Adhesion of Polymeric Binders to Glass Fiber G. D. Andreevska’ and Yu. A. Gorbatkina Institute of Chemical Physics, Academy of Sciences of the USSR, rlfoscow, USSR
A procedure and apparatus have been developed for determining adhesion of polymeric binders directly to the fiber surface. Use of the method yielded vast experimental data characterizing the adhesion strength as a function of the polymer type and its chemical structure. Adhesion of polymeric binders to the pure and modified glass fiber surface has been estimated. Adhesion strength and tensile strength of glass-reinforced plastics have been compared.
Physical and mechanical properties of glass-reinforced plastics depend essentially on how tightly the resin adheres to a glass fiber. It is important, therefore, to reliably estimate various resins as adhering directly to the surface of a fiber. Adhesion strength is difficult to measure with bulk glass samples because they are fragile and have low mechanical strength, aiid strongly adhering resins destroy them cohesively. Besides, the bulk samples should be carefully ground and polished so that the glass surface is affected mechanically and chemically by solid particles of a grinding powder or by acid-polishing suspensions. Therefore, it seemed promising to study the resins as adhering t o a clean and smooth surface of flame-polished glass fibers. A procedure has been developed in the Reinforced Plastics Laboratory (Institute of Chemical Physics, the USSR Academy of Sciences) to determine adhesion directly a t the fiber. The procedure is essentially as follows. Two metallic wires (or two glass fibers 120-150 p in diameter) are positioned in parallel, one precisely above the other. A segment of each fiber is covered by the resin studied. A thin fiber 7-20 p in diameter normal t o the thick fibers is stretched between them. Thicker fibers are drawn together 50 that the thin fiber is entirely immersed in the resin covering them. See Figure 1. The procedure produces a resin layer located on a very short interval of the thin fiber (Shiryaeva and Andreevska, 1962; Andreevska and Shiryaeva, 1963; Gorbatkina et al., 1963). The sample is tightened into a paper or metallic foil frame and left for about 20 hr in the air, then polymerized in an oven, monitoring thermal conditions previously determined for the resin. The cooled sample is measured by an eyepiecemicrometer microscope for its adhesion length, 1, and fiber diameter, d. The contact surface is a cylinder surface. The breaking machine measures the force necessary to shear the fiber from the resin layer. Adhesion strength of a sample is calculated from:
T,
=
P/S
=
P/?rdl
where T , is adhesion strength, kg/cm2; P is the shearing force, emz). kg; S is contact surface, em2 (8is about 1 X Adhesion strength, To,is calculated by
T,
Ti/n
= n
=1
+ AT
T o whom correspondence should be addressed. 24
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1972
where AT is a correction, owing to cohesive destruction of the fibers, which occurs partially in the experiments. AT is seldom in excess of 7-10ojO of the entire adhesion, To. The data processing is reported in detail in the paper by Gorbatkina and Khazanovich (1967). The procedure above applies to synthetic fibers or thin metallic wires as well. The determination of adhesion requires many samples. Therefore an apparatus (Figure 2) (Andreevska et al., 1964) has been designed to prepare about 100 samples in 6 hr. The results obtained with the above procedure are fairly reproducible. We have used this method to study the effect of various factors on the adhesion strength during the formation or destruction (Andreevska, 1966b) of adhesive mountings; the results provide an estimation of “order of adhesion” of various resins; for example, epoxy resins (which were or were not modified by other resins) adhere to the surface of alumoborosilicate fibers a t about 320-480 kg/cm*, phenolformaldehyde resins do so a t 200-300, polyester ones a t 100-150, and silicon resins a t 100-300 kg/cm2, depending on the influence of resin structure or type of solvent. Polymeric adhesion to glass fiber is affected by various chemical, physical, or mechanical variables which operate during the formation, as well as destructioii of adhesive mountings. One of the most important variables is the chemical structure of a polymeric binder or the relative content of polar functional groups or mobility and flexibility of polymeric fragments constituting the net structure of a thermoset resin. Table I presents the adhesion of various resins t o a pure surface (unsized) of alumoborosilicate E-glass fibers. The resins in the table (epoxy, phenolic, polyester) are most often used to obtain glass-reinforced plastics. Also adhesion as affected by the type or concentration of the groups introduced into the resin is demonstrated. The results are averaged over 60-80 determinations carried out with polymeric binders of each type. The variation is about 7-9%; accuracy is 3-5%. The results demonstrate that adhesion is affected significantly by chemical “composition” of a polymeric binder-e.g., adhesion strength is affected by the relative content of epoxy groups in a polymer. Adhesion of a resin characterized by higher content of epoxy groups is approximately 12% greater than that of a resin with a lower content of the groups. A resin modified by DEG-1 (a product obtained from diethylene glycol condensed with epichlorohydrin containing about 25% epoxy groups) has a distinctly better adhesion.
Table 1. Adhesion of Certoin Polymeric Binders to Gloss Fiber
Wt%
Composition of polymeric binder
Epoxy resin (1517Yc epoxy groups) Epoxy resin (18-2070 epoxy groups) The same, modified by DEG-1 Thiocol (polysulfide) Thiocol (polysulfide) Thiocol (polysulfide) Glycidyl ether Glycidyl ether Phenolformaldehyde resin The same, modified by Furfurol Polyvinyl butyral Epoxy resin Polyester resin The same, modified by epoxy resin
Adherion strength, kg/cme
327 366 15 5 10 23 40 60
453 396 386 283 389 297 300 281 240 376 158 295
I Figure 1.
Thin fiber as located in a resin
(I.
Thin fiber. b. Resin layer on thick fibers. d. Diameter of thin fiber. I. Length of adhesive mounting. P. Force necenory to extrudefiberfrom resin
~
Small amounts of thiocol introduced into an epoxy polymer improve its adhesion, which may be explained by a greater relaxation and lower residual stress of the polymer. Further increase in the thiocol content decreases the adhesion because mechanical properties of the epoxy-thiocol resin depreciate (e.g., epoxy resin containing 5% thiocol has a tensile strength of about 800 kg/cm2, 10% thiocol corresponds to 700 kg/cm2, and 23’% to a value as low as 560 kg/cm2). Adhesion of an epoxy polymer containing an active diluent, glycidyl ether, depends on the dilution. The ether contains active epoxy groups and therefore improves the adhesion. This is true, however, only when the amouut of ether is below optimal. This may be explained by the fact that higher dilutions make collisions of the functional groups less probable and therefore a solidifier is less active. As a result, the hardened polymer acquires a loose structure resulting in less adhesion. Phenolformaldehyde resins adhere to fibers somewhat weaker than epoxy resins. Their adhesion strength depends on the composition and varies significantly when modifying the latter by other polymers-e.g., polyvinyl butyral introduced into a resol resin decreases its adhesion (owing to a decrease in the number of methylol and hydroxy groups in the system) but iucreases the elasticity. Phenol resins modified by an epoxy resin obtain a markedly geater adhesion strength. Adhesion of polyester resins is comparatively low; it almost doubles after the resins have been modified by epoxies. Active organo-silicon monomers introduced into polymeric binders (or modifying the fiber surface) improve the adhesion to glass fibers, but the lubricating sizes used in manufacturing the glass yarn usually decrease it. However, resins cbaracterized by a higher shrinkage may develop a positive effect of such sizes. Table I1 supplies the data. These results show that active organo-silicon monomers introduced into a polymeric binder (or modifying the surface of a fiber) improve the adhesion significantly. The table shows that resins characterized by low shrinkage suffer 15-2570 reduction in adhesion to glass fibers treated with lubricating sizes, while the adhesion of resins with high shrinkage may be improved (Andreevska et a%,1965). A soft plastifying layer (size) formed on the fiber surface dissipates stresses, hence increases adhesion; this increase is as high as 45% with polyester resins.
Figure 2. 1.3.
2. 4. 5.
Apparatus for routine sampling Standord plate Small drums for stretching thin fibers smoii jaw3 for metal wirer Gloss fibers
Usually, comparatively low mechanical strength of glassreinforced plastics based on silicon resins is thought to be caused by poor adhesion of the resins to the fiber. It is of interest, therefore, t o measure adhesion of silicon resins directly t o a pure surface of a glass fiber. Table I11 shows our da.ta (Ivanova-Mumzhiyeva, 1968; Ivanova-Mumahiyeva et al., 1968) obtained with certain organic polysiloxanes adhering to glass fibers. It also demonstrates adhesion as affected by a solvent. The data demonstrate that silicon resins often adhere to a pure fiber glass surface as well as phenolic or even epoxy resins. Adhesion inherent in the silicon resin-to-glass system may be governed by the fact t,hat both components contain similar “structural unit,s.” The same is true with hydrophobic films firmly attached to the surface and formed on a glass fiber surface modified by organo-silicon monomers. Adhesion of silicon resins is strongly affected by a specific polymer structure. Table I11 shows that adhesion of silicon resins definitely depends on solvent polarity. Certain resins adhere more strongly in nonpolar than polar solvents; this may be attributed to a higher resin-to-solvent affinity. Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. I , 1972
25
?;one Lubricating size None Aminoethoxysilaiie 2% “amino” introduced into resin Lubricat,ing size
348 302 376 438
None
240
~~niiiioetlioxysilaiie 2% “amino” introduced into resin
264 302
strengths of 15,000-19,000 lrg/cm2, values cor high adhesion of the resins (about 350-480 k pheiiolic resins, the unidirectioiial glass-reinforced illastics have tensile strengths of about 9000-12,000 kg/cmS, and this correlates wit’h adhesion strength of the resins, 200-300 kg/cm2. At the same time, glass-reinforced plastics obtained from silicon resins have mechanical strengths twice as low as those from phenolic resins, even though adhesion of the former is not loxer than that of the latter resins. We beliere that low mechanical strength of the plast,ics obtained from silicon resins may be explained by a weak molecuhr interaction of their structures rather than adhesion of the resins-e.g., molecular interaction in polymethylsilosaiie is half that in polyvin ylchloride. To sum up, techniques and apparatus reported here provide a considerable body of evidence concerning adhesion of various resins to a pure and modified surface of glass fibers under conditions which match the fibers’ interacting with polymeric films in real reinforced systems.
Lubricating size
180
Conclusions
Table II. Adhesion of Polymeric Binders of Various Shrinkage to Pure and Modified Glass Fiber Surfaces Fiber surf ace modified b y
Resin
Adhesion strength, kg/cm2
Shrinkage of about 3-47,, Epoxy Epoxy Epoxy phenol Epoxy phenol Epoxy phenol Epoxy phenol Phenolformaldehyde polyvin ylbut yral Phenolformaldehyde polyvin ylbutyral Phenolformaldehyde polyvinylbut,yral Phenolformaldehyde polyvinylbutyral
442 323
Shrinkage of about 6-1070 Polyest er Polyester Epoxy pol yesteracrylate Epoxy polyesteracrylate Epoxy polyesteracrylate Epoxy polyesteracrylate
None Lubricating size None Aminoethoxysilane Vinylethoxysilaiie Lubricating size
158 240 355 418 406 348
Table 111. Adhesion of Silicon Polymers to Glass Fiber Polymer
Solvent
Adhesion strength, kg/cm2
Polyniethylsiloxane
Polar (alcohol, acetone) Polymethylsiloxane Nonpolar (benzene, toluene) Polyphenylsiloxaiie Polar Nonpolar Polyphenylsiloxane Polar Polymethylphen ylsiloxane (hlethyl groups prevailing) Nonpolar Polymethylphenylsiloxane Polar (Phenyl groups prevailing) Nonpolar
300 320 107 150 320 198 291 217
The higher adhesion of polymethylsiloxane as compared with polyphenylsiloxane may be explained by the fact that methyl groups which are smaller than phenyl groups influence the coiled structural units to be pulled (straightened up). This may occur when heating (thermally solidifying) the adhesive mountings and may help release polar groups of certain units of the chain. The groups, in turn, will interact with active sites present on the fiber surface, thus increasing adhesion. The table also shows adhesion of polymethylphenylsiloxanes in which either methyls or phenyls prevail. Again, higher adhesion in the methyl-group-prevailing polymers may be due to the smaller size of methyl as compared with phenyl. Since glass-reinforced plastics are characterized by a tremendously developed glass fiber surface, the more strongly the surface adheres t o the matrix, the stronger the composite. Consequently, mechanical properties of glass-reinforced plastics depend significantly on how strong the reinforcing fibers adhere to a polymeric binder-e.g., unidirectional glassreinforced plastics (hndreevska, 1966a,b, 1967; Andreevska and Zelensky, 1969) obtained from epoxy resins have tensile 26
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
The procedure and apparatus for determining adhesion of polymeric binders directly to t,he fiber surface provide reliable data characterizing adhesion strength in conditions very similar to real-world fiber-reinforced systems. Direct addition of various active agents to the resin, as well as modification of the glass-fiber surface by means of such agents, improves the adhesion strength. The quantitative correlation between the mechanical strength of oriented glass-reinforced plastics and the adhesion strength of polymeric binders of various types used as a matrix has been shown. Acknowledgment
The authors acknowledge the help of the Science Officer of our laboratory, V. G. Ivanora-Mumzhiyeva, in studying silicon resin adhesion. literature Cited
Andreevska, G. D., 2-me Congres des plastiq. renforces., Paris, France, p 249, 1967. Andreevska, G. D., Plast. Kaut., 6 , 347 (1966a). Andreevska, G. D., “Vysokoprochney Orientirovannye Stecloplastiki” (Oriented Glass Reinforced Plastics of High Strength, USSR, in Russian), “Xauka” Publishers (The Science), pp 153-261, 1966b. Andreevska, G. D., Gorbatkina, Yu. A,, Zamotova, A. V., et al., Mechan. Polim. (Mechanics of Polymers, USSR,in Russian), 1, 93 (19651. Andreevska, G. D., Shiryaeva, G. V.,Vysokomol. Soed. (High Polymers, USSR,in Russian), 5 , 1735 (1963). Andreevska, G. D., Shiryaeva, G. V., Illinskiy, A. >I., Standartizatsiya (Standardization, USSR,in Russian), No. 11, 13 ( 1964) . Andreevska, G. D., Zelensky, E. S., 24th Annual Technical Conference, SPI Reinforced Plastics, Washington, D. C., USA, Sect. 19-E, 1969. Gorbatkina, Yu. A., Shiryaeva, G. V., Andreevska, G . D., Zh. Fiz. Khim., 37, 237 (1963). Gorbatkina, Yu. A,, Khazanovich, T. N., Sb. “Fiziko-khiniiya i Mekhanika Stekloplastikov” (Physical Chemistry and >lechanics of Glass Reinforced Plastics, USSR,. in liussiari), Collective Papers, “Nauka” Publishers (The Science), p 64, 1967. Ivanova-Murnzhiyeva, V. G., Sb. “Yubileynaya Konferentsiya Molodykh Uchenykh po Teoretich. Problemam Fiziki” (Symposium on Young Scientists on Theoretical Physics, USSR,in ltussian), Collected Papers, NIITEKhIhI Publishers, p 89, 1968. Ivanova-hlumzhiyeva, V. G., Andreevska, G. D., Gorbatkina, Yu. A,, Sb. “Adgesiya i Prochnost Adgezionnykh Soyedineniy” (Adhesion and Adhesive Bonds Strength, USSR, in Russian), Collected Papers, DKTP Publishers, p 92, 1968. Shiryaeva, G. V., Andreevska, G. D., Plast. M a s s y , No. 4, 43 (1962). RECEIVED for review May 22, 1970 ACCEPTEDMay 19, 1971
