PVC softeners are one of the most important components of flexible PVC formulations. It is worth mentioning that about 80% of the plasticizers used in the polymer industry are used only in the PVC industry. In addition, flexible PVC formulations have about 40-60 percent by weight of additives, and a large share of these additives is related to softeners.
For this reason, different types of PVC softeners have been investigated in this chapter. In order for a plasticizer to work well in PVC formulations, it must be able to spread evenly in the mixture, which is done through mixing with heat.
Mechanism of action of PVC plasticizers
Different theories have been presented to explain the mechanism of action of plasticizers in PVC formulations. By placing softening molecules between polymer chains, polymer-polymer interactions and van der Waals forces between them are reduced and prevent the formation of hard networks.
This facilitates the movement of polymer chains and subsequently reduces the glass transition temperature of PVC.
Another theory is another theory. Based on this theory, polymer and plasticizer create a three-dimensional network due to the presence of weak intermolecular forces, which can have a suitable and flexible reaction against external stress. Some theories also pay attention to the amount of free volume, because with the increase of free volume, there will be more space for the movement and movement of polymer chains.
When the polymer is at a temperature higher than the glass transition temperature, by heating more, with the help of molecular fluctuations, its free volume will increase and the formulation will become flexible. Of course, the amount of free volume can be increased by changes in the structure of the main chain, such as placing more side groups. By adding softener, the PVC chains are separated from each other and softener molecules are placed between them, the free volume increases and the PVC becomes flexible.
In all these theories, if the plasticizer concentration is low, the PVC-plasticizer interactions determine the way the system reacts.
With the help of this point, it is possible to explain the phenomenon of anti-softening, which occurs in amounts less than 15 phr of the softener.
The presence of PVC softeners in the PVC formulation has a great effect on its processability, so that the PVC formulation with the softener, due to its lower viscosity, has good flowability and with less residence time in the forming processes. , is less destroyed. This type of formulation can be used for long and complex shaped sheets such as offset sheets, cross heads, blown films and film sheets and very wide sheets.
The melt viscosity of the PVC formulation without softener is relatively high, and this imposes an additional load on the equipment and causes an increase in the torque at lower speeds of the extruder spiral. For this reason, special attention should be paid to the strength of the extruder spiral axis for these mixtures.
The amount and type of softener is effective on the properties and processability of the formulation, and in general, with the increase of the molecular volume of the softener, its volatility and compatibility with PVC decreases.
Types of PVC softeners
One type of classification of softeners is that they are divided into two types: first and second. The first type of softeners have more stability and durability in the formulation and product and do not easily come out of the product under heat and with the passage of time or proximity to different liquids and solvents. But second type softeners have limited compatibility with PVC and for this reason they are used together with a first type softener such as dioctyl phthalate. Softeners of the second type have little optical and thermal stability and are mostly used to reduce the price. In the following, the explanations related to the most important and most used PVC softeners are presented.
This group is one of the most widely used PVC softeners and is produced from the reaction of phthalic anhydride with alcohols. Phthalates are actually esters of phthalic acid that have good compatibility, suitable processability, low volatility, good light and temperature stability, and a relatively cheap price, and for this reason, they are widely used.
This group of softeners are used to create proper flexibility at low temperatures.
The most common members of this group are dioctyl adipate (DOA) and diisononyl adipate (DINA).
Phosphoric acid esters are used as softeners for formulations that must have good flame resistance. The most widely used members of this group are triaryls, which produce aromatic structures such as isopropyl phenol and tertiobutyl phenol.
This group of softeners has good optical and thermal stability in PVC and is resistant to washing. Epoxy softeners have a synergistic effect with most metal stabilizers such as zinc and cadmium compounds. Epoxidized soybean oil (ESO) is one of the most widely used members of this group, which are often used in food and pharmaceutical grade PVC formulations.
This group of softeners has low volatility and good durability in PVC due to relatively high molecular weight. This compound is often in the form of polyesters that are obtained by reacting a dihydric alcohol (such as glycol) with a dicarboxylic acid.
Trimethiates are obtained from the esterification of triphthyl anhydride with 2-ethylhexyl or isononyl alcohol and are used in electrical insulators due to their low volatility. The most important members of this group are trioctyl trimethyate (TOTM) and triisononyl trimethyate (TINTM).
Sometimes special softeners are used to prepare food grade formulations that have high chemical resistance and durability. Glycolate esters, nitrile elastomers, polyurethanes and ethylene-vinyl acetate copolymers are important members of this group.