Key Information

Polyolefin films (mostly blown or cast polyethylene (LLDPE) and polypropylene (PP) - but also PVC and to some extent PET) tend to adhere to each other due to strong van der Waals interaction or electrostatic charges when being in close contact (adjacent layers). The closer the distance between two layers, the stronger the adherence which occurs typically when material is wound up after film production. Once in close contact, it is nearly impossible to move the layers relatively to each other due to resulting high coefficient of friction. The higher the temperature, pressure, and contact/processing time, the higher the tendency to stick to each other. Additionally low-molecular-weight polymers (oligomers) might migrate from bulk to the surface and add as “tackifier.”

To avoid the adherence of layers due to a close contact, particulate matter is introduced into the film in a highly diluted concentration. By that measure a micro-rough surface is created, and contact area of film layers is minimized; the distance between the layers is maximized and adherence suppressed.

Typically this is done by using masterbatches with concentrations up to 50wt% of particulate matter which are added during processing of the film. Final concentration of the anti-blocking or anti-stick agent in the film material is typically in the range of 0.1% up to 1.0%, depending on the application. Minerals used for this application should have little to no impact on the mechanical properties of the film; must not deteriorate transparency, haze, color, and gloss of the film; and must be compatible with the film production process. Several minerals are used for this purpose: talc, calcined kaolin, cristobalite, precipitated silica, diatomaceous earth, mica, calcium carbonates, calcium sulfate (anhydrite), magnesium carbonate, magnesium sulfate, and feldspars. The big advantage of using inorganic anti-blocking additives is that they do not migrate and are not released from the polymer matrix. Especially when films are subjected to food contact or weathering, most inorganic anti-blocking additives (except carbonates) are more persistent in comparison to the migrating organic ones. On the other hand - since they cannot be dissolved in the polymer - it is more difficult to obtain optimum optical properties. In this respect the match of refractive index between polymer and inorganic additive - besides the particle-size distribution (which causes the wanted roughness but on the other hand unwanted light scattering of the surface) - governs transparency as well as haze. Even if the average refractive index of the mineral matches the one of the polymer, the mineral will intensively scatter the light if it has pronounced birefringence. Birefringence is the dependence of refractive index on the direction of a light beam relative to the axis of the crystal lattice. The refractive index of polyethylene (dependent whether it is LD or HD) ranges between 1.51 and 1.54, the one of polypropylene typically around 1.49. Both materials are isotropic (without birefringence) when clarified properly. Table 1 shows data of refractive indices and birefringence of feldspathoids, Table 2 the data of other minerals used for anti-blocking. Looking at those tables, the use of the carbonates calcite and magnesite in films is prohibitive when aiming at optimum transparency due to their high birefringence.

For inorganic additives, average particle sizes chosen are in the range of a fraction of the film thickness, and special care is taken to ensure an absence of oversize particles to avoid blocking of nozzles or partial film rupture during processing (especially when films are blown). Top cuts are chosen typically to exceed the film

Table 1 Typical data of the pure feldspathoid mineral species (modified from (Robinson 2003)

Table 2 Typical data of the pure mineral species

thickness a bit so these particles extend surfaces on both sides of the film. Special care must be taken since some minerals (like talc) act as nucleating agents, changing completely mechanical and optical properties of the film even in small quantities by inducing its crystallization. Also absence of impurities (like iron oxides), promoting the oxidization of the polymer, has to be taken into account. Finally hardness in combination with particle size of the mineral governs wear at least on extruders used to produce the masterbatch and later on equipment used to produce the films.

Organic alternatives used for anti-blocking or anti-stick are amides, fatty acid amides, fatty acids, salts of fatty acids, silicones, or others. They work with different mechanisms compared to the inorganic anti-blocking additives, migrating to the film surface upon cooling and forming a release layer. Sometimes combinations of inorganic and organic anti-blocking additives are used. Organic additives typically have lower anti-blocking ability in comparison to the inorganic ones but better slip effect.

The extent of blocking between films can be measured according to ASTM 3354-89. The force (actually expressed by a weight) to disjoin two adjacent films with a contact area of 100 cm² is measured. The efficiency of anti-blocking is described by the coefficient of friction (COF) by ASTM 1894. Here the force between the two polymer sheets or of one sheet against a standardized surface (steel) necessary to make the sheet slide over the second surface is measured.

Particle forms of anti-blocking additives are used.

Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 show the typical particle forms of inorganic anti-blocking agents used in polyolefin films.

Special care has to be taken for the choice of additive combinations in polyolefins since all additives can show specific interaction with each other. Worst case interaction occurs if one additive deactivates to effect of another additive. Those adverse effects or how to circumvent them by proper choice of anti-blocking additive has been described by Imerys (2014). They compared hydrous with calcined clay in antiblocking applications concerning their surface interaction with organic substances. Starting with inverse gas chromatography, they calculated the free energies of adsorption on those two minerals for various organic substances (e.g., amines as model substance for HALs). Strong interaction of alkenes or polar materials with hydrous clay was attributed to the Lewis acidic site (hydroxyl groups), whereas the calcined material (by calcination metal hydroxyl groups are converted into metal

Figure 1 SEM of feldspar flour 44% orthoclase,

45% albite, 4% anorthite, 7% quartz

Figure 2 SEM of feldspar flour 85% albite, 3% orthoclase, 4% anorthite, 8% quartz

Figure 3 SEM of feldspar flour 87% orthoclase, 7% albite, 4% kaolinite, 2% quartz

Figure 4 SEM of nepheline syenite flour 25% nepheline, 73% feldspars, 2% analcime, natrolite

Figure 5 SEM of anhydrite

Figure 6 SEM of calcined clay (kaolin)

Figure 7 SEM of

precipitated silica

Figure 8 SEM of cristobalite flour

Figure 9 SEM of

diatomaceous earth

Figure 10 SEM of

muscovite mica

Figure 11 SEM of calcite

oxide sites) showed much less interaction. They conclude that the reduction of surface polarity by calcination reduced the interaction/adsorption and thus the deactivation of other additives by the anti-blocking material. Subsequently they compared hydrous clay, calcined clay, talc, natural silicas, diatomaceous earth, and synthetic silica in haze and blocking force at 1,500 and 3,000 ppm loading level. Calcined clay imparted lower haze in comparison to the other additives (hydrous clay, talc, diatomaceous earth, as well as natural and synthetic silica). Lowest blocking force (best anti-blocking ability) was observed conversely to haze, with synthetic silica showing the best behavior in that respect.

Smith (2006) published the interaction and its effect on film fracture on LLDPE films of several combinations of polymer-processing aids (PPAs) and different antiblocking additives. The three PPAs chosen, necessary to prevent film fracture, were all FKMs (fluoroelastomers) based on copolymers of vinylidene fluoride and hexafluor- opropylene pure or mixed with other processing aids. The five anti-blocking additives

Table 3 Physical properties of mineral flours used for anti-blocking in polyethylene

chosen were diatomaceous earth, surface-treated talc, talc, a mineral mixture of sodium and potassium aluminosilicates, and a synthetic, amorphous silica. He concludes that PPAs based on FKM are effective if they contain additional processing aids, enhancing incompatibility between FKM and the LLDPE. By using this technology, the interaction with most anti-blocking additives in this study could be overcome. However synthetic silica proved to be highly interactive with the PPAs requiring a much higher dosage level of the PPAs to perform properly. Interestingly he found that there was no difference between pure and surface-treated talc in view of the interaction to PPAs.

A comparison of different, common mineral products for anti-blocking has been published by HPF (HPF The Mineral Engineers 2013). Table 3 shows the properties of mineral flours used in comparison.

Films were produced by blow-film extrusion using a 3% polyethylene mineral masterbatch (MFI 2.0 g/10 min) - feldspar additionally in a 60% masterbatch. The concentration of the mineral in the final film was 3,000 ppm; film thickness was 50 |im, MFI (190 °C/2.16 kg) of 0.75 g/10 min.

By dynamic friction coefficient measurement according to DIN EN ISO 8295/ ASTM D1894, differences in anti-blocking behavior were recorded. The lower the friction coefficient, the less blocking occurs and the better the anti-blocking performance. Table 4 summarizes the data.

Optical properties were determined by means of haze and transparency measurement according to ASTM D1003 and gloss according to DIN 67530/ASTM D2457. Table 5 shows the results.

• • Anhydrite showed excellent anti-blocking, but optical properties (esp. haze) are comparably poor.
• • Cristobalite and nepheline syenite showed comparable good results.
• • Feldspar (which was orthoclase in this case) showed best anti-blocking and optical properties.