Dual ovenable containers

Polyethylene terephthalate) (PET) has become a major synthetic polymer during the past forty years. Significant commercial markets [1] have been developed for its application in textile and industrial fibers, films, and foamed articles, containers for carbonated beverages, water and other liquids, and thermoformed applications (e.g. dual ovenable containers). 

Dual ovenable containers must be able to perform under freezable conditions as well as in both microwave and conventional ovens. Materials commonly used for dual ovenables are temperature-resistant plastics such as crystallized PET (CPET), thermoset polyesters, and polycarbonate. CPET, the dominant material, has a temperature stability up to 232°C. 

Formed multilayer barrier packages, eg, trays, bowls, and cans from coextruded PP/PVDC/ PP, are being developed for soups and entrees that can be stored at ambient temperatures and heated to serving temperature in a microwave oven. Coextrusions of high performance, high temperature polymers, such as polyether-imide and polysulfone, for dual-oven containers capable of withstanding conventional bake-oven temperatures of 204-232°C are being tested. 

Attention has been paid to the possible use of multilayer coextrusion of engineering resins for dual ovenable containers, as multilayer packaging could increase shelf stability and thus eliminate the need for freezing or refrigeration. One multilayer combination in commercial use is a sheet of Lexan polycarbonate sandwiched between layers of Ultem polyetherimide. Thermoformed trays of this material are reported to have better break resistance at lower temperatures and higher stiffness at temperatures up to 232°C, as compared to CPET. They are also insert, compatible with polyester lidding materials, and have lower oxygen permeability than PET materials. 

Most plastics used in dual ovenables are two to six times more expensive than the aluminum containers they replace in frozen food applications. Less expensive plastic containers are made of molded fiber and polyester and paper laminates. However, due to the popularity of the microwave oven, in many cases microwaveable-only containers, instead of dual ovenables, are used. For example, Campbell uses a polypropylene microwaveable-only container for its Soup du Jour, thereby reducing the material costs considerably. 

Wheel machines are designed specifically for dedicated products, such as picnic plates, dual-ovenable food trays, and takeaway coffee lids [8]. The wheel is a horizontal cylinder that contains dozens to hundreds of female or negative molds. The sheet is continuously extruded through the sheet die directly onto the slowly turning wheel. Vacuum pulls the sheet into the cool molds. As the wheel turns, the formed products are cooled. When the products are sufficiently cooled, the sheet containing the products is stripped from the wheel and fed to a conventional in-hne trimmer. 

During the 1800s, benzene was of limited commercial value, finding use mainly as a solvent. But after the invention of the internal combustion engine and the automobile, it was found that motors ran better when the fuel contained benzene. This added a new economic incentive to recover all of the benzene possible from the steel industry s coke ovens. However, just prior to World War II, the importance of benzene as a chemical intermediate started to be recognized. These dual incentives (gasoline and chemical intermediate) led to new and improved benzene processes based on petrochemistry rather than coal. 

The analysis starts when a small quantity of sample in liquid or gaseous state is injected. The dual role of the injector is to vaporise the analytes and to mix them uniformly in the mobile phase. Once the sample is vaporised in the mobile phase, it is swept into the column, which is usually a tube coiled into a very small section with a length that can vary from 1 to over 100 m. The column containing the stationary phase is situated in a variable temperature oven. At the end of the column, the mobile phase passes through a detector before it exits to the atmosphere. Some gas chromatograph models have their own power supply, permitting them to be used in the field (see Fig. 2.16). 

The combustion efficiency of the GC-combustlon system (Figure 1) was tested by comparing the methane concentration of the gas atmosphere In the vacuum line that resulted from the Injection of 2 ml of methane under normal operating conditions (three separate trials) to the methane concentration of the vacuum-line gas atmosphere after Injection of v2 ml of methane with the combustion furnace turned off (three separate trials). The trap Immediately downstream from the combustion oven was Immersed In a Dewar flask containing an Isopropyl alcohol slush ( v-90 C). The resulting methane concentrations, which were obtained with a GC equipped with dual flame Ionization detectors. Indicated a combustion efficiency > 99X. 

FIGURE 16.15 Gas chromatograms of a 1-mL headspace sample of a volatile mixture containing acetaldehyde, methanol, acetone, ethanol, isopropanol, and w-propanol. GCFID conditions dual capillary columns RTxBAC-1 RTx BAC-2 30-m x 0.33-mm-i.d. X 1.0-p.m film carrier gas helium = 7 mL/min oven = 40°C injection = 150°C, detector = 200°C loop = 70°C. (Courtesy of New Jersey State Police.)... 

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