- Email: [email protected]
Investigations on cure kinetics and thermal degradation of stereolithography Renshape™ 5260 photosensitive resin
Polymer Testing 27 (2008) 698–704
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Characterisation
Investigations on cure kinetics and thermal degradation of stereolithography RenshapeÔ 5260 photosensitive resin G.V. Salmoria a, *, P. Klauss a, A.T.N. Pires b, J. Roeder b, V. Soldi b a b
´polis, SC, Brazil CIMJECT, Mechanical Engineering Department, Federal University of Santa Catarina, UFSC, 88040-900 Floriano ´polis, SC, Brazil POLIMAT, Chemistry Department, Federal University of Santa Catarina, UFSC 88040-900 Floriano
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2008 Accepted 17 May 2008
The stereolithography resin RenshapeÔ 5260 was characterized by infrared and nuclear magnetic resonance spectroscopy, indicating the presence of epoxy and acrylic reactive groups as well as aromatic and aliphatic ether groups. Infrared spectroscopy was also used to evaluate the curing process and identify the components of the thermal degradation of liquid and cured resins. Kinetics studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking, due to the reaction order of the curing process. The cured resin had only one degradation stage with activation energy of 90–250 kJ mol1 in the weight loss fraction of 0.10–0.92, whereas a second degradation stage was observed for the liquid resin above the 0.6 weight loss fraction. This second degradation stage gives Ea values in the range of 70–140 kJ mol1. The higher weight loss temperature of the cured resin compared with the liquid resin and the single decomposition stage observed for the former suggest a different decomposition mechanism and stability of the cured resin at higher temperatures compared to the liquid resin. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Photopolymer characterization Cure kinetics Thermal degradation
1. Introduction Designers and engineers use modern technologies to evaluate their product designs, such as rapid prototyping technologies used to build better prototypes in shorter times. Stereolithography (SL) is a rapid prototyping and manufacturing technique used to manufacture high accuracy parts. SL resin is exposed to ultraviolet (UV) laser beam radiation, which induces a localized polymerization process to solidify the liquid resin layer-by-layer, to form manufactured parts directly from 3D computer data [1,2]. Many kinds of photosensitive resins (photopolymers) used in stereolithography are composed of a complex mixture of epoxy and acrylate oligomers and additives. The SL resins have specific properties in terms of viscosity,
* Corresponding author. E-mail address: [email protected] (G.V. Salmoria). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.05.008
curability, elastic modulus, tensile strength, toughness and others, depending on their composition [3–6]. The building parameters, photoinitiator and resin photosensitivity properties and composition are the determining factors in the degree of cure of the objects manufactured by stereolithography. As the optimization of these parameters is difficult, a post-curing process is necessary, which can be carried out by ultraviolet radiation or thermal treatment. This process increases the degree of cure and, consequently, improves the thermal, mechanical and chemical properties of the resin [4,7]. Previous kinetics studies have indicated that DSM SomosÒ 7110 SL resin undergoes solvent-free polymerization, which is diffusion-controlled during the chemical crosslinking due to the reaction order of the cure reaction being very close to unity, allowing rapid photopolymerization [6]. Decomposition analysis has shown that cured resin has a higher weight loss temperature than liquid resin. Also, the single decomposition stage of the former, compared
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
2. Experimental 2.1. Materials and cure procedures The photopolymer RenshapeÔ 5260, used in the stereolithography process, was employed as the study material and used without prior purification. A stereolithography machine, model SLA-250/30 (3D Systems Incorporation, USA), with a He–Cd UV laser (325 nm) was used to produce the specimens. For the cure kinetics experiments, the samples were maintained in an UV chamber with eight fluorescent Philips TLK 40 W/05 lamps, with a radiation range of 300–360 nm. 2.2. Infrared and nuclear magnetic resonance spectroscopy The infrared spectra of the liquid and cured resins were obtained using a 16 PC Perkin Elmer spectrophotometer, performing 20 scans at a resolution of 4 cm1. The sample film of liquid resin was analyzed on a silicon cell. Nuclear magnetic resonance analysis was performed with a Varian 400 MHz spectrometer at room temperature. All spectra were obtained using 30% volume of resin and DCCl3 as the solvent. DEPT and 2D NMR (1H–13C heteronuclear and COSY) spectra were also recorded. 2.3. Cure kinetics and thermal decomposition In order to investigate the cure kinetics, a sample film of resin on the silicon cell was maintained in the UV chamber. FTIR spectra were recorded as a function of time on the Perkin Elmer spectrophotometer. The thermogravimetric analysis was conducted using a Shimadzu TGA-50 thermogravimetric analyzer. Nonisothermal experiments were performed in the temperature range 25–550 C at different heating rates (5, 10 and 20 C min1). The sample weights ranged from 13 to 20 mg and the nitrogen flow-rate was 50 cm3 min1. The thermogravimetric data were analyzed through the Ozawa method to determine the kinetics parameters, using the associated TGA-50 software. The apparent activation energy was derived from the slope of the dependence of the heating rate on the reciprocal of absolute temperature, at a certain mass loss. In the infrared spectral analyses of the gas products, the samples were placed in a ceramic sample holder and heated in a Lindberg/Blue MOD, TF5503A, tubular oven under nitrogen flow (50 cm3 min1). The gas evolution analysis was conducted at a heating rate of 10 C min1, from room temperature up to pre-established temperatures, to obtain the thermogravimetric curves of each system.
3. Results and discussion 3.1. Characterization of the resin Fig. 1 shows the FTIR spectra of the liquid and cure resins in the 4000–500 cm1 region. In the liquid resin spectrum, a broad band at 3503 cm1 can be observed, corresponding to the hydroxyl stretching vibration, and strong bands at 2929 cm1and 2863 cm1 due to the C–H3 and CH2 stretching vibrations, respectively. The band at 1731 cm1 corresponds to the C]O stretching of the ester group, and the band at 1634 cm1 to the C]C present in the acrylate group. The bands at 1265, 909 and 801 cm1 correspond to the –C–O–C– stretching vibration of the ether groups present in epoxy molecules. The bands at 1180 and 1104 cm1 correspond to the C–O group. All of these bands are characteristic of epoxy and acrylate groups. In the cured resin spectrum the band at 1634 cm1, corresponding to the C]C of the acrylate group, showed a reduction due to the reticulation process when the two spectra were compared. The 1H NMR liquid resin spectrum is shown in Fig. 2. Chemical shifts related to TMS are reported in Table 1. The region of 1.0–2.7 ppm is attributed to the alkyl hydrogen of the CH3, CH2 and CH groups. The shifts at 2.8 and 3.0 ppm are related to the –OCH2 and –OCH protons of the epoxy group, respectively. The spectrum shows protons present at 5.7, 5.9 and 6.3, characteristic of the terminal vinyl group. The protons related to the aromatics can be observed at 6.6 and 7.0 ppm in low quantity when compared with DSM SomosÒ 7110 [6], characteristic of the p-substituted aromatic groups, which confer better mechanical and thermal properties to the cured resin. The other chemical shifts correspond to the signals of the epoxy and acrylate group in aliphatic structures. The 13C NMR liquid resin spectrum in Fig. 3 shows the CH2 signals for different groups, as reported in Table 1. The regions at 17 and 20–30 ppm relate to CH3 and CH2 aliphatic carbons and that at 30–40 ppm to C–H carbons. The chemical shifts present at 43 and 50 ppm relate to carbons of the epoxy groups –OCH2 and –OCH, respectively.
Líquid
Transmittance
with the two decomposition stages of the latter, suggests a change in the decomposition mechanism due to modifications in the chemical composition induced by the cure [6]. This study investigated the chemical group composition, cure process and thermal degradation of liquid and cured RenShapeÔ 5260 resin used in the stereolithography process, using infrared and nuclear magnetic resonance spectroscopy, and thermal and gas evolution analyses.
699
Cured
4000
3000
2000
1000
Wavenumber (cm-1) Fig. 1. Infrared spectra for the liquid and cured resin.
700
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 2. 1H NMR (400 MHz) liquid resin spectra.
The signals at 49 and 51 ppm and at 60–72 ppm relate to –OCH2 and –OCH groups in aliphatic structures. The presence of signals at 110 and 130 ppm and 113 and 122 ppm can be attributed to olefin and aromatic carbons, respectively. The carbonyl group signals were within the 165– 204 ppm range relating to the ester group. The liquid resin DEPT spectrum is shown in Fig. 4, where the signals recorded above the baseline correspond to CH3 and CH carbons, and below the baseline to CH2 carbons. The assignment of the carbon signals to a particular type of carbon can be established using this method. By correctly choosing the experimental parameters it is possible to obtain information on the methyne, methylene and methyl carbons. Fig. 5 shows the 2D NMR heteronuclear liquid resin spectrum that reinforces the attribution given in Table 1. In the experiments, each resonance frequency signal is related to those of its neighbors and, thus, the molecular connectivity can be determined. The IR and NMR spectroscopic analyses showed the presence of epoxy and acrylic functional groups in the RenshapeÔ 5260 liquid resin, as well as aromatic and aliphatic ether groups. 3.2. Cure kinetics of the liquid resin The mechanical properties of parts manufactured for stereolithography depend on the degree of cure of the Table 1 1 H and 13C NMR signals related to different molecular groups 1
H (ppm)
1 1.2–2.2 2.3–2.7 2.8 3.0 2.9–3.1 3.2–4.2 6.6–7.0 5.7–6.3 5.9
Group
13
CH3 CH2 CH OCH2 (epoxy) OCH (epoxy) OCH2 OCH CH (phenyl) ]CH ]CH2 C]O
17 20–30 30–40 43 50 49–51 60–72 110 and 126 127 131 160 and 170
C
photosensitive resin. Knowledge of the relationship between resin structure and properties is useful in the SL tool manufacturing process, application and quality control (5, 6 and 8). Fig. 6 shows the IR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min. The difference in the peaks at wavenumber 2990 cm1, which represent the epoxy group, of the two spectra is evident. The peak area assigned to the epoxy groups at 2990 cm1 decreased with the network formation via the chemical crosslinking of resin components and can be used to follow the cure conversion of the resin. A predominant peak at wavenumber 2930 cm1, CH3 group stretching, could be used as a standard for comparison, and the Gaussian deconvolution of the peaks permitted an evaluation of the area under the peak. The peak area assigned to the C]C stretching vibration of acrylic at 1634 cm1 decreases with the network formation, as for the epoxy group. The band area of the carbonyl group stretching at 1729 cm1 was used as the internal correction during the curing process and the Gaussian deconvolution of the peaks permitted an evaluation of the peak area, as shown in Fig. 7. Fig. 8 shows the polymerization profile (decrease in the relative intensity of the band) of the epoxy and acrylate monomers, obtained by infrared spectroscopy, displayed for different ultraviolet radiation exposure times. Analysis based on chemical conversion usually requires different extents of reaction for each independent step in a multiplereaction sequence, unless a can be defined for all of the reactions that involve the key component. Because the epoxy and acrylate groups in the resin participate in several chemical reactions, such as the network formation, the fractional conversion (a) can be defined as the total conversion based on all of the possible reactions. The fractional conversion can be described as [6]:
a[
At A0
(1)
where At and A0 are the peak area at time t and at the initial time, respectively, of the infrared spectrum at 2990 cm1
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 3.
701
13
C NMR (400 MHz) liquid resin spectra.
(epoxy) and 1634 cm1 (acrylate). The determination of the kinetics parameters is based on the general nth order rate expression as follows [6]:
resin conversion curve as a function of time, using the integration of Eq. (3) into the reaction order n equal to 1, resulted in straight lines [6]
dðaÞ [ kðaÞn L dt
lnðaÞ [ Lkt
(2)
where k is the kinetics constant and n is the reaction rate at a certain temperature. The plot of the data for the liquid
(3)
for the ultraviolet radiation (see Fig. 9), showing firstorder kinetics. The values of the kinetics constant are 6.8 102 min1 for acrylate groups and 1.8 102 for
Fig. 4. DEPT spectrum of the RenshapeÔ SL 5260 resin.
Fig. 5. Heteronuclear 1H–13C correlation spectrum of the RenshapeÔ SL 5260 resin.
702
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 6. FTIR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min, showing the peak at 2990 cm1, related to epoxy group.
3.3. Thermal degradation of the liquid and cured resin The thermogravimetric curves (TG and DTG) for the liquid resin are shown in Fig. 10. The liquid resin showed a first loss stage within a temperature range of 150–350 C, related to 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 505 C. Parameters such as the maximum degradation temperature, determined considering the derivative curves, and the percentage of mass loss in each stage of degradation were 288 and 430 C, and 50% and 44%, respectively, for the liquid resin. The corresponding solid residue can be attributed to ash and inorganic additives. Fig. 11 presents the thermogravimetric curves (TG and DTG) for the cured resin where only one mass loss stage can be observed. The maximum degradation temperature and the mass loss for the cured resin were 430 C and 94.40%, respectively, indicating higher stability for cured specimens, in comparison with liquid resin.
Fig. 7. FTIR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min, showing the peak at 1634 cm1, related to acrylate group.
In Fig. 12, the activation energy values for liquid and cured resin are plotted as a function of the weight loss fraction (a). The activation energy ranges were 50–100 and 70– 140 kJ mol1, corresponding to the 0.03–0.59 and 0.61–0.92 weight loss fractions, respectively. The cured resin had
-0,5 acrylate epoxy
-1,0 -1,5
ln α
epoxy groups. Thus, the acrylate monomer reacts 3.7 times faster than the epoxy monomer. According to Decker (1999), this reaction via a radicalar (acrylate) polymerization mechanism is about 3 times faster than via a cation (epoxy) mechanism [9].
-2,0 -2,5 -3,0 -3,5 0
5
10
Time (min)
15
20
25
Fig. 8. The cure conversion ln a, as a function of time, by ultraviolet radiation process.
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
260
DTG 0,0
100
240
90 80 60 50 40
-0,4
30
200
Ea (kJmol-1)
-0,2
DTG (mg/min)
TG (%)
Liquid resin Cured resin
220
70
703
180 160 140 120 100 80
20 430°C
10 0 0
100
200
300
60
TG
288°C
40
-0,6 400
500
600
20
Temperature (°C)
0,0
0,2
0,4
0,6
0,8
1,0
Weight loss fraction (α) Fig. 9. TG and DTG curves for liquid resin. Fig. 11. Values of activation energy versus a for the liquid.
activation energy values from ca. 90 to 250 kJ mol1 in the a range 0.10–0.89, according to the curve profile. The activation energy of the decomposition process was determined by the Ozawa method, based on the Arrhenius equation [8,10]. As observed, for the liquid resin there were two decomposition stages, and the activation energy values correspond to different decomposition mechanisms. The profile of the curve reveals that the thermal stability of the liquid resin is lower in the first stage (49.50– 33.41 kJ mol1) and increases to higher values in the second stage (59.40–153.76 kJ mol1). The low value in first stage was attributed to the existence of weak points in the polymer chains, whereas the higher value of the second stage was associated with high degrees of random scission of the main chain. The cured resin had activation energy values in the range of 95.90–242 kJ mol1, corresponding to 0.1–0.9 weight fraction, respectively, indicating greater thermal stability when compared with the liquid resin. 3.4. Infrared analysis of gas decomposition products
to the C]O anhydride group, also observed in DSM SomosÒ 7110 decomposition [6]. As reported in the literature, a cyclical anhydride is a common product found in the acrylate ester decomposition reactions [11,12]. The bands observed at 2927 and 2869 cm1 are related to the C–H stretching. The presence of a band at 1727 cm1 suggests the formation of volatile products related to the C]O, such as alkyl esters and aldehydes. The highest gas evolution occurred from 300 C to 550 C, as can be noted by the band intensities in the FTIR spectrum. The absorption bands at 2358 and 2170 cm1 are related to the products CO2 and CO, respectively, indicating the scission of strong links in the backbone. The absorption band at 3453 cm1 is associated with the OH group, and that at 1459 cm1 is related to the CH2 group of the aliphatic products formed by the scission of the polymeric main chain. The band at 1030 cm1 indicates the presence of the C–O groups. The absorption bands at 815 and 665 cm1 are associated with C]C and C]O, respectively. A new band at 3017 cm1 (symmetric ]CH2) appeared
Fig. 13 shows the FTIR spectra for the evolved gas products of the liquid resin within a temperature range of 100– 550 C. In this spectrum a low quantity of gas products can be observed up to 300 C, assigning the band at 1866 cm1
100°C 150°C
1866
200°C
TG (%)
-0,02
60
-0,04
40
-0,06
DTG(mg/min)
80
2358
240°C
0,00
Absorbance
DTG
100
2927 2869
300°C
TG 0 200
300
400
1459
1030 815 665
400°C 450°C 3735 3017
500°C 550°C
430°C 100
1727
350°C 3453
-0,08
20
2170
500
Temperature (°C) Fig. 10. TG and DTG curves for cured resin.
600
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Fig. 12. Infrared spectrum of the decomposition of the liquid resin.
704
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
212°C 1866
240°C 270°C
Absorbance
300°C
29272869
2358 1727
350°C 380°C
1079
2170 1866 1459
2706
440°C
912 813
3018
500°C
665
References
550°C
4000
3500
with an activation energy between 95.90 and 242 kJ mol1 in the weight fraction of 0.1–0.9, respectively. The liquid resin showed activation energies for two decomposition stages; the first stage in the range of 49–33.41 kJ mol1, and the second stage at 59.40–153.76 kJ mol1. The results of this study indicate that the cured resin has a greater stability and different decomposition mechanism to the liquid resin, observed through the higher weight loss temperature of the cured resin and the single decomposition stage. This result was also observed in previous studies carried out with DSM SomosÒ 7110 resin.
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Fig. 13. Infrared spectrum of the decomposition of the liquid resin.
which indicates that at high degradation temperatures the formation of unsaturated volatile products also occurs. The FTIR spectra for the gas products of the cured resin are shown in Fig. 15 for different decomposition temperatures. The gas products were the same as those evolved in the degradation of the liquid resin. The band at 912 cm1, related to C–C, was not observed in the decomposition of the liquid resin. 4. Conclusions The kinetics studies showed that the value of the kinetics constant is 6.8 102 min1 for acrylate groups and 1.8 102 for epoxy groups. Thus, the acrylate monomer reacts 3.7 times faster of the epoxy monomer. The decomposition studies by thermogravimetric analysis showed that the cured resin has one decomposition stage
[1] P.F. Jacobs, Rapid Prototyping and Manufacturing, Fundamentals of Stereolithography, Society of Manufacturing Engineers, Dearborn, MI, 1992. [2] P.F. Jacobs, Stereolithography & Other RP&M Technologies: From Rapid To Rapid Tooling, Booknews Inc., USA, 1996, April. [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. Technol. 89–90 (1999) 211–217. [4] J.Y.H. Fuh, Y.S. Choo, L. Lu, et al., Post-cure shrinkage of photo-sensitive material used in laser lithography process, J. Mater. Process. Technol 63 (1997) 887–891. [5] G.V. Salmoria, et al., Stereolithography Somos 7110 resin: mechanical behavior and fractography of parts post-cured by different methods, Polymer Test. 24 (2005) 157–162. [6] G.V. Salmoria, et al., Stereolithography Somos 7110 photosensitive resin: study of curing kinetic thermal degradation, J. Process. Mater. Techonol. 168 (2005) 164–171. [7] 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 prototyping. Part I. Mechanical properties in the green state, J. Mater. Process. Technol. 67 (1997) 41–45. [8] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881. [9] C. Decker, High-speed curing by laser irradiation, Nucl. Instrum. Methods Phys. Res. B 151 (1999) 22–28. [10] T. Ozawa, J. Therm. Anal. 7 (1975) 601. [11] M. Coskun, H. Erten, K. Demirelli, Thermal degradation of poly [3-(1cyclohexyl)azetidinyl methacrylate], Polym. Degrad. Stab. 69 (2000) 245–249. [12] M. Coskun, C. Soykan, M. Ahmedzade, K. Demirelli, Preparation and thermal degradation of poly (p-substituted phenacyl methacrylates), Polym. Degrad. Stab. 72 (2001) 69–74.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Characterisation
Investigations on cure kinetics and thermal degradation of stereolithography RenshapeÔ 5260 photosensitive resin G.V. Salmoria a, *, P. Klauss a, A.T.N. Pires b, J. Roeder b, V. Soldi b a b
´polis, SC, Brazil CIMJECT, Mechanical Engineering Department, Federal University of Santa Catarina, UFSC, 88040-900 Floriano ´polis, SC, Brazil POLIMAT, Chemistry Department, Federal University of Santa Catarina, UFSC 88040-900 Floriano
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2008 Accepted 17 May 2008
The stereolithography resin RenshapeÔ 5260 was characterized by infrared and nuclear magnetic resonance spectroscopy, indicating the presence of epoxy and acrylic reactive groups as well as aromatic and aliphatic ether groups. Infrared spectroscopy was also used to evaluate the curing process and identify the components of the thermal degradation of liquid and cured resins. Kinetics studies indicated that this solvent-free polymerization was diffusion-controlled during the chemical crosslinking, due to the reaction order of the curing process. The cured resin had only one degradation stage with activation energy of 90–250 kJ mol1 in the weight loss fraction of 0.10–0.92, whereas a second degradation stage was observed for the liquid resin above the 0.6 weight loss fraction. This second degradation stage gives Ea values in the range of 70–140 kJ mol1. The higher weight loss temperature of the cured resin compared with the liquid resin and the single decomposition stage observed for the former suggest a different decomposition mechanism and stability of the cured resin at higher temperatures compared to the liquid resin. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Photopolymer characterization Cure kinetics Thermal degradation
1. Introduction Designers and engineers use modern technologies to evaluate their product designs, such as rapid prototyping technologies used to build better prototypes in shorter times. Stereolithography (SL) is a rapid prototyping and manufacturing technique used to manufacture high accuracy parts. SL resin is exposed to ultraviolet (UV) laser beam radiation, which induces a localized polymerization process to solidify the liquid resin layer-by-layer, to form manufactured parts directly from 3D computer data [1,2]. Many kinds of photosensitive resins (photopolymers) used in stereolithography are composed of a complex mixture of epoxy and acrylate oligomers and additives. The SL resins have specific properties in terms of viscosity,
* Corresponding author. E-mail address: [email protected] (G.V. Salmoria). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.05.008
curability, elastic modulus, tensile strength, toughness and others, depending on their composition [3–6]. The building parameters, photoinitiator and resin photosensitivity properties and composition are the determining factors in the degree of cure of the objects manufactured by stereolithography. As the optimization of these parameters is difficult, a post-curing process is necessary, which can be carried out by ultraviolet radiation or thermal treatment. This process increases the degree of cure and, consequently, improves the thermal, mechanical and chemical properties of the resin [4,7]. Previous kinetics studies have indicated that DSM SomosÒ 7110 SL resin undergoes solvent-free polymerization, which is diffusion-controlled during the chemical crosslinking due to the reaction order of the cure reaction being very close to unity, allowing rapid photopolymerization [6]. Decomposition analysis has shown that cured resin has a higher weight loss temperature than liquid resin. Also, the single decomposition stage of the former, compared
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
2. Experimental 2.1. Materials and cure procedures The photopolymer RenshapeÔ 5260, used in the stereolithography process, was employed as the study material and used without prior purification. A stereolithography machine, model SLA-250/30 (3D Systems Incorporation, USA), with a He–Cd UV laser (325 nm) was used to produce the specimens. For the cure kinetics experiments, the samples were maintained in an UV chamber with eight fluorescent Philips TLK 40 W/05 lamps, with a radiation range of 300–360 nm. 2.2. Infrared and nuclear magnetic resonance spectroscopy The infrared spectra of the liquid and cured resins were obtained using a 16 PC Perkin Elmer spectrophotometer, performing 20 scans at a resolution of 4 cm1. The sample film of liquid resin was analyzed on a silicon cell. Nuclear magnetic resonance analysis was performed with a Varian 400 MHz spectrometer at room temperature. All spectra were obtained using 30% volume of resin and DCCl3 as the solvent. DEPT and 2D NMR (1H–13C heteronuclear and COSY) spectra were also recorded. 2.3. Cure kinetics and thermal decomposition In order to investigate the cure kinetics, a sample film of resin on the silicon cell was maintained in the UV chamber. FTIR spectra were recorded as a function of time on the Perkin Elmer spectrophotometer. The thermogravimetric analysis was conducted using a Shimadzu TGA-50 thermogravimetric analyzer. Nonisothermal experiments were performed in the temperature range 25–550 C at different heating rates (5, 10 and 20 C min1). The sample weights ranged from 13 to 20 mg and the nitrogen flow-rate was 50 cm3 min1. The thermogravimetric data were analyzed through the Ozawa method to determine the kinetics parameters, using the associated TGA-50 software. The apparent activation energy was derived from the slope of the dependence of the heating rate on the reciprocal of absolute temperature, at a certain mass loss. In the infrared spectral analyses of the gas products, the samples were placed in a ceramic sample holder and heated in a Lindberg/Blue MOD, TF5503A, tubular oven under nitrogen flow (50 cm3 min1). The gas evolution analysis was conducted at a heating rate of 10 C min1, from room temperature up to pre-established temperatures, to obtain the thermogravimetric curves of each system.
3. Results and discussion 3.1. Characterization of the resin Fig. 1 shows the FTIR spectra of the liquid and cure resins in the 4000–500 cm1 region. In the liquid resin spectrum, a broad band at 3503 cm1 can be observed, corresponding to the hydroxyl stretching vibration, and strong bands at 2929 cm1and 2863 cm1 due to the C–H3 and CH2 stretching vibrations, respectively. The band at 1731 cm1 corresponds to the C]O stretching of the ester group, and the band at 1634 cm1 to the C]C present in the acrylate group. The bands at 1265, 909 and 801 cm1 correspond to the –C–O–C– stretching vibration of the ether groups present in epoxy molecules. The bands at 1180 and 1104 cm1 correspond to the C–O group. All of these bands are characteristic of epoxy and acrylate groups. In the cured resin spectrum the band at 1634 cm1, corresponding to the C]C of the acrylate group, showed a reduction due to the reticulation process when the two spectra were compared. The 1H NMR liquid resin spectrum is shown in Fig. 2. Chemical shifts related to TMS are reported in Table 1. The region of 1.0–2.7 ppm is attributed to the alkyl hydrogen of the CH3, CH2 and CH groups. The shifts at 2.8 and 3.0 ppm are related to the –OCH2 and –OCH protons of the epoxy group, respectively. The spectrum shows protons present at 5.7, 5.9 and 6.3, characteristic of the terminal vinyl group. The protons related to the aromatics can be observed at 6.6 and 7.0 ppm in low quantity when compared with DSM SomosÒ 7110 [6], characteristic of the p-substituted aromatic groups, which confer better mechanical and thermal properties to the cured resin. The other chemical shifts correspond to the signals of the epoxy and acrylate group in aliphatic structures. The 13C NMR liquid resin spectrum in Fig. 3 shows the CH2 signals for different groups, as reported in Table 1. The regions at 17 and 20–30 ppm relate to CH3 and CH2 aliphatic carbons and that at 30–40 ppm to C–H carbons. The chemical shifts present at 43 and 50 ppm relate to carbons of the epoxy groups –OCH2 and –OCH, respectively.
Líquid
Transmittance
with the two decomposition stages of the latter, suggests a change in the decomposition mechanism due to modifications in the chemical composition induced by the cure [6]. This study investigated the chemical group composition, cure process and thermal degradation of liquid and cured RenShapeÔ 5260 resin used in the stereolithography process, using infrared and nuclear magnetic resonance spectroscopy, and thermal and gas evolution analyses.
699
Cured
4000
3000
2000
1000
Wavenumber (cm-1) Fig. 1. Infrared spectra for the liquid and cured resin.
700
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 2. 1H NMR (400 MHz) liquid resin spectra.
The signals at 49 and 51 ppm and at 60–72 ppm relate to –OCH2 and –OCH groups in aliphatic structures. The presence of signals at 110 and 130 ppm and 113 and 122 ppm can be attributed to olefin and aromatic carbons, respectively. The carbonyl group signals were within the 165– 204 ppm range relating to the ester group. The liquid resin DEPT spectrum is shown in Fig. 4, where the signals recorded above the baseline correspond to CH3 and CH carbons, and below the baseline to CH2 carbons. The assignment of the carbon signals to a particular type of carbon can be established using this method. By correctly choosing the experimental parameters it is possible to obtain information on the methyne, methylene and methyl carbons. Fig. 5 shows the 2D NMR heteronuclear liquid resin spectrum that reinforces the attribution given in Table 1. In the experiments, each resonance frequency signal is related to those of its neighbors and, thus, the molecular connectivity can be determined. The IR and NMR spectroscopic analyses showed the presence of epoxy and acrylic functional groups in the RenshapeÔ 5260 liquid resin, as well as aromatic and aliphatic ether groups. 3.2. Cure kinetics of the liquid resin The mechanical properties of parts manufactured for stereolithography depend on the degree of cure of the Table 1 1 H and 13C NMR signals related to different molecular groups 1
H (ppm)
1 1.2–2.2 2.3–2.7 2.8 3.0 2.9–3.1 3.2–4.2 6.6–7.0 5.7–6.3 5.9
Group
13
CH3 CH2 CH OCH2 (epoxy) OCH (epoxy) OCH2 OCH CH (phenyl) ]CH ]CH2 C]O
17 20–30 30–40 43 50 49–51 60–72 110 and 126 127 131 160 and 170
C
photosensitive resin. Knowledge of the relationship between resin structure and properties is useful in the SL tool manufacturing process, application and quality control (5, 6 and 8). Fig. 6 shows the IR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min. The difference in the peaks at wavenumber 2990 cm1, which represent the epoxy group, of the two spectra is evident. The peak area assigned to the epoxy groups at 2990 cm1 decreased with the network formation via the chemical crosslinking of resin components and can be used to follow the cure conversion of the resin. A predominant peak at wavenumber 2930 cm1, CH3 group stretching, could be used as a standard for comparison, and the Gaussian deconvolution of the peaks permitted an evaluation of the area under the peak. The peak area assigned to the C]C stretching vibration of acrylic at 1634 cm1 decreases with the network formation, as for the epoxy group. The band area of the carbonyl group stretching at 1729 cm1 was used as the internal correction during the curing process and the Gaussian deconvolution of the peaks permitted an evaluation of the peak area, as shown in Fig. 7. Fig. 8 shows the polymerization profile (decrease in the relative intensity of the band) of the epoxy and acrylate monomers, obtained by infrared spectroscopy, displayed for different ultraviolet radiation exposure times. Analysis based on chemical conversion usually requires different extents of reaction for each independent step in a multiplereaction sequence, unless a can be defined for all of the reactions that involve the key component. Because the epoxy and acrylate groups in the resin participate in several chemical reactions, such as the network formation, the fractional conversion (a) can be defined as the total conversion based on all of the possible reactions. The fractional conversion can be described as [6]:
a[
At A0
(1)
where At and A0 are the peak area at time t and at the initial time, respectively, of the infrared spectrum at 2990 cm1
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 3.
701
13
C NMR (400 MHz) liquid resin spectra.
(epoxy) and 1634 cm1 (acrylate). The determination of the kinetics parameters is based on the general nth order rate expression as follows [6]:
resin conversion curve as a function of time, using the integration of Eq. (3) into the reaction order n equal to 1, resulted in straight lines [6]
dðaÞ [ kðaÞn L dt
lnðaÞ [ Lkt
(2)
where k is the kinetics constant and n is the reaction rate at a certain temperature. The plot of the data for the liquid
(3)
for the ultraviolet radiation (see Fig. 9), showing firstorder kinetics. The values of the kinetics constant are 6.8 102 min1 for acrylate groups and 1.8 102 for
Fig. 4. DEPT spectrum of the RenshapeÔ SL 5260 resin.
Fig. 5. Heteronuclear 1H–13C correlation spectrum of the RenshapeÔ SL 5260 resin.
702
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
Fig. 6. FTIR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min, showing the peak at 2990 cm1, related to epoxy group.
3.3. Thermal degradation of the liquid and cured resin The thermogravimetric curves (TG and DTG) for the liquid resin are shown in Fig. 10. The liquid resin showed a first loss stage within a temperature range of 150–350 C, related to 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 505 C. Parameters such as the maximum degradation temperature, determined considering the derivative curves, and the percentage of mass loss in each stage of degradation were 288 and 430 C, and 50% and 44%, respectively, for the liquid resin. The corresponding solid residue can be attributed to ash and inorganic additives. Fig. 11 presents the thermogravimetric curves (TG and DTG) for the cured resin where only one mass loss stage can be observed. The maximum degradation temperature and the mass loss for the cured resin were 430 C and 94.40%, respectively, indicating higher stability for cured specimens, in comparison with liquid resin.
Fig. 7. FTIR spectra of the resin exposed to UV light for 0 (liquid) and 21 (cured) min, showing the peak at 1634 cm1, related to acrylate group.
In Fig. 12, the activation energy values for liquid and cured resin are plotted as a function of the weight loss fraction (a). The activation energy ranges were 50–100 and 70– 140 kJ mol1, corresponding to the 0.03–0.59 and 0.61–0.92 weight loss fractions, respectively. The cured resin had
-0,5 acrylate epoxy
-1,0 -1,5
ln α
epoxy groups. Thus, the acrylate monomer reacts 3.7 times faster than the epoxy monomer. According to Decker (1999), this reaction via a radicalar (acrylate) polymerization mechanism is about 3 times faster than via a cation (epoxy) mechanism [9].
-2,0 -2,5 -3,0 -3,5 0
5
10
Time (min)
15
20
25
Fig. 8. The cure conversion ln a, as a function of time, by ultraviolet radiation process.
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
260
DTG 0,0
100
240
90 80 60 50 40
-0,4
30
200
Ea (kJmol-1)
-0,2
DTG (mg/min)
TG (%)
Liquid resin Cured resin
220
70
703
180 160 140 120 100 80
20 430°C
10 0 0
100
200
300
60
TG
288°C
40
-0,6 400
500
600
20
Temperature (°C)
0,0
0,2
0,4
0,6
0,8
1,0
Weight loss fraction (α) Fig. 9. TG and DTG curves for liquid resin. Fig. 11. Values of activation energy versus a for the liquid.
activation energy values from ca. 90 to 250 kJ mol1 in the a range 0.10–0.89, according to the curve profile. The activation energy of the decomposition process was determined by the Ozawa method, based on the Arrhenius equation [8,10]. As observed, for the liquid resin there were two decomposition stages, and the activation energy values correspond to different decomposition mechanisms. The profile of the curve reveals that the thermal stability of the liquid resin is lower in the first stage (49.50– 33.41 kJ mol1) and increases to higher values in the second stage (59.40–153.76 kJ mol1). The low value in first stage was attributed to the existence of weak points in the polymer chains, whereas the higher value of the second stage was associated with high degrees of random scission of the main chain. The cured resin had activation energy values in the range of 95.90–242 kJ mol1, corresponding to 0.1–0.9 weight fraction, respectively, indicating greater thermal stability when compared with the liquid resin. 3.4. Infrared analysis of gas decomposition products
to the C]O anhydride group, also observed in DSM SomosÒ 7110 decomposition [6]. As reported in the literature, a cyclical anhydride is a common product found in the acrylate ester decomposition reactions [11,12]. The bands observed at 2927 and 2869 cm1 are related to the C–H stretching. The presence of a band at 1727 cm1 suggests the formation of volatile products related to the C]O, such as alkyl esters and aldehydes. The highest gas evolution occurred from 300 C to 550 C, as can be noted by the band intensities in the FTIR spectrum. The absorption bands at 2358 and 2170 cm1 are related to the products CO2 and CO, respectively, indicating the scission of strong links in the backbone. The absorption band at 3453 cm1 is associated with the OH group, and that at 1459 cm1 is related to the CH2 group of the aliphatic products formed by the scission of the polymeric main chain. The band at 1030 cm1 indicates the presence of the C–O groups. The absorption bands at 815 and 665 cm1 are associated with C]C and C]O, respectively. A new band at 3017 cm1 (symmetric ]CH2) appeared
Fig. 13 shows the FTIR spectra for the evolved gas products of the liquid resin within a temperature range of 100– 550 C. In this spectrum a low quantity of gas products can be observed up to 300 C, assigning the band at 1866 cm1
100°C 150°C
1866
200°C
TG (%)
-0,02
60
-0,04
40
-0,06
DTG(mg/min)
80
2358
240°C
0,00
Absorbance
DTG
100
2927 2869
300°C
TG 0 200
300
400
1459
1030 815 665
400°C 450°C 3735 3017
500°C 550°C
430°C 100
1727
350°C 3453
-0,08
20
2170
500
Temperature (°C) Fig. 10. TG and DTG curves for cured resin.
600
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Fig. 12. Infrared spectrum of the decomposition of the liquid resin.
704
G.V. Salmoria et al. / Polymer Testing 27 (2008) 698–704
212°C 1866
240°C 270°C
Absorbance
300°C
29272869
2358 1727
350°C 380°C
1079
2170 1866 1459
2706
440°C
912 813
3018
500°C
665
References
550°C
4000
3500
with an activation energy between 95.90 and 242 kJ mol1 in the weight fraction of 0.1–0.9, respectively. The liquid resin showed activation energies for two decomposition stages; the first stage in the range of 49–33.41 kJ mol1, and the second stage at 59.40–153.76 kJ mol1. The results of this study indicate that the cured resin has a greater stability and different decomposition mechanism to the liquid resin, observed through the higher weight loss temperature of the cured resin and the single decomposition stage. This result was also observed in previous studies carried out with DSM SomosÒ 7110 resin.
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Fig. 13. Infrared spectrum of the decomposition of the liquid resin.
which indicates that at high degradation temperatures the formation of unsaturated volatile products also occurs. The FTIR spectra for the gas products of the cured resin are shown in Fig. 15 for different decomposition temperatures. The gas products were the same as those evolved in the degradation of the liquid resin. The band at 912 cm1, related to C–C, was not observed in the decomposition of the liquid resin. 4. Conclusions The kinetics studies showed that the value of the kinetics constant is 6.8 102 min1 for acrylate groups and 1.8 102 for epoxy groups. Thus, the acrylate monomer reacts 3.7 times faster of the epoxy monomer. The decomposition studies by thermogravimetric analysis showed that the cured resin has one decomposition stage
[1] P.F. Jacobs, Rapid Prototyping and Manufacturing, Fundamentals of Stereolithography, Society of Manufacturing Engineers, Dearborn, MI, 1992. [2] P.F. Jacobs, Stereolithography & Other RP&M Technologies: From Rapid To Rapid Tooling, Booknews Inc., USA, 1996, April. [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. Technol. 89–90 (1999) 211–217. [4] J.Y.H. Fuh, Y.S. Choo, L. Lu, et al., Post-cure shrinkage of photo-sensitive material used in laser lithography process, J. Mater. Process. Technol 63 (1997) 887–891. [5] G.V. Salmoria, et al., Stereolithography Somos 7110 resin: mechanical behavior and fractography of parts post-cured by different methods, Polymer Test. 24 (2005) 157–162. [6] G.V. Salmoria, et al., Stereolithography Somos 7110 photosensitive resin: study of curing kinetic thermal degradation, J. Process. Mater. Techonol. 168 (2005) 164–171. [7] 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 prototyping. Part I. Mechanical properties in the green state, J. Mater. Process. Technol. 67 (1997) 41–45. [8] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881. [9] C. Decker, High-speed curing by laser irradiation, Nucl. Instrum. Methods Phys. Res. B 151 (1999) 22–28. [10] T. Ozawa, J. Therm. Anal. 7 (1975) 601. [11] M. Coskun, H. Erten, K. Demirelli, Thermal degradation of poly [3-(1cyclohexyl)azetidinyl methacrylate], Polym. Degrad. Stab. 69 (2000) 245–249. [12] M. Coskun, C. Soykan, M. Ahmedzade, K. Demirelli, Preparation and thermal degradation of poly (p-substituted phenacyl methacrylates), Polym. Degrad. Stab. 72 (2001) 69–74.