Understand the toughening and modification of plastics
TIME：2021-10-20
For many kinds of modified plastics technology, toughening technology has been studied and concerned by academic and industrial circles, because the toughness of materials often plays a decisive role in the application of products. Here are some questions about plastic toughening:
How to test and evaluate the toughness of plastics?
What is the principle of plastic toughening?
What are the factors affecting the toughening effect of plastics?
What are the toughening methods of plastics?
How to understand that toughening must first be compatibilized?
one
Characterization of toughness of plastics
Toughness, as opposed to rigidity, is an attribute reflecting the difficulty of object deformation. 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), flexural strength and flexural modulus of the material; On the contrary, the greater the toughness, the greater the elongation at break and impact strength. Impact strength is the strength of the spline or workpiece under impact. It usually refers to the energy absorbed by the spline before fracture. The impact strength shows different values with the spline shape, test method and sample conditions, so it can not be classified as the basic property of the material.
There are many methods of impact test, which can be divided into normal temperature impact, low temperature impact and high temperature impact according to the test temperature; According to the stress state of the sample, it can be divided into bending impact - simply supported beam and cantilever impact, tensile impact, torsional impact and shear impact; According to the adopted energy and impact times, it can be divided into one impact test with large energy and multiple impact tests with small energy. Different impact test methods can be selected for different materials or different applications, and different results can be obtained. These results cannot be compared.
two
Toughening mechanism of plastics
Plastic toughening can be divided into flexible toughening agent toughening and rigid toughening agent toughening. The toughening mechanism includes elastomer direct energy absorption theory, yield theory, crack core theory, multiple crazing theory, crazing shear band theory, crazing branching theory, Wu's theory and so on. The craze shear band theory is widely accepted because it can successfully explain a series of experimental facts.
According to the crazing shear band theory, in the blend system of rubber toughened plastics, the role of rubber particles mainly has two aspects:
On the one hand, as the center of stress concentration, a large number of crazes and shear bands are induced in the matrix;
On the other hand, control the development of crazing so that the crazing can be terminated in time without developing into destructive cracks.
The stress field at the end of crazing can induce shear band and terminate crazing. When the crazing extends to the shear zone, it will also prevent the development of crazing. When the material is subjected to stress, the generation and development of a large number of crazes and shear bands consume a lot of energy, so as to improve the toughness of the material. Crazing macroscopically shows the phenomenon of stress white hair, while the shear band is related to the formation of thin neck, which is different in different plastic substrates.
three
Factors affecting the toughening effect of plastics
1. Properties of matrix resin
The results show that improving the toughness of matrix resin is beneficial to improve the toughening effect of toughened plastics. Improving the toughness of matrix resin can be achieved by increasing the molecular weight of matrix resin and narrowing the molecular weight distribution; The toughness is improved by controlling whether it is crystallized, crystallinity, crystal size and crystal form. For example, adding nucleating agent to PP can improve the crystallization rate and refine the grains, so as to improve the fracture toughness.
2. Characteristics and dosage of toughening agent
A. Particle size of dispersed phase of toughening agent - for elastomer toughened plastics, the optimal value of particle size of dispersed phase of elastomer is also different due to different characteristics of matrix resin. For example, the optimum rubber particle size in hips is 0.8 ~ 1.3 μ m. The optimum particle size of ABS is 0.3 μ M, the optimum particle size of PVC modified ABS is 0.1 μ M or so.
B. Glass transition temperature of toughening agent - the lower the glass transition temperature of general elastomer, the better the toughening effect;
C. The interfacial strength between toughening agent and matrix resin - the effect of interfacial bond strength on toughening effect is different in different systems;
D. Elastomer toughener structure - related to elastomer type, 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. Bonding force between two phases
The two phases have good bonding force, which can effectively transfer the stress between the phases, so as to consume more energy. The macro comprehensive performance of plastics is better, especially the improvement of impact strength. Generally, this binding force can be understood as the interaction force between two phases. Graft copolymerization and block copolymerization are typical methods to increase the two-phase binding force. The difference is that they form chemical bonds through chemical synthesis, such as graft copolymers hips and ABS, block copolymers SBS and pur.
For toughening agent toughened plastics, it belongs to the method of physical blending, but its principle is the same. The ideal blend system should be that the two components are partially compatible and form phases respectively. There is an interface layer between them. In the interface layer, the molecular chains of the two polymers diffuse each other and have an obvious concentration gradient. By increasing the compatibility between the blend components, they have good adhesion, so as to enhance the diffusion, diffuse the interface and increase the thickness of the interface layer. This is not only the toughening of plastics, but also the key technology for the preparation of polymer alloys - polymer compatibility technology!
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What are the methods of toughening plastics?
1. Rubber elastomer toughening
EPR (ethylene propylene diene), EPDM (ethylene propylene diene), butadiene rubber (BR), natural rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR), etc. It is suitable for toughening and modification of all plastic resins.
2. Thermoplastic elastomer toughening
SBS, SEBS, Poe, TPO, TPV, etc. It is mainly used for toughening polyolefins or non-polar resins. Compatibilizers need to be added when toughening polymers containing polar functional groups such as polyester and polyamide.
3. Toughening of core-shell copolymer and reactive terpolymer
ACR (acrylate), MBS (methyl acrylate butadiene styrene copolymer), PTW (ethylene butyl acrylate glycidyl methacrylate copolymer), E-MA-GMA (ethylene methyl acrylate glycidyl methacrylate copolymer), etc. It is mainly used for toughening engineering plastics and high temperature resistant polymer alloys.
4. Blending toughening of high toughness plastics
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. Toughening by other methods
Nano particle toughening (such as nano CaCO3), sarin resin (DuPont metal ionomer) toughening, etc.
General purpose plastics are generally obtained by free radical addition polymerization. The molecular main chain and side chain do not contain polar groups. When toughening, good toughening effect can be obtained by adding rubber particles and elastomer particles; Engineering plastics are generally obtained by condensation polymerization. The side chain or end group of molecular chain contains polar groups. When toughening, functional rubber or elastomer particles can be added to improve the toughness.
five
How to understand that toughening must first be compatibilized
Generally speaking, plastics absorb and dissipate energy in the process of interface debonding, cavitation and matrix shear yield under the action of external force. Except that non-polar plastic resins can be toughened by directly adding elastomer particles with good compatibility (similar compatibility principle), other polar resins need effective compatibilization to achieve the purpose of final toughening. When several types of graft copolymers mentioned above are used as toughening agents, they will have strong interaction with the matrix, such as:
(1) Toughening mechanism with epoxy functional group: after ring opening of epoxy group, addition reaction occurs with hydroxyl, carboxyl or amine group at the end of polymer;
(2) Core-shell toughening mechanism: the outer functional groups are fully compatible with the components, and the rubber has toughening effect;
(3) Ionomer toughening mechanism: the physical crosslinking network is formed by the complexation between metal ions and carboxylic acid groups of polymer chain, so as to toughen.
In fact, if the toughening agent is regarded as a kind of polymer, this compatibilization principle can be extended to all polymer blends. When preparing useful polymer blends in industry, reactive compatibilization is a technology we must use. At this time, the toughening agent has a different meaning, and the titles of "toughening compatibilizer" and "interfacial emulsifier" are particularly vivid!
