Current Methodologies for Testing Degradability of Agricultural

Mulches and Future Approaches

Douglas G. Hayes

Dept. Biosystems Engr. and Soil Sci.

University of TennesseeKnoxville, TN 37996-4531

SCRI PLANNING MEETING: SPECIALTY CROPS / DEGRADABLE MATERIALSOctober 30, 2008University of Wisconsin, River Falls

Goals for Testing• Comparison of material props of mulches• Evidence of degradation• Quantitative data of degradation time course

– Comparison of materials, procedures– Comparison to the literature

• Understanding of underlying degradation mechanism, relationships between– Physical changes– Chemical changes– Protective performance of mulch / cover

Systems for Testing

R.J. Muller, 2003

Quantities to be Measured• Physical

– Structural integrity– Mechanical strength– Location of cleavage sites

• Chemical– Loss of mass– Change of average molecular weight (MW)– Distribution of MW– Identifying functional groups being cleaved;

underlying kinetic mechanism

I. Physical Testing1. Color

2. Light Transmission

3. Scanning Electron Microscopy, SEM

4. Differential Scanning Calorimetry, DSC

a. Glass Transition Temperature, Tg

b. Melting Point Temperature, Tm

c. Gel Content

5. Tensile Strength, Elongation, Strain Energy

6. Weight g m-2, thickness

7. Dynamic Rheology: G’ and G” moduli

8. Surface Area by ??

Color Change•Colorimeter = Simple measurement

•Workup: recovery of mulch, removal of excess soil•3 filters ~response similar to eye, •(Spectrophotometer: amount of light reflected or transmitted at each wavelength)

•Color Change Parameter:•E = [ (L-L0)2 + (a-a0) 2 + (b-b0) 2]0.5

•L = brightness•a = red•b = yellow

•Color change may represent•Adsorption of soil•degradation

Darkening

Yellowing

Kijchavengkul et al, 2008

Light Transmission (%T)

Kijchavengkul et al, 2008

•Workup = same as for colorimetry•Spectrophotometry, 400-700 nm, Transmission

•Depicts changes in openness between fibers•As % T Increases, weed formation is more probable•Decrease of %T with time: soil adsorption?

SEM

SEM of 20S3 (Promot MZM + Molasses) at 500X

b) SB PLA Control at 500X

•Qualitative information on soil adsorption, fiber degradation

•Wadsworth et al, unpublished, 2008

Differential Scanning Calorimetry (DSC) (ASTM D-3418)

Kijchavengkul et al, 2008

• -60oC 160oC @ 10oC/min

• Tm peak broadens upon degradation when x-linking occurs

• Gel formation: Xg = 1-exp(k tn) (Avrami Eq) -Increase of Xg ~ cross-linking (X-ray diffraction can support; Alt: ASTM D-2765)

Tm

T for transitionto amorphous

Tensile Strength (Strength at Breakage), Elongation (under Load at Breakage) and

Strain Energy (ASTM D 882)

Ho et al. 1999Kijchavengkul et al, 2008

Increase of Brittleness

Tensile Strength (Strength at Breakage), Elongation (under Load at Breakage) and

Strain Energy (ASTM D 882)

Ho et al. 1999Kijchavengkul et al, 2008

•Samples stored in dessicators at a fixed, common, relative humidity for ~48 hr

•Sample swatches should be made both in the direction parallel and perpendicular to extrusion

•Breakage ~ physical disintegration into fragments

II. Chemical Testing

1. Size Exclusion Chromatography (SEC) = Gel Permeation Chromatography (GPC)

2. FTIR Spectroscopy

3. NMR Spectroscopy

4. MALDI-TOF Mass Spectroscopy

5. Gas Chromatography-Mass Spec (GC/MS) (of residuals in soil or water)

6. CO2 formation / O2 consumption

0

0.5

1

1.5

2

7 8 9 10 11

Retention Time, min.

De

tec

tor

Sig

na

l

LauricAcid

Oligo(Ricinoleic Acid)

RandomCo-Polymeriz.Of R + w-PDL

PDL (RID)

ELSD

Oligo(R) + PDL

Oligo(R) + Lauric Acid

GPC Analysis of Ricinoleyl / w-Pentadecanoic acyl Co-Polymers

Kelly and Hayes, 2006

GPC

Ho et al, 1999

•Workup: Dissolve mulch in solvent (CHCl3; Ionic Liquid?)

•Dilute w/ mobile phase (THF)

•Often, multiple columns linked in series

•RI or light scattering detector (via static LS) Mw

•Chromatogram Mn, PDI; area per mass of net

+N NCl-

+N N

O-

O

1-Butyl-3-methylimidazolium chloride[Bmim][Cl]

1-Ethyl-3-methylimidazolium acetate[Emim][Ac]

GPC Calibration of Star Polymers

100

1000

10000

8 8.5 9 9.5 10 10.5

Retention Time, min

MW

PE

TMP

DD

Typically, Polystyrene or Polyethylene glycol standards used

Kelly and Hayes, 2006

FTIR-Attenuated Total Reflectance (-ATR)

Kijchavengkul et al, 2008

•Loss of intensity at 1710 cm-1 (C=O stretching) and 1270 cm-1 (C-O stretching chain scission of ester group

1H-NMR

a. b.c.

d.e.

f.

g.

h.

i.

C

CH2OHHOH2C

CH2OHHOH2C

40-42 ppm

60-65 ppm

e.

Pentaerythritol

OOH

COHCH

CH

CH

a.d.

132-143 ppm

179 ppm

71.5 ppm

Ricinoleic Acid

C

CH2OOH2C

CH2OOH2C

173-175 ppmb.

73.7 ppm

c.

Pentaerythritol-poly(ricinoleic acid)Polyester

+ H2O

Kelly and Hayes, 2006

Useful for low-MW oligo’s

MALDIRicinoleic acid + Pentaerythritol

0500 1000 1500 2000 2500 3000 3500 m/z

200

400

600

800

1000

1200

1400

a.i.

Useful for low-MW oligo’s, co-polymer

Kelly and Hayes, 2006

Use of Chemical and Physical Data

• Physical and chemical data compared: how are structural disintegration and lowering of MW interrelated?

• Phys and chem data compared to performance as a protective agent

• Phys and chem data compared to environmental changes in the field (T, sunlight, r.h., etc.)

References

• Ho, KL, et al, J Environ Poly Degr 7, 167 & 173 (1999)

• Kelly, AR, Hayes, DG, J Appl Poly Sci 101:1646-1656 (2006)

• Kijchavengkul, T., et al., Chemosphere 71: 942 & 1607 (2008)

• Muller, RJ, in: Biopolymers, Volume 10, General Aspects and Special Appli cations, A. Steinbuchel, ed. Weinheim, Germany: Wiley, 2003.