The secret of PVC toughening agent is here
There are many factors that determine the impact resistance of PVC products, including the molecular weight of PVC, the surface condition of the material, the thickness of the material, the temperature of use, the type and structure of the impact modifier, the particle size, the content, and the stability under UV. Synthetic process (dispersion mechanism). The thickness of the shell in the impact modifier of the core-shell structure, as well as the interaction with other components of the formulation (including fillers, heat stabilizers, lubricants). Impact modifiers not only increase the impact strength, but also affect some other properties of PVC materials.
Under normal circumstances, the impact strength of PVC increases with the increase of PVC molecular weight. There are also ultra-high molecular weight PVC with molecular weight over 2500 on the market. Its impact resistance is better than general-purpose PVC resin (SG-1~SG-8). Therefore, in PVC products with low impact resistance, high molecular weight PVC can be selected without additional impact modifiers, but molding processing problems require extra attention. Adding the same number of parts and grades of impact modifier to PVC resin of different polymerization degree, the obtained impact strength is positively correlated with the molecular weight of PVC, that is, under the same conditions, the modified high molecular weight PVC resin has better impact resistance. excellent.
Rigid PVC is a brittle material. Surface defects or flaws increase the probability of impact fracture. A variety of factors can form PVC surface defects, such as large particle size fillers, surface embossing and other surface treatment processes, as well as surface degradation of PVC resin caused by weathering or environmental factors. When the PVC material is stressed, impact energy is generated, causing the crack to originate from the surface defect and propagate to the inside until it disappears. There are many ways to eliminate the impact energy generated. If the propagating crack encounters a cavity or the force of propagation acts on a large surface area, it is beneficial to eliminate the generated force. Nano-scale calcium carbonate uses a similar mechanism. To achieve toughening effect. In addition, there is a theory that nanoparticles change the crystallinity of PVC (PVC crystallinity is about 10%, which belongs to amorphous polymer materials) and enhance it, thereby greatly improving the resistance to impact, or nano When the particles are blended with PVC, a shear band is produced, and the shear band can absorb more impact energy.
The particle size of some impact modifiers is determined, and the final particle size and scale of some impact modifiers are related to the blending process. A typical example is chlorinated polyethylene CPE. The melting temperature is a major factor in controlling melting and impact. The CPE dispersion is relatively complicated. The melting temperature of CPE is lower than that of PVC, which is 110-130 ° C. The CPE melts first during the process, and the molten CPE is coated on the surface of the unmelted PVC to form a network. As the temperature increases, the PVC begins to melt, at which point two coexisting systems of PVC and CPE are formed. Usually, the amount of CPE added is small, phase transformation occurs in the above system, CPE becomes dispersed in the continuous PVC melt, and the dispersion scale of CPE has an important influence on the impact properties of the material. Under the same amount of addition, when the particle size of the molten CPE is 1 μm, the obtained modified PVC has the best impact performance.
The nature of CPE is related to the chlorine content. Traditionally, 35% chlorine-containing CPE has been used because of its better rubber elasticity and excellent compatibility. Recently, chlorine-containing 25% CPE has also been used because it promotes melting and increases melt strength. In addition, ordinary PVC heat stabilizers can also be used for CPE without the addition of other special stabilizers.
Core-shell impact modifiers are also widely used in PVC products. Among them, the core provides impact resistance and the shell improves the adhesion between the PVC and the impact modifier particles. MBS is a core made of butadiene-styrene copolymer, a shell made of methyl methacrylate-styrene copolymer; an acrylic impact modifier, AIM, which is made of acrylic acid and polystyrene. The methyl acrylate is a shell, and the siloxane-acrylate has a multilayer structure in the core. Similar to ABS, MBS has good compatibility with PVC and can be used as an impact modifier for PVC. However, in the ABS and MBS formulations, most of them are used for indoor products because of their lack of weather resistance. MBS can be used for translucent to transparent products, while AIM can only be used for translucent products. In order to improve the optical performance of AIM, it must be redesigned. When used in a transparent product, the core is made of an acrylic-styrene copolymer. Comparing silicone-containing and all other acrylic impact modifiers, PVC-containing silicone ester products have excellent low-temperature impact properties, which means that the addition of silicone grease in the acrylic impact modifier provides excellent weatherability and Thermal stability, outdoor PVC products have improved impact resistance.