VISIBLE LIGHT CURABLE FREE RADICAL COMPOSITIONS CONTAINING n-ARENE METAL COMPLEXES

1. DESCRIPTION

FIELD OF THE INVENTION

This invention relates to free radical polymerisable compositions and to the use of π-arene metal complexes in a photoinit ator system for such compositions. The invention is applicable to both light-sensitive compositions which are curable on exposure to light and light-activatable anaerobic compositions which are activated on exposure to light so that they are curable on subsequent exclusion from air.

BACKGROUND ^" OF THE INVENTION

There are many known free radical polymerisable compositions which are curable or activatable on exposure to ultraviolet light. However the use of UV-light is disadvantageous and open to some risks in commercial practice. It would be preferable therefore to use visible light as the energy source. Some sensitising agents or photoinitiators such as fluorenone peresters and 1 ,2-diketone/amine combinations are known for rendering acrylic compositions sensitive to visible light but the resulting compositions will only cure under limited conditions and it is difficult to achieve a tack-free surface on the cured composition.

It is known to use organometallic complexes as initiators in cationic polymerisable compositions. European Patent No 0 094 915 Ciba-Geigy AG

A) A cationically polymerisable organic material and

B) At least one compound with -formula I:

where a stands for 1 or 2 and n and q independently of each other each stand for an integer from 1 to 3, M is the cation of a monovalent to tri- valent metal from groups IVb to Vllb, VIII or lb of the periodic system (CAS notation), m is an integer corresponding to tne valence of L + q, Q is a haloge and L is a metal or nonmetal with a valence of 2 or 7, R is a π-arene and R is a π-arene or the anion of a π-arene.

European Patent Application 0109851 Minnesota Mining and Manufacturing Company described a composition comprising a cationically polymerisable material and a catalytically effective amount of an ionic salt of an organometallic complex cation as polymerisation initiator, said ionic salt of an organometallic complex cation being capable of adding an intermediate strength nucleophile or upon photolysis of liberating at least one coordina¬ tion site, said metal in said organometallic complex cation being selected from elements of Periodic Groups IVB, VB, VIB, VIIB and VIII.

There is nothing in the prior art to suggest that the ir-arene metal com¬ plexes of European Patent No 0094915 would have particular util ty and advantage in free radical polymerisable compositions.

SUMMARY OF THE INVENTION

The present invention provides a polymerisable composition comprising

1) free radically polymerisable monomeric material

2) a ir-arene metal complex of formula I as ^' defined above

3) a free radical polymerisation initiator.

The free radically polymerisable material is preferably one or more monomers and/or prepolymers containing acrylate or methacrylate groups; or a co¬ polymerisable monomer combination based on a polythiol and a polyene.

The ir-arene metal complex is preferably a complex salt of a π , π iron arene cation and a non-πucleophilic anion, which is of the formula:

R ⁶

Fe ⁺ LZ ^' VII

'7

R ⁷ -

where R is a η arene such as benzene, alky!benzenes (e.g. toluene, xylene, mesitylene, and cumene), alkoxybenzene (e.g. methoxybenzene), halobenzenes (e.g. chlorobenzene), haloalkylbenzenes (e.g. p-chlorotoluene) , naphthalene, alkoxynaphthalenes, alky!naphthalenes, halonaphthalenes (e.g. methoxy- naphthalene, methylnaphthalene, chloronaphthalene), biphenyl, indene, pyrene

or dipheπylsulfide; R is the anion of a cyclopentadieπyl compound such as cyclopentadiene, acetylcyclopentadiene, benzoyl cyclopeπtadiene, butyl- cyclopentadiene, a ylcylopentadiene, indene and the like, L is di- to heptavalent metal or metaloid, Z is a halogen and is equal to 1 Plus the valence of L. Examples of the ions LZ _k - include SbF _β ~, BF , AsF , and PFg . Examples of such compounds include:

The cumyl compound is available from Ciba-Geigy Ltd., Basle, Switzerland, under the number CG 24-61. The other three compounds are described in

Meier, et al , Paper FC 85-417, presented at the Radcure Europe 85 Conference,

May 6-8, 985. Further compounds of this type are described in EP 0 094 915.

The free radical polymerisation initiator is most suitably a peroxide or hydro- peroxide compound such as benzoyl peroxide, cumene hydroperoxide, methyl- ethylketoπe peroxide and the like.

Both the π-arene metal complex and initiator are suitably employed at levels of 0.01 - 10%, preferably 0.1 - 8% , and more particularly 0.5 - 5% by weight of the composition.

The compositions according to the invention include photosensitive compo¬ sitions which are curable on exposure to visible or UV- light and anaerobic compositions which are activatable on exposure to visible or UV light. The composition may also incorporate one or more cationically polymerisable monomers of the kind described in European Patent 0094915.

The compositions of the invention may also contain compatible imides. How¬ ever in the application entitled "Thiolene compositions based on bicyclic ene compounds" filed on even date herewith there are described a curable thiol-ene formulation comprising: a) . a polyth ol b) a polyene compound having a plurality of groups of the formula

where Q is CR' ₂ , 0, S, NR' or S0 ₂ , R' is H or alkyl, and m is 0-10, and c) an effective amount of a free radical initiator which is a mixture of a peroxide or hydroperoxide compound and a n5, η6 arene complex salt of the formula VII as defined above. In this application we make no claim to such a composition.

The acrylic or methacrylic monomer material may be any of such materials which are known in the art. The thiol-ene components may likewise be any of such materials which are known in the art such as in US Patents 2767 156; 3 661 744; 3 740844; 4 119 617 and 4157 421.

The composition may also contain another organometallic complex, particu¬ larly a metallocene such as ferrocene in an amount of 0.1-10 by weight of the composition.

It is surprising that the compositions of the present invention are so sensitive to visible light. Current visible-light sensitive coating compositions based on free radical polymerisable monomers and sensitizers or photoinitiators selected from fluorenone peresters or 1,2-diketone/amine combinations will not cure when exposed to light from low intensity fluores¬ cent tubes (approx. 30 ) or from high intensity tungsten halogen lamps (e.g. Philips 650 PF 813). Compositions of the present invention cure rapidly on exposure to such light sources and (in the case of acrylic monomers) give tack-free surfaces at the high intensities, and even at low intensities on long exposures.

Existing light activatable anaerobic compositions require activation with UV light. Anaerobic compositions according to the present invention may be activated by visible light, e.g. high intensity tungsten/halogen lamps or low intensity fluorescent lighting tubes.

Imide-containiπg compositions cannot be photocured with conventional photo¬ initiators in thick sections due to light screening by the imide. The present invention overcomes that problem; sections of imide-containing compositions according to the invention, with a thickness of several mm's, have been successfully cured with visible light.

DESCRIPTION OF THE PREFERRED EMBODIMENT

EXAMPLE 1

A stirred mixture of mesitylene (24 g; 0.2 m) ferrocene (37.2 g; 0.2 ) , aluminium powder (5.4 g; 0.2 m) , aluminium chloride (53.2 g; 0.4 m) and methylcyclohexane (500 mis) were refluxed under ₂ for 20 hrs. After cooling the reaction mixture was poured onto 300 g of crushed ice and water. The aqueous layer was separated and the organic layer extracted with 3 x 100 mis water. The combined aqueous extracts were filtered and added to the original aqueous layer. The combined aqueous extracts were washed with 2 x 90 is portions of cyclohexane and added to a warm solution of potassium hexafluorophosphate (36.8 g) in 250 mis of water. A green- yellow coloured preci ate formed which was removed by filtration and dried in a vacuum desiccator to yield 23.3 g of crude product. Recrystallization from a mi ture of acetone and ether gave 14.2 g of a pure product identi-

6 5 fied from elemental analysis as (η -mesitylene)(η -cyclopentadienyl) iron hexafluorophosphate:

Calculated Found

% Carbon 43.52 42.59 % Hydrogen 4.40 4.23 % Halogen 29.53 30.00 % Phosphorous 8.03 8.35 % Iron 14.51 12.68

The gel permeation chromatogram (G.P.C.) (m crostyragel columns) showed only one component. The UV/visible light spectrum showed absorption maxima at λ = 410 nm (t= 180) and λ = 242 nm (e = 530) and a shoulder at λ = 450 nm (ε - 120). The compound decomposed without melting at 236°C.

EXAMPLE 2

A stirred mixture of anisole (130 mis), ferrocene (9.3 g) , aluminium powder (1.35 g) and aluminium chloride (13.3 g) was heated at 85°C for 16 hours under a nitrogen at osphene. This was followed by a further heating period at 100°C for 3.5 hours. After cooling the mixture was slowly added to 300 mis of a mixture of ice and water. The entire mix¬ ture was filtered and the aqueous layer separated. To this was added a warm solution of 9.2 g of potassium hexafluorophosphate in 100 mis of water. The solution was cooled to 2°C for 16 hours during which time a green-yellow solid precipitated. This material was collected by filtra¬ tion and dried to yield 2.3 g of a green-yellow compound believed to be

6 -[

(n -methoxybenzene)(η -cyclopentadienyl) iron hexafluorophosphate.

EXAMPLE 3

The following blend of free radically polymerizable monomers was prepared by mixing the components together until a homogenous mixture resulted:

Blend A

Acrylate Ester Terminated Prepolymer (Synocure 3100,

Cray Valley Products) 75.19 g Acrylate Ester Monomer (Tone M-100, Union Carbide) 10.60 g Acrylic Acid 3.61 g N- inyl-2-Pyrrolidone 10.60 g

A photosensitive composi tion was prepared by dissolving into this monomer blend Z% ( _α ⁶ -cumene) (π ⁵ -cyclopentadienyl ) iron hexafluorophosphate (CG024-61 Ciba Geigy) and 2% cu ene hydroperoxide. Thin films of the photosensi tive composition, 75u in thickness were prepared on cleaned 4 x T" microscope glass slides. The coated slides were then independently exposed to UV and visible light from a number of sources and the times required to obtain tack-free through-cured coatings determined. The results obtained are as follows:

Light Source Distance from Source (cms) Cure Tim

200 W/Inch Medium Pressure Hg Lamp (LI) 20 5 sees.

(UVA LOC 1000W, Loctite Deutschland GmbH)

UVA LOC 1000 fitted with poly- (L2) 20 60 sees. carbonate filter 650M Tungsten Halogen Lamp (L3) 6 60 sees.

(Philips PF 813) 30W Fluorescent Tube (L4) 1 90 mins.

(Thorny-

Light source (LI) produces high intensity ultraviolet light over the wave¬ length spectrum 200-400 nm along with high intensity visible light at wave¬ lengths > 400 nm.

(L2) produces visible light only at wavelengths > 400 nm.

(L3) produces high intensity vi sible light at wavelengths > 400 nm along with lower outputs in the UV regϊon principally between 300 and 400 nm.

(L4) is a low intensity source of visible light. It produces no UV radiation.

The results of this experiment demonstrate the effectiveness of the inventive photoinitiator system for curing acrylate-type compositions using visible light.

EXAMPLE 4

A photosensitive composition was prepared by dissolving the following ingredients together:

Ingredient Weight Percent

Blend A (Example 3) 96 (n -mesitylene)(n cyclopentadienyl) iron hexafluorophosphate (Example 1) 2

Cumene hydroperox de 2

Coatings of the composition were prepared as described in Example 3. Tack-free surface and through curing times were measured following irra diation from the various light sources as described also in Example 3. The results obtained are as follows:

Light Source Distance from Lamp (cms) Cure Time

LI 20 5 sees

L2 20 ^' 3. 5 mins

L3 6 70 sees

L4 1 4 hrs

EXAMPLE 5

A photosensitive composition was prepared by blending the following ingredients together:

Ingredient Weight Percent

Blend A (Example 3) 96

(n 6-anisol e) (η 5 -cyclopentadienyl ) iron

PFg (Example 2) 2

Cumene hydroperoxi de 2

Coatings of the compositi on were prepared as described in Example 3. Tack-free surface and through curing times were measured following i rradiation from various l ight sources as described also in Exampl e 3. The resul ts obtained are as follows:

Light Source Distance from Lamp (cms) Cure Time

LI 20 7 sees

L4 1 approx. 12 hrs

EXAMPLE 6

Compositions similar to that described in Example 3 were prepared except that the (n -cumene)(η -cyclopentadienyl) iron hexafluorophosphate (ir-arene) and cumene hydroperoxide (CHP) were independently omitted. The compositions were exposed to light sources (LI) - (L4) and cure examined as described in Example 3. The results obtained are as follows:

Light Source Distance (cms) Cure Time

No ir-arene No CHP

LI 20 > 60 sees > 60 sees

L2 20 > 3 mins > 3 mins

L3 6 > 2 mins > 2 mins

L4 1 ^• > 160 mins > 160 mins

The results of this experiment demonstrate that both the it-arene and CHP are necessary to effect the cure of monomer Blend A.

EXAMPLE 7 (Comparative Examp ^' i A)

2,2-0imethoxy-2-phenylacetophenone (IRGACURE 651, Ciba Geigy) and 2-methyl- rl-[4-(methylthio) phenylj -2-morpholinopropan-l-one (IRGACURE 907, Ciba Geigy) are well known commercially-available UV sensitive free radical photo¬ initiators. UYE-10H (GeneraTElectric) is a well known commeπcally-avail- able triarylsulfonium salt cationic photoinitiator.

Compositions containing these photoinitiators were prepared as follows:

Composition, wt I

5A 5B 5C 50 5E 5F

BLEND A 98 96 98 - 98 96

IRGACURE 651 2 2 - - - -

IRGACURE 907 - - 2 2 - -

UVE-IOH - 2 2

Cumene Hydroperoxide - 2 - 2 - 2

Coatings of the compositions were exposed to light sources (LI) - (L4) and the cure examined as described in Example 3. The results obtained are as follows:

Light Source Distance Cure Time

(cms) 5A 5B 5C 5D 5E 5F

LI 20 1 sec 1 sec 1 sec 1 sec 7 sees 7 sees

L2 20 >3 mins >3mins >3 mins >2 mins >3 mins >3 mins

L3 6 >2 mins >2 mins >2 mins >2 mins >2 mins >2 mins

L4 1 >3 hrs >3 hrs >1.5 hrs >4 hrs >70 mins >70 mins

These results demonstrate that conventional free radical or cationic photo¬ initiators do not function, either alone or in conjunction with cumene hydro¬ peroxide as visible light sensitive photoinitiators of acrylate monomers.

EXAMPLE 8 (Comparative Example B)

The 4-tert-butylperoxy ester of fluoren-9-one (J BPF) alone or 1,2 diketones, such as benzil or camphorquinone (CQ) , in combination with amine co-initiators are well known visible light photoinitiators of free radical polymerization.

Compositions containing these photoinitiators were prepared as follows:

Composition in parts by weight

6A 6B 6C

BLEND A (Exampl e 3) 100 100 100 t-BPF 2 - -

Cumene hydroperoxide 2 - -

Benzi l - 3.3 -

2,2 ' -(p-tolylimino)diethanol - 3.3 -

CQ - - 2

N,N-dimethyl -p-toluidine ^{— —} 2

Coatings of the compositions were prepared as described in Example 3, and exposed to light sources LI - L4. The cure Was examined also as described in Example 3 and the results obtained are as follows-:

Light Source Distance from Source (cms) Cure Time

6A 6B 6C

LI 10 (*20) 5 sees 5 sees * 7 sees L2 10 (*20) > 60 sees > 60 sees * > 180 sees L3 6 > 60 sees > 60 sees > 120 sees L4 1 > 5 hrs > 2.5 hrs > 3.5 hrs

By comparing the results obtained with these prior art compositions to the results obtained with the inventive composition described in Example 3 it is clear that the latter compositions show an improved curing performance when irradiated with only visible light (L2 and L4) or with light rich in visible light (L3).

13

EXAMPLE 9

A radiation activatable anaerobic composi tion was prepared by mixing the followi ng ingredi ents together in the fol lowing weight percentages :

Adhesive Component Weight Percent

Monomer X 22.4

Monomer Y 48.6

2-hydroxypropy methacrylate 25.0

Cumene hydroperoxide 2.0 (n 6-cumene)(η5-cyclopentadienyl) iron- hexafluorophosphate 1.0

Ferrocene 1.0

Monomer X is the urethane acrylate reaction product of toluene diisocyanat and the hydroxypolyoxypropylene derivative of trimethylolpropane (commer¬ cially available under the trade name Pluracol TP 2450) having unreacted isocyanate functionality capped with hydroxyethyl methacrylate.

Monomer Y is the urethane acrylate prepared by reacting two moles of toluen diisocyanate wi h one mole of hydrogenated bisphenol A, diluting the reacti mixture with methyl methacrylate and ^" further reacting it with two moles of hydroxyethyl methacrylate in the manner disclosed in Example V of US Patent No. 3,452,988.

Pairs of mild steel grit blasted 1apshears2.5cms wide, were coated on one face with a film of the above composition to a thickness of approx. 0.5 mm. The coated lapshears were then exposed to visible light from a Thorn 30W, Warmwhite fluorescent light tube for different periods of time. The coated panels were placed 1 cm directly under the light tube. After the irradiati period, small pieces of stainless steel wire, 0.125 mm diameter and bent in a U-shape were embedded into the adhesive coating of one panel of the pair. The coated surfaces of each pair were then firmly placed in contact to give in each case a \ inch overlap in the length direction of the lapshears for an adhesive bond to develop. In each case the bondline gap of the join

was determined by the wire diameter, viz. 0.125 mm. The bonds were left for 84 hours at room temperature to cure. The bond strengths were meas¬ ured in a tensile shear mode using conventional tensile testing equipment. The results are summarised as follows:

Exposure Time Tensile Shear Breaking

2 Sees. Strength, daN/cm

0 0

30 0

60 46.5

The result demonstrates that irradiation with visible light had activated the adhesive composition for bonding to occur.

EXAMPLE 10

Light sensitive cαnpositions were prepared by blending together the follow¬ ing ingredients in parts by weight:

8A 88

Acrylate ester terminated prepolymer

(Synacure 3100, Cray Valley Products) 70 70

3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ERL-4221 , Union Carbide) 30 30

(n- -cumene) (n -cyclopentadienyl) iron hexafluorophosphate 2 -

Cumene hydroperoxide 2 -

Di olyliodonium hexafluorophosphate - 2

2,2-dimethoxy-2-phenylacetophenone - 1

15

Coatings of the composi tions were prepared as described in Example 3 and exposed to various l ight sources . The cure was examined as described i n Exampl e 3 and the resul ts obtai ned are as fol lows :

Light Source Distance from Light (cms) Cure Time

8A 8B

LI (UV + VIS) 30 5 sees 3 s L3 (Mainly VIS) 6 50 sees > 80 s L4 (VIS. only) 1 90 mins No cur

Composition 8A of the present invention is clearly polymerisable to a tack- free coating by irradiation with visible light, whereas prior art composition 8B is sensitive only to ultraviolet light.

EXAMPLE 11

4-allyloxy-3-methoxypropenylbenzene was prepared by refluxing a stirred mix¬ ture of isoeugenol (164 g), allylbro ide (133 g) and potassium carbonate (278 g) in dry acetone (700 is) for 18 hours. Removal of the solids and solvent gave a crude product (208 g) which was vacuum distilled to yield 130 g of pure 4-allyloxy-3-methoxy-prop-l-enylbenzene (b.p. 104-118°C at 0.1 mbar).

EXAMPLE 12

Light sensi tive thiol/ene composi tions containing the diene descri bed in Example 11 were prepared as fol lows :

Component Weight X

10A 10B IOC

4-al lyloxy-3-methoxyprop-l -enylbenzene 44 45 45 Pentaerythritol tetrakis (β-mercaptopropionate

(Q43 from Thiokol) 52 53 53 (n -cumene) (n- -cyclopentadienyl ) iron hexafluorophosphate 2 Cumene hydroperoxide 2 2,2-dimethoxy-2-pheπylacetophenone

Coatings of the cαnpositions were prepared as described in Example 3 and the cure examined also as described in Example 3. The results obtained are as follows:

Light Source Distance from Source (cms) Cure Time

10A 10B IOC

LI (UV and Visible) 10 >2 mins >2 mins 15 sees L4 (Visible only) 1 90 sees >2 hrs > 1.5 hrs

On irradiation with visible light only (Source L4) , composition 10A of the present invention is clearly superior to prior art composition IOC. The results obtained with composition 10B demonstrate the advantage of incorporating a peroxidic material into the formulation. Composition 10A is not suitable as a UV sensitive material however due probably to a UV light induced acid catalysed dimerization of the s-methyl substituted styrene function of the diene component. The development of an intense green colour in this composition during UV irradiation is an indication of this reaction.

EXAMPLE 13

A light sensitive thiol/ene composition was prepared by dissolving to¬ gether the following materials:

Weight %

SPILAC T-510-7 (Showa Highpolymer Co., Tokyo) 96 (n 6-cumene)(n5-cyclopentadienyl) iron hexafluorophosphate 2

Cumene hydroperoxide 2

Coatings of the composition were prepared and the cure examined following irradiation as described in Example 3. The following results were obtained:

Light Source Distance from Source Cure Time

LI 20 1 sec

L2 20 5 sees

L4 1 50 sees

When the hydroperoxide was omitted from the formulation the curing times were as follows:

LI 1 sec

L2 6 sees

L4 No cure after 1.5 hrs irradiation

When the conventional photoinitiator 2,2-dimethoxy-2-phenylacetophenone was used instead of the π-arene in the above formulation the composition failed to cure under visible light sources L2 (>60 sees) and L4 (>15 hours),

EXAMPLE 14

A light sensitive thiol/ene composition was prepared by blending ±ogether the following ingredients in parts by weight:

Ingredient Parts by Weight

Trially!-s-triazinetrione (triallyl isocyanurate) ^" 40.8

Tetra thiol Q-43 ( f_ Example 12) 56.9

Phosphorous acid 0.1

2,6-di-tert-butyl- -methylphenol 0.2

2,2,dimethoxy-2-phenylacetophenone 2.0 (η6-cumene)(n5-cyclopentadienyl) iron hexafluorophosphate 2.0

Cumene hydroperoxide 2.0

A coating of the composition was prepared and the cure examined following irradiation 1 cm under the visible light source L4 as described in Example 3. The coating was found to be fully cured after 90 sees exposure. A similar composition but without the ττ-arene and hydroperoxide ingredients failed to cure under the same conditions.

EXAMPLE 15

A solution of 17.8 g of methyl -5-norbornene-2 ,3-di carboxyl i c anhydride and 5.4 g of 1 ,3-phenylenedi amine in 80 is of acetone was stirred at room

temperature under nitrogen for 0.5 hours. 0.1 g of nickel acetate and 2.5 mis of triethylamine were then added and the mixture heated to reflux temperature. 12.7 g of acetic anhydride were then added and the mixture heated for a further 3 hours. 80 mis of water were added and the mixture was chilled to 2 ^g C. At this stage a brown viscous resin precipitated and the solvent mixture was decanted. The resin was dissolved in 90 mis of dichloromethane, washed with 2 x 100 ml portions of water and dried over sodium sulphate. Removal of the solvent under reduced pressure yielded 18.3 g of a brown viscous resin which was shown by gel permeation chroma- tography to consist of one major component, higher in molecular weight tha either of the starting compounds and a minor component identified as the starting anhydride. The major component is believed to be the bis(norborn ene i ide) having the formula:

EXAMPLE 16

A light sensitive composition was prepared by dissolving the following ingredients together:

Blend A (Example 3) 96

(n -cumene)(n -cyclopentadienyl) iron Fg 2 tert-butyl peroxybenzoate 2

Coatings of the composition were prepared as described in Example 3. Tack free and through curing times were measured following irradiation from the various light sources as described also in Example 3. The results obtaine are as follows:

PCT/US87/02619

20

Light Source Distance from Source (cms) Cure Time

LI 20 5 sees

L2 20 120 sees

L3 6 90 sees

L4 1 120 mins

EXAMPLE 17

Light sensitive compositions were prepared by blending together the following ingredients in parts by weight:

17A 17B

Blend A (Example 3) 80 80 bis(norbornene imide) of Example 15 20 20 (n -cumene)(n -cyclopentadienyl) iron hexafluorophosphate 2 _ Cumene hydroperoxide 2 2 2,2-dimethoxy-2-phenylacetophenone - 2

A sample of each composition was poured into an open-topped cylindrical mould as described in Example 16 and irradiated under light source LI, 30 cms from the lamp, for 60 sees. After this time composition 17A was found to have completely gelled through the 4 mm depth whereas composition 17B had cured only to a depth of 0.5 mm, the rest of the material remain¬ ing liquid. A similar result was obtained when the samples were poured into an open-topped cylindrical mould 8mm in diameter and 4mm in depth and the filled mould exposed to UV/visible light from a SUPERLITE 201 (high pressure mercury light source supplied by Lumatec GmbH, Munich, W. Germany5 through a 1m long light guide. The light system was fitted with a filter which cut-off all visible light with wavelengths greater than 530nm. Thesampleswere placed 10 mm directly below the tip of the lightguide. After 3 m ns. exposure, sample 17A had completely cured through 4mm depth forming a solvent insoluble hard gel, whereas Sample 17B had cured only in a surface layer.