ACRYLITE® Cell Cast (GP) Polymerizable Cement
Proper cementing of acrylic sheet is a vital step in manufacturing museum quality displays. When exhibiting valuable works of art, ACRYLITE® sheet helps make the display case as perfect as the artwork. This tech brief gives advice to achieve excellent joints.
The following information is given regarding the use of chemically reactive, two-part polymerizable cements such as ACRIFIX® 2R0190.
Two Component Adhesives
The cure is brought about by a chemical reaction, polymerizing adhesives are polymers dissolved in monomers which cure when a hardener is added.
Polymerizing adhesives fill better and, therefore, impose fewer demands on the accuracy of fit between the parts. They must be used where high strength and good resistance to environmental conditions are required.
Other two-component adhesives like epoxy resins, isocyanates (polyurethane), phenolics, and aminoplastics are not suitable for bonding this sheet to itself or other materials because their adhesion is very low.
Workshop & Equipment
The following equipment is desirable when cementing facilities are being considered for polymerizable adhesives:
- Flat Working Surface- Cover work area with glass or polyester film. Vapor removal is necessary. Solvent vapors are heavier than air.
- Weighing Scale- The scale must have an accuracy of about one gram for weighing out the adhesive.
- Mixing Vessels- Use round beakers made of glass, polyethylene, or other materials which are insoluble and do not swell.
- Stirrer- Stir small amounts with glass or polyethylene insoluble rods.
- Vacuum Chamber (optional)- A small metal, glass, or plastic chamber which can hold a beaker of the adhesive and is capable of withstanding a vacuum of 3 psi.
- Vacuum Pump (optional)- A small vacuum pump capable of evacuating the above chamber to a vacuum of 3 psi.
- Syringes or Squeeze Bottle- Disposable syringes or squeeze bottle for applying the adhesive.
- Jigs & Fixtures- For mass production, it is advisable to use gluing jigs which can be used repeatedly. Parts can be attached with clips, clamps, lead weights, or vacuum.
- Sealing & Spacing- Use polyester tape to contain the adhesive.
Area bonds may be produced either horizontally or vertically. Thin sheets and thick blocks can both be bonded in the horizontal position. Blocks can be bonded vertically.
For horizontal bonding, the degassed cement (bubble free) is poured on the panel approximately one-third the distance on the side as shown in Figure 1A. The adhesive may be held on the base with adhesive polyester tape. Any bubbles formed when the adhesive is poured should be removed.
Starting from one edge, the top plate is placed in such a way that the adhesive moves evenly between the plates.
With thin sheets, the relatively high viscosity of the adhesive itself is sufficient to prevent the cement from being squeezed out from the area being bonded through the weight of the cover.
Thick blocks should be placed about .050 inches from the base by suitable spacers (polyethylene cords) as shown in Figures 1B and 1C.
If bubbles form while the cement is applied, they can be sucked out with fine capillaries as long as the adhesive is still soft. Bubbles can also be extracted by pricking them with a fine steel wire and then pulling them out at high speed. This technique, as shown in Figure 2, is most suitable when bonding large areas.
When areas are vertical, they are bonded by spacing the sheets or blocks with a soft elastic cord so as to provide a chamber at the top. The prepared adhesive is then poured into the chamber.
The minimum thickness of the chamber is .060 to .080 inches which allows the cement to flow without bubbles.
Bonding vertically has the following advantages:
- An adhesive with bubbles can still be poured because the bubbles will rise to the top.
- Since the adhesive area is thicker, blocks with variations in the thickness can be bonded.
- Placement of the top plate, difficult in the horizontal method, is eliminated.
- Adjoining surfaces may be covered with adhesive polyester film. When the cement starts to set (still just soft), remove the polyester film with any remaining cement. This is shown in Figure 3.
To create butt joints the panels are fixed to a flat support, leaving a gap between them of at least .03 inches. The underside and faces of the gap are sealed with adhesive tape with a non-adhesive center strip. See Figure 4.
The adhesive is introduced into the one open side of this joint by using a suitable syringe. Avoid bubbles.
The shape of the bonding gap depends primarily on the thickness of the panels that will be butt joined. When the tensile stress is to be uniaxial, V-butt joints with a 60° aperture angle provide the highest tensile strength as shown in Table 1. However, aperture angles of this size are only practical when the sheets are thin.
|Sheet Thickness (inches)||Aperture angle of the seam||Minimum thickness of the glue line (inches)|
|Approx .220”||Butt joint with single V groove- 30°||.03”|
|Approx 1.25”||Butt joint with single V groove-15°||.03”-.06”|
The shrinkage of the adhesive during polymerization and heat conditioning is about 15-20% by volume. Therefore, to prevent a concave surface, the gap should be sufficiently over-filled with adhesive.
Bonding must be done in suitable jigs to ensure that the bonding materials are fixed during bonding and the cure.
When creating T-joints, spacing the bonding gap uniformly is difficult, especially when the adhesive joints of thin materials are long. In addition, there is an adhesive bead on either side of the joint. Solvent cements are better for bonding thin sheets with a blunt edge. A beveled edge is generally useful for polymerizing adhesives because it allows one sheet's edge to be placed directly on top of the other. In practical trials, the relationship between the wedge angle and the material thickness, as shown in Figure 5, has given best results.
In order to produce an adhesive bead, the horizontal sheet should project as shown in Figure 6.
Finishing the Adhesive Joint
A bonded part should be heat conditioned (annealed) before finishing the adhesive joint. If not, it could form constrictions when it is later heat conditioned and the constrictions would impair the bond strength.
Concave grooves are planed with a router mounted on a sliding carriage.
When higher demands are imposed on the optical quality of the bond, the finished surfaces may be additionally sanded with fine, wet, abrasive paper or fine steel wool and then polished.
To keep the work of finishing to a minimum, the amount of adhesive in the joint is best kept low so that only a little of it projects after heat conditioning.