In the report below EZ Cool is being referred to as Low-E, which is the manufacturers brand name.

 

Hybrid Bus Body Panel Insulation Research & Development

Senior Project Report 

Stefan Weiss

March 18, 2013

ETEC 432

 

Abstract

As the Hybrid Bus Project moves into the vehicle-build stage, it will be necessary to determine a suitable insulation material for the body panels of the bus. This project was implemented to decide which available material will provide the insulation properties needed, comply with FMVSS and APTA standards and guidelines, and minimize the weight added to the structure of the bus.

The insulation material selected will have to:

·         Reduce 85 dB(A) exterior road noise to 65 dB(A) interior noise.

·         Maintain an interior temperature range of 65°-80°F with an exterior temperature range of 10°-90° and a HVAC system installed.

·         Burn at a rate ≤4 inches per minute.

·         Not pose any health hazards.

Multiple test panels were assembled and tested for thermal resistance, sound transmission loss, and burn rate. The tests that were conducted were based off of existing ASTM tests but were scaled down to account for the lack of material available for this project.

The results or the thermal resistance testing showed that the materials with the higher densities performed better than the lower density materials; although, the difference was minimal between the materials.

The sound transmission loss results showed that the panel assemblies with an air gap reduced road noise more than the rigid panels. However, the rigid panels were more effective at reducing powertrain noise which has a lower frequency wavelength than road noise.

Upon conducting a vertical burn test on all of the materials, it was concluded that certain materials could not be used for the Hybrid Bus due to the fact that they burn too fast. Average burn rates were calculated for all of the materials. On a side note, the extreme flammability of the polypropylene matrix in Twintex was confirmed; although, the burn rates for a 2-ply sheet as well as the honeycomb assemblies were slow enough to pass the standard.

In conclusion, the material chosen to insulate the Hybrid Bus is a 3/16” thick polyethylene foam with polished aluminum facings called Low-E. It is manufactured by JSP and distributed by Harbour Supply in Springtown, Texas. When applied to the entire bus, this material will only add approximately 17.5 pounds to the curb weight of the bus and will cost approximately $530.

Introduction

As the Hybrid Bus Project moves further into the vehicle-build stage it will be necessary to determine a suitable insulation material for the body panels of the bus. Whenever composite materials are used for weight reduction there are often negative effects that occur as well. These effects are commonly increased noise transfer and vibration due to the density of the material. To combat these negative effects, an insulation material must be selected and incorporated into a body panel design. The selected material must provide sufficient thermal and acoustic insulation to provide a comfortable interior atmosphere in the bus. The body panel design must be cost efficient and easy to process to ensure manufacturing feasibility in the future. The Hybrid Bus Team would also like to make the bus’ body as recyclable as possible which significantly limits the design of the body panels but helps to support the overall goal of sustainability within the project. If the body panels include many different materials that need to be separated before they can be recycled, it should require minimal effort and should not require use of any unnecessary equipment or harmful chemicals.

Objectives

·         80 dB(A) exterior noise reduced to 65 dB(A) interior noise

o    APTA TS 5.8

·         Powertrain noise less than 75 dB(A) for driver and less than 80 dB(A) for passengers

o    APTA TS 5.8

·         Maintain a 65°-80° F interior temperature with an exterior temperature of 10°-95° F (with HVAC system installed and fully operational)

o    APTA 5.4.8.1

·         Insulation material is recyclable

·         Insulation material is non-toxic

·         Insulation material burns at a rate less than 4 inches per minute

o    FMVSS 302 & FTA Docket 90A

Functions

·         Provide in-cabin comfort by reducing noise and moderating temperature

·         Comply with standards and guidelines

Constraints

·         Cost

·         Less than 300 lbs. added to the curb weight of the vehicle

·         Less than 3 inch wall thickness

·         Less than 15 minutes added to panel processing time

Proposed Solution

The solution to this problem starts with narrowing down the options of materials by researching what’s out there and being used in vehicles already, as well as new materials, and constructing a decision matrix to eliminate unsuitable materials for this project. Once several materials have been selected, test panel assemblies can be made and tested for thermal resistance, sound transmission loss, and burn rate. The testing apparatus is built out of polyethylene packing foam, hot glue, and lumber to test the panels for thermal resistance and sound transmission loss. The burn rate test requires a fume hood, a set of U-shaped clamps, stand with clamps, and a propane torch. Once all of the tests are complete and the results plotted in Excel, analysis can be done and a material selected for use in the Hybrid Bus.

Methodology

Material Selection

Material selection was done by researching existing and available materials that are currently being used to insulate vehicle and other structures. Information on these materials was acquired from manufacturers of the materials as well as websites and home improvement stores. Once enough information was gathered on several materials, a weighted decision matrix was created to determine which materials were more suitable for the Hybrid Bus body panels. The decision matrix can be seen in APPENDIX A. Objectives that were included in the decision matrix, in order of importance, were: non-toxicity, flame resistance, R-value, density, cost, ease of recyclability, ease of installation, and sound absorption. The decision matrix also defined which materials could be processed with the Twintex (sandwich panel), materials that require adhesive assembly (layered panel), or materials that do not have structural integrity and would require standoffs with mechanical fasteners (insert panels). The materials that resulted in the highest score were: Low-E (PE core w/ Al facings), fiberglass batt, polyethylene packing foam, and expanded polypropylene.

The Low-E material was acquired from Harbour Supply in Springtown, Texas. It consists of a 3/16” polyethylene foam core with polished aluminum facings. It is mainly used to insulate homes and hot rod vehicle projects. The term “Low-E” refers to the low E-value of the material which means that it has excellent radiant heat resistance. Hot rodders like it for both the low E-value and its road noise cancellation characteristics.

Two densities of the expanded polypropylene were acquired from ARPRO, 1.3 lb/ft^3 and 3.7 lb/ft^3. The reason why EPP was chosen was for its excellent insulation and toughness properties but also because it is the same material as the matrix of Twintex T PP.

Fiberglass batt insulation has been proven to be one of the best insulation materials available and is also very inexpensive and available. A small package of R13 fiberglass batt was purchased from Lowes for under $5.00.

The polyethylene packing foam was left at the TDC and available to use, so it was included in the testing and used to fabricate the testing apparatus for the thermal resistance and sound transmission loss testing.

Apparatus Fabrication

The apparatus that was used for the thermal resistance testing and the sound transmission loss testing was based off of ASTM C-1363-11 but was scaled down to accommodate smaller test panels. A testing apparatus was built out of polyethylene packing foam, hot glue, and lumber. It has a “hot side” and a “cold side,” separated by a 2’x2’ test panel. The hot side has an extra wall with an air gap in between. It is designed this way to direct more heat or sound through the test panel as well as shield the heat or sound from any outside elements that might interfere with testing. A frame was built out of scrap lumber to keep the walls of the apparatus from bowing outwards. The lid had a 2’ slit in the center to fit the test panels and create a tight seal between the hot and cold sides. During testing, weights were placed on top of the lid of the apparatus to create a better seal and prevent any heat or sound pressure from escaping.

Panel Fabrication

The process of manufacturing Twintex face sheets by vacuum bagging is as follows:

  1. Clean the aluminum tool with a razor, scouring pad, and isopropyl alcohol.
  2. Cut out necessary materials (Twintex, peel-ply, breather, and bagging).
  3. Apply sealant tape around the perimeter of the tool.
  4. Lay the materials on the tool in this order:
    1. Twintex
    2. Peel-ply
    3. Breather
    4. Bagging
  5. Fold an extra piece of breather and insert it in one of the corners of the tool with the vacuum fixture on top.
  6. Carefully seal the bag with the sealant tape by peeling the paper strip away and applying pressure.
  7. Process the part in the processing oven at 370° F for 60 minutes.

Once the Twintex face sheets were processed they - along with the preprocessed PP honeycomb panels and insulation materials - were cut to 2’x2’. The layered style panels were assembled with a light coating of 3M Super 77 spray adhesive and pressure from some scrap metal which was placed on the panels while the spray adhesive set up. The insert style panels (Low-E and fiberglass batt) were assembled by drilling holes in the corners of the Twintex and honeycomb panels and securing the two together with mechanical fasteners with 2” standoffs in between. For the Low-E panel, the material was adhered to the panels with the Super 77 before assembly. For the fiberglass batt panel, the material was laid out evenly on the honeycomb panel with the bolts and standoffs sticking up. The Twintex panels was then fastened on top and secured with nuts and washers.

Thermal Resistance Testing

Thermocouples were placed on either side of the test panel (which divides the apparatus in the center), and one in the center of both the hot and cold sides. A space heater was used to heat up one side of the apparatus. The heater was set to LOW and ¾ voltage which heated the hot side of the apparatus to 170°-180°F. Each test ran for 25 minutes and temperatures from both sides of the test panel were recorded every minute. The Katz Cart at the TDC was used for its thermocouple reader capabilities which allowed rapid switching between thermocouples without having to unplug any wires. Each material was tested three times. Each time a material was tested again, the panel was disassembled and reassembled using different Twintex and honeycomb panels to account for manufacturing inconsistencies.

Sound Transmission Loss Testing

The same apparatus from the previous test was used for this test as well. Speakers were placed in one half of the box and a sound level meter in the other. A tone generator app on an iPad was used to output the correct frequency through the speakers (average road noise is 1000 Hz and average powertrain noise is 500 Hz) and the volume was adjusted to achieve 85 or 105 dBA respectively. The sound pressure level was measured through each panel twice. Computer software was used on a laptop outside of the apparatus to read real-time sound pressure levels so that the apparatus could remain closed during the test. The source sound pressure was calibrated by placing the sound level meter in the “hot side” and tuning the frequency and amplitude (volume) until the sound pressure level was correct.

Burn Rate Testing

Description: C:\Users\Stefan\Pictures\2013\2013-03-09 3-9-2013\3-9-2013 184.JPGTwo 14” U-shaped aluminum clamps were fabricated to hold samples of each material for a vertical burn test. The actual FMVSS 302 test cabinet could not be procured, so the PET lab fume hood was used instead. The clamps, with a piece of insulation material, were attached to a chemistry apparatus stand and held in place while a propane torch was used to burn the end of the sample for 15 seconds. Once the flame reached 1½” from the bottom – where it was burned – a time was started. Once the flame stopped travelling or reached 1½” from the top of the clamp, the timer was stopped and the distance was measured in inches. The burn rate of the material could then be calculated using the following formula:

Text Box: Burn Rate test setup
 

Results

Thermal Resistance Testing

This graph shows the average hot side temperature versus the average response of the three tests from each material test panel. The results have been adjusted to all start at 80° F to show a better comparison between the materials. The non-adjusted version of this chart, as well as the tables of values used to generate the graph, can be found in APPENDIX B.


 

Sound Transmission Loss Testing

1000 Hz @ 5/8 Volume [ROAD NOISE]

Material

Source (dBA)

Measured (dBA)

 

Material

Source (dBA)

Measured (dBA)

HD EPP

85

51.3

 

HD EPP

85

51.5

LD EPP

85

55.8

 

LD EPP

85

55.8

PE FOAM

85

53.5

 

PE FOAM

85

53.7

LOW-E

85

43.7

 

LOW-E

85

43.9

FIBERGLASS

85

43.1

 

FIBERGLASS

85

43.5

HONEYCOMB

85

52.1

 

HONEYCOMB

85

52.1

TWINTEX

85

53.9

 

TWINTEX

85

53.9

AIRGAP

85

43.1

 

AIRGAP

85

43.3

The table above shows the results of the sound transmission loss testing for reduction of road noise which has an average frequency of 1000 Hz. The 3 best performing materials are highlighted in green and the worst performing material is highlighted in red.

 

 

513.6 Hz @ MAX Volume [POWERTRAIN NOISE]

Material

Source (dBA)

Measured (dBA)

 

Material

Source (dBA)

Measured (dBA)

HD EPP

105

63.8

 

HD EPP

105

63.9

LD EPP

105

64.3

 

LD EPP

105

64.2

PE FOAM

105

65.9

 

PE FOAM

105

65.8

LOW-E

105

71.2

 

LOW-E

105

71.2

FIBERGLASS

105

68.2

 

FIBERGLASS

105

68.2

HONEYCOMB

105

66.3

 

HONEYCOMB

105

66.3

TWINTEX

105

76.2

 

TWINTEX

105

76.1

AIRGAP

105

66.7

 

AIRGAP

105

66.8

The table above shows the results of the sound transmission loss testing for reduction of powertrain noise which has an average frequency of 500 Hz. For the purposes of this test, and to create exactly 105 dBA, the frequency was adjusted to 513.6 Hz. The 3 best performing materials are highlighted in green and the worst performing material is highlighted in red.

 


 

Burn Rate Testing

Material

Test #1

Test #2

Average Burn Rate

Distance

Time

Burn Rate

Distance

Time

Burn Rate

HD EPP

10

50.4

11.90

10

58.7

10.22

11.06

LD EPP

1.25

13.8

5.43

0.5

9.6

3.13

4.28

PE Foam

10

54.2

11.07

10

60.1

9.98

10.53

Low-E

0.5

8.4

3.57

0.25

4.5

3.33

3.45

Fibers

0.25

3.8

3.95

0.125

3

2.50

3.22

2 Ply TPP

10

195

3.08

10

185.2

3.24

3.16

Honeycomb

10

339

1.77

10

318.7

1.88

1.83

The table above shows the results of the burn rate testing. The materials highlighted in red did not pass the test. The material highlighted in yellow barely did not pass the test and could possibly pass with more testing since the two tests performed had such different results. The materials highlighted in green passed the test and should be able to be used in vehicular applications.

Analysis

The results of the thermal resistance testing showed that, for the most part, all of the chosen materials performed equally and that 2-plies of Twintex, PP honeycomb, or a combination of the two were not sufficient for insulating the body panels. More accurate results could have been acquired if the test was run for a longer duration than 25 minutes. The graphs of the results seem to show that the cold side of the testing apparatus was only beginning to heat up when the tests finish. The reason why the tests were not ran longer than 25 minutes was that by the time this trend was noticed, half of the testing was already completed and there was not enough time to redo all of that testing. The trends of the materials’ performance can be somewhat accurately predicted though and the results that were acquired are sufficient.

The results of the sound transmission loss testing showed that panel assemblies with an air gap of some sort reduce road noise better than the panels with a rigid foam core; although, all of the panels performed well enough to meet the APTA guideline of reducing an 85 dBA noise to a 65 dBA noise through the body panel. Different frequencies seem to resonate with different thicknesses and densities of material which can cancel the loudness of the noise and can lead to some odd results. For instance, a 154.6 Hz frequency tone is reduced more by a single panel of PP honeycomb than any of the test panels fabricated for this project.

The results of the burn rate testing showed that the expanded polypropylenes and the polyethylene packing foam burned too fast. The polypropylene honeycomb and 2-ply Twintex laminate burned intensely and created dark smoke but the flame propagation was slow enough to pass the test (≤4” per minute). Low-E (EZ Cool) and the fiberglass batt were self-extinguishing and created very little smoke. Attention should be given to the fact that the polypropylene matrix in Twintex is extremely flammable.


 

A decision matrix was created to determine the most suitable material based on a number of factors. The matrix is scored 1-5, 1 being the best and 5 being the worst. So, the lower the final score the better.

Low-E (EZ Cool) is the most suitable insulation material for the Hybrid Bus based on the table(s) above. It has the worst score for thermal resistance; however, all of the material performed very similarly with only a few degrees of difference. Even with the worst thermal resistance, Low-E (EZ Cool) came out on top. It is also the easiest material to install and can be shaped easily with a utility knife.

Outline of Time Spent on the Project

·         Research                                                             20 Hours

·         Material Acquisition                                        5 Hours

·         Panel Fabrication                                             8 Hours

·         Apparatus Fabrication                                    12 Hours

·         Thermal Resistance Testing                         21 Hours

·         Sound Transmission Loss Testing              13 Hours

·         Burn Rate Testing                                            8 Hours

·         Analysis                                                                10 Hours

·         Presentation                                                      15 Hours

·         Report/Poster/Journal                                  22 Hours

·         TOTAL                                                                   134 Hours

*All hours are logged in lab journal*

Problems Encountered

The largest setback was having to track down the correct driver for the Amprobe software that came with the sound level meter. It took the better part of two mornings on the phone with Amprobe to finally find a website that had a free download of the newest driver.

Another setback was having Heath Techna back out of their offer to help with the burn rate testing. This did not cost too much time but it caused the burn rate testing to be delayed until the week before the presentation of the project.

Actual vs. Proposed Timeline

The proposed timeline for the project involved starting and possibly finishing the project over the summer of 2012. However, there was not enough time left over after the regular tasks of the internship position with the Hybrid Bus Project were complete. Plus, it would have been unethical to work on the project while getting paid to work on other tasks. The proposed timeline can be seen in APPENDIX C and APPENDIX D. The actual timeline of this project was over the winter quarter of 2013 and can be seen in the figure below.


 

Actual vs. Proposed Costs

The proposed cost of this project was $293.87. The actual cost of the project was just under $450 mostly due to the purchase of a premium sound level meter. The breakdown of the actual cost can be seen in the table below.

Ethical Considerations

There are certain ethical considerations that need to be accounted for when selecting an insulation material for the Hybrid Bus. The first is that the material must be nontoxic during its use as an insulation material, which means that it does not irritate the passengers’ skin or respiratory system and does not off gas any harmful volatile organic compounds (VOCs). Some materials are only harmful during processing, such as polyurethane foam spray; however, it should be considered that the manufacturers will be wearing protective clothing and respirators while the passengers will not. The second ethical consideration to account for is the flame resistance of the insulation material. FMVSS 302 states that a vehicular insulation material must burn at a rate of less than 4 inches per minute. This would allow any emergency responders to travel to the scene of an accident and have time to extinguish any flames that are present before anyone is seriously burned. The third ethical consideration is recyclability. The Hybrid Bus Team would like the Hybrid Bus to be as recyclable as possible to conform to their overall goal of sustainability. The Hybrid Bus Team has already been successful in the recycling of Twintex T PP panels; however, if another material is fused or bound to the Twintex it would create difficulties in the recycling process.

 

Recommendations for Future Work

If further work was to be done on this project, it would be advised that different air gap widths be tested for sound transmission loss with the Low-E (EZ Cool) material. Also, one could try only using one layer of the Low-E (EZ Cool) on either the Twintex laminate within the panel assembly or only on the honeycomb. Another crucial test would be to see if the self-extinguishing nature of the Low-E would be enough to extinguish flame propagation across Twintex or PP honeycomb. Based on the results of the burn rate test, some form of flame retardant for Twintex should be researched or developed.

Conclusion

Based on the testing results and the cost and densities of each material, Low-E was chosen as the most suitable insulation material for the Hybrid Bus. Low-E is an affordable, lightweight, and effective insulation material that will satisfy the needs of the Hybrid Bus Project. It is easy to install, only adds approximately 17½ pounds to the bus’ curb weight, and will cost roughly $530 to insulate the entire vehicle.

Appendices

Appendix A – Material Decision Matrix

Appendix B – Thermal Resistance Testing Results

 


 

Appendix C – Original Manufacturing Plan from proposal document

 

Appendix D – Proposed Gantt Chart

Description: K:\Sr Project\Definition\SR Project Gantt.jpg