Enhancement of the Mechanical Properties of a Zeolite Based

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Enhancement of the mechanical properties of a zeolite based composite coating on aluminium substrate by silane matrix modification Luigi Calabrese, Lucio Bonaccorsi, Angela Caprì, and Edoardo Proverbio Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00844 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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Industrial & Engineering Chemistry Research

Enhancement of the mechanical properties of a zeolite based composite coating on aluminium substrate by silane matrix modification Luigi Calabrese*†, Lucio Bonaccorsi†† , Angela Caprì†, Edoardo Proverbio† † Department of Engineering, University of Messina, Contrada di Dio, 98166 Messina, Italy †† Department of Civil Engineering, Energy, Environment and Materials, University Mediterranea of Reggio Calabria, Salita Melissari, 89124 Reggio Calabria, Italy

Corresponding author: Luigi Calabrese e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Adhesion and impact properties of a silane-zeolite composite coating on aluminium substrates, have been investigated. The composite film was prepared with different silane matrix formulations (changing bi-functional and tri-functional silane ratios). The results showed high hydrophobicity of the modified silane-zeolite coating and good adhesion performances for the sample with lower bi-functional silane amount. Furthermore the composite coating with mixed formulation of silane matrix evidenced good impact properties, with better results for the sample despite the unmodified silane coating with higher amount of bi-functional silane.

KEYWORDS Aluminium, silane, coating, zeolite, mechanical characterisation


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Introduction Since the ban of chromium containing coating the demand for high performance substrates for corrosion protection of aluminum alloys is still pressing

1,2

. In the last decade promising

technologies based on sol–gel process to obtain hybrid protective coatings were adopted

3–6

. In

particular the inert character of silane-based hybrid coatings in corrosive media attracted considerable interests as an environmental-friendly chemistry application 7. The silanes are organosilicon monomers with general formula X3Si(CH2)nY, where X is usually a hydrolyzable alkoxy group (e.g. CH3O or C2H5O) and Y is a functional group (such as amino, vinyl, hydroxyl and glycidyl organo-functional groups). To guaranty a good reactivity of silane molecules, the hydrolysis of their alkoxy groups in alcohol solution is necessary to obtain reactive silanol groups (Si-OH). These silanol groups can interact with the hydroxyl compound on metal surfaces, forming strong Si-O-Me bonds. The Si-O-Me bonds are responsible of the good adhesive performance of the silane coating on metal substrate

8,9

. Furthermore the silanol

groups allow silane molecules condensation by forming siloxane groups, Si-O-Si, so giving rise to a highly cross-linked three-dimensional structure. The so formed silane films, due to the dense –Si–O–Si– network, act as effective barrier layers hindering the diffusion of aggressive species towards the metallic substrate

10–12

. Unfortunately, the low thickness of this coating and the

absence of any active electrochemical behavior leads to a short life in aggressive media 13. An enhancement of mechanical and physical properties is necessary to improve durability of hybrid organic–inorganic polymer materials prepared via the sol–gel. A possible approach to increase the protective properties of the coatings, is the addition of functional fillers

14–18

to the

silane film in order to improve barrier properties and at the same time use the filler as container for corrosion inhibitors.


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In particular Deflorian et al.

19

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successfully applied high-performance silane sol–gel films on

galvanized steel by employing neat cloisite and cerium oxides grafted montmorillonite nanoparticles. Similar results were obtained in

14

where the impact of addition of cloisite

nanoparticles on the protective behavior of the hybrid organic–inorganic silane layer both in the presence and absence of cerium nitrate was assessed. Similarly zeolite fillers could be used effectively as containers of corrosion inhibitors in the composite coatings silane matrix for the active corrosion protection of metal substrates. Dias et al.

20

used ion exchange zeolite

microparticles as reservoirs for Ce (III). The zeolites were introduced into silica–zirconia sol–gel films, improving the barrier properties of the coating and conferring active corrosion protection to a AA2024 substrate. Calabrese et al. 21 obtained similar results by using a not ion exchanging zeolite as micro-container of cerium corrosion inhibitor. However, a careful mechanical characterization of the coatings is fundamental in surface engineering design to have stable and effective performances during service life also in specific environmental conditions. The mechanical behavior of composite hybrid coatings are strictly related to the silane matrix used in the formulation as and its interaction with filler and substrate 22,23

. Calabrese et al. in

24

showed as the micro-structure of the silane matrices have significant

influence on the mechanical and electrochemical properties of the composite coating. According to 12, long aliphatic chains could enhance performances and durability of zeolite based composite coating. Cambon et al.

25

investigated mechanical properties and corrosion behavior of cerium

doped silane coatings pointing out an excellent correlation between the evolution of the barrier effect and mechanical properties of the coating with the increase in cerium concentration into the sol. An intrinsic relationship between mechanical, adhesion and physical barrier properties of coatings was, therefore, identified.


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Furthermore, hybrid materials with good anti-corrosion performances, adhesion strength and impact resistance can be used for corrosion- and abrasion resistant coatings as confirmed by Jena et al.

26

in experimental tests on silane-ZnO hybrid coatings on stainless steel substrate. In this

work the addition of a zeolite filler to silane coatings was proposed considering the high chemical affinity of the filler with the silane matrix in order to optimize the performance of the composite films. Silane functionalization of zeolite surface enhances the hydrophobic behavior 27 and exalts barrier properties of the composite coating

28–30

. However, a non-optimal chemical

interaction between silane matrix and structural/functional filler may result in defected or unhomogeneous surface, affecting significantly the mechanical and electrochemical performance of composite coatings

15,31

. While highly cross-linked three-dimensional network of siloxane

chains can promote adhesive bond performance of the silane coating on metal substrate

32

. The

terms of service could induce stresses on the silane zeolite coating, which must demonstrate good adhesion and effective mechanical stability even at static and dynamic loads

33

. In this

concern the silane zeolite composite coating could offer potential application in air conditioning system where good mechanical stability and durability is a crucial aspect necessary to ensure a reliable and long lasting coating 34. Based on these considerations, the aim of the present work was to asses the effect of silane matrix modifications on the performances of a zeolite composite coating, with the purpose to better understand the relationship between composite three-dimensional structure and mechanical performances. In this concern, in this work a composite coating characterized by a mixed silane matrix (n-propyltrimethoxysilane and dimethyldimethoxysilane, respectively called S3 and S2) filled with zeolite SAPO-34 particles on Aluminium 6061 substrates was investigated. Bi-layer coatings, characterized by an inner pure silane layer (S3) and an outer


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silane-zeolite composite film, were prepared. Different silane matrices combinations (S3+S2 with 2:1, 1:1 and 1:3 hydroxyl stoichiometric ratios) were studied. All depositions were obtained by using dipping sol-gel technique. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses were used to characterize the morphology and polymerization of deposited films. Mechanical stability of the silane-zeolite coating may be affected by different typologies of mechanical stresses during its lifetime (static or dynamic loads). For this reason, a single mechanical test is not sufficient to prove the overall mechanical stability of the coated layer. The adhesion of the coatings with the substrate was evaluated by tape test on cross-cuts and pull-off test. Furthermore, drop-weight impact tests were performed in order to evaluate the mechanical stability of the composite coating under impulsive stress conditions. The results evidenced as the addition in small amount of S2 compound in a S3 structure enhances the adhesion and impact properties of the coating.

Experimental Details Sample Preparation N-propyl-trimethoxy-silane (a tri-functional silane compound, S3, MW=120.22, density=0.88 g/cm3) and dimethyl-dimethoxy-silane (a bi-functional silane compound, S2, MW=164.2 density=0.93 g/cm3), both supplied by Aldrich, purum >97%, were used in this work as filming agent. The silane compounds as supplied are characterized by methyloxy groups that by hydrolysis were converted in hydroxyl terminated groups, according to literature procedure 35, in order to be able to perform the crosslinking process. Hydrolysis was performed in presence of distilled water and ethanol (ethanol/water/silane 90/5/5 % v/v). The pH was adjusted to 4.2 with


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the addition of acetic acid to control the self condensation. Different ratios between bi-functional and tri-functional silane compounds were realized (table 1). The solution was then magnetically stirred, at 25°C, for 24 h. Zeolite SAPO34 powder, synthesized in laboratory as described in 36 (mean crystals size about 3 µm) were added to the silane solution. The dispersion was initially homogenized in an ultrasonic bath for 15 min and then aged for 24h before the dip coating procedure. Strips of Aluminium 6061, with size 20x40x2 mm, were obtained from commercial sources. The surface of the samples was mechanically grinded with emery paper up to grade 500, then degreased in a diluted alkaline solution (0.1 N NaOH) for 60s, washed in distilled water and finally with acetone. The sol-gel films were applied on aluminium coupons by dip-coating, using a withdrawal rate of 3 mm/min. A by-layer dipping procedure was proposed

11

to develop the

composite coating. After the first immersion in a S3 silane solution for 1 min the sample was cured at 80°C for 20 min. In this way the silane layer can be used as primer to better anchor the composite zeolite based coating. Then, a second immersion was performed in a S2-S3 silane/zeolite solution for 1 min followed by a final curing in oven for 12h at 80°C. A summary of the samples used in this work is reported in table 1. 80%wt zeolite content was chosen in order to exalt the adsorption performances of the zeolite filler for its effective use in adsorption heat pumps. At the same time this was identified as an optimal compromise to have effective mechanical and protective performance of the coating 31. Coating morphology and surface coverage grade were evaluated by optical and scanning electron microscope (ZEISS SEM/FIB crossbeam 540). Silane cross-linking process was evaluated by FTIR analysis using a Thermo Nicolet Nexus 670 spectrometer on composite


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coatings. The spectra were recorded from 500 to 4000 cm-1 on portion of coating scratched from the sample surface and embedded in KBr pellets.

Table 1: Samples summary

Sample code

Coating type

Zeolite/Silane Part of S2 silane Part of S3 silane Thickness [wt %]

[wt]

[wt]

[µ µm]

S3

Silane

--

--

1

ZS3

composite

80

--

1

13-18

ZS2S3-1:2

composite

80

1

2

13-18

ZS2S3-1:1

composite

80

1

1

13-18

ZS2S3-3:1

composite

80

3

1

13-18

Adhesion Test Qualitative and quantitative adhesion tests were performed on the zeolite silane coating. The purpose was to have more specific information about the interfacial interaction between the aluminium substrate and the bilayer composite coating. To have a statistical significant distribution of data an average of five replicas for each batch was performed.

Tape test on cross-cuts Tape test on cross-cuts (according to ASTM D3359) were carried out to assess the coating adhesion to the substrate. On the coating surface a grid was achieved with cuts at a distance of 1 mm each. Then, an additional number of cuts at 90°, centered on the original cuts, were made to


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obtain a square grid. A tape was placed over the grid ensuring a good contact with the coating surface. Within 90±30s of application, the tape was removed by seizing the free-end and rapidly folding on itself (not yanking) at an angle of about 180°. Samples were evaluated using a polarized light microscope. The removed grid area, calculated by digital image analysis, was rated according to the ASTM specifications, from 0B (low adhesion) to 5B (high adhesion).

Pull-off Test The tensile adhesion strength of the composite coatings on Al6061 substrate was measured by a DeFelsko PosiTest AT-M pull-off tester. An aluminium dolly (10 mm diameter) was glued onto the coating surface using a cyanoacrylate adhesive and cured at room temperature for 24 h. A clamping fixture was designed to avoid the misalignment during the uniaxial tensile test. The fracture surface of sample and dolly were evaluated by stereo-microscope. Occasionally small cavities were seen on the adhesive surface after pull-off, these cavities could have altered the stress distribution and effective area of contact and consequently were excluded.

Drop weight impact test The flexibility of coatings was roughly estimated by drop weight impact test, according with the set-up reported in

33

. The impact test consisted in hitting a coated sample by a metallic sphere

(diameter 24 mm, weight 56 g) initially suspended by an electromagnet and then allowed to freely fall down from different heights. In this way it was possible to quantify the impact energy transferred to the sample. The sample was placed on a proper support with a tilt angle of 45°, to avoid the rebound of the falling sphere over the sample. After impact, the specimen was carefully


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removed from the support for post-impact optical damage evaluations. The failure impact energy was identified for damaged area with diameter about 1.3 mm)

Results and Discussion

FTIR on composite coatings Figure 1 shows the FTIR spectra of the composite coatings at different S2 content. We can identify three separate regions, typical of silane compounds, related to Si-O, C-H and O-H groups present in the molecular structure of the coating constituents. In particular: Si-O-Si bond: The widespread peak observed between 1000 and 1200 cm-1 is associable to the asymmetrical vibration of Si-O-Si bond in the silane network

37,38

. This bond is the

discriminating factor to evaluate the completion of the silane cross-linking process

39

. In fact,

during the reticulation process, silanols groups tend to react each other by condensation involving the formation of a siloxane film by the following reaction: R − Si (OH )3 + R − Si (OH )3 → R − Si (OH )2 − O − Si (OH )2 − R + H 2O

(1)

Consequently the intensity of this peak is related with the formation of covalent bonds between oxygen atoms with silanes molecules and zeolite surfaces

40

obtaining a crosslinked silane-

zeolite network. The presence of Si-O-Si groups is responsible of the good hydrophobic and barrier properties of the silane coatings. In details this widespread peak could be divided in two sub-peaks: the first one at high wavelength (about 1200 cm-1) associated with weak and flexible external bonds in the silane film

41

. Instead the peak at lower wavelength (near to 1000 cm-1)

could be associated with internal less flexible bonds, usually characterized by a higher crosslinking density

42

, or with the interfacial Si-O-Al bonds between the silane film and the


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metal aluminium substrate 43. Similarly, some adsorptions in the range 800-720 cm−1 are usually attributed to symmetric stretching motions of the oxygen atoms 44. This latter absorption region, could be used as confirmation of the good condensation achieved by silane matrix considering that the peak between 1000-1200 cm-1 can be shielded by the presence of the zeolite filler that have a broad absorption peak in the same region 45, prejudging its interpretation to discriminate the difference in the silane structures. However the same peak could be also been suggested to be originating from SiOCH3 group due to incomplete hydrolysis of silane 44. C-Hx bonds: The CH2 and CH3 groups in the alkyl and methyl groups, respectively in the organic chains of S3 and S2 silanes, can be identified analyzing two specific peaks, in the range 2950-2850 cm-1 associated with the molecular stretching of these bonds. In particular peaks at higher wavelengths are related to methyl groups, instead the peaks at lower wavelengths are related to methylene groups

46,47

. A second less relevant peak was observed at 1420-1380 cm-1,

due to bending of these bonds 48. O-H bond: The spectrum shows a stretching vibration region at about 3000-3800 cm-1, which is related mainly with the presence of water adsorbed by the zeolite filler that have a relevant hydrophilic behavior to moisture. This region can be also related with hydrogen bonds (mainly at lower wavelengths) and Si-OH bonds (mainly at higher wavelengths). Considering the high amount of zeolite powder in the composite coating this peak is not able to discriminate the silane matrix modifications because the moisture absorption by zeolite hides the hydrogen bonds and hydrolyzed silane groups bands. The peak at about 1630 cm−1 was attributed to the bending mode of water (O–H) 49. The peak at 890 cm-1 is due to symmetric stretching of the unreacted SiOH group, produced as a result of the hydrolysis process of silanes

50

. Consequently analyzing

the intensity of this peak it is possible to evaluate the amount of unreacted monomers and


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conversely the condensation level of the silane matrix. A summary of FTIR band assignments for the investigated composite silane film is reported in table 2.

Table 2: FTIR band assignments for composite silane films Band position (cm−1)

Band assignment

Note

Reference

2950-2850

-CH3, -CH2

stretching

46-47

1420-1380

-CH3, -CH2

bending

48

1260

Si-CH3, Si-CH2

symmetric bending

50

1200-1000

Si-O-Si, Si-O-Al

SiO asymmetric stretching

37,38,41,42,44

890

-SiOH

stretching

50

800

Si-CH3, Si-CH2

rocking bending

38,51

800-720

Si-O-Si

symmetric stretching

44, 45

Comparing the spectra at varying S2 amount it is possible to evidence as increasing the amount of dimethyl-dimethoxy-silane the peak at 2870 cm-1, due to CH2 group progressively disappears. The methylene group is present only in the N-propyl-trimethoxy-silane, consequently increasing the amount of bi-functional silane its contribute progressively is reduced. Conversely the peaks at about 2960 cm-1, related to methyl groups, slightly increase increasing the S2 amount. This is justified considering that the S2 molecules have two methyl groups despite the S3 molecules characterized by only a terminal methyl group in its alkyl chain. Similar considerations could be extrapolated analyzing the peak at about 1260 cm-1, related to Si-CH3 and Si-CH2 groups

50

.

Although in this case we would not have a clear indication to the presence of the large Si-O band that significantly affects its interpretation. Better results can be deduced analyzing the peak at 800 cm-1 related to rocking bending of Si-CH3 and Si-CH2 groups


38,51

. This peak progressively

12

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increases at increasing S2 amount, confirming the trend evidenced in the vibrating stretching peaks of CHx groups. Further considerations can be deduced by analyzing the shape of the broad peak at 1000-1200 cm-1 associated with Si-O bonds. At low content of S2 a main peak at about 1200 cm-1 was observed. The S3 silane matrix has a high crosslink density, due to the tri-functional reactivity of its constituent, implying, after reticulation, a very rigid network. Increasing the amount of bifunctional silane on the matrix formulation a slight increase of peak intensity was observed. This could be associated with an increase in Si-O-Si bonds. Analyzing the peak observed at 890 cm-1, associated with stretching of Si-OH bonds, we observe a progressive reduction with increasing of S2 content. This implies that in the composite coating realized with only tri-functional silane compound some unreacted silanol groups are presents. Consequently the ZS3 does not reach a complete crosslinking. This behavior could be justified considering that after gelation, these residual unreacted constituent remains entrapped in the three-dimensional network of the silane, due to the low mobility of the molecular chains in the rigid structure of the ZS3 composite coating. Conversely, the addition of bi-functional silane allows a reduction of unreacted groups and an improvement of the condensation degree, probably due to a higher flexibility of the silane network that favors the interaction between unreacted silanol groups. Only with high content of S2 (ZS2S3-3: 1) there is again a slight increase of unreacted silanol groups (peak at 890 cm-1), due to an excess of S2 that remains unreacted in the matrix bulk.

Morphological analysis Figure 2 shows the SEM micrographs at 20,000 magnifications of the coated samples. The samples morphology revealed a mainly regular and homogeneous surface without cracks. This


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confirm that good interaction between the silane and zeolite filler has been reached, favoring the formation of a mechanically-stable coating after the matrix crosslinking. Anyway, locally some not compacted areas in the composite coating surface could be detected. Occasionally “valleys” and “hills” can be identified, mainly on samples with bifunctional silane compound, as consequence of a local not optimal stratification of the zeolite domains. Furthermore, the coating surfaces contain some little inhomogeneities, evidenced as darker area in the micrographs, due to local superficial voids. However, as evidenced in a previous work, the cavities are not sufficiently deep to reach the underlying aluminum substrate 31. Figure 3 shows the cross section of the ZS2S3-1:1 coating. It is possible to observe that the thickness of the silane interlayer is about 1 µm. The composite silane-zeolite layer, instead, has a thickness of about 13-18 µm. All coating batches have shown similar thicknesses.

Adhesion properties In figure 4 reference images of the grid after tape test for each batch are reported. As evidenced in figure 3 all samples are characterized by a quite homogeneous morphology, confirming their good zeolite-silane interlocking. Therefore, the discriminating contribution to the bond strength variation is the chemical bond at the interface between coating and substrate, that is formed during the coating deposition process. In the coating the chemical bond between the aluminium and silanol functional groups have a fundamental role promoting the interfacial interaction and influencing the adhesion behavior of the different tested composite coatings

52

. In order to

quantify the amount of detached area an image analysis was performed by an on-purpose created (in Python language) script . The blank and white (BW) converted images are reported as reference in figure 4.


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The results of tape test on cross-cuts are summarized in figure 5. The addition of S2 in the silane matrix formulation significantly influences the percentage of detached area. In particular the ZS2S3-3:1 specimens evidenced lower adhesion properties. A large detachment of the coating (above 20%, standard index 3B according to ASTM D3359) occurred during tape test on cross-cuts. The sample with low amount of S2 (ZS2S3-1:2), instead, exhibited very affordable bonding performance with the lowest detached area percentage (associated with the standard index 3B, according to ASTM D3359). Slight higher detached area was observed for ZS3 and ZS2S3-1:1 samples (respectively 9.7% and 11.7%). Although the tape test is able to evaluate qualitatively the adhesion performance of the coating, further information can be obtained by pull-off test, which provide quantitative information about the tensile adhesion resistance of the coating. The pull-off tests evidenced as the addition of low amount of silane S2 (sample ZS2S3-1:2) increases the tensile adhesion of about 20% compared with ZS3 sample (figure 6). Furthermore, increasing silane S2 content on silane matrix formulation a significant progressive reduction of the adhesion performance was observed (ZS2S3-1:1 and ZSS3-3:1 samples). These results are consistent with ones from tape test, where very poor adhesion performances were found for the specimen ZS2S3-3:1. The good adhesion properties observed for coatings with low S2 amount could be explained considering a better interaction between the silane and the zeolite grains. Following that, higher stresses are necessary to induce crack propagation and decohesion of the zeolite grains. Instead, very high S2 content reduces significantly the amount of hydroxyl groups of silane matrix, therefore inhibiting the interaction of the composite coating with the silane interlayer. As a consequence the adhesive failure mechanism of the coating is favored at low stress level.


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The surface of fracture resulting from pull-off tests was then evaluated using a digital microscope Hirox KH-8700, in order to obtain more information on the adhesion performance of each composite coating. It is important to make a distinction between “cohesion” and “adhesion”. Cohesion is defined as the internal strength of an adhesive as a result of a variety of interactions within the adhesive components. Adhesion, instead, is the bonding of the adhesive to a substrate, due to a variety of possible interactions. Based on these consideration, an adhesive fracture occurs at the coatingsubstrate interface while a cohesive fracture occurs within the coating. Figure 7 shows the characteristics of the fracture surface (in terms of adhesive or cohesive failure) expressed as the percentage area. For all the coating batches, except for ZS3 coating, a predominatly adhesive failure was observed. Only a small portion of the detached surface evidenced a cohesive fracture. In ZS3 samples the fracture was mainly cohesive (adhesive 38% and cohesive 62%). This failure mechanism occurs when the bulk strength of the coating is much less than the adhesive strength. All silane modified coatings, instead, evidenced mainly an adhesive fracture mode. This failure mechanism occurs when the bulk strength of the material is much higher than the adhesive strength. For these samples crack starts at the coating-aluminium interface and progressively propagates generating complete debonded areas. The very high cohesive energy between silane and zeolite filler favored the coating detachment at the coating/metal interface, resulting in an adhesive failure mechanism. Furthermore, it should be noted that increasing the S2 amount in the silane matrix a progressive increase of cohesive fracture mode can be observed (the cohesive failure percentage for ZS2S3-1:2 and ZS2S3-3:1 samples are respectively 9% and 28%). This behavior can be


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justified considering a significant reduction of silanol groups due to a large amount of bifunctional silanes instead of tree-functionals, reducing the crosslinks between silanes and zeolite filler. This slightly reduces the cohesive resistance between composite constituents. As previously discussed, in ZS3 samples a mixed adhesive/cohesive fracture was observed. The combination of crack formation and propagation within the coating bulk or at the silane layer interface takes place. This behavior was probably influenced by a reduced cohesive resistance of this coating batch that favored a larger amount of cohesive crack propagation than other batches. SEM images with local EDX analysis of a pull-off fracture surface are reported in figure 8. The dark areas are related with an adhesive fracture (the EDX spectrum evidence the presence Al substrate). White/gray areas are instead related with cohesive fracture. The local EDX spectrum in this area evidence the typical peaks of Si, Al and P elements, that are the constituents of SAPO34 zeolite, confirming the cohesive fracture of the coating. A scheme of the failure mechanism occurred on the silane-zeolite composite coating is reported in figure 9. Based on FTIR results, the silane S3 matrix (ZS3) have a high cross-link density, due to the trifunctional activity of its constituent, which implies a very rigid structure with residual unreacted constituent that remains entrapped on the three-dimensional network of the silane matrix 31. This stimulates locally the reduction cohesive strength inducing a mixed adhesive/cohesive fracture mode. Conversely, the ZS2S3 system, thanks to S2 molecules is characterized by a greater flexibility of the molecular chains that allows a greater reticulation index (defined as percentage of reacted silane compounds) reducing the number of unreacted hydroxyl groups on the silane network, as


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confirmed by FTIR spectra, enhancing the silane-zeolite cohesion. An excess of bi-functional silane compound induces a progressive increase of cohesive fracture. This behavior could be explained considering a harmful reduction of crosslinks in the silane network.

Drop weight impact test Figure 10 shows the drop weight impact test results, showing the failure impact energy of the coatings at varying S2 content. Good impact performances were observed for ZS3 and ZS2S31:2 samples. The former, in particular, evidenced a critical damage energy of about 225 mJ while the latter was above to 270 mJ. The worst results were observed for the ZS2S3-3:1 sample with an impact resistance lower of about 40% than the ZS2S3-1:2 samples. With the purpose to better clarify the impact behavior of the silane modified composite coating, the damaged area at increasing impact energy was plotted in figure 11. For impact energy under 30 mJ no damage effects on the coated surfaces were observed. The damage diameter increased by increasing the impact energy. The sample ZS2S3-1:2 have always lower damage diameter compared with other coatings. ZS3 samples, although characterized by a brittle behaviour evidenced good impact performances due to their good adhesion properties with the aluminium substrate (as evidenced in figures 5 and 6). The ZS2S3-3:1 sample showed a quite higher damage diameter evolution at increasing impact energy, probably due to a non affordable cohesive and adhesive resistance of this coating that prejudices its impact mechanical stability. ZS2S3-1:1 samples, instead, although evidenced lower adhesion performances compared with ZS3 showed comparable damage energy values. It was supposed that the impact performances of these samples could be mainly related with the higher chain flexibility induced by the polycondensation of S2 silane that suffered partially the impact strain, suggesting that the metal/coating adhesion plays a less relevant role on the impact damage of the coating. The


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dashed interpolated curves shown in Figure 11 show an asymptotic behavior with increasing impact energy. In particular, all samples evidence at high impact energy a damage diameter value of about 1.4 mm. This is due to the fact that the impact energy is high the depth of the damaged area can reach the coating thickness. At these values of impact energy there is a compression collapse of the composite coating and the observed impact resistance is the combination of two effects: the first related to the coating performances and the second related to the elastic properties of the aluminum substrate. The achievement of this threshold value identifies the minimum energy contribution for obtaining the complete compression collapse of the coating induced by drop weight test.

Conclusions In this work the effect of the addition of a bi-functional silane (S2) in a tri-functional silane (S3) matrix on the physic-mechanical properties of a zeolite composite coating was evaluated. The adhesion properties (performed by tape test on cross-cuts, pull-off test ) of the coating are good. The best results are obtained for the coating with a lower bi-functional silane amount, ZS2S3-1:2, which evidenced an increase of the tensile adhesion of about 20% compared with ZS3 sample. At the same time increasing the S2 amount a progressive increase in the fracture adhesive character was observed. Furthermore, despite the unmodified silane coating, the composite coating with mixed formulation evidenced good impact properties, with better results for the sample with higher amount of bi-functional silane (about 30% higher that ZS3 sample). However an excess of S2 compound (ZS2S3-3:1) induced an abrupt decrease of adhesive and impact mechanical performances (respectively about -80% and -40% compared with ZS2S3-1:2) due to a harmful reduction of crosslinks in the silane network.


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Figure and table caption list Figure 1. FTIR spectra of silane-zeolite coating at increasing bi-functional S2 silane in the matrix formulation. Figure 2. SEM images at 20.000x magnifications of a) ZS3 b) ZS2S3-1:2 c) ZS2S3-1:1 d) ZS2S3-3:1 samples. Figure 3. SEM images of the cross section of ZS2S3-1:1 samples at 1100x magnifications. Figure 4. Tape test grid images of composite coating by varying S2 content. Figure 5. Percentage of detached area by tape test of composite coating by varying S2 content. Figure 6. Pull-off strength of composite coating by varying S2 content. Figure 7. Adhesive/cohesive fracture mode (%) of composite coatings varying silane matrix. Figure 8. SEM with EDS images of pull-off fracture surfaces. a) ZS3 mixed adhesive-cohesive fracture; b) ZS2S3-3:1 adhesive fracture. Figure 9. Scheme of adhesive/cohesive fracture mode of composite coatings varying silane matrix Figure 10. Damage energy for drop-weight impact tests for composite coating at varying S2 content. Figure 11. Damage diameter for drop-weight impact tests for composite coating at varying S2 content.

Table 1: Samples summary. Table 2: FTIR band assignments for composite silane films


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Enhancement of the Mechanical Properties of a Zeolite Based

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