H ERCULES POW DER C O M P A N Y , W IL M IN G T O N , DEL.
Four low-vlscosity nitrocelluloses with viscosities o f 19, 27, 33, and 40 centipoises, all below RS ’ /«-second type, were evaluated in five compositions with each o f four com m er
cial nonoxidizing alkyd resins for Sward rocker hardness and for tem perature-change resistance after 6 m onths o f weathering at M ia m i, Fla., and after a combined outdoor exposure o f 3 m onths at M iam i and 3 years at W ilm ington, Del. T h e data furnish evidence that satisfactory lacquers o f the autom obile finishing type can be m ade by com bin
ing nitro ellulosc having viscosities as low as 27 centipoises with nonoxidizing alkyd resins. This lim itation does not apply to resins such as drying oil types which are good film formers, where still lower-viscosity nitrocellulose is used lo obtain quick-setting lack-free films.
N
ITROCELLULOSE lacquers based on RS ‘ /^second and RS ‘ /(-second viscosity types have been widely used for many years because of properties not possessed by other coating materials. Chief among them are rapid drying to a hard film without the necessity for subsequent baking, toughness which imparts good resistance to temperature change, hardness which makes such lacquers scuff- and print-resistant, ease of refinishing, and formation of pleasing films which have excellent weathering resistance.Since these low-viscosity-type nitrocelluloses (RS l/ r and '/(-second) produce coatings with such excellent properties, it became of practical interest to develop still lower-viscosity types, and to establish the lowest limit of viscosity which would still retain the good qualities exhibited by RS */r- and ‘ /(-second types. Consequently, a series of viscosity types below RS
‘ /(-second were developed and evaluated in lacquers. Although this investigation was carried out primarily to establish the lowest practical viscosity type of nitrocellulose for use in lacquers, without emphasis on determining the highest solids, it is obvious that the use of a lower-viscosity-type nitrocellulose makes pos
sible lacquers of higher solids content. This is important in view of the growing interest in lacquers of increased solids con
centration. It is also apparent that the recent trends toward high solvency and hot-spray formulation and application are ap
plicable to lacquers containing lower-viscosity-type nitrocellu
loses than ‘ /(-second with consequent higher-solids concentration.
Investigations are in progress to obtain specific information on these points to supplement the data reported here.
PA NE L E X PO SU R E S
M a t e r i a l s a n d F o r m u l a t i o n s . A preliminary investiga
tion was carried out with a variety of resias and formulations to serve as the basis for selection of typical formulas, resins, and nitrocellulose types for a more critical study. This preliminary work led to a choice of four typical resins and five formulas, based on lacquer films having good color retention and good buf
fing properties of reasonable hardness. Representative com
mercial nonoxidizing alkyd resins were chosen. Their choice does not imply that equally good, or possibly better, resins are not available. Four nitrocelluloses with viscosities ranging from 19 to 40 centipoises were selected. On the same scale, RS x/r second nitrocellulose has a viscosity of 125 centipoises and RS
‘/(-second, 50 centipoises. Table I lists the nitrocelluloses, resins, and formulas selected, together with the pigment and solvent compositioas.
August, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 757
L a c q u e r s a n d T e s t P a n e l s . All lacquers were formulated to have a viscosity of 40 centipoises (determined by the Hercules horizontal capillary viscometer at 25° C., 2) when sprayed. The pigment was ground in a nitrocellulose-base solution in a ball mill for 48 hours. This base solution was then used to formulate the five selected lacquer compositions by adding the appropriate plasticizer and resin and by subsequent thinning with solvent to the selected viscosity level for spraying, coats of lacquer, allowing each to air-dry before application of the next successive coat. The final films were force-dried 2 hours at 70° C., and allowed to stand overnight beforo polishing. All panels were given edge and back protection by painting with the following Parlon composition containing Alcoa aluminum pow
der (in parts by weight): formulation to proper brushing viscosity for easy application.
T e s t M e t h o d s . The Sward rocker hardness (I) was measured on all panels by the standard method without modification.
Temperature-chango resistance data were obtained on films of all compositions which had been weathered previously for 6 months in Florida. The test consisted of fifty cycles of 2-hour alternating periods at —15° and + 11 0° F. in air, followed by sixteen cycles of 2-hour alternating periods at + 110° in air and —15° F. in glyc
erol. This procedure was followed because, with only two ex
ceptions, the alternating cold and warm exposures in air did not
cause any failure. T o obtain additional information, a more se
vere (rapid) change in temperature was considered necessary.
Consequently, the panels which had survived the fifty cycles in air were given additional temperaturc-change exposure by plung
ing the warm panels at + 11 0°, directly into glycerol at —15°
F. This direct contact with a liquid coolant was more effective in revealing differences in temperature-chango resistance.
All panels exposed to outdoor weathering were placed at an angle of 45° with the vertical, facing south, both at Wilmington,.
Del., and Miami, Fla. One set was exposed 6 months in Florida and then given the temperature-change resistance test described.
Another similar set was exposed 3 months in Miami, followed by an additional 3 years at Wilmington. The latter set was ob
served after the 3 months in Florida for chalking, dirt collection, and gloss after repolishing, The panels were then given an ar
bitrary rating based on visual inspection; on a scale of 1 to 5, inclusive, 1 was good and 5 very bad. This same set of panels was inspected periodically during the additional 3-year exposure at Wilmington for rusting, cracking, blistering, and chalking.
T a b l e I. on 12.2% (by weight) solutions of nitrocellulose in Hercules horizontal capil
lary viscometer at 25° C ., using a solvent com posed of: 25% denatured
Beckosol 1308 Formulas Glyptal 2471 Formulas Glyptal 2477 Formulas Rezyl 99 Formulas
Centipoises 1 2 3 4 5 1 2 3 4 5
Tem p.-Change Resistance after 6-Montl
19 66 + 58 58 57 58 66 66 66 + 66 20
Centipoises ing tion Gloss b ing tion Gloss*» ing tion
W ith Beckosol 1308
758 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 8 IN TE R P R E T A T IO N O F DATA
The Sward hardness data in Table II demonstrate that the viscosity of the nitrocellulose has no direct bearing on the hard
ness of the lacquer films. On the other hand, both formulation and the Dature and quantity of resin influence hardness. In general, lacquers with hardness ratings in the range of 19 to 30 had the best buffing characteristics.
The temperature-change resistance results given in Table II show that ten of the thirteen failures were with 19-centipoise ni
trocellulose. Since this test was considered relatively severe, the results should serve as an indication that lacquers with good temperature-change resistance can be made with any low-vis- cosity nitrocellulose having a viscosity of 27 centipoises or higher. Formula 5, with a ratio of 1.5 parts of resin to 1 part nitrocellulose, produced more than twice as many failures as any
T a b l e IV. C o m b i n e d R a t i n g s f r o m D a t a in T a b l e III ( L o w e s t N u m b e r s A r e B e s t ) Rating b y Viscosity Type of RS
Nitrocellulose Rating b y Resin Rating b y Formula
Dirt Dirt Dirt
Centi- Chalk- collec Chalk- collec- Chalk- collec
poises ing. tion Gloss Type ing tion Gloss No. ing tion Gloss
19 68 61 28 Beckosol 1308 96 56 30 1 60 47 2 2
lulose Months Months Months Months Months
Viscosity, T o first T o com - T o first T o com - T o first T o com T o first T o com T o first T o com
Centi m it' plete rust- plete . rust- plete rust plete rust plete
poises ing failure® t ing failure® ing failure® ing failure® ing failure®
W ith Beckosol 1308
* By chalking. Letters in parentheses indicate failure b y cracking (cr) and b y blistering (bl) instead of b y chalk-ing. Values followed by + sign indicate good condition and satisfactory coatings even after 37 months of out-door weathering.
nitrocellulose viscosity and dirt collection. The choice o f resin has a direct bearing on chalking, dirt collection, and gloss after repolishing. Formula 2, which contains most plasticizer, and formula 5, which contains most resin, chalked most. Formulas 4 and 5, with more resin than the others, collected most dirt.
Gloss after repolishing was affected slightly by formulation.
Following the 3-month exposure in Florida, the panels were exposed for an additional 3 years at Wilmington (Table V). Al
though the results are not entirely consistent, a large majority of the early failures occurred with 19-centipoise-type nitrocel
lulose, and these failures were largely due to cracking, an indica
tion of embrittlement. With these exceptions, failure of a ma
of the formulas containing a higher proportion of nitrocellulose.
This indicates that the toughening action imparted by nitrocel
lulose is reduced somewhat when the proportion of nitrocellulose to resin is reduced from 1:1 to 1:1.5. Different resins also have a bearing on the temperature-change resistance.
A visual rating of the panels, after exposure for 3 months in Florida, for chalking, dirt collection, and gloss after repolishing is recorded in Table III. A regrouping and totaling of the nu
merical ratings in Table III is shown in Table IV. With this compilation it is easier to make a general comparison of the effects of the three variables—nitrocellulose viscosity, resin, and formu
lation. The numbers indicate the degree of failure, with the low
est totals representing the best results.
Chalking and gloss after repolishing appear to be slightly poorer for 19-centipoise nitrocellulose than for the three higher- viscosity types tested, with little difference between 27-, 33-, and 40-centipoise types. No correlation appeared to exist between
containing 19-centipoise nitro
cellulose, and essentially no dif
ference appeared in the higher viscosities, it may be concluded t h a t , w ith th e p a r t ic u la r formulas used, the low limit of viscosity of nitrocellulose for good weathering properties is in the neighborhood of 27 centi
poises. There appears to be little choice among the resins. Con
sidering formulation, the trend appears to be toward more rapid chalking as the resin concen
tration is increased.
The data showing that many of the lacquers afforded protec
tion to the metal for 3 months in Florida plus an additional 3- year continuous exposure in Wilmington is believed ample evidence that satisfactory lac
quers of the automobile finishing type can be made by combin
ing low-viscosity nitrocelluloses within the 27- to 40-centipoise range with nonoxidizing alkyd resins. Attention is drawn to
cate any viscosity limitations on low-viscosity nitrocelluloses in combination with oxidizing-type alkyds or with mixtures of oxidizing and nonoxidizing types. Investigations are now being carried out which may furnish information on these points.
A C K N O W LE D G M E N T
The author wishes to acknowledge the work done by his as
sociates in the Cellulose Products Department of Hercules Powder Company in obtaining the data presented here.
L IT E R A T U R E C ITE D
(1) Gardner, H . A ., "P h ysical Exam ination o f Paints, Varnishes, Lacquers and C olors” , 9th ed., p. 117, W ashington, Inst, of Paint and Varnish Research, 1939.
(2) Speicher, J. K ., and Pfeiffer, G . H ., Colloid Symposium Mono*
graph, 5, 2 7 0-2 (1928).
Przsxh txd before the Division of Industrial and Engineering Chemistry at the 107th Meeting of the Am ib ic a n Ch x m ic a i Society, Cleveland, Ohio.