Plasticizers and Plasticization Methods

Plasticizers are relatively non-volatile organic substances (mainly liquids). When incorporated into a plastic or elastomer, they help improve the polymer’s:

• Flexibility
• Extensibility and,
• Processability

Plasticizers increase the flow and thermoplasticity of a polymer by decreasing the viscosity of the polymer melt, the glass transition temperature (Tg), the melting temperature (Tm) and the elastic modulus of the finished product without altering the fundamental chemical character of the plasticized material.

Use of Plasticizers

Plasticizers are among the most widely used additives in the plastic industry. They are also usually cheaper than other additives used in polymer processing.

Plasticizers are most often used in PVC, the third largest polymer by volume after PP and PE. In turn, PVC is used in a wide range of products. Examples include:

• Unplasticized PVC (or rigid PVC) is used in applications such as pipes, siding, and window profiles.
• Plasticized PVC (or flexible PVC) finds applications in automotive interior trim, cables, PVC films, flooring, roofing and wall coverings, etc.

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However, a significant amount of plasticizers are also used in polymers like acrylics, PET, polyolefins, polyurethanes, etc. Plasticizers are also sometimes used in rubbers but in these cases they are used as extenders.

Continue reading and click to explore more about:

• Plasticization Methods
• Processing with Plasticizers
• Loss of Plasticizers Plasticizer Exudation
• Classification of Plasticizers
• Applications of Plasticizers

Plasticization Methods

There are two main principal methods exist for plasticization:

1. 1. Internal Plasticization
      A polymer can be internally plasticized by chemically modifying the polymer or monomer so that the flexibility is increased. It involves copolymerization of the monomers of the desired polymer (having high Tg) and that of the plasticizer (having low Tg) so that the plasticizer is an integral part of the polymer chain. The most widely used internal plasticizer monomers are:
      • Vinyl acetate
      • Vinylidene chloride
      But this technique is limited: every copolymer is only suited to certain flexibility requirements 

      Also, the complexity of the reaction can lead to longer reaction times and increased costs. Internally plasticized materials show temperature dependence and dimensional instability at high temperatures.

1. External Plasticization
   This is the most commonly used method of plasticization because low cost liquid plasticizers give the formulator freedom in developing formulations for a range of products (from semi-rigid to highly flexible depending on the quantity). The most widely used external plasticizers include esters formed from the reaction of acids or acid anhydrides with alcohols. There are two main groups of external plasticizers:
   • • A primary plasticizer enhances elongation, softness and flexibility of polymer. They are highly compatible with polymers and can be added in large quantities. For example: up to 50% of vinyl gloves are made up of plasticizers, which make the PVC flexible and soft enough to wear.
   • • A secondary plasticizer is one that typically cannot be used as the sole plasticizer in a plasticized polymer. Secondary plasticizers may have limited compatibility with the polymer and/or high volatility. They may or may not contain functional groups which allow them to solvate the polymer at processing temperatures. Secondary plasticizers are variously used for:
       • Cost reduction
       • Viscosity reduction
       • Solvency enhancement
       • Surface lubricity augmentation, and
       • Low temperature property improvement
   • Extenders are a subset of secondary plasticizers. They are commonly employed with primary plasticizers to reduce costs in general purpose flexible PVC. They are mostly low cost oils having limited compatibility in PVC. They are added to reduce cost and in some cases to improve fire resistance. Examples of extenders include naphthenic hydrocarbons, aliphatic hydrocarbons, chlorinated paraffins (fire resistance) and others.

Processing with Plasticizers

Suspension PVC (S-PVC) Process is the common method to manufacture PVC:

• PVC obtained in the form of particles with size 50-200 microns
• Lower flexible PVC formula costs
• PVC particles obtained are mixed with plasticizers & can be extruded in pellets which are further used for processing via extrusion, calendaring, injection molding…
• Processing equipment is typically very expensive

Incorporation of an external plasticizer in PVC polymer enhances its flexibility. Addition of plasticizer chiefly involves five distinct steps:

• Plasticizer mixed with resin
• Plasticizer penetrates and swells the resin particles
• Polar groups in the PVC resin are freed from each other
• Plasticizer polar groups interact with the polar groups on the PVC chain
• PVC structure is re-established Upon cooling, with full retention of plasticizer

Loss of Plasticizers Plasticizer Exudation

The incompatibility between polymer and plasticizer can cause exudation. There are several factors which can lead to migration of plasticizer out of plastics surface (or into or onto a substrate to which it is held in intimate contact) like temperature change, humidity change, mechanical stress, weathering, etc.

Loss of plasticizer can lead to less flexibility, embrittlement, and cracking.

Classification of Plasticizers

Plasticizers are commonly classified based on their chemical composition. It is possible to understand the influence of structural elements (e.g. different alcohols in a homologous series of phthalates, adipates, etc.) on the properties of plasticizers and their effect on base polymers.

Different plasticizers affect different physical and chemical properties of materials. Therefore, you need a particular plasticizer to change properties in a certain direction to meet requirements.

There are several general chemical families of plasticizers that are used for polymer modification. Among them, the most commonly used are:

1. 1. Phthalate Esters – They are produced by esterification of phthalic anhydride or phthalic acid obtained by the oxidation of orthoxylene or naphthalene. Most commonly used phthalate plasticizers include:
      1. DEHP: Low molecular weight ortho-phthalate. Still the world’s most widely used PVC plasticizer
      2. DINP, DIDP: High molecular weight ortho-phthalates

1. 1. Aliphatic dibasic acid Esters – These include chemicals such as glutarates, adipates, azelates and sebecates. They are made from aliphatic dibasic acids such as adipic acid and alcohols.

1. 1. Benzoate Esters – They are esterification products of benzoic acid and selected alcohols or diols.

1. 1. Trimellitate Esters – They are produced by esterification of trimellitic anhydride (TMA) and typically C8 – C10 alcohols

1. 1. Polyesters – They are formed by the reaction of many combinations of dicarboxylic acids and difunctional alcohols.

1. 1. Citrates – They are tetraesters, resulting from the reaction of one mole of citric acid with three moles of alcohol. Citric acid’s lone hydroxyl group is acetylated.

1. 1. Bio-based Plasticizers – They are based on epoxidized soybean oil (ESBO), epoxidized linseed oil (ELO), castor oil, palm oil, other vegetable oils, starches, sugars etc.

1. Others – Includes Phosphates, Chlorinated Paraffins, Alkyl Sulfonic Acid Esters and more

When added to polymerresin, these plasticizers provide following benefits:

• They make a product softer, improve flexibility
• The processing becomes possible or easier
• Plasticized products do not break easily at cold temperatures

Applications of Plasticizers

Over 90% of the plasticizers used in thermoplastic polymers are used in PVC. The plasticized polymer market and the plasticized PVC market are largely one and the same although some plasticizer is also used in acrylic polymers, polyurethanes, polystyrene even polyolefins.

Major end uses include:

 

	Film and Sheeting Products made from flexible PVC film and sheet include roofing membranes, geomembranes, upholstery, luggage, advertising signs, swimming pool liners and other.
	Flooring Flexible PVC flooring products include resilient sheet flooring, vinyl composition tile, luxury vinyl tile, vinyl backed carpet tile.
	Wire and Cable Flexible PVC is a good electrical insulator with good processability and a useful service temperature range hence it is the perfect material for electrical applications such as insulation and jacketing for electrical conductors, insulation for fiber optic cables.
	Coated Fabrics PVC synthetic coated fabrics offer weather-resistance, and have excellent strength and durability. Used in industries which support architecture, lifestyle, sports, advertising, defense, mining, food & agriculture, automobiles and transportation. Products include tarpaulins, tents outdoor furniture and others

 

Other Applications of Flexible PVC:

• Consumer goods – Apparel, footwear, packaging
• Medical – Blood bags, IV tubing, biohazard containment structures, other medical devices
• Non-PVC – Small amounts of PVC type plasticizers are used in other polymers including acrylics, polyurethanes, polystyrene

Most plasticized PVC products are durable goods, products with long service lives. Phthalates, because of their low volatilities, low water solubilities, good resistance to sunlight and temperature extremes, good compatibility with the PVC polymer and generally good resistance to biodegradation, are well suited for use in such products. Examples include:

• Flexible PVC roofing membranes
• Geomembranes
• Wire and cable insulation

Phthalate plasticizers have been very difficult to replace in these end uses. Likewise, because of their relatively high solvating strength for the PVC polymer and relatively low viscosities, phthalate plasticizers have been found to be more easily processed in flexible PVC compounds as compared to at least some of the phthalate replacements.

Note however, that nearly half of the 7 million tons of plasticizer used annually is DEHP and DEHP, a general purpose type plasticizer, can be replaced in many products.

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