2-Methyl-1,3-propanediol (MPO) is a non-toxic, liquid glycol used in a broad range of industries as a chemical derivative, resin intermediate, reactive diluent, viscosity reducing agent, solvent, carrier, emollient, humectant, and heat transfer fluid.
To begin, let’s take a look at the chemical-branched diol structure of MPO compared with three alternatives: mono-propylene glycol (MPG), dipropylene glycol (DPG), and neopentyl glycol (NPG), as shown below. Please note that DPG is a mixture of three isomeric chemical structures, with each of the three isomers having the hydroxyl functionality connected differently. The di-secondary and the secondary-primary DPG isomers are the dominant isomers; the most reactive di-primary DPG isomer exists only at very low levels.
Figure 1: Chemical structures of MPO, NPG, MPG, and DPG (all 3 isomers) are shown
MPO, MPG, and DPG are all branched aliphatic diols which inhibit crystallization tendencies and exhibit a broad range of solvency. Formulators use MPO, MPG, and DPG as chemical intermediates in the manufacture of high-performance polyurethanes, polyester polyols, unsaturated polyester resins, saturated polyesters, and alkyd resins. In these resin systems, they contribute flexibility, reduced crystallinity, increased transparency, and hydrolytic stability. Formulators use MPO, MPG, and DPG in large volumes because of their lower costs, good reactivity, and compatibility characteristics with other raw materials. Polyester resins based on these glycols exhibit lower viscosities versus polyester-based glycols such as ethylene glycol (EG), 1,4 butane diol (BDO), and neopentyl glycol (NPG).
NPG has structural similarities to MPO and MPG, but the key difference of the double pendant-methyl groups in the centre of the C3 chain of NPG gives rise to different physical and chemical properties.
Many polyester resin formulations contain NPG as the sole glycol component, or feature NPG in conjunction with a modifying glycol. Polyesters synthesized from NPG with saturated or unsaturated di-acids are used in coating resins, paints, lubricants, plasticizers, and fiberglass-reinforced plastics applications. NPG achieves high performance in resins due to its resistance to oxidation, its nonpolar chemical nature, and steric retardation of hydrolysis.
The chemical structure and resulting physical properties of NPG create some processing challenges. At ambient temperatures, NPG is a white crystalline solid and must be melted or dissolved to chemically process.
NPG is available commercially in molten liquid form, as a 90% slurry in water, and in solid flake form. With a melting temperature of 127°C, transportation of the molten form is an expensive and hazardous challenge. As a slurry, the 10% water used to mobilise the NPG must be removed during processing, which impacts processing time and energy costs. The solid flake form tends to agglomerate in storage, which leads additional processing difficulties.
We observe three advantages in substituting MPO into PMG and DPG - the 3 ‘P’s
We observe three advantages in substituting MPO into NPG - the 3 ‘P’s
Figure 3: Viscosity change with temperature data of MPO and NPG Adipates of 1000 and 2000 mw
Recent tightness in the supply-demand balance for MPG, DPG, and NPG has led formulators to select MPO as a viable alternative glycol. Using MPO in place of MPG and DPG can bring added value in processing. These benefits include lower reaction temperatures due to higher reactivity, as well as in performance, hydrolysis, abrasion, and UV resistance.
Using MPO as an additive to NPG polyester production could bring significant processing benefits. Replacing NPG completely with MPO has the key processing benefit of ease of handling.
Replacing NPG with MPO completely will produce a new material with different properties: a softer material with a lower Tg and a lower viscosity, but with similar UV resistance and hydrolytic stability. These properties lend themselves well to high solids content, liquid-applied coatings applications, such as sprayable coatings and elastomers, where solvents are becoming increasingly more controlled by emissions legislation.
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