Belt process makes PP sheet run faster and clearer.

A new extrusion process from Japan reportedly makes extremely clear polypropylene sheet without resorting to orientation. It's said to do so at higher speeds and over a wider range of thicknesses than conventional processes. The secret is a steel cooling belt that presses the sheet against the main cooling roll. The belt holds the sheet under pressure for a distance that is 10 to 200 times longer than is provided by a conventional polishing roll or air knife. Longer cooling under pressure produces smoother, glossier sheet and allows the line to run faster, according to IPEC Co. Ltd. of Tokyo, developer of the patent-pending "belt process." This process was first exhibited at the International Plastics Fair (IPF) in Tokyo in 1994 (see PT, April '94, p. 55). Five Japanese firms have been using it commercially to make PP and other sheet since 1993. The technology is now available in the U.S. through I-Pec USA, Ramsey, N.J. Norio Kasai, president of I-Pec USA, says the belt process can also be used to make PS, PET, PE, EVA, ABS, acrylic, and polycarbonate sheet.

BEATS OTHER METHODS

According to IPEC technical director and co-founder Keita Morita, the preferred traditional means of obtaining high-clarity PP sheet has been the polishing-roll method, in which the melt is cast vertically from a slot die between two metal rolls. Because of the very small area of nip contact, line speeds must be relatively low in order to obtain high-quality sheet, says Morita. He also says the polishing-roll method is not suited to making sheet thinner than 0.3-0.4 mm. That process also requires maintenance of a bank of melt above the nip. This requirement makes the process hard to stabilize and causes lengthy start-up times, according to Morita.

Alternatively, the air-knife process is capable of running faster, but it yields lower clarity, in Morita's view. The low pressure applied by this method is not sufficient to "iron out" thickness variations or uneven smoothness, he says. Unbalanced cooling by air on one side of the sheet and a roll on the other not only limits clarity but also restricts line speeds to 20 m/min max. and 10 m/min where sheet appearance is important. In addition, limited pressure and cooling capabilities reportedly restrict sheet thickness to less than 0.5 mm.

By contrast, the IPEC process is said to provide higher clarity than the polishing-roll method and higher output rates than even the air-knife process (see Table 1). The belt process reportedly also handles a wider range of thicknesses than either method (Table 2).

High gloss is provided by compressing the sheet over a relatively long distance between the mirror-finish main roll and the special mirror-finish steel belt made by Sandvik of Sweden. The belt smooths out any thickness irregularities in the sheet as it emerges from the die. Morita says this is an advantage over a third method of PP extrusion - that of direct casting into a water bath without passing over a roll. This method, however, cannot correct any thickness variations, which will cause uneven cooling and thereby lead to growth of crystallites that impair transparency, according to Morita.

Besides imparting the desired thickness and smoothness, the steel belt also forms a solidified skin layer on the sheet before it enters the quench bath. The steel belt is maintained at the proper quench temperature by its contact with the main driving roll. The latter is heated/cooled with oil, as is the main cooling/polishing roll. Contact with the cooling roll and belt immediately forms a clear, amorphous solid skin on both sides of the PP sheet, while the center remains molten. The sheet then enters a water bath to complete the quenching process. Subsequently, the sheet is dried and passed over four to six annealing rolls.

Since the belt process requires no bank of melt as does the polishing-roll method, the process is said to be easy to stabilize and quick to start up (10-15 min is reportedly typical). Kasai says the system is relatively simple to operate and does not require much additional training. Unlike the polishing-roll process, sheet thickness can be adjusted quickly and "on the fly" by simply changing the take-off speed. Also, the belt hold-down roll can be raised or lowered to change the length of the arc of the compression zone in order to adjust sheet quality and line speed.

TABLE 1 - TEST RUNS OF 0.3-MM PP SHEET
(Random Copolymer)

                    Belt Process   Polishing Roll   Air Knife

Line speed, m/min     20              5                10
Haze, %              1.7              5                15
Gloss, %             140            125                90

[TABULAR DATA FOR TABLE 2 OMITTED]

WHAT IT COSTS

Equipment for the IPEC belt process costs around 1020% more than other polypropylene sheet extrusion systems, Kasai reports. Floor-space requirements are comparable to s standard line. With the belt process, he notes, processors may be able to attain superior clarity using standard PP homopolymer rather than more costly and less stiff random copolymer.