Stabilizers are basically the chemical agents used in polymer processing to stabilize a chemical compound by the reaction of another chemical agent. There are mainly two types of stabilizers.
Primary stabilizers are chemicals which are capable of reacting with labile structures and accept HCI to inhibit the initiation of degradation.
Secondary stabilizers inhibit or reduce the degradation of PVC by accepting thermo-oxidative groups.
These stabilizers are effective in the propagation step. Stabilizers that react with oxygen or peroxide are called antioxidants.
PVC can be protected with a variety of stabilizer systems. The choice of stabilizer depends on the probable decomposition mechanisms involved. Primary and secondary stabilizers are generally used in combination. The primary stabilizers are mainly acid acceptors such as metallic soaps. Typical examples are, metallic soaps such as cadmium, barium, and zinc stearates which also act as lubricants. These chemicals simply react with HCl and are able to reduce further degradation. Calcium and zinc stabilizers are low in toxicity and they are often prepared in the form of a dispersion in epoxidized oil. Barium and cadmium stabilizers are mainly solid products those which are liquid contain phosphite groups.
The compatibility of PVC stabilizers containing heavy metals and synergetic effects of metal stabilizers are reviewed. Organotin compounds such as sulphur containing thio-tin compounds, octyl-tin compounds also act as partial plasticizers and improve melt flow properties. The mechanism of stabilization is more complicated. There are three main groups of organotin stabilizers. Carboxylate derivatives, sulphur containing organotins, carboxylate-mercaptides most organo-tin stabilizers could be used in transparent applications. Octyl-tin compounds are regarded as non toxic. The amount of tin and ligand structure affects the stabilizing efficiency.
Tribasic lead sulphate is the stabilizer which also provides opacity and gives white pigmentation. It is a cheap popular stabilizer but presents toxic it problems. It does not alter the melt rheology but it is an effective heat stabilizer and the opacity it produces assist in protection against UV degradation. Other types of stabilizers which mainly fullfill antioxidant effect are also used including phosphites, nitrogen containing stabilizers such as N, N’diphenyl thiourea, p-diketons, polyols, phenol derivatives. Stabilizers are added in the concentration range of < 1 to 3 phr.
The stability of PVC could also be improved by co-polymerization or polymerizing in the presence of stabilizer .
Ultraviolet (UV) Stabilizers
UV stabilizers are chemicals which reduce or inhibit degradation of polymers resulting from UV radiation. The absorption of UV radiation by PVC excites PVC molecules if the molecules stay at this state for sufficiently long, the photolytic scission of certain C-Cl bonds occur and eventually cause breakage of chemical bonds which consequently degrades the polymer.
The PVC compounds intended for outdoor applications require UV stabilizers. The function of UV stabilizers could be described by simply absorbing the UV radiation and re-emitting it as heat without deterioration.
The basic chemical groups of UV stabilizers used for PVC are benzophenones, berrzotriazoles, salicylates, acrylonitriles, hindered amines and pigments. Hindered amine light-stabilizers are the most commonly used UV stabilizers (-40% of light stabilizer market) . Benzophenones UV stabilizers, which are effective in thin films import little color and have low toxicity. These type of UV stabilizers, for example hydroxybenzophenones form quinoid structure upon absorption of light. The quinoid reverts back to hydroxybenzophenone with loss of energy as heat. Substituent groups affect the UV absorption and volatility of UV stabilizers.
The benzophenones are comparatively low priced UV stabilizers but discolor during processing and weathering. Benzotriazoles show higher absorption than benzophenones at equivalent concentrations. These stabilizers also form quinoid structures. They have lighter color than benzophenones but more expensive. The properties of these type of UV stabilizers in film application has been reviewed. Although they are not used much, salicylates, benzoates, oxanilides and various pigments such as carbon black and titaniumdioxide are used for specialty PVC compounds. Carbon black functions as a UV absorber and titanium dioxide as a light-scatterer and opacifier. The UV stabilizers are very efficient at low concentrations. They are commonly used in 0.2-0.3 phr range.
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