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Do you know the methods for testing the toughness of modified recycled plastics?

2022-07-21 Page view : 18 views

Modified plastics, refers to the general purpose plastics and engineering plastics based on the filling, blending, strengthening and other methods of processing modified plastic products to improve the performance of flame retardancy, strength, impact resistance, toughness and other aspects.

Today, modified plastics are playing an increasingly important role in national life, especially in the automotive and home appliance sectors, where they play an irreplaceable role. For the many types of modified plastics technology, plastic toughening has been studied and focused on by academics and industry, as the toughness of the material often plays a decisive role in the application of the product. In this article, the Recycling Information Station will explain a few things about plastic toughening.

The stiffer the material, the less likely it is to deform, the more ductile the material, the more likely it is to deform.

Toughness, as opposed to rigidity, is a property that reflects the ease of deformation of an object. The more rigid the material, the less likely it is to deform, and the more ductile it is, the more likely it is to deform.

Generally, the greater the rigidity, the greater the hardness, tensile strength, tensile modulus (Young’s modulus), bending strength and bending modulus of the material; conversely, the greater the toughness, the greater the elongation at break and impact strength.

Impact strength is the strength of a specimen or part to withstand an impact, and is often broadly defined as the amount of energy absorbed by a specimen before it breaks. Impact strength has different values depending on the form of the strip, the test method and the conditions of the specimen, and therefore cannot be classified as a fundamental material property.

Results obtained from different impact test methods are not comparable.

Impact test methods are many, based on the test temperature: room temperature impact, low-temperature impact and high-temperature impact three; based on the specimen stress state, can be divided into bending impact – simple beam and cantilever beam impact, tensile impact, torsional impact and shear impact; based on the energy used and the number of impacts, can be divided into a large energy impact and small energy impact test multiple times. Different materials or different applications can choose different impact test methods and get different results, which are not comparable.

The mechanism of plastic toughening and the factors influencing it are analyzed and summarized
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1. Silver stripe-shear band theory

In blended systems of rubber-toughened plastics, the role of rubber particles is twofold.
On the one hand, acting as a center of stress concentration and inducing a large number of silver lines and shear bands in the matrix.
On the other hand, the development of the silver pattern is controlled so that it is terminated in time without developing into a destructive crack.

The stress field at the end of the silver stripe can induce a shear band that terminates the silver stripe. It also prevents the development of the silver grain when it extends into the shear band. The generation and development of large amounts of silvering and shear banding during stressing of the material consumes a large amount of energy, resulting in an increase in the toughness of the material. Silvering is macroscopically manifested as a stress whitening phenomenon, while shear banding is associated with fine necking, which varies in different plastic substrates.

For example, HIPS matrix toughness is small, silver-grained, stress whitening, silver-grained volume increases, the transverse dimension is basically unchanged, no fine neck in tension; toughened PVC, matrix toughness is large, yielding is mainly caused by the shear band, there is a fine neck, no stress whitening; HIPS/PPO, silver-grained, shear band are occupied by a considerable proportion, fine neck and stress whitening phenomenon at the same time.

2. Factors affecting the toughening effect of plastics

(1) Research on the properties of the matrix resin shows that improving the toughness of the matrix resin is conducive to improving the toughening effect of the toughened plastic, and that improving the toughness of the matrix resin can be achieved through the following means.

Increase the molecular weight of the base resin so that the molecular weight distribution becomes narrower; improve toughness by controlling whether or not it crystallizes, as well as crystallinity, crystal size and crystal shape. For example, nucleating agents are added to PP to increase the rate of crystallization and refine the grain size, thereby improving fracture toughness.

(2) Properties and dosage of toughening agents

A. Influence of dispersed phase particle size of toughening agent – For elastomeric toughened plastics, the best value of the dispersed phase particle size of elastomer differs depending on the characteristics of the base resin. For example, the best value of rubber particle size in HIPS is 0.8-1.3μm, the best particle size of ABS is about 0.3μm, and the best particle size of PVC-modified ABS is about 0.1μm.
B. Influence of toughening agent dosage – there is an optimum value for the amount of toughening agent to be added, which is related to the particle spacing parameter.
C. The effect of the glass transition temperature of the toughening agent – generally the lower the glass transition temperature of the elastomer, the better the toughening effect.
D. Influence of toughener interfacial strength with the matrix resin – the influence of interfacial bond strength on the toughening effect varies from system to system.
E. Influence of elastomer toughener structure – related to elastomer type, degree of cross-linking, etc.

(3) The bond between the two phases has a good bond between the two phases, can make the stress can be effectively transferred between the phases and thus consume more energy, the better the overall performance of the macro plastic, especially the impact strength of the most significant improvement. Graft copolymerization and block copolymerization are typical methods of increasing the bonding force between two phases, the difference being that they are chemically bonded by means of chemical synthesis, such as graft copolymers HIPS and ABS, and block copolymers SBS and polyurethane.

For toughening agents to toughen plastics, it is a physical blending method, but the principle is the same. The ideal blending system should be two components both partially compatible and each into a phase, there is an interface layer between the phases, in the interface layer of the two polymer molecular chain diffusion, there is a clear concentration gradient, by increasing the compatibility between the blended components, so that they have a good bonding force, and then enhance the diffusion of the interface diffusion, increase the thickness of the interface layer. This is the key technology for the toughening of plastics and the preparation of polymer alloys – polymer compatibility technology!

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