4-E cure systems.

This article will cover 4-E cure systems. The four Es represent: Environmentally-safe: economical: exposure-less: and easy to use

Environmental

The environmental focus with cure systems has been primarily to reduce or eliminate nitrosamine generating curatives. However, there are other environmental and health issues with rubber curatives and rubber additives besides nitrosamines. In addition to nitrosamines, other suspected carcinogens, skin sensitizers, inhalation hazards and physical hazards, and how the risks to the worker and environment can be reduced or eliminated, will also be addressed.

The issue of nitrosamines and the rubber industry has been discussed, debated and studied for over 15 years. Lately, the subject has gathered new interest with nitrosamines and nitrosating chemicals being required to be reported on IMDS (International Material Data System). This system also requires reporting on certain chemicals with health and environmental risks, including skin sensitizers, cancer suspect chemicals, heavy metals and chemicals that pose health risks. The European rubber part manufacturers have already eliminated nitrosamine generating accelerators and curatives from their compounds primarily due to the German regulations on cancer suspected nitrosamines. Most rubber manufacturers refer to non-nitrosamine cure systems as not containing chemicals that will generate any of the nitrosamines on the list adopted by Germany. Higher molecular weight nitrosamines and sterically hindered nitrosamines are not included on this list because the secondary amines needed to generate these nitrosamines are not volatile enough to get airborne at ambient temperatures. However, some manufacturers of pharmaceutical rubber parts require a completely non-nitrosamine cure system that does not generate secondary amines of any kind. Nitrosamines on the German regulated list are shown in table 1.

Nitrosamines are formed by the reaction between secondary amines that are split off from the rubber curatives and oxides of nitrogen that are in the air (figure 1).

[FIGURE 1 OMITTED]

Nitrosamine sources include:

* Chemicals that are nitrosamines and can split off NO, such as N,N-dinitroso pentamethylenetetramine (a blowing agent) and nitrosodiphenylamine (a retarding agent).

* Rubber chemicals that have absorbed NOx on their surface. These include carbon blacks, light-colored fillers such as silica and metal oxides with large surface areas formed at high temperatures in the presence of air.

* Mixtures of molten salt consisting of alkali nitrates and nitrites that are used with salt bath vulcanization systems.

* Oxides of nitrogen, at increased concentration, in the atmosphere. This is caused by fork-lift trucks and other machinery powered by internal combustion engines.

* Some common foods and beverages, such as bacon and beer, contain much higher levels of nitrosamines than the average rubber plant worker is exposed to.

The formation of nitrosamines is dependent upon the secondary amine that is given off by the break-up of the rubber accelerator being volatile enough to come out of the rubber compound and react with NOx in the air. Secondary amines with lower boiling points are more volatile. The boiling point of the secondary amine is dependent on the chemical structure of its -R groups:

* Secondary amines based on lower molecular weight -R groups are more volatile and come out of the compound more readily and at a faster rate than higher molecular weight secondary amines.

* Steric hindrance and branching within the -R groups also affect the boiling point of the secondary amine.

Some of the curatives that are used in the rubber industry split off secondary amines and can therefore produce carcinogenic nitrosamines. These include accelerators from the thiuram, dithiocarbamate and morpholine compound families, as shown in figure 2.

[FIGURE 2 OMITTED]

Thiurams are a good group of accelerators to examine the effects of structure on the formation of nitrosamines (figure 3).

[FIGURE 3 OMITTED]

Some areas that have measurable levels of nitrosamines include: Injection molding units; salt bath vulcanization and testing equipment; vulcanization units such as curing presses; warehouses; compound mixing areas; and finished products, such as the inside of an automobile with EPDM weatherstrip cured with nitrosamine generating cure systems.

Some commonly used materials in the rubber industry that include some cancer suspect chemicals or contain materials that could generate cancer suspect chemicals include:

* Sulfenamides and thiazoles--these accelerators generate 2-mercaptobenzothiazole (MBT) as the active curative and also have small amounts of free MBT present. MBT was put on the Section 313 list because NTP has it listed as a suspect cancer causing substance.

* DOTG (diorthotolyl guanidine)--DOTG contains up to 1% orthotoluidine, which is listed by IARC as a Group 2B possible human carcinogen

* Clays and talc--may contain some crystalline silica (quartz) which IARC and NTP classifies as a Group 1 carcinogen when inhaled.

* Aromatic oils--classified as a Group 1 (proven human carcinogen) by IARC.

* Antimony oxide--classified as an IARC Group 2B carcinogen.

* N-phenyl-1-naphthylamine (PANA)--an antioxidant that is a cancer suspect.

* Phenol-formaldehyde, methylated melamine-formaldehyde (HMMM) and resorcinol-formaldehyde resins--can contain residual amounts of phenol (IARC Group 3 suspected carcinogen) and formaldehyde (Group 2A--IARC and NTP Group 2, OSHA--suspected).

* Carbon black--Group 2B possible human carcinogen (IARC).

* Ethylene thiourea (ETU)--reasonably anticipated (NRC).

Heavy metals are now being watched for both environmental and health issues. Some of the health issues with heavy metals are cancer, nervous system damage and inhalation poisons. Some of the materials in the rubber industry containing heavy metals are: Antimony oxide (filler) contains lead, arsenic and antimony; chromium compounds (colorants); copper compounds (CDMC, CMBT, CDBC accelerators); nickel compounds (NDBC) accelerator, antidegradant; tellurium (TDEC accelerator); iron (colorant); titanium dioxide--contains titanium; zinc oxide--contains zinc and residual amounts of lead; and zinc compounds (zinc accelerators such as ZMBT, ZDMC, ZDBC, ZBEC, zinc stearate, zinc containing slab dips and many other zinc containing compounds).

Skin sensitizers are chemicals that can cause an immediate allergic reaction and/or sensitization over time. Skin sensitizers automatically get a 2 rating on the HMIS scale (moderate health risk). Chemicals that are skin sensitizers are quite common in the rubber industry. They include: thiazole accelerators--MBTS and MBT; p-phenylene diamine antidegradants--6PPD, DPPD, IPPD, etc.; azodicarbonamide foaming agents; trifunctional acrylic esters; hexamethylenetetramine (HMT)--methylene donor; resorcinol and resorcinol-formaldehyde resins--adhesion promoter; thiourea accelerators; and some thiuram and dithiocarbamate accelerators can cause sensitization and/or a temporary allergic reaction if alcohol is consumed before or after handling.

Most every chemical can be classified as a skin, eye and/or respiratory irritant. Some individuals are more susceptible to irritants than others. Overexposure to chemical irritants over time can cause allergic reactions and sensitization.

Many chemicals are corrosive and can cause severe skin burns, eye ulceration and respiratory damage. Usually these chemicals have a HMIS health rating of 3 (serious hazard). Just a few of these chemicals include: Calcium oxide (a desiccant) can cause skin burns; resorcinol and resorcinol based resins can cause skin burns; 1,8-diazabicylo-5,4,0-undecene-7 (DBU); zinc-O,O-di-n-butylphosphorodithioate (ZBPD); and m-phenylenedimaleimide.

Many chemicals used in the rubber industry are hazardous by inhalation, including:

* Highly toxic by inhalation--accelerators CDMC, ZDMC, TDEC, PbDMC.

* Toxic by inhalation--TBzTD, antimony oxide, fumed silica, alkylated melamine-formaldehyde resin (HMMM);

* Other inhalation hazards--amorphous silica, talc, clays, zinc oxide.

* Any chemicals that produce dust that can be inhaled can be considered potential inhalation hazards. This includes the vast majority of the rubber chemicals.

Other chemicals are physical hazards: Most organic rubber chemicals can form explosive dust/air mixtures in dusty plant environments; flammable solids such as sulfur, p,p'-oxybis(benzene sulfonylhydrazide) [OBSH], azodicarbonamide; flammable liquids; and liquids that are used in small amounts and may have to be heated and weighed by hand.

Many rubber chemicals are also hazardous to the environment. Most sulfenamides, thiazoles, guanidines, thiurams and some dithiocarbamate accelerators are IMDG Class 9 environmentally hazardous materials and toxic to aquatic organisms. Wastes that contain heavy metal compounds (Te, Zn, Cu, Ni, etc.), such as TDEC, ZBEC, CMDC and zinc oxide are hazardous wastes per the Superfund law 40 CFR 302.4 and Section 313 Toxic Chemicals 40 CFR 372.65. Thiurams are particularly hazardous to aquatic organisms, and many are classified by DOT as marine pollutants. These are just some of the hazardous waste chemicals. Many rubber additives contain small amounts of hazardous waste chemicals that would cause them to be classified as hazardous for disposal. Individual state and local laws may also classify certain chemicals as hazardous.

Economics--reducing the high cost of non-nitrosamine cure systems

Many non-nitrosamine curatives have been developed in the last 15 years, but their costs have been very high compared to their conventional nitrosamine generating counterparts.

The newer non-nitrosamine curatives are also higher in molecular weight, requiring higher amounts to get the cure state that is needed, and they are slower than their nitrosamine counterparts. The combination of the above has resulted in non-nitrosamine cure systems costing two and even four times as much as conventional nitrosamine cure systems. Some examples of costs are shown in table 2.

Many of the faster non-nitrosamine accelerators, such as ZBEC and TBzTD, have limited solubility levels in many polymers. For example, ZBEC and TBzTD are soluble in EPDM rubber at 0.5 phr or less. The major disadvantages of most non-nitrosamine cure systems are high cost; bloom; slow cure rate; low state of cure; and objectionable smell (most dithiophosphates).

Chem Technologies has spent the last three years developing non-nitrosamine cure systems for customers. Through our research and development, we have developed a cost-effective non-nitrosamine cure booster/activator, identified as CT(GP)100. CT(GP)100 is a proprietary blend designed to increase the cure rate and state of cure of non-nitrosamine cure systems. It has a very low odor. It was tested against a commonly used non-nitrosamine cure booster, zinc diamine dicyanato, sold under the Rhein Chemie tradename of Geniplex 80 (80% active). Zinc diamine dicyanato has an ammonia-like odor. This study was done in both dense and sponge EPDM compounds (table 3).

Results

Rheometer maximum torque

Both CT(GP)100 and zinc diamine dicyanato cure boosters increased the maximum torque significantly compared to the control. Overall, CT(GP)100 showed slightly higher torque values than zinc diamine dicyanato (figure 4).

[FIGURE 4 OMITTED]

Rheometer Tc50

Both CT(GP)100 and zinc diamine dicyanato cure boosters increased the cure rate compared to the control (figure 5).

[FIGURE 5 OMITTED]

Rheometer Tc90 cure time

Both CT(GP)100 and zinc diamine dicyanato cure boosters decreased the Tc90 cure time compared to the control (figure 6).

[FIGURE 6 OMITTED]

Mooney scorch, Ts5

Both CT(GP)100 and zinc diamine dicyanato cure boosters decreased the Ts5 Mooney scorch compared to the control, indicating a faster cure rate (figure 7).

[FIGURE 7 OMITTED]

Tensile strength

The compounds with the cure boosters had slightly higher tensile compared to the control compound (figure 8).

[FIGURE 8 OMITTED]

Elongation

The compounds with the cure boosters had similar elongation at break to the control in the EPDM dense compound. In the EPDM sponge compound, the zinc diamine dicyanato compound had slightly higher elongation, while CT(GP)100 had slightly lower elongation than the control (figure 9).

[FIGURE 9 OMITTED]

100% modulus

The compounds with the cure boosters had higher modulus values compared to the control in the EPDM dense compound. In the EPDM sponge compound, the zinc diamine dicyanato compound had similar modulus to the control, while CT(GP)100 had higher modulus (figure 10).

[FIGURE 10 OMITTED]

Rheometer curves--EPDM dense compounds

The rheometer curves at 160[degrees]C and 175[degrees]C clearly showed that both boosters increased the rate and state of cure. The zinc diamine dicyanato showed some slight reversion at 175[degrees]C. The rheometer curves at 190[degrees]C show that there is some reversion in all three compounds, with the zinc diamine dicyanato showing the most reversion.

Rheometer curves--EPDM sponge compounds

The rheometer curves show that the compounds with the boosters had a higher rate and state of cure.

Cure system cost comparison

Figure 11 compares the non-boosted control to other cure systems that have boosters. The cost index takes into account not only the cost of the ingredients, but also the phr level. If the control cure system is boosted with 0.5 phr TBzTD and an additional 0.7 phr ZBPD, the cost index jumps to 174. This combination was not included in the study, but is an example of what the cost would be if traditional non-nitrosamine accelerators were added. If 0.7 phr of CT(GP)100 is added, the cost index is only 134. If 0.75 phr of zinc diamine dicyanato is added, the cost index jumps to 185.

Exposure-less and easy to use dispersions

With the new focus on environmental issues and less exposure of plant workers to chemicals, purchasing dispersions in pre-weighed batch inclusive bags becomes more attractive. There are advantages in using not only pre-weighed chemicals, but in multi-ingredient dispersion of the rubber chemicals. Most organic rubber chemicals show a slight eutectic reaction when they are combined together with a small amount of plasticizer that is compatible with the rubber compound. Multi-ingredient dispersions disperse quicker and more thoroughly in the rubber compound, allowing for economic advantages and a better final product.

Figure 12 shows that the combined curatives lower the overall melt to below the mixer drop temperature of rubber compounds. This allows a melting dispersion into the rubber, as opposed to a mechanical dispersion, as would be the case with the higher melting point accelerators such as MBTS.

[FIGURE 12 OMITTED]

Figure 13 shows the difference between a non-treated powder blend, a 90% dust suppressed powder blend and a 75% bead blend.

[FIGURE 13 OMITTED]

To demonstrate the advantages of incorporating dispersions into rubber versus dry powders, a study was done on an open laboratory mill. Table 4 shows blends that were added to separate batches of a black rubber compound, and the time of incorporation was measured. The blends were added at equal activity levels. The times show that dispersions greatly decreased the incorporation time on the open mill, and that the multi-ingredient dust suppressed powder and bead dispersions incorporated as fast as single ingredient polymer-bound dispersions.

The dispersions usually come packaged in low melt, batch-inclusive bags that can be added directly to the mixer. Multi-ingredient and single ingredient dispersion product forms include: 90% active dust suppressed powders for internal mixers; 75% active beads for the best dispersion in internal mixers and also for mill addition: 80% active multi-ingredient polymer-bound slab; and 72% active absorbed liquid concentrates for sate addition of liquids.

The individual bags of dispersion can come packaged in throwaway corrugated packaging or in recyclable returnable containers.

Preblended dispersions allow for reduced number of raw materials in the plant; reduced air-borne dust; reduced worker contact with chemicals; less loss of chemicals, allowing for reduced chemical usage; and reduced packaging waste, since preblends typically come in batch inclusive bags. If returnable containers are used, there is no disposal of bags, cardboard containers or pallets.

Most preblended dispersions are treated with a plasticizer, which typically reduces the chemical's health hazards. For example, ZDMC (zinc dimethyldithiocarbamate) is a severe inhalation hazard. After treatment in our blends, the inhalation potential of ZDMC is reduced and it is in a form that is no longer regulated. Another example is sulfur, a flammable solid. Once sulfur has been mixed into our treated blends, it is no longer a flammable solid. The net results are less regulatory reporting and a potentially healthier work force.

Preblended dispersions also offer quality and economic advantages as well.

* Purchasing--reduced number of suppliers and items purchased, allowing for closer control over quality issues such as CpKs, etc.

* Raw material maintenance--fresher materials, because preblends come in sealed batch inclusive bags that are a barrier to moisture.

* Compound staging area--fewer raw materials allowing for fewer weigh-up errors.

* Mixing--faster mix cycles and reduced energy consumption. Faster and better dispersion into the compound is often the result of the effect of a lower melt point when the chemicals are mixed together with a binder.

* Processing--the use of preblended dispersions results in a more consistent, more homogenous compound from the mixer, which in turn results in more consistent product from the processing lines. Batch to batch consistency is also dramatically improved.

* Final product--better appearance, high quality and less scrap.

* The net result of the above advantages is an overall reduction in the amount of chemicals used.

Conclusions

The CT(GP)100 non-nitrosamine cure booster increases both the rate and state of cure, has a low odor and can be used to optimize performance and economics when designing a non-nitrosamine cure system.

Preweighed multi-ingredient dispersions can reduce environmental impact, provide improved economics, reduce/ eliminate worker exposure and are easy to use.

Table 1--German regulated list

N-nitrosodi-n-butylamine (NDBA)
N-nitrosodiethylamine (NDEA)
N-nitrosodi-i-propylamine
N-nitrosodiethanolamine
N-nitrosodimethylamine (NDMA)
N-nitrosodi-n-propylamine
N-nitrosoethylphenylamine (NEPhA)
N-nitrosomethylphenylamine (NMPhA)
N-nitrosopiperidine
N-nitrosomethylethylamine
N-nitrosomorpholine (NMOR)
N-nitrosopyrolidine (NPVR)

Table 2

                   Nitrosamine    Non-nitrosamine

Thiuram               TMTD             TBzTD
Cost/lb.              $1.10           $10.00
Dithiocarbamate       ZDBC             ZBEC
Cost/lb.              $1.10            $7.00
Sulfur donor          DTDM             DTDC
Cost/lb.              $3.00           $14.00

Table 3

EPDM dense rubber masterbatch--highly loaded with
carbon black/oil (550 total phr)

Non-nitrosamine base cure system of:

Sulfur                                    1.15 phr
ZBEC                                      0.50 phr
ZBPD                                      1.35 phr
MBTS                                      1.25 phr

Cure booster
1. Control                                    None
2. CT (GP) 100                            0.70 phr
3. Zinc diamine dicyanato (80% active)    0.75 phr

EPDM sponge rubber masterbatch--medium loaded
with carbon black/oil (250 total phr)

Non-nitrosamine base cure system of:
Sulfur                                    1.15 phr
ZBEC                                      0.80 phr
ZBPD                                      1.35 phr
MBTS                                      1.25 phr
OBSH                                      1.15 phr

Cure booster
1. Control                                    None
2. CT (GP) 100                            0.70 phr
3. Zinc diamine dicyanato (80% active)    0.75 phr

Note: all phr (parts per hunitrred rubber) are 100%
active except where indicated otherwise

Test procedures
The curatives and cure boosters were added to the
  masterbatch on a lab 2-roll mill
MDR rheometer @ 160[degrees]C, 1750[degrees]C and 190[degrees]C,
  ASTM D5289
Mooney, ML 1+4 @ 121[degrees]C, ASTM D1646
Durometer A hardness, ASTM D2240
Stress strain, ASTM D412

Table 4--blends added to separate batches

100% active powder blend (compound back-rolled)
  4:10
90% active dust suppressed powder blend dispersion
  0:45
80% active dust suppressed bead blend dispersion
  0:35
80% active polymer bound single ingredient dispersion
  0:40

Table 5--data

TP-2004-003                          2             3            4

                                  Control      CT(GP)100      Zinc
                                 no booster                  diamine
                                                            dicyanato

                                   Dense         Dense        Dense
                                    EPDM         EPDM         EPDM
                                  compound     compound     compound

MDR rheometer @ 160[degrees]C, ASTM D5289
Maximum torque                      1.16         1.37         1.37
  (Newton-meter)
Tc10% (minutes)                     1.71         1.25         1.25
Tc50% (minutes)                     3.18         2.84         2.72
Tc90% (minutes)                     7.86         7.49         7.42
Ts1 (minutes)                       0.69         0.60         0.59
Ts2 (minutes)                       2.21         1.51         1.60

MDR rheometer @ 175[degrees]C, ASTM D5289
Maximum torque                      1.37         1.46         1.42
  (Newton-meter)
Tc10% (minutes)                     0.84         0.63         0.65
Tc50% (minutes)                     1.78         1.41         1.32
Tc90% (minutes)                     6.11         4.39         4.01
Tsl (minutes)                       0.46         0.39         0.40
Ts2 (minutes)                       0.99         0.72         0.76

MDR rheometer @ 190[degrees]C, ASTM D5289
Maximum torque                      1.17         1.29         1.25
  (Newton-meter)
Tc10% (minutes)                     0.49         0.43         0.41
Tc50% (minutes)                     0.86         0.75         0.71
Tc90% (minutes)                     2.75         1.84         1.77
Ts1 (minutes)                       0.30         0.36         0.27
Ts2 (minutes)                       0.57         0.48         0.47

Mooney ML 1+4 @ 121[degrees]C, ASTM D1646
Ts5 (minutes)                       15.0         11.3         10.4
Viscosity @ 4 minutes               36.6         38.4         37.8
  (MU)

Hardness, ASTM D2240
Durometer A (points)                 67           67           67

Stress strain, ASTM D412
Tensile (MPa)                       9.15         9.80         9.86
Elongation @ break (%)              576           557          530
100% modulus (MPa)                  2.58         2.85         2.81
200% modulus (MPa)                  5.04         5.61         5.70
300% modulus (MPa)                  6.59         7.30         7.47

TP-2004-003                          9            12           10

                                  Control      CT(GP)100      Zinc
                                 no booster                  diamine
                                                            dicyanato

                                   Sponge       Sponge       Sponge
                                    EPDM         EPDM         EPDM
                                  compound     compound     compound

MDR rheometer @ 160[degrees]C, ASTM D5289
Maximum torque                      1.26         1.40         1.34
  (Newton-meter)
Tc10% (minutes)                     0.81         0.62         0.62
Tc50% (minutes)                     1.72         1.43         1.55
Tc90% (minutes)                     6.25         5.41         5.51
Ts1 (minutes)                       0.52         0.39         0.42
Ts2 (minutes)                       0.97         0.74         0.73

MDR rheometer @ 175[degrees]C, ASTM D5289
Maximum torque                      1.21         1.33         1.29
  (Newton-meter)
Tc10% (minutes)                     0.50         0.41         0.40
Tc50% (minutes)                     0.93         0.79         0.84
Tc90% (minutes)                     3.38         2.59         2.74
Tsl (minutes)                       0.35         0.28         0.29
Ts2 (minutes)                       0.59         0.47         0.46

MDR rheometer @ 190[degrees]C, ASTM D5289
Maximum torque                      1.13         1.23         1.15
  (Newton-meter)
Tc10% (minutes)                     0.33         0.29         0.28
Tc50% (minutes)                     0.56         0.50         0.51
Tc90% (minutes)                     1.52         1.20         1.14
Ts1 (minutes)                       0.24         0.20         0.20
Ts2 (minutes)                       0.39         0.34         0.34

Mooney ML 1+4 @ 121[degrees]C, ASTM D1646
Ts5 (minutes)                       4.8           3.1          3.6
Viscosity @ 4 minutes               38.7         57.7         45.3
  (MU)

Hardness, ASTM D2240
Durometer A (points)                 53           55           52

Stress strain, ASTM D412
Tensile (MPa)                      12.63         12.92        14.12
Elongation @ break (%)              564           471          675
100% modulus (MPa)                  1.52          1.90         1.43
200% modulus (MPa)                  3.41          4.49         2.95
300% modulus (MPa)                  5.46          6.90         4.64

Figure 11--cure system cost index
comparison

Control
No booster       100
Boosted
0.5T BzTD
+0.7 ZBPD        174
Boosted
0.7 CT(GP)100    134
Boosted
0.75 zinc
diamine
dicyanato        185

Note: Table made from bar graph.