No. 1 of 19

Polymers for Geosynthetics

by

Dr. Don Bright

The Tensar Corporation

The information presented in this document has been reviewed by the Education Committee of the International Geosynthetics Society and is believed to fairly represent the current state of practice.

However, the International Geosynthetics Society does not accept any liability arising in any way from use of the information presented.

Presentation

• Polymers• Principal polymers in Geosynthetics• Principal polymer chemical configurations• Polymerization• Molecular weight & its importance• Elements of deterioration and degradation• Controlling impact of deteriorative elements

Polymers: By Definition

• Polymers are macromolecular structures formed by the chemical union (polymerization) of many (poly) repeat mono units (mers) of a specific chemical configuration.

• The polymerization process results in a long molecular structure of the monomer unit.

H H H H H H H H H H H H H H H H H H

| | polymerization | | | | | | | | | | | | | | | | C = C = - C - C - C - C - C - C - C - C - C - C - C - C - C - C - C - C - | | | | | | | | | | | | | | | | | | H H H H H H H H H H H H H H H H H H

Monomer Polymer

Polymer Chains

Polymers By Classification

• Polymers are either inorganic or organic with the latter being the more predominant.

• Inorganic polymers comprise only a few compounds.

• Organic polymers are principally derivatives of petroleum.

• Organic polymers are classified as natural, semisynthetic, or synthetic.

Classification Of Polymers

• Inorganic– Siloxanes– Silicones– Sulfur Chains

• Organic– Natural Polymers

• Polysaccharides

• Insulin

• DNA

– Semisynthetic Polymers

• Rayon

• Cellulose Acetate

– Synthetic Polymers

Synthetic Polymers

• Synthetic polymers are subdivided into principal categories:

– Thermoset Polymers

– Thermoplastic Polymers

Thermoset Resins

• Are polymers, that once are fully cured, cannot be resoftened with heat and reprocessed.

• Examples– Epoxies– Phenolics– Rubbers– Elastomers

Thermoplastic Resins

• Are polymers that can be resoftened repeatedly with heat and reprocessed.

• Examples– Polyolefins– Vinyl polymers– Polyesters– Engineering polymers– Fluorocarbons

Examples Of Polyolefins

• Polyethylene

• Polypropylene

• Polybutylene

Examples Of Vinyl Resins

• Poly(vinyl chloride) – Rigid Grade: Pipe– Plasticized Grade: Geomembrane – Plastisol Grade: Coating

• Poly(vinyl dichloride) [clear food wrap]

• Poly(vinyl butyral) [Windshield Laminate]

Examples Of Polyesters

• Poly(ethylene terephthalate) (PET)– Geotextiles– Geogrids– Tire Cord Tread Belting

Examples Of Engineering Resins

• Polyamide [NylonTM]

• Polycarbonate [LexanTM]

• Poly(methy methacrylate) [PlexiglassTM]

Examples Of Fluorocarbons

• Polytetrafluroethylene [PTFE Plumbers Tape]

• Polychlorotrifluoroethylene [wire coating]

Grades Of Polyethylene(ASTM D 1248)

• Low Density Polyethylene (LDPE) 0.910 < Density < 0.925

• Medium Density Polyethylene (MDPE) 0.926 < Density < 0.940

• High Density Polyethylene (HDPE) 0.941 < Density < 0.965

Grades Of Polypropylene

• Homopolymer

• Impact Copolymer (with > 7% PE in PP)

• Random Copolymer (with < 7% PE in PP)

Chemical Configurations

Polyethylene

H H

| |

-- -- C - C -- --

| |

H H

Polypropylene

H H

| |

-- --- C -- C --- --

| |

H CH3

PE & PP Polymerization

• Addition Polymerization

• A Random Process

• Broad Molecular Weight Distribution

Broad Molecular Weight Distribution

|_____________________| |__ short__|

chain length

|_______________________long chain length___________________|

|___intermediate chain length__|

|____________| |__________________________|

|___________________________________|

|_________________| |________________________________________|

Chemical Configuration

Poly(ethylene terephthalate) (PET)

H H | |

- C - C - O - C - - C - O -

| | || || H H O O

PET Polymerization

• Condensation Polymerization

• Generation of water molecules

• Narrow Molecular Weight Distribution

Narrow Molecular Weight Distribution

|____________________Longest Chain Length @ 2X _________________|

|_Shortest Chain Length @ 1X _|

Molecular Weight Distribution Ratio 2:1

Environmental ExposureNeed To Consider

• Weathering

• Chemical degradation– Oxidation– Hydrolysis

• Biological degradation

Weathering

• Exposure to:– Ultraviolet light– Temperature– Oxygen– Humidity– Airborne Agents

• Chemical• Biological

Consequences Of Weathering

• Product Deterioration– Physical Properties

• Density, Appearance, Integrity– Mechanical Properties

• Tensile Strength and Creep Resistance• Polymer Degradation

– Molecular Breakdown– Lower Molecular Weight– Free Radical Group Formation

Protection From Weathering

• Polyolefins– Addition of Carbon Black and/or– UV Stabilizer Package

• Polyester– UV Stabilizer Package or– Protective Coating with UV Stabilizer

Package

Effect of Carbon Black on Resistance to UV Lightfor Polyethylene and Polypropylene

0

25

50

75

100

0 1 2 3 4 5 6

Carbon Black Content (Wt %)

Per

cen

t of P

rote

ctio

n (%

)

Polyethylene

Polypropylene

Oxidative Degradation

• Degradation of a polymer through its reaction with oxygen

• Dependent upon:– Product exposed surface area– Product manufacturing process– In-use environment oxygen

concentration• Susceptible geosynthetic polymers

– Polyolefins: PE and PP

Consequences Of Oxidation

• Product deterioration– Physical properties

• Density, appearance, integrity– Mechanical properties

• Tensile strength and creep resistance• Polymer degradation

– Molecular breakdown– Lower molecular weight– Free radical group formation

Controlling Oxidation

• Antioxidant: inhibitor of oxidation process

• Polymer and product configuration dictates:– Antioxidant package

• Chemical composition• Mechanism of prevention

– Concentration / loading

Hydrolytic Degradation

• Molecular breakdown due to reaction of a specific monomeric chemical structure with water or water vapor

• Susceptibility is dependent upon– Molecular Weight, MW– Carboxyl End Groups, CEG

• Susceptible Geosynthetic Polymers– Polyesters (PET)

Consequences Of Hydrolysis

• Product deterioration– Physical properties

• Density, appearance, integrity– Mechanical properties

• Tensile strength and creep resistance• Polymer degradation

– Molecular breakdown

– Reduces molecular weight (MWn)

– Increases carboxyl end groups (CEG)

Controlling Hydrolysis

• Selection of molecular weight (MWn)– Coated geogrids & high strength geotextiles :

MWn > 25,000– Non Woven Geotextiles: MWn < 20,000

• Selection of carboxyl end groups (CEG)– Coated geogrids & high strength geotextiles :

CEG < 30– Non Woven Geotextiles: 40 < CEG < 50

• In-use environment 3 < ph < 9

Biodegradation

• Micro-organisms – Bacteria, fungi, algae

• Physical degradation• Molecular deterioration

• Macro-organisms– Rodents, insects

• Physical degradation

Controlling Biodegradation

• Not a concern for the molecular weight grades of PE, PP, PET, and PVC used in geosynthetics.

• Microorganisms are known to attack and digest additives used to plasticize some base polymers.

Summary

• Polymers• Principal polymers in Geosynthetics• Principal polymer chemical configurations• Polymerization• Molecular weight & its importance• Elements of deterioration and degradation• Controlling impact of deteriorative elements