Get products out faster with the right additives.
1 Engineered Fillers for Polyolefins
Beyond resin extension or property enhancement, mineral fillers are finding a new role as productivity aids. Fine-particle, surface-treated calcium carbonates in pure or masterbatch form have been developed specifically to increase output of polyolefin blown-film extrusion and injection or blow molding. At modest loadings of 10-20%, the filler does not greatly change product properties but does boost thermal conductivity so that the melt cools and solidifies much more quickly. Major proponents of this technology are film and bag producer Heritage Plastics and filler supplier ECC International.
Film & coating run faster
Heritage pioneered the use of calcium carbonate in blown film about 12 years ago. It buys fine-ground calcium carbonate from suppliers such as ECC, Georgia Marble, J.M. Huber, and Omya. Heritage makes a pellet concentrate for its own use and for sale to other processors. Designated HM10, it contains 75% of a 1-micron calcium carbonate in an LLDPE carrier. It's aimed at LLDPE blown film and, more recently, has been used in HMW-HDPE. Another recent development is an LDPE version of the concentrate, called H-Tec, for extrusion coating.
Frank Ruiz, technical director, says Heritage has about three dozen film customers using its mineral filler. He estimates at least 500 million lb/yr of film is made with mineral reinforcement - primarily for trash bags.
Ruiz says 10-20% filler increases LLDPE blown-film output by 20-40% through faster bubble cooling. In recent work with HMW-HDPE, addition of 15% filler concentrate provided a 15% increase in output rate. Similar results have been achieved in LDPE extrusion coating. Says Ruiz, "With 30% calcium carbonate, you increase the coating output rate by 30%, you can run as thin as 0.25 mil with no web tear, and adhesion of LDPE to most substrates is enhanced."
A few years ago, ECC launched its FilmLink 400 calcium carbonate, designed for use in thin PE films. It is sold to compounders who customize concentrates or compounds for film producers. The filler is used in non-clear applications, such as t-shirt and trash bags and agricultural films.
Technical director David Skellhorn says addition of 10-20% FilmLink can raise output 20-40%. In LDPE and LLDPE blown films, higher dart impact, puncture resistance, tear strength, and antiblocking also result.
Mold caps & bottles faster
Last year at NPE, ECC launched Zytocal, a pellet concentrate of 90% calcium carbonate in a "universal" carrier for polyolefin (mainly HDPE) injection and blow molding. It can be added directly at the molding machine without a special mixing screw. A newer version, Zytocal SI, is an 85% concentrate containing finer particles that reportedly do a better job of preserving impact strength in thin-wall parts.
At Zytocal loadings of 10-20%, better thermal conductivity speeds cooling. "Cycle-time reductions of 15% are quite typical," Skellhorn says, noting that easier flow with Zytocal can be as important as faster cooling. For one closure, 20% Zytocal shortened cycle time by 30%, from 12.75 sec to 8.96 sec. A structural-foam pallet with 16.5% Zytocal saw a 14% saving as the cycle shrank from 179.3 sec to 154.6 sec.
In blow molding, productivity of a bleach bottle rose 14%, from 21 to 24 cycles/min, with 16.5% Zytocal. Cycle time for a thick blow molded panel dropped 27%, from 105 sec to 77 sec.
Zytocal has other benefits for blow molders. Skellhorn notes easier trimming and detabbing with less neck distortion, better surface finish from lower melt fracture, reduced pest-mold distortion, and improved wall-thickness uniformity from reduced die swell. What's more, 10% Zytocal increases HDPE stiffness by about 10%. For one shampoo bottle, this permitted thin-walling by 10%. Finally, label application is easier, Skellhorn claims. "The reduction in temperature of the mold wall reduces out-gassing and shrinkage, which affects label integrity. We've seen rate increases as high as 30% on large wheel blow molders based on this effect."
2 Blowing Agents For Unfoamed Parts
A newer role for chemical blowing agents is as processing aids in unfoamed injection molding. Reduced cycle times and elimination of sink marks have been the major benefits.
Dennis Keane, marketing manager of B.I. Chemicals, estimates that about 10% of injection molders are currently using blowing agents - particularly endothermic types - for these purposes. He believes that 50-60% of injection molders could obtain cycle-time reductions of 5-10% with endothermic blowing agents in solid parts.
Gas acts as a solvent
When exposed to heat, endothermic blowing agent additives decompose and liberate C[O.sub.2] gas. At pressures above 1700 psi, C[O.sub.2] is a "supercritical fluid" that readily dissolves in the plastic melt and acts as a viscosity reducer, permitting longer flow and lower melt temperatures. Michael Reedy, president of Reedy International, says his customers typically can reduce melt temperatures 10-20 [degrees] F by using only 0.5-1.0% of Reedy's Safoam endothermic concentrates.
Lower viscosity allows processors to fill intricate tools faster with less injection pressure. Lower melt temperatures mean there is less heat to remove from the part. The C[O.sub.2] gas also absorbs heat, cooling the part from the inside while the mold cools it from the outside, notes marketing manager Dennis Mead of Phoenix Color & Compounding. After molding, the C[O.sub.2] eventually leaves the plastic through evaporation.
According to Reedy, the best way to use an endothermic blowing agent in unfoamed injection molding is to have a large nozzle opening and small mold vents. This allows you to fill very fast and maintain high pressure on the air in the tool cavity, preventing the C[O.sub.2] from causing splay. It is also important that the tool is designed to fill from thin to thick sections - rather than the more typical reverse route.
If the goal is to reduce sink marks, Reedy advises molders to drop their packing and holding pressures in order to allow the internal C[O.sub.2] gas pressure to overcome the shrinkage force of the plastic. Typical packing pressures are only 5075 psi, and hold time may be eliminated altogether, Reedy says.
An example from Reedy is a molder of HIPS disposable shaver parts in a 25-cavity, center-gated tool. The original process utilized 1450 psi injection pressure and a 9-sec cycle time. The parts on the perimeter had less "crisp" definition than those in the center. Use of 1% of Safoam FP20, a 20%-active blowing-agent concentrate, allowed use of only 950 psi injection pressure and a 7-sec cycle, and all parts were of equal quality.
Robert Heinold, director of polypropylene products at A. Schulman Inc., reports that PP compounds containing endothermic blowing agents can reduce cycle times in unfoamed parts. Last year, Schulman acquired foamable PP compounds from Eastman Chemical Co. Heinhold says PP molders now frequently use such products at 2% or lower levels to reduce the cost of molding thick and ribbed PP parts by eliminating sink marks. With less risk of sinks, holding times and overall cycles can be trimmed. In one case, the molding cycle of a PP wheel was cut more than half, Heinold says.
Engineering resins benefit
GE Plastics polymer scientist Adeyinka Adedeji says the company is working with blowing agents as flow promoters for thin-wall molding of Noryl PPE alloys, ABS, PC, PC/ABS, PC/PBT, and PC/PET. Recent data show that blowing agents improve flow in PPE alloys by reducing the glass-transition temperature, therefore decreasing the viscosity of the material. Says Adedeji, "You can tell there is potential for reduced cycle times, since you can fill your mold faster." He adds that the blowing agent's effect on Tg is only temporary, unlike traditional flow promoters such as mineral oil, which depress the HDT of finished parts. "With blowing agents, once the gas leaves the part as it cools, the HDT of the thermoplastic is unaffected."
Adedeji notes that both endothermic and exothermic blowing agents (like azodicarbonamides) can improve flow in thin-wall parts. The latter generate heat, raising the melt temperature and aiding flow. A recent study shows that as walls become thinner, the flow enhancement of exothermic blowing agents slightly exceeds that of endothermics.
3 Process Aids for Extrusion Blow Molding
Fluoroelastomer-based processing aids have long been used in extruded PE films to enhance output by reducing melt fracture ("sharkskin") and die build-up, as well as extrusion pressure, torque, and temperature. The newest additives of this type reportedly condition dies more quickly, are more effective, and cost less to use. They boost productivity not only in film but in extruded pipe and cable. Moreover, they work well with conventional PE and newer metallocene resins and even with polypropylene.
More economical & versatile
DuPont Dow Elastomers offers modified fluoropolymer additives that reportedly increase processing flexibility and lower cost. Viton FreeFlow RC for blown film and SC for other extrusions are alloys of a fluoroelastomer, a special Teflon fluoroadditive, and an ethylene-copolymer ionomer. The ionomer protects the fluoropolymer from die-surface contamination. Moreover, its aggressive adhesion limits interaction of the fluoropolymer components with other additives - HALS uv stabilizers, amine antistats, talcs, and calcium stearates - that can render conventional fluoropolymer additives ineffective. These additives are said to be less costly to use than previous fluoropolymers.
DuPont tested the new additives in LLDPE agricultural film containing talc and HALS - both of which typically interact with fluoroadditives. The cost of using the new fluoropolymer alloy was less than 25% of that for conventional processing aids.
Technical-service representative Rich Chapman adds that these additives allow processors to adjust temperature, pressure, and speed. "By lowering temperature, you get improved flow, which can translate to improved output. It allows you to go from a 1 MI resin to 0.50-0.75 MI and produce a much stronger film at a lower gauge."
These alloys reportedly work well in conventional and metallocene LLDPE blown and cast film, extrusion coating, and blow molding. HDPE applications include blown film, raffia, pipe, and cable. Recently, they have found use in PP raffia and fiber.
Dyneon also reports significant productivity increases with its Dynamar PPA family of fluoroelastomer processing aids in blown and cast film, pipe, and tubing of LLDPE, mLLDPE, HDPE, and PP. Higher-temperature uses - such as cast film, HDPE, and PP extrusion - are made possible by the newest Dynamar grade, FX5911. It is a fluoro-thermoplastic with a melting point of 159-171 C. FX5911 is naturally free-flowing, so it needs no silica or mineral treatment.
Technical-service specialist John Horns says that in test runs of LLDPE films, Dynamar PPA increased output by 30-35% or more with narrow die gaps. In HDPE film and pipe, the increases approached 50%.
Dyneon is developing new grades that reportedly perform better with HMW-HDPE and are more resistant to interaction with HALS and amine antistats.
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|Title Annotation:||Heritage Plastics and ECC International increase production with engineered mineral fillers|
|Author:||Sherman, Lilli Manolis|
|Date:||Jul 1, 1998|
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