Property modification of a silicone acrylic pressure-sensitive adhesive with oligomeric silicone urethane methacrylate

European Polymer Journal 112 (2019) 320–327

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Property modification of a silicone acrylic pressure-sensitive adhesive with oligomeric silicone urethane methacrylate

T

Hee-Woong Parka,b, Ji-Won Parkc, Jung Hyun Leeb, Hyun-Joong Kimc, Seunghan Shina,d,

⁎

a

Green Chemistry & Materials Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea c Research Institute of Agriculture and Life Sciences, College of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Republic of Korea d Department of Green Process and System Engineering, Korea University of Science & Technology (UST), Cheonan, Chungnam 31056, Republic of Korea b

ARTICLE INFO

ABSTRACT

Keywords: Pressure-sensitive adhesive Silicone macro-azo-initiator Silicone urethane methacrylate Low surface energy substrate Peel strength

A silicone acrylic pressure-sensitive adhesive (SPSA) was synthesized using a silicone macro-azo-initiator as an adhesive for low surface energy substrates. The SPSA had a high peel strength value on an SUS (steel use stainless) substrate due to its low surface energy; however, it had very low peel strength values on low surface energy substrates such as PP and PE. In this study, a silicone urethane dimethacrylate (SiUDMA) oligomer was synthesized as a modifier for the SPSA. The SiUDMA had a much lower viscosity and a slightly higher surface energy than the SPSA. The addition of SiUDMA increased the peel strength of the SPSA regardless of the substrate. In the case of the PP and PE substrates, the increase of peel strength with SiUDMA addition was remarkably large when compared to the SUS substrate because SiUDMA significantly reduced SPSA viscosity and improved its fluidity, consequently facilitating the wetting of the rough surfaces of PP and PE. The rough surfaces of PP and PE could also aid the spreading of SiUDMA-modified SPSA and enlarge the interfacial area. On the other hand, UV curing of SiUDMA and the SiUDMA-modified SPSA resulted in a serious decrease in the peel strength on the PP and PE substrates due to the decrease in the fluidity of the SPSA material, but the SAFT of SPSA increased with UV irradiation.

1. Introduction A pressure-sensitive adhesive (PSA) is a semi-solid material that can easily adhere to various dissimilar surfaces using light pressure with a short contact time and can be removed without residue. Among the diverse types of PSAs, acrylic PSAs are most widely used for their many advantages, such as excellent resistance to oxidation, water resistance, a lack of yellowing, and optical clarity. However, acrylic PSAs have poor bonding properties for low surface energy substrates, low temperature flexibility, and high temperature stability [1–5]. To overcome these drawbacks of acrylic PSAs, polydimethylsiloxane (PDMS) is frequently used as a copolymer because the Si-O units in PDMS provide a low surface energy (21–22 mN/m), optical clarity, nontoxic and non-flammable properties, and good thermal and oxidative stability. However, it is difficult to make a homogeneous mixture by directly mixing PDMS with acrylic PSAs because the two compounds have low compatibility. Therefore, many approaches have been attempted to solve this problem [6–11].

A water-based emulsion system using a specific emulsifier has previously been proposed to solve the compatibility problem between acrylic- and silicone MQ resin-based PSAs, and the emulsion copolymerization of an acrylic monomer and vinyl terminated PDMS has also been reported to prepare an acryl-silicone hybrid PSA, which had good weather and heat resistance [12,13]. Pang et al. designed a hydrophobic and thermally stable acrylic PSA by introducing an epoxy terminated PDMS as a siloxane chain crosslinker [14]. Incorporating the PDMS silicone unit into the acrylic PSA significantly improved both wettability and thermal stability, although the adhesion properties of the material decreased slightly. The compound 3-acryloxypropyl methyldimethoxysilane has also been used as the silicone unit; this silane coupling agent was reacted with butyl acrylate and acted as a crosslinking point to generate acrylic PSAs with high shear and physical properties [15]. As an alternative to the emulsion method, a silicone macro-azo-initiator (MAI) has been developed for incorporating silicone units as block copolymers [16,17]. Silicone containing block-copolymers has an

⁎ Corresponding author at: Green Chemistry & Materials Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea (S. Shin). E-mail address: [email protected] (S. Shin).

https://doi.org/10.1016/j.eurpolymj.2019.01.021 Received 9 October 2018; Received in revised form 4 January 2019; Accepted 8 January 2019 Available online 09 January 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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interesting combination of surface and bulk properties. Since the MAI has an azo group between the PDMS chains, it was used for PMMA polymerization for the first time. The PMMA-containing siloxane blocks was more hydrophobic and resistant to water than PMMA without the siloxane [18]. However, MAI has rarely been used in PSA synthesis. In one example, Kim et al. used MAI as an initiator for PSA synthesis and reported that changing the MAI content from 3.33 to 26.7 phr both improved the thermal stability and decreased the peel strength of the PSA [19]. In addition, despite the silicone block inclusion by MAI, the peel strength of PSA on a low surface energy substrate such as Teflon was not noticeably improved [20]. In this work, we synthesized a silicone acrylic pressure-sensitive adhesive (SPSA) using MAI as an initiator to develop a useful PSA for low surface energy substrates such as PP or PE. To modify the adhesion property of the SPSA, we synthesized silicone urethane dimethacrylate (SiUDMA), which is an oligomeric silicone compound having urethane linkages in the main chain and methacrylate groups at each chain end, used to overcome the poor compatibility caused by direct mixing of PDMS. Different quantities of SiUDMA were added, and UV curing was performed to generate a semi-interpenetrating polymer network (semiIPN) structure to improve the probe tack, 180° peel strength, and thermal stability of the SPSA. The results were evaluated based on the viscosity and surface free energy change caused by the addition of SiUDMA to the SPSA material.

purchased from Shinetsu Co. Ltd. (Japan), and its basic properties are shown in Table 1. All chemicals were used without purification. 2.2. Synthesis of silicone urethane dimethacrylate (SiUDMA) Silicone urethane prepolymer synthesis was performed according to Scheme 1. IPDI (14.12 g) and DBTDL (0.067 g, 0.1 wt% of the prepolymer) were added to a two-necked flask equipped with a mechanical stirrer. The mixture was stirred for 15 min at 50 °C under an N2 atmosphere, and then 50 g of PDMS was slowly injected for 20 min. The reaction mixture was stirred for 40 min. To introduce the methacrylate functionality, 2.84 g of HEMA was slowly injected for 20 min. After 40 min, the reaction mixture was cooled to room temperature. 2.3. Synthesis of silicone acrylic pressure-sensitive adhesive (SPSA) The acryl monomers (2-EHA, AA, and IBA) and solvent (EA) were added to a 3-neck flask equipped with a mechanical stirrer and a reflux condenser. The mixture was stirred at 80 °C, and then MAI was dissolved in EA before being slowly added for 20 min. After 6 h, the solution was cooled to room temperature (see Scheme 2). The composition of the silicone acrylic PSA (SPSA) is shown in Table. 2. The synthesized SPSA was composed of 43% of solid content. GPC was used to determine that the polymer weight average molecular weight (Mw) is 423,000 and its number average molecular weight (Mn) is 63,000. It has a broad polydispersity (PDI = 6.71).

2. Experimental 2.1. Materials The silicone macro azo initiator (MAI), VPS-1001, was purchased from Wako-chemical Co. Ltd. (Japan) (see Fig. 1). 2-Ethylhexyl acrylate (2-EHA, 98%), acrylic acid (AA, 99%), isobornyl acrylate (IBA, > 85%), 2-hydroxyethyl methacrylate (HEMA, 99%), isophorone diisocyanate (IPDI, 98%), dibutyltin dilaurate (DBTDL, 95%), and ethyl acetate (EA, 99.8%) were purchased from Aldrich (St Louis, MO, USA). Darocur 1173 (2-Hydroxy-2-methylpropiophenone, used as a photoinitiator, 97%) was purchased by Ciba Specialty Chemicals Ltd. (Switzerland). Carbinol-terminated polydimethylsiloxane (PDMS, KF6000) was

2.4. Preparation of UV-crosslinked pressure-sensitive adhesive tape Solutions of the modified SPSA were coated onto the corona-treated PET films using a Baker applicator to make PSA tapes, and then tapes were put into the oven at 100 °C for 20 min to remove the solvent. The thicknesses of the dried PSA tapes were approximately 50 μm. After covering the dried PSA tapes with a fluorine-coated releasing film, UV curing of the PSA tapes was performed using 0–2000 mJ of UV dose (Scheme 3). Fig. 1. Chemical structure of MAI (VPS-1001). Its molecular weight was 70,000–90,000, and the molecular weight of PDMS unit was 10,000. The azo group content was 2.2–3.4%.

Table 1 Basic properties of carbinol terminated PDMS (KF6000). PDMS

Viscosity (cP)

OH group value (mg KOH/g)

Molecular weight

Si-O repeat number (approximately)

KF6000

35

∼120

935

∼9

Scheme 1. Synthesis of silicone urethane dimethacrylate (SiUDMA). 321

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Scheme 2. Synthesis of silicone acrylic pressure-sensitive adhesive (SPSA).

the sample viscosity. Each sample contained approximately 43 wt% solid contents (see Table 3). The viscoelastic properties of each SPSA were measured using an Advanced Rheometric Expansion system (ARES, G2 TA Instruments). The SPSAs were set on an 8 mm-diameter-round plate, and the gap between the plates was 1 mm. The plates were twisted by various strains (0.05–40%) and frequencies (1 Hz) at temperatures ranging from −40 to 80 °C with a heating rate of 5 °C/min. To measure the gel fraction of SPSAs, approximately 1 g of sample was packed in 20 mesh metallic paper and stirred in EA for 1 day at room temperature. After filtration, the remaining solid was dried in an oven at 80 °C until a constant weight was obtained. The gel fraction was calculated by the following equation:

Table 2 Basic composition of SPSA. Chemical

Weight ratio

2-Ethyl hexyl acrylate Acrylic acid Isobornyl acrylate VPS-1001(MAI) Ethyl acetate

71.43 4.76 19.05 4.76 132.56

2.5. Measurements The Fourier transform infrared (FTIR) spectra of SiUDMA were obtained by transmittance mode using a NICOLET 6700 (Thermo electron Co., MA, USA). The sample was dried in a vacuum oven at 40 °C for 24 h to remove the residual chemicals. The sample was coated on a KBr pellet, and infrared absorption was measured. The measurement range was 650–4000 cm−1, and resolution was 4 cm−1. Proton nuclear magnetic resonance spectroscopy (1H NMR, Bruker Avance 300 Bruker) was used to obtain further structure analysis data. The spectrum was obtained using deuterated chloroform (CDCl3) as a solvent at 25 °C. Chemical shifts for spectra were referenced in ppm by using tetramethylsilane as the internal standard. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) were measured by gel permeation chromatography (GPC, PL-GPC 220 Agilent Technologies) at 25 °C. The column set was calibrated with polystyrene narrow standards. The samples were dissolved in a tetrahydrofuran (THF) solution at a concentration of 0.5 wt%. The contact angle was measured by contact angle goniometer (Phoenix 300, Surface & Electro-Optics). One drop each of distilled water and diiodomethane was dropped on the PSA tape, and the change in angle was measured every minute (for up to 10 min). The temperature was 24 ± 1 °C, and relative humidity was 50 ± 2%. The surface free energy was calculated using the Owens-Wendt method [21]. The solution viscosities of SPSAs and SiUDMA were measured using a viscometer (DV-II, Brook field). For temperature stabilization, the samples were left at 25 °C for 20 min in the viscometer. After stabilization, a suitable rpm speed was selected for the spindle according to

Gel fraction (%) = (W1/W0) × 100 where W0 was initial sample weight and W1 is the weight after the process. The SPSA’s adhesion properties were evaluated by measuring probe tack, peel strength, and the shear adhesion failure test. Probe tack and 180° peel strength were measured using SurTA system (ChemiLab, Korea). The probe tack debonding speed was 0.5 mm/s, and maximum debonding force was measured at 25 °C. The size of the cylinder-shaped stainless steel probe was 1/8 in. The 180° peel strength was measured as follows: SPSA films were attached to a high surface energy substrate (Stainless steel (type 304)) or low surface energy substrates (polypropylene (PP) and polyethylene (PE)), and rolled with a 2 kg rubber roller twice. The samples were left at room temperature for 20 min before measurement. The peeling speed was 300 mm/min, and measurements were repeated three times. The surface roughness of a substrate was characterized using 3D digital multifunctional microscope HIROX MPS (HIROX, Japan) with MXB 2500REZ lens and diffuse adapter (HIROX, Japan). The measurement was performed with confocal mode and 20 nm precision and the surface roughness was calculated by removing the surface deviation form and waviness using Gaussian filter. The shear adhesion failure test was conducted using a 25 mm × 25 mm size rectangular specimen that was attached to a stainless substrate (SUS304). The specimens were stored at room temperature for 24 h. After loading with a 1 kg weight, the specimens were

Scheme 3. Synthesis of SiUDMA-modified SPSA. 322

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GPC indicates that most of the SiUDMA has 3 repeating units in its structure.

Table 3 Formulation of UV-Crosslinked SPSA containing SiUDMA. Sample Code.

SPSA

SiUDMA

Darocur 1173 (Photoinitiator)

UV (mJ)

SPSA SPSA20-0 SPSA20-300 SPSA20-800 SPSA20-2000 SPSA40-0 SPSA40-300 SPSA40-800 SPSA40-2000

100 80

– 20

0 5 part

60

40

0 0 300 800 2000 0 300 800 2000

3.2. Adhesion properties of SiUDMA-modified SPSA The probe tacks of the SiUDMA-modified SPSAs were measured to see the effect of SiUDMA on the adhesion properties of the SPSA. As seen in Fig. 4, the probe tack of the SPSA increased upon addition of SiUDMA but decreased with increasing UV doses. When the UV dose was 2000 mJ/cm2, the probe tack of the SPSA modified with 40 wt% of SiUDMA was lower than that of the SPSA modified with 20 wt% of SiUDMA. Considering that the tack value of the PSA depends on bulk properties, such as modulus and viscosity, as well as geometric properties, such as thickness and surface roughness [17,22], the surface free energies (SFEs) and viscosities of SiUDMA, SPSA and SiUDMA-modified SPSA were analysed to explain these results. Fig. 5 shows the contact angles of water and diiodomethane and the SFEs of the SPSA series. As seen in Fig. 5, the contact angle of the liquid drops gradually decreased with measuring time due to the soft surface of the SPSA film, while the SFEs of the SPSA film increased from 25.5 to 38.3 mN/m with time. The SFEs of SPSA measured just after dropping increased from 25.5 to 32.7 and increased to 33.4 mN/m by adding the SiUDMA (whose SFE was 35.0 mN/m as a fully cured film). The SFEs of SiUDMA-modified SPSAs increased slightly, but they were still much lower than that of SUS304. However, the viscosity of the unmodified SPSA was remarkably higher than that of the SiUDMA-modified SPSAs (7731 cP for SPSA vs 5670 for SPSA20-0, and 3104 cP for SPSA40-0; see Fig. 6) because SiUDMA has very low viscosity (880 cP) compared with SPSA. Considering both the SFE and viscosity results, the increase of the probe tack by the addition of SiUDMA is mainly due to the decreased viscosity of the SiUDMA-containing SPSA materials. Fig. 4 also shows that probe tack sharply decreased with increasing UV dose. This is easily understood by the viscosity increase of SPSA due to the crosslinking of SiUDMA by UV irradiation. To verify the crosslinking reaction of SiUDMA caused by UV irradiation, the gel fraction of pristine and SiUDMA-modified SPSAs were measured and compared (Fig. 7). When the UV dose was 2000 mJ/cm2, the gel fraction of SPSA40 became 74.5%, and this countervailed the profit from the low viscosity of SiUDMA. As a consequence, the probe tack of SPSA40-2000 was lower than that of SPSA20-2000. Fig. 8 shows viscoelastic properties of pristine and SiUDMA-modified SPSA as a function of temperature. The storage modulus (G′) of SPSA decreased with increasing quantities of SiUDMA. This means SPSA40-0 and SPSA20-0 have higher mobility than SPSA. These results are well correlated to the results of viscosity and probe tack. The glass transition temperatures (Tgs) determined by tan δ curves of SPSA40-0, SPSA20-0, and SPSA were −24.6, −18.8, and −15.7 °C, respectively. This is due to the low molecular weight of SiUDMA that contains PDMS as a soft segment. Based on the single Tg peak observed in tan δ curves, we can expect that there was no significant problem in compatibility between SiUDMA and SPSA. Fig. 9 shows the 180° peel strengths of SiUDMA-modified SPSAs. All samples showed adhesive failure irrespective of substrates. In the case of the SUS substrate, the peel strength of SPSA was very large, and it approximately doubled just by the addition of SiUDMA. The peel strength of both SiUDMA-modified SPSAs decreased with increasing UV irradiation. As discussed in the probe tack results, higher concentrations of SiUDMA seemed to expedite wetting by reducing the SPSA viscosity. Since the SFEs of SPSAs were marginally changed by UV irradiation as seen in Fig. S1, the sharp decrease in peel strength by UV irradiation is mainly due to the fluidity decrease by the crosslinking reaction of the SiUDMA molecules. Considering the peel strengths (Fig. 9) and the initial tack and peel force results (Fig. S2), it can be assumed that the viscosity of SiUDMA-modified SPSA was increased with UV dose and became larger than that of SPSA at 2000 mJ/cm2 of UV dose. The SiUDMAs form a semi-IPN structure between the SPSA polymer chains

held in the oven and heated from 25 to 200 °C with a ramping speed of 0.4 °C/min. Thermo-gravimetric analyses of samples were performed using a TGA 8000 PerkinElmer. The samples were heated from room temperature to 600 °C with a heating rate of 10 °C/min in a nitrogen atmosphere. 3. Results and discussion 3.1. Characterization of SiUDMA The progress of the reaction to synthesize SiUDMA was monitored by FTIR. As seen in Scheme 1, the NCO peak should disappear after the reaction is complete. The FTIR spectra (shown in Fig. 2), show that the band at 2250 cm−1, assigned to NCO, completely disappeared within 1 h after the 2nd step of the reaction started. As the reaction proceeded, the NCO and OH peaks of IPDI and HEMA disappeared, while the C]C peak of HEMA appeared and increased in intensity. Further structural identification of SiUDMA was performed using 1H NMR spectroscopy. Although it is difficult to assign each peak in the NMR spectrum clearly, specific peaks for the methacrylate of HEMA (C]CH2, CH3eC]CH2), peaks for PDMS (SieCH3, SieCH2e), and peaks for IPDI (eCH2e, CeCH3) were clearly observed (see Fig. 3). Both the FT-IR and 1H NMR results clearly indicate that SiUDMA was successfully synthesized. The molecular weight of SiUDMA was measured by GPC. Its weight average molecular weight (Mw) was 8800 and number average molecular weight (Mn) was 4200. Its polydispersity was 2.08. The calculated molecular weights of SiUDMA based on its chemical structure are approximately 3953 (m = 3) and 5110 (m = 4). The Mn value obtained by

(a) (b) 1

(c)

3

2

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 2. FT-IR spectra of SiUDMA; (a) 10 min after the 1st step, (b) 10 min after the 2nd step, and (c) final product. Peak 1 is assigned to the NCO stretching frequency of IPDI, peak 2 is the C]C stretching frequency of HEMA, and peak 3 is the OH stretching frequency of HEMA. 323

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Fig. 3. 1H NMR spectrum of SiUDMA.

350 300

Probe Tack (gf)

the liquid but also increases the interfacial area. In Fig. 9, the SPSA had a much lower peel strength on PP or PE than on SUS because PP or PE has a lower spreading energy than SUS, although the PP or PE substrate had a relatively rough surface (0.154 ± 0.005 μm for PP and 0.233 ± 0.016 μm for PE vs Ra = 0.123 ± 0.003 μm for SUS). This also implies that SPSA is too viscous to increase its spreading tendency by surface roughness. As already stated, SPSA is a high molecular weight polymer having many PDMS (Mw = 10,000 g/mol) blocks in the chain as well as at chain end and it has higher viscosity than SiUDMA-modified SPSAs. When the SiUDMA-modified SPSAs were used, the increase of peel strength was very large on the PP or PE substrates compared to that on the SUS substrate despite the surface energies of SiUDMA-modified SPSAs being slightly increased. This large increment seems to have resulted from the low viscosity of SiUDMA-modified SPSAs and the rough surfaces of the PP or PE substrates. As previously mentioned, surface roughness could help the spreading of the adhesive and enlarge the interfacial area, which is facilitated by the low viscosity of SiUDMAmodified SPSAs. It can be easily comprehended that the steep decrease of peel strength on the PP or PE substrates by UV irradiation is ascribed to the increase in the viscosity of the SiUDMA-modified SPSAs due to crosslinking.

20 wt% 40 wt%

250 200 150 100 50 0

SPSA

0

300

800

2000

2

UV dose (mJ/cm ) Fig. 4. Probe tack of SiUDMA-modified SPSA as a function of UV dose. The light grey colour is 20 wt% of SiUDMA and grey is 40 wt% of SiUDMA added system.

by UV irradiation (see Scheme 3). Aubery already reported that if PSA fails in cohesive mode, an increase in crosslinking density raises peel force, but in interfacial mode, crosslinking density decreases peel force. [23] Higher crosslinking density is considered the main factor for decreased peel strength with increased shear strength. Meanwhile, the SPSA had an extremely low peel strength on the low surface energy substrates such as PP (∼31 mN/m) and PE (∼32 mN/m) (see Fig. 9(b) and (c)). However, the SiUDMA-modified SPSAs had approximately 30–50 times higher peel strength than the pristine SPSA, which was much larger increase than that on the SUS substrate. Since the same adhesives were used, these results should be interpreted by the two main factors of adhesion: one is surface energy, and the other is surface topology. For example, spreading energy, which is determined by the surface tension of each material, should be large enough for good adhesion, and high surface roughness is also required to achieve good adhesion. The spreading energy on the SUS substrate is much larger than that on PP or PE because SUS has a large surface tension difference with SPSA compared to PP and PE. Thus, SPSA spreads easily on the SUS substrate to give high peel strength. The surface roughness of a substrate, on the other hand, not only improves the spreading tendency of

3.3. Thermal stability of SiUDMA-modified SPSA The thermal stability and shear resistance of the SPSAs were determined using the shear adhesion failure test (SAFT). The SAFT test measures the temperature and time until the PSA fails when a 1 kg weight was attached to it with heating from 25 to 200 °C at a rate of 0.4 °C/min. As shown in Fig. 10, the thermal stability of SPSA films increased with the increasing UV dose. However, SPSA20-0 and SPSA40-0 samples showed worse stability than pristine SPSA because, as mentioned previously, low molecular weight SiUDMA had a negative effect on the cohesion of the SPSA. Therefore, SPSA40-0 had slightly lower stability than did SPSA20-0. However, with UV irradiation, the PSA40 series showed better stability than PSA20 series with the same UV dose condition because the PSA40 series had more methacrylate groups than the SPSA20 series, which induced a more crosslinked structure for the semi-IPN. 324

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SPSA20-0

70

35 30

60

25

50 40

0

2

4 6 Time (min)

8

20

10

40

Surface energy (mN/m)

Water DIM SFE

80

100

Contact angle (o)

Contact angle (o)

40

90

Surface energy (mN/m)

SPSA 100

90

35

80 Water DIM SFE

70 60

30 25

50 40

0

2

4 6 8 Time (min)

10

20

Contact angle (o)

100

40

90

35

80 70

30

Water DIM SFE

60

25

50 40

0

2

4 6 Time (min)

8

Surface energy (mN/m)

SPSA40-0

20

10

Fig. 5. Contact angles with water and diiodomethane (DIM) of SPSAs and calculated surface free energy (SFE) as a function of measuring time.

(a)

9000

6000 4500

G' (Pa)

Viscosity (cP)

7500

3000

SPSA

SiUDMA SPSA20-0 SPSA40-0

106

102 -6 0 -4 0 -2 0

Fig. 6. Solution viscosities of SPSA, SiUDMA, and SiUDMA-modified SPSAs.

(b)

100

0

20

40

60

o

Temperature ( C) 1.5 SPSA SPSA20-0 SPSA40-0

SPSA20 SPSA40

75

80 100

1.0

50

Tan

Gel fraction (%)

SPSA SPSA20-0 SPSA40-0

104

1500 0

108

25

0.5 0

0

500

1000

1500

2000

-60 -40 -20

2

UV dose (mJ/cm )

0

20

40

60

Temperature (oC)

Fig. 7. Gel fractions of SiUDMA-modified SPSAs with different UV dose.

80 100

Fig. 8. Viscoelastic curves of the SPSA with different SiUDMA contents.

325

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1800 (a) 1500

Peel Strength (gf /25mm)

20 wt% 40 wt%

1200 900 600 300 0

SPSA

0 300 800 2000 UV dose (mJ/cm2) Peel Strength (gf/25mm)

Peel Strength (gf /25mm)

H.-W. Park et al.

900 (b)

20 wt% 40 wt%

600 300 0

SPSA

900 (c)

0 300 800 2000 UV dose (mJ/cm2)

20 wt% 40 wt%

600 300 0

SPSA

0 300 800 2000 UV dose (mJ/cm2)

Fig. 9. Peel strength of SiUDMA-modified SPSA as a function of UV dose with different substrates; (a) SUS (b) Polypropylene (c) Polyethylene. The light grey colour is 20 wt% of SiUDMA and grey is 40 wt% of SiUDMA added system.

20 wt% 40 wt%

*

100 80

150

Weight (%)

Temperature (oC)

200

100 50 0

60 SPSA SIUDMA SPSA20-0 SPSA20-300 SPSA20-800 SPSA20-2000

40 20 0

SPS A

0

300

800

2000

0

UV dose (mJ/cm2) Fig. 10. SAFT (shear adhesion failure test) results of SiUDMA-modified SPSA. *The 2000 mJ/cm2 sample reached 200 °C and lasted for more than 30 h.

100

200

300

400

500

Temperature (oC)

600

Fig. 11. TGA curves of SPSA, SiUDMA, and SiUDMA-modified SPSAs.

Further identification of the modified SPSA’s thermal stability was obtained by thermogravimetric analysis (TGA) measured from 25 °C to 600 °C. Fig. 11 shows the weight loss curves of SPSA, SiUDMA, and the SPSA20 series with different UV doses. The low molecular weight SiUDMA degraded the fastest, and its Td5 (temperature at 5% weight loss) was 258.7 °C. SiUDMA showed a two-step degradation. The first step is the depolymerization stage of a urethane functional group, and the second step is the decomposition of the soft segment (PDMS) and the hard segment (IPDI). SPSA20-0 had lower thermal stability than pristine SPSA due to the SiUDMA addition. Its Td5 was 281 °C, which was lower than that of SPSA (Td5 = 299.8 °C). Although SPSA20-0 showed fast degradation below 400 °C due to the low thermal stability and plasticizing behaviour of SiUDMA, it had better thermal stability than SPSA above 400 °C, and the thermal stability above 400 °C was further improved by increasing the UV dose.

4. Conclusion We successfully synthesized and characterized SiUDMA, a silicone urethane acrylate oligomer to improve the adhesion properties of the silicone acrylic pressure-sensitive adhesive (SPSA) prepared using a macro-azo-initiator containing a silicone unit. The SiUDMA contained about three PDMS units and had a lower viscosity than the SPSA (880 cP vs 7731 cP at 25 °C in 43 wt% solution). The addition of SiUDMA to the highly viscous SPSA significantly increased the probe tack of the material by reducing its viscosity. In the 180° peel test, the peel strength of SiUDMA-modified SPSA on the SUS, PP, and PE substrates increased significantly with addition of SiUDMA. When low surface energy substrates such as PP and PE were used, the peel strength dramatically increased mainly due to the low viscosity of SiUDMA-modified SPSAs and the rough surfaces of the PP and PE substrates (30–50 times higher than pristine SPSA). However, the peel strength decreased steeply with 326

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UV curing due to the viscosity increase. These results indicate that low viscosity and low surface energy of adhesive are both very crucial for good adhesion with low surface energy substrates. On the other hand, the thermal stability of the SiUDMA-modified SPSA decreased slightly due to the addition of low molecular weight SiUDMA and its plasticizing effect. However, SiUDMA-modified SPSA that was irradiated at 300 mJ/cm2 had improved probe tack, peel strength and SAFT value compared with pristine SPSA. Notwithstanding the relatively limited sample, this work offers valuable insight into the development of low surface energy pressure-sensitive adhesives for silicone-coated circuit boards and skin-adhered devices in the near future.

[8]

[9] [10]

[11]

Acknowledgements This study was supported by the Fundamental R&D Program for Core Technology of Materials (10049089, 10076899) funded by the Ministry of Trade, Industry and Energy (MOTIE, Republic of Korea) and by the R&D Program (kitech EO-18-0003) funded by Korea Institute of Industrial Technology, Republic of Korea.

[12]

Appendix A. Supplementary material

[15]

[13] [14]

[16]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.01.021.

[17]

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