Chapter 5B Flashcards
What does the shear strength test measure?
It measures the maximum load required to completely separate the movable part of a specimen from the stationary part through shearing action
What is the fatigue and flexing test?
It determines how many cycles a material can withstand before fracturing. The test factors include temperature, stress, frequency, amplitude, and mode of stressing.
- Important for producing integral hinges and one-piece box and lid containers.
What is measured in the abrasion resistance test?
The test measures the wear resistance of a material by assessing the amount of material lost when an abrader rubs against the sample.
- Amount of material loss indicates how well the sample resists the abrasive treatment.
Why are thermal properties tested in plastics?
To ensure correct processing and to confirm the material can stand for specific service temperatures of applications.
Define specific heat and its importance
The amount of heat required to raise the temperature of a unit mass by 1°C or 1 K. It helps calculate costs for processing and forming materials.
What is thermal expansion?
It measures the change in length, area, or volume per unit temperature rise.
Materials without fillers have higher thermal expansion coefficients.
high thermal expansion prevents use of plastics in some application.
Uses: - To predict shrinkage in injection molded part.
- To fit parts over one another.
What is deflection temperature, and how is it measured?
It is the highest continuous operating temperature that a material can withstand without significant deflection. The sample is placed on supports, and force is applied as the temperature rises.
_ T raised until sample chars, blisters, distorts.
Deflection temperature use?
- Measurement of relative service temperature for a material when used in load-bearing parts.
- Represents a value which can be used to compare different materials.
- Applied in many aspect of products using thermoplastic.
- Higher hear deflection temperature means faster molding process in injection molding process.
How is flammability measured?
Measure the ability of a material to support combustion.
- A plastic strip is ignited, and the flame source is removed. The test records the time and amount of material consumed.
What is the glass transition temperature (Tg)?
temperature at which a plastic transitions from a hard, glassy state to a soft, rubbery state.
What factors affect the glass transition temperature?
Backbone flexibility, bulky side groups, molecular weight, polar groups, crosslinking, and the presence of plasticizers.
How does backbone flexibility affect Tg?
A more flexible backbone lowers Tg
. For example, polydimethylsiloxane Tg =−127°C) is highly flexible.
What is the function of a plasticizer, and how does it affect Tg?
A plasticizer increases flexibility, reduces the melting temperature, and lowers viscosity.
- Spacing out polymer chains, increase free volume, so polymer can slide past each other.
- reducing Tg.
Plasticizer can evaporate from polymer:
- Tg increase
- Product rigid.
