PTMEGs with a broader MWD have higher fractions of both low-MW diol molecules and high-MW chains compared with narrow MWD PPG polyols. The short chain diol fraction in PTMEG can become part of the hard-block segment thus increasing the concentration of the hard phase and significantly increasing the modulus and hardness of the elastomer. An increase in the hard phase volume generates additional restraints on the mobility of the soft segments, thereby reducing the rate and extent of cold and stress-induced crystallization of the PTMEG. Using PTMEG 1000 (vs. PTMEG 2000) would result in a higher fraction of the low-MW diol species contributing to the hard segment and thus have higher impact on suppressing blend crystallization.
The high molecular weight fraction associated with a broad MWD leads to an increase in the resiliency and rebound properties and an improvement in phase separation within the PUR elastomer. Accordingly, the addition of PTMEG to a PPG polyol enhances hardness, tensile properties and the resiliency and rebound properties of the associated PUR elastomer. Addition of PPG polyols into a blend results in lower Durometer, softer polyurethane elastomers. PPG polyols result in a dilution in the low-MW diol fraction of the PTMEG polyol with a reduction in the hard-block concentration.
Polyether Polyol Blends in Adhesive Applications
The balance of tack, adhesion and cohesion forces are core elements in the design of adhesives. In Pressure Sensitive Adhesives (PSAs), tack, shear strength and peel strength are additional controlling properties. While the selection of polyol employed and hard block-soft block ratio are used in optimizing the adhesive properties of a TPU, blending PTMEG with PPG polyols provides another dimension to the design of adhesives.
The principles laid-out above help to guide the selection of polyol blend ratios in optimizing the required adhesive properties. For the reasons cited above, PTMEG, which is crystallizable, show greater adhesion and cohesion characteristics in a PUR. PPG polyols enhance tack and adhesion to hydrophobic surfaces and improve handling characteristics like viscosity. Altering the ratio of PTMEG and PPG polyols in blends allows an optimization of adhesive properties to meet various performance criteria. In one study, excellent combinations of tack, shear, and peel properties were obtained in a PSA produced from a 50:50 blend of PTMEG and PPG polyols. 2
Polyol Blends in Cast PUR Elastomers
In one shot cast polyurethane elastomers, reactivity, and cure rates/demold times in a polyol/MDI/BDO system are much faster with a PTMEG polyol vs. a PPG polyol. This reflects the 100% primary hydroxy di-functionality in a PTMEG. PPG polyols made with a conventional KOH catalyst contain 3-4 mole percent of mono-hydroxy concentrations (the other terminus being allyl unsaturation) and lower reactivity secondary hydroxyl functionality. Virtually all final elastomer properties improve with the addition of PTMEG in a blend with a PPG polyol. This includes hardness, tensile and tear strengths, resiliency and rebound properties, abrasion, tensile and compression set. There are multiple factors associated with these improvements: 1) the overall superior mechanical properties of the linear PTMEG structure, 2) stress-induced crystallization at high levels of PTMEG, 3) fewer chain ends due to the high 2.0 functionality of the PTMEG diol, and 4) an increase in the overall polymer MW achievable. For these reasons, PPG polyols produced by the KOH process are more often used in polyurethane foam applications and less so in cast and TPU elastomers. Double Metal Cyanide (DMC) catalysts produce PPG polyols with high difunctionality and low mono-unsaturation contents. DMC catalysts are used for high MW and elastomer grade PPG polyols.
Studies have reported that as little as 10-20 weight percent of a PPG polyol in PTMEG begins to affect the stress crystallization of PTMEG polyols, with an associate increase in ultimate elongations.3
Polyol Blends in TPUs and Prepolymer Cast Elastomers
In prepolymers and TPUs, the low viscosity of the PPG polyols can offer processing advantages. With regard to the mechanical properties, the same trends as discussed for cast elastomers apply to prepolymer based systems and TPUs. The overall magnitude of the property differences can be bridged using blends of PTMEG and PPG polyols in elastomeric polyurethanes.
Polyurethane Elastomer Summary
Blending of PTMEG and PPG polyols allow optimizing of properties and costs in polyether polyurethane formulations. Addition of PTMEG to PPG polyols enhances tensile, tear and rebound/resilience properties, dynamic properties, and adhesion-cohesion in adhesives. The PPG polyols can improve tack, "cold-hardening" characteristics, softness, and processing in PTMEG elastomers, while reducing raw material costs.
These polyol blends can create value in polyurethane application such as adhesives, cast polyurethane elastomers, TPUs and polyurethane dispersion coatings.