Why Proper Design is Essential to the Successful Use of Acrylics
When it comes to the proper use of acrylic plastic, the design is almost as important as the product itself. That’s because poor design can lead to weaknesses that lead to inferior acrylic performance.
In this article, we are going to take a look at architectural panel design considerations using the “Handbook of Acrylics” as our reference resource.
Here’s what that book has to say about acrylic panel design considerations:
As with the conversion of nearly all materials into products, it is necessary to understand the physical properties of the material and observe the recommended practices for its use.
With plastics, in particular, it is necessary to give careful consideration to the methods or processes which are used to shape and join the materials, as the possibilities are quite diverse and the material’s performance can be altered by the selected process.
Design of the process is often as important as the design of the product itself. Other considerations include the following:
1. Thermal changes must be given careful attention, as properties of plastic are much more sensitive to such changes than are most other materials.
2. Properties of plastics are time dependent, and parameters such as load duration and load-application rate in combination with thermal changes must be carefully evaluated.
3. Plastics exhibit a rather unique failure mechanism known as crazing, which can be attributed to the action of stress, solvents, and environmental exposure. This degradation can result in failure by fracture of the part and, as such, it must be understood if the problem is to be avoided.
Crazing is defined as the formation of fine cracks which can extend over or under the surface of a plastic. The cracks are difficult to see, and when magnified they appear as a three-dimensional lacy network.
They appear on the tension surface of an object and are normally oriented perpendicular to the direction of maximum tensile stress. When crazing occurs in a random fashion, it is normally the result of solvent action and is known as solvent crazing.
Crazing reduces the load-carrying capability of a material and acts as a stress riser. Residual monomer can act as a solvent and result in crazing.
It is an irreversible process and cannot be eliminated, except by the physical removal of the crazed material through grinding and polishing.
To salvage a part which has been crazed, the part can be annealed and the craze physically removed. This results in an overall thickness reduction, but it has the advantage of removing the stress concentration caused by a residual tensile stress developed during fabrication.
There are many examples of the improper design of products made of plastic: the radio case which becomes distorted when left in the sun or the plastic case which takes abusive treatment when first purchased but which breaks after a few months.
These problems can be avoided through cautious and considered application of good design principles, which are developed through a good understanding of the material’s properties and are available from suppliers, either in written form or through direct contact.
Let’s look at some specific design considerations:
In most applications, acrylics are joined to some other structure by conventional bolting or riveting, clamping, or adhesive bonding.
The effects of the differential thermal expansion between members and the basic mechanical properties which affect structural performance must be considered if the bond is to hold.
Acrylic can also be joined to other acrylic elements to form complex shapes by direct bonding to form butt, scarf, lap, or offset joints. Acrylic adhesives or solvent-based cements are used.
Each joint has features which make it particularly suitable for special types of loading.
The adhesives used to form these joints can be either reactive or solvent and are selected on the basis of the grade of acrylic being bonded.
Reactive adhesives are commonly acrylic based and are used on cross-linked grades. Solvent adhesives rely on their ability to soften the parent material and are used with low molecular weight, non-crosslinked grades.
The solvent adhesives can be used as either straight solvents or as combinations of solvent and acrylic materials. These adhesives have varying degrees of strength and can be optically transparent; however, in most cases they yellow or otherwise discolor with age.
Before and after bonding, it is recommended that the acrylic be annealed to remove stresses and absorbed moisture. If this is not done, the joints will degrade with time.
Annealing is a time-at-temperature phenomenon and is dependent upon the thickness and grade of the acrylic: the thicker the sheet, the longer the annealing time.
Annealing temperatures are normally above the heat-distortion temperature of the material, i.e., from 200 to 230°F (93 to 110°C), and the time is that period necessary to bring the material to thermal equilibrium.
Cooling is a necessary part of the operation and must be controlled to prevent part warpage caused by residual stress formation.
The cooling period is normally considered to be complete when the midplane temperature is below the glass transition temperature of the material.
Acrylic can be bolted or riveted to substructures, but normally edge attachment materials are first bonded to the acrylic to provide a way to distribute the fastener load (Figure 2.21).
Acrylic is “notch” sensitive and differential thermal expansion must be considered.
In summary, the weak link in the structural chain is usually the joining mechanism, which means that careful attention must be given to the method used, the structural design, the acrylic’s notch sensitivity, and the effects of differential thermal expansion.
Here’s another important design consideration:
In many applications, it is necessary to laminate acrylic to other acrylic layers or different materials, such as glass or reinforced plastic. This need arises when the properties or the acrylic composite structure offer design advantages which cannot be achieved when a single material is used. One example is an aircraft windshield which is composed of glass, a flexible interlayer, acrylic, an interlayer, and acrylic. This provides a structure which is lightweight, resistant to bird impacts and windshield-wiper abrasion, and fail-safe.
Another example is the use of as-cast acrylic, an interlayer, and stretched acrylic to form a window which can tolerate high temperatures on the as-cast acrylic face while experiencing only moderate temperatures on the stretched acrylic side.
Many times it is desirable to place decorative films, wire grids, reflective coatings, or other elements within an acrylic composite to achieve various effects.
Here’s one more thing to consider about design:
Long-term critical pressures are essential for the design of windows in underwater habitats, deep ocean simulators, and saturated diving chambers, where the magnitude of hydrostatic pressure is constant and applied for long periods of time.