(8 Copyright 1993 American Chemical Society
JULY 1993 VOLUME 9, NUMBER 7
Letters Silicones on Glass Surfaces. 2.t Coupling Agent Analogs Leanne G. Britcher, David C. Kehoe, Janis G. Matisons,*Roger St. C. Smart,and A. Geoffrey Swincer School of Chemical Technology, University of South Australia, The Levels, South Australia 5095, Australia Received November 30,1992. In Final Form: April 22, 1993 A siloxane with hydrolyzable alkoxy groups in ita side c w s is found to attach to E-glass fibers as effectivelyas a trialkoxysilane. However, a hydroxyl-terminatedpoly(dimethylsiloxane),while interacting with the E-glass fibers from solution, is almost totally removed from the glass surface by subsequent washing with a variety of organic solvents. Diffuse reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy data are used to establish the interaction of such siloxanes with the glass surface, and the strength of such an interaction is indicated by the extent of removal of the adsorbed species after washing with various solvents.
Introduction The deleterious effects of water on the mechanical properties of many glass-reinforced composites are well doc~mented.l-~ Diffusion and reaction of water at the glass-matrix interface are responsible for the resultant delamination. To overcome such problems, coupling agents are used to generate a water-resistant interface between the organic polymer and inorganic ~ubstrate.”~ These coupling agents must be able to react or interact with both the glass surface and the polymer.”12 ~~
* To whom correspondence should be addressed. For part 1, see ref 17. (1) Bader, M. G.; Bailey, J. E.; Bell, I. In Ceramics in Severe Environments; Kriegel, W. W., Palmour, H., Eds.; Materiala Science kearch; Plenum Prese: New York, 1972; Vol. 5. (2) Atkine, A. G.J. Mater. Sci. 1975, 10, 819. (3) Outwater, J. 0. J. Adhes. 1970,2, 242. (4) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (5) Rosen, M. R.; Goddard, E. D. Proceedings of the 34th Annual Technical Conference, SPI Rshforced PlasticslCompositesh t . , 1979; Sect. 19-E. (6) Angst, D. L.;Simmons, G. W. Langmuir 1991, 7,2236. (7) Park, J. M.; Subramanian, R. V. J. Adhes. Sci. Technol. 1991,5, 459. (8) Pmtano, C. G.;Wittberg, T. N. Surf. Interface Anal. 1990,15,498. (9) Arklee, B. InH a Silicon Compound Register and Review, 5th ed.; Anderson, R., Larson, G. L., Smith, C., Eds.; HI% America Inc.: Piscataway, NJ,1991; p 59.
Silane coupling agents are commonly applied to fibers to improvethe overallperformanceof reinforced composite materials.4v6 These are applied from dilute aqueous solutions, partial hydrolyzates, or organic solvents (generally an alcohol),”12 and most have undergone initial hydrolyzation and oligomerizationprior to interactingwith the chosen s u b ~ t r a t e . ~Silanes J~ may interact with glass fiber surfaces initially through hydrogen bonding, with subsequent condensationand lateral reactions generating siloxane structures (Figure 1). It is also possible, in some systems, that lateral polymerization occurs without the formation of bonds to the surface.13 The siloxane film formed on the glass fiber surface consists of multiple layers.47J1J2 Two factors which may influence the structure of the silane coupling agent interphase are (i) the pH of the solution, and (ii) the drying conditions employed.“ The amount of silane coupling agent adsorbed by the oxide surfaceis greatly affectedby the pH of the applied solution. (10) Wang, D.;Jonea, F. R.; Denison, P. J. Adhes. Sci. Technol. 1992, 6, 79. (11) Allen,K. W. J. Adhes. Sci. Technol. 1992,6, 23. (12) Drown,E. K.; Al Moussawi,H.;Dnal, L.T.J. Adhes. Sci. Technol. 1991,6, 865. (13) Tripp, C. P.; Hair,M. L Langmuir 1991,8,1120. (14) Iehida, H. In Adhesion Aspects of Polymeric Coatings; Mittal,K. L., Ed.; Plenum Press: New York, 1983; pp 45-106.
Q143-1463/93/ 24Q9-16Q9$Q4.OO/Q 0 1993 American Chemical Society
Letters
1610 Langmuir, Vol. 9, No.7, 1993 a) RSi(OR)3 + 3
Ha0
-=
RSI(OH), + 3 R'OH
-
b) 3 RSi(OHh
R
I
R
R
l
l
HO-SI-0-SI-0-SI-OH
b
b
o1
A
A
A
+ ZH20
I
CH2-Si
c , p ' I ' ' I ' HO-SI-O--I-O-SI-OH
I
l
0 /
0 +
R
l I
0 A
H
H
H
H
P
P
P
I
l
l
I
l
-M-O-M-O-M-
R
I
I
HO-SI-0-SI-0-SI-OH
-
I
,k
H(
0
I
l
l 0.
0.
b)
,h
HI
0 I
-M-O-M-O-M-
I
,k
H:
l
0. 0 I
l
GLASS SURFACE
)
F
t
I
R
l
HO--SI-O-SI-O-?-OH
R
R
I
l
R
l
HO-SI-0-SI-0-SI-OH I
l
0 I
II
0 I
l l
GLASS SURFACE
l
l
I
l
r ie-OH
l l
+ 2H20 -M-0-M-0-M-
O
215.
R
0 I
11:
Figure2. Structuresof the siloxanes: (a) siloxane coupling agent analog, dp = 213,average empirical composition of m = 176 and n = 27; (b)poly(dimethylsiloxane),hydroxyl-terminated, dp =
HEAT
-M-O-M-O-M-
-I/-0 Me
l
GLASS SURFACE d
HO
(CCH,CH,OCH,),
l
GLASS SURFACE
Figure 1. Alkoxysilane coupling agenta on glass: (a) hydrolysis of alkoxy groups, (b) condensation, (c) hydrogen bonding with glass silanols, (d) surface bond formation. Basic or acidic conditions increase the rate of hydrolysis and condensation of the silane,15and thereforewill increase the amount of silane adsorbed. The surface potential of the oxide substrate also varies with the pH of the applied solution, affecting the orientation of the adsorbed silane 1a~ers.I~ The situation is even more complex on E-glass fibers, where surface microheterogeneitiesmay exist and the surface potential is not simply an averaging of the component oxide potentials." Drying the silane-treated fibers will lead to the formation of siloxane bonds between the couplingagent and the surface (Figure 1). The number of siloxane bonds with the surface is influenced by the temperature and duration of the drying procedure.4J4 It is well established that more than a monolayer of silane coupling agent is required on the fiber in order to optimize the strength of the resulting ~omposite.~J Only in this way is it possible to generate an interpenetrating network of the coupling agent (residenton the fiber)within the polymer matrk4J There is, however, an optimum thickness of coupling agent which if not achieved results in a substantial decline in the overall performance of the A large flexible polymeric backbone will comp~eite.~J enable the interphase to adjust to the steric constraints imposed by the oxide surface and display a continuous reactive surface to the polymer matrix. Furthermore, the ratio of hydrophilicto hydrophobicgroupsmay be adjusted so as to optimize the number of polar group interactions for maximum dry strength and d u r a b i l i t ~ . ~Arkles J~ et d.l5have indicated that, in order to control and reproduce silane modification of oxide surfaces, it is necessary to carefullycontrol hydrolyzation and oligermerizationrates and pathways. In addition, it is also necessary to control the degree of cross-linking and size of polymeric silane segmentsto ensure their interpenetration into the polymer matrix for optimum composite proper tie^.^ Siloxanes, like silanes, are also capable of adhering to avariety of s u r f a c e ~ . l ~They - ~ ~ are strongly water resistant (16) Arkles, B.;Steinmetz,J. R.; Zazyczny, J.; Mehta, P. J. Adhes. Sci. Technol. 1992,6, 193. (16) Bell,J. P.; Schmidt, R. G.; Malofeky, A.; Mancini, D. J. Adhes. Sci. Technol. 1992, 5, 927.
polymers,2O and should, in principle, also be able to give water-resistant interfacesbetween glass fibers and organic resins in composite materials. They display considerable backbone fle~ibility,2'-~~ so that they could adjust to the availability of the reactive sites on Blase surfaces. The investigation of siloxanes bearing the appropriate functional groups may therefore lead to a whole new class of coupling agents, with all the advantages of silanes, but with greater control and reproducibility in surface modification. An earlier study17looked at the interactions of siloxanes bearing a variety of functional groups with glass Surfaces. The interactionsbetween these siloxanesand E-glassfibers were assessedby X-ray photoelectron spectroscopy(XPS), and compared to that of a common silane coupling agent, vinyltrie(2-methoxyethoxy)silane. In no instancedid these siloxanescontain alkoxy groups, so such siloxanes should not have attached strongly to the E - g h fiber surface. Surprisingly, the siloxanes bearing amino, hydrido, and methacryl functional groups covered the surface of the E-glase fibers at least as effectively as vinyltris(2-methoxyethoxy)silane,resisting removalby a variety of organic solvents. Siloxanes containing epoxy and imino groups however did not attach effectivelyand were easilyremoved. The initial results with these functionalized siloxanes have now prompted us to synthesize and examine the attachment to E-glass fibers of a siloxane "coupling agent analog" bearing a large number of alkoxy groups. This allows a comparison to be made among this siloxane coupling agent analog (Figure 2b), the vinyltris(2-methoxyethoxy)silaneused in the earlier study, and a hydroxylterminated poly(dimethylsiloxane) (Figure 2a). Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) and X-ray photoelectron spectroscopy ( X P S ) were again used to analyze the treated E-glass fibers. This study is part of an ongoing investigationinto the chemistry and application of siloxanes as coupling agents. (17) Bennett,D. R.; Matisons, J. G.; Netting, A. K. 0.; Smart,R. St. C.; Swincer, A. G. Polym. Znt. 1092,27, 147. (18) Auroy, P.;Auvray,L.; W e r , L. J. Colloid Interface Sei. 1992,150 (l),187. (19) Cosgrove, T.;Preetidge, C. A.; Vincent B. J. Chem. SOC.,Faraday Trans. ISSO, 86 (9), 1377. (20) NOH,W . The Chemistry and Technology of Siliconeu, 2nd d.; Academic P m : New York, 1pp 442,470. (21) Litxinov, V. M.; Lavrukhin, B. D.; Zhdanov, A. A. Polym. Sci. USSR.Ser. A 1986.27.2474. ,~ ,~ (22jLitvinov, v . M.;~avrukhin,B. D.; zhdanov, A. A. Polym. Sci. USSR,Ser. A 1986,27,2482. (23) Dubchak, I. L.; Pertein, A. I.; Zhdanov, A. A. Polym. Sci. USSR, Ser. A 1986,27,2340. (24) Viallat, A.; Cohen-Addad,J. P.; Pouchelon,A. Polymer 1986,27, 843.
Langmuir, Vol. 9, No. 7,1993 1611
Letters
Experimental Section Untreated E g k fiber (waterwashed only)was obtained from ACI (Australia)and uaed without any further modification. A hydroxyl-terminated poly(dimethyKioxane) (PDMS, M, = 16000, General Electric Co.), dichloromethane, and acetone (Chemplex) were used as supplied. Toluene (Ace Chemicals) and hexane (Shell)were distilled and stored over molecular sieves. Vinyltrb(2-methoxyethoxy)silane (UnionCarbide)was distilled prior to use. The siloxane coupling agent analog was prepared by hydroeilylation of vinyltris(2-methoxyethoxy)silane onto the appropriate hydridoeiloxane,synthesizedas outline eleewhere." The treatment of E g k fibers with either siloxanewas similar. The fibers were immersed in toluene containing either 5.3 % (w/ w) PDMS or 3% (w/w) siloxane coupling agent analog, and allowed to stand for 18 h at raom temperature. The solutions were then decanted, and the treated E g k fibers washed successively with (1)dichloromethane, (2)toluene, (3) hexane, (4)acetone, and (5)dichloromethane. The fibers were then oven dried (3 h, 100 O C in air). A sample of the PDMS-treated fibers was taken after the fmt dichloromethane wash and also oven dried (3h, 100 O C in air). Vinyltris(2-methoxyethoxy)silane was applied as a 3.8% (w/w) solution in ethanol, and the treated fibers were similarly washed with organic solventa and oven dried.'' All FTIR spectra were measured at room temperature with a single-beam BIORAD Model FTS 65 spectrometer in the wavenumber region from 4000 to 400 cm-', at a resolution of 4 cm-I, wing a MCT detector. Each sample was mounted in a Spectra Tech diffuse reflectance apparatus and scanned 256 times, with the fibers parallel,and at an angle of 90° with respect to the direction of the IR beam (for maximum signal to noise ratio). The XPS data were obtained using a Perkin-Elmer PHI 5100 X P S system with a concentric hemisperical analyzer and a Mg Ka X-ray source operating at 300 W, 15 kV, and 20 mA. Pass energies of 78 and 18 eV were used for survey and elemental spectra,respectively. Atomic concentrationswere obtained wing standard sensitivity factors.% The angle between the sample surface and the analyzer was fued at 45O,and that between the source and analyzer was fixed at 54.6'. The pressure during analysis ranged from 10-8to 1O-gTorr. Results and Discussion The interactions between silane coupling agents and inorganic oxide, fiier surfaces have been well studied. A number of spectroscopic techniques have been used to examine surface modification of fiers, though IR spectroscopy and X-ray photoelectronspectroscopy (XPS)are the two techniques commonly employed to characterize these modified surfaces. XPS is capable of providing chemical information to a depth of only a few nanometers, so it is a particularly powerful technique for analyzing modified surfaces on a variety of Analysis of glass surfacesusing transmissionor ATR (attenuated total reflectance) IR spectroscoy is difficult, because of the strong incipient scattering.29 Diffuse reflectance Fourier transform IR spectroscopy (DRIFT), however, relies on the scattering properties of the sample, and so has been successfully used to analyze coatings on glass fibers.30 Characteristic IR vibrations of the coatings below 1600 cm-*are difficult to identify when using DRIFT, because of the strong Si-O and B-O absorbances of E-glass An overlayer of KBr is reported to alleviate this problem,3O but it is not that easy to reproduce this (26) Lipp, E.; Smith, A. L. In The Anulytical Chemistry of Silicones; Smith, A. L.; Ed.;Wiley New York, 1991; pp 49,326-333. (26) Duplock, S . K.; Matisons,J. G.; Swincer, A. G.; Warren, R. F. 0. J. Zmrg. Orgammet. Polym. 1991, 1, 361. (27)Britcher, L. G.; Kehoe, D. C.; Matisons, J. G.;Swincer, A. G. Unpublished results. (28) Briggs, D.; Seah, M. P. Practical Analysis by Auger and X-ray Photoelectron Spectroscopy; Wiley New York, 1983. (29) lehida, H.; Koenig, J. L. Polym. Eng. Sci. 1978,18, 128.
I >
-1 5180
1
38111
I
I
I
I
I1
1
3018 29~13 2981 2U i U 2UOB 2?b0 2780
1
,
2650
2600
i
2550
I
2500
Yawnumbere
Figure 3. IR spectraof (a)PDMS, (b)PDMS on E g h fibers, dichloromethane wash only, (c) PDMS on E - g k fibers, full solvent wash, and (d) %glass fibers.
technique, and in any case it was not required in this study because the siloxanes investigated had characteristic vibrations which could be readily monitored away from this region. Unlike silica, E-glass fiber exhibits only a very broad Si-OH so the interactions between the coupling agent and the active sites on this substrate (the surface silanols) are not easily interpreted. The effects of pH on adsorption of coupling agents are known to be quite complex for E-glass fiber Surfaces (uide supra). These effects were not examined in this study, because these siloxanes(asin the previous in~estigationl~) must be applied to the glassfibers from neutral nonaqueous solvents (the siloxanes being immiscible with water). Viyltris(2-methoxyethoxy)silane was applied from dry ethanol, and lH NMR confiied no oligomerization or hydrolysis occurred in this solvent. A direct comparison of adsorption onto glass surfaces can therefore be made between the siloxanes and this trialkoxysilane. The siloxane-treatedfibers were immediatelysubjected to a sequential washing regime consisting of (i) dichloromethane, (ii) toluene, (iii) hexane, (iv) acetone, and (v) again dichloromethane. Siloxanes are known to be very soluble in these solvents;= therefore, any weakly bound or physisorbed species will be removed by this procedure. The relative amount of siloxane(asdetermined by DRIFT and XPS) still present after washing was used to indicate the effectivenessof the attachment, Le., the generation of the stable couplingagent interphasenBc%BBBryfor optimum composite properties (uide supra). The fibers were then dried for 3 h a t 100 OC so that all traces of residual solvent would be removed. PDMS contains only two silanol groups separated by a long dimethylsiloxanebackbone, thus limiting the interaction with either the E-glass fiber surface or itself. It was anticipated that ita attachment to E-glassfiberswould be poor, and so it should be readily removed on washing. Figure 3a showsthe IR absorbancespectrum of neat PDMS in the region 2500-3100 cm-l. The DRIFT spectra of PDMS-treated E-glass fiber washed only with dichloromethane, PDMS-treated fiber subjected to the full solventwashing sequence, and untreated E g h fiber are also shown (Figure3b,c,d, respectively). Untreated E-glass fiber shows only the first overtone of the B-O stretching vibration at 2671 cm-l (see Figure 3d),S2whereas the neat siloxane, PDMS, displays both the asymmetric and symmetricC-H stretchingvibrations of the methyl groups, at 2963 and 2906 cm-l, respectively (see Figure 3a).% (30)Culler,S. R.; McKenzie, M. T.; Fina,L. J.; Iehida, H.; Koenig, J. L. Appl. Spectrosc. 1984,38,791. (31) Condrata R. A., Sr. In Introduction to G b r Science; Pye, L. D., Stevens, H. J., La Course, W. C., Eds.; Plenum Presa: New York, 1972; p 101. (32) Haalnnd,D. AppI. Spectrosc. 1986,40 (8), 1162.
1612 Langmuir, Vol. 9, No. 7, 1993
Letters
Table I. Atomic Concentration of Treated and Untreated E-GlueFiberr by element
(line)
c (1s)
0 (1s) Si(2p) Ca(2p)f Al(2p) B (1~)s Na (18)s F (1s)
calcda 62 19 8.3 6.2 3.6
0.54
untreated Eglaas fiber
PDMSb-treated fiber (initial)c
29 (OY 50 (68) 12 (17) 2.8 (3.9) 3.5 (4.8) 0.60 (0.95) 0.94 (1.2) 0.40 (0.55)
atomic concentration ( % ) PDMS-treated vinyltria(2-methoxyethoxy)fiber ( f i a l ) d silanstreated fiber
54
36
31 14 0.43
