Chapter 5C Flashcards
What are some physical properties of plastics?
Density and relative density, mold shrinkage, and viscosity.
How is density measured, and what is its importance?
Density is mass per unit volume (kg/m³). It helps determine material characteristics like strength and weight
Define mold shrinkage and explain its importance.
Mold shrinkage is the ratio of the decrease in length to the original length. It affects the size of molded parts, which designed larger than the desired finished part during mold design.
- Shrinkage continues for some time after molding.
- To allow complete post-mold shrinkage, do not take measurements until 48 hours.
What is viscosity, and why is it significant?
Viscosity is the resistance of a liquid to flow. More sluggish the liquid, the higher viscosity.
It is important for processing and handling plastics (transferring resin and injecting polymer to the mold)
What are the main environmental properties of plastics?
Chemical resistance, weathering, permeability, and water absorption.
Chemical resistance
- Ability of the plastic to resist attack from chemical reagents.
- Type of bonds, energy needed to break these bonds are important in considering the chemical reactivity and solubility of plastics.
What factors affect the chemical resistance of plastics?
Fillers, plasticizers, stabilizers, colorants, catalysts, and the type of bonds and molecular structure.
What is weathering in plastics, and what factors influence it?
Weathering refers to material degradation due to environmental exposure \ UV. Influencing factors include solar radiation, temperature, humidity, pollutants, and their combinations.
Weathering Tests
Exposure to heat, moisture and sunlight
Accelerated weathering tester
Rated on: - Color, gloss change, cracks and crazing, Loss of physical properties.
Why is permeability important in plastics?
It determines the volume or mass of gas or vapor penetrating a film in 24 hours, crucial for food packaging and preventing contamination
What and how is water absorption tested?
- Some plastics absorb moisture
- Dry bfr entering any process that involve heating and melting to avoid bubble inside the material.
Tests: using thermogravimetric principles, samples are weighed, heated, and reweighed to measure water absorbed.
Disadvantages: - May release other gas as well.
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What are some important optical properties of plastics?
Gloss, transparency, clarity, haze, color, and refractive index.
How is specular gloss measured?
Using a glossmeter to measure light reflection at specific angles.
Define luminous transmittance.
The ratio of transmitted light to incident light. It measures how much light passes through a plastic.
What factors determine color perception in plastics?
Light source, object properties, and observer perception. Color is described using value, chroma, and hue.
