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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•Wadsworth et al, unpublished, 2008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.