Polyurethane Additives

Polyurethane additives are the minor components of a formulation, usually under a fifth of its weight, that control reaction speed, foam structure, fire behaviour and processing. Two systems built on the same polyol and isocyanate can perform completely differently because of their additive package. This guide covers each additive family, what it does, typical dosage ranges and the defects it causes when it is wrong.

01What Polyurethane Additives Do

Additives turn a slow, uncontrolled reaction between a polyol and an isocyanate into a reproducible industrial process. Without a catalyst most polyol-isocyanate mixtures react too slowly for a production line; without a surfactant a foam would form large, irregular bubbles and collapse.

In a two-component system the additives are almost always pre-blended into the polyol side (the A component in European usage, often called the B side in North America), because isocyanates react with water, amines and alcohols. The isocyanate side normally contains only the isocyanate or a prepolymer. Dosages are expressed in phr, parts per hundred parts of polyol by weight.

These ranges are general industry values intended for orientation. A supplier's technical data sheet and line trials always take precedence.

02Catalysts

Polyurethane catalysts accelerate the two competing reactions in a foaming system: the gelling reaction between isocyanate and polyol, and the blowing reaction between isocyanate and water. The ratio of gelling to blowing activity decides whether the foam rises and cures in step.

If blowing runs ahead of gelling, gas is produced while the polymer is still too weak to hold it, and the foam collapses. If gelling runs ahead, the cells close and set before expansion is finished, which leads to high density, tight foam and, in flexible foam, shrinkage.

Tertiary Amine Catalysts

Tertiary amines are the workhorse catalysts in foam. Triethylenediamine (TEDA) is a classic gelling catalyst; bis(2-dimethylaminoethyl) ether (BDMAEE) and pentamethyldiethylenetriamine (PMDETA) are strongly blowing-selective. Dimethylcyclohexylamine (DMCHA) is widely used in rigid foam as a balanced catalyst.

• Delayed-action catalysts are amines blocked with an organic acid. They stay inactive while the mixture flows into a mould or panel cavity and switch on as the exotherm raises the temperature. They improve flowability without slowing the final cure.
• Reactive (non-fugitive) amines carry a hydroxyl or amine group that bonds into the polymer. They reduce odour and volatile emissions, which matters for automotive interiors and mattresses.

Metal Catalysts

Metal catalysts are strong gelling catalysts. Stannous octoate is standard in flexible slabstock foam. Organotin compounds such as dibutyltin dilaurate have long been used in coatings, adhesives, sealants and elastomers, but several organotins face regulatory restrictions in the EU, so bismuth and zinc carboxylates are increasingly used as alternatives in CASE systems.

Potassium carboxylates (potassium octoate, potassium acetate) and certain quaternary ammonium salts promote the trimerisation of isocyanate into isocyanurate rings. This is the chemistry behind PIR (polyisocyanurate) foam, which is made at a high isocyanate index and has better fire behaviour and thermal stability than standard PUR foam.

Cream, Gel And Tack-Free Time

Catalyst performance is measured through the reaction profile, recorded in a cup test or with an instrumented foam rise analyser:

• Cream time: seconds from mixing until the mixture changes colour and starts to rise.
• Gel (string) time: the point at which the polymer forms strings when touched with a spatula; the network is now load-bearing.
• Tack-free time: the surface is no longer sticky.
• Rise time: expansion is complete.

Line speed on a continuous panel line, demould time in a shoe mould and the pot life of a coating all derive from this profile. Pluskim supplies standard catalysts in its 4000 series alongside system-specific packages; the Pluskim catalyst page describes how the gelling and blowing balance is adjusted to a given line.

03Silicone Surfactants

Silicone surfactants control the cell structure of polyurethane foam. They are usually polyether-polysiloxane copolymers: a silicone backbone with polyether side chains, tuned so that the molecule sits at the gas-liquid interface.

A surfactant works in three stages:

1. Nucleation: during mixing it lowers surface tension so that a large number of very small air bubbles are dispersed. These become the seeds of the cells; more seeds mean finer cells.
2. Stabilisation: while the foam rises, it prevents thin cell walls from rupturing and the bubbles from merging (coalescence) before the polymer has gained strength.
3. Cell opening or closing: at the end of the rise, the surfactant's structure influences whether cell windows open (needed in flexible foam for breathability and to prevent shrinkage) or stay closed (needed in rigid insulation foam to keep the low-conductivity gas inside).

Rigid foam surfactants favour fine, closed cells, which directly lowers thermal conductivity. Flexible slabstock surfactants are selected for a wide processing window, and HR moulded foam uses weaker stabilisers because HR polyols are inherently more stable. Typical doses run from about 0.5 to 3 phr. Silicone surfactants from Pluskim are offered for both rigid and flexible foam.

04Blowing Agents

Blowing agents create the gas that expands polyurethane into foam. They are either chemical, generating gas by reaction, or physical, evaporating from the heat of the reaction.

Water (Chemical Blowing)

Water reacts with isocyanate to form an unstable carbamic acid, which releases carbon dioxide and leaves a urea linkage. It is used in practically every foam. In flexible slabstock foam, water is the main blowing agent at roughly 2 to 5 phr; each additional part of water lowers density but also consumes isocyanate and adds urea hard segments, making foam firmer. In rigid foam, water is often combined with a physical agent.

Physical Blowing Agents

• Hydrocarbons: cyclopentane, n-pentane and isopentane are low-cost, have negligible ozone depletion potential and very low global warming potential, and are standard in refrigerators and continuous panels. They are flammable, so plants need explosion-protected equipment.
• Hydrofluoroolefins (HFOs): HFO-1233zd(E) and HFO-1336mzz(Z) have very low global warming potential and give low thermal conductivity. They are used where flammability rules out pentanes, for example in spray foam and some appliance and panel applications.
• Methyl formate is used in some spray, integral skin and rigid systems as a low-GWP option.

From CFCs To Today: The Regulatory Path

Until the late 1980s, CFC-11 was the dominant blowing agent for rigid foam. The Montreal Protocol phased out CFCs because of their ozone depletion, and the industry moved to HCFCs such as HCFC-141b, which are themselves being phased out. The later Kigali Amendment and regional rules such as the EU F-gas regulation restrict high-GWP HFCs, which is why hydrocarbons, HFOs, water and methyl formate dominate new formulations. Pluskim states that it uses next-generation blowing agents that do not harm the ozone layer.

05Flame Retardants

Flame retardants reduce the ignitability and flame spread of polyurethane, which is essential in building insulation, furniture foam in some markets and transport applications.

• TCPP (tris(1-chloro-2-propyl) phosphate) is the most widely used additive flame retardant in rigid foam. It also lowers viscosity.
• TEP (triethyl phosphate) is a halogen-free phosphate, often combined with TCPP.
• Expandable graphite swells into an insulating char layer when heated and is used in flexible and rigid foams that must meet demanding fire tests.
• Reactive flame retardants are phosphorus- or halogen-containing polyols that are built into the polymer and therefore cannot migrate out.

In rigid foam, chemistry matters as much as additives: PIR foam made with a high isocyanate index and aromatic polyester polyols forms a stable char and needs less flame retardant than PUR foam of the same density. Fire performance is always proven by testing the finished product, for example Euroclass classification under EN 13501-1 in Europe or the ASTM E84 tunnel test in the United States, where Pluskim reports being the first Turkish brand approved.

06Chain Extenders And Crosslinkers

Chain extenders are short difunctional molecules that react with isocyanate to form the hard segments that give elastomers, shoe soles and integral skin foams their hardness and strength. Crosslinkers have three or more reactive groups and increase network density.

• Monoethylene glycol (MEG) and 1,4-butanediol (BDO) are the standard chain extenders in shoe sole and cast elastomer systems.
• Diethylene glycol (DEG) is used as a chain extender and is also a building block of polyester polyols.
• Glycerol is a trifunctional crosslinker and a common polyol initiator.
• Diethanolamine (DEOA) is the classic crosslinker in HR moulded foam, giving early green strength so seats can be demoulded quickly.

Increasing chain extender raises hardness and modulus but shortens the processing window; the balance is set together with the isocyanate index. The polyols guide explains how these glycols relate to polyol design.

07Plasticizers, Fillers, Colorants And Release Agents

Plasticizers

Plasticizers soften the polymer and reduce viscosity. DOTP (dioctyl terephthalate) is a common non-ortho-phthalate plasticizer in shoe soles, elastomers and sealants. Excess plasticizer can migrate to the surface, weaken adhesion and slow demoulding.

Fillers And Glass Microspheres

Fillers reduce cost or tune properties. Hollow glass microspheres lower density, reduce shrinkage and improve dimensional stability in putties, coatings and syntactic parts, typically at a few percent by weight. Mineral fillers such as calcium carbonate are used in sealants and some elastomers.

Colorants And Pigment Pastes

Pigments are dispersed in a polyol or plasticizer carrier as a paste so that they mix evenly at low dose. Colour pastes must not contain water or reactive groups that upset the formulation.

Mould Release Agents And Cleaning Solvents

External release agents (wax, silicone or water-based) are sprayed onto the mould so moulded parts such as soles, integral skin and HR seats release without damage. Internal release agents are blended into the system. Consumption, surface finish and paint adhesion all depend on the agent chosen; Pluskim supplies low-consumption mould release agents designed for polyurethane. Dedicated solvents are used to flush mixing heads and lines; they are a maintenance chemical, not a formulation component.

08Troubleshooting Foam Defects Through The Additive Package

Most processing defects in polyurethane foam are caused by an imbalance in the additive package or by temperature and moisture, rather than by the base polyol or isocyanate. The table lists the usual first suspects.

Changes should be made one variable at a time and checked with a reaction profile. Raw material quality also plays a role: a polyol with high water content behaves like a formulation with extra blowing agent. The article on the true cost of cheap raw materials covers how inconsistent lots show up on the line.

09Choosing Additives For A System

Additives are chosen as a package for a specific polyol, isocyanate, process and climate, not individually from a catalogue. A catalyst that works in a summer trial may give a different profile in winter; a surfactant tuned for a low-pressure machine may behave differently on a high-pressure line.

• Start from the target: density, hardness, thermal conductivity, fire class, demould time or line speed.
• Fix the polyol and isocyanate first (see isocyanates and raw materials by application).
• Set the catalyst and surfactant package through cup tests, then confirm on the production machine.
• Recheck after any raw material change, including a new supplier of the same additive.

Systems houses carry out this work as part of supplying a formulated system, which is often faster than developing an additive package in-house. The main polyurethane raw materials guide explains where additives sit in the overall formulation.

10Sources And Further Reading

1. UNEP OzonAction, About the Montreal Protocol: unep.org
2. ISOPA, product information on diisocyanates and polyols: isopa.org
3. PU Europe, polyurethane insulation industry association: pu-europe.eu
4. American Chemistry Council, Center for the Polyurethanes Industry: americanchemistry.com
5. Pluskim, polyurethane catalysts: pluskim.com

11Frequently Asked Questions

12Related Reading

• Polyols For PolyurethanePolyols for polyurethane explained: polyether, polyester, polymer and bio-based grades, OH value, functionality and how to choose one per application.→
• Isocyanates For Polyurethane: MDI, TDI And PrepolymersIsocyanates for polyurethane: MDI, polymeric MDI, TDI, aliphatics and prepolymers, NCO content, isocyanate index, safe storage and EU training rules.→
• Polyurethane Raw Materials By ApplicationWhich polyols, isocyanates and additives each application uses: rigid and flexible foam, footwear, coatings, adhesives and binders, in one clear table.→