Actually, compounds that can make the adhesive film more flexible are known as toughening agents. Some thermosetting resin adhesives, including unsaturated polyester resin adhesives, phenolic resin, and epoxy resin, exhibit high brittleness and low elongation after curing. It is easy for the bonded portion to split and expand quickly in response to external stress, which weakens the adhesive layer and renders it unsuitable for structural bonding. Therefore, it is necessary to make efforts to lessen brittleness, boost toughness, and enhance load-bearing strength. A toughening agent is anything that can increase toughness and decrease brittleness without impacting the adhesive’s other essential qualities.
The majority of toughening agents have active groups that can interact chemically with the resin. After curing, they are not entirely compatible and occasionally phase separate. This will produce a toughening effect that is more ideal and maintain or slightly lower heat distortion temperature. Impact resistance has improved dramatically. The stiffness, strength, and heat deformation temperature are significantly decreased when low-molecular liquids or plasticizers are introduced to the resin. As a result, the structural bonding criteria cannot be met. Therefore, there is no comparison between plasticizers and tougheners.
Various toughening agent types
It can be separated into thermoplastic elastomer tougheners and rubber tougheners.
Liquid polysulfide rubber, liquid acrylate rubber, liquid polybutadiene rubber, liquid nitrile rubber, ethylene-propylene rubber, and styrene-butadiene rubber are the main varieties of rubber tougheners.
A type of synthetic material called a thermoplastic elastomer may be plasticized and molded at high temperatures and has the elasticity of rubber at room temperature. As a result, this kind of polymer combines the traits of thermoplastics and rubber. It can be utilized as a matrix material for composite products as well as a toughening agent. Currently employed as tougheners for composite materials, these materials mostly comprise polyurethane, styrene, polyolefin, polyester, syndiotactic 1,2-polybutadiene, and polyamide goods. Styrene and polyolefin are the ones.
Other tougheners: Low-molecular polyamides and low-molecular inactive tougheners, including phthalates, are other tougheners appropriate for composite materials. Plasticizers are another name for inactive tougheners, which are substances that do not contribute to the resin’s curing reaction.
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Different toughening mechanisms are used by various types of toughening chemicals. In order to incorporate certain flexible segments, liquid polysulfide rubber might react with epoxy resin. This lowers the epoxy resin’s modulus and increases toughness at the expense of heat resistance. The bonding strength actually diminishes when liquid nitrile rubber is used as a toughening agent for epoxy resin when it is cured at room temperature; the toughening and bonding effects are only more pronounced when the epoxy resin is cured at medium and high temperatures. Prior to curing, carboxyl-terminated liquid nitrile rubber toughened epoxy resin is compatible. However, after curing, the phases separate to create a “island structure” that can absorb impact energy without significantly lowering heat resistance. Without creating a phase separation structure, the T-99 multifunctional epoxy curing agent cures epoxy resin to incorporate flexible segments into the cross-linked structure. It increases tensile strength without significantly lowering heat resistance.The semi-interpenetrating network polymer created by the thermoplastic resin’s continual penetration into the epoxy resin network boosts the toughness of the epoxy resin-cured product.
Nanoparticles range in size from 1 to 100 nm, have a large specific surface area, and have strongly unsaturated surface atoms, which results in a high surface activity. At the contact, the epoxy group and the nanoparticles generate a much stronger van der Waals force that is capable of causing microcracks and absorbing energy. Both nano-SiO2 and nano-clay have the ability to start and stop silver fractures. Nanoparticles also possess a high degree of stiffness. When expanding fractures come into contact with nanoparticles, they will spin or deflect, absorbing energy for the purpose of toughening. Additionally, nanoparticles and resin work well together to enhance the matrix’s capacity to disperse and absorb impact energy, leading to an improvement in toughness.
selection criteria
You can only get good overall performance by choosing the proper toughener based on the type of resin and the intended use of the adhesive.
Liquid silicone rubber, polysulfide rubber, polyether, polysulfone, polyimide, nano calcium carbonate, nano titanium dioxide, and other materials are among the adhesives made from epoxy resins.
Carboxyl nitrile rubber, liquid nitrile rubber, polyvinyl butyral, polyethersulfone, and polyphenylene ether ketone are examples of phenolic resin adhesives. Polyvinyl alcohol and carboxyl nitrile latex are used as toughening agents in water-soluble phenolic resin;
Acrylic rubber, carboxyl nitrile rubber, chloroprene rubber, chlorosulfonated polyethylene, ABS resin, and other materials are frequently used in quick-setting acrylic structural adhesives;
The acrylic rubber, ABS, SBS, SEBS, etc. should be used as the -cyanoacrylate adhesive;
Liquid nitrile rubber, polyvinyl butyral, polyvinyl acetate, etc. should be used as adhesives for unsaturated polyester resin;
Adhesives made of urea-formaldehyde resin can also contain polyvinyl alcohol and polyvinyl acetate emulsion.
Summary
At room temperature, many polymers become brittle, severely decreasing their usefulness. For instance, polystyrene is easy to produce, transparent, and electrically insulating, but it needs to be reinforced with rubber to increase toughness and resistance. Impact power. A toughening agent, commonly referred to as an impact modifier, is a type of additive that improves the toughness of plastics.
A sort of additive known as a toughening agent can lessen the fragility and increase the impact resistance of composite materials. Active and inactive toughening agents are two groups into which it can be separated. Its molecular chain has active groups that can interact with the matrix resin, making it an active toughening agent. It can create a network structure and expand a portion of the flexible chain, enhancing the composite materials’ resistance to impacts. Tougheners that do not participate in chemical reactions but are nevertheless compatible with the matrix resin are known as inactive tougheners.
