ANNUAL REVIEW
Adhesives C. CLEMENT ANDERSON New adhesive resins are continually being synthesized and formulated to meet the ever-increasing demands for materials which will withstand extreme environmental conditions. The ability of a resin to perform satisfactorily in a specific adhesive application is dependent upon the fundamentals of polymer science. The landing gear door of the F- 7 7 7 fighter contains a honeycomb core bonded with Plustilock adhesives
ver the past several years, use of adhesives has increased tremendously. During these years, adhesives have supplanted mechanical fasteners in a wide range of applications. Aircraft, skis, surgical tapes, automobiles, food packaging, and footwear all use adhesives to some extent. T h e adhesives are as varied as the applications in which they are used, and new adhesive resins are continually being synthesized and formulated to meet the ever-increasing demands for materials which will withstand extreme environmental conditions, will be more easily applied, and will provide higher strengths than materials now in use. I n most cases, regardless of use, the adhesive is a synthetic high polymer. The ability of a resin to perform satisfactorily in a specific adhesive application is dependent upon the fundamentals of polymer science, such as hydrogen bonding, cohesive energy density, flexibility, molecular organization, crystallization, cross-linking, and wetting (37). Chemists are now using these fundamentals to design a polymer which will satisfy a given bondin5 requirement. The manufacture of adhesives is thus becoming more sophisticated and specialized since it is almost impossible to make an adhesive resin to provide an acceptable performance in several areas of application. However, even with the increased number of new resins developed for adhesive use, the adhesive formulator continues to play a n important role since tackifiers, flow control agents, cross-linking agents, wetting agents, and fillers are still needed to some extent in the new resins. Not only are new resins and processes being developed, but chemists are also improving old adhesive systems.
0
High Temperature Adhesives
New adhesive resins are often developed in response to the needs of industry where the environmental conditions are too severe for existing materials. Such a response has led to the development of polymers for use as adhesives a t high temperatures. These polymers which have been developed contain aromatic and heterocyclic rings and are stable a t temperatures in excess of 500" F. (78, 33). All of these highly aromatic and heterocyclic resins are insoluble and infusible under ordinary conditions and, to be applicable for adhesive application, must be used as a prepolymer. Even these prepolyniers are only sparingly soluble and are used in solvent-free formulations. The prepolymer must be heated to 150' to 400' C. to assure sufficient flow for subsequent wetting of the substrate. Pressure, as much as 200 p.s.i. in some cases, also promotes flow and more intimate contact with the substrate. Therefore, the primary drawbacks of these new adhesive resins are the drastic processing conditions necessary to produce a strong bond (75). However, these resins have outstanding high temperature oxidation resistance and when fully cured can withstand prolonged exposure at 500' F. The most promising of these high temperature resins are the polyirnides and the polybenzimidazoles (PBI) (Figure 1). Table I (3) compares the two leading high temperature adhesives with an VOL. 5 9
NO. 8
AUGUST
1967
91
P o l y benzimidazole
n
0
H epoxy/phenolic. The polyimide adhesi1.e was cured a t 575" F., the polybenzimidazole at 600" F., and the epoxy/phenolic at 350' F. The bonds were not postcured. The polyimide is more heat resistant than the polybenzimidazole (Figure 2). Even though thc polybenzimidazole has the higher tensile shear strength at lower temperatures, it cannot Withstand prolonged exposure at high temperatures and, therefore, the polyimide is more suitable for applications in supersonic aircraft, However, both of the aromatic resins arc superior to other adhesives systems for high temperature applications.
+?mN)-& N
N
'/
1
Poly b enzothioazole 0
0
Polyimide Neoprene Cements
In past years, neoprene adhesives which were a formulation of neoprene, a phenolic resin (such as p-tertbutyl phenolic resin), and magnesium and zinc oxides would have a tendency to separate into two distinct phases. This separation was described as phasing, and neoprene cements would undergo this phenoriienon in a relatively short time. Investigators have indicated that stable dispersions of small insoluble particles can be prepared if a layer of molecules, such as polymer molecules, can be adsorbed on their surface. TVlien this adsorbed layer is sufficiently thick, it prevents the insoluble particles from approaching each other and thus froin aggregating (76). Investigators ha\-e recently sho\vii that this typc of stabilization can be achieved in neoprene adhesive systerns. \t'hen the phenolic resin is absent from the system, the dispersion is stabilized by the adsorbed neoprene chains; however, when the phenolic resin is added, much of the adsorbed neoprene is replaced by the smaller and more polar phenolic. T h e commercially available phenolic resins had a number average molecular weight of approximately 900. Therefore, in a phenolic resin there is a considerable amount of low molecular weight species. TVhen these low molecular weight spccies are too small to prevent the oxide particles from forming agglomerates, a second phase is formed. Investigators have shown that when the low molecular weight phenolic is initially removed by some suitable means, the resulting adhesive forniulation is stable and no phasing is observed after several months on the shelf ( 9 ) . T h e same workers experimented with many neoprene formulations and concluded that phasing is primarily dependent upon the molecular weight distribution of the phenolic resin. IVhen there is an appreciable ariiourit of low molecular weight species present, phasing will occur. When these species are removed by fractionation or by further condensation of the phenolic resin, phasing is retarded or stopped completely as can be seen in Table 11. Since the disclosure of this research, the companies 92
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Polyquinoxaline 0
Figure 7 .
Polyimides und polybenzimidazoles
2000
.-
P
C
0
10
20
30
40
50
60
Hours Heot Ageing ut 700°F
Figure 2. Pol) benzimidazole and pobiniide adhesives heat aEinLg
which produce and supply phenolic resin to the adhesives industry have now placed on the market resins which are nonphasing when incorporated into neoprene adhesive formulations. Hot Melt Adhesives
There has been a steady increase in the consumption of hot melt adhesives over the past several years in such applications as furniture, shoes, counter tops, assembly of plastics, and in packaging. The growth of hot melts in packaging can be seen by past statistics and forecasts for 1970. Currently, 50 million pounds of hot melt resin are used by the packaging industry, includinq resin used as adhesives and as coatings. Of that figure, about 20% is for adhesives. By 1970 it is projected that 250 million pounds of hot melt resin will be used, approximately 50 million pounds as adhesive ( 1 9 ) . This acceptance of hot melt adhesives is due to the development of adhesive resins that have been specially tailored for hot melt application and new types of equipment to facilitate the application. Currently, ethylene-vinyl acetate copolymers are used almost universally as hot melt adhesives and coatings, because a wide range of compositions are available which are compatible with most other ingredients. The resin contributes flexibility, strength, and cohesiveness. The other ingredients usually present in a hot melt formulation are paraffin or microcrystalline waxes, which are inexpensive fillers and moisture resistant, and natural or synthetic resins such as polyterpenes or rosin esters, which act as wetting agents and add tack and fluidity. A new synthetic resin has recently been developed for use in hot melt formulations. This resin is a terpene-urethane which has been developed recently as an additive for vinyl acetate-ethylene hot melt systems. This resin when properly formulated provides excellent resistance to heat for extended periods of time with good retention of color. The terpene-urethane resin provided good adhesion for the hot melt and it is in this area where the new resin might be preferred over a usual terpene resin (77). Because of the Treat success of vinyl acetate-ethylene copolymers in hot melt applications, a new family of ethylene copolymers have been developed. These polymers, composed of ethylene and ethyl or isobutyl acrylate, are commercially available in a wide range of melt indices and comononier contents. The effect of the isobutyl or ethyl acrylate in the polymer is to increase compatibility with the other components, improve the flexibility and toughness of the resin, and improve adhesion. The incorporation of the acrylic ester permits maximum grease resistance and low temperature flexibility. Table I11 (6, 33) compares the properties of some of the ethylene copolymers on the market today. Another new adhesive is also based on ethylene ( 2 3 ) . The ionomer resins which have recently become available corninercially have excellent adhesion to a wide range of substrates, including metals such as copper, bronze, tin, steel, and aluminum; and nonmetals such as glass, cotton, paper, wood, and some plastic films (7). This resin may be applied in the melt or in a film form for laminating.
TABLE I. SHEAR COMPARISON OF T E N S I L E STRENGTH W I T H TEMPERATURE
Tensile Shear Strength Formulated
Unjormulated
1
77’ F.
EPOXY /
I
2500
2200 1400
I
I
3900 3500
4000 2100 0
100 hr. 200 hr. 600
2000
900 0
1 0 hr. 24 hr. 60 hr.
...
TABLE II. P H A S I N G E L I M I N A T E D BY REMOVAL OF LOW MOLECULAR W E I G H T SPECIES
I
Au. Mol. Wt.
Resin
Commercial
Av. mol. wt. Phasing, 70vol. as clear layer Months at 24’ C. 1 10
900
TABLE 1 1 1 .
COMPARISON OF E T H Y L E N E COPOLYMERSa
E-EA
% co-
monomer 18-22 2-3 Melt index 0.928 Density Tensile, 2000 p.s.i. Elongation, 700 % Yield strength, 6700 p.s.i. Hardness, Shore “D” 35 Vicaf softening pt., O F. Low temp. flexibility,
c.
140
Modified
E-iBA
E-VA
E-VA
28-32 2-3 0.937
18-22 2-3 0.921
28-32 2-3 0.925
27-29 5-6 0,953
18 2.5 1.941
1185
2000
1290
>1840
2750
810
750
770
>?BO
850
405
820
257
390
27
41
26
30
IO?
143
100
105
< -70
:-70
< -70
E , ethylene; E A , e//iyl acrylate; L B A ,isobutyl acrylate; V A , uinyl
VOL. 5 9
NO. 8
AUGUST
390
-
acetate
1967
93
The properties of the resin (Table IV) are to a great extent due to the presence of strong intercliain forces which develop between ionized carboxyl groups and metallic ions. Unlike covalent bonds in conventionally cross-linked resins, the iononier bonds are thermally reversible permitting melt fabrication to be performed with conventional technology. Since the resin has excellent adhesion to many substrates, it can be used for laminating aiid in “sandwich” constructions. The polymer is extremely resistant to bases and only slowly attacked by strong acids. Hot melt adhesives are also vying for use in applications where tliermosettiiig adhesives are normally used. The phenoxl--type resins combine thermoplasticity with many of the outstanding properties of a thermosetting resin. The resin has the following structure :
The phenoxy resin has a chemical structure similar to epoxy resins; however, the phenoxy is a high molecular weight thermoplastic polymer which needs no further conversion and has an infinite shelf life (29) (Table \I), The phenoxy resins can form strong bonds to metals in seconds at a temperature of 600’ to 650’ F. At these bondiiig conditions, a tensile lap shear strength of 3500 to 4000 p.s.i. can be obtained. The same bond can be made in 2 to 3 min. at 500’ F. and in 30 min. at 375’ F. The phenoxy resins can also be plasticized with many of the common plasticizers and still maintain a high percentage of their strength, while their bonding temperature is siibstantially reduced. An effective hot melt is one into which a polyvinyl ether is blended into the phenoxy. At present, the phenoxy resin is gaining in popularity as a hot melt adhesive in applications where a strong bond, formed in a matter of minutes, is needed. One-Package Structural Adhesive
Considerable research and development is being done in the production of a one-package, tlzermosettiiig structural adhesive. There is a wide range of methods for achieving such an adhesive. The most promising method appears to be the development of a latent curiizg system for epoxy resins, and numerous patents have appeared which disclose the various types of curing systems. Diacidhydrazides (30) are able to cure epoxy resins in half an hour and are stable a t room temperatures for 4 months or more. \Vlien isophthalpldihydrazide was used at a concentration of 2.5 plir. (parts per 100 of rubber or resin) to cure an epoxy, such as ERL-2774, a room temperature shear strength of 2950 p.s.i. was achieved ; the shear strength was maintained between -55’ C. and 150’ C. A dihydrazide-dicyandiamide hardener system ( 4 ) provides a higher degree of cure and a higher degree of cross-link density than either a dihydrazide or dicyandiamide alone. Such an epoxy system is also stable for a few months at room ternperature. Dihydrazides have also been mixed with diallylnielamine for use as latent epoxy hardeners (25). 94
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
T A B L E IV. PROPERTIES OF AN ETHYLENE COPOLYMER
i Density, g./’cc. Melt index Tensile strength, p.s.i. Elongation, yo Yield strength, p.s.i. Softening pt. Vicat, O F. Max. processing temp., ’ F Upper service temp., n. load,
T A B L E V.
Surlyi A - 7650 0.95 1.8-2.4 4000 450 1800 170 600 160
F.
PROPERTIES OF P H E N O X Y RESINS
I Density, g./cc. Melt flow Tensile strength, p.s.i. Elongation, yo Softening temp., F.
Phenoxy Resin 1. I 8 2.5 9000-9500 50-100 21 2
A wide range of latent triazine hardeners has also been synthesized which alone or lvith dicyandiarnide produce relatively rapidly cured epoxy resins. Another latent hardening system for epoxy resins consists of a polycarboxylic acid anhydride and an accelerator such as urea or thiourea (22). Gelation occurs in approximately 1 5 min. at 150’ C. This type of system, however, is not as latent as previous systems and the shelf life does not appear to be more than 3 months. Another new hardener for epoxy resins, which has recently been developed, is that of a boroxine-amine adduct (21). This adduct is characterized by low catalytic activity at room temperature and produces a flexible and impact rcsistant cured resin. These two properties are usually absent in normal BFa-aniine complexes. These epoxy systems wdl cure in 1 hr. at 150’ C. The latency appears to be limited, for there is a large increase in viscosity during the first I 5 days storage at rooin ternperature. Still another latent epoxy system is one which utilizcs amine salts of hydrofluoboric acid ( I ) . These salts whcn mixed with epoxy resins ill gel the resin within 1 or 2 min. at 160’ C., but will remain unreactive at room temperature for an extended period of time. rncchanisrn for this cure has recently heen proposed ( 1 2 ) . Unfortunately, the use of this curing agent in adhesive formulations has not been successful since the cured resin does not possess the necessary flexibility. A final curing system for epoxy resins wortliy of mention is the hardener system, a complex of aromatic amines aiid inorganic salts such as cadniiurii bromide or zinc bromide (73). A coinplex of zinc broinidc and an aromatic amine such as p-phenylenediamine will cure C. Clement Anderson i s Senior Research Chemist in the Coatings and Resins D i v i s i o n of PPG Industries, Research and Development Center, S f r i n g d a l e , Pa. This is the Jirst review on adhesives by Dr. d n d e r s o n . AUTHOR
Release Agent
Cure Time, M i n .
Glycerin Hexylene glycol Polyethylene glycol 400 Polypropylene glycol 150 a
M = 7000; M M = 1,000,000;
Temp., a F.
5-8
180
15 15 15 15 15
212 325 212 325 325
Viscosity at 25' C.a Cured
Initial
1 da.
Yes Yes Yes No Yes Yes
1.5 MM
...
I
I
1.9 M
2 MM
...
... 10 M 10 M
38da.
...
25 'M
15'M
1
78 da.
...
... 10 M
12da.
20'M
... 25 'M 20 M
40 'M 30 M
.
.
50 'M 35 M
I
...
T h e ahoue data are the result o f initial exploration; therefore, the cure time and temberature have not been obtimized.
a n epoxy resin in 3 min. a t 150' C. Such one-package epoxy systems are stable for 3 months a t ambient temperatures. All of the methods mentioned are attempts to produce one-package epoxy systems which may or may not be applicable for adhesive use. At present, a n epoxy adhesive is on the market which will cure in several minutes a t 350' F. and exhibits a high shear strength; however, the shelf life is limited to only a few months at room temperature. Therefore, the need for a fast curing, long shelf life one-package epoxy still exists. A latent curing system can be mechanically separated from the bulk of the resin by encapsulation of the hardener. Ideally, in a resin-catalyst system, the catalyst is microencapsulated and the capsules are mixed into the resin, This mixture can then be stored indefinitely since the active components are separated by the walls of the microcapsules. At the time of application the active ingredient can be released from the capsule by external pressure, heat, or chemical agent. Various methods of encapsulating reactive ingredients have appeared in the literature (74, 24, 32), and such capsules have been very successful in other areas. However, in one-package adhesive systems, which incorporate a n active hardener, there are various difficulties which
must be overcome before the potential of such a system is realized. I n systems in which an organic solvent is encapsulated there is very little loss of the encapsulated material through the unreactive capsule wall (26) ; however, in cases where a reactive material is encapsulated, there is a tendency for the active ingredient to attack the capsule wall. Once a n encapsulating material is developed which is impervious to reactive ingredients, the capsule method for separating reactive components in a one-package system will be very attractive. A special type of encapsulation is provided by the use of molecular sieves. Molecular sieves can isolate a reactive chemical or catalyst until such time as it is needed either for reaction or to initiate reaction (20). Any molecule small enough to enter the adsorption cavities can be preloaded within the sieve and released when needed. T h e reactive catalyst can be released by heating. I t can be seen from Figure 3 that the release is over a narrow temperature range (20). A number of hydroxyl bearing release agents have also been tested. They will preferentially displace amine hardeners a t elevated temperatures. The latency of the epoxy system can be seen in Table V I (27). An epoxy system such as the one shown has a lap shear strength of 3000 to 3500 TABLE
VII. COMPARISON O F SURFACE T R E A T M E N T O F POLYMER F I L M S
Tensile Strength, P. s.I.
Temperature
Ti
Untreated G-80 polytetrafluoroethylene film Treated G-80 polytetrafluoroethylene film, 10 min. Untreated G-80 polytetrafluoroethylene film Treated G-80 polytetrafluoroethylene film, 10 min. Untreated Marlex polyethylene film Treated Marlex polyethylene film Untreated Marlex polyethylene film Treated Marlex polyethylene film
Temp. Joint Formation,
c.
50
50
600
50
50
150
2400
150
400 2200
50 50
1000
150 150
2700
Figure 3. Rate of release for catalyst adsorbed i n molecular sieve VOL. 5 9
NO.
a AUGUST 1967
95
p.s.i. The sieves contain between 15 and 207, by weight of the active ingredients and therefore a large number of siel-es are needed to contain a stoichiometric ratio of hardener to epoxy resin. Also the loaded sieves are very sensitive to moisture, and premature release of hardener will result if precautions are not taken to exclude all water. The sieves are also applicable in urethane systems which ordinarily are marketed in twopackage systems (28). New Theories
Not all of the advances in adhesives and adhesion have been made in the development of new inaterials or the improvement of old ones. Investigators have developed a process of substrate treatment to improve the performance of adhesives used to bond the material. Moreover, bulk properties of the substrate such as color, tensile strength: and elongation are unaffected by the treatment; and the wettability of the surface is unchanged. 'iVhereas earlier methods of surface treatment produced changes in the wettability of the substrate to permit the adhesive to spread and have a more intimate contact with the surface of the substrate, this method causes no such change on the surface, and the critical surface tension remains the same. This process for the treatment of low surface energy substrates is called Casing which stands for cross-linking by activated species of inert gases. These investigators have indicated that strong adhesive bonds cannot be made with low surface energy polymers, such as polyethylene, because of wliat is termed a weak boundary layer at the surface of the polymer (10,7 7 ) . This weak boundary layer is composed of low molecular weight species w7hich cause adhesive joint failure because of poor cohesive strength. Even when an adhesive spreads spontaneously, the weak boundary layer permits only tlie formation of weak bonds. The technique of casing involves impinging electronically excited species of rare gases upon the surface of the substrate. LVlien the excited spccies come into contact with the polymeric substrate, they cause radicals to form because they abstract hydrogen atoms. These radicals immediately react to form a tightly cross-linked matrix on the surface of the polymer. As a result of the high cross-link density, the cohesive strength is tremendously increased and the weak boundary layer has been removed. When adhesive bonds are made with the treated polyrners, the adhesive joints which are formed are considerably stronger than joints made with the untreated polymers. The investigators have indicated that the contact time of the activated gases with the polymeric substrate is as short as 1 sec. and there is a considerable increase in adhesive joint strength. Much of the work reported has been with treatment of polyethylene and Teflon. Gases which, when activated successfully, Cross-linked the surface of the polymer included helium, argon, krypton, neon, hydrogen, and nitrogen. Table VI1 shows the great improvement of tensile strength. 96
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
In subsequent work, the investigators have indicated that the surface cross-linking takes place only in arnorphous regions of the polymer and that strong joints are obtained when the cross-linked matrix is only 50C A. thick. Nylon (Z),when treated with activated heliurn, also produces a cross-linked surface from 100 to several hundred angstroms thick. As with polyethylene and polytetrafluoroethylene, nylon when trcated gave strong adhesive joints with conventional adhesives such as epoxy resins. Treated nylon fibers may thus be useful in the tire industry. I n bonding materials such as rubber and nylon, the weakest link has been the low mechanical strength in the surface region of nylon. By treating the nylon fiber with activated species of inert gases, this weakness is removed and a strong rubber-to-nylon bond can be formed. At the moment, only small areas can be treated at a given time, and it has been successful with a limited number of polymers ; however, more polymers are being tested and equipment is being developed so that fibers can be continuously treated. Although commercialization of this process is still in the future, it certainly appears that this method is potentially irnportant in permitting strong bonds to be formed between materials which until now could not be formed. Over the past year, significant progress has been made in the improvement of existing adhesives and in the invention of new resins which are the response for the need of higher performance adhesives. REF E R ENCES (1) Buck, B. I . : e t n l . (to Yarslcy Rewarrh l ~ h . )Brit. , Parent 963,058 (.Tuly 8, 1964). (2) Chern. Enq. Neit,i 44, 58-9 (1966). (3) Courrrighr, J. R., Ikeda, C . K., hbsrracr, Soc. Plasrics Engrs., Inc., J u n e 1964. (4) Dowi-;A. R . , Jr. (to [Vcstinghousc i.1ectric C o . ) , U S. Parent 3,294,748 (Dcc. 27, 1966). (5) Dow C o . , '.Lthylcne-Copolymer Information Bulletin (1966)," Form No.
170-231-3111-766. 1966. (6) Du Pont Co.,E , I., T e c h . Informarion Bull., .'Ll\nx 460," Form KO.A 32412, September 1963. (7) D u Ponr Co., E. I., T e c h . Vcwslettcr S o , I, "Laminarian with Surlyn A , Ionarner Resin." Bull. No, A-50740, Ailgust 1966. (8) Fincke: J . K., e t el., T e c h . R e p r . AFML-TR-66-188, M a y 1966. (9) Garrett, R . R., Lawrence, R . D., Adhesive and Sealant Council, Inc., Proc. 1966 Sprinq Sem-inar (March 1966). (10) Hiinsen, R. H., Schonhorn, H., ACS Preprint, ..Org. Coatings Plastics Chem. h-0. 1," 26, 162-67 (1966;. (11) I b i d . , K O .2, p. 266. 112) Harris, J. J . . T e a i n . S. C . , J . '4,bpl. Poij~,Sci. 10, 523 (1966). (13) Herbert, P.,e t a / . (to Borden Co.1, U . S . P a t r n i 3,310,602 (March 21, 1967). Lee, H. R.( t o Gcncral Elrcrric C o . ) . I b t d . , 3,264,248 (-\us. 2, 1966). Levine, H . H., Adheiriesdge 8, 37 (1965). (16) Mackor, E. L., J . Coilold Scr. 7, 535 (1952). (17; h i a r t i n , R . A . , T r a v e r , C. R., "Resins in Hoc M e l t Coatings," T A P P I Meeting, Fch. 24, 1966. (18) hiarvel. C., J . Poi). Sci. 50, 511 ( 1 9 6 1 ) . (19) . , "dud. PackoPinP 40. 113 11966). . (20) I\-eddenriep, R. J . , Adhesives and Sealan: Council, Inc., Proc. 1966 Spring Seminar ( M a r c h 1966). (21) Pollnow, G. F.; Haworrh, D. T. ( t o .\ilk Chalmcrs Co.), U. S , Patent 3,284,408 (Nov. 8, 1966:. (22) P r a r t , R.J. (to .411is Chalmers C o . ) ,h d . . 3,294,749 (Dec. 27, 1 9 6 6 ) . (23) Rees, R. \\-. ( t o E. I . d u Pont Co.), Zbid., 3,264,272 (Aug. 2 , 1966). (24) Reyes, Z.(to Inrernarional Business Machine C o r p . ) . Ibid., 3,173,878 (March 16, 1965). (25) Schurb, J , N. (to Minnesota hfining a n d Xlanufaciurine C o . ) , Zbid., 3,030,247 (April 17, 1 9 6 2 ) . (26) Twiss, S.: .\PPI. Polymer Symp. No. 3, 455 (1966). (27) Union Carbide Corp.. "Chemical-Loadcd Molecular Sieves as Epoxy Curing Agents a n d Catalysts,'' Form Xo, F-2767. (28) Union Carbide Corp., "Moleculdr Sieves and Chemical-loaded hlulccular Sieres in Polyurethane Systems," Form S o . F-2766. ( 2 9 ) U n i o n Carbide Corp., "Phenoxy Product Data," Form No. J-2421-A. (30) \%'PRY, R. Id, (to Minneiotn Mining a n d Manufacturing C o , ) , U. S . P a l r n t 2,847,395 (Aug. 12, 19.58:. (31) IVeiss, P., PoljmerS&. I'and C : N o . 12, 169-83 (1966). (32) IFIler, B. (to Dunlop R u b b e r G o . ) , S o u t h African Patcnt 31518 (July 15, 1964,. (33) I'eagcr, R . E., i\ppl. Polymer S y m p . X o . 3, 369 (1966) l
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I
