Adhesives

Question 1

Definition

Answer 1

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.

Question 2

Selecting the Bond

Answer 2

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

Question 3

Adhesive Properties

Answer 3

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

Question 4

Manufacture

Answer 4

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.

Question 5

Successful Joints

Answer 5

Design of joint
Substrate and conditions in service
Surface preparation
Adhesion selection

Joint loading: shear, cleavage, peel, tension, compression

Question 6

Pre-Treatment

Answer 6

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

Question 7

Adhesive Product Selection Process

Answer 7

Look at subsrates to bond first
Cure speed, pot life, gap fillings, colour

Chemical resistance, temperature resistance

Specific approvals

Question 8

Challenges with MMA

Answer 8

Strong odour

Flammable and highly toxic

Surface prep

Fast cure and bond performance