GB2216136A - Novel processes for bonding polycarbonate substrates and adhesive compositions useful therein - Google Patents

Description

"NOVEL PROCESSES FOR BONDING POLYCARBONATE SUBSTRATES AND ADHESIVE

COMPOSITIONS 22 16 1 -36 USEFUL THEREIN" The present invention relates to the art of bonding a transparent polycarbonate article to another polycarbonate substrate (which may or may not also be transparent).

Since the invention is particularly directed to the task of bonding or sealing polycarbonate vehicle headlamp assemblies, it will be described in detail by reference thereto.

Automotive headlamp assemblies in general consist of two parts: a headlamp housing in which the lamps are seated; and a transparent front cover. The seal bonding the transparent cover to the housing must meet rigid requirements which in general may be described as stability against wather and normal environmental use and conditions (other than collision) over the life of the vehicle.

In the evolution of material selection for such headlamp assemblies, it has now become common to manufacture the two components from polycarbonates. While the front cover will of course be transparenti the housing will be opaque with the interior having a reflective coating for increased illumination.

-Prior to the present invention,, such headlamp assemblies have in general been bonded or sealed by one of two methods: (1) a two-part thermally curable urethane adhesive; or (2) vibration welding. 1 -1 the former, the two-part urethane composition is mixed and applied, the parts are assembled and the resulting 2 assembly is then passed through an oven for a time and at a temperature to ef f ect curing. In the vibration welding system, the assembled parts are subjected to vibrationy e.g. sonic vibration. to melt and weld the surfaces together.

Both systems require capital expenditure for the necessary equipment. In addition,, the vibration welding technique is limited by the geometric shape of the articles to be assembled. More importantly, neither system is sufficiently rapid to satisfy completely the manufacturing needs of an automotiveassembly plant.

For these reasons, there is a great need in the automotive industry for a system for manufacturing polycarbonate headlamp assemblies more rapidly and with less capital expense and equipment.

In accordance with the present invention, the aforementioned objects are achieved in an elegant and highly efficacious manner by emloying the photocurable compositions which will be described in detail hereinafter.

Photopolymerization. or radiation curing. as it is sometimes called, is in general well known. It has been referred to in the literature as "the quiet revolution". From a modest beginning in the early 1970's when it was employed primarily in printing. papermaking, and filling and coating flat wood stock. this "quiet evolution" in the technology has transformed it into a crowded art replete with patent literature. In recent years. this revolution has expanded the technology into the electronics industry, e.g. for photoresists and connectors, and for encapsulating components and sealing circuit boards, as well 3 as into the fields of paints, fibre optics, jewellry, and dental and medical applications. Today, radiationcured inks, coatings and adhesives are found in such diverse items as furniture, beverage cans, plastic containers, microchips and a host of other products.

Of the four major types of radiation curing known in the art,, namely, ultraviolet (UV), electron beam (EB), infrared (IR) and visible light (VL) curingr UV and EB have dominated the market, UV being most predominant. For example, in the early 1980's, UV curing was found in about 85% of the radiation-cured applications. EB, a relatively high capital cost system, was the curing agent in nearly all the remaining applications, particularly for thicker, heavily pigmented coatings. IR curing, which was developed firstr has been almost entirely supplanted by UV. Most recently, visible light curing has found application, mainly in such areas as dental restoration and tooth fillings.

In general, the purpose behind the development of radiation curing has been to eliminate the need for solvents and for ovens and the like for removing these solvents. Thus, radiation-cured inks, coatings and adhesives have afforded such advantages over their solvent-containing counterparts as being more economical, faster and safer. These advantages account primarily for the "quiet revolution" in the technology.

Typically, radiation-curable compositions, e.g. adhesive compositions,, will include at least one suitable polymer or oligomer, a photoinitiator and a liquid monomer in which the various other ingredients are soluble, the monomer being cross- linkable with the polymer in the presence of the curing actinic radiation IT 4 which, as mentioned, is UV light for most applications. In general, irrespective of the source of curing radiation employed, these liquid compositions may be characterized as being solvent-free and possessing an excellent shelf life, in that they may be stored in the absence of curing radiation for long periods of time.

The liquid monomers which have heretofore been suggested for use in radiation curable compositions include both mono- and poly-functional monomeric materials. Mixture of mono- and poly-functional monomers provides a useful way of varying the degree of hardness of the cured composition.

As examples of monomers heretofore employed, mention may be made of butanediol dimethacrylate, butoxyethyl methacrylate, butyl methacrylate, diethylaminoethyl methacrylate, diethylene glycol dimethacrylate, dimethyl-aminoethyl methacrylate, ethylene glycol dimethacrylate, 2-ethylhexyl methacrylate, ethoxyethyl methacrylate, glycidyl methacrylate, hydroxyethyl meth- acrylate, hydroxypropyl methacrylate, methyl methacrylate, neopentyl glycol dimethacrylate, polyethylene glycol dimethacrylate, tert.-butyl- aminoethyl methacrylate, triethylene glycol dimethacrylate, te'. _rahydrofurfuryl methacrylate and trimethylolpropane trimethacry- late. Other monomers which may be used include acrylates such as butylene glycol diacrylate, nbutylacrylate, diethylaminoethyl acrylate, 1- ethylhexyl acrylate, ethoxyethyl acrylate, hexanediol diacrylate, polyethylene glycol diacrylate, phenoxyethyl acrylate, pentaerythritol. triacrylate, trimethylolpropane triacrylate, triethylene glycol diacrylate, etc.; acrylamides such as N-isobutyloxymethyl acrylamide, Nmethylolacrylamide, N,Nl-dimethylacrylamide, N,N,- I methylene bisacrylamide, etc.; allyl monomers such as allyl glycidyl ether, allyl methacrylate, diallyl phthalate, etc.; as well as various other monomers known in the art, including vinyl monomers, , glycidyl ethers and the like.

The list of useful resins and oligomers includes: e poxy acrylate resins, oligoester acrylates, unsaturated polyesters, urethane acrylics, polymethylstyrene, styrene maleic resins, unsaturated polybutadiene hydroxyl terminated, tall oil and rosin derived resins, liquid polyamides, unsaturated alkyds, phenolics, vinyl esters, bisphenol type polyesters,, halogenated polyesters, furan resins and the like.

The photoinitiators employed will vary according to the source of curing radiation. In typical UV systems, monomethyl ether of benzoin or a higher alkyl benzoin ether may be used as the activator along with or without a peroxide polymerization catalyst. Absorption of UV radiation causes the ether to decompose into free radicals which then initiate the polymerization reaction. On the other hand, visible light activated products usually employ camphorquinone in combination with an organic amine, e.g. N, N dimethylaminoethyl methacrylate. The diketone absorbs radiation in the 420-460 nm range, producing an excited triplet state which, in conjunction with the amine,, results in ion radicals.

While not intended to be fully comprehensive, the foregoing survey will serve to illustrate the extensive state of the art pertaining to photopolymerization and the long list of suitable monomers, resins and oligomers available for selection by the polymer chemist wishing to formulate a photopolymerizable 6 composition.

As was mentioned previously, the patent literature is replete with references to various radiation-curable compositions. including combinations of polymers and monomers, which can be employed in such compositions. The following patents are illustrative of the state of the art.

U.S. Patent 41073,777 issued to O'Neill et al contains a detailed disclosure of unsaturated, water- dispersible polyester adhesives, films and textile finishes.

U.S. Patent 4,082,710 issued to Vraneken et al discloses specified isocyanate-modified compounds consisting of the reaction products of an organic iso- cyanate with compounds with multiple acrylic radicals, which isocyanate- modified compounds can be polymerized in the presence of visible or UV light. They may be used either singly or mixed with other materials, such as inert non-copolymerizable polymers, reactive copolymerizable polymers, copolymerizable oligomers, inert plasticizers, inert organic solvents, copolymerizable olefinically-unsaturated monomer compounds and various adjuvants.

U.S. Patent 4,,181,752 issued to Martens et al relates to pressure-sensitive adhesives obtained by subjecting a solventless radiation- sensitive acrylatecontaining polymerizable mass to radiation in the near UV region. Disclosure is made of procedures for the free radical copolymerization in the presence of UV of an acrylate, i.e. an acrylic acid ester of an alkanol and a monomer such as acrylic acid, methacrylic acid, acrylamide, acrylonitrile, methacrylonitrile, Nsubstituted acrylamides, hydroxy acrylates, N-vinyl 7 pyrrolidone, maleic anhydride and itaconic acid.

U.S. Patent 4,,206,025 issued to Vraneken relates to specified acrylic polyesters that can be cured by UV or EB. These polyesters are described as meaning poly- merizable organic compounds obtained by the polyesterification of dicarboxylic acid with a stoichiometric excess of OH groups of di- and polyhydric alcohols, the excess hydroxyl groups being then esterified with acrylic acid or one of its functional derivatives.

U.S. Patent 4,530g,746 issued to Azuma et al relates to photopolymerizable resin compositions comprising a monomer, e. g. a vinyl-end capped monomer, showing slight shrinkage upon polymerization. Specifically, the disclosed compositions comprise: (a) the aforementioned monomer; (b) at least one epoxy- acrylate, 1,2-polybutadiene, polyester or organopoly siloxane resin having one or more acryloyloxy groups or methacryloyloxy groups on their molecular end or ends; and (d) a photosensitizer.

U.S. Patent 4,,533,446 issued to Conway et al discloses an anaerobic adhesive composition activatable by UV or visible radiation comprising: (a) an anaero bically polymerizable acrylate ester monomer, (b) a compound which decomposes upon exposure to ultraviolet or visible light to release a strong acid, (c) a peroxy free radical initiator, and (d) an activator of anaerobic polymerizatin which,, in the presence of a strong acid, reacts with the peroxide initiator to catalyze polymerization of the monomer.

As mentioned earlier, visible light curable adhesives have to date found industrial application primarily in the dental artsf e.g. restoration and fillings. In general, such compositions employ one or 8 more methacrylic monomers one or more oligomers or polymers, and of course the photoinitiator system. As examples of illustrative patents pertaining thereto, mention may be made of U.S. Patent Nos. 4,407,984; 5 41439,380; 414591,193; 4r525,, 256; 4 15 6 3 r 15 3 and 4,,581,389. In accordance with the present invention, the aforementioned objects are accomplished by employing a visible light curable adhesive composition consisting essentially of: (1) one or more polyfunctional acrylic monomers; (2) a high molecular weight polymer e.g. an elastomer that is essentially nonreactive in the composition, which functions to increase its viscosity and which further functions as an impact modifier and to improve the flexibility of the cured adhesive or, alternatively a polyfunctional aliphatic urethane acrylate polymer; and (3) a catalyst or photoinitiator activatable by visible light.

As was mentioned previously, the present invention relates broadly to the art of bonding a transparent polycarbonate substrate to another polycarbonate substrate. More particularly, it relates toan improved adhesive system for providing polycarbonate automotive headlamp assemblies.

In manufacture, such headlamp, assemblies consist of two essential components, namely a housing for the headlamp and a transparent front cover or plate through which light from the headlamp within the housing is transmitted. These two components must then be sealed together in a manner sufficient to meet the strict requirements imposed by automotive manufacturers, which requirements are in turn predicated upon the environmental conditions associated with the operation of the j 9 vehicle containing these headlamps.

At present, there are essentially two different systems in commercial production for achieving the requisite standards for stability in usage of the assembly.

The first system utilizes a two-part thermally curable urethane adhesive. In manufacture, the two part adhesive is admixed and applied to one or both surfaces to be bonded and the respective surfaces are then brought into contact to complete the assembly. The headlamp assembly is then conveyed into an oven operating at- a temperature and for a time sufficient to provide the adhesive bond. While this adhesive system is completely satisfactory in terms of the quality of the product produced, it nevertheless suffers from certain manufacturing deficiencies, chief of which is the time required for heating to complete the seal. Other disadvantages include the energy consumption and factory space required for the oven. All of the above may of course be translated into cost factor in automotive assembly plants.

The second system for producing headlamp assemblies utilizes the technology known as vibration welding wherein the parts are assembled and subjected to vibration. e.g. sonic vibration, to melt and weld the surfaces together. This system also requires time and energy. Moreoveri vibration weldingwill inherently cause stress and weakening of the bonded area.

Consequently,, there has been a long felt need in the automotive industry to find an adhesive which will provide in seconds the requisite seal and, ideally, do so without the need f or ovens or the like energy and space consuming equipment.

For such an adhesive to be useful, as previously stated, it must meet certain rigid qualifications. To meet these standards,, it must possess the following physical characteristics:

(1) shear adhesion strength at 230C -- a bond strength of 1393 pounds per square inch (psi) or have substrate failure; (2) shear adhesion strength at -290C -- a bond strength of 1393 psi or have substrate failure; and (3) creep resistance at 880C -- it must have no slippage after 24 hours.

In addition to these functional characteristics, any adhesive contemplated for commercial use for the headlamp assembly should also be transparent and otherwise innocuous and aesthetically acceptable.

In order to justify transition in production from one of the two functionally acceptable systems heretofore described which are currently in production, a new adhesive system should offer significant manufacturing advantages, e.g. satisfy the primary task of appreciably lowering the production time for mass producing the assemblies.

In accordance with the invention, the foregoing objectives are accomplished with a novel visible light curable adhesive composition.

As previously discussed, photopolymerizable adhesive compositions are well known in the art. However,, most of the prior art relates to UV curinc which is not suitable for the polycarbonate bonding herein contemplated in view of the fact that the poly- carbonates are not sufficiently transparent to UV light, even though they transmit light within the visible spectrum. Conversely, while visible light curable adhesives have achieved some notoriety in recent years, their industrial application has been e ssentially restricted to dental composites and restoration materials. In such dental applications,, methacrylates are employed and methacrylates as a class do not cure sufficiently fast to satisfy fully the objectives of this invention.

Consequently, this invention can be said to relate to a novel use for visible light curable adhesive compositions as well as to specified compositions of this general description which are suitable for bonding polycarbonates in general and in the preparation of polycarbonate headlamp assemblies in particular.

The novel compositions of this invention consist essentially of: (1) one or more polyfunctional acrylic monomers; (2) one or more polymers as previously descried; and (3) a catalyst or photoinitiator system activatable by visible light. Optionally,, it is contemplated that these compositions may additionally contain minor amounts of other materials. Thus, while the above-mentioned materials constitute the essential components, as much as 10% by weight of additional monomers and/or polymers may be included, where found desirable or expedient to do so. [Somewhat greater amounts as high as 15% by weight have not been tested but are believed to be operative.) As examples of useful polyfunctional acrylic monomers, mention may be made of 1,6-hexanediol diacrylate,, tripropylene glycol diacrylate, trimethylolpropane triacrylate, tetraethylene glycol 12 diacrylate, pentaerithritol triacrylate, butylene glycol diacrylater triethylene glycol diacrylate, etc. For headlamp assemblies, 1,6- hexanediol diacrylate is preferred and consequently has been selected for the most extensive testing.

As noted, useful polymers include non-reactive elastomers, i.e. rubber compounds which do not react with the acrylic monomer during curing as well as polyfunctional aliphatic urethane acrylate polymers which will so react. The rubber compounds or elastomers, when employed, function to increase the viscosity to a workable level as well as to function as an impact modifier and to improve the flexibility of the cured adhesive. One useful class of elastomers is that known in the art as polyacrylic rubbers or sometimes referred to as acrylic elastomers. As examples of the materials of this description,, mention may be made of "Hycar" (trademark of 13.F. Goodrich Company), a class of "chemically saturated" rubbers made from acrylic acid esters, a particularly preferred one being Hycar 4051CG, an off-white slab form,high polymer, specific gravity=1.10, Mooney viscosity, ML-1 + 4 min., 1000C=25-40, solution viscosity, 20% in MEK (unmilled) LVF Brookfield viscosity=6,000 max.

Another useful class of elastomers providing the above-noted functions are the chlorosulphonated poly ethylene synthetic rubbers such as "Hypalon" (trademark of DuPont), white chips, specific gravity=1.14, Mooney viscosity, ML=1 + 4 at 1000C=28.

It is also contemplated that reactive elastomers may be employed, e.g. those of the "Kraton G" series (trademark of Shell Chemical Company for a series of mainly triblocks characterized as being two-phase 13 polymers consisting of polystyrene domains in a rubbery polyethylene-butylene matrix).

As examples of polyfunctional aliphatic urethane acrylates which can be employed as the polymeric component, mention may be made of "Chemlink" (trademark of Sartomer Company for a group of aliphatic urethane acrylate oligomers), e.g. Chemlink 9504, specific gravity 25 C=1.17, viscosity, cps 700F=167,000, 1000F=10,500, 1400F=3,250, and Chemlink 9505, specific gravity=1.20, viscosity, 1000F=145,,000, 1400F=35,000; CMD 8800 (trade designation of Celanese), a nonvolatile acrylated urethane based on aliphatic isocyanate, specific gravity=1.05, viscosity, 250C=semisolid, 650C=9,800 cpst and understood from Celanese trade literature to contain 10% of the monomer, ethoxyeoxy thyl acrylate; etc.

It will be appreciated that if a non-reactive polymer is employed, e.g. an elastomer of the foregoing description, the adhesive formed upon curing will comprise a mixture of that polymer and the polymer obtained by polymerization of the monomer or monomers contained in the curable composition. On the other hand, if a polyfunctional polymer such as those described above is employed, the adhesive formed by curing will comprise a copolymer of that polymer and the monomer(s) in the curable composition.

The visible light curative system may comprise the reagents heretofore known in the art for this purpose and which per se comprise no part of this invention. A particular useful system per se known in the art for catalyzing polymerization in the presence of visible light comprise a mixture of camphorquinone (d-213 bornanedione) and an organic amine, preferably N,N-di- 14 methylaminoethyl methacrylate. Generally, the catalyst combination may be employed in a ratio on the order of 1:10 camphorquinone:amine, although the ratio may be varied.

The proportions of polymer to monomer in the aforementioned compositions may vary and are not capable of precise quantification. Generally speaking, appreciably lesser amounts of the high molecular weight elastomer will be employed than would the alternate difunctional aliphatic urethane acrylate polymer.

By way oil illustration, a workable viscosity for the curable compositions [viscosity suitable for applying the formulation without undue flowing] would be in the range of 2000-20,000 cps. To achieve this viscosity, the minimum proportion of elastomer:

monomer(s) will be on the order of 1:10. Greater amounts of elastomer may of course be employed, the upper limitation being the selected range of workable viscosity. 20 When employing the aforementioned polyfunctional polymer, greater amounts are usedr as previously mentioned. For example, the range of polymer:monomer may be on the order of 1:1 to about 7:3 by weight. As will be apprec-ated, the compositions of this invention must be contained in a lighttight environment prior to use. When packaged or stored in the dark, they possess excellent shelf life.

Curing can be effected in any light source,, including ordinary room lighting, which will prcvide visible radiation, e.g. at the 460nm wave length. Preferably, howLver, curing is effected with a high intensity blue light source, e.g. with a lamp emitting on the order of 30 watts/inch energy between 400 and 470 nm. Visible light sources of this description are commercially available and thus well known in the art. The visible light curable compositions of this invention possess sufficient viscosity, e.g. between 5 2000 and 45,000 cps so that they can be applied without "running". They may be applied by any of the known methodsl for example, by coating, brushing, or with a syringe or other instrument. When applied and exposed to visible light curing radiation, they are character- ized by a rapid initial gel, e.g. a few seconds, and complete cure may be obtained in less than ten seconds following exposure to the curing radiation.

In contrast thereto, methacrylics as a class, e.g. those employed in the dental adhesive art, require a 1.5 much longer curing time,, e.g. on the order of 120 seconds. These longer curing times are not satisfactory for the uses envisioned by the present invention.

The invention may be put into practice in various ways and a number of specific embodiments will be deu;4ibed by way of example to illustrate the invention and with reference to the accompanying drawing.

The following examples show by way of illustration and not by way of limitation the preparation of visible light curable compositions contemplated by this invention.

The Figure is an exploded perspective view of an illustrative automotive headlamp assembly to which this invention is particularly directed.

EXAMPLE 1

19.0 kg of Hycar 40:1CG were completely dissolved in a mixture of 68.8 Kg of 1,6-hexanediol diacrylate and 10.0 kg ethoxyethoxyethyl acrylate at a temperature 16 of 450C by mixing for about f ive hours. While main taining the temperature, 200.0 grams of camphorquinone were added and the resulting mixture was stirred for about two hours until solution was complete. 2.0 kg of N,N-dimethylaminoethyl methacrylate were then added and the mixture was stirred for one hour in the dark to provide a visible light curable adhesive composition, Brookfield viscosity of 40,000 cps at 250C, having the following proportions of ingredients:

% by Weiqht Hycar 4051 CG 19.0 1,6-hexaned-'Lol diacrylate 68.8 Ethoxyethoxyethyl acrylate 10.0 Camphorquinone 0.2 N,N-dimethylaminoethyl methacrylate 2.0 In similar manner, the following additional compositions were prepared:

EXAMPLE 2 % by Weiqht Chemlink 9505 68.6 1,6-hexanediol diacrylate 29.2 Camphorquinone 0.2 N,N-dimethylaminoethyl methacrylate 2.0 EXAMPLE 3 % by Weiqht Chemlink 9504 68.6 1,6-hexanediol diacrylate 29.2 Camphorquinone 0.2 N,N-dimethylaminoethyl methacrylate 2.0 17 EXAMPLE 4 % by Weiqht CMD 8800 55.9 1,6-hexanediol diacrylate 35.7 Ethoxyethoxyethyl acrylate 6.2 Camphorquinone 0.2 N N-dimethylaminoethyl methacrylate 2.0 The polymeric component in each of Examples 2 to 4 is of a wax-like consistency and consequently needs to be melted. e.g. in 500C oven before attempting to admix it with the other components.

EXAMPLE 5 % by Weiqht Hypalon-30 19.15 1,6-hexanediol diacrylate 76.65 Camphorquinone 1.0 NtN-dimethylaminoethyl methacrylate 3.2 EXAMPLE 6 % by Weiqht Hypalon-30 19.6 1,6-hexanediol diacrylate 78.2 Camphorquinone 0.2 N,N-dimethylaminoethyl methacrylate 2.0 EXAMPLE 7 % by Weiqht Hypalon-30 19.8 1j6-hexanediol diacrylate 79.05 Camphorquinone 0.15 N,N-dimethylaminoethyl methacrylate 1.0 The novel adhesives of this invention are in general characterized as providing high lapshear test results and very good impact test results. They 18 provide very strong adhesive bonds to polycarbonates (and to polymethyl methacrylate substrates as well).

They are further characterized as providing good fracture toughness, chemical and moisture resistance, as well as excellent resistance to temperature changes and mechanical abrasion.

While,,. as heretofore described,, they have great utility in bonding the polycarbonate surface of a transparent object to another polycarbonate substrate, of particular interest to this invention is to seal polycarbonate headlamp assemblies in a matter of a few secorids and in manner which meets very rigid test criteria.

The drawing illustrates a typical such headlamp assembly. As shown therein. the headlamp assembly will comprise a transparent front cover 10 and a headlamp housing 12. The front cover 10 has a flange 14 extending around its internal periphery and a face 16 which of course is transparent and preferably has a surface which will refract and maximize transmission of light emitted from the headlamp. The housing 12 is shown to have a pair of central openings 18 in its interior surface 20 in which the headlamps are adapted to be seated and through which the terminal base portion of the lamps extend externally for electrical contact; an exterior surface 22 defining the dimensions of the housing; and a grooved front peripheral portion 24 adapted for accommodating the flange 14 of the front cover. Typically, the inner surface 20 of the housing contains a reflective metal coating for increased efficiency of the headlamp.

To seal the headlamp assembly in accordance with this invention, a quantity of adhesive is first 19 positioned substantially uniformly throughout the groove 2 4. The f lange 14 is then seated within the adhesive-containing groove to position the two component parts in juxtaposition and visible light is applied for a time and of an intensity to initiate photopolymerization and subsequent adhesive formation to seal the two parts togehter. This may be accomplished in less than ten seconds following exposure with a high intensity blue light source, as previously described.

Headlamps of the type shown in the drawing were sealed with thecompositions of Examples 1 to 7, respectively, and subjected to testing. The test results, including, where applicable, the manufac- turer's specifications, are shown in Tables 1A and 1B. TABLE 1A

1 Lapshear @ R.T. (230C) Lapshear at -290C (Mfg. spec - 1393 psi (Mfg. spec - 1393 psi Example or substrate failure) or substrate failure) 1 422 25 - sub 2 450 108 - sub 2 753 42 - sub 387 139 - sub 3 610 19 - sub 855 116 - sub 4 6915 92 - sub 517 295 - sub 391 27 - sub 430 - sub 6 369 74 sub 280 28 - sub 7 403 64 - sub 310 57 - sub Notes on Table 1A 1 Test measurements are in pounds per 0.5 inch2 not pounds per square inch in accorance with 30 manufacturer's instructions.

2 sub = substrate failure (passes specification).

TABLE 1B

Ex. Creep Resist- Elongation & Stretch Shore D ance @ 880C at Break Hardness (Mfg. spec no slippage after 24 2 hours) % lbs/0.1 in-Z hcLZcmA 1 >400 hrs 4.2 65 55 2 >400 hrs 5.5 335 77 3 >400 hrs 5.1 180 60 4 >400 hrs 5.4 305 72 >2000 hrs 1 100 70 6 >2000 hrs 2 120 73 7 >2000 hrs 2 95 70 In the foregoing description, mention has been made of bonding a transparent polycarbonate article (e.g. the front panel of the headlamp, assembly shown in the drawing) to another polycarbonate substrate (e.g. the headlamp, housing). However, it will be appreciated that the respective polycarbonatle substrates need not be surfaces of articles consisting only of polycarbonates. The articles to be bonded may be laminates of different materials rather than a single sheet or molded article of polycarbonate. The only criteria for applicability of the present invention is that at least one of the two articles be transparent to visible radiation to permit curing and that the respective surfaces to be bonded consist of polycarbonate.

While reference has been made throughout the fore going description to bonding polycarbonate to polycar bonate,, as previously mentioned very strong bonds of polymethyl methacrylate substrates may also be obtained. Thus, in its broadest aspects, the invention 21 is directed to bonding polycarbonate and/or polymethyl methacrylates substrates to one another.

Since certain changes may be made without departing from the scope of the invention herein involved it is intended that all matter shown in the accompanying drawing and described in the foregoing s pecification, including the examples, shall be interpreted as illustrative and not in a limiting sense.

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Claims (33)

1. A method for bonding two substrates, each being polycarbonate or polymethyl methacrylate, at least one of the said substrates f urther being transparent to visible light, the said method comprising the steps of:

(1) contacting the said substrates to be bonded with a visible light curable adhesive composition consisting essentially of (a) at least one polyfunctional acrylic monomer; (b) a polymer selected from the group consisting of elastomers and polyfunctional aliphatic urethane acrylates; and (c) a catalyst or photoinitiator system activatable by visible light to initiate curing and adhesive formation; and (2) thereafter, subjecting the said composition to visible light to bond the said respective substrates together.

2. A method as claimed in Claim 1 in which at least one of the said substrates is polycarbonate.

3. A method as claimed in Claim 1 or Claim 2 for bonding opposed surfaces to two polycarbonate articles at least one of which is transparent to visible light.

4. A method as claimed in Claim 3 in which the said articles when bonded comprise an automotive headlamp assembly.

5. A method for forming an automotive headlamp P I 1% 23 assembly from molded component parts comprising a transparent polycarbonate cover and a polycarbonate headlamp housing, the said component parts being adapted to be placed in superposition and, when bonded, to provide the said headlamp assembly, the said method comprising the steps of:

(1) applying to the surface of at least one of the said components to be bonded a layer of light curable adhesive composition consisting essentially of (a) at least one polyfunctional acrylic monomer; (b) a polymer selected from the group consisting of elastomers and polyfunctional aliphatic urethane acrylates; and (c) a catalyst or photoinitiator system activatable by visible light to initiate curing and adhesive formation; (2) bringing the said components to be assembled into superposition whereby the respective surfaces to be bonded are contacted by opposed sides of the said applied layer of curable adhesive composition; and (3) subjecting the said applied layer of curable composition to visible light to form the said adhesive and thereby bond the said respective component parts to form the said headlamp assembly.

6. A method as claimed in any one of Claims 1 to in which the said monomer is 1,6-hexanediol diacrylate.

7. A method as claimed in any one of Claims 1 to 6 in which the said composition further includes no more than about ten percent by weight of an additional monomer.

24

8. A method as claimed in Claim 7 in which the said additional polymer is ethoxyethoxyethyl acrylate.

9. A method as claimed in any one of Claims 1 to 8 in which the said polymer is an elastomer which is essentially non- reactive in the said composition, the said elastomer being present in an amount sufficient to increase the viscosity of the said compostion.

10. A method as claimed in Claim 9 in which the minimum proportion of elastomer to monomer, by weight, is about 1:10.

11. A method as claimed in any one of Claims 1 to 8 in which the said polymer is a polyfunctional aliDhatic urethane acrylate.

12. A method as claimed in Claim 11 in which the said polymer is selected from the group consisting of aliphatic urethane acrylate oligomers and acrylated urethanes based on aliphatic isocyanate.

13. A method as claimed in Claim 11 or Claim 12 in which the ratio, by weight, of polymer to monomer is about 1:1 to about 7:3.

14. A method as claimed in any one of Claims 1 to 13 in which the viscosity of the said curable composition is from about 2000 to about 45000 cps.

15. A method as claimed in any one of Claims 1 tv 14 in which the said photoinitiator system comprises camphorquinone and an organic amine.

16. A visible light curable adhesive composition adapted for bonding polycarbonate or polymethyl methacylate substrates, the said composition consisting essentially of: (a) at least one polyfunctional acrylic monomer; (b) a polymer selected from the group consisting of elastomers and polyfunctional aliphatic urethane acrylates; and (c) a photoinitiator system activatable by visible light to initiate curing and adhesive formation, the said monomer(s) and polymer(s) being at least 90 percent by weight of the total weight of monomers and polymers in the said composition.

17. A composition as claimed in Claim 16 in which the said polymer is an elastomer which is essentially non-reactive in the said composition, the said elastomer being present in an amount sufficient to increase the viscosity of the said composition.

18. A composition as claimed in Claim 16 or Claim 17 in which the minimum proportion of elastomer to monomer, by weight, is about 1:10.

19. A composition as claimed in any one of Claims 16 to 18 in which the viscosity of said curable composition is from about 2000 to about 45000 cps.

20. A composition as claimed in any one of Claims 16 to 19 in which the said elastomer is selected from the group consisting of polyacrylic rubbers, and 26 chlorosulphonated polyethylene synthetic rubbers.

21. A composition as claimed in any one of Claims 16 to 19 in which the said polymer is a polyfunctional aliphatic urethane eacrylate.

22. A composition as claimed in Claim 21 in which the said polymer is selected from the group consisting of aliphatic urethane acrylate oligomers and acrylated urethanes based on aliphatic isocyanate.

23. A composition as claimd in any one of Claims 16 to 22 in which the said monomer is 1,,6-hexanediol diacrylate.

24. A composition as claimed in any one of Claims 16 to 23 in which the the ratio, by weight, of polymer to monomer is about 1:1 to about 7:3.

25. A composition as claimed in any fne of Claims 16 to 24 in which the viscosity of4said curable composition is from about 2000 to about 45000 cps.

26. A visible light curable adhesive composition having viscosity from about 2000 to about 45000 cps and consisting essentially of:

(a) 1,6-hexanediol diacrylate monomer; (b) at least one polymer selected from the group consisting of polyacrylic rubbers, chlorosulphona ted polyethylene synthetic rubbers, aliphatic urethane acrylate oligomers and acrylated urethanes based on aliphatic isocyanate, the said monomers and the said polymer(s) being at least 90 27 percent by weight of the total weight of monomers and polymers in the said composition; and (c) a photoinitiator system activatable by visible light to initiate curing and adhesive formation.

27. A composition as claimed in any one of Claims 16 to 26 in which the said photoinitiator system comprises camphorquinone and an organic amine.

28. A composition as claimed in Claim 27 in which the said amine is N,Ndimethylaminoethyl methacrylate.

29. A composition as claimed in any one of Claims 16 to 28 further including ethoxyethoxyethyl acrylate monomer.

30. A composition as claimed in Claim 16 substantially as specifically described herein with reference to the examples.

31. A method as claimed in Claim 1 substantially as specifically described herein with reference to the examples o:t the drawing or both.

32. A headlamp assembly whenever made by a method as claimed in any one of Claims 1 to 15 or Claim 31.

33. An article whenever made by a method as claimed in any one of Claims 1 to 15 or Claim 31.

Published 1989 atThe Patent Office, State House.6671 High Holborn, London W01B 4TP. kuxther copies maybe obtainedfrom The Patent Office. Sales Branch, St Mary Cray, Orpington, Kant BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87