Investigation Of Effects Of Alum And Potassium Sesquicarbonate On The Fire Characteristics Of Flexible Polyurethane Foam

Raw materials used for polyurethane foam polymers [43]

In manufacturing polyurethane polymers, two groups of at least bifunctional substances are needed as reactants; compounds with isocyanate groups and compounds with active hydrogen atoms. The physical and chemical character, structure and molecular size of these compounds influence the polymerization reaction as well as ease of processing and final physical properties of the finished polyurethane. In addition additives such as catalysts, surfactants, blowing agents, cross linkers, flame retardants, light stabilizers and fillers are used to control and modify the reaction process and performance characteristics of the polymer [39]. The raw materials include the following:

Isocyanates

Isocyanates with two or more functional groups are required for the formation of polyurethane polymers. Only the diisocyanates are of interest for polyurethane manufacture and relatively few of these are employed commercially. Volume wise, aromatic isocyanates account for the vast majority of global diisocyanate production. Aliphatic and cycloaliphatic isocyanates are also important building blocks for polyurethane materials but in much smaller volumes. There are a number of reasons for this; first, the aromatically linked isocyanate group is much more reactive than the aliphatic one. Secondly, aromatic isocyanates are more economical to use. Aliphatic isocyanates are used only if special properties are required for the final product. Even within the same class of isocyanates, there is a significant difference in reactivity of the functional group based on steric hindrance. In the case of 2, 4-toluene diisocyanate, the isocyanate group in the para position to the methyl group is much more reactive than the isocyanate group in the ortho position. The most important ones used in elastomer manufacture are the 2, 4- and 2, 6- toluene diisocyanates (TDI), 4,4-dicyclohexylmethane diisocyanates (MDI) and its aliphatic analogue 4, 4- dicyclohexylmethane diisocyanate (H12MDI) xylene diisocyanate (XDI) etc. Some various monoisocyanates used commercially are n - butyl, n - propyl, n - phenyl and 4 - chloro and 3, 4 -dichlorophenyl isocyanates which are used for substituted ureas and carbamates [44].

Isocyanates can be made in many ways using the Curtius, Hoffman and Lossen rearrangements which may involve nitrene as an intermediate but are not satisfactory for large scale operation.

Curtius Reaction

R

NaN3  -N2

COCl RCON3 RCON RNCO

Hoffman Rearrangement NaOBr       -HBr

RCONH2        RCONHBr      RCON RNCO

Lossen rearrangement

R'COOR2 NH2OH R2OH + RCONHOH H2O RCON RNCO

The use of azides in the Curtius reaction is hazardous and the utility of Hoffman and Lossen rearrangement is limited to preparation of isocyanates. An isocyanate takes part in very many reactions but are difficult to prepare in high yield and purity. Aromatic isocyanates are made by phosgenation of the corresponding amines or amine hydrochlorides in an inert medium (o- dichlorobenzene) the reaction proceeding in two stages: first, at room temperature or some what higher temperature to generate the carbamyl chloride and HCl; further treatment with phosgene at temperature of 150 - 170ËšC then forms the isocyanate.

RNH2 COCl2 RNHCOCl + HCl RNH2 RNH2HCl + RNCO
RNH2HCl COCl2 RNCO + 3HCl

Polyols

Polyols are higher molecular weight materials manufactured from an initiator and monomeric building blocks. They are easily classified as polyether and polyester polyols. Polyether polyols contain the repeating ether linkage – R-O-R- and have two or more hydroxyl groups as terminal functional groups. They are manufactured commercially by the catalyzed addition of epoxies (cyclic ethers) to an initiator (active hydrogen containing compounds) such as water, glycols. Polyester polyols are made by the polycondensation of multifunctional carboxylic acids and hydroxyl compounds. They can be further classified according to their end use as flexible or rigid polyols depending on the functionality of the initiator and their molecular weight. Flexible polyols have molecular weights from 2,000 to 10,000 (OH group from 18 to 56) while rigid polyols have molecular weights from 250 to 700 (OH group from 300 to 700).

Polyols for flexible applications use low functionality initiators such as dipropylene glycol (f = 2) or glycerine (f = 3) while polyols for rigid applications use high functionality initiators such as sucrose (f = 8), sorbitol (f = 6) and, mannich bases (f = 4). Graft polyols (also called filled or polymer polyols) contain finely dispersed styrene acrylonitrile or polyurea (PHD) polymer solids chemically grafted to a high molecular weight polyether backbone. They are used to add toughness to microcellular foams and cast elastomers PHD polyols are used to modify the combustion properties of HR flexible foam.

Polyester polyols fall into two distinct categories according to composition and application

Conventional polyester polyols are based on virgin raw materials and manufactured by the direct polyesterification of high purity diacids and glycols such as adipic acid and 1, 4- butanediol. Other polyester polyols are based on reclaimed raw materials and are manufactured by transesterification (glycolysis) of recycled polyethyleneterephthalate (PET). They bring excellent flammability characteristics to polyisocyanurate (PIR) board stock and polyurethane spray foam insulation [44].

1.15.2.1 Polyethers

Polypropylene glycols and poly tetramethylene glycols are the polyethers commonly used in solid polyurethanes. The manufacturing process in both cases involves the addition polymerization of the monomeric epoxide.

2nCH2 (CH2)4CO      + HOROH

HO (CH2)5COO R OOC(CH2)5 OHn

The reaction is rapid and has the advantage that no water is produced as a by product. Low molecular weight polyester with functionalities e.g. f = 2.4 or f =3 have been made of cross linking agent for use with polyurethanes. Polyester based polyurethanes are less expensive and have better oxidative and high temperature stability than polyether based urethanes.