Adhesive Usage in Plastic Fabrication

There are many ways to use adhesives in plastic fabrication. You can walk into 10 different fabricators and see 10 different methods of doing practically the same thing, each achieving the same end result. Regardless of what one considers the ÒproperÓ plastic fabrication technique, the same basic principles are found throughout. Once you have determined what the finished product will look like and selected the fabrication materials, the next task is to choose the right bonding agent for the job, which can be quite confusing given the multitude of adhesive options available (e.g. reactive adhesive, solvent cement, etc.1) To make the selection process a little bit easier, start by answering these simple questions:

• What materials are going to be bonded?
• What are the assembly conditions?
• Clean up and start up of operations interrupted by the accident.
• What are the conditions under which the assembled parts will be placed?
• Do you know your desired viscosity or cure time?

What materials are going to be bonded?

It is always important to verify the materials you are attempting to bond.

• Some plastic substrates (e.g. olefins such as polyethylene and polypropylene) will not bond together, regardless of what adhesive is used.
• Acrylics have different grades. Some acrylic materials may not bond to solvent cements due to increased resistance to the cements. Instead, a two-component, reactive adhesive is more suitable for bonding these surfaces.
• Same plastic material can easily be joined together with the right solvent cement. However, joining dissimilar plastic materials together will require the user to better understand the physical properties of these plastics. For example, there is a substantial difference in thermal expansion among various plastics. When joints between dissimilar plastics are exposed to rising and falling temperatures, each plastic type expands and contracts by different amounts, and a large stress is placed on the joints. The reactive adhesive and/or solvent cement used in joining dissimilar plastics must be able to withstand such stress.

Never hesitate to contact your adhesive supplier for their recommendations. If needed, they should also be able to provide you with data to support their claims. Most suppliers have selection guides making it easier for you to choose the appropriate adhesive for your plastic fabrication needs.

What are the assembly conditions?

The temperature and humidity conditions in the fabrication area and the temperature of the parts to be assembled are important.

• The ideal room temperature for fabricating acrylic sheet is between 70¡F and 75¡F.
• Low relative humidity condition is preferred. Excessive moisture may cause the assembled joints to become cloudy.
• It is not recommended to solvent cement in room temperatures under 60¡F or over 100¡F. Low temperatures will increase the surface hardness of the plastic, reduce the solvent cement action and increase the soak time (i.e. duration where plastic is dipped in the solvent cement prior to bonding). Crazing, a network of fine cracks on or under the plastic surface, may occur under such condition. At high temperatures, the solvent cement easily volatilizes causing the cement to be less effective in bonding plastic parts.

It is also important to work in a well ventilated area or area with appropriate vapor removal system. Solvent vapors from adhesives are often heavier than air. Some solvents are highly volatile and may be flammable. Excessive exposure to solvent vapors may cause drowsiness, dizziness, intoxication and/or nausea.

What are the conditions under which the assembled parts will be placed?

Before choosing the proper adhesive for your plastic fabrication project, always consider the conditions where the final assembled parts will be placed. Are the parts going to be outdoor, inside in a machine or in a showroom? What are the temperature ranges and climate conditions? Is it in an area where temperatures are extreme or variable? The right adhesive should be able to handle the temperature and climate stresses as well as maintain an efficient bond.

Do you know your desired viscosity or cure time?

The faster you can bond the parts, the more finished products you can produce. This may be true, but are you jeopardizing the quality of your assembled parts? Fast cure time is not always better. If you have a big surface to cover, you do not want the adhesive 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 ÒspottyÓ joint coverage. Do you have to fill any gaps in the fabrication piece? ÒWater-Thin,Ó low viscosity solvent cement is the most popular cement used because of its fast cure time and lower cost. However, it will not fill gaps and attempting to use it for gap-filling purpose will result in joint failure (e.g. bubbles in the joint area and weak bonding strength). More viscous solvent cements or reactive adhesives are recommended for such purposes.

Reactive Adhesives

Reactive adhesives are usually packaged into two components: a base resin and a catalyst. To use the adhesive, carefully follow the mixing ratio and direction as specified by the manufacturer. Adding too much or too little catalyst to the base resin will negatively affect the adhesive performance.

• Assemble: Prepare the plastic parts and ensure that they are closely fitted and held firmly in place with masking tape or a clamp. Depending on the type of acrylic material and fabrication method, the possibility of crazing may be reduced if the parts are annealed (i.e. the process of heating and slow cooling to make the parts tougher and less brittle) prior to adhesive application.
• Mix: Mix the reactive adhesive components together per manufacturer specification. Read the product guideline and identify the adhesiveÕs Òworking timeÓ (i.e. the time frame which the mixed adhesive is effective and can be used for application). The Òworking timeÓ ranges from a few minutes to several hours depending on the base resin and catalyst used.
• Apply: Immediately apply the mixture to the plastic parts. Lightly press the parts together to remove air bubbles. Do not squeeze the parts too hard as to force the adhesive out of the joint.
• Cure: Let the assembled parts cure according to the manufacturer specification before processing and fabrication. Some reactive adhesives are packaged as a dispensing gun with replaceable resin and catalyst cartridges. The mixing ratio is pre-determined making the adhesive in this packaging format much easier to use; just dispense and assemble.

Solvent Cementing Techniques

There are two basic solvent cementing techniques: capillary action method and the soak or dip method. The capillary action method is by far the most common method used due to its speed and ease of use.

• Assemble: The plastic parts must be closely fitted with no visible gaps and held firmly in place with masking tape or a clamp.
• Inject Cement: The cement is dispensed with a syringe or applicator bottle with a 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 thicker cements, use a 14 gauge needle. The capillary action will draw the cement between the parts.
• Cure:An 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 bonding strength is reached within 24 to 48 hours and will continue to build over several weeks. In the soak or dip method, the plastic parts must also fit closely together.
• Soak/Dip: Pour a moderate amount of solvent cement into a pan. Dip the edge of one of the parts to be joined into the cement. Only dip the edge. Exposing too much area of the plastic to the cement will result in a weak, slow setting joint. Soak thin sheet plastic for approximately 20 seconds whereas thick sheet plastic should be soaked for 30 seconds, depending on the solvent cement being used. Remove and hold the sheet at a slight angle to allow excess cement to drain off.
• Assemble: 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 cement to work on the surface of the piece that was not dipped. After 30 seconds, apply slight pressure to squeeze out any air bubbles. Be careful not to apply too much pressure as the cement can also be squeezed out resulting in a ÒdryÓ joint (i.e. weak bonding strength).
• Cure: Maintain the joined parts in firm contact with each other for 10 to 30 minutes and do not move the parts. Similar to the capillary action method, wait 3 to 4 hours before processing and fabrication. High bonding strength is reached within 24 to 48 hours and will continue to build over several weeks. Other cementing tools include eye droppers for use with Òwater thinÓ cements and brushes or rollers for more viscous, bodied cements.

Potential Solvent Cementing Problems

A common problem in plastic fabrication, particularly when using acrylics, is crazing, a network of fine cracks running on or slightly under the plastic surface. Acrylic is prone to physical stress during original sheet casting and fabrication of finished products (e.g. machining and polishing). If subjected to high stress for a long period of time, acrylic will eventually craze. The tendency to craze also increases when stressed materials are exposed to solvent cementing. To minimize crazing, avoid placing additional stress on acrylic parts during solvent cementing.

• Carefully prepare the joining parts. Parts must fit accurately and smoothly without being forced.
• Annealing of the parts before cementing may minimize the stress.
• The joining edges should be machined smooth to fit each other, but should not be buffed since this tends to round edges.
• Avoid flame polishing or disk sanding acrylic parts where solvent cement contact may occur. Flame polished edges will craze upon contact with solvent cement.

Another common problem related to solvent cementing is poor bonding strength. The main causes are improper fit of parts, excessive clamping pressures, and/or poor technique. Properly follow the cementing techniques mentioned in the previous section should improve the bonding strength between the assembled parts.

Safety Precautions

Many, but not all, adhesives can be harmful if sufficient concentrations are inhaled for extended period of time, absorbed through the skin or swallowed. When using adhesives in your plastic fabrication project, safety should always be the number one priority. Always practice proper safety procedures:

• Wear appropriate chemical-resistant gloves when handling adhesives to avoid skin contact. If the skin does come in contact with an adhesive, the exposed area should be washed thoroughly with soap and water. Follow additional safety procedures as provided by the manufacturer.
• Work in a well-ventilated area and avoid breathing the adhesive vapor. Vapor exposure can be reduced by using the adhesive product in a cartridge format or applicator bottle with easy open/close access. When the soak or dip solvent cementing method is used, ensure the workplace is properly ventilated as solvent cements can easily volatilize from the soaking pan with a large surface area.
• Adhesives are flammable. Always keep them away from open flame or sources of high heat. There should be no smoking in the area where the adhesives are stored or used.
• Take special care when mixing chemicals (e.g. multicomponent adhesives, raw chemicals, etc.) Pay attention to the proper mixing order and carefully follow each productÕs direction of use. Deviation from the proper mixing order may result in a violent chemical reaction with potential for explosion or fire.
• Keep copies of the Material Safety Data Sheets (MSDS) for all adhesives and other chemicals in an easily accessible location. The MSDS contain detailed information about the Manufacturer, Hazardous Ingredients, Chemical Physical Data, Fire and Explosion Hazard Data, Health Hazard Data, Reactivity, Spill/Leak Procedure, Special Protection Information and Special Precautions. Consult the MSDS whenever you have questions about the products being used.

Written by SCIGRIP Americas, the manufacturer of Weld-On¨ Industrial Assembly Adhesives and Solvent Cements for plastics and other materials used in the fabrication industry. For additional information, contact WELD-ON Assembly Adhesives, Customer Service & Technical Service, 500 Distribution Parkway, Collierville, TN 38017, 901-853-5001, Fax: 901-853-5008, E-mail: info@scigrip-americas.com, Web: www.scigrip.com.

1 Reactive adhesive: A two- or three-component adhesive system that chemically reacts when components are mixed together, to form an adhesion with the surfaces of the plastic joints. Solvent cement: An adhesive, formulated with resins and solvents, initially dissolves the plastic surfaces and then ÒweldsÓ the plastic joints together (i.e. the adhesive become a part of the plastic).