POLYURETHANE COMPOSITES: A VERSATILE THERMO-SET POLYMER MATRIX FOR A BROAD RANGE OF

APPLICATIONS

Guido Bramante1, Luigi Bertucelli1, Andrea Benvenuti1 and Kevin J. Meyer2

1. Dow Italia SRL, Via Carpi 29, Correggio 42015 (RE) Italia2. The Dow Chemical Company, Freeport, Texas 77541 USA

ABSTRACT

Polyurethane resin is currently a minor player in the composite industry, with unsaturatedpolyesters and epoxies having the largest shares among thermosetting materials1. However, theoutlook for this chemistry is positive due to the quest for fast, automated and quality compositeproduction. Polyurethanes have recently been applied to a broad range of technologies forcomposite production. This paper will focus on the study of the mechanical properties oflaminates obtained by the continuous method of pultrusion using the same glass fiberreinforcement and comparing the performance of different thermosetting resins. Specific focus isput on the contribution of the polymer matrix to the final properties of the composite.

1. INTRODUCTION

1.1 Background

Vehicles and Transportation Architectural/Infrastructure

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Short fibers Chopped fibers Endless (mats)

Increasing Fiber length

Short fibers Chopped fibers Endless (rovings)

Increasing Fiber length

Figure 1. Polyurethane Composite Manufacturing Techniques

Polyurethane composites can be grouped as a function of the density of the polymericmatrix and the length of the fiber reinforcement. These two variables also determine the type andcomplexity of the manufacturing technology. Stiffness varies greatly across these two variables

1 “Growth Opportunities in Global Composites Industry 2013-2018” Lucintel Market Report. March 2013.

ranging from self-supporting mineral-filled polyureas suitable for vertical body panels forvehicles, to stiff quasi-structural long-fiber reinforced polyurethanes, to truly structural partsusing preformed fiber-mat and/or continuous fiber reinforcements as shown in Figure 1.

The development of the polyurethane composite platform started with the development ofsuitable formulations for fabrication processes using chopped fibers. The versatile FiberComposite Spray (FCS) technology quickly gained momentum due to its relatively low initialcapital requirements (open mold) and good automation level. Later on the Long-Fiber Injection(LFI) technology was vastly developed for vehicles (e.g. tractor bonnets) and industrialapplications (e.g. manhole covers)2.

Recent efforts have focused on continuous fiber reinforced composites due to the improvedproperties that they are capable of providing3. Structural Reaction Injection Molding (S-RIM)and Resin Transfer Molding (RTM) proved to be a promising technology for polyurethanes asdue to the closed nature of the processes4. The process enables the production of structural partscharacterized by excellent strength-to-weight ratio and are capable of producing highlyorthotropic mechanical properties. Recently a polyurethane system for the Pultrusion Processusing a closed injection method was developed where fast speeds, good surface luster and lowpull forces were demonstrated. All this, coupled with the absence of diluents, makespolyurethanes a real and attractive option for a clean and environmentally friendly pultrusionprocess.

1.2 Polyurethanes in Pultrusion and their Value Proposition

The pultrusion market is still heavily dominated by polyester (85%), vinyl ester (7%), epoxy(5%), phenolic resins (2%) and others, with glass fiber being by far the predominantreinforcement5. There exists a market pull in the pultrusion market for new resins that couldbring improve performance and productivity and provide a more environmentally friendlyprocessing solution. Polyurethanes can be run as fast as polyesters but yield mechanical andthermal performance in the neighborhood of epoxies. Excellent mechanical properties ofpolyurethane composites open the space to redesign existing profiles and, in some cases,allowing a redesign of the glass reinforcement package. Additionally, polyurethanes offer analternative to chemistries that use volatile organic compounds.

Due to polyurethanes fast reactivity, the processing of polyurethanes requires a modificationto the conventional open bath impregnation approach used for polyesters, vinyl esters andepoxies. A closed injection system is best used for the application of polyurethane chemistry tothe pultrusion process. The closed injection system is comprised of a 2K low pressure mixingmachine connected to a specifically designed impregnation box that is attached to the entrance ofthe die as shown in Figure 2.

2 Mazumdar, S.K. Composites Manufacturing: Materials Product and Process Engineering. Taylor and FrancisGroup. CRC Press (2002).3 Hyer, M.W. Stress Analysis of Fiber Reinforced Composite Materials. DEStech Publications, Inc. (2009).4 Strong, B.A. Fundamentals of Composites Manufacturing. Society of Manufacturing Engineers; Second Edition(2008).5 AIMPLAS Instituto tecnologico del Plastico, 2010 “Principales Avances en Poltrusion y SMC”, pp 9

Figure 2. Pultrusion manufacturing techniques.

2. EXPERIMENTATION

2.1 Experimental Objectives

Polyester and vinyl ester pultruded profiles properties have been extensively characterizedover the years and published elsewhere6. The objective of this work is to update the existingliterature on the matter by the mechanical analysis of two new polyurethane systems. The firstsystem is a VORAFORCE® toughened Epoxy system7. The second system is a polyurethanesystem under the same trade name. Vinyl ester and unsaturated polyester resins extensively runin the pultrusion market were selected as controls. Pultruded profiles using the mentioned resinswere produced on the same pultrusion line with the same type of glass fiber glass reinforcementso that the only variable in the experiments is the resin chemistry. Additionally, Speed runs werecarried out on the pultrusion line with both systems to demonstrate the potential for productionspeed using polyurethane versus epoxy.

2.2 Sample Preparation

Pultruded bars of full rectangular cross-section 200mm x 3.5mm were produced using aDurapul 2408 pultrusion line with reciprocating pullers and hydraulic grips. The selection ofthermoset resins comprised the polyurethane system (PU) mentioned above, a toughened epoxysystem (EP) and two well know resins systems in pultrusion: an unsaturated Isophthalicpolyester system (UP) and vinyl ester system (VE). The PU samples were the only onesproduced using closed injection all the others were run with open bath. A GRACO HFR mixingunit was connected to the chamber to deliver the reaction mix. The same glass fiberreinforcement structure (GFR) was used for all the prototypes which was composed by threelayers as follows (from top to bottom): one continuous filament mat (CFM) of 300 g/m2, 200 E-glass direct rovings of 4400 TEX, and again a CFM of 300 g/m2, all from Owens Corning. The

6 Hyer, M.W. Stress Analysis of Fiber Reinforced Composite Materials. DEStech Publications, Inc. (2009).7 VORAFORCE is a trademark of The Dow Chemical Company.

reinforcement was calculated for a target content of 75% glass fiber weight fraction. No fillerswere added to the resins. The production speed was kept constant at 0.5m/min. Three heatingzones equipped with electrical heating plates (Z1, Z2, Z3) were used.

The temperature set-point on the pultrusion die varied according each resin requirement andis summarized in the Table 1. The degree of curing was estimated as the relative variationbetween the 1st cycle Tg and 2nd cycle Tg determined by dynamic mechanical thermal analysis(DMTA) with a 3 point bending geometry, a 3°C/min heat rate and a ramp from 40°C and200°C.

ID Z1 Z2 Z3Tg [°C]DMTA

1st Cycle

Tg [°C]DMTA 2nd

Cycle

% REL Tg1st and 2nd

CycleVoraforce® TP 1200/1260 PU 100 195 185 111 131 18.0Voraforce® TP 201/254 EP 110 167 180 113 118 4.4Isophthalic-Polyester system UP 105 160 Off 169 173 2.4Vinyl Ester system VE 95 113 Off 107 125 16.8

Table 1: Production parameters for the pultruded laminates

2.3 Testing and Methods

The mentioned set of composite laminates was first tested to determine the glass fiberweight fraction since a comparison of mechanical properties can only be fair if the glass fiberfraction is similar. The second consideration to make is that composite materials are highlyanisotropic. This means that properties are strongly affected by the direction that the load isapplied within the material. Composites are a subclass of anisotropic materials that are classifiedas orthotropic. Orthotropic materials have properties that are different in three mutuallyperpendicular directions8. Having this in mind, a single pultruded sheet can be considered alaminate comprising a top lamina composed by a glass fiber continuous filament mat (CFM), anintermediate lamina composed by direct rovings, and finally a third one composed by glass fiberCFM.

Since pultruded laminates are orthotropic materials it is required to define a coordinatesystem to determine the directions in which the coupons will be cut and results reportedafterwards. We will define the 0º degree direction (simply indicated as “0°”) as the directionalong the roving’s length (also referred as “machine direction” since it is the pulling direction ina pultrusion line). The orthogonal direction or crosswise is symbolized as the 90º direction(simply “90°”). For some specific testing the ± 45º direction was also considered.

A summary of the testing protocol proposed in this work is summarized in Table 2. It isimportant to note for each of the test the dominant composite constituent based on the directionof the load applied. For example, for the unidirectional tensile, it is expected that the load at 0ºwill be mostly dominated by the glass fiber roving tensile properties. Opposite to that, in 90ºdirection, and assuming that contribution to the tensile strength of the continuous mat will be thesame in all the four prototypes, is mostly the polymeric resin the key constituent affecting theresults. For each of the mechanical testing five coupons per type of material were tested and

8 F.C. Campbell. Structural Composite Materials. ASM International (2000).

averaged unless said otherwise. All the testing has been conducting in standard atmosphericconditions (25C and 50%HR).

2.3.1 Composite Constituent Weight Fraction

For the determination of the glass weight fraction, the ASTM D3171 was applied whichuses a muffle to burn-out all the organic components of the composite (600C x 4 hours)9. Theremaining residue, containing the reinforcement, is then filtered, washed, dried, cooled, andweighed (to the nearest 0.001 g). Then the weight percent of the reinforcement is calculated asthe percentage of the original weight.

2.3.2 Tension, Flexure and Compression

The tensile test was executed using a MTS 100kN dynamometer with extensometer. Toimprove gripping on clamps and to avoid undesired slippage a metallic grid was used betweenthe composite surface and the jaws of the clamps. Coupons were not tapered. A total of 8coupons were tested per sample. The flexural test was executed using a “3 point being”configuration in the same dynamometer. For the compression test, a Zwick/Roell Z100 THdynamometer of 100kN capacity was used. The coupons were instrumented with electronicstrain gauge extensometers (macroXtens with resolution up to 0,0006 mm) in both directions(0º/90º) in order to be able to determine the strain and the compression moduli. The clampingfixture was of the type IITRI (Illinois Institute of technology Research Institute) in a load cell of100kN.

2.3.3 Shear

Out plane (inter-laminar) and in-plane shear properties (V-notched) were performed. Theinter-laminar shear was conducted by the short beam (ILSS) method10. This method is a variantof the 3 point bending flexural test where the span to thickness ratio of the probe is restricted to4:1 forcing the shear stress to attain failure before tension or compression. Rectangular couponsare cut having its length parallel to the direction of the rovings (0°). ILSS is easy to run andsimple and due to this is extensively applied as quality control tool but do not provide pureuniform shear stress state.

On the other hand, the V-notched method, also referred as Iosipescu is one of the fewexisting methods that provide uniform shear stress state11. This was the rationale to include it inthe testing protocol. For the in plane shear quantification the V-notched shear test was performedat different directions (0°, 90° and 45°).

2.3.4 Structural Properties: Impact Resistance, Open Hole Strength, Bearing Strength andScrew Pull-Out Strength

Composite materials are very sensitive to out-of-plane loading because they are muchweaker in the thickness direction than in the plane of lamination (for pultrusion the plane

9 ASTM Standard D3171, 2015, “Standard Test Methods for Constituent Content of Composite Materials.” ASTMInternational, West Conshohocken, PA.10 ASTM Standard D2344, 2013, “Standard Test Method for Short Beam Strength of Polymer Matrix CompositeMaterials and Their Laminates.” ASTM International, West Conshohocken, PA.11 Gibson, R.F. Principles of Composite Material Mechanics, CRC Press, 3rd Edition (2012).

dictated by the production direction). Consequently, composite materials subjected to traverseimpact may suffer significant damage, resulting in deterioration of their overall load-carryingcapacity.

From the available group of tests that simulates the impact of a foreign object in the traversedirection, the instrumented free fall dart impact test was chosen. It permits to register the entireload-time and load-energy curves. An Instron Dynatup 9250HV with an instrumented darthaving a spherical dart nozzle of 20mm diameter and a weight of 24kg was used. Total coupondimension were 95x95mm. Three coupons per type of composites were tested. The height of thedart was adjusted to cover different levels of impact energy 25J, 35J and 40J.

Post operations like drilling, screwing and gluing are important in pultrusion structural partproduction. The precedents testing fail to address the effect of holes or cutouts in laminates. Theopen-hole test and bearing strength are proper tests (among others) intended to quantify thebehavior of the composite under concentration of stresses induced by the defect (hole, cutouts orscrewing). A modified EN ISO 527-4 procedure was also performed to quantify the requiredpulling force to pull out a screw previously tightened to the laminate12. A screw having a hookwas driven in the central part of the composite laminate. An Instron of 30kN with a 10kN loadcell capacity was equipped with a hook on the top clamp, and a support structure was fixed to thebottom clamp, in such a way that the screw could be pulled out of the composite in areproducible way.

2.3.5 Microscopic Analysis Using Scanning Electron Microscopy

Electron microscopy analysis was done on fractures samples. An ESEM QANTA 200 FEI,available at Modena University was used for the scanning electron microscopy (SEM)characterization. Acquisition parameters were: distance 11mm, energy 20KeV, spot size 3.5 and4.5.

Test type Test orientation Standard ASTMDominating Composite Constituent

Fiber Matrix

Tensile0°

ASTM D3039X

90° X

Flexural0°

ASTM D790X

90° X

Compression0°

ASTM D695/D3410X X

90° X XDart Impact Out of plane ASTM D3763 X

Open hole tensile 0º/90° ASTM D5766 X XShort Beam

Interlaminar shear0° ASTM D2344 X

Notched InplaneShear (Iosipescu)

0°/45º/90º ASTM D5379 X

Ultimate BearingStrengh

0° and 90° ASTM D5961 X X

Screw Pull OutStrength

Out of plane EN ISO 527-4 Mod X

Table 2: Testing Protocol Summary

12 ISO Standard ISO 527-4:1997 “Determination of tensile properties -- Part 4: Test conditions for isotropic andorthotropic fibre-reinforced plastic composites.”

3. RESULTS

3.1 Mechanical Analysis of Samples

3.1.1 Constituent Weight Fraction

The results of the determination of the fiber content, expressed as fiber weight fraction, wereall approximately 75% target and then ensuring representative samples.

3.1.2 Tension, Compression and Flexural

The tensile results in Figure 1 show that in the 0º direction (lengthwise) little or nodifferentiation among the prototypes exists. Most of the load is supported by the reinforcement.In the perpendicular direction, the polymeric matrix dominates and is precisely were epoxy andpolyurethane show the highest values of strength and modulus. Flexural results are in line withthe tensile testing (Figure 2). On the compression test, the results seem to indicate a similarbehavior for the PU, EP and VE samples with lower values for UP (Figure 3).

Figure 1: Tensile Results in 0° and 90° Directions

Figure 2: Flexural Results in 0° and 90° Directions.

Figure 3: Mechanical Response to Uniaxial Compression in 0° and 90° Directions

3.2 Shear Strength

One critical aspect on the integrity of the composite structure is the reduced tendency todelamination. Delamination may occur due to different reasons, for example due to impact,drilling, nailing, etc. Delamination cause severe reductions on the load carrying capacity of the

composites and must be avoided [2]. The reduced tendency to delamination can be quantifiedindirectly through the characterization of the behavior of the composite structure when is testedunder shear. The results in Figure 4, clearly show a superior behavior of PU vs the othersamples. Particularly, VE and UP coupons resulted greatly damaged as can be seen on thepictures taken from the observation of the coupons under optical microscopy. In line with theshort beam shear test results, the in plane shear V-Notched method indicated a cleardifferentiation of PU, EP and VE vs UP as seen in the plots in Figure 5.

Figure 4: Inter-laminar Shear Results and Optical Microscopy Analysis on Shear FractureAccording to ASTM D2344

Figure 5: In Plane Shear Testing by the V- Notched Method.

3.3 Structural Properties: Impact Resistance, Open Hole Strength, Bearing Strength andScrew Pull-Out Strength

As mentioned, the integrity of the composite structure subjected to a traverse impact is ofinterest in view of its durability or life span. The curves of force vs time in the instrumented dartimpact test permitted a good discrimination between the different composite materials studied.The mechanism of damage for out of plane impact is complex and normally there are severalmodes involved such as delamination, matrix cracking and fiber breaking that can occurseparately or simultaneously. Figure 6 shows the plot of force-time at different impact energies.A perfect elastic material would recover completely after impact and would give a “bell-shaped”

curve of Force vs time. Any discontinuity in the Force vs time plot indicates a fracture. The areaof delamination/fracture is also a qualitative indication of damage. Analyzing the impactedsamples at 40J impact energy, the failure mode for UP seems to be mostly delamination, whereasEP seems to exhibit predominantly cracking along the roving’s direction. VE exhibited mostlymixed delamination and cracking. PU seems to have the less area of damage and the best impactresistance among the samples tested.

Figure 6: Structural Integrity Assessment of Impact Resistance.

Most of the composite parts will undergo an assembly stage for form a final structure.Methods include gluing, bolting screwing, nailing, drilling. The open-hole test has as objective tointroduce a defect (hole) that acts as a point for stress concentration. The calculus of the stressignores the reduction of the area caused by the hole. Figure 7 shows the results on open-holestrength on the 90º direction. Coupons not breaking in the perimeter of the hole were rejected asdictated by the norm. For the big majority of the coupons the fracture was initiated in the hole(stress concentration) and in the direction of the fibers (as expected). For the polyester the failurealso included certain extent of delamination.

Figure 7: Structural Integrity Assessment of Open-Hole Strength.

Figure 8: Open-Hole Results and Fracture Modes

On bolted parts, bearing stress is generated by direct contact between the outer surface ofthe bolt or pin and the inner surface of the hole in the composite specimen in which the bolt orpin is inserted. Bearing strength is an important indication of the structural stability of thecomposite structure. Measurements have been made on single shear tensile loading in the twoorthogonal directions. Higher bearing strength has been found for Epoxy, followed in order ofmagnitude by polyurethane, vinyl ester and polyester. For the polyester samples it was observedsome extent of delamination on top of cracking (see Figure 8).

Figure 9: Screw Pull Out force and Testing Device

Composite structures often have poor out of plane transverse strength. The plot in Figure 9shows comparative data for screw pull out force. The results are aligned with previousmechanical analysis in favor of PU and EP as materials offering more resistance to the traversepull strength.

3.4 Scanning Electron Microscopy Analysis

Figure 10: Scanning Electron Microscopy Analysis on Unidirectional (0º) Fractured Samples.

SEM analysis was executed on fractured samples (unidirectional tensile at 0º direction) asshown in Figure 10. The images are indicating a good level of bonding for PU, EP and VE whereit can be clearly observed the “finger print” of the fibers left on the resin. The images obtainedfor UP might suggest an easier detachment of the fibers from the matrix, evidenced by asmoother finger print. The interpretation of the authors is that the level of fiber bonding is lower

for the polyester samples vs the other samples analyzed (for the given glass fiber type and sizingpresent in the samples).

3.5 Speed Test

Pultrusion speed depends on multiple factors such as: cross-section complexity,reinforcement complexity, resin reactivity profile, internal mold release type. A key processingparameter in pultrusion is the control of the pulling force. As the pultrusion speed increases thepulling force tends to increase as well. As with any other new resin technology, polyurethanechemistry and processing must be learned and mastered to optimize profile production. Theintention of this section then is therefore to demonstrate, under the boundary conditions used inthe experiment, the maximum pulling speed that can be achieved using the polyurethane system.

The experiment was conducted as follows. The pultrusion speed was slowly increased insteps starting from 0.5 m/min. The die temperatures were increased after each speed step changeto get a profile with good surface luster (free of powder). When the surface of the profileachieved the right luster and was considered commercially acceptable a new speed change on thespeed was done. This sequence was executed until it was not possible to get a good surface onthe profiles even increasing the temperature of the die. The test was conducted in two differentpultrusion lines using two different reinforcements as follows:

-EXPERIMENT “A”: The experiment was run using a Durapul 2408 pultrusion line(Martin Pultrusion Group) with reciprocating pullers and hydraulic grips. Pultruded bars of fullrectangular cross-section 200mm x 3.5mm were produced using 300 g/m2 mats and 200 rovingsof 4400TEX to totalize a glass fiber weight fraction of 75%. Mats and rovings were from OWC.

-EXPERIMENT “B”: The experiment was run in a PULTREX 10T machine withreciprocating pullers and hydraulic grips. The cross section of the profile was 2.5mm x180mmand the reinforcement was unidirectional, using 160 rovings of 4800TEX for a target glass fiberweight fraction of 84%. Rovings were from 3B Fiberglass.

In the experiment “A” it was possible to run as fast as 1.5m/min having an excellent surfaceluster and more than acceptable pulling force values (< 1T). In the experiment “B” the maximumspeed achieved was 2.3m/min and was conditioned by limitations on the pultrusion line and noton the resin system. The distance from the exit of the die till the first grip was not long enough asto permit enough cooling of the profile. Based on the feedback received from differentpultruders, a pultrusion speed range demonstrated for PU of about 1.5 to 2m/min is in line withtypical Polyester pultrusion speeds and is 3-4X time faster than Epoxy.

4. CONCLUSIONS

The main objective of this work was to quantify the contribution of different polymer matrixto the final mechanical properties of composites made by pultrusion using glass fiberreinforcement. A mechanical testing protocol including the most relevant testing was used. Wesaw broad improvement in properties using polyurethane as the matrix in non-fiber dominatedtesting. Of particular relevance are crosswise tensile and flexural strength, dart impact resistance,inter-laminar shear strength, and screw pull-out force, where polyurethane has betterperformance in comparison to polyester and vinyl ester controls. Additionally, it is observed that

polyurethane properties fall in the range of the toughened epoxy system. Additionally, pultrusiontrials conducted in two different pultrusion lines using different reinforcement, demonstratedpotential production speeds for polyurethane in the range of 1.5m/min to 2m/min, which may behigher than those potentially achieved for other incumbent systems.