Amorphous polyamide having excellent oxygen barrier properties

Abstract: An amorphous polyamide resin is provided having the combined properties of a high glass transition temperature and excellent oxygen barrier properties. The polyamide is obtained from the reaction products of para-xylene diamine, adipic acid and isophthalic acid, and finds particular utility as a container layer for hot fill packaging of fruit juices.

Description: This invention relates to amorphous polyamides and, more particularly, relates to amorphous polyamides obtained by reacting p-xylylene diamine with adipic acid and isophthalic acid. Retort packaging is common in the fruit juice industry where hot juices, typically at temperatures ranging from 80°C to 120°C, are filled directly into the final disposable commercial packaging. By filling with hot juice, the container is simultaneously filled and sanitized. The use of polyamides as layers in such retort packaging has generally been unsuitable for a number of reasons, namely, many polyamides are crystalline in nature, lacking the level of clarity necessary for aesthetically pleasing clear containers; some polyamides lack a glass transition temperature sufficiently high enough to maintain the shape of the container and prevent deformation thereof during hot fill; and some polyamides lack the oxygen barrier properties necessary to prevent oxidation and discoloration of fruit juices held by containers made therefrom. Conventional polyamides include those set forth in NL Pat. No. 7,212,060; U.S. Pat. No. 4,018,746; U.S. Pat. No. 4,482,695; and U.S. Pat. No. 4,640,973; these polyamides generally suffer from either having a relatively low glass transition temperature, employing relatively high cost monomers, or exhibiting relatively low levels of clarity. Accordingly, one object of the present invention is to provide a polyamide which has the combined properties of a high glass transition temperature, excellent clarity, and a low oxygen transmission rate. Another object of the present invention is to provide an amorphous polyamide resin suitable as a container layer for hot filled fruit juices or other hot filled, oxygen sensitive liquids. SUMMARY OF THE INVENTION The present invention provides a novel polyamide which is transparent, has a glass transition temperature which is sufficiently high to handle hot fill packaging, and has oxygen barrier properties which are sufficient to protect fruit juice from oxidation and discoloration, thereby permitting containers made therefrom to provide extended shelf lives to fruit juices contained therein. The polyamide of the present invention is obtained from reaction products of para-xylylene diamine, adipic acid and isophthalic acid. DETAILED DESCRIPTION OF THE INVENTION The present polyamide is prepared by reacting a diamine with a dicarboxylic acid. The term dicarboxylic acid is meant to include diaryl esters and dihalides thereof. The diamine is para-xylylene diamine; and the dicarboxylic acid is a mixture of respective amounts of isophthalic acid and adipic acid. The isophthalic acid is present at a level of from 80 mole% to 40 mole% based on the total moles of dicarboxylic acid; more preferably at a level of from 80 to 60 mole% thereof; and most preferably at a level of 80 mole% thereof. Preferably the adipic acid is present at a level of from 20 to 60 mole% based on the total moles of dicarboxylic acid; more preferably at a level of from 20 to 40 mole% thereof and most preferably at a level of 20 mole% thereof. The desired amorphous polyamide is obtained from the reaction products of the para-xylylene diamine, isophthalic acid and adipic acid. The diamine and dicarboxylic acid are reacted in mole ratios of from between 48:52 and 52:48 and preferable in a mole ratio of 50:50. The para-xylylene diamine has the following structural formula: and can be prepared by oxidizing p-xylylene in the presence of ammonia to obtain a cyano compound which upon hydrogenation forms para-xylylene diamine. These reactions can be represented as follows: This reaction produces high yields, often greater than 90% yields, of para-xylylene diamine thereby potentially permitting low-cost production of para-xylylene diamine. Isophthalic acid is represented by the following chemical structure: and has the properties of being in the form of colorless crystals and has a melting point of 345°C to 348°C. Isophthalic acid can be derived from the oxidation of m-xylene. Adipic acid has the chemical formula of HOOC(CH₂)₄COOH and is commercially available. The polyamides of the present invention are prepared by methods generally known in the art for producing polyamides such as by conventional salt processes which involve reacting a diamine with a dicarboxylic acid in a solvent to form a solid polyammonium salt which precipitates from the reaction mixture. The solid is then heated successively in an autoclave reactor under pressure to control explosive initial dehydration and subsequently under reduced pressure to effect conversion of the polyammonium salt to a polyamide. The diamine and dicarboxylic acid component are preferably reacted in approximately stoichiometric quantities. The polyamides may also be prepared by a melt process involving the reaction of the diamine and a diaryl ester of the dicarboxylic acid. A suitable melt process is set forth in Fox et al U.S. Patent 4,567,249. The polyamide resin of the present invention has first moieties represented by the following formula: ; and second moieties represented by the following formula: wherein, the first moieties are present in the polyamide at a level of from 80 mole% to 40 mole% based on the total moles of first and second moieties in the polyamide and the second moieties are present at a level of from 20 mole% to 60 mole% based on the total moles of first and second moieties in the polyamide. More preferably, the polyamide has from 80 mole% to 60 mole% of the first moieties based on the total moles of first and second moieties and from 20 mole% to 40 mole% of the second moieties based on the total moles of first and second moieties. Most preferably, the polyamide consists essentially of 80 mole% of the first moieties based on the total moles of first and second moieties and 20 mole% of the second moieties based on the total moles of first and second moieties. The most preferred polyamide resin has a glass transition temperature of at least 170°C and an oxygen transmission rate of at most about 1.94 x 10⁻⁵ cm³-µm/m²-24hr-Pa (0.50 cc-mil/100 in²-day-atm). The preferred polyamide is obtained from the reaction products of a mixture consisting essentially of about 50 mole% of para-xylylene diamine based on the total moles of diamine and dicarboxylic acid; from 40 mole% to 20 mole% of adipic acid based on the total moles of diamine and dicarboxylic acid; and from 10 mole% to 30 mole% of the isophthalic acid based on the total moles of diamine and dicarboxylic acid. The polyamide resin of the present invention is clear, amorphous having no distinct melting point and preferably has an intrinsic viscosity in deciliters per gram (dl/g) of between about 0.40 and about 1.60 as measured in 40:60 weight mixture of tetrachloroethylene and phenol; preferably a glass transition temperature of at least 130°C, more preferably a glass transition temperature of between 150 and 180°C, and most preferably a glass transition temperature of about 174°C; and preferably has an oxygen transmission rate of at most about 3.88 x 10⁻⁵ cm³-µm/m²-24hr-Pa (1.0 cc-mil/100in²-24hr-atm), more preferably has an oxygen transmission rate of at most about 1.94 x 10⁻⁵ cm³-µm/m²-24hr-Pa (0.5 cc-mil/100in²-24hr-atm), and most preferably has an oxygen transmission rate of at most about 1.16 x 10⁻⁵ cm³-µm/m²-24hr-Pa (0.3cc-mil/100in²-24hr-atm). The oxygen transmission rate or permeability of the resin is measured as cubic centimeters of oxygen permeating a 25.4 µm (1 mil) thick sample, 6.45 m² (100 inches square), for a 24-hour period under an oxygen partial pressure difference of 1.013 x 10⁵ Pa (1 atmosphere) at 30°C. The polyamide resin of the present invention may be made into film or sheet, may be used as a laminating layer between two permeable layers of film, may be used to make molded containers by coextrusion blow molding or other molding techniques, and may be employed to form a coextruded layer. Containers employing a layer of the polyamide resin of the present invention are suitable for the hot fill of fruit juices therein. Hot filling of juices involves filling of the container with juices having temperatures of from about 80°C to about 120°C. These high temperatures permit simultaneous filling and sanitizing of the interiors of the containers. The high glass transition temperature of the resin enables the containers to keep their shapes and avoid deformation during hot fill. The oxygen barrier properties of the resin enable the containers to provide long shelf life to the fruit juices therein by preventing oxidation thereof by atmospheric oxygen, the amorphous character of the polyamide resin results in containers made therefrom being clear thereby enhancing the aesthetics of the container, allowing the consumer to see the true appearance of the juice to be purchased, and the potentially relatively low cost diamine and diacids employed potentially enhance the cost competitiveness of containers made from the resin. Preferably containers employ the amorphous polyamide as an intermediate layer between inner and outer layers of aromatic polycarbonate. Suitable containers may be made by coextrusion blow molding. Suitable aromatic polycarbonate resins are made by reacting phosgene with bisphenol A. The invention is further described in the examples which follow. These examples are illustrative of the invention and are not to be construed in limitation thereof. Table 1 sets forth polyamides obtained by reacting para-xylylene diamine with isophthalic acid, adipic acid and mixtures thereof. Note that the polyamides of examples 1, 5 and 6 were crystalline whereas the polyamides of examples, 2, 3 and 4 were amorphous having no distinct melting point. Table 2 sets forth polyamides obtained from reacting various diamines with various diaryl esters. Note that the polyamides derived from para-xylylene diamine typically had higher glass transition temperatures than corresponding polyamides derived from meta-xylylene diamine. Examples 1, 5 to 9, 11, 13, 14, 15, 17 and 18 are comparative.

Claims: A polyamide consisting of first moieties having a formula of : , and second moieties having a formula of : said first moieties being present in said polyamide at a level of from 80 mole % to 40 mole % based on the total moles of first moieties and second moieties in said polyamide, said second moieties being present at a level of 20 mole % to 60 mole % based on the total moles of first moieties and second moieties in said polyamide. The polyamide of Claim 1 wherein said polyamide consists of from 80 mole % to 60 mole % of said first moieties based on the total moles of first and second moieties, and from 20 mole % to 40 mole % of said second moieties based on the total moles of first and second moieties. The polyamide of Claim 2 wherein said polyamide has a glass transition temperature of at least 170°C and an oxygen transmission rate of at most 1.94x10⁻⁵cm³-µm/m²-24hr-Pa (.50 cc-mil/100 in²-day-atm) measured as cubic centimeters of oxygen permeating a 25,4 µm thick sample, 6,45 m² for a 24-hour period under an oxygen partial pressure difference of 1,013x10⁵ Pa at 30°C.

Inventors: FOX DANIEL WAYNE, PETERS EDWARD NORMAN, RIDING GEOFFREY HENRY, WILLARD FRED G, FOX, DANIEL WAYNE, PETERS, EDWARD NORMAN, RIDING, GEOFFREY HENRY, WILLARD, FRED G.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1990
Main classification: C08G69
IPC codes: C08J5, B65D81, C08G69