PROPERTIES OF CELLULOSE AS APPLIED IN PLASTIC MATERIALS' W. E. GLOOR Hercules Powder Company, Wilmington, Delaware
IT
IS the object of this article to show how the generally accepted facts concerning the structure of cellulose, developed and proved during the past quarter century, are applied and used in the large plastics industry now based upon this raw material and its derivatives. In reviewing the data oht,ained by the toil of a great many workers in this complicated field, the details have been simplified into a few broad generalizations, which will be discussed for those cellulose derivatives most widely used. In the first place, it is accepted that cellulose and its derivatives in the ideal state, such as is obtained when dilute solutions are prepared in appropriate solvents, are long-chain molecules built up of recurring small chemical units. In this state, they are analogous in structure to the synthetic long-chain polymers, such as polystyrene, methyl methacrylate, and the various vinyl polymers. It is accepted also that the l'ength of these chains plays no small part in determining the properties of the plastics prepared from them; this point will be illustrated with data obtained with a few selected systems. In speaking of chain length, it is assumed that the length mentioned will be thought of as average chain length, measured by viscometric means in dilute solutions and, hence, based upon weight-average constants. The intrinsic viscosity of materials mentioned in this paper can be obtained by dividing the particular degree of polymerization (D.P.)by the appropriate constant given by Kraemer (18). Secondly, since each anhydroglucgse unit of the cellulose chain contains three hydroxyl groups, their substitution with various acyl and alkyl groups offers a great opportunity for variation as to the kind, amount, and distribution of substituents along the cellulose chain. A third group of factors, not often emphasized as contributing to the properties of cellulose derivatives in the ideal state but very important in the selection of cellulosic materials and modifiers for actual use, may be classified under the heading of colloidal properties. These include such factors as the particle size and porosity of the materials, their affinity for solvents and plasticizers, their penetrability by plasticizers and solvents, and the influence of solvation-desolvation equilibria (7). A fourth factor, often neglected in the discussions of the scientific aspects of high polymers but which is frequently of supreme importance in the industrial appli-
'Presented before the New Brunswick Group of the North New Jersey Section of the Amerioan Chemical Society on November 20, 1946. 214
cation of cellulose plastics, involves their purity. 'The purity of the final product will depend in part upon the effects of small amounts of substituents in the chains: these may have been placed there as a result of side reactions during the processes of making the derivative, or they may be just plain "dirt." Whatever the procesr by which these suhstituents have been introduced, theil. effects are real and must be taken into account. The influence of these four factors-chain length, substitution, colloidal properties, and contaminant snhstituentswill now be discussed for the various ?ommercial cellulose plastics. For the purpose of this survey, no consideration will be given to the use of cellulose derivativesin solution processes, attention being focused mainly on the hot-molding plastics. CELLULOSE FILLERS
Consideration of cellulose-filled thermosetting phrtics, even though brief, may seem out of place in this article; however, cellulose-filled plastics constitute the largest proportion of all phenolic and urea plastins. The long chain length found in the wood fiber, chopped rag, or laminated plastic provides the tough reinforcement necessary to give impact strength to these products. The chain length of these materials, ranging from D.P. (degree of polymerization) 750 to 1500, provides a tough skeleton which will withstand the heat. of molding. While the materials used are not esterified and hence retain their well-knok absorbency for wat,el.: this absorbency factor is largely controlled by the aajin which the resin is impregnated in them. This is onts important colloidal aspect of their use. Another is t h d r particle size. The finely divided wood flours are used in plastic compounds which require considerabli~ flow; in these the molten-resin carries the reinforcer along with i t during molding. Long fibers or cords, or even chopped bits of fabric, are used to fill the h i d impact grades of molding powders, and care must he taken in processing on rolls and in such an operation a,s transfer molding that the long fibers are not broken u]) to such an extent that their reinforcing power is lost. The penetration of the fibers by the resins also involves such factors as the condition of the surface of the fiber.. and the extent to which the resinification has heen carried out prior to impregnation of the fibers. Purity of the cellulose is involved especially in the selection d fihers for the reinforcement of light colors in the nre:i plastics. Turning now to the class of thermoplastic materids commonly included under the term cellulosics, the
MAY, 1942
nitrate, acetate, and acetate butyrate esters and the ethyl ether are the cellulose derivatives widely used in the plastics field.
215
CmL"L0,.
",.Te
c. a,,
%., ",,*"
CELLULOSE NITRATE
The use of cellulose nitrate as a base for plastics of the celluloid type was the first large-scale application of the material, antedating its use in smokeless powder and probably leading to theidea of using gelatinized cellulose nitrate as a low-burning propellant (19). Plastic-grade cellulose nitrate today has a nit,rogen content of 10.9 to 11.2 per cent, corresponding to a substitution of 1.95 to 2.08 hydroxyls by nitrate groups per anhydroglucose unit, which is considerably less highly nitrated than the material of 2.7 substitution used for ballistic purposes. The permutoid nature of the nitration reaction used in preparing the material tends to give a very uniformly nitrated material which shows no significant variation in the degree of nitration of various portions when fractionated. It has been shown by Mahoney and Purves ( I S ) that in the lower-nitrated materials (below acetone-solubility range of 10.5 per cent nitrogen)more than half of the nitrate esterification is located on the primary hydroxyl groups; in the more highly esterified, technically useful materials, however, it appears that the distribution of nitration in any one chain is well predicted by Spurlin's (17) chart, for equilibrium r e actions. Figure 1shows a section of t,en anhydroglucose unit,s in a cellulose chain with the substituents distributed according to this picture. It must be emphasized that careful preparation of the material is required, especially on a large scale, to insure that all portions of the material being nitrated react uniformly, and the industry has worked out solubility tests which indicate the degree of uniformity of nitration of a particular batch. If careful preparation of material is not carried out, the finished product will not work properly and many un-nitrated fibers will be sho,m to be present when a sample is looked a t under polarized light. Since only.one grade of material is used i f t h e preparation of plastics, little information exists as to the properties of cellulose nitrate plastics save from the qualitative standpoint. This indicates that material of higher degree of nitration does not weld as well, nor extrude wit,h as good a surface as does the plastic grade (10.9 to 11.2per cent N). With respect to average chain lengt,h of the material,
PORmNS OF ESTERIFID CELUIUISE WAINS 10 UNITS LONG
Fi.".. 1. Distribution of Nitration and Asetylation i n Any On. Celldoe. Chain as Predicted from Spudin's (17) Chart for Equilibrium Reactions
two are most often found in use today. Cellulosenitrate generally used for sheet, rod, and tube manufacture has a D.P. of about 7.50. The other, with a somewhat smaller D.P., about 550, finds use chiefly in extrusion work where the lower D.P. permits more effective control of the process. Less than 30 per cent of the polymerization is lost in processing the material into its finished form (Table 1). Regarding the colloidal problems, the commercial practice of shipping the material damped with ethanol (amounting to 35 per cent by weight of the shipment) insures that each fiber will be wet with enough latent solvent so that it readily forms a paste when the camphor and solvent are added to it in the mixer. From here on, however, the processor must carefully regulate his manipulations so as to keep the ingredients homogeneously mixed, yet in a state which permits the desired forming operations to proceed. This is an especially important problem today when the scarcity of camphor has compelled producers to incorporate other materials, such a's castor oil derivatives, into the formulations, since it has been found that good welding and surface can be obtained with such ingredients only when a more active solvent is added to the mix. It should be pointed out that the combination of camphor and alcohol, at the temperatures used for working up nitrate, is still a solvent for cellulose nitrate, while replacement. of part of the camphor by a less active material requires that some volatile solvent be added to keep the solvency a t the required level. At present, the specific affinity of cellulose nitrate for camphor is believed to be a large factor in thissolvent action; yet it. is of interest that the attraction must be a weak one when compared with the primary valence forces which bind together t,he component unit,s of the molecular chains, since the camphor may be removed by exTABLE 1 tract,ion or by precipitating a solution in a large volume Effect of Molding Operations o n Degree of Polymerization of nonsolvent for the cellulose nitrate. of Cellulose Nitrate Plastics* Regarding purity, it is well known that successful In Plaslies production of cellulose nitrate for plastics work requires Intrinsic D.P. or practically complete removal of the sulfate esters which &faterial , viscosity chain length may be formedby side reactions in the preparation of tlellulose nitrate (11.1 per cent. N) 1.85 500 the material. The usual methods of hydrolysis of sulAfter extrusion into rod 1.61 435 After baking into sheet 1.50 405 fate esters must be modified so as to produce no harmful ' Measured upon 2 per cent solutions of cellulose nitrate, in effects upon the cellulose nitrate. In addition, careful neeton&,and oalcullttcd by means of Marbin's equation (15) usmg selection of raw materials, even including the acid Kramner's constant of 270. mixes, is required to obt,ain the quality material de-
216
JOURNAL OF CHEMICAL EDUCATION PLASTICIZER
-44%
". 2.0
2.2 2.4 2.6 2.6 3.0 NUMBER OF HYDROXYLS SUBSTITUTED
Figure 2.
Effsct of Substitution of Cdlulosa Acetate on Flow Temperatun, of Plsstics
Plsrtiriied with 1: 1 dimethyl a d diethy1 phthehe..
manded by today's markets. It is believed that one of the most important factors in keeping production of cellulose nitrate for plastics a t the current high level has been the successful solution of these problems by the industry.
The effect on flow temperature is seen to be rather slight-a small rising trend with increase in acetylation. Hardness, on the other hand, falls off rapidly as the cellulose acetate reaches the upper range of acetylation. Figure 4 summarizes this variation by comparing the flow temperature, a t two selected levels of hardness, with degree of acetylation. Figure 5 indicates the variation of impact strength (2) with degree of acetylation; it is seen that this property, in a given formulation, remains constant or rises slightly with degree of esterification, until a substitution above 2.65 is reached, a t which point it falls off abruptly. This point will be discussed later in this section. The influence of acetylation upon water absorption values has been discussed elsewhere (11). In many formulations, water absorption has been found to decrease linearly as the degree of' acetylation is increased. This is understandable in terms of the number of available hydroxyl groups.
CELLULOSE ACETATE
Turning now to cellulose acetate, there are two main grades of interest to the plastics industry. The earliest form used was based upon an acetylation to 52 to 54 per cent combined acetic acid content, corresponding to a substitution of 2.2 and 2.35 of the three hydroxyls on each anhydroglucose unit. This is in the range where cellulose acetate shows its lowest melting point. All of the early varieties of cellulose acetate plastics were based on this material, and today it still comprises from 50 to 60 per cent of the material sold to the industry. To obtain better water resistance, a new grade of cellulose acetate of combined acetic acid content ranging from 56 to 58 per cent (2.5 to 2.7 hydroxyls substituted per anhydroglucose unit) has been developed. The effect of degree of acetylation upon the properties of cellulose acetate plastics will be discussed briefly. Figure 2 indicates the variation in flow Qemperature(5) with degree of acetylation of a series of cellulose acetate samples, of D.P. between 275 and 295, compounded with several d i e r e n t ratios of mixed phthalate plasticizers. Figure 3 shows the same set of plastics for which a hardness measurement on the Rockwell machine ( 1 ) is compared with degree of esterification.
u Y
5 300 W
U
Figun 3.
I
2.0 2.2 2.4 2.6 2.8 NUMBER OF HYDROXYLS SUBSTITUTED
Effect of Substitution of Cellulose Acetate on Hsrdn... of Plastic.
Plasticized with 1: 1 dimethgl m d didhyl phthdatea
280
3
9 2601 2.6 2.8 2.0 2.2 2.4 NUMBER OF HYOROXYLSSUBSTlTUTEO
.,
F i g 4 EffBct of Substitution of Cellulose Acetate on Flew Tempe..*".. I t Con.t.nt PI-tic Hudne.. D.P. of cdlulacetats = 275 to 295. PI-tid~ed with 1: 1 dimethyl and diethy1 phthdstea to give the h..dn.horn ,.
. 3
As regards the degree of substihtion and the d i s . . tribution of substituents in cellulose acetate, it must be remembered that, in commercial acetylation, the material is taken first to the triacetate stage, and the cellulose goes from a fibrous stage into a smooth solution. The degree of acetylation of the finished cellulnse acetate is controlled by hydrolyzing this triacetate in an^ acid medium back to the desired degree of substitution. Since this hydrolysis is curied out in a homogeneous medium, the product resulting from the permutoidtype reaction will show a distribution of substituents along the chain falling in line with Spurlin's predict,ion 117). ,Figure 1 also shows a portion of a cellulose chain. acetylated to a substitution of 2.4. One factor, complicating this admittedly overshnplified picture, is the effect of the hydrolysis catalyst, usually sulfuric acid, upon the course of the hydrolysis. A great excess of sulfuric acid as the catalyst would be expected to form n certain amount of sulfate ester; i t might be expected that this ester group would hydrolyze off faster from primary groups than secondary hydroxgls and t h u ~in\-
e
HARDNESS 3 0
P
I
HARDNESS 40
MAY, 1947
217
ening and hardening types of plasticizers. The drop upon successive re-injections is still less. Present practice, where reground stock is used, is, largely, to mix 20 to 40 per cent of reground stock with fresh molding powders; this practice is seen to he technically sound. TABLE 2 Effect of Molding Operations on Degree of Polymerization of Cellulose Acetate Plastics* 2.0 2.2 2.4 2.6 2.0 NUMBER OF HYDROXYLS SUBSTITUTED
nmre5.
E E ~ C of ~substitution of c.11~1Impact strength of Plastis.
Pl-tidzed
with 1: 1 dimethyl and di.th,.l
~ ~ . t . t .on
phthdat..
fluence the course of the subsequent hydrolysis. However, published data indicate that commercial cellulose acetate shows only slightly more than one-third of the free hydroxyls (6,9)in the primary or &position in the anhydroglucose units. It appears, therefore, that variations caused by these differences in the properties of cellulose acetate used in plastics are but slight. Chain length of the cellulose derivative is of especial significance in the acetates used for plastics. Materials offered commercially for this purpose range in D.P. from 240 to 360. The usual injection-moldmg powders are based upon cellulose acetate of D.P. ranging from 255 to 300, while some of the materials of high acetylation and special products of unusually high viscosity may show D.P.'s as high as 350. The latter are used where tdughness is of greatest interest, as in the highimpact grades of molding powder. Figure 6 indicates how flow temperature, hardness, and impact strength vary with the degree of polymerization of the cellulose acgtate for materials of 53 per cent combined acetic acid content (2.2 to 2.3 acetyl groups per anhydroglucose unit). The effect on hardness is probably less noticeable in actuality than it appears to be by measurement; however, with this formula ove? the range of polymerization studied, the hardness appears to increase from 38 to 49 (Rockwell M Scale) as the D.P. rises. The increase in flow temperature for the formulation presented is of great significanceand indicates a definite linear rise in flow temperature with polymerization. The effect on impact strength is most interesting and characteristic; there is a definite falling off in impact strength below a D.P. of 240, with only a slight rise above that chain length. The existence of a definite break point in toughness or flexural strength as the chain length decreases below a certain value also has been observed for cellulose nitrate films CIO), omegadecanoic acid polymers ( 5 ) , and other materials. Another characteristic of much technical interest is the extent to which the degree of polymerization is lowered by the molding process. Table 2 indicates how materials plasticized with various common plasticizers behave with respect to loss of D.P. upon molding. It is seen that with the present-day materials, this amounts to scarcely 10 per cent of the total with both the tough-
UPONMOLDING INTO PLASTICS The fist three formulations contain toughening plasticizers, the last two hardening plasticizers. D.P. of D.P. of original D.P. i n injectismPlastic formulation, cellu2ose molding molded pw cent compositions acetate puwder material 75 Cellulose acetate 18.75 Diethyl phthalate 290 6.25 Dimethyl phthalste 72: Cellulose acetate 300 21 Diethyl phthalate 7 Dimethyl phthalate Cellulose acetate 74 21 KP45 (diethylene glycol dipropionrate) 300 5 Triphenyl phosphate Cellulose acetate
266
272
292
288
...
292
'
* Csloulated from absolute viscosity measurements upon 3 per cent cellulose acetate solutions in acetone, using Msstin's equation (15) and Kraemer's constant of-230. A further interesting illustration of the nay in which the properties of cellulose acetate plastics can be improved through control of the degree of polymerization lies in the recently developed series of cellulose acetates found to produce improved impact strength a t low temperatures. One of the most often-cited limitations of cellulose acetate plastics is that, a t temperatures below O°F., the high impact strength exhibited a t normal temperatures becomes progressively less as the temperature falls. Through the development of acetylation methods which preserve the chain length of the cellulose acetate to an extent not previously thought possible, it 5
I:
Z
5
", = -
g 3
; . X 2-
6s
2.
zoo-
i.0-
z y . > B P o 2 01
; =
"
PO0
0
2.0
O P 01 C U l N LENGTH
320 0 a0 OF CLLUILODC ACETATE
I
Figure 6. Variation in Properties of Cellulosa Asetato Plastic with Chain Lensth of Flake Used Substitution of s d l u l o ~ eacetate = 2.3 to 2.4. Plasticimsd with 31 per ecnt 1 : l dimsthyl and diethy1 phthdlt..
JOURNAL OF CHEMICAL EDUCATION
218
has been found that the material produced by this kind of reaction has a substantially bett,er impact strength a t subzero temperatures than any acetate used commercially. Figure 7 contains data showing the impact strength found when a cellulose acetate of 2.25 to 2.4 substitution was compounded with phthalate plasticizers to give a relatively hard formulation. Data are presented from the standpoint of the degree of polymerization of the original cellulose acetate used in preparing the plastic and of the chain length remaining in the molded piece after injection molding. It appears that the impact strength of the ordinary materials can a t least be doubled a t a temperature of -40°F. simply by using the higher-viscosity cellulose acetate material. The data in Table 3 show that the effect persists even though the plasticizers used are of the stiffening variety. It is also evident that a slight improvement LOW-~emperature Impact Strength of Cellulose Acetate (2.3 Substitution) Plastics D
orzgt
Plastic jonnulation, per cent compositions
cellulose acetate
75 Cellulose acetate 18.75 Diethyl phthalate 450 6.25 Dimethyl phthalate Same formulation 300 72 Cellulose aeetate 21 Diethyl phthalate 490 Dimethvl ~hthdate 7 .. Same formulation 300 74 Cellulose acetate KP45 (diethylene glycol dipro21 pionate) 490 5 Trinhenvl nhosnhate Same formulation 300 74 Cellulose acotato 21 Triaeetin 490 5 Triphenyl phosphat,e Same formulation 300
.
"
.
.
A
.
Same formulation
P
3W
Izod i m p 1 at -40 F., ft.-lb./in. of nolch 0.42 0.25 0.59 0.25 0.69 0.27 0.31 0.18
0.21
in the heat resistance of plastics based upon t,his material manifests itself, as is indicated by the flow temperat.ures and heat distortion temperatures of the materials. This is also consistent with the effect of chain length upon flow temperat,nre presented in Figure 6. In considering the colloidal aspects of cellulose acetate as they affect plastic properties, it is found, first, that they affect the preparation of the plastics. The two main processes in use involve either (1) the preparation of paste with solvent in a jacketed, sigmabladed, heavy-duty mixer, followed by rolling to sheets, filtering and block-pressing, extruding or granulation into molding powder, and drying, or (2) the hot-colloiding of the flake in a Banbury-type mixer or on hot rolls. With the first method, the density of the flake may have some bearing upon the uniformity of the gel obtained, and it is necessary to precipitate a light, porous particle of uniform density. The conditions of the aggregationdisaggregat,ionequilibrium pointed out by Doolittle (7) apply here, as aell as do the facts about the effect of film-casting conditions upon strength of film, pointed out by Russian workers (16). In addition to these considerations, the hot process requires an optimum particle size to insure rapid penetration or diffusionhy the plasticizer into the individual particles. The data given in Figure 5 also are of interest in this connection, since they illustrate the effectof the affinity of the plasticizer for the cellulose chain upon the strength of the resulting plastic. It is seen that, a t a constant ratio of plasticizer to acetate, the impact strength drops off sharply above.@substitution of 2.65. This may be interpreted to mean that some attraction, of the order of a hydrogen bond, exists between the polar portions-that is, carbonyl groups and the like-of the plasticizer molecules and the hydroxyl groups which stud the cellulose chains until a point is reached where the chains have a greater tendency to stick to each other than to stick to the plasticizer. It has been shown by Fuller and coworkers (4) that materials approaching the triacetate have a greater tendency to form crystalline aggregates than those of lower suh, stitution; from the practical, macroscopic standpoint, the unwillingness of these plastic mixes to absorb plasticizer may well be explained by this sort of mechanism. The fact, emphasized in connection with Table 2, that the slight loss in polymerization upon molding is practically the same with both hardening- and toughening-type plasticizers again indicates that the difference in action of the two probably resides in their molecular structure, which is lightly bonded to perhaps several chains. In this connection also, one explanation of the effect of longer chains in enhancing low-temperature impact strength may be advanced. It is well known that the degrees of polymerization, usually expressed by intrinsic viscosities, represent an average chain length with individual fractions distributed over quite a wide range of D.P. It would be expected, in such a material, that the plasticizer may completely dissolve the shorter-
MAY, 1047
length fractions, while it may only solvate or incompletely swell the longer chains. In the usual material of average degree of polymerization of about 300, a greater amount of this low-weight, presumably soluble material would he expected to be present than in the highstrength material of average degree of polymerization of about 500. Upon cooling, the solutions of the former would be expected to become quite rigid, while in the latter material, with less of this interchain solidified gel, the solvated chains would preserve their flexibility down to lower temperature levels. The slight improvement in high-temperature properties may well be explained upon the basis of less thermal agitation in the longer chains than in the shorter ones as the temperature increases. With respect to purity, it has been stressed many times in the literature (14) that combined sulfate esters or partial salts thereof are harmful to the heat stability of cellulose acetate intended for hobmoldimg use. It is one of the characteristics of all plastic-grade acetate that a low content of sulfate, either free or combined, be present. Acidity of any kind, in fact, is harmful, although in controlled quantities on certain types of acetate it may be added to enhance the color stability of the molding material. It is generally known that a material with more acid present than allowable often molds to give a very light color (18)hut produces a yery brittle, degraded product. On the other hand, addition of basic ions, such as those of the alkali metals, to.an acid product will produce a material which, while remaining strong, will discolor very badly when hotmolded. Production of the best pastel shades requires a cellulose acetate of extreme purity in these categories. The use of cellulose acetate plastics in injection molding also requires that the material be essentially of t h e same degree of substitution; mixtures of materials of widely varying acetylation cause difficulty in the preparation of homogeneous molding materials and also tend to produce more hazy plastics. A furth'er point to be considered under this subject of purity is +he incompatibility of the various cellulose derivatives, such as acetate with acetate butyrate, ethyl cellulose with acetate, etc. Contamination of one material with small quantities of another may occur in a molding shop unless particular care is taken to see that i t does not. It might be concluded, from experience with solutions where a small amount of one material may be mixed with another to form an apparently homogeneous sol, that this would not matter; in practice, however, it must be avoided, as the small quantities of plasticizer present in plastics are definitely not sufficient to bring two dissimilar materials into a homogeneous condition.
219
ing to 0.95 acetyl group and 1.6 butyryl groups per anhydroglucose unit. The same considerations discussed under acetylation apply as well to this material. The hydrolysis is somewhat complicated by the fact that the acetyl groups are hydrolyzed much faster than the butyryl groups introduced during the process of making the mixed triester. The degree of polymerization of cellulose acetate butyrate used in plastics is ahout 350. The effect of chain length upon plastic properties appears to be about the same as with cellulose acetate. One or two novel points arise in considering the colloidal aspects of this mixed ester. Since in this material the proportion of cellulose residue, exclusive of hydroxyl, is only 35.1 per cent as contrasted with 42.2 per cent in the usual plastics grade of cellulose acetate, it is readily understood that the material itself is essentially softer than the acetate. As a consequence it will require less added plasticizer to reach a given degree of plasticity, as measured, for instance, by the flow temperature. Since the chains are separated by a greater distance because of the bulkier butyrate groups, it is also readily understood why the material can be softened by plasticizers with bulkier substituents, such as diamyl or dibutyl phthalates, while in the cellulose acetates, only groups not larger than those of diallyl phthalate can be accomodated. Another influence of the colloidal properties ofcelluloseacetate hutyrate may be related to degree of crystallization in the molded state. Baker, Fuller, and Pape (4) have shown that, while both cellulose triacehte and tributyrate can be quenched to an amorphous condition by fast cooling of a melt and the former may be chaliged into a Crystalline condition either b y hobannealing or when exposed to moisture, the -latter changes only when exposed to moisture. This factor doubtless influences the favorable behavior of both the acetate hutyrate and some of the harder plastics based upon the higher-substituted cellulose acetate when exposed to humid, tropical conditions. Regardimg purity, the same conditions mentioned for cellulose acetate apply to this ester with even greater force; slight amounts of acidity, for example, may cause odor development during molding.
ETHYL CELLULOSE The only cellulose ether of commercial importance in the plastics industry is ethyl cellulose. Two grades are most frequently used: one has a degree of substitution of 2.21 to 2.28, corresponding to an ethoxyl content of 44.5 to 45.5 per rent; the other, of ethoxyl content ranging from 46.8 to 48.5 per cent, contains from 2.37 to 2.50 ethoxyls suhstituted per anhydroglucose unit. CELLULOSE ACETATE BUTYRATE However, commercial practice in this n e d y begun use The mixed ester, cellulose acetate butyrate, is largely of cellulose ethers in hot-molding plastics has not yet used in injection-molding and hot-extrusion types of become well standardized. This is illustrated by the plastics where its ready moldability in large sections is fact that some users prefer material of a substitution of particular value. The material used commercially even lower than that cited, while others use material has an acetyl content of about 13.5 per cent and a intermediate in substitution to those quoted. Matebutyryl content of ahout 36.5 per cent (S), correspond- rials vith a substitution of 2.3 to 2.5 are in the minimum
JOURNAL OF CHEMICAL EDUCATION
220
Figure 8.
Effoct of Substitution of Ethyl Cellulose on P..partien of Plastics
D.P.
of ethyl celluloee = 540 t o 600. Pllatic composition contains 84 pep cont ethyl cellulose and plastici=e? end ?..in.
melting point range of ethyl cellulose. Figure 8 indicates the effect of ethylation upon the flow temperature of a typical ethyl cellulose plastic composition where the only variable is degree of substitution. The same figure shows similar data for hardness and impact strength. I t is seen that the commercial materials (2.21 to 2.50 substitution) cover the minimum range of the flow-temperature curve and also include the maximum range of impact strength. The latter observation should not be taken as final, however, since molding conditions, under which these particular plastics were held a t 100 to 125'F. higher than for the flow temperature measurements, may not have produced sufficient fusion on the high and low ends of the ethoxyl scale to have produced the maximum fusion. There does not seem to be the sharp falling off in impact strength as the degree of ethylation increased, as was observed with ~elluloseacetate. This point will be considered more fully when the colloidal effects are discussed. Regarding chain length, that of the ethyl cellulose used in plastics is in the range of D.P. 540 to 600, which is somewhat longer than that of the plastic-type cellulose acetates. Properties of ethyl cellulose are strongly influenced by the chain length used, as is indicated by Figure 9. In this figure are shown the variation in flow temperature, hardness, and impact strefgth for plastics containing the same proportion of ingredients as shown in Figure 8, but based upon ethyl cellulose of substitution 2.21 to 2.28 with the chain length variations as indicated. It is seen that, as mit,h rdli~loseacetate, the
-
6
IMPACT. 70.r
J/ MO
L10
-
-
D.P. oj original elhyl Charpy, Charpy, Izod, cellulose 7 7 ° F . -40°F. -40°F.
Material
F"ky$$5bsubst,itution
840 555
6.8 6.6
3.4 2.2
3.1 2.1
600
9.1 7.1
3.4 2.3
1.9 1.4
8.1 7.5
3.3 2.2
2.6 1.3
7.9 6.3
3.0 2.2
2.7 2.0
:. ..
Formula A: 2.35 substitution
Formula.B: E/C,2.35 subst,itution
IMX)
600 lWO 600
F0;7;1&2:5 Fmmulas: Ethyl cellulom Plasticizers Stabilizer
.
.,
.,
A 84 16 0.25
B 87 13 0.25
C 95 5 0.25
400
300
-0
chain
r00
celluloseon
~ f f e c of t h n g t h of ~ t h y l Properties of PI-tics
-
MATERIAL: Ethyl ocllulaso of 2.35 substitution, 2.14 intrinsic viscosity. FORMULA: 84 parts ethyl cellulose, 16 parts plasticisers, and st,abiliaer as shown below per 100 parts of plastic. Dala With 0.6 part Wilh 1.0 part stabilizer stabilizer
*
DP OR CHAIN LENGTH OP EIHYLCEUULOSE
rig"r. 9.
Im.paet slrenglh, jt.-lb./in. of notch
~
8.0
* $'
TABLE 4 Illnuen=* of Chain Length of Ethyl Cellulose on LowTemperature Impact Strength
TABLE 5 P e ~ m k n c eof Chain Length of Ethyl Cellulose during Suceessi-re Re-injections of Ethyl Cellulose Plastics aao 9
4
flow temperature and impact strength are strongly dependent upon the chain length of the ethyl cellulose used, nith hardness rising only slightly as the degree of polymerization increases. As with cellulose acetate, the low-temperature impact strength of ethyl cellulose, already at a high level in the customary materials used in plastics, can he further enhanced by increasing the chain length of the ether. This is illustrated by Table 4 for material of 2.25 and 2.35 substitution. A further point of interest lies in the extent to which ethyl cellulose can retain its chain length and impact strength upon successive re-injection. As shown by the data of Table 5, three successive re-injections of the entire plastic fail to reduce the chain length or impact strength below a useful level; hence, the re-injection of portions of material with fresh plastic appears to be sound practice with this cellulose ether as it was with the rellulose esters. Turning now to the colloidal properties of ethyl cellulose, a diversity of materials are found to be used as plasticizers because of the excellent compatibility of ethyl cellulose, especially in the ethoxyl region near the
suhtitutions callulose 2.a5. pleetie composition oontailu 84 per cent -thy1 cellulose.
Properlies i n mlding powder Origind ethyl cellulose After 1st injection After 2nd injection After 3rd inject,ian
.
D.P. 640 580 580 576
Charpy impact, 77-F.
... 7.8 5.5 5.6
D.P.
Charpy impact, 77°F.
600 592 576 580
8.2 5.3 5.4
...
MAY, 1947
minimum of the melting-point or flow curve, with many classes of high-boiling organic compounds. These include (1) esters which produce rather soft, readily flowable materials and (2) hydrocarbons, both aromatic and aliphatic, which also produce flowable plastics but of somewhat increased rigidity. The material of higher ethylation, being more oil-sensitive and of high flow temperature, is seldom used. Material of the lower ethylations, down to 2.21 ethoxyl, is most often used with ester plasticizers or with mixtures of esters and hydrocarbons. A slight advantage in impact strength a t low temperatures is found by using material of 2.4 to 2.5 substitution, perhaps because of the better solubility of these materiak in most plasticizers. This fact may also explain the poorer impact strengths noted in Figure 9 for the materials in the extreme ranges of ethoxyl conten6 shown. With respect to purity, the ethylation process makes no use of sulfuric acid, as do the esterification procedures; however, a d i e r e n t set of problems are encountered. This material must be carefully protected from oxidative degradation both during processing and molding; this involves the removal of metallic ions, which might catalyze oxidation during the purification, as well as the proper protection of the material during molding by means of common antioxidants, such as diphenylamine or menthylphenol. Such materials also serve to eliminate acidic products of heat degradation which might also adversely affect the properties of the material. Another factor which must be watched for during processing is that the batch of ethyl cellulose is all of the same degree of ethylation, since inclusion of a material of much hiaher or lower ethylation than the average of a batch will tend to give fused aggregates during processing of the 5ake into molding aggregates and also during its molding. SUMMARY
In summary, this paper gives a discussfon of the importance of four of the factors found to be critical in the study of cellulosic materials used in plastics. These factors are degree of esterification, degree of polymeriza-
tion or chain length, colloidal properties, and purity. The influence of these factors has been illustrated by practical examples taken from studies involving the use of cellulose esters (the nitrate, acetate, and acetate butyrate) and of the ether, ethyl cellulose, in hobmolding plastics. ACKNOWLEDGMENT
It is a pleasure to acknowledge the assistance of Messrs. W. 0.Bracken, F. E. Piech, and Wayne Schrag of the Hercules Parlin Plant staff in obtaining much of the data upon which the examples of this paper were based. LITERATURE CITED (1) A.S.T.M. Designation D229, Am. Sac. Testing Materiab. Standards on Plastics, p. 294 (Oct., 1943). (2) A.S.T.M. Designations D25643T, ibid., p. 212 (Oct., 1943). (3) A.S.T.M. Designation D569, a d . . D. 197 (Oct.. 1943). (4, . . BAKER.W. o..c. S. FULLER; AND N. R. PAPE.J: Am. Chem. S O C . , ' 776 ~ ~ ,(1942). %bid.,55, 4717 (5) C ~ O T R E RW. S , H., m n F. J. VANNAT~A, f193R). ~-...,.
(6) CRADIER, F. B., AND C. B. Pmrns, i W . , 61, 3458 (1939). (7) DOOLITTLE, A. K., Ind. Eng. Chem., 36, 239 (1944).. C. R., AND L. W. A. MEYER,ibid., 32,1053 (1940). (8) FORDYCE, (9) GAUDNEE,T. S., AND C. B. PWVES, J. Am. Chem. Sac., 64, 1539 (1942). (10) GWOR,W. E., Ind. Eng. Chem., 27, 1162 (1935). (111 For examole. see W. E. GLOOR."Cellulose and cellulose derivatives," in "High ~o~ymkrs,"Vol. V, Interscience Publishers, he., New York, 1943, pp. 1095-6. (12) KRAEMER,E. O., Ind. Eng. Cha.,30, 1200 (1938). J. F.,AND C. B. PmVES, J. Am. Chem. So&, 64, (13) MAHONEY, 9 (1942). '!Cellulose and cellulose M u , C. J., AND C. R. FORDYCE, derivatives,".in "High Polymers," Vol. V, Interscience Publishers, Inc., New York, 1943, p. 675. MARTIN, A. F.,Am. Chem. Soc. Meeting, Memphis, Tennessee, April 20-24, 1942. MOIBEEV,A. A., AND Z. A. ROQOYIN,J. Applied C h . U.S.S.R., 14, 579-86 (1941). S P ~ I . ~H., M., J. A n . Chem. Sdc., 61, 2222 (1939). STERN,R. L.,U. S. Patents 2,280,863 and 2,286,041 (issued April 28,1942 and June 9,1942, respectively, and assigned to Hercules Powder Company). R, of ExplcVANGELDER,A. P., AND H. S C E L A ~"History sives Industry in America," Columbia University Press, New York, 1927, p. 785.
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