Plasticization is a process that refers to a change in thermal and mechanical properties of a polymer which involves: lowering of temperature at which deformation can be affected with very small forces, decrease in viscosity, increase the toughness, and lowering of the second-order transition temperature. These effects can be produced by compounding a polymer with a low molecular weight polymer or introducing a comonomer into the original polymer which forms secondary bonds with chains and spreads them apart reducing crystallinity and increasing chain flexibility. Hence, plasticizers are low molecular weight polymers or compounds or comonomer incorporated into the polymer to achieve plasticization.
You can visualize the plasticizer use in our Article, “Blending of Aramid and Plasticizers” .
Usage of Plasticizers in Industry
The below table gives a brief idea about additives used in the USA market in the synthesis of polymers. Plasticizers being one of the most commonly used additives are less expensive and easier to play with.
Most commonly plasticizers are used in the formation of film and sheets. Due increase of polymer window and door applications, plasticizer usage in profiles also increased substantially.
Attributes of plasticizers
There are a number of attributes a plasticizer should have so that they can perform their function without deteriorating the properties of the polymer. Good compatibility with polymers is one of the major attributes required from a plasticizer which depends on solubility and polarity, molecular weight, and structural configuration of a plasticizer. Another important factor is resistance to migration and leaching which requires a plasticizer having a high boiling point and low viscosity. They should be nontoxic so that it can fulfill health and safety requirements.
Considering the above discussion criteria for plastic additives selection for a given polymer can be described as follows:
- Compatibility with the polymer.
- Desired thermal and mechanical properties.
- Resistance to migration and leaching.
- Resistance to chemicals, water, and UV radiations.
- Effect of plasticizer on rheological properties of a polymer.
- Volume-cost analysis.
Types of plasticizers
Plasticizers can be either internal or external, internal plasticizers are inherently part of polymer but external ones are added into the polymer which is not attached to the chains with primary bonds and can be lost by evaporation, migration, or extraction. There is often a marked temperature dependence of properties for both types of plasticizers but this is more pronounced in internal
On the basis of the compatibility of plasticizer, the external plasticizer can be classified further into two types: primary plasticizers and secondary plasticizers. Primary ones have sufficient compatibility with the polymer and interact directly with polymer chains providing desired modifying effect. On the other hand, secondary ones have limited compatibility and will exude from polymer if used alone. They are used as a part of a primary plasticizer system to meet secondary requirements i.e. cost, low-temperature properties, and permanence.
Commonly Used Plasticizers
The history of plasticizers goes back to 1912 when triphenyl was discovered and used as additives. It replaced camphor and castor oil which were used for plasticization purposes in the early days; they were unsatisfactory for many ends uses. Tricresyl phosphate was also an important early discovery that is still in use today. Tributyl phosphate and glycerin acetate were also used as plasticizers but faded soon due to their volatility.
In 1920 phthalic acid esters found application as a plasticizer and remained as the largest class in the 21st century. Dibutyl phthalate (DBP) and Di (2-Ethylhexyl) phthalate are some of the materials used since 1930. But then food legislation, health, and safety requirements played an important part in the selection of materials for plasticizers. Initially, a few fatty acid esters, benzoates, tartrates, and chlorinated hydrocarbons were available to meet the new safety requirements.
The most commonly used plasticizers worldwide are esters of phthalic acid and they account for 92% of the plasticizer industry. The reason behind their extensive use is the presence of desired properties in plasticizers i.e. good fusion properties, nonvolatility at ambient conditions, high elasticity, and low cost.
But certain issues are also faced by phthalates. The shorter chain length phthalates are easier to formulate as they can defuse faster but they are nonvolatile. The branched phthalates are not very volatile but they increase the viscosity and reduce the plasticization effect. Certain health-related issues of phthalates have also been found by recent studies. Their use has been criticized in medical plastic bags and dialysis tubing as leach out and can cause endocrine disruption.
Phosphates are another class of them used in polyvinyl chloride. Their flame retardant properties make them primary plasticizers for such applications. Trimellitates, paraffinic sulfonic acid and phenyl esters, polyesters, chlorinated hydrocarbons, aliphatic/aromatic monocarboxylic acid esters such as benzoates, and a variety of elastomers have also been used as plasticizer for years.
The use of polymers and plastics in specialty applications has evolved the plasticizer industry over past few decades. Many novel plasticizers have been manufactured for special applications i.e. citric acid esters, oligomers and polymeric plasticizers, epoxidized soybean oil, PVC/EVA (ethylene vinyl acetate) graft polymers and terpolymers.
Plasticizer – related Challenges
Despite of all good properties possessed by material used as plasticizer there are many technical challenges related to them. These challenges are:
- Migration out of polymer:
- Solid–solid migration
- Liquid leaching
High temperatureflexibility Low temperaturelubricity
- Health and environmental effects
- Flammability concern
- Compatibility with new polymers
- Stability to ultraviolet rays
- Improved material lifetime
Several approaches have been used to meet the challenges related to plastic additives. Surface modification and cross-linking of plasticizer
In the present work, the approach we have used to produce a plasticization effect in aramids to enhance their processability is the incorporation of hyperbranched polyamide ester. As mentioned in the previous section, hyperbranched polymers are highly branched and multifunctional polymers. Due to the high level of branching, hyperbranched polymers have excellent solubility and low viscosity. They possess almost all desired properties required in a plastic additive. The branched structure permits to design a thermally stable plasticizer by incorporating bulky functional groups. Hence, they can be used as plasticizers for aramids to enhance their processability without deteriorating other properties.
You can study about Plasticizers from Handbook of Plasticizers.