USOO7041746B2

(12) United States Patent (10) Patent No.: US 7,041,746 B2

Dzikowicz (45) Date of Patent: May 9, 2006
(54) ACCELERATOR SYSTEM FOR SYNTHETIC 5,594,073 A * 1/1997 Crepeau et al. .......... 525/3318
POLYISOPRENE LATEX 6,618,861 B1 9/2003 Saks et al. ................... 2,161.7
6,828.387 B1 * 12/2004 Wang et al. ...... ... 525/329.3
(75) Inventor: Robert Thomas Dzikowicz, Monroe, 2002/0173563 Al 11/2002 Wang et al. .................. 524/25
CT (US)
FOREIGN PATENT DOCUMENTS
(73) Assignee: R.T. Vanderbilt Company, Inc.,
Norwalk, CT (US) GB 1185896 * T 1968
GB 1185896 3, 1970
(*) Notice: Subject to any disclaimer, the term of this JP 55-144037 A * 11, 1980
patent is extended or adjusted under 35
U.S.C. 154(b) by 27 days. OTHER PUBLICATIONS
(21) Appl. No.: 10/743,210 JP55-144037 (English translation).*
Neoprene Latex, E.I. duPont deNemours & Co., Inc., 1962,
(22) Filed: Dec. 22, 2003 J. Carl.
Polychloroprene Latexes, The Vanderbilt Latex Handbook,
(65) Prior Publication Data Third Edition, R. T. Vanderbilt Company, Inc. 1987, J. Fitch.
Structural Characterization of Vulcanizates. Part III. The
US 2005/OO65249 A1 Mar. 24, 2005 cis-1,4-Polyisoprene Tetramethylthiuram Disulfide-Zinc
O O Oxide System, Journal of Applied Polymer Science, vol. 8,
Related U.S. Application Data pp. 581-602 (1964), C.G. Moore and A.A. Watson.
(60) Provisional application No. 60/505,510, filed on Sep. * cited by examiner
24, 2003.
(51) Int. Cl. Primary Examiner Ling-Sui Choi
CSF 36/08 (2006.01) Assistant Examiner—Rip A Lee
CSK 5/39 (2006.01) (74) Attorney, Agent, or Firm Norris, McLaughlin &
CSK 5/405 (2006.01) Marcus
CSK 5/47 (2006.01)
(52) U.S. Cl. ............................... 525/333.1: 525/332.6; (57) ABSTRACT
525/332.7:524/571:524/201:524/202:524/203;
524/211:524/216 The i 1 h
(58) Field of Classification Search ................ 524/572, e year t tO Athi t system, E"E
524/201, 202, 203, 211, 216, 892; 2/162.7: E. em Ior syn i. E. S. e
525/333.1, 332.6, 332.7 accelerator system comprises aluniocarpamate and unlourea
See application file for complete search history. and can produce synthetic polyisoprene films having a
tensile strength of about 3,000 psi to about 5,000 psi at low
(56) References Cited curing temperatures. The invention also pertains to a method
for curing synthetic polyisoprene latex.
U.S. PATENT DOCUMENTS
5,466,757 A * 11/1995 Watanabe et al. ........... 525/352 18 Claims, No Drawings
 US 7,041,746 B2
1. 2
ACCELERATOR SYSTEM FOR SYNTHETIC SUMMARY OF THE INVENTION
POLYISOPRENE LATEX
The invention concerns an accelerator system (i.e., an
CROSS REFERENCE TO RELATED accelerator composition) for synthetic polyisoprene latex
APPLICATIONS that can produce films having a tensile strength of about
3,000 psi to about 5,000 psi (about 20.7 MPa to about 34.5
This application claims the benefit of U.S. Provisional MPa) at low curing temperatures. The synthetic polyiso
Application No. 60/505,510 filed Sep. 24, 2003. prene films have a low modulus and can be balanced for pot
life or maturation (compound viscosity increases) and ten
BACKGROUND OF THE INVENTION
10 sile strength. The accelerator system comprises:
1) dithiocarbamate, and
1. Field of Invention 2) thiourea
The invention pertains to an accelerator system, Such as The invention also relates to synthetic polyisoprene latex
an accelerator system particularly Suited for synthetic poly 15 compositions containing the accelerator system, and prod
isoprene latex. The accelerator system comprises dithiocar ucts prepared therefrom.
bamate and thiourea. The accelerator system can, for A method for curing the synthetic polyisoprene latex
example, produce synthetic polyisoprene films having a composition comprising the accelerator system is also
tensile strength of about 3,000 pounds per square inch within the scope of the invention. The method generally
(“psi') to about 5,000 psi at low curing temperatures. comprises the steps of forming a film from a composition
2. The Prior Art comprising synthetic polyisoprene latex and an accelerator
system having dithiocarbamate and thiourea and heating the
Synthetic polyisoprene latex is chemically very similar to film at low temperature (e.g. about 90° C. to about 140° C.)
natural rubber latex. However, the physical properties of the for up to about 30 minutes.
Vulcanized films have never equaled those of the natural 25
product. Natural latex films can be cured quickly (about 15 DETAILED DESCRIPTION OF THE
to 20 minutes) at low temperatures (about 90 to 100° C.) INVENTION
with a variety of accelerators to produce tensile strengths of
about 4,000 psi to 5,000 psi (about 27 to 35 MPa). Poly 30
Any dithiocarbamate may be used in the accelerator
isoprene latex films having high tensile strengths prior to the system, such as dithiocarbamates available from the R. T.
invention described herein, were very difficult to cure, Vanderbilt Company, Inc., Norwalk, Conn., USA (“Vander
primarily because no suitable accelerator System was avail bilt'). Examples of dithiocarbamates useful in the invention
able to promote the sulfur crosslinking. Regardless of the are sodium dithiocarbamate or Zinc dithiocarbamate, par
accelerator system selected for curing, only tensile strengths 35
ticularly, from Vanderbilt, Zinc dibutyldithiocarbamate
of about 500 psi to 2,000 psi (3 to 10 MPa) were achieved. (“ZDBC', available under the trade name BUTYL
U.S. patent application Ser. No. 2002/0173563A1 ZIMATE(R), zinc diethyldithiocarbamate (“ZDEC', avail
describes a process for making dipped articles from latex able under the trade name ETHYL ZIMATE(R) and zinc
involving the use of an accelerator system comprising Zinc dibenzyldithiocarbamate (“ZBEC', available under the
diethyldithiocarbamate (“ZDEC), Zinc 2-mercaptoben 40 trade name BENZYL ZIMATE(R).
Zothiazole (“ZMBT), and diphenyl guanidine (“DPG'). Blends of dithiocarbamates and other materials may be
ZDEC with the ZMBT is a popular accelerator system for used in the accelerator system. Non-limiting examples of
natural rubber latex. Only the addition of DPG allows this such blends are the combination of the zinc dithiocarbamate
accelerator system to achieve cured films with tensile with ZMBT (available from Vanderbilt under the trade name
strengths in excess of 3,000 psi (20 MPa). 45 ZETAX(R), and a blend of sodium dibutyldithiocarbamate
U.S. Pat. No. 6,618,861 concerns a polyisoprene latex (“NaDBC), sodium mercaptobenzothiazole (“NaMBT)
compound that includes an accelerator System of 2.0 parts and 1.3 dibutylthiourea, which is referred to in this Speci
per hundred (“phr) tetramethylthiuram disulfide fication as “WB-7.
(“TMTD'), 0.2 phr zinc 2-mercaptobenzothiazole Any thiourea may be used in the accelerator system,
(“ZMBT'), 0.2 phr zinc dibutyldithiocarbamate (“ZDBC), 50 including but not limited to trimethyl, diethtyl, ethylene,
0.2 phr 1,3-diphenyl-2-thiourea and 0.2 phr zinc dieth diphenyl and the like. The thiourea may be dibutylthiourea,
yldithiocarbamate (“ZDEC). However, after curing, this in particular 1,3-dibutylthiourea, available under the trade
accelerator system provides tensile strength only of about name THIATER U from Vanderbilt.
1,900 psi. The accelerator system used in a synthetic polyisoprene
Thiourea (thiocabanilide) and thiourea/guanidine accel 55 latex composition achieves a Surprising improvement over
erator systems are known for advancing the cures of poly the art, since synthetic polyisoprene films having higher
chloroprene latex compounds. Sodium dithiocarbamate?thi tensile strengths (e.g., about 3,000 psi to about 5,000 psi)
uram accelerators are also recommended for the same can be obtained, without the necessity for a thiuram or a
purpose. (J. Carl, Neoprene Latex, E. I. duPont deNemours guanidine, such as TMTD and DPG. In particular, by using
& Co., Inc., 1962 and J. Fitch, Polychloroprene Latexes, The 60 dithiocarbamates at amounts greater than about 0.2 phr, and
Vanderbilt Latex Handbook, Third Edition, R. T. Vanderbilt up to about 4.0 phr, including about 0.5 phr to about 1.5 phr:
Company, Inc., 1987). However, these accelerator systems and thiourea, at amounts greater than 0.2 phr and up to about
are recommended to advance the bisalkylation crosslinks of 4.0 phr, including about 0.5 phr to about 1.5 phr, a much
Zinc chloride initiated by the metal oxides. They do not stronger latex product, having a higher tensile strength, can
presume to significantly affect Sulfur crosslinking as in 65 beachieved, without the necessity of a further additive such
polyisoprene latex where no allylic chlorine atoms are as TMTD and/or DPG. The tensile strengths for synthetic
present. polyisoprene films that can beachieved with the accelerator
 US 7,041,746 B2
3 4
system of the invention are similar to that which can be EXAMPLES
obtained with the use of conventional accelerator systems in
natural rubber. Synthetic polyisoprene latex having the composition set
The accelerator composition may also comprise a thiaz forth in Table 1 was formulated by methods known in the art.
ole, such as ZMBT, sodium 2-mercaptobenzothiazole or All components set forth in Table 1 are reported as parts per
combinations thereof. Use of the thiazole, however, is
optional. ZMBT, when used in an accelerator system with a hundred (phr). Different accelerator systems, which are
thiourea, but without the dithiocarbamate, results in a film identified in Table 2 were added and the latex compositions
having an unacceptable low tensile strength of less than were cured at different conditions as set forth in Table 2. The
2,000 psi. 10 curing conditions were 30 minutes at 100° C., 18 minutes at
Very low modulus compounds. Such as are required for 120° C. and 6 minutes at 140° C.
latex gloves, can be obtained with the accelerator System of The amount of each component of the accelerator system
the invention. For example, latex gloves with 300% modulus is set forth in parts per hundred. Thiourea and dithiocar
ranges of between about 190 psi to about 230 psi (about 1.3 15 bamate levels were varied from 0.33 phr to 1.0 phr. ZDEC/
MPa to about 1.6 MPa can be made). A latex compound that ZMBT was used at a level of 0.5 phr each as was the WB-7
increases in Viscosity upon standing is said to precure. The that contained a similar ratio of NaDBC/NaMBT. The
assumption is that Vulcanization takes place in the colloidal compounds and results in the tables illustrate the nature of
state. Therefore, products manufactured with precured or the acceleration properties of the accelerator systems. Levels
matured compound tend to achieve higher physical proper
ties and/or a faster cure rate than compounds that are of 0.1 to about 4.0 phr of the individual accelerator com
processed shortly after they are mixed. This can be advan ponents in combinations with each other can give satisfac
tageous or disadvantageous depending on the variables of tory results.
the manufacturing process. Latex compounds that continue
to precure to a point where the increasing viscosity renders Examples 1A and 1B
25
them unusable are said to have exceeded their pot life. The
accelerator Systems of the invention allow for optimizing the Satisfactory tensile strength results were obtained
precure or maturation rate by selecting the type(s) and whether the dithiocarbamate/thiazole salts are zinc as in the
amount(s) of dithiocarbamate used with the thiourea. For ZDEC/ZMBT blend or sodium as in WB-7. Also, the
instance, a polyisoprene latex compound accelerated by a different cure time/temperature conditions produced films
30
combination of ZBEC with a thiourea will have a long pot with satisfactory physical properties. All viscosities are
life. The viscosity will not increase substantially upon satisfactorily low, as well.
standing for two or more weeks. The same is true when an
accelerator system of ZDEC and ZMBT is used with a Examples 2A and 2B
thiourea. Polyisoprene latex films having an accelerator
system containing ZDBC with a thiourea cure to very high 35
physical properties, but their colloidal compounds tend At lower cure temperatures, the ZDBC/thiourea combi
toward diminished pot life. However, by decreasing the nation produced films with much higher tensile strength than
amount of the ZDBC, very satisfactory pot lives can be the ZBEC/thiourea combination but there was little differ
achieved.
40
ence at the higher cure temperature. However, the Viscosity
The accelerator system can be used to satisfactorily cure of the ZDBC/thiourea combination increased significantly
polyisoprene latex compounds with the dithiocarbamate/ upon standing for two weeks whereas the ZBEC/thiourea
thiourea accelerator combinations at oven temperatures as combination did not.
low as about 100°C. The curing temperature may be around
90° C. to about 140°C., such as about 100° C. to about 120° 45 Examples 3A and 3B
C. Generally, the curing times will vary with oven tempera
ture and the curing times may be up to about 30 minutes Example 3A is colloidally stable with a 350 cps viscosity
long, however, acceptable synthetic polyisoprene films can after standing almost a week and produces films with very
be obtained by curing at the relatively low temperatures of
about 100° C. to about 120° C. for about 18 minutes to about high tensile strength. These films can be satisfactorily vul
30 minutes. At higher temperatures (such as about 130° C. 50 canized at all three of the time/temperature conditions. Also,
to about 140°C.); satisfactory cures can be obtained in up the 300% modulus is 230 psi or less in each case, which is
to about 10 minutes, preferably up to about 6 minutes. a very important consideration for the latex glove industry.
In an embodiment of the invention, the accelerator system Example 3B illustrates the effect of decreasing the dosage
comprises: 55
of the accelerator system. In Example 3B, the total amount
1) dithiocarbamate, a blend of dithiocarbamates or dithio of accelerator is /2 the amount in Example 3A, (1 phr vs. 2
carbamate in combination with thiazole, at greater than phr). Viscosities are lower so the compound is more stable.
0.2 phr to about 4.0 phr (preferably about 0.5 phr to Tensile strengths are satisfactory.
about 1.5 phr); and
2) thiourea at greater than 0.2 phr to about 4.0 phr 60 TABLE 1.
(preferably about 0.5 phr to about 1.5 phr). Dry phr Supplier
The accelerator composition of the invention may also
64% IR 401 Kraton (R)
consist essentially of dithiocarbamate, a blend of dithiocar Polyisoprene Latex 1OO Polymers, Inc.
bamates or dithiocarbamate in combination with thiazole, 33% Darvan (R) WAQ O.25 R. T. Wanderbilt
and thiourea. The invention further relates to a latex com 65 33% Zinc Oxide O.S various
position including an accelerator system as described above, 50% Sulfur 1.5 various
which latex composition is free of TMTD and/or DPG.
 US 7,041,746 B2
6
3. The composition of claim 1 wherein the dithiocarbam
TABLE 1-continued ate is selected from the group consisting of Sodium dithio
Dry phr Supplier
carbamate, Zinc dithiocarbamate and combinations thereof.
4. The composition of claim 3 wherein the zinc dithio
50% VANOX (R) SPL 2 R. T. Wanderbilt carbamate is selected from the group consisting of Zinc
33% DARVAN SMO O.S R. T. Wanderbilt dibutyldithiocarbamate, zinc diethyldithiocarbamate, Zinc
Accelerators As noted below in Table 2 dibenzyldithiocarbamate and combinations thereof.
Notes: 5. The composition of claim 1 further comprising thiaz
KRATON (R) is a registered trademark of the KRATON (R) Polymers Group ole.
of Companies having offices in Houston, Texas, USA. 10 6. The composition of claim 5 wherein the thiazole is
DARVAN (R) and VANOX (R) are registered trademarks of Vanderbilt. selected from the group consisting of Zinc 2-mercaptoben
Zothiazole, sodium 2-mercaptobenzothiazole, or combina
TABLE 2 tions thereof.
7. The composition of claim 1 wherein the thiourea is 1.3
Compound Example Example Example 15 dibutyl thiourea.
Number 1A 1B 2A, 2B 3A 3B
8. The composition of claim 1 wherein the amount of
thiourea is about 0.5 phr to about 4 phr.
ZDECZMBT 1 9. The composition of claim 1 wherein the amount of
ZDBC
ZEBC
1
1
O.67
O.67
O.33
O.33
thiourea is about 0.5 phr to about 1.5 phr.
WB-7 2 10. A method for curing synthetic polyisoprene latex
1,3 dibutylthiourea 1 1 1 O.67 O.33 comprising the steps of forming a film from a composition
Viscosity, cps comprising synthetic polyisoprene latex and an accelerator
after mix 250 250 250
system having about 0.5 phr to about 4.0 phr dithiocarbam
after 1 day 3OO 275 ate and greater than 0.2 phr to about 4.0 phr thiourea wherein
after 2 days 350 275 25 the accelerator system does not contain tetramethyithiuram
after 3 days 275 400 300 disulfide or diphenylguanidine and heating the film at a
after 6 days 350 287.5 temperature of about 90° C. to about 140°C. for up to about
after 1 week
after 2 weeks 3OO 2000 275 30 minutes wherein the synthetic polyisoprene latex cured
after 3 weeks film has a tensile strength of about 3,000 psi to about 5,000
after 4 weeks 2SO-350 30 pS1.
(est) 11. The method of claim 10 wherein the dithiocarbamate
Cure (a) 100° C. for 30 is selected from the group consisting of sodium dithiocar
minutes
bamate, Zinc dithiocarbamate and combinations thereof.
Tensile Strength, psi 4020 476O 2830 S12O 2800 12. The method of claim 11 wherein the Zinc diothiocar
300% Modulus, psi
Elongation, % 83O 900 84O
230
860
170
1OOO
35 bamate is selected from the group consisting of Zinc dibu
Cure (a) 120° C. for 18
tyldithiocarbamate, Zinc diethyldithiocarbamate, zinc diben
minutes Zyldithiocarbamate and combinations thereof.
13. The method of claim 10 wherein the accelerator
Tensile Strength, psi 3890 SOSO 4500 278O 442O 3950 system further comprises thiazole.
300% Modulus, psi 220 190 40 14. A latex glove comprising synthetic polyisoprene latex
Elongation, % 1OOO 940 87O 810 890 930
Cure (a) 140° C. for 6 cured in accordance with the method of claim 10.
minutes 15. The method of claim 10 wherein the amount of
Tensile Strength, psi 461 O 3890 491O 451O 4560 2990
dithiocarbamate is about 0.5 phr to about 1.5 phr.
16. The method of claim 10 wherein the amount of
300% Modulus, psi 210 200
Elongation, % 96O 980 880 850 910 820 45 thiourea is about 0.5 phr to about 4 phr.
17. The method of claim 16 wherein the amount of
thiourea is about 0.5 phr to about 1.5 phr.
18. A method for curing synthetic polyisoprene latex
What is claimed is: comprising the steps of forming a film from a composition
1. A composition comprising synthetic polyisoprene latex
50 comprising synthetic polyisoprene latex and an accelerator
system having dithiocarbamate and 1.3 dibutyl thiourea
and an accelerator system having about 0.5 phr to about 4.0 wherein the accelerator System does not contain tetram
phr dithiocarbamate and greater than 0.2 phr to about 4.0 phr ethyithiuram disulfide or diphenylguanidine and heating the
thiourea wherein the composition does not contain tetram film at a temperature of about 90° C. to about 140°C. for up
ethyithiuram disulfide or diphenylguanidine and a polyiso 55 to about 30 minutes wherein the synthetic polyisoprene latex
prene film formed from heating and curing the composition cured film has a tensile strength of about 3,000 psi to about
has a tensile strength of about 3,000 psi to about 5,000 psi. 5,000 psi.
2. The composition of claim 1 wherein the amount of
dithiocarbamate is from about 0.5 phr to about 1.5 phr.