Since the advent of metallocene-catalyzed plastomer and LLDPE film resins, improving processability while retaining their inherently high physical and optical properties has been an industry target.
Since the advent of metallocene-catalyzed plastomer and LLDPE film resins, improving processability while retaining their inherently high physical and optical properties has been an industry target. Additives have been found to play a role here—though a somewhat limited one. No new additive chemistries have yet been developed to meet the particular needs of metallocene film resins. The same applies to other PE film grades with similar attributes that have emerged from new non-metallocene single-site catalysts, enhanced Ziegler-Natta processes, and other proprietary catalyst technologies from suppliers such as Equistar Chemicals, Nova Chemicals, and Eastman Chemical.
However, resin makers have been working side by side with additive and masterbatch suppliers to come up with different formulations of traditional PE film additives, including slip and antiblock agents, antifogs, antistats, and processing aids.
Carriers and loadings
Says Shing Li, new-business director at Ciba Specialty Chemicals, “These additives work by migrating to the surface of the film to change the surface energy. Metallocene film resins may be morphologically different because they are more uniform in molecular architecture. This tends to retard migration of these additives to the surface. Formulating these additives so that they can migrate better to the surface is the key.”
Some masterbatch suppliers have focused on developing additive concentrates in compatible carriers—e.g., mLLDPE in place of conventional LDPE or LLDPE. The aim is to minimize or eliminate any loss of strength, including dart and Izod impact and tear resistance, and to retain the excellent optical properties also inherent in such film resins. Optimized concentrates also often have higher-than-usual active-ingredient loadings to overcome the migration problem.
Says Paul Albee, technical director at masterbatch producer Polyvel, Inc., “One of the things that come with the low softening point or heat-seal temperature of mLLDPE and some other low-density, high-performance film resins is that they tend to have a more tacky surface. Their coefficient of friction (COF) is high and as a result they adhere to other films, as well as to themselves and to metal surfaces of production equipment. The aim is to use products that decrease the film’s COF, reducing this sticking.”
Not surprisingly, fluoroelastomer processing aids have also been getting a lot of play in processing mLLDPE films. Because of their narrow molecular-weight distribution and higher melt viscosity, mLLDPE film resins are more prone to melt fracture. Some new fluoroelastomer additive formulations allow offered efficiency while minimizing cost.
Why carrier resin counts
Where mLLDPE is used as a minority blend component (e.g., 30-40%) to enhance physical properties of LDPE in monolayer structures, the consensus is that a conventional LDPE-based concentrate works fine. However, the trend in the U.S. is toward using 100% mLLDPE layers in multilayer film structures. Ampacet Corp., a pioneer in the development of concentrates based on metallocene-resin carriers, strongly recommends their use for multilayer film applications where mLLDPE or a plastomer is used as the skin or heat-seal layer. Director of technology Dr. Victor Mimeault says examples are fresh-produce packaging and meat wraps that require very high gloss, low haze, and very high clarity.
Food-packaging film converter Barrier Films, Inc. of Sparks, Nev., is a specialist in the use of plastomers and mLLDPE film resins. It has been using some of the new additive concentrates with mLLDPE carriers and high additive loadings. Applications engineer Roland Planeta says mechanical properties of mLLDPE can be compromised by even a small amount of LDPE carrier resin. “We’ve seen no problems when using these newer additive concentrates, and the clarity can be much better.”
Comments Rich Hall, technical services manager for food packaging at ExxonMobil Chemical Co., “We think that it’s an advantage in some cases to have an mLLDPE carrier in additive concentrates when processing mLLDPE resins such as our Exceed grades. Generally, if you are only adding 1-2% of the masterbatch, it shouldn’t make much difference what the carrier is. But it will make a difference with addition levels closer to 5% and up.”
(Sources at Dow Plastics declined to comment on this topic, stating that they could not discuss any polyolefins issue during merger negotiations with Union Carbide.)
Oscar Mascarenhas, product manager at A. Schulman, says use levels of an additive concentrate such as an antiblock sometimes must be a lot higher to counteract the low density of the material and tackiness of the film. Mascarenhas recommends a high-clarity antiblock in mLLDPE carrier for use in a 100%-mLLDPE, high-clarity sealant layer. Where clarity is not an issue, and the mLLDPE is used in a blend, a standard antiblock concentrate will do, he says.
At Nova Chemicals, senior additives specialist Tony Tikuisis says his firm’s experiments show that even 1-2% of a masterbatch with an incompatible carrier can adversely affect physical properties. “We have seen up to 20-30% reduction in key physical properties such as dart impact.” Nova is working with masterbatch producers like Ingenia (formerly WedTech, Inc.) to develop additive concentrates with compatible carriers for use with Nova’s new Advanced Sclairtech octene LLDPEs.
Adds Mark Mack, technical director at Equistar Chemicals, “The carrier resin should have rheological properties similar to those of the resin you are processing. A rule of thumb is that when you have a lot of difference in melt viscosity, it’s impossible to mix the two components.” Generally, you want the resin and additive viscosities to differ by no more than a factor of two to three, Mack says, so the components can be well mixed under the shear-rate conditions of a given process.”
Dan Gray, marketing manager for Impact mLLDPE resins at Phillips Chemical Co., points out one consequence of metallocene resins’ narrow MWD: “Because these resins are harder to process and more prone to melt fracture, lots of people have been looking at the fluoroelastomer processing aids.”
Adds Rich Chapman, technical director for Viton FreeFlow fluoropolymers at DuPont Dow Elastomers, “We see mLLDPE being used more and more on its own and not as a blend, so that processing aids are viewed as the way to go. Instead of opting for a low-viscosity LDPE or LLDPE carrier, many film converters are now looking for a processing-aid concentrate in a mLLDPE carrier—despite the higher viscosity—so it won’t jeopardize film strength and clarity.”
What’s available now
A handful of companies currently offer additive concentrates tailored to mLLDPE. Included are slip masterbatches based on erucamide or oleic acid amides, antiblock products based on synthetic or natural silicas (e.g., talc and diatomaceous earth), antifog concentrates based on fatty-acid esters, and antistats based on long-chain amines and short-chain amides.
For example, Ampacet’s line of FDA-sanctioned additive concentrates with metallocene-based carriers currently includes two slip masterbatches. Grade 100329 is a 5%-erucamide, slow-blooming product and 100436 is a 5%-oleamide fast-blooming slip agent.
Ampacet also has five highly loaded antiblock concentrates. Three contain diatomaceous earth (DE): 100342 and 100371 with 20% DE of different particle sizes, and 100940 with 25% DE. Product 100524 has 50% talc, and 100902 contains 70% calcium carbonate. Two slip/antiblock products are also now available: 100407 has 5% erucamide and 35% DE; 100897 has 3.5% erucamide and 35% DE.
Also offered by Ampacet are 100676 processing-aid concentrate, with 3% fluoropolymer; 101035 antioxidant/process-aid combo, with 13% hindered phenol, 6% phosphite, and fluoropolymer; and 101042 uv concentrate with 12% HALS. Three highly loaded white concentrates (50%, 70%, and 80% TiO2) and a 50% carbon-black product round out the line.
Within the last two years, Polyvel, Inc., has introduced highly loaded slip, antistat, and antifog concentrates in mLLDPE carriers. Marketing director Eric Albee says these products are 20-25% active versus 3-5% for conventional additive concentrates. “Film converters can use 1-2% of these high-activity products, versus having to put 5-10% of a conventional concentrate in the mLLDPE. They are about two-and-a-half times higher in cost but have five times the activity level.”
Polyvel’s metallocene-resin concentrates include slip additive RE25M, a 25%-erucamide, FDA grade that boasts excellent dispersion and superior slip characteristics. At low use levels (0.25-0.5% active ingredient), it has been shown to significantly reduce the COF of blown film.
Polyvel also developed the VF Series of antifog blends, which are customized for multilayer food-packaging films for frozen foods and cold-meat wrap. They are 20-40% loaded concentrates and are used at 2-5% levels (0.4-0.6% active ingredient). According to Paul Albee, mono- and diglycerides have been found to be very effective in mLLDPE films. “They migrate better to the surface and have improved thermal stability as compared with sorbitan esters, which tend to be more static in the film.”
Polyvel’s antistat concentrates based on mLLDPE include VSE-10 for food packaging and VS-EO9M for electronic-packaging cast film. Both are highly loaded (20%) and engineered for use at only 2.5-3.0%—about half the use level of conventional products.
A. Schulman is another pioneer in this field, having developed several Polybatch additive concentrates since 1995 for metallocene film resins. Examples of standard products include antiblocks EXT-203T (20% talc), EXT-2017S (20% natural silica), and EXT-20212, (10% glass spheres). Use levels for these products are generally 1-5%.
According to Schulman’s Mascarenhas, the company has found the synthetic antiblocks to be more efficient because they allow for selecting the best particle size for a particular application. Nova’s Tikuisis cautions that while synthetic silicas provide excellent optics, their antiblocking efficiency is generally not as high as DE or talc. “In-house testing has shown that the optical and blocking properties of synthetic silicas tend to be diametrically opposed (i.e., either good blocking at the expense of poor optics or good optics with poor blocking,” he reports.
Schulman also offers combination slip/antiblock products. These include EXT-4226TSE, which combines natural silica and talc with a proprietary slip agent, and EXT-4220TS, which has the same two antiblocks with erucamide slip agent. Both concentrates are said to provide good antiblock performance with low abrasion and reasonable cost.
Polybatch slip products for metallocene resins include EXT-209 (5% oleamide) and EXT-209E (5% erucamide). Schulman also offers process-aid concentrates such as EXT-206F (3% fluoropolymer), as well as custom uv concentrates. The latter typically contain up to 20% HALS or uv absorber. They are used in multi-layer window-masking film.
More recently, Shulman has made custom antifog and antistat masterbatches in mLLDPE carriers. Mascarenhas says the antifog products are aimed at high-clarity applications, such as fresh-produce bags.
As part of its newly expanded line of pellet additive masterbatches, sold globally under the CESA name, Clariant Masterbatches Div. is offering custom formulations for mLLDPE and other new film resins. Business director Manfred Dicks says most of the effort is in antistat formulations.
Processing aids advance
Certain fluoroelastomer processing aids have been found to work especially well with mLLDPE resins. In blown film, where most of the interest is, their purpose is to reduce melt fracture and gel formation. In cast film, the aim is primarily to eliminate die build-up as well as to reduce gels.
Sources at both Dyneon and DuPont Dow Elastomers say that sales of these products are on the upswing as a result of growing use of plastomers, mLLDPE, and other new high-performance LLDPE film resins. Randy Stone, marketing manager for Viton FreeFlow fluoropolymers at DuPont Dow Elastomers, says the company has geared new development efforts to these film resins. “Typically, they all have narrow MWD with less branching and much better physical and optical properties.”
Adds technical director Rich Chapman, “Although there are certainly differences among them, the biggest difference between them and conventional LLDPE is that they are generally more difficult to process. The processing aid allows converters to adjust their equipment so they can run these materials at higher shear rates, lower temperatures, and low pressures.”
Two products from the company—Viton FreeFlow SC and RC—are geared to mLLDPE and similar low-density film resins. Chapman says these patented formulas are designed to work more efficiently with minimal cost by allowing a processor to use more process aid but less of the more costly fluoroelastomer component. An acid copolymer that cleans the die surface is used in the SC product, while the RC version contains a combination of acid copolymer and glycol. “You might use up to 1000 ppm (0.1%) of the processing aid, but only 250-300 ppm is the fluoroelastomer,” Chapman explains.
From Dyneon, Dynamar FX9613 and FX 5920A are the two process aids that have proved most successful with both resin producers and film extruders, according to Dynamar marketing supervisor Susan Woods. FX5920A is a patented combination of polyethylene glycol and fluoroelastomer that offers lower interaction with other additives, such as antiblocks.
More recently, these products have been demonstrated to work well for gel suppression at only 200-500 ppm (0.02-0.05%) levels. In one experiment, PPA FX5920A was shown to significantly reduce the number of gels in blown film of a very low-density plastomer. Since this is a more difficult material to process, results are expected to be even better in mLLDPE or other new LLDPEs.
Within the last year, Algroup Lonza launched Glycolube ML-1, a fatty-ester lubricant for mLLDPE blown film. Experiments showed it could reduce the extruder amps/rpm required to process a 1-MI mLLDPE and increase the lb/hr per amp. The inherently high sealability of this mLLDPE was not compromised by the process aid.
A developmental variant of this additive appears to be synergistic with fluoroelastomers and has the potential of reducing use levels of the latter by nearly half to under 400 ppm vs. 600-800 ppm previously, says Lonza technical director Joe Williams. Substantial cost savings could result, as the new product will sell for around $1/lb, whereas fluoroelastomers cost around $20/lb.