Do you know your desired viscosity or cure time?
What Substrates are Going
to be Bonded?
It is always important to verify the materials you are
attempting to bond. There are some substrates that will
not bond together, regardless of what adhesive you
choose. Common examples of this include olefins such as
polyethylene and polypropylene. Also, certain seemingly
straightforward materials like acrylic have different grades
which make it harder to be bonded with certain cements
due to increased resistance. You may not be able to use a
solvent cement and instead use a two-component adhesive
on these surfaces. Remember, it all comes down to
the substrate.
You also need to take into consideration the differences
between bonding “like” materials to that of bonding “dis-similar”
substrates. Without proper consideration, these
joints can be much more difficult to bond and may result
in failure. However, to counter the myth that “dissimilar”
plastics cannot be joined, they most certainly can be. It
just takes more understanding of the cements and the
plastics to be joined. In most cases there is a substantial
difference in thermal expansion between different plastics.
When joints between dissimilar materials are
exposed to rising and falling temperatures, the materials
expand and contract by different amounts, placing large
stress on any bond between them. Your cement needs to
be able to take that into consideration.
Also, you should never hesitate to contact your adhesive
supplier for their recommendations. They should be able
to provide you with not only recommendations, but also
data if needed to support their claim. Also, many have
charts to make it easy for you to decide which adhesive
you should choose depending on your substrates.
What are the Conditions Under Which
the Cementing Will Take Place?
There are many factors in determining the conditions
under which the cementing will occur. Things to consider
include: whether the fabrication will take place inside or
outside, the temperature during cementing, the humidity
level and having proper ventilation.
The temperature and humidity conditions in the
cementing area and the temperature of the parts to be
cemented are important. The ideal temperature for
cementing acrylic sheet is between 70ºF and 75ºF. It is
not recommended to cement in room temperatures under
60ºF or over 100ºF. “Water-Thin” or very low viscosity
cements should not be used at temperatures below 60ºF.
Low temperatures will increase the surface hardness of
the substrate, reduce the solvent action and increase the
soak time, in turn increasing the possibility of crazing. The
most desirable conditions are normal room temperatures
and low relative humidity. Excessive moisture may cause
the joints to become cloudy. It is often recommended to
reduce the cloudy appearance in these conditions add a
small amount of acetic acid to your solvent to slow down
the process and reduce the blushing effect. An example of
this is Weld-On® #5.
It is also important to work in a well ventilated area.
Ventilation or vapor removal is necessary, as solvent
vapors are heavier than air. Some solvents are highly
volatile and may be flammable. If not ventilated, excessive
exposure to vapors may cause drowsiness, dizziness,
intoxication or nausea.
What are the Conditions Under Which
the Assembled Parts Will be Placed?
Before choosing an adhesive, you should always consider
the conditions under which the final assembled parts
are going to be placed. Are the parts going to be outdoors,
or inside in a machine or showroom? If the parts
will be outside, what are the temperature ranges and climate
conditions? Is it in an area where temperatures are
extreme or variable? These are all conditions that must
be answered before you choose the proper cement.
Do you Know Your Desired
Viscosity or Cure Time?
When determining your viscosity and cure time, many
questions must be answered. Remember, faster is not
always better. The common thought is the faster you can
cement the more parts you can produce. This may be true,
but are you jeopardizing the quality of your parts? If you
have a big surface to cover, you do not want the cement to
cure on one area before you finish applying to the entire
area. This often will result in a joint failure due to lack of
adhesion in certain areas. You will have “spotty” joint coverage.
Also you must determine if you have any gaps to fill.
“Water-Thin” cements are the most popular solvent
cements due to speed and often price but they will not fill
gaps. If used in areas where a bodied cement or gap filling
cement is needed, a lack of adhesion and failure may occur.
Often these are areas where bubbles are seen because the
water thin cement will flow over the top of the part.
Cementing Techniques
The two basic cementing techniques for solvent cementing
are the Capillary Action Method and the Soak or Dip
Joint Method. Between these two methods, the capillary action is by far the most common method used for both
its speed as well as ease of use.
Under the capillary method, the parts must be closely
fitted with no visible gaps. Often pieces are held firmly in
place with masking tape or a clamp. The cement is dispensed
with a syringe or applicator bottle with needle
along the edge of the joint. For water thin cements, it is
recommended to use a 23 or 25 gauge needle, and for
slightly thickened cement use 14 gauge. The capillary
action will draw the cement between the parts. Initial
bond will begin to form in as quickly as 30 seconds and
the time for the joint to set will vary from approximately
2 to 5 minutes depending on the solvent used, temperature
and humidity. Wait 3 to 4 hours before processing
and fabrication. High strength is reached within 24 to 48
hours and will continue to build over several weeks.
In the soak or dip method, the parts must also fit closely.
Pour a moderate amount of solvent cement into a dip
or soak pan. Dip the edge of one of the pieces to be joined
into the solvent. Only dip the edges, exposing too much
area to the solvent will result in a weak, slow setting joint.
Thin sheet should remain in the solvent for approximately
20 seconds and thick sheet should be soaked for 30
seconds, depending on the solvent being used. Remove
and hold the sheet at a slight angle to allow excess solvent
to drain off. Carefully place the soaked edge precisely in
place on the part to be joined. Hold together for 30 seconds without applying pressure. This allows the solvent to
work on the surface of the piece that was not dipped.
After 30 seconds, apply slight pressure to squeeze out any
air bubbles, however too much pressure will squeeze out
the cement resulting in a dry joint. When the pieces are
joined, allow to maintain firm contact for 10 to 30 minutes,
not allowing the parts to move at this time. Similar
to the capillary method, wait 3 to 4 hours before processing
and fabrication. High strength is reached within 24 to
48 hours and will continue to build over several weeks.
Other methods for applying water thin cement can be
done using eye droppers and body cement can use brushes,
rollers, etc.
Potential Cementing Problems
When determining potential causes for joint failure, there
are many factors that may come into play. Preparation of
the joint is very important when cementing parts. The
parts must fit accurately and smoothly without forcing. If
the parts do not match or fit well, the edges to be cemented
should be machined smooth to fit but should not be
buffed since this tends to round edges. More specifically,
flame polished edges will craze upon contact with a solvent
cement. The proper preparation of edges is necessary to
ensure that little or no internal stress is present in the
material. Such stresses will cause crazing on contact with
solvent cements, which brings us to our two most common cementing problems. The two most common problems
found with cementing are crazing and poor joint strength.
Crazing is a network of fine cracks running, on or slightly
under, the surface of plastic materials. Acrylic plastic
will eventually craze when subjected to high stresses for
a long period of time. The tendency to craze is greatly
increased when the stressed material is exposed to solvent
or solvent vapors. Stress is often introduced into a
part during casting of the sheet or during fabrication
operations such as machining or polishing. The possibility
of crazing may be greatly reduced by minimizing internal
stresses through annealing of the parts before
cementing. Also to avoid crazing, avoid flame polishing or
disk sanding where solvent contact may occur. Potential
culprits for weak cement joints are improper fit of parts,
inadequate mixing of cements, excessive clamping pressures
or poor technique.
Safety Precautions
When using any cements and adhesives, safety should
always be your # 1 priority. It is always important to be in
the habit of practicing proper safety procedures. Many,
but not all cements may be harmful if sufficient concentrations
are inhaled for extended periods of time,
absorbed through the skin or swallowed. To limit the
amount of skin contact with the materials, wear gloves
when handling solvents. If the skin does come in contact,
the exposed area should be washed thoroughly with soap
and water. As stated earlier, all cements should be used in
well ventilated areas, again remembering that most
vapors from the cementing process are heavier than air.
Often volatilization can be reduced by using products in
cartridge forms which reduce the need to hand mix components,
or by the use of a closed applicator bottle. Both
of these applications keep the solvents themselves, as well
as their vapors, in a more controlled environment. Other
methods such as the dip and soak method discussed earlier
are more prone to producing objectionable vapors.
Some types of cement are also flammable. All cements
should be kept away from open flame or sources of high
heat. There should be no smoking in any area in which the
cements are stored or used.
Special care must be taken when mixing chemicals,
which not only goes for mixing 2- or 3-component items
but also for mixing raw chemicals. Attention to the proper
order must be taken into consideration. Deviation from
the proper order of mixing may result in a violent reaction
causing potential explosion or fire.
You should always have a copy of the Material Safety
Data Sheets (MSDS) for all cements and adhesives that
you keep on hand. They should be kept in a location that
is easily accessible and should be consulted to whenever
in doubt about a product. The MSDS will include sections
on: the Manufacturer, Hazardous Ingredients, Physical
Data on the product, Fire and Explosion Hazard Data,
Health Hazard Data, Reactivity, Spill or Leak Procedures,
Special Protection Information and Special Precautions.
The most important thing to always remember is to use
common sense. Treat all adhesives and solvent cements
with respect and use only for their intended purposes.
Written by Chad Hogancamp, Industrial Adhesives
Manager, IPS Corporation, manufacturers of Weld-On®
industrial cements and adhesives for plastics and other
materials used in the fabrication industry.
For more information, contact IPS Corporation, 455 W. Victoria
Street, Compton, CA 90220, 800-421-2677, Fax: 310-898-
3392, E-mail: chadh@ipscorp.com, Web: www.ipscorp.com.
References used:
Working with Acrylics, Craftics Inc. For your free copy,
call Craftics Inc. at 505-338-0005.
