PPG polyols

The PPG polyols are also available with ethylene oxide capping, which pro-... 

Most moisture-curing liquid adhesives utilize poly(oxypropylene) (PPG) polyols, as shown above. These raw materials produce among the lowest-viscosity prepolymers but may not have sufficient modulus at higher temperatures for some applications. A certain percentage of polyester polyols may also be utilized to boost performance, but these may cause a large increase in viscosity, and so they are more often used in conjunction with polyether polyols to provide a high-performance adhesive with workable viscosities. Poly(butadiene) polyols may be utilized for specific adhesion characteristics. 

A study was done measuring the thermal oxidative stability of polyurethanes made from PPG polyols, varying the isocyanate curative. Oxygen absorption was... 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

When the diisocyanate and polyol are being reacted together, the diisocyanate is kept in excess as long as possible. The overall conditions are kept slightly acidic to reduce side reactions taking place. The acidity is a must when there is more polyol than diisocyanate present. The older-style PPG polyols may contain traces of potassium hydroxide (KOH). The KOH is a very active alkaline. Even small traces have a strong effect. 

This paper describes work which we have carried out on TPU elastomers derived from poly(oxypropylene)glycols (PPG polyols) and from poly(oxyethylene-oxypropylene)glycols as the polyol components of the elastomers. Our interest in these materials is based on the lower cost of these polyols, especially since the polyol comprises about 45 to 65 percent by weight of the raw materials used in preparing most TPU elastomers. 

Materials. 1,4-Butanediol was vacuum dried (60°C., 2mm Hg) for six hours and then stored over molecular sieves. The PPG polyols used were PPG 1025 and PPG 2025 from Union Carbide Corporation. Tipped PPG polyols were supplied through the courtesy of F. J. Preston of the Olin Corporation Research Center, New Haven, Connecticut. Polyols were vacuum dried (100°C., 2 mm. Hg) for one hour immediately prior to use. The MDI, Isonate 125 MF from Upjohn Company, was stored at 50°C. and decanted prior to use so that only water white, clear material was used. The BHEHQ was from Eastman Chemical Products. 

Polymers prepared from untipped PPG polyols have some inherent deficiencies. Table 1 shows data on injection molded TPU elastomers based on PTMG and PPG polyols. 

Effect of Oxyethylene Content on Stability. The processing instability of TPU elastomers based on PPG 2000 can be overcome by the use of ethylene oxide "tipped" PPG polyols, especially those containing high levels of oxyethylene groups, e.g., 30% to 45% by weight. The improved stability of the polymers based on such polyols is demonstrated in Table 2. 

Figure 6. Effect of NCO/OH ratio on hardness build-up for polymer made from tipped PPG polyol (45% EO) at NCO/OH ratios of 1.01 (0), 1.03 (%), 1.05...

Figure 8. DSC cooling curves of TPU elastomers (based on 2000 M.W. tipped PPG polyol (45% EO) varying in NCO/OH ratio).

Table 9.1 Monol (unsaturation), functionality and mole % monol of conventional PPG polyols ...

In this chapter, the dramatic effect that monol content or polyol functionality has on processability and properties of polyurethane cast elastomers is discussed. This effect is shown for elastomers prepared by both the prepolymer and one-shot processes. Further improvements in elastomer processability and formulating latitude can be achieved by incorporating oxyethylene moieties into the polyol backbone. For one-shot elastomer processes, ultra-low monol PPG polyols capped with ethylene oxide have been commercialised. 

MDI/BDO Cured Elastomers Based on Ultra-Low Monol PPG Polyols... 

The reactivity of PPG polyols can be increased by reacting with EO, which ring opens to form a primary hydroxyl group. A 100% primary hydroxyl content cannot be achieved using this approach since EO has a tendency to form chains instead of distributing evenly to each secondary hydroxyl group however, it is well known that a 100% primary content is not required to achieve one-shot processability. The effect of the primary hydroxyl content on the processability of a one-shot elastomer system is discussed next. 

The previous studies led to the development of three products, a 2250-MW diol (Acclaim Polyol 2220), 4000-MW diol (Acclaim Polyol 4220) and a 6000-MW triol (Acclaim Polyol 6320). Table 9.4 shows the typical properties of these new higher-reactivity, ultra-low monol PPG polyols. The behaviour of these products in one-shot systems is discussed below. 

When comparing ultra-low monol PPG polyols with PTMEG it is essential to keep in mind the fundamental differences between these two high-performance polyether polyols in order to obtain the maximum benefit in urethane applications. Table 9.13 summarises these differences in terms of their chemical structure, hydroxyl type (reactivity), MWD and crystallisability. 

Table 9.13 Comparison of fundamental characteristics of ultra-low monol PPG polyols and PTMEG ...

So one can maximise the benefit from ultra-low monol PPG polyols in urethane systems such as MDI/BDO cured elastomers by understanding their fundamental differences in comparison to other high-performance polyols like PTMEG. A detailed look at the effect of the polyol MWD on the mechanical and dynamic properties of polyurethanes follows. 

A less well-understood factor, which affects the properties of polyurethanes, is the polyol MWD. Ultra-low monol PPG polyols have very narrow MWD (polydispersity < 1.1) compared to other high-performance polyols such as PTMEG or polyester polyols (polydispersity 1.7 to 2.4) as illustrated by the gel permeation chromatography (GPC) curve in Figure 9.10. As expected, a narrower MWD results in an order-of-magnitude... 

In formulating polyurethane/urea systems using ultra-low monol PPG polyols, it was found that polyol MWD has a major effect on polymer properties. A broadened MWD is required to achieve a property profile similar to a PTMEG-based system. The MW-distribution effect is illustrated using TDI prepolymers cured with MBOCA, TDI moisture-cured prepolymers and aqueous polyurethane/urea dispersion coatings. 

Ultra-low monol PPG polyols have excellent properties and ease of processing. The elimination of monol enables polymers to be produced having much higher final MW, i.e., improved properties, as well as significantly faster MW buildup during polymer formation, i.e., shorter demould times and excellent green strength. 

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