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 [Nylon TM ] Polycarbonate [Lexan TM ] Poly(methy methacrylate) [Plexiglass TM ]

Examples Of Fluorocarbons Polytetrafluroethylene [PTFE Plumbers Tape] Polychlorotrifluoroethylene [wire coating]

Grades Of Polyethylene (ASTM D 1248) Low Density Polyethylene (LDPE) < Density < Medium Density Polyethylene (MDPE) < Density < High Density Polyethylene (HDPE) < 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 CH 3

PE & PP Polymerization Addition Polymerization A Random Process 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

|____________________Longest Chain 2X _________________| |_Shortest Chain 1X _| Molecular Weight Distribution Ratio 2:1

Environmental Exposure Need 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

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 (MW n ) –Increases carboxyl end groups (CEG)

Controlling Hydrolysis Selection of molecular weight (MW n ) –Coated geogrids & high strength geotextiles : MW n > 25,000 –Non Woven Geotextiles: MW n < 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