FAQ |  MPO: The Alternative Glycol You May Not Know

2-Methyl-1,3-propanediol (MPO) is a non-toxic, liquid glycol alternative 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.

What glycols can MPO replace?

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

What are the similarities between MPO, MPG and DPG?

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).

What are the similarities between MPO and 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.

Substituting MPO into formulations containing MPG & DPG – what are the advantages?

We observe three advantages in substituting MPO into PMG and DPG - the 3 ‘P’s 

1. Price: The supply-demand balance for MPG and DPG has recently changed, sparking cost increases, and causing formulators to seek alternatives to MPG & DPG. 
   
   MPO is less susceptible to the market dynamics that contribute to the fluctuations in pricing and availability of MPG & DPG.
   
   
2. Processing: There is a real basis for considering MPO as a value-added alternative to MPG and DPG, based on similarities in solvency, high reactivity, physical characteristics, and end-use performance. 
   
   MPO has both a low melting point (-54°C) and a high boiling point (+212°C) which, coupled with the primary hydroxyl functionality, gives it a broader range of processing temperatures compared with MPG and DPG in the production of polyester resins. The reactivity of MPO enables lower reactor temperatures, which leads to lower-color polyester resins, as well as significant potential energy savings. Overall, MPO affords higher reaction rates and greater reaction completeness.  We estimate the esterification rates with MPO are about four to five-fold faster than with DPG with aromatic acids like IPA and TPA, or adipic acid. Furthermore, MPO reacts cleanly without side reactions seen with many other glycols, and MPO is also less hygroscopic in nature than DPG.
   
   
3. Performance: MPO, MPG, and DPG contain polar hydroxyl functionality and non-polar methyl groups, which promote a reduction in crystallization and a wide range of solvency, viscosity reduction characteristics, and intermolecular compatibility.  Further, MPO contributes to increases in UV resistance and abrasion resistance. These attributes lead to utility in a wide range of applications, as a carrier, solvent, reactive diluent, or as chemical intermediates.
   
   MPO offers superior resistance to hydrolysis compared to DPG in polyester resins. The data in the graph below represents hydrolytic stability testing of 1000 molecular weight polyester-adipate resins, when 10% water is added to the polyester resin, and subjected to a temperature 90°C.  Please note that EG and DEG refer to ethylene glycol and diethylene glycol respectively.
   
   Figure 2: Comparing the hydrolysis of various glycol-adipates

Substituting & blending MPO into formulations containing NPG – what are the advantages?

We observe three advantages in substituting MPO into NPG - the 3 ‘P’s

1. Price: NPG’s supply-demand balance has recently changed, sparking cost increases and causing formulators to seek alternatives to NPG. 
   
   MPO is less susceptible to the market dynamics that contribute to the fluctuations in pricing and availability of NPG.
   
   
2. Processing: MPO has both a lower melting point (-54°C) and a higher boiling point (+212°C) than NPG (+127°C and +208°C respectively), which gives it a broader range of processing temperatures when producing polyester resins.
   MPO does not require any special high-temperature storage versus molten NPG and contains no water versus NPG slurries.
   
   MPO makes a good companion glycol in NPG polyester production due to the susceptibility of NPG vapors to crystalise on cold spots on reactor vessel walls (deposition); the MPO solvates the NPG, preventing deposition. This leads to improved consistency of production, higher yields, and lower color.
   
   
3. Performance: MPO’s singular pendant methyl group, versus NPG’s dual-pendant methyl groups, promotes a reduction in crystallization and a wide range of solvency, viscosity-reduction characteristics, and intermolecular compatibility.  Further, MPO produces softer polyester resins with lower glass transition temperatures (Tg), but it does so without sacrificing the hydrolytic stability or UV resistance typical of NPG based polyesters.
   
   The graph below compares the viscosities over typical processing temperature ranges of MPO and NPG-adipates of 1000 and 2000 molecular weight

Figure 3: Viscosity change with temperature data of MPO and NPG Adipates of 1000 and 2000 mw

Conclusions

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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