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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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