How to extrude plastomer blown film.
A world of new opportunity is opening up for blown film processors. Historically, film processors have been forced to choose between processing and performance when evaluating resins for demanding blown film applications. The arrival of polyolefin plastomers (POPs) has changed that. These new types of ethylene copolymers, produced by "single-site" metallocene catalysts, were first introduced commercially two and a half years ago. POPs for film applications can offer the strength, toughness, and processability of a linear polymer with added flexibility, impact, optical clarity, and heat-sealability similar to polar ethylene copolymers.
Because of their linear structure, POPs generally process similarly to LLDPEs and can run on existing LLDPE equipment. However, there are subtle differences that must be understood in order to process these resins efficiently. Most important, the lower melting points of POPs require some adjustment of barrel-temperature profile and/or screw design, especially as polymer density is reduced.
Because there are different families of POPs on the market, the following processing guidelines are tailored specifically to Affinity-brand POPs from Dow Plastics.
UNDERSTANDING PLASTOMER RHEOLOGY
Although they process similarly, there are a number of key differences between Affinity POPs and LLDPEs that can influence processability. These differences involve molecular-weight distribution (MWD), long-chain branching incorporation, and short-chain branching distribution.
POPs exhibit narrow MWD, which delivers excellent tensile strength, puncture resistance, dart impact, and optics, as well as good low-temperature performance. Narrow MWD, however, also increases the torque required to process polymer through the extruder and die.
HOW DO RHEOLOGY INDEX (DRI) AFFECTS BLOWN FILM EXTRUSION
(2-mil film made from 1.0 MI POP on a 2.5-in., 24:1 Egan at 160 lb/hr with a 70-mil die gap and 2.5 BUR)
Screw Melt Extruder DRI Pressure, psi Temp., F Amps
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
In the case of Affinity POPs, the latter effect is offset by long-chain branching, which makes the resin more shear-sensitive at higher shear rates, as shown in Fig. 2. This reduces the torque necessary to push the resin through the extruder. The presence of long-chain branching also can be expected to improve bubble stability compared with other POPs of similar MWD.
A narrow distribution of short-chain branching, provides lower melting points for POPs than LLDPEs. The particular POP resin composition, such as the percent comonomer content or density, will largely dictate the melt-temperature profile and cooling requirements for optimum output rates. For example, POPs with higher comonomer contents have significantly lower melting and freezing points than do LLDPE or ULDPE resins and consequently require higher cooling capacities.
The unique polymer design of Affinity POPs, which incorporates narrow MWD together with long-chain branching, provides improved processing through the extruder. The benefits of this molecular design are that Affinity POPs process with less melt fracture and at lower amps, temperatures, and pressures than competitive metallocene-catalyst resins - and do so without the use of LDPE blends or processing aids. These advantages are particularly beneficial if extruder horsepower is limited.
The Dow Rheology Index (DRI) is a measure of the rheological effects of long-chain branching on Affinity POPs. As the DRI increases, processability improves. As shown in the table on the facing page, when the amps decrease, so do the pressure and melt temperature.
DRI is a useful tool for comparing Affinity POPs with the same melt index. It helps you select a resin with processability best suited to your extrusion process. However, DRI should not be used to compare polymers of different melt indices or MWDs, nor to compare POPs with blends of POPs and other resins or processing aids.
The following are basic recommendations for screw design, barrel settings, die gaps and pressures, bubble cooling, and blow-up ratios.
* Screws: The key consideration is to match the screw design to the melting characteristics of the POP in order to obtain the appropriate output at the desired temperature with good melt quality. For lower-density POPs (less than 0.900 g/cc), processors should consider a metering screw with a mixing section and with a slightly longer transition or compression length (five to seven "turns" or diameters), as shown in Fig. 4. Barrier screws, which are generally employed with LLDPEs, have also been used successfully with POPs. The limitation on using barrier screws is that they tend to be low-shear, high-efficiency screws. Moderate- to high-shear barrier-screw designs work better with Affinity POPs.
* Barrel temperatures: Barrel-temperature profiles must also be optimized to provide uniform melt quality. Because POPs have lower melting points than LLDPEs, barrel profiles should be different. Generally, the feed-zone temperature must be reduced when processing POPs. We suggest feed-zone temperatures of 285-320 F (140-160 C).
In order to effectively manage bubble cooling, we suggest use of barrel cooling to reduce melt temperature. Affinity POP blown film should be extruded at 375-445 F (190-230 C) for optimal properties and processability.
* Die gaps and pressures: Moderate-pressure dies (2900-4440 psi) can generally be used for POPs. Die gaps used for processing LLDPEs have been found adequate for POPs. Affinity resins can be mn on a wide range of die gaps, from 0.03 to 0.11 in. (0.75-2.8 mm), depending on the specific die output rate and film properties desired, the polymer selected, and/or the amount of melt fracture encountered.
* Bubble cooling: As noted in Fig. 6, the crystallization temperatures of POPs decrease with density. More cooling capacity is therefore required to maintain control of frost-line height, as compared with LLDPEs. Refrigerated air and internal bubble cooling (IBC) are beneficial and will improve operating rates.
* Blow-up ratio: Apart from film thickness, BUR is the most significant determinant of film properties. Affinity POPs have been run at a wide range of BURs, from 1.8 to 3.5. Higher BUR aids bubble cooling and provides more balanced film orientation.
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|Date:||Feb 1, 1995|
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