Ultrasonic welding of fibre reinforced thermosetting resin sections

Abstract

Process for forming a permanent join between two sections of fibrous material contained in a thermosetting resin matrix, said process comprising overlaying the two sections and subjecting the overlaid sections to ultrasonic welding to form a permanent join between the two sections, wherein there is no significant change in the sub-ambient Tg of the fibrous material contained in the thermosetting resin matrix in the region of the permanent join.

Claims

1. A process for forming a permanent joint between two sections of fibrous material contained in a thermosetting resin matrix, said process comprising overlaying the two sections and subjecting the overlaid sections to ultrasonic welding to form a permanent joint between the two sections, wherein there is no significant change in the sub-ambient Tg of the fibrous material contained in the thermosetting resin matrix in the region of the permanent joint; wherein the thickness of the permanent joint is from 50% to 150% of the average thickness of each of the two overlaid sections before they are overlaid; wherein the length of the overlaid section is from 3 mm to 200 mm; wherein the ultrasonic welding is carried out by contacting the area to be ultrasonically welded with a sonotrode while it is supported by an anvil; wherein at least one of the sections of fibrous material contained in a thermosetting resin matrix is shaped to remove a portion of the material in the area to be joined before the sections are overlaid, preferably wherein both sections of fibrous material contained in a thermosetting resin matrix are shaped to remove a portion of the material in the area to be joined before the sections are overlaid; and, wherein both sections to be joined are shaped in complimentary castellated or saw-tooth patterns.

2. The process according to claim 1, wherein the sonotrode operates at a frequency of from 15 to 70 kHz.

3. The process according to claim 2, wherein the sonotrode operates with an amplitude of from 5 to 150 μm.

4. The process according to claim 3, wherein the sonotrode operates with a pressure of from 0.01 to 0.6 MPa (0.1 to 6 bar).

5. The process according to claim 4, wherein the sonotrode operates for a welding period of from 0.01 to 30 seconds.

6. The process according to claim 5, wherein the sonotrode is operated with a hold time of from 0.1 to 10 seconds following completion of the weld time.

7. The process according to claim 6, wherein the sonotrode and anvil are arranged so that when the sonotrode is at its closest point to the anvil, the gap between the sonotrode and the anvil is from zero to 150% of the average thickness of each of the two overlaid sections before they are joined.

8. The process according to claim 7, wherein both sections of fibrous material contained in a thermosetting resin matrix are shaped, and the amount of material removed from each shaped section is from 25 to 75%.

Description

EXAMPLES

(1) The invention will now be illustrated by reference to the following Examples and Figures, in which:

(2) FIG. 1 illustrates the structure of 3 exemplary shaped joins according to the present invention,

(3) FIG. 2 illustrates a castellated shaped section of material to be joined according to a process of the present invention;

(4) FIG. 3 illustrates two pieces of saw-tooth shaped sections of material suitable for joining according to a process of the present invention in 3 arrangements; and

(5) FIG. 4 illustrates two pieces of saw-toothed sections of material suitable for joining according to a process of the present invention in 3 further arrangements.

(6) The materials shown in FIG. 1 represent two narrow sections of composite material joined using three different shaped joints. In the first join (S), the two sections of composite material have flat (unshaped) ends and are overlaid to provide a region for joining having a uniform thickness. In the second join (P), the two sections of composite material each have ends that have been shaped into a triangle (saw-tooth), and have been overlaid so that the bases of each of the triangles meet, i.e. so that the triangular end of each section overlies a flat region of the other section. In the third join (SP), the two sections of composite material each have ends that have been shaped to produce two peripheral straight sections and a central triangle (saw-tooth), and have been overlaid so that the straight sections of each section meet and the triangular ends of each section overlie flat regions of the other section.

(7) The material shown in FIG. 2 represents a section of composite material having a width of 52 mm. One end of the material has been shaped to form a castellated arrangement having 10 mm long and 4 mm wide projecting sections separated by 2 mm wide recessed sections.

(8) The materials shown in FIG. 3 represent 2 sections of composite material each having ends shaped in a saw-tooth pattern comprising triangles having a base width of 6 mm and a height of 20 mm. In a) the two sections of material are arranged with the peaks of the first section of material facing the troughs of the second section of material (PtoV) but with no overlap of the materials. In b) the first section of material has been advanced towards the second section by 40 mm, so that the there is a combined 40 mm region of overlap of the two sections of material including two regions having the thickness of one of the sections and a central region having the thickness of both sections. In c) the first section has been advanced towards the second section by 50 mm, creating a 50 mm region of overlap again including two regions having the thickness of one of the sections and a larger central region having the thickness of both sections. As will be appreciated, it is also possible to produce a smaller area of overlap, for example by advancing the first section by only 30 mm towards the second section, and this will result in a 30 mm region of overlap again comprising two sections having the thickness of one of the sections but comprising a smaller central region having the thickness of both sections.

(9) The materials shown in FIG. 4 represent 2 sections of composite material each having ends shaped in a saw-tooth pattern comprising triangles having a base width of 6 mm and a height of 20 mm. In a) the two sections of material are arranged with the peaks of the first section of material facing the peaks of the second section of material (PtoP) but with no overlap of the materials. In b) the first section of material has been advanced towards the second section by 40 mm, so that the there is a combined 40 mm region of overlap of the two sections of material including two regions having the thickness of one of the sections and a number of central regions having the thickness of both sections. In c) the first section has been advanced towards the second section by 50 mm, creating a 50 mm region of overlap again including two regions having the thickness of one of the sections but now having a single large central region having the thickness of both sections. As will be appreciated, it is also possible to produce a smaller area of overlap, for example by advancing the first section by only 30 mm towards the second section, and this will result in a 30 mm region of overlap again comprising two sections having the thickness of one of the sections and a number of central regions having the thickness of both sections, but also including regions having no material and therefore no thickness

Example 1

(10) Ultrasonic welding was carried out to join sections of prepreg formed from unidirectional carbon fibres impregnated with curable epoxy resin. The prepreg comprised Intermediate Modulus type A carbon fibre 12 k, impregnated with epoxy resin M21EV, the resin content was 34% by weight and the fibre areal weight was 268 gm.sup.−2. The individual components and the prepreg are available from Hexcel Composites Limited. Tapes were prepared with 3 widths: 6.5 mm, 12.5 mm and 25.5 mm.

(11) Ultrasonic welding was carried out using a Telsonic Ultrasonic Generator series SG-22 2000 (20 kHz frequency) comprising the following components:

(12) Telsonic ultrasonic welder press, with an MPS4 controller

(13) Convertor SE 50/40-4-20 KHz idle peak to peak amplitude of 20 microns at 100% generator amplitude set point

(14) Booster attached to the convertor boosting amplitude by 1:2.2

(15) Titanium alloy 6A1-4V Sonotrode, 270 mm long×30 mm wide connected to the booster with design such to boost amplitude by: 1:2

(16) The welding parameters held constant were:

(17) Generator frequency—20 KHz

(18) Generator amplitude set point—100%

(19) Convertor amplitude—20 microns peak to peak

(20) Booster—increasing amplitude by 1:2.2

(21) Sonotrode—boosting amplitude to 1:2

(22) Processing amplitude—88 microns peak to peak

(23) Gap: The gap between the sonotrode and the anvil, it is set to the thickness of a single section of material whilst the sonotrode is not under ultrasonic motion

(24) Holding time under pressure (after ultrasonic welding process is complete and Sonotrode vertical motion stopped)—5 seconds

(25) The following parameters were varied, as shown in Tables 1 and 2:

(26) Pressure (the downward pressure applied on the sonotrode during the welding process), Ultrasonic welding time, Material overlap length and Material width.

(27) The results are shown in Tables 1 and 2. The thickness of the joined sections was measured either on a cross section of the joint or on the side of the joint (after being brought down to its original width) with an optical microscope (a Keyence microscope, at magnifications of 5 to 175 times) or a Mitutoyo QuantuMike micrometer.

(28) During the joining of the two strips of prepreg, there was fibre spreading, i.e. a proportion of the fibre and resin was displaced, resulting in a wider portion at the joint. This increase in width was cut out manually with a knife to bring back the width to its original width (i.e. 6.5 mm, 1.5 mm or 25.5 mm).

(29) TABLE-US-00001 TABLE 1 Sample number 1 2 3 4 5 6 7 8 Initial width (mm) 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 Overlap join (mm) 5 10 5 10 5 10 5 10 Pressure applied (bar) 1 1 2 2 1 1 2 2 Welding time (s) 1 1 1 1 2 2 2 2 Initial thickness strip 1 304 314 310 325 307 316 292 305 (μm) Initial thickness strip 2 313 326 305 307 318 290 302 325 (μm) Average thickness of 309 320 308 316 312 303 297 315 sections to be joined (μm) Average thickness at join 301 323 223 282 288 302 157 156 after ultrasonic welding (μm) Thickness of join as 97.4 100.9 72.4 89.2 92.3 99.7 52.9 49.5 percentage of average thickness of sections to be joined (%)

(30) TABLE-US-00002 TABLE 2 Sample number 9 10 11 12 13 14 15 16 Initial width (mm) 12.5 12.5 12.5 12.5 25.5 25.5 25.5 25.5 Overlap join (mm) 5 10 5 10 5 10 5 10 Pressure applied (bar) 1 1 1 1 1 1 2 2 Welding time (s) 1 1 2 2 1 2 1 2 Initial thickness strip 1 (μm) 338 315 308 274 301 378 284 325 Initial thickness strip 2 (μm) 330 314 336 260 289 295 303 289 Average thickness of 334 315 322 267 295 337 294 307 sections to be joined (μm) Average thickness at join 328 307 283 301 371 350 324 285 after ultrasonic welding (μm) Thickness of join as 98.2 97.5 87.9 112.7 125.8 103.8 110.2 92.8 percentage of average thickness of sections to be joined (%)

(31) The results in Tables 1 and 2 show that in almost every case join thicknesses of within plus or minus 50% of the average thickness of the non-joined sections were achieved, and that in many cases join thicknesses of plus or minus 20%, or even as low as plus or minus 5% were achieved. In comparison, joining the same materials by conventional methods, such as by use of a mechanical clamping device operated at 65° C. for up to 2 minutes would be expected to produce join thicknesses of approximately 200% of the average thickness of the non-joined regions.

Example 2

(32) Sections of the prepreg materials used in Example 1 but cut to widths of 6.35, 12.7 and 25.4 mm were joined using the same equipment as set out in Example 1. The fixed welding parameters were the same as those used in Example 1, and the joint overlap, pressure applied and welding time were varied as shown in Table 3. The tensile strength of the joined materials was tested using an Instron Test machine with a 400N load cell. Samples were maintained with mechanical wedge grips, and tested at a speed of 2 mm/min, at room temperature. The joints were broken at ambient temperature (approximately 22° C.). The results are shown in Table 3.

(33) TABLE-US-00003 TABLE 3 Initial 6.35 6.35 6.35 12.7 12.7 12.7 25.4 25.4 width (mm) Overlap join 5 10 5 5 10 5 5 10 (mm) Pressure 1 1 2 1 1 2 1 1 applied (bar) Welding 1 1 2 1 1 2 1 1 time (s) Average 190 240 140 270 280 170 310 380 thickness at join after ultrasonic welding (μm) Tensile 19 84 54 79 147 54 206 346 strength (N)

(34) The results in Table 3 show that the joins produced by the process according to the present invention were effectively permanent joins, in that they were all sufficiently strong to withstand normal handling of the joined prepregs.

Example 3

(35) Sections of the prepreg material used in Example 1 but cut to a width of 12.7 mm were joined using the same equipment as set out in Example 1. The fixed welding parameters were the same as used in Example 1, and the joint overlap was 10 mm, the pressure applied was 1 bar and the welding time was 1 second. The resin chemistry at the joined regions was investigated by DSC performed on a Mettler Toledo DSC (with run parameters of −50° C. to 350° C., at a temperature increase rate of 10° C./min), to check for localised cure or aging, and the sub-ambient Tg was also measured. Two joints were tested in duplicate and compared to an adjacent area of non-welded material (Standard 1). The results are shown in Table 4.

(36) TABLE-US-00004 TABLE 4 Sample Tg ° C. ΔH J/g Join 1 run 1 1.7 133 Join 1 run 2 −0.3 143 Mean: Join 1 0.7 138 Join 2 run 1 −0.7 181 Join 2 run 2 0.6 179 Mean: Join 2 0.0 180 Standard 1 run 1 2.5 212 Standard 1 run 2 1.6 197 Mean: Standard 1 2.1 205

(37) The results in Table 4 show that joining the materials by the method of the present invention did not significantly change the sub-ambient Tg or case localised cure or aging.

Example 4

(38) Further joins were made in using prepregs corresponding to the materials used in Example 1, but all having a width of 12.7 mm. The joins were formed using the apparatus and set parameters described in Example 1. Overlap was fixed at 10 mm, welding time was fixed at 0.5 seconds, and a pressure of 1 bar was used (one sample “S2” was joined at 0.75 bars to show the effect of pressure).

(39) Thickness of the join was measured with a micrometre, and tensile strength was measured with an Instron machine with a 400N load cell, using the methods described in previous the Examples.

(40) Three different join shapes were tested, namely Straight (S), Saw-tooth (P) and Straight-saw-tooth (SP), as shown in FIG. 1.

(41) The results are shown in Tables 5 and 6.

(42) TABLE-US-00005 TABLE 5 Sample 1 2 3 4 5 6 7 Joint Pattern code S S S S S P P Initial width (mm) 12.7 12.7 12.7 12.7 12.7 12.7 12.7 Overlap join (mm) 10 10 10 10 10 10 10 Pressure applied 1 1 1 1 1 1 1 (bar) Welding time (s) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Initial thickness strip 295 290 290 295 290 290 290 1 (μm) Initial thickness strip 290 290 295 295 290 285 290 2 (μm) Average thickness of 292.5 290 292.5 295 290 287.5 290 sections to be joined (μm) Average thickness at 290 285 288 284 290 285 285 join after ultrasonic welding (μm) Thickness of join as 99.19 98.3 98.5 96.3 100 99.1 98.3 percentage of average thickness of sections to be joined (%) Tensile strength (N) 110 10 115 97 77 100

(43) TABLE-US-00006 TABLE 6 Sample 8 9 10 11 12 13 14 Joint Pattern code P SP SP SP S S S Initial width (mm) 12.7 12.7 12.7 12.7 12.7 12.7 12.7 Overlap join (mm) 10 10 10 10 10 10 10 Pressure applied 1 1 1 1 0.75 0.75 0.75 (bar) Welding time (s) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Initial thickness strip 285 285 290 295 280 285 290 1 (μm) Initial thickness strip 285 290 290 290 280 290 290 2 (μm) Average thickness of 285 287.5 290 292.5 280 287.5 290 sections to be joined (μm) Average thickness at 285 285 285 285 310 315 315 join after ultrasonic welding (μm) Thickness of join as 100 99.1 98.3 97.4 110.7 109.6 108.6 percentage of average thickness of sections to be joined (%) Tensile strength (N) 115 104 110 117

(44) The results in Tables 5 and 6 show that in almost every case join thicknesses of within plus or minus 10% of the average thickness of the non-joined sections were achieved, and that in most cases join thicknesses of plus or minus 5% were achieved. The results in Tables 5 and 6 also show that in most cases the joins produced by the process according to the present invention were effectively permanent joins, in that they were all sufficiently strong to withstand normal handling of the joined prepregs.

Example 5

(45) In this example, sections of a prepreg of width 250 mm were joined. The prepreg material was the same as the prepreg material used in Example 1.

(46) To allow for fibre and resin spreading a pattern was cut at the end of both sections to be joined. The pattern used was a castellated pattern, with approximately 33% of the material overlapping taken off, as shown in FIG. 2. The legs produced by the cutting were 10 mm long with a width of 4 mm, and the gap between each leg was 2 mm. The material was cut using the Zund automatic cutter.

(47) The 4 mm wide legs were overlapped one above the other with another piece of prepreg that had the exact same pattern, with the 2 mm gaps left as spaces for the prepreg to spread into and the overlaid sections were ultrasonically welded to make a permanent join using the ultrasonic weld press described in Example 1.

(48) The overlap was 10 mm, the pressure used was 2 bars, and the welding time of 1 second.

(49) The average thickness, measured on a cross section of the joint with a Keyence microscope (mag 5-175 times) was 420 microns.

(50) Measures on the single ply non treated area gave an average of 300 microns.

(51) The increase thickness at the joint was approximately 40%.

(52) Samples were then cut out throughout the width of the master tape, to test the tensile strength of samples similar to slit tape products.

(53) The average tensile strength for 6 mm wide strips was 30N, i.e. an effectively permanent join was formed.

Example 6

(54) Samples of the prepreg material described in Example 1 were cut to widths of 12.7 mm and joined using the equipment described in Example 1. The fixed welding parameters were the same as those described in Example 1 and the joint overlap, pressure applied and welding time were varied, as shown in Table 7. Joint thickness, joint strength and sub-ambient Tg in the joint regions were measured using the methods described in the previous Examples, and the results were compared to the results for adjacent non-welded regions of material (Ref 1). The results are shown in table 7.

(55) TABLE-US-00007 TABLE 7 Sample number Ref 1 1 2 15 Initial width (mm) 12.7 12.7 12.7 12.7 Overlap join (mm) / 10 10 25 Pressure applied (bar) / 1 1 1 Welding time (s) / 0.5 0.5 2 Initial thickness strip 1 (μm) / 282 288 285 Initial thickness strip 2 (μm) / 281 280 280 Average thickness of sections to / 281.5 284 282.5 be joined (μm) Average thickness of join after / 290 290 300 ultrasonic welding (μm) Thickness at join as percentage / 103.0 102.1 106.2 of average thickness of sections to be joined (%) Tensile strength (N) / 101 102 315 Tg (° C.) −4.13 −3.01 −3.83 /

(56) The results in Table 7 show that the processes of the present invention can provide very strong joins having thicknesses very close to the thickness of the individual unjoined materials, i.e. joint areas varying only slightly in thickness from unjoined areas (i.e. from 2.1% to 6.2% thicker), without significantly altering the chemistry of the material in the area of the join (sub-ambient Tg changed by from 0.3° C. to 1.12° C. only).

Example 7

(57) Samples of a prepreg material comprising the same components as the material described in Example 1 but having a reduced fibre areal weight of 194 g/m.sup.2 were cut to widths of 6.35 mm and joined using the equipment described in Example 1. The fixed welding parameters were the same as those described in Example 1 and the joint overlap, pressure applied and welding time were varied, as shown in Table 8. Joint thickness, joint strength and sub-ambient Tg in the joint regions were measured using the methods described in the previous Examples, and the results were compared to the results for adjacent non-welded regions of material (Ref 2). The results are shown in table 8.

(58) TABLE-US-00008 TABLE 8 Sample number Ref 2 1 2 3 4 Initial width (mm) 6.35 6.35 6.35 6.35 6.35 Overlap join (mm) / 5 10 15 25 Pressure applied (bar) / 0.75 0.75 0.75 0.75 Welding time (s) / 0.1 0.15 0.2 0.4 Initial thickness strip 1 / 221 225 226 222 (μm) Initial thickness strip 2 / 220 224 222 224 (μm) Average thickness of / 220.5 224.5 224 223 sections to be joined (μm) Average thickness at join / 195 219 215 215 after ultrasonic welding (μm) Thickness at join as / 88.4 97.6 96.0 96.4 percentage of average thickness of sections to be joined (%) Tensile strength (N) / 46 97 136 260 Tg (° C.) −1.55 0.22 −0.11 −0.25 −0.14

(59) The results in Table 8 show that the processes of the present invention can provide very strong joins having thicknesses very close to the thickness of the individual unjoined materials, i.e. joint areas varying only slightly in thickness from unjoined areas (i.e. 88.4% to 97.6% of the unjoined thickness), without significantly altering the chemistry of the material in the area of the join (changing by from 1.3° C. to 1.77° C. only).

Example 8

(60) Samples of prepreg material comprising the same components as the materials described in Example 1 but having a reduced fibre areal weight of 194 g/m.sup.2 were prepared having a width of 150 mm. In some cases the ends of the materials were straight (S) and in some cases the ends were formed into saw-tooth patterns comprising triangles having a base width of 6 mm and a height of 20 mm. Sections of each of the materials were overlaid on correspondingly shaped sections of material with various overlap lengths. For the saw-tooth shaped materials two different configurations were used, either point to valley (PtoV) as shown in FIG. 3, or point to point (PtoP) as shown in FIG. 4. Joining was carried out as described in Example 1 using the same fixed parameters but with the following variable parameters, Pressure: 2 bar, ultrasonic welding time: 2 seconds, material overlap length as shown in Table 9, material width: 150 mm, joint pattern as shown in Table 9. In addition, multiple welding cycles (two or three, as shown in Table 9) were carried out moving the material to be joined with respect to the sonotrode between each cycle to ensure that all of the area of overlap plus 5 to 10 mm either side of the overlaid region was subjected to welding.

(61) Following welding the thickness of the joined regions was measured using a micrometer, and the joined materials were slit into 6.35 mm wide tapes.

(62) The strength of the joints were measured in the slit materials using a Texture Analyzer, Model: TA XTplus (available from Stable Micro systems) having a 400N load cell (capable of measuring loads of up to 350N). The load testing was carried out at 2 mm/min at approximately 8° C. The results are shown in Table 9.

(63) TABLE-US-00009 TABLE 9 Sample number 1 2 3 4 5 Initial width (mm) 150 150 150 150 150 Joint Pattern PtoV PtoV S PtoP PtoV Overlap join (mm) 50 50 10 40 30 Pressure applied (bar) 2 2 2 2 2 Welding time (s) 2 2 2 2 2 Welding cycles 2 3 2 3 2 Initial thickness strip 1 (μm) 222 223 222 224 222 Initial thickness strip 2 (μm) 224 223 221 222 222 Average thickness of 223 223 221.5 223 222 sections to be joined (μm) Average thickness at join 343 322 325 254 254 after ultrasonic welding (μm) Thickness of join as % of 154 144 147 114 114 average thickness of sections to be joined (%) Tensile strength test 1 (N) >350 >350 >350 200 >350 Tensile strength test 2 (N) >350 >350 >350 >350 >350 Tensile strength test 3 (N) >350 >350 310 >350 >350

(64) The results in Table 9 show that wide sections of material may be joined by the processes of the present invention, and that even with long regions of overlap the thickness of the joins is in most cases within plus or minus 50% of the average thickness of the non-joined sections, and with thickness of less than plus or minus 20% of the average thickness possible in some cases by shaping the ends of the sections to be joined. The joined materials may be slit to form narrow tapes and the strengths of the joins in the narrow tapes can still exceed 300N, or even 350N.