Corona-Discharge Treatment of Polymeric Films, : Chemical Studies JAMES F. CARLEY* Lawrence Livermore Laboratory Livermore, Calqornia 94550 and P. THOMAS KTZE** University of Colorado Boulder, Colorado 80309 Polymeric films, chiefly polyethylenes, were subjected to corona-discharge treatment in a continuous treater at commercial rates in a program covering wide ranges of the main processing factors (2). Electron-spin-resonance measurements on freshly treated films found no free radicals. Reactions of the treated surfaces with a free-radical compound, diphenyl picryl hydrazyl (DPPH) were studied, focusing mainly on the rate effects. The evidence indicates that corona treatment produces ' fairly stable peroxide structures ofthe forms RO,R andr0,r on polyethylene surfaces. RO,R reacts rapidly with DPPH alone, while R02R undergoes a slower reaction after addition of the catalyst, triethylene diamine. DPPH is capable of detecting as few as lol3 peroxide groups per square centimeter. Activation energies were 12 kcalimole for the uncatalyzed reaction and 16 kcal/mole for the amine-catalyzed reaction. As with the physical effects reported earlier (2), the production of peroxides is most strongly dependent on the energy delivered to the film during treatment. This energy is proportional to the quotient of corona current and web speed, US. Regression analysis showed that air-gap thickness, relative humidity, and number of electrodes used also were significant factors, while dielectric thickness and corona frequency were not. We found thats, the polar component of surface energy of the treated film, which is nearly zero for untreated polyethylenes, is exponentially related to the concentrations of both RO,R and RO,R with a correlation coefficient for 92 specimens tested of 0.88. We believe this is the first strong evidence linking treatment factors, at commercial levels of treatment, to chemistry of the treated surface and linking: both of those to changes in physical behavior of the surface. NTRODUCTON lectrical-corona discharge has been in use for about E three decades for the improvement of adhesion of inks and adhesives to polymeric films, yet has received relatively little fundamental study. Although we are still striving to understand the mechanisms of adhesion phenomena, energy concepts are generally accepted. Particle energies in corona discharge are on the order of 10 electron-volts (ev), far above the energies typical of covalent bonds. Energies of carbon-carbon and carbon-hydrogen bonds, for example are 2.54 and 3.79 ev, respectively. Free radicals formed on the film surface rearrange to form functional groups that directly enhance wetting and adhesion. * To whom corrrspondence should he addressed. ** Present address: BM Corporation, Boulder, Colorado Nearly all prior work, in contrast to this study, was done at treatment intensities (energy applied per unit area treated) orders of magnitude above those used commercially. Preceding work is reviewed comprehensively in Kitze's thesis (1); only a few most relevant papers are mentioned here. n an earlier report (2), we dealt with the effects of treatment factors on the physical characteristics of films surface energy, wetting and peel-adhesion strength. We will be concerned here mainly with the chemical effects. McKelvey (3) reviewed the literature for polyethylene surface activation up to about 1960. He found that most investigators agreed that polar groups were formed on the surface, probably by oxidation. n his discussion of corona-discharge treatment, the film, placed between electrodes, was treated as the dielectric of a capacitor that charges and discharges during each cycle of alter- 330 POLYMER ENGNEERNG AND SCENCE, MARCH, f980, Vol. 20, No. 5
Corona-Discharge Treatment of Polymeric Films, 11: Chemicul Studies nating applied potential. Porter (4), in his kinetic analysis of corona-discharge activation, found that the rate of activation was directly related to current. He also postulated that current, and hence treatment, would be directly related to frequency of applied field. n our work, however, variation of frequency from 800 to 3000 hz had no significant effect on any aspect of treatment. Hines (5) studied the chemical changes occurring at low treatment levels on polyethylene by analyzing surface scrapings from treated and untreated films. nfrared evidence indicated the presence of hemiformal, formal, polyformaldehyde, vinylidene, peroxide, and others. Bradley and Fales (6) stated that oxygen-containing plasmas cause formation of ketone, peroxide, and other groups on polymer surfaces. They used peroxidized surfaces for grafting vinyl monomers. Bradley and Heagney (7) refined an iodometric test to analyze for very low levels of surface peroxides on films. On polyethylene terephthalate they found an increase of about 5 x 1014 peroxide groups per square centimeter after corona-discharge treatment. Attempts to determine the species formed during corona treatment have been relatively unsuccessful because of the small amount of material involved at commercial treatment levels. Our own attempts at ATR (attenuated-total-reflectance) infrared analysis resulted in detecting no chemical changes at these levels. Except for Hines (5), other investigators who detected carbonyl and unsaturation with R applied treatment times and energies several orders of magnitude greater than commercial levels. The existence of stable free radicals on surfaces exposed to corona discharge has been shown for polystyrene by electron-spin resonance (ESR) by Kelen and Dick (8), but they found no free radicals with polyethylene. Kim, Evans, and Goring (9) also reported no evidence of free radicals by ESR on polyethylene treated in air corona. nitially, in this work, we speculated that free radicals might be formed on the treated surface and could play a significant role in the observed phenomena. Attempts to react the surface for a few seconds after treatment with a solution of diphenyl picryl hydrazyl (DPPH), a freeradical scavenger, showed no significant immediate reaction. Also, samples less than an hour old gave no detectable organic-free-radical signal with electron-spin resonance. Fortunately, a reactive species created by the corona treatment did react, later, with the DPPH solution in a significant way. Efforts to mod$ the reaction of the original species present to gain additional clues as to its identity revealed a way to analyze for a second species. What we think occurs in corona treatment is production of free radicals by hydrogen-atom abstraction, interaction with atmospheric oxygen by these radicals to form peroxy radicals, and subsequent rearrangement and reaction of the peroxy radicals. Ozone is present and must be considered a likely reactant. Organic peroxides have been studied (10) and their formation and reactions are reasonably well known. Based on this literature, the observed phenomena of films treated in air coronas, and our own analytical results, we have developed some ideas on the chemistry of this process. We deal in particular with polyethylene but believe that other polymers react similarly. One of the first things that happens on the film surface during corona discharge is at least partial destruction of existing peroxide structures (probably cross-links between chains). Carbonyl is likely to form from secondary alkyl peroxides (10, p. 54). mprovement in heatsealability at low treatment levels (as is observed) would certainly be expected with reduced surface crosslinking. The reactions for this step are as follows: R <H-R 0 0 0 0 R <H-R corona 1-2R <H--R+SR <--R + Hz Simultaneous reaction of the polymer chains with the corona could be expected to form alkyl radicals; these are most likely to be either secondary (1) R~--CH~-R+R --~H-R + H. (2) or tertiary, when attack is at a branch point. Addition of oxygen to alkyl radicals is a zero-activation-energy reaction which is extremely fast (11), giving a peroxy radical: R. + 02-+ROz* (3) Peroxy radicals tend to combine, also rapidly, to form peroxides. Recent evidence suggests (12) that an intermediate tetroxide (or possibly a cage complex) exists: 2ROZ**R04R+R02R + 0 2 (4, 5) Alkyl peroxy radicals are also known to have other fates, ofcourse (10, p. 159). Cyclic ethers can be formed with elimination of hydroxyl radicals. ntramolecular rearrangements and scissions can give alkenes and carbonyls in a variety of ways. Other products from peroxy-radical reactions include hydrogen, short-chain alkanes, carbon dioxide, and formaldehyde. Almost all of these have been observed (13, 14) on corona-treated polyethylene or in gaseous reaction products from the corona. The reaction of ozone with hydrocarbons (especially alkenes) can form ozonides and, subsequently, ozonide-degradation products, including other peroxides, carbonyl compounds, and olefins (10, p. 39). Ozonides are cyclic trioxides: R CHSH-R 0-0-0 + 03-+R <H----CH-R +R <H 0 4 CH-R \/ 0 (6) (7) The relative rates of the various reactions and the reaction conditions will be extremely important in determin- POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5 33 1
James F. Carley and P. Thomas Kitze ing what the final surface composition will be after treating. For example, in our studies at relatively brief (20 ms) treatment at low applied-energy levels, some ozonide appears and the initial peroxide level decreases somewhat. At short times and high energies both ozonide and peroxide become more evident. This could result from either corona-induced decomposition of the ozonide or direct reaction between the RO,. and R-. For long corona-treatment times and moderate energy levels, we see some increase in ozonide and more increase in ROzR using our analytical technique. Extremely long times (minutes to hours) have allowed other investigators to use infrared analysis to pick up peroxide and alkoxy (RO-) decomposition products: carbonyl, ethers, unsaturation, etc. The lives of these surface groups upon storage after treatment have also been studied. The unstable group, which we attribute to ozonide, has a half-life of 6 to 8 h. The short duration of heat-sealability advantages from low levels ofcorona treatment (14,15) may be due to the presence of these groups. The peroxide groups are very stable; we have estimated the half-lives of surface peroxides at several months. DPPH REACTON WTH TREATED FLM The analytical procedure developed during this study reacts the treated polymer surface with dilute solutions of DPPH in benzene. DPPH, a stable free radical whose structure is diagramed below, has been frequently used as a scavenger in free-radical analysis, for example, in the study of peroxide-decomposition rates (16). Structure The intense coloration of DPPH at extremely low concentrations makes it very attractive as a reagent for trace analysis by spectrophotometry. We assume that decomposition of each peroxidic group gives two radical segments. Each of these in turn is assumed to be captured by a DPPH molecule to form a stable product. We also make use of the fact that tertiary amines cause decomposition of alkyl peroxides in which a carbon atom attached to a peroxide oxygen also has a hydrogen (10, p. 255). n this study we used triethylene diamine, which has the structure diagramed below. CHZ--CHz / \ N-CH~-CH~-N \ / CHZ--CHz n the preliminary work with the reaction of DPPH and film, certain observations were made that guided development of a proposed reaction mechanism. All of the following were made with polyethylene (PE) unless noted otherwise. 1. DPPH concentration showed an exponential decay (half-life = 6 h) to a lower level. The change in concentration was approximately proportional to amount of treatment. After the exponential decay, there was an almost constant rate of usage of DPPH with time; in many cases this rate appeared to be the same as obtained with untreated film (where no significant exponential decay occurred). 2. Use of a tertiary amine (triethylene diamine) as a catalyst gave a rapid acceleration of DPPH usage initially. Also it gave a curve of concentration vs time very similar in shape to the curve obtained without amine. However, much more usage of DPPH resulted and the half-life was dependent on the catalyst level used. 3. Use of DPPH at three concentration levels (0.0250 g/l, 0.0124 g/l, and 0.00625 g/l) gave practically identical curves of DPPH usage vs time at the zero and low amine levels. 4. DPPH solution using benzene as the solvent showed no detectable change in concentration over a few days time. 5. DPPH solution in benzene showed a slow reaction with amine, the reaction rate increasing with increase in concentration of either DPPH or amine. (At the high concentration of amine we could see an effect of DPPH concentration, but at the low amine concentration we could not.) 6. Untreated film showed a low, essentially constant, rate of DPPH usage without amine. With amine, however, untreated film gave an exponential decay to lower DPPH concentrations. At low treatment levels this exponential decay of DPPH usage became less than for untreated film. But at high treatment levels, the usage became much higher than for untreated film. 7. Analysis of an untreated, highly branched film, a polyethylene-vinyl acetate (PE-VA) copolymer, showed a reaction similar to that of untreated PE when no amine was present. With amine, the untreated PE-VA film gave a very high exponential decay of DPPH usage. On treatment, the rate of DPPH usage and amount of usage were about the same whether amine was present or not. With high levels of treatment usage of DPPH with amine on treated film can actually be somewhat lower than on untreated film. 8. Reaction of DPPH with treated film (PE) with no amine present until completion of the exponential decay and then addition of amine to that sample gave total DPPH usage equivalent to that obtained by adding amine to the sample initially. The exponential-decay rate on the sample after delayed-amine was added looked nearly the same as the decay rate where amine was added initially. 9. Addition of cobalt naphthenate, a hydroperoxide accelerator, caused retardation of DPPH usage. 10. ESR measurements within an hour of treatment showed no detectable free radicals. Details of sample preparation for these measurements are given in (1, p. 112). 11. The rate of usage of DPPH increased with temperature; the activation energy without amine was found to be about 16 kcal/mole. With the catalyst, the activa- 332 POLYMER ENGNEERNG AND SC NCE, MARCH, 1980, Vol. 20, No. 5
Corona-Discharge Treatment of Polymeric Films, 11: Chemical Studies tion energy was about 12 kcal/mole. 12. Reaction of treated film with SOz and subsequent examination with ATR infrared showed no detectable sulfate bond. SOz should react with hydroperoxides to give stable sulfates (17). The form of DPPH usage data suggested that two reactions would have to be considered: 1) a first-order peroxide decomposition, and 2) a side reaction to account for the gradual, almost constant, usage after the exponential-decay period was finished. The side reaction was observed to occur in a way that suggests possibilities for its cause: impurities from the film, reaction of DPPH with catalyst (when used), or reaction with other species from corona treatment. n any event, we can subtract out the DPPH usage for the side reaction if we make the approximation that its rate of DPPH usage is constant. This leads to the following: k ROxR~2ROy* (relatively slow) (8) 2ROy. + DPPH+Products (fast) (9) kl = first-order rate constant for peroxide decomposition. For the rate-controlling step: -- dcp = k,c, dt where C, = peroxide concentration, t = time. Usage of DPPH due to peroxide decomposition is: where CDppH = DPPH concentration. Using the initial condition that at t = 0, Cp = GPO, the initial peroxide concentration, one obtains Then For the side reaction Cp = Cpoe-klt (12) ( dci'ph) = -kz side where k, = zero-order rate constant for the side reaction. Total DPPH-usage rate is ntegrating and using the initial condition that at t = 0, CDppH = CDPP~,~, the initial concentration of DPPH, we find that the drop in DPPH concentration, X, is given by X = CDppH,O - CDppH = kzt + 2C (1 - e-klt) (16) Using X-vs-t data, kz, Cpo, and kl can be evaluated from a least-squares fit of the data. n our analysis of the process data we used several k,'s and solved for the kz and Cpo of each sample at each k,. Then the k, that minimized the residual sum of squares of all the data was chosen and a final k, and Cpo calculated for each sample. POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5 For the amine-catalyzed reaction, the catalyst is thought to work by abstracting tertiary hydrogens, with subsequent rearrangement (splitting) to radical species. R CHz--CHz / \ H4-B + N<HZ--CHz-N+2R{ \ / R CHz--CHz (17) B can be a peroxide or possibly other side-chain functional group. The radical products, of course, are scavenged by DPPH. Peroxides are thought to split at the oxygen bond during their rearrangement (10, p. 225). The analysis of this decomposition and associated side reactions gives the same results as the analysis of the uncatalyzed reaction. However, one additional factor must be considered; the peroxides that decompose without catalysis still react and we think their rate is also accelerated by the catalyst. Because of this acceleration, we can simplify our analysis by assuming that at all but very short times, DPPH usage by these species is complete and of a constant value. The equations then become: X, = 0 at t = 0 X, = CDPPH,~ - CDppH - 2Cpo for t > 0 (18) = kz,t + 2Cp0,(1 -,+lat) - 2Cp0 (19) The subscript a refers to the amine-catalyzed reaction. Solution of these equations for kza, GPO,, and k,, is carried out in the same manner that was used for the uncatalyzed reaction. Derivation of equations used for computation as well as the Fortran V computer program are in the Appendix of Ref. (1). The technique as developed is sensitive to about 1 x 1013 peroxide groupslcm' and can be carried out using the following extremely simple procedures. Film samples to be analyzed for unstable species are, after treatment, hand cut to convenient size, usually 100 cm2 (6 x 16.67 cm), rolled on a glass rod, and inserted into 13- x 100-mm screw-top test tubes. About 6 ml of DPPH in benzene (approximate concentration 0.61 g DPPH/) is accurately pipetted into the tube and the tube is stoppered and shaken to completely wet out the film surface. The solution is transferred to a spectrophotometer tube for determination of DPPH concentration at desired times. To analyze for the stable peroxide requires addition of a drop of stock amine solution before shaking. Transmittance readings were generally taken twice a day over a four day period. Some commercially used additives for flexible-film materials (for example, some antioxidants) appear to react with DPPH and interfere with the analytical procedure. EXPERMENTAL DETALS The films used in this work were described in Ref. (2) p. 328, with the bulk of the work devoted to a PE homopolymer containing no additives. Diphenyl picryl 333
James F. Carley and P. Thomas Kitze hydrazyl was obtained from Aldrich Chemical Company, catalog No. D21-140-0, and was used as received. Benzene was Mallinckrodt s spectrophotometric, analytical-reagent grade, used as received. Triethylene diamine was Dabco 33-LV from Houdry Process and Chemical Company, a 33.3 percent solution in dipropylene glycol. Concentrated solutions of DPPH in benzene were made up, usually at 0.025 41; these were diluted to 0.0125 41 for most runs. Stock catalyst solution consisted of 1.0 g of Dabco 33-LV diluted to 10 ml with benzene. One drop of catalyst solution is calculated to deliver 0.8 mg of triethylene diamine. A Bausch & Lomb Spectronic 20 spectrophotometer was used with 0.500-in. diameter tubes for transmittance measurements at 525-nm wave length, the wave length of strongest absorption. Beer s law holds well in the concentration range used (18); 0.0125 g/l of DPPH in benzene gave 36.7 percent transmittance. Films were air-corona treated on commercial-type equipment described in Part (2). The main process study was based on what might be termed a sliding central-composite design. The central-composite design involved four main factors, web speed, dielectric thickness, air-gap thickness and corona frequency, each controlled over about a four-fold range. The sliding part consisted of repeating this design at four to six levels of corona current, which was important and was easy to change. Since the corona power, with the Lepel SST- 1700 generator and the treater used, is proportional to corona current (), the quotient of current and web speed, /S, is an accurate measure of the energy applied per unit area of treated film. A Mylar carrier web was used to prepare the small samples required for testing; these 6- by 15-in. sections were attached to the carrier with double-sided tape. (The carrier was included as part of the dielectric layer.) Other experimental factors studied in separate experiments were relative humidity, number of electrodes, and electrode spacing. When the analytical test was run, DPPH solution was added about four minutes after treatment; this was about the average time needed to cut samples and place them in screw-top test tubes. The rest of the same sample was then used for tape-peel specimens, if desired, which took another few minutes to prepare. Another sample was treated for wetting tests; the ASTM wipe test was run within a minute or two of treatment and contactangle measurements were usually completed within fifteen minutes of sample treatment. During the process study, chemical analyses usually were carried out only on the samples produced at the moderate and highest power settings (25 and 35). All reported values of R03R (no catalyst) and R02R (amine catalyst used) were calculated using optimum values for k, and k,, in Eqs 16 and 19. The optimum values were determined from the process-study data by the least-squares search described earlier. RESULTS AND DSCUSSON We present in Fig. 1 data from the central point of the design showing surface concentration of peroxides as a function of applied current. t is apparent from this data 7 i i ;i 0 0 10 20 30 40 50 CURRENT, MA Fig. 1. Peroxide concentrutionfirst fulls slightly, then rises with coronu current ut central point of process study. Conditions were: weh speed = 100 ftlnrin, dielectric thickness = 14.2 mils, uir gcip = 60 mils and frequency = 1900 Hz. that significant amounts of stable peroxides are already on the film surface before treatment; low levels of treatment by corona discharge at least partially destroy these structures. Then, at increased treatment levels, more of the stable peroxide groups result than were originally present. Earlier exploratory work in development of the analytical test repeatedly showed the same effect. These observations fit the observed heat-sealing behavior well-low levels of treatment allow lower temperatures to be used for sealing but high levels of treatment make heat-sealing dimcult. We think this phenomenon associated with heat sealing is the result of existence of varying amounts of peroxide crosslinks on the surface. t should be noted that both the treated and untreated sides of the samples are in contact with the DPPH solution; if reduction in peroxide groups occurs only on the treated side at low treatment levels, then a high proportion are apparently destroyed on that side. The autohesion phenomena presented by Kim, Evans, and Goring (9) should be reconsidered with this finding in mind. Breakdown of peroxide crosslinks by the nitrogen corona at short times of treatment should enhance autohesion, as reported, even though surface energy is not increased much. We can speculate that at 334 POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5
Corona-Discharge Treatment of Polymeric Films, : Chemical Studies moderate treatment times, some direct crosslinking has occurred between chains to interfere with autohesion but surface energy is still low. At high treatment levels, increased surface energies give increased autohesion. With oxygen-containing coronas, unstable peroxide groups can give rise to covalent bonds between the layers (trace amounts ofoxygen could be operating in the nitrogen corona as well). The behavior of the unstable species (no acceleration of decomposition with cobalt naphthenate and no formation of sulfate on exposure with SOz) precluded it being hydroperoxide, so we looked at DPPH reaction at other temperatures to try to match the kinetic behavior to that of known peroxides. The activation energy for the decomposition of ozonide has been estimated at 17 kcal/mole (10, p. 128). Our measurements of activation energy for decomposition of the more unstable type of species is about 16 kcal/mole in the 23 to 45 C range. The measurement is based on values of initial slope of DPPH concentration versus time (without catalyst) at 23 and 45 C; the ratio of slope at 45 C to slope at 23 C was 6.3. Film for these tests was treated at a relatively high level (current = 45 ma, speed = 25 fpm)l Amine catalysis of secondary peroxides (or other peroxides having a hydrogen on the carbon attached to oxygen) has also been studied (10, p. 255); activation energy for this decomposition is about 12 kcal/mole. We found that raising the temperature of the catalyzed reaction from 23 to 45 C increased the initial rate of DPPH usage by a factor of 4.0. The activation energy calculated for this acceleration is 12 kcal/mole; hence kinetic evidence has been found for types of peroxides which could logically arise on the corona-treated surface. Data used for activation-energy calculations is presented in Table 1. Absorbencies were calculated for each point (these are proportional to concentration) and graphed. The initial slopes are, of course, proportional to the rate constants and were used for the activationenergy calculations. The uncertainty of the transmittance measurements makes the probable error in the activation energies about ten percent. Process Study The full tabulation of conditions and results of the process study on PE homopolymer comprises 210 pages in the Appendix of Ref. (1). Experimental errors were Table 1. Data for Activation Energy Estimation of DPPH Reaction Temperature = 23.0% Time, h 0.0 0.50 1.02 2.30 6.72 18.90 Percent transmittance 40.2 40.3 40.5 40.8 41.3 42.3 With amine Time, h 0.0 0.50 1.02 2.33 6.75 18.93 Percent transmittance 40.2 41.7 42.3 43.8 45.5 48.2 Temperature = 450 C Time, h 0.0 0.35 0.82 2.10 6.50 18.70 Percent transmittance 40.8 41.6 42.4 42.5 44.2 46.2 With amine Time, h 0.0 0.32 0.87 2.15 6.59 18.75 Percent transmittance 40.8 43.8 45.8 47.6 51.6 56.3 measured by completely repeating certain run sets at later dates, including three repetitions of the set at the design's central point (see Fig. 1). Eighty film samples were reacted with DPPH. Analysis of the results was done by regression using a stepwise multiple-regression program that was part of the BM 360 System Scientific Subroutine Package. We arrived at the functional form used for regression analysis by the following process. We assumed that the extent of chemical change on the surface is chiefly proportional to the energy applied. Our electrical studies (1) had shown that corona power in our system was proportional to current, so the energy applied per unit of area treated is proportional to current divided by web speed, ZS. We also expected that the effectiveness of the corona energy in causing chemical changes would depend somewhat on process parameters. We saw them as generating a function that modifies the direct dependence of chemical-species production on applied energy. For the ozonides, then, we have R0& = f(d, F, G, H, N, S) * (Z/S) (20) in which RO& is the ozonide concentration, groups/cm2, D = dielectric thickness, mils, F = corona frequency, hz, G = air-gap thickness, mils, H = relative humidity, percent, N = number of electrodes used, S = web speed, ft/min, and Z = corona current, ma. The modifying function had the flexible form Z(uJi + bjxf), but only those terms were retained that delivered a reduction of one percent or more in the total variance, an amount that in this work was statistically significant at the 90 to 95 percent level of significance. The equation for RO& that resulted from the regression work with the 80 sets of observations made during the process study is R03R = -0.64 + (Z/S) (8.39 + 0.021s - 0.036G - 0.0011H2) (21) Throughout the work some experiments which had already been run were repeated. These replications provided an estimate of experimental-error variance for RO& of 0.524 with 16 degrees of freedom (df). t was, therefore, possible to test our model for lack of fit. The analysis of variance for this regression is Sum of Mean Source df squares square F Total around mean 79 188.919 - Accounted for by regression 4 163.143 40.786 120.3 Residual 75 25.776 0.344 Experimental error 16 8.389 0.524 Lack of fit 59 17.387 0.302 <1 Having found the lack-of-fit mean square to be smaller than the experimental error, we pooled the two to produce the residual mean square, against which the mean square for regression was tested. The residual mean square corresponds to a standard error of estimate of 0.59 x 10l3 gp/cm2. TheZS term of the model accounted for 78 percent of the total sum of squares; the other three POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5 335
James F. Carley and P. Thomas Kitze factors accounted for only 8 percent more. The fit to the data provided by E9 21 is shown graphically in Fig. 2. The same approach was taken with the peroxide values. The regression equation below accounts for 85 percent of the RO& sum of squares. Again, the lion's share of this was due to 11s. ROzR = 1.52 + (US) (15.27-0.142G + 2.30N) (22) The lack-of-fit mean square here, though larger than the experimental error, was not significantly larger at the 95 percent level. The pooled standard error is 1.19 x 1013 gplcm'. From these equations we can see that more of both types of peroxides are formed in thinner air gaps and, of course, at higher applied energies. As might be expected, the correlation coefficient between RO& and RO& is high, 0.826. Note that the frequency of the corona discharge and dielectric thickness had no significant effect on the production of either RO& or RO&. Aging of Corona-Treated Film Changes in the chemical activity of the unmodified polyethylene (UCC1) film surface as a function of time after corona treatment were investigated at two treatment levels. The treated samples were stored at ambient conditions until testing; all were treated at the same time. Treatment was carried out in the same manner used for the process study; frequency was 1900 hz, dielectric thickness was 0.0199 in., and gap was 0.060 in. The low treatment level was at a corona current of 16.4 ma and 100 fpm speed while the higher treatment level was done at 30.7 ma and 25 fpm. Calculated chemical test results are summarized in Table 2. The RO& results show about the same decay rate of that species as seen in the analytical test with DPPH present. The groups seen after amine catalysis of their decomposition remain in high proportion after relatively long times. a 8 8 ", RO3R CALC'D FROM EQ 21 0 2 3 4 5 6 Fig. 2. Scatter diagram for ROa measurements. Diagonal line represents exuct equality with Eq 21. 3 Table 2. Chemical Test Results for Aged Samples Time after treatment, days 0.0 0.1 0.3 1.0 8.0 13.0 30.0 For low treatment level: R03R, lol3 groups/crn2 0.8 0.1 0.0 o* 0.3 0' RO,R, 1013 groups/crn2 4.9 4.4 5.5 6.6 4.4 2.4 For high treat me nt leve : RO,R, groups/cm2 5.1 3.5 1.7 1.6 1.1 0.3 RO,R, lol3 groups/crn* 11.0 10.0 7.3 6.6 6.1 7.2 * The least-squares solutions of the transmittance/concentratlon equations actually gave slightly negative, but chemically meaningless, values for these samples. All negative values were treated as zero in this work. Relationship Between Peroxide Concentrations and Physical Properties Over the years that polyethylene surfaces have been treated to improve their adhesion behavior, most workers have felt confident that the adhesion improvement must be connected with changes in polymer chemistry at the surface. However, for one reason or another, as noted in the ntroduction, earlier studies produced little quantitative evidence linking the physical properties of treated PE surfaces with the chemistry of identically treated samples of the same films. There has been even less evidence linking both these aspects with the variables of commercial treatment. n Part of this work (2), we presented equations showing the dependence of the polar component of surface energy, ASTM Wipe (a related surface-energy phenomenon) and ASTM peeladhesion strength on process factors. We have just offered such equations for the concentrations of two peroxide groups on the same surfaces. While the two sets of equations have some characteristics in common, they are far from being exactly alike in form and in the relative magnitudes of the coefficients. We have, therefore, examined the relationships between physical properties of the surfaces and the peroxide-group concentrations. Surface energy for polymer surfaces has two components, a polar component and a dispersive component, whose sum equals the surface energy. Of these the polar component, 6, was shown in Part to increase sharply during corona treatment and to be closely linked with improvement of peel-adhesion strength. Figure 3 is a plot of 6 vs RO& concentration for 80 runs of the process study plus 12 samples that were tested after aging, six ofwhich were treated at a low energy level (11s = 0.66 ma ftlmin) and six which were given twice that energy. Though there is considerable scatter here, particularly at the lowest concentrations where we are approaching the limits of the analysis, it is clear that the dependence is not linear. The picture suggests a fastrising exponential that flattens toward an asymptote at the higher RO& values. Also, since all our work showed that no RO& was found on films that had not been treated, the contribution of RO& to 6 should logically be zero when RO& is zero. After a few trials, we came up with the following simple equation which provided about as good a fit as the scatter will permit. 336 POLYMER ENGN ERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5
Corona-Discharge Treatment of Polymeric Films, 11: Chemical Studies RO,R, 10'~ gp/cm2 P 1 1 1 0 2 3 4 5 6 7 Fig. 3. This plot shows the relationship between polar component of surface energy und ozonide concentration on lowdensity PE films without additives, treated at commercial levels. Filled circles are for aged specimens. $ = 20(1 - e -m) (23) This is the equation of the curve shown on the plot. Figure 4 shows the corresponding plot for RO& for the same 92 samples. n this case the rate of rise at the low end is less rapid and the approach to an asymptote is less definite. Although untreated PE film that contains no additives is much like ordinary paraffin and has essentially no polar component of surface energy, we repeatedly found, as exemplified in Fig. 1, that there was a small concentration of ROzR present. t seems reasonable, therefore, that some threshold level of R0& must be reached (in the absence of R0&) before 6 becomes measurable. The plot suggests that this threshold is 1 x l o 0 5 10 15 20 Fig. 4. The polar component of surface energy, which increases sharply with increasing level of treatment and constitutes most of the surface energy on treatedfilms (2), is also closely linked with peroxide (R0,RJ concentrution. 1013 gp/cm2. Using this value and the same asymptote as before, 20, we obtained a satisfactory fit with the equation yg = 241 - e-(r02r-1)/3 ) (24) which is the curve shown on the plot. As we noted earlier, R0& and RO& values are closely correlated, so one might suppose that either of these equations would suffice for estimating $. However, we noticed that the aged samples, for which the points are plotted as filled circles on both figures, fell mostly above the curve infig. 3 and mostly below that of Fig. 4. Also, some severely deviant points on one figure were not so on the other, for example, the three lowest points between R 08 = 1 and 2 in Fig. 3. We also believed that both kinds of peroxide groups should contribute to surface polarity and thus, to $. We therefore decided to fit an additive combination of the above two forms, allowing the criterion of least squares to determine the two coefficients. The result was 6 = 9.52(1 - e - m ) + 10.25(1 - e-(r02r-1)13 1 (25) This equation gave a coefficient of determination for all 92 points of 0.78, corresponding to a correlation coefficient of 0.88. The 99 percent confidence interval for the true correlation here is from 0.83 to 0.94 (19), so we feel that the final quantitative link between surface chemistry and physical properties of treated surfaces has been demonstrated beyond reasonable doubt. The form of Eq 25, which was superior in a number of ways to several others tried, may be useful to other researchers attempting to elucidate the mechanism linking the concentrations of chemical species to physical properties. n Part of this study (2), we showed that ASTM wipe, a standard test of surface wettability, was very closely related to $. We also showed that the ASTM Peel strength can be accurately estimated from the interaction term for the surface energies of the treated film and the pressure-sensitive adhesive tape used in the test. However, two of the elements of that interaction term are the polar and dispersion-force components of the tape surface, which were constant in our tests. The third element was the dispersion-force component of the treated film, s, which was shown to be only slightly affected by treatment. Thus, practically all the variation of the interaction term is attributable to the one remaining element, $. These facts tell us that, if we were to prepare scatter diagrams for ASTM wipe and peel strength vs R 08 and R0&, we would have high and significant correlations in those systems, just as we did for yc. Other Films Modified polyethylene (UCCS) was corona-discharge treated at several levels. Unfortunately, one of the additives in this material reacted with the DPPH, making analysis for peroxides impossible. We believe the antioxidant may have been the culprit because modified PE-VA did not show the interference and contained the same slip additive. POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5 337
James F. Carley and P. Thomas Kitze Several other films were tested at only one treatment level each. Two of these were low-density polyethylenes from Dow, one with a low content of slip additive (Dow No. 1) and one with medium slip-additive content (Dow No. 2). Polyethylene terephthalate (PET, Mylar A) from DuPont and commercial polypropylene films from Hercules complete the list of other films treated. Data and results from these tests are listed in the Appendix of Ref. (1). Treatment level used may be high compared to commercial levels. DPPH usage with the PE-VA films showed behavior digerent from that observed with polyethylene. With PE-VA, very high amounts of DPPH were reacted in the case of both untreated and treated films with catalyst, but after treatment DPPH usage was about the same whether catalyst was present or not. We believe that with untreated film surfaces, the amine catalyst can cause splitting of the acetate linkage giving products that react with DPPH and that after treatment many of these same positions on the carbon chain have already been reacted into unstable peroxides by the corona discharge. There is also a possibility that some relatively stable peroxy radicals could be formed at these branch points; we made no ESR measurements with this polymer. Whatever the causes, very poor fit to the kinetic model used for analysis of DPPH data resulted. The slip additives in the modified sample of PE-VA covered up these effects (probably quite literally) to some extent, and its behavior was more like that of PE. Little interference was noted in the DPPH reaction with either of the Dow films; the film with more slip additive gave higher usage after treatment. This suggests that oxidation of the additive occurs more rapidly than oxidation of the polymer. Substantial amounts (5 x 1013 gp/cm2) of surface peroxides were also found on untreated PET film; heatsealability improvements by treatment have been commented on by Bradley and Fales (6). They attribute the improvement to thermal decomposition of unstable peroxides to form oxy radicals; these can react at the interface to give enhanced bonding. Breakdown of existing stable peroxide crosslinks by the corona during treatment could also contribute to better heat sealability. The polypropylene film used also contained additives that interfered with the DPPH reaction and we were not able to use our analytical technique. CONCLUSON n summary, we have produced evidence for the existence of chemical species that could logically arise from interaction of corona discharge in air with polymer surfaces. The concentrations of these species appear to depend directly on applied energy as measured by the quotient of corona current divided by web speed, and to decrease with increasing air gap and humidity. Dielectric thickness and frequency had no detectable effects. The concentrations of peroxides were strongly correlated (positively) with the adhesion and wetting properties observed; R02R and ROa accounting for 78 percent of the observed variation in the polar component of surface energy. Nevertheless, it should be pointed out that other chemical species which do not react with DPPH undoubtedly are formed by corona treatment and could also affect physical properties. However, ESR measurements showed that none of the active species present were organic free radicals. n addition, the reactions that are occurring with corona treatment in air can, at high levels of treatment, cause degradation of physical properties of the surface layer, probably due to chain scission. Our adhesion data (1) showed this effect. ACKNOWLEDGMENTS We are grateful to Professors P. L. Barrick, L. F. Brown, S. J. Gill, G. E. Gless, and R. E. West of the University of Colorado, who gave generously of their time and expertise in conferences. Thanks, too, to Professor R. E. Eckert of Purdue University, who, while on sabbatical at CU, suggested the use of DPPH for this work. The BM Corporation s generous underwriting of a Resident Study Fellowship and equipment, supplies, and services made it all possible. REFERENCES 1. P. T. Kitze, Ph.D. Thesis, Univ. of Colorado (1973). Available from University Microfilms, Ann Arbor, Mich. 2. J. F. Carley and P. T. Kitze,Polym. Eng. Sci., 18,326( 1978). 3. J. M. McKelvey, Polymer Processing, John Wiley, New York (1962). 4. J. C. Porter, Sc.D. Thesis, Washington University of St. Louis (1960). 5. R. A. Hines, An nvestigation ofchemical Changes Occurring on Surface of Polyethylene During Treatment, 132nd National ACS Meeting, New York (September 8-13, 1957). 6. A. Bradley and J. D. Fales, Chem. Technol., 1,232-237 (Apr. 1971). 7. A. Bradley and T. R. Heagney,Anul. Chem., 42,894 (1970). 8. A. Kelen and W. Dick, Prepkints Papers ntern. Symp. Free Radicals, 5th, Uppsala, 31-1, 2 (1961). 9. C. Y. Kim, J. Evans, and D. A.. Goring, J. Appl. Polym. Sci., 15, 1365-75 (1971). 10. D. Swern, Editor, Organic Peroxides, Vol., Wiley- nterscience, New York (1970). 11. J. J. Levetsky, F. J. Lindsey, and W. S. Kaghan, SPE J., 20, 1305-08 (1964). 12. P. D. Bartlett and G. Guaraldi, J. Am. Cheni. Soc., 89,4799 (1954). 13. E. J. Lawton, P. D. Zemany, and J. S. Bolwit, J. Am. Chem. Soc., 76, 3437 (1954). 14. K. Rossman, J. Polym. Sci., 19, 141-144 (1956). 15. F. Buchheisterand M. Seifel, U.S. Patent 3,424,735(Jan. 28, 1969). 16. C. E. H. Bawn and S. F. Mellish, Truns. Furaduy Soc., 47, 1216 (1951). 17. J. Mitchell, Jr. and L. R. Perkins,Appl. Polym. Symp., No. 4 (1967). 18. R. J. Salloum, Ph.D. Thesis, Purdue University (1970). 19. W. H. Beyer(Ed.), Handbook oftables for Probability and Statistics, p. 158, Chemical Rubber Co., Cleveland (1966). 338 POLYMER ENGNEERNG AND SCENCE, MARCH, 1980, Vol. 20, No. 5
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