WO2013070502A1

WO2013070502A1 - Multilayer polymer structure

Info

Publication number: WO2013070502A1
Application number: PCT/US2012/063164
Authority: WO
Inventors: Charles C. Crabb; Thomas H. Richards; Florence Mehlmann
Original assignee: Arkema France
Priority date: 2011-11-07
Filing date: 2012-11-02
Publication date: 2013-05-16

Abstract

Multilayer polymer structures have at least three layers. These layers include a polar capstock layer, an olefinic substrate layer, and a tie layer containing a heterogeneous olefin-acrylate copolymer. The heterogeneous olefin-acrylate copolymer is found to have increased adhesion when compared to a relatively homogeneous olefin-acrylate of the same monomer composition. The multi-layer thermoformable structure comprises at least one polyolefin-based layer, at least one polar polymer-containing capstock layer, and at least one tie layer comprising a heterogeneous olefin-acrylate copolymer, wherein said tie layer(s) is directly adjacent to, and in between the polyolefinbased layer and the capstock layer.

Description

MULTILAYER POLYMER STRUCTURE

FIELD OF THE INVENTION

The invention relates to multilayer polymer structures having at least three layers. These layers include a polar capstock layer, an olefinic substrate layer, and a tie layer containing a heterogeneous olefin-acrylate copolymer. The heterogeneous olefin-acrylate copolymer is found to have increased adhesion when compared to a relatively homogeneous olefin-acrylate of the same monomer composition. BACKGROUND OF THE INVENTION

Multi-layered polymeric structures are used to take advantage of the properties of two or more different polymers. The multi-layer polymeric structures (or sheets) are found in parts used in many applications, including the automotive industry;

communications such as telephones, radio, TV, cassettes, etc.; power tools;

appliances; business machines; toys; furniture; medical devices, building and construction, etc.. When preparing multilayer structures, the layers of the structures must adhere securely to each other for good performance. If the layers of the structure do not adhere well to each other, a special adhesive, or in other cases a tie layer, may be used to join the layers of the multilayer structure together.

The multilayer structures of the invention may be produced by any methods available in the art, such as by co-extrusion techniques, lamination techniques, thermoforming, injection molding, blow molding or any combination thereof. Co- extrusion is a process in which two or more molten polymeric compositions are simultaneously extruded through a feedblock die or, alternatively, through a multi- manifold die, to form a laminar structure with different functional properties in each layer. A feedblock die can be used to feed a multi-manifold die in a single process, to provide excellent flexibility in the manufacture of the multilayer structures.

Lamination is the process of bonding together prefabricated sheet or film layers, or prefabricated and extruded sheet or film layers, by the use of adhesives, or by a combination of heat and pressure. Alternatively, hot melt lamination or thermal lamination brings two or more molten polymer layers together outside the extrusion die, usually at a nip roll or at the top roll of a roll stack.

Multilayer structures formed by blends of different polymer compositions are known. Examples of such multilayer structures include those disclosed in U.S. Pat. Nos. 5,264,280, 5,374,680 and 5,385,781. Generally speaking, these patent references disclose multilayer structures having olefinic core layers and capstock layers containing polymers of vinyl aromatic compounds such as polystyrene.

Polyolefin substrate layers (and especially thermoplastic polyolefin (TPO)) with an acrylic capstock layer provide an excellent combination of the strength and low-cost value of a TPO, with the weatherable and appealing appearance of an acrylic capstock layer. Unfortunately, non-polar TPO and polar acrylics do not adhere well to each other, and a tie layer must be found that can adhere these layers together. US 6,455,171, US 6,420,050, and US 2008/0220274 disclose multilayer structures which provide the physical properties of an olefinic core layer and the scratch and chemical resistive properties of an acrylic capstock layer. The tie layers disclosed are either an olefinic acrylate copolymer, or a block copolymer of vinyl aromatic monomer with aliphatic conjugated diene, partially hydrogenated diene, or olefin monomer. These references are silent on the means by which the olefin- acrylic copolymer is synthesized, or any mention of monomer distribution within the copolymer, only broad over-all monomer levels. Examples use olefin-acrylics from Eastman and elfAtochem. Each of these manufacturers use an autoclave polymerization process.

A paper by Chou, et al. ("High Flexibility EMA made from High Pressure Tubular Process", 2002 ANTEC) describes differences in monomer distributions between ethylene/methyl acrylates (EMA) made by autoclave reactors and those made in tubular reactors where monomers and initiator are added at multiple locations along the reactor. The autoclave reactor, benefitting from a uniform mixture of monomers, produces a relatively homogeneous distribution of comonomer in the polymer chains. In contrast, the tubular reactor lacks back-mixing of the monomers, and has a long polymerization time, leading to a broader comonomer distribution among the polymer chains. The more homogeneous autoclave-produced polymer has a melting point that decreases essentially linearly with monomer content, while the more heterogeneous tubular reactor polymer has a higher melting point despite a high MA content.

Mechanical property measurement showed that the tubular reactor polymer produced more flexible EMA polymer than the autoclave, for similar monomer content. The article does not mention any adhesive effects of the heterogeneous polymer compared to the more homogeneous polymer.

Heterogeneous olefin-acrylate copolymers have been found to have good adhesion to polyesters (US 5,532,066 and US 2004/0001960) There is a need to develop tie layer compositions with improved adhesion of a polar capstock layer to an olefinic polymer substrate.

Applicant has now surprisingly found that tie layers containing heterogeneous olefin- acrylic copolymer provide much better adhesion than homogeneous olefin- acrylic copolymers for adhering a variety of polar capstock films to an olefinic substrate.

SUMMARY OF THE INVENTION

The invention relates to a multi-layer thermoformable structure comprising: a) at least one polyolefin-based layer,

b) at least one polar polymer-containing capstock layer, and

c) at least one tie layer comprising a heterogeneous olefin-acrylate copolymer, wherein said tie layer(s) is directly adjacent to, and in between the polyolefin- based layer and the capstock layer.

DETAILED DESCRIPTION OF THE INVENTION

All percentages used herein are weight percentages unless stated otherwise, and all molecular weights are weight average molecular weights unless stated otherwise. All references listed are incorporated herein by reference.

The invention relates to a multilayer polymer structure containing at least a capstock layer, a tie layer, and a substrate layer.

Capstock

The multilayer structure of this invention contains at least one polar capstock layer. Polar capstock layer polymers include, but are not limited to styrenic-based polymers, acrylic -based polymers, polyesters, polycarbonate and thermoplactic polyurethane (TPU). Preferred capstock layer polymers are styrenic and/or acrylic- based. The acrylic -based layer comprises either an acrylic polymer, or a vinyl cyanide- containing compound,- for example an acrylonitrile-butadiene-styrene (ABS) copolymer, an acrylonitrile-styrene-acrylate (ASA) copolymer, or styrene acrylonitrile (SAN) copolymer. "Acrylic polymer" as used herein is meant to include polymers, copolymers and terpolymers formed from alkyl methacrylate and alkyl acrylate monomers, and mixtures thereof. The alkyl methacrylate monomer is preferably methyl methacrylate, which may make up from 50 to 100 percent of the monomer mixture. 0 to 50 percent of other acrylate and methacrylate monomers or other ethylenically unsaturated monomers, included but not limited to, styrene, alpha methyl styrene, acrylonitrile, and crosslinkers at low levels may also be present in the monomer mixture. Suitable acrylate and methacrylate comonomers include, but are not limited to, methyl acrylate, ethyl acrylate and ethyl methacrylate, butyl acrylate and butyl methacrylate, iso-octyl methacrylate and acrylate, lauryl acrylate and lauryl methacrylate, stearyl acrylate and stearyl methacrylate, isobornyl acrylate and methacrylate, methoxy ethyl acrylate and methacrylate, 2-ethoxy ethyl acrylate and methacrylate, dimethylamino ethyl acrylate and methacrylate monomers. Alkyl (meth) acrylic acids such as methacrylic acid and acrylic acid can be useful for the monomer mixture. Most preferably the acrylic polymer is a copolymer having 70 - 99.5 weight percent of methyl methacrylate units and from 0.5 to 30 weight percent of one or more Ci-g straight or branched alkyl acrylate units.

Styrenic-based polymers include, but are not limited to, polystyrene, high- impact polystyrene (HIPS), acrylonitrile-butadiene- styrene (ABS) copolymers, acrylonitrile-styrene-acrylate (ASA) copolymers, styrene acrylonitrile (SAN) copolymers, methacrylate-butadiene- styrene (MBS) copolymers, styrene -butadiene- styrene block (SBS) copolymers and their partially or fully hydrogenenated derivatives, styrene-isoprene- styrene (SIS) block copolymers and their partially or fully hydrogenenated derivatives, and styrene-methyl methacrylate copolymers (S/MMA). A preferred styrenic polymer is ASA. The styrenic polymers of the invention can be manufactured by means known in the art, including emulsion polymerization, solution polymerization, and suspension polymerization. Styrenic copolymers of the invention have a styrene content of at least 10 percent by weight, preferably at least 25 percent by weight.

In one embodiment, the capstock layer polymer has a weight average molecular weight of between 50,000 and 500,000 g/mol, and preferably from 75,000 and 150,000 g/mol, as measured by gel permeation chromatography (GPC). The molecular weight distribution of the acrylic polymer is monomodal or multimodal and the polydispersity index is higher than 1.5.

In one embodiment, the acrylic-based layer is a blend of an acrylic polymer and 5 to 80 wt%, preferably 10 to 40 wt%, of a polyvinylidene fluoride polymer or copolymer thereof. In one embodiment, the multilayer structure of the invention contains two or more polar capstock layers, and two or more tie layers, such as a five- layer structure of polar capstock/tie layer/polyolefin-based polymer/tie layer/polar capstock layer. The structure could have an acrylic layer on one side, and a styrenic layer on the other side. In a structure in which multiple acrylic layers and/or multiple tie layers are used in layers non-adjacent to each other, the acrylic or styrenic layers and tie layers can be of the same of different compositions, though in a preferred embodiment the multiple acrylic or styrenic layers and tie layers are the same. In another embodiment, the polar capstock layers may be composed of two or more acrylic layers or two or more styrenic layers directly in contact with each other. In another embodiment, the tie- layer may be composed of two or more tie layers directly in contact with each other.

The capstock layer of the invention has a thickness of from 0.025 to 3 mm, and preferably from 0.075 to 0.5 mm. Substrate

The polyolefin-based layer, herein also referred to as a substrate layer, is thicker than the acrylic based layer(s) and tie layer(s) combined. It could contain one or more different polyolefin layers, and a polyolefin layer could be a blend of two or more different polyolefins. The polyolefins employed in the semicrystalline or crystallizable olefin polymers can be homopolymers, copolymers, terpolymers, or mixtures thereof, etc., containing one or more olefin monomeric units. In a polyolefin- based layer, the polyolefins are generally present in an amount from 30 to 100% by weight, preferably at least 55%, and more preferably at least 60% by weight. The polyolefin of this invention excludes cyclic olefin copolymer (COCs). It is common for one or more of the polyolefin layers to contain rework - material that has already been processed into an article, such as a film or sheet. The rework polyolefin is then granulated and blended with virgin polyolefin prior to re-extrusion. The rework may contain non-polyolefin components.

Polymers of alpha-olefins or 1 -olefins are preferred in the present invention, and these alpha-olefins may contain from 2 to about 20 carbon atoms. Alpha-olefins containing 2 to about 6 carbon atoms are preferred. Thus, the olefin polymers may be derived from olefins such as ethylene, propylene, 1-butene, 1-pentene, 4-methyl-l- pentene, 1-octene, 1-decene, 4-ethyl-l-hexene, etc. Examples of polyolefins include polypropylene, polyethylene, and ethylene propylene copolymers. In one embodiment, the polyolefins include polypropylene and ethylene- propylene polymers and copolymers. Propylene polymers may be semi-crystalline or crystalline in structure. The number average molecular weight of the propylene polymers is preferably above about 10,000 and more preferably above about 50,000. In addition, it is preferred in one embodiment that the apparent crystalline melting point be above about 75°C. and preferably between about 75° C. and about 250° C. The propylene polymers useful in the present invention are well-known to those skilled in the art and many are available commercially. Polypropylene are the preferred propylene polymers. Thermoplastic polyolefins (TPO) are an especially preferred substrate layer.

In one embodiment, the multilayer structure of the invention contains two or more substrate layers.

Tie-layer

The tie layer or layers used between the polar capstock layer(s) and the olefinic substrate layer(s) contains a heterogeneous olefinic-acrylic copolymer. In the case of more than one tie layer, at least one must contain a heterogeneous olefinic- acrylic copolymer.

By "heterogeneous" olefinic-acrylic copolymer is meant a copolymer having a broad comonomer distribution within polymer chains, for a given overall monomer ratio - some chains containing a higher level of olefin monomer units than others, and some containing a higher level of acrylate monomer units - while the average monomer ratio over all of the changes is the same as a homogeneous polymer. For example, a heterogeneous olefinic-acrylate copolymer could have long runs of olefin monomer units. The mechanical beta relaxation region, as measured by dynamic mechanical spectroscopy (DMS) also known as dynamic mechanical analysis (DMA) may show two distinct peaks, indicating two distinct compositions. More commonly, the peak is broadened, or a shoulder is present indicating a more continuous heterogeneous composition.

In a specific embodiment, an ethylene- alkyl acrylate copolymer having an alkyl acrylate content of X weight percent, X being equal to or greater than 15 and being based on the total weight of ethylene and alkyl acrylate in the copolymer, having an average melting point temperature in degrees Celsius (°C), as measured by differential scanning calorimetry, of greater than the value obtained from the expression: 114.44 - 1.2X. A specific example being when the olefin-acrylate polymer is an ethylene-methyl acrylate copolymer having a methyl acrylate content of Y weight percent (Y equal to or greater than 15) based on the total weight of ethylene and methyl acrylate in the copolymer, the heterogeneous copolymer would have an average melting point temperature in degrees Celsius (°C), as described in EP

0605643, measured by differential scanning calorimetry, of greater than the value obtained from the expression: 120 - 1.61Y.

The olefin-acrylate copolymers generally contain 40 - 95 weight percent and preferable 60 - 85 weight percent of one or more olefin monomer(s), and 5-60 weight percent and preferably 15-40 weight percent of one or more acrylate monomer(s). Useful olefin comonomers include, but are not limited to, alpha-olefins containing 2 to 6 carbon atoms such as ethylene, propylene, 1-butene, 1-pentene, 4-methyl-l- pentene, 1-octene, 1-decene, 4-ethyl-l-hexene, etc. Examples of polyolefins include polypropylene, polyethylene, and ethylene propylene copolymers. Poly ethylene is especially preferred.

Useful acrylates include, but are not limited to the acrylates listed for the acrylic capstock layer. The C_1-6 acrylates are preferred, especially methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, and hexyl acrylate. Methyl acrylate and ethyl acrylate are especially preferred.

Examples of suitable olefin acrylate copolymers include, but are not limited to ethylene methyl acrylate copolymers (EMA), ethylene butyl acrylate copolymers (EBA), ethylene-n-butyl acrylate-maleic anhydride, ethylene-ethylacrylate- maleic anhydride, ethylene- glycidyl methacrylate, ethylene-ethyl acrylate, ethylene-methyl acrylate-glycidyl methacrylate. Ethylene vinyl acetate based copolymers and olefin/(meth)acrylic acid copolymers (optionally fully or partially in salt form) are also useful for the tie layer.

Heterogeneous olefin acrylates can be made by means known in the art, including, but not limited to a continuous tubular reactor, and an autoclave reactor with multiple feeds, and described in US 2004001960 and US 5,532,066 respectively. In a preferred embodiment, the heterogeneous olefin acrylate is made in a tubular reactor in which monomer and initiator are added at multiple locations along the reactor.

The heterogeneous olefin acrylate polymer could be blended with a homogeneous olefin acrylate copolymer of the same or different composition. In such a blend, at least 10 percent by weight, preferably 25 percent, more preferably 50 percent and even more preferably at least 70 percent of the blend is heterogeneous olefin acrylate copolymer

In one embodiment, the olefin acrylate is blended with one or more polymers or oligomers selected from the groups of polydienes, polyolefins, polyesters (such as polylactic acid), acrylics, and styrenics, which can contain functional moieties such as epoxide, carboxylic acid, carboxylate, amine, amide, siloxane, silicone, urethane, anhydride. By "oligomer" as used herein, is meant to include organic molecules with a weight average molecular weight of 200 g/mol to 20,000 g/mol, as measured by gel permeation chromatrography. "Polymer" as used herein, is meant to include organic molecules with a weight average molecular weight higher than 20,000 g/mol, preferably higher than 50,000 g/mol, as measured by gel permeation

chromatrography. The acrylic polymer is as defined above, and can be the same or different from the acrylic polymer capstock layer or the acrylate in the olefin- acrylate copolymer of the tie layer. In one embodiment this acrylic polymer has a weight average molecular weight of less than 200,000 g/mol, and preferably less than 100,000 g/mol, and more preferably less than 80,000 g/mol. In another embodiment, the added polymer is an acrylic polymer comprising at least 20% of butyl

methacrylate as a comonomer. In another embodiment, the polymeric or oligomeric material contains covalently bonded functional moieties. Functional moieties include, but are not limited to, epoxide, carboxylic acid, carboxylates, anhydride, amide, amine, siloxane, silicone, urethane, and amino groups. The average weight molecular weight of this material is preferably lower than 20,000 g/mol, more preferably lower than 10,000 g/mol. In yet another embodiment, the added component is an oligomer of (meth) acrylate and / or styrene, copolymerized with a glycidyl (meth)acrylate or a (meth)acrylic acid monomer (e.g. JONCRYL from BASF). The functionalized oligomers are intended to be materials that improve adhesion without being too volatile. Functional oligomers includes, for example, an epoxidized C₁₈ olefin.

The ratio of the olefin- acrylate copolymer to the added polymer or oligomer of the tie layer blend is from 20 - 99 weight percent of the olefin- acrylate copolymer, preferably 80-95 weight percent, to respectively 1 - 80 weight percent of the added polymer or oligomer, and preferably 5 to 20 wt percent. The components of the blend can be combined by means known in the art, including but not limited to dry blending, solvent blending, and melt blending in an extruder. The tie layer of the invention has a thickness of from 0.025 to 2 mm, and preferably from 0.075 to 0.5 mm.

The polar capstock layer(s), polyolefin-based substrate layer(s) and tie layers may contain one or more impact modifiers, fillers or fibers, or other additives of the type used in the polymer art. Examples of impact modifiers include, but are not limited to, core-shell particles and block or graft copolymers. Examples of additives include, for example, UV light inhibitors or stabilizers, lubricant agents, heat stabilizers, flame retardants, synergists, pigments and other coloring agents. Examples of fillers employed in a typical compounded polymer blend according to the present invention include talc, calcium carbonate, mica, matting agents, wollastonite, dolomite, glass fibers, boron fibers, carbon fibers, carbon blacks, pigments such as titanium dioxide, or mixtures thereof. In one embodiment, the acrylic polymer is blended with a polyvinylidene fluoride polymer or copolymer, or with an aliphatic polyester - such as polylactic acid. Examples of matting agents include, but are not limited to, cross-linked polymer particles of various geometries, The amount of filler and additives included in the polymer compositions of each layer may vary from about 0.01 to about 70% of the combined weight of polymer, additives and filler. Generally amounts from about 5% to about 45%, from about 10% to about 40%, are included.

The fillers may be treated with coupling agents to improve the bond between the fillers to the resin. For example, the fillers can be treated with materials such as fatty acids (e.g., stearic acid), silanes, maleated polypropylene, etc. The amount of coupling agent used is an amount effective to improve the bond between the fillers with the resin.

Manufacture

The multi-layer structure of the invention can contain three or more layers, and can be made by any method known to the art. This includes separate formation of the layers, followed by lamination, coextrusion of all layers - which is preferred, or a combination of coextrusion and lamination.

The multilayer structure can have any given geometry, including but not limited to, a flat sheet, a rod, or a profile. The multilayer structure exhibits excellent structural integrity, excellent surface appearance, high impact strength, high scratch resistance, and excellent resistance to UV rays. It has been found that a homogeneous tie layer (autoclave reactor-produced) has much less adhesion than a heterogeneous tie layer (tubular reactor-produced) as measured by a 90° peel test, e.g. ASTM D6862, for the same overall monomer content. The heterogeneous tie layer provides increased adhesion to the polar capstock layer, olefinic substrate, and preferably both.

EXAMPLES

Example 1 - Comparing adhesive strength of coextruded sheet made with homogeneous vs. heterogeneous tie layer

Three-layer coextrusion was performed using a TPO substrate (Lyondell-

Basell E3400), an acrylic capstock (SOLARKOTE-H300 from Arkema) and three different ethylene- methyl acrylate copolymers, each containing 24% methyl acrylate.

A. 24% MA MI = 2 g/10 min M.P. = 70°C autoclave

B. 24% MA MI = 0.5 g/10 min M.P. = 70°C autoclave

C. 24% MA MI = 2 g/10 min M.P. = 91°C tubular

Where MI is melt flow rate per ASTM 1228 measured at 190°C and 2.16Kg weight, and M.P. is the melting point as measured by differential scanning calorimetry.

Samples A and B were made via an autoclave process giving a homogeneous product. Sample C, having the same average composition, was made via a tubular reactor and has a heterogeneous composition.

Adhesion of the capstock was measured using a 90° peel test per ASTM

D6862.

Material Peel Strength

A. 0.72 lbf/in

B. 0.78 lbf/in

C. 13.2 lbf/in

Claims

What is claimed is:

1. A multi-layer thermoformable structure comprising:

a) at least one polyolefin-based layer,

b) at least one polar capstock layer, and

c) at least one tie layer comprising at least one material selected from the group consisting of heterogeneous olefin-acrylate copolymers, homogeneous or heterogeneous ethylene vinyl acetate based copolymers, and homogeneous or heterogeneous olefin/(meth) acrylic acid copolymers that can be fully or partially in salt form, wherein said tie layer(s) is directly adjacent to, and in between the polyolefin-based layer and the capstock layer.

2. The multi-layer thermoformable structure of claim 1, wherein said tie layer comprises at least one heterogeneous olefin-acrylate copolymer.

3. The multi-layer thermoformable structure of claim 1, wherein said polar capstock layer comprises a polymer selected from the group consisting of styrenic- based polymers, acrylic-based polymers, polyesters, polycarbonate and thermoplactic polyurethane (TPU).

4. The multi-layer thermoformable structure of claim 1, wherein said polar capstock layer comprises a styrenic-based polymer or an acrylic-based polymer.

5. The multi-layer thermoformable structure of claim 1, wherein said polyolefin- based layer is a thermoplastic polyolefin.

6. The multi-layer thermoformable structure of claim 1, wherein said polar capstock layer comprises a polar polymer having a weight average molecular weight of between 50,000 and 500,000 g/mol , as measured by gel permeation

chromatography.

7. The multi-layer thermoformable structure of claim 3, wherein said styrene- containing layer comprises an styrene copolymer selected from the group consisting of acrylonitrile-butadiene- styrene (ABS) copolymers, acrylonitrile- styrene- acrylate (ASA) copolymers, styrene acrylonitrile (SAN) copolymers, methacrylate-butadiene- styrene (MBS) copolymers, styrene-butadiene- styrene block (SBS) copolymers and their partially or fully hydrogenenated derivatives, styrene-isoprene- styrene (SIS) block copolymers and their partially or fully hydrogenenated derivatives, and styrene- methyl methacrylate copolymers (S/MMA)

8. The multi-layer thermoformable structure of claim 1, wherein said polar capstock layer further comprises one or more impact modifiers, fillers, polyvinylidene fluoride polymers, polylactic acid, or fibers.

9. The multi-layer thermoformable structure of claim 1, wherein said polyolefin- based layer comprises at least 30 weight percent of one or more polyolefin materials.

10. The multi-layer thermoformable structure of claim 2, wherein said

heterogeneous olefin acrylate copolymer comprises from 10 to 100 weight percent of the tie layer.

11. The multi-layer thermoformable structure of claim 2, wherein said heterogeneous olefin-acrylate comprises from 5 to 60 weight percent of acrylate monomer units and from 40 to 95 weight percent of olefin monomer units.

12. The multi-layer thermoformable structure of claim 2, wherein said

heterogeneous olefin acrylate copolymer is an ethylene-alkyl acrylate copolymer having an alkyl acrylate content of X weight percent (X equal to or greater than 15) based on the total weight of ethylene and alkyl acrylate in the copolymer, wherein said heterogeneous copolymer has an average melting point temperature in degrees Celsius (°C), as measured by differential scanning calorimetry, of greater than the value obtained from the expression: 114.44-1.2X.

13. The multi-layer thermoformable structure of claim 2, wherein said

heterogeneous olefin acrylate copolymer is an ethylene-methyl acrylate copolymer having a methyl acrylate content of Y weight percent (Y equal to or greater than 15) based on the total weight of ethylene and methyl acrylate in the copolymer, wherein said heterogeneous copolymer has an average melting point temperature in degrees Celsius (°C), as measured by differential scanning calorimetry, of greater than the value obtained from the expression: 120 - 1.6Y.

14. The multi-layer thermoformable structure of claim 2, wherein said

heterogeneous olefin acrylate copolymer is selected from the group consisting of ethylene methyl acrylate copolymers (EMA), ethylene butyl acrylate copolymers (EBA), ethylene-n-butyl acrylate-maleic anhydride, ethylene-ethylacrylate- maleic anhydride, ethylene- glycidyl methacrylate, ethylene-ethyl acrylate, ethylene-methyl acrylate-glycidyl methacrylate.

15. The multi-layer thermoformable structure of claim 1, wherein said tie layer has a thickness of from 0.05 to 2.5 mm, and the acrylic or styrenic layer has a thickness of from 0.05 to 2.5 mm.

16. The multi-layer thermoformable structure of claim 1, wherein said tie layer further comprises as a blend with said olefin acrylate copolymer, at least one polymer or oligomer selected from the group consisting of polydienes, polyolefins, polyesters, acrylics, and styrenics.

17. The multi-layer thermoformable structure of claim 16, wherein said at least one polymer or oligomer contains functional moieties selected from the group consisting of epoxide, carboxylic acid, carboxylate, amine, amide, siloxane, silicone, urethane, anhydride.

18. The multi-layer thermoformable structure of claim 17, wherein the ratio of said olefin acrylate copolymer to said at least one polymer or oligomer in the tie-layer blend is from 20-99 to 1-80.

19. The multi-layer thermoformable structure of claim 1, wherein said olefin acrylate copolymer is polymerized in a continuous tubular reactor.