Phenomena of Cross-linking in Polymers

Posted on April 12, 2020 by Ahmed Affan Sheikh | 160 Views

Introduction

In this blog, we delve into polymer crosslinking, a crucial process behind many specialty polymers in the chemical market. Previously, we explored polymer molecular weights and how they influence material properties. While this remains fundamental, high-performance applications require further crosslinking to enhance durability, strength, and other key characteristics.

Cross-links betiveen Polymer chains A B ant C

The phenometsa of cross-linking is the interaction and reaction between two or more polymer chains, resulting in creation of chemical bonds between them, leading to the creation of a unique final polymer material with a 3D network structure. This structure can be considerett a single interconnected mega molecule with an immensely high molecular weighof

Cross-linking Triggers

The methods to trigger cross-linking are diverse, ranging from heat and radiation to the introduction of catalysts and crosslinking agents. Yet, the outcome remains the same a network of chemical bonds creating a new polymer architecture.

Crosslinking can take place between chains of the base polymer in case of heat, radiation, or catalyst as a trigger. However cross-linking more commonly takes place between chains of base polymer and cross-linker, where again heat, radiation, or catalyst can be incorporated as additional triggers.

Cross-linkers are special chemicals that contain groups capable of bonding with the chemical groups in the base polymer. These cross-linkers come in various molecular structures and chemistries, from small molecules to large polymers. What makes cross-linking fascinating is that, with the vast array of molecular structures and weights found in base polymers, there are countless ways to combine them with cross-linkers, leading to a wide range of potential product characteristics in different polymer categories.

Polymer Behavior During Crosslinking

As a cross-linking trigger is introduced, the polymer system is left with a certain ‘pot life’ or ‘open time’. This is effectively the maximum time allowed for the user to use the chemical mix before its properties start to irreversibly change. This pot life can range from several months in 1K systems, to a few minutes to a few days in multi- component systems. As the pot life of a polymer mix ends, it undergoes the following changes:

Increased Viscosity: As the polymer chains crosslink and form a network, the material’s viscosity often increases, making it less fluid and more solid-like.
Heat Generation: In some curing processes, heat is generated as a byproduct of the chemical reactions. This heat can be used to facilitate further crosslinking and curing.
Physical Changes: The polymer material may undergo physical changes, such as becoming more rigid, less soluble in solvents, and exhibiting improved dimensional stability.
Irreversible Process: Crosslinking and curing are typically irreversible processes. Once the covalent bonds are formed, it is challenging to revert the material to its original state.

Degree of Cross-linking

The degree of cross-linking in polymers refers to the extent or density of crosslinks within a polymer network

The degree of cross-linking can vary widely in polymers based on the crosslinking temperature and the time given at that temperature. The degree of cross-linking is typically expressed as a percentage of available bonding sites that have undergone crosslinking.

In multi-component systems, at room temperature, approximately 70-90% degree of cross-linking is achieved. The ‘cure time’ for multi-component systems tells how much time will it take for the chemical mix to achieve the maximum degree of cross- linking at that temperature.

However, as stated earlier that heat is also a trigger, if the cross-linking reaction is allowed to take place for some time at high temperatures, even 100% degree of cross-linking can be achieved.

Stoichiometric Ratios of Base Polymers and Crosslinkers in multi-component Systems

In a multi-component system the number of base resins and cross-linkers can exceed one. In the process of crosslinking polymers in multi-component systems, achieving precise control over the stoichiometric ratios of the base polymers and crosslinking agents is crucial. The stoichiometric ratio defines the proportion in which these components should be mixed to ensure effective crosslinking and to avoid the consequences of imbalanced formulations. To determine the appropriate stoichiometric ratios, a deep understanding of the molecular structure and molecular weight of both the base polymers and the crosslinkers is essential

.Molecular Structure: The molecular structure of the base polymers and crosslinkers determine the availability of active sites for crosslinking reactions. Active sites are specific chemical groups or functional moieties that are capable of forming covalent bonds with other molecules. It is imperative to identify and quantify these active sites in both the base polymers and crosslinkers

Molecular Weight: The molecular weight of each component provides information about the number of repeating units or monomers in a polymer chain. This information is critical for calculating the number of active sites per unit weight in each component

Calculating Active Sites: Once the number of active sites in both the base polymers and crosslinkers is known, it becomes possible to determine the stoichiometic ratio required for effective crosslinking. This is achieved by ensuring that an equal number of active sites in the base polymers and crosslinkers are available for crosslinking reactions. In industrial terms, the stoichiometric ratio is typically expressed as the mass ratio, which specifies how many grams of one component should react with a certain number of grams of the other.

If the number of active sties is very low, the resulting cross-linked network is flexible or elastomeric, while it the number of active sites is high, the resulting cross-linked network is very compact and rigid.

Elastomer

Highly cross-linked thermoset

Low cross-link density elastomer vs high cross-link density thermusel

Importance of Accurate Stoichiometric Ratios: An inaccurate stoichiometric ratio can lead to two poteritial scenarios: excess base polymer chains or excess cross- linker chains.

If an excess of any of these two is present, not all the active sites available for reaction will be utilized for crosslinking. This can result in the formation of unreacted or dangling base polymer chains or cross-linker chains within the crosslinked matnx These unreacted chains may negatively impact the final properties of the crosslinked polymer, such as mechanical strength, thermal stability, and chemical resistance.

Balancing the stoichiometric ratio is, therefore, essential to achieve the desired properties in the crosslinked polymer Precise control over the ratio ensures that all available active sites are utilized for crosslinking, leading to a well-defined and optimized crosslinked structure with the desired combination of properties.

Advantages of Cross-linking

Crosslinking in polymers offers several advantages that make it a crucial process in various industrial applications.

Improved Mechanical Strength: Crosslinked polymers typically have higher tensile strength, flexural strength, and resistance to deformation compared to their non-crosslinked counterparts. This makes them suitable for applications requiring durability and load-bearing capacity.
Improved Adhesive Properties: Crosslinked adhesives often have better bonding strength and adhesive properties. They can adhere more strongly to various substrates, making them valuable in bonding arid sealing applications.
Enhanced Thermal Stability: Crosslinked polymers exhibit improved resistance to high temperatures. They have higher glass transition temperatures (Tg) and can withstand elevated temperatures without softening or deforming. This makes them suitable for applications in high-temperature environments.
Chemical Resistance: Crosslinked polymers are often more resistant to chemical attack (acids, bases, solvents) and degradation. The three- dimensional network structure created by crosslinking helps protect the polymer chains from chemical penetration, making them suitable for use in corrosive or chemically aggressive environments.
Improved Electrical Properties: Some crosslinked polymers exhibit excellent. electrical insulation properties, making them suitable for use in electrical and electronic applications where high dielectric strength is required.
Controlled Release: in drug delivery systems and controlled-release applications crosslinking can be used to control the release rate of substances encapsulated within the polymer matrix.
Reduced Environmental Impact: Crosslinked polymers can be more environmentally friendly in certain applications. For example, they may resist microbial degradation and remain stable for longer periods, reducing the need for frequent replacement or maintenance.

Generally, in all the above case, the higher the degree of cross-linking, the better the mentioned property.

The degree of crosslinking can be adjusted to tailor the polymer’s properties to specific application requirements. This flexibility allows engineers and material scientists to design materials with precise characteristics

Relevant Industrial Examples

Crosslinked Amino Resin: Used in the production of melamine kitchen-ware as well as melamine foam, a versatile maternal known for its excellent sound- absorbing and cleaning properties.
Crosslinked Epoxy Resin: Used in high-performance adhesives, coatings. composites, and castings due to its exceptional strength and thermal stability.
Crosslinked Phenolic Resin: Also known as Bakelite, it is utilized in high- temperature applications, such as brake pads and insulation materials, due to its superior thermal stability.
Crosslinked Polyethylene (PEX): Crosslinking enhances the chemical and thermal resistance of polyethylene, making it suitable for hot and cold water pipes and radiant heating systems.
Crosslinked Polyethylene Terephthalate (PETXL): PETXL is used in beverage bottles that require higher temperature resistance and enhanced durability.
Crosslinked Polypropylene (XLPP): XLPP pipes and fittings are employed in plumbing systems due to their improved chemical resistance and strength.
Crosslinked Polyurethane: Used in automotive parts, durable foams, flexible molds, and high performance coatings and adhesives due to its enhanced toughness and resistance to wear and tear.
Crosslinked Polyvinyl Acetate (PVACXL): PVACXL is used in woodworking adhesives and coatings, offering superior water resistance and adhesive strength.
Crosslinked Polyvinyl Chloride (CPVC): CPVC pipes are suitable for hot water distribution systems and industrial applications due to improved chemical resistance and elevated temperature tolerance.
Crosslinked Silicone Rubber: Found in automotive seals, gaskets, and medical devices due to its excellent flexibility and resistance to extreme temperatures.
Crosslinked Styrene Butadiene Rubber (XSBR): XSBR is employed in tire manufacturing for its enhanced wear resistance and durability.
Crosslinked Unsaturated Polyester: Used in the construction of boat hulls and automotive parts, providing excellent strength and resistance to environmental stressors.

Conclusion

Crosslinking in polymers plays a pivotal role in tailoring material properties for a wide array of industrial applications. This versatile process enables the creation of materials with Improved mechanical strength, enhanced thermal stability, superior adhesive properties, and resistance to various environmental factors. The choice of base polymer and cross-linker, additional cross-linking triggers, degree of crosslinking, and stoichiometric ratios are all critical parameters that allow engineers and scientists to precisely design materials to meet specific application requirements in diverse industries.

As industries evolve and new challenges arise, the art and science of crosslinking will continue to be a cornerstone of materials engineering. By carefully controlling all parameters in a cross-linked polymer system, we unlock the potential to create materials that meet the stringent demands of modern technology and design, all while contributing to sustainability and efficiency. In a world where materials are the buliding blocks of progress, crosslinking remains a powerful tool to shape a better future.