WO2016172011A1

WO2016172011A1 - Process for making pla stereocomplex

Info

Publication number: WO2016172011A1
Application number: PCT/US2016/027938
Authority: WO
Inventors: Kevin Terence MCCARTHY; James Russell VALENTINE
Original assignee: Natureworks Llc
Priority date: 2015-04-24
Filing date: 2016-04-15
Publication date: 2016-10-27

Abstract

PLA stereocomplex is made by crystallizing a blend of a high-L-PLA resin and a high-D-PLA resin in the presence of steam or subcooled water at a temperature of 120 to 200°C. The process allows for rapid generation of PLA stereocrystallites. Surprisingly, the formation of PLA stereocrystals is highly favored over lower-melting PLA homocrystals, even when the crystallization step is performed at temperatures below 160°C. The process has the further advantage of producing crystallized articles having very high levels of crystallinity.

Description

PROCESS FOR MAKING PLA STEREOCOMPLEX This invention relates to a process for making polylactide stereocomplex.

PLA (polylactide or polylactic acid) is a thermoplastic resin useful for making a variety of products such as fibers, thermoformed articles such as deli- trays, food packaging and bottles, among many others. PLA can be made by polymerizing lactic acid directly via polycondensation or by first converting lactic acid to lactide (a cyclic diester of lactic acid) and conducting a ring-opening polymerization of lactide using suitable initiator and catalyst. Lactic acid is produced industrially by the fermentation of sugars, which are an annually renewable resource unlike the petrochemicals which form the feedstocks of the vast majority of other commodity plastics. Another advantage of PLA is that it is compostable. Under appropriate conditions in an industrial composting facility, PLA will undergo hydrolysis to generate lower molecular weight oligomers that are consumed by microorganisms to generate carbon dioxide, water and humus.

Lactic acid contains a chiral carbon atom, and so exists in L- (S-) and D- (R-) enantiomeric forms. Each repeating unit formed when lactic acid is polymerized to form PLA (either directly or indirectly through the intermediate production of lactide) similarly exists in either the L- or D- form. The proportions and distribution of the L- and D- units has a profound effect on the properties of the polymer. When one of the enantiomer types is highly predominant, the PLA is capable of being crystallized to form a semi-crystalline material. When both types of enantiomeric units are present in significant amounts and are randomly distributed, the PLA is an amorphous material that can be crystallized to a small extent, if at all, and even then only with difficulty. The line of demarcation between the crystallizable grades of PLA and the amorphous ones is at approximately 8- 10 mole-% of the less predominant enantiomer. Random copolymers that contain at least 8-10 mole-% of each of the L- and D- enantiomers tend to be amorphous types. Random copolymers that contain less than about 8- 10% of either the L- or D- enantiomer tend to be more easily crystallizable. PLA resins become progressively easier to crystallize as the mole ratio of the predominant enantiomer increases towards 100%.

PLA resins crystallized in the foregoing manner exhibit melting temperatures in the range of 150 to 185°C. Such crystals, whether they originate from the crystallization of PLA resins of high D- or high L- content, are sometimes referred to as "homocrystals" or "PLA homocrystals" .

The presence of the homocrystals imparts some heat resistance to an appropriately manufactured part, in that the homocrystals help reduce softening in an article made from the PLA resin when it is exposed to elevated temperatures up to the crystalline melting temperature of the homocrystals. This makes the article more resistant to distorting (measured, for example, as the heat distortion temperature (HDT)), softening and/or sticking to itself or other things when exposed to moderately high temperatures. In addition, there is evidence that the hydrolytic stability of PLA is increased when crystalline domains are present. Thus, resistance to both heat and hydrolysis can be improved by increasing the crystalline content.

The increase in hydrolytic stability appears to be connected to the rate of water diffusion through the polymer. Water diffusion into crystalline domains is much slower than in amorphous domains due to the tight packing of polymer chains in homocrystals. The slow diffusion of water into crystalline domains restricts the hydrolysis reactions mainly to the surface of the crystallites. On the other hand, water can diffuse rapidly into amorphous domains, and so hydrolysis can occur throughout those domains. As a result, hydrolytic stability increases with increasing crystallinity.

The melting temperature of PLA homocrystals is not high enough to provide the heat resistance needed in some applications. Thus, the use of PLA resins is largely restricted to applications in which at most only moderately high temperatures are encountered.

It has long been known that a mixture of a poly(L-lactide) resin and a poly(D-lactide) resin can under certain circumstances form a second type of crystalline structure that has a melting temperature of about 205°C to as high as about 240°C. These crystalline structures are sometimes referred to as "stereocrystals" or "PLA stereocrystals" ; a PLA composition containing such stereocrystals is referred to as a "stereocomplex" or "PLA stereocomplex" . In principle, PLA stereocomplex formation represents a route to further improving the heat resistance of PLA articles. PLA stereocomplex articles also have a significant advantage in hydrolytic stability, which is important when the article is used in a wet or humid environment. Despite these potential advantages, PLA stereocomplexes have found very few practical applications. A reason for this is difficulty in processing a mixture of high-L PLA and high-D PLA into useful semi-crystalline articles using the economical and efficient melt processing methods favored by the plastics industry.

Crystallization of a PLA resin, whether of a single resin by itself to form homocrystals or of a mixture of a poly(L-lactide) and a poly(D-lactide) to form stereocrystals, does not occur instantaneously. Instead, crystallization requires the article to be held within a specific temperature range— between the glass transition temperature and the melting temperature of the crystallites— in which crystallization can take place. Thus, crystallinity is not an inherent property of any PLA resin, but instead an artifact of its heat history. In any melt-processing operation, the article is by necessity cooled through the required range, and some crystallization almost always will take place during this cooling step.

Unfortunately, the rate of crystallization of PLA is generally quite slow, especially at temperatures near the glass transition temperature and near the crystalline melting temperature. The slow crystallization requires that the article be held on the manufacturing equipment and within the necessary temperature range for a prolonged period to provide the time needed for the crystals to form. This slows the manufacturing process, decreases equipment utilization rates and increases costs.

When a mixture of poly(L-lactide) and poly(D-lactide) is crystallized, homocrystals and stereocrystals form in competition with each other across a wide temperature range that extends from the glass transition temperature of PLA to the homopolymer melting temperature. Thus, a mixture of a high-L PLA resin with a high-D PLA resin often will crystallize to form crystals of each type. This is disadvantageous because when more of the polymer forms homocrystals, fewer high-melting stereocrystals form, and as a result the heat-stability of the finished article is compromised. It is often desirable to crystallize the article to form many stereocrystals and relatively few, if any, homocrystals.

It is possible in principle to form stereocrystals exclusively by crystallizing between the melting temperatures of the homocrystals and stereocrystals (for example, at approximately 185- 15°C), but as a practical matter this approach is not useful. Stereocomplex formation is especially slow at these temperatures. For this reason, stereocomplex crystallization is almost always carried out below 160°C, at temperatures where competing homopolymer crystallization also occurs at a fast rate.

To fully realize the benefits of good heat resistance and hydrolytic stability, a large portion of the mass of the PLA resin blend needs to be converted to stereocrystals. The amount of crystallinity (homocrystals, stereocrystals or both) in a PLA resin can be expressed as the enthalpy of melting of the crystals in Joules per gram of PLA resin in the sample (i.e., the combined weights of the high-L PLA resin and the high-D PLA resin), which is abbreviated herein as J/g. The theoretical enthalpy of melting of stereocrystals is 130 J/g. That is, if the entire mass of a PLA resin blend were fully formed into stereocrystals, its enthalpy of melting would be 130 J/g. In practice, enthalpies of melting of PLA stereocomplex products are much lower than this. It is very difficult to obtain even 65 J/g of stereocrystals in PLA. This corresponds to a crystallization of 50% or less of the total mass of the resin. The remainder is in the form of lower- melting PLA homocrystals and amorphous regions, which are less effective in promoting thermal and hydrolytic stability. Better thermal and hydrolytic stability would be obtained if more of the mass of a PLA resin could be converted to stereocrystals.

US 7,771,493 describes a process for crystallizing granules of a poly(L- lactide) resin. Thermoplastic resins such as PLA are commonly manufactured in the form of small pellets so they can be packaged and handled easily. When first manufactured, PLA resins are usually highly amorphous. In amorphous form, PLA resins are highly prone to "blocking", i.e., sticking together to form an agglomerated mass. Therefore, PLA resins are in most cases heat-treated in hot dried air or nitrogen to partially crystallize them. However, the heat-treating step itself can cause the polymer granules to block. In the process set forth in US 7,771,493, the heat-treatment step is performed in water. The water acts as a more efficient heat transfer medium, and provides a means to keep the heated granules from contacting each other and sticking. Once the surface of the particles has crystallized (after a short period of at most five minutes), the granules are removed from the water, dried and then further crystallized in the absence of water. In the process of US 7,771,493, the contact time in water is kept to a minimum because the PLA granules are highly susceptible to hydrolysis before the crystallization step is completed. US 7,771,493 does not describe any method for forming PLA stereocomplex. What is wanted is a fast and convenient method of forming PLA stereocomplex articles. The process should provide reasonably rapid crystallization rates, and produce stereocrystals preferentially to homocrystals. The process preferably uses moderate temperatures to minimize thermal degradation and energy consumption.

The invention is in one aspect a process for making a PLA stereocomplex, comprising the steps of

a) blending a high-D PLA starting resin and a high-L PLA starting resin to form a solid blend; and

b) performing a crystallizing step by exposing the solid blend to a temperature of 120 to 200°C while in contact with steam or subcooled water or both steam and subcooled water for a time sufficient to form a crystallized blend, wherein at least 15 J/g of crystallites having a melting temperature of 205 to 240° C form in the solid blend during the crystallizing step and the solid blend is not melted during the crystallizing step.

This invention offers several important advantages. Stereocrystals form rapidly, even at the lower crystallization temperatures within the foregoing range. The rapid crystallite formation is believed to be due in part to rapid heat transfer from the steam or subcooled water to the sample, as well as significantly enhanced chain mobility due to water plasticization. Surprisingly, the higher- melting stereocrystals are formed preferentially to homocrystals, even when the crystallization step b) is performed at temperatures at or below the homocrystal melting temperature, at which temperatures homocrystals would be expected to form in competition with stereocrystals. When homocrystals are present, either in the starting sample or due to homocrystal formation during the crystallization step b), continued heating under the step b) conditions often reduces or even eliminates their presence while continuing to develop the desired stereocrystals.

Even more surprisingly, little hydrolysis of the resin blend is seen in this process, even though the crystallization step b) may be performed for a somewhat prolonged period, and even in cases in which the starting resin blend is highly amorphous and would ordinarily be expected to be susceptible to hydrolysis. The final product often exhibits excellent hydrolytic stability.

Yet another advantage of the invention is that the crystallization step b) can be separated easily from earlier steps in which the blend is processed into some useful physical shape such as, for example, a powder, fiber, film, molded article, etc. Thus, the blend can be formed and melt-processed into the desired form, and then cooled to below the glass temperature to "quench" it, thereby producing the PLA article in its glassy state but which nonetheless contains at most a small amount of stereocrystals and usually at most a small amount of homocrystals. An advantage of doing this is that the processing equipment used to shape the article does not need to be occupied by the shaped article until the crystallization step is completed. The small amount of crystallization that typically occurs before the quenching step is completed often is sufficient to allow the article to maintain its shape during subsequent handling, storage and/or transportation, until such later time as the crystallization step b) is performed. That small amount of crystallization may prevent "blocking" during that handling, storage and/or transportation. The ability to separate the melt- blending and shaping steps from the crystallization step allows for reduced cycle times and increased equipment utilization rates during the melt-blending and shaping steps, because the part does not need to occupy the processing equipment during the crystallization step.

In some embodiments, the crystallization step b) is performed under conditions of use of the PLA blend. In certain end-use applications, a PLA blend may be exposed to conditions of temperature and moisture as described herein. In such cases, it is often unnecessary to perform the crystallization step b) prior to putting the article into service. Instead, crystallization step b) can occur as the PLA article is being used. In this way, the cost of performing a separate crystallization step or of performing the crystallization step on production equipment can be avoided or at least minimized.

An example of such an end-use application is in the treatment (e.g., hydraulic fracturing) of a subterranean formation such as an oil and/or gas well. In such a treatment process, the PLA blend, possibly in the form of fibers, is introduced into the subterranean formation prior to performing the crystallization step. Conditions in such a subterranean formation often include temperatures of 120 to 200°C, and steam or subcooled water is typically present. Under such conditions, the crystallization step takes place in the subterranean formation, with little hydrolysis occurring during the crystallization step, to produce a material that, once crystallized, is highly resistant to hydrolytic degradation. For the purposes of this invention, the terms "poly lactide", "polylactic acid" and "PLA" are used interchangeably to denote polymers having repeating units of the structure -OC(=0)CH(CH3)- ("lactic repeating units"). The PLA resin preferably contains at least 80%, such as at least 90%, at least 95% or at least 98% by weight of the lactide repeating units. These polymers are readily produced by condensation polymerization of lactic acid or by ring-opening polymerization of lactide.

In this invention, a "high-D PLA resin" is one in which the D- enantiomer constitutes at least 90%, preferably at least 95%, of the lactic repeating units in the polymer. Up to 100% of the polymerized lactic repeating units in the high-D PLA resin may be the D-enantiomer. In some embodiments, at least 96%, at least 97% or at least 98% of the lactic repeating units in the high-D PLA resin are the D-enantiomer.

Similarly, a "high-L PLA resin" is one in which the L-enantiomer constitutes at least 90%, preferably at least 95%, of the lactic repeating units in the polymer. Up to 100% of the polymerized lactic repeating units in the high-L PLA resin may be the L-enantiomer. In some embodiments, at least 96%, at least 97% or at least 98% of the lactic repeating units in the high-L PLA resin are the L-enantiomer.

The high-L PLA and high-D PLA starting resins used in the invention each have molecular weights high enough for melt processing applications. A number average molecular weight in the range of 20,000 to 500,000, as measured by gel permeation chromatography against a polystyrene standard, is generally suitable. Somewhat higher and lower values can be used in some circumstances. The molecular weight of the high-D PLA and high-L PLA resins may be similar to each other (such as a number average molecular weight difference of 20,000 or less). It is also possible that the molecular weights of the high-D PLA and high-L PLA resins differ by a larger amount.

The starting PLA resins each preferably are substantially linear, having an average of no more than 0.5, preferably no more than 0.2, long-chain branches, per molecule. Long-chain branches for purposes of this invention are chains having at least 4 carbon atoms and/or ether or ester oxygen atoms in the chain.

Either or both of the starting PLA resins may further contain repeating units derived from other monomers that are co-polymerizable with lactide or lactic acid, such as alkylene oxides (including ethylene oxide, propylene oxide, butylene oxide, tetramethylene oxide, and the like), cyclic lactones, hydroxy acids such as glycolic acid, glycolide, or cyclic carbonates. Repeating units derived from these other monomers can be present in block and/or random arrangements. Such other repeating units preferably constitute no more than 5% or no more than 2% by weight of the PLA resin, if they are present at all. The PLA resins are most preferably essentially devoid of such other repeating units.

The PLA resins may also contain residues of an initiator compound, which is often used during the ring-opening polymerization process to provide control over molecular weight. Such initiator residues include, for example, residues of water, alcohols, glycol ethers, polyhydroxy compounds of various types (such as ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, glycerine, trimethylolpropane, pentaerythritol, hydroxyl-terminated butadiene polymers and the like). Lactic acid or a lactic acid oligomer also can be used as an initiator, in which case the initiator residue forms lactic units in the resin. The initiator residues preferably constitute no more than 5%, especially no more than 2%, of the weight of the starting PLA resin, except when the initiator is lactic acid or a lactic acid oligomer, in which case a greater proportion of initiator residues can be present due to the lactic repeating units in such residues.

A particularly suitable process for preparing the starting PLA resins by polymerizing lactide is described in U. S. Patent Nos. 5,247,059, 5,258,488 and 5,274,073.

This preferred polymerization process typically includes a devolatilization step by which volatile materials such as water, residual monomer and lactic acid oligomers are partially or entirely removed from the polymer. The polymerization catalyst is preferably deactivated or removed, and it is preferred not to add other materials to the starting resins that significantly catalyze transesterification reactions between the starting resins.

The starting PLA resins each preferably have a free lactide content of 1% by weight or less, more preferably 0.5% by weight or less and especially 0.2% by weight or less. Lactide content can be measured using a method in which the polymer is first dissolved in a suitable solvent like methylene chloride, the polymer is then precipitated upon addition of a non-solvent such as cyclohexane, and the presence of lactide is analyzed for in the supernatant by gas chromatography with a flame ionization detector. In this invention, the starting high-D PLA resin and high-L PLA resin are mixed to form a blend. The proportions of the two resins can range widely to as low as 90: 10 or as high as 10:90 by weight. However, stereocrystal formation is more favored when the weight ratios of the high-D PLA and high-L PLA resins are 25:75 to 75:25. In addition, the melting temperature of the stereocrystals that form in step b) tends to become higher as the weight ratio of high-D PLA and high-L PLA resins becomes closer to unity. A more preferred weight ratio is 30:70 to 70:30 and an even more preferred weight ratio is 40:60 to 60:40. A weight ratio of 45:55 to 55:45 or 48:52 to 52:48 is especially preferred.

The blending step preferably is performed by heating the starting resins to a temperature sufficient to melt them. The starting resins typically are supplied to the blending step in the form of pellets, powders and/or flakes. The temperature preferably is at least 180°C and more preferably is above the stereocrystal melting temperature (i.e., at least 230°C, preferably at least 240°C). The starting resins can be heated and melted separately and then combined, or heated together as a mixture. The combined starting resins typically are mechanically mixed to intimately blend the starting resins. Single- screw extruders, twin-screw extruders, accumulating extruders and the like are suitable apparatus for performing the blending step, among others. After mixing, the blend is cooled until it solidifies.

Other blending methods, such as solution blending, can be used, but are less desired due to the need for solvents and the need to remove the solvent from the blended material to form a solid before performing subsequent operations.

The blend preferably contains at least 95%, more preferably at least 98% by weight of the starting PLA resins. It may contain up to 100% by weight of the starting PLA resins.

The blend of high-D PLA resin and high-L PLA resin may be entirely amorphous at the start of the crystallization step b). However, it generally will be partially crystallized (due, for example, to some crystallization that occurs during cooling from the melt blending temperature and/or removal of solvent), and thus will contain an amorphous phase and a crystalline phase. The blend may contain PLA homocrystals, stereocrystals or both. The total amount of crystallinity, including PLA homocrystals and stereocrystals may be, for example, 1 to 60 J/g. The blend of starting PLA resins is crystallized by exposing it to a temperature of 120 to 200°C while in contact with steam and/or subcooled water. A preferred temperature range for the crystallization step is 120 to 180°C. The blend is held under such conditions for a time sufficient to produce, during the crystallization step, at least 15 J/g of stereocrystals, i.e., PLA crystallites having a melting temperature of 205 to 240°C. The crystallization step may be performed until at least 30, at least 50, or at least 70 J/g of stereocrystals are produced during the crystallization step. The stereocrystals produced are in addition to any stereocrystals that might be present in the blend at the start of the crystallization step.

At crystallization temperatures of 120 to 160°C, it has been found that PLA homocrystals may form in competition with stereocrystals, especially early in the crystallization process. However, as the crystallization step is continued for longer periods, the amount of homocrystals decreases whereas the amount of stereocrystals continues to increase. This is a very surprising effect of the invention, which is believed to be related to the presence of water. Although the invention is not limited to any theory, it is believed that homocrystals in contact with water will exhibit a lowering of their crystalline melting temperature, which allows the homocrystals to melt at the temperature of the crystallization step, even though this temperature is below the normal crystalline melting temperature of the homocrystals. This melting allows the newly liberated and mobile polymer chains to re-form into stereocrystals. Therefore, even though PLA homocrystals may be present at the start of the crystallization step, and more may form in early stages of the crystallization step, further heating in the presence of steam and/or subcooled water results in a loss of some or all of these PLA homocrystals. Conversely, the stereocrystals are more resistant to the effect of water, and continue to form. Moreover, the rapid formation of stereocrystals is believed to at least partially account for the surprisingly small amount of mass loss that occurs, despite the hot and humid conditions.

When the crystallization step b) is performed at a temperature of about

120 to 160°C, the time of the crystallization step may be 15 minutes or more, such as 15 minutes to 1 hour, to reduce the amount of PLA homocrystals. Depending in part on the duration of the crystallization step, the blend after the crystallization step may include some quantity of both PLA homocrystals and stereocrystals. At crystallization temperatures above about 160°C, stereocrystals form almost exclusively. 15 J/g of stereocrystals typically form in a matter of seconds and 50 J/g of stereocrystals often form within 15 minutes at these crystallization temperatures. Therefore, at these temperatures, it is preferred to conduct the crystallization step b) for a period of, for example, 15 seconds to 15 minutes, preferably 1 to 10 minutes. In addition, PLA homocrystals that may be present at the start of the crystallization step tend to disappear rapidly. Therefore, shorter times are needed at crystallization temperatures above 160°C. Surprisingly, little hydrolysis occurs despite the higher temperature and presence of water; again, this is attributed to the rapid development of large amounts of stereocrystals, with much slower diffusion of water than in amorphous domains.

The temperature in the crystallization step is above the boiling point of water at 1 atmosphere pressure. Accordingly, the water is present as steam or as a subcooled liquid, by which it is meant that the water is under super- atmospheric pressure sufficiently great that the water is condensed into a liquid at the particular temperature. A portion of the water may be in each of the gaseous and subcooled liquid states.

The blend may be formed into a specific shape as suitable for its intended end-use application before performing the crystallization step, if desired. In some embodiments, the blend may be formed into pellets, into a powder, or into a high surface area article such as a film or fiber, prior to performing the crystallization step. A high surface area has been found to favor rapid crystallization, possibly because water penetrates more rapidly into high surface area material. Thus, the blend in some embodiments is in a physical form having a surface area of at least 0.0001 m²/g, at least 0.001 m²/g, at least 0.01 m²/g or at least 0.05 m²/g, as measured by BET gas sorption methods, when the crystallization step is performed.

An application of particular interest for this invention is in treating subterranean formations. In the oil and gas industry, PLA resins are sometimes used in the form of fibers or powders in hydraulic fracturing operations. In hydraulic fracturing, a viscosity-modified aqueous fluid is pumped down the well and into the surrounding formation under high pressure that creates fractures within the rock. The viscosity modified fracturing fluid contains a particulate solid, called a proppant, along with PLA fibers. PLA fibers have been proposed to help suspend the proppant in the fracturing fluid and facilitate the transport and placement of proppant within the length of fractures produced at high pressures. Once the high fracturing pressure is reduced, the proppant keeps the fractures open, so that oil and gas can escape into the wellbore after the PLA hydrolyzes. See U. S. Patent Nos. 6,949,491, 7,267, 170 and 7,275, 596 for more information regarding the use of PLA in hydraulic fracturing.

According to an embodiment of this invention, the PLA is a blend of a high-D PLA resin and a high-L PLA resin as described before, but which has not undergone the crystallization step. The conditions in the subterranean formation include the presence of steam and/or subcooled water and a temperature of 120 to 200° C. When the blend is introduced into the formation, it comes in contact with the steam or subcooled water and is exposed to the aforementioned temperature range. The crystallization step takes place in the subterranean formation. This produces a crystallized blend as described before, which is more stable to hydrolytic degradation, but nonetheless degrades over time as desired.

Other possible uses for polylactide resins in subterranean applications are in the production of diverting agents and porous cements. Diverting agents are typically used to temporarily plug high permeability regions in a wellbore, and thereby divert fluid flow to less permeable regions which may then be hydraulically fractured, for example. Such diverting agents eventually hydro lyze and return permeability to the initial value leaving behind no residue. Porous cements are sometimes wanted as well casings and gravel packs, again for the purpose of allowing production fluids to pass through and enter the well. One way of accomplishing this is to include particles of acid-soluble carbonate compounds in the cement composition. A polylactide resin can be included in the cement composition. The resin becomes trapped in the cement as it hardens and then degrades to produce an acid that dissolves the carbonate compound and produces the desired pores. In embodiments of this invention, the polylactide resin is a blend as described here, and which has not undergone the crystallization step. The crystallization step takes place in the well, where the requisite temperate conditions and presence of steam or subcooled water exist. As before, the crystallized material is stable to hydrolytic degradation, but nonetheless hydrolytically degrades over time.

The invention further provides a means by which the hydrolytic degradation rate can be controlled, or "tuned" to a specific value as may be desirable in a particular application. In the treatment of subterranean formations, as described above, hydrolytic degradation of the polylactide is important and even necessary, but not immediately after the resin is put into place. Instead, it is necessary that the resin remain in place for a certain period, which may be minutes to hours to days or even weeks, before it degrades and is solubilized. The hydrolytic degradation rate depends at least in part on the amount of stereocrystals and to some extent on their melting temperature. Thus, by manipulating these parameters, it is possible to increase or decrease the rate of hydrolytic degradation under specific conditions.

Preferably, the product after the crystallization step b) contains no more than 20 J/g, preferably no more than 15 J/g, more preferably no more than 5 J/g, of homocrystals and at least 50 J/g, preferably at least 65 J/g of stereocrystals. The amount of stereocrystals in some embodiments is at least 80 J/g, at least 100 J/g or at least 110 J/g; in these embodiments, the amount of homocrystals may be less than 10 J/g, less than 5 J/g, or less than 2 J/g. Total crystallinity (homocrystals plus stereocrystals) may be at least 75 J/g, at least 90 J/g or at least 110 J/g in specific embodiments. These levels of crystallinity are very high. The ability to produce very high levels of crystallinity, in particular high amounts of stereocrystals, is a significant advantage of the invention.

The amount of stereocrystals that form can also be partially controlled by varying the weight ratio of the high-D PLA resin and the high-L PLA in the starting blend. In general, the closer the ratio of these resins is to 50:50 by weight, the higher amount of stereocrystals that can form. Thus, by varying the ratios of the high-D PLA and high-L PLA resin, one can specify the maximum amount of stereocomplex crystallinity that can be produced in the crystallization step. As one of the starting resins becomes more predominant in the blend, the amount of stereocrystals that can form becomes lower.

The melting temperature of the stereocrystals may be from 205 to 240°C. In some embodiments, the stereocrystal melting temperature is at the high end of this range, such as from 220 to 240°C. An advantage of the invention is that highly ordered stereocrystals, having these higher melting temperatures, form readily.

The melting temperature of the stereocrystals can be partially controlled through the optical purity of the starting resins. The range of melting temperatures is believed to be due at least in part to the "optical purity" of the crystallites, i.e., how well the crystallites are ordered. More "ordering", and thus a higher crystalline melting temperature (such as above 220 to 240°C), is favored when both of the starting PLA resins are highly optically pure, i.e., the high-D PLA resin has very few L-units and the high-L PLA has very few D-units. When higher- melting stereocrystals are desired, the high-D PLA resin may have no more than 2 mole% or no more than 1 mole% of L-units and the high-L PLA resin may have no more than 2 mole% or no more than 1 mole% of D-units.

Conversely, less "ordering" and a lower crystalline melting temperature (such as 205 to 220°C), is favored when one or both of the starting PLA resins are less optically pure, having, for example, 2 to 8 mole% or 3 to 6 mole% of the non- predominant enantiomer (L-units in the case of the high-D PLA and D-units in the case of the high-L PLA resin). Through selection of the optical purity of the starting PLA resins, therefore, one can at least partially adjust the melting temperature of the stereocrystals that form in the crystallization step, within the broad range of 205 to 240°C.

The following examples are provided to illustrate the invention, and are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

Examples 1-20 and Comparative Samples C1-C16

PLA powder Blend A is made by melt-blending equal amounts of a high-L

PLA resin containing 99.5% L-lactic repeating units and 0.5% D-lactic repeating units, and high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L-lactic repeating units, and cooling the resulting mixture quickly to below the glass transition temperature. The material is then ground into a powder. This blend contains <3 J/g of PLA homocrystals and <3 J/g of stereocrystals.

PLA powder Blend B is made by melt-blending equal amounts of the same high-L PLA resin and high-D PLA resin, and cooling the resulting mixture to below the glass transition temperature. The material is ground into a powder, and then heated in a convection oven to 105°C under air and held isothermally for 15 minutes. After cooling, the blend contains 23 J/g of homocrystals and 29 J/g of stereocrystals.

PLA powder Blend C is made by melt-blending equal amounts of the same PLA resins, and cooling the resulting mixture to below the glass transition temperature. The material is ground into a powder, and then heated in convection oven to 180° C under air and held isothermally for 15 minutes. After cooling, the blend contains 30 J/g of homocrystals and 41 J/g of stereocrystals.

PLA powder Blend D is made by melt-blending equal amounts of a high-L PLA resin containing 98% L-lactic repeating units and 2% D-lactic repeating units, and a high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L-lactic repeating units, and processing the blend into continuous, drawn fiber. The resulting fibers contain 10 J/g of homocrystals and 47 J/g stereocrystals.

Duplicate samples of Blends A-D are crystallized at various temperatures (180°C, 163°C, 150°C, 135°C and 121°C), in the presence of sub-cooled water. For comparison, under hot dry air at the four lower temperatures.

Crystallizations under sub-cooled water are performed by weighing 1 gram of material into a glass pressure vessel and adding 50 mL deionized water. The vessels are sealed and immersed in an oil bath equilibrated at the desired temperature, allowing 15 minutes for sample to reach desired temperature. The samples are then either removed or maintained in the heated bath for an additional time as indicated in the following Tables 1-5. After removal from the bath, the samples are cooled to room temperature, filtered and vacuum dried below the glass transition temperature. No agglomeration of the sample particles and fibers is observed. The various crystallized materials are designated as Examples 1-20, as identified further in Tables 1-5 below.

The hot air crystallizations are performed in a convection oven.. The samples are placed into pans and then held in a hot air convection oven equilibrated at the desired temperatures, again allowing 15 minutes for sample to reach the desired temperature. After removal from the oven, the samples are cooled to room temperature. These crystallized materials are designated Comparative Samples C1-C16, as set identified further in Tables 1-5 below.

The crystallized samples from both the sub-cooled water and hot air crystallizations are then analyzed by Differential Scanning Calorimetry (DSC) to determine the amounts of homocrystals and stereocrystals and the corresponding melting temperatures. In the DSC method, the sample is heated from 0 to 275°C at 50°C/min, using a Mettler 822e DSC or equivalent instrument. Measurements are taken on the first upheat.

The samples crystallized in sub-cooled water are also reweighed to determine the amount of mass lost during the crystallization step. Mass loss is indicative of the amount of hydrolytic degradation that occurs during the crystallization step to generate low molecular weight lactic acid oligomers (Mn <1000 Da) soluble in crystallization medium. Results are as indicated in Tables 1-5.

Table 1. Examples 1-4, 180° C Crystallization Step Temperature

At a crystallization temperature of 180°C, each of the four blends achieve very high levels of stereocrystal content within 15 minutes. The theoretical heat of fusion for stereocrystals is 130 J/g. The values of 104 to 116 J/g of stereocomplex crystals in the treated samples correlates to a crystallization of 80 to 90% of the mass of the sample. This is an extremely high level of crystallinity, which is very difficult to achieve in other manners. Traditional annealing of such blends in air generally do not generate more than 40-50 J/g of either homocrystals or stereocrystals, whereas thermal treatment at high temperatures in water generates much higher levels of crystallinity, in which stereocrystals predominate. The stereocrystal melting temperature is above 220°C in all cases except Example 4, where it is slightly below that value. In Example 4, the lower stereocrystal melting temperature is at least partially attributable to the lower optical purity of the high-L PLA starting resin. The treated samples have no measureable PLA homocrystals, even in the cases of Examples 2, 3 and 4, where the respective starting blends contain appreciable amounts of PLA homocrystals. The mass loss in each case is only 5% or less, which indicates that little hydrolysis to water soluble fragments occurs during the crystallization step.

Table 2. Examples 5-8, and Comparative Samples C1-C4, 163°C

Crystallization Step Temperature

When crystallized in subcooled water at 163°C, Blends A, B and D (Examples 5, 6 and 8) contain only stereocrystals after 15 minutes, and in each of these cases 63% or more of the mass of the sample has crystallized into stereocrystals. With an additional 30 minutes of crystallization at this temperature, well over 90% of the mass has formed into stereocrystals. In each of Blends B and D (Examples 6 and 8), homocrystals present at the start of the crystallization step fully disappear within the first 15 minutes of heating. Blend C still contains some PLA homocrystals after 15 minutes of heating, but less than at the start of the crystallization step. After 15 minutes, the amount of stereocrystals has increased from 41 to 93 J/g. An additional 30 minutes of heating fully removes the homocrystals and increases the stereocrystals to 114 J/g, or about 90% of the mass of the sample.

Comparative Samples C1-C4 show the effect of crystallizing the same blends, at the same temperature, in dry air. The blends are unable to develop more than 50 J/g of stereocrystals, while generating significant amounts of homocrystals. With the exception of Sample C4, the relative amounts of homocrystals and stereocrystals generated in Samples C1-C3 are very similar.

Table 3. Examples 9-12 and Comparative Samples C5-C8, 150°C

Cr stallization Ste Tem erature

Table 4. Examples 13-16 and Comparative Samples C9-12, 135°C

Cr stallization Ste Tem erature

Table 5. Examples 17-20 and Comparative Samples C13-16, 121° C Cr stallization Ste Tem erature

At the lower crystallization temperatures of Examples 9-20, the development of stereocrystals and the disappearance of homocrystals proceeds more slowly. In many cases, homocrystals are seen to form during early stages of the crystallization step, as seen in Examples 9, 10, 13, 17, 18 and 19. Stereocrystals form in all cases during early stages of the crystallization step. As the crystallization step is continued, the homocrystals partially or entirely disappear, and the amount of stereocrystals continues to increase. Although the final levels of stereocrystallinity are lower in these cases than for Examples 1-8, at least 68 J/g are produced in each instance. This corresponds to >50% of the theoretical heat of fusion of stereocomplex was achieved in Examples 9-20. Despite the slower rate of stereocrystal formation at these temperatures, their melting temperature is in almost all cases quite high. Mass loss is very low in these samples, indicating that little hydrolysis to water soluble fragments occurs under these conditions.

Comparative Samples C5-C16 again fail to develop significant stereocrystal content, and generally greater amounts of homocrystals than do the corresponding examples of the invention originating from crystallization in sub- cooled water.

Examples 21-24

PLA powder Blend E is made by melt-blending 60% of a high-L PLA resin containing 99.5% L- lactic repeating units and 0.5% D-lactic repeating units, and 40% high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L- lactic repeating units, and cooling the resulting mixture quickly to below the glass transition temperature. The material is then ground into a powder. This blend contains <3 J/g of homocrystals and <3 J/g of stereocrystals.

PLA powder Blend F is made by melt-blending 60% of a high-L PLA resin containing 99.5% L- lactic repeating units and 0.5% D-lactic repeating units, and 40% high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L- lactic repeating units, and cooling the resulting mixture quickly to below the glass transition temperature. The material is ground into a powder, and then heated in a convection oven to 105°C under air and held isothermally for 15 minutes. After cooling, the blend contains 34 J/g of homocrystals and 17 J/g of stereocrystals.

PLA powder Blend G is made by melt-blending 70% of a high-L PLA resin containing 99.5% L-lactic repeating units and 0.5% D-lactic repeating units, and 30% high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L- lactic repeating units, and cooling the resulting mixture quickly to below the glass transition temperature. The material is ground into a powder. This blend contains <3 J/g of homocrystals and <3 J/g of stereocrystals.

PLA powder Blend H is made by melt-blending 70% of a high-L PLA resin containing 99.5% L-lactic repeating units and 0.5% D-lactic repeating units, and 30% high-D PLA resin containing 99.5% D-lactic repeating units and 0.5% L- lactic repeating units, and cooling the resulting mixture quickly to below the glass transition temperature. The material is ground into a powder, and then heated in a convection oven to 105°C under air and held isothermally for 15 minutes. After cooling, the blend contains 40 J/g of homocrystals and 17 J/g of stereocrystals.

Each of Blends E-H are subjected to sub-cooled water crystallization as in previous examples, at a 163°C crystallization temperature. Results are as indicated in Table 6.

Table 6. Examples 21-24, 163° C Crystallization Temperature

Blends E-H contain unequal amounts of the high-D and high-L PLA resins. As such, the theoretical amount of stereocomplex that can form is lower in each case, because the amount of stereocrystals that can form is limited to twice the weight of the less-predominant resin. For Blends E and F, for example, the theoretical amount of stereocrystals that can form is approximately 0.8 X 130 J/g, or 104 J/g. For Blends G and H, for example, the theoretical amount of stereocrystals that can form is 0.6 X 130 J/g, or 78 J/g. As the data in Table 6 shows, these theoretical levels of stereocomplex formation are achieved with this invention. Significant homocrystallization takes place only when the amounts of high-D and high-L PLA resins become greatly unequal (Blends G and H)

Claims

WHAT IS CLAIMED IS:

1. A process for making a PLA stereocomplex, comprising the steps of a) blending a high-D PLA starting resin and a high-L PLA starting resin to form a solid blend; and

b) forming a crystallizing step by exposing the solid blend to a temperature of 120°C to 200°C while in contact with steam or subcooled water or both steam and subcooled water for a time sufficient to form a crystallized blend, wherein at least 15 J/g of crystallites having a melting temperature of greater than 200 to 240°C as measured by DSC form in the solid blend during the crystallizing step and the solid blend is not melted during the crystallizing step.

2. The process of claim 1, wherein in step b), the amount of crystallites having a melting temperature of greater than 200 to 240°C is increased in the solid blend by at least 30 J/g during the crystallizing step.

3. The process of claim 1, wherein in step b), the amount of crystallites having a melting temperature of greater than 200 to 240°C is increased in the solid blend by at least 50 J/g during the crystallizing step.

4. The process of claim 1, wherein in step b), the amount of crystallites having a melting temperature of greater than 200 to 240°C is increased in the solid blend by at least 75 J/g during the crystallization step.

5. The process of any of claims 1-4, wherein, after step b), the solid blend contains less than 25 J/g of crystallites having a melting temperature of 150 to 190°C.

6. The process of claim 5, wherein, after step b), the solid blend contains no more than 10 J/g of crystallites having a melting temperature of 150 to 190°C.

7. The process of claim 6, wherein, after step b), the solid blend contains no more than 5 J/g of crystallites having a melting temperature of 150 to 190°C.

8. The process of any of claims 1-7, wherein, after step b), the solid blend contains more than 90 J/g of crystallites having a melting temperature of 220- 240°C.

9. The process of any of claims 1-8, wherein at the start of step b), the solid blend contains up to 50 J/g or crystallites having a melting temperature of 150 to 190°C.

10. The process of claim 9, wherein the amount of crystallites in the solid blend that have a melting temperature of 150 to 190°C is reduced during step b).

11. The process of any preceding claim, wherein after step a), the blend is introduced into a subterranean formation, and step b) is performed in the subterranean formation.

12. The process of any preceding claim, wherein at least a portion of the water is a subcooled liquid.

13. The process of any preceding claim, wherein at least a portion of the water is steam.

14. The process of any preceding claim, wherein the temperature in step b) is up to 180°C.

15. The process of any preceding claim, wherein the temperature in step b) is up to 165°C.

16. The process of any preceding claim, wherein the temperature in step b) is at least 135°C.

17. The process of any preceding claim, wherein step b) is performed for at least 10 minutes.

18. The process of any preceding claim, wherein step b) is performed for at least 30 minutes.

19. The process of any preceding claim, wherein at the start of step b), the blend has a surface area of at least 0.001 m²/g.

20. The process of any preceding claim, wherein the blend formed in step a) is in the form of fibers.

21. A method for treating a subterranean formation, comprising a) introducing a solid blend of a high-D PLA starting resin and a high-L PLA starting resin into the subterranean formation;

b) in the subterranean formation, heating the solid blend to a temperature of 120 to 200°C while in contact with steam, subcooled water or both steam and subcooled water to form a crystallized solid blend having crystallites with a melting temperature of greater than 200 to 240°C as measured by DSC.

22. The process of claim 21, wherein at least a portion of the steam or subcooled water is a ground water.

23. The process of claim 22, wherein at least a portion of the steam or subcooled water is water contained in a treatment fluid introduced into the subterranean formation prior to, simultaneously with or after the introduction of the blend of a high-D PLA starting resin and a high-L PLA starting resin into the subterranean formation.

24. The process of any of claims 21-23, further comprising

c) degrading the crystallized solid blend into a fluid in the subterranean formation.

25. The process of any of claims 21-24, wherein at the start of step b), the blend has a surface area of at least 0.001 m²/g.

26. The method of any of claims 21-25, wherein the blend is in the form of fibers.