PRESSURE-SENSITIVE ADHESIVE BASED ON ACRYLONITRILE BUTADIENE RUBBERS

Abstract

Pressure-sensitive adhesives based on acrylonitrile-butadiene rubbers having good shock-resistance combined with easy removability of the adhesives from the substrate shall be provided. This is solved by a pressure-sensitive adhesive which comprises at least one acrylonitrile-butadiene rubber; and at least one tackifier resin, and is characterized in that the acrylonitrile content of the acrylonitrile rubber is less than 30 wt %; the pressure-sensitive adhesive comprises at least one rosin resin as tackifier resin; and the entirety of all tackifier resins has an acid number of 6-150. The pressure-sensitive adhesive can be used for bonding parts in mobile electronic devices.

Claims

1. A pressure-sensitive adhesive comprising at least one acrylonitrile-butadiene rubber; and at least one tackifier resin, wherein the acrylonitrile content of the acrylonitrile rubber is less than 30 wt %; the pressure-sensitive adhesive comprises at least one rosin resin as tackifier resin; and the entirety of all tackifier resins has an acid number of 6-150.

2. A pressure-sensitive adhesive according to claim 1, wherein the acrylonitrile content in the acrylonitrile-butadiene rubber is at least 10 wt %.

3. A pressure-sensitive adhesive according to claim 1, wherein the pressure-sensitive adhesive comprises tackifier resins to a total extent of 40 to 70 wt %, based on a total weight of the pressure-sensitive adhesive.

4. The pressure-sensitive adhesive according to claim 1, wherein the acrylonitrile rubber is a solid acrylonitrile rubber.

5. The pressure-sensitive adhesive according to claim 4, wherein the pressure-sensitive adhesive further comprises at least one liquid acrylonitrile-butadiene rubber, the acrylonitrile content in the liquid acrylonitrile-butadiene rubber being at least 10 and less than 30 wt %.

6. The pressure-sensitive adhesive according to claim 1, wherein the entirety of all tackifier resins has an acid number of at least 9.

7. The pressure-sensitive adhesive according to claim 1, wherein the pressure-sensitive adhesive is free of oils and plasticizers.

8. A method of bonding parts in mobile electronics devices comprising bonding the parts with a pressure-sensitive adhesive according to claim 1.

Description

EXAMPLES

Test Methods

[0106] Unless otherwise indicated, the measurements are carried out under test conditions of 23±1° C. and 50±5% relative humidity.

Impact Resistance; z-plane (DuPont Test)

[0107] A square sample with a frame shape was cut from the adhesive tape under investigation (external dimensions 33 mm×33 mm; border width 2.0 mm; internal dimensions (window cutout) 29 mm×29 mm). This sample was adhered to a polycarbonate (PC) frame (external dimensions 45 mm×45 mm; border width 10 mm; internal dimensions (window cutout) 25 mm×25 mm; thickness 3 mm). On the other side of the double-sided adhesive tape, a PC window measuring 35 mm×35 mm was adhered. The bonding of PC frame, adhesive tape frame and PC window took place in such a way that the geometric centers and the diagonals lay in each case one above another (corner to corner). The bond area was 248 mm.sup.2. The bond was pressed under 10 bar for 5 s and stored for 24 hours with conditioning at 23° C./50% relative humidity.

[0108] Immediately after storage, the bonded assembly of PC frame, adhesive tape and PC window with the protruding edges of the PC frame was clamped into a sample holder in such a way that the assembly was oriented horizontally and the PC window was beneath the frame. The sample holder was subsequently inserted centrally into the intended receptacle of the DuPont impact tester. The impact head, weighing 150 g, was inserted such that the circular impact geometry with a diameter of 24 mm impacted centrally and flush on the window side of the PC window.

[0109] A weight with a mass of 150 g, guided on two guide rods, was dropped vertically from a height of 5 cm onto the assembly thus arranged of sample holder, sample and impact head (measuring conditions 23° C., 50% relative humidity). The height of the falling weight was raised in steps of 5 cm until the impact energy introduced caused destruction of the sample as a result of the impact stress, and the PC window parted from the PC frame.

[0110] In order to be able to compare experiments with different samples, the energy was calculated as follows:

E[J]=height[m]*mass weight[kg]*9.81 kg/m*s.sup.2

[0111] For each product, five samples were tested, and the average energy value was reported as a characteristic value for the impact resistance. A value of at least 0.45 J is considered to indicate good impact resistance.

Push-Out Strength (z-plane)

[0112] The push-out test provides information on the extent to which the bond of a component in a frame-shaped body, such as of a window or display in a housing, is resistant.

[0113] A rectangular, frame-shaped sample was cut out of the adhesive tape under investigation (outside dimensions 33 mm×33 mm; border width 2.0 mm in each case; inside dimensions (window opening) 29 mm×29 mm, bond area 248 mm.sup.2 on each of the top and bottom sides). This sample was adhered to a rectangular PC plastic frame (PC=polycarbonate) (outside dimensions 40 mm×40 mm; border width of the long borders 8 mm in each case; border width of the short borders 10 mm in each case; inside dimensions (window opening) 24 mm×24 mm; thickness 3 mm). Adhered on the other side of the double-sided adhesive tape sample was a rectangular PC sheet with dimensions of 35 mm×35 mm. The full bond area of the adhesive tape available was utilized. The PC frame, adhesive tape sample and PC window were bonded such that the geometric centers, the bisecting lines of the acute diagonal angles and the bisecting lines of the obtuse diagonal angles of the rectangles each lay on top of one another (corner on corner, long sides on long sides, short sides on short sides). The bond area was 248 mm.sup.2. The bond was pressed at 248 N for 5 s and stored for 24 hours with conditioning at 23° C./50% relative humidity.

[0114] Immediately after storage, the adhesive assembly composed of PC frame, adhesive tape and PC sheet was placed with the protruding edges of the PC frame onto a frame structure (sample holder) in such a way that the assembly was oriented horizontally and the PC sheet pointed downward, hanging freely.

[0115] A plunger is then moved through the window of the PC frame, perpendicularly from above, at a constant speed of 10 mm/s, so that it presses centrally onto the PC plate, and a record is made of the respective force (determined from respective pressure and contact area between plunger and plate) as a function of the time from the first contact of the plunger with the PC plate until shortly after the plate has fallen (measuring conditions 23° C., 50% relative humidity). The force acting immediately prior to the failure of the adhesive bond between PC plate and PC frame (maximum force Fmax in the force-time diagram in N) is recorded as the answer of the push-out test.

Push-Out Strength for Removability (EtOH/Water)

[0116] For the determination of removability of the adhesive tape under the influence of EtOH/water, the specimens manufactured for the push-out strength test as described hereinbefore were stored at 65° C. and 90% relative humidity in a 75:25 (vol %) mixture of ethanol and water. For the ethanol/water storage a closeable vessel was used in order to prevent loss of ethanol by evaporation. Following storage, the specimens are first rinsed off with distilled water and then stored for 24 hours at 23° C. and 50% relative humidity. Only then the push-out strength test as described above was repeated.

[0117] The drop of the maximum force F.sub.max in % is given as the result; a high drop (at least 35%) indicates good removability of the tape.

Preparation of the PSAs

[0118] The pressure-sensitive adhesives (PSAs) set out in the examples were homogenized as solvent-based compositions in a kneading apparatus with a double-sigma kneading hook. The solvent used was butanone (methyl ethyl ketone, 2-butanone). The kneading apparatus was cooled by means of water cooling.

[0119] In a first step, the solid acrylonitrile-butadiene rubber was pre-swollen with the same amount of butanone at 23° C. for 12 hours. This preliminary batch was then kneaded for 2 hours. Subsequently, again, the amount of butanone selected above and, optionally, the liquid NBR rubber were added in two steps with kneading in each case for 10 minutes. Thereafter the tackifier resin was added as a solution in butanone with a solids content of 50%, and homogeneous kneading was continued for 20 minutes more. The final solids content is adjusted to 30 wt % by addition of butanone.

Production of the Test Specimens

[0120] The PSA was coated onto a PET film which was 23 μm thick and etched with trichloroacetic acid by means of a coating knife on a commercial laboratory coating bench (Sondermaschinen Oschersleben GmbH). The butanone was evaporated in a forced air drying cabinet at 105° C. for 10 minutes. The slot width during coating was adjusted so as to achieve a coat weight of 50 g/m.sup.2 following evaporation of the solvent. The films freed from the solvent were subsequently lined with siliconized PET film and stored pending further testing at 23° C. and 50% relative humidity.

Examples

[0121]

TABLE-US-00001 TABLE 1 Acrylonitrile-butadiene rubbers used Mooney viscosity ACN content ML 1 + 4, 100° C. Name [wt %] [MU] Nipol ® 401 18.5 73 to 83 Nipol ® 401LL 18.5 32 to 44 Nipol ® DN 28.0 45 to 55 2850

TABLE-US-00002 TABLE 2 Tackifier resins used Softening Name Chemical basis Manufacturer point OH/acid no. Dertophene ® T 110 Terpene-phenolic DRT 112° C. OH no. 50 resin Dertophene ® T Terpene-phenolic DRT 95° C. OH no. 35 resin Dertophene ® H150 Terpene-phenolic DRT 118° C. OH no. 145 resin Picco ® AR85 Aromatically Eastman 87° C. — modified hydrocarbon resin Eastman Picco ® AR100 Modified aromatic Eastman 101° C. — hydrocarbon resin Foral ® 85-E Hydrogenated Eastman 85° C. Acid no. 9 rosin ester Foral ® 105-E Hydrogenated Eastman 101° C. Acid no. 14 rosin ester Foral ® AX-E Fully hydrogenated Eastman 80° C. Acid no. 165 rosin

TABLE-US-00003 TABLE 3 Plasticizers used ACN content/ chemical nature Viscosity Name [wt %] [MU] Nipol ® 1312LV 26 9.000-16.000 (Brookfield, [mPa*s], spindle 4, 12 rpm, 50° C. Indopol ® H-100 Polybutene (PIB) 200-235 cSt (100° C.)

TABLE-US-00004 TABLE 4 Composition of the examples according to the invention Example no. 1 2 3 4 5 6 Raw material Initial mass of solids [wt %] Nipol ® 401 57 57 57 50 45 57 Foral ® 85-E 43 21.5 21.5 50 55 21.5 Foral ® 105-E 21.5 Foral ® AX-E 21.5 Dertophene ® 21.5 H150

TABLE-US-00005 TABLE 5 Composition of the comparative examples Comparative Example no. 7 8 9 10 11 12 13 14 15 16 Raw material Initial mass of solids [%] Nipol ® 401 57 57 57 57 63 57 57 Nipol ® 401LL 57 54 Nipol ® DN 53 2850 Dertophene ® 43 T 110 Dertophene ® 43 41 T Dertophene ® 39 43 37 H150 Picco ® AR85 43 43 Picco ® AR100 43 Foral ® AX-E 43 Nipol ® 8 1312LV Indopol ® H- 5 100

TABLE-US-00006 TABLE 6 Test results of the examples according to the invention Example no. test 1 2 3 4 5 6 DuPont [J] 0.63 0.54 0.54 0.50 0.49 0.56 Push-out [N] 133 93 131 135 124 187 Push-out 39 40 46 61 82 44 drop [%]

TABLE-US-00007 TABLE 7 Test results comparative examples Example no. test 7 8 9 10 11 12 13 14 15 16 DuPont [J] 0.47 0.38 0.57 0.15 0.15 0.35 0.38 0.61 0.44 0.56 Push-out [N] 165 — 174 314 107 260 198 158 165 173 Push-out 7 — 14 87 84 76 47 33 35 30 drop [%]