Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-04-09
2001-01-09
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S202000, C428S407000, C525S307000, C525S308000, C525S309000, C525S310000, C525S316000
Reexamination Certificate
active
06172135
ABSTRACT:
The present invention relates to impact modified (meth)acrylic polymers and articles formed therefrom.
(Meth)acrylic polymers, for example poly (methyl methacrylate), PMMA, are well known. However, such polymers are often relatively brittle, that is they are not resilient and have poor resistance to sudden impact, thereby limiting their general use.
Conventionally, to improve the impact resistance of such polymers, impact modifying polymers are blended with the (meth)acrylic polymer. These impact modifying polymers typically possess a Tg that is lower than that of the (meth)acrylic polymer and usually a Tg of less than 0° C.
Commonly, the impact modifying polymers are prepared and used in the form of so-called multistage core-shell particles. The multistage core-shell particles are blended, for example melt blended, with the (meth)acrylic polymer to form a composition containing about 40% by weight of the core-shell particles.
Extensive activity has centred on achieving the optimum configuration, that is the number and relative thickness of the core and of each shell, and also the composition of the core and of each shell in order to maximise the impact resistance of the resulting blend. Such configurations and compositions have increasingly become more sophisticated and complex thereby leading to increased difficulty and expense of manufacture of the core-shell particles and, hence, the resultant blends. Consequently, impact modified (meth)acrylic polymers are often only used for specialised applications where the need for their superior optical properties outweigh their general expense.
EP-A-0606636 teaches that, in order to achieve improved toughness over that provided for by the addition of an impact modifier in the form of a three stage core-shell, an (meth)acrylic polymer has to be blended with certain specific polysiloxanes. The three stage core-shell particles disclosed consist of an inner core of PMMA which is cross-linked with 1,4 butane diol dimethacrylate; a first shell of a copolymer consisting of 82% w/w of n-butyl acrylate and 18% w/w of styrene; and a second shell of PMMA. The inner core represents 15% w/w of the particle; the first shell 65% w/w of the particle; and the second shell 20% w/w of the particle. The core-shell particles are blended at a level of 40% w/w with a (meth)acrylic polymer. The (meth)acrylic polymer consists of a copolymer containing 99% w/w of methyl methacrylate and 1% w/w of methyl acrylate. The n-butyl acrylate/styrene copolymer content of the blend is calculated as 26% w/w. Improvements in notched impact resistance of up to about 38% are indicated as having been achieved by the additional use of the specified polysiloxanes to achieve a maximum notched impact value of 2.8 kJ.m
−2
.
GB-A-2039496 is directed towards the preparation and use of a four stage core-shell particle. Typically, the inner core and second shell are a butyl acrylate/styrene copolymer containing 80% w/w of butyl acrylate, 18% w/w of styrene and 2% of allyl methacrylate graft cross-linker. Typically, the first shell is a methyl methacrylate/ethyl acrylate copolymer containing 94.6% w/w of methyl methacrylate, 5% w/w of ethyl acrylate and 0.4% w/w of allyl methacrylate. The third shell is a methyl metacrylate/ethyl acrylate copolymer containing 95% w/w of methyl methacrylate and 5% w/w of ethyl acrylate. The first and third shells together represent 25% w/w of the particle. Comparative examples show the preparation of a three stage core-shell particle. In the three stage core-shell particle the butyl acrylate/styrene copolymer core has been omitted so that the particles now have a methyl methacrylate/ethyl acrylate core and the methyl methacrylate/ethyl acrylate content of the particles is 25 to 35% w/w. It is shown that, when the particles are blended at a level of 50% with an (meth)acrylic polymer consisting of 95% w/w methyl methacrylate and 5% w/w ethyl acrylate thereby giving a butyl acrylate/styrene copolymer content in the blend of 37.5% w/w (using the four stage core-shell particles) and 27.5% w/w (using the three stage core-shell particles), the four stage core-shell particles gave rise to a 26% increase in impact toughness.
U.S. Pat. No. 5,286,801 teaches that the impact strength of an (meth)acrylic polymer, consisting of a copolymer containing 99% methyl methacrylate and 1% methyl acrylate, is improved by the use of a five stage core-shell particle in which the core, second shell and fourth shell are formed from a methyl methacrylate/ethyl acrylate copolymer containing 95.4 to 95.8% w/w methyl methacrylate, 3.9 to 4.6% w/w ethyl acrylate and 0 to 0.3% w/w allyl methacrylate; and the first and third shells are a n-butyl acrylate/styrene copolymer containing 80.4% w/w butyl acrylate, 17.6% w/w styrene and 2% w/w allyl methacrylate. The methyl methacrylate/ethyl acrylate copolymer representing 34.5% w/w of the total particle. Comparative examples relate to a three stage core-shell particle in which the core and second shell are formed from a methyl methacrylatelethyl acrylate copolymer containing 95.9 to 96% w/w methyl methacrylate, 4% w/w ethyl acrylate and 0 to 0.1% w/w allyl methacrylate; and the first shell is a n-butyl acrylate/styrene copolymer containing 80.4% w/w butyl acrylate, 17.6% w/w styrene and 2% w/w of allyl methacrylate. The methyl methacrylate/ethyl acrylate copolymer representing 35.5% w/w of the total particle. When blended at a level of 39% with an (meth)acrylic polymer, to give a n-butyl acrylate/styrene content of 25.5% w/w (using the five stage core-shell particles) and 25.2% w/w (using the three stage particles), the best five stage core-shell particles gave an unnotched Charpy Impact strength of 81 kJ.m
−2
which represented an increase of 19% over that achieved by the comparative three stage core particles.
A C Archer et al, Proceedings of the Churchill Conference on Deformation, Yield and Fracture of Polymers, Cambridge, April 1994, analysed the effect that the number of stages together with the size and content of each stage of various 2 to 4 stage core-shell particles had on the impact strength of (meth)acrylic polymers, as typified by a copolymer containing 92% mol/mol methyl methacrylate and 8% mol/mol butyl acrylate. The general conclusions were that the impact resistance of the blend increased rapidly with increasing volume fraction of n-butyl acrylate/styrene copolymer until the volume fraction was in the range 0.1 to 0.2. However, increasing the volume fraction above 0.2 caused a decrease in impact resistance.
Surprisingly, it has now been found that a three stage core-shell particle can be produced which when blended into a relatively brittle (meth)acrylic polymer can imbue the blend with a significantly higher impact resistance than that which has been hitherto achieved with conventional multistage core-shell particles in comparable blends.
Accordingly, in a first aspect, the present invention relates to a multistage core-shell particle consisting of a core, a first shell and optionally a second shell, substantially free from vinylically unsaturated compounds having at least two equally reactive double bonds, wherein
(i) the core contains a first (meth)acrylic polymer;
(ii) the first shell contains a low Tg polymer comprising 0 to 25% by weight of a styrenic monomer and 75 to 100% by weight of an (meth)acrylic monomer, the (meth)acrylic monomer capable of forming a homopolymer having a glass transition temperature (Tg) in the range from −75 to −5° C., and which first shell represents more than 65% by volume of the combined volume of the core and first shell;
(iii) the second shell, when present, contains a second (meth)acrylic polymer which may be the same or different from the first (meth)acrylic polymer; and
(iv) the core and first shell together contain from 0.5 to 1.0% by weight of a graft-crosslinker.
In a second aspect, the present invention provides a composition comprising a matrix of a third (meth)acrylic polymer containing residues of core-shell particles obtainable from a plurality of mu
Fraser Ian Muir
Treadgold Jonathan Mark
Imperial Chemical Industries plc
Pillsbury Madison & Sutro LLP
Rajguru U. K.
Seidleck James J.
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