The epidemic has made polypropylene quite popular, and topic of "polymer" has regained unprecedented attention. Gradually, in response to problems that arise in production, we also find that our theoretical knowledge is more and more insufficient. Learn some knowledge about polymer materials every day, and sooner or later you will become an "expert" in field of polymers. materials. Today we will talk about our views on strengthening of polypropylene.
1. Why polypropylene should be reinforced and modified
Polypropylene (PP) is a thermoplastic synthetic resin obtained by polymerization of propylene monomer. It is widely used in fields of automotive, packaging and daily necessities because of its excellent comprehensive properties. The polypropylene used in our normal processing and molding process mainly forms α crystalline form, as shown in Figure 1. α-crystalline polypropylene has higher flexural modulus and tensile strength, but lower toughness. This greatly limits further application in some areas with high requirements for toughness (such as car bumpers, various pipes and plastic cases of some household appliances, etc.), this is reason why we need to strengthen ordinary PP.
Fig.1. Polarized images of various types of α-crystals
Secondly, method of strengthening and modifying polypropylene
Polypropylene is modified to improve impact resistance in two ways: chemical modification and physical modification, as shown in Table 2-1.
Table 2-1 Modification of polypropylene to improve impact resistance
Chemical modification: Chemical modification can enhance strength and modify polypropylene in terms of molecular chains, which can greatly improve toughness and toughness of polypropylene. And in terms of "similar compatibility", raw materials used for modification are mainly organic substances, which can improve compatibility between polymers and solve problem of physical modification deposition to a certain extent. It can be said that modification of PP to improve strength is ideal means . However, in terms of economic efficiency, chemical-thermal modification makes high demands on equipment, and process is complex, requiring qualified personnel, large capital investments, high cost of polypropylene products and a long production cycle, so further development and application is limited.
Physical modification: mixing modification is most common modification method, which has advantages of simple operation, low cost and fast effect; method of inducing polypropylene to form β-crystals through crystallization can also greatly improve toughness of polypropylene, and shows great development potential in some applications.
3. An example of increasing strength of polypropylene
3.1 Composite of modified polypropylene and talc powder with increased POE impact resistance
The hardening mechanism of POE is in accordance with silver shear band theory. When POE is mixed with polypropylene substrate, since POE is a polyolefin thermoplastic elastomer, it will be distributed in particulate continuous phase of PP, that is, POE. The dispersed phase of microfibrillated particles (commonly known as "island") is randomly distributed within or between spherulites of brittle matrix continuous phase (commonly known as "sea"), forming a "sea-island" structure with good interfacial interaction. When polypropylene is subjected to an external force, POE particles can evenly distribute external impact stress over its spherical surface, reducing stress acting on brittle matrix.
Fig. 3-1 Influence of polyester content on mechanical properties of polypropylene composites
Yuan Haibing. Investigation of composites from modified polypropylene and POE talc [J]. China Plastics, 2018, 32(03):33-36.
It can be seen from Figure 3-1 that as POE content increases, tensile strength and bending strength of composite material gradually decrease, bending modulus also decreases continuously, and notched toughness increases continuously. The main reason for decrease in mechanical properties is that POE is a block polymer, when added to a polypropylene substrate, molecular chains of polypropylene are destroyed, which leads to a decrease in crystallinity of polypropylene. POE itself Compared to PP, physical and mechanical properties will be much lower, which will reduce strength of entire system.
Polypropylene, as one of "five general-purpose plastics", has been widely used in industry and everyday life due to its advantages, but we also have to deal with disadvantages of polypropylene materials. The strength and fragility indicated in article is one aspect. Therefore, hardening modification of polypropylene is a very important topic. Whether it is chemical modification or physical modification, simple and effective hardening modification methods should be a key research direction, and it still needs to be constantly studied by majority of scientific and technical personnel. .