Читать книгу Poly(lactic acid) - Группа авторов - Страница 52

Vapor pressure of anhydrous lactic acid has been measured by relatively few authors. Due to the challenges in purifying lactic acid, data are somewhat scattered. Table 2.3 summarizes data and the accepted data are plotted in Figure 2.1. Data are accepted based on consistency between values from multiple researchers by plotting with the expected approximate linear behavior on the coordinates of Figure 2.1. Many of the data are measurements provided by researchers using lactic acid for chemical synthesis. The accepted data are well represented by Equation 2.1 for vapor pressure P ^sat (in units of Pa) that is extrapolated in the figure for comparison with lactide vapor pressure data at higher temperatures. Care should be taken in using the extrapolation above 405 K because the vapor pressure may have a curvature on the log P ^sat vs. 1/T plot that is not captured by the extrapolation. Oligomerization and subsequent water evaporation are likely to occur during vapor pressure measurements, complicating the interpretation of the results.

TABLE 2.1 Physical Properties of Lactic Acid

Property	Value	Isomer	Reference
Molar mass (g/mol)	90.078	D, L, rac	—
Melting point (°C)	52.7	D, L	[6]
	16.8	rac	[6]
Refractive index [n]D 20	1.4265	rac	[7]
pK a (22°C, I = 0.1 M KCl)	3.73	rac	[8]
Octanol : water partition coefficients (log K ow)	−0.93	L	[9]

TABLE 2.2 Physical Properties of Lactide

Property	Value	Isomer	Reference
Molar mass (g/mol)	144.13	D, L, meso, rac	—
Melting point (°C)	53–54	meso	[10]
95–98	L	[10]
95–98	D	[10]
122–126	rac	[10]
Specific rotation L‐lactide
[α]D 20	−266 (c 1.00 dichloromethane)	—	[11]
[α]D 20	−299 (c 1.00 toluene)	—	[11]

TABLE 2.3 Vapor Pressure of Anhydrous Lactic Acid

T (K)	P sat (Pa)	Source	Accepted(A) or Rejected(R)
351.15	13.332	[13]	R
352.65	39.997	[14]	R
355.15	66.66	[15]	A
358.15	133.32	[15]	A
369.15	133.32	[16]	R
377.15	599.95	[17]	A
392.15	1599.9	[15]	A
395.15	1866.5	[15]	A
395.15	1999.8	[18]	A
395.25	1599.9	[19]	R
400.65	2666.4	[20]	A
405.15	3333.1	[21]	A

FIGURE 2.1 Selected vapor pressures of anhydrous lactic acid selected from literature as summarized in Table 2.3.

(2.1)

Witzke [1] evaluated lactide vapor pressure data from a Boehringer Ingelheim patent [12], as well as data provided by Cargill. The Boehringer Ingelheim patent provides vapor pressure data and a distillation to separate meso‐lactide from D‐ and rac‐lactide. The Witzke thesis provides plots superimposing the Cargill data with the patent data but does not tabulate the Cargill data. The Boehringer Ingelheim vapor pressures are provided in the approximate range of 100 to 225°C and are represented by:

(2.2)

(2.3)

The equations and the patent data are provided in Figure 2.1. Witzke was able to reproduce the patent distillation to separate meso‐lactide from D‐ and rac‐lactide using intermediate temperatures and a high reflux ratio.