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70 Hour Heat Resistance of Materials - ASTM D395 (As a percentage of lost dimension - lower is better)

212F248F

275F302F

HNBR

Nitrile

Neoprene0%10%20%30%40%50%60%70%80%90%

Handbook for Rubber Materials in Automotive Air Conditioning

Applications

Santech Industries, Inc. 2450 Handley Ederville Road Fort Worth, Texas 76118 USA

November 1993

Santech Vision

Our focus is to be the leader in Sealing Technology by recognizing current and future customer needs.

Santech is committed to an ethical and professional work environment where individuals are empowered to make their own decisions and improvements.

Table of Contents Air Conditioning Applications Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Simplified Air Conditioning System. . . . . . . . . . . . . . . . . . . 2 Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Chemical Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Materials and Sealing Theory Rubber Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Seal Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Testing Types of Relevant Testing . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Analysis of Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Summary of Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Cost Analysis of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Original Equipment Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Exposing Myths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Recommendations for Rubber Materials . . . . . . . . . . . . . . . . . . . 17 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1993 Santech Industries, Inc. All Rights Reserved.

Santech Industries, Inc. 2450 Handley Ederville Road Fort Worth, TX 76118 USA

817-589-1212

Santech Industries Page 1 Rubber Handbook

Air Conditioning O-Ring Applications All current rubber applications in mobile air conditioning are non-moving (static). They can be divided into several types of seal gland designs. The cut-away drawings below illustrate some of the designs.

Face Seal

Before Compression During Compression

Springlock Fitting

Spring O-Ring

During Compression

Double Groove Face Seal

During Compression

Static ID/OD Shaft Seal

During Compression


Simplified Air Conditioning System The following illustration depicts a simplified system divided into three parts and named for descriptive purposes:

HIGH SIDE - The discharge side of the compressor through the hottest half of the condenser.

LIQUID LINE SIDE - From the coolest half of the condenser through the orifice tube or expansion valve.

LOW SIDE - From the orifice tube or expansion valve to the suction side of the compressor. Including the evaporator.

Compressor Condenser

Expansion Valve

Evaporator

High Side

Liquid Line Side

Low Side

Each section of the system has a different temperature and pressure. Description Temperature Pressure High Side High High Liquid Line Side Moderate High Low Side Low Low The highest pressures are on the high side and the liquid line side. The highest temperature is on the high side. Note that the temperature drops after the liquid goes through the condenser.


Temperatures It is important to understand that operating temperatures in the actual underhood environment vary from vehicle to vehicle and are not the same as temperatures on the test stand. An air conditioning system designed for a mobile application

would normally only see temperatures below 200°F. Factors effecting this are the heat generated by the compressor and the normal pressure/temperature relationship of the refrigerant used in the system. When a system is operating on a test stand or in an application with few outside heat influences, then the system temperatures will be below 200°F. Truck and agricultural applications would be good examples with expectations of the lower temperatures. Unfortunately, automobile applications do not generally fall in the lower temperature range. Higher temperatures in the 250-290°F are generated by external heat sources. Small

engine compartments, exhaust manifold, engine block, and lack of ventilation are the general external heat factors. The major factor in external heat is the mounting of the compressor, hoses, and other system components and their proximity to the exhaust manifold. Several of the original equipment manufacturers presented some high temperature figures that will give indications of what certain vehicles and systems might see. I have listed them below: Manufacturer Higher Ranges Notes: Chrysler 250-270°F When the system is next to the exhaust

manifold Ford 250-300°F 250°F when the system is in perfect

charge. Up to 300°F when the system is undercharged. Some applications run up to 250°F with the system off.

General Motors 290°F Have measured 290°F at the compressor high pressure switch port.

Volvo 248°F + The system is designed to operate under 248°F during normal driving.

The higher temperature ranges are on the high side of the system and are vehicle dependent. However, a rebuilt compressor or the replacement seals furnished to the repair shop could be put into one of these vehicle-specific applications. Normal engineering discipline would encourage design of the seal material for the highest temperature application.

300

200

100

F

300

200

100

F


Chemical Environment Refrigerants: Refrigerants used by the original equipment manufacturers in automotive air conditioning systems are R-12 and R-134a. Some truck and trailer refrigeration units operate with R-22 and others. Other blends of refrigerants and chemicals that can work as refrigerants are available to the aftermarket. Since all automotive manufacturers in the world have chosen R-134a as the replacement for R-12 and since each of the other alternatives require different system changes, this discussion will be limited to R-12 and R-134a with their appropriate lubricants. Lubricants: Each refrigerant has lubricants designed to work with it in the system. R-12 uses mineral oil. All automobile manufacturers are using Polyalkylene Glycol Oils (PAG) with R-134a in new production. Some automobile manufacturers, (Volvo, Jaguar, and Rolls Royce) are recommending Polyol Ester Oils (ESTER) be used with R-134a in conversions from a R-12 system to a R-134a system. Refrigerant/Lubricant Combinations: The three combinations that were investigated are: R-12 with Mineral Oil R-134a with PAG R-134a with ESTER

Summary of Applications: ♦ All rubber applications in the systems are static ♦ Rubber may be exposed to three possible refrigerant/lubricant mixtures ♦ The system temperature is vehicle dependent ♦ The highest temperatures are on the high side ♦ Automotive applications are higher temperature than test stand or truck


Rubber Materials There are three rubber materials that account for almost all rubber used in automotive air conditioning applications. The three materials are HNBR, Nitrile, and Neoprene. The table below describes their ASTM designation and common names:

NBR Nitrile, Buna-N, Nitrile Butadiene Rubber HNBR Hydrogenated Nitrile Butadiene Rubber, Highly Saturated Nitrile, HSN CR Neoprene, Neoprene W, Chloroprene, Polychloroprene

Specifying the type of rubber is similar to specifying a type of cake. Chocolate cake can have thousands of variations based on the recipe. Rubber types can also have thousands of recipes based on how they are compounded. A rubber compound is composed of many ingredients and will have different properties based on how the recipe is designed. A rubber recipe is listed below for a simple Nitrile compound:

Ingredient Number of PartsNitrile Polymer 100.0Zinc Oxide 5.0Stearic Acid 1.0SRF Carbon Black 80.0Sulfur 0.5TMTD 1.5CBS 1.5Naugard 445 2.0 Total 191.5

In the example above there are even hundreds of different grades of the Nitrile Polymer that could be specified. Each different grade would give different chemical resistance to oils and different temperature resistance characteristics. Santech's common Nitrile compounds have an average of 12 ingredients. Materials are compounded specifically for the application. Temperatures, chemical resistance, abrasion resistance, rubber hardness, and other factors contribute to the design of the compound. General characteristics of the rubber material can only be used as a general guideline. The specific compound that is considered for the application must be tested to determine the actual performance. Testing of actual compounds for air conditioning applications will be presented in the Test Results section of this handbook.


Seal Force What is Seal Force?

Static Face Seal

ÐÐÙÙÙ Ù

Ð ÐFs

Before Compression During Compression Fs = Seal Force Fs >= System Pressure Rubber materials seal gas and fluids by exerting outward force when they are squeezed in an application. The seal force of the seal must be greater than the system pressure that it is sealing. The seal force will decline as the seal material is degraded by temperature or by chemicals. Below is an actual R4 compressor manifold connection application:

In this application the O-rings are squeezed by 20% when the manifold is connected. This 20% squeeze represents .027 inches of the .139 inch O-ring thickness. When an O-ring is subjected to 300°F for 70 hours with a similar squeeze (25%) during tests, the O-ring loses part of its thickness and becomes flat due to temperature degradation. These tests are described in the Types of Relevant Testing Section of this Handbook. The following table summarizes the data: Significant Data

SuctionConnection

Manifold

Compressor

DischargeConnection

Manifold


Original O-Ring Size: .139 inches Groove Depth: .112 inches Squeeze on Ring: .027 inches Squeeze as Percentage: 20% percentage Comparing expected loss of thickness, due to temperature degradation, the following table displays the projected loss of thickness and the possible thickness left to exert sealing force. Comparing HNBR, Nitrile, and Neoprene at 300°F for 70 Hours. Description HNBR Nitrile Neoprene Compression Set 40% 77% 88% Lost Dimensions .011 .021 .024 Squeeze on Ring .016 .006 .003 The squeeze left to seal with Nitrile and Neoprene materials at this temperature would not be sufficient for proper seal engineering. Lower temperatures yield different results: Comparing HNBR, Nitrile, and Neoprene at 212°F for 70 Hours. Description HNBR Nitrile Neoprene Compression Set 14% 27% 27% Lost Dimensions .004 .007 .007 Squeeze on Ring .023 .020 .020 These results are acceptable for normal seal engineering. Chemicals cause certain rubber materials to swell. All three rubber materials swell with certain refrigerant/lubricant combinations. Swell of less than 10% is generally considered favorable in static applications where constant contact with the chemical is maintained. Removal of the chemical may cause the material to revert back to its original size losing some of its volume. Swelling of the material can help replace some of the lost dimensions due to temperature.

Summary Seal Force plays an important role in system design. It is imperative that the seal design engineer considers temperature, chemical degradation, and seal squeeze during material selection.


Types of Relevant Testing There are three basic tests that give an excellent indication of performance of rubber materials in automotive air conditioning applications: ♦ Compression Set (Heat Aging) to ASTM D395Note1 ♦ Fluid Immersion to ASTM D471 ♦ Immersion in Refrigerant/Lubricant mixtures to Santech QP409 All three of these tests may be performed with standard test specimens in the form of slabs and buttons or with the end product. When these tests are performed with O-rings, ASTM D1414 contains additional specific information on each ASTM test. ASTM D395 Compression Set (Heat Aging)

The test fixture is constructed of two or more plates. Plates are bolted together with a spacer between each plate to maintain specified distance. The four holes in the plates are for bolts. The elastomer, to be tested, is compressed

25% for the specified time at a specified temperature. When the test is complete, the specimens are removed and allowed to relax. The specimens are measured and compared to measurements from before the test. The compression set test results are expressed as a percentage of the compressed area. A low compression set value is good and a high compression set value is bad. The table below gives examples of .100 cross section O-rings with a .075 spacer:

Original Dimension

Compressed Section

Dimension After

Dimension Lost

Compression Set

.100 .025 .090 .010 40%

.100 .025 .078 .022 88% The following picture depicts O-rings and their theoretical shapes as they go through the testing. The After Test picture represents a 40% loss of compression set. Note 1: ASTM is the American Society for Testing and Materials


O-Ring Configuration

0102030405060708090

100

Before Test During Test After Test

Compressed Section↓↑

ASTM D471 Fluid Immersion Two specimens are selected for each material to be tested. Properties of one specimen are examined before the test. The other specimen is immersed in the fluid for the specified time at a specified temperature. The properties of the specimen are examined and compared to the properties prior to the test. The differences are reported depicting the change in properties. Properties generally shown are hardness change, tensile change, elongation, and volume change. QP409 (Santech Internal) Immersion in Refrigerant/Lubricant Mixtures

Two specimens are selected for each material to be tested. Properties of one specimen are examined before the test. The other specimen is immersed in a refrigerant/lubricant mixture within a pressure vessel for the specified time at a specified temperature. The properties of the specimen are examined and compared to the properties prior to the test. The differences are reported depicting the change in properties. Properties generally shown are hardness change, tensile change, elongation, and volume change.


Test Results The following tests were performed on samples of the following materials: Abbreviation Description of the Material HNBR Santech Industries ST7470 Compound (Size 211 O-Ring) Nitrile Santech Industries ST4470 Compound (Size 211 O-Ring) Neoprene A Specific Neoprene Compound (Size 211 O-Ring)

Original Properties HNBR Nitrile NeoprHardness, Shore A, pts. - ASTM D2240 77 74 78Tensile Strength, PSI - ASTM D412 1272 1936 2040Ultimate Elongation, % - ASTM D412 159 321 296

Heat Aging Compression Set ASTM D395 Method B, 25% Compression

HNBR Nitrile Neopr

70 hours at 100 ± 2 C (212°F) (%) 14 27 2770 hours at 120 ± 2 C (248°F) (%) 17 47 4970 hours at 135 ± 2 C (275°F) (%) 28 73 7470 hours at 150 ± 2 C (302°F) (%) 40 77 88

Immersion in Refrigerant R-134a with PAG Santech Procedure QP 409, 168 h at 23 ± 2 C

HNBR Nitrile Neopr

Hardness Change (pts.) -2 -3 -1Tensile Change (%) +8 -12 -22Elongation Change (%) +17 -8 -8Volume Change (%) +3 +8 +4Visual Evaluation OK OK Note1

Immersion in Refrigerant R-134a with ESTER Santech Procedure QP 409, 168 h at 23 ± 2 C

HNBR Nitrile Neopr

Hardness Change (pts.) -2 -4 -2Tensile Change (%) +28 -15 -25Elongation Change (%) +18 -17 -11Volume Change (%) +7 +3 -3Visual Evaluation OK OK Note1

Immersion in Refrigerant R-12 with MIN. OIL Santech Procedure QP 409, 168 h at 23 ± 2 C

HNBR Nitrile Neopr

Hardness Change (pts.) -1 +1 -4Tensile Change (%) 0 -13 -48Elongation Change (%) +9 -12 -30Volume Change (%) +2 +7 +14Visual Evaluation OK OK Note1Note 1: Lost blue coating Note 2: All testing was done on o-rings following ASTM D1414


Analysis of Test Results 70 Hour Heat Resistance of Materials - ASTM D395 (As a percentage of lost dimension - lower is better)

212F248F

275F302F

HNBR

Nitrile

Neoprene0%10%20%30%40%50%60%70%80%90%

Based on heat resistance of the materials, Nitrile (ST4470 Series) and Neoprene would only be recommended to operate around 200°F with occasional heat spikes to 250°F. HNBR (ST7470 Series) would be acceptable around 250°F with heat spikes to 300°F.


Volume Swell in Refrigerant/Lubricant Mixtures (QP409)

HNBR Nitrile Neoprene-4%

-2%

0%

2%

4%

6%

8%

10%

12%

14%

HNBR Nitrile Neoprene

R12/Min. Oil R134a/PAG R134a/ESTER

On the basis of this one test, HNBR (ST7470) would appear to be the most stable, Nitrile (ST4470) would also be acceptable, and this particular Neoprene would be marginal. This particular Neoprene compound swelled 14% in the R12/Mineral Oil and shrank 3% in the R134a/ESTER.

Summary of Test Results HNBR could be used in any area of the air conditioning system with any of the combinations of refrigerants and lubricants. Nitrile and Neoprene should only be used in the lower temperature applications with any of the combinations.


Cost Analysis of Materials

Fittings Mid-Size Shell0

0.2

0.4

0.6

0.8

1

1.2

Fittings Mid-Size Shell

Comparison of Materials per $.10 of Nitrile

HNBRNitrileNeoprene

Size Range HNBR * Nitrile * Neoprene * Fittings .06 each .01 each .05 each Mid-Size .17 each .03 each .07 each Shell .99 each .17 each .45 each * Average of multiple sizes in each group. Some sizes will be more and others will be less. Use only as a guide. The cost of synthetic rubber materials is dependent on two major factors; the cost to synthesize the base rubber material and the world-wide volume. HNBR is a fairly new material and is just beginning to be widely used. Nitrile and Neoprene have been used for decades. HNBR is costly to manufacture, Neoprene is moderately priced and Nitrile is inexpensive. The world-wide volume of Nitrile is very high. HNBR and Neoprene are lower volume. The automobile manufacturers also have to justify cost for any upgrade in material. This justification can be based on safety, customer satisfaction, or on reduction of warranty repairs. The correct elastomer must be chosen based on the ultimate cost to the business enterprise and the consumer. With the cost difference between HNBR versus Nitrile and with the fact that most original equipment manufacturers have switched to HNBR materials, evidently justification was found for the switch.


Original Equipment Materials Original Equipment Manufacturers can be divided into two distinct categories: automobile manufacturers and component manufacturers. The following tables will list the automobile manufacturers and compressor manufacturers. Automobile Manufacturers (system connections) Company Mat'l Color Application Notes BMW HNBR Green Chrysler HNBR Green Ford HNBR Green General Motors Neopr Black Honda HNBR Black Mercedes HNBR

Nitrile Green Black

HNBR on the high side of the system Nitrile in the other sections.

Nissan Nitrile Black Toyota HNBR Black Volvo HNBR Yellow Other manufacturers were contacted and not all responded by publication time. Compressor Manufacturers Company Mat'l Color Application Notes Cal Sonic HNBR Black Chrysler HNBR

HNBR GreenBlack

Green for service applications Black in internal applications.

Climate Control CCI

Neopr Nitrile

Black Black

Compressor is only used in truck, bus, and agriculture applications under 200°F

Diesel Kiki HNBR Nitrile

Black Black

HNBR where units see high heat Nitrile in shell seals

Ford HNBR Nitrile

Green Black

Green for service applications Black Nitrile for shell on FX15(FS10)

General Motors HNBR Neopr

Black Black

HNBR for high pressure switch R4, HR6, and V5. Neoprene all other applications.

Nippondenso HNBR Nitrile

Black Red

HNBR (RBR) for R134a applications Nitrile for R12 applications

Sanden HNBR HNBR Nitrile

Black Green Black

HNBR for R134a applications. New HNBR in the USA will be green. Nitrile for R12 applications

Every automobile manufactured by the above manufacturers contains HNBR somewhere in the air conditioning system (Nissan uses Cal Sonic compressors). This will give an indication of the prominence of HNBR in air conditioning systems.

Exposing Myths


Nitrile will not work? This statement is absolute nonsense. CCI, Diesel Kiki, Ford, Mercedes, and Nissan all use some Nitrile in R-134a applications.

It is important to differentiate between the base material Nitrile and a specific compound. Some Nitrile compounds would not be compatible with R-134a/lubricant combinations, and some HNBR and Neoprene compounds would also not be compatible. Published test reports can show general compatibility or non-compatibility for "Nitrile", "HNBR", or "Neoprene" but remember that the test must be performed on the specific compound.

Neoprene is better than Nitrile? Neoprene is the material General Motors uses for most applications. It is also the material that CCI has chosen because of their lower temperature requirements and the additional need of R-22 compatibility in many of their applications. Each of these two manufacturers have chosen Neoprene based on their applications. The Neoprene and Nitrile compounds that were tested in our laboratory both have operating temperatures around 200°F with upper temperature limits around 250°F for short duration. The chemical compatibility of the Nitrile was very good and the cost of Nitrile is much lower than Neoprene. In these lower temperature applications where Nitrile and Neoprene are both valid choices, why would you use Neoprene at a higher price? Original Equipment Manufacturers are changing to Neoprene? The aftermarket was told that the original equipment manufacturers are changing to Neoprene. This is also nonsense. General Motors has used Neoprene in their systems for over 15 years. There was a Nitrile compound in use on a captured O-ring application that was hardening. This application was changed to Neoprene 1-2 years ago. The high pressure switch port O-ring in the R4, HR6, and V5 can experience temperatures up to 290°F and were switched to HNBR this year. Climate Control Inc. switched to Neoprene on their compressors during the R-134a conversion. Neoprene offers superior compatibility to R-22 which is used in many of their applications. For R-134a applications, where Neoprene is not required, CCI switches the material on their front bellows to Nitrile because the material is "more forgiving."

212F248F

275F302F

HNBR

Nitrile

Neoprene0%10%20%30%40%50%60%70%80%90%


Some Neoprene compounds are definitely an option in R-134a applications but the industry is not changing to Neoprene materials. If a general statement could be made concerning the world-wide trend of manufacturers in automotive air conditioning, it would be, "The industry is changing to HNBR materials."


Recommendations for Rubber Materials All Non-Compressor Applications (Use Green HNBR O-Rings for All Applications) ♦ Cost is minimal because of ring size. ♦ Mechanic can install in any part of the system. ♦ Color coded for visual identification. Compressor Applications (Option 1) (Use Green HNBR O-Rings for All Applications) Advantages: ♦ Use the higher temperature material as a selling point. ♦ Assure that the compressor will operate in almost any vehicle application. Disadvantage: ♦ The cost of the HNBR material is higher that Nitrile. Compressor Applications (Option 2) (Use Green HNBR O-Rings for High Temperature Applications Use Green HNBR O-Rings for any Visual Applications Use Nitrile for Low Temperature Applications) Advantage: ♦ Cost is minimal because large rings in Nitrile are low cost. Disadvantage: ♦ Vehicle specific testing to identify low temperature applications is critical.

Diesel Kiki compressors, the Ford FS10 compressor, and GM compressors are the only compressors that utilize some low temperature materials as original materials in specific applications.


Acknowledgments Santech Industries would like to thank the following companies for their contributions to this handbook: ARI American Refrigeration Institute BMW Cal Sonic Castrol Chrysler Climate Control Inc. Diesel Kiki Dupont Ford Motor Company General Motors Goodyear Honda ICI LubraSol Mercedes Nippondenso Nissan Sanden Toyota Volkswagon Volvo Zeon Chemicals Zexel

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