Field of the Invention:
This invention relates to aqueous emulsions containing copolymers
of vinyl acetate and ethylene in which vinyl acetate is the major component. Copolymers
of the invention are utilised in paper coating compositions and demonstrate good
rheology and dry pick strength with enhanced wet pick strength.
Background of the Invention:
Paper coating compositions usually comprise an aqueous polymer binder
emulsion, a pigment and optionally other additives common in the technology. The
present invention is specifically directed to novel emulsions based on copolymers
comprising a vinyl C1 to C4 alkanoate component forming at least about 60% by weight
of the copolymer, more usually at least about 70% by weight and an alkylene component,
eg. ethylene. Vinyl acetate is an example of a vinyl ester of a C1 to C4 alkanoic
acid to which the invention is generally directed. Coating compositions of the
invention will contain a pigment, for example clay, which is compounded with the
latex binder and used in coating a cellulosic web for example a paper or paperboard
web. The characteristics of the latex binder component are significant in the ease
of preparation of the paper coating composition, its application and on the qualities
of the coated product.
The coating composition will be coated on to a paper substrate, dried
and calendered. Usually the composition will be applied at a level to give a loading
of about 18 to about 24 g/m² solids on one surface. The copolymer particles will
bind those of the pigment filler to provide cohesive strength and contribute to
the adhesion between the coating and paper substrate. The coating, which will
usually have a thickness of about 3 microns to about 10 microns imparts gloss and
whiteness to the paper and provides a suitable surface for receiving ink.
When the coating is subjected to successive printing operations,
for example in offset lithography, the coating must provide acceptable wet and
dry pick strengths. When the first colour is printed any pull by the tacky ink
giving failure of the coating is termed 'dry pick'. Failure when the coated paper
passes to the second inking stage, when ink is applied to prewetted areas, is
termed 'wet pick'. Subsequent inking steps can also give rise to wet pick failure.
General description of the Invention:
The invention provides the use in paper coating of an aqueous copolymer
emulsion having a solids content from about 20% to about 70% by weight and comprising
- i) from about 60%, preferably from about 70%, to about 90% by weight of a vinyl
C1 to C4 alkanoate,
- ii) from about 5% to about 20% of a C1 to C4 alkylene monomer, and
- iii) from about 5% to about 20% by weight, preferably from about 8% to about
15% by weight, of vinyl esters having the general formula R&sub1;R&sub2;R&sub3;
CCOOCHCH&sub2; wherein R&sub1;R&sub2; and R&sub3; are each alkyl groups having
at least one carbon atom and R&sub1;+R&sub2;+R&sub3; is from 6 to 9.
This product has good rheology and dry pick strength and enhanced
wet pick strength which is attributable to the presence of vinyl ester (iii) monomers.
There is a general requirement in paper coating to improve these characteristics.
The invention extends to a pigmented paper coating composition comprising
- i) to 100 parts of pigment and
- ii) from about 3% to about 30% by dry weight of an aqueous emulsion as defined
In the emulsions prepared for use in paper coatings the polymerisation
system will be substantially free of protective colloids. Colloidal materials may
be added subsequently to the paper coating composition.
Although vinyl acetate is the preferred vinyl alkanoate monomer because
of its availability, cost and known reactivity, other vinyl esters within the class
defined are usable in particular vinyl formate, propionate, butyrate and isobutyrate.
The preferred alkylene is ethylene but other ethylenic hydrocarbons,
for example propylene, butylene and isobutene are usable.
Optionally the copolymer may contain minor monomer components added
to provide specific benefits, examples are sodium vinyl sulphonate, acrylic acid,
methacrylic acid, acrylamide, hydroxy functional acrylates, vinyl silanes and
vinyl halides. A favoured comonomer is a polyethylencially unsaturated compound
selected from triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl
fumarate, divinyl benzene and diallyl phthalate.
Methods for preparing the copolymer emulsions of the invention are
well characterised in the literature. Polymer Synthesis (vols I and III) by Sandler
& Karo (Academic Press 1974) and Preparative Methods of Polymer Chemistry
(2nd Ed) by Sorenson and Campbell (Interscience 1968) provide preparative information.
Methoden der Organischen Chemie (Houben-Wey) Band XIV published by George Thieme
Verlag Stuttgart (1961) also provides preparative descriptions.
Preferably at least one surfactant used in the polymerisation process
contains a C14 to C20 alkylene moiety and examples are an oleyl propanol amide
sulphosuccinate obtainable from Witco of USA under the trade name Emcol K8300
and the potassium salt of the sulphonation product of oleic acid obtainable from
Lankro Chemicals of Manchester England under the trade name Lankropol OPA.
The paper coating composition comprises a pigment, for example clays,
hydrated silica clays, and other conventional ingredients. The clays used include
Kaolin group clays and hydrated silica clays, specific clays are disclosed in
"Kaolin Clays and their Industrial Uses" by J. M. Huber Corp. (1949) New York chapters
10 to 16.
Other pigments, for example calcium carbonate, titanium dioxide,
blanc fixe, lithopone and zinc sulphide may be used in addition to clay. In general
the paper coating composition will comprise
Parts by weight
dispersing agent eg. sodium pyrophosphate
0.1 to 0.5
latex (on solids basis)
3 to 30
Cobinder eg. starch or casein
0 to 25
Defoamer, eg. a hydrocarbon oil
0 to 0.2
Water to provide desired solids level.
The coating compositions produced with the aid of the latexes of
the invention will be applied to fibrous paper webs using conventional means for
example trailing blade coaters, air knife coaters and roll coaters.
The applicants are aware of the following disclosures relating to
EPA 0295727 (Shell) describes polymer emulsions containing vinyl
acetate, ethylene and versatates (Veova 9) for use in paint compositions to give
non tacky surfaces which are alkali resistant. These benefits are demonstrated
in comparison with Veova 10. The description suggests the emulsions are usable
in paper coating and other fields, but there is no disclosure of the wet pick
benefit identified in the present application.
EPA 0172354 (Air Products) describes vinyl alkanoate/Ethylene emulsions
for paper coating which provide enhanced dry pick strength and gloss. The additional
monomers are unsaturated acids/esters and dicyclopenta-dienyl acrylate. Vinyl versatates
are quoted as examples of the alkanoates but there is no suggestion to use them
as a third monomer with VA/E. VA/E is stated to lack sufficient wet pick resistance
for offset printing.
GB 1144316 (Dunlop) describes alkali-soluble polymer emulsions formed
from quaternary alpha carbon carboxylic, vinyl carboxylate and unsaturated carboxylic
monomers. pH and viscosity stability derive from the tertiary monomer.
- i) Dry pick strength values (cm/sec) were measured using a coating composition
on Reeds Aircoat woodfree paper using an IGT Dynamic Pick Tester (AC2 Model) with
25 Kg load and low viscosity ink. The composition tested contained:
* obtainable from English China Clay of St Austell England.
** Obtainable from Finn Forest Chemicals of Cheam, England.
*** Allied Colloids of Bradford, England.
Pigment SPS clay*
to 100 parts
Emulsion under test
24 parts of 50% solids emulsion
Water retention aid**
Water to give 45% solids
- ii) The wet pick strength was measured with the apparatus of the dry pick method.
0.3 mls of ink was applied to each roller and distributed for 15 minutes. A test
strip of paper coated with the test composition to a level of 20g/m² (solids) was
placed in the device. The inked roller was contacted with the test strip and adjusted
to a printing force of 35 Kgf. A drop of tap water was placed on the test strip
immediately in the path of the roller so ink is applied over a layer of water spread
by progress of the roller.
The degree of resistance to pick is judged at a qualitative level
i.e. good - fair - bad.
Specific description of the invention:
Examples of the invention will now be given to illustrate but not
limit the invention.
In the examples several functional monomers and surfactants were
used and these are represented by abbreviations and letters in the table of results
in order to simplify the presentations.
Functional monomers: A172 is vinyl tris trimethoxy silane obtainable from Union
Carbide of USA.
DAM is diallyl maleate.
EHM is ethyl hydrogen maleate
SVS is sodium vinyl sulphonate
Surfactants: A is sodium nonylphenol 20 EO sulphate obtainable from Lankro Chemicals
of Manchester England as Perlankrol RN75.
B is sodium oleyl propanol amide sulphosuccinate obtainable from Witco of USA as
C is sodium dodecyl benzene sulphonate obtainable from Lankro Chemicals as Arylan
D is disodium salt of an ethoxylated alcohol half ester of sulphosuccinic acid
obtainable from Cyanamid of Wayne New Jersey USA as Aerosol A102.
A surfactant solution (initial charge) comprising 37% aq. solution
of Emcol K8300 (74.2g), 30% aq solution of Aerosol A102 (53.2g) and ammonium persulphate
(4.3g) dissolved in deionised water (999g) was prepared and charged to a 4 litre
pressure vessel. The contents are heated to 68°C. At 68°C the reactor was purged
twice with nitrogen, once with ethylene and pressurised with ethylene to 450 lbs/sqin
gauge with stirring. The temperature was adjusted to 76°C while 10% of the liquid
monomer phase was pumped to the reactor.
The continuous addition of persulphate solution (5.3g in 300g of
deionised water) was begun and the reaction mass was held at 76°C for 45 minutes.
At the end of this period, the continuous addition of the remainder (90%) of the
monomer phase was started. The persulphate solution was added over 6.5 hours and
the monomer phase over 5.0 hours. The pressure was maintained at 450 lbs/sq in
gauge until the desired quantity of ethylene had been reacted, this was after
four hours from start of continuous persulphate additon.
After completion of the addition of monomer, the reaction mass was
held at 80°C for 45 mins, at the end of which the addition of persulphate solution
was stopped. The contents of the reactor was cooled to 40°C, followed by pumping
of finishing off stage (4.6g of each in 70g of deionised water) to the reactor,
in separate streams, over 30 minutes.
The contents of the reactor were then cooled to 25°C and discharged
to a degassing tank.
Ammonia solution was used to adjust the pH of the emulsion latex
to 8. The emulsion had a solids content of 51.6% by wt.
9 is vinyl ester of versatic acid in which R&sub1;+R&sub2;+R&sub3;=7 and is obtainable
from Shell Chemicals of Chester England.
Parts per hundred monomer
Vinyl Acetate (VA)
Veova 9** (VV9)
The result of the dry and wet pick tests on a coating composition
containing the resulting emulsion are given in Table I.
Examples II to XII
Additional examples were performed using the process steps of Example
I but varying the quantities of the base monomers, i.e. vinyl acetate, VeoVa 9
and ethylene, the functional monomers and the stabilising surfactants. These emulsions
were also tested for wet and dry pick strengths.
The emulsion products had solids contents comparable to that obtained
in Example I.
The applicants have found the addition of a vinyl ester of the class
exemplified by VeoVa 9 to a vinyl acetate/ethylene emulsion polymer system improves
the wet pick resistance of a paper coating composition.
* Comparative Example
** functional monomer present
NM - not measured