Stabilization of Polyvinyl Chloride-Type Plastics J
J
J. G . HENDRICKS AND E. L. WHITE National Lead Co. Research Laboratories, 105 York St., Brooklyn 1, N. Y. T h e quality and serviceability of polyvinyl chloride-type plastics are functions of the constituents of the plastic. Numerous stabilizers are available for use in assisting to achieve and retain desirable properties. The purpose of this paper is to determine the heat stability and weathering resistance imparted by these stabilizers in a basic formulation and to indicate the effect of plasticizer and resin types on these properties. The results of tests on 91 individual stabilizers are reported. Stabilizer effectiveness ranges from nil to varying degrees of heat stability or weathering resistance to combinations of both. Stabilizing action depends, to a first approximation, on the kind of metal in the stabilizer, when present, thus providing a convenient method of classification. Plasticizer type, based on chemical structure and ease of oxidation, also exerts a major influence on both heat stability and weathering resistance. Resin type is of particular importance in weathering. The fact that stabilizers, plasticizers, and resins all exert great influence on the stability of a vinyl plastic and that these materials may be readily classified according to general stability provides a greatly simplified and highly useful approach to the compounding of vinyl plastics of desired characteristics.
T
HE tremendous growth of vinyl plastics in the past few years
is due in large measure to the general improvement in quality and serviceability of such plastics. Major advances in this phase of the technology have come from new stabilizers and plasticizers, but even more from the increasingly effective use of Stabilizers, plasticizers, resins, and other constituents of vinyl plastic products. This study deals primarily with the evaluation of stabilizers for polyvinyl chloride-type plastics both individually and according to a convenient classification in a basic compound. The effects of resins, plasticizers, and other constituents are considered briefly and will be reported in some detail at a later date.
Evaluation of light stability requires special comment for several reasons, foremost of which is the status of artificial-light testing apparatus for polyvinyl chloride-type plastics. Modern carbon arc machines, which are generally preferred, have a dangerously low degree of correlation with actual sunshine exposure in regard to both order of magnitude and order of merit of different sampleB. Some of the older, less imposing machines, such as the BWM-C Weather-Ometer, give much more reliable results primarily because of greater distance from the arc to the sample and lower temperatures, but even then results so obtained are not always accurate. So-called Hot Fade-Ometers are not considered reliable in spite of the increasing number of performance specifications based on their use as a matter of convenience. One of the most important contributions that could be made to vinyl plastics technoloa would be the development of suitable artificial-light testing apparatus. In outdoor weathering there is considerable variation in radiation, particularly from season to season. Apparent stability is influenced by rainfall, in some cases to a marked extent. This does not apply particularly to the water-soluble stabilizers, as might be imagined, but rather to those stabilizers and stabilizerplasticizer combinations which tend to fail, in the relatively early stages a t least, by surface spotting or discoloration. A severe rainfall at an appropriate time or times may greatly extend the outdoor life of such compounds by washing away degraded materials that would otherwise accelerate deterioration of the whole plastic. EFFECT OF STABILIZER
The selection of a stabilizer system is of course a critical step in the development of a vinyl plastic product. Primarily because other components of the plastic influence stability and stabilizer action to a very marked degree and also because various end uses and effects are desired, there is no one best stabilizer or stabilizer system for all compounds. Such factors must be considered when stabilizers for a specific compound are selected. Regarding this problem, the action of stabilizers in unfilled dioctyl
PROCEDURE
Unless otherwise stated, the basic recipe that was used consisted essentially of 100 parts by weight of polyvinyl chloride-acetate (95 to 5 ) , 50 arts of di-2-ethylhexyl phthalate, and 5 parts of stabilizer. Tge compounds were premixed in a beaker and fused and sheeted in a 5-minute cycle a t 250' F. (275' F. for homopolymer) on a two-roll 12-inch even speed mill. Heat stability was judged by the degree and kind of color changes occurring on exposure of 12-mil films exposed for I hour a t 325' F. and for 0.5 and 1.0 hour a t 340' F. and also on 75-mil sheets ress polished a t 300' F. (320' F. for homopolymer) and exposea for 1to 8 hours in a circulating air oven at 300" F. Artificial light stability tests were conducted on 12-mil films exposed in a BWM-C Weather-Ometer using 9200 P X globes and a t white and black panel temperatures of 120' and 155' F., respectively, and with the water off. Samples were mounted on white cardboard and portions removed periodically until failure. Outdoor weathering tests were made a t Sayville, L. I., by exposure of 6-mil films fastened on painted wooden fences at 45' southern exposure. Specimens were collected weekly until the film became brittle. Failure was judged on the basis of spottin severe stiffening, brittleness, or discoloration designated as A,
C, and D, respectively, in the tables. Results of weathering tests are reported in terms of langlies (radiation in gram-calories per square centimeter), rather than in terms of time. Radiation in the past year at Sayville corresponded to about 120,000langlies.
TABLE I. EFFECT OF STABILIZER TYPEON STABILITY Type
Average Heat Stability
2335
Hours
Average Outdoor Exposurea, L X 10-3 A D B C
None Very poor loo0 Very poor 300 h Fair '150 A Very poor 500 A. D
16 47 39 86
43 35 29 33 39 46 52 16 21 32
... ... ,..
... , . .
,.. ,
..
78 72 61 75 63 43 23 67
io
69 58 61 69 44 GI
>110
> 110 60 > 120 53
Mixed Cadinium Stabilizers 70 Mark X I Vanstay €1 VS-2 1%-26
CoMetal Ba Ba Ba Sn
Sn
Fair Good Fair+ Poor Poor
225 350 300 230 700
A .1
.I I .I
STABILIZERS ON
STABILITY \veacherOmeter Hours
Heat Stability
Stabilizer
Outdoor I1HO ,
.
>110 97 >110 >l50 21 75 >60 82
,
75 69 61
... ..
56
.. ..
, , ,
.. ... ,,
...
>110 >110 63 61 53 53 (also sticky) 90
,
85 81 39
...
and natural weathering. Thus the apparently substantinl general increase in light stability of cadmium stabilizers over barium stabilizers a3 indicated by the Weather-Ometer results is not substantiated by actual weathering tests. Furthei examples of marked inconsistencies of this nature are to be found in the results of the various organic stabilizers IiPted in Tablf. 111.
TABLE 111. EFFECT O F STABILIZER COMBINATIOXS q Stabilizer n-22 Plumh-0-Si1 B Plumb-0-Si1 B D-22 Plumb-0-Si1 13 JCX Plumb-0-Si1 B JCX Trihase Vanstas 16 Tribase
Type Tin Normal lead Sormal lead Tin Sorinal lead Cadiilium 50% ' T o r m a llead SO%} 2admiuin Baric lead Sodium 50%) Basic lead
75%'0\ 25%J
Vanstay 16 Staflex YX
50%;
Staeex YX QMXA
60%' 40%j
gnrx.1 a
.1. Spotting. C . Emhrittlement.
Sodium Cadmium Barium Cadmium Barium
Heat Stability Good Good+ Very good
-_
OK S T h B I L I T Y
~ ~ Ometer Hours 890 A 250 A 1000 A
OutdCJIJl Exposure", ~ ~ 1
A 3P 16
C
JR
61
55
71
Good A 250.1 16 Very poor 280 D , ,550 d 33 Very poor 250 A 28
:2
61
Excellent Good Very good+
100 A 1600 4 >1200
29 58 75
53 97 145
Very poor Fair Good
250 A
43
68
350.1 200 A
43 21
37 78
~
~
~
-
October 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
'
The more commonly used tin stabilizers generally impart improved heat stability and somewhat better clarity and light stability than the cadmium salts. As in the case of the cadmium salts, the Weather-Ometer results tend to appear better than the outdoor exposure results. Although the heat stability is good, it requires careful attention since many of the tin salts fail by turning black within a time temperature range that is regarded as critical. As shown in Table 111, combining tin with certain normal lead salt stabilizers alleviates this tendency. The sodium stabilizers impart a remarkable degree of light stability and weathering resistance. As a class thehe materials have the disadvantage of high moisture content or absorption which may be reflected in clouding of the film or gassing, the latter particularly in paste resins. Extraction by water does not seem to be too valid an objection in view of the excellent weathering characteristics but supplementary heat stabilization is required. Excellent combinations of heat stability and weathering resistance can be obtained by using carefully selected sodium stabilizers with some of the basic lead salt stabilizers, as shown in Table 111. The normal lead salt stabilizers are of considerable commercial importance in that they provide maximum heat stability consistent with translucence. Supplementary light stabilizers, preferably selected from the tin salts, are normally required. The basic lead salt stabilizers as a group are far superior in heat stability to all other types of stabilizers. Weathering resistance varies considerably but is generally better than predicted by Weather-Ometer results. The actual weathering resistance imparted by Tribase is far better than that which might be anticipated from Weather-Ometer results, for example. Most of the basic lead salts are opaque to greater or lesser extents. Because of their excellent electrical properties, the basic lead salts are required, for all practical purposes, in primary vinyl electrical insulation. The lubricant stabilizers have been considered simply as stabilizers in the foregoing discussion. Because of their importance to both processing and stability, the results obtained with these agents have been collected in Table IV. ST.4BILIZERS TABLEIV. STABILITY O F LUBRICANT Stabilizer Calcium stearate ' 621 Barinac Cadmium stearate HC n-Lead stearate DS-207 a A. Spotting. D. Discoloration. C. Embrittlement.
Heat Stability Very poor Fair Fair Very poor Fair Fair Very good
+
weatherOmeter Hours 50 50 300 300 300 150 750
Outdoor Exposurea, L x 10-3 A D C 14 8 28 25 , .. 35 42 .,. 72 16 ... 60
... 43 47
. .. .,.
...
. .. 82
>110
A detailed study of various stabilizer combinations is not within the scope of the present study. Such combinations are of considerable importance since they may be synergistic, additive, or poorer than the individual components. Several key combinations are shown in Table I11 to illustrate their importance. The above considerations are necessarily somewhat generalized. Other components play important roles in the stability of individual plastic compounds. Key factors include the types and relative amounts of resins, plasticizers, fillers, and colorants (1). Some of these will be considered briefly at this time and at greater length a t a later date. EFFECT O F RESIN
The effect of the resin itself on stability is often very substantial. This is shown in Table V which condenses the stability characteristics of a calendering grade polyvinyl chloride resin, a calendering grade polyvinyl chloride-acetate resin, and a paste
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resin with several stabilizers. These data indicate that in some cases the homopolymer weathering life may be twice that of a copolymer-based plastic, which may in turn have twice the weathering life of a paste resin. This is not true in all cases, but it does emphasize the fact that the resins play an important role in stability which should be considered in compounding a resin selected to meet processing or product requirements. A second important factor is also illustrated in Table V. Some stabilizer combinations are more effective in one resin base than another. For example, the addition of DS-207 to Tribase doubles its weathering life in a homopolymer-based plastic but not in a copolymer-based product.
TABLE V. EFFECTOF RESINSON WEATHERING Stabilieer
Homopolymera A D C
..
Copolymer A D C
A
55
8
8
28
..
21
48
43
..
E6B (organic) C-2 (calcium) S N (strontium) 25 BVS (barium) 40 SCX (cadmium) .. D-22 (tin) Vanstay 16 (sodium) Plumb-0-Si1 B (normal lead) 39 Tribase (basic lead) 35 Tribase-DS-207 (basic lead) 72 Dyphos (basic lead) 130 a A. Spotting D . Discoloration. C. Emb+ttlement. G. Gassing.
.. .. ..
.
69
58
.
16
. 6 1 , 5 3
Paste D C
G
0
8 . . 2 8 . .
63
29
150
30
,.
48
28
..
53
>160
110
,.
..
>120
,.
.. ..
1 4 . , 2 5 , ,
,.
EFFECT OF FILLERS
Fillers are often incorporated in vinyl plastics for specific functions aside from cost. A notable example is the use of certain clays that substantially improve electrical properties. The effect of fillers on stability varies widely depending on the specific filler, impurities present, etc. Perhaps the most common effect is a moderate reduction in heat stability and improved light stability. In general uncoated fillers or fillers coated with saturated material8 are preferred, and reasonable care should be taken to select fillers free from iron, zinc, and other materials detrimental to polyvinyl chloride. EFFECT OF PLASTICIZERS
Plasticizers are a major factor in the stability of vinyl plastics because of interaction with both resin and stabilizer. Table VI compares the average stability of several types of plasticizers in polyvinyl chloride homopolymer resin stabilized with dibutyl tin dilaurate. This classification is based on an extensive study of the effect of plasticizers on stability and is derived from the chemical constitution and general properties of plastics as well as influence on stabilization. Its purpose is to illustrate the marked effect choice of plasticizer has on stability.
TABLE
VI. EFFECTOB PLASTICIZER TYPEON STABILITY
Plasticizer Type Unsaturated ether esters Unsaturated esters Ether esters Phosphate esters Chlorinated compounds Polymeric Aryl esters Alkyl esters a A. Spotting. D. Discoloration. B. Stiffening. C. Embrittlement.
Heat Stability Very good Fair Fair Verypoor PoorFair+ Fair+ Good
WeatherOmeter Hours 50 C 200B 300B 275 D 175 A, D 650A 375 A, D 650A
Outdoor Exposurea, L x 10-3 A D B C . . . . . , 10 22 . . . . . . 14 27 . . . . . . 18 3.5 20 20 , , . 60 6 6 . . , 28 30 . . , . . . 4 6 10 10 . . . 36 30 . . . . . , 51
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The unsaturated and/or ether ester plmticizers fail on relatively short outdoor exposure by stiffening which in this case is incipient brittleness. Basic lead salts should not be used when these plasticizers predominate (3)since they impair heat but not light stability, The normal lead salt stabilizer Plumb-0-Si1 is particularly effective with this type plasticizer because of its stabilizing action and its tendency to minimize the characteristic spewing of these esters (3). The phosphate esters have poor heat stability and most often fail by discoloration on exposure to light. Chlorinated compounds offer several very desirable characteristics a t low cost but require the use of Dyphos for satisfactory stability ( 2 ) . The polyesters feature low volatility at some sacrifice in other physical properties. Low volatility of the plasticizer does not of itself mean long life or permanence of the plastic product, however. Proper stabilization against heat and light is required as well. Most of the previously considered plasticizers were esters, but contained chemical modifications influencing their stability. The remaining simple esters may be classified further according to whether aryl or aliphatic groups predominate. Those in which at least two of the three groups, in the general case of a dibasic acid wter, are aliphatic show a pronounced advantage in
Vol. 43, No. 10
stability as well as in general plasticizing action and have the greatest flexibility of all plasticizers in regard to choice of stabilizers, SUMRI4RY
The several types of stabilizers have been evaluated and key factors influencing stability and stabilizer action have been evaluated. The effective formulation of raw materials into quality plastic products requires careful choice of resin, plasticizers, stabilizers, and modifiers. The individual characteristics of the components are no less important than the question of how specific interactions will affect the whole. Stabilizers should be chosen with these principles in mind. LITERATURE CITED
(1) Clark, W. K., ModernPZmtics, 26, 97 (July 1949). (2) Hendricks, J. G., White, E. L., and Bolley, D. S., IXD. ENr,. CHEM..42. 899 (1950). (3) Kational Lead Co:, “Handbook on Stabilizers for Vinyl Resins,” X e w York, 1949. RECEIVED Afsrch 2, 1951. Presented before t h e Division of Paint, Varnish, a n d Plastics Chemistry at the 119th Meeting of t h e AMERICAN CXE\XICAI, SOCIETY, Boston, Mass.
ance of idal Coatin A STATISTICAL ANALYSIS JOHN M. LEONARD AND A. L. PITiMAN Naval Research Laboratory, Wushington, D. C. o n e aspect of the moisture- and fungusproofing of military electronic equipment is the application of organic coatings. These coatings have been developed and specified with little knowledge of their merits under conditions of end use. The toxicities to fungi of approximately one hundred different toxic agents dispersed in varnish and lacquer vehicles have been studied by the exposure of coated cotton-braid wires in a tropical jungle. The results indicate that most toxicants are inadequate, especially at the concentrations now employed. Variance analysis of a portion of the data permits estimations of the influences of the variables considered singly and in combination; and it yields information that will be especially useful in future experiments.
T H E use of toxic agents to supprebv mMew in paints is an old practice. The motives are usually esthetic-suppression of the unsightly disfigurement of a s u r f a e e - x prophylactic-the control of deleterious organisms as in a dairy (II), tannery ( S ) , or brewery ( 4 , IO). Much less attention has been paid to the problem of mildew control on clear coatings, unpigmented varnishes, and lacquers. The use of such vehicles emerged during the last war, when considerable quantities of them were used for the prohetion of susceptible circuit elements in a variety of electronic equipment, especially that which was shipped to the South Pacific theater of operations. The coatings were applied with the pri,may motives of impeding moisture absorption and of creating Donwetting surfaces with high electrical resistance. To suppress &mildewone of several different toxic agents was commonly included in the coating formulation.
With special reference to fungicidal properties, various coating materials qualify on the basis of laboratory test only. Procedures for evaluating the fungus-inhibitive potencies of the roatings have been studied considerably by laboratories of the military establishment and by its contractors (1,6). Though it is common knowledge that such tests are unrealistic, there is a striking dearth of information regarding actual fungus-suppressive qualities under conditions which approximate end use. To supply the deficiency, the experiment described herein was undertaken. EXPERIMENTAL
Of the two vehicles selected, one was a 33-gallon length phpnolic tung oil varnish described under Specification AN-TT-V-I 18 and the other an alkyd resin-modified nitrocellulose lacquer, the composition of which is given in Specification AN-G51. These vehicles were not selected as being necessarily optimum materials for the purpose intended but rather as being representative. Each is someahat susceptible to fungus growth and since a n important motive for the investigation was to explore toxic agents, it was deemed desirable to use vehicles of moderate susceptibility so as better to differentiate superior tovic agents from mediocre ones. Approximately 100 different toxic agents vvere ground in ball mills into the two vehicles at 10% concentration based on the nonvolatile content. Concentrations of 1 and 5% were prepared by diluting portions of the 10% grinds with appropriate amounts of vehicle. In reasonable simulation of an end use the coatings were applied by dip t o short lengths of cotton braided wire. Pickups based on the weight of the cotton braid approximated 100%. All specimens were prepared in triplicate and suspended horizontally in shallow wooden trays as shown in Figure 1. The arrangement of the specimens was carefully randomized so that in practically no case did a single tray contain two identical specimens, a procedure which tends to minimize the influence of fortuitous variations in exposure conditions. The trays were exposed in a small hut in a Panama jungle with conditions which
