U.S. patent application number 13/077503 was filed with the patent office on 2011-07-21 for water-based resin compositon and articles made therefrom.
This patent application is currently assigned to ALLEGIANCE CORPORATION. Invention is credited to Ida Berger, Seong F. Chen, Shiping WANG, Wei C. Wong, Yun-Siung T. Yeh.
Application Number | 20110178234 13/077503 |
Document ID | / |
Family ID | 41063758 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110178234 |
Kind Code |
A1 |
WANG; Shiping ; et
al. |
July 21, 2011 |
WATER-BASED RESIN COMPOSITON AND ARTICLES MADE THEREFROM
Abstract
An aqueous elastomer dispersion includes a dispersed phase and
an aqueous phase. The dispersed phase includes an elastomer
including curable aliphatic conjugated-diene elastomers, such as
polyisoprene, and a minor amount of at least one additive. The
aqueous phase includes water and other optional components in
either a soluble state or a dispersion state. The aqueous elastomer
dispersion may be prepared by dissolving an elastomer, such as
rubber, and additives in a solvent mixture and then converting the
resulting solution into an aqueous emulsion. The aqueous emulsion
is concentrated and the solvent is stripped from it to yield a
dilute latex. The dilute latex that is obtained is concentrated
again. Articles made from the aqueous elastomer dispersion include
medical gloves, condoms, probe covers, dental dams, finger cots,
catheters and the like.
Inventors: |
WANG; Shiping;
(Libertyville, IL) ; Chen; Seong F.; (Gelugor,
MY) ; Berger; Ida; (Worcester, MA) ; Yeh;
Yun-Siung T.; (Libertyville, IL) ; Wong; Wei C.;
(Kedah, MY) |
Assignee: |
ALLEGIANCE CORPORATION
McGaw Park
IL
|
Family ID: |
41063758 |
Appl. No.: |
13/077503 |
Filed: |
March 31, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12403901 |
Mar 13, 2009 |
|
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13077503 |
|
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61036605 |
Mar 14, 2008 |
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Current U.S.
Class: |
524/571 |
Current CPC
Class: |
C08J 3/07 20130101; C08L
9/10 20130101; C08J 3/05 20130101; C08K 5/0008 20130101; C08C 1/075
20130101; C08J 2309/00 20130101; C08K 5/0008 20130101; C08L 21/02
20130101 |
Class at
Publication: |
524/571 |
International
Class: |
C08L 9/10 20060101
C08L009/10 |
Claims
1. A method of preparing an aqueous elastomer dispersion,
comprising the steps of: preparing an elastomer/additive solution
comprising an elastomer, at least one additive, and a solvent
system; emulsifying the elastomer/additive solution to form an
emulsion; concentrating the emulsion to form a concentrated
emulsion; removing substantially all of the solvent system from the
concentrated emulsion to form a substantially solvent-free
dispersion; and optionally concentrating the substantially
solvent-free dispersion.
2. The method of claim 1, wherein the elastomer is selected from
the group consisting of polyisoprene, polychloroprene,
styrene-butadiene copolymers, acrylonitrile-butadiene copolymers,
styrene block copolymers, and butyl rubber.
3. The method of claim 2, wherein the elastomer has a molecular
weight of about 100,000 to about 3,000,000.
4. The method of claim 2, wherein the at least one additive is
selected from the group consisting of sulfur/sulfur donors,
peroxide curing/crosslinking agents, vulcanization accelerators,
vulcanization activators, antioxidants, stabilizers, plasticizers,
antiozonants, pigments, fillers, antimicrobial agents, indicators,
and additional polymers.
5. The method of claim 4, wherein an amount of the at least one
additive is about 0.5-49% by weight of total solids of the aqueous
elastomer dispersion.
6. The method of claim 1, wherein the solvent system includes a
first solvent and second solvent.
7. The method of claim 6, wherein the elastomer is soluble in the
first solvent, and the at least one additive is soluble in the
second solvent.
8. The method of claim 6, wherein the first solvent includes one or
more solvents, and the second solvent includes one or more
solvents.
9. The method of claim 1, wherein the emulsifying step comprises
adding an emulsifier.
10. The method of claim 9, wherein the emulsifying step further
comprises mixing with a high shear mixer.
11. The method of claim 1, wherein one or both of the concentrating
steps comprise adding a creaming agent.
12. The method of claim 11, wherein one or both of the
concentrating steps further comprise separating a concentrate and
an aqueous solution.
13. The method of claim 1, wherein one or both of the concentrating
steps is carried out by centrifuging using a continuous centrifugal
separator.
14. The method of claim 1, wherein the removing step comprises
evaporating the solvent system from the concentrated solution.
15. The method of claim 1, wherein the removing step comprises
heating the concentrated solution to evaporate the solvent
system.
16. The method of claim 1, further comprising the step of adjusting
the pH of the aqueous elastomer dispersion.
17. The method of claim 1, wherein the total solids content of the
aqueous elastomer dispersion is about 45-65% by weight.
18. The method of claim 1, further comprising the step of making an
article from the aqueous elastomer dispersion.
19. The method of claim 18, wherein the article is selected from
the group consisting of gloves, condoms, probe covers, dental dams,
finger cots, and catheters.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a novel water-based resin
composition that includes an aqueous elastomer dispersion. Further,
the invention relates to a method of preparing the aqueous
elastomer dispersion and articles made from the aqueous elastomer
dispersion which provide substantial improvements in performance
when compared to those made from conventionally prepared
water-based resin compositions.
BACKGROUND OF THE INVENTION
[0002] The use of water-based resin dispersions for making articles
such as a film, coating, and adhesive provides several advantages
when compared to its solvent-based counterpart. In particular,
water-based dispersions are environmentally-friendly, easy to
formulate, control temperature, and regulate pre-maturation, and
have adjustable viscosities.
[0003] However, there are also several disadvantages to using a
conventional water-based resin dispersion. For example, there are
limits on both the compounding of the dispersion and the resulting
article. After compounding, a barrier forms between the elastomer
dispersion and the remaining components such as dispersions of
curing ingredients and additional polymer dispersions. During the
film forming process, the remaining components must overcome this
barrier by leaving the confines of their own dispersion (water,
surfactant, and emulsifier), traveling through the aqueous medium
of the compounding formulation, crossing the surface of the
dispersion of the elastomer particle, and finally penetrating into
the elastomer particles. The likelihood of this occurring is even
smaller for higher molecular weight components. This obstacle also
leads to a limitation on the nature of the components that may be
used.
[0004] The second disadvantage is the limitations in the
performance of the articles prepared from a conventional elastomer
dispersion. There is limited miscibility between the elastomer and
the remaining components, particularly when both have different
inherent structures. This causes the elastomer and the components
to remain in their bulk states and inhibits their ability to
diffuse and mix in a homogenous manner. As a result, articles made
from conventional elastomer dispersions have inferior
properties.
[0005] The third disadvantage is the complexity of formulating
conventional elastomer dispersions and a lack of flexibility in
altering the formulations while attempting to improve the
performance of articles made from the conventional elastomer
dispersions.
[0006] Thus, there is a need to develop a new elastomer dispersion
that can reduce or eliminate these disadvantages. This will result
in a dramatic improvement in the performance of the resulting
articles.
SUMMARY OF THE INVENTION
[0007] The invention relates to a novel aqueous elastomer
dispersion, its method of preparation, and articles made from the
aqueous elastomer dispersion. The aqueous elastomer dispersion of
the invention includes a dispersed phase and an aqueous phase. The
dispersed phase includes an elastomer such as polyisoprene and a
minor amount of at least one additive. The aqueous phase includes
water and other optional components dissolved in it.
[0008] The aqueous elastomer dispersion may be prepared by
dissolving an elastomer and additives in a solvent mixture and then
converting the resulting solution into an aqueous emulsion. The
aqueous emulsion is concentrated by removing the aqueous phase
material and the solvent is stripped from the concentrated emulsion
to yield a dilute latex. The dilute latex that is obtained is
preferably concentrated again to a desired solid content.
[0009] Articles made from the aqueous elastomer dispersion include
medical gloves, condoms, probe covers, dental dams, finger cots,
catheters and the like.
DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a flowchart illustrating a preferred method for
preparing the aqueous elastomer dispersion of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The present invention, which relates to a novel aqueous
elastomer dispersion, its method of preparation, and articles made
from the aqueous elastomer dispersion formulation, overcomes the
obstacles discussed above.
[0012] The following terms and definitions are provided to clarify
the meaning of specific terms:
[0013] A colloidal "dispersion" is an intermediate between a true
solution and a mixture, or suspension. It can also be considered an
"emulsion," which consists of two liquid phases, a "dispersed
phase", microscopic globs, which are distributed throughout the
"dispersing phase". In oil in water dispersion (O/W), the
dispersing phase is also named as aqueous phase. The emulsion of
the present invention is generally referred to as a synthetic
colloidal polymer, wherein the polymer may be prepared via emulsion
polymerization (nitrile, polychloroprene), coordination
(Ziegler-Natta) polymerization (cis-polyisoprene) or anionic
polymerization (cis-polyisoprene).
[0014] "Latex" was originally referred to as a sap from a rubber
tree for making rubber products. Thus, dispersions, emulsions and
latex are all considered to be kinetically stable, colloidal
systems and these terms are used interchangeably for water-based
elastomer systems in the invention.
[0015] "Vulcanization" refers to a specific curing process of
rubber that involves the addition of sulfur and is normally used in
solid rubber process. It is an irreversible chemical crosslinking
reaction in which polymer molecules are linked to other polymer
molecules by atomic bridges composed of sulfur atoms.
[0016] "Curing" refers to either a chemical reaction that results
in a higher molecular weight or a physical process that is
associated with a solidified phase change. For the process of the
present invention, curing and vulcanization may be used
synonymously when referring to a process utilizing sulfur or sulfur
donors.
[0017] A conventional "curing process" of an elastomer is a
crosslinking reaction using curing ingredients including (1)
curing/crosslinking agents such as elemental sulfur and/or various
insoluble sulfur donors which release sulfur when heated, and/or
peroxide curing agents, (2) accelerators which behave like
catalysts for the reaction and may be categorized as either primary
(e.g., mercaptobenzothiazole MBT and mercaptobenzothiazole
disulfide MBTS) or secondary accelerators (e.g., thiuram,
dithiocarbamate and guanidine), and (3) activators such as metallic
oxides and metallic salts (e.g., zinc oxide).
[0018] "Creaming" is one of several methods (centrifuging,
electro-decantation and evaporation) developed for concentrating
newly-made elastomer dispersions. This process can concentrate a
low-solid emulsion to, for example, an above 50% solid emulsion.
The creaming process depends on the difference in specific gravity
between water (1.0) and the elastomer polymer (e.g. about 0.91) or
buoyancy of the elastomeric polymer by gathering the particles
which are surrounded by serum, near the bottom or near the top of
the emulsion. When the separation is complete, the aqueous serum
phase is removed, leaving the concentrated emulsion cream phase.
Creaming can be accelerated by the addition of certain creaming
agents such as sodium alginate, polyvinyl alcohol or cellulose
derivatives.
[0019] The aqueous elastomer dispersion of the invention includes a
dispersed phase and an aqueous phase. The dispersed phase includes
an elastomer and a minor amount of at least one additive. The
aqueous phase includes water soluble components which are used in
conventional elastomer dispersion formulations, such as
surfactants, pH adjusting agents and other water soluble adjuvants.
The amount of water soluble components in the aqueous phase of the
elastomer dispersion is typically not more than about 10%, and is
preferably about 2-10% by weight of total dispersion solids.
[0020] The elastomer may be a curable aliphatic conjugated-diene
including, but not limited to polyisoprene, polychloroprene,
styrene-butadiene copolymers, acrylonitrile-butadiene copolymers,
styrene block copolymers, and butyl rubber. The preferred elastomer
of the invention is polyisoprene, which may be either natural or
synthetic. Any synthetic, linear, high molecular weight
polyisoprene with a molecular weight of about 100,000 to about
3,000,000 that is commercially available can be used.
[0021] The at least one additive in the dispersed phase of the
aqueous elastomer dispersion may include at least one of curing
ingredients, non-curing ingredients, and additional polymers, to be
discussed below, with the same, similar or different chemical
structures from the elastomer. The total amount of additive(s) used
is about 0.5-49% by weight of total dispersion solids.
[0022] The curing ingredients may include any such ingredients
found in conventional elastomer dispersion compounding
formulations. For example, the curing ingredients may include, but
are not limited to, sulfur/sulfur donors, accelerators (primary and
secondary), and sulfur-curing (or vulcanization) activators and
peroxide curing/crosslinking agents which are known to those
skilled in the art. There is no limitation on the chemistry and the
number of curing ingredients, but they should be capable of
dissolving or dispersing in the solvent system of the elastomeric
solution which is described in detail below. The chemical structure
of the curing ingredients and their activity do not have to be kept
intact throughout the emulsification process, but the incorporation
of curing ingredient(s) should not adversely affect the desirable
performance of the resulting article made by the dispersion of the
present invention.
[0023] When curing using sulfur, the main curing agent preferably
comprises elemental sulfur (generally believed to be in the form of
S8, but not so limited). This may be used alone or in combination
with a sulfur donor. A sulfurless system can also be used, but this
requires a sulfur donor. Sulfur donors may include, but are not
limited to thiuram polysulfides such as tetramethylthiuram
disulfide and tetraethylthiuram disulfide, which also function as
vulcanization accelerators, and xanthogen polysulfides such as
butylxanthogen disulfide, CPB, diisopropyl xanthogen polysulfide
DIXP, and diisopropyl xanthogen disulfide.
[0024] Peroxides may include, but are not limited to, dibenzoyl
peroxides such as one manufactured by R.T. Vanderbilt as Varox
A-75, which has a curing temperature of 120.degree. C./20 minutes
for polyisoprene latex, dicumyl peroxides such as one manufactured
by Akzo Nobel as Perkadox BC-40B, which has a curing temperature of
polyisoprene rubber of 120.degree. C./20 minutes, and combinations
thereof.
[0025] Accelerators may include, but are not limited to,
dithiocarbamates such as zinc dimethyl dithiocarbamate (ZDMC), zinc
diethyldithiocarbamate (ZDEC), zinc dibutyl dithiocarbamate (ZDBC),
zinc dibenzyl dithiocarbamate (ZBEC) and zinc pentamethylene
dithiocarbamate (ZPD), thiazoles such as 2-mercaptobenzothiazole
(MBT), sodium 2-mercaptobenzothiazole (SMBT) and zinc
2-mercaptobenzothiazole (ZMBT), thiuram sulfides such as
tetramethyl thiuram disulfide (TMTD), tetraethyl thiuram disulfide
(TETD and tetrapentamethylene thiuram disulfide (TPTD), guanidines
such as diphenylguanidine (DPG) and di-o-tolyguanidine (DOTG), and
thioureas such as thiourea and diphenyl thiourea. One or more
accelerators may be used to formulate the elastomer dispersion of
the invention.
[0026] Activators may include, but are not limited to, zinc oxide,
magnesium oxide and lead oxide. Zinc oxide is the most commonly
used vulcanization activator. A single accelerator or a synergistic
combination of accelerators may be used.
[0027] Including curing ingredients such as sulfur/sulfur donors,
accelerators and/or activators in the emulsification process
improves the curing mechanism and modifies the topological features
of cured articles. For example, it provides manufactured articles
with such qualities as a thinner film, a lower level of chemical
residue, and an increased curing efficiency compared with a
manufactured article made conventionally by resin dispersion
containing the elastomer only and curing ingredients added as
aqueous dispersions.
[0028] Most of the non-curing ingredients used in aqueous elastomer
dispersion formulations of the art are solid, moisture sensitive
and have a high molecular weight. This causes the resulting article
made from the elastomer dispersion to have such problems as low
efficiency of ingredient use, discoloration, property
deterioration, and processing difficulties. These problems can be
overcome with the aqueous elastomer dispersion of the invention. In
a preparation of the elastomer dispersion of the invention,
non-curing ingredients may be included in the formulation without a
series of physiochemical processes such as aggregation, diffusion
and migration (which occur with formulation of a conventional
elastomer dispersion). Consequently, the performance of the
resulting article, such as glove, is improved.
[0029] Any non-curing ingredients that are conventionally used in
elastomer dispersion compounding formulations may be used in the
aqueous elastomer dispersion of the invention. For example, the
non-curing ingredients may include, but are not limited to,
antioxidants, stabilizers, plasticizers, anti-ozone agents,
pigments, fillers, antimicrobial agents, indicators, and additional
polymers.
[0030] Suitable antioxidants that may be added to the aqueous
elastomer dispersion may include, but are not limited to hindered
phenols such as butylated hydroxytoluene
(2,6-di-tert-butyl-4-methylphenol) and thiodiethylene
bis-di-t-butyl-4-hydroxyphenyl propionate, hindered polyphenolics
such as butylated reaction products of p-cresol and
dicyclopentadiene, hindered phenol/hindered polyphenolics such as
trimethyl-tris (di-t-butyl-4-hydroxybenzym)-benzene or octadecyl
di-t-butyl-4-hydroxyphenyl propionate, amines such as a blend of
6PPD with methyl styrene and bis-alpha-dimethylbenzyl diphenyl
amine, mixtures such as zinc mercaptotulumimidazole/phenolic,
triazinone derivatives such as triazinone-phenol mixtures,
polyaromatic amines such as poly(m-anisidine), phenolic antioxidant
hydrazides such as phenolics with anhydride copolymer, phenolics
such as 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), cresols such
as 2,4-dimethyl-6-(1-methylcyclohexyl)-p-cresol, and styrenated
phenols. It is preferred that hindered polyphenolics are used.
[0031] Colloidal stabilizers including alkalis for pH adjustment,
surfactants and alkaline caseinates such as sodium caseinate may
also be added to the aqueous phase.
[0032] Suitable plasticizers that may be added to the aqueous
elastomer dispersion may include, but are not limited to, fatty
salts, mineral oils and ester plasticizers.
[0033] An antiozonant may be added to the aqueous elastomer
dispersion because ozone severely damages some elastomeric
articles, such as highly unsaturated polyisoprene articles. When
included in the aqueous elastomer dispersion of the invention,
certain high molecular weight polymers, such as paraffin wax, EPDM
and hydrogenated polydiene can provide such articles with excellent
ozone resistance. Other examples of antiozonants that may be used
in the invention may include, but are not limited to alkyl/aryl
p-phenylenediamines such as
N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine 6PPD,
organoclay-antiozonant complexes such as smectite-containing clay
with alkyl-aryl-p-phenylenediamine, functionalized benzotriazoles
such as N,N-disubstituted para-phenylenediamine, triazines such as
tris (N-1,4-dimethylpentyl-p-phenylenediamino) 1,3,5-triazine and
tris (N-alkyl-p-phenylenediamino) 1,3,5-triazine, and
p-phenylenediamines such as
N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD). In addition,
polymers including waxes such as paraffinic wax (MW=300-500),
microcrystalline wax (MW=600-700) (with paraffinic wax) and low MW
PE wax (MW=100-1100), polymeric antiozonants such as polymeric
diphenyldiamine, and ozone inert polymers such as EPDM and
brominated isobutylene/para-methylstyrene copolymer (BIMSM) may be
used as antiozonants. It is preferred that waxes are used.
[0034] Suitable pigments that may be added to the aqueous elastomer
dispersion may include a wide range of natural pigments such as
titanium dioxide and iron oxides, and synthetic pigments.
[0035] Suitable fillers that may be added to the aqueous elastomer
dispersion may include, but are not limited to, inorganic fillers
such as clays, calcium carbonate, talc, and silica and organic
fillers such as crosslinked polymethyl methacrylate, finely divided
urethane resin particles and polyethylene microsphere.
[0036] Infection prevention can be effectively achieved when an
antimicrobial agent is dispersed in the aqueous elastomer
dispersion of the invention. The resulting article, such as an
antimicrobial glove, is safer for use because the antimicrobial
agent does not leach out to any appreciable degree (e.g. at least
95% is retained in the article after storage at room temperature
and a relative humidity of 60% for three months), as it is
intimately mixed/dispersed with the polyisoprene. Examples of
antimicrobial agents that may be used in the polyisoprene
dispersion of the invention include, but are not limited to,
water-insoluble organic phenol compounds such as
5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan),
2-hydroxy-2,4,6-cycloheptatrien-1-one, o-phenylphenol,
2-benzyl-4-chlorophenol, chlorophenol, chlorothymol, and
para-chloro-meta-xylenol (PCMX), inorganic silver compounds such as
silver ion-zeolite particle and silver sulfadiazine, zinc compounds
such as zinc pyrithione, copper compounds such as copper oxide,
fatty esters such as glycerol monocaprate and glycerol monolaurate,
silicone-ammonium polymers, nitrogen-containing polymers such as
polyhexamethylene biguanide and hydroxybenzoate-containing polymers
such as poly(methacrylate) containing hydroxybenzoate. It is
preferred that organic phenol compounds are used.
[0037] Indicators may also be included in the aqueous elastomer
dispersion of the invention to provide a signal of status change of
the latex article, for example, when a glove is breached. Various
indicators are used based on their physical and chemical properties
and function by different mechanisms. For example, the moisture
indicators may include, but are not limited to, inorganic salts
such as cobalt chloride, metal ozonide and ozonide ester, colorants
such as triarylmethane, azo dye and alphazurine, phthalein dyes
such as thymolsulfonephthalein and phenolphthalein, organic salts
such as dyes (phenolphthalein, thymol blue, m-cresol purple) along
with transition metals and solvatochromic dyes such as betaine. The
microbe indicators may include, but are not limited to,
solvatochromic dyes such as betaine/Reichardt's dye,
zwitterion/merocyanine and pyridinium iodide, acid pH dyes such as
bromcresol purple and phthalein, and anthocyanin dyes. The blood
indicators may include, but are not limited to, dye intermediates
(weak bases) such as O-tolidine OTO and O-dianisidine
(3,3'-dimethoxybenzidine), acid-base indicators such as
phenolphthalein, and chemiluminescents such as
5-amino-2,3-dihydro-1,4-phthalazinedione (Luminol). Organic salts,
solvatochromic dyes and acid-base indicators are preferred.
[0038] An additional polymer(s) may be used in the dispersed phase
of the aqueous elastomer dispersion. Such polymers are preferably
selected such that when the additional polymer(s) is/are dissolved
in the solvent system, at least part of the polymeric molecule is
miscible with the elastomer. The differences between the elastomer
and an additional polymer may include chemical composition, chain
configuration, polymer conformation, molecular weight/molecular
weight distribution and combinations thereof. The additional
polymer(s) may be involved in the curing process or not, but its
existence should not compromise final performance of the
manufactured article. Whether or not polyisoprene is used as the
curable aliphatic conjugated-diene elastomer, the additional
polymer(s) may include, but is/are not limited to, liquid
polyisoprene, lower molecular weight polyisoprene, isomers or
analogue of polyisoprene, a terminal-functionalized derivative of
polyisoprene, a copolymer of polyisoprene, polychloroprene, a
styrene-butadiene copolymer, acrylonitrile-butadiene copolymers,
styrene block copolymers, butyl rubber, and modified forms of these
polymers. The aqueous elastomer dispersion of the invention may
have a range of 0.5 to 49% of the additional polymer in the
dispersed phase.
[0039] A flowable liquid polyisoprene with a molecular weight of
about 25,000-50,000 may be used as a processing aid in an elastomer
formulation and can be co-dispersed with conventional high
molecular weight polyisoprene, as described in the present
invention. In addition to its application as barrier sealing,
adhesive and elastomeric coating (See U.S. Pat. No. 7,335,807), it
can also improve film forming and provide thin films such as gloves
when included in the polyisoprene dispersion of the invention.
Examples of non-limiting liquid polyisoprene materials may include,
but are not limited to depolymerized rubber DPR and synthetic
liquid polyisoprene such as Claprene L-IR-30.
[0040] Lower molecular weight polyisoprenes function similarly to
reactive plasticizers so they can modulate the curing process.
These low molecular weight polyisoprenes can make the article
thinner, softer and increase tactile sensitivity. Typically, the
molecular weight of such polymers is in the range of about
20,000-100,000. They may include but are not limited to Lycopene,
Isolene, Synotex 800, and Natsyn 2210.
[0041] Structural isomers include cyclized
polyisoprene/3,4-polyisoprene, trans-1,4-polyisoprene, and branched
polyisoprene/star-shaped elastomers. Cyclized
polyisoprene/3,4-polyisoprene, such as Synotex 800 and Isogrip, can
improve the tear resistance, wet grip, tensile strength and lower
modulus for manufactured articles due to its cyclized structure.
Trans-1,4-polyisoprene, for example TP-301, provides a different
chain configuration for film stability against film aging and
dimensional changes. Branched and star-shaped polyisoprene, such as
Shellvis 250, provides an improved fit for articles, such as
gloves, and a decreased tackiness due to a unique time-temperature
relaxation property of its non-linear structure (See Terminal
relaxation and diffusion of entangled three-arm star polymers:
Temperature and molecular weight dependencies, Journal of Polymer
Science Part B: Polymer Physics 35 (15) 2503-2510 1997,
incorporated herein by reference). Their effectiveness depends on
the branching index and a value greater than 1 is preferred (See
Rheological Properties of 1,4-Polyisoprene over a Large Molecular
Weight Range, Macromolecules, 37 (21), 8135-8144, 2004,
incorporated herein by reference).
[0042] An example of an analogue elastomer is a hydrogenated
polyisoprene available as LIR 920. It can provide a manufactured
article, such as a glove, with a low modulus, low aging
degradation, good fit and recovery upon stretch. This is due to its
soft aliphatic chain to provide lubrication in the mobility of the
polymeric chain when forming a film.
[0043] Terminal-functionalized derivatives of the polyisoprene may
include, but are not limited to acrylated polyisoprene,
carboxylated polyisoprene, epoxidized polyisoprene, sulfonated
polyisoprene, and hydroxylated polyisoprene. Acrylated polyisoprene
may include but are not limited to HEMA- and acrylic acid-modified
polyisoprene. They can provide a manufactured article, such as a
glove, with an improved chlorination process by way of decreasing
the overall chlorination level. Carboxylated polyisoprene can
provide improved stability to the aqueous elastomer dispersion of
the invention. Additionally, they provide the manufactured article
with improved film forming properties. Its hydrophilic nature lends
itself to absorbing moisture and providing the manufactured article
with breathability. Sulfonated polyisoprene provides a manufactured
article with such properties as antistatic, antimicrobial, improved
wet donning, and anti-coagulation properties.
[0044] Copolymers of polyisoprene that may be used in the dispersed
phase of the aqueous elastomer dispersion may include, but are not
limited to, di-block and tri-block copolymers. Examples of di-block
copolymers may include polyisoprene-b-polybutadiene,
polystyrene-b-polyisoprene, polyisoprene-b-polymethyl methacrylate,
and polyisoprene-b-polyacrylic acid. Examples of tri-block
copolymers may include styrene-isoprene-styrene (SIS) and
styrene-b-(ethylene-co-butylene)-b-styrene (SEBS). The copolymers
may be dissolved in a solvent such as toluene before being
incorporated in the dispersed phase. Such copolymers provide the
resulting manufactured article with an improved barrier.
[0045] A generalized formulation for the vulcanization of
water-based latex and the addition of various additives such as the
one shown in Table 1 below may be used to prepare the aqueous
elastomer dispersion of the invention. With the exception of the
stabilizers, all of the ingredients in the formulation of Table 1
are in the form of aqueous dispersions (for solids) or aqueous
emulsions (for liquids).
TABLE-US-00001 TABLE 1 Parts by dry weight rubber (phr) Ingredient
Range Preferred Rubber latex 100 100 Sulfur and/or Sulfur donors
0.1-5.sup. 0.5-2 Accelerators 0.1-10 0.5-5 Activators 0.1-5.sup.
0.5-2 Antioxidants 0.1-5.sup. 0.5-2 Stabilizers as required
Plasticizers 0-15 .sup. 0-5 Antiozonants 0-5 0.5-2 Pigments 0-5
.sup. 0-2 Fillers 0-30 0-10 Antimicrobial Agents 0-5 .sup. 0-1
Indicators 0-5 .sup. 0-1 Additional Polymers 0-49 0.5-25
[0046] The formulation in Table 1 may be used for natural rubber
latex or in currently available commercial synthetic rubber
latexes. Generally, using additives as described above in various
combinations or concentrations provides superior properties that
are not attainable with currently available synthetic latexes. The
formulation of Table 1 may be modified to make the aqueous
elastomer dispersion of the invention. In particular, for certain
applications, a lower concentration of the additives may be desired
to provide better or similar properties.
[0047] In one embodiment of the invention, the crosslinking agent,
preferably sulfur, is not added into the latex as this may cause
premature excessive crosslinking of the rubber in the latex
particles. In other embodiments of the invention, the crosslinking
agent, preferably sulfur and/or sulfur donors, is/are added as an
aqueous dispersion during compounding of the aqueous latex before
use. Similarly, since the vulcanization activator, preferably zinc
oxide, is inorganic, it does not dissolve in an organic solvent and
cannot be incorporated into the latex dispersion. Thus, the
vulcanization activator may be added as an aqueous dispersion
during compounding of the aqueous latex before use.
[0048] A method for preparing the aqueous elastomer dispersion from
an elastomer such as dry rubber, with additives in the dispersed
phase according to the invention is illustrated in the flowchart of
FIG. 1. The method is described as follows and in the Examples. The
basic steps outlined in FIG. 1 are not meant to limit the scope of
the invention and may be repeated or performed in different orders
to prepare various types of latexes according to the invention.
[0049] As shown in FIG. 1, the starting elastomer, such as rubber,
is optionally processed to reduce its size so that it can easily
dissolve in a solvent. A preferred range of sizes for the rubber
particles is about 1-3 mm.
[0050] Then, a solution of the elastomer and a minor amount of at
least one additive in a solvent system is prepared. The solvent
system provides a medium for homogenously mixing the elastomer and
the additional additive prior to emulsification. The additives that
are to be included in the dispersed phase are dissolved in a
solvent or a mixture of solvents. The solvents for the additives
are not limited, but should be mixable with solvents that will be
used to dissolve the elastomer. Suitable solvents for additives may
include but are not limited to dichloromethane (DCM), chloroform,
carbon tetrachloride, acetone, and alcohols.
[0051] The concentrations of the accelerators added may be based on
the formulation for vulcanizing the rubber provided in Table 1 and
are optimized to provide desired properties of the vulcanized
rubber. A sufficient amount of solvent is added to ensure that the
additives are still soluble with the solvent for the rubber. The
additives may be first dissolved in the additive solvent and then
combined with the rubber solvent or may be dissolved in a mixture
of the additive solvent and the rubber solvent.
[0052] The rubber is dissolved in the solution containing the
solvent(s) and the additives. Alternatively, the rubber may first
be dissolved in its own solvent before adding it to the mixture of
additives and their solvents. Suitable solvents for elastomers may
include but are not limited to, pentane, hexane, heptane, pentene,
toluene, cyclopentane, and cyclohexane. The solvents for the
elastomers may be organic liquids that have a boiling point of less
than 100.degree. C., and preferably less than 70.degree. C.,
preferably at atmospheric pressure or if necessary, with the
application of a partial vacuum. The solvent system may include a
single solvent or a solvent mixture including a co-solvent.
[0053] The concentration of the rubber in the solvent is governed
by the solubility of the rubber in the solvent. It is preferred to
dissolve as much rubber as possible in the solution. However, its
concentration should preferably be no more than 80%, more
preferably no more than 60%, and most preferably no more than 50%
of the maximum limit of solubility because at maximum solubility,
the solution may be too viscous for it to be mixed with the
emulsifier solution and to be broken down into small droplets to
form an emulsion.
[0054] The vessel containing the rubber/additive/solvent mixture
may be closed to minimize evaporation and stirred until the rubber
is completely dissolved. The temperature of the solvent is
preferably maintained at about 25-35.degree. C., more preferably at
about 28-30.degree. C.
[0055] The resulting solution containing the rubber, additives, and
solvent is then converted into an aqueous emulsion by a process
known to those of skill in the art, such as those described in U.S.
Pat. Nos. 3,250,737, 3,285,869, 3,971,746, 3,968,067 and 6,329,444,
hereby incorporated by reference. The process may include
emulsifying the polymer solution using an emulsifier composition,
removing the solvent or liquefying the polymer solution, and
combining it with an aqueous medium under conditions that are
favorable to stabilizing the emulsion. Alternatively, the
water-based dispersion may be formed by following the steps of
emulsifying the polymer solution using an emulsifier composition in
an aqueous medium and then removing the solvent, as outlined in
FIG. 1.
[0056] In general, the emulsifier should be capable of forming a
stable emulsion that can withstand the heat during evaporation of
the solvent and the subsequently formed latex could be destabilized
by alkaline earth metal ions, such as calcium ions (e.g., from
calcium nitrate used for coagulant dipping to make gloves).
Emulsifier agents may include, but are not limited to, sodium
dodecyl sulfate, sodium dodecyl benzene sulfonate, and carboxylic
acid soaps such as caprylic acid soap, capric acid soap, lauric
acid soap, oleic acid soap and rosin acid soap or mixture of these.
Suitable emulsifiers should preferably have low foaming
propensities so that excessive foaming does not occur during the
various process steps of isolating polyisoprene latex concentrate.
The weight of the emulsifier solution is typically about 1-5 wt/vol
%, preferably 1-3 wt/vol %, based on the volume of the solvent. For
example, an emulsifier solution of 2% Nekal BX dry (BASF, 68%
sodium alkyl naphthalene sulphonate) in water adjusted to a pH of
10.5-11.0 with, e.g., 3% potassium hydroxide solution may be
used.
[0057] The latex emulsion is then prepared by pouring the
rubber/additive/solvent solution into the emulsifier solution which
is preferably stirred, e.g., with a laboratory Silverson L4RT mixer
fitted with a Square Hole High Shear Screen.TM. at about 3500-4500
rpm. The mixer speed is varied until the foams that are formed
break. In an exemplified process, after stirring for about 5
minutes at 3500-4500 rpm, the mixer speed may be increased to about
5000-7000 rpm and stirred for about 10 minutes. The mixer speed may
then be reduced to about 500-1000 rpm and stirred for about 5
minutes during which time the foams that are formed break. If
excessive foaming occurs after some time of high speed shearing
(5000-7000 rpm), the mixing speed may be reduced to about 1000 rpm
and stirring continues until the foams break. Then, the mixing
speed may be increased back to 5000-7000 rpm and mixing continues
until the total time of shearing at this speed is about 10
minutes.
[0058] The stirring speed is not limited to the ranges mentioned
above, but is chosen so that the final mean diameter of the
elastomer particles is similar to those of commercial latex
products. For example, commercial polyisoprene latex products have
a volume mean diameter of about 1 micrometer. Typically, a volume
mean particle diameter of about 0.5 to 1.5 micrometers is
desirable. Higher stirring speeds result in a final elastomer latex
particle having a smaller particle diameter. The range of speeds
can be broadened, depending on the desired application. For
example, about 5000 rpm-8000 rpm should give the desired final
particle size depending on the total time of high shear mixing. A
slightly shorter time is required when the higher speed is used,
and a lower speed will result in larger particle size.
[0059] Next, the aqueous emulsion is concentrated, whereby at least
20%, preferably at least 30%, and more preferably at least 40% of
the aqueous phase is removed resulting in a first emulsion
concentrate. The emulsion is preferably concentrated using a
creaming agent to increase the rate of separation. The creaming
agent may include, but is not limited to sodium alginate, ammonium
alginate, methyl cellulose, locust bean gum, gum tragacanth, and
carrageenin.
[0060] The emulsion containing the creaming agent is stirred at a
rate of about 1500 rpm for 5 minutes, for example. The mixture may
then be filtered through a muslin cloth and poured into a
separatory funnel. After leaving the mixture in the separatory
funnel for about 16-20 hours, the emulsion separates into two
layers. The upper layer includes a concentrated emulsion and the
lower aqueous layer includes water and possibly a small amount of
emulsion, which is drained off.
[0061] Other ways to concentrate the emulsion include using a
continuous centrifugal separator (for example, made by Alpha Laval,
Westphalia or Sharples), evaporation using a thin film evaporator
such as Luwa thin-layer evaporator, or ultrafiltration using
membranes. On an industrial scale, the emulsion may be kept in a
tall cylindrical tank and at an appropriate time, the lower layer
of skim may be drained out leaving an upper layer of concentrated
emulsion for further processing.
[0062] Once the aqueous layer of the concentrated emulsion is
removed, at least 90%, preferably at least 95%, more preferably at
least 98%, and most preferably at least 99% of the solvents are
stripped from the concentrated emulsion to yield a dilute latex.
The solvents may be stripped from the concentrated emulsion by,
e.g., heating in a laboratory rotary evaporator, such as a Buchi
rotary evaporator in which an evaporating flask is partially
immersed in a water bath at a temperature of about 50-55.degree. C.
The evaporating flask may be rotated at about 150 to 200 rpm so
that a fresh thin film of the emulsion is continuously formed
thereby increasing the surface area for evaporation to occur.
[0063] The solvent vapor may be condensed and recovered by
circulating ethylene glycol at a temperature of about -15 to
-25.degree. C. through the cooling coil of the evaporator and also
by immersing the receiving flask of the evaporator in a bath of
ethylene glycol at the above temperature.
[0064] Typically, a temperature of about 10-15.degree. C. above the
boiling point of the solvents evaporates off the solvents fairly
rapidly. However, to evaporate off traces of residual solvent at
the end of the evaporation process, the solvents may be heated to a
higher temperature of about 20-25.degree. C. above their boiling
points. The temperature range can be broadened. For example, if the
boiling point of the solvent is high, e.g. about 60-100.degree. C.
or greater than 100.degree. C., then it would be desirable to apply
a partial vacuum to the emulsion so that the boiling point of the
solvent is reduced to less than about 60.degree. C.
[0065] As an alternative to using a rotary evaporator to remove the
solvent, any suitable device including a thin film evaporator such
as a Luwa thin-layer evaporator, for example, may be used. On an
industrial scale, a large scale rotary evaporator may be used.
[0066] After stripping off the solvent, the emulsion containing
latex may be concentrated again to remove any remaining aqueous
components by creaming, e.g., using sodium alginate. For example,
about 0.1% sodium alginate (as a 2% aqueous solution) based on the
volume of the emulsion may be added to the emulsion and mixed well
by stirring. The mixture may then be transferred into a separatory
funnel and left overnight for about 16-18 hours during which time
the mixture separates into two layers. The upper layer contains
concentrated latex and the lower layer typically contains a
slightly turbid aqueous solution and possibly some latex. The lower
aqueous layer is drained off and a concentrated latex is obtained.
Preferably, in this second concentration step, the total solids
content of the dilute latex is increased by at least three times,
preferably at least four times, and more preferably at least five
times. For example, the total solids content may increase to about
45-65%.
[0067] The concentrated latex may be kept in a separatory funnel
for further separation. If this is desired, after a certain amount
of time, the lower and predominantly aqueous layer may be drained
off again. The latex may then be filtered through a muslin cloth.
The total solids content obtainable from this process is preferably
about 40% to about 70% by weight, depending on how long the latex
is left to separate. Taking into account transportation costs, a
higher total solids content of latex is generally preferred since
transporting latex with higher total solids content means
transporting more rubber and less water.
[0068] For making gloves, the solids content of compounded latex
(i.e. after vulcanizing ingredients and other additives have been
added to the latex) is about 25% to 40% depending on the thickness
of the gloves required. Typically, gloves with a higher thickness
require higher total solids content.
[0069] After the latex is concentrated to the desired solids
content, the pH of the obtained latex may be adjusted, e.g., with
an about 5% potassium hydroxide solution. The pH may be adjusted
based on the intended application. For example, the pH range of
polyisoprene latex for making gloves is about 10 to 11.
[0070] A method for preparing a polyisoprene latex according to the
steps of FIG. 1 and the description above will be described in
detail below in Example 1.
Example 1
Polyisoprene Latex with Additives Wingstay L, ZDEC and DPG
[0071] In this example, the additives Wingstay L, an antioxidant
(obtained from Goodyear), and zinc diethyl dithiocarbamate (ZDEC)
and diphenyl guanidine (DPG), vulcanization accelerators (both
obtained from Flexsys), are first dissolved in solvents. The
solutions are prepared by dissolving 0.4 g Wingstay L in 60 mL DCM,
dissolving 0.2 g DPG in 10 mL DCM and dissolving 0.1 g ZDEC in 10
mL DCM and then adding these into 600 mL of pentane (obtained from
Merck) in a 1 liter beaker.
[0072] 20 g of polyisoprene rubber (Kraton IR) is cut into small
pieces and slowly added into a beaker that is continuously stirred
and contains the additive solution which includes 600 mL pentane
and 80 mL DCM solvent that contains predissolved 2 phr Wingstay L,
1 phr DPG and 0.5 phr ZDEC. The concentration in Example 1 is about
2.9 weight polyisoprene/volume solvent. A higher concentration of
about 6% weight/volume should be feasible.
[0073] The beaker is tightly covered, for example, with a
polyethylene sheet to minimize evaporation and the
rubber/additive/solvent mixture is stirred until the rubber is
completely dissolved. The temperature of the solvent is maintained
at about 25-32.degree. C., more preferably about 28-30.degree.
C.
[0074] The rubber/additive/solvent mixture is converted to an
aqueous emulsion using a suitable emulsifier and a high shear
mixer. An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium
alkyl naphthalene sulphonate) is prepared be dissolving 18 g Nekal
BX dry in 900 mL water and adjusting the pH to 10.5-11.0 with,
e.g., 3% potassium hydroxide solution. The polyisoprene latex
emulsion is then prepared by slowly pouring the
rubber/additive/solvent solution into the emulsifier solution which
is stirred, e.g., with a laboratory Silverson L4RT mixer fitted
with a Square Hole High Shear Screen.TM. at about 3500-4500
rpm.
[0075] After stirring for about 5 minutes at this speed, the mixer
speed is increased to about 6000-7000 rpm and stirred for about 10
minutes. The mixer speed is then reduced to about 500-1000 rpm and
stirred for about 5 minutes during which time the foams that are
formed break. Then, the mixing speed may be increased back to
6000-7000 rpm and mixing continues until the total time of shearing
at this speed is about 10 minutes.
[0076] The emulsion is concentrated by adding 0.45 g sodium
alginate dissolved in 25 mL water into it and stirring at about
1500 rpm for 5 minutes. The mixture is filtered through a muslin
cloth and is then poured into a separatory funnel. After leaving
the mixture in the separatory funnel for about 16-20 hours, the
emulsion separates into 2 layers. The upper layer includes a
concentrated emulsion (about 700-750 mL) and the lower aqueous
layer includes water and possibly a small amount of emulsion, which
is drained off.
[0077] The solvents (in this example, pentane and DCM) are then
stripped from the concentrated emulsion by, e.g., heating in a
laboratory rotary evaporator, such as a Buchi rotary evaporator in
which an evaporating flask is partially immersed in a water bath at
a temperature of about 50-55.degree. C. The evaporating flask is
rotated at about 150 to 200 rpm so that a fresh thin film of the
emulsion is continuously formed thereby increasing the surface area
for evaporation to occur.
[0078] After stripping off the solvent, the emulsion containing
polyisoprene latex is again concentrated by creaming, e.g., using
sodium alginate. About 0.1% sodium alginate (as a 2% aqueous
solution) based on the volume of the emulsion is added to the
emulsion and mixed well by stirring. The mixture is then
transferred into a separatory funnel and left overnight for about
16-18 hours during which time the mixture separated into two
layers. The upper layer contains concentrated latex and the lower
layer contains a slightly turbid aqueous solution with a little
latex. The lower aqueous layer is drained off and a concentrated
latex is obtained.
[0079] The concentrated latex is kept in the separatory funnel for
further separation. After about 7 days, the lower and predominantly
aqueous layer is drained off again. The latex is filtered through a
muslin cloth, and its pH is adjusted to 10.6, e.g., with a 5%
potassium hydroxide solution.
[0080] Another example of preparing a polyisoprene latex according
to the steps of FIG. 1 will be described in detail below in Example
2.
Example 2
Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG and MBT
[0081] A solution containing four additives is prepared by
dissolving 0.44 g Wingstay L (obtained from Goodyear, 2 phr) in 60
mL DCM (obtained from Merck), dissolving 0.22 g DPG (obtained from
Flexsys, 1 phr) in 20 mL DCM, dissolving 0.11 g ZDEC (obtained from
Flexsys, 0.5 phr) in 20 mL DCM, dissolving 0.11 g MBT (obtained
from Merck, 0.5 phr) in 50 mL acetone (obtained from Merck) and
then adding the solutions one at a time to 600 mL pentane (obtained
from Merck, solvent for polyisoprene) in a 1 liter beaker.
[0082] 22 g of polyisoprene rubber (Kraton IR) is cut into small
pieces and slowly added into the continuously stirred solvent
mixture containing the four predissolved additives described above.
The beaker is tightly covered with a piece of polyethylene sheet to
minimize evaporation and the rubber/additives/pentane/DCM/acetone
mixture is stirred until the rubber is completely dissolved. The
temperature of the solvent mixture is preferably maintained at
about 25-32.degree. C., more preferably about 28-30.degree. C.
[0083] An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium
alkyl napthalene sulphonate) is prepared be dissolving 18 g of
Nekal BX dry in 900 mL water and adjusting the pH to 10.5-11.0
with, e.g., a 3% potassium hydroxide solution.
[0084] A polyisoprene rubber/additives/solvent emulsion is then
prepared by slowly pouring the rubber/additive/solvent solution
into the emulsifier solution which is stirred, e.g., with a
laboratory Silverson L4RT mixer fitted with a Square Hole High
Shear Screen.TM. at about 4000-4500 rpm. After stirring for about 5
minutes at this speed, the mixer speed is increased to about
5800-6000 rpm and stirring continues for about 10 minutes. The
mixer speed is then reduced to about 500-1000 rpm and stirring
continues for about 5 minutes during which time the foams that are
formed breaks. After the foams break, the mixing speed is increased
back to 5800-6000 rpm and mixing continues at this speed for about
10 minutes.
[0085] The emulsion is concentrated by adding 0.5 g of sodium
alginate (obtained from Kimitsu Chemical Industries, Japan)
dissolved in 25 mL of water and stirring at about 1500 rpm for 5
minutes. The mixture is filtered through a muslin cloth and then
poured into a separatory funnel. After leaving the mixture for
about 16-20 hours, the emulsion separates into 2 layers. An upper
layer is a concentrated emulsion (about 720 mL) and a lower aqueous
layer may contain a small amount of emulsion, which is drained
off.
[0086] The solvents (in this case pentane/DCM/acetone) are stripped
off from the concentrated emulsion by, e.g., heat evaporation in a
Buchi rotary evaporator by heating it in an evaporating flask
partially immersed in a water bath at a temperature of about
50-55.degree. C. The evaporating flask is rotated at about 150 to
180 rpm so that a fresh thin film of the emulsion is continuously
formed, thereby increasing the surface area for evaporation to
occur. When the evaporation slows down (after about 2 hours), the
temperature of the heating water may be increased to 60-65.degree.
C. to complete the evaporation (about 3 hours). The total
evaporation time is typically about 5 hours.
[0087] After stripping off the solvent, the emulsion (which is
polyisoprene latex) is concentrated again by creaming, e.g., using
sodium alginate. About 0.15% sodium alginate (as a 2% aqueous
solution) based on the weight of the emulsion is added to the
emulsion and mixed well by stirring. The mixture is then
transferred into a separatory funnel and left for 11 days whereby
the mixture separated into two layers. An upper layer contains
concentrated latex and a lower layer contains a slightly turbid
aqueous solution, potentially with a small amount of latex. The
lower aqueous layer is drained off and a concentrated latex is
obtained. The latex is filtered through a muslin cloth and its pH
adjusted to 10.9, e.g., with 5% potassium hydroxide solution.
[0088] The aqueous elastomer dispersions of the invention, prepared
for example as described in Examples 1 and 2, exhibit properties
such as solid content, particle size, and pH comparable to
conventional elastomer dispersions that are commercially available.
To demonstrate this feature, samples of the polyisoprene latex with
additives Wingstay L, DPG and ZDEC, prepared in accordance with
Examples 1 and 2, were tested and compared with two different
samples of the widely available Kraton.RTM. IR401 PI latex
products. The data summarized in Table 2 below indicate that the
properties of the aqueous polyisoprene latex of Examples 1 and 2,
such as volume mean particle diameter, solids content and pH, are
comparable to the Kraton polyisoprene product.
TABLE-US-00002 TABLE 2 Aqueous PI Aqueous PI Kraton Kraton
Properties Latex, Latex, IR401 Lot IR401 Lot of Latex Example 1
Example 2 B1070119 B1070629 Volume Mean 0.45 0.49 0.94 1.04
Particle Diameter (.mu.m) Total Solids 51.6 52.97 64.8 65.6 Content
(%) pH 10.6 10.9 10.7 10.7 Note: Particle size measured using
Mastersizer S from Malvern Instruments Ltd.
[0089] Additional examples of preparing polyisoprene latex
according to the steps of FIG. 1 described above but having
increased concentrations of accelerators compared to those of
Examples 1 and 2 are described in detail below in Examples 3-5. A
latex from Example Control, prepared by a different method, is also
tested for comparison with the latexes of Examples 3-5.
Examples 3-5
And Control Polyisoprene Latex with Varying Amounts of Additives
Wingstay L, ZDEC, DPG and ZMBT
[0090] In Examples 3-5, varying amounts of Wingstay L, ZDEC, and
DPG, as shown in Table 3 below, are incorporated with polyisoprene
latex into a single dispersion following the method described above
in reference to FIG. 1. Example Control has the same formulation as
Example 3, as shown in Table 3, but is not prepared according to
the methods of the present invention. Instead, Example Control is
prepared according to the method described in Example 1 of U.S.
Pat. No. 6,828,387 (see col. 7, line 45 to col. 8, line 67). In
U.S. Pat. No. 6,828,387, each of the accelerators was formulated
into a separate dispersion and was added individually to a solution
containing latex in water. In Example Control, accelerators ZDEC,
DPG, and ZMBT and Wingstay L that were already in dispersion form
were added to latex in an aqueous solution and were then mixed
together.
[0091] The latex particle sizes for Examples 3-5 were increased
from the values given in Example 1 and 2 by stirring the emulsions
at a lower speed. For Examples 3 and 4, the high shear stirring
speed was 5000 rpm+/-100 rpm for 10 minutes instead of 5900
rpm+/-100 rpm for 10 minutes. For Example 5, the stirring speed was
5500 rpm+/-100 rpm for 10 minutes. The particle sizes for Examples
3-5 are given in Table 3 below.
[0092] About 25 g of the latex from each of Examples 3-5 and
Control was weighed in a beaker and stirred with a magnet. Further,
S, ZnO, ZMBT were added as aqueous dispersions and sodium caseinate
was added as an aqueous solution to each latex in amounts shown in
Table 3 below. For Control, Wingstay L, ZDEC and DPG were also
added as dispersions. Water was added to dilute the compounded
latex to about 45% total solids content and the pH of the
compounded latex was adjusted to about 10.5 with a dilute potassium
hydroxide solution. Each latex mixture was then filtered through a
muslin cloth. The mixtures were stirred continuously for about 18
hours at an ambient temperature of about 28.degree. C.
[0093] Latex films were prepared by casting about 6-8 g of latex on
leveled glass plates having an enclosed area of about 60 mm by 100
mm. The latex was left to dry for about 24 to 48 hours. An
anti-tack agent such as calcium carbonate or starch powder was
applied to the dried film and the films were stripped off the glass
plates. The films were then leached in water at 70.degree. C. for 1
hour and allowed to dry in the air for 30 minutes. The films were
then cured in a hot air oven at 135.degree. C. for about 20
minutes. Dumbbell specimens of the cured films were cut for
physical testing using ASTM Die D and tested according to the ASTM
D412-98a test method. The results of tests of various physical
properties are shown below in Table 3.
TABLE-US-00003 TABLE 3 Example 3 4 5 Control Ingredients (phr)
Wingstay L 2.0 2.0 2.0 2.0 ZDEC 0.5 0.75 0.75 0.5 DPG 1.0 1.0 1.0
1.0 Sodium caseinate 0.75 0.75 0.75 0.75 S 1.25 1.25 1.25 1.25 ZnO
0.5 0.5 0.5 0.5 ZMBT 0.5 0.5 0.69 0.5 Tested Properties Tensile
strength (MPa) -- 9.4 18.0 21.2 Stress at 500% 0.7 1.3 1.7
Elongation (MPa) Ultimate Elongation (%) 1210 1080 1022 Mean Volume
Diameter (.mu.m) 0.75 0.68 0.64 N/A
[0094] The cured film of Example 3 was soft and sticky and could
not be tested. As shown in Table 3, increasing the amount of ZDEC
incorporated into the latex particles from 0.5 phr in Example 3 to
0.75 phr in Example 4 gave a cured film with a tensile strength of
9.4 MPa. The film of Example Control exhibited higher tensile
strengths than that of Example 4.
[0095] Increasing the amount of ZDEC incorporated into the latex
particles from 0.5 phr to 0.75 phr and the amount of ZMBT during
compounding from 0.5 phr to 0.69 phr in Example 5 resulted in cured
films with physical properties comparable to that demonstrated by
Example Control, which was prepared using the known method
described in Example 1 of U.S. Pat. No. 6,828,387. Overall, the
results in Table 3 indicate that the novel PI latex prepared
according to the present invention may provide good physical
properties such as tensile strength. ASTM D3577-06 Type II
classification standard for surgical gloves requires a minimum
tensile strength of 17 MPa, a maximum stress at 500% elongation of
7.0 MPa, and a minimum ultimate elongation of 650%.
[0096] Another approach is to select accelerators that have low
solubility in the aqueous phase, such as those having a longer
hydrocarbon chain or a higher molecular weight. Examples of such
accelerators may include, but are not limited to, zinc dibenzyl
dithiocarbamate, zinc dinonyl dithiocarbamate, zinc di-isononyl
dithiocarbamate, and zinc N-dodecyl-N-isopropyl dithiocarbamate.
Such accelerators have higher solubility in the rubber phase and
lower solubility in the aqueous phase.
[0097] In U.S. Pat. No. 6,828,387, the amounts of ZDEC, ZMBT and
DPG added to the mixture were in the range of about 0.50 to 1.00
phr. In the present invention, the amounts of each of these
accelerators may be higher. In particular, the amount of ZDEC may
be in the range of about 0.2 phr to about 7.0 phr, preferably about
0.5 phr to about 3.0 phr, the amount of DPG may be in the range of
about 0.2 to 10.0 phr, preferably about 0.5 phr to about 4.0 phr,
and the amount of ZMBT may be in the range of about 0.2 phr to
about 10.0 phr, preferably about 0.5 phr to about 4.0 phr.
[0098] In the latex made according to the process of the present
invention, the additives are already incorporated into the
elastomer before compounding. This is because the additives are
dissolved in a suitable solvent, combined with the polyisoprene
solution, and then made into a dispersion. This method differs from
other known methods, such as the method recited in Example 1 of
U.S. Pat. No. 6,828,387 and in Example Control above, where
separate dispersions/emulsions of each of the components (elastomer
and additives) are obtained from suppliers and are then combined
together.
[0099] Dispersions/emulsions are, by nature, thermodynamically
unstable systems, and in a multiple formulation process, there are
many challenges to obtaining a desirable uniform mixture. In
particular, it is difficult to match the stabilization of each of
the individual dispersions/emulsion systems, and because of this, a
broad particle size distribution often results. The method of the
present invention may overcome this disadvantage.
[0100] Further, the process for converting the solution containing
these three additives into latex is the same as that for converting
a solution of polyisoprene without the additives into latex. Thus,
this method does not require extra energy. This process allows for
a savings in energy, equipment cost, materials (a lower amount of
surfactant is needed to stabilize the dispersion), and labor
required for dispersing these three ingredients when compared to
the method recited in Example 1 of U.S. Pat. No. 6,828,387.
[0101] In addition to Wingstay L, ZDEC and DPG, copolymers of
polyisoprene may be incorporated into the dispersed phase of the
polyisoprene latex by the method described below in Examples 6 and
7, which follow the method outlined in FIG. 1.
Example 6
Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG and SIS
(Quintac 3421)
[0102] The copolymer of polyisoprene used in this example was a
styrene-isoprene-styrene (SIS) diblock copolymer obtained from Zeon
Chemicals under the trade name Quintac 3421. Quintac 3421 has a
styrene content of 14%. A solution was prepared by dissolving 0.44
g Wingstay L (obtained from Goodyear, 2 phr) in 60 mL DCM (obtained
from Merck), dissolving 0.22 g DPG (obtained from Flexsys, 1 phr)
in 20 mL DCM, dissolving 0.165 g ZDEC (obtained from Flexsys, 0.75
phr) in 20 mL DCM, and dissolving 3.3 g Quintac 3421 (obtained from
Zeon Chemicals, 15 phr) in 30 mL toluene (obtained from Merck). The
solutions were then added one at a time to 600 mL pentane (obtained
from Merck, solvent for polyisoprene) in a 1 liter beaker. The
solution of Quintac 3421 in toluene was very viscous and 3 mL of
toluene was used for rinsing the residue sticking on the glass
container.
[0103] g of polyisoprene rubber (Kraton IR) was cut into small
pieces and slowly added into the continuously stirred solvent
mixture containing the four predissolved additives described above.
The beaker was tightly covered with a piece of polyethylene sheet
and tied with a rubber band to minimize evaporation and the
rubber/additives/pentane/DCM/toluene mixture was magnetically
stirred for about 3 hours whereby the rubber was completely
dissolved. The temperature of the solvent mixture was maintained at
about 28-30.degree. C.
[0104] An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium
alkyl napthalene sulphonate) was prepared be dissolving 18 g of
Nekal BX dry in 900 mL water and adjusting the pH to 10.5-11.0 with
a few drops of 3% potassium hydroxide solution.
[0105] A polyisoprene rubber/additives/solvent emulsion was then
prepared by slowly pouring the rubber/additive/solvent solution
into the emulsifier solution which was stirred with a laboratory
Silverson L4RT mixer fitted with a Square Hole High Shear
Screen.TM. at 4400-4600 rpm. After stirring for 5 minutes at this
speed, the mixer speed was increased to 4900-5100 rpm and stirring
continued for about 10 minutes. The mixer speed was then reduced to
about 1000 rpm and stirring continued for about 5 minutes during
which time the foams that were formed break. No excessive foaming
was noted under these conditions.
[0106] The emulsion was concentrated by adding 0.5 g of sodium
alginate (obtained from Kimitsu Chemical Industries, Japan)
dissolved in 25 mL of water and stirring at about 1500 rpm for 5
minutes. The mixture was filtered through a muslin cloth and then
poured into a separatory funnel. After leaving the mixture for
about 16-20 hours, the emulsion separated into 2 layers. An upper
layer was a concentrated emulsion (about 810 mL) and a lower
aqueous layer contained a small amount of emulsion, which was
drained off.
[0107] The organic solvents pentane/DCM/toluene were stripped off
from the concentrated emulsion by heat evaporation in a Buchi
rotary evaporator by heating the emulsion in an evaporating flask
partially immersed in a water bath at a temperature of about
45.degree. C. for 3 hours followed by 50.degree. C. for 4 hours.
The evaporating flask was rotated at about 210 rpm so that a fresh
thin film of the emulsion was continuously formed, thereby
increasing the surface area for evaporation to occur. The solvent
vapor was condensed and recovered by circulating ethylene glycol at
a temperature of about -15.degree. C. to -25.degree. C. through the
cooling coil of the evaporator and also immersing the receiving
flask of the evaporator in a bath of ethylene glycol at the above
temperature. After that, toluene which has a higher boiling point,
was evaporated at about 60.degree. C. under reduced pressure of
about 100 torr for 4 hours. The weight of the concentrated latex
was 246.2 g.
[0108] The latex was further concentrated by creaming using sodium
alginate. 0.25 g sodium alginate dissolved in 15 mL water was added
to the latex and mixed well by stirring. The mixture was then
transferred into a separatory funnel and left for 10 days during
which the mixture separated into two layers. An upper layer
contained concentrated latex and a lower layer contained a slightly
turbid aqueous solution with a little latex. The lower aqueous
layer was drained off and a concentrated latex was obtained. The
latex was filtered through a muslin cloth and its pH was adjusted
to 10.95 with 5% potassium hydroxide solution. The weight of
concentrated latex obtained was 42.16 g with a total solids content
of 55.4%. The mean volume diameter of the particles was 0.69
micrometers.
[0109] This latex was compounded with 0.75 phr sodium caseinate
added as aqueous solution, 1.25 phr sulfur added as dispersion, 0.5
phr zinc oxide added as dispersion, and 0.69 phr ZMBT added as
dispersion. Water was added to dilute the mixture to a total solids
content of about 45% and the pH of the compounded latex was
adjusted to about 10.5 with a dilute potassium hydroxide solution.
The latex was filtered through a muslin cloth into a beaker and was
then covered and stirred with a magnet for 20 hours at ambient
temperature (26-28.degree. C.). Latex films were prepared by
casting 7 to 8 g of latex on leveled glass plates having an
enclosed area of 60 mm by 100 mm. The latex was left to dry for
about 48 hours. Calcium carbonate powder was applied on the
surfaces of the dried films and the films were stripped off the
glass plates. The films were then leached in water at 70.degree. C.
for 1 hour and were hung to dry in the air for 30 minutes. They
were then cured in hot air oven at 135.degree. C. for 20
minutes.
[0110] Dumbbell specimens of the cured films were cut for physical
testing using ASTM Die D and tested according to ASTM D412-98a
method as shown in Table 4 below.
Example 7
Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG and SIS
(Vector 4111A)
[0111] The copolymer of polyisoprene used in this example was an
SIS triblock copolymer with the trade name Vector 4111A (obtained
from Dexco Polymers) that has a styrene content of 18%. A solution
was prepared by dissolving 0.44 g Wingstay L (obtained from
Goodyear, 2 phr) in 60 mL DCM (obtained from Merck), dissolving
0.22 g DPG (obtained from Flexsys, 1 phr) in 20 mL DCM, dissolving
0.165 g ZDEC (obtained from Flexsys, 0.75 phr) in 20 mL DCM,
dissolving 3.3 g Vector 4111A (obtained from Dexco Polymers, 15
phr) in 30 mL toluene (obtained from Merck) and then adding the
solutions one at a time to 600 mL pentane (obtained from Merck,
solvent for polyisoprene) in a 1 liter beaker. The solution of
Vector 4111A in toluene was very viscous and 3 mL of toluene was
used for rinsing the residue sticking on the glass container.
[0112] 22 g of polyisoprene rubber (Kraton IR) was cut into small
pieces and slowly added into the continuously stirred solvent
mixture containing the four predissolved additives described above.
The beaker was covered tightly with a piece of polyethylene sheet
and tied with a rubber band to minimize evaporation and the
rubber/additives/pentane/DCM/toluene mixture was magnetically
stirred for about 3 hours whereby the rubber was completely
dissolved. The temperature of the solvent mixture was maintained at
about 28-30.degree. C.
[0113] An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium
alkyl napthalene sulphonate) was prepared by dissolving 18 g of
Nekal BX dry in 900 mL water and adjusting the pH to 10.5-11.0 with
a few drops of 3% potassium hydroxide solution.
[0114] A polyisoprene rubber/additives/solvent emulsion was then
prepared by slowly pouring the rubber/additive/solvent solution
into the emulsifier solution which was stirred with a laboratory
Silverson L4RT mixer fitted with a Square Hole High Shear
Screen.TM. at 4400-4600 rpm. After stirring for 5 minutes at this
speed, the mixer speed was increased to 4900-5100 rpm and stirring
continued for about 10 minutes. The mixer speed was then reduced to
about 1000 rpm and stirring continued for about 5 minutes during
which time the foams that were formed break. No excessive foaming
was noted under these conditions.
[0115] The emulsion was concentrated by adding 0.5 g of sodium
alginate (obtained from Kimitsu Chemical Industries, Japan)
dissolved in 25 mL of water and stirring at about 1500 rpm for 5
minutes. The mixture was filtered through a muslin cloth and then
poured into a separatory funnel. After leaving the mixture for
about 16-20 hours, the emulsion separated into 2 layers. An upper
layer was a concentrated emulsion (about 800 mL) and a lower
aqueous layer contained a small amount of emulsion, which was
drained off.
[0116] The organic solvents pentane/DCM/toluene were stripped off
from the concentrated emulsion by heat evaporation in a Buchi
rotary evaporator by heating the emulsion in an evaporating flask
partially immersed in a water bath at a temperature of about
45.degree. C. for 3 hours followed by 50.degree. C. for 4 hours.
The evaporating flask was rotated at about 210 rpm so that a fresh
thin film of the emulsion was continuously formed, thereby
increasing the surface area for evaporation to occur. The solvent
vapor was condensed and recovered by circulating ethylene glycol at
a temperature of about -15.degree. C. to -25.degree. C. through the
cooling coil of the evaporator and also immersing the receiving
flask of the evaporator in a bath of ethylene glycol at the above
temperature. After that, toluene, which has a higher boiling point,
was evaporated at about 60.degree. C. under reduced pressure of
about 100 torr for 4 hours. The weight of the concentrated latex
was 241.0 g.
[0117] The latex was further concentrated by creaming using sodium
alginate. 0.25 g sodium alginate dissolved in 15 mL water was added
to the latex and mixed well by stirring. The mixture was then
transferred into a separatory funnel and left for 10 days whereby
the mixture separated into two layers. An upper layer contained
concentrated latex and a lower layer contained a slightly turbid
aqueous solution with some latex. The lower aqueous layer was
drained off and a concentrated latex was obtained. The latex was
filtered through a muslin cloth and its pH was adjusted to 10.99
with a 5% potassium hydroxide solution. The weight of concentrated
latex obtained was 41.52 g with a total solids content of 54.27%.
The mean volume diameter was 0.57 micrometers.
[0118] 25 g of the latex was compounded with 0.75 phr sodium
caseinate added as aqueous solution, 1.35 phr sulfur added as
dispersion, 0.60 phr zinc oxide added as dispersion, and 0.75 phr
ZMBT added as dispersion. Water was added to dilute the mixture to
a total solids content of about 45%, and the pH of the compounded
latex was adjusted to about 10.5 with a dilute potassium hydroxide
solution. The latex was then filtered through a muslin cloth into a
beaker and was covered and stirred with a magnet for 40 hours at
ambient temperature (about 26-28.degree. C.). In this example, the
latex was stirred for 40 hours instead of 20 hours as in Example
6.
[0119] The compound was converted to a cured film, as described in
Example 6 above. Dumbbell specimens of the cured films were cut for
physical testing using ASTM Die D and tested according to ASTM
D412-98a method.
[0120] Physical properties of the cured latex films of Examples 6
and 7 were tested and compared with the ASTM D3578-05 Standard
Specification for Rubber Examination Gloves, Type II
classification. The results are summarized in Table 4.
TABLE-US-00004 TABLE 4 Tensile Stress at 500% Ultimate strength
elongation, M500 elongation (MPa) (MPa) (%) Example 6 15.2 1.5 1070
Example 7 16.4 1.4 1090 ASTM D3578-05, 14.0 (minimum) 2.8 (maximum)
650 (minimum) Type II
[0121] As indicated in Table 4, the cured polyisoprene-SIS latex
films of Examples 6 and 7 met the ASTM D3578-05 standards.
Moreover, the stress (or modulus) at 500% elongation, M500, were
surprisingly low and comparable to that of the cured polyisoprene
films discussed above (see Example Control above which has M500 of
1.7 MPa). A low modulus is a desirable property because the
resulting product feels soft and comfortable and provides good
tactile sensitivity to the user. The M500 values of the films of
Examples 6 and 7 were significantly lower than that of natural
rubber gloves which may have an M500 value of about 3.9 MPa (range
2.6-5.0 MPa). Polyisoprene gloves typically have M500 values of
about 2.0 MPa (range 1.4-2.5 MPa).
[0122] The method of the present invention may incorporate up to
20%, preferably up to 30%, more preferably up to 40%, and most
preferably up to 49% (by weight of total dispersion solids) of SIS
to the dispersed phase. Typically, SIS cannot be milled down to
form dispersions such as the ones used for sulfur, vulcanization
accelerators, antioxidants, etc. because it is not a hard solid and
a commercial SIS latex is not available. Thus, SIS cannot be
blended with polyisoprene latex and made into a product using
conventional methods.
[0123] An advantage of using SIS in combination with elastomers
such as polyisoprene is that SIS is significantly less expensive
than polyisoprene and the amount of polyisoprene used in the latex
may be reduced by incorporating SIS, thus reducing the cost of the
raw materials. By incorporating SIS in the dispersed phase,
improved tear strength, puncture strength, solvent resistance, and
degradation resistance properties, as well as processing advantages
may be obtained.
[0124] The method described in Examples 6 and 7 is not limited to
the addition of SIS to elastomers such as polyisoprene. As noted
above, other polymers may also be incorporated with the elastomer
using the process of the invention. In addition, SIS may be
combined with the elastomer without additives. An elastomer-SIS
latex without such additives may then be converted to a cured film
by using the methods discussed above.
[0125] Advantages of the latex of the invention, process of making
the latex, and articles manufactured from the latex include
improved film forming (less surfactant/emulsifier compared to
conventional composition), performance modification of gloves and
other articles (protection of sensitive additives), and enhanced
formulation properties (formulation stability and compatibility).
In addition, the latex provides improved bulk properties of gloves
and other articles (softness, barrier and oxidative degradation
resistance), reduced surface contamination, e.g. cleaner glove
(less impurity migration to surface), increased formulation
efficiency (availability of additive without diffusion loss),
improved maturation process (no diffusion process of
ingredients/additives), and reduced manufacturing costs (less
individual additive production).
[0126] The aqueous polyisoprene latex of the invention demonstrates
benefits over the Kraton product because elastomeric articles made
from the inventive aqueous polyisoprene latex provide substantial
improvements in performance in the various applications since a
wider range of additives may be included in the latex. The
improvements in the performance depend on the nature and amount of
the additives dissolved in polyisoprene solution prior to
emulsification. For example, the advantages of an article such as a
glove made from the aqueous elastomer dispersion of the invention
include mechanical properties such as barrier performance,
touch-feel sensation, tactile sensitivity, tear resistance, and
instrument gripping, and enhanced glove surface performances such
as antimicrobial activity, antistatic property, a clean glove
surface, and sweat absorption. Thus, according to the invention,
additives may be selected to improve specific properties of
products in a wider range of applications than previously
possible.
[0127] The aqueous water-based elastomer composition of the
invention may be processed to make a variety of articles including,
but not limited to medical gloves, condoms, probe covers (i.e., for
ultrasonic and transducer probes), dental dams, finger cots,
catheters and the like. Methods for processing the composition may
include, but are not limited to coagulant dipping, dispersion
dipping, drying, leaching and oven curing.
* * * * *