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Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch015
Thermal Shock Resistance of Toughened Epoxy Resins Masatoshi Kubouchi and Hidemitsu Hojo 1
2
Department of Chemical Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan Department of Industrial Engineering and Management, Nihon University, College of Industrial Technology, 1-2-1 Izumi-cho, Narashino-shi, Chiba-ken 275, Japan
1
2
The thermal shock resistance of epoxy resin specimens toughened with carboxy-terminated poly(butadiene-acrylonitrile)
(CTBN)
and poly-
glycol were tested using a new notched disk-type specimen. The new thermal shock testing method consists of quenching a notched disk-type specimen and applying a theoretical analysis to the test results to determine crack propagation conditions. For both toughened epoxy resins, this test method evaluated improvements in thermal shock resistance. The thermal shock resistance of epoxy resin toughened with CTBN
exhibited a maximum at a 35 parts per hundred resin content
of CTBN.
The epoxy resin toughened with polyglycol exhibited im-
proved thermal shock resistance with increasing glycol content.
THE PROBLEM OF CRACK RESISTANCE is increasing in importance with the demand for high reliability resin products. The uses for epoxy resin range from large electrical products to miniature electronic parts. The broad spectrum of use reflects the excellent insulation resistance of epoxy resins. Practical methods for testing the thermal-stress crack resistance of epoxy resin are heat cycle tests, such as the Olyphant washer test (J), that investigate crack initiation. These relatively rapid tests use a specimen containing a metal insert: a washer or bolt embedded near the center of an encapsulating resin disk. In this type of specimen, the stress condition is 0065-2393/93/0233-0365$06.00/0 © 1993 American Chemical Society
Riew and Kinloch; Toughened Plastics I Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
366
R U B B E R - T O U G H E N E D PLASTICS
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complicated and difficult to analyze, which makes comparison with failure mechanisms in commercial applications difficult. A new test method is proposed that evaluates the crack resistance of epoxy resin by using a thermal shock method with a notched disk-type specimen (2), followed by application of an analytical method based on linear fracture mechanics (3). Because this testing method consists of simple fracture modes, a quantitative analysis can be applied. This chapter examines the applicability of the new testing and evaluation method to toughened epoxy resin modified with carboxy-terminated poly(butadiene-acrylonitrile) ( C T B N ) or plasticizer.
Experimental Method The new thermal shock tests are conducted by quickly cooling the preheated notched disk-type specimen. Consideration of simple specimen geometry and stress analysis follows the same procedures as Manson (4).
Materials and Specimen. Two bisphenol-type epoxies (epoxy equiv wt = 184-194 and 370-435) are used as the matrix resin, and acid anhydride is added as a hardener. The disk-shaped specimen is made by casting. The shape and size (60-mm diameter; 10 mm thick) of the disk-type specimen are shown in Figure 1. A sharp notch is introduced at the base of the slot (0.3 mm wide) using a razor blade. Toughened epoxy specimens modified with carboxy-terminated poly(butadiene-acrylonitrile) ( C T B N ; mol wt = 3400) or polyglycol plasticizer were also tested. A larger specimen size (120 mm diameter; 10 mm thick) was used for testing polyglycol-toughened epoxy. The physical and mechanical properties at room temperature of the respective epoxy resins are shown in Table I. Test Method. As shown in Figure 1, the specimen is fixed both top and bottom with balsa insulators, which cause the specimen to suffer lateral thermal cooling shock. The disk specimen cools from the surface to the center along the radius. Then expansion difference between the outer and inner parts of the specimen creates mode-I fracture mechanics at the tip of the specimen notch. The 5 min cooling period corresponds to the time necessary to maximize the stress intensity factor at the notch tip. For the lowest temperature, a dry ice-pentane cooling system (approximately 200 K) is used. Temperature difference is created by changing the initial temperature from 280 to 380 K. The crack initiation due to thermal shock is detected by visual observation after cooling.
Riew and Kinloch; Toughened Plastics I Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
KUBOUCHI &
367
Hojo Thermal Shock Resistance
Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch015
15.
holder Figure 1. The test specimen.
Theoretical Analysis In this analysis it is assumed that the temperature distribution is not influ enced by the notch and that the thermal stress distribution is a quasi-static state. Based on the elastic stress analysis and linear fracture mechanics, the stress intensity factor Κ at the notch tip is obtained as follows: τ
2]fc R Κ, = 1.12-7=-
σ
r
,
0
9
άξ
Riew and Kinloch; Toughened Plastics I Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
(1)
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R U B B E R - T O U G H E N E D PLASTICS
Table I, Physical and Mechanical Properties of Epoxy Resin at Room Temperature CTBN-Toughened, Plasticizer-Modified, Modifier Content (phr) Modifier Content (phr) a
Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch015
Property Tensile strength (MPa) Young's modulus (GPa) Poisson's ratio (m/m) Fracture toughness (MPa\/m) Rockwell hardness (H scale) Thermal conductivity (J/m s Κ) Coefficient of linear thermal expansion (1/K)
b
0
20
35
50
0
10
20
82.5
68.0
55.6
39.3
81.2
79.2
65.9
3.56
2.78
2.07
1.85
3.29
2.78
2.71
0.36
0.40
0.37
0.40
0.39
0.34
0.35
1.24
2.06
2.18
1.51
1.87
1.93
2.16
75
57
27
64
81
19
58
0.24
0.23
0.23
0.23
0.20
0.20
0.20
5.24
7.69
8.93
8.72
4.91
7.08
6.80
E P + Me-THPA; EP equivalent 184-194. EP + PA; EP equivalent 370-435.
Û fe
a Ε AT ( 1 1 ,r = 7 r { - ô / T*rdr + - τ / Τ r
σ
θ
R
(1 - ν ) \R J
θ
2
r J 2
0
0
where ν is Poisson's ratio, α is the coefficient of thermal expansion, Ε is the elastic modulus, R is the radius of the disk specimen, Δ Τ is the temperature difference ( = T — T , where Τ is temperature and subscripts i and f indicate initial and final), c is the notch length, ξ is the coordinate onto the notch, r is the distance from the center of the disk specimen, and T * is the nondimensional temperature ( = ( Τ — Τ^/ΔΤ). Equation 1 is normalized by aEjïï /(l — v) and the nondimensional stress intensity factors (Kf) are defined as f
x
(l-v)K,
1
αΕν/R
ΔΓ
exp
e al
CT
c
il Γ
ϋ
*Ical
w VR
=
\
fi
K
( T*rdr R
(3)
