CAR Material Selection & Properties 3 — Questions and Answers
Question 1: What is the significance of a material's Curie temperature in appliance recycling metal sorting?
- It indicates the melting point of the metal
- Above this temperature, ferromagnetic metals lose their magnetic properties (Correct answer)
- It determines the metal's scrap market value
- It measures resistance to chemical corrosion
Correct answer: Above this temperature, ferromagnetic metals lose their magnetic properties
Understanding the Curie temperature is important because magnetic separation efficiency changes if materials are heated during processing.
Question 2: Which refrigerant oil type requires separate collection because it is miscible with HFC refrigerants?
- Mineral oil
- Polyolester (POE) oil (Correct answer)
- Naphthenic oil
- Paraffinic oil
Correct answer: Polyolester (POE) oil
POE oil is used with HFC refrigerants like R-134a and must be properly drained and managed due to its hygroscopic nature and environmental concerns.
Question 3: Acrylonitrile butadiene styrene (ABS) plastic in appliance housings is best identified by which characteristic?
- It floats in water and has resin code #5
- It sinks in water and has resin code #7 or #9 (Correct answer)
- It has a greasy feel and resin code #4
- It is transparent and has resin code #1
Correct answer: It sinks in water and has resin code #7 or #9
ABS has a density greater than water (~1.05 g/cm³) and is typically coded #7 (other) or #9 on the resin identification system.
Question 4: Lead solder used in older appliance circuit boards is hazardous primarily due to its:
- High flammability when heated
- Toxicity and classification as a RCRA hazardous waste (Correct answer)
- Strong magnetic field interference
- Corrosive reaction with plastic housings
Correct answer: Toxicity and classification as a RCRA hazardous waste
Lead is classified as a hazardous substance under RCRA, and circuit boards with lead solder must be managed accordingly to prevent soil and groundwater contamination.
Question 5: What property makes aluminum an energy-efficient choice for heat exchanger fins in air conditioners?
- Low thermal conductivity compared to steel
- High thermal conductivity combined with low density (Correct answer)
- High magnetic permeability for induction heating
- High melting point for high-temperature applications
Correct answer: High thermal conductivity combined with low density
Aluminum's high thermal conductivity (~205 W/m·K) and low density make it ideal for fins that must efficiently transfer heat while minimizing weight.
Question 6: In appliance recycling, 'zorba' refers to:
- A mixture of shredded non-ferrous metals after steel removal (Correct answer)
- Pure aluminum recovered from compressors
- The foam fraction separated from refrigerators
- Copper wire stripped from motor windings
Correct answer: A mixture of shredded non-ferrous metals after steel removal
Zorba is the industry term for a mixed non-ferrous metal fraction (mainly aluminum with copper, zinc, and other metals) produced after magnetic and ferrous removal.
Question 7: Which material property determines whether a plastic component will sink or float during density-based separation in recycling?
- Tensile strength
- Specific gravity relative to the separation medium (Correct answer)
- Melting point
- Color and opacity
Correct answer: Specific gravity relative to the separation medium
Sink-float separation exploits differences in specific gravity: plastics denser than the medium sink, while less dense plastics float for sorting.
CAR Material Selection & Properties 3
What is the significance of a material's Curie temperature in appliance recycling metal sorting?