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New amorphous polyesters designed for extrusion and thermoforming.

New Amorphous Polyesters Designed for Extrusion and Thermoforming

Polyester has a long history of successful use in packaging, especially food packaging. Biaxially oriented Mylar polyester films were introduced in 1952, and the polyester bottle for carbonated beverages was commercialized in 1978.

Today, the use of amorphous polyester (APET) sheet in thermoforming is one of the fastest growing segments of packaging. Thermoformed cups, trays, clamshells, blisters, and organizers are used in a variety of food packaging applications and for medical products and hardware. Cups for cold beverage service, especially on airlines, ahd tubs for delicatessen products are two of the newest and fastest growing uses. Trays for bakery products are the next likely area of rapid growth. In these products, APET is replacing crystal polystyrenes and PVC. APET usage is estimated at 60 to 70 million lbs/yr, and double-digit growth over the next five years has been forecast.

Polyester Advantages

Polyester's advantages over polyolefin as a packaging material include its moderate oxygen and water barriers; excellent barrier to flavors, odors, and most solvents; and good resistance to fats and oils. Polyester neither absorbs flavors nor imparts off-tastes in food packaging. APET sheet has excellent optical properties--high gloss, transparency and sparkle, and very low haze. Exposure to gamma ray irradiation during sterilization has little effect. At dosages up to 5 megarads, there is no yellowing and little loss of physical properties.

Polyester processing is safe because no toxic fumes or gases are evolved and it contains no chlorinated monomers. It is perceived as an environmentally friendly plastic and is recyclable. Clean edge trim, trim press skeletons, and defective parts are easily recycled in the sheeting operation. Polyester is believed to be one of the cleanest plastics in use today.

Polyester Disadvantages

Polyester properties that reflect toughness--tear, flex, and impact strength--may be somewhat less than those of polyolefins. Moisture barrier is also less but is adequate for many applications in the thicknesses used. Because it is usually easily crystallized, polyester is not considered as a heat-sealing material.

Polyester is not normally thought of as a barrier polymer because its oxygen barrier is much less than that of high barrier polymers such as PVDC, EVOH, and nitrile. However, when compared to polyolefin, its oxygen barrier is quite high. Because polyester serves as both the structural and barrier materials, its use in greater thicknesses makes the barrier adequate for many products.

Polyester Processing

Homopolymer PET, as produced, has little melt strength and is difficult to process on conventional three-roll stacked sheet lines and cast film lines. This difficulty has been reduced by special die designs and use of a top roll that is smaller than the middle roll, and by a secondary solid phase polymerization step in the resin manufacturing process that raises the intrinsic viscosity (IV) and thus the melt strength of the polymer. However, the high IV resins are more viscous and require more extruder power or higher barrel temperatures. The result may be lower throughput rates and lower line speeds as well as increased power usage.

Many potential APET sheet applications do not require the physical properties of the high IV resins. The following describes the physical properties and processing characteristics of a new class of polyesters developed by Du Pont for improved sheet and cast film production.

HMV Polyesters

Selar PTHMV (high melt viscosity at low shear rate) resins are polyesters that have been chemically modified to have the high melt strength and "polyethylenelike" rheology needed for improved extrusion processing. The three commercial resins are Selar [R] PT 7001, homopolymer polyester; Selar [R] PT 8202, medium copolymer polyester; and Selar [R] PT 8307, high copolymer polyester. Their thermal properties are shown in the Table.

Other copolymers are under development to provide a range of melting temperatures and crystallization behaviors that can be used to tailor structures to meet specific needs in the APET market. All HMV polyesters comply with U.S. Food and Drug Administration (FDA) Regulation 21 CFR 177.1630 for food contact.

Rheology

The HMV resins exhibit a more non-Newtonian melt viscosity response to shear rate than standard homopolymer polyesters. In Fig. 1, the melt viscosity vs. shear rate curves for standard homopolymer polyesters with IVs of 0.65, 0.72, and 0.84 are relatively flat while the HMV melt viscosity curve shows much more shear sensitivity. The unique rheology of the HMV polyester is quite clear when its melt viscosity curve is compared with that of the 6.5-MI low-density polyethylene (LDPE) in Fig. 1. The HMV melt viscosity shows a typical temperature dependence (Fig. 2).

The higher viscosities of the high-melt-strength, high-IV standard polyesters used in sheet extrusion require an increase in extruder drive power, which causes an increase in extrusion pressure and shear heating. Because there is little viscosity reduction at high shear in the extruder (Fig. 1), at a given screw speed, reductions in drive power, and melt pressure, can be accomplished only by increasing stock temperature. This, however, decreases melt strength and extruder output. Figure 1 shows that the HMV polyesters process through the extruder like a 0.72-IV resin, yet they have the high melt strength desirable in sheet casting.

The "polyethylene-like" processability of the HMV polyester can also be seen in the relatively high melt swell as the polymer leaves the die. Melt swell is related to the viscoelastic properties of the polymer and contributes to the improved processability in film and sheet. Melt swell was measured by extruding a strand at a shear rate of 117 [sec.sup.-1] from a Kayeness rheometer into a buoyant solution and then comparing the diameter of the strand with the diameter of the capillary. The results given below clearly show the melt swell of HMV polyester to be more like LDPE than conventional polyesters.

Resin %Swell 0.65-IV polyester 6 0.85-IV polyester 15 HMV polyester 42 6.5-MI LDPE 35

Crystallinity Behavior

The crystallization behavior of the resin is important in APET sheet production and thermoforming. The crystallization rate will determine the operating temperature of the three-roll stack and may affect the amount of amorphous regrind that can be processed through the drying hopper. Crystallinity in APET sheet causes hae and makes thermoforming difficult or impossible. However, in drying regrind, rapid crystallization is desirable to prevent agglomeration or "caking" in the drying hopper.

Homopolymers have a higher crystallization rate than copolymers. The type and amount of copolymer content and other components determine the rate of crystallization. HMV polyesters offer the range of crystallinity behavior needed for a variety of APET applications. Crystallization times at 135 [degrees] C are shown below.

Resin Time, sec 0.8-IV homopolymer 120 0.8-IV copolyester 250 PT 7001 homopolymer 95 PT 8202 copolymer 150 PT 8307 copolymer 500

Extrusion of HMV Resins

The extruder conditions for the HMV polyesters are much the same as with other APET resins used for sheet but less demanding than those required for PETG, another amorphous polymer. They are easily processed on a w ide variety of extruders. An extruder with an L/D of 30:1 is preferred, to provide more melting time and a more uniform melt temperature. However, extruders with L/Ds ranging from 24:1 to 32:1 have been used successfully.

For optimum performance, the screw should be designed for the rheology and melting characteristics of the HMV polymer and the desired throughput from the particular extruder. Because of the polyethylene-like processing characteristics of the HMV polymers, good results have been obtained using a general-purpose screw with a 3:1 compression ratio. HMV polyesters also run well on double-flighted screws designed for PETG and on polystyrene screws.

The extruder barrel temperature and downstream equipment temperatures should be optimized for each extrusion system. Screw cooling is not required but may be used if available. Reversed temperature profiles are usually used for polyesters. The feed zone temperature depends on the melting point of the resin and the incoming material temperature. The temperature profile for a typical 4.5-in, 30:1-L/D extruder with a barrier-flighted polyester screw is: feed zone 290 [degrees] C, 285 [degrees] C, 280 [degrees] C,275 [degrees] C, 270 [degrees] C, with all downstream equipment at 270 [degrees] C. A melt temperature range of 270 [degrees] C to 280 [degrees] C is normal.

A gear pump and melt mixer feeding a coathanger-type flex lip die is suggested for best sheet gage uniformity.

Drying and Recycling

As with all polyesters, HMV resins must be properly dried before extrusion to prevent the hydrolytic degradation that results in loss of melt strength and physical properties of the sheet. HMV polyesters are supplied as fully crystallized pellets, which permits drying at high temperatures without bridging or agglomerating in the drying hopper. An in-line, dehumidifying dryer system with an insulated hopper is recommended. Typical drying conditions are 165 [degrees] C air at a -30 [degrees] C dew point with an air volume of 1 cfm/lb per hour of resin usage. HMV polyesters, like most others, should be dried to <0.01% moisture content and preferably to 0.005%.

If properly dried, HMV polyesters are easily recycled in sheet operations with littles loss of IV or melt strength. In a typical operation, 20% to 40% amorphous regrind can be blended with virgin resin pellets and successfully crystallized and dried at 150 [degrees] C. Higher levels of amorphous regrind should be evaluated carefully to prevent "caking" or a "melt down" in the drying hopper.

Sheet Production

Standard polyesters with an IV of 0.80 or greater have sufficient melt strength for sheet extrusion. Standard homopolymer PETs with IVs of 0.75 or less tend to pour out of the die like water and sag excessively. Figure 1 shows that HMV polymers have a melt strength equivalent to that of standard homopolymer PETs in the 0.80- to 0.90-IV range. The slope of the HMV melt viscosity curve and its relatively high melt swell indicate a polyethylene-like rheology.

These properties of the HMV polymers allow a higher drawdown ratio and production of thinner sheet than is possible with 0.80-IV polyesters. In one case, the lower limit for nip polished sheet had been 10 mils. With PT 7001 HMV polyester, 8-mil sheet was easily produced by reducing extruder speed. Even thinner 6-mil sheet was produced by increasing the line speed to the maximum.

Coextruded Sheet

HMV APET resins have created new possibilities for the sheet producer. The range of melt temperatures available in the HMV polyester family allows coextrusion with heat-sensitive EVOH polymers and Bynel [R] coextrudable adhesive resins to produce a five-layer, high-barrier, transparent, thermoformable APET-based sheet. Five-layer sheet has been made with a 0.75-mil EVOH layer and a total thickness of 11 mils.

In heat-sealing applications, a two-layer coextruded sheet can be made using the 7001 homopolymer for the structural layer and the lower melting 8307 copolymer for the sealing layer.

Thermoforming

The HMV APET sheet is easily thermoformed using vacuum, pressure, and plug-assist methods. Sheet made from 8202 copolymer forms well at the same conditions used for PETG, while sheet made from 7001 homopolymer may require slightly higher over temperatures. The high melt strength of the HMV resins reduces the amount of sheet sag during the heating phase. It also helps in producting uniform part thickness in high-aspect-ratio forming. Cylindrical parts with a depth-to-diameter ratio of 3:1 have been easily formed.

Sheet temperatures for best forming lie in the 90 [degrees] C to 120 [degrees] C range (195 [degrees] F to 250 [degrees] F). As with other APET sheet, moderate heat settings over a longer length give better results than high heats over short lengths.

Mold temperatures of about 35 [degrees] C to 50 [degrees] C (95 [degrees] F to 120 [degrees] F help prevent "markoff" or cold draw marks in the side walls of formed parts, especially in deep, vertical-walled parts. The forming cycle time for HMV APET sheet is about the same as for other APETs and oriented polystyrene, but much shorter than for polyvinyl chloride sheet.

Cutting e quipment for APET should be well maintained and sharp. Steel rule dies should be made from a good grade of steel. Aluminum striker plates are not recommended. Punch and die sets should be tight fitting and heavy enough to prevent flexing to avoid "angel hair" and ragged cutting. A progressive or scissortype cut is preferred.
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
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Author:Strum, William L.
Publication:Plastics Engineering
Date:Apr 1, 1991
Words:2061
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