Types of Heat Stabilizers for Vinyl Compounds

There are several basic groups of heat and light stabilizers currently offered to the vinyl industry which include:

Mixed Metals
Organic acid salts (liquid and solid), consisting of any one or a combination of barium, calcium, cadmium (disappearing), and zinc. Typically, C8 to C18 straight chain or branched chain aliphatic carboxylic acids are used. Aromatic (alkyl benzoic) acids once used are no longer in favor due to toxicity concerns.

Organotin Compounds
The physical and chemical properties of this organotin heat stabilizers solely depend on the nature of the chemical groups linked to the tin atom. organotin mercaptides offer excellent thermal performance and therefore, are employed as the most efficient thermal stabilizers.
Organotin stabilizers are well compatible with the other additives used in PVC therefore minimizing processing challenges. organotin mercaptides also offer outstanding color retention in plasticized and rigid PVC processing.

Lead Salts and Soaps (Liquid and Solid)
Heat Stabilizers based on lead salts and soaps offer exceptional long-term heat stability. These heat stabilizers are considered as one of the most cost-effective form of stabilizers for PVC. PVC compounds when stabilized with lead heat stabilizers show:

-Excellent heat and light stability
-Exceptional mechanical and electrical properties
-Display a wider processing range

Apart from these benefits, lead heat stabilizers have some limitations associated with them as well. Lead stabilizers when used in PVC windows lead to their discoloration.
Leads offer the best electrical properties mainly due to the insolubility of lead chlorides formed during stabilization. Currently, lead is under pressure for possible replacement by special mixed metal systems, in secondary and decorative wire insulation. However, primary insulation is still best stabilized by lead.

Calcium or Zinc-based Compounds
Calcium or Zinc-based stabilizers usually contain calcium stearate and small quantities of zinc soaps like zinc octoate.
Calcium/zinc-based stabilizers that are used for rigid PVC applications are generally available in liquid/ powder forms. Such heat stabilizers improve color stability during processing of PVC and maintain it during the lifetime of the PVC article.


Organic and Miscellaneous Types

Organic heat stabilizers include alkyl/aryl phosphites, epoxy compounds, beta-diketones, amino crotonates, nitrogen heterocyclic compounds, organosulfur compounds (i.e. ester thiols), hindered phenolics, and polyols (pentaerythritols). These types currently are being heavily researched, and their use at the expense of metal-containing stabilizers is expected to grow significantly.
A minor group consisting of carboxylic or mercaptoester salts of antimony, strontium, potassium.
Calendered, extruded, or molded and plastisol flexible vinyl compounds are often stabilized with mixed metal Ba/Zn, Ca/Zn heat stabilizers. Ca/Zn systems are mostly employed in applications requiring USFDA approval for direct or indirect food contact.
In North and South America, and parts of the Far East, rigid vinyl compounds for extrusion and molding are frequently stabilized with organotin mercaptides and lead or mixed metal systems are used for this purpose in Europe.