Application of Polyurethane in Spare Tire Cover Boards

The automotive spare tire cover board (also known as trunk floor carpet or luggage compartment carpet) is installed inside the vehicle trunk and primarily serves the following functions:

1. Concealment: It separates the spare tire compartment from the trunk, concealing the spare tire and onboard tools, ensuring a flat appearance and consistent layout of the trunk. This determines that the spare tire cover board cannot be small in area.

2. Load-bearing capacity: It needs to bear a portion of the items stored in the trunk. This requires the spare tire cover board to be sturdy, reliable, and durable.

3. Noise insulation and vibration damping: The spare tire cover board can block part of the noise generated by the chassis and rear wheels, making it an integral part of the vehicle's NVH (Noise, Vibration, and Harshness) system engineering. This requires the material of the spare tire cover board to possess certain sound-absorbing and vibration-damping capabilities.

Throughout the development history of spare tire cover boards, various materials have been used, including PHC honeycomb panels, PU composite panels, PP injection-molded panels, PP honeycomb panels, SMC panels, and hard fiberboards, each with different performance characteristics. Due to factors such as poor high-temperature resistance of PP, excessive weight, insufficient environmental friendliness, or poor noise insulation and vibration damping effects of other materials, their market shares are gradually declining.

According to a document shared by BASF, PHC honeycomb panels offer the best overall performance, meeting current market demands for lightweight, environmentally friendly, and sturdy products, and are currently one of the mainstream materials used.

Today, we will focus on the topic and introduce two production processes for spare tire cover boards related to polyurethane: PHC honeycomb panels and PU composite panels.

1. PHC Honeycomb Panels

PHC (Paper Honeycomb) panels are made based on the principle of natural honeycomb structures. Corrugated base paper is bonded with adhesive to form countless hollow regular hexagons, creating an integral load-bearing structure—the paper core—which is then laminated with facing paper on both sides, resulting in a new type of sandwich-structured, environmentally friendly and energy-saving material.

When applied as spare tire cover boards, PHC honeycomb panels are combined with polyurethane composite material spraying technology (PU-R-CSM). The paper honeycomb core is reinforced with fiber materials such as fiberglass, and then sprayed with polyurethane for effective composite molding, forming a polyurethane composite material that is stronger and lighter than traditional panels.

The specific process flow, referencing Wuxi Gissing Muqiao, is as follows:

(1) The un-stretched honeycomb paperboard is properly processed using a stretching and shaping device;

(2) Fiberglass mats are evenly laid on both sides of the honeycomb paperboard;

(3) Polyurethane is evenly sprayed on both sides of the above substrate. The total spray amount depends on the manufacturer's strength and load-bearing standards, generally ranging from 250 to 600 g/m²;

(4) The substrate is sent to a hot press mold for compression molding. The upper and lower mold temperatures are set at 100–130°C, pressure at 8–12 MPa, and hot pressing time at 280–350 seconds;

(5) The pressed substrate is removed and cooled at room temperature on a cooling fixture for 40–60 seconds;

(6) After cooling and shaping, hot melt adhesive is sprayed on one side at 90–130 g/m², and non-woven fabric is laminated;

(7) Accessories are installed, inspected, and packed.

Finished product reference parameters:

Parameter                                 Value

Paper honeycomb core thickness 12 mm

Chopped fiberglass mat           600 g/m²

PU spray amount                         600 g/m²

Substrate thickness                         12 mm

Substrate density                         0.25 g/cm³

This process offers advantages such as convenient processing, fast curing, low odor, high automation, low labor intensity, and significant product superiority. It is currently the most advanced production technology for spare tire cover boards.

2. PU Composite Panels

The structure of spare tire cover boards made from PU composite panels is very similar to that of automotive headliners, and the production process is also similar.

The structural design is as follows: The polyurethane (PU) layer is located in the middle, with adhesive fiberglass layers on both the upper and lower sides. The upper adhesive fiberglass layer is covered with a surface fabric layer, while the lower adhesive fiberglass layer is covered with a non-woven fabric layer.

The process flow, also referencing Wuxi Gissing Muqiao, is as follows:

(1) Prepare all layers of materials. The PU layer is generally made of microcellular PU;

(2) Adhesive fiberglass layers are laid on both sides of the PU layer, and non-woven fabric is laid under the bottom fiberglass layer;

(3) The above substrate is sent to a hot press mold for compression molding, then removed and cooled at room temperature;

(4) The molded composite panel is sent to an oven for heating. The upper layer is then bonded with the surface fabric layer and sent to a mold for compression molding.

Note: The adhesive fiberglass layer is made by combining fiberglass with polypropylene (PP) or high-density polyethylene (HDPE) through high-temperature, high-pressure, and cooling compression. It offers strong adhesion and eliminates the need for additional adhesives, reducing VOC emissions.

This type of spare tire cover board features light weight, high strength, good thermal insulation, and excellent sound absorption and vibration damping.

As mentioned earlier, the spare tire cover board is a large-area automotive interior component and is part of the vehicle's NVH system engineering. It not only requires specific process flows and performance indicators but also undergoes evaluation of its VOC and odor contribution values. Optimization is carried out from both raw materials and synthesis processes. The following are reference VOC and odor indicators for finished spare tire cover boards:

(Note: The VOC and odor reference indicators mentioned in the original text were not provided in numerical form, so they are not translated here. If you have specific data, I can add them.)

Summary

From the above, we can see that the current advanced production technologies for spare tire cover boards fully leverage the advantages of polyurethane materials: lightweight, easy to process, good thermal insulation, sound absorption and vibration damping, and environmental friendliness. Based on the many benefits these new technologies bring to automotive lightweighting, safety, and environmental protection, they are also widely used in other automotive applications. For example, PHC technology is also used in rear shelf panels, floor panels, sunroofs, and parcel shelves, while PU composite panels are also used in headliners and carpets.