Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Patent
1990-01-04
1991-02-05
Nielsen, Earl
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
528361, 528365, 525113, C08L 6300, C08L 6310
Patent
active
049905872
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of preparing toughened thermoset epoxy polymers from elastomer-containing epoxy resin formulations.
Epoxy resins are used extensively as matrix resins for fiberreinforced composite materials and as adhesives where advantage is taken of favourable properties such as high modulus, low creep and reasonable elevated temperature performance. However, characteristics such as these require a cured epoxy resin having a highly crosslinked structure which results in a tendency towards brittleness and poor resistance to crack growth.
Although a number of techniques have been developed to increase the toughness of cured epoxy resins (also referred to in this specification as epoxy polymers), probably the most successful has concerned the use of carboxyl-terminated butadiene acrylonitrile (CTBN) rubbers. The addition of such elastomers to uncured epoxy resins has produced cured rubber-modified epoxy resins exhibiting microstructures consisting of relatively small rubbery particles dispersed in, and bonded to, a matrix of crosslinked epoxy resin. Numerous studies have conclusively shown that such two-phase morphologies yield an epoxy polymer having a considerably higher toughness compared to the unmodified system with only minor reductions in other important properties such as modulus and glass transition temperature. Such attributes have resulted in rubber-modified epoxy resins being seriously considered for structural adhesive and composite matrix applications.
The properties of cured rubber-modified epoxy resins depend to a large degree on the factors controlling the critically important phase separation process. Of fundamental importance is the need to achieve an optimum compatability balance. Good initial compatability between the rubber and the uncured epoxy formulation together with a capacity for molecular incompatability at a later stage in the cure reaction, prior to gelation, are the essence of the technique.
Many factors have been shown to influence the phase separation characteristics of rubber-modified epoxy resin formulations and thus influence mechanical properties. Notable examples include the acrylonitrile content of the rubber modifier and the quantity and type of curing agent employed. The latter can influence the epoxy/rubber compatability balance as well as exert a degree of control over the rate of gelation and cure.
Cure conditions (temperature and time) are known to influence the mechanical properties of both unmodified cured epoxy resins and cured epoxy resins modified by the inclusion of elastomers. In particular Manzione et al (Journal of Applied Polymer science, Vol. 26, 889-905 (1981)) and Levita et al (Polymer (1985) 26 p1110-1116) have investigated the effect of cure conditions on epoxy resin formulations based on diglycidyl ether of bisphenol A and pieridine curing agent which are modified with, respectively, carboxyl-and amine-terminated butadiene-acrylonitrile liquid rubbers. Although not usually employed in commercial formulations, piperidine offers certain advantages in this type of formulation over other curing agents in that
(i) a pre-reaction between the epoxy compound and the amine or carboxyl-terminated rubber are not required prior to cure, and
(ii) it encourages the formation of a clearly defined two phase cured product, in which the amount of rubber remaining in the cured epoxy matrix phase surrounding the rubbery particles is minimised thereby preventing the formation of a product which is flexible.
Both groups of investigators reported optimum cure conditions of 120.degree. C. for 14-16 hours to provide products of maximum toughness. However, such lengthy cure times are a clear disadvantage in the commercial use of adhesives where much shorter cure times are required to facilitate the speedy production of assembled structure. Levita et al, who used cure times in excess of 14 hours in all their reported work, found that for epoxy formations modified with ATBN (amine-terminated butadiene-acrylonitile rubber), toughness increased with cure te
REFERENCES:
patent: 3673274 (1972-06-01), Tomalia et al.
patent: 4025578 (1977-05-01), Siebert
patent: 4119592 (1978-10-01), Murphy
patent: 4476285 (1984-10-01), Crabtree et al.
Pavlikova et al., "Modification of Epoxy Resins by Carboxyl-Terminated Liquid Rubbers", Int. Polymer Science and Technology 1, No. 11 (1980).
Manzione et al., "Rubber-Modified Epoxies", Journal of Applied Polymer Science 26, 889-905 (1981).
Shaw Steven J.
Tod David A.
Nielsen Earl
The Secretary of State for Defence in Her Britannic Majesty's Go
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