Polypropylene, abbreviated as PP, is a polymer formed by the addition polymerization of propylene. It is a white waxy material with a transparent and light appearance. The chemical formula is (C 3 H 6 ) n, and the density is 0.89 to 0.91 g/cm 3. Flammable, melting point 164~170℃, It softens at around 155°C and has an operating temperature range of -30 to 140°C. It is resistant to corrosion by acids, alkalis, salt solutions and a variety of organic solvents below 80°C, and can decompose under high temperature and oxidation. Polypropylene is a thermoplastic synthetic resin with excellent performance. It is a colorless, translucent, lightweight general-purpose thermoplastic plastic with chemical resistance, heat resistance, electrical insulation, high-strength mechanical properties and good high-wear resistance processing properties, which is widely used in the production of clothing, blankets and other fiber products, medical equipment, automobiles, bicycles, parts, pipelines, chemical containers, etc. It is also used in food and medicine packaging.

On October 27, 2017, the World Health Organization’s International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and polypropylene was included in the list of Class 3 carcinogens.

History

Time	Development
1954	G. Natta was the first to polymerize propylene into polypropylene (using aluminum-titanium chloride as a catalyst) and established the theory of directional polymerization , which attracted people’s attention.
1957	Italy’s Montecatini and the United States’ Hecules have established 6,000 t/a and 9,000 t/a polypropylene production facilities respectively.
Late 1960s to mid-1970s	Polypropylene has entered a period of great development.
1980s to present	Polypropylene production ranks first among synthetic resins and is now second only to polyethylene.
1962	China began researching polypropylene production technology.
Since the 1980s	Polypropylene is developing rapidly in China. China has introduced some advanced polypropylene production technologies and equipment, and has established a number of large and medium-sized polypropylene production facilities in Yanshan, Yangzi, Liaoyang, etc. A large number of small-scale bulk polypropylene production facilities have also been built in various places, which have played a certain role in alleviating the contradiction between supply and demand. The substantial increase in production scale has prompted China’s polypropylene resin production to enter a rapid development stage.
2012	China’s PP production capacity reached 12.967 million tons.
2015	China’s PP production capacity is 20.13 million tons/year.

Physical and chemical properties

1. Odorless, tasteless, non-toxic. It is the lightest of the commonly used resins;

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 7 times of folding and bending without damage, and the impact strength is poor at low temperature. The tensile strength of PP is generally 21-39 MPa; bending strength is 42-56 MPa, compression strength is 39-56 MPa, elongation at break is 200%~400%, notched impact strength is 2.2-5 kJ/㎡, and low-temperature notched impact strength is 1-2 kJ/㎡. Rockwell hardness R95~105.

3. Good heat resistance, continuous use temperature can reach 110-120℃.

4. Good chemical stability. It does not react with most chemicals except strong oxidants. Solvents cannot dissolve PP at room temperature. Only some halogenated compounds, aromatic hydrocarbons and high-boiling point aliphatic hydrocarbons can make it swell. It has excellent water resistance.

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 main chain of PP, the hydrogen on the tertiary carbon atoms is easily attacked by oxygen. Therefore, PP has poor weathering resistance and antioxidants or UV absorbers must be added.

7. Mice were gavaged 1 to 5 times at a dose of 8g/kg and no obvious symptoms of poisoning occurred. Rats inhaled the decomposition products of polypropylene heated to 210-220℃ 30 times, each time for 2 hours, and developed symptoms of irritation to the eye mucosa and upper respiratory tract. Similar to polyethylene, its recycled products are prohibited from being used to hold food.

Production process

Polypropylene resin is one of the four general-purpose thermoplastic resins ( polyethylene, polyvinyl chloride, polypropylene, and polystyrene ). It is produced through polymerization reaction using propylene as the raw material and ethylene as the comonomer.

The process methods used to produce polypropylene in the world are mainly divided into the following categories: solvent method, solution method, liquid phase bulk method (including liquid phase and gas phase combination method) and gas phase bulk method. The characteristics of each process are briefly described as follows:

Solvent polymerization

The solvent method (also known as the slurry method or mud method, slurry method) was the earliest polypropylene production process adopted. However, due to the deashing and solvent recovery steps, the process is long and complicated. With the advancement of catalyst research technology, the solvent method has stagnated since the 1980s and has been gradually replaced by the liquid phase bulk method.

Process characteristics: (1) Propylene monomer is dissolved in an inert liquid solvent (such as hexane ) and solvent polymerization is carried out under the action of a catalyst. The polymer is suspended in the solvent in the form of solid particles, and a kettle-type stirred reactor is used; (2) There are deashing and solvent recovery steps. The process is long and complex, the equipment investment is large, and the energy consumption is high. However, the production is easy to control and the product quality is good; (3) The polypropylene particles are separated by centrifugal filtration and then dried by air boiling and extruded into granules.

Solution polymerization

Process characteristics: (1) Use high-boiling point straight-chain hydrocarbons as solvents, operate at a temperature higher than the melting point of polypropylene, and the resulting polymer is completely dissolved in the solvent and distributed uniformly; (2) High-temperature gas stripping method evaporates and removes the solvent to obtain molten polypropylene, which is then extruded and granulated to obtain pellet products; (3) The only manufacturer is Kodak in the United States.

Liquid phase bulk method

The liquid-gas combined liquid-phase bulk polypropylene production process is a new process developed in the middle and late stages of polypropylene production. This production process was introduced seven years after the industrial production of polypropylene began in 1957.

The liquid phase bulk process is used to produce polypropylene. The catalyst is directly dispersed in liquid propylene without adding any other solvents to the reaction system to carry out a liquid phase bulk polymerization reaction of propylene. The polymer continuously precipitates from the liquid phase propylene and is suspended in the liquid phase propylene in the form of fine particles. As the reaction time increases, the concentration of polymer particles in the liquid phase propylene increases. When the propylene conversion rate reaches a certain level, the unpolymerized propylene monomer is recovered by flash evaporation to obtain a powdered polypropylene product. This is a relatively simple and advanced industrial production method for polypropylene. The liquid phase bulk process represents a new technology and a new level in the international production of polypropylene in the 1980s.

Process characteristics: (1) No solvent is added to the system, propylene monomer is liquid-phase bulk polymerized in a tank reactor, and ethylene and propylene are gas-phase copolymerized in a fluidized bed reactor; (2) The process is simple, with less equipment, less investment, and low power consumption and production costs; (3) Homopolymerization uses a tank stirred reactor (Hypol process) or a loop reactor (Spheripol process), and random copolymerization and block copolymerization are both carried out in a stirred fluidized bed.

A typical example of liquid phase bulk process is BASELL’s Spherizone liquid phase bulk process. Spherizone is a gas phase circulation technology that uses Ziegler-Natta catalysts to produce polymers that maintain toughness and processing properties while having high crystallinity, rigidity and more uniformity. It can produce highly uniform multi-monomer resins or bimodal homopolymers in a single reactor. The Spherizone cycle reaction has two interconnected zones, and different zones play the role of gas phase and liquid phase loop reactors in other processes. These two zones can produce resins with different relative molecular weights or monomer composition distributions, expanding the performance range of polypropylene.

The core equipment of this process is the MZCR (multi-zone circulation reactor system) reactor R 230 system. The reactor consists of two parts: a riser and a downcomer. In the riser, the polymer is blown upward by the reaction gas to form fluidization, and is sent to the upper part of the downcomer to pass through the cyclone separator, and the powder is collected in the downcomer. The reaction gas is circulated by a centrifugal compressor through an external pipeline, and the reaction heat is removed by a circulator cooler on the external circulation pipeline. The reactor product is discharged through a valve installed at the bottom of the downcomer. After the discharged powder is degassed at high and low pressures, it is directly steamed and dried to obtain a powder product when producing homopolymers and random copolymers. When producing impact-resistant products, the powder after high-pressure degassed is discharged into a gas-phase fluidized bed reactor. This reactor still uses the Spheripol II gas-phase reactor system. The copolymerization reactor is a vertical cylindrical container with spherical heads at the top and bottom and a fluidized bed at the bottom. The main material is stainless steel and the inner surface is polished.

The current maximum single-line production capacity of this process has reached 450,000 tons/year. The ethylene content of the MZCR (multi-zone circulation reactor) impact copolymer products can be as high as 22% (rubber content greater than 40%), and terpolymer products containing ethylene and 1-butene can also be produced.

Gas phase bulk method

Process characteristics: (1) No solvent is introduced into the system, and propylene monomer is polymerized in the gas phase in the reactor; (2) The process is short, the equipment is small, the production is safe, and the production cost is low; (3) The polymerization reactor includes fluidized bed, vertical stirred bed and horizontal stirred bed.

A typical example of the gas phase bulk method is the Unipol gas phase process of DOW Chemical Company. The Unipol gas phase polypropylene process is a gas phase fluidized bed polypropylene process developed by Union Carbide Corporation (UCCP) and Shell in the 1980s. It is a successful transplantation of the fluidized bed process used in polyethylene production to polypropylene production. The process uses a high-efficiency catalyst system, with the main catalyst being a high-efficiency carrier catalyst and the co-catalyst being triethylaluminum and an electron donor.

The UNIPOL process is simple, flexible, economical and safe. It can produce a full range of products including homopolymers, random copolymers and impact copolymers with only a small amount of equipment. The operating conditions can be adjusted within a large operating range to maintain uniform product performance. The small amount of equipment used reduces the maintenance workload and improves the reliability of the device. Due to the limitations of the fluidized bed reaction kinetics itself and the low operating pressure that reduces the storage capacity of materials in the system, the process is safer to operate than other processes and there is no risk of equipment overpressure in the event of an accident.

This process does not discharge any liquid waste and emits very little hydrocarbons into the atmosphere, so the impact on the environment is very small. Compared with other processes, this process is easier to meet various strict environmental, health and safety regulations. Another notable feature of this process is that it can be used in conjunction with super-condensed state operation, namely the so-called super-condensed gas phase fluidized bed process (SCM). This technology can increase the existing production capacity by 200% by increasing the proportion of liquid phase in the reactor to 45%. Since the liquid content is not the basic factor for the instability of the fluidized bed and the formation of polymer agglomerates, the key operating variables of this technology are the density of the expanded bed and the ratio of the expanded bulk density to the settled bulk density. Since super-condensed state operation can most effectively remove the heat of reaction, it can increase the production capacity of the reactor by more than 2 times without increasing the volume, which is very significant for investment savings. The ethylene content of the impact copolymer product can be as high as 17% (rubber content greater than 30%).

The core equipment of this process is the gas phase fluidized bed reactor, circulating gas compressor, circulating gas cooler and extrusion granulation unit. The fluidized bed reactor is a hollow container with an expansion section at the top and a distributor at the bottom. The operating pressure of the first reactor is 3.5MPaG and the temperature is 67°C. The operating pressure of the second reactor is 2.1MPaG and the temperature is 70°C. The circulating gas compressor is a single-stage, constant speed, centrifugal compressor.

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 electrical, magnetic, optical, thermal, and combustion functions and actual needs, polypropylene modification has become the most active and fruitful field in the current development of plastic processing.

PP chemical modification

By copolymerization modification, cross-linking modification, grafting modification, adding nucleating agents, etc., the polymer components and macromolecular structure or crystal configuration of polypropylene can be changed to improve its mechanical properties, heat resistance, aging resistance and other properties, enhance its comprehensive performance and expand its application field.

1. Copolymerization modification

Copolymerization modification is a modification performed during the propylene monomer synthesis stage using catalysts such as metallocene. When the monomer is polymerized, the added olefin monomer copolymerizes with it 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 the metallocene catalyst uses an irregularly shaped transition state with certain restrictions as a single active center to achieve precise control of the relative molecular mass and its distribution, the content of the copolymer monomer, the distribution on the main chain, and the crystal structure of the polymer.

2. Graft modification

PP (polypropylene) resin molecules are non-polar crystalline linear structures with low surface activity and no polarity. There are disadvantages such as poor surface printability; poor coating adhesion; difficulty in blending with polar polymers; and incompatibility 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, so as to overcome the disadvantages of difficult blending, compatibility and adhesion. Under the action of the initiator, the grafting monomer undergoes a grafting reaction during melt mixing. The initiator decomposes to produce active free radicals when heated and melted. When the active free radicals encounter unsaturated carboxylic acid monomers, the unstable bonds of the unsaturated carboxylic acid monomers are opened and reacted with the active free radicals of PP to form grafting free radicals, which are then terminated by molecular chain transfer reactions. Common grafting modification methods for PP include: melting method, solution method, solid phase method, suspension method, etc. After grafting, the hydrogen atoms in the PP molecular chain are replaced, making it more polar. These polar groups enhance the compatibility of PP and significantly improve its heat resistance and mechanical properties.

3. Cross-linking modification

Crosslinking modification mainly involves modifying linear or branched polymers into network-structured polymers through crosslinking. Crosslinking modification of PP (polypropylene) can improve its mechanical properties, heat resistance and morphological stability, and shorten the molding cycle. The main methods of crosslinking modification of polypropylene are chemical crosslinking modification and radiation crosslinking modification. The main differences between them are different crosslinking mechanisms and active sources. Chemical crosslinking modification is achieved by adding crosslinking aids, and radiation crosslinking modification is mainly achieved through strong radiation or strong light. Due to the thickness requirements of radiation crosslinking modification on PP, this method is difficult to popularize. At present, the silane grafting crosslinking method has developed rapidly because it can prepare materials with excellent performance. The PP produced by the silane grafting crosslinking method has high strength, good heat resistance, high melt strength, strong chemical stability and good corrosion resistance.

PP physical modification

During the mixing and kneading process, organic or inorganic additives are added to the PP (polypropylene) matrix to obtain PP composite materials with excellent performance, mainly including: 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 added to the polymer to improve the heat resistance of PP, reduce costs, increase rigidity, and reduce molding shrinkage, but the impact strength and elongation of PP will also decrease. Glass fiber is an inorganic non-metallic whisker with excellent performance. It is low in price, good in insulation, strong in heat resistance, good in corrosion resistance, and high in mechanical strength. It is widely used. The performance of PP modified by glass fiber filling is significantly improved. However, when the glass fiber addition reaches about 30%, the mechanical properties of the material can be significantly improved. If the addition amount is too large, part of the glass fiber will not be fully impregnated, which will deteriorate 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. In addition, as the amount of glass fiber added increases, the flowability of the composite material decreases, resulting in difficulties in the performance of the PP molding process.

(2) Blending modification

The modification method is to blend PP (polypropylene) with polyethylene, engineering plastics, thermoplastic elastomers or rubber to improve the performance of PP. The blending modification is completed in processing equipment such as internal mixers, open mixers, extruders, etc. The process is easy to control, with a short production cycle and low cost. It can improve the colorability, processability, antistatic properties, impact resistance and other properties of PP. 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 blended modified PP are still not ideal. During blending modification, shear force may cause a part of the macromolecular chain to be cut off to form free radicals and form grafted or block copolymers. These new copolymers can also effectively increase the volume of PP.

PP modification technology has greatly improved the mechanical properties of composite materials, greatly expanded the application field of PP, improved the cost-effectiveness of products, promoted the engineering process of PP, and expanded the application of PP from general plastics to engineering plastics, greatly broadening its application range. In recent years, the research and development of PP modification technology has been rapid, and more and more new technologies have been applied to PP modification. The comprehensive performance of PP has been significantly improved, and the application field has been continuously expanded. The development prospects are very broad.

(3) Enhancement modification

Adding fibrous materials to plastics can significantly increase the strength of plastic materials, so it is called reinforcement modification. Materials with a large diameter-to-thickness ratio can significantly increase the bending modulus (rigidity) of plastic materials, which can also be called reinforcement modification.

The reinforcing materials used in the reinforcement modification of PP (polypropylene) are mainly glass fibers and their products, in addition to carbon fibers, organic fibers, boron fibers, whiskers, etc. In glass fiber reinforced PP, the most commonly used glass fibers are alkali-free glass fibers and medium-alkali glass fibers, among which alkali-free glass fibers are used in the largest amount. The diameter of the glass fiber is controlled in the range of 6 to 15 μm, and the length of the glass fiber must be guaranteed to be 0.25 to 0.76 mm, so that the performance of the product can be guaranteed and the glass fiber can be well dispersed. It is generally believed that the modification effect can only be achieved when the length of the glass fiber in the product is greater than 0.2 mm. The glass fiber content (mass fraction) is preferably between 10% and 30%, and the performance decreases when it exceeds 40%. In addition, the addition of organic silane coupling agents can form a good interface between the glass fiber and PP, thereby 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 has good water vapor resistance, chemical corrosion resistance and creep resistance, it can be used as an engineering plastic in many occasions, such as fan blades, heater grilles, impeller pumps, lampshades, electric furnaces and heater shells, etc.

While the production volume of polypropylene is growing rapidly, its performance is also constantly being innovated, which makes its application breadth and depth constantly changing. In recent years, some new varieties of polypropylene with more unique properties have been introduced, such as transparent polypropylene and high melt strength polypropylene, either through improvements in the polymerization reaction or measures taken during granulation after polymerization.

Transparent modification

The crystallization of PP (polypropylene) is the main cause of opacity. By rapidly freezing the crystallization tendency of PP, a transparent film can be obtained. However, for products with a certain wall thickness, the core layer cannot be quickly cooled and frozen because heat conduction takes time. Therefore, for products with a certain thickness, we cannot expect to improve transparency by rapid cooling. We must start from the crystallization law of PP and the influencing factors.

Modified PP obtained through certain technical means can have excellent transparency and surface gloss, and can even be comparable to typical transparent plastics (such as PET, PVC, PS, etc.). Transparent PP is more superior in that it has a high heat deformation temperature, which is generally 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 been rapidly developed around the world in recent years, and its application areas range from household daily necessities to medical devices, from packaging products to heat-resistant utensils (for microwave heating).

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

• Use metallocene catalyst to polymerize transparent PP;
• Obtain transparent PP through random copolymerization;
• Adding a transparent modifier (mainly a nucleating agent) to ordinary polypropylene improves its transparency.

High melt strength polypropylene

One of the disadvantages of polypropylene is its low melt strength and poor sag resistance. Usually, amorphous polymers (such as ABS and PS) have rubber-like elastic behavior over a wide temperature range, while semi-crystalline polypropylene does not. This disadvantage makes polypropylene unable to be thermoformed over a wide temperature range. Its softening point and melting point are very close. Once the melting point is reached, the melt viscosity drops sharply, and the melt strength also drops significantly. This leads to problems such as uneven wall thickness of the product and collapse of the extruded foam cells during thermoforming, which greatly limits the application of polypropylene in some aspects. High melt strength polypropylene (HMSPP) refers to polypropylene whose melt strength is not very sensitive to temperature and melt flow rate, and has great development and application prospects.

HMSPP is a resin containing long-chain polypropylene, which is grafted during post-polymerization. The melt strength of this homopolymer is 9 times that of ordinary polypropylene homopolymer with similar flow properties. When the density and melt flow rate are similar, the yield strength, flexural modulus, heat deformation temperature and melting point of HMSPP are higher than those of ordinary polypropylene, but the notched 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 thermoformed parts to be demolded at a higher temperature, shortening the molding cycle and making it possible to produce containers with a larger stretch ratio and thinner walls on ordinary thermoforming equipment.

Under a constant strain rate, the melt flow stress of HMSPP begins to increase gradually and then increases exponentially, showing obvious strain hardening behavior. When strain occurs, the tensile viscosity of ordinary polypropylene decreases, while that of HMSPP remains stable. The strain hardening ability of HMSPP can ensure that it maintains uniform deformation during molding stretching, while ordinary PP always starts to deform from the weakest or hottest part of the structure when stretched, resulting in various defects in the product or even failure to form.

At present, there are two main methods for preparing HMSPP: one is to modify polypropylene with other compounds through reaction, and the other is to modify polypropylene with other polymers through blending. The specific implementation methods mainly include radiation method, reaction extrusion method, and grafting method initiated during polymerization. In the process of preparing HMSPP, there are two major problems: the degradation and gelation of polypropylene, and the competition between polymer grafting and monomer homopolymerization, and the competition between β-bond breaking and crosslinking and branching of the polymer main chain. The main factor affecting the melt strength of polymer is its molecular structure. In the case of polypropylene, its melt strength is determined by its relative molecular mass and its distribution and whether it has a branched structure. Generally speaking, the larger the relative molecular mass and the wider the relative molecular mass distribution, the greater its melt strength. Long chain branches can significantly improve the melt strength of grafted polypropylene.

HMSPP special resin solves the problem of ordinary polypropylene being difficult to thermoform. It can be used to form thin-walled containers with a large stretch ratio on ordinary thermoforming equipment. It has a wide processing temperature range, is easy to master the process, and has uniform container wall thickness. It can be used to make microwave food containers and high-temperature steaming and sterilization containers. Ordinary polypropylene mixed with HMSPP has a higher processing temperature and processing speed than pure ordinary polypropylene, and the transparency of the film made is also better than that of ordinary polypropylene. This is mainly because HMSPP has the characteristics of tensile strain hardening, and its long chain branches have the effect of refining the crystal nucleus.

The strain hardening behavior of HMSPP is the key factor in achieving high stretch ratios and fast coating speeds. Using HMSPP can achieve higher coating speeds and thinner coating thicknesses. HMSPP has higher melt strength and tensile viscosity, and its tensile viscosity increases with increasing shear stress and time. The strain hardening behavior promotes the stable growth of the pores, inhibits the destruction of the micropore walls, and opens up the possibility of polypropylene extrusion foaming.

Although the research on high melt strength polypropylene only started in the late 1980s, its various excellent properties, reasonable price advantages and wide range of applications have been recognized worldwide. It has a trend of gradually replacing traditional PS and ABS and developing into engineering plastics, and 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 both absolute quantity and the breadth and depth of its applications. As a modified plastics industry, the high cost-effectiveness, multifunctionality and engineering of polypropylene are always important tasks ahead.

Applications

Usage allocation

Injection molding products account for 50% of the total consumption in Europe and the United States, mainly used as automobile and electrical appliance parts, various containers, furniture, packaging materials and medical equipment, etc.; films account for 8% to 15%, polypropylene fibers (commonly known as polypropylene in China) account for 8% to 10%; pipes and plates for construction account for 10% to 15%, and others account for 10% to 12%. At present, the amount used in China for woven products accounts for 40% to 45%, followed by films and injection products, which account for about 40%; polypropylene and others account for 10% to 20%.

China mainly uses polypropylene as a material in food packaging, household goods, automobiles, optical fibers and other fields. The largest area of ​​polypropylene use in China is woven bags, packaging bags, strapping ropes and other products, accounting for about 30% of total consumption. In recent years, with the development of polypropylene injection molding products and packaging films, the proportion of polypropylene used in woven products has declined, but it is still the area with the largest polypropylene consumption. Injection molding products are the second largest polypropylene consumption area in China, accounting for about 26% of total consumption, and it is also one of the areas with the largest demand for polypropylene in the future. Another major consumption area of ​​domestic polypropylene is film, accounting for about 20% of total consumption, mainly BOPP (biaxially oriented polypropylene film). In the next few years, the proportion of textile products will gradually decrease, while the proportion of injection molding products, pipes and sheets will increase. According to experts’ predictions on the development of the polypropylene industry, China’s demand for polypropylene may reach about 23.7 million tons by 2020. Textile products, injection molding products, and films are still the main demand areas for polypropylene in China, while the annual demand for pipes, sheets, fibers and other fields is growing rapidly, and the china’s demand for polypropylene is also growing rapidly. The market for special materials such as high-speed graphic BOPP films, tubes, thin non-woven fabrics, and highly transparent food containers has good development prospects.

Machinery and automobile manufacturing parts

Polypropylene has good mechanical properties and can be directly manufactured or modified to manufacture various mechanical equipment parts, such as industrial pipes, agricultural water pipes, motor fans, infrastructure templates, etc. Modified polypropylene can be molded into bumpers, anti-scratch strips, car steering wheels, instrument panels and interior decoration parts, greatly reducing the weight of the car body and achieving the purpose of energy saving.

Electronic and electrical industrial devices

Modified polypropylene can be used to make insulating shells of household appliances and washing machine liners, and is commonly used as insulating materials for wires and cables and other electrical appliances. The polypropylene composite material prepared by mixing 60-80 parts by weight of homopolymer polypropylene, 20-40 parts of ethylene-vinyl alcohol copolymer, and 1-10 parts of compatibilizer (the reaction product of polypropylene maleic anhydride grafted product and ethylene-vinyl alcohol copolymer) at 170°C-190°C has high toughness and an impact strength of up to 210J/m. It has high gas barrier properties and a water vapor permeability rate of nearly 2000g·μm/(m2 · 24h). When preparing barrier films, traditional film-making processes can be used for production, which is relatively simple and has low production costs.

Construction Industry

Polypropylene fiber is the lightest of all chemical fibers, with a density of (0.90~0.92) g/ cm3. It has the advantages of high strength, good toughness, good chemical resistance and antimicrobial resistance, and low price. Polypropylene reinforced with glass fiber or modified with rubber or SBS is widely used in the production of construction templates. Foamed polypropylene can be used to make decorative materials.When an earthquake occurs, the failure mode of polypropylene fiber ceramsite concrete is plastic failure, without fragments falling off. Polypropylene fiber ceramsite concrete is safer than plain ceramsite concrete.

Agriculture, fisheries and food industry

Polypropylene can be used to make greenhouse air canopies, ground films, culture bottles, agricultural tools, fish nets, etc., and to make food turnover boxes, food bags, beverage packaging bottles, etc. It can be reactively blended with waste PET (polyethylene terephthalate) to make multifunctional waste PET, and in-situ fiber-forming composite materials can be made by in-situ fiber-forming of multifunctional waste PET and polypropylene. The composite material has structural characteristics such as waste PET forming special-shaped microfibers, and a moderately flexible and strongly bonded interface between waste PET microfibers and PP matrix resin. The toughness and rigidity of the in-situ fiber-forming composite material prepared by compounding waste PET and PP are significantly higher than those of PP, and the reproducibility of mechanical properties is quite good. Recycling waste PET, which is a large amount of waste generated in China every year, has significant economic and social benefits.

The eastern coastal areas of China have vast marine tidal flats with typical saline soil characteristics. There are studies on the use of polyacrylamide (PAM) in conjunction with three types of forage grasses to implement soil and water conservation in coastal saline soil areas. PAM was applied under biological measures. It has a good promoting effect on the three types of forage grasses to improve the soil’s ability to resist erosion. The application of PAM can reduce soil erosion and increase rainwater interception; low doses (1g/m3) are given priority, and the soil and water conservation benefit per unit mass of PAM is the highest, which can reduce annual erosion by 42.8%~46.7%, inhibit total soil evaporation by 28.7%~40.4%, increase soil water loss by 5.0%~12.4%, reduce water loss rate by 1.83%~3.25%, and promote soil water holding capacity; in the early stage of forage growth. Increase rainwater interception by 16.5%~33.8%. The synergistic effect of PAM is conducive to inhibiting soil evaporation and enhancing rainwater interception capacity.

Textile and printing industry

Polypropylene is the raw material for synthetic fibers. Polypropylene fibers are widely used to make lightweight, beautiful, and durable textile products. Images printed using polypropylene materials are particularly bright, colorful, and beautiful.

Other Industries

In the chemical industry, polypropylene can be used to prepare various corrosion-resistant pipelines, storage tanks, valves, special-shaped packings in packing towers, filter cloths, corrosion-resistant pumps and linings of corrosion-resistant containers; in medicine, it can be used to make medical devices; polypropylene can also be developed and applied in the energy field through grafting, compounding and blending processes.

Waste PP recycling technology

Polypropylene (PP) is currently the second largest general-purpose plastic. With the development of industries such as construction, automobiles, home appliances and packaging, waste PP has become one of the largest waste polymer materials in recent years. At present, the main ways to deal with waste PP are: incineration for energy supply, catalytic cracking to prepare fuel, direct utilization and recycling. Considering the technical feasibility, cost, energy consumption and environmental protection factors in the process of treating waste PP, recycling is currently the most commonly used, effective and most recommended way to treat waste PP.

Due to the influence of light, heat, oxygen and external forces during use, the molecular structure of PP will change, and the products will become yellow, brittle, or even cracked, resulting in a significant deterioration in PP toughness, dimensional stability, thermal oxygen stability and processability. Directly using waste PP to manufacture products is difficult to meet the requirements of processing and use.

Therefore, the recycling technology of waste PP is constantly developing. By alloying with other polymers or compounding with fillers, the processing performance, thermal properties, physical and mechanical properties of waste PP can be significantly improved, thus achieving high performance of waste PP.

Alloying

Alloying is the process of mixing waste PP with other polymer materials to prepare a macroscopically uniform material. By selecting different polymer materials for alloying, the processing performance, physical and mechanical properties of waste PP can be improved. For example, the use of elastomers can significantly improve the impact toughness of waste PP.

A study on the mechanical properties and thermal deformation behavior of waste PP/RU composite rubber (natural rubber and styrene-butadiene rubber each account for 50%) blends found that first plasticizing the RU composite rubber into fine rubber particles and then evenly dispersing them in the waste PP continuous phase can significantly improve the impact strength and elongation at break of the waste PP, but will lead to a decrease in the rigidity and thermal deformation resistance of PP.

Since most elastomers are incompatible with waste PP and have poor interfacial bonding, phase separation occurs during processing and use, affecting its performance. In order to improve the interfacial compatibility of waste PP alloys and enhance interfacial bonding, many scholars have conducted extensive research and discovered two compatibilizers that can enhance the interfacial bonding of blended materials and improve the storage modulus, loss modulus and system viscosity of blended materials.

Vulcanizers can improve the impact and tensile strength, melt viscosity, elongation at break and ductility of the blended materials; the addition of peroxide crosslinking agents can further improve the compatibility of the blended materials and increase the impact and tensile strength of the blended materials, but lead to a slight decrease in elongation at break.

Composite

Composite material is the process of mixing waste PP with non-polymer materials to prepare composite materials. It is the main way to achieve high performance and functionalization of waste PP. Composite material can improve its physical and mechanical properties such as rigidity, strength, thermal and electrical properties, and reduce costs.

According to filler composition, fillers can be divided into inorganic fillers and organic fillers.

Inorganic filler compound

Inorganic fillers commonly used in PP composites can be used to composite with waste PP, such as calcium carbonate, talc, montmorillonite, metal oxides, fly ash and glass fiber. Studies have found that although these inorganic fillers can significantly improve the rigidity of waste PP and reduce costs, they have a large difference in polarity with waste PP, high surface energy and poor compatibility, resulting in a decrease in the elongation at break and impact toughness of the composite material.

Organic filler compound

Common organic fillers include wood powder and wood fiber, starch, wheat straw, hemp fiber and waste newspapers. There is research on the microporous foaming technology of waste PP filled with wood fiber. The results show that when the melting temperature is 180°C and the holding pressure is 12.5MPa, the microporous structure is evenly distributed. The microporous structure can extend the propagation path of the cracks and absorb external impact energy, thereby improving the impact strength.

Natural fiber is an emerging filler material for waste PP. Due to its high water absorption and incompatibility with waste PP, surface treatment is the main method to achieve high performance of natural fiber filled waste PP composites. In addition, waste polyester can also be used to modify waste PP. Some scholars have studied the crystallization behavior of β-nucleated waste PP/waste polyester fabric composites. The results show that waste polyester and β-nucleating agent have heterogeneous nucleation effects on the crystallization of waste PP, increase the crystallization temperature of waste PP, and induce the formation of β crystals.

Hybridization

Hybrid composite is the process of preparing composite materials by filling polymers with two or more fillers. Due to the limitations of a single filler, hybrid composite can improve the comprehensive performance of polymers through the complementary advantages and synergistic effects of different fillers. Therefore, research on the preparation and related properties of composite materials filled with mixed fillers has attracted attention. The fillers involved mainly include mixed inorganic fillers and mixed inorganic/organic fillers.

Alloy Composite

In order to give full play to the advantages of alloying and compounding, some researchers have begun to combine alloying and compounding to further improve and enhance the physical and mechanical properties of waste PP, and realize the high performance and industrialization of waste PP, such as organic fillers and elastomers, inorganic fillers and elastomers combined to modify waste PP.

The research results in this regard show that the fracture of waste PP and talc-filled waste PP composites at low temperatures is brittle behavior, and the addition of EOC (ethylene-octene copolymer) can significantly improve the impact resistance of the composites; the dynamic mechanical behavior of EOC-toughened talc-filled waste PP composites does not change with the increase in the number of recycling times.