Phosphorus, Sulfur, and Silicon and the Related

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Phosphorus, Sulfur, and Silicon and the Related Elements

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Hydrolysis-Condensation Kinetics of Different Silane Coupling Agents Marie-Christine Brochier Salona; Mohamed Naceur Belgacema a LGP2, Grenoble INP-Pagora, BP 65, Domaine Universitaire, St Martin d'Hères, France Online publication date: 19 February 2011

To cite this Article Brochier Salon, Marie-Christine and Belgacem, Mohamed Naceur(2011) 'Hydrolysis-Condensation

Kinetics of Different Silane Coupling Agents', Phosphorus, Sulfur, and Silicon and the Related Elements, 186: 2, 240 — 254 To link to this Article: DOI: 10.1080/10426507.2010.494644 URL: http://dx.doi.org/10.1080/10426507.2010.494644

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Phosphorus, Sulfur, and Silicon, 186:240–254, 2011 C Taylor & Francis Group, LLC Copyright ISSN: 1042-6507 print / 1563-5325 online DOI: 10.1080/10426507.2010.494644

HYDROLYSIS-CONDENSATION KINETICS OF DIFFERENT SILANE COUPLING AGENTS Marie-Christine Brochier Salon and Mohamed Naceur Belgacem

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LGP2, Grenoble INP-Pagora, BP 65, Domaine Universitaire, St Martin d’Hères, France GRAPHICAL ABSTRACT

Abstract The hydrolysis kinetics of 14 alkoxy silane coupling agents were carried out in an ethanol:water 80:20 (w/w) solution under acidic conditions and were monitored by 1H, 13C, and 29Si NMR spectroscopy. Acidic conditions were selected in order to enhance the silanol formation and to slow down the self-condensation between the resulting hydrolysed silanol groups. In situ 29Si NMR spectroscopy allowed the determination of the intermediate species as a function of the reaction time. Thus, the following silane coupling agents were studied: 3-methacryloxypropyl trimethoxy silane (MPMS), 3-mercaptopropyl trimethoxy silane (MRPMS), 3-cyanopropyl triethoxy silane (CPES), triethoxy vinyl silane (VES), trimethoxy (2-phenylethyl) silane (PEMS), octyl triethoxy silane (OES), trimethoxy (7-octen-1-yl) silane (OEMS), 3-aminopropyl triethoxy silane (APES), 3-aminopropyl trimethoxy silane (APMS), 3-(2-aminoethylamino)propyl trimethoxy silane, (DAMS), 3-[2-(2-aminoethylamino)-ethylamino]propyl trimethoxy silane (TAMS), 4-amino-3,3-dibutyl trimethoxy silane (ADBMS), trimethoxy [3-(phenylamino)propyl] silane (PAPMS), and

Received 28 March 2010; accepted 15 May 2010. The authors thank Yves Gentil from GE Bayer Silicones (Switzerland) for the gift of the ADBMS silane coupling agent used in this work. Address correspondence to Mohamed Naceur Belgacem, LGP2, Grenoble INP-Pagora, BP 65, Domaine Universitaire, F-38402 St Martin d’Hères, France. E-mail: [email protected] 240

HYDROLYSIS-CONDENSATION KINETICS

241

triethoxy-3-(2-imidazolin-1-yl) propyl silane (IZPES). A parameter quantifying the grafting potentiality of each silane coupling agent towards OH-rich solid substrates (such as cellulose) was established as a function of the nature of the alkoxy groups (methoxy or ethoxy), as well as that of the fourth substituent (vinyl, aminopropyl, etc.) of the silane studied. Keywords Grafting activity; hydrolysis; kinetics; self condensation; pling agents

29Si

NMR; silane cou-


INTRODUCTION Functional trialkoxysilanes, R Si(OR)3 , are widely used in numerous industrial applications as coupling agents to enhance the adhesion between polymeric matrices and inorganic solids.1–5 The mechanisms associated with the involved coupling reactions are ensured thanks to the presence of two types of reactive moieties borne by these chemicals. In fact, on one hand, alkoxy groups OR (as such or after hydrolysis) enable the silane reagent to be anchored onto the surfaces bearing hydroxyl groups,6–8 whereas organic functionality R (amine, methacrylic, vinylic, cyano, phenyl, etc.) improves their compatibility, or even induces their copolymerization with organic matrices, thus enhancing the interfacial adhesion between the matrix and the reinforcing elements.9,10 Some systematic investigations have already been undertaken involving different coupling agents. This article summarizes the most relevant silanes studied up to now by our group and aims to establish comparison of their behavior towards hydrolysis. The final goal of such an investigation strategy is to better understand the kinetics behavior of this family of grafting agents and to establish clear-cut conditions favoring the grafting of cellulose substrates with silanes. In fact, when subjecting cellulose to grafting with prehydrolyzed silanes, consecutive and competitive reactions (hydrolysis and self condensation) occur, which make the study of cellulose–silane interaction impossible, without good knowledge of the mechanisms and the kinetics associated to the silane under investigation. Our group has already shown that in situ NMR (1H, 13C, 29Si) spectroscopy (in solution and/or in solid state) was found to be a very powerful tool in understanding hydrolysis-solvolysis kinetics and mechanisms.11 29Si NMR was particularly useful because it offers the possibility of establishing the nature of the different species present in the reaction medium. The feasibility of this method was demonstrated for two amino silanes (APES and TAMS) and for a methacrylic derivative (MPMS),11,12 under different pH conditions. These results were confirmed by observing the behavior of two other silanes, namely 3-mercaptopropyl trimethoxy silane (MRPMS) and a purely aliphatic derivative, octyl triethoxy counterpart (OES). OES was found to undergo hydrolysis very slowly and needed big quantities of triethylamine (as a catalyst) to initiate the release of ethanol, thus favoring the formation of silanol groups. Even with these steps taken the solution contained only initial nonhydrolyzed silane and three-dimensional self-condensates.13 Such a behavior strongly limits the possibility of using OES in such a context. Amino silanes seemed to display a particular behavior, which has encouraged us to study eight silane coupling agents bearing nitrogen atoms.14 Only γ -amino silanes exhibited solvolysis reaction, i.e., the exchange of the alkoxy group with the corresponding radical of the alcholic solvent. In this work, it was also established that, on one hand, if the amine function is linked to aromatic structures or if it is sterically hindered, the formation of condensed units is restricted, thus favoring the formation of silanol moieties, considered as potential functions to graft OH-rich materials. On the other hand, if the nitrogen atom is engaged in a


242

M.-C. BROCHIER SALON AND M. N. BELGACEM

non-amino function (imidazolin), the concentration of hydrolyzed moieties is much higher than that of condensation products. Amino-, vinyl-, mercapto-, methacryloxy-, and glycidoxy-propyltrialkoxysilanes (GPS) are very widely used in surface modification applications.15,16 In this study, we avoided the study of GPS, because the epoxy function it contains may undergo hydrolysis reactions, thus complicating the kinetic reactions. Nevertheless, it will be worth studying such a molecule in the upcoming investigations. Alkoxysilanes offer the advantage of providing various structures (especially concerning the fourth organic substituent), e.g., amino, vinyl, mercapto, methacryloxy, isocyanato, glycidoxy substituents, etc. Thus, the silane to be used as a grafting agent of the reinforcing elements (cellulose fibers) is chosen depending on the matrix selected to prepare the composite materials. For examples, amino silanes are suitable when the chosen matrix is an epoxy-based polymer, whereas the mercapto or methacryloxy counterparts are associated with vinylic matrices. This article intends to summarize the hydrolysis-condensation reaction of 14 silanes coupling agents in acidic medium. A new tool for comparing the grafting efficiency of the studied silanes with OH-rich substrates (such as cellulose) is proposed. It is based on determining a parameters associated with the reaction time at which the concentration of the most reactive species towards cellulose reaches its maximum value for each studied silane. EXPERIMENTAL Materials The organo-functional trialkoxy silanes (see Table 1) 3-methacryloxypropyl trimethoxy silane (MPMS), 3-mercaptopropyl trimethoxy silane (MRPMS), 3cyanopropyl triethoxy silane (CPES), triethoxy vinyl silane (VES), trimethoxy (2-phenylethyl) silane (PEMS), octyl triethoxy silane (OES), trimethoxy (7-octen-1-yl) silane (OEMS), 3-aminopropyl triethoxy silane (APES), 3-aminopropyl trimethoxy silane (APMS), 3-(2-aminoethylamino) propyl trimethoxy silane (DAMS), 3-[2(2-aminoethyl-amino)ethylamino]propyl trimethoxy silane (TAMS), trimethoxy[3(phenylamino)propyl]silane (PAPMS), and triethoxy-3-(2-imidazolin-1-yl) propyl silane (IZPES) were Fluka products of high purity. 4-Amino-3,3-dibutyl trimethoxy silane (ADBMS) was provided by Bayer, to which we are indebted. They all were used as received. Hydrolysis of Silane Coupling Agents The hydrolysis and condensation reactions of the silanes were carried out in a mixture of ethanol-D6 and deuterated water CD3 CD2 OD:D2 O, 80:20 w/w at a concentration of 10% w/w, and acetic acid-D4 5% w/w versus solvents (added to the quantity necessary for amine neutralization). The reaction kinetics was set up by in situ 1H, 13C, and 29Si NMR spectroscopy. All the hydrolysis reactions were conducted at a temperature of 25◦ C in ethyl alcohol independently from the alkoxy nature of the silane studied. Solution NMR Kinetics All the compounds and solvents were directly weighted into clean tubes. D2 O was first added to ethanol-D6 , and the zero reaction time was set immediately after the addition


243

Table 1 The silane coupling agents used in this work Silane

Formula

APES APMS DAMS

TAMS


ADBMS

PAPMS

IZPES

CPES

MPMS

MRPMS PEMS

VES OES OEMS

of the silane. The kinetic studies were carried out directly in NMR tubes by following the evolution of the relevant NMR signals. 1H, 13C, and 29Si NMR spectra of the silane solutions were performed on Varian Unity 400 and Mercury 400 spectrometers equipped with a 5 mm probe for 1H and 13C and operating at 399.956 and 100.572 MHz, respectively. All


244


chemical shifts were measured with a coaxial insert tube containing the tetramethylsilane (TMS) solution as an external reference. The spectral widths were calibrated to obtain the best resolution for proton and carbon spectra. Concerning silicon, a 10 mm BB probe operating at 79.455 MHz was used to optimize the signal:noise ratio in order to minimize the acquisition times, thus allowing us to set up the kinetics of the relatively fast reaction. The spectral width was 12 kHz, and the relaxation delay 100 sec with a proton decoupling only applied during the acquisition time, in order to avoid negative NOE. The number of scans was increased with time. The T1 measurements were made with the inversion-recovery method. 29 Si NMR spectroscopy is of a great interest to acquire further information about the structure of the oligomeric species formed during the hydrolysis and condensation reactions. The chemical shift of silicon is determined by the chemical nature of its neighboring atoms and moieties, namely the number of siloxane bridges attached to a silicon atom. M, D, T, and Q structures are the commonly used notation corresponding to one, two, three, and four Si O bridges, respectively. 29 Si NMR also provides a way to monitor the hydrolysis and the condensation reaction of silicon alkoxides. For example, a T structure bears only one organic Si-R’ side group and three siloxane bridges, which could be differentiated between Si OSi and Si OR groups. According to Glasser and Wilkes,17–21 the following nomenclature is used for Mi, Di, Ti, and Qi, where the index “i” refers to the number of O Si groups bound to the silicon atom of interest. In fact, Ti corresponds to R Si( OSi)i OR(3-i) , which gives rise to possible four T peaks,21 namely T0, T1, T2, and T3, as sketched in Scheme 1. It is also well known that the transition from Si OR to the corresponding silanol Si OH gives a shift to lower field (because of the electron-donating properties of OR), and the extent of the shift increases with the molecular weight of the alkyl group attached,22–24 e.g., 5–6 and

R'

OR

RO

OR

Si RO

R'

OR

RO

Si

Si O

RO

R'

Si O

R'

R'

R'

Si

Si

Si

RO

OR O

O

OR

O Si RO

OR R'

T3 structure three dimensional Scheme 1 Presentation of Ti silane structures.

OR Si

O

T2 structure linear link

T1 structure dimer or chain end

RO

R' R'

OR


245

2–3 ppm for ethyl and methyl groups, respectively. In our study, all the intensities of the 29 Si signals are measured with a precision of 10%.

RESULTS AND DISCUSSION Kinetics of Hydrolysis


All the kinetic curves showing the evolutions of the species present in the reaction medium were previously shown elsewhere.11–14,25 Figure 1 presents the curves associated

Figure 1 IZPES acetic acid–catalyzed hydrolysis: 29Si NMR spectra (below), kinetics curves (above).

246



Table 2 Silane hydrolysis in alcohol at 25◦ C

Silane

T0R t(0%)

APES

4h

T0H max - t

T1 max - t

33% - l h

55% - 2 h

APMS

10 min 35% - 20 min

66% - l h

DAMS

20 min

TAMS

lh

55% - l0 min

75% - l h

ADBMS

l h 30 65% - 20 min

78% - 2 h

56% - l0 min 65% - 30 min

PAPMS

l0 h

50% - l h

76% - 5 h

IZPES

l0 h

40% - 2 h

60% - 6 h

OES

> 48 h 63% - 15 h

60% - 48 h

OEMS

> 48 h

82% - 2 h

70% - 34 h

VES

18 h

60% - 4 h

70% - 24 h

MRPMS

lh

98% - l h

70% - 48 h

PEMS

5h

92% - 3 h

72% - 36 h

MPMS

18 h

75% - 3 h

80% - 40 h

CPES

18 h

68% - 5 h

70% - 30 h

T2 - t0 T3 - t0 max 24 h - (max 48 h) max 24 h - (max 48 h) 1h 48% - (45%) 20 min 45% - (43%) 20 min 59% - (47%) 4h 19% - (64%) lh 59% - (65%) 2h 47% - (60%) 3h 62% - (64%) 17 h 2% - (9%) 18 h 4% - (12%) 8h 20% - (33%) 8h 7% - (12%) l0 h 8% - (15%) 14 h 16% - (20%) 8h 13% - (29%)

5h 47% - (51%) 2h 50% - (56%) 2h 40% - (50%) 4h 25% - (30%) 8h 19% - (25%) 11 h 15% - (18%) 12 h 13% - (20%) —

Function 1 Amino 1 Amino 2 Amino 3 Amino 1 Amino Phenyl amino Imidazole Alcane

—

Alkene

—

Vinyl

—

Mercapto

—

Phenyl

—

Methacryloxy

—

Cyano

t(0%): Silane disappearance time—t0 : entity apparition time. max—t: Entity maximum of intensity and corresponding time. max 24 h: Entity maximum of intensity after 24 h (48 h) reaction time.

with IZPES hydrolysis, as deduced from the kinetics monitored by 29Si NMR spectroscopy in quantitative mode, i.e., sufficient relaxation delays and proton decoupling only during the acquisition to avoid negative NOE, as discussed before. As the reaction medium remains completely homogeneous, i.e., all the components are totally soluble during the observation, the sum of the peaks intensities is equal to 100% at any reaction time. For each entity, the quantity of each reactive moiety can be expressed as a percentage with respect to the amount of the total species present in the solution (reaction medium). Table 2 summarizes these data for the previously studied 14 different alkoxy silane coupling agents.11–14,25 Here our intention is to establish a parameter displaying the grafting potentiality of each silane, avoiding excessively fastidious comparisons between the kinetic curves, NMR spectra, or the data compiled in the form of a table (e.g., Table 2), because such work is not always sufficient, but in most cases borrowing. Thus, such a parameter (ASi OH ) is obviously linked to the active free silanol units available for further grafting of OH-rich surfaces. Therefore, ASi OH displays somehow the potentiality of grafting OH-rich surfaces of any silane under investigation, as a function of


247

time. Then, it becomes easy to predict the time of hydrolysis at with the grafting efficiency reaches its maximum value, for each coupling agent. Beari et al. have developed the following Equation (1) to determine the proportion of active silanol functions26:

In fact, the grafting reaction between silane coupling agents and hydroxy functions (e.g., from polysaccharides) may occur only when the former is hydrolyzed, thus bearing silanol (Si OH) end groups. This is the reason why in the ASi OH expression, neither T0R nor T3 concentrations appear, since these two moieties do not possess any free OH (alkoxy and siloxane moieties, respectively). The ASi OH parameter is a function of only T0H , T1, and T2, which generate 3, 2, and 1 OH functions, respectively. The maximum OH concentration possible is obtained with 100% T0H units. It is worth noting that this equation is only a function of the number of free OH units. It does not take into account the accessibility (so called “steric hindrance”) of each free OH group, which could be crucial for further consideration in a heterogeneous grafting reaction, which constitutes the major limitation of this semi-empirical equation. It is then obvious that the accessibility of free OH units from a T0 entity (the monomer) is higher than that belonging to T2 moiety, which moreover could eventually be included in a three-dimensional network. Such limitation could be overcome by assigning a coefficient (associated with the steric effect) for each reactive moiety (T0H , T1, and T2), but such an approach remains semiempirical, since the criteria of the coefficient chosen can be only arbitrary. The processing of the data concerning the kinetics of hydrolysis of the 14 studied silanes used in this work with the Equation (1) are presented in Table 3 and Figure 2, which

mole%


Active Si − OH functions (mol%) 3 ∗ mol%T 0 + 2 ∗ mol%T 1 + 1 ∗ mol%T 2 = ASi−OH = 3

100% 90% OES OEMS VES CPES MPMS MRPMS PEMS APES APMS DAMS TAMS ADBMS PAPMS IZPES

80% 70% 60% 50% 40% 30% 20% 10% 0% 0

6

12

18

24

30

36

42

48

time (h) Figure 2 The evolution of ASi OH versus reaction time of the 14 alkoxy silanes deduced from acetic acid–catalyzed hydrolysis kinetics curves.

248

OES

0% 0% 0% 0% 7% 17% 25% 30% 37% 42% 52% 64% 68% 72% 73% 75% 78% 78% 77% 77% 78% 77% 76% 74% 73%

Time (h)

0,00 0,25 0,33 0,50 1,00 2,00 3,00 4,00 5,00 6,00 8,00 10,00 12,00 14,00 16,00 18,00 20,00 22,00 24,00 28,00 32,00 36,00 40,00 44,00 48,00

Table 3 ASi

0% 0% 2% 33% 51% 82% 83% 84% 85% 84% 83% 79% 81% 77% 76% 73% 74% 73% 73% 71% 70% 69% 66% 66% 64%

0% 0% 2% 12% 24% 44% 62% 71% 72% 77% 77% 77% 73% 72% 70% 69% 68% 65% 65% 63% 62% 60% 59% 58% 57%

VES 0% 10% 10% 11% 23% 44% 64% 70% 78% 80% 81% 81% 80% 77% 74% 75% 72% 70% 69% 67% 64% 63% 63% 62% 59%

CPES 0% 0% 10% 17% 57% 78% 81% 83% 84% 85% 84% 83% 80% 78% 77% 76% 74% 73% 71% 68% 67% 66% 65% 63% 62%

MPMS 0% 0% 0% 39% 64% 87% 94% 93% 94% 92% 89% 86% 82% 80% 79% 77% 77% 75% 74% 72% 69% 68% 68% 68% 68%

PEMS 0% 25% 59% 84% 99% 97% 95% 94% 92% 91% 89% 87% 85% 83% 82% 80% 78% 78% 76% 75% 74% 72% 71% 70% 70%

MRPMS 0% 23% 31% 41% 55% 61% 56% 54% 44% 40% 35% 29% 27% 23% 23% 21% 20% 19% 20% 19% 19% 19% 18% 18% 15%

APES 0% 55% 74% 67% 65% 49% 34% 37% 33% 27% 28% 26% 24% 21% 21% 19% 18% 18% 18% 17% 17% 16% 16% 16% 16%

APMS 0% 78% 77% 69% 57% 47% 40% 36% 31% 26% 25% 25% 21% 20% 19% 19% 19% 19% 16% 17% 17% 14% 15% 14% 14%

DAMS 0% 81% 74% 63% 63% 57% 54% 48% 45% 39% 38% 37% 36% 37% 33% 32% 32% 31% 30% 28% 27% 27% 27% 26% 23%

TAMS 0% 72% 85% 79% 72% 66% 59% 57% 55% 52% 50% 49% 48% 42% 39% 35% 33% 34% 34% 34% 33% 32% 30% 28% 30%

ADBMS

0% 4% 18% 30% 44% 62% 61% 64% 59% 59% 55% 54% 50% 48% 47% 42% 37% 38% 38% 37% 36% 34% 31% 32% 32%

IZPES

values (in%) deduced from acetic acid–catalyzed hydrolysis kinetic curves monitored by 29Si NMR spectroscopy for the 14 alkoxy silanes

OEMS

OH


0% 19% 36% 52% 72% 67% 67% 65% 61% 59% 58% 56% 54% 53% 50% 49% 45% 45% 41% 42% 40% 38% 38% 36% 34%

PAPMS

mole%


249

100%

80%

60%

40%

a 0% 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

time (h) mole%


20%

100%

80%

60%

OES OEMS VES CPES MPMS MRPMS PEMS APES APMS DAMS TAMS ADBMS PAPMS IZPES

40%

20%

b 0% 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

time (h)

Figure 3 The evolution of ASi OH versus reaction time (for 6 h) deduced from acetic acid–catalyzed hydrolysis kinetics curves: (a) amino silanes, (b) other functionalized silanes.

shows that all the curves display a similar profile, i.e., a first fast ascendant portion reaching a maximum value at the beginning of the reaction, followed by a slower decreasing part of the curve indicating a gradual lost of the potentiality of grafting of the silane studied. This is the consequence that, in all cases, the first step of the reaction is the silanol formation, which is the predominant coefficient in Equation (1). The first rising part of curves gives some indications about hydrolysis itself. The faster the hydrolysis is, the higher the curve slope and vice versa. The kinetics of hydrolysis is also closely related to the time necessary to obtain a maximum value of ASi OH , denoted as ASi OHmax . A high level value for ASi-Ohmax indicates that at the corresponding reaction time, there is little

250 Table 4a = [ASi


APES APMS DAMS TAMS ADBMS PAPMS IZPES

M.-C. BROCHIER SALON AND M. N. BELGACEM OHmax —ASi OH(t) ]/

ASi

OHmax .

Loss of active Si OH units for the amino alkoxy silanes

6h

12 h

24 h

48 h

−34% −64% −66% −51% −39% −18% −8%

−56% −68% −73% −55% −44% −25% −21%

−67% −75% −80% −63% −60% −44% −40%

−75% −79% −82% −71% −65% −53% −50%

competition between the two competitive reactions, and that a maximum of silanol groups is present in the reaction medium. The second dropping down part of the curves conveys the importance of condensation reactions. This reaction is undesirable in our context, because it consumes the reactive species for cellulose grafting. A very fast decrease is associated with rapid formation of three dimensional units and a rapid decrease of the concentration of free silanol groups. The general feature of Figure 2 shows two different behaviors of alkoxy silanes: amino silanes and other functionalized ones. After 1 day of reaction time, ASi OH of all amino silanes is below 25%, whereas those related to the non-amino–based derivatives are above 55%. When zooming the two parts of the curve (Figure 3), more details could be noticed: The first difference is that tor the amino silanes, ASi OH rises extremely fast. The ASi OHmax values were reached very rapidly for DAMS and TAMS, the two quickest silanes, i.e., less than 1 h for the reaction. The triethoxy derivatives APES and IZPES displayed a slower increase and gave the ASi OHmax values after 2 h of reaction time (see Figure 4, for example). For the other silanes, MRPMS also gave a ASi OHmax value after 1 h of reaction time, whereas for all the other silanes, 3–6 h were necessary to reach their corresponding ASi OHmax values, with the exception of the slowest silane studied here, OES, which reaches its ASi OHmax value after 20 h of reaction time. The second big difference was observed in the second part of curves. An extremely drastic and fast decrease was observed for amino silanes since they lost between half and three-quarters of their ASi OHmax values within 6 h of reaction time (Table 4a, Figures 5 Table 4b = [ASi alkoxy silanes OES OEMS VES CPES MPMS PEMS MRPMS

OHmax —ASi OH(t) ]/

ASi

OHmax .

Loss of active Si OH units for the other functionalized

6h

12 h

24 h

48 h

0% −1% 0% 0% 0% −2% −8%

0% −5% −5% −2% −6% −12% −15%

−2% −14% −16% −15% −17% −21% −24%

−7% −24% −26% −27% −28% −28% −30%

mole%


251

a

100% 90% 80%

MRPMS

70%

PEMS

60%

MPMS OEMS

50%

ADBMS PAPMS

40%

TAMS DAMS APMS

20% 10% 0%

0

6

12

18

24

30

36

42

48

time (h) mole%


30%

100%

b

90% 80% 70%

CPES

60%

VES 50%

OES IZPES

40%

APES

30% 20% 10% 0% 0

6

12

18

24

30

36

42

48

time (h) Figure 4 The evolution of ASi OH versus reaction time (for 48 h), deduced from acetic acid–catalyzed hydrolysis kinetics curves: (a) trimethoxy silanes, (b) triethoxy silanes.

and 6). During the same reaction time (Table 4b, Figures 5 and 6), the other functionalized silanes, MRPMS and PEMS for example, lost only 11% and 4% of their ASi OHmax values, respectively. For the majority of the non-amino–bearing compounds, the decrease is slow and regular. Losing of the quarter of their ASi OHmax values takes about 24 h, except for OES, which lost 3% within a 24 h reaction time. The difference in behavior between methoxy and ethoxy silanes was observed previously.14 Ethoxy groups underwent the hydrolysis less readily compared to methoxy


252


Figure 5 Losses (in%) of the ASi OH at different reaction times, deduced from acetic acid–catalyzed hydrolysis kinetics curves: (a) amino silanes, (b) other functionalized silanes.

homologues. As a general rule, the time necessary to reach ASi OHmax values is longer, and absolute values are lower for all the triethoxy-derivatives when compared to trimethoxy silanes. In this context, the amino silanes could be compared between themselves. Thus, for example, the ASi OHmax value related to APES hydrolysis (see Figure 4) was about 61%, and it was reached after 2 h reaction time (Figure 3), whereas the ASi OHmax value associated with APMS was about 74% after 33 min reaction time. At the beginning of the reaction, the ASi OH evolution related to DAMS was comparable to that of ADBMS, but the former presents a drastic decrease after reaching its ASi OHmax value, probably because of two factors: (i) the presence of the two amino groups, which enhance condensation reactions, and (ii) the absence of the steric effect induced by the two methylene groups as side moieties in the fourth substituent. The consequence of the steric hindrance effect on ASi OH parameter became more observable from the sixth hour of reaction time, as shown in Figure 2. Thus, the curves associated with TAMS and ADBMS started to be separated from those related to APMS, DAMS, and APES homologues. For all the other functionalized silanes, the mercapto-bearing derivative, MRPMS, is the most reactive at the beginning of the reaction (ASi OHmax = 98% after 1 h). PEMS is less reactive requiring a longer delay (3 h), but it also gave a quite high ASi- OHmax value (94%) The less efficient one is vinyl silane VES (ASi OHmax = 77% after 6 h reaction time), mainly



253

Figure 6 The values of ASi OH parameter for the 14 alkoxy silanes, deduced from acetic acid–catalyzed hydrolysis kinetics curves (maximum—at the end of the observation).

because of two reasons: (i) it is an ethoxy derivative, and (ii) the vinyl function enhances the competitive condensation reactions. In fact, it was established that upon hydrolysis of silane coupling agents under basic conditions, the self-condensation reactions of the hydrolyzed silanol groups proceed with an enhanced rate.11,13 Such a behavior was observed in two cases: (i) when adding a catalytic amount of triethyl amine, and (ii) when using aminobearing silane coupling agents. Therefore; one could expect that the nucleophilic character associated with vinyl function may play a similar role. The silane OES behaved differently. Because of its lowest reactivity towards hydrolysis, its ASi OHmax value is similar to that of VES (see Figure 6), but at the end of the reaction, OES provides more free OH units, because no organic functions are present to enhance condensation reaction, as is the case with VES. On the other hand, among the five ethoxy derivatives studied, OES gave the highest and the most stable value of ASi OH after 48 h reaction time. There are some similarities in the behavior of MPMS and OEMS, and for that related to CPES and VES. For the all the non-amino silanes the ASi OHmax values obtained are quite reasonable, i.e., higher than 60% after 1 day of reaction time. Although, the time needed for the pre-hydrolysis step seems to be long, the concentration of reactive species in the reaction medium is acceptable. CONCLUSION The hydrolysis reaction of 14 silane coupling agents was studied in view of their exploitation as reactive media to graft cellulosic substrates. A new helpful tool, based on establishing a parameter closely related to the concentration of OH groups (arising from hydrolyzed siloxane to yield silanol functions), was proposed for the coupling agents studied. The kinetics of hydrolysis monitored by 29Si NMR spectroscopy was found to be a reliable tool to better understand the nature of the species present in the reaction

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media. In fact, according to the reaction mechanisms previously established, the efficient grafting could occur under experimental conditions favoring the formation of the maximum concentration of silanol groups. Work is under progress in order to confirm these hypotheses.


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