Auto-Reparation of Polyimide Film Coatings for Aerospace Applications Challenges & Perspectives

© 2012 Abadie et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Auto-Reparation of Polyimide Film Coatings for Aerospace Applications Challenges & Perspectives


Introduction
In the aerospace industry metal parts are substantially replaced by polymer matrix composites presenting many advantages compared with the metal parts they are replacing. However composite damage is very difficult to detect because it often forms a sub-laminate locations, invisible to the naked eye. To overcome these problems, research in the last two decades has led to the development of self healing polymeric materials that mimic some features found in biological systems. The development of self-healing polymeric materials, those that practically imitate self-healing process of wounds is a new and challenging problem for space applications to protect different types of devices made with composite materials and represents a new paradigm in materials design [1]. The pioneer works in autoreparation of composites were developed by White S. R et al [2][3][4][5][6]. Their originality was to suggest the encapsulation of monomer -dicyclopentadiene and Grubbs catalysor disseminated uniformly into the composites. Under loading cracks are formed and microcapsules open up releasing monomer and initiator which fulfill the cracks by polymerization and crosslinking reactions and allow the composite to retrieve his original structure without losing its initial mechanical properties - Figure 1.
The concept is strongly dependant on the reactivity of the monomers used as well as the initiator systems [7].
A sub-orbital spaceflight (above 100 km) or a low Earth orbit (LEO) impose on the composites the following conditions : absence of oxygen, temperature variable from -120°C (dark side) to +300°C (solar side), UV and heavy ions radiation, low pressure 10 -4 Pa. For example, the International Space Station is in a LEO that varies from 320 km (199 miles) to 400 km (249 miles) above the Earth's surface. The methodology developed was to test the feasibility of self-healing by using an encapsulated multifunctional acrylates, i.e. trimethylol propane triacrylate (TMPTA) in presence of a radical photoinitiator (Darocur ® 1173).
The novelty of the work was to develop new polymeric film composites, based on Polyimide containing in the same reservoir microencapsulated active monomer and generator of active species, to prevent destruction of the damaged sample.
The new concept tested was to crosslink the monomer in presence of a radical photoinitiator which, upon exposure to UV, produces active centres, permitting the self-healing of the damaged composites. [8]. A schematic of our system is briefly described in Figure 2.
Three major problems have been studied: 1. Develop a suitable microcapsule which could, under loading, liberate the active principle in presence of initiator.
2. Distribute homogeneously the microcapsules into the polymeric film used as coatings 3. Test the new super-composite film and its auto-reparation once micro cracks have been formed, to prevent crack growth and macro damage of the composite.

Microencapsulation
Microcapsules shell should be resistant to high temperatures (> 300 °C) and have average diameters ranging 1-20 µm in order to put them in the host material.
Different models of microcapsules have been tested: Poly(urea/formaldehyde), Polyurethane and Silica gel

Synthesis of poly(urea-formaldehyde) microcapsules
The healing agent (trimethylol propane triacrylate TMPTA monomer) and the photoinitiator (Darocur  1173) presented in - Figure 3, are encapsulated in poly(urea-formaldehyde) by in situ polymerization - Figure 4.  These microcapsules are carried out by in situ polymerization in direct emulsion (Oil/Water). At room temperature, deionized water and aqueous solution of ethylene maleic anhydride (EMA) are mixed in a reactor. The reactor is placed in a temperature-controlled oil bath on a programmable hot plate with external temperature probe and the solution is agitated with a mechanical stirrer. Under agitation, urea, ammonium chloride and resorcinol were dissolved in the solution. The pH is raised from ~2.6 to 3.5 by drop-wise addition of sodium hydroxide (NaOH) and hydrochloric acid (HCl). One or two drops of 1octanol are added to eliminate surface bubbles. Then, the healing agent (TMPTA monomer) and the photoinitiator (Darocur  1173) are added slowly in the solution to lead to a stable emulsion. After 10 minutes of stabilization, an aqueous solution of formaldehyde (37 wt. %) is incorporated in the emulsion to obtain 1:1:9 molar ratio of formaldehyde to urea. The emulsion is covered and heated at 55 °C for 5 hours. After 5 hours of continuous agitation, the mixer and hot plate are switched off. Once cooled to ambient temperature, particles are filtered on Büchner and are washed with a water/ethanol solution (50 wt. %/50 wt. %). Then, particles are dried in air at room temperature for 24-48 h - Figure 5. . The shell of microparticles presents two type of surface: one has a smooth surface -6 (a), or a rough surface - Figure 6 (b). The Figure 6 (c) illustrates a poly(urea-formaldehyde) microcapsule which has been ruptured under physical constraint (pressure).  Infrared Analysis -The comparison between infrared spectra of UF empty microparticles - Figure 7 (a) and UF microparticles loaded in TMPTA - Figure 3 (b), shows the presence of new bands, in particular at 1722 cm -1 (C=O stretching vibration), 983 cm -1 (wagging of the =CH2 group), and 808 cm -1 (twisting of the =CH2 group), characteristic bands of TMPTA which indicates that this compound has been encapsulated in UF microcapsules.

Weight loss (%)
Temperature °C Thermogravimetric Analysis -Empty UF microcapsule shows - Figure 8, a weight loss (around 10 %) until 110 °C corresponding to the evaporation of free water in microcapsules, and an important decreasing in weight (around 80 %) starting from 180 °C due to the decomposition of urea-formaldehyde microparticles. Therefore the urea formaldehyde could not be used to self-heal spatial device composites because of polymer decomposition below 300 °C (maximum temperature in space).

Synthesis of poly(urethane) microcapsules by interfacial polycondensation
The TMPTA monomer and the Darocur® 1173 photoinitiator are encapsulated in polyurethane microcapsules by in situ polymerization. Microcapsules are obtained by in situ polymerization in direct emulsion (Oil/Water). At room temperature, deionized water and polyvinyl alcohol PVOH (3 wt. %) are mixed in a reactor equipped with a mechanical stirrer. Then, an organic solution of monomer (Hexamethylene diisocyanate HMDI/chloroform) containing the monomer (TMPTA) and photoinitiator (Darocur  1173) to be encapsulated is added to the aqueous solution leading to a stable Oil/Water emulsion. After 5 minutes of stabilization, a hydrophilic monomer (Ethylene Diamine EDA in excess) is added to the emulsion - Figure 9. The reaction is continued until stabilization of the pH (around 5 hours).   Infrared Analysis -The comparison between Infrared spectrums of the empty polyurethane microparticles - Figure 12 (b), and polyurethane microparticles loaded in TMPTA - Figure  12(c) shows the presence of new bands, in particular at 1721 and 983 cm -1 and confirm that TMPTA monomer has been encapsulated in polyurethane microcapsules.    Figure 13, presents a decreasing in weight (above 200°C) due to the deterioration of polymer. Therefore, this polymer is not a good candidate as well to encapsulate healed agent for aerospace applications.

Synthesis of silica-gel microcapsules by sol-gel polymerization process
The TMPTA monomer and the Darocur 1173 photoinitiator are encapsulated in silica gels microcapsules by sol-gel polymerization. 3-(trimethoxysilyl)propyl methacrylate (MPTS), tetraethoxysilane (TEOS), trimethylol propane triacrylate (TMPTA), Darocur ® 1173, silica surfactant PEGSi (compatibility agent to increase the stability between the shell and the aqueous solution) and ethanol are mixed with a magnetic stirrer in a bottle. Then, an aqueous solution of ammoniac (16 wt. %) is added drop by drop to the mixture. After 10 minutes of continuous agitation, this solution is added to an aqueous solution of polyoxyethylene 1,2-nonylphenyl ether (Igepal CO-720 (NP12) at 1 wt. %. After 2 hours of reaction, the suspension is filtered under vacuum. Microparticles are collected and are dried in air at room temperature for 24-48 h. Figure 14 illustrates the schematic road to synthesis of silica-gel microcapsules loaded with TMPTA monomer [9].
Scanning Electron Microscopy -Silica microcapsules are studied by ESEM. The spherical particles have average diameters ranging from 1-30 µm, shown in - Figure 15. The shells of microparticles present a smooth surface.  For that we have used silica-gel microcapsules (empty microcapsules and microcapsules loaded with TMPTA) with TEOS precursor alone (not methacrylate chain in this compound). The comparison between Infrared spectra of the empty silica-gel microparticles with TEOS precursor alone - Figure 17 (a) and silica-gel microparticles loaded with TMPTA - Figure 17 (c), shows the presence of new bands, in particular at 1721, 983 and 807 cm -1 . These bands, characteristic of the TMPTA monomer, confirm the presence of TMPTA encapsulated in silica-gel microcapsules.
Thermogravimetric Analysis -The thermogravimetric analysis of silica-gel particles (without TMPTA) indicates a good thermal resistance (above 320°C) of silica-gel membranes - Figure 18. We have encapsulated with success TMPTA monomer by different processes of microencapsulation. Two of three techniques used cannot be applicable for aerospace application as the microcapsules are not thermal resistant above 300 °C, presenting a temperature of degradation below 200°C. However, silica membrane is much more resistant due to their degradation temperatures exceeding 300°C.

Dispersion into the polyimide film
Polyimide films are based on dianhydride of dicyclodecene tetracarbonic acid and oxydianiline ODA - Figure 19. The dianhydride was synthesized by Solar-irradiation technology of a mixture of benzene/toluene fraction with maleic anhydride to produce highly reactive monomer -benzene adduct (AB) - Figure 20.    Table 1 below. Table 1. Simulator parameters PI films loaded with 15 wt. % of Si-microcapsules loaded with TMPTA monomer exposed 30 min are equivalent to 6 month exposure at the real space condition over geostationary orbit of the earth.

Mechanical performance of the composite film
The capsules were the defects of the structure in which cracks mainly grew from. The increases in capsules concentration led to a more brittle fracture as shown in Figure 25. Tensile curves were measured using a universal "Shimadzu" Autograph AGS machine. The test samples were strips with working size of 5x30 mm. The thickness of samples was measured by a magnetic "PosiTector 6000" instrument.
Then, a circular hole (diameter = 0.3 mm) was made in each specimen with the least (0.5 wt. %) and greatest capsules concentration (5 wt. %): Specimens pi-1 and pi-4 respectively. The holes were made by special drill press.
According to the theory of elasticity, the stress concentration coefficient near circular hole is equal to 3 ; in fact the real stress near the hole is approximately one. So, the circular hole might be considered as the standard stress concentrator.
Stress -strain curves for Specimens pi N°1 (0.5 wt. %) and pi N°4 (1.5 wt. %) with hole are represented in Figure 26. The experiment showed that for Specimens pi N°1, the crack passed through the hole. So stress concentration near capsules was less than 3. But for Specimens pi N°4 the crack went through the hole. With the help of this method we can separate films with capsules into two classes: with effective stress concentration less and more than 3.
Using the polarizing microscope POLAR 312, we have detected stress fields near the capsules, even before the loading. It must be taken into account that heat input might be needed for stress relaxation to occur. On Figure 27, stress fields are represented by bright spot. Around some capsules, internal strain remained even after rupture of the specimen. In the experiment the film was elongated manually under the microscope by special micromachine.
Stress fields were detected near the hole too as depicted in Figure 28. The increasing strain near the hole was illustrated in the photos placed in order. The cracks grew through each hole visually. For low concentration of capsules, cracks led to rupture. For high concentration, big crack arose from capsules. It meant that local concentration of stress depended on overlapping of stress fields near capsules too. Theoretically, such interaction appeared when distance between capsules was less than 5 capsule diameters - Figure 29.
These observations allowed us to conclude that stress fields near capsules are the predominant effect; capsules concentration influenced the fracture when stress fields overlapped near capsules. Test pieces were cut from polyimide films. Straight samples, 10 mm in width and 20 mm in length, were tested in tension at 2 mm/min speed with "Shimadzu" "Autograph" machine. The thickness of samples varied from sample to sample and was approximately 0.12 mm. After testing, failed samples were studied under an optical LOMO Mikmed-2 microscope. Hitachi S-520 scanning electronic microscope was used to get a high-magnification image of samples' top and side surfaces, and fracture surfaces of samples were studied with microscopes. Figure 30 shows the typical stress-strain curves for pure PI film (without particles containing healing resin). The diagram is typical for plastic materials. Initially, the material deformed elastically, and yielding at strain more than 5 % was observed. The Young modulus of fracture deformation of samples was very high, around 130 %. Figure 30. Typical stress-strain curve for pure PI film (without spherical particles) Figure 31 shows typical stress-strain curves for PI containing 10 wt. % of particles with healing resin. The Young modulus of composite polymer was calculated from the slope of the stressstrain curves to be 2.6 GPa. The curves were similar to that of pure PI. However, the fracture strain was lower at approximately 50 %, due to the stress concentration near particles. In each sample, two types of cracks were made. First type of cracks was through-thethickness. These cracks were made by applying 100 g force on a changeable scalpel blade as shown in Figure 32a-c.
The resulting crack is shown in Figure 33 below where spherical particles of different size are observed.  Figure 34a shows a typical stress-strain curve for composite PI sample with a crack similar to that in Figure 33. The curve is similar to that of composite PI without a crack. The fracture strain was about 18 %, which was typical for ductile materials. However, fracture strain was lower than that in samples without a crack.   In this case the crack was on the surface and not deep. These cracks were made by putting 100 g force on a curved scalpel blade. The crack was not wide with low opening, and could not be measured with an optical microscope. Figure 36 shows typical stress-strain curves for samples with crack subjected to UV irradiation. In this case, two different types stress-strain curves were observed. Figure 36. Two types of stress-strain curves for samples with surface cracks made as shown in Figure  35 and irradiated with UV light The first one is shown by the curve 1 in the Figure 36. In this case the fracture strain is equal to approximately 30 %. This value is higher than that before UV irradiation (20-22 %). The stress-strain curve of the second type is shown by the curve 2 in the Figure 36. In this case the fracture strain is approximately 55 %, which was much higher than in the first case.
In the composite samples two layers were observed: the first one was pure PI and the second was PI filled with spherical particles. Samples with double-layered structure can be observed in Figure 37. In the right photo, a round particle can be noticed Study of the fractured samples showed that the layer of the composite PI debonded from the pure PI layer, and this explained higher fracture toughness in this case. Local cracking and debonding of composite surface layer under tensile load can be observed in Figure 38.

Formulation for low orbital spaceflight
In the condition of low orbital spaceflight (under 30 km), the presence of oxygen might reduce the photospeed of the crosslinking when radical process is used. To overcome this problem we have investigated the use of epoxies initiated by a cationic process of which kinetic is non sensitive to the presence of oxygen [10][11][12][13].
For that purpose we have considered a multifunctional epoxy formulations based on dicyclopentadiene and phenolic epoxy, such as solid Epiclon-HP720 - Figure 40.  The final objective was to formulate a system based on epoxy having the highest photoreactivity for self-healing applications.
The Photo-Differential Scanning Calorimetry (photo-DSC) [10] was used to study the cure kinetics of UV-initiated photo-polymerization of epoxy resin monomers and vinyl ether in presence of cationic photo-initiator, mainly Cyracure UVI-6976, former Cyracure UVI-6974, which consists of a mixture of dihexafluoroantimonate of S, S, S', S'-tetraphenylthiobis (4,1-phenyllene) disulfonium and hexafluoroantimonate of diphenyl (4-phenylthiophenyl) sulfonium (CAS no. 89452-32-9 and 71449-78-0) at 50 wt. % in propylene carbonate - Figure  44 [11]. We have studied the effect of the temperature for different solvents on the rate coefficient k according to the simplified Sestak and Berggren equation [14] and use the Arrhenius equation to calculate their activation energy Ea - Table 2. Photocuring kinetic of UV-initiated cationic photo-polymerization of divinyl ether shows a very low activation energy comparing to the two epoxies studied above. The Epiclon HP-7200/ Rapi-Cure DVE-3 system has been optimized by Differential Photo Calorimetry (DPC) in presence of cationic photoinitiator Cyracure UVI-6976.
We have shown that the mixture of a multi-functional epoxy resin Epiclon HP-7200 and divinyl ether Rapid-Cure® DVE-3 for the ratio Epiclon/DVE (60 wt. % / 40 wt. %), is a valuable candidate for UV encapsulation and for further self-healing test.

Conclusions
Silica-oragnic microcapsules based on 50 wt. % MPTS and 50 wt. % TEOS have been selected to encapsulate the monomer trimethylol propane triacrylate TMPTA and the photoinitiator Darocur ® 1173. The microcapsules prepared have diameter size of 4.00  0.30 µm in optimal synthesis conditions: reaction time of 2 h and stirring rate of 450 rpm. These silica-gel macrocapsules have been chosen over urea/formaldehyde or polyurethane due to their thermal stability up to 375 °C.
We have demonstrated the feasibility of self-healing of Polyimide films as coating using a photo-radical mechanism to crosslink the TMPTA monomer.
Mechanical properties of film coatings have been investigated and found that -Stress fields near capsules are the essential factor -Large through-the-thickness and side cracks are not healed -Short surface cracks are healed and Debonding of the surface layer containing spherical particles is the most effective mechanism of healing.
For low orbital spaceflight (under 30 km) where there is the presence of small amount of oxygen, the photoreactivity of different formulations based on epoxies and vinyl ethers have been optimized by DPC, showing the feasibility of self-healing in using a photo-cationic mechanism.