WO2011018513A1

WO2011018513A1 - Infrared absorbing dye-containing cyanoacrylate compositions and laser welding method using such compositions

Info

Publication number: WO2011018513A1
Application number: PCT/EP2010/061830
Authority: WO
Inventors: Ciaran Bernard Mcardle; Maria Parals
Original assignee: Loctite (R & D) Limited; Henkel Ag & Co. Kgaa
Priority date: 2009-08-13
Filing date: 2010-08-13
Publication date: 2011-02-17

Abstract

The present invention relates to infrared absorbing dye-containing cyanoacrylate compositions and a method of using such compositions, such as in forming a weld between at least two surfaces of one or more workpieces, over a joint region, using a laser.

Description

INFRARED ABSORBING DYE-CONTAINING CYANOACRYLATE COMPOSITIONS AND LASER WELDING METHOD USING SUCH COMPOSITONS

BACKGROUND

Field

[0001] The present invention relates to infrared absorbing dye-containing cyanoacrylate compositions and a method of using such compositions, such as in forming a weld between at least two surfaces of one or more workpieces, over a joint region, using a laser.

Brief Description of Related Technology

[0002] Cyanoacrylates are well-known monomers that are quite useful as instant adhesives. The "instant" nature of the adhesives is derived from their high reactivity that relates to their molecular structures. Cyanoacrylates are unique in their ability to instantly polymerise when required yet remain stable when stored or not in use. Nevertheless, such reactive

materials may be easily destabilized, which makes formulating cyanoacrylate compositions as one part products quite tricky.

[0003] In the past, cyanoacrylate compositions have been colored with dyes. For instance, European Patent No. 105 062 speaks to certain fluorescent dyes that are soluble in the cyanoacrylate monomers. These dyes are C.I. Solvent Green 5, C.I. Acid red 50 and C.I. Acid Red 52.

[0004] U.S. Patent No. 6,689,826 (Wojciak) is directed to and claims a cyanoacrylate composition having a cure indicator comprising a cyanoacrylate component; and a dye dissolved in the cyanoacrylate component to provide a solution having a first color, where a resultant cured composition has a second color. The dyes are fluorescent and may be selected from fluorescein, diiodofluorescein, tetrabromofluorescein,

tetrabromotetrachlorofluorescein, and combinations thereof.

[0005] Included among dyes in the ^Λ826 patent are xanthenes and anthraquinones . Fluorans are a preferred class of xanthene dyes suitable for use in the ^Λ826 patent. Particularly

preferred fluorans comprise fluorescein (D&C Yellow #7), dibromofluorescein (D&C Orange #5), diiodofluorescein (D&C Orange #10), tetrabromofluorescein (D&C Red #21), and

tetrabromotetrachlorofluorescein (D&C Red #27) .

[0006] Preferred antrhaquinone dyes in the ^Λ826 patent are 7, 16-dichloro-6, 15-dihydro-5, 9, 14, 18-anthrazine-tetrone (D&C Blue #9), the disodium salt of 2, 2' - [ ( 9, 10, -dihydro-9, 10-dioxo- 1, 4-anthracenediyl) diimino] bis- [ 5-methylbenzenesulfonic acid (D&C Green #5), 1, 4-bis (4 ' -methylanilino) anthraquinone (D&C Green #6), and l-hydroxy-4- ( 4-methylanilino) anthraquinone (D&C Violet #2) .

[0007] International Patent Publication No. WO 2006/074890A1 refers to cyanoacrylate compositions containing radio-opaque dyes such as iodo-substituted phenols.

[0008] Because of the reactive nature of the cyanoacrylate monomer, only specific dyes and dye classes may be formulated into stable one part cyanoacrylate compositions.

[0009] Laser welding is a known method for welding together materials constructed from plastics. The method involves positioning of two plastic substrates, that must be held in intimate contact with one another. The substrate on the side addressed by the laser must be transparent to visible light and the other substrate may be opaque, to visible light. Part of the region of contact between the two plastic substrates is then exposed to a laser beam. The laser beam passes through the first plastic substrate on the laser address side and may be absorbed by the second plastic substrate which is opaque. The temperature of the absorbing plastic substrate increases, causing the region of contact between the two plastic substrates to melt, thereby forming a weld.

[0010] This method however requires at least one substrate to be opaque to visible light.

[0011] In International Patent Publication No. WO 00/20157, a method of forming a weld between workpieces over a joint region is provided. The method of the PCT ^Λ157 patent document includes exposing the joint region to incident radiation having a wavelength outside the visible range so as to cause melting of the surface of one or both workpieces at the joint region, and allowing the melted material to cool, thereby welding the workpieces together. The method also includes providing a radiation absorbing material at the joint region in one of the workpieces or between the workpieces which has an absorption band matched to the wavelength of the incident radiation so as to absorb the incident radiation and generate heat for the melting process, the absorption band being substantially outside the visible range so that the material does not affect the appearance of the joint region or the workpieces in visible light.

[0012] In order to form the welded joint between the

workpieces in the PCT ^Λ157 patent document, the workpieces are mechanically clamped together. This is an essential processing step because local fusion of the substrates may only occur when they are in intimate contact. Clamping in this way is

cumbersome, increases cost, restricts degrees of 3-dimensional freedom relating to shaped parts, increases processing time and decreases throughput efficiency. -A-

[0013] Accordingly, it would be desirable to provide a laser welding method that did not require clamping to the workpieces while the laser welding operation was being performed.

SUMMARY

[0014] The present invention relates to infrared absorbing dye-containing cyanoacrylate compositions.

[0015] The present invention relates also to a method of forming a weld between at least two surfaces of one or more workpieces, over a joint region, using a laser and infrared absorbing dye-containing cyanoacrylate compositions.

[0016] According to the method aspect of the invention, therefore, a method of forming a weld between at least two surfaces of one or more workpieces over a joint region is provided where the method includes providing a composition comprising an infrared radiation absorbing material in a reactive carrier; effecting instant bonding the two surfaces to be subsequently welded into intimate contact; exposing the joint region to incident radiation having a wavelength outside the visible range so as to cause melting of the surface of one or both workpieces at the joint region, and allowing the melted material to cool thereby welding the workpieces together.

[0017] In the case where the reactive carrier is a liquid, instant bonding would be effected on contact of the two surfaces to be subsequently welded. In the case where the reactive carrier is a solid form, such as a powder or film, instant bonding would be effected following mild heating in a general sense (as opposed to local fusion effected by a focused laser) . DETAILED DESCRIPTION

[0018] As noted the present invention relates to infrared absorbing dye-containing cyanoacrylate compositions, and a method of forming a weld between at least two surfaces of one or more workpieces, over a joint region, using a laser and infrared absorbing dye-containing cyanoacrylate compositions.

[0019] The infrared absorbing dye-containing cyanoacrylate composition includes at least one α-cyanoacrylate monomer of the formula :

H₂C =C COOR¹

CN

where R¹ represents a straight chain or branched chain alkyl group having 1 to 12 carbon atoms (which may be substituted with a substituent such as a halogen atom or an alkoxy group) , a straight chain or branched chain alkenyl group having 2 to 12 carbon atoms, a straight chain or branched chain alkynyl group having 2 to 12 carbon atoms, a C3-C12 cycloalkyl group, a C6-C20 aralkyl group or any C5-C20 aryl group. Specific examples of R¹ are a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a pentyl group, a neopentyl group, a hexyl group, an allyl group, a methallyl group, a crotyl group, a propargyl group, a cyclohexyl group, a benzyl group, a phenyl group, a cresyl group, a

2-choroethyl group, a 3-choropropyl group, a 2-chorobutyl group, a trifluoroethyl group, a 2-methoxyethyl group, a 3-methoxybutyl group and a 2-ethoxyethyl group. Ethyl cyanoacrylate is a particularly desirable choice for use in the inventive

compositions and is a common component of a low viscosity instant adhesive. Neopentyl cyanoacrylate is a particularly desirable choice for use in the inventive compositions and is a solid form monomer; melting such a material at approximately 45°C develops an instant adhesive. [0020] A single α-cyanoacrylate monomer or a mixture of two or more of these α-cyanoacrylate monomers can be used.

Generally, the above α-cyanoacrylate monomer used alone as an adhesive, and one or more components such as those set forth below, are used to formulate a commercial composition. The additional components include accelerators; anionic

polymerization inhibitors; radical polymerization inhibitors; additives, such as plasticizers, heat stabilizers and toughening agents .

[0021] The infrared absorbing dye-containing cyanoacrylate composition includes an amount of the α-cyanoacrylate monomer in the range of from about 50 to 99.5% by weight, such as 60 to 90% by weight, desirable 75% by weight, based on the total weight of the composition.

[0022] The infrared absorbing dyes useful in the present invention are preferably compatible with the α-cyanoacrylate monomer and thus creates the ability to formulate a one part composition. In other instances where the infrared absorbing dye is incompatible with the α-cyanoacrylate monomer, the infrared absorbing dye may be dispensed onto a substrate to be welded either before or after the α-cyanoacrylate monomer is dispensed onto that substrate or another substrate to be laser welded to the first substrate.

[0023] The infrared absorbing dyes should having the

following attributes:

• A narrow, absorption band near 800 nm (or longer

wavelengths, depending on the laser used) with a high molar absorption coefficient.

• Little to no absorption in the region 400-700 nm.

• Preferably adequate solubility in the reactive carrier. [0024] Examples of such infrared absorbing dyes include the cyanine dyes, the squarylium dyes, and the croconium dyes.

In addition, metallized azo dye (such as the ones disclosed in U.S. Patent No. 4,892,584), rare earth metal chelates (such as the ones disclosed in U.S. Patent No. 5,837,042), polyester with phthalocyanines, naphthalocyanines, or squarines copolymerized therewith (such as the ones disclosed in U.S. Patent No.

5,990,197), and uncomplexed metal phthalocyanine (such as the ones disclosed in U.S. Patent No. 6,149,719). See also

International Patent Publication No. WO 02/083798 and U.S.

Patent No. 5, 093,147.

[0025] A tris (dialkylaminophenyl) aminium dye coupled with one anion, a nickel dithiolene, a tetrakis (dialkylaminophenyl) aminium dye coupled with one anion, a cyanine, a squarylium, and croconium.

[0026] Other useful dyes include nickel dithiolate, nickel dithiolene, cyanine, squarylium and croconium. Preferred dyes comprise tris (dialkylaminophenyl) aminium (or, tris [4- dialkylamino) phenyl] ammoniumyl) , tetrakis (dialkylaminophenyl) aminium (or, N,N-bis (4-dialkylaminophenyl) -N- [4- (N, N-bis (4- dialkylaminophenyl) -amino) phenyl] aminium), and tetrakis

(dialkylaminophenyl) diimonium (or, 2 , 5-cyclohexadiene-l, 4- diylidene-bis [N, N-bis (4-dialkylaminophenyl) -ammonium] ) .

[0027] The tris (dialkylaminophenyl) aminium dye may be represented by the following structure:

X-

where :

Ri through R₆ each independently represent a substituted or unsubstituted alkyl group of 1 to 8 carbon atoms; and

X^~ represents an anion.

[0028] Particularly useful forms within the above structure include those where Ri through R₆ each independently represent a methyl, ethyl, propyl or butyl group. More particularly, Ri through R₆ may each independently represent an n-propyl or i- propyl group; or an n-butyl, i-butyl, or t-butyl group.

[0029] Optionally, Ri and R2 may join to form a ring, or R₃ and R4 may join to form a ring, or R5 and R₆ may join to form a ring .

[0030] The anion, X^~, is represented by hexafluoroantimonate, hexafluorophosphate, hexafluoroarsenate, perchlorate or

tetrafluoroborate . [0031] The tetrakis (dialkylaminophenyl) aminium dye may be represented by the following structure:

where :

Ri through R₈ each independently represent a substituted or unsubstituted alkyl group of 1 to 8 carbon atoms; and

X^~ represents an anion.

[0032] Particularly useful forms within the above structure include those where Ri through R₈ each independently represent a methyl, ethyl, propyl or butyl group. More particularly, Ri through R₈ may each independently represent an n-propyl or i- propyl group; or an n-butyl, i-butyl, or t-butyl group.

[0033] Optionally, Ri and R2 may join to form a ring, or R3 and R4 may join to form a ring, or R5 and Re may join to form a ring, or R₇ and R₈ may join to form a ring.

[0034] As above, the anion, X^~, is represented by

hexafluoroantimonate, hexafluorophosphate, hexafluoroarsenate, perchlorate or tetrafluoroborate . [0035] The tetrakis (dialkylaminophenyl) diimonium dye may be represented by the following structure:

where :

Ri through Rs each independently represent a substituted or unsubstituted alkyl group of 1 to 8 carbon atoms; and

X^~ represents an anion.

[0036] Particularly useful forms within the above structure include those where Ri through R₈ each independently representing a methyl, ethyl, propyl or butyl group. More particularly, Ri through R₈ may each independently represent an n-propyl or i- propyl group; or an n-butyl, i-butyl, or t-butyl group.

[0037] Optionally, Ri and R2 may join to form a ring, or R₃ and R4 may join to form a ring, or R5 and Rε may join to form a ring, or R7 and R₈ may join to form a ring.

[0038] And as above, the anion, X^~, is represented as

hexafluoroantimonate, hexafluorophosphate, hexafluoroarsenate, perchlorate or tetrafluoroborate . [0039] Cyclic planar compounds consisting of four pyrrole rings bridged to each other by methyne carbon atoms and chelated with a metal ion bearing a +2 charge, such as Pt⁺², Cu⁺², or Zn⁺², palladium porphyrin, metalloazaporphyrins, by cyclic planar compounds consisting of four pyrrole rings bridged to each other by nitrogen atoms and chelated with a metal ion bearing a +2 charge, such as Pt⁺², Cu⁺², Zn⁺², or Pd⁺², Fischer Base dyes, indolene molecules comprising a benzene ring fused to a pyrrole ring with the N in a position adjacent to the juncture.

[0040] A dienyl group is attached at the pyrrole carbon adjacent the N and is terminated with numerous molecular moieties which are conjugated with double bonded structures and with various alkyl substituents on the indolene ring.

[0041] The infrared absorbing dyes may be used as discrete components, or in blends of dyes. It is also possible that a blend of dyes containing a non-infrared dye, such as a radio- opaque dye, provided these non-infrared dyes do not confer substantial coloration to the part and of course are compatible with the cyanoacrylate monomer should it be mixed together therewith prior to application on a substrate. Such added non- infrared dyes may be useful in nondestructive testing or examination of subsequent welds.

[0042] Infrared dyes absorb radiation in the region beyond 780 nm with high efficiency. That is, these dyes have high extinction coefficients at one or more wavelengths in that spectral region. When this radiation is absorbed from a laser source, the dye dissipates the absorbed energy principally as heat via vibronic relaxations. The heat is localised to the dye and surrounding environment. Here, the surrounding environment is a polymer material, and thus melting occurs at the surface interface between the dye and polymer. If an infrared radiation transmissive polymer material is adjacent to this surface, the melting will cause a weld to occur.

[0043] The workpieces to be joined by the laser welding method using the infrared absorbing dye-containing cyanoacrylate compositions are constructed of a plastic material, examples of which include polycarbonate ("PC") , polymethylmethacrylate ("PMMA"), polyamides, and polyesters.

[0044] The laser should be oriented to pass through for instance an upper substrate to the joint is described in U.S. Patent No. 5,893,959, the contents of which are incorporated herein by reference. The laser address wavelength should be efficiently coupled into strong absorption bands provided by the infrared dye. Suitable laser wavelengths are known to those skilled in the art and may be derived from frequency doubled or tripled Nd:YAGs, Argon ion, Cu vapor, Ruby, HeNe, Krypton ion (647 nm) , diode and dye pumped lasers.

[0045] To the surface of at least one of the substrates to be joined is applied the infrared absorbing dye, which may be in the reactive carrier. If liquid, the reactive carrier may be applied by dip coating, dye infusion, painting, spraying, printing, dry burnishing, paste application and the like. If solid, the reactive carrier may be applied as a preformed film or a powder. The reactive carrier, whether applied as a component of the composition or separately from the reactive carrier, will bond the surfaces to be joined at least

temporarily so that a laser may expose the joining region to radiation. Bonding will occur instantly from the reactive liquid carriers, or on demand after mild bulk heating from the solid reactive carriers.

[0046] Typically, the infrared radiation will have a

wavelength of 780 nm or more, typically up to 1500 nm. [0047] According to the invention, therefore, a method of forming a weld between at least two surfaces of one or more workpieces over a joint region is provided where the method includes providing a composition comprising an infrared

radiation absorbing material in a reactive carrier; effecting an instant bond between the surfaces at room temperature or above room temperature, exposing the joint region to incident

radiation having a wavelength outside the visible range so as to cause melting of the surface of one or both workpieces at the joint region, and allowing the melted material to cool thereby welding the workpieces together.

[0048] The infrared absorbing dye-containing cyanoacrylate composition -- when in one part form -- is applied on at least one surface of one or more workpieces at the to-be-formed joint region (s ) .

[0049] With reference to the figures, Figure 1 shows a first plastics workpiece 1 and a second plastics workpiece 2

positioned in overlapping contact so as to define a joint region indicated at 3. In the joint region 3 between the plastics workpiece 1,2 in the composition of the reactive liquid carrier and the radiation absorbing material. The joint is welded by exposing the joint region 3 to a laser beam generating infrared radiation .

[0050] The first plastics workpiece 1 is transmissive to radiation from the radiation beam 4 and may or may not transmit visible light. In this respect, transmissive means that the plastics workpiece 1 absorbs less than a predetermined portion of the incident radiation. Accordingly, the plastics workpiece 1 may be transparent or translucent to radiation in the visible spectrum, or may reflect such radiation but typically will not be totally absorbent (i.e., black) . [0051] The plasties workpiece 2 also may or may not be transmissive to radiation in the visible spectrum and may or may not absorb infrared radiation directly.

[0052] The radiation beam 4 has a wavelength outside the visible spectrum but in a range which will be absorbed by the dye .

[0053] That is, infrared radiation having a wavelength between 780-1500 nm. Accordingly, the laser 5 may be an Nd:YAG laser, or a diode laser.

[0054] When the joint region 3 is exposed to the radiation beam 4, the additive will absorb the radiation. This causes the workpiece, to which the infrared absorbing composition is applied, to heat up melting the plastics workpieces 1,2 in the joint region 3, whereby on cooling the workpieces weld together. When the weld is formed, because the materials at the weld are transmissive to radiation in the visible spectrum, the weld itself will make little or no change to the visible appearance of the component. Welding occurs as a result of the heat

generated giving melting of the plastic material up to a depth of typically 0.2mm. Where compatible material is in intimate contact interdiffusion of molecules and hence welding will occur. It is well known that cyanoacrylate monomers may be prepared by ready thermal depolymerisation of its polymer, so that heat absorbed in the infrared containing polycyanoacrylate may also cause interdiffusion of cyanoacrylate molecules into the substrate. The heat generation at the weld interface is controlled by the absorption coefficient of the dye layer, and the processing parameters. The main parameters are laser power, which is typically between 1OW and 500W, the welding speed

(typically 5-200mm/sec) and the spot size of the laser beam (0.5-lOmm wide) . Processing can also be carried out with a fixed laser array, which would irradiate the joint area for a defined time.

[0055] Typical absorbent additives can be selected from chemical groups such as metal phthalocyanine dyes, metalated azo dyes and metalated indoaniline dyes. Table 1 below provides a set of examples of matched sources and materials:

Table 1

[0056] The invention also provides a method for nondestructive testing of a laser welded plastic joint enabled by imaging from dye components contained in the region of the weld. The method includes the steps of examining the welded region of intact welded parts by an X-Ray imaging system.

[0057] The invention also provides a method for providing insigna in a plastic welded part enabled by imaging from dye components contained in the region of the weld. The method includes the steps of examining the welded region of intact welded parts by way of a spectrometer tuned to sense absorbivity of dye components in the welded region.

EXAMPLES

[0058] Example 1: PMMA sheet welding: Two clear sheets of polymethylmethacrylate approximately 3mm thick are lap welded using the ClearWeld™ process with a Nd:YAG laser. A 10-15μm MMA film containing typically 0.01-0.1 wt % infrared absorbing dye is placed at the interface. The two pieces are clamped together and welded with an applied power of IOOW at speeds in the range 0.1-1. Om/min . The laser beam will be approximately 6mm in diameter and the film 5mm wide.

[0059] Example 2: PMMA sheet welding: Two clear sheets of polymethylmethacrylate approximately 3mm thick are lap welded using the ClearWeld™ process in combination with adhesive bonding. The MEK based ClearWeld™ absorber dye is applied to only one coupon and the cyanoacrylate monomer to the other, prior to bonding and laser welding. The amount of absorber applied is 50 nl/mm². The laser welding is carried out using a Nd:YAG laser and set at IOOW power, 3 m/min weld speed.

[0060] Example 3: PMMA sheet welding: Two clear sheets of polymethylmethacrylate approximately 3mm thick are lap welded using the ClearWeld™ process in combination with adhesive bonding. The MEK based ClearWeld™ absorber dye is applied to one coupon and the cyanoacrylate monomer to the other, bringing the two coupons together prior to bonding and laser welding. The amount of absorber applied was 80 nl/mm². The laser welding is carried out using a Nd:YAG laser and set at IOOW power, 3 m/min weld speed.

[0061] Example 4: PMMA sheet welding: Two clear sheets of polymethylmethacrylate approximately 3mm thick are lap welded using the ClearWeld™ process in combination with adhesive bonding. 10 ml of MEK based ClearWeld™ absorber is mixed with 20 g of cyanoacrylate. Two drops of this mixture are applied to one coupon and the two coupons are brought together and laser welded. The laser welding is carried out using a Nd:YAG laser and set at IOOW power, 3 m/min weld speed.

[0062] Results between examples: Tensile tests were carried out at strain rate 1 mm/min. Example 4 showed a higher joint strength of 1,51 kN, comparing to example 1 (1,42 kN) , example 2 (1,29 kN) and example 3 (1,14 kN) . [0063] Example 5: Infrared dye-containing cyanoacrylate compositions: The infrared absorbing dye is dissolved in dry acetone forming a dye solution. The dye solution is

subsequently mixed with cyanoacrylate monomer forming the dye- containing cyanoacrylate composition. Two drops of the dye- containing cyanoacrylate composition is applied to one PMMA coupon and then brought together to another PMMA coupon forming an instant joint (about 2 seconds) . Some of the infrared dyes include the ADS 830AT (American Dye Source), S0268 or S2007 (FEW Chemicals), E1125 (Epolight) and IR-27 (Sigma-Aldrich) .

Standard dye concentrations in dye-containing cyanoacrylate compositions are from 0,001% to 0,02%.

Claims

WHAT IS CLAIMED IS:

1. A composition comprising an infrared absorbing dye in a reactive carrier.

2. A composition according to Claim 1 wherein the reactive carrier is in a liquid or a solid form.

3. A composition according to Claim 1 wherein the reactive carrier is a cyanoacrylate .

4. A composition according to Claim 1 wherein the dye is ADS 830AT (American Dye Source), S0268 or S2007 (FEW Chemicals), E1125 (Epolight) , IR-27 (Sigma-Aldrich) , a cyanine dye, or a squarylium dye, or a croconium dye, or a metallized azo dye, or a rare earth metal chelate dye, or a phtalocyanine dye, or a naphthalocyanine dye, or a squarine copolymerized therewith or an uncomplexed metal phthalocyanine dye.

5. A method of forming a weld between at least two surfaces of one or more workpiece(s) over one or more joint region (s), steps of which comprise:

providing two or more surfaces of one or more workpiece(s) to be joined;

providing a composition comprising an infrared absorbing dye in a reactive carrier;

applying the composition to at least one of the surfaces to be joined;

joining the surfaces to be joined to form a joint region; and

exposing the joint region to incident radiation having a wavelength outside the visible range so as to cause melting of the surface of one or both workpieces at the joint region.

6. A method according to Claim 5 further comprising allowing the melted material to cool thereby welding the workpieces together .

7. A method according to Claim 5, wherein the reactive carrier is a cyanoacrylate .

8. A method according to Claim 7, wherein the cyanoacrylate is the cyanoacrylate composition includes at least one

α-cyanoacrylate monomer of the formula:

H₂C =C COOR¹

CN

where R¹ represents a straight chain or branched chain alkyl group having 1 to 12 carbon atoms (which may be substituted with a substituent such as a halogen atom or an alkoxy group) , a straight chain or branched chain alkenyl group having 2 to 12 carbon atoms, a straight chain or branched chain alkynyl group having 2 to 12 carbon atoms, a cycloalkyl group, an aralkyl group or any aryl group.

9. A method according to Claim 5, wherein the infrared absorbing dye is provided as a coating on the substrate prior to the application of cyanoacrylate.

10. A method according to Claim 5, wherein the radiation absorbing material is exposed to radiation through one of the workpieces .

11. A method according to Claim 5, wherein at least one of the workpieces is made of a plastic material.

12. A method according to Claim 5, wherein the radiation absorbing material has a lower limit of an absorption band above 700 nm.

13. A method according to Claim 12, wherein the absorption band defines the range 780-1100 nm.

14. A method according to Claim 12, wherein the absorption band defines the range 820-860 nm.

15. A method according to Claim 12, wherein the absorption band lies in the infrared range.

16. A method according to Claim 12, wherein the radiation is a provided by a laser beam.

17. A method according to Claim 12, further comprising a non- infrared dye.

18. A composition according to Claim 17, wherein the non- infrared dye is a radio-opaque dye.

19. A method for non-destructive testing of a laser welded plastic joint enabled by imaging from dye components contained in the region of the weld, steps of which comprise examining the welded region of intact welded parts by an X-Ray imaging system.

20. A method for providing insigna in a plastic welded part enabled by imaging from dye components contained in the region of the weld, steps of which comprise examining the welded region of intact welded parts by way of a spectrometer tuned to sense absorbivity of dye components in the welded region.