Today, the role of modified plastics in national life is becoming more and more important, especially in the areas of automobiles, home appliances, etc. For plastics with many types of modified plastic technology, plastic toughening technology has been academically And research and attention in the industry, because the toughness of the material often has a decisive influence on the application of the product. This article will answer several questions about plastic toughening:
1. How to test and evaluate plastic toughness?
2, What is the principle of plastic toughening?
3, What are the commonly used tougheners?
4, What plastic toughening methods are there?
5. How to understand that toughening must be added first?
First, the performance characterization of plastic toughness
- The greater the rigidity, the less likely the material to deform, and the greater the toughness, the more likely it is to deform.
Relative to toughness and rigidity, it is an attribute that reflects the degree of difficulty of deformation of an object. The greater the rigidity, the less likely the material to deform. The greater the toughness, the more likely it is to deform. Generally, the greater the rigidity, the hardness of the material, the tensile strength, and the pull. The modulus (Young's modulus), bending strength, and flexural modulus are all greater; on the contrary, the higher the toughness, the greater the elongation at break and the impact strength. The impact strength is the strength of the spokes or parts subjected to impact. , Usually refers to the energy absorbed by the spline before it breaks. The impact strength varies with spline morphology, test methods, and sample conditions. Therefore, it cannot be attributed to the basic properties of the material.
- The results of different impact test methods cannot be compared
There are many methods for impact test, depending on the test temperature: There are three types of impact at room temperature, low temperature impact and high temperature impact; according to the force state of the sample, it can be divided into bending impact - simply supported beam and cantilever beam impact, tensile impact, torsional impact And shear shock; According to the energy and the number of impacts, can be divided into a large energy impact and multiple energy impact test. Different materials or different uses can choose different impact test methods, and get different results, these The result cannot be compared.
Second, the plastic toughening mechanism and influencing factors
(A) Craze-shearing 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, induced matrix produces a large number of silver lines and shear bands;
On the other hand, controlling the development of the silver streaks causes the silver streaks to stop in time without developing destructive cracks.
The stress field at the end of the silver strea can induce a shear band and terminate the silver streaks. When the silver streaks extend into the shear band, it also prevents the development of the silver streaks. When the material is stressed, a large number of silver streaks and shear bands are produced. And the development consumes a lot of energy, so that the toughness of the material is improved. The macroscopic appearance of the silver streak is the discovery of the stress white image, while the shear band is related to the production of the neck, and it performs differently in different plastic matrices.
For example, the HIPS matrix has less ductility, silver striations, whiter stress, increased silver veining volume, substantially the same lateral dimension, no neckline stretching, toughened PVC, high matrix toughness, and yield is mainly caused by shear bands. There are thin necks, no stress whitish; HIPS/PPO, silver lines, shear bands all occupy a considerable proportion, and neck and stress whitish phenomena occur simultaneously.
(b) There are three main factors that affect plastic toughening effect:
1, the characteristics of the matrix resin
Studies have shown that improving the toughness of the matrix resin is beneficial to improving the toughening effect of the toughened plastic. The toughness of the matrix resin can be improved by the following ways:
Increase the molecular weight of the matrix resin to make the molecular weight distribution narrow; by controlling the crystallinity and crystallinity, the crystal size and crystal shape increase the toughness. For example, adding a nucleating agent in PP increases the crystallization rate and refines the crystal grain. Improve fracture toughness.
2, the characteristics and amount of toughening agent
A. Effect of Particle Size of Toughening Agent Dispersion Phase - For elastomer toughened plastics, the characteristics of the matrix resin are different, and the optimum value of the particle size of the dispersed phase of the elastomer is not the same. For example, the best value of rubber particle size in HIPS. For 0.8-1.3μm, the optimal particle size of ABS is about 0.3μm, and the optimal particle size of PVC modified ABS is about 0.1μm.
B. Effect of the amount of toughener - There is an optimal value for the amount of toughener added, which is related to the particle spacing parameter;
C. Effect of Toughening Agent Glass Transition Temperature - The lower the glass transition temperature of the general elastomer, the better the toughening effect;
D. Influence of interfacial strength between toughener and matrix resin - Effect of interfacial bond strength on toughening effect Different systems differ;
E. Effect of elastomer toughener structure - related to elastomer type, cross-linking degree, etc.
3, the binding force between the two phases
The good binding force between the two phases can enable the effective transmission between phases to consume more energy when stress occurs, and the overall plastic performance on the macro is better. Among them, the improvement of impact strength is the most significant. The binding force can be understood as the interaction force between the two phases. Graft copolymerization and block copolymerization are typical methods for increasing the binding force of the two phases. The difference is that they form chemical bonds, such as graft copolymers, by chemical synthesis methods. HIPS, ABS, Block Copolymer SBS, Polyurethane.
For toughening toughened plastics, it is a method of physical blending, but the principle is the same. The ideal blending system should be that the two components are both partially compatible and phase-forming, and there is an interfacial layer between the phases. The molecular chains of the two polymers in the interfacial layer diffuse with each other and have a distinct concentration gradient. By increasing the compatibility between the blending components, they have good binding force, which in turn enhances diffusion and diffuses the interface, increasing the interface. The thickness of the layer. And this, that is, plastic toughening is also the key technology for the preparation of polymer alloys - polymer compatibility technology!
Third, what are the plastic toughening agents? How to divide?
(A) How to divide the toughening agent commonly used in plastics
1. Toughening of rubber elastomers: EPR (ethylene propylene diene monomer), EPDM (ethylene propylene diene monomer), butadiene rubber (BR), natural rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR), etc. Suitable for toughening modification of plastic resin used;
2. Toughening of thermoplastic elastomers: SBS, SEBS, POE, TPO, TPV, etc.; used for toughening of polyolefins or non-polar resins, for toughening of polymers containing polar functional groups such as polyesters, polyamides, etc. Need to add compatibilizer;
3, Toughening of core-shell copolymers and reactive terpolymers: ACR (acrylic esters), MBS (methyl acrylate-butadiene-styrene copolymers), PTW (ethylene-butyl acrylate-methyls) Glycidyl acrylate copolymer), E-MA-GMA (ethylene-methyl acrylate-glycidyl methacrylate copolymer), etc.; used for toughening of engineering plastics and high temperature polymer alloys;
4, high toughness plastic blending toughening: PP / PA, PP / ABS, PA / ABS, HIPS / PPO, PPS / PA, PC / ABS, PC / PBT, etc.; polymer alloy technology is to prepare high toughness engineering plastics Important approach
5, Toughening in other ways: Toughening of nanoparticles (such as nano-CaCO3), toughening of Sarin resin (DuPont metal ionomer);
(b) In actual industrial production, the toughening of modified plastics can be divided into the following situations:
1, The toughness of the synthetic resin itself is insufficient, and the toughness needs to be improved to meet the use requirements, such as GPPS, homopolymerization PP, etc.;
2. Significantly improve the toughness of plastics, realize super-toughness, and long-term use requirements in low-temperature environments, such as super tough nylon;
3, The resin is filled, the flame retardant and other modifications cause the material's performance to decrease. At this time, effective toughening must be performed.
General plastics are generally obtained by free radical addition polymerization. The molecular main chain and side chains do not contain polar groups. Adding rubber particles and elastomer particles when toughening can achieve better toughening effect; engineering plastics It is generally derived from condensation polymerization. The side chains or end groups of the molecular chains contain polar groups. Toughening can be achieved by adding functionalized rubber or elastomer particles with higher toughness.
Types of toughening agents commonly used in resins
The key to toughening plastics is to increase capacity—pro, what do you think?
In general, when the plastic is subjected to an external force, the plastic desorbs at the interface, voids, and absorbs and absorbs energy during the process of shear and yield of the matrix. In addition to the toughening of the non-polar plastic resin, the elastomer can be directly added to the elastomer. In the case of particles (similar compatibility principle), other polar resins need effective compatibilization to achieve the final toughening purpose. When the above-mentioned several types of graft copolymers are used as toughening agents, they will strongly interact with the matrix. Role, for example:
(1) Toughening mechanism with epoxy functional group: After the ring opening of the epoxy group, an addition reaction occurs with the polymer hydroxyl group, carboxyl group or amine group;
(2) Core-shell type toughening mechanism: The outer functional group is fully compatible with the components, and the rubber has a toughening effect;
(3) Ionomer toughening mechanism: A physical crosslinked network is formed by the complexation between the metal ions and the carboxylates of the polymer chain, thereby playing a role in toughening.
In fact, if the toughening agent is regarded as a class of polymers, this principle of compatibilization can be extended to all polymer blends. In the following table, when industrially preparing useful polymer blends, the reaction Sexual expansion is a technology that we must apply. At this point, the toughening agent has a different meaning. The term 'toughening compatibilizer' and 'interfacial emulsifier' are especially appealing!
Examples of polymer blends with industrial value and their capacity expansion
X—reported reports of such blends are less reported; none—indicates that useful polymer blends are not required for effective compatibilization; reactivity 2—indicates blends can be blended between blends In-situ generation of useful graft or block copolymers to increase the compatibility between the components
In summary, plastic toughening is equally important for both crystalline and amorphous plastics. From general plastics, engineering plastics to special engineering plastics, the heat resistance is gradually increased, the cost price is rising, and so is the resistance to tougheners. Heat resistance, aging resistance, etc. put forward higher requirements. At the same time, it is also a big test for plastics modification and toughening technology. The most important and most critical one is to maintain good compatibility with the matrix and components!