Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 429-433
429
Homogeneous Composites of Ultrahigh Molecular Weight Polyethylene and Minerals. 2. Properties' Edward G. Howard," Barbara L. Glarar, and John W. Collette Central Research and Development Department, E. I. Du Pont de Nemours & Company, Experlmental Station, Wilmington,Delaware 19898
Homogeneous mineraVultrahigh molecular weight polyethylene composites have been made with metal oxides, clays, silica, silicates, carbonates, sulfates, etc., that possess high modulus and low-temperature Impact resistance. Composites made from alumina trihydrate have considerable flame resistance.
Introduction The previous paper discussed the preparation of homogeneous composites of ultrahigh molecular weight polyethylene and minerals by polymerization of ethylene on a catalytically active mineral surface (Howard et al., 1981). The process provides a route to highly mineral-filed polyethylene which cannot be made by melt blending. These composites have an unusual combination of high stiffness, impact resistance, and in some cases fire resistance. Many inorganic compounds have been used as substrates for the polymerization including metal oxides, clays, silicas, silicates, carbonates, sulfates, etc. (Howard, 1980). The technology has also been extended to carbon blacks (Adelman and Howard, 1979),and certain polymeric compounds such as polyacrylonitrile (Howard, 1978b). This paper discusses the effect on composite properties of mineral content, polymer characteristics, and mineral characteristics (e.g., mineral size, shape, thermal treatment, and chemical composition). Composite Properties Modulus. Minerals are frequently added to polymers to provide increased stiffness or modulus. We used the Einstein-Guth-Gold equation (EGG) (eq 1)to compare
M = ~ o ( +i 2.58 + 14.182) Mo = modulus of polymer M = modulus of blend
(1)
8 = volume fraction of filler
the results from polymerization on the filler with melt blending and to determine the effect of mineral shape and content on composite moduli. the EGG equation predicts the increase in viscosity of a fluid on addition of noninteracting spheres and has been used by Furter (1964) to analyze modulus response in filled polyethylenes. The results are shown in Figure 1. As predicted by the EGG equation, the modulus for all filled composites rises sharply with increasing filler volume fraction. The modulus for gibbsite filled composites closely follows the theoretical EGG curve. Melt blends of polyethylene and gibbsite also follow this curve, indicating that no unusual reinforcing action results from polymerization on the filler. The moduli for composites derived from calcined clay and mica are significantly higher than that from the gibbsite, in line with the decreasing sphericity of these fillers. Table I shows some specific data and also illustrates the effect of mineral shape on heat distortion temperature Contribution No. 2746.
(HDT). The HDT is determined both by the total mineral content and modulus so that the more reinforcing clay has a higher HDT than calcium carbonate. Impact Properties The most unusual property of the composites is their toughness or resistance to impact at high filer loading. We have used the notched Izod strength (at -18 "C)and the Gardner falling dart impact (at 25 "C) to characterize toughness. Table I shows the effect of various minerals on impact strength. The plate-like particles of mica reduce strength markedly, whereas clay, calcium carbonate, and gibbsite show only slight differences. All are substantially tougher than melt blends of polyethylene with these filers (Howard et al., 1976, 1981). Under impact, these tough composites fail in a ductile fashion in contrast to typical blends. This is shown in Figure 2. A melt blend of 50 wt % calcium carbonate and polyethylene show brittle fracture in a falling dart Gardner Test at an impact strength of 80 in.-lb (0.9 J);microcracks are visible even at 20 in.-lb (0.2 J). Similar results are observed with clay, alumina hydrate, and other particulate minerals. In contrast a composite with 68% gibbsite shows ductile yielding and no fracture up to 240 in.-lb (26 J); calcium carbonate composites behave similarly. The Importance of Molecular Weight The impact properties of 50% clay composites are very sensitive to molecular weight (Figure 3). Both Gardner and Izod impact rise sharply as the ?)inh increases from about 12 to 20 (corresponding to an increase in molecular weight from 2 X lo6 to about 5 X lo6). A similar increase in impact strength is observed with unfilled polyethylene as the molecular weight increases from lo5 to lo6 (Figure 3). The latter has been attributed to a decrease in crystallinity and a corresponding increase in the amorphous fraction present in the unfilled polymer. Other factors must be important in the composites as there is little change in the degree of crystallinity after the molecular weights exceed 1 X lo6. Some of the variation in impact properties with different fillers and different catalysts can be attributed to changes in the matrix molecular weight. Impact-Modulus Balance The property combination of high impact strength, ductile fracture, and increased modulus which can be achieved in these composites has been long sought after. Composites derived from clay or gibbsite offer particularly attractive balance of stiffness and toughness. This is shown by the comparisons with some commercially available plastics and with glass reinforced resins in Figure
0196-4321 ~81/1220-0429$01.25/0 0 1981 American Chemical Society
4SO Ind. Eng. Chem. Rod. Res. Dev.. Vd. 20. No. 3,
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Table I. Effect ofMineral Particle ShaDe notched Izod mineral CaCO, AI,O,.3H,O
mineral shape
calcined kaolin mica
mineral modulus initial content. % GPa (kpsi) 63.5 64 64 44
irregularly spherical ruptured stacked plates thin plates
2.8 (400) 3.8 (550) 4.8 (700) 6.2 (900)
(-18%) HDT (ft-lb/in. (1.82 MPa), J/m ofnotch) "C 517 19.7) 83 88 373 i7) ' 91 373 (7) 65 80 (1.5)
Table 11. Kaolin Particle Size
average particle size, rm mineral content, %
0.5'
modulus (i). GPa (kpsi) tensile MPa (psi) elongation (b), 90 hod, -18 "C, Jlm (It-lblin. of notch) HDT, "C (1.82 MPa)
Harwick 50R
Harwick GK.
64.7 4.8 (700) 2 2 (3250) 3.0 239 (4.5) 70.0
9.OC
72.5 6.2 (900) 1 7 (2500) 23.0 80 (1.5) 97.0
Georgia Kaolin Hydrite MP.
v Figure 1. Relative modulus VB. volume percent filler. 4. The composites offer a high level of tough" a t equal
modulus or increased stiffness at equal impact strength relative to these established materials. Tensile Properties The tensile properties are unusual for a filled plastic. The stress-strain curve (0.2 in./min) for a typical tough
Figure 2. Ductile and brittle fractures: top, eompceite of 123% ALOSBHIO; bottom, melt blend of 50% CaCOIland Alathon 4070. Numben, indicate impact force (in.-lb).
2.0b 72.4 1.6 (670) 19 (2700) 80.0 223 (4.2) 74.0
composite ( F i i 5) shows a poorly defmed yield followed by a long draw region. In this draw region, microvoids are formed, the cross seetion of the tensile bar changes relatively little and the density decreases. This is shown in Figure 6;after 550% elongation the original crm sections of 12.5 X 3 nun hasonlyshrunk20% to 10 X 3 nun. These tensile characterktica resemble those of impact polrstyrene where crazing followed by void formation has been considered a factor in the increased impact strength Bucknall (1977) has pointed out that rigid particles can induce yielding in brittle polymers in the same way as rubber particles do, and that the distinctive feature of rubbertoughened plastics is their postyield behavior. The adhesion of the matrix to the rubber phase is an important factor in this behavior. Figure 7 is a SEM of a tensile fracture of a polyethylene/clay composite which clearly shows the voids formed during the fracture and the polymer strands adhering to the mineral. A further example is shown in Figure 8, which is a side view of a one way oriented film. Sample A is a melt blend (40% clay in HDPE);sample B is a composite of similar composition. Void formation is evident in both but the composite shows greater adhesion of the polymer to the mineral. Mineral Factors Mineral Particle Size. Mineral particle size effects are demonstrated with kaolinite and are summarized in Table II. The lowest modulus is obtained with an average particle size of 2.0 rm. Smaller or larger particles give a higher modulus; the elongation changes follow the same pattern. The 9.0-rm clay particles improved heat deflection temperature but reduced impact resistance. Mineral Thermal Treatment. The thermal treatment of kaolinite has a profound effect on composite propertiea Kaolin clay is irreversibly altered by heating at 5O(t600 O C . There is an endothermic loss of water and a change in ita X-ray diffraction pattern; at 950 "C, a strong exothermic event occurs, and concurrently, the stacks are fractured and disrupted, the degree of which depends on the heating rate. The effects of these on composite properties is shown in Table 111, using as a basis of comparison the amount of clay that can be added before the -18 "C hod falls below 1.0. Unaltered kaolin can be used to a loading of about 75%. For 600 "C kaolinite this value is reached with only 58%, while with loo0 "C kaolin, 70% is tolerated. The data of Table III illustrate that this effect is related to the polymer's molecular weight.
I d . E N . Chem.Rod. Res. h.. Vol. 20, No. 3, 1981 4Sl
I
150% R A Y 1
v
rl
7,"h
Figure 3. Impact strength-effect of molecular weight.
i
I
Figure 7. SEM tensile fracture. Polyethylene50% clay. Teasile fracture (R.T.).
3."
2.8
*.I
5.5
C".
Figure 4. Izod impact vs. flexural modulus.
35% C L A V P E BLEND EDGE FRACTURE SURFACE FILM DRAWN 5X ny 5X
40% CLAV-PE DIRECT C W P O S l l E EDGE FRACTURE SURFACE
Figure 5. Stressatxain m e .
F l L n DRAMN
5X
BY
5X
Figure 8. SEM oriented films: top. polyethylene/clay mixture: bottom, polyethylene/clay mmposite.
Figure 6. Elongated tensile bar.
Unusually high tensile strength composites can be prepared with calcined gibbsites (Howard, 1978a). The gibbsite loses approximately 2 mol of water at 225270 OC to yield a high surface area AlZO3.XHzO(X 250
OC
>250 'C
43% maximum weight loss
36% maximum weight loss
70% hydrated alumina 46% clay/ 23% hydrated alumina 80% hydrated alumina polycarbonate polysulf one ' UHMW PECpe ' s d
heat evolution factor
flame spread factor (Fa) 3.1 3.2
12.9 12.3
flame spread index (I,)b 40 39
13.5 7.5 3.0 16.3
14 32 11.5 255
(Q)
1.1 5.0 3.5 15.5
ASTM E162. I, = F , X Q. ' Howard (1978b), Lexan 9600/9800, flame retarded polycarTable 11. bonate resin, General Electric Co. e AC 1220 ultrahigh molecular weight polyethylene. Allied Chemical Co. Table VI. Smoke and Char Formation AraDahoe Chamber
Figure 10 shows a plot of oxygen index (0.1,)vs. mineral content for clay, calcined clay, and gibbsite, Al2O3.3H20. There is little difference between clay and calcined clay, indicating that, although the clay can lose the elements of water
>400 O C
A1203.2Si02.2H20 A1203.2Si02 it has little effect on the 0.1.; it is likely that the burning temperature is well below the 400 "C required to dehydrate this mineral. Gibbsite increases the 0.1. particularly above 60% filler; very surprisingly, half the gibbsite can be replaced with clay with only a slight decrease in 0.1. This indicates that both the high filler level and the evolution of water are important. Gibbsite based composites also perform well in the radiant panel test (ASTM-E162). This test measures the rate at which the flame travels down the surface and the amount of heat evolved. The results are in Table V. As the data illustrate, the flame spread index of 70-8070 gibbsite filled composites approaches or equals that of polycarbonate and polysulfone. A major factor in this low index is the slow rate of propogation of the flame across the surface of the sample. Smoke and char formation are an equally important aspect of combustibility in living areas. The gibbsite fiied composites have a very low tendency to form smoke (Table VI). The characteristics result from the low combustible polymer content in addition to the absence of smoke promoting flame retardants.
rigid PVC polycarbonate PE Composites' 52% calcined clay 58% Al20,*3H,O 80% Al,0,*3H,O 80% Al,0,.3H,O (0.0625 in.)
%smoke
%char
10.Za
9.0a
7.6' 18.8'
8.2 2.8 1.5 2.2
4.5 3.2 2.9 3.0
a 30-s burn; all others 60-s burn. Lexan 9600 (flame retarded polycarbonate) available from General Electric Co. Sample thickness 3.18 m m (0.125 in.) unless otherwise noted.
Miscellaneous Composites As discussed by Howard et al. (1981), certain minerals have to be treated with acid before polymerization. The member of this group of minerals that received the most attention is calcium carbonate. Very high loadings are possible, and the product responds to annealing by a large enhancement of heat deflection temperature as shown in Table VII. The composites from ferric oxide are unusual because of their high densities as well as being potentially magnetic. Also, the physical data show that high elongations do not necessarily follow from good impact resistance (Table VIII) which is in contrast for most of our other products. Fiber Substrates The technique described in these papers can also be
I d . Eng. Chem. Rod. Res. Dev.. Vd. 20. No. 3, 1981 433
Table VII. Calcium Cerbonate/UHMW PE Compoeitei calcium carbonate," % 61.0 75.0 polymer inherent viscosity 30.0 tensile (max), MPa (psi) 22 (3150) modulus (i), GPa (kpsi) 4.07(590) elongation (b) 330 Izod impact (-18 "C), 477 (9.0) Jlm ift-lh/in. of notch) ~~~~
~~
29.7 17 (2400) 5.18(750) 100 191 (3.6)
1 m -
~~.~ .... .....
1.82 MPa (264 ps$,'*C not annealed annealed
68.0
83 110
Georgia Marble Gamma Sperae. Table VIII. Polyethylene Composites with Ferric Oxide 67 711 % Fe.0. tensiietmax), MPa (psi) 53 (3350) 2; (3200) modulus (i), GPa (kpsi) 3.9 (560) 6.5 (940) Izod, -18 "C, >800 ( > 1 5 ) 201 (3.8) J/m (ft-lblin. of notch)
elongation (b), % density (calcd), g/mL
HDT, 1.82 MPa(264 psi),
85 21 68
1 2.6
POLYETHYLENE GLASS FIBER COATED GLASS FIBER Figure 11. Polyethylene on glass fibers: uncoated fiber and polyethylene coated fiber.
Table X. Polyethylene/FabricComposites.
"C
Polvacrvlanitrile
Table IX. Polyethylene/Fabric Comvosite. G l m Fabric %
tensile ( m a ) . MPa (psi) modulus (i). GPa (kpsi)
eloneation
Izod;-18°C, J/m (ft-lblin. of notch) density (calcd), g/mL
polyacrylonitrile 81% 68.3 (9900) 21.3 (3100)
n~5
13ct2.5) 1.81
applied to glass and polyacrylonitrile fibers (Howard, 1978b). Because glass is chemically basic, it is necessary to coat it with an acidic substance before ethylene can be polymerized on its surface. The polyethylene/glass initial product is uniformly coated (Figure 11) and can be hot pressed to a uniform, stiff, strong structure (Table IX). Composites from polyacrylonitrile fabric are noteworthy because they have an unusual balance of physical prop. erties and passes a low density for a reinforced composite (Table X). The hot pressed sheets are resistant to delamination by boiling water. Fabrication The composites are t y p i d y isolated from the reaction as free-flowing powders and can be fabricated in a number of ways. Hot compression molding was used to develop most of the physical properties given in this paper. A simple alternate procedure is to cold press and sinter. These powders can be pressed under 5000-10000 psi at r w m temperature to complicated forms possessing considerable green strength. Simply heating these objects above the melting point of the composite's polyethylene component causes the object to shrink about 10% to very strong, tough parts. There is no melt flow and the sharp edges and comers of the green form persist throughout the sintering operation.
continuous fiber. 90 density (calcd), g / k
tensile (max). MPa (psi) modulus (i), GPa (kpsi) elongation (b), 5% hod, - 18 "C, J/m (ft-lb/in. of notch) HDT. 1.82 MPa (264 psi), "C
72 1.07 63 (9200) 3.2(460) 45 345 (6.5) 77
91 1.08 48 (8370) 4.2 (610) 26 376 (7.1) 86
Fabricated sheet made by a continuous compression molding process can be formed by hot or cold pressure assist methods. Cold formed sheet recovers to the original shape when heated, but hot formed objects are dimensionally stable to about 125 "C. Literature Cited Adsman. R.: Howd. E. G. US. Patnt 4 151 1%. 1979. -mi. c. E. "~aghasd m-. ~pplsdscw pu~ahsn. LM.: Ldon 1977: see W l y chspta 7. Furter. W. F. Can. J. (xam. Eng. 1084.77, am.B. L.; ~ovard.E. G.: &nene. J. w. Rs mm. n78. s,430. Howard. E. G.; GLa2ar. 0. L.; Upsmmb. R. D.; Msd)oMld. R. N.: Tubxk. C. W.; coktta.J. W. Ind. Eng. C m . Fmd. Rep. aSv. p r m a m d e h mls ISSW. 1981. Howard. E. 0.; Uarer. 0. L.; CdLme. J. W. Absbacts. sodstv of PLlStlc Engineers. "4Tedmical Cantasnoe. Oct 5-7. 1976. p 38. Howard. E. 0. U.S. Patent4097U7. 1978a. Howard. E. G. U.S.Patent 4 126647. i978b. Howard.E.G. US. Palem4187210. 1980.
Receiued for review April I , 1980 Accepted March 30,1981 presented at the ACS/csI Chemical Conpreas, Honolulu, Hawaii, April 1979. Division of Industrial and Engineering Chemistry.
