Thermal stabilizers are widely used and come in various types. Each type has different applications in plastic modification. In flame-retardant PET, polyester PBT and halogen-free flame-retardant agents can be added. Flame retardants are characterized by non-leaching, environmental friendliness, good heat resistance, and non-dripping properties. A thermal stabilizer refers to a substance that enhances the toughness of polymer materials.
Materials to enhance the flexibility of adhesive film layers. Certain thermosetting resin adhesives are more brittle, prone to cracking and rapid propagation under external forces, leading to adhesive layer failure and lack of fatigue resistance, rendering them unsuitable for structural bonding. Therefore, it is necessary to reduce brittleness, increase toughness, and improve load-bearing strength. Substances that can reduce brittleness, increase toughness, and not affect the other main properties of the adhesive are called toughening agents.
Typically containing active groups that can undergo a chemical reaction with resins, they are not fully compatible upon curing and may require phase separation, resulting in a more ideal toughening effect. This can maintain or only slightly reduce the heat deflection temperature while significantly improving impact resistance. The addition of some low-molecular liquid substances, known as plasticizers, can reduce brittleness but will drastically lower rigidity, strength, and heat deflection temperature, failing to meet structural bonding requirements. Therefore, plasticizers and toughening agents are entirely different. Toughening agents can be categorized into rubber-based tougheners and thermoplastic elastomer-based tougheners.
Certain linear high molecular weight compounds are soluble in resins, contain active groups, and can participate in the curing reaction of resins, enhancing the elongation at break and impact strength. However, their heat deflection temperature decreases. These substances are known as flexibizers. Common types include liquid polysulfide rubber and liquid nitrile rubber. Due to their proper blending with resins, they can be used to produce structural adhesives, thus they are also categorized as toughening agents. While flexibilization and toughening are related yet distinct concepts, they are actually difficult to strictly differentiate. Theoretically, toughening is different from flexibilization as it does not soften the material as a whole but transforms the epoxy resin solid into a multiphase system. The toughening agent aggregates into spherical particles, forming a dispersed phase within the continuous phase of the cross-linked network of the epoxy resin, resulting in a sudden increase in crack resistance and a significant improvement in fracture toughness, with minimal loss in mechanical properties and thermal resistance.
Different types of toughening agents possess distinct toughening mechanisms. Liquid polysulfide rubber can react with epoxy resin, introducing some flexible chain segments, reducing the modulus of the epoxy resin, enhancing toughness, but compromising its heat resistance. Liquid nitrile rubber, as a toughening agent for epoxy resin, exhibits minimal toughening effect and actually decreases adhesive strength during room temperature curing; only at medium to high temperatures does it show significant toughening and adhesive properties. Liquid nitrile rubber with terminal carboxyl groups toughens epoxy resin, being compatible before curing and separating into phases afterward, forming a "sea-island" structure. This structure can absorb impact energy while maintaining the heat resistance.
Thermoplastic resins are continuously interpenetrated within the epoxy resin network, forming a semi-interpenetrating network polymer, thereby enhancing the toughness of the cured epoxy resin.
Dura-enhancers are widely used and come in various types, each with distinct applications in plastic modification. In flame-retardant PET, polyester PBT and halogen-free flame-retardant agents can be added, which offer non-migration, environmental friendliness, good thermal resistance, and non-dripping properties. Dura-enhancers refer to substances that can impart better toughness to polymer materials.
