Muntin bars are designed to accent residential and commercial windows, doors, and storefronts with the look of a true divided lite for a fraction of the cost. This design approach is called SDL – simulated divided lite. There are several methods for attaching the grille or a muntin bar to the glass; screw-in, glaze-in, snap-in, or tape applied. This blog post will help the owner, architect, OEM, or glazier to understand the different types of muntin bar attachment tapes and the relative strengths and weaknesses of each.
Muntin Bars and Grilles
Muntin bars and the assembled grilles can be made from a variety of materials including PVC (vinyl), wood, aluminum (painted or anodized), and bronze. The bars can be supplied in lineal form to OEMs who can cut them to length and fit the bars themselves, delivered as broken down kit with one grille per bag, or in a fully assembled pattern ready to install.
The bars themselves come in a variety of widths, heights, profiles, and colors. Some design examples are show below.
Muntin Tape Options
The two most common types of double coated foam tapes used for muntin bar attachment are acrylics and polyethylene.
Acrylic Foam Tapes
Acrylic foam tapes include the well known VHB brand from 3M and the Norbond brand from Saint Gobain. Other high quality acrylic foam tapes include the AS series from Scapa and the ACX brand from Tesa.
The 3M VHB closed cell acrylic tapes are monolithic. This means that the adhesive itself is “foamed” creating a foam/adhesive matrix that is uniform throughout the thickness of the product. The other manufacturing technique is coating an acrylic adhesive onto both sides of a closed cell acrylic foam core. These acrylic foam tapes all exhibit viscoelastic behavior meaning that the “viscous” behavior allows the tape to absorb energy such as sudden wind gusts while the elastic behavior allows the tape to recover (an analogy would be stretching a rubber band and then letting it return to its’ normal shape).
Polyethylene (PE) Foam Tapes
Also referred to generically as polyolefin foam tapes, these consist of a foam core made for irradiated, cross liked polyethylene that can be modified with the addition of EVA or other polymers to form a tough, versatile product. Additional coatings can be added to the foam core to create an ultra-smooth surface for superior wet out characteristics on the muntin bars and the insulated glass unit (IGU). High quality polyethylene foam tapes are produced by Adhesives Research, Berry Plastics (Adchem), Adhesives Applications, Mactac, and Scapa.
Although the PE foam tapes will not exhibit the same viscoelastic behavior as the acrylic foam tapes, they have low water absorption properties and exhibit good dynamic shear and tensile properties with a favorable price point that make them suitable for many muntin bar applications.
Muntin Tape Design Considerations
As mentioned, muntin bars are often made from PVC, wood, and aluminum. The muntin bars for residential windows are typically smaller in width, height, and weight than muntins for commercial windows. Commercial muntin bars are usually made of aluminum (often painted) and are usually wider and have a higher profile making them much heavier.
Another consideration is coefficient of thermal expansion (CTE). Simply put, this is the change in length or volume of a material for a unit change in temperature.
Muntin bars will shrink and expand as the temperature cycles down during the night and up during the day. This movement has to be accounted for when selecting a tape that will move easily with the bar and grille assembly.
One of the most important steps in assembling and installing the grille is to make sure that bars are not cut too long so that they ride up onto the window sash or an adjacent muntin bar. It is recommended to leave 1/32” of clearance so that the bar has space to move as thermal expansion happens. This becomes more critical in commercial window applications where the buildings are usually higher and any failures can be catastrophic if the bond is compromised.
Aside from temperature, wind loads, rain, blowing sand and contaminants, and cleaning chemicals need to be taken into account when considering tape selection.
Muntin Bar Cleaning Process
Adhesive tapes are not fussy when it comes to surface contamination. They will gladly adhere to oil, grease, dust, or anything else all while compromising the high bond strength you’re looking to achieve.
The easiest and safest way to clean the bars is to use isopropyl alcohol (IPA). 50% to 70% IPA is a great way to quickly clean excess dirt and contamination from the muntin bar. If lint-free wipes can be used for cleaning, even better.
Silane Coupling Agents for the IGU – Do Your Need Them?
Many of the PE foam tape manufacturers claim that no silane pretreatment is necessary for their tapes and as a practical matter, residential windows manufacturers have used PE foam tapes without silane pretreatment for many years without issue.
Let’s look at the science. Glass is known to be a “water loving” surface. The technical name for this behavior is hydrophilic. This water layer is so small that it can only be detected by analytical instruments but it is there and its’ presence can undercut or compromise the bond of an adhesive or sealant over time.
The chemical name for the class of silane coupling agents that are most effective as a glass pretreatment is 3-Glycidoxypropyl trimethoxysilane resin. The important thing to understand is that this class of silanes possesses both organic and inorganic reactivity that allows them to “couple” organic materials such as foam tapes with inorganic surfaces such as glass. The silane changes the nature of the glass from hydrophilic (water loving) to hydrophobic (water hating). These properties enable the tape and glass to withstand physical, chemical, weather, and thermal degradation.
Silane pretreatment is essential when attaching munitn bars to commercial windows and curtain wall systems. The added stability afforded by the use of the silane reduces the risk of de-bonding in high humidity or moist conditions.
We’re sometimes asked how a silane can possibly work if the concentration in the cleaning liquid is so small (<1%). Think of baking muffins or biscuits. You use a fair amount of flour and other ingredients but only a small amount of baking powder since it is highly reactive (the dry acid and base in the powder react with moisture to make carbon dioxide). You don’t need a lot to get a big result!
Installation Steps for Tape Applied Muntin Bars and Grilles
- Place the window unit on a horizontal work area; preferably away from any dust generating process such as sawing.
- Dry fit the grille into the window by preassembling the grille and making sure it fits with needed clearances on all parts of the grille.
- Clean the glass and pretreat using the silane coupling agent directions.
- Create a “pig tail” by peeling back a 2-3 inches of the plastic release liner (this technique does not work with paper release liners) on each end of the bars. This allows the bars to be positioned may permit a small amount of repositioning especially with the acrylic foam tapes.
- Once the grille is positioned properly, pull on the “pig tail” and work your way along the bar removing the liner and pressing firmly without bending the bar.
- The unit can be handled immediately but the tape will continue to build bond strength over 72 hours. 50-70% of the bond strength is immediate, 90% after 24 hours, and 100% after 72 hours.
Contact Tom Brown, Inc. for More Information on Tape Options for Muntin Bars
If you would like sample rolls of acrylic or PE foam tapes, additional data on tape performance, silane coupling agent recommendations, or any further application guidance, please contact Tom Brown, Inc.