Antiblock Additives

Antiblock additives are often incorporated into packaging films to prevent them sticking together. Their effectiveness relates directly to the roughness they impart on a film surface while minimising loss in optical properties. They are often used in association with slip additives such as erucamide. Common antiblock additives include crystalline silica, diatomaceous earth and talc (magnesium silicate). 

Antiblock additives can be seen on the surface of films using optical microscopy or SEM. Identification can normally be achieved with internal reflection IR spectroscopy (e.g., with a germanium crystal to minimise sampling depth) or using an X-ray attachment with the electron microscope. 

The optimal quantity of additive formulated into the resin varies with the type of polymer, the specific additive and its desired effect. For example, the amount of antiblock additives can be as high as several percent for some applications. ("Blocking" may be defined as the tendency of sheets of polyethylene film to "stick" together.) However, in most cases, the quantities of additives needed to achieve its function are in the range of 0.05 to 1% (500 to 10,000 ppm). 

Sachtosil CF, P V - controlled process results in synthetic-like material used as antiblocking additive... 

Gulf markets three highly transparent EMA resins that are designated 2205, 2255, and TD-938. Grade 2255 is the same base resin as 2205, except that it contains lubricant and antiblocking additives. Gulf literature for these resins Indicate the following features ... 

Inorganic antiblocking additives are frequently added in concentrations larger than required because they perform other functions, such as surface matting and plasticizer absorption. 

Ultrathene UE 672-000. Excellent impact strength, heat-sealability, good toughness and flexibihty, contains antioxidant and high levels of slip and antiblock additives. 

Markarian J. Slip and antiblock additives surface medication for film and sheet. Plastics Addit Compound [trade journal—Elsevier Ltd.) November/... 

Fatty acid amides are an important class of polymer additives used as slip agents or lubricants to prevent imwanted adhesion. Actually, the addition of such amides for the purpose of antiblock additives for polymer sheets, in glass laminates has often been found to adversely affect the optical characteristics of the polymer sheet, as well as adhesion of the polymer sheet to glass. 

The detailed mechanism of how organic antiblock additives work is not yet understood. It is thought that a barrier layer is formed on the plastic film surface, thus inhibiting the two adjacent plastic film layers adhesion. Their usage is limited. Organic antiblock additives were partially discussed above and will not be further mentioned here. 

Blocking is a surface effect between adjacent film layers that stick to one another. Blocking is quantified by the force needed to separate two film layers under controlled conditions. Two situations where blocking is an issue are the opening of blown film tubes after extrusion and film layer separation after packing and storage. Antiblock additives are used to overcome these issues. 

Antiblock additives can be divided into two classes inorganic and organic. Chemically inert, inorganic antiblock additives migrate to the film surface and partially stick out of the surface to create a microroughness of the film surface. Figure 1.21 illustrates this principle. 

Figure 1.21 Antiblock additives maintain film separation.

Antiblocking agents are additives that inhibit the stickiness (blocking) of polymer surfaces. Blocking is most apparent in films, particularly blown films. After a blown film bubble has passed through the nip roller and the two halves have been pressed together, they may tend to block. Antiblocks help the film to be easily pulled apart. Diatomaceous earth is a commonly used antiblock additive. 

Blocking is the term used for the tendency of two pieces of film to stick together, such as after the two sides of a blown film bubble are pressed together through the nip rollers. Blocking in films can lead to difficulties in handling and conversion or can simply bean inconvenience to customers. Many processors use an antiblock additive during blown film production to minimize this tendency. 

The choice of antiblocking additive depends on the polymer, the desired film quality, and whether there is a pigment. Several inorganic substances are used, including synthetic amorphous precipitated silica, diatomaceous earths, nepheline syenite, calcined clay, coated calcium carbonate, magnesium carbonate, magnesium sulphate, mica, talc and various zeolites. Calcium carbonate particles are approximately spherical, but silica ones are irregular mica forms sheets and talc is plate-like. 

Antiblock suppliers have therefore been concerned in recent years with achieving the best overall balance of properties, including haze, yellowness and coefficient of friction. Both sUp and antiblocking additives tend to lower the permeabihty of the film towards oxygen, carbon dioxide and water vapour. 

A number of other companies such as Dover Chemical and Akzo Nobel have developed improved slip and antiblock additives. Ampacet markets a non-migrating slip agent for use in multilayer films where different coefficients of friction are needed in different directions. Antistat performance is provided on the irmer layer, and slip or antiblock performance on the outer one. 

Kenite, White flux-calcined diatomite, antiblock additive, CeUte Corp., do World Minerals Inc. 

The market for uPVC film is diminishing under environmental pressure and consequently the use of particulate fillers is also diminishing. Most of the film is transparent for food packaging, display, blister packs and so on, and kaolins and other silicate minerals are used as antiblocking additives, without detriment to colour and transparency of the film. 

For pigments, antiblocking additives, and other additives required at low concentrations, the masterbatch route is normally technically the best and most reliable. It is also the best for minimising cleaning and facilitating automatic and accurate dosing. 

Antiblocking agents for film resins are summarized in Table 14.1. They find application for linear low density poly(ethylene), poly-(propylene), poly(vinyl chloride), and poly(ethylene terephthalate) in amounts of 1000-4000 ppm. These antiblocking additives are also used for a heat-sealable outer skin of a multilayer film (3). 

Typically, injection molding grades are used for masterbatches. One of the main masterbatches used in the polyolefins industry is LDPE-based silica masterbatches, since Si02 is used in film applications as an antiblocking additive. 

There are some areas of overlap in usage of different chemical groups but specific compounds frequently differ in each application (fatty acid amides are a classical example of compounds used in all three applications). One obvious general difference is that inorganic materials are only used as antiblocking additives. Chemical classification is used in the next chapter, which discusses general properties of antiblocking, release, and slip additives. 

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