CA1326626C

CA1326626C - Primed polyolefin surfaces for epoxy adhesives

Info

Publication number: CA1326626C
Application number: CA000539274A
Authority: CA
Inventors: James J. Kobe; Frank J. Armatis, Jr.; J. Thomas Simpson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1986-06-11
Filing date: 1987-06-10
Publication date: 1994-02-01
Anticipated expiration: 2011-02-01
Also published as: AU7412087A; AU600278B2; JPS6311339A; KR950010586B1; KR880000495A

Abstract

Primed Polyolefin Surfaces for Epoxy Adhesives Abstract An article is provided which comprises a polyolefin first substrate adhered to a second-substrate with an epoxy adhesive. The polyolefin first substrate has graft-polymerized thereto at least one monomer selected from the group consisting of sulfonic and carboxylic acids and alkyl esters thereof containing fewer than eight carbon atoms in the ester group, amides and mono- and di-substituted amides, alcohols, amines, and 5-, 6-, and 7-membered heterocyclic rings having at least one oxygen and/or nitrogen atom in the ring, with the proviso that said monomer contain an ethylenically unsaturated polymerizable moiety. The graft-polymerized monomers are in a layer less than 5 micrometers in thickness and contact the epoxy adhesive.

Description

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Description PRIMED POLYOLEFIN S~R~ACES FOR EPO~Y ADZ~ESIVE5 Technical Fiel_ This invention relates to polyolefin substrates 5Z having a primed surface whZich provides improved boZnding of -.
~pOZxy resin adhesives thereto and a method for adhering primed polyolefin substrates with epoxy resin adhesives.

BackZ~round ~or many years there has existed a desire fcZr polyolefin materials which can be bonded to other maZterial~
e.g., metals, ceramics and wood, or to themselves with epoxy ' resin adhesZives~ Bonding of such material s wi~h epoxy resin adhe~Zives is often dZesirable, particularly when the bonded material iZ6Z to be subjected to conditions o~ high :
temperature and high humidity.
Z Vntreated polyolefin substrates, e.g.~ :
Z, polyethylenel polypropylene or polyallo~er which is a copolymer of polyethylene and polypropylene, are generally unreceptive to epoxy adhesives alnd much effort ha~ been j 20 expended i~ the search for a technique which will easily and reliably increase adhesion without siqni~icantly changing the properties of the substrate. Surface modification techni~ues for enhancing adhesive bonding ~hich have been examined are treatment with helium gas plasma, oxygen gas ¦ 25 plasma and chromic acid. These treatments and other surface modification procedures have a common shortcoming in poor durability. Light rubbing of th~ surface causes a decrease l in the effect, the altered surface being easily abraded.
'! Irradiation of pZolyolefin substrates, such as with an elZlQlctron beam, to improve the adhesion of various coatings i5 al o known and has been dZisclosed in U.S.

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Patents No. 4,041,192 (Heger et al.), No~ 4,148,839 (Fydelor), No. 3,252,880 (Magat et al.) and No. 4,179,401 (Garnett et al.)O
Polyolefin substrates have been pro~id2d with a treated surface for improved adhesion of pre~sure-sensitive adhesives. Such pressure-sensitive adhesive~ include rubber-type adhesives sueh as those based on natural rubber::
and ~ynth2tie rubbers, e.g., styrene-butadiene, rhloroprene, neoprene, and isobutylene rubbers and synthetic resin-type~ :~
adhesives such as acrylic, polyvinyl chloride, polyvinyl acetate, and pslyvinyl butyral adhesives. U.S. Patent No.

3,628,987 (Nakata et al.) discloses a pressure-sensitive ~;
j adhe~ive film wherein the film surface to which the adhesiveis adhered has graft-polymerized thereto a vinyl monomer or diene monomer, the adhesive havins a solubility para~eter near that of polymers of the vinyl or diene monom~r. U.S.
Patent No. 4,563,388 (~onk et al.~ discloses a polyolefin substrate having graft-polymeri:æed thereto at lea~t one ~, monomer s~lected from the group consisting of acrylic acid, methacrylic acid and ester~ thereof; acrylamide;
j methacrylamide; 6terically non-hindered tertiary alkyl acrylamide~ and methacrylamides having three or les~ carbon atoms in the alkyl group; and N--vinyl pyrrolidone, and firmly adherently bonded to the graft-polymerized monomer an :!, 2s acrylic type normally tacky and pressure-sensitive adhesive.
:¦ A s~ries of articles, "Surface Modifica~ion ~fJ Polyethyle~e by Radiation-Induced Grafting for Adhe~ive .:
:l Bondlng. I. Relatlonship Between Adhesive Bond Strength and ::-, Surface Composition," (S. Y~makawa, J. Ap~l, Polym Sci, 20, ;, 30 3057-3072 (1976J; "II. Relationship Between Adhesive Bond Strength and Surface Structure," (S. Yamakawa et al., i Macror~olecu1ea, 9, 754-758, tl976); "III. Oxidativ~ . i ;' Degradation and Stabilization of Grafted Layer," (S. . J--I Yamakawa et al., J. Appln. Polym. Sci., 22, 2459-2470 ~': 35 (1978~ IV. Improv~ment in Wet Peel Strength," (S. Yamakawa 'A' , ''.
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et al~, J. Appl~ Pol~m. Sci., 25, 25-39 (1980), and "V.
Comparison with Other Surface Treatments," (S. Yamakawa et al., J. Appl. Po~ym. Sci., 25, 40-49 ~1980), disclose grafting of methyl acrylate (followed by saponification), vinyl acetate, acrylic acid, acrylamide, and methylolacrylamide to polyethylene by vapor-phase mutual grafting or liquid-phase preirradiation at thicknesses of grafted ~onomer of more than 10 micrometers to improve -:
adhesion of epoxy adhesives.

1 0 SumM~y The present invention provides a polyolefin first substrate firmly adhered to a second substrate with an epoxy resin adhesive, ~aid polyolefin first substrate having graft-polymerized thereto at least one monomer selected from ~-the group consisting of sulfonic and carboxylic acids and alkyl ester~ thereof containing fewer than eight carbon atoms in the 2ster group, amide~; and mono- and di-~ubstituted amides, alcohols, amines, epoxides, and 5~, :
6-, and 7-membered heter w yclic rings having at least one oxygen and/or nitrogen atom in the ring, with the proviso tAat said monomer contain an ethylenically unsaturated , poly~erizable moiety, and that t:he graft-polyme~ized A~ monomers are in a layer less than 5 micromet~rs in thickn~ss and eontactin~ said epoxy resin adhesive.
~he present invention also provides a method for adhering a polyolefin ~irst substrate to a second substr~te with an epoxy resin adhesive co~prising the steps of a) coatin~ on said polyolefin first substrate a liquid monomer selected from the group consisting of .
sulfonic and carboxylic acids and alkyl esters '. ~hereof containing fewer than eight carbon atoms in the ester group, amides and mono- and di-substituted ~ amides, alcohols, amines, epoxides, and 5-, 6-, and ., 7-membered heterocyclic rings having at least one , 35 oxygen and/vr nitrogen atom in the ring, in a layer .~ :
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less than 5 micrometers in thickness with the ~-proviso that said monomer contain an ethylenically unsaturated polymerizable moiety;
b) irradiating the coated sub~trate with ioni2ing radiation to graft polymerize said monomer to said substrate;
c) coating an epoxy resin adhesive on at least one of aid graft-polymerized monomer surface of said - --first substrate and (ii) a surface of said second ~, 10 substrate;
d) contacting said surfaces (i) and tii~; and e) curing said epoxy resin adhesive.
Surprisingly excellent peel values, i.e., greater than 25 lb/in t440 N/dm), result when the grafted monomer 1, 15 layer is les than 5 micrometer~ thic~. Good peel adhesion i~ also retained after exposure to water for a period of about one week.

Detailed Description The substrate o~ the invention is a polyolefin material, i.e., polyethylene, polyalkyl-~-olefins, and I blend~ thereof, uch a~ polyethy~lene, polypropylene, or i polyallomer which i~ an ethylen~/propylene copolymer.
~hese polyolefin materials can also contain s~all a~ount~
'~ i.e., up to about 10 weight percent of modifying monomers, 0.g., ~ros~linkabl~ terpolymers, such a dicyclopentadiene and norborneene~ a~d ionomers which enhance ~trength, such as acrylic acid. These terpolymers and ionomer~ are~not .. -3 present on the surface of the polyole~in material~ in suficient~quantl~ties to affect epoxy adhe~iv~ adhesion to 303 ~the polyole in materials~
' :~ The thicknes~ og the polyolefin ~at2rial is d*ter~ined by the end usç o~ the adhes~ve-coated product, typical thicknesses being in the ranye of about 0.025 to about~5.0 milli~eters. The polyole~in material may contain ~5~ additives, such as carbon black, calcium carbonate, silica, 2 ~

titanium dioxide, crosslinking agents, dispersants and extrusion aids, which are known in the art.
Various specific monomers suitable for graft-polymeriæation onto polyolefin substrates to promote j 5 adherence of epoxy adhesives to the substrate include acrylic acid, dimethylacrylamide, diethylacrylamide, dipropylacrylamide, hydroxyethylacrylate, butanedioldiacrylate, hexanedioldiacrylate, diethylaminoethylacrylate, glycidylmethacrylate, j 10 isobutylmethacrylate, cyclohexylmethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, ~3 pentaerythritol tetraacrylate, hydantoin hexacrylate, ~ 2-vinyl-4,4-dim~thylazlactone, ~nd N-vinyl-pyrrolidone.
3 15 The mGnOmer composition generally contains about 60 to about 100 weight percent monomer. The monomer composition applied to the polyolefin substra~e may contain various additive such as crosslinking agents, surfactant~, alcohols, and acids.
~l 20 Crosslinking agents, some of which may also be monomers suitable for grafting onto th0 substrate themselv~s, may be added to further enhance heat resistance of the product. Examples of useful crosslinking agents include polyethyleneglycol diacrylate, pentaerythritol tetraacrylate, tetr3ethylene glycol dimethacrylate, trimethylolpropan~ tri~crylate, trimethylolpropane trimethacrylate, allyl methacry:Late, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, thiodiethylene glycol diacrylate, and triallyl cyanurate. The preferred I 30 crosslinking agents include polyethylene glycol diacrylate, :~1, tetraethylene glycol dimethacrylate, trimethylol propane ~`~ triacrylate, and thiodiethylene glycol diacrylate.
;, Crosslinking agents may be present in monomer compositions in amounts o~ about 0 to about 80 percent by weight, '' 35 pref~rably 0 ts about 40 percent by weight.
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Surfactants may be present in the monomer composition to improve the uniformity of the coating on the substrate. ~-The thickness of the monomer composition is preferably less than about 5 micrometers, more preferably le~s than about 1 micrometer and most preferably less than about 0.5 micrometer. ~
For uniform application of the monomer to the ~-polyolefin ~ubstrate, the substrate is preferably treated to enable a polar liquid to wet the surface. This can be achieved by treating the substrate with an oxidizing agent such a~ nitric acid or chromic acid, or treating the ~ substrate with flame, plasma discharge or corona discharge.
j The preferred method of treatment is corona discharge.
The mono~er may be graft-polymerized to the substrate by use of actinic radiation such as x-rays, beta ray~, gamma rays, ultraviolet light, vi~ible light and 1ectron beam irradiation. The pre~erred method of graft~polymerization is by irradiation with an electron J 20 beam. The eleçtron beam dosag~ is typically greater th~n d -! about 0.05 Mrads, preferaby greater than about 0.5 ~rads, I
and more preferably in the range of about 2 to about 10 Mrad~. -The sub&trate to which the polyolefin material is i 25 to be adhered can be any substrate which bonds well with ¦~ epoxy adhesive~. Such substrates include, for example, :-metals such as aluminum, teel, galvanized ~teel, stainless ~teel, copper, and titanium, pla~tics, other than polyolefins, uch as polycarbonate, polyvinyl chloride, polyester and fiber-reinforced polye~ter, pol~methylmethacrylat~, and acrylonitrile-butadiene-styrene, and particularly those plastics which are high-temperature impact modified, natural and synthetic rubbers, such as polychloroprene, styren~-butadiene rubber, and nitrile j~ 35 rubber, ceramics, wood, painted surfaces, glas~ primed with silane and polyole~in having a monomer graft-polymerized !~ thereto according to the present invention.

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, Epoxy adhesives useful in the present invention include any one- or two-part epoxy adhesives a~ long as the curing temperature required does not exceed the softening temperature of the polyolefin substrate. In use the epoxy adhesive may be applied to either the polyolefin substrate or to the substrate to which ~he polyolefin material iæ to be adhered or to both substrates. ~ypical epoxy adhesives include Scotch-Weld~ brand Epoxy Adhesives 1838 and 2216 ~/A, each available from Minnesota Mining and Manufacturing Company The advantages of the invention are illustrated by the following examples, it being understood that numerous variations will be well wi~hin the ability of those skilled in the art.
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`I 15 Example 1 ., .
Into a polyallomer resin, available from ~ennessee Ea6tman under the trade design~tion "Tenite" 5321E, was l milled 1~ carbon black by weight. The composition was then:l extruded as a sheet having a thickness of about 0.5 millimeter, after which one surface was coron~-treated by pas6ing the sheet through a 1 millimeter air gap between a ~ chlorinated polyethylene-coated electrode and a grounded l aluminum cylinder. The frequency of the unit was 16 kHz, the total input power being 1.5 kw and the exposure time of the sample being on the order oE 0.6-1.0 sec. Ov~r the .l corona-treated surface was then applied a blend of ~0 parts j by weight N,N-dimethyl acrylamide, 20 part by weight ~I trimethylol propane triacrylate, and 0.5 part by weight of a ~`l fluorochemical acrylate oligomer ~a wetting and lev*ling agent commercially available from 3M Company as "FC 430"). -::
The blend, which had a viscosity of 2D3 CpS at 25C, was applied to the coro~a-treated polyallomer sur~ace using a No. 20G quad knurled gravure roll in combination with a i doctor blade, minimizing the coating thickness to less than ::
, ~ 35 one micrometer. The coated surface was then exposed to :`, ' ,', '', . `", ,~:

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electron beam irradiation at a dosage of about 4 Mrads and a voltage of 150-250 kV in a nitrogen atmosphere. The thickness of the grafted layer was found to be less than 0.4 micrometer using attenuated total reflection-infrared -:-spectro~copy. . : -~
A cold-rolled steel panel (4" x 12" x 0.32" (lO cm x 30 cm x 0.8 cm), designated 80-730F cold-rolled steel, 40, unpolished? 17603001, available from ~dvanced Coating Technologie~, Inc., Hillsdale, Michigan~ was wiped three ::.
times with methyl ethyl ketone to remove a~y surface oil present. Scotch-Weld ~3brand Epoxy Adhesive 1838-L B/A
translucent was prepared ~y mixing together 6 parts by -- .
weight base and 5 par~s by weight accelerator. The adhesive wa~ coated on the steel panel and a one-inch wide x six-inch long strip of polyallomer having the N,N-dimethyl acrylamide/tri~ethylolpropane triacrylat~ grafted thereto wa~ placed on the adhe~ive with the grafted sur~ace :
contacting the adhesive and an end of the polyallomer extending beyond the plate. A 4.5 lb (2 kg) roller was ¦ 20 rolled acros6 the ~teel plat0/adhesive/polyallomer zompo~ite ::
twice. The co~posite, in which the epoxy adhesive layer wa6 ..
about 0.004 inch (0.1 mm) thick, was placed in an oven at 170F (77C) or 2 hours. The oven was then turned of~ and the compo6ite wa6 ~llowed to cool overnight (16 hour~).
Adhesion was determined by pulli.ng the polyallomer from steel plate at a 90~ angle and a rate of 12 inches (30 cm) per minute. No ~dhesive failure occurred~ The polyallomer adherend failed at a force of about 40 lb/in (700 ~/d~
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g Com~arative Exam~les 1-3 In Comparative Examples 1-3, untreated polyallomer, polyallomer which had been corona-treated as in :
Example 1, and polyallomer which had been corona-treated and subjected to electron beam radiation as in Example 1 but with no monomer present, respectively were tested for adhesion as in Example 1. The resultc are shown in Tablç 1.

¦ ~xample 2-19 n Examples 2-19, various monomers set forth in . lO Table 1 were graft-polymerized onto polyallomer as described in Example 1 except that the monomer was applied by wiping the polyallomer with a paper towel moistened with the monomer and the dosage of electron beam irradlation was j about 5 Mrads. In each of Examples 2-12 and 14-19, 95 parts by weight of the indicated monomer was mixed with 5 parts by weight trimethylolpropane triacrylate, as crosslinking agent and in Example 13 the 'crimethylolpropane triacrylate was applied as a 50 weight percent solution in acetone.
Composites were prepared as in E~xample 1 and tested ior adhecion. ~he results are set t~orth in Table 1.

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Table 1 -;:-Adhesion Example Monomer ~/dm?
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2 acrylic acid PAB*
3 dimethylacrylamide PAB

4 diethylacrylamide PAB :::
dipropylacrylamide ~AB
6 hydroxyethylacrylate PAB
~ 7 butanedioldiacrylate PAB
:l 8 hexanedioldiacrylate PAB `~ g diethylaminoethylacrylate P~
1 15 10 glycidyl methacrylate P~B
1 , 11 i~obutylmethacrylate P~
¦ 12 cyclohexylmethacrylate PAB
3 13 trimethylolpropane triacrylate pAs 14 trimethylolpropane trimethacrylat~ 26 . 20 15 p~ntaerythritol triacrylate PA~
I 1~ pentaerythritol tetraacrylate PAB
.~ 17 hydantoin h~xacrylate 33 i 18 2-vinyl-4,4-dimethylazlactone PAB
:~ 19 N-vinylpyrrolidone PAB

* polyallomer adherend break Examples 20 and 21 ..
j To 20 mil tO.S mm~ thick polyallomer sheet -.
.. ~ material wa~ gra~ted a monomer mixture of 80 parts by weight di~thylacrylamide~ 20 parts by weight trimethylolpropane triacrylate, and 0.5 parts FC 430 as in Example lo Cold-roll2d mild ~teel plates (3" x 7" x 1/8" t7.5 cm x 17.5 ::

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fi cm x 2.2 cm) Type 1018 available from Paper Calmenson h Co., St. Paul, Minnesota) were abraded by hand using Grade 35 sandpaper, followed by Grade 80 sandpaper, and then a Grade 80 abrasive bristle brush, and then degreased with methyl ethyl ketone. The steel plates were then coated with Scotch-Weld~ brand Epoxy Primer 194~, available from Minnesota Mining and Manufacturing Co., at a thickness of about 1-2 mil (0.025-0.05 mm) and allowed to dry for about ten minutes.
In Example 20, Scotch-Weld ~ brand Epoxy Adhesive 1838-L B/A Translucent, available from Minnesota Mining and Manu~acturing Co. was prepared by mixing equal volumes of base and accelerator. The adhesive was coated on the primed steel plate. The grafted surface of a 1 inch x 8 inch piece of th~ polyallomer was laid over the adhesive ~s in Example l and a second steel plate was placed on top of the polyallomer to effect uniform contact between the polyallomer material and the adhesive. The thus-formed composite was allowed to cure at room temp~rature (73F, 23C) for three days. The top steel plate was then removed and the somposite was soaked in water ~t room temper~ture for 13 days. When the compo~ite was tested using the 90D
peel test as in Example 1, the polyallomer adherend broke I be~ore any bond failure occurred.
', In ~xample 21 a composite was prepared and tested ~, 25 as in Example 20 except that the adhesive used was Scotch-Weld ~ brand 2216 B/A Structural Adhesive (2 part~ by '~ volume base ~ixed with 3 parts by volume accelerator).
I Again, the polyallomer adherend broke before any bond 1 failure oocurred.
Various modifications and alterations of this ' invention wîll be apparent to those skilled in the art - without dep~rting from the scope and spirit of this ! invention and this invention should not be restricted to that set forth herein for illustrative purposes.

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Claims

1. An article comprising a polyolefin first substrate adhered to a second substrate coated with an epoxy adhesive, said polyolefin first substrate having graft-polymerized thereto at least one monomer selected from the group consisting of sulfonic and carboxylic acids and alkyl esters thereof containing fewer than eight carbon atoms in the ester group, amides and mono- and di-substituted amides, alcohols, amines, epoxides, and 5-, 6-, and 7-membered heterocyclic rings having at least one oxygen and/or nitrogen atom in the ring, with the provisos that said monomer contain an ethylenically unsaturated polymerizable moiety and that said graft-polymerized monomers are in a layer less than 5 micrometers in thickness and contacting said epoxy adhesive.

2. The article of claim 1 wherein said graft-polymerized monomers are dimethylacrylamide and trimethylolpropane triacrylate.

3. The articles of claim 2 wherein said monomers are present in a ratio of about 80 parts by weight dimethylacrylamide to about 20 parts by weight trimethylolpropane triacrylate.

4. The article of claim 3 further comprising a wetting agent in said graft-polymerized layer.

5. The article of claim 1 wherein said polyolefin first substrate is polyallomer.

6. The article of claim 1 wherein the thickness of said graft-polymerized monomer layer is less than about 1 micrometer.

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7. The article of claim 1 wherein the thickness of said graft-polymerized monomer layer is less than about 0.5 micrometer.

8. The article of claim 1 wherein said second substrate is metallic.

9. The article of claim 1 wherein said article has a peel value of at least 25 lb/in (440 N/dm).

10. The article of claim 1 wherein the strength of the bond between said epoxy and said graft-polymerized surface of said polyolefin substrate is greater than the strength of a polyallomer sheet having a thickness of 0.5 millimeter.

11. A method of adhering a polyolefin first substrate to a second substrate with an epoxy adhesive comprising the steps of a) coating on said polyolefin first substrate a liquid monomer selected from the group consisting of sulfonic and carboxylic acids and alkyl esters thereof containing fewer than eight carbon atoms in the ester group, amides and mono- and di-substituted amides, alcohols, amines, epoxides, and 5-, 6-, and 7-membered heterocyclic rings having at least one oxygen and/or nitrogen atom in the ring, in a layer less than 5 micrometers in thickness, with the proviso that said monomer contain an ethylenically unsaturated polymerizable moiety;
b) irradiating the coated substrate with ionizing radiation to graft polymerize said monomer to said substrate;

c) coating an epoxy adhesive on at least one of (i) said graft-polymerized monomer surface of said first substrate and (ii) a surface of said second substrate;
d) contacting said surfaces (i) and (ii); and e) curing said epoxy adhesive.

12. The method of claim 11 wherein said monomer is coated at a thickness of less than about 1 micrometer.

13. The method of claim 11 wherein said monomer is coated at a thickness of about 0.5 micrometer.

14. The method of claim 11 wherein said ionizing radiation is electron beam radiation.

15. The method of claim 14 wherein said electron beam radiation is at a dosage of about 0.05 Mrads to 0.5 Mrads.

16. The method of claim 11 wherein said monomers are dimethylacrylamide and trimethylolpropane triacrylate.

17. The method of claim 16 wherein said monomers are present in a ratio of about 80 parts by weight dimethylacrylamide to 20 parts by weight trimethylolpropane triacrylate.

18. The method of claim 17 further comprising a wetting agent coated with said liquid monomer.

19. The method of claim 11 further comprising corona-treating said polyolefin first substrate prior to coating said polyolefin first substrate with said liquid monomer.