Color Uniformity in Low Pressure c/
Laminates PAUL FRAM, THOMAS F. DUNNE, AND FRED LEONARD Army Prosthetics Research Laboratory, Walter Reed Army Medical Center, Washington 12, D . C .
A
S I N other fields of production, the importance of color uniformit,y in thomanufacture of low pressurc laminates is well recognized. The uniformity and reproducibility of color is often used as a guide for product control and frequently has a marked influence on sales acceptance of a product. The development of polymerizing resins suitable for low-pressure curing (1, 3, 7 , 9, 11) ha8 had particular impact on the orthopedic and prosthetic industry. Several years of intensive study (4) in the development of low pressure laminates for prosthetic components has firmly established the position of thermosetting low pressure resins in this industry. To the amputee population, the cosmetic appearance of the prosthesis (10) is of great, importance, approaching that, of function, and for this reason an investigitt'ion was conducted t o deterinine the factors responsible for color variations and to establish satisfactory color shade guides in laminated prostheses for the art,ificial limb manufacturing industry. Considerable difficulty is encountered a t present in supplying uniformly colored devices, such as an artificial arm. The limb shops generally assemble an upper extremit'y prosthesis by combining laminate components which are st.ock items supplied by various manufacturers with custom-fit>tedlaminate components such as the shoulder sockets and above-elbow stump sockets. On? reason for the nonuniformity in color is the wide discrepancy among the suppliers and limb shops concerning the choice of a flesh color. Most of the commercially available polycstcr-styrene liquid resins, which are used in prosthetic work, cure to a light amber color. These liquid resins are supplied under such trade names as Paraplex, Laminac, Fosterite, Vibriri, Selectron, and Marco. (.:ombinations of such resins as Laminae 4116 and 4134 (American Cyanamid Co.) as well as mixtures of Paraplex P-43 and P-13 (Rohm & Haas Co.) produce laminates with excellent physical properties for application in prostheses. The 70 to 30 weight ratio mixture of Paraplex P-43 and P-13 resins was selected for this investigation. Laminates of this resin mixture have excellent impact strength and freedom from cracking. I n service, t'hese lamiuates withstand severe abuse, are free from t'he absorption of odors of perspiration, are easily cleaned with either soap and water or organic solvents, and show no indication of being toxic when in direct contact with the skin. I n addition, i t is possible in a short period of time t,o cure the laminate structure t'o a uont,acky surface without excessive evolution of heat. The choice and amount of reinforcing material may also influence the final laminate appearance a~ well as the physical proprrticis. Practically any type of fibrous material may be incorporated into a laminated struct,ure. In the manufacture of upper and lower extremity prostheses, cotton and nylon stockinet have b t r n very succcssfully employed. The stockinet, because of its two-way stretch, can be readily fitted over irregular contours, thereby producing a smooth replica of the desired form. The most, generally employed reinforcing material for laminate prostheses is cotton stockinet, which is readily available in many sizes a t low unit cost. Nylon, o n the other hand, is more satisfactory from the standpoint of color uniformity, as i t is white and wquires no further treatment, while the cotton stockinet is usually unbleached and has a somewhat yellow tinge. Nylon is rhemically inert and has good resistance to high temperature. S ylon st,ockinet was therefore stllcctcd in many of these experi-
mental laminates, but unbleached cotton was included becauw of its wide acceptance in the industry. I'K0CISI)IJKE
Gincc it was believed that inconsisteneics and variations in t,he laminating procedures were partly responsible for nonuriiformity of color, investigation was undertaken to evaluate individually the effect of each phase of thr laminating procedure on the final color. A control color was prepared from the following procrdure which was established as the stimdard: Pigment concentrates of hlapico iron oxide pigments (Columbian Carbon Co.) were ink-milled, using tightly spaced rolls, and then manually mastcrbatched before incorporation into t3he liquid resin. The titanium dioxide, a white pigment (Titauium Pigment Corp.), was included in t,he color masterbatch for the purpose of hiding somewhat the nonuniformity introduced h y the stockinet. The colored resin masterbatch was agitated with a high-speed rotary-blade stirrer to ensure thorough pigment dispersion, Catalyst, 2% by weight of Luperco ATC (Novadel Agene Corp.), containing 50% benzoyl peroxide in tricresyl phosphate, was added to a carefully weighed portion of the colored liquid resin and dissolved by stirring. Then, for each 100 grams of reein, 7 drops of Naugatuck Promoter No. 3 (Naugatuck Chemical Co.) was added and the mixture was stirred manually until homogeneous. The forms used for the laminate preparation were made of plaster of Paris in the shape of the frustum of a cone. These molds were first coated witjh a film of polyvinyl alcohol and then wrapped with four layers of reinforcing stockinet. Just prior to lamination, t'he stockinet-covered mold was slipped into a tapered tube of polyvinyl alcohol film, and then the resin was poured into the small end of the tube and worked into the stockinet. When the stockinet, was thoroughly soaked in resin, the polyvinyl HIcoho1 bag was tied off above and helow the mold. The excess refiin was strung from the stockinet by using a taut string to apply pressure to the lay-up ~ n forciug d the resin to the bottoni of i,he polyvinyl alcohol bag. The laminate was allowed to rpmain for 1 hour at room tempcruture, and after the polyvinyl alcohol cover was punctured above itnd below the mold, curing was continued in a circulating-air oven a t 65" C. for 45 minutes. After the lay-up was cooled to room temperature, the polyvinyl alcohol bag was stripped, and the plaster of Paris mandrel waR cracked and removed from the laminate. The laminate color wafi evaluated by means of the Hunter color and color-difference meter, a tristimalus colorimeter which measures color on three scales, Kd rePresent,ing luminous reflectance. a t8heamount of red and green. and b the amount of y e l l o ~and blue. This instrument gives a uniform measurement of the visual difference between colors (8). The Hunter color-difference meter was standardized agitinst a huff tile standard the spectral reflectance characteristics of which were measured with a Beckman spectrophotometer in the wave-length region of 400 to 650 mr. Using the t'en selected ordina1,e method for I.C.I. Illuminant C (6), the tristimulufi values were calculated, and from these, the corresponding Hunter readings, Rd. a, and b were obtained (6). The color characteristics of the nt>andardbuff tile were Rd, 34.5; a, 8.5; and b, 20.3.
393
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
394
Vol. 45, No. 2
EFFECT OF PROCESS VARIABLES
According to the observations listed in Table I, some conclusions were formulated with regard to the effect of processing variables on the color of the cured laminate. One of the significant factors mas the amount of proniotcr, as shown by the increase in vellow content, b value, n-ith :in inc~rcawin promoter content. Increasing conccntrationg of ciitnl) s t effected relatively minor
Cata-
Promoterb,
lpsta.
Drops/ 100
Cure Wt. % Grains Resin 1 Ilr. at room temp. and 45 min. at, GZ' C . 2 7 4 7 2 None 2 14 2 7 Rooin tenip. 2 7 1 fIr. at 65" c. 2 7 5 Luperco ATC. b Xsugatuck Promoter No. 3.
Stockinet Material
Final Color b, €Iunter Meter Rd n b
Nylon Nylon
30.2 27.3 32.8 30.7 Cotton(un- 27.5 bleached) Nylon 28.1 Sylon 34.6 Nylon .Vylon
9 . 1 13.8 6 . 3 14 4 9.7 9.7 9 . 3 15.2 8.1 16.9 6.8 8.2
12.8 16 3
of the approximate amount of pigment required to achieve a laminate oi the desired color shade (Figures 1 and 2). In Figure 1. 500 mg. of pigment concentrate mixture per 100 grams of resin included 9 mg. of 50% LIapico Brown S o . 418 concentrate, 45 mg. of 331/3% Jlapico Lemon Yellow concentrate, I
10
Imq
I5
20
25
SO
MAPICO RED X516 CONTENT in% concentrate in loop rer;m)
of 33
Figure 1. Effect of Red Pigment on Laminate Red Color Component
changes in the laminate color, the niost significant of which ivas a loss in brightness, noted as a decrease in Rd. This was especially true in t,he case of the nylon stockinet. The effect on color, hoxever, \vas not as critical as that caused by promoter concentration. The laminate color appeared to be particularly susceptible to variations in curing conditions. Higher oven temperatures as Fell as premature oven t'retitnient caueed an increase in both yellon. content and luminous reflectance. During the early etagcs of polymerization or gelling of the resin the color is highly sensitive to a rapid increase in tempcr~tture. It was concluded that to obtnin reproducible color beiore placing the laminate in the oven it was necessary to allo\T- partial polynierization to occur a t room temperature. PIGXIENT C O X T E l T
Once the procedure was established for obtaining consistcnt, color reproduction in laminated prostheses, the next step was an assessment and standardization of the color shade guides. It was desired t o develop first a color guide for the fabrication of both flesh-colored nylon and cotton-reinforced laminates, and, sccondly, a pigment concentrate 17-hich yielded a reasonably stable suspension when dispersed in the laminating resin. I n order to establish the relationship between the concentration of pigment in the laminating resin and t,he resulting color, tKo series of pigment mixtures n-ere carefully formulated iising the folloxing ingredient,e: Kt.
Titanox White, RCHT-X Mapico Red KO. 516 l\Iapico Lemon Yellow llapico Brown No. 418
yo in Paraplex G-60
,
I n one series the amount of Mapico Red No. 516 pigment concentrate mas varied, while in a second series the independent viuiable was the amount of Mapico Lemon Yellow concentrate. Laminates, reinforced with four la3 crs of nylon stockinet, were piepared using 0.50 gram of the pigment mixture per 100 grams of resin, and by means of the Hunter color-difference meter the resulting colors were determined. Thus data rvere obtained kvhich readily lent themselves to a graphical intci prctLttion,enabling calculation
r
1 IO
20
30
40
ao
MAPICO L E M O N YELLOW CONTENT (rngot 33 113% L O ~ C D ~ I I Din I1000 ~ roiinl
Figure 2. Effect of Letnon Yellow Pigment on Laminate Yellow Component
and enough -50% titanium dioxide concentrate to give a total of 500 ~ n g . In Figure 2 , 500 mg. of pigment concentrate mixture per 100 grams of resin included 9 mg. of 50% Mapico Brown No. 418 concentrate, 15 mg. of 311/3% Mapico Red h-0. 516 concentrat'e, and enough 50% titanium dioxide concentrate to give a total of 500 mg. The mnge of color shades chosen for investigation was based on previous information obtained in this laboratory as a result of a measurement of skin color values for several hundred individuxls. Ranges of a values from 7 to 10 and of b values from 13 t o 16 were found to tipproximate closely the flesh color of the human hand, provided the Rd value waz between 25 and 30, The plot (Figure 1) of the a value or red color component as a. function of llapico Red content exhibited a curve with a slope n-hich ciwreased rapidly with increasing pigment content. On the other hand, the plot of the b value. yellow color component, of the series of laminates as a function of Mapico Lemon Ycllorv content gave approximately a linear relationship. iln explariation lor this difference in behavior is found in the comparison of tho yespwtive a and b values for the undiluted red and yellow pigment concentrates with the ranges of red and yellow color values covered k>yeach graph. The a value of 12, found for the RIapico Bed pigment concentrate, is close t,o the laminate red color range of 7 to 10, so that the data fall on a curve which approaches asymptotically the limiting cdor value. I n contrast, the b value of the Xapico Lemon Yellow concentrate is 24, which is sufficiently higher than the range of the b values in Figure 2, so that the plot, approxiniat,es a straight line rather than a curve.
February 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 111. PIGWENT MIXTUREFOR COTTON-REINFORCED
TABLE 11. PIGMENT MIXTUREFOR NYLON-REINFORCED LAMINATE Pigment Concentrate"
0
Pigment Content,
%
Grams Pigment Conoentrate/100 Grams Mixture
Parts
W!&t
Pigments were ink-milled in Paraplex P-13.
395
LAMINATE Pigment Concentratea Titanox White RCHT-X Mapico Red No. 516 Mapico Lemon Yellow Mapico Brown No. 418 a
Pigment Content,
%
80 33'/a 33'/3 50
Grams Pigment Concentrate/100 Grams Mixture 03.30 4.06 1.04 1.60
Parts
W2ht 100 4.35 1.11 1.71
Pigments were ink-milled in Paraplex P-13.
TABLEIV. EFFECTOF PROCESS VARIABLES O N CURE TIMEOF LIQUIDPOLYESTER-STYRENE RESINS NYLON-REINFORCED LAMIXATE COLORSTANDARD.The formula for compounding the pigment concentrate used in obtaining the flesh color selected for the nylon-reinforced laminate is described in Table 11. The dispersion of 0.50 gram of this pigment mixturein 100 grams of the liquid resin should result in thedesired laminate color shade for which Hunter color-difference meter measurements are Rd,29.0; a, 9.2; and b, 15.2. COTTON-REINFORCED LAMINATE COLOR STARDARD.The problem of duplicating the color standard obtained for nylon in laminates reinforced with unbleached cotton stockinet was readily solved. The adjustment in yellow and red color components could be predicted if the color difference between the cotton and nylon laminates were known, using the same pigmented resin and also the relationships between color and pigment content shown in Figures 1 and 2. The color standard selected for cotton stockinet has readings on the Hunter color-difference meter of Rd, 29.8; a, 9.9; and b, 16.0. The formula for compounding the pigment concentrate mixture for this standard is described in Table 111. DISCUSSION
Previously reported attempts ( 4 ) to color resins in the liquid state for purposes of improving the appearance of the prosthesis have been attended with certain difficulties. Some drawbacks attributed to the pigmentation were a resulting brittleness in the cured laminate, color changes caused by oxidation of the pigments by the catalyst, and inhibition of polymerization by some pigments. Generally, the following two basic types of colorants are used in the plastic field. Colored igments, typified by iron oxides, titanium dioxide, barium sulgte, mercuric oxide, and a variety of other metallic salts. Organic dyes, which o n e their coloring effect to the existence of chromophore groups in the organic molecule. Of these, the inorganic pigments may be used in applications in which the decreased electrical insulation and loss of transparency are not important. On the other hand, the organic dyes may be applied in smaller amounts where higher dielectric values and transparency are desired. A variety of other conditions, including compatibilities, wettability, absence of injurious chemical effect, stability on curing, and dispersibility, must be satisfied before the dye or pigment is acceptable in any resin formulation, It was decided from some of the above considerations that the inorganic pigments should be suitable for application in lowpressure laminates for prosthetic components. The pigments selected for most of this study included a series of Mapico pigments (Columbian Carbon Co.), reported to be pure varieties of iron oxides with a particle size less than 1 micron and to have eyceptionally high resistance to color change in outdoor exposure tests. Under normal conditions of use these pigments did not reduce the colored liquid resin stability and did not noticeably affect the cure of the resin. Furthermore, stable dispersions of the masterbatched pigment concentrate in the liquid resin could be easily obtained by stirrirrg,
Paraplex P-43/parsplex P-13 70/30 70/30 70/30 70/30 60/40 60/40
a b 0
Formulation Promoterb, Catalysta, diops/lOP % grams resin 0 0 1 0 1 7 1 7 1 7 1 7
Pigment concentrate, % 0 0 0
0 0 0.5
Temp. of $we, C.
Time t o Cure, Rlin.
65 65 65 25
40
30 30
..
E
!O
00
35 25
Benzoyl peroxide (Luperco ATC). Naugatuok Promoter No. 3. Incomplete cure after 60 minutes a t 65' C.
No deleterious effect by the pigment concentrates was observed on polymerization of the resin to a suitable structure. In Table IV are listed the times required to cure the liquid resin t o a rigid solid a t room temperature and a t 65' C. These data were obtained by employing the technique described by Burnett ($), in which a drop of resin is placed between two glass slides. The two slides are placed a t right angles to each other and squeezed together, essentially sealing the resin from the inhibiting effect of oxygen in the air. The cure time is determined when it is no longer possible to move the slides a minute distance relative to one another. It was found that the catalyzed pigmented resin gelled into a rigid structure slightly faster than the unpigmented one. The stability to flocculation of pigment dispersions in liquid resin is of practical importance. It was found that the use of Paraplex G-60 as the pigment vehicle was unsatisfactory in this respect. The tendency to settle out was somewhat slower with disllyl phthalate as the vehicle. When Paraplex P-13, the less viscous of the two laminating resins, was evaluated, the amount of pigment which settled in a period of 1 week was very insignificant and the pigment was easily redispersed. On the basis of these results, it &as concluded that Paraplex P-13 was a suitable pigment vehicle. There are, of course, many pigment dispersions suitable for low pressure laminates which are commercially available from such companies as American Cyanamid Co., New Pork, N. Y., Pittsburgh Plate Glass Co., hlilwaukee, Wis., and Claremont Pigment Dispersions Corp , Brooklyn, N. Y . When the pigments are milled directly into one of the laminating resins it is advisable for good shelf-life to store the concentrates in a cool place. It was concluded that to obtain consistent and reproducible color in low pressure laminates a rigidly adhered-to procedure of formulation and fabrication is required. Variation in nearly any phase of the procedure can have a profound effect on the final color achieved. It is emphasized that particular care should be exercised in the concentration of promoter and the curing treatment, since these factors definitely arc influential in determining laminate color. ACKNOWLEDGMENT
The many suggestions and valuable guidance of W. A. Carnelli, consultant in art and color to the Advisory Committee on Artificial Limbs, National Research Council, is gratefully acknon.1edged.
INDUSTRIAL AND ENGINEERING CHEMISTRY
396
(1) B e a v e r , E. AI., IND. Ex:. CH ( 2 ) RurIlett, R. E,, et (LL, (to Gel 2,628,178 (Feb. 10, 1953).
(6) H a r d y , A. C., “ H a n d b o o k of Coloriiiii,ti y , ”1). 30, (’anihridgr. T h e Teclinology Press. 1936. ( 7 ) I-Iarris, R . 11. (to :hnerirall (’yanaiilid (‘o..). L-. i;.h t 527 (:ipril 19, 1949). (8; H u n t e r , I
