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.


Erucamide vs. Oleamide Slips

Oleamide and erucamide fatty acid derivatives are the most common slip agents used in polyethylene film. Oleamide is derived from mono-unsaturated C18 oleic acid while erucamide is the amide of C22 mono-unsaturated erucic acid. Each offers distinct benefits and advantages when used in specific processing situations.

 

The rate of diffusion of slip additive to the surface of the polymer film varies as a function of chemical structure. The lower molecular weight oleamide migrates to the film surface more rapidly and is often referred to as a 'fast blooming' slip (see Figure 1). Although 'slow blooming' erucamide develops its lubricating effect at a slower rate initially, after 4-6 hours it produces lower COF values than equal quantities of oleamide (see Figure 1 and Figure 2).

Antiblocking performance is another key characteristic of amide slips. ' Blocking' is the tendency for polyethylene film sheets to adhere to one another as they are separated vertically; antiblocking is therefore the ability to reduce inter-sheet adhesion characteristics. In most cases, the effectiveness of amorphous and crystalline silicas, diatomaceous earths and talc antiblocks far outweigh the marginal antiblock properties provided by amide slip agents. However, because inorganic antiblocks increase haze levels and can negatively affect other optical and physical film properties, some high clarity film applications rely exclusively on the slip agent to provide the necessary reduction in blocking force. In these cases, erucamide provides superior antiblocking compared to equal levels of oleamide. Stearamide (another fatty acid amide) provides even better antiblocking, but since it does not function effectively in reducing COF, it is usually used in combination with oleamide or erucamide to achieve a desired balance of slip and antiblock characteristics.

Thermal stability is also an important feature to consider when selecting a slip agent. Erucamide has a lower vapour pressure and is less volatile than oleamide, making it more suitable for higher temperature processing. Lower volatility means that more of the slip will stay on the film surface and not end up venting off as smoke or depositing on metal die surfaces. In addition, erucamide is less prone to thermal oxidation during processing and contributes less color (i.e. yellowing) and odor to the final product. The chemical stability of erucamide also translates into lower rates of oxidation, rancidification and color formation during product storage.

Oleamide is often the slip agent of choice for in-line bag converting operations where a low COF is needed in a short period of time. Because of its faster migration, a lower level of oleamide can be used in place of erucamide. Since oleamide is generally less costly, the savings are two fold. The slower blooming rate of erucamide can be an advantage in roll stock applications, where film that is too slippery causes winding difficulties and telescoping of rolls. The slower exudation of erucamide can also result in more effective in- line corona treatment. Because of its low volatility and chemical stability, erucamide is also ideally suited for most high quality food packaging applications.


Formulating Guidelines:

Optimization is the key to formulating with erucamide and oleamide slips. As previously stated, too much slip of any kind in roll stock can result in a telescoping effect which renders the rolls unusable. Excessive slip levels can also have an adverse effect on corona treatment levels and may interfere with adhesion of aqueous-based printing inks or cause problems in subsequent laminating operations.

Most conventional LDPE and LLDPE film resins exhibit COF values in the range of 0.60 to 0.75 while values of 0.80 to 0.95 are often typical of highly amorphous polyolefin elastomers such as EVA, EMA, EnBA, VLDPE and ULDPE. For most packaging film applications, reducing the COF levels to 0.2 or less results in significant improvements in film quality, increases film throughput rates and enhances handling properties such as rapid rewinding. Slip levels of 500 to 3000 ppm are typically required to decrease COF values to this level.

Since changes in primary resin characteristics such as melt index, density, comonomer content and type can greatly affect the performance of oleamide and erucamide slips, optimization studies are highly recommended. When formulating for standard 1-3 mil polyethylene film applications, 750 to 1000 ppm of slip is a good starting point for testing. Typical COF values for LDPE film as a function of amide concentration are shown in Figure 3.

In general, slip addition levels should be increased:

when using an amorphous polymer with a low degree of crystallinity (i.e. low density values)

to offset the higher surface-to-volume ratio when down-gauging films

when processing at higher temperatures in order to compensate for loss by volatilization (particularly      important with oleamide slip)

to offset physical absorption of the slip additive and increases in surface roughness when incorporating      high levels of solids in the film (i.e. fillers, pigments, antiblocks, etc.)

The use of oleamide and erucamide slip masterbatches are highly recommended in order to achieve uniform slip addition at low incorporation levels. Colortech offers two slip concentrates based on oleamide and erucamide chemistries for use in the manufacture of polyethylene blown and cast films. Colortech 10002-12 Oleamide Slip and 10003-12 Erucamide Slip consist of a 5.0% loadings of amide slip agent in a 5 melt index linear low density carrier resin. Figure 4 illustrates the addition rates for Colortech 10002-12 Oleamide Slip and 10003- 12 Erucamide Slip needed to achieve specific slip additive levels in finished film.


Storage and Shelf Life

As fatty acid derivatives, both oleamide and erucamide slip agents are susceptible to oxidation reactions that can lead to color and odor formation. Under proper storage conditions, Colortech 10002-12 Oleamide Slip and 10003-12 Erucamide Slip concentrates will remain stable for up to one year following manufacture. To maintain optimum product characteristics, these concentrates should be stored in closed containers in a cool, dry environment. Good inventory management practices can help to minimize problems with out-of- date materials.

 

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