Surf actant-Treated Minerals as Reinforcing Fillers ALAN S. MICHAELS Massachusetts I n s t i t u t e of Technology, Cambridge, Mass.
T
The results of this work have led to the conclusion that treatment of mineral fillers with surfactants improves the properties of filled resinous compositions by facilitating dispersion of t h e solid in the resinous matrix, but may weaken compositions by reducing the strength of the resin-to-filler bond if used in excess, or if added to systems in which the filler disperses readily without surface modification. Surface treatment of mineral fillers appears to have great promise for permitting production of very highly filled useful plastic products; in this application, surfactants are believed to enhance the uniformity of distribution of the resin on the filler particles by rendering the solid preferentially resinwettable. Precoating of filler with resin also facilitates densification of the composition under heat and pressure, and leads to development of higher strength and abrasion resistance. The use of finely divided mineral solids-e.g., clay, limestone, silica, etc.-as fillers for plastics, resins, and elastomers enjoys wide popularity as a means for increasing the elastic modulus, reducing plasticity, increasing thermal stability, and reducing the cost of many plastic products. Most of these fillers, however, fail to make a substantial contribution t o the cohesive strength and tear and abrasion resistance of the resins in which they are incorporated, in many cases they actually have a detrimental effect on these latter properties. In certain instances a filler is found which, in combination with a given resin, yields a product of superior physical properties, particularly with regard t o tensile strength and abrasion and tear resistance. Such a solid is termed a “reinforcing filler,” of which the most often-cited example is carbon black in combination with natural rubber, butadiene styrene copolymers, and a limited number of other polymers of predominantly hydrocarbon character. The unusual feature of the phenomenon of reinforcement is that the process is highly specific and selective: that is, a
H E object of this investigation was to determine the effect of treatment of the surfaces of certain mineral fillers with selected surface-active compounds upon the physical properties of resinous or elastomeric compositions containing such fillers. Mineral solids used were kaolinite and wollastonite. Surfaceactive agents employed were a fatty quaternary ammonium salt, and a fatty amine, Polymers used were GR-S, a poly(viny1 chloride)-vinylidene dichloride copolymer, plasticized polystyrene, and an epoxy resin. Compositions were prepared by suspending the filler in water; adding the surface-active agent; adding the resinous material as an aqueous emulsion; coagulating the solids, filtering off the water, and drying; and compressionmolding and/or -curing the product a t elevated temperature and pressure. Limited surface treatment of the filler in compositions containing 50% by weight of GR-S produced under certain conditions as much as 60% increase in tensile strength and 70% increase in 100% modulus. Excess treatment with surface-active agents resulted in a reduction in strength and modulus. Effects were more marked with kaolinite as the filler than with wollastonite. Similar tests on compositions containing 50% by weight of poly(viny1 chloride) showed a loss in strength owing to surface treatment, along with a decrease in modulus and an increase in ultimate elongation. Proper surface treatment of kaolinite with a fatty quaternary ammonium salt (Hyamine 1622) was found t o permit the preparation (by the above method) of a dense, tough, and strong oomposition containing only 10% GR-S by weight. The tensile strength of this product was over twice that obtained with a similar composition prepared with untreated filler, and its abrasion resistance, three times greater. Treatment of the filler with excess surface-active agent was found to cause reduction in both strength and abrasion resistance. Attempts to produce kaolinitepolystyrene products containing only 10% resin revealed that, whereas surface treatment of the filler with Hyamine 1622 reduced tensile strength, such treatment improved abrasion resistance and greatly improved retention of strength after water immersion. Further study of treated kaolinite-polystyrene formulations, in which resin content was varied from 10 to 25% by weight, revealed that maximum strength was obtained a t a resin content of about 20% and that strength is extraordinarily sensitive t o the degree of densification of the compound (obtained during molding) a t the lower resin contents. The results indicated that, by more effective densification techniques, very strong products might be obtained which contain as little as 12% resin. Epoxy resin-kaolinite products containing 10% resin were prepared, employing both surface-inactive (diethylenetriamine) and surface-active (dimethylaminomethylphenol) compounds as resin curing agents. Use of the former curing agent led t o the formation (on oven curing) of a porous, ceramiclike product of relatively high strength (- 1000 pounds per square inch) whose properties were not improved by compression molding. The latter curing agent led t o the production of a water-repellent powder which compression molded easily t o yield a dense, nonporous solid of very high (- 1600 pounds per square inch) strength and good resiliency.
Better plastic and elastomeric materials for building uses can be developed by pretreating their mineral fillers with surfactants:
...GR-S and polystyrene compounds show improved strength and abrasion resistance
...resilient yet
highly filled plastics become practical because of better distribution of the resins
...plastics
containing 50% filler (where untreated filler is difficult to disperse) show added strength and stiffness
297
298
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 2
to-matrix adhesive force is large, the filler volume loading is low (so that there is essentially no contact between filler & . particles), the ultimate strength of the 2 1200 compound (based on rupture cross sec85 tion) should be equal t o (or slightly eo0 greater than) that of the pure matrix. If, however, the adhesive force is large, k 400 and the volume loading of filler is high enough so that interparticle contact i,+ frequent, then the u1t)imate strength of the mixture may be primarily determined by the magnitude of the interfacial (filler matrix) area per unit volume of mixture; this quantity is roughly proportional t o the volume loa.ding of filler, and inversel;). proportional t o the filler particle diarneter. Under these circumstances, tlic matrix serves more as an adhesive for ’. the filler particles than as a suspending r; 4 medium, and inasmuch as the strength of 4 thin adhesive bonds frequently can ex3 ceed greatly that of bulk adhesive (1, 10, 11), the ultimate strength of such n g mixture may, under appropriate con0 i0Ol I I I I 0, ditions, far exceed that of the ma0 5 /O 15 20 25 M E 4 PRIM4C / I O 0 G F I L L F R M E 4 HYAMINE / I O 0 E FILLER trix. Figure 2. Physical properties of 5070 Figure 1. Physical pmperties of 5Oq‘ I n the foregoing, it has been assumed filler-5Oqo GR-S compositions filler-5OqO GR-S compositions that the filler is dispersed in the maHyamine 1622-treated filler Primac-treated filler trix as discrete mrticles. If, on the other hand, the filler is present as agglomerates of particles, the strength of such a composition vi11 be determined by the relative magmgiven filler may exert a pronounced reinforcing action on one tudes of the strength of the agglomerates, the Ptrength of adheresin, but not, on others, and one form of the same filler may he sion of the matiis t o the surfaces of the agglomerates, and the far less effective in reinforcement than another. bulk strength of the matrix. Fine particle agglomerates usually Research on the reinforcement of rubber and related substances have low cohesive strength, so that in all probability failure nil1 with carbon black has been extensive, and much information has occur first within the agglomerates. Such failure, coupled v itf. been accumulated regarding the mechanism of and factors affectthe fact that particles within the agglomerates are not in contact ing reinforcement in these systems ( 7 , lg-14, 16). The major with the matrix, would be expected t o lead to a composition whose carbon black properties influencing reinforcement appear t o be strength nmuld be lower than one in which the same filler n-as particle size, degree of particle dispersion in t’he polymer matrix, completely dispersed. and particle eurface characteristics. I n general, very small Many commonly employed mineral fillers, which have no particle size, high degree of particle dispersion, and high adsorpsignificant reinforcing ability, are of small enough particle size tivity of the solid for the polymer substance, favor high reinforcet o be potentially capable of reinforcement. The major obment. Despite the large amount of n-ork done on carbon black, jects of this investigation m r e t o select a fen- of these minrelatively little study has been made of the other filler materials, erals and, by appropriate surface treatments, t o attempt t o although investigation of calcium carbonate (8)!colloidal silica, convert them into reinforcing fillers in combination with a few and amine-treated montmorillonoid clays ( “Bentones”) as fillers selected resins: t o determine by what mechanism surface treatlor rubber and other hydrocarbon polymers (J,16) has indicated ment alters the reinforcing ability of such fillers; and t o demonthat the same generalizations apply to the reinforcing action strate the possibility of preparing verv highly filled composiof these mineral fillers as t o carbon black. tions with excellent physical properties by appi o p i a t e modificaThe effect of incorporation of a filler (be it a reinforcing filler tion of the filler particle surfaces. or not) on the physical properties of elastomeric or visco-elastic substances can be partiall>-explained as follon-s: If part,icles of PNTER\IEDIATE FILLER LQADIRGS WITH high elastic modulus are dispersed through a Ion- elastic-modulus COIBTIRUOUS RESHU PH4SE matrix, it is obvioiis that the modulus of the illistiire ( a t low When a finely divided mineral solid is blended with Theory. stresses) ill be higher than t,hat of the matrix, solely because of a resin, the physical properties of the composition can be exthe decrease in volume content of matrix substance. Hence, pected t o be controlled by the state of aggregation of the sohd the modulus of the composition would be expected to increase particles, and the strength of adhesion between solid and resin. rapidly wit,h the volunie loading of filler up t o fairly high filler If the filler is agglomerated, the composition n-ill in all probability loadings. The tensile strength of the material, however, will be relatively weak because of breakdown of agglomerates under depend on lactors other than mere fillcr volume, of which the most stress, and be relatively soft-i.e., have a low tangent modulus important may he the force of adhesion of the matrix t’o the because of the distortability of the agglomerates. Similarly, particle surfacee. If the adhesion is very lov-far lower than the the abrasion resistance of such a composition will be poor, since etrength of the matrix alone-then the particles ~vvlllbecome defiller agglomerates will “spa11 out” under abrasive action, leaving tached from the matrix at Ion- stresses, and the ultimate strength the pure resin behind. If the filler-resin bond is weak, this n-auld of t,he compound nil1 be roughly equal to the product of the t,ensile be expected t o yield a compound of lor\- Etrength and abrasion strength of the pure matrix and the fraction of the rupture cross resistance, and t o result in a rather Ion modulu? of elasticity, section occupied by continuous matrix suhstance. XI the particle1600
i
Y,
2
B
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1956
c:
299
3000
Gi
2000
L
l
l
m 1 I w1 I
9 000
I
1
b -KAOL/NfTEr
d
$-
sE
2000
2
IO00
k
0
0 -WOLLASTONITE
I
0
2
4
8
8
10
MEO. ~ Y A ~ / N io0 E G. F U E R
0
1
8
4
MEO.
,
/2
PRIMAC / 100 G.
I
I6
20
FILLER
Figure 3. Physical properties of 50% filler-50Y0 poly(viny1 chloride-vinylidene dichloride) compositions
Figure 4. Physical properties of 50% filler-5Oq0 poly(viny1 chloride-vinylidene dichloride) compositions
Hyamine 1622-treated filler
Primac-treated filler
since detachment of resin from filler under stress would result in the applied load being carried by the low modulus resin alone. Both the state of aggregation of the filler, and the strength of the filler-resin bond, are determined by the forces acting at the filler-resin interfaces. Most mineral fillers are ionic crystalline solids with very intense residual fields of force present on the particle surfaces. Such substances have a high adsorptive affinity for water and other polar compounds, and disperse re1ativeI.i. easily in high polarity liquid media. While they also have a relatively high adsorptive affinity for low polarity compounds, they will not adsorb such substances in the presence of water (or other polar compounds). Furthermore, they do not disperse readily in low polarity liquid media, largely because their residual surface forces are better satisfied by interparticle cohesion than by adhesion t o the surrounding medium. If, however, such solids are dispersed in water, and treated with appropriate surface-active compounds which bond more firmly t o the solid surface than water itself, i t is possible t o render t o solid organophilic, and thus t o enhance materially its dispersibility in low polarity organic media. For example, most natural silicates and aluminosilicates behave as anionic colloids in aqueous dispersion under normal conditions. When small quantities of cationic surfactants-e.g., fatty amine or quaternary ammonium salts-are added t o such dispersions, they are adsorbed almost quantitativeIy by the solid, cause flocculation, and render t h e solid preferentially oil-wettable and -dispersible. Materials treated in this fashion should, therefore, disperse far more readily in low polarity resins than their untreated counterparts. Clearly, however, treatment of this sort is bound t o reduce the residual force-field intensity at the particle surfaces, and thus reduce adsorptive affinity for both polar and nonpolar
24
compounds. Reduction in water adsorption is, of course, desirable from the standpoint of filler action, since this tends t o minimize interference of water with resin-to-filler adhesion. On the other hand, an across-the-board reduction in adsorptivity is undesirable, as this tends t o reduce the adhesion between resin and filler. One would anticipate, therefore, that surfactant treatment of a mineral filler would enhance the properties of a composition in which poor properties were due to inadequate particle dispersion or presence of adsorbed moisture, but would worsen the properties of a composition under conditions where the major effect of surface treatment was t o weaken resin-tofiller adhesion. The latter effect would be expected to predominate with a relatively high polarity resin in which the filler disperses readily without surface treatment. After the filler has been treated with an appropriate surfaceactive agent, it becomes necessary to select a technique of incorporation of filler with resin which takes maximum advantage of its improved dispersibility in the resin matrix. Inasmuch as the mineral solid is believed t o be most effectively surface treated in aqueous suspension, it appears logical t o blend it (after treatment) with the desired resin while it is present in an already well-dispersed state in water. Such blending can be accomplished by adding t o the aqueous filler dispersion an aqueous resin dispersion-i.e., a latex or emulsion. If coagulation of either the resin or the filler can be prevented during this blending operation (or, if this proves impossible, if the blending of the two components can be carried out very rapidly with strong agitation) the resulting suspension will contain an intimate mixture of resin and filler particles, the homogeneity of which will be dependent primarily upon the relative sizes of the two particulate phases. [For example, if the resin globules are 0.1 micron in diameter, the filler particles are 1 micron in diameter, and 20
300
INDUSTRIAL AND ENGINEERING CHEMISTRY
volumes of resin are added per 100 volumes of filler, the final mixture will consist of suspension in which each filler particle is surrounded (on the average) by 200 resin particles.]. Addition of a coagulant to this system will result in both the deposition of resin particles on the surfactant-treated filler particle surfaces, and agglomeration of the resin particles, the former process presumably predominating under conditions where blending is excellent and the filler concentration is high. ( I n the example cited, 200 resin particles would be more than adequate to cover completely the surface of a filler particle.) I n such a case, the coagulum so obtained would be expected to consist of a filler resin mixture of extraordinary homogeneity, n-hich would require very little in the way of additional mechanical manipulation in order to disperse the filler as highly as possible in the resin phase. Clearly, however, the accomplishment of this objective is dependent upon the avoidance of premature coagulation of either dispersed phase during blending, since this Tvould yield a grossly inhomogeneous final coagulum. This technique of blending filler and resin has been used throughout this investigation with generally good results and little difficulty; with very highly filled products, where blending of filler and resin by ordinary methods (roll or Banbury milling) is out of the question, the technique has been a virtual necessity. Experimental. To test the above hypothesis, a series of compounds was prepared ( 6 ) employing two resins, a butadienestyrene elastomer and a poly(viny1 chloride)-vinylidene dichloride copolymer; two mineral fillers, kaolinite (R.T . Vanderbiit) and wollastonite (G. L. Cabot); and two cationic surfactants, Hyamine 1622 [(diisobutylphenoxyethoxyethyi) dimethylbenzylammonium chloride ] and Primae JMA-T (a branchedchain, primary fatty amine acetate), both products of Rohm & Haas, Inc. The compounds were prepared by dispersing the fillers in water with the aid of a small quantity of sodium tetraphosphate, adding the surfactant in prescribed amount with agitation, adding the resins as latices to the treated filler dispersion (along with compounding ingredients where required) with strong agitation, coagulating the mixture with acetic acid and filtering off tho coagulum, drying the coagulum a t moderate temperature, sheeting out the composition on a rubber mill, and molding and/or curing in sheet form in a standard vuloanizing press. Compounds were prepared a t 50% by weight filler loading and subjected to tensile test. The salient results of this study are shown in Figures 1 t o 4. Results and Discussion. K i t h the GR-S compounds, treat,ment of the fillers causes an initial increase in tensile strength n-ith increasing concentration of surfactant, followed by a decrease a t higher concentrations. Accompanying this effect are a n initial drop in ultimate elongation followed by a rise and an initial increase in stiffness (1007, modulus) followed by a drop. These trends appear t o hold for both fillers, and for both surfactants. Kaolinite appears to be the better filler for this elastomer, and the effects of surface treatment are much more marked for this mineral. There is also a significant difference in the action of the two surfactants 011 the fillers, Primae giving the stronger products, but IIyamiiie having a more pronounced stiffening effect. The maximum strengths are observed a t surfactant concentrations where maximum flocculation of the fillers in aqueous dispersion is found to occur. With poly(viny1 chloride)-kaolinite compounds, however, it is seen that surfactant treatment of the fillers causes reduction in strength, increase in ultimate elongation, and reduction in stiffness (initial tangent modulus). With poly(vinp1 chloride)wollastonite compositions, treatment of the filler with a small quantity of Hyamine 1622 appears to improve strength slightly, and to stiffen the product appreciably, whereas Primae does not exert this effect. At the higher surfactant concentrations, wollastonite and kaolinite exhibit the same trends. The noted differences in behavior between GR-S and poly(vinyl chloride) compounds can be explained, it is believed, in
Vol. 48, No. 2
800
600
400
2 MEQ
4 ffYAh4NVE / I O 0 C
6 FILLER
Figure 5. Physical properties of 90% kaolinite-lQ% GR-S compositions Effect of Hyamine 1622 Concentration
terms of dispersion of the filler and filler-resin adhesion. Visual examination of GR-S compositions containing untreated solids reveals that the fillers are poorly dispersed; the products are mottled in appearance, and discrete agglomerates of filler particles can be observed. Products prepared with surfactant-treated fillers become progressively more homogeneous in appearance as the surfactant concentration is increased. Yet poly(vinyl chloride) compositions containing untreated fillers are homogeneous, and surfactant treatment appears to have no visible effect on the homogeneity of the composition. I t is likely that the observed initial increase in strength and stiffness of GR-S compositions affected by surfactant treatment is caused by improved filler dispersio duction in strength and decrease in stiffness face treatment can be attributed t o a reduction in resin-to-filler adhesion. But with poly(viny1 chloride), whose polarit,y evidently alone is high enough to disperse the filler particles, surfactant treatment serves primarily to reduce filler-to-resin adhesion, with consequent reduction in strength and stiffness. Why a small amount of Hyamine 1622 appears to incrcase the strength and stiffness of poly(vinyi chloride)-wollastonite formulations is obscure. .4t the Hyamine 1622 Concentration on kaolinite corresponding to optimum strength of GR-S compositions, roughly 20% of the surface of the filler is calculated to be covered with adsorbed surfactant. This implies that only a relatively small fraction of the particle surface need be modified to permit dispersion in loa polarity media, leaving the majority of the unmodified (and highly adsorptive) surface to provide strong adhesion to the resin. Differences in the effect of the two minerals on composition properties can be primarily attributed to differences in particle size. Kaolinite is a far finer filler than wollastonite, and is therefore more likely t o remain agglomerated in low polarity media. Surfactant treatment is thus likely to improve to a greater extent dispersion of kaolinite than of wollastonite. -41~0,since the finer filler will have the greater interfacial area of
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1956
contact with the resin per unit volume of composition, reduction of resin-to-filler adhesion by surfactant treatment would be expected t o have a greater detrimental effect on kaolinite-composition properties than on those containing wollastonite, as the data indicate.
Table I.
Physical Properties of 90% Kaolinite10% Polystyrene Formulations Tensile Strength, Lb./Sq. Inch After 24 hours water Original immersion
Filler Treatment Meq./100 Gram
Density Grams/’
None Hyamine 1622
1.73
475
100
0.80
1.80 1.78 1.71
460 400
160
0.56 0.53
1.0 2.0 3.0 4.0
co.
..
400
325
200 225
..
Abrasion Loss, Grams/Sq. Inoh/Min.
0.48
0.71
Differences in the effect of the two surfactants appear to be related t o the firmness of adsorption of the additive by the mineral and the structure of the hydrophobic part of the surfactant molecule, and thus its compatibility with resin phase. Hyamine 1622 evidently is adsorbed more strongly by the minerals than is Primac, inasmuch as the Hyamine concentration level, at which maximum tensile strength is obtained in GR-S formulations, is about one sixth that of Primac. However, the alkylphenyl ether chain on Hyamine 1622 would appear t o be less soluble in or compatible with GR-S than is the saturated hydrocarbon chain on Primac, with the result t h a t the former compound should have greater detrimental effect on resin-filler adhesion (when present in high concentrations on the filler) than the latter. Conversely, the hydrophobic part of Hyamine 1622 should be more compatible with chlorinated vinyl polymers such as poly(viny1 chloride) than that of Primac, and hence the latter compound should have a more detrimental effect on the strength of poly(viny1 chloride) compositions. The data of Figures 1 and 2 seem t o verify this hypothesis. HIGH FILLER LOADINGS WITH DISCONTINUOUS RESIN PHASE
Theory. A platy mineral such as kaolinite, in which the primary particles are found t o have a length-to-thickness ratio of about 10, should in theory be compactible t o extremeIy high densities-densities approaching that of the crystalline solid itself (specific gravity of kaolinite is about 2.6). Were such compaction to be carried out in the presence of a uniformly distributed binding resin, it should be possible t o prepare dense, strong compositions containing very small quantities of resinous binder. Preparation of such a product by ordinary techniques, however, is virtually impossible, first because the clay cannot be completely disaggregated into primary particles, and second, because no simple blending technique will achieve the extremely highly uniform distribution of binder required. Since surfactant treatment of kaolinite appears t o enhance its dispersibility in low polarity media and t o render the mineral hydrophobic, i t is possible that the major obstacles t o preparation of such a composition might be overcome by dispersing the clay in water, adding t o the dispersion a n appropriate surfactant, adding resin in latex form, and coagulating the mixture. As the clay is initially in a high degree of dispersion in water, essentially complete exposure of the primary particle surfaces can be expected; treatment with the surfactant, followed by addition of submicron particles of a thermoplastic resin in latex form, should result in the deposition of the filler particles of a fairly uniform layer of resin. Coagulation and drying of the solid, followed by heating and compression, should lead to marked densi-
301
fication (the fused resin acting as the interparticle lubricant) and a coherent product. Because perfect orientation of the clay platelets in such a composition is exceedingly unlikely, the final molded product will contain (if the resin content is low) an appreciable fraction of void space. The structure of this body can thus be envisioned as a skeleton of platy particles spot welded together a t frequent points by submicroscopic globules of resin. The strength of such a bpdy obviously will be a very great function of its bulk density, since the number of spot welds per unit volume increases rapidly with reduction in void space. GR-S Formulations. A series of compositions containing 90% by weight of kaolinite and 10% by weight of GR-S (plus compounding ingredients) was prepared by the aqueous dispersion method outlined above (9),using Hyamine 1622 as the treating agent for the clay. The coagulum obtained after acetic acid addition was filtered, compacted manually into a block, and dried for 4 hours at 100’ F. The resulting product was a weak, porous solid. This block was then placed in a vulcanizing press, coldpressed for 10 minutes at 2000 pounds per square inch, and then vulcanized a t 295’ F. and 2000 pounds per square inch for 105 minutes. The slabs so produced were cut into tensile and abrasion test specimens, and these properties measured (Figure 5). The effect of surfactant treatment of the filler on the properties of the composition is obvious from these data, the trends paralleling those found for lower-filler-content products. Of particular note is the fact that the benefits t o be gained from filler treatment in very highly loaded stocks are far greater than in less highly filled compositions. What the experimental data do not reveal, however, is the amazing change in appearance and handling characteristics of the product due t o the surfactant treatment. The product containing untreated kaolinite was a white, brittle, and unglazed ceramiclike product with very poor impact characteristics, shattering like china when struck with a sharp blow. The product containing 2.6 meq. of Hyamine 1622 per 100 grams of kaolinite was a brownish-yellow glossy solid with properties akin t o hardwood: It had good impact characteristics, and could be drilled, sawed, and carved with a sharp knife. A nail could be driven into the face of the sheet without cracking it, but attempts to drive nails edgewise into the sheet caused splitting along a line parallel t o the face. Abrasion of the sheet produced crumbs of composition comparable t o sawdust, rather than a fine powder obtained with the untreated clay-filled formulation. Treatment of kaolinite with Hyamine 1622 (up t o a concentration of about 2.6 meq. per 100 grams) clearly aids wetting-out of the clay by the resin and thus compaction of t h e clay under pressure, as evidenced by the marked increase in density of the molded product. Evidently, increase in surfactant concentration continues t o aid wetting or dispersion up t o a concentration of about 3.9 meq. per 100 grams, since density continues t o increase up t o this level of treatment. Above this level, no significant beneficial effect on densification is apparent. The gradual deterioration in strength and abrasion resistance, and increase in elongation, above this level of treatment is believed, as noted earlier, t o be caused by a weakening of the filler-to-resin bond. The fact t h a t the improvement in properties caused by surfactant treatment is due primarily t o improved wetting or dispersion of filler by the resin is shown in Figure 6. These photomicrographs, taken with a Leitz Ultropak of small samples of the various formulations which (prior t o molding) were shaken with benzene t o produce dilute suspensions, reveal a gradual improvement in particle dispersion up t o a Hyamine concentration of about 3.9 meq. per 100 grams, and no significant change thereafter. Effect of Resin Type on Polystyrene Formulations. T o determine whether the effects of surfactant treatment noted for GR-S compositions were of general character, a series of compounds was prepared (6) substituting a plasticized polystyrene latex
INDUSTRIAL AND ENGINEERING CHEMISTRY
302 Hyamine 1622
Vol. 48, No. 2 Nyamine 1622
0.0 meq.
1 . 3 meq.
2.6 meq.
3.9 m e q .
5.2 meq.
6.6 meq.
Figure 6.
Effect of Hyamine treatment on degree of dispersion of kaolinite in GR-S Ilyamine 1622 concn., meq./100 grams of
(Koppers Polystyrene Latex P ) for the GR-S latex. The cakes obtained after drying were compression molded a t 3000 pounds per square inch and 275' F. for 30 minutes. Results are shown in Table I. These results indicate that surfactant treatment of the filler causes a deterioration of strength, and does not have the beneficial effects on ease of densification of polystyrene-kaolinite compositions that it does on GR-S compositions. The improvement In abrasion resistance however, is significant, and the best results again are seen to occur (as noted earlier) a t a Hyamine 1622 concentration of about 3.0 meq. per 100 grams of clay. Why aurfactant treatment failed to improvedensity and strength significantly with this resin is not clear; it is believed that the fluidity of the resin a t molding temperature, and the technique of molding-i.e., rate of compression, and temperature history of the sample during molding-are important factors controlling final density and strength, Of particular note is the effect of surfactant treatment of the filler on the tensile strength of the composition after water immersion. The product prepared with untreated filler suffers about an 80% loss in strength on immersion; this appears clearly t o be caused by selective adsorption of water by the clay, ~ i t h consequent detachment of the resin from the clay surface. On the other hand, as the concentration of surfactant on the clay is increased, the wet strength of the composition improves significantly and regularly. By rendering the clay organophilic, the surfactant reduces the adnorption of water by the mineial, and
,
olny
delays absorption of n-ater into the voids of the composition, bot>h actions serving to stabilize the resin-to-filler bonds. It may he concluded, therefore, that even in cases where surfactant treatment' of filler causes reduction in dry strength of a highly filled product, such treatment may nevertheless be desirable since it tends t o improve markedly the resistance of the product to det'erioration in water. Effect of Resin Concentration and Density on Polystyrene Formulations. I n highly filled compositions, the properties of the product would be expected to be very sensitive to minor variations in resin content, as this variable controls both the frequency of interparticle bonding by resin and the dependency of strength upon densification. To clarify the interrelationship between strength, density, and resin content, a series of compositions was prepared by the method already outlined ( 2 ) , using kaolinite treated with 3.0 meq. of Hyamine 1622 per 100 grams and plasticized polystyrene at concentrations of 10 t o 25% by weight. Minor variations in molding conditions caused differences in product density, which was measured by mercury displacemenr;. The effects of both resin content and density on strength are shown in Table I1 and Figure 7 . Average tensile strength (disregarding density as an independent variable) increases with resin content, passing through a maximum in the neighborhood of 20% resin; densities much cloaer t o the theoretical are attainable a t the higher than a t the lower resin contents; and the increase in strength with increase in density is far greater a t lorn-er resin contents. I t is clear from
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1956
Table 11. Effects of Resin Content and Density in Kaolinite-Polystyrene Compositions Resin Content, % by
wt .
Density Grams/
cc.
Per Cent of Theoret- Ultimate ical Elongation, Density %
Theoretical" Density
Tensile Strength, P.S.I.
12
1.48 2.22 67 4.0 230 1.53 69 3.8 496 1.63 73 6.0 840 15 1.51 2.15 70 4.8 590 1.59 74 4.5 640 1,64 76 5.0 670 20 1.52 2.02 76 7.2 770 1.67 83 5.5 830 25 1.80 1.92 94 10 768 1,84 96 10 703 1.88 98 9 703 a Theoretical density = density of sample calculated assuming no air voids: density of kaolinite, 2.6 grams/cc.; density of polystyrene, 1.07 grams/cc.
4
*
1
I
I
800
a
2
600
3
-? ru
9 2
400
zoo
~
50
70
60 PER C E N T
OF
BO
THEORETICAL
90
100
DENSITY
Figure 7. Tensile strength of Hyarninetreated kaolinite-polystyrene compositions as function of density and resin content
Figure 7 that, were it possible by appropriate manipulation during molding (or by some alternative compaction technique) t o obtain compositions of 12 % resin content with densities comparable with that achieved in the GR-S formulation (about 2.02 grams per cc.), ultimate tensile strengths in the neighborhood of 2000 pounds per square inch should result. Whether differences in ease of compaction of GR-S-kaolinite and polystyrene-kaolinite compositions are caused by differences in the rheological properties of the two resins, in the uniformity of deposition of resin on the filler particles, or in the dispersibility of treated kaolinite in the two resins, remains t o be determined POLAR RESIN BINDERS WITH DISCONTINUOUS RESIN PHASE
Theory. The highly filled compositions described have contained, as binders, resins of predominantly hydrocarbon character whose ability to disperse, wet-out, and adhere strongly t o mineral filler particles is limited. Such compositions have been shown, under certain circumstances, to exhibit improved physical properties by treatment of the filler with substances which improve dispersion and wetting of the solid by the resin. If, on the other hand, a relatively polar-binding resin is employed, the need of preliminary surface treatment of the filler might be expected t o disappear; in fact, if the adhesion of such a resin t o the filler particles is good, precoating of the mineral surface with a fatty substance should actually weaken the final product.
303
However, t o produce a uniform, high strength Composition a t filler loadings as high as 90% by weight, it is essential t o have the resin distributed as uniformly as possible throughout the mass. This can best be accomplished, it is believed, by blending resin and filler together in the form of aqueous dispersions. Since, under these circumstances, water undoubtedly will wet the filler particles preferentially, adhesion between the mineral and resin particles cannot develop until the water is essentially completely removed. This will impose a limitation on the uniformity of resin distribution, and also will prevent effective interparticle lubrication by the resin during subsequent hotpressing of the dry mixture. But if the filler can be treated with an agent which will permit wet-out of the mineral by the resin in the presence of water and be compatible or chemically reactive with the resin as well, then blending of resin and filler in aqueous dispersion will lead to a coating of the filler particles by resin, and development of a strong filler resin bond. Under these circumstances, resin distribution will be nearly perfect, as will interparticle lubrication; hence the physical properties of a composition prepared in this fashion should surpass those prepared without proper filler treatment. The epoxy resins fall into the category of moderately polar compounds which display excellent adhesion t o many solid surfaces, notably metals and glass; hence their adhesion t o mineral solids such as kaolinite would also be expected t o be high. These resins can be cured or cross linked by addition of polyamines such as diethylenetriamine, or of primary, secondary, or tertiary aromatic amines. Polyamines are miscible with water and exhibit no significant capillary activity; although they are adsorbed by kaolinite as polyvalent cations, they do not appear t o increase the hydrophobicity of this mineral. The aromatic amines, however, have very limited water-solubility (except at low pH), are capillary active, and appear t o adsorb on kaolinite and render the solid hydrophobic. Thus, epoxy resin-kaolinite compositions containing polyamines as curing agents should be considerably different in character than those prepared with aromatic amines, the latter being stronger and denser. Experimental. Compositions containing 90% by weight of kaolinite and 10% of an epoxy resin (Shell Chemical Co.'s Epon 834) were prepared ( S ) , one using a polyamine as a curing agent, and the other an aromatic amine. Slightly different procedures were used t o prepare these two products. Kaolinite was dispersed in water, and heated Epon 834 added with strong agitation. A quantity of diethylenetriamine (DETA) equivalent t o 20% of the weight of resin was then added. The suspension was filtered, and the cake dried and cured as indicated in Table 111. T o the aqueous kaolinite suspension was added with agitation a mixture consisting of 79y0 Epon 834, 16% dimethylaminomethylphenol (Rohm & Haas' DMP-10) and 5% xylene. The resulting slurry was filtered, dried at moderate temperature, and cured as indicated in Table 111. Results and Discussion. Compositions prepared with diethylenetriamine, alter drying a t room temperature, resembled unfired ceramic in that they were porous and coherent. Press curing this material a t elevated temperature caused a marked improvement in strength, but little densification; it was observed that merely oven curing the dry material a t 110" F. without application of pressure resulted in the development of comparable strength. The final product was, in either case, a brittle, porous solid resembling unglazed porcelain Compositions prepared with dimethylaminomethylphenol yielded after drying a t room temperature a noncoherent, waterrepellent powder of low density resembling talc. Oven curing this powder a t 110' F. without pressure did not convert it into a cohesive solid. However, when heated t o 350' F. and compressed, the powder was found t o flow and compact readily, yielding a dense, glossy solid of extraordinary strength. Diethylenetriamine-containing compositions dry t o coherent
304
INDUSTRIAL AND ENGINEERING CHEMISTRY
solids and do not benefit by compression molding, evidently because the kaolinite, being water wet, undergoes marked shrinlrage because of capillary forces during dehydration. Once the water is removed, the filler particles are not sufficiently lubricated t o slide and pack more closely under pressure, although droplets of resin distributed between the particles are able to spot weld the structure into a coherent mass after being cured. Thus, whether curing takes place a t elevated pressure or not makes little difference in the properties of the product. With dimethylaminomethylphenol-containing compositions, hovever, the shrinkage forces produced by dehydration are largely eliminated because of the preferential wetting of the solid by the resin-Le., the air-water-solid contact angle is increased; consequently a noncoherent powder of low density results. The filler particles, now rather uniformly coated with thin films of resin, can under heat and pressure slip over one another and conipact t o high density, yielding a strong, nonporous product. In other words, the resin-coated filler is essentially a molding powder, the major difference being that only about 10% of the material is fusible. The possibility of precoating mineral fillers n-ith other thermoplastic or thermosetting resins by a technique of this sort, and thus to produce moldable fillers, is enticing. If the quantity of aminophenol present in this formulation is computed in terms of its concentration with respect t o the clay present, the figure is found to be 2 grams (about 13 meq.) per 100 grams of clay, or considerably in excess of the optimum concentration found for I-Iyaniine 1622. This suggests that use of a lower concentration of aminophenol in epoxy-kaolinite compositionsmight yieldstill better results than those herein reported. Unfortunately, the aminophenol serves two functions in this formulation, and a significant reduction in its concentration greatly prolongs the cure of the resin. This does not, however, preclude the possibility of using a mixture of aminophenol and a surface-inactive curing agent such as diethylenetriamine; such systems definitely warrant further study. The importance of density of the molded product again is evident in Table 111. There are clearly two factors controlling the extent of densification of these highly filled products: One is the ability of the composition to be compacted, which is determined by the uniformity of distribution of resin and the degree of resinfiller interaction-i.e., dispersion and wetting; the other is the mechanics of the compaction process. It has become evident in this work that compression molding is not the most efficient may to achieve orientation and close packing of platy materials such as kaolinite. Possibly processes such as calendering or extrusion of the composition, followed by compression and heating, would yield products of higher density and hence with superior physical characteristics. CONCLUSIONS
The results of this study lead to the following conclusions. Treatment of mineral fillers such as kaolinite or wollastonite with small controlled quantities of selected surface-active agents is accompanied by improvement in strength and stiffness of plastic or elastomeric products containing 50% by weight of filler, provided the untreated filler does not disperse readily in the resin n-ith which it is compounded. Excess treatment of the filler is detrimental t o physical properties. Surfactant treatment causes reduction in strength and stiffness of compounds based on resins in which the filler disperses readily without treatment. Surfactant treatment of mineral fillers permits the preparation of very highly filled (90% by weight of filler) plastic compositions Kith good physical properties. The role of filler treatment in this application appears to be enhancement of uniformity of distribution of the binding resin, and a preferential wetting of filler particles by resin. Filler treatment also appears t o improve water resistance of these highly filled compositions. The strength of highly filled plastic compositions is markedly dependent on the densification achieved during fabrication.
Vol. 48, No. 2
Table 111. C u r i n g Agent arid C o n c e n t r a t i o n Effect o n 90% Kaolinite-107’ Epon 834 Compositions Ciirin Agent and Eoncn. ( % on Resin) 20% D E T A
20% DhI1’-10
Tensile Curing Density, Strength Conditions Grams/Cc. Lb./Sq. Inch Dried 20 hrs. a t room temp. t o 1 76 1040 28T0 moisture Dried at 350° F. f o r 10 min. Prrss cured 1 hr. at 350° F. Dried 38 hrs. ox-er CaCln to 1 72 660 < 1Yo moisture Press cured 1 hr..a t 35OC F. Dried at 360’ F. for 15 min. 1 38 210 Press cured 4 5 inin. a t 350’ 1:. 1 51 395 1.74 1410 Dried 20 hrs. oi-er CaC12 to 1 81 630 < 1% moisture Press cured 1 hr. a t 350’ F. 2.01 1660
Whereas surfactant treatment of the filler appears to facilitate densification, the mechanics of the compaction process are of critical importance in obtaining high quality products. ACKSOW LEDGM ENT
This paper is a contribution of the Llassachusetts Institute of Technology Soil Stabilization Laboratory, representing a part, of a program of basic research sponsored by industrial contributions and the Department of the Army, Corps of Engineers, Research and Development Laboratories. The author gratefully acknowledges the financial and technical assistance given by the sponsoring organizations, the advice and criticism of the staff of the laboratory, and the cooperation of R.H. Carter in preparing the diagrams. L I T E R A T U R E CITED
Baldauf, G. H.. “Strength Behavior of Adhesive Bonds,” Sc.D. thesis, Llassachusetts Institutc of Technology, Cambridge, Mass., 1949. (2) Bolta, C. C., “Production and Properties of Polystyrene-Clay Products,” S.B. thesis, Massachusetts Institute of Technology, Cambiidge, Mass., 1955. (3) Bonner, F. J . , “Epoxy Resins as Binders for High Filler Content Plastics,” S.B. thesis, Alassachu-etts Institute of Technology, Cambridge, Mass., 1955. Carter, L. M’ , Hendricks, J. 0.. and Bailey, D. S.,“Elastomer Reinforced with a Modified Clay,” U. S. Patent 2,531,396 (Nov. 28, 1950). Deery, H. J., “Compounding of Surfactant Treated Kaolinite with Polystyrene,” 1‘I.S. thesis, Ilassachusetts Institute of Technology, Cambridge, Mass., 1955. Deis, L. P., “Surfactant-Treated Fillers as Reinforcing Agents for Rubbers and Plastics,” L I S . thesis, 1Iassachusetts Institute of Technology, Cambridge, hlasa., 1953. Gutoff, E. B., “Rubber Filler Interactions,” 8c.D. thesis, ;\Iassachusetts Institute of Technology, Cambridge, Mass., 1952. Kambara, S., J . Soc. Chem. I n d . ( J a p a n ) 45, 967-70 (1942). Kuhn, J. O., “Use of Surface Active Agent Treated Clays in Reinforcing Rubber,” M.S. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1953. McBain, J. W., and Lee, W. B., J. Phvs. Chem. 32, 1178 (1928). hferrill, E. W., “Certain Cohesive and ildhesive Characteristics of Thermoplastic High Polymers,” i3c.D. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1947. Parkinson, D., and Blanchard, A . F., Trans. Tnst. Rubber I n d . 23, 259-79 (1948); Rubber Chem. and Technol. 2 2 , 118-37 (1)
I1 949) --, . \--
Schaeffer, W. D., Polley, M. H.. and Smith, W. R . , J . Phys. & Colloid Chem. 54, 227-39 (1950). Schaeffer,W. D., and Smith, W. R., IND. ENG.CHEM.47, 128690 (1955). Shaver, R. G., “Elastomer Reinforcement with Organophilic Clays,” M.S. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1952. Sweitzer, C. W., Goodrich, W. C., and Burgess, K. A , , Rubber Age ( N . Y . ) 65, 651-62 (1949). RECEIVED for review July 30, 1955.
ACCEPTED November 18, 1955.
