Physical Modification of PP and Its Progress

Although PP has many advantages, it has substantial weaknesses in terms of resistance to low-temperature impact, aging, mold shrinkage, flammability, and dyeing. Therefore, it has been subjected to great structural and engineering plastic materials. For this reason, various modifications must be made to PP in order to broaden its scope of application and to allow modified PPs to enter the ranks of engineering plastics from general plastics.

PP modification includes two major categories: one is chemical modification and the other is physical modification. The physical modification is described below.

Physical modification

The physical modification is to add other inorganic materials, organic materials, other plastic varieties, rubber varieties, thermoplastic elastomers, or some additives with special functions in the PP matrix, and obtain excellent performance through mixing and mixing. PP composites.

Physical modification is roughly divided into: fill modification, reinforcement modification, blend modification, functional modification, and the like.

The physical modification process is simple and easy, and generally does not require complicated or special equipment. The original equipment of the plastic molding processing factory can be used. However, in the case of modified products requiring special properties, special equipment needs to be added. For magnetic materials, for example, an enhanced magnetic field device needs to be added to the extruder head.

The following describes several physical modification methods and their development.

1. Fill modification

A certain amount of inorganic fillers and organic fillers are added to the PP resin to improve certain properties of the product and reduce the material cost.

PP-filled inorganic fillers are commonly used: mica powder, calcium carbonate, talc, wollastonite, carbon black, gypsum, red mud, lithopone, barium sulfate, etc. Common organic fillers are: wood flour, rice hull flour, peanuts Shell powder and so on.

The performance of mica-filled PP is related to the ratio, mixing process, coupling agent type, and particle size of mica. Generally, the amount of mica added is 40% or less, the particle size of mica is more than 300 mesh, the amount of titanate coupling agent is about 30% of mica, and the amount of silane coupling agent may be less, acrylic When the surface treatment agent, the amount can be increased to 5-10%, the type of mica has little relationship with the thermal properties and mechanical properties of the composite material, the aspect ratio of mica is not too large, and the particle size of mica is the mechanical property of the material. Basically, the performance is not greatly affected, the particle size is small, and the impact at room temperature, low temperature impact strength and heat distortion temperature are slightly increased. The greater the aspect ratio of mica, the better the enhancement.

When mica is filled with PP, it is important to not destroy the aspect ratio of mica as much as possible to make the mica evenly dispersed. Therefore, static mixers, pin-type mixing screws, and twin-screw extruders are used to help improve the filling effect. .

When the wollastonite is filled with PP, the amount of wollastonite is preferably 30-40%, and the grain size is preferably 300-325 mesh. After the filling, the tensile strength of the composite material is reduced, but at room temperature and low temperature Notched impact strength increased, such as the normal temperature notched impact increased from the original 4.5 to 5.9 kJ/m2, the low temperature notch impact increased from the original 1.8 to 2.2 kJ/m2, and the unnotched impact strength decreased. . From this point of view, the filling of wollastonite helps to overcome the weakness of polypropylene's notch sensitivity.

Treating wollastonite with a silane coupling agent is beneficial to improve the mechanical properties of the composite material, and the molding processability of the material can also be improved.

When other talcum powder, red mud, heavy calcium carbonate, etc. are filled with PP, the viscosity increases more, but as the shear rate increases, this viscosity increase phenomenon gradually weakens, generally available surface treatment agents such as polyethylene wax, Fatty acid salts and the use of twin-screw extruder can improve the molding process performance.

When organic filler wood flour or corn cob core is used, it is better to choose an aspect ratio of more than 15 to improve toughness and load distortion.

When low-calcium talc is used, it refers to talc content of 10-20%. PP composites can replace ABS or high-impact polystyrene. High-filling means that talc content exceeds 30%. It is mainly used for low appearance of products. Mainly require the heat distortion temperature, modulus and other properties of the product.

At present, the fillers in the filling and modifying process increasingly tend to be ultra-fine, and ultra-fine calcium carbonate and ultra-fine talc powder have appeared, with a particle size of 1250 mesh and 2500 mesh.

In addition, the development trend of filling modification is that the surface modification technology of the filler is continuously improving, and many surface treatment agents are developed, not only acting as a coupling agent, but also having a dispersant, a wetting agent, a viscosity reducing agent, and a molding processing aid. , increase the toughness and other effects, so you can increase the amount of filling, can reach more than 60% of the amount of filling.

2. Enhanced modification

Reinforced modified PP can replace engineering plastics. The reinforcing materials selected are glass fiber, asbestos fiber, single crystal fiber, and mica, boron, silicon carbide, etc., and mica in filler modification. When talc is well treated, it can also be used as a reinforcing material.

Glass fiber reinforced PP is widely used. One is the mixing method, ie, the short glass fiber is shaped like a filler. The other is a cladding method. The method is similar to the wire production method, ie, it is installed on a single screw extruder. A pack of heads, glass fiber into the head repeatedly, P} from the hopper into the screw, the two together, and then granulation can be.

Commonly used coupling agents are silanes, or the addition of chlorinated organics or peroxides in the coupling agent system improves PY performance. It is also treated with maleimide, phthalic anhydride, pyromellitic anhydride, benzophenone tetrahydride, and the like.

When glass fiber reinforcement is modified, it should be noted that the glass fiber is uniformly dispersed in the PP matrix. If the dispersion is not uniform, the "accumulation area" and "gap area" of the glass fiber appear in the mixed system, and these tend to cause stress. Focus on reducing product strength. Another problem with the orientation of glass fiber is also an important issue, and special care should be taken in injection molding.

3. Blend modification

Blend modification refers to the use of blends of other plastics, rubbers, or thermoplastic elastomers with PP to fill in larger spheres in PP to improve the toughness and low temperature brittleness of PP. To this end, a plastic having a lower glass transition temperature relative to PP, such as LDPE, HDPE, is added so that the PP crystalline phase and the amorphous LDPE constitute a two-phase continuous penetration hook. This through structure acts as a tough network to transmit and disperse impact energy.

For example, blends of PP/HDPE are generally expected to have a good melt index of the two raw materials. HDPE's content is generally less than 25%. From the blend's flow curve, HDPE plays a “plasticizing effect”. ".

Adding rubber or elastomer is the main way to modify PP, such as in the PP can be added to the world: butadiene rubber, ethylene propylene rubber, styrene-butadiene block copolymer (SBS) ethylene propylene diene ternary copolymer (EPDM), natural rubber, polyisobutylene rubber, etc.

Sometimes the use of ternary blend toughening system, the effect is better, such as PP / LDPE / ethylene propylene rubber, PP / LDPE / EVA, PP / LDPE / EPDM, when the low temperature brittleness can be reduced to about -40 °C.

In general, the amount of rubber added is about 10%, and the amount of LDPE is also about 10%.

For example PP and polyurethane blends, PP and polyurethane and SBS ternary blend.

The basic formula: PP/PU=90:10 for binary blending and PP/PU/SBS=90:5:5 for ternary blending.

Process conditions: 45mm extruder, aspect ratio 20:1, barrel temperature 180, 210, 205°C, nose temperature 180°C, screw speed 15-20 rpm.

Observed under the microscope, an "island" structure with rubber as the dispersed phase was formed.

Sometimes in ternary blending, 20-30% amount of HDPE is used as an insert of PP spherulites, which can make the PP spherulites incomplete and be divided into wafers. HDPE is also known as a reinforcing compatibilizer. Polystyrene is sometimes used as the third component.

In the ternary blending process, a two-step blending process is sometimes used. That is, the master batch is firstly prepared, that is, first the PP and the elastomer SBS are first made into a master batch at a ratio of 1:1, and then the master batch is added to the master batch. The required HDPE and the remaining PP material.

When powder PP is used, some antioxidants and auxiliary antioxidants should be added to the formula.

4. Other modifications

Flame retardant modification: PP is a flammable material with a decomposition temperature of 320-400°C, a light-off temperature of 350°C, a horizontal burning rate of 37 mm/min, and an oxygen index of 18.5. There are two kinds of PP flame retardant methods: First, a large number of inorganic fillers with flame retardant effects such as aluminum hydroxide, magnesium hydroxide, but must be added to 70-80% in order to make products with flame retardant, is bound to affect the processability And other properties of the product. The second method is to use flame retardants, such as phosphorus-containing, halogen-containing antimony compounds. This method has a small amount of addition, about 10%, but it is poor in durability and corrosion of the molding equipment.

At present, an intumescent flame retardant system has been developed. It is composed of a carbonization agent, a carbonization accelerator, and a foaming agent. Its flame retardant effect is superior to the above two methods. Such as using ammonium polyphosphate and melamine, according to the amount of 3:1, it has a significant effect on the flame retardant effect of PP.

Anti-static modification: Adding carbon black and other conductive fillers in PP can reduce the surface resistance of the products to 10 ohms or less.

Others are modified with a nucleating agent or electroplating surface treatment, that is, modification of secondary processing, adding antioxidants in PP, anti-UV agents to improve heat, photoaging properties, and the like.