Article pubs.acs.org/IECR
Characteristics of Sonar Encapsulant Materials Fabricated from Electron-Beam-Irradiated Polyurethane Elastomers Coated with UVCurable Polyurethane Acrylates Jisun Kim, Hyungu Im, and Chang Keun Kim* School of Chemical Engineering & Materials Science, Chung-Ang University, 221 Huksuk-dong, Dongjak-gu, Seoul, 156-756, Korea ABSTRACT: The use of thermoplastic polyurethane (TPU) elastomers as encapsulant materials for undersea sonar devices is frequently limited because they do not satisfy the stringent requirements for undersea use. To develop desirable polymeric materials having lower swelling ratios in paraffin oil and seawater than TPU, as well as better mechanical strength, we produced encapsulant materials composed of cross-linked TPU and a coating layer prepared from polyurethane acrylate (PUA) oligomers cured using ultraviolet (UV) irradiation. The electron-beam irradiation of the TPU was adapted to fabricate cross-linked TPU. The encapsulant materials developed in this study exhibited a higher tensile strength and lower swelling ratio than standard TPUs. Reduction in the swelling ratio of the former reached up to 95% of the latter after being impregnated with seawater. Changes in the tensile strength of the former with seawater impregnation were negligible.
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INTRODUCTION Thermoplastic polyurethane (TPU) elastomers composed of methyl diphenyl diisocyanate (MDI) as hard segments and poly(tetramethylene glycol) (PTMG) as soft segments are frequently used as encapsulant materials for undersea sonar devices.1−7 Since sonar devices are dragged behind submarines or surface ships and the encapsulant is filled with a paraffin oil to protect against the ingress of seawater, encapsulant materials should have low swelling ratios in both seawater and paraffin oil without significant deterioration of their properties. However, TPU elastomers prepared from MDI and PTMG do not satisfy the stringent requirements for materials to be used as sonar encapsulants, as they exhibit swelling in seawater and paraffin oil and degraded mechanical properties when immersed in these liquids over long periods.8,9 To develop materials that can overcome the drawbacks of standard TPU elastomers, TPU blends with poly(acrylonitrile-co-butadiene) elastomers (NBR) and TPU composites with hollow glass microspheres (HGM) have been studied in our laboratory.8,9 TPU/NBR blends and TPU/HGM composites exhibited better mechanical strength and lower swelling ratios in seawater and paraffin oil than did standard TPUs. The new materials showed enhanced mechanical strength as compared to TPU blends and composites after being impregnated with seawater and paraffin oil, as required. The amount of seawater and paraffin oil impregnated in a TPU could be reduced by changing the linear chain structure to a network structure with cross-linking reactions. It is known that the solubility of a low molecular weight gas or liquid in a polymer matrix can be reduced by decreasing the free volume of the polymer matrix and by hindering chain motion.10−13 Since cross-linked TPU might have less free volume and exhibit more hindered chain mobility than pristine TPU, the use of TPU with a network structure as an encapsualant material is a promising approach in protecting against the loss of mechanical strength caused by impregnation. In addition, the coating of © 2013 American Chemical Society
TPU with materials having a high barrier characteristic against impregnation is another promising strategy.14 In this study, cross-linked TPUs, which are coated with polymeric materials having a high barrier property, were prepared as encapsulant materials for sonar devices. Electron beam (e-beam) irradiation was performed to fabricate crosslinked TPU, and the cross-linked TPU was then coated with polymers formed from polyurethane acrylate oligomers using ultraviolet (UV) irradiation.15−22 The mechanical and barrier properties of these materials were explored by comparing them with those of pristine TPU.
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MATERIALS AND PROCEDURES Materials. Commercially available thermoplastic polyurethane (TPU) elastomer (grade Skythane R-185A) was supplied by SK Chemicals (Seoul, Korea). This TPU contains 2 mol of 4,4′-diphenylmethane diisocyanate (MDI) as a hard segment, 1 mol of poly(tetramethylene glycol) (PTMG, Mw = 1000 g/mol) as a soft segment, and 1 mol of 1,4-butanediol as a chain extender. The number average molecular weight and weight average molecular weight of the TPU determined by gel permeation chromatography (GPC) using polystyrene standards were 175 000 and 337 000 g/mol, respectively. UV-curable polyurethane acrylate (PUA) was adapted as a coating material. Two kinds of UV-curable PUAs (PUA-1 and PUA-2) synthesized specially for this study were supplied by Miwon Specialty Chemicals (Seoul, Korea). According to the supplier, the PU oligomer in the PUAs consisted of isophorone diisocyanate (IPDI) as a hard segment and polytetramethylene glycol (PTMG) as a soft segment. The isocyanate end groups of the PU oligomer were reacted with 2-hydroxyethyl acrylate to provide two acrylate functional groups at the ends of PUA-1, Received: Revised: Accepted: Published: 1908
November 21, 2012 December 30, 2012 January 9, 2013 January 9, 2013 dx.doi.org/10.1021/ie303208p | Ind. Eng. Chem. Res. 2013, 52, 1908−1915
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constant frequency of 1 Hz. The specimens were cooled under liquid nitrogen and then heated from −100 to 150 °C at a rate of 5 °C/min under nitrogen. Specimens for tensile testing were prepared in accordance with American Standards Testing Method (ASTM) specification D412. Tensile tests were performed using a universal testing machine (UTM, model UTM-301, R&B Corp., Daejon, Korea) at a cross head speed of 500 mm/min. The tensile property values reported represent the average of five tests. The swelling ratios of the TPUs were examined in seawater and paraffin oil. The specimens were cut into small pieces (length × width × thickness = 1 cm × 3 cm × 0.3 cm) and immersed into a seawater bath (or a paraffin oil bath) at 30 °C. Changes in the weights of the specimens were measured as a function of impregnation time. Each experiment was performed at least five times, and the reported data are the means of the obtained values. The swelling ratio (SR) was calculated using the equation
and those were reacted with pentaerytritol triacrylate to provide six acrylate functional groups at the ends of PUA-2. The characteristics of the PUAs and TPUs are listed in Table 1. 1Table 1. Materials Used in This Study abbrev
no. of reactive end groups
molec wt (g/mol)
1.05 1.19
polyurethane
TPU
polyurethane acrylate-1 polyurethane acrylate-2
PUA-1
2
M̅ n = 175 000 M̅ w = 337 000 5100
PUA-2
6
1900
density (g/cm3) 1.10
Hydroxycyclohexyl phenyl ketone (Ciba Specialty Chemicals Co.) was used as a photoinitiator. Paraffin oil used for the swelling test was supplied by SK Energy (grade YU-8, average molecular weight = 500 g/mol, Seoul, Korea). According to the supplier, this oil is a long-chain normal paraffin that is bonded with about seven methyl side groups. Synthetic seawater for the swelling test was prepared in accordance with ASTM specification D1141. Procedures. Specimens for tensile strength, swelling, and ebeam radiation tests were prepared by compression molding. The pelletized TPU was poured into a mold (length × width × thickness = 5 cm × 5 cm × 0.3 cm) and placed in a compression molding machine operating at a plate temperature of 180 °C and a holding pressure of 12 MPa. The TPU was kept at 180 °C for 5 min and then cooled to room temperature for 1 h by natural convection. After being molded, the specimens were placed in an oven at 80 °C for a minimum of 24 h prior to use for experiments. Electron beam irradiation (2.5 MeV) on the TPU sheet was performed by Ebtech Co. (Deajon, Korea) using lab-made e-beam equipment connected to a van de Graaf electron accelerator (2.5 MeV). The absorbed doses varied from 50 to 300 KGy at a dose rate of 5 MGy/h. Electron beams were used to irradiate the TPU sheet under oxygen atmosphere at a controlled temperature of 30 °C. The e-beam-irradiated TPUs were coated with PUA-1, PUA-2, or a mixture of the two at a thickness of 100 μm. Note that the coated PUAs contained 0.1 wt % of 1-hydroxycyclohexyl phenyl ketone as a photoinitiator. The TPUs coated with PUAs were polymerized by irradiating UV light from a light source (intensity 600 mW/cm2) under nitrogen-purged conditions for 2 min. The molecular structures of the e-beam-irradiated TPUs (eTPUs) and PUA were confirmed by FT-IR (Magna 750, Nicolet) and 1H NMR (Varian VNS, 600 MHz) analyses. The FT-IR spectra were collected over 30 scans in the 4000−500 cm−1 region through measuring the transmittance at a resolution of 4 cm−1. 1H NMR was registered with chloroform-d (CDCl3, 100.0 atom % D, Aldrich Chemical Co.) as the solvent and tetramethylsilane (TMS, 99.9+%, NMR grade, Aldrich Chemical Co.) as the internal standard. The molecular weights of the e-beam-irradiated TPUs were determined by gel permeation chromatography (GPC, Waters 515-2410, Waters) using polystyrene standards and tetrahydrofuran (THF) as a mobile phase. The cross-sectional morphology of the PUAcoated TPUs were observed using a scanning electron microscope (FE-SEM, model Sigma, Carl Zeiss). The glass transition temperatures (Tg) and the storage moduli of the cured PUAs were measured by dynamic mechanical analysis (DMA, SS6100, Seico Instruments) at a
SR =
Wt − Wo × 100 Wo
(1)
where Wo and Wt are the weights of the specimen before impregnation and after impregnation for time t, respectively.
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RESULTS AND DISCUSSION Electron-Beam-Irradiated TPU. The standard TPUs used here exhibited high swelling in seawater and paraffin oil and degraded mechanical properties over long periods of immersion in these liquids. To develop a new material that can address these drawbacks, molecular structure modification of the TPUs was performed by irradiating them with an e-beam. The debonding and bonding reactions that occurred were investigated with XPS and GPC. Figure 1 shows XPS wide scans of the pristine TPU and the TPU with a 300 KGy absorbed dose (e-TPU-300, Figure 1a), the fits of the C1s of the pristine TPU (Figure 1b) and e-TPU-300 (Figure 1c), the fits of the O1s of the pristine TPU (Figure 1d) and e-TPU-300 (Figure 1e), and the fits of the N1s of the pristine TPU (Figure 1f) and e-TPU-300 (Figure 1g). As shown in Figure 1a, C1s, N1s, and O1s peaks were observed in the spectra of both the pristine TPU and the e-TPU-300. The relative contents of C, O, and N in TPU changed with the absorbed e-beam doses. Figure 2 shows the carbon and oxygen contents of the pristine TPU and the e-beam-treated TPUs as a function of absorbed ebeam dose. The carbon content decreased with increasing dose, while the oxygen content increased with increasing dose. The change in nitrogen content was similar to that of the carbon content, meaning that oxidation reactions occurred during ebeam radiation. As shown in parts d and e of Figure 1, the O1s peaks of the TPUs could be deconvoluted into two binding energies at 531.6 (ether, −C−O−C−) and 533.5 eV (carbonyl, −CO). The content of ether groups increased with increasing absorbed dose, while that of the carbonyl groups slightly decreased with increasing absorbed dose. This result indicates that the oxygen supplied during e-beam radiation was mainly engaged in forming ether linkages in the TPU. The N1s peak of the pristine TPU could be deconvoluted into two binding energies at 399.6 (urethane linkage, −NHCO) and 401.4 eV (cyanate, −NCO), while that of e-TPU-300 exhibited a single binding energy for a urethane linkage. Note that a deconvoluted N1s peak corresponding to cyanate was observed when the absorbed doses were less than or equal to 150 KGy. This means that the cyanate groups that remained 1909
dx.doi.org/10.1021/ie303208p | Ind. Eng. Chem. Res. 2013, 52, 1908−1915
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Figure 1. XPS scan spectra of pristine TPU and e-TPU-300: (a) wide scan spectra of pristine TPU and e-TPU-300, (b) C1s fitting curve for pristine TPU, (c) C1s fitting curve for e-TPU-300, (d) O1s fitting curve for pristine TPU, (e) O1s fitting curve for e-TPU-300, (f) N1s fitting curve for pristine TPU, and (g) N1s fitting curve for e-TPU-300.
unreacted during TPU synthesis reacted during e-beam radiation to form urethane linkages.
Changes in the molecular weights of TPUs were examined with GPC in order to confirm the structure modification caused 1910
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linking reactions induced the formation of an insoluble fraction, while the chain scission reactions resulted in a decrease in the molecular weight of the TPU. The structural modification of the TPU caused by e-beam irradiation might lead to changes in mechanical properties due to cross-linking and chain cleavage. Figure 4 illustrates the
Figure 2. Changes in the carbon and oxygen content of the e-TPUs as a function of absorbed e-beam dose.
by e-beam irradiation. An insoluble fraction was observed when dissolving irradiated specimens in THF. Figure 3 shows
Figure 4. Changes in the tensile strengths and elongation at break of the e-TPUs as a function of absorbed e-beam dose.
tensile strengths of pristine TPU and e-TPUs calculated from the stress−strain curve at specific strains. Note that the tensile test for e-TPU-300 was not performed because e-TPU-300 specimens contained bubbles caused by e-beam irradiation. The tensile strength of the TPU at a specific strain increased with increasing absorbed e-beam dose. This result might stem from an increase in the cross-linking density. The elongation at break decreased with increasing e-beam dose when the absorbed ebeam dose was greater than 50 kGy. Encapsulant materials must have low swelling ratios in seawater and paraffin oil in order to protect against the degradation of mechanical properties. Figure 5 shows the swelling ratios of TPUs in seawater and paraffin oil. The swelling ratio increased with increasing impregnation time before approaching a constant value. The swelling ratios of the pristine TPU and the e-TPUs increased with increasing impregnation time over 6−7 days, reaching equilibrium in both liquids after about 1 week of impregnation. It was assumed that the swelling ratio reached equilibrium when it did not change further with impregnation time. The equilibrium swelling ratios of the e-TPUs were smaller than those of pristine TPU. e-TPU-100 exhibited the smallest swelling ratios in both mediums. The swelling ratios of e-TPU-100 in paraffin oil and seawater were about 58% and 65% smaller than those of the pristine TPU. An increase in the insoluble portion caused by the cross-linking reaction as the irradiation dose increased may help reduce the swelling ratio, while a decrease in the molecular weight of the soluble portion of the TPU in THF with increasing irradiation dose might be unfavorable in reducing the swelling ratio. Figure 6 shows the changes in the densities of the e-TPUs as a function of irradiation dose. eTPU-100, which has the highest density among the e-TPUs, might contain the lowest free volume. Because of this, e-TPU-
Figure 3. Changes in the molecular weights and insoluble fraction of the e-TPUs as a function of absorbed e-beam dose.
changes in the molecular weights and insoluble fraction of the TPUs as a function of absorbed e-beam dose. The insoluble fraction of the e-beam-irradiated TPU increased as the absorbed e-beam dose increased and TPU-e-300 did not dissolve in THF. When a 50 KGy e-beam dose was applied to the TPU (e-TPU-50), e-TPU-50 totally dissolved in THF, and its molecular weight was higher than that of pristine TPU. However, analysis of the soluble fractions showed a decrease in molecular weight above 50 kGy. These results indicate that the e-beam irradiation induced simultaneous chain scission and cross-linking reactions. It is known that soft segment scissions occur preferentially on methylene in the α-position of the PTMG ether group by the primary formation of hydroperoxide.15,16 The alkoxy radicals formed by the release of hydroxyl radicals recombined with TPU chains to initiate chain scission and cross-linking reactions. As a result, the TPU cross1911
dx.doi.org/10.1021/ie303208p | Ind. Eng. Chem. Res. 2013, 52, 1908−1915
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100 exhibited the least swelling when impregnated in both liquids. Figure 7 shows the changes in the tensile strengths of
Figure 5. Changes in the swelling ratios of the pristine TPU and eTPUs as a function of impregnation time in (a) seawater and (b) paraffin oil.
Figure 7. Changes in the tensile strengths of the pristine TPU and eTPU-100 at 100% elongation as a function of impregnation time in (a) seawater and (b) paraffin oil.
TPU and e-TPU-100 as a function of impregnation time in paraffin oil and seawater. The tensile strength of the TPUs decreased with increasing impregnation time for 1 week, after which additional changes were negligible. The tensile strength reduction ratios of e-TPU-100 were smaller than those of the pristine TPU after impregnation with both liquids. Since e-TPU maintained a higher tensile strength than the pristine TPU after impregnation with both liquids, e-TPU is a better choice for use in sonar encapsulants. PUA Coating on e-Beam Radiation of TPU. Materials with lower swelling ratios in seawater and paraffin oil than eTPUs might be desirable for sonar encapsulants. Electronbeam-irradiated TPUs were coated with polymers fabricated from PUAs polymerized by UV curing reactions to produce sonar encapsulants exhibiting better swelling behavior and mechanical strength than e-TPUs. Figure 8 shows 1H NMR spectra of PUA-1 and PUA-2. All peaks in the 1H NMR spectra could be assigned from information related to the raw materials used for synthesis. The resonance peaks between δ = 0.97 and
Figure 6. Changes in the densities of the e-TPUs as a function of absorbed e-beam dose.
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dx.doi.org/10.1021/ie303208p | Ind. Eng. Chem. Res. 2013, 52, 1908−1915
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Figure 8. 1H NMR spectra and molecular structures of the PUAs: (a) PUA-1 and (b) PUA-2.
Figure 9. FT-IR spectra of (a) PUA-1, (b) the polymer formed from PUA-1 by UV irradiation for 30 s, and (c) the polymer formed from PUA-1 by UV irradiation for 2 min.
3.0 ppm in Figure 8a stem from methyl groups and methylene groups in the cyclic ring of IPDI and the ethylene of PTMG (−CH2CH2−), respectively. The peaks at 3.31 and 3.45 ppm are due to the protons in the methylene oxide groups in PTMG (−CH2O−). The peaks at δ = 5.69, 6.1, and 6.4 ppm were assigned to the terminal vinyl groups (CH2CH−) in acrylates. The molecular structures of the PUAs were analyzed by comparing the vinyl groups in the acrylates with the cyclic rings in IPDI. The expected molecular structures of the PUAs obtained from the 1H NMR spectra and molecular weight information are presented in Figure 8. Note that PUA-1 contains two terminal acrylates while PUA-2 contains six terminal acrylates.
Figure 9 shows FT-IR spectra of PUA-1 (Figure 9a), PUA-1 after 30 s UV curing (Figure 9b), and PUA-1 after 2 min of UV curing (Figure 9c). As shown in Figure 9a, the FT-IR spectrum of PUA-1 exhibits a stretching peak at 1635 cm −1 corresponding to the CC groups in the acrylates. This peak, which is still present in the FT-IR spectrum of PUA-1 after 30 s of UV curing (Figure 9b), was not observed in the FT-IR spectrum of PUA-1 after 2 min of UV curing (Figure 9c). This indicates that the UV curing reactions of the PUAs were complete after 2 min of UV irradiation. PUA-2 exhibited similar curing behavior to PUA-1. On the basis of this, the UV irradiation time of the PUA curing reactions was fixed at 2 min. Figure 10 shows the storage moduli of UV-cured PUA-1, PUA1913
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Figure 10. Changes in the storage moduli of polymers formed from PUA-1, PUA-2, and their mixture as a function of temperature.
2, and their mixture as a function of temperature. Cured PUA-1 exhibited a glass transition at about 33 °C (onset temperature of the glass transition). However, cured PUA-2 did not exhibit a glass transition because the cross-linking density of PUA-2 is higher than that of PUA-1 and the molecular weight of PUA-2 is lower than that of PUA-1, as shown in Table 1. The swelling behavior of e-TPU-100 coated with UV-cured PUAs (e-TPU-100/PUAs) in seawater and paraffin oil was similar with that of TPE or e-TPU. Note that the thickness of the coating layer was fixed at about 100 μm, as shown in Figure 11a. The swelling ratio increased with increasing impregnation time before approaching a fixed value. The equilibrium swelling ratios of e-TPU-100/PUAs were nearly the same, regardless of the type of coating material, as shown in Figure 11b. The swelling ratios of the e-TPU-100/PUAs in both liquids were lower than those of e-TPU-100. As a result, the swelling ratios of the e-TPU-100/PUAs in seawater and paraffin oil were reduced by about 95% and 82%, respectively, as compared to pristine TPU. The tensile strengths of e-TPU-100 coated with UV-cured PUA-1 (e-TPU-100/PUA-1) were examined as a function of impregnation time in seawater. The tensile strength of e-TPU-100/PUA-1 before impregnation with paraffin oil or seawater was nearly the same as that of e-TPU-100. Changes in the tensile strength of e-TPU-100/PUA-1 with seawater impregnation time were not observed, while about a 10% reduction in tensile strength was observed after impregnation with paraffin oil. In summary, encapsulant materials with enhanced tensile strengths that can be maintained after seawater and paraffin oil impregnation can be fabricated by treating pristine TPU with e-beam irradiation, followed by coating with polymers formed from PUA using UV irradiation.
Figure 11. (a) Cross sectional morphology of e-TPU-100 coated with a polymer formed from PUA-1 (e-TPU-100/PUA-1) and cured by UV irradiation and (b) equilibrium swelling ratios of various TPUs after impregnation with seawater and paraffin oil.
due to UV irradiation were confirmed by XPS, GPC, 1H NMR, and FT-IR. The e-beam irradiation on the TPU induced simultaneous chain scission and cross-linking reactions. The tensile strength of the e-TPU at a specific strain increased with increasing irradiation dose, while the swelling ratio of the eTPU reached a minimum when the density of the e-TPU (eTPU-100) reached a maximum dose of 100 KGy. The tensile strength of pristine TPU and e-TPU decreased with increasing impregnation time and then approached a constant value. The tensile strength reduction ratio of the e-TPU after impregnation with paraffin oil or seawater was smaller than that of pristine TPU. The e-TPU-100 coated with PUAs (e-TPU-100/PUAs) exhibited a lower swelling ratio in paraffin oil and seawater than e-TPU-100. The swelling ratio of e-TPU-100/PUA-1 in seawater was reduced by about 95% as compared to pristine TPU. As a result, the tensile strength of e-TPU-100/PUA-1 did not change with impregnation time in seawater. In conclusion, encapsulant materials for underwater applications with lower swelling ratios in paraffin oil and seawater and better tensile strength than pristine TPU can be produced by treating pristine TPU with e-beam irradiation followed by coating with a polymer formed from PUA using UV irradiation.
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SUMMARY To develop thermoplastic elastomers satisfying the stringent requirements for use as encapsulant materials in underwater sonar devices, encapsulant materials composed of an e-beamirradiated TPU and a coating layer prepared from PUAs cured by UV irradiation were produced. Changes in the molecular structures of TPU due to e-beam irradiation and in the PUAs
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 1914
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crosslinked polyurethane−acrylate composite. J. Appl. Polym. Sci. 2012, 124, 958. (22) Han, W.; Lin, B.; Yang, H.; Zhang, X. Synthesis and properties of UV-curable hyperbranched polyurethane acrylate oligomers containing carboxyl groups. Polym. Bull. 2012, 68, 1009.
ACKNOWLEDGMENTS This work was supported by an Agency for Defense Development grant funded by the Korean government (ADD-09-01-07-21).
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