Basic Concept Q&A on Polyurethane(Part Eight)

1. Characteristics of Horizontal and Vertical Foaming

Horizontal Foaming Characteristics:

Edge Film Lifting Method: This method enhances the traditional horizontal foaming machine by adding a side paper pulling device, allowing the foam edges and center to rise synchronously, resulting in a near-flat top foam block.

Balanced Pressure Plate Method: Features top paper and a top cover plate.

Overflow Trough Method: Uses an overflow trough and a descending conveyor belt.

Vertical Foaming Characteristics:

Can produce large cross-sectional foam blocks with a smaller flow rate. In contrast, horizontal foaming machines require a flow rate 3-5 times greater to achieve the same cross-sectional blocks.

Due to the large cross-section of foam blocks, there are no upper or lower skins, and the edge skin is relatively thin, significantly reducing cutting losses.

Requires less floor space, with a building height of about 12-13 meters, resulting in lower plant and equipment investment costs compared to horizontal foaming processes.

Allows easy production of cylindrical or rectangular foam bodies by changing the hopper and mold, particularly suitable for producing cylindrical foam blocks for rotary cutting.

2. Basic Points for Selecting Raw Materials for Soft Foam Preparation

Polyol:

For general block foam, polyether polyol with a molecular weight of 3000-4000 is mainly used, primarily tri-functional polyether.

High resilience foams often use tri-functional polyether with a molecular weight of 4500-6000. Increased molecular weight improves foam tensile strength, elongation, and resilience but decreases reactivity.

Increased functionality of polyether accelerates the reaction, enhancing the cross-linking degree of polyurethane, resulting in higher foam hardness and lower elongation.

Isocyanate:

Toluene diisocyanate (TDI-80) is the main isocyanate for polyurethane soft block foams. TDI-65, with relatively lower activity, is used only for polyester-type polyurethane foams or specific polyether-type foams.

Catalyst:

Block soft foam catalysts are mainly divided into two categories: organometallic compounds, with stannous octoate being the most common, and tertiary amines, with bis(dimethylaminoethyl) ether being commonly used.

Foam Stabilizer:

Non-silicone surfactants are mainly used in polyester-type polyurethane block foams, while organosilicon-alkylene oxide copolymers are mainly used in polyether-type block foams.

Blowing Agent:

Only water is used as a blowing agent when manufacturing polyurethane soft block foams with a density greater than 21 kg/m³. Low-density formulations use low-boiling compounds like dichloromethane (MC) as auxiliary blowing agents.

3. Effect of Environmental Conditions on Block Foam Properties

Temperature Influence:

The foaming reaction of polyurethane accelerates with the rise in material temperature, potentially causing scorching or fire hazards in sensitive formulations.

Air Humidity Influence:

As humidity increases, part of the isocyanate groups in the foam react with moisture in the air, reducing foam hardness and increasing elongation. The increased urea groups also enhance the foam’s tensile strength.

Atmospheric Pressure Influence:

At higher altitudes, the density of the foam decreases significantly for the same formulation.

4. Main Differences in Raw Material Systems for Cold and Hot Molded Soft Foams

Cold Cure Molding:

Uses raw materials with high reactivity, curing primarily through the heat generated by the system itself, completing the curing reaction in a short time. The mold can be demolded within a few minutes after material injection.

Hot Cure Molding:

Uses raw materials with lower reactivity, requiring external heating to complete the reaction. The foam product needs to be fully cured in an oven before demolding.