Chemistry of Electrical Discharge Polymerization

30 Chemistry of Electrical Discharge Polymerization P E T E R M . HAY

1

Downloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch030

Central Research Laboratories, J. P. Stevens & Co., Inc., Garfield, N . J.

Organic compounds were polymerized by aTesladischarge acting on a mixture of nitrogen and the vapor of the organic compound at one atmosphere. The polymer was collected as a deposit on a moving plastic film passing between the electrodes. Easy solubility of most coatings indicated little cross-linking. Elemental analysis of the coatings found carbon, hydrogen, and nitrogen. Infrared spectra of polymers from benzene, toluene, and styrene were all similar and indicated the presence of oxygenated or nitrogenated groups in addition to an aromatic structure. It was tentatively concluded that the mechanism of reaction is fragmentation of the monomer by the electrical discharge followed by a complex recombination reaction.

>Tphe conversion of volatile organic compounds into liquid and solid A

products by the action of a high-voltage gas discharge has been

observed by many people over the past 100 years or more (5).

The

literature of the past 40 years contains a large number of references to the effects of electric discharge on organic compounds and a complete review of all of these is beyond the scope of this paper. Many of the investigators were interested in the synthesis of derivatives having about the same molecular weight as the starting compounds and a few (2, 4, 11,

18)

mentioned high-boiling liquid or solid by-products. Other investigators collected and analyzed solid products formed in electric discharge from butane (2), benzene (10, 14, 18), and tetrabromomethane (6) and other compounds, but little has been reported on the molecular structure of such products. Even in the older references the solid products have been referred to as polymers. 1

Present address: Sandoz Inc., Hanover, N. J. 350 In Chemical Reactions in Electrical Discharges; Blaustein, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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Electrical Discharge Polymerization

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In a few instances solid polymers have been made by electric discharge from vinyl monomers which are commonly polymerized by other means. Thus, styrene (8,13,16), vinyl acetate (15), methyl methacrylate (8), and numerous alkenes have been polymerized by exposure to discharge.

The products usually were compared with polymers made from

the same monomers by conventional catalysis and the close similarity of infrared spectra was used as evidence for similar molecular structure. Recently some companies have been reported to be working on the application of electric discharge polymerization to coating of containers (3), steel strip (21),

or fabric (I).

These references all have indicated

that the coatings would be formed under vacuum conditions. This imDownloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch030

poses severe process limitations. The work described in this paper was carried out at atmospheric pressure, with the electrical discharge taking place in a mixture of nitrogen and volatile organic compounds, some of them vinyl monomers. One expectation was that the ionization of a vinyl monomer would start a vinyl polymerization reaction by an ionic mechanism or that an organic ion would decompose to a free radical and start a free-radical vinyl polymerization. The function of the nitrogen was merely for dilution of the organic monomer so that the reaction could be carried out in simple equipment at atmospheric pressure.

The basic

scheme was to lead a strip of fabric through a discharge polymer-polymerization zone and thereby coat it with deposit of polymer. It was expected that the rate of polymerization would be small and for this reason some attention was paid to additives which might have the effect of increasing the yield of polymer. Brominated and chlorinated compounds were tried in view of their known influence on the course of other polymerization reactions (17,

20).

Experimental Methods A l l polymerizations were carried out at room temperature in a mixture of organic vapors and nitrogen at a total pressure of one atmosphere. The gases were delivered to the reaction zone through the system shown schematically in Figure 1. Nitrogen passed through liquid monomer in one bubble tube and through additive in another tube. The nitrogen and entrained vapors entered the enclosed reaction chamber at one corner and exited to the atmosphere at the opposite corner. The ratio of monomer to additive was determined by weighing the tubes before and after the experiment. Details of the reaction chamber are shown in Figure 2. Polymerization was initiated by corona discharge between two cylindrical, parallel, insulated electrodes ( A ) made by lining the inner surfaces of borosilicate glass tubing with aluminum foil. The glass tubing had a wall thickness of 2.5 mm. and an outside diameter of 62 mm. The two glass surfaces were separated by a gap of 4 mm. The alternating current high voltage was supplied by a Tesla generator, manufactured by Lepel H i g h Fre-

In Chemical Reactions in Electrical Discharges; Blaustein, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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quency Laboratories, Inc. ( M o d e l H F S G - 2 ) . The peak voltage, as esti mated by spark length in air, was about 20,000 volts. Because of the irregular wave-form in a Tesla circuit it was not considered practical to determine instrumentally the power consumed in the corona gap so no observations of chemical efficiency were possible. A moving strip of flexible substrate, (B) was positioned in the electrode gap. As shown in Figure 2, this substrate, which in most cases was 2-mil (50 microns) poly (ethylene terephthalate ) film, was formed into a closed loop 47 cm.

το ATMOSPHERE

REACTION C H A M B E R

Figure 1.

Figure 2.

Vapor polymerization-schematic layout

Vapor polymerization on moving substrate


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long and 5 cm. wide and was moved by the rotating roller, C . Vapors entered through D and exited through E . Polymer which formed in the corona zone deposited both on the glass electrode coverings and on the moving substrates. The substrate strip was dried and weighed before and after the polymerization. The coated strips were also baked in a circulating air oven and reweighed. The final weight gain was taken as the yield. In some cases polymer was removed from the glass surfaces by solvent and recovered for infrared or chemical analysis.

Results


Survey of Polymerizable Compounds. A number of volatile organic compounds were subjected to corona discharge with a wide variety of results.

In almost all cases a deposit of oily or solid brown material

formed on the substrate strip. The weight gain resulting from 15 minutes of corona polymerization ranged from tenths of a milligram to over 30 milligrams. W h e n the coated strips were heated at 1 5 0 ° C . for 10 minutes, part of the added weight was lost.

Some of this loss is thought to be

unreacted monomer which was absorbed by the substrate and coating. Another part of it could be very low molecular weight products of the corona reaction or be the result of thermal decomposition of the coating. The compounds which gave the heaviest coatings after heating (taking into account the amount of monomer volatilized), included triallylamine, acrylonitrile, toluene, and styrene.

As the list in Table I shows

it is not necessary for the monomer to be a vinyl compound in the strict sense of the word for a non-volatile polymer product to form in corona discharge.

Toluene, benzene, benzotrifluoride, and even acetone gave

measurable yields. There seemed to be no pattern of relationship between the structure of a monomer and its yield in corona polymerization. Table I.

Corona-Polymerizable Compounds

Acetone Acrylic Acid Acrylonitrile Allylamine Benzene Benzotrifluoride

Ethyl Acrylate 1-Octene Styrene Toluene Vinyl Acetate 4-Vinylcyclohexene

Effect of Additives. It was found that many halogenated

organic

compounds, when added to the monomer being volatilized, had the effect of giving a higher yield.

Chloroform, bromoform, and iodoform were

more effective in increasing polymer yields from styrene than some other additives. The yields after oven heating are given in Table II, expressed as percent of styrene vaporized. It is interesting to note that the yield


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is usually increased more by a moderate amount of additive and not so much by a larger amount.

Bromine itself reacted immediately with the

styrene in the bubble tube but produced the largest yield of all additives tested. Chlorine and iodine seemed to have the reverse or no effect. Table II. Effect of Halogenated Additives on Corona Polymerization of Styrene Yield of Polymer as a Percent of Styrene Used


Additive

Percent Added

Chloroform Bromoform Bromoform Bromoform Bromoform Bromoform Iodoform

0.5 0.5 1.0 2.5 5.0 10.0 5.0

Yield

Additive

2.24 1.16 1.42 1.97 1.92 1.20 2.12

Chlorine Bromine Bromine Bromine Iodine None

—

Percent Added Yield 0.56 1.90 3.20 1.67 0.97 1.07

7 1 5 10 5 0

—

—

Other additives that enhanced the yields of styrene polymer were carbon

tetrachloride,

l,2-dibromo-l,l,2,2-tetrafluoroethane,

tane and 2-bromobutane.

1-bromobu-

Yields from monomers other than styrene were

not all increased by halogenated additives; some even were decreased. It was impossible to develop any rational relationship between monomer structure and susceptibility of the monomer to yield enhancement

by

halogenated additive. The results given above were all derived from experiments in which the additive and the monomer were mixed and volatilized from a single bubble tube. Thus, the exact composition of the vapor was not known. A new set of experiments was carried out using separate bubble tubes for monomer and additive. The weight changes of the tubes during a run were used to calculate mole ratios of additive to styrene. The results of experiments with four additives are shown in Figure 3. The conversion to polymer of styrene without additives was 0.75 to 0.9%.

As increasing

amounts of bromoform, 1-bromobutane or 2-bromobutane were added, the conversion increased and then fell off again. The pattern of points in the case of 1-bromobutane was widely scattered but most of the points lay well above the level for styrene itself. W i t h 2-bromo-2-methylpropane as an additive there was no significant increase in conversion. The weight gain from the additives alone, with no styrene, were all low compared with styrene. In considering chemical explanations for corona polymerization, both free radical and ionic intermediates are possibilities. Experiments were run with various additives to styrene that might be expected to inhibit each kind of reaction through combination with the active intermediate but no clear-cut reduction in yield was observed. Benzoquinone at 1 and 2 mole % gave normal yields. Water and butylamine were extensively


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355

studied but, as Figure 4 demonstrates, the tendency for these supposed cation scavengers to depress the conversion is slight and not clear-cut. Butylamine by itself gave a surprisingly high yield.

Ammonia, triethyl-

amine, acetone, and carbon dioxide as additives had no substantial effect


on yield.

Figure 3.

Effect of hrominated additives on styrene poly merization Ο Styrene with 1-bromobutane Δ Styrene with 2-bromobutane • Styrene with 2-bromo-2-methylpropane φ Styrene with bromoform

Analytical Results Some evidence was collected on the chemical nature of some of the products. The products deposited on the moving film and on the glass electrode covers were usually easily dissolved in common solvents like acetone, chloroform, and benzene, indicating low molecular weight and that there was little cross-linking. Infrared spectra were obtained of polymers made from benzene, toluene, styrene, and styrene mixed with 1-bromobutane. The electrode deposits were dissolved in chloroform and films were cast on salt plates by evaporating the solvent. A l l the transmission spectra were similar to one another, as can be seen in Figures 5 and 6, and they


356

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Ζ

LU ϋ

tu ÛL

oc UJ

Σ Ο û.

1-0

ζ ο


ζ ο ο

0.5f

0.1

0.2

0.3

0.4

0.5

0.6

0.7$

1.0

MOLE FRACTION OF ADDITIVE Figure 4.

Effect of potential inhibitors on styrene polymerization φ Styrene with water Δ Styrene with butylamine 1000

700

900 E-T0LUENE F-BENZENE G-STYRENE

D-CONV. POLYSTYRENE

Figure 5.

Infrared spectra of corona discharge polymers

had many of the same general features as the spectra published by Jesch, Bloor, and Kronick (13)

and by Williams (21 ) for spectra of the insoluble

coatings formed by glow discharge at low pressure.


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357

When compared with the spectrum of conventional polystyrene all the spectra show many of the same strong absorbances ( two in the range 700-770 cm." , three in the range 1400-1650 cm." , and two in the range 1

1

2800-3000 cm." ) but, at the same time, lack the distinct absorption 1

patterns of polystyrene found in the regions from 850 to 1400 cm." and 1

1800 to 1950 cm." . In addition, all of the experimental spectra contain 1

absorption bands not found in polystyrene—e.g., 1220-1250 cm." , 1700 1

cm." , 3400-3500 cm." . It is likely that these additional bands come from 1

1

oxygen and nitrogen groups such as hydroxyl, amino, acid, ester, and


amide.

Figure 6.

Infrared spectra of corona discharge polymers

The nitrogen-containing groups can arise from the reaction of active nitrogen in the discharge whereas the oxygen-containing groups may be formed by reaction of traces of water or oxygen in the discharge or they may result from combination of water and oxygen of the air with reactive groups left in the discharge polymers. A n attempt to obtain spectroscopic evidence of such a post-reaction was not successful. As shown in Figure 6, there was little alteration in the spectrum of a styrene polymer upon aging overnight in the air. The structure of the product from a mixture of styrene and 1-bromobutane, as far as is shown by the infrared spectrum of Figure 6, is the same as that from styrene alone. The infrared spectra give evidence that substituted aromatic rings are a major component of these solids, along with various groups containing nitrogen or oxygen. Although a polystyrene structure (a long


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IN ELECTRICAL

DISCHARGES

carbon chain with pendant phenyl groups on alternate carbons) is a possi bility, there is no direct evidence that this is the chemical structure of these products. The close similarity between spectra of products of ben zene, toluene, and styrene suggests that all are of similar structure, pos sibly recombination products of fragments from a relatively severe break down of the respective monomers. Other evidence of chemical composition was found in elemental analysis. T h e styrene product had considerably less carbon than styrene itself, as seen in Table III. It also contained a significant amount of nitrogen and, as determined by difference, a large amount of oxygen. Heating the polymer in air did not change the composition significantly. Downloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch030

Styrene polymer made in the presence of 25 to 30 weight-%

1-bromo

butane had almost the same analysis plus a significant bromine content. Table III.

Elemental Analysis of Corona Polymers

Monomer Styrene, unheated product Styrene, oven-heated Styrene plus bromobutane unheated product oven-heated product Styrene, calculated Bromobutane, calculated

Br

Ο (by difference)

C

H

Ν

72.23 73.46

6.86 6.68

3.2 3.39

— —

17.71 16.47

73.74 73.29 92.26 35.06

6.95 6.69 7.74 6.62

3.59 3.56

6.08 6.97

9.64 9.49

— —

—

— —

58.32

Discussion The salient features of the information presented above might be summarized as follows.

In a mixture of organic vapor and nitrogen

subjected to high-frequency, high-voltage, electrodeless

discharge, the

nitrogen, the organic compound, and trace amounts of oxygen and water are activated and combine chemically to yield products of higher molecu lar weight than the starting materials.

These products condense on any

solid surface available and may even undergo further chemical reaction within themselves and with more monomeric material which is not elec trically activated. The chemical mechanism of this series of reactions must be complex, possibly initiated by simple ionization through collision with a rapidlymoving electron followed by expulsion of two secondary electrons: N + e~ -> No + 2e~ or R H + e- -> R H + 2e~ 2

+

+

These are the products ordinarily found in mass spectrometry and on exposure to gamma or beta radiation. These activated cationic prod ucts might activate vinyl polymerization or undergo other

secondary


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reactions, yielding neutral free radicals or even anions, either of similar structure or in the form of fragments and combination or rearrangement products.

In the present case there exists the further possibility that

reaction products become reactivated,

since they are formed in the

presence of the high-voltage field, and undergo further reaction. The simplest mechanism to consider would be polymerization of the vinyl monomers to long-chain products after initiation by some active species.

The fact that non-vinyl compounds gave good yields may be

explained through a mechanism involving some fragmentation of every monomer molecule and combination of the fragments to products of higher molecular weight. This leads to the likelihood that vinyl monomers Downloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch030

are polymerized by this non-vinyl mechanism, rather than by an ionic or free-radical polymerization reaction. The effect of various additives on a styrene polymerization reinforces the tentative conclusion that vinyl polymerization is not taking place. Even though yields were increased by halogenated additives they were not decreased by additives expected to act as scavengers for free-radical species (benzophenone) or ionic species (butylamine or water).

Under

an assumed mechanism of fragmentation and rapid recombination to condensed products, halogenated compounds additives may serve to increase the efficiency of energy transfer from the electric field to the monomer. The fact that nitrogen is found in these discharge polymers might have been expected from previous reports on the chemistry of active nitrogen (7, 9, 12),

some of which referred to solid products containing

nitrogen that were formed by mixing electrically-activated nitrogen with unactivated organic molecules.

In the case of the present report, the

situation is complicated by the possibility of having both the nitrogen and the organic molecule be activated by the corona discharge. It must be said that the chemical system reported here is sufficiently complex not to allow complete analysis. In addition, although the apparatus and conditions used in the experiments reported here proved adequate for coating a moving substrate, the properties and usefulness of the coatings were not suitable for practical use. Perhaps further research work on the complex chemical mechanism will lead to the means for improving the properties of the coatings.

Acknowledgments The able assistance of Roger Kolsky and Walter Miner and the helpful analytical interpretations of Elliot Baum are gratefully acknowledged. Special thanks are due Paul Stam for his interest in and support of this research.


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May

25,

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Chemistry of Electrical Discharge Polymerization

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