In view of the poor impact resistance, weather resistance and surface decoration of polypropylene at low temperatures, as well as the gap between the functionality of electricity, magnetism, light, heat and combustion and the actual needs, polypropylene has been modified to become the most active and fruitful field in plastic processing.
Chemical modification of PP
Through copolymerization modification, cross-linking modification, grafting modification, and addition of nucleating agent, the mechanical properties, heat resistance, aging resistance and other properties of polypropylene can be improved by changing the polymer components and macromolecular structure or crystal configuration of polypropylene, so as to improve its comprehensive performance and expand its application field.
(1) Copolymerization modification
Copolymerization modification is carried out with metallocene and other catalysts in the stage of propylene monomer synthesis. When monomers are polymerized, the added olefin monomers are copolymerized with them to obtain random copolymers, block copolymers and alternating copolymers. The mechanical properties, transparency and processing fluidity of homopolymer PP are improved. The complex formed by metallocene catalyst takes the transition state with irregular shape restricted as a single active center, so as to accurately control the relative molecular weight and its distribution, the content of comonomer, the distribution on the main chain and the crystal structure of polymer.
(2) Graft modification
The molecular structure of PP (polypropylene) resin is nonpolar crystalline linear structure, with low surface activity and no polarity. Poor surface printability; Poor coating and bonding; It is difficult to blend with polar polymers; It is difficult to be compatible with polar reinforcement fiber and filler. Graft modification is to introduce polar groups into its macromolecular chain to improve the blending, compatibility and adhesion of PP, so as to overcome the shortcomings of difficult blending, compatibility and adhesion. Under the action of the initiator, the grafting monomer conducts grafting reaction during melting mixing. The initiator decomposes to produce active free radicals when heated during melting. When the active free radicals encounter unsaturated carboxylic acid monomers, the unstable bond of unsaturated carboxylic acid monomers is opened to react with PP active free radicals to form grafting free radicals, which are then terminated through molecular chain transfer reaction. The common graft modification methods of PP include melting method, solution method, solid phase method, suspension method, etc. The hydrogen atom in the grafted PP molecular chain was replaced and showed strong polarity. These polar groups enhanced the compatibility, heat resistance and mechanical properties of PP.
(3) Crosslinking modification
Crosslinking modification is mainly to modify linear or dendritic polymers into reticular polymers by crosslinking. Crosslinking 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. Their main differences lie in different cross-linking mechanisms and different active sources; Chemical crosslinking modification is to realize polypropylene modification by adding crosslinking additives. Radiation crosslinking modification is mainly realized by strong radiation or strong light. Due to the requirements of radiation crosslinking modification on PP thickness, it is difficult to popularize this method. At present, silane grafting crosslinking method is developing rapidly because it can produce materials with excellent properties. PP produced by silane grafting crosslinking method has high strength, good heat resistance, high melt strength, strong chemical stability and good corrosion resistance.
PP physical modification
In the process of mixing and mixing, organic or inorganic additives are added to PP (polypropylene) matrix to obtain PP composites with excellent performance, mainly including filling modification, blending modification, etc.
(1) Filling modification
During PP molding, silicate, calcium carbonate, silicon dioxide, cellulose, glass fiber and other fillers are filled in the polymer to improve the heat resistance, reduce the cost, improve the rigidity and reduce the molding shrinkage of PP, but the impact strength and elongation of PP will also decrease. As an inorganic non-metallic whisker with excellent performance, glass fiber is widely used because of its low price, good insulation, heat resistance, corrosion resistance and high mechanical strength. The properties of PP filled with glass fiber have been significantly improved, but the mechanical properties of the material can be significantly improved only when the glass fiber content reaches about 30%; If the amount of glass fiber is too large, some glass fibers will not be fully impregnated, the bonding property between the polymer matrix and the glass fiber interface will become worse, and the mechanical strength of the composite will decline. With the increase of the amount of glass fiber, the flow property of the composite will decline, which will lead to the difficulty of PP molding processing performance. [12]
(2) Blending modification
PP is blended with polyethylene, engineering plastics, thermoplastic elastomer or rubber to improve the performance of PP. Blending modification is completed in internal mixer, open mill, extruder and other processing equipment. The process is easy to control, the production cycle is short, and the cost is low. It can improve the colorability, processability, anti-static, impact resistance and other properties of PP. Polymer blending can integrate the outstanding properties of each component and make up for the deficiencies in the properties of each component. The comprehensive properties of the blends are significantly improved, but the low temperature resistance and aging resistance of PP modified by blending are still not ideal. During blending modification, shear force may cause part of macromolecular chains to be cut off to form free radicals and form graft or block copolymers. These new copolymers can also effectively play a compatibilizing role in PP.
PP modification technology has doubled the mechanical properties of composite materials, greatly expanded the application field of PP, improved the cost performance ratio of products, promoted the engineering process of PP, and also expanded the application range of PP from general plastics to engineering plastics. In recent years, the research on PP modification technology has developed rapidly. More and more new technologies have been applied to the modification of PP. The comprehensive properties of PP have been improved significantly, and the application fields have been expanded continuously. The development prospect is very broad.
(3) Enhanced modification
Adding fibrous materials to plastics can significantly improve the strength of plastic materials, so it is called reinforcement modification. The materials with large diameter thickness ratio can significantly improve the bending modulus (rigidity) of plastic materials, which can also be called reinforcement modification.
The reinforcement materials used in the reinforcement and modification of PP are mainly glass fiber and its products, as well as carbon fiber, organic fiber, boron fiber, whisker, etc. Among glass fiber reinforced PP, alkali free glass fiber and medium alkali glass fiber are mostly used, among which alkali free glass fiber is the most used. The diameter of glass fiber shall be controlled within 6-15 μ Within the range of m, the length of glass fiber must be kept within 0.25~0.76mm, which can not only ensure the product performance, but also make the glass fiber disperse well. It is generally believed that only when the glass fiber length in the product is greater than 0.2 mm can the modification effect be achieved. The content of glass fiber (mass fraction) is 10%~30%, and the performance will decline when it exceeds 40%. In addition, the addition of organosilane coupling agent can form a good interface between glass fiber and PP, and improve the bending modulus, hardness, load deformation temperature, especially the dimensional stability of the composite system.
As glass fiber reinforced PP can improve mechanical strength and heat resistance, and its water vapor resistance, chemical corrosion resistance and creep resistance are very good, it can be used as engineering plastics in many occasions, such as fan blades, fan grid, impeller pump, lampshade, electric furnace and heater shell.
While the production quantity of polypropylene is developing rapidly, its performance is also constantly changing, which makes the breadth and depth of its application constantly changing. In recent years, some new varieties of polypropylene with more unique properties, such as transparent polypropylene, high melt strength polypropylene, have come out, either by improving the polymerization reaction or by taking measures in the granulation after polymerization.
Transparent modification
The crystallization of PP (polypropylene) is the main cause of opacity. Transparent films can be obtained by using the crystallization trend of quenched frozen PP. However, for products with a certain wall thickness, the core layer cannot be rapidly cooled and frozen due to the time required for heat conduction. Therefore, the transparency of products with a certain thickness cannot be improved by quenching. We must start with the crystallization law and influencing factors of PP.
The modified PP obtained by certain technical means can have excellent transparency and surface gloss, even comparable to typical transparent plastics (such as PET, PVC, PS, etc.). Transparent PP has the advantage of high thermal deformation temperature, generally higher than 110 ℃, and some even reach 135 ℃, while the thermal deformation temperature of the above three transparent plastics is lower than 90 ℃. Due to the obvious performance advantages of transparent PP, it has been rapidly developed in the world in recent years. Its applications range from household products to medical devices, from packaging products to heat-resistant containers (for microwave heating).
The transparency of PP can be improved through the following three ways:
(1) Transparent PP was polymerized with metallocene catalyst;
(2) Transparent PP was obtained by random copolymerization;
(3) Transparent modifier (mainly nucleating agent) is added to ordinary polypropylene to improve its transparency.
High Melt Strength Polypropylene
One of the disadvantages of polypropylene is its low melt strength and poor sag resistance. Generally, amorphous polymers (such as ABS and PS) have elastic behavior similar to rubber in a wide temperature range, while semi crystalline polypropylene does not. This disadvantage causes that polypropylene can not be hot formed in a wide temperature range, and its softening point is very close to the melting point. Once it reaches the melting point, the melt viscosity drops sharply, followed by a significant decline in the melt strength, resulting in uneven wall thickness of products during hot forming, collapse of extruded bubble holes and other problems, greatly limiting the application of polypropylene in some aspects. High melt strength polypropylene (HMSPP) is a kind of polypropylene whose melt strength is not sensitive to temperature and melt flow rate.
HMSPP is a kind of polypropylene with long branched chains in the resin. The long branched chains are initiated for grafting in post polymerization. The melt strength of this homopolymer is 9 times that of ordinary polypropylene homopolymer with similar flow characteristics. With similar density and melt flow rate, the yield strength, bending modulus, hot deformation temperature and melting point of HMSPP are higher than that of ordinary polypropylene, but the notch impact strength is lower than that of ordinary polypropylene.
Another feature of HMSPP is that it has a higher crystallization temperature and a shorter crystallization time, which allows hot formed parts to be demoulded at a higher temperature, so as to shorten the molding cycle. It can be made into containers with large draw ratio and thin wall on ordinary hot forming equipment.
At constant strain rate, the melt flow stress of HMSPP increases gradually, and then increases exponentially, showing obvious strain hardening behavior. When strain occurs, the tensile viscosity of ordinary polypropylene decreases, while HMSPP remains stable. The strain hardening ability of HMSPP can ensure that it can maintain uniform deformation during forming and stretching, while ordinary PP always deforms from the weakest or hottest part of the structure when being stretched, resulting in various defects of the products, even failure to form.
At present, there are two main preparation methods of HMSPP: one is reactive modification of polypropylene with other compounds, and the other is blending modification of polypropylene with other polymers. The specific implementation methods mainly include radiation method, reactive extrusion method, and induced grafting method during polymerization. In the process of preparing HMSPP, there are two major problems: the degradation of polypropylene and gel problems. At the same time, there is competition between polymer grafting and monomer homopolymerization, and polymer backbone β The competition between bond breaking, cross-linking and branching. The main factor affecting the strength of polymer melt is its molecular structure. As far as polypropylene is concerned, its melt strength is determined by its relative molecular weight, its distribution and whether it has a branched chain structure. Generally, the larger the relative molecular weight is, the wider the relative molecular weight distribution is, and the higher the melt strength is. The longer the branch chain, the higher the melt strength of grafted polypropylene is.
HMSPP special resin solves the problem of common polypropylene hot forming difficulty. Thin wall containers with large draw ratio can be formed on common hot forming equipment. The processing temperature range is wide, the process is easy to master, and the wall thickness of the container is uniform. It can be used to make microwave food containers and high-temperature cooking sterilization containers. The ordinary polypropylene mixed with HMSPP has higher processing temperature and processing speed than pure ordinary polypropylene, and the transparency of the film is also better than ordinary polypropylene. This is mainly because HMSPP has the characteristics of tensile strain hardening, and its long branch chain has the function of refining the crystal nucleus.
The strain hardening behavior of HMSPP is the key factor to obtain high tensile ratio and fast coating speed. Higher coating speed and thinner coating thickness can be obtained by using HMSPP. HMSPP has high melt strength and tensile viscosity. Its tensile viscosity increases with the increase of shear stress and time. The strain hardening behavior promotes the stable growth of cell, inhibits the damage of microporous wall, and opens up the possibility of polypropylene extrusion foaming.
Although the research of high melt strength polypropylene started in the late 1980s, its various excellent properties, reasonable price advantages and wide application range have been recognized worldwide, and it has a trend to gradually replace the traditional PS and ABS and develop into engineering plastics. Its development and utilization prospects are broad.
Polypropylene is one of the most important general-purpose plastics. It is the fastest growing variety in terms of absolute quantity, breadth and depth of application. As a modified plastic industry, the high cost performance, multifunction and engineering of polypropylene are always important tasks in front of us.