Technical Bulletins

Selecting Amide Slip Concentrates for Polyethylene Film Applications

 

The primary function of fatty acid amide additives in the production of polyethylene blown and cast film is to convey slip and antiblock properties to the film surface. In order to make the best use of slip additives in formulating for polyethylene film applications, it is helpful to consider some basic concepts of polymer morphology.

Although LDPE and HDPE are common ethylene-based homopolymers used in film production, their respective manufacturing routes result in very different molecular architectures that help to determine each resins unique surface properties. The high degree of long and short chain branching inherent in LDPE resins give rise to a molecular structure that cannot readily order itself on cooling. As a result, LDPE exhibits a low degree of crystallinity with crystalline regions embedded in a highly amorphous matrix. One consequence is that LDPE resins often display 'sticky' surfaces that tend to 'block' or adhere together under the influence of temperature and pressure. In addition, LDPE surfaces do not slide over one another easily and may adhere to other surfaces during processing or storage. HDPE homopolymer resins are essentially linear in nature with little or no side chain branching. Because they can achieve relatively high levels of crystallinity on cooling, HDPE resins generally exhibit hard, slippery surfaces that resist blocking. UHMW-HDPE resins combine extremely high molecular weights with a high degree of crystallinity producing surfaces that are often referred to as “self lubricating” because of their low inherent coefficient of friction.

 

In addition to basic homopolymer polyethylenes, the processing industry also makes extensive use of ethylene co- and terpolymer resins. These include the familiar butene, hexene and octene LLDPEs, polar copolymers such as EVA, EBA, EMA, and EMMA, as well as entire new families of VLDPEs, ULDPEs, reactor TPOs and metallocene resins currently being commercialized. Comonomer incorporation levels can range from 0.2% to more than 35% by weight.

One of the underlying rationales for incorporating comonomers into the backbone of polyethylene is to modify the long and short chain branching distribution and alter the polymer morphology. Copolymerization can result in “softer” resins with better optical properties, improved tensile and impact strengths, superior low temperature characteristics and enhanced heat sealability. Predictably, it also increases inherent surface friction and tack, making these materials more difficult to process and convert.

Addition of primary fatty acid amides such as erucamide and oleamide to the polymer resin can alleviate many of these processing problems. Amide slips provide an internal reservoir of lubrication that migrates to the surface of the polymer film shortly after exiting from the die. Migration occurs because the slip agent has a limited chemical compatibility (i.e. solubility) in the polyethylene resin matrix. During processing, the amide is solubilized in the amorphous melt but as the resin cools and begins to crystallize, the slip is literally squeezed out of the solidifying polymer matrix. Molecular orientation processes occurring at the blown film frost line or in subsequent tentering operations can accelerate the rate of migration. Once on the surface, the amide forms a soft lubricating layer, filling in valleys and imperfections. This layer effectively separates adjacent film surfaces, preventing sticking and blocking. More importantly, it also reduces the force required to move coated surfaces tangentially across one another, thereby lowering the coefficient of friction (COF).

COFs for polyethylene film are usually expressed for steady-state sliding conditions and are properly referred to as kinetic coefficients of friction. Standard test procedures for the measurement of kinetic and static COF values are found in ASTM D1894-90.

Experience has shown that high COF LDPE and LLDPE films extruded without the use of slip additives can exhibit problems such as sticking and pulling in the nip rolls and collapsing frame, resulting in wrinkled and unusable product. These problems often result in lower production rates and high scrap generation. Incorporation of a suitable amide slip provides lubrication for both cast and blown films, increasing thoughput and significantly reducing handling problems during converting.

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