Photoinitiated Polymerization - American Chemical Society

Chapter 26

Photocurable Pressure-Sensitive Adhesives Using Alkyl Oxetane

Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch026

Hiroshi Sasaki Corporate Research Lab, Toagosei Company Ltd., 1-1 Funami-cho, Minato-ku, Nagoya-shi, Aichi 455-0027, Japan

As cationically photopolymerizable monomers, oxetanes have been shown to possess many good properties, such as safety (AMES Test negative) and high reactivity. In addition to these properties, alkyl oxetanes, having alkyl side chains, exhibited high compatibility with other monomers and very low viscosities. Using 3-ethyl-3-(2-ethylhexyloxy)methyl oxetane (EHOX), Photocurable PSA formulations in combination with epoxy monomers and a hydrogenated petroleum resin as a tackifier were investigated. The adhesion properties of the cured PSA sheets were evaluated and correlated with the results of viscoelastic measurements. With the aid of EHOX, uniform and clear PSA formulations with low viscosity and high reactivity were achieved. The cured PSA sheets were shown to possess good adhesive properties. Substitution of EHOX with a aliphatic mono-epoxide (UVR-6216) resulted in poorly reactive formulations and poor performance as cured PSA sheets. With aid of a cycloaliphatic diepoxide (UVR6110), the heat resistance properties of the cured PSA sheets were drastically improved.

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© 2003 American Chemical Society

Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Pressure sensitive adhesives (PSAs) are widely used not only in industrial applications but also in daily life, such as packaging, tapes, labels and so on, because of their excellent characteristics - easy to stick to many kind of substrates without any activation and easy to peel. Among the many types of PSAs, solvent- or water-borne systems are most popular. With increased environmental pressures, replacement of solvent-born systems is becoming gradually more important and solvent-free systems are highly desired. To achieve a solvent-free system, UV-curable PSAs have been proposed as environmental-friendly technologies. UV-curable acrylic monomer systems, that polymerize through radical polymerization has been mainly evaluated (i). As oxygen is known to inhibit the radical polymerization, nitrogen atmosphere or lamination should be applied to complete the radical polymerization of acrylic monomers. At the same time, the residual unreacted monomers are known to cause an odor problem and skin irritation (2). To solve this problem with acrylic radical systems, photoinitiated cationic polymerization systems have been introduced. For example», a cationically curable PSA formulation consisting of a rather low molecular weight heterotelechelic-polymer and a tackifier was introduced (3). Although the UV-cured PSA sheet of this formulation was reported to possess good adhesive properties, some problems seem to remain. The viscosity of the formulation is rather high and temperatures greater than 80°C are required when applying the coating to substrates. The formulation range is also narrow due to the low compatibility of the polymers used. These problems could be a barrier to the industrial applications of this system. Oxetanes have been shown to possess many good properties, such as safety (AMES Test negative) and high reactivity (4). In addition to these properties, alkyl oxetanes, having alkyl side chains, exhibit high compatibility with other monomers and very low viscosity. Using alkyl oxetanes, low viscosity and highly reactive UV-curable PSA formulation seem to be possible. In this paper, UV-curable PSA formulations using 3-ethyl-3-(2ethylhexyloxy)methyl oxetane (EHOX) in combination with the monomers listed on Scheme 1 were investigated and the adhesive properties of the cured PSA sheets correlated with their viscoelastic character.

Experimental Materials and experimental conditions used in this study are listed below.

Materials.

All materials listed below were used as received. Monomers used in this study were shown in Scheme 1.


298


•

EHOX: 3-Ethyl-3-(2-ethylhexyloxymethyl)oxetane from Toagosei Co. Ltd.

•

UVR-6216: 1,2-Epoxy-hexadecane from Union Carbide Co.

•

EKP-207: Heterotelechelic linear polymer from KRATON Polymers.

• •

UVR-6110: Cycloaliphatic diepoxidefromUnion Carbide Co. Rhodosil 2074: Iodonium salt cationic photoinitiator from Rhodia Co.

•

P-90: Hydrogenated petroleum (C-5) resin (softening point is 90°C) from Arakawa Chemical Ind.

EHOX

UVR-6216

Scheme 1. Monomers used in this study

Sample Formulations. The PSA formulation was prepared by mixing monomers, photoinitiator, and tackifier in the desired composition at 40°C. The viscosity of formulations was measured by Brookfield viscometer at 25°C.

Photo-DSC measurement. Heat of polymerization during cationic photopolymerization was measured by DSC220C (Seiko Instruments Inc.) equipped with a UV-1 lighting unit (200 W Hg-Xe lamp, Seiko Instruments Inc.). Samples (2 to 3 mg) were applied to an aluminum pan and irradiated with U V light under an air atmosphere. The light intensity was adjusted to 20 mW/cm at 365nm using ND filter. 2


299

Viscoelasticity Measurements. A lmm-thickness PSA formulation was cured using a UV irradiator equipped with 120 w/cm of high pressure Hg lamp for 1 minute. The total irradiated UV energy under this condition was 2.85 J/cm . The viscoelasticity of the cured PSA sample was measured by the parallel plate method using a dynamic mechanical spectrometer (RDS Π: Rheometrics Inc.). The modulus dependency versusfrequencywas measured in the frequency range of 0.1 to 10 rad/sec at 25, 15, 5 and -5°C respectively. A master curve (25°C, 0.1 to 100 rad/sec.) for the modulus versus frequency was made by a horizontal shift of these curves according to the shift factor derived from WLF equation. Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch026

2

Adhesion properties. The PSA formulation was coated onto 50 μηι PET sheet (25 μηι thickness) using an applicator. The PSA sheet was prepared by curing the coated liquid formulation using a conveyer type UV irradiator equipped with 120 W/cm of high pressure Hg lamp at a conveyer speed of 10 m/min. The total irradiated UV energy was 135 mJ/cm . The adhesion properties of the PSA sheet, such as peel adhesion (180°) to stainless steel (SUS) or polyethylene (PE) at 25°C and a holding power at 40°C were measured according to JIS-Z0237. The probe tack at 25°C was measured according to ASTM D-2979. Conditions for determination of the shear adhesion failure temperature (SAFT) were: 25mm X 25mm contact area, 500g load, heating rate, 0.4°C/min. 2

Results and Discussion The properties of the PSA formulations using EHOX, EKP-207, P-90 (tackifier) and Rhodosil 2074 (photoinitiator) are shown in Table I. The compatibility of each component for these formulations was good and clear solutions were obtained. The properties of a PSA are known to depend primarily on the viscoelastic nature of the adhesive mass (5). As the rate of deformation of a PSA during bonding is considered to be low, the modulus at low frequencies should be depressed, so that the composition is soft enough to flow and wet the substrate in a short time. On the other hand, during peeling, the deformation rate is high


300


because the thickness of the adhesive is low. The modulus of a PSA should be elevated to maintain the adhesive layer at a high testing rate. In the formulation of ordinary PSAs, an elastomer or rubbery polymer provides the elastic component, while a tackifier constitutes the viscous component. To function properly, a tackifier must be reasonably compatible with the base polymer. If the compatibility is low, at higher amounts of resin, the modulus of a PSA formulation increases across the entire frequency range of 0.1 to 100 rad/sec. For compatible formulations, as the concentration increases, the modulus is depressed at low frequencies and increases in the higher frequency region.

Table I. PSA Formulations and Their Adhesive Properties RM

EHOX UVR-6216 EKP-207 Rhodorsil 2074 P-90 Viscosity * (cps) 180°Peel SUS (g/inch) ΡΕ Holding Power (hrs) 1

2)

3)

S

A

F

T

4)

(

o

C )

5)

Probe Tack (gf)

I 80 20 1 60 390 220 40 >24 164 579

2 80 20 1 80 760 620 160 >24 126 665

3 80 20 1 100 1480 1050 370 >24 117 490

4

80 20 1 120 2870 1210 618 >24 109 371

5 60 20 20 1 120 2400 680 360 >24 91 320

6

40 40 20 1 120 2020 450 460 >24 80 345

7 80 20 1 120 1470 J) _6) _6) _6) _6)

1) Measured at 25°C 2) 25μηι coating was cured with 120 W/cm high pressure Hg Lamp at 10 m/min. conveyor speed and measured according to JIS Z-0237 3) Measured at 40°C according to JIS Z-0237 4) 500g of load and heating rate 0.4°C/min. 5) Measured according to ASTM D-2979 6) Not measured due to insufficient cure (R.N.1 to 4) The viscosity of the formulations increased with the addition of P-90. Using EHOX, the viscosity of the formulations containing up to 120 phr of P-90 was still low. The storage modulus (G') and loss modulus (G") plots against frequency (0.1 to 100 rad/sec.) for the formulations are shown in Figure 1. With higher amounts of P-90, the storage modulus was depressed at low frequencies and increased at high frequencies, which is due to the good compatibility of the tackifier. The loss modulus increased across the entire range. Tan δ versus frequency plots for the formulations are shown in Figure 2. The value of the Tan δ increased with addition of P-90. These high values of Tan δ should contribute to the stress relaxation during bonding and peeling.


301

ce Ρ*

ta Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch026

ο

0.1

1 10 Frequency (rad/sec.)

100

f

Figure 1. Storage (G ) and loss (G") modulus plot for R.N.I to 4

ce

0.1

1 10 Frequency (rad/sec.)

100

Figure 2. Tan δ plot for R.N. 1 to 4


302


In the formulation of PSAs, the adhesion properties are known to be optimized by the quantity of tackifier used (5). To some extent, the peel strength and tack increases as the content of tackifier is increased, and reaches a maximum. The optimum content of tackifier for tack is lower than that for peel strength. The peel strength increased with the addition of P-90, and the probe tack reached a maximum at 80 phr. On the other hand, the SAFT was reduced with higher amounts of P-90. This drop can be explained by a decrease in the storage modulus at low frequencies. (R.N.4 to 7) With the addition of UVR-6216, the viscosity of the formulations was reduced. Complete substitution of EHOX with UVR-6216 resulted in poor photopolymerization (R.N.7). The heat of polymerization of formulations was measured using photo-DSC and the results were shown in Figure 3. In the cationic photopolymerization of R.N.4, a higher exotherm was observed in the early stages of the irradiation and decreased smoothly after the peak. This smooth decrease can be explained by the fast propagation rate


303


because of the high basicity of oxetane ring. With the addition of UVR-6216 to EHOX, the peak height was reduced (R.N.5 and 6). A smaller exotherm was seen for R.N.7 and the heat generation lasted during the entire period of irradiation. This long heat flow could be explained by the slow polymerization rate of the oxirane ring. As the content of UVR-6216 is increased, both the storage modulus (G') and loss modulus (G") was depressed across the entire frequency range (Figure 4). This decrease in modulus could be explained by the presence of residual monomer, which can work as a softener, due to low reactivity of UVR-6216. All the adhesive properties were also depressed with a high content of UVR-6216.

J3 *3 •a ο

0.1

1

10

100

Frequency (rad/sec.)

Figure 4. Storage (C) and loss (G") modulus plot for R.N.4 to 6 Table II shows the formulations with cycloaliphatic diepoxide (UVR-6110) and adhesion properties of the cured PSA sheets. The heat resistance improved remarkably (high SAFT) with the addition of 2 wt% of UVR-6110. On the other hand, the 180° peel strength and probe tack were reduced slightly as the amount of UVR-6110 was increased. Figure 5 shows the storage modulus (G') of the cured PSA layer with 4% of UVR-6110 and without it as a function of temperature. Without UVR-6110, a steep decrease of the storage modulus around 80°C was observed, while the G' curves for the cured PSA layer with UVR-6110 showed a long plateau zone extending to higher temperatures. The high SAFT value for the PSA layer with UVR-6110 can be explained by this high modulus at elevated temperatures.


304 Table IL PSA Formulations using UVR-6110 and Their Adhesive Properties 1 }

R.N.

1

2)


9 78 20 2 1 110 2250 1020 464 202 467

8

EHOX EKP-207 UVR-6110 2074 P-90 Viscosity * (cps) 180°Peel SUS (g/inch) PE SAFT (°C) Probe Tack (gf) J)

4)

80 20

1 110 2050 1190 510 109 510

10

11

76 20 4 1 110 2480 854 327 >205 510

74 20 6 1 110 2630 651 102 >205 570

1) Conditions and procedures are the same as in Table I.

10'°

10» ce

ïio

8

XJ O

S

7

Ά 10

5

10

-50

0

50

100

Temp. (°C) Figure 5. Storage Modulus

curves ofR.N.S

and 10

Conclusions Using the monofunctional alkyl oxetane, EHOX, Photocurable PSA formulations in combination with epoxy monomers and a hydrogenated petroleum resin as tackifier were investigated. The viscoelasticity and adhesion


305 properties of the cured PSA formulations were also evaluated. The characteristics are listed below. • • •


•

A wide variety of formulations was achieved using EHOX possessing an excellent compatibility with other monomers. With aid of EHOX, uniform and clear PSA formulations with low viscosity and good reactivity were achieved. The cured PSA sheets of the formulation were proved to possess good adhesive properties. The use of à cycloaliphatic diepoxide improved the heat resistance properties of the cured PSA sheet.

Due to these superior characteristics, oxetanes should contribute to expand the possibility of UV curable PSAs.

References 1. 2. 3. 4. 5.

Zollner, S., RadTech Europe, 1999, 543 Barnes, C.E., J. Am. Chem. Soc., 1945, 67, 217 Erickson, J.; Zimmermann, E.; Southwick, J.; Kiibler, K., Adhesive age, November, 1995, 18 Sasaki, H., RadTech North America, 2000, 61 Butler, G.L., Natural Rubber. Hand book of PSA technology 2nd ed., V A N NOSTRAND RHEINHOLD, 1989, 396 - 456


Photoinitiated Polymerization - American Chemical Society

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