Quantification of the Flexibility of Flame Retardant Insulation and Jacket Materials When Designing Wire & Cable Products

Transcription

1 Quantification of the Flexibility of Flame Retardant Insulation and Jacket Materials When Designing Wire & Cable Products Paul Lorigan T & T Marketing, Inc. Easton, PA plorigan@ttmarketinginc.com Abstract This paper examines Standard Test Method for Rubber Property - Durometer Hardness ASTM D for correlation to Flexural Properties of filled and unfilled wire and cable compounds per Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials ASTM D Keywords: Shore A Hardness, Flexural Modulus, Chord Modulus, Flame Retardant, Non-halogen 1. Introduction When selecting wire and cable insulation and jacket materials for flexible constructions, design engineers typically seek softer materials. The softness/hardness is most often quantified by Durometer Testing in the Shore A scale. In the course of developing new generation low smoke zero halogen compounds, it was empirically noted that certain materials seemed more flexible than their hardness data indicated. This work was undertaken to understand the correlation between hardness and the appropriate flexural modulus of these materials. Unfilled materials are compared to various flame retardant compounds for hardness and the most relevant flexural modulus. Flame retardant materials based on halogen/antimony, metal hydrates and ammonium polyphosphate intumescent systems are tested at their effective flame retardant levels for hardness and the appropriate flexural modulus. Average mass and volume fractions of the filler loadings are examined for correlation. Correlation coefficients are calculated on the aggregate data for immediate hardness and 15 s delay. Observations and conclusions are made for the various polymer systems and flame retardant technologies. 2. Experimental 2.1 Shore A Hardness This test method permits hardness measurements based on the initial indentation and after a 15 second delay. This test method is based on the penetration of an indentor when forced into the material under specified load. The indentation hardness is inversely related to the penetration and is dependent on the elastic modulus and viscoelastic behavior of the material. This test method is an empirical test intended for control purposes. No simple relationship exists between indentation hardness determined by this test method and any fundamental property of the material tested. (1) A Shore A Durometer equipped with a stand is shown in Figure 1. It is positioned on top of an additional metal stand to more easily read the gauge. To achieve reproducible results, the unit should be level and the indentor placed very gently onto the material. The Shore A gauge specifications are shown in Figure 2. 1

2 Many still use Shore hardness as the standard proxy for flexibility. A semiempirical relationship exists between Young s modulus and Shore hardness by the equation: E = ( S) ( S) Where E = Young s modulus and S = Shore hardness This relation is inaccurate for materials which permanently deform when strained or having a non-hookean stress/strain curve. Figure 1. Shore A Durometer with Stand 2.2 Flexural Property Measurement ASTM D is a test method that utilizes a 3 point loading system to a supported beam. A bar of rectangular cross section rests on two supports and is loaded by means of a loading nose midway between the supports. The specimen is deflected until rupture occurs in the outer surface or until a maximum strain of 5%. (2) Flexural modulus is mathematically the ratio of flexural stress (psi) to corresponding flexural strain (% extension). That is, the pressure produced by the material with each unit of extension, thus, our modulus is given in units of psi. Figure 3. Flexural Modulus Figure 2. Shore A Indentor Dimensions Until the flexural modulus standard was established, Shore hardness served as the only proxy to quantitatively measure flexibility of a material. The materials evaluated in this study have significant thermoplastic deformation when strained. In other words, they are not perfectly elastic. Materials which are highly 2

3 elastic have a stress/strain curve which is Hookean. chamber to the tester. Instantaneous values were used to capture the temperature effect. Chord Modulus values were also generated using a Perkin Elmer Dynamic Mechanical Analyzer (DMA 7e). Values were generated at -40 C, -20 C, 0 C, 20 C and 40 C. 3. Data 3.1 Ambient Testing Figure 4. Hookean Curve The stress strain curves of the thermoplastic elastomer compounds and olefin materials evaluated in this study are non-hookean and have the shape shown below. Product Shore A (immed.) Shore A 15s Delay Modulus (Chord 0.5% - 1%) [psi] AVERAGE Unfilled EP/PP Copolymer ,190 Unfilled TPV EP/PP Homopolymer ,080 Halogenated TPE EP/PP Homopolymer ,653 Halogenated TPE EP/PP Homopolymer ,441 Halogenated TPV EP/PP Homopolymer ,855 Halogenated TPV EP/PP Homopolymer ,784 Mag Hydroxide Olefin ,421 Mag Hydroxide Olefin ,965 Intumescent TPV Metallocene ,825 Intumescent TPV EP/PP ,149 Intumescent TPE EP/PP ,387 Table 1. Shore A Data & Chord Modulus Figure 5: Typical Stress/Strain Curve TPE A more appropriate modulus calculation for non-hookean curves is to pick two strain points, connect them with a line and calculate the slope of that line. This treatment is called Chord Modulus. Any two points can be agreed and chosen for specification work. For this study 0.5% and 1.0% were chosen as the strain points. The effect of temperature on both Shore A Hardness and Chord Modulus was also examined. A common cold box and oven were used for to condition samples for hardness tester. Samples were moved as quickly as possible from the conditioning Product FR Mass Fraction FR Volume Fraction Crystalline Content Amorphous Content Unfilled EP/PP Copolymer 0.0% 0.0% Unfilled TPV EP/PP Homopolymer 0.0% 0.0% Halogenated TPE EP/PP Homopolymer 42.4% 11.6% Halogenated TPE EP/PP Homopolymer 45.5% 12.4% Halogenated TPV EP/PP Homopolymer 32.0% 8.6% Halogenated TPV EP/PP Homopolymer 32.0% 8.6% Mag Hydroxide Olefin 65.0% 27.5% 33 0 Mag Hydroxide Olefin 60.0% 25.4% 38 0 Intumescent TPV Metallocene 35.0% 34.7% 5 43 Intumescent TPV EP/PP 35.0% 34.7% Intumescent TPE EP/PP 35.0% 34.7% Table 2. Compound Characterization 3.2 Temp Effects Hardness & Flexibility. Temp. C mpp TPV Intum Hal TPE Hal TPE Unfilled TPV Olefin Mg(OH) Intum TPE

4 Table 3. Shore A Instant vs. Temperature Temp. C mpp TPV Intum 120,344 23,331 11,666 9,632 6,047 Hal TPE 126,958 76,498 36,213 21,083 12,002 Hal TPE 69,362 26,797 9,649 6,486 3,915 Unfilled TPV 48,376 22,851 14,270 10,875 7,708 Olefin Mg(OH) 2 291, ,121 48,578 29,174 12,913 TPE Intum 170,941 77,929 36,233 18,979 12,254 Table 4. DMA Chord Modulus vs Temp. 4. Analysis 4.1 Correlation Analysis (Linear) Parameters Correlation Coefficient Shore A 15s delay vs Chord Modulus 0.76 Chord Modulus vs Crystalline Content FR Only 0.75 Shore A Inst vs Chord Modulus 0.72 Shore A 15s delay vs Amorphous Content Shore A 15s delay vs Crystalline Content FR Only 0.69 Chord Modulus vs Crystalline Content 0.65 Shore A 15s delay vs Crystalline Content 0.60 Chord Modulus vs Amorphous Content Shore A 15s delay vs Amorphous Content FR Only Chord Modulus vs Amorphous Content FR Only Shore A 15 s delay vs FR Volume Fraction 0.22 Chord Modulus vs FR Volume Fraction 0.22 Shore A 15 s delay vs FR Mass Fraction 0.12 Chord Modulus vs FR Mass Fraction 0.01 Table 5. Correlation Analysis Good correlation is noted between Shore A Hardness and Chord Modulus. The correlation is stronger if the 15s delay hardness value is used (0.75). However, another perspective on the correlation data is a value of 0.75 indicates a less than perfect correlation. If we take a closer look at specific data sets we can see that if Shore A alone is used to estimate flexibility, erroneous conclusions can be made. Table 6. Specific Modulus Deltas vs Hardness In Table 6 we can see two compounds that have identical Shore A measurements (96.5) but differ in Chord Modulus by 33,728 psi. The difference in flexibility is a factor of 1.8X, almost twice as rigid. The second data set in Table 6, differ in Shore A by less than one point (0.8) but one compound is 2.2 X more rigid than the other. A third data set has a difference in Shore A of 0.2, within the standard deviation of the test, and a difference in Chord Modulus of 7,798 psi or a more modest 21% difference in flexibility. Good correlation is indicated between the crystalline content of the compounds and both hardness and flexibility. Likewise there is correlation (negative) between high amorphous content and both hardness and flexibility. In other words, the more amorphous the compound, the greater the expectation that the compound will be soft and flexible. Once again, care must be taken to realize the correlation is not 1.0 so therefore the individual compound needs to be tested without a general assumption being made. Unexpected results from this study were the lack of correlation between flame retardant level (both mass and volume fraction) and either hardness or flexibility. Product Shore A 15s Delay Modulus (Chord 0.5% - 1%) [psi] AVERAGE Δ psi % Mag Hydroxide Olefin ,421 Intumescent TPV EP/PP ,149 33,728 56% Intumescent TPV Metallocene ,825 Intumescent TPE EP/PP ,387 19,562 45% Halogenated TPV EP/PP Homopolymer ,855 Halogenated TPE EP/PP Homopolymer ,653 7,798 79% Figure 6. Shore A vs. Temperature From the above illustration it can be seen that with exception of one compound tested, 4

5 Shore A Hardness is not greatly affected by temperatures from -25 C to 43 C. Chord Modulus data generated on the DMA shows greater differentiation between compounds. Particularly pronounced is the increase in Chord Modulus of a magnesium hydroxide filled olefin compound as the temperature drops from -20 C to -40 C. and thank you to intern student Evan Pretti (Lehigh University) for commissioning our DMA tester, development of test methods and data generation. Thank you to Dan Weaver who masterfully oversees our compounding, extrusion, and training, and safeguards the accuracy and precision of our work. Finally thank you to the IWCS for reviewing and approving this paper for publication. 7. References (1) ASTM D (Reapproved 2010) Standard Test Method for Rubber Property Durometer Hardness (2) ASTM D Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials Figure 7: DMA Chord Modulus vs. Temp. 8. Author 5. Conclusions 1. Flexural Modulus testing is a better quantification of material flexibility than Shore Hardness testing. 2. Flexural Modulus testing utilizing a Dynamic Mechanical Analyzer is effective in quantifying changes in flexibility over a range of service temperatures. 3. Greater crystalline content generally correlates with greater hardness and rigidity. 4. Greater amorphous content generally correlates with increased flexibility and softness. 5. No correlation is noted between flame retardant filler loadings and hardness or flexibility. 6. Acknowledgements Thank you to intern student Benjamin Lorigan (The Pennsylvania State University) for Mechanics research and math modeling Paul graduated from Gannon University in 1980 with a B.S. in Chemistry. He has done graduate work at Ball State University, Lehigh University and Western University in London, Ontario. Other positions held include President - EnerKem Marketing, 5

6 Business Manager - AT Plastics, Technical Director BP Performance Polymers, Compound Development Engineer Reichhold Polyolefin and Vinyl Materials Division, and Compound Development Engineer Anaconda Wire & Cable Company. Paul trained and worked for several years in BP Chemical s Wire & Cable Compound Skill Center in Meyrin, Switzerland as well as BP s Polymer Science Branch in Grangemouth, Scotland. Paul is responsible for the research, development and technical service of wire and cable compounds distributed and manufactured by T & T Marketing. He works in T & T s laboratory located in Easton, PA. 6