How to choose plastic modifier

At present, there is a tendency to move away from simple, unmodified plastic resins. Plastics are being broadened beyond their original characteristics. Recycled plastics need to be regenerated and the mix needs to be enhanced. Therefore, the use of modifiers is wise and critical. Today, modifiers are no longer used solely to improve impact strength. Their use also includes heat distortion modifiers and processing aids.

The choice of modifier first considers the polymer. Is the processed polymer crystalline or amorphous? Is the yield stress high or low? Is it easy to stress crack?

The first question related to the degree of crystallinity. Plastics such as nylon and PBT are not susceptible to impact modification because their crystalline regions do not produce any inherent yield or cracking tendency as sites where cracking occurs. Amorphous plastics such as polycarbonate (PC) and polystyrene (PS) are easy to modify.

The principle of impact modification is that the impact energy is absorbed by the plastic deformation in the initial stage of cracking and before it propagates. There are two deformation mechanisms here: 1 yielding or stretching; 2 silver lines, micro-crack structures formed by very fine micro-wires absorb energy in time to the resin.

If the plastic is easily deformable, ie it is mainly deformed by yielding, it is necessary to reduce its yield stress. To do this, it is not necessary to substantially reduce the overall tensile properties, but to uniformly disperse the rubber particles in the resin to form a stress concentration point. During the impact, as the stress increases, the rubber particles can feel each other's presence and produce local yield before the cracks are generated. The plastic's ability to yield determines how much rubber it needs. Therefore, many small uniform particles are ideal. Particles with a size of 0.12-1.0 microns are used in easily deformable resins. To minimize the modifiers needed, the best particle size should be chosen.

If a resin such as PS or SAN is deformed by a crack generating mechanism, 0.3-4 micron rubber particles are required. Large rubber particles can prevent cracks from developing into cracks, which will transfer energy from one crack to two or three new cracks, which will absorb more impact energy.

Type of rubber impact modifier

Body rubbers such as EPDM, EEA and EVA must be mechanically dispersed into the resin to obtain the desired particle size. However, when rubber and plastic resins are not similar, doing so is ineffective. The reason for this is that in further melt processing such as molding, surface tension can cause the rubber to separate from the mechanical dispersion, resulting in poor surface morphology and sizing. This problem has been successfully solved by the following method: Putting EPDM into PP, both have almost equal solubility coefficients, and the mixed thermodynamic energy exceeds the separated surface tension energy.

Grafting the body rubber can overcome some of the aforementioned drawbacks. Currently, rubbers with grafted polymer chains sold by suppliers can increase the compatibility of the rubber and the parent resin. Examples of this are EPDM-grafted SANs. The mechanical mixing of the two is critical. Scaling and granules are a problem because the grafts cannot fully interact with the parent resin at these sites. This material can be used in PCs or SANs.

Block polymer rubber When the rubber and good trim polymer are connected together, a network structure appears in the rubber particles, that is, has compatibility. At this point it can be seen that its performance has been further improved. Therefore, in block polymers such as SBS, although the particle size is difficult to control, it can still be well dispersed. This type of SBS is very effective in PS and adhesives.

Functional body/block polymer rubber When the bulk rubber is grafted onto a reactive functional group and then reacted with the parent rubber, a more desirable situation can be obtained. Examples of this are EPDM grafting to maleic anhydride, SBS grafting to maleic anhydride, and the like. The functional group reacts with a matrix resin such as nylon, and the matrix resin becomes its compatible material, that is, an ideal surfactant for dispersing particles.

Rubber grafted latex At this point, the desired particle size is determined and fixed in place. ABS (acrylonitrile/butadiene/styrene) and MBS (methyl methacrylate/butadiene/styrene) impact modifiers need only to be dispersed. Grafting imparts cohesiveness to the matrix and compatibility with the matrix, and dispersed particles do not agglomerate during processing. Mild cross-linking maintains the integrity of the particles and results in 0.08-0.5 micron particles. However, at present, this emulsification process cannot produce particles of 0.6 μm or more. This modifier is mainly used in PVC.

PVC modifiers are used in glassy amorphous PVC modifiers according to their function and modification characteristics and can be divided into 6 groups (see Table 1, page 106): 1 High-impact impact modifiers: for opaque Impact resistant mixture.

2 Transparent Impact Modifier: This modifier is used when optical properties and impact resistance are required.

3 thermal deformation modifier: used to improve the PVC mixture processing temperature range.

4 Ordinary modifiers: used to improve impact resistance, high temperature strength and low temperature flexibility.

5 Weatherability Modifiers: The use of such modifiers in outdoor applications can prevent UV photodegradation.

6 Processing aids: Improve the melt properties of PVC by reducing melting time.

Impact modifiers such as ABS and MBS have a synergistic effect on the improvement of PVC impact resistance. Therefore, adding a small amount of modifier in PVC can obtain high impact resistance and increase the flexibility of PVC without significantly changing the mechanical properties of the mixture. The molecular weight of PVC determines the amount of impact modifier. The higher the molecular weight, the less the amount of modifier required. The final use of the product determines the molecular weight required for the PVC mix. For example, low-molecular-weight PVC is best processed by in-mold processing; high-molecular-weight PVC is selected by tubular extrusion. Typical applications for high-impact impact modifiers are for PVC pipes, injection molding compounds, and calendered opaque films and sheets.

Transparent impact modifiers provide some additional properties such as light transmission, haze, and yellowness index in the PVC mix as opaque modifiers. Low optical properties such as white and discoloration. In the preparation of emulsions of ABS and MBS modifiers, the optical properties needed to maintain transparency are obtained by equalizing the refractive index of PVC and modifiers; by controlling the particle size of the rubbery matrix at 1000-3 000 A The impact resistance is obtained within a narrow range of distribution; the compatibility/incompatibility balance (impact resistance) is obtained by the solubility parameters of the grafted S/AN or MMA/S. Typical applications for such modifiers include transparent calendered films, packaging sheets, and blown PVC bottles.

Thermal deformation modifiers can increase the effective heat application temperature of PVC. Each addition of a modifier can increase IT. Adding a thermal deformation modifier to PVC also increases stiffness, minimizing the impact of tensile strength, but often weakens the impact strength. Such modifiers usually consist of poly-α-methylstyrene/acrylonitrile (AMSAN) or glutarimide. For AMS polymers, the thermal deformation of PVC is increased due to the steric hindrance of its methyl group attached to styrene. Because of its heterocyclic structure, glutarimide polymers can increase the stiffness of the polymer chains, thereby increasing the thermal deformation of PVC precursors. Thermal deformation modifiers include vinyl siding, heat-resistant profiles, and automotive instrumentation gaskets that require molded-fastness.

Common modifiers are semi-rigid modifiers for semi-rigid PVC blends and are typical ABS modifiers, containing less butadiene and more ungrafted full-rigid poly-S/AN. These modifiers have two phases of rigidity and rubberiness, allowing semi-rigid compounds to have a variety of properties. The butadiene rubber phase can increase the crack resistance at low temperatures, and the high molecular weight rigid S/AN has processability such as thermoformability and good performance retention. Typical uses of common modifiers include automotive dashboard sheets, luggage ABS decking materials, and automobile profiles.

Weatherable impact modifiers prevent UV photodegradation. As with MBS and ABS, butadiene modifiers are not suitable for outdoor use unless they are protected by an ultraviolet protection layer on the outside. At the double-stranded site of butadiene, UV light can break its unsaturated carbon chain skeleton and make it brittle by oxidation and other degradation reactions. The function of modifiers with strong UV degradation is similar to that of MBS and ABS, but they have acrylic acid ester or 2-ethylhexyl acrylate. The polymer chains of these components do not contain double-strands. There is a place where the degradation reaction starts. These modifiers are commonly referred to as acrylic modifiers and are mainly used for PVC siding, window profiles, and other applications that require weathering. They have some impact resistance when used outdoors, but the effect is not as effective as ABS or MBS.

Another weatherability modifier that can be used outdoors is CPE (chlorinated polyethylene). The modifier is not very effective and the modification effect is not very good. The toughness of the PVC precursor is increased by a mechanism similar to that of plasticization (or an interpenetrating network).

Adding processing aids to the PVC compound increases melt and melt performance. Typical processing aids are very high molecular weight polymers such as MMA/EA, styrene, MMA/S/AN or S/AN. It is mainly used in PVC blends, and its amount is generally 1 part or less for PVC dry blends. Its function is to promote the melting of the mixture by increasing the friction between the PVC and the inner surface of the metal of the mixing device. In PVC foaming, the control of the melt viscosity is very efficient due to the high molecular weight polySAN and polyMMA/S/AN. In the plastics industry, these different modifiers perform their duties and require impact resistance for each specific polymer. Comparison of various aspects such as fluidity, cost, stability, and particle size control can correctly select modifiers.

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