PV junction box mounting and sealing advances

New advances in liquid and tape sealing systems can help solar photovoltaic panel manufacturers address the challenges of cost reduction and connection reliability for junction box mounting and sealing. Brent Ekiss, Fabrico, shares the pros and cons of available materials.

October 31, 2011 — As new generations of solar modules become an increasing challenge for manufacturers, material converters will be called upon for more rigorous adhesives and materials testing. As growth in the solar market continues through 2011, solar panel manufacturers are exploring ways to decrease costs, improve manufacturing efficiency, and meet expectations for longer effective panel lifetimes. Customer demands for 25-year warranties are putting pressure on manufacturers to improve junction box mounting and sealing technologies while also increasing manufacturing efficiency.

Whether a solar panel manufacturer is working with monocrystalline silicon, polycrystalline silicon, amorphous silicon, or thin film photovoltaic technologies, junction box mounting and sealing requires:
1) Reliable bonding throughout a range of temperatures and conditions;
2) Easy inclusion in the manufacturing and assembly process;
3) Resistance to degradation from UV light and extreme temperature cycling;
4) Very low moisture vapor transmission rate (MVTR);
4) Good adhesion to metal, glass, TPT, and fluorocarbons.

New advances in both liquid and tape technologies are providing solar panel manufacturers with a range of solutions for junction box mounting and sealing that can meet their needs.

Junction box mounting and sealing

Historically, there are two primary ways to mount and seal a junction box (see Figure) using adhesives: liquid adhesives, such as silicone RTV, or adhesive tapes, such as acrylic foam tape. Both mounting and sealing methods have their pros and cons.

Figure. Solar panel and junction box illustration. SOURCE: Fabrico.

Liquid adhesives

Liquid adhesives have been used in junction box mounting since the industry?s beginning.  They provide for strong durable bonds with good resistance to temperature extremes, plus high moisture resistance. Moisture intrusion into the electrical connections within a junction box can cause tremendous damage. Liquid adhesives can be used with junction boxes for both potting and mounting and sealing.

Silicone RTV is a traditional potting and mounting adhesive, applied both manually or as part of an automated dispensing process. Silicone RTV (room temperature vulcanization) has good moisture vapor transmission rate (MVTR) plus the following characteristics:
1) Bonds to glass, metals, coated materials, and plastics;
2) able to withstand UV radiation and harsh weather resistance;
3) Has a flexible joint to withstand movement and temperature cycling;
4) Has high long-term durability;
5) Heat resistance from -40&#176C to +150&#176C.

Because liquids may be incorporated into dispensing stations to provide an in-line manufacturing solution, they can help reduce materials costs and processing time through automation. Automated adhesive dispensing equipment eliminates the possibility for error in manual or semi-automatic processing. Processing systems in automated dispensing arrangements can minimize excessive adhesive application and waste or residue.

New advances in silicones for junction box potting and mounting are now offering fast-fixturing — with fixture strength in just two to three minutes, and handling strength in under twenty minutes. Elasticity for these silicones has also been improved to provide more flexibility for mounted junction boxes. In addition to faster curing at room temperature, silicones provide a water proof seal with strong adhesion to back sheet material such as PVF and PET.

In reference to edge seal applications, research is continuing into the use of molten/liquid butyl in addition to two-part silicone liquids. Liquid butyl can allow for accurate and repeatable application to panel edges.

Liquid structural adhesives, elastomers, and thermally and electrically conductive adhesives and materials are undergoing continuous research as panel manufacturers investigate new technologies for meeting customer warranties and streamlining production time, effort, and costs.

While silicone RTV provides many advantages for mounting and sealing, there are also familiar drawbacks:
1) Curing times can run anywhere from 10 minutes, to several hours, to days, and this can slow down the panel manufacturing process;
2) Silicone RTV use can be messy (even with dispensing equipment) and time and labor devoted to clean-up can be significant;
3) Inconsistent application (even using dispensing equipment) can also cause quality, durability and WVTR issues.

Available tapes

Adhesive tapes including acrylic foam tapes. Some solar panel manufacturers find that their processes benefit more from mounting junction boxes using adhesive tapes. In these cases, pre-cut rolls of double-sided adhesive tapes are used to eliminate the time-consuming application of liquid adhesives, the investment in dispensing technology, and, above all, the time required for the liquid adhesives to cure. In addition to slowing down production in their processes, the effects of discovering a problem might mean hundreds of panels could be rejected and scrapped.

Polyethylene and polyurethane foam tapes. Polyethylene (PE) and polyurethane foam tapes have long been used by solar manufacturers for edge and frame sealing and for attaching junction boxes. These tapes are made in different grades and thicknesses. Typical thicknesses are  0.8mm, 1mm, and 1.55mm, with a thickness on tolerance of ±20%, which is normal for a blown foam.

PE foam tapes are coated on both sides with adhesive using a transfer lamination process. The foam is corona treated so the adhesives will key into the foam. The transfer lamination process gives a variance in the performance of the tape. The quality and consistency of the corona treatment of the foam and the control on the lamination process can affect tape performance. Delamination of the adhesive can occur when these processes are not controlled properly.

PE tape is very useful in applications where gap filling is required and the bond is not subjected to a lot of stress. PE tapes are easily applied and can fit well into the typical solar panel manufacturing process. They are also cost-effective materials.

The compressive strength of PE foam tapes, however, is low. Cell rupture can occur with very little force and foam cells don?t recover well from compression. The internal cohesive strength of foam is poor and tears can occur. Flex strength during elongation and maximum static load are also low. When subjected to repeated expansion and contraction as a result of high and low temperature exposure and cycling, as well as being used with different materials like glass, aluminum, and plastic, the foam will degrade and break down over time leading to leakage and water absorption.

There have been issues with PE foam tapes in solar applications where the foams fail after eight to nine years of service life. Selecting the appropriate materials is a critical decision for solar manufacturers whose products are expected to last longer than 25 years.

Acrylic foam tape. High strength acrylic foam tape provides an attractive alternative for solar applications. The acrylic looks and feels like foam but in most cases, is an acrylic with air bubbles and glass beads injected into it. It also gives the tape a viscoelastic effect that will stretch and retract to its original shape without breaking the bond. This provides the excellent expansion/contraction capabilities necessary for solar use without any adhesion loss.

The tapes have excellent load bearing characteristics with conformability, high tensile strength, high shear and peel adhesion, and have high resistance to plasticizer migration. Several manufacturers have tapes that have UL 746C recognition. Acrylic foam tapes also have excellent durability as well as solvent and moisture resistance.

Relative shear strength is very high, adhesive strength tensile in N/cm2 is 110, with an adhesive strength 90° peel N/10mm of 35.0. Standard slitting tolerance is ±1/32″ (0.8mm) with precision slitting tolerances of ±1/64″ (0.44mm). The tape can be die-cut in limitless shapes and sizes.

Additionally, acrylic foam tapes can resist very high wind forces and snow loads. In addition, they are more than capable of withstanding very high UV exposure for long periods without degrading or discoloring. Unlike PE foam tapes, they withstand temperature extremes: -40°C to +160°C.

Acrylic foam tapes also deliver moisture, dust, and air sealing for frame bonding, edge sealing, and junction box mounting. The tape can be precision die-cut for use as a gasket for a wide range of junction box sizes and shapes. It bonds well to polycarbonates, PPE, and other thermo-plastics.

Eliminated curing time allows some manufacturers to keep the production line moving — there?s no need to wait for liquid adhesives to cure.

The selection of the appropriate junction box sealant technology will always depend on the specific manufacturing process. Tapes offer new possibilities for solar panel manufacturers. However, like every option, it has its drawbacks, the largest being its higher cost.

Role of the materials converter

The role of the converter is to help solar industry manufacturers select the best sealing and mounting solutions for their materials and manufacturing process. In many cases this can include a combination of both liquid and tape adhesives, especially in junction box mounting and sealing where the trend is toward larger and heavier junction boxes. In such a case, the tape might be recommended to hold the junction box in place while the silicone RTV is also applied.

A converter with extensive materials and adhesives expertise and in-depth knowledge of the solar panel manufacturing industry will be able to work closely with the manufacturer?s design engineers and process engineers to develop and implement a solution that is cost-effective and durable, able to meet the 25-year performance guarantees that may be required, no matter how harsh the operating environment.

A converter can be a problem solver as well, from coming up with the latest advances in liquid or tape adhesives, to making recommendations and researching new formulations for specific characteristics.

As new generations of solar modules become an increasing challenge for manufacturers, material converters will be called upon for more rigorous adhesives and materials testing. They will ensure compliance with governing standards as well as provide expertise and support to optimize costs, minimize rejects, and meet manufacturing schedules. Only with that level of assistance from converters and other vendors can solar panels evolve to the point of competing with, as opposed to augmenting, conventional methods of power generation.  
 
Brent Ekiss is a technical field representative at Fabrico, 4175 Royal Drive, Suite 800, Kennesaw, GA 30144 USA; ph.: 630-728-3055; bekiss@fabrico.com.

Watch Fabrico’s free, on-demand webcast: Advances in Materials Converting and PSA Tapes, Including Bus Bar Tapes, for Solar Manufacturing

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