Thermal degradation behaviors of polydimethylsiloxane-graft-poly(methyl methacrylate)

Thermochimica Acta 573 (2013) 32–38

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Thermal degradation behaviors of polydimethylsiloxane-graft-poly(methyl methacrylate) Hao Li a , Shumei Liu a,b,∗ , Jianqing Zhao a,b,∗∗ , Dahua Li a , Yanchao Yuan a a b

College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China The Key Laboratory of Polymer Processing Engineering, Ministry of Education, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 3 July 2013 Received in revised form 8 September 2013 Accepted 11 September 2013 Available online 1 October 2013 Keywords: Polydimethylsiloxane Polyacrylate Thermal degradation Kinetics

a b s t r a c t Polydimethylsiloxane-graft-poly(methyl methacrylate) (PDMS-g-PMMA) was synthesized by emulsion polymerization. Thermal degradation behaviors of PDMS-g-PMMA were studied by using thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR) and pyrolysis-gas chromatography–mass spectrometry (PY-GC–MS). It was found that a two-stage degradation happened due to PMMA graft chain and PDMS main chain, respectively, and the grafting ratio of PMMA can be calculated through the first-stage weight loss. The 5% weight loss temperature (T5% ) of PDMS-g-PMMA increased with increasing PDMS contents. The apparent activation energy (Ea ) of thermal degradation was evaluated by Kissinger–Akahira–Sunose method and the average value of Ea of PMMA chain in PDMS-g-PMMA was larger than that of pure PMMA. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Polydimethylsiloxane (PDMS) containing Si O Si main chain and methyl side groups processes many excellent properties, such as high flexibility, high hydrophobicity and excellent thermal stability. It is widely used in the aerospace, construction, electronics, automotive and medical field as silicone, silicone oil and silicone rubber [1,2]. But its low surface energy, poor solvent resistance and poor compatibility with other polymers limit its applications to some degree [3–5]. The combination of PDMS with polyacrylates can substantially increase the adhesion, strength, solvent resistance of PDMS, and also maintain the original excellent heat resistance, weather resistance etc. The research on PDMSpolyacrylate copolymer has attracted much interest during the past few decades [6–10]. It is well known that PDMS possess high thermal stability [11]. The 5% weight loss temperature (T5% ) in degradation of trimethylsilyl-terminated polydimethylsiloxane (TMSPDMS) and dimethylvinylsilyl-terminated polydimethylsiloxane (DMVSPDMS) attained 450 ◦ C and 360 ◦ C, respectively [12]. However, the thermal stability of PDMS is reduced by the incorporation of PMMA with poor heat resistance [13–16]. S.D.

∗ Corresponding author at: College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China. Tel.: +86 20 22236818; fax: +86 20 22236818. ∗ ∗ Corresponding author at: College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China. Tel.: +86 20 87113576; fax: +86 20 87113576. E-mail addresses: [email protected] (S. Liu), [email protected] (J. Zhao). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.09.014

Smith et al. [13] combined methyl methacrylate-endcapped polydimethylsiloxane with methyl methacrylate to prepare poly(methyl methacrylate)-graft-polydimethylsiloxane (PSX-gPMMA) through solution polymerization method. The graft copolymers were less thermally stable than PDMS homopolymer, but they were more thermally stable than PMMA homopolymer. Most of polysiloxane-polyacrylate copolymers mentioned above were prepared by solution polymerization. In fact the studies on the preparation of those copolymers by emulsion polymerization have received more attention in recent years [17–20]. Many kinds of polysiloxane-polyacrylate copolymers with different properties can be obtained by controlling the latex particle size distribution and the ratio of the monomers. The copolymers with a core-shell structure have better film-forming property, stability, adhesion and mechanical properties. H.S. Lee et al. [21] prepared latex particles with a polysiloxane-poly(acrylateco-styrene) copolymer core and a poly(acrylate-co-styrene) shell, the copolymer had excellent adhesion force and strength. The polysiloxane-polyacrylate copolymers were generally used as coating and film. The existing studies mainly focused on the control of latex particle size and core-shell structure. Actually, these copolymers after demulsification can be used as impact modifiers in flame-retardant modification of polymer materials due to the presence of silicon element [22]. The high thermal stability is demanded with the impact modifier applications. To our knowledge, however, there were few reports on the relationship between the thermal stability property and the structure of polysiloxane-polyacrylate graft copolymers. In this study polydimethylsiloxane-poly(methyl methacrylate) graft copolymer (PDMS-g-PMMA) was prepared by emulsion

H. Li et al. / Thermochimica Acta 573 (2013) 32–38

polymerization. Its structure and thermal properties were studied by using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and thermo gravimetric analyzer (TGA). Then thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR) and pyrolysis-gas chromatography-mass spectrometry (PY-GC–MS) were used to demonstrate the thermal degradation mechanism.

33

CH3 Si O HC

CH3

CH3 +

4 CH2

O

Si

Si

H3C

+

4

CH3

CH3 O

Si CH3

CH3

N2, 85oC

CH3

8h

2. Experimental 2.1. Materials

CH3 Octamethyl cyclotetrasiloxane (D4), tetramethyl tetravinyl cyclotetrasiloxane (VD4) and hexamethyldisiloxane (MM) were provided by Tianci Organic Silicon Company (China) and were used as received. Methyl methacrylate (MMA), dodecylbenzene sulfonic acid (DBSA) and poly(ethylene glycol) monooctylphenyl ether (OP-10) were purchased from Aladdin Industrial Corporation (China). MMA was purified by distillation and K2 S2 O8 was recrystallized from water just before use.

H3C

Si

CH3 O

CH3

CH3

Si

O

HC

CH2

O

Si

m

CH3 n

CH3

CH3

CH3

N2, 85oC

Si

MMA

6h

2.2. Synthesis of PDMS-g-PMMA

2.3. Characterization FTIR spectra were recorded on powder pressed KBr pellets using a VERTEX70 (Bruke, Germany) spectrometer. 1 H NMR spectra were recorded on an AVANCE300 (Bruke, Germany) spectrometer using CDCl3 as solvent. TGA was performed on a TG209F1 (Netzsch, Germany) thermogravimetric analyzer and dynamic experiments ran at heating rates of 5, 10, 20 and 30 ◦ C min−1 under a nitrogen flow of 20 ml min−1 . TGA-FTIR apparatus was a combination of a T27 (Netzsch, Germany) infrared spectrometer with a STA449C (Netzsch, Germany) thermal analysis instruments. PY-GC–MS measurement system was a CDS5150 (Shimadzu, Japan) pyrolysis apparatus coupled with a

CH3

CH3 H3C

Si

O

Si O

CH3

CH3

CH3

Si O

m

n

Si

CH3

CH3

CH3

CH PMMA

H2 C

Fig. 1. The reaction scheme of PDMS-g-PMMA synthesis.

2010PLUS (Shimadzu, Japan) gas chromatography mass spectrometer. 3. Results and discussion 3.1. Characterization of PDMS-g-PMMA In order to confirm the occurrence of grafting between PDMS and PMMA, the FTIR spectra of pure PDMS, PMMA and PDMS(67)g-PMMA are shown in Fig. 2. Compared with the spectrum of the

(a)

Si-CH3 Transmittance (%)

PDMS-g-PMMA was synthesized by emulsion polymerization. The distilled water, surfactants (DBSA and OP-10) and monomers (D4, VD4 and MM) were added into a four-necked flask equipped with a mechanical stirrer, a thermometer, a reflux condenser and a nitrogen inlet. Before the reaction, nitrogen was added into the flask to remove oxygen. The reaction was carried out for 8 h at 85 ◦ C with stirring at about 500 rpm. After neutralized with NaOH solution to stop the reaction, the final latex of PDMS was obtained. Then a certain quality of MMA was added into PDMS latex. The free-radical copolymerization of PDMS with MMA was performed at 85 ◦ C for 6 h using a K2 S2 O8 initiator. The graft copolymers were precipitated in ethanol and dried under reduced pressure. The product was purified by extensive extraction with acetone to remove any unreacted MMA and nongrafted PMMA, then purified in hexane to remove the non-grafted polysiloxane and surfactants. Through this procedure, a series of PDMS-g-PMMA with different PDMS contents was obtained and designated as PDMS(33)-g-PMMA, PDMS(50)-g-PMMA, PDMS(67)g-PMMA and PDMS(86)-g-PMMA according to weight contents of reactant PDMS in the feed. The reaction scheme is shown in Fig. 1. For comparative purposes, the pure PDMS was obtained from the latex of PDMS mentioned above. The latex was directly dried at 80 ◦ C, then washed with water to remove the surfactants, and then dried again. PMMA was prepared by free radical polymerization of MMA at 75 ◦ C with K2 S2 O8 initiator.

(b)

(c) C=O

Si-CH3 4000

3000

2000

1000 -1

Wavenumbers (cm ) Fig. 2. FTIR spectra of: (a) PDMS; (b) PMMA; (c) PDMS(67)-g-PMMA.

34

H. Li et al. / Thermochimica Acta 573 (2013) 32–38

Fig. 3.

1

H NMR spectrum of PDMS(67)-g-PMMA.

pure PDMS, a new peak at 1730 cm−1 appears due to carboxyl group in the spectrum of PDMS(67)-g-PMMA. Compared with that of pure PMMA, the characteristic absorptions at 1268 cm−1 for C H deformation of Si CH3 , a doublet between 1100 and 1000 cm−1 for Si O Si asymmetric stretching vibration, and at 805 cm−1 for C Si C asymmetric stretching vibration are also observed. The 1 H NMR spectrum of PDMS(67)-g-PMMA is shown in Fig. 3. The signal peaks located at ı 0.02–0.16 ppm account for methyl proton attached to silicon. The two peaks at ı 0.86 ppm and ı 1.04 ppm account for the protons of ordinal ␣, ␤-methylene (␣-CH2 to the Si atom). Additional three peaks at ı 1.61 ppm, ı 1.85 ppm and ı 3.62 ppm account for the protons of methyl, methylene and methoxy in PMMA chain. The peak of solvent CDCl3 locates at ı 7.29 ppm. All confirms that PMMA are successfully grafted onto PDMS main chain. 3.2. TG analysis TGA curves of PMMA, PDMS and PDMS(67)-g-PMMA at a heating rate of 20 ◦ C min−1 under nitrogen are shown in Fig. 4. It can be found that T5% of PDMS(67)-g-PMMA is 357 ◦ C, which is higher than that of PMMA(294 ◦ C), but lower than that of PDMS (472 ◦ C). DTG curves are analyzed with a least-square fitting routine and deconvoluted through multiple Gaussian calculations as Fig. 5. The peak temperature (Tp ), the rate of the weight loss (dWt /dt) and weight loss (W) for each degradation stage are listed in Table 1.

100

Mass (%)

80

c

60

DTG curve of PMMA shows clear evidence of the existence of two-stage degradation, Tp1 and Tp2 are 280 ◦ C and 378 ◦ C, respectively. The weight loss (6.0%) in the first stage degradation may originate from scissions of the unstable head-to-head linkages, and the weight loss (90.9%) in the second stage may arise from scissions at chain-end initiation from vinylidene ends formed by disproportionation during polymerization and the random scission of C C within PMMA chain [13,23,24]. PDMS shows three-stage degradation in DTG curve, Tp1 , Tp2 and Tp3 are 505 ◦ C, 558 ◦ C and 604 ◦ C, respectively. The volatilization of some low molecular weight products with poor thermal stability, mostly six- and eight-membered cyclic siloxy compounds accounts for the least stable stage. The weight loss (8.4%) in the second stage is consistent with the calculated vinyl content (8.0%) according to the feed ratio and the scissions of the unsaturated side chain should account for the proportion of weight loss. The weight loss in the most stable stage is 65.2%, primarily attributed to the heterolytic cleavage of the Si O main chain bonds [12]. Two-stage degradation can be observed and two peaks are fitted in DTG curve of PDMS(67)-g-PMMA. Tp1 and Tp2 are 395 ◦ C and 470 ◦ C, respectively. Its Tp1 is higher than Tp2 (378 ◦ C) of pristine PMMA, but its Tp2 is lower than Tp1 of PDMS, maybe caused by the graft copolymerization. It is found that the weight loss (37.1%) in the first degradation stage is relatively close to the charged percent of MMA (35%) and the weight loss of the second degradation stage (61.1%) is relatively close to the charged percent of PDMS (65%). 3.3. TGA-FTIR analysis TGA-FTIR was performed to find out the products corresponding to every stage degradation of PDMS(67)-g-PMMA. FTIR spectra of their evolved volatile components at typical temperatures of stage 1 and stage 2 are shown in Figs. 6 and 7, respectively. The vibrational bands at 1730, 1170, 2970 cm−1 associated with the C O, C O, CH3 stretching mode can be obviously observed in Fig. 6. All spectra in stage 1 are quite similar to the spectrum of MMA, indicating that the degradation arises from PMMA graft chain. The intensity of these peaks increases with the temperature and reaches to the maximum at 400 ◦ C, but decreases above 400 ◦ C. The characteristic peak of C O at 1730 cm−1 almost disappears at 440 ◦ C, which signifies the completeness of degradation of PMMA graft chain. The vibrational bands at 1268 and 805 cm−1 associated with the Si CH3 symmetric deformation and stretching vibrations, and the Si O Si asymmetric stretching vibration in the range 1100–1000 cm−1 develop from 440 ◦ C. The peak intensity also increases first, reaches to the maximum at 460 ◦ C, then decreases. It is obvious that the degradation product in the second stage originates from PDMS chain. It can be regarded that two stages of the degradation process are independent of each other and PDMS chain follows to degrade only after most of PMMA chains decomposed at ca. 440 ◦ C. For this reason, the grafting ratio of PMMA can be calculated through the first stage weight loss from TGA curves.

b

3.4. PY-GC–MS analysis

a

40

Sample

20 a b c

T /º C 5%

294 PMMA PDMS(67)-g-PMMA 357 PDMS 472

0 200

400

600 o

Temperature ( C) Fig. 4. TGA curves of (a) PMMA, (b) PDMS(67)-g-PMMA and (c) PDMS at 20 ◦ C min−1 under nitrogen.

PY-GC–MS measurement was carried out to discuss in detail degradation processes on the basis of chemical information about volatile products. The sample was placed in the quartz capillary sample holder at the beginning of each series of analyses. The pyrolysis temperature was programmed in two steps to correspond to two-stage degradation of PDMS-g-PMMA. The pyrolyzer initially was set at 50 ◦ C for 60 s, programmed to 440 ◦ C at a rate of 10 ◦ C ms−1 and held for 30 s; the next higher temperature, 700 ◦ C, was programmed and the sample was used throughout the series. As a comparison the sample was directly programmed to

H. Li et al. / Thermochimica Acta 573 (2013) 32–38

35

Table 1 Characteristic parameters of thermal degradation of PMMA, PDMS and PDMS(67)-g-PMMA. Stage 1

Stages

PMMA PDMS PDMS(67)-g-PMMA

Stage 2

Stage 3

Tp1 (◦ C)

dWt /dt (%/min)

W (%)

Tp2 (◦ C)

dWt /dt (%/min)

W (%)

Tp3 (◦ C)

dWt /dt (%/min)

W (%)

280 505 395

1.7 3.1 11.6

6.0 17.8 37.1

378 558 470

28.4 10.8 43.9

90.9 8.4 61.1

604

19.7

65.2

700 ◦ C in another mode. The assigned structures based on mass spectroscopy and measured content for significant pyrolyzate are listed in Table 2. As can be seen from the table, the main pyrolyzate are methyl methacrylate and cyclic siloxanes. When programmed to 440 ◦ C firstly, the relative yield of MMA is 60.9% accompanied with some cyclic siloxanes. But when programmed from 440 ◦ C to 700 ◦ C, the relative yield of MMA is just 8.1%. And the pyrolyzate are mainly cyclic siloxanes in this pyrolysis process. When the sample is directly programmed to 700 ◦ C, the relative yield of MMA is 58.7%, which is very close to the 60.9% programmed to 440 ◦ C. All indicates that PMMA chains decomposed mostly before 440 ◦ C, and the degradation of PDMS chain happens only after PMMA chains decomposed mostly.

Table 2 Measured contents for significant pyrolyzate of PDMS(67)-g-PMMA. Pyrolysis temperature/◦ C

Pyrolysis productsa

Sample 1

MMA D3 D4 D5 D6 D7 D8 Others

Sample 2

440

440–700

700

60.9 0 0.8 5.2 5.8 4.1 1.0 22.2

8.1 14.3 8.1 9.8 14.1 11.5 2.3 31.8

58.7 5.9 2.4 2.4 3.8 3.2 1.0 22.6

a D3, D4, D5, D6, D7 and D8 are cyclic siloxanes with different silicon atom numbers in the structure.

0

-5

0

280ºC -5

505 ºC

DTG(%/min)

-15

-20

-10

558º C -15

-25 -20

-30 100

604 ºC

378ºC 200

300

400

500

600

300

400

500

Temperature(º C)

Temperature(º C)

(a)

(b)

0

-10

DTG(%/min)

DTG(%/min)

-10

395 º C

-20

-30

-40

470º C

-50

-60 200

300

400

500

600

Temperature(º C)

(c) Fig. 5. DTG and its fitted curves of (a) PMMA, (b) PDMS and (c) PDMS(67)-g-PMMA.

600

700

36

H. Li et al. / Thermochimica Acta 573 (2013) 32–38 Table 3 Grafting ratio results of PMMA for four PDMS-g-PMMA from TGA curves. Graft copolymers

Feed ratio of PDMS/MMA

Theoretical PMMA proportiona

Grafting ratio of PMMA

PDMS(33)-g-PMMA PDMS(50)-g-PMMA PDMS(67)-g-PMMA PDMS(86)-g-PMMA

0.5/1 1/1 2/1 6/1

66.7% 50.0% 33.3% 14.3%

64.0% 50.0% 35.0% 14.0%

a Theoretical PMMA proportion is calculated through the feed ratio of PDMS and MMA.

The method examines the multiple heating rate kinetics. Ea of degradation is calculated from TGA curves based on series of experiments performed at various heating rates (ˇ). Ea is calculated from the slopes of the lines in the plot of ln(ˇ/T2 ) versus 1/T using the following expression: Ea = −slope × R

Fig. 6. FTIR spectra of the evolved volatile components at typical temperatures of (a) stage 1 and (b) stage 2 of PDMS(67)-g-PMMA.

3.5. Thermal degradation kinetics analysis The apparent activation energy (Ea ) of thermal degradation can be determined using the method of Kissinger–Akahira–Sunose [25] according to the following equation:

 ln

ˇ T2

 =

Const − Ea RT

(R represents gas constant)

A series of PDMS-g-PMMA with different PDMS contents was prepared by varying the feed ratio of PDMS and MMA as listed in Table 3. TGA curves of the copolymers under nitrogen are shown in Fig. 8. All graft copolymers clearly show obvious two-stage degradation. T5% of the graft copolymers increases from 333 ◦ C to 389 ◦ C as PDMS content increases from 33.3% to 85.7%. It is evident that the thermal stability of the graft copolymers enhances with PDMS contents. The first stage weight loss, corresponding to the grafting ratio results of PMMA in PDMS-g-PMMA, is close to the theoretical proportion of PMMA calculated by the charged amounts. Ea of thermal degradation of PDMS-g-PMMA series is calculated through the Kissinger–Akahira–Sunose method and the plots of Ea versus ˛ (˛ represents the conversion degree) of the two degradation stages are shown in Fig. 9. As can be seen, at the lower conversions (up to ˛ = 0.2), the value of Ea increases with ˛ increasing for the first degradation stage. After ˛ = 0.2, in the conversion range of 0.2 ≤ ˛ ≤ 0.8, the Ea value is almost stable. The value of Ea relatively constant with respect to conversion degree in the range of 0.1 ≤ ˛ ≤ 0.8 for the second degradation stage. The average value of Ea (Ea,av , 0.2 ≤ ˛≤ 0.8) of thermal degradation of PMMA graft and PDMS main chain for four graft copolymers is summarized in Table 4 compared to that of pure PMMA and PDMS. It is found that Ea,av of PMMA degradation in four PDMS-g-PMMA is larger than that of pure PMMA, and PDMS(50)-g-PMMA is provided with the largest Ea,av . The thermal stability of PMMA is enhanced by PDMS probably due to a retarding effect of the PDMS chain on scissions of PMMA chain. In addition, Ea of PDMS chain in PDMS-g-PMMA is quite equivalent and Ea,av of PDMS chain increases as PDMS content 100

Si-O-Si

d

80

Si-CH3 Absorbance

c

Mass (%)

(b)

C-O

60

b a

40

C=O

PDMS(33)-g-PMMA

b

PDMS(50)-g-PMMA

343

c

PDMS(67)-g-PMMA

357

d 0 100

PDMS(86)-g-PMMA

389

20

(a) 4000

3500

3000

2500

T 5%/º C

Sample a

2000

1500

1000

500

-1

Wavenumbers (cm )

200

300

333

400

500

600

700

o

Temperature ( C) Fig. 7. FTIR spectra of the evolved volatile components of PDMS(67)-g-PMMA at (a) 400 ◦ C and (b) 460 ◦ C.

Fig. 8. TGA curves of four PDMS-g-PMMA at 20 ◦ C min−1 under nitrogen.

H. Li et al. / Thermochimica Acta 573 (2013) 32–38

37

300

Second stage

250

PDMS(86)-g-PMMA PDMS(67)-g-PMMA PDMS(50)-g-PMMA PDMS(33)-g-PMMA

200

Ea (kJ/mol)

Ea (kJ/mol)

250

150

100

First stage PDMS(86)-g-PMMA PDMS(67)-g-PMMA PDMS(50)-g-PMMA PDMS(33)-g-PMMA

50

150

0 0.0

0.2

0.4

0.6

0.8

200

1.0

Conversion degree, α

0.0

0.2

0.4

0.6

0.8

1.0

Conversion degree, α

Fig. 9. The plot of Ea versus ˛ for the two degradation stages of PDMS-g-PMMA.

Table 4 Ea,av of PMMA and PDMS chain calculated by Kissinger–Akahira–Sunose method. Polymers

Ea,av of PMMA chain (kJ mol−1 )

Ea,av of PDMS chain (kJ mol−1 )

PMMA PDMS PDMS(33)-g-PMMA PDMS(50)-g-PMMA PDMS(67)-g-PMMA PDMS(86)-g-PMMA

183.1 ± 8.1 N/A 236.5 ± 1.6 255.2 ± 1.8 227.2 ± 4.2 225.1 ± 5.2

N/A 209.4 ± 3.4 197.2 ± 2.2 224.1 ± 1.7 213.3 ± 4.0 172.5 ± 7.7

increases from 33.3% to 66.7%. Ea,av of PDMS chain for PDMS(86)g-PMMA is lower than others maybe because the flexible PDMS chain with better heat dissipation reduces the temperature of the graft copolymer [9,14]. Besides, Ea,av of PMMA chain degradation is larger than that of PDMS chain degradation in graft copolymer, which is opposite to Ea,av value of pure PMMA and PDMS, maybe due to the existence of defect in PDMS main chain caused by the first degradation of PMMA graft chain. 4. Conclusion A series of PDMS-g-PMMA with different PDMS contents was synthesized by emulsion polymerization. TGA analyses indicated that the graft copolymers all possess greatly improved thermal stability, their degradation temperatures at 5% weight loss were up to 330 ◦ C, and enhanced as PDMS contents. A two-stage thermal degradation behavior due to PMMA graft chain and PDMS main chain was confirmed by TGA-FTIR and PY-GC–MS results. Ea,av of thermal degradation of PMMA chain in PDMS-g-PMMA evaluated by Kissinger–Akahira–Sunose method was larger than that of pure PMMA. PDMS-g-PMMA has a great potential as impact modifier in flame-retardant modification of polymer materials. Acknowledgments This research work was supported by a grant from the Cultivation Fund of the Key Scientific and Technical Innovation Project, Education Department of Guangdong province (cxzd1008) and Joint Fund of NSFC with Guangdong Provincial Government (U1201243). References [1] A.J. O’Lenick, Silicones-basic chemistry and selected applications, J. Surfactants Deterg. 3 (2000) 229–236.

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