What they are, what they are used for and how to choose stabilizing additives for recycled PVC

It is important to know that pure PVC does not lend itself to almost no application: for this reason in the transformation processes additives are always added to the PVC that protect the polymer during processing so as to prevent its degradation and also allow to improve the characteristics of the resulting artifact according to its intended use.

The formulation of the material is in fact defined considering three fundamental aspects:

– Type of processing: the material must be able to withstand the stresses and temperatures involved in the process, be in the right shape (dry-blend, granule, latex, etc.), be sufficiently stable and have adequate properties for the type of processing;

– Final application: the final use of the product, stresses, hostile environments, or even particular limitations imposed, for example, on contact with food or in the medical field, must be taken into consideration;

– Cost: always important aspect; depending on the quantity and type of additives.

A typical formulation, for rigid PVC, includes resin, thermal stabilizer (avoids degradation), process aids (improve the characteristics of the melt and the workability) and the lubricant.

For plasticized PVC, a similar base is used, but plasticizers are added. Other additives are dyes and fillers.

The fillers are inserted mainly to reduce the quantities of PVC for the same volume and therefore to reduce costs, but also affect the properties by increasing the hardness and rigidity of the finished product.

An additive must neither evaporate during transformation nor exude towards the surface during the use of the product.

This means that the additive must have a low vapor pressure at high temperatures and must not precipitate or crystallize by migrating from the polymer matrix during aging.

While insoluble additives, such as fillers and pigments, do not give rise to these migration phenomena, on the contrary, soluble additives, such as low molecular weight plasticizers, are susceptible to migration phenomena both during transformation and during use , and can even act as a vehicle for the migration of other additives present in smaller quantities.

Let’s take a closer look at stabilizers

As is already known, the main disadvantage in the use of PVC is its thermal instability; in fact at about 100 ° C it undergoes a degradation called dehydrochlorination, ie it releases hydrochloric acid. This causes a lowering of the mechanical properties and a discoloration.

The transformation of PVC into products always requires the addition of thermal stabilizers which avoid and reduce the propagation of thermal degradation, due to the development of hydrochloric acid of PVC during the gelation and processing phase.

These products also allow to improve the resistance to sunlight, heat and atmospheric agents of the product. Finally, stabilizers have a strong impact on the physical properties of the mixture as well as on the cost of the formula.

Generally they are added to 1% to PVC and remain firmly anchored to the polymer matrix.

The choice of the appropriate thermal stabilizer depends on several factors: the technical requirements of the product, the current regulations and costs.

The most common stabilizers are generally dispersed in an organic co-stabilizer which increases their stabilization characteristics. The main stabilizers are: tin stabilizers, cadmium stabilizers, lead stabilizers, barium / zinc stabilizers, Ca / Zn stabilizers, organic stabilizers.

Stabilizers Ca / Zn

Recently developed and with excellent success, they are proposing themselves as valid substitutes for lead stabilizers on a practical level and also on an economic level.

Their operation is based on the same principles as lead stabilizers, but, unlike these, they do not cause environmental or human health problems.

To improve the efficiency of these stabilization systems, sometimes other elements such as aluminum or magnesium-based compounds are added. For some applications it is necessary to use co-stabilizers such as polyols, soybean oil, antioxidants and organic phosphates.

Depending on the type of stabilizing system, final products with a high degree of transparency, good mechanical and electrical properties, excellent organoleptic properties and a high degree of impermeability can be obtained.

Hand in hand with the Ca / Zn stabilizers, calcium-organic systems are being developed which alongside the many positive sides:

good processability, good thermal stability linked to the absence of Zn (the excess of which could trigger a sudden degradation of the product) have some negative sides such as the color tone of the base (tending to yellow).

Organic stabilizers

To date, organic stabilizers are not considered to be primary stabilizers and, even less, particularly powerful. Some are used because of low toxicity, others are used as co-stabilizers in combination with primary stabilizers.

A particularly important representative who falls into this family of lubricants is epoxidized soybean oil. Epoxidized soybean oil is composed of 10% stearic acid and palmitic acid, for the rest of partially epoxidized polyunsaturated fatty acids.

It is used in formulations in quantities ranging from 2 to 5 phr based on the function it will have. In a quantity less than 2 phr it will have a co-stabilizing function, in a higher quantity it will also have a lubricating function.

Automatic translation. We apologize for any inaccuracies. Original article in Italian

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