Outdoor Exposure

 

Flexible Poly (Vinyl Chloride) For Long Outdoor Life

JOHN H. OREM and J. KERN SEARS
Monsanto Chemical Company
St. Louis, Missouri 63166

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A review of the compounding requirements for producing flexible PVC with quite satisfactory outdoor life is presented. Testing criteria and the importance of incorporated additives of specific types are reviewed. Data on multi-location outdoor weathering are shown. The prime importance of thickness and additive migration is proposed.

Plasticized polyvinyl chloride's ability to withstand outdoor exposures is influenced by many factors. These include the flexibility, the thickness of the fabricated product, the additives which are incorporated into the formulation and the amount of thermal degradation that is initiated during compounding and fabrication. This latter point, while being of less importance, must be remembered. At Monsanto Company, for many years we have been exposing plasticized PVC in Florida, Arizona and more recently Puerto Rico.

In this paper, we will give bench marks for the service life under different conditions which may be expected from clear and from pigmented flexible PVC. These formulations will utilize general purpose plasticizers, primarily DOP (di-2-ethyl hexyl phthtalate). Following papers will discuss our intensive evaluations of more varied formulations.
 

TERMS AND THE STANDARD FORMULATION

One sun hour is an accumulated hour during which the sun is shining with an intensity of O.823 Langleys per min ( g cal/cm²/min). Much of our testing has been, and continues to be, done in Miami, Florida. In this area there are approximately 110 sun hours per mouth. This is a total of 1200 to 1300 sun hours per year.

A Langley is 1 gram calorie/cm²/minute. In Miami, Florida, with a 45° due south exposure, the specimens receive approximately 150,000 Langleys per year.

One watt is 0.07 gram calories per minute.

Failures of exposed specimens are measured or recorded in several different ways including:

Time-To-Fifth Spot. Clear films show incipient failure by development of very small random brown spots characteristic of UV degradation. The "first failure" is recorded as the time to the fifth spot on these clippings. The first few spots may appear fortuitous in some cases and unrelated to general failure.

Final Failure. Spotting and accompanying embrittlement continue until the film loses integrity and tends to tear from the rack or until it is completely brown. This is the "final failure" time.

Brittleness Temperature. The brittleness temperature is measured before exposure and then on the samples as they are received at regular intervals from the exposure site. Measurements are run up to 22° C. The ASTM 1790-62 (Masland Cold Crack) method, with the use of semi-micro specimens, is followed in determining brittleness temperatures.

Elongation. Elongation is measured on the samples before exposure. As the samples are received from the testing site, their elongations are again measured.

The data for this paper was collected from unsupported film that was exposed direct to the sun facing due south at 45° from the horizontal.

Today, Monsanto Company's Plasticizer Research Laboratory utilizes the following as its starting standard formulation for evaluating plasticizer performance under outdoor conditions.
 

PVC “s” type (IV 1.13)	100.0
Plasticizer	50.0
Epoxy stabilizer	3.0
Barium cadmium liquid stabilizer	2.0
Phosphite stabilizer	0.1
UV absorber	1.0
Stearic acid	0.5

THICKNESS

Throughout this discussion, we will be showing the influence of thickness on the weathering of flexible PVC films and sheets. Darby and Graham (1) attribute this to the volatilization and leaching of the stabilizers. Degradation commences on the surface where radiation intensity is the greatest. The thicker sections provide a larger reservoir of stabilizer. Stabilizer readily, and constantly, migrates from the reservoir to the film's surface. Thicker films and sheets having larger reservoirs contain more of the preventative-stabilizer. This results in the longer life.

In Fig. 1, the actual results of our Florida aging program show the benefits of thickness. This work was done using DOP as the plasticizer and includes the results of exposed formulations with and without ultraviolet light absorber. The benefits of UV absorbers will be discussed later during our comments on stabilizers. From the actual results that went up to a 20 mil (0.5 mm) thickness, we extrapolated to 60 mil (1.5 mm) thickness; this is shown in Fig. 2. 

PLASTICIZERS

All plasticizers, and all plasticizer concentrations, do not perform in the same manner. Generally, more volatile plasticizers will yield films with a shorter outdoor life expectancy. Clear films, including a UV absorber, plasticized at 50 phr (parts per hundred resin) were exposed in 4, 10 and 20 mil thicknesses in Florida. In this evaluation, four general purpose plasticizers were studied. Two were highly branched diisodecyl phthalate (DIDP) and diisononyl phthalate (DINP). One was singly branched, DOP. The fourth plasticizer, heptyl-nonyl-undecyl phthalate, was essentially linear. This study revealed the benefit of using the less branched phthalate plasticizers for products to be used outdoors. DIDP and DINP, the two highly branched plasticizers, plasticized 4 mil films were entirely brown after 24 mo exposure. The 10 and 20 mil specimens using DIDP browned at 30 months. Heptyl-nonvl-undecvl phthalate and DOP, however, had not shown any browning after 36 months.

A limited outdoor Feathering study in Florida by Darby and Graham (2) showed 35 phr plasticizer to be the most beneficial for long-term durability. This was based on work using two plasticizer systems without UV absorber. One was DOP and the other plasticizer system was 90 percent DOP and 10 percent 2-ethyl hexyl diphenyl phosphate. Also seen in this particular study was the synergistic influence of the phosphate plasticizer in thin films of 4 mil thickness. Looking first at DOP, and using elongation as the measure for outdoor life, we find these 4 mil films plasticized at 20 phr had a life of 13 months. The 35 phr plasticizer level survived for 23 months. At a 50 phr plasticizer concentration, the outdoor life fell to 15 months, and at 70 phr it was only 11 months.

Now, we see the influence of the phosphate synergism on plasticizer level and the outdoors. At the lower two concentrate where films are relatively stiff, the service life increase from the addition of this phosphate plasticizer is 9 to 15 percent. Of considerable interest, the soft and flexible films, those with 50 and 70 phr of plasticizer, have a dramatic increase in life expectancy. The small addition of phosphate plasticizer yields a 50 percent increase in outdoor serviceability. Later work from the same laboratory shows the response of films containing a UV absorbers and plasticized at 50 phr, to outdoor aging when the plasticizer system is varied from all DOP to all 2-ethyl hexyl diphenyl phosphate. We see this benefit of the phosphate synergism in 20 mil thicknesses as well as in the thin 4 mil films. The optimum level of phosphate plasticizer is 10 to 15 percent of the total plasticizer system.
 

STABILIZERS

When formulating flexible PVC films and sheets for outdoor use, the complete stabilizer system must be considered. Darby and Graham (2) showed the influence of epoxidized soybean oil, several metal salts of organic acids, a phosphate ester and an ultraviolet absorber, 2-hydroxy-4-methoxy benzophenone, on weathering. For background, we will briefly review this work.

The influence of stabilizers on outdoor durability of flexible PVC was measured using epoxy cadmium stabilizers individually and in synergistic mixtures with 2-hydroxy-4-methoxy benzophenone. "Epoxy-cadmium" indicated a synergistic mixture of an epoxy compound, a barium cadmium salt of an organic acid and a phosphate ester. When only the epoxy compound and barium cadmium salt were used, decomposition occurred quite early. This was seen by discoloration, serious tack formation and the loss of elongation. The addition of an ultraviolet light stabilizer, such as 2-hydroxy-4-methoxy benzophenone, had essentially no benefit in this stabilizer system. However, triphenyl phosphite by itself yielded a longer life than either of the above stabilizers. When 2-hydroxy-4-methoxy benzophenone was added to the triphenyl phosphate, there was a large improvement in the weathering life of the film. Although the epoxy and barium cadmium constituents were not necessarily needed to achieve good outdoor durability, they are definitely required to insure adequate heat stability during the processing of the flexible polyvinyl chloride.

Our data were obtained with formulations using conservative stabilizer levels.
 

PIGMENTS AND COLORANTS

Outdoor life expectancy of flexible PVC may be aided through pigmentation. H. C. Jones (3) found increasing increments of anatase and rutile titanium dioxide definitely improved reflectance characteristics of plasticized PVC. His accelerated weathering studies revealed rutile titanium dioxide to be decidedly superior to the anatase as a light stabilizing agent.

In his work, Mr. Jones was interested not only in whether the pigments absorbed in the UV range, but also what happens to the absorbed energy. He found, "When an untreated rutile absorbs UV the absorbed energy goes into a photochemical reaction that liberates active oxygen. For this reason, rutile for use in plastics is given a surface treatment which inhibits this photochemical reaction and causes the absorbed UV energy to be dissipated as heat.

DeCoste and Hansen (4) showed colored pigments strongly assisting in the maintenance of mechanical properties by protecting the plasticized polyvinyl chloride compositions from degradation. Combinations of pigments are beneficial to long-term outdoor aging because they can shield in the visible and the ultraviolet range - the use of rutile titanium dioxide with a selected colored pigment was quite good.

Many exposures have revealed the benefits of titanium dioxide in films of three thicknesses: 4 mils (0.1mm), 10 mils (0.25mm), 20 mils (0.5mm). Without titanium dioxide and without an ultraviolet absorber all failed in 22 months. However, with titanium dioxide the 4 mil film had a life of 32 months, and the 10 mil film's life was 47 months. More pronounced, was the effect on the 20 mil flexible sheet; with titanium dioxide, it survived 76 months, more than 6 years, in Florida.

Similar outdoor studies were made with blue and black films. Both colorants, phthalocyanine blue at 0.9 phr and channel black at 1 phr, definitely extend the weathering life of flexible PVC films and sheets. The 20 mil blue film survived 80 months, over six and a half years, before failing the room temperature brittleness test.
 

Table 1. Weather-Ometer Exposures of Flexible PVCa Containing Titanium Dioxides
Exposure, days	Anstaso		Ruttie
	Rb	bb	R	b
	1 P.H.R.		1 P.R.R.
0	73.4	2.4	78	2.5
10	71.7	0.8	78.1	2.2
20	65	7.3	75.3	4.9
30	36.5	20.3	73.8	6.7
40	24.1	15.2	79	7.9
50	20,1	11.8	69.11	9.9
60	19.1	10.2	67.6	10
	3 P.H.R.		3 P.H.R.
0	86.8	2.7	89,4	3.5
10	82	1.5	88.1	4
20	78.8	4.6	64.4	7.4
30	70	11.3	82.4	9.1
40	44.7	19.5	80.8	10.5
50	40.8	16.2	79.4	11.8
60	38.8	14	79.9	11.9
	5 P.H.R.		5 P.H.R.
0	89.5	2.6	91.4	3.4
10	83.9	1.7	90	4.7
20	80.9	4.6	86.2	7.8
30	72.5	9.8	84.1	9.7
40	49.8	17.4	81.9	11.2
50	46.6	13.9	80.8	12.5
60	45.2	12.6	81.2	12.7
a Formula (parts by weight): PVC resin (Geon 101), 100:plasticizer (DOP), 50:stabilizer (Mark M), 3:lubricant (stearic Acid), 0.5 titanium dioxide, as indicated.
B Hunter color measurements (reflectance and “b” value) made over standard black background.

As has been stated many times, the use of black pigments is encouraged to achieve the ultimate in outdoor life with flexible PVC products. Twenty mil sheets withstood 5 years of Florida exposure before reaching the 0° C brittleness temperature. Extrapolating this, we would expect a 60 mil, black pigmented, flexible PVC sheet to withstand more than ten years outdoor exposure in most environments.
 

CONCLUSION

Flexible PVC can be produced that will provide quite satisfactory outdoor life. In formulating, one should attempt to use a plasticizer concentration in the range of 35 parts per hundred parts of PVC. Consider using a good phosphate ester as 10 percent of the plasticizer system to take advantage of the synergistic effect, particularly where plasticizer levels above 35 phr have to be used. Wherever possible, use some pigmentation; black formulations will give the longest outdoor life. Blues significantly extend life. The incorporation of treated rutile titanium dioxide is very beneficial. Ultra-violet light absorbers must be included in clear films. In addition to the epoxy and barium cadmium containing stabilizers, include a phosphate ester in the stabilize system. The thicker the film, the longer will be its expected outdoor life.

Throughout this work we have reported the weathering life and extrapolated life expectancy using brittleness temperature along with other methods of measurement. Cold Crack measuring is very sensitive to surface deterioration and, therefore, quite sensitive to weathering. To conserve exposed samples, we use semi-micro specimens (32mm x 4mm) for determining the Masland Cold Crack temperature.
 

REFERENCES

1. Joseph R. Darby and Paul R. Graham, "Outdoor Durability of Plasticized Polyvinyl Chloride," Modem Plast., January 1962.

2. Ibid., Darby and Graham

3. H. C. Jones, "Improving Weatherability of Plastics With White Pigments. Modern Plas., January 1972.

4. J. B. DeCoste, and R. H. Hansen, "Colored Poly(Vinyl Chloride) Plastics for Outdoor Applications." SPE J., 18,431 (1962.)

 

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