Factors determinants en el disseny d’un Motor Stirling

Paper and paperboard represent by far the most widely used package material both in the United States and around the world. Because of its derivation from cellulose fibers, paper per se is not a barrier to moisture or oxygen and so it is generally combined with other materials such as plastic or even aluminum foil to render it effective in packaging applications. Most of this category is comprised of paperboard rather than paper, with the dividing boundary being 0.010 inch caliber or gauge, paper being below the line and paperboard being above 0.010 inch.

The two basic types of paperboard are virgin or that originating directly and primarily from trees and their wood, and recycled, or that whose raw material is used paper and paperboard. Generally virgin pa- perboard, i.e., from tree wood, is cleaner and more uniform, and has the greatest strength per caliper (unit gauge). Furthermore, it accepts the barrier material for coating more easily than does recycled paper- board. On the other hand recycled paperboard may, if desired, have a superior surface for printing. Recy- cled paperboard has been used as a secondary (non- food contact) package material for many decades, with the origins of the material being largely trim- mings and scrap from paper, paperboard, and corru- gated fiberboard converting plants.

Because paperboard is moisture sensitive, for dairy products packaging it is generally necessary to pro- tect the paperboard, which then functions primarily as a structural material. Among the coatings used are low-density polyethylene applied by hot melt extru- sion over the entire surface. Polyethylene is an ex- cellent moisture and water barrier to protect the base paperboard.

Paperboard is used in dairy product packaging as the substrate for both gable top and aseptic brick/block-shaped cartons to contain fluids. In the latter application it is extrusion laminated with plas- tic and aluminum foil to foster a long time shelf life. Coated paperboard is also used to fabricate cups and trays to contain semisolid dairy products such as yo- gurt and cottage cheese.

Probably the major dairy products packaging ap- plication for paperboard, however, is in three-layer form in corrugated fiberboard cases used for distribu-

tion. The corrugated structure consists of three layers of two outer flat sheets called liners of paperboard, usually virgin, plus an inner fluted sheet or medium that can be either virgin or recycled. The corrugated structure offers vertical and horizontal compression and impact strength to protect the contents, usually primary packages.

Increasingly, the printing on corrugated fiberboard liners is being improved to permit the cases to be used as retail displays or even as consumer packages and multipacks.

Metal

Metal is most often used for cylindrical cans, which are either thermally processed for microbiological stability, e.g., evaporated milk, or internally pressur- ized with carbon dioxide as for beer, and carbonated beverages. Aluminum is by far the most important metal used for can fabrication, being the primary metal for beer and carbonated beverage cans, and increasingly used for still beverage cans such as for juices and aseptically canned milk, but only sparsely for food cans except for shallow pet food and fish cans. In the past aluminum cans with easy open tops were used to contain milk-based puddings and yo- gurts that were filled aseptically. This application has been replaced by barrier plastic cups with pee- lable flexible lidding materials. Almost all aluminum cans are two-piece. More recently, impact/extrusion- formed aluminum bottles are being applied for dairy products. Bottles are narrow neck structures, usually closed with metal screw closures, but sometimes with polypropylene closures.

Steel represents the major metal used for food cans, usually being coated with chromium oxide and later coated with a thermoset plastic to protect the metal from corrosion.

Aluminum is also used in very thin or foil gauges −ca 0.00035 inch or below, as a flexible or semirigid packaging laminant to impart oxygen and water vapor barrier to the lamination. In this form, because it is fragile, the aluminum must be protected from damage by plastic or paper.

Glass

Glass is historically the oldest packaging material still in use. Glass is the best barrier known and by far the most inert to product contents. Furthermore, in appropriate structures, glass has the greatest vertical compressive strength. On the other hand, glass is very

heavy per unit of contents contained, is energy inten- sive to manufacture, and, as is well known, is prone to breakage with impact. Glass may be fabricated into bottles and jars, almost all of which require plastic or metal devices to close. Although glass was the most widely used material for packaging fluid milk and its fermented analogues during the first half of the twentieth century, its dairy products applications during the past two decades have dwindled to virtu- ally zero. Occasionally, a few dairies offer yogurt in glass to convey a high quality image, but most dairies shun glass as a hazardous material in production and packaging operations.

Plastic

Plastic is the newest package material having been developed during the last century and having come into prominence only since the 1950s. In actuality, the term “plastic” describes a family of materials re- lated by their common derivation from petrochemical sources. Each is quite different in properties relative to packaging requirements, and so no single plastic material is capable of being universally employed. All plastic package materials are characterized by their lightweight, relative ease of fabrication, low cost, and ability to be tailored for specific end ap- plications. Together, by weight, all plastics comprise about 20% of package materials, but because of their low densities, protect far larger volumes of contents than any other package materials, perhaps 60–70% of all foods and dairy products.

Polyethylene

The most commonly used packaging plastic is polyethylene, which may be obtained in high, medium, and low densities with variations now avail- able on each of these. Low-density polyethylene (LDPE) is tough, flexible, easily formed after heat- ing, lightweight, and forgiving as a heat sealant. It is an excellent water and water vapor barrier, but a poor oxygen and flavor barrier. LDPE’s most com- mon uses are as flexible pouches and bags, and as the heat sealable extrusion coatings on paper, paper- board, and aluminum foil. Thus LDPE is the coating on gable top fluid cartons, the laminant on aseptic bricks and blocks, and the heat seal coating on many flexible lidding materials.

High-density polyethylene (HDPE) is a semirigid, somewhat stiff translucent easily thermoformable plastic. With fairly good heat resistance, HDPE has

excellent moisture and water resistance but is a very poor gas barrier. HDPE is used to form bottles for milk and many other liquids, as well as a wide variety of other products with modest barrier requirements. HDPE may also be formed into cups, tubs, or trays to contain yogurt and cottage cheese.

Polyester

Polyethylene terephthalate polyester (PET) has been available as a specialty film packaging material for many years, but only since the late 1970s did it enter as a significant package material. A modest oxygen and water vapor barrier, in package form after ori- entation, PET is tough and transparent. PET’s ma- jor packaging applications today are for carbonated beverage bottles, with other bottles and jars as for drinkable yogurt, salad dressing, peanut butter, etc., thermoformed cups and tubs, etc., in the semirigid category are secondary applications at present. PET may also be formed into films that are tough and di- mensionally stable and, therefore, are quite good as laminants to protect aluminum foil or for lidding-type flexible closures. In partially crystallized form PET may be fabricated into trays for dual oven (microwave and conventional conduction–convection) reheating.

Polypropylene

In oriented film form, polypropylene is an excellent, economic, tough, transparent, high-moisture barrier, low-gas barrier package material, which has captured almost the entire quality flexible packaging mar- ket. Among the packages being made with oriented polypropylene (OPP) are potato chip pouches, bak- ery goods overwraps, and candy bar wraps. Because of its relatively high temperature resistance (up to 250◦F), polypropylene resin is combined with other higher barrier packaging materials to produce multi- layer plastic bottles and cans such as for ketchup or for “bucket-type” cans for microwave reheating. For economic reasons (i.e., when the commodity price is favorable), polypropylene may be injection molded into tubs and cups to contain fermented dairy prod- ucts such as yogurt and cottage cheese.

Polystyrene

Polystyrene is a plastic with a relatively poor oxygen and water vapor barrier but good structural proper- ties. Being inexpensive and easy to form by sheet extrusion and thermoforming or injection molding

methods, polystyrene has been one plastic of choice for cup/tub containment of yogurt, cottage cheese, etc., since the demise of wax-coated paperboard dur- ing the late twentieth century.

Oxygen-Barrier Materials

Most of the above plastics are not good oxygen bar- riers. To obtain the oxygen barrier, two plastic resins, polyvinylidene chloride (PVDC), and ethylene vinyl alcohol (EVOH) are employed commercially. PVDC is the older of the two and has excellent water vapor and fat resistance but is relatively difficult to fab- ricate, as well as being questioned on environmen- tal grounds for its hydrogen chloride content. Much PVDC is used in emulsion-coating form on films to achieve oxygen barrier in films used for processed meats and cured cheese.

EVOH is a better oxygen-barrier material and is easier to fabricate but is very sensitive to moisture. The economics of both high-oxygen barrier mate- rials dictate that they be combined with other less expensive structural plastic resins. Thus EVOH is usually coextruded (i.e., forced with another plastic through a common die) with polypropylene to pro- duce films, sheets, or coatings. The EVOH is pro- tected from environmental or product moisture in these applications. In addition to its involvement in “bucket-style” cans, EVOH is also an increasingly important material to protect food and beverage con- tents from flavor interaction with packaging mate- rials. With many food and beverage contents now being held for prolonged periods up to a year at am- bient temperature, the probability of adverse prod- uct plastic interactions, largely flavor changes, is relatively high. Consequently, an intermediate high- barrier material such as EVOH serves to minimize such interactions in packages such as those for chilled juices.