Types of High Rebound Soft Foam

Currently, the production methods can be broadly divided into two types: the first involves reacting high-activity, high molecular weight polyether polyols with various crosslinking agents and diisocyanates; the second involves reacting chemically grafted polyethers with diisocyanates. Both methods achieve good results.

1. High-Activity Polyether Type
This type of high rebound foam plastic primarily uses high molecular weight, high-activity polyether polyols, with appropriate crosslinking agents, which react with diisocyanates.

There are two ways to manufacture high-activity polyether polyols. One is by introducing ethylene oxide at the ends of the polyether, transforming the end groups into primary hydroxyl groups. Generally, primary hydroxyl groups are 3-4 times more reactive than secondary hydroxyl groups, thus achieving the goal of cold curing. Typically, for high rebound soft foam plastics, the relative molecular weight of the high-activity polyether polyols should be around 4500-6500, with a primary hydroxyl content of 60%-85%. The primary hydroxyl content does not reach 100% due to:

• The steric effects caused by the composition and configuration of the base polyether.
• The impact of reaction co-polymerization rates caused by the already formed primary hydroxyl groups.
• The use of excessive ethylene oxide, which reduces the miscibility between polyether and isocyanates, and increases the moisture absorption of the resulting foam products. Therefore, an appropriate ethylene oxide ratio must be selected.

Another type of high-activity polyether involves introducing amine groups at the ends of the polyether. This is achieved by reacting polyether polyols with ammonia under pressure. Amine groups react at the highest rate with isocyanates, thus also achieving cold curing. However, due to the harsh conditions required for amination, this method is less commonly used in industry. Currently, most manufacturers use primary hydroxyl-based high-activity polyethers.

2. Grafted Polyether Type
This type of high rebound foam product uses grafted polyether (also known as polymer polyol) as the raw material. The polyether is grafted with acrylonitrile, styrene, or a mixture of both, which increases the stiffness of the molecular chains, thereby improving the rebound and compression load performance of the foam products. This type of foam has advantages such as a simple formulation, ease of operation, high mechanical strength, and excellent fatigue resistance. It is widely used for seat cushions, pillow cores, and other shock-absorbing materials. By varying the formulation, products of different densities and hardness can be produced.

Low-density, low-hardness high rebound foam products are mainly used for bedding and chair backs; HR foam products with a density of around 27 kg/m³ are typically used for chair backs and cushions; ultra-low-density foam products with a density of around 17.6 kg/m³ are mainly used for pillow production, replacing wood wool or other fiber-filled pillow cores.

Low-density, high-hardness (i.e., high compression load) high rebound foam products are characterized by: products with a density of 32-38.4 kg/m³, when deformed by 25%, achieving a compression hardness of over 140N; when deformed by 65%, achieving a compression hardness of 320-360N. These products are mainly used to manufacture cushions and mattresses that require higher load-bearing capacity.

The advantage of the high-activity polyether type is that polyether production is simple and relatively inexpensive, but the operational range for formulations is narrower, and TDI 80/20 must be used, otherwise, shrinkage may occur. Additionally, the mechanical properties of the product are not as superior as those made with grafted polyethers. In contrast, grafted polyether polyols are more expensive, have lower foaming operation requirements, and can use TDI mixtures such as 80/20 or 65/35. The resulting foam products have higher strength, making them especially suitable for manufacturing products with high compression load requirements.

To meet the increasingly stringent requirements of the automotive industry for polyurethane soft foam plastics, including low fogging, low VOC, low density, and high resistance to moisture and heat aging, PU researchers worldwide have conducted extensive research in recent years, particularly in high rebound foam plastics. Notable new products include full MDI HR foam, low-VOC HR seat cushions, and HR foams with high resistance to moisture and heat aging and low fogging.

For low-density HR foams, the automotive industry's demand for lightweighting and cost reduction must be balanced with maintaining foam performance. That is, while the density of HR foam is low, its physical properties still meet the automotive industry's comprehensive performance requirements for foam seat cushions and backrests. Currently, foreign companies typically start with specially synthesized polyether polyols to meet these requirements.