Both polyurethanes and epoxy resins help protect a photovoltaic junction box from the main causes of product degradation.
Junction boxes or enclosures are designed to provide protection from environmental stresses and contaminants. They must preserve the long-term electrical performance integrity of the module and prevent tampering with the connections and electronics inside.
Junction boxes are constructed of various metallic or non-metallic materials. Regardless of the junction box’s construction, PV module manufacturers must consider long-term functional protection of the connections inside the enclosure. If degradation and eventual failure of the connections occur, module manufacturers and system operators will be faced with potentially large losses from increased maintenance costs, system downtime and warranty claims, along with the loss of customers and revenue.
Environmental stresses may lead to junction-box failure over time. Despite case construction, outdoor exposure to variable weather conditions - including rain, snow and salt spray - can lead to water penetration of the box, moisture-vapor permeation across seals and moisture condensation onto the connections in the box. Soluble salts in the moisture vapor can also be deposited onto the connections and cause corrosion and electrical failures.
Potting-Compound Chemistries
No matter where or how the degradation or damage to a junction box occurs, the end results are the same - uncertainty and a down system. One way to help ensure the reliability and performance of the grid and electrical connections contained in a junction box is to encapsulate the connections with a thermosetting resin, referred to as an encapsulating or potting resin.
The goal of potting is to protect the electrical connections from premature failure by covering them with a solid resin encapsulant and preventing moisture intrusion. Potting materials keep dusts, salts and moisture away from the connections, even if the junction box allows moisture in (whether through a poor seal or cracking of the junction box itself). Encapsulating resins can be formulated to meet Underwriters Laboratories (UL) and/or Canadian Standards Association (CSA) requirements.
Potting compounds also protect the electrical connections contained in junction boxes from impact stresses that can occur in transit and during installation. The encapsulant absorbs the stress of the impact that may have otherwise strained the electrical connections.
Although there are many types of thermosetting resins, the two most common types are epoxies and polyurethanes polyurethanes. Each formulation has its specific application and physical properties, along with price. It is important to understand the individual junction box’s requirements and investigate the pros and cons of each type of material. To do so, it is helpful to look at their chemistries and to compare their key properties.
Generally, epoxy resin refers to the reaction products of molecules containing multiple epoxy functionality with various chemical hardeners to produce a solid thermoset material. Examples of epoxy hardeners include polyamides, aliphatic amines and anhydrides.
Polyamide hardeners provide better adhesive qualities, aliphatic amine hardeners provide better chemical resistance and anhydrides provide better high-temperature resistance. The type of hardener used with the epoxy, as well as any fillers or modifiers used in formulating, will determine the physical properties the polymer displays when cured.
Polyurethane refers to the reaction products of a monomer containing at least two isocyanate (-NCO) groups with another monomer that contains at least two hydroxyl (-OH) groups. A simpler explanation of polyurethanes is that a polyol is combined with an isocyanate to produce a polyurethane. Most polyurethanes used in potting and encapsulating applications are based on polymeric MDI.
Just like epoxy resins, the physical characteristics of the cured polyurethane polymer will be dependent on the various polyols, isocyanates, fillers and modifiers used in the formulation.
Solar J-Box Encapsulant Product-selection guidelines
The case can be made for either epoxy resin or polyurethane as a suitable junction-box encapsulant. Both technologies will hold up under the environmental conditions that the junction box will encounter, as both technologies have been successfully used for outdoor and automotive encapsulation for decades.
Understanding the differences in the materials can help in making technology decisions. For example, a rigid epoxy resin is much more difficult to tamper with than a soft, flexible polyurethane. On the other hand, polyurethane may not crack or pull away from a junction box when subjected to extreme thermocycling conditions.
One key difference between a polyurethane and an epoxy is the gel speed or cure time. Polyurethanes can be made to gel in seconds, making them excellent candidates for use in automated, fast-paced production lines. Epoxy resins have a longer pot life or open time before gelling, which can cause a bottleneck when it comes to production.
Selecting a technically competent supplier that can help in selecting and testing the encapsulant is a logical first step in product selection. The supplier should be experienced in the formulation chemistry, dispensing and process design, along with the market application. This will reduce project completion time and facilitate success.
Communicating all the expected performance requirements related to the potting compound to the supplier is also critical to the project’s success. Ensure that the supplier understands the processing requirements and restrictions of your operation.
Explain all concerns about the environments in which the modules and the junction box will operate. Discuss costs up front, as selecting and testing a potting material that is too expensive is impractical.
A knowledgeable supplier will help identify the key properties and requirements of the application, such as the junction box’s construction material and how well the proposed potting compound adheres. The supplier will also help with meeting the requirements of certification under the governing authority such as UL or CSA.
Jon Zarnstorff is solar projects manager at Epic Resins, a Palmyra, Wis.-based custom formulator of epoxy resin and polyurethane compounds. He can be contacted at jzarnstorff@epicresins.com or (262) 506-4413.
