How to Select the Right Plasticizer for Polymers?

Plasticizers have to meet several requirements mentioned below.
 

• First and most important, they need to be compatible and permanent to stay durably in the final article.
• Plasticizers also need to withstand heat and shear during processing and sustain severe aging conditions.
• Mixing plasticizers with PVC and processing the blend need to be cost-effective as well.
• Plasticizers act as processing aids and need to be suited to various conversion processes like extrusion, calendaring, dipping, spread coating, etc.
• Long shelf life and durability over harsh aging conditions (UV resistance, migration and extraction resistance) are key attributes to consider when selecting a plasticizer.
• Finally, plasticizers need to meet specific industry requirements (e.g., cable rating, automotive specifications) or specific approvals (such as toys, food, and medical)^[4].

Plasticizers are typically colorless and odorless liquid esters designed to give a permanent softening effect. Their major function is to impart flexibility and workability of the resin. Plasticizers play an important role in the processing of PVC by:
 

• Improving internal lubrication
• Reducing internal friction, and
• Adhesion to metal surfaces

They decrease the processing temperature, the melt viscosity while improving filler and pigment dispersion^[4].

Plasticizers increase flexibility by lowering the glass transition temperature of polymers, decrease tensile strength but increase elongation and change the thermal properties so that the polymer is more easily worked by heat and perform better at low temperatures^[1]. 

 

There are two main groups of plasticizers:
 

Internal Plasticizers
 

• Internal plasticizers are part of the PVC molecule (where a second monomer is copolymerized into the polymer structure).
• Internal plasticizers have generally too narrow of a temperature processing range to be useful. This is why external plasticizers are the most important commercially.

External Plasticizers
 

• External plasticizers can be monomeric or polymeric.
• Examples for polymeric plasticizers are EVA, CPE, NBR and terpolymers or polyesters mainly based on adipic acid, diols and mono alcohols.
• Examples for monomeric plasticizers include phthalates, adipates, benzoates, and citrates.

They can be further divided into:
 

• Primary (GP or general-purpose) suited to a very wide range of applications and processes.
• Secondary plasticizers enhance processing or enhancing specific flexible PVC properties (such as low temperature and flame retardancy). They are typically less compatible compared to primary plasticizers.
• Finally, extenders providing cost-reduction but having low compatibility limits.
   

Phthalates account for over 80% of the world consumption. (See figure below)

Global Plasticizer Consumption in 1000 MT by Plasticizer Type in 2017^[2]

LMW Phthalates can be further divided into o-phthalates and terephthalates (para-phthalatesor p-phthalates).
 

• Terephthalates offer lower levels of compatibility with PVC.
• Ortho-phthalates (o-phthalates) based on phthalic acid can meet the performance requirements of a broad range of applications.
  • LMW o-phthalates have 3 to six carbon atoms in the backbone and include:
    • Di-butyl phthalate (DBP)
    • Di-ethylhexyl phthalate (DEHP)
    • Butyl benzyl phthalate (BBP), and
    • Di-isobutyl phthalate (DIBP)
• HMW o-phthalates are carbon backbones greater than 7 and include:
  • Di-isononyl (DINP), and
  • Di-isodecyl phthalates (DIDP)
     

The overall compatibility, volatility, and solubility temperature of o-phthalates as the function of the number of carbon atoms present in the alkyl chain of these esters are shown below. The figure highlights that an optimum between compatibility, volatility, and solubility can be found with alkyl chains containing 9 and 10 carbons. 
 

The Red Curve Shows Volatility, the Blue Curve Compatibility, and the Green Curve Shows the Solubility Temperature.

Plasticizers like DINP and DIDP would typically offer the lowest volatility among the general-purpose plasticizers and good retained mechanical properties after aging. They combine low density and efficiency for volume cost advantages while providing formulators with the needed permanency and resistance to extraction, maximizing the retention of mechanical properties over time.

In addition to phthalates, several specialty plasticizers are offered to meet different needs like, for example:
 

• Aliphatics for low temperature applications like adipates, sebacates and azelates
• Cyclohexanoates for low solubility in fat
• Trimellitates for high temperature resistance
• Benzoates to boost gelation
• Phosphates to improve flame retardancy
• Polymerics to improve extraction resistance
• Citrates for toys and food contact
• Aromatic sulfonates for saponification resistance, and
• Various specialty bio-based plasticizers
   

All these plasticizers address niche requirements, while HMW plasticizers, like DINP and DIDP, offer the broadest range of use and performance in the general-purpose market.

The LMW o-phthalates (DOP, DBP, BBP, DIBP) are classified as reproductive toxicants and are SVHC (Substances of Very High Concerns) under REACH, subject to authorization and restrictions^[11].On the other hand, HMW phthalates have been found to be safe by multiple regulatory agencies across the globe over the last few years.

The ECHA RAC carried out a robust scientific weight of evidence evaluation on DINP and concluded that it did not warrant classification for reprotoxic effects under the EU's Classification, Labelling and Packaging (CLP) regulation.

The European plasticizer market has been adapting to the regulatory and market pressures with an important major shift from the use of these classified LMW phthalates to the use of non-classified HMW phthalates and other plasticizers. DINP's positive regulatory outcome in Europe will support back conversion from alternative and less performing plasticizers.

DINP and DIDP are among the very few plasticizers that were thoroughly evaluated under REACH^[7].

 

Sustainability is a very complex topic. VinylPlus®, the voluntary sustainable development program of the European PVC industry, has a renewed ten-year commitment to sustainability.
 

• The program establishes a long-term framework for the sustainable development of the industry by tackling a number of critical challenges in the EU-28, Norway and Switzerland.
• The VinylPlus® program was developed bottom-up through industry workshops and through an open dialogue with all stakeholders, including NGOs, regulators, public representatives, and users of PVC. Each of the five key challenges is based on The Natural Step^[9] (TNS, a Swedish NGO) System Conditions for a Sustainable Society.
   

Other considerations may also include:
 

• Global availability
• Efficient production (reduced manufacturing complexity, reduced steps involved, reduced by-products, impurities and increased energy efficiency)
• Volume and processing cost advantages (doing more with less) needing no or lower amounts of additional secondary plasticizers and also safety in current applications.

DINP and DIDP possess many of the attributes mentioned. The annual progress report of VinylPlus®^[7] shows the progress of the PVC industry moving to more sustainable plasticizers^[7].