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organic nanocomposites as new basic raw materials for adhesives and sealants Part 2
ARTICLE IN PRESS
International Journal of Adhesion & Adhesives 26 (2006) 567–570 www.elsevier.com/locate/ijadhadh
Functionalized inorganic/organic nanocomposites as new basic raw materials for adhesives and sealants Part 2$ F. Bauera, U. Deckera, H. Ernstb, M. Findeisenb, H. Langgutha, R. Mehnerta, V. Sauerlandc, R. Hinterwaldnerd, a
Institut fu¨r Oberfla¨chenmodifizierung, Permoserstr. 15, D-04318 Leipzig, Germany b Universita¨t Leipzig, Linne´str. 5, D-4103 Leipzig, Germany c Bruker Daltonik GmbH, FahrenheitstraX e 4, D-28359 Bremen, Germany d Hinterwaldner Consulting & Partner (GbR), Marktplatz 9, D-85614 Kirchseeon, Germany Accepted 1 November 2005 Available online 19 January 2006
Abstract A high content of nano-sized silica, alumina and titania was embedded in epoxy Novolac adhesives. By grafting a glycidyloxypropyl functionalized trialkoxysilane on different nanoparticles polymerization-active fillers were prepared. 29Si NMR and MALDI-TOF mass spectrometry revealed oligomeric siloxane structures present on the surface of coated nanoparticles. In heat-induced polymerization reactions these surface-modified nanoparticles form covalent crosslinks to the epoxy resin, thus efficiently modifying the viscoelastic properties. The reinforced nanocomposites revealed shifts of the glass transition temperatures of about 20 K pointing to the improved thermal stability. r 2005 Elsevier Ltd. All rights reserved. Keywords: Adhesives; Sealants; Hot-melt adhesives; Nanocomposites
1. Introduction Advanced adhesives must often meet competing requirements, for instance, they have not only to guarantee a firm bond but also must cushion stress situation by improved toughness. To bring such contradictory material properties into harmony with each other, the incorporation of nanoparticles, having average particles sizes between 5–50 nm, in a variety of matrix resins enabled the development of a rather new class of materials (nanocomposites) which are challenges for adhesive raw materials manufacturers. Heat-curing epoxies and numerous acrylates curable by ultraviolet radiation or beams of electrons can be used as the base resin. Due to their small size and large surface area nanoparticles are unique fillers yielding totally different effects and $ STICK! 4th European Congress on Adhesive and Sealant Raw Materials 2005. Corresponding author. Tel.: +49 8091 53990; fax: +49 8091 539920. E-mail address: [email protected] (R. Hinterwaldner).
0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.11.001
improved physical properties compared to conventional fillers with sizes in the mm range. Of particular importance, nanoparticles present in the polymer must be well distributed in order to change the polymer structures. However, physical mixing of inorganic nanopowders and organic resins may lead via particle aggregation to high viscosity even at low filler content. This particular complication can be prevented with the aid of organosilanes which change the hydrophilic nature of the oxide surfaces into a hydrophobic one. Surface modification by trialkoxysilanes is commonly used and extensively described in the literature [1,2]. With the aid of these coupling agents the dispersibility of the inorganic filler in organic media is improved and, hence, the suitability for many applications. In addition, modification of nanoparticles by acrylic, vinyl or epoxy functionalities enables the formation of chemical bonds between both components, which is expected to guarantee a durable interconnection between the two incompatible phases. Polymerization-active nanoparticles are expected to be centers of crosslinking reactions
ARTICLE IN PRESS 568
F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
intensifying the stiffness of nanocomposites as well as improving the viscoelastic properties of nanoadhesives. For example, studies of organic and inorganic nanopowders revealed modification effects like glass temperature shifts of about 10 K and mechanical reinforcement [3–5]. In previous studies, application of modified silica nanoparticles in radiation curable acrylate formulations has shown to result in nanocomposite films with improved scratch and abrasion resistance [6] and reinforced nanoadhesives with improved glass transition temperature of about 50 K [7]. In this paper, the modification of silica, alumina, and titania nanoparticles by trialkoxysilanes as well as the reinforcement of epoxy adhesives will be described.
The grafting procedure of trialkoxysilanes, e.g. glycidyloxypropyltrimethoxysilane (GLYMO), methacryloxypropyltrimethoxysilane (MEMO), and propyltrimethoxysilane (PTMO), onto the surface of oxide nanoparticles, e.g. silica (AEROSIL OX50), alumina (Aluminiumoxid C), and titania (P25, all available from Degussa), was described in detail earlier [8,12]. The coated nanoparticles were characterized by temperature-programmed oxidation (TPO), 29Si NMR, IR and MALDI TOF mass spectrometry. Embedding of surface-modified fillers led to reinforced epoxy adhesives, based Epoxy Novolac DEN 425 (Dow), with a high fill ratio of oxide nanoparticles of about 30 wt%. These nanoadhesives were heat cured at 160 1C with about 3 wt% hardener component (ANCAMINE 2014AS, Huntsman).
Scheme 1. Embedding of polymerization-active nanoparticles, formed by grafting of GLYMO onto the surface of alumina, into epoxy Novolac resins.
100 PTMO/silica GLYMO/silica MEMO/silica
95
weight (%)
2. Experimental
90
85
80
3. Results 75
Surface modification by trialkoxysilanes implies both hydrolysis of alkoxysilanes to silanols and condensation of silanols formed with terminal OH groups present on the particle surface. For silanes provided with olefinic or epoxy functionalities, e.g. methacryloxypropyltrimethoxysilane (MEMO), vinyltrimethoxysilane (VTMO), and glycidyloxypropyltrimethoxysilane (GLYMO), a polymerization-active siloxane shell is formed around the nanoparticles which results in crosslinking reactions with the acrylate network as depicted in Scheme 1. 4. Characterization of grafted nanoparticles After modification the silane coverage of silica nanoparticles modified by GLYMO, MEMO, and PTMO has been determined by TPO (Fig. 1). For all silanized samples, the overall loss of weight as a function of the temperature revealed burning at about 300 1C. The maximum weight loss of MEMO, GLYMO, and PTMO modification was observed to be 24, 16, and 9 wt% respectively. Assuming a schematic grafting via a tridentate silane structure, a monomolecular surface coverage should be obtained
100
200
300 400 500 temperature (°C)
600
700
800
Fig. 1. TPO profile of silica nanoparticles grafted by PTMO, GLYMO, and MEMO.
already by about 4.3 wt% GLYMO. The observed organic content of silica modified by MEMO and PTMO is likewise higher than the expected content due to monomolecular silane coverage. The translation of the content of organics into molecular surface coverage is 5.5, 3.7, and 5.6 molecules/nm2 for MEMO, GLYMO, and PTMO modification, respectively. Compared to 2–3 OH groups per nm2 on nanosized silica [9] the observed coverage is higher than a monomolecular one and can be taken to suggest grafting of precondensed silane structures. To characterize silanized oxide surfaces 29Si NMR spectroscopy was proven to be the standard method [10]. With respect to condensed or grafted organosilanes, signals are typically observed in the 45y50 ppm, 55y 60 ppm and 65y70 ppm regions and are assigned to mono(T1)-, bi(T2)-, and tri(T3)-fold Si–O-linked silicones, respectively.
ARTICLE IN PRESS F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
569
TiO2
-40
-50
-60 chemical shift [ppm]
-70
-80
Fig. 2. 29Si NMR spectra of GLYMO modified alumina, titania, and silica nanoparticles in the presence of GLYMO.
a
signal intensity [a.u.]
e f
d
c
b
80 70 60 50 40 30 20 10
17
18
19
3402.049
16
3066.873
2576.743
15
3224.986
14
2907.853
13
2398.672
12
20
2000
2733.822
1711.474
11
2221.693
1500
SiO2
10
2045.576
Al2O3
9
1377.372
signal intensity [a.u.].
signal intensity [a.u.]
1535.415
8
1869.520
Monomer units 7
2500 m/z
3000
3500
Fig. 4. MALDI-TOF mass spectrum of GLYMO modified silica nanoparticles. All molecules were observed.
solvated nanoparticles. Fig. 3 shows the 13C NMR spectrum of silica nanoparticles after modification by GLYMO. Broad bands between 7 and 10 ppm reveal a down filed shift of the carbons nearest to silicon at 5.1 ppm (SiCH2–CH2–CH2O(CH2CH2O)) indicating their significantly lower segmental mobility of the silane chain as well as a specific interaction with the silica surface.
0
5. MALDI-TOF MS investigations
with silica
neat GLYMO
15
13
11
9 7 chemical shift[ppm]
5
3
Fig. 3. Part of 13C NMR spectrum of GLYMO and a dispersion of GLYMO-modified silica nanopowder within GLYMO used as solvent. Insert: full scale spectrum.
The intense signal at 42.0 ppm (Fig. 2) reveals the presence of neat GLYMO (used as solvent). Whereas T1 and T2 structures of grafted or condensed trialkoxysilanes are observed to a similar quantity on alumina, titania, and silica nanoparticles, highly condensed T3 silicon atoms were rarely formed during in-situ silanization. These findings obtained in the presence of an excess of silane disagree with observations under typical in-situ silanization conditions (using the resin as solvent) where high proportions of T3 silicones were obtained [8]. In addition, the interaction between filler and trialkoxysilanes can be verified by 13C NMR spectroscopy of
Activated by laser irradiation, MALDI TOF mass spectrometry clearly showed the presence of GLYMO on the surface of silica nanoparticles (Fig. 4). The formation of GLYMO oligomers composed of more than 20 monomeric silane units was indicated by molar masses of 43600 Da. Moreover, mass signals differing in 18 Da indicated different degrees of trialkoxysilane condensation. The observed MS pattern is in agreement with a proposed a ladder-like structure of grafted trialkoxysilanes [6]. In addition to improved dispersibility of inorganic fillers after silane modification, crosslinking reactions between filler and resin can be achieved by trialkoxysilanes provided with polymerization-active functionalities. Both the nanoparticles modified by GLYMO and the epoxy Novolac resin can be identified by IR bands of the characteristic oxiran ring structure between 800–950 cm1. As shown in Fig. 5, the band at 910 cm1 assigned to epoxy structures disappeared after heat curing at 160 1C and revealed, thereby, the formation of polymeric network. 6. Material characterization Nanoparticles modified with respect to the chemical nature of the base resin can be crosslinked with the monomeric and oligomeric matrix components during curing. These copolymerization reactions are expected to have strong effects on the viscoelastic properties of the nanocomposite if the nanoparticles are homogenously
ARTICLE IN PRESS F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
570
Table 1 Onset and glass transition temperature of epoxy adhesives reinforced by 30 wt% GLYMO modified nanoparticles
910
Epoxy Epoxy Epoxy Epoxy
(a) 1750
1500
1250
1000
Tg (1C)
57 81 83 80
66 85 92 86
modified titania, there is an apparent increase of the storage modulus E0 in the temperature range 60 1C to 100 1C. This apparent stiffening of the sample can be attributed to perturbation resulting from stress release within the sample leading to changes in sample geometry [11].
(b)
2000
resin resin+TiO2 resin+Al2O3 resin+SiO2
Tonset (1C)
750
wavenumber[cm-1]
Fig. 5. IR spectrum of epoxy Novolac resin filled with 30 wt% alumina nanoparticles modified by GLYMO before (a) and after heat curing (b).
2.0 epoxy resin e epoxy + TiO2 epoxy + Al2O3 e epoxy + SiO2
E′-Modulus (109 Pa)
1.5
1.0
0.5
7. Conclusions After surface modification by glycidyloxypropyltrimethoxysilane, about 30 wt% silica, alumina, and titania nanoparticles have been incorporated in epoxy Novolac adhesives. The coated nanoparticles were studied by 29Si NMR and MALDI-TOF mass spectrometry which revealed anchoring of polysiloxane chains. Due to the polymerization-active surface coverage of nanoparticles, crosslinking reactions occurred during heat curing between the modified nanoparticles and the epoxy resin formulation. Hence, the reinforcement of epoxy adhesives yielded an improvement of viscoelastic properties compared with the neat epoxy Novolac system. For all the nanocomposites, increased stiffness and a shift of the glass transition temperature of about 20 K have been observed. References
0 0
20
40
60
80 100 120 temperature(°C)
140
160
180
200
Fig. 6. Temperature dependence of the storage modulus E0 of a epoxy resin and the corresponding nanocomposites with GLYMO modified titania, alumina, and silica.
distributed in the resin matrix. Indeed, in DSC measurements of an epoxy resin filled with 30 wt% GLYMO modified titania, alumina, and silica, both the modulus as well the glass transition temperature increased compared to the neat epoxy resins (Fig. 6). As expected, the incorporation of polymerization-active nanoparticles yield an increased density of the polymeric network and higher stiffness. The reinforced composites revealed a shift of the glass transition temperature DTg of about 20 K (Table 1) pointing to the particular thermal stability of alumina nanocomposites. For the nanocomposites with GLYMO
[1] Allen KW. J Adhesion Sci Technol 1992;6:23. [2] Zhang S, Schindler B, Nicholson G, Bayer E. J High Resolut Chromatogr 1995;18:579. [3] Landry CJT, Coltrain BK. J Macromol Sci A 1994;31:1965. [4] Lan T, Pinnavaia TJ. Chem Mater 1994;6:2216. [5] Dufresne A, Cavaille JY, Helbert W. Macromolecules 1996;29:7624. [6] Bauer F, Gla¨sel H-J, Decker U, Ernst H, Freyer A, Hartmann E, Sauerland V, Mehnert R. Progress in Organic Coating 2003;47:147. [7] Bauer F, Gla¨sel H-J, Hartmann E, Langguth H, Hinterwaldner R. Int J Adhes Adhes 2004;24:519. [8] Bauer F, Ernst H, Decker U, Findeisen M, Gla¨sel H-J, Hartmann F, et al. Macromol Chem Phys 2000;201:2654. [9] Michael G, Ferch H. Schriftenreihe Pigmente: Grundlagen von AEROSILs (Technical Bulletin # 11), Degussa AG, Frankfurt a.M. 1998. [10] Albert K, Bayer E, Pfleiderer B. J Chrom 1990;506:343. [11] Mano JF. Thermochimica Acta 1999;332:171. [12] Gla¨sel HJ, Hartmann E, Mehnert R, Hinterwaldner R. Europ Patent 1 1999;123:354 assigned to Institut fu¨r Oberfla¨chen-modifizierung e.V. and Hinterwaldner Consulting & Partner (GbR).
International Journal of Adhesion & Adhesives 26 (2006) 567–570 www.elsevier.com/locate/ijadhadh
Functionalized inorganic/organic nanocomposites as new basic raw materials for adhesives and sealants Part 2$ F. Bauera, U. Deckera, H. Ernstb, M. Findeisenb, H. Langgutha, R. Mehnerta, V. Sauerlandc, R. Hinterwaldnerd, a
Institut fu¨r Oberfla¨chenmodifizierung, Permoserstr. 15, D-04318 Leipzig, Germany b Universita¨t Leipzig, Linne´str. 5, D-4103 Leipzig, Germany c Bruker Daltonik GmbH, FahrenheitstraX e 4, D-28359 Bremen, Germany d Hinterwaldner Consulting & Partner (GbR), Marktplatz 9, D-85614 Kirchseeon, Germany Accepted 1 November 2005 Available online 19 January 2006
Abstract A high content of nano-sized silica, alumina and titania was embedded in epoxy Novolac adhesives. By grafting a glycidyloxypropyl functionalized trialkoxysilane on different nanoparticles polymerization-active fillers were prepared. 29Si NMR and MALDI-TOF mass spectrometry revealed oligomeric siloxane structures present on the surface of coated nanoparticles. In heat-induced polymerization reactions these surface-modified nanoparticles form covalent crosslinks to the epoxy resin, thus efficiently modifying the viscoelastic properties. The reinforced nanocomposites revealed shifts of the glass transition temperatures of about 20 K pointing to the improved thermal stability. r 2005 Elsevier Ltd. All rights reserved. Keywords: Adhesives; Sealants; Hot-melt adhesives; Nanocomposites
1. Introduction Advanced adhesives must often meet competing requirements, for instance, they have not only to guarantee a firm bond but also must cushion stress situation by improved toughness. To bring such contradictory material properties into harmony with each other, the incorporation of nanoparticles, having average particles sizes between 5–50 nm, in a variety of matrix resins enabled the development of a rather new class of materials (nanocomposites) which are challenges for adhesive raw materials manufacturers. Heat-curing epoxies and numerous acrylates curable by ultraviolet radiation or beams of electrons can be used as the base resin. Due to their small size and large surface area nanoparticles are unique fillers yielding totally different effects and $ STICK! 4th European Congress on Adhesive and Sealant Raw Materials 2005. Corresponding author. Tel.: +49 8091 53990; fax: +49 8091 539920. E-mail address: [email protected] (R. Hinterwaldner).
0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.11.001
improved physical properties compared to conventional fillers with sizes in the mm range. Of particular importance, nanoparticles present in the polymer must be well distributed in order to change the polymer structures. However, physical mixing of inorganic nanopowders and organic resins may lead via particle aggregation to high viscosity even at low filler content. This particular complication can be prevented with the aid of organosilanes which change the hydrophilic nature of the oxide surfaces into a hydrophobic one. Surface modification by trialkoxysilanes is commonly used and extensively described in the literature [1,2]. With the aid of these coupling agents the dispersibility of the inorganic filler in organic media is improved and, hence, the suitability for many applications. In addition, modification of nanoparticles by acrylic, vinyl or epoxy functionalities enables the formation of chemical bonds between both components, which is expected to guarantee a durable interconnection between the two incompatible phases. Polymerization-active nanoparticles are expected to be centers of crosslinking reactions
ARTICLE IN PRESS 568
F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
intensifying the stiffness of nanocomposites as well as improving the viscoelastic properties of nanoadhesives. For example, studies of organic and inorganic nanopowders revealed modification effects like glass temperature shifts of about 10 K and mechanical reinforcement [3–5]. In previous studies, application of modified silica nanoparticles in radiation curable acrylate formulations has shown to result in nanocomposite films with improved scratch and abrasion resistance [6] and reinforced nanoadhesives with improved glass transition temperature of about 50 K [7]. In this paper, the modification of silica, alumina, and titania nanoparticles by trialkoxysilanes as well as the reinforcement of epoxy adhesives will be described.
The grafting procedure of trialkoxysilanes, e.g. glycidyloxypropyltrimethoxysilane (GLYMO), methacryloxypropyltrimethoxysilane (MEMO), and propyltrimethoxysilane (PTMO), onto the surface of oxide nanoparticles, e.g. silica (AEROSIL OX50), alumina (Aluminiumoxid C), and titania (P25, all available from Degussa), was described in detail earlier [8,12]. The coated nanoparticles were characterized by temperature-programmed oxidation (TPO), 29Si NMR, IR and MALDI TOF mass spectrometry. Embedding of surface-modified fillers led to reinforced epoxy adhesives, based Epoxy Novolac DEN 425 (Dow), with a high fill ratio of oxide nanoparticles of about 30 wt%. These nanoadhesives were heat cured at 160 1C with about 3 wt% hardener component (ANCAMINE 2014AS, Huntsman).
Scheme 1. Embedding of polymerization-active nanoparticles, formed by grafting of GLYMO onto the surface of alumina, into epoxy Novolac resins.
100 PTMO/silica GLYMO/silica MEMO/silica
95
weight (%)
2. Experimental
90
85
80
3. Results 75
Surface modification by trialkoxysilanes implies both hydrolysis of alkoxysilanes to silanols and condensation of silanols formed with terminal OH groups present on the particle surface. For silanes provided with olefinic or epoxy functionalities, e.g. methacryloxypropyltrimethoxysilane (MEMO), vinyltrimethoxysilane (VTMO), and glycidyloxypropyltrimethoxysilane (GLYMO), a polymerization-active siloxane shell is formed around the nanoparticles which results in crosslinking reactions with the acrylate network as depicted in Scheme 1. 4. Characterization of grafted nanoparticles After modification the silane coverage of silica nanoparticles modified by GLYMO, MEMO, and PTMO has been determined by TPO (Fig. 1). For all silanized samples, the overall loss of weight as a function of the temperature revealed burning at about 300 1C. The maximum weight loss of MEMO, GLYMO, and PTMO modification was observed to be 24, 16, and 9 wt% respectively. Assuming a schematic grafting via a tridentate silane structure, a monomolecular surface coverage should be obtained
100
200
300 400 500 temperature (°C)
600
700
800
Fig. 1. TPO profile of silica nanoparticles grafted by PTMO, GLYMO, and MEMO.
already by about 4.3 wt% GLYMO. The observed organic content of silica modified by MEMO and PTMO is likewise higher than the expected content due to monomolecular silane coverage. The translation of the content of organics into molecular surface coverage is 5.5, 3.7, and 5.6 molecules/nm2 for MEMO, GLYMO, and PTMO modification, respectively. Compared to 2–3 OH groups per nm2 on nanosized silica [9] the observed coverage is higher than a monomolecular one and can be taken to suggest grafting of precondensed silane structures. To characterize silanized oxide surfaces 29Si NMR spectroscopy was proven to be the standard method [10]. With respect to condensed or grafted organosilanes, signals are typically observed in the 45y50 ppm, 55y 60 ppm and 65y70 ppm regions and are assigned to mono(T1)-, bi(T2)-, and tri(T3)-fold Si–O-linked silicones, respectively.
ARTICLE IN PRESS F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
569
TiO2
-40
-50
-60 chemical shift [ppm]
-70
-80
Fig. 2. 29Si NMR spectra of GLYMO modified alumina, titania, and silica nanoparticles in the presence of GLYMO.
a
signal intensity [a.u.]
e f
d
c
b
80 70 60 50 40 30 20 10
17
18
19
3402.049
16
3066.873
2576.743
15
3224.986
14
2907.853
13
2398.672
12
20
2000
2733.822
1711.474
11
2221.693
1500
SiO2
10
2045.576
Al2O3
9
1377.372
signal intensity [a.u.].
signal intensity [a.u.]
1535.415
8
1869.520
Monomer units 7
2500 m/z
3000
3500
Fig. 4. MALDI-TOF mass spectrum of GLYMO modified silica nanoparticles. All molecules were observed.
solvated nanoparticles. Fig. 3 shows the 13C NMR spectrum of silica nanoparticles after modification by GLYMO. Broad bands between 7 and 10 ppm reveal a down filed shift of the carbons nearest to silicon at 5.1 ppm (SiCH2–CH2–CH2O(CH2CH2O)) indicating their significantly lower segmental mobility of the silane chain as well as a specific interaction with the silica surface.
0
5. MALDI-TOF MS investigations
with silica
neat GLYMO
15
13
11
9 7 chemical shift[ppm]
5
3
Fig. 3. Part of 13C NMR spectrum of GLYMO and a dispersion of GLYMO-modified silica nanopowder within GLYMO used as solvent. Insert: full scale spectrum.
The intense signal at 42.0 ppm (Fig. 2) reveals the presence of neat GLYMO (used as solvent). Whereas T1 and T2 structures of grafted or condensed trialkoxysilanes are observed to a similar quantity on alumina, titania, and silica nanoparticles, highly condensed T3 silicon atoms were rarely formed during in-situ silanization. These findings obtained in the presence of an excess of silane disagree with observations under typical in-situ silanization conditions (using the resin as solvent) where high proportions of T3 silicones were obtained [8]. In addition, the interaction between filler and trialkoxysilanes can be verified by 13C NMR spectroscopy of
Activated by laser irradiation, MALDI TOF mass spectrometry clearly showed the presence of GLYMO on the surface of silica nanoparticles (Fig. 4). The formation of GLYMO oligomers composed of more than 20 monomeric silane units was indicated by molar masses of 43600 Da. Moreover, mass signals differing in 18 Da indicated different degrees of trialkoxysilane condensation. The observed MS pattern is in agreement with a proposed a ladder-like structure of grafted trialkoxysilanes [6]. In addition to improved dispersibility of inorganic fillers after silane modification, crosslinking reactions between filler and resin can be achieved by trialkoxysilanes provided with polymerization-active functionalities. Both the nanoparticles modified by GLYMO and the epoxy Novolac resin can be identified by IR bands of the characteristic oxiran ring structure between 800–950 cm1. As shown in Fig. 5, the band at 910 cm1 assigned to epoxy structures disappeared after heat curing at 160 1C and revealed, thereby, the formation of polymeric network. 6. Material characterization Nanoparticles modified with respect to the chemical nature of the base resin can be crosslinked with the monomeric and oligomeric matrix components during curing. These copolymerization reactions are expected to have strong effects on the viscoelastic properties of the nanocomposite if the nanoparticles are homogenously
ARTICLE IN PRESS F. Bauer et al. / International Journal of Adhesion & Adhesives 26 (2006) 567–570
570
Table 1 Onset and glass transition temperature of epoxy adhesives reinforced by 30 wt% GLYMO modified nanoparticles
910
Epoxy Epoxy Epoxy Epoxy
(a) 1750
1500
1250
1000
Tg (1C)
57 81 83 80
66 85 92 86
modified titania, there is an apparent increase of the storage modulus E0 in the temperature range 60 1C to 100 1C. This apparent stiffening of the sample can be attributed to perturbation resulting from stress release within the sample leading to changes in sample geometry [11].
(b)
2000
resin resin+TiO2 resin+Al2O3 resin+SiO2
Tonset (1C)
750
wavenumber[cm-1]
Fig. 5. IR spectrum of epoxy Novolac resin filled with 30 wt% alumina nanoparticles modified by GLYMO before (a) and after heat curing (b).
2.0 epoxy resin e epoxy + TiO2 epoxy + Al2O3 e epoxy + SiO2
E′-Modulus (109 Pa)
1.5
1.0
0.5
7. Conclusions After surface modification by glycidyloxypropyltrimethoxysilane, about 30 wt% silica, alumina, and titania nanoparticles have been incorporated in epoxy Novolac adhesives. The coated nanoparticles were studied by 29Si NMR and MALDI-TOF mass spectrometry which revealed anchoring of polysiloxane chains. Due to the polymerization-active surface coverage of nanoparticles, crosslinking reactions occurred during heat curing between the modified nanoparticles and the epoxy resin formulation. Hence, the reinforcement of epoxy adhesives yielded an improvement of viscoelastic properties compared with the neat epoxy Novolac system. For all the nanocomposites, increased stiffness and a shift of the glass transition temperature of about 20 K have been observed. References
0 0
20
40
60
80 100 120 temperature(°C)
140
160
180
200
Fig. 6. Temperature dependence of the storage modulus E0 of a epoxy resin and the corresponding nanocomposites with GLYMO modified titania, alumina, and silica.
distributed in the resin matrix. Indeed, in DSC measurements of an epoxy resin filled with 30 wt% GLYMO modified titania, alumina, and silica, both the modulus as well the glass transition temperature increased compared to the neat epoxy resins (Fig. 6). As expected, the incorporation of polymerization-active nanoparticles yield an increased density of the polymeric network and higher stiffness. The reinforced composites revealed a shift of the glass transition temperature DTg of about 20 K (Table 1) pointing to the particular thermal stability of alumina nanocomposites. For the nanocomposites with GLYMO
[1] Allen KW. J Adhesion Sci Technol 1992;6:23. [2] Zhang S, Schindler B, Nicholson G, Bayer E. J High Resolut Chromatogr 1995;18:579. [3] Landry CJT, Coltrain BK. J Macromol Sci A 1994;31:1965. [4] Lan T, Pinnavaia TJ. Chem Mater 1994;6:2216. [5] Dufresne A, Cavaille JY, Helbert W. Macromolecules 1996;29:7624. [6] Bauer F, Gla¨sel H-J, Decker U, Ernst H, Freyer A, Hartmann E, Sauerland V, Mehnert R. Progress in Organic Coating 2003;47:147. [7] Bauer F, Gla¨sel H-J, Hartmann E, Langguth H, Hinterwaldner R. Int J Adhes Adhes 2004;24:519. [8] Bauer F, Ernst H, Decker U, Findeisen M, Gla¨sel H-J, Hartmann F, et al. Macromol Chem Phys 2000;201:2654. [9] Michael G, Ferch H. Schriftenreihe Pigmente: Grundlagen von AEROSILs (Technical Bulletin # 11), Degussa AG, Frankfurt a.M. 1998. [10] Albert K, Bayer E, Pfleiderer B. J Chrom 1990;506:343. [11] Mano JF. Thermochimica Acta 1999;332:171. [12] Gla¨sel HJ, Hartmann E, Mehnert R, Hinterwaldner R. Europ Patent 1 1999;123:354 assigned to Institut fu¨r Oberfla¨chen-modifizierung e.V. and Hinterwaldner Consulting & Partner (GbR).