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A review of additives for plastics: slips and antiblocks.

This is the second article in a four-part series that looks at the range of additives commonly used in plastics to improve their appearance and performance. These primers are meant to create a common base of information on additives within the industry.

Plastic films need certain surface properties if they are to be of use in packaging and other applications. Two properties loom large in their effect on film processability: coefficient of friction (COF) and blocking. COF affects how a film slides over itself and other surfaces, while blocking, or the tendency of adjacent film layers to be attracted to and adhere to each other, makes it harder to work with films on rolls, stacked as sheets or opening bags for filling.

Slips and antiblocks are widely used to bring these properties under control. This article focuses on polyolefin films, given their widespread use in flexible packaging.

Keeping Friction Within Bounds

Although many substances can lower COF in polyolefin films, commonly used commercial slips are usually unsaturated, long-chain amides. The common chemistries are oleamide and erucamide, called primary amides, which are popular because they migrate rapidly, create low COF at modest addition rates, and are relatively inexpensive. Erucamide is often preferred because it is derived from non-animal ingredients and has better thermal stability than oleamide.

Primary amides have limited thermal stability and volatize at temperatures above 400[degrees]F, as is typically seen in cast-film production (usually 500[degrees]F to 550[degrees]F) or extrusion coating (usually above 600[degrees]F). In such cases, secondary amides or non-migratory slips are used.

The COF of low-density polyethylene (LDPE) or linear LDPE (LLDPE) films depends on slip-agent concentration and film thickness. Low-slip films have a COF of 0.5 to 0.8, which corresponds to a slip content of 100 to 400 ppm. Medium-slip films have a COF of 0.2 to 0.4 (500 to 600 ppm slip addition), and high-slip, a COF of 0.05 to 0.2 (700 to 1000 ppm slip addition). Most flexible packaging applications call for COF between 0.2 and 0.4.

Primary amides are relatively small molecules, so they migrate or bloom to the surface of a film quickly after extrusion. Rapid diffusion is best when the film formed is immediately converted to bags and other products. Roll stock stored for a time before being converted needs a slip that diffuses more slowly. This keeps COF from decreasing rapidly in order to avoid telescoping and winding problems.

COF decreases with time after a film is formed as the slip migrates from the bulk to the surface until equilibrium is reached between the surface and the bulk. It often takes 24 to 48 hours for surface COF to stabilize. If the surface slip layer is lost, it usually gets replenished by slip from the bulk of the film.

Molecules of secondary amides are almost twice the size of those of primary amides, so they are less volatile at high temperatures. They also migrate more slowly, making it easier to control COF, and generally interfere less with secondary operations, like printing and sealing, than primary amides.

The performance of primary and secondary amides depends on many factors:

* Slip concentration: The higher the slip concentration, the faster the diffusion rate.

* Type of polymer: Slips migrate more slowly in crystalline polymers, such as HDPE and polypropylene. EVA and other polar polymers tend to interact chemically with slips and slow their migration. Highly amorphous polymers, like metallocene LLDPE and tacky polymers like EVA, need more slip addition to reach the same COF as less-tacky or less-amorphous polymers.

* Other additives: Since antistats, antifogs, and other migratory additives migrate to a film's surface, they compete with slips during diffusion and for surface sites. Depending on the chemistries involved, other additives may hinder slip migration or be hindered by it.

* Corona treatment: This method promotes slip migration to the treated side by burning off surface slip, which creates a slip concentration gradient, and by forming surface polar sites due to polymer oxidation as a result of the high energy of the treatment. Such migration can cause printing problems.

* Winding tension: Slip migrates more slowly in film wound at higher roll tension than at lower roll tension.

* Film thickness: The thicker the film, the longer it takes for slip to come to equilibrium. Thinner films need a higher slip addition rate than thicker films to yield the same COF level.

The choice of a slip masterbatch formulation depends on migration rate, thermal stability, and other properties needed. In general, slip masterbatches are made utilizing carriers, such as LDPE, LLDPE, and mLLDPE, which correspond to the make-up of the final film. These masterbatches usually comprise 5% to 10% of the slip agent.

Non-migratory slips have a higher molecular weight than secondary amides and do not diffuse through a polymer matrix. As a result, COF undergoes minimal change after film extrusion (Fig. 1), so this type of slip needs to be added only to the skin layer of multilayer films. This differs from migratory slips, which are added to all film layers. Non-migratory slips work well at the high temperatures involved in cast films and shrink tunnels and do not adversely affect heat-sealing properties. They provide for COFs of about 0.2 to 0.4, depending on the addition rate and the polymer system.

[FIGURE 1 OMITTED]

A recent development involves a grade that offers consistent slip, which meets the need to hold COF steady after polyethylene-based films are laminated to those made of PET or other polymers using polar adhesives, such as those based on polyurethane. Such adhesives attract and bind slip, drawing it away from the polyethylene surface. This can raise COF to 1.0 or more and prevent film from moving freely in vertical form-fill-seal and other equipment. The minimal interaction with polar adhesives makes converting more predictable.

Making Films That "Repel"

Film producers counter the attractive forces between layers of polyethylene, polypropylene, and other films by adding either inorganic or organic antiblocking agents. Inorganic antiblocks contain mineral particles that limit film-to-film contact by forming small bumps on film surfaces. These additives have a relatively low cost and are most often used in large-volume, commodity applications. They work best in thin films. One drawback: as particulates they often reduce film clarity.

Inorganic antiblocks most often contain natural silica (diatomaceous earth), talc, or calcium carbonate, although zeolite, synthetic silica, ceramic, kaolin, and mica are sometimes used. Talc often has the lowest cost-performance ratio, while diatomaceous earth often provides the best way to minimize total inorganic content while having a minimal effect on film's optical properties.

Factors to consider when using inorganic antiblocks include particle size, shape, quality of dispersion, and film gauge. Other factors include antiblock hardness, refractive index, and specific gravity (Table 1). Hardness affects machine wear, especially for concentrate producers, while the difference in refractive index between inorganic particles and the polymer determines how an antiblock affects haze. Iron content is also important because it can degrade organic additives like slips, antistats, and antifogs and even affect the polyolefin.

Organic antiblocks usually cost more than inorganic ones, so they are added in higher-value films needing high clarity. Commonly used organic antiblocks include compounds of the amide, organic stearate, and metallic stearate families, and such materials as silicone and polytetrafluoroethylene.

Although antiblocks are tried-and true materials, the field continues to advance. One new area involves high-clarity slip antiblocks that combine advanced mineral antiblocks with a slip and a proprietary clarifying agent. These grades overcome the increase in haze caused by traditional inorganic antiblocks. One of them actually turns this situation around by making blown and cast LLDPE films as much as 65% clearer than the original resin. It also reduces coefficient of friction and improves gloss as much as 50%.

These grades reduce the light scattering that causes haze by using a mineral antiblock having uniform spherical particles and a narrow particle size distribution (Fig. 2a). Conventional mineral antiblock particles are quite irregular in shape and size (Fig. 2b), which leads to poor film optics.

[FIGURE 2 OMITTED]

The ability to increase clarity and gloss allows film producers and converters to reduce the level of the more expensive LDPE they add to LLDPE film to improve clarity. It also lets them create higher-value shrink, stretch, and other films at little or no added cost. Trials with this antiblock were conducted in blown and cast LLDPE films. In one case, addition of 1% to 3% of this specially formulated slip antiblock masterbatch, Ampacet 102286, in a 1.25-mil blown LLDPE film improved haze versus film made with just the base resin. The 2% masterbatch, for instance, reduced haze by 53% (Fig. 3). This addition level also increased gloss by 54% and reduced COE In a 2-mil cast LLDPE film, a 1% addition rate cut haze by 62%.

[FIGURE 3 OMITTED]

Another new grade, a clarity antiblock, combines a diatomaceous earth antiblock with an organic antiblock configured to work with this mineral. It preserves darity in lower-density plastomer resins, which tend to be tackier and more likely to block. Since this product is needed at lower levels than traditional antiblocks, it offers an advantage to film producers. Compared with one of the industry's best selling antiblocks, this material improved blocking efficiency by nearly 50% in olefin films, while reducing both haze and coefficient of friction.

Summary

Slips and antiblocks are used in nearly all packaging films, especially those made of polyolefin. Without them, most films would be difficult to process since they would not slide easily over themselves or elements in converting, packaging, and printing equipment and would tend to adhere when pressed together on take-up rolls or when stacked after cutting.

The universe of slip and antiblock additives is quite large, encompassing primary, secondary, non-migratory, and other slips, as well as organic and inorganic antiblocks and their combinations. The number of masterbatches available is even greater because the polymer carrier in which these additives are placed can be keyed to the polymer in the film. This diversity gives film producers, converters, and packagers the ability to choose slip and antiblock packages tailored to their specific needs.

By Dr. Prakash Patel and Dr. Nilesh Savargaonkar

Ampacet Corporation, Tarrytown, N.Y.
Table 1. Selected Properties of Mineral Antiblocks.

Property Silica Talc CaC[0.sup.3]

Mohs Hardness 7-8 1 3
Refractive Index 1.48 1.59 1.60
Specific Gravity 2.3 2.8 2.7
Acid Resistance Good Good Poor
Alkali Resistance Good Good Fair
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Title Annotation:NORTH AMERICA
Comment:A review of additives for plastics: slips and antiblocks.(NORTH AMERICA)
Author:Patel, Prakash; Savargaonkar, Nilesh
Publication:Plastics Engineering
Geographic Code:1USA
Date:Jan 1, 2007
Words:1752
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