In the field of industrial material applications, polypropylene (PP) sheets are widely used in multiple scenarios due to their outstanding characteristics such as light weight, strong corrosion resistance, and convenient processing. For example, products such as food packaging boxes that require safety and non-toxicity, or parts such as automotive interior decorations that require material strength, can all be seen. Specifically, its composition will directly affect the material properties, such as key indicators such as tensile strength and light transmittance, as well as the most important safety issues, especially in the field of packaging that directly contacts food, which requires strict compliance with hygiene standards.
The main components of PP sheets: polypropylene (PP)
In the field of industrial material applications, polypropylene (PP) sheets are widely used in multiple scenarios due to their outstanding characteristics such as light weight, strong corrosion resistance, and convenient processing. For example, products such as food packaging boxes that require safety and non-toxicity, or parts such as automotive interior decorations that require material strength, can all be seen. Specifically, its composition will directly affect the material properties, such as key indicators such as tensile strength and light transmittance, as well as the most important safety issues, especially in the field of packaging that directly contacts food, which requires strict compliance with hygiene standards.
Regarding the core composition of PP sheets, polypropylene as the main component has unique molecular characteristics. From the perspective of chemical structure, it is simply a long chain structure formed by connecting many propylene (C₃H₆) small molecules. According to the different arrangements of molecular chains, it can be divided into three types: the first is isotactic structure, that is, the molecules are arranged neatly like soldiers in a line. In this case, the material will be harder but easy to crack, such as plastic containers that need to withstand a certain pressure; the second is syndiotactic structure, where the molecular chains are arranged alternately. This structural arrangement will bring different physical properties; the third is a disordered random structure. Although it looks more transparent, the actual strength performance is not ideal.
From the perspective of physical properties, the degree of crystallization of this material directly affects the use effect. For example, materials with high crystallinity are more suitable for scenes that require high temperature resistance, because their melting point can usually reach 160-170 degrees Celsius, which can fully meet the needs of thermal processing. In terms of weight performance, its density is about 0.03-0.06g/cm³ lower than that of polyethylene (PE). This subtle difference is particularly important in the design of lightweight products, such as our common beverage bottle caps or medical syringe parts.
The expansion of the composition of PP sheets mainly involves the addition of additives and material modification
In terms of the use of additives, the main goal is to improve the fluidity of the material during processing, such as adding lubricants to make the melt smoother, or using tougheners to improve the impact resistance of the material, or using fillers to replace some raw materials to save costs. Things like plasticizers are mainly used to make the material softer and more flexible, but it should be noted that phthalates cannot be used in food packaging. There are also antioxidants, such as hindered phenols such as BHT, which are commonly used to prevent aging problems during high-temperature processing.
The selection of additives will have many effects. Simply put, when selecting additives, the processing should consider temperature adjustment. For example, the use of plasticizers may reduce the molding temperature, but the size of the parts is prone to deformation. In terms of performance, the addition of toughening agents does make it resistant to falling, but the tensile strength of the material may be compromised. Environmental protection requirements are also a key point. Some stabilizers containing heavy metals are now explicitly banned by many countries.
the composition difference between copolymer PP and homopolymer PP
Homopolymer PP is like plastic made of a single raw material. It has high crystallinity and is therefore hard, suitable for parts such as car bumpers that need to withstand collisions. Copolymer PP will be mixed with auxiliary materials such as ethylene or butene, and is divided into two types according to different arrangements: random copolymers have excellent transparency and flexibility, and can be used as food preservatives; block copolymers have outstanding impact resistance, and are very suitable for parts that are often bumped, such as refrigerator shells.
The effect of the composition ratio on performance can be understood as follows: the more copolymer monomers are added, the more elastic the material is, but the strength will decrease. For example, when the ethylene content is added to 10% to 15%, the impact resistance can be increased tenfold, but the material strength may drop by 20% to 30%. In this case, the formula ratio needs to be adjusted according to the specific application scenario in industrial production.
Changes in the composition of recycled PP sheets
1. Chemical effects of repeated processing
· After multiple processing, the molecular chains inside the material will break and branch, which will directly lead to an increase in melt fluidity, that is, it is more difficult to control the morphology during processing.
2. Problems with residues and decomposition products
· Under high-temperature oxidation conditions, substances such as carbonyl and decomposition products such as hydroxyl will be produced, which will reduce the purity of the material.
3. Solutions to performance degradation
· For example, the tensile strength may decrease by about 30%, and the color will become yellowish. At this time, it is necessary to add a compatibilizer or a reinforcing agent (such as some special additives) to improve it.
Composition standards for food-grade and industrial-grade PP sheets
1. Core standards for food-grade materials
· It must comply with international certification standards such as FDA, and substances such as phthalates cannot be used.
· Migration testing mainly depends on whether harmful components will be precipitated when the material comes into contact with food.
2. Application characteristics of industrial-grade materials
· Allowing the use of some non-food-grade additives, which is cheaper and suitable for scenarios such as building boards that do not require high safety.
3. Key points of certification testing
· Food-grade materials should focus on testing the content of heavy metals, and also conduct migration experiments simulating food contact environments.
Conclusion
1. Correlation between ingredients and performance
· The structural characteristics of polypropylene determine the basic performance, such as strength or transparency, while additives and modified treatments give the material more possibilities.
2. Future development direction
· Bio-based material direction: Use renewable resources such as corn starch as raw materials to reduce carbon emissions in the production process.
· Degradable direction: Add starch or PLA ingredients to the material so that the discarded material can be decomposed in the natural environment
