Medical gloves abstract
The invention is the manipulation of the zinc oxide content and
sulfur content of nitrile butadiene rubbers and selected vulcanization
conditions that can be achieved economically with common production
facilities. The manipulation of these components affects the relaxation
property of gloves formed by this material. Produced are gloves
that have a relaxation property, higher than 50%, and a low modulus
(approximately 3 Mpa). The glove maintains decent ultimate tensile
strength (>20 Mpa) and elongation (>500%). The glove is produced
by a vulcanization process, which lasts from 5 to 60 minutes at
temperatures ranging from 300.degree. F. to 400.degree. F. The tensile
strength and elongation are well above the ASTM requirements for
medical gloves. The current ASTM requirements are ASTM D412-92.
Thanks to sufficient vulcanization, the films produced provide satisfactory
protection from viral penetration. The tearing strength is also
better because of the lower modulus.
Medical gloves claims
What is claimed is:
1. An elastomer formulation for a glove comprising: a nitrile butadiene
rubber latex; Sulfur ranging from about 2 to about 5 parts per hundred
parts of the nitrile butadiene rubber latex; and Zinc oxide, said
zinc oxide, present in an amount greater than 0.5 parts per hundred
and less than 1.0 parts per hundred parts nitrile butadiene rubber,
wherein said elastomer has a relaxation property greater than 50%.
2. The elastomer formulation for a glove as claimed in claim 1
further comprising: a secondary accelerator ranging from 0.5 to
about 2 parts per hundred parts of the nitrile butadiene rubber
latex.
3. The elastomer formulation for a glove as claimed in claim 2
wherein the secondary accelerator is selected from the group consisting
of: zinc dibutyldithiocarbamate; zinc diethyldithiocarbamate; and
zinc 2-mercaptobenzothiazole.
Medical gloves description
[0001] This application is a continuation-in-part of Ser. No. 09/877034
filed Jun. 11 2001 and Ser. No. 10/120796 filed Apr. 12 2002.
BACKGROUND OF THE INVENTION
[0002] Natural rubber latex gloves provide excellent protection
from numerous dangerous pathogens as well as many harsh chemicals.
The natural rubber latex glove manufacturing industry mushroomed
in early 1980s, especially in the Far East. However, soon after
that, it was recognized that the inherent proteins of natural rubber
latex would cause allergic reactions (Type I) to occur in certain
people. In rare cases, the allergic reaction could be fatal. Therefore,
for those people, alternatives, to natural rubber latex gloves,
must be provided.
[0003] Although a series of synthetic materials including nitrile
butadiene rubber (NBR), polychloroprene (CR), polyurethane (PU),
polyisoprene (IR), polyvinyl chloride (vinyl, PVC), polyethylene
(PE), etc. as well as many of their blends and copolymers have been
used as alternatives to natural rubber latex, the overall performance
and the cost of the alternatives are not quite satisfactory. Among
the alternatives, nitrile butadiene rubber is the most popular one,
an elastic glove at a reasonable cost.
[0004] U.S. Pat. No 5014312 and Reissue Patent RE 35616 both
issued to Tillotson et al, cover nitrile butadiene rubber gloves.
The patents address relaxation properties. The stress (or modulus)
of the material under constant strain at-six minutes should be less
than 50% of its initial value. Most of the nitrile gloves currently
commercially available have their relaxation property clustered
about 40%, although that could be varying from 30.about.45%. Other
gloves might have improved tensile strength, or elongation, or fewer
additives that could cause Type IV allergic reactions (ZnO, etc.).
None of them have displayed relaxations at six minutes that could
exceed 50%.
[0005] Tillotson et al compared relaxation properties between natural
rubber latex and nitrile films, but has not shown that the relaxation
property of nitrile films can be tuned within a certain range via
proper formulation.
[0006] U.S. Pat. No. 6031042 issued to Lipinski reveals a nitrile
rubber formulation. The formulation contains no zinc oxide and only
1.0 PHR sulfur resulting in a relaxation property of only approximately
40%. No consideration of adjusting the formulation to tune the relaxation
property of the glove is contemplated.
[0007] U.S. Pat. No. 6566435 issued to Teoh et al discloses a
nitrile latex formulation containing less than 0.5 PHR zinc oxide
and sulfur. No contemplation is given to a product with a zinc oxide
content of greater than 0.5 parts per hundred and less than 1.0
parts per hundred with a relation property tuned to above 50%. More
over, the proposed formula restricted the latex to be used. It must
consist of a carboxylated content between 2.about.6%. We found the
content of the carboxylated group is not as critical as they claimed.
Conversely, we used a product that contains 7% of carboxylated group
and the relaxation property can still be much higher than 50%. We
also tested the sample formula in this patent to make some films.
The films do not age well.
[0008] Relaxation property is not an ASTM required quality control
parameter for gloves. But together with modulus, another non-ASTM
required quality control parameter for gloves; they can characterize
the performance and the tactile sensation of a glove. The higher
the relaxation property, the better the glove will fit a hand's
shape. Otherwise, the glove becomes loose after a while. But if
high relaxation were combined with high modulus, the glove would
quickly cause finger fatigue. Natural rubber latex gloves has a
(relaxation >80%, and a 300% modulus (<2 MPa), while nitrile
butadiene rubber gloves show lower relaxation (typical 40%) and
a much higher modulus at 300% (>7 Mpa).
[0009] Relaxation property is an intrinsic characteristic of material
nature. Most nitrile butadiene latexes manufactured via emulsion
polymerization would yield a relaxation of about 40%, as evidenced
by the nitrile gloves currently available. This inherent property
is predominantly caused by polymer chain structure, which would
be determined by the polymerization mechanism. Different nitrile
butadiene rubber vendors might have different controlling parameters
and procedures, but their products have very little differentiation
due to the fact that they all use emulsion polymerization for economic
reasons.
[0010] Nitrile butadiene latexes, produced via polymerization mechanisms
other than emulsification, namely for dipping applications could
have quite different structure, and thus different relaxation profiles,
but there are no such products that are commercially available right
now because of cost. Once the polymer chain structure has been predefined
in the polymerization, there is little one can do to manipulate
it. It is an objective of the invention to tune this parameter (relaxation)
to above 50%. Meanwhile, the other mechanical properties must meet
ASTM requirements.
[0011] To evaluate the performance of a nitrile glove, relaxation
is only one of many physical properties. Tensile strength, modulus,
elongation, before and after aging, are all very important; as required
by ASTM. Gloves depending on the designed application can emphasize
different characteristics.
[0012] Three components in formulation are critical to affect these
desired properties. First of all, sulfur is the crosslinker. Secondly,
zinc oxide is the so-called primary activator for sulfur vulcanization.
Lastly, the so-called secondary accelerators include zinc dibutyldithiocarbamate
(ZDBC or BZ), zinc diethyldithiocarbamate (ZDEC or EZ), zinc 2-mercaptobenzothiazole
(ZMBT), etc.
[0013] Through experimentation, we find out that ionic crosslinking
will lead to strong tensile strength and good aging resistance,
as desired. However, it also results in low elongation, high modulus,
and low relaxation, as undesired. Zinc oxide free would result in
highest relaxation, lowest modulus, and longest elongation. But
the films don't age well. The films show fairly poor aged elongation
(<400%).
[0014] Sulfur content is critical to relaxation. We made two series
of films with sulfur content 1 PHR and 3 PHR but changed the zinc
oxide from 1 PHR to 0. The correlation is clear. At 1 PHR of sulfur,
relaxation would go from 35% to 45% by decreasing zinc oxide from
1 PHR to 0. At 3 PHR of sulfur, relaxation would go from 45% to
55%.
[0015] Having reduced zinc oxide, usually one has to add some more
secondary accelerators to compensate, so that sulfur vulcanization
can occur timely as desired. However, excessive secondary accelerators
not only cause concerns on Type IV allergy, but also accelerate
aging of the products during storage.
[0016] Varying the ratio of these three components, one can have
optimized formulations for different desired applications.
SUMMARY OF THE INVENTION
[0017] The invention is the manipulation of the zinc oxide content,
greater than 0.5 PHR to less than 1.0 PHR, sulfur content, 2 PHR
to 5 PHR, and secondary accelerators, 0.5 PHR .about.2 PHR, of nitrile
butadiene rubbers and selected vulcanization conditions that can
be achieved economically with common production facilities. The
manipulation of these components affects the relaxation property
of gloves formed by this material. Produced are gloves that have
a relaxation property, higher than 50%, and a low modulus (approximately
3 Mpa). The glove maintains decent ultimate tensile strength (>20
Mpa) and elongation (>500%). And the glove must age well, aged
elongation >400% and aged tensile strength >20 MPa. The tensile
strength and elongation are well above the ASTM requirements for
medical gloves. The current ASTM requirements are ASTM D412-92.
Thanks to sufficient vulcanization, the films produced provide satisfactory
protection from viral penetration. The tearing strength is also
better because of the lower modulus.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Compounding:
[0019] Carboxylated nitrile butadiene rubber undergoes two kinds
of crosslinking in normal formulation and vulcanization. The first
one is where carboxylated acid groups are linked to each other via
a reaction with zinc oxide at room temperature. This is the so-called
ionic crosslinking. The second kind of crosslinking is where unsaturated
butadiene blocks are crosslinked via a conventional sulfur system
at elevated temperatures. This is also referred as covalent crosslinking.
One of our preferred formulations, produce films whose properties
are comparable to those of natural rubber latex (300% modulus <2
Mpa, tensile strength >20 Mpa, and simultaneously elongation
>500%). But, the relaxation at six minutes cannot be higher than
45%. Therefore, zinc oxide was adjusted to prevent a performance
reduction. The resulting formulation is tabulated as following:
1 INGREDIENTS PHR Nitrile Butadiene Rubber 100 22'-methylene-bis-(4-methyl-6-butylphenol)
0.5 Zinc Oxide 0.85 Zinc dibutyldithiocarbamate (BZ) + Zinc 2- 0.7
mercaptobenzothiazole Sulfur 3.0 Potassium hydroxide 1.0 Titanium
dioxide 0.5
[0020] Mechanical Properties:
[0021] It was found it is possible to keep relaxation >50% at
even higher ZnO levels via choosing proper nitrile latex and varying
production parameters. As anticipated, the higher ZnO level is,
the lower the relaxation and the tougher the glove is. Nevertheless,
we now think a further optimized formula ought to contain higher
ZnO level to give longer shelf life of the gloves.
2 Aged Description Relaxation (R) Elongation (%) ZnO 0 66 434 ZnO
0.5 62 470 ZnO 0.7 58 493 ZnO 0.85 53 515 ZnO 1.0 48 520
[0022] The correlation between ZnO and relaxation and aged elongation
was formulated. 3.0 PHR of sulfur was used in all these data presented.
[0023] As a result, we found zinc oxide formulations with percentages
ranging from greater than 0.5% to 1.0%, and vulcanization conditions
yield higher relaxation properties, greater than 50%, and an improved
soft glove. Each formulation had a sulfur component of 3.0 PHR.
This combination shows a more balanced performance, not only relaxation
>50% but also an extraordinary aged elongation >500%.
[0024] The zinc oxide compound exhibited relaxation greater than
50%. Powdering and/or leaching had no significant impact on glove
properties. The formulations produce improved compounds and optimized
vulcanization conditions (temperature and duration). The new formulations
and procedures are easily realized economically under common nitrile-glove
production lines. Depending on the desired applications, both powdered
and powder free gloves could be produced in the same formula.
[0025] In a formulation with zinc oxide and sulfur, two kinds of
crosslinking mechanisms govern the carboxylated NBR vulcanization.
At room temperature, zinc oxide reacts with carboxylated groups
to form ionic crosslinking, resulting in low relaxation. On the
other hand, sulfur crosslinking, especially multi-sulfur crosslinking
between double bounds of polybutadiene chains results in high relaxation.
Varying the ratio of these two components, one can tune the relaxation
to a certain level. The previously disclosed zinc oxide embodiment
achieved at relaxation as high as 62%, compared to 40% with a normal
zinc oxide and sulfur formula. With limited zinc oxide content of
greater than 0.5 parts zinc oxide per 100 parts NBR and less than
1.0 parts zinc oxide per 100 parts NBR and 3.0-5.0 parts of sulfur
per 100 parts NBR, a relaxation greater than 50% can still be achieved.
The affect of the varied ratio of the zinc oxide and sulfur is summarized
on the following table:
3 Zinc Oxide Sulfur Parts Per 100 parts Parts Per 100 parts NBR
NBR Relaxation (%) 1 3 48 0.85 3 53 0.70 3 58 0.5 3 62
[0026] As can be seen, lowering the amount of zinc oxide increases
the relaxation property. These elastomers were made under the same
vulcanization process disclosed above.
4 Zinc Oxide Sulfur Parts Per 100 parts Parts Per 100 parts NBR
NBR Relaxation (%) 0 1 48 0 1.5 50 0 2 54 0 2.5 54 0 3 59 0 3.5
61 0 5 62
[0027] When the formula are free of zinc oxide, it seems relaxation
almost increases linearly with sulfur content at the beginning.
Then, it level off after sulfur reached 3 PHR. Therefore; we also
collected data for the formula with zinc oxide.
5 Zinc Oxide Sulfur Parts Per 100 parts Parts Per 100 parts NBR
NBR Relaxation (%) 0.5 1 48 0.5 2 55 0.5 3 60 0.5 5 64 1 5 51 1.5
5 41
[0028] Instead of reach a plateau after 3 PHR of sulfur, the relaxation
seems to continue to increase. And relaxation 50% is achieved even
when zinc oxide is 1 PHR. But the other properties are not so desirable
any more. The elongation before aged is less than 500% and modulus
is >8 MPA, very tough.
[0029] As aforementioned, the third component in the formula is
the level of secondary accelerators. When zinc oxide, the primary
activator of sulfur vulcanization, level is low, relatively higher
secondary accelerators are needed. Otherwise, to ensure sufficient
vulcanization requires substantial heat or prolong vulcanization
duration. However, excessive accelerators also result in poor aging
resistance.
6 Aged Accelerators Relaxation (R) Elongation (%) 1.0 56 450 1.5
61 407 2.0 64 377
[0030] The sulfur is 3 PHR and zinc oxide is 0.5 PHR for these
three films. The aging was conducted at 100.degree. C. for 22 hours,
as ASTM specified.
[0031] Although under the same vulcanization conditions, more accelerators
result in higher relaxation, which is also desired, the aged elongation
is clearly changing to undesired direction.
[0032] As mentioned previously, relaxation is an intrinsic property
of the polymer structure and in carboxylated NBR; the most predominant
factor is the chain structure of polybutadiene. In commercially
available carboxylated NBR synthesized via emulsion polymerization,
the polybutadiene block consists of three types of double bonds:
12; cis 14; and trans 14. More cis 14 structure yields high
relaxation. It is possible to synthesize polybutadiene with cis
14 dominant structures in an organic solvent. In this way, it is
possible to achieve higher relaxation, > than 80%, with the normal
formula. For an NBR, which is not carboxylated, there is no reaction
between the zinc oxide and carboxylated groups and therefore, the
content of zinc oxide may not matter. Natural rubber may have a
relaxation property > than 80% with 1 PHR of zinc oxide.
[0033] While the invention has been described with respect to a
preferred embodiment, variations, modifications would be apparent
to one of ordinary skill in the art without departing from the spirit
of the invention. |