Barrier polymers Polyamide, EVOH and PVDC

Polyamide

Polyamides (PA) or nylons are condensation polymers, linear, thermoplastic polyamides that contains amide group as a recurring part of the polymer chain.  In general they are clear, thermoformable, strong and tough over a broad range of temperatures.

They can be made from the condensation of diamines and dibasic acids or from the condensation of amino-acids which contain both amine and acid functional groups in a single molecule.  Nylons are identified by numbers corresponding to the number of carbon atoms in the monomers. Two numbers are needed in the case of the condensation of diamines and dibasic acids and only one number indicates how many C atoms are in the amino acid.  For example:

 

Number of C atoms in the diamine                6	Number of C atoms in the diacid 6	Name of the nylon Nylon 66
Number of C atoms in the amino acid 6		Name of the nylon Nylon 6

 

Polyamides (PAs) nylon 6 and nylon 66 are widely used as gas barrier materials in flexible packaging applications.  They are not often used as a gas barrier in rigid plastics packaging applications.  The gas permeability of nylon 6 and nylon 66 increases at high humidity.  They are also chemical resistant raw materials and suitable for solvent barrier applications.  Therefore, both nylon 6 and nylon 66 are used in multilayer HDPE bottles for agricultural chemicals and other solvent based chemicals. 

 

Polyamides have strong intermolecular forces.  These forces are combined with crystallinity to yield tough, high melting thermoplastic materials (For example, Nylon 66 has a melting point of 269°C).

Nylons can be processed using conventional extrusion process.  Films can be produced either the cast film or blown film processes.  During the film production, different degrees of crystallinity are obtained depending on the temperature and rate of quenching.  When the cooling rate is increased, a less crystalline nylon is obtained since the polymer was not given sufficient time to form crystals.  The decrease in crystallinity produces a more transparent and more thermoformable film.  Biaxial orientation of nylon films provides better crack resistance, mechanical and barrier properties.     

Blow molding is used to produce industrial containers, fuel tanks and oil reservoirs, as well as some other containers.  Thermoformed polyamides are also used for disposable pharmaceutical packs, and meat and cheese packaging.

 

Polyamides are used in coextrusion with other plastic materials.  Polyolefins are commonly used in the coextrusion processes to provide heat sealability and moisture barrier, and also to reduce cost.  Multilayer films containing nylon layer are used in vacuum-packing of processed meats.  Also, PVDC coating on PA is available for improved gas barrier properties.  PA is also used to extrusion coat paperboard to get heavy duty paperboard. 

 

 

EVOH

Ethylene vinyl alcohol (EVOH) is produced by a controlled hydrolysis of ethylene vinyl acetate copolymer.  The highly polar OH groups increase the intermolecular forces, while the ethylene groups maintain molecular mobility.  The polymer has randomly distributed ethylene and vinyl alcohol units.  The ratio of ethylene to vinyl alcohol determines the end product characteristics.  The lowest ethylene or the highest vinyl alcohol content can be used to get very high barrier properties whereas the highest ethylene content or the lowest vinyl alcohol leads to better flexibility.  Processors should decide the desired processing characteristics in order to decide on the ethylene/vinyl ratio that is suitable to their needs.

 

 

 

Structure of EVOH

 

EVOH is a moisture sensitive polymer.  The presence of water decreases the oxygen barrier properties of the material.  Therefore, EVOH is usually incorporated into packaging structures as a buried inner layer in a coextrusion surrounded by polyolefins or other good water vapor barrier polymers.  These structures contain an adhesive or tie layer between the EVOH and the polyolefin to provide adequate adhesion between EVOH and polyolefin. 

 

 

Properties of some EVOH copolymers

 

 

Property	EVOH 32 %, Ethylene	EVOH 44%, Ethylene
Density, g/cm3	1.19	1.14
Tensile Strength, MPa	88	68
Tm, °C	181	164
Tg, °C	70	55
Heat Seal Temperature, °C	179-238	177-238
Oxygen Permeability, cm3 µm2 day atm 0% RH 65% RH	0.004 0.013	0.24 0.045
WVTR, g µm/m2 day at 38°C 90% RH	2.5	2.8

 

Processing and applications of EVOH

Processing method	Sample application	Sample structure
Cast coextrusion	Processed meat, cheese	PET/EVOH/EVA
Blown extrusion	Red meat	LLDPE/EVOH/LLDPE
Lamination	Condiments	OPP/EVOH/LDPE
Coextrusion coating	Aseptic packaging	LDPE/paperboard/EVOH/Ionomer
Thermoforming	Yogurt	PP/EVOH/PP
Coextrusion blow molding	Ketchup	PET/EVOH/PET

 

PVDC

Polyvinylidene chloride (PVDC) is a transparent and almost colorless polymer.  PVDC can be obtained directly ethylene and chlorine or by the further chlorination of vinyl chloride with removal of hydrogen chloride by alkali treatment.  It is polymerized in suspension or emulsion processes.  The chemical formula CH₂=CCl₂, polymer repeating unit structure -[CH₂-CCl₂-]

Vinylidene chloride homopolymers and copolymers were first produced as Saran, a registered trademark of Dow Chemical.  Their low water vapor and gas permeability make them ideal for food packaging.  The polymers are based on vinylidene chloride (VCD) and comonomers such as vinyl chloride, methyl acrylate and vinyl nitrile. 

 

PVDC has a melting point around 400°C but it decomposes at 205°C, producing hydro chloric acid HCl in a manner similar to PVC.  Therefore, these conditions make PVDC homopolymer impossible to melt process.  By adding comonomers, the melting point is decreased to a range of 140-175°C, making melt processing feasible. 

Addition of comonomers also reduces crystallinity and the crystalline melting point, permitting processing at lower temperatures.  For example, vinyl chloride and methyl acrylate are used for extrudable resins, in amount from 6 – 28 %.  All commercially available PVDC resins are copolymers.           

 

The main applications of PVDC are in food packaging as barrier materials to moisture, gases, flavors and odors.  PVDC is also used in pharmaceutical and cosmetic packaging.  The structures containing 10 – 20 % of PVDC copolymer are used as shrinkable films.  PVDC is also used as a barrier material in thermoforming applications.