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Epoxidized hydroxy-terminated polybutadiene — synthesis, characterization and toughening studies
Epoxidized hydroxy-terminated polybutadiene synthesis, characterization and toughening studies P.B. Latha, K. Adhinarayanan and R. Ramaswamy (Vikram Sarabhai Space Centre, India) Received 23 June 1993; revised 6 October 1993 Hydroxy-terminated polybutadiene was epoxidized using performic acid generated in situ. Different grades of epoxidized hydroxy-terminated polybutadiene (EHTPB)were
prepared by adjusting reaction conditions such as temperature, time and molar ratios of formic acid and hydrogen peroxide. The EHTPBwas characterized by chemical and spectroscopic methods. EHTPB was found to be a good toughening agent for epoxy resins, which are brittle at room temperature. The mechanical properties of a toughened epoxy with EHTPBas the toughening agent were evaluated. Lap shear strength and T-peel strength were observed to increase with increasing EHTPBcontent, pass through a maximum at an EHTPBcontent of about 10 parts per 100 parts epoxy resin (pphr) and then decrease. The enhancement of mechanical properties was attributed to the higher toughness produced by the dispersed rubber particles. At higher EHTPBcontent, the rubber phase became continuous, and the system exhibited a fall in mechanical properties due to the flexibilization effect. The glass transition temperature (Tg) of the cured epoxy resin was not affected by the incorporation of up to lO pphr EHTPBbut higher levels Of EHTPBdecreased T o due to flexibilization.
Key words: adhesives; t o u g h e n e d epoxies; flexibilization; epoxidized h y d r o x y terminated polybutadiene
Epoxy resins are highly crosslinked polymers that are brittle and glassy at room temperature. These materials typically exhibit high moduli, near linear elastic stress/ strain behaviour, very good adhesion, excellent chemical resistance, and good electrical insulation = 3. Practical uses of epoxies include load-bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is their poor resistance to fracture. For structural adhesives, a high glass transition temperature (Tg) is an essential requirement for maintaining adhesive strength at high temperatures. All unmodified cured epoxy resins with relatively high T~ have one drawback - - their brittleness which can produce failure of the adhesive joint 4.
One method of improving the fracture properties of epoxy resins is to incorporate reactive liquid rubbers into the formulations. Extensive studies 5 7 have been reported on toughness enhancement of cured epoxy resins without significant reduction in thermal and mechanical properties by the addition of low levels of reactive liquid rubbers such as carboxy-terminated polybutadiene-acrylonitrile (CTBN), epoxidized hydroxy-terminated natural rubber (EHTNR),etc. 8 10. A small amount of rubber in the form of discrete particles in a glassy thermoset resin, such as an epoxy, can greatly improve crack resistance and impact strength 11 ' 12 . This improvement is achieved without significantly decreasing the thermal or mechanical properties. The theory13 is that, during the curing cycle,
0143-7496/94/01/0057-05 (c~ 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 1 JANUARY 1994
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elements of reactive liquid polymer precipitate from the matrix and lbrm a homogeneously dispersed particle phase. This two-phase morphology is the key to the toughening mechanism of the host matrix. The twophase structure results in higher toughness because the mechanical energy is uniformly distributed by the rubber particles, thereby reducing the local stress concentration. The result is that a higher external load is required to break the specimen. This improvement is achieved without reducing the thermal properties of the base epoxy resin since the epoxy matrix contains very little or no rubber t4. In this work epoxy resin based on the diglycidylether of bisphenol A (DGEBA) was toughened using different grades of epoxidized hydroxy-terminated polybutadiene (EHTPB). The epoxidation of hydroxy-terminated polybutadiene (HTPB) was performed mainly to increase the polarity and hence the compatibility with the epoxy resin. This is an essential condition for any reactive liquid elastomer to exhibit phase separation during curing. The novelty of the work lies in the fact that the incorporation of EHTPB into the epoxy resin system resulted in increase of lap shear bond strength, impact strength and tensile strength of the cured system. This is similar to what was previously reported during modification of DGEBA epoxy resin using CTBN, amine-terminated polybutadiene acrylonitrile (ATBN), mercapto-terminated polybutadiene acrylonitrile (MTBN), etc. Epoxidation of unsaturated elastomers is well known~5, ~6. In the process of preparing EHTPB we followed the peroxy acid epoxidation of HTPB, the peroxy acid being generated in situ using aliphatic acid and hydrogen peroxide. EHTPB was characterized by chemical and spectroscopic methods. 2,4,6-Tris(N,Ndimethylaminomethyl) phenol was used as the curing agent. This curing agent is known for its ability to induce homopolymerization of epoxy resins, resulting in cured systems possessing better high temperature properties:. The epoxy resin was cured at ambient temperature and then postcured at high temperature. Mechanical properties such as tensile strength and elongation and bonding properties such as lap shear strength on A1/AI were evaluated for the toughened epoxy system. The impact strength and Tg of the toughened system are also discussed. Experimental Materials
Araldite GY250 epoxy resin (Ciba Ltd) was chosen as the control resin; its epoxy value is 5.5eqkg -1. 2,4,6Tris(N,N-dimethylaminomethyl) phenol (HY960, Ciba) was curing agent. The HTPB chosen for epoxidation was produced at Vikram Sarabhai Space Centre, India; hydroxyl value = 34.5 mg KOH g- 1, density = 0.9 gcm 3, viscosity = 40-60 Poise and number-average molecular weight (Mn) = 2730. Synthesis and characterization of EHTPB
HTPB was epoxidized using performic acid generated in situ by the reaction of hydrogen peroxide with formic acid. HTPB (30 g) was mixed with formic acid (26 g) in toluene (300 ml) and stirred well in a water bath at 25°C. To this hydrogen peroxide (32 g; 30%)
58
INT.J.ADHESlON AND ADHESIVES J A N U A R Y 1994
was added dropwise m order to recycle the lornlic acid used. This was necessary to reduce the acid content and thus prevent opening of the epoxide rings lk~rmcd. which are very sensitive to acids. Four different types of EI-|TPB with different epoxy values were prepared by adjusting experimental parameters such as temperature, time and molar ratios of acid and hydrogen peroxide (see Tables 1 3). The epoxy value of EHTPB was determined using the pyridine hydrochloric acid method. EHTPB (0.5 g) was refluxed with pyridine hydrochloric acid (20 ml). Upon cooling, the mixture was titrated with NaOH (0. I N) using phenolphthalein as indicator. Infra-red (IR) spectra of the samples showed a sharp peak corresponding to epoxy groups at 910cm t confirming epoxidation of the double bonds in HTPB. Resin formulations
Different formulations containing 0 to 20 parts by weight EHTPB per 100 parts by weight GY250 resin (pphr) were made. These formulations were mixed with 6 parts by weight of 2,4,6-tris(N,N-dimethylaminomethyl) phenol per 100 parts by weight of bisphenol epoxy resin, cured at ambient temperature and then postcured at 120°C for 1 h.
Table 1.
Effect of H202 on epoxy value of EHTPB Molar ratio
Epoxy value (eq kg-1)
HTPB
HCOOH
H202
1 1 1 1 1
1 1 1 1 1
0.5 1 1.5 2 3
Table 2.
0.6 1 1.5 1.75 1.6
Effect of formic acid on epoxy value of EHTPB Molar ratio
Epoxy value (eq kg- 1)
HTPB
HCOOH
H202
1 1 1 1
1 2 3 4
2 2 2 2
1.75 2.1 1.5 1.25
Table 3. Variation of epoxy value of EHTPB with temperature of reaction* Temperature (°C)
Epoxy value (eq kg -1)
25 30 40 50 60 70
2.1 2.4 2.6 3 3.2 2.9
* M o l a r ratio HTPB : H C O O H : H202 - 1 : 2 : 2
Evaluation of adhesive properties
Impact strength determination
Lap shear bond strength specimens were made according to ASTM-D-1002 specifications. B 51 SWP aluminium alloy containing 0.8% Mg, 0.4% Fe, 0.6% Mn and 1% Si was used for evaluating bonding properties. Aluminium adherends were abraded with emery paper, etched with chromic acid solution, rinsed with distilled water, dried at 100°C for 30 min and then cooled. The resin formulations were then applied over the area to be bonded and joined using contact pressure. These bonded specimens were kept at ambient temperature for a period of 24 h and then cured at 120°C for 1 h. The lap shear strength of the bonded specimens was determined in an Instron 4202 testing machine using a crosshead speed of 10 mm min 1. Five specimens were made and tested for each formulation. Results are given in Tables 4 ~ .
The unnotched Charpy impact test was employed to obtain the impact energy. Test specimen dimensions were 125 × 10 × 10mm in all cases and testing was conducted at room temperature. Data are given in Tables 4-6.
Evaluation of mechanical properties
To determine the tensile strength, elongation and modulus, dumb-bell shaped specimens were cast from each formulation and allowed to cure at ambient temperature and then postcured at 120°C for 1 h. Tensile strength evaluation was carried out on the Instron 4202 testing machine at a crosshead speed of 10 mm min -1 . The gauge length was 45 mm. Results are given in Tables 4-6.
Table 4. No
1 2 3 4 5 6 7 8
Table 5.
Glass transition temperature
Differential scanning calorimetry (DSC) was used to determine the Tg of samples cured at 120°C using a DSC-20-Mettler TA 3000 instrument at a heating rate of 10°Cmin 1. Data are presented in Tables 4-6. Morphology
Morphology of the cured resin was studied using scanning electron microscopy (SEM). Micrographs of the fractured surface of tensile test specimens were obtained using a stereoscan 250 MK-3 Cambridge instrument. Specimens were cut, mounted on an aluminium stub using a conductive paint and sputtercoated with gold.
Discussion
Unmodified cured epoxy resins, like other thermosets with high Tg, are brittle at room temperature.
Mechanical properties of GY250 toughened with EHTPB having epoxy value of I eq kg -1 EHTPB content (pphr)
Tensile strength (MPa)
Elongation (%)
0 2 3 4 5 10 15 20
52.0 52.4 54.9 56.3 57.9 61.0 59.3 55.9
4.7 4.9 5.2 3.9 4.9 5.5 6.2 7.0
Lap shear strength (MPa)
Impact strength (J m -1 )
Tg (K)
9.8 11.3 12.0 12.5 13.0 16.2 13.8 11.8
150 200 260 260 300 320 290 270
388 387 387 387 388 387 372 370
Mechanical properties of GY250 toughened with EHTPB having epoxy value of 3 eq kg -1
No
EHTPB content (pphr)
Tensile strength (MPa)
Elongation (%)
Lap shear strength (MPa)
Impact strength (J m -1)
Tg (K)
1 2 3 4
5 10 15 20
57.9 63.2 50.5 46.5
7.0 8.0 9.0 9.5
15.0 16.7 12.7 12.2
320 320 290 280
386 387 373 369
Table 6. agent
Mechanical properties of epoxy resin with 10 pphr EHTPB having different epoxy values as toughening
No
Epoxy value (eq kg 1)
Tensile strength (MPa)
Elongation (%)
Lap shear strength (MPa)
Impact strength (J m -1)
Tg (K)
1 2 3 4
1.0 2.0 2.5 3.0
61.3 67.1 63.7 63.2
5.5 7.9 8.0 8.0
16.2 18.6 16.9 16.7
320 340 310 320
388 387 387 387
INT.J.ADHESION AND ADHESIVES JANUARY 1994
59
Improving their crack resistance by the addition of a reactive liquid rubber to the uncured neat epoxy system is a good solution for the problem of brittleness Is. Epoxy resins toughened with reactive liquid polymers consist of a continuous rigid epoxy phase and a dispersed rubber phase. The rubber phase is formed in situ during the early stage of the cure reaction. The precise chemical composition of the rubber particles is not known. We propose that our particle composition is a mixture of linear copolymers of EHTPB~:~poxy resin and homopolymer of epoxy resin. The rubber domains impart ductility to the system which in turn improves the toughness and crack resistance by absorbing strain energy. We have demonstrated a technique by which a two-phase thermoset system is produced to form a toughened system. The initial solubility of the reactive liquid rubber in the epoxy resin before cure is a critical condition not only lbr chemical reaction but also for the in ,s'itu formation of rubber domains. G o o d solubility with good reactivity results in a system with small rubbery domains I'~. HTPB is not miscible with bisphenol epoxy resin. Introducing epoxy groups into the double bonds o f HTPB increases the polarity of HTPB and, since both systems contain epoxy groups, miscibility is also increased. This increascd miscibility helps in the phase separation of the rubber domains and thus in effective toughening of the epoxy resin. Peroxy acid assisted epoxidation is a convenient method to incorporate oxygen into diene blocks. It can be achieved either by a preformed peroxy acid or by the it~ silu method using aliphatic acids and hydrogen peroxide. Based on our experimental trials, the in situ method was selected because of the easy control of the epoxidation reaction. F o u r different EHTPB samples with different epoxy values were prepared by epoxidizing HTPB with performic acid generated in situ by reacting formic acid ( H C O O H ) with hydrogen peroxide (H202). HO [ C H e C H - C H HTPB
CH~ ],, O H
HCOOH H~O:
O
/\ HO [ CH2-CH- CH CH2 ],, OH + H C O O H + H20 EHTPB The EHTPB samples thus prepared were characterized for epoxy value and by Im spectroscopy (Tables 1 3). Initially, the effect of molar ratio of hydrogen peroxide on epoxidation was studied. The optimum epoxy value of 1.75 was achieved when the molar ratio of H,O~ was 2 with respect to HTPB and H C O O H . Similarly, the molar ratio of H C O O H was varied and its effect on the epoxy value of EHTPB was studied. It was observed that the maximum epoxy value was obtained with a H C O O H molar ratio of 2. When the amount of acid was increased further, keeping the hydrogen peroxide molar ratio constant, the epoxy value was found to decrease. This was due to opening of the oxirane rings by the formic acid regenerated from the performic acid after HTPB epoxidation. The maximum epoxy value of EHTPB was obtained with a molar ratio of HTPB:HCOOH:H~O-, of 1:2:2. Another parameter studied was the effect of temperature on epoxidation, keeping the above molar
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INT.J.ADHESION A N D ADHESIVES J A N U A R Y 1994
ratio constant. The epox,v value incrcascd gradually with increasing temperature of the epoxidation reaction, reaching a maximmn of 3.2 cq kg ~ at 60 (', and then decreased (Table 3). Similarly, the optimum reaction time was found to bc 3 h; at Iongcr timcs the epoxy value was l\)und to decrease. Thcsc findings arc again attributed to the opening of the cpoxide rings to form hydroxy and formate ester groups at high temperature and upon prolonged COlllacl with the regenerated formic acid. The I)GEBA type epoxy resin and hardener were mixed with EHTPB and the bonding properties wcrc evaluated. The epoxidized hydroxy-termmatcd polybutadiene was capable of reacting ~ith the hardener in the same manner as the epoxy resin. The EHTPB bisphenol epoxy combination ,aas cured at high temperature with 2 , 4 , 6 - t r i s ( N , N - d i m e t h y l a m i n o m e t h y l ) phenol. The curing mechanism ~-° inwHved attack by the tertiary amine on the epoxy, resulting in a crosslinked thcrmoset polymer. Lap shear strength values increased, showed a maximum and then decreased with increasing EHTPB content. The maximum appeared in the vicinity of l0 pphr t:,HTPB. There was a substantial improvement m lap shear strength from 9.8 MPa for the neat system to 18.6 MPa t\~r the toughened one (Tables 4 and 6). The maximum value of impact strength was also obtained at 10 pphr EHTPB. Toughening of the cured epoxy resin by the incorporation of Hrrl'B was evidenced by the Charpy impact test. Above the optimum EHTPB content a t'all in impact strength was observed. Impact strength was increased from 150 to 340J m i (Tables 4 and 6). Tensile strength was l\mnd to increase from 52 to 67 MPa upon the addition of 10 pphr EHTPB: at higher contents it decreased (Tables 4 and 6). Tg values were unaffected by the incorporation of 5 to 15 pphr I~HTPB in the epoxy hardener system, but decreased at EHTPB contents above 15 pphr (Tables 4 and 6). This illustrates that at low levels o f EHTPB there is phase separation and little EHTPB in the epoxy matrix. Since the epoxy matrix contains relatively little clastomer, the thermal properties (Tt,) arc the same as that of the unmodified epoxy. Above 10 pphr EHTPB the 7) values decrease due to flexibilization of the epoxy resin by EHTPB; the discontinuous rubber phase becomes continuous beyond 15 pphr EttTPB. Both the mechanical and adhesive bonding properties were increased by the addition of low levels o f EHTPB, reached a maximum at 10 pphr EHTPB and then started to decrease. The same phenomenon was noticed at each different EItTPB epoxy value (compare Tables 4 and 5). For a series of samples containing 10pphr EHTPB of different epoxy values, the mechanical properties of the cured epoxy resin increased with increasing epoxy value up to a maximum of 2, after which no improvement was observed (Table 6). The I~HTPBwith epoxy value = 1 may not provide sufficient crosslinking to increase the mechanical properties, whereas the crosslinking may be higher than required for EHTPB with epoxy value = 2.5 and 3, compared with the optimum case at epoxy value = 2. These results can be explained by the toughening and flexibilization caused by the addition of EHTPB to the
aggregation (Fig. l(c)). Because the rubber does not phase separate it can lead to flexibilization of the cured epoxy resin, with a consequent decrease in rigidity. Conclusions
Low-level addition of epoxidized hydroxy-terminated polybutadiene (EHTPB), a reactive liquid polymer, to an epoxy resin cured at high temperature with 2,4,6tris(N,N-dimethylaminomethyl) phenol resulted in significantly improved mechanical and bonding properties compared with the neat system. Maximum mechanical properties were obtained at a level of 10 parts EHTPBper 100 parts epoxy resin (pphr). The enhancement of bulk mechanical properties was attributed to the higher toughness produced by the dispersed rubber particles, At higher EHTPBcontent, the rubber phase became continuous and the system exhibited a flexibilization effect. The glass transition temperature was unaffected by the addition of EHTPB up to 10 pphr. The results of this study demonstrate that epoxy resins containing low levels of EHTPB, cured at high temperature with 2,4,6-tris(N,N-dimethylaminomethyl) phenol, can be used to formulate structural adhesives. References 1 2 3 4 5 6 7
8 9 10 11 12 13 14 Fig. 1 SEM micrographs of tensile fracture surfaces of epoxy resin cured with 2,4,6-tris(N,N-dimethylaminomethyl) phenol at ambient temperature followed by postcure at 120C for 1 h: (a) without EHTPB; (b) containing 10pphr EHTPB; and (c) containing 15pphr EHTPB
epoxy resin. The lack of improvement in mechanical and bonding properties above 10 pphr EHTPB may be explained by considering the change in morphology of the fractured surface of the cured epoxy resin. Unmodified cured epoxy resin failed in a brittle manner (Fig. l(a)). A two-phase microstructure consisting of relatively small rubber particles dispersed in the epoxy matrix was observed for the modified formulation containing 10 pphr EHTPB with epoxy value of 2 (Fig. l(b)). The cavities of broken rubber particles that can be seen in this figure result in higher toughness by means of a crack-terminating mechanism and fracture surface cavitation. On increasing lhe concentration of EHTPB from 10 to 15 pphr, the second phase became indistinguishable from the matrix due to
15 16 17 18
19
20
Manzione, L.T., Gillham, J.K, and McPherson, C.A. J App/Polym Sci26 (1981) pp889-905 Manzione, L.T., Gillham, J.K, and McPherson, C.A. J Appl Polym Sci 26 (1981) pp 907-917 Bartlet, P., Pawcaulr, J.P. and Sautercan, H. J Appl Polym Sci 30 (1985) p p 2955-2966 "Advances in Polymer Science', Vol 17 (Springer-Verlag, 1975) Sultan, J.N. and McGarry, C.J. Polym Engng Sci 13 (1979) p 29 Kunz, S.C., Sayre, J.P. and Assinte, R.A. Polymer23 (1982) p 1897 Drake, R.S. and Siebert, A.R. "Rubber Modified Thermoset Resins" ACS Syrup Ser No 208 (American Chemical Society, Washington, DC, 1983) Sasidharan Achary, P., Latha, P.B. and Ramaswamy, R. J Appl Polym Sci41 (1990) pp 151-162 Gupta, S.K. and Latha, P.B. lot Natural Rubber Conf, Bangalore, India, 1992 (Indian Rubber Institute) Alan, R. and Siebert, A.R. "Rubber Modified Thermoset Resins" op. cit. Riew, C.K. and Smith, R.W. J Polym Sci, Part A-I 9 (1971) p 2737 Mansion, J.A. and Sperting, LH. "Polymer Blends and Composites" (Plenum Press, New York, 1976) Bitner, J.L., Rushford, J i . , Rose, W.S., Hunston, D.L. and Riew, C.K. J Adhesion 13 (1981) p 13 Siebert, A.R. and Riew, C.K. "The Chemistry of Rubber Toughened Epoxy Resins I; 161st Nat Meetg, ACS Org Coatings Plast Div, March 1971 (American Chemical Society, Washington, DC) Jelling, I.R. Rubber Chem Techno158 (1985) p 86 Rause, C., Paurat, R., Cheriat, R., Ledran, F. and Danyard, J.C. J Polym Sci (C) 16 (1969) p4687 Lee, H. and Neville, K. "Handbook of Epoxy Resin' (McGraw Hill New York, 1967) pp21-23 McGarry, F.J. and Willner, A.M. 'Toughening of an epoxy resin by an elastomeric second phase' Research Report R 68-8 (Massachusetts Institute of Technology, Cambridge, 1968) Riew, C.K., Rowe, E.H. and Siebert, A.R. 'Toughness and brittleness of plastics' (B.F. Goodrich Co, Research and Development Center, OH) Patrick, R.L. (Ed) in "Treatise on Adhesion and Adhesives' (Marcel Dekker, New York, 1969) No 1.2, p29
Authors
The authors are with the Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum, 695022, India. Correspondence should be addressed to P.B. Latha.
INT.J.ADHESION AND ADHESIVES JANUARY
1994
61
prepared by adjusting reaction conditions such as temperature, time and molar ratios of formic acid and hydrogen peroxide. The EHTPBwas characterized by chemical and spectroscopic methods. EHTPB was found to be a good toughening agent for epoxy resins, which are brittle at room temperature. The mechanical properties of a toughened epoxy with EHTPBas the toughening agent were evaluated. Lap shear strength and T-peel strength were observed to increase with increasing EHTPBcontent, pass through a maximum at an EHTPBcontent of about 10 parts per 100 parts epoxy resin (pphr) and then decrease. The enhancement of mechanical properties was attributed to the higher toughness produced by the dispersed rubber particles. At higher EHTPBcontent, the rubber phase became continuous, and the system exhibited a fall in mechanical properties due to the flexibilization effect. The glass transition temperature (Tg) of the cured epoxy resin was not affected by the incorporation of up to lO pphr EHTPBbut higher levels Of EHTPBdecreased T o due to flexibilization.
Key words: adhesives; t o u g h e n e d epoxies; flexibilization; epoxidized h y d r o x y terminated polybutadiene
Epoxy resins are highly crosslinked polymers that are brittle and glassy at room temperature. These materials typically exhibit high moduli, near linear elastic stress/ strain behaviour, very good adhesion, excellent chemical resistance, and good electrical insulation = 3. Practical uses of epoxies include load-bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is their poor resistance to fracture. For structural adhesives, a high glass transition temperature (Tg) is an essential requirement for maintaining adhesive strength at high temperatures. All unmodified cured epoxy resins with relatively high T~ have one drawback - - their brittleness which can produce failure of the adhesive joint 4.
One method of improving the fracture properties of epoxy resins is to incorporate reactive liquid rubbers into the formulations. Extensive studies 5 7 have been reported on toughness enhancement of cured epoxy resins without significant reduction in thermal and mechanical properties by the addition of low levels of reactive liquid rubbers such as carboxy-terminated polybutadiene-acrylonitrile (CTBN), epoxidized hydroxy-terminated natural rubber (EHTNR),etc. 8 10. A small amount of rubber in the form of discrete particles in a glassy thermoset resin, such as an epoxy, can greatly improve crack resistance and impact strength 11 ' 12 . This improvement is achieved without significantly decreasing the thermal or mechanical properties. The theory13 is that, during the curing cycle,
0143-7496/94/01/0057-05 (c~ 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 1 JANUARY 1994
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elements of reactive liquid polymer precipitate from the matrix and lbrm a homogeneously dispersed particle phase. This two-phase morphology is the key to the toughening mechanism of the host matrix. The twophase structure results in higher toughness because the mechanical energy is uniformly distributed by the rubber particles, thereby reducing the local stress concentration. The result is that a higher external load is required to break the specimen. This improvement is achieved without reducing the thermal properties of the base epoxy resin since the epoxy matrix contains very little or no rubber t4. In this work epoxy resin based on the diglycidylether of bisphenol A (DGEBA) was toughened using different grades of epoxidized hydroxy-terminated polybutadiene (EHTPB). The epoxidation of hydroxy-terminated polybutadiene (HTPB) was performed mainly to increase the polarity and hence the compatibility with the epoxy resin. This is an essential condition for any reactive liquid elastomer to exhibit phase separation during curing. The novelty of the work lies in the fact that the incorporation of EHTPB into the epoxy resin system resulted in increase of lap shear bond strength, impact strength and tensile strength of the cured system. This is similar to what was previously reported during modification of DGEBA epoxy resin using CTBN, amine-terminated polybutadiene acrylonitrile (ATBN), mercapto-terminated polybutadiene acrylonitrile (MTBN), etc. Epoxidation of unsaturated elastomers is well known~5, ~6. In the process of preparing EHTPB we followed the peroxy acid epoxidation of HTPB, the peroxy acid being generated in situ using aliphatic acid and hydrogen peroxide. EHTPB was characterized by chemical and spectroscopic methods. 2,4,6-Tris(N,Ndimethylaminomethyl) phenol was used as the curing agent. This curing agent is known for its ability to induce homopolymerization of epoxy resins, resulting in cured systems possessing better high temperature properties:. The epoxy resin was cured at ambient temperature and then postcured at high temperature. Mechanical properties such as tensile strength and elongation and bonding properties such as lap shear strength on A1/AI were evaluated for the toughened epoxy system. The impact strength and Tg of the toughened system are also discussed. Experimental Materials
Araldite GY250 epoxy resin (Ciba Ltd) was chosen as the control resin; its epoxy value is 5.5eqkg -1. 2,4,6Tris(N,N-dimethylaminomethyl) phenol (HY960, Ciba) was curing agent. The HTPB chosen for epoxidation was produced at Vikram Sarabhai Space Centre, India; hydroxyl value = 34.5 mg KOH g- 1, density = 0.9 gcm 3, viscosity = 40-60 Poise and number-average molecular weight (Mn) = 2730. Synthesis and characterization of EHTPB
HTPB was epoxidized using performic acid generated in situ by the reaction of hydrogen peroxide with formic acid. HTPB (30 g) was mixed with formic acid (26 g) in toluene (300 ml) and stirred well in a water bath at 25°C. To this hydrogen peroxide (32 g; 30%)
58
INT.J.ADHESlON AND ADHESIVES J A N U A R Y 1994
was added dropwise m order to recycle the lornlic acid used. This was necessary to reduce the acid content and thus prevent opening of the epoxide rings lk~rmcd. which are very sensitive to acids. Four different types of EI-|TPB with different epoxy values were prepared by adjusting experimental parameters such as temperature, time and molar ratios of acid and hydrogen peroxide (see Tables 1 3). The epoxy value of EHTPB was determined using the pyridine hydrochloric acid method. EHTPB (0.5 g) was refluxed with pyridine hydrochloric acid (20 ml). Upon cooling, the mixture was titrated with NaOH (0. I N) using phenolphthalein as indicator. Infra-red (IR) spectra of the samples showed a sharp peak corresponding to epoxy groups at 910cm t confirming epoxidation of the double bonds in HTPB. Resin formulations
Different formulations containing 0 to 20 parts by weight EHTPB per 100 parts by weight GY250 resin (pphr) were made. These formulations were mixed with 6 parts by weight of 2,4,6-tris(N,N-dimethylaminomethyl) phenol per 100 parts by weight of bisphenol epoxy resin, cured at ambient temperature and then postcured at 120°C for 1 h.
Table 1.
Effect of H202 on epoxy value of EHTPB Molar ratio
Epoxy value (eq kg-1)
HTPB
HCOOH
H202
1 1 1 1 1
1 1 1 1 1
0.5 1 1.5 2 3
Table 2.
0.6 1 1.5 1.75 1.6
Effect of formic acid on epoxy value of EHTPB Molar ratio
Epoxy value (eq kg- 1)
HTPB
HCOOH
H202
1 1 1 1
1 2 3 4
2 2 2 2
1.75 2.1 1.5 1.25
Table 3. Variation of epoxy value of EHTPB with temperature of reaction* Temperature (°C)
Epoxy value (eq kg -1)
25 30 40 50 60 70
2.1 2.4 2.6 3 3.2 2.9
* M o l a r ratio HTPB : H C O O H : H202 - 1 : 2 : 2
Evaluation of adhesive properties
Impact strength determination
Lap shear bond strength specimens were made according to ASTM-D-1002 specifications. B 51 SWP aluminium alloy containing 0.8% Mg, 0.4% Fe, 0.6% Mn and 1% Si was used for evaluating bonding properties. Aluminium adherends were abraded with emery paper, etched with chromic acid solution, rinsed with distilled water, dried at 100°C for 30 min and then cooled. The resin formulations were then applied over the area to be bonded and joined using contact pressure. These bonded specimens were kept at ambient temperature for a period of 24 h and then cured at 120°C for 1 h. The lap shear strength of the bonded specimens was determined in an Instron 4202 testing machine using a crosshead speed of 10 mm min 1. Five specimens were made and tested for each formulation. Results are given in Tables 4 ~ .
The unnotched Charpy impact test was employed to obtain the impact energy. Test specimen dimensions were 125 × 10 × 10mm in all cases and testing was conducted at room temperature. Data are given in Tables 4-6.
Evaluation of mechanical properties
To determine the tensile strength, elongation and modulus, dumb-bell shaped specimens were cast from each formulation and allowed to cure at ambient temperature and then postcured at 120°C for 1 h. Tensile strength evaluation was carried out on the Instron 4202 testing machine at a crosshead speed of 10 mm min -1 . The gauge length was 45 mm. Results are given in Tables 4-6.
Table 4. No
1 2 3 4 5 6 7 8
Table 5.
Glass transition temperature
Differential scanning calorimetry (DSC) was used to determine the Tg of samples cured at 120°C using a DSC-20-Mettler TA 3000 instrument at a heating rate of 10°Cmin 1. Data are presented in Tables 4-6. Morphology
Morphology of the cured resin was studied using scanning electron microscopy (SEM). Micrographs of the fractured surface of tensile test specimens were obtained using a stereoscan 250 MK-3 Cambridge instrument. Specimens were cut, mounted on an aluminium stub using a conductive paint and sputtercoated with gold.
Discussion
Unmodified cured epoxy resins, like other thermosets with high Tg, are brittle at room temperature.
Mechanical properties of GY250 toughened with EHTPB having epoxy value of I eq kg -1 EHTPB content (pphr)
Tensile strength (MPa)
Elongation (%)
0 2 3 4 5 10 15 20
52.0 52.4 54.9 56.3 57.9 61.0 59.3 55.9
4.7 4.9 5.2 3.9 4.9 5.5 6.2 7.0
Lap shear strength (MPa)
Impact strength (J m -1 )
Tg (K)
9.8 11.3 12.0 12.5 13.0 16.2 13.8 11.8
150 200 260 260 300 320 290 270
388 387 387 387 388 387 372 370
Mechanical properties of GY250 toughened with EHTPB having epoxy value of 3 eq kg -1
No
EHTPB content (pphr)
Tensile strength (MPa)
Elongation (%)
Lap shear strength (MPa)
Impact strength (J m -1)
Tg (K)
1 2 3 4
5 10 15 20
57.9 63.2 50.5 46.5
7.0 8.0 9.0 9.5
15.0 16.7 12.7 12.2
320 320 290 280
386 387 373 369
Table 6. agent
Mechanical properties of epoxy resin with 10 pphr EHTPB having different epoxy values as toughening
No
Epoxy value (eq kg 1)
Tensile strength (MPa)
Elongation (%)
Lap shear strength (MPa)
Impact strength (J m -1)
Tg (K)
1 2 3 4
1.0 2.0 2.5 3.0
61.3 67.1 63.7 63.2
5.5 7.9 8.0 8.0
16.2 18.6 16.9 16.7
320 340 310 320
388 387 387 387
INT.J.ADHESION AND ADHESIVES JANUARY 1994
59
Improving their crack resistance by the addition of a reactive liquid rubber to the uncured neat epoxy system is a good solution for the problem of brittleness Is. Epoxy resins toughened with reactive liquid polymers consist of a continuous rigid epoxy phase and a dispersed rubber phase. The rubber phase is formed in situ during the early stage of the cure reaction. The precise chemical composition of the rubber particles is not known. We propose that our particle composition is a mixture of linear copolymers of EHTPB~:~poxy resin and homopolymer of epoxy resin. The rubber domains impart ductility to the system which in turn improves the toughness and crack resistance by absorbing strain energy. We have demonstrated a technique by which a two-phase thermoset system is produced to form a toughened system. The initial solubility of the reactive liquid rubber in the epoxy resin before cure is a critical condition not only lbr chemical reaction but also for the in ,s'itu formation of rubber domains. G o o d solubility with good reactivity results in a system with small rubbery domains I'~. HTPB is not miscible with bisphenol epoxy resin. Introducing epoxy groups into the double bonds o f HTPB increases the polarity of HTPB and, since both systems contain epoxy groups, miscibility is also increased. This increascd miscibility helps in the phase separation of the rubber domains and thus in effective toughening of the epoxy resin. Peroxy acid assisted epoxidation is a convenient method to incorporate oxygen into diene blocks. It can be achieved either by a preformed peroxy acid or by the it~ silu method using aliphatic acids and hydrogen peroxide. Based on our experimental trials, the in situ method was selected because of the easy control of the epoxidation reaction. F o u r different EHTPB samples with different epoxy values were prepared by epoxidizing HTPB with performic acid generated in situ by reacting formic acid ( H C O O H ) with hydrogen peroxide (H202). HO [ C H e C H - C H HTPB
CH~ ],, O H
HCOOH H~O:
O
/\ HO [ CH2-CH- CH CH2 ],, OH + H C O O H + H20 EHTPB The EHTPB samples thus prepared were characterized for epoxy value and by Im spectroscopy (Tables 1 3). Initially, the effect of molar ratio of hydrogen peroxide on epoxidation was studied. The optimum epoxy value of 1.75 was achieved when the molar ratio of H,O~ was 2 with respect to HTPB and H C O O H . Similarly, the molar ratio of H C O O H was varied and its effect on the epoxy value of EHTPB was studied. It was observed that the maximum epoxy value was obtained with a H C O O H molar ratio of 2. When the amount of acid was increased further, keeping the hydrogen peroxide molar ratio constant, the epoxy value was found to decrease. This was due to opening of the oxirane rings by the formic acid regenerated from the performic acid after HTPB epoxidation. The maximum epoxy value of EHTPB was obtained with a molar ratio of HTPB:HCOOH:H~O-, of 1:2:2. Another parameter studied was the effect of temperature on epoxidation, keeping the above molar
60
INT.J.ADHESION A N D ADHESIVES J A N U A R Y 1994
ratio constant. The epox,v value incrcascd gradually with increasing temperature of the epoxidation reaction, reaching a maximmn of 3.2 cq kg ~ at 60 (', and then decreased (Table 3). Similarly, the optimum reaction time was found to bc 3 h; at Iongcr timcs the epoxy value was l\)und to decrease. Thcsc findings arc again attributed to the opening of the cpoxide rings to form hydroxy and formate ester groups at high temperature and upon prolonged COlllacl with the regenerated formic acid. The I)GEBA type epoxy resin and hardener were mixed with EHTPB and the bonding properties wcrc evaluated. The epoxidized hydroxy-termmatcd polybutadiene was capable of reacting ~ith the hardener in the same manner as the epoxy resin. The EHTPB bisphenol epoxy combination ,aas cured at high temperature with 2 , 4 , 6 - t r i s ( N , N - d i m e t h y l a m i n o m e t h y l ) phenol. The curing mechanism ~-° inwHved attack by the tertiary amine on the epoxy, resulting in a crosslinked thcrmoset polymer. Lap shear strength values increased, showed a maximum and then decreased with increasing EHTPB content. The maximum appeared in the vicinity of l0 pphr t:,HTPB. There was a substantial improvement m lap shear strength from 9.8 MPa for the neat system to 18.6 MPa t\~r the toughened one (Tables 4 and 6). The maximum value of impact strength was also obtained at 10 pphr EHTPB. Toughening of the cured epoxy resin by the incorporation of Hrrl'B was evidenced by the Charpy impact test. Above the optimum EHTPB content a t'all in impact strength was observed. Impact strength was increased from 150 to 340J m i (Tables 4 and 6). Tensile strength was l\mnd to increase from 52 to 67 MPa upon the addition of 10 pphr EHTPB: at higher contents it decreased (Tables 4 and 6). Tg values were unaffected by the incorporation of 5 to 15 pphr I~HTPB in the epoxy hardener system, but decreased at EHTPB contents above 15 pphr (Tables 4 and 6). This illustrates that at low levels o f EHTPB there is phase separation and little EHTPB in the epoxy matrix. Since the epoxy matrix contains relatively little clastomer, the thermal properties (Tt,) arc the same as that of the unmodified epoxy. Above 10 pphr EHTPB the 7) values decrease due to flexibilization of the epoxy resin by EHTPB; the discontinuous rubber phase becomes continuous beyond 15 pphr EttTPB. Both the mechanical and adhesive bonding properties were increased by the addition of low levels o f EHTPB, reached a maximum at 10 pphr EHTPB and then started to decrease. The same phenomenon was noticed at each different EItTPB epoxy value (compare Tables 4 and 5). For a series of samples containing 10pphr EHTPB of different epoxy values, the mechanical properties of the cured epoxy resin increased with increasing epoxy value up to a maximum of 2, after which no improvement was observed (Table 6). The I~HTPBwith epoxy value = 1 may not provide sufficient crosslinking to increase the mechanical properties, whereas the crosslinking may be higher than required for EHTPB with epoxy value = 2.5 and 3, compared with the optimum case at epoxy value = 2. These results can be explained by the toughening and flexibilization caused by the addition of EHTPB to the
aggregation (Fig. l(c)). Because the rubber does not phase separate it can lead to flexibilization of the cured epoxy resin, with a consequent decrease in rigidity. Conclusions
Low-level addition of epoxidized hydroxy-terminated polybutadiene (EHTPB), a reactive liquid polymer, to an epoxy resin cured at high temperature with 2,4,6tris(N,N-dimethylaminomethyl) phenol resulted in significantly improved mechanical and bonding properties compared with the neat system. Maximum mechanical properties were obtained at a level of 10 parts EHTPBper 100 parts epoxy resin (pphr). The enhancement of bulk mechanical properties was attributed to the higher toughness produced by the dispersed rubber particles, At higher EHTPBcontent, the rubber phase became continuous and the system exhibited a flexibilization effect. The glass transition temperature was unaffected by the addition of EHTPB up to 10 pphr. The results of this study demonstrate that epoxy resins containing low levels of EHTPB, cured at high temperature with 2,4,6-tris(N,N-dimethylaminomethyl) phenol, can be used to formulate structural adhesives. References 1 2 3 4 5 6 7
8 9 10 11 12 13 14 Fig. 1 SEM micrographs of tensile fracture surfaces of epoxy resin cured with 2,4,6-tris(N,N-dimethylaminomethyl) phenol at ambient temperature followed by postcure at 120C for 1 h: (a) without EHTPB; (b) containing 10pphr EHTPB; and (c) containing 15pphr EHTPB
epoxy resin. The lack of improvement in mechanical and bonding properties above 10 pphr EHTPB may be explained by considering the change in morphology of the fractured surface of the cured epoxy resin. Unmodified cured epoxy resin failed in a brittle manner (Fig. l(a)). A two-phase microstructure consisting of relatively small rubber particles dispersed in the epoxy matrix was observed for the modified formulation containing 10 pphr EHTPB with epoxy value of 2 (Fig. l(b)). The cavities of broken rubber particles that can be seen in this figure result in higher toughness by means of a crack-terminating mechanism and fracture surface cavitation. On increasing lhe concentration of EHTPB from 10 to 15 pphr, the second phase became indistinguishable from the matrix due to
15 16 17 18
19
20
Manzione, L.T., Gillham, J.K, and McPherson, C.A. J App/Polym Sci26 (1981) pp889-905 Manzione, L.T., Gillham, J.K, and McPherson, C.A. J Appl Polym Sci 26 (1981) pp 907-917 Bartlet, P., Pawcaulr, J.P. and Sautercan, H. J Appl Polym Sci 30 (1985) p p 2955-2966 "Advances in Polymer Science', Vol 17 (Springer-Verlag, 1975) Sultan, J.N. and McGarry, C.J. Polym Engng Sci 13 (1979) p 29 Kunz, S.C., Sayre, J.P. and Assinte, R.A. Polymer23 (1982) p 1897 Drake, R.S. and Siebert, A.R. "Rubber Modified Thermoset Resins" ACS Syrup Ser No 208 (American Chemical Society, Washington, DC, 1983) Sasidharan Achary, P., Latha, P.B. and Ramaswamy, R. J Appl Polym Sci41 (1990) pp 151-162 Gupta, S.K. and Latha, P.B. lot Natural Rubber Conf, Bangalore, India, 1992 (Indian Rubber Institute) Alan, R. and Siebert, A.R. "Rubber Modified Thermoset Resins" op. cit. Riew, C.K. and Smith, R.W. J Polym Sci, Part A-I 9 (1971) p 2737 Mansion, J.A. and Sperting, LH. "Polymer Blends and Composites" (Plenum Press, New York, 1976) Bitner, J.L., Rushford, J i . , Rose, W.S., Hunston, D.L. and Riew, C.K. J Adhesion 13 (1981) p 13 Siebert, A.R. and Riew, C.K. "The Chemistry of Rubber Toughened Epoxy Resins I; 161st Nat Meetg, ACS Org Coatings Plast Div, March 1971 (American Chemical Society, Washington, DC) Jelling, I.R. Rubber Chem Techno158 (1985) p 86 Rause, C., Paurat, R., Cheriat, R., Ledran, F. and Danyard, J.C. J Polym Sci (C) 16 (1969) p4687 Lee, H. and Neville, K. "Handbook of Epoxy Resin' (McGraw Hill New York, 1967) pp21-23 McGarry, F.J. and Willner, A.M. 'Toughening of an epoxy resin by an elastomeric second phase' Research Report R 68-8 (Massachusetts Institute of Technology, Cambridge, 1968) Riew, C.K., Rowe, E.H. and Siebert, A.R. 'Toughness and brittleness of plastics' (B.F. Goodrich Co, Research and Development Center, OH) Patrick, R.L. (Ed) in "Treatise on Adhesion and Adhesives' (Marcel Dekker, New York, 1969) No 1.2, p29
Authors
The authors are with the Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum, 695022, India. Correspondence should be addressed to P.B. Latha.
INT.J.ADHESION AND ADHESIVES JANUARY
1994
61