Cured Polvvinvl Formal Sheet J
J
THERMAL PROPERTIES M. M. SPRUNG, F. 0. GUENTHER, AND M. T. GLADSTONE' General Electric Research Laboratory, The Knolls, Schenectady, N . Y .
P
thereof-for example, a mixture oi butanol and toluene is more effective than either alone. A sample of 15-953 Formvar (polyvinyl formal) used in much of this work was obtained from the Shawinigan Resins Corp., Springfield, Mass. In the sample designation, the first number refers t o the viscosity of the polyvinyl acetate from which the formal is prepared, the second to the per cent hydrolysis of the polyvinyl acetate (this step must precede formalization), and the letter refers to the type of stabilizing agent used. The E-type stabilizers can be effectively destroyed by suitable heat treatment. Rubber milling differential rolls can conveniently he used in the preparation of polyvinyl formal sheet. The polyvinyl formal powder, mixed with 50 to 100% of its weight of volatile plasticizer, is fed t o moderately cold rolls and macerated without external heat until a uniform sheet results. The rolls are then warmed slowly as mixing is continued. For most of the combinations studied a final roll temperature between 65' and 85" C. is optimum for the most common grade of commercial polyvinyl formal. The end point is best determined by weight loss; FABRICATION ANI) CURE for a particular plasticizer optimum weight loss can only be deterThe preparation of cured polyvinyl formal sheet involves mined by experience. It is difficult t o handle sheeb that retain thermal or chemical crosslinking reactions, whereby the norless than 10 to 20% plasticizer; the rest must be removed submally thermoplastic character of the vinyl polymer is appreciably sequent to milling. lessened. The steps involved include the incorporation of a temThe simplest way t o remove residual volatile plasticizer would porary plasticizing agent and later its complete removal. Subbe by oven drying to constant weight, but in general this method is inoperable. A polyvinyl formal sheet dried a t as low a temsequently, the sheet is cured until optimum properties are nerature as 70" to SO" C. achieved. The removal of before pressing retains a forvolatile products (both midable degree of "elastic added plasticizers and curmemory," and heatirig such ing products) from a plastic a pressed sheet, even to sheet that is continuously SHEET POLYVtNYL FORMAL moderate temperatures increasing in colloidal com(looo t o 125" C.) results in plexity and particularly in serious mechanical distormelt viscosity involves the can be formed using rubber tion. Accordingly, it is nechazards of blistering, uncompounding techniques essary to press the sheet even shrinkage, and surface while it is still rather heavily roughness. These undesirplasticized. Removal of the able effects must, of course, can be improved by crossresidual liquid can then genbe minimized by proper linking erally be accomplished withchoice of conditions. out undue distortion. Some The best solvents for should find use in many elecvolume shrinkage is natupolyvinyl formal are chlorally t o be expected. rinated aliphatic hydrocartrical insulating problems Because of the tenacity bons, aliphatic and cyclowith which polyvinyl foraliphatic ethers, cyclic acemal retains many volatile tals, and phenols. Most of these are good plasticizers. plasticizers, it is not convenient t o use liquids of Some liquids that are not solvents are also quite effective as plasticizing agents. These even moderately low vapor pressures, unless it is desired to retain include certain aliphatic mono- and polyhydroxy compounds, them as permanent plasticizers. Thus, it was found inipracaliphatic ketones, ether-alcohols, aromatic chloro compounds, ticable t o completely remove butyl ('ellosolve (6-butoxyethanol), esters, and a variety of polyfunctional compounds. Some which has a normal boiling point of 171 ' C. and a vapor pressure plasticizer combinations are better than any of the components a t 25" C. of 0.6 mm. 1 Present addreae, Technical Department, Behr-Manning Gorp., Troy, Table I indicates some plasticizers that can be used and gives optimum milling conditions for each. N. Y.
OLYVINYL formal and mixtures of polyvinyl formal with phenol-formaldehyde condensates have been employed commercially for a number of years where a combination of toughness, flexibility, abrasion resistance, thermal stability, and good chemical and electrical characteristics are required. Polyvinyl formal alone is commonly employed as a molding powder or in laminates. An outstanding example of the usefulness of a polyvinyl formal-phenolic combination is Formex magnet wire insulation, where a very thin film of cured resin provides protection against thermal and mechanical hazards and also ensures . against electrical breakdown (7). A sheet material fabricated from polyvinyl formal would be of great interest and utility. Cured polyvinyl formal sheet should be useful in systems operating a t temperatures well above 100" C., where the uncured polymer is relatively useless. The limited solubility and high melt viscosity of technically useful grades of polyvinyl formal, however, have presented difficult barriers against the accomplishment of this objective.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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'I'able 1.
F a b r i c a t i o n of PolSvinyl Formal with Aid of Volatile Plasticizers
(100 gratne of h,-tv,ie pol3 x ins1 formal 111 each exwiimenti Pla,t1c 1 1 e 1 __ Vapor Boiling press. \Vt., point, a t 250 c., .C~,.P~!~:TLAH roinponent grama ' C./760 min. Initial Final Dioxane 100 101 ' 29.0 Room 70-80 temp. 100 116 15.2 60 75-80 Methyl isobutyl ketonr 68-73 100 171 80- 65 0.6 Butyl Cellosolve 75-80 280 73-80 50 ... Glyceiyl diacetate 55-60 44.0 75-80 100 78.4 E t h b l alcohol 44.0 72-80 78.4 90 5 5 -ti0 E t h y i alcohol %35-25gn 10 0.01 83.7 100 62.0 Koo1n temp. 58-63 68-73 79.6 72 100 40 Methyl ethyl ketone 40 69-74 8 2 . 5 60 33 b Isopropyl alcohol 110.8 20 33 23.8 20 100 ~
___
Curing Schedules. The cure of a polyvinyl formal sheet involves a series of both physical and chemical changeb. As residual solvents and any ahsorbed moisture are removed, chemical reactions are initiated, leading t o the formation of other volatile products; these include formaldehyde, acetic acid, carbon dioxide, water, and possibly formic, acid. The curing reactions result in improved physical and chemical properties, but overcuriny lead5 again to deterioration in many physical properties and ma\- eventuallx- cause serious chemical degradation. The following curing wliedule was normall\ used : 1.5 hours a t 100' C., 1 hour a t 325' C.: 0.5 hour a t l50* C., and 10 min. at 200" C. This schedule will subsequently be referred to t ~ the s standard cure. Sheets were ( w e d on perforatcd copper eweening in air convection ovens vapable of maintaining temperatures within 3 ~ 2 ' . Short oven (wrw a t the higher of thew temperatures would naturally be used, were it not for the desirability of producing blister-free sheets, which can only be r e a l i 4 by starting a t relatively low temperatures and gradually incrrnsing them. The allowable starting temperature and rate of temperature rise depends 011 the t r p e of fugitive plasticizer used, 011 total volatile content, and on thickness--for example, a 15-mil sheet may require seveial times longer treatment a t the lower temperatures than an 8- to 10-mil sheet. CROSSLINKING AGEYTS FOR POLYVINYL FOR\I iI
Ordinary polyvinj 1 formal contains both uIireacteci it( etate and hydroxyl groups. On heating, carbon-carbon double honcls and other unsaturated centers will be expected in primary transformation products. Since such unsaturated centers are susceptible t o further attack either by ovygen or by free radicds, the use of peroxides for speeding up and controlling the cure is suggested. The presence of free hydroxy gioupp suggests the possibility of crosslinking by means of hydroxyl 1 eagents, such as dibasic acids and acid anhydrides, dialdehydes, diisoryanstes, and ovides or salts of polyvalent metals; or reactive polymerizable molecules such aa the ineth>lol phenols present in alkalicatalyyed phenol-aldehyde reaction products; or the methylol amines and amides present in Ltniline-formaldeh)-de, ureaformaldehyde, and melamine-formaldehyde reaction products. Treatment with catalytic amount8 of moderately strong acid is another reasonable approach. Accelerating Cure with Peroxides and with Acids. The follorn-ing peroxides (1 to Z P 2 , based on the weight of polyvinyl formal) were tried: tl~l~enzoyl peronde (Lucidol), laurovl peroxide, tert-butyl perbenzoatth ( EP-4) and tert-butyl hydroperoxide Test buttons (1/1 inch thiczk by 3/4 inch in diameter) were prepared from the milled sheet by compression molding (20 minutes at 180' C.) and were te8ted for flow a t 150" C. Laurogl peroxide and tert-butyl hydroperoxide were ineffective. EP-4 and dibenzoyl peroxide considerably improved the flow re-
&'i%
!L 18 13'
18 30 10 14.5
21.5 12.5 13. R
Vol. 41, No. 2
sistance; however, there was some gas evolution from these samples on heating. Acids of known strength were milled into Etype polyvinyl formal in small, controlled amounts, using dioxane as the fugitive plasticizer. Sheet R pressed from these products (2 minutes a t 150" C.) were oven cured a t 150' C. for an hour, then molded into standard l/d X 3/4 inch diskp (20 minutes a t 180" C.). Alternately, the acid waR dissolved in acetone and added to a weighed quantity of polyvinyl formal po\T-der wet with more acetone. After mixing thoroughlv, the solvent was removed, first under vacuum and the final traces in a n oven a t 100" C. The plastic mass was thinned by passing briefly through rolls, broken up mechanically, and molded into disks (20 minutes a t 180' C.).
Flow Measurements. In the flow apparatus the standard disk ( 1/4 X 3/4 inch) was held on a small metal platform. Pressure was applied by means of a 6-kg. weight, supported on a rack-and-arm arrangement so that it could either rest freely on the specimen or be held above it. A micrometer, attached through a spindle, measured instantaneous compression, flow, or return, I n the test, the specimen was first allowed t o come t o temperature, allowing for thermal expansion. The weight was then applied and readings were taken for 10 minutes; then the weight was removed, and the specimen was allowed t o recover for 10 minutes. A flow and recovery versus time curve can be plotted if desired. Data obtained are shown in Table 11.
Table 11.
Flow D a t a a t 150" C.
(Acid accelerated cure; acetone soak method) Polyvinyl Flow, Acid KO.X LO', Formal, % 70 None ... 56-58 Sebacic Approx. I 1.0 49-53 %Ethyl hexoii. 4 1.0 36 o-Nitrobenaoic 32 1.0 37-40 Salicylic 106 1.5 51 730 1.0 17-18 Phosphoric 28-33 Maleic 1000 1.0 1.0 20-24 Trichloroacc tic 13000 2.0 15 y-Tolue~~esulfonic Very strong 1.0 1--4
Permanent Set, % 44--46 37-42 18
20-21 6 7.0-7.1 11.5-12 2 6.5-8.2 3.8 0
Crosslinking with Glyoxal. In a typical experiment, 370 grams of E-type polyvinyl formal. 83.5 grams of 30c7, aqueous glyoxal, and 37 grams of formic acid (a weak acid with only slight catalytic effect) were dissolved in 1480 grams of dioxane. Thii mixture was maintained under a reflux condenser a t 100" C. with stirring for 3 days. The viscosity continually increased, and finally virtual gelation ensued. The solvent was removed by working the sample on differential rolls, leaving a composition from which a smooth preased sheet was easily obtained. Later the compounding operations and chemical reactions were earried out simultaneously on differential rolls. Glyoxal-treated polyvinyl formal (and, incidentally, polvvinyl butyral) sheets showed considerahly reduced flow a t elevated temperatures. For example, for disks molded a t 150' C. and tested a t 125" C. (while still retaining some solvent) the flow was reduced from 70 to 74% to 2 t o 5% for polyvinyl formal and from 51 t o ,55% t o 2.5 t o 4,570 for polyvinyl butyral. A rapid, simple test for measuring relative flow way subsequently used. A spindle was attached t o a steel disk about 2l/2 inches in diameter, and cylindrical steel pieces weighing 250 grams each were pierced to allow them t o slip on the spindle. Standard x 3/4 inch disks, molded a t 150" C., were placed on the base plate and the weights set on top. This assembly wits allowed t o stand in an oven a t a given temperature for 10 minutes; the permanent set u as determined micrometrically and the specimen moved t o a second oven, readings repeated, etr.
February 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
The data obtained indicate that the glyoxal treatment is quite effective and also emphasize the superiority of the higher molecular weight samples and of the less permanent (E-type) stabilizers. For example, the permanent set in I O minutes a t 150' C. was 63.5y0 for an untreated, molding grade sample; 47.5% for a transiently stabilized (E-type) sample; 11% for a molding grade sample treated with 6% by weight of glyoxal; 8.7% for a similarly treated high molecular weight, permanently stabilized sample; and 3.9% for the treated, transiently stabilized sample. To determine the importance of the free hydoxyl groups, a number of special samples of E-type polyvinyl formal were studied. The data shown in Table I11 indicate that the polyvinyl alcohol content of the polyvinyl formal must be appreciably in excess of 2% for the crosslinking agent t o be effective. The sample with zero polyvinyl acetate content was also more SURceptible to distortion than normal E-type polyvinyl formal.
Table 111.
Effect of Hydroxyl Content
(Poly\inyl formal milled with 6 % by weight of glyoxal, as 30% solution, in dioxane: molded disk subjected to 250-gram weight for 10 minutes at temperature given) Total Pernianent Set, % * Composition, % . rSaniplr PVOAca PVOHb 100' C. 125' C. 150' C. 200' C. 5.2 10.1 12.5 36.5 4I3 0.0 a.2 0.0 7.2 46.4 ti 1 . 2 ... 13.0 C 16.8 2.0 5.2 36.7 50.7 ... 11.3 2.2 2.6 2.6 2.6 D 13.4 2.2 ... 17.6 2.2 2.2 :E 11.2 cl Polyvinyl acetate. 0 Polyvinyl aloohol.
Glyoxal-treated pressed sheets could be cured according to the standard schedule. However, it was usually possible t o begin the cure at 125' or even at 150' C. since the effect of the glyoxal treatment was to decrease greatly the incidence of blistering and bubble formation. Astonishingly little-Le., 0.75% by weight of glyoxal-can thus pronouncedly affect the properties of the milled sheet. The glyoxal-modified products, when fully cured and solvent-free, were, however, more brittle than untreated polyvinyl formal and somewhat more subject to retention of elastic memory. Crosslinking with Diisocyanates. Diisocyanates, which react readily with alcohols by simple addition, yielding urethanes, are effective crosslinking agents for other resin systems (4, 6). Hexamethylene diisocyanate, O=C=N(CH2)6N=C=O, and were studied. Both toluene diisocyanate, C&C&(N=C=0)2, react rapidly with the residual hydroxyl groups of polyvinyl formal. The same general technique was used as in the experiments with glyoxal. These samples were very stable against flow, even up t o 200" C. However, considerable elastic memory was again noted. Similar observations were made with pressed sheets. As with glyoxal-modified sheets, the cure could be begun a t 125" or 150" C. since difficulty with blistering was again greatly reduced. The reactions are, however, very difficult to control. The undesirable effects mentioned previously, manifestations of elastic memory, are more pronounced with toluene diisocyanate as crosslinking agent than with hexamethylene diisocyanate. This is Understandable since the former is not only more reactive but also constitutes a much more rigid molecule. Hence, by geometric consideration, i t can introduce rather great strains when it forms molecular bridges. Crosslinking with a Phenol-Formaldehyde Resin. An alkalicatalyzed phenolic resin was prepared from a commercial cresol mixture (Koppers No. 620 cresylic acid) and paraformaldehyde. E-type polyvinyl formal was compounded with 10 to 50% by weight of this resin, using dioxane as plasticizer. Sheets pressed a t 150" C. for 5 minutes, using 12-mil shims, showed interesting properties. They were easily pressed, retained a high degree of flexibility and strength after cure, and had good resistance to
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distortion and mechanical penetration a t elevated temperatures. There was, however, difficulty in minimizing bubble formation during cure, and it was necessary t o start a t temperatures lower than 100" C. and slowly work upwards. Because of the superiority of these sheets, many of the subsequent detailed measurements involved a similar phenolic modified product. STUDIES O F THERMAL STABILITY
The following phenomena were studied in considerable detail: 2. Changes in flexibility (crease resistance) 2. Changes in area (dimensional stability) 3. Weight loss 4. Formation of specific decomposition products Weight loss was measured in an open system with free wvess of ambient gas-Le., air or dichlorodifluoromethane (Freon-12)and in a closed system of small volume under vacuum or containing air, dichlorodifluoromethane, or nitrogen. Freon-12 was chosen as an example of a chemically inert gas of commercial interest to the chemical, electrical, and refrigeration industries. Most of these observations concern polyvinyl formal or polyvinyl formal-phenolic sheet prepared in the laboratory under controlled conditions. In addition, two samples of calendered sheet were furnished by the Minnesota Mining and Manufacturing Co. The cresol-formaldehyde resin was prepared as follows *
9 mixture of 432 grams (4.00 moles) of three-de ree boiling range m-cresol-p-cresol mixture, 108 grams (3.46 moyes) of 96% paraformaldehyde, and 14.4 grams of purified triethanolamine were caused to react a t 98.0' to 101.5' C. for 110 minutes, a t which time 95% of the available formaldehyde was consumed (determined by analysis) (6). The resin was cooled rapidly and dissolved in an equal weight of absolute ethanol. This solution was kept at room temperature for several months N ithout appreciable increase in viscosity. Milling and Pressing Procedure. Unmodified polyvinyl formal sheets 3 and 3A were prepared as follows: To 200 grams of E-type polyvinyl formal was added 200 granm of absolute Ethanol. The batch was milled on differential rolls a t an initial roll temperature of 50" to 55' C. and a final temperature of 63' to 68" C. A sheet weighing 225 grams was obtained-l2.5% plasticizer (ethanol) was retained, based on the polyvinyl formal. I t was then pressed between flat plates for 5 minutes a t 150" C. and a t about 3500 pounds per square inch, using IO-mil shims. Phenol-aldehyde modified polyvinyl formal sheets 4 and 4A were prepared as follows. T o 200 grams of E-type polyvinyl formal was added 200 grains of ethanol and 50 grams of a 50% solution of m, p-crrsol-
Table I V .
Curing Schedule for Milled and Pressed Sheets Time, Minutes Sheets Sheets 3 and 3A 4 and 4 A 25 27 30 15 6 10 30 30 5 5 30 30 7 2 15 15 5 5 15 15 16 18 30 30 10 9 5 5 216 229 196 213 19.5 28.5 11.8 9.1
Temp., O C. 30-100 100 100-110 110 110-115 115 115-120 120 120-125 125 125-150 150 150-200 200 Total time, minutes Weight after curing, grams Weight loss grams Weight loss: % Weight loss (excess over calculated plasticizer con5.0" 1.9 tent), % '1 'The nJditional weight loss of 3.1% for polyvinyl formal-phenolic A e e t is i)resuniahly owing to loss of volatile coniponents of tho henolic resin. l h e cured sheet thus contains somewhat less tlian 10% phcno&e c o n i ~ o n ~ n f . ~
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Figure 1. Distribution of flexibility (90 total values) Polyvinyl formal 0 , polyvinyl lormal-phenolic 0
formaldrliyde resin in ethanol. T h e batch was compounded on ditferential rolls at a n initial tempcrature of 50' to 55' C. and a final temperature of 65" to 75' C. This sheet weighed 252 granis, retaining 12.0% plasticizer (ethanol ), based on the combined weights of polyvinyl formal and creeol-formaldehyde resin used. In pressing, 12-mil shims were used to compensate for the greater softness of this sheet. Curing Schedule. The pressed sheets, (gross weights, after standing overnight, 215.5 and 241.5 grama) were heated in a high velocity air oven, according t o the schedule shown in Table IV. This schedule, established by enipirical considerations, must be carefully adhered to because of the critical nature of the physical and chemical changes involved, as was emphasized earlier. The cured sheets were immediately cut into smaller pieves and transferred, within a matter of minutes, to a desiccator, 5% here they n ere stored over Drierite desicr:iiit. Samples 5 and 5A were :ippi o\imntcly 14-mil calendered 4icets. Sheet 5 contained 4% tricreq 1 p1iosph:ite nnd 1% hydrocurbon oil; sheet 5A contained 10% ~~licnol-forinaldeh~*de resin in addition t o these minor ingledients. The volatile plasticizer was diacetone alcohol. Samples weie cuied rapidly in an air convection oven. The total curing time v a s 2 hours, including 15 initlutes a t 100" and 125' C., 30 minutes a t 150" C., 5 minutcts a t 200" C., and the remninder bridging between these stated temper:itures. The total weight loss as 21.5% for sheet 5 and for sheet 5.4. These samples were also cut into sm:ill sections :tnd stored over Ilrieiite in n desiccator.
Figure 3.
Loss of Flexibility in Air. For the present purpose, flexibility is defined as the number of times the sperimen can be t)erlt sharply, through an angle of 180"-i.e., Doo on either side of normal-while held firmly at thc point of bending, before a break ocrurs. This test was performed simply by holding a strip of the plastic (about to 1/2 inch wide by 1 t o 5 inches long) in a small, smooth-jawed bench vice and fiesing continuously through 180" until a break occurred. The precision and reproducibility of the measurements are illustrated by Table V and Figures 1 and 2. These data indicate that the normal probability relation is rather closely adhered to, and that good reproducibility can be achieved if at least 30 measurements are made each time. Figures 3, 3A, 4, and 4d show the rate of loss of flexibility 15 Ith time for samples 5 and 5 d and for samples 3 and 4 R hen heated in 175O, a convection air oven a t temperatures of IOO", 125O, la", and 200" C. The oveii used was capable of maintaining a tenipeiitture ~ i t l i i n11' C. Each of the expcrimentd points is based on at least 30 fle\ilrilitJ measurements. These data illustrate strikingly the increase in phj sical stability which results from the addition of a phenolic resin t o polvvinyl foimal. [-sing the time at which the flexibility decreases to of its original value as an index, the improveinent in flexibility retention iu between 300 and 400% both for sheet 4 coinpared to sheet 3 and for sheet 5A compared to sheet 5 . These relations are more clearly depicted in Figures 5 and 6, where the time required for 50% loss in flexibility is plotted as a function of the reciprocal of ahsolute temperature.
Sheet 5 0 , sheet 5.4 0
figure 4.
Loss of flexibility in air oven
Sheet 3 0 , sheet 4 0
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1955
Flexibility D a t a
Table V.
Flexibility, No. CreasesM-Min. Av.
Sheet NO.
Thickness, Mils
5 5 5A 5A 5A 5A 3 4
13.2-14.8 13.2-14.8 11.5-12.5 11.5-12.5 11.5-12.5 11.5-12.5 12.5-14.5 11.5-13
Table VI. Aging Time, Years 0.46 0.46 0.46
0.46 0.49 0.79 0.33 0.67 1.01 1.83
Total Measurements
in 10 18
1
15 14 18
in
14
C h a n g e in Flexibility on Aging at Room Temperature Flexibilitya,
% ' of Original Conditions
4
In room air In room air; heated 3 0 niin. a t 15W C. before testing I n room air. heated 3 min. a t ZOOo C.'before testing In room air; conditioned 96 hours a t 50% relative humidity before testing Uncured sheets stored in room air. given standard cure befire testing In room air Desiccator, over Drierite In desiccator In desiccator In desiccator
.. ..
.. 88
..
..
3
5
5
'
4
..
66
67
..
71
59
70
59
68
66
73 69
64 58
..
.. .. .. ..
97 88 86 86
..
.. .. I .
Values for sheets 3 and 4 are average of 90 tests; others are a irerages of 30 tests. Q
If these curves are extrapolated, they suggest rather extraordinary stability at room temperatures. It was therefore interesting t o collate information concerning room temperature aging of polyvinyl formal sheets under controlled conditions. Such data, shown in Table VI, qualitatively confirm the results of extrapolation for the molded sheet (samples 3 and 4) but suggest a relatively higher rate of deterioration at room temperature for the calendered sheet containing tricresyl phosphate (samples 5 and 5A). An impractically long time would, of course, be required for a better comparison. It is of interest that the flexibility could not be restored by annealing at 150' t o 200' C. or by conditioning at 50% relative humidity. Dimensional Changes. Figurcs 7 and 8 show changes in area that occur on heating polyvinyl formal and polyvinyl formalphenolic sheets in air at 150' C. The data of Figure 8 were obtained with the aid of a planimeter. Those in Figure 7, which are more accurate, were obtained by holding the specimen (meas-
309
urine; about one square inch in area) 111 a small clamp having smooth jaws made of Lucite, acrylic resin, and of such size that the specimen protruded about 1/16 inch beyond the jaws on all sides. The actual measurement was then made with a micrometer caliper. The data obtained indicate that a sheet cured a t high temperatures on a wire screen (or similar structure) relaxes almost instantaneously at 150' C. with a n accompanying shrinkage of 3 t o 5%. Thereafter, on continued heating, a further, very slow decrease in area occurs, which effect can be cot related with weight loss; both weight loss and continuous area change (shrinkage) are the result of the loss of volatile decomposition products. Accordingly, the long-time dimensional changes are greater for polyvinyl formal than for similar polyvinyl formal-phenolic sheets. Other experiments, data for which are not shown here, indicate that samples kept a t room temperature and high humidity do not undergo further appreciable area changes, once the initial stresses have been relieved. Weight Loss in an Air Oven. Beachell, Fotis, and Hucks ( 1 ) recently studied the weight loss behavior in air of films of uncured polyvinyl formal cast on glass plates from tetrachloroethane solution. They believe that the weight loss represents a summation of oxidation and degradation reactions. The observed rates were very high; the time required for a 25y0 weight loss was about 100 hours a t 150' C., 25 hours at 175' C., 8 hours a t 200' C., and 2.3 hours a t 225' C. Cured polyvinyl formal films are much more stable (as will appear from data t o be shown presently). The only volatile decomposition product identified by Beachell and associates was formaldehyde, but the mechanism postulated also assumed the formation of water. Bukey (B) and Callinan (8) have described some effects of thermal aging on the electrical properties of polyvinyl formal resins. Aging of polyvinyl formal coated wires (Formex) for periods up t o 45 days at 150' C. in air led t o an increase in the ' C.) 10,000-cycle power factor peaks (occurring at about 0 from about 2.5% t o nearly 4.5%. Simultaneously, the dielectric strength decreased from about 4000 volts per mil t o 2000 volts per mill (root mean square voltage). When polyvinyl formal
I 50
loo
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I50
I
I
200
280
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Figure 5 (left). Sheet 5 0 , sheet 5A Figure 6 (right). Sheet 3 0 . sheet 4
8
Time r e q u i r e d for 50% loss of flexibility in air oven
Figure 7 (above). Sheet 3 0 sheet 4 0 Figure 8 (below). Sheet 5 .,'sheet SA 0
C h a n g e in area at 150' C. in air
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 2
T I Y E I N HOURS
Figure 9.
Sheet 3 0 , sheet 4 0
Figure 11.
Sheet 5 @, sheet 5.A 0
Weight loss a t 100-200" C. i n air oven
films cast from tetrachloroethane solution were aged in air a t 175' C. for 24 hours, an increase in dielectric constant a t 10,000 cycles from about 3.3 to 4.0, meaaured a t room temperature, was observed. There was a concomitant rise in the brittle point from 70" to 100" C . Similar measurements made on films of General Electric Formvar enamel aged 24 hours a t 225' C. in air (polyvin) 1 formal-phenolic films) showed the dielectric constant to increase from about 5.0 to 8.0 while the brittle point rose from 110" to 135" C. A relatively slower change was noted a t 175" C. In the present work, samples were removed from the oven a t varying times, cooled in a desiccator, and weighed rapidly. I n calculating the weight loss, a correction ( u s u a l l y 0.1 t o 0.2%) was applied, r e p r e s e n t i n g the amount of moisture unavoidably picked up during the short time the sample was exposed t o the atmosphere during handling. Figure 9 i-epresents data for sheets 3 and 4, which inI L-__ volved no additives 210 220 230 240 250 260 270 other than the phe, ,ne nolic resin and the fugitive plasticizer Figure 10. Weight loss rate i n air oven cs. reciprocal temperature (ethanol). T h e w i d e l y divergent Sheet 3 @, sheet 4 0 data are plotted 1o g a r i t h m i c a 1 1 y to permit quick visual comparisons. The curves indicate strikingly the greater stability of the phenolic modified sheets toward degradation in air, particularly at the lower temperatures. Actually, there was no appreciable weight change for sheet 4 a t 100" C. in air until after 3000 hours (4 months), a t which time the polyvinyl formal sheet (No. 3) was decomposed to the extent of about 2%. Figure 10 is an Arrhenius plot of these data, taking the approximatelv constant rates of weight loss between 1 and 2% as the index The apparent activation energy for the processes involved in these decompositions is about 25,000 cal. per mole for polyvinyl formal and about 27,000 cal. per mole for polyvinyl formal phenolic. These values compare with 21,000 cal. per mole given by Beachell, Fotis, and Hucks ( 1 ) for uncured polyvinyl formal films.
Figure 11 summarizes data obtained with sheets 5 and 5A which originally contained a hydrocarbon oil and tricresyl phosphate in addition to the normal constituents. The data are again compressed by using a logarithmic scale. Keight losses for these sheets are consistently higher than for sheet,s 3 and 4, reflecting the loss of oil and plasticizer as well as volatile decomposition products. Otherwise, the general relations are similar t o those previously observed. At the l o n e r temperatures, the phenolic modified sheet is much moi'e stable than the unmodified sheet; but this is no longer significantly true at higher temperature, especially during early stages where plasticizer remnants contribute heavily t o the products that are lost. Weight Loss in a Closed System. For this series of esperi.trips of cured plastic sheet, weighing ahout 2 grams each, were weighed rapidly and insert,ed into borosilicate glass tubes "4 inch in diameter, which, when sealed, had a volume of about 100 ml. A standard tapered glass joint \vas sealed on the open end, which was constricted, and the tubes were attached to a vacuum line. The system was pumped down to about 100 microns, and the gas to he used (ai?, nitrogen, or Freon) was'admitted through a drying tuhe. The sj.atem was then pumped down to 50 microns and oiie a,t,niosphere of gas readmitted; then to 10 microns (or lower) for an hour. Xext, 700 mm. of dry gas was admitted, and the t , u h were 3e:tled off a t the con-
8 I T Y I
WURS
Figure 12. Weight loss in Freon 12 a t 128' C. Sheet 3 0
- - - -,
sheet 4
-.
---o----
sheet 3 in open air oven
strictions. For those experiments carried out in vacuum, the procedure was the same as for air, except that the tubes were finally sealed in a vacuum after a final period of pumping of a t least an hour a t 10 microns or lower. The tubes were placed in constant temperature ovens at 128", 150°, 1 7 5 O , 1 8 5 O , or 200" C., removed a t approximate time intervals, coolcd, and opened. The plastic strip was weighed and then heated to constant weight in air a t 150" C. (usually for one hour) in order to remove volatile decomposition products absorbed by the film. Weight loss figures were corrected by subtracting from each the amount lost
INDUSTRIAL A N D ENGINEERING CHEMISTRY
February 1955
P
"i-
'Or
-- -- - - -
--
--- -- -.- - - - -
Figure 13. Loss at 150' C . Sheets 3 and 4 in vacuum Figure 14. Loss at 175' C. Sheet 3 0, sheet 4 X. -* , Experiments in vacuum 0 A -; sheets 3 and 4 in Freon 0 X , in nitrogen -; in Freon 0 CiI sheets 3 and 4 in open air oven 0 X ; sheets 0 0 ; in air 0 X 3and4inairO X -; in open air oven 0 X
0X
311
---
-c.x- -.-,- -
Figure 15. LOSSat 185O and 4 in vacuum 0 x 3 and 4 in nitrogen 0
Sheets 3 sheets
Weight loss i n sealed tubes
accurate weighing. The aluminum shield, 8, and heater, 9, were necessary to prevent deposition of paraformaldehyde a t the exit of the reactor. Tube 3 was a safety trap. A few milliliters of water were added, because it was observed in a preliminary experiment that paraformaldehyde deposited here also if this trap was dry. Tubes 4 and 5 each contained 20 ml. of water and tubes 6 and 7 five ml. of 0.1N sodium hydroxide diluted to 20 mi. The procedure for loading the tube, measuring weight loss, and analyzing the decomposition products, follows:
' ,' ao
Figure 16. Sheet 3
I
I
I
100 HOURS
I50
200
Weight loss i n sealed tubes a t 200' C.
--------,
0, sheet 4 X; in Freon in open air oven
in air
-,
by the unaged film when heated in air a t 150' C. for an hour (usually 0.2 to 0.3%). These data are summarized in Figures 12 to 16. The weight loss observed in a circulating air oven is reproduced in each case for comparison. In general, the rates observed in nitrogen, vacuum, and Freon a t lower temperatures were about the same, within experimental accuracy. The rates in air were considerably higher and much higher still in presence of a large excess of air. The superiority of the phenolic resin modified material is evident throughout these experiments. For example, a t 175' C. (Figure 14), the polyvinyl formal-phenolic sheet (No. 4)is more stable in the presence of a limited volume of air than is the polyvinyl formal sheet (No. 3) in Freon. Figure 17 is an Arrhenius plot of the averaged results obtained in vacuum, nitrogen, and Freon. The apparent activation energies obtained graphically from Figure 17 are 26,000 and 27,000 cal., respectively, for polyvinyl formal and polyvinyl formal-phenolic sheets, about the same as in the presence of air. Composition of Products Formed in a Stream of Dry Freon. I n these experiments, specimens of plastic sheet were heated in a stream of pure, dry Freon-12 gas (dichlorodifluoromethane) a t temperatures of 150', 170', 190", and 200' C. The apparatus is shown schematically in Figure 18, and also the method of mounting the sample in a weighable glass assembly. The sheet was mounted on glass rods with glass helices as spacers and sealed in tube 1 (Figure 18). This method of mounting made it possible t o have 35 t o 40 square inches of sheet exposed t o the gas in a container of convenient size for
Approximately 10 grams of the sheet material in the form of disks la/^, inches in diameter with four properly spaced holes were accurately weighed and stacked on the glass rods with glass helix spacers. This assembly was dried for 15 minutes a t 150' C. to remove absorbed moisture and then sealed in tube 1. The loaded t u b e was dried 45 minutes a t 150" C., a n d t h e weight a t this time was used as the zero value. The tube was flushed with Freon, m o u n t e d in the rapidly stirred silicone oil bath in a t h e r m o s t a t which was held within f0.1" C. of the des i r e d temperature, and then connected to the series of traps shown in Figure 18. T h e necessary heat-up period was determined to be 15 minutes by placing a mercury thermometer in the middle of the disks. This experiment was done on a used sample by I/T x l o 5 t cutting off the top of the tube and drillFigure 17. Weight loss rate ing a hole in the 1/T disks t o admit the For linear rates beyond 1% reaction; thermometer. T h e avera ed values for vacuum nitrogen average reading of and &eon; sheets 3 and 3A' 0, sheet: three Anschutz ther4 and 4A X mometers was taken. A gas flow rate of approximately 2 grams per hour was obtained by passing the gas from a Mathieson Co. cylinder through a pressure-reducing valve, then through three orifices mounted in series. Each orifice was an 8-mil hole, inch deep, constricted by insertion of a 7-mil nickel wire. At the completion of a heating period, tube 1 was removed and cooled in silicone oil, with Freon flow continuing. A nitrogen line was then attached and pure nitrogen passed over the
*
312
Figure 18.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Apparatus for studying decomposition products formed
sample, whilc the silicone oil was removed in a benzene vapor degreaser. The tube was then stoppered, and weighed after a storage period of 30 minutes in a desiccator. The contents of traps 3, 4, and 5 \?-ere combined, acetic acid was determined by titrating with 0.1N NaOH, and formaldehyde was determined by the bisulfite method (6). For cresol determination, it wm first necessary to make the solution alkaline and to oxidize the sulfite present to sulfate with hydrogen peroxide. Excess peroxide was decomposed with manganese dioxide and heat, and the manganese dioxide was removed by filtration, Cresol was then determined by bromination with bromidebromate solution (8). Bromination results were not reproducible to better than 2 m1. of O.1N BrO,-, in spite of the fact that prepared standards of cresols, contaminated with formaldehyde and bisulfite, gave good reproducibility. The presence of interfering substances was partially substantiated when titration of the volatile decomposition products from a polyvinyl formal sheet containing no cresol-formaldehyde resin required 1.7 to 2.8 cc. of O.1N BrQa-. This interference reduces the accuracy of the cresol determination considerably. Carbon dioxide was determined by titration of the S a O H absorption traps with 0. lit' sodium hydroxide first to a phenolphthalein and then to a methyl orange end point. All carbon dioxide results had two corrections applied. One correction of 0.0011 gram of carbon dioxide was independent of heating time and represented carbon dioxide piclrup from handling. h second correction of 0.000021 gram of carbon dioxide per hour wab applied, after a blank experiment on Freon passed through a glass tube a t 200" C. for 93.5 hours, and then into liquid absorbent, gave an analysis of 0.0031 gram of carbon dioxide. No acidic decomposition products from Freon were observed in either the water or sodium hydroxide traps.
Discussion. The results of this series of measurements are shown in Figures 19 to 22. The first of these figures indicates total weight loss rates versus time a t various temperatures. These curves in general are quite different, from those shown in Figures 12 to 16. In the experiments carried out in sealed tubes, the weight loss rates tend toward constancy after about 1% total reaction. I n the gas stream, constancy is achieved much later. Although the final rates are comparable, the intermediate rates are much higher in the open system. During the early stages of the reaction and especially at the lower temperatures, the instantaneous rates in the flowing gas stream may be five or even ten times those found
Vol. 47, No. 2
in the closed system. At later stages, however, these rates are more nearly equal. In a closed system, some of the decomposition products may exhibit an inhibitory effect or may retard reversible steps in the reaction through mass action effects. Figure 20 is an Arrhenius plot of the composite data obtained in these runs. The ordinates are reciprocal temperature, the abscissas are time, and the additional parameter considered is per cent weight loss. Approximately parallel straight lines are shown. Since the lines in Figure 20 are essentially parallel, a single activation energy may be assumed. This is about 35,000 calories, an appreciably higher figure than was calculated from the data obtained in a closed system. Possibly in the presence of a flushing gas the total activation energy contains a contribution from a diffusion piocess in addition t o the activation energy of the chemical reactions involved. However, from a practical poiiit of view, the two sets of data are in over-all good agreement, since the extrapolated rates obtained rtt 10aO C. are within a factor of 1.5 to 2 of one another. The composition of the products formed a t varying stages of the reaction is shown in Figures 21 and 22. I n the polyvinyl formal sheet, formaldehyde and acetic acid together account for about 50% of the product a t any time (about 25% each). These percentages, in general, increase as the reaction proceeds, except a t the higher temperatures, when formaldehyde tends t o constitute a constant fraction of the total. Carbon dioxide is evolved throughout the reaction, a somewhat puzzling phenomenon, constituting, on the average, about 5% of the total. A possible source of carbon dioxide might be the rearrangement of a chain segment bearing a hydroperoxide group, introduced during cure of the sample in air. Por example:
I
0 I / CH:.
d COP
+ HzO
However, this reaction should not continue at a seemingly constant rate, since the concentration of the hydroperoxide groups should decrease rapidly. Another conceivable explanation of carbon dioxide formation is an end group reaction, which,
/
, /
I
*-
Figure 19. Total weight loss in stream
of Freon
Sheet 3A sheet 4A 0, additional pointa (which would be. o f ft h e figure) were obtained at 150' C. as follows: for sheet SA, 2.469% a t 610 hours; for sheet 4A, 2.05% a t 760 hours
Figure 20. Total weight loss V S . 1 / T in Freon stream Sheet 3A
-.
o - - - - sheet
4A
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1955
313
however, should also decrease rapidly t 2euC \ with time. It is possible that the car\ I \ bon dioxide is a decomposition product 4\ of formic acid, formed by partial oxida\ tion of formaldehyde. Since formic \ acid, if present, was not differentiated \ analytically from acetic acid, this hy9 pothesis could not be verified experii Yr" mentally. The so-called "unaccounted ,{i for" portion of the reaction product does decrease rapidly with time. It is initially about 60% and ultimately only about 30% of the total. It is Figure 21. Polyvinyl formal (sheet 3A) Figure 22. Polyvinyl formal-phenolic (sheet 4 A ) assumed that the main constituent is water. However, other products may Products formed in a Freon stream be present in small amounts. The behavior of thepolyvinylformalphenolic sheet is significantly different in some respects (Figure 22). Formaldehyde and acetic acid both are initially low but increase again t o about 25% of the total. Carbon dioxide is produced in slowly increasing percentages, again comprising roughly 5 % of the total. The remainder or unaccounted for portion is over 80% of the total during early stages of the reaction, Figure 23. Polyvinyl f o m a l Figure 24. Polyvinyl formal-phenolic decreasing to 30 to 40% a t later Sheet Sample Solvent Symbol Sheet Sample Solvent Symbol stages. I n this case, a major com11 E 8 F Dioxane 0 Dioxane 0 ponent of t h i s fraction is "cresol" 16 E Dioxane 17 E Dioxane 0 15 E Ethanol rn Ethanol 14 F (presumably small amounts of unreEthanol 3 E 4 E Ethanol 7A (2 7 acted cresol and smaller amounts of 4 A E Ethanol low molecular weight cresol-formalWeight loss in air oven at 150" C. dehyde reaction products). Analyses for free cresol, carried out as described earlier, are summarized in Table VII. p-Cresol, probably the major component of the phenolic infrom it of small amounts of strong acid (HCl). Considerable gredients collected, is likely to be accompanied by some mwork has, however, been done with dioxane, which is relatively cresol and some higher phenols. Since m-cresol has a higher reeasy to remove without blistering the sheet. A number of other activity toward bromine than p-cresol and the higher phenols ethers and acetals have also been found suitable for preparation have lower reactivities (9),a fair approximation may be obtained of polyvinyl formal sheet. This list includes p-ethoxyethanol by assuming that p-cresol constitutes the bulk of this fraction, (Cellosolve), p-methoxyethanol (methyl Cellosolve), dimethyl as was done in Table VII. Using these figures for free cresol, a dioxane, isopropylglycidyl ether, and dioxolane. However, sambetter estimation of the water found may be made, simply by ples prepared with these solvents (all of which are relatively easily peroxidized) might be less stable thermally than those prepared subtracting from the unaccounted for values given in Figure 22 the cresol values given in Table VII. These results, found in the with alcohols, ketones, etc., as fugitive plasticizers. A careful comparison of sheets prepared with ethanol and dioxane was last line, indicate that, as a maximum, between one third and one half of the total reaction products may be considered to be therefore made. water. These values are comparable t o those found for the Two samples of E-type polyvinyl formal, having the following total unaccounted for portion in the polyvinyl formal sheet characteristics, were used during this work: (Figure 21), and the reasonable assumption may therefore be Sample E Sample F made that this entire portion is water. Polyvinyl acetate, viscosity 15 15 Some Solvent Effects. Two of the best solvents for polyvinyl Polyvinyl acetate 12.0 12.9 Polyvinyl alcohol: 6.0 6.9 formal are ethylene dichloride and dioxane. The former was 0.9 Moisture % avoided during this work because of the possibility of formation Viscosity'(2 g./1W rn1.1, cp. 5.9 416
:
2
Table VII.
Analyses for Free Cresol in Reaction Products-Sheet 4A 1900 c. 170° C. 150' C.
Total weight loss % Total weight of broducts, gram 0 . lNBrOa-used, ml. Bromineutiliaed, moles X
104
0.97
1.17
7.0
2.7
1017
50.0
1.35 31.0 49.0
5.35 48.0 34.5
Phenols as p-cresol, moles x 104 3.5 Phenolsasp-cresol % 40.0 Water (corrected ' max.).
%
1.45
2.22
3.21
1.14
2.05
0.0968 0.0479 0.1201 0.1073 0.1017 0.1228 0.0985 14.0 5.5 21.4 2.7 3.0 23.8 1.4 1.35
1.5
0.675 0.75 6.8 5.5 53.0 34.5
11.9
0.75
5.95 52.0 48.0
0.375 4.3 47.5
Polyvinyl formal and polyvinyl formal-phenolic sheets (15% cresol-formaldehyde resin present initially) were prepared and cured in air according t o the standard curing schedule. Figures 23 and 24 show the results obtained when the cured sheets were aged further in a n air convection oven at 150' C. It is clear that a polyvinyl formal sheet prepared with ethanol as fugitive plasticizer is more stable when heated in air than one prepared similarly with dioxane. The easily peroxidized solvent (dioxane) probably introduces hydroperoxide groups or similar labile points
Vol. 47, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
314
along the polymer chain. Other peroxidizing solvents might be expected to behave similarly. There is no detectable effect due to the viscosity of the sample. The situation is not so clear when one considers the phenolic modified sheets. Dioxane again clearly introduces lability. However, the results are much harder to check in this system. It would appear, on the basis of the few runs made, that the lower viscosity sample, F, is lesss table than the standard sample, E. ACKh-OWLEDGMENT
The flow apparatus used in this B’ork was designed by 1cI. M. Stafford of this Laboratory, whom we wish to thank for its use. The authors are also indebted to R.N. Crozier of the Shawinigan Resins Corp., Springfield, Mass. for furnishing the special samples.
LITERATURE CITED
(1) Beachell, H. C., Fotis, P., and Hucks, J., J . Polymer Sci., 7,
353 (1951). (2) Bukey J. R., Naval Research Laboratory Rept. P-2915,July 29,
1946. (3) Callinan, T. J., Zbid.,C-3191,October 30, 1947. (4) Hochtlen, A., Kunststofe, 40, 221 (1950). (5) Xolthoff, I. AI., and Stenger, V. R., “Volumetric Analysis,” 2nd rev ed., Vol. 11, p. 220, Iriterscience Publishers, New York, 1941. (6) Xuller, E., Bayer, O., and coworkers. Angew. Chem., 64, 523 (1952). (7) Patnode, W. r , Flynn, E. J., and Weh, J d.,Elec. Eng., 58, 379 (1939). (8) Sprung. M . X , ISD. ENG.CHEX.,ANAL.ED.,13,35 (1941). (9) Sprung, M. SI., J. Am. Chem. Soe.. 63, 334 (1941). RECEIVED for review May 8, 1954.
ACCEPTEDOctober 1, 1954.
Weathering of Polyvinyl Chloride J. B. DECOSTE AND V. T. WALLDER Bell Telephone Laboratories, Inc., Murray H i l l , N . J .
For telephone equipment, vinyl coatings
. . . are proving excellent for indoor u s e
. . . are desirable for outdoor applications Formulations are reported with excellent outdoor weathering properties; light shielding pigments are important I
WIRE
and cable form a vital part of the telephone plant, and coatings used in their construction must be durable if reliable service is t o be realized. The wire and cable in the telephone system represents a long-time investment, and a service life of 20 years is not an unusual requirement. Vinyl coatings iii central offices and other protected locations give every indication that they will continue to maintain their physical and electrical properties in a satisfactory manner. Durability in these coatmgs has been achieved through careful choice of the plasticizers, stabiliiers, and resins used in their formulation. Applications are developing wherein it would be desirable to employ vinyl-coated wires in outdoor locations. The ease with which vinyl compositions mav be furnished in a range of colors and their high abrasion resistance are the immediate reason for extending their use. Before such a step could be taken, however, it was necessary to assess the general weatherability of polyvinyl chloride formulations. The results of weathering tests on a group of commercial grades of wire extrusion compounds were examined t o determine the nature of the problem, When it appeared possible to use plasticized vinyls outdoors, weathering studies were undertaken on experimental compositions
in an attempt to determine the relationghip of compositional factors to weathering. From this information, it has been possible to develop vinyl compounds with improved weatherability for use on wire and cable. EXPERIMENTAL RI ETWOD S
Natural Weathering. Three outdoor sites were chosen to obtain the weathering action of several different climates. One of these is in a suburban location about 20 miles west of New York City a t Murray Hill, N. J.; another is in sandy pine voods a t Perrine, Fla., close to Miami; and the third is in the Gila Desert a t Aztec, Ariz., which is between Phoenix and Yuma. A brief summary of the climatological factors in the vicinity of I each location is presented in Table I (93). The atmospheric conditions a t the Florida and Arizona locations caused specimens to degrade a t a faster rat,e than in New Jersey. This is in substantial agreement with the observat’ions of Yustein, Winans, and Stark ($3) who noted that a vinyl chloride copolymer darkened most rapidly in New Mexico and Panama. Specimens exposed in the Florida location received an abundance of- both sunshine and moisture. The surface of specimens from this location weathered clean but the underside, where water c d d collect, often showed green or black organic growth. The specimens returned from iirizona were covered with a light coat’ingof fine sand which was removed easily with a moist cloth. This was in contrast t o an adherent sooty coating of dirt’ t’hat accumulated on t h e New Jersey specimens. The Arizona location produced the severest impairment of physical properties for a given exposure time. The major factors appeared t o be high temperature and .an abundance of sunshine. The question of a suitable unit for exposure measurement has been the subject of much discussign. The most common unit is the day, although Clark ( 4 ) prefers the sun hour, while the Society of Plastics Industry (19) has used a solar energy unit (the langley) to measure the exposure of vinyl films. When our studies were begun, the calendar year appeared t o be the most desirable unit to use. It is appreciated, however, that some
