Epoxy
Various epoxy adhesives are available. The different curing mechanisms are:
- 2 component hot or cold curing
- 1 component hot curing
-
UV-curing.
Standard epoxy adhesives are brittle and show low peel strength. To improve
toughness, modified epoxy adhesives have been developed. The use temperature
varies between - 40°C and 80°C (-40°F-180°F) for cold curing systems. Hot
curing epoxies can normally be used up to 150°C (300°F).
In general, large deviations in lap shear bonding strength are found,
depending on the particular combination of adhesive and material.
With some plastics, pretreatment can give a considerable improvement. Oils and
grease negatively affect the adhesion of epoxies.
Polyurethane
Polyurethane adhesives are relatively inexpensive and show good adhesion.
Varieties exist from elastomeric to rigid. Several types of curing mechanisms
are available:
- 1 component thermosetting
- 2 component catalyzed
- reactive
hot melts.
Polyurethane adhesives are tough and show a high peel strength. They can be
used at temperatures between -80°C and 100°C (-110°F-210°F).
Adhesion to engineering plastics is good. Degreasing is often sufficient to
obtain the required bonding strength.
Acrylic
Acrylics are flexible and tough. Fast curing takes place at room temperature.
Care should be taken when joining amorphous thermoplastics such as Xantar, as
environmental stress cracking may occur. Several systems are available:
- 1 component UV-curing used for transparent plastics
- 2 component
premix
- 2 component no-mix.
Use temperature is between -55°C and 120°C (-70°F-250°F). Acrylics show
excellent peel strength and are tough.
Good adhesion is obtained on amorphous thermoplastics. Pretreatment may
improve the lap shear bonding strength considerably.
Cyano-acrylic
Cyano-acrylics are fast curing systems but rather brittle, which results in
low peel strength and impact properties in the joint. Rubber modified
cyano-acrylics have been developed to improve toughness.
A very high lap shear bonding strength can be obtained with most engineering
thermoplastics. Unfilled polyesters (Arnite PET and PBT) show moderate
results. Effective primers are available to improve the bonding strength on
polyolefins.
Silicone
Silicone adhesives react under the catalytic effect of water. Humidity in the
air or some moisture on the surface of the parts is sufficient. The reaction
times are relatively long, compared to cyano-acrylics. Silicone adhesives
offer a high elasticity.
UV Cure
UV curable adhesives use ultraviolet light to initiate polymerization and
contain no solvents. Curing time is short, typically 3 to 10 seconds. UV
curable adhesives have a high bond strength and can easily be applied to
transparent materials like Xantar polycarbonate.
Hot melt
Hot melt adhesives are thermoplastics, available as pellets, or in block, tape
or foil shape. The adhesive is heated above the melting temperature and
applied to the surfaces to be bonded with special equipment like rollers,
nozzles or calendars. The bond is formed after the melt cools to a solid. The
operating equipment must operate fast for effective bonding. These adhesives
are fairly viscous, solvent free, and have good gap filling abilities.
