Permeability and Other Film Properties of Plastics and Elastomers

~~ Plastics

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6

There are two related properties derived from Pick's first law and measured to assessthe barrier properties of plastic films and similar materials. These properties are the permeability coefficient and the vapor transmission rate. Pick's first law states that the volume (V) of a substancethat penetrates a barrier wall is directly proportional to the area (A) of the wall, partial pressure differential (p) of the penetrant, and time (t); and inversely proportional to the wall thickness (s), if the wall is homogeneous in the direction of penetration. The coefficient p in the equation representing Pick's first law, V = p {A .p. t)/s, is the permeability coefficient. Pick's first law applies only to permanent gasesthat obey Henry's law on proportionality of penetrant solubility in the barrier to the partial pressure of the ,penetrant. Therefore, the permeability coefficient can be measured only for permanent gases, i.e., gases that become liquid at pressures and temperatures far from normal (1 atm and DoC, respectively). These gasesinclude air, oxygen, argon, and carbon dioxide. The permeability coefficient can be measured according to ASTM standard 01434. A convenient unit of measurement for the permeability coefficient in the metric system is (cm3 .mm)/(m2 .day. atm). Since the permeability coefficient is dependent on temperature, a test temperature must be reported. The vapor transmission rate (VTR) is measured for the vapors of substances, such as water and acetone, that are liquid at pressures and temperatures close to normal. Vapors do not obey Henry's law, and the vapor transmission rate is not proportional to the pressure differential in Pick's first law. To account for this fact, Pick's first law for vapors is expressed as W = VTR {A .t)/s, where W is the weight of the penetrant. The vapor transmission rate can be measured according to ASTM standard 03985. A convenient unit of measurement for the vapor transmission rate in the metric system is (g. mm/ m2 .day). Since the vapor transmission rate is not proportional to the pressure differential, the latter must be stated to make test values of the vapor transmission rate meaningful. (It is customary to substitute a pressure differential with a relative humidity differential in reporting the water vapor transmission rate.) A test temperature must also be reported for the vapor transmission rate because of its dependence on temperature. The vapor transmission rate is influenced by the affinity between the vapor and the barrier material, and by processes that may occur during permeation, such as swelling of the barrier material. Vapor transmission rate values cannot be converted into permeability coefficient values. The following table gives conversion factors for the common units of measurement of the permeability coefficient and the vapor transmission rate. To convert a value from a common unit to the convenient metric unit for the permeability coefficient, (cm3. mm)/(m2. day. atm), or for the vapor transmission rate, (g. mm)/(m2. day), multiply this value by a factor provided in the corresponding column, taking into account instructions in the Note column.

Appendix

IV -Permeability

Units

Conversion

PERMEABILITY UNITS CONVERSION TABLE

1x10' m'/sac. atm

m3.om/an'.

1x10'

0.8400008401 set. atm

0.840000%401

a'(STP) . cml cm*, sac . atm an' (STP) mill 100 &I2/ day

8.64WOOe408 3.9370080-01

I

an' (STP) . mml ma/day an'. 0.1 mmJ m' . alm . day an' .0.5 mm/ 100 in' day an'. 100mmlm'~day, bar an'. 15ml m' . atm day an' 2Oml m' . day . atm an' .25m/ m' atm , day an'. miV 100 In' alm . day

1.000000em 1.--01 7.750015a+OQ

. sec

rniwm'

an' . miV m'

2 000000e-02 2.500000%-02 3.397008%-01

etm

. atm . d~;.

2.1945609407

I

an'. mm/an'. atm dav 8

I

ana mm/ m* atm dav ~.

I

8.7544809410 l.MMXOfi%+MI

I

1.a1325osm

.

.

I

I

.

una.mm/m', day, bar

1.013250B-03

an'. mmlm'. P a , day an'.mm/m'.sec.atm

8.64ooM)e404

an' . mm/ m' . sac . o m H ~ ana/1 0 0 in'. elm. dav

I I

an'/ m';rtm. day an'/ ma . day an'/ m' . day. bar an'/m'. day. bar, 100 an'. mil/ 100 in'. bar. dav ~

I

I

I

I

I

I

I

I

I

I

I

I -1

8.586397e+08 1.013250%+00 1.550M)3e401

1 1

1.000000e401

1,4

1.000000e400

1,4

1.01325W+Oo

1

1.013290%-02

1

I

g . 25mlm'. day

I

1.137749%+05

I

I

I

I 2.500000%-02

I

I

a.3(kn/ma.dav

3.00Mxx)e-02

g . miV 100 in'. aim day

3.937008%-01

3

3

.

i

3.9891730-01

. .

It'. miVn' poi day a.0.5mmJm'~dav

I

1.01325oe405

an'.N/m'.bar,dav

I

2.540000e-02 1.MxK)oe40l

an'.rnm/an'.kPa.aec

1 una mrn/m* bar, dav

I

1S00000e-02

8

I

an'.

4

1.013250eMO

g mW 100 In'

bar day

3.989173e-01

a . mlV 1 W In'

dav

3.397008%-01

In'. mW 100 in*. rtm dry

6.451800.*00

.

mg mil/ in'. dry

3.937008.-02

mg. mmtm' PI day ml mW m' rlm dry

2.54OWOWZ

ma.mm/m*. P i . dry

1.0132M)r+02

mn'. mm/m'. e Pr

8.75448Ol.03

mn'/m.MPr.day N . cm'/m'. bar. day

1. 0 1 3 2 ~ . 0 1

1

5

1.0182xk+02

3

2.540000.42

e

NOTES TO TABLE

-

!&&&& Values for the penneebHityweffkient and the vapor transmlsslon rate in most of the above units may be In the range of several powers of magnitude. However, these velues are usually given in an easy-tomd decinpl fomu\t @ractrCai unb),with the magnitude factor stated In a table or graph title or in the notes. Care rhould be taken, when convertkrg,to account for Me factor.

1. The conversion factor b qpllCable only H the Rkn thichess is lmowq multiply the vatwfactor productby the nlm tMcknesa (N) in mm. 2 Theorlginai unlt, 1 x 10 (tothe power of 10) cm3/un2/mm/seClcmHgl is Incorrect, Nehouldbe 1 x(10tothepowerof 10) 01113. mm/ cm?/seclcmHg or 1 x 1010cm3. mm/cm*.88c cmHg. a

3. UnN of prewre(e.g., etm) in the orlgld unIt can be Ignoredtor Ihemeasurement8condvcted at m a l pressure (1 ab);otherwise the conversion factor Is not valid and the value cannot be convefted. 4. The conversion factor is applicable only H the pressure dmerentiel Is lolown; dMde the value-factor product by the pressure in afm. 5. Theorlghsl unit,

100 in*/day, Is krcorrect, kshould be g

.mW 100 krVday or g

mW 100 Inz. day.

6. The original unit, g/ day/ mY mm, is incorrect, It should be g mml day/ m2. a

7. The original unIt, g/ day/ 100 in?/ mil, is Inconect, It should be g .mW day/ 100 in? 8. The original unit, g/ mV mil, is tnconect, It shouM be g In days.

. mW m2; the conversion factor is appikableonly If the t&neis known; divide the value-factor product by the time

9. The orlglnal unlt has a factor comprising a real number with a positive power of magnlhrde,which is krcorrect. The power of magnitude should be negative. The m m h factor has been revised to account for this error.

Glossary of Terms ABS See acrylonitrile butadiene styrene polymer. ABS nylon alloy See acrylonitrile butadiene styrene polymer nylon alloy. ABS PC alloy See acrylonitrile butadiene styrene polymer polycarbonate alloy.

acrylonitrile butadiene styrene polymer nylon alloy A thermoplastic processed by injection molding, with properties similar to ABS but higher elongation at yield. Also called ABS nylon alloy. acrylonitrile butadiene styrene polymer polycarbonate alloy A thermoplastic processed by injection molding and extrusion, with properties similar to ABS. Used in automotive applications. Also called ABS PC alloy.

ABS resin See acrylonitrile butadiene styrene polymer. accelerant See accelerator. accelerator A chemical substance that accelerates chemical, photochemical, biochemical, etc. reaction or process, such as crosslinking or degradation of polymers, that is triggered and/or sustained by another substance, such as a curing agent or catalyst, or environmental factor, such as heat, radiation or a microorganism. Also called accelerant, promoter, cocatalyst. acetal resins Thermoplastics prepared by polymerization of formaldehyde or its trioxane trimer. Acetals have high impact strength and stiffness, low friction coefficient and permeability, good dimensional stability and dielectric properties, and high fatigue strength and thermal stability. Acetals have poor acid and UV resistance and are flammable. Processed by injection and blow molding and extrusion. Used in mechanical parts such as gears and bearings, automotive components, appliances, and plumbing and electronic applications. Also called acetals. acetals See acetal resins. acetone A volatile, colorless, highly flammable liquid with molecular formula CH3COCH3. Acetone has autoignition temperature 537 °C, mixes readily with water and some other solvents and is moderately toxic. Acetone dissolves most thermoplastics and some thermosets. Used as organic synthesis intermediate, e.g., in the manufacture of bisphenol A and antioxidants, as solvent in paints and acetate fiber spinning and for cleaning of electronic parts. Also called dimethyl ketone, 2-propanone. acrylate styrene acrylonitrile polymer Acrylic rubber-modified thermoplastic with high weatherability. ASA has good heat and chemical resistance, toughness, rigidity, and antistatic properties. Processed by extrusion, thermoforming, and molding. Used in construction, leisure, and automotive applications such as siding, exterior auto trim, and outdoor furniture. Also called ASA. acrylic resins Thermoplastic polymers of alkyl acrylates such as methyl methacrylates. Acrylic resins have good optical clarity, weatherability, surface hardness, chemical resistance, rigidity, impact strength, and dimensional stability. They have poor solvent resistance, resistance to stress cracking, flexibility, and thermal stability. Processed by casting, extrusion, injection molding, and thermoforming. Used in transparent parts, auto trim, household items, light fixtures, and medical devices. Also called polyacrylates. acrylonitrile butadiene styrene polymer ABS resins are thermoplastics comprised of a mixture of styrene-acrylonitrile copolymer (SAN) and SAN-grafted butadiene rubber. They have high impact resistance, toughness, rigidity and processability, but low dielectric strength, continuous service temperature, and elongation. Outdoor use requires protective coatings in some cases. Plating grades provide excellent adhesion to metals. Processed by extrusion, blow molding, thermoforming, calendaring and injection molding. Used in household appliances, tools, nonfood packaging, business machinery, interior automotive parts, extruded sheet, pipe and pipe fittings. Also called ABS, ABS resin, acrylonitrile-butadiene-styrene polymer.

acrylonitrile copolymer A thermoplastic prepared by copolymerization of acrylonitrile with small amounts of other unsaturated monomers. Has good gas barrier properties and chemical resistance. Processed by extrusion, injection molding, and thermoforming. Used in food packaging. acrylonitrile-butadiene-styrene polymer See acrylonitrile butadiene styrene polymer. activation energy An excess energy that must be added to an atomic or molecular system to allow a process, such as diffusion or chemical reaction, to proceed. adsorption Retention of a substance molecule on the surface of a solid or liquid. alcohols A class of hydroxy compounds in which a hydroxy group(s) is attached to a carbon chain or ring. Alcohols are produced synthetically from petroleum stock, e.g., by hydration of ethylene, or derived from natural products, e.g., by fermentation of grain. The alcohols are divided in the following groups: monohydric, dihydric, trihydric and polyhydric. Used in organic synthesis, as solvents, plasticizers, fuels, beverages, detergents, etc. amorphous nylon Transparent aromatic polyamide thermoplastics. Produced by condensation of hexamethylene diamine, isophthalic and terephthalic acid. annulus test An ozone resistance test for rubbers that involves a flat-ring specimen mounted as a band over a rack, stretched 0 to 100% and subjected to ozone attack in the test chamber. The specimen is evaluated by comparing to a calibrated template to determine the minimum elongation at which cracking occurred. anthraquinone An aromatic compound comprising two benzene rings linked by two carbonyl (C=O) groups, C6H4(CO)2C6H4. Combustible. Used as an intermediate in organic synthesis, mainly in the manufacture of anthraquinone dyes and pigments. One method of preparation is by condensation of 1,4-naphthaquinone with butadiene. antioxidant A chemical substance capable of inhibiting oxidation or oxidative degradation of another substance such as plastic in which it is incorporated. Antioxidants act by terminating chain-propagating free radicals or by decomposing peroxides, formed during oxidation, into stable products. The first group of antioxidants include hindered phenols and amines; the second - sulfur compounds such as thiols. Ar See argon. area factor The ratio between the total area of pore openings on the surface of a membrane that is in contact with the incoming flow of a penetrant, to the area of this surface. argon A chemically inert, tasteless, colorless, noncombustible monoatomic gas. Argon is often used to characterize permeability of polymeric films, as carrier gas in gas chromatography, as inert gas shield in welding, in electric bulbs such as neon, lasers and as a process environment. Also called Ar.

aroma barrier A plastic film or its component preventing the escape of aromatic volatiles from foodstuffs or cosmetics seal-packaged in the film. aromatic polyester estercarbonate A thermoplastic block copolymer of an aromatic polyester with polycarbonate. Has higher heat distortion temperature than regular polycarbonate. aromatic polyesters Engineering thermoplastics prepared by polymerization of aromatic polyol with aromatic dicarboxylic anhydride. They are tough with somewhat low chemical resistance. Processed by injection and blow molding, extrusion, and thermoforming. Drying is required. Used in automotive housings and trim, electrical wire jacketing, printed circuit boards, and appliance enclosures. ASA See acrylate styrene acrylonitrile polymer. ASTM C177 An American Society for Testing of Materials (ASTM) standard test method for the measurement of steady-state heat flux in a flat-slab specimen by a guarded-hot-plate apparatus. The method provides for the calculation of thermal transmission properties based on the flux measurements. The measurements are carried out without heat flux reference standards on a variety of solids under different environmental conditions. The apparatus consists of the top and bottom isothermal cold plates with an isothermal heater placed between them, and two specimens placed between the cold plates and the heater, all enclosed in an insulated chamber. The heater has a metered heating core area surrendered by a primary guard. ASTM D96 An American Society for Testing of Materials (ASTM) standard test method for determining water vapor transmission of materials such as paper, plastic film and sheeting, fiberboards, wood products, etc., that are less than 31 mm in thickness. Two basic methods, the Desiccant Method and the Water Method are used. The specimens have either one side wetted or one side exposed to high humidity and another to low humidity. In the Desiccant Method, the specimen is placed air-tight on a test dish with a desiccant that is weighed to determine the gain of weight due to water vapor transmission. In the Water Method, the water is placed in the dish that is weighed to determine the loss of water due to evaporation through the specimen.

ASTM D256 An American Society for Testing of Materials (ASTM) standard method for determination of the resistance to breakage by flexural shock of plastics and electrical insulating materials, as indicated by the energy extracted from standard pendulum-type hammers in breaking standard specimens with one pendulum swing. The hammers are mounted on standard machines of either Izod or Charpy type. Note: Impact properties determined include Izod or Charpy impact energy normalized per width of the specimen. Also called ASTM method D25684. See also impact energy. ASTM method D256-84 See ASTM D256. ASTM D412 An American Society for Testing of Materials (ASTM) standard methods for determining tensile strength, tensile stress, ultimate elongation, tensile set and set after break of rubber at low, ambient and elevated temperatures using straight, dumbbell and cut-ring specimens. ASTM D471 An American Society for Testing of Materials (ASTM) standard method for determining the resistance of nonporous rubber to hydrocarbon oils, fuels, service fluids and water. The specimens are immersed in fluids for 22-670 hours at -75 to 250°C, followed by measuring of the changes in mass, volume, tensile strength, elongation and hardness for solid specimens and the changes in breaking strength, burst strength, tear strength and adhesion for rubber-coated fabrics. ASTM D570 An American Society for Testing of Materials (ASTM) standard method for determining relative rate of water absorption of immersed plastics. The test applies to all kinds of plastics: molded, cast, laminated, etc. The specimens are immersed for 2 to 24 hours or until saturation at ambient temperature, or for 1/2 to 2 hours in boiling water. The absorption is calculated as a percentage of weight gain.

ASTM D638 An American Society for Testing of Materials (ASTM) standard method for determining tensile strength, elongation and modulus of elasticity of reinforced or unreinforced plastics in the form of sheet, plate, moldings, rigid tubes and rods. Five (I-V) types, depending on dimensions, of dumbbell-shaped specimens with thickness not exceeding 14 mm are specified. Specified speed of testing varies depending on the specimen type and plastic rigidity. Note: Tensile properties determined include tensile stress (strength) at yield and at break, percentage elongation at yield or at break and modulus of elasticity. Also called ASTM method D638-84. See also tensile strength. ASTM D638, type IV See ASTM D638. ASTM method D638-84 See ASTM D638. ASTM D696 An American Society for Testing of Materials (ASTM) standard test method for the measurement of the coefficient of linear thermal expansion of plastics by using a vitreous silica dilatometer. The test is carried out under conditions excluding any significant creep or elastic strain rate and effects of moisture, curing, loss of plasticizer, etc. The specimen is placed at the bottom of the outer dilatometer tube and the tube is immersed in a liquid bath at a desired temperature. ASTM D774 An American Society for Testing of Materials (ASTM) standard test method for determining the bursting strength of paper having a bursting strength less than 200 points and a thickness less than 0.6 mm. The bursting strength is determined as the hydrostatic pressure in pascals required to produce rupture when applied at a controlled rate through a rubber diaphragm to a circular area 30.48 mm in diameter of the material held rigidly and initially flat but free to bulge under the increasing pressure. ASTM D882 An American Society for Testing of Materials (ASTM) standard test method for determining the tensile strength and percentage elongation at break and at yield, elastic modulus, tensile energy to break and other tensile properties of thin plastic film and sheeting of less than 1.0 mm in thickness, according to two different methods. Method A employs a constant rate of specimen grip separation, whereas Method B employs a constant rate of motion of one grip and a variable rate of motion of another, which is attached to a pendulum weighing head. ASTM D1004 An American Society for Testing of Materials (ASTM) standard test method for determination of the initial tear resistance of flexible plastic films and sheeting. The test is preformed at very low rates of loading, e.g., 51 mm/min, to measure the force required to initiate tearing. The specimen geometry in this test produces a stress concentration in a small area of the specimen. The maximum stress, usually found near the onset of tearing, is recorded in newtons or poundsforce. ASTM D1006 An American Society for Testing of Materials (ASTM) standard practice for conducting exterior exposure tests of house and trim paints on new wood. Painted testing panels (boards or plywood) are exposed for several years on vertical fences facing both north and south and visually examined for failures at prescribed intervals (1-6 months). ASTM D1434 An American Society for Testing of Materials (ASTM) standard test method for determining gas transmission rate, permeance and permeability (for homogeneous materials) of plastic film, sheeting, laminates and plastic-coated papers or fabrics under steady-state conditions. The sample is mounted in a gas transmission cell to form a barrier between 2 chambers. One chamber contains the test gas at a high pressure, and the other chamber receives gas at a lower pressure. The transmission rate is monitored either by the increase in pressure in the receiving chamber (Method M) or by a change in volume of gas (Method V). ASTM D1708 An American Society for Testing of Materials (ASTM) standard method for determining tensile properties of plastics using microtensile specimens with maximum thickness 3.2 mm and minimum length 38.1 mm, including thin films. Tensile properties include yield strength, tensile strength, tensile strength at break, elongation at break, etc. determined per ASTM D638.

ASTM D1709 An American Society for Testing of Materials (ASTM) standard test method for determining resistance of polyethylene film to impact by the free-falling dart. The impact resistance is measured as the energy that causes 50% failure rate of the film. The energy is calculated as the product of dart weight and dropping height. There are 2 test methods (A and B) using darts with different diameters of their hemispherical head and different dropping heights. ASTM D1894 An American Society for Testing of Materials (ASTM) standard test method for determining coefficients of starting and sliding friction (static and kinetic coefficients, respectively) of plastic film and sheeting when sliding over itself or other substances under specified conditions. ASTM D1922 An American Society for Testing of Materials (ASTM) standard test method for determining the resistance of flexible plastic film or sheeting to tear propagation. The resistance is measured as the average force, in grams, required to propagate tearing from a precut slit through a specified length, using an Elmendorf-type pendulum tester and 2 specimens, a rectangular type and one with a constant radius testing length. ASTM D2167 An American Society for Testing of Materials (ASTM) standard test method for determining in-place density and unit weight of compacted or firmly bonded soil using a rubber balloon apparatus. The calibrated apparatus is used to determine the volume of an excavated soil by measuring the drop in the level of the liquid that fills the apparatus and the void (through expansion of the rubber balloon) when the external pressure is applied. ASTM D2176 An American Society for Testing of Materials (ASTM) standard test method for determining the folding endurance of paper by the MIT tester. The tester has an oscillating folding head with a clamping jaw and a clamping jaw that can move in a direction perpendicular to the axis of rotation of the folding head. The endurance is reported as the number of double, 135 ° folds made at a given tension before fracture. Also called TAPPI T511. ASTM D2240 An American Society for Testing of Materials (ASTM) standard method for determining the hardness of materials ranging from soft rubbers to some rigid plastics by measuring the penetration of a blunt (type A) or sharp (type D) indenter of a durometer at a specified force. The blunt indenter is used for softer materials and the sharp indenter - for more rigid materials. ASTM D2457 An American Society for Testing Materials (ASTM) standard test method for the measurement of specular transparency of clear and colorless thin plastic sheeting. The transmittance is measured as the ratio of the radian flux transmitted by a specimen to that incident on it, in essentially the same direction. The test results are greatly influenced by the design characteristics of the instrument, e.g., the angular width of the receptor aperture that should be 50% vinylidene chloride and other monomers such as methyl methacrylate or vinyl chloride. They are conveniently available as latexes for use in waterborne coatings, as solvent-soluble resins (for use in solvent-based coatings) and as (co)extrusion powders. The permeabili~

ofPVDC

0.08 g .mm/m

.day

is quite low.

For example, water vapor transmission

at 38°C and 90% RH.

The permeability

of PVDC

rate ofa PVDC film is 0.01 -

decreases with

increasing

mole

fraction of vinylidene chloride due to an increase in crystallinity .Conversely, its toughness, flexibility at low temperatures and heat sealing properties improve with decreasing mole fraction of vinylidene chloride. PVDC has good chemical resistance and can be sealed by heat or high-frequency current. The main limitations of PVDC include corrosiveness during processing and limited stability to heat and sterilization by radiation. Its narrow processing temperature range prevents its co-extrusion with nylons and polycarbonate that require high processing temperatures. PVDC latexes are mainly used for coating plastic and cellulose films and extrudable PVDC is used in heatshrinkable and co-extruded films. Co-extruded PVDC has a good adhesion to polyvinyl chloride, ethylenevinyl alcohol copolymers Ethylene

Vinyl

Alcohol

and acrylics. Copolymers

Ethylene-vinyl alcohol copolymers (EVOH) are obtained by hydrolysis of ethylene-vinyl acetate copolymers and contain 32 or 44 mol% ethylene. EVOH containing >50 or