Chapter 4B Flashcards
What factors should be considered when selecting an adhesive?
Materials to be bonded, compatibility of adherends and adhesives, processing requirements, and service conditions.
Why should flexible materials like rubber not be joined with rigid, brittle adhesives?
Rigid bonds may crack, reducing bond strength. Flexibility differences should be internal stress into the glue line.
What issues arise from incompatible adhesives?
Problems include corrosion of metallic parts, plasticizer migration from flexible plastics, and adverse effects of solvents on plastic adherends.
What are the limitations of thermoplastic adhesives?
They fail under low sustained loads, soften upon heating, and cannot withstand vibratory stress for long periods, Unsuitable for structural application.
Thermoset adhesives?
For structural applications, Rigid bond, retain their strength at elevated temperature.
- Poor under peel stress
Which adhesives are suitable for high-temperature applications above 120°C?
Only certain thermoset adhesives, such as silicones, are suitable.
What factors influence the effectiveness of adhesion?
Joint design, surface wetting, and surface treatment.
Why is joint design important in adhesive bonding?
It determines how well the joint withstands applied loads and stresses in service.
What are the four basic types of stresses in joints?
Shear: Even stress across whole bonded area. Not completely attach 2 phases.
Tension: Stress evenly distributed over the joint area.
- cleavage: Stress not uniform but concentrated at one side of joint
- peel: One or both must be flexible
What is peel stress, and why should it be avoided?
Peel stress places high stress on the joint’s boundary line. It is avoided as it weakens the bond.
What is wetting in adhesive bonding?
Wetting is the process of spreading adhesive over a surface to maximize contact. Good wetting occurs when the contact angle is less than 90°.
How can pressure improve wetting?
Pressure enhances wetting by ensuring better interfacial contact, resulting in stronger adhesion.
Why is surface treatment necessary for adhesion?
It ensures the surface is clean, smooth, and chemically receptive, promoting better adhesion.
How should metal surfaces be treated for bonding?
Using solvent cleaning, abrasive blasting, chemical surface alteration, or a combination of these methods.
What is the proper preparation for wood surfaces?
Sanding, planing, or machining away contamination, followed by cleaning with air pressure or solvent.
What steps are common in preparing non-metal surfaces?
Solvent cleaning, chemical alteration, and abrasive cleaning.
What are some methods for applying liquid adhesives?
Brushing, flowing, spraying, roll coating, knife coating, silk screening, and melting.
What are the advantages and disadvantages of brushing adhesives?
Advantage: Suitable for specific areas and complex shapes.
Disadvantage: Limited control over film thickness and uneven application.
Why is spraying a preferred method for large surfaces?
It provides better film thickness control and covers uneven contours efficiently.
What is roll coating best suited for?
Large, flat materials requiring continuous adhesive application and uniform coverage.
How does silk screening ensure precise adhesive application?
It applies adhesive to specific areas using stencils or patterns.
What are common issues with inadequate bonding?
Starved joints, dry joints, wet joints, joint misalignment, poor surface treatment, and non-uniform glue lines.
What types of tests are performed to evaluate adhesives?
Strength tests (shear, tensile, impact, cleavage, peel) and durability tests (weathering, aging, water resistance).
