Effect of Formulation Parameters on Performance of Polyisocyanurate Laminate Boardstock Insulation

SACHCHIDA N. SINGH, JODY S. FIFE, SHEILA DUBS AND PAUL D. COLEMAN

Huntsman Advanced Technology Center 8600 Gosling Road The Woodlands, TX 77381

ABSTRACT

The continued success of polyisocyanurate laminate boardstock insulation (polyiso) in the construction industry can be attributed to many factors. These include its superior thermal resistance, good structural properties and excellent fire performance, but above all, its cost-effectiveness. Polyisocyanurate insulation boards have always been positioned as the lowest installed cost insulation per unit of R-value while meeting the structural and fire requirements of the building codes and blowing agent mandates of the Environmental Protection Agency. Such attributes are essential for maintaining a competitive edge over alternative insulation materials.

With conversion to pentane blowing agents fully completed a few years ago, the focus of the board manufacturers and material suppliers has shifted to formulation and process optimization. Learning from ongoing commercial production using pentanes, and building on prior published works, this study systematically looks at the effects of selected polyurethane formulation and processing variables on the key foam and laminate board performance characteristics. Formulation variables examined include isocyanate index, polyol, and fire retardant. In particular, this study looks at interplay between polyol hydroxyl number, fire retardant level and isocyanate index on board performance. Measured performance characteristics include structural and thermal properties of laminate boards and its fire performance, including some Factory Mutual Calorimeter test.

This work suggests that the performance of the polyiso board is strongly dependent on weight % of isocyanate in the foam formulation. The structural performance of the board as expressed by compressive strength and dimensional stability is satisfactorily met only when the weight % of the isocyanate in total foam is 57% or higher irrespective of the OH # of the polyol at the tested foam density. Reducing the weight % of isocyanate in the foam formulation is unlikely to improve cost-effectiveness as a higher density would then be needed to meet all of the performance requirements. Increasing the fire retardant level is found to be beneficial to improving the fire properties, but high levels of non-reactive fire retardant plasticize the foam and thus increase the dimensionally stable density.

INTRODUCTION

With a production of over 5.4 billion board feet in 2004, polyiso board is the insulation of choice for commercial roofing in USA [1]. It represents over 50% of all insulation used in new roof construction and at least 40% of all insulation used in re-roofing applications. Such wide acceptance by the construction industry is attributable to the fact that no other competitive product can match its combination of thermal resistance, fire performance, structural integrity and cost-effectiveness. The polyiso industry has been able to maintain its position of the lowest installed cost insulation per unit R-value through all the blowing agent and other changes. This drive for lowest cost while meeting the performance requirements has meant constant re-examination of all the factors that contribute to cost and performance. Recent high energy prices have added a new dimension to this re-examination. Higher energy prices spur the demand for more energy-efficient building practices which translates to higher demand for insulation. Higher energy prices have generally gone hand in hand with higher petrochemical prices, which generally mean higher raw material costs for polyiso. Higher demand along with concurring higher raw material cost creates a unique situation for re-examination of all factors contributing to cost and performance.

Raw material costs are by far the largest component of the polyiso product variable cost. Among the various components of polyiso board, the foam formulation is the one that is generally optimized to yield lowest cost per unit R-value while meeting the mechanical and fire requirements. Polyiso boards are currently manufactured by a continuous lamination process, using aromatic polyester polyols, high functionality polymeric MDI, fire retardant, catalyst, surfactant,

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and water and pentane as a blowing agent. Column 2 of Table 1 shows a typical formulation currently in use. High functionality polymeric methylenediphenyl diisocyanate (HF-PMDI) such as RUBINATE® 1850 isocyanate is the largest single component of the foam formulation and thus the single largest cost. Though the isocyanate index which determines the amount of the HF-PMDI in the formulation has always been in play in any formulation optimization, lately there have been initiatives to lower the hydroxyl number of the aromatic polyester polyol, which in turn lowers the amount of HF-PMDI at the same index [2-4]. Similarly, though the type and level of fire retardant has always been a variable in formulation optimization, there has lately been an emphasis by some to increase the amount of fire retardant and simultaneously lower the amount of HF-PMDI [2,5].

The objective of this paper is to critically examine the role of high functionality polymeric MDI, aromatic polyester polyol and fire retardant in meeting all the performance requirements of the polyiso board. This is done with the aim of finding the most cost-effective approach to manufacture board.

Table 1. Typical Formulations Standard OH Polyol Low OH Polyol Low OH polyol w/ high FR PHP % Total PHP % Total PHP % Total Aromatic Polyester Polyol Std. OH of 240 100 30.50 Low OH of 208 100 33.18 100 32.4 TCPP 10 3.05 9.2 3.05 15 4.9 K-octoate (15% K) 6.0 1.83 5.5 1.83 5.66 1.83 K-acetate (10% K) 1.0 0.30 0.92 0.30 0.94 0.30 JEFFCAT® PMDETA catalyst 0.2 0.06 0.18 0.06 0.19 0.06 Silicone surfactant 2.0 0.61 1.84 0.61 1.88 0.61 Iso- and/or n-pentane 22.0 6.71 20.22 6.71 20.72 6.71 Water 0.4 0.12 0.37 0.12 0.38 0.12 Total Polyol Side 141.6 43.19 138.25 45.87 144.76 46.90 RUBINATE® 1850 isocyanate 186.2 56.81 163.2 54.13 163.9 53.10 Isocyanate Index 260 260 260 Aromatic in foam, wt % 37.6 36.6 35.9 Isocyanurate in foam, wt % 10.7 10.2 10.0 Nitrogen in foam, wt % 5.8 5.5 5.4 Chlorine in foam, wt % 1.0 1.0 1.6 Phosphorous in foam, wt % 0.3 0.3 0.5 Non-reactive/mono-functional in polymer, wt %

8.1 8.4 10.2

Requirements on Polyiso Boards

The physical property requirements for polyiso insulation are specified in ASTM C 1289 - 06 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. This standard specifies the compressive strength, dimensional stability, flexural strength, thermal resistance, moisture resistance to name a few of the many performance requirements on the boards. Though each of the requirements are critical to the use of board, it is generally understood that compressive strength, dimensional stability, thermal resistance and fire performance are the properties that determine the formulation and operating board density.

Though polyiso insulation boards are available in a range of compressive strengths, as per ASTM C 1289, a minimum of

16 psi on the faced product in the thickness direction is required for all polyiso products. Generally, it has been a practice of the board manufacturer to target a compressive strength of 20 psi. Dimensional stability of polyiso board is tested at three different environmental conditions: -40oF, 158oF/97% relative humidity and 200oF in accordance with ASTM D2126 on 12” by 12” by board thickness specimen. In addition, our prior work has demonstrated that the most severe field exposure condition on board is simulated by exposure to -40oF under the Dimvac test conditions on 4” by 4” by 1” core foam [6].

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Even though, ASTM C 1289 only specifies the minimum thermal resistance of the board measured after 180 days of conditioning at room temperature, most polyiso manufacturers have adopted the Long Term Thermal Resistance (LTTR) as the exclusive means to market thermal performance of roof insulation. LTTR is measured using ASTM C 1289-06, Annex A, by predicting the 15 year time-weighted average R-value.

In order to meet the requirements of model building codes throughout the USA, polyiso products must meet a series of

fire performance tests developed by ASTM, Factory Mutual Research Corporation (FMRC) and Underwriters Laboratories (UL). The exact fire performance test that the polyiso board must meet depends on building location, roof design and insurance underwriters of the property. Reference 7 gives an overview of many of the widely used tests. Polyiso roof insulation is known for being the only foam plastic board product that meets the strict standards of both FM Approvals (Standard 4450) for Class 1 roof systems and UL (UL1256) without the use of an additional thermal barrier layer between the insulation and the supporting steel deck. As such any formulation re-examination must retain this quality. It is generally understood that FMRC Calorimeter (FM Standard 4450/4470), used to assess the fuel contribution rate of a roof assembly so as to reduce the potential to spread fire on the underside of the roof deck, is the most challenging test to meet to get a Class 1 rating. Exterior combustibility tests conducted in accordance with ASTM E 108 on a roof assembly and ASTM E 84 flame spread and smoke developed ratings on core foam are two other tests that present some challenge.

In addition, polyiso must have good adhesion to faces, low water absorption, low water vapor transmission, prescribed

flexural strength and tensile strength perpendicular to board surface. Material Requirements

Before we explore the most cost-effective approaches to meet the above requirements on polyiso boards, let us discuss the structural and thermal stability requirements on the polymer to meet such performance. During FM Calorimeter testing, temperatures at the underside of the steel deck have been reported to go from near ambient temperature to 1000oF within minutes and then creep as high as 1500-1600oF in the 30 minute duration of the test [5, 8, 9]. Such temperatures are well above the decomposition temperature of most organic polymers including Polyiso foam, a low density poly(isocyanurate-urethane) material. The decomposition temperature of the isocyanurate linkage, the most thermally stable bond in polyiso foam, is in the range of 570-620oF [9, 10]. Other linkages such as urethane, ester, urea, allophanate, and uretonimine break-down at lower temperatures. It is the char forming ability of the polyiso foam that keeps the asphalt on the roof assembly from melting and dripping into the calorimeter and not contributing to the allowable BTU limits of the FM Calorimeter test. Polyiso foam board must form a dimensionally stable char as a result of thermal decomposition so that the char maintains a continuous surface and does not form wide and deep cracks due to excess shrinkage. The char must have low thermal conductivity so as to prevent heating up the coverboard and asphalt. Molten asphalt would find its way into the calorimeter through the build-in joints and any cracks thorough the coverboard and char layer. Intumescent and oxidation resistant char would be beneficial but not essential.

Surprisingly there has not been any study to understand the mechanism or composition of char formation in the FM calorimeter. A review of the general polymer literature suggests that char yield should have a direct relationship to the number of aromatic ring structures in the polymer back-bone [9, 11]. Literature also suggests that the char is primarily a carbonaceous residue and is best described as a conglomerate of loosely linked small graphitic regions, i.e., aromatic carbons [12]. A recent study of the composition of char in polyiso foam exposed to direct flame also suggests the presence of primarily aromatic carbon [13]. Though the polyiso foam board is not exposed to direct flame in the FM Calorimeter, it is in the ASTM E 84 and E 108 tests.

All this suggests that while investigating the polyiso formulation to meet the fire requirements of board, we should consider not only the isocyanurate content of the foam but also the aromatic content. Pentane blown formulations have generally used a fire retardant (FR) to meet all the fire requirements [2]. Despite numerous efforts to find alternatives, phosphate esters, in particular tris(2-chloro-isopropyl)-phosphate (TCPP), have remained the FR of choice for the US boardstock industry due to their unique cost-performance balance. The fire retardation mechanism of phosphate esters is fairly well characterized and generally works as free radical scavenger in the gas phase and char promoter in the condensed phase [14, 15]. It is generally accepted that phosphate esters go through the following steps:

Phosphoric acid ester → phosphoric acid → metaphosphoric acid → poly(metaphosphoric) acid. Through their strong dehydrating and dehydrogenating action, these acids accelerate the carbonization of the polymer

and formation of a carbonaceous charred structure [14-16]. It is likely that the aromatic content and the cross-link density of the polymer (and thus inherent thermal stability) plays a role in the effectiveness of the above char formation mechanism.

Mechanical properties of the polyiso foam board, i.e., compressive strength and dimensional stability, follow many of the rules of general polymer science. Those rules suggest such properties are enhanced by the formation of a long polymer

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chain, high cross-link density, presence of high main-chain aromatic content, and absence of non-reactive or chain-terminating mono-functional additives. Early polyisocyanaurate foams from the 1970’s were too brittle and the introduction of di-functional aromatic polyester polyols, especially those of decreasing OH # from 350 to 250, improved conversion, reduced friability and paved the way for the development of present day urethane modified polyisocyanurate foam [3, 8]. The use of roughly 240 hydroxyl aromatic polyester polyol as the sole polyol in polyiso foam became a standard in the mid-80’s and has performed well through all the changes in blowing agents. Formulation Parameters

Table 2 shows the characteristics of the key reactive components of a typical polyiso foam. The aromatic content is defined as weight % of aromatic ring alone, i.e., phenyl content. This is different than phthaloyol (an aromatic ring with two carbonyl groups) content which is how the polyester polyol industry sometime defines aromatic content. It is very unlikely that the carbonyl attached to the phenyl ring would participate in or facilitate char formation. Nitrogen content is listed here as it is thought to lower flammability [17]. The weight % of non-reactive and mono-functional additives is listed too, as non-reactives would plasticize the foam polymer and mono-functional species would terminate the chain, thus lowering the cross-link density and molecular weight. The amount of such additives in aromatic polyester polyols varies significantly depending on the supplier and the exact polyol. Ten weight % is on the lower end of the typical level of such additives in polyols. HF-PMDI with its average functionality of 2.9 is the only network forming component in the polyiso formulation. Of course, isocyanates promote additional cross-linking by formation of isocyanaurate, allophanate, biuret and uretonimine all of which are network forming.

Table 2. Typical properties of key reactive components HF-PMDI Polyol K-octoate Water Aromatic content, wt. % 55.5 ~20 0 0 Nitrogen content, wt. % 10.2 0 0 0 Number average functionality 2.9 < 2.0 <2 2 Equivalent weight 137 250 - 300 165 9 Non-reactive & mono-functional, wt. % 0 >10 ~30 0

The effect of decreasing the hydroxyl number of the polyol on the aromatic and isocyanurate content of the foam at a fixed index and blowing level is illustrated in Figure 1. Blowing level, defined as the volume of gas generated per unit weight of polymer, is fixed to compare foams at the same density. Table 1 shows the impact of decreasing the hydroxyl number along with the effect of increasing the FR content on weight % of nitrogen, chlorine, phosphorous and non-reactives. Clearly all these parameters change significantly when OH # of the polyol or the FR content is changed.

In this study, we used a full factorial experimental design with formulation variables as shown in Figure 2. The choice of fixed weight % of HF-PMDI as opposed to fixed index is exactly that, a choice. The findings of the study are not dependent on this choice.

Board Manufacture

Boards from 12 different formulations shown in Table 3 and 4 were produced on a commercial fixed gap laminator in the USA using raw materials that are commercially available. In order to limit the quantity of boards produced, only four batches of polyol side, one each with two polyols at each level of TCPP were made and then the weight % of isocyanate was changed by adjusting the relative flow rates through the pumps. The amount of pentane in the formulation was controlled independently in order to insure a constant core foam density throughout the 12 formulations. All this meant that the amount of the additives such as catalyst and surfactant changed a bit among the boards made using the same polyol and FR level. At the same isocyanate level though, e.g. compare A-57 to C-57 or B-55 to D-55, the level of all components except the independent variables remained the same.

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34

35

36

37

38

150 200 250

OH # of Polyol

Arom

atic

con

tent

, w

t %

9

10

11

12

Isoc

yanu

rate

, wt %

Aromatic Content Isocyanurate

Figure 1. Effect of the OH # of a polyol on aromatic and isocyanurate content of a foam at a fixed index of 260

All boards were made using typical production conditions (line speed, laminator/facer/chemical temperatures, and stacking sequence). All laminates had a nominal thickness of 1.5” and used an ASTM C1289, Type II black facer typical of roofing boards. A minimum of 30 minutes of laminator run time was used for each formulation to assure a properly lined up process before collecting boards for testing. All laminates used for testing were bundle cured for 24 hours in the middle of the bundle, sorted for shipment and then stored indoors as a bundle for many days prior to any testing.

Figure 2. Schematic of the full factorial experimental design RESULTS AND DISCUSSIONS

Whenever applicable, foam laminates were tested using the standard test method described in the polyiso material standard ASTM C 1289 – 06. Two tests not described by ASTM but widely used in industry, namely Hot Plate fire test and Dimvac dimensional stability tests were also used. A full factorial modeling of the data, which included analysis of variance and assessment of main effects and interactions among the variables were performed using a commercial statistical analysis software. A critical p-value of 0.05 was chosen to determine whether a property was significantly affected by the chosen variable or not. Tables 3 and 4 show average values of all the results along with the formulations.

All of the 12 formulations processed well giving fine celled foam, with soft, straight knit-lines and no visible post-growth. The reactivity for all systems were fairly close to each other and typical of current commercial practice. The low hydroxyl polyol seems to process better, with less frothiness, better flow, and easier line up of the product. The core foam density and the defaced board density were fairly constant for all the runs. This was purposely done by fine tuning the level of pentane added in-line so as to compare the effect of the control variables without interference from density. The core foam density was 1.56 ± 0.02 pcf and the defaced board density was 1.74 ± 0.04 pcf. Such densities are likely to be on the low end of the commercial production range and were targeted to enable us to discriminate among the variables chosen here.

OH# 208

OH # 240

% Isocyanate

% T

CPP

Hyd

roxy

l # o

f pol

yol

57 55 53

5.2

2.6

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Table 3: Formulations Using Standard Hydroxyl Aromatic Polyester Polyol (APP) Standard OH Polyol + Low TCPP Standard OH Polyol + High TCPP Formulation, % total system A-57 A-55 A-53 B-57 B-55 B-53 APP, Std. OH of 240 30.15 32.02 33.87 27.85 29.56 31.28 TCPP 2.44 2.60 2.75 4.88 5.18 5.48 K-octoate (15% K) 1.76 1.87 1.98 1.77 1.88 1.98 K-acetate (10% K) 0.29 0.31 0.33 0.29 0.31 0.33 JEFFCAT® PMDETA catalyst 0.06 0.07 0.07 0.07 0.07 0.07 Silicone surfactant 0.65 0.69 0.73 0.65 0.69 0.74 Water 0.13 0.14 0.15 0.13 0.14 0.14 Pentane 7.52 7.32 7.12 7.36 7.18 6.98 Total Polyol Side 43.00 45.00 47.00 43.00 45.00 47.00 RUBINATE® 1850 isocyanate 57.00 55.00 53.00 57.00 55.00 53.00 Isocyanate Index 260 236 215 278 252 230 Aromatic in foam, wt. % 37.5 36.7 36.0 37.0 36.3 35.5 Isocyanurate in foam, wt % 10.7 9.7 8.7 11.1 10.1 9.1 Nitrogen in foam, wt % 5.8 5.6 5.4 5.8 5.6 5.4 Chlorine in foam, wt % 0.79 0.84 0.89 1.58 1.68 1.78 Phosphorous in foam, wt % 0.23 0.25 0.26 0.46 0.49 0.52 Non-reactive/mono- in polymer, wt % 7.6 8.1 8.6 10.0 10.6 11.2 Physical Properties Density, pcf Core foam 1.6 1.5 1.6 1.6 1.6 1.6 Defaced laminate 1.7 1.7 1.7 1.7 1.7 1.7 Compressive Strength, psi Laminate across thickness 20.3 18.1 17.2 18.8 16.8 17.4 Dimensional stability, Laminate 2 weeks @ -40oF, Rank* 1-4 1 2 2-3 1-2 2 2-3 Dimensional stability, core foam 2 weeks @ -40oF, Rank* 1-4 1 1 1 1 1 1 2 wks @ -40oF after Dimvac, Rank* 1-4 1 2-3 2-3 1-2 2 2-3 2 wks @ 70oF/97%RH, % linear change 3.1 3.7 6.3 3.1 3.8 6.3 Thermal Resistance, ft2.h.oF/Btu.in Initial, 6.1 6.1 6.1 6.0 6.0 6.0 Predicted LTTR for 2.5” 5.7 5.7 5.7 5.6 5.6 5.7 Fire Performance FM Calorimeter Pass NT NT NT NT NT Hot Plate Weight retention, % 79 79 77 83 80 79 Thickness retention, % 74 71 65 74 80 85 Butler Chimney Weight retention, % 66 63 59 85 83 79 Extinguish time, sec. 19 19 22 12 12 14 Oxygen Index 22.8 22.6 22.6 23.5 23.4 23.0 NBS max. smoke density 70 73 84 32 33 48

* Rank 1- No distortion, 2 – Some edge collapse, 3 - Severe edge collapse, 4 – Severe distortion NT – Not Tested

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Table 4: Formulations Using Low Hydroxyl Aromatic Polyester Polyol (APP) Low OH Polyol + Low TCPP Low OH Polyol + High TCPP Formulation, % total system C-57 C-55 C-53 D-57 D-55 D-53 APP, Low OH of 208 30.15 32.02 33.87 27.85 29.56 31.28 TCPP 2.44 2.60 2.75 4.88 5.18 5.48 K-octoate (15% K) 1.76 1.87 1.98 1.77 1.88 1.98 K-acetate (10% K) 0.29 0.31 0.33 0.29 0.31 0.33 JEFFCAT® PMDETA catalyst 0.06 0.07 0.07 0.07 0.07 0.07 Silicone surfactant 0.65 0.69 0.73 0.65 0.69 0.74 Water 0.13 0.14 0.15 0.13 0.14 0.14 Pentane 7.52 7.32 7.12 7.36 7.18 6.98 Total Polyol Side 43.00 45.00 47.00 43.00 45.00 47.00 RUBINATE® 1850 isocyanate 57.00 55.00 53.00 57.00 55.00 53.00 Isocyanate Index 291 264 241 310 282 256 Aromatic in foam, wt. % 37.5 36.7 36.0 37.0 36.3 35.4 Isocyanurate in foam, wt % 10.7 9.7 8.7 11.1 10.1 9.1 Nitrogen in foam, wt % 5.8 5.6 5.4 5.8 5.6 5.4 Chlorine in foam, wt % 0.79 0.84 0.89 1.59 1.68 1.78 Phosphorous in foam, wt % 0.23 0.25 0.26 0.46 0.49 0.52 Non-reactive/mono- in polymer, wt % 7.6 8.1 8.5 10.0 10.5 11.2 Physical Properties Density, pcf Core foam 1.6 1.6 1.6 1.6 1.6 1.5 Defaced laminate 1.8 1.8 1.8 1.8 1.8 1.7 Compressive Strength, psi Laminate across thickness 19.7 18.2 17.3 19.7 18.2 18.9 Dimensional stability, Laminate 2 weeks @ -40oF, Rank* 1-4 1-2 2 2 1-2 1-2 2-3 Dimensional stability, core foam 2 weeks @ -40oF, Rank* 1-4 1 1 1 1 1 1 2 wks @ -40oF after Dimvac, Rank* 1-4 2 2-3 3 1 1-2 1-2 2 wks @ 70oF/97%RH, % linear change 3.1 3.9 5.6 3.0 4.1 6.6 Thermal Resistance, ft2.h.oF/Btu.in Initial, 6.0 5.8 5.8 5.8 5.9 5.8 Predicted LTTR for 2.5” 5.6 5.4 5.4 5.4 5.6 5.5 Fire Performance FM Calorimeter Pass Fail Predict Fail + NT Pass NT Hot Plate Weight retention, % 79 77 77 76 79 76 Thickness retention, % 77 71 69 77 75 70 Butler Chimney Weight retention, % 64 56 52 71 65 61 Extinguish time, sec. 21 23 24 17 18 20 Oxygen Index 22.6 22.6 22.3 23.2 23.2 22.8 NBS max. smoke density 52 65 84 36 46 52

Rank 1- No distortion, 2 – Some edge collapse, 3 - Severe edge collapse, 4 – Severe distortion + test aborted at 27th minute out of 30 minute

NT – Not Tested

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Structural Properties of Polyiso Board The compressive strength of the laminate in the thickness direction is key to the ability to properly install the roof assembly. It is generally targeted to be 20 psi or higher. Figure 3 shows the average compressive strength and it is higher for boards containing higher weight % of HF-PMDI. Mathematical modeling of the compressive strength data suggests that increasing the isocyanate content of the foam and reducing TCPP content would lead to higher strength. There also seems to be a statistically sound interaction between polyol and TCPP level with higher level of TCPP with the low hydroxyl polyol giving a higher strength.

16

18

20

57 55 53

Isocyanate (HF-PMDI), wt%

CS

in ri

se d

ir., p

si

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH +High TCPP

Figure 3. Compressive strength of the laminate in thickness direction

Cold dimensional stability of the polyiso board as measured by aging 12” by 12” by full-faced boards for two weeks at -40oF showed little change in the measured dimensions but the edges of many samples shrank. Figure 4 contains pictures of the samples showing varying degree of edge shrinkage. Results were similar for boards made using Low OH polyol, i.e., C and D samples. Tables 3 & 4 give an average visual ranking of the samples based on criteria defined at the bottom of the table. An analysis of the numerical rank data suggests that a higher isocyanate level is the only statistically plausible way to avoid edge shrinkage at the chosen board density. Of course, higher board density would be the other but that was not evaluated here. Figure 4. Picture of 12” by 12” laminate after two weeks at -40oF

Dimensional stability of the core foam was evaluated under three sets of conditions, namely two weeks at -40oF, 2 weeks at 158oF/97% relative humidity both in accordance with ASTM D 2126 and two weeks at -40oF after Dimvac conditioning. Dimvac test condition exposes the foam to a greater pressure difference as CO2 in foam is allowed to escape while no air is allowed to diffuse in foam [6].

None of the core foam samples showed dimensional change of higher than 1% after two weeks at -40oF under the

conventional dimensional stability test, but many shrank significantly under the Dimvac condition. This is an extremely

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important finding as it suggests that under the nominal testing conditions used by quality control laboratories, all of the foam boards made in this study would appear dimensionally stable. However, under a more severe test condition, a condition still very plausible in the field, some boards would shrink. One such scenario that would simulate the Dimvac condition is making of the boards in late summer, the board being exposed to weeks of high temperature in the warehouse or in trucks and then being used in the field in a relatively cool climate, all in one season [6]. Statistical analysis on visual rank results suggests that higher isocyanate (HF-PMDI) content is the only statistically sound way to reduce/eliminate shrinkage. Similar to the findings with compressive strength, there seems to be a statistically sound interaction between polyol and TCPP level with the higher level of TCPP and the low hydroxyl polyol giving less shrinkage.

0

2

4

6

8

57 55 53

Isocyanate (HF-PMDI), w t%

% L

inea

r cha

nge

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 5. Average linear change in length and width direction after 2 weeks at 158oF/97% relative humidity

Figure 5 above shows the average linear change in length and width direction of core foam after exposure to 158oF/97% relative humidity. A higher isocyanate level is the only statistically sound way to reduce expansion of the foam sample under these conditions.

Thus it is clear that the structural performance of the board as expressed by compressive strength and dimensional stability is satisfactorily met only at 57% isocyanate (HF-PMDI) content in the formulation irrespective of the OH # of the polyol at the tested foam density. At lower isocyanate content in the foam, the density would need to be higher to avoid any structural performance issues. Keep in mind, the index will have to be as high as 310 to get 57% isocyanate when using lower OH polyol and higher level of FR. Lower FR level does improve structural performance but that could hurt fire performance. It does appear that a unique combination of low hydroxyl polyol and high level of TCPP gives unusually improved structural performance. It is likely that the lower viscosity of this combination improves mixing and thus network forming capability. This may be a topic for future work.

Thermal Resistance of Boards

Initial and 2.5” predicted LTTR values are listed in Table 3 & 4 and the LTTR values are shown in Figure 6. LTTR values were measured following Annex-A of ASTM C 1289 – 06, with a slice thickness of 9 mm. Statistical analysis suggest

5.2

5.4

5.6

5.8

57 55 53

Isocyanate (HF-PMDI), w t %

LTTR

val

ue

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 6. Predicted LTTR values for 2.5” thick board in ft2.hoF/Btu.in

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that isocyanate and TCPP level has much lower impact on initial and LTTR values as compared to the hydroxyl # of the polyol. Lower hydroxyl polyol gave a lower initial and LTTR value by about 2 - 4%. Further measurements to predict the LTTR value of 1.5” thick product are ongoing. Fire Performance of Boards - FM Calorimeter

Given the high cost and limited availability of the FM Calorimeter, only selected boards were tested using a decision tree. The roof assembly used for the test consisted of a FM approved steel deck, 1.5” thick polyiso board fastened to the deck, 0.5” wood fiber coverboard adhered to the insulation using hot asphalt applied at a rate of 20-25 lb/sq (100 sq. ft.), four-ply fiberglass felt reinforced built-up roof cover with each ply adhered using hot asphalt applied at a rate of 20-25 lb/sq, and a flood coat of asphalt at a rate of 60 lb/sq. This roof assembly is widely used by the polyiso industry as it is considered the most difficult to pass. However, once attained it means that the board can be used in a wide variety of roof assemblies including three-ply built up and membrane, provided other approval bodies and building code criteria are met.

Table 5 shows the result of all the FM Calorimeter tests conducted here. The allowable maximum average rate of fuel contribution for the three, five, ten and 30 minute time intervals is 410, 390, 360 and 285 Btu /ft2 /min. respectively for a Claas 1 roof assembly. The boards made using 57% isocyanate, low level of TCPP and either the standard (Board A-57) or the low hydroxyl (Board C-57) polyol passed the FM Calorimeter test whereas the board made using the low hydroxyl polyol, low level of TCPP but only 55% isocyanate failed. Actually the board C-55 using low hydroxyl polyol, low level of TCPP but only 55% isocyanate was tested twice, first in the beginning of the test series and then at the end, and it failed both times. The board C-53 made using low OH polyol, low level of TCPP but only 53% isocyanate was tested but an instrument failure in the calorimeter stopped the test during the 27 minutes of the 30 minute long test. Comparison of the flue outlet temperature and the residual foam char of the board C-53 with that of other boards, especially C-55, unambiguously suggests that C-53 would have failed the test if taken to completion. This along with the poor structural performance of boards made using 53% isocyanate led us to not retest C-53. Another board, D-55 made using the low hydroxyl polyol, 55% isocyanate but the higher level of TCPP did pass the FM Calorimeter test.

Table 5. FM Calorimeter test results Board ID C-57 C-55 C-55 repeat C-53 D-55 A-57

Isocyanate (HF-PMDI), wt% system 57 55 55 53 55 57 Aromatic polyester polyol type Low OH Low OH Low OH Low OH Low OH Std. OH TCPP, wt % system 2.44 2.60 2.60 2.75 5.18 2.44 Maximum average rate of fuel contribution , Btu /ft2 /min.

3 min. rate (410 max to pass) 228 450 469 204 375 5 min. rate (390 max to pass) 226 421 452 188 368 10 min. rate (360 max to pass) 216 340 386 151 317 30 min. rate (285 max to pass) 139 206 230 108 177 Rating Pass Fail Fail

Test aborted at 27th min.

out of 30 min

Predict fail Pass Pass

Given the high cost to conduct the FM Calorimeter test, above results were supplemented by a variety of small scale laboratory tests, such as Hot plate, Butler Chimney, Oxygen index, and NBS smoke density which have traditionally been used in the industry to compare fire performance of boards. Recently there has been a proliferation of small scale tests but surprisingly, no data has been published showing any correlation of results from the small scale tests with the FM Calorimeter [5, 18, 19].

The hot plate test is among the widest used to correlate with the FM calorimeter and Figure 7 compares the thickness retention results on full thickness boards. Statistical analysis of the data suggests that the thickness retention is increased by going to higher isocyanate and TCPP content with OH # of polyol making a very minor impact. When we compared the thickness retention in the hot plate test with that of residual char in the FM Calorimeter test, we concluded that the hot plate test used in this study was not severe enough. For example, the board made using formulation C-55, i.e., low OH polyol, low TCPP with 55% isocyanate, had a char thickness of only ~ 50% after the end of the FM calorimeter test, but the corresponding number in the hot plate test was ~70%. A more severe hot plate test is being designed.

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60

70

80

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57 55 53

Isocyanate (HF-PMDI), wt %

Thic

knes

s re

tent

ion,

%

Std OH + low TCPP Std OH +High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 7. Thickness retention in a Hot Plate test on full thickness boards

The Butler Chimney test is widely used to compare the performance of materials to direct flame fire tests such as ASTM E 84. The temperature of the flame of the burner used in the Butler Chimney has been recorded to be 960 ± 5oC [13]. Therefore, the weight retention and extinguish time in this test show the effect of a 960 ± 5oC open flame applied vertically to bare foam in air. Weight retention, i.e., char yield and the extinguish time of the flame after the burner is removed from the bottom of the foam is given in Tables 3 & 4. As can be seen from Figure 8, and confirmed by data analysis, raising the FR level had the most impact on improving weight retention, followed by increasing the isocyanate content. Lower hydroxyl polyol gave a lower char yield as compared to standard hydroxyl.

40

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57 55 53

Isocyanate (HF-PMDI), wt%

But.

Chi

m. w

t. re

t, %

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 8. Weight retention in the Butler Chimney test on core foam Oxygen Index is a widely used test in the polymer industry and it measures the minimum oxygen concentration in a

gaseous oxygen-nitrogen mixture necessary to support the combustion of a vertical sample burning downwards. Small changes in the index can indicate large differences in fire performance, e.g., the oxygen index for pentane is 15.6 whereas the same for polypropylene is only 17.4. Results from the oxygen index test are consistent with findings from the Butler Chimney test.

The NBS smoke density test is widely used to assess the smoke generation in the ASTM E 84 test and Figure 10 shows the results of this study. All the maximum smoke density numbers are well below what would be needed to meet the maximum smoke density requirements of the ASTM E 84 test.

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21

22

23

24

57 55 53

Isocyanate (HF-PMDI), wt %

Oxy

gen

inde

x

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 9. Oxygen index on core foam It is clear from all the fire test results that the FM calorimeter requirements and most likely ASTM E 84 and ASTM E-108 requirements as suggested by small scale tests simulating open flame fire, can be satisfactorily met when isocyanate content of the foam is 57% even with low amounts of TCPP and irrespective of the OH # of the polyol. As the weight % of the isocyanate in the formulation is reduced, good fire performance are achieved only when FR level is relatively high.

20

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57 55 53

Isocyanate (HF-PMDI), wt%

Max

imum

sm

oke

dens

ity

Std OH + Low TCPP Std OH + High TCPP Low OH + Low TCPP Low OH + High TCPP

Figure 10. Maximum smoke density in the NBS smoke test on core foam Overall Findings

The results of this study suggest that higher isocyanate (HF-PMDI) content in the foam formulation improves the structural, thermal and fire performance of the polyiso board whereas increasing the FR content improves fire but deteriorates structural performance. Lowering the hydroxyl number of the polyol at a fixed isocyanate level seems to have a neutral impact at best.

Much of these findings can be explained on the basis of basic polymer science. Increasing the isocyanate amount increases the aromatic content, the nitrogen content, isocyanurate content, and average functionality and decreases the amount of non-reactive and chain-terminating mono-functional additives. This makes the foam more cross-linked and thermally stable allowing it to meet the dimensional stability requirements at lower density and the FM Calorimeter requirements at lower density and/or thickness. To the polyiso formulator, it has been always known that a high isocyanurate content is advantageous as it imparts several desirable properties to the foam, low flammability, low smoke evolution, good thermal stability and good dimensional stability. What may not have been clear is that high isocyanurate content in foam brings along higher aromatic content, nitrogen content, average functionality and lower non-reactives which together yield the better properties. Lowering the hydroxyl number of polyol while keeping the index the same may

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appear to keep the isocyanurate content unchanged, but it does not. By virtue of its definition, index only calculates excess –NCO equivalents as a multiple of total –OH groups. Thus when the hydroxyl number of the polyol is lowered, the total number equivalents of –OH decreases and thus excess –NCO equivalent decreases which in turn lowers the isocyanurate content at the same index.

The hydroxyl value of the polyol had been constant until recently, and thus it made no difference whether one used a fixed index or fixed weight % of isocyanate. However, with the hydroxyl value of polyols changing, it would be more prudent to formulate keeping the weight % of isocyanate constant rather than the index. Thus when looking to improve the cost-effectiveness of polyiso foam, one should remember that board density dominates all other factors affecting cost. Any efforts to optimize the isocyanate (HF-PMDI) content of the foam should be checked to ensure that they do not inadvertently increase the overall cost of the polyiso foam by raising the minimum stable density of the foam.

Increasing the FR level may be beneficial to improving the fire performance, but it plasticizes the foam and thus increases the dimensionally stable density [20]. Certainly some level of FR is essential to meeting the fire requirements with pentane blown foam [2], and at those low levels it may not act as a plasticizer. Thus one needs to carefully look at the cost-performance balance when adding fire retardant, while keeping in mind the dimensionally stable density. It is prudent to determine the dimensionally stable density using not just the conventional tests, but also tests such as Dimvac which simulate a “worst case” scenario. This would minimize the chance of cost prohibitive surprises.

The results of selected FM Calorimeter tests along with those of small scale laboratory tests demonstrate that higher isocyanate content improve all aspects of fire performance and lowering the OH # of polyol hurts the fire properties even at the same isocyanate level let alone the same index. We must remember, foam failure in the FM Calorimeter test is a self-accelerating process and thus small differences in char forming ability is likely to make a large difference in the outcome.

It does appear that the lower hydroxyl polyols may ease processing of the foam by having less froathing, and better flow. It is not clear whether the underlying cause for the easier processing is lower OH# of the polyol, better pentane compatabilizers, or viscosity modifiers added to these polyols.

There are many what if questions that a polyiso formulator may raise at this point. What if the functionality or the aromatic content of the polyol was raised, simultaneous to lowering the OH number? What if the FR was reactive, or even cross-link forming? Though some of these possibilities are limited by other constraints such as viscosity requirements on the polyol side and pentane compatibility, there is always room for further innovation. CONCLUSIONS

The performance of the polyiso board is strongly dependent on the weight % of isocyanate in the foam formulation. This is because of all the attributes, such as high aromatic and nitrogen content, high inherent functionality, and ability to form thermally stable, cross-linking bonds, that high functionality polymeric MDI such as RUBINATE® 1850 isocyanate brings to the formulation. In other words, high thermal stability of polyiso foam is related not just to high cross-link density and high thermal stability of the isocyanurate structure but also to accompanying high aromatic content. If the urethane content in the foam is increased, the advantages of the isocyanaurate linkages would decrease. Maintaining high isocyanurate and aromatic content is advantageous as it imparts several desirable properties to the foam, low flammability, low smoke evolution, good thermal stability and good dimensional stability.

The structural performance of the board as expressed by compressive strength and dimensional stability is satisfactorily met only when the weight % of the isocyanate is 57% irrespective of the OH # of the polyol at the tested foam density. At a lower weight % of isocyanate in the foam, the density would need to be higher to avoid any structural performance issues. Keep in mind, the index will have to be as high as 310 to get 57% isocyanate when using lower OH polyols and higher levels of TCPP. Lower TCPP levels do improve structural performance but also would hurt fire performance.

The fire performance of the board as measured by the FM calorimeter can be easily met when isocyanate content of the foam is 57% even with low amounts of TCPP and irrespective of the OH # of the polyol. As the weight % of the isocyanate in the formulation is reduced, good fire performance is achieved only when the FR levels is relatively high. But higher FR level do hurt structural performance at the same density [20].

This work suggests that it may be more prudent to formulate keeping the weight percent of isocyanate in the total foam constant rather than the index. Thus when looking to improve the cost-effectiveness of polyiso foam, one should remember that board density dominates all other factors affecting cost. Any efforts to optimize the isocyanate (HF-PMDI) content of the foam should be checked to ensure that they do not inadvertently increase the overall cost of the polyiso foam by raising the minimum stable density of the foam. A plausible route to decrease density is by using alternative blowing agents as suggested in our paper at the last API [21].

All information contained herein is provided "as is" without any warranties, express or implied, and under no

circumstances shall the authors or Huntsman be liable for any damages of any nature whatsoever resulting from the use or

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reliance upon such information. Nothing contain in this publication should be construed as a license under any intellectual property right of any entity, or as a suggestion, recommendation, or authorization to take any action that would infringe any patent. The term "Huntsman" is used herein for convenience only, and refers to Huntsman Corporation, its direct and indirect affiliates, and their employees, officers, and directors.

RUBINATE® and JEFFCAT® are registered trademark of Huntsman Corporation or an affiliate thereof in one or more, but not all, countries. ACKNOWLEDGEMENTS

The authors wish to thank Tom Piazza and David Dorsey for their invaluable help in producing this paper. The authors would also like to thank all the associates at Huntsman Advanced Technology Center who helped set up the laboratories and testing facilities in time to make this work possible. REFERENCES 1. PIMA. 2005. “Polyiso Foam Roof Insulation: It’s More than R-value, It’s Meeting the Codes,” Advisories and other

documents on www.polyiso.org 2. Berrier, R. E., S. N. Singh, and J. S. Costa. 1998. “Hydrocarbon Blown Rigid Polyurethane Foam for the Boardstock

Industry – A Novel Approach,” Proceedings of the Polyurethanes Expo ’98, pp. 5-13. 3. Gilbert, D. R.. 1985. “Index Versus Flammability in Urethane Modified Isocyanurate Foam Systems,” Proceedings of

the SPI 30th Annual Technical Conference, pp. 172-175. 4. David Shieh, R. Donald, J. Luna, and A. DeLeon. 2006. “Low OH number Polyester Polyols for Lamination,”

Proceedings of the Polyurethanes Expo ’06. 5. Feske, B. and J. Canaday. 2001. “Optimization of Flame-Retardants for Rigid PIR Foams: A new Screening

Apparatus, and Correlation to Large-Scale Flammability Tests,” Proceedings of Polyurethanes Expo 2001, pp. 627. 6. Singh, S. N., J. J. Lynch and D. Daems. 1995. “Techniques to Assess the Various Factors affecting the Long Term

Dimensional Stability of Rigid Polyurethane Foam,” Proceedings of the Polyurethane 1995, pp. 11-19. 7. Ross, L. and J. Hagan. 2002. “Polyurethane Products: Overview of US Model Building Code Fire Performance

Requirements,” Proceedings of Polyurethanes Expo 2002, pp.217-233. 8. DeLeon, A. 1981. “Isocyanaurate Foam in F.M. Class I Steel Roof Deck Construction,” Proceedings of the SPI 25th

Annual Technical Conference, pp. 45-50. 9. Tideswell, R. B. 1982. “Development of a Factory Mutual Class I Isocyanurate Foam Laminate,” Proceedings of the

SPI 26th Annual Technical Conference, pp. 314-19. 10. Simon., J, F. Barla, A. Kelemen-haller, F. Farkas, and M. Kraxner. 1988. “Thermal Stability of Polyurethanes,”

Chromatographia, Vol 25, No. 2 pp. 99-106. 11. Nelson, G.L. 1995. “Surfaces and Char,” in Fire and Polymers II, ACS Symposium Series 599, G. L. Nelson, ed.

Washington, ACS pp. 159-160. 12. Factor, A. 1990 “Char Formation in Aromatic Engineering Polymers,” Chapter 19 in Fire and Polymers, G. L. Nelson,

ed. Washington, ACS pp. 274-287. 13. Tang, Z., M. M. Maroto-Valer, J. M. Andresen, J. W. Miller, M. L. Listermann, P. L. McDaniel, D. K. Morita, and W.

R. Furlan. 2002. “Thermal Degradation of Rigid Polyurethane Foams Prepared with Different Fire Retardant Concentrations and Blowing Agents,” Polymer 43. pp. 6471-6479.

14. Bleuel, e., U. Totermund, C. Seitz, P. Boehme, and M. Reichelt. 2002. “Fundamentals of Flame Retardation: The Burning Process and the mode of Action of Flame Retardants,” Proceedings of Polyurethanes Expo 2002, pp. 234-243.

15. Matuschek, G. 1995. “Thermal Degradation of Different Fire Retardant Polyurethane Foams,” Thermochimica Acta 263, pp.59-71.

16. Tokuyasu, N., K. Fujimoto, and T. Hamada. 2004. ‘Halogen Free Flame Retardant Technology for Polyurethane Foam,” Proceedings of Polyurethanes 2004, pp. 234-243.

17. Chamberlain, D. L. 1978. “Flame Retardancy of Polymeric Material,” W. C. Kuryla and A. J. Papa, eds. Vol. 4, New York, Marcel Dekker, Inc. pp. 109-168.

18. Williams, W., G. R. Alessio, and S. V. Levchik, “Development of a Bench-scale and Micro-scale Test to Correlate with FM 4450 Calorimeter Test,” Proceedings of Polyurethanes Expo 2005. pp. 409-414.

19. Falloon, S. and R. Roark. 2003. “Novel Flame Retardants for Hydrocarbon Blown Polyisocyanurate Foam,” Proceedings of Polyurethanes Expo 2003. pp. 42-45.

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20. Prociak, A., J. Pielichowski, M. Modesti, F. Simioni, and M. Checchin. 2001. “Influence of Different Phosphorous Flame Retardants on Fire Behaviour of Rigid Polyurethane Foams Blown with Pentane,” Polymery, 46, nr 10. pp. 692-696.

21. Singh, S. N., M. Ntiru-Karamagi, and M. Ritchie. 2005. “Optimizing Polyiso Blowing Agents,” Proceedings of Polyurethanes Expo 2005. pp. 402-408.

BIOGRAPHIES Sachchida N. Singh

Sachchida is currently a Scientist Fellow for the Polyurethanes business at Huntsman. Since joining in 1987, he has held increasingly responsible positions in the technology development departments of the business. He has worked in many different application areas of polyurethane chemistry and technology and has lately spent significant efforts in the rigid foam sector. He has a doctoral degree in Materials Science and Engineering from Massachusetts Institute of Technology and a Master of Science degree in Chemical Engineering from Rensselaer Polytechnic Institute.

Jody S. Fife

Jody S. Fife is currently a Technical Service Representative for the Polyurethanes business at Huntsman. Since joining in 1992 he has worked in many different positions with the production facility in Geismar, LA. He has held positions such as Laboratory technician, Foam technician, and Equipment Inspector. His latest position has been a technical service representative of polyurethanes chemistry in the rigid foam sector. He has a Bachelor of Science in Management and Chemistry from Southeastern Louisiana University.

Sheila Dubs

Sheila Dubs is currently a Senior Technical Service Representative for the Polyurethanes business at Huntsman. Since joining the business in 1999, she has worked in many different application areas, namely, automotive, flexible foam, elastomers, and rigid foam, in a variety of technical service and development roles. She earned a Bachelor of Science in Chemical Engineering degree from University of Michigan.

. Paul D. Coleman

Paul is currently a Technical Manager for the Polyurethanes business at Huntsman. Since joining the business in 1986, he has held a variety of positions in different application areas of the rigid foam technical group. He earned a Bachelor of Science in Chemical Engineering (BSChE) degree with a concentration in Economics from Tufts University.

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