Study on a Novel Ion-Conductive Compound Plasticizerfor Soft and Antistatic PVC Materials

Jiliang Wang, Wanqing Yang, Jingxin LeiState Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University,Chengdu 610065, China

Ion-conductive plasticizers (ICP) composed of dibutylphthalate (DBP) and butyl 2-poly(ethylene glycol) phthal-ate (BPEGP)/lithium bisoxalato borate (LiBOB) were suc-cessfully synthesized. The composites blended of poly(vinyl chloride) (PVC) and ICP were fabricated in a Haaketorque rheometer. FTIR, surface resistivity measure-ment, and mechanical test were used to investigate thecomprehensive properties of the PVC/ICP composites.The results show that all the synthesized ICP can reducethe surface resistivity of the PVC/ICP composites to105 V sq21 orders of magnitude as the content of ICPreaches 50 phr. The increasing temperature enhancesboth the mobility of PEG molecular chains and the diffu-sion of lithium cations, and thus effectively improves theantistatic ability of the PVC/ICP compounds. With twoexceptions of PVC/ICP compounds which include thosemade of PEG800 and PEG1000, the temperature depend-ence of the surface resistivity of PVC/ICP does not obeythe Arrhenius relationship. The introduction of ICP intoPVC matrix would improve the antistatic ability of thecomposites remarkably. Meanwhile, the mechanicalproperties of the composites are reduced to some rea-sonable extent. POLYM. ENG. SCI., 50:57–60, 2010. ª 2009Society of Plastics Engineers

INTRODUCTION

Poly(vinyl chloride) (PVC) is one of the most common

commodity plastics, which has been widely used in the

automobile, building construction, packaging fields, etc.,

because of its low cost, easy method of preparation, and

the broadening of the properties range [1–3].

The surface resistivity of typical PVC ranges from

1013 to 1016 O sq21, which would limit its large applica-

tions in some special fields, in which the antistatic ability

is required. To enhance the antistatic ability of PVC com-

posites, both antistatic agents, including ionic and non-

ionic surfactants, and conductive fillers such as carbon

blacks, metal fibers, are commonly used [4–6]. Unfortu-

nately, the former cannot endow the compounds with per-

manent antistatic ability, the latter suffers the problem of

filler migration or metal decay in polymer matrix and sur-

face, which would bring about the damage to electronic

devices and reduce the usage of the end products [7].

Moreover, the color of the PVC composites filled with

traditional conductive fillers is too black, and thus it is

difficult to obtain a colorless product by this method.

Dioctyl phthalate and dibutyl phthalate (DBP) are the

most common plasticizers for the preparation of semi-soft

to all-soft PVC composites. However, it seems that a par-

ticular plasticizer with some extent permanent ion-con-

ductive ability (or so-called in situ) has not been previ-

ously reported. In this article, we have developed a novel

transparent ion-conductive plasticizer (ICP) composed of

DBP and butyl 2-poly(ethylene glycol) phthalate

(BPEGP)/lithium bisoxalato borate (LiBOB) with neither

traditional antistatic agents nor conductive fillers. The

synthesized BPEGP owns both the similar molecular

structure with DBP to ensure their compatibility and the

coordination ability to coordinate with lithium cations.

FTIR, surface resistivity measurement, and mechanical

properties of the PVC/ICP composites were studied.

EXPERIMENTAL

Materials

DBP was purchased from Qilu Petrochemical

(Shandong, China), and used without further treatment.

Poly (ethylene glycol) (PEG), supplied by Kelong Chemi-

cal Reagent (Chengdu, China), was dried under vacuum

at 608C for 24 h before use. LiBOB was synthesized by

the method referred in the literature [8]. All the other

chemicals used in this experiment were analytically pure.

PVC (SG-II) and compound stabilizer (Baeropan SMS

318) were provided by Tianyuan (Yibing China) and

Baerlocher (Germany), respectively.

Synthesis of the Ion-Conductive Plasticizer

All the ion-conductive plasticizers (ICP) were synthesized

by the ester-exchange reaction at 1808C under the protection

Correspondence to: Jingxin Lei; e-mail: jxlei@scu.edu.cn

DOI 10.1002/pen.21510

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2010

of nitrogen, in which 2-ethyl-hexanoic acid tin salt was used

as the catalyst. When the theoretically stoichiometric butanol

was obtained by distillation from the reactor, heating was

stopped for �30 min and then continued to maintain the tem-

perature at around 808C. The sets content of LiBOB and DBP

were added into the reactor under vigorous stirring for 1 h,

the resulting compound was filtered and the slimy filtrate was

used as the end plasticizer. The synthesis route of BPEGP

was shown in Scheme 1.

Preparation of the Antistatic PVC/ICP Composites

PVC (100 phr) powder, synthesized ICP (50 phr), and

the compound stabilizer (5 phr) were mixed in a high-

speed mixing chamber at room temperature for 5 min, the

mixing speed was 1500 rpm. The resulting mixture was

dried at 608C for a given time (more than 2 h) until ICP

penetrated into the PVC matrix sufficiently, and then used

to prepare the PVC/ICP composites in a Haake torque

rheometer equipped with an electrically heated mixing

head and two noninterchangeable rotors. The processing

temperature, rotor speed, and blending time were set at

1608C, 30 rpm, and 8 min, respectively.

Characterization

FTIR Analysis. Pure DBP, synthesized BPEGP, and Liþ

coordinated BPEGP were characterized by using a Nicolet560

FTIR spectrometer with a resolution setting of 4 cm21. The

scanning range was altered from 400 to 4000 cm21.

Conductivity Measurement. A surface resistivity meter

(ZC46A, Shanghai, China) was used to measure the sur-

face resistivity of the PVC/ICP composites at ambient

environment. The surface resistivity was also measured

from 30 to 908C under a low relative humidity (RH) of

12% by using the same instrument. The 1-mm thick

sheets of the PVC/ICP composites were prepared by using

compression molding at 1808C, and then used for the con-

ductivity measurements.

Tensile Tests. The sheets which were prepared for the

surface resistivity measurement were also used for the

tensile tests by using an Instron4302 at a tensile rate of

100 mm min21, and the dimensions 25 mm 3 6 mm 3 1

mm dumbbell samples were prepared for tests.

RESULTS AND DISCUSSION

FTIR Spectroscopy

The ester exchange reaction and the coordination effects

between ethylene oxide (EO) groups and Liþ ions (EO/Li ¼6/1) are confirmed by infrared spectroscopy illustrated in Fig.

1a for pure DBP, and in Fig. 1b and c for the synthesized

BPEGP and Liþ ions coordinated BPEGP compound ICP,

respectively. Figure 1a shows typical DBP spectra without an

absorption band at near 3479 cm21, which is the characteristic

absorption band of the ��OH asymmetric stretching of PEG

end groups. In Fig. 1b, an obvious absorption band at 3479

cm21 is observed. Furthermore, the intensity of the absorption

band at around 1119 cm21 originating from the

��C��O��C�� stretching in PEG ethylene oxide groups

increases. The characteristic features belong to the ��CH2��and ��CH3 groups at near 2870, 2960, and 1459, 1351 cm21

are also observed. The characteristic spectra in Fig. 1b indi-

cate that the ester exchange reactions between DBP and PEG

have taken place successfully.

FTIR analysis is also used to investigate the interac-

tions between polymer molecular chains, having ether (or

ester) oxide groups, and alkali salts [9–11]. The wave

number of the carbonyl stretching is found to remain at

near 1726 cm21 in Fig. 1c, indicating that the Liþ ions

are almost not coordinated with the carbonyl groups of

BPEGP. The changes of absorption bands at about 1351

and 1119 cm21 imply that Liþ ions in this system prefer-

entially coordinate with ether oxygen groups of the PEG

SCHEME 1. Schematic synthesis route of the ionic-conductive plasticizer.

FIG. 1. FTIR of (a) pure DBP, (b) synthesized BPEGP, and (c) BPEGP

coordinated with LiBOB.

58 POLYMER ENGINEERING AND SCIENCE—-2010 DOI 10.1002/pen

molecules, which is in good agreement with the analysis

in the early reported literature [12].

Surface Resistivity Measurement

The effect of molecular weight (MW) of PEG used in

the ester exchange reaction on the surface resistivity of

the PVC/ICP composites are shown in Fig. 2. As can be

seen from Fig. 2, the surface resistivity of the composites

without BPEGP/LiBOB reaches 1013 O sq21 orders of

magnitude. All the synthesized BPEGP/LiBOB plasticiz-

ers with different MWs (400–1000 g/mol) are able to

effectively reduce the surface resistivity of the PVC/ICP

compounds to about 5 3 105 O sq21 at ambient environ-

ment. The surface resistivity of the PVC/ICP composites

ranges from 5.15 to 5.75 3 105 O sq21 as MW of PEG

alters from 400 to 1000 g/mol, revealing that MW of

PEG in BPEGP is not the most important factor affecting

the surface resistivity of the PVC/ICP composites. The

reason behind this is probably accounted for the fact that

the ratio of EO/Li in all ICP samples is a constant (6/1),

and this ratio is not fluctuated with the increase in MW.

Figure 3 demonstrates the effect of temperature on the

surface resistivity of different PVC/ICP blends under a

RH of 12%. The experimental data indicate that the sur-

face resistivity of all samples apparently decreases with

increasing temperature. It can be attributed to the fact that

both the mobility of PEG molecular chains and the diffu-

sion of lithium salts enhance with temperature, which will

be greatly favorable to the enhancement of the antistatic

ability. Figure 3 also reveals that with two exceptions of

PVC/ICP compounds which include those made of

PEG800 and PEG1000, the relationship between tempera-

ture and the surface resistivity of PVC/ICP does not obey

the Arrhenius equation. Moreover, over the whole

testing temperature range, it is evident that the surface

resistivity of the PVC/ICP composites prepared by using

PEG1000 based ICP is higher than that prepared by

PEG800 based ICP.

Tensile Strength and Elongation at Break Results

To find out the effect of MW of PEG on the mechani-

cal properties of the PVC/ICP composites, the tensile

strength and elongation at break tests were carried out at

ambient environment. As can be seen from Fig. 4, when

the MW of PEG is lower than 800, the increasing MW

decreases the tensile strength of the PVC/ICP compounds

from 22 to 11 MPa. The tensile strength of all samples

except for BPEGP prepared by PEG400 is lower than that

of the pure DBP-plasticized PVC material whose tensile

strength approaches 19.8 MPa. On the other hand, the

elongation at break of all PVC/ICP samples blended of

different MWs of PEG is not so good in comparison with

that of the PVC/DBP (100 phr/50 phr) compounds. The

elongation at break of the composites increases from

109.23 to 155.49% when the MW of PEG increases from

400 to 1000 g/mol. The probable reasons behind the me-

chanical properties reduction mentioned earlier are prob-

ably accounted for:

FIG. 2. Effect of the molecular weight of PEG on the surface resistiv-

ity of the PVC/ICP composites.

FIG. 3. Effect of temperature on the surface resistivity of the PVC/ICP

composites.

FIG. 4. Effect of molecular weight on the mechanical properties of the

PVC/ICP composites.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2010 59

1. The content of ICP which includes pure DBP and

BPEGP/LiBOB is too large to maintain an optimum

mechanical property of the PVC/ICP blend.

2. The existence of a concentrated alkali salt (EO/Li

¼ 6/1) may weaken the interactions between the

plasticizer and the PVC polymeric matrix, which will

also influence the mechanical properties remarkably.

The relevant mechanism of that can be illustrated in

Scheme 2.

CONCLUSIONS

BPEGP can be successfully synthesized by ester

exchange reaction of DBP and PEG with different molec-

ular weights. ICPs blended of DBP and the synthesized

BPEGP/LiBOB are able to efficiently reduce the surface

resistivity of the PVC/ICP composites to 105 O sq21

orders of magnitude as BPEGP/LiBOB reaches 25 phr.

The surface resistivity of the PVC/ICP compounds having

an ion-conductive ability is able to be reduced 1 or 2

orders of magnitude as the temperature ranges from 30 to

908C. The introduction of the ion-conductive plasticizer

(i.e., BPEGP/LiBOB) to PVC matrix leads to the reduc-

tion of the mechanical properties of the composites. How-

ever, such ICPs are able to be used for the preparation of

many soft PVC products, especially, which need the per-

sistent antistatic ability and good appearances of the end

manufactures. Furthermore, we are endeavoring to over-

come the mechanical properties reduction of the PVC/ICP

composites by using more suitable molecular design and

selecting a moderate salt concentration. And we have al-

ready developed a superior PVC/ICP (100 phr/60 phr)

compound with both lower surface resistivity (about 107

O sq21) and excellent mechanical properties which almost

obtain the level of pure DBP-plasticized PVC composites.

The relevant details will be reported later.

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SCHEME 2. Schematic interactions among lithium cations, DBP, and

BPEGP.

60 POLYMER ENGINEERING AND SCIENCE—-2010 DOI 10.1002/pen