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Cross-linking agents are also known as cross-linkers or hardeners. They are substances used to create chemical bridges between polymer chains. This process is known as cross-linking and it improves the durability and elasticity of the resultant material. Cross-linking agents are also quite common with adhesives, sealants, and coating materials.
Polyfunctional aziridine
Aziridine is a three-membered nitrogen-containing heterocycle ring that belongs to the epoxide family of compounds often used as a coating crosslinker. Aziridines are highly reactive because of the strain in the ring structure, allowing them to react with nucleophiles such as amines, alcohols, acids, and carboxyls. Polyfunctional aziridines, characterized by multiple aziridine rings in a single molecule, enhance cross-linking density and improve coatings' mechanical performance and chemical resistance properties. These aziridines are mainly from commercial sources such as resins that have been modified or from monomeric aziridines.
Carbodiimides
These compounds contain the unique coupling functional group -(N=C=O)2- where two isocyanate-like functional groups are connected by a NN bond. They react with acid and hydroxyl functional groups liberating CO2 and forming urethanes that polymerize and crosslink. They are mainly used in acrylic and polyurethane-based coatings to improve chemical resistance, heat, and water.
Polyfunctional anhydrides
They are originating from polycarboxylic acids by removing water and characterized by at least two anhydride functional groups in a single molecule. These anhydrides cure epoxy resins by opening the epoxy ring, linking the resin molecules and anhydride together, and forming a cross-linked network. These compounds improve the adhesives' mechanical properties and chemical resistance and reduce curing time and temperature.
Blocked isocyanates
They are engineering isocyanates (reactive functional groups) that have been blocked or trapped by reacting with a blocking agent comprising a hydroxyl-containing compound or a nucleophile. They are unblocked under process conditions (heat) to react with hydroxyl-terminated resins to form polyurethanes. They are used for delayed curing allowing flexibility during processing and application while preventing premature reaction.
Taurines
They are amino sulfonic acids containing a sulfonyl (SO2) group and an amino (NH2) within an inset alkyl side group such as β- or γ-aminopropionic acid. Taurines and their derivatives such as sulfocholic acids and cytosine are small biomolecules that serve as cross-linking agents and chelators during collagen crosslinking. They interact with collagen fibers to form covalent bonds and promote denser, stiffer, and less extensible fiber structure.
When selecting cross-linking agents for a particular application, kindly consider the following factors:
Compatibility
The crosslinking agent should be chemically compatible with the base polymer or resin system to ensure proper integration and reaction. For instance, in polyurethane coatings, the crosslinking agent must have -NCO and -OH functional groups that can react to form extended networks. Similarly, acrylic coatings use crosslinkers having multiple functional groups that react with hydroxyls such as MF and BF).
Functionality
Select crosslinking agents with higher functional group numbers for greater crosslinking density. High functionality crosslinkers produce more extensive networks with improved mechanical properties such as chemical resistance, temperature, tensile strength, and rigidity. Lower-functionality crosslinkers are suited for softer, more flexible coatings where less crosslinking is preferred.
Curing conditions
Consider the temperature and time required for curing. Some crosslinkers react at room temperature, making them easier to use in processes that are sensitive to curing temperatures such as in varnishes applied on wooden furniture. Others may require higher temperatures or prolonged time which may not be ideal. Select crosslinkers that match the curing conditions of the target application.
Shelf stability and pot life
Also consider the shelf stability of the coating with the crosslinking agent added and the pot life during application. Pot life is the period the paint remains workable before it cures. Some crosslinkers such as blocked isocyanates improve stability and elongate pot life by blocking isocyanates and preventing them from prematurely reacting with hydroxyls. Certain fast-reacting crosslinkers may reduce pot life and instability.
Molecular weight
Molecular weight influences the viscosity, mobility, and diffusion of the crosslinking agent within the coating. Higher molecular weight crosslinkers increase viscosity, reducing the tendency of paint to run during application and migration. They also improve the formation of thicker films that protect well. Lower molecular weight crosslinkers have better mobility and diffusion within the coating structure, improving film uniformity and interaction, but may cause defects such as poor exfoliation.
APT and environmental considerations
The coatings' performance tests and HVAC systems' safety tests are considered. Select crosslinkers that are VOCs free or have minimal VOC content to avoid harmful emissions that contribute to global warming and ozone layer depletion. Such crosslinkers will also accommodate regulatory compliance with environmental protection agencies.
Crosslinking agents have the following features:
Functional groups
Crosslinking agents are organic molecules with multiple active or functional groups to react with paint resins. Common functional groups include aldehyde (-CHO), isocyanate (-NCO), epoxy (-O), and vinyl (-C=CH2). Polyfunctional resins used serve as crosslinking agents that contain multiple active hydrogen atoms interacting with crosslinking agents having carbonyl groups to form a dense three-dimensional network within the resin). These resins are obtained by phenol and formaldehyde condensation to produce Bakelite and are the backbone of the resultant resin. These functional groups are responsible for the chemical properties and crosslinking behavior of the agents.
Preferential path
This refers to the tendency of crosslinking agents to migrate towards and preferentially react with certain regions or components within the coating system. For instance, towards the resin or higher density regions instead of lower density or toward a lower functional group region. High functional carbon-based crosslinking agents preferentially reacted with -NH and -OH functional groups produce high crosslinking density at the expense of far less pathway and poor film formation. Conversely, functional groups that have less affinity toward lower pathway functional groups to form thick films but cause poor exfoliation.
Crosslinking density
This is a measure of how many crosslinks exist within a polymer network and is influenced by polymer molecular weight, the number of functional groups in crosslinking agents, and their types. It was found that higher crosslinking density improved coatings' mechanical properties, elasticity, water, and chemical resistance. Conversely, lower crosslinking density improved flexibility and higher coating tendency.
Mobility
Refers to the ease with which crosslinking agents diffuse within the coating system. It favored higher mobility since the agents could easily interact with functional groups and form crosslinks. Conversely, it lowered mobility due to high viscosity hindered diffusion causing poor exfoliation. During the early stages of film formation, less mobile agents react with functional groups to form crosslinks that hinder further mobility, leading to dry coatings (premature reaction) and poor exfoliation). On the contrary, too much mobility caused by hindered agents inhibited reaction and low exfoliation instead.
Prototropy & tautomerism
Prototropic is the transfer of a proton from one atom to another with simultaneous bond formation or breaking and occurs in swift steps such as seconds. It leads to formation of isomers having different functional groups at distinct locations within the molecule. Tautomerism is the rearrangement of electrons accompanied by the aforementioned proton transfer. It occurs in slower steps resulting in resonance stabilizing isomers. Tautomeric experiments were found to be a vital factor in influencing crosslinking density by stabilizing high functional groups where lower polarity is experienced.
Crosslinking agents have the following therapeutic potentials:
Cancer treatment
Cisplatin is a prominent cancer chemotherapeutic drug containing a platinum metal complex that acts as a crosslinker. It treats various cancers such as lung, bladder, testicular, and ovarian by forming DNA-platin adducts that bind DNA strands together, inhibiting DNA repair and replication ultimately resulting in cell death. Similarly, metal-based drugs that mimic biological cofactors and targeting biomolecules for drug delivery are developed through crosslinking.
Dialytic membranes
Epichlorohydrin crosslinked agarose membranes are used in medical dialyzers for the removal of toxic substances and waste from patients undergoing dialysis. Recrosslinked chitosan hydrogels serve as effective membranedialyzers with better viscodilatant, antimicrobial, and drug delivery properties.
Tissue engineering
Collagen cross-linking agents such as glutaraldehyde and genipin are used to produce resilient, biodegradable, biocompatible scaffolds that promote cell attachment, proliferation, and differentiation in tissue engineering for organ and soft tissue regeneration. Similarly, chitosan and hyaluronic acid scaffolds utilized in wound healing are produced by crosslinking.
Osteogenic implants
Calcium phosphate and hydroxyapatite cross-linked chitosan scaffolds are developed to mimic extracellular matrix enhancing bone regeneration and integrating with natural bone tissue. Similarly, collagen crosslinking agents such as EDC transform collagen into osteogenic xylose and ribose tissue engineered implants for bone repair and regeneration.
Maintaining and caring for cross-linking agents involves proper storage, handling, and monitoring of conditions to ensure their effectiveness and longevity. Here are some key maintenance tips:
Store properly
The agents should be stored in tightly sealed containers to prevent contamination and exposure to air which may cause degradation. Also store in a cool, dry place, away from direct sunlight and extreme temperatures. Such conditions were found to retard crosslinking agents' activity and efficacy. Additionally, blocked isocyanates and other crosslinkers that are prone to moisture absorption and hydrolysis should be stored in tightly sealed containers or desiccants exposed to moisture to preserve activity.
Labeling
Crosslinking agents have different functional groups and are made from distinct materials. Thus, care should be taken when labeling to avoid using the wrong agent with the wrong resin system which may lead to disastrous consequences. Every label in every container should matter and include details such the date of receipt, date of use, name of the product, and characteristic effects if they tend to forget easily.
Avoid contamination
Always use clean, dry tools when measuring and handling crosslinking agents to avoid contamination which may alter their chemical composition and effectiveness. Contaminated agents may lead to poor coatings performance and hazardous chemicals. Also avoid direct contact with skin as some crosslinkers may be irritating or toxic.
Monitor potency
Regularly check the consistency, color, and appearance of crosslinking agents and their containers. Any noticeable change may signify contamination, degradation, or chemical reaction and are not safe to use. Similarly, regularly check labels for storage littered with dust and wear as well as verify expiration dates. Some expired agents may have adverse effects when used.
Proper disposal
Some crosslinking agents have harmful effects on users and chemical nature. Examples include carbodiimides and aziridines. Dispose of them properly to avoid hazardous effects on human beings and the environment. Create a hazardous waste disposal plan as well as disposal procedures for contaminated items (like personal protective equipment). Follow local regulations regarding the disposal of chemicals.
protect against moisture
Some crosslinking agents like isocyanates are highly moisture-sensitive and can react with water to form urea and carbamate which retards and lowers their effectiveness. Thus, moisture control is crucial. Always work in low-humidity environments when handling moisture-sensitive agents and store them in tightly sealed containers. Consider using desiccants or moisture-barrier containers to ads to protect these agents from exposure to humidity.
Regular inspection
Frequently inspect crosslinking agents for signs of degradation such as color changes, precipitate formation, and odor or changes in consistency. Such degradation agents are no longer effective when used.
A1: Crosslinking agents are molecules containing multiple functional groups reacting with a target polymer and forming covalent bonds or crosslinks. They are also called crosslinkers and play a critical role in enhancing the structure's mechanical properties, elasticity, and chemical resistance.
A2: Crosslinking agents maintain and stabilize pharmaceutical compounds. They are incorporated during formulation processes where they enhance the stability and solubility. For instance, glutaraldehyde is a crosslinking agent for stabilizing biological tissues like organs used for transplantation in medical procedures and treats various cancers.
A3: The key factors influencing the selection of crosslinking agents are compatibility, functional groups, curing conditions, pot life and environmental considerations, and crosslinking density.
A4: During paint formulation, crosslinking agents react with paint resins to form a dense three-dimensional network structure improving the paint's coating properties. These properties include elasticity, mechanical strength, chemical resistance, and temperature.
