Interesting Differences between Closed-Cell and Open-Cell Backer Rods

Due to exposure against different weather elements, concrete can develop cracks. In order to save the cracks from worsening further, an option would be Backer Rods. A backer rod is a closed cell, flexible material manufactured through plastic extrusion. A gas is injected into the material to create plastic foam. The result can be best described as similar to a tube inflated under slight pressure since gas is trapped under its cellular structure. This kind of backer rod is considered as superior to the traditional usage of sand, wood, fiberboard, jute, rope and twisted paper as filler before the final concrete sealer is applied. A backer rod has three primary purposes.

• To form a base against which sealant is applied
• To force uncured sealant out laterally under tooling pressure resulting in 100% contact of sealant to sidewall or substrate
• To dictate the cross-sectional configuration of the sealant bead

Closed cell backer rods

The first kind of backer rod introduced was the closed cell, flexible foam material made from polyethylene plastic. However, this backer rod was less than perfect and there were many drawbacks which required additional research to eliminate the problems. One of the more serious issues was outgassing wherein much of the rigidity of the rod is lost when the cells become ruptured or deflated. Another problem is that the rod loses its ability to regain its original round shape once it has been deformed. Early applications of the Backer Rods made used of scrap material coming from furniture shops. The polyurethane materials coming from the furniture industry had a very high degree of resiliency and were able to retain its shape even in the most severe usage.  It therefore became the ideal material for the Backer Rods.

Open cell backer rods

The round open cell backer rod was later introduced in the market and was made from a special formulation of polyurethane. In comparison to the previous closed cell backer rod, the open cell backer rod was more flexible as it was able to retain its rigidity based on its chemical and physical properties. In the closed cell, rigidity is the result of gases trapped in the exterior. Open cell backer rods are also easier to install because they are softer and more pliable than the closed-cell versions. Large diameter pieces can be compressed by as much as 75% of their diameter so that they can fill in gaps that vary in thickness in certain areas. Not only are fewer sizes used in varying width joints but in some particular cases one size will work the full length of the joint. The desired end result in the use of Backer Rods results in a satisfactory seal that exceeds all expectations.

Between the open cell and closed cell Backer Rods, the former is has much higher heat resistance. Open cell backer rod has been used successfully in hot-applied sealant material of up to 500 degrees Fahrenheit without any negative effects. The maximum allowable temperature for closed cell backer rod as recommended by manufacturers is 160 degrees Fahrenheit.