Hybrid nanocomposite coatings for application in ...

Solid State Phenomena Vol. 151 (2009) pp 234-239 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland

Hybrid nanocomposite coatings for application in construction materials: tribological study C. Silvestre1, a, MJ López-Tendero1,b , M. Cruz-Yusta1,c, N. Baeza2,d, C. Guillem2,e, S. San Juan1,f, J.M. Lloris1,g, E. Tamayo3,h 1

Unidad Técnica de Investigación de Materiales, Instituto Tecnológico de la Construcción AIDICO, Benjamín Franklin, 17, 46980 Paterna, Valencia, España

2

Unidad Técnica del Mármol, Instituto Tecnológico de la Construcción, AIDICO, Camí de Castella nº 4, 03660 Novelda, Alicante, España 3

Instituto de Ciencia de los Materiales, Universidad de Valencia, Polígono La Coma s/n, 46980, Paterna, Valencia, España a

b

c

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected], g [email protected], [email protected]

d

Keywords: hybrid, sol-gel, coating, tribological properties, construcion applications

Abstract. The Nanocomposite group of AIDICO and the Nanomaterials Group of University of Valencia collaborate in the research of new organic-inorganic hybrid materials with application in construction products. Tribological properties and good thermal stability are some of the required specifications for such applications. Some hybrid coatings based on simultaneous formation of epoxy and silica networks are introduced in this work. Elastic modulus and hardness obtained from TDMA analyses and indentation techniques, together with thermal stability by TGA, are the main properties analysed in order to fulfil the requirements for construction coating applications. New routes for obtaining these coatings based on sol-gel have been explored. Introduction Hybrid Organic-Inorganic materials [1-3] is an emergent research line in the field of coatings for applications in high mechanical and durability performance building materials [4]. Inorganic coatings are known for their high resistance towards heat and damage, having the disadvantage of being very brittle. Organic coatings, on the other hand, are flexible, adhere well to substrates, and can be easily modified or functionalized; but they are lacking a high resistance to damage or heat. Most of the organic/inorganic hybrid materials are nanocomposite materials where the inorganic part and the organic entities interact at molecular level at nanoscopic domain. The result hybrid shows a synergistic combination of totally new properties different from the ones inherent to the two initial unmodified components. [5] The definition of a hybrid material includes a wide range of materials: ranging from mono-phase polymer networks, where the hybrid composition reefers to the presence of functional groups of different nature with respect to the main component [6], to self-assembling or host-guest superstructures [7]. At this point it is relevant to clarify that the term “hybrid” is not the appropriated one for describing a system based on mixtures of pure organic or pure inorganic component [8]. Although there are many different approaches for obtaining hybrid materials, the hybrids are always processed under a temperature range that the organic compounds can withstand. Due to that, most common methods are based on the use of sol-gel technology [9] combined with organic polymer chemistry, due to the fact that the so-called sol-gel process enables the synthesis of ceramic materials at temperatures as low as room temperature. The first hybrids based on organic polymer chemistry reported in literature were synthesized using polydimethylsiloxane (PDMS) and TEOS as a silica precursor [10]. The next step in hybrid

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materials development was to apply the same strategy to carbon backbone polymers [11]. Wilkes et al. worked in this area calling their result hybrids ceramers [12]. They applied a number of oligomers and polymers in combination with various metal alkoxides in the sol-gel process. The polymers were modified with silane coupling agents in order to ensure a covalent interaction between the organic and the inorganic phase. On later researches the interaction between organic and inorganic phase in hybrid materials was achieved via hydrogen bonding or via the reaction of hydroxyl groups of organic compounds with the alkoxy groups of the sol-gel precursors. This work presents the first results of a project that aims to understand the relationship between the structure and the syntheses parameters, processing, and mechanical properties of hybrid materials used in the development of coatings that could be applied on construction products in order to increase their tribological properties. Experimental Materials. Tetraethoxysilane (TEOS, Aldrich Chemical Company) was used for the preparation of the precursor sols and was employed without further purification. Distilled water was used for hydrolysis and 0.996 N HCl (Fluka) and Amonium hydroxide 0.996 N (Fluka) were used as catalysts. The selected epoxy prepolymer was diglycidyl ether of bisphenol A (DGEBA, DER331) from Air Products, having an epoxy molecular mass (EMM) of 190 g/mol. An aliphatic amine, Anquamine 401 (Air Products) was also used as a crosslinker or hardener agent. In a previous study the proportion resin/hardener was adjusted to 100/60 [13]. Preparation of hybrids. The usual procedure for obtaining epoxy hybrids is the direct mix of TEOS precursors with Ratio prepolymers and crosslinkers, but this does not TEOS/epoxy Prehydrolysis allow the required hydrolysis to yield reactive wt % hydroxyls. Another problem in common methods is the loss of inorganic precursor due to 10:90 pH= 2 STHA-1 evaporation process during curing with 15:85 pH= 2 STHA-2 temperature. This work analyzes different 20:80 pH= 2 STHA-3 procedures based on pre-hydrolysis of TEOS, 25:75 pH= 2 STHA-4 both in acid (pH 2) and alkaline (pH 12) 10:90 pH= 12 STNA-1 water/TEOS mixtures (molar ratio 5:1), that 15:85 pH= 12 STNA-2 were left stirring for 1 hour. The resulting pre20:80 pH= 12 STNA-3 hydrolized TEOS was mixed with the epoxy 25:75 pH= 12 STNA-4 polymer at 6500 rpm during 5 minutes, and this Table 1. Hybrid polymer formulations. mixture was then kept under agitation at 350 rpm during 3 hours. The following step was the addition of water diluted crosslink agent (50 wt%) to the previously prepared TEOS/epoxy mixture. Molded pieces were cured at 60 ºC during 2 hours and postcured at 80 ºC during 1 hour. Table 1 collects all hybrid formulations with variable TEOS/epoxy compositions that were prepared based on a total mass of TEOS+polymer at varying weight ratios. The ratios of TEOS/epoxy compositions varied from 10:90 to 25:75 wt% for both prehydrolysis processes at pH 2 and 12. Dynamical mechanical thermal analysis (DMTA). The viscoelastic properties of the cured hybrid materials were studied using a Dynamical Mechanical Temperature Analyser (DMTA) Mettler Toledo DMA/SDTA861, using the two bending-point mode with a heating rate of 3 ºC/min. under a temperature range from -50 to 180ºC. The materials were cured at 60 ºC during 2 hours and postcured at 80 ºC during 1 hour. The test specimens had the following dimensions: 2x10x20 mm3. Thermogravimetry (TGA). TGA studies were carried out in a Thermogravimetric analyzer (TGA) TGA/SDTA 851e (Metler Toledo). About 20 mg of cured hybrid material was heated up

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Nanocomposite Materials

from 25ºC to 700ºC using a heating rate of 10ºC/min. All experiments were carried out in air atmosphere (flow rate 80mL/min). Vickers hardness. For the Vickers hardness tests, samples were prepared by applying 60 microns thick hybrid coatings on construction substrates. A pyramid-shaped diamond indenter was pressed against hybrid coating surfaces during 10 seconds using a load of 100 grams. Differential Scanning Calorimetry (DSC). DSC measurements were performed under nitrogen flux using a DSC 822e equipment (Mettler Toledo). The following conditions where used: temperature range from -60 to 180ºC at 10ºC/min heating rate. Results and discussion The thermal properties of the cured hybrid formulations were analyzed using a thermogravimetric analyzer. The TGA curves of pure epoxy resin in presence of an increasing amount of TEOS prehydrolized, at acid and basic media, are shown in figures 1 and 2 respectively. A rapid weight loss was observed around 300-350 ºC for the hybrid polymers synthesized at pH 2 with the exception of hybrid STHA-4 which showed to be more stable and decomposed more rapidly around 400 ºC. For hybrids obtained at pH 12 a rapid weight loss was also achieved at a different temperature, around 350-400 ºC. The decrease in thermal stability of the hybrid polymers synthesized at pH 2 could indicate a less extensive silicate network formation respect to the ones synthesized at pH 12.

Hybrid polymers synthesized at pH 2 decompose in three stages, while hybrids synthesised at pH 12 undergo decomposition largely in one step. This could be due to the fact that the more extensive silicate network formed at pH 12 undergoes decomposition in essentially a single step.


E' (GPa)

Using differential calorimetry methods, the obtained Tg values (Table 2) showed higher values for all the hybrid formulations than the epoxy polymer used as reference (Tg value of neat resin 82 ºC). For hybrids formulated under acid conditions the higher increments of 12 and 13 ºC (referred to the neat epoxy) correspond respectively to hybrids STHA3 and STHA4 having more TEOS contents. For the hybrids obtained at pH basic the Tg values are all close to 90 ºC that implies an increment of 8ºC.

14.0

STHA

12.0

STNA

10.0 8.0 6.0 4.0 2.0 0.0 0

5

10

15

20

25

% TEOS in total m ass TEOS+EPOXY

Fig. 3. Storage modulus evolution with TEOS content

30

Ao STHA 1 STHA 2 STHA 3 STHA 4 STNA 1 STNA 2 STNA 3 STNA 4

Ratio TEOS/epoxy wt% 0 10:90 15:85 20:80 25:75 10:90 15:85 20:80 25:75

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DSC Tg (ºC) 82 90 90 94 95 89 89 90 89

Table 2. DSC results of Hybrid formulations

Fig. 3 shows the variation of the storage modulus (E’ measured at a constant temperature of 20 ºC) versus % of TEOS in epoxy hybrids obtained at pHs 2 and 12. The variation of storage modulus is non-linear. At the lowest content of inorganic phase precursor TEOS (10 %) there is not noticeable increase in the modulus value compared to the one of the neat epoxy resin (E’= 4.5 GPa). A little increment of TEOS content form 10 to 15 % produces a modulus increase to 6.9 GPa and 7.7 GPa in hybrids obtained at basic and acid conditions respectively. Higher contents of TEOS precursor yield to very close modulus values for acid and basic conditions (9.2 and 9.5 GPa respectively). Further investigation is required to provide more accurate values of E’.

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HV Increment (%)

16 14 12 10 8 6 4 2 0 STHA1 STHA2 STHA3 STHA4 STNA1 STNA2 STNA3 STNA4

Hybrid polymers obtained in acid conditions showed a quasi-lineal increase of Vickers micro-hardness following the increase in content of TEOS precursor, as it would be expected since more silica network is formed. Increments higher than 10 % were achieved for all the prepared hybrids except for the hybrid STNA1 prepared in alkaline conditions. (Fig 4)

Fig.4. % Vickers micro-hardness increment vs neat resin

In all cases the hybrids showed higher VH values for acid prepared samples that for their corresponding basic prepared ones. A maximum value of 25 for VH was achieved when a 25 % of TEOS precursor was employed and was prehydrolized in acid conditions. Conclusions Two different prehydrolisis procedures at pH 2 and pH 12 were conducted for obtaining hybrid TEOS/epoxy composites, using Anquamine crosslinker and incorporating different contents of inorganic phase precursor TEOS (ratio TEOS/epoxy wt % from 10:90 to 25:75). It can be concluded that, in general, acid prehydrolisis leads to an increase in storage modulus and Vickers microhardness value for hybrid coatings applied on constructions substrates. Thermal characterization by TGA analyses showed that under basic prehydrolysis conditions the hybrids showed a better thermal stability, starting decompositions at higher temperature (300 ºC) than those under acid conditions (350 ºC). There are not big differences between the Tg values for acid and basic conditions, although they are slightly high for acid prehydrolizes samples with 20 and 25 % TEOS content. Mechanical and thermal results seem to be promising for further coating development that could be applied to the construction sectors, nevertheless further work must be achieved to analyse the properties in the interphase between the construction substrate and hybrid coatings. Adhesion properties and reinforcing mechanical properties in construction samples will be presented after conclusion of the project. References [1] B.M. Novak. Adv. Mater., 5(6):422-433, 1993. [2] E. Amerio, M. Sangermano, G. Malucelli, A. Priola, B. Voit. Polymer 46 (2005) 11241–11246. [3] S. Karataş, C. Kızılkaya, N. Kayaman-Apohan, A. Güngör. Progress in Organic Coatings 60 (2007) 140–147. [4] V. Morote-Martínez, V. Pascual-Sánchez, JM Martín-Martínez. European Polymer Journal 44 (2008) 3146–3155. [5] S. Yano, K. Iwata, K. Kurita. Materials Science and Engineering C 6 (1998) 75-90. [6] P. Cardiano, S. Sergi, M. Lazzari, P. Piraino, Polymer 43 (2002) 6635. [7] S. Föster, M. Antonietti, Adv. Mater. 10 (1998) 195. [8] J.G. Tsavalas, JW Gooch, F.J. Schork, J. Appl. Polym. Sci. 75 (2000) 916.


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[9] J. Brinker and G.W. Scherer. Sol-Gel Science: the physics and chemistry of sol-gel processing. Academic Press, 1990. [10] B.M. Novak, m.w. Ellsworth and C. Verrier. Nanostructured organic-inorganic hybrid materials synthesized through simultaneous processes. ACS Symp. Series, 585:86-96,1995. [11] S. Kohjiya, K. Ochiai, and S. Yamashita. Preparation of inorganic/organic hybrid gels by solgel process. J. Non-Cryst. Solids, 119:132-135.1990. [12] G.L. Wilkes, A.B. Brennan, H.H Huang, D. Rodrigues, and B. Wang. The synthesis, structure and property behaviour of inorganic-organic hybrid network materials prepared by sol-gel process. Mater. Res. Soc. Symp. Proc, 171:15-29, 1990. [13] M.J. López-Tendero, I. Beleña, E. Martínez-Tamayo. Hybrid nanocomposites based on interpenetrating organic-inorganic polymers as construction materials In Proceedings, 2nd International Symposium on Nanotechnology in Construction, Bilbao, Spain (2005).

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Nanocomposite Materials doi:10.4028/3-908454-10-7 Hybrid Nanocomposite Coatings for Application in Construction Materials: Tribological Study doi:10.4028/3-908454-10-7.234