A PVC stabilizer cannot be selected solely by static thermal stability time, dosage, or price. The same system may work well in one formulation but produce an unsatisfactory colour in another.
The reason is that a stabilizer system performs several functions at once, while the requirements for these functions depend on the formulation, processing technology, and end-use conditions of the product.
What Functions Does a Stabilizer System Perform?
The main thermal degradation process in PVC involves HCl elimination and the formation of conjugated polyene sequences. As the process develops, the colour of the material changes, mechanical properties deteriorate, and processing stability is affected. Oxidation reactions may also take place in the polymer and the organic components of the formulation.
A stabilizer system may include:
- a primary thermal stabilizer;
- co-stabilizers;
- primary and secondary antioxidants;
- internal and external lubricants;
- acid scavengers;
- UV absorbers and light stabilizers;
- other functional additives.
Antioxidants do not replace the thermal stabilizer. Their role is to limit thermo-oxidative processes and strengthen the protection of PVC and other organic components of the compound. The need for an antioxidant package and its composition depend on the specific application of the material.
How Primary Antioxidants Work
Primary antioxidants interrupt oxidation chain reactions. The most widely used are hindered phenols and, for certain applications, aromatic amines. They form complexes, dissipate energy as heat, and dissociate to regenerate the parent molecule. This enables an antioxidant to act repeatedly, which explains its effectiveness at low concentrations.
Bulky substituents next to the phenolic group stabilise the resulting phenoxy radical. Increasing the molecular weight of an antioxidant can also reduce its volatility and tendency to migrate. However, an antioxidant does not work indefinitely: it is gradually consumed while protecting the polymer.
How Secondary Antioxidants Work
Hydroperoxides are formed during oxidation. Their radical decomposition produces new active species that accelerate further degradation.
Secondary antioxidants decompose hydroperoxides mainly through non-radical pathways. This group includes organophosphites, organophosphonites, and sulfur-containing compounds, including thioethers.
Phosphites are particularly useful for protecting the material during mixing and processing. They can improve initial colour and reduce compound damage caused by temperature and shear. Thioethers are more often used where longer-term thermo-oxidative protection of the finished product is required.
Why Primary and Secondary Antioxidants Are Used Together
A primary antioxidant interrupts the radical chain, while a secondary antioxidant reduces the formation of new radicals during hydroperoxide decomposition. Because of their different mechanisms, their combination can be more effective than either component on its own.
However, a combination does not always provide automatic synergy. Possible effects include:
- a change in the initial shade;
- reduced effectiveness of one component;
- insufficient stability during extended processing;
- antioxidant migration;
- formation of coloured reaction products.
Therefore, an antioxidant package should be tested in the complete formulation, not assessed solely by the properties of its individual components.
Why One Antioxidant Package Does Not Suit Every Product
Outdoor Profiles
For window profiles, siding, and other light-coloured outdoor products, initial colour and weathering resistance are particularly important.
An antioxidant should not produce strongly coloured reaction products or interfere with other formulation components. At the same time, an antioxidant alone is insufficient: long-term protection also depends on the thermal stabilizer, pigment system, UV absorbers, and light stabilizers.
Plasticized PVC
In cable compounds, membranes, and flexible products, the antioxidant system must account not only for PVC but also for the plasticizer, lubricants, and other organic components.
Important factors include:
- prolonged thermal load;
- retention of mechanical properties;
- electrical properties;
- volatility;
- migration;
- properties after thermal ageing.
Transparent and Decorative Products
For transparent PVC, colour, absence of haze, and compatibility of all components are critical. Even an effective antioxidant may be unsuitable if it reduces transparency or forms coloured products during processing.
Food-Contact and Medical Applications
For products that contact food or medical media, selection is further limited by requirements for composition, migration, and permitted applications.
If the product undergoes radiation sterilization, changes in colour, structure, and mechanical properties after irradiation must be assessed separately. High-energy radiation can generate free radicals and cause simultaneous crosslinking and degradation of polymer chains.
What to Check When Comparing Stabilizers
Static thermal stability measured in the laboratory is not enough. The comparison should cover the behaviour of the compound during processing and the properties of the finished product.
Depending on the application, the following are evaluated:
- initial colour and colour change during processing;
- static and dynamic thermal stability;
- plasticization and torque;
- product surface quality and deposit formation;
- mechanical properties;
- colour and properties after thermal ageing;
- weathering resistance;
- migration, odour, and emissions;
- compliance with specific regulatory requirements.
The first trial should preferably be carried out without replacing other components at the same time. Otherwise, it is impossible to determine whether the change in results is related to the stabilizer, lubricants, pigment, filler, or processing conditions.
Why a Stabilizer Should Not Be Selected by Price Alone
The difference in the price of a stabilizer package usually represents only part of the cost of the finished product. An incorrect system choice can result in much greater losses:
- yellowing and unstable colour;
- a shorter production run;
- lower throughput;
- increased scrap;
- deterioration of properties after ageing;
- customer claims and a shorter product service life.
It is therefore more appropriate to compare not the cost per kilogram of a PVC thermal stabilizer, but the cost of saleable output with the required processing and performance characteristics.
How a Stabilizer System Is Selected
There is no universal stabilizer because manufacturers of window profiles, cable compounds, flooring, and transparent films solve different tasks.
System selection requires consideration of:
- PVC type and K value;
- the presence and amount of plasticizer;
- filler loading and pigmentation;
- processing technology;
- temperature and shear load;
- colour and transparency;
- operating conditions;
- ageing requirements and regulatory approvals.
Revada Plastic selects the stabilizer system and antioxidant package based on the current formulation, equipment parameters, and finished-product requirements. The final solution is confirmed through laboratory testing and industrial trials, not only by a thermal stability value or product price.
