Types and properties of polypropylene modification

In view of the poor impact resistance of polypropylene at low temperatures, poor weather resistance, poor surface decoration, and the gap 

between its functionality and actual needs in terms of electricity, magnetism, light, heat, combustion, etc., polypropylene was modified to 

become At present, plastic processing is the most active area of development and has achieved the most fruitful results.

1. PP chemical modification

Through copolymerization modification, cross-linking modification, graft modification, adding nucleating agents, etc., the polymer components 

and macromolecular structure or crystal configuration of polypropylene are changed to improve its mechanical properties, heat resistance, and 

aging resistance. and other properties, improve its comprehensive performance and expand its application fields.

① Copolymerization modification

Copolymerization modification is a modification carried out in the propylene monomer synthesis stage using catalysts such as metallocenes. 

When the monomers are polymerized, the added olefin monomers are copolymerized with them to obtain random copolymers, block copolymers, 

alternating copolymers, etc. The mechanical properties, transparency and processing fluidity of homopolymer PP are improved. . The complex 

formed by the metallocene catalyst uses a transition state with an irregular shape and certain restrictions as a single active center to achieve 

precise control of the relative molecular weight and distribution, comonomer content, distribution on the main chain and polymer crystal form structure.

② Graft modification

The PP resin molecules have a non-polar crystalline linear structure, low surface activity and non-polarity. There are shortcomings such as poor 

surface printability, poor coating adhesion, difficulty in blending with polar polymers, and difficulty in compatibility with polar reinforcing fibers 

and fillers. Graft modification is to introduce polar groups into its macromolecular chain to improve the blending, compatibility and adhesion 

of PP and overcome the shortcomings of difficulty in blending, compatibility and adhesion. Under the action of the initiator, the grafting monomer 

undergoes a grafting reaction during melting and mixing. The initiator decomposes to generate active free radicals when heated, melted, and heated. 

When the active free radicals encounter the unsaturated carboxylic acid monomer, the unsaturated carboxylic acid monomer is After the unstable 

bond of the acid monomer is opened, it reacts with the active free radicals of PP to form graft free radicals, which are subsequently terminated 

through a molecular chain transfer reaction. Common graft modification methods of PP include: melt method, solution method, solid phase method, 

suspension method, etc. The hydrogen atoms in the graft-modified PP molecular chain are replaced and become more polar. These polar groups 

enhance the compatibility of PP and greatly improve its heat resistance and mechanical properties.

③ Cross-linking modification

Cross-linking modification mainly involves modifying linear or dendritic polymers into polymers with a network structure through cross-linking. 

Cross-linking modification of PP can improve its mechanical properties, heat resistance and morphological stability, and shorten the molding cycle. 

The main methods of cross-linking modification of polypropylene include chemical cross-linking modification and radiation cross-linking modification. 

The main differences between them are different cross-linking mechanisms and different active sources; chemical cross-linking modification is achieved 

by adding cross-linking auxiliaries to polypropylene. Modification, radiation cross-linking modification is mainly achieved through strong radiation 

or strong light. Due to the requirements on the thickness of PP for radiation cross-linking modification, it is difficult to popularize this method. At present, 

the silane graft cross-linking method is developing rapidly because it can produce materials with excellent properties. The PP produced by the silane 

graft cross-linking method has high strength, good heat resistance, high melt strength, strong chemical stability, and corrosion resistance good.

PP GRANULES

2. PP physical modification

During the mixing and kneading process, organic or inorganic additives are added to the PP matrix to obtain PP composite materials with excellent 

performance, which mainly include: filling modification, blending modification, etc.

① Filling modification

During the PP molding process, fillers such as silicate, calcium carbonate, silica, cellulose, and glass fiber are filled into the polymer to improve the heat 

resistance of PP, reduce costs, increase rigidity, and reduce molding shrinkage. However, the impact strength and elongation of PP will also decrease. 

As an inorganic non-metallic whisker with excellent properties, glass fiber has low price, good insulation, strong heat resistance, good corrosion resistance, 

high mechanical strength, and is widely used. The properties of PP modified by glass fiber filling have been significantly improved. However, when the 

added amount of glass fiber reaches about 30%, the mechanical properties of the material can be significantly improved; when the added amount is too large, 

some of the glass fibers will not be fully impregnated, which will worsen the bonding performance of the interface between the polymer matrix and the glass fiber, 

resulting in a decrease in the mechanical strength of the composite material, and as the amount of glass fiber added increases, the flow performance of the composite 

material decreases, resulting in difficulty in the performance of the PP molding process.

② Blending modification

Blending PP with polyethylene, engineering plastics, thermoplastic elastomers or rubber is a modification method to improve the performance of PP. 

Blending modification is completed in processing equipment such as internal mixers, open mills, and extruders. The process is easy to control, the production 

cycle is short, and the cost is low. It can improve the coloring, processability, antistatic properties, and resistance of PP. Impact resistance and other properties. 

Polymer blending can combine the outstanding properties of each component and make up for the deficiencies in the performance of each component. 

The comprehensive performance of the blend is significantly improved, but the low-temperature resistance and aging resistance of the blend-modified PP are still not ideal. 

During blending modification, shearing force may cause part of the macromolecular chains to be cut off to form free radicals and form graft or block copolymers. 

These new copolymers can also effectively compatibilize PP.

PP modification technology has doubled the mechanical properties of composite materials, greatly expanded the application fields of PP, improved the cost 

performance of products, promoted the engineering process of PP, and also expanded the application of PP from general plastics to the field of engineering plastics, 

greatly broadening its application scope. In recent years, the research on PP modification technology has developed rapidly. More and more new technologies 

are used in PP modification. The comprehensive performance of PP has been significantly improved, the application fields are constantly expanding, 

and the development prospects are very broad.

③ Enhance modification

Adding fibrous materials to plastics can significantly improve the strength of plastic materials, so it is called enhanced modification. Materials with a large 

diameter-to-thickness ratio can significantly increase the flexural modulus (stiffness) of plastic materials, which can also be called enhanced modification.

The reinforcing materials used in the reinforcement modification of PP are mainly glass fibers and their products, in addition to carbon fibers, organic fibers, 

boron fibers, whiskers, etc. Among glass fiber reinforced PP, the most commonly used glass fibers are alkali-free glass fiber and medium-alkali glass fiber, 

of which the alkali-free glass fiber has the largest amount. The diameter of the glass fiber is controlled within the range of 6 to 15 μm, and the length of the glass 

fiber must be guaranteed to be between 0.25 and 0.76 mm. This can not only ensure the performance of the product, but also ensure good dispersion of the glass fiber. 

It is generally believed that the modification effect can only be achieved when the length of glass fiber in the product is greater than 0.2 mm. The glass fiber 

content (mass fraction) is preferably between 10% and 30%. When it exceeds 40%, the performance will decrease. In addition, adding organosilane coupling agents 

can form a good interface between glass fiber and PP, improving the flexural modulus, hardness, load deformation temperature, and especially dimensional stability of the composite system.

Since glass fiber reinforced PP can improve mechanical strength and heat resistance, and glass fiber reinforced PP has good water vapor resistance, chemical 

corrosion resistance and creep resistance, it can be used as engineering plastics in many occasions, such as fan blades, Heater grilles, impeller pumps, lampshades, 

electric stoves and heater housings, and more.

While the production quantity of polypropylene is developing rapidly, it is also constantly innovating in performance, so that the breadth and depth of its application 

are constantly changing. In recent years, it has either been improved during the polymerization reaction or measures have been taken during granulation after polymerization. 

Some new varieties of polypropylene with more unique properties have emerged, such as transparent polypropylene, high melt strength polypropylene, etc.

3. PP transparent modification

The crystallization of PP (polypropylene) is the main cause of opacity. By using the crystallization tendency of PP to freeze quickly, a transparent film can be obtained. 

However, for products with a certain wall thickness, due to the time required for heat conduction, the core layer cannot be cooled and frozen quickly. Therefore, 

for products with a certain thickness, we cannot expect to use rapid cooling to improve the transparency. We must start from the crystallization rules and influencing factors of PP.

Modified PP obtained through certain technical means can have excellent transparency and surface gloss, even comparable to typical transparent plastics (such as PET, PVC, PS, etc.). 

What is more advantageous about transparent PP is its high heat deformation temperature, which can generally be higher than 110°C, and some can even reach 135°C, 

while the heat deformation temperatures of the above three transparent plastics are all lower than 90°C. Due to its obvious performance advantages, transparent PP 

has developed rapidly around the world in recent years. Its application fields range from household necessities to medical equipment, from packaging supplies to 

heat-resistant utensils (for microwave heating).

The transparency of PP can be improved through the following three ways:

(1) Use metallocene catalyst to polymerize transparent PP;

(2) Transparent PP is obtained through random copolymerization;

(3) Add transparent modifiers (mainly nucleating agents) to ordinary polypropylene to improve its transparency.

Polypropylene is one of the important general-purpose plastics. It is the fastest growing variety in terms of absolute quantity and the breadth and depth of applications. 

As a modified plastics industry, the high cost performance, multi-functionality and engineering of polypropylene are always important tasks ahead.