Blowing agents are broadly categorized into two types: physical and chemical.
1. Chemical Blowing Agents (CBAs)
These are solid compounds that decompose when heated, releasing gases (like N2, CO2, NH3) that form bubbles within the rubber matrix. They are the most commonly used in rubber processing.
- Azodicarbonamide (ADCA or Azo):
- Function: One of the most widely used chemical blowing agents. It decomposes at temperatures typically between 150-205°C, releasing primarily nitrogen gas. It’s known for producing fine, uniform cell structures. Different grades are available with varying decomposition temperatures and gas yields.
- Application in Rubber Shoes: Widely used in EVA (Ethylene-vinyl acetate) and rubber compounds for shoe midsoles, outsoles, and other foamed components to achieve lightweight, cushioned, and shock-absorbing properties.
- Key Results of Missing ADCL (Blowing Agent/Crosslinker)
- Dense, Non-Foamed Product: Without ADCL, the EVA won’t expand properly during heating and molding, leading to a solid, dense piece of plastic instead of the intended foam.
- Poor Flexibility & Cushioning: The characteristic soft, springy feel and shock absorption of EVA foam will be lost, making the part hard and brittle.
- Sink Marks & Voids: If the material shrinks unevenly or air gets trapped due to poor flow, you might see sink marks (dents) or internal voids, especially in thicker areas, as the dense material cools.
- Short Shots: The lack of foaming pressure might prevent the molten EVA from filling the entire mold cavity, leading to incomplete parts.
- Surface Issues: You might see duller surfaces, poor texture, or even burn marks if the material overheats trying to flow without the aid of foaming.
- Why It Happens
- ADCL Function: ADCL (Azodicarbonamide) decomposes under heat, releasing gas (like nitrogen) that creates the closed-cell structure in EVA foam, and it also acts as a crosslinking agent, improving elasticity and strength.
- Molding Process: In injection molding, this expansion happens under pressure, creating the desired foam density and properties. Missing the ADCL means no gas is released, and crosslinking is reduced.
- In essence, you’ll get a poorly performing, hard plastic part instead of the intended functional EVA foam product. .
- Considerations: Exothermic decomposition (releases heat), can produce a characteristic odor, and may have regulatory considerations regarding its decomposition byproducts (e.g., formamide).
- Dinitrosopentamethylenetetramine (DNPT):
- Function: Decomposes at lower temperatures (around 190-194°C) compared to some ADCA grades, also releasing nitrogen and carbon monoxide. It’s an economical option for producing sponges and expanded rubber.
- Application in Rubber Shoes: Used in various cellular rubber products for footwear where low cost and good expansion are desired.
- Considerations: Highly flammable, releases fumes that resemble smoke upon decomposition, and can be mildly irritating to the eyes and skin.
- p-Toluenesulfonyl Semicarbazide (TSS):
- Function: A high-temperature decomposition blowing agent (around 200-225°C) suitable for engineering plastics and rubbers that require higher processing temperatures. It offers controlled expansion.
- Application in Rubber Shoes: Less common for standard shoe components, but might be used in specialized rubber formulations requiring higher processing temperatures and specific foam properties.
- Sodium Bicarbonate (NaHCO3) / Ammonium Bicarbonate (NH4HCO3):
- Function: These are inorganic, endothermic blowing agents (absorb heat during decomposition). They decompose to release carbon dioxide (CO2) and water. Often used in combination with an acid activator (e.g., citric acid) to lower the decomposition temperature and control gas release. They are considered more environmentally friendly as their decomposition products are non-toxic.
- Application in Rubber Shoes: Can be used, especially in conjunction with activators, for producing foamed rubber, particularly natural rubber latex foam, where non-toxic and odorless properties are desired.
- Considerations: Can have poor dispersibility in polymers if not surface-treated, and may lead to open, uneven cell structures if not properly controlled.
2. Physical Blowing Agents (PBAs)
These are typically gases or volatile liquids that expand when heated or when pressure is reduced, creating a cellular structure. Historically, CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) were used, but due to their environmental impact, they have largely been phased out. Modern PBAs include:
- Inert Gases (e.g., Nitrogen, Carbon Dioxide):
- Function: Injected under high pressure into the molten polymer, they dissolve and then expand as the pressure is released (e.g., during extrusion or injection molding), forming a foam.
- Application in Rubber Shoes: Used in processes like gas injection molding for foamed rubber components.
- Considerations: Requires specialized equipment for injection.
- Volatile Liquids (e.g., aliphatic hydrocarbons like petroleum ether, expandable microspheres):
- Function: Low-boiling point liquids that vaporize and expand when heated during processing. Expandable microspheres contain a liquid core that volatilizes and expands the sphere when heated.
- Application in Rubber Shoes: Can be used for specific foaming applications, particularly expandable microspheres for fine cell structures and reduced density.
- Considerations: Flammability and toxicity depending on the specific liquid.
Laboratory Testing Parameters Before Batch Production:
Before large-scale batch production of rubber shoe components, laboratory testing of the blowing agent and the rubber compound is crucial to ensure desired foam properties and consistent quality.
- Blowing Agent Characterization:
- Decomposition Temperature: Measured to ensure the blowing agent decomposes at the desired processing temperature of the rubber compound. Techniques like Differential Scanning Calorimetry (DSC) can be used.
- Gas Volume/Yield: Determines the amount of gas produced per unit weight of the blowing agent. This influences the final density of the foam.
- Particle Size: Affects dispersibility in the rubber matrix and ultimately the uniformity and size of the cells.
- Moisture Content: High moisture can affect decomposition and foam quality.
- pH (for aqueous suspensions): Relevant for certain blowing agents or if used in latex systems.
- Purity: Ensures consistent performance.
- Rubber Compound Testing (with blowing agent incorporated):
- Mooney Viscosity (ASTM D1646): Measures the flow characteristics of the uncured rubber compound. This is critical for understanding how the material will process and whether it can hold the generated gas bubbles before curing.
- Cure Properties (Rheometry – ASTM D5289): Using a Moving Die Rheometer (MDR) or Rubber Process Analyzer (RPA):
- Scorch Time: Indicates the time available before the onset of vulcanization (curing), crucial for processing without premature foaming or curing.
- Cure Rate: How quickly the rubber cures, influencing the “gas capture window” where bubbles are formed and stabilized.
- Torque Development: Provides insight into the overall cure characteristics.
- Foaming Behavior: RPAs can also be used to observe the foaming profile of the compound, including gas generation and cell development during simulated processing conditions.
- Density/Specific Gravity (ASTM D792): Measures the density of the foamed rubber. This is a primary indicator of the effectiveness of the blowing agent and the desired weight reduction.
- Tensile Strength and Elongation at Break: Measures the mechanical strength and flexibility of the foamed rubber, indicating its durability and performance under stress.
- Hardness (Shore A or C for foams): Measures the resistance to indentation, important for shoe cushioning and support.
- Compression Set: Measures the ability of the foam to recover its original thickness after being subjected to a compressive force, crucial for long-term cushioning performance in shoes.
- Cell Morphology (Microscopy – SEM): Scanning Electron Microscopy (SEM) can be used to visually inspect the cell size, distribution, and open/closed cell content. This directly impacts properties like cushioning, insulation, and durability.
- Aging Tests (e.g., Heat Aging): Simulates the effects of environmental exposure on the foamed rubber’s properties over time.
By meticulously testing these parameters, manufacturers can select the appropriate blowing agent, optimize its concentration, and adjust processing conditions to achieve the desired properties for rubber shoe components, ensuring both performance and consistent quality in batch production.