WO2002018136A2

WO2002018136A2 - Polymeric composites of chlorotrifluoroethylene for use in architectural load-bearing structures

Info

Publication number: WO2002018136A2
Application number: PCT/US2001/027292
Authority: WO
Inventors: Katherine M. Sahlin; Laura Socha; John Effenberger
Original assignee: Chemfab Corporation
Priority date: 2000-09-01
Filing date: 2001-08-31
Publication date: 2002-03-07
Also published as: AU2001287018A1; WO2002018136A3

Abstract

The invention relates to novel amorphous, high molecular weight polymeric composites of CTFE-VF2 for use in architectural load-bearing structures. These composites include a termonomer comprising a vinyl ester or acid with an alpha hydrogen. The composites of the invention may be used in a variety of long-term structure end use applications where resistance to UV exposure, staining, chipping, cracking and flaking are desirable.

Description

POLYMERIC COMPOSITES OF CHLOROTRIFLUOROETHYLENE FOR USE IN ARCHITECTURAL LOAD-BEARING STRUCTURES

FIELD OF THE INVENTION

The present invention relates generally to polymeric composites that are used in the construction of architectural load-bearing structures. In particular, the invention concerns the unique advantages of amorphous copolymers of chlorotrifluoroethylene (CTFE) and vinylidene fluoride (VF₂), which improve the translucency of the composite without compromising the integrity of the composite in its long-term structural end use. The integrity of the composite can be judged by its ability to retain strength and elongation as well as its ability to resist dirt, cracking, chipping and flaking after exposure to UN light.

The invention is the description of the structure and properties of a class of CTFE- VF₂ polymers which make them suitable as a polymer matrix material in composites for architectural structural load bearing structures. This polymer was incorporated in U.S. Patent o. 5,759,924 (incorporated herein by reference) and this description sets forth the particular polymer and its advantageous properties.

BACKGROUND OF THE INVENTION

Structural and civil engineers have used tensioned, plastic-coated or rubber-coated fabric as load-bearing structures in building design for many years. A load-bearing structure is a building component that accommodates the application of external mechanical forces (or loads) without losing its physical integrity. A typical load-bearing structure is composed of a frame constructed of arches and/or beams. Load-bearing structures incorporating coated fabrics were employed initially in the design of air-supported shelters for travelling exhibits, and as enclosures for microwave antennae. More recently, coated-fabric, load-bearing structures have evolved into prestressed (tensioned) members with tensioning provided by stretching a coated fabric over the arches and beams of the structure. As a prestressed member, the internal tension in the stretched fabric provides additional resistance to deformation when another load is applied to the structure.

In the earliest structures incorporating coated fabrics, the fabric served as a reinforcement to control the shape of the structure and to facilitate load-bearing behavior in the structure. The most common coating materials included rubbers such as neoprene, and plastics, such as polyvinyl chloride or polyurethane. The most commonly used woven fabric reinforcements were simple, plain-woven fabrics of nylon or polyester yarns.

Prior art coated-fabrics for load-bearing structures typically incorporate additives to the coating material to protect the structure from the environment. For example, additives may be incorporated into the coated fabric to reduce the ultra-violet burden of sunlight on both the coating polymers and the fibrous reinforcement and thus enhance the outdoor durability of the coated fabric. While such additives protect the coated fabric from the environment, they also substantially reduce or eliminate the translucency of the coated fabric. Light is transmitted through the coated fabric by passing through the myriad of tiny gaps or openings in the coated fabric (so called windows in the woven fabrics).

It would be desirable to provide a polymeric composite for use in architectural applications that has improved ability to retain its physical properties after being exposed to UN light.

SUMMARY OF THE INVENTION

The polymer of the invention is of the class of copolymers containing CTFE and

VF₂. The distinguishing features of this polymer that make it suitable for architectural membrane materials are as follows: high molecular weight (MW); a level of VF₂ sufficient to render the polymer amorphous; presence of vinyl ester or acid with an alpha hydrogen which acts to retard the formation of crystalline domains.

This combination of features is uniquely suited to the UV light exposure expected in an outdoor application. Copolymers of CTFE and VF₂ which are lower in MW and which contain sufficiently low VF₂ concentration to be considered crystalline and not totally amorphous are not suitable candidates for such applications. The presence of a vinyl ester or acid with an alpha hydrogen further acts to retard the formation of crystalline domains and appears to be instrumental in enabling the polymer to withstand the UV exposure expected in such applications.

The recrystallization of the polymers considered unsuitable for outdoor exposure reduce the tensile strength of the matrix as well as reduce the elongation of the matrix. The increased crystallinity and larger sized crystallites that are formed upon exposure to UV light at hot environmental temperatures can produce stress risers in the polymeric matrix material. The preferred composite has a high molecular weight for thermal stability that allows for increased end use temperatures and less degradation for a given set of processing conditions; and a VF₂ content (approximately 17% by weight or more) sufficient to produce an amorphous copolymer. This amorphous nature also allows for a more conformable material with a wider processing window. The CTFE-VF₂ blends alone still recrystallize with UN exposure. The preferred composite further comprises a termonomer (a vinyl ester or acid with an alpha hydrogen) which suppresses the tendency of the polymer to recrystallize upon exposure to elevated temperatures and UN. Vinyl propionate (in the amount of approximately 1.5% by weight) was found to be an effective termonomer.

While many CTFE-VF₂ copolymers remain clear (no hazing) after exposure to

UV, the inventive polymer showed significant improvements in retaining elongation and tensile strength after exposure to UV and heat compared to other CTFE-VF₂ copolymers.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram comparing tensile elongation retention after QUV exposure at 45 °C between commercial extruded PCTFE-VF₂ films of intermediate MW and films cast of low and high MW PCTFE-VF₂.

Fig. 2 is a diagram comparing crystallinity as a function of QUV exposure at 45°C and 70°C between commercial extruded PCTFE-VF₂ films of intermediate MW and films cast of low and high MW PCTFE-VF₂.

Fig. 3 is a diagram comparing tensile elongation retention after QUV exposure at

45°C between high MW coatings of the invention and non-amorphous, high MW coatings.

Fig. 4 is a diagram comparing tensile strength retention after QUV exposure at 45°C between high MW coatings of the invention and non-amorphous, high MW coatings.

DETAILED DESCRIPTION OF THE INVENTION

The use of CTFE-VF₂ fluoropolymer resins to achieve water white coatings on architectural membranes is of interest in the market. This resin material imparts higher light transmission, as compared to current materials, such as PTFE, in the interstitial areas of the coated fabric membrane. While many compositions exhibit some translucency, only the fully amorphous, high molecular weight CTFE-VF₂ polymers of the invention would be suitable to tolerate the outdoor environment of such an application.

Referring to Fig. 1, the percent of original tensile strength after QUV exposure at 45 °C is compared between commercial extruded PCTFE-VF₂ films of intermediate MW (Aclar 22 A) and films cast of low and high MW PCTFE-VF₂. Low MW films degrade to 0% of their original elongation after 325 hours of exposure at 45 °C. High MW films retain 80% of original elongation after 200 hours and 40% after 500 hours of exposure at 45°C. All of these films recrystallize to some degree after UV exposure (see Figure 2) although the high MW sample exhibits fair tensile strength retention. Also demonstrated in Fig. 2, the increase in crystallinity is accelerated under UV light when the temperature of exposure is increased to 70°C.

Referring to Figs. 3 & 4, to counteract this recrystallization, higher proportions of VF₂ were incorporated to render the starting material amorphous and vinyl propionate was added to the polymerization of the high MW polymer to retard the tendency to recrystallize. The high MW film with 9% VF₂ and no added vinyl propionate demonstrates the same characteristics as shown in Figure 1. The high MW film made with the 6% VF₂ and 3% vinyl propionate loses to a considerable degree both elongation and strength. The slightly crystalline film cast from the <17% VF₂, 1.5% vinyl propionate termonomer retains 8% of its original elongation after exposure for 3600 hours at 45°C, while the >17% VF₂, 1.5% vinyl propionate film retains 100% of its elongation at the same conditions.

An embodiment of the present invention comprises a substrate and an amorphous, high MW coating disposed on the substrate, the coating comprising a fluoropolymer comprising CTFE, VF₂, and a termonomer, wherein the termonomer comprises either a vinyl ester or an acid with an alpha hydrogen. The substrate in this embodiment is typically a woven fabric, preferably fiberglass or polyester. The VF₂ in this embodiment is present in the amount of approximately 15% or more and preferably approximately 17% or more by weight of the polymer. The termonomer in this embodiment is present in an amount of approximately between 1.0% and 3.0% and preferably approximately 1.5% by weight of the coating. Such composite preferred for such an application also has a translucency of at least about 23% of normally incident visible light.

The present invention comprises coatings of high molecular weight. The MW of the polymer of the present invention can be determined and/or measured in a number of ways known in the relevant art. One common method involves determining the intrinsic viscosity (IV) of the material and comparing it to the intrinsic viscosity of material of known molecular weight. For example, the coating of the invention may be dissolved in a solvent, such as orthochlorobutyltrifluoride (OCBTF), at approximately 135°C, and the flow of the resultant viscous material measured and compared to the flow rates of material of a known molecular weight. Measured in such a way, the high MW coatings of the present invention have an intrinsic viscosity of approximately 2 dl/g and a molecular weight of approximately 800,000 to 1,000,000.

EXAMPLES

Films of various copolymer constructions were placed in a QUV tester, which provided the UV exposure. Chamber temperature was set at either 45°C or 70°C. Water was present, but not cycled in a particular pattern. Samples were removed after various exposure times and tested for tensile strength, elongation at break, crystallinity and crystallite size.

EXAMPLE 1

Films cast of low and high MW PCFTE-VF₂ were compared to a commercial extruded PCFTE-VF₂ film of intermediate MW. Data for tensile elongation retention after QUV exposure at 45°C is in Figure 1. The low MW semi-crystalline cast film degraded quickly to 0% of original elongation after 316 hours of exposure at 45 °C. The high MW cast film retained 82% of its elongation after 204 hours and 41% after 508 hours of exposure at 45°C. From this comparison it is clear that the low MW material disintegrates with exposure to heat and UV, however, all of the films embrittled upon exposure. Although there is fair strength retention for the high MW cast film, a comparison of crystallinity data indicates that all of these films recrystallize after UV exposure (see Figure 2). The increase in crystallinity is accelerated in the UN when the temperature of exposure is increased to 70°C, as seen in Figure 2.

EXAMPLE 2

Additions of higher proportions of VF₂, to the point of rendering the polymer amorphous, as well as the addition of vinyl propionate were made to the high MW copolymer to counteract this recrystallization. Films were cast of these polymers and tested after exposure in the QUV. Data for tensile elongation retention and tensile strength retention after QUV exposure at 45°C are in Figures 3 and 4 respectively. The film with 9% VF₂ and no added vinyl propionate is the same as that shown in Figure 1. The film made with the 6% VF₂ and 3% vinyl propionate showed comparable loss of elongation and strength while the film cast from the <17% VF₂, 1.5% vinyl propionate termonomer retained 94% of its elongation after exposure for 576 hours at 45°C. The <17% VF₂ termonomer is lower in crystallinity than the 6% VF₂ termonomer in this example. The film cast from the >17% VF₂, 1.5% vinyl propionate termonomer retained 94% of its elongation after 1130 hours. A comparison of Figures 3 and 4 illustrates that retention of tensile elongation is a dramatic indicator of the different material responses to UV which can be obtained when varying the crystallinity of these compositions.

Claims

WHAT IS CLAIMED IS:

1. A polymeric composite for use in outdoor load-bearing structures, the composite comprising: a substrate; and at least one amorphous, high molecular weight polymer coating layer disposed over the substrate; the coating layer comprising a fluoropolymer comprising CTFE, VF₂, and a termonomer, the termonomer comprising either a vinyl ester with an alpha hydrogen or an acid with an alpha hydrogen.

2. The composite according to claim 1, wherein the VF₂ is present in the polymer amount of approximately 15% or more by weight of the polymer.

3. The composite according to claim 1, wherein the termonomer is present in the polymer in the amount of about 1-3% by weight of the polymer.

4. The composite according to claim 1, wherein the substrate is comprised of fiberglass or polyester.

5. The composite according to. claim 1, wherein the polymeric composite exhibits a limiting oxygen index (LOI) of greater than 50%.