ENHANCED THERMAL RESISTANCE

TECHNICAL FIELD

[0001] This invention relates generally to roofing products, and more particularly to roofing membranes and geomembranes formulated to have enhanced thermal resistance.

BACKGROUND

[0002] Thermoplastic polyolefin (TPO) based roofing membranes are one of many types of roofing membranes available on the market today. TPO may be a melt blend or reactor blend of a polyolefin plastic, such as a polypropylene polymer, with an olefin copolymer elastomer (OCE), such as an ethylene- 3Q propylene rubber (EPR) or an ethylene-propyl ene-diene rubber (EPDR). Examples of commercially available TPO membranes include SURE WELDTM (Carlisle Inc.), GENFLEXTM (Omnova Solutions, Inc), ULTRAPLYTM (Firestone Building Products) and EVERGUARD TPOTM (OAF). Novel stretchable TPO membranes are disclosed in U.S. Patent No. 7,666,491, which is incorporated by reference herein.

[0003] TPO based membranes may also be used as geomembranes to cover landfills, storage ponds of water, and the like. Such TPO membranes may have a higher polypropylene content than a similar TPO roofing membrane and sometimes they are referred to as fPP (flexible polypropylene) membranes or sheets.

[0004] TPO-based roofing membranes may comprise one or more layers. A TPO membrane may comprise base- (bottom) and cap- (top) layers with a fiber reinforcement scrim (middle) sandwiched between the other two layers. The scrim may be a woven, nonwoven, or knitted fabric composed of continuous strands of material used for reinforcing or strengthening membranes. The scrim is generally the strongest layer in the composite. The fabric can contribute significantly to the tensile strength of the roofing membrane and provide for dimensional stability. In an example, the fabric reinforcement comprises a polyester yarn based scrim. Glass fiber based scrims may also be used for situations where additional weight and/or improved dimensional stability are desired.

BRIEF SUMMARY

[0010] An exemplary embodiment of a thermoplastic polyolefin composition may include a thermoplastic polyolefin resin and a UV stabilizer package comprising 0.001 weight percent to 3 weight percent ultrafine titanium dioxide. In an embodiment, the UV stabilizer package may comprise from 0.5 weight percent to 2 weight percent ultrafine titanium dioxide. In an exemplary embodiment, the UV stabilizer package may comprise from 1.5 weight percent to 2 weight percent ultrafine titanium dioxide.

[0011] According to an embodiment of the present disclosure, a thermoplastic polyolefin roofing membrane may comprise a cap layer made of a first thermoplastic polyolefin composition, the first thermoplastic polyolefin composition comprising thermoplastic polyolefin resins and a UV stabilizer package. The disclosed roofing membrane may also include a core layer made of a second thermoplastic polyolefin composition and a scrim layer disposed between the cap layer and the core layer. In an embodiment, the UV stabilizer package of the first thermoplastic polyolefin composition comprises from 0.001 weight percent to 3 weight percent ultrafine titanium dioxide.

[0012] According to an embodiment of the present disclosure, a method of manufacturing a thermoplastic polyolefin membrane may include providing a first thermoplastic polyolefin mixture comprising a thermoplastic polyolefin resin and a UV stabilizing package comprising 0.001 weight percent to 3 weight percent ultrafine titanium dioxide. The disclose method may further include extruding a first thermoplastic polyolefin layer from the first thermoplastic polyolefin mixture.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] Figure 1 illustrates a perspective, side view of an exemplary TPO membrane in accordance with the present disclosure.

DETAILED DESCRIPTION

[0014] Figure 1 is a perspective, side view of a portion TPO membrane 100. The TPO membrane 100 comprises a cap layer 102 and a core layer 104. A scrim layer 106 is substantially sandwiched between the cap layer 102 and the core layer 104. Each of the cap layer 102 and core layer 104 is made of a TPO sheet. When installed on the roof of a house, the TPO membrane 100 may be oriented such that the cap layer 102 is facing upward toward the direction of sunlight and the core layer 104 is facing downward toward the roof. As such, the cap layer 102 may be formulated with additives such as UV stabilizers, UV absorbers, antioxidants, process and thermal stabilizers, fire retardants, white pigments or color pigments, and any other additives known in the art. The formulation of the cap layer 102 may provide for long-term stability in an outside environment and provide an aesthetic appearance if containing color pigments. While the core layer 104 may be formulated to include additives, the core layer 104 may be configured to include no additives to reduce costs. In embodiments in which the core layer 104 includes additives, the formulation for the core layer 104 may be the same as the formulation for the cap layer 102. In another embodiment in which the core layer 104 includes additives, the formulation for the core layer 104 may be different from the formulation for the cap layer 102.

[0015] A method of preparing TPO membrane 100 may comprise first mixing the additives with TPO to effect a desired formulation and forming a sheet of TPO by extrusion. As such, the additives may be distributed throughout the TPO sheet. To form the TPO membrane 100, scrim 106 may be disposed between the TPO sheets 102 and 104, and the three layers may be laminated together at an elevated temperature such that the two TPO layers 102 and 104 are fused and welded together through the interstices of the scrim 106. [0016] Due to the rising energy cost, installing solar panels over a commercial roof may provide significant energy saving for the building owner. Solar panels may be rigid solar panels as well as flexible solar panels. Flexible solar panels may be installed on the roof by directly laminating the backside of the solar panels to the top side of roofing membranes. A significant amount of heat may be generated by the solar panels, especially if they have dark colors. The heat in turn may be transferred to the surface of roofing membranes that are underneath of the flexible solar panels. Consequently, the surface temperature of roofing membranes underneath the solar panels can reach over 200F in hot climates. In a similar manner, geomembranes may have flexible solar panels laminated or otherwise attached to their cap layers.

[0017] In embodiments in which the TPO membrane 100 is installed on the roof, a flexible solar panel (not shown) may be laminated to the cap layer 102 of the TPO membrane 100. As such, the cap layer 102 may be subject to the heat generated by the solar panel. Evan without a flexible solar panel installed over the TPO membrane 100, the absorption of intense light during the summer can also subject the TPO membrane 100 to a high temperature.

[0018] A disadvantage of conventional TPO roofing membranes is that they are designed to withstanding field temperature below 180F. If TPO roofing membranes are exposed to a field temperature higher than 180F for an extended period (such as a couple of years), the TPO membranes may degrade quickly due to UV radiation along with thermal-oxidative degradation caused by high-temperature exposure. Such degradation may lead to surface cracking of the TPO membranes and therefore premature field failure or roof leaking.

[0019] The cap layer 102 and core layer 104 of the TPO membrane 100 may be formulated to include TPO resins. Additives may be included in the TPO cap layer 102 to enhance the resistance to degradation. In some embodiments, additives may also be included in the TPO core layer 104. Suitable additives may include a UV stabilizer package. A UV stabilizer package may include any combination of one or more below ingredients: 1) UV stabilizers functioning as free radical scavengers (e.g., Tinuvin XT-850, which is a Hindered Amine Light Stabilizer ("HALS")); 2) antioxidants functioning as inhibitors of thermo-oxidative degradation at a broad temperature range for long-term thermal stabilizers (e.g., Irganox 1010, which is a sterically hindered phenolic antioxidant); 3) process or thermal stabilizers functioning as inhibitors of thermo-oxidative degradation during extrusion process (e.g., Irgastab FS301, which is a system comprised of a phosphate processing stabilizer Irgafos 168 and a high molecular weight hydroxylamine Irgastab FS042); and/or 4) UV absorber functioning to absorb UV light and dissipate it as thermal energy (e.g., Tinuvin 328, which is 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol).

[0020] In addition to a UV stabilizer package, the TPO cap layer 102 and/or core layer 104 may further include other additives. For example, the formulation for TPO cap layer 102 and/or core layer 104 may include rutile titanium dioxide (Ti0 ₂ ) and at least one fire retardant. In an embodiment, the formulation for TPO cap layer 102 may include 1- 10 weight percent rutile Ti0 ₂ . In another embodiment, the formulation for TPO cap layer 102 may include 2-4 weight percent rutile Ti0 ₂ , The use of Ti0 ₂ in a pigment package has been described in U.S. Patent Pub. No. 2008/0050559, which is incorporated by reference herein. Ti0 ₂ may also be included in a UV stabilizer/antioxidant package as described in U.S. Patent Pub. No. 2004/0157075, which is incorporated by reference herein.

[0021] One of ordinary skill in the art would appreciate the difference in particle size between rutile Ti0 ₂ and ultrafine Ti0 _2, such as the DuPont™ Light Stabilizer 210 ("DLS 210"), which in some embodiment may have a mean particle size of 130 nm. By comparison, rutile Ti0 ₂ generally has a mean particle size ranging from around 250 nm or more.

[0022] A surprising benefit of including ultrafine Ti0 ₂ in a TPO formulation is an unexpected improvement in heat resistance. Disclosed in the present disclosure are surprising heat resistance data observed in TPO formulations comprising ultra fine Ti0 ₂ together with other UV stabilizer package components. According to an embodiment of the present disclosure, an exemplary heat- resistant TPO formulation comprises TPO resins and a UV stabilizer package that includes 0.001 to 3 weight percent ultrafine Ti0 ₂ . The exemplary heat-resistant TPO formulation may further comprise rutile Ti0 ₂ and a fire retardant. In addition to the ultrafine Ti0 ₂ , the U V stabilizer package in the heat resistant formulation may include any suitable components described in the present disclosure. In an embodiment, the UV stabilizer package may include Tinuvin XT-850, Tinuvin 328, Irgastab FS301. In another embodiment, UV stabilizer package may include Tinuvin XT-850, Tinuvin 328, Uvasorb HA88FD (a HALS), Irgastab FS301 , and Irganox 1010. It is to be appreciated that various combinations of UV stabilizer packages components in various amounts may be used together with ultrafine Ti0 ₂ to provide various heat-resistant TPO formulations in accordance with the principles of the present disclosure.

[0023] An exemplary heat-resistant formulation according to the present disclosure is provided in Table 1 below, and the heat resistance of such a formulation is compared to that of comparative examples A, B, and C. Based on these four different TPO formulations for the cap layer 102, four different test TPO membranes were prepared. Each test TPO membrane has a thickness of 80 mm. All four illustrative formulations include TPO resins and 3% rutile Ti0 ₂ . The heat-resistant formulation 1 includes a UV stabilizer package comprising 2% DLS 210, along with 1.7% Tinuvin XT-850, .5% Tinuvin 328, and .5% Irgastab FS301. The UV stabilizer package in comparative example A includes only 2% DLS 210 without additional additives. Compared to the novel UV stabilizer package in the heat-resistant formulation 1, the UV stabilizer package in comparative example B does not include any ultrafine Ti0 ₂ but is otherwise the same. Compared to the novel UV stabilizer package in the heat-resistant formulation 1 , the UV stabilizer package in comparative example C does not include any ultrafine Ti0 ₂ but includes an additional .25% of Irganox 1010.

[0024] The test TPO membranes were tested according to the 280F oven test method, which comprises the following steps: 1) set the oven temperature to 280F for a forced air oven which is well calibrated; 2) put 1 by 2.75 inch piece of the test TPO membrane into the oven; 3) after oven aging for a time period, take the sample out of the oven to cool it down to room temperature; 4) wrap the sample around 3 inch mandrel to examine the TPO cap layer side to see if there are visible cracks under 10X magnification; and 5) if there is visible cracks under 10X magnification, record numbers of days the TPO sample being aged inside of 280F oven.

[0025] Table 1 : Thermal Resistance of 80 mil TPO Membrane

[0026] Comparing heat-resistant formulation 1 to comparative example A, which includes only ultrafine Ti0 ₂ in its U V stabilizer package, the data in Table 1 shows that heat-resistant formulation 1 improves the heat resistance by 636%. The heat-resistance data for comparative examples B and C shows that while a conventional UV stabilizer package comprising three or four different components may provide some heat resistance, the addition of ultrafine Ti0 ₂ synergistic ally allows for at least a 39% improvement in heat resistance.

[0027J According to an embodiment of the present disclosure, an exemplary heat-resistant formulation comprises TPO resins and a U V stabilizer package that includes 0.25 - 2 weight percent ultrafine Ti0 ₂ . According to another embodiment of the present disclosure, an exemplary heat- resistant formulation comprises TPO resins and a UV stabilizer package that includes 0.5 - 2 weight percent ultrafine Ti0 ₂ . According to yet another embodiment of the present disclosure, an exemplary heat-resistant formulation comprises TPO resins and a UV stabilizer package that includes 1.5 - 2 weight percent ultrafine TiO ₂ .

[0028] Shown in Table 2 is the correlation between the amount of ultrafine Ti0 ₂ and the corresponding effectiveness in resisting heat-induced degradation, Exemplary heat-resistant formulations 2-5 are identical except the amount of ultrafine Ti0 ₂ (DLS210) is increased from .5% to 2%. The days it took for cracks to develop did not increase linearly with increases in the concentration of ultrafine Ti0 ₂ . While each formulation still provides improved heat resistance over comparative examples D and E, there appears to be an optimal heat resistance at around 1.5% to 2% ultrafine Ti0 ₂ .

[0029] Table 2: Thermal Resistance of 60 mil TPO Membrane

[0030] It is to be appreciated that the optimal range of concentration of ultrafme Ti0 ₂ may vaiy as the composition of the UV stabilizer package varies. It is to be further appreciated that the optima! concentration of ultrafme Ti0 ₂ may vary as the mean particle size of the ultrafme Ti0 ₂ varies. Ultrafine Ti0 ₂ suitable for the formulations of the present disclosure may have a mean particle size of 165 nm or less. In an embodiment, ultrafine Ti0 ₂ having a mean particle size between 125 to 150 nm may be used. In another embodiment, ultrafine Ti0 ₂ having a mean particle size between 1 10 to 165 nm may be used. It is to be appreciated that the embodiments provide herein are merely exemplary, and may be adjusted in accordance with the principles of the present disclosure.

[0031] Turning back to Fig. 1, the cap layer 102 may be formulated according to any of the heat- resistant TPO formulations disclosed in the present disclosure. In an embodiment, the entirety of the cap layer 102 may formulated with the heat-resistant TPO formulation of the present disclosure. In another embodiment, only a portion of the cap layer 102 is formulated with the heat-resistant TPO formulation of the present disclosure. For example, the cap layer 102 may include a first sub-layer (not shown) adjacent to the outer surface of the cap layer 102, and in an embodiment, the cap layer 102 may be configured to have the first sub-layer formulated with the heat-resistant TPO formulation of the present disclosure.

[0032] While various embodiments in accordance with the disclosed principles have been described above, it should be understood that they have been presented by way of example only, and are not limiting, Thus, the breadth and scope of the invention(s) should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.

[0033] Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a "Technical Field," such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the "Background" is not to be construed as an admission that technology is prior art to any mvention(s) in this disclosure. Neither is the "Summary" to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to "invention" in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings herein,