Plastics Compounding: A Deep Dive into Polymer Additives
Plastics Compounding and Additives
What is Plastics Compounding?
Plastics compounding involves customizing raw plastic materials using various additives to meet specific color, property, and performance requirements.
Compounding Ingredients for Polymers
Various additives modify base polymers, expanding their applications. Key ingredients include:
- Resins: Act as binders, holding constituents together. Determine molding methods. Resin content in the final product ranges from 30% to 100%.
- Plasticizers: Increase flexibility, impart flame resistance, and reduce chemical/solvent resistance. Examples include vegetable oils and fatty acid esters.
- Fillers or Extenders: Reduce costs, increase tensile strength and hardness, and reduce flexibility. Examples include mica, sawdust, and chalk.
- Lubricants: Impart a glossy finish and prevent sticking to molds. Examples include waxes and soaps.
- Pigments: Provide color. Dyes create transparent colors, while pigments produce opaque colors. Examples include organic and inorganic dyes.
- Catalysts: Used in thermosetting plastics to accelerate polymerization. Examples include hydrogen peroxide (H₂O₂), zinc, ammonia, and its salts.
- Stabilizers: Prevent decomposition and discoloration at molding temperatures. Examples include stearates of lead (Pb), calcium (Ca), barium (Ba), lead silicates, and lead chromate (PbCrO₄).
Rubber Vulcanization and Properties
Vulcanization
Vulcanization is the process of improving natural rubber’s tensile strength and quality by heating it with sulfur.
Structural Changes After Vulcanization
- Cross-linking: Sulfur creates bridges between polymer chains, forming a three-dimensional network.
- Increased Molecular Weight: Cross-link formation increases the overall molecular weight.
- Reduced Solubility: Rubber becomes less soluble in solvents.
Properties of Vulcanized Rubber
- Enhanced Elasticity
- Greater Tensile Strength
- Improved Durability
- Better Heat Resistance
- Ozone and Weather Resistance
Styrene-Butadiene Rubber (SBR)
Key Properties of SBR
- Widely used due to availability and affordability.
- Offers high tensile strength, abrasion resistance, and resilience.
- Good low-temperature flexibility and resistance.
- Resistant to organic acids, water, chemicals, alcohols, ketones, and aldehydes.
- Crack resistant, accommodating large amounts of fillers.
SBR Additives
- Carbon black: Increases strength, abrasion, and UV resistance.
- China clay: Used in non-black rubbers; adds strength and reinforcement.
- Calcium carbonate: Reduces the final product amount.
- Silica: Increases thermal conductivity, dimensional stability, and electrical insulation.
- Filaments: Reduces the finished product’s stretchability.
Uses of SBR
- Car tires (lesser proportions, even in heavy-duty and high-performance tires) due to high heat resistance.
- Lighter-duty tires use cold emulsion SBR.
- Specialty applications (e.g., motorcycle treads, radial car tires) use solution SBR (higher cost).
- Automotive parts (e.g., drive couplings).
- Industrial applications: wire insulation and cabling, belting, roll coverings, haul-off pads, hoses, seals, gaskets, abrasion resistance, and metal adhesion.
- Commercial applications: shoe soles, molded rubber goods, carpet backing adhesive.
Production and Processing of SBR
Production begins by mixing elastomers with additives, followed by shaping using various processing methods. Compounding additives with SBR usually involves sulfur for vulcanization and reinforcing fillers to enhance mechanical properties and reduce costs by extending the rubber.
Kevlar (Aromatic Polyamide)
Preparation
Kevlar is produced via polycondensation of terephthaloyl chloride (aromatic dichloride) and 1,4-phenylenediamine (aromatic diamine).
Properties
- High Strength (5x stronger than steel, 10x stronger than aluminum)
- Heat Resistance
- Chemical Resistance (except some strong acids)
- Low-Temperature Performance (maintains strength at -196°C)
Applications
- Aerospace and Aviation (structural components)
- Automotive Industry (tires, brake pads)
- Protective Gear (bulletproof vests, ropes, cables, helmets)
Silicones
Properties of Silicones
- Low thermal conductivity and chemical reactivity.
- Low toxicity.
- Water repellent; forms watertight seals.
- High resistance to oxygen, ozone, and UV light.
- Electrically insulative and conductive properties.
- High gas permeability and thermal stability.
- Superior solvents for organic compounds.
Preparation of Silicones
Silicones are prepared from pure silicon (obtained by reducing silicon dioxide with carbon at high temperatures): SiO₂(s) + 2C(s) → Si(s) + 2CO(g)
Production generally involves three stages:
- Synthesis of chlorosilanes
- Hydrolysis of chlorosilanes
- Condensation polymerization
Burning silicone in oxygen produces solid silica (SiO₂), char, and various gases. The dispersed powder is sometimes called silica fume.
Phenol-Formaldehyde Resin
Preparation Method
Condensation reaction of phenol and formaldehyde (using a catalyst) forms a thermosetting resin (e.g., Novolak or Bakelite).
Properties
- Thermosetting (cannot be remolded after curing)
- High Strength
- Heat and Chemical Resistance
- Good Electrical Insulation
Applications
- Adhesives (wood products, laminates)
- Molding Compounds (electrical insulators, automotive parts)
- Coatings (paints, varnishes)
- Composites (reinforced plastics)
Polyurethane Rubber
Preparation Method
Polyurethane rubber is created by reacting polyols (long-chain alcohols) with diisocyanates (TDI or MDI) using catalysts and additives.
- Polyols and diisocyanates react to form a prepolymer.
- The prepolymer is cured with agents (water or amines) to form the final rubber.
Properties
- Elasticity
- Durability
- Chemical Resistance
- Temperature Resistance
- Customizable Hardness
Applications
- Sealants and Gaskets (automotive, industrial)
- Footwear (soles, cushioning)
- Flexible Hoses (automotive, hydraulic systems)
- Coatings (protective coatings, adhesives)
- Medical Devices (prosthetics)