PET Crystallinity Modification

Polyethylene Terephthalate (PET) is produced from esterification of mono ethylene glycol (MEG) with terephthalic acid (TPA), or from transesterification of mono ethylene glycol (MEG) with dimethyl terephthalate (DMT). But, what would happen if you substituted 2-methyl-1,3-propanediol (MPO) for mono ethylene glycol (MEG) in the PET polymerization process?

MPO Glycol Modified Polyethylene Terephthalate (PET)

Substituting low levels of  2-methyl-1,3-propanediol (MPO) for mono ethylene glycol (MEG) in the PET polymerization process lowers the melting temperature and reduces the degree of crystallinity of PET. 

This modification enhances clarity, transparency, and flexibility by decreasing the crystallization characteristics (temperature and rate) of conventional PET homopolymers.  Such modifications are especially important in applications such as deep dyable polyester fibers, high clarity blow-molded bottles, and clear, thermoformable film and sheet.

MPO glycol is a low toxicity, high boiling, low freezing (-54°C) and low viscosity liquid that is compatible with conventional melt-phase and solid-state PET polymerization technologies. The modification is performed by adding MPO within the glycol feed during PET polymerization.  Small amounts of the “methyl branched” MPO results in irregular units which are responsible for partially disrupting crystallization.  MPO glycol can be added in the proper proportion to achieve the desired crystallization kinetics for intended applications, while enhancing other attributes of PET homopolymers.  For example, higher elongation rates and percent elongations are beneficial in sheet thermoforming and fiber extrusion.  Higher clarity is desirable in bottle and container applications.

The controllable effects of MPO glycol on PET crystallization kinetics are similar to those achieved in APET with isophthalic acid (IPA) and PET-G and PCTG with 1,4-cyclohexane dimethanol (CHDM).  MPO glycol is a smaller molecule (lower molecular weight) and requires lower substitution weights to achieve performance characteristics associated with APET and PET-G.  The relationship of MPO substitution to the resulting crystalline characteristics (% crystallinity, Tm, Tc, crystallization rate constants, etc.) has been determined for MPO glycol modification of PET.  It was found that the effects of MPO, CHDM or IPA substitutions are similar when compared on a molar basis in the copolymer.  On a weight basis, the required levels of MPO are lower vs. IPAa and CHDM.  As a di-primary diol, the copolymerization rates in the PET process are high, with the end-proportion of MPO remaining in the copolymer being higher than in the feed ratio (i.e. a high incorporation efficiency).

The structural representation below shows the differences between the basic PET homopolymer and the structural alteration of the more flexible methyl branched and 1,3-propanediol chain.

As delineated above, incorporating the MPO into the PET chain reduces the glass transition temperature (Tg) and crystallization temperature (Tc), and causes crystallization rates to be slower at any temperature.   By reducing the tendency to stress crystallize, high spinning speeds can be achieved along with other spin line improvements.  The isothermal crystallization rate constants (k) vs. temperature for a series of MPO modified PETs are shown in the chart below.

Chart Source: S.A.Schwartz, et.al., PET Modification with MPDiol glycol, Chemical Fibers International, Volume 53, pp.445, December ,2003

When melt spun above a certain speed (typically 3500 m/min), PET fibers tend to crystallize in the spin line itself, which renders the yarn properties useless in post-processing.  Above a certain speed, the crystallization rates become so high that the fibers tend to undergo a cohesive failure at the freeze line.  MPO delays the onset of stress-induced crystallization to higher stress levels and, consequently, permits higher speeds and attainment of finer denier fibers.  These fibers, processed at higher speeds, retain a majority of their physical properties.

Improved dyability is another advantage afforded by MPO modification.  MPO-co-PET more efficiently absorbs dye in a dyebath vs. a PET homopolymer.  In the example below, a 2.7 wt.% MPO-co-PET dyed significantly deeper with a higher rate of dyeability and higher color strength (K/S).

MPO modification of fiber grade PET is an expanding commercial use for MPO and can offer advantages over the use of IPA as a crystallization modifier for PET. 

Data Sources: 1. S.A.Schwartz, MPDiol-co-PET: A Novel Copolymer for Fiber Applications, The Polyester & PET Chain 8^th World Congress, Maach Business Services, December 1, 2003.

2.  S.C.Chen, et. al., Process for Producing Polyester Fiber Having Improved Dyeability, Elongation and Strength, U.S. Patent, 5,916,677, June 29, 1999.

MPO Modification for Molding & Extrusion Grades of PET

MPO modified PET can be formulated with enough MPO content to slow crystallization rates so that the co-polyesters remain amorphous under melt processing conditions. 

Levels of MPO in PET, under 15 mole %, lead to crystallization rate modifications that can be tuned for controlling crystallization in the stretch blow molding process for bottles and in thick sheets for thermoforming.  Above this range, the MPO-co-PET is amorphous and does not crystallize.  

Low levels of MPO effectively retards the crystallization rates of PET so that crystallization during processing can be control, for example in stretch blow molding or deep-draw sheet thermoforming.  The desired crystallization rates can be fine-tuned depending on the molding and extrusion applications and conditions.  The chart below shows the crystallization rate half times vs. the weight substitution of MPO or CHDM in the glycol feed in the PET process.  The data was generated by DSC at 170 °C from the glassy polymer.

At 5 weight % of MPO in the glycol feed, the Tm of the PET copolymer is about 230 °C.  At the same 5 weight % of CHDM in the glycol feed, the resulting PET-G has a Tm of ~ 242 °C.  This shows the greater influence of MPO on the depression of crystallinity characteristics in a PET copolymer.  In polyesters, an “odd-even effect” has been observed for the repeating units in the polymer chains. For an even number of carbon atoms in a chain, as in EG-PET, modeling has shown better molecular chain alignment in the polymer.  For an odd-number of carbon atoms in a chain, for example with 2-methyl-1,3-propanediol, molecular overlap of the ester moieties is displaced, resulting in a lower crystalline order. This effect is in addition to the steric effect of the bulk methyl branch in the MPO ester unit and leads to the higher efficiency of MPO as a PET chain modifier vs. other monomers.

Summary

Low levels of MPO can effectively fine-tune the crystallization rates of PET so that crystallization kinetics during processing can be controlled, for example in melt-spun fibers, stretch blow molding or thermoformable sheets.  In fiber spinning, the increased substitution of MPO glycol reduces the rate of crystallization at high spinning speeds.  MPO modification leads to higher productivity and increased spinning speeds with the potential to produce finer denier filaments.  At levels under 15 mole %, MPO substitution can control crystallization in the stretch blow molding process for high clarity bottles and in thick sheets used for deeper draw thermoforming.

Benefits of MPO vs. IPA and CHDM for PET modification include 1) lower levels of MPO can achieve desired crystallization characteristic; 2) the high reactivity and incorporation rates for MPO in conventional melt-phase and solid-state PET polymerization technologies, 3) ability to fine-tune crystallization characteristics by varying MPO levels; 4) low cost and ease of handling; and 5) the performance benefits achievable including increased productivity, clarity, flexibility and post-processing properties like thermoforming, deep-dying of fibers, etc.

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