Despite its significant advantages, the limitations of PP Honeycomb must also be acknowledged in applications:
- Limited High-Temperature Resistance: This is the primary constraint. Standard PP honeycomb typically has a continuous service temperature range of -40°C to +80°C, with short-term peaks up to 100°C. Beyond this temperature, PP softens, causing core instability and a decrease in the overall stiffness of the sandwich structure. Therefore, it is unsuitable for areas near engines or high-temperature baking environments.
- Must be Used in a Composite Structure: The surface of the honeycomb core consists of open cells. Its local strength is very low under direct pressure (it can be indented with a finger). It must be firmly bonded with rigid face sheets (skins) to form a sandwich structure before it can bear out-of-plane loads. This means it cannot function as a standalone structural component.
- Connections Require Special Treatment: At the edges of the sandwich panel or where fasteners need to be installed, the honeycomb core cannot provide sufficient pull-through strength or support. Typically, “potting” or “insert” techniques (filling the cells with high-strength resin or embedding solid blocks) are used to create reliable connection points.
- Combustibility: Pure PP is a combustible material. Although face sheets in a sandwich structure often provide some fire barrier, for applications with strict fire regulations (e.g., rail transportation, marine interiors), Flame-Retardant PP Honeycomb (FRPP Honeycomb) with added flame retardants must be used, which increases cost.
- Sensitivity to Extreme Solvents: While chemical resistance is generally good, PP may still swell or experience stress cracking when exposed to certain strong polar solvents (e.g., concentrated nitric acid, chlorinated hydrocarbons) at elevated temperatures. Material selection should be evaluated for specific chemical environments.
