Polypropylene, abbreviated as PP, is a polymer formed by polyaddition of propylene. It is a white waxy material with a transparent and light appearance。The chemical formula is (C3H6)n, and the density is 0.89~0.91g/m³.Flammable, melting point 164~170 °C,It softens at about 155°C, and the operating temperature range is -30~140°C.It can resist the corrosion of acids, alkalis, salts and a variety of organic solvents below 80 °C, and can decompose under high temperature and oxidation.Polypropylene is a kind of thermoplastic synthetic resin with excellent performance, mainly including homopolymerization, copolymerization and impact three types of products, widely used in injection molding, pipes, films, fibers, etc., the main production process is liquid phase body method, liquid phase body gas phase combination process and gas phase process and other three categories.It is widely used in the production of clothing, blankets and other fiber products, medical equipment, automobiles, bicycles, parts, transportation pipelines, chemical containers, etc., and is also used in food and pharmaceutical packaging.
Physicochemical properties
1. Odorless, tasteless and non-toxic. It is one of the lightest resins commonly used;
2. Excellent mechanical properties, including tensile strength, compressive strength and hardness, outstanding rigidity and bending fatigue resistance, the movable hinge made of PP can withstand more than 7 × 10 to the power of 7 folds and bends without breaking, and the impact strength is poor at low temperatures. The tensile strength of PP is generally 21-39 MPa; The flexural strength is 42-56 MPa, the compressive strength is 39-56 MPa, the elongation at break is 200%~400%, the notched impact strength is 2.2-5 kJ/m2, and the low-temperature notched impact strength is 1-2 kJ/m2. Rockwell hardness R95~105;
3. Good heat resistance, continuous use temperature up to 110-120°C;
4. Good chemical stability, except for strong oxidants, it does not interact with most chemicals; The solvent cannot dissolve PP at room temperature, only some halogenated compounds, aromatic hydrocarbons and high boiling point aliphatic hydrocarbons can make it swell, and the water resistance is particularly good;
5. Excellent electrical properties, good resistance to high-frequency electrical insulation, and good electrical insulation in humid environments;
6. Since there are many tertiary carbon atoms with methyl groups on the backbone of PP, the hydrogen on the tertiary carbon atoms is susceptible to oxygen attack, so the weathering resistance of PP is poor, and antioxidants or ultraviolet absorbers must be added
7. Mice were gavaged 1~5 times at a dose of 8g/kg, and no obvious poisoning symptoms were caused. The decomposition products of polypropylene heated to 210~220 °C were inhaled 30 times for 2 hours each time, and the symptoms of ocular mucosa and upper respiratory tract irritation appeared. As with polyethylene, it is forbidden to use its recycled products for food.
Polypropylene modification
In view of the poor impact resistance, poor weather resistance, poor surface decoration of polypropylene at low temperatures, and the gap between the functionality and actual needs in electricity, magnetism, light, heat, combustion, etc., the polypropylene has been modified, which has become the most active and fruitful field in the current plastic processing development.
- PP chemical modification
Through copolymerization modification, cross-linking modification, grafting modification, adding nucleating agent, etc., the polypropylene polymer components and macromolecular structure or crystal configuration are changed to improve its mechanical properties, heat resistance, aging resistance and other properties, improve its comprehensive properties, and expand its application fields.
(1) Copolymerization modification
Copolymerization modification is the modification of propylene monomer synthesis stage by using catalysts such as metallocene. When the monomer is polymerized, the added olefin monomer is copolymerized with it, and the random copolymer, block copolymer and alternating copolymer are polymerized, and the mechanical properties, transparency and processing fluidity of homopolymer PP are improved. The complexes formed by metallocene catalysts are used as a single active center in a transition state with a certain restriction of irregular shape, so as to achieve precise control of the relative molecular weight and its distribution, comonomer content, distribution on the backbone and the crystal structure of the polymer.
(2) Grafting modification
The PP (polypropylene) resin molecule has a non-polar crystalline linear structure, low surface activity, and no polarity. Poor surface printability; Poor coating and adhesion; Difficult to blend with polar polymers; Disadvantage that it is difficult to be compatible with polar reinforcing fibers and fillers. Graft modification is to introduce polar groups to its macromolecular chain to improve the blendability, compatibility and adhesion of PP, and to overcome the shortcomings of difficult blending, incompatibility and bonding. When the active free radical encounters the unsaturated carboxylic acid monomer, it promotes the unstable bond of the unsaturated carboxylic acid monomer to open and react with the PP active free radical to form the free radical, which is then terminated by the molecular chain transfer reaction. The common grafting modification methods of PP are: melting method, solution method, solid phase method, suspension method, etc. The hydrogen atoms in the grafted modified PP molecular chain are replaced and show strong polarity, and these polar groups enhance the compatibility of PP, and the heat resistance and mechanical properties are greatly improved.