Views: 1 Author: Site Editor Publish Time: 2023-03-15 Origin: Site
For many types of modified plastic technologies, plastic toughening technology has been studied and paid attention to by enterprises and academics, because the toughness of materials often plays an important or even the most critical impact on the application of some products. Another point is that more and more injection molded parts need both toughness and good rigidity, so is there a strong engineering plastic with good toughness, rigidity and strength? Today, with this problem that is very related to many practical applications, let's talk about the toughness and rigidity of plastics, we may wish to start from the point of plastic toughening:
1. How to test and evaluate the toughness of plastics?
2. What is the principle of plastic toughening?
3. What are the commonly used flexibilizers?
4. What are the toughening methods of plastics?
5. How to understand that toughening must first increase capacity?
1. Performance characterization of plastic toughness
Toughness is the opposite of rigidity, is a property that reflects the difficulty of deformation of the object, the greater the rigidity, the less likely it is to deform, and the greater the toughness, the easier it is to deform. But there are also some engineering plastics, while the toughness is very good, the rigidity is also very good, this comprehensive performance of the material is a strong engineering plastic. Generally, the greater the rigidity, the greater the hardness, tensile strength, tensile modulus (Young's modulus), bending strength, and flexural modulus of the material; Conversely, the greater the toughness, the greater the elongation at break and the greater the impact strength. Impact strength is expressed as the strength of the spline or part subjected to impact, and usually refers to the energy absorbed by the spline before it breaks. Impact strength exhibits different values depending on spline morphology, test methods, and specimen conditions, and therefore cannot be classified as a fundamental property of the material.
There are many methods of 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 force state of the specimen, it can be divided into bending impact - simple support beam and cantilever impact, tensile impact, torsional impact and shear impact; According to the energy used and the number of impacts, it can be divided into one impact 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, which cannot be compared.
2. Toughening mechanism and influencing factors of plastics
Silver-shear band theory
In the compounding system of rubber toughened plastics, the role of rubber particles mainly has two aspects:
On the one hand, as the center of stress concentration, it induces the matrix to produce a large number of silver lines and shear bands;
On the other hand, controlling the development of silver grain allows the silver grain to terminate in time without developing into destructive cracks. The stress field at the end of the silver grain can induce the shear band and terminate the silver grain. When the silver grain extends to the shear band, it also prevents the development of the silver grain. When the material is stressed, the generation and development of a large number of silver grains and shear bands consume a lot of energy, so that the toughness of the material is improved. Silver streak macroscopically manifests as stress gray hair phenomenon, while shear bands are associated with fine neck generation, which behaves differently in different plastic substrates.
For example, the HIPS matrix has less toughness, silver graining, stress whitishing, silver graining volume increases, the transverse size is basically unchanged, and there is no fine neck stretching; Toughened PVC, large matrix toughness, yield is mainly caused by shear belt, fine neck, no stress whitening; HIPS/PPO, silver grain, shear band all account for a considerable proportion, fine neck and stress whitening phenomenon at the same time.
There are three main factors affecting the toughening effect of plastics
1. Characteristics of matrix resin
Studies have shown that improving the toughness of the matrix resin is conducive to improving the toughening effect of toughened plastics, and improving the toughness of the matrix resin can be achieved by the following ways:
Increase the molecular weight of the matrix resin to narrow the molecular weight distribution; Toughness is improved by controlling whether crystallization is made, as well as crystallinity, crystal size, and crystal form. For example, a nucleating agent is added to PP to increase the crystallization rate and refine the grains, thereby improving the fracture toughness.
2. Characteristics and dosage of flexibilizer
A. Influence of the particle size of the dispersed phase of the flexibilizer - for the toughened elastomer plastic, the characteristics of the matrix resin are different, and the optimal value of the particle size of the elastomer dispersed phase is also different.
B. Effect of the amount of flexibilizer used – there is an optimal value for the amount of toughener added, which is related to the particle spacing parameter;
C. The influence 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 of flexibilizer and matrix resin - the influence of interfacial bonding strength on toughening effect varies from system to system;
E. The influence of the structure of the elastomer toughener - related to the type of elastomer, the degree of crosslinking, etc.
3. The bonding force between the two phases
The two phases have good adhesion, so that when stress occurs, it can be effectively transferred between phases to consume more energy, and the better the comprehensive properties of plastics on the macroscopic level, especially the improvement of impact strength is the most significant. Usually this binding force can be understood as the interaction force between the two phases, grafting copolymerization and block copolymerization is a typical way to increase the binding force of the two, the difference is that they form chemical bonds through chemical synthesis.
For toughened plastics, it is a physical blending method, but the principle is the same. The ideal blending system should be that the two components are both partially compatible and separate phases, there is an interface layer between the phases, and the molecular chains of the two polymers in the interface layer diffuse each other, with obvious concentration gradients, and increase the compatibility between the blended components to make them have good bonding force, and then enhance the diffusion to make the interface diffuse and increase the thickness of the interface layer. This is not only plastic toughening, but also an important technology for the preparation of polymer alloys - polymer compatibility technology.
3. What are the plastic toughening agents? How is it divided?
How are the commonly used toughening agents for plastics divided?
1. Toughening of rubber elastomer: EPR (diethylene propylene), EPDM (ethylene propylene), cis-butadiene rubber (BR), natural rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR), etc.; Suitable for toughening modification of plastic resins used;
2. Toughening of thermoplastic elastomers: SBS, SEBS, POE, TPO, TPV, etc.; It is mostly used for polyolefin or non-polar resin toughening, and compatibilizers need to be added when toughening polymers containing polar functional groups such as polyesters and polyamides;
3. Toughening of core-shell copolymer and reactive terpolymer: ACR (acrylates), MBS (methyl acrylate-butadiene-styrene copolymer), PTW (ethylene-butyl acrylate-glycidyl methacrylate copolymer), E-MA-GMA (ethylene-methyl acrylate-glycidyl methacrylate copolymer), etc.; Mostly used in engineering plastics and high temperature resistant polymer alloy toughening;
4. High toughness plastic blending and toughening: 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 toughening: nanoparticle toughening (such as nano CaCO3), sarin resin (DuPont metal ionomer) toughening, etc.;
In actual industrial production, the toughening of modified plastics is roughly divided into the following situations:
1. The toughness of the synthetic resin itself is insufficient, and it is necessary to improve the toughness to meet the needs of use.
2. Greatly improve the toughness of plastics, and realize the requirements of ultra-toughening and long-term use in low temperature environments, such as ultra-tough nylon;
3. After the resin is filled, flame retardant and other modifications, the performance of the material deteriorates, and effective toughening must be carried out at this time.
General-purpose plastics are generally obtained by free radical addition polymerization, the molecular main chain and side chain do not contain polar groups, and rubber particles and elastomer particles can be added to obtain better toughening effect when toughening; Engineering plastics are generally obtained by condensation polymerization, and the side chains or end groups of the molecular chain contain polar groups, which can be toughened by adding functional rubber or elastomer particles to have higher toughness.
Types of flexibilizers for commonly used resins
Generally speaking, plastics in the process of interface debonding, cavitation, matrix shear yield when subjected to external force to absorb, dissipate energy, except for non-polar plastic resin toughening can directly add its compatible elastomer particles (similar compatibility principle), other polar resins need effective capacity increase to achieve the purpose of final toughening. The previously mentioned grafted copolymers interact strongly with the matrix when used as flexibilizers, such as:
(1) Toughening mechanism with epoxy functional group: after the epoxy group is opened, it has an addition reaction with the polymer end hydroxyl, carboxyl group or amine group;
(2) Core-shell toughening mechanism: the outer functional group is fully compatible with the components, and the rubber plays a toughening effect;
(3) Ionomer-type toughening mechanism: physical cross-linking network is formed by the complexation between metal ions and carboxylate of polymer chains, thereby playing a role in toughening.
In fact, if the toughening agent is considered as a class of polymers, this capacitance principle can be extended to all polymer blends. As shown in the table below, when preparing useful polymer blends in industry, reactive capacity enhancement is the technology we must use, at this time the toughening agent has a different meaning, the title of "toughening compatibilizer" and "interface emulsifier" is particularly imaged!
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