Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Nanotechnologies applied to the restoration and maintenance ofwooden built heritage

C. Bertolini Cestari & T. MarziDipartimento di Progettazione Architettonica e Disegno Industriale, Politecnico di Torino, Torino, Italy

S. InvernizziDipartimento di Ingegneria Strutturale e Geotecnica, Politecnico di Torino, Torino, Italy

J.M. TullianiDipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Torino, Italy

ABSTRACT: The paper presents a preliminary experimental campaign, which aims at investigating the possibleapplication of carbon nanotubes to the mechanical improvement of timber structures. Different wood species areconsidered, as well as different solvents or resins as a dispersing media for carbon nanotubes.The experimentationis carried out on purpose exploiting exclusively the capillary properties of wood, in view of a possible on siteapplication of the technique. Although still at a preliminary stage, these first results appear to be promising, andthe research on the procedure is still in progress.

1 INTRODUCTION

In contrast with other fields of civil engineering andbuilding recovery technology, the application of com-posite nanomaterials employed to reinforce woodenstructures is a little known technique which requires,albeit partially, to be fully tested before being appliedon a large scale. This holds true mainly for existingwood components (elements); with regards to newwooden elements, there is a much greater wealth ofexperience.

The use of nanomaterials and composite materi-als to strengthen wood was suggested seven yearsago by several American researchers who studiedthe effects of carbon nanotubes-based composites onthe mechanical characteristics of reinforced woodenelements [1].

In Italy, given the greater scope of recovery andconservation issues, research has focussed on method-ologies for consolidating existing ancient structures.Experiments carried out on antique timber and existingstructures are very complicated since the mechani-cal characteristics of the timber vary on account ofthe inevitable presence of membrane defects (knots,oblique wood fibre, lesions, etc.). However, experi-mental results have shown a clear trend in terms ofefficiency concerning said recovery techniques.

The development of polymers reinforced withnanoparticles is one of the most promising approachesin the field of future engineering applications. Theunique properties of some nanoparticles (carbon blackand carbon nanotubes) and the possibility of com-bining them with traditional reinforceing elements(fibreglass, carbon fibre or Kevlar) have generatedan intense research program in the nanocompositessector [1–2].

Carbon black is made up of particles with a diam-eter of 30 nanometres (nm) and is commonly usedas a charge to make polymers conductive in orderto avoid the accumulation of electric charges. Car-bon nanotubes have a diameter of several nanome-tres and their length measures several micron; theyhave very good potential for improving the electri-cal and mechanical properties of polymers, even with0.1% weight content compared to epoxy resin [1–2].Indeed, the resistance to traction of single wall car-bon nanotubes or SWCNT can reach up to 600 GPaand the elastic modulus range between 1 and 5TPa[1]. The difficulties lie in transferring these remark-able mechanical, thermal and electrical properties ofnanotubes to the polymer matrix. Consequently thecorrect dispersion of the nanotubes in the polymerand timber interface, between the reinforcement andthe polymer matrix, is crucial [1–4].The dispersion

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of carbon nanotubes in the matrix is complex giventhe widespread specific surface of the nanoparticles(1000 m2/g or more) which tend to favour the forma-tion of agglomerates. Different techniques for dispers-ing said materials in solvents (acetone, ethanol. . .) inaddition to ultrasounds, mechanical shaking or a com-bination of the two techniques, have already been triedout [1–7].

The interface adhesion can be improved by chem-ically functionalising the nanotubes surfaces; thisgenerates strong covalent-type of bonding [2, 3, 6,7]. The bonding between nanotubes and polymersallow the strain to be transferred from one stage toanother.

Amino groups have been used for this purpose anddouble wall carbon nanotubes or DWCNT with orwithout a functional group surface can be purchasedon the market (Nanocyl, Namur, Belgium) [8].

A picture of said experimental difficulties is pro-vided by the results obtained with epoxy resins whereby adding SWCNT in the best cases the mechani-cal resistance to bending stress is slightly increased,although in most cases, resistance is often reduced[5]. Recently, fabrics containing up to 39% in SWCNTweight impregnated with epoxy resin have been pro-duced; once again the results obtained fell belowexpectations [9].

On the other hand, significant progress in mechan-ical resistance values has been made with poly-methyl methacrylate (PMMA), polyvynilic alcoholsand with polystyrene-based composites [5]. Promis-ing results have also been achieved with polyurethaneresin composites with up to 10% in weight ofnanotubes [10].

Summarizing, carbon fibre nanotubes provide anumber of advantages:

– they are morphologically and chemically compati-ble both with polymer resin, used as bonding mate-rial, and with wood, they are anatomically similar tostrong piping bonded with a thermoplastic matrixand equipped with strong dissipative capacity withregards to fracturing energy.

– nanotubes allow the polymer bonding matrix toimprove it own inbuilt deformation capacitiesconsiderably thanks to the transformation of ahomogeneous bulk with vitreous behaviour intoa micro-reticule with high level of porosity anddeformability;

– the mechanical characteristics of the resin-fibrecompound are considerable on account of the highspecific resistance of the fibres which ensure greatcohesive strength combined with high ductility.The combination of the two produces significantcreep resistance without the composite, loosing anydeformation capacity;

– the tubular structure of nanofibres has great per-meability to vapour potential: this is an important

characteristic when dealing with large surfacestreated with glue, and especially in the case of woodsince any accumulations of humidity must be easyto disperse in order to avoid biotic degradation.

2 MATERIALS AND METHODS

The first step of the experimentation was dedicated toassess the efficiency of different impregnation tech-niques.The carbon nanotubes were dispersed by meansof an ultrasonic probe for different time in ethanol andacetone. Such solvents are supposed to act as a trans-port medium to bring the carbon nanotubes directlyinto the channels of the wood microstructure.

Specimens with section equal to 2.5 cm × 2.5 cmwere sunk in the suspensions for different times, asshown in Figure 1. We decided to exploit only the cap-illarity natural phenomenon instead of vacuum or otherpressure assisted impregnation techniques.This choicewas adopted in view of a future onsite application inexisting timber structures.

The following wooden species have been selectedfor the analysis: fir, Douglas pine, oak and larch. Somesamples were obtained from new timber, other fromeighteenth century structures.

Nanocyl multiwall carbon nanotubes were used forthe process. For economical reasons, the series 7000,not functionalized (Table 1), was used to set up the pro-cess, while series 3101 functionalized with carboxylgroups were used for the mechanical tests (Nanocyl®-3101 series are purified to greater than 95% carbon andthen functionalized with COOH groups).

In a second step, an epoxy resin (MAPEI epojet,[11]) was used for the dispersion in order to get a prod-uct for mechanical improvement that could be appliedon the wood surface by painting, or act as a reinforcedglue to connect different timber part.

Epojet is a two component solvent-free epoxy adhe-sive. The pre-measured portions (Part A = resin andPart B = hardener) must be mixed together beforebeing used. Once mixed, Epojet becomes a liquidwith low viscosity very suitable for injection. Themix ratio between part A and B is 4 to 1. Thisresin has been selected because of its low Brook-field viscosity (respectively 500 and 320 mPa.s forpart A and B).

The efficiency of the impregnation procedure hasbeen assessed with observation at the SEM micro-scope. The overall mechanical improvement of thetimber specimen is going to be evaluated from compar-ison of mechanical tests performed on un-reinforced,impregnated and resin painted samples. The bendingstrength has been evaluated by the three-points bend-ing test carried on with displacement control.The spanof the samples was 370 mm and the load was appliedwith a velocity of 12 N/s.

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Figure 1. Impregnation test at different times: progressionof the solution absorption.

Figure 2. TEM image of Nanocyl carbon nanotubes.

Table 1. Carbon nanotubes characteristics.

Method ofProperty Unit Value measurement

Average diameter nanometer 9.5 TEMAverage length microns 1.5 TEMCarbon purity % 90 TGAMetal oxide % 10 TGASurface area m2/g 250 BET

Figure 3. Detail of the three-point bending test.

3 RESULTS AND DISCUSSIONS

3.1 Mechanical characterization of un-reinforcedwood

The un-reinforced behavior of four different timberspecies have been characterized, from a mechanicalpoint of view, with standardized three-point bend-ing tests. A picture of the test procedure is shown inFigure 3.

The oak old samples displayed the most fragilebehavior, while Douglas pine appear to be sensitive

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Figure 4. Load displacement diagrams for four differentunreinforced timber species.

Figure 5. SEM image of the larch sample as such.

to local buckling phenomena. The peak load rangedbetween 1340 N and 2770 N, provided that the crosssection of the samples was 25 mm times 25 mm. Theload displacement diagrams of each test is shown inFigure 4.

A direct comparison with reinforced samples willbe provided in the future.

3.2 Carbon nanotubes dispersion in solvents

Four different suspensions have been prepared withdifferent nanotubes (series 7000) concentrations. Fur-ther, each suspension has been used for impregnationas shown in Figure 1. In order to assess the efficiencyof the impregnation the higher part of the specimenwas cut and observed with the SEM. As a reference,also slices of not impregnated wood were observed(Figure 5).

The first suspension was prepared by dispersingabout 0.1% carbon nanotube in ethanol, after 20 min-utes of ultrasonication with an ultrasonic probe. Adroplet of this suspension was sampled and set uponthe SEM sample holder for observation. As illustrated

Figure 6. Clustering of carbon nanotubes in the firstdispersion.

Figure 7. Better dispersion of nanotubes in the seconddispersion.

in Figure 4 the dispersion was unsuccessful, since thenanotubes remained agglomerated.

It is worth noting that the morphology of the clusterin Figure 6 is also due to the poor purity of the adoptedcarbon nanotubes as reported in Table 1. Due to thisreasons, the impregnation was not proceeded.

In the second dispersion we decided to increasethe sonication time up to two hours, and the car-bon nanotube content (1%). The resulting suspension(Figure 7) was more satisfying, but we could not pro-ceed with the impregnation due to high viscosity ofthe suspension (which was close to that of a paste).

The third ethanol suspension was obtained with0.5% carbon nanotube content and two hours son-ication. This proportion allows a good dispersionpreserving a moderate viscosity, and was chosen forfurther impregnation. Unfortunately, although the cap-illary process is active, there was no effective transport

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Figure 8. SEM image of the top larch sample section: nonanotubes appear to be penetrated the wood microstructure.

Figure 9. SEM image of the top larch sample sectionimpregnated with acetone solution: almost no nanotubesappear to be penetrated the wood microstructure.

of nanotubes inside the sample, as observed with theSEM (Figure 8).

Because of this unsuccessful trials, we decided tochange the solvent and adopt acetone in place ofethanol.

The acetone solution of 0.5% carbon nanotubeweight content was sonicated for two hours andthe resulting dispersion was good preserving lowviscosity.

The wood sample was sunk into this suspension fortwenty-four hours. Also in this case, almost no car-bon nanotubes are observable at the top section of thesample from the SEM image (Figure 9).

In each case, the SEM analysis has been pushed upto the maximum allowable resolution (10000×) butwe were not able to observe not even CNT clusters.

Figure 10. SEM image of CNT agglomeration in the epoxyresin (the resin was fractured to emphasize the defectspresence).

3.3 Carbon nanotubes dispersion in epoxy resin

The main difference between epoxy resins and the sol-vents described above pertains the higher viscosity.Therefore, first of all it has been necessary to find theoptimal carbon nanotube content keeping constant thetwo hours sonication duration.

Proved that although the dispersion in the solventwas efficient, the overall impregnation technique wasnot working satisfactorily, we moved to investigate thedispersion using resin as a dispersing medium.

After some trials, we adopted a carbon nanotubesweight content equal to 0.3% with respect to the resin(A + B fraction).

We decided to disperse first the nanotubes in theresin component B, which has the lowest viscosity.Then the two components were mixed together duringthirty minutes mechanical stirring at room tempera-ture. The mix was poured into a mould and let curingfor six hours at sixty degrees.

Unfortunately, this precaution was not sufficient toavoid clustering of carbon nanotubes, as evidenced byFigure 10, and in more detail Figure 11.

3.4 Mechanical behavior of reinforced wood

A preliminary assessment of the efficiency of the rein-forcing procedure has been carried out in the followingway. First some of the timber samples were coated withresin only, then other were coated with the CNT rein-forced resin obtained after four hours sonication. TheCNT content was increased up to 0.5% with respect tothe resin.

All the coated samples were cured at room tem-perature for seven days, according to the manufac-turer recommendations, and to simulate an in-situintervention.

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Figure 11. SEM detail image of CNT agglomeration in theepoxy resin.

Figure 12. Load displacement curves for the eighteenth cen-tury oak samples. Comparison between un-reinforced, resincoated, and CNT reinforced resin coated.

Figure 13. Load displacement curves for the Hemlok firsamples. Comparison between un-reinforced, resin coated,and CNT reinforced resin coated.

The preliminary results obtained from the few sam-ples tested till now are in some case very promising,in other case not so unambiguous.

In the case of the eighteenth century oak samples,we observe an increase of about 19% in the peak load

when only resin is used for the coating. If CNT rein-forced resin is used for the coating, the gain in the peakload raises up to 42%.

On the other hand, as far as the Hemlok fir samplesis concerned, an increase of about 45% is obtained incase of resin coating, regardless the presence of a CNTreinforcing in the resin.The influence of the CNT rein-forcing appears even to be slightly disadvantageous,but this could be also due to the fact that we still havetoo few results to compare.

In general, the comparison must be continuedconsidering more samples and more timber species.

4 CONCLUSIONS

In the paper a preliminary experimental campaign isdescribed, which aims at investigating the possibleapplication of carbon nanotubes to the mechanicalimprovement of timber structures. Different woodspecies were considered, as well as different solvents orresins as a dispersing media for carbon nanotubes. Theexperimentation was on purpose carried out exploitingexclusively the capillary properties of wood, in viewof a possible on site application of the technique.

As far as the present results concern, the solvent dis-persions appears, if properly designed, effective fromthe point of view of getting an optimal dispersion.None of them, unfortunately, appear to be efficientfrom the point of view of impregnation exploiting thesolely capillary phenomenon.

On the other hand, the epoxy resin dispersions arestill problematic since the aspect of carbon nanotubesclustering has not been solved yet.

The first results concerning the assessment of thetimber retrofitting with CNT resin coating are some-what positive, since they provide an increase in theflexural strength which is greater or at least equal tothe one obtained with the resin alone.

These first results appear to be challenging, and theresearch on the procedure is still in progress [12].

ACKNOWLEDGEMENTS

The research was carried out also with the economi-cal support of the Italian Ministry of University andResearch, through a PRIN project co-ordinated atnational level by C. Bertolini Cestari.

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