What are the characteristics and basic properties of pp plastic?

Source:pulixin By Administrator Posted:2022-05-21
PP is the abbreviation of polypropylene, which is one of the most active plastic varieties. Understanding its structure and characteristics is very necessary for rational design of formula structure and rational selection of processing. Let's take a look at the characteristics and basic properties of pp plastics.
 

First, the characteristics of pp plastic

1. Good heat resistance. PP products can be used in a high temperature environment below 100 degrees for a long time, and the temperature will not be significantly deformed when the temperature reaches about 150 degrees.

2. The density is small, 0.81---0.91g/㎝3, which is the smallest density among commonly used plastics. PP material has good mechanical properties, high heat resistance, good chemical properties, almost no water absorption, no reaction with most chemicals, pure texture, non-toxic, and good electrical insulation.

3. The disadvantages of PP plastic are low dimensional accuracy, insufficient rigidity and poor weather resistance. It has the phenomenon of post-shrinkage, and the product is easy to age, become brittle and deform.

4. The shrinkage rate is large, the shrinkage rate is 1.2-----2.5%, and it can be changed to about 0.7% by adding 30% glass fiber.

5. Drying is not required if stored properly.

6. The back pressure of the melt adhesive can be 5bar, and the back pressure of the toner material can be appropriately increased.

7. The melting point is 160-175℃, and the decomposition temperature is 35O℃. However, during injection processing, the temperature setting cannot exceed 275°C, and the mold temperature is 50-90°C. For high mold temperature with higher size requirements, the core temperature is more than 5°C lower than the cavity temperature.

8. The injection pressure adopts higher injection pressure and holding pressure, and the holding pressure is about 80% of the injection pressure. Switch to holding pressure at about 95% of the full stroke, and use a longer holding time.

9. Injection speed, in order to reduce internal stress and deformation, high-speed injection should be selected, but if the patterned surface appears light and dark stripes diffused by the gate, low-speed injection and higher mold temperature should be used.

10. Mold. Generally, the straight diameter of the runner is 4-7mm, the length of the needle gate is 1-1.5mm, and the diameter can be as small as 0.7mm. The shorter the length of the edge gate, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and gradually increases with the length of the melt flow in the cavity, the mold must have good exhaust. , The vent hole is 0.025mm deep to 0.038mm, and the thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use a large and round nozzle and a circular runner, and the thickness of the reinforcing ribs should be small.

11. The viscosity of pp melt decreases significantly with the increase of shear rate, and is less affected by temperature.
 

Second, the basic performance of PP

1. PP physical properties Non-toxic, odorless, tasteless milky white high crystalline polymer, the relative molecular mass is about 80,000-150,000. Low density: 890-910kg/m3, which is one of the lightest varieties of plastics. Strong hydrophobicity: the water absorption rate in water for 24h is only 0.01%. The formability is good, but the shrinkage rate is large, and the thick-walled products are easy to dent. Product surface gloss is good, easy to color.

2. Mechanical properties PP has high crystallinity and regular structure, so it has excellent mechanical properties. However, at room temperature and low temperature, due to the high regularity of its molecular structure, the impact strength is poor. The most prominent performance of PP is the resistance to bending fatigue.

3. Thermal properties PP has good heat resistance, and the products can be sterilized and sterilized at temperatures above 100 °C, and will not deform at 150 °C without external force. The embrittlement temperature is -35 °C, and embrittlement occurs below -35 °C, and the cold resistance is not as good as PE. An increase in isotacticity or a decrease in molecular weight contributes to the improvement of thermal properties.

4. Chemical stability The chemical stability of PP is very good. In addition to being corroded by concentrated sulfuric acid and concentrated nitric acid, it is relatively stable to other chemical reagents. However, low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons and chlorinated hydrocarbons can soften and swell polypropylene, and its chemical stability increases with the increase of crystallinity. Therefore, PP is suitable for making various chemical pipelines and accessories. Corrosion effect is good.

5. Electrical properties: PP has excellent high-frequency insulation properties. Since it hardly absorbs water, its insulation properties are not affected by humidity. It has a high dielectric coefficient and can be used to make heated electrical insulation products with the rise of temperature. It also has a high breakdown voltage and is suitable for use as electrical accessories, etc. Good resistance to voltage and arc, but high static electricity. Due to the existence of side methyl groups in PP, tertiary carbon atoms appear alternately on the molecular chain, and tertiary carbon atoms are easily oxidized when in contact with copper ions, resulting in poor oxidation resistance and radiation resistance of polypropylene. Therefore, copper inhibitors must be added in applications that are in contact with copper.

6. Weather resistance PP is very sensitive to ultraviolet rays and needs to be added with additives to improve its aging resistance. Commonly used are UV absorbers and UV quenchers. What needs to be pointed out here is the brittle-ductile transition temperature of PP.