A Novel Repair Method of Carbon Fiber Reinforced Plasticswith Reinforcing Fibers Intact

Y. Tsukada, Y. Suzuki, H. Takahashi and J. Mizuguchi

Fiber Innovation Incubator, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan

A novel repair method of carbon fiber reinforced plastics (CFRPs) has been developed by means of our novel technology based upon thethermal activation of oxide semiconductors (TASC). TASC means the appearance of significant catalytic effects when the semiconductor isheated at about 350­500°C. This technology allows us to remove the polymer matrix alone in damaged areas of CFRP in the presence ofthermally activated semiconductors while retaining the embedded fibers intact. The polymer-eliminated area is then refilled with an epoxy resinto complete the repair. In parallel, characterization of the carbon fiber has also been carried out by optical microscope, scanning electronmicroscope (SEM), X-ray diffraction (XRD), as well as thermogravimetric analysis and differential thermal analysis (TGA/DTA). The analysisrevealed that no noticeable deterioration of the carbon fiber is recognized, but the sizing agent for the fiber is found to be eliminated by TASC.[doi:10.2320/matertrans.M2013148]

(Received April 15, 2013; Accepted July 23, 2013; Published September 6, 2013)

Keywords: fiber reinforced plastic, glass fiber, carbon fiber, thermal activation of semiconductors, repair of fiber reinforced plastic (FRP)

1. Introduction

Fiber reinforced plastics (FRPs) are characterized by highmechanical strength as well as lightweight, and thus widelyused in construction related areas such as ships, air planes,automobiles, bicycles and so on.1,2) However, once FRPs arepartially damaged, there is no way to repair it with embeddedreinforcing fibers intact. An alternative method called thescarf method is currently used in such a way as tomechanically remove the damaged area (i.e., both polymerand reinforcing fibers) by a disk grinder, followed by anapplication of a prepreg together with a binder tape.3)

However, the mechanical strength of the repaired part isgreatly diminished because the reinforcing fibers wereentirely severed during the repair process. In the presentpaper, we will report on our novel repair method withembedded reinforcing carbon fibers intact, utilizing thethermal activation of oxide semiconductors (TASC).

The TASC technology developed by us means theappearance of significant oxidative, catalytic effects whenthe semiconductors are heated at about 350­500°C.4) TASCallows us to entirely decompose any polymers (i.e., boththermoplastic and thermosetting polymers) into H2O and CO2

in an instance. TASC creates, at first, a vast number ofoxidative holes upon heating. Then, the holes capture bondedelectrons from polymer molecules, leaving behind unstablecation radicals in the polymer, as shown in Fig. 1.5) Then, theradical travels throughout the polymer chain at elevatedtemperatures to make the whole polymer unstable. Thisinduces a radical splitting (Fig. 1), resulting in the fragmen-tation of a giant molecule into small pieces such as ethylene,propane, etc. The fragmented molecules react then withoxygen in air to give rise to H2O and CO2 (i.e., completecombustion). The TASC technology has successfully beenapplied to the complete decomposition of FRPs and theirrecycling in our previous investigations.6,7)

Then, an attempt has been made in the present inves-tigation to repair partially damaged FRPs with our TASCtechnology. The repair process is schematically shown in

Fig. 2. The crucial point is to locally and intensively heat thedamaged area to remove the polymer matrix alone whileretaining the embedded reinforcing fibers intact. To achievethis, an IR lamp, flame, or laser can be used in practice. In ourexperiment, an IR lamp was used together with an ellipsoidalmirror, using TiO2 as a semiconductor.

Although our method is still at the early stage of thedevelopment, we believe that our TASC method can basicallybe applied to the repair of a defect area where both fibers and

Fig. 1 Fragmentation of a polymer into small molecules by radicalsplitting. A seed radical created by oxidation is then multipliedspontaneously to induce the radical splitting.

oxide semiconductor

FRP Plate

Damaged area

application of a semiconductor layer

heating with IR

resin application

exposed fibers

Fig. 2 Schematic diagram of the repair process.

Materials Transactions, Vol. 54, No. 10 (2013) pp. 2059 to 2063©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

polymer matrix are damaged, and/or to the area wherethe matrix alone is damaged while the fibers remain intact.Anyway, the TASC method can keep the repair area to aminimum.

2. Experiment

2.1 Materials, experimental setup and experimentalprocedure

Carbon fiber FRPs (i.e., CFRP) were prepared in thefollowing way. Carbon fiber textiles (Torayca Cloth CO6343from Toray industries, Inc.) were impregnated with an epoxyresin of XNR6815 and a hardener XNH6815 from NagaseChemteX Corporation, using a Vacuum Assisted ResinTransfer Molding machine. Then, the plate of a dimensionof 200mm © 100mm © 2mm was cured at room temper-ature for 24 h, followed by post curing at 50°C for 12 h.

Powdered TiO2 (ST-01) was obtained from ISHIHARASANGYO KAISYA, Ltd. TiO2 was dispersed in a ketonicsolvent. Then, a thin dispersion layer of TiO2 was applied byspraying onto an FRP plate through an aluminum mask ofabout 30mm in diameter (film thickness: about 7 µm).

An IR lamp equipped with an ellipsoidal mirror (model:NQIR-S3-60/75/30-24/0.3: 24V 300W) was purchasedfrom Kashima Co., Ltd. The spot size of the lamp was about10mm in diameter at the focal distance. The lamp was placedabove the TiO2-coated CFRP together with the aluminummask as shown in Fig. 3.

The FRP plate was irradiated in such a way as to stepwisecover the whole TiO2-coated area by moving the IR lamp.The applied voltage on the lamp was about 7­10V and theirradiation time for each spot was about 3­5min. This localheating entirely removed the polymer matrix alone. Afterthat, a minute amount of remaining TiO2 was eliminated byair blow.

Resin infusion was carried out onto the polymer-eliminated area, using a compact vacuum-assisted manifoldequipped with a small resin injector. The same epoxy resinwas used for infusion and cured under the same conditions asdescribed above.

2.2 Equipment for characterization of materialsReflection spectra of the FRP plate were measured in

the IR region by means of a microscope IR spectrometer(MFT-2000 from JASCO), and in the visible region by a

microscope-spectrophotometer from Carl Zeiss (UMSP 80),using a silicon carbide as the reflection standard.

A digital microscope of MHX-2000 and a scanningelectron microscope (SEM) of VE-8800, both of which arefrom Keyence Corporation, were used for optical and SEMimages, respectively. Powder X-ray diffraction experimentswere made on an R-AXIS RAPID-F diffractometer fromRigaku. Raman spectra were measured by an NRS-3100 laserRaman microscope-spectrophotometer from JASCO Corp.Thermogravimetric analysis (TGA) and differential thermalanalysis (DTA) measurements were carried out in order tostudy the detachment of sizing materials from the reinforcingfibers. TGA/DTA measurements were performed in air witha heating rate of 10°C/min, using a Rigaku Thermo Plus8230.

3. Results and Discussion

3.1 Elimination of the polymer matrix and the refill ofmatrix resin

Figures 4(a) and 4(b) show the CFRP plate before andafter heat treatment. Figure 4(c) is a magnified picture of thecarbon fiber fabric. The carbon fiber textile looks reallyundamaged. The thickness of the polymer-removed area isabout 1mm, and this can easily be controlled by changingthe heating power. In addition, the termination of thepolymer elimination was easily recognized by the changein reflectivity in IR and visible spectra shown in Figs. 5(a)and 5(b), respectively. The reflection spectrum before

CFRP Plate

Al mask

IR lamp

TiO2

30 mm

Thermocouple

Fig. 3 Experimental setup for the repair system.

(a)

(b)

(c)

Fig. 4 Pictures for CFRPs: (a) before treatment, (b) after treatment and(c) magnified view of the exposed carbon fiber textile.

Y. Tsukada, Y. Suzuki, H. Takahashi and J. Mizuguchi2060

treatment shown in Fig. 5(a) exhibits characteristic CH andCH2 stretching bands around 2800­3000 cm¹1 as well as theCC stretching bands due to the aromatic ring at about 1400and 1600 cm¹1. These are due to epoxy resins. A sharpabsorption band at about 930 cm¹1 can also be assigned to theC­O­C ether bond of the three-membered ring at the terminalgroup. Then, these characteristic bands began to disappearwith irradiation time, accompanied by the reduction inreflectivity. Finally, the reflectivity was ceased to reduce andremains unchanged. The present flat spectrum is in goodagreement with that of the original carbon fiber.

Figure 5(b) is the reflection spectra in the visible region,showing no characteristic bands. This is due to that fact thatthe epoxy resin is nearly transparent in the visible region. It isobvious that the reflectivity is remarkably reduced upon heattreatment due to the increased roughness caused by polymerelimination and that the extent of reduction saturates about13% reflectivity. This suggests that the reflectivity changecan be a good measure of the termination of the polymerelimination, allowing us to use a conventional, inexpensivedensitometer for the detection of the termination point.

Figure 6 shows the repaired FRP completed with the refillof the epoxy resin. The resin was found to get into the carbonfibers with high affinity. That is, the polymer-eliminatedcarbon fibers were excellently impregnated with the epoxyresin, although the sizing agent of the carbon fibers hadtotally been removed by the TASC treatment, as described inthe following section [Figs. 10(a) and 10(b)]. It is also notedthat the present repair treatment left a ring mark at the edge ofthe repaired part.

3.2 Characterization of the recovered carbon fiberThe carbon fibers before use for CFRP and after recovery

from CFRP by TASC has previously been characterized and

reported in Ref. 7). The summary of the result is describedbelow. Figures 7(a) and 7(b) show the SEM images for onesingle carbon fiber before use and after recovery, respectively.No noticeable difference is recognized in appearance betweenthem. Figure 8 shows the Raman spectra of carbon fibersbefore use and after recovery. The Raman spectra werecharacterized by two peaks at about 1600 and 1350 cm¹1.

Fig. 5 Reflection spectra before and after treatment: (a) IR region and(b) visible region.

10 mm

Repaired area

Fig. 6 Microscope picture of the repaired FRP after the refill of matrixresin.

Fig. 7 SEM images for one single carbon fiber: (a) before use, and (b) afterrecovery.

Fig. 8 Raman spectra of one single carbon fiber: (a) before use, and(b) after recovery.

A Novel Repair Method of Carbon Fiber Reinforced Plastics with Reinforcing Fibers Intact 2061

The Raman shift at 1600 cm¹1 is characteristic of the highlyoriented pyrolytic graphite (G band); whereas the peak atabout 1350 cm¹1 (D band: so to speak disorder-induced peak)appears when the carbon includes some amorphous ordiamond-like components.8) No significant difference isfound in peak position between both spectra. Figure 9 showsthe X-ray diffraction diagrams for carbon fibers before useand after recovery. The diffraction peaks are assigned asshown to the graphite structure (space group: P6_3mc) inaccordance with the structural report.9) The diffractionintensity of the (002) plane at about 2ª = 26° is slightlyhigher in the sample before use than that after recovery. Allthe rest remains intact.

Figures 10(a) and 10(b) show the TGA/DTA curves forcarbon fibers before use and after recovery, respectively. Thesample before use exhibits a weight loss of about 1.3% atabout 300°C, as also accompanied by a change in heat flowin the DTA curve. This suggests the detachment of the sizingmaterial. (The sizing materials are usually coated on the fibersurface so as to enhance compatibility and adhesion betweenfibers and polymer matrix.) On the other hand, in recoveredcarbon fibers [Fig. 10(b)], no noticeable change is observedin weight loss in the temperature range between RT and500°C. This confirms that no sizing material exists any moreon the surface of the carbon fiber, suggesting that the re-coating of a sizing material is necessary when the fiber isreused. However, nowadays, FRPs based on thermoplasticsare, more and more, in fashion, where no sizing materials areused because it often disturbs the quality of FRP.

3.3 Repair of glass fiber FRPsSimilar results to those of CFRPs were also obtained

with glass fiber FRP (i.e., GFRP). Characterization of therecovered glass fibers is also given in Ref. 7).

3.4 Applicability of the TASC method and furtherdevelopment

The TASC-repair method is still at the early stage of thedevelopment. Nevertheless, we are confident that the presentmethod can basically be applied to the repair of a defect areawhere both fibers and polymer matrix are damaged, and/orto the area where the matrix alone is damaged while thefibers remain intact. Anyway, the TASC method can keep therepair area to a minimum.

TASC-repair is also possible even to the thickness of about2 cm without causing carbonization of the polymer matrix tooccur. That is, the matrix is entirely decomposed into H2Oand CO2 accompanying no monomer evolution.

The repair result described above appears to be encourag-ing for the repair of partially damaged FRP. It is also possibleto extend the present result to a large area by scanning an IRlamp for X-Y directions. Heating by laser, flame, or rodheater is also possible. However, the use of an IR lamptogether with an ellipsoidal mirror appears to be the best atthe moment because of its low cost and easy manipulation.

4. Conclusions

A novel repair method of CFRPs has been developed,utilizing our novel TASC technology. The method ischaracterized by retaining embedded reinforcing fibers intact.The reclaimed fibers are found to be undamaged as shown bySEM, XRD and Raman spectra; whereas the sizing agentwas entirely removed according to TGA/DTA and XPSmeasurements. The repair process is simple and inexpensive,and thus opens up a new horizon for the repair of partiallydamaged FRPs.

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2) S. J. Pickering: Compos. Part A 37 (2006) 1206­1215.

Fig. 9 XRD of carbon fibers: (a) before use, and (b) after recovery.

Fig. 10 TGA curves of carbon fibers: (a) before use, and (b) after recovery.

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