Structure A, PP
As one of most actively modified plastics, polypropylene is very necessary to understand its structure and characteristics for intelligent design of formula structure, reasonable choice of processing and application.
First, let's look at PP structure.
Figures 2.1, 2.2 and 2.3 are schematic diagrams of various polypropylene structures from which various excellent properties of polypropylene are derived.
Figure 2.1 Molecular structure of polypropylene
Figure 2.2 Structural diagram of atoms of a branched chain of polypropylene
Figure 2.3. The structure of polypropylene spherulite - photograph from a polarized microscope
B, PP base performance
Polypropylene is divided into conventional polymerization and random polymerization, but since conventional polymerization polypropylene is mainly used in manufacturing, this chapter mainly deals with conventional polymerization polypropylene.
(a) Physical properties of polypropylene
A tasteless, odorless, non-toxic, milky white, highly crystalline polymer with a relative molecular weight of approximately 80,000–150,000.
Low density: 890–910 kg/m3, one of lightest types of plastic.
Strong hydrophobicity: coefficient of water absorption in water within 24 hours is only 0.01%.
Good formability, but shrinkage rate is high and thick-walled products sag easily.
The product has a good surface gloss and is easy to paint.
(b) Mechanical properties
PP has a high degree of crystallinity and regular structure, so it has excellent mechanical properties. However, at room temperature and low temperature, due to high uniformity of molecular structure, impact strength is low. The most prominent characteristic of polypropylene is its resistance to flexural fatigue. Among them, isotacticity and molecular weight (MFI) have a big impact on performance. The specific impact is shown in Figure 2.4.
It can be seen from figure that isotacticity increases, strength, rigidity and hardness increase, impact resistance decreases.
Molecular weight increases (MFI decreases), strength increases, stiffness and hardness decrease, impact resistance improves.
Figure 2.4 Relationship between different isotacticity, molecular weight and mechanical properties
(c) Thermal characteristics
PP has good heat resistance, product can be sterilized at temperatures above 100°C, and at 150°C it does not deform without external influence. The embrittlement temperature is -35°C, embrittlement occurs at temperatures below -35°C, and frost resistance is lower than that of polyethylene.
Increasing isotacticity or decreasing molecular weight improves thermal properties.
(d) Chemical stability
The chemical stability of polypropylene is very good, except that it can be corroded by concentrated sulfuric acid and concentrated nitric acid, it is relatively resistant to various other chemicals, but low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons can. Polypropylene softens and swells , and its chemical stability improves with increasing crystallinity, so PP is suitable for making various chemical pipes and fittings, and has good anti-corrosion effect.
(e) Electrical properties
Polypropylene has excellent insulating properties at high frequencies. Since it practically does not absorb water, insulating performance is not affected by humidity. It has a high dielectric coefficient and can be used for manufacture of electrical insulating products with heating when temperature rises. It also has high breakdown voltage and is suitable for use as electrical accessories, etc. Good voltage resistance and arc resistance, but high static electricity.
Due to presence of side methyl groups in PP, tertiary carbon atoms alternately appear in molecular chain, and tertiary carbon atoms are very easy to oxidize when in contact with copper ions, resulting in poor oxidation resistance and radiation resistance of polypropylene. Therefore, when in contact with copper, it is necessary to add copper inhibitors.
(f) weather resistance
PP is very sensitive to UV rays and additives must be added to increase its aging resistance. Commonly used are UV absorbers and UV quenchers.
The following should be noted here:The brittle-plastic transition temperature of polypropylene. Many studies and materials have shown that polypropylene has an apparent brittle transition temperature. Year on raw materials of PP and PP/POE, PP/POP three systems for application of resistance to changes in ambient temperature found that: PP has no obvious brittle-ductile transition temperature, experiment showed that regardless of PP raw materials or hardened material, change in strength tensile is linear and corresponds to change equation Y = A-BX, coefficient kthe correlations of regression fit reaches 0.991; impact strength has a significant change at about 25°C, and then impact strength decreases smoothly, bending strength increases significantly at about 5°C (see Fig. 2.5). Although author is unable to carry out experiments at lower temperatures due to experimental conditions, current conclusions have been confirmed by experiments of N. Rao and C. O'Brien.
Figure 2.5. Changes in mechanical properties of polypropylene materials with temperature changes
C, polypropylene processing rheology
Figure 2.6 shows relationship between shear rate and apparent grade of polypropylene. It can be seen from figure that polypropylene is both shear sensitive and heat sensitive, but polypropylene is more shear sensitive.
Figure 2.6 Relationship between shear rate and apparent viscosity of polypropylene
Therefore, attention should be paid to judicious use of these relationships in processing.
Source: "Practical Guide to Rubber and Plastic Technology"
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