There Is Always Something You Don’t Know about The Mechanism of Elastomer Toughening Polymers

01 Types of Elastomers

1.1 Thermoplastic Elastomers

Thermoplastic elastomers have the characteristics of being able to be molded after being heated to the process temperature, and can be heated and molded multiple times. It is a material that has the characteristics of both rubber and thermoplastic plastics. Thermoplastic elastomers have a variety of possible structures. The most fundamental one is that there must be at least two mutually dispersed polymer phases. At normal use temperature, one phase is a fluid, so that the temperature is higher than its glass transition temperature, and the other phase is a solid, so that the temperature is lower than its glass transition temperature or equal to the glass transition temperature, and there is interaction between the two phases.

That is, a polymer material that shows rubber elasticity at room temperature and can be plasticized and molded at high temperature, has mechanical properties and use properties similar to rubber, and can be processed and recycled as thermoplastic plastics. It builds a bridge between plastic and rubber. Therefore, thermoplastic elastomers can process rubber products as quickly, effectively and economically as thermoplastic plastics. In terms of processing, it is a plastic; in terms of properties, it is also a rubber. Thermoplastic elastomers have many advantages over thermosetting elastomers. Thermoplastic elastomers include hydrogenated ethylene-butylene rubber, polyamide, vinyl acetate, polyolefins, polyurethanes and other specific varieties.

02 Mechanism of elastomer-enhanced toughened polymers

2.1 Direct energy absorption theory

When the material is stretched or impacted, cracks will appear. At this time, the elastomer distributed in the matrix polymer spans across the cracks. If the cracks need to develop, the elastomer molecules must be stretched. This stretching effect will absorb a lot of energy, thereby increasing the strength of the material.

2.2 Crack Core Theory

As a stress concentration point, the elastic body generates a large number of small cracks instead of a small number of large cracks. The energy required for the expansion of many small cracks is greater than that required for the expansion of large cracks. At the same time, the stress fields of a large number of small cracks will interfere with each other, thereby weakening the front stress of crack development, thereby alleviating the development of cracks or causing crack termination.

2.3 Silver-streaking-shear band theory

01 The concept of silver grain and shear tape

Experiments show that the deformation mechanism of polymers includes two processes: one is the shear deformation process, and the other is the silver-streaking process. Shear deformation is just a simple change in the shape of the material, and the intermolecular cohesive energy and the density of the object remain basically unchanged. The silver-streaking process greatly reduces the density of the material.

When a polymer material is subjected to force, it will produce elastic deformation. When the external force is greater than the yield stress, an irreversible deformation will occur. This deformation requires many chain segments to move independently. Under certain conditions, such as strain softening of the polymer material or structural defects, local stress concentration may occur, thus generating local shear deformation. This phenomenon is called "shear band".

Another yield deformation mechanism of polymer materials is the silver-streaking mechanism. When a polymer is subjected to stress, it turns white, which is called the silver-streaking phenomenon. The reason for stress whitening is the generation of silver streaks. The local area where silver streaks are generated in the polymer material is called the silver streak body, or silver streaks for short. The reason for silver streaking is stress concentration caused by structural defects and uneven material structure. Interaction between silver streaks and shear bands

Under the action of stress, polymer materials will produce shear bands and silver streaks at the same time. The result of the interaction between the two will become an important factor affecting the deformation and even destruction of polymer materials. In the deformation of polymer materials, the two mechanisms of shear and silver streaks exist simultaneously and interact with each other, which makes the polymer materials change from brittle failure to ductile failure.

There may be three ways for the interaction between shear bands and silver streaks: first, silver streaks are healed or terminated when they encounter existing shear bands. This is because the high orientation of macromolecules in the shear bands limits the development of silver streaks; second, new shear bands are triggered at the tips of silver streaks where stress is highly concentrated, and the newly generated shear bands in turn terminate the development of silver streaks; third, shear bands reduce the initiation and growth rate of silver streaks.

02 Silver streaks and shear band theory

This theory believes that the main reason for the toughening of elastomers is the large-scale generation of silver streaks and shear bands and the result of the interaction between silver streaks and shear bands.

The first important role of elastomer particles is to act as a stress concentration center, inducing a large number of silver streaks and shear bands. The generation and development of a large number of silver streaks and shear bands requires a lot of energy. The proportion of silver streaks and shear bands is related to the properties of the matrix. The greater the toughness of the matrix, the higher the proportion of shear bands; at the same time, it is also related to the deformation rate. When the deformation rate increases, the proportion of silver streaks will increase.

The second important role of elastomer particles is to control the development of silver streaks and terminate them in time. During the external force process, the elastomer particles deform, not only generating a large number of small silver streaks or shear bands, absorbing a lot of energy, but also terminating the silver streaks in time without developing into destructive cracks.

The characteristics of the silver-craze-shear band theory are that it not only takes into account the role of elastomer particles, but also affirms the influence of the continuous phase performance of the resin. At the same time, it clarifies the dual functions of silver-craze, that is, the generation and development of silver-craze consumes a lot of energy, which can increase the fracture energy of polymer materials; silver-craze is also the precursor of cracks and material damage. However, the defect of this theory is that it ignores the interaction between the continuous phase of the matrix and the dispersed phase of the elastomer. It should be said that the interfacial properties of the polymer multiphase system have a great influence on the performance of polymer materials.

NANJING HAISI is professional manufacturer of thermoplastic elastomers extruder, we supply Twin Screw Extruder, Single Screw Extruder, Plastic Recycling Extruder, Two Stage Extruder, Lab Scale Extruder, Mixing and Feeding Machine, Cooling and Pelletizing Machine, Crushing Machine and so on.