Brief introduction and physical and chemical properties of PP plastic

PP plastic, chemical name: polypropylene

English name: Polypropylene (referred to as PP)

Specific gravity: 0.9-0.91 g/cm3 Molding shrinkage: 1.0-2.5% Molding temperature: 160-220℃.

Ingredient structure

PP is a crystalline polymer. Among the commonly used plastics, PP is the lightest, with a density of only 0.91g/cm3 (smaller than water). Among the general-purpose plastics, PP has the best heat resistance, its heat distortion temperature is 80-100 ℃, and it can be boiled in boiling water. PP has good stress cracking resistance and high flexural fatigue life, commonly known as "folding glue". The comprehensive performance of PP is better than that of PE material. PP products have light weight, good toughness and good chemical resistance. Disadvantages of PP: low dimensional accuracy, insufficient rigidity, poor weather resistance, easy to produce "copper damage", it has the phenomenon of post-shrinkage, and after demolding, it is easy to age, become brittle, and easy to deform.

In daily life, the commonly used fresh-keeping boxes are made of PP materials.

Molding characteristics

1. Crystalline material, low hygroscopicity, easy to melt and rupture, and easy to decompose after long-term contact with hot metal.

2. The fluidity is good, but the shrinkage range and shrinkage value are large, and shrinkage holes, dents, and deformations are prone to occur.

3. The cooling speed is fast, the pouring system and cooling system should dissipate heat slowly, and pay attention to controlling the molding temperature. It is easy to orientate when the material temperature is low and high pressure. When the mold temperature is lower than 50 degrees, the plastic parts are not smooth, and it is easy to produce poor welding and flow marks. Warp deformation is easy to occur above 90 degrees

4. The thickness of the plastic wall must be uniform to avoid lack of glue and sharp corners to prevent stress concentration.

Process characteristics

PP has good fluidity and good molding performance at the melting temperature. PP has two characteristics in processing: First, the viscosity of PP melt decreases significantly with the increase of shear speed (it is less affected by temperature). ); Second: The degree of molecular orientation is high and the shrinkage rate is large.

The processing temperature of PP is better around 200-300℃, it has good thermal stability (decomposition temperature is 310℃), but at high temperature (270-300℃), it may degrade if it stays in the barrel for a long time. Since the viscosity of PP decreases significantly with the increase of shear speed, increasing the injection pressure and injection speed will improve its fluidity and improve shrinkage deformation and depression. The mold temperature should be controlled within the range of 30-50 °C. The PP melt can pass through a very narrow die gap and appear draped. During the melting process of PP, it needs to absorb a large amount of fusion heat (larger specific heat), and the product is hotter after being ejected from the mold. PP material does not need to be dried during processing, and the shrinkage and crystallinity of PP are lower than those of PE.

Horizontal comparison

The performance comparison of PP and several other main general-purpose plastics

Plastic type PP PE PVC PS ABS

Density Minimum Less than water Larger Slightly higher than water Slightly higher than water

Rigidity better poor good good good

Shrinkage rate generally poor good good good

Toughness poor at low temperature good poor poor good

Strength High Low High High High

Heat resistance good average poor poor poor

Chemical stability good good good good

Weather resistance poor poor general general poor

Toxicity Non-toxic Non-toxic Can be non-toxic Non-toxic Non-toxic

Adhesive Adhesion Poor Poor Good Fair Average

Heat sealing performance average good average average

Formability good good troublesome good good

Performance overview

1. Density

PP has the smallest density among all synthetic resins, only 0.90~0.91g/cm3, which is about 60% of the density of PVC. This means that more products of the same volume can be produced with the same weight of raw material.

2. Mechanical properties

The tensile strength and rigidity of PP are relatively good, but the impact strength is poor, especially the impact resistance is poor at low temperature. In addition, if the article is formed with orientation or stress, the impact strength can also be significantly reduced. Although the impact strength is poor, after modification such as filling or reinforcement, its mechanical properties can compete with higher-cost engineering plastics in many fields.

3. Surface hardness

The surface hardness of PP is low among the five types of general-purpose plastics, only slightly better than PE. When the crystallinity is high, the hardness also increases accordingly, but it is still not as good as PVC, PS, ABS, etc.

4. Thermal properties

Among the five general-purpose plastics, PP has the best heat resistance. PP plastic products can work for a long time at 100 ° C, and when there is no external force, PP products will not deform when heated to 150 ° C. After using a nucleating agent to improve the crystalline state of PP, its heat resistance can be further improved, and it can even be used to make utensils for heating food in microwave ovens.

5. Stress cracking resistance

Residual stress in the molded product, or the product works under continuous stress for a long time, will cause stress cracking. Organic solvents and surfactants can significantly promote stress cracking. Therefore, the stress cracking tests were all carried out in the presence of surfactants. Commonly used adjuvants are alkyl aryl polyethylene glycols. The test shows that the stress cracking resistance of PP when soaked in surfactant is the same as that in air, and it has good resistance, and the smaller the melt flow rate of PP (the larger the molecular weight), the stronger the stress cracking resistance.

6. Chemical stability

PP has excellent chemical stability and is inert to most acids, alkalis, salts and oxidants. For example, it is stable in concentrated phosphoric acid, hydrochloric acid, 40% sulfuric acid and their salt solutions at 100 °C, and only a few strong oxidants such as oleum can make it change. PP is a non-polar compound, very stable to polar solvents, such as alcohols, phenols, aldehydes, ketones and most carboxylic acids will not make it swell, but it is easy to dissolve or swell in some non-polar organic solvents.

7. Air tightness (gas barrier)

PP has a certain permeability to oxygen, carbon dioxide and water vapor. Compared with nylon (PA) and polyester (PET), it has a significant gap. For high-barrier plastics, such as PVDC, EVOH, etc., it is even worse. . However, its air tightness is quite good compared to other non-plastic materials. By adding barrier materials or coating the surface with barrier plastics, its air tightness can be greatly improved.

8. Aging performance

There are tertiary carbon atoms in PP molecules, which are easily broken and degraded under the action of light and heat. Unstabilized PP is heated at 150 ° C for more than half an hour, or exposed to sunlight for 12 days, and it will become significantly brittle. The PP powder without stabilizer will be seriously degraded when placed indoors in the dark for 4 months, giving off a distinct sour smell. Adding more than 0.2% antioxidant before granulation of PP powder can effectively prevent the degradation and aging of PP during processing and use. Antioxidants are divided into two categories: free radical chain reaction terminators (also known as primary antioxidants) and peroxide decomposers (also known as secondary antioxidants). will have a good synergistic effect. The currently recommended B215 antioxidant is the main antioxidant 1010 (phenols) and the auxiliary antioxidant 168 (phosphite) in a ratio of 1:2. In order to prevent photoaging, it is necessary to add ultraviolet absorber to PP, which can convert the ultraviolet absorption and excitation of wavelength 290~400nm into non-destructive longer wavelength light. For PP plastic products buried in the soil or used indoors to avoid light, only the main and auxiliary antioxidants can be added, and there is no need to add ultraviolet absorbers.

9. Electrical properties

PP is a non-polar polymer with good electrical insulation, and PP has extremely low water absorption, and electrical insulation will not be affected by humidity. The dielectric constant and dielectric loss factor of PP are very small and are not affected by frequency and temperature. The dielectric strength of PP is high and increases with temperature. These are beneficial for electrical insulating materials in wet and hot environments. On the other hand, the surface resistance of PP is very high, and antistatic treatment must be carried out in some occasions.

10. Good processing performance

PP is a crystalline polymer, and its particles will not melt below a certain temperature, unlike PE or PVC, which softens as the temperature increases during the heating process. Once a certain temperature is reached, the PP particles melt rapidly and can be completely transformed into a molten state within a few degrees. The melt viscosity of PP is relatively low, so it has good fluidity in molding processing, especially when the melt flow rate is high, the melt viscosity is smaller, and it is suitable for injection molding of large thin-walled products, such as the inner tub of a washing machine. After PP leaves the die, if it is slowly cooled in the air, larger grains will be formed, and the transparency of the product will be low. If it is quenched in water (as follows to make a film by the water-cooling method), the molecular motion of PP is rapidly frozen, and crystals cannot be formed, and the film at this time is completely transparent. The molding shrinkage rate of PP is relatively large, reaching more than 2%, which is much larger than that of ABS plastic (0.5%). The molding shrinkage rate of PP can vary with the type and amount of other materials added, which needs to be carefully considered when making injection molded products with matching dimensions.