1. Definition of Plastics

Plastics are macromolecular compounds polymerized from monomers via addition polymerization or polycondensation reactions. They feature moderate deformation resistance, between fiber and rubber. Plastics mainly consist of resin plus additives such as fillers, plasticizers, stabilizers, lubricants and colorants.
Resin serves as the primary component, accounting for 40% to 100% of the total weight of plastics. The fundamental properties of plastics are mainly determined by the nature of the resin, while additives also play a vital role. Some plastics are basically composed of synthetic resin with few or no additives, such as acrylic glass and polystyrene.

2. Plastic Classification

Plastics are important synthetic organic polymer materials with a wide range of applications. Understanding the composition and classification of plastics helps us use plastic products rationally, supports sorted plastic recycling, and reduces "white pollution".
Plastic classification systems are diverse:
- By application: general-purpose plastics, engineering plastics, specialty plastics
- By physical & chemical properties: thermosetting plastics, thermoplastics
- By molding process: compression molding, laminating, injection molding, blow molding, extrusion, casting, reaction injection molding plastics
Below is the classification based on molecular structure and mechanical properties.

Classification by Molecular Structure

1. Polyolefin plastics: LDPE, HDPE, LLDPE, PP, EEA, EVA, PB-1, TPX
2. Polystyrene-based plastics (listed separately due to wide variety and importance, also part of polyolefins): PS, HIPS, ABS, AAS, ACS, MBS, AS
3. Vinyl plastics: PVC
4. Acrylic plastics: PMMA
5. Nylon (Polyamide): transparent nylon, MC nylon, PA6, PA66, PA610, etc.
6. Poly(phenylene ether) & polyesters: POM, PPO, NORYL, PPS, PSF, PC, PET, PBT, polyarylate, polyarylsulfone
7. Cellulose plastics: CN, CA, CAP, CAB, EC, CEC, HEC
8. Polyurethane: TPU
Molecular structure largely determines material characteristics. Grouping plastics by molecular structure helps readers better understand material performance.

Classification by Mechanical Performance

1. Low overall mechanical performance: General plastics — PE, PP, EEA, EVA, PVC
2. Medium overall mechanical performance: Engineering plastics — PS, HIPS, ABS, AAS, ACS, MBS, AS, BS, PMMA
3. High overall mechanical performance: Structural engineering plastics — PA, POM, NORYL, PC, PET, PBT
Material properties define suitable applications. There is no rigid boundary between general plastics and engineering plastics. Some general engineering plastics can also be used for structural parts when meeting mechanical requirements.

3. Synthesis Processes of Common Plastic Materials

3.1 Polyethylene Terephthalate (PET)

PET is a high-molecular polymer with chemical formula: -[OCH₂-CH₂OCOC₆H₄CO]ₙ-. It is synthesized through esterification and dehydration polycondensation of terephthalic acid and ethylene glycol. Three mainstream production routes are adopted globally: DMT method, PTA method and EO method.

(1) DMT Process
Dimethyl terephthalate (DMT) reacts with ethylene glycol (EG) via transesterification, followed by polycondensation to form PET.
Step1: DMT reacts with ethylene glycol or 1,4-butanediol under catalysts (zinc, cobalt, manganese acetate or mixtures with antimony trioxide, dosage: 0.01%~0.05% of DMT mass). Methanol by-product is continuously removed. BHET or BHB ester is generated.
Step2: BHET undergoes polycondensation in pre-polycondensation and final polycondensation reactors at 270℃ and 270~280℃ respectively. Small amounts of stabilizers are added to improve melt thermal stability. The polycondensation runs under high vacuum (residual pressure ≤ 266 Pa) with strong stirring to obtain high molecular weight polyester.

(2) PTA Process
High-purity terephthalic acid (PTA) directly esterifies with ethylene glycol or 1,4-butanediol to generate BHET, then polycondensation proceeds. The core challenge is uniform mixing of PTA and glycol monomers, accelerating reaction and suppressing etherification.
Compared with DMT route, it eliminates DMT production, refining and methanol recovery. It can produce polymers with higher molecular weight and good thermal stability for high-end products such as tire cord. However, high purity requirement for PTA increases raw material refining costs.

(3) EO Process
Ethylene oxide directly reacts with PTA to produce BHET before polycondensation. It skips ethylene glycol production, features high equipment utilization and easy product refining.
Disadvantage: The addition reaction requires pressure of 2~3 MPa, demanding high-standard equipment. Due to explosion, flammability and toxicity risks, this route has been phased out (once adopted in Japan).

3.2 Polyvinyl Chloride (PVC)

PVC is produced by free-radical polymerization of vinyl chloride monomer (VCM). The polymerization degree n ranges from 500 to 20000. PVC belongs to synthetic resin, one of three major synthetic materials (synthetic resin, synthetic fiber, synthetic rubber).
Four polymerization methods: bulk polymerization, emulsion polymerization, suspension polymerization, solution polymerization.
(1) Bulk Polymerization
No water or dispersant is required. Two-stage reaction:
Stage 1: VCM monomer, initiator and additives are fed into pre-polymerizer. Under heating and strong stirring, pre-polymerization proceeds under constant pressure & temperature. Reaction stops at 8%~12% VCM conversion to form seed particles.
Stage 2: Seeds are transferred to polymerization vessel. Extra VCM, additives and initiator are added for continuous polymerization under low-speed stirring. Reaction terminates at conversion 60%~85% (depends on formulation). Unreacted monomer is degassed and recovered. Steam stripping removes residual VCM, then PVC powder goes through classification, homogenization and packaging.
(2) Emulsion Polymerization
Final product is PVC paste resin (E-PVC) for plastisol. Two-stage production:
1. VCM emulsion polymerization generates PVC latex, suspension of 0.1~3 μm primary PVC particles in water.
2. Latex is spray-dried to get paste resin, secondary particles of 1~100 μm (mostly 20~40 μm). When blended with plasticizer and sheared, secondary particles break into fine particles to form stable PVC plastisol.
(3) Suspension Polymerization
Water, suspending agent and antioxidant are loaded into reactor, then VCM monomer is added and dispersed into tiny droplets under stirring. Monomer-soluble initiator is added, then temperature rises for polymerization at 45~70℃.
Low-temperature polymerization (42~45℃) produces high molecular weight PVC; high-temperature polymerization (62~71℃) yields low/ultra-low molecular weight PVC.
A two-step suspension polymerization technology has been developed abroad to boost productivity. The first stage reaches polymerization degree ~600, then fresh monomer is added for second-stage reaction. It shortens cycle time, and the resin shows good gelling, molding and mechanical performance.
Suspension PVC is the dominant manufacturing method thanks to flexible grade adjustment, easy process control, low operation cost and large-scale production capability.
A series of specialty PVC resins have been developed: ultra-high / ultra-low DP resin, high bulk density resin, skin-free resin, radiation-resistant, medical-grade and heat-resistant PVC. Development of specialty grades marks the advancement of suspension PVC industry.

3.3 Polypropylene (PP)

5 main PP production technologies: solution process, slurry process, bulk process, gas-phase process and bulk-gas phase combined process.
Advanced global technologies are gas-phase and bulk-gas combination. They produce homopolymers and random copolymers, then use tandem gas reactors for impact copolymers. These technologies support large-scale plants (≥200,000 tons/year), cost efficiency, diversified and high-performance products, and are widely applied.
China’s industrial PP production started in the 1970s. After decades of development, multiple processes (batch bulk, bulk, bulk-gas, gas-phase) coexist with production lines of various sizes.

4. Common Defects in Plastic Molding

1. Sink Marks
Sink marks often occur during molding, mainly caused by excessive lubricant. Too much lubricant leads to uneven mold temperature, non-uniform cooling and inconsistent solidification of molten plastic, resulting in sink marks on finished parts. Plastic pellets are melted and injected into molds, shaped under pressure. Uneven cooling creates indentations on final products.
2. Short Shot
Short shot happens when molten plastic cannot fully fill the mold cavity or there are large gaps between melt and mold. The incomplete filling causes rough surfaces or notches on plastic parts. Strict monitoring of injection and raw material screening can reduce this defect and improve product quality.
3. Flash
Flash appears at the edge of molded plastic components, caused by over-injection of plastic melt. Insufficient mold clamping force or poor venting also triggers flash defects.

5. The 7 Plastic Resin Identification Codes

You may notice triangular recycling symbols printed at the bottom of plastic containers, marked with numbers from 1 to 7. Below is the detailed explanation of each code.

#1 PET (Polyethylene Terephthalate)

Typical applications: mineral water bottles, carbonated beverage bottles.
PET resin has good mechanical performance; its impact strength is 3~5 times higher than many other films. It is fold-resistant, transparent, UV-blocking and glossy. PET bottles are also called PET bottles.
Max heat resistance: 65℃, minimum cold resistance: -20℃. It is only suitable for room-temperature or cool liquids. Filling hot liquid or direct heating causes thermal deformation and release of harmful substances. Long-term use over 10 months may release potential carcinogens.
PET bottles are disposable. Discard them after use via recycling. Do not reuse as water cups or storage containers.

#2 HDPE (High Density Polyethylene)

Typical applications: detergent & shower product containers, white medicine bottles, skincare packaging, supermarket shopping bags.
HDPE is highly crystalline non-polar thermoplastic resin. Melting point:120~160℃, normal service temperature ≤100℃; industrial grades can resist up to 110℃. Food-grade HDPE bags can hold food.
Containers for cleaning supplies can be reused after cleaning, but residual detergent is hard to remove and breeds bacteria. It is recommended not to reuse HDPE containers long-term. Dispose of waste HDPE via recycling classification.

#3 PVC (Polyvinyl Chloride)

Typical applications: raincoats, plastic films, building materials.
PVC is amorphous white powder with low branching degree, specific gravity ~1.4, glass transition temperature 77~90℃. It starts decomposing around 170℃. PVC is sensitive to heat and light. When heated above 100℃ or exposed to prolonged sunlight, it decomposes and releases hydrogen chloride, triggering auto-catalytic degradation, discoloration and rapid decline in mechanical properties.
PVC features good plasticity and low cost, but two toxic substances may exist: unpolymerized vinyl chloride monomer and harmful plasticizer additives. These toxic ingredients easily leach out under high temperature or oil contact and may cause cancer. Do not heat PVC products.

#4 LDPE (Low Density Polyethylene)

Typical applications: cling film, plastic film.
LDPE, also known as high-pressure polyethylene. Ethylene monomer is polymerized under high pressure with initiators. After separation of unreacted ethylene, melt extrusion, pelletizing, drying and blending are performed. It is suitable for most thermoplastic molding processes.
LDPE is widely used for film products, injection parts, medical devices, food packaging and blow molded hollow articles.
Cling film made of LDPE has limited heat resistance. It melts above 110℃ and releases non-degradable plastic agents. When wrapped food is heated in microwave, grease dissolves harmful substances from the film. Remove cling film before microwave heating.

#5 PP (Polypropylene)

Typical applications: woven bags, microwave-safe food containers.
PP is semi-crystalline thermoplastic plastic, featuring high impact resistance, strong mechanical performance, resistance to many organic solvents, acid and alkali corrosion. It is widely used in industry, and even adopted for Australian banknotes.
PP can withstand 130℃, and it is the only plastic material safe for microwave heating. It can be reused after thorough cleaning.
Attention: Some microwave containers use PP for the body but PS (Code 6) for lids. PS has good transparency but poor heat resistance. Remove the lid before putting into microwave.

#6 PS (Polystyrene)

Typical applications: instant noodle bowls, disposable fast food boxes.
PS is synthesized via addition polymerization of styrene monomer. It resists acid, alkali, salt, mineral oil, organic acid and low-carbon alcohol, but softens or dissolves in acetone, esters and aromatic solvents. It has good insulation, transparency, high refractive index, water resistance and excellent colorability. The melt flows well for molding and extrusion.
PS can be made into instant noodle cups and foam food containers. It tolerates hot and cold temperature, but **DO NOT microwave it**, otherwise chemicals may leach out. It is also not suitable for strong acid (orange juice) or strong alkali, which can decompose PS.

#7 PC & Other Resins

Typical applications: water bottles, space cups, baby bottles.
PC is widely used for baby bottles and sports bottles. It has drawn public attention due to bisphenol A (BPA).
In theory, BPA will not remain if fully converted during manufacturing. However, residual unreacted BPA may leach into food or drinks. Higher temperature accelerates BPA release.
Recommendations for #7 containers: Do not heat, avoid direct sunlight. Do not wash in dishwashers. Clean new containers with baking soda and warm water, air-dry at room temperature. Discard containers with scratches or damage, as cracks harbor bacteria. Avoid repeated use of aged plasticware.