Present invention relates to a photografting method, in which one or multi layer(s) of films or sheets, with intermediate layer(s) of a photo¬ polymerizable composition, the intermediate layer(s) is/are grafted onto adjacent polymer surfaces. The present invention also relates to a process for preparing laminate or composite material and laminates or composite materials made by the procesε. The present invention relates to a use of the laminates or the composite materials .

This invention is based on our previous invention of " surface photografting " of polymer sheets, films, filaments and yarns ( B. R&nby, Z. M. Gao, A. Huh and P. Y. Zhang, Polvmer Preprints. ACS, 27:2, 38-39 (1986)). The invention has been developed as a batch process, where initiator and monomer in vapor phase are transferred and initiated by UV iπadiation to react at the substrate surface, and a continuous ^p rocess, where initiator and monomer in liquid phase are transferred to the substrate surface by presoalάng immediately before the irradiation with UV light ( B. Rinby, Z. M Gao. A Huh and P. Y. Zhang, Modification of Polymer Surfaces by Graft Copolymenzation.in "Chemical Reactions on Polymers ", J. L. Benham and J F Kinstle. εαs. AC> Symp.Ser. ____ 168-185 (1988)).

The continuous surface photografting is a rapid and efficient process for sheet, film, filament and yarn. The process modifies the substrate surface with a very thin layer of grafted polymer (2 to 8 nm). Using hydrophilic monomers like acrylic acid, acrylamide or 4- vinyl pyridine, the modified surface of an inert substrate polymer shows increased wetting with water ( lower contact angle), increased adsoφtion of dyes (by a factor of 5 to 15) and increased adhesion to epoxy resins (by a factor of 3 to 6) Fuπher results are presented in a printed dissertation by Zhang Pei Yao, Surface Modification tn Continuous Graft Copolymerization. Royal Institute of . Stockholm Sweden, October 14, 1988. The substrates studied are commercially available samples like polyethylene tape film, polypropylene film strips, polyethylene fibers and polypropylene fibers, which are surface photografted using the continuous process with acrylic acid and acrylamide as monomers, benzophenone as initiator and an aliphati ketone, e.g. acetone, as solvent. Zhang ^* s disseπation contains four papers which ar printed in J. Applied Polymer Science 4Q, 1647 (1990), 41, 1459 and 1469(1990) an 43, 621(1991).

In another series of surface photografting experiments, the batch process was applied to commercial polymer film samples of polystyrene and LD and HD polyethylene, using acrylic acid, glycidyl acrylate and glycidyl methacrylate as monomers. These studies are presented in a printed dissertation by Klas Allmer, Surface Modification of Polymers. Royal Institute of Technology, Stockholm, Sweden, October 7, 1988. The grafted layers were analyzed by ESCA and reflection IR spectra and contact angle measurements. When glycidyl monomers were used, the grafted polymer chains retained the epoxy groups which in a secondary process were reacted with amines, photostabilizers and heparin. In this way increased photo-oxidative stability of polypropylene film was obtained by the very thin surface layer of hindered amine, hindered phenol and dihydroxy benzophenone grafted onto the substrate. The surface of polyethylene grafted with glycidyl monomer was further modified with short chains of poly(ethylene glycol) attached by main valence bonds in a secondary reaction. The substrate surface was covered with a hydrogel of PEG which reduced the adsorption of protein, i.e as blood-clotting, by a factor of about 8. A surface of polyethylene grafted with glycidyl monomer was further reacted with heparin in a secondary process, which reduced the thrombus formation of the surface in contact with blood by a factor of about 4. Allmer ^" s dissertation contains five papers which are printed in J. Polymer Science, Polymer Chem. Ed. 26, 2099(1988), 27. 1641 3405 , and 3419 ( 1989) and 28, 173 ( 1990)

In the surface photografting process with vapor and liquid transfer of initiator ana monomer, respectively, the initiation occurs at the substrate surface by abstraction ot hydrogen. Benzophenone molecules (BP) absorb UV quanta by a (π— π*) transition ot the ketone group(CO) and abstract hydrogen. The radicals formed at the substrate surface add monomer and form grafted polymer chains. A review of our research work on surface photografting with reference to the original papers is presented in a lecture, B Ranby, Surface Modification of Polymers by Photoinitiated Graft Polymerization, Makromol. Chem., Macromol. Symp. 63, 55-67 (1992).

A patent has been applied for a continuous surface photografting process, " Treatment of Ultrahigh Molecular Weight Polyolefin to Improve Adhesion For Resin". US Patent

No. 5, 039, 549 of Aug. 13, 1991 (application filed Nov. 14, 1990) by H. X. Nauyen, G. Riahi, R.C. Bennett and G.M. Wood and assigned to Allied-Signal Inc . The experimental procedure by presoaking the substrate in initiator and monomer and UV irradiation on line in a reaction chamber as descnbed in die patent, agrees with the description in our papers published in 1986 and 1988 including the effects of the surface modification: improved wetting of the surface and increased adhesion to resins for preparaαon of composite matenals. It is surpπsing that a US Patent is granted on a process which was previously described in the literature and well known The paper "UV Grafting of Extented Chain Polyethylene Fibers" by G Riahi, G Wood, H Nguyen and A. Poursartip, printed in Composite Interfaces, Vol 1 , No 1 , pp 55-73 (1993), describes die surface photografung method without giving a single reference to our papers from 1986 and 1988 A US Patent No 5,002,582 ( 1991 ) by P E Guire, S.G.Dunkirk, M.W. Josephson and M.J. Swanson, descnbes a modificanon process for polymer surfaces which essenually is the same as surface photografting in our papers from 1986 and 1988. The biomedical applications mennoned in the patent are largely the same as those descnbed in K Allme s pπnted disseπation, published on October 7 1988, e.g obtaining permanent wetting and funcuonal groups on the polymer surface for immobilizing hepaπn, anubodies and other biomolecules The patent gives no reference to our papers on surface photografting and the applicauons descnbed there

The batch process for surface photografting is slow wuh UV lrradiauon times ot a few minutes, because the transfer of initiator and monomer in vapor phase is inefficient The conπnuous process is much faster with UV irradianon umes of 5 to 10 seconds because the transfer of ln ator and monomer in liquid phase is efficient and fast

In the photografting process for lamination, initiator and monomer form a liquid composition between two substrate films of which the top film is transparent to the ln ating UV irradiation With an assembly of three substrate films with the reactive liquid composition in two layers between the films, the two top films should be transparent to the lnitiaung UV irradiation From our previous research work, it is kiiown

that all polymer substrate surfaces containing secondary and tertiary bonded hydrogen ( bond energy 95 and 91 kcal/mole, respectively) are surface photografted in our two processes. Allylic hydrogens are most easily abstracted due to the low bond strength (88 kcal/mole). Ethylenic hydrogens are not abstracted by die excited benzophenone due to the high bond strength (104 kcal/mole). The photografung lamination is initiated at die polymer surfaces in contact with the liquid composiuon. The grafted chains grow until they are terminated by combinauon of two chain end radicals or by combmauon of a chain end radical widi a ketyl radical (from benzophenone)

In the photografting process for lamination ln ator and monomer are liquids at die grafting temperature. No solvent should be added The reacung composiuon is enclosed between two substrate films which excludes the atmospheπc oxygen Therefore, no men atmosphere is necessary for the process

When the reacting composition is a photoinitiator dissolved in reactive monomer, the two substrate surfaces are grafted and die polymeπzed composition forms an mtermediate layer which fills the space between the two films It forms a strong. adhesive bond between the two films and can also be a barπer for a penetrant With acrylic acid as monomer, the grafted polyacrylic acid is an efficient oxygen barπer (Examplel) The intermediate layer of die grafted polymer in this case consists of two layers, grafted onto each substrate surface. Therefore, the two substrate films are separated if the grafted laminate is immersed in a solvent for the grafted polymer, e g , hot water for grafted acrylic acid.

Widi a small amount of a polymer (e g 2 wt %) dissolved in the monomer, the viscosity of the composition increases which makes it easier to apply and retain between the substrate films. With a small amount of crosslinker added to the composition, e g 5 wt % trimethylolpropane-triacrylate to acryhc acid, the intermediate grafted layer forms a gel which is anchored by the grafting onto the surface of the two substrate films The crosslinking due to the additives contπbutes to the strength of the grafted laminates and prevents separation by solvents. The crosslinking has only a minor effect on the barπer propeπies for small molecules like oxygen

Example 1

A photopolymerizable composition A (PCA) was prepared by dissolving 5 g benzophenone and 2 g cellulose acetate (40%) in 100 g acrylic acid. A low density polyeώylene (LDPE) film 188 μm ick and a polyethylene terephthalate (PET) film 256 μm thick were cut to pieces of 60 X 60 mm square. Of the PCA 10 μl was applied by a microsyringe on the surface of the PET film and die LDPE film was deposited on top. To make the liquid PCA layer even, a clear quartz plate 8 mm thick covering the laminate and a brass weight of 1,588 kg was placed on top of the laminate. After a few seconds the brass weight was removed. The laminate widi the quartz plate on top was irradiated for 25 sec. at 55°C with a 2 kW high pressure mercury lamp HPM 15 (from Philips) at a distance of 15 cm from lamp to laminate. The lamination by photografting is performed as shown in Fig. 1 for LDPE film on top of PET film. The adhesion of the two films was measured as a 90° peel test in an Instron instrument at an extension rate of 20 mm / min.. The grafted laminate failed by breaking of the LDPE film and not by separation in the PCA adhesive layer. The average strength measured was 1052 N/m (= die strength of the LDPE film).

After immersion of die photografted laminate in hot water (80°C) for 4 hours, the LDPE and PET films could be separated, each film surface covered with a swollen layer of grafted polyacrylic acid. After washing with hot water to remove homopolymer. thε films were dried and weighed. The grafting efficiency was 80 %, which agrees with earlier measurements for continuous surface photografting of acrylic acid onto an LDPE surface.

Example 2

A photopolymerizable composition B (PCB) was prepared by dissolving 5 g benzophenone, 2 g poly(v nylacetate) (PVAc) with Mn 500,000 and 5 g mmethylol- propanetriacrylate (PMPTA) in 100 g acrylic acid. The same assembly of LDPE and PET films and the same experimental process as in Example 1 was used, this time with 10 μl of PCB between the films. The laminate was lπadiated with UV hght for 35 sec under the same conditions as in Example 1. In 90° peel tests this photografted laminate showed similar properties as the laminate in Example 1 It failed by breakage of die LDPE film. In tiiis case die two films could not be separated after treatment in hot water This means that the polymeπzed PCB layer was grafted onto both film surfaces and crosslinked to a gel

Example 3

A film of LDPE 32 μm thick was cut to pieces 60 X 60 mm square Of the photopolymenzable composition PCA 25 μl was applied as an intermediate layer between two LDPE film samples using the same procedure as in Example 1. The assembly was πradiated with UV light for 30 sec. under the same conditions as in Example 1. The oxygen permeability was measured in an OX-TRAN 2/20 MH instrument from MOCON, Minneapolis, .Minn., USA. The permeability value 12,0 cc Q-^J day m ² was obtained for the photografted laminate and the value 948 cc O / day m2 for a blank sample, e.g two LDPE films without grafted layer. The photografted layer of

poly(acrylic acid) is a most efficient C^ barrier.

Example 4

A photopolymerizable composition C (PCC) was prepared by dissolving 5 g benzophenone and 2 g poly(vinyl acetate) PVAc widi Mn 500,000 in 100 g of a monomer mixture of 20 vol. % acrylic acid and 80 vol. % butyl acrylate. An LDPE film 63 μm thick was cut to samples 95 X 95 mm square, an assembly of two film samples with 25 μl of PCC as intermediate layer was prepared by die same procedure as in Example 1. The assembly was irradiated for 60 sec. at 55°C under a 2 kW HPM lamp at a distance of 15 cm. oxygen permeability P was measured by Terra Pak Materials R & D Inc., Lincolnshire, IL, USA, using a proprietary method. For the photografted sample. P=226 cc O ₂ / day m ² was obtained while a blank sample of two films gave P = 440 cc O ₂ / day m2. In this case the grafted intermediate layer is a rubber-like polymer with rather poor barrier properties.

Example 5

To evaluate the barrier properties of a bulk photografted layer between two LDPE films (30 to 35 μm tiiick, commercial blown film) a seπes of laminates were prepared The photopolymerizable composition PCA was used, descnbed in Example 1. with a thickness of 7 μm. The laminates were prepared as descnbed in Example 1 and irradiated for 30 sec. at 55°C with a HPM 15 UV lamp from Philips at a distance of 15 cm. The oxygen permeability of the photografted laminates and the blank samples of two LDPE films without PCA layer was measured in a MOCON transmission analysis instrument OX-TRAN 2/20 with air (21% O ₂ ) and pure oxygen (100% 0 ₂ ) and given as cc/day m ² .

Table 1. Permeability measurements

Laminate LDPE // 7 μm PCA // LDPE (photografted)

Blank LDPE /LDPE

Permeability P cc / day m

No. Sample

Air Pure O ₂

1 Laminate 18.0 85.7

2 Blank 686 3267 3 Laminate 13.0 62.0 4 Blank 705 3357 5 Laminate 1 1 .0 52 4 6 Blank 715 3405

The oxygen permeabdity for blank samples vanes by about ±2% which is less than the vananon in thickness of the film ±7%.

The oxygen permeability for the laminates vanes by about ±25% which indicates a large vaπation in tiiickness of the photografted layer The photografted layer is an efficient 0 ₂ barπer. It decreases tiie permeability of the laminates for oxygen tacto- 40 to 60

Example 6

A seπes of oxygen permeability measurements were made for commercial film samples used later as laminates in photografting experiments (Example 7) These measurements were made by Tetra Pak Materials R & D Inc., Lincolnshire, IL, USA, using a proprietary method and the results given as P-values cc O ₂ / day m ² (Table 2), measured for pure oxygen.

Table 2 Permeability measurements for blank samples of commercial films of low density polyethylene (LDPE), oriented polypropylene (OPP), polyethylene terephthalate

(PET), polyvinylidene chloride (PVDC) and polyvinyl alcohol (PVA)

No. Film sample Thickness P-value P* (μm) (cc O ₂ /d y,m2) (ccθ ₂ /day,m ² ,μm])

BYl _^ l LDPE/LDPE 125+125 1195 299,750

BYL-2 LDPE/LDPE 63+63 2097 264,222

BYL-3 OPP/OPP 15+15 2279 68,370

BYL-4 PET 125 8.9 1 1 12.5

BYL-6 PVDC 10 27 270

BYL-5 PVA 12 0.1 1.2

P* Values in the last column are P x Thickness (μm) and represent a specific permeation rate for oxygen (die accuracy is about 3 figures).

Example 7

A series of lamination experiments with the commercial film samples described in Example 6, Table 2, using photografting with the photopolymerizable composition C (PCC), were prepared as shown schematically in Fig. 2. A PVA film is laminated between an LDPE film (top) on a PET film (bottom). The permeability of the resulting photografted laminates was measured by Tetra Pak Materials R & D, Inc., using a proprietary method and given as cc 0 ₂ / day m ² . The reactive PCC contains 5 g benzophenone and 2 g poly(vinylacetate) dissolved in a monomer mixture of 80 g butylacrylate and 20g acrylic acid . The UV iπadiation was made with a 2 kW HPM 15 lamp for 60 sec. at 55°C. For the laminates the film samples used are identified in Table 2 with the thickness in μm both for the films and the grafted layer of PCC marked as in Table 3. Thickness values for films and grafted layers in order are given in bracket.

Table 3 Permeability of photografted laminates

No. Laminate: Film samples ,Thickness in μm P (cc O-i/m ² day)

BYB-1 PET // PVA // LDPE (125/15/12/15/125)* 0.1

BYB-2 LDPE // PVA // LDPE (125/10/12/10/125) 0.2

BYB-3 LDPE // PVDC ( 125/12/10) 125

BYB- 1DPE // PVDC // LDPE (63/8/10/8/63) 78

BYB-5 LDPE // PVDC // PET (63/6/10/6/125) 8.6

BYB-6 LDPE // PVDC ( 125/6/10) 126

BYB-7 LDPE // PVA //PET (63/12/12 12/125) 0.1

BYB-8 LDPE // PVA // LDPE (63/7/12 7/63) 0.1

BYB-9 LDPE // PVA // PET ( 125/5/12/5/125) 0.2

BYB-10 LDPE // LDPE (63/12/63) 1077

BYB-1 1 PET // LDPE ( 125/10/63) 10.1

BYB-12 LDPE // PET (125/10/125) 16.3

BYB-13 LDPE // PVA // PET ( 125/6/12/6/125) 0.1

BYB-14 LDPE //PVA //LDPE (63/15/12/15/63) 0.1

BYB-15 PET // PVDC (125/6/10) 8.2

BYB-16 PET // PVDC (125/10/10) 7.7

BYB-17 OPP // PVDC (15/7/10) 62

BYB-18 LDPE // PVDC (63/10/10) 54

BYB-19 OPP // PVDC // OPP (15 7/10/7/15) 116

* This means: PET film 125μm, PCC layer 15μm, PVA film 12μm, PCC layer 15μm, LDPE film 125μm.

Films of LDPE, OPP, PVDC and PVA but not films of PET are sufficiently transparent to UV irradiation of wavelength 250—400 nm for photografting as measured in separate experiments. Therefore, the laminates cannot be photografted with UV inadiation through a PET film which has been observed in the photografting of the

laminates in Table 3.

The laminaπon process by photografting lends itself to be developed as a continuous process similar to the connnuous surface photografting as first descnbed in our papers in 1986 and 1988 and later copied by several authors Apphcation of a photoreacove composiuon on moving sheets or films, assembly to mulnlayered laminates and UV irradiauon of the laminates for photografung would be a combmation of known technologies

Conclusions

A new mediod for preparation of laminates and composite matenals has been invented It is named "laminanon photografting" and involves UV-irradiated grafting of two or more polymeπc films or of fiber reinforcement between two or more polymer films with intermediate layers of a photopolymenzable composition (PC) in liquid state

The lamination photografung process is based on our previous invenoon of "surface photografting" for surface modification of polymer sheet, film, filament and yarn

In the lamination photografting process, the PC is deposited or injected to foπn a thin layer (usually 5 to 15 μm thick) between the two films to be photografted with or without fiber reinforcement enclosed.

The assembly is irradiated widi UV light of wavelengths 250 to 400 nm (near UV) from one or both sides.

Of the film materials used for lamination photografting, polyethylene, polypropylene, polyvinylidene chloride and polyvinyl alcohol but not poly(ethylene terephthalate) are transparent enough to the near UV irradiation used (250 to 400 nm).

With a reactive vinyl monomer or a mixture of two or more vinyl monomers and benzophenone as initiator, photografting occurs on both film surfaces, forming a layer which swells with a solvent and allows separation of die two grafted films of which both after separation are covered with a swollen layer of grafted polymer.

When the monomer/initiator solution contains a small amount of multifunctional vinyl or allyl monomer as crosslinker, die grafted layer forms a crosslinked gel which may swell but does not dissolve. Crosslinking gives, therefore, a more permanent laminate than grafting without crosslinker added.

When a small amount of a soluble polymer is added to d e monomer/initiator mixture, (the polymerizable composition PC), the viscosity of the PC solution increases which makes it more easy to apply and retain as an intermediate layer during photografting. The addition of crosslinker and soluble polymers gives grafted layers of higher strength but has only a minor effect on the permeability for small molecules like oxygen.

Lamination by photografting of proper monomers can give efficient barrier properties to a laminate, e.g.a 7 μm layer of photografted poly(acrylic acid) between two 30 to 35 μm thick films of LDPE decreases the permeability for oxygen by a factor 40 to 60.

Lamination by photografting is an efficient method to laminate films of very low permeability, e.g. PET, PVDC and PVA films, onto or between carrier films of high permeability, e.g. polyethylene and polypropylene. Laminates of high mechanical strength and very low permeability have been prepared in this way. Many combinations of films and photografting compositions are possible to obtain specific properties.

Films of PVA and ed ylene-vinyl alcohol copolymers (EVOH) have barrier properties which are much affected by d e water content, i.e. the humidity of the penetrant.

Lamination by photografting is a simple and efficient method to protect the PVA and

EVOH films e.g. by photografting lamination with HDPE or PP films on both sides.

Lamination of high-strength polyethylene fibers (Spectra 900 from Allied-Signal

Comp.,USA) as reinforcement between two films of LDPE by photografting with benzophenone as initiator and acrylic acid as monomer has been made using the same procedure as previously described. Mechanical testing showed increased strength, indicating that both fiber and film surfaces were grafted. The results were not reproducible due to incomplete wetting of e fiber surface and air bubbles which appeared and were difficult to remove. Odier combinations of fiber, film and monomer composition could be more successful in lamination by photografting.