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On evaluation of the polar contribution to contact material surface energy
Journal of Food Engineering
12 (1990) 239-247
Short Communication On Evaluation of the Polar Contribution to Contact Material Surface Energy
ABSTRACT
related to the polar contribution to surface energy were measured for 20 different surfaces. Interpretation of these properties is discussed, and the method of their evaluation is compared to other approaches to suface characterizarion. The results indicate that the contact angle analysis used here avoids the potential ambiguity associated with those analyses most commonly used.
Properties
NOTATION b k w, w:,
W!
Ys, YL
Yd7 YP e Jts
Ordinate intercept in Fig. 1 (mJ m-“) Slope in Fig. 1 (dimensionless) Work of adhesion between a solid and liquid phase (J m- “) Dispersive (d) and polar (p) components of W,(J m- *) Surface or interfacial energy at a solid-vapor (S) and liquidvapor (L ) interface (J rnm2; also referred to as surface or inter-facial tension, N m- ’ ) Dispersive (d) and polar (p ) components of surface or interfacial energy (J m-*) Contact angle (“) Equilibrium spreading pressure (J m- ‘) INTRODUCTION
In a previous paper (McGuire & Kirtley, 1988) some theoretical considerations and limitations associated with the evaluation of food contact surface energetics were described. Measurement of surface energetics by analysis of contact angle data was reviewed, and the need for evaluation of the dispersive and polar components of the work of adhesion between 239 Journal
of Food
Engineering
0260-8774/90/$03.50
Publishers Ltd, England. Printed in Great Britain
- 0
1990
Elsevier Science
J. McGuire
240
a diagnostic liquid and the solid surface was emphasized. The work of adhesion, IV,, is the free energy change per unit area associated with separation of an interface. The dispersive component of W. is related to the geometric mean of the dispersive force contributions of the solid and liquid; its measurement was shown to be straightforward. However, the methods used to determine the polar component of W, have no theoretical basis. A first step towards both a useful and theoretically sound surface quantification was presented which included evaluation of the relationship between the polar contribution to W, and the polar force contribution of the diagnostic liquid surface tension, i.e. the relationship between WI: and y:. Preliminary results suggested this relationship is linear. Further, the value of the constant of proportionality, k, for a given surface was suggested to be related to the polar component of solid surface energy, ri, and to provide an index of contact material interactability with components of fluid food. In a later communication (McGuire & Kirtley, 1989) further support was lent to the notion that k is unique for a given surface and not a function of the diagnostic liquid series used to generate the data required for its calculation. It is the purpose of this note to present results of an evaluation of the polar surface character exhibited by a variety of materials, and to illustrate how analysis of contact angle data by the present method avoids the potential ambiguity associated with the most commonIy used analyses.
MATERIALS
AND METHODS
Anhydrous ethanol (EM Science, Cherry Hill, NJ) was dissolved into independent aqueous solutions at concentrations ranging from 0 through lOO%, and the surface tension of each solution was measured by the ring method. The dispersive and polar components of surface tension, r$ and $, were then evaluated for each solution by methods detailed earlier (McGuire & Kirtley, 1988). The solid materials evaluated are listed in Table 1. Most of the materials listed are commonly used in food contact as processing surfaces or packaging barriers; germanium (Ge) is an infrared-transmitting crystal used in surface spectroscopy. Paraffin and PE, were supplied as laminates on paper; all remaining polymers were supplied as flat strips or films. Grade # 304 stainless steel was cut into small rectangular sheets and polished to a mirror finish; the glass samples were simple microscope slides. Materials manufactured by the International Paper Company were actually obtained from the NC State University aseptic processing and packaging pilot plant in Raleigh, NC. Unless otherwise
Food contact surface characterization
241
TABLE 1 Surfaces Selected for Contact Angle Analysis Supplier
Surface
Paraffin
Northern Engineering Portland, OR
Viton
International
Polytetrafluoroethylene
(PTFE)
Polyethylene (PE), PE2
Paper Co., Raleigh, NC
Universal Plastics Co., Portland, OR International Paper Co., Raleigh, NC (not Hz0 equilibrated) Northern Engineering and Plastics Corp., Portland, OR (not H,O equilibrated) Northern Engineering Portland, OR
PC
and Plastics Corp.,
and Plastics Corp.,
High density (HD) PE
Universal Plastics Co., Portland, OR
Linear low density (LID) PE
Molded Container
Ultra-high mol. wt (UHMW) PE
Universal Plastics Co., Portland, OR
Glycol-modified terephthalate
Eastman Chemical Products, Inc., Kingsport, TN
polyethylene (PETG)
Corp., Portland, OR
Ethylene vinyl alcohol (EVOH)
International
Paper Co., Raleigh, NC
Polypropylene
Universal Plastics Co., Portland, OR
Nylon, Nylon, Nylon,
Kraft, Inc., Glenview, IL Universal Plastics Co., Portland, OR Du Pont Co., Wilmington, DE
Ge
Analect Instruments,
Acetal
Universal Plastics Co., Portland, OR
# 304 Stainless steel (ss)
Alaskan Copper and Brass Co., Portland, OR
Glass, Glass?
Erie Scientific Co., Portsmouth, NH Becton, Dickinson & Co., Parsippany, NJ
Utica, NY
noted, all surfaces were water-equilibrated prior to contact angle analysis as previously described (McGuire & Kirtley, 1988). Evaluation of solid surface properties The dispersive component of solid surface energy, $j, was measured and the contact angle data for the aqueous series of ethanol solutions were
J. McGuire
242
TABLE 2 Polar Surface Properties of Selected Materials
Material
k (dimensionless)
W W,,,, (ew (2)) 04~)
W!:water (experimental) WW)
Paraffin
0
0
0
- 1.77
Viton
0
0
0
- 2.67
PTFE
0
0
0
PE, PE, PE,
0 0.243 (O-123) 0.625 (0.049)
-i-38 - 3.52
(2.75) (0.878)
0 5.65 19.7
- 3.39 4.08 19.1
HD PE
0.864 (0.059)
- 6.45
(O-658)
25.6
- 2.55
LLD PE
0.868 (O-064)
- 5.20
(0.783)
27.0
12.5
UHMW PE
0.956 (0.060)
- 7.41
(0.666)
28.1
PETG
a969 (0.068)
EVOH
0.998 (0.117)
- 1.12
Polypropylene
1.01
(0.031)
Nylon, Nylon, Nylon,
1.10 1.23 1.31
Ge
- 21.4
4.97
36-o
34.6
(2.61)
36.0
38.5
- 8.06
(0.481)
29.5
28.2
(0.102) (O-045) (0.045)
- 3.82 - 6.86 - 16.0
(2.29) (O-803) (1.00)
37.0 38.8 32.7
39.1 40.3 33.3
1.28
(0.074)
- 4.55
(1.43)
43.0
45.6
Acetal
1.39
(O-045)
(0.801)
35.1
37.1
# 304 ss
1.76
(0170)
- 8.47
(3.80)
56.9
60.9
Glass, Glass,
1.91 2.04
(0.103) (0.169)
-4.51 - 6.49
(1.97) (3.78)
66,4 69.3
70.4 73.3
O-044 (1.52)
- 165
recorded for each material by methods detailed earlier (McGuire & Kirtiey, 1988). The value IV! was calculated for each drop formed on each surface as follows: wy:= yL(1+ cos e) - 2( y”ly$1’2
(1)
where yt , 0, yf, and y$ are known for each situation described by a drag of liquidin contact with any one material. Wgwas then plotted against y,_ for each of the 20 surfaces; a linear fit to the data was usually obtained. The slope of each line (k) and its intercept (b) were recorded.
Food contact surface characterization
243
RESULTS AND DISCUSSION Table 2 lists polar surface properties evaluated for each of the materials tested, along with the standard errors associated with each estimate of k and b (in parentheses). The magnitude of k generally agrees with the expected polar character of each solid surface. The negative values of the intercept have been suggested to be a measure of JZ~,the reduction in surface energy of the solid resulting from adsorption of vapor from the diagnostic liquid (Dann, 1970). It should be noted that the values of k reported here for paraffin, Viton, PTFE and PE, are defined as zero. Unlike the other surfaces, these four surfaces exhibited no polar character. Indeed, values of Wi determined with diagnostic liquids from the ethanol-water series were invariably less than or equal to zero. Therefore, the slope of such a line is suggested to have no physical significance. Polyethylene is relatively nonpolar; the reason for the pronounced difference in polar character exhibited by the different polyethylenes in comparison to paraffin, Viton and PTFE is not obvious. However, based on contact angle data recorded for four different series of polar liquids, Dann (1970) reported Teflon and paraffin as exhibiting no polar character, whereas polyethylene was observed to exhibit some polar character. Additionally, eqn (1) can be applied to data reported by Kaelble (1970) to show that polyethylene is again observed to be polar relative to PTFE and paraffin. Although the present data is in qualitative agreement with the previous statements, in each case the contact angle data were recorded under different conditions and with different procedures. Moreover, the exact chemical nature of the surfaces of the commercial preparations used in the present study is not known. Quantitative comparison of the present data with the previous data would consequently be difficult to interpret. However, it is certainly important to note the influence of surface equilibration with water, as illustrated by data presented for PE, , PE,, and PE, . Specifically, polyethylene apparently exhibits increased polar character upon water-equilibration. For solids that interact by dispersion forces only, all theoretical and experimental evidence predicts that adsorption of high energy material cannot reduce the surface energy of a low energy material. For example, the surface energy of paraffin cannot be reduced by adsorption of the aqueous solutions selected here as diagnostic liquids. The fact that a given liquid has a contact angle greater than zero degrees on a given low energy solid surface is evidence that the liquid surface energy is greater than that of the solid and ns should be zero. Consequently, b has been defined as zero for paraffin, Viton, PTFE and PE, in Table 2.
J. McGuire
244
Two important points warrant consideration with respect to evaluation of the properties listed in Table 2. First, yi is not proportional to (yP2; rather, it is directly proportional to yL. Second, the graphical method used here yields a generally better fit to the data than the graphical method (Zisman plot method) used to evaluate the critical surface tension, yC. These points are clearly illustrated in a comparison of Figs 1 and 2. Figure 1 is an application of eqn ( 1) to nylon2 and glass. Figure 2 is a Zisman plot constructed with the same data used in Fig. 1. With respect to nylon, and glass, greater confidence should be associated with estimates of k and b relative to estimates of yC. Table 3 lists the coefficients of determination achieved using the straight line method described here compared with those evaluated from Zisman plots created for each material. In general, a better fit is achieved with the present method. The value of the polar component of the work of adhesion between any given solid surface and water ( IV! water) can be recorded with the
0
10
30
yLP,
40
zJ,nl~
Fig. 1. Linear relationship achieved between the polar component of the work of adhesion and the polar component of liquid surface tension for nylon, and glass.
Food contact surface characterization
245
0.6 -
0.4’ 20
30
40
50 y,,
60
70
90
mJ/m2
Fig. 2. Relatively poor fit achieved by plotting the cosine of contact angles measured on nylon, and glass against the surface tension of the corresponding diagnostic liquid (the Zisman plot method).
following equation: w!
water = k( YE water) + b
(2)
The value W[ waterprovides an index of surface hydrophilicity; since it can be calculated with the straight line equation above, actual contact angle data for water need not be used. This avoids the rather serious problems associated with using pure water as a diagnostic liquid (Andrade, 1985). Indeed, contact angle data for water only occasionally falls on the straight line determined by the W; versus 7:: relationship, just as it only occasionally falls on the line constructed in a Zisman plot. For each of the 20 surfaces tested, Table 2 lists values of Wi,,,,, calculated by eqn (2), and calculated by a single application of eqn (1 ), using data for water only. Data in’Table 2 suggest that experimentally determined values of Wi wateragree with calculated values for relatively highenergy surfaces, and fall below those calculated by eqn (2) for lower-energy surfaces, including all of the polyethylenes.
246
J. McGuire
Coefficients
TABLE 3 of Determination Achieved Using the Present Method for Evaluation and b, and the Zisman Plot Method for Evaluation of yc
of k
? (present method)
? (Zisman plot method)
PE,
0.565 O-964
0.827 O-898
HD PE
O-964
0,974
LLD PE
O-969
0.967
UHMW PE
0.969
0.976
PETG
0.986
0.978
EVOH
O-96 1
0.798
Polypropylene
0.991
0.851
Nylon, Nylon, Nylon,
0.975 0.992 O-997
0.771 0.856 O-929
Ge
0.983
0.831
Acetal
0.994
0.847
# 304 ss
0.973
O-380
Glass, Glass,
O-986 0.980
0.463 o-514
S&ace
PEz
Finally, a comment should be made on current thoughts associated with what are referred to here as polar interactions. Fowkes (1985) suggests acknowledgment of acid-base interactions rather than polar interactions, in which two polar groups may interact only when one is acidic and the other basic. Acknowledging the complexity of fluid foods, the constituents of which exhibit acidic sites, basic sites, and sites of different relative acidities and basicities, it is suggested that general polar interactions will always be present if the solid surface exhibits any degree of polar character. However, more work in this area is probably warranted. ACKNOWLEDGMENTS Technical Station.
Paper
No. 8644
of the Oregon
Agricultural
Experiment
Food contact surface characterization
This work was supported Center, Logan, Utah; Oregon Universities cooperating.
247
by the Western Dairy Foods Research State, Utah State, and Brigham Young
REFERENCES Andrade, J. D. (1985). Surface and interface analysis of polymers - polymer surface dynamics. In Fouling and Cleaning in Food Processing, ed. D. Lund, E. Plett & C. Sandu. University of Wisconsin, Madison, pp. 79-87. Dann, J. R. (1970). Forces involved in the adhesive process. II. Nondispersion forces at solid-liquid interfaces. J. Colloid Znterjke.Sci., 32,32 l-3 1. Fowkes, F. M. ( 1985). Interface acid-base/charge-transfer properties. In Sueace and Interfacial Aspects of Biomedical Polymers. Volume 1: &face Chemistry and Physics, ed. J. D. Andrade. Plenum Press, New York and London, pp. 337-72. Kaelble, D. H. (1970). Dispersion-polar surface tension properties of organic solids. J. Adhesion, 2,66-81. McGuire, J. & Kirtley, S. A. (1988). Surface characterization for prediction of food particle behavior at interfaces: theoretical considerations and limitations. J. Food Engrg, 8,273-86. McGuire, J. & Kirtley, S. A. (1989). On surface characterization of materials targeted for food contact. J. Food Sci., 54,224-6.
Joseph McGuire Departments of Agricultural Engineering and Food Science &Technology, Oregon State University, Cowallis, Oregon 97331-3906, USA (Received 30 August 1989; revised version received 8 February 1990; accepted 14 March 1990)
12 (1990) 239-247
Short Communication On Evaluation of the Polar Contribution to Contact Material Surface Energy
ABSTRACT
related to the polar contribution to surface energy were measured for 20 different surfaces. Interpretation of these properties is discussed, and the method of their evaluation is compared to other approaches to suface characterizarion. The results indicate that the contact angle analysis used here avoids the potential ambiguity associated with those analyses most commonly used.
Properties
NOTATION b k w, w:,
W!
Ys, YL
Yd7 YP e Jts
Ordinate intercept in Fig. 1 (mJ m-“) Slope in Fig. 1 (dimensionless) Work of adhesion between a solid and liquid phase (J m- “) Dispersive (d) and polar (p) components of W,(J m- *) Surface or interfacial energy at a solid-vapor (S) and liquidvapor (L ) interface (J rnm2; also referred to as surface or inter-facial tension, N m- ’ ) Dispersive (d) and polar (p ) components of surface or interfacial energy (J m-*) Contact angle (“) Equilibrium spreading pressure (J m- ‘) INTRODUCTION
In a previous paper (McGuire & Kirtley, 1988) some theoretical considerations and limitations associated with the evaluation of food contact surface energetics were described. Measurement of surface energetics by analysis of contact angle data was reviewed, and the need for evaluation of the dispersive and polar components of the work of adhesion between 239 Journal
of Food
Engineering
0260-8774/90/$03.50
Publishers Ltd, England. Printed in Great Britain
- 0
1990
Elsevier Science
J. McGuire
240
a diagnostic liquid and the solid surface was emphasized. The work of adhesion, IV,, is the free energy change per unit area associated with separation of an interface. The dispersive component of W. is related to the geometric mean of the dispersive force contributions of the solid and liquid; its measurement was shown to be straightforward. However, the methods used to determine the polar component of W, have no theoretical basis. A first step towards both a useful and theoretically sound surface quantification was presented which included evaluation of the relationship between the polar contribution to W, and the polar force contribution of the diagnostic liquid surface tension, i.e. the relationship between WI: and y:. Preliminary results suggested this relationship is linear. Further, the value of the constant of proportionality, k, for a given surface was suggested to be related to the polar component of solid surface energy, ri, and to provide an index of contact material interactability with components of fluid food. In a later communication (McGuire & Kirtley, 1989) further support was lent to the notion that k is unique for a given surface and not a function of the diagnostic liquid series used to generate the data required for its calculation. It is the purpose of this note to present results of an evaluation of the polar surface character exhibited by a variety of materials, and to illustrate how analysis of contact angle data by the present method avoids the potential ambiguity associated with the most commonIy used analyses.
MATERIALS
AND METHODS
Anhydrous ethanol (EM Science, Cherry Hill, NJ) was dissolved into independent aqueous solutions at concentrations ranging from 0 through lOO%, and the surface tension of each solution was measured by the ring method. The dispersive and polar components of surface tension, r$ and $, were then evaluated for each solution by methods detailed earlier (McGuire & Kirtley, 1988). The solid materials evaluated are listed in Table 1. Most of the materials listed are commonly used in food contact as processing surfaces or packaging barriers; germanium (Ge) is an infrared-transmitting crystal used in surface spectroscopy. Paraffin and PE, were supplied as laminates on paper; all remaining polymers were supplied as flat strips or films. Grade # 304 stainless steel was cut into small rectangular sheets and polished to a mirror finish; the glass samples were simple microscope slides. Materials manufactured by the International Paper Company were actually obtained from the NC State University aseptic processing and packaging pilot plant in Raleigh, NC. Unless otherwise
Food contact surface characterization
241
TABLE 1 Surfaces Selected for Contact Angle Analysis Supplier
Surface
Paraffin
Northern Engineering Portland, OR
Viton
International
Polytetrafluoroethylene
(PTFE)
Polyethylene (PE), PE2
Paper Co., Raleigh, NC
Universal Plastics Co., Portland, OR International Paper Co., Raleigh, NC (not Hz0 equilibrated) Northern Engineering and Plastics Corp., Portland, OR (not H,O equilibrated) Northern Engineering Portland, OR
PC
and Plastics Corp.,
and Plastics Corp.,
High density (HD) PE
Universal Plastics Co., Portland, OR
Linear low density (LID) PE
Molded Container
Ultra-high mol. wt (UHMW) PE
Universal Plastics Co., Portland, OR
Glycol-modified terephthalate
Eastman Chemical Products, Inc., Kingsport, TN
polyethylene (PETG)
Corp., Portland, OR
Ethylene vinyl alcohol (EVOH)
International
Paper Co., Raleigh, NC
Polypropylene
Universal Plastics Co., Portland, OR
Nylon, Nylon, Nylon,
Kraft, Inc., Glenview, IL Universal Plastics Co., Portland, OR Du Pont Co., Wilmington, DE
Ge
Analect Instruments,
Acetal
Universal Plastics Co., Portland, OR
# 304 Stainless steel (ss)
Alaskan Copper and Brass Co., Portland, OR
Glass, Glass?
Erie Scientific Co., Portsmouth, NH Becton, Dickinson & Co., Parsippany, NJ
Utica, NY
noted, all surfaces were water-equilibrated prior to contact angle analysis as previously described (McGuire & Kirtley, 1988). Evaluation of solid surface properties The dispersive component of solid surface energy, $j, was measured and the contact angle data for the aqueous series of ethanol solutions were
J. McGuire
242
TABLE 2 Polar Surface Properties of Selected Materials
Material
k (dimensionless)
W W,,,, (ew (2)) 04~)
W!:water (experimental) WW)
Paraffin
0
0
0
- 1.77
Viton
0
0
0
- 2.67
PTFE
0
0
0
PE, PE, PE,
0 0.243 (O-123) 0.625 (0.049)
-i-38 - 3.52
(2.75) (0.878)
0 5.65 19.7
- 3.39 4.08 19.1
HD PE
0.864 (0.059)
- 6.45
(O-658)
25.6
- 2.55
LLD PE
0.868 (O-064)
- 5.20
(0.783)
27.0
12.5
UHMW PE
0.956 (0.060)
- 7.41
(0.666)
28.1
PETG
a969 (0.068)
EVOH
0.998 (0.117)
- 1.12
Polypropylene
1.01
(0.031)
Nylon, Nylon, Nylon,
1.10 1.23 1.31
Ge
- 21.4
4.97
36-o
34.6
(2.61)
36.0
38.5
- 8.06
(0.481)
29.5
28.2
(0.102) (O-045) (0.045)
- 3.82 - 6.86 - 16.0
(2.29) (O-803) (1.00)
37.0 38.8 32.7
39.1 40.3 33.3
1.28
(0.074)
- 4.55
(1.43)
43.0
45.6
Acetal
1.39
(O-045)
(0.801)
35.1
37.1
# 304 ss
1.76
(0170)
- 8.47
(3.80)
56.9
60.9
Glass, Glass,
1.91 2.04
(0.103) (0.169)
-4.51 - 6.49
(1.97) (3.78)
66,4 69.3
70.4 73.3
O-044 (1.52)
- 165
recorded for each material by methods detailed earlier (McGuire & Kirtiey, 1988). The value IV! was calculated for each drop formed on each surface as follows: wy:= yL(1+ cos e) - 2( y”ly$1’2
(1)
where yt , 0, yf, and y$ are known for each situation described by a drag of liquidin contact with any one material. Wgwas then plotted against y,_ for each of the 20 surfaces; a linear fit to the data was usually obtained. The slope of each line (k) and its intercept (b) were recorded.
Food contact surface characterization
243
RESULTS AND DISCUSSION Table 2 lists polar surface properties evaluated for each of the materials tested, along with the standard errors associated with each estimate of k and b (in parentheses). The magnitude of k generally agrees with the expected polar character of each solid surface. The negative values of the intercept have been suggested to be a measure of JZ~,the reduction in surface energy of the solid resulting from adsorption of vapor from the diagnostic liquid (Dann, 1970). It should be noted that the values of k reported here for paraffin, Viton, PTFE and PE, are defined as zero. Unlike the other surfaces, these four surfaces exhibited no polar character. Indeed, values of Wi determined with diagnostic liquids from the ethanol-water series were invariably less than or equal to zero. Therefore, the slope of such a line is suggested to have no physical significance. Polyethylene is relatively nonpolar; the reason for the pronounced difference in polar character exhibited by the different polyethylenes in comparison to paraffin, Viton and PTFE is not obvious. However, based on contact angle data recorded for four different series of polar liquids, Dann (1970) reported Teflon and paraffin as exhibiting no polar character, whereas polyethylene was observed to exhibit some polar character. Additionally, eqn (1) can be applied to data reported by Kaelble (1970) to show that polyethylene is again observed to be polar relative to PTFE and paraffin. Although the present data is in qualitative agreement with the previous statements, in each case the contact angle data were recorded under different conditions and with different procedures. Moreover, the exact chemical nature of the surfaces of the commercial preparations used in the present study is not known. Quantitative comparison of the present data with the previous data would consequently be difficult to interpret. However, it is certainly important to note the influence of surface equilibration with water, as illustrated by data presented for PE, , PE,, and PE, . Specifically, polyethylene apparently exhibits increased polar character upon water-equilibration. For solids that interact by dispersion forces only, all theoretical and experimental evidence predicts that adsorption of high energy material cannot reduce the surface energy of a low energy material. For example, the surface energy of paraffin cannot be reduced by adsorption of the aqueous solutions selected here as diagnostic liquids. The fact that a given liquid has a contact angle greater than zero degrees on a given low energy solid surface is evidence that the liquid surface energy is greater than that of the solid and ns should be zero. Consequently, b has been defined as zero for paraffin, Viton, PTFE and PE, in Table 2.
J. McGuire
244
Two important points warrant consideration with respect to evaluation of the properties listed in Table 2. First, yi is not proportional to (yP2; rather, it is directly proportional to yL. Second, the graphical method used here yields a generally better fit to the data than the graphical method (Zisman plot method) used to evaluate the critical surface tension, yC. These points are clearly illustrated in a comparison of Figs 1 and 2. Figure 1 is an application of eqn ( 1) to nylon2 and glass. Figure 2 is a Zisman plot constructed with the same data used in Fig. 1. With respect to nylon, and glass, greater confidence should be associated with estimates of k and b relative to estimates of yC. Table 3 lists the coefficients of determination achieved using the straight line method described here compared with those evaluated from Zisman plots created for each material. In general, a better fit is achieved with the present method. The value of the polar component of the work of adhesion between any given solid surface and water ( IV! water) can be recorded with the
0
10
30
yLP,
40
zJ,nl~
Fig. 1. Linear relationship achieved between the polar component of the work of adhesion and the polar component of liquid surface tension for nylon, and glass.
Food contact surface characterization
245
0.6 -
0.4’ 20
30
40
50 y,,
60
70
90
mJ/m2
Fig. 2. Relatively poor fit achieved by plotting the cosine of contact angles measured on nylon, and glass against the surface tension of the corresponding diagnostic liquid (the Zisman plot method).
following equation: w!
water = k( YE water) + b
(2)
The value W[ waterprovides an index of surface hydrophilicity; since it can be calculated with the straight line equation above, actual contact angle data for water need not be used. This avoids the rather serious problems associated with using pure water as a diagnostic liquid (Andrade, 1985). Indeed, contact angle data for water only occasionally falls on the straight line determined by the W; versus 7:: relationship, just as it only occasionally falls on the line constructed in a Zisman plot. For each of the 20 surfaces tested, Table 2 lists values of Wi,,,,, calculated by eqn (2), and calculated by a single application of eqn (1 ), using data for water only. Data in’Table 2 suggest that experimentally determined values of Wi wateragree with calculated values for relatively highenergy surfaces, and fall below those calculated by eqn (2) for lower-energy surfaces, including all of the polyethylenes.
246
J. McGuire
Coefficients
TABLE 3 of Determination Achieved Using the Present Method for Evaluation and b, and the Zisman Plot Method for Evaluation of yc
of k
? (present method)
? (Zisman plot method)
PE,
0.565 O-964
0.827 O-898
HD PE
O-964
0,974
LLD PE
O-969
0.967
UHMW PE
0.969
0.976
PETG
0.986
0.978
EVOH
O-96 1
0.798
Polypropylene
0.991
0.851
Nylon, Nylon, Nylon,
0.975 0.992 O-997
0.771 0.856 O-929
Ge
0.983
0.831
Acetal
0.994
0.847
# 304 ss
0.973
O-380
Glass, Glass,
O-986 0.980
0.463 o-514
S&ace
PEz
Finally, a comment should be made on current thoughts associated with what are referred to here as polar interactions. Fowkes (1985) suggests acknowledgment of acid-base interactions rather than polar interactions, in which two polar groups may interact only when one is acidic and the other basic. Acknowledging the complexity of fluid foods, the constituents of which exhibit acidic sites, basic sites, and sites of different relative acidities and basicities, it is suggested that general polar interactions will always be present if the solid surface exhibits any degree of polar character. However, more work in this area is probably warranted. ACKNOWLEDGMENTS Technical Station.
Paper
No. 8644
of the Oregon
Agricultural
Experiment
Food contact surface characterization
This work was supported Center, Logan, Utah; Oregon Universities cooperating.
247
by the Western Dairy Foods Research State, Utah State, and Brigham Young
REFERENCES Andrade, J. D. (1985). Surface and interface analysis of polymers - polymer surface dynamics. In Fouling and Cleaning in Food Processing, ed. D. Lund, E. Plett & C. Sandu. University of Wisconsin, Madison, pp. 79-87. Dann, J. R. (1970). Forces involved in the adhesive process. II. Nondispersion forces at solid-liquid interfaces. J. Colloid Znterjke.Sci., 32,32 l-3 1. Fowkes, F. M. ( 1985). Interface acid-base/charge-transfer properties. In Sueace and Interfacial Aspects of Biomedical Polymers. Volume 1: &face Chemistry and Physics, ed. J. D. Andrade. Plenum Press, New York and London, pp. 337-72. Kaelble, D. H. (1970). Dispersion-polar surface tension properties of organic solids. J. Adhesion, 2,66-81. McGuire, J. & Kirtley, S. A. (1988). Surface characterization for prediction of food particle behavior at interfaces: theoretical considerations and limitations. J. Food Engrg, 8,273-86. McGuire, J. & Kirtley, S. A. (1989). On surface characterization of materials targeted for food contact. J. Food Sci., 54,224-6.
Joseph McGuire Departments of Agricultural Engineering and Food Science &Technology, Oregon State University, Cowallis, Oregon 97331-3906, USA (Received 30 August 1989; revised version received 8 February 1990; accepted 14 March 1990)