LOW Tg LIGNIN

Abstract

Lignin has a weight average molecular weight of at least 6,000 daltons and comprising (a) from 2% to 10% of a low molecular component having a weight average molecular weight (M.sub.w) of from 300 to 1500 daltons, and (b) from 10% to 50% of a high molecular weight component having a weight average molecular weight (M.sub.w) of at least 10,000 daltons; and exhibiting a T.sub.g of from 100° C. to 130° C. when measured by differential scanning calorimetry.

Claims

1. A lignin having a weight average molecular weight (M.sub.w) of at least 6,000 daltons, wherein the lignin comprises: (a) from 2% to 10% of total low molecular weight components, wherein low molecular weight components are those having a weight average molecular weight (M.sub.w) of from 300 to 1500 daltons, and (b) from 15% to 50% of total high molecular weight components, wherein high molecular weight components are those having a weight average molecular weight (M.sub.w) of at least 10,000 daltons; wherein the lignin exhibits a T.sub.g of from 100° C. to 130° C. when measured by differential scanning calorimetry.

2. The lignin of claim 1, wherein the T.sub.g is from 105° C. to 125° C.

3. A lignin having a weight average molecular weight of at least 6,000 daltons, wherein the lignin comprises: (a) about 4% of an F1 fraction, and (b) about 42% of an F5 fraction; wherein the fractions are determined by the Kringstad fractionation process; and wherein the lignin exhibits a T.sub.g of from 100° C. to 130° C. when measured by differential scanning calorimetry.

4. The lignin of claim 3, wherein the T.sub.g is from 105° C. to 125° C.

5. The lignin of claim 3, further comprising about 2% of an F2 fraction.

6. The lignin of claim 3, further comprising about 26% of an F3 fraction.

7. The lignin of claim 3, further comprising about 26% of an F4 fraction.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0013] FIG. 1 is a graph showing T.sub.g vs. M.sub.w for a number of the inventive lignins.

[0014] FIG. 2 is a chart of a Kingstad fractionation of a softwood lignin.

[0015] FIG. 3 is a chart of a Kingstad fractionation of one embodiment of the lignin of the present invention.

[0016] FIGS. 4-6 are GPC data for representative samples of the lignin of the present invention.

[0017] FIGS. 7-8 are DSC curves for representative samples of the lignin of the present invention.

[0018] FIG. 9 is a P NMR graph of a comparative lignin embodiment.

[0019] FIG. 10 is a P NMR graph of an inventive lignin embodiment.

[0020] FIG. 11 is a C NMR graph of a comparative lignin embodiment.

[0021] FIG. 12 is a C NMR graph of an inventive lignin embodiment.

DETAILED DESCRIPTION

[0022] The inventor has discovered a softwood kraft lignin that has a low T.sub.g. The lignin has an average molecular weight of at least 6,000 daltons and comprises (a) from 2% to 10% of a low molecular component having a weight average molecular weight (M.sub.w) of from 300 to 1500 daltons, and (b) from 10% to 50% of a high molecular weight component having a weight average molecular weight (M.sub.w) of at least 10,000 daltons; and exhibits a T.sub.g of from 100° C. to 130° C. when measured by differential scanning calorimetry.

[0023] During heating, 10-50 chain molecules start to move co-ordinately, giving rise to the glass transition temperature (T.sub.g). The glassy state is the region where molecules are rubbery, meaning that it is possible to stretch the material and snap it back to its original length. Glass transitions are influenced by the free volume between polymer chains, the freedom of molecular side groups, branches, chain stiffness and chain length among other factors. These properties are influenced by the polarity of the units as well as their covalent bonds.

[0024] Without being bound by theory it is believed the amounts and molecular weights of the two fractions cause the T.sub.g to be in the range of 100° C. to 130° C. The T.sub.g is fairly constant over a wide range of molecular weights (M.sub.w) in contrast to reported lignin T.sub.g which rise rapidly with a rise in molecular weight.

[0025] It should be noted that the there appears to be little difference in the chemical content of the inventive softwood lignin and other softwood lignins as shown by .sup.31P NMR spectroscopy and quantitative .sup.13C NMR characterization.

[0026] For the purposes of this application a softwood lignin from the Backhammar mill in Sweden was used as a comparative lignin.

[0027] In this application the following methods were used:

[0028] Glass Transition

[0029] Glass transitions were measured on a TA Instrument Q200 Digital Scanning calorimeter (DSC) using Aluminum T-Zero Hermetic Pans. 7-10 mg lignin was ground to a fine powder and dried in vacuo at 95° C. with Drierite. The method employed involved cooling the samples at 15.00° C./min from room temperature to −75.00° C., heating at 15.00° C./min to 200.00° C., cooling at 15.00° C./min to −75.00° C., and a final heat at 15.00° C./min to 200.00° C. Glass transitions were observed in the final heat cycle. DSC spectra were obtained at Weyerhaeuser Technology Center.

[0030] Measurement of the glass transition (T.sub.g) can show a high dependence on variability in the DSC method which is used to collect the data (i.e. heating rate and sample size). Because of this, it is important to maintain consistent sample size and method for all samples. There are additional factors which can skew DSC results. This includes, but is not limited to, plasticization by residual water or other solvents. For this reason, it's important to fully dry the lignin prior to running DSC. Different analysis methods of the DSC curve can attribute to T.sub.g variability. T.sub.g is reported as ½ the value of ΔC.sub.p for the transition.

[0031] FIGS. 7 and 8 are DSC curves for two embodiments of the inventive lignin. The three temperatures in each of the graphs are, in order, the upper softening point, the glass transition temperature T.sub.g and the lower softening point.

[0032] Molecular Weight

[0033] The lignin samples were acetylated to allow dissolution in tetrahydrofuran (THF) for GPC analysis. The lignin samples (˜100 mg) were stirringly acetylated with 2 mL of acetic anhydride(pyridine (1/1, v/v) at room temperature for 24 hours. After acetylation, the acetylated lignin sample was then dissolved in THF for GPC analysis using Agilent 1200 series liquid chromatography containing ultraviolet (UV) detector. The sample was filtered through a 0.45 μm membrane filter prior to injection. 20 μl of sample was automatically injected. GPC analyses were carried out using a UV detector on a 4-column sequence of Waters™ Styragel columns (HR0.5, HR2, HR4 and HR6) at 1.00 ml/min flow rate. Polystyrene standards were used for calibration. WinGPC Unity software (Version 7.2.1, Polymer Standards Service USA, Inc.) was used to collect data and determine molecular weight profiles. GPC Analysis was performed at the Institute of Paper Science and Technology (IPST).

[0034] FIGS. 4-6 are GPC curves for three embodiments of the inventive lignin.

[0035] Kringstad Solvent Fractionation Technique

[0036] 500 O.D. grams of water washed lignin was washed sequentially with methylene chloride, n-propanol, methanol and methanol/methylene chloride (7/3, v/v). For each step, the dry lignin was dispersed into 2 liters of solvent while stirring and stirred at room temperature for 30 minutes. The slurry was filtered and the insoluble material was resuspended in an additional 2 liters of solvent and stirred for 30 minutes at room temperature before being filtered again. At this point, the undissolved material was rinsed with an additional 1 liter of solvent. The undissolved material was ground to a fine powder and dried in vacua at 95° C. in the presence of Drierite. The filtrates Were combined and concentrated under reduced under pressure. The resulting solid was ground into a fine powder and dried under the same conditions. This solvent extraction resulted in five different lignin fractions. The molecular weight increases through the fractions, F1 being the lowest molecular weight and F5 being the highest molecular weight.

[0037] F1=methylene chloride soluble fraction.

[0038] F2=n-propanol soluble fraction

[0039] F3=methanol soluble fraction

[0040] F4=methanol/methylene chloride soluble fraction; 70/30

[0041] F5=final undissolved residue

[0042] There is a difference in the fractions in a comparative softwood lignin and in the lignin of the present invention as shown by two representative samples. This is shown in FIGS. 2 and 3. The weight percent of the F1 fraction was 1% for the comparative lignin and 4% for the inventive lignin. The weight percent of the F2 fraction was 1% for the comparative lignin and 2% for the inventive lignin. The weight percent of the F3 fraction was 42% for the comparative lignin and 26% for the inventive lignin. The weight percent of the F4 fraction was 37% for the comparative lignin and 26% for the inventive lignin. The weight percent of the F5 fraction was 19% for the comparative lignin and 42% for the inventive lignin. This is a comparison of an embodiment of a comparative softwood lignin and an embodiment of the inventive lignin.

[0043] The inventive lignin has a weight average molecular weight (M.sub.w) of at least 6,000 daltons and the lignin comprises (a) from 2% to 10% of a low molecular component having a weight average molecular weight (M.sub.w) of from 300 to 1500 daltons (the F1 component), and (b) from 10% to 50% of a high molecular weight component having a weight average molecular weight (M.sub.w) of at least 10,000 daltons (the F5 component). The F2-F4 fractions comprise the rest of the lignin.

[0044] .sup.31P NMR (Nuclear Magnetic Resonance)

[0045] The samples were dried under vacuum for 24 hours at 40° C. and accurately weighed out into 2 ml vial (˜20 mg). The .sup.31P-NMR spectra of samples were characterized by using a Bruker 400 MHz DMX NMR spectrometer. The dried samples were dissolved in a solvent of pyridine/CDCl.sub.3 (1.5/1 v/v) and phosphorylated with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP). The cyclohexanol served as the internal standard and chromium acetylacetonate as relaxation agent. The spectra were recorded 25 s pulse delay, 128 acquisitions at room temperature.

[0046] The results are shown in Table 1 and in FIGS. 9-10.

TABLE-US-00001 TABLE 1 C-5 substituted Aliphatic Phenolic Guaiacyl Carbox- OH OH OH p-hydroxyl ylic OH mmol/g mmol/g mmol/g mmol/g mmol/g lignin lignin lignin lignin lignin Comparative 1.83 1.75 1.88 0.22 0.48 Inventive 1.89 1.70 1.91 0.25 0.43

[0047] .sup.13C NMR

[0048] The same samples were analyzed using quantitative .sup.13C-NMR with a Broker 400 MHz Avance/DMX NMR spectrometer. The lignin sample (˜0.1 g) was dissolved in DMSO (0.5 ml). The .sup.13C-NMR spectrum was recorded under quantitative conditions employing inversed-gated decoupling pulse, a 90° pulse, 12 s pulse delay at 50° C. 12,288 scans were accumulated for each spectrum. The integral between 160-107 ppm was set as the reference, assuming it includes six aromatic carbons. Manual phasing and baseline corrections were carried out before integration.

[0049] The results are shown in Table 2 and FIGS. 11 and 12.

TABLE-US-00002 TABLE 2 Chemical shift, Compar- Inven- ppm Groups ative tive 160~140 C.sub.Ar—O(oxygenated C) 2.08 2.02 141~123 C.sub.Ar—C (substituted C) 1.92 1.97 123~107 C.sub.Ar—H (un-substituted C) 2.00 2.00 90~78 C.sub.β 0.25 0.26 78~67 C.sub.α 0.36 0.34 61.1~58.5 C.sub.γ in β-O-4 without α-C═O 0.19 0.17 58.0~54.0 Methoxyl OCH.sub.3 0.83 0.80 54.0~52.6 C.sub.β in β- β& β-5 0.09 0.09

[0050] NMR Results (Expressed as per Aromatic Ring)

[0051] The inventive lignin used for the tests was recovered from Southern Pine Kraft black liquor by acidification with CO.sub.2 which resulted in the precipitation of some of the lignin. The lignin was separated via filtration and washed further with acidified water before being filtered and dried. The resulting lignin showed high purity with ash levels less than 0.5%. Chemical analysis was performed using both .sup.31P-NMR and quantitative .sup.13C-NMR and was shown to be very comparable to another industrial softwood Kraft lignin.

[0052] Industrial lignin samples were fractionated according to the Kringstad fractionation method and the M.sub.w of the fractions were determined. The T.sub.g of the lignin samples was also determined.

[0053] Fresh samples and aged samples of the industrially produced lignins were tested. The results are shown in FIG. 1, which also plots the M.sub.w of literature references. It can be seen that the present lignin has a remarkably constant T.sub.g over a wide range of molecular weights (M.sub.w) in contrast to the literature references which show a rapidly rising T.sub.g as the molecular weight increases.