POE price increase again?
As one of the commonly used toughening materials, this dynamic undoubtedly gives modification companies headaches. Strength and toughness are two important mechanical performance indicators, so plastic toughening modification has been a major direction of plastic modification, and nowadays by Strength and toughness are two important mechanical performance indicators, so plastic toughening modification has been a major direction of plastic modification, and nowadays by adding toughening agent is still the most direct and effective means of material toughening.
So in addition to the use of elastomer toughening, what other options? Today I take you to organize, toughening modification what are the feasible ideas.
Commonly used toughening agents can be divided into two categories: active toughening agents and inactive toughening agents. are those whose molecular chains contain active groups that can react with the matrix resin, which can form a network structure and increase part of the Non-reactive tougheners are a class of tougheners that are well compatible with the base resin, but do not participate Non-reactive tougheners are a class of tougheners that are well compatible with the base resin, but do not participate in the chemical reaction.
Elastomeric toughening materials
Almost all flexible materials can be used as toughening agents for brittle materials, the key is the compatibility between the multi-phase components, only a good compatibility toughening system to achieve the required toughening purpose, and otherwise it is counterproductive. Commonly used elastomeric tougheners are classified by their high or low glass transition temperature.
(1) High impact resistant resin
Such as chlorinated polyethylene (CPE), methyl methacrylate-butadiene-styrene copolymer (MBS), acrylate copolymer (ACR), styrene-butadiene block copolymer (SBS), hydrogenated SBS (SEBS), POE, EVA, etc.
(2) High impact resistant rubber
Such as ethylene propylene rubber (EPR), EPDM, NBR, NR, SBR, BR, CR, etc.
In addition, if TPU is used as a toughening agent for PA66, the notched impact strength of PA66/TPU composite is improved substantially, and the tensile properties of the composite are only slightly decreased. The mechanical properties reach the best when the addition amount of TPU is 5%. The crystallinity decreased by 50%, indicating that the system has obvious toughening effect on PA66.
Classification by internal structure of elastomer.
(1) Predetermined elastomer class
They are core-shell structured polymers, with a soft elastomer core and a polymer with a high glass transition temperature shell. Such as MBS, ACS, MABS, etc.
(2) Non-predetermined elastomer class
It is a net polymer, and its impact modification is modified by solvent action mechanism, such as CPE, EVA, etc.
(3) Transitional elastomer class
The structure is between predetermined and non-predetermined elastomer, such as ABS, etc.
Rigid toughened materials
Inorganic rigid toughening materials (RIF), mainly ultrafine inorganic fillers and special fillers.
Ultra-fine inorganic filler refers to the particle size between 0.1-5μm, the modification effect of such filler with the increase of filler addition, the blending system of the tensile strength and impact strength decreases smoothly, unchanged or slightly increased, while other properties continue to rise.
Special fillers include super fibers (CF), needle/sphere fillers, alkaline earth metal salts and rare earth minerals, etc.
Superfibers such as carbon fibers, boron fibers, quartz fibers, organic fibers, carbon nanotubes and various whiskers. The dispersion of superfibers in polymers must be carried out by relatively specific and targeted methods.
Needle-like filler such as needle-like wollastonite, which has a certain toughening effect.
Generally, it is treated with silane coupling agent or compound coupling agent for 10-15min at 50-80℃, and the addition amount is recommended 10-15%. The wollastonite without treatment does not have toughening effect.
The spherical fillers include glass microspheres, glass hollow microspheres, wollastonite beads, plastic beads, ceramic beads, etc.
The alkaline earth metal salts mainly include barium sulfate, the filling amount in a wide range, the impact strength of the filling material has different degrees of improvement, at the addition of about 60%, the impact strength reaches the peak.
Rare-earth minerals are mainly rare-earth oxides, alkyl rare-earth compounds and rare-earth salts, such as rubidium oxide and cerium fluoride. The toughening mechanism of rare earth minerals is generally related to their nucleation, i.e. rare earths are used as nucleating agents to improve the crystalline structure of the toughening system and achieve the toughening purpose. Mainly used in the toughening of crystalline polymers, such as PP, PA, POM, etc.
(2) Organic rigid toughened materials (ROF)
Such as PMMA, PP, PS, MMA/S and SAN, etc. With the dual function of toughening and strengthening, the effect of toughening and strengthening is parabolic.
When a polymer material is subjected to impact loading when the material is damaged (fracture), the size of its toughness depends on the size of the impact energy absorbed by the material and its ability to resist crack expansion. So how do the reinforcing materials and the matrix play a role in toughening in polymers?
1. Elastomer toughening mechanism
Elastomer directly absorbs energy, when the specimen is impacted will produce micro-cracks, when the rubber particles across both banks of the crack, the crack to develop must stretch the rubber, rubber deformation process to absorb a lot of energy, thereby increasing the impact strength of plastic.
2. Yield theory
Rubber toughened plastic high impact strength mainly comes from the matrix resin yield deformation occurred a lot, the matrix resin yield deformation of the reason is that the coefficient of thermal expansion and Poisson’s ratio of rubber are greater than that of plastic, the cooling stage in the molding process of thermal contraction and deformation process of the transverse contraction of the surrounding matrix hydrostatic tension stress, so that the matrix resin free volume increases, reducing its glass transition temperature, easy to produce plastic deformation and improve toughness. It is easy to produce plastic deformation and improve the toughness. On the other hand, it is caused by the stress concentration effect of rubber particles.
3. Cracking core theory
Rubber granules act as stress concentration points, creating a large number of small cracks instead of a few large ones, and it takes more energy to extend numerous small cracks than to extend a few large ones. At the same time, the stress fields of a large number of small cracks interfere with each other, weakening the leading stresses for crack development, which, in turn, slows down the crack development and leads to the termination of the cracks.
4. Multiple silver pattern theory
Due to the extremely high number of rubber particles in the toughened plastic, a large number of stress concentrates trigger a large number of silver patterns, which can dissipate a large amount of energy. The rubber particles are also silver stripe terminators, and the small particles cannot terminate the silver stripe.
5. Silver stripe-shear band theory
This is an important theory that is generally accepted in the industry. Numerous experiments have shown that the polymer deformation mechanism consists of two processes: one is the shear deformation process and the other is the silver rippling process. The shear process includes both diffuse shear yield deformation and the formation of local shear bands. Shear deformation is only a change in the shape of the object, the cohesion energy between molecules and the density of the object is basically unchanged. The silver striation process, on the other hand, causes the density of the object to decrease significantly. On the one hand, there are cavities in the silver grain body, indicating that silver grainization has caused certain damage to the material, which is a precursor of sub-microscopic fracture damage; on the other hand, silver grain consumes a lot of energy in the formation and growth process, which restrains the expansion of cracks and improves the toughness of the material, which is one of the mechanical mechanisms of polymer toughening. Therefore, the correct understanding of silver striation phenomenon is the core of understanding the deformation and fracture process of polymer materials, and is one of the keys to carry out the design of co-blended modified plastics, especially toughened plastics.
6. Cavitation theory
Cavitation theory refers to the phenomenon of cavitation inside the rubber particles or at the interface layer between the rubber particles and the substrate under the action of three-dimensional stress in the process of low temperature or high speed deformation. The theory that: rubber-modified plastic under the action of external forces, the disperse phase rubber particles due to stress concentration, resulting in the interface between the rubber and the matrix and itself to produce cavitation, once the rubber particles are cavitated, the hydrostatic tension stress around the rubber is released, the stress state of the thin matrix ligament between the cavities, from three-dimensional to one-dimensional, and the plane strain into plane stress, and this new stress state is conducive to the shear band formation. Therefore, cavitation itself cannot constitute a brittle-tough transition of the material; it only leads to a shift in the stress state of the material, which triggers shear yielding, prevents further crack expansion, and improves the material toughness.