Modified plastics are playing an increasingly important role in national life, and plastic toughening technology has been receiving attention from academic research and industry, today we will learn about plastic toughening issues such as:
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, and what are the methods?
4. how to understand the toughening must first increase the capacity?

I. Characterization of plastic toughness

The greater the rigidity of the material is less prone to deformation, the greater the toughness is more prone to deformation.
Toughness and rigidity is relative to reflect the degree of difficulty of the object deformation of a property, the greater the rigidity of the material is less prone to deformation, the greater the toughness of the material is more prone to deformation.
Usually, the greater the rigidity, the material's hardness, tensile strength, tensile modulus (Young's modulus), bending strength, bending modulus are larger; conversely, the greater the toughness, the greater the elongation at break and impact strength.
Impact strength is expressed as the strength of a specimen or component to withstand an impact, and is usually generalized to mean the amount of energy absorbed by the specimen before rupture occurs. Impact strength exhibits different values depending on the form of the strip, the test method and the conditions of the specimen, and therefore cannot be categorized as a fundamental property of the material.
The results obtained by different impact test methods cannot be compared.
Impact test methods are many, according to the test temperature: room temperature impact, low temperature impact and high temperature impact three.
According to the specimen force state, can be divided into bending impact - simply supported beam and cantilever beam impact, tensile impact, torsion impact and shear impact.
According to the energy used and the number of impacts, can be divided into a large energy impact and small energy impact test.
Different materials or different uses can choose different impact test methods, and get different results, these results can not be compared.

Second, the plastic toughening mechanism and impact factors

(A) silver grain - shear band theory
In the rubber toughened plastic blending system, the role of rubber particles have two main aspects:
On the one hand, as the center of stress concentration, inducing the matrix to produce a large number of silver grain and shear band;
On the other hand, to control the development of silver grain so that the silver grain will be terminated in time without developing into destructive cracks. The stress field at the end of the silver grain can induce shear bands and terminate the silver grain. It also prevents the silver grain from developing when it extends into the shear band. The creation and development of a large number of silverstreaks and shear bands in a material subjected to stress consumes a large amount of energy, which results in an increase in the toughness of the material. Silvering is macroscopically manifested as the phenomenon of stress whitening, whereas shear banding is associated with the generation of thin necks, which behave differently in different plastic matrices.
For example, HIPS matrix toughness is small, silvering, stress whitening, silvering volume increases, the transverse size is basically unchanged, stretching without fine neck; toughened PVC, the matrix toughness is large, yielding is mainly caused by the shear band, there is a fine neck, no stress whitening; HIPS/PPO, silvering, shear band have a considerable proportion of the fine neck and stress whitening phenomenon occurs at the same time.
(ii) factors affecting the effect of plastic toughening are mainly three points
1. Characteristics of the base resin
Research shows 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 realized through the following ways:
Increase the molecular weight of the matrix resin, so that the molecular weight distribution becomes narrow; through the control of whether the crystallization, as well as the degree of crystallization, crystal size and crystal type to improve the toughness. For example, the addition of nucleating agents to PP improves the crystallization rate and refines the grain size, thus improving the fracture toughness.
2. Toughening agent properties and dosage
①. Toughening agent dispersed phase particle size - for elastomer toughened plastics, the characteristics of the base resin is different, the optimal value of the elastomer dispersed phase particle size is not the same. For example, the optimal value of rubber particle size in HIPS is 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.
②. Influence of toughening agent dosage - There exists an optimal value for the amount of toughening agent added, which is related to the particle spacing parameter;
③. The effect of the glass transition temperature of the toughening agent - generally the lower the glass transition temperature of the elastomer, the better the toughening effect;
④. Influence of interfacial strength between toughening agent and base resin - the influence of interfacial bonding strength on toughening effect varies from system to system;
⑤. Influence of elastomer toughening agent structure - related to the type of elastomer, crosslinking degree, etc..
3、Binding force between two phases
Good bonding between the two phases can make the stress can be effectively transferred between the phases to consume more energy, the better the overall performance of the plastic on a macro level, in particular, the most significant improvement in impact strength. Usually this bonding force can be understood as the interaction force between the two phases, graft copolymerization and block copolymerization is a typical method to increase the bonding force of the two phases, the difference is that they are formed through the method of chemical synthesis of chemical bonds, such as graft copolymer HIPS, ABS, block copolymer SBS, polyurethane.
For tougheners toughening plastics, it belongs to the method of physical blending, but the principle is the same. Ideal blending system should be both partially compatible with the two components and their respective phase, the existence of an interface layer between the two polymers in the interface layer of the molecular chain of the diffusion of each other, there is a significant concentration gradient, by increasing the compatibility between the components of the blend, so that it has a good bonding force, and then enhance the diffusion of interfacial dispersion to increase the thickness of the interfacial layer. This is the plastic toughening is also the preparation of polymer alloys where the key technology - polymer compatibility technology!

Third, what are the plastic toughening agents? How to divide?

(A) the characteristics of the base resin
1, rubber elastomer toughening: EPR (binary ethylene-propylene), EPDM (EPDM), cis-butadiene rubber (BR), natural rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR), etc.; for the plastic resin used in toughening modification;
2, thermoplastic elastomer toughening: SBS, SEBS, POE, TPO, TPV, etc.; mostly used for polyolefin or non-polar resin toughening, used for polyester, polyamides and other polymers containing polar functional groups need to be added to toughening compatibilizer;
3, core-shell copolymers and reactive terpolymers toughening: ACR (acrylates), MBS (methyl acrylate - butadiene - styrene copolymer), PTW (ethylene - butyl acrylate - glycidyl methacrylate copolymer), E-MA-GMA (ethylene - methyl acrylate - glycidyl methacrylate copolymer) and so on. Glycerol ester copolymer), etc.; mostly used in engineering plastics and high temperature polymer alloy toughening;
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 an important way to prepare high toughness engineering plastics;
5, other ways of toughening: nanoparticle toughening (such as nano-CaCO 3), Sarin resin (DuPont metal ionomer) toughening and so on;

(B) in the actual industrial production, modified plastics toughening is probably divided into the following cases:
1, the synthetic resin itself toughness is insufficient, need to improve the toughness to meet the demand for use, such as GPPS, homopolymerization PP, etc.;
2, greatly improve the toughness of the plastic, to achieve ultra-tough, low temperature environment for long-term use requirements, such as ultra-tough nylon;
3, the resin has been filled, flame retardant and other modifications caused by the decline in the performance of the material, at this time must be effective toughening.
General-purpose plastics are generally obtained by free radical addition polymerization, the molecular main chain and side chain does not contain polar groups, toughening rubber particles and elastomer particles can be added to obtain a better toughening effect;
Engineering plastics are generally obtained by polycondensation polymerization, the molecular chain of the side chain or end group contains polar groups, toughening by adding functionalized rubber or elastomer particles higher toughness.

Fourth, the key to toughening plastics is to increase capacity

Generally speaking, the plastic in the external force to interface de-adhesion, cavitation, matrix shear yielding process of absorption, dissipation of energy, in addition to non-polar plastic resin toughening can be added directly to its compatibility with elastomer particles (similar to the principle of compatibility), other polar resins need to be effective in order to achieve the purpose of effective capacitance to achieve the ultimate toughening. The previously mentioned types of graft copolymers, when used as toughening agents, will have strong interactions with the matrix, for example:
1. epoxy-functionalized toughening mechanism: the epoxy group opens the ring and reacts with the hydroxyl, carboxyl or amine group at the end of the polymer.
2. Core-shell toughening mechanism: the outer functional group is fully compatible with the components, and the rubber is toughened.
3. Ionomer-type toughening mechanism: with the help of metal ions and polymer chain complexation between the carboxylic acid root to form a physical cross-linking network, so as to play the role of toughening.
In fact, if the toughening agent is regarded as a class of polymers, it is possible to extend this principle of capacitation to all polymer blends. Industrial preparation of useful polymer blends, reactive capacitance is a technology we must use, at this time the toughening agent has a different meaning, “toughening compatibilizer”, “interfacial emulsifier” designation appears to be extraordinarily image!
In summary, plastic toughening whether for crystalline plastics or amorphous plastics is equally important, and from general-purpose plastics, engineering plastics to special engineering plastics and its heat resistance gradually improved, the cost of prices continue to rise, so that the toughening agent heat resistance, aging resistance and other higher requirements, but also on the plastics modification and toughening technology is a big test, and the most important and most critical one is to maintain a good compatibility with the base and the components! The most important and the most critical one is to maintain good compatibility with the matrix and components!