Polyolefin 'plastomers' arrive for film.

The first grades of a new generation of polyolefins made by so-called "single-site" metallocene catalyst technology have just been commercialized in North America and Europe. Dow Plastics, Midland, Mich., has trademarked the new resins "Affinity." The initial entries are film resins--low-density linear ethylene-octene copolymers with narrow molecular-weight distribution, which is said to confer better mechanical properties than those of previous LDPEs or LLDPEs. At the same time, Affinity resins possess long-chain branching, reportedly giving them very good processability comparable to conventional LDPE. (LDPE is the only other PE to possess long-chain branching, but not Affinity's narrow MWD.)

These initial products of Dow's "Insite" catalyst technology are dubbed "plastomers," signifying that they are ore rubbery than traditional plastics, though not true elastomers. (Commercializing Insite-derived polyolefin elastomers for wire and cable, automotive and other uses is a parallel effort at Dow.)

The first five commercial grades include three for blown film and two for cast film (see Table 1). Dow says it's actively selling these grades "into a dozen applications," ranging from diaper liners to heat-seal layers for food packaging.

[TABULAR DATA 1 OMITTED]

In fact, the ability quickly to generate new application-specific resins is one of the attractive features of this highly predictable, highly controllable new catalYst technology. Through the use of computerized "artificial intelligence" and molecular modeling, Dow says it is able today to tailor a new resin to a particular customer's requirements and get the first pilot-plant samples into the customer's hands within just two weeks. Dow aims to get that lead time for samples down to a mere six days.

Dow is currently serving domestic and foreign customers from its first 125-million-lb/yr Insite-technology plant, which came on stream recently in Freeport, Texas. Dow expects to have a second plant operating within a year. Conversion can be done within six months and at modest cost, Dow says, by converting an existing solution-process reactor train to the new technology. If market growth proceeds as expected, Dow anticipates converting a new train as often as every 18 months.

NEW RHEOLOGY YARDSTICK

Conventional viscosity measures are inadequate for understanding how Affinity resins process, Dow says, because of their unique combination of narrow MWD and longchain branching (LCB).

An example is the traditional meltflow ratio [I.sub.10]/[I.sub.2] (or HLMI/MI) used to characterize the processability of LLDPEs and HDPEs. Changes in that flow ratio correlate with changing MWD in LLDPE and are typically accompanied by changes in extruder specific output (Ib/hr/rpm), extractables, and molded part warpage. However, these variables are unchanged by differences in [I.sub.10]/[I.sub.2] ratio among Affinity resins because they all have the same narrow MWD ([M.sub.w]/[M.sub.n] is about 2). In the case of susceptibility to draw resonance in cast-film lines, Affinity resins show the opposite relationship to flow ratio--i.e., whereas a higher flow ratio means an LLDPE line must run at lower maximum speed to avoid draw resonance, Affinity resins can be run even faster.

Dow came up with what it calls the "Dow Rheology Index" or DRI to characterize its new resins. DRI is essentially independent of MI. DRI values are inversely proportional to the low-shear viscosity of an Affinity resin. Increasing DRI value correlates with a greater degree of LCB and also with improved extrudability, as reflected in higher melt strength, drawdown, and bubble stability coupled with lower extruder amps, backpressure, melt temperature, neckin, and draw resonance (see Table 2). The accompanying graph shows how higher DRI and LCB correlate with higher low-shear viscosity and lower high-shear viscosity. According to Dow, raising the MI of an Affinity resin lowers the overall viscosity curve, while raising the DRI increases low-shear viscosity (melt-strength) nd decreases high-shear viscosity (improving umpability).

TABLE 2--EFFECTS OF |DOW RHEOLOGY
INDEX' ON BLOWN FILM EXTRUSION
Dow Rheology   Screw        Melt    Extruder
Index(1)       Pressure(2)  Temp.   Amps(3)
               (psi)        (F)
0.5            5970         507     136
1              5009         476     116
2              4203         447     99
3              3793         430     90
5              3332         411     80
7              3060         398     74
10             2796         385     68
(1) Index to compare Dow metallocene resins; higher values correlate with
greater long-chain branching. (2) On a 2.5-in., 24:1 Egan extruder. (3) For 2-mi
l
film at 160 lb/hr with 70-mil die gap.

DRI can range from zero for a narrow-MWD resin with no LCB (like other metallocene polyolefins on the market) to a maximum of 30. The numbers are mainly important in a relative sense, as a means for Dow or a customer to select the best Affinity resin for a specific application. Since DRI was developed specifically for the unique rheology of Affinity resins, it may not be directly applicable to other polymers.

In general, Dow recommends that extruders, screws, coex feedblocks, and dies for LLDPE cast or blown films should work well with Affinity resins. With such equipment, the new resins may be expected to give lower amps and backpressure. And, because they are more resistant to melt fracture, they can be processed through narrow die gaps, which can enhance film properties and bubble cooling.

FIVE NEW FILM GRADES

The first five grades target sealant and lamination layers in blown and cast film to replace specialty sealant materials like EVA and ionomer. All are FDA approved for direct food contact except PL 1880, which is approved for indirect contact. They cost from 50 to over $1/lb, with initial sealant grades in the 50-80[cts.]/lb range.

* Affinity HF 1030 is aimed at cast embossed film for diapers, adult incontinence products, and medical disposables. Its dart impact reportedly beats that of Dowlex LLDPE, LDPE, and blends of the two. Secant modulus is comparable to an LLDPE/LDPE blend. Drawdown is excellent. (CIRCLE 26)

* PL 1845 is for sealant layers in cast coex film. It's intended for multilayer bags or pouches for liquid, dry, fresh or frozen foods. (CIRCLE 27)

* FM 1570 is a blown film sealant resin for coextruded or laminated films in coffee packaging, bulk liquid packaging, and stand-up food pouches. Its dart impact and puncture resistance in 2-mil monolayer films reportedly exceed those of Dowlex and other LLDPEs. Haze level is lower than those of LLDPEs or Dow's Attane ULDPE. (CIRCLE 28)

* PL 1840 and 1880 are coex blown film sealant grades. PL 1840 is for multilayer bags or pouches for liquid, dry, fresh or frozen food. PL 1880 targets more demanding packaging, like high-speed form-fill-seal pouches, medical pouches, and processed meat and poultry packages. Their hot-tack strengths generally exceed those of EVA, ionomer and ULDPE sealants, and PL 1880, in particular, does so at much lower seal temperatures. Dart impact and puncture resistance of both new resins are greater than those of EVA or ionomers. Drop tests of VFFS-formed pouches showed less leakage than pouches of ULDPE, EVA or ionomer. Haze level of PL 1880 is lower than 9% or 18% EVA and ionomers; PL 1840 is comparable to or better than all except 18% EVA. In organoleptic (taste and odor) tests, PL 1840 came out ahead of EVAs, while PL 1880 surpassed sodium and zinc ionomers. (CIRCLE 29)

* FW1650, a PVC replacement grade, was developed for the European market, where environmental concerns are driving a switch from PVC films. It targets food overwrap, especially for red meat. (CIRCLE 30)

Dow spokesmen say "stay tuned" for more new Insite-based resins aimed at these applications under development: extrusion coating sealants, skin packaging, shrink and label film, gaskets and cap liners, injection molded goods, medical products, foams, and impact modifiers for polyolefins (see also Technology Newsfocus).