D. SATAS
Porous Sprayed Sheets a nd Coatings
Leather-like or textile-like materials can be built up, with the porous nature apparent in this cross-section
38
INDUSTRIAL
A N D ENGINEERING CHEMISTRY
I actively the past few years, plastic films have been extended into a wide variety of uses, both n
,
porous
in sheeting and in coatings. Some porous films, especially films of poly(viny1 chloride), have been used for quite some time in such specialized applications as battery separators (5). At present, the utilization of porous plastics in the apparel and footwear industries is of special importance. Large amounts of various elastomers in sheet and film form, including coated textiles, are consumed by these industries. Nonporous products are not entirely satisfactory for uses in which breathability or ability to transmit water vapor is required for comfort. Entrance of the porous plastic materials into these areas not only improves the suitability and the functioning of plastic sheeting already used, but also replaces some of the established materials used for apparel and footwear. It is not surprising, therefore, that a considerable activity is noticeable in this area-particularly in regard to processes for introducing porosity into polymeric film and coatings.
A spraying process for preparation of porous elastomeric sheeting and coatings with properties found only in woven fabrics or leather is described in this article. Spraying of polymer solutions onto various substrates is a widely practiced method of coating. The versatility of this method is not fully appreciated or utilized, especially in the preparation of porous coatings. Porous coatings have been prepared by spraying solutions of chlorosulfonated polyethylenes; in these, porosity was largely caused by the voids produced by escaping solvent vapor (2). Spraying has also been used to prepare both organic and inorganic fibers. The manufacture of fibers from inorganic .materials, such as glass, has been extensively used for many years. D. E. Till (6) has sprayed polymer solutions in order to produce fibers and fibrous mats. V. A. Wente and coworkers (8)have reported the preparation of extremely fine polymeric fibers and fibrous mats by the spraying of polymer melts.
One process (3),the subject of this article, is suitable for the preparation of porous sheets and coatings consisting of irregularly shaped polymer particles. The particles are deposited in such a manner as to deform them into voids of the previously deposited layer, with substantial coalescence. The resulting structure has density, integrity, and strength while possessing porosity due to numerous interconnecting openings. A cross section of such a sheet is shown in Figure 1 below. For comparison, Figure 4 shows a cross section of porous plastic sheet prepared by removing a soluble granular material from polymer matrix, and Figure 5 shows a structure of porous sheet obtained by foaming. In the spray process, the thickness of the coating is built up by successive spraying. It is possible, therefore, to obtain a sheet with 'one relatively smooth surface of reduced porosity and another surface consisting of much coarser particles. Figures 6 and 7 show such surfaces. The spray may also be directed onto a patterned surface, the negative of which is reproduced on the surface of the
that atomization by means of introducing the liquid into an air stream of sufficiently high velocity produces a smaller average particle size than does spinning disk or impingement atomization (7). The air stream serves other purposes in addition to atomization. It assures a rapid removal of solvent, and it also propels the particles so that they may impinge the target at high velocity. For these reasons, our work was carried out primarily with air atomizers. The specific design is of minor importance, and many commercially available air spray guns are suitable. During the short time period between the atomization and particle coalescence on the target surface, many important changes occur which determine the properties and appearance of the coating. A large part of the solvent evaporates, the viscosity of the solution particles increases, the particles (which are somewhat elongated by the drag forces) are deformed upon impingement on the target, and the particles partially coalesce with the earlier deposit without completely losing their identity. The size of
porous sheet. Thus, leather-lie or other three dimensional surface appearances can be obtained without further embossing.
the particles, their shapes, and the degree of coalescence are the most important variables which determine porosity, hand, strength, and other physical properties of the sprayed coating or unsupported sheet. The type of polymer and solvents used, as well as process conditions, influences these variables, and it is very important to properly balance all of these facton. Polymer Solution. The solution viscosity and the velocity of the atomizing air stream are the main variables which determine the drop size of the spray. For most polymers it is necessary to use solutions of about 10% solids concentration; these are of low enough viscosity to obtain desirable drop size ( 7 5 - 5 0 . ~ diam.). The effect of solution viscosity on the nature of the sprayed sheet is illustrated in Table I. As the solution viscosity increases, the atomization becomes poorer and the deposited particles are of larger size, contributing to the surface coarseness. Larger particles also retain more solvent and therefore flow
PrOebSS
The preparation of porous polymer sheets by spraying requires a careful balance of the conditions in order to obtain these desired properties: porosity, sufficient tensile and tear strength, and bend modulus such that the material will have a textile-lie or leather-like hand. Breakup of a liquid stream can be due to liquidoriginated instability, or to friction and turbulence created by a second fluid. I n general, it is recognized
D . satas is Research Chemical Engineer at The Kendall Co., Research Center, Barrington, Ill. H e wishes to thank W . 0 . Elson and R . M. IIocy f o r their support, and A . Smilga and Z . Hrozencik for performance of many experiments. AUTHOR
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NO. 4
APRIL 1965
39
TABLE 1.
Vtrcosity,
cpr.
EFFECT OF VISCOSITY ON POLYURETHANE SHEET Tarde Sfrtglh, Air Pmrobilify,a Surface Lb./sq. In. Sec./lW cc. A~peamnct
34
150
66
115 785 1090
180 690
0.5 1.2
Smooth Smooth
6.9 Nonporous
Came Game
more after deposition; thus the product becomes less porous. The rate of solvent evaporation during the short time interval before the droplet of polymer solution reaches the target depends on the solvent volatility. Since the degree of coalescence in the final product depends on viscosity of the spray particles at the time of impact, and thus on the solid concentration in the drop at that time, the selection of proper solvents or solvent combinations is very important. In one specific case it was shown that the solid concentration in the spray droplets increased from 14 to 5047% during the short period of time between atomization and impact upon the target. In many cases it is convenient to control this by addition of certain amounts of a high-boiling solvent which evaporate slowly, and impart enough fluidity to the particles. A typical example is shown in Figure 2. The polymer is 45-55 blend of butadieneacrylonitrile copolymer and poly(viny1 chloride). Acetone was used as a solvent. The viscosity of the atomized droplets at the time of impact is determined by the amount of plasticizer and high-boiling solvent-cyclohexanone in this specific example. The amount of plasticizer is again fixed, since it remains permanently in the product and determines its flexibility. Therefore the amount of
cyclohexanone used is the free variable, in this case, which allows one to regulate the degree of coalescence of the particles during the impact. When the amount of cyclohexanoneis increased above 65% of the weight of the resin, a product of low porosity is obtained because of excessive particle coalescence. If the amount of cyclohexanone is decreased below 40%, a product of poor strength and high porosity is obtained because of low degree of coalescence. Atomization Air. The velocity and amount of atomization air are critical factors in the process of preparation of porous sprayed coatings. In order to obtain an adequate break-up of the polymer solution, a certain minimum air velocity is required. The size of particles and ligaments produced not only influence the texture of the coating, but also affect the drying rate and therefore the degree of coalescence and porosity of the product. In addition to affecting particle size, the amount of air used for atomization of the solution also has an effect on the drying rate. High polymer-to-air weight ratios tend to yield wet coatings of low porosity, while a low ratio yields a highly porous but weak structure. The air stream propels the polymer solution particles, and the force with which they impinge on the substrate to be coated depends on their mass and velocity, which again influences the degree of coalescence among individual particles and ligaments. Obviously, the distance between the spray and the target or the time the particles are in flight similarly affect the degree of coalescence among the particles. Figure 9 illustrates the effect of the distance on the properties of the coating for given atomization conditions. The polymer used was a blend of butadieneacrylonitrile copolymer and poly(viny1 chloride). The effect of high-boiling solvent (cyclohexanone) is also shown. Decrease of the distance causes the deposition of wet spray as the velocity of spray particles is higher at shorter
c A
Figure 2. Mosf suifablc composition for prepmation of buIadinuonylomtila copolyrnn milk poly(ldnyl ckloridc) blend in wow IscrthnIika shuts. A--plnrriciM, E ~ c l o h x a w n r C-latonur , blend 40
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Figure 3.
W i d - u p and of rhc sprq cwring opncrtion
Fieure 4
Figure 5
Fieure 6
Figure 7
Figure 4. Cross section of the porous plastic matrix remaining after removal of soluble granules, 245X. Figure 5. Cross section of a foamed sheet, 4 7 X . Figure 6. Surface of a smooth @rayed sheet, 75X. Figure 7. Surface of a rough @rayed sheet, 16x. Figure 8. Porous urethane elastomer surface, 7 7 X
Figure 8
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4 APRIL 1965
41
m distances between the spray gun and the target. Both of these factors tend to increase the degree of coalescence, and the resulting coating may become nonporous. Increasing the distance to greater than 30 in. in the given example causes the formation of loosely compacted coating, which is of low tensile strength and lacks uniformity and smoothness because of poor adhesion among the pprayed particles. It is believed by other investigatorsf7) that at distances greater than one foot from the spray nozzle no important changes in drop size distribution occur. An attempt was made to intercept polymer solution particles in flight
at various distances. In order to minimize the distortion of the particles on impact, a Petri dish filled with transparent gel was waved across the spray. Because of the irregularity of particle shape, sizes were only estimated, and the particles caught in the gel were classified into two groups : elongated fiber-like particles, and shot-lie particles. The observations, summarized in Table 11, suggest that agglomeration of the particles occurs during the flight. The particles caught close to the spray nozzle were primarily of a fibrous character, while the Ones intercepted further away were primarily shot. Of course, the tendency to agglomerate would depend greatly upon the distance between the particles in the spray and might not be observed in case of low solution-to-air weight ratio sprays where the particles are widely scattered in the air stream. Products
sfrengfh and porosity of thc product. A 4 . 0 , B4.95, C-7.42, D-7.89gramr of cyclohcxanoncpn gram of ilartomn blend
DiSloW,
Rmga of
Nozdt-fo-Tmgsl, Cm.
Fibn DimnarSrs,
x
cm.
10
Range of Shot Dimetns, cm. x 70-9
10-8
Mainly fibers Mainly fibera.
25
0.1 0.14.2 0.14.2
35 60
0.,14.5 2-10
2-3, aggregates started to appear 2-10 5-12, proportion of
79
2-10
20, predominantlyheavy
100
5-12
20
18
aggregates inacasca
aggregates
TABLE 111.
W
IER VAPOR A N D AIR
I Mofniol Experimental Experimental Chamois Washable garment leather
Watn vapor
Ai,
Tl07U-
mitlrd,
Trammis~ion,
Thickness, Mils
Watn Absorbed,
G./Sp.
G./Sq. In.
In. HI.
SW.
34 11 33
0,00225 0.00003 0.0204
102 115 79
0.4 61 2
38
0.0120
72
I
Nonporous
a Gurlcy's D-mstn readings. Time in seconds required lo h a m i t 400 CC. of air through a 7 q.in. men of a conrton6prcrrwc of 20 or.
42
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
The spraying technique is adaptable for manufacture of a large variety of products. A leather-lie material was prepared by this process using a blend of butadieneacrylonitrile copolymer and poly(viny1 chloride). Leather grain on the surface can be.obtained by spraying the polymer solution onto a belt with a negative pattern. The sheet can be vulcanized for additional strength and it accepts the leather grain rather faithfully from the belt. The air permeability of these leather-lie products was evaluated by Gurley's Densometer, and water vapor transmission by Thwing-Albert Vapometer cut (Thwing-Albert Instrument Co.) according to Joint Army-Navy Specification JAN-P 127. Table I11 shows some of these data. Water vapor transmission of the leather-like products is of the same order of magnitude as leather. However, the transfer mechanism in porous polymer films appears to be mainly molecular diffusion, and the tests performed with Thwing-Albert Vapometer cup support this assumption ( 4 ) . Porous coatings when properly formulated and applied on textile backings yield a breathable, waterproof construction useful for many applications. Figure 3 shows a textile coating operation in progress. Polyurethane elastomers such as Estanes are especially suitable for sprayed coatings because the polymer is soluble and has good mechanical properties uncured. Porous coatings of these polymers have good abrasion resistance and when applied over a stretchable backing form composite fabrics suitable for manufacture of elastic garments. The surface of such a coating is shown in Figure 8. LITERATURE CITED (I) Marshall, W. R., Jr., "Afomiratim and Spray Ww," C h . E q . Pmg. MmrM Sn.50, No. 2,41 (1951). (2) &he, 1. D., E. I. duPonl dc Ncmoun & Co., he., BL363 (h. 1959). (3) Satu, D. (to Kcndall Company), F-h PPtmt 1.298,959 (June 12, 1962). (4) sa-, D.,cam,n.0.. J . AM. ~ d ~ ~ s~. ws( it9 i6 2. ) . (5) S U n w , H.I., l m d d o n / o ~tka El#n~rolnnd Elmradr lndvlhlu P, 9 (No". 1956): (6) Till, D. E.,M d m Tetila Mq. 40. 36 (ocr. 1959). (7) W d q M. A,, Woraham. D. H., "Atomkition in HiEh Vdaity Air Strcama.' En0 Research & EnSincerin~ Co., Bumblcbec %ria Report, No. 221, 145 (May 1958). (8) Wmte, V. A,, Boon
