4 Polyether Polyols for Elastic Polyurethanes
OCH 2 CHOH
CH2
O O
O
C C
+
CH2 CH2
(4.71)
80 70 60 50 40 30 20 10 0
1
0 2 3 4 5 6 7 8 9 10 EO (%)
Primary hydroxyl (%)
HBF4
KOH
11121314151617
Figure 4.27 The effect of the catalyst nature on the primary hydroxyl content
It is very interesting that by using alkaline-earth catalysts in the ethoxylation reaction (Ca, Sr or Ba alcoholates or carboxylates), a narrower distribution of EO sequences
per hydroxyl group resulted, compared to use of alkaline catalysts. For example, with barium alcoholate as catalyst approximately 80–85% primary hydroxyl, at 15% EO as terminal block, are obtained with polyether triols (MW of 5,000 Da), compared to 65–75% primary hydroxyl obtained in the presence of KOH. The explanation of this behavior is the occurrence of a template effect, by the complexation of the potassium cation with the poly[EO] chains formed:
+ n O
O O-
K+
O-K+ O
O (4.72)
The potassium cation is retained, by complexation, at the same chain end, the alcohol-alcoholate equilibrium is perturbed, and EO reacts preferentially with this template structure and the resulting primary hydroxyl content decreases. In the case of bivalent cations with two positive charges, the alcoholate anion is linked more strongly by electrostatic forces and the coordination of the cation with the formed poly[EO] chains takes place to a much smaller extent.
In conclusion, the ethoxylation catalyst nature has an important influence on the primary hydroxyl content. A higher primary hydroxyl percentage than in the classical reaction catalysed by KOH is obtained by the ethoxylation of the intermediate polyether polyols in acidic catalysis or with alkaline-earth alkoxides or carboxylates [25–29].
4.1.4.1.5 Removing Propylene Oxide before the Ethoxylation Reaction The last step in the synthesis of the intermediary propoxylated polyether, before ethoxylation, is the degassing step, the elimination of the unreacted PO by vacuum distillation. It was observed experimentally that if the PO is not efficiently removed in the degassing step, the resulting primary hydroxyl content is lower. The explanation is very simple: EO is much more reactive than PO and reacts first. After the addition of the majority of EO, the remaining PO (the less reactive monomer), reacts at the end of chain, transforming part of the primary hydroxyls into secondary hydroxyls.
In conclusion, in order to obtain high percentage primary hydroxyls, it is necessary to remove very efficiently the remaining PO after the synthesis of the intermediate
Polyether Polyols for Elastic Polyurethanes The polyether diols, block copolymers of PO–EO with terminal poly[EO] block are obtained absolutely identically to the previously described EO capped polyether triols, the difference being that the propoxylated intermediate is a propoxylated polyether diol.
The most important polyether, PO–EO block copolymer structures, having terminal poly[EO] block (structure a) and internal poly[EO] block (structures b and c), are presented in Figure 4.28.
CH2
CH2
CH2
CH2
CH2
CH2
a)
b)
c)
OH
CH OH
OH OH OH OH OH
CH OH
OH EO n
CH
= = PO n
Figure 4.28 The structures of polyether triol block copolymers PO–EO: a) terminal poly[EO)] block; b) poly[EO] block linked to the starter; and c) internal poly[EO]
block
The most important polyether triol, PO–EO block copolymers with poly[EO] block, used in practice, are:
a) Polyether triols, based on glycerol, PO and EO (terminal block) with a MW of 3,000 Da (Table 4.10);
b) Polyether triols, based on glycerol, PO and EO (terminal block) with a MW of 5,000 Da (Table 4.11);
c) Polyether triols, based on glycerol, PO and EO (terminal block) with a MW of 6,000 Da (Table 4.12); and
d) Polyether triols, based on glycerol, PO and EO (internal block) with a MW of 3,600 Da (Table 4.13).
Table 4.10 Characteristics of polyether triol, based on glycerol PO–EO block copolymers (terminal block) with a MW of 3,000 Da
Characteristic Unit Value
Aspect – Viscous liquid
MW daltons 3,000
Functionality Hydroxyl groups/mol 3
EO % 5
Primary hydroxyl % 30–35
OH# mg KOH/g 53–59
Viscosity (25 °C) mPa.s 400–550
Unsaturation mequiv/g 0.035–0.04
Acidity mg KOH/g Max. 0.05–0.1
Water content % Max. 0.05–0.1
Na + K ppm Max. 5
Colour APHA Max. 30–50
Application: Hot moulded flexible PU foams for car seating
The polyether triols (PO–EO block copolymers with terminal poly[EO] block) are very reactive polyols due to the presence of a high percentage of primary hydroxyls.
These polyether polyols with terminal poly[EO] block, are used preferentially for
Polyether Polyols for Elastic Polyurethanes
Table 4.11 Characteristics of polyether triol, based on glycerol PO–EO block copolymers (terminal block) with a MW of 4,700–5,000 Da
Characteristic Unit Value
Aspect – Viscous liquid
MW daltons 4,700–5,000
Functionality Hydroxyl groups/mol 3
EO % 13–15
Primary hydroxyl % 65–75
OH# mg KOH/g 33–39
Viscosity (25 °C) mPa.s 750–1,000
Unsaturation mequiv/g 0.06–0.065
Acidity mg KOH/g Max. 0.05–0.1
Water content % Max. 0.05–0.1
Na + K ppm Max. 5
Colour APHA Max. 30–50
Applications: Cold moulded high resilience flexible PU foams for car seats, semiflexible and integral skin PU foams, high resilience slabstock flexible foams
Table 4.12 Characteristics of polyether triol, based copolymers (terminal block) with a MW of on glycerol PO–EO block 6,000–6,500 Da
Characteristic Unit Value
Aspect – Viscous liquid
MW daltons 6,000–6,500
Functionality Hydroxyl groups/mol 3
EO % 13–15
Primary hydroxyl % 75–85
OH# mg KOH/g 27–29
Viscosity (25 °C) mPa.s 1,000–1,200
Unsaturation mequiv/g 0.08–0.09
Acidity mg KOH/g Max. 0.05–0.1
Water content % Max. 0.05–0.1
Na + K – –
Colour APHA Max. 30–50
Applications: cold moulded high resilience flexible PU foams, semiflexible and integral skin PU foams, microcellular elastomers (shoe soles)
Table 4.13 Characteristics of polyether triol, based on glycerol PO–EO block copolymers (internal poly[EO] block) with a MW of 3,400–3,600 Da
Characteristic Unit Value
Aspect – Viscous liquid
MW daltons 3,400–3,600
Functionality Hydroxyl groups/mol 3
EO % 10–12
Secondary hydroxyl % 94–96
OH# mg KOH/g 450–650
Viscosity (25 °C) mPa.s 500–650
Unsaturation mequiv/g 0.045–0.055
Acidity mg KOH/g Max. 0.05–0.1
Water content % Max. 0.05–0.1
Na + K ppm Max. 5
Colour APHA Max. 30–50
Application: Continuous slabstock flexible PU foams
The polyols with internal poly[EO] block, used sometimes in slabstock flexible PU foams, give flexible PU foams with lower compression strength than the random copolyether polyols PO–EO, having the same MW and EO content. The poly[EO]
block has a softening effect due to the solvation of urea and urethane bonds. These polyethers with internal poly[EO] units are called ‘internally activated’ polyether polyols. In spite of the presence of terminal low reactivity, secondary hydroxyl bonds (94–96%), a reactivity increase takes place, explained by the catalytic effect of some urethane and urea bonds which are partially solubilised by the poly[EO] block. These urethane and urea groups have a well known catalytic effect on the reaction between isocyanate groups and hydroxyl groups, leading to a self-acceleration effect. As an immediate consequence, the polyether polyols with internal poly[EO] block are more reactive in the foaming process than the random copolyethers of PO–EO of the same MW and EO content.
As an example, a group of polyether diols (block PO–EO copolymers with terminal poly[EO] block) are the polyethers derived from PG (or DPG), PO and EO of MW of 2,000 Da and approximately 15–20% EO as a terminal block (Figure 4.29).
Polyether Polyols for Elastic Polyurethanes
Poly[EO] Poly[PO] Poly[EO]
HO OH
Figure 4.29 General structure of polyether diols block copolymers PO–EO
In Sections 4.1 and 4.1.1–4.1.4, the chemistry of polyether polyol synthesis, the mechanism and kinetics of alkylene oxide polyaddition to hydroxyl groups and the most important structures of polyalkylene oxide polyether polyols for elastic PU–PO homopolymers, random PO–EO copolymers and PO–EO block copolymers – were discussed.
Tables 4.9–4.14 show some general characteristics of polyether polyol PO–EO block copolymers, such as MW, OH#, functionality, viscosity and colour, but also some other characteristics such as unsaturation, EO content, and potassium and sodium content which are specific for polyether polyols.
Table 4.14 Characteristics of polyether diols, block copolymer PO–EO, with terminal poly[EO] block, with a MW of 2,000 Da
Characteristic Unit Value
Aspect – Viscous liquid
MW daltons 2,000
Functionality Hydroxyl groups/mol 2
EO % 18–20
Primary hydroxyl % 65–70
OH# mg KOH/g 53–59
Viscosity (25 °C) mPa.s 400–550
Unsaturation mequiv/g 0.03–0.04
Acidity mg KOH/g Max. 0.05–0.1
Water content % Max. 0.05–0.1
Na + K ppm Max. 5
Colour APHA Max. 30–50
Application: PU elastomers, microcellular elastomers (shoe soles)
Unsaturation (standard test methods American Society for Testing and Materials, ASTM D4671 [111] and International Organization for Standardization, ISO 17710 [112] represents the amount of terminal double-bonds in polyether polyols.
One usual method is chemical determination of the double-bond content based on the quantitative reaction of mercuric acetate with double-bonds in methanol. The reactions involved are:
C + +
C Hg(CH3COO)2 CH3OH Hg OCOCH3
OCH3
C +
+ +
C CH3COOH
CH3COOH NaOH CH3COOK H2O (4.73)
It is observed that 1 double-bond generates 1 mol of acetic acid which is neutralised with an equivalent quantity of NaOH of known concentration. This stoichiometry permits an easier calculation of the double-bond content.
The unsaturation is expressed in milliequivalents of double-bonds per 1 g of polyether (mequiv/g). Recent methods for determination of terminal unsaturation in polyether polyols are based on 1H-NMR and 13C-NMR spectroscopic methods [88].
In the 1H-NMR spectra of polyethers, the protons linked to the carbon atoms of allyl and propenyl double-bonds have specific chemical shifts:
CH2 CH* CH2O 5.70