New Slip Agents Provide Extended Performance

SpecialChem - Apr 1, 2008

Compiled by Gordon Graff

Keyword Abstract: Slip agents, antiblocking, polyolefins, lubricants, siloxanes, PET, polylactic acid, PLA, biodegradable, hightemperature, surface treatments, fatty acid amides, oleamides, erucamides, packaging, ionomers, additives, films, processing aids, multilayer, Ampacet, Chemtura, Clariant, Croda, Multibase, Uniqema, PolyOne, Spartech, Sukano.

Higher temperature processing, a rising demand for multilayer films, and increased use of specialty plastics films are driving the evolution of new types of slip agents – additives which reduce the coefficient of friction of polymer surfaces for easier processing and handling. The most widely used slip agents are fatty acid amides. Incorporated into polyolefins and other polymers before or during processing, these additives gradually migrate to the surface of films, forming a thin lubricating layer that makes it easier for the film surfaces to slide past each other. Now, traditional slip agent formulations are giving way to products with new chemistries and modes of action.

Heat Resistance

Because processing of polymers is increasingly taking place at higher and higher temperatures, the thermal and oxidative stability of slip agents has become an important issue. Degradation of slip agents by heat and oxygen can result in loss of slip properties, as well as unwanted colors and odors. Among amide slip agents, erucamides have better thermal properties than oleamide agents. (Both are typically added to polyolefins at levels between 500 and 1500 ppm.) In particular, the erucamides resist hightemperature oxidation, and create fewer volatiles than oleamides on film surfaces during processing. (Less volatiles means the agents stay on the film surfaces longer.) In general, oleamides and erucamides with fully saturated carbon chains exhibit better thermal stability than those with unsaturated molecules. Meanwhile, secondary amide and bisamide slip agents are more thermally stable than primary amides such as oleamides and erucamides.

Without a slip agent, one polyethylene film sticks to another (left). With a slip agent added (right) the films move freely due to reduced friction.

Without a slip agent, one polyethylene film sticks to another (left). With a slip agent added (right) the films move freely due to reduced friction.
(Source: Uniqema.)

Many producers, including Chemtura, Uniqema, Croda and others, have introduced slip agents targeted at the hightemperature processing market. Additionally, the products often have antiblocking properties, inhibiting the adhesion of film layers to each other.

Grades
Amide type
Chemical identity
Slip
Antiblock
Max. use temp.
E
Primary
Erucamide
Excellent
Good
<270°C
ER
Primary
Erucamide, refined
Excellent
Good
<270°C
VRX
Primary
Oleamide, refined vegetable grade
Excellent
Medium
<230°C
SR
Primary
Stearamide, refined
Medium/low
Excellent
<220°C
SRV
Primary
Stearamide, refined vegetable grade
Medium/low
Excellent
<220°C
BR
Primary
Behenamide, refined
Low
Excellent
<280°C
203
Secondary
Oleyl palmitamide
Medium
Medium
<300°C
212
Secondary
Stearyl erucamide
Medium
Good
<330°C
EBS
Bis amide
Ethylene bisstearamide
Low
Excellent
<310°C
EBO
Bis amide
Ethylene bisoleamide
Medium
Good
<300°C

Slip agents with longerchain molecules, as well as secondary and bisamides are the most tolerant of high processing temperatures, as illustrated by the Crodamide line.
(Source: Croda Chemicals Europe, Ltd.)

As slip agent molecules become larger their migration rates to film surfaces become slower. Secondary amides, for example, are preferred over primary amides when it is desirable to slow down migration rates. Reducing migration rates is often useful in multilayer films, where it is desirable to confine the slip properties to one layer.

Definition
Coefficient of Friction
PPM Slip Content*
Low Slip
0.50 0.80
200 400
Medium Slip
0.20 0.40
500 600
High Slip
0.05 0.20
700 1000
*For tackier resins (e.g. EVA, other copolymers, lower density polyolefins such as 'metallocenes', ULDPE & VLDPE), higher levels may be needed.
Higher dosages of slip agents result in lower coefficients of friction.
(Source: Ampacet.)

NonMigratory Agents

In recent years, new classes of slip agents have been introduced that do not migrate at all. These additives usually consist of very large molecules that have only limited ability to migrate. One such line, from Multibase, consists ultrahigh molecular weight (UHMW) polydimethylsiloxanes.  Incorporated into polyolefins, and deposited on film surfaces during extrusion, the nonmigratory slip agents remain on the surfaces, reducing the coefficient of friction immediately. (By contrast, it can take anywhere from a day to a week for amide slips to reach their full effectiveness, because they must gradually diffuse through an extruded film to reach the surface.) The rapid action of nonmigrating slips is useful with high speed packaging operations, as well as in stretch films, and extrusions of very tacky resins. Nonmigrating agents can be added to the outer layers of multilayer films, but are not required in the inner layers, which saves on costs. Nonmigrating slip agents are also relatively heat stable, allowing them to resist the temperatures of hot filling and shrink tunnels.

For PET Films and Molded Goods

New slip agents are also emerging for PET molded goods and extruded sheet. PET has a high surface friction, which can cause scratching, sticking and static in molded parts. PET sheet can exhibit mold release problems during thermoforming and can stick to rollers during fabrication. Nested PET thermoformed articles can be difficult to separate. Conventional amide slip agents can also produce colored byproducts when added to PET. New slip agent formulations, or combined slip and antiblocking agents, are now available to minimize or eliminate these problems. Most of the formulations are proprietary. Among the suppliers of such agents are Clariant Masterbatches, Croda, and Sukano.

Other recently introduced slip agents are targeted at packaging films made with ionomers. One supplier of these classes of additives, Ampacet, says its products, sold in the form of masterbatches, are also suitable for ethylene vinyl acetate, ethylenemethyl acrylate and nylons. These additives are certified as compliant with the U.S. Food and Drug Administration (FDA) regulations, and can be used in food packaging films, especially for meat and cheese products.

Biodegradable packaging, such as this PLA vegetable tray, is a growing market for slip agents
Biodegradable packaging, such as this PLA vegetable tray, is a growing market for slip agents.
(Source: Sukano.)

Biopolymer Additives

Slip agents designed for use in biopolymers are one of the newest product developments. They are offered by such suppliers as Sukano, Clariant, PolyOne, and Spartech. The use of these agents is increasing with the growth of biodegradable packaging, particularly polylactic acid (PLA). PLA has a tendency toward yellowing and haze during processing; it also frequently sticks to rollers during film manufacturing. In addition, the material is very brittle, which can inhibit cutting. The new PLA masterbatches, some with both slip and antiblocking properties, are formulated to counteract these tendencies. They are finding widespread uses in degradable agricultural film, food packaging trays, and drinking cups.

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