Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1993-07-19
1995-03-28
Jagannathan, Vasu S.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525305, 525304, 522120, 523104, 523113, 524773, 623 10, 424422, 424437, 264222, C08F26502, C08F26506, A61F 218, B29C 3340, C08J 328
Patent
active
054018064
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to elastomer forming systems which are mouldable at ambient temperatures and which have a low reaction exotherm on setting, to kits comprising the components of such systems and to elastomer materials formed from them. These systems are particularly applicable to techniques for taking impressions of sensitive parts of human and animal bodies and for formation of prostheses. More particularly they are useful in the fabrication of earmoulds and ear impressions for use in hearing aid applications.
Dimensional accuracy of audiological materials, satisfactory handling characteristics and adequate service life are all fundamental requisites of an effective hearing aid fitment programme. Existing commercially available audiological materials have several limitations in so far as they show a high degree of dimensional instability and those whose shrinkage values are within acceptable clinical limits are difficult to handle and expensive. Two-stage earmould materials (eg Molloplast-.beta.) that provide good acoustic seal when used together with an impression material of superior dimensional stability (e.g. Otoform AK) are difficult to work with. Venting, surface polishing, bevelling and trimming are difficult to achieve and tubing tends to collapse when mounted within them.
Hard acrylic, which is durable, easily workable in terms of venting and surface polishing, etc, fails to provide good acoustic seal where high gain is required and comfort cannot be ensured due to its texture. Available acrylic based material, cold-cure acrylic poly(methylmethacrylate), which is used for one stage earmould making suffers from the following limitations:
(i) The setting reaction is highly exothermic, leading to potential discomfort and even damage to the aural tissues;
(ii) High degree of shrinkage (23% by volume for the monomer; 8% by volume for monomer/polymer mix) on setting, leading to dimensional inaccuracy of the impression, with consequences for the accuracy of the final earmould;
(iii) Rigidity, causing red, sore and inflamed ears, resulting from physical irritation by the roughness of the surface of the finished mould;
(iv) Strong fumes from the monomer cause headaches unless a ventilated room is used and
(v) Excessive dryness of the skin is caused when handling the materials (see Bulmer, British Journal of Audiology, 7, 5-8 (1973).
Poly (methyl methacrylate) cold-cure materials have been formally withdrawn from audiological applications in the developed world for such reasons.
Although previous studies (eg, Combe and Nolan, Scandinavian Audiology, 18 67-73 (1989) have recognised the need to formulate products that meet with audiological requirements, further investigations into polysulphides and acrylic based materials for impression taking were discouraged due to their reported handling characteristics and lack of stability.
The present invention provides novel acrylate based materials, particularly homopolymers and copolymers of the higher methacrylates, and their use, inter alia, as audiological and prosthetic moulding materials. These materials provide the advantages of being (i) bio-compatible with tissues, (ii) easily mixed to the required consistency without handling the material, (iii) low in cost, (iv) having a suitable shelf life, (v) much lower in exotherm and shrinkage on setting and (vi) ambient temperature cure.
Previous uses of higher methacrylates have included formation of so-called soft acrylics as soft lining materials for dentures, maxillo-facial prostheses and soft earmould materials (Parker and Braden, Journal of Dentistry (1982) 10, 149-153. There have also been investigations of the properties of cross-linked higher poly -(methacrylates) for testing as ear mould materials (Davy and Braden, Biomaterials (1987) 8, 393-396.
Typical formulations produced by these workers comprised components which would be processed by the dough technique whereby a powder of suspension polymerised copolymer is mixed with liquid monomer to form a dough.
Components used in those formulation
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K. W. M. Davy, "The Mechanical Properties of Elastomeric Poly(alkyl methacrylate)s", Biomaterials, vol. 8, No. 5, 12 Feb. 1987, Guildford GB, pp. 393-396.
Braden Michael
Okpojo Allison O.
Jagannathan Vasu S.
The Secretary of State for Health in Her Britannic Majesty's Gov
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