The effects of polyol unsaturation levels on the properties of urethane sealants.

For many decades, production of propylene oxide polyols for the urethane industry has utilized KOH as the catalyst for the propaxylation of polyhydric initiators. Unfortunately, a side reaction occurs wherein abstraction of a methyl hydrogen from propylene oxide results in the formation of the alkoxide, which subsequently competes with the polyhydric initiator for propylene oxide and produces a monofunctional ether moiety. The competing reactions are shown in figure 1.

As the molecular weight of the growing chains increases, the addition of propylene oxide monomer to monol begins to predominate. Unsaturation content increases as a function of molecular weight, and the equivalent weight of the multifunctional species is effectively capped at approximately 2,O00. A 6,200 mw propylene oxide triol synthesized through KOH catalysis (with a small ethylene oxide cap for reactivity) will contain between 40-45% monol content on a molar basis. In addition, the actual functionality of the triol is approximately 2.1.

The presence of this monofunctional species is detrimental in elastomeric polyurethane applications in numerous ways. During polyurethane formation, high molecular weights can not be attained because the monol acts as a polymerization chain stopper. In addition, the unreacted polyol chain end (carbon-carbon double bond) acts as an internal plasticizer. The true functionality of a polyol is decreased from theoretical as the unsaturation content increases. As an example, table 1 shows the trend of decreasing functionality with increasing unsaturation in a 6,200 mw propylene oxide triol synthesized using KOH as catalyst.

Table 1 - functionality vs. unsaturation in
6,200 mw propylene oxide triols
Unsaturation         Functionality    % Monol
(meq KOH/g polyol)                     (molar)
    0.100                2.11            44.4
    0.050                2.43            28.4
    0.020                2.77            11.7

Unsaturation also plays a role in decreasing the weather-ability of polyurethane. It is well known that polymers containing double bonds are highly susceptible to degradation via UV radiation. In polyols, the terminal unsaturation should be an active participant in the propagation of the UV-initiated free radical reactions which lead to crosslinking and chain scission. Evidence for increased crosslinking after UV exposure will be discussed. The results indicate a dependence on unsaturation content.

The intent of our work was to quantify the effect of polyol unsaturation content on the tensile properties of polyurethane sealants and elastomers. In high-extensibility sealants, a triol/diol blend is normally used in order to achieve the required balance of properties. Sealants used in the construction industry must possess high elongations, reasonable tensile strength and low resistance to applied stress (modulus). The triol provides the covalent crosslinking necessary to achieve elastomeric properties. In this work it was found that even a partial reduction (~ 50%) of the unsaturation content of the triol used in sealants results in a significant gain in tensile properties through increased functionality. In elastomers, lowering the unsaturation of the diol improved the physical properties of TDI-based cast elastomers. However, significant impact on tensile properties was seen only when unsaturation was reduced to very low levels. The tensile properties of films cast from the sealant polyol blend show the effect of increased crosslinking associated with reduced unsaturation. And finally, long term UV testing of elastomers reveals that a reduction in the unsaturation content results in an increased retention of elastomeric properties.

Experimental

Sealants

A 6,200 MW, commercially-available, propylene oxide product was used as the triol in the control sample. The normal unsaturation level of this polyol is about 0.10 meq KOH/g polyol. The molar content of monol is between 40-45%. The functionality is approximately 2.1. Two analogs were synthesized to lower levels of unsaturation, 0.05 and 0.02 meq KOH/g polyol respectively. The synthetic routes to the analogs involve a catalysis other than KOH. Low unsaturation, all-PO diols at 4,000 mw were also produced in the same manner. TDI prepolymers were made from 50/50 (equivalents), triol/diol blends. The prepolymers were subsequently used in a sealant formulation which was designed to give maximum extensibility. Prepolymer synthesis steps were as follows:

* 1.0 eq. TDI;

* 0.249 eq. triol;

* 0.249 eq. 2,000 mw diol;

* polyol blend vacuum dried and degassed at 95[degrees]C for two hours

* blend added to TDI, under nitrogen, at 40[degrees]C, temperature raised to 80[degrees]C for 1.5 hours;

* the % NCO was checked through titration to insure completion of the reaction.

The prepolymer was combined with a 1,000 mw diol (chain extender), 25% by weight talc (filler), a tin catalyst, and defoamer to form a sealant. The mixture was cured at 70[degrees]C for four hours, and then was post-cured at ambient temperature, 50% relative humidity for one week before testing.

Elastomers

TDI prepolymers were made from diols using standard techniques. The prepolymers were synthesized to a 6% free NCO content, which was measured by titration. Individual prepolymers were vacuum dried and degassed at 95[degrees]C for two hours, mixed with 5-10% acetone and dibutyltindilaurate (DBTDL) catalyst, placed under vacuum again for five minutes, and cast onto glass plates. The moisture-cured materials were allowed to cure at 50% relative humidity, 23[degrees]C, for two weeks before testing.

Films

In order to further elucidate the effect of unsaturation on functionality, films were cast from the prepolymer blends (50/50, triol/diol) used in the sealant formulation. The prepolymer blends were degassed at room temperature. A small amount of acetone (< 5%) and DBTDL were added, and the mixture was placed under vacuum for an additional five minutes. The films were cast onto glass and cured under the same procedure used with the elastomers.

Results and discussion

Sealants

The sealant formulation used in this work is typical in its basic formulation. Left out were additives normally used in commercial applications such as UV absorbers and pigments. Due to the high molecular weight between crosslinks, the formulation exposes the polyol contribution in providing structural physical properties. The negative effects of unsaturation were revealed in the testing of the urethane sealants. Table 2 shows the tensile properties and hardness of sealants made from triols possessing different unsaturation levels.

[TABULAR DATA 2 OMITTED]

The control sealant, made from a commercial 6,000 mw propylene oxide triol synthesized with KOH, did not cure into a testable material in this formulation. This is understandable considering its functionality (2.1) and unsaturation content (45% molar monol). However, when the unsaturation content of the triol was reduced by 50% (to 0.050, sample 2), the sealant cured and developed reasonable properties. As expected, reducing the unsaturation even further (to 0.020, sample 3) improved the sealant even more. Hardness, tensile strength and modulus increased as unsaturation decreased. High elongations were maintained. A peculiar yielding behavior is observed between 100% and 300% elongation of the sealant made from the 0.020 unsaturation triol. Based on the data, the improvements in physical properties are a direct result of the additional crosslinking provided by the reduced unsaturation of the triols.

The increased functionality of propylene oxide polyols with reduced unsaturation allows for an expansion of current polyurethane capabilities. Polyols can be synthesized to higher molecular weights when methods other than KOH catalysis are used (refs. 2 and 3). In order to test the effects of increased molecular weight in sealants, a 4,000 mw diol with reduced unsaturation was substituted for the 2,000 mw diol in the sealant formulation. It was felt that a reduced unsaturation diol, combined with the reduced unsaturation 6,200 mw triol, would produce a sealant with superior properties. The data in table 3 reveal the relationships between molecular weight, unsaturation content and the tensile properties of high molecular weight sealants.

[TABULAR DATA 3 OMITTED]

The advantages of increased molecular weight are obvious from the samples using the 4,000 mw diol (2 and 4) show improved tensile strength over those using the 2.000 mw diol (1 and 3). Equivalent, or lower, hardness values are also evident in the higher molecular weight samples. Elongations remain high in all samples. The yielding behavior of sample 3 between the 100% and 300% modulus is no longer evident in sample no. 4, which uses the 4,000 mw diol. As expected, the best properties are seen in the sealants with the lowest triol unsaturation. It is apparent that as triol functionality is increased, tensile properties improve. It is important to note, however, that partial reduction of triol unsaturation to 0.050 (meq KOH/g polyol) gives a cured sealant with reasonable tensile properties, while the control sample, with a 0.100 unsaturation content, does not possess enough functionality to be used in this formulation.

Elastomers

Reducing the unsaturation of high molecular weight, propylene oxide diols allows for the production of soft cast elastomers with improved physical properties. Currently, polytetramethylene oxide (PTMEG) polyols are utilized in polyether-based urethane elastomers with hardness values below 80 Shore A. KOH-synthesized propylene oxide diols with equivalent weights greater than 1,000 give very poor properties in elastomers because of a lack of functionality. Such a commercial product would have a functionality of less than 1.7. This value renders the diol useless in traditional elastomers, where functionality must be greater than 1.9 in order to achieve the molecular weight necessary for good physical properties. However, propylene oxide polyols have a significant cost advantage over PTMEG polyols. It is therefore natural to assume that they would be utilized commercially to a greater extent if their functionality was increased (ie., through reduced unsaturation) to an appropriate extent.

It is now known that reducing the unsaturation of propylene oxide diols to very low levels (~ 0.02) enables them to deliver surprisingly good physical properties in low-hardness elastomers. In fact, studies reveal that elastomers made from such diols show equal or better physical properties than similar PTMEG-based elastomers (ref. 4). In our study, propylene oxide diols of 4,000 mw were produced at different levels of unsaturation. Table 4 shows the effect of unsaturation on TDI-based (6% NCO), moisture-cured cast elastomers made from the diols.

[TABULAR DATA 4 OMITTED]

The data show that the physical properties improve only slightly when the unsaturation of sample 2 is reduced by 50% to 0.050 (sample 3). It is obvious that a functionality increase from 1.67 to 1.82 does not build sufficient molecular weight. However, when the diol functionality exceeds 1.9 (sample 4), properties rise dramatically. and approach those of the 2,000 mw diol. In fact, the 100% modulus and the tear strength are significantly higher in sample 4 than in sample 1. Elongation, tear strength and tensile strength increase in samples 2, 3 and 4 as unsaturation is decreased. The data show that in elastomers, where the diol is typically the sole polyol component, the unsaturation content must be reduced to very low levels in order for the materials to be useful. It is also apparent from the data that low unsaturation propylene oxide diols of >2,000 mw can be used in polyurethane with good results.

Films

Moisture-cured films were made from the TDI prepolymer blends used to produce the sealants in table 2. The diol/triol ratio of the blends is 50/50 by equivalents. Tensile testing of the cured films was done to examine the effects of increasing triol functionality. The data are shown in table 5.

[TABULAR DATA 5 OMITTED]

The physical properties reveal the effect of changing crosslink density within the material. The elongation is the highest for sample 1, the least crosslinked material, and declines in samples 2 and 3 as crosslinking increases. Tensile strength goes through a maximum, and then declines with increasing crosslinking. This trend is consistent with elastomer theory.

Environmental resistance

To date, little has been published on the effect of polyol unsaturation on the resistance to UV radiation of polyurethane products like sealants. Most of the chemical literature has focused on polymers with internal double bonds like polybutadiene and SBR. In such materials, UV-initiated free radical mechanisms ultimately lead to chain scission and crosslinking (ref. 5). In the case of polyols used in polyurethane, it would seem logical that the terminal double bonds would participate in the propagation of free radical reactions. To test this idea, we subjected two cast elastomers with different unsaturation content to 1,250 hours of exposure to UV radiation in a QUV testing apparatus. The test alternates a four hour exposure to UV radiation with a four hour condensation cycle. Water immersion tests (30 days, 70[degrees]C) were run concurrently in order to eliminate the effects of hydrolytic degradation. Both samples showed a roughly equivalent loss of properties in the water immersion test. On the other hand, the QUV test revealed major differences in property retention. Table 3 lists the results of tensile testing done before and after W exposure.

The data are quite clear in showing the relationship between unsaturation content and polymer degradation in the form of free radical-induced crosslinking. The crosslink density of sample 1 has increased to the point where 72% of the original elongation is lost. Sample 2 shows only a 40% loss of original elongation, and still retains reasonable elasticity with 483% elongation-until-break. Sample I became noticeably stiffer than sample 2, and its tear strength was almost completely lost. The data suggest the role unsaturation plays in weatherability. The conclusion can be drawn that polyols with reduced unsaturation will improve material durability in applications requiring intense exposure to W radiation .

Conclusion

The reduction of unsaturation in polyols used in the production of elastomeric urethane materials provides large benefits in the form of improved physical properties and environmental resistance. Diols must possess very low levels of unsaturation to be of practical benefit. In triols, even a partial reduction (~ 50%) in unsaturation results in noticeable gains in the properties of sealants. Reduction of triol unsaturation to very low levels provides even further gains. The combination of high molecular weight, low unsaturation triols and diols gives tough, highly extensible materials which perhaps will be competitive with silicones in the 50% movement capability sealant market. Commercialization of propylene oxide-based diols with very low unsaturation will provide a new tool for formulators. Formulators will be able to incorporate a higher molecular weight, soft segment than was previously possible in elastomers. The weatherability of urethane products will benefit from the increased retention of elastomeric properties provided by polyols with lower unsaturation content.

Table 6 - change in elastomer(*) elongation with
exposure to UV radiation
Unsaturation   Original     After 1.250   % loss
               elongation   hrs. UV       elongation
1.0.050           573          163           72%
2.2.015           790          483           40%
(*)4,000 mw propylene oxide diol, TDI, 6% NCO, moisture cure.

References

(1.) H. Kaczmarek, Polymer Bulletin, 34, 211-218 ( 1995). (2.) U.S. Patent 4,637,851. (3.) U.S. Patent 4,985,491. (4.) U.S. Patent 5.340,902 (5.) J.F. Rabek, "Photostabilization of polymers," Elsevier, Ch. 1 ( 1990).

Acknowledgements

"The effect of polyol unsaturation levels on the properties of urethane sealants" is based on apaper presented at SPI's Polyurethane 1995.

"Anew era for MDI: flexible PU slabstock foam" is based on apaper presented at SPI's Polyurethane 1995.

"Crosslinking structure and properties of poly(urethane-urea) elastomers" is based on apaper presented at the Rubber Division's October 1995 meeting.