Water Vapor Permeability of Packaging Materials (LDPE) and Fabricated Package Systems
By
Kirtiraj Kundlik Gaikwad MS: Packaging
School of Packaging Michigan State University, East Lansing USA
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Water Vapor Permeability of Packaging Materials (LDPE) and Fabricated Package Systems
Kirtiraj K Gaikwad, Maria Rubino School of Packaging, Michigan State University, East Lansing 48825 MI USA
ABSTRACT
Low-density polyethylene (LDPE) films were investigate to the water vapor transmission rate (WVTR) of packaging material, fabricated package. And also to evaluate the influence factor of temperature and Relative humidity on WVTR. The permeation of water vapor through film was evaluated by two different approaches i.e. for WVTR of pouch by ASTM E96-08 at 37.50C 85%RH and for package system (ASTM F 1249) at 200C &400C with 60%RH &100 %RH. It was seen that as the temperature increase the value of permeability increases, The value of experimental and predicted permeability of the LDPE was compared and analyzed. Key words: LDPE, water vapor transmission, ASTM 96-08, Desiccants.etc
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1 INTRODUCTION One of the biggest concerns in the food industry is food deterioration by the penetration of moisture, oxygen, and, to a lesser degree, organic vapors. Storing food in the proper environment slows the deterioration and better preserves the food. However, the food's useful lifetime is much further extended by the use of polymeric packaging films such as polyethylene, polystyrene, vinyl polymers, and various cellulose-based films. Generally, these polymers serve very well as barriers between the environment and the food. (Smith et al 1991) Plastic films are used extensively in food packaging due to economic considerations and advantages in physical, mechanical, and chemical properties over other packaging materials. However, moisture may permeate plastic packaging materials during storage, resulting in a loss of food quality. For example, change in texture of food, change in color, thereby affecting the sensory quality of a food. (Maria Rubino et al 2001) So, there is a growing need for reliable methods of measuring water vapor transmission rate through films that are highly permeable to water vapor (Y. HU et al). Sorption and diffusion of different permeant molecules, such as water or organic vapor, through polymeric films may affect the barrier and mechanical properties of the polymer. (Maria Rubino et al 2001) We can use the permeation properties of the packaging material and storage conditions to predict the shelf life of the production Moisture subsequently adsorbs upon the surface of totally or partially dehydrated foods and penetrates into the food, while oxygen may react with the food surface. Eventually, both events lead to browning and rancidity. The time required for these processes is in part dependent upon the physical and transport properties of the polymer. Therefore, the selection of the polymer to be used for a particular food packaging application strongly influences its useful lifetime. (Smith et al 1991) LDPE is used in food packaging because of their properties, like excellent transparency, outstanding gloss, Good hot tack and sealing properties etc. So It is important to carry study on WVTR of LDPE since it have good demand in market. Standard method for Water vapor transmission described by ASTM E 96-08. It involves Desiccant and Water method we used desiccant method for our experiment. The Desiccant method requires a desiccant, usually anhydrous calcium chloride, to maintain the inside of the dish at 0% relative humidity (RH). The prescribed procedure requires the WVTR to be taken from the slope of net weight gain Qt versus time plotted at steady state. We also calculate Transmission rate, permiance, permeability coefficient etc as per ASTM 96-08 procedure. We also used ASTM F 1249 Method to measure a continues plastic film. The objective of the present study was to examine the Water vapor transmission rate of of both packaging material (LDPE) and fabricated package. And also to evaluate the influence factor of temperature and Relative humidity on WVTR
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2 Materials and Methods
Part 1: Determination of the water vapor transmission rate of LDPE pouch (ASTM E96) Materials: LDPE (low density polyethylene) film were provided by School of Packaging, Heat-sealing machine, Desiccant(W.A.Hammond Dorietile company ltd.USA), Electronic weigh balance (OHAUS Corporation calibrate, USA), Humidity chamber with 37.80C and 85% RH ( Hot Pack Corp., Philadelphia, PA,USA Model No: 417532),
Methods: In this part of experiment LDPE film taken. LDPE films which has 1.25 mil thicknesses. Then the film is cut into 10*10 cm2 * 2 to make a pouches for experiment. For accurate measurement is taken by scale. After cutting into10*10 cm2 Recorded the thickness and area of empty pouches and also weight. After that it sealed from three sides by heat sealing machine and one side is kept open for to fill desiccant. Total four pouches made from LDPE film and now pouches filled by 21.68059 gm desiccants approximately. Afar filling with desiccant pouch properly sealed on heat sealing machine. Then all four pouches that is three with desiccants and one is without desiccants is collect in tray and kept in conditioning chamber. After that weight change reading is taken after every two day of interval for all four pouches. Day by day we noticed that weight of pouches is increasing in some cases its slightly decreasing. After ten days we noticed that there is no weight change of pouches. And after that we gathered all reading and start analysis.
Fig: LDPE pouch 10*10 cm2 & Heat Sealing Machine
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By applying 𝑄𝑡 = (𝑇𝑡 − 𝑇𝑖 ) − (𝐶𝑡 − 𝐶𝑖 ), we measure the weight change. Finally, 𝑇𝑡 is the weight of pouch with desiccants in different time, 𝑇𝑖 is the initial weight of pouch, 𝐶𝑡 is the weight of empty pouch, and 𝐶𝑖 is the initial weight of empty pouch. Based on above equation we plot, of 𝑄𝑡 VS time graph. After draw a graph, the slope of the line is the value of 𝑄𝑡 / t. So we can respectively calculate the value of Transmission Rate (=𝑄𝑡 / A*t), the value of permance (Transmission Rate/ΔP), and the value of permeability (Permance* thickness). Eventually, we have gotten the permeability value of LDPE in RH 85% and 37.8 C).
Part 2: Water vapor transmission rate of Package system (ASTM F 1249) Materials: LDPE film, PERMATRAN W3/31, Grease for water vapor, Distilled Water, Nitrogen Gas, Method: Before start the tests exposed the apparatus to high moisture levels, outgas the system to desorb residual moisture. Then two samples of LDPE film of 50 cm2 area cut. We did experiment in four different conditions that are 20 C and 40 C temperature and 60%RH and 100%RH with one replicate of each sample. First the LDPE film were Cut the film using template for PERMATERAN W3/3. After cutting we measured the thickness of the film sample. After that checked the amount of nitrogen gas, flow rate, temperature. Mount the LDPE film to two cells with grease for water vapor. Setting the program to test two different temperatures 200C and 400C and two different RH 60% and 100% we can directly get the transmission rate of Cell A and Cell B in four different conditions that is 400C RH60%, 400C RH100%, 200C RH60%, and 200C RH100%
Fig: PERMATERAN W3/3
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To calculate the transmission rate, permeance and permeability of all sample in different RH and different Temperature we have formula for all Transmission Rate =𝑄𝑡 / A*t, Permance =Transmission Rate/ΔP, and Permeability = Permance* thickness. From this value we can calculate average permeability coefficient. After getting Permeability, we can plot a line in the graph of In Permeability VS 1/T from this graph we get slope, Ea (Activation Energy) / R(R=8.314 J/mol* K). From which we can calculate the activation energy. Then will get four Ea (Cell A, Cell B in RH60%, and Cell A, Cell B in RH100%).
Part 3: Data Interpretation: Material: Microsoft Excel By run T-test calculation, we will get a P-value. If the P-value< 0.05, that means null hypothesis In this experiment, null hypothesis is Ea at RH60%=Ea at 100% is fail to reject, which implies that activation energy is not affected by RH, otherwise, P-value>0.05 means activation is affected by RH. If null hypothesis is fail to reject, we can reasonably use the formula of P2= P1*exp^ Ea*(I/T11/T2)/R to predict the permeability at 37.8C by using the permeability at 400C.And finally we will compare the result of predict permeability at 37.8C with the experimental P from ASTM E-96.
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3 RESULTS Part 1: Determination of the water vapor transmission rate of LDPE pouch (ASTM E96) 1) Experimental Readings are as show in Table No 1 Table No: 1 25 Jan Sample
(Initial weight) (gm)
28 Jan (gm)
30 Jan (gm)
4 Feb (gm) 1 Feb (gm)
Pouch + desiccant A
21.98709
22.3767
22.566
22.9920
23.3947
Pouch + desiccant B
21.68059
22.1265
22.3822
22.6527
23.1937
Pouch+ desiccant C
21.74289
22.2579
22.6021
22.9528
23.2931
Empty Pouch D
0.8700
0.8680
0.8555
0.8481
0.8668
With the help of (ASTM E 96-08) equation we can calculate weight change of pouches. The equation is for weight change is as follow,
𝑸𝒕 = (𝑻𝒕 − 𝑻𝒊 ) − (𝑪𝒕 − 𝑪𝒊 ) Where, Qt = net weight gain of desiccant inside the pouch Tt = weight of the pouch with desiccant at time t Ti = the initial weight of the pouch with desiccant Ct = weight of the control pouch at time t Ci = the initial weight of the control pouch By Using above formula we can calculate the weight change So, calculation for example Pouch +Desiccants sample A of 28th Jan Qt = (22.3767-21.98709) – (0.8680-0.8700) = 0.39161 Applying same formula for above value in table so we get below reading in Table 2
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Table No 2:
Sample Pouch + desiccant A Pouch + desiccant B Pouch+ desiccant C
Day 3 Wt change (gm) 0.3916
Day 5 wt change (gm)
Day 10 wt change (gm)
0.5943
Day 7 wt change (gm) 1.0268
0.448
0.7162
0.9941
1.5164
0.5171
0.8738
1.2319
1.5535
1.4178
Now we have Qt for each pouch according to day that is time so now we can plot the graph for net weight gain of desiccant inside the pouch Qt verses time t. graph is as follow.
Qt vs time t 2 y = 0.1526x - 0.0348
Qt
1.5
Pouch+ DeciccantA Pouch+ Decicant B
y = 0.1487x + 0.1149
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Pouch+ Decicant C Linear (Pouch+ DeciccantA)
0.5 y = 0.1522x - 0.0936
Linear (Pouch+ Decicant B)
0 0
2
4
6
8
10
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Linear (Pouch+ Decicant C)
Time t
From above graph we get a slope, which we are using to calculate Transmission rate. So, No we are calculating WVTR, Permeance and permeability coefficient are as follow 1) Pouch + Desiccant A:
Q Slope (= t t ) q 𝐓ransimission Rate (Flux)= = At A (=10cm×10cm×2sides) =
0.1522g/day =0.003044 (g/day*cm^2) 100 cm2 *2
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Permeance=
Transimission Rate ∆p 0.003044 = =0.00007353545 (g/day*cm2 *mmHg) 48.7mmHg*0.85
Permeability = Permeance × Thickness =0.00007353545*1.25=0.00009191931(g*mil/day*cm^2*mmHg) 2) Pouch + Desiccant B: 𝐓ransimission Rate (Flux)=
q
Q Slope (= t )
t = A (=10cm×10cm×2sides) 0.1526g/day = =0.003052 (g/day*cm^2) 100 cm2 *2
Permeance=
At
Transimission Rate ∆p 0.003052 = = 0.00007372871 (g/day*cm2 *mmHg) 48.7mmHg*0.85
Permeability = Permeance × Thickness =0.00007372871*1.25=0.00009216088(g*mil/day*cm^2*mmHg) 3) Pouch + Desiccant C:
Q Slope (= t t ) q 𝐓ransimission Rate (Flux)= = At A (=10cm×10cm×2sides) 0.1487g/day = =0.0007435(g/day*cm^2) 100 cm2 *2 Permeance=
Transimission Rate ∆p 0.0007435 = = 0.0000179611(g/day*cm2 *mmHg) 48.7mmHg*0.85
Permeability = Permeance × Thickness =0.0000179611*1.25=0.00002245137(g*mil/day*cm^2*mmHg) Calculation for Average Permeability P is as follows: 0.00009191931+0.00009216088+0.00002245137 / 3 P = 0.00006884385(g*mil/day*cm^2*mmHg)
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Unit Conversion of permeability coefficient for SI Unit:
For Pouch+ Desiccant A: = (9.19193E-05)*(0.001*2.54)/ (86400*100000*133.323) = 2.02685E-19(kg*m/Pa*sec*m^2)
PermiabilityCoeficient (g*mil/day*cm^2*mmHg) PermiabilityCoeficient (kg*m/Pa*sec*m^2)
Pouch+ Desiccant A 9.19193E-05
Pouch+ Desiccant B 9.21609E-05
Pouch+ Desiccant C 2.24514E-05
2.02685E-19
2.03218E-19
4.9506E-20
Part 2: Water vapor transmission rate of Package system (ASTM F 1249)
Calculation of WVTR, Permeance, permeability coefficient of film
For 40C 60%: Cell A:
WVTR:
Permeance=
Per
6.343525+6.290430+6.156934/3 = 6.26362966 (gm/m2-day) Transimission Rate ∆p 6.263629 = = 0.18877724532 (g/day*cm2 *mmHg) 55.3mmHg*0.6
Permeability = Permeance × Thickness = 0.188777*1.25=(g*mil/day*cm^2*mmHg)
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So From above equation we can get WVTR, Permeance, permeability coefficient of film as shown In below table.
Average
60%RH 100 % RH Cell A Cell B Cell A Cell B Cell A Cell B Cell A Cell B 50 cm2 50 cm2 50 cm2 50 cm2 1.25 1.25 1.25 1.25 LDPE LDPE LDPE LDPE 20 C 40 C 20C 40C 1.431576 1.45172 6.343525 6.316577 2.663229 2.599164 16.04899 16.49935 1.151483 1.167214 6.29043 6.286842 2.664515 2.589984 16.15097 16.45501 1.255238 1.271873 6.156934 6.06122 2.769256 2.645124 16.04011 16.53333 1.279432333 1.29694 6.26363 6.221546 2.699 2.611424 16.08002 16.4959
Permeance(g/day*cm^2*mmHg) Permiability Coeficient (g*mil/day*cm^2*mmHg) Permiability Coeficient (kg*m/Pa*sec*m^2)
0.121850698 0.123518 0.188777 0.187509 0.154229 0.149224 0.290778 0.298298 0.152313373 0.154397 0.235972 0.234386 0.192786 0.18653 0.363472 0.372873 3.35856E-16 3.4E-16 5.2E-16 5.17E-16 4.25E-16 4.11E-16 8.01E-16 8.22E-16
Area Thickness Material Temp WVTR (gm/m2-day)
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Plot and select the Calculation of activation energy under the different RH%
Temp lnP 1/T
-34.6 0.00315 -34.7
60%RH Cell A 20 C
Cell B
Cell A 40 C
-35.6298496 -35.6163 0.00341122 0.003411
0.0032
0.00325
Cell B
100 % RH Cell A 20C
Cell B
Cell A 40C
Cell B
-35.1921 -35.1988 -35.3942 -35.4272 -34.7601 -34.7346 0.0031930.003193 0.003411 0.003411 0.003193 0.003193
ln P vs 1/T 0.0033
0.00335
0.0034
0.00345
-34.8 Cell B 60% RH
-34.9
lnPermiabilituy
-35
-35.1
y = -2910.6x - 25.465
Cell A 100% RH Cell B 100% RH Cell A RH 60%
-35.2 -35.3 -35.4
y = -3179.2x - 24.582
-35.5 y = -1916.1x - 29.08 -35.6 -35.7
y = -2009.4x - 28.775
1/T (K)
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Now from above graph we got slope (-Ea/R) for each RH%, and we know R value (8.314) now we can calculate activation energy for each RH%. So for Cell A 60%RH Activation Energy = - (-2009.4 *8.314)/1000 = 16.7061516KJ Cell B 60%RH Activation Energy =-(1916.1 *8.314)/1000= 15.93046KJ Cell A 100%RH Activation Energy= (-2910.6*8.314)/1000 = 24.19873KJ Cell B 100%RH Activation Energy= (-3179.2*8.314)/1000 = 26.43187 KJ
Unit Conversation of Permeability coefficient for SI Unit For Cell A 20 60%RH, = (0.152313373)*(0.001*2.54)/ (86400*100000*133.323)=3.35856E-16kg*m/Pa*sec*m^2 From above calculation we got following SI unit conversion
60%RH Cell A Temperature 20 C Permeance(g/day*cm^ 0.12185 2*mmHg) 0698 Permeability 0.15231 Coefficient 3373 (g*mil/day*cm^2*mm Hg) Permeability Coeficient 3.35856 (kg*m/Pa*sec*m^2) E-16
Cell B
0.123 518 0.154 397
Cell A 40 C 0.188 777 0.235 972
0.187 509 0.234 386
100 % RH Cell A 20C 0.154 229 0.192 786
3.4E16
5.2E16
5.17E16
4.25E16
Cell B
Cell B
Cell B
0.149 224 0.186 53
Cell A 40C 0.290 778 0.363 472
4.11E16
8.01E16
8.22E16
0.298 298 0.372 873
Part 3: F-test: F-test for 60%RH and 100%RH is 0.425667 which is higher than 0.005 so we need to compare data by T-test.
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T-test In this study T-test is to conclude whether Activation Energy will be affected by Relative Humidity or not. In T-test, I select two trials. The first trial is two different values of Activation Energy in RH 60%, and the second trial another two values of Activation Energy in RH 100%. We test null hypothesis Ea in RH 60% = Ea in RH 100% (a=0.05).After calculating we got the Pvalue in T-test-value=0.0168<0.05, this means the null hypothesis fail to reject. So we can say that, Activation Energy is not influenced by RH. We can use activation Energy in RH 60% to predict the P in RH 85%, because activation is not affected by RH. Another factor, temperature also doesn’t influence Ea. Because Ea/R is the slope of in Permeability VS 1/T, in the same line, different points are in different temperatures, but they are in the same line, which means they have the same slope. So Ea is not affected by T. Depending on this analysis, we will use the average of Ea. The average of Ea is 20.816KJ. Using this formula P2= P1*exp^ Ea*(I/T1-1/T2)/R, The permeability in 40C RH 100% is 8.01E-16. We can know the Permeability in 20C. P2= 8.018E-16 *e^20816*(1/319-1/3411)/8.314=5.7045369E-15kg*m/Pa*sec*m^2 From above calculations, our experimental permeability at 37.8C and RH85% is 5.22E-16 kg*m/Pa*sec*m^2, and the predicted experimental permeability at 37.8C and RH85%is 5.7045369E15kg*m/Pa*sec*m^2. In these two values I do not found much difference, so we can say that method in this experiment is correct. And the conclusion that activation energy is not affected by RH
5 DISCUSSION: Application of WVTR in food packaging: WVTR is very important in food packaging. A critical function of flexible packaging is to keep dry products dry (potato chips, pretzels, fortune cookies) and moist products moist (cheese, muffins, chewing gum). Without protective packaging, products will quickly gain or lose moisture until they are at equilibrium with the environmental relative humidity. At this point, crispy products are soggy, and chewy products are hard and dry. WVTR is the standard measurement by which films are compared for their ability to resist moisture transmission. Lower values indicate better moisture protection. With the Water vapor transmission rate testing, effective quality control of food can be guaranteed so as to improve the storage, transportation and shelf life results and prolong shelf life span of the product
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Temperature dependence of permeability coefficient: Partial pressure influence on temperature and RH, so that mean RH affects the permeability of coefficient P. For the mechanism, if we increase the temperature the molecules in polymeric packaging material is start moving due to this more space is generate in material. So that there are more holes create in material and due to this water vapor molecular pass from hole and permeation occur in large quantity. Relative Humidity dependence of Activation energy: LDPE is hydrophilic material which means readily absorbing or dissolving in water. Hydrophobic is repelling, tending not to combine with, or incapable of dissolving in water. They they have low activation energy and gets easily disturbed with the increase in the moisture. Whereas the activation energy of hydrophobic energy is high and it is less affected by the increase in the humidity. Error Analysis: - Pouch: The thickness of the pouch can vary from place to place, which can vary the permeation process of the pouch as well as predicted value. -Weighing Balance: There is chance of error generation in the measuring of the weight of empty (control pouch) and pouch filled with desiccants -Humidity Chamber: - The regular opening and closing of the humidity chamber of the Chamber door can change the inner condition of the chamber which can deviate our end WVTR results of the pouch. - Human Error: - There is great chance of error generation by human component in calculation part and collection of the data.
6 REFERENCES 1) Y. HU et al, Measurement of Water Vapor Transmission Rate in Highly Permeable Films Journal of Applied Polymer Science, Vol. 81, 1624–1633 (2001) 2) Maria Rubino et al, Permeation of Oxygen, Water Vapor, and Limonene through Printed and Unprinted Biaxially Oriented Polypropylene Films. J. Agric. Food Chem., 2001, 49 (6), pp 3041–3045 3) Smith, J. S.; Peppas, N. A. Mathematical analysis of transport properties of polymer films for food packaging. VII. Moisture transport through a polymer film with subsequent adsorption on and diffusion through food. J. Appl. Polym. Sci. 1991, 43, 1219−1225 4) Standard Test Methods for Water Vapor Transmission of Materials ASTM International 15
Designation: E96/E96M – 10 5) Standard Test Method for Water Vapor Transmission Rate through Plastic Film and Sheeting Using a Modulated Infrared Sensor ASTM Designation: F1249 – 06 (Reapproved 2011)
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