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SEM-EDX analysis and TOF-SIMS 3D imaging of a textile/rubber interface undergoing fatigue loading C. Valantin a,∗ , R. Benoit b , M.P. D. a , F. Lacroix a , E. Gomez c , P. Phalip c , J. Morcel d , D. Tricoche d , N. Aït Hocine e a

LMR, CERMEL, University of Tours, 29 rue des Martyrs, 37300 Joué-lès-Tours, France CRMD, University of Orléans, UMR 6619 – CNRS, 1b rue de la Férollerie, 45071 Orléans Cedex 2, France c Hutchinson Research Center, Rue Gustave Nourry, BP31, 45120 Châlette-sur-Loing, France d Hutchinson, Joué-lès-Tours, France e LMR, INSA Centre Val de Loire, 3 rue de la Chocolaterie, BP 3410, 41034 Blois Cedex, France b

a r t i c l e

i n f o

Article history: Received 1 June 2015 Received in revised form 1 November 2015 Accepted 3 November 2015 Available online xxx Keywords: Polyamide cord–rubber composite Fatigue damage Interphase analysis ToF-SIMS SEM-EDX Nanoindentation

a b s t r a c t Unidirectional textile–rubber composites must exhibit a strong interface to reach admissible lifetime expectancy. Usually, this is achieved by coating the textile with Resorcinol-Formaldehyde-Latex (RFL). Nevertheless, former SEM observations of our composite revealed microcracks at the RFL/rubber interface. They appeared to propagate through fatigue loading, in correlation with hardening of RFL and interfacial rubber, both highlighted by nanoindentation tests. This mechanical damage was not correlated to chemical structure degradation which limited the development of more reliable interfacial materials’ formulations. In this perspective, EDX and ToF-SIMS are used in the present study to characterize RFL coated textile and composite’s textile/rubber interface compositions, before and after different numbers of cycles of fatigue loading. Those analyses allow the identification of textile contaminants, potentially responsible for composite’s initial interfacial microcracks: inorganic grains, polysiloxanes, fatty acids, latex surfactants and esters contaminants, attributed to diffusion from the RFL layer or from textile sizing. Concerning RFL hardening, four potential mechanisms can be raised: VP-latex complexation with metallic ions coming from rubber formulation, RF post-condensation or disproportion, migration of latex surfactants and thermo-oxidative degradations. This last mechanism seems to be the main one responsible for the hardening of interfacial rubber. © 2015 Elsevier B.V. All rights reserved.

1. Introduction For industrial goods as tires, hoses or belts, a strong interface between rubber and unidirectional continuous textile reinforcements (the cords) is needed to reach admissible mechanical properties and fatigue life. In this perspective, surface properties of the textile are usually enhanced with Resorcinol–Formaldehyde–Latex (RFL) through a dipping process, as RFL biphasic structure favors textile adhesion with rubber. More precisely, RFL latex phase is expected to co-crosslink with rubber during the composite molding [1–3] whereas its resorcinol-formaldehyde thermosetting resin phase is mainly aimed to provide textile bonding, thanks to its hydroxyl groups.

∗ Corresponding author. Tel.: +33 247361207. E-mail address: [email protected] (C. Valantin).

As textile/rubber adhesion is an important criteria to improve fatigue life of such composites, authors quantified it mainly using mechanical tests, as peeling [4] or pulling out tests [5–7]. This approach was sometimes combined with surface analysis of the treated cord [8–10]. Despite all of this, questions remain concerning interfacial compositions after composite molding and the effects of thermo-mechanical aging on adhesive properties. To our knowledge, no interfacial analyses were directly carried out for such textile–rubber composites. Only some ToF-SIMS or XPS characterizations of steel or brass cord–rubber systems are mentioned in the literature [11–13]. In this work, we propose to study polyamide cord–rubber composites strongly fatigue loaded at high temperature. Before falling apart, these composites exhibit a textile pull-out at the edges, assumed to be caused by interfacial weaknesses. Indeed, sectional SEM observations of initial composites, introduced in a previous study [14], revealed 15 ± 2 ␮m long cracks at the RFL/rubber interface. They appeared to propagate with fatigue loading, leading

http://dx.doi.org/10.1016/j.apsusc.2015.11.027 0169-4332/© 2015 Elsevier B.V. All rights reserved.

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to 60% delamination after 44 million cycles. Moreover, through nanoindentation tests, such crack propagation was correlated with hardening of interfacial materials (RFL and closest rubber) [14]. Therefore, with the prospect to suggest new material formulations to improve fatigue resistance of our composites, the present work investigates on RFL/rubber interfacial compositions. The aim is to point out contaminants potentially responsible for initial microcracks and physicochemical phenomena causing local fatigue hardening. For that, composites from the same batch are fatigue loaded for different times before a characterization of interfacial compositions by ToF-SIMS depth profiling. Such analysis is combined with nanoindentation tests and EDX analyses. For reference, RFL film and RFL treated cord are also characterized by EDX and ToF-SIMS. 2. Experimental 2.1. Materials The composite studied is made of a synthetic rubber matrix reinforced by continuous textile fibers. Textile reinforcements provided are made of polyamide microfilaments twisted together to reach a diameter around 1 mm. They are joined together thanks to a dipping in a toluene solution of 4,4-methylenediphenylisocyanate (4,4-MDI). After a second dipping, RFL coats the whole structure with a layer from 2 to 20 ␮m thick (Fig. 1). The latex used in this work is a terpolymer styrenebutadiene-vinylpyridine (VP-latex). To make the composites, continuous textile reinforcements are coated with a solvent-based rubber solution to get a good building tack. Then, they are stretched and placed in parallel between two rubber layers. The all structure is vulcanized under pressure at 180 ◦ C for 15 min, leading to 1.7 mm thick composites (Fig. 1). 2.2. Fatigue loading of the composites

2.3. Nanoindentation on composites Nanoindentation is carried out on composite cross-sections with a NanoTest Vantage from Micro Material Ltd., using a Berkovich tip. Each indent is characterized by a loading, dwelling and unloading phase. The maximum load is set to 10−3 N. It remains constant during a dwelling phase of 10 s, applied to dissipate viscous effects. Loading and unloading rates are ±0.01 mN s−1 . An optical microscope is used to precisely locate the indents on the different composite materials. For the determination of the indentation modulus or reduced modulus Er , the Oliver and Pharr method is used [16], following equation (1): √ S 



2

Ac

Ac ≈ 24.56 h2c hc = hmax − ε

(2) Pmax S

(3)

with ε being a dimensionless geometric parameter (0.75 for a Berkovich tip) [17]. Young’s modulus E can then be determined from equation (4) [18]: 1 − i2 1 1 − 2 + = Er E Ei

(4)

where  is the Poisson’s ratio of the tested material and i , Ei are respectively Poisson’s ratio and Young’s modulus of the indenter (0.07 and 1140 GPa for the diamond tip used here). For rubbers and polymeric materials tested in this study, Ei » E and local  values are unknown, especially after fatigue loading. Consequently, to plot moduli profiles across textile/rubber interface, Er is preferred to E. 2.4. Scanning electron microscopy and energy dispersive X-rays (SEM-EDX) SEM-EDX analyses are carried out on composite cross-sections. Beforehand, EDX analyses of RFL films (dried overnight at room temperature), RFL treated cords surfaces and cross-sections (after polyester resin embedding) have been used as references. All surfaces are sputter coated in a vacuum evaporator, with platinum for pictures or with carbon for EDX analyses. A Zeiss ULTRA Plus or a JEOL JSM-6480LV equipped with an Energy Dispersive System PGT Sahara detector are used. Quantitative analysis is performed at 15 kV and 2 nA, for 240 s, at a working distance of 11 mm. 2.5. Time-of-flight secondary ion mass spectrometry

Composites from the same batch are fatigue loaded on industrial test rigs at 100 ◦ C and 24 Hz, close to the conditions of use. Severe local tensile/compressive conditions are applied along the textile reinforcements’ axe to accelerate textile/rubber damage by amplifying interfacial shear. The fatigue tests are stopped at different times (0, 15, 29 and 44 million cycles), all before the pull out failure.

Er =

For a perfect indenter on a material not involving pile-up, as it is the case in this work:

(1)

where S = dP/dh is the initial slope of the unloading curve (software calculated), P is the applied force and Ac is the projected contact area of the tip at the contact depth hc .

Two types of ToF-SIMS experiments are carried out on the composites: mappings on cross-sections and depth profiles from the rubber to the RFL. Beforehand, analyses of RFL films and RFL treated cords surfaces are used as references. TOF-SIMS analyses are carried out with an ION-TOF GmbH TOF SIMS 5. They are performed using a 25 keV pulsed Bi+ LIMG source focused in 50 × 50 to 150 × 150 ␮m2 areas. Depth profiles are acquired in the dual-beam mode. Etching is performed with an O2 + source at 2 keV in 300 × 300 ␮m2 areas. An electron flood source is used for charge compensation. After acquisitions, Regions Of Interest (ROIs) are sometimes performed to analyze one specific area. 3. Results and discussions 3.1. Nanoindentation: fatigue loading effect on interfacial moduli At first, the effects of fatigue loading on composite properties are investigated through nanoindentation tests. To visualize indentation moduli (Er ) gradients, across RFL/rubber interface, profiling is carried out on different composites cross-sections: before fatigue loading or after 15 and 44 million cycles. At least three profiles are acquired per fatigue time. Because of RFL thickness heterogeneity, clearly pointed out in light microscopy (Fig. 2a), indentation moduli profiles obtained have not been averaged. For each fatigue time, some typical results are illustrated in Fig. 2b–d. On initial composites (Fig. 2b), indentation moduli of the rubber matrix are higher close to the RFL. That could testify

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Fig. 1. Chemical composition and structure of the studied composite. (TEM picture from [15] and proposed formula from [1]).

of a region of restricted mobility and higher crosslinking density, favored by rubber curing agents’ diffusion, in accordance with literature [1,19,20]. Indeed, combining AFM moduli measurements with EDX analysis on aramid cord–NR composite, Wennekes observed such interfacial hardening and attributed

it to an interfacial enrichment in sulfur [1]. Concerning the RFL layer, the average indentation modulus is 1.5 GPa. Looking closer, the values show an exponential increase, from ∼1 GPa near the rubber matrix to ∼2 GPa, close to polyamide fibers.

Fig. 2. a. Example of textile/rubber interface investigated. b–d. Indentation moduli profiles across the textile/rubber interface, on composites before and after fatigue loading.

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Fig. 3. RFL/rubber microcrack after 44 millions of fatigue loading cycles. Fig. 4. EDX composition comparisons of RFL coated cord and references films.

After 15 million cycles of fatigue loading (Fig. 2c), moduli profile points out another trend inside the RFL layer: higher indentation moduli are reached close to the interface with rubber. Inside the rubber matrix itself, a strong hardening is also highlighted close to the RFL. Those evolutions could be explained by fatigue loading, causing thermal aging and/or inter-diffusions at the RFL/rubber interface, submitted to shear stress concentration and local heating. After 44 million cycles (Fig. 2d), the hardening of interfacial materials is confirmed. Indentation modulus of the RFL even reaches 10 GPa. 3.2. EDX and ToF-SIMS characterization of RFL coated cords and RFL films To correlate mechanical properties evolutions discussed above, to compositional changes at the RFL/rubber interface, EDX and ToFSIMS profiling are acquired in the composites, before and after fatigue loading. Before that, as RFL formulations are proprietary, EDX and ToFSIMS analyses are carried out on RFL films and on RFL coated textile provided, in order to identify atomic and fragmental tracers. They will be particularly useful to locate RFL/rubber interface on further composite analyses. Moreover, such analyses are expected to highlight surface contaminants. They could be responsible for microcracks observed at the RFL/rubber interface, just after composite molding, and propagating with fatigue until composite failure (Fig. 3). 3.2.1. EDX analysis EDX spectra (not shown here) point out no significant difference between surface and bulk composition of the RFL. Spectra are dominated by C detection but three other characteristic atoms are present: K, Na and S. Comparing the RFL coated cord surface to reference films (Fig. 4.), K, Na and S elements are mainly coming from the latex phase and could be attributed to polymerization catalysts, like potassium persulfate, or latex globules surfactants. Na and K could also come from the bases KOH, NaOH or Na2 CO3 , frequently added in RFL bath formulations, to obtain the desired pH, prevent coagulation of the latex or to favor resorcinol formaldehyde condensation [1,2]. Some traces of other atoms are detected in the RFL layer of coated cord (Fig. 4) and could come from contaminants, fillers or additives: Mg, Ca, Cl, Si, I and Al. For example, Ca and Mg atoms could be attributed to CaCO3 sometimes used to catalyze RF condensation [21] or more certainly to latex fillers frequently used to increase flexibility, to accelerate

the drying step or to reduce the cost of latex compounds. They could also come from latex coagulants, as CaCl2 or MgCl2 [22]. Indeed, Cl is not detected in latex or RF films so it could have been added during RFL bath formulation process, after latex and RF mixing step. Concerning iodine, it is not detected on RFL film surface and could then come from diffusion from the greige fiber. EDX analyses inside polyamide microfiber confirm such hypothesis. This element could then be attributed to potassium iodide, potentially used as heat stabilizer in polyamide formulations [23]. Silicon may have been added to the RFL as silicates latex thickeners [22]. However, this atom is more intensely detected on cord surface than on top of RFL film which could be one evidence of a surface contamination caused by cord processing, storage or handling. 3.2.2. ToF-SIMS analysis ToF-SIMS experiments are also performed on RFL coated cord and RFL films surfaces, to obtain more detailed information on RFL chemistry and to highlight potential contaminants. In the positive ion mode, ToF-SIMS profile obtained on top of one fiber of the RFL coated cord is illustrated in Fig. 5. The intensities of each fragment detected are normalized to total. Three zones can be identified. In the first one, different contaminants are detected: - Polydimethylsiloxane (PDMS), with characteristics fragments SiCH3 + (43u), SiC3 H9 + (73u) or Si2 C5 H15 O+ (147u) [9]. - Polymethylphenylsiloxane (PMPS), with characteristic fragment C6 H5 Si+ (105u). - Metallic overconcentrations (Mg+ , Al+ , Ca+ . . .). - Traces of high masses peaks at 127/155/497/525/553/581u. Those last peaks could potentially be attributed to the different octanoic and decanoic esters of pentaerythrityl tetracaprylate/tetracaprate (Fig. 6), following [M–MAO ]+ or [M–MAD ]+ fragmentation pattern. M corresponds to the molecular weight of one ester and MAO , MAD , to molecular weights of octanoïc and decanoïc acids respectively. All those surface contaminants could have different origins. Esters and polysiloxane (PDMS and PMPS) are potentially caused by handling or mechanical processing of the cords. They could also come from diffusions from the greige textile surface, being the lubricant part of sizing formulations, used in the textile industry to make fiber spinning easier [9]. Metallic over-concentrations could be the result of handling pollutions but also diffusion phenomena from the bulk of the RFL layer.

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Fig. 5. ToF SIMS depth profile of positive fragments on top of a RFL coated textile.

In the second zone of ToF-SIMS profile (Fig. 5), EDX results are confirmed and characteristic fragments of the RFL coat are K+ , Na+ , Ca+ , Mg+ , Al+ and Si+ . However the higher sensitivity of the technique allows the detections of other mono elemental fragments as F+ , Cs+ or Ti+ . Typical fragments of the latex phase are also present in this zone, as C5 H3 + (63u), C7 H5 + (89u), C8 H7 + (103u), C9 H7 + (115u) for the polystyrene block, C7 H8 N+ (106u) for polyvinylpyridine block and C4 H5 + (53u), C5 H7 + (67u), C6 H7 + (79u) for polybutadiene block [24–29]. RF resin is not so easily detected. However, it brings aromatic fragments like C6 H5 + (77u) or C7 H7 + (91u) [30], oxygenated fragments as OHx + (x from 1 to 3) and some metallic ions as Na+ , Al+ , Mg+ , Fe+ or Cs+ . Even traces of Zn+ , B+ , Li+ are detected and could be attributed to residues of condensation catalysts [31]. In the third zone, N+ and CH3 N+ detection increases are linked with the entrance in a polyamide microfiber. In the negative ion mode, ToF-SIMS profile obtained on top of one fiber of the RFL coated cord can be divided into two zones (Fig. 7). In the first zone, identification of previous contaminants is confirmed and new ones are detected: - PDMS with SiCH3 O− (59u), SiO2 − (60u) and SiCH3 O2 − (75u) [9]. - Overconcentration of heteroatoms Cl− , F− , S− ,. . . that could be combined into grains with former metallic ions.

- Traces of octanoic and decanoic esters of pentaerythrityl tetracaprylate/tetracaprate with C8 H15 O2 − (143u) and C10 H19 O2 − (171u) fragments. - Traces of fatty acids soaps of palmitic, linoleic, oleic, stearic and dehydroabietic acids, frequently used to stabilize synthetic latexes [32,33] with C16 H31 O2 − (255u), C18 H31 O2 − (279u), C18 H33 O2 − (281u), C18 H35 O2 − (283u) and C20 H27 O2 − (299u). - Traces of alkyl sulfate or alkylbenzenesulfonate surfactants with HSOx − /SOx − (x from 1 to 4), Cx H2X−1 SO4 − and Cx Hx -C7 H7 SO3 − [34]. The last two kinds of contaminants are also detected when the surfaces of a latex film or a RFL coated cord are analyzed (Fig. 8). In the second zone of ToF-SIMS profile, characteristic fragments of VP latex (Cx N− ) and RF resin (Cx Hy Oz − ) are highlighted. NaOH− and KOH− fragments are also present and could be attributed to bases used in RFL formulation as previously explained. HSOx − /SOx − fragments, attributed to latex surfactants, are also more intensely detected, confirmed by a mapping of SO3 − and CNO− fragments, on a sectioned embedded cord (Fig. 9), the second ion being representative of polyamide microfibers. Such strong detection of SO3 − ion in the RFL layer suggests that surfactants are trapped during its drying on the cord. As explained by Voyutskii [35], their distribution could have a strong impact on the mechanical properties of the final RFL coating. They could form segregated domains by accumulating between latex globules or dissolve into the matrix, acting as plasticizer.

Fig. 6. Structure and molecular weights of the different esters of pentaerythrityl tetracaprylate/tetracaprate.

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Fig. 7. ToF SIMS depth profile of positive fragments on top of a RFL coated textile.

Fig. 8. Comparative spectra in positive mode of a RFL film and a RFL coated textile (200–340u).

Fig. 9. ToF-SIMS mapping of CNO− and SO3 − fragments on RFL coated cord section.

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Fig. 10. 3D ToF-SIMS imaging of RFL contaminants a. PDMS and b. metallic ions (∼80 × 80 × 2 ␮m3 ).

Iodine is also clearly detected in this zone, confirming previous EDX analysis and a diffusion phenomenon from polyamide microfibers. To sum up, ToF-SIMS analyses of RFL coated cords and RFL, RF or latex films allowed the identification of RFL tracers that will be useful for further interfacial analyses: mainly K+ , Na+ , NaOH− , KOH− , Cl− , F− , S− , SO3 − but also Ca+ , Mg+ , Al+ , Cx Hy N+ , OHx + . . .. Moreover, they have highlighted surface pollution that could explain initial adhesion weaknesses observed at the RFL/rubber interface of the textile reinforced composite studied: inorganic grains, polysiloxanes (PDMS and PMPS), fatty acids, surfactants and esters. Both could restrict RFL wetting or curing agent diffusion and induce initial RFL/rubber microcracks. For example, PDMS pollution, illustrated with the SiC3 H9 + characteristic fragment in Fig. 10a, forms segregated domains on the RFL layer (represented by Na+ ). Moreover, metallic ions form grains (Fig. 10b) which dimensions (∼20 × 20 ␮m2 ) match more or less the microcracks lengths measured, initially, at the RFL/rubber interface (15 ± 2 ␮m). Some contaminants, like PDMS or the ester, are already detected when analyzing cord surface one step before RFL coating (Fig. 11). They could then be caused by manufacturing conditions (industrial processing, human handling or storage) or by a diffusion from fiber sizing, formulation added on top of the fibers to make polyamide spinning easier [9]. On the contrary, metallic grains are not detected

Fig. 11. ToF-SIMS imaging (∼100 × 100 × 5 ␮m3 ).

of

a

cord

one

step

before

RFL

coating

at this step. As a result, they could be attributed to diffusions from the heterogeneous RFL layer itself. 3.3. Interfacial composition on initial composites Having previous RFL tracers in mind, EDX and ToF-SIMS analyses are carried out on composites cross-sections, to highlight interfacial compositions before fatigue loading and to attempt to link them to the nanoindentation moduli profile illustrated in Fig. 2b. 3.3.1. EDX analysis EDX profiles (Fig. 12) and mappings (not shown here) point out three typical elements for the RFL layer: K, Cl and S, confirming previous ToF-SIMS analyses. They also allow the identification of two characteristic elements of the rubber matrix: Zn and Ca. They are attributed to CaCO3 peroxide support [36] and zinc poly(methylmetacrylate) reinforcing domains [37,38], respectively. Moreover, the rubber solution applied on the textile just before composite molding (Fig. 1), can be easily highlighted inside the composite with a strong silicon detection, mainly explained by the silicon dioxide fillers added to the formulation. 3.3.2. ToF-SIMS analysis In the positive ion mode, equivalent profiles can be obtained by ToF-SIMS, alternating etching and analysis from the rubber to the RFL layer. They allow 3D imaging of the interface as illustrated in Fig. 13. RFL/rubber interface is clearly highlighted. It corresponds to an increase in K+ detection and a decrease in Si+ one, in accordance with EDX analysis. Above the interface, Si+ overconcentration is observed and is attributed to the rubber solution layer, rich in silicon dioxide. Surprisingly, such ions seem linked with Zn+ ones whereas zinc monomethacrylate is added in less proportion into the rubber solution than into the rubber matrix. This discrepancy could be explained by a diffusion phenomenon occurring during composite molding. Such diffusion would corroborate the higher moduli measured in this area by nanoindentation (Fig. 2) and even the higher Zn+ detection in the RFL layer after molding (normalized intensity of 2 × 10−3 against 5 × 10−5 for the initial coated cord (Fig. 5)). Some other metallic ions are detected in the rubber matrix. For example, Ca+ sporadic accumulations are noticed (Fig. 13). They could be attributed to CaCO3 grains of peroxide support and are associated with metallic ions like Fe+ and Ti+ , which could be residues of an accelerating system. All those metallic elements are also strongly present at the RFL/rubber interface and appear having diffused inside the RFL layer during composite molding. Indeed, Ca+

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Fig. 12. EDX profile across textile/rubber interface on an initial composite.

is detected with an intensity of 2 × 10−1 in the RFL against 2 × 10−4 for the coated cord before molding (Fig. 5). Regarding Al+ , Mg+ , V+ or Cr+ metallic ions (Fig. 13), they form grains, dispersed all over the matrix and could be attributed to contaminants or catalyst residues of such synthetic rubber [39,40]. Some of these metallic ions could also come from diffusion from the RFL surface, where they were strongly detected before (Fig. 10b). Those metallic ions could be harmful to interfacial fatigue resistance by catalyzing thermo-oxydative degradation reactions [41–44], already favored by local high temperature and shear forces. This radical-induced degradation mechanism could be a cause of interfacial materials’ hardening that will be investigated hereon.

3.4. Interfacial compositions on fatigue loaded composites To follow interfacial compositions evolutions and explain local hardening phenomena, a ToF-SIMS profile is acquired on a composite fatigue loaded for 15 million of cycles (Fig. 14). In accordance with initial interfacial composition, the RFL layer is highlighted with a decrease in Si+ detection and a relative constancy of Ca+ intensity. However, K+ appears having diffused into the rubber layer and Al+ , Mg+ , Ti+ or Zn+ metallic ions’ drop is less sharp at the interface, also indicating diffusion from the rubber into the RFL layer. Such diffusion phenomena, probably accelerated by local higher temperatures, could explain the appearance of Cx Hy Nz M+ or

Fig. 13. 3D Imaging of the RFL/rubber interface on composite before fatigue loading (∼55 × 55 × 20 ␮m3 ).

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Fig. 14. Postive Tof-SIMS profile across RFL/rubber interface on a composite fatigue loaded for 15 million of cycles.

Fig. 15. a. Metallic and b. Kx Nay SOz + fragments detection evolution with fatigue loading, inside the RFL layer.

Kx Nay SOz + fragments on either side of the interface (Fig. 15). The firsts could result from the complexation of vinylpyridine groups of VP-latex [45–47], by metallic ions coming from the rubber matrix, the seconds ones could be due to latex surfactants migrations and rearrangements. Both mechanisms could explain RFL hardening with fatigue loading, the first with the creation of strong crosslinks and the second with the reduction of the plasticizing effect of latex surfactants. To test such hypotheses and look further into the phenomena responsible for RFL and more generally interfacial materials’ hardening, ToF-SIMS imaging is performed on transversal cuts of composites after 0, 15 and 29 million cycles. Each material composition is then highlighted by choosing ROIs on the ion image obtained. Analyzing the RFL layer, metallic diffusion from the rubber is confirmed (Fig. 15a) as well as the increase of Kx Nay SOz + species (Fig. 15b). Furthermore, a decrease of CH3 O+ , H3 O+ , CH2 O− , OH− fragments is noticed (Fig. 16). It raises a new hypothesis to explain RFL hardening: residual water vaporization and/or post reaction of RF resin by condensation of residual methylol groups (-CH2 -OH) [48] or dissociation of methylether bridge (-CH2 -O-CH2 -) [2]. The first reaction releases water, which could explain calcium oxygenation detected between 0 and 15 million cycles of fatigue loading (Table 1). Finally, to explain RFL and more generally interfacial materials’ hardening, a last hypothesis can be considered: thermo-mechanical degradation, favored by local high temperature, shear and metallic

ions diffusions. Indeed, EDX analyses of the RFL, 1 ␮m from the interface with rubber, have highlighted an increase in oxygen content with fatigue loading (Fig. 17). That could be explained by local R–O–R or R–O–O–R bonds creations, following the oxidation mechanism described by Bolland [44]. On the rubber side, to quantify potential R–R crosslinking caused by thermo-mechanical degradation, branching pointers can be calculated for rubber material close to the RFL layer. This calculation is based on the hypothesis that crosslinking increases the intensity of branched high-mass fragments Cx Hy + (x > 5). For example, in a high-mass cluster with x = 6, that we will call C6 cluster, we can find C6 H11 + fragments in both linear and branched forms (Fig. 18). The greater abundance of branched high-mass fragments relative to low-mass linear fragments is attributed to the stabilization of the positive charge by the tertiary carbon atoms at the branching point. Following Van Ooij and Brinkhuis [27], Briggs [28] or Lianos et al. [49] methods, branching ratios are calculated in the rubber, Table 1 Evolution with fatigue of intensity ratios between calcium based fragments. Number of fatigue loading cycles (millions)

0 15

Intensity fragments ratios

CaO+ /Ca+

CaOH+ /Ca+

CaH2 O+ /Ca+

2 × 10−3 5.5 × 10−2

1 × 10−4 9 × 10−2

1 × 10−5 2 × 10−2

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Fig. 16. CH3 O+ , H3 O+ , CH2 O− , OH− fragments detection evolution with fatigue loading, inside the RFL layer.

degradation. That could explain local hardening highlighted by nanoindentation. 4. Conclusions The textile–rubber composites studied showed microcracks at the RFL/rubber interface, propagating with fatigue loading. With the help of nanoindentation tests, such propagation was correlated with a local hardening. Then, with the prospect to propose new material formulations to improve fatigue life span of the composites, combined EDX and ToF-SIMS analyses have been used. They allowed:

Fig. 17. Oxygen content evolution with fatigue loading, inside the RFL layer, 1 ␮m from the interface with rubber (EDX).

Fig. 18. Linear (a) and branched (b) fragments of C6 H11 + [49].

Fig. 19. Branching pointers evolutions with fatigue loading for the rubber 1 ␮m from the interface.

1 ␮m from the interface, and plotted versus number of cycles in Fig. 19. Whatever the method used, ratios show a small increase with fatigue. Such results seem to confirm the hypothesis of an interfacial rubber crosslinking caused by thermo-mechanical

- identifying contaminants on RFL coated cord, potentially responsible for RFL/rubber initial microcracks. - giving information about interfacial compositions after composite molding. - correlating interfacial compositions evolutions with fatigue loading to the local hardening. Concerning the first point, inorganic grains, polysiloxanes (PDMS and PMPS), fatty acids, surfactants and esters were identified on RFL coated cords. Potentially due to manufacturing conditions, most of those surface contaminants could also be attributed to diffusion phenomena from the RFL layer itself or from polyamide fiber sizing. Concerning initial composition of the RFL/rubber interface, discrepancies were highlighted. They may have been caused by diffusions occurring between RFL and rubber during molding. That could explain why zinc was detected in the RFL layer, only after composite molding. On the rubber side, strong detection of zinc were attributed to poly(zinc monomethacrylate) reinforcing domains and may explain higher nanoindentation moduli values measured locally. Concerning aging mechanisms responsible for interfacial materials hardening, EDX analyses gave little information whereas ToF-SIMS highlighted diffusion phenomena and appearance of new fragments. It allowed raising four different hypotheses to explain RFL hardening: - VP-latex complexation with metallic ions coming from rubber formulation. - RF post-condensation or disproportion. - Latex surfactants migrations. - Thermo-mechanical degradation leading to local oxygen enrichment.

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Please cite this article in press as: C. Valantin, et al., SEM-EDX analysis and TOF-SIMS 3D imaging of a textile/rubber interface undergoing fatigue loading, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.11.027