Optimization of Recovery and Relaxation of Acrylic Pressure-Sensitive

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General Research

Optimization of Recovery and Relaxation of Acrylic PressureSensitive Adhesives by using UV Patterning for Flexible Displays Jong-Ho Back, Dooyoung Baek, Kyeng-Bo Sim, Gyu-Young Oh, Seong-Wook Jang, Hyun-Joong Kim, and Youngdo Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05208 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Industrial & Engineering Chemistry Research

Optimization of Recovery and Relaxation of Acrylic Pressure-Sensitive Adhesives by using UV Patterning for Flexible Displays

Jong-Ho Backa, Dooyoung Baeka,b, Kyeng-Bo Sima, Gyu-Young Oha, Seong-Wook Janga,b, HyunJoong Kima,b,*, Youngdo Kimc

a Laboratory

of Adhesion and Bio-Composites, Program in Environmental Materials Science,

Seoul National University, Seoul 08826, Republic of Korea b Research

Institute of Agriculture and Life Sciences, College of Agriculture and Life Sciences,

Seoul National University, Seoul 08826, Republic of Korea c Samsung

Display Co., Ltd., Yongin 446-711, Republic of Korea

* Corresponding

Author:

Prof. Hyun-Joong Kim Tel.: +82-2-880-4784; Fax: +82-2-873-2318; E-mail: [email protected] (www.adhesion.kr)

ACS Paragon Plus Environment

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Abstract For flexible displays, recovery and relaxation of acrylic pressure-sensitive adhesives (PSA) must be enhanced; however, only a few studies have focused on their optimization. High crosslinking density of the PSA leads to improved recovery but deteriorates the stress relaxation; thus, it is difficult to perform optimization by simply controlling the crosslinking density. Herein, it was determined that a UV-patterned PSA with both high and low crosslinking densities in a single layer enables the optimization of both recovery and relaxation. By introducing the UV-patterned PSA, the elasticity and recovery largely deteriorated but the stress relaxation was significantly improved compared to that of the non-patterned PSA and this effect was enhanced with increase in the applied strain. Thereby the recovery and relaxation were well optimized with both values above 71% only at 300% strain. The recovery and relaxation of PSA had, respectively, a positive and negative correlation with the storage modulus.


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1. Introduction Pressure-sensitive adhesives (PSAs) are polymeric materials that permanently tacky at room temperature in dry film form. PSAs firmly adhere to various substrates and have the ability to bind two different substrates.1, 2 PSAs are classified by their composition such as acrylate, silicone, rubber, and so on. In particular, acrylic PSA has many advantages including high transparency, adhesion performance, UV-curable property, and resistance to aging, and thus, it has been widely used for various applications such as in labelling, protective films, medical application, automotive parts, and display.3-10 In display applications, PSA is widely used for the assembly of the display components.11-15 As the display has developed from a rigid plate shape to having a curved shape or flexible nature, the flexibility of display components has been investigated.16-18 PSA, one of the important components of a flexible display (Figure 1), has also been investigated in order to characterize and enhance flexibility19 because poor flexibility of PSA may lead to serious defects such as delamination during the period of use of the display. Our research group previously attempted to determine the factors influencing the flexibility of a PSA, such as molecular weight and crosslinking density. The effect of crosslinking density on the flexibility of PSA was evaluated by comparing the recovery and relaxation of PSA; however, the optimization of the two properties has not been investigated extensively yet.20-22


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Figure 1. Structure of flexible display and its composition. The recovery of PSA is important for flexible displays because a high recovery of PSA prevents it from sagging under repeated folding–unfolding conditions. Further, relaxation is also required because low stress relaxation causes high levels of residual stress, which can lead to delamination, stress corrosion, cracking, and reduction in fatigue life of the assembled display.23 Despite the significance of recovery and relaxation for the PSA for flexible displays, only a few studies have attempted to optimize the two properties. Lee et al., determined that increasing the crosslinking density of PSA resulted in enhancement of recovery but degradation of relaxation.22 Zhang et al., also suggested that crosslinking lead to improvement in the elasticity and recovery of the PSA but caused a reduction in the relaxation.24 However, all these studies only focused on characterizing the recovery and relaxation according to the crosslinking density, rather than optimizing the two properties. In this research, we prepared a UV-patterned PSA containing both high and low crosslinking densities into a single PSA layer to optimize both the recovery and relaxation. The crosslinking density of PSA was controlled by a UV dose and the curing behavior and mechanical strength of the PSA were estimated. Further, the recovery and relaxation for non-patterned and patterned PSAs were evaluated.


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2. Materials and methods 2.1 Preparation of UV-patterned acrylic PSA Acrylic oligomer was synthesized by bulk polymerization of the acrylic monomer.11, 25, 26 The specific composition of the oligomer is presented in Table 1. The crosslinking agent (polyethylene glycol 200 dimethacrylate, SartomerTM SR210, Sartomer, USA) and photo-initiator (2-hydroxy-2methyl-1-phenyl-propan-1-one, Irgacure 1173, BASF, Germany) were added as 1 wt.% of the synthesized oligomer.

Table 1. Composition of acrylic oligomer. Classification

Acrylic monomer

Photo-Initiator

Composition

Content (wt.%)

2-Ethylhexyl acrylate

64

Isobornyl acrylate

19

Acrylic acid

3

Methyl methacrylate

4

2-Hydroxyethyl acrylate

10

2-Hydroxy-2-methyl-1-phenyl-propan-1-one

0.1 phr

Acrylic PSA was prepared by UV curing of the mixture, the curing mechanism of which is shown in Scheme 1 (a). The PSA was coated on a silicone release film by film applicator with 50 µm. Through UV irradiation (intensity=100 mW/cm2), a radical was formed from the photoinitiator, followed by coupling reaction with C=C double bond in both crosslinking agent and acrylic oligomer. After the first UV irradiation, the pre-cured PSA, which had low crosslinking density, was prepared into a film form. The minimum UV dose for the formation of the PSA film from viscous liquid state was 750 mJ/cm2, and thus, we set the UV dose for the pre-cured PSA as 750 mJ/cm2 in the process. As shown in Scheme 1 (b), the pre-cured PSA with the masking film


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was exposed to a second UV irradiation. The masking film had a line pattern—the UV radiation is blocked by the lines, although it can penetrate the regions in which a line is not printed. The masking film was prepared by printing the line pattern on back side of the silicone release film. Therefore, after exposure to the second UV irradiation with the masking film, the line patterned PSA was prepared, which included both high and low crosslinking densities in a single PSA layer. By controlling the second UV irradiation, we prepared the patterned PSA that had different crosslinking densities for a highly cured region. All samples were cured by a UV conveyor with constant speed. UV dose of the UV conveyor was fixed as 250 mJ/cm2 and total UV dose was controlled by the number of passing onto UV conveyor. (a) 1st UV irradiation

2nd UV irradiation

Cured PSA

Pre-cured PSA

Before curing

(UV dose was set as 750 mJ/cm2)

Photo-initiator Crosslinking agent

(b)

Crosslinking point Acrylic oligomer

Locating masking film

UV

UV

PSA PET film or release film

1st UV irradiation

Pre-cured PSA

(750 mJ/cm2)

(UV dose was set as 750 mJ/cm2)

2nd UV irradiation (250, 500, 750, 1000, and 1250 mJ/cm2) Top view Width = 1 mm

2nd UV irradiation 2

(250, 500, 750, 1000, and 1250 mJ/cm )

Patterned masking film

Top view

H: Highly cured region, high crosslinking density (1000, 1250, 1500, 1750, and 2000 mJ/cm2)

L: Pre-cured region, low crosslinking density (750 mJ/cm2)

H

Non-patterned PSA

Removing masking film

Top view

H L H L H L

UV patterned PSA

Scheme 1. Preparation of UV-patterned PSA: (a) Curing mechanism of PSA and (b) process of UV patterning of PSA.


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2.2 Fourier transform infrared spectroscopy and gel fraction As a radical is generated by the photo-initiator via UV irradiation, it reacts with the C=C bond so that the acrylic PSA is cured. By comparing the conversion of the C=C bond, we could quantitatively estimate the reaction by using FT-IR (JASCO FTIR-6100). As the C=C bond of acrylate shows an FT-IR peak at 810 cm-1, the conversion of the C=C bond was evaluated using Equation (1): 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) =

(𝐴810)0 ― (𝐴810) (𝐴810)0

× 100,

(1)

where (A810)0 and (A810) denote the intensity integration of the 810 cm−1 peak at 0 mJ/cm2 and after curing, respectively. The gel fraction is the weight ratio of the total polymer weight to the crosslinked polymer weight, and it was estimated using the following steps: Cured PSA was soaked in toluene for 24 h and then filtered using a 200-mesh filter. After drying the filtered mesh, the weight filtered by the mesh was measured to determine the crosslinked polymer fraction. The remaining mass was the linear polymer, which penetrated the mesh. Therefore, the gel fraction was calculated by dividing the crosslinked polymer weight by total weight.

2.3 Lap shear test and adhesion test We used a polyethylene terephthalate (PET) film (75 μm thick) as the substrate for the lap shear test specimen, and the adhesion area was 25 mm × 25 mm. A universal testing machine (AllroundLine Z010, Zwick, Germany) pulled the specimen in the tensile direction at 1 mm/s. The maximum stress for the fracture was determined as the lap shear strength and the strain was evaluated by dividing the displacement by the thickness of the PSA (50 µm).


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The peel strength and probe tack were investigated by using a texture analyser (TA-TX plus, Micro Stable Systems, UK). The peel strength was measured at an angle of 180° with a crosshead rate of 5 mm/s crosshead rate. The backing film of the PSA was the 75-μm PET film. Probe tack, initial adhesion strength, was measured at a constant separation rate of 10 mm/s after initial contact (1 N for 10 s).

2.4 Dynamic mechanical analysis To characterize the viscoelastic property of the UV-patterned and non-patterned PSAs, we implemented a dynamic mechanical analysis (DMA, Q800, TA Instruments, USA). The flatshaped test specimens were prepared by curing the PSA in a flat shaped silicone mold in order to obtain the PSA into free-standing shape without any substrate. The flat-shaped test specimens were fixed under tension: clamp with 0.1% strain and frequency of 1 Hz. The temperature was increased from 220 K to 380 K at a constant rate (5 K/min). In the case of the UV-patterned PSA, the pattern direction was parallel to the tensile direction of the DMA. All samples were conducted once, not including repeat test.

2.5 Strain recovery and stress relaxation The DMA (Q800, TA Instruments, USA), equipped with a tension clamp, was used to evaluate the strain recovery and the stress relaxation of the PSA. As shown in Figure 2 (a), the test specimen was prepared by using polycarbonate substrate (PC, 30 mm × 6 mm) and the adhesion area was 20 mm × 6 mm. Strain was applied in the tensile direction so that the PSA experienced shear strain between the polycarbonate substrates. Constant shear strain was applied for 10 min and then, the force was decreased to zero for 5 min. The shear strain values (200%, 300%, and 400%) were


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varied for various bending radii of the flexible display module. When the shear strain was increased to 500%, some samples were broken during the experiments so that we selected the shear strain values from 200% to 400%. The strain and recovery time were the same as used in previous research by our group.22 As shown in Figure 2 (b), the strain was naturally recovered for 5 min and we calculated the strain recovery using Equation (2): 𝑆𝑡𝑟𝑎𝑖𝑛 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 (%) =

𝑆0 ― 𝑆𝑡 𝑆0

× 100,

(2)

where St and So denote the strains at the recovery time t of 6 and 300 s, and at the beginning of recovery (t = 0 s), respectively.


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Shear strain PSA (50 µm thick)

PC

(a)

S0 Strain (%)

St (t=6 s)

St (t=300 s) 0

5

10

15

Time (min)

(b)

0.20

0.15

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fi (Initial stress) 0.10

Ff (Final stress)

0.05

0.00 0

5

10

15

Time (min)

(c)

Figure 2. Methodology for evaluating recovery and relaxation of PSA: (a) Test specimen, (b) strain recovery, and (c) stress relaxation. As shown in Figure 2 (c), the stress was relaxed during the constant strain condition, and it was calculated using Equation (3): 𝑆𝑡𝑟𝑒𝑠𝑠 𝑟𝑒𝑙𝑎𝑥𝑎𝑡𝑖𝑜𝑛 (%) =

𝐹𝑖 ― 𝐹𝑓 𝐹𝑖

× 100,

(3)

where Fi and Ff denote the initial and final stress under constant strain, respectively.


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All samples were conducted once, not including repeat test.

3. Results and discussion 3.1 Curing behavior of PSA Acrylic PSA was cured by UV irradiation and it was evaluated using FT-IR and gel fraction. In the FT-IR analysis, the intensity of the C=C bond peak (810 cm-1) was weakened as the UV dose was increased, which indicates that the C=C bond reacted with the radical that was formed by UV irradiation (Figure 3 (a)). In other words, as the UV dose was increased, the conversion, which is the parameter indicating the quantity of the curing reaction, was increased. In addition, the gel fraction of the PSA also increased with UV irradiation (Figure 3 (b)). From these results, it was determined that the crosslinking degree was easily controlled by changing the UV dose for the PSA. Therefore, by separating the UV penetration area and blocking area, it was possible to prepare the line-patterned PSA, which contained the two regions with different crosslinking densities.

0.10

2

UV dose (mJ/cm ) 0 250 500 750 1000 1250 1500 2000

Conversion (%)

Absorbance

0.15

0.05

100

100

80

80

60

60

40

40

20

0.00 830

20 Conversion Gel fraction

0

820

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800

Gel fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

500

1000

1500

0

2000

2

UV dose (mJ/cm )

-1

Wavenumber (cm )

(a)

(b)

Figure 3. Curing behavior of PSA: (a) FT-IR and (b) conversion and gel fraction. 3.2 Lap shear strength and adhesion performance of PSA


11


The mechanical strength of acrylic PSA was evaluated through a lap shear test and adhesion test. As the UV dose was increased, the acrylic PSA was crosslinked so that the lap shear strength was enhanced (Figure 4 (a)). Therefore, it is suggested that the highly cured region of UV-patterned acrylic PSA has a higher mechanical strength than that of the pre-cured region of the UV-patterned acrylic PSA (see Scheme 1 (b)). The peel strength of acrylic PSA was enhanced by UV irradiation because of the increase in cohesion strength (Figure 4 (b)). The cohesion of acrylic PSA continuously increased with increase in the UV irradiation, which decreased the wetting of PSA to the substrates; this implied that the peel strength of acrylic PSA decreased at a high UV dose. The probe tack also decreased as the

0.4

2

1

0.2

0.0 500

Lap shear strength Strain at break 1000

1500

2000

2500

12

25

0

3000

Peel strength (N/25 mm)

3

20 8 15

10 4

Probe tack (N)

0.6

4

Lap shear strength (MPa)

UV dose was increased, for the same reason as the decrease in the peel strength.

Strain at break (10 %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

0 500

Peel strength Probe tack 1000

1500

2000

2500

0

3000

2

2

UV dose (mJ/cm )

UV dose (mJ/cm )

(a) (b) Figure 4. (a) Lap shear strength and (b) adhesion performance of PSA. 3.3 Viscoelastic property of UV-patterned PSA In order to compare the viscoelastic properties of non-patterned and UV-patterned PSA, the storage modulus and damping factor (tan δ) were estimated by DMA (Figure 5). As the UV dose was increased, the storage modulus of non-patterned PSA was enhanced in the entire temperature range (Figure 5 (a)). There was no significant difference in the peak intensity of the damping factor;


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however, with UV irradiation, the peak was slightly shifted to the higher temperature region, which indicates increase in the glass transition temperature (Figure 5 (b)). The storage modulus and glass transition temperature of UV-patterned PSA, containing both high and low crosslinking density, were also increased with the increase in UV dose of the highly cured region (Figure 5 (c) and (d)). However, especially in the glass state, the storage modulus was lower than that of non-patterned PSA, because the pre-cured region (UV dose = 750 mJ/cm2), included in the patterned PSA, had the lowest storage modulus.

4

10 10

2

10

1

10

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

1.5

Tan 

Storage modulus (MPa)

2.0

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

3

1.0

0

10

0.5 -1

10

-2

10

220

260

300

340

0.0 220

380

260

(a)

2.0

2

UV dose (mJ/cm ) 1000 1250 1500 1750 2000

3

10

2

380

1

10

2

UV dose (mJ/cm ) 1000 1250 1500 1750 2000

1.5

Tan 

10

340

(b)

4

10

300

Temperature (K)

Temperature (K)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

0

10

0.5 -1

10

-2

10

220

260

300

340

380

0.0 220

260

Temperature (K)

300

340

380

Temperature (K)

(c) (d) Figure 5. Viscoelastic property of non-patterned and patterned PSA: (a) Storage modulus and (b) damping factor of the non-patterned PSA, (c) Storage modulus and (d) damping factor of the patterned PSA. The preparation procedure of the UV-patterned PSA is illustrated in Scheme 1. 3.4 Recovery property of UV-patterned acrylic PSA


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PSA is a polymeric material and hence it could be naturally recovered after deformation. Figure 6 shows the recovery behavior and strain recovery of non-patterned PSA. Strain was immediately recovered above 50% for 6 s and gradually recovered until 300 s. The elasticity and recovery property were related to the storage modulus of the polymeric material24 and hence the strain recovery of PSA was enhanced by UV irradiation because of the increase in its storage modulus. In our previous work, strain recovery of PSA was improved by increasing crosslinking density of PSA and it supported above results.20, 22 As the applied strain was increased from 200% to 400%, strain recovery was decreased, which indicates that high strain resulted in high degree of plastic deformation.

As shown in Figure 7, compared to non-patterned PSA, UV-patterned PSA had lower strain recovery because UV-patterned PSA contained the pre-cured region (750 mJ/cm2) that has low storage modulus. However, the strain recovery of patterned PSA was lower than that of nonpatterned PSA with dose of 750 mJ/cm2. This might result from unknown factors such as the interface and Poisson’s ratio of two different crosslinking density regions. Poisson’s ratios of polymeric materials could be changed by the crosslinking density of the polymer network and under high strain condition.27, 28 Under stretched condition, two regions with different Poisson’s ratio were differently deformed so that recovery could be disturbed. Therefore, it could be considered that these factors had influence on the recovery behavior of the patterned PSA. Also, as the applied strain was increased from 200% to 400%, the difference of strain recovery between non-patterned and patterned PSA was widened. When the applied strain was increased to 400%, both non-patterned and patterned PSA were largely deformed than strain of 200% and 300%. Especially, in the case of the patterned PSA, as the PSA was largely deformed, difference of the


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deformation behavior between highly cured and pre-cured region was increased due to their different Poisson’s ratio. This phenomenon could disturb the recovery of the patterned PSA so that the difference of strain recovery between non-patterned and patterned PSA was widened at strain of 400%.

400

200

100

(d) 80

Strain recovery (%)

300

Strain (%)

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

(a)

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20

0 0

5

10

Recovery time (sec) 6 300

0 600

15

800

1000

Time (min)

(b) Strain (%)

300

200

100

2000

1800

2000

1800

2000

(e)

0 10

60

40


0 600

15

800

1000

1200

1400

1600 2

Time (min)

UV dose (mJ/cm )

400

100

2

(c) 300

200

100

(f) 80

Strain recovery (%)

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

60

40

20

0 5

1800

80

20

0

1600

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Strain recovery (%)

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

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1400 2

2

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1200

UV dose (mJ/cm )

400

Strain (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

15


0 600

800

1000

1200

1400

1600 2

Time (min)

UV dose (mJ/cm )

Figure 6. Recovery behavior of PSA: Strain = (a) 200%, (b) 300%, and (c) 400%. Strain recovery of PSA: Strain = (d) 200%, (e) 300%, and (f) 400%. All samples were conducted once, not including repeat test.


15


2

* Strain recovery at 750 mJ/cm was 73%.

100


(a)

H L H L H L

H Non-patterned PSA

80

Strain recovery (%)


60

40

20

(b)

UV patterned PSA

0 900

Non-patterned PSA Patterned PSA 1200

1500

1800

2100

2

UV dose (mJ/cm )

2

2


100


100

80

80

Strain recovery (%)

Strain recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

0 900

(d)


1500

1800

2100

0 900

2


1500

1800

2100

2

UV dose (mJ/cm )

UV dose (mJ/cm )

Figure 7. Strain recovery of non-patterned and UV-patterned PSA: (a) Schematic illustration of non-patterned and UV-patterned PSA, strain = (b) 200%, (c) 300%, and (d) 400%. All samples were conducted once, not including repeat test. 3.5 Relaxation property of UV-patterned acrylic PSA The stress of viscoelastic materials is relaxed under the constant strain condition because of their dissipation property that originates from the mobility of the polymer chain. Figure 8 shows the relaxation behavior and stress relaxation of PSA. As high degree of UV irradiation was applied, stress relaxation was generally decreased. Likewise, in our previous work, stress relaxation of PSA was decreased with increase in crosslinking density of PSA and it was due to reduction in the mobility of the polymer chain.22 When the applied strain reached 400%, the maximum stress was increased but it was largely relaxed so that the applied strain of 400% had the highest stress


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relaxation. This suggests that excessively high strain caused enhancement of the dissipation of PSA because high strain facilitates movement of the polymer chain. 0.08

(d)

80

Stress relaxation (%)

0.06

Stress (MPa)

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

(a)

0.04

0.02

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0.00 0

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(e) 80


(b) Stress (MPa)

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UV dose (mJ/cm )

Time

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0.02

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(c) Stress (MPa)

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0.02

60

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0.00 0

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0 600

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1000

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1400 2

UV dose (mJ/cm )

Time (min)

Figure 8. Relaxation behavior of PSA: Strain = (a) 200%, (b) 300%, and (c) 400%. Stress relaxation of PSA: Strain = (d) 200%, (e) 300%, and (f) 400%. All samples were conducted once, not including repeat test.


17


2

* Stress relaxation at 750 mJ/cm was 62%.

100


(a)

H L H L H L

H Non-patterned PSA

80



60

40

20

(b)

UV patterned PSA

0 900


1500

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2100

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UV dose (mJ/cm )

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2


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)


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2

UV dose (mJ/cm )

UV dose (mJ/cm )

Figure 9. Stress relaxation of non-patterned and UV-patterned PSA: (a) Schematic illustration of non-patterned and UV-patterned PSA, strain = (b) 200%, (c) 300%, and (d) 400%. All samples were conducted once, not including repeat test. Figure 9 shows the difference of stress relaxation between non-patterned and UV-patterned PSA. By containing the pre-cured region (750 mJ/cm2), stress relaxation was significantly improved in patterned PSA than in non-patterned PSA because of the high dissipation property of the pre-cured region. As strain was increased from 200% to 400%, stress relaxation of patterned PSA was higher than that of the low crosslinking density region (750 mJ/cm2) and the difference of stress relaxation between non-patterned and patterned PSA was expanded. Similar to strain recovery, high strain promotes the differences of deformation behavior between highly cured and pre-cured region due to their different Poisson’s ratio. While this phenomenon disturb the strain recovery of the patterned PSA, it significantly improved stress relaxation of the patterned PSA.


18

Page 19 of 38

According to Figure 7 and Figure 9, by introducing patterning on the PSA, strain recovery was deteriorated but stress relaxation was improved and this effect was enhanced with increase in the applied strain. However, strain recovery was too deteriorated at strain of 400% and stress relaxation was not enhanced enough at strain of 200%. It shows that the optimization of the recovery and relaxation was only seen at 300% strain condition.

o

* G' = Storage modulus at 25 C (MPa)

o


100 y = 10.9x + 89.8 2 (R =0.70)

y = 32.0x + 78.1 2 (R =0.18)

40

20 Non-patterned PSA Patterned PSA -1.2

y = -28.1x + 61.2 2 (R =0.24)

80

60

0 -1.6

100


80

Strain recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.8

-0.4

0.0

0.4

60

40

y = -18.6x + 42.2 2 (R =0.65)

20

0 -1.6

Non-pattern 2 Pattern with 750 mJ/cm -1.2

-0.8

-0.4

log (G')

Log(G')

(a)

(b)

0.0

0.4

Figure 10. (a) Strain recovery and (b) stress relaxation storage modulus (Strain = 300%). All samples were conducted once, not including repeat test. As shown in Figure 10, we plotted the strain recovery and stress relaxation as a function of the storage modulus (G’). We found that the strain recovery of PSA has a positive linear relation with log(G’) and the stress relaxation of PSA has a negative linear relation with log(G’). In the case of non-patterned PSA, strain recovery and stress relaxation were changed with the storage modulus but these two properties could not be optimized by controlling the storage modulus. On the other hand, in the case of UV-patterned PSA, the trend lines of strain recovery and stress relaxation crossed each other, which means that recovery and relaxation could be optimized by UV patterning. Especially, at the range of log(G’) from -0.2 to -0.4, both of the strain recovery and the stress relaxation were well optimized above 71%. As this range of the log(G’) is inside of the range of


19


Page 20 of 38

general PSA’s log(G’), so called Dahlquist criterion and Chang’s window1, 29-31, it supported the possibility of industrial application of it. Additionally, a preliminary simulation of the stress distribution was performed, as shown in Figure 11. Type of the simulation is finite element analysis and we set the modulus of H (Highly cured region) and L (Pre-cured region) as 1 MPa and 0.7 MPa, respectively. At initial condition, strain and force were both zero and the strain was applied to 300%. The maximum and overall stress were lower for patterned PSA than for non-patterned PSA. This observation supports that patterned PSA has better dissipation property than non-patterned PSA. However, as the simulation did not consider many unknown factors such as interface between the patterned lines and different Poisson’s ratio of both lines, it was difficult to precisely explain the above results based on simulation.

(a)

L

H

L

H

L

H: Highly cured region L: Pre-cured region H H L H L L

H

(c) (b) Figure 11. Simulation of stress distribution under the constant strain (300%) condition: (a) Schematic illustration for simulation and stress distribution of (b) non-patterned PSA and (c) UV-patterned PSA. 4. Conclusions


20

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We used UV curable acrylic PSA that was used for assembly of display. The curing behavior, mechanical strength, adhesion performance, viscoelastic, recovery and relaxation property of the PSA were determined. PSA with high crosslinking density had improved elasticity and recovery but poor dissipation and stress relaxation than PSA with low crosslinking density. We found that the strain recovery of PSA had positive linear relation with log(G’) but the stress relaxation had negative linear relation with the same. Importantly, we found that the patterned PSA, containing both high and low crosslinking density, had lower strain recovery but higher stress relaxation than non-patterned PSA. Therefore, while the strain recovery and stress relaxation of non-patterned PSA could not be optimized by varying the storage modulus, those of the patterned PSA could be well optimized. Although we suggest patterning of PSA as an optimization method for recovery and relaxation in this paper, there are many other factors including pattern width, strain-pattern angle, chemical composition of PSA, etc., that need to be considered. In further research, it is necessary to investigate these factors for better optimization of the recovery and relaxation of PSA.

Acknowledgements Funding: This work was financially supported by Samsung Display Co., Ltd. There are no conflicts of interest to declare.

References (1) Satas, D., Handbook of Pressure Sensitive Adhesive Technology. 3rd ed. Van Nostrand Reinhold New York 1999. (2) Benedek, I., Pressure-Sensitive Adhesives and Applications. CRC Press 2004.


21


Page 22 of 38

(3) Czech, Z., Synthesis and cross-linking of acrylic PSA systems. J. Adhes. Sci. Technol. 2007, 21, 625. (4) Czech, Z.; Pełech, R., The thermal degradation of acrylic pressure-sensitive adhesives based on butyl acrylate and acrylic acid. Prog. Org. Coat. 2009, 65, 84. (5)Lee, S.-W.; Lee, T.-H.; Park, J.-W.; Park, C.-H.; Kim, H.-J.; Song, J.-Y.; Lee, J.-H., Curing Behaviors of UV-Curable Temporary Adhesives for a 3D Multichip Package Process. J. Electron. Mater. 2014, 43, 4246. (6) Czech, Z.; Kowalczyk, A.; Kabatc, J.; Świderska, J., UV-crosslinkable acrylic pressuresensitive adhesives for industrial application. Polym. Bull. 2012, 69, 71. (7) Czech, Z., Development of solvent-free pressure-sensitive adhesive acrylics. Int. J. Adhes. Adhes. 2004, 24, 119. (8) Tan, H. S.; Pfister, W. R., Pressure-sensitive adhesives for transdermal drug delivery systems. Pharm. Sci. Technol. Today 1999, 2, 60. (9) Townsend, B. W.; Ohanehi, D. C.; Dillard, D. A.; Austin, S. R.; Salmon, F.; Gagnon, D. R., Characterizing acrylic foam pressure sensitive adhesive tapes for structural glazing applications— Part I: DMA and ramp-to-fail results. Int. J. Adhes. Adhes. 2011, 31, 639. (10) Czech, Z., Solvent-based pressure-sensitive adhesives for removable products. Int. J. Adhes. Adhes. 2006, 26, 414. (11) Park, C.-H.; Lee, S.-J.; Lee, T.-H.; Kim, H.-J., Characterization of an acrylic pressuresensitive adhesive blended with hydrophilic monomer exposed to hygrothermal aging: Assigning cloud point resistance as an optically clear adhesive for a touch screen panel. Reac. Funct. Polym. 2016, 100, 130.


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(12) Lee, J.-G.; Shim, G.-S.; Park, J.-W.; Kim, H.-J.; Han, K.-Y., Kinetic and mechanical properties of dual curable adhesives for display bonding process. Int. J. Adhes. Adhes. 2016, 70, 249. (13) Chang, E.; Holguin, D., Curable optically clear pressure-sensitive adhesives. J. Adhes. 2005, 81, 495. (14) Wang, T.; Lei, C. H.; Dalton, A. B.; Creton, C.; Lin, Y.; Fernando, K. S.; Sun, Y. P.; Manea, M.; Asua, J. M.; Keddie, J. L., Waterborne, nanocomposite pressure‐sensitive adhesives with high tack energy, optical transparency, and electrical conductivity. Adv. Mater. 2006, 18, 2730. (15) Ahn, B. K.; Kraft, S.; Wang, D.; Sun, X. S., Thermally stable, transparent, pressure-sensitive adhesives from epoxidized and dihydroxyl soybean oil. Biomacromolecules 2011, 12, 1839. (16) Li, H. U.; Jackson, T. N., Flexibility testing strategies and apparatus for flexible electronics. IEEE Trans. Electron Devices 2016, 63, 1934. (17) Paik, K. W.; Kim, J.-H.; Kim, Y. R. In Flexibility of anisotropic conductive films (ACFS) bonded CIF (Chip In Flex) package for wearable electronics applications, Proc. Tech. Program Pan Pac. Microelectron. Symp. 2016, 1. (18) Lu, S.-T.; Chen, W.-H., Reliability and flexibility of ultra-thin chip-on-flex (UTCOF) interconnects with anisotropic conductive adhesive (ACA) joints. IEEE Trans. Adv. Packag. 2010, 33, 702. (19) Salmon, F.; Everaerts, A.; Campbell, C.; Pennington, B.; Erdogan-Haug, B.; Caldwell, G. In 64‐1: Modeling the Mechanical Performance of a Foldable Display Panel Bonded by 3M Optically Clear Adhesives, SID Int. Symp. Dig. Tech. Pap. 2017, 48, 938.


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(20) Lee, J.-H.; Lee, T.-H.; Shim, K.-S.; Park, J.-W.; Kim, H.-J.; Kim, Y., Adhesion performance and recovery of platinum catalyzed silicone PSAs under various temperature conditions for flexible display applications. Mater. Lett. 2017, 208, 86. (21) Lee, J.-H.; Lee, T.-H.; Shim, K.-S.; Park, J.-W.; Kim, H.-J.; Kim, Y.; Jung, S., Molecular weight and crosslinking on the adhesion performance and flexibility of acrylic PSAs. J. Adhes. Sci. Technol. 2016, 30, 2316. (22) Lee, J.-H.; Lee, T.-H.; Shim, K.-S.; Park, J.-W.; Kim, H.-J.; Kim, Y.; Jung, S., Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications. Int. J. Adhes. Adhes. 2017, 74, 137. (23) Chen, C.-J.; Lin, K.-L., Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum. Thin Solid Films 2000, 370, 106. (24) Zhang, X.; Ding, Y.; Zhang, G.; Li, L.; Yan, Y., Preparation and rheological studies on the solvent based acrylic pressure sensitive adhesives with different crosslinking density. Int. J. Adhes. Adhes. 2011, 31, 760. (25) Czech, Z.; Wesolowska, M., Development of solvent-free acrylic pressure-sensitive adhesives. Eur. Polym. J. 2007, 43, 3604. (26) Czech, Z.; Milker, R., Solvent‐free radiation‐curable polyacrylate pressure‐sensitive adhesive systems. J. Appl. Polym. Sci. 2003, 87, 182. (27) Shokuhfar, A.; Arab, B., The effect of cross linking density on the mechanical properties and structure of the epoxy polymers: molecular dynamics simulation. J. Mol. Model. 2013, 19, 3719. (28) Ren, W.; McMullan, P. J.; Griffin, A. C., Stress–strain behavior in main chain liquid crystalline elastomers: effect of crosslinking density and transverse rod incorporation on “Poisson's ratio”. Phys. Status Solidi B 2009, 246, 2124.


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(29) Chang, E., Viscoelastic windows of pressure-sensitive adhesives. J. Adhes. 1991, 34, 189. (30) Chang, E., Viscoelastic properties of pressure-sensitive adhesives. J. Adhes. 1997, 60, 233. (31) Gdalin, B. E.; Bermesheva, E. V.; Shandryuk, G. A.; Feldstein, M. M., Effect of temperature on probe tack adhesion: extension of the dahlquist criterion of tack. J. Adhes. 2011, 87, 111.


25


Page 26 of 38

For Table of Contents Only Patterned masking film

H

UV PSA PET film

H H

L L

L

Removing masking film H L H L UV patterned PSA

H: High crosslinking density L: Low crosslinking density


26

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41



Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Page 28 of 38

Shear strain

PSA (50 µm thickness)

PC

(a) S0

St (t=6 sec) Strain (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


St (t=300 sec) 0

5

10

15

Time (min)

(b) 0.20

0.15

Stress (MPa)

Page 29 of 38

Fi (Initial stress) 0.10

Ff (Final stress)

0.05

0.00 0

5

10

Time (min)

(c) ACS Paragon Plus Environment

15


0.10

2

UV dose (mJ/cm ) 0 250 500 750 1000 1250 1500 2000

Conversion (%)

Absorbance

0.15

0.05

100

100

80

80

60

60

40

40

20

0.00 830

20 Conversion Gel fraction

0

820

810

800

0

500

1000

1500 2

UV dose (mJ/cm )

-1

Wavenumber (cm )

(a)

(b)


2000

0

Gel fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 30 of 38

Page 31 of 38

3

0.2

1

Peel strength (N/25 mm)

2

12

20 8 15

10 4 5

Lap shear strength Strain at break 0.0 500

Peel strength Probe tack 0

1000

1500

2000

2500

3000

0 500

0 1000

1500

2000

2500 2

2

UV dose (mJ/cm )

UV dose (mJ/cm )

(a)

(b)


3000

Probe tack (N)

0.4

25

4

Lap shear strength (MPa)

0.6

Strain at break (10 %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41



4

10

10

2

10

1

10

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

1.5

Tan 


2.0

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

3

1.0

0

10

0.5 -1

10

-2

0.0 220

10

220

260

300

340

380

260

(a) 10

2

2

1

10

UV dose (mJ/cm ) 1000 1250 1500 1750 2000

1.5

Tan 

10

380

2.0

2

UV dose (mJ/cm ) 1000 1250 1500 1750 2000

3

340

(b)

4

10

300

Temperature (K)

Temperature (K)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 32 of 38

1.0

0

10

0.5 -1

10

-2

10

220

260

300

340

380

0.0 220

Temperature (K)

260

300

Temperature (K)

(c)

(d)


340

380

Page 33 of 38

400

200

100

(d) 80

Strain recovery (%)

(a) Strain (%)

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

300

60

40

20

0 0

5

10


0 600

15

800

1000

Strain (%)

200

100

2000

1800

2000

1800

2000

(e) 60

40

20

0 10


0 600

15

800

1000

1200

1400

1600 2

Time (min)

UV dose (mJ/cm )

400

100

2

300

200

100

(f) 80

Strain recovery (%)

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

(c)

60

40

20

0 5

1800

80

Strain recovery (%)

(b) 300

0

1600

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

5

1400

UV dose (mJ/cm )

400

0

1200

2

Time (min)

Strain (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


10

15


0 600

800

1000

1200

1400

1600 2

Time (min)

UV dose (mJ/cm )



2


100

Strain recovery (%)

80

60

40

20

(a)

(b) 0 900


1500

1800

2100

2

UV dose (mJ/cm )

2

2


100


100

80

80

Strain recovery (%)

Strain recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 34 of 38

60

40

20

60

40

20

(c) 0 900

(d)


1500

1800

2100

0 900

2


1500

1800 2

UV dose (mJ/cm )

UV dose (mJ/cm )


2100

Page 35 of 38

0.08

(d) 80


0.06

Stress (MPa)

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

(a)

0.04

0.02

60

40

20

0 600

0.00 0

5

10

15

800

1000

1200

1400

1600

1800

2000

1600

1800

2000

1600

1800

2000

2

UV dose (mJ/cm )

Time

0.08

(e) 80


(b) Stress (MPa)

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

0.06

0.04

0.02

60

40

20

0 600

0.00 0

5

10

15

800

1000

1200

1400 2

UV dose (mJ/cm )

Time (min)

0.08

100

2

UV dose (mJ/cm ) 750 1000 1250 1500 1750 2000

(f) 80


(c) 0.06

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


0.04

0.02

60

40

20

0.00 0

5

10

15

0 600

800

1000

1200

1400 2

UV dose (mJ/cm )

Time (min)



2


100


80

60

40

20

(a)

(b) 0 900


1500

1800

2100

2

UV dose (mJ/cm )

2

2


100


100

80


80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 36 of 38

60

40

60

40

20

20

(c) 0 900

(d)


1500

1800

2100

0 900


1500

1800 2

2

UV dose (mJ/cm )

UV dose (mJ/cm )


2100

Page 37 of 38

o

o


100 y = 10.9x + 89.8 2 (R =0.70)

y = -28.1x + 61.2 2 (R =0.24)

80

60 y = 32.0x + 78.1 2 (R =0.18)

40

60

40

y = -18.6x + 42.2 2 (R =0.65)

20

20

Non-pattern 2 Pattern with 750 mJ/cm

Non-patterned PSA Patterned PSA 0 -1.6


100


80

Strain recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


-1.2

-0.8

-0.4

0.0

0.4

0 -1.6

-1.2

-0.8

-0.4

log (G')

Log(G')

(a)

(b)


0.0

0.4

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 38 of 38

(a)

L

H

L

H

L

H: Highly cured region L: Pre-cured region

H H L H L L

H

(b)

(c)


Optimization of Recovery and Relaxation of Acrylic Pressure-Sensitive

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