Plastics Additives

 

Plasticizers

Plasticizers improve the flexibility and durability of plastics. They are used in a number of plastics. However, almost 90% of all plasticizers are used for the production of flexible PVC. Plasticizers are essentially non-volatile solvents. The most common plasticizers are phthalatic, trimellitic, benzoic, and adipic acid esters. These low molecular weight compounds function as spacers between polymer chains and reduce the rotational energy barrier, thus improve the flexibility of the plastic. Plasticizers also lower the processing temperature and change many other physical and mechanical properties. For example, they lower the glass transition temperature, the Youngs modulus, and the melt viscosity.

 

Impact Modifiers

Impact modifiers are added to brittle plastics to improve the shock absorption ability which is an important requirement for plastics used in engineering applications. Impact modifiers are sometimes added to tougher plastics as well that lost some of their inherent toughness due to the detrimental effects of some additives. Typical impact modifiers are (functionalized) elastomers such as butadiene rubber, and (crosslinked) acrylics; block copolymers such as SBS, NBR, and EPDM; and core-shell rubbers with a soft rubber core and a hard shell such as ABS, NBR core-shell graft copolymers.

 

Lubricants and Flow Promoters

These are additives that improve the processability of the resin and prevent damage to the molding equipment by reducing friction (external lubricants), and by lowering the bulk viscosity (internal lubricants). Typical external lubricants are metallic soaps, fatty acids, paraffin and low MW polyethylene. These compounds migrate to the interface between the molten resin and the metal surface of the processing equipment and reduce friction. Internal lubricants promote the flow of the resin and facilitate mold filling. Common internal lubricants are fatty esters, EVA waxes and oxidized polyethylene. A third class of processing additives is mold release agents (de-molding agents), which keep the plastic product from sticking to the mold and reduce surface imperfections. Silicone oils, graphite, zinc stearate, and molybdenum disulfide are often used for this purpose if the aforementioned lubricants do not provide sufficient mold release.

 

Fillers / Extenders

Fillers are important additives. They are usually inexpensive, inert, inorganic materials that simply add bulk to the plastics, i.e. they reduce raw material cost by lowering the resin consumption (extenders). However, there are other benefits of mineral fillers; for example, they can improve the moldability and stability of the resin. In addition they increase the heat-deflection temperature and reduce the thermal expansion when heated. The most common minerals commonly used as fillers include calcium carbonate, talc, silica, clay, mica, kaolin, calcium sulfate, glass fibers, glass beads, carbon black, alumina trihydrate and wollastonite. Fillers are also added to paints, coatings, adhesives, and sealants.

 

Flame Retardants

Flame retardants reduce the flammability of plastics. They can be divided into inorganic and organic compounds. The most common inorganic flame retardants are aluminum hydroxide (also known as aluminum trihydrate or ATH), magnesium hydroxide, zinc borate, antimony oxides and hydromagnesite (hydrated magnesium carbonate mineral). The most common organic flame retardants are chlorinated and brominated compounds. Their effectiveness increases with the halogen content which can be as much as 80 percent by weight of halogen. Other common organic flame retardants are organophosphates and organophosphonates. Sometimes a combination of two flame retardants is used which increases their efficiency.

 

Antioxidants & Other Stabilizers

These compounds slow down degradation of plastics during processing and service when exposed to excessive heat, oxygen, ozone and radiation. The two main classes of antioxidants are free-radical scavengers and peroxide scavengers. The free-radical scavengers are sometimes called primary antioxidants whereas peroxide scavengers are often called secondary antioxidants or hydroperoxide decomposers. The most widely used primary antioxidants are sterically hindered phenols and the most common secondary antioxidants are trivalent phosphorus compounds (phosphites). They are often used in combination to achieve a synergistic inhibition effects. Other important stabilizers are hindered amine light stabilizers (HALS) and UV light absorbers, which are effective inhibitors against free radical or UV induced degradation during service.

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