Slip and antiblock additives: surface medication for film and sheet

Modifying the surface of polymer film and sheet through the use of slip and antiblock additives can either improve performance in fabrication or downstream packaging equipment, or the end-use performance of the polymer.

Slip and antiblock additives modify the surface of polymer film and sheet to improve performance in fabrication equipment, downstream packaging equipment, or end-use. Slips and antiblocks are most commonly used in polyolefins, especially polyethylene. The additives are also used in polypropylene, PVC, polystyrene, nylon, and PET. Applications include bags, wrap and packaging, with both single-layer and multi-layer constructions. For many uses, additives must have appropriate direct or indirect food contact approvals. Slip and antiblock may be added directly by the resin producer, but are also commonly added by the processor via masterbatches in order to tailor additive levels for specific needs and allow accurate dosing at low levels.

Slip

Slip additives reduce friction between films and between film and converting equipment, which improves movement through extrusion lines and downstream packaging operations. The effect of slip additives is measured by sliding one film across another and calculating from the resistive force a static or kinetic Coefficient of Friction (CoF). Slip agents cause film to slide more easily and have a lower CoF.

Amide slips
Fatty acid amides are the most commonly used slip additives. Because amides are incompatible with the polymer, they migrate or bloom to the surface and form a crystalline structure that decreases friction. Amide slips include primary amides, secondary amides, and bisamides. Erucamide is the most widely used amide, followed by oleamide. When choosing an amide slip, formulators consider migration rate, efficiency in reducing CoF and thermal stability.
Migration rates vary with amide type, polymer type, slip concentration and use temperature. Primary, unsaturated erucamide and oleamide migrate quickly and are the most efficient slip agents for polyolefins, typically able to reduce CoF to 0.2 or less. Larger-molecule amides with very slow migration rates may be used in multi-layer films to limit slip effect to one layer. For example, secondary amides like oleyl palmitamide and stearyl erucamide will migrate through a PP-copolymer skin layer, but not through a PP-homopolymer core layer, notes Adam Maltby, plastics applications manager at Croda.
Besides CoF reduction, thermal oxidative stability is becoming more important as processors increasingly use polymers with higher processing temperatures, comments Keith Hughes, global marketing manager for thermoplastics at Chemtura. Oxidative degradation of slip can result in slip property loss, increased colour, and odour. Amides with higher thermal stability are also less volatile and will remain in the polymer even at higher processing temperatures. Saturated versions of erucamide and oleamide offer better thermal stability than their unsaturated counterparts, and, in general, erucamides are more thermally stable than oleamides. Secondary amides and bisamides are even more thermally stable. They are used in the higher temperature cast film process and in engineering polymers such as styrenics. Croda’s new Incroslip G has higher thermal stability than previous grades, resulting in greater colour stability and durability on the film surface, useful for maintaining CoF over a long storage life, notes Mr. Maltby. Croda’s Incroslip amide grades undergo additional processing steps that improve stability and colour and remove taste and odour. Although these grades were originally developed for water bottle closures, they are also finding use in organoleptically sensitive film packaging such as cheese wrap.
When evaluating any additive, including slips, the formulation should be tested as a whole in order to understand potential interactions and optimize additive levels, advise suppliers. Polar additives may interact with amides, slowing the expected bloom rate. Other additives may speed bloom rate, resulting in too much slip at the surface. Some antiblocks may absorb amide slips. Migratory additives like stearates, antistats, or antifogging agents can compete with slips for space on the film surface. “Formulating is often a trial-and-error procedure. For example, a processor may need to adjust slip level first to obtain the desired CoF, then add in antistat and see what effect it has,” explains Mr. Maltby.
Besides interactions with other additives, amide slips can be affected by downstream processes such as adhesive laminating, which changes the film’s polarity and draws slip away from the surface. Ampacet’s new slip masterbatch for laminated films maintains a consistent CoF before and after lamination, and is more cost-effective than non-migratory slips, says the company.
Excessive slip, whether caused by unexpected interactions or by overdosing, can cause significant problems. These include powdery residue on rollers, printing problems, or ‘wet blocking’, in which excess slip makes the film surface very smooth, allowing layers to fuse together. “A very common failure mode we see is processors adding too much slip,” comments Brian McKinley, strategic business manager of films at Ampacet. Because slip migrates to the surface, thicker films need lower dosage levels. Slip-only masterbatches make it easy for processors to adjust slip level with film thickness changes, as well as with changes in resin, pigment and other additives. “Single-component masterbatches have greater flexibility and are preferred by processors that have many formulations. Combined masterbatches, with slip and antiblock or multiple components, are used by processors with a limited number of formulations or with feeder limitations,” explains Alam Shah, director of technology at Spartech.

Non-migratory slips
Although amides are entrenched in most applications because of their high efficiency and good cost structure, non-migratory, high-molecular weight organic materials like siloxanes are being used in speciality applications. Non-migratory slips offer immediate CoF reduction that is not affected by high storage or use temperatures such as in shrink tunnels or hot-filling. Non-migratory slips can be used in the outer layers of multi-layer film, which helps to mediate cost, notes Ampacet. Multibase’s ultra-high molecular weight (UHMW) poly-dimethyl siloxane finds use in areas such as stretch films, thin films with very high packaging speeds, and in film extrusion of very tacky resins that benefit from immediate CoF reduction, says Jean-Pierre Patte, performance enhancement market leader with Multibase, a Dow Corning company. UHMW siloxanes do not affect sealability and are acceptable for most printing requirements. Although UHMW siloxanes do not migrate, they may transfer from one layer to another contacting layer in a roll of multi-layer film.

Antiblock

Antiblocks, as their name suggests, prevent blocking - adhesion of one layer of film to another, caused by contact under pressure and heat. Blocking makes it difficult to unwind a film roll or open a bag. Inorganic mineral antiblocks, dispersed throughout the film, roughen the film surface on a microscopic level so that adjacent film layers do not stick to each other. Inorganic antiblocks, such as talc and silica, are widely used. Formulators may also use organic antiblocks, including amides, stearates, silicones, and polytetrafluoroethylene.

Inorganic antiblocks
Talc, the most widely used inorganic antiblock with about 40% of the global market volume, is particularly popular in North America and Asia-Pacific, notes the Townsend consultancy. Diatomaceous earth (DE), which is a natural silica mineral, has about a quarter of the global market and is widely used in Europe. Synthetic silica has another quarter of the market and is widely used in China and the Asia-Pacific region. Other minerals such as kaolin and calcium carbonate are also used.
In clear films, an antiblock’s effect on film clarity can be just as important as preventing blocking. Demand for antiblocks for high clarity films has been increasing, driven by growth in flexible packaging and shrink film, as well as the use of high clarity resin technologies such as metallocene-catalyzed LLDPE, says Mr. McKinley. Clarity is related to refractive index, particle size distribution and loading level. Higher clarity is achieved when the refractive index of the antiblock most closely matches the refractive index of the polymer. The challenge for inorganic antiblock is that larger antiblock particles have higher blocking resistance but smaller particles yield better clarity. “Managing the particle size distribution and top size helps balance the optical and antiblocking properties of the film,” says Vipul Joshi, marketing manager at Specialty Minerals. Traditionally, synthetic and natural silicas or organic antiblocks have been used for clarity applications, but formulators have other options. Specialty Minerals offers Optibloc®, a proprietary mineral designed for excellent clarity and antiblocking, in three different particle size grades. Specialty Minerals is also introducing a next-generation product, Optibloc® 300. “Optibloc 300 will offer the antiblocking capabilities of silicas with higher optical clarity.” says Mr. Joshi. Unimin Corporation’s high clarity Minbloc® antiblocking additives are produced from derivatives of high purity natural mineral feedstocks that offer near transparency in optical grade polyolefin resins and are distinguished by lack of absorption or interference with other process additives, says the company. Kaolins are used in PET and polyolefins to provide a balance of antiblocking effect and clarity, says supplier Imerys. Ampacet introduced a clarity antiblock masterbatch containing a spherical, inorganic antiblock that reduces haze compared to DE. Another masterbatch combines this antiblock with a PE nucleator to decrease haze significantly and improve gloss, says Mr. McKinley. He adds that Ampacet also offers a masterbatch that combines DE with a large-molecule organic antiblock to provide both slip and antiblocking properties with a step up in clarity compared to DE alone.
In addition to clarity, interactions with other additives are a concern. Antiblocks can absorb additives such as antioxidants, slip or processing aids. Natural silicas have very low additive interaction, while synthetic silicas and uncoated talc have higher additive absorption. Surface-coated talcs, such as Specialty Mineral’s Polybloc®, have both reduced absorption of other additives and improved dispersion. Grades of Imerys’ calcined and uncalcined kaolins are designed for particularly low interaction with slip and processing aids by adjusting particle size, surface area and other attributes, says Dr. Cesar Agra-Gutierrez, technical development manager for film and packaging at Imerys.
Compounders and processors may also consider mineral hardness. “Abrasion caused by antiblocks can be an issue in masterbatch and film production,” notes Mr. Joshi of Specialty Minerals. As the softest natural mineral, talc is least abrasive to equipment.
Natural silica products typically contain some crystalline silica, which in fine-powder form is considered a respiratory hazard that requires controlled handling during compounding. World Minerals, part of the Imerys Group, recently introduced Superfloss CSC, a DE antiblock that contains less than 1% crystalline silica, for processors desiring low-crystalline-silica antiblocks.

Organic amide antiblocks
For clarity applications, organic amide antiblocks are occasionally used alone but are more often used to reduce the inorganic antiblock level. Amides must have time to migrate to the surface before antiblocking and slip properties take effect. In soft polymers like EVA where inorganic antiblock particles are not as effective, amides do work well, notes Mr. Maltby. Of the amides, stearamide and behanamide provide the most efficient antiblocking properties. Behanamide is used in BOPP film, for example, to provide slip and antiblocking properties while maintaining clarity.

Slip and antiblock for PET and PLA sheet

Slip and antiblock use in sheet is growing with the use of new polymer grades such as speciality PET and PETG, speciality PP, and PLA, notes Kirk Jacobs, business director at Clariant Masterbatches. He notes that masterbatch producers play a significant role in the PET sheet market, since PET resin suppliers do not usually formulate specifically for sheet. Slip and antiblock prevent sheet from sticking to rollers during initial fabrication, improve stacking of sheets, aid in mould release after thermoforming, and allow ‘de-nesting’ of stacked, thermoformed packages. Maintaining high clarity, along with slip and antiblock effects and printability, is key.
Suppliers have developed new and improved products for this growing market. Croda’s Incromax PET 100, a slip additive designed for PET and PLA, recently obtained US FDA food contact approval and is in the process of extending its food approval status in Europe. While amides tend to produce colour in PET due to catalyst interactions and high processing temperatures, Incromax does not produce colour, says the company. Clariant introduced CESA®-block 1501 for PET and 1601 for PETG, and also developed customer-specific masterbatches. Sukano offers a highly concentrated, combined slip and antiblock masterbatch for PET, produced using proprietary compounding technology that maintains high clarity, says Thomas Weigl, sales and marketing director at Sukano. Sukano notes that slip and antiblock additives have several advantages over silicone coatings used to reduce CoF of PET packaging. Silicone coatings can interfere with printing and other downstream processes, contaminate regrind, and are restricted from food contact applications in some European countries because of environmental, health and safety concerns, explains Mr. Weigl.
Biopolymers like PLA are beginning to grow in packaging. Key factors for additives used in PLA include compatibility with the resin to maintain clarity and obtaining the appropriate, controlled rate of migration, says Mr. Jacobs. New CESA-slip, CESA-block, and CESA-nucleant masterbatches for biopolymers like PLA have been developed by Clariant for the PLA packaging and film industry, adds Mr. Jacobs. Sukano’s slip and antiblock masterbatches for PLA film and sheet include optical brighteners. These masterbatches have food contact approval and are certified as suitable for composting, notes Mr. Weigl. PolyOne’s new OnCap Bio additive masterbatches and Spartech’s masterbatches include slip and antiblock masterbatches for biodegradable polymers such as PLA.

Contacts:
Akzo Nobel; www.akzonobel-polymerchemicals.com
Ampacet; www.ampacet.com
Chemtura Corporation; www.chemtura.com
Clariant Masterbatches; www.clariant.com
Croda; www.croda.com
Dow Corning; www.multibase.com
Imerys; www.imerys.com
PolyOne; www.polyone.com
Rio Tinto Minerals; www.riotinto.com
Spartech; www.spartech.com
Specialty Minerals; www.specialtyminerals.com
Sukano; www.sukano.com
Townsend; www.townsendpolymer.com
Unimin; www.antiblocks.com
World Minerals; www.worldminerals.com