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Classification of plasticizers

29.03.2016
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Plasticizers (softeners) are softening substances which reduce the intensity of intermolecular interaction and increase the mobility of polymer chains. As a result lower glass transition temperature, hardness, strength and increased flexibility of polymer is achieved . Plasticizers also act as an internal lubricant. They reduce friction between chains as they slide against each other in opposing directions, thus making their processing easier. Plasticizers demonstrate low volatility and various degrees of miscibility with polymers. They are primarily functional substances that change the physical properties of PVC and other polymers, creating a whole new world of flexible and durable products.

Their chemical structure is very diverse. They may belong to different classes of organic compounds, such as hydrocarbons, fluorinated substances, esters, ketones, alcohols, amines, fats, oligomers and others. The division of plasticizers is made according to their use or their chemical class, or depending on the molecular weight (monomeric, polymeric). Monomeric plasticizers include esters with a molecular weight of less than 500. These include: mono-, di-, and tri-esters of acids or the following anhydrides: terephthalic, phthalic, adipic, phosphoric, sebacic, citric, benzoic acid, trimellitic and monoalcohols. In contrast, polymeric plasticizers have a molecular weight of more than 500. They form in a reaction between difunctional acids (usually adipic, sebacic acid) and ethyl glycol or propylene glycol. The most important feature is their very low migration from the softened plastic and their low volatility. More about mono- and polymeric plasticizers and examples of their application will be presented in the next article.

 
One of the most common divisions is made on the basis of their solvation properties and their compatibility with PVC, where they are further divided into primary and secondary plasticizers.

 
Primary plasticizers are compatible with PVC and those containing polar groups are characterized by high solvation capabilities. They are also defined as real solvents. These include sulphates and esters of alkylsulphite acids, alcohols and phenols. Diesters of ortho-phtalic acid produced from phthalic anhydride and OXO alcohols containing from 4 to 13 carbon atoms in a molecule are the most common. They cause the polymer to gelate quickly, which facilitates the processing of PVC in typical temperatures. Similar properties are demonstrated by terephthalic diesters and alcohols containing 4-8 carbon atoms in the molecule, which thanks to the so-called safe, non-ortho-phthalic application profile, are becoming more and more popular among PVC manufacturers. The most important plasticizers belonging to the phthalic esters group include: Bis(2-ethylhexyl) phthalate (DEPH= DOP, Oxoplast® O); diisononyl phthalate (DINP)-with similar plasticizing properties to DEHP but worse softening performance, obtained from phthalic anhydride (PA) and isononyl alcohol; diisodecyl phthalate (DIDP), which is less volatile compared to DEHP (higher molecular weight) and is obtained from PAN and isodecyl alcohol. This group also includes: diizoheptyl phthalate (DIHP); diisotridecyl phthalate (DTDP); dibutyl phthalate (DBP); diisobutyl phthalate (DIBP); benzyl butyl phthalate (BBP); di iso undecyl phthalate (DIUP) and phthalic esters of linear alcohols with varying chain lengths.

 
Non-phthalate plasticizers also need mentioning. DEHT (Oxoviflex®), i.e. 1,4-benzenedicarboxylic acid bis(2-ethylhexyl) ester produced by Grupa Azoty S.A. is the response to the ever-growing interest in non-phthalate plasticizers on the PVC processing market. This product is obtained by reacting 2-EH with PTA, which is also used to produce PET bottles. DEHT can easily replace higher and lower ortho-phthalate plasticizers. It has very good physical and chemical properties compared to other plasticizers. Due to its functional parameters, it easily blends in with the specific applications currently reserved for the mass-produced DINP and DEHP plasticizers. Oxoviflex® has been registered in accordance with Regulation (EC) No 1907/2006 (REACH). More information about non-phthalate plasticizers will be included in the articles to come.

 
Secondary plasticizers contain groups which are less polar. They demonstrate limited solubility and compatibility with PVC, that is why they are often used in mixtures with primary plasticizers. Such mixtures show a reduced tendency to migration, but increased strength at reduced temperatures and resistance to precipitation. This group includes aliphatic and aromatic chlorinated hydrocarbons (e.g. chlorinated paraffins) as well as epoxy esters of unsaturated fatty acids obtained from plants (e.g. epoxidised butyl and n-hexyl esters of unsaturated fatty acids). They are used to give the mixture special properties: flexibility at low temperatures, reduced flammability and a lower price of the product.
Thanks to such a wide range of diverse groups of plasticizers, the number of possible PVC applications and the expectations as to the properties of the finished product are constantly growing. It is noted that the final choice of plasticizer or its composition, as well as its quantitative share in the blend is always going to involve achieving a product with an advantage of desired properties over those that are less sought for. Some things to consider include price, compatibility, efficiency and impact on properties, which was discussed in the previous article.