Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-04-05
2002-05-07
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S832000, C526S200000, C526S932000
Reexamination Certificate
active
06384132
ABSTRACT:
This invention relates to environmentally friendly (optionally structured) aqueous architectural coating compositions, for example water-resistant paints (or varnishes or woodstains) suitable for use at ambient temperatures (say up to 35° C. and usually above 0 or 5° C.) in decorating surfaces of architectural structures such as buildings or furniture or fittings found in buildings. The invention allows starch to be used commercially in making these aqueous compositions so reducing their dependency on materials obtained from non-renewable resources such as petrochemicals. It also permits the use of smaller amounts of certain expensive co-monomers.
All aqueous architectural paints contain film-forming binder polymer. As a coating of liquid paint dries on a surface, this film-forming binder polymer serves to form a film (i.e. a dried coat) of paint which bonds to the surface and also binds together all the non-volatile components of the paint including particularly any pigments, opacifiers and extenders present.
A wide variety of binder polymers are available, but those most commonly used in aqueous architectural paints are of three broad types obtained from mono-ethylenically unsaturated monomers and known colloquially as the “acrylics”, the “vinyls” and the “styrenics”. The “acrylics” are usually copolymers of at least two alkyl esters of one or more mono-ethylenically unsaturated carboxylic acids (e.g. methyl methacrylate/butyl acrylate copolymer) whilst the “vinyls” usually comprise copolymers of a mono- vinyl ester of a saturated carboxylic acid and at least one of either an acrylic monomer or a different mono-vinyl ester. The “styrenics” are copolymers containing styrene (or a similar mono-vinyl aromatic monomer) together with a copolymerisable monomer which is usually an acrylic. Such binder polymers require the use of monomers which are obtained from petrochemical feedstocks whereas nowadays it is environmentally desirable to use as much as possible of material obtainable from renewable resources. In addition, some of the monomers essential to the binder polymer contain sterically bulky groups and are therefore relatively expensive.
Starch is one of the most abundant renewable resources because it is commercially available from such crops as potato, wheat, maize (both waxy and non-waxy), rice, sago, sorghum and tapioca. Moreover, it has been known since the 1960's that starch-containing film-forming polymer binders could be made by polymerising mono-ethylenically unsaturated monomers at ambient pressure in the presence of aqueous dispersions of starch whereupon binder polymers were formed which consisted of chains of polymerised mono-ethylenically unsaturated monomers chemically associated with the starch and this reduced the amount of material in the binder which comes from non-renewable resources. The precise nature of the association between the chains of polymerised monomers and the starch has not yet been unequivocally determined, but it is widely believed that the chains graft onto the starch. Such starch-containing binders have been used successfully in adhesives, in coating paper and in sizing textiles but instability problems led to phase separation and unpredictable increases in viscosity which meant that the starch-containing binders were not commercially suitable for use in making aqueous water-resistant architectural paints.
In 1981, British Patent Specification GB 2 075 525A disclosed that long term stability of aqueous dispersions of graft copolymers of starch and “vinyl” monomers could be improved by using a starch which had been modified by
a) subjecting it to acid and/or enzymatic hydrolysis either before or after the hydrolysis and
b) reacting the starch with molecules which bring about the positive introduction into the starch of (preferably non-ionic or cationic) groups such as carbamylethyl, alkyl, benzyl, benzalkyl, hydroxyalkyl, cyanoalkyl, acyl or dialkylamino alkyl groups.
Very stable aqueous dispersions of starch-containing film-forming binders were then obtained which GB 2 075 525A suggested could be used in adhesives, paper coating, textile sizing and in the making of water-based (i.e. aqueous) paints, although it did not disclose water-resistant architectural paints.
Starch-containing film-forming binders substituted with such specified non-ionic or cationic groups appear to have been very successful in the fields of adhesives, paper coating and textile sizing but they are not well suited for use in making aqueous water-resistant architectural paints owing to a high water-sensitivity as is illustrated by the loss of abrasion resistance which occurs when dried coats of the paints are subjected to the British Standards wet scrub resistance test described later. Water-sensitivity is of course a significant problem when a dried coat of paint is to be exposed to rain or frequent water-condensation.
A secondary disadvantage is that the conventional substituted starch binders are not very effective in creating the structure which is sometimes wanted in aqueous architectural paints, especially thixotropic paints. Structure can be created in various ways including an interaction between a so-called “structuring agent” (which is typically a titanium or zirconium chelate or a structuring clay) and the binder polymer plus any cellulosic thickener which may be present in the paint. Structuring clays are clays such as laponite or bentonite clays. These three types interact in some way which is not fully understood to create a gel. Gel structures reduce the tendency for a paint to drip or splash and so they make painting less messy and allow brushes and rollers to be loaded with greater volumes of paint. During application of the paint to a surface by brush or pad, the paint experiences high shear forces which temporarily destroy the gel structure and allow the paint to be spread easily over the surface. This is known as “thixotropy”.
A binder polymer for a modern aqueous architectural paint should preferably be capable of manufacture in conventional plants, nearly all of which operate at ambient pressure and it should be able to form aqueous dispersions which are stable under high shear forces such as are encountered during application of a paint by brush or pad. The paints should be environmentally friendly and able to give dried coatings which are sufficiently water-resistant to permit use outdoors or in humid locations such as found in kitchens and bathrooms. Accordingly, it is an object of this invention to provide an environmentally friendly aqueous coating composition which contains starch which has been modified to permit the formation of stable dispersions of binder polymer but which retains good water-resistance in the dried paint. An optional object is to provide a starch-containing binder polymer which can be used in a thixotropic paint.
Accordingly, this invention provides an environmentally friendly (optionally structured and possibly thixotropic) aqueous architectural coating composition which includes film-forming binder polymer composed of modified starch chemically associated with chains of copolymerised monomers at least 93 wt % (and preferably 95 to 100 wt) of which are selected from mono-ethylenically unsaturated monomers wherein
a) the starch has been modified by the introduction of carboxylic acid groups optionally converted to an inorganic salt,
b) up to 50 wt % of the starch-containing binder polymer is provided by the modified starch and
c) not more than 7 mol % (preferably 0 to 3 mol %) of the copolymerised monomers are derived from carboxylic acid monomers.
Granular amylopectin is the predominant form of starch in most natural sources and for this reason the starch prior to modification generally has a weight average molecular weight of at least 5×10
5
and usually over 1×10
6
. The preferred starches are obtained from potatoes, maize (corn) or from waxy maize. Potato and maize starch is relatively cheap and contains over 70 wt % amylopectin whereas waxy maize starch is more expensive but contains even more (of
Horley Susan Mary
Wheeler Stephen Arthur
Imperial Chemical Industries Plc
Pezzuto Helen L.
Schmitz Thomas M.
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