Mechanical, chemical and acoustic properties of new hybrid ceramic-polymer varnishes for musical instruments

Journal of Non-Crystalline Solids 355 (2009) 132–140

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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Mechanical, chemical and acoustic properties of new hybrid ceramic-polymer varnishes for musical instruments R. Rodriguez a,*, E. Arteaga b, D. Rangel a, R. Salazar c, S. Vargas a, M. Estevez a a

Centro de Fisica Aplicada y Tecnologia Avanzada, UNAM, Apdo Postal 0-1010, Queretaro, Qro. 76000, Mexico Escuela de Lauderia, INBA, Queretaro, Qro. 76000, Mexico c Lab. Mecatronica & MEMS, UPAEP, 11 Poniente 2304, Puebla, Pue. 72160, Mexico b

a r t i c l e

i n f o

Article history: Received 22 June 2008 Received in revised form 1 October 2008 Available online 20 November 2008 PACS: 81.05.Lg 81.05.Qk 81.07.Wx 82.35.Gh 82.35.Lr

a b s t r a c t Novel ceramic–polymer hybrid varnishes were designed to protect the wood surface of musical instruments. These hybrid coatings consist of chemically functionalized silica nanoparticles and synthetic solvent-based acrylic- and alkyd–polyurethanes. The nanoparticles were added to increase the abrasion resistance. An alkoxide was used to increase the number and reactivity of OH’s groups on the wood surface improving the adhesion with the coating through a chemical link between them. The properties of the synthetic coatings were compared with those of a traditional varnish (based on alcohol and natural resins) to obtain a better performance. Two types of woods were used: maple and spruce. The samples were characterized by UV–Vis, mechanical and abrasion tests, water’s absorption, acoustic properties, chemical resistance and SEM. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Acoustic properties and phonons Chemical properties Chemical durability Films and coatings Measurement techniques SEM S100 Nanoparticles, colloids and quantum structures Nano-composites Nanoparticles Optical properties Absorption FTIR measurements Oxide glasses Silica Titanates Polymers and organics

1. Introduction Wood has been, from ancient times, a material of great importance to mankind, not only because of its abundance in nature or of its beautiful appearance, but also mainly due to its important properties and wide range of applications. Wood has been, and it is right now, one of the primary building materials used by the human being, and its cellular structure provides it with excellent properties such as low weight (density), high mechanical strength * Corresponding author. Tel.: +52 5 623 4153; fax: +52 5 623 4165. E-mail address: [email protected] (R. Rodriguez). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.10.001

(in tension and compression), low thermal conductivity, good acoustic properties, and high endurance to outdoor conditions (when it is properly treated). It is also a highly versatile material because it can be bent or twisted in special and complicated shapes and is readily worked, fastened, and finished. Its final surface is pleasant to touch and its patterns are of great beauty [1–6]. There are external agents that can damage untreated wood surface by changing its texture and appearance: weather conditions (temperature, humidity, UV radiation, etc.), living organisms (fungus, bacteria, etc.), handling (stain, scratching, abrasion, human sweat, etc.), and so on. The human perspiration is particularly aggressive due to its acid character with a pH as low as 3.5; it

R. Rodriguez et al. / Journal of Non-Crystalline Solids 355 (2009) 132–140

contains, among other compounds, chlorides and sulfates that can stain and damage the wood’s surface [7–9]. When unprotected wood is exposed to wetting and sunlight, it can exhibit rather serious cracking, probably as a consequence of a quick moisture loss, with its corresponding dimensional changes; this attack is especially notorious in polluted weathers where chemical compounds can damage, in some cases severely, the wood’s surface. Then, it is a common practice to cover the wood surfaces with different kinds of coatings to protect them. Occasionally, coatings may contribute to the deterioration in the wood’s properties: a deficient application on exposed wood parts may retain excess of water favoring rotting conditions [10–13]. An important use of wood as a construction material is in the fabrication of musical instruments: its cellular structure, density and constituents are partially responsible for its important acoustic properties [14]. Wood is generally an economical building material from renewable sources; however, fine woods appropriated for the construction of fine musical instruments are rare and expensive. Today, a significant number of musical instruments involve, in their manufacture, wood pieces: their flexibility allows transmitting, in an appropriate way, mechanical vibrations (musical notes): vibrational normal modes are strongly influenced by the wood morphology and density [15,16]. Then, the protection of wood surface of fine musical instruments is of primary importance in the performance and conservation, for future generations, of these invaluable instruments. Any damage (physical or chemical) on the wood surface can modify the acoustic response of the instrument in addition to its aesthetic aspect. Therefore, one important constituent in the fabrication of musical instruments is the protective coating [17,18]. The middle age was the golden age of the musical instrument construction; in this age, many of the varnishes were developed for violins, violas, cellos, guitars, etc. Nowadays, in the manufacture of musical instruments these traditional varnishes, based on alcohol and natural resins and were developed hundreds of years ago, are still in use [5,19,20]. At present, practically all coatings are based on synthetic polymers and show excellent properties in many senses. However, one of the weakest points of many polymeric materials is the lack of wearing resistance. Commercial products, mainly polymer and waxes, offer several attributes, essentially an improvement in the final appearance of the wood surface, but none protect against wearing. Then, if a polymer-based coating is going to be used to protect wood surfaces against wearing, scratching or any mechanical action, it has to be added with hard ceramic nanoparticles to provide the coating with this property [21]. The interaction between the ceramic particles and the resin is important because the aggregation of the nanoparticles can reduce the wearing protection making cloudy the varnish due to the light scattering effects. In this work, new hybrid ceramic–polymer nanocomposites were designed to protect wood surfaces of musical instruments against scratching, abrasion and chemical attack. These coatings possess high adhesion with the wood surface, high gloss and transparency. The mechanical, chemical and acoustic properties of these new varnishes were compared with the corresponding properties of one traditional varnish that was prepared with an old recipe. 2. Experimental 2.1. Materials Two different types of woods widely employed in the construction of musical instruments were used: spruce and maple. Wood pieces of different sizes were prepared for several tests: 14.5  1.0  0.25 cm for acoustic and mechanical, 2.5  2.5 

133

0.25 cm for abrasion, and 1.0  1.0  0.25 cm for chemical resistance and water’s absorption. For the UV–Vis analysis, the coating was applied on optical flat glasses. In all cases, five pieces of each type of samples were prepared for reproducibility. Three different types of coatings were prepared: (a) alkyd–polyurethane prepared using alkyd hydroxylated resin (Reichhold, Mex) catalyzed with poly-isocyanate (Bayer, Mex), (b) acrylic– polyurethane prepared using acrylic hydroxylated resin (Bayer, Mex) catalyzed with the same compound, and (c) a traditional coating prepared using turpentine gum and linseed oil, both products were obtained from a drugstore. Toluene (Baker, Mex) was used as a solvent for the synthetic coatings. Silica nanoparticles (Degussa, Ger) of 16 nm were added to synthetic coatings at concentrations: 0, 3, and 5 wt%. An anti-adherent (AA), dimethyl polysiloxane (Degussa, Ger), was also added at concentrations: 0, 1, and 1.5 wt%; this anti-adherent produces coatings with good resistance to oil- and water-base compounds, increasing the protection to the musical instrument and providing a glossy finish. For comparison purposes, a commercial varnish (Comex, Mex) specially designed for wood application was also included in the abrasion resistance test; for this coating the primer suggested by the manufacturer was applied. 2.2. Wood surface preparation In all cases the wood surfaces were ground using a sandpaper Fandeli No. 600 until a smooth flat surface was obtained; the wood dust was removed using a clean soft brush. A chemical modification was performed on the wood surfaces in order to increase the number and reactivity of the OH’s groups that are going to react with the NCO’s groups of the poly-isocyanate to link chemically the varnish with the wood and form the polyurethane; in this way it is also possible to reduce the penetration of the coating into the wood’s pores reducing, as much as possible, any modification in the wood’s density and in the mechanical properties which render, finally, in the acoustic response. To achieve this goal, the wood pieces were painted with titanium isopropoxide (Aldrich Chem., USA). This alkoxide is a very active compound that reacts rapidly with the primary hydroxyl of the cellulose (the main constituent of these types of woods) producing three labile OH’s groups per each primary hydroxyl, as shown in the following schematic reaction:

ðCelluloseÞ  OH þ R  O  TiBðORÞ3 ! ðCelluloseÞ  O  TiBðORÞ3 þ ROH

ð1Þ

where R stands for the isopropyl group. Because titanium alkoxide is very reactive and hydroscopic compound, it reacts with the moisture in the air producing the hydrolysis of the other three isopropoxy groups resulting in the following reaction:

ðCelluloseÞ  O  TiBðORÞ3 þ 3H2 O ! ðCelluloseÞ  O  TiBðOHÞ3 þ 3ðROHÞ

ð2Þ

As can be seen, the role of this coupling agent is to increase the number and reactivity of the hydroxyl groups of the cellulose ring. These groups, together with the OH’s groups of the hydroxylated resin, can react with the poly-isocyanate to link chemically the coating with the wood. The modified wood pieces were placed on a stove at 50 °C for 1 h to finish the chemical reaction and to remove the water and alcohol produced during the reaction; some of the alkoxide (which does not react with the cellulose) reacts with itself due to the presence of moisture producing a white powder (titania), which is removed with a clean and soft brush. The wood’s surface appearance remains practically unchanged after this treatment.

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2.2.1. Ceramic functionalization The silica particles contain silanol groups („Si–OH) on their surfaces. Because these are hydrophilic groups, it is difficult to disperse the particles in organic solvents. Then, the silica surface has to be chemically modified anchoring on the surface hydrophobic groups (such as methyl groups); these groups make easier the particle dispersion into the resin providing an effective protection against wearing, and leaving the final coating practically transparent. The chloro trimethyl silane (Aldrich Chem., Co) was used to modify the surface of the silica particles. This chemical modification changes the hydroxyl groups of the silica particles by methyl groups, according to the following reaction:

ðSilica ParticleÞ  OH þ Cl  SiBðCH3 Þ3 ! ðSilica ParticleÞ  O  SiBðCH3 Þ3 þ HCl:

ð3Þ

As can be noticed from this reaction one hydroxyl group was substituted by three methyl groups (CH3). In this way, the silica particles become more oleophilic and improve the compatibility with the resin, allowing a better dispersion; in this way if is possible to obtain a homogeneous and transparent coating with a high content of ceramic particles (up to 5%) and without the pale white color caused by the light scattering effects produced by the large aggregates. The chemical modification was carried out as follows: silica nanoparticles were suspended in toluene at a concentration of 30 wt% using a high shear rate stirring. A solution of chloro trimethyl silane in toluene at 3 wt% was also added drop-by-drop, to the silica particles with strong agitation. The mixture was heated until the reflux conditions were reached and maintained in these conditions for 2 h. At the end, a suspension of functionalized silica nanoparticles in toluene was obtained. 2.2.2. Hybrid coating preparation Both hydroxylated resins (alkyd and acrylic) were prepared in a similar way: the resin was mixed with the functionalized silica nanoparticles at different concentrations (0, 3 and 5 wt%) using a high shear rate stirrer; in this moment it is possible to add pigments or additives (such as levellers). The resin with the nanoparticles was mixed with poly-isocyanate in a proportion 4:1 by wt% to form the corresponding polyurethane. Once the poly-isocyanate is mixed with the resin, the system has to be applied in less than an hour. Under these conditions, the coating dries to tact after 3 h. These two types of hybrid coatings, at different silica concentrations, were applied on the functionalized wood surface. 2.2.3. Abrasion experiments The abrasion experiments were performed according to the ASTM-D-1242-95 norm (Tabers’ method) on 2.5  2.5  0.3 cm wood plates that are properly covered with the respective coatings. On a steel disk plate of 25.5 cm radius rotating at 80 rpm, a Fandeli sandpaper No. 600 was glued and a weight of 20 g was placed on top of the wood plates. All abrasion experiments were performed in dry conditions and at room temperature. The weights were determined using an analytical balance with a precision of 1  104 g. The initial weight of the covered plates was determined; after 20 s of abrasion the disk was stopped, the plates and the sandpaper were cleaned by removing the wood dust with a clean and soft brush, and the plates were weighted; this process was repeated for 400 s. 2.2.4. Water’s absorption The water’s absorption was determined by completely covering small pieces of wood (1.0  1.0  0.25 cm) with the coating under analysis and by immersing them in distilled water for 50 days. Every day the samples were removed from the water, dried,

weighted and replacing them back to the water. The weight was obtained using a balance with an error of ±0.005 g. 2.2.5. Chemical resistance The aggressive product that is commonly found near or in contact with the musical instruments is the human sweat. The human perspiration, with a pH as low as 3.5, contains, among other compounds, chlorides and sulfates. To determine the chemical resistance, the coating was applied on the surface of optical flat glass slides and were immersed in three different liquids: sodium hydroxide (NaOH) at pH = 12, nitric acid (HNO3) at pH = 2, and sulfuric acid (H2SO4) at pH = 2, for 4 h. After this, the slides were rinsed with distilled water, dried in air and were placed in the UV–Vis apparatus for analysis. 2.2.6. SEM characterization Scanning Electron Microscopy (SEM) images were obtained from the interface wood-coating. The samples were prepared by freezing the coated wood with N2 liquid and fractured, and the new exposed surface was coated with carbon, as a standard procedure for SEM preparation, and analyzed. 2.2.7. Acoustic characterization Samples of 14.5  1.0  0.25 cm were used to determine the effect of the varnishes in the acoustic response for both types of woods. Exactly the same piece of wood was characterized before and after the coating application in order to take into account small differences in wood pieces that look apparently identical. One end of the sample was held horizontally and firmly tightened in a solid piece of aluminum (5  5  6.5 cm), while the rest can move freely in a cantilever configuration. The wood sample was excited using an electromechanical device containing an electromagnetic coil that pulls vertically the sample out of its equilibrium position: 0.18 cm for spruce and 0.54 cm for maple. These differences in the initial displacement are due to the differences in the mechanical properties of these types of woods: maple is more rigid than spruce. The sample is released to start the oscillation freely. The acoustic signal produced by the wood was collected by an omnidirectional microphone with high sensitivity, low noise and wide dynamic range: a flat response from 20 Hz to 5 kHz. The microphone was placed near (0.1 cm) the free end of the sample to obtain a high signal amplitude. The signal from the microphone was coupled directly to a Digital Oscilloscope Tektronix TDS 3032 of 300 MHz equipped with an inter-build pre-amplifier that couples the impedances to optimize the signal transfer. The signal was recorded in real time together with its FFT; for each run, 10 000 points were recorded. The error introduced by this instrument is lower than 0.6%. These data were analyzed using a harmonic exponential-decay function to obtain relaxation times, amplitudes and frequencies of oscillation. The data acquisition was synchronized with the trigger signal of the exciting mechanism to measure in the same way all samples allowing a comparison between the signals from uncovered and covered samples. In this way, it was possible to determine the influence of the varnish in the acoustic response of the wood. Each sample was measured three times to see the reproducibility of the data. The whole apparatus was placed in a close and hermetic anechoic box (40  40  20 cm) acoustically isolated to avoid the external signals that can reach the microphone. 2.2.8. UV–Vis characterization UV–Vis analysis was performed to determine the possible changes in the coating color produced by chemical degradation in samples immersed in acids and bases. The coated glass slides were placed in a two beam UV–Vis spectrometer Spectronic Genesis 2PC. In the reference an uncoated glass slice was placed.

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0.025

12

-5

Weight Lost (x 10 g/s)

0.02

Rate of Weight Lost (x 10-5 g/s)

Alk 0%Cer 0%AA Alk 0%Cer 1%AA Alk 0%Cer 1.5%AA Alk 3%Cer 0%AA Alk 3%Cer 1%AA Alk 3%Cer 1.5%AA Alk 5%Cer 0%AA Alk 5%Cer 1%AA Alk 5%Cer 1.5%AA

0.015

0.01

0.005

10

8

6

4

2

Acr 5-1

Acr 5-1.5

Traditional

Commercial

Ak5-1

Ak5-1.5

Traditional

Acr 5-0

Ak5-0

Ak3-1.5

Acr 3-1

Acr 3-1.5

Acr 3-0

Fig. 1a. Weight lost as a function of the abrasion time for alkyd coating with different concentrations of ceramic and AA.

Acr 0-1

0

Acr 0-0

400

Alk 5-1

350

Alk 5-1.5

300

Alk 5-0

250

Alk 3-1

200

time (s)

Alk 3-1.5

150

Alk 3-0

100

Alk 0-1

50

Alk 0-1.5

0

Alk 0-0

0

Sample Fig. 2. Rate of weight lost for all samples.

0.025 Alk 0%Cer 0%AA Alk 0%Cer 1%AA Alk 0%Cer 1.5%AA Alk 3%Cer 0%AA Alk 3%Cer 1%AA Alk 3%Cer 1.5%AA Alk 5%Cer 0%AA Alk 5%Cer 1%AA Alk 5%Cer 1.5%AA

0.015

0.01

0.005

0 0

50

100

150

200

250

300

350

400

Water Absorption after 50 Days (g)

0.5

-5

Weight Lost (x 10 g/s)

0.02

0.4

0.3

0.2

0.1

Ak3-1

Ak3-0

Ak0-1.5

Ak0-1

Ak0-0

Ac5-1.5

Ac5-1

Ac5-0

Sample

3. Results Figs. 1a and 1b show the wearing profiles for alkyd- and acrylic– polyurethane at different concentrations of silica nanoparticles and AA. Each one of these profiles is the average over three different samples similarly prepared. The wearing data were taken up to 400 s; however, only the first 10 points (200 s) were considered for the fitting to a straight line. The slope of the linear fitting corresponds to the rate at which the weight is lost. These rates are shown in Fig. 2 for all samples. The two numbers in the sample’s name correspond to the ceramic and AA concentrations, respectively. The results obtained for the water’s absorption for all samples are reported in Fig. 3. Here, the increment in weight is reported after an immersion in water for 50 days. These values correspond to the average taken on five different samples. The standard deviation was calculated for all data resulting between 3.5 and 6%. Figs.

Ac3-1.5

Ac3-1

Ac3-0

Ac0-1.5

0

Ac0-1

Fig. 1b. Weight lost as a function of the abrasion time for acrylic coating with different concentrations of ceramic and AA.

Ac0-0

time (s)

Fig. 3. Water’s absorption after 50 days of immersion.

4a–4c show the UV–Vis spectra of the coatings without and with chemical attack with H2SO4; the attacks with HNO3 and NaOH were not so severe as compared with the sulfuric acid, and are not shown. The results shown in these figures correspond to Ak5, Ac5, and traditional samples. These samples were selected because they showed good performance practically in all aspects. The acoustic analysis is reported in Figs. 5–9. In Figs. 5a, b, the signals recorded by the microphone for the maple sample without and with the coating Ac3-0 are reported, respectively. The results for other samples (not shown) are similar. In these figures, the points correspond to the experimental data and the continuous line to the fitting using the equation:

I ¼ I0 et=t0 sinðxt þ /Þ;

ð4Þ

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H2SO4

H2SO4 4

Alkyd 5-0 Without Chemical Attack Alkyd 5-0 With Chemical Attack Alkyd 5-1 With Chemical Attack Alkyd 5-1.5 With Chemical Attack

0.14

Traditional Without Chemical Attack Traditional With Chemical Attack

3.5

0.12

Absorbance (au)

3

Absorbance (au)

0.1

0.08

0.06

0.04

2 1.5 1

0.02

0 400

2.5

0.5

450

500

550

600

650

0 400

700

450

500

Wavelength (nm)

650

700

Fig. 4c. Absorbance in the visible region for traditional coating immersed in H2SO4 for 4 h.

H2SO4

0.15

1

AAc3 without varnish

Acrylic 5-0 Without Chemical Attack Acrylic 5-0 With Chemical Attack Acrylic 5-1 With Chemical Attack Acrylic 5-1.5 With Chemical Attack

0.1

0.8

0.05

Intensity (au)

Absorbance (au)

600

Wavelength (nm)

Fig. 4a. Absorbance in the visible region for alkyd coating immersed in H2SO4 for 4 h.

0.6

0.4

0 -0.05 -0.1

y = m1*exp(-m2*m0)*sin(m3*m0... Value Error m1 0.090697 0.00083275 m2 7.5376 0.10356 m3 426.24 0.10546 m4 -2.3449 0.0095267 Chisq 2.549 NA R 0.85475 NA

-0.15 0.2

-0.2

0 400

550

-0.25 450

500

550

600

650

700

0

where I0 (=m1), t0 (=1/m2), x (=m3), and / (=m4) are the amplitude of the oscillation, the relaxation time, the frequency of oscillation, and the phase, respectively. The amplitude of oscillation for coated and uncoated samples is reported in Figs. 6a and 6b for spruce and maple, respectively. A similar analysis for the relaxation time for spruce and maple is reported in Figs. 7a and 7b. Finally the frequencies, as obtained from Eq. (1), for the spruce and maple samples are shown in Figs. 8a and 8b. The signal from the microphone that arrives at the oscilloscope was analyzed in frequency domain using the FFT option in the apparatus. Only six frequencies are reported, the first one being the most intense. Frequencies higher than 700 Hz have a very small contribution and were not considered here. In Figs. 9a and 9b, the amplitudes, averaged over all synthetic coatings, of the first six normal modes for spruce and maple samples, respectively are reported. In Fig. 10, a typical SEM micrograph showing the coating-

0.1

0.15

0.2

0.25

0.3

0.35

time (s)

Wavelength (nm) Fig. 4b. Absorbance in the visible region for acrylic coating immersed in H2SO4 for 4 h.

0.05

Fig. 5a. Oscillation amplitude for maple without coating. The continuous line corresponds to a fitting using Eq. (1).

wood interface for the alkyd–polyurethane is shown. This figure shows the wood to the right in dark gray color and the coating to the left in light gray color. 4. Discussion The analysis of the wearing data reported in Figs. 1a and 1b was carried out only in the first 200 s: for higher times the hybrid coating can be wore out and the wood surface appears to change the rate at which the weight is lost. The first point (t = 0 s) was also eliminated from the linear fitting procedure because at the beginning the coating surface has small irregularities, which renders in a non-uniform pressure applied on the surface; after the first 20 s these irregularities are smoothed, and the surface is uniformly ground. During the first 200 s, the samples lost weight linearly.

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Maple

0.15 0.14

AAc3 with varnish

Without Varnish With Varnish

0.1 0.12

0.05

Amplitude (au)

Intensity (au)

0.1

0 -0.05 -0.1

y = m1*exp(-m2*m0)*sin(m3*m0... Value Error m1 0.072935 0.00083865 m2 6.8209 0.11924 m3 426.61 0.12046 m4 -2.8111 0.011727 Chisq 2.7514 NA R 0.7998 NA

-0.15 -0.2

0.08

0.06

0.04

0.02

-0.25

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

AAc0-0

AAc3-0

AAc5-0 AAk0-0

time (s) Fig. 5b. Oscillation amplitude for maple with coating. The continuous line corresponds to a fitting using Eq. (1).

AAk5-0

A-T

Fig. 6b. Oscillation amplitude obtained from the fitting data with Eq. (1) for maple.

Spruce

0.14

AAk3-0

Sample

Spruce

0.25

Without Varnish With Varnish

Without Varnish With Varnish

0.12

0.2

Relaxation time (s)

Amplitude (au)

0.1

0.08

0.06

0.04

0.15

0.1

0.05 0.02

0

0 PAc0-0

PAc3-0

PAc5-0

PAk0-0

PAk3-0

PAk5-0

P-T

Sample Fig. 6a. Oscillation amplitude obtained from the fitting data with Eq. (1) for spruce.

The abrasion resistance, i.e. the inverse of the rate at which the weight is being lost with the time was obtained from the initial slope of the wearing profiles. Fig. 1b shows the wearing profiles for the acrylic–polyurethane; the abrasion resistance was determined in a similar way. From Fig. 2, it is possible to observe that the alkyd samples have a higher wearing resistance relative to the corresponding acrylic samples. As expected, the wearing resistance increases when the concentration of nanoparticles is increased: the silica particles are very rigid and are consequently very resistant to abrasion process. This figure shows that the slope is also reduced when the AA concentrations are increased; this is because an increment in the AA concentration reduces the friction coefficient producing a slipping effect which diminishes the sanding produced by the sandpaper. The sample with the highest abrasion resistance (i.e. with the lowest slope) was the alkyd sample with 5% ceramic and 1.5% AA with a rate of weight lost of 2.63  105 g/s. All the alkyd samples with 5% ceramic had a better performance respect to all acrylic

PAc0-0

PAc3-0

PAc5-0

PAk0-0

PAk3-0

PAk5-0

P-T

Sample Fig. 7a. Relaxation time obtained from the fitting data with Eq. (1) for spruce.

samples. The best acrylic sample (3% ceramic and 1.5% AA) had a rate of weight lost of 3.41  105 g/s. The worst sample was the commercial varnish with a slope of 11.26  105 g/s, followed by the traditional coating with a slope of 7.63  105 g/s. The synthetic coatings improve significantly the abrasion resistance relative to the traditional coatings, providing a better protection to the musical instrument. The results of the water’s absorption reported in Fig. 3 show that all synthetic coatings behave in a similar way with a tendency of a better performance for the acrylic samples. The sample with the lowest absorption (0.091 g) was the acrylic Ac3-0, while for the alkyd samples the lower absorption (0.125 g) corresponds to Ak3-1. The sample with the highest absorption was the traditional coating with 0.445 g. Even when the weight was determined precisely and the samples together with the sandpaper was cleaned with a soft brush after each weight determination, some small silicon carbide particles are incrusted in the coating and some wood dust stick to the sandpaper, reducing the reproducibility of the

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Maple

0.2

Maple

Without Varnish With Varnish

500

0.15

400

Frequency (Hz)

Relaxation time (s)

Without Varnish With Varnish

0.1

300

200 0.05

100

0

0 AAc0-0

AAc3-0

AAc5-0

AAk0-0

AAk3-0

AAk5-0

A-T

AAc0-0

AAc3-0

AAc5-0

AAk3-0

AAk5-0

A-T

Sample

Sample Fig. 7b. Relaxation time obtained from the fitting data with Eq. (1) for maple.

Fig. 8b. Frequency obtained from the fitting data with Eq. (1) for maple.

Spruce

800

AAk0-0

Spruce

0.03 Without Varnish With Varnish

With Varnish

Without Varnish

700

0.025

Amplitude (au)

Frequency (Hz)

600 500 400 300

0.02

0.015

0.01 200

0.005

622.3

514.8

412.3

Sample

309.4

P-T

207

PAk5-0

103.9

PAk3-0

650.4

PAk0-0

532.3

PAc5-0

426

PAc3-0

321.4

PAc0-0

214.7

0

0

107.2

100

Frequency (Hz)

Fig. 8a. Frequency obtained from the fitting data with Eq. (1) for spruce. Fig. 9a. Frequency spectrum for spruce without and with varnish.

wearing experiments. The error associated with this determination can be as large as 9%; these errors are reported in Fig. 3. The UV–Vis analysis for samples attacked with sulfuric acid is shown in Figs. 4a–4c. In Fig. 4a, it is possible to see that the absorbance for the sample Ak5-0 without chemical attack (reference); as can be noticed, the absorbance is very low (<0.01 au) in the visible region. The absorbance was increased when the sample was chemically attacked reaching the values between 0.07 and 0.1 au; that is an indication of a small degradation suffered by the polymeric coating. Because the attacked coatings absorb more in the blue region, they show some very light brown-yellow color. For the acrylic coating (Fig. 4b), the chemical attack was more severe because the absorbance increases from 0.01 au for the un-attacked acrylic coating Ac5-0 to values in the range from 0.1 to 0.8 au, representing a significant increment in the absorbance. This increment is produced by the degradation of the acrylic coating by the sulfuric acid; in this case, a more intense brown-yellow color was obtained. The traditional coating, with an originally brown-red color, when it

was immersed in sulfuric acid, suffers a severe attack which increased the absorption in the visible region to 0.6 au. The acoustic signals for maple reported in Figs. 5a and 5b were fitted using a harmonic exponential-decay function (Eq. (1)) to obtain amplitudes, frequencies and relaxation times. As can be noticed from these figures, the average amplitude of oscillation was reduced from 0.0907 to 0.0729 when the coating was applied. This reduction in amplitude can be due to an increment in the wood’s rigidity and in its weight: the oscillation energy is proportional to the product of mass and the square of the amplitude, then when the coating was applied, the mass of the sample was increased and, for the same energy, the amplitude has to be reduced. Additionally, the relaxation time was also increased from 0.133 to 0.147 s, meaning that the sound (acoustic signals) suffers less attenuation, i.e. the coating reduces the energy dissipation maintaining the oscillation for longer times. The frequency of oscillation remains practically constant (with an error less than 0.09%) before and after

R. Rodriguez et al. / Journal of Non-Crystalline Solids 355 (2009) 132–140

Maple

0.035

With Varnish

Without Varnish 0.03

Amplitude (au)

0.025

0.02

0.015

0.01

458

408.8

318.8

209.7

140.6

70.46

472.1

432.7

351

212

141.3

0

70.69

0.005

Frequency (Hz) Fig. 9b. Frequency spectrum for maple without and with varnish.

139

whereas for maple the change was ±0.7%. The oscillation frequency is practically unaffected by the presence of the coating. The vibration of the wood samples in cantilever configuration has more than one single frequency. Fig. 9a shows the frequency spectrum for spruce obtained from the FFT analysis performed by the oscilloscope; as mentioned, the error introduced by the instrument is smaller than 0.6% and it is not shown in the figures. As can be noticed, the fundamental frequency remains practically unchanged in amplitude (from 0.0269 to 0.0273 au) and with a slightly shift to low frequencies (from 107.2 to 103.9 Hz), while the second normal mode was enhanced by the presence of coating and also slightly shifted to low frequencies (see Fig. 9a). The higher modes remain practically unchanged in amplitude but all shifted to low frequencies. For maple (Fig. 9b) the first two normal modes were attenuated in amplitude by the presence of coating, but their frequencies remain unchanged; the amplitudes for higher normal modes suffer small changes and the frequencies are shifted to low values for the coated sample. Summarizing, in all cases the frequencies or remain unchanged or are slightly shifted to low frequencies; for spruce there is a tendency to maintain the amplitudes, whereas for maple there is an attenuation in amplitudes. The SEM image shown in Fig. 10 shows the wood-coating interface. As can be noticed, there are no empty spaces between the coating and the wood surface; this is an indication of good compatibility and consequently of good adhesion between phases; the presence of empty spaces can be signal of incompatibility and segregation between phases. Similar images (not shown) were obtained for other coatings applied on both types of woods. 5. Conclusions

Fig. 10. SEM micrograph for an alkyd coating. The wood is at the right size (dark gray color) and the coating at the left (light gray color).

the coating application, meaning that the coating did not produce any modification in the frequency of oscillation. Similar results (not shown) were obtained for spruce. The analysis of the acoustic signals using Eq. (1) is reported in Figs. 6–8. Figs. 6a and 6b show the changes in amplitude for spruce and maple samples covered with all coatings. There is a tendency for spruce samples to increase the amplitude, whereas for maple the tendency is the opposite. The deviation from the average is ±6% for spruce and ±12% for maple. These behaviors in amplitude can be due to the differences in the mechanical properties of the woods and the coatings. Figs. 7a and 7b show the relaxation times for spruce and maple using different coatings. This is a measure of how long the vibration of the wood piece is kept before being attenuated by dissipative mechanisms. The deviation between coated and uncoated samples was ±9% for spruce and ±7% for maple. Figs. 8a and 8b show the frequency of oscillation obtained from Eq. (1) for spruce and maple, respectively. As can be observed, the frequencies suffered small changes before and after the coating process: for spruce the change in the frequency was of ±3.1%,

A new hybrid ceramic–polymer composite was designed as a coating for musical instruments. The addition of ceramic particles to polymeric resins produces coatings with excellent properties: high abrasion resistance, good adhesion to wood, high hydrolytic stability, good resistance to acids and bases, and low interference with the wood’s acoustic response. It was possible to increase the wearing resistance of the alkyd–polyurethane coating by a factor of 4.3 relative to the commercial coating by just adding 5% of ceramic and 1.5% AA. In the acrylic case, the wearing resistance was increased by a factor of 3.3 relative to the commercial coating, adding 3% ceramic and 1.5% AA. The use of an alkoxide as an adhesion promoter provides an interface with no empty spaces that render in a good adhesion between the substrate and the coating, as shown by the SEM images. The water’s absorption was also improved using synthetic coatings, which posses a high hydrolytic stability. The alkyd–polyurethane shows an excellent chemical resistance to strong acids and bases. The acoustic analysis showed that the coating practically has no influence on the frequency but produces a small signal attenuation (around ±9%). References [1] J. Kopac, S. Sali, J. Mater. Process. Technol. 133 (2003) 134. [2] G. Tsoumis, Science and Technology of Wood, Van Nostrand Reinhold, New York, 1991. [3] J. Bodig, B. Jayne, Mechanics of Wood and Wood Composites, Van Nostrand Reinhold, New York, 1992. [4] I.M. Egenberg, A.K. Holtekjølen, E. Lundanes, J. Cultural Heritage 4 (2003) 221. [5] P.K. Järvelä, O. Tervala, P.A. Järvelä, Int. J. Adhes. Adhes. 19 (1999) 295. [6] E. Byskov, J. Christoffersen, C.D. Christensen, Int. J. Solids Struct. 39 (2002) 3649. [7] A.E. Daniels, J.R. Kominsky, P.J. Clark, J. Hazard. Mater. 87 (2001) 117. [8] P.H. Winfield, A.F. Harris, A.R. Hutchinson, Int. J. Adhes. Adhes. 21 (2001) 107. [9] T. Fischer, L. Nylander-French, G. Rosén, Am. J. Contact Dermatitis 6 (1995) 56. [10] T. Ozdemir, S. Hiziroglu, J. Mater. Process. Technol. 186 (2007) 311. [11] C.E. Frazier, J. Ni, Int. J. Adhes. Adhes. 18 (1998) 81. [12] S.B. Elvy, G.R. Dennis, J. Mater. Process. Technol. 48 (1995) 365.

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[13] H. Oka, H. Hamano, S. Chiba, J. Magn. Magn. Mater. 272–276 (2004) E1693. [14] S. Sali, J. Kopac, Improvement of guitar sound by machining, in: Proceedings of the Eighth DAAAM, International Symposium, Dubrovnik, Croatia, 1997, p. 293. [15] F.C. Beall, Int. J. Adhes. Adhes. 9 (1989) 21. [16] J.M. Biernacki, F.C. Beall, Int. J. Adhes. Adhes. 16 (1996) 165. [17] S. Nami Kartal, Won-Joung Hwang, Y. Imamura, J. Mater. Process. Technol. 198 (2008) 234.

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