Surface Discoloration in an Acrylic Spa

We are occasionally asked what has caused a localized discoloration in a spa.  Most of the time, this change in color appears as a bleached surface.  There have even been examples where tiny blisters were present in the bleached area   In every case we have investigated, the root cause has been localized exposure to a strong sanitizing chemical.

The majority of spas are produced from  a  cross-linked, high molecular weight acrylic polymer sheet, a material that is very resistant to chemicals.  However, no product is perfect when it involves chemical resistance.   Spas require the use of sanitizing chemicals to maintain water chemistry.   Manufacturers provide recommendations on what products to use, and how to use them.  Failure to comply with these recommendations can lead to problems.  Even if a product might be used in a swimming pool, it may have issues when used in a spa.

Pure acrylic is clear with transparency better than most glass.  Pigments are added to the liquid acrylic to create the attractive colors and effects.  New colors often use exotic pigments.  Sanitizers for water are oxidizing agents used to kill bacteria, algae, etc.  Some are stronger than others.  The most common spa sanitizers are compounds of chlorine or bromine.   Some of these products have been found to bleach the acrylic pigments, and cause discoloration.  Manufacturers recommendations are always designed to prevent concentrated chemicals from long contact with the acrylic surface, and the plumbing system.  Water treatment chemicals for spas are best when in liquid form since they are easier to control concentration.  All examples of spa surface discoloration, that have been reported to MTP, have been caused by  one of these water treatment products.

Inorganic chlorine compounds such as calcium hypochlorite and sodium hypochlorite  commonly used sanitizers for swimming pools and spas.  However, they are both effected by sunlight, so they require the addition of cyanuric acid to stabilize the chlorine in outdoor pools and spas.  Organic chlorine compounds are products combined with cyanuric acid. Sodium dichlorostriazinetrione (“dichlor’) and trichloro-s-triazinetri-one (‘trichlor) are both popular stabilized chlorines for water treatment of outdoor pools.  Most of the spa discoloration cases we have seen have been the result of the use of “tri-chlor”.  Specifically, they have been due to the use of this chemical in the form of solid tablet.  This product dissolves slowly in water, and is a very strong oxidizer as well as increases the acidity of the water.  When these tablets are in contact or close proximity to the acrylic surface, it will attack the acrylic pigments and cause bleaching, and sometimes blistering.  Di-Chlor is a weaker chemical, and should be the stabilizer of choice for spas and is available in granular form.  Furthermore, when any sanitizer or acid is added to spa water, the circulating pump and jets should be running to assure rapid mixing of the chemical into the water.  This avoids creating localized regions of highly concentrated chemicals.

These problems have been verified by one of the acrylic sheet manufacturers.  They performed extensive testing of several water treatment products, and found that ‘trichlor” tablets can cause the bleaching similar to the examples that we have  observed and have been reported to us. They also found that when combined with hot water, it would cause blistering.  Some cases reported to MTP showed a red, rust-like material in the bleached area.  Their tests duplicated this phenomena when there was a source of iron in the water.   High concentration of the oxidizing chemical is a prerequisite for the discoloration.  This is facilitated by tablet form chemicals, since they can lay on the acrylic surface while dissolving.  If water is not being circulated, there will be a very high concentration at the interface of the acrylic and the tablet.

The two photos above show discoloration and blistering from the incorrect use of chemicals like “tri-chlor”.

Some people like to use floaters with a chemical inside.  Although it is a convenient way to treat water, it is not recommended, since it can cause problems.  The floater can rest against the side of the spa, and since it is in close proximity, it can cause bleaching of the surface.

Poor chemical control can also cause problems within the plumbing system by reacting with sealants, o-rings, etc.  There have  been reports of other spa components being compromised the use of bromine sanitizers, and ozonators (direct ozone addition).   Any chemical should be quickly dissolved and diluted with water to avoid problems.   It is very important for a spa owner to maintain total water chemistry (e.g. pH, chlorine, total solids, etc) as recommended by the manufacturer.  Various problems can occur if chemistry gets out of control.  For example, if chlorine level is too high the vapors  can bleach the under surface of a spa cover.   Excessive chemicals in water can also damage the jets, controls and fixtures.  Check with your manufacturer for their recommendations on how to maintain water chemistry in your spa.

Sometimes, the bleached surface can be removed by sanding and re-polishing, so surface appearance can be recovered.  However,  avoid repeating the factors which caused the original discoloration.

Following are some photos that provide examples to chemical bleaching (chemical burns).  The owner of at least one of these samples reported seeing pits in the bleached area.   There are chemicals that have been known to attack cross-linked acrylic.  One example is Wintergreen oil.  Although, the normal mode of failure from chemical attack on an acrylic surface is stress cracking, failure can occur in various ways.  Failure in a spa caused by wintergreen oil have been seen where it looked like a liquid had been poured on the surface , and it ran down the side.  The acrylic was discolored, and microscopic crazing was present.  Refer to information on stress-cracking in the website, http://www.multitechproducts.com.   So, unless a chemical has been approved for use in a spa, avoid using it.

The pictures above illustrate these problems in solid color and marble (multiple colored patterns) color spas.

The apparent defect can also occur in “granite” spas as shown below.    Again, they can be repaired using a sanding procedure.  You would start by using 80 or 100 grit sandpaper until the discolored area is removed, and you see the original color.  Then you would finish by using 320 grit wet/dry paper, and finally 400 grit.  If granite texture needs to be regained, you can use Multi-Tech Products K2000 clear topcoat with a brush.  Use the procedure provided to add texture.

 

 

 

 

 

 

 

 

The picture below gives an example of the worst case of chemical attack we have seen on an acrylic spa.  This severe, widespread discoloration would not lend itself to sanding, buffing, and polishing to recover the original color(s).  Read the message at the link for more information on cause and refinish options on this case.

https://multitechproducts.wordpress.com/2013/11/04/282/

 

 

 

 

 

Some general information on chemicals used for spas follows:

Calcium hypochlorite is available in granular or tablet form. It provides 65% available chlorine by weight and remains stable if stored in a dry, cool area. The chemical can be dissolved and introduced as a liquid, or it can be added in dry form. When
applied directly, it may cause a temporary cloudiness. Direct applications should be broadcast evenly over the water surface to avoid bleaching.
This chemical, when contaminated by or mixed with an organic compound, can produce a fire. A good rule is never to mix calcium hypochlorite with another chemical or store it in anything but the original container. Mix the chemical into water not water into the chemical. Calcium hypochlorite should not be handled with bare hands and must bekept off the operator’s clothes. As a chlorinating agent, calcium hypochlorite will slightly increase water pH. It’s pH is 11.8.

Sodium dichloro-s-triazinetrione and trichloro-s-tria zinetrione – chlorine compounds that contain cyanuric acid, and are used to stabilize chlorine.  The dichlor is more soluble and provides 56% or 62% available chlorine, depending on formulation.   The dichlor compound has little effect on pH, while trichlor is extremely acid (pH 2.8 -3.0).  Dichlor can be added directly to the water. The major effect of cyanuric acid on hypochlorous acid (HOCI) is to keep it from being decomposed by ultraviolet light contained in sunlight. Because it is readily decomposed by LTV light, the dosage of a chlorinating agent loke calcium hypochlorite that is sufficient for an indoor pool/spa is dissipated rapidly in an outdoor pool/spa.  Cyanuric acid bonds with the available chlorine in a manner that does not use up the chlorine.  At high stabilizer levels (over 100 ppm), chlorine’s efficiency may be reduced. The operator should consult the local codes and manufacturers’ recommendations on the proper use of stabilizers.

Regards,

Ken Wolfe (Chemical Engineer & Consultant)