Plasticization: Polymers and additives boosting flexibility

The term plasticization refers to the softening and increase in flexibility of a polymer. This change is due to the addition of specific additives, especially plasticizers.

 

Role of plasticizers in plasticization

A plasticizer is a non-volatile substance incorporated in a plastic or elastomer. It changes the thermal and mechanical properties of the material. The expected property changes in a plasticized plastic or elastomer are as follows:
 

• lowers rigidity/elastic modulus
• increases elongation to break
• increases toughness/impact strength
• may reduce the melt viscosity
• reduce the glass transition temperature (Tg)

Types of plasticization

Plasticization is classified into two main groups such as internal and external.

 

Internal plasticization

These components are inherent to the structure of the polymer molecule. For instance, a second monomer is added through copolymerization. This leads to reduced order within the polymer structure. So, it becomes more challenging for the chains to closely align. This results in the softening of the polymer, i.e., a decrease in Tg or modulus. Typically, the internal plasticizer is a monomer chosen for its polymer's favorable low-temperature properties.

Internally plasticized systems, comprising simple random copolymers intended for flexible plastic applications. They often exhibit a disappointingly narrow temperature range of use. This is because they undergo a more pronounced softening compared to analogous externally plasticized systems or polyblends.

Another form of internal plasticization involves introducing side chains. These include either substituents or grafted branches. This phenomenon is well-documented in polyacrylates and polymethacrylates. It reduces the forces between polymer chains due to the bulkiness of substituent groups.

Another example is the alkylation of polyamides. They result in elastic fibers by decreasing crystallinity and Tg through the reduction of intermolecular forces. These fibers serve as precursors to contemporary elastic fibers.

 

External plasticization

External plasticizers are significant in commercial uses. They provide more optimal property combinations. Compared to adding plasticizers during polymerization, they give manufacturers more flexibility in formulation.

External plasticizers have a low vapor pressure. They interact with the polymer at high temperatures without a chemical reaction. The interaction happens through their solvent or swelling ability.

It's essential to differentiate between solvent plasticizers and nonsolvent plasticizers.
 

• For amorphous polymer, any plasticizer acts as a solvent plasticizer. This means that under appropriate conditions, the polymer would dissolve in the plasticizer.
• For crystalline or semicrystalline polymers, certain compounds can penetrate both regions. So, they are called true plasticizers or primary plasticizers.
• If only amorphous areas are penetrated, the compound acts as a nonsolvent plasticizer. Also called a secondary plasticizer or softener. These softeners sometimes work as diluents for primary plasticizers.
   

Figure 1: Internal Plasticization (L) and External Plasticization (R)⁵

 

Several theories and explanations have been developed to observe the characteristics of the plasticization process. The mechanism of plasticizer interaction with polymer chains is explained through several theories.

 

Lubrication theory

The lubrication theory says: As the system is heated, plasticizer molecules diffuse into the polymer. This weakens forces between polymer chains.

Acting as shields, the molecules reduce interactive forces between polymers. This prevents rigid network formation. Lowered intermolecular or van der Waals forces along the polymer chains enhance flexibility, softness, and elongation.

 

Gel theory
 

According to the gel theory, plasticized polymer is seen as an intermediate state, neither solid nor liquid. It is held together by a 3D network of weak secondary bonding forces between the plasticizer and the polymer. These forces can be overcome by external stresses, allowing the plasticized polymer to flex, elongate, or compress.

 

Free volume theory

Free volume, measuring internal space within a polymer, affects flexibility. Increasing free volume provides more space for molecule or polymer chain movement. This is achieved by:
 

• modifying the polymer backbone such as adding side chains or end groups, or
• introducing small molecules with flexible end groups

The free volume theory combines aspects of both lubricity and gel theories of plasticization.
 

Figure 2: Schematic Representation of Plasticization Mechanism of Polymer with Plasticizer⁵

Mechanistic explanation

This approach focuses on the interactions between the plasticizer and resin macromolecules. It assumes plasticizer molecules are not permanently bound to the resin. They can self-associate and associate with the polymer at specific sites. Due to the weak nature of these interactions, there's a dynamic exchange process where one plasticizer is replaced by another. Different plasticizers yield distinct effects based on the strengths of their interactions.

 

Which polymer properties affect material flexibility?

Molecular structure
 

The arrangement of polymer chains influences a material's capacity to bend and deform. Longer and more flexible chains correlate with increased flexibility.

 

Inclusion of specific additives

Plasticizers play a pivotal role in this enhancement by:
 

• diminishing intermolecular forces between polymer chains,
• effectively lubricating the molecular structure, and
• facilitating smoother movement and deformation

Which plasticizer properties impact plasticization efficiency?
 

Compatibility with polymer

For a plasticizer to be effective, it must have two structural components.
 

• The polar part should bind reversibly with the polymer.
• The non-polar part adds free volume and contributes shielding effects at other polar sites on the polymer chain.
   

Maintaining a balance between the two parts is critical to prevent compatibility issues. The solubility parameter methods are useful tools for estimating plasticizer compatibility.

 

Molecular weight

There exists an optimum range of molecular weights for plasticizer efficiency. Lower Mw plasticizers have a noticeable plasticization effect than their higher counterparts.

 

Polarity

Good compatibility requires matching polar plasticizers with polymers that have polar groups. The distance of the polymer's polar groups also influences the plasticizer polarity needed. If the forces between plasticizer molecules exceed plasticizer-polymer interactions, no plasticization happens. For example, as observed with glycerol and PVC.

Polar groups on flexible aliphatic chains also improve plasticization than stiff aromatic chains. The increased mobility of aliphatic groups enables more effective plasticization.

 

Molecular structure

Extended plasticizer molecules lower Tg effectively than bulkier molecules containing ring structures. For example, aliphatic chains have higher flexibility than aromatic chains.

 

Effect of glass transition temperature

• Concentration: In most systems, the reduction of Tg is directly proportional to the concentration of plasticizer in the polymer.
• Branching: Comparisons between branched and linear plasticizer molecules have been made. Linear molecules are more efficient in lowering Tg than their branched counterparts.
• Internal mobility (flexibility): This parameter plays a crucial role in determining plasticizer efficiency. This is true for various polymers unless polymer crystallinity is involved. The reduction of Tg is proportional to the temperature difference between the Tg of the polymer and plasticizer.

Limiting factors of plasticization of polymers

Plasticization improves the flexibility and processability of polymers. Certain factors can counteract or restrict this enhancement.

 

Incompatibility

Certain polymers may have limited compatibility with specific plasticizers, hindering their integration. They also reduce the desired plasticization effects.

 

Intermolecular interaction
 

The primary role of a plasticizer is to insert itself between polymer chains. But, the attractive forces between polymer molecules pose a significant challenge. The chemical and physical structure of the polymer influences these forces.

Considering the intermolecular forces between plasticizer and polymer is essential. The following interactions must be of similar magnitudes for effective plasticization:
 

• plasticizer-plasticizer,
• plasticizer-polymer, and
• polymer-polymer

Crystallinity

Plasticizer molecules face greater difficulty penetrating crystalline regions compared to amorphous regions. This is because minimal free space exists between polymer chains of crystalline polymers.

 

Volatility
 

Highly volatile plasticizers may evaporate over time. They lead to loss of plasticization and a decrease in material flexibility.

 

Plasticizer migration
 

Plasticizer migration refers to the undesired movement of a plasticizer outside a compound. The movement occurs through gas volatilization, liquid extraction, or solid migration. This phenomenon occurs when there is limited interaction between the polymer and the plasticizer.

Impact of plasticizer migration on product performance
 

• The primary purpose of a plasticizer is to enhance a product's flexibility. When plasticizers migrate out of the product, flexibility is compromised, leading to embrittlement.
• Plasticizer migration can trigger the migration of other additives (e.g., UV stabilizers, antioxidants, etc.). This results in the deterioration of the overall performance.
• Additionally, plasticizer migration may impact the material's long-term stability.
• Aesthetic effects can also manifest in various scenarios:
  • Fogging: Outgassing of plasticizers in automotive dashboards causes surrounding glass to fog up.
  • Paint issues: Migrating plasticizers can harm the finish and create problems during repainting.
  • Leaching: Pigment is carried away with migrating plasticizers. This causes it to 'leach' down the substrate.

How to prevent plasticizer migration?
 

1. Use specialist plasticizers: Those with high molecular weight or a high degree of branching impede movement.
2. Use reactive plasticizers: These chemically graft into the polymer matrix, making it challenging for the plasticizer to migrate.
3. Surface coating: Applying a coating prevents plasticizer migration from the surface.

Plasticizer extraction
 

Some applications involve contact with fluids or other materials. They may result in the extraction of plasticizers from the polymer matrix. This extraction process can diminish the plasticization effect over time. Hence understanding these elements is critical for optimizing the performance of plasticized materials.

 

There are criteria to consider besides fundamental requisites like:
 

• low volatility,
• temperature or light stability, and
• minimal or no odor

One of these, as mentioned earlier, is intermolecular forces. Further essential considerations are outlined below.

 

Solvent power
 

The plasticizer must have a high solvent power for the polymer, particularly in crystalline polymers.
 

• A solvent-type plasticizer is necessary to penetrate both ordered and disordered regions.
• A nonsolvent plasticizer (softener) can only enter amorphous regions.
   

When a low molecular weight compound penetrates crystalline regions, properties may deteriorate. For example, properties dependent on crystallinity such as tensile strength and modulus. In cases where these properties are crucial, using only a secondary plasticizer or softener might be more helpful.

 

Compatibility

The plasticizer should show polymer compatibility across both processing and usage temperature ranges. Exposure to common substances or conditions (like water, oil, oxygen, or sunlight...) should not disrupt the compatibility balance.

Factors influencing compatibility include:
 

• polarity,
• molecular weight, and
• shape of the plasticizer

Efficiency

Plasticizer efficiency relates a desirable modification of a product's properties to the amount of plasticizer required for this effect. For instance, the efficiency of various plasticizers in "plasticizing" a given polymer may be expressed in terms of the depression of the glass temperature by a given mole or volume fraction of the plasticizer.

Plasticizer efficiency depends on factors such as:
 

• size,
• molecular weight, and
• rate of diffusion in the polymer matrix

The higher the diffusion rate, the greater the efficiency. But this can lead to higher volatility.

 

Permanence

The permanence of a plasticizer means its tendency to remain in the plasticized material. It is influenced by:
 

• the size of the plasticizer molecule and
• its rate of diffusion in the polymer

Larger molecules with lower vapor pressure and volatility tend to exhibit greater permanence. This explains the popularity of certain polymeric plasticizers, despite their higher cost. Factors like polarity and hydrogen bonding also affect the plasticizer's vapor pressure. However, there is often a trade-off since a high diffusion rate, enhancing plasticizer efficiency, results in lower permanence.

Selecting a plasticizer usually involves a compromise, as meeting all the above requirements is often challenging. This trade-off is illustrated in Figure 3.

 

Figure 3: Schematic Representation of Relationships Between Three Important Properties of the Plasticizer: Compatibility, Efficiency, and Permanence¹

Incorporating plasticizers into polymers can influence diverse material characteristics. They improve their adaptability and performance across various applications. The plasticizer's type and concentration can alter the properties of the final flexible product.

The selection of a plasticizer involves finding a balance between:
 

• meeting performance criteria and
• controlling product costs

Table 1 outlines key properties to consider in vinyl plastics plasticized with Diisononyl phthalate (DINP) plasticizer.
 

Hardness

Plasticizer efficiency denotes a plasticizer's capacity to impart softness to the product. It is quantified as a ratio of the slope in the hardness-versus-plasticizer-concentration relationship. This correlation, expressed in phr (parts per hundred resin), is depicted in Figure 4.
 

Figure 4: Hardness Versus Plasticizer Concentration in PVC²

Several types of plasticizers mentioned in the graph include:
 

• Di-2-ethylhexyl phthalate (DOP),
• Diisodecyl phthalate (DIDP),
• Ditridecyl phthalate (DTDP),
• Tris(2-ethylhexyl) trimellitate (TOTM), and
• Di-2-ethylhexyl adipate (DOA)

In a specific group of esters sharing a common acid group, the plasticizer efficiency rises when the molecular weight of the plasticizer decreases in PVC. Additionally, enhanced plasticizer efficiency is observed with a more linear alcohol chain.

 

Low-temperature flexibility

Incorporating a plasticizer into a PVC product extends the lower limit of its useful temperature range. The factors influencing the low-temperature characteristics of a polymer include:
 

• level and the type of plasticizer
• concentration of the plasticizer

Although the effectiveness varies among different plasticizer chemistries. The improved low-temperature flexibility depresses the glass transition temperature (Tg) of the polymer.Low-polarity plasticizers without aromatic moieties provide more rotational freedom vs. their higher-polarity counterparts of similar molecular weights.

Aliphatic diesters of adipic, azelaic, and sebacic acids prove highly effective. The alcohol part of the esters contributes to greater linearity in the plasticizer. This correlates with superior low-temperature performance. Figure 5 illustrates the low-temperature flexibility (Tf) for PVC plasticized with various esters.
 

Figure 5: Low-temperature Effects in Plasticized PVC²

Flame retardancy
 

The inclusion of plasticizers like phthalates, adipates, and trimellitates can impact flammability.

Certain plasticizer families serve to inhibit the burning of plasticized PVC. These include brominated phthalate plasticizers, triaryl, and alkyl aryl phosphates. These products are blended with other plasticizers to strike a balance between flame resistance, physical properties, and cost.

Chlorinated paraffins, acting as secondary plasticizers reduce flammability and smoke. Their high volatility and compatibility issues limit their use to specific end-uses.

You can also achieve flame resistance by incorporating additives into flexible PVC formulations. These additives include antimony trioxide, alumina trihydrate, molybdenum ammonium octanoate, and zinc borate.
 

Color

Plasticized plastics maintain their color, as most commercial-grade plasticizers are colorless ("water white"). Avoid using colored plasticizers to prevent undesirable coloration in flexible PVC compositions. Select 30+ plasticizers offering color stability in our database.

 

Durability
 

Plasticizers prevent the development of microcracks in the polymer matrix. They thereby enhance material durability and resistance to mechanical stress. Well-chosen plasticizers can also improve resistance to UV exposure and chemical attack. This contributes to the long-term stability of the polymer.

 

Dielectric properties

In specific applications, plasticizers can influence the dielectric properties of polymers. This makes them suitable for use in electrical insulating materials.

 

Compatibility
 

Plasticizers can enhance compatibility between polymers and various additives or fillers. This results in more homogeneous and stable formulations.


Browse our extensive database to select plasticizers compatible with various polymers:
 

Toxicity

Plasticizers such as phthalates can leach out and dissolve into fluids. They can come into contact with skin, raising concerns about potential health effects.⁴

 

Environment
 

The release of plasticizers into the environment can occur during various stages. These include manufacture, distribution, fabrication, product use, and disposal. Most releases occur during the process of incorporating plasticizers into the PVC polymer.⁴

The future of plasticizers hinges on the availability of cost-effective options without unacceptable health and environmental risks.⁴