Anti-blocking Agents for Plastics

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Why does film blocking occur?

When making plastic films, the various film-converting operations (wrapping and winding for example) proceed at high speeds and pressures, increasing the incentive for the plastic films to stick together. The ‘sticking’ or ‘blocking’ is due to the presence of high frictional forces between the films which are the result of an array of intermolecular forces.

What is anti-blocking?

Simply put, anti-blocking agents reduce frictional forces between the film layers in contact, thereby improving the handling and conversion of the film. Nowadays, anti-block agents are well established and the market is saturated with various technologies ranging in cost, complexity, and efficiency.

Inorganic vs. Organic ant-blocking agents

Antiblock agents can be broadly separated into organic and inorganic additives. The two types are often used synergistically.

Figure 1: Adding an anti-blocking agent to plastic carrier bags or bin bags as pictured above, can help stop them sticking together so that they remain easy to open and use.

Inorganic anti-block agents

Inorganic anti-block agents work by a physical mechanism. They consist of inorganic particles which protrude from the film surface and act as ‘spacer bars’ between the film layers. By decreasing the contact area between the film layers, the overall friction is reduced, making it easier to separate the films from each other (Figure 1). Inorganic anti-blocking agents with small particle sizes are preferred for thin films, while coarser particles are preferred for thicker films. Inorganic anti-block materials include talc, calcium carbonate, natural silicas, synthetic silicas, aluminium silicates, and anhydrites.

Figure 2: Packaging films are easier to open when small amounts of anti-blocking agents are added. Inset: Microscopic image showing the protrusion of inorganic anti-blocking agents from the film surface. (magnification 200x).

Organic anti-block agents 

Organic anti-block agents are materials that migrate to the film surface. Also referred to as ‘release agents’ or ‘slip agents’, these materials impart lubricity to the surface. Organic materials used for anti-blocking include amide waxes, stearates, silicones, PTFEs and erucamides. These materials are significantly more costly than inorganic anti-block agents and hence used in more demanding applications. Wax additives will be covered in a subsequent blog post.

How to select a suitable inorganic anti-blocking agent for a film application?

The appropriate inorganic anti-blocking agent for any given application depends on the requirements of the final film product (Table 1) as well as the polymer type. Several factors must be considered, including the required anti-blocking performance, film thickness, filler processability, wear on equipment, and regulatory aspects such as food contact legislation. To fabricate a film product with high perceived quality, the optical properties of the plastic film are also paramount. These include transparency, clarity (sharpness of image), and lack of haze (milky appearance).  

Table 1: The selection of a suitable anti-blocking agent depends on a range of parameters.

Low-cost fillers such as calcium carbonate have a negative effect on the optical and mechanical properties of the polymer film. For non-demanding applications such as black bin bags, calcium carbonate is often sufficient. For higher quality films, in particular where transparency is important, more sophisticated inorganic anti-blocking agents are much more suitable.

Clear films require anti-blocking agents which have minimal effect on the optical properties of the film, whilst providing an effective reduction in the coefficient of friction. For food packaging, additional food packaging regulations must be met. Below we have outlined a few materials from our portfolio which meet these requirements. The materials can be further broken down into natural and synthetic inorganic anti-blocking agents.

Natural inorganic antiblocking agents

Trefil 1313 by HPF Quarzwerke and Satintone W by BASF are both natural, cost-effective, and high-performance inorganic antiblocking agents. They are brightly coloured and finely powdered products which are easy to incorporate and offer good anti-blocking performance. Typical addition levels are around 3000 – 6000 ppm.

Trefil 1313 is a natural anhydrite which belongs to the anhydrous sulphates, with a naturally high whiteness. Satintone W is even finer and is therefore recommended for thinner films. Satintone W is an aluminosilicate which has been calcined to give it an extra bright appearance which further limits the degree of yellowness typically imparted by other natural materials such as calcium carbonate. The calcination process also makes Satintone W extra easy to disperse thereby lowering extruder pressures. Due to their high purity, Satintone W and Trefil 1313 both carry food-contact approval and have therefore become well-established materials in food packaging as well as general purpose packaging and agricultural film applications.

Figure 3: Electron microscope image of the structure of Trefil 1313 showing its regular blocky nature.

Synthetic inorganic anti-blocking agents

Synthetic inorganic anti-blocking agents are chemically pure and geometrically well-defined materials that generally offer higher performance compared to their natural counterparts. Synthetic silicas are the most common and can be produced in a pyrogenic process (fumed silica) or via the precipitation of silicate salts (precipitated silica). Both processes result in synthetic silicas that are completely amorphous (no crystalline fraction present) which can be a concern with natural anti-blocking agents.

While synthetic silica products have a higher intrinsic cost compared to natural inorganic anti-blocking agents, they can be used in much lower concentrations, largely offsetting costs. Typical addition levels are around 500-2000 ppm for common polymers such as PP, LDPE, LLDPE, and PET. Besides the high anti-blocking efficiency, the high absorption capacity of these materials (due to the porosity) is also advantageous. For example in PVC films, precipitated silicas can absorb migrating PVC plasticisers which would otherwise contribute to the coefficient of friction.

Due to the low addition levels required with synthetic silicas, these materials are often introduced via masterbatch. Let-down can be performed in many different types of processing machines, such as extruders, calendars, and planetary mixers.

Spherilex® from Evonik is an example of precipitated silica that has been specifically designed and optimised for anti-blocking in polymer films. The particles are extremely spherical in nature and prepared with a narrow particle size distribution (Figure 4). This means that every particle counts toward the antiblocking performance and as a result, the overall addition levels can be lowered, which in turn benefits optical clarity of the film.

Figure 4: The particle size distribution of SPHERILEX® 30 AB and SPHERILEX® 60 AB is extremely well defined. Inset: SEM micrograph of SPHERILEX® 60 AB illustrating the round shape of the particles.

Summary

In conclusion, a broad range of anti-blocking technologies are available nowadays. The most suitable material is highly dependent on application requirements and at Lawrence Industries we would be glad to assist you with this - get in touch with our technical sales team to discuss your project (01827 314151) or request a sample on our product page.