Optimization of a flame-retarded polypropylene composite

H. Dvira, M. Gottlieba,*, S. Darenb, E. Tartakovskyb

aChemical Engineering Department and Stadler Minerva Center for Mesoscopic Macromolecular Engineering,

Ben Gurion University, Beer Sheva 84105, IsraelbDead-Sea Bromine Group Ltd, POB 180 Beer Sheva, Israel

Received 25 April 2002; received in revised form 3 March 2003; accepted 11 March 2003

Abstract

Composite materials demand constant improvements in mechanical and flame retardant (FR) properties. The goal of this project

is to study the effect of additives on these properties in polypropylene composites containing glass fibers, pentabromobenzyl acry-late (PBBMA) as a primary FR and magnesium hydroxide as a secondary FR. Optimal composition is reached by means of sta-tistical design of experiments (DOE) rather than by ‘‘trial and error’’ approach. The DOE approach allows minimization of thenumber of experiments, investigation of the influence of each additive and the mutual interactions between additives. It also allows

here, prediction of optimal sample properties better than 8% from experimental values. The optimal composition exhibits improvedmechanical and FR properties. Both FRs reduce the impact strength while enhancing flame retardancy. Glass fibers increase themodulus, but have only a moderate effect on the impact strength due to poor adhesion with PP. The interpretation of the effect of

glass fibers on the flammability is inconclusive.# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Glass fibers; A. Polymer-matrix composites; B. Mechanical properties; C. Statistics; Flame retardancy

1. Introduction

Polypropylene (PP) is a polymer of great importancein the industrial sector due to its low density, high waterand chemical resistance, ease of processability and beingone of the most cost-effective polymers available today.It is employed in numerous applications from non-woven fibers to the automotive industry. Its relativelylow price is the driving force to improve its perfor-mance, to broaden its versatility and to enable it tocompete with other polymers used nowadays for moredemanding applications. PP as the polymer hostrequires the incorporation of additives and reinforce-ment in order to enhance its mechanical properties. Oneof the most commonly used reinforcement agents arechopped strand silane surface treated glass fibers. Theglass fibers are surface modified since PP has a non-polar nature, which hinders its interaction and adhesionto polar fillers such as glass fibers. The surface treat-ment improves the compatibility of the glass with the PP

and hence improves the mechanical properties of thecomposite [1]. PP, in common with most syntheticpolymers, is flammable and may therefore be hazardousnear an open flame. It may also enhance fire propaga-tion due to its tendency to drip when hot. Therefore, theaddition of a flame retardant is an essential requirementif we wish to achieve a PP based composite with goodflame resistance. Pentabromobenzylacrylate (PBBMA),used as a flame retardant in PP composites, is a crystal-line monomer with a chemical structure shown inFig. 1a. PBBMA may polymerize by free radical poly-merization of the unsaturated acrylic bond initiatedthermally during reactive extrusion to producepoly(pentabromobenzylacrylate) (PBBPA) [2] whosestructure is shown in Fig. 1b. The fire retardancymechanism of this FR and its suitability for use with PPhas been discussed previously [3–5].A fraction of PBBMA reacts to a variable degree [3]

with PP to form either a graft (PP-g-PBBPA), or it mayattach to low molecular weight PP chains as an end-group. The grafting of PBBMA onto PP may increasethe polarity of the PP and thus enhance its adhesion topolar fillers, a role that is presently performed by modi-fiers such as maleic anhydride. More details can be

0266-3538/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0266-3538(03)00170-2

Composites Science and Technology 63 (2003) 1865–1875

www.elsevier.com/locate/compscitech

* Corresponding author. Tel.: +972-8-646-1486; fax: +972-8-

6472916.

E-mail address: moshe@inca.bgu.ac.il (M. Gottlieb).

found in our previous work [3–5] which dealt with theextent of thermal polymerization, grafting of the FRonto PP, and the amount of residual monomers left inthe system. The influence of other additives on thereaction and resulting morphology was also reported inthese references.The addition of PBBMA imparts flame retardant

properties to the PP composite and possibly improvesits filler compatibility, as previously discussed. Yet, italso has adverse effects on the mechanical properties ofthe composite. Glass fibers may balance these antag-onistic effects. Although added as reinforcement agent,glass fibers are also expected to improve flame retardantproperties by diluting the host polymer. The addition ofsecondary additives may further improve or deterioratethe mechanical and flammability properties of the com-posite. Application requirements, which often demanddifferent combinations of properties, may dictate the useof a specific composite material whose constituents willact synergistically to meet the needs of the application.The effect of each individual additive is easily deter-mined by means of several simple experiments. How-

ever, synergistic/antagonistic interactions between thedifferent additives are not obvious and require in-depthinvestigations. Usually, in designing a polymer posses-sing a specific set of properties, it is necessary to balancebetween the product costs and its properties, whichshould be at least equivalent if not better than those ofthe polymer host. There can never be a single idealcomposite, since each additive contributes its ownparticular attributes. Thus, it is of practical importancefirst of all, to understand better the effect of differentadditives and the interactions between them in the mostefficient way and secondly, to be able to forecast com-posite properties based on the knowledge acquired.In this paper, we describe a method for the determi-

nation of the contribution of each of the additives (for-mulation variables) by means of the experimental designfollowed by statistical analysis of mechanical and flameretardant properties (responses). In this approach, thesystem is considered as a black box that is affected by anumber of independent controllable input variableswhose actions are transformed to the changes in thesystem responses. The variation of the measuredresponse caused by the modification of controllableinput variables can be translated to a predictive mathe-matical model. The model developed can then be usedto predict response within the operating range of con-trollable variables.In the next section the details of the experimental

methods are described. The statistical methods used todesign the experiments and analyze the data aredetailed in Section 3. The effect of the different vari-ables on the tested properties, their statistical sig-nificance, interactions between variables and theoptimization of composite formulation are discussed inSection 4.

2. Experimental

2.1. Materials

The materials are all technical grade, commerciallyavailable and were used as received. A typical formulationwas prepared by compounding the following materials:Polymer matrix:Polypropylene (3120MN1, Appryl, France) d=0.9 g/

cm3, fusion temp=163 �C.Additives:1. A. PBBMA (FR1025M, DSBG, Israel) primary

FR;

B. Antimony trioxide (ATO). A masterbatch of

80% Sb2O3 and 20% Low Density Poly(-ethylene), characteristic diameter 1.5 mm(L0112 Kafrit, Israel) is added as FR syner-gist. For optimal FR activity, it is added in1:3 ratio of ATO:PBBMA.

Fig. 1. Chemical structure of the FR: (a) monomer (PBBMA), (b)

polymer (PBBPA).

1866 H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875

2. Irganox B225, a blend of Irganox 1010-hindered

phenol type and Irgafos 168-phosphate type(Ciba Giegy, Germany). It is an antioxidant,which is added in order to prevent PP oxidationduring compounding (the commercial PP abovecontains �800 ppm of this antioxidant).

3. Magnesium Hydroxide, coated with 2% stearic

acid (FR20, DSBG, Israel). It is used as multi-functional filler with intrinsic flame retardantproperties. It serves as a secondary FR.

4. Chopped Glass Fibers with characteristic length

4.5 mm and characteristic diameter �10 mm,d=2.6 g/cm3 (P968, Vetrotex, France). Serves asthe primary filler added in order to improvemechanical properties. The fiber surface is trea-ted by the manufacturer with a silane-basedcoupling agent for better compatibility with PP.

5. Maleic Anhydride modified PP (PP-g-MAH).

(Exxelor VM42E, Exxon Mobile, USA)d=0.9g/cm3, used as a compatibilizer, gives thePP a somewhat polar nature. Only the graftedpolymer is used, no free MAH is added to thecomposite.

In addition small amounts (0.2 wt.%) ofPoly(ethylene) wax (AC-6A, Allied, USA) were addedin order to reduce shear stresses developed duringextrusion.

2.2. Compounding of composites

The five components listed above have been selectedas controllable input variables for the optimization of thecomposite. The relative amounts of these additives andthe way these amounts were selected will be described inthe section dealing with the experimental statisticaldesign. Once the amounts to be used had been deter-mined, the composite was compounded as follows:All formulations were compounded in a twin screw

extruder (Berstorff ZE25 co-rotating, open vent, extruderL/D=32) at 250 �C, and 350 rad/min angular velocity ofthe screws rotation, and subsequently pelletized. Thepellets were dried overnight, then injected by means ofan Arburg Allrounder 320S injection molding machineinto molds of the required specimen shape for theappropriate tests (e.g. ‘dog-bone’ shape for tensile test,rods for UL94V flammability test etc.). A nozzle tem-perature of 240 �C and an injection pressure of 1300 barwere employed.

2.3. Flame retardancy tests

2.3.1. Vertical UL94 (UL94V)Rectangular rod shaped samples with dimensions of

127�12.7�3.2 (or 1.6) mm are exposed vertically to amethane gas burner flame for 10 s, as required by

UL94V. The sample is ignited at the bottom and burnsupward. The time required for the flame to self extin-guish after burner removal is measured and the occur-rence of dripping onto a piece of cotton placedunderneath the sample is recorded. The test is repeatedfor five different samples. If none of the samplesburns for more then 10 s and the drips do not ignitethe cotton, the material is classified as V-0. If noneburns for more then 30 s and the drips do not ignitethe cotton, the material is classified as V-1, but if thecotton is ignited the material classified as V-2. If anyof the samples burns for more than 30 s or if theentire sample is consumed, the material is classifiedas non-rated (NR).

2.3.2. LOI (limiting oxygen index)Samples shaped as rods or strips with dimensions

75�6.5�3 mm were enclosed in an open-vent chamberin a mixed oxygen/nitrogen environment. The sample isignited at its upper end with a hydrogen flame. Thesample burns from the top downwards. The atmospherecomposition that permits steady burning is determined.The limiting oxygen index is the minimum fraction ofoxygen in the gas mixture that will sustain burning for 3min or 2 inches, (whichever comes first). The system wascalibrated using a standard sample with a defined LOIvalue (ASTM D-2863-77). It should be reminded that itis incorrect to assume that material with an oxygenindex over 21% can not burn in practice despite the factair contains only 21% oxygen. Such an assumption dis-regards the fact that burning proceeds upward causingpreheating so that materials with oxygen index over21% will burn in air.

2.4. Mechanical tests

2.4.1. Izod impact reversed notchedThe sample used here is the same as in UL94V test

above. A standard notch (2.5 mm long and 0.25 mmnotch tip radius) is applied to the specimen. For thereversed Izod, the notch is in the opposite direction tothe striking hammer. The samples were examined in aZwick apparatus. The striking energy of the pendulumwas 1 J (ASTM D-256-81).

2.4.2. Tensile strengthThe sample used in the tensile test has a ‘dog-bone’

shape in order for either necking or failure to occur inthe center of the specimen and not near the clamps. Thesample is loaded at a constant rate and the stressapplied is recorded. The samples were tested in a Zwick1435 apparatus. Measurement procedure and calibra-tion was performed according to ASTM D-638-95.Before experiments were carried out optimal testingconditions were determined. It was determined that thetensile modulus for our samples should be obtained by

H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875 1867

tests carried out in the range of extension forcesbetween 200 and 600 N.

3. Statistical experimental design

3.1. Choice of the operating ranges for the controllablevariables

The amounts of five additives were chosen as con-trollable input variables for the experimental design:glass fibers, PBBMA/ATO, PP-g-MAH, Irganox andMg(OH)2. The range of concentrations of each variableis shown in Table 1. The two FRs: PBBMA as a pri-mary FR and Mg(OH)2 as a secondary one, were selec-ted for the following reasons. A flame-retarded PPcomposite based on Magnesium Hydroxide alonerequires extreme FR loading up to 65% by weight, inorder to achieve acceptable flame retardancy (V-0 clas-sification). Such loading will result in a composite withvery poor mechanical properties. The same is true forPBBMA, which requires a loading of approximately30% if used alone [6]. The incorporation of both FRs inPP allows the use of lower amounts of FRs with bettermechanical properties and at reduced cost, with iden-tical flame retardancy properties.The percentage of the PP itself has been set to a con-

stant level of 38.5% in all formulations. Higher con-centrations of PP in the composites result inunacceptable flame retardancy as can be seen in Table 2.These data show the narrow window available for PPconcentration to obtain a formulation with acceptableflame retardant properties. For example, the samplecontaining 38.5% PP is classified as V-0. Increasing thePP concentration at the expense of Mg(OH)2, by a mere1.5% would result in improved mechanical propertiesbut causes total flammability failure. The high sensitivityof flame retardancy to small changes in the matrix com-position makes the use of statistical design an invaluablemethodological approach. The ‘best’ compositionobtained prior to the application of the statistical analy-sis serves as the starting point for the statistical analysisand will be referred to as the ‘‘best initial formulation’’.On the basis of this narrow window, we attempt to

optimize other components in the composite such that

best possible mechanical and flame retardant propertiesare obtained. This implies that the amounts of thesecomponents in the composite are varied in a regularmanner between maximum and minimum values basedon prior knowledge.

1. The concentration of glass fibers was varied

between 15 and 25% due to the superiormechanical properties achieved within theselimits.

2. The primary FR PBBMA was used in combi-

nation with ATO as an FR synergist at an opti-mal constant ratio of 3:1 [7]. The selected upperlimit for this FR (15%) is the value in the V-0formulation in Table 2. Economical argumentsset this as an upper bound for the FR. The lowerlimit (10%) is believed to be the lowest value thatcan still achieve V-0 flame retardancy in thissystem and even that only in the presence ofother components.

3. The amount of PP-g-MAH is varied within the

range from 1 to 2%. It is customary to add 2% ofmaleic anhydride to PP. The decision to use asmaller amount of PP-g-MAH as the lower limitwas based on the assumption that PBBMA mayalso graft onto PP and play the same surface-modifying role as maleic anhydride.

4. The fourth variable is the antioxidant Irganox

B225, which is a radical scavenger and mayinterfere with the polymerization of thePBBMA in the composite. Therefore the lowerlimit was set to zero and the upper limit wasbased on the amount customarily used in PPcomposites.

5. The amount of the last variable—the secondary

FR (Mg(OH)2), was set by the overall amount ofthe other components to add up to 100%.

Table 1

Constrains for controllable variables in DOE

Variable

Upper limit

[+] (% w/w)

Lower limit

[�] (% w/w)

Glass fibers

25 15

PBBMA+ATO (3:1)

20 13.3

MAH

2 1

Irganox

0.3 0

Mg(OH)2

25 14.15

Table 2

Initial formulations

Component

Formulation No.

Units

A1 A2 A3 A4

PP

% 38.5 40.0 42.0 44.0

Glass fibers

% 20.0 20.0 20.0 20.0

PBBMA

% 14.0 14.0 14.0 14.0

ATO

% 4.6 4.6 4.6 4.6

MAH

% 1.0 1.0 1.0 1.0

PE Wax

% 0.2 0.2 0.2 0.2

Irganox

% 0.3 0.3 0.3 0.3

Mg(OH)2

% 21.4 19.9 17.9 15.9

UL-94 Rating (3.2 mm)

V0 NRa NRa NRa

Max. flaming time

s 10 19 55 50

Total flaming time

s 49 86 178 242

Cotton ignition

No No No No

LOI

% 25.0–25.5 25.0–25.5 25.0–25.5 24.0–24.5

a NR, Non Rated in UL94 classification.

1868 H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875

3.2. Statistical methods

The optimization of the formulation compositionswith respect to their desirable characteristics (calledresponses) is performed using the technique of mixturedesign. The outcome of the experimental design is amathematical expression for the value of the responsesas a function of the different ingredients in the compo-site (named factors). The total number of experimentalpoints is the smallest number of experiments that stillenable the formulation of this mathematical expressionwhich is termed the response surface and can be repre-sented by the following quadratic regression formula [8]:

R ¼Xq

i¼1

bixi þ�Xq

i<j

bijxixj

where R, the experimentally measured response; xi,concentration of the ith ingredient (the values of xi areprescribed by the statistical design and are known foreach experimental plan); bi, the regression coefficients,values of which are chosen to achieve the best fitting ofthe model to the given experimental data; and q, thenumber of ingredients in the mixture.The terms of the type bixi express the direct influence

of the ith ingredient on a certain response R (‘‘maineffects’’ of factors): the higher the value of bi, the greaterthe influence of the ingredient on the response. Theterms of the type bijxIxj express the so-called interactionsbetween factors. These interaction terms reflect the non-additivity in the influence of pairs of factors on a givenresponse: the higher the value of bij, the higher thedegree of non-additivity in the combined influence ofthe ith and jth factors on the response.The statistical significance of the factor effects is

obtained by comparison of the bi values with the esti-mate of the random noise in the system. This estimate isobtained as the standard deviation in a series of repe-ated experiments built into the experimental plan. Themain effects are estimated by the approximate t-test inthe framework of the mixture linear model. The secondorder interactions between variables are estimated by a t-test in the framework of a mixture quadratic model. Thelevel of significance for all statistical tests, both for themain effects and second order interactions, was chosento be �=0.05. In other words, if a p-value in a test forthe significance of a certain factor is smaller than 0.05,this factor is considered statistically significant at�=0.05 level of significance. One should remember, thatthe p-value represents the smallest level of significancethat would lead to rejection of the null-hypothesis (i.e.there is no effect of the controllable factor on theresponse under investigation) while this hypothesis istrue. The smaller the p-value obtained in the analysis ofthe influence of a certain factor on the response underinvestigation, the higher the probability that this factor

does influence the response. Factor interactions of thirdor higher orders are assumed to be negligible, as is rou-tinely practiced in the DOE process.The design of experiments of choice was a two-level

plan, i.e. the concentration of each factor (ingredient)was examined at two levels, the high and the low levelsof concentration. Specifically, the experimental plan wasa D-optimal two-level mixture design for five variables.The statistical analysis of the chosen D-optimal mixturedesign was performed by ‘‘Design Expert’’ software[Design-Expert1 V 5.0.8, Stat-Ease-Corp., MinneapolisMN 55413-9827 USA].The control factors (i.e. concentrations of the addi-

tives) and their experimental constraints (high [+] andlow [�] levels) are presented in Table 1.The experimental matrix included 20 runs: 15 runs for

the quadratic model, 2 runs for the curvature estimationand 3 replicate runs for the estimation of the randomnoise [9]. The experimental plan was created and ana-lyzed by the ‘‘Design Expert’’ software. The full experi-mental matrix is presented in Table 3. The experimentswere performed in a completely randomized sequence asset by the software. The final outcome of the analysis isan optimal composition with its associated desirabilityvalue (1.0—highly desirable, 0—highly undesirable) andpredicted properties.

4. Results and discussion

The results of all the mechanical and flame retardancytests performed are listed in Tables 4 and 5.

4.1. Statistical analysis of the factors and the interac-tions on the mechanical properties

In general, the statistical analysis demonstrates theexistence of statistically significant effects, both ‘‘maineffects’’ and ‘‘interactions’’. The following paragraphspresent the results of the statistical analysis of the cho-sen D-optimal mixture design and the discussion ofinfluence of main effects and of interactions on thecomposite properties.

4.1.1. ModulusOnly PP-g-MAH has a statistically significant influ-

ence (p-value=0.01) on the composite modulus at themain-effect level of analysis. Maleic anhydride is pri-marily used as a modifier in order to improve the adhe-sion between the polymer host and the glass fibers byfacilitating the interaction between the non-polar PPand polar filler. Such interactions are known to increasethe modulus of PP composite [10]. On the other hand,PP-g-MAH is known to adversely affect the modulus ofbulk PP by hindering PP crystallization [11]. Under nor-mal circumstances the improved adhesion overcomes the

H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875 1869

negative effect on crystallization resulting in overallincrease in modulus. Contrary to our expectations, ahigher maleic anhydride concentration resulted here in alower composite modulus. A possible interpretation isas follows: the FR replaces PP-g-MAH at the fiber sur-face hence reducing the effectiveness of PP-fiber inter-action. As a result, we hypothesize that in the presentformulation the maleic anhydride favors the PP bulk on

the PP-glass fibers interface. In the PP bulk the graftedMAH groups of the PP-g-MAH interfere with PP crys-tallization. This interpretation is not supported by anydirect experimental observation but is indirectly inferredfrom DSC measurements, which show that maleicanhydride lowers the amount of crystalline PP [3]. Theheat of fusion measured by DSC reflects the degree ofcrystallinity of a composite. When the heats of fusion

Table 4

Flame retardancy results of the mixture design matrix

Formulation No.

Units

1 2 3 4 5 6 7 8 9 10

UL-94 Rating (3.2 mm)

V1 V0 V1 V0 V1 V0 V0 NRa V0 NRa

Max. flaming time

s 26 9 19 6 23 2 0 48 0 32

Total flaming time

s 143 27 83 9 64 4 0 182 0 120

Cotton ignition

No No No No No No No Yes No No

UL-94 Rating (1.6 mm)

NRa V1 NRa V0 V2 V0 V0 NRa V0 NRa

Max. flaming time

s 32 19 40 5 26 4 5 55 5 39

Total flaming time

s 227 71 142 25 85 12 13 266 15 161

Cotton ignition

Yes No Yes No Yes No No Yes No Yes

LOI

% 24.0–24.5 25.5–26.0 25.0–25.5 26.0–26.5 26.5–27.0 25.0–25.5 26.0–26.5 24.5–25.0 25.0–25.5 24.5–25.0

Units

11 12 13 14 15 16 17 18 19 20

UL-94 Rating (3.2mm)

V0 NRa V1 V0 V1 V1 V1 V0 V0 NRa

Max. flaming time

s 7 40 13 1 13 12 24 4 10 60

Total flaming time

s 30 134 41 1 44 42 65 6 45 257

Cotton ignition

No No No No No No No No No Yes

UL-94 Rating (1.6mm)

V0 NRa V2 V0 V1 NRa NRa V0 V1 NRa

Max. flaming time

s 9 53 20 2 19 47 31 3 10 45

Total flaming time

s 48 241 100 5 102 128 86 8 54 222

Cotton ignition

No Yes Yes No No No No No No Yes

LOI

% 25.5–26.0 24.0–24.5 24.0–24.5 25.5–26.0 25.0–25.5 25.0–25.5 25.5–26.0 26.0–26.5 26.0–26.5 24.0–24.5

a NR, Non Rated in UL94 classification.

Table 3

Experimental matrix for the mixture design

Component

Formulation No.

Units

1 2 3 4 5 6 7 8 9 10

PP

% 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50

Fibers

% 25.00 21.68 25.00 17.78 15.00 25.00 15.00 23.35 15.00 22.85

PBBMA

% 10.00 12.50 12.50 13.90 14.63 15.00 14.74 10.00 14.74 10.00

ATO

% 3.30 4.15 4.15 4.64 4.87 5.00 4.91 3.30 4.91 3.30

MAH

% 1.50 1.00 1.00 1.77 2.00 1.50 2.00 1.00 2.00 2.00

PE Wax

% 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

Irganox

% 0 0.30 0.15 0.22 0.30 0 0 0.30 0 0.30

Mg(OH)2

% 21.50 21.67 18.50 22.98 24.50 14.80 24.65 23.35 24.65 22.85

Units

11 12 13 14 15 16 17 18 19 20

PP

% 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50 38.50

Fibers

% 20.00 20.85 25.00 18.65 25.00 18.10 25.00 15.15 20.00 22.85

PBBMA

% 15.00 10.00 12.50 12.50 15.00 12.30 15.00 15.00 15.00 10.00

ATO

% 5.00 3.30 4.15 4.15 5.00 4.10 5.00 5.00 5.00 3.30

MAH

% 1.00 2.00 2.00 1.00 1.50 1.50 1.50 1.00 2.00 2.00

PE Wax

% 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

Irganox

% 0.30 0.15 0 0 0.30 0.30 0.30 0.15 0.15 0.30

Mg(OH)2

% 20.00 25.00 17.65 25.00 14.50 25.00 14.50 25.00 19.15 22.85

1870 H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875

and crystallization were analyzed as responses in theframework of DOE, an increase in maleic anhydrideconcentration was shown to decrease the heat of fusionand even more significantly, the heat of crystallization(p-value=0.06).

4.1.2. Maximal strengthAt the main-effect level of analysis we find again that the

addition of maleic anhydride (in the form of PP-g-MAH)lowers the mechanical properties of the composite, in thiscase the maximal strength (p-value=0.02). This is ascri-bed, as before, to the lowering of PP crystallinity.In addition to the main-effect we also find here sub-

stantial negative interactions of second order:

1. Glass fibers enhance maximal strength at low

concentrations of maleic anhydride in a morepronounced way than at high concentrations(p-value=0.05).

2. The addition of large amounts of PBBMA over

the entire range of PP-g-MAH concentrationsresults in decreased values of maximal strengthrelative to those observed with low amounts ofPBBMA (p-value=0.06).

3. The addition of PP-g-MAH in presence of

Mg(OH)2 results in lower maximal strength(p-value=0.08).

All of the above indicate a poor modifying effect ofmaleic anhydride in the presence of the FR apparentlydue to its reduced role in adhesion at the polymer–fillerinterface. Maleic anhydride may interact with filler sur-faces via a reaction between its functional group andOH groups on Mg(OH)2 or glass fibers [10]. PBBMAmay also react with the fillers via a radical mechanismand hence compete with the maleic anhydride for siteson the surface of those fillers. This may force the maleicanhydride away from the filler surface and into thebulk of the PP matrix. It may be concluded that sinceboth the glass fibers and the Mg(OH)2 are surfacemodified to render them less polar, the addition of the

maleic anhydride (PP-g-MAH) as compatibilizer wasunnecessary in this case and had negative effects asresult of rejection by the other additives.

4.1.3. Izod impactStatistical analysis of factor-influences on impact

strength, as determined by the notched Izod Impact test,failed to reveal any significant effects. This was due to,the large scatter in the data (the range of measuredvalues was 15 (a.u.) with an absolute standard deviationof �8) that may have masked any effects present.Statistical analysis of the reversed notched Izod

Impact test results showed that all of the factors underinvestigation had significant influence as main effects onthis response. There were no statistically significantinteractions.Increasing the PBBMA concentration causes a reduc-

tion in impact strength (p-value=0.04). We have deter-mined [3,4] that in its monomeric form the FR isinsoluble in the matrix, while in its polymeric form itmodifies the morphology of PP. It is difficult at thispoint to determine the exact mechanism by which theFR reduces the impact strength of the composite. Yet, itmay be attributed either to the change in morphology,to a plasticizing effect of the monomeric FR, to achange in interaction between adjacent PP chains due toFR grafting, or to a combination of these effects.Increasing the concentration of Mg(OH)2 particles

also affects adversely (p-value=0.02) the impactstrength. The presence of high loading of solid particlesembedded in the polymer matrix results in reducedability to absorb impact energy. This is due to the fillereffect, which damages matrix continuity, and each par-ticle is a site of stress concentration, which can act as amicrocrack initiator [12]. Mg(OH)2 is sized with stearicacid, which comprises a long carbon chain that adheresto the polymer matrix. It may be argued that thesechains reduce stress concentration in the particle vici-nity, but apparently its effect is insufficient to overcomethe negative effect of the high loading of particles. Thus,the net result is a decrease in impact strength.

Table 5

Mechanical properties results of the mixture design matrix

Formulation No.

Units

1 2 3 4 5 6 7 8 9 10

Modulus

MPa 6297 6288 6488 4777 4874 6521 4508 6396 4245 5690

Max. strength

MPa 39.2 40.9 41.7 37.6 37.9 42.3 36.4 41.5 36.2 39.6

Impact

J/m 78.7 87.8 101.5 100.0 103.4 98.7 90.6 97.5 89.4 90.3

�

J/m 2.4 5.4 2.5 8.9 2.8 4.2 9.1 8.4 2.8 6.8

Units

11 12 13 14 15 16 17 18 19 20

Modulus

MPa 5683 5373 5803 5082 6032 4890 6021 4354 5047 5444

Max. Strength

MPa 40.4 37.6 39.7 36.6 40.4 36.7 40.3 36.7 38.2 39.1

Impact

J/m 95.0 92.5 82.2 83.7 101.2 93.1 93.1 95.6 85.6 96.9

�

J/m 7.2 10.2 5.9 5.6 10.0 4.6 4.6 6.5 5.2 4.4

H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875 1871

The addition of maleated PP also reduces compositeimpact strength (p-value=0.07). This may be attrib-uted, as before, to its effect on the morphology andchain conformation in the PP matrix.Glass fibers have the most unexpected effect observed

in the reduction of impact strength within the 15–25%loading range (p-value=0.02). The increase of glassfiber concentration from low level to high level causes asmall, but statistically significant decrease (i.e. gentlenegative slope) in impact strength. This is in contrast tothe large expected increase in impact strength and theincrease observed in PP composite, which contain onlyglass fibers. Glass fibers are presumed to disperse theimpact energy over a large volume, especially when thefibers are surface treated with silane coupling agents toimprove direct interaction with the PP matrix. In addi-tion, glass fibers have the role of fracture propagationinhibitors [13]. We have shown [4] that in this compo-site, despite the sizing, there is poor adhesion andtherefore the glass fibers act as an additional solid fillerwith a detrimental effect on composite impact resis-tance, as discussed above.Increasing the Irganox B225 concentration increases

the impact strength (p-value=0.02). This may beattributed to the prevention of PP oxidation. PP oxida-tion gives rise to end products such as ketones, which inturn may lead to chain degradation and reduction inmolecular weight resulting in loss of mechanical proper-ties. In addition, Irganox, as a radical scavenger, shouldinhibit secondary undesirable reactions of free radicalsformed during a mechanical fracture of the chains.

4.2. Statistical analysis of the factors and the interac-tions on the flame retardant properties

Prior to the discussion of flame retardant properties, wewill briefly review the flammability tests carried out.For a given product, the fire hazards in different areas

of application (e.g. building, transportation, electricalconsumer products, textile etc.) are so different thatflammability information from one category, does notapply to another. Even within one category there islarge variability between specific applications. Standardlaboratory tests try to remove unimportant parametersin a certain situation and focus on the relatively impor-tant ones. However, these tests do not predict materialbehavior in actual fire, because they are limited to smalllaboratory scale fire simulations. These tests are inten-ded only to compare between flammability properties ofdifferent materials under similar fire conditions. Onecannot compare results obtained by different testsbecause of lack of correlation due to the different pur-pose and the different fire situation partially reproducedin each test. These flammability tests also differ in differ-ent countries based on local regulations and standards. Inour particular case, the two tests employed are classified

as combustibility tests. The LOI test which is commonfor materials used in coverings and linings, and in cablesand wires for the construction industry, expressesflammability performance by a numerical value—theoxygen index (cf. Section 2). UL94V is commonly usedin applications other than building materials, especiallyin the electrical consumer products industry (plasticparts in devices and appliances). UL94V classifies aproduct by its flammability based on a qualitativescale [14]. PP the polymer used here, is classified asNR in UL94V and has a LOI value of 17–17.5.In the evaluation of UL94V results, only the total

flaming time was subjected to statistical analysis. Asexplained in the Experimental section above, each test isrepeated for five different samples of the same compo-sition. The flaming time results for V-0 class samples arerelatively reproducible with low dispersion (for examplein case of a 3.2 mm thick sample the typical flamingtime is ��1 s with standard deviation of ��1.6 s). Asclassification deteriorates the results are prone to bemore erratic with higher dispersion of flaming times(V-1: ��10 s, � �4.5, s; NR: ��26 s, ��14 s). There-fore, in order to reduce variability and scatter in theresults, we have chosen the total flaming time which isconsiderably more reproducible, as the response for thestatistical analysis rather than other measures such asthe maximal flaming time.The UL94V measurement was performed on samples

of 3.2 and 1.6 mm thickness. Although test results maydiffer between samples of different thickness, the resultsof the statistical analyses yield identical effect of allinput variables on the flammability irrespective ofthickness.Statistical analysis of LOI indicates that variations of

PBBMA and Mg(OH)2 concentrations within the selec-ted ranges have no statistically significant effect. This ismost surprising. In contrast, UL94V statistical eval-uation of the FRs shows that increasing the PBBMAconcentration has the most significant effect in loweringthe total flaming time (p-value=0.003). This implies asuperior flame retardant property, as expected. Increas-ing the Mg(OH)2 concentration also reduces the totalflaming time, as would be expected from a secondaryflame retardant (p-value=0.05). The expected effect ofboth FRs on the flammability of the samples as deter-mined by the UL94V test is in contradiction to the lackof any significant effect measured by the LOI test. Itimplies that the LOI test is not sensitive enough to dif-ferentiate between the flammability levels of the com-positions tested here.Statistical evaluation of the LOI shows that increasing

Irganox B225 concentration increases slightly the LOI(p-value=0.09). Whereas, UL94V evaluation showsthat increasing Irganox concentration results in highertotal flaming time (p-value=0.04). The higher flamingtime may be attributed to the role of Irganox as an

1872 H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875

inhibitor of the PBBMA radical polymerization.PBBPA, which is formed in situ during extrusion, is welldispersed in the PP matrix whereas the PBBMA isundissolved in the PP matrix [4]. The homogeneousdispersion of the PBBPA in the matrix allows for betterBr release and therefore better flame retardancy. Irga-nox may inhibit the PBBMA radical polymerizationand hence increase total flaming time. In addition, thehigh extent of polymerization caused by the low con-centration of Irganox, may increase the viscosity of thecomposite and subsequently, decrease the flowabilityduring UL94V test. In view of the larger p-valueobtained in the LOI statistical analysis, the conflictingresult from the UL94V experiment and the smaller sen-sitivity of LOI test we consider the increased FR prop-erty obtained by the latter as questionable.Increasing the PP-g-maleic anhydride concentration

results in a lower LOI (p-value=0.03). No statisticallysignificant effect was found in UL94V. This effect is dueto the higher chemical activity of PP-g-MAH and lowermolecular weight PP-g-MAH chains, which have highdecomposition rate.Increasing the glass fibers concentration lowers to

some extent the total flaming time (p-value=0.1). Incontrast, increasing the glass fiber concentration lowersthe LOI (p-value=0.08). Decreased total flaming time isattributed to a straightforward dilution effect of thepolymer matrix by the glass fibers. By the same reason-ing, we would also expect an increase in LOI, contraryto our experimental findings. In the following we shalltry to explain this surprising effect. One possiblemechanism for enhanced flammability is based on thesubstantially different thermal expansion coefficients ofglass-fibers and the PP matrix. Upon cooling of thecomposite after the compounding, unequal contractionleads to a ‘‘tunnel effect’’ that may cause the formationof air pockets around the glass fibers. The air trappedbetween the glass fibers and the polymer matrix will notonly enhance the burning, but also contribute to thepoor adhesion between the glass and the PP implied inthe discussion of mechanical properties above. Theproposed mechanism does not address the question ofpolymer creep into the air pockets eliminating even-tually their existence. Further experimentation isrequired to test this hypothesis.It is observed that the two flammability tests (LOI

and UL94V) result in a contradiction as to the effects ofglass fibers. This contradiction may be ascribed to thedifferent nature of these tests [15]. Brominated flameretardants work by releasing HBr gas into the flamewhere it acts as a free radical scavenger. In order toquench the free radical chain reactions that constitutethe burning process, the HBr has to achieve criticalminimum concentration. The chimney-like constructionof the LOI apparatus and the continual gas flow preventthe HBr from reaching this critical concentration. On

the other hand, the relatively static atmosphere aroundthe burning UL94V sample allows this critical HBrconcentration to be achieved. This is the source of dif-ference between the two test methods.The other possible explanation for increased flamm-

ability in LOI and decreased flammability in UL94Vis based on the order of magnitude higher thermalconductivity and thermal diffusivity of the glass fibersrelative to the PP matrix. Using this argument, theglass-fibers play the role of heat conductors. They con-duct the heat faster into the composite and therebycontributing to further PP decomposition and release ofcombustible gases. This progressive breakdown alsoresults in bromine release. Since as explained earlier, theLOI is performed in a continuously rejuvenated environ-ment due to oxygen flow, the bromine atoms in the gasphase are being continually withdrawn from the flamingarea, thus preventing them from reaching that criticalconcentration, which would quench the free radical reac-tions. The UL94V is performed in a stagnant environmentwhere the bromine atoms are able to perform their func-tion, and the opposite statistical influence is observed.Finally, glass fibers have a large effect on UL94V test

results by the imparted increase in the viscosity of theheated composite. By reducing the flowability of thecomposite, an anti-dripping effect is achieved, whichleads to better UL94V evaluation. Dripping and flow-ability are also affected by the thickness of the sample.There was no indication of second order interactions

in the LOI statistical analysis. The UL94V statisticalanalysis revealed several mutual interactions. The inter-action between PBBMA and glass fibers is significant(p-value=0.04): the combination of a high level con-centration of PBBMA with a low level concentration ofglass fibers causes a significant decrease in total flamingtime. The interaction between PBBMA and Mg(OH)2 isalso significant (p-value=0.05): again the combinationof high concentration of PBBMA with low concen-tration of Mg(OH)2 causes a significant decrease in totalflaming time. This is a somewhat puzzling result sinceone would expect that the higher the total amount ofFRs (primary and secondary) the lower would be theflaming time since there is no evidence for adverse effectof one FR on the other. Thus, the results from the sta-tistical analysis may hint at an indirect effect (e.g.increased melt viscosity, reduced polymerization ofPBBMA etc.), which is responsible for the negativeinteraction parameter obtained from the statistical ana-lysis. We are unable to offer an explanation for thisresult at this point.

4.2.1. Search for optimal compositionOn the basis of the statistical analysis presented above,

we tried to construct the optimal composition of the PPformulation. The most crucial step in the optimizationprocess is the ranking of the composite properties by

H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875 1873

their importance. The basic concept of optimizationmeans compromise in some values in order to reachhigher values in others. The properties with respect towhich this optimization was performed were chosen asmaximal reversed notched impact strength (representa-tive mechanical property) and minimal total flamingtime, which provides V-0 in UL94V test (representativeflame retardant property).After conducting all 20 experiments of the D-opti-

mal mixture design, the software constructs an alge-braic equation (statistical model), which connects themeasured responses with the values of factors underinvestigation. This statistical model can be employed forsearching the optimal conditions, once the most impor-tant responses are specified. The optimization procedureperformed by the software yielded three formulations,which were expected to meet the optimization require-ments. The predicted optimal formulations are pre-sented in Table 6, together with the experimentallymeasured responses for these ‘optimal’ formulations.The main feature of these results is the similarity of theseproposed compositions as well as the measured char-acteristics of these formulations. To appreciate the pre-dictive power of the statistical model, we have comparedthe predicted characteristics of the best-proposed for-mulation (desireability=0.995; Table 6 sample B1) withexperimentally obtained characteristics. The results ofthis comparison are presented in Table 7. All predictedresponses, with the exception of the modulus, are in

extremely good agreement with the experimentallyobtained values. The confidence limits of the predictedvalues for the responses do not exceed 8% (see Table 7).Specifically, the confidence limits for the impact, max-imal strength and LOI are equal to �8, �4.8 and�2%, respectively. In contrast, the experimental mod-ulus was markedly higher than that predicted by themodel. We are unable to explain this discrepancy.Table 8 presents the characteristics of four formula-

tions: the ‘‘best initial formulation’’ (A1), the best pre-dicted formulation by the DOE model (B1) andadditional two formulations found in the course of DOEstudy which seem to have highly desirable properties.The two latter samples have been added to ascertain thatthe formulation chosen by the statistical procedure asoptimal formulation is indeed the best available. One mayobserve that the best formulation predicted by the sta-tistical model is a desirable compromise betweenmechanical and flammability characteristics: relatively

Table 6

‘Optimal’ formulations and test results

Component

Formulation No.

Units

B1 B2 B3

PP

% 38.50 38.50 38.50

Fibers

% 22.31 24.10 24.77

PBBMA

% 15.00 15.00 15.00

ATO

% 5.00 5.00 5.00

MAH

% 1.00 1.30 1.41

PE Wax

% 0.20 0.20 0.20

Irganox

% 0 0 0

Mg(OH)2

% 17.99 15.90 15.12

UL-94 Rating (3.2mm)

V0 V0 V0

Max. flaming time

s 0 1 1

Total flaming time

s 0 1 2

Cotton ignition

No No No

UL-94 Rating (1.6mm)

V0 V0 V0

Max. flaming time

s 1 2 2

Total flaming time

s 6 4 3

Cotton ignition

No No No

LOI

% 25.5–26.0 25.5–26.0 25.0–25.5

Modulus

MPa 6953 6834 7203

Max. strength

MPa 41.8 42.1 41.6

Impact

J/m 90.6 91.9 89.4

�

J/m 3.8 4.7 5.7

Table 7

Comparison between model prediction and experimental results of

formulation B1

Response

Predicted results Experimental results

Mean

A 95%

confidence

interval

Experimental

value

A 95%

confidence

interval

Modulus

5908 5578–6237 6953 (av.) 6470–7436

Impact

90.0 82.7–97.2 90.6 (av.) 87–94.2

Max. strength

42.3 40.3–44.4 41.8 (av.) 40.9–42.8

Total flaming

time (1.6 mm)

7

(�48)a–55 6 –

Total flaming

time (3.2 mm)

(�22)a

(�68)a–25 0 –

LOI

25.4 24.9–25.9 25.5 - 26 –

a Physically equivalent to value of zero.

Table 8

Properties of the optimal formulations

Initial

formulation

The DOE optimal formulations

A1

The best

predicted

The best formulations in the

DOE experimental matrix

B1

6 4

Impact

103�3 90�4 99�4 100–9

Modulus

6333 6953 6521 4777

UL94

(1.6 mm)

T=109 V1

T=6 V0 T=12 V0 T=25 V0

UL94

(3.2 mm)

T=49 V0

T=0 V0 T=4 V0 T=9 V0

LOI (%)

25.0–25.5 25.5–26.0 25.0–25.5 26.0–26.5

1874 H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875

high impact and V-0 rating (i.e. low total flaming time).The comparison of the latter formulation with the initialformulation demonstrates the compromise character ofthe process optimization: by accepting a slight decreasein impact value a dramatic improvement in flame retar-dancy is obtained. It is instructive to examine in detail thetwo additional compositions selected as superior for-mulations. Both of them are found to be inferior to (B1)the best predicted formulation. Total flaming time (1.6mm) in these formulations is 12 and 25 s, respectively, asopposed to 6 s for the optimal formulation. However, ifwe take into account the high value of the standard errorof the predicted mean for this response (22 s), we areobliged to reject these formulations, since the total flamingtime in UL94V=(V-0) is not allowed to exceed 50 s.Indeed, these two formulations fail the UL94V testregarding V-0 at the �=0.05 confidence level, if we takethe total flaming time as a sum of the predicted mean (12or 25 s) and twice the standard error (22�2=44 s).

5. Conclusions

The technique of statistical experimental design hasbeen employed to obtain an optimal formulation ofhigh loading polypropylene composite with suitablemechanical and flame retardancy properties. The DOEtechnique helps optimize a set of flame retardant addi-tives, which guarantee acceptable flame retardant prop-erties with minimal negative effect on mechanicalproperties. Moreover, it allows the study of the influ-ence of individual additives and the mutual interactionsin the composite within the chosen concentrationworking range. DOE provides a route to obtain theoptimal formulations, and to get the abovementionedinformation with the minimum number of experiments.An optimization of different additive loads was per-

formed and the final polypropylene composite exhibitedimproved mechanical and flame retardant properties. TheDOE scheme allows a prediction of most sample proper-ties within 8% from the actual experimental values. In atypical PP composite under investigation, we observedthat maleic anhydride lowers the modulus and impactstrength. This was attributed to the failure of maleicanhydride to enhance adhesion at the interface betweenPP and the different fillers and its effect was reduced to aplastizing effect in the bulk. As expected, PBBMA andMg(OH)2 decrease the composite impact strength andenhance flame retardant properties. Glass fibers increasethe modulus as a reinforcing agent, but have only amoderate effect on impact strength within the investi-gated range, probably due to poor adhesion with PP.Contradictory results have been obtained for the effect ofglass fibers on flame retardant properties. Differentexplanations for these results have been suggested, butno definitive conclusion is possible at this point.

Acknowledgements

The Ariel program for French-Israeli cooperation hasfinancially supported this work. MG acknowledges thefinancial support of the German-Israel Foundation andthe Israel Basic Science Foundation. We are grateful tothe staff of the Plastics Application Laboratory atDSBG for their help in carrying out the mechanical andflammability tests.

References

[1] Berger ES, Petty EH. Organofunctional silanes. In: Katz HS,

Milewski JV, editors. Handbook of fillers for plastics. New York:

Van Nostrand Reinhold Company; 1987.

[2] Muskatel M, Utevski L, Shenker M, Daren S, Peled M, Charit Y.

Flame retardant polypropylene fibers with good dyeability. J App

Polym Sci 1997;64(3):601–6.

[3] Dvir H, Gottlieb M, Daren S. Thermal polymerization of a

brominated flame retardant in a glass fiber reinforced poly-

propylene—quantitative analysis. J App Polym Sci 2003;88(6):1506–

15.

[4] Dvir H, Goldraich M, Gottlieb M, Daren S, Lopez Cuesta J.

Distribution of a brominated acrylate flame retardant in poly-

propylene. Polymer Degradation and Stability 2001;74:465–74.

[5] Dvir H, Goldraich M, Gottlieb M, Daren S. Imaging the dis-

tribution of a brominated acrylate FR in polypropylene. In:

Conference Proceedings—Conference on Polymer Modification,

Degradation and Stabilization, Palermo, Italy, September 2000.

Available: http//modest.unipa.it/conferences/2000/html/symp2/

O_2_Tu_1230.pdf.

[6] Montezin F, Lopez-Cuesta JM, Gaudon P, Crespy A. Flame

retardant and mechanical properties of a copolymer PP/PE con-

taining brominated compounds/antimony trioxide blends and

magnesium hydroxide or talc. In: Conference Proceeding—

Eurofillers 97, Manchester, UK, 1997, p. 399–402.

[7] Touval I. antimony oxide. In: Katz HS, Milewski JV, editors.

Handbook of fillers for plastics. New York: Van Nostrand Rein-

hold Company; 1987.

[8] Cornell JA. experiments with mixtures. New York: John Wiley &

Sons; 1990.

[9] Bowles ML, Montgomery DC. How to formulate the ultimate

margarita: a tutorial on experiments with mixtures. Quality

Engineering 1997–1998; 10(2): 239–253.

[10] Constable RC, Humenix JA, Adur AM. Glass fiber reinforced

polypropylene- performance enhancement with acid modified poly-

propylene. In: Conference Proceeding—ANTEC 89, 1389–1393.

[11] Karian HG, editor. Handbook of polypropylene and poly-

propylene composites; 1999.

[12] Mareri P, Bastide S, Binda N, Crespy A. Mechanical behaviour

of polypropylene composites containing fine mineral filler: effect

of filler surface treatment. Composites Science and Technology

1998;58(5):747–52.

[13] Karger-Kocsis J, Harmia T, Czigany T. Comparison of the frac-

ture and failure behavior of polypropylene composites reinforced

by long glass fibers and by glass mats. Composites Science and

Technology 1995;54(3):287–98.

[14] Troitzsch J. International plastics flammability handbook. New

York: Hanser Publishers; 1990.

[15] Hornsby PR,MthuphaA. Analysis of fire retardancy inmagnesium

hydroxide filled polypropylene composites. Plastics and Rubber

and composites processing and Applications 1996;25(7):347–55.

H. Dvir et al. / Composites Science and Technology 63 (2003) 1865–1875 1875