[0001] The invention relates to a process for forming multilayer structures having a metal layer. More particularly, the invention relates to a process for forming multilayer structures having a metal layer and an SBS-containing tie layer, at low temperature and contact time.

BACKGROUND OF THE INVENTION

[0002] Over the past decade, the transportation, building and storage industries have increasingly relied upon composite materials to displace construction designs composed entirely of wood or metal. However, while the use of all-metal construction for transportation panels in semi-trailers, truck bodies, and portable storage containers has declined as a result of the acceptance of composite materials, aluminum, steel and other metals are still quite extensively used as part of the composite because of the durability and strength they provide. These applications benefit from processes where the metal layer is bonded to other layers in the structure in as efficient manner as possible. Similar benefits exist in other applications, such as adhering nails to nail-collation tape, wire and cable products, where an aluminum wire is typically adhered to the outer layer of low density polyethylene, or in composite multilayer pipe applications, where an aluminum interior layer is sandwiched between interior and exterior layers of polyethylene, cross-linked polyethylene or polypropylene.

[0003] Various adhesives are known to bond layers of dissimilar materials, such as olefin polymers to EVOH or metals, and are useful in multilayer applications. Adhesives containing ethylene-propylene rubbers are widely used commercially in such applications. Styrene-based adhesives have also been used. U.S. Pat. No. 8,184,298 discloses adhesives containing unmodified styrene-based elastomers in multilayer structures containing polyethylene and polypropylene. U.S. Pat. Appi. Publ. 2007/0187569 discloses adhesives containing styrene-isoprene-styrene polymers in multilayer structures containing styrene polymer layers. U.S. Pat Appi. Publ. 2009/0171022 discloses adhesives containing blends of styrene-isoprene- styrene and styrene-butadiene-styrene polymers in multilayer structures containing styrene polymer layers. U.S. Pat. Pubi. 2008/0163978 discloses extrusion and lamination processes where metal and polyoiefin layers are bonded using a tie-layer containing two functionalized ethylene polymers. However, a continuing need exists for processes that can efficiently and effectively bond metal layers to other materials in the more demanding conditions of reduced temperature and contact time.

SUMMARY OF THE INVENTION

[0004] The present invention relates to a process for producing a multilayer structure comprising contacting an adhesive layer having a first side and a second side, on its first side with a metal layer having a first side and a second side, on its first side at a temperature of 325°F to 425°F for a contact time of 0.5 to 5.0 seconds, thereby adhering the adhesive layer to the metal layer. The adhesive layer comprises 45 to 85 wt% of an ethylene-C4-8 a- oiefin linear low density copolymer; 5 to 25 wt% of a polymer grafted with an ethylenicaily unsaturated carboxylic acid or acid derivative, the polymer selected from polyethylene resins or elastomers; and 0 to 30 wt% of a styrene-butadiene-styrene triblock copolymer having an Ml of 5 to 50,

DETAILED DESCRIPTION OF THE INVENTION

[0005] it has unexpectedly been found that it is possible to produce a multilayer structure having a metal layer by a process of contacting a metal layer and an adhesive layer containing a styrene-butadiene-styrene triblock copolymer at reduced temperature and contact times. The process provides advantages in lower operating costs and higher production rates, while retaining desired levels of product adhesion.

Contacting the adhesive and metal layers.

[0006] The adhesive layer and metal layer can be contacted by any method typically practiced by those skilled in the art, to form an A/M bi-layer structure, where "A" is the adhesive layer and "M" is the metal layer. Preferably, the adhesive layer is contacted with the metal layer in an extrusion lamination or compression molding process where the adhesive and/or metal are heated and pressed together. Extrusion lamination can include hot roll or calendar lamination processes, where the adhesive and metal layers are drawn onto one or more heated rollers. The heated rolls heat the adhesive and metal layers and press them together. Alternately, the adhesive layer and/or the metal layer can be passed over an open flame as in a flame lamination process, the adhesive layer being softened by the flame and/or by the heat transferred from the metal, with the metal and adhesive layers then being pressed together. In sheet extrusion processes, a polyethylene or polypropylene sheet layer is extruded and then brought into contact with the adhesive layer. The polymer/adhesive multilayer structure is brought into contact with the metal layer. The residual heat from extrusion of the sheet layer softens the adhesive which then bonds the polymer sheet to the metal layer.

[0007] In compression molding processes, a multilayer structure containing an adhesive layer and a metal layer is first placed in an open, heated mold cavity. Preferably, the multilayer layer structure to be molded is pre-heated. The moid is closed with a top force or plug member, pressure is applied to force the multilayer structure into contact with all mold areas, and heat and pressure are maintained until the molding material has cured.

[0008] Preferably, the adhesive and metal are adhered by a lamination process.

[0009] The adhesive layer and the metal layer are contacted at a temperature from 325T to 425°, more preferably, 35G°F to 420°F, and most preferably 390 to 410°F. Preferably, the adhesive layer and the metal layer are contacted at a pressure of 10 psig to 50 psig. In extrusion lamination processes, the pressure refers to the nip roil pressure. In compression molding processes, the pressure refers to the pressure exerted on the mold.

[0010] The contact time for the adhesive layer and the metal layer is 0.5 to 5.0 seconds. Preferably, the contact time is 0.5 to 2.5 seconds, most preferably 0.7 to 1 .2 seconds. For the purposes of this specification, the term contact time is defined as the period during which the adhesive layer and metal layer are pressed together at a temperature of at least 325°F and a pressure of at least 10 psig. Preferably, the level of adhesion achieved between the adhesive layer and the metal layer Is at least 3.5 pounds per linear inch (PLI).

Metal Layer

[0011] For the purposes of this specification, the term metal layer can include aluminum and its alloys, such as aluminium, Ainico, duraluin, AA- 8000, and magnaiium; antimony; bismuth and its alloys, such as cerrosafe, wood's metal and rose metal; cobalt and its alloys, such as megallium, satellite, ultimet, vitallium, copper and its alloys, such as arsenical copper, beryllium copper, billon, brass, bronze, constantan; gold and its alloys such as eiectrum, tumbaga, rose gold and white gold; iron and its alloys, such as carbon steel, stainless steel, and galvanized steel; lead and its alloys, such as solder, terne, and molybdocba!kos; magnesium and its alloys such as magnox and elektron; manganese; nickel and its alloys, such as alumel, german silver, chromel, hastelloy, inconel, monei metal, nichrome, nicrosil, nisil, nitinol, cupronickei, and ainico; platinum; palladium; silver and its alloys, such as billon, sterling silver, Britannia silver, goloid, eiectrum, argentium sterling silver, shibuichi, and platinum sterling; tin and its alloys, such as britannium, and pewter; titanium and its alloys, such as beta c and 8ai-4v; tungsten; or zinc and its alloys such as zamak. Preferably, the metal is selected from aluminum and its alloys, copper and its alloys, carbon steel, stainless steel or galvanized steel. Most preferably, the metal is selected from aluminum and galvanized steel.

[0012] Preferably, the metal layer has a thickness of about 10 to 30 mils, more preferably from 15 to 25 mils.

Adhesive Layer

[0013] The adhesive layer comprises a styrene-butadiene-styrene tribiock copolymer (SBS), a polyethylene resin grafted with an ethylenicaliy unsaturated carboxylic acid or acid derivative, and an ethylene-C^e a-o!efin linear low density copolymer. SBS tribiock copolymers useful for the invention are commercially available thermoplastic elastomers which preferably contain from 25 to about 80 wt% styrene. More preferably, the SBS tribiock copolymers contain 30 to 55 wt% styrene. Most preferably, the SBS tribiock copolymers contain 40 to 50 wt% styrene. The melt indexes (Ml) of the SBS copolymers are from 5 to 50 dg/min, determined in accordance with ASTM D 1238 (200° C; 5 kg). Preferably, the melt indexes are in the range from 10 to 35 dg/min, more preferably from 20 to 30 dg/min.

[0014] The grafted polymer is obtained by reacting unsaturated carboxyiic acids and carboxylic acid anhydrides, or derivatives thereof, with a polyethylene resin or a plastomer, under grafting conditions. The grafting monomers, i.e., acid, anhydride or derivative, are incorporated along the polyethylene or plastomer backbone. When the polymer to be grafted is a polyethylene resin, the resin includes ethylene homopolymers and copolymers of ethylene with propylene, butene, 4-methyl pentene, hexene, octene, or mixtures thereof. Preferably, the polyethylene to be grafted is HDPE or LLDPE. Most preferably, the polyethylene to be grafted is HOPE.

[0015] When the polymer to be grafted is a plastomer, the plastomer includes copolymers of ethylene with about 2.5 to 13 mole % of a C4-8 a- olefin having densities of 0.85 to 0.92 g/cm3. The piastomers are produced using metailocene or single-site catalysts. Preferably, the plastomer is an ethylene copolymer of butene-1 , hexene-1 or octene-1 . Such piastomers are commercially available from Dow Chemical Company in its ENGAGETM line of polymers or from Exxon Chemical Company's in its EXACTTM line of products.

[0016] The carboxyiic acids or anhydrides useful as grafting monomers include compounds such as acrylic acid, ma!eic acid, fumaric acid, citaconic acid, mesaconic acid, maieic anhydride, 4-methyl cyc!ohex-4-ene-1 ,2- dicarboxylic acid or anhydride, bicycio(2.2.2)oct-5-ene-2,3-dicarboxylic acid or anhydride 2-oxa-1 ,3-diketospiro(4,4)non-7-ene, bicyclo(2.2.1 )hept-5-ene-2,3- dicarboxylic acid or anhydride, tetrahydrophtha!ic acid or anhydride, x- methylbicyclo(2.2.1 }hept-5-ene-2,3-dicarboxylic acid or anhydride, nadic anhydride, methyl nadic anhydride, himic anhydride, and methyl himic anhydride. Maieic anhydride is a particularly useful grafting monomer. Acid and anhydride derivatives which can be used to graft the polyethylene or polypropylene include diaikyi maieates, diaikyi fu ma rates, diaikyl itaconates, dialkyl mesaconates, diaikyi citraconates and aikyl crotonates. [0017] Grafting is accomplished by thermal and/or mechanical means in accordance with known procedures, with or without a free-radical generating catalyst such as an organic peroxide, where the grafted sample is prepared by heating a mixture of the polyolefin and graft monomer(s), with or without a solvent, while subjecting it to high shear. Preferably, the grafted products are prepared by melt blending the polyethylene or polypropylene in the substantial absence of a solvent, in the presence of the grafting monomer in a shear- imparting reactor, such as an extruder. Twin screw extruders such as those marketed by Werner-Pfieiderer under the designations ZSK-30, ZSK-53, Z8K- 83, ZSK-90 and ZSK-92 are especially useful for carrying out the grafting operation. Preferably, the amount of acid or acid derivative comonomer(s) grafted onto the polyethylene or polypropylene ranges from 0.1 to 4 weight percent, preferably from 0.5 to 3.0 weight percent. Preferably, when ma!eic anhydride is grafted onto HDPE or LLDPE, the grafted maleic anhydride concentration is 0.5 to 4 weight percent, most preferably 1 to 2.5 weight percent. Melt indexes of the modified ethylene polymers as measured by ASTM D 1238, at 190°C, 2.18 kg, are preferably 1 to 20 dg/min, more preferably 5 to 18 dg/min.

[0018] The grafting reaction is carried out at a temperature selected to minimize or avoid rapid vaporization and consequent losses of the graft monomer and any catalyst that may be employed. The graft monomer concentration in the reactor is typically about 1 to about 5 wt.% based on the total reaction mixture weight. A temperature profile where the temperature of the polyolefin melt increases gradually through the length of the extruder/reactor up to a maximum in the grafting reaction zone and then decreases toward the reactor exit is preferred. The maximum temperature within the reactor should be such that significant vaporization losses and/or premature decomposition of any peroxide catalyst are avoided. The grafting monomer and any catalyst used are preferably added in neat form to the extruder/reactor.

[0019] The ethylene-C4-8 α-o!efin linear low density copolymer used in the tie layer composition has a density of 0.910 to 930 g/cm3, as measured by ASTM D 792, and a melt index as measured by ASTM D 1238, condition 190/2,16, from 0.1 to 25 dg/min, preferably 0.5 to 5 dg/min, most preferably 1 to 3 dg/min.

[0020] The adhesive layer compositions can further comprise additives such as stabilizers, UV absorbers, metal deactivators, thiosynergists, peroxide scavengers, basic co-stabilizers, acid scavengers, nucleating agents, clarifiers, conventional fillers, dispersing agents, plasticizers, lubricants, emuisifiers, pigments, flow-control agents, optical brighfeners, flame-proofing agents, antistatic agents, blowing agents, and mixtures thereof, which can be added in amounts well known to those skilled in the art.

[0021] The adhesive layer contains 50 to 85 wt%, preferably 67 to 80 wt% of the ethy!ene-C4-8 a~olefin linear low density copolymer; 5 to 25 wt%, preferably 8 to 15 wt% of the polymer grafted with an ethylenically unsaturated carboxylic acid or acid derivative; and 10 to 30 wt%, preferably 10 to 20 wt%, more preferably 12 to 18 wt% of the styrene-butadiene-styrene triblock copolymer.

Multilayer structures

[0022] Multilayer structures can be formed which include the metal/adhesive bi-layer structure. For example, an additional adhesive layer can be bonded to the metal layer to form an A/M/A structure, where A is the adhesive layer and M is the metal layer. Additionally, polyolefin layers can be included in the multilayer structure, e.g., a P/A/M/A or P/A/M/A/P structures can be formed where the metal layer is the core layer, and P is a polyolefin layer individually selected from polypropylene or polyethylene. Alternately, additional layers can be added to the A/M bilayer structure so that one or more of the outside layers is a metal, e.g., P/A/M, A/P/A/M, M/A/P/A/M. Formation of these multilayer structures can be formed either in a single step, i.e., all the layers are contacted together simultaneously, or separately.

Polyoief in Layers

[0023] When polyolefin layers are used in the multilayer structure, they can be polyethylene, polypropylene or both. When the polyolefin layer is polypropylene, the polypropylene layer (PP) preferably includes a layer selected from a homopoiymer of propylene, a copolymer of propylene with ethylene or C4-C10 alpha-oiefins where the ethylene or C4-C10 alpha-olefin comonomers are present in amounts up to 10 wt% or mixtures thereof. The propylene homopoiymers and copolymers can be produced using either Ziegler Natta or single-site catalysts, e.g., metallocene catalysts. When the propylene polymer is a copolymer, it preferably contains 2 to 8 wt% ethylene as a comonomer. More preferably, the propylene polymer layer is a propylene homopoiymer.

[0024] When the polyolefin is polyethylene, the polyethylene layer (PE) is preferably selected from ethylene homopoiymers, ethylene copolymers where the comonomer is chosen from C4-8 a-olefins, or mixtures thereof. The polyethylene layer can also include co-extruded structures of polyethylene with other ethylene copolymers such as ethy!ene-vinyi acetate copolymer and ethylene methyl acrylate copolymer. Ethylene homopoiymers and ethylene- C4-8 α-olefin copolymers include very low density polyethylene (VLDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE) and high density polyethylene (HDPE). VLDPE is defined as having a density of 0.860 to 0.910 g/cm3, as measured by ASTM D 792. LDPE and LLDPE are defined as having densities in the range 0.910 to 0.930 g/cm3. MDPE is defined as having a density of 0.930 to 0.945 g/cm3. HDPE is defined as having a density of at least 0.945 g/cm3, preferably from 0.945 to 0.969 g/cm3. The ethylene homopoiymers and copolymers preferably have melt indexes, as measured by ASTM D 1238, condition 190/2.16, from 0.01 to 400 dg/min, preferably, from 0.1 to 200 dg/min., more preferably from 1 to 100 dg/min. Preferably, the polyethylene layer is LDPE. The polyolefin layer can be filled with filler, talc, glass fibers, mica, flame retardents or foaming agent.

[0025] The following examples illustrate the invention; however, those skilled in the art will recognize numerous variations within the spirit of the invention and scope of the claims.

[0026] Adhesive compositions of the examples were prepared using the following components: LLDPE-1 LLDPE having a density of 0.918 g/cm ³ and an MS of 2 dg/min.

Mgraft-1 HDPE grafted with 1.9 wt% maleic anhydride, having an

Ml of 9.5 dg/min and a density of 0.952 g/cm ^' \

Mgraft-2 HDPE grafted with 1.6 wt% maleic anhydride, having an

Ml of 6.0 dg/min and a density of 0.952 g/cm ³ .

Mgraft-3 An ethylene-butene plastomer, commercially available from The Dow Chemical Company, and having an Ml of 0.5 dg/min and a density of 0.901 g/cm ³ was grafted with 1 .9 wt% maleic anhydride. Grafting was conducted in a

ZSK-40 twin screw extruder without addition of peroxide. The resulting Ml of the grafted plastomer was 15.6 dg/min.

Mgraft-4 A grafted plastomer was prepared as with Mgraft-3 but using 50 ppm of Luperox 101 , an organic peroxide commercially available from Arkema Co. The resulting Ml was 8.6 dg/min.

EPR-1 Vistalon 722, an ethylene-propylene copolymer commercially available from ExxonMobil Chemical, containing 72 wt% ethylene and 28 wt% propylene, and having an Ml of 1 dg/min.

SBS-1 Vector 6241 A, a styrene-butadiene-styrene tribiock copolymer commercially available from Dexco Polymers LP, containing 43 wt% styrene and having an Ml of 23 dg/min.

ADD-1 A 50/50 blend of Irganox 1010 and Irgafos 168, both commercially available from BASF. .

CONTROL EXAMPLE 1

[0027] In Control Example 1 , a 5-mii cast adhesive film was prepared by first melt blending in a ZSK-18 extruder an adhesive composition containing 74.8 wt% LLDPE-1 , 10 wt% Mgraft-1 , 15 wt% SBS-1 , and 0.2 wt% ADD-1 , and then extruding the blended mixture in a Kiilion KL-100 extruder having a temperature profile of 145°C, 155°C, 160°C and 170°C.

[0028] The adhesive film and a 22 mil aluminum strip were cut into 1 .5" X 3" coupons, and a sample arranged, where the top layer was the adhesive film and the bottom layer was the aluminum. The bars of a Sentinel heat sealer were preheated before heat sealing, by closing the heat-sealing bars for three seconds with only the top bar heated. The sample was then heat sealed at 450°F and 40 psig for a contact time of one second. Following heat sealing, the coupons were cut into 1 " X 3" strips for adhesion testing. Adhesion was determined as pounds per linear inch (PSJ) on the heat-sealed sample by ASTM method D1878 in an Instron tensile tester, by measuring the force required to separate the layers in a T~Peei configuration at a cross head speed of 10 in/minute.

EXAMPLE 2

[0029] Example 2 is identical to Control Example 1 except that the heat seal temperature was at 400°F.

EXAMPLE 3

[0030] Example 3 is identical to Example 2 except that the sample was heat sealed for a contact time of three seconds.

COMPARATIVE EXAMPLE 4

[0031] Comparative Example 4 is identical to Control Example 1 , except that in the adhesive composition, EPR-1 was used instead of SBS-1.

COMPARATIVE EXAMPLE 5

[0032] Comparative Example 5 is identical to Example 2 except that in the adhesive composition, EPR-1 was used instead of SBS-1 .

COMPARATIVE EXAMPLE 6

[0033] Comparative Example 6 is identical to Example 3 except that in the adhesive composition, EPR-1 was used instead of SBS-1 .

[0034] Table 1 summarizes the adhesion values of Control Example 1 , Examples 2 and 3, and Comparative Examples 4-6.

Table 1

[0035] Table 1 demonstrates that the process of the invention using samples containing an SBS/grafted PE-based adhesive not only provides good adhesion values at low contact temperatures and time, but also demonstrates improved adhesion retention at low contact temperatures over a range of contact times, relative to EPR-based adhesives.

CONTROL EXAMPLE 7

[0036] Control Example 7 is identical to Control Example 1 except that in the adhesive composition, MGraft-2 was used instead of Mgraft-1 . EXAMPLE 8

[0037] Example 8 is identical to Example 2 except that in the adhesive composition, Mgraft-2 was used instead of Mgraft 1.

EXAMPLE 9

[0038] Example 9 is identical to Example 3 except that in the adhesive composition, Mgraft-2 was used instead of Mgraft-1 .

COMPARATIVE EXAMPLE 10

[0039] Comparative Example 10 is identical to Comparative Example 4 except that in the adhesive composition, Mgraft-2 was used instead of Mgraft- 1.

COMPARATIVE EXAMPLE 1 1

[0040] Comparative Example 11 is identical to Comparative Example 5 except that in the adhesive composition, Mgraft-2 was used instead of Mgraft- 1

COMPARATIVE EXAMPLE 12

[004] J Comparative Example 12 is identical to Comparative Example 6 except that in the adhesive composition, Mgraft-2 was used instead of Mgraft- 1.

[0042] Table 2 summarizes the adhesion values of Control Example 7, Examples 8 and 9, and Comparative Examples 10-12.

Table 2

[0043] Table 2 demonstrates that the process of the invention using samples containing an SBS/grafted PE-based adhesive not only provides good adhesion values at low contact temperatures and time, but also demonstrates improved adhesion retention at low contact temperatures over a range of contact times, relative to EPR-based adhesives.

CONTROL EXAMPLE 13

[0044] Control Example 13 is identical to Control Example 1 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft-1.

EXAMPLE 14

[0045] Example 14 is identical to Example 2 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft-1 .

EXAMPLE 15

[0046] Example 15 is identical to Example 3 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft-1 ,

COMPARATIVE EXAMPLE 18

[0047] Comparative Example 18 is identical to Comparative Example 4 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft- 1.

COMPARATIVE EXAMPLE 17

[0048] Comparative Example 17 is identical to Comparative Example 5 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft- 1

COMPARATIVE EXAMPLE 18

[0049] Comparative Example 18 is identical to Comparative Example 6 except that in the adhesive composition, Mgraft-3 was used instead of Mgraft- 1.

[0050] Table 3 summarizes the adhesion values of Control Example 13, Examples 14 and 15, and Comparative Examples 16-18.

Table 3

[0051] Table 3 demonstrates that the process of the invention using samples containing an SBS/grafted plastomer-based adhesive not only provides good adhesion values at low contact temperatures and time, but also demonstrates improved adhesion retention at low contact temperatures over a range of contact times, relative to EPR-based adhesives.

CONTROL EXAMPLE 19

[0052] Control Example 19 is identical to Control Example 1 except that in the adhesive composition, Mgraft-4 was used instead of Mgrafi-1 .

EXAMPLE 20

[0053] Example 20 is identical to Example 2 except that in the adhesive composition, Mgraft-4 was used instead of Mgraft-1 .

EXAMPLE 21

[0054] Example 21 is identical to Example 3 except that in the adhesive composition, Mgraft-4 was used instead of Mgraft-1 .

COMPARATIVE EXAMPLE 22

[0055] Comparative Example 22 is identical to Comparative Example 4 except that in the adhesive composition, Mgraft-4 was used instead of Mgraft- 1

COMPARATIVE EXAMPLE 23

[0056] Comparative Example 23 is identical to Comparative Example 5 except that in the adhesive composition, Mgraft-4 was used instead of Mgraft- 1.

COMPARATIVE EXAMPLE 24

[0057] Comparative Example 24 is identical to Comparative Example 6 except that in the adhesive composition, Mgrafi-4 was used instead of Mgraft- 1

[0058] Table 4 summarizes the adhesion values of Control Example 19, Examples 20 and 21 , and Comparative Examples 22-24,

Table 4

[0059] Table 4 demonstrates that the process of the invention using samples containing an SBS/g rafted plastomer-based adhesive not only provides good adhesion values at low contact temperatures and time, but also demonstrates improved adhesion retention at low contact temperatures over a range of contact times, relative to EPR-based adhesives.