We mentioned earlier that there are two types of material used for shell
surfaces -- OAS and Acrylic. The shell surface alone is not strong enough to
withstand the enormous weight of the water in a spa, so an equally important
component is the shell understructure. The understructure is the primary source
of support and structural integrity for a spa.
There are differences between the rigidity characteristics of spa shell
materials and understructure materials, but when the two are bonded securely
together they are very strong. Since the majority of this strength is due to the
understructure, you should understand the different types available when you’re
looking a new spa.
When OAS is used as the shell surface material, it’s most common for the
understructure to include a thick layer of solid foam. This is no ordinary foam,
though, because when hardened it works just like fiberglass to provide
structurally sound support. The disadvantage of solid foam is that when it’s
applied by the manufacturer it covers the plumbing lines and spa fixtures. This
can make it much more difficult to access and service these components.
When acrylic is used as the shell surface material, it’s most common for the
understructure to use a combination of fiberglass and additives. Three factors
contribute to the quality of an acrylic understructure:
• Thickness of the fiberglass
• Quality of the fiberglass
• Quality of the manufacturing process.
Thickness is a key factor in the spa’s structural integrity and longevity, but
the thicker the fibreglass the more expensive it is to manufacture. Spa makers
can save hundreds of dollars by using just a thin layer of fibreglass, so you
should verify the understructure construction before you decide on a spa. A good
way to do this is to ask your spa dealer to show you a cross section of the spa
shell.
The quality of the manufacturing process is related to how the fiberglass is
applied to the acrylic shell. The most durable spas come from manufacturers who
hand roll each layer of fiberglass to the acrylic shell. This is an expensive,
labor intensive process but it’s the only way to prevent deterioration and
delamination of the acrylic shell.
Delamination is a term that was mentioned earlier. It refers to what happens
when two bonded materials (the shell surface and the bonding agent) expand and
contract at different rates. The result is blisters, cracks, and bubbles in the
shell. The primary causes of deterioration due to delamination are cheap,
inferior materials and improper application.
Acrylic shells are most prone to delamination, but OAS shells can deteriorate as
well. This shows up as crazing, or spider cracking, in the spa. A reputable
manufacturer will only design and sell spas made with materials that minimize
and virtually prevent any type of deterioration.
