Blocking is a common problem encountered by manufacturers of polyolefin films and coatings. There is an array of antiblock types available. The overview covers the fundamental reasons for considering and using antiblock additives. A review of the most commercially important grades is offered, with general guidelines relative to the user’s needs. Natural silica (diatomaceous earth) and talc prove to be of greatest interest for most commodity applications

 

Blocking ist the adhesion of two adjacent layers of film. it is a problem most associated with polyethylene and polypropylene films (either blown or cast), and to a lesser extent in extrusion coated or laminated products

 

it is thought that blocking of adjacent film layers occurs due to the presence of Van der Waal’s forces between the amorphous regions of the polymer. There forces increase with reduced distance between the two layers, thereby increasing blocking when two layers are pressed together (e.g. winding onto a takeup roll or stacking of finished, converd films). Another possible reason for blocking is the presence of low molecular weight species (such a oligomers), which tend to migrate to the surface of the film

 

The most effective method for combating these handling issues is to add an antiblock additive. An antiblock additive present in the resin microscopically protrudes from the film surface. This creates asperities (i.e. “little pumps”), which help to minimize the film-to-film surface contact, increasing the distance between the two layers, thereby minimizing blocking

 

The blocking between adjacent layers results in increased friction (resistance to motion), and the addition of an antiblock generally contributes to a reduction in the film-to-film coefficient of friction (COF). COF is a measure of the relative difficulty with which one surface will slide over an adjoining surface. The greater the resistance to sliding, the higher the COF value – e.g. “low-slip” or “no-slip” film, sometimes referred to as “high COF” film

Electrostatic charges are typically caused by friction between two materials. Static charging can disturb the continuity many processes. E.g. Static charging can hinder the opening of the tubular film during packaging processes. Often the static charging is the limiting factor for higher speed in these processes. In addition, a reduced static charging avoids dust pick up by the packed goods. As a result, the materials can be stored and eventually displayed in a more attractive way

 

Internal antistatic additives are incorporated in the polymer matrix. The controlled incompatibility causes migration to the surface. There it forms a polar layer that absorbs water from the atmosphere. This layer is able to conduct / dissipates the charges (higher conductivity/ lower resistance and shorter charge decay time)

 

Important here is a balanced incompatibility to control the migration: a too high incompatibility gives a low anti-static performance, a too high incompatibility causes a greasy surface

 

An alternative is the addition of conductive fillers as carbon black. These create a conductive path in the matrix. Here no migrating additives are used. The anti-static properties are stable in time

Exposure to sunlight, and in some cases, even light from artificial sources can have adverse effects on the usefulness of polyethylene products or package contents. UV radiation can break down the chemical bonds in a polymer as well as have adverse effects on packaged food, beverages, pharmaceuticals, and non-perishables. This process called photodegradation will ultimately cause cracking, chalking, and the loss of physical properties such as impact strength, tensile strength, elongation and other properties. It can also alter the chemical composition of packaged goods as well as dyes contained within

 

Ultraviolet Absorbers – UVAs

UVAs slow down the degradation process by absorbing harmful UV radiation and dissipating it as thermal energy This ability results in their widespread use as a UV filter, e.g., protecting what’s behind the package. UVAs are also used synergistically with UV inhibitors for long term weathering applications. There are many non-pigmented applications in today’s market requiring this protection including food, beverage, and non-perishables. A minimum pathway of 6-8 mils is required for UVAs to perform efficiently. UVAs are sacrificial, so the gauge and expected lifespan of the protected product must be considered when making a recommendation. Enough UVA must be present in the reservoir (matrix) to meet the requirements. There are two UVA chemistries which are predominant today

 

Hindered Amine Light Stabilizers – HALS

Hals are the most effective of the light stabilizers (UVIs) for polyolefins. They are available in a wide range of molecular weighs and structures suitable for almost any application. HALS function by trapping free radicals formed during the auto-oxidation process and hindering their proliferation. HALS are colorless, highly efficient at very low concentrations, and may be used from a cyclic process wherein the HALS are regenerated, rather than consumed, during the stabilization process

The guage of the product, geographic location of exposure, and the chemical environment are the most critical factors in choosing the type and amount of HALS. The chemistries available are differentiated by volatility, migration characteristics and chemical resistance

Antioxidants are used to protect polymers from degradation, both during processing (=short term stabilization) and during use (=long term stabilization). Polymer degradation generally results in gel formation and changing melt flow index, loss of mechanical and esthetical properties (crazing, gloss reduction, chalking), and discoloration

 

Generally during polymer degradation free radicals are formed, resulting in uncontrolled chain reactions. Anti-oxidants are used to decrease the amount of free radicals by different mechanisms e.g. by avoiding the formation of free radicals using complexing agents or hydroperoxide decomposers, or by decreasing the amount of radicals using radical scavengers or hydrogen donors

The appearance of a product is very important. Plastic products can begin to yellow, giving them a color that negatively affects their appearance and hurts customer appeal. Optical brighteners absorb light in the UV and violet region (340-370 nm) and re-emit that light in the blue region (420-470 nm). This can enhance the appearance by making it appear cleaner, brighter and more white

 

Optical brightener additives work to brighten the colors of your application to help hide the effects of yellowing. Through custom color concentrates, 3p’s additives can enhance the color of your plastic products and keep them from visibly aging. 3p can customized an optical brightener solution based on your polymer needs

Processing Aid masterbatches contain special-based Polymer Processing Additives. Because of the incompatibility of this additive with the plastics and the high affinity for the metal, a thin coating is formed in the extruder die. The friction between the molten plastic and the metal die-wall reduces, resulting in a reduced stress on the plastic. This eliminates the melt fracture during the extrusion of (m)LLDPE rich blends at high shear rates (output rates). The head pressure reduces. The production can be optimized. The films have a higher gloss

 

Additives. Because of the incompatibility of this additive with the plastics and the high affinity for the metal, a thin coating is formed in the extruder die. The friction between the molten plastic and the metal die-wall reduces, resulting in a reduced stress on the plastic. This eliminates the melt fracture during the extrusion of (m)LLDPE rich blends at high shear rates (output rates). The head pressure reduces. The production can be optimized. The films have a higher gloss

 

In addition Processing Aid Masterbatch products used in PE, PP, PS, … will minimize any die lip build-up. Less stops will be required for cleaning of the die lips, resulting in a higher productivity, thus reduced cost

 

The coating prevents the plastic to remain

 

Typical Benefits by using Processing Aid Masterbatch

 

Elimination of melt fracture in (m) LLDPE

 

Reduce operating pressures

 

Increase output

 

Higher levels of LLDPE in LDPE blending

 

Better mechanical properties

 

Down gauging

 

Improve processing of lower MI resins

 

Extrusion through narrower die gaps

 

Less machine direction drawing

 

Better balance of properties

 

More efficient cooling –> better optical properties

 

Improved gauge control

 

Reduce Die Build-up: Longer run-times in between cleaning stops

 

Reduction in gel formation during extrusion

 

Faster color change

 

Increase gloss and improve surface smoothness

The processing of plastics films will be disturbed by a high Coefficient of Friction. This tendency to adhere to metal surfaces and to other film surfaces will be a limiting factor for high speed processing applications as packaging, handling of films, etc

 

Slip additives are added to reduce the surface coefficient of friction of polymers and are used to enhance either processing or end applications. Typically slip additives have a low compatibility with the polymer which allows migration to the film surface. A good control of the compatibility controls the migration speed and thus the speed of acting performance can be engineered