Thermal stabilizers
Thermal stabilizers must prevent or alleviate the decomposition of PVC resin. The selection of thermal stabilizer varieties in the formulation generally depends on the requirements of the product. For example:
Lead salt stabilizers are mainly used in rigid products. Lead salt stabilizers have good thermal stability, excellent electrical properties, and low cost. However, they are highly toxic, prone to contaminating products, and can only produce opaque products.
In recent years, composite stabilizers have emerged in large numbers, posing a risk of replacing single-component stabilizers. The characteristics of composite stabilizers include strong specialization, minimal pollution, and simplified processing for manufacturing enterprises. However, due to the lack of unified standards, the differences between composite stabilizers from various manufacturers are significant.
Barium-cadmium stabilizers are a type of thermal stabilizer with good performance. They are widely used in PVC agricultural films. Typically, barium-cadmium zinc and organic phosphates are used in combination with antioxidants.
Calcium-zinc stabilizers can serve as non-toxic stabilizers used in food packaging, medical devices, and pharmaceutical packaging. However, their stability is relatively low, and when used in large quantities, transparency decreases, and they are prone to frosting.
Organotin stabilizers have good performance and are suitable for both PVC rigid and transparent products. Especially, octyltin has become an indispensable stabilizer for non-toxic packaging products, albeit at a higher cost.
Epoxy stabilizers are usually used as auxiliary stabilizers. When used in conjunction with barium-cadmium and calcium-zinc stabilizers, they can improve light and heat stability. However, they are prone to migration. Other auxiliary stabilizers include polyols and organic phosphates.
In recent years, rare earth stabilizers and hydrotalcite-based stabilizers have also appeared. Rare earth stabilizers are characterized by excellent processing performance, while hydrotalcite is a non-toxic stabilizer.
Principles of Thermal Stabilizer Selection
In the formulation of rigid PVC, when the plasticizer content is low or absent, the amount of stabilizer should be correspondingly increased to achieve good stability.
(1) Opaque rigid products commonly use tribasic lead sulfate and dibasic lead phosphite, with a synergistic effect when used together in a ratio of 2:1 or 1:1, with a total addition of 3 to 5 parts. Alternatively, composite lead stabilizers are commonly used nowadays.
(2) For transparent rigid products, lead salts are not used. Instead, metal soap stabilizers other than Pb and Ca, as well as organic tin, organic antimony, and rare earth stabilizers, are commonly chosen. Metal soap stabilizers are added in quantities of 3 to 4 parts, while organic tin stabilizers are added in quantities of 1 to 1.5 parts.
For soft PVC and PVC paste product formulations, the plasticizer content is high, and the processing temperature is low, so the amount of stabilizer can be reduced appropriately.
(1) Opaque flexible products commonly use lead salts (1 to 2 parts) in combination with metal soap (1 to 2 parts).
(2) Semitransparent flexible products commonly use several metal soaps in combination, with an addition of 2 to 3 parts.
(3) Transparent flexible products commonly use organic tin (0.5 to 1 part) in combination with metal soap (1 to 2 parts). Alternatively, organic antimony and rare earth stabilizers can replace organic tin.
In non-toxic PVC formulations, the selection of thermal stabilizers:
(1) Lead salt stabilizers should be avoided.
(2) Metal soap stabilizers other than Pb and Cd, such as calcium-zinc stabilizers, can be chosen.
(3) Non-toxic organic tin stabilizers can be chosen.
(4) Organic antimony and rare earth stabilizers can be chosen.
(5) Non-toxic epoxy stabilizers in auxiliary stabilizers can be chosen.
The synergistic effect of primary stabilizers: In a PVC formulation, several primary stabilizers are often used together because different primary stabilizers have a synergistic effect.
(1) Tribasic lead sulfate and dibasic lead phosphite have a synergistic effect, with a synergistic ratio of 2:1 or 1:1.
(2) Different metal soaps have a synergistic effect, with the thermal stability order of metal soaps as follows: Cd, Zn > Pb > Ba, Ca.
(3) There is a synergistic effect between metal soaps and organic tin stabilizers, and both are often used together in transparent formulations.
(4) Some rare earth stabilizers have a synergistic effect with organic tin, reducing costs when replacing organic tin.
Synergy between primary and auxiliary stabilizers:
(1) Metal soaps and epoxy
(2) Metal soaps and polyols
(3) Metal soaps and β-diketones
(4) Some rare earths and epoxy
(5) Metal soaps and organic phosphates
When using thermal stabilizers in conjunction with other additives, some stabilizers themselves do not have lubricating properties, such as lead salts, organic tin, organic antimony, and rare earths, so lubricants need to be added separately in the formulation. Some stabilizers themselves have lubricating properties, such as metal soaps, so lubricants can be omitted or reduced in the formulation.
Sulfur-containing organic tin stabilizers and organic antimony stabilizers should not be used in conjunction with Pb- and Cd-containing stabilizers, as they will cause sulfur contamination when used together.
Antioxidants
Antioxidants in PVC products undergo oxidation during processing and use due to heat and ultraviolet light, which is related to the production of free radicals.
The main function of primary antioxidants is chain-breaking termination or free radical scavenging. Their main function is to combine with free radicals to form stable compounds, terminating chain reactions.
The main antioxidant used for PVC is bisphenol A. There are also auxiliary antioxidants or hydrogen peroxide decomposers, such as triphenyl phosphate and benzoylphenyl isooctyl phosphate. The combined use of primary and auxiliary antioxidants can exert a synergistic effect.
Ultraviolet Absorbers
PVC products used outdoors are sensitive to ultraviolet radiation within the sensitive wavelength range, causing PVC molecules to become excited or chemical bonds to be broken, leading to the generation of free radical chain reactions and promoting PVC degradation and aging. To improve UV resistance, UV absorbers are often added.
Commonly used UV absorbers for PVC include triazine-5, UV-9, UV-326, TBS, BAD, and OBS. Triazine-5 has the best effect, but it imparts a slight yellow color to the film. Adding a small amount of phthalocyanine blue can improve this.
In PVC agricultural films, UV-9 is commonly used, with a typical usage of 0.2 to 0.5 parts. TBS, BAD, and OBS, which are salicylic acid derivatives, have mild effects and, when used in combination with antioxidants, provide good aging resistance.
For opaque products, weather resistance can generally be improved by adding masking titanium dioxide. However, if UV absorbers are added at this time, a large amount is required, which may not be cost-effective.