Property enhancement of melt processible rubber by blending with thermoplastics....

With ever increasing property requirements coupled with severe economic constraints, and the limitations of commercially available elastomers, thermoplastic elastomers and thermoplastics, the rubber chemist/materials engineer/part designer has an enormous challenge to select the appropriate material for a specific application.

In the thermoplastic elastomer (TPE) arena, the rubber chemist may desire a product that has the rubbery nature of a melt processible rubber (MPR), the toughness of a thermoplastic urethane (TPU), the low temperature properties of a copolyester (COP) and the cost of a plasticized polyvinylchloride (PVC). Since no such product exists, the chemist must consider compromise because until now most assumed that TPES could not be altered and one had to be satisfied with the properties of the particular grade supplied. Fortunately, all of these resins are compatible and may be blended in all proportions to achieve a broad range of performance modifications and enhancements to the basic MPR properties.

Using MPR as a base, a study was initiated, using a statistically designed process with ECHIP (ref. 1) software, to provide insight into how blending with TPU, COP and PVC affects physical and chemical properties. This article covers the results of this study and shows that a large amount of information can be developed with minimal experimentation.

Even though MPR has many desirable properties, this study shows that its properties can be further enhanced by blending with TPU to improve toughness, abrasion and oil resistance. Compression set, tear resistance and low temperature properties are enhanced by blending with COP. Plasticized PVC lowers the material cost and slightly improves tear resistance when added to MPR.

Using the various graphics and plots provided in this article as guidelines and compounding expertise, the chemist can develop proprietary compounds, tailored to meet specific end use requirements.

Experimental design process

ECHIP is a state-of-the-art software package that provides a user friendly program for performing statistically designed experiments. Experimental design is a systematic approach to performing experiments and analyzing the results. The objective is to determine the effects of variables on the responses with a minimum number of experiments.

Benefits are: fewer experiments, predictable relationships, increased product knowledge, quicker answers, increased efficiency, reduced cost. Typically, experimental design falls into one of three categories: screening, response surface or mixtures.

Screening is used when there is a large number of variables, to determine which are most important. It is good for initial investigations and will determine the effects of variables over a range of values as you go from the low to the high values. The types of screening designs available in ECHIP are: factorial, Plackett-Burman and linear. We have found the ECHIP version of linear design to be best suited for our use.

Response surface is used to determine the relationship between the variables and the responses where there are a smaller number of variables (less than 7 or 8). Standard response surface designs available in ECHIP are: linear with center point, quadratic, central composite in cube, central composite in sphere and cubic. Choice of design usually depends on the number of variables.

Mixtures is a response surface technique used when variables are proportions of a mixture [sum of the variables equal to 1(100%)]. This is the design used for the blend (mixture) study covered in this article. Standard outputs from ECHIP are the effects table, contour plot and 3D graphics. The effects table identifies the important variables and the degree of the effect (variables can be ranked from smallest to largest effect). The contour plots and 3D graphics give a visual interpretation of the data which are easy to understand without training in statistics.

Variables and responses

The four components (variables) used in this blend study (figure 1) were: melt processible rubber, then-noplastic urethane, copolyester TPE and flexible polyvinylchloride. The resulting properties (responses) measured include:

Property Test method Symbol

Hardness, durometer A ASTM D 2240 SA Tensile strength, MPa ASTM D 412 TEN Elongation at break, % ASTM D 412 ELONG Volume swell in ASTM No.

1 oil (7 da @ 100[degrees]C), % ASTM D 471 VS1 Volume swell in ASTM No.

3 oil (7 da @ 100[degrees]C), % ASTM D 471 VS3 Volume swell in water

(7 da @ 100[degrees]C), % ASTM D 471 VSW Compression set B

(22 hr @ 100[degrees]C), % ASTM D 395 CS Trouser tear strength,

k N/m ASTM D 470 TT Graves tear strength

(Die C), k N/m ASTM D 624 GT Clash-berg stiffness

Temp. @ 69 MPa, [degrees]C ASTM D 1043 CB Tabor abrasion (CS-17,1000g load), mg/1,000 cycles TA Cost, compared to MPR (MPR = 1.0) COST

Figure 1 - blend study - experimental design

Variables Product Responses

MPR [right arrow] Blend [right arrow] Hardness (SA) TPU [right arrow] [right arrow] Tensile Str. (TEN)

[right arrow] Elongation (ELONG) COP [right arrow] [right arrow] Volume swell (VS) PVC [right arrow] [right arrow] Etc.

Design

Table 1 shows the design generated by ECHIP for the four variables. A total of 16 blends were required (six single plus five duplicates). The left hand column gives the run number and experiment sequence - run 7 is completed first, run 10 second, etc. Reading across gives the blend proportions - for example the composition of run 7 is a 50/50 blend of TPU and COP.

         Table 1 - computer design

Run No.            Variables

          MPR     PVC     TPU     COP

   7      0.00    0.00    0.50    0.50
  10      1.00    0.00    0.00    0.00
  13      0.50    0.50    0.00    0.00
   4      0.50    0.00    0.50    0.00
   5      0.50    0.00    0.00    0.50
  13      0.50    0.50    0.00    0.00
   1      0.00    0.00    1.00    0.00
  14      0.00    1.00    0.00    0.00
  10      1.00    0.00    0.00    0.00
   5      0.50    0.00    0.00    0.50
   9      0.00    0.50    0.00    0.50
   4      0.50    0.00    0.50    0.00
   8      0.00    0.50    0.50    0.00
  15      0.00    0.33    0.33    0.33
   1      0.00    0.00    1.00    0.00
   2      0.00    0.00    0.00    1.00

Sample preparation

All compounds were blended and pelletized in a 30 mm twin screw extruder to insure that the blends were uniform. Initially we intended to soften the pellets on a 2-roll mill and compression mold the sheeted-off material into 2 mm plaques. This technique worked well for all samples except TPU, COP and the blends of both with PVC. Because of the difficulty of breaking the pellets down on the 2-roll mill the TPU/PVC and COP/PVC blends required preheating in a mixer prior to sheeting-off of the 2-roll mill. TPU and COP could not be compression molded easily because of the high melt temperature and resulting sticking to the mold. Consequently, we injection molded plaques of the same thickness (2 mm) and averaged the machine and cross-machine direction properties to get properties comparable to those of the compression molded plaques. As in many designed experiments, all of the elements do not necessarily work as theoretically intended and procedures had to be modified to complete the process. We believe that, even though we had to alter our sample preparation in a few cases the overall results are valid.

Results

The sample plaques were tested for physical properties and chemical resistance. The resulting data were inputed into the ECHIP program. The data were analyzed by ECHIP and a statistical table was generated. The R-squared values (shown in table 2) gave values ranging from 86 to 99% indicating that most of the variability was explained by the model.

        Table 2 - R-squared

Variable   RSQ([R.sup.2])   Variable   RSQ([R.sup.2])

SA         0.885            CS         0.969
TEN        0.978            TT         0.959
ELONG      0.975            GT         0.994
VS1        0.978            TA         0.862
VS3        0.951            CB         0.953
VSW        0.855            COST       0.999

For most chemists, contour plots and 3D graphics may be more useful than the statistical effects table for interpreting the data. Since contour plots show only three variables at one time, effects of the fourth variable can be seen by comparing several of the plots using common axes as a base. The contour plot and 3D plot are complementary - the 3D plot gives a clear perspective of the surface contour whereas the actual values are more easily picked off the contour plot.

Plots and graphics were generated for all of the properties - figure 2 is shown as an excellent example, it shows how tensile strength is affected by blend composition. It is interesting to note that the effect is not linear, as one might expect.

The units on the triangular axes of both contour plots and 3D graphics are proportioned of a particular mixture, i.e., 1.0 MPR is 100% melt processible rubber, 1.0 TPU is 100% thermoplastic urethane, 0.5 on the MPR-TPU axis is a 50/50 blend. As you move above the MPR-TPU axis toward the PVC apex you start to include PVC in the blend, i.e., a point in the center of the triangle is a 1/3/1/3/1/3 blend of MPR/TPU/PVC. The units on the isoline on the contour plot and the vertical axis (TEN) is that for tensile strength (MPa), i.e., the numbers 10, 18, 26, etc., are tensiles of 10 MPa, 18 MPa, 26 MPa. Figures on other properties are available.

Property interpretation

Generally speaking, as hardness of a material increases, the stiffness increases within a resin family, but does not always hold true when comparing different materials. An increase in tensile strength usually suggests improved material toughness and/or strength. Higher elongation is usually desirable provided the material does not go beyond its yield point. MPR is very elastic (rubber-like) and does not exhibit a yield point like TPU, COP or PVC. An increase in volume swell is not necessarily bad provided it is not accompanied by weight loss and shrinkage after removal from the fluid - on the other hand, volume decrease usually indicates that some of the material is being dissolved or extracted such as plasticizer from flexible PVC. Compression set is a property normally associated with rubbers - the lower the number the better recovery from a compressive force. Thermoplastics usually have a very high compression set (90-100%), rubbers 30% or less and TPRs somewhere between (usually less than 70%). Clash-Berg stiffness temperature is the temperature where a given material requires 69 MPa force to twist a test bar - usually the lower the number the more flexible the material is at low temperatures. Tear strength (both trouser and Graves) measures resistance to a tearing force - the higher the force the better. Tabor abrasion measures the weight loss from the material as an abrasive wheel rotates over it for a given number of cycles - the higher the weight loss die poorer the material's resistance to abrasion, conversely the lower the number the better.

Contour plot and 3D graphics for all of the variables and responses were analyzed and show that when blending MPR with TPU, COP or PVC:

* Hardness increases with TPU and COP (COP > TPU) and is essentially unchanged with PVC.

* Tensile increases with TPU and COP (TPU > COP) and is essentially unchanged with PVC.

* Elongation goes through a maximum for TPU, goes through a minimum for COP, and is essentially the same for PVC. Therefore to increase elongation one would blend in up to 50% TPU or more than about 67% COP.

* Resistance to ASTM No. 1 oil decreases with increasing amounts of PVC, which causes higher shrinkage from the extraction of its plasticizers, COP causes less extraction and swells slightly at concentrations above 50%, and TPU reduces extraction. TPU appears to give the best improvement.

* Resistance to ASTM No. 3 oil decreases with PVC indicating that extraction of its plasticizers is occurring. COP causes increased swell whereas TPU decreases swell. TPU appears to give the best improvement.

* Water resistance is only slightly affected by blending MPR with COP. Swell goes through a maximum with both TPU and PVC (40-50%). Therefore adding COP would have less effect.

* Compression set increases with both TPU and PVC (TPU > PVC). COP goes through a minimum at about 40%; therefore it would appear that blending in small amounts of COP would improve compression set.

* Tear resistance improves with both trouser and Graves for all (COP > TPU > PVC).

* Abrasion resistance improves with TPU and PVC. Improves with more than about 30% COP. TPU appears to give best improvement.

* Low temperature flexibility increases slightly with COP and TPU (COP > TPU) and decreases with PVC.

* Cost is reduced with PVC but increases with TPU and COP (COP > TPU)

Summarizing, the chemist can enhance properties of melt processible rubber (MPR) by blending with:

* TPU to improve toughness, abrasion and oil resistance.

* COP to improve compression set, tear resistance and low temperature properties.

* PVC to lower cost and slightly improve tear resistance.

Summary

Melt processible rubber has the most rubberlike characteristics of any commercially available thermoplastic elastomer. It has good rubbery toughness over a broad temperature range and is resistant to weathering and attack by oils and other fluids. However, the rubber chemist may want to enhance one or more of the properties to achieve a combination of properties not available in any single commercial grade. The compatibilities of copolyester thermoplastic elastomer, thermoplastic urethane and flexible PVC with MPR in all proportions enables the chemist to blend any combination of these resins with MPR to the cost/performance balance desired. This article summarized the effect of blending MPR with various combinations and proportions of these resins and provides general guidelines for property modifications and/or enhancement. Also, it shows how the ECHIP computer program can be utilized to provide a large amount of information with a minimum number of experiments. Expanded property effects along with the effect on processing and/or blending with other compatible materials would be a useful extension of this study.

References

[1.] ECHIP Software, version 5.1, license number 11089; ECHIP Incorporated, Hockessin, Delaware 19707.