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Stereolithography Somos 7110 photosensitive resin: study of curing kinetic and thermal degradation
Journal of Materials Processing Technology 168 (2005) 164–171
Stereolithography Somos 7110 photosensitive resin: study of curing kinetic and thermal degradation G.V. Salmoria a, b , V.J. Gonzalez d , C.H. Ahrens b , V. Soldi c , A.T.N. Pires c, ∗ b
a LABMAT, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´ opolis, SC, Brazil CIMJECT, Departamento de Engenharia Mecˆanica, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´opolis, SC, Brazil c POLIMAT, Departamento de Qu´ımica, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´ opolis, SC, Brazil d Departamento de Pol´ımeros, Instituto Universit´ ario de Tecnologia de Valencia, Venezuela
Received 14 November 2003; received in revised form 19 November 2004; accepted 24 November 2004
Abstract Different types of photopolymers, also called photosensitive resins, have been used in the stereolithography process to obtain specimens with specific size and application. The Somos 7110 resin evaluated in this study was characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, indicating the presence of epoxy and acrylic functional groups, as well as, aromatics and aliphatic ether groups. Infrared spectroscopy was also used as the technique to evaluate the curing process and identify the components of thermal degradation of liquid and cured resin. Thermogravimetric analysis was other complementary procedure used to quantify the components formed in the degradation process and determines the related physicochemical parameters. The kinetic studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking, due to the reaction order of the curing process, which was very close to unity. The cured resin had only one degradation stage with activation energy of 100–125 kJ mol−1 in the weight loss fraction of 0.20–0.85, in contrast to the second degradation stage observed for liquid resin above the 0.5-weight loss fraction. This second degradation stage gives Ea -values in the range 210–260 kJ mol−1 . The higher weight loss temperature of the cured resin compared with the liquid resin and the single decomposition stage suggest different decomposition mechanisms and a stability of the cured resin at higher temperatures compared to the liquid resin. © 2005 Elsevier B.V. All rights reserved. Keywords: Photopolymer characterization; Curing kinetics; Thermal degradation
1. Introduction Photopolymers, also called photosensitive resins, are employed in rapid prototype processes, stereolithography (SL), to build a part in a layer-by-layer photopolymerization [1–3]. Many kinds of resins are commercially available with specific requirements such as low viscosity, high heat deflection temperature or significant flexibility. The resin properties, such as humidity tolerance, low-curl, dimensional precision, cure processing behavior and mechanical strength of the part, have relevant importance. The degree of curing of the object obtained by stereolithography depends on the building parameters and resin ∗
Corresponding author. Fax: 55 48 331 9711. E-mail address: [email protected] (A.T.N. Pires).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.11.012
photosensitivity. Due to difficulties in optimizing these parameters, a post-curing process by ultraviolet radiation or by thermal treatment is required to increase the degree of cure improving thermal, mechanical and chemical properties [4,5]. For functional prototypes such as molds for the injection of thermoplastics (“rapid tooling”) or metallurgy models (“investment casting”) [6–9], knowledge of the composition components and curing kinetics of the resin, such as mechanical and thermal properties of the part is important to estimate the performance of the stereolithography mold or model under the working conditions. This work investigated the curing kinetic process and the thermal degradation of liquid and cured Somos 7110 resin, as well as characterized of the main chemical groups of the liquid resin by spectroscopic techniques.
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
2. Experimental 2.1. Materials and curing procedure The photopolymer DSM Somos® 7110 was employed as a resin in a stereolithography process and was used without further purification. A stereolithography machine, SLA250/30 model, from 3D Systems Incorporation, USA, with a He–Cd UV laser (325 nm) was used to produce specimens with dimensions of 4.50 and 0.45 mm diameter and height, respectively. The ultra-violet post-curing process was carried out in a chamber with eight fluorescent lamps Phillips TLK 40 W/05, with a radiation range of 300–360 nm. A MicroQu´ımica MQBEP 2000 MP furnace was employed in the thermal post-cure of the specimens. 2.2. Infrared and nuclear magnetic resonance spectroscopy FTIR spectra of the resin or specimens were obtained on a 16 PC Perkin Elmer spectrophotometer, performing 20 scans at a resolution of 4 cm−1 . The sample films of liquid resin on KBr plates or a gas cell for the gaseous product analyses were employed. NMR experiments were performed on a Bruker 200 MHz spectrometer at room temperature. All spectra were obtained from 30% volume of resin using DCCl3 as a solvent. DEPT and 2D NMR (1 H 13 C heteronuclear and COSY) spectra were also recorded. 2.3. Curing kinetics The sample films of liquid resin on KBr plates were maintained in a furnace at 398 K or in a UV chamber and FTIR spectra recorder as a function of time. An analogous procedure was carried out at 423 K. 2.4. Thermal decomposition The liquid resin and the specimen degradation was analysed with a Shimadzu 50 thermogravimetric analyser under nitrogen atmosphere. Non-isothermal experiments were performed in the temperature range of 298–873 K at different heating rates (5, 10 and 20 K min−1 ) for each sample. The nitrogen flow was maintained at 50 cm3 min−1 and samples of ca. 10 mg were used for each experiment. The thermogravimetric data were analysed through the Ozawa method to determine the kinetics parameters, through the associated TGA-50 software. The apparent activation energy was derived from the slope of the dependence of the heating-rate upon the reciprocal absolute temperature, at defined mass loss. For the FTIR analysis of the gas products evolved from the degradation process, samples of ca. 200 mg were heated (10 K min−1 ) in a tubular oven (Lindberg/Blue mod ¨ under nitrogen flow (50 cm3 min−1 ). The spectra TF5503A)
165
of the gas products were obtained during the degradation process at different temperatures in the tubular oven connected to the gas cell of the FTIR equipment.
3. Results and discussion 3.1. Characterization of the resin The FTIR spectra of liquid and cured resin in the 4000 to 500 cm−1 region are shown in Fig. 1. In the liquid resin spectrum, a broad band corresponding to the hydroxyl stretching vibration at 3450 cm−1 , and a strong band due to the C H stretching vibration at 2925 and 2850 cm−1 . The band at 1723 cm−1 corresponds to the C O, and those at 1634 cm−1 and 1620 cm−1 to the C C. The bands at 1173, 914 and 788 cm−1 correspond to the C O stretching vibration, and that at 760 cm−1 to the CH2 angular deformation. Such bands are inherent to epoxy and acrylic groups as well. The epoxy rings converted to ether groups during the curing process raises the band at 1250 cm−1 and causes the disappearance of bands at 914 and 788 cm−1 . The band intensity corresponding to the C C of the acrylic group also show a reduction due to reticulation process as observed in Fig. 1. The 1 H NMR liquid resin spectrum is shown in Fig. 2, with corresponding signals of protons in different electromagnetic environments. It is possible to identify protons of methyl, methylene, olefin and aromatic groups. Chemical shifts relative to TMS are reported in Table 1. The predominant signals corresponding to the aliphatic protons could explain the lower value of the glass transition temperature being lower than that of the aromatic epoxy resin. (Tg = 328 K, determined by DSC). The hydrogen chemical shift within a range of 6.7–7.1 ppm due to aromatic protons can be attributed to additives or monomer units that can confer better mechanical and thermal properties to cured resin. The 13 C NMR spectrum shows signals of CH2 for different groups, as reported in Table 2. The presence of the signals at 127 and 131 ppm and at 113 and 127 ppm are attributed
Fig. 1. Infrared spectra for the liquid and cured resin.
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Fig. 2. 200 MHz 1 H NMR liquid resin spectrum. Table 1 NMR signals refering to different molecular groups
1H
Chemical shift (ppm)
Structural assignments
0.9–1.1 1.3–2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.6 4.2 5.8 and 6.4 6.1 6.7 and 7.1
H of CH3 H of CH2 H of CH2 H of CH (epoxy) H of CH2 (epoxy) H of CH2 O H of CH O H of CH3 O H of O CH2 CH2 O H of O CH2 O H of CH2 H of CH H aromatic
to olefin and aromatic carbons, respectively. The carbonyl group signals were identified within the 165–175 ppm range relating to the ester group. The assignment of the carbon signals to a particular type of carbon can be established using the DEPT method. By correctly choosing the experimental parameters it is possible to obtain information about methyne, methylene and methyl carbons. Fig. 3 shows the liquid resin DEPT spectrum, where Table 2 13C NMR signals relating to different molecular groups Chemical shift (ppm)
Structural assignments
8–5 and 30 20–30 34 36 51 54 68 131 127 113 and 127 165, 172, 174 and 175
C of CH3 C of CH2 C of CH (epoxy) C of CH2 (epoxy) C of CH O C of CH3 O C of O CH2 CH2 O C of CH2 C of CH C aromatic C of C O
the signals recorded above the base line correspond to CH3 and CH carbons, and below the base line to CH2 carbons. In the 2D NMR heteronuclear 1 H 13 C correlation experiments, each resonance frequency signal is related to those of its neighbors and thus molecular connectivities can be determined. Fig. 4 shows the 2D NMR heteronuclear liquid resin spectrum that reinforces the attribution denoted in Tables 1 and 2. The analysis of NMR spectra indicates the presence of epoxy and acrylic groups in the liquid resin in agreement with the FTIR spectrum. The IR and NMR spectroscopy analysis showed the presence of epoxy and acrylic functional groups, as well as, aromatic and aliphatic ether groups in the Somos 7110 resin. The aliphatic structure groups explain the value of 328 K for the Tg . 3.2. Curing kinetics of the liquid resin The curing process is of particular importance in the manufacture of parts that can influence the macroscopic properties of the final product. The basic parameter governing the state of material is chemical conversion. Therefore, knowledge of the rate of curing kinetics and how it changes with cure temperature or light irradiation is important and useful for predicting the chemical conversion achieved after a curing program. The area peak assigned to the C C stretching vibration of acrylic groups at 1634 cm−1 decreases with the network formation via the chemical crosslinking of resin components and can be used to follow the cure conversion of the resin. Due to the impossibility of obtaining the same sample dimensions, the band area of the carbonyl group stretching at 1730 cm−1 was used as the internal correction during the curing process and the Gaussian deconvolution of the peaks permitted an evaluation of the peak area (Fig. 5). Analysis based on chemical conversion usually requires different extents of reaction for each independent step in a
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
167
Fig. 3. DEPT spectrum of the SOMOS 7110 resin.
multiple-reaction sequence, unless ␣ can be defined for all of the reactions that involve the key component. Because the acrylic groups in the resin participate in several chemical reactions such as the formation of a network, the fractional conversion (α) can be defined as the total conversion based on all of the possible reactions [10]. The fractional conversion can be described as: α=
At Ao
(1)
Fig. 4. Heteronuclear 1 H
13 C
where At and Ao are the peak area at time t and at initial time, respectively, of the infrared spectrum at 1634 cm−1 . The determination of the kinetics parameters is based on the general nth order rate expression as follows: −
d(a) = k(α)n dt
(2)
where k is the kinetics constant and n is the reaction rate at temperature T.
correlation spectrum of the SOMOS 7110 resin.
168
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Fig. 6. The cure conversion ln α, as a function of time, by thermal process at ()398 K and. () 423 K, and by () ultraviolet radiation. Fig. 5. Infrared spectrum of the resin film after 40 min at 398 K showing the peak at 1634 cm−1 .
Fig. 7. TG and DTG curves for (a) liquid and (b) cured resin.
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
169
The plot of data from the cure conversion of liquid resin as a function of time, using the integration of Eq. (2) into the reaction order n equal to 1: ln(α) = −kt
(3)
demonstrated straight lines for the different cure conditions, i.e., thermal and ultraviolet radiation as shown in Fig. 6. These different curing conditions showed first order kinetics. The complete conversion of the acrylic groups did not occur under the reaction conditions that were investigated in this study. The value of the kinetics constant is equal to 2.58 × 10−2 min−1 for the thermal curing at 398 K. The cure process was accomplished by thermal curing at 423 K and ultraviolet radiation curing with corresponding values for the kinetics constant of 3.49 × 10−2 min−1 and 3.66 × 10−2 min−1 , respectively. The similarity between the values for the kinetics constant for the thermal cure at 423 K and the ultraviolet cure, suggests that under these conditions, the cure process occurs through a similar mechanism. The solvent-free polymerizations were diffusion-controlled during the chemical crosslinking stage as indicated the apparent reaction order equal to unity. 3.3. Thermal decomposition of the liquid and cured resin The thermogravimetric curves (TG and DTG) for the liquid resin are shown in Fig. 7. The liquid resin showed a first
Fig. 8. Values of Ea vs. α for the liquid and post-cured resin.
mass loss stage within a temperature range of 423–623 K, related to the acrylic group degradation, as will be discussed in the next section. The second peak, in the DTG curve corresponds to the complete polymer degradation at temperatures up to 773 K. Parameters such as the maximum degradation temperatures, determined considering the derivative curves, and percentage of mass loss in each stage of degradation were 553 and 731 K, and 35 and 55%, respectively, for the liquid resin. The corresponding solid residue must be attributed to ash and inorganic additives.
Fig. 9. Infrared spectrum of the decomposition of the liquid resin.
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Fig. 10. Infrared spectrum of the decomposition of the cured SOMOS 7110 resin.
Different behavior was observed in the thermogravimetric curve for the cured resin with only one mass loss stage, as shown in Fig. 6. The maximum degradation temperature and the mass loss for the cured resin were 720 K and 90%, respectively, indicating higher stability for cured specimens, in comparison with liquid resin. The activation energy (Ea ) was determinated by the Ozawa method [11,12], based on the Arrhenius equation. In Fig. 8 the Ea -values for liquid and cured resin were plotted against weight loss fraction (αd ). As observed, for cured resin, the Ea changed from ca. 100 to 125 kJ mol−1 in the αd range 0.20–0.85. Due to the existence of two degradation stages for the liquid resin, the profile of the plot of activation energy as a function of the weight loss fraction presents two ranges of Ea values corresponding to different degradation mechanisms. The Ea -value ranges were 60–110 and 210–260 kJ mol−1 corresponding to the 0.06–0.33 and 0.50–0.90 weight loss fractions, respectively.
common product found in the methacrylate ester decomposition reactions [13,14]. The highest gas evolution occurred from 633 to 783 K, as can be noted by the band intensities. The absorption bands at 3000 to 2900 cm−1 are associated with C H stretching, 2350 and 2150 cm−1 are related to the products CO2 and CO, respectively. The absorption bands at 1740 and 1650 cm−1 are associated with C O from esters and organic acids, respectively, due to scission of the main carbon chain in macromolecules. The difference in the band intensity of products with the temperature is in agreement with the thermogravimetric analyses (Fig. 7). The Fig. 10 shows the FTIR spectrum for the evolved gas products of the cured resin, with bands at 2980 and 1200–1000 cm−1 related to C H aliphatic groups and C O bonds, respectively, that were not observed in the decomposition of the liquid resin due to the curing process up to 613 K. The other gas products from the cured resin were the same as those evolved in the degradation of the liquid resin.
3.4. Infrared analyses of gas degradation products 4. Conclusion The gas products of thermal degradation were analyzed by infrared spectroscopy to evaluated thermal stability of the liquid and cured resin. Fig. 9 shows the FTIR spectrum for the evolved gas products of the liquid resin within a temperature range from 393 to 883 K. In this spectrum we can observed the low quantity of gas product up to 633 K, assigning bands at 1850 and 1100 cm−1 to C O and C O groups, respectively. As reported in the literature, a cyclical anhydride is a
The kinetics studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking due to the reaction order of the cure reaction being very close to unity, allowing rapid photopolymerization. The value of the kinetics constant is equal for 2.58 × 10−2 min−1 for the thermal curing at 398 K. The curing process was accomplish by thermal curing at
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
423 K and ultraviolet radiation curing with corresponding values for the kinetics constant of 3.49 × 10−2 min−1 and 3.66 × 10−2 min−1 , respectively. The results show the cured resin having only one degradation stage with activation energy of 100 to 125 kJ mol−1 in the weight loss fraction of 0.20–0.85, in contrast to the second degradation stage observed from liquid resin above the 0.5-weight loss fraction. This second degradation stage shows the Ea -value range to be 210–260 kJ mol−1 . However, the higher weight loss temperature of cured resin compared with liquid resin and the single decomposition stage suggest different decomposition mechanisms and a higher temperature stability for cured resin in comparison to the liquid resin. References [1] P.F. Jacobs, Jacobs, “rapid prototyping and manufacturing, in: Fundamentals of Stereolithography, Society of Manufacturing Engineers, Dearborn, MI, 1992. [2] P.F. Jacobs, Stereolithography and Other RP&M Technologies: From Rapid To Rapid Tooling, Booknews Inc., USA, 1996. [3] J.Y.H. Fuh, L. Lu, C.C. Tan, Z.X. Shen, S. Chew, Processing and characterizing photo-sensitive polymer in the rapid prototyping process, J. Mater. Process. Tech. 89–90 (1999) 211–217.
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[4] J.Y.H. Fuh, Y.S. Choo, L. Lu, et al., Post-cure shrinkage of photosensitive material used in laser lithography process, J. Mater. Process. Tech. 63 (1997) 887–891. [5] C.M. Cheah, A.Y.C. Nee, J.Y.H. Fuh, L. Lu, Y.S. Choo, T. Miyazawa, Characteristics of photopolymeric material used in rapid prototypes. Part I. Mechanical properties in the green state, J. Mater. Process. Tech. 67 (1997) 41–45. [6] N. Hopkinson, P. Dickens, Predicting stereolithography injection mould tool behavior using models to predict ejection force and tool strength, Int. J. Prod. Res. 38 (16) (2000) 3747–3757. [7] J.S. Colton, Y. LeBaut, Thermal effects on stereolithography injection mold inserts, Polym. Eng. Sci. 40 (6) (2000) 1360–1368. [8] A.E. Palmer, J.S. Colton, Failure mechanisms in stereolithography injection molding tooling, Polym. Eng. Sci. 40 (6) (2000) 1395–1404. [9] P.M. Dickens, Conversion of RP Models to Investiment Casting, Rapid Prototyping J. 1 (4) (1995) 4–11. [10] N.A. Stjohn, G.A. George, Cure kinetics and mechanisms of a tetraglycidyl-4,4 -diaminodiphenylsulphone epoxy-resin using near IR spectroscopy, Polymer 33 (1992) 2679. [11] T. Ozawa, A new method of analyzing thermogravimetric data, B. Chem. Soc. Jpn. 38 (11) (1965) 1881–1889. [12] T. Ozawa, Critical investigation of methods for kinetic-analysis of thermoanalytical data, J. Therm. Analysis 7 (3) (1975) 601–617. [13] M. Coskun, H. Erten, K. Demirelli, Thermal degradation of poly[3(1-cyclohexyl)azetidinyl methacrylate], Polym. Degrad. Stabil. 69 (2000) 245–249. [14] M. Coskun, C. Soykan, M. Ahmedzade, K. Demirelli, Preparation and thermal degradation of poly(p-substituted phenacyl methacrylates), Polym. Degrad. Stabil. 72 (2001) 69–74.
Stereolithography Somos 7110 photosensitive resin: study of curing kinetic and thermal degradation G.V. Salmoria a, b , V.J. Gonzalez d , C.H. Ahrens b , V. Soldi c , A.T.N. Pires c, ∗ b
a LABMAT, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´ opolis, SC, Brazil CIMJECT, Departamento de Engenharia Mecˆanica, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´opolis, SC, Brazil c POLIMAT, Departamento de Qu´ımica, Universidade Federal de Santa Catarina, UFSC, 88040-900 Florian´ opolis, SC, Brazil d Departamento de Pol´ımeros, Instituto Universit´ ario de Tecnologia de Valencia, Venezuela
Received 14 November 2003; received in revised form 19 November 2004; accepted 24 November 2004
Abstract Different types of photopolymers, also called photosensitive resins, have been used in the stereolithography process to obtain specimens with specific size and application. The Somos 7110 resin evaluated in this study was characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, indicating the presence of epoxy and acrylic functional groups, as well as, aromatics and aliphatic ether groups. Infrared spectroscopy was also used as the technique to evaluate the curing process and identify the components of thermal degradation of liquid and cured resin. Thermogravimetric analysis was other complementary procedure used to quantify the components formed in the degradation process and determines the related physicochemical parameters. The kinetic studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking, due to the reaction order of the curing process, which was very close to unity. The cured resin had only one degradation stage with activation energy of 100–125 kJ mol−1 in the weight loss fraction of 0.20–0.85, in contrast to the second degradation stage observed for liquid resin above the 0.5-weight loss fraction. This second degradation stage gives Ea -values in the range 210–260 kJ mol−1 . The higher weight loss temperature of the cured resin compared with the liquid resin and the single decomposition stage suggest different decomposition mechanisms and a stability of the cured resin at higher temperatures compared to the liquid resin. © 2005 Elsevier B.V. All rights reserved. Keywords: Photopolymer characterization; Curing kinetics; Thermal degradation
1. Introduction Photopolymers, also called photosensitive resins, are employed in rapid prototype processes, stereolithography (SL), to build a part in a layer-by-layer photopolymerization [1–3]. Many kinds of resins are commercially available with specific requirements such as low viscosity, high heat deflection temperature or significant flexibility. The resin properties, such as humidity tolerance, low-curl, dimensional precision, cure processing behavior and mechanical strength of the part, have relevant importance. The degree of curing of the object obtained by stereolithography depends on the building parameters and resin ∗
Corresponding author. Fax: 55 48 331 9711. E-mail address: [email protected] (A.T.N. Pires).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.11.012
photosensitivity. Due to difficulties in optimizing these parameters, a post-curing process by ultraviolet radiation or by thermal treatment is required to increase the degree of cure improving thermal, mechanical and chemical properties [4,5]. For functional prototypes such as molds for the injection of thermoplastics (“rapid tooling”) or metallurgy models (“investment casting”) [6–9], knowledge of the composition components and curing kinetics of the resin, such as mechanical and thermal properties of the part is important to estimate the performance of the stereolithography mold or model under the working conditions. This work investigated the curing kinetic process and the thermal degradation of liquid and cured Somos 7110 resin, as well as characterized of the main chemical groups of the liquid resin by spectroscopic techniques.
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
2. Experimental 2.1. Materials and curing procedure The photopolymer DSM Somos® 7110 was employed as a resin in a stereolithography process and was used without further purification. A stereolithography machine, SLA250/30 model, from 3D Systems Incorporation, USA, with a He–Cd UV laser (325 nm) was used to produce specimens with dimensions of 4.50 and 0.45 mm diameter and height, respectively. The ultra-violet post-curing process was carried out in a chamber with eight fluorescent lamps Phillips TLK 40 W/05, with a radiation range of 300–360 nm. A MicroQu´ımica MQBEP 2000 MP furnace was employed in the thermal post-cure of the specimens. 2.2. Infrared and nuclear magnetic resonance spectroscopy FTIR spectra of the resin or specimens were obtained on a 16 PC Perkin Elmer spectrophotometer, performing 20 scans at a resolution of 4 cm−1 . The sample films of liquid resin on KBr plates or a gas cell for the gaseous product analyses were employed. NMR experiments were performed on a Bruker 200 MHz spectrometer at room temperature. All spectra were obtained from 30% volume of resin using DCCl3 as a solvent. DEPT and 2D NMR (1 H 13 C heteronuclear and COSY) spectra were also recorded. 2.3. Curing kinetics The sample films of liquid resin on KBr plates were maintained in a furnace at 398 K or in a UV chamber and FTIR spectra recorder as a function of time. An analogous procedure was carried out at 423 K. 2.4. Thermal decomposition The liquid resin and the specimen degradation was analysed with a Shimadzu 50 thermogravimetric analyser under nitrogen atmosphere. Non-isothermal experiments were performed in the temperature range of 298–873 K at different heating rates (5, 10 and 20 K min−1 ) for each sample. The nitrogen flow was maintained at 50 cm3 min−1 and samples of ca. 10 mg were used for each experiment. The thermogravimetric data were analysed through the Ozawa method to determine the kinetics parameters, through the associated TGA-50 software. The apparent activation energy was derived from the slope of the dependence of the heating-rate upon the reciprocal absolute temperature, at defined mass loss. For the FTIR analysis of the gas products evolved from the degradation process, samples of ca. 200 mg were heated (10 K min−1 ) in a tubular oven (Lindberg/Blue mod ¨ under nitrogen flow (50 cm3 min−1 ). The spectra TF5503A)
165
of the gas products were obtained during the degradation process at different temperatures in the tubular oven connected to the gas cell of the FTIR equipment.
3. Results and discussion 3.1. Characterization of the resin The FTIR spectra of liquid and cured resin in the 4000 to 500 cm−1 region are shown in Fig. 1. In the liquid resin spectrum, a broad band corresponding to the hydroxyl stretching vibration at 3450 cm−1 , and a strong band due to the C H stretching vibration at 2925 and 2850 cm−1 . The band at 1723 cm−1 corresponds to the C O, and those at 1634 cm−1 and 1620 cm−1 to the C C. The bands at 1173, 914 and 788 cm−1 correspond to the C O stretching vibration, and that at 760 cm−1 to the CH2 angular deformation. Such bands are inherent to epoxy and acrylic groups as well. The epoxy rings converted to ether groups during the curing process raises the band at 1250 cm−1 and causes the disappearance of bands at 914 and 788 cm−1 . The band intensity corresponding to the C C of the acrylic group also show a reduction due to reticulation process as observed in Fig. 1. The 1 H NMR liquid resin spectrum is shown in Fig. 2, with corresponding signals of protons in different electromagnetic environments. It is possible to identify protons of methyl, methylene, olefin and aromatic groups. Chemical shifts relative to TMS are reported in Table 1. The predominant signals corresponding to the aliphatic protons could explain the lower value of the glass transition temperature being lower than that of the aromatic epoxy resin. (Tg = 328 K, determined by DSC). The hydrogen chemical shift within a range of 6.7–7.1 ppm due to aromatic protons can be attributed to additives or monomer units that can confer better mechanical and thermal properties to cured resin. The 13 C NMR spectrum shows signals of CH2 for different groups, as reported in Table 2. The presence of the signals at 127 and 131 ppm and at 113 and 127 ppm are attributed
Fig. 1. Infrared spectra for the liquid and cured resin.
166
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Fig. 2. 200 MHz 1 H NMR liquid resin spectrum. Table 1 NMR signals refering to different molecular groups
1H
Chemical shift (ppm)
Structural assignments
0.9–1.1 1.3–2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.6 4.2 5.8 and 6.4 6.1 6.7 and 7.1
H of CH3 H of CH2 H of CH2 H of CH (epoxy) H of CH2 (epoxy) H of CH2 O H of CH O H of CH3 O H of O CH2 CH2 O H of O CH2 O H of CH2 H of CH H aromatic
to olefin and aromatic carbons, respectively. The carbonyl group signals were identified within the 165–175 ppm range relating to the ester group. The assignment of the carbon signals to a particular type of carbon can be established using the DEPT method. By correctly choosing the experimental parameters it is possible to obtain information about methyne, methylene and methyl carbons. Fig. 3 shows the liquid resin DEPT spectrum, where Table 2 13C NMR signals relating to different molecular groups Chemical shift (ppm)
Structural assignments
8–5 and 30 20–30 34 36 51 54 68 131 127 113 and 127 165, 172, 174 and 175
C of CH3 C of CH2 C of CH (epoxy) C of CH2 (epoxy) C of CH O C of CH3 O C of O CH2 CH2 O C of CH2 C of CH C aromatic C of C O
the signals recorded above the base line correspond to CH3 and CH carbons, and below the base line to CH2 carbons. In the 2D NMR heteronuclear 1 H 13 C correlation experiments, each resonance frequency signal is related to those of its neighbors and thus molecular connectivities can be determined. Fig. 4 shows the 2D NMR heteronuclear liquid resin spectrum that reinforces the attribution denoted in Tables 1 and 2. The analysis of NMR spectra indicates the presence of epoxy and acrylic groups in the liquid resin in agreement with the FTIR spectrum. The IR and NMR spectroscopy analysis showed the presence of epoxy and acrylic functional groups, as well as, aromatic and aliphatic ether groups in the Somos 7110 resin. The aliphatic structure groups explain the value of 328 K for the Tg . 3.2. Curing kinetics of the liquid resin The curing process is of particular importance in the manufacture of parts that can influence the macroscopic properties of the final product. The basic parameter governing the state of material is chemical conversion. Therefore, knowledge of the rate of curing kinetics and how it changes with cure temperature or light irradiation is important and useful for predicting the chemical conversion achieved after a curing program. The area peak assigned to the C C stretching vibration of acrylic groups at 1634 cm−1 decreases with the network formation via the chemical crosslinking of resin components and can be used to follow the cure conversion of the resin. Due to the impossibility of obtaining the same sample dimensions, the band area of the carbonyl group stretching at 1730 cm−1 was used as the internal correction during the curing process and the Gaussian deconvolution of the peaks permitted an evaluation of the peak area (Fig. 5). Analysis based on chemical conversion usually requires different extents of reaction for each independent step in a
G.V. Salmoria et al. / Journal of Materials Processing Technology 168 (2005) 164–171
167
Fig. 3. DEPT spectrum of the SOMOS 7110 resin.
multiple-reaction sequence, unless ␣ can be defined for all of the reactions that involve the key component. Because the acrylic groups in the resin participate in several chemical reactions such as the formation of a network, the fractional conversion (α) can be defined as the total conversion based on all of the possible reactions [10]. The fractional conversion can be described as: α=
At Ao
(1)
Fig. 4. Heteronuclear 1 H
13 C
where At and Ao are the peak area at time t and at initial time, respectively, of the infrared spectrum at 1634 cm−1 . The determination of the kinetics parameters is based on the general nth order rate expression as follows: −
d(a) = k(α)n dt
(2)
where k is the kinetics constant and n is the reaction rate at temperature T.
correlation spectrum of the SOMOS 7110 resin.
168
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Fig. 6. The cure conversion ln α, as a function of time, by thermal process at ()398 K and. () 423 K, and by () ultraviolet radiation. Fig. 5. Infrared spectrum of the resin film after 40 min at 398 K showing the peak at 1634 cm−1 .
Fig. 7. TG and DTG curves for (a) liquid and (b) cured resin.
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The plot of data from the cure conversion of liquid resin as a function of time, using the integration of Eq. (2) into the reaction order n equal to 1: ln(α) = −kt
(3)
demonstrated straight lines for the different cure conditions, i.e., thermal and ultraviolet radiation as shown in Fig. 6. These different curing conditions showed first order kinetics. The complete conversion of the acrylic groups did not occur under the reaction conditions that were investigated in this study. The value of the kinetics constant is equal to 2.58 × 10−2 min−1 for the thermal curing at 398 K. The cure process was accomplished by thermal curing at 423 K and ultraviolet radiation curing with corresponding values for the kinetics constant of 3.49 × 10−2 min−1 and 3.66 × 10−2 min−1 , respectively. The similarity between the values for the kinetics constant for the thermal cure at 423 K and the ultraviolet cure, suggests that under these conditions, the cure process occurs through a similar mechanism. The solvent-free polymerizations were diffusion-controlled during the chemical crosslinking stage as indicated the apparent reaction order equal to unity. 3.3. Thermal decomposition of the liquid and cured resin The thermogravimetric curves (TG and DTG) for the liquid resin are shown in Fig. 7. The liquid resin showed a first
Fig. 8. Values of Ea vs. α for the liquid and post-cured resin.
mass loss stage within a temperature range of 423–623 K, related to the acrylic group degradation, as will be discussed in the next section. The second peak, in the DTG curve corresponds to the complete polymer degradation at temperatures up to 773 K. Parameters such as the maximum degradation temperatures, determined considering the derivative curves, and percentage of mass loss in each stage of degradation were 553 and 731 K, and 35 and 55%, respectively, for the liquid resin. The corresponding solid residue must be attributed to ash and inorganic additives.
Fig. 9. Infrared spectrum of the decomposition of the liquid resin.
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Fig. 10. Infrared spectrum of the decomposition of the cured SOMOS 7110 resin.
Different behavior was observed in the thermogravimetric curve for the cured resin with only one mass loss stage, as shown in Fig. 6. The maximum degradation temperature and the mass loss for the cured resin were 720 K and 90%, respectively, indicating higher stability for cured specimens, in comparison with liquid resin. The activation energy (Ea ) was determinated by the Ozawa method [11,12], based on the Arrhenius equation. In Fig. 8 the Ea -values for liquid and cured resin were plotted against weight loss fraction (αd ). As observed, for cured resin, the Ea changed from ca. 100 to 125 kJ mol−1 in the αd range 0.20–0.85. Due to the existence of two degradation stages for the liquid resin, the profile of the plot of activation energy as a function of the weight loss fraction presents two ranges of Ea values corresponding to different degradation mechanisms. The Ea -value ranges were 60–110 and 210–260 kJ mol−1 corresponding to the 0.06–0.33 and 0.50–0.90 weight loss fractions, respectively.
common product found in the methacrylate ester decomposition reactions [13,14]. The highest gas evolution occurred from 633 to 783 K, as can be noted by the band intensities. The absorption bands at 3000 to 2900 cm−1 are associated with C H stretching, 2350 and 2150 cm−1 are related to the products CO2 and CO, respectively. The absorption bands at 1740 and 1650 cm−1 are associated with C O from esters and organic acids, respectively, due to scission of the main carbon chain in macromolecules. The difference in the band intensity of products with the temperature is in agreement with the thermogravimetric analyses (Fig. 7). The Fig. 10 shows the FTIR spectrum for the evolved gas products of the cured resin, with bands at 2980 and 1200–1000 cm−1 related to C H aliphatic groups and C O bonds, respectively, that were not observed in the decomposition of the liquid resin due to the curing process up to 613 K. The other gas products from the cured resin were the same as those evolved in the degradation of the liquid resin.
3.4. Infrared analyses of gas degradation products 4. Conclusion The gas products of thermal degradation were analyzed by infrared spectroscopy to evaluated thermal stability of the liquid and cured resin. Fig. 9 shows the FTIR spectrum for the evolved gas products of the liquid resin within a temperature range from 393 to 883 K. In this spectrum we can observed the low quantity of gas product up to 633 K, assigning bands at 1850 and 1100 cm−1 to C O and C O groups, respectively. As reported in the literature, a cyclical anhydride is a
The kinetics studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking due to the reaction order of the cure reaction being very close to unity, allowing rapid photopolymerization. The value of the kinetics constant is equal for 2.58 × 10−2 min−1 for the thermal curing at 398 K. The curing process was accomplish by thermal curing at
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423 K and ultraviolet radiation curing with corresponding values for the kinetics constant of 3.49 × 10−2 min−1 and 3.66 × 10−2 min−1 , respectively. The results show the cured resin having only one degradation stage with activation energy of 100 to 125 kJ mol−1 in the weight loss fraction of 0.20–0.85, in contrast to the second degradation stage observed from liquid resin above the 0.5-weight loss fraction. This second degradation stage shows the Ea -value range to be 210–260 kJ mol−1 . However, the higher weight loss temperature of cured resin compared with liquid resin and the single decomposition stage suggest different decomposition mechanisms and a higher temperature stability for cured resin in comparison to the liquid resin. References [1] P.F. Jacobs, Jacobs, “rapid prototyping and manufacturing, in: Fundamentals of Stereolithography, Society of Manufacturing Engineers, Dearborn, MI, 1992. [2] P.F. Jacobs, Stereolithography and Other RP&M Technologies: From Rapid To Rapid Tooling, Booknews Inc., USA, 1996. [3] J.Y.H. Fuh, L. Lu, C.C. Tan, Z.X. Shen, S. Chew, Processing and characterizing photo-sensitive polymer in the rapid prototyping process, J. Mater. Process. Tech. 89–90 (1999) 211–217.
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