Investigation Of Effects Of Two Flame Retardants On The Fire Characterisit Ics Of Flexible Poly Ether Foam

Table 2 lists the components which are commonly involved in a formulation and gives a typical range of quantities for each component utilized. As can be seen from the table, the quantities of all components listed are based on the amount of polyol utilized in the formulation. For example, water is typically used in the range of 1.5 – 7.5 parts per hundred polyol (pphp). However, the isocyanate a dded to the formulation is usually reported by an index number. An isocyanate index of 100 indicates that there is a stoichiometric amount of isocyanate added to react with functional groups from the polyol, water, and cross – linkers added in the formulat ion. In the following subsections, each type of component will be discussed in detail.
2.7.6.1    Isocyanates
The two most common sources of isocyanate functionalities in foam production come from toluene diisocyanate (TDI) and diphenylmethane diisocyanate ( MDI), of which the former is more commonly used in North America, where as the latter one has a greater market demand in European countries [59]. TDI exists in two isomeric forms, as shown below both of which are used in foam production. The two isomers differ mainly in two ways. Firstly, as indicated, the relative reaction rates of the different isocyanate groups on each molecule differ considerably [60].

The relativity of the ortho position in the 2,4 isomer is approximately 12% of the relativity of the para position due to the steric hindrance caused by the methyl group. However, when the reaction temperature approaches 100°C, steric hindrance effects are overcome and both the position reacts at nearly the same rate. In comparison, the NCO groups on 2,6 TDI have equal reactivities though the reactivity of the second isocyanate group dro ps by a factors around 3 after the first group reactions. The second way in which the two isomers differ is that the 2,6 isomer is symmetric as compared to the 2,4 isomer and therefore is expected    to    form    hard    segments    with    better    packing characteristics [ 61,62].
2.7.6.2    Polyols
The        soft        phase    of    polyurethane    foam    is    usually        a polyfunctional alcohol or polyol phase which on reacting with isocyanate groups covalently bonds with urea hard segments    through        urethane    linkages.        Glycols        such    as ethylene glycol, 1,4 – butanediol, and 1,6-hexanediol are relatively much lower in molecular weight as compared to the polyols used in flexible foam production. These are more commonly used for chain extension to form hard segments (in polyurethane elastomers) and therefore will be referred to as    ‘chain            extender’.        Polyols    used        for        flexible    foam formulations are higher molecular weight ( C. 3000 to 6000 g/mol) and have average functionalities in the range of 2.5 – 3 [57]. Polymerization processes allow production of a wide range of polyols, differing in molecular weight, functionality, reactivity, and chain structure [58]. Selecting the right polyol is an important issue, and the choice is governed by the desired foam properties and economics.
The first polyether polyol which was sol d for the production of flexible polyurethane foams was polyoxytetramethylene glycol [58]. Although the use of this polyether polyol resulted in good overall foam properties, extensive use of the same was restricted due to the high costs involved. At prese nt, there are two kinds of polyols commercially available for flexible foam production, hydroxyl terminated polyethers and hydroxyl terminated polyesters. The polyether polyols are produced by ring opening propoxylation or ethoxylation onto a variety of starting materials called initiators. Around ninety percent of the flexible polyurethane foam market utilizes polyether polyols based on propylene oxide in comparison to polyester polyols, because of their lower cost, better hydrolysis resistance, and greate r ease in handling [57]. Also, polyurethane foams, due to their low density cellular structure, expose a large surface area to the atmosphere. This further makes polyether polyols advantageous over polyester polyol due to the known greater
hydrolytic stability of the polyether backbone. Finally, polyether based flexible foams contribute lower Tg values, are softer and more resilient, making then suitable candidates for bedding and seating applications [57].

The common polyether polyols used in flexible foam production utilize ethylene oxide (EO) and propylene oxide (PO) as the repeat units. The polyols produced are typically random hetero-copolymers of EO and PO, though in some cases where high reactivity of the polyol is required, the polyol is EO end -capped. This is because primary hydroxyl groups are approximately three times more reactive towards isocyanates as compared to secondary hydro xyl groups [57]. The reason behind producing polyols utilizing both EO and PO monomers is argued as follows. Though , polyols based solely on PO have relatively low reactivities, they are superior as compared to all -EO based polyols in terms of possessing lower water absorption. On the other hand, EO based polyols become important where water solubility is required. Thus by making polyols incorporating both repeat units, the resultant polyol gives a balance of required properties, i.e., lower water swelling is obtained due to the PO repeat units in the backbone, where as the EO repeat units provide good mixing of the water, isocyanate, and the polyol. In addition, if end -capped with the primary EO groups, the polyol has a high reactivity which is importance for production of high resiliency (HR) foams.