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Part 1: Stabilizers

So, you have a thermoplastic project in mind.  You know what polymer you’d like to use and have a great vision of where you want this endeavor to lead.  The next step?  Deciding which additive you will need to enhance your project.  Typically, one or more additives are incorporated into thermoplastic products.  Many are added at the time of manufacture, while others are introduced by the converter during extrusion or injection molding.  The choice of additive depends largely on the application for which the material is intended.  The chemistry and process variables for the specific thermoplastic must also be considered before choosing the right additives for its anticipated end use. 

Additives may be divided into subgroups based on their functionality.  The most common of those subgroups are stabilizers, fillers, and colorants.  Here we will provide a more in-depth look at the first of these additive groups, stabilizers. 

STABILIZERS:

• Antioxidants protect polymers against the negative impacts of oxidative degradation such as chain scission and a loss in mechanical properties.  This occurs when a polymer reacts with atmospheric oxygen, or it can be caused by a reaction with oxygen free radicals during high-temperature processes.  Most commonly, free radical scavengers, such as hindered phenols, are used in cases where atmospheric oxygen is prevalent.  These are referred to as primary antioxidants.  Secondary antioxidants are incorporated to protect a material during processing.  Phosphite esters are frequently used for this purpose because they react with free oxygen to form organophosphates.

• UV stabilizers are a class of additive which protect the plastic component from photodegradation.  Similar to oxidation, photodegradation can result in a loss of properties as well as the discoloring of a material.  Once again, free radical scavengers are effective in stopping the degradation process even after it has begun.  A class referred to as HALS (hindered amine light stabilizers) are very common for use in preventing/stopping photodegradation. .  Additionally, UV absorbers such as benzophenones and benzotriazoles are used to absorb ultraviolet light which they then reemit as energy via a bathochromic shift.   Lastly, quenchers, such as metal chelates, are often used to absorb energy from excited molecules then dissipate it at a lower frequency.

• Flame Retardants are another common and wide-ranging additive in the plastics industry.  Like wood, combustion of thermoplastics occurs when free hydrogen forms during thermal degradation.  Mineral flame retardants such as aluminum and magnesium hydroxide are additives that breakdown in an endothermic reaction which helps cool the flame zone.  Organohalogens (primarily bromine) and organophosphates are reactive when they thermally degrade.  This causes these compounds to react with free hydrogen forming hydrogen bromide or phosphoric acid which reduces the availability for combustion.  Since they react in the solid state, organophosphates are commonly used in intumescent systems along with compounds that release nitrogen gas to form an insulation char along the burn area.

Careful consideration must be given when choosing the right additive for an application.  Some products may have food contact requirements or other regulatory restrictions.  While effective as a flame retardant, bromine compounds produce smoke which inhibits vision and may not be suitable for some environments.  Choosing the right UV absorber is dependent on the specific plastic because the wavelength causing degradation changes from material to material.  Free radical scavengers must be compatible and dispersed to the molecular level. 

While the above group are among the most common additives currently used in the plastics industry, this is certainly not a full listing of your possibilities for additives.  Special applications may require the incorporation of less familiar additives such as an antiozonant to protect a material from the harmful effects of ozone.  Antimicrobials are growing in popularity and are used in plastics exposed to the environment to resist the growth of microorganisms on the surface of the plastic.   Whatever the end use, a complete understanding of the requirements and materials are critical to the success of any program. 

Have a question about which additive to use in your application?  Contact our expert team today!