FLEXIBLE SLABSTOCK FOAM RAW MATERIAL AND THEIR FUNCTIONS

MAKING OF FOAM BLOCKS


A large proportion of flexible polyurethane foam is making in blocks, either discontinuously in simple boxes or in a continuous process known as slabstock foaming. In this process the raw materials, polyol, di-isocyanine, water, auxiliary blowing agents, catalysts, surfactant and other additives are mixed continuously into an in-line mixer and the mixed reactants, still in liquid form, are poured into a continuous paper and/or polyethylene mould on moving conveyor. In the paper mould, the liquids foam and expand to form a continuous block of foam. This blcok is out into sections, stored for at least a day to cure and cool, and then cut in variety of ways into the shapes required. The main uses for this foam are mattresses, furniture’s, automotive seating and textile laminating.

THE RAW MATERIALS AND THEIR FUNCTIONS


The essential components of flexible foam formulation are:

1)Polyol
2)Di-isocyanate
3)Water
4)Auxiliary Blowing Agents (ABA’s)
5)Catalysts: tertiary amines and tin salts
6)Silicone surfactants.

To these may be added optionally colours, fire retardants, combustion modifiers, fillers and other materials.

Taking each of the essential components in turn:

Polyol

Most flexible slabstock foam is made from polyther polyols.

The are essentially propylene oxide and ethylene oxide copolymers with a tri-functional initiator and are therefore trials. The polyols for standard block foams have hydroxyl values in the range 46-56 mgKOH/g (nominal molecular weights of 3500 to 3000 respectively).

Di-isocyanate

The most common di-isocyanate for flexible foams is toylene di-iscocyanate, also called toluene di-siocyanate or simply TDI. Commercial grades of TDI are mixtures of the 2,4 and 2,6 isomers in controlled proportions.The most frequently grade used to manufacture flexible polyurethane slabstock foams, consists of 80 parts of the 2,4 isomer and 20 parts of the 2,6 isomers.

Blowing agents

The primary blowing agent, which causes the foam to expand, is carbon dioxide, generated by the reaction between water and di-isocyanate. An auxiliary blowing agent may be used in combination with water to produce foams below 21 kg/m3 density or to produce soft foams at all densities. These auxiliary blowing agents are low boiling point liquids

Catalysts

Tertiary amines

These catalysts accelerate and control the rate of the water/di-isocyanate reaction.
Examples are:

Di-methyl diamine, often referred to as TEDA or DABCO TM , the original Houdry trade name; now marketed by Air Products and now most commonly supplied as DABCO TM33LV – a 33% solution of TEDA in dipropylene glycol
Bis (dimethylaminoethyl) ether 70% in di-propylene glycol; most commonly known as NIAZ TM A1, the original trade name of Union Carbide Co; now marketed by Witco.

Tin salts

These catalysts are specific for the reaction between polyol and di-isocyanate. Almost universally used is stannous octoate (the tin II salt of 2-ethyl-hexoicacide), often referred to as tin catalyst, SnOct or t-9, the original trade name of M & T Chemicals Inc.; now marketed as DABCO TM T-9 by Air Products.
The control accurately the small quantities required, the stannous octoate as supplied is normally diluted with polyol before use or can be purchased in an already diluted form. An alternative catalyst for use especially where polyol pre-blend (containing water, amines and silicone) are made is di-butyl tin dilaurate commonly known as DBTDL, DBTL or DABCO TMT-12.

Silicone Surfactant

A surfactant is essential to control of the foaming process. In slabstock foams this is siloxane based and is commonly referred to as ‘silicone’. It has two functions:
 To assist the mixing of the components to form a homogeneous liquid.
 To stabilize the bubbles in the foam during the expansion so preventing collapse before the liquid phase polymerises.

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