Three technologies for plastic toughening
Nowadays, the role of modified plastics in national life is becoming more and more important, especially in the fields of automobiles and home appliances. For the modified plastics technology, plastic toughening technology has been studied and paid attention by academics and industry, because the toughness of materials often has a decisive influence on the application of products. This article will answer a few questions about 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 toughening agents?
4. What are the toughening methods for plastics?
5. How to understand that toughening must first increase capacity?
First, the performance characterization of plastic toughness
——The greater the rigidity, the less likely the material is to deform. The greater the toughness, the easier it is to deform.
The toughness is opposite to the rigidity. It is an attribute that reflects the difficulty of deformation of the object. The larger the rigidity, the less likely the material is to deform. The greater the toughness, the easier 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; conversely, the greater the toughness, the more the elongation at break and the impact strength Big. Impact strength is expressed as the strength of the spline or workpiece that is subjected to impact, and generally refers to the energy absorbed by the spline before it is broken. The impact strength varies with the shape of the spline, the test method and the conditions of the sample, and therefore cannot be classified as the basic properties of the material.
——The results obtained by different impact test methods are not comparable
There are many methods for 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 stress state of the specimen, it can be divided into bending impact-simple beam and cantilever beam impact, tensile impact and torsional impact. And shear shock; according to the energy and the number of impacts, it can be divided into one impact of large energy and multiple impact tests of small energy. Different impact tests can be selected for different materials or different applications, and different results can be obtained. These results cannot be compared.
Second, plastic toughening mechanism and influencing factors
(1) Silver-Shear Band Theory
In the blending system of rubber toughened plastics, the role of rubber particles is mainly twofold:
On the one hand, as the center of stress concentration, the induced matrix generates a large amount of silver streaks and shear bands;
On the other hand, controlling the development of silver streaks causes the silver streaks to terminate in time without developing into destructive cracks.
The stress field at the end of the silver streak can induce a shear band to terminate the silver streaks. The development of silver streaks is also prevented when the silver streaks extend to the shear band. The generation and development of a large amount of silver streaks and shear bands when the material is subjected to stress consumes a large amount of energy, thereby increasing the toughness of the material. The macroscopic representation of silver streaks is characterized by stress whitening, while the shear bands are related to the formation of thin necks, which behave differently in different plastic matrices.
For example, the HIPS matrix has less toughness, silver streaking, whitening stress, increased volume of silver streaks, substantially unchanged lateral dimensions, and no necking for stretching; toughened PVC, high toughness of the matrix, and yielding mainly caused by shear bands. There are thin necks and no stress whitening; HIPS/PPO, silver streaks and shear bands all occupy a considerable proportion, and the neck and stress whitening phenomenon are simultaneously produced.
(2) There are three main factors affecting the toughening effect of plastics.
1. Characteristics of matrix resin
Studies have shown that increasing the toughness of the matrix resin is beneficial to improve the toughening effect of the toughened plastic, and improving the toughness of the matrix resin can be achieved by the following means:
The molecular weight of the matrix resin is increased to make the molecular weight distribution narrow; the toughness is improved by controlling the crystallinity and crystallinity, crystal size, crystal form, and the like. For example, the addition of a nucleating agent to the PP increases the crystallization rate and refines the grains, thereby increasing the fracture toughness.
2. Characteristics and dosage of toughening agent
A. Effect of the particle size of the toughener dispersed phase - For elastomer toughened plastics, the properties of the matrix resin are different, and the optimum values ​​of the dispersed phase of the elastomer are also different. For example, the optimum particle size of rubber in HIPS is 0.8-1.3 μm, the optimum particle size of ABS is about 0.3 μm, and the optimum particle size of PVC-modified ABS is about 0.1 μm.
B. Effect of the amount of toughening agent - there is an optimum value for the amount of toughening agent added, which is related to the particle spacing parameter;
C. Effect 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 between toughener and matrix resin - the effect of interfacial bond strength on toughening effect varies from system to system;
E. Effect of elastomer toughener structure - related to elastomer type, degree of crosslinking, etc.
3. Binding between two phases
The two phases have a good bonding force, which can make the effective transmission between the phases when the stress occurs, and consume more energy. The overall performance of the plastic on the macro is better, especially the impact strength is most remarkable. Usually, this binding force can be understood as the interaction force between two phases. Graft copolymerization and block copolymerization are typical methods for increasing the binding force of two phases. The difference is that they form chemical bonds by chemical synthesis, such as Branch copolymer HIPS, ABS, block copolymer SBS, polyurethane.
For toughening agents toughening 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 partially compatible and each phase is formed, and an interfacial layer exists between the phases. In the interfacial layer, the molecular chains of the two polymers interdiffuse each other, and there is a significant concentration gradient. The compatibility between the components makes them have a good bonding force, which in turn enhances the diffusion and diffuses the interface and increases the thickness of the interface layer. And this, that is, plastic toughening is also the key technology for the preparation of polymer alloys - polymer compatible technology!
3. What are the plastic toughening agents? How to divide?
(1) How to divide the toughening agent commonly used in plastics
1. Rubber elastomer toughening: EPR (diethylene glycol), EPDM (ethylene propylene diacetate), butadiene rubber (BR), natural rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR), etc. Suitable for toughening modification of the plastic resin used;
2, thermoplastic elastomer toughening: SBS, SEBS, POE, TPO, TPV, etc.; used for polyolefin or non-polar resin toughening, for polyester, polyamide and other polymers containing polar functional groups toughen When a compatibilizer is added;
3. Toughening of core-shell copolymer and reactive terpolymer: ACR (acrylate), MBS (methyl acrylate-butadiene-styrene copolymer), PTW (ethylene-butyl acrylate-methyl Glycidyl acrylate copolymer), E-MA-GMA (ethylene-methyl acrylate-glycidyl methacrylate copolymer), etc.; 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 the preparation of high toughness engineering plastics. Important route
5. Toughening by other means: toughening of nanoparticles (such as nano-CaCO3), sanding resin (DuPont metal ionomer) toughening, etc.;
(2) In actual industrial production, the toughening of modified plastics is roughly divided into the following cases:
1. The synthetic resin itself has insufficient toughness, and it is necessary to improve the toughness to meet the use requirements, such as GPPS, homopolymer PP, etc.;
2, greatly improve the toughness of plastics, to achieve the requirements of super toughening, long-term use of low temperature environment, such as super tough nylon;
3. The resin is filled, flame retardant, etc., which causes the performance of the material to decrease. At this time, effective toughening must be carried out.
General-purpose plastics are generally obtained by free radical addition polymerization. The main chain and side chain of the molecule do not contain polar groups. When toughening, rubber particles and elastomer particles can be added to obtain better toughening effect; Generally, it is obtained by condensation polymerization, and the side chain or terminal group of the molecular chain contains a polar group, and the toughness can be obtained by adding a functionalized rubber or elastomer particles toughness.
Toughener types of commonly used resins
The key to plastic toughening is to increase capacity - pro, what do you think?
In general, plastics absorb and dissipate energy during the process of external decompression, voiding, and matrix shear yielding. In addition to the toughening of non-polar plastic resins, elastomers with good compatibility can be directly added. For particles (similar to the principle of compatibility), other polar resins require effective compatibilization to achieve ultimate toughening. When the above-mentioned types of graft copolymers act as toughening agents, they will have strong interaction with the matrix, for example:
(1) Toughening mechanism with epoxy functional group: the ring-forming reaction of the epoxy group with the hydroxyl group, carboxyl group or amine group of the polymer end;
(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-type toughening mechanism: a physical cross-linking network is formed by the complexation between the metal ions and the carboxylate of the polymer chain, thereby functioning as a toughening.
In fact, if the toughening agent is considered as a class of polymers, this compatibilization principle can be extended to all polymer blends. As shown in the table below, when industrially preparing useful polymer blends, reactive compatibilization is a technique we must use. At this time, the toughening agent has a different meaning, "toughening compatibilizer", "interface The name "emulsifier" is particularly attractive!
Industrial polymer blend examples and their compatibilization methods
X - less reported in the literature for such blends; none - means that useful polymer blends are obtained without effective compatibilization; reactivity 2 - means blending between blends In-situ formation of useful graft or block copolymers to improve compatibility between components
In summary, plastic toughening is equally important for crystalline plastics and amorphous plastics, and the heat resistance of general-purpose plastics, engineering plastics, and special engineering plastics is gradually increasing, and the cost price is also rising, thus resisting toughening agents. Heat and aging resistance have put forward higher requirements, and it is also a big test for plastic modification and toughening technology. The most important and most important one is to maintain good compatibility with the matrix and components!
Check Valve,Stainless Steel Check Valve,Duckbill Check Valve,Marine Check Valve
Jiangsu Hida Marine Valve Co., Ltd , https://www.haidavalve.com