Over 50 % of the PET film produced in the world is used as a photographic filmbase. The manufacturers of these materials, mainly Agfa-Gevaert, Eastman Kodak, du Pont de Nemours, Fuji, Minnesota Mining & Manufacturing, and Konishiroku have long been interested in PET film recovery. An important motivation for the efforts made by these companies is the fact that photographic films are usually coated with one or more layers containing some amount of rather expensive silver derivatives, which have been recovered since the early 20th century, when cellulosics were used as a film base. Silver recovery makes PET-base recovery more economical.5'6 In a typical way of operation, PET film recycling is coupled with the simultaneous recovery of silver, as represented in Figure 2.
In the first step of the process, photographic emulsion layers containing silver are washed with, for example, NaOH, and after separation, silver is recovered on one side, and cleaned PET-waste on the other side.2 Important in this process is that the washed PET-film scrap is clean enough to be recovered by direct re-extrusion, although careful analysis remains necessary.
Direct recycling of PET-waste in the molten state, before re-extrusion to PET-film, is of course the most economical process thinkable, as recovered PET-scrap can be substituted for virgin PET-granulate without requiring any additional steps. It is well-known that PET in the molten state gives rise simultaneously to polymer build-up and to polymer degradation, so that reaction conditions for this process have to be controlled very carefully in order to obtain an end-product with desired physical, chemical and mechanical properties, like color, molecular weight, and molecular weight distribution.
A large number of reaction parameters have to be kept under permanent control (temperature, environmental atmosphere, holding time in a melt state, amount of impurities, type of used catalysts and stabilizers, etc.). The order of addition of the PET flakes is very important. A typical flowsheet of a batch-PET-process7 is represented in Figure 3. In such a process, the PET-flakes can be added after polymerization, before the melt enters the film extruder screw (Figure 3, indication 1). Such a procedure, however, has two main drawbacks:
In order to eliminate these disadvantages, several alternative operation modes have been worked out in the past. A method to add recycled PET during
the esterification step (Figure 3, indication 2) has been described by du Pont.8 In such a way filtration can take place in the low-viscosity phase, and volatiles can still be eliminated during the prepolymerisation phase.
Although PET-recycling by direct re-use is by far the most economical process, it is only useful in practice for well characterized PET-wastes, having exactly known chemical composition (catalysts, stabilizers, impurities). Therefore, this process is the most suited for the recovery of in-production wastes, but it may not be ideal for customer-recollected PET-film. An industrial process for X-ray film-recycling was worked out by the IPR-company9 and introduced to the market under the name REPET on the basis of a triple motivation:
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