Controlled alloying of CSM roofing membranes with thermoplastic resins.

A roof can be defined as the covering and supporting framework on the top of a building which is designed and installed to provide shelter from the elements and to sustain the building's interior environment. Primitive roofs were made of thatched natural fibers pitched steeply to allow water to quickly run off (ref. 1). Initially, flat roofs were only acceptable in hot dry climates. However, the discovery of new materials and construction techniques have allowed flat roofs to be used in practically every climate.

Today there are a number of options that the roofing industry have to consider when selecting a roofing system. The following are but a few of the considerations:

* Architectural - Which may include color, slope, design (shape), weight and energy conservation.

* Installation requirements - How should the system be applied and under what conditions?

* Performance and durability - Ability to accommodate movement due to thermal and mechanical strains; ability to function at high and low temperatures; resistance to environmental elements, such as rain, snow, sunlight, ozone and air pollutants; ability to shed water; life expectancy.

Although the traditional built-up roofing (BUR) remains the most commonly used roofing construction, the use of single-ply roofing membranes continues to grow at a rapid rate. The built-up roof typically involves applying alternate layers of hot bitumen (asphalt or coal tar based) and bitumen saturated felts. The felt is used to impart strength and dimensional stability to the bitumen.

One estimate has indicated that single-ply roofing membranes now account for over 40% of the annual low-sloped roofing market. There have been a number of reasons for the increased interest in single-ply roofing systems, not the least of which are:

* The desire to increase design flexibility and color ability;

* Increased incidents of failures occurring in built-up roofs;

* The increased emphasis on energy savings;

* The longevity of single-ply roofing membranes; and

* Ease of installation versus built-up roofing practices

Experimental

The three polymers selected for this study were Elvax 360, an ethylene vinyl acetate, Hypalon 45, a chlorosulfonated polyethylene, and Hypalon CP-2500, chlorinated polyethylene (CM). The Hypalon 45 (CSM) was selected because of its historical performance in the single-ply membrane industry and because:

* It is more thermoplastic than the other CSM grades;

* Has good low temperature flexibility;

* Better uncured strength;

* Lower viscosity at processing temperatures; and

* Good chemical and oil resistance.

The chlorinated polyethylene (CM) was selected because of its similarity to the chlorosulfonated polyethylene (table 1). Like the chlorosulfonated polyethylene, both have a chlorine content of 25%, but the chlorinated polyethylene has a higher viscosity, a slightly higher crystallinity temperature (70[degrees]C versus 50[degrees]C) and a different physical form.

  Table 1 - CMS and CM basic properties

Polymer type                    CSM        CM
Specific gravity                 1.7        1.10
Chlorine content, %                25         25
Mooney viscosity
  (ML 1 + 10 @ 100[degrees]C       37     120(*)
Crystallinity temperature
[T.sub.c],[degrees]C               50         70
Physical form                   Chips     Powder

(*) Note: Tested using ML 2 + 10 @ 121[degrees]C

The EVA selected has a 25% vinyl acetate content, a crystallinity temperature of 60[degrees]C, and a 2.0 melt index (table 2). Although other EVAs might be considered, the one selected has a high molecular weight and outstanding toughness. Typically, these materials have excellent water resistance and adhesion characteristics.

  Table 2 - EVA basic properties
Specific gravity                                            0.95
Crystallinity temperature ([t.sub.c]),[degrees]C              60
Tensile strength, MPa (psi)                         26.2 (3,800)
Elongation, %                                       800 to 1,000
Hardness, Shore A                                             85
Melt index                                                   2.0
Vinyl acetate content, %                                      25

The formulation used in the designed experiment is shown in table 3. All blends evaluated in this study were mixed in an internal mixer, using an up-side down mixing procedure. This mixing procedure involves charging the mixer with all the ingredients (except the polymers) and adding the polymers last. A sweep-down is done when the compound reaches 82[degrees]C (180[degrees]) and is then dumped at 116[degrees] to 121[degrees]C (240 to 250[degrees]F).

  Table 3 - formulations evaluated
                                            P.H.R
Chlorosulfonated polyethylene           100 to 15
Chlorinated polyethylene                 70 to 15
Ethylene vinyl acetate                   60 to 10

Magnesium hydroxide                           6.0
Calcium carbonate                            80.0
Rutile titanium dioxide                      30.0
Polymeric hindered phenol                     1.0
Stearamide                                    1.0
Polyethylene glycol                           1.3
Oligomeric hindered amine                     0.8

Except where noted, the stress-strain tests were conducted on 1.3 x 0.191 x 15.2 cm. (0.5 x 0.075 x 6.0 inch) strips, died from cold pressed slabs. Cold pressing is performed by sheeting the compound to standard thickness, dieing out preforms, placing the preforms in a 110[degrees]C (230[degrees]F) mold, bumping the press twice, turning on the cold water to reduce the mold temperature and then removing the slabs when the mold temperature reaches 24[degrees]C (75[degrees]F). All tensile tests are conducted at 12.7 cm./min. (5 in./min) extension rates.

All seaming was conducted on calendered sheets, using a hand held heat seaming gun and a roller to press the two heated surfaces together. A 2.5 cm. (1 in.) wide area was not adhered and 2.5 x 15.2 cm. (1 x 6 in.) samples were died from the adhered sheets so that 90' peel adhesion tests could be conducted.

Discussion

In the industrial roofing market, the two synthetic rubbers that have come to dominate the rubber portion of the single-ply roofing membrane (SPRM) are EPDM and CSM. Although both synthetic rubbers have found their niche in the marketplace, there is a continuous need to upgrade and/or to bring new and improved features to the industry. Therefore, this work was designed to investigate the potential of blending three polymeric materials - CSM, EVA and CM to improve the following parameters:

* Calendering ability or processing;

* Physical strength (i.e., tensile, elongation, tear, etc.);

* Heat seaming ability and seaming "open time";

* Environmental aging.

With the exception of the EVA, both CSM and CM have physical performance requirements that are listed in ASTM D-5019, "Standard specification for reinforced non-vulcanized polymeric sheet used in roofing membrane," (tables 4 and 5). This specification provides material requirements for both the non-reinforced coatings and the fabric reinforced membrane. For this study, I have concentrated primarily on the non-reinforced properties, thus eliminating the influence of the fabric reinforcement typically used between the cover and bottom plies of a single-ply roofing membrane. Not evaluating the full construction assembly allows one to more effectively judge the contributions the polymeric covers.

  Table 4 - ASTM D-5019 specification for CSM

(Internally fabric reinforced)
Thickness (min.), mm (in.)                0.9 (0.036)
Break strength (min.), N (lbf)              560 (125)
Elongation, %                                      15
Tear strength (min.), N (lbf)                100 (25)
Low temperature bend @ -40[degrees]C             Pass
Linear dimen. change (max.), %                    2.0
Ply adhesion (min.), N/m (lbf/in.)          1,050 (6)
Ozone resist., no cracks @ 50 pphm               Pass
Weather resistance, no cracks                    Pass
Hydrostatic resist. (min.), kPa (psi)     1,100 (160)
  Table 5 - ASTM D-5019 specification

Cover coating - no reinforcement)
          Types                          1 (CSM)      2 (CM)
Properties
Tensile strength (min.), MPa (psi)      4.8 (700)   2.9 (1,000)
Elongation (min.), %                          300           300
Tear resistance (min.), N/m (lbf)       600 (150)     748 (170)
Ozone resist. @ 100 pphm,
 no cracks                                   Pass          Pass
Water absorption, (max.), %                    10            10

As noted in table 3, a single basic formulation was used to evaluate the effect of different polymeric blend ratios. This was done by varying the ratios of the three polymers and keeping the base formulation the same. The CSM was varied from 100 to 15 phr, the CM from 70 to 15, and the EVA from 60 to 10 phr. Although the polymer levels changed, their ratios totaled 100 phr. The formulation chosen contained:

* Magnesium hydroxide - as the acid acceptor;

* Calcium carbonate as a filler;

* Rutile titanium dioxide as a pigment and UV screen;

* Tetrakis (methylene(3,5-di-tert-butyl-4 hydroyhydrocinnamate) methane as an antioxidant;

* Stearamide for release and as an anti-blocking agent;

* Polyethylene glycol as a processing aid;

* An oligomeric hindered amine for additional ultra-violet light protection.

No curing agent was introduced into the compounds, to allow for in service moisture curing, which permits a longer heat seaming shelf life or seaming "open time."

Processing characteristics

The effect of the CSM/CM/EVA ratios on Mooney viscosity at 100[degrees]C, after a two minute preheat, using the large rotor can readily be seen in figure 1. As expected, the 10 minute readings are basically influenced by the ratios of the polymers and their individual viscosity. Therefore, as we increase the level of EVA, the viscosity of the compounds decreases. CM having the highest individual viscosity at 100[degrees]C, has just the opposite effect.

The influence of the individual polymers on surface smoothness during calendering can be seen in figure 2. The compounds were evaluated on a four roll, inverted "L" shaped calender. The top two rolls were maintained at 99[degrees]C (210[degrees]F), the middle roll at 91[degrees]C (195[degrees]F), and the lower roll at 27[degrees]C (80[degrees]F). Visually rating the surface smoothness from 0 to 5, with 0 being the worst and 5 being the best, the amount of CSM and EVA had the most influence on the surface smoothness. Based on the slope of the lines, however, the level of EVA is most influential. Visual observations also suggest that the level of CSM helped the rheology of the compounds, by improving flow across the nip of the calender rolls. This may be rationalized by the differences in polymer crystallization temperatures to and viscosities. The [t.sub.c] of the CSM is approximately 50[degrees]C, while the [t.sub.c] of CM is 70[degrees]C and that of the EVA is 60[degrees]C.

Although subjective, the addition of EVA also imparts a glossier surface to the compounds, thus providing the compounds with better visual aesthetics. Calender release or the ease at which the stock can be pulled off the rolls (non-sticking) at processing temperatures, was also rated using the same scale. The effect of the polymers on this processing characteristic was almost identical to the relationship found for calender surface.

Physical properties

One of the benefits of using CSM in single-ply membranes is its ability to continue to improve in physical properties with time by curing. "Once installed on the roof, the pendant sulfonyl chloride group on the CSM polymer backbone", as shown in figure 3, "can hydrolyze in the presence of moisture to the corresponding sulfonic acid and slowly react at ambient temperatures with divalent metal oxides or hydroxides to form metal sulfonate cross-links. This results in progressive surface curing which provides added strength and durability. The acidic by-products of the first step are neutralized by additional metal oxide or hydroxides (ref. 2).

As a consequence of this reaction, shelf life of 100% CSM single-ply membrane rolls should be monitored carefully to prevent heat seaming problems in the field. Even though this is a concern, the 100% CSM membranes can be solvent welded if necessary.

Looking at the tensile properties, alloying the polymers gives the compounder a greater latitude for improving the room temperature tensile strength. Using the same triangular plotting technique, room temperature tensile strengths of the compounds were significantly influenced by the ratio of the CM and EVA (figure 4). It was also noted that the modulus values, especially the modulus at 25% elongation, followed the same pattern. Even though this relationship did not significantly change when the same properties were measured at 50[degrees]C (122[degrees]F), the 25% moduli decreased to approximately half of their original room temperature values for all compounds. Using cross-head travel as a measure of the elongation, triangular plots of the original values readily indicate that the CM polymer had the most significant effect at room temperature. As the ratio of the CM is increased, the compound elongation decreased. Again, these tests were conducted with a cross-head speed of 12.7 cm./min. (5 in./min.), and because of the nature of the materials, it is expected that had the tests been conducted at 5.1 cm./min. (2 in./min.) the absolute values would have been greater, but the same relationship between diem would be true.

The relationships of the original tear strengths at room temperature, 50[degrees]C and 70[degrees]C, were not as straightforward as observed with the tensile or stress-strain tests (figures 5, 6 and 7). Original tests at room temperature and at 70[degrees]C suggest that there is a region that exhibits high tear values. This region seems to occur where the polymer ratios are 20/60/20, (CSM/EVA/CM) for the room temperature test data and in the 40/30(30 (CSM/EVA/CM) area for the tests conducted at 70[degrees]C. Overall, however, the tear measurements under all three conditions tend to suggest that as the amount of CSM is decreased the tear values increase. Because tear strength is a significant concern to the roofing industry, this is an area of "cause and effect" that could be studied in greater detail.

After the compounds were mixed, they were allowed to sit at ambient room conditions for two weeks and then calendered into sheets. From the sheets, two 30.5 x 15.2 cm (12 x 6 in.) panels were cut for seaming with a hand held heat gun. After the seaming operation, the bonded panels were allowed to condition for 24 hours and then three 2.5 x 15.2 cm. (1 x 6 in.) strips were died from each assembly. These strips were then put into a tensile tester and a 90[degrees] peel adhesion test was conducted (see figure 8). The results indicate that EVA had the most positive effect on adhesion, and that the CM had the most deleterious effect. Of greater significance, however, is the ability of the EVA to extend the time period in which the compounds can be heat seamed without significant loss in sewn strength.

Aged properties

As noted earlier, it is important that single-ply roofing have a long service life. In the past, 100% CSM single-ply roofing membranes have demonstrated excellent resistance to various fluids, UV and ozone, along with good low temperature flexibility. Therefore, it is essential that these properties be considered.

The fluid resistance of both CM and CSM is greatly dependent upon their chlorine content. As the level of chlorine increases in these polymers, die oil resistance increases. In the case of the EVA, the oil resistance tends to improve with decreasing levels of vinyl acetate. For example, in ASTM #3 oil an EVA with a 25% vinyl acetate content would tend to have a volume swell of 16.5% versus a volume swell of 3.2% for an EVA with 9.3% vinyl acetate content. The EVA chosen for this study has a vinyl acetate content of 25%. As anticipated, the degree of volume increase in IRM #902 oil and vegetable oil, after 70 hours at 50[degrees]C, is almost exclusively dependent on the ratio or level of EVA in the compound.

When aged in distilled water for seven days at 50[degrees]C, it was difficult to draw any conclusions regarding the volume swell, since all the compounds tested had a very narrow range (i.e. 2.9% to 4.3% swell). Aging for longer periods of time, however, seem to indicate the following:

* The degree of volume swell in water is inversely related to the level of EVA and CM in the compounds. As their levels are increased, the amount of volume swell decreases.

* As the level of CM in the polymer blend increases, the percent change in tensile strength increases. The level of EVA in the blend has the least effect on tensile loss.

To evaluate heat aging performance, the compounds were aged for four weeks at 50[degrees] and 70[degrees]C. Under both conditions, as the level of CM and EVA increased, the percentage change in properties decreased. However, when one considers what is occurring within the compound, die influence of the CSM is not unexpected. As noted earlier, the compounds were designed to take advantage of the ability of the CSM to cure while in service. Therefore, heat aging under these conditions would tend to accelerate the curing process in the CSM. Typically, it is expected that as the CSM cures, the elongation would decrease and that the tensile would go through a maximum and then decline. Unlike the CSM, the CM would remain fairly unaffected by these aging conditions, since there is nothing in the formulation to drive it to cure. It is further surmised that the EVA in the blend is truly demonstrating its heat aging capabilities under these conditions.

After 2,000 hours in the QUV tester at 80[degrees]C, it was visually observed that none of the test compounds showed signs of crazing, but there were variations in discoloration. The discoloration observed was not severe, and tended to be a slight yellowing. Subjectively rating the amount of yellowing from 0 to 5, with 5 representing no discoloration, suggests that higher levels of both CM and EVA yield the least discoloration. None of the compounds exhibited ozone cracking, under 20X magnification, after being aged for 144 hrs. in 100 pphm of ozone at 40[degrees]C and under 20% elongation. And, all the compounds passed the -40[degrees]C bend test for low temperature flexibility.

Summary

The scope of this study addresses many of the properties and concerns of the single-ply membrane roofing industry. It has been shown that die use of CSM, CM and EVA at die proper ratio can significantly improve important calendering characteristics by improving the surface appearance and calender roll release. This study describes the advantageous contributions that each of the polymers deliver to the compound and the subsequent single-ply membrane toward meeting the needs of the roofing industry such as:

* How the CSM cures in service to provide continuous improvement in physical characteristics;

* How the level of CM can improve heat aging and fluid resistance; and

How improvements in heat seaming characteristics can be obtained with the proper level of EVA

From this study, one can estimate the best tri-blend balance for optimum properties and it would consist of the following:

* A chlorosulfonated polyethylene (CSM) level between 55 and 70%;

A chlorinated polyethylene (CM) level between 10 and 20%; and

* An ethylene vinyl acetate (EVA) level between 10 and 35%.

Figure 1 - Mooney viscosity ML2 + 10 @ 100[degrees]C

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Figure 2 - calender surface roll temperatures

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Figure 4 - stress/strain - original tested @ R.T.

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Figure 5 - Die "C" tear - original tested @ R.T.

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Figure 6 - Die "C" tear - original tested @ 70[degrees]C

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Figure 7 - Die "C" tear - original tested @ 50[degrees]C

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Figure 8 - seam peel adhesion tested @ R.T.

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References

[1.] F. W. Doherty and A.L Shloss: "Single-ply synthetic roofing membranes." Paper presented to the ASTM Committee D-8, Detroit, Michigan, June 24, 1981. [2.] F. W. Doherty: "Hypalon synthetic rubber for durable single-ply roofing membranes." Paper presented at the winter Rubber Division, ACS Meeting, 1987.