layers is monitored in real time using photothermal radiometry. The curing mate- rials studied are two adhesives and a UV curing resin. It is shown that the curing.
Eur. Phys. J. Special Topics 153, 203–205 (2008) c EDP Sciences, Springer-Verlag 2008 DOI: 10.1140/epjst/e2008-00428-2
THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS
Photothermal monitoring of curing in multilayered systems M. Zambrano-Arjona1 , R.A. Medina-Esquivel2 , and J.J. Alvarado-Gil1 1 2
Centro de Investigaci´ on y de Estudios Avanzados del IPN-Unidad M´erida, Antigua Carretera a Progreso Km. 6, CP 97310, M´erida, Mexico Centro de Investigaci´ on y de Estudios Avanzados del IPN-Unidad Queretaro, Libr. Norponiente #2000, Fracc. Real de Juriquilla CP 76230 Quer´etaro, Mexico
Abstract. In this work the process of a curing layer buried inside two external layers is monitored in real time using photothermal radiometry. The curing materials studied are two adhesives and a UV curing resin. It is shown that the curing process can be observed in all the cases as an increase in the thermal diffusion coefficient of the buried sample. It is shown that the kinetic behavior of the adhesives is very similar, probably due to the fact that evaporation is the main mechanism that generates the curing contrary to what happens in the UV curing resin where the light flux is the determining factor that induces the polymerization.
1 Introduction In a wide variety of applications, curable materials are used frequently in such a way that they remain buried inside some other materials, this is specially important in the electronic industry [1–3]. Therefore it is important to have methods to determine how the curing process evolves under such conditions. In this work the curing of two types of adhesives and a UV curing resin is investigated [3]. In this work the process of curing is studied by means of the infrared emission of the samples after being illuminated by a modulated laser beam. It is shown that the UV curable dental resin shows a measurable change in PTR signal that can provide us with the thermal diffusivity evolution; on the other hand the cold welding shows a strong change in PTR signal that indicates a stronger process of polymerization. The dynamic behaviour of the curing of the buried layers for the three kinds of materials is compared.
2 Materials and methods The samples were prepared forming a sandwich, in which the central layer was the curable material. Three curable materials were studied: A cold welding adhesive (61-A www.itwpolymex.com), a polyvinyl acetate adhesive (Resistol 850) and a UV curing resin. In the case of the cold welding and polyvinyl acetate adhesives, two external thin graphite foils (200 µm) were used. In the case of the UV curing resin, the top layer is a 200 µm graphite layer and the bottom Corning glass slide 150 µm thick, partially transparent to the UV. The adhesives experience curing by the continuous evaporation of their solvents and the UV curing resin was illuminated through the transparent layer using a UV light source (Translux EC/Translux ECS) to induce the polymerisation. A schematic diagram of the experimental photothermal radiometry (PTR) setup is shown in Figure 1. Radiation from a modulated solid state laser (680 nm) is directed onto the
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The European Physical Journal Special Topics
Fig. 1. PTR experimental setup. 0.0053 -107
Glue 850
Glue 850
0.0052
-108
Data: SampleA17Hz_B Model: Logistic Chi^2 = 4.5299E-10 A1 0.00479 ±4.3528E-6 A2 0.00527 ±7.1782E-6 x0 1070.3186 ±22.92652 p 1.4621 ±0.04955
0.0050
0.0049
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PTR Signal (Volts)
0.0051
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-110
0.0048 -111
0.0047 -112
0.0046 0
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Fig. 2. Time evolution of the photothermal amplitude and phase of the polyvinyl acetate.
sample surface. The emitted IR radiation from the sample surface is collected and focused onto the detector using two off-axis paraboloidal mirrors. The detector is a liquid N2 cooled HgCdTe (Judson) element with an active area of 1 mm2 and sensitivity range between 2–12 µm. The detector signal is preamplified (PA-300 Judson) before been sent into the lock-in amplifier. The two lock-in amplifier outputs, amplitude, and phase, are recorded as functions of time.
3 Results and discussion Figures 2, 3 and 4 show the experimental data for amplitude and phase for the different layered systems studied as a function of time. It can be observed that the system with cold welding shows a much higher change than the observed for the polyvinyl acetate and UV-curing resin. It can be observed that the cold welding adhesive experiences a much higher change than the other materials. In order to analyze the time evolution of these systems, a logistic equation is used, given by: A1 − A2 (1) y = A2 + p, 1 + (t/t0 ) with A1 , A2 constants that define the maximum and minimum signal, and t0 is the time at which the slope changes and p measures slope at the point of maximum derivative. Fitting the experimental data using equation 1, the following results are obtained, for the cold welding adhesive t0 = 1149 and p = 1.33, for the polyvinyl acetate adhesive t0 = 1070 and p = 1.46, and for the resin t0 = 169, p = 6.89. These results indicate that the kinetics for both adhesives
Photoacoustic and Photothermal Phenomena
0.0007
Cold welding
205
Cold welding
160
0.0006
0.0004
Data: soldfrio1_B Model: Logistic Chi^2 = 1.44E-11 A1 0.00014 A2 0.00074 x0 1149.27287 p 1.331 ±0
0.0003
0.0002
Phase (Grade)
Amplitud (Volts)
140 0.0005
±0 ±4.1338E-7 ±2.15653
120
100
80
0.0001
60 0
1000
2000
3000
4000
5000
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Fig. 3. Time evolution of the photothermal amplitude and phase of cold welding adhesive. 0.00058
55
0.00056
Dental Resin (UV curable)
50
Dental Resin (UV curable)
0.00054
45
Phase (Grade)
PTR Signal (Volts)
0.00052 0.00050 0.00048 0.00046 Data: GraResVid17Hz_B Model: Logistic Chi^2 = 2.4633E-10 A1 0.00042 ±3.1823E-6 A2 0.00053 ±3.3066E-6 x0 169.02109 ±5.46627 p 6.8331 ±0
0.00044 0.00042 0.00040
0
50
100
150
200
Time (seconds)
250
300
350
35 Data: GraResVid17Hz_C Model: Logistic Chi^2 = 4.62995 A1 20.3844 ±0.44134 A2 48.59569 ±0.41798 x0 158.80478 ±2.50608 p 6.9 ±0
30 25 20 15
0.00038 -50
40
400
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0
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Time (seconds)
Fig. 4. Time evolution photothermal of the amplitude and phase of dental resin UV curing.
is similar; even though the change in the amplitude for the cold welding one is much higher when the material goes from the non-cured situation to the cured one. On the other hand the kinetics of the resin is very different due to the fact that the process depends mostly on the light energy flux into the sample [4].
4 Conclusions The kinetics of systems of curing materials buried between two thin plates has been studied using photothermal radiometry. The real time evolution analysis shows that the two types of adhesives, even though they have a very different, follow a similar behaviour, influenced mainly by the fact they are systems that cure by evaporation. The time evolution of the UV-curing resin, on the other hand depends mainly in the UV-light energy flux that receives the sample. Due to the fact that the measurements are performed in the transmission mode, the results indicate that the material is becoming a better diffuser with time, thus shifting the statistical centerpoint of the thermal-wave source closer to the detection location (back-surface), resulting in increasing amplitude and phase lag closer to the detection spot.
References 1. D.P. Almond, P.M. Patel, Phothermal Science and Techniques (Chapman & Hall, London, 1996) 2. J.L. Pichardo, J.J. Alvarado-Gil, J. Appl. Phys. 89, 4070 (2001) 3. C. May, Epoxy Resins in Chemistry and Technology, CRC, 2nd edn. (Marcel Dekker Inc., New York, 1988) 4. E.C.R. Coloiano et al., International Conference on Photoacosutic and Photothermal Phenomena, edited by H. Vargas, E. Correa da Silva, L.C. Moura Miranda (2004), pp. 793–795
