BACKGROUND OF THE INVENTION Production of panels (plate type units) for construction of nylon heat exchangers is carried out by a twin sheet thermoforming process with internal gas assist. A phenol solution is applied to the inner, mating surfaces of the two sheets of nylon film employed to effect bonding. This basic concept is discussed in the DuPont"Zytel"Handbook and refinements are presented in US Patents 4,935,462 and 5,039,370 which form the basis for panel production in accordance with US 5,195,240. While bonds formed by this process using phenol are strong and durable, the odor of phenol is undesirable and may be unacceptable during manufacture or in the finished article.

BRIEF DESCRIPTION OF THE DRAWINGS The sole figure of the drawing, Fig. 1, is a plan view of an embodiment of a heat exchanger panel made in accordance with the invention.

SUMMARY OF THE INVENTION The present invention uses multilayer films for formation of panels, wherein the inner layers, or the mating surfaces, of the films used in the twin sheet thermoforming process with internal gas assist are made of a polyamide modified to be thermally bondable or heat-sealable, eliminating the need for phenol. These films can be produced by coextrusion of a thin bondable layer (0.013-0.076 mm) preferably of a blend of polyvaprolactam (19-40% by weight) polymer prepared from equimolar amounts of 2-methylpentamethylene diamine and 1,12-dodecanedioic acid (20-40% by weight) such as"D-12"high

performance nylon sold by DuPont, a copolymer of ethylene and a vinyl carboxylic acid, partially neutralized by a Group 1A, 2A or 3A metal ion (15-58% by weight) such as"Surlyn"ionomer sold by DuPont, and a polymeric grafting agent (3-16% by weight) such as"EBAGMA"grafting agent sold by DuPont, optionally containing heat stabilizers, pigments and other know adjuvants, onto a main structural layer (2-10 mil) of polyamide film. Formulations for the bondable inner layer are further described in US Patent Application No. 60/014,150, filed March 25,1996, incorporated herein by reference. Panel formation can be carried out with the same equipment, under minor modifications to the process.

The polyamide-based composition of the inner layer of each sheet is a heat formable laminating film made from a multi-phase thermoplastic resin composition comprising the following main components: i) at least one polyamide resin selected from aliphatic and semi- aromatic polyamides that can be either semi-crystalline or amorphous in structure having a number average molecular weight of at least about 5000; ii) at least one polyamide resin comprising at least one pendant alkyl branch having 1 to 3 carbon atoms within at least two amide linkages along the polymer backbone and at least one sequence of at least seven consecutive carbon atoms, excluding carbon atoms in pendant alkyl branches, if any, within at least two amide linkages along the polymer backbone, the melting point of the polyamide being less than 200°C; the polyamides of I) and ii) having graft sites and forming the continuous phase of the composition; iii) at least one ethylene copolymer, E/X/Y, where E is ethylene and is at least 50 % by weight of E/X/Y, X is from 1-35 % by weight of an acid containing unsaturated mono-carboxylic acid, and Y is 0-49 % by weight of a moiety derived from at least one alkyl acrylate, alkyl methacrylate, alkyl vinyl ether, carbon monoxide, sulfur dioxide, or mixtures thereof where the alkyl groups contain 1-12 carbon atoms, and further wherein the acid groups in the acid-containing moiety are neutralized from 0-100% by weight of a metal ion;

iv) at least one polymeric grafting agent which contains reactive groups selected from at least one of epoxides, isocyanates, aziridines, silanes, alkyl halides, alpha-halo ketones and aldehydes, or oxazoline, which reacts with the acid-containing moieties in component iii) and additionally reacts with the graft sites of components i) and ii), and the weight percent of the monomer (s) containing the reactive groups is 0.5-15 weight percent of the polymeric grafting agent, and the remainder of the polymeric grafting agent contains at least 50 % by weight of ethylene and from 0-49 % by weight of a moiety derived from at least one alkyl acrylate, alkyl methacrylate, alkyl vinyl ether, carbon monoxide, sulfur dioxide, or mixtures thereof where the alkyl groups contain 1-12 carbon atoms.

The components are combined as follows: from about 17 to about 54% by weight of component i), from about 1 to about 40% by weight of component ii), from about 5 to about 69% by weight of component iii), and from about 0.5 to about 45 % by weight of component iv); such that the sum of components i) and ii) equals from about 29 to about 72% by weight.

In another embodiment of the invention there is provided a novel multi- phase resin composition comprising as the main components: from about 17 to about 54% by weight of (i) at least one polyamide resin selected from aliphatic and semi-aromatic polyamides that can be either semi-crystalline or amorphous in structure having a number average molecular weight of at least about 5000, wherein the semi-crystalline polyamides have a melting point greater than 200°C; from about 1 to about 40 % by weight of (ii) at least one polyamide resin comprising at least one pendant alkyl branch having 1 to 3 carbon atoms within at least two amide linkages along the polymer backbone and at least one sequence of at least seven consecutive carbon atoms, excluding carbon atoms in pendant alkyl branches, if any, within at least two amide linkages along the polymer backbone, the melting point of the polyamide being less than 200°C;

the polyamides of I) and ii) having graft sites and forming the continuous phase of the composition; and with the proviso that the sum of components i) and ii) is from about 29 to about 72% by weight; from about 5 to about 69% by weight of (iii) at least one ethylene copolymer, E/X/Y, where E is ethylene and is at least 50 % by weight of E/X/Y, X is from 1-35 % by weight of an unsaturated mono-carboxylic acid, and Y is 0- 49 % by weight of a moiety derived from at least one alkyl acrylate, alkyl methacrylate, alkyl vinyl ether, carbon monoxide, sulfur dioxide, or mixtures thereof where the alkyl groups contain 1-12 carbon atoms, and further wherein the acid groups in the acid-containing moiety are neutralized from 0-100% by weight of a metal ion; and from about 0.5 to 45% by weight of (iv) at least one polymeric grafting agent which contains reactive groups selected from at least one of epoxides, isocyanates, aziridines, silanes, alkyl halides, alpha-halo ketones and aldehydes, or oxazoline, which grafting agents react with the acid-containing moieties in component iii) and additionally react with the graft sites of components i) and ii), and the weight percent of the monomer (s) containing the reactive groups is 0.5-15 weight percent of the polymeric grafting agent, and the remainder of the polymeric grafting agent contains at least 50 % by weight of ethylene and from 0-49 % by weight of a moiety derived from at least one alkyl acrylate, alkyl methacrylate, alkyl vinyl ether, carbon monoxide, sulfur dioxide, or mixtures thereof where the alkyl groups contain 1-12 carbon atoms.

In a preferred form of the invention, the end group balance of the low temperature nylon has been found to affect the processing and properties of the final film product. In other words, it has been found that high performance nylon, specifically D12, with balanced or carboxyl rich end groups in the formulation reduces filter pressure drops and melt viscosities during film production, and improves film dimensional stability during heating-compared to the incorporation of D12 having amine-rich ends.

A preferred form of the present formulation comprises from about 17 to about 54% by weight, more preferably, from about 18 to about 47% by weight,

and most preferably, from about 19 to about 40% by weight of Nylon 6 (component i); from about 1 to about 40% by weight, more preferably about 10 to about 40% by weight; most preferably from about 20 to about 40% by weight of Nylon D12 (low temperature nylon), (component ii); from about 5 to about 69%, more preferably from about 11 to about 58% by weight, and most preferably from about 15 to about 48% by weight of ethylene E/X/Y (component iii); from about 0.5 to about 45% by weight, more preferably from about 2 to about 28%, most preferably from about 3 to about 16% by weight of EBAGMA (component iv); with the total amount of nylon ranging preferably from about 29 to about 72% by weight, more preferably from about 38 to about 71% by weight, and most preferably from about 45 to about 70% by weight.

It should be noted that for the ranges set out above these may be applied to the various generic components also.

In another preferred form of the invention, the formulation comprises from about 55 to about 80% by weight of components (i) and (ii), with the nylon components I) and ii) taken together always in the majority (the iii) component which can be"SURLYN"ionomer produced by DuPont, and the iv) coponent which can be"EBAGMA"comaptibilizer produced by DuPont, are in the minority), but component (i) may range from about 20 to about 60% by weight, and component (ii) may range from about 10 to about 35 % by weight.

In every instance, the formulations disclosed herein may include antioxidants, heat stabilizers or mixtures thereof. Typically these comprise from about 0.05 to about 5.0% by weight, preferably from about 0.05 to about 2.0% by weight. Organic heat stabilizers have been found to be better than the metal halide heat stabilizers, such as CuI/KI, in terms of retention of film physical properties after oven aging for one hour at 200°C. "Irganox"1010/1098 blend of two hindered phenolic antioxidants produced by Ciba Specialty Chemicals is a preferred example of such a material. This substance also reduces filter pluggage and reduces pressure during the production of the film.

Other optional ingredients may be selected from flame retardants, anti- blocking agents, slip additives, pigments or dyes, processing aids, plasticizers

and ultra-violet blocking agents. These may be used in suitable quantities as are well known to those skilled in the art.

All of the resin formulations set forth above may be formed into films using techniques well known in the art. Such films form part of the novel aspects of the present invention with respect to the novel resin formulations noted above.

It is to be understood that the terms layer, sheet, and film are used interchangeably herein. The term layer may encompass monolayer and multilayer films as well.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In this invention, some elements of the technology of US Patent 5.195,240 are used without the need for solvent bonding which is required by that patent.

Fig. 1 is an embodiment of a heat exchanger panel. In this embodiment, orifices 1 and 3 of the inlet and outlet header means, generally indicated by 2 and 4, are located at opposite ends of the heat exchanger panel, which in the embodiment shown is elongated. Such an elongated heat exchanger panel is particularly adapted to be located in, for instance, the radiator of an automobile.

In Fig. 1, the inlet and outlet header means 2 and 4 are connected by a series of substantially linear fluid flow passages 5 and 6 that are interspaced with similarly elongated sections in which the panels of the heat exchanger are sealed together; in particular embodiments, the linear fluid flow passages have a width of less than 12.5 mm and especially in the range of 1.2-2.5 mm EXAMPLES A film was produced by coextrusion of a layer (0.051 mm) of a blend of"Zytel"FN 727 flexible nylon produced by DuPont) (70% by weight), and polymer prepared from 2-methylpentamethylene diamine and 1,12-dodecanedioic acid (30% by weight) onto a main structural layer (0.153 mm) of nylon 6,6 blended with CFE8005 polyolefinic toughener sold by DuPont (20% by weight).

The film so formed was found to give seal strength of 16.1 kg/linear cm (14 lb/linear inch) when sealed at 180C, pressure of 117 kg/cm2 (40 psi) and dwell time of 10 seconds. Heat exchanger panels formed from this film according to US 5,195,240, omitting the phenol/benzyl alcohol coating, at 175C and dwell time of 6-20 seconds, were well formed and required internal air pressure of 205 kg/cm2 (70 psi) to rupture, compared to 234-293 kg/cnr' (80-100 psi) for panels formed according to US 5,195,240. Although this is a lower level of strength, it has the advantage of being done without solvents and is useful for making heat exchangers.

"Zytel"FN 727 partially-grafted flexible nylon, produced by DuPont, is a blend by weight of 40% nylon 6; 46%"Surlyn"9320 ionomer produced by DuPont; 10%"EBAGMA"EP4934-6 compatibilizer produced by DuPont; 2% zinc stearate; and 2%"Irgonox"1010 hindered phenolic antioxidant produced by Ciba Specialty Chemicals. It is in US Patent 5,091,478-Saltman et al., incorporated by reference.

"CFE8005"polyolefinic toughener, produced by DuPont, can be made as a blend by weight of 75.8% nylon 6,6; the functional equivalent of 17.2% Fusabond MF416D EP rubber, grafted with maleic annhydride, compatibilizer produced by DuPont; 4.4% carbon black 40% concentrate in nylon 6;"DER 732"diepoxy ethylene oligomer with a MW of about 300 produced by Dow Chemical; and 1500 ppm sodium hypophosphite. It is in US Patent 4,174,358-Epstein.