Tetrafluoroethylene-Perfluoro (propyl vinyl ether) Copolymers. As Water-Resistant, Thin-Film Adhesives Joseph P. Reardon* and William A. Zisrnan Laboratory for Chemical Physics. Naval Research Laboratory. Washington. D. C. 20375
Copolymers of tetrafluoroethylene and perfluoro(propy1 vinyl ether) used as thin-film, hot-melt adhesives form strong, highly water-resistant bonds to polytetrafluoroethylene, glass, sapphire, aluminum, and steel. Thanks to their low surface tensions, the copolymer melts can wet nearly any substrate. Strong adhesion to the substrate surface is effected by virtue of dispersion forces alone, thereby eliminating the need for chemical compatibility of the materials or for coupling agents. Joints made with synthetic sapphire are apparently totally unaffected by exposure to water, a finding which points up the paramount importance of water imperviousness of both the adhesive and adherend in achieving truly waterproof bonds.
The ideal adhesive for demanding structural applications would combine high-temperature strength with resistance to hydrolytic and oxidative attack. Moreover, it would not have the rigidity which makes ceramics and highly cross-linked polymers susceptible to catastrophic failure upon crack initiation. An adhesive material which approaches this ideal state of affairs has been found in the form of random copolymers of tetrafluoroethylene with minor proportions of perfluoro( propyl vinyl ether), C F F C F ( O C F ~ C F ~ C F ~(Harris ) and McCane, 1964). Unlike the well-known homopolymer of tetrafluoroethylene, which has a softening point but no true melting point, these copolymers are true thermoplastics. We have found that they make poor substrates for ordinary adhesives, yet when they themselves are used as hot-melt adhesives, they adhere tenaciously to a wide diversity of surfaces. The key to this two-faced nature lies in the low surface energies of both the solid and the melt (Reardon and Zisman, 1974). The critical surface tensions of wetting of the solids have been found to range from 16 to 19 dyn/cm, with the result that common adhesives cannot wet them and, consequently, cannot form strong adhesive bonds with them. The low surface tensions of the molten copolymers, however, enable them to wet and spread over nearly any solid surface with which they come into contact, thereby fulfilling a necessary condition for good adhesion. The various chemical and physical attributes of these copolymers suggest corresponding benefits for adhesive applications, as summarized in Table I. Experimental Section Materials. Experimental resins were supplied in the form of 0.030 in. thick compression-molded sheets by the Plastics Department of E. I. duPont de Nemours and Co. Three were chosen for this study, each containing 1.5 mol % perfluoro( propyl vinyl ether) comonomer but differing in the viscosity of the melt, as shown in Table 11. The 0.030-in. sheets were reduced to a thickness of 0.006 in. by further molding them between Pyrex plates, as described in the next section. The resultant films, when freed from the Pyrex plates, were cleaned in hot trichlorotrifluoroethane (Freon T F ) in a Soxhlet extractor. Bonding Procedure. When joints were to be made, the copolymer film and adherend, each freshly cleaned, were assembled and immediately transferred to a Wabash hydraulic press. The 12 x 14-in. platens of the press maintained constant pressure a t f l psig and controlled temperature to *1"C during bonding. Specimens were held a t
330°C for 30 min and then were slowly returned to room temperature (-2 deg/min). Pressure was maintained throughout to assure spreading of the viscous melt and to prevent shrinkage voids from forming during cooling. Adhesive Joints. A. Pyrex/Copolymer/Pyrex. Pyrex plates 0.375 in. thick were cleaned by immersion for a few hours in a hot bath of 1:l nitric-sulfuric acid, followed by thorough rinsing in distilled water and drying in a clean oven a t 115°C. A square of 0.030 in. thick copolymer was sandwiched between the freshly cleaned Pyrex plates and placed in the hydraulic press. (To give the desired thickness of copolymer film, for later use in making lap shear joints, 0.005-in. shim stock was inserted a t the edges of the plates.) The plates were held under a load of a few hundred pounds during the bonding process. After they had returned to room temperature, no amount of twisting or prying could separate them. The resin film proved to be such a tenacious adhesive that only in several instances could the plates be loosened by immersion in water for two weeks or so. For the most part, the only practicable way of separating them was to cycle them between boiling and cold water for several hours until stress from differential thermal expansion caused the copolymer film to release its grip. The same experiment using tetrafluoroethylene-hexafluoropropylene copolymer (0.030-in. compression-molded Teflon F E P 160) in place of the tetrafluoroethylene-perfluoro(propy1 vinyl ether) copolymers produced no adhesion between the Pyrex plates and the film, a finding which agreed with observations of the manufacturer (Du Pont, 1968). B. Sapphire/Copolymer/Sapphire. Suspecting that the eventual failure of the Pyrex joints might be due to the diffusion of water through the glass into the bond interface, we substituted sapphire, a material impervious to water penetration. Two windows of Linde clear synthetic sapphire (0.125 in. thick, 1.5 in. in diameter) were cleaned first by abrading with fine alumina and then by the acid bath technique described for Pyrex. A small piece of film of copolymer I was melted between them and allowed to cool under 100 psig. A strong joint resulted, the adhesive film measuring 0.0015 in. Immersion in water for more than a month failed to weaken the joint, which was finally loosened only after several boiling water-cold water cycles. A second, identical joint showed no signs of weakening after several weeks of immersion. C. PTFE/Copolymer/PTFE. I t was observed that molten copolymer would spontaneously spread on polytetraI nd. Eng. Chem., Prod. Res. Develop.. Vol. 13, No. 2, 1974
119
Table I. Advantages of the Copolymers of Tetrafluoroethylene and Perfluoro (propyl vinyl ether) in Adhesion Applications as Suggested by Their Chemical and Physical Properties
Property
Typical range of values
Utility
Definite melting pointa Low melt surface tensionb Low critical surface tensionb
300-310 "C 18 dyn/cm (estimated) 16-19 dyn/cm
Chemical inertnew
Same as for PTFE
High tensile strength.' (ASTM D638-68)
4300 psi a t 25°C 3400 psi at 100°C
Hot-melt adhesive Able to wet nearly all substrates Resistant to penetration of liquids into the adhesive/adherend interface Stable to corrosive environments and weathering Structural joints with high-temperature capability
M I T flex endurance& (7-8 mil compressionmolded film)
1500 psi a t 25OOC 50,000-500,000 cycles
Usable for joints subject to flexing and vibration
a McCane (1973). Reardon and Zisman (1974). c Heat-aging tests show no degradation of tensile properties after more than 2000 hr a t 285OC.
BOND REGION X I INCHES
Table 11. Copolymers Studied as Thin Film Adhesives Code
Melt flow number, g/10 mina
I I1 I11
2 6 12
/ 112
\
(1
ASTM D2116-66.
fluoroethylene (PTFE) and, when solidified, could not be pried off. This observation prompted us to make some lap shear joints using 0.125 in. thick PTFE and copolymer I1 a t 320°C (above the melting point of the copolymer but below the softening point of the PTFE). When the 1 in. wide, 0.5 in. overlap joints were pulled on an Instron testing machine at 0.05 in./min, the PTFE yielded indefinitely yet the bond region suffered hardly any distortion (see Figure 1).One joint was subjected to immersion in boiling water for several days and to repeated thermal shock generated by cycling it between boiling water and a Dry Iceacetone mixture ca. -7O"C), but neither treatment had any detectable effect on the joint. D. 1100 Aluminum/Copolymer/ 1100 Aluminum. Lap shear joints were made using 0.063 in. thick 1100 aluminum (commercially pure aluminum, 99.0+% Al) with either 0.5 or 1.0 in. overlap. Aluminum panels 4 X 6 in. were prepared for bonding by wiping them clean with acetone-soaked tissue, soaking them in reagent grade toluene, rinsing with acetone, etching for 10 min in a 68°C chromate bath (conc. HzS04, sodium dichromate, and distilled water in 10:1:30 parts by weight), rinsing in tap and distilled water, and drying for 15 min in a clean oven at 115°C. Upon their removal from the drying oven, the panels and copolymer film were immediately assembled and placed under the hydraulic press. The bonded panels were cut into 1 in. wide strips on a band saw and the end pieces discarded. In an attempt to relieve stresses which may have been set up in the bond region of the joints, the specimens were annealed for 4 hr a t 285°C. (Unfortunately, annealing may have itself introduced stresses by reason of thermal expansion differences between the metal and the film. As discussed below, for instance, steel joints when annealed a t 260°C for 1 hr showed a 16% decrease in strength.) Table I11 gives variations in the preparation of the joints and the resultant ultimate tensile shear strengths, uur as determined under standard testing conditions (0.05 in./ min crosshead speed, 25°C and 50% R.H.) on an Instron tester. The tensile force required to break these joints was greater than the yield strength of the aluminum. At the breaking point, the original 5 or 5.5 in. of exposed specimen ( i e . , not counting the two 1-in. segments in the In120
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 2, 1974
RESIDUAL COPOLY COPOLYMER COATING THE P T F Ec
'PTFE
Figure 1. A typical lap shear joint of PTFE and copolymer I1 after having been pulled on the Instron testing machine. 4
I01
ibi
Figure 2. Mode of failure of lap shear joints of 0.063 in. thick 1100 aluminum and 0.001 to 0.003 in. thick copolymer film: (a) during tensile loading; (b) after joint failure. stron grips) had stretched by about 0.25 in. and bent 5-10" from the vertical in the region of the bond. The resultant mode of failure, as illustrated in Figure 2, was a peeling of the copolymer adhesive film, the film adhering to the end of each aluminum strip, and finally a ripping apart of the film across the middle of the bond region. E. 2024 Aluminum/Copolymer/2024 Aluminum. Table IV gives pertinent information on lap shear joints made from three thicknesses of 2024-T3 alloy aluminum (containing approximately 4.5% Cu, 1.5% Mg, and 0.6% Mn). The method of preparing the joints was essentially the same as that for 1100 aluminum except that a bonding pressure of 8 psig was used for all specimens. To make the 0.125 in. thick specimens, however, the alloy was sheared into 1 in. wide strips; these strips were then bonded as a way of avoiding possible stresses in the bonded area caused by band saw vibrations. This approach, as the values of uu show, seemed to be counterproductive, most probably because of uneven heat distribution during bonding across the overlap area resulting from exposure of this area on four sides. The 0.063 in. thick specimens were annealed a t 285°C for 4 hr with no adverse effects on bond strength, as can
Table 111. Tensile Shear Strengths of Lap Shear Joints Made with 0.063 in. Thick 1100 Aluminuma
Bonding pressure, Psig 33 16 33 33 16 16
Average Range of u“, psi nu, psi None 1070 1020-1110 None 860 830-900 1.o None 740 720-760 1.o Water immersionhc 435 380-490 1.o None 730 725--735 1.o Water immersion* 710 705-715 0.5 8 None 960 920-1010 1.o 8 Water immersionb 680 640-740 Panels were first bonded and later cut into 1 in. wide strips on a band saw. The specimens were then annealed for 4 hr a t 285OC. Joints were immersed in distilled water for 24 hr a t room temperature (22OC). Plus immersion in boiling water for 5 hr. Copolymer I1 I1 I1 I1 I1 I1 I11 I11
Overlap, in. 0.5 0.5
Post-bonding treatment
Number of specimens
Table IV. Tensile Shear Strength of Lap Shear Joints Made with 2024-T3 Aluminum Alloy
Copolymer
Metal thickness, in.
I I I I I11 I11 I11 I11 I11
0.094 0.094 0.125 0.125 0.063 0.063 0.094 0.094 0.125
Overlap, in. 0.75 0.75 0.75 1.o
0.50 1.o
0.75 0.75 0.75
Assembly Panels Panels Strips Strips Panels Panels Panels Panels Strips
No. of speciPost-bonding treatment mens None Water immersiona None None Annealedb Annealedb None Water immersiona None
Average uu, psi
Range of vu, psi
Predominant mode of failure
1480 1280 940 870 1560 1400 1420 1120 740
1410-1540 1120-1360 800-1070 750-1050 1440-1810 1250-1550 1190-1600 910-1250 710-800
Peel Mixed Mixed Burst Peel Peel Mixed Burst Mixed
&Jointswere immersed in distilled water for one week a t room temperature (22’C). b At 285OC for 4 hr. Table V. Tensile Shear Strengths of Lap Shear Joints Made from 0.055 in. Thick 1020 Mild Steel and Copolymer I iOverlaD 0.75 in.: Bonding Pressure, 20 wig) As Bonded After immersion in water for 1 week No. of
Suecimen Untreated steel
Code 1 2 2aa
3 Pre-oxidized steel Q
IP 3P
Assembly Strips Panels Panels Panels Strips Panels
specimens Av uu, psi 6 1820 11 2330 5 1970 5 1040 5 1410 5 2260
Range of nu, psi 1540-2070 1920-2450 1810-2110 930-1170 1360-1430 1970-2450
No. of specimens Av u”, psi 3 1500
Range of uu, psi 1040-1960
...
...
... ...
...
5b 4 3b
810 870 1750
640-960 730-950 1550-1880
Joints were annealed at 26OOC for 1hr shortly before testing. 0.1 % diisopropylammonium nitrite added as rust inhibitor.
be seen by comparing their uu values with those of the unannealed 0.094 in. thick specimens. The modes of failure among those specimens have been characterized in Table IV in three ways: (1) peel: the same mechanism as for the specimens with 1100 aluminum except that the film pulled free of one of the two strips instead of ripping across the middle; (2) burst: the adhesive film suddenly broke apart, leaving white fragments of ordinarily clear film adhering to each face of the two metal strips; (3) mixed: usually some peeling froxp the edges inward with “burst” failure in the center of the bond region. In the case of the 0.063 and 0.094 in. thick joints, there was some slight bending of the metal strips (2-3”) in the overlap area, an observation which is compatible with the higher yield strength of the 2024 alloy over that of the softer 1100 aluminum. F. Steel/Copolymer/Steel. Lap shear joints with 0.75 in. overlap were fabricated from 0.055 in. thick 1020 coldrolled, mild steel ( a carbon steel containing a maximum of 0.25% carbon), either untreated or pre-oxidized. Preoxidizing was accomplished by heating the solventcleaned steel at 330°C for 1 hr in a circulating-air oven so as to impart a durable straw-to-blue colored oxide coating. Such an oxide has been used to promote maximum
adhesion of electrostatically deposited PTFE on steel (FitzSimmons and Zisman, 1958) and hence was of interest in relation to these copolymers. In Table V specimens are differentiated according to whether they were bonded from pre-sheared 1 in. wide strips of steel or from 4 x 11 in. panels which were later cut into 1 in. wide specimens on a band saw. The pre-sheared strips were wiped clean with acetone-soaked tissues. extracted with Freon TF, baked in the oven if they were to be pre-oxidized, extracted with benzene, and immediately transferred to the press for bonding. The steel panels, being too large for our Soxhlet extractors, were cleaned with acetone, then scrubbed in an aqueous solution of Tide and rust inhibitor (0.1% diisopropylammonium nitrite), rinsed with distilled water and acetone, and placed in a 115°C oven for quick drying. Some caution is needed in interpreting the tensile strength values given in Table V. The joints were made in three separate batches on three different days. For instance, sets 1 and l p ( p for “pre-oxidized”) were processed in one batch, and the same was true for 2 and 2a ( a for “annealed”) and for 3 and 3p. Sets 2 and 3, except for being processed on different days, were made to the same specifications; the surprising discrepancy between their Ind. Eng. Chern., Prod. Res.
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121
tensile strengths prevents us from comparing joints prepared in different batches. At the heart of this problem is probably the great susceptibility of clean mild steel to oxidation, so that seemingly insignificant variations in handling the steel can produce considerable differences in the end product. This hypothesis is supported by the fact that the mode of failure of all the steel joints was essentially the same. Inspection of the broken joints revealed that along each exposed edge of the bond overlap area ran a border 1 to 2 mm wide where the steel had darkened considerably and where peel failure had occurred. In the presheared specimens this border ran along four sides; in specimens cut from bonded panels it was found only along the top and bottom of the overlap area. In the interior of the bond region failure was principally cohesive, a frosted white layer of copolymer covering the face of each steel strip. Here and there in the layer of frosted copolymer (as seen under a 7 0 microscope) ~ the film had ripped a hole in the layer opposite it, bringing with it fragments of the steel beneath. In contrast to the oxide-darkened metal a t the exposed edges, the color of the steel beneath the white film of copolymer was unchanged from before bonding. G . Peel Specimens. Three specimens were fabricated by bonding 1 in. wide strips of 0.020 in. thick 1100 aluminum to stainless steel plates with copolymer 111. The aluminum was cleaned as previously described. The substrates of 304 stainless steel were degreased with methyl ethyl ketone, polished with fine alumina powder until water exhibited a zero contact angle on it, and briefly airdried. Bonding was done in a Carver press a t 320°C and 56 psig. The aluminum strips were peeled from the copolymer film in a 90O-peel jig on a table-model Instron a t a rate of 0.5 in./min. As the specimens were peeled, the peeling force fluctuated greatly, giving a maximum of 6.8 kg. Figure 3 summarizes the results of post-mortem examinations of the peel specimens. The various surfaces were probed with droplets of n-hexadecane deposited from a freshly flamed platinum wire. Large contact angles were a good indication of residual copolymer film. A film was detected by the droplets on the underside of the aluminum strip; although invisible to the eye, even under microscopic investigation (70X), it was also readily detectable by the very slick but dry surface sensed by one’s fingertips. Such an invisibly thin film left on the peeled aluminum strip strongly suggests a mechanism of cohesive failure in the film just below the metal/copolymer interface. Another type of peel specimen was made in which strips of copolymer film were to be directly peeled from a metal substrate. Strips of either copolymer I or copolymer 111 measuring 0.030 in. thick, 1 in. wide, and several inches long were bonded to substrates of 1020 steel a t 330°C and 20 psig. The strips of copolymer, reduced in thickness to about 0.020 in. by the bonding procedure, adhered so tenaciously to the steel substrates that they tore instead of peeling. Results and Discussion Our study shows that the copolymers of tetrafluoroethylene and perfluoro(propy1 vinyl ether) have strong potential as thin-film, hot-melt adhesives. For such applications three main points need to be discussed. A. Ability to Wet Adherend. An important criterion of good adhesion is that the adhesive be able to wet the substrate. The copolymers studied here vividly demonstrated their ability to wet all types of solid materials (provided the solid is sufficiently heat stable to accept bonding) when, in the molten state, they spread spontaneously on solid PTFE, a material of very low surface energy and thus unwettable by most liquids. Moreover, the ability of 122
Ind. Eng. Chern., Prod. Res. Develop., Vol. 13,No. 2, 1974
a
,ALUMINUM
TAB (UNBONDED)
BONDED AREA e:50-
\
\
COPOLYMER FILM AINLESS STEEL
Figure 3. A typical Al/copolymer/stainless steel peel specimen as probed with drops of n-hexadecane after peeling.
the copolymers to adhere without any sort of chemical or hydrogen bonding further underscores their wide range of applicability. B. Strength of Joints. The copolymers, when used for bonding aluminum to aluminum or steel to steel, produced strong joints with reasonably good water resistance. The methods employed to prepare the metals for bonding were chosen mainly as convenient ways of getting flat, fairly smooth, and clean surfaces. Without a doubt, considerably greater adhesive strengths and improved water resistance could be achieved by a study of surface preparations and various other parameters of the bonding procedure. For example, chromate etching of aluminum, such a s was used by us, produces a porous outer layer of oxide which promotes adhesion through mechanical interlocking of adhesive and substrate (Bright, et al., 1971). Thus, the lower tensile shear strength of water-soaked aluminum joints is probably attributable to penetration of water through this porous outer layer into the bond interface, Water would disrupt the dispersion forces between adhesive and substrate, thereby reducing their adhesion to mere mechanical interlocking of the two materials. The result, a larger degree of adhesive failure (“peel”) would be reflected in lower uu values, as was indeed observed. Similarly, the particular kind of oxide on a steel surface certainly has a heavy bearing upon the strength of the bond and its mode of failure. In general, optimization of the bonding parameters can be expected to be a result of reducing bond failure to pure cohesive failure, where bond strength depends only on the inherent high mechanical strength of the copolymer adhesive itself. C. Imperviousness to Water. The copolymers are essentially impervious to water. Bonded to another waterimpervious material, they would be expected to yield totally water-resistant joints, which we found to be the case with sapphire bonded to sapphire with copolymer. Specimens made with Pyrex instead of sapphire succumbed to the action of water within a few weeks of immersion, which was not a surprising result considering the readily hydrated nature of the silica glasses. Possibly water molecules penetrated into the adhesive/adherend interface by diffusion through bulk glass rather than by two-dimensional migration across the interface itself. Such evidence suggests that the key to totally water-resistant joints lies in both adhesive and adherend being impervious to water. As an operating principle this would dictate the design of surface preparation of the adherend as well as the basic selection of materials. Initial results, then, show great promise for these perfluorinated thermoplastic resins as hot-melt, thin-film adhesives. Their unique set of physical and chemical properties gives them a range of applicability few other polymers can begin to match. Our work with them is continuing, with emphasis on their role in water-impervious joints. Hopefully the investigation will lead to a better understanding needed for their optimum utilization.
Literature Cited Bright, K., Malpass, B. W., Packham, D. E.,, Brit. Polym. J., 3, 205 (1971). t E. I. duPont de Nemours 8 Co., Plastics Department, Application Data Sheet No. 2.016. 1908. FitzSimmons, V . G., Zisman, W . A,, Ind. Eng. C h e m . , 50,781 (1958).
Harris, J. F.. Jr., McCane, D. I . (to E. I . duPont de Nemours & Co.). U. S.Patent 3,132,123 (May5, 1964). McCane, D. I., privatecommunication, 1973. Reardon, J. P., Zisman, W. A.. submitted for publication, 1974.
Received for review November 23, 1973 Accepted February 23, 1974
Starch-Filled Polyvinyl Chloride Plastics-Preparation
and Evaluation
Richard P. Westhoff, Felix H. Otey, Charles L. Mehltretter, and Charles R. Russell* Northern Regional Research Laboratory. Agricultural Research Service. U .S.Department of Agriculture. Peoria. ill. 67604
Starch was added as a filler to polyvinyl chloride (PVC) formulations to produce plastics that are potentially biodegradable. The most uniform and transparent plastics resulted when the filler was added either by coprecipitating a cross-linked starch xanthate-PVC emulsion or by concentrating a starch-PVC emulsion to produce a dry mixture of the two components. Physical properties of the plastics closely parallel those containing inorganic fillers, except for improved clarity. The starch-filled plastics were readily attacked by a mixture of microorganisms commonly occurring in soil.
Various techniques were studied for incorporating corn starch into polyvinyl chloride (PVC) formulations in a continuing effort to produce biodegradable plastics (Otey, et al., 1972). Earlier studies demonstrated that starch and starch-derivatives can be used as reactive fillers in polyurethane plastics (Otey, et al., 1969). Buchanan and associates (1968) developed methods for adding starch to rubber formulations by coprecipitating cross-linked starch xanthate and latex rubber. Three techniques evaluated here were coprecipitation of cross-linked starch xanthate with a PVC latex, co-concentration of a starch and PVC latex, and dry mixing of starch and PVC. Experimental Section Coprecipitation. In a 2-1. beaker mix 162 g (1 mol) pearl corn starch with 1200 ml of water. Add 250 ml of 2 N NaOH with good stirring to produce a uniform gel. Cool the starch gel to 20°C, add 6 ml (0.1 mol) of carbon disulfide with stirring, and store the suspension at 5 to 10°C for at least 2 hr. The product is a clear viscous solution containing about 0.07 xanthate group per anhydroglucose unit. To 320 g (32 g dry basis) of the starch xanthate solution, add 300 ml of water and 135 g (76 g dry basis) of PVC latex (B. F. Goodrich Chemical Co. Geon 151) and stir for 30 min. Add 1.5 g of NaN02 and stir for 10 min. With good stirring add a water solution of 5% alum until the suspension maintains pH 4. Filter the resulting curd, wash three times with distilled water, and dry in a forced air oven a t 40-50°C for 16 hr. Hammer mill the dry product (29.6% starch, 70.4% PVC) to a fine powder. Co-Concentration. In a 1-1. beaker heat with stirring a mixture of 21.8 g of corn starch and 400 ml of water at 90°C for about 20 min. To the starch paste add 86.2 g (48.4 g dry basis) of Geon 151 and stir for 10 min. Pour the starch-PVC blend into a large glass tray and dry in a forced air oven at 40-50°C for 16 hr. Hammer mill the dry product (31% starch, 69% PVC) to a fine powder. Dry Mix. Whole corn starch and commercial pregelatinized corn starch (A. E. Staley Manufacturing Co. Star-
amic 213) were each pin-milled to an average particle size of 20 p , vacuum dried at 90°C, and then mixed with powdered PVC (Geon 126). Plastic Formulation. Dry composites of starch-PVC, prepared as described above, were combined with dioctyl phthalate (DOP) and dibutyltin dilaurate stabilizer (2% of total mixture) and blended on a rubber mill at 250°F for 10 min. The mixtures were then molded in a 1 x 6 x 0.072 in. aluminum cavity at 140°C and 6000 psi for 10 min. Test Methods. Plastics were tested by the following ASTM procedures: tensile strength and per cent elongation, D 638-641'; shore D hardness, D 1706-61; and fungi resistance, D 1924-70. The mixed fungus spore suspension contained Aspergillus niger, Penicillium funiculosum, Trichoderma sp., and Pullulariapullulans. Per cent water absorption is the amount of water absorbed when the samples were soaked for 2 weeks at room conditions divided by original sample weight times 100. Clarity was determined with a Martin Sweets Co. color brightness tester, Model S2. Samples were placed over a beam of light and covered with a white background and then a black one. The reflectance values read for the black background divided by the reflectance for a white background times 100 are the reported clarity values. These values should be considered relative to the control only and not as having any numerical meaning. The method served as an alternative to visual observation. The lower the value the more transparent the specimen. Results and Discussion Properties of plastics formulated by the three methods used to incorporate three levels of DOP and various amounts of starch are recorded in Table I. Generally the addition of a starch filler to PVC systems lowered tensile strength and per cent elongation of the plastics and increased hardness in a pattern somewhat similar to inorganic fillers (Sarvetnick, 1969). Strength properties were retained best when the starch was incorporated through coprecipitation or co-concentraInd. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 2,1974
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