Nowadays, the role of modified plastics in national life is becoming more and more important, especially in the fields of automobiles and home appliances. For the modified plastics technology, plastic toughening technology has been studied and paid attention by academics and industry, because the toughness of materials often has a decisive influence on the application of products. This article will answer a few questions about plastic toughening:

The greater the rigidity, the less likely the material is to deform. The greater the toughness, the easier it is to deform.

The toughness is opposite to the rigidity. It is an attribute that reflects the difficulty of deformation of the object. The larger the rigidity, the less likely the material is to deform. The greater the toughness, the easier it is to deform. Generally, the greater the rigidity, the greater the hardness, tensile strength, tensile modulus (Young's modulus), flexural strength, and flexural modulus of the material; conversely, the greater the toughness, the more the elongation at break and the impact strength Big. Impact strength is expressed as the strength of the spline or part subjected to impact, and generally refers to the energy absorbed by the spline before it is broken. The impact strength varies with the shape of the spline, the test method and the conditions of the sample, and therefore cannot be classified as the basic properties of the material.

The results obtained by different impact test methods are not comparable.

There are many methods for impact test. According to the test temperature, there are three kinds of normal temperature impact, low temperature impact and high temperature impact. According to the stress state of the specimen, it can be divided into bending impact-simple beam and cantilever beam impact, tensile impact and torsional impact. And shear shock; according to the energy and the number of impacts, it can be divided into one impact of large energy and multiple impact tests of small energy. Different impact tests can be selected for different materials or different applications, and different results can be obtained. These results cannot be compared.

Plastic toughening mechanism and influencing factors

(1) Silver-Shear Band Theory

In the blending system of rubber toughened plastics, the role of rubber particles mainly has two aspects:

On the one hand, as the center of stress concentration, the induced matrix generates a large amount of silver streaks and shear bands;

On the other hand, controlling the development of silver streaks causes the silver streaks to terminate in time without developing into destructive cracks.

The stress field at the end of the silver streak can induce a shear band to terminate the silver streaks. The development of silver streaks is also prevented when the silver streaks extend to the shear band. The generation and development of a large amount of silver streaks and shear bands when the material is subjected to stress consumes a large amount of energy, thereby increasing the toughness of the material. The macroscopic representation of silver streaks is characterized by stress white, while the shear bands are related to the formation of thin necks, which behave differently in different plastic matrices.

For example, the HIPS matrix has less toughness, silver streak, whitening stress, increased volume of silver streaks, and the transverse dimension is basically unchanged. The stretch has no neck; toughened PVC, the toughness of the matrix is large, and the yield is mainly caused by the shear band. There are thin necks and no stress whitening; HIPS/PPO, silver streaks and shear bands all occupy a considerable proportion, and the neck and stress whitening phenomenon are simultaneously produced.

(2) There are three main factors affecting the toughening effect of plastics.

1, the characteristics of the matrix resin

Studies have shown that increasing the toughness of the matrix resin is beneficial to improve the toughening effect of the toughened plastic, and the toughness of the matrix resin can be achieved by:

The molecular weight of the matrix resin is increased to make the molecular weight distribution narrow; the toughness is improved by controlling the crystallinity and crystallinity, crystal size, crystal form, and the like. For example, the addition of a nucleating agent to the PP increases the crystallization rate and refines the grains, thereby increasing the fracture toughness.

2. Characteristics and dosage of toughening agent

A. Effect of the particle size of the toughener dispersed phase - For the elastomer toughened plastic, the properties of the matrix resin are different, and the optimum value of the dispersed phase of the elastomer is also different. For example, the optimum particle size of rubber in HIPS is 0.8-1.3 μm, the optimum particle size of ABS is about 0.3 μm, and the optimum particle size of PVC-modified ABS is about 0.1 μm.

B. Effect of the amount of toughening agent - there is an optimum value for the amount of toughening agent added, which is related to the particle spacing parameter;

C. Effect of the glass transition temperature of the toughening agent - the lower the glass transition temperature of the general elastomer, the better the toughening effect;

D. Effect of interfacial strength between toughener and matrix resin - the effect of interfacial bond strength on toughening effect varies from system to system;

E. Effect of elastomer toughener structure - related to elastomer type, degree of crosslinking, etc.

3, the binding between the two phases

The two phases have good bonding force, which can make the effective transmission between the phases when the stress occurs, and consume more energy. The overall performance of the plastic is better, especially the impact strength is most remarkable. Usually this binding force can be understood as the interaction force between two phases. Graft copolymerization and block copolymerization are typical methods for increasing the binding force of two phases. The difference is that they form chemical bonds by chemical synthesis, such as Branch copolymer HIPS, ABS, block copolymer SBS, polyurethane.

For toughening agents toughening plastics, it is a method of physical blending, but the principle is the same. The ideal blending system should be that the two components are partially compatible and each phase is formed, and an interfacial layer exists between the phases. In the interfacial layer, the molecular chains of the two polymers interdiffuse, and there is a significant concentration gradient, by increasing the blending. The compatibility between the components makes them have a good bonding force, which in turn enhances the diffusion and diffuses the interface and increases the thickness of the interface layer. And this, that is, plastic toughening is also the key technology for the preparation of polymer alloys - polymer compatible technology!