What are the methods of plastic toughening modification?

What are the methods of plastic toughening modification?

01 Toughness characterization of plastics

Toughness is opposite to rigidity, which is an attribute reflecting the degree of difficulty in deformation of an object. The greater the rigidity, the less likely the material is to deform, and the greater the toughness, 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 spline or workpiece under impact, usually refers to the energy absorbed by spline before rupture. Impact strength varies with spline shape, test method and sample condition, so it cannot be attributed to the basic properties of the material.

There are many methods of impact test, according to the test temperature divided into three kinds of normal temperature impact, low temperature impact and high temperature impact; According to the stress state of the sample, it can be divided into bending impact - simply supported beam and cantilever beam impact, tensile impact, torsional impact and shear impact; According to the energy and impact times, it can be divided into large energy single impact test and small energy multiple impact test. Different impact test methods can be selected for different materials or different uses, and different results can be obtained, which cannot be compared.

02 Toughening mechanism of plastics

Plastic toughening is divided into flexible toughening agent and rigid toughening agent. The toughening mechanism includes direct energy absorption theory of elastomers, yield theory, crack core theory, multiple silver striation theory, silver striation - shear zone theory, silver striation theory, Wu's theory, etc. The silver stripe-shear zone theory is widely accepted because it can successfully explain a series of experimental facts.

According to the silver-shear band theory, the rubber particles have two main functions in the rubber toughened plastics blends: on the one hand, as the stress concentration center, the rubber particles induce the matrix to produce a large number of silver-shear bands;

On the other hand, controlling the development of silver streaks allows them to stop in time without developing destructive cracks.

The stress field at the end of the silver streak can induce shear bands and terminate the silver streak. It also stops the development of silver lines when they extend into the shear zone. When the material is subjected to stress, the generation and development of a large number of silver lines and shear bands consume a lot of energy, thus improving the toughness of the material. The macroscopic appearance of silverization is stress gray hair, while the shear band is related to thin neck, and its behavior is different in different plastic substrates.

03 Factors affecting toughening effect of plastics

1、Properties of matrix resin

The results show that increasing the toughness of matrix resin is beneficial to improving the toughening effect of toughened plastics. The toughness of matrix resin can be improved by increasing the molecular weight of matrix resin to narrow the molecular weight distribution; The toughness can be improved by controlling whether crystallization, crystallinity, crystal size and crystal type. For example, the addition of nucleating agent to PP increases the crystallization rate and refines the grains, thus increasing the fracture toughness.

2、Characteristics and dosage of toughening agent

A. Particle size of dispersed phase of toughening agent——For elastomer toughened plastics, the optimum particle size of dispersed phase of elastomer varies with the properties of matrix resin. For example, the optimal particle size of HIPS rubber is 0.8 ~ 1.3 μm, the optimal particle size of ABS is about 0.3 μm, and the optimal particle size of PVC modified ABS is about 0.1 μm.

B. Glass transition temperature of toughening agent ——Generally, the lower the glass transition temperature of elastomers, the better the toughening effect;

C. Interface strength between toughening agent and matrix resin ——The effect of interfacial bond strength on toughening effect is different for different systems.

D. The structure of elastomer toughening agent is related to the type of elastomer, crosslinking degree, etc;
E. Amount of toughening agent - There is an optimal amount of toughening agent, which is related to the particle spacing parameter；
3、The binding force of the two phases

The two phases have good binding force, which can make the stress can be effectively transmitted in the phase, so as to consume more energy. The overall performance of plastic is better in macro, especially the improvement of impact strength is the most significant. Generally, this bonding force can be understood as the interaction force between two phases. Graft copolymerization and block copolymerization are typical methods to increase the bonding force between two phases. The difference is that they form chemical bonds through chemical synthesis, such as graft copolymers HIPS and ABS, and block copolymers SBS and PUR.

For toughening agent toughened plastic, it belongs to physical blending method, but its principle is the same. Ideal blend system should be two components was partly compatibility and the respective phase, and there is a layer interface, the interface layer, two kinds of polymer molecular chain diffusion concentration gradient, obviously increased by blending component, the compatibility between the good binding force, thus enhancing diffusion of interfacial diffusion, increase the thickness of the interface layer. And this is the plastic toughening is also the preparation of the key technology of polymer alloy - polymer compatible technology!

04 What are the methods of plastic toughening?

1、Rubber elastomer toughened EPR (diethylpropylene), EPDM, butadiene rubber (BR), natural rubber (NR), isobutene rubber (IBR), nitrile rubber (NBR) and so on. It is suitable for toughening and modifying all plastic resins.

2、Thermoplastic elastomer toughened SBS, SEBS, POE, TPO, TPV, etc. It is mainly used for toughening polyolefin or non-polar resin, and compatibilizers should be added when toughening polymers containing polar functional groups such as polyester and polyamide.

3、Core-shell copolymer and reactive terpolymer toughened ACR (acrylate), MBS (methyl acrylate - butadiene-styrene copolymer), PTW (ethylene-butyl acrylate - glycidyl methacrylate copolymer), E-MA-GMA (ethylene-methyl acrylate - glycidyl methacrylate copolymer). Used for engineering plastics and high temperature resistant polymer alloy toughening.

4、High toughness plastic blending toughened PP/PA, PP/ABS, PA/ABS, HIPS/PPO, PPS/PA, PC/ABS, PC/PBT, etc. Polymer alloy technology is an important way to prepare high toughness engineering plastics.

5、Other ways of toughening nanoparticles (such as nano CaCO3), sarin resin (Dupont metal ionomer) toughening, etc.
General plastics are generally obtained by free radical addition polymerization, molecular main chain and side chain do not contain polar groups, when toughening adding rubber particles and elastomer particles can obtain better toughening effect; And engineering plastics are generally obtained by condensation polymerization, molecular chain side chain or end group contains polar groups, toughening by adding functional rubber or elastomer particles with high toughness.

Toughening Agent Type for commonly used Resin

05 How to understand toughening must first increase tolerance?

In general, plastic under external force in the interface adhesion, hollowing out, substrate shear yield of the process of absorption and dissipation of energy, in addition to nonpolar plastic resin toughening can be directly added into good compatibility with elastomer particles (similar compatible principle), all the other polarity resin need effective capacity to achieve the goal of eventually toughening. When the graft copolymers mentioned above are used as toughening agents, they all have strong interactions with the matrix. For example:

(1) Toughening mechanism with epoxy functional group: addition reaction occurs between epoxy group and polymer terminal hydroxyl, carboxyl or amine group after ring-opening;

(2)   Core-shell toughening mechanism: the outer functional groups are fully compatible with the components, and rubber has toughening effect;

(3)   Toughening mechanism of ionomer: physical crosslinking network is formed by the complexation between metal ions and carboxylic acid of polymer chain, so as to toughen.
In fact, the principle of compatibilization can be extended to all polymer blends if toughening agents are considered as a class of polymers. In the industrial preparation of useful polymer blends, reactive compatibilization is the technology we must use, at this time the toughening agent has a different meaning, "toughening compatibilizer", "interface emulsifier" is particularly vivid!