Polypropylene (PP), as a thermoplastic polymer, is widely used in the field of plastics. Due to different catalysts and polymerization processes, the properties and uses of the resulting polymers are also different. PP has many useful applications properties, but still lacks inherent toughness, especially below its glass transition temperature. However, its impact resistance can be improved by adding impact modifiers.
1. PP Homopolymerization
Polypropylene (PP), as a thermoplastic polymer, began commercial production in 1957 and was the first stereoregular polymer. Its historical significance is further reflected in that it has always been The fastest growing major thermoplastic, its total national output reached 3 million tons in 2004. It is widely used in the field of thermoplastics, especially in fiber and filament, film extrusion, injection molding, etc.< /p>
1.1 Chemistry and properties
PP is used in metal-organic stereoregular catalysts (Ziegler-Natta type), such as δ-TiCl3-(C2H5)2AlCl or TiCl3-(C2H5)3Al (Efficiency 300-900 g polypropylene/g TiCl3), the propylene monomer is synthesized under controlled temperature and pressure conditions. Due to the different catalysts and polymerization processes used, the molecular structure of the resulting polymer has three different types of three-dimensional structures. The chemical structures and quantities are also different. These three structures refer to isotactic polymers, syndiotactic polymers and atactic polymers. In isotactic polypropylene (the most common commercial form), the methyl groups are all in the polymerization state. This structure easily crystallizes on the same side of the backbone. The isotactic form of crystallinity gives it good resistance to solvents and heat. The catalyst technology used during the previous decade enabled the formation of non-isotactic isomers. To the minimum extent, it eliminates the need to separate useless random components and simplifies the production steps. There are two main processes for producing polypropylene: one is the gas phase method; the other is the liquid propylene slurry method. In addition, there are some old-fashioned slurry process units in operation, which use a liquid saturated hydrocarbon as the reaction medium.
In comparison, high-density and low-density polyethylene have higher densities, which are equivalent to Low melting point and lower flexural modulus, that is, stiffness. These performance differences lead to different end uses. Stiffness and easy orientation make polypropylene homopolymers suitable for making various fibers and for stretch tapes, while their higher resistance Thermal properties allow them to be used to make rigid high-pressure vessels and appliances and molded parts for automobiles.
The main factors affecting the processing and physical properties of polypropylene homopolymers include: molecular weight (usually expressed as flow rate Represented); molecular weight distribution (MWD for short); stereoregularity and additives. The average molecular weight of polypropylene ranges from about 200 000 to 600 000. Molecular weight distribution is usually expressed in terms of the weight average molecular weight () and number average molecular weight () of the polymer. The ratio of Sensitive to shear, that is, when the applied pressure increases, its apparent viscosity decreases. Polypropylene with a wide molecular weight distribution is more sensitive to shear than one with a narrow distribution, so materials with a wide molecular weight distribution are more sensitive to shear during the injection molding process. Easy to process. Some specific uses, especially fibers, require a narrow range of molecular weight distribution. The molecular weight distribution is related to both the catalyst system and the polymerization process. Peroxide is commonly used for chemical cracking in the extrusion process behind the reactor. Narrowing the molecular weight distribution range. This process is called the controlled rheology (CR) process.
Compared with polyethylene, isotactic polypropylene’s unique molecular structure and spiral crystals lead to Chains are more susceptible to oxidative degradation by light and heat. Under normal processing and end-use conditions, polypropylene is subject to random chain scission, resulting in reduced molecular weight and increased flow rates. All commercial grades of polypropylene contain stabilizers to Protect materials during processing and provide satisfactory end-use performance. For special applications, in addition to antioxidants and UV inhibitors, other additives must be added. For example: lubricants and anti-sticking agents are added to film formulations , to reduce the friction coefficient and prevent the film from adhering to itself. Add antistatic to the packaging material to eliminate static charges. In order to improve transparency or shorten the model cycle, a nucleating agent is required. Homopolymer resins are usually classified by flow rate and end use. Flow rate depends on both average molecular weight and molecular weight distribution. Certain special application requirements
The flow rate is as high as 400 dg/min, while the flow rate of ordinary commercial homopolymers is within the range of 0.5-50 dg/min. The flow rate is usually the most important factor in determining processing characteristics.
1.2 Processing and Application
The combination of polypropylene's excellent flow properties and wide range of flow rates, as well as other unique polymer properties, gives it excellent processing properties. The lower flow rate can meet the requirements of extrusion belts, belts It can also meet the processing requirements of shaped filaments and monofilaments, and can also make the finished product have tensile strength and low elongation, while maintaining sufficient lateral integrity to minimize splitting and dust flying on the winding machine guide device. To offset their characteristic low transverse strength and tendency to break (fibrillation), more highly oriented film-to-fiber products, such as coarse-denier textiles, strings, and cords, typically require flow rates in the range of 7 to 20. Decorative strip products containing foaming agents are extruded from polypropylene with a flow rate close to 10, so that the melt strength and orientation ability can be appropriately balanced. This polymer can produce a moderate degree of orientation With a smooth satin-like surface effect, the product has sufficient transverse strength to delay breakage. The extrusion of nonwovens and multifilament products requires a low viscosity, free-flowing material, so polypropylene with extremely high flow rates is used For these purposes.
Casted PP films are widely used in graphics and artwork. In addition, the film can be biaxially oriented and thermally set, giving it excellent mechanical and thermal properties and can be used in various properties. In terms of laminates and packaging materials, the tubular water cooling process can be used to process PP into extruded blown films and single-layer films. Extruded sheets for thermoforming require the use of materials with low flow rate formulas, so that they have Sufficient melt strength. When using PP extruded profiles, lower flow rate processing performance is always better. Profile extrusion is usually limited to smaller cross-sections so that it can be quenched with water to ensure that the product has sufficient toughness. PP can also be extruded Molded into tubular products, such as beverage straws and drinking water pipes. PP is also used in cable coatings.
Injection molding, which is second only to extrusion in terms of dosage, is well adapted to the characteristics of polypropylene. PP is good The flow properties and tough mechanical properties of PTFE are used to produce many different types of products with inherently tough mechanical properties. Good processability and excellent stress fracture resistance produce excellent molded seals .Generally speaking, materials formulated with low flow rates are used to produce thick-walled products and those requiring toughness. Materials with high flow rates are used to produce thin-walled parts and products requiring rapid processing.
1.3 Market
p>PP homopolymer can use various processing techniques to produce a wide range of products.
Extruded products are the largest market for PP consumption, and textile fibers and monofilaments are the largest among them. part. For a long time, PP has been the main raw material for manufacturing fibers because of its coloring ability, resistance to abrasion and chemicals, and favorable economic conditions. Oriented and non-oriented films occupy the second place in the extruded products market It has a large share and is an area that continues to grow.
Next, injection molded products are the second largest market for PP homopolymers, including containers, sealers, automotive applications, household products, toys and Many other consumer goods and industrial end uses. Many blow molded containers use polypropylene because of its good moisture barrier properties and sufficient cleanliness. In view of the new demand for plastic products in the future, PP homopolymer will continue to remain Growth. Good economic conditions, good mechanical properties, light weight, strong coloring ability and easy processing will make PP continue to be the material of choice for many applications in this century.
2. Resistance Impact PP Polymer
PP has many useful properties, but lacks inherent toughness, especially below its glass transition temperature. However, by adding impact modifiers , which can improve its impact resistance. The traditional modified elastomer is usually ethylene-propylene rubber. It is generally believed that the rubber particles spread throughout the semi-crystalline polypropylene matrix can form many stress concentration points on the interface to prevent local deformation. , and fracture propagation. Impact modifiers have always been added during final mixing, and more recently, on-site synthesis of elastomer components has gained commercial importance. Furthermore, a new series of impact modifiers is being promoted Agents to replace ethylene-propylene rubber, namely Flexomer polyolefins, Exact plastic elastomers and Insite polymers. These are olefin polymers that fill the gap between very low density polyethylene and traditional ethylene-propylene elastomers.
2.1 Chemistry and Properties
Isotactic PP homopolymer is polymerized from propylene under the catalysis of Ziegler-Natta catalyst system. The ethylene-propylene rubber component is synthesized in a series of reactors or purchased in advance and then mixed with PP homopolymer in the extruder. The resulting impact-resistant polypropylene is granulated and sold. The impact-resistant PP homopolymer produced on-site can be selected by Appropriate catalyst composition and reactor conditions to accurately control its important properties. Catalyst composition and reactor conditions determine the crystallinity of the matrix resin, the composition and quantity of the rubber component and the overall molecular weight distribution.
Impact-resistant PP is one of the lightest thermoplastics, with a density of less than 1 and a lower price per pound of product than PET, PBT, high-impact polystyrene and ABS. The unit volume cost of impact-resistant PP is based on specific volume. Lower than those resins above and polyvinyl chloride (PVC). Only HDPE is comparable in this regard. Impact-resistant PP is usually processed at moderate temperatures, ranging from 350 to 550°F. Impact-resistant polypropylene*** Polymers have a broad spectrum of melt flow rates, typically ranging from less than 1 to about 30. Resins with the highest melt flow rates are usually made by "visbreaking" materials with lower melt flow rates. Also It is to perform a one-step reaction on the material after coming out of the reactor to reduce the average molecular weight, thereby producing products with higher melt flow rates. Impact-resistant polypropylene polymers have high resistance to chemicals and environmental stress fracture After treatment, the material can have excellent Izod impact strength and low Ghanal impact properties. Izod impact strength ranges from 0.5 to greater than 15 ft·lb/in; at -40°F, Ghana Impact strengths range from 15 to more than 300 in. lbs.
The rubber component provides impact strength to polypropylene, but reduces the stiffness and thermal deformation of impact-resistant polypropylene relative to homopolymers. Temperature. Filled impact-resistant polypropylene polymers can withstand higher temperatures without deformation. Fillers are typically fiberglass, mica, talc, and calcium carbonate. End users of these polymers should be aware of each specification. For products, trade-offs need to be made between different melt strengths, melt flow rates, stiffness and heat deflection temperatures.
2.2 Uses
The main commercial uses of impact-resistant polypropylene are Injection molded parts in automobiles, household products, and appliances. Its impact resistance, low density, colorability, and processability make it an ideal material. Medium impact resin grades with higher melt flow rates have higher flow Performance, this feature is particularly useful when injecting large parts such as automotive panels.
High impact resistance Resins with low melt flow rates (generally less than 2) can be converted into excellent puncture resistance Film, the impact resistance and steam sterilization resistance of this film are suitable for disposable medical waste bags. Extruded sheets can be processed into large and thick parts by thermoforming, such as: guard plates and automobiles in the automotive industry. Tail trunk lining. The mechanism by which the elastomer component improves the impact resistance of polypropylene can induce stress whitening when the material is impacted. Most uses are based on the dispersion of the elastomer component in the polypropylene matrix. Based on the opposite concept, a new type of bumper is being developed. The result is a molecular composite structure.
Notes
Polypropylene The English name for the polymer of propylene is polypropylene, abbreviated as PP Homopolymer A polymer formed by the polymerization of a monomer is called a homopolymer.
Polymers Polymers are substances with particularly large molecular weights. Common molecules are called small molecules. Generally composed of several or dozens of atoms, the molecular weight is also between tens and hundreds. For example, the molecular weight of water molecules is 18, and the molecular weight of sulfur dioxide is 44. Polymers are different, and their molecular weight must be at least greater than 10,000. High The molecules of molecular substances are generally composed of thousands, tens of thousands or even hundreds of thousands of atoms, and its molecular weight is calculated in tens of thousands, hundreds of thousands, or even hundreds of millions. The "high" of a polymer refers to its high molecular weight.
Polymer polymers are divided into natural polymers and synthetic polymers. Natural rubber, cotton, etc. are all natural polymers. Synthetic polymers mainly include: chemical fibers, synthetic rubber and synthetic resins (plastics) ), also known as the three major synthetic materials. In addition, the main components of most coatings and adhesives are also synthetic polymers. Synthetic polymers are also called polymers. Such as: polypropylene, polyethylene
Ethylene, etc.
Polymerization of two or more monomers or monomers and polymers is called polymerization, and the product obtained by polymerization is Polymers. Block polymers, graft polymers, random polymers, regular polymers, etc.