Recent Advances in the Vapor Deposition Polymerization of p

42 Recent Advances in the Vapor Deposition Polymerization of p-Xylylenes

Downloaded by CORNELL UNIV on September 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch042

WILLIAM F. GORHAM Union Carbide Chemicals and Plastics, Research and Development Department, P.O. Box 670, Bound Brook, N. J. 08805

The process for preparing linear poly-p-xylylenes by pyrolytic polymerization of di-p-xylylenes has been extended to include the formation of p-xylylene copolymers. Pyrolysis of mono-substituted di-p-xylylenes or of mixtures of substituted di-p-xylylenes results in formation of two or more p-xylylene species. Copolymerization is effected by deposition polymerization on surfaces at a temperature below the threshold condensation temperature of at least two of the reactive intermediates. Random copolymers are produced. Molecular weight of polymers produced by this process can be controlled by deposition temperature and by addition of mercaptans. Unique capabilities of vapor deposition polymerization include the encapsulation of particulate materials, the ability to replicate very fine structural details, and the ability of the monomers to penetrate crevices and deposit polymer in otherwise difficultly accessible structural configurations.

A new general synthetic method for preparing linear poly-p-xylylenes was reported recently (6, 7, 8, 9, 10, 11, 13). This new method involves the vacuum pyrolysis of di-p-xylylene or substituted di-p-xylylenes at temperatures of 6 0 0 ° - 7 0 0 ° C . to form p-xylylenes and the subse quent condensation and spontaneous polymerization of these reactive species to form a family of linear polymers. The over-all reaction scheme is illustrated at the top of p. 644. This process was shown to be general, and a large family of substi tuted poly-p-xylylenes prepared in which X equals hydrogen, halogen, alkyl, cyano, ester, acyl, and other groupings. Advantages of this process (6, 7, 8, 9,10,11, 13) over previous routes (4) to poly-p-xylylene include 643 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

644

ADDITION A N D CONDENSATION POLYMERIZATION

PROCESSES

on condensation


100%

III the quantitative nature of both reactions, the formation of linear rather than crosslinked polymers, and the absence of byproducts and telomers. While all substituted di-p-xylylenes are pyrolyzed under substantially identical conditions, the temperatures of condensation and polymerization varied substantially, depending on the p-xylylene derivative under study. It was established that there is a threshold condensation temperature, Tc, above which the rate of condensation-polymerization was very slow under the system conditions (50-100μ) normally used. The TVs for several p-xylylene monomers were established as follows: Monomer

Tc, °C.

p-Xylylene 2-Methyl-p-xylylene 2-Ethyl-p-xylylene 2-Chloro-p-xylylene 2-Acetyl-p-xylylene 2-Cyano-p-xylylene 2-Bromo-p-xylylene Dichloro-p-xylylene

30 60 90 90 130 130 130 130

In this series Tc is related to both molecular weight of the monomer and the polarity of the substituent(s) attached to the aromatic ring. It appears that the monomer must be able to condense on a surface and be available in sufficiently high concentration to react with its neighbors to form initially diradical intermediates such as IV containing at least three and probably four monomer units.

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

42.

GORHAM

•CH

2

645

p-Xylylenes

7\y_cH -CH -V/ 2

2

>-CH —CH -// 2

2

\\-CH

2


IV Once this intermediate is formed, growth appears to be very rapid through addition of monomer units which condense near either radical site. High molecular weight polymers are then formed by a free radical addition mechanism. This polymerization route was first proposed by Szwarc (17). There is general agreement (4) that the original Szwarc proposal represents the most probable path. An excellent paper discussing the polymerization of p-xylylene has been published by Errede, Gregorian, and Hoyt (2). Growth is terminated by coupling of radicals from two growing polymer molecules or by the reactive sites becoming buried in the poly mer matrix. Polymers formed by the di-p-xylylene process have been shown (6, 7, 8, 9 , 1 0 , 1 1 , 1 3 ) to be living polymers and exhibit radical con centrations of 5-10 Χ 10" mole of free electrons per mole of p-xylylene. 4

Pyrolysis of monosubstituted di-p-xylylenes, such as acetyl-di-pxylylene (V) results in formation of two reactive p-xylylenes with dif ferent Tes. The two species were separated as their polymers by using the principle of threshold condensation temperature. The pyrolysis vapors containing the two monomers VI and VII were passed initially through a zone maintained at a temperature low enough to permit rapid condensation and polymerization of acetyl-p-xylylene, but substantially above the Tc of p-xylylene which passed through the first zone and polymerized in a final zone maintained at ambient temperature. In a sense, the monomers were fractionated on the basis of volatility, and the monomers were isolated in the form of their polymers. These transfor mations are illustrated at the top of p. 646. Copolymerization Studies The earliest work on the copolymerization of p-xylylene type mono mers was reported in the brilliant pioneering studies of M. Szwarc (17). He and Roper (5) studied the pyrolysis of mixtures of p-xylylene and pseudocumene and concluded that mixtures of p-xylylene and 2-methylp-xylylene were formed which copolymerized on condensation. Appar ently no extensive work on copolymerization or the nature of the products was conducted.



646


PROCESSES

Other groups (I, 15) have also conducted studies on the copoly merization of substituted p-xylylenes by pyrolysis of mixtures of sub stituted p-xylenes. Problems associated with the stability of many sub stituent groups at the elevated pyrolysis temperatures (ΞΞΞ 900°C.) required to convert p-xylenes to p-xylylenes have, however, prevented a general study of copolymerization. The formation of p-xylylene monomers by pyrolysis of substituted di-p-xylylenes is a quantitative, clean process and provides an excellent starting point for generating mixtures of monomers and for studying their copolymerization. The formation of linear poly-p-xylylenes by the di-pxylylene route is also a major advantage in characterizing the products formed. Considerable flexibility is available in the di-p-xylylene process with regard to preparation of intermediates for generating monomers. Intro duction of a substituent on one ring of di-p-xylylene (as, for example, acetyl-di-p-xylylene) provides a starting material which on pyrolysis yields two distinct monomers. Alternatively, disubstituted products, for


42.

GORHAM

647

p-Xylylenes

example dichloro, dibromo, or dialkyl derivatives, provide intermediates which on pyrolysis yield pure sources of a single monomer. The prepara tion of a broad range of substituted di-p-xylylenes has been reported (12, 13, 16,18,19,20).


In considering the copolymerization of substituted p-xylylenes, such as X and XI, an important question is whether the growing polymer chain XII (which has just added a unit of monomer X ) shows any preference for reacting with X or XI. The chain-propagating step involves addition of a radical to a highly reactive monomer to form a covalent bond and a

XII new radical site. Owing to the great reactivity of these molecules in the condensed phase, it is probable that XII shows little or no preference but will react with whichever is available at the radical site. Since the substituent groups Ri and R are quite far removed from the reaction site, it is also probable that steric and electronic effects are not nearly as important as in vinyl copolymerizations. 2

Assuming the above statements are correct, any two p-xylylene spe cies should be capable of copolymerization in any desired ratio. This is somewhat of a simplification since it has been observed that for each substituted p-xylylene there is a definite ceiling condensation temperature above which it will not condense and polymerize at any appreciable rate. Thus, if the monomer does not condense, it is not available for copoly merization. This was demonstrated in the studies described earlier of the pyrolysis of acetyl-di-p-xylylene and separation of the monomers VI and VII on the basis of widely differing Tc's. Therefore, to study copolymerization one must either utilize two monomers with approximately the same vapor pressure characteristics or maintain the initial polymerization zone at a temperature below the threshold condensation temperature of the more volatile species. Studies were conducted on the copolymerization of several sets of monomers.

American Chemical Sodety Library N.W.

Platzer; Addition and Condensation Polymerization 1155 16 th St., Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

648

ADDITION A N D C O N D E N S A T I O N

POLYMERIZATION

PROCESSES


Experimental Conversion of Di-^-xylylenes to Poly (£-xylylenes). The following equipment and procedure were used to convert di-p-xylylenes to poly-pxylylenes. The pyrolysis reactions were carried out in a 24-inch section of 1-inch i.d. 96% silica tubing. The first 6 inches of the tube served as a distillation zone, and the following 18-inch section as the pyrolysis zone. The pyrolysis tube was connected by appropriate glass connections to a glass deposition chamber. Deposition chambers ranging in diameter from 1 to 6 inches and in length from 8 to 20 inches were used. The end of the chamber was connected by tubing through a dry ice trap to a 4 cu. ft./min. mechanical pump. A thermocouple or Pirani vacuum gage probe was placed in the system between the dry ice trap and the pump to record pressure and pressure changes. In each experiment, a measured quantity of di-p-xylylene, ringsubstituted derivatives, or mixtures were placed in a porcelain boat, and the boat was placed in the distillation zone. The system was then closed and evacuated to 1-100 μ, depending on the derivative in question. The di-p-xylylene was distilled at the rate of 1 gram every 3 or 4 minutes through the pyrolysis zone. To achieve this rate, the distillation zone was maintained at temperatures ranging from 140° to 220°C., depending on the derivative. The pyrolysis zone was heated to 600°C., the tem perature being measured by the thermocouple in the middle of the furnace on the outside of the tube. The pyrolysis gases were then led into a deposition chamber. The glass joints leading from the pyrolysis zone to the deposition chamber were maintained at about 200°C. to prevent premature polymerization. The deposition chamber was usually held at room temperature, although with some derivatives it was heated as high as 100 °C. to permit deposition of polymer over a fairly broad area. A pressure rise of 5-100 μ is generally observed during a pyrolytic polymerization. At the end of the run the pressure falls back to the base pressure. The heating jacket on the distillation section of the pyrolysis tube is removed at this point to ensure that complete distillation of the charge has occurred. When it is established that the distillation is com plete, the vacuum is broken, the equipment is dismantled, and the polymeric film is recovered from the walls of the deposition chamber. Preparation of Mixtures of ^-Xylylenes and Separation into Polymer on the Basis of Tc. The distillation and pyrolysis steps were conducted as described above for di-p-xylylene and substituted di-p-xylylenes. The pyrolysis gases were led immediately into a deposition zone which con sisted of a 24-inch section of 1-inch i.d. glass tubing. The initial 15-inch section of this tubing (Zone A) was heated to 9 0 ° - 1 0 0 ° C . The final 9-inch section of the tubing (Zone B) was maintained at room tempera ture. The end of the deposition zone was connected via rubber tubing through the dry ice trap to the pump. Preparation of Copolymer of Chloro- and Butyl-^-xylylene. Mix tures of dichloro-di-p-xylylene and butyl-di-p-xylylene were prepared and melted to ensure homogeneity. The mixture was placed in the distillation zone, and the reaction was carried out in the usual fashion. The co polymers formed in the initial polymerization zone which was maintained at 9 0 ° - 1 0 0 ° C . Poly-p-xylylene formed in the final, air-cooled zone. At


42.

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649

p-Xylylenes

the end of each run, the copolymers were removed mechanically and extracted with ether. In each run, the pyrolysis temperature was 600°C., the distillation temperature 100°-125°C., and the pressure of the system was 0.2 mm. The runs lasted 10-12 minutes. The following data were obtained: Run fc

B

C

D

0.8 0.15 0.88 0.70 10 22.5 12 220-225

1.6 0.15

1.0 0.0 0.97 0.85 0 25.8 0 270-280


A 0.8 Dichloro-di-p-xylylene (gram) 0.3 Butyl-di-p-xylylene (gram) 0.95 Weight of crude copolymer (gram) 0.55 Weight of extracted polymer (gram) 18.5 Theoretical % butyl" 18.9 Percent chlorine in polymer 25 Calculated % butyl 210-215 Polymer melting point, °C. 6

1.35 5.5 22.8 11 230-235

° Based on monomer charge. Based on chlorine analysis of copolymer. 6

Results and Discussion Copolymerization of Chloro- and Butyl-£-xylylene. Mixtures of dichloro-di-p-xylylene (XIII) and butyl-di-p-xylylene (XIV) were pyrolyzed to form chloro-p-xylylene (XV), butyl-p-xylylene (XVI), and p-xylylene (VII). In the initial polymerization zone (ca. 9 0 ° C . ) chlorop-xylylene (XV) and butyl-p-xylylene (XVI) condensed and polymerized. Since this temperature is above the Tc of p-xylylene at 0.3 mm., pxylylene molecules passed through and condensed and polymerized in the final, air-cooled zone. In this way it was possible to study the copoly merization of chloro- and butyl-p-xylylenes starting from the mixtures of the three monomers XV, XVI, and VII. Poly(chloro-p-xylylenes) containing (in theory) about 5, 10, and 20% butyl-p-xylylene were prepared in this way (see illustration on p. 650). The crystalline melting points of the products (XVII) were in the range 2 2 0 ° - 2 5 0 ° C . compared with 290°C. for pure poly(chloro-pxylylene). Solubility characteristics of the products and a sample of poly(chloro-p-xylylene) were studied by heating in a-chloronaphthalene. The solution temperature is the minimum temperature required to dissolve the product with slow heating. The gel temperature is that at which the solution of the product in a-chloronaphthalene sets to a gel on gradual cooling. Results were as follows: Run Theoretical % butyl Solution temp., °C. Gel temp., °C.

A 18.5 170 65

Β 10 185 68

C 5.5 210 71

D 0 215 105



650

ADDITION

A N D CONDENSATION

P O L Y M E R I Z A T I O N

PROCESSES

The product from Run A was found to be completely soluble in s-tetrachloroethane at room temperature, whereas pory(chloro-p-xyrylene) is insoluble in this solvent at all temperatures. These marked changes in solubility characteristics are considered excellent evidence for the formation of random copolymers rather than block copolymers or mixtures of homopolymers. Chlorine analyses of the products were in the range expected for the copolymers. All experimental evidence indicated that copolymers were obtained rather than a mixture of two homopolymers. Copolymerization of Ethyl- and Chloro-^-xylylene. The copolymerization of these monomers was studied by pyrolysis of a mixture of 1.35 grams of dichloro-di-p-xylylene and 0.27 gram of ethyl-p-xylylene at 600°C. and initial condensation of the pyrolysis products in a zone main tained at 90°C. and a pressure of about 0.3 mm. Under these conditions, chloro- and ethyl-p-xylylene would be expected to condense and poly merize, while p-xylylene would pass downstream to condense and poly merize in the final air-cooled zone. At the end of the run, the product was removed from the 90°C. zone, extracted with ether, and dried. A total of 1.5 grams of tough, high molecular weight product was obtained. Since the product weighed more than the initial charge of dichloro-pxylylene, either a copolymer or a mixture of homopolymers was formed. Physical property measurements provide strong evidence for the formation of a random copolymer rather than formation of either block


42.

651

p-Xylylenes

GORHAM

copolymers or mixtures of homopolymers. Stiffness-temperature measure ments of the "copolymer" and of poly(chloro-p-xylylene) and poly (ethylp-xylylene) are shown in Figure 1. The presence of only one glass

\ ΙΟ

\

^^^.^Polychloro-p-Xylylene

5

\ \

\90/IOchoro/ethyhp-Xylylene ^^Copolymer

>ν \

CO

ïecant Modulus, ρ


10°

ΙΟ

4

^ ^ \ \

- \

^Polyethyl-^p-Xylylene

\

s?

\

1 ΙΟ

\

3

ι ιη2

I

I

i

50

100

150

ι ^ l

l

ι

200

250

300

Temperature,°C. Figure 1.

Temperature stiffness measurements of poly(p~ xylylenes)

transition ( 6 5 ° C ) , a lower level of crystallinity, and only one melting point (250°C.) are interpreted to indicate the formation of a random copolymer. It is interesting to note that incorporation of approximately 10 mole % ethyl-p-xylylene lowers the melting point of poly(chloro-pxylylene) from 290° to 250°C. The incorporation of 10 mole % of 2-ethyl-p-xylylene as a block copolymer would be expected to have little or no effect on the crystalline melting point, and a mixture of two homo polymers would exhibit two melting points.



652

PROCESSES

Copolymerization of Chloro- and Dichloro-£-xylylene. Trichlorodi-p-xylylene (XVIII) was obtained by chlorination of di-p-xylylene with three molar equivalents of chlorine. Pyrolysis yielded monomers X V and XIX, which were condensed and polymerized on a 90 °C. surface. A quantitative yield of product was obtained. The product was trans parent, tough, self-extinguishing, had a softening point above 280°C., and exhibited the correct elemental analysis for copolymer XX. Owing to the low solubility of the chlorinated poly-p-xylylenes, no attempts


CI

CI XIX were made to fractionate the product. This is an example of formation of a copolymer from a single starting material. Copolymers of ethyl- and chloro-p-xylylene were also prepared by pyrolytic polymerization of mixtures of dichloro-di-p-xylylene and diethyl-di-p-xylylene at 50 °C. This is an example of preparation of a copolymer by pyrolysis of a mixture of two disubstituted di-p-xylylenes. All available evidence indicates that copolymers can be prepared from any two p-xylylenes provided that the polymerization is conducted


42.

GORHAM

p-Xylylenes

653

at a temperature at which each monomer will condense and polymerize separately. The results of characterization studies of the copolymers indicate that random copolymers are formed. The question of whether the monomers add in a head-to-head or head-to-tail fashion remains to be investigated.


Related Studies Control of Molecular Weight. Studies have been conducted on tech niques for controlling the molecular weight of poly-p-xylylenes produced from di-p-xylylenes by the vacuum pyrolysis route. Earlier work by Szwarc (17), Errede (3), and Auspos (I) indicated that very reactive chain transfer agents were required to achieve a significant effect in the polymerization of p-xylylene derived from p-xylene. This general picture was confirmed in the present study. The effect of potential chain transfer agents was studied by cocondensation with the polymerizing p-xylylene species. It was estab lished that it was necessary to match volatility of the agent with that of the p-xylylene. Apparently, the agent must be in the condensed phase near a polymerizing site to participate in a chain transfer reaction result ing in termination of one growing chain. Both aliphatic and aromatic mercaptans were useful in achieving significant changes in molecular weight as measured by the RV (reduced viscosity of 0.2% solution in α-chloronaphthalene at 150°C.) of the product. Concentrations of 0, 1, and 2% of β-naphthylmercaptan in polymerizing chloro-p-xylylene re sulted in poly (chloro-p-xylylene) with RVs of 1.34,1.09, and 0.65, respec tively. Similarly, concentrations of 0, 7, and 14% dodecylmercaptan in polymerizing chloro/butyl-p-xylylene resulted in 90/10 chloro/butyl-pxylylene copolymer with RVs of 2.2, 1.02, and 0.85, respectively. Yields of carbon tetrachloride-insoluble polymer were lowered by the presence of the mercaptans, indicating the probable formation of soluble telomers. Unique Capabilities. A number of unique features and capabilities of the p-xylylene polymerization process have been uncovered and eluci dated. The reactive monomers will condense and polymerize on any solid surface placed in the condensation (deposition) zone. The chemical nature of the surface is unimportant, and many materials, including strong acids, bases, the alkali metals, metal hydrides, and chemically reactive compounds such as resorcinol, have been coated when placed in a deposition chamber. The polymerization of p-xylylenes on condensation is extremely rapid and appears to proceed from gaseous monomer to solid polymer without passing through a viscous stage. The monomer behaves as a reactive plasma which surrounds solid objects placed in the deposition chamber.



654


PROCESSES

Figure 2. Mesh coated with poly(chloro-p-xylylene). Polymer is transparent inner rings. 16.5 X

Figure 3.

Sawtooth coated with poly(chloro-p-xylylene). 16.5 X

For these reasons uniform thickness films are deposited on all surfaces. Unique features which result are the deposition of uniform thickness coatings on sharp edges, in crevices, and around the inside of holes. A metallizing mask was coated with 1.5 mils of poly (chloro-p-xylylene). Figures 2 and 3 are magnifications of the coated saw tooth and coated


42.

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p-Xylylenes

655


mesh. It is noteworthy that uniform thickness coatings are deposited over and around these structural features. In essence these structural features are replicated in the coating. In a further elaboration of this feature the ability of the monomer to penetrate between closely positioned glass slides and into deep crevices was investigated. In the initial experiment, several pairs of 3 inch X 3 inch glass slides were placed in a polymerization zone, the distance between the parallel surfaces of the plates varied from 0.017 to 0.125 inch, and sufficient chloro-p-xylylene was introduced to deposit 1.0 mil poly (chloro-p-xylylene) on the walls of the chamber and exposed surfaces. The plates were placed perpendicular to the flow of the gaseous mono mer. At the end of the experiment the samples were removed, and the thickness of the coating at the center point on the inside surface of the plates was examined with the following results: Distance Between Plates, inch

Coating Thickness at Center, mil

1

1

0.125

0.8

0.0625

0.68

0.0312

0.34

0.017

0.04

In a second series of experiments, 3 inch X 3 inch glass slides were joined at one end, the side openings were closed with tape, and the angle of the open end (0) varied from 90° to 1°. This experiment was designed to investigate the degree of penetration of the monomer-polymer into a crevice. The polymerization was conducted to deposit 1.0 mil poly(chloro-p-xylylene) on the walls of the chamber. The samples were re moved at the end of the run, and the thickness of the film at the bottom point of the 3-inch crevice was measured. Θ, Angle of Opening (°) 90 20 10 5 1



656

ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES

In both experiments a surprising degree of penetration of the polymerizable monomers into a difficultly accessible point was achieved. In the crevice experiment the thickness of the deposited polymer at the bottom of the crevice in all runs was more than 5 0 % that deposited on the outer surface, except where the angle of opening was only 1 ° of arc. Particle Encapsulation. A unique capability of the p-xylylene vapor deposition process has been uncovered in the area of encapsulation (12) of particulate solids. The particles or granules to be encapsulated are placed in a container which in turn is placed in the deposition chamber, and the nozzle from the pyrolysis tube is inserted in the mouth of the bottle. During the run the monomers pass from the pyrolysis zone through a nozzle into the bottle and polymerize on the surface of the tumbling particles or granules. Polymer is also formed on the inner surface of the bottle which is rotated at 5 0 - 1 5 0 r.p.m. Relatively simple equipment (Figure 4 ) has been used to study this phenomenon. Polyethylene Bottle Containing Particulate Material Vacuum Chamber Pyrolysis Furnace

LJ r

YW

Τ

Sublimation Furnace

Figure 4.

Drive Motor u

Ο Ι

/ Pyrolysis Tube Vacuum

Bottle Holder Attached to Shaft

Apparatus for encapsulating particulate materials

Particles as small as 5 0 - 1 0 0 mesh can be encapsulated. It is essential to keep the particles in continuous motion—e.g., by simple rotation of the bottle to maintain a tumbling bank—to prevent agglomeration and sticking. Since no solvents are involved and no liquid viscous stage is encountered during the polymerization, coherent, uniform coatings are deposited around each particle. Particulate materials which have been studied in the encapsulation process include sodium chloride, sodium dichromate, lithium, sodium, lithium hydride, zinc, and lithium aluminum hydride. Typical experi ments are described below. E X A M P L E 1 : ENCAPSULATION OF L I T H I U M A L U M I N U M HYDRIDE WITH POLY (CHLORO-P-XYLYLENE). In the distillation zone were placed 5 . 0

grams of dichloro-di-p-xylylene. In a 4-oz. polyethylene bottle were placed 1 0 . 0 grams ( 4 0 0 pellets) of lithium aluminum hydride ( L A H ) . L A H was obtained from Metal Hydrides, Inc., as 1/8-inch diameter pellets. The bottle was positioned in the coating chamber, the system was


42.

GORHAM

657

p-Xylylenes

evacuated to 0.01 mm., the bottle was rotated at 75 r.p.m., and the coat ing was conducted over a 30-minute period. At the end of the run, the bottle was removed, and a total of 11.27 grams encapsulated pellets was recovered.


E X A M P L E 2: E N C A P S U L A T I O N O F 3/16-INCH S O D I U M H Y D R O X I D E P E L LETS W I T H P O L Y ( C H L O R O - P - X Y L Y L E N E ) . Fifty grams of sodium hydroxide

pellets were encapsulated with polymerizing chloro-p-xylylene generated by pyrolysis of 5.0 grams of dichloro-di-p-xylylene over a 15-minute period. The bottle was rotated at 60 r.p.m. during the run. A pyrolysis temperature of 660°C. and system pressure of 50 μ were employed. A total of 51.97 grams of encapsulated pellets was recovered at the end of the run. EXAMPLE

3:

EXAMPLE

4:

E N C A P S U L A T I O N O F 20-MESH

ZINC

ENCAPSULATION OF L I T H I U M

WITH

GRANULES

WITH

P O L Y ( C H L O R O - P - X Y L Y L E N E ) . Ten grams of 20-mesh zinc granules were encapsulated with polymerizing chloro-p-xylylene generated by pyrolysis of 2.0 grams of dichloro-di-p-xylylene over an 8-minute period. A pyroly sis temperature of 660°C. and a system pressure of 40 u were employed. The bottle containing the granules was rotated at 60 r.p.m. during the run. A total of 10.35 grams of encapsulated granules was recovered. POLY(CHLORO-P-

X Y L Y L E N E ) . Pellets of lithium were prepared by cutting approximately 1/4 inch lengths from 1/8 inch diameter lithium wire. Grease from the surface was removed by washing in heptane for 5 minutes,filteringand transferring the pellets to a 4-oz. polyethylene bottle. All handling and transfer operations were conducted under argon. A total of 0.42 gram of lithium pellets was encapsulated with polymerizing chloro-p-xylylene produced by pyrolysis at 660°C. of 1.5 grams of dichloro-di-p-xylylene over an 8-minute period. The pressure of the system during the encapsu lation was 0.01 mm. The weight of the encapsulated particles was 0.465 gram. Two pellets were tested for perfection of encapsulation by placing in water and measuring hydrogen evolution. There was no measurable evolution after a 30-day test period. Surprisingly high levels of protection were achieved in the encapsu lated products when poly (chloro-p-xylylene) was used as the encapsulant. In a typical experiment, 150 grams of sodium hydroxide pellets were encapsulated with 2.5% polymer. The pellets were 3/16" in diameter and of rather irregular contour. On the average there were 10 pellets per gram. Protection of the encapsulated pellets was measured by choos ing 50 coated pellets from the production at random. Ten test tubes were filled with 20 ml. of distilled water, and a portion of universal p H indicator was added. Five pellets were placed in each tube. One uncoated pellet immersed in the test solution resulted in a p H change from 5 to 13 within ten seconds (color change of yellow to blue). p H changes in tubes containing encapsulated pellets were monitored. A tube is con sidered to have "failed" if the p H of the solution changes to any value above 7. This indicates an imperfection or pinhole in at least one of the pellets. In this test, pellets coated with 2.5% encapsulant exhibited no failures in any tube after 5 days, indicating a high level of protection.




658

PROCESSES

Samples of commercial 20-mesh zinc were encapsulated with 3.5 and 7.5% poly (chloro-p-xylylene). No problems with sticking or agglomera tion were encountered. The perfection of encapsulation was examined by immersion of 1 gram of encapsulated zinc from each run in 3N hydrochloric acid and measuring hydrogen evolution. One gram of uncoated zinc liberates 375 ml. of hydrogen within 30 seconds in this medium. There was no hydrogen evolution from any of the encapsulated samples during a 30-day test period, again indicating a very high level of protection and perfection of the coatings on the individual coated particles. Attention was then turned to encapsulation of a more reactive spe cies, lithium aluminum hydride (Example 1). Several batches were encapsulated readily with poly (chloro-p-xylylene) utilizing the standard tumbling process. Products with from 1-20 wt. % coating were prepared. To evaluate the perfection of encapsulation, the coated pellets were immersed in pure methanol. Evolution of hydrogen from one or more points indicates the presence of a pinhole or other imperfection in the coating. Since the encapsulated pellets are denser than methanol, pin holes in individual pellets are detected easily by evolution of a stream of hydrogen bubbles rising from the pellet to the surface. Such pellets are removed. The test is continued for 15 minutes, and the percent of pinhole-free pellets is calculated. Typical data are presented in Table I.

Table I. Effect of Weight Percent Poly (chloro-^-xylylene) Coating on Percent of Pinhole-Free L A H Pellets Run

Wt. % Coating

% of Pellets Pinhole-Free

A Β C D

5.5 9.5 11.2 15.5

82.5 94 98.25 100

Table II. Effect of Weight Percent Poly (chloro-f-xylylene) Coating on L A H Pellets to Long Term Protection in 50/50 Methanol/Water

Run

Wt. % Coating

Number of Pellets Tested

A Β C D Ε

1 5.7 9 15 19

20 20 20 20 5

Duration of Test, days 1% 21 26 29 150

Total Hydrogen Evolution, ml.


500 85 30 13 0

42.

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p-Xylylenes

659


It is seen that the percentage of pinhole-free pellets increases from 82 to 100% as the weight percent of coating is increased from 5-15%. A second test conducted with the pellets was to measure long term protection against hydroxylic solvents. This was accomplished by im mersing 20 pellets of each of several batches in 50/50 methanol/water and measuring hydrogen evolution with time. One uncoated pellet in this medium liberates 85 ml. of hydrogen within 15 seconds. Table II summarizes the important data. Acknowledgments It is a pleasure to acknowledge the expert technical assistance of Charles E . White of the Union Carbide laboratories in the performance of this work. Physical property measurements and interpretations were conducted by Alexander Brown and Neale Merriam. It is also a pleasure to acknowledge guidance gained from many stimulating and provocative discussions held with William E. Loeb of these laboratories and D . J. Cram of the University of Southern California. Literature Cited (1) Auspos, L. Α., Hoel, L. Α., Hubbard, J. K., Kirk, W. M., Jr., Schaefgen, J. R., Speck, S. S., J. Polymer Sci. 15, 9 (1955). (2) Errede, L. Α., Gregorian, R. S., Hoyt, J. M., J. Am. Chem. Soc. 82, 5218 (1960). (3) Errede, L. Α., Hoyt, J. M., J. Am. Chem. Soc. 82, 436 (1960). (4) Errede, L. Α., Szwarc, M., Quart. Rev. (London) 12, 301 (1958). (5) Ibid., p. 314, Ref. 33. (6) Gorham, W. F., J. Polymer Sci. Pt. A-1, 3027 (1966). For Patent Refer ences to this work, see Literature Cited Nos. (7-11) and (13) below. (7) Gorham, W. F., British Patent 883,939 (Dec. 7, 1961). (8) Ibid., 883,941 (Dec. 7, 1961). (9) Gorham, W. F., German Patent 1,085,673 (July 21, 1960). (10) Gorham, W. F., U. S. Patent 3,342,754 (Sept. 19, 1964). (11) Ibid., 3,288,728 (Nov. 29, 1966). (12) Ibid., 3,117,168 (Jan. 7, 1964). (13) Ibid., 3,221,068 (Nov. 30, 1965). (14) Gorham, W. F., Willard, H., U. S. Patent 3,300,332 (Jan. 24, 1967). (15) Kaufman, M. H., Mark, H. F., Mesrobian, R. B., J. Polymer Sci. 13, 3 (1954). (16) Pollart, D. F., U. S. Patent 3,164,625 (Jan. 5, 1965). (17) Szwarc, M., J. Polymer Sci. 6, 319 (1951). (18) Yeh, Y. L., U. S. Patent 3,349,142 (Oct. 24, 1967). (19) Yeh, Y. L., U. S. Patent 3,155,712 (Nov. 3, 1964). (20) Yeh, Y. L., U. S. Patent 3,153,103 (Oct. 13, 1964). RECEIVED

March 14, 1968.


Recent Advances in the Vapor Deposition Polymerization of p

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