Mono- and Bis- Maleimide Resins in Preimpregnated Fibres

: Fibres are preimpregnated by solutions of mono- and bis-maleimides with comonomers. Imides alone polymerize to resins with too low energies of fracture. In the presence of Methylene dianiline or Aniline Diphenylmethylenebismaleimide reacts via Michael addition to equimolar addition products. When fibres are preimpregnated with these addition compounds alone or in combination with flame retardants, B-prepregs are obtained, which after curing show high glass temperatures and sufficient energies of fracture. 2.5 moles of Diphenylmethylene bismaleimide and 1mol Methylene dianiline or 1mole Aniline react to resins with glass temperatures of 350 and 380°C and energies of fracture of 75 and 100J/m 2 . The resins contain no carcinogen or blood harming free amines. A one pot reaction starting from Methylene dianiline and Maleic anhydride is possible and more economic. As polymerized bismaleimides possess their imide bonds in the side chain, they are strictly spoken no polyimides with imide bonds in the main chain. Diphenylmethylenebismaleimide and styrene as copolymer react to an insoluble crosslinked polymer. Fibres are preimpregnated with equimolar mixtures of 2- Bromophenylmaleimide and styrene. After curing laminates with 285°C glass temperature and sufficient energies of fracture are obtained. The heat resistant resins are charring polymers and display higher Limiting Oxygen Indices, when the heats of combustion are increased. Differential Scanning Calorimetry determines the temperatures and the heats of glass transition, which indicate that the glass temperature is raised, when the enthalpy is increased and the entropy reduced, which is achievable by Diphenylbismaleimide and 2- Bromophenylmaleimide with their large side groups appropriate for intermolecular forces and steric hindrance.


INTRODUCTION
Polyimides PIs are thermo resistant polymers with imide groups in the main chain. The most popular route to PI is based on the reaction of aromatic diamines with dianhydrides. PI is offered as thermoplastic granules, as thermoset powders or as semi finished products like preimpregnated fibres so-called prepregs, circuit boards, coil coatings, composites, varnishes or fibres for fire protective clothes. Since 1955 they are commercially available and since then many investigations documented in numerous publication have been performed . On the one hand PIs compete with engineering and commodity plastics and on the other hand with iron and aluminium. PIs show residues between 25 to 50% and belong to the group of charring plastics. They are highly related to graphite, electric conductive polymers and organic colour pigments, which all have in common a conjugated double bond structure. They resist against heat up to 250-350°C and are expensive with prices of 15 to 100€/kg. The commodity plastics polyethylene PE, polystyrene PS and the engineering plastic polyamide PA are products with prices from 1 to 4€/kg and with heat stabilities from 100 to 240°C. They belong to the Bismaleimides are the reaction product of diamines with maleic anhydride. They possess two double bonds. In the presence of amines or other H donating compounds they perform the Michael reaction and react to the precursor resin.
1957 the patent US 2,818,405 [23] was granted to DuPont, in which liquid diamines and bismaleimides reacted to elastomers. 1968 in the patent US 3,380,964 from Rhone Poulenc [24] homo and copolymerization of BMI are claimed. In the same year the synthesis of polyaminobismaleimides from bismaleimides and aromatic diamines is described in Brit 1,190,718 [25]. The early market leader Rhone Poulenc withdraws Kerimid based on the carcinogenic Methylene dianiline MDA. Ciba Geigy, now BASF, develops Matrimid 5292 a 1:1 mixture of diphenylbismaleimid DPBM and diallyl bisphenol A claimed in the patent US 4,100,140 [26].
Aniline, which reacts with formaldehyde to Methylenedianiline MDA, is the cheapest amine in the list of diamines in Table 2, but as mono amine aniline can be only applied in the Michael reaction. MDA and Naphthene diamine NDA are key products for polyurethanes and are available in large quantities. Also 5(6) Amino 1-(4'aminophenyl) 1, 3 trimethylindane DAPI, a reaction product of two molecules alpha Methylstyrene, is moderate in price. m-Aminophenol and m-Aminobenzoic hydrazide are building parts in commercial products.
Diamines can be used as reactants for the synthesis of bismaleimides and or as ingredients for the Michael reaction. Mono amines can only react in the synthesis of mono maleimides or in the Michael reaction. Mono maleimides and styrene polymerize to heat resistant styrene maleimide polymers. When the same diamine is used in the synthesis of bismaleimide and in the Michael reaction, the two stage reaction can be performed in a one pot reaction starting from one mole diamine and two moles maleic anhydride for the bismaleimide reaction followed by the Michael reaction between bismaleimide and diamine to the BMI resin.
Many bismaleimides BMI, which are based on MDA and other diamines such as those given in Table 2 are described in the literature [3,33]. Their data as far as available are represented in Table 3 and 3a: Their formulas, molecular weights MW, heats of formation Hf, heats of melting and polymerization hm/hpol as well as combustion and decomposition hcomb and hdec, the temperatures of melting, of polymerization, of glass transition and decomposition Tm, Tpol, Tg and Tdec and the amount of residue Rdec are summarized.
The data for the heats of formation Hf and of combustion hcomb are calculated according to van Krevelen [42,43].
The heats of decomposition hdec as well as the residuals after degradation Rdec result from balances of weights and heats.
The most important measure for the achieved thermo resistance is to be seen in the glass transition    Under the assumption that a high Tm will lead to a high Tg bismaleimides based on2,5 Bis(aminophenylthio)thiadiazol bismaleimide BDT with the highest Tm of 265°C followed by p-BAPS 4,4`bis (4-aminophenoxyphenyl)sulfone with Tm= 250°C and 2 (4-Aminophenylthio) 4(4-aminophenyloxo) thiazol bismaleimide BT with Tm= 205°C are those, which promise high Tg. Interesting is the fact that 1-(4aminophenyl)-1, 3 trimethylindane bismaleimide DAPI evolves about half the heat of polymerization. With Tm =90°C m-BAPS, though nearly related to p-BAPS, has the lowest temperature of fusion. The observed or expected advantages in properties seem not enough to justify a switch over from MDA to a substitute. Therefore a critical look is thrown on BMI based on MDA with the aim of reduction of toxicity and costs of manufacture: The precursor DPBM/MDA 2.5/1 molar ratio with very low MDA free content is produced in a one pot reaction starting from MDA and MSA or the cheap aniline is integrated into the prepolymer DPBM/A 2.5/1 instead of MDA. Equimolar mixtures of Styrene and 2-Bromophenylmaleimide applied on fibres are prepregs, which cure to polymers with high glass temperature and sufficient fracture energy.

EXPERIMENTAL
In Table 4 the ingredients used and their providers were summarized.
For the production of DPBM from MDA and maleic anhydride three different procedures were described:
Closed loop thermal dehydration in toluene and dichloromethane Daman Global Bismaleimide Market by Manufacturers (2017) [17] 3. Azeotropic distillation in toluene WO 97/47597 [39,40]  98g MSA were dissolved in 200g DMF. A solution of 99g MDA in 125g DMF was added. The mixture was stirred for 2 hours at RT. In this time MSA and MDA reacted to the amidoacid. Then 0.5g methane sulfonic acid was added and the temperature was raised to 100°C and a vacuum of 270mbar was applied. A distillate comprising 17% water and 83% DMF evaporated. When 18g water was removed, imidization was finished and the temperature was reduced to 60°C. A solution of 39g MDA in 95g DMF was added. When the viscosity had risen from 5 to 16cSt at 25°C, the solution was cooled to RT. The content was stirred for 1h and then precipitated in 1000g water. The yellow precipitate was filtered, carefully washed with MSA acidic water followed by washing with distilled water. After filtration a yellow cake was obtained, which was dried at 70°C under vacuum. The yield was 213g or 98% of theory. The free MDA content was less 0.1%. Discoloration by formation of aniline black was not observed.

Preparation of BMI Resin 2.5m DPBM and 1m MDA with a Free MDA Content of Less than 0.1% [27]
In a first step DPBM was synthesized [31].
In a second step the Michael reaction took place: 2.5m DPBM and 1m MDA to the precursor in 40%/ 55% solution at 60°C.
A 4l vessel was charged with 1256/942g DMF and 687/945g DPBM. The vessel was stirred and heated to 40°C. When all DPBM was solved 151/207g MDA was added. The whole vessel was floated with N 2 . The content was heated to 60°C and kept at that temperature for 3h, until the viscosity had increased 5to 16cSt (40%) or 15 to 60cSt (55%) and the IR band for primary amine band at 3338 cm -1 had disappeared.
Then the vessel was cooled to 30°C, and the content was precipitated into MSA acidic cold water (pH= 2-3). After filtration the retained cake was washed with MSA acid water (pH=2-3) followed by washing with distilled water. The yield amounted 98% and the content of MDA free was less 0.1%

Preparation of BMI Resin 2.5m DPBM and 1m
Aniline A with a Free A Content of Less than 0.1% The reaction was performed in analogy to the reaction of N-phenylmaleimide with aniline [38].
A 4l vessel was charged with 852.5g DMF and 945g DPBM. The vessel was stirred and heated to 40°C. When all DPBM was solved, 98g A were charged. The whole vessel was floated with N 2 . The content was heated to 60°C and kept at that temperature for about 3h, until the viscosity reached 14cSt and the primary amine band had disappeared. Then the vessel was cooled to 30°C. The content was precipitated into MSA acidic water cold water. After filtration the cake was washed with MSA acid water (pH=2-3) followed by distilled water. The yield amounted 1022g or 98% of theory and the content of free A was less 0.1%.

Preparation of Copolymers from Styrene with Phenylmaleimide, Xylylmaleimide, 2-Brom Phenylmaleimide and DPBM
A stirred vessel was loaded with 0.1mole styrene, 0.1 mole maleimide and 400 ml toluene and heated to reflux. Then 0.2% Di tert-butylperoxide and 0.2% AiBN were added. After 2h reflux at 110°C the solvent was eliminated in a Rotavapor. The residue was washed with acetone and dried at 100°C under vacuum for 6h.

Preparation of B Prepregs Comprising BMI Resins with 2.5m DPBM and 1m MDA or 1m A or Styrene-2-Bromophenylmaleimide Resin as well as FRs in Addition in a Tube Reactor Combined with an Impregnation Device for Glass Tissues
The reactor consisted of a stirred 10l vessel with an outlet on the bottom equipped with a pump leading into a copper tube with static mixing elements. A KDM small dosage balance Engelhardt dowitec fed the vessel continuously with a powder mixture comprising 82% DPBM and 18% MDA or 90.6% DPBM and 9.4%A and 0-20% FR and an inlet provided DMF for a 55% solution. The tube with 1.3cm diameter and a length of 50m had a volume of 6.6 l. For the sake of space saving the tube was wounded and placed in a water bath. On one end of the tube was the pump for dosage and on the other end a nozzle with a slit with 1m breadth and a variable width of about 1.3mm. Through the nozzle the solution entered a dipping bath tub. The glass tissue was continuously transported into the bath filled with solution. There it was converted over a diverting roller. The loaded tissue left the bath and entered an IR drying funnel of 1m length. In the funnel it was diverted and passed once more the bath. After twice dipping it entered again the drying funnel and was once more diverted. The temperature in the drying funnel was 130°C and the drying distance after the last dip was 2*1m. The impregnated tissue left the dryer in the B state.
Prepregs with 50% resin and 50% glass content were produced from the glass tissue US 2116 (Tissa Comp.) with a basic weight of 100g/m 2 .
The monomers for the resins 1, 2 and 3 were applied as 55% solutions with a densities at 60°C of 1.05g/cm 3  In addition flame retardants FR were applied: the halogen flame retardants Trisbromophenyl maleimide TBP, Ethylene bis tetrabromophthalimide EBTBP as well as the halogene free flame retardants Bicyclopentaerythritol phosphate BCPP, equimolar mixture of dicyandiamide and melamine DCDA/M and melamine polyphosphate MPP.

Manufacture of Laminates from B-Prepregs
Laminates with 1.6mm thickness, which are appropriate samples for the UL 94 test, were produced on a Langzauner 220T vacuum press. 14 layers of B stage prepregs were piled and put into the press. Then the press was closed under a pressure of 15bar. Within 30 minutes the press was heated to 180°C and kept at that temperature for 1 hour. In every case about 5% flux of resin was detected. Then the laminate with 1.6mm thickness and about 45% resin content was taken out and cured at 200°C for 48 hours for optimizing the properties.

Characterization of Products
Thermogravimetric Analysis TGA, Thermomechanic Analysis TMA as well as Differential Scanning Calorimetric DSC measurements were performed on a Mettler Toledo TMA/SDTA 840 with TGA/SDTA 851 Modul. Chemicals or extruded samples were placed in aluminium oxide crucibles of 900 µl volume (ME 511119, 960) with 12 mm diameter covered by punctured lids. The TGA measurements took place under nitrogen or air with 80 ml/min at a heating rate of 5-50 K/min. The TMA measurements were performed in aluminium oxide crucibles with 7 mm diameter and 4.6 mm height covered by 6 mm diameter lids in air at a heating rate of 5-50 K/min under nitrogen. The heats of combustion hcomb were measured in a calorimeter according to DIN 51900 Teil 2 (1977) Heizwertbestimmung and calculated. In DSC measurements the chemicals were heated at different heating rates under nitrogen. In this manner the heats and temperatures of fusion hf and Tf as well as those of decomposition Tdec and hdec were determined. TGA measurements under air or nitrogen allowed the determination of residues R.MDA free was determined by gas chromatography Werkvoorschrift Nr. 3002WV-0192 BASF Antwerpen N.V. (1992). The kinematic viscosity was measured in an Ubbelohde viscosimeter according DIN 51562. Fourier-transform absorption spectroscopy was performed on a Bruker IFS 45 spectrometer with the following conditions 4000-400 cm -1 spectral band, liquid state in couvettes.

RESULTS
The Reaction of 2.5moles DPBM with 1mole MDA to BMI Resin DPBM and MDA were dissolved in DMF in 40 and 55% concentration. The reaction was followed at temperatures between 40 and 120 °C by the content of free MDA and increase in viscosity as given in Figure 1 showing the hyperbolic curves for the free MDA calculated for solid in order to eliminate the influence of concentration and the corresponding viscosity for a 55% solution.
In Figure 1 the curves were of hyperbolic shape, indicating that the product MDA free*v was constant. In Table 6 the viscosities for 40% and 55% solution were listed in dependence of the conversion c.
Equilibrium was observed for the Michael addition. At each temperature the reaction stopped at a definite amount of MDA free. Therefore the equilibrium with the constant Ke was set up in equation 1: 1-c moles MDA and 2.5-c moles DPBM were in equilibrium with c moles MDADPBM at the conversion c.
In the Arrhenius plot of MDA free or Ke in dependence of temperature activation energy EA=18kJ/mole was obtained.
When the equilibrium state was reached, the mixture was precipitated in maleic acidic water. After precipitation the equilibrium in solution was no longer valid, MDA free and maleic acid reacted to a soluble salt remaining in the filtrate.
The reaction between 2.5m DPBM and 1m MDA was followed in solution by determining the free MDA content and the viscosity at 25°C as measure for the molecular weight. FTIR, DSC and C 13 NMR data in the literature [47][48][49][50] indicated that DPBM reacted with MDA to MDADPBM in a quick reaction. At temperatures higher than 150°C DPBM in excess reacted to the polymer. In IR a quick loss of 50% primary amine and maleimide were observed and instead of maleimide the succinic group occurred. From  these results it was concluded that one double bond of DPBM reacted with one MDA in a quick reaction.
In Figure 2 the Michael reaction was described as formula. The double bond content of 100% for DPBM was reduced to 82% in the mixture and reached 32% after reaction to the addition product DPBMMDA.
The balances of weight and heat were set up. The heats of formation were calculated from the heats of combustion according to van Krevelen [42,43]: For the endothermic reaction a heat consumption of 18kJ/mole was introduced, which had been derived from MDAfree or Ke in dependence of temperature. The reactions of MDADPBM and 1.5 DPBM to 2.5MDADPBpol were followed by DSC measurements in Figure 3.
In Figure 3 the heat flux was measured for DPBM and for the adduct comprising 2.5m DPBM and 1m MDA at constant heating rate. DPBM fused at 150°C under 90J/g heat uptake and polymerized at 250°C under heat evolution of-265J/g, which was higher than those of -197J/g [3] and -181J/g [44] given in the literature. For BMI a very small heat of fusion was detected at 80°C and at 230°C the heat of polymerization amounted -190J/g. The heat of polymerization of one double bond 85 kJ/mole [46] was introduced into the balance of weight and heat. Under the assumption that in a first step the molten adduct MDADPBM polymerizes and in a second step the molten DPBM polymerized with only one double bond, the following balances were set up:   Giulio [47] investigated the same composition 2.5DPBM/1MDA and observed at 164°C after 5h a double bond reduction from 70 to 40% comparable with F1: 82 to 32% and at 233°C after 5h a residual double bond content of 30% comparable with F2: 32 to 21%.
When Hpol =85kJ/mole was introduced into the balance, measured and calculated heats of polymerization were hpol ex/cal= -0.19/-0.19kJ/g. Under the assumption that DPBM polymerized with only one double bond to a linear chain with a double bond content of 50%, the Succinimide to Maleimide ratio 1/1 was expected In Figure 5 Poly DPBM with the Succinimide/ Maleimide ratio =1/1 was depicted.  According to Loustalot [48] after 24h at 200°C the ratio of Succinimide to Maleimide was 2/1. In this case a much higher heat of polymerization was obtained by calculation according F5a. In nitrogen DPBM exerted the first degradation at 490°C and the second at 600°C and 70% residue, in air DPBM degraded at 450°C. Under nitrogen BMI started to decompose at 400°C followed by a second step of degradation at 500°C and 75% residue, in air BMI initiated degradation at 400°C and 450°C at 70% residue.
The events under nitrogen could be expressed by balances of weight and mass.   The experimental hdec ex were obtained from TGA measurement, which were run with different heating rates. The experimental DSC data of Figure 6 were compared with those calculated: DPBM decomposed in N 2 under heat uptake 0.40/0.38kJ/g and BMI degraded in N 2 under heat uptake 0.42/0.43kJ/g.
In Table 8 commercial and experimental products with various ratios MDA/DPBM were collected and compared in respect to their energies of fracture GIC and their heat stabilities quantified in Tg and Tdec.
In Figure 7 the temperature of degradation Tdec, the glass transition Tg, the heat of polymerization hpol and the energy of fracture G IC were correlated with the amount of MDA.
According to Figure 7 BMI comprising 2.5m DPBM and 1m MDA with 18% MDA was a fair compromise between 90J/m 2 energy of fracture and 340°C glass transition temperature. The critical energy of fracture GIC in dependence of the MDA content showed sigmoide shape. At low MDA concentration the relation between G IC and the weight fraction of MDA followed the parabolic equation 2 G IC = 3600*(w%MDA/100) 3 equ.2

The Reaction Product of 2.5moles DPBM and 1mole A
The Michael reaction was also performed with the less expensive aniline A in order to obtain a 2.5DPBM/1A adduct, which was further polymerized to the resin A2.5DPBMpol.
The reaction was transformed in balances of heat and weight using the same heats of reaction for addition and polymerization already used with MDA. h reac ex/cal = /0.018kJ/g In the past Technochemie offered BMI resins based on m Aminobenzoic hydrazide ABH and m-Aminophenol AP.
In Table 9 the properties of BMI based on MDA, A (Aniline), ABH (m-Aminobenzoic hydrazide) and AP (m-Aminophenol) were compared. According to Table 9 the highest glass temperature Tg was achieved by BMI based on A.

Styrene-Maleimide Copolymers MMI
BMI resins were more related with styrene monomaleimide copolymers than with polyimides, because both carry their imide bond in the side chain. From Phenylmaleimide, Xylylmaleimide and 2-Bromophenyl maleimide styrene copolymers were synthesized with the aim of later prepreg production. Equimolar mixtures of styrene and monomaleimides reacted to linear polymers with alternating styrene and maleimide molecules in the chain, as described in Figure 8.
Also 1m DPBM was reacted with 1m styrene and a cross linked, insoluble product was obtained. The polymers with monomaleimides were soluble and  showed all well defined glass transition temperatures. The insoluble polymer with DPBM exerted a melting point at the melting point of DPBM, but evolved only half of the heat of melting. Therefore with high probability one could assume that one half of DPBM had reacted with both double bonds under cross linking and the other half remained not dissolved and melted as DPBM. All products were characterized by DSC measurements in Figure 9.
The literature postulated for the addition product of styrene or alpha methyl styrene with DPBM a cyclic structure [3]. In Figure 10 this structure was depicted.
A second DPBM should be attached in position C 1 under proliferation of the chain.
The obtained thermoplastic polymers MMI were characterized with their properties, which were given in Table 11.
BrPhenyl the 2-Bromophenylmaleimide Styrene copolymer showed the highest Tg and was chosen for   the manufacture of prepregs. In the case of BMI the Bprepreg consisted of 50% tissue and 50% (51%MDADPBM +49%DPBM). In the case of MMI the B-prepreg comprised 50% tissue and 50% (29%Styrene 71% Bromophenylmaleimide). After curing the B-pregregs to laminates the resins had to be completely polymerized, in order to obtain optimized properties.
As in all balances of weight and heat 85kJ/mole was introduced as heat of polymerization: In Figure 9 the glass temperatures were observed as steps in the heat flow curves. The sigma shaped steps with turning points were characterized by their height W/g and width T= 20*t at heating rate of 20K/min. The width did not differ very much and was about 10°C in any case or t= 60/20=30sec. By differentiation a maximum was obtained with the coordinates height/2 = (W/g)/2 and width= 30sec.
The area under the maximum was the heat flow integral and could be determined by integration ʃ d(W/g)*30 or approximately by the area of the triangle with the height (W/g)/2 and the width 30sec.

0W/g
In Table 12 the so determined heats of glass transition Hg, the glass temperatures Tg and the calculated entropies Sg were collected. As dT did not differ very much the value 10°C was taken in every case, Stoliarov [49,50] investigated charring polymers by DSC measurements and found for PEI a "melting heat or glass transition heat" of 1.0J/g, which fits to the obtained value of 1.7J/g. The observed heats and entropies of glass transition were much smaller than those for melting points. The sterically hindered Xylyl and BrPhenyl copolymers exerted higher Sg values than the non hindered Phenyl. The styrene-Phenyl polymer showed a yellow colour, whereas the styrene Xylyl and BrPhenyl polymers were of white colour. The lack of colour was caused by a breakdown of the conjugated system. For polymers, which change from liquid to solid state without heat development, the glass temperature was equal to the Kauzmann temperature [51,52]. In this case the enthalpy Hg as well as the entropy of glass transition Sg were zero. For polystyrene, PEI, PAI and for equimolar styrene maleimide the glass temperature was not equal to the Kauzmann temperature.
The mentioned PEI and PAI are well known under the brand names Ultem 1000 and Torlon 42030 L and styrene maleimide polymers are sold under the brand names Malecca by Denka Chemicals and styrene maleic acid anhydride copolymers under the brand name Dylarc by Nova Chemicals and Xirane by Polyscope Polymers. The styrene DPBM styrene polymer was cross linked and non soluble. No step for the glass temperature could be detected in the DSC. The observed melting point at 150°C and 0.035kJ/g heat of melting could be explained under the assumption that both double bonds of DPBM reacted with styrene S and half of DPBM remained non reacted and non dissolved so that the expected heat of melting was 0.090kJ/g*(358/462)/2 = 0.035kJ/g.
Heat resistant polymers compete with engineering plastics like polyamides as well as with metals like iron and aluminium. In Table 16 enthalpies and entropies were listed for iron, aluminium and crystalline polymers in comparison.
The thermoplastic polyamides achieved their high melting temperature by their high enthalpies caused by hydrogen bonds.

FR-BMI Laminates and their Burning Behaviour
Halogen free and bromine FR from Table 17 were applied by dry powder mixing to the 2.5m DPBM/1m MDA BMI adduct, which thereafter was dissolved in DMF to obtain a 55% solution, which was used in the tube reactor combined with an impregnation device. The solution passed the tube in 4.4 h. The temperature of reaction was 60°C and the output into a bath was 1.4l/h. The reacted solution filled a bath, in which the glass fabric US 2116 was led with a speed of 7.5m/h. After twice dipping and drying a B-prepreg with about 50% resin content was obtained. The brominated flame  In Figure 11 TBrP melted at 142°C under heat consumption of 0.05kJ/g and decomposed at 300°C with -0.004 and 0.01kJ/g heat of decomposition, ETBr melted under heat uptake of 0.065 kJ/g at 460°C followed by exothermic decomposition with -0.05 and -0.04 kJ/g heat evolution.
The applied FRs were different in their mechanism of activity. Tribromophenylmaleimide TBrP was integrated into the chain during curing. Ethylene bistetrabromrophthalimide was non reactive. In the case of fire both halogene FRs split off bromine, which interfered with the radical chain of combustion. Bicyclopentaerythritole phosphate BCPP was intumescent and built a cellular char under heat. The mixture of dicyandiamide and melamine DCDA/M evolved non burning gases and reacted to melamine C 3 H 6 N 6 and further to C 3 N 4 . Under fire Melamine polyphosphate MPP changed to ammonia, water and C 3 N 4 *(P 2 O 5 )0.5.
In Figure 12 DCDA/M melted at 210°C with 0.14kJ/g heat uptake. At 270°C a reaction took place with -0.36kJ/g heat evolution. Decomposition occurred at 320-430°C with 0.55kJ/g heat of decomposition. The intumescent FR BCPP showed two exotherm peaks at 310 and 350°C with each -0.1kJ/g evolution of heat. MPP degraded at 410°C with of 1.1kJ/g heat of decomposition.
The flame retardants bicyclopentaerythritol phosphate C 5 H 9 O 5 P, melamine polyphosphate C 3 H 6 N 6 .HPO 3 and the equimolar 16 layers of prepregs were staggered one upon the other in a vacuum press in order to achieve samples with 1.6mm thickness for the UL 94 test. The conditions of manufacture were 15bar pressure, within 30 minutes heating to 180°C for 1 hour. The laminates with 1.6mm thickness and 45% resin content were cured at 200°C for 48 hours. In the UL94 the after flaming time took 22 seconds for the virgin samples. When 5% flame retardants were added, the after flaming time was reduced under the necessary limit of 10 seconds.
Iron possessed the highest density followed by aluminium and graphite whereas plastics were the lightest materials. Polymers with residues, the so called charring polymers, solidified in the glassy state. The ladder polymer PAN and cellulose could be charred to carbon fibres. The burning behaviour was characterized by the LOI value. Graphite showed the highest LOI value. The highest heat of combustion hcomb exerted aluminium.
When only charring polymers without halogen from Table 15 were taken into consideration and their heats of combustion hcomb were put in relation with the inverse LOI value. The data were situated more less on a straight line, which could be described by equation 5. Equation 5 described the situation that charring polymers accumulated more and more carbon approaching graphite with high heat of combustion and low burning.

CONCLUSIONS
In Table 16 and 16a commercial thermoplastic polyimides, polybenzimidazoles as representatives of heat resistant polymers and polyamide 66 as a representative of engineering plastics are summarized. The thermodynamic approach allows the determination     of volatile products and of the amount of heat produced during degradation. PA 66 is a non charring polymer Rdec=0%, polyimides and polybenzimidazoles are charring polymers, which degrade to residues Rdec>0%. The polyimides as well as polybenzimidazole PBI exert exotherm heats of decomposition hdec. High heat resistance and easy workability are contradicting and have to be balanced in each case. PBI shows improved heat resistance but is difficult to handle. The thermoplastic copolyimides PAI, PEI, PEstI and PSI as well 2-Bromphenylmaleimide styrene copolymer reach glass temperatures about 10°C higher than 269°C the melting temperature of polyamide 66. The quantitative treatment of the glass transition temperature determines the enthalpy of glass transition hg about 0.5-1.1J/g, which is small in comparison with the enthalpy of melting hm about 70-300J/g. Nevertheless the small differences in the calculated enthalpies and entropies indicate that high values of enthalpy due to intermolecular interactions and low values of entropy caused by steric hindrance lead to high Tg. According to Table 16 and 16a thermoplasts with glass temperatures in the range of 210 to 280°C degrade at temperatures of 400 to 460°C. Their properties have to be compared with those of thermosets in Table 17.
Thermosets exert higher glass temperatures of 300 to 700°C and degrade between 550 and 450°C. Resins based on 2.5mDPBM/1mMDA and 2.5m DPBM/1mA belong to thermosets like Kapton or Vespel and PI, which comprises benzophenone dianhydride and bis aniline ether. In comparison with thermoplastics all thermosets have the advantage of low processing temperatures but the disadvantage of time consuming curing and processing risks like bubble formation and varying physical properties. PolyDPBM has a high Tg of 435°C and a high Tdec of 475°C but the energy of fracture is too low for practical application. DPBM and BMIs polymerize to linear polymers with imide bonds situated in the side chains. Therefore strictly spoken they are no polyimides, which carry their imide linkages in the main chains. In order to liberate BMIs from their niche existence, cost reduction is needed. A switch from 2.5mDPBM/1mMDA to 2.5mDPBM/1mA has cost reduction as consequence. As both substances are similar in toxicity their monomer content has to be very low. MDA is under the suspicion of causing cancer and A is a poison for blood, their amounts have to be as small as possible. The absence of free amine is achieved by precipitating the reaction mixture in MSA acidic water. In the unsolved state the equilibrium between bounded and free amine is no longer valid. The water soluble amine-maleic acid salt can be washed out completely. The process of manufacture is optimized, when the addition product (DPBM)MDA is synthesized from the reactants MDA and maleic anhydride MSA in an one pot reaction without isolating DPBM. The precursor resin 2.5DPBM)1MDA has a glass temperature of 335°C, a temperature of degradation of 470°C and a energy of fracture of 100J/m 2 . The precursor resin 2.5DPBM)/1A has a glass transition temperature of 380°C, a temperature of degradation of 430°C and an energy of fracture of 75J/m 2 . As the reaction of DPBM with styrene to a soluble resin fails, 2-Bromophenylmaleimide and styrene are applied on tissues for the production of prepregs. After curing a glass transition temperature of 285°C is obtained. Flame retardants reduce the after flaming time and shift the weight loss curve from that under air to that under nitrogen. The flame retardants though very different in mechanism exert similar efficiency, an amount of 5% is sufficient to reach after flaming times below 10 seconds for a V-0 classification. BMIs belong to the group of charring polymers, for which a correlation between the heat of combustion and the inverse limiting oxygen index is obtained, quite opposite to that valid for non charring polymers. The correlation predicts that polymers with high heats of combustion are low burning. The quantitative treatment of the glass transition temperature showed that the enthalpy of glass transition is much smaller than the enthalpy of melting. Nevertheless the data indicate that high glass temperatures are combined with high enthalpies due to intermolecular bonding and low entropies caused by steric hindrance.