Polyurethane Properties: Tailoring PUR Hard Block Segments

Polyurethanes are a special class of segmented block copolymers, consisting of alternating sequences of soft and hard segments. The soft segment is generally based on polyether or polyester polyols with glass transitions (Tg) well below room temperature, while the hard segment is composed of a diisocyanate and chain extender. The hard segment is often crystalline.

Polyurethane Properties: Soft and Hard Segments

Chain extenders are functional, low molecular weight compounds.  The chemistry of the hard and soft segments acts synergistically to produce a wide range of properties such as elasticity, hardness, tensile and flex strength, resiliency, and durability, etc.  The soft segment provides elasticity, toughness, and resiliency, while the hard segment contributes strength, hardness, and elevated temperature performance.

Tailoring Polyurethane Properties through Hard Block Variations

The chemical structures of the hard and soft segments, as well as their ratios and chain lengths, have a substantial effect on overall polyurethane properties.  With regard to the hard block characteristics, the isocyanate of choice and the chain extender strongly influence the end mechanical, physical, thermal, and processing properties of a polyurethane, through their ability to drive hard block phase separation and to promote crystallization and intermolecular hard segment bonding.

While many combinations of diisocyanates, diols, and diamines have been explored in hard block segments, methylene diisocyanate (MDI) is the most-used diisocyanate, while 1,4-butanediol (BDO) remains as the most common chain extender.   Diamines are primarily used with the lower reactivity toluene diisocyanate (TDI)-based polyurethane elastomers.  In this article, we will  examine how the structural characteristics of the hard block segment can be used to tailor the properties of a polyurethane elastomer.  Gantrade markets many of the urethane intermediates to be discussed here.

MDI/Diol Hard Block Chemistry

The first criterion for an effective hard block segment is linearity; the second criterion is that hard block structure ideally contains an even number of carbon atoms (number of CH₂ groups) in the diol segment.  Molecular level chain-packing of the individual hard segments of polyurethanes benefits the overall performance properties of the resulting elastomers.    

BDO Chain Extender

As a chain extender, 1,4-butanediol (MW: 90.12, EW: 45.06, Hydroxyl Value: 1245 mg KOH/g.) is a versatile liquid diol intermediate with reactive primary hydroxyl functionality and a linear structure that lends itself to formulating polyurethanes with a good balance of hardness, high strength and durability, and thermal stability. BDO yields crystalline urethane hard segment domains with MDI that efficiently microphase separately, to afford tough elastomeric networks.  In thermoplastic polyurethane (TPU) compositions, uncrosslinked BDO-MDI domains can be melted, since they willflow at elevated temperatures to allow thermoplastic processing.   The associated networks re-form when cooled, to yield tough elastomeric compositions. 

Particularly outstanding is the high degree of crystallinity and melt enthalpy (heat required to melt the crystalline hard segment) in MDI-BDO hard segments of a polyurethane.  Applications include a vast array of performance-cast urethane parts like wheels, rollers, belts, recreational equipment, pipe linings and pipeline products, pump liners and parts, shaker screens, and hydro-cyclones.  Other applications include high-performance adhesives, sealants and coatings, as well as RIM and TPU parts, footwear, appliance parts, and automotive components.  Polyurethanes based on MDI/BDO systems perform well in aggressive environments, such as exposures in mining, mineral, and oil field operations and in medical devices 

In the table below, we compare BDO as a chain extender to other conventional diols.  The melting enthalpy of the MDI/BDO segment is noteworthy.

The mechanical properties of a representative polyurethane based on a PTMEG 2000/MDI prepolymers and BDO as the chain extender are referenced below.  The table shows the effect of increased hard block content (higher NCO percentage) on properties. The tensile properties, tear strength, and hardness all increase with increasing hard block concentrations.

The influence of the molecular weight of the PTMEG polyol and the weight ratio of the PTMEG soft segment and MDI/BDO hard segment in polyurethanes have been assessed.  As the hard-to-soft block ratio increases in a polyurethane, the Shore A hardness increases as does the tensile modulus, tensile strength, tear strength and compression set.  The elongation at break and hydrolysis-resistance increases with an increase in the PTMEG content.

In a systematic study, E. Pechhold and G. Pruckmayr observed the following effects of the molecular weight of the PTMEG and the soft segment content on the physical-mechanical properties of PTMEG/MDI/BDO polyurethane cast elastomers:                                               

• The degree of phase separation of the soft from the hard segment increases as the molecular weight of the PTMEG polyol increases. The better phase separation gives improved resilience and hysteresis properties, as well as better low temperature ductility.
• The Tg of PTMEG/MDI/BDO elastomers decreases as the molecular weight of the PTMEG increases from PTMEG 650 to PTMEG 2000.  Phase separation efficiency improves with higher molecular weights of the PTMEG.  This dynamic is due to reduced hard block miscibility in the soft segment at the higher PTMEG molecular weights.
• The Tg of the soft block is dependent on the soft-to-hard block ratio.  Particularly at soft segment contents of 50 percent or less, the lower molecular weight grades show less efficient segment phase separations, resulting in higher Tgs.  We have provided data in the figure below, where the Clash-Berg temperature is used as a surrogate for the Tg.

• Bashore rebound/resilience increases with the weight percent of PTMEG in the PUR, and to a lesser extent, with the PTMEG molecular weight.
• The hydrolytic stability of PTMEG/MDI/BDO elastomers was notably better with higher molecular weight PTMEG grades, and when the amount of the hard segment concentration was highest.  {See E. Pechhold and G. Pruchmayr, Rubber Chemistry and Technology, 55, 76 (1982)}

MPO Chain Extender

2-Methyl-1,3-propanediol (MPO) is an alternative to BDO in MDI, especially in systems where lower durometer polyurethane elastomers are of interest.  MPO has a methyl-branched structure and an odd-number (3) of carbon atoms between the hydroxy end-groups.  The structures of MPO and BDO are compared below:

MPO demonstrates a unique performance at low temperatures and excellent hydrolytic stability.  As a monomer, it is less hygroscopic with a very low freezing point at -54℃.  A comparison with BDO as a chain extender is provided in the following chart. Note the Durometers, rebound, tensile, and tear characteristics of both chain extenders.  MPO can help to  both reduce costs and improve productivity. Of note are the Shore A Durometer values in the low 60s range.

For softer polyurethane applications, such as fabric coatings, paddings, and spray coatings, MPO displays versatility as a substitute for BDO.

HQEE & HER Chain Extenders

Compared to BDO as a chain extender, MDI-polyurethane elastomers cured with HQEE exhibit improved performance at elevated temperatures, higher hardness, increased tear strengths, and higher resilience. The MDI/HQEE systems replicated many of the performance attributes of MOCA/TDI systems, with the HQEE/MDI elastomers exhibiting better high temperature and moisture resistance properties. For this reason, MDI/HQEE systems are often recommended as alternatives to the TDI /MOCA systems.

However, there are difficulties associated with processing HQEE, due to its high melting point (+102 °C/230 °F), as well as the fact that HQEE does not super-cool (HQEE will quickly crystallize below its melting point).  Accordingly, in processing HQEE, the melting tank and all process lines must be heated to 110°C and insulated to prevent cold spots. The prepolymer should be preheated to around 100 °C before mixing with the HQEE. Otherwise, the HQEE will crystallize in the mix, causing “starring” in the cured urethane elastomer.  The temperature of the mold should also be above 110°C.

An alternative to HQEE is the structurally similar aromatic diol, resorcinol bis(2-hydroxyethyl) ether (HER).  HER exhibits a lower melting point (89 °C) and super-cools, which improves overall handling and processing.  These characteristics of HER afford significant processing advantages, including a wider processing temperature window, which facilitates casting, coating, and molding operations.

The structural similarities of HER versus HQEE are revealed in the chemical structures shown below:

Comparative performance studies in MDI systems have shown that HER and HQEE produce MDI polyurethane elastomers with similar properties. The data below compares the basic physical properties of HER versus HQEE in MDI elastomers. The data shows that HER and HQEE extended MDI elastomers have comparable properties in a polyester-based MDI elastomer (also polyether systems).

Applications for MDI/HQEE or MDI-HER elastomers include industrial wheels and tires, sport and amusement park wheels, pipe linings and coatings, rollers, industrial roll covers, gaskets, seals, and other high-performance end-uses.

Ortho-Rich MDI in Hard Block Segment

When referencing monomeric MDI as the isocyanate, formulators generally mean the 4,4’-isomer. 4,4’-MDI is a symmetrical, linear diisocyanate with the two identical NCO groups exhibiting equal reactivities with diol/polyols.  A commercially available isomer of 4,4’-MDI is 2.4’-MDI or ortho, para-MDI, offered in mixtures up to around 50% 2,4’-isomer or greater.  These grades are called ortho-rich MDI or MDI-50 (Cosmonate® PI, Lupranate® MI, Suprasec® 3050).  The structure of the 2,4’-isomer is asymmetric and the NCO groups are dissimilar, with the 2-isomer exhibiting lower reactivity for steric reasons.  These structural differences are shown in the images below:

MDIs with two differently reactive isocyanate groups are advantageous in making prepolymers with low degrees of chain extension, and therefore low prepolymer viscosities.  Further, ortho-rich monomeric MDI can be stored and handled as a liquid at room temperature.  The asymmetrical characteristic of ortho-rich MDI allows production of semi-amorphous polyurethane elastomers using BDO as a chain extender, with low levels of crystallinity and chain packing, very slow crystallization kinetics, low hardness-greater softness values, low compression set, and high resiliency.  An ortho-rich polyester prepolymer chain-extended with BDO exhibited lower Shore A hardness values and significantly higher tear strengths with low compression set values.

Major applications for ortho-rich MDIs are in laminating and construction adhesives, sealants, coatings, flexible molded high resiliency foam, sports flooring, and cast elastomers.

Chain Extension with TMP in BDO

Trimethylolpropane (TMP) can be included in the diol composition of a chain extender to function as a crosslinker.  The general level of TMP in a butanediol mixture is six percent or lower.  At low levels of TMP, tensile strength, hardness, tear, abrasion, elongation, resiliency, and other properties improve.  At higher levels, a reduction in tensile properties and elongation can be observed, but resiliency, compression set, and abrasion properties can continue to improve. 

In addition to TMP, other crosslinking triols include polycaprolactone triols (Placcel®303 and 305) and propoxylated TMP triols.

Conclusion

The choice of isocyanate and the chain extender strongly influence the resulting mechanical, physical, thermal and processing properties of a polyurethane, through the resulting hard-block characteristics, phase separation, crystallinity, and intermolecular hard-segment bonding. While many combinations of diisocyanates, diols, and diamines have been used commercially, hard block segments based on methylene diisocyanate (MDI) and 1,4-butanediol (BDO) represent one of the most versatile systems.  Understanding the chemical variables associated with this class of hard block segments allows tailoring of the properties of the resulting polyurethane elastomers.  Variables include selection of the specific MDI type, the chain extender composition, the ratios of hard-block to soft-block segments and the segment lengths.

To learn more about tailoring your polyurethane properties to specific market applications, contact the team at Gantrade today.