Adhesives Flashcards
Definition
Synthetic, material that securely bonds two substrates with sufficient strength to transfer high loads of >7MPa with longevity and without failure due to temperature weathering and chemical immersion.
Selecting the Bond
Adhesive selection:
Component/bond design
Tolerances and accuracy
Dimensions
Production methods
Training workers
Variable Factors:
Adhesive storage
Substrate quality
Pre-treatment quality
Post-pre-treatment life
Mixing and Application
Open or Assembly time
Assembly and Curing
Adhesive Properties
Viscosity: dictates the application methods
High: difficult to remove from the container, will stay in a thicker bond line
Low: can flow too much and run off the surfaces, can penetrate and seal cracks on assemblies
Thixotropy: an indication of a material’s decreases in apparent viscosity over time while under stress
Indicate an adhesive’s ability to fill gaps
Give a product resistance to sagging on vertical surfaces
Characterise the spreadability
Lap Shear Strength: gives a measure of the ultimate load and a way of comparing adhesive strength
Record the type of failure
Cohesive failure: Adhesive remains on both substrates, indicating a strong bond
Adhesive failure: adhesive remains on one substrate, indicating the
Tensile Strength:
Elongation: the amount of strain required to break a specimen
Ultimate tensile strength: the max amount of stress a specimen can withstand before breaking apart
Peel Strength: used to measure a material’s bond strength and is the average load per unit width of borderline required to separate the material when tested at angles (180/90)
An important indication of an adhesive’s ability to provide a permanent bond in a demanding environment it shows the polymer can withstand difficult service conditions such as vibration or dynamic loading
Curing Profile: Gel time, work life and pot life are all the amount of time, at room temperature, from initial mixing until the mixture has such a high viscosity that it can no longer be stirred /or used.
They can be increased or decreased by cooling or heating the resin and/or hardener
Once a thermoset resin has solidified, full cross-linking of the polymer can be achieved by heating or “post-curing” the adhesive in an oven to achieve maximum Tg /mechanical properties.
Heat Resistance: Glass transition temperature (Tg) is an approximation of the point at which a thermoset material starts to “change phase”…from a “glassy” to a “rubbery” material. Generally, the maximum service the temperature of an adhesive is calculated using the Tg.
Mixing
Mix ratio: the amount of hardener to be added to a given amount of resin (pbw or pbv)
Optimum adhesion strength and durability depend on the use of the correct mix ratio
Mixing creates an irreversible chemical bond
Manufacture
Archimedes screw: To use an Archimedes screw for mixing adhesives, you would typically place the adhesive material in a container or a mixing chamber. As the screw rotates, it picks up the adhesive and moves it along the helical surface. This action facilitates the blending of the adhesive components, ensuring a more uniform and consistent mixture.
Successful Joints
Design of joint
Substrate and conditions in service
Surface preparation
Adhesion selection
Joint loading: shear, cleavage, peel, tension, compression
Pre-Treatment
Minimum:
Cleaning
Vacuum and de-dusting
Degreasing (e.g.. acetone)
Industrial standard:
Grinding
Grit blasting
Peel ply (composite and plastics)
Use of primer
Advanced:
Anodising
Etching
Corona, flame,
Plasma
Adhesive Product Selection Process
Look at subsrates to bond first
Cure speed, pot life, gap fillings, colour
Chemical resistance, temperature resistance
Specific approvals
Challenges with MMA
Strong odour
Flammable and highly toxic
Surface prep
Fast cure and bond performance