How to Enhance Polymer Properties Through Crosslinking?

Do you know what makes crosslinked polymers an edge above linear polymers?

Linear polymers have long chains held by weaker van der Waals force or hydrogen bonding. They are thermoplastic because heat can break the bonds between the polymer chains. This in turn allows the chains to flow past each other so that the material is remolded.

Crosslinking involves the formation of covalent bonding between adjacent polymer backbone. It creates an interconnected three-dimensional structure (Figure 1). The chemical bonds in the crosslinked polymer are stronger making them thermosetting in nature.

The key difference in linear and crosslinked polymers leads to variations in:

• Mechanical properties,
• Thermal properties, and
• Chemical properties

This makes crosslinked polymers helpful in scenarios where high strength and stability are paramount. Figure 1 below shows a structure of linear and crosslinked polymers.



Figure 1: Schematic Representation of Linear and Crosslinked Polymer
Read on to learn more about:

1. Types of Crosslinking
2. Effects of Crosslinking on Polymer Properties
3. Applications of Crosslinked Polymers
4. Advantages and Disadvantages of Crosslinking
5. Future Trends in Crosslinking Technology

Types of Crosslinking

There are different types of crosslinking in polymers. They can be broadly categorized into three types:

Chemical crosslinking

Chemical crosslinking involves the formation of covalent bonds between polymer chains. This creates a permanent 3D network. The mechanism relies on chemical reactions like:

• condensation,
• addition, or
• free radical polymerization

Figure 2: Schematic Representation of Chemical Crosslinking Forming Covalent Bonds¹

This process can be initiated by heat, catalysts, or other chemical agents like:

→ Peroxides: Benzoyl peroxide, dicumyl peroxide.

→ Diisocyanates: Hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI).

→ Epoxy resins: Diglycidyl ether of bisphenol A (DGEBA), triglycidyl isocyanurate (TGIC).

→ Sulfur: Used in rubber vulcanization to form a bridge between polymer chains.

→ Platinum catalysts: Used in silicone crosslinking by the addition-cure method.

Physical crosslinking

Physical crosslinking relies on non-covalent interactions like:

• hydrogen bonding,
• van der Waals forces, or
• crystalline domains

Unlike chemical crosslinking, physical crosslinks are reversible. It is dependent on external factors like temperature, pressure, or solvent conditions. This method is used where flexibility and responsiveness to external stimuli are required.


Figure 3: Schematic Representation of Physical Crosslinking¹

Examples of physical crosslinking include:

• Physical entanglements: Long chains become entangled in high molecular weight polyethylene. This leads to increased viscosity and improved mechanical properties.


• Hydrogen bonding agents: Polyvinyl alcohol (PVA) forms hydrogen bonds.


• Crystalline structures: Polyethylene with crystalline regions.


• Ionic interactions: Ionic interactions contribute to physical crosslinking in polyelectrolytes. For example, in sodium polyacrylate.


• Self-assembly and supramolecular interactions: In supramolecular polymers, non-covalent interactions contribute to physical crosslinking. For example, π-π stacking or host-guest interactions.

Radiation-induced crosslinking

Radiation-induced crosslinking is done with high-energy radiations to induce crosslinking in polymers. For example, gamma rays, ultraviolet (UV) rays, and electron beams. The energy from radiation breaks existing bonds and generates reactive species. These form new crosslinks.


Figure 4: Radiation-induced Crosslinking of Polymers²

Examples of radiation-induced crosslinking polymers are:

Thermoplastics Elastomer Acrylic acrylate Silicone acrylate

Polyester acrylate Urethane acrylate Epoxy acrylate Polyether acrylate

Understanding crosslinking mechanisms is crucial for tailoring polymer properties. The method chosen depends on the desired properties and applications.


Explore our extensive database to select suitable crosslinking agents:

Effects of Crosslinking on Polymer Properties

Crosslinking influences the properties of polymers. The controlled introduction of crosslinks enables fine-tuning of polymer properties. It helps to achieve desired performance characteristics such as:

• improved mechanical strength,
• enhanced thermal stability,
• increased chemical resistance,
• modified solubility, and
• processing characteristics

Here's how crosslinking impacts each of these properties:

Mechanical properties

Generally, crosslinking enhances the tensile strength and toughness of polymers. This is due to the creation of a 3D network that resists deformation under stress. The modulus or stiffness of crosslinked polymers is generally increased. This provides greater structural integrity. This is in comparison to their non-crosslinked counterparts.

Crosslinked polymers tend to be harder due to the reinforcement of the polymer matrix. This makes them more resistant to abrasion and wear. Some levels of crosslinking increase stiffness. Whereas, moderate crosslinking can maintain or improve elasticity. This depends on the specific polymer and crosslinking method.

Select from 200+ crosslinking agents that show good mechanical/physical performance

Thermal properties

Melting point

Crosslinked polymers have higher melting points compared to their linear counterparts. The 3D network restricts molecular motion. It requires more energy to break intermolecular bonds.

Glass transition temperature (Tg)

Crosslinking can raise the Tg. This indicates an increased thermal stability and a more rigid amorphous phase in the polymer.

• At low degrees of cross-linking the shift in Tg is very small.
• At high degrees of cross-linking, the shift is very large. It is sensitive to small changes in the number-average molecular weight of polymer between cross-linked points.

In most systems, the shift in Tg is not independent of the chemical composition of the polymer. The chemical composition of the polymer gradually changes. This is due to the increased incorporation of crosslinking agents into the network structure.

Select from 80+ crosslinking agents in our database that exhibit thermal resistance

Chemical properties

Crosslinked polymers generally exhibit enhanced chemical resistance. This is due to the reduced mobility of polymer chains and decreased susceptibility to chemical attack. The 3D network formed by crosslinking provides a barrier. This is against the diffusion of solvents and chemicals into the polymer matrix.

A good rule of thumb for resins is:

• Polyester and epoxy resins - They have resistance to the chemicals with a pH between 1 and 11 scale.
• Vinyl ester resins - They resist degradation of pH generally from 8 to 14.

Additionally, any chemical rule of thumb with petroleum-based solvents may create an issue. Various crosslinked resins show different chemical properties which are detailed below.

Let’s explore how different crosslinked polymers exhibit chemical resistance.

Vinyl ester resins

Vinyl ester resins offer excellent resistance to acids, alkalis, hypochlorites, and many solvents. Novolac and other high molecular-weight vinyl ester resins offer:

• better stability in high temperatures and
• improved resistance to sulfuric acid

Vinyl esters also resist a wide pH range and high temperatures.³

Isophthalic polyesters

Isophthalic polyesters have poorer chemical resistance than vinyl esters. They have more chemical resistance than polyesters. They are less expensive than vinyl esters and other polyesters. They provide adequate performance in a specific exposure environment.

Chlorinated polyesters

Chlorinated polyesters are the best choice for strong chromic acid. They are not good in alkaline conditions but excel in strong mineral and oxidizing acids.

Bisphenol A fumarate resins

Bisphenol A fumarate resins offer broad resistance, such as in strong alkaline environments.

Select from 60+ crosslinking agents in our database that exhibit chemical resistance

Solubility

Crosslinked polymers are insoluble or have reduced solubility compared to their linear counterparts. The extensive network limits the ability of solvent molecules to penetrate. Thus they can not break polymer-polymer interactions.

In certain cases, partial crosslinking may result in a material that is soluble in specific solvents. This allows controlled swelling or dissolution. For example, less cross-linked rubbers could swell. Whereas, highly cross-linked rubbers and thermosetting materials may not swell in any solvent.

Processing characteristics

Viscosity

The viscosity of crosslinked polymer melts is higher. This is because the molecular weight increases with broad molecular weight distribution. Crosslinked thermoplastics can lead to a drastic reduction in melt flow index (MFI). This impacts the ease of processing during manufacturing processes.

Figure 5 presents the effect of dicumyl peroxide (DCP) crosslinker concentration on the MFI of the polyolefin. It can be observed that the MFI values decrease as the DCP concentration increases. The decrease in MFI values is because of crosslinking. This extends the molecular chains and decreases the mobility of the chains.

Figure 5: Effect of Dicumyl Peroxide (DCP) Content on Melt Flow Index (MFI) of Polyolefin⁴

Melt stability

Crosslinked polymers may exhibit improved melt stability. They can resist degradation or flow during processing at elevated temperatures. The crosslinked polymers are more thermally stable than linear polymers.

Furthermore, the cross-linked biopolymers have a broader molecular weight distribution. Thus, they show improved melt strength. This allows faster processing on industrial extrusion or foaming lines. They exhibit minimal thermal degradation.

Curing time

In the case of thermosetting polymers, the crosslinking process is a crucial step in the manufacturing process. It influences the curing time and conditions required for shaping the final product.

Figure 6 shows how various properties of a melamine resin (a typical thermosetting polymer) change with the degree of cure. The strength and heat-distortion temperature increase with crosslinking but the modulus remains constant. These two factors tend to counteract one another.

Thus, the tensile strength can increase with cross-linking if the flaw size decreases at a faster rate than the surface-energy term. The strength and toughness of a material are increased by an increase in the surface area generated during fracture. Thus, rough fracture surfaces and many cracks rather than a single fracture crack may be desirable.


Figure 6: Properties of an Unfilled Melamine Resin as a Function of Curing Time and Extent of Crosslinking Reaction⁵
In the above figure, L_m is a function of the degree of reaction during the curing process. Thus, crosslinking increases with curing time.


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Applications of Crosslinked Polymers

The crosslinked polymers find applications in rubbers, elastomers, and thermosetting plastics. This shows how crosslinking can adapt materials to meet specific performance needs. The crosslinked structures enhance mechanical strength, durability, and resistance to environmental factors. Thus, making them indispensable in various industries.

Rubbers

The crosslinked structure of rubber is achieved by vulcanization. It improves the mechanical properties of tires. It enhances their tensile strength, hardness, and wear resistance. Thus, providing longevity and improved performance on road surfaces.

The crosslinked network ensures that the rubber maintains shape and elasticity even under challenging conditions. This contributes to better traction and durability.


Select the commercial grades of crosslinking agents for various types of rubbers available in our database:

Elastomers

Crosslinked elastomers are used in hoses and seals due to their ability to withstand dynamic conditions. The crosslinked structure maintains flexibility and resilience. This makes them suitable for applications that involve repeated compression and expansion.

They also offer chemical resistance and prevent degradation over time. This ensures the longevity of hoses and seals in various environments.

Thermosets

Epoxies

Crosslinked epoxy resins are essential in adhesives, coatings, and composites. The 3D crosslinked network provides superior:

• mechanical strength,
• chemical resistance, and
• dimensional stability

Epoxies are used in the aerospace and construction industries. Here, their durability, adhesion, and resistance to various environmental factors are crucial.

Polyurethanes

Crosslinked polyurethanes find applications in diverse areas. For example, foams, coatings, and flexible/rigid plastics. The crosslinked structure allows for tailoring the material's properties.

They provide flexibility, impact resistance, and hardness. Polyurethanes are used in furniture, automotive interiors, and insulation materials.

Unsaturated polyesters and vinyl ester

The applications of unsaturated polyester resin are varied. The polyester resins represent one of the absolute compounds used in a wide range of industries. The most important applications are:

• fiber-reinforced composites and polymer concrete
• buttons,
• artificial marble, and gel coats

The main areas of polyester consumption include wind energy, marine, pipe and tank, transportation, and construction industries.

Phenolics

Phenolic resins are good for high-temperature applications. They are used in parts that must meet fire safety standards. Phenolic resins are used in a wide range of applications. For example:

• electronics and ballistics
• offshore water pipe systems and mine ventilation
• aerospace, rail, and mass transit

Silicones

Crosslinked silicones are valued for their thermal stability, flexibility, and biocompatibility. They are used in medical applications. For example, in the manufacturing of implants and medical devices due to their inert nature and lack of toxicity.

In electronics, their thermal stability makes them suitable for high-temperature environments. The crosslinked structure ensures that silicones maintain their properties under challenging conditions.


Select commercial grades of crosslinking agents suitable for various thermosets:

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Other crosslinked plastics

Bismaleimides

Bismaleimides have glass transition temperatures above 260°C. Their continuous-use temperature is 200–230°C.

They find applications in high-performance materials (where temperatures reach above 150–180°C). For example, structural composites and adhesives for aircraft, aerospace, and defense applications.

Cyanate esters

Cyanate esters have very good processing characteristics, high-temperature stability, and lower water absorption than epoxies and bismaleimides. They can be used as:

• composites for aircraft,
• encapsulants⁶ for microelectronics,
• printed circuit boards, etc.

Their low dielectric loss⁷ and water absorption are the main advantages concerning epoxies. However, these features come at a price.


Select the suitable crosslinking agents compatible with other polymers such as:

Advantages and Disadvantages of Polymer Crosslinking

Understanding the advantages and disadvantages of crosslinking is essential. This allows the selection of the appropriate polymer for specific applications. The choice depends on the properties and performance requirements of the final product. Table 1 summarizes the key advantages and disadvantages of crosslinking polymers.

Advantages	Disadvantages
Strength: Crosslinking enhances the tensile strength and mechanical properties of polymers. It creates a 3D network. This results in a material that is more robust and resistant to deformation under stress.	Brittleness: Crosslinked polymers are generally less flexible than their non-crosslinked counterparts. The rigid 3D network limits the movement of polymer chains. This results in reduced flexibility and ductility.
Thermal stability: Crosslinked polymers exhibit improved thermal stability and resistance to chemical degradation. The covalent bonds formed during crosslinking offer stability to the polymer structure. This makes it more durable in various environments.	Recycling challenge: Crosslinked polymers can pose challenges in recycling processes. The covalent bonds formed during crosslinking make it difficult to revert the material to its original state. Thus, limiting the options for recycling.
Chemical resistance: Crosslinked polymers have reduced solubility and enhanced resistance to solvents. The interconnected network of polymer chains restricts the penetration of solvent molecules. Thus, making the material more resistant to chemical dissolution.	Processing difficulties: The processing of some crosslinked polymers can be very challenging. Several factors contribute to increased manufacturing complexity. These include higher viscosity, longer curing times, and specific processing conditions.
Dimensional stability: Crosslinked polymers maintain their shape and dimensions under different temperature and pressure conditions. This dimensional stability is beneficial where a specific form is critical to maintain.	Low temperatures brittleness: Crosslinked polymers may exhibit increased brittleness at low temperatures. The rigid structure formed by crosslinking can lead to a reduction in impact resistance. This occurs in applications requiring flexibility at lower temperature ranges.
Durability: Crosslinked materials are known for their durability and longevity. The improved mechanical and chemical properties contribute to the robustness of the material. Thus, extending its lifespan in various applications.	Limited reversibility: The irreversible nature of many crosslinking reactions limits the reversibility of the process. Once crosslinked, it is challenging to modify or undo the crosslinked structure. Thus, restricting the material's adaptability in certain applications.

Table 1. Advantages and Disadvantages of Polymer Crosslinking

Future Trends in Crosslinking Technology

The emerging trends in crosslinking technology reflect a broader shift towards sustainability, precision engineering, and the development of materials with advanced functionalities.

As research progresses, advancements contribute to the creation of innovative materials. Such materials have improved performance and a reduced environmental footprint. The developments are focused on the:

• reduction of volatile organic compounds,
• replacement of hazardous poly(isocyanates) with noxious crosslinkers,
• acid rain resistance and scratch resistance in automotive topcoats,
• low-temperature cure for saving energy, and
• higher performance

Use of bio-based crosslinking agents

There is a rise in demand for the use of renewable or bio-based materials as crosslinking agents. Several materials are being explored for their ability to crosslink polymers sustainably. These include:

• Biopolymers,
• Natural extracts, or
• Bio-derived compounds

This trend aligns with the development of eco-friendly and biodegradable materials. This is in response to environmental concerns. Some biobased crosslinking agents are 2,5-Furandicarboxaldehyde, bio-based epoxy resin, bio-based isocyanates, gallic acid-based compounds, etc.

Crosslinking of water-based coating

In water-based systems, research in crosslinking technology has become active. This is as per the market growth of waterborne coatings. Compared to solvent-based coatings, waterborne coatings are easy to apply. They also offer high performance.

Recent developments in new crosslinking systems for water-based coatings are focused on:

• the ambient temperature cure of emulsion polymers or water-solubilized polymers
• the low-temperature cure for cathodic electrocoating

Self-healing crosslinked materials

Polymeric materials are susceptible to mechanical damage, such as degradation or crack damage. This results in loss of functionality and reduction of mechanical properties. Self-healing materials are designed to repair damage and extend the service life of the material.

Crosslinked polymers have dynamic covalent bonds or reversible interactions. This allows the healing of microcracks or damage over time. This technology has applications in various sectors. For example, coatings, adhesives, and structural materials. This contributes to increased durability and reduced maintenance.

Microfluidic-based crosslinking techniques

Microfluidic platforms are being employed in crosslinking processes. This offers precise control over reaction conditions. This in turn enables the production of microscale and nanoscale crosslinked structures. This advancement allows the creation of materials with:

• enhanced properties,
• improved homogeneity, and
• novel functionalities

Microfluidic-based crosslinking is relevant in several applications. For example, biomedical applications, drug delivery systems, and advanced materials design.

Light-induced crosslinking

The use of light as a trigger for crosslinking reactions is known as photopolymerization. This method has gained traction and offers several advantages:

• rapid processing,
• spatial and temporal control, and
• reduced energy consumption

Light-induced crosslinking is being explored for several applications. For example, in 3D printing, precise control over the crosslinking process is essential.

Smart crosslinking for responsive materials

Researchers are developing smart crosslinking strategies that respond to external stimuli. For example, temperature, pH, or mechanical stress. These responsive materials can undergo reversible changes in their crosslinked structure. This is based on environmental conditions that lead to adaptive properties. Applications include sensors, actuators, and materials with tunable properties.

Hybrid crosslinking approaches

Hybrid crosslinking approaches involve combining multiple crosslinking mechanisms or agents. This helps to achieve synergistic effects. For example, combining chemical and physical crosslinking or incorporating nanoparticles with crosslinking functionalities. These hybrid approaches offer enhanced control over material properties and multifunctionality.

Conclusion

Crosslinking is crucial as it offers a spectrum of options for materials design with its advantages and applications. The profound impact of crosslinking on polymer properties demonstrates its significance across industries. It enhances the durability of tires through vulcanization. It also ensures the stability of adhesives in aerospace.

Despite the benefits of crosslinking, it is important to recognize its challenges. There are a few limitations like brittleness, processing issues, and reversibility. These must be considered when choosing the right polymer for specific uses. Understanding this balance is vital for informed decision-making.

Looking towards the future, crosslinking technology is poised for exciting developments. The shift towards sustainability is reflected in the exploration of bio-based crosslinking agents. This aligns with the global push for eco-friendly materials. The dynamic nature of ongoing research is highlighted by the innovations in:

• water-based coatings,
• self-healing materials,
• microfluidic-based crosslinking techniques,
• light-induced crosslinking, and
• smart crosslinking for responsive materials

Hybrid crosslinking approaches combine different mechanisms or agents. They exemplify the multifaceted nature of future developments. These strategies aim to provide enhanced control over material properties. This paves the way for materials with advanced functionalities.

The future landscape of crosslinking technology is shaped by various measures. This includes emphasis on reducing environmental impact, improving precision engineering, and creating materials with superior performance.