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A new grade of natural rubber latex: NC360.

One of the greatest challenges for the natural rubber latex industry has been the latex protein allergy problem. Natural rubber latex is produced from a milky white liquid extracted from the hevea brasiliensis tree. The tree incorporates antioxidants, carbohydrates, lipids and proteins with the desirable polymer polyisoprene. It is these proteins which have been implicated in the cause of these allergies. In this article we will discuss a new grade of natural rubber latex developed in response to the latex protein allergy issue. The product is called NC360 latex. We will discuss the properties of NC360 and compare them to standard grades of natural rubber latex. Then we will discuss producing gloves using NC360. We will cover the processing challenges we have encountered producing a glove that will meet ASTM standards. We will review the physical properties of the gloves produced from NC360. We will demonstrate that NC360 is a commercially, viable product, and with NC360 we enable the natural rubber latex industry to produce safer products.

Properties

All grades of NR latex start with a material called field latex that is tapped from the hevea brasiliensis tree. This material is only 33% natural rubber along with several other naturally occurring chemicals. A typical composition of field latex is listed in table 1.
Table 1 - composition of field latex

 %

Total solids content 36
Dry rubber content 33
Proteinous substances 1 - 1.5
Resinous substances 1 - 2.5
Ash Up to 1
Sugars 1
Water Ad. 100


There is a significant amount of non-rubber solids (NRS) present in field latex, about 3.0 - 6.0%. The proteins that can cause latex allergies are part of the NRS portion. The field latex is processed into commercial grades of NR latex. This usually involves concentrating the field latex to higher total solids. The main methods of concentration are centrifuging and creaming. Ammonia and other preservatives are added for preservation. ASTM/ISO standards have established standards for three different types of latex based on concentration method and preservative systems. The specifications are listed in table 2.
Table 2 - requirements for specified latex types

 Type 1 (*) Type 2 Type 3
 (**) (***)

Total solids, min., % 61.5 66.0 61.5
Dry rubber content (DRC), min., % 60.0 64.0 60.0
Total solids minus dry rubber 2.0 2.0 2.0
 content, max., %
Total alkalinity calculated as 0.60 min. 0.55 min. 0.29 max.
 ammonia, as % latex
Sludge content, max., % 0.10 0.10 0.10
Coagulum content, max., % 0.050 0.050 0.050
KOH number, max. 0.80 0.80 0.80
Mechanical stability, s, min. 650 650 650
Copper content, max., % of 0.0008 0.0008 0.0008
 total solids
Manganese content, max., % of 0.0008 0.0008 0.0008
 total solids
Color on visual inspection No pronounced blue or gray
Odor after neutralization with No putrefactive odor
 boric acid

(*) Centrifuged natural latex preserved with ammonia only or by
formaldehyde followed by ammonia.

(**) Creamed natural latex preserved with ammonia only or by
formaldehyde followed by ammonia.

(***) Centrifuged natural rubber latex preserved with low
ammonia with other necessar perservatives.


A review of the ASTM specifications from table 2 and the typical properties in table 3 highlights the differences between the NC360 and the three types of commercially available NR latex. These differences are significant as they affect the production of gloves from the NC360.
Table 3 - typical properties of commercially available natural
rubber latex and NC360

 Type 1 Type 2 Type 3 NC360

Total solids, % 61.7 67.5 61.7 65.8
Dry rubber content (DRC), % 60.2 66.3 60.2 65.5
Total solids minus dry rubber
 content, % 1.5 1.2 1.5 0.3
Total alkalinity calculated as
 ammonia, as % latex 0.70 0.60 0.24 0.60
Sludge content, % 0.05 0.10 0.05 <0.01
Coagulum content, % 0.010 0.005 0.010 0.001
KOH number 0.62 0.65 0.67 0.40
Mechanical stability, s 1,200 1,500 1,000 1,500
Copper content, % of total
 solids 0.0003 0.0003 0.0003 <0.0001
Manganese content, % of total
 solids 0.0001 0.0001 0.0001 <0.0001
Color on visual inspection No pronounced blue or gray
Odor after neutralization with
 boric acid No putrefactive odor

Extractable protein content -
 cast films

ASTM D5712-95 ([micro]g
 protein/g of latex film 1,900 860 2,000 <28
LEAP - ELISA, ([micro]g of
 protein/g of latex film 1,100 870 1,200 1.2


The non-rubber solids (NRS) are the difference between total solids content (TSC) and dry rubber content (DRC). NC360 has a very low level of NRS. This is a very important difference. By removing the non-rubber solids from the latex we also removed the protein. Unfortunately, there are many naturally occurring chemicals in latex that have beneficial properties to the latex and material made from the latex. The proteins help facilitate vulcanization, the lipids react with ammonia to produce soap which gives the latex mechanical stability, the resins are a very effective natural antioxidant which gives products good aging properties. This significantly lower NRS results in the TSC not meeting the ASTM specification for type 2 latex even though the DRC meets the limit. This is not an issue in glove production since most formulations lower the TSC of the compound to 50%.

Mechanical stability is a measure of how much shear force the latex can handle before the latex destabilizes and forms coagulum. To become a commercially viable product, NC360 must have enough mechanical stability to survive the rigors of pumping, transporting, mixing and compounding. Several components of the non-rubber solids play a role in developing mechanical stable latex. The lipids react with ammonia to form soaps in natural rubber latex. The proteins also play a role in stability. Since we have removed these materials, we had to find the proper mix of surfactants that would give us the requisite amount of mechanical stability.

The KOH and VFA numbers are used to indicate the state of preservation of natural rubber latex. Natural rubber latex is susceptible to bacterial contamination. As bacteria grow and reproduce in the latex they produce volatile fatty acids such as formic, acetic and propinoic. These fatty acids are metabolic by-products of the bacteria so by measuring its content in the latex you have a measure of bacterial activity. The KOH number measures the total acid content in the latex. The VFA measures only the amount of volatile fatty acids in the latex. Bacterial activity can adversely affect NR latex properties rendering the latex useless in industrial processes. The NC360 must remain stable from the time of production until its delivery and use at the customer. In table 4, we show that NC360 remains very stable up to one year after production. This is another benefit of removing the NRS. The nutrients the bacteria need to survive have been removed from the NC360.
Table 4 - changes in KOH and VFA number

Latex type Type 1 Type 2 NC360

Initial KOH number 0.45 0.55 0.33
KOH number after 3 months 0.66 0.68 0.33
Initial VFA number 0.008 0.007 0.006
VFA number after 3 months 0.008 0.007 0.005


Two other tests of interest are the ZOV and the protein content. The ZOV, or zinc oxide viscosity test, is a measure of chemical stability of NR latex. It can indicate sensitivity toward zinc oxide. It is measured as a viscosity increase after zinc oxide is added to the latex. Zinc oxide is a very common ingredient in natural rubber latex compounding. It is a cure activator for NR latex. It enables you to efficiently cure natural rubber latex products. Latex that is sensitive to zinc oxide can go through dramatic increases in viscosity up to 10 times the original viscosity. This can make it very difficult to produce any product with NR latex. NC360 is very insensitive to zinc oxide. Using the ZOV test we have found the viscosity unchanged or actually dropped up to an hour. NC360 will remain stable through the compounding process.
Table 5 - ZOV results, cps (ref. 1)

Time, min. Type 1 Type 2 NC360

 2 100 140 120
 5 110 190 120
 10 120 200 120
 20 130 210 130
 30 132 220 132
 40 144 230 140
 50 144 244 150
 60 155 260 154
Table 6 - compound formulations

Trial 1 2 3

Total solids, % 30.00 33.00 32.74
Soap (non-ionic), phr 0.2
Sulfur, phr 0.800 0.645 0.800
ZnO, phr 0.7 0.8 0.8
Accelerator
(dithiocarbamate), phr 1.00 1.12 1.12
Precure, chloroform no. 2+ 2+ 2+
Cure time, min. 15 - 20 15 - 20 15 - 20
Cure temp., [degrees] C 80 - 110 80 - 110 80 - 110
Leach time, min. 2 2 2
Leach temp., C 45 - 55 55 - 60 55 - 60
Aqueous coagulant
Calcium nitrate, % 8.0 12.0 12.0
Calcium carbonate, % 3.0 2.5 2.5


Since the advent of the allergy issue, it has become very important to measure the protein content of latex and latex products. ASTM has established D5712-95 for measuring protein in NR. The LEAP-ELISA test method is another test method used to measure protein content. Table 3 shows typical protein results for the three ASTM types and NC360. The results are the amount of extractable protein from air dried cast films.

Experiment

The novel properties of NC360 presented several processing difficulties. To test NC360 in a commercial environment, we ran three different trial runs on standard equipment.

Results and discussion

The results, from the first trial are shown in table 7. The first trial run used a standard glove formulation on a commercial glove line. Observations made during the trial run were:

* Small coagulum was found in the dipping tank. Adjustments to the surfactants will be necessary.

* At the same curing time and temperature, the gloves are under-cured, thus the gloves had low modulus. Due to the absence of the non-rubber solids, the cure package will need adjustments to compensate.

* The compound was insensitive to calcium nitrate. Increase the calcium nitrate solution concentration to get better film pick up.
Table 7

Physical properties Unaged Aged 22 hrs.
 @ 100 [degrees] C

Tensile strength, MPa 18.5 17.8
Elongation, % 850 820
Modulus @ 300%, MPa 2.7 2.9


With adjustments to the formulation, we were able to overcome the technical problems encountered in trial 1. In trials 2 and 3, we needed to see if the gloves produced could meet ASTM standards on physical properties. In trial 2, the physical properties are near the minimum. Upon aging they failed to retain their physical properties to meet the ASTM requirements. The protein content had an increasing trend during the trial. This is only seen in the D5712-95 test results. The LEAP-ELISA test results do not show this same behavior. This could indicate that the D5712-95 is picking up false positives from the compounding ingredients. It also shows that the compounding ingredient concentrations are increasing in the leach tank during the trial run. Gloves from the end of the run were not leached as effectively. For trial 3, the formulation was changed to correct for these problems. The refresh rate for the water in the leach tank was also increased. The gloves' physical properties on unaged and aged meet the ASTM specifications. The D5712-95 results do not show protein content increasing with time, but the average content is significantly higher. This is probably due to the increased accelerator content. The LEAP-ELISA test results have a lower average in trial 3 (table 9) than trial 2 (table 8).
Table 8

 Glove Time into Protein content
 type run

 ASTM LEAP
 D5712-95 ELISA
 [micro]g/g [micro]g/g

1 Powder 1 hr. <28 4.5
2 Powder 3 hrs. <28 4.4
3 Powder 15 hrs. <28 4.4
4 Powder 21 hrs. <28 5.3
5 Powder free 1 hrs. <28 23.8
6 Powder free 15 hrs. 42 17.6
7 Powder free 18 hrs. <28 3.3
8 Powder free 21 hrs. <28 23.0
9 Powder free 24 hrs. <28 27.5
Table 9

 Aged 22
 hrs. @
 Unaged 100
 [degrees] C

 Modulus Tensile Elongation Modulus
 @ 300% strength at break @300%
 Date Time (MPa) (MPa) (%) (MPa)

 1 6/3/99 4:30 PM 1.16 25.21 895 0.86
 2 6/3/99 1:00 AM 1.19 19.84 862 0.87
 3 6/4/99 9:00 AM 1.20 24.58 860 0.83
 4 6/4/99 4:00 PM 1.27 31.49 840 0.88
 5 6/4/99 2:20 AM 1.27 25.36 846 0.92
 6 6/5/99 9:00 AM 1.40 25.41 824 0.86
 7 6/5/99 4:30 PM 1.20 24.42 801 0.87
 8 6/5/99 3:25 AM 1.29 25.26 852 0.96
 9 6/6/99 9:20 AM 1.24 24.17 840 0.89
10 6/6/99 1:00 AM 1.29 24.27 829 0.86

 Aged 22 hrs. @ 100
 [degrees] C Protein content

 Tensile Elongation ASTM LEAP
 strength at break D5712-95 ELISA
 Date Time (MPa) (%) [micro]g/g [micro]g/g

 1 6/3/99 4:30 PM 20.32 945 102 0.4
 2 6/3/99 1:00 AM 19.25 894 51 0.5
 3 6/4/99 9:00 AM 16.12 849 84 0.5
 4 6/4/99 4:00 PM 17.72 820 33 0.7
 5 6/4/99 2:20 AM 19.79 879 69 0.5
 6 6/5/99 9:00 AM 19.16 730 72 0.5
 7 6/5/99 4:30 PM 16.96 873 84 0.5
 8 6/5/99 3:25 AM 19.13 892 105 0.5
 9 6/6/99 9:20 AM 17.75 908 96 0.5
10 6/6/99 1:00 AM 18.26 873 78 0.6
Table 10

 Aged 22
 hrs. @
 Unaged 100
 [degrees] C

 Modulus Tensile Elongation Modulus
 @ 300% strength at break @ 300%
 Date Time (MPa) (MPa) (%) (MPa)

 1 6/3/99 4:30 PM 1.16 25.21 895 20.32
 2 6/3/99 1:00 AM 1.19 19.84 862 19.25
 3 6/4/99 9:00 AM 1.20 24.58 860 16.12
 4 6/4/99 4:00 PM 1.27 31.49 840 17.72
 5 6/4/99 2:20 AM 1.27 25.36 846 19.79
 6 6/5/99 9:00 AM 1.40 25.41 824 19.16
 7 6/5/99 4:30 PM 1.20 24.42 801 16.96
 8 6/5/99 3:25 AM 1.29 25.26 852 19.13
 9 6/6/99 9:20 AM 1.24 24.17 840 17.75
10 6/6/99 1:00 AM 1.29 24.27 829 18.26

 Aged 22 hrs. @ 100
 [degrees] C Protein content

 Tensile Elongation ASTM LEAP
 strength at break D5712-95 ELISA
 Date Time (MPa) (%) [micro]g/g [micro]g/g

 1 6/3/99 4:30 PM 945 0.86 102 0.4
 2 6/3/99 1:00 AM 894 0.87 51 0.5
 3 6/4/99 9:00 AM 849 0.83 84 0.5
 4 6/4/99 4:00 PM 820 0.88 33 0.7
 5 6/4/99 2:20 AM 879 0.92 69 0.5
 6 6/5/99 9:00 AM 730 0.86 72 0.5
 7 6/5/99 4:30 PM 873 0.87 84 0.5
 8 6/5/99 3:25 AM 892 0.96 105 0.5
 9 6/6/99 9:20 AM 908 0.89 96 0.5
10 6/6/99 1:00 AM 873 0.86 78 0.6


References

(1.) Latex Foam Rubber, E.W. Madge, John Wiley & Sons, NY, 1962) Data from Hoe Hai San table 5 pp. 64-65.
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Comment:A new grade of natural rubber latex: NC360.
Author:Flannery, Mathew
Publication:Rubber World
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Nov 1, 2000
Words:2702
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