. Along with the low toxicity citricacid-based structure, this leaching resistance is why Citroflex

0

5

10

15

20

25

30

35

40

Perc

enta

ge o

f pla

stic

izer

lost

, [(M

o-M

)/Mo]

% [thtdPh+][deca-]

DEHP

DIDP

[tbam+][doss-][thtdPh+][Cl-][thtdPh+][mes-]TOTM

[thtdPh+][dbs-]

Citroflex B6Citroflex A6[thtdPh+][Tf2N-]

0 1 2 3 4 5 6 7 8 9 10 11 12Time, t (days)

Fig. 5. Percentage of plasticizers lost due to leaching in DI water from 20 wt% plasticized PVC samples at 50 �C for 10 days.


plasticizers are often used in food-contact packaging applica-tions. Those were the only two samples whose appearanceswere not altered after the experiment. All other samples be-came partially opaque or white owing to loss of plasticizer,partial phase separation and the heated environment. DEHP,which is under scrutiny because of leaching from medicalplastics and consumer goods [7e10], performed particularlypoorly as more than 30 wt% of the initial plasticizer contentwas leached. The highest amount of weight loss was observedwith the [thtdPhþ][deca�]-plasticized samples. However, anal-ysis of the leaching data for [thtdPhþ][deca�] was complicatedby a color change from light yellow to light brown by the endof the test, which may well have occurred due to thermal deg-radation of the sample at elevated temperature (50 �C) used toaccelerate leaching.

Elastic moduli of the samples, after the 10-day leaching ex-periment, were found to have increased (Table 5). Disregard-ing the [thtdPhþ][deca�]-plasticized samples due to thermaldegradation, the greatest changes in elastic moduli were ob-served for the phthalate and trimellitate plasticizers. WhileCitroflex A6, Citroflex B6 and some of the ILs showed almostno change in moduli after leaching, some other ILs showedmoderate increase in elastic moduli, yet less than those ofthe phthalate and trimellitate plasticizers. Thus, there wereseveral IL-plasticized PVC samples where the ILs sufferedcomparable or less leaching than the traditional plasticizersin aqueous media, and also retained greater flexibility thanthe latter ones after leaching.

In Fig. 6, relative change in elastic moduli of plasticizedPVC samples is shown with respect to plasticizer loss. Fromleaching point of view, the best plasticizers are those that donot leave the polymer. Thus, plasticizers at the left end, e.g.,Citroflex A6 and Citroflex B6 are good plasticizers, and so is[thtdPhþ][Tf2N�]. A second consideration for good plasticiza-tion that withstands leaching is whether the modulus changesmuch after the leaching. From this standpoint, many of theionic liquids showed good behavior, in that the flexibility ofthe PVC samples changed the least. However, another way oflooking at the data is that the plasticizers with high plasticizingefficiency will cause the greatest changes in modulus after plas-ticizer loss due to leaching. Thus, the phthalate and trimellitate

Table 5

Change in elastic moduli (at 25 �C) of plasticized PVC samples after leaching

in DI water for 10 days at 50 �C

Plasticizer content Modulus before

leaching, E0 (GPa)

Modulus after

leaching, E (GPa)

E/E0

20% DEHP 0.279� 0.012 0.925� 0.028 3.315� 0.17

20% DIDP 0.388� 0.026 1.080� 0.113 2.784� 0.35

20% TOTM 0.552� 0.042 1.290� 0.021 2.337� 0.20

20% Citroflex A6 0.546� 0.074 0.580� 0.109 1.062� 0.25

20% Citroflex B6 0.887� 0.009 0.896� 0.003 1.010� 0.04

20% [tbamþ][doss�] 0.755� 0.013 1.640� 0.225 2.172� 0.30

20% [thtdPhþ][Tf2N�] 0.929� 0.073 1.823� 0.044 1.962� 0.16

20% [thtdPhþ][Cl�] 0.855� 0.009 0.915� 0.038 1.070� 0.05

20% [thtdPhþ][deca�] 0.582� 0.025 1.465� 0.078 2.517� 0.17

20% [thtdPhþ][dbs�] 1.023� 0.086 1.215� 0.205 1.188� 0.22

20% [thtdPhþ][mes�] 1.098� 0.194 1.770� 0.033 1.612� 0.29

plasticizers, [tbamþ][doss�] and [thtdPhþ][Tf2N�], can bethought of as having good plasticizing effectiveness. However,if the plasticizers are phase separated from the PVC, they arenot having an appreciable effect on the modulus; thus, evena large leaching percentage will result in little change in themodulus e.g., [thtdPhþ][Cl�] and [thtdPhþ][deca�] are proba-bly not forming uniform samples, and when some of these ILsleach out, less of a change in the mechanical properties of PVCwas observed.

3.5. Migration

Migration of plasticizers from flexible plastics to othersolid materials is highly dependent on the diffusivity of theplasticizer in its host polymer and diffusivity is inversely influ-enced by strong polymereplasticizer interactions. Therefore,ILs, with the possibility of additional hydrogen bond forma-tion with PVC on top of the polar interactions, were expectedto show noticeable resistance to migration compared to the tra-ditional plasticizers. Even though diffusivity increases as adirect result of increased plasticizer concentration and temper-ature, it should have been dependent mainly on the type ofplasticizer under the experimental condition of 40 �C [6]. InTable 6, the percentage of plasticizers migrated fromplasticized to unplasticized PVC sheets during the two-weekexperimental period is shown. The traditional plasticizers, es-pecially the phthalates, were more prone to migration when incontact with an unplasticized surface. Also the relative migra-tion of the three traditional plasticizers matched the trend ob-served elsewhere [35]. The ILs, having distinct polar groups,were fairly compatible with the polar ends of PVC. Forsome of the ILs, the bulky structure might have been an addi-tional factor that resisted migration. [thtdPhþ][Cl�] showed nodetectable migration. It could be the coordinating electroneg-ative [Cl�] anion which made the IL an inherent part of thepolymer network, or there could have been any special kindof bonding that took place, as anticipated earlier from the un-expected thermal stability observations. Thus ILs, in general,

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35 40Percentage of plasticizer lost

Rat

io o

f Ela

stic

Mod

uli o

f 20

wt%

Pla

stic

ized

PVC

Sam

ples

afte

r and

bef

ore

Leac

hing

, E/E

o

[thtdPh+][Tf2N-]

Citroflex A6Citroflex B6

[thtdPh+][dbs-]

TOTMDIDP

DEHP

[thtdPh+][mes-]

[thtdPh+][Cl-]

[tbam+][doss-][thtdPh+][deca-]

Fig. 6. Relative change in elastic moduli with plasticizer loss due to leaching

for different plasticized PVC samples.


were found to be superior to the traditional plasticizers as mi-gration resistant entities in the polymer system investigated.

4. Conclusions

To the best of our knowledge, this is the first major journalpaper focused on plasticizing aspects of ILs in PVC systems. Itis true that the current plasticizer industry is rich enough to of-fer numerous plasticizers capable of rendering a wide range ofend-use properties of PVC materials. But different plasticizershave their shortcomings which often make their use inappro-priate for certain applications. It has, therefore, become a com-mon practice in the vinyl industry to use plasticizer blends toprovide the optimum balance of desired properties [6]. Oftensecondary plasticizers, extenders and other additives areused in conjunction with primary plasticizers.

For a plasticizer to effectively plasticize PVC, the polarsegment should bind itself reversibly with PVC, while thenon-polar part should impart free volume in the polymer net-work, thus contributing to shielding effects between chains tooffer lubricity. Due to the structural similarity of many ILswith traditional phthalate and trimellitate plasticizers, andalso based on preliminary results obtained [27e33], ILshave been proposed to be potential plasticizers for PVC. Inthis paper, the potential of ILs as replacement of traditionalplasticizers was investigated for several applications. ILsshowed excellent performance in producing flexible plastics.Many of them produced PVC more flexible than those plasti-cized by traditional plasticizers. Some ILs proved adequate forlong-term exposure to moderately high temperatures. Notably,though, all but one of the ILs tested proved to destabilize thePVC at high temperatures. This in sharp contrast with our ear-lier reports on IL-plasticized PMMA [27e30], and may indi-cate a wide range of performance characteristics that isdependent on both the IL and polymer chemistry. IL plasti-cizer behavior under far-range UV exposure was not signifi-cantly different than the traditional plasticizers and theirleaching and migration characteristics were far better thanthe plasticizers currently used in medical and commodity plas-tics. Based on the results obtained here, it can be stated that

Table 6

Percentage migration of plasticizers from plasticized PVC to unplasticized

PVC sheets

Plasticizer content Percentage of

plasticizer migrated

20% DEHP 11.7� 1.25

20% DIDP 10.7� 3.23

20% TOTM 6.90� 0.70

20% [bmimþ][PF6�] 8.09� 0.42

20% [tbamþ][doss�] 8.07� 2.04

20% [tbtdPhþ][dbs�] 2.86� 2.86

20% [tbtdPhþ][mes�] 1.07� 1.04

20% [thtdPhþ][Tf2N�] 1.28� 0.32

20% [thtdPhþ][Cl�] 0.00� 0.00

20% [thtdPhþ][deca�] 3.63� 0.90

20% [thtdPhþ][dbs�] 1.45� 1.44

20% [thtdPhþ][mes�] 5.52� 2.28

ILs have properties that add to the diversity of choices whileselecting plasticizers. They may provide superior performancefor PVC to offer flexibility, lengthen material lifetime and re-duce diffusional plasticizer loss. Considering that the majorityof plasticizers are used in materials produced in calenderingoperations [3], there are significant losses of plasticizersthrough volatilization under processing conditions throughtheir end-use applications. So, high temperature stable ILsmay be able to minimize plasticizer loss in addition to render-ing specific advantages over traditional plasticizers from thevery early stage of applications.

The performance of a plasticizer is defined in terms of com-patibility, processability, cost, efficiency, low temperatureproperties, permanence and some specific properties (insula-tion, smoke and flame retardancy, odor/taste, toxicity). Unfor-tunately, it is neither possible to predict the performancecharacteristics of a plasticizer simply by defining their physicaland/or chemical properties, nor the mechanical behavior ofplasticized polymers can be predicted from the solvency char-acteristics of the plasticizers. Based on currently known anionsand cations, there could be as many as one trillion accessible ILstructures [44] which dramatically widens the possibility of ILapplications. At the same, this wide variety of ILs often makesit difficult to screen out the most appropriate or suitable one forspecific application. During the last decade, ILs have emergedas highly useful solvents in chemistry with large potential forindustrial use. But their commercial application has been lim-ited by a dearth of in-depth knowledge about the chemistry andthermodynamics involved in IL systems and their high cost.The results obtained so far show immense prospects for ILsas plasticizers, but a thorough thermodynamic analysis of dif-ferent IL-PVC systems is yet to be performed. The current plas-ticizer market has been well-established for more than 50years. Approximately 87% of industrial plasticizers used arecomprised by phthalate esters [1], and nearly half of this vol-ume is for general purpose applications [6]. Therefore, evenwith the ongoing controversy regarding the health and safetyof phthalates, it will not be easy for ILs to compete againstthem in commodity applications. The cost of ILs, in particular,may relegate the use of ILs to high-end specialty products. AllILs may neither be good plasticizers for all polymers, nor be-nign in all aspects, but with the tunability of IL structures itis possible to design ILs with desired characteristics to beused as plasticizers for specific applications. Specifically, con-sidering the unique thermodynamic properties of ILs and thepossibility of forming numerous ILs with different combina-tions of anions and cations, ILs could revolutionize the plasticindustry, once the thermodynamics of ILePVC interactions arebetter understood.

Acknowledgments

The authors would like to thank Dr. Jim Davis Jr., Dr. WilliamM. Reichert, Dr. Rex Ren, Cytec Canada, Inc., and Sachem, Inc.for providing ILs, Dr. David E. Nikles, Dr. Robin D. Rogers andDr. Mark L. Weaver for experimental facilities and ACS-PRF


Grant# 37742-G7 and the University of Alabama GraduateCouncil Fellowship for funding the project.

References

[1] Krauskopf LG. Monomerics for polyvinyl chloride (phthalates, adipates,

and trimellitates). In: Edenbaum J, editor. Plastics additives and modifiers

handbook, section III: plasticizers. New York: Van Nostrand Reinhold;

1992.

[2] Gachter R, Muller H, editors. Plastics additives handbook. 3rd ed. New

York: Carl Hanser Verlag Munchen Wien; 1990.

[3] Krauskopf LG, Godwin A. Plasticizers. In: Wilkes CE, Summers JW,

Daniels CA, editors. PVC handbook. Munich: Carl Hanser Verlag; 2005.

[4] Tullo AH. Cutting out phthalates. Chem Eng News 2005;83(46):29e31.

[5] Murphy J. Additives for plastics handbook. 2nd ed. New York: Elsevier

Science Ltd; 2001.

[6] Krauskopf LG. Plasticizers: types, properties and performance. In:

Nass LI, Heiberger CA, editors. Encyclopedia of PVC, vol. 2. New

York: Marcel Dekker Inc; 1988.

[7] Rahman M, Brazel CS. The plasticizer market: an assessment of tradi-

tional plasticizers and research trends to meet new challenges. Prog

Polym Sci 2004;29(12):1223e48.

[8] Kay J. Cal-EPA to pass rule on chemical used in plastics e agent found

in hospital, toys, may cause birth defects. San Francisco Chronicle; Oct

21, 2003.

[9] Hileman B. EU bans three phthalates from toys, restricts three more.

Chem Eng News 2005;83(28):11.

[10] Hileman B. FDA suggests replacing DEHP in plastics. Chem Eng News

2002;80(37):6.

[11] Holbrey JD, Turner MB, Rogers RD. Selection of ionic liquids for green

chemical applications. In: Rogers RD, Seddon KR, editors. Ionic liquids

as green solvents: progress and prospects. American Chemical Society

symposium series, vol. 856. Washington, DC: Am Chem Soc; 2003.

p. 2e12.

[12] Huddleston JG, Visser AE, Reichert WM, Willauer HD, Broker GA,

Rogers RD. Characterization and comparison of hydrophilic and hydro-

phobic room temperature ionic liquids incorporating the imidazolium

cation. Green Chem 2001;3(4):156e64.

[13] Holbrey JD, Seddon KR. The phase behavior of 1-alkyl-3-methylimida-

zolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J Chem

Soc Dalton Trans 1999;13:2133e9.

[14] Marszall MP, Baczek T, Kaliszan R. Reduction of silanophilic interac-

tions in liquid chromatography with the use of ionic liquids. Anal

Chim Acta 2005;547(2):172e8.

[15] Liu H, He P, Li Z, Liu Y, Li J. A novel nickel-based mixed rare-earth

oxide/activated carbon supercapacitor using room temperature ionic liquid

electrolyte. Electrochim Acta 2006;51(10):1925e31.

[16] Freemantle M. Ionic liquids make splash in industry. Chem Eng News

2005;83(31):33e8.

[17] Hong K, Zhang H, Mays JW, Visser AE, Brazel CS, Holbrey JD, et al.

Conventional free radical polymerization in room temperature tonic

liquids: a green approach to commodity polymers with practical advan-

tages. Chem Commun 2002;13:1368e9.

[18] Bradaric CJ, Downard A, Kennedy C, Robertson AJ, Zhou Y. Industrial

preparation of phosphonium ionic liquids. In: Rogers RD, Seddon KR,

editors. Ionic liquids as green solvents: progress and prospects. American

Chemical Society symposium series, vol. 856. Washington, DC: Am

Chem Soc; 2003. p. 41e56.

[19] Li Z, Liu H, Liu Y, He P, Li JH. A room-temperature ionic-liquid-tem-

plated proton-conducting gelatinous electrolyte. J Phys Chem B 2004;

108(45):17512e8.

[20] Shin JH, Henderson WA, Scaccia S, Prosini PP, Passerini S. Solid-state

Li/LiFePO4 polymer electrolyte batteries incorporating an ionic liquid

cycled at 40 �C. J Power Sources 2006;156(2):560e6.

[21] Poole SK, Shetty PH, Poole CE. Chromatographic and spectroscopic

studies of the solvent properties of a new series of room-temperature liq-

uid tetraalkylammonium sulfonates. Anal Chim Acta 1989;218:241e64.

[22] Armstrong DW, He L, Liu YS. Examination of ionic liquids and their in-

teraction with molecules, when used as stationary phases in gas chroma-

tography. Anal Chem 1999;71(17):3873e6.

[23] Zhao Z, Qiao W, Wang X, Wang G, Li Z, Cheng L. Effects of kinds of

ionic liquid catalysts on alkylations of 1- and 2-methylnaphthalene with

alkenes. Appl Catal A Gen 2005;290(1e2):133e7.

[24] Park S, Kazlauskas RJ. Biocatalysis in ionic liquids e advantages beyond

green technology. Curr Opin Biotech 2003;14(4):432e7.

[25] Chiappe C, Leandri E, Lucchesi S, Pieraccini D, Hammock BD,

Morisseau C. Biocatalysis in ionic liquids: the stereoconvergent hydroly-

sis of trans-b-methylstyrene oxide catalyzed by soluble epoxide hydro-

lase. J Mol Catal B Enzym 2004;27(4e6):243e8.

[26] Yang Z, Pan W. Ionic liquids: green solvents for nonaqueous biocatalysis.

Enzyme Microb Tech 2005;37(1):19e28.

[27] Scott MP, Brazel CS, Benton MG, Mays JW, Holbrey JD, Rogers RD.

Application of ionic liquids as plasticizers for poly(methyl methacrylate).

Chem Commun 2002;13:1370e1.

[28] Scott MP, Benton MG, Rahman M, Brazel CS. Plasticizing effects

of imidazolium salts in PMMA: high temperature stable flexible

engineering materials. In: Rogers RD, Seddon KR, editors. Ionic liquids

as green solvents: progress and prospects. American Chemical Society sym-

posium series, vol. 856. Washington, DC: Am Chem Soc; 2003. p. 468e77.

[29] Scott MP, Rahman M, Brazel CS. Application of ionic liquids as low

volatility plasticizers for PMMA. Eur Polym J 2003;39(10):1947e53.

[30] Rahman M, Benton MG, Scott MP, Brazel CS. Room temperature ionic liq-

uids as environmentally benign plasticizers and reaction media for

polymerization reactions. Proceed Green Chem Eng Confer 2003;7:180e3.

[31] Shoff HW, Rahman M, Brazel CS. Leaching and migration resistance of

phosphonium-based ionic liquids as PVC plasticizers: a comparative

study of traditional phthalate and citrate plasticizers with ionic liquids.

Polym Prepr 2004;45(1):295e6.

[32] Rahman M, Brazel CS. Effectiveness of phosphonium, ammonium, and

imidazolium-based ionic liquids as plasticizers for poly(vinyl chloride):

thermal and ultraviolet stability. Polym Prepr 2004;45(1):301e2.

[33] Rahman M, Shoff HW, Brazel CS. Ionic liquids as alternative plasticizers

for PVC: flexibility and stability in thermal, leaching and ultraviolet En-

vironments. In: Brazel CS, Rogers RD, editors. Ionic liquids in polymer

systems: solvents, additives and novel applications. American Chemical

Society symposium series, vol. 913. Washington, DC: Am Chem Soc;

2005. p. 103e18.

[34] Armstrong DW, Zhang LK, He L, Gross ML. Ionic liquids as matrixes

for matrix-assisted laser desorption/ionization mass spectrometry. Anal

Chem 2001;73(15):3679e86.

[35] Wilson AS. Plasticisers: principles and practice. London: The Institute of

Materials; 1995.

[36] Crosthwaite JM, Muldoon MJ, Dixon JK, Anderson JL, Brennecke JF.

Phase transition and decomposition temperatures, heat capacities and vis-

cosities of pyridinium ionic liquids. J Chem Thermodyn 2005;37(6):

559e68.

[37] Stepek J, Vymazal Z, Dolozel B. Mod Plast 1963;40(10):146e7. 205.

[38] DIN 53408. Testing of plastics: determination of solubility temperature

of polyvinyl chloride (PVC) in plasticizers. Berlin; June 1967.

[39] Matthews G. PVC: production, properties and uses. London: Institute of

Metals; 1996.

[40] Kaczmarek H, Swiatek M, Kaminska A. Modification of polystyrene and

poly(vinyl chloride) for the purpose of obtaining packaging materials degrad-

able in the natural environment. Polym Degrad Stab 2004;83(1):35e45.

[41] Strelkova LD, Fedoseyeva GT, Minsker KS. Photochemical degradation

of polyvinylchloride. Polym Sci USSR 1976;18(9):2360e3.

[42] Hill DJT, Lewis DA, O’Donnell JH, Whittaker AK. The crosslinking

mechanism in gamma irradiation of polyarylsulfone: evidence for

Y-links. Polym Adv Technol 1998;9(1):45e51.

[43] Odian G. Principles of polymerization. 3rd ed. New York: John Wiley &

Sons, Inc; 1991.

[44] Stepnowski P, Muller A, Behrend P, Ranke J, Hoffmann J, Jastorff B.

Reversed-phase liquid chromatographic method for the determination

of selected room-temperature ionic liquid cations. J Chromatogr A

2003;993:173e8.

Documents

Ionic liquids: New generation stable plasticizers for poly(vinyl chloride)