How to choose a plastic modifier

At present, there is a trend to get rid of simple, unmodified plastic resins. Plastics are being broadened beyond their original characteristics. Recycled plastics need to be regenerated and the mix needs to be reinforced. Therefore, the choice of modifiers is sensible and critical. Today, modifiers are no longer just used to improve impact strength, but their use also includes thermal deformation modifiers and processing aids.

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

The first issue related is crystallinity. Plastics such as nylon and PBT are not susceptible to impact modification because the crystalline region does not have any inherent tendency to yield or crack as a site where cracks occur. Amorphous plastics such as polycarbonate (PC) and polystyrene (PS) are susceptible to modification.

The principle of impact modification is to absorb impact energy through plastic deformation before the crack occurs and before it spreads. There are two kinds of deformation mechanisms: 1 yielding or stretching; 2 silver streaks, the microcrack structure formed by the very fine microfilaments absorbs energy when it spreads to the resin.

If the plastic is easily deformed, that is, it is mainly deformed by yielding, it is necessary to lower its yield stress. In order to do this, it is not necessary to greatly reduce the overall tensile properties, but the rubber particles are uniformly dispersed in the resin to form a stress concentration point. During the impact process, as the stress increases, the rubber particles can feel each other's existence and produce local yield before the crack is generated. The ability of a plastic to yield depends on how much rubber it needs. Therefore, many small, uniformly dispersed particles are most desirable. Particles having a size of 0.12 to 1.0 μm are used in the easily deformable resin. In order to minimize the amount of modifier required, the optimum particle size should be chosen.

If a resin such as PS or SAN is deformed by a crack generating mechanism, rubber particles of 0.3 to 4 μm are required. Large rubber particles can prevent cracks from developing into cracks, transferring energy absorbed by one crack into two or three new cracks, which absorbs more impact energy.

Type of rubber impact modifier

Body rubbers such as EPDM, EEA, and EVA must be mechanically dispersed into the resin to achieve the desired particle size. However, when the rubber and the plastic resin are not similar, this is ineffective. The reason for this is that in further melt processing such as molding, surface tension can cause rubber to be separated from mechanical dispersion, resulting in inferior surface morphology and scaling. This problem has been successfully solved by the following method: EPDM or the like is placed in the PP, the solubility coefficients of the two are almost equal, and the thermodynamic energy of the mixing exceeds the surface tension energy of the separation.

The grafted body rubber overcomes some of the aforementioned drawbacks. Currently, rubbers with grafted polymer chains sold by suppliers increase the compatibility of the rubber and the matrix resin. An example of this is EPDM grafted SAN, the mechanical mixing of which is critical, and the scaling and granules are a problem because at these locations the graft does not interact sufficiently with the parent resin. This material can be used in PC or SAN.

Block polymer rubber When the rubber and the well-trimmed polymer are joined together, a network structure appears in the rubber particles, that is, compatibility is obtained. At this point, it can be seen that its performance has been further improved. Therefore, in a block polymer such as SBS, although the size of the particles is difficult to control, good dispersion can be obtained. This type of SBS is very effective for use in PS and adhesives.

Functional Body/Block Polymer Rubber A more desirable situation is obtained when the bulk rubber is grafted onto a reactive functional group and, in turn, reacts with the parent rubber. Examples of this are EPDM bonding to maleic anhydride, SBS grafting to maleic anhydride, and the like. The functional group reacts with a parent resin such as nylon, and the matrix resin becomes a compatible, that is, an ideal surfactant for particle dispersion.

Rubber Graft Emulsion At this point, the desired particle size is determined and fixed at a certain location. ABS (acrylonitrile/butadiene/styrene) and MBS (methyl methacrylate/butadiene/styrene) impact modifiers need only be dispersed. Grafting imparts cohesiveness and compatibility with the parent, and the dispersed particles do not agglomerate during processing. Mild cross-linking maintains particle integrity and results in particles from 0.08 to 0.5 microns. However, this emulsification process currently does not produce particles above 0.6 microns. This modifier is mainly used in PVC.

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

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

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

4 common modifier: used to improve impact resistance, high temperature strength and low temperature flexibility.

5 Weatherability modifier: The use of this modifier in outdoor applications prevents UV photodegradation.

6 processing aids: improve the melt properties of PVC by reducing the melting time.

High-efficiency impact modifiers such as ABS and MBS have synergistic effects on the improvement of PVC impact resistance. Therefore, the addition of a small amount of modifier to the PVC results in high impact resistance and increases the flexibility of the 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 end use of the product determines the molecular weight required for the PVC mix. For example, low molecular weight PVC is best processed by die-casting; high molecular weight PVC is processed by tubular extrusion. Typical applications for high efficiency impact modifiers are for PVC pipes, injection molded compounds and calendered opaque films and sheets.

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

Thermal deformation modifiers increase the effective heat use temperature of PVC. Each additional addition of a modifier can increase IT. The addition of a thermal deformation modifier to the PVC also increases stiffness and minimizes the effects of tensile strength, but often weakens the impact strength. Such modifiers typically 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 the methyl groups attached to the styrene. The glutarimide polymer increases the polymer chain stiffness due to its heterocyclic structure, thereby increasing the thermal deformability of the PVC matrix. Examples of thermal deformation modifiers include vinyl siding, heat-resistant profiles, and automotive instrument liner sheets that require mold fastness.

Ordinary modifiers are semi-hard modifiers for semi-rigid PVC blends. They are typical ABS modifiers with less butadiene and more ungrafted fully rigid poly S/AN. These modifiers have a rigid and rubbery two phase that allows the semi-rigid mixture to have a variety of properties. The butadiene rubber phase can increase the low temperature crack resistance, and the high molecular weight rigid S/AN has the processing properties such as thermoformability and good performance retention. Typical use of common modifiers are automotive instrument panel sheets, luggage ABS cover materials, and automotive profiles.

Weatherable impact modifiers prevent UV photodegradation. Like MBS and ABS, butadiene modifiers are not suitable for outdoor use unless the outer layer has an ultraviolet protection layer. In the double-stranded portion of butadiene, ultraviolet light can break its unsaturated carbon chain skeleton and make the modification brittle by oxidation and other degradation reactions. Modifiers with strong UV degradation are similar to MBS and ABS, but they have acrylic acid or 2-ethyl-hexyl acrylate technology. These components do not contain double chains in the polymer chain. There is a site where the degradation reaction starts. These modifiers are commonly referred to as acrylic modifiers and are used primarily in PVC siding, window profiles, and other applications where weathering is required. They have some impact resistance when used outdoors, but they are not as effective as ABS or MBS.

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

Adding processing aids to the PVC compound increases melt and melt properties. Typical processing aids are polymers of very high molecular weight such as MMA/EA, styrene, MMA/S/AN or S/AN. It is mainly used in PVC blending materials, and its amount is generally 1 part or less for PVC dry blending. 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 equipment. In PVC foaming, it is very effective to control the melt viscosity due to high molecular weight poly SAN and poly MMA/S/AN. In the plastics industry, these different modifiers perform their duties and require impact resistance for each specific polymer. The comparison of various aspects such as fluidity, cost, stability and particle size control can correctly select the modifier.

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