Classification and physical properties of plastics

March 30, 2022

By siedery

This article introduces the classification and properties of commonly used plastics in detail. This knowledge is necessary for everyone engaged in the injection molding industry because it is related to the material selection of product development and design.

  1. What is plastic?
  2. the source of plastic
  3. the development process of plastics
  4. the classification of plastics
  5. Physical properties of plastics
  6. the characteristics of plastic
  7. the plastic coloring
  8. Recycling of water mouth material

Firstly. What is plastic?

Plastic is an artificial polymer organic compound (resin) with plasticity.

Plastic refers to the organic synthetic resin as the main component, with or without adding other matching materials (additives) and the composition of man-made materials. It is usually molded into a device with a certain shape under heating and pressure.

Common plastic products
Common plastic products

Plasticity refers to the property that, like clay, deforms when force is applied and does not return to its original state when the external force is removed. The so-called elasticity (elasticity), refers to the application of a certain degree of force deformation, but the removal of the force applied to restore the original state, this property is called elastic (for example: rubber), with elastic objects called elastomers (such as: elastic band); plastic is the use of this plasticity generated when heated, processed into various shapes.

Second, the source of plastic

Plastic is made of low molecular organic compounds (such as: ethylene, propylene, styrene, vinyl chloride, vinyl alcohol, etc.) polymerized under certain conditions into a polymer organic compounds (polymers).

The molecules that make up plastics are polymer compounds (polymers) because they have a molecular weight of more than 10,000 molecules. In general, plastic molecules contain carbon (C) atoms and hydrogen (H) atoms, and some plastics contain small amounts of oxygen (O) and sulfur (S) atoms in their molecular structure. The basic raw materials of plastics are low molecular carbon and hydrogen compounds, which are refined and synthesized from petroleum, natural gas or coal cracking products to produce man-made resins.

Third, the development process of plastics

The development and production time of different plastics:

  • 1920-1933: polystyrene (PS), polyvinyl chloride (PVC), polyethylene (PE), ethylene vinyl acetate copolymer (EVA), polypropylene (PP)
  • 1935: Acrylonitrile-butadiene-styrene copolymer (ABS)
  • 1939: Polyethylene terephthalate copolymer (ABS)
  • 1949: Polyamide (Nylon)
  • 1950:Poly(methyl methacrylate)(Acrylic)
  • 1956:Polyformaldehyde (POM)
  • 1958:Polycarbonate (PC)
  • 1964:Poly(dimethylene oxide)(PPO)
  • 1965:Polysulfone (PSF)
  • 1975: Polybutylene terephthalate (PBT)
  • 1985:Liquid crystal plastic (LCP)
  • 1994:PET modified to injection molding grade (PETG)
  • 1995:Super-elastic polyoxymethylene (TPOM)
  • 1996: Transparent rigid polyvinyl chloride

After 1960, the application and processing technology of plastics has been developed by leaps and bounds.

Fourth, the classification of plastics

At present, the plastic has developed to more than 300 kinds, the most commonly used plastics are more than a dozen.

Classification and industrial application of common plastics
Classification and industrial application of common plastics

1. Classification according to the application of plastics

Generally, they are divided into general-purpose plastics and engineering plastics:

General purpose plastics can only be used as general non-structural materials, their production volume, relatively low price, general performance, mostly used for making daily necessities. (Such as: PE, PP, PVC, PS, PMMA, EVA, etc.)

Engineering plastics are high mechanical properties and high temperature resistance, corrosion resistance, can be used as structural materials, with excellent overall performance (including: mechanical properties, electrical properties, heat resistance, chemical resistance, etc.), can be in a wider temperature range and a longer period of time to maintain this performance, and can be subjected to mechanical stress and more demanding chemical, physical environment in the long-term use. The seven recognized engineering plastics are: ABS, PC, POM, PA, PET, PBT, PPO, etc. The production of engineering plastics is relatively small and expensive. In addition, there are functional plastics (such as LCP, artificial organs, etc.), nano-plastics, degradable plastics, etc.

2, According to the crystalline form of plastic classification

Generally divided into crystalline plastics and amorphous plastics.

Crystalline plastic refers to the plastic in which the molecules can produce a certain geometric structure under appropriate conditions (e.g.: PE, PP, PA, POM, PET, PBT, etc.), most of which belong to the partially crystalline state.

Amorphous plastics are those plastics whose molecular shapes and molecular arrangements are not crystalline but disordered (e.g. ABS, PC, PVC, PS, PMMA, EVA, AS, etc.), and the mechanical properties of non-crystalline plastics are the same in all directions (isotropic).

3, Classified according to the basic behavior presented when subjected to heat

Generally divided into thermoplastics and thermosets.

Thermoplastics are plastics that can be repeatedly heated and softened and cooled and hardened within a specific temperature range (such as: ABS, PP, POM, PC, PS, PVC, PA, PMMA, etc.), which can be recycled.

Thermosetting plastics are plastics that become non-fusible when heated and no longer have plasticity when heated again and cannot be recycled (such as: phenolic resins, epoxy resins, amino resins, polyurethane, expanded polystyrene, etc.).

4, According to the classification of plastic light-transmitting

Generally divided into transparent plastics translucent plastics and opaque plastics.

Light transmission rate of 88% or more of the plastic called transparent plastics (such as: PMMA, PS, PC, Z-polyester, etc.), commonly used translucent plastics are: PP, PVC, PE, AS, PET, MBS, PSF, etc., opaque plastics are mainly POM, PA, ABS, HIPS, PPO, etc.

5, Classified by the hardness of plastic

Generally divided into hard plastics, semi-hard plastics, and soft plastics.

Common hard plastics are ABS, POM, PS, PMMA, PC, PET, PBT, PPO, etc;

Semi-rigid plastics are: PP, PE, PA, PVC, etc;

Soft plastics are soft PVC, K rubber (BS), TPE, TPR, EVA, TPU, etc.

6, Category according to the chemical structure of plastics

  • Polyolefin (such as: LDPE, MDPE, HDPE, LLDPE, UHMWPE, PP, etc.)
  • polystyrene class (such as PS, AS, BS, ABS, MBS, HIPS, etc.)
  • polyamide class (such as: PA6, PA66, PA610, PA1010, etc.)
  • polyether (such as PC, POM, PSF, PPO, etc.)
  • polyester (such as PBT, PET, etc.)
  • acrylate (such as PMMA)

Five, Physical properties of plastics

1. Specific gravity (density) ★

The specific gravity of plastic is at a certain temperature, the weight of the sample and the weight of the same volume of water, the unit is g/cm3, commonly used in liquid buoyancy method for the determination of the method.

2. Water absorption ★

Water absorption of plastic is the specified size of the specimen immersed in distilled water at a certain temperature (25 ± 2) ℃, after 24 hours of the amount of water absorbed; absorption of water affects its size and shape, the water absorption rate expressed by weight, often expressed in %.

3. Breathability

Permeability refers to a certain thickness of plastic film under an atmospheric pressure, a square meter area, the volume of gas permeable in 24 hours (cm3) value, but the amount of air permeability and thin wax thickness, area, time, temperature, air pressure difference, etc…

4. Moisture permeability

Moisture permeability refers to the permeability of water vapor through plastic film, the basic principle and definition is the same as air permeability.

5. Transparency

The ratio of the luminous flux through the object and the luminous flux onto the object is called transmittance; the ratio of scattered light to all transmitted light in the direction of incident light is called haze or turbidity. Haze is usually translucent and has the nature of diffuse transmission of the incident light.

6. Tensile Strength

Tensile strength refers to the specified test temperature, humidity and tensile speed, along the longitudinal direction of the specimen applied tensile load, to determine the maximum load when the specimen is damaged.

7. Compressive Strength

Compressive strength is the strength of the specimen to rupture (for brittle materials) or yield (for non-brittle materials) by applying a compressive load to the specimen.

8. Bending strength

Bending strength is the strength of the specimen in two points of support, the application of concentrated load, the specimen deformation or until rupture.

9. Impact strength

Impact strength refers to the impact of the specimen broken, the joules consumed per unit area, for some high impact strength of plastic, often in the middle of the specimen with the required size of the notch, which can reduce the joules it needs when broken.

Different test methods are available for different specimens: drop ball impact test, high speed tensile impact test.

10. Friction coefficient

Friction coefficient is the ratio of friction force to positive pressure. A positive pressure is added to the specimen to determine the ratio of dynamic to static when the specimen is moving rigidly.

11. Abrasion

Abrasion is the process of mechanical damage caused by the continuous separation of particles from the friction surface in the process of friction of plastics, resulting in a continuous change in the size of the friction parts, also known as wear or abrasion.

12. Hardness

Plastic hardness refers to the plastic resistance to other hard objects pressed into the performance, the general Rockwell hardness and Shore hardness two.

Shore hardness refers to the depth of indentation by the indenter pin under the specified pressure and time.

Shaw indenters can be divided into two categories, namely: A and D type. The applied load weight is 1.0, 5.0 kg, and the indentation time is 15 seconds. Type A is suitable for soft plastics, and Type D is suitable for semi-hard plastics; when using Type A, the measurement is more than 95% of the range, it should be changed to Type D. When Type D measures more than 95% of the range, it is necessary to change to Rockwell indentation.

13. Fatigue strength

Fatigue strength refers to a static destructive force and a small amount of alternating cycles in the environment, so that the strength of plastic damage; fatigue load sources are tensile, bending, torsion, impact, etc..10. Friction coefficient

Friction coefficient is the ratio of friction force to positive pressure. A positive pressure is added to the specimen to determine the ratio of dynamic to static when the specimen is moving rigidly.

14. Abrasion

Abrasion is the process of mechanical damage caused by the continuous separation of particles from the friction surface in the process of friction of plastics, resulting in a continuous change in the size of the friction parts, also known as wear or abrasion.

15. Hardness

Plastic hardness refers to the plastic resistance to other hard objects pressed into the performance, the general Rockwell hardness and Shore hardness two.

Shore hardness refers to the depth of indentation by the indenter pin under the specified pressure and time.

Shaw indenters can be divided into two categories, namely: A and D type. The applied load weight is 1.0, 5.0 kg, and the indentation time is 15 seconds. Type A is suitable for soft plastics, and Type D is suitable for semi-hard plastics; when using Type A, the measurement is more than 95% of the range, it should be changed to Type D. When Type D measures more than 95% of the range, it is necessary to change to Rockwell indentation.

16. Fatigue strength

Fatigue strength refers to a static destructive force and a small amount of alternating cycles in the environment, so that the strength of plastic damage; fatigue load sources are tensile, bending, torsion, impact, etc..

17. Creep

Creep refers to a certain temperature, humidity conditions, the plastic in the continuous action of a fixed external force, the characteristics of the change over time, the characteristics of this deformation with the increase in load and increase, with the reduction of load and decrease, the deformation is gradually restored. The sources of creep are tensile creep, compression creep, bending creep, etc.

18. Durable strength

Durable strength is the ability of plastic to withstand static load for a long time from high to low as a function of time. For example: the strength of the plastic before the load is 1000 hours, and after the load may only be between 50% and 70%.

19. Coefficient of linear expansion

The coefficient of linear expansion is the number of centimeters of elongation per centimeter of plastic when the temperature increases by 1 degree Celsius. The coefficient of linear expansion of plastics is generally about ten times that of steel.

19. Specific heat

Specific heat is the unit of heat required to raise 1 gram of plastic by 1 degree Celsius.

20. Thermal conductivity

Thermal conductivity refers to the unit of heat that can be passed through a unit area and thickness of plastic. The thermal conductivity of plastic is very small, only about one percent of steel, so it is a good insulation material.

21. Heat resistance

Plastic heat resistance is a characteristic that reflects the relationship between the temperature of the plastic part and the amount of deformation, heat resistance is more important for temperature-related plastic parts.

22. Glass transition temperature

Plastic from the molten flowable temperature down to the solid temperature is called the glass transition temperature, at this time the molecular chain segments basically can not move, the internal rotation of the chain link fastening is also very difficult, only a little movement between the atoms stretch and have ordinary elastic deformation, so at this time the plastic will have a great brittleness.

23. Embrittlement temperature

When pressure is applied to a plastic at a certain low temperature, it will be destroyed with little deformation, and this temperature is the embrittlement temperature.

24. Decomposition temperature

Decomposition temperature refers to the temperature of the plastic when the macromolecular chain breaks when heated, and is one of the indicators to identify the heat resistance of plastics; when the temperature of the melt exceeds the decomposition temperature, most of the melt will show a yellowish color, and the strength of the product will be greatly reduced. 

25. Melt index

Melt index (MFI) MI refers to thermoplastic at a certain temperature and pressure, the melt in 10 minutes time through the small hole in the tester out of the weight of the melt, the unit is expressed in grams / 10 minutes.

Six, the characteristics of plastic

1. Advantages of plastics

  • easy to process, easy to manufacture (easy to shape)
  • Can be colored as needed, or made into transparent products
  • can be made of light and high strength products
  • No rust, not easy to corrosion
  • Not easy to transfer heat, good heat preservation performance
  • can be made both conductive parts, but also to make insulating products
  • excellent vibration damping, sound-deadening performance, good light transmission
  • low cost of product manufacturing

2. Disadvantages of plastic

  • poor heat resistance, easy to burn
  • With the change of temperature, the nature will also change greatly
  • Mechanical strength is low
  • easy to be corroded by special solvents and drugs
  • Poor durability, easy to age
  • Easy to be damaged, also easy to be contaminated with dust and dirt
  • Poor dimensional stability

Seven, the plastic coloring

The application varieties of colorants are: dry powder (color powder), color seeds, color masterbatches, liquid color paste, etc. They are divided into two categories: organic pigments and inorganic pigments.

Colorants need to have the following good performance: strong coloring power, strong covering power, good dispersibility (compatibility), good heat resistance, good light resistance, good migration resistance, good solvent resistance, good chemical resistance, low shrinkage rate, etc. As the customers’ requirements for the color of plastic parts become more and more demanding, the application of color masterbatches or pulling granules is becoming more and more widespread.

Eight, Recycling of water mouth material

The general thermoplastic water mouth material can be recycled, the experiment proves that the proportion of water mouth material added within 25%, the plastic performance (strength) of the impact is not obvious (10% below. The control and recycling of sprues is the research topic of plastic industry, the use of hot runner mould is the creation of reducing sprues. The number of times and the proportion of the recycling of the water mouth material, the color strength of plastic products have different degrees of impact, production to strictly control the amount of added water mouth material.

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