Polyether, a primary material, reacts with isocyanates to form urethane, the backbone reaction in foam production. Under equivalent functionality, increasing molecular weight enhances foam tensile strength, elongation, and resilience, while reactivity decreases for similar polyethers. With the same equivalent value (molecular weight/functionality), higher functionality accelerates reactions, increasing urethane crosslinking, foam hardness, and reducing elongation. The average functionality of polyols should be above 2.5 to ensure adequate foam recovery under compression.
Excessive polyether usage, equivalent to reducing other ingredients (TDI, water, catalysts, etc.), can lead to foam cracking or collapse. Insufficient polyether results in harder foam with reduced elasticity and poor touch.
1. Blowing Agents
For polyurethane foam with density greater than 21, use only water (chemical blowing agent) for high-density formulations. Low-boiling compounds like dichloromethane (MC) are used in low-density or ultra-soft formulations as auxiliary blowing agents. Auxiliary blowing agents decrease foam density and hardness, requiring increased catalyst usage. Their gasification absorbs some reaction heat, slowing curing and avoiding core burning risks.
Foaming ability is expressed by the foaming index (IF), calculated using the formula: IF = m(water) + m(F-11)/10 + m(MC.)/9 (100 parts polyether).
Water, acting as a blowing agent, reacts with isocyanates to form urea, releasing CO2 and heat, contributing to chain growth. Excessive water reduces foam density, increases hardness, weakens pore pillars, and lowers load-bearing capacity, leading to foam collapse and cracking. If water exceeds 5.0 parts, adding a physical blowing agent is necessary to absorb some heat and prevent core burning.
Water scarcity reduces catalyst usage, but density increases.
2. Toluene Diisocyanate (TDI)
TDI 80/20, a mixture of 2,4- and 2,6-isomers, is commonly used for flexible foam. The TDI dosage is calculated as (8.68 + m(water) × 9.67) × TDI index, with a typical index of 110-120. Increasing isocyanate index enhances foam hardness, but beyond a point, hardness plateaus, tear and elongation strength decrease, and foam exhibits large pores, rising closed-cell content, reduced rebound, prolonged surface stickiness, and extended curing time, leading to core burning risks. A low isocyanate index results in foam cracking, poor rebound, low strength, significant permanent compression, and a damp surface.
3. Catalysts
Amines, typically A33, accelerate isocyanate-water reactions, influencing foam density and bubble opening. Excessive amine leads to foam cracking and porosity, while inadequate amounts cause shrinkage and closed-cell foam. Tin, often in the form of stannous octoate (T-9), catalyzes gelation reactions. Excess tin accelerates gelation, increasing viscosity and reducing rebound and breathability, causing closed-cell phenomena. Proper tin usage yields well-expanded open-cell foam. Reduced tin results in insufficient gelation, leading to cracking, edge or top cracks, and flashing.
Reducing amines or increasing tin strengthens polymer bubble film, reducing hollow or cracking phenomena during gas evolution.
Whether polyurethane foam exhibits an ideal open-cell or closed-cell structure depends on balancing gelation and foaming reaction rates during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formulation.
4. Foam Stabilizers (Silicone Oil)
Silicone foam stabilizers disperse polyurea well in the foaming system, acting as "physical cross-linking points." They significantly increase the early viscosity of foam mixtures, preventing cracking. With emulsification enhancing inter-component solubility and reduced liquid surface tension, silicone surfactants aid nucleation during gas dispersion, facilitating the production of small gas bubbles. This controls foam pore size, structure, and improves foaming stability, preventing dimples and cracks, providing foam elasticity, and controlling pore size and uniformity. Higher foam and POP usage leads to greater silicone oil usage.
High silicone oil usage results in increased foam wall elasticity, finer pores, and closed-cell formation.
Low silicone oil usage leads to foam cracking, collapse after foaming, larger pore size, and increased coalescence.
5. Temperature Impact
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Polyurethane foaming reactions accelerate with increasing material temperature, posing core burning and ignition risks in sensitive formulations. Maintaining constant temperatures for polyol and isocyanate components is essential. Foam density decreases with increasing material temperature during foaming. In the same formulation, higher temperatures in summer result in accelerated reactions, leading to decreased foam density, hardness, increased elongation, and improved mechanical strength. Increasing the TDI index in summer can compensate for hardness reduction.
6. Humidity Impact
Increased humidity reduces foam hardness due to isocyanate-water reactions. Adjusting TDI levels during foaming can compensate for humidity effects. Excessive humidity may cause high curing temperatures, leading to core burning.
7. Atmospheric Pressure Impact
In the same formulation, foaming at high altitudes results in lower foam density.
Note:
(1) Gelation and foaming reactions compete during foam formation, with foaming reactions generally faster than gelation reactions.
Gelation reaction: Formation of urethane (with -OH groups).
Foaming reaction: Reactions involving water, producing urea and generating bubbles.
(2) Nucleating Agents: Substances causing bubble formation, such as fine solid particles, liquids, foam stabilizers, or microbubbles already dissolved in materials, including air or nitrogen dissolved in polyols and isocyanates, carbon dioxide, foam stabilizers, carbon black, and other fillers. More nucleating agents result in more bubbles and smaller pores.
As temperature rises, gas solubility in liquids decreases, resulting in more bubble formation or growth of previously formed bubbles. A longer milky white time favors the growth of larger air bubbles.
Increasing the catalyst amount shortens the milky white time, as the competition between gelation and bubble formation reactions results in fine-pored foam.
(3) Whether foam exhibits an ideal open-cell or closed-cell structure depends on the balance between gelation and gas expansion rates during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formulation.
In regard to polyurethane foam catalysts, two main types are used: tertiary amines and metal-based catalysts. Tertiary amines are commonly referred to &#;blowing&#; catalyst as they are selective toward isocyanate/water reaction whereas metal-based catalysts are considered as &#;gelation&#; catalysts as they are preferential toward the isocyanate/polyol reaction. Reaxis supplies a wide range of tin-based, bismuth-based, and zinc-based metal catalysts.
The choice of catalysts depends on the specific foam type, raw materials used, desired final properties and, increasingly, environmental drivers. Metal-based catalysts, like tertiary amines are more selective toward specific reactions involved in polyurethane synthesis. Some common Reaxis products used in polyurethane foams include:
Inorganic tin products such as REAXIS® C129 (Stannous Octoate) and REAXIS® C125 (Stannous Neodecanoate) are commonly used in the production of flexible foams. The benefits of these tin chemical products include high active metal content, good ligand compatibility, long history of commercialization and no organotin content.
Organotin carboxylate products such as REAXIS® C216 (Dioctyltin Dilaurate), REAXIS® C218 (Dibutyltin Dilaurate), REAXIS® C233 (Dibutyltin Diacetate) and REAXIS® C325 (Dimethyltin Dineodecanoate) are commonly used in both rigid and flexible foams. Primarily used to promote the isocyanate and polyol reactions, these catalysts will also partially catalyze the water and crosslinking reactions.
Organotin-sulfur products such as REAXIS® C319 (Dibutyltin Dimercaptide), REAXIS® C322 (Dibutyltin bis-(2-ethylhexyl Mercaptoacetate)) and REAXIS® C214 (Dioctyltin bis-(isooctyl Mercaptoacetate)) are commonly used in rigid foam applications where enhanced front-end reaction delay and improved hydrolytic stability, versus organotin carboxylates, is required.
The use of bismuth and zinc-based metal catalysts is increasing as regulations tighten on organotins. Common zinc and bismuth catalyst products include REAXIS® C716 (Bismuth Neodecanoate), REAXIS® C (Bismuth Neodecanaote) REAXIS® C616 (Zinc Neodecanaote) and the bismuth-zinc metal blend, REAXIS® C708 (Bismuth/Zinc Neodecanoate) and REAXIS® C717, (Bismuth/Zinc Octoate). Our line of non-carboxylate-based bismuths include REAXIS® C739P50 and REAXIS® C739E50 provide for improved isocyanate/polyol selectivity and hydrolytic stability and like tin catalysts, bismuth carboxylates are more selective toward the isocyanate/polyol reaction. The selectivity towards the crosslinking reactions is unique to the zinc carboxylate-based catalysts.
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