We have all had the experience of walking into the grocery store's produce section and reaching for the roll of plastic bags. You tear along the perforated line and begin looking for an opening, only to find the bag is sticking together, making your task more difficult than it should be. The bag won’t open because the adhesion between the two film layers is strong. This is called blocking, and it can be a common problem with the polyolefin films used to make produce bags. A specific amount of force is needed to separate the film layers from each other. An antiblock (AB) is typically a finely ground mineral, used as an additive, to reduce adhesion between the film layers. It does this by creating a micro rough surface between layers. Thus, a produce bag with just the right amount of AB should open with ease.
The more particles distributed on the film surface, the more pronounced the surface roughness and antiblocking effect. Likewise, larger particles tend to increase in antiblocking efficiency than smaller particles.
I can see clearly now the haze is gone
In applications for high transparency films (e.g., shrink wrap, garment bags, etc...), the AB’s refractive index (RI) must closely match that of the polymer. A common example demonstrating mismatched RI is partially filling a glass cup with water then adding a pencil inside the cup. The pencil appears to bend at the interface of air and water. When a light ray leaves the air and enters a new medium, like water, it bends away from its original path, thus the appearance of “the bending pencil.” Clarity is directly related to the difference in the RI of the AB additive and the polymer. When the RI is constant through the material in the viewing direction, the light does not bend, and optical clarity can be achieved.
When the RI of the AB matches the RI of the polymer, the resulting film will have a higher clarity or lower haze. If the RI of the AB is not close to the RI of the polymer, there is less clarity or more haze within the manufactured film. The larger particles exhibit a higher antiblocking effect, although they negatively affect the optical properties. In contrast, the smaller particles result in better clarity but a decrease in antiblocking properties.
Minerals are a bag’s best friend
Common inorganic ABs are diatomaceous earth (DE), talc, nepheline syenite, and synthetic silica. DE is one of the most effective ABs when used at a low loading. Talc and nepheline syenite have great antiblocking efficiency and are considered great economic alternatives to DE. For optimal optical clarity, synthetic silica is great, but it is an expensive choice suitable for the highest clarity film. Some may prefer calcium carbonate as a low-cost AB, although it is not commonly used. Typically, its effectiveness is low and can lead to haze and poor clarity in polyethylene films.
Although the size, shape, and type of AB influence its performance, processors should also consider other aspects like loading, resin type, and processing. ABs can also be surface treated (e.g., surface treated talc), inhibiting the adsorption of other additives (e.g., slip). The surface treatment makes ABs possible to pair with a slip, giving a slip-antiblock (SLAB) masterbatch that can reduce the coefficient of friction (COF) of films.
Ask a Colortech Sales Representative for more information on our Antiblock products. Our Technical Service Team will select the right mineral grade for your packaging manufacturing needs. Eliminate the hassle by combining the additives you need in one pellet, such as asking for our SLAB combination products.