Polymer Nanocomposites for Infrastructure Rehabilitation

Transcription

1 Polymer Nanocomposites for Infrastructure Rehabilitation M.R. Kessler and W.K. Goertzen 1 Abstract. Polymer matrix composites (PMCs) are becoming increasingly important in the structural repair and rehabilitation of damaged infrastructure from pipelines to buildings to bridges. For example, composite overwraps are used to repair corroded steel pipelines because the repair can be completed in a relatively short amount of time and the fluid transmission in the piping system can remain undisrupted while the repair is being made. Often in these applications, a primer and filler adhesive is used to fill defects in the substrate so that load can be adequately transferred to the continuous fiber composite. In this work we discuss various nano-scale reinforcements such as fumed silica, alumina, nanoclay, and carbon nanotubes as additives to this filler adhesive in order to improve mechanical properties and to tailor the thermal expansion of the composite to match the underlying substrate being repaired. The thermal expansion mismatch is especially important in applications where temperature fluctuations are present. We highlight our results from rheology, thermal expansion, and dynamic mechanical analysis testing of nanosilica/cyanate ester composites and show that the incorporation of the nano-scale fillers can result in improvement of the thermo-mechanical behavior of the composites. 1 Introduction Corrosion has a costly and deleterious effect on aging infrastructure throughout the world. As such, considerable attention has been focused on innovative techniques to arrest corrosion in the carbon steel found in bridges, pipelines and pipework, water and wastewater systems, and electric power generation facilities and M.R. Kessler Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, USA mkessler@iastate.edu W.K. Goertzen Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, USA

2 242 M.R. Kessler and W.K. Goertzen to restore the structural integrity of these systems, especially pipelines and bridges [1-4]. Many of these repair technologies utilize fiber-reinforced polymer matrix composites. In damaged pipelines, composite overwraps can be used for timely, costeffective repair of external corrosion (as shown in Figure 1) without the need to disrupt fluid transmission in the piping system while the repair is being made. In order for the composite overwrap to be effective, a putty (filler adhesive) is used to fill the defect region to allow a uniform surface for the outer composite wrap to be applied. The filler adhesive is the medium by which the pipe pressure is transferred to the outer fiber-reinforced composite wrap. Because of the processing and performance requirements on the filler adhesive, nanoparticles may be added to the thermosetting resin to increase the thixotropy of the prepolymer resin and the thermomechanical properties of the cured adhesive. In our work we have investigated systems with fumed nanosilica, nanoalumina, multiwalled carbon nanotubes, and carbon nanofibers. Fig. 1 Typical composite repair system for damaged pipeline showing (top left) damaged pipe with external corrosion damage, (top right) repaired pipe with composite overwrap installed, (bottom) cross-sectional schematic showing metallic pipe, filler adhesive, and external composite wrap

3 Polymer Nanocomposites for Infrastructure Rehabilitation 243 The steps involved in making the repair are shown in Figure 2. First the prepolymer resin is mixed with appropriate curing agents and nanoscale fillers (using a combination of high shear mixing and ultrasonication) and applied to the steel substrate to restore the original dimension of the pipe. Next the primer is applied to the remaining substrate surface. The outer composite overwrap is applied after first impregnating the reinforcement (in this case a carbon fiber fabric) by hoop wrapping the reinforcement around the defect region followed by curing of the thermosetting polymer matrix. Fig. 2 Steps in repairing a damaged pipe using a composite overwrap. The epoxy putty used in step 3 to fill the defect is rheologically engineered with nanosilica (Photo courtesy of Jeff Wilson) 2 Nano-fillers There are several nanoscale fillers that may be added to the thermosetting prepolymer prior to application on the repair substrate. The purpose of adding these fillers is to (1) increase the thixotropic behavior of the prepolymer to prevent sag of the putty before the outer composite is applied and cured; (2) increase the stiffness and strength of the cured network, thereby increasing the load that is transferred to the outer structural composite; and (3) reduce the thermal expansion mismatch between the polymer filler putty and the underlying substrate. This last purpose is especially important for materials which operate at elevated temperatures such as

4 244 M.R. Kessler and W.K. Goertzen Fig. 3 Thermosetting filler adhesive (cyanate ester resin) cured in a simulated steel defect. The cracks in the polymer are a result of the large thermal expansion mismatch between the polymer and the steel substrate systems that utilize pressurized steam and other process piping, pressure vessels and storage tanks, heat exchangers, burners, furnaces, and industrial exhaust systems. Figure 3 below illustrates the large strains that can develop in the polymer filler due to coefficient of thermal expansion (CTE) mismatch. The incorporation of nanoscale filler dispersed at a molecular level results in an ultra-large interfacial area per unit volume between the nano-filler and the matrix polymer. It is this large internal interfacial area, coupled with the nanoscale dimensions constraint on the polymer matrix that is largely responsible for the unique features in polymer nanocomposites compared to polymers filled with conventional microscale filler. There are numerous nanoscale fillers that can be considered in composites for infrastructure rehabilitation applications; however, many of them share common features with regard to processing, morphology, and reinforcement effect. Several of these nanoscale fillers are discussed next. 2.1 Metallic Oxides Nanosilica, Nanoalumina, Nanotitania Much of the work we have performed to date with modifying the filler adhesive with nanoparticles has been with fumed nanosilica and nanoalumina. Fumed nanosilica is made by a vapor phase flame hydrolysis process of silicon tetrachloride. In this process, SiO 2 molecules condense and form spherical primary nanoparticles from 5 to 40 nm. These primary particles form mostly aggregates (primary particles sintered together) that are about 0.2 to 0.3 µm in diameter [5]. Fumed silica is used extensively as an agent to reinforce and modify the rheological properties of liquids, adhesives, and elastomers. Thermosetting polymers such as epoxies [6-10], polyurethanes [11-12], and polyesters [13] have used fumed silica to modify rheological (thixotropy, sag resistence, and anti-settling agent) and end-use mechanical properties. Figure 3 below shows TEM micrographs of silica aggregates used in the present work. Nanoscale aluminum oxide particles and nanoscale titanium dioxide particles are processed by a similar flame hydrolysis process as the fumed silica, but with other metallic chloride precursors. TEM micrographs of nano-alumina particles used in the present study are shown in Figure 5.

5 Polymer Nanocomposites for Infrastructure Rehabilitation 245 Fig. 4 TEM of fumed silica aggregates. The image on the left has a primary particle size of 12 nm (AEROSIL 200). The image on the right has a primary particle size of 40 nm (AEROSIL OX 50). The scale bar is 200 nm Fig. 5 TEM of nanoalumina particles used in this work (image courtesy of Mufit Akinc) 2.2 Nanoclay Perhaps the most widely investigated nanoparticles in polymer composites are montmorillonite nanoclays. The key to obtaining well dispersed, effective nanocomposites with clays is to obtain exfoliation of the particles, which is complicated by the coupling of the particles due to surface charges and self-attraction. Nanoclays are often referred to as crystals or tactoids, but they are actually composed of thousands of silicate layers (platelets), geometrically stacked like a deck of cards. The surface of a platelet has a relative positive charge (cations). These charges can be shared between adjacent platelets and promote adhesion of the platelets. In addition, bonding of the platelets can also occur by weak van der Waals bonds, further promoting the deck of cards cubic structure and preventing mixing in organic solutions. In order to promote compatibility of the particles in organic materials, such as thermosetting prepolymers, the surfaces of the particles are typically made hydrophobic. This is usually accomplished by modifying the surface with an organic surfactant, such as ammonium cations that have an alkyl chain [14]. Basically, these chains act as a tie layer one end of the molecule has an affinity for the cation surface of the platelet and the other end has an affinity for organic molecules. This helps the particles to be mixed into an organic solution. If the

6 246 M.R. Kessler and W.K. Goertzen clay particles are sufficiently exfoliated and well dispersed in the polymer matrix some of the material properties that are significantly enhanced include strength, stiffness, and permeability (moisture susceptibility). 2.3 Carbon Nanotubes and Nanofibers Multi-walled carbon nanotubes [15] and single-walled carbon nanotubes [16,17] are now nearly 15 years old. Since their discovery and synthesis a decade and a half ago, much interest has been shown by researchers and business leaders within the polymer and composites community. The high strength and elastic modulus, and low CTE (α axial ~ K 1, α transverse ~ K 1 ) [18] combined with the high aspect ratio of the nanotubes make them ideal candidates for nanoreinforcement in polymer matrix composites for infrastructure rehabilitation. Because of the extremely high strength of the individual nanotubes, failure of nanocomposites nearly always occurs at the interface between the matrix and the nanotubes, and adequate dispersion in the host matrix can be an issue during processing. However, when these obstacles are overcome (such as by chemical functionalization of the nanotube surface), the benefits of adding carbon nanotubes to polymers include increased dimensional stability, conductivity, improved thermal properties (T g and flame resistance), improved mechanical properties (strength and stiffness), and significantly reduced thermal expansion coefficients [19] even at relatively low loading levels. Figure 6 below shows TEM micrographs of a single multiwalled carbon nanotube and the nanotubes dispersed in a thermosetting polymer matrix used by our research group. Fig. 6 TEM images of (left) an individual multiwalled carbon nanotubes and (right) carbon nanotubes dispersed in a thermosetting polymer matrix (images courtesy of Wonje Jeong) Carbon nanofibers (CNFs) are similar to carbon nanotubes (CNT), but can be produced at a lower cost. CNFs with diameters ranging from 50 to 200 nm are larger than CNTs but smaller than continuous carbon fiber.

7 Polymer Nanocomposites for Infrastructure Rehabilitation Experimental The remainder of the paper will focus on our results in modifying a filler adhesive for pipeline repair applications with nanoscale fumed silica. The adhesive was a bisphenol E cyanate ester (BECy), EX-1510 from Bryte Technologies (Morgan Hill, CA) cured with a polymerization catalyst (EX-1510-B) at the manufacturer s suggested loading of 3 phr (parts per hundred resin). Hydrophilic fumed silica, Aerosil 200 (referred to as 12 nm) and Aerosil OX 50 (referred to as 40 nm), was supplied by Degussa (Frankfurt, Germany) shown previously in Figure 4. Cyanate ester/silica suspensions were prepared by slowly adding the fumed silica during mixing of the filler putty pre-polymer with a high-shear mixer and further mixed briefly with a sonic dismembrator (3.2 mm diameter probe tip, frequency of 23 khz, power ranged between 16 and 18 W during sonication). Figure 7 shows the difference in dispersion in cured composites with and without ultrasonic mixing. Portions of the mixed resins were analyzed using parallel plate oscillatory rheology. After resin was mixed, it was degassed at 60 C for 1 h under vacuum and then placed in a convection oven for the final curing process (180 C for 2 h, 250 for 2 h, ramp of 1 C/min between each step). Samples were machined from the solid block of material for dynamic mechanical analysis and thermomechanical analysis. Characterization equipment included a Q400 thermomechanical analyzer, a Q800 dynamic mechanical analyzer with gas cooling accessory, and a AR 2000ex controlled stress rheometer with environmental test chamber all from TA Instruments (New Castle, Deleware). Fig. 7 TEM images showing the effect of sonication on dispersion for 5 phr 40 nm silica in cured cyanate ester resin (left) no sonication performed before curing the composite and (right) sonication at approximately 17 Watts for 75 s before cure. Scale bar is 2 microns 4 Results and Discussion Figure 8 shows the intense shear thickening and pseudoplasticity in the suspension with increasing filler content. It also shows that the system with the smaller primary particles (12 nm) has a greater level of thixotropy, likely due to increased surface-surface interactions from hydrogen bonding and subsequent agglomeration.

8 248 M.R. Kessler and W.K. Goertzen Viscosity (Pa-s) increasing silica loading Shear Rate (1/s) BECy 1 phr, 12 nm silica 2 phr, 12 nm silica 5 phr, 12 nm silica 6.72 phr, 12 nm silica G', G" (Pa) G G (408 s) G' - 5 phr, 12 nm silica G G (4578 s) G' - 30 phr, 40 nm silica G" - 5 phr, 12 nm silica G" - 30 phr, 12 nm silica 100 Visc. - 5 phr, 12 nm silica Visc phr, 40 nm silica Time (s) Fig. 8 Rheological behavior of the nanosilica/becy suspension prior to curing showing, (left) the viscosity versus shear rate for the 12 nm silica suspension, (right) the timedependent viscosity (thixotropy) in both the 12 nm and 40 nm suspension (results adapted from ref. [20]) 10 1 Complex Viscosity, η * (Pa-s) Storage Modulus (MPa) BECy 1 phr, 40 nm silica 5 phr, 40 nm silica 20 phr, 40 nm silica 49.2 phr, 40 nm silica Temperature ( C) increasing silica content Tan Delta BECy 1 phr, 40 nm silica 5 phr, 40 nm silica 20 phr, 40 nm silica 49.2 phr, 40 nm silica Temperature ( C) Fig. 9 Dynamic mechanical behavior of nanocomposites with 40 nm fumed silica (results adapted from ref. [21]) Fig. 10 Thermomechanical analysis (TMA) data showing the reduction in thermal expansion with increasing silica loading (results adapted from ref. [22]).

9 Polymer Nanocomposites for Infrastructure Rehabilitation 249 Figure 9 shows the increase in the glassy and rubbery moduli with increased loading of the 40 nm nano-silica and the decrease in damping with silica loading. The results indicate that the incorporation of fumed silica has a pronounced effect on the modulus of the nanocomposites. The decrease in damping was used in Ref. [21] to estimate the interphase thickness in the polymer nanocomposite. The coefficient of thermal expansion for the nanocomposites decreased from 63.5 ppm/k for the neat resin to 46.3 ppm/k when 20.7 vol% nanosilica (40 nm) was incorporated into the resin (see Figure 10). 5 Conclusions Polymer nanocomposites are beginning to be used in civil infrastructure rehabilitation, specifically in the dimensional restoration filler (filler adhesive) used in the repair of damaged pipelines and pipework. The addition of nano-scale fumed silica increased the thixotropic behavior of the prepolymer (reducing unwanted sag in the resin), decreased the compliance (inverse of the stiffnes) and thermal expansion of the nanocomposites. References 1. Mableson, R., Patrick, C., Dodds, N., Gibson, G.: Refurbishment of steel tubulars using composite materials. Plastics, Rubbers, and Composites 29(10) (2000) 2. Cuthill, J.: Advances in materials, methods, help gain new users. Pipeline & Gas Journal 229(11), (2002) 3. Meier, U.: Composite Materials in bridge repair. Appl. Compos. Mater. 7, (2000) 4. Radford, D.W., et al.: Composite repair of timber structures. Constr. Build. Mater. 16(7), (2002) 5. Product Technical Data, CAB-O-SIL M-5, Cabot Corporation, Billerica, MA, USA (2000) 6. Miller, D.G.: Improving rheology control of epoxy hardeners. Adhes Age 29, (1986) 7. Kang, S., et al.: Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol-gel process. Polymer 42(3), (2001) 8. Preghenella, M., Pegoretti, A., Migliaresi, C.: Thermo-mechanical characterization of fumed silica-epoxy nanocomposites. Polymer 46(26), (2005) 9. Jana, S.C., Jain, S.: Dispersion of nanofillers in high performance polymers using reactive solvents as processing aids. Polymer 42(16), (2001) 10. Wichmann, M.H.G., Cascione, M., Fiedler, B., Quaresimin, M., Schulte, K.: Influence of particle surface treatment on mechanical behavior of fumed silica/epoxy resin nanocomposites. Compos. Interface 13(8-9), (2006) 11. Torro-Palau, A.M., Fernandez-Garcia, J.C., Orgiles-Barcelo, A.C., Martin-Martinez, J.M.: Characterization of polyurethanes containing different silicas. Int. J. Adhes Adhes 21, 1 9 (2001)

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