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Applied Surface Science 258 (2012) 8861– 8866

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

ntistatic modification of polypropylene by incorporating Tween/modifiedween

nna Zhenga,∗, Xiang Xua, Huining Xiaob, Na Lia, Yong Guana, Shuzhao Lia

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237,hinaDepartment of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C1

r t i c l e i n f o

rticle history:eceived 5 March 2012eceived in revised form 15 May 2012ccepted 19 May 2012vailable online 28 May 2012

a b s t r a c t

Tween, one type of non-ionic surfactants, was used as inner antistatic agent of polypropylene (PP) and theantistatic performance of PP/Tween sheets were evaluated by surface resistivities (�s) and water contactangles. The influence factors including hydrophile–lipophile balance (HLB) of Tween, addition amount,process conditions and atmospheric humidity were investigated in detail and the results showed Tween40 with HLB at 15.7 provided PP an optimum antistatic effect with surface resistivity of 1010 �/sq, water

◦

eywords:weenolypropylenentistaticrosslinkingurface treatment

contact angle of 57 , and surface energy of 49.5 mN/m. The �s of PP/Tween sheets declined approximately1 order of magnitude with 10% increasing of relative humidity. In order to improve the washing enduranceof antistatic PP, a combination of T40 and cationic PP (CPP) were blended with PP and PP/CPP/T40 sheetsrevealed improved washing durability, with �s below 1011 �/sq after ultrasound washing. In addition,T40 with double bonds was synthesized and UV crosslinking of modified T40 on PP surface also presenteddefinite effects on water resistance.

. Introduction

Polypropylene (PP) is one of the most widely used thermal-lastics in packaging material, textile, electric appliances, andutomobile parts. However, PP products are easily accumulatedlectrostatic charges due to the inherent high insulativity, caus-ng troubles such as electric shocks and damages to the electronic

achines [1–3]. In particular, the biaxially oriented polypropyleneBOPP) films are easy to attract dusts and cling together due to thelectrostatic charges generated by friction, thus their productivitys influenced significantly. In order to eliminate the static, the wayf blending antistatic agents into the PP matrix is popular underurrent circumstance [2,3].

Many researchers are engaged in looking for novel antistaticgents, such as carbon nanotubes [4,5], intrinsically conductiveolymers (ICP) [6–10], ionic polymers [11], and so on. Such conduc-ive additives eliminate the electrostatic by forming a continuousonducting network inside the plastic matrix [12]. The drawbacksie on large quantities of conductive additives (usually above 5%)eing needed to form transmission paths, so the reduction of the

ptical transparency and alteration of the mechanical propertiesften limit the application range, especially for BOPP films.

∗ Corresponding author. Tel.: +86 18917949223.E-mail address: zan@ecust.edu.cn (A. Zheng).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.05.105

© 2012 Elsevier B.V. All rights reserved.

Up to now, the widely used antistatic agents in industry are atype of low molecular weight surfactants, which are the “soap-like”compounds with a hydrophobic and a hydrophilic part [13], includ-ing cationic antistatic agents (e.g. dimethyl octadecyl hydroxy ethylammonium nitrate), anionic antistatic agents (e.g. alkyl phosphatediethanol amine salt), non-ionic antistatic agents (e.g. polyethy-lene glycol octadecyl ether) and ampholytic antistatic agents (e.g.1-carboxyethyl-2-alkyl imidazoline sodium salt). Among them,non-ionic antistatic agents are more popular due to their non-toxicity, good thermal stability, and high transparency. Whenblended with plastics with low addition amount (usually less than1%), such surfactants migrate to the surface and enhance the surfaceconductivity by attracting a layer of water.

However, the antistatic agents with low molecular weight areeasily removed and washed out from the surface, consequently lim-iting their effect over time. In order to immobilize the antistaticagents, UV crosslinking of modified surfactants on the surface isinvestigated in current research. Such method has scarcely beenreported and is potential to be applied in industry due to its con-venience.

According to the previous research [14], thehydrophile–lipophile balance (HLB) of the surfactant is of greatsignificance to its antistatic performance. In order to find antistatic

agents with proper HLB, Tween and Span surfactants, which havedifferent HLB values, were investigated as antistatic agents forPP. As shown in Scheme 1, Tween and Span surfactants are non-ionic surfactants with similar polysorbate structures, which are

8862 A. Zheng et al. / Applied Surface Sci

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It is generally accepted that the antistatic mechanism for non-ionic antistatic agents is that, the low molecular weight agentsmigrate to the plastic surface and improve the surface conduc-tivity by attracting a layer of water [14]. It can be concluded that

Scheme 1. Structures of Tween and Span surfactants.

ominated by the alkyl groups such as T20 (laur-), T40 (palmic-),60 (stearic-) and T80 (oleic-). T65 and T85 have three stearic andleic groups in one molecule, respectively, while Span surfactantsave no polyether in comparison to Tween. Such surfactants showifferent hydrophilic–hydrophobic properties and are widelysed in food additives [15], cosmetics [16], detergents [17] andelf-assembly [18]. However, being used as antistatic agents for PPs rarely reported.

In this work, Tween and Span surfactants were blended with PPo fabricate antistatic PP sheet samples. The antistatic properties ofheet samples were evaluated by means of surface resistivities andater contact angle. The influence factors of antistatic properties,

ncluding structure, process conditions, and relative humidly (RH),ere elaborated. In order to improve the washing endurance, two

easible methods including combination with cationic polypropy-ene and surface crosslinking of modified T40 were presented.

. Experimental

.1. Materials

PP (T36F) was purchased from China Petroleum and Chemicalorp. Tween (T20, T40, T60, T80, T65, T85), Span (S20, S40), acry-

oyl chloride, and acrylic acid (AA) were supplied from Sinopharmhemical Reagent Co., China. Cationic polypropylene master batchCPP) contained 2% guanidine groups, was purchased from Shang-ai Fuyuan Rubber-plastics Co. Ltd. used without further treatment.,4,6-Trimethyl benzoyl diphenylphosphine oxide (TPO) was useds the photo initiator with absorption wavelength at 299, and66 nm.

.2. Modification of T40

A mixture of 0.1 mol T40 and 0.33 mol acryloyl chloride weredded into a 100 mL round bottom flask equipped with magnetictirring. After continuous stirring for 24 h at room temperature, theroduct was purified by rotary evaporation to remove the unre-cted acryloyl choride, denoted as MT40. Its molecular structureas characterized by FTIR using a Nicolet 5700 infrared spectrom-

ter (Thermo Electron Scientific Instruments Corp., USA), and 1HMR using a Varian Unity 400 proton and multinuclear magnetic

esonance spectrometer.

.3. Preparation of antistatic PP sheet samples

Prescribed percent of PP, Tween (or Span) and antioxygen 10100.2 wt%) were blended in a internal mixer (SU-70, Changzhou

uyan Science and Technology Corp., China) at 180 ◦C, 60 rpm for

min. The blends were moulded into 160 mm × 180 mm × 1 mmheees firstly with a 25 t plate vulcanization machine XQLB-50 × 350 at 200 ◦C, 7.5 MPa for 5 min, followed by cold pressing

ence 258 (2012) 8861– 8866

at room temperature, 10 MPa for 5 min. The sheet samples werecut into 80 mm × 90 mm × 1 mm sheets for testing.

To fabricated the antistatic PP samples with immobilized anti-static agents, PP, antistatic agent combination (MT40, TPO) andantioxygen 1010 (0.2 wt%) were made into sheet samples firstly andthen the sheets were treated under a 500 W UV light for a prescribedtime.

2.4. Characterizations of PP sheet samples

Surface resistivities of PP/Tween sheets were measured using aZC-36 high resistance meter (Shanghai Precision Instruments Co.,China) at 23 ◦C, relative humidity 65% (65% RH), 500 V voltage,referred to China national standard GB/T1410-1989. The reportedvalues are the average of the measurements of three sheets. Contactangles were measured with an optical contact angle meter JC2000D(Shanghai powereach Corp., China) at ambient temperature. Waterdroplets were dropped carefully onto the surfaces and the averagevalue of five measurements at different positions of the sample wasadopted as the contact angle.

The wash durability of sheet samples was evaluated by com-parison of their �s before and after washing. With respect towashing the sheet samples suffered ultrasonic processing in waterfor 30 min at the frequency of 40 kHz, and then exposed in air to bedried at 23 ◦C, 65% RH for 24 h.

3. Results and discussion

3.1. Influence of hydrophile–lipophile balance of surfactant onantistatic performance

Fig. 1 shows �s of PP sheets incorporated 0.5% Tween andSpan surfactants, respectively. It can be seen that the �s of PPsheets contained Tween or Span significantly depend on thehydrophile–lipophile balance (HLB) and have decreased 1–4 ordersof magnitude in comparison to the neat PP sheet. According tothe military handbook DOD-HDBK-263 [19], the antistatic mate-rial has a �s between 109 and 1014 �/sq. The industry standardsANSI/EIA-541-1988 (USA) and ANSI/ESD S541-200 (USA) alsodefined the electrostatic dissipative (ESD) material having the �s

at 105 to 1012 �/sq and 104 to 1011 �/sq respectively. Therefore,the PP samples incorporated 0.5% Tween has reached the antistaticrequirement, while the Span and PEG1000 samples have limitedantistatic performance. In particular, T40 provides an optimumantistatic effect (�s = 6.3 × 109 �/sq) measured after moulding.

Fig. 1. �s of PP/Tween (0.5%) sheets and PP/Span (0.5%) sheets measured in the firstand 21st day at 23 ◦C, 65% RH.

A. Zheng et al. / Applied Surface Science 258 (2012) 8861– 8866 8863

Table 1�s, contact angles and surface energies of PP/T40 sheets.a

Addition of T40 �s (�/sq) Contact angle (◦) Surface energy(×10−3 N/m)

Water Diiodomethane

N/A 1.22 × 1015 94.6 56.1 30.90.2% 5.69 × 1013 79.0 53.6 35.10.5% 1.50 × 1010 57.1 40.4 49.6

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attract enough water from the atmosphere, resulting in poor anti-static performance. With the relative humidity increasing, moreand more water is attracted by surfactants, leading to higher surfaceconductivities.

1.0% 2.41 × 1010 59.0 40.0 48.7

a Measured at 23 ◦C, 50% RH.

he antistatic ability of non-ionic surfactant is determined by itsobility and hygroscopicity. The mobility depends on the compat-

bility with PP, and the compatibility is influenced by the HLB ofurfactants. When the HLB value is low, the surfactant with highydrophobicity has a strong interaction with PP. As a result, theobility slows down and the �s remains little changed. With the

ncreasing of HLB value, the interaction between surfactant and PPecreases, leading to a faster migration of surfactant and betterntistatic performance of plastics. However, when the HLB value isore than 16.7, the surfactant is too hydrophilic to have enough

ompatibility, causing aggregation inside the PP matrix. There-ore, the antistatic performance declines. According to this analysis,40, with a hydrophobic tail (C15H31–) and four hydrophilic arms,rovides a proper hydrophile–lipophile balance, so that it can benriched on the plastic surface rapidly and improve the surface con-uctivity conspicuously. While the HLB value is too low (e.g. S40)r too high (e.g. PEG1000), the surfactants present slight antistaticffect due to their slow migrations.

After three weeks, most sheet samples reveal a slight decreasen the �s, indicating that the antistatic agents inside the matrixould migrate to the surface with the time prolonged. �s of PP/T40

heet remains unchanged, suggesting that T40 surfactant is satu-ated on the PP surface after being moulded.

.2. Influence of addition amount on antistatic performance

Table 1 displays �s, contact angles and surface energies ofP/T40 sheets incorporated 0.2%, 0.5% and 1% T40, respectively.urface energies calculated via a harmonic-mean method [20] fromater contact angles and diiodomethane contact angles can reflect

he polarity of sample surface. Compared with neat PP sample,P/T40 (0.5%) presents excellent antistatic performance with �s at.50 × 1010 �/sq. When the addition amount increases to 1%, thes as well as the surface energy reveal a slight change, indicating

hat T40 surfactant is saturated on the PP surface with 0.5% addi-ive. In other words, the outermost surface of PP is crammed with40 molecules, while larger addition amount cannot increase theolarity of PP surface. With respect to PP/T40 (0.2%), T40 surfactant

s not saturated on the PP surface and the polarity declines, result-ng in a lower surface energy and a higher �s. In summary, withhe addition amount of T40 increasing, the antistatic performancef PP/T40 sheet samples improves at the beginning and reaches alateau in the end, with the turning point of addition between 0.2%nd 0.5%.

.3. Influence of process conditions on antistatic performance

The influence factors of process conditions including hot pressime and temperature were investigated as shown in Fig. 2. Whenhe hot press time is less than 7 min, the �s of PP/T40 (0.5%) sheets

ecrease with temperature increasing. While the hot press time isrolonged to 12 min, the distinction of temperature is slight, andhe antistatic performance declines in comparison to samples hotressed for 7 min.

Fig. 2. �s of PP/T40 (0.5%) sheets prepared in different temperatures and hot presstimes, measured at 23 ◦C, 50% RH.

It is noted that the component with similar surface energy isprone to be enriched on the interface, forming a transition layer[20–22]. During the process of hot press, the polar componentwould be enriched on the PP surface due to the “inductive effect” ofthe steel mould. This is one important driving force of the migrationdiscussed above. With the temperature increasing, the viscosity ofthe melt decreases and the mobility of T40 molecules is enhanced,thus the �s decreases. In this case, the prolongation of the hot presstime would accumulate more antistatic agents on the surface, lead-ing to an improvement of the antistatic performance. However, T40molecules would lose antistatic ability when contacting with thesteel mould for a long time due to the decomposition.

3.4. Influence of atmospheric humidity on antistatic performance

Fig. 3 depicts the relationship of relative humidity and the �s

of PP/Tween sheets. It can be seen the �s is significantly depen-dent on the relative humidity. When the atmospheric humidity isbelow 28% RH, �s are constant at high values (above 1013 �/sq).While the relative humidity is above 28% RH, the �s show almostlinear decreases with the relative humidity increasing. The aver-age slope calculated by fitting the curves is −11.52, demonstratingthat every 10% increase of RH would cause about 1 order of mag-nitude reduction of �s. The results are well in accordance withprevious research [23]. As known, the electrostatic charges are dis-sipated from the “water layer” attracted by antistatic agents. Such“water layer” has a dynamic balance with vapour in the environ-ment. In low atmospheric humidity, the antistatic agents cannot

Fig. 3. �s of PP/Tween (0.5%) sheets measured in different relative humidities.

8864 A. Zheng et al. / Applied Surface Science 258 (2012) 8861– 8866

Table 2Water contact angles of PP/Tween (0.5%) sheets measured at different relativehumidities/◦ .

50% RH 40% RH 20% RH

T20 0.5% 61.3 73.3 69.3T40 0.5% 57.0 55.0 56.6

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Fig. 5. FTIR spectra of T40 and MT40.

Table 3�s of PP/Tween and PP/CPP/T40 sheets before and after washing.a

Antistatic agent Before washing(�/sq)

After washing(�/sq)

T20: 0.5% 5.05 × 1010 1.39 × 1015

T40: 0.5% 7.07 × 109 1.22 × 1015

T60: 0.5% 1.32 × 1010 1.43 × 1015

T85: 0.5% 5.88 × 1010 1.18 × 1015

CPP/T40: 10%/0.5% 5.13 × 109 6.92 × 1011

CPP/T40: 10%/1% 1.55 × 1010 1.2 × 1012

9 12

T60 0.5% 64.6 54.3 62.0T85 0.5% 58.6 53.8 59.1

Water contact angles of PP/Tween samples incorporated of 0.5%ween were measured in the atmospheric humidity 50% RH, 40%H and 20% RH. As shown in Table 2, the water contact angles reveallight changes with the variation of atmospheric humidity. It is notifficult to understand that the water contact angle is dependent onhe cohesive energy density and the polarity of the material, whichre not influenced by the humidity. This demonstrates the presencef a layer of surfactants on the PP surface, which is not influencedy the atmospheric humidity.

.5. Improvement of the washing durability by combination withationic polypropylene

In order to improve the washing durability of PP/T40 sheets, aombination of T40 and cationic polypropylene (CPP) was blendedith PP, and the �s of PP/T40/CPP sheets are shown in Fig. 4. CPP is

ne type of functionalized polypropylene master batch contained% cationic guanidine groups. In comparison to the PP/CPP (5%)ample, the introduction of T40 improved the antistatic perfor-ance, with �s changed from 1014 �/sq to 1010 �/sq. In addition,

he introduction of CPP also reveals a slight improvement for PP/T40

0.5%) sheets, indicating CPP and T40 display a synergistic effect inhe antistatic performance.

The washing durability of PP/Tween and PP/CPP/T40 sam-les was evaluated by comparison of their �s before and after

ig. 4. �s of PP/CPP/T40 sheets with variation of (a) CPP and (b) T40, measured at3 ◦C, 50% RH.

CPP/T40: 5%/0.5% 6.03 × 10 1.15 × 10CPP/T40: 20%/0.5% 4.68 × 109 1.62 × 1011

a Measured at 23 ◦C, 50% RH.

washing. As shown in Table 3, both PP/Tween and PP/CPP/T40samples exhibit excellent antistatic properties before washing,with �s between 109 and 1011 �/sq. After ultrasonic cleaning,PP/Tween samples lose the antistatic performance completelywhile PP/CPP/T40 samples remain to be antistatic, with �s between1011 and 1012 �/sq. Obviously, Tween on the surface is easilywashed off due to its low molecular weight, while CPP, whose polargroups are grafted on the polypropylene, remains and plays a partin electrostatic dissipation.

3.6. Immobilizing T40 by UV crosslinking

3.6.1. Synthesis of MT40As is discussed above, PP/Tween samples have poor washing

durability since the low molecular weight surfactants are easily

washed off. In order to improve the washing durability, immobiliz-ing the surfactants on the surface is investigated by UV crosslinkingmethod. As shown in Scheme 2, T40 was modified with acryloylchloride to obtained double bonds. The modified T40 (MT40) was

Fig. 6. 1H NMR spectrum of MT40.

A. Zheng et al. / Applied Surface Science 258 (2012) 8861– 8866 8865

Table 4The variation of antistatic agent layer exposed under UV light.

Antistatic agent Components UV exposure

5 s 30 s

AS 0 MT40: 100% Uncrosslinked UncrosslinkedAS 1 MT40/TPO: 90%/10% Crosslinked, thin film Crosslinked, thin filmAS 2 MT40/TPO/AA: 85%/5%/10% Crosslinked, thin film Crosslinked, thin film

Table 5�s of UV-irradiated and untreated PP/AS sheetsa before and after washing/�/sq.

Antistatic agent Dosage (%) Untreated UV exposure 30 s

Before washing After washing Before washing After washing

AS 0 1.0 2.4 × 1010 3.2 × 1014 7.2 × 1010 1.5 × 1014

AS 1 0.5 2.9 × 1011 5.2 × 1014 2.0 × 1011 1.2 × 1013

1.0 3.8 × 1010 4.4 × 1014 2.9 × 1011 5.7 × 1012

1.5 1.3 × 1010 7.1 × 1013 6.1 × 109 3.3 × 1011

AS 2 0.5 6.5 × 1011 9.8 × 1014 1.1 × 1012 1.8 × 1013

1.0 1.6 × 1010 7.8 × 1013 2.0 × 1010 1.5 × 1011

1.5 1.5 × 1010 2.6 × 1013 1.3 × 1010 1.6 × 1012

a Measured at 23 ◦C, 50% RH.

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Scheme 2. S

lended with PP to fabricate PP/MT40 sheets, and the PP/MT40heets were irradiated under UV light to crosslink the MT40.

The structure of MT40 is characterized by FTIR and 1H NMR anal-sis as shown in Figs. 5 and 6, respectively. Compared with T40, the

ew peak at 1635 cm−1 corresponds to the double bond (see Fig. 5),nd the rest peaks reveal no difference with T40, indicating a suc-essful synthesis. In the 1H NMR spectrum, the signals appearedn 0.8–1.4 ppm, 3.4–3.8 ppm and 5.8–6.5 ppm are attributed to the

Fig. 7. Proposed mechanism for antist

sis of MT40.

alkyl protons, ethyl ether protons and vinyl protons, respectively,which also provide the evidence of the designed structure.

3.6.2. The crosslinking effect of MT40

The crosslinking effect of MT40 was investigated by coating

MT40 or its complex on the glass slide, and then the glass slidewas exposed under UV light for 5 s or 30 s. As is shown in Table 4,antistatic agent incorporated of TPO (AS1, AS2) can be crosslinked

atic PP with washing durability.

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866 A. Zheng et al. / Applied Surfa

ithin 5 s, while antistatic agent without TPO (AS0) cannot berosslinked even being exposed for 30 s. The results indicate that its possible for the MT40 layer accumulated on the PP surface to berosslinked in the presence of TPO.

.6.3. Immobilizing MT40 by UV crosslinkingTable 5 shows the �s of UV-irradiated and untreated PP/AS

heets with different addition amounts before and after wash-ng. It can be seen that before washing both UV-irradiated andntreated sheets reveal good antistatic performance with �s below011 �/sq. After washing, the untreated samples lost the antistaticffect, while a part of the UV-irradiated samples including PP/AS11.5%) and PP/AS2 (1%) present definite antistatic effect with �s at011 �/sq. This indicates a portion of MT40 molecules were immo-ilized on the surface by crosslinking.

Fig. 7 depicts antistatic PP with washing durability. As shown,he MT40 molecules enriched on the surface are easily washed off.

hile the samples are irradiated with UV light, the photoinitiatorPO decompose into free radicals, causing the crosslinking of theouble bonds. The crosslinked MT40 has high molecular weight andlenty of alkyl “anchors”, preventing it to be washed off. However,uch crosslinking is inadequate and further investigation will bearried on in future.

. Conclusions

In summary, we demonstrate that Tween surfactants are effec-ive antistatic agents for PP. The antistatic performance of PP/Tweenamples are influenced by the HLB of Tween, addition amount, pro-ess conditions and atmospheric humidity. In particular, PP/T400.5%) sample exhibits an optimum antistatic ability with surfaceesistivity of 1010 �/sq, water contact angle of 57◦, and surfacenergy of 49.5 mN/m. A combination of T40 and CPP has improved

he washing durability of antistatic PP samples, with �s at 1011 �/sqfter ultrasound clean. In addition, UV crosslinking of modified T40n PP surface also presents definite effects on the improvement ofashing durability.

[

ence 258 (2012) 8861– 8866

Acknowledgements

The research was supported by Shanghai Leading AcademicDiscipline Project (B502) and Shanghai Key Laboratory Project(08DZ2230500).

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