New advances in millable urethanes.

Millable urethanes are polymers that are known for their excellent abrasion and strength properties, while being able to be processed on conventional rubber equipment. Existing millable urethanes are primarily used in applications that take advantage of these properties.

A newly developed polymer also takes advantage of (and improved upon) urethane's excellent permeability and fluid resistance for automotive applications.

A second polymer has been developed for FDA applications requiring contact with wet foods. Applications for this polymer can include belting for food processing, milking inflations, etc.

Development of these polymers and compounding studies for specific applications will be discussed. Compounding techniques for achieving high hardness compounds with excellent processibility and properties will also be evaluated.

Polyurethanes can be tailored to meet specific application requirements, from very hard cellular foams to very soft and flexible gels. This can be done by the correct choice of raw materials to be used in the reaction. Polyurethanes are prepared from polyhydroxyl-containing species and polyisocyanates. The polyhydroxyl materials usually are selected from polyetherols or polyesterols. These make up the soft segment of the polymer. Other hydroxyl containing species in the mix are chain extenders. These are part of the hard segment. The other part that makes up the hard segment is the diisocyanate.

The polyols employed are made up of either polymers of propylene and ethylene oxide or tetrahydrofuran. For millable gums, polytetrahydrofuran polyols such as polytetramethyleneetherglycol (PTMEG) are employed because of the superior physical properties and hydrolytic stability that result. Polyester polyols are usually adipates of lower molecular weight diols, such as ethylene glycol, propylene glycol, butanediol (BDO) or hexanediol.

Chain extenders are usually chosen from the same list of raw materials used to prepare the polyesters, depending on the end polymers' performance needs. Examples of chain extenders are ethylene glycol, diethylene glycol, butanediol and hexanediol.

There are several commercially available diisocyanates used in the production of millable urethanes. They include:

* MDI--methylene diphenyl diisocyanate;

* TDI--toluene diisocyanate; and

* [H.sub.12] MDI--methylene dicyclohexane diisocyanate.

Diisocyanates are chosen depending upon the specific requirements of the application. For example, [H.sub.12] MDI is typically used for polymers requiring UV stability ([H.sub.12] MDI has no aromatic character and hence has good resistance to UV radiation).

Millable gums, in general, are "under-indexed" thermoplastic urethanes. What this means is that there is a stoichiometric deficiency of isocyanate groups compared to hydroxyl groups in the overall reaction. Also, in order to properly cure these materials like rubber, labile hydrogens must be present to radically cure with the correct promoter. This can be accomplished by the use of MDI as the isocyanate or by introducing carbon-carbon double bonds in the chain extender.

Urethanes, in general, have excellent properties in many areas, including abrasion resistance, tear strength, oil resistance and ozone resistance. A comparison of urethane to several other commonly used polymers is shown in table 1.

This article discusses two new polymers, one developed for automotive applications and one for food handling applications. It also discusses methods of formulating high hardness compounds having properties that have been traditionally difficult to achieve in millable urethanes.

Experimental

Preparation of the urethanes was accomplished by weighing 3 kg of the appropriate polyol into a one gallon can. The can was then blanketed with nitrogen or argon. A pre-weighed amount of chain extender was then added to the can, along with enough tin catalyst to make about 10 ppm tin level in the final gum. The contents of the can were then agitated on high speed for 30 minutes to ensure a homogeneous mixture. If a polyester was employed, 45 g of a carbodiimide stabilizer (Stabaxol P200) was added to react with any residual acid groups on the polyester. For polyester-based gums, the can was sealed and allowed to stand in a 77[degrees]C oven overnight to allow the reaction to take place. Polyether based gums can be used immediately. The isocyanate demand was calculated to bracket several index points above and below 1:1 NCO/OH. This was to ensure that usable Mooney viscosity material was produced. (This was done at stoichiometries from 0.96:1.0 to 1.03:1.0). 275 grams of pregum mixture (mixture of polyol, chain extender and catalyst) was weighed into a one-pint can whose insides had been previously coated with a mold release agent. The appropriate amount of diisocyanate was then added to the can and the contents were mixed at 1,000 rpm with a Jiffy mixer for 60 seconds. The can was then degassed for a short period of time to remove any air that had become mixed into the liquid and capped. When all cans were complete, they were put into a 104[degrees]C oven for 72 hours to allow the gum to form. The cans were then removed from the oven and allowed to cool. Once cooled, the gum was removed from the can and milled to get samples for Mooney viscosity. Once the usable Mooney viscosity was determined for this gum, the remaining pregum was reacted with the appropriate amount of diisocyanate in the same way to make a larger quantity of material for evaluation purposes. Mooney viscosities were all determined on an Alpha Technology Mooney MV2000 instrument.

Compounds were mixed on a two-roll mill in a single-pass mix procedure. Samples were tested for cure characteristics with a Tech-Pro MDpt using procedures in ASTM D6204. Samples were cured in an electric press to prepare samples for testing. ASTM test methods were used for all tests. Abrasion testing was done per ASTM D5963, commonly known as the D1N abrasion test.

Millable urethane polymer for low permeability automotive applications

Urethanes have very good resistance to permeability of gases, approaching that of butyl rubber (IIR). A comparison of the nitrogen permeability of several commercial grades of millable urethane vs. CR, NBR and IIR is shown in figure 1. Polyester millable urethanes have very good permeability, comparable to slightly better than NBR, but defensive to IIR. The millable polyether polyurethane EU-34 has higher (poorer) permeability, similar to that of CR. Because the requirements of several automotive applications require the optimum resistance to nitrogen permeability, a program to develop an improved polymer was undertaken.

Development of AU-28 millable urethane

The requirements for automotive applications such as hydropneumatic suspension parts include both low temperature flexibility properties and very low nitrogen permeability. To minimize permeability, the soft segment needed to be made as amorphous as possible. This meant that a high ethylene glycol-containing polyester should be chosen. However, too high an ethylene glycol content would mean that we would lose low temperature properties due to the high crystallinity. We conducted a series of experiments where the polyester backbone was varied to include a varied ratio of ethylene to propylene glycol. The molecular weight was varied between 2,000 and 3,000. In this fashion, we were able to pick the correct polyester structure that gave us the low temperature properties we needed, as well as the low air permeability (table 2). The polymer with an 80:20 ratio of ethylene glycol:propylene glycol and a 2,000 molecular weight was chosen, as it had the required low temperature properties and a balance of other properties (including permeability, tested on other samples not shown here).

Figure 2 shows the permeability of the new polymer, AU-28, compared to the other polymers. It shows an improvement over the previously best millable urethanes for permeability, and gets closer to the permeability of IIR.

[FIGURE 2 OMITTED]

Applications

The polymer AU-28 was developed for applications that can take advantage of its excellent permeability and low temperature properties, along with excellent oil resistance. One such application is a hydropneumatic suspension system where nitrogen gas, in conjunction with a hydraulic fluid, provides cushioning for automobiles.

Table 3 shows a comparison of the properties of the compounded AU-28 vs. values that were targeted for this application. For some dynamic applications, a lower modulus compound is desirable. Table 4 shows a lower modulus compound, achieved by using the less-reinforcing black N990. Physical properties, including permeability, compression set and abrasion resistance, are excellent.

AU-28, because of its excellent permeability, oil resistance and other properties, is being looked at for other applications such as ball joints and diaphragms.

Millable urethane polymer for food handling applications Preparing a gum that meets specific requirements for food contact applications, as indicated by 21CFR177, is very restrictive. For 21CFR177.1680, the raw materials can be chosen from the list published by the federal government. The soft segment, chain extender and isocyanates portions are all listed; so that if one prepares a material from the ingredients on the list, the end product should be certifiable under that requirement. For instance, AU-66 is prepared from a polyester, chain extender and isocyanate that are on the list of certifiable raw materials in 21CFR177.1680. If the end user produces an item using AU-66 and with additional ingredients on the list, they should be able to get that item certified under that title. Urethanes meeting 21CFR177.1680, however, can only be used for dry food applications.

The requirements for 21CFR177.2600 are much more stringent, however. Only two combinations of raw materials for polyurethanes are allowed. They are:

* Polyether urethanes prepared from PTMEG, BDO and MDI; or,

* polyester urethanes prepared from butylene adipate, BDO and MDI.

Since wet food handling applications would, by definition, need good resistance to aqueous solutions, the most desirable polymer would be a polyether; so that is the polymerization route taken here. As an aside, it was not possible to prepare a usable millable gum using the adipate due to the highly crystalline nature of the polyester. This crystallinity translated into a very hard, barely millable material.

Several combinations of molecular weight and chain extender level resulted in the final polyether millable urethane product (table 5). As can be seen, the addition of the lower molecular weight PTMEG to the 2000 molecular weight material made the polymer easier to process and gave much improved properties. The best polymer was the blended polyol with the "Y" level of BDO, and this polymer is called EU-26.

Applications

Belting--general (conveyor) belting Polymers with acceptance for wet food applications have great utility in the handling of foods such as grains, meats and other items. Important properties for these applications are tear resistance, flex resistance and resistance to any fluids, fats and oils which may come in contact with the belt. EU-26, being an ether urethane, is inherently good for resistance to water; and urethanes are generally very good for resistance to fats and oils that are in foods.

Table 6 shows a base compound for a conveyor belt and a slightly higher modulus and hardness compound that was achieved by increasing the silica and methacrylate coagents. Tear strength was slightly improved in the harder compound, while tensile strength, abrasion resistance and compression set were lower.

In an effort to improve tear resistance, levels of peroxide (dicumyl peroxide) were evaluated. Table 7 shows that lower peroxide levels gave improved tear strength and abrasion resistance. Curing at a lower temperature, 145[degrees]C vs. 15 I[degrees]C, also improved tear strength with no significant change in other properties.

A range of silica levels from 25 to 45 parts was evaluated to develop compounds for other applications. The data in table 8 show the expected trends in properties. Tensile strength, compression set and abrasion resistance were somewhat lower with higher silica, while tear strength was improved.

Milking inflations

Rubber milking inflations or milking liners for milking cows and other mammals are commonly made from nitrile or silicone rubber, although liners made of natural rubber and blends of natural rubber with SBR or NBR are also used. Important properties for this application are good flex life, good aging resistance, good resistance to milk products and good abrasion resistance. Silicone rubber gives longer life in use than NBR or NR (and blends), primarily because of its better resistance to milk fats and aging. Urethane rubber, because of its outstanding abrasion resistance, may lead to longer life products for this application.

A compound developed for milking inflations is shown in table 9. All ingredients are in compliance with 21CFR17-7.2600.

Other new developments

High hardness compounds with good properties, processibility It has always been desirable to be able to make millable urethane compounds with high hardness (90+ durometer A) that can compete with cast urethane products, as cast urethanes have excellent properties at these high hardnesses. High hardness compounds with millable urethanes have always been troublesome to compound and manufacture. Each of the three types of cure systems gave its own problems:

Sulfur cure systems rely on high filler loadings to get high hardness, which typically results in very high viscosity and scorchy compounds. Resins or thermoplastic materials can be blended to help increase hardness, but they are often difficult to blend with the millable urethane, and properties are usually compromised, most typically compression set.

Isocyanate cures can be used with certain polymers to achieve high hardness with very good properties, but scorchiness and extremely short shelf life (several days or less) limit their practical use.

Peroxide cure systems are most commonly used for high hardness compounds, usually with the inclusion of a liquid trifunctional methacrylate (TMPTMA) to increase hardness, along with a moderate level of reinforcing filler. The liquid coagent allows reasonable viscosity in these hard compounds. The downside of this approach is that compounds can tend to have marginal properties due to the high degree of crosslinking with the TMPTMA and peroxide, giving low elongation and low tear strength.

Recent work with high levels of tri-functional coagent, along with high levels of a di-functional coagent, have shown promise in giving good processing compounds in the 90+ durometer A hardness range that maintain good strength and elongation properties.

A high hardness compound based upon a clear millable urethane, EU-97, using high methacrylate coagent levels, is shown in table 10. By reducing the reinforcing filler (fumed silica) and adding the di-functional methacrylate DEGDMA, compound viscosities are greatly reduced, while physical properties are maintained. Because of the inherent clarity of the polymer, transparent or translucent compounds can be made that have the same look as castable urethane materials.

A study was done evaluating various levels of the two coagents TMPTMA and DEGDMA in polyester millable urethane AU-66M. This black compound had a constant level of N-550 carbon black and both of the coagents were varied from 0-20 parts. Data are shown in table 11 with some of the key properties charted in figures 3-11 and discussed below.

[FIGURE 3-11 OMITTED]

Scorch, as measured by the time to a one-point rise in the torque of the curemeter, and the tc90, time to 90% cure, both show that the TMPTMA is the primary factor. TMPTMA gave faster cure times and shorter scorch times with higher levels, while the DEGDMA had a lesser effect on these properties (figures 3 and 4).

[FIGURE 4-5 OMITTED]

Hardness was affected significantly by both coagents (figure 5), although the TMPTMA gave about a 70% higher hardness increase than did the DEGDMA (20 parts of TMPTMA hardened 22 points; 20 parts of DEGDMA hardened 13 points).

100% modulus and elongation at break showed higher modulus and lower elongation with higher levels of TMPTMA, but increasing the DEGDMA had a very minor effect on these properties (figures 6 and 7).

[FIGURE 6-7 OMITTED]

Tensile strength showed improvement with higher levels of TMPTMA, but a decline with higher levels of DEGDMA (figure 8). Tear strength showed a decline as levels of TMPTMA increased, especially at low DEGDMA levels (figure 9).

[FIGURE 8-9 OMITTED]

Abrasion resistance testing generally showed poorer abrasion resistance as the levels of either coagent increased. The exception is with low levels of TMPTMA (less than 5 parts): Here, the abrasion resistance improved with increasing levels of DEGDMA (figure 10), giving superb abrasion resistance with 10-20 parts of DEGDMA.

[FIGURE 10 OMITTED]

Higher levels of TMPTMA gave better (lower) compression set and DEGDMA gave poorer (higher) set (figure 11).

Summary

Millable urethanes are specialty polymers that have found use in applications that need the unique combination of properties that urethanes offer including excellent abrasion resistance, tensile and tear strength, oil resistance and ozone resistance. Urethanes are inherently very good for resistance to gas permeability as well, but this property was improved upon with development of a new polymer which combines excellent nitrogen penneability with excellent low temperature flexibility. Another new polymer filled a gap in the range of commercial polymers by being developed for wet food handling requirements. These two new polymers expand the range of applications where millable urethanes could be used.

The use of blends of tri-functional methacrylates with difunctional methacrylates, at relatively high levels, has been found to give hard compounds (90+ durometer A) with excellent properties, which have been difficult to achieve in the past. This also expands the range of applications for millable urethanes, making them more competitive with cast urethanes in high hardness applications

Table 1

Property/           Urethane   Polychlor-   Nitrile   Natural    EPDM
Material                           oprene              rubber

Tensile strength           E          G-E         F       G-E       F
Hardness range,        15-95        10-95    20-100    30-100   25-90
 duro. A
Max service              100          120       120       100     175
 temp. [degrees]C
Ozone                      E          F-G         P         P       E
 resistance
Cut resistance             E            G         F         G       G
Tear strength              E            G         F         G       F
Compression              F-G            G       F-G         F       G
 set
Abrasion                   E          G-E         F         G       F
 resistance
Heat build-up            F-G            E         P         E     F-G

E--excellent; G--good; F--fair; P--poor

Table 2

Ethylene glycol:          70:30   80:20   70:30   80:20   60:40
 Propylene glycol ratio
Molecular weight          2,000   2,000   3,000   3,000   3,000

Cured physical
 properties                                                      Target
Durometer A                  65      65      65      62      46      68
100% modulus, MPa           3.8     3.5     3.9     3.2     0.7     4.8
2000% modulus, MPa         14.1    13.0    15.6    12.4     1.0    16.8
Tensile strength, MPa      23.9    26.0    27.4    29.4    17.1    33.1
Elongation, %               265     279     269     306    >500     289
Tear, die C, kN/m          21.2    20.8    17.9    21.5    20.8    24.0
Bashore resilience, %        29      33      47      47      NT      40

Gehman low temperature,
 [degrees]C
TR-10                       -30     -31     -36     -35     -33
TR-30                       -27     -28     -32     -32     -30     -31
TR-50                       -24     -25     -29     -29     -25     -28
TR-70                       -19     -20     -26     -24     -18     -25

Table 3

Cured properties              Target   AU-28

Durometer A                       72      71
100% modulus, MPa                3.4     2.7
200% modulus, MPa                9.5     7.8
300% modulus, MPa               17.4    15.5
Tensile strength, MPa           30.8    28.7
Elongation, %                    467     469
Tear, die C, kN/m               38.5    32.4
Bashore resilience, %             35      31
Compression Set, 22 h./            4       8
  80[degrees]C, %
Abrasion loss, [mm.sup.3]         88      84
Gehman T10, deg. C               -31     -31
Nitrogen permeability,           6.5     5.7
  cm2/sec-atm x [10.sup.-8]
Fluid resistance ASTM oil
  #1, 70 h./100[degrees]C
Hardness change, points            0       0
Volume increase, %                 5       6

Table 4

Millable urethane AU-28 (Millathane 28)   101.5
Stearic acid                                0.3
Process aid--Struktol WB222                 1.0
Low melt polyethylene AC617A                1.0
N990 black                                 20.0
TAC (triallyl cyanurate)                    0.5
Varox DBPH-50                               4.0

Cured Properties                          128.3
Durometer A                                  62
100% modulus, MPa                           2.2
200% modulus, MPa                           5.3
300% modulus, MPa                          13.3
Tensile strength, MPa                      27.3
Elongation, %                               446
Tear, die C, kN/m                          22.6
Bashore resilience, %                        44
Abrasion loss, [mm.sup.3]                  77.4
Compression set, 22 h./80[degrees]C, %      7.2
Nitrogen permeability at 80[degrees]C,      3.2
  cm2/sec-atm x [10.sup.-]9

Table 5

PTMEG MW                2,000    2,000   2,000    2,900   2,900

BDO level                   X   X*0.33   X*0.5   X*0.33       X
Durometer A                         50
100% modulus, MPa                  1.2
200% modulus, MPa         Too      1.7     Too      Too     Too
300% modulus, MPa        hard      2.4    hard     hard    hard
Tensile strength, MPa              7.8
Elongation, %                      575
Tear, die C, kN/m                 23.6

PTMEG MW                2,000/   2,000/    2,000/
                          250      250       250

BDO level                   Y    Y*0.9    Y*0.75
Durometer A                66       67        56
100% modulus, MPa         1.8      2.0       1.2
200% modulus, MPa         3.6      3.9       1.8
300% modulus, MPa         6.2      6.8       3.2
Tensile strength, MPa    21.9     16.4      20.4
Elongation, %             461      421       647
Tear, die C, kN/m        23.8     22.8      23.8

Table 6

EU-26 (Millathane 26)                     100    100
Stearic acid                              0.5    0.5
Precipitated silica (HiSil 243 LD)         30     35
Titanium dioxide                            2      2
Process aid Struktol WB222                  1      1
BGDMA (SR 297)                             --      5
TMPTMA (SR-350)                             8     10
DiCup 40C                                   5      5
Press cure 8 minutes at 160[degrees]C
Hardness, durometer A                      76     83
100% modulus, MPa                         2.5    3.7
200% modulus, MPa                         4.8    6.8
300% modulus, MPa                         8.9   11.9
Tensile strength, MPa                    22.4   19.2
Elongation, %                             452    394
Tear, die C, kN/m                        28.5   32.4
Abrasion loss, [mm.sup.3]                  68     97
Compression set, 22 h./70[degrees]C, %     26     29

Table 7

EU-26 (Millathane 26)                    100     100     100
Stearic acid                             0.3     0.3     0.3
Precipitated silica (Ultrasil VN3)        35      35      35
Titanium dioxide                           3       3       3
Process aid Struktol WB222                 1       1       1
DEGDMA (SR-231)                            5       5       5
TMPTMA (SR-350)                            8       8       8
DiCup 40C                                  3       4       5
Press cure at 145[degrees]C, minutes      20      20      21
Durometer A                               80      80      80
100% modulus, MPa                        2.3     2.7     2.9
200% modulus, MPa                        3.6     4.4     5.1
300% modulus, MPa                        6.0     7.5     8.8
Tensile strength, MPa                   22.4    25.5    25.9
Elongation, %                            615     565     510
Tear, die C, kN/m                       45.2    42.5    40.3
Press cure at 151[degrees]C, minutes      17      16      16
Durometer A                               80      80      80
100% modulus, MPa                        2.6     3.0     3.5
200% modulus, MPa                        4.4     5.4     6.5
300% modulus, MPA                        7.5     9.8    11.8
Tensile strength, MPa                   23.0    26.3    23.6
Elongation, %                            530     480     425
Tear, die C, kN/m                       39.4    38.7    33.4
Abrasion loss, [mm.sup.3]              74-78   77-86   81-82
  145[degrees]C-151[degrees]C cures

Table 8

EU-26 (Millathane 26)                       100     100     100
Stearic acid                                0.3     0.3     0.3
Precipitated silica (Ultrasil VN3)           25      35      45
Titanium dioxide                              3       3       3
Process aid Struktol WB222                    1       1       1
DEGDMA (SR-231)                               5       5       5
TMPTMA (SR-350)                               8       8       8
DiCup 40C                                     4       4       4
                                          146.3   156.3   166.3
Press cure at 151[degrees]C, minutes         22      20      19
Durometer A                                  77      80      84
100% modulus, MPa                           2.7     3.1     3.2
200% modulus, MPa                           4.8     5.1     4.9
300% modulus, MPa                           8.3     8.3     7.8
Tensile strength, MPa                      25.2    24.3    22.8
Elongation, %                               510     515     555
Tear, die C, kN/m                          34.0    39.7    48.5
Bashore resilience, %                        43      41      36
Abrasion loss, [mm.sup.3]                    91     105     151
Compression set. 22 hr./70[degrees]C, %      28      27      33

Table 9

EU-26 (Millathane 26)                      100
Stearic acid                               0.3
N774 black                                  10
Water-washed clay (Polyfil HG90)             5
Low melt polyethylene AC617A                 1
VVO (Akrofax 11LG)                           5
TMPTMA (SR-350)                              3
DiCup 40C                                  4.8
Process aid Struktol WB222                   1

Press cure at 11'/160[degrees]C
Durometer A                                 55
25% modulus, MPa                           0.7
100% modulus, MPa                          1.3
200% modulus, MPa                          1.9
300% modulus, MPa                          3.3
Tensile strength, MPa                     19.9
Elongation, %                              643
Tear, die C, kN/m                         24.2
Bashore resilience, %                       58
Compression set, 22 hr./70[degrees]C, %     23

Table 10

EU-97 (Millathane 97)                      100    100    100
Fumed silica (Wacker HDK N20)               45     35     30
DBEEA (TP-95)                                2      2      2
Process aid AC617A low-melt PE               2      2      2
Stearic acid                               0.5    0.5    0.5
PEG (Carbowax 3350)                        2.5      2      2
Vinyl silane (Silquest A-172)                2    1.5    1.5
AO Irganox 1010                           0.25   0.25   0.25
Dicumyl peroxide                             1      1      1
TMPTMA (SR-350)                             20     20     20
DEGDMA (SR-231)                             --     10     20
Mooney viscosity, ML(1+4)/100[degrees]C    146     52     36

Cured properties
Hardness, durometer A                       91     94     95
100% modulus, MPa                          6.5   12.0   13.7
200% modulus, MPa                         12.0     --   21.1
Tensile strength, MPa                     21.1   16.9   22.7
Elongation, %                              315    180    220
Tear, die C, kN/m                         35.2   28.8   30.4
Compression set, 22 hr./70[degrees]C        32     26     21
Bashore resilience, %                       50     43     43

Table 11

AU-66M (Millathane 66M)      101.5   101.5   101.5   101.5   101.5
Stearic acid                   0.3     0.3     0.3     0.3     0.3
N-550 black                   40.0    40.0    40.0    40.0    40.0
DBEEA (TP-95)                  5.0     5.0     5.0     5.0     5.0
Process aid Struktol WB222     1.0     1.0     1.0     1.0     1.0
Process aid AC617A PE          1.0     1.0     1.0     1.0     1.0
Varox DBPH 50                  5.0     5.0     5.0     5.0     5.0
DEGMA (SR-231)                10.0    20.0      --    20.0      --
TMPTMA (SR-350)               10.0      --      --    20.0    20.0
Total                        173.8   173.8   153.8   193.8   173.8

Mooney viscosity
 ML(1+4)/100[degrees]C          40      35      76      23      43

MDR,30'/175[degrees]C
 ML, dNm                       0.9     1.0     1.2     0.6     1.0
 MH, dNm                      28.7     7.7    13.0    63.5    48.5
 ts1, minutes                  0.3     0.5     0.7     0.2     0.2
 t50, minutes                  2.8     2.4     2.2     0.7     1.5
 t90, minutes                  6.5     6.4     5.5     4.1     4.8

Press cure minutes at
  175[degrees]C                  7       6       6       4       5
 Durometer A                    84      80      67      95      89
 Durometer D                    32      25      20      50      40
 100% modulus, MPa             6.5     2.2     1.9    10.1     9.5
 200% modulus, MPa            14.0     4.3     3.0    16.1    18.6
 300% modulus, MPa            19.0     7.0     9.9      --      --
 Tensile strength, MPa        21.1    12.3    19.2    17.7    23.4
 Elongation, %                 365     570     635     216     270
 Tear, die C, kN/m            35.4    38.9    43.9    38.2    30.5
Abrasion resist.
  [mm.sup.3] loss               65      43      67     127      81
Bashore resilience, %           39      38      50      38      38

Compression set, %
 22 hr./70[degrees]C            22      61      18      21      12
 22 hr./100[degrees]C           28      68      37      23      22
 22 hr./125[degrees]C           48      80      62      24      22