SPHERILEX® - next generation antiblocking agents for polymer films

Blog Archive | 8 minutes  | Author: Koen Nickmans , Ph.D.

The Need for Antiblocking in Polymer Films

The sticking together of adjacent film layers – blocking - is an age-old problem encountered by manufacturers of plastic film. The film layers are packed together tightly during winding operations or other types of handling, and subsequently held together by an array of intermolecular forces causing a high coefficient of friction (CoF). These forces make the films impossible to separate and the film products difficult to open.

Antiblock agents act to reduce the CoF between film layers and are therefore essential additives in the production of plastic films. They can be broadly separated into organic and inorganic additives, and they are often used synergistically.

 

Antiblocking Options Available

Inorganic antiblock agents consist of inorganic particles which are usually rough and ill-defined. They act to reduce the CoF by physically protruding from the film surface. The particles help to decrease the contact area between the film layers reducing the overall friction, making it easier to separate the films from each other (Figure 1). By physically separating the film layers, they also decrease the various attractive intermolecular interactions. Inorganic antiblock materials include silica, kaolin, mica, and calcium carbonate. However, the variable particle size, shape, colour, and impurities of these naturally derived materials have a negative impact on the optical properties of the films, such as haze. This limits the use of inorganic antiblock agents to commodity applications.

Organic antiblock agents are materials that migrate to the film surface. Also referred to as ‘release agents’ or ‘slip agents’, these materials are of a chemical nature. They add lubricity to the surface, which is responsible for decreasing the CoF. Materials include amides, stearates, silicones, and PTFE. These materials are often significantly more costly than inorganic antiblock agents and hence are restricted to more demanding applications.

antiblocking agents stopping polymer film from sticking to itself

Figure 1. Packaging films are easier to open when small amounts of precipitated silica are added. They act as inorganic antiblocking agents. Inset: Microscopic image showing the protrusion of particles from the film surface. (magnification 200x).

 

So, how does one get effective anti-blocking performance in thin, clear films, while keeping the cost down?

 

EVONIK Silica Products for Antiblocking

Precipitated silicas, sold by Evonik under the trade names SIPERNAT®, AEROSIL® and SPHERILEX®, offer the best of both worlds. These inorganic materials function by the previously described physical mechanism of antiblocking. Yet they are synthetically produced, resulting in a chemically pure and well-defined product; making them highly efficient. 

SIPERNAT® 44 MS is a precipitated silica that is well established in the film manufacturing industry. Incorporation of a small amount this material into a film (< 0.5%) is enough to significantly lower the CoF and achieve an antiblocking effect (Figure 2). Preferably the precipitated silica is introduced via a masterbatch. These masterbatches are then used in processing machines such as extruders, calendars, and planetary mixers. For processing, the use of an extruder equipped with a vent zone is recommended, as precipitated silica contains some physisorbed water.

Addition of precipitated silica (and any other inorganic particle) has an adverse effect on the haze, which decreases the transparency of the film. In practice, the CoF and haze are therefore adjusted to the product and manufacturing procedure concerned.

Haze is caused by the scattering of light by particles at the film surface and at the particle/film interface. A narrow particle size distribution is critical toward minimizing haze. Essentially, a narrow particle size distribution ensures that each particle is of the correct dimensions to contribute to the physical antiblocking mechanism, reducing the total amount of particles necessary to sufficiently lower the CoF. This is why synthetically derived precipitated silicas are superior inorganic antiblocking agents, and it is why SIPERNAT® 44MS is broadly considered as best-in-class with regard to the trade-off between CoF and haze.

 

SPHERILEX are highly efficient antiblocking agents

Figure 2. Left: The Coefficient of Friction (CoF) in an LDPE film is significantly reduced by the addition of antiblocking agent Sipernat 44 MS. Right: The addition of antiblocking agent has a limited but adverse effect on the haze, negatively impacting film transparency.

 

State-of-the-art = SPHERILEX® Silica Antiblocking Agents from EVONIK

Now, Evonik has introduced the next generation of precipitated silicas for antiblocking. SPHERILEX® 30 AB and SPHERILEX® 60 AB are produced by a new patented manufacturing process that is unique to Evonik. The process is continuous in nature, as opposed to the traditional batch process of producing precipitated silica, allowing the production of narrowly defined particle sizes and uniquely round shape particles (Figure 3).

SPERILIEX® 60 AB, with a mean particle size of 6 microns, has been designed for blown and cast films (eg. PE, PP, and PVC). SPHERILEX® 30 AB, with a lower particle size of 4 microns, has been designed for very thin films and biaxially oriented film applications (e.g. PE, BO(PET), BO(PA), and BO(PP)).

 

SPHERILEX particles have a tight particle size distribution and spherical shape

Figure 3. 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.

 

The particle size distribution of SPHERILEX® is extremely well-defined due to the continuous production process, which involves state-of-the-art sieve technology. Large particles are essentially absent (characterized by the wet sieve residue (less than 100 ppm larger than 25 µm). As a result, every particle contributes to the antiblocking performance, resulting in low addition levels and low haze. Compared to precipitated silicas of the previous generation, a lower CoF can be achieved at a similar or improved haze. Lower addition levels also enable cost engineering in-formulation.

Figure 4 shows this relationship in more detail for the SPHERILEX® materials - incorporated into an HDPE blown film (left) and a PP cast film (right). In both experiments, the precipitated silica was combined with a small amount of slip agent (Erucamide). The SPHERILEX® materials are easily combined with slip agents and other film additives since they exhibit reduced oil absorption. In each case, the SPHERILEX® grades show an improvement in the reduction of CoF whilst minimizing haze - making these materials ideal for antiblocking applications requiring high transparency.

 

SPHERILEX low haze antiblocking agents

Figure 4. Antiblocking performance of SPHERILEX 30 AB and SPHERILEX 60 AB. Left: PE blown film with 0.15% antiblock and 0.05% slip agent (Erucamide). Right: PP cast film with 0.20% antiblock and 0.1% slip agent.

 

There are further benefits associated with swapping out traditional precipitated silicas with the next generation products: SPHERILEX® 30 AB and SPHERILEX® 60 AB.  For example, the specialized manufacturing process results in a reduction of crystal bound water (< 2 % loss on drying), which is commonly present in precipitated silica. As a result, outgassing during extrusion is minimized. Furthermore, the spherical shape of the particles allows for easy dispersion in most polymer matrices, with minimal impact on viscosity. This allows for the possibility of high masterbatch loadings. The abrasion of extrusion equipment is also diminished due to the ball-bearing effect caused by these regular and round particles.

 

Summary

In conclusion, precipitated silicas are essential inorganic antiblocking agents for high transparency polymeric films, which act by protruding from the film surface. SPHERILEX® 30 AB and SPHERILEX® 60 AB are new antiblocking products from Evonik which offer next generation CoF reduction alongside minimal impact on haze. Get in touch with our technical sales team to discuss your project (01827 314151) or request a sample on our product page.

Author: Koen Nickmans , Ph.D.

Koen studied chemistry at the Catholic University of Leuven in Belgium with a specialisation in polymers. He subsequently obtained a PhD in the field of responsive polymeric coatings from the Eindhoven University of Technology in the Netherlands. He has been with Lawrence Industries since 2019 as a technical sales manager, covering all areas involving polymers: thermoplastics, thermosets, and elastomers.