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 ETWODS
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
February 1955 Table I.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Climatological Factors a t Outdoor Test Stations
Latitude Annual precipitation inches Av. relative humidit;, % 7:30 A.M. 1:30 P.M. Temw., F. A+. daily max. January July Av. daily min. January July Available sunshine. %
New York City 40' 40' N 42.9 73 58
Miami, Fla. 26' 45' N 55.8
Phoenix, Ariz. 33' 25' N 7.7
86 61
57 30
315
Loss of plasticizer results in increased stiffness. Chemical decomposition of the polymer is reflected by loss of tensile strength. The most useful single index of weatherability appears t o be the per cent retention of the original elongation. This reflects t o some extent changes in both plasticizer content and chemical decomposition. PRELIMINARY OUTDOOR EXPERIENCE
38.3 82.0
74.3 87.0
64.9 103.6
24.6 66.8
62.2 76.2
38.8 77.3
60
66
85
variation does occur in the weather from one year t o another, and it is helpful in comparative tests t o furnish the dates when exposures were made. At the various exposure sites racks were located facing south in order t o obtain the maximum amount of sunlight on the specimens. The Florida and Arizona racks were constructed with a 30' angle to the horizontal while the New Jersey racks were a t 45O. Accelerated Weathering. A substantial portion of the present work is based on accelerated weathering data. It was necessary to obtain such data as a temporary basis on which to make decisions until outdoor information was available. The outdoor results obtained t o date confirm t h a t artificial tests predict with a high degree of reliability the order of weather resistance for plasticized olyvinyl chloride (PVC) systems. T h e a r t i z i a l weathering machine used was a modified X-1-A National weathering unit employing a bare carbon arc and has been described previously (81). Specimens were mounted in this test without backing. Specimen Preparation. The polyvinyl chloride compositions were prepared on a 6 X 12 inch, two-?rollmill operated a t 160" C. A vinyl chloride homopolymer having a n average chlorine content of 57.7% by analysis and a specific viscosity of 0.58 (ASTM D-1243 a t 20' C.) was used in all the experimental compositions. Sheets, compression molded a t 170' C. t o a thickness of 0.030 inch, were used for exposure tests. Test specimens were die punched for stress-strain evaluation following exposure. This procedure had not been adopted when the commercial compounds reported herein were exposed. These materials were exposed as 1-inch dumbbell specimens (ASTM die C) and were 0.075 inch in thickness. Sheet specimens for outdoor exposure were mounted on perforated aluminum panels which in turn were fastened t o the exposure racks. The free area of the perforations was small so t h a t most of the specimen was in contact with the aluminum. This condition may have accelerated aging since i t has been shown t h a t aluminum backing, a good reflector of ultraviolet radiation] significantly increases the rate of degradation of clear vinyl film (19). Evaluation. Where wire and cable are concerned, the continued usefulness of the plastic as a protective coating is of primary importance. Emphasis in this investigation has been placed, therefore, on the retention of physical properties. Superficial changes, such as the spewing of plasticizers with the development of tack, obviously cannot be overlooked. Homever, when these changes do occur, a loss in physical properties generally ensues and would be detected in a stress-strain test. Stress-strain measurements were made in accordance with ASTM Designation D-412 with the exception that a modified T-50 specimen (ASTM Designation D 5 9 9 4 0 T ) was used. Tensile strength, elongation] and modulus a t 50% (5-50) and at 1 0 0 ~ (8-100) o elongations were the parameters used t o measure degradation. Principal changes in physical properties of plasticized PVC occur through plasticizer volatilization and chemical decomposition. No single physical parameter may be completely depended on as a n index of weatherability. A modulus measurement is most sensitive for following changes in plasticizer concentration.
I n 1945, a series of typical insulating and jacketing PVC compositions was exposed outdoors a t Murray Hill, N. J. The stress-strain data compiled from these samples over a period of 4 years' exposure showed t h a t there was a wide difference in the weathering resistance of these materials. I n general, those compositions pigmented with carbon black showed the highest order of resistance t o weathering, followed by the lighter-colored compounds. The colorless transparent compositions aged very poorly and showed little promise as outside wire and cable coatings. A summary of the data for the three types of materials is given in Figure 1,
Y E A R S OUTDOORS A T MURRAY H I L L , N.J.
Figure 1. Natural weathering data on polyvinyl chloride insulating and jacketing compositions
Compositions based on both vinyl chloride homopolymers and low vinyl acetate (5%) copolymers were included in this series of commercial extrusion materials. No marked difference, however, was observed in the weatherability of the two types of resin compositions. From this it was concluded that minor differences between resins was not as critical a factor as certain of the other compositional variables. Compounds containing both monomeric and polymeric plasticizers were represented in this study. Sufficient evidence was obtained to show t h a t with selected monomeric plasticizera it should be possible t o formulate either light-colored or black compounds for long-time outdoor service. Polymeric plasticizers in light-colored or in transparent compositions appeared to be of less value for long-time outdoor service. From the results of this study it was concluded that black weather resistant PVC compositions could be formulated and that colored compositions could be developed. Therefore] an investigation was begun to study the compositional variables that affect aging in nonblack PVC compounds. To date, these have included stabilizers, pigments, and plasticizers. These studies
INDUSTRIAL A N D ENGINEERING CHEMISTRY
316 350-49
351-49
352-49
353-49
354-49
DOP
355-49
. .
356-49
-
I_
357-49
_-
4-50
.
359-49
I
..
DOP/octyl diphenyl phosphate 4-50: tribasic lead sulfate
360-49
361-49
362-49
363-49
364-49
Polyester
Figure 2.
Three and one half years' exposure at Aztec, Ariz.
PVC resin Plasticizer (polyester 40 parts) Stabilizer Lead stearate Clay TiOe, anatase
Pnrts by Wt. 65.0 35.0 5.0 0.3 5.0 0.5
have not been exhaustive but have provided sufficient information to guide the formulation of suitable outdoor coatings for m-ire and cable. Rather than develop specific colors it was decided to approach the problem by first developing a white cdmpound with suitable weathering characteristics. When this was accomplished pigments could be added to the base t o obtain desired colors. STABILIZERS
Vol. 47, No. 2
nal color by acting as oxidation catalysts in t'he destruction of these chromophoric groups. The results of outdoor exposure tests begun in 1950 on a group of experimental compositions in which the stabilizer was the principal variable are given in Table 11. Each stabilizer was tested in the presence of three representat'ive types of plasticizers. The general formulation, given in the t'able, is typical of an extrusion grade of flexible vinyl insulation. These data show that the elongation retention of many of the compositions fell off sharply with weathering. Exposure in Arizona left very few materials in a serviceable condition. Florida exposure was less severe while the climate of S e w Jersey had the least effect. The various stabilizers gave markedly different results with each of the plasticizers. K O stahiliaer was found that provided significant prot.ect.ion to the polyester plasticized compound in all locations. Only dibasic lead phosphite was effective in dioctyl phthalate (D0P)while dibasiclead phosphite, tribasic lead sulfate, and the litharge (Pb0)-basic silicate oi lend (BSWL) m i x t u r e were effective with the octyl diphenyl phosphate-DOP blend. Basic carbonate of white lead (BCTVL) proved effective only with the octyl diphenyl phosphate-DOP blend and then only in the temperate climate of New Jersey. The organic sodium phosphate and the barium-cadmium ricinoleate were particularly ineffectual as stabilizers in all locations and Kith all pla,sticizer combinations. Bent specimens of this series that were exposed in Arizona are shown in Figure 2 . All these specimens were originally white and highly flexible before weathering. A careful examination of the photograph will disclose that many of the specimens have cracked and darkened as a result of exposure. Those compositions which best retained their original color also showed the highest elongation retention values. Light Absorption Properties of Stabilizers. The value of shielding organic materials from the high energy radiation in sunlight is well appreciated. The use of carbon black in cellulose acetobutyrate and polyethylene for this purpose has been demonstrated ( I , $ 1 ) . It is natural to expect that some portion if not all the light protection to be derived from stabilizers may be through their shielding ability. To investigate this point a series of compositions was prepared with a number of stabilizers and rutile titanium dioxide as a reference material. The amount of shbilizer or pigment was 0.8% by volume on the basis of the compound. Specular transmission measurements from 330 to 700 mp were
The conventional approach to imparting weather resistance to PVC is through the addition of materials t h a t have become known as stabilizers. Systems of PVC and plasticizer without other additives are not inherently resistant to the weather. After one year or less of outdoor exposure, they ordinarily degrade as evidenced by darkening or embrittlement to a point where they would be of no practical importance as wire or cable coatings Practical experience with stabilizers shows that they may impart color stability and retard physical changes when PVC is exposed to light and to heat. -4 Table 11. Elongation Retention of Weathered PVC Compositions Containing number of authors have directed Various Stabilizers their attention t o the use of (31/1 years' outdoor expoaure) stabilizers in PVC and have disStabilizer Series Base Formula cussed the mechanism whereby Polyvinyl chloride 65.0 such stabilizers protect PVC Plasticizer 35.0 Stabilizer As indicated from heat and light (7, 10-13, 0.3 Lead stearate Clay 5.0 16, 17). The bulk of the staTitanium dioxide, anatase 0.2 bilizers used in vinyl wire and Retained Elon:;ation, % cable coatings are basic lead Octyl Diphenyl Di-9-ethyl hexyl park Phospliate/DOPa Phthalate (DOP) Polyester Ab salts. These substances are by New FlorAriXerv FlorAriNew FlorAripotential hydrochloric acid acida zona Stabilizer Wt. Jersey ida zona Jersey ida zona Jersey ceptors, and as has been shown Basic carbonate of white lead 5 81 42 Brittle 46 25 9 47 31 7 by Scarborough and his associLitharge ates (18),hydrogen chloride is a 43 0 32 17 Brittle 84 70 54 56 Basicsilicateof lead catalyst for the dehydrochloriDibasic lead phosphite 5 86 79 61 57 21 7 56 66 87 nation process in PVC. Boyer Organic sodium phos( 3 ) and Fox and his coworkers Phte .. .. 53 25 24 44 29 25 Brtsic silicate of lead : :} '' (8) have shown that polyene Tribasio lead sulfate 5 . 0 88 86 68 .. .. .. .. .. chromophores resuIt from therCadmium ricinoleate mally degraded PVC, and Barium ricinoleate : 17 12 Brittle 30 11 Brittle 17 Brittle Brittle Druesedow and Gibbs (6) cona 1 . 1 mixture b weight cluded that heavy metal stabilb Dibasic acid-g$col poliester, 40 parts; same as polyester A in Table V. izers probably preserve origi-
i} 4"
..
E}
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1955 U V LITO R AL- E T + - -
VISIBLE RADIATION
PTS. BY W T PVC RESIN
RUTILE
I
I
1 TITANIUM 1
MINERAL O I L STEARIC ACID STABILIZER OR PIGMENT I
0.5 0.8% (BY VOI
I
317
of the rate of oxidation in the presence of this stabilizer. The test with polyethylene was made a t 140' C. on an intimate mixture of 2% of the stabilizer with polyethylene, prepared by hot milling. It is concluded from these results that dibasic lead phosphite has no measurable antioxidant activity, and its chief value in vinyls for outdoor use lies in screening ultraviolet radiation. PIGMENTS
Pigments are ordinarily added to vinyls for such purposes as providing color, altering physical or electrical properties, and reducing cost. Observations presented in Figure 1 for insulating and jacketing compositions demonstrated that the) also increase weather resistance. To explore this possibility further, a series of compositions containing a number of white pigments, extender pigments or fillers] and carbon black were exposed t o both natural and accelerated weathering. The experimental compositions used in this series were prepared by adding various percentages of the pigments to the following formulation: Pigment Series Base Formula (272-50) .Polyvinyl chloride resin Di-2-ethylhexyl phthalate Basic silicate of lead Lubricant Stearic acid Mineral oil
100
1
100 0 (parts by wt.) 65 0 6 5
0 5 2 0
0 CARBON B L A C K
1
1
1
1
I
8
9
10
II
12
90
z 0 80 F
U
Figure 3.
L i g h t absorption properties of lead stabilizers
9
70 60
PVC RESIN
n
ILI
2_ 50
made employing a Beckman Model B spectrophotometer, The m w u r e m e n t s were made on calendered film of the order of 0.001 lnch in thickness mounted between quartz slides. This information is presented in Figure 3 where the transmission characteristics of the various films are expressed in terms of light absorption coefficients. The light absorption coefficient has been shown by Hame and his associates (9) to be a useful index of weatherability for polyethylene pigmented with carbon black. The curves in Figure 3 show that there are definite differences among the absorption properties of the various stabilizers. Generally speaking, the higher the level of the absorption curves, particularly a t the shorter wave lengths, the better the protection afforded. The absorption oi dibasic lead phosphite takes a pronounced upward sweep in the ultraviolet region which undoubtedly has a bearing on the favorable results obtained with this substance. It would be interesting to carry these determinations to a t least 290 mp, the lower limit of sunlight] t o determine !f the indicated trends are real since light below 350 my is credited with being the most damaging t o vinyl polymers (4). Since the structure of dibasic lead phosphite indicates that it might function as an antioxidant or free radical trap, its activity in this direction also was investigated. The oxygen absorption method employed by Biggs and Hawkins ( 8 ) in studying the oxidative aging of polyethylene was employed for evaluating antioxidant activity. The oxygen uptake of diiso-octyl phthalate d o n e and in the presence of 2% by weight of this stabilizer, was measured a t 105' C. The oxygen absorption was the same in both cases, and there was no evidence that the dibasic lead phosphite inhibited oxidation. The curves for oxygen uptake with time showed no induction period and significant amounts of oxygen were absorbed immediately. Another experiment with dibasic lead phosphite in polyethylene also showed no inhibition
s 40 t-
5 30 V
20 n
IO 0 0
I
2 3 4 5 6 PER C E N T PIGMENT BY
7
VOLUME IN PVC BASE
Figure 4. Weatherability based on 3-year exposure data f r o m New Jersey, Florida, and Arizona Basic silicate of lead, because of its poor light screening, was chosen as the stabilizer in order to better evaluate the protective action of the pigments. I t s primary function was to provide thermal protection during processing. It is important t o note that the results obtained in this study reflect only what may he expected with di-2-ethylhexyl phthalate and that other plasticizers may not act the same.
Natural Weathering. The elongation-retention properties of the pigmentation series based on 3 years' (1951-53) exposure data from New Jersey, Florida, and Arizona are plotted in Figures 4 and 5. From these curves, obtained from averages of at least three specimens of each sample from each location, it is apparent t h a t both the nature and concentration of the pigment has a major effect on weathering. I n general, most pigments provide maximum protection at a concentration of about 2% by volume. T h e principal exception t o this is antimony oxide for which no maximum is reached up t o a concentration of 3.6%. All the pigments included in this study exerted some positive protective action. Of the two extender-type pigments ex-
100
1 1 '
1
The spread in aging resistancc with the various rutile pigments suggests that, they can protect by shielding but can also promote degradation unless suitably treated. Ostensibly, these pigments have the same light absorption properties but different surface calcination treatments. All tests, however, indicate that despite any possible adverse photosensitivity effects, an over-all benefit is derived from the use of t,itanium dioxide. The complete strcss-strain data before and after natural vieatliering for the rutile titanium dioxide, (Al, Si, Zn) composition (303-50)and for the one with channel black (322-50) are given in Table 111. KO changes of any serious magnitude occurred in mj- of the properties. The moduli for both oom-posit,ions generally increased, which means that some stiffening has occurred. The small decrease in tensile strength occurring with titanium dioside and the increase for carbon black are typical trends with these pigment's. The tensile strength changes in compositions with titanium dioxide have been interpreted to mean that some cthemical degradation has occurred which weakens the structure of the plastic. In compositions with carbon black, the changes are similar to those observed due to volatilimtion of pladcixer. Effect of Climate. The relative effect of the climates a t the three outdoor test sites may be qualitatively judged from the appearance of t,he weathered specimens shown in Figures 6 and 7. The portion of the control specimen missing from the New- Jersey column was consumed in analytical t'ests to he described. All the specimens, with the esception of the one containing carbon black, were originally white or near white. A comparison of the appearance of the weathcred specimens shows darkening to be more extensive among those from Arizona. Antimony oxide, which gave reasonable protection in New Jersey and Florida, failed in Arizona. Several of the titanium dioxides, which gave
Q C A R B O N B,LACK
1 0
Vol. 47, No. 2
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
318
P V C RESIN DIOCTYL PHTHALATE BASIC L E A D S I L I C A T E
65 6.5
2 3 4 5 6 7 8 9 10 P E R CENT TITANILM DIOXIDE a y VOLUME IN P V C B A S E
Figure 5 . Weatherability based on 3-year exposure d a t a from New Jersey, Florida, a n d Arizona amined, calcium carbonate appeared to have some advantage over clay, but both pigments supplied negligible protection. Fine particle size grades of both these pigments with an average diameter between 1 and 5 microns were used. Zinc oxide and antimony oxide provided definite protection which was expected from their known performance in finishes (16). The use of zinc oxide in PVC is not considered good practice because of the possibility of dehydrochlorination, although this effect does not appear to have occurred in this work. Antimony oxide is employed as a flame retardant in plasticized PVC, and there would be no technical ohFiew iection to its use. The best weather resistJersey ance was obtained from the composition containing 3.6% by volume of a channel black having an average particle size of 20 m+ This carbon black is similar to the grade that has been shown to provide the best weathering protection for polyethylene ( 2 1 ) . Of the white compositions, the best weatherability was obtained with a treated grade of rutile titaninni dioxide. Titanium Dioxide. The protection afforded by various titanium dioxides varies considei ably. Grades of titanium dioxide that provide the best durability in oleoresinous binders also give the best results in vinyl plastics (22). Referring to Figure 5, the anatase grade is inferior to the various rutile grades Among the rutile grades the one treated most conipletely to reduce chalking gave the best results. This grade conforms to the Federal Specification for titanium dioxide TT-T-425, Type 111, chalk-resisting, silicon-aluminum-zinc-treated rutile. The use of titanium dioxide in oiganic binders has some controversial aspects. It has been described by DeCroes and Tarnblyn (6) a s an oxidation photosensitizer that may be controlled by inhibitors. The cyclic reduction and oxidation of titanium compounds has been given as an explanation for the disintegration of organic binders in which these materials have been used (1.4). Treatment of titanium dioxide during manufacture with sniall amounts of aluminum, silica, and zinc, materially reduces these adverse effects.
.
.
Florida
Arizona Sumple
so.
_. .
Vol.
2;2-50
Pigment a None
273-50
CaCoi
2.6
mn-30
Clay
2.6
3 13-50
Ph904
1.1
510-50
ShrOi
1.2
291-50
zno
1.3
30:3-.50
Ti02 rutileI,
Zd,Al, Si
-
....
1.1
Channel hladc 3 .6 10 pari.s/100 p a n s PVC raaain
321-50
by wt.
Figure 6 . Three- year exposure of vinyl compositions
February 1955
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
319
substantial protection elseTable 111. Stress-Strain Properties of a Titanium Dioxide and a Carbon Black where, also developed surface Composition after 3 Years’ Outdoor Exposure discoloration in Arizona. (303-50) Titanium Dioxide (322-50) Carbon Black Superficial observations, borne Rutile (Al, Zn,Si), 1.8% by Vol. 20 mg Channel, 3.6% by bd. out by elongation-retention New New Orig. Jersey Florida Arizona Orig. Jersey Florida Arizona measurements for this group Tensile stren th, lb./sq. inch 2810 2835 2685 2445 2780 2340 2955 3490 of specimens, are given in Elongation 305 360 315 270 315 355 325 310 S-50 modufus, lb./sq. inch 685 580 695 905 740 505 930 860 Table IV. S-lOOmodulua, lb./sq. inch 1175 1140 1175 1400 1170 1295 1270 1555 The order among the compounds with respect to weather resistance was about the same through the use of accelerated testing equipment. Data from at the various locations. IIowever, the degree of degradation differed significantly, This is shown in the elongation-retention these tests are presented here for the purpose of showing the that may be Obtained from such tests. de@;reeOf averages given below. The normal solar radiation in the vicinity Accelerated weathering data for the pigmentation series are of each location is also given t o show the relationship of this cliplotted in Figures 8 and 9 on the same basis as t h a t for natural weathering. A comparison of the natural aging data of Figures matic factor to the degradation of plasticized PVC. 4 and 5 and the accelerated data shows that a high degree of Av. Retained correlation exists between the two tests. The shape of the reElongation, % tained elongation curves is similar, and the general order of the Sola: Pigment Stabilizer Radiation0 pigments with respect to their protective efficiency is about the series, series, (Av./Day), same. The generally lower elongation-retentions observed for 3 years 31/1 years Langleys the 1800-hour accelerated weathering period indicate that it is Murray Hill, N. J. 76.1 55 0 285 6 equivalent to something greater than 3 years of outdoor exposure. Perrine, Fla. 58.9 36 5 405 7 Aztec, Ariz. 42 8 21.4 543.2 Zinc oxide compositions weathered much better outdoors than would have been predicted from accelerated tests. Minor exU. S. Weather Bureau data: langley = 1 g . cal./sq. om. ceptions occurred in the case of calcium carbonate and clay in which their order was reversed. It is because of such exceptions The rate of degradation a t Perrine, Fla., is about one and one , results in accelerated tests should only be used as a that half times as fast, and the rate at Aztec, Ariz., is about twice as temporary basis for judgment until supplemented by the outdoor ( 4 ) or until such time as Our knowledge of the weathering testsJJ fast as that a t Murray Hill, N . J. These rates of degradation are process increases t o the point where artificial weathering instruroughly proportional to the average daily solar radiation ments can be made that will supply completely dependable infora t each location. On the basis of these results, therefore, Arizona mation. probably has the severest climate to which PVC may be subjected WEATHERED POLYVINYL CHLORIDE within continental United States.
4
. .
Spotting. POlYvinYl chloride compositions, when weathered, often develop brown spots in the early stages of deterioration. These gradually increase in size until they join together and finally cover the whole New Jersey Florida Arizona surface of the specimen. The initiation of these spots may be due to the dispersion of Sample No. TiOa voi. % the lead salt stabilizers. Chemical analysis 272-50 None of the spotted areas have been made in order t o obtain some insight into the degradation process. These tests were made on the 27260 control specimen from the pigmentation series that had been exposed outdoors for 297-50 Anatase 1.8 one year in New Jersey. Examination of the weathered sample revealed that the surface of the specimen was covered with hard brown spots surrounded 300-50 Rutile, A1 1.7 by softer, off-white material. The brown spots were tested quantitatively for lead and were found to contain twice the amount present in the lighter colored areas. Evi347-50 Rutile, AI, dence of the uneven distribution of the lead si 1.7 salt was obtained from tests on both the aged and unaged compound. Treatment of the aged sample with a caustic solution caused the removal of the surface discoloration, in303-50 Rutile, 2x1, AI, Si 1.7 dicating that the brown spots were probably acidic. Subsequent testing of these areas with sodium sulfide produced black spots within the body of the specimen An 5-51 Rutile, DOP unaged sample was similarly treated and masterbatch 1.1 black spots were also visible beneath the mrface. From these results it appears that the npotting which developed is associated with 306-50 Rutile, CaSOh 1.2 areas containing a high localized concentration of the lead salt. For obtaining the best weather resistance, it would appear Figure 7. Three-year exposure of vinyl compositions
Accelerated versus Natural Aging. Valuable preliminary weathering information on vinyl compositions has been obtained
...
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
320
Table IV. Elongation Retention of Weathered PVC Compositions Containing Pigment Expt. No.
Retained Elongation After 3 Years' Outdoor Exposure, yo New Jersey Florida Arizona
Pigment By Volumen None 365 Calcium carbonate 2.6 Clay (calcined) 2.6 Lead sulfate 1.1 Antimony oxide 1.2 360 Zinc oxide 1.3 370 Channel black 3.6 315 335 Anatase, A1 1.8 370 Rutile A1 1.7 375 Rutile' A1 Si 1.7 305 Rutile: Al: Si, Zn 1.7 375 1.7 Rutileb 2.2 355 Calcium Extended Rutile0 Calculated on basis of whole composition. Commercial paste masterbatch of a rutile TiOp; 70% pigment, 30% DOP. 30% rutile TiOn, 70% CaSOc.
272-50 273-50 288-50 313-50 310-50 291-50 322-50 297-50 300-50 347-50 303-50 5-51 306-50
a b c
Orig. Elongation, %
...
desirable to keep the lead content as low as possible consistent with processing needs. Improving the uniformity of the dispersion in order t o avoid localized agglomerations should also assist weatherability. Solubility. The solubility of a number of specimens, highly degraded by exposure to the weather, have been examined. The purpose of these tests was to determine if the degradation process was one of chain build-up through cromslinking. Ordinarily, only a small amount of crosslinking between chains will render a polymer insoluble. The test was made by refluxing about 0.1 gram of the aged specimen in 50 ml. of boiling methyl isobutyl ketone for one hour. Samples from the pigmentation series that had been aged 3 years in Arizona were examined in this fashion. Tests were made on some of the badly weat,hered of this series which are shown in Figures 6 and 7, including those with experimental numbers 272-50, 273-50, and 288-50. Some of the better samples also were examined. These included 303-50 and 291-50. All samples were found to be completely soluble. These results confirm others that, have been made on a wide variety of weathered PVC compositions. All the weathered samples were soluble. The only exception t o this has been with compositions containing carbon black which even before aging are difficult to dissolve. From t,hese solubility tests it would appear that no crosslinking occurred and that outdoor degrada-
18 57 16 36 72 88 113 88 89 95 118 99
11 46 20 12 28 74 103 54 72 81 103 79 83
Vol. 47,No. 2 tion of PVC proceeds by a process not involving crosslinking to any real extent. PLASTIC1ZERS
10 19 Brittle 23 18 80 98 30 54 33 89
The
desired
flexibility in
PVC is obtained through the
addition of plasticizers which solvate the resin. The percentage of plasticizer used in flexible wire and cable coatings is substantial (30 to 40y0of the total 61 100 42 composition), and therefore its resistance t o oxidation and weathering becomes of interest. Loss of plasticizer compatibility in the rcsin often occurs as PVC systems weather. When this occurs the plasticizer collects on the exposed surfaces while a loss of flexibility occurs in the substrate. Oxidation of the plasticizer t o a less compatible structure is generally considered as the cause of the trouble. Oxidation Tests. A light-catalyzed oxidation test is useful for examining plasticizers for their resistance to oxidation as a screening test prior t o incorporation with resin. This test is made by subjecting the materials to oxygen and light and measuring the rate of oxygen uptake a t 25' C. The light source used is a carbon arc with a Corex D glass globe which filters out radiation below 300 mp. The relative consumption of oxygen with time is plotted in Figure 10 for a number of different types of plasticizers. The chlorinated hydrocarbon tested was a viscous liquid essentially aliphatic in nature and about 60% chlorine by weight. The polyester curve is an average one for polyesters A and B, referred t o later. The scale used to indicate the relative rate of oxidation is equivalent to the oxygen uptake (cc.) per gram of plasticizer multiplied by 100. It is believed that for piimary plasticizers in PVC systems only the most oxidation resistant materials should be used consistent with the properties desiwd. The information in Figure 10 shows that di-2-ethylhex) 1 phthalate, octyl diphenyl phosphate, and dicapryl sebacate are three of the best plasticizers in this respect. The study of dicapryl sebacate has been limited, however, because this
PERCENT TITANIUM P I G M E N T BY VOLUME I N PVC BASE
Figure 8.
Accelerated aging of pigmented vinyl compositions
Figure 9.
Accelerated aging of titanium dioxidepigmented vinyl compositions
February 1955 Table V.
INDUSTRIAL AND ENGINEERING CHEMISTRY 2500
Weight Loss after Accelerated Weathering
32 1
(A)
I
1
CHANNEL
NATURAL
I
Weight LOSS after 2000 Hours’ Accelerated Weathering, % Natural White Black 14.2 6.5 6.9 14.9 8.6 7.5 14.5 11.2 34.0 9.9 21.2 8.2 8.2 8.1 9.4 3.2 7.7 3.9 10.1 18.5 11.9 2.6 1.0 4.1
a
b
Plasticiser (272-50) Di-2-ethylhexyl phthalate Diiso-octyl phthalate Trioctyl phosphate Octyl diphenyl phosphate Tricresyl phosphate Polyester Aa Polyester B 6 Acrylonitrile copolymer Dibasic acid-glyool polyester. Monobasic acid-modifled polyeeter.
PERCENT
plasticizer has a volatility higher than that considered desirable for use in weather-resistant compositions. Plasticizer Weathering. A series of PVC compositions was prepared with a group of primary plasticizers to determine the effect of the type of plasticization on weatherability. Representative examples from among the phthalates, phosphates, polyesters, and an acrylonitrile copolymer were included. Experimental formulations were prepared by substituting the various plasticizers for the DOP in the pigmentation series base formula (272-50). In addition to the natural formulations, titanium dioxide and carbon black compositions also were tested; 10 parts by weight of each of the pigments were employed. The carbon black was a fine particle size channel black and a rutile titanium dioxide treated with Al, Si, and Zn was used. Accelerated weathering has been completed on this series, but no report is available a t this time on the outdoor tests. Weight change with accelerated weathering as well as the usual stressstrain measurements were followed. The dimensions for the weight change specimens were 1 X 3 X 0.075 inch.
Figure 11. Relationship of weight loss to aging time and retained elongation of plasticized vinyls
P L A S T I C I Z E R 65 BSWL 6.5 LUBRICANT 25 PIGMENT 10
POLYESTER
1
I
1
1
PHOSPHATE ESTER
OCTYL PHTHALATE ESTER
INCREASING STIFFNESS
PER CENT RETAINED ELONGATION
Figure 12.
H O U R S OF LIGHT EXPOSURE A T 25OC.
MIN. WAVELENGTH 300
Figure 10.
mp
Light-catalyzed oxidation of plasticizers
-
5 - 5 0 MODULUS RATIO (AGED/ORIGINAL)
Stress-strain properties after 2000 hours’ accelerated weathering
The weight measurements showed that the breakdown of the PVC compositions was in all cases accompanied by some weight loss. The magnitude of the loss varied widely with the nature of the plasticizer. Examples of the losses encountered are given in Table V. No general relationship between the weight loss and the retention of physical properties has been observed. Each plasticizer system is different in this respect and varies, depending upon the kind of pigmentation. The relation of the weight loss for five dioctyl phthalate type plasticized PVC compositions with respect to weathering time is shown in Figure 1lA. The data for the black and white compositions are so nearly alike that a single curve suffices t o describe these materials. Losses in the natural compounds were higher at any given time than for those with pigments. I n both, the curves display a fairly steady rate of IOBS, tending slightly t o decrease with time. The relation of weight loss t o elongation retention is shown in Figure 11B. The natural material suffers a much greater change in elongation for a given weight loss than either of those with pigment. The change in elongation of the natural compositions is far greater than could be accounted for by simple plasticizer volatilization. The changes for the pigmented materials are also somewhat greater than could be accounted for in this fashion, but the difference is not nearly so large. Condensed elongation retention and S-50 modulus data are given
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
322
in Figure 12 for the group of plasticizers examined. Average values are given for each of the plasticizer types in white and black formulations. All the natural samples had degraded a t the end of 2000 hours' exposure t o the point where they had no measurable elongation. None of the plasticizers conferred any significant weatherability t o the natural PVC compositions. The addition of titanium dioxide and carbon black improved the weatherability of all of the plasticized systems. The improvement with black is much more pronounced and general than with titanium dioxide. Where white or nonblack coatings are required, dioctyl phthalate would be the best plasticizer to use. Where black compounds may be employed, any one of the types examined can be formulated into coatings with outdoor weatherability, but the polyesters and the acrylonitrile copolymer deserve special consideration. CONC LU SIOXS
Natural and accelerated weathering tests show that polyvinyl chloride compositions can be formulatcd that are suitablc for long-time outdoor service. The exclusion of sunlight from the body of the plastic represents the maior formulation problem. Unprotected polyvinyl chloride systems degrade rapidly when exposed to the weather. This degradation may be retarded through the incorporation of suitable basic lead salts capable of acting as light screens. A relationship has been demonstrated to exist between light absorption of lead stabilizers and the improvement in weathering that is obtained. The use of lead salts alone is not believed t o be sufficient to sustain polyvinyl chloride compositions for long-time outdoor service in all climates. Further protection through light-shielding pigments is considered necessarv. The best protection is provided by a small particle size carbon black. This pigment gives protection t o compositions prepared from a variety of plasticizers. Where color is important, a rutile titanium dioxide treated with aluminum, zinc, and silicon gives a white base with good weatherability to which colorants may be added. Titanium dioxide is most effective with octyl phthalate type plasticizers. ACKNOW'LEDGMENT
It is a pleasuie to acknowledge the assistance of G. F. Brown, who prepared most of the compositions tested, and of Patricia
as
Vol. 47, No. 2
Langille and Margaret Woodring, !Tho obtained most of the phlsical test data. H. V. \J7adlow carried out the microanalytical work involved. R. H. Eiickson and W. L. Hawkins furnished the oxidation data that have been presented. The authors also received much encouragement from discussion of this work a i t h B. S.Biggs and W.0. Baker. LITERATURE CITED
(1) Biggs, B. S., Bell System Tech. J . , 30, 1078 (1951). (2) Biggs, B. S., and Hawkins, W. I,.,Modern Plastics, 31, 121 (September 1953). (3) Boyer, R. F., J . Phys. & CoIZoid Chem., 51, 80 (1947). (4) Clark, F. G., h a . ENG.CHEM.,44, 2697 (1962). (5) Decroes, G . C., and Taniblyn, J. Til', Alodern Plastics, 29, 127 (Aoril1952'r. (6) Drk&edow, D., and Gibbs, C. T., I h z d . , 30, 123 (June 1953). (7) Escales, E., Kunststoje, 41, 327 (1951). (8) Fox, V. W., Hendricks, J . G., and R a t t i . H . J., IKD.EXG.CHEM., 41,1774(1949). 19) Haine, W. A,, Smith, E. F., and Smith, N. K., Am. Inst. Eleo. Engrs., Misc. Paper 52-205, presented a t general meeting, Ninneapolis, Minn., June 1952. (10) Hendricks, J. G., U. 8. Patent 2,579,572 (December 25, 1951). (11) Hendricks, J . G., White, E. L., I Y D . ENG.CHCM.,43, 2336 (1951). Hendricks, J. G., a n d White, . :1 I,., Wire and W i r e I'~oduct8, 27,1053 (1952). Hendricks, J. G., White, E. L.. and Bolley, D. S.,1x0. EXQ. C ~ ~ ~ . , 4 2 , (1950). 899 Jaeobsen, A. E.,Ibid., 41,523 (1949). Mack, G. P., Modern Plastics, 3 1 , 160 (Sovember 1963). Vol. Mattiello, .J. J . , "Protective and Decorative Coatings." 11, pp. 372, 423, Wiley, New York, 1942. (li) Ilosenberg, A., Kunststofe, 42, 41 (1953). (18) Scarborough, A . L., Kellner, TT. L.. Rizao. P. W., Modern Plastica,29,111 (May 19521. (19) Society of Plastics Industry, Group 111, Committee on Exposure Test Methods, prelirninaiy report, Oct. 1, 1963. (20) U. S. Dept. of Commerce, Bureau of Census, Statistical Abatract of t h e United States, p. 148, 1952. (21) Wallder, V. T.. Clarke. 1%'. (J., aiid coworkers, Isn. ENG.CHEM., 42,2320 (1950). 0 1 , Paint & Varnish Production ( 2 2 ) Werthan, P., Ofic. Dig. Ped c l u b s . 2 9 2 , 3 1 1 (1949). (23) Yustein, S. E., Winans, It. R.. and Stark, H. J., Am. S o r . Testing Materials, preprint KO. 89, 1953. RECEIVED for review April 30, 1954. ;ICCEPTED October 19, 1954. Presented a t 11Ieeting-in-hliniature, North .Jersey and S e w York Sections, ACS. Neiyarlc, N . J . , Jan. 25, 1964, and L-e\\. Y o l k , N . Y., Feb. 12, 1954.
sins Co
ELECTRICAL AND PHYSICAL PROPERTIES OF FTV-2.5 AND OTHER RESINS PAUL EI-IRLICII1, R. W. TUCKER, AXD P. $. FRANIBLIN Diamond Ordnance Fuze Laborutories, Pushington 25, D . C .
EVELOPMENT of resins lor casting or potting electronic equipment has been a continuing project of the National Bureau of Standards. As described in a preceding publication ( 7 ) , the requirements for these resins include. first, lorn7 electrical losses over a wide range of frequencies and temperature and, secondarily, low viscosit,y in the monomeric form: low curing temperatures and times? low shrinkage on polymerization, small coefficient of thermal expansion, high tensile, compressive, and impact strength, nonc.orrosiveness, and chemical stability. Crosslinked polystyrene has many of the required properties but is slow in curing and exhibits high shrinkage during polymerization. In order to overcome these limitations, a t least in part, the National Bureau of Standards has developed several casting 1
Present address, Plastics Division, Monsanto Chemical Co., Springfield,
Mass.
irsin8 which are essential13 copol? i i i c i b of styrpne with the comonomer chosm to acceleiate cuniig The first of the copolymers develsped, NBS casting resin, uses 2,5-dichlorostyrene, which has a higher polymerization rate than styrene, as the comonomer. The formulation consists of 33% 2,5-dichlorostyrene, 21.5% poly-2,5-dichlorostyrene,and 11% polystyrene which keep the shrinkage on polymerization to a minimum; 21 % styrene, 13 % hydrogenated terphenyl n-hich reduces brittleness; 0.5% of a 40% solution of divinylbenzene which prevents melting a t elevated temperatures, and a catalyst. The method of preparation a n d some of the properties of this resin have been described ( 7 ) Because inhibitom have been removed, batches of the resin mixture require refrigeration for storage.
