IMPACT ABSORPTION SHEET AND DOUBLE-SIDED PRESSURE-SENSITIVE ADHESIVE SHEET

Abstract

The present invention aims to provide an impact-absorbing sheet having high impact resistance and excellent resistance against sebum. The present invention also aims to provide a double-sided adhesive sheet including the impact-absorbing sheet. Provided is an impact-absorbing sheet including an impact-absorbing layer, the impact-absorbing layer having a maximum value of loss tangent tan δ of 0.7 or more at a frequency of 1.0×10.sup.3 to 1.0×10.sup.6.5 Hz at 23° C. and having a degree of crystallinity of 2% or higher.

Claims

1. An impact-absorbing sheet comprising an impact-absorbing layer, the impact-absorbing layer having a maximum value of loss tangent tan δ of 0.7 or more at a frequency of 1.0×10.sup.3 to 1.0×10.sup.6.5 Hz at 23° C. and having a degree of crystallinity of 2% or higher.

2. The impact-absorbing sheet according to claim 1, wherein the impact-absorbing layer contains an olefin elastomer having a crystalline structure.

3. The impact-absorbing sheet according to claim 1, wherein the impact-absorbing layer has an OD value of 7 or less.

4. The impact-absorbing sheet according to claim 1, further comprising an outer layer integrally laminated on at least one surface of the impact-absorbing layer, wherein the outer layer has a tensile modulus of elasticity of 200 MPa or more.

5. The impact-absorbing sheet according to claim 4, wherein the outer layer and the impact-absorbing layer have a difference in solubility parameter (SP value) of 2 or less.

6. A double-sided adhesive sheet comprising: the impact-absorbing sheet according to claim 1; and an adhesive layer integrally laminated on both surfaces of the impact-absorbing sheet.

7. The double-sided adhesive sheet according to claim 6, wherein the impact-absorbing layer has a 25% compressive strength of 930 kPa or less.

8. The double-sided adhesive sheet according to claim 6, which is used for bonding and fixing a component of a portable electronic device to a device body.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0120] FIG. 1 is a schematic view illustrating a drop impact test of double-sided adhesive sheets obtained in examples and comparative examples.

DESCRIPTION OF EMBODIMENTS

[0121] The present invention is more specifically described in the following with reference to, but not limited to, examples.

EXAMPLE 1

(1) Preparation of Impact-Absorbing Sheet

[0122] An amount of 100 parts by weight of an olefin crystal-ethylene-butylene-olefin crystal (CEBC) block polymer (DYNARON 6200 available from JSR Corporation) and 3 parts by weight of carbon black were used as materials to constitute an impact-absorbing layer. Low density polyethylene (LDPE) (PE in Table 1) was used as a material to constitute outer layers.

[0123] The materials to constitute an impact-absorbing layer and the material to constitute outer layers were melted at 200° C. These molten resins were laminated in a multilayer die while being extruded (co-extruding temperature: 200° C.) The extrudate was then cooled to give an impact-absorbing sheet in which an outer layer having a thickness of 10 μm was laminated on each side of a non-foam impact-absorbing layer having a thickness of 60 μm.

[0124] The impact-absorbing layer was cut to 5 mm×30 mm and chucked at the longitudinal sides (30 mm sides) at a chuck gap of 15 mm in a dynamic viscoelasticity measuring apparatus (Rheogel-E4000 available from UBM). The tensile viscoelastic modulus was measured within the range of −60° C. to 100° C. at a temperature increase rate of 5° C./min, and a master curve was established at a reference temperature of 23° C. to calculate the maximum value of loss tangent tan δ at a frequency of 1.0×10.sup.3 to 1.0×10.sup.6.5 Hz at 23° C. The frequency at which the local maximum of the loss tangent tan δ occurred was also determined.

[0125] Separately, the impact-absorbing layer was cut to 30 mm×30 mm. The impact-absorbing layer was irradiated with X-rays using an X-ray diffraction device (SmartLab available from Rigaku Corp.). In the obtained diffraction data (diffraction profile), a scattering region derived from an amorphous portion and a scattering region derived from a crystalline portion were separated. The degree of crystallinity was calculated as the ratio of the integrated crystalline scattering intensity to the total integrated scattering intensity.

[0126] The transmittance was also measured with a haze meter (NDH4000 available from Nippon Denshoku Industries Co., Ltd.) and the OD value of the impact-absorbing layer was calculated.

[0127] The SP values of the impact-absorbing layer and the outer layers were calculated based on the constitutional units of the polymers constituting the impact-absorbing layer and the outer layers and the Fedor's equation.

[0128] The tensile moduli of elasticity of the outer layers and the impact-absorbing sheet were calculated according to the ASTM D638 method.

(2) Preparation of Adhesive A

[0129] A reaction vessel was charged with ethyl acetate as a polymerization solvent and the ethyl acetate was bubbled with nitrogen. The reaction vessel was heated while nitrogen was flowed thereinto, thereby starting reflux. Subsequently, to the reaction vessel was added a polymerization initiator solution prepared by diluting 0.1 parts by weight of azobisisobutyronitrile, as a polymerization initiator, 10 times with ethyl acetate. Then, 63.5 parts by weight of butyl acrylate, 33.5 parts by weight of 2,2,2-trifluoroethyl acrylate, and 3 parts by weight of acrylic acid were dropwise added over two hours. After the dropwise addition, the polymerization initiator solution prepared by diluting 0.1 parts by weight of azobisisobutyronitrile, as a polymerization initiator, 10 times with ethyl acetate was again added to the reaction vessel, and the polymerization reaction was allowed to proceed for four hours. Thus, a (meth)acrylate copolymer-containing solution was obtained.

[0130] To the obtained (meth)acrylate copolymer-containing solution was added TETRAD-C (available from Mitsubishi Gas Chemical Company), as a crosslinking agent, in an amount of 1 part by weight relative to 100 parts by weight of the (meth)acrylate copolymer. Thus, an adhesive A was obtained.

(3) Production of Double-Sided Adhesive Sheet

[0131] The adhesive A was applied with an applicator to a 75-μm-thick release PET film release-treated with silicon. The adhesive A was dried at 110° C. for three minutes to form an adhesive layer having a thickness of 35 μm. This adhesive layer was bonded to the impact-absorbing sheet using a silicon roller to give a one-side adhesive sheet. Both surfaces of the impact-absorbing sheet were corona-treated in advance with a corona treatment device (“CT-0212” available from Kasuga Denki, Inc.) under the conditions of 270 W and 18 m/min.

[0132] In the same manner, the release PET film on the opposite surface of the impact-absorbing sheet was removed, and the same adhesive layer as above was bonded to the surface. The adhesive layers were then aged at 40° C. for 48 hours. Thus, a double-sided adhesive sheet in which each surface was covered with a release PET film was obtained.

(EXAMPLE 2

[0133] A double-sided adhesive sheet was obtained as in Example 1 except that the CEBC block polymer and a styrene-ethylene-propylene-styrene (SEPS) block copolymer (SEPTON 2063 available from Kuraray Co., Ltd.) were used (CEBC:SEPS ratio=8:2) in the impact-absorbing layer instead of the CEBC block polymer alone.

EXAMPLE 3

[0134] A double-sided adhesive sheet was obtained as in Example 2 except that the CEBC:SEPS ratio was 6:4.

EXAMPLE 4

[0135] A double-sided adhesive sheet was obtained as in Example 2 except that the CEBC:SEPS ratio was 4:6.

EXAMPLE 5

[0136] A double-sided adhesive sheet was obtained as in

[0137] Example 1 except that polypropylene (PP) was used in the outer layers.

EXAMPLE 6

[0138] A double-sided adhesive sheet was obtained as in Example 1 except that the amount of carbon black was changed to 1 part by weight relative to 100 parts by weight of the block polymer so as to adjust the OD value to 2.5.

EXAMPLE 7

[0139] A double-sided adhesive sheet was obtained as in Example 1 except that the amount of carbon black was changed to 6 parts by weight relative to 100 parts by weight of the block polymer so as to adjust the OD value to 6.6.

EXAMPLE 8

[0140] A double-sided adhesive sheet was obtained as in Example 1 except that in the preparation of the adhesive, the monomers added dropwise were changed to 23.5 parts by weight of butyl acrylate, 23.5 parts by weight of ethyl acrylate, 50 parts by weight of 2,2,2-trifluoroethyl acrylate, and 3 parts by weight of acrylic acid to give an adhesive B.

EXAMPLE 9

[0141] A double-sided adhesive sheet was obtained as in Example 2 except that the adhesive B was used instead of the adhesive A.

EXAMPLE 10

[0142] A double-sided adhesive sheet was obtained as in Example 1 except that a single layer was melt-extruded using only the materials to constitute the impact-absorbing layer, and that no outer layer was formed.

EXAMPLE 11

[0143] A double-sided adhesive sheet was obtained as in Example 1 except that a softener (polybutene, number average molecular weight: 1,350) was added (70 parts by weight relative to 30 parts by weight of CEBC) to the impact-absorbing layer.

EXAMPLE 12

[0144] A double-sided adhesive sheet was obtained as in Example 11 except that the amount of the softener added was changed (50 parts by weight to 50 parts by weight of CEBC).

EXAMPLE 13

[0145] A double-sided adhesive sheet was obtained as in Example 12 except that the softener was changed to a petroleum softener (paraffin oil) (Diana Process Oil PW90, available from Idemitsu Kosan Co., Ltd.).

COMPARATIVE EXAMPLE 1

[0146] A double-sided adhesive sheet was obtained as in Example 1 except that a styrene-ethylene-propylene-styrene (SEPS) block copolymer was used in the impact-absorbing layer instead of the CEBC block polymer (CEBC:SEPS ratio=0:10).

COMPARATIVE EXAMPLE 2

[0147] A double-sided adhesive sheet was obtained as in Example 2 except that the CEBC:SEPS ratio was 2:8.

COMPARATIVE EXAMPLE 3

[0148] A double-sided adhesive sheet was obtained as in Example 1 except that a polyethylene terephthalate (PET) film (E5100 available from Toyobo Co., Ltd., thickness: 75 μm) was used as the impact-absorbing sheet.

<Evaluation>

[0149] The following evaluations were performed on the double-sided adhesive sheets obtained in the examples and the comparative examples. Table 1 shows the results.

(1) Drop Impact Test

<Preparation of Test Device>

[0150] FIG. 1 is a schematic view illustrating a drop impact test of the double-sided adhesive sheets obtained in the examples and the comparative examples. A piece having outer dimensions of 46 mm wide and 61 mm long and inner dimensions of 44 mm wide and 59 mm long was punched out of the obtained double-sided adhesive sheet to prepare a frame-shaped specimen having a width of 1 mm. Next, as shown in FIG. 1(a), the specimen 41, with the release paper removed, was attached to a 2-mm-thick polycarbonate plate 43 having a square opening of 38 mm wide and 50 mm long in its center portion. The specimen 41 was attached such that the square opening was positioned substantially at the center. A polycarbonate plate 42 of 55 mm wide, 65 mm long, and 1 mm thick was then attached from above the specimen 41 such that the specimen 41 was positioned substantially at the center. A test device was thus assembled. A pressure of 5 kgf was then applied for 10 seconds from the side of the upper polycarbonate plate of the test device, whereby the upper and lower polycarbonate plates and the specimen were pressure-bonded. The workpiece was left to stand at room temperature for 24 hours.

<Evaluation on Drop Impact Resistance>

[0151] As shown in FIG. 1(b), the test device prepared above was turned upside down and fixed to a support. An iron ball 44 of a size that can pass through the square opening and a weight of 300 g was dropped through the square opening. The height from which the iron ball was dropped was gradually increased so as to measure the iron ball drop height at which the specimen was peeled off from the polycarbonate plate due to the impact of the iron ball drop.

[0152] A rating “oo (Excellent)” was given when the height was 70 cm or higher. A rating “o (Good)” was given when the height was 50 cm or higher and lower than 70 cm. A rating “x (Poor)” was given when the height was lower than 50 cm.

(2) Evaluation on Resistance Against Sebum (Measurement of Oleic Acid Swelling Ratio)

[0153] A plane rectangular test piece (20 mm×40 mm) was cut out from each of the impact-absorbing layers obtained in the examples and the comparative example before the lamination of the outer layers. The weight of the test piece was measured. The test piece was immersed in oleic acid under the conditions of a temperature of 40° C. and a humidity of 90% for 24 hours, and taken out from the oleic acid. The surface of the test piece was washed with ethanol. Then, the test piece was dried at 70° C. for three hours. The weight of the dried test piece was measured, and the oleic acid swelling ratio of the impact-absorbing layer was calculated using the following equation (1):

Swelling ratio (% by weight)=100×(W.sub.5)/(W.sub.4)   (1)

[0154] (W.sub.4: weight of test piece before immersion in oleic acid, W.sub.5: weight of test piece after immersion in oleic acid and drying).

[0155] An impact-absorbing layer having an oleic acid swelling ratio of preferably 100 to 300% by weight, more preferably 100 to 200% by weight can be determined to exhibit high resistance against oleic acid, a main component of sebum. In Table 1, a rating “o (Good)” was given when the oleic acid swelling ratio was 100% by weight or more and a rating “x (Poor)” was given when the impact-absorbing layer was dissolved due to the immersion (oleic acid swelling ratio was lower than 100% by weight).

(3) Evaluation on Processability (Punching Evaluation)

[0156] Each of the double-sided adhesive sheets obtained in the examples and the comparative examples, together with the release paper, was cut in the thickness direction by moving the cutter of a cutting machine upward and downward, whereby a frame-shaped piece was punched out. The presence or absence of partial detachment of the adhesive layers from the release paper and wrinkles were visually determined.

[0157] A rating “o (Good)” was given when neither the partial detachment of the adhesive layers from the release paper nor wrinkles were observed. A rating of “Δ (Fair)” was given when partial detachment or wrinkles were observed but the frame shape obtained by punching was maintained. A rating “x (Poor)” was given when the frame shape obtained by punching was not maintained due to partial detachment or wrinkles.

(4) Evaluation on Adhesiveness (Evaluation of Adhesive Area)

[0158] One side of each of the double-sided adhesive sheets obtained in the examples and the comparative examples was attached to a glass plate having a thickness of 2 mm in such a manner that the double-sided adhesive sheet and the glass plate were completely bonded (adhesive area was 100%). Next, an acrylic plate having a thickness of 3 mm was provided. The other side of the double-sided adhesive sheet was pressure-bonded to the acrylic plate by reciprocating a 10-kg roller once. An image of the specimen was captured from the acrylic plate side with a digital camera (1920×1080 pixels, a magnification of 4 times). The obtained image was binarized into a black and white image (the threshold was ½ of the maximum concentration). The proportion of the area of the black portion to the area of the entire double-sided adhesive sheet was calculated as the adhesive area proportion.

TABLE-US-00001 TABLE 1 Example Example Example Example Example Example Example Example 1 2 3 4 5 6 7 8 Impact- Degree of crystallinity 6 5 4 2 6 6 6 6 absorbing (X-ray diffraction) (%) layer Loss Maximum 0.90 1.06 1.38 1.55 0.90 0.90 0.90 0.90 tangent value at 1.0 × 10.sup.3 to tanδ 1.0 × 10.sup.6.5 Hz Frequency of 5.70 5.67 5.62 5.55 5.70 5.70 5.70 5.70 local maximum log (freq. (Hz)) CEBC:SEPS ratio 10:0 8:2 6:4 4:6 10:0 10:0 10:0 10:0 CEBC:softener weight ratio — — — — — — — — OD value 4.9 4.9 4.9 4.9 4.9 2.5 6.6 4.9 SP value (cal/cm.sup.3).sup.1/2 8.27 8.4 8.52 8.65 8.27 8.27 8.27 8.27 25% compressive strength (kPa) 1000 930 850 770 1000 1000 1000 1000 Outer Material PE PE PE PE PP PE PE PE layer Tensile modulus of elasticity 200 200 200 200 1600 200 200 200 (D638 method) (MPa) SP value 8.56 8.56 8.56 8.56 8.02 8.56 8.56 8.56 Impact- Tensile modulus of elasticity 180 180 180 170 1350 180 180 180 absorbing (D638 method) (MPa) sheet Adhesive layer A A A A A A A B Impact Drop impact test results (cm) 50 55 63 70 50 50 50 53 resistance Evaluation ∘ ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ Resistance Swelling ratio (% by weight) 120 161 193 236 120 120 120 120 against 40° C./90% RH sebum Evaluation ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Process- Punching evaluation ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ability Adhesive- Adhesive area ratio (%) 73 80 82 87 69 75 76 78 ness Compar- Compar- Compar- Example Example Example Example Example ative ative ative 9 10 11 12 13 Example 1 Example 2 Example 3 Impact- Degree of crystallinity 5 6 5 5 5 0 1 35 absorbing (X-ray diffraction) (%) layer Loss Maximum 1.06 0.90 0.98 1.12 1.12 2.07 1.72 0.09 tangent value at 1.0 × 10.sup.3 to tanδ 1.0 × 10.sup.6.5 Hz Frequency of 5.67 5.70 5.12 4.31 4.31 5.50 5.52 — local maximum log (freq. (Hz)) CEBC:SEPS ratio 8:2 10:0 10:0 10:0 10:0 0:10 2:8 — CEBC:softener weight ratio — — 3:7 5:5 5:5 — — — OD value 4.9 4.9 4.9 4.9 4.9 4.9 4.9 — SP value (cal/cm.sup.3).sup.1/2 8.4 8.27 7.36 7.59 7.59 8.9 8.78 13.17 25% compressive strength (kPa) 930 1000 900 410 410 450 650 <1000 Outer Material PE — PE PE PE PE PE — layer Tensile modulus of elasticity 200 — 200 200 200 200 200 — (D638 method) (MPa) SP value 8.56 — 8.56 8.56 8.56 8.56 8.56 — Impact- Tensile modulus of elasticity 180 120 180 160 160 170 170 3600 absorbing (D638 method) (MPa) sheet Adhesive layer B A A A A A A A Impact Drop impact test results (cm) 58 77 65 75 50 90 73 10 resistance Evaluation ∘ ∘∘ ∘ ∘∘ ∘ ∘∘ ∘∘ x Resistance Swelling ratio (% by weight) 161 120 102 115 115 (Dis- (Dis- 103 against 40° C./90% RH solved) solved) sebum Evaluation ∘ ∘ ∘ ∘ ∘ x x ∘ Process- Punching evaluation ∘ Δ ∘ ∘ ∘ ∘ ∘ ∘ ability Adhesive- Adhesive area ratio (%) 82 88 85 92 95 92 88 67 ness

INDUSTRIAL APPLICABILITY

[0159] The present invention can provide an impact-absorbing sheet having high impact resistance and excellent resistance against sebum. The present invention can also provide a double-sided adhesive sheet including the impact-absorbing sheet.

REFERENCE SIGNS LIST

[0160] 41 specimen (frame shape) [0161] 42 polycarbonate plate (thickness: 1 mm) [0162] 43 polycarbonate plate (thickness: 2 mm) [0163] 44 iron ball (300 g)