FI126585B

FI126585B - FIRE-RESISTANT THERMOPLASTIC COMPOSITE

Info

Publication number: FI126585B
Application number: FI20135502A
Authority: FI
Inventors: Eetta Saarimäki; Outi Härkki
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2013-05-13
Filing date: 2013-05-13
Publication date: 2017-02-28
Also published as: EP2997089A4; FI20135502A; WO2014184429A1; EP2997089A1

Description

FLAME RESISTANT THERMOPLASTIC COMPOSITE FIELD OF THE INVENTION

The present invention relates to novel thermoplastic composites and methods of manufacturing the same. The inventive thermoplastic composites have improved flame resistance and good processability properties, and they are less harmful to environment than current thermoplastic composites.

BACKGROUND

Thermoplastics are linear or branched polymers that become flowing under pressure and at elevated temperatures. The polymer chains of thermoplastics have the capability to slide past one another when heated or sheared, allowing the thermoplastic to become mouldable in melt state. When cooled, the thermoplastics adapt a rigid structure. PVC, polyvinyl chloride, is a widely used thermoplastic. Its benefits are the low cost and versatility in processing and performance, such as mechanical properties and weather and UV-stability. PVC is used e.g. in cable insulation and in manufacturing of pipes and profiles. The hardness and other mechanical properties of PVC can be controlled by using plasticizers. PVC can be produced e.g. as a hard PVC or soft PVC. Soft PVC composites can be used to coat technical fabrics that can e.g. be used in storage houses, rolling doors and shielding in diverse equipment and stock.

Combustion of PVC produces a large amount of smoke. Flame retardancy of PVC is good compared to most thermoplastics. However, attempts are made in the field for improving the flame retardancy of PVC materials for example by adding flame retardants, such as alumina trihydroxide (ATH) or antimony trioxide (ATO), in order to improve the flame retardancy of PVC products. ATO is widely used as a PVC flame retardant synergist (FR) because of its smoke suppression effect in combination with the primary flame retardant. According to the International Antimony Association, 37% of Sb203 production worldwide (120.000 tons in 2010) is used in plastic PVC FR systems. PVC compositions are used in various applications that require processing of the PVC composites into various shapes. Therefore the mechanical properties such as tensile strength and elasticity of PVC compositions used in the applications that has to meet certain criteria typical for the particular application. Using large amounts of filler material such as flame retardants is detrimental to mechanical properties and processability of PVC compositions and it is often preferable to keep the amount of such fillers low.

Growing environmental awareness and stricter legislative requirements for flame retardants are the driving forces for developing new environmentally friendly flame retardant polymeric materials. In this respect the widely used ATO is not an optimal FR agent. ATO is classified in EU with the risk phrase R40 limited evidence of carcinogenic effect and with hazard statement H351 suspected of causing cancer by inhalation. Therefore, there is a need for less harmful alternatives for ATO in thermoplastics FR systems. CN102120856 discloses a PVC nanocomposite comprising aluminium hydroxide and molybdena, zinc borate, antimony oxide, aluminium hydroxide or magnesium hydroxide as fire retardants.

Despite some progress in the field, there still exists a need to provide environmentally friendly improved flame resistant PVC materials that allow easy processing.

SUMMARY

An object of the invention was to provide environmentally friendly improved flame resistant thermoplastic composites.

Another object of the invention was to provide a new method for manufacturing environmentally friendly improved flame retardant thermoplastic composites.

Another object of the invention was to develop flame retardant thermoplastic composites that are easy to process and compatible with processes that are used to manufacture conventional thermoplastic products.

Another object of the invention was to reduce the amount of flame retardant components in the thermoplastic composites to allow better processability of the thermoplastic products wherein the thermoplastic composite is used. Nano-sized filler particles allow lower amount of FR filler and improved processability of the composite.

Aspects of the invention relate to thermoplastic composite having improved flame resistance. In an embodiment the flame resistance of thermoplastic composite is improved by using a two-component system wherein aluminium trihydroxide (ATH) functions as the first FR and the second, synergistic, FR is selected from hydrotalcite, organoclay such as natural montmorillonite, or kaolin.

In one aspect the thermoplastic composite comprises or contains ATH as the first FR and kaolin as the second, synergistic, FR.

In one aspect the thermoplastic composite according to the invention is essentially free from molybdenum oxide, antimony, antimony trioxide, antimony pentoxide, antimonite, zinc borate, aluminium hydroxide, and magnesium hydroxide. In a particular aspect the inventive PVC composite is free from antimony.

In one aspect the invention provides use of aluminium trihydroxide as the first FR and micro and/or nanometer-sized mineral filler particles as the second, synergistic, FR in a two-component FR system for manufacturing a thermoplastic composite with improved flame retardancy.

In one aspect the invention provides a method of manufacturing a flame resistant thermoplastic composite having a two-component FR system by mixing PVC components with aluminium trihydroxide as the first FR and a second, synergistic, FR selected from kaolin, hydrotalcite and/or organoday into an essentially homogeneous dispersion; and melting the homogeneous dispersion while mixing.

In one aspect of the invention the fist or the second FR have an average particle size in the nanometre scale.

In one aspect of the invention the thermoplastic is PVC. As is evident to a person skilled in the art, the aspects of the invention are not limited to PVC only, but any thermoplastic is equally well suited for the two component FR system according to the invention.

Other aspects of the invention relate to products coated with the thermoplastic composite according to the invention.

The characterizing features of the invention are presented in the appended claims. DEFINITIONS

Unless otherwise specified, the terms, which are used in the specification and claims, have the meanings commonly used in the field of thermoplastic industry. Specifically, the following terms have the meanings indicated below.

The terms "flame retardant filler" and "flame retardant" refer to particulate agents, compounds and material used typically in the thermoplastic industry to prevent or retard combustion of the thermoplastic material. Examples of such agents include molybdenum oxide, zinc borate, antimony oxide, aluminium hydroxide, aluminium trihydroxide, magnesium hydroxide, and antimony trioxide, antimony pentoxide, antimonite or other antimony-containing agents.

The term "flame retardant composite", refers to a thermoplastic composite which has flame retardant filler in the composition to improve the flame retardancy of the material.

The term PVC composite refers to polyvinyl chloride composite.

The term HRR refers to heat release rate characterized by cone calorimeter.

Unless otherwise noted, all percentages are by weight.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a SEM image of the PVC composites manufactured in the Examples. A: PVC FRI; B: PVC FR6; C: SEM of hydrotalcite particles according to the invention.

Figure 2 shows the heat release of PVC composite prepared according to Example 1. A: PVC FR reference and sample PVC FRI; B: PVC FR reference and samples PVC FR3, PVC FR4, PVC FR5 and PVC FR6.

Figure 3 shows the smoke production of PVC composites prepared according to Example 1. A: PVC FR reference and sample PVC FRI; B: PVC FR reference and samples PVC FR3, PVC FR4, PVC FR5 and PVC FR6.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have surprisingly found that excellent flame resistance of thermoplastic composites, such as PVC composites, can be achieved by a two-component FR system comprising ATH as the first FR and the second, synergistic, FR component is selected from kaolin, hydrotalcite and organoclay. The second FR has a synergistic effect with the first FR and the two-component FR system obviates the need for other FRs, such as antimonous agents.

Synergistic flame retardant fillers

Mineral fillers kaolin, hydrotalcite and/or organoclay as flame retardants were incorporated into PVC via melt mixing. Hydrotalcite and/or kaolin were used as the synergistic FR and ATH as the primary FR in two-component FR system. The results show that both kaolin and hydrotalcite can replace and reach the flame resistance effect of antimony trioxide in FR PVC composites. Each of the two agents lowers the HRR slowing the flame spread in the composite. Also a clear smoke suppression effect can be observed. Substitution of the traditional ATH flame retardant with a nano-size ATH filler does allow lower overall loading of the FR without compromising the performance of the composite, in particular its mechanical properties and processability. The present invention provides non-toxic, environmentally less harmful and economical alternatives to antimony trioxide in PVC composites.

In one embodiment the synergistic FR may be hydrotalcite. Hydrotalcite is chemically aluminium magnesium hydroxy carbonate. As is well known in the art, hydrotalcite may occur in various forms wherein the ratio of aluminium oxide to magnesium oxide may vary. The AbCteiMgO may be 37.5:62.5. Hydrotalcite according to the invention may be obtained from various sources.

In one embodiment the hydrotalcite is nano-sized and/or micron-sized. The hydrotalcite may have an average particle size of about 4.7pm. Even though the measured average particle size may be about 3-6 pm, it is well known in the field that the actual particle size and dimensions of the individual hydrotalcite particles may vary from nanometre scale particles to larger (see Fig. 1C which shows the presence of particles having a diameter in the nanometre scale even though the average particle size of the hydrotalcite is 4.7pm). Consequently, the hydrotalcite particles according to the invention may comprise hydrotalcite particles in the range of 10nm-100pm.

In one embodiment the hydrotalcite particles are coated with an agent preventing agglomeration of the particles. Such agents may comprise stearic acid or any other commonly known anti agglomeration surface modification which is compatible with the thermoplastic composite according to the invention and which prevents agglomeration of hydrotalcite particles. The anti-agglomeration coating enhances homogeneous dispersion of the hydrotalcite particles throughout the thermoplastic composite which improves the synergistic FR effect of the hydrotalcite.

In one embodiment, the synergistic FR may be kaolin which has the chemical formula Al2Si20s(0H)4.

In one embodiment the kaolin is nano-sized and/or micron-sized. The kaolin may have an average particle size of about l-3pm. Even though the measured average particle size may be about l-3pm, it is known in the field that the actual particle size of the individual kaolin particles may deviate from the average particle size and such a product may contain kaolin particles having a particle size in nanometre scale and in the micrometre scale. Consequently, the kaolin particles according to the invention comprise kaolin particles in the range of 10nm-100pm.

In one embodiment the loading level of the second FR is kept below 8%by weight, e.g. 7% by weight, 6% by weight, 5% by weight, 4.5% by weight, 4% by weight, 3.5% by weight, 3% by weight, 2.5% by weight, 2% by weight, 1.5% by weight, or 1% by weight. The particular loading level naturally depends on the other properties of the thermoplastic composite, such as the loading level of other filler particles like ATH, the average particle size of ATH and the second FR, and the desired properties such as hardness and elasticity of the PVC composite. Generally a lower loading level of fillers is beneficial to mechanical properties and processability of the thermoplastic composite.

The filler of the invention (such as ATH, kaolin, hydrotalcite or organoclay) may be in the form of a dry product, suspension or paste. In one aspect the filler may be in the form of a dry product, such as powder. The particle size of the filler may be controlled before mixing with the other components of the thermoplastic composite, or it may be controlled after mixing the components.

The filler may optionally comprise other additives known in the art and generally used in the manufacture of thermoplastic composites, such as plastisizers, heat stabilizers, UV stabilizers, lubricants, processing aids, impact modifiers, thermal modifiers, fillers, biocides, and pigments.

Method for manufacturing the thermoplastic composite

In one embodiment the thermoplastic composite according to the invention is manufactured using a method which comprises mechanical mixing of the thermoplastic powder with plasticizer, additives and fillers followed by melt state mixing using extrusion process, a two roll mill or a calendering process. 1. Mixing PVC components, PVC powder, plastisizers and fillers, such as processing aids and anti-oxidant, with ATH and the second synergistic FR into a homogeneous mixture in powder form. 2. Melting the homogeneous mixture while compounding. 3. Optionally processing the composite into a form targeted to certain application e.g. surface layer, tube or cable.

In the first step the components may be mixed in any order or simultaneously. In one embodiment the PVC components are mixed first and the FR components milled to the desired particle size are added subsequently. Suitably the thermoplastic components are milled to the desired particle size and mixed into a homogeneous dispersion. The mixing can be carried out in a high speed mixer, such as Papenmeyer type NTHKV5 high speed mixer or a similar blade mixer. Suitably the mixing is carried out until the components are evenly dispersed throughout the mixture. In one embodiment both ATH and the second synergistic FR are milled simultaneously into the selected particle size prior mixing. In another embodiment ATH and the second synergistic FR are milled to the selected particle size separately prior mixing.

In the second step melting is carried out at a temperature which ensures melting of the compound. Suitably the temperature can be about 180°C and the mixing is carried out using a suitable mill, such as a two roll mill LRM-S-110/E+W from the company LabTech. The milling parameters are selected such that they ensure even distribution of the fillers throughout the thermoplastic composite during melting and milling. Suitably the melt mixing is continued for a time sufficient to thoroughly melt the compound, such as about 4 minutes.

Optionally, the mechanic properties such as or tensile strength or impact strength of the resulting thermoplastic composite is tested. A skilled person is readily able to select an appropriate method to test such properties using techniques commonly used in the art, such as tensile testing machine.

Optionally, the PVC composite produced in the method above is directly applied onto a substrate, such as a fabric, and cooled.

Optionally, the thermoplastic composite produced in the method above is extruded into pellets, granules or extruded into a profile or tube.

Suitably normal atmospheric pressure is used in the method.

The second synergistic FR is suitably selected from kaolin, hydrotalcite and organoclay (montmorillonite). The substrate is suitably finely divided particulate material.

Suitably, the ATH may be at least partially in nanometer size (nano-ATH) and/or the second synergistic FR may be of nanometer size. When nano-ATH is used, the loading level of nano-ATH may be lower and similar flame retardancy can be achieved than when using micron-sized ATH with higher loading levels. Suitably, ATH may be present in an amount of 25% or less by weight, e.g. between 1% and 25%, or about 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2% or 1% by weight. Preferably ATH is present in an amount of more than 10% but less than 25% by weight. When nano-ATH is used, both nano-ATH and the second synergistic FR may be present in an amount below 10% by weight, e.g. about 9%, 8%, 7%, 6%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5%, 1% or 0.5% by weight. Generally the amount of nano-ATH and the second synergistic FR is lower than that of the ATH.

The flame testing was performed in the present application by measuring the heat release rate (HRR) using the CC-1 cone calorimeter equipment from Gowark Ltd. The selected method measures oxygen consumption derived from oxygen concentration and the flow rate of the combustion product stream. Even though the present application uses the method above to test flame retardancy of the PVC composite, other similar methods are as well applicable and a person skilled in the art is readily able to select a suitable method.

If desired, in order to improve performance of the PVC composite materials other fillers and additives may be used.

Scanning electron microscope images of PVC composite according to the invention is illustrated in Figure 1.

Use

The PVC composite according to the invention may suitably be used in the manufacture of textiles surface layer, pipes, cables, tubes and profiles for construction, electrical appliances (housing, protective cover etc.).

One aspect of the invention relates to a thermoplastic composition comprising a two component flame retardant system wherein the flame retardant system comprises aluminium trihydroxide as the primary flame retardant and the second, synergistic, flame retardant is selected from the group consisting of kaolin, hydrotalcite and organoclay.

In another aspect the thermoplastic composition comprises aluminium trihydroxide having an average particle size of less than 5pm, such as less than 4pm, less than 3pm, less than 2pm, about 1pm, or less than 1pm.

In another aspect the second synergistic flame retardant has an average particle size of less than 5pm, such as about 4.5pm, about 4pm, about 3.5pm, about 3pm, about 2.5pm, about 2pm, about 1.5pm, about 1pm, or less than 1pm.

In another aspect the loading level of aluminium trihydroxide is less than 25% by weight, preferably more than 10% by weight and less than 25% by weight.

In another aspect the loading level of the second synergistic flame retardant is less than 8% by weight, e.g. 7.5%, 7%, 6.5%, 6%, 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5%, 1% or 0.5% by weight and, optionally, nano aluminium trihydroxide is present in an amount between 0.5% and 5%.

In another aspect the second synergistic flame retardant is kaolin optionally having a particle size between lOnm and 10pm, preferably between lOOnm and 5pm.

In another aspect the thermoplastic composition is essentially antimony free.

In another aspect the second synergistic flame retardant is coated with an antiagglomeration agent, such as stearic acid.

In another aspect the flame retardant system contains aluminium trihydroxide as the first flame retardant and kaolin as the second synergistic flame retardant, and kaolin optionally has a loading level of less than 5% by weight, and wherein the kaolin particles have an average particle size of less than 1pm.

In another aspect the thermoplastic composition is PVC.

In another aspect the invention relates to use of aluminium trihydroxide as the first flame retardant and a mineral filler as the second synergistic flame retardant in a two-component flame retardant system for manufacturing a thermoplastic composite with improved flame retardancy.

In another aspect, in the use the second synergistic flame retardant is selected from hydrotalcite, kaolin and organoclay.

In another aspect, in the use the thermoplastic composite is essentially free from antimony.

In another aspect, in the use the second synergistic flame retardant has an average particle size of less than lOpm, preferably an average particle size of less than 5pm, about 4.5pm, about 4pm, about 3.5pm, about 3pm, about 2.5pm, about 2pm, about 1.5pm, about 1pm, or less than 1pm.

In another aspect, in the use the second synergistic flame retardant is kaolin.

In another aspect, in the use the first flame retardant and the second synergistic flame retardant have an average particle size of lOnm - 10pm.

In another aspect, in the use the thermoplastic composition is PVC.

In another aspect the invention relates to a method for manufacturing flame resistant thermoplastic composite wherein the method comprises 1. Mixing thermoplastic components with aluminium trihydroxide as the first flame retardant and a second synergistic flame retardant comprising hydrotalcite, kaolin and/or organoclay into an essentially homogeneous dispersion 2. Melting the homogeneous dispersion while mixing.

In another aspect, in the method of manufacture above the first flame retardant has an average particle size of less than 10pm and the second synergistic flame retardant is kaolin having an average particle size of less than 5pm, preferably less than 1pm.

In another aspect the aluminium trihydroxide in step 1 of the method of manufacturing has an average particle size of less than 5pm, such as less than 4pm, less than 3pm, less than 2pm, about 1pm or less than 1pm.

In another aspect the second synergistic flame retardant in in step 1 of the method of manufacturing has an average particle size of less than 5pm, such as about 4.5pm, about 4pm, about 3.5pm, about 3pm, about 2.5pm, about 2pm, about 1.5pm, about lpm, or less than 1pm.

In another aspect the loading level of aluminium trihydroxide in step 1 of the method of manufacturing is less than 25% by weight, preferably more than 10% by weight and less than 25% by weight.

In another aspect the loading level of the second synergistic flame retardant in step 1 of the method of manufacturing is less than 8% by weight, e.g. 7.5%, 7%, 6.5%, 6%, 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5%, 1% or 0.5% by weight and, optionally, nano aluminium trihydroxide is present in an amount between 0.5% and 5%.

In another aspect the second synergistic flame retardant in step 1 of the method of manufacturing is kaolin optionally having a particle size between lOnm and 10pm, preferably between lOOnm and 5pm.

In another aspect the thermoplastic composition in step 1 of the method of manufacturing is essentially antimony free.

In another aspect the second synergistic flame retardant in step 1 of the method of manufacturing is coated with an anti-agglomeration agent, such as stearic acid.

In another aspect the thermoplastic composition in the method of manufacturing is PVC.

The following examples are illustrative of embodiments of the present invention and they are not meant to limit the invention in any way.

EXAMPLES

The following materials were used in the examples:

Antimony trioxide Sb203 (ATO) was supplied by Campine NV, average particle size of 0.7-1.5pm. Aluminium hydroxide (ATH) was used as micron-size flame retardant. ATH Hymod® M9400SP was supplied by Huber, having the average particle size of 1.1pm. This ATH is a vinyl functionalized surface treated grade. Nano-sized aluminium hydroxide (nano-ATH) was supplied by Sasol, the trade name Disperal BT. Hydrotalcite (HT) was also supplied by Sasol, the trade name of PURAL MG 63 MC hydrotalcite. The average particle size of this stearic acid coated hydrotalcite was 4-6pm.

Table I. Analytical data of hydrotalcite used in the PVC composite according to the invention.

Kaolin was a sample from Finnish origin and it was grinded at VTT to the average particle size of l-3pm. PVC was supplied by Solvay S.A., the trade name of PVC-SolVin®. Plasticizer and selected stabilizers were mixed to all PVC compounds equally prior to melt compounding by laboratory scale two roll mill.

Analysis methods:

Secondary electron images were taken with JEOL JSM-6360 LV SEM operated at acceleration voltage 15kV. SEM samples were prepared by fracturing soft PVC composite mat in liquid nitrogen. Approximately 20nm thick gold layer was sputtered on the sample.

Prior to flame testing the sample thickness was measured. The thickness of the PVC composite coated fabrics varied between 0.68 and 0.78mm (average of at least 4 measurements).

Cone calorimeter used to determine the fire behaviour of the samples was a CC-1 equipment form Gowmark Ltd. The test was performed according to the ISO 5660-1 standard. The method evaluates ignitability, combustibility and smoke production. The measured heat release rate (HRR) is determined by the measurement of the oxygen consumption derived from the oxygen concentration and the flow rate in the combustion product stream. The tested specimens were exposed to a heat flux density of 50kW/m2. Tests were conducted with the sample in a horizontal position. EXAMPLE 1

Manufacturing of PVC-coated fabric samples

Kaolin had the original particle size of 15-20pm. It was ground with Hosokawa- Alpine laboratory scale air jet milling method (Type: 100AFG/ZPS/ATP Multi Processing System Alpine) and simultaneously graded to average particle size of l-3pm.

Preliminary mixing for all the PVC-composites was carried out using Papenmeyer (Type: NTHKV5) high speed mixer. Temperature of the mixture did rise during the mixing due to the friction between the blade mixer and the compound. PVC-compounds were melt mixed using a two roll mill machine (Type: LabTech LRM-S-110/E+W). Temperature was set to 180°C. Mixing time was minimum 4 minutes. Rotating speeds of the rolling cylinders were set to the ratio of 3:5 and the roll gap was adjusted to 0.25mm to ensure even distribution of flame retardants in the composition. After mixing the samples were removed onto a polyester base fabric, straight from the rolling cylinder.

Aluminium hydroxide ATH is the primary FR component in the PVC composite. The formulas of the studied composites are presented in Table 2. The performance of the PVC composite with ATO is used as a reference. Results of kaolin and hydrotalcite as the second synergist FR are reported below.

By replacing part of the micron size ATH with nanosize ATH seems to lower the rate of heat release and the smoke production. A lower FR content in the composite with the performance at the same level can be reached. Prior to cone calorimeter measurements single flame test (SFS-EN 11925-2 method) was used to evaluate the flame retardancy of the composites.

Table 2. Composition and flame retardancy performance of tested PVC composites. The improvement in the flame retardancy is indicated with marking + or ++, marking ++ denoting the highest improvement.

Even distribution of the filler particles in the composite was achieved by two roll mill melt processing. Figures 1A and IB show SEM images of the samples PVC FRI and PVC FR6. The shape of kaolin and hydrotalcite filler particles is flaky. Hydrotalcite has a double layered metal hydroxide structure, scale of the thickness of the flakes being nanometric. Nano-ATH was detected to be in the form of spherical agglomerates (when supplied) with a diameter of some pm or less. The same size and form can be detected in the composites (Figure IB).

Figure 2A shows the rate of heat release (abbreviated as HRR) of the reference sample of PVC containing ATO and ATH (PVC FR, reference), and sample FRI wherein kaolin is the synergist agent (PVC FRI). The time to ignition appears to be identical with the two samples tested, but the HRR is significantly lower for the PVC composite containing kaolin as the synergist agent. Figure 2B describes the performance of hydrotalcite as a synergist to ATH, compared to ATO synergist. Hydrotalcite synergist does lower the rate of heat release significantly (PVC FR3). Replacing part of the primary ATH by nano-ATH has a positive effect on the HRR (PVC FR5 vs PVC FR4). When the loading of primary ATH is substantially lower in the presence of hydrotalcite and the nano-ATH, the FR performance of the composite still remains at the same level (PVC FR6).

Figures 3A and 3B present the smoke production rate of the composites determined by cone calorimeter. Kaolin works as a more effective smoke suppressant compared to ATO. Also hydrotalcite is an effective smoke suppressant in PVC composites.

Formation of the char layer during combustion plays an essential role in mineral filler loaded FR composites. It is suggested in the literature that the residual Al- and Mg-oxides can contribute to the formation of an insulative charred layer acting as a barrier to protect the compound when hydrotalcite is present [4].

References [1] T.R. Hull, A. Witkowski, L.Hollingbery, Polym. Degradation and Stability, 96 (2011), 1462-1469.

[2] H. Lu, L. Song, Y. Hu, Polym. Adv. Technol., 22 (2011), 379-394.

[3] M. Hai Yuan, S. PingAn, F. ZhengPing, Science China Chemistry, 54 (2011), 302-313.

[4] X. Wang, Q. Zhang, Polym. Int., 53 (2004), 698 - 707.

[5] Y.Q. Hua, Q. Qin, Polym. Mater. Sci. Eng., 19 (2003) 172-.

[6] F.R. Costa, U. Wagenknecht, G. Heinrich, Polym. Degradation and Stability, 92 (2007), 1813-1823.

[7] C. Manzi-Nsuti, P. Singtipya, E. Manias, M. M. Jimenez-Gasco, J. M. Hossenlopp, C. A. Wilkie, Polymer 50 (2009) 3564-3574.

[8] B. Yong-zhong, H. Zhi-ming, L. Shen-xing, W. Zhi-xue, Polym. Degradation and Stability, 93 (2008), 448-455.

Claims

A thermoplastic composition comprising a two-component flame retardant system, characterized in that the thermoplastic material is PVC and the flame retardant system consists of a primary flame retardant and a second synergistic flame retardant, wherein the primary flame retardant is aluminum trihydroxide and the added amount of aluminum trihydroxide is greater than 10% by weight and less than 25% by weight, and the added amount of the second synergistic flame retardant is less than 6% by weight, and the second synergistic flame retardant has an average particle size of less than 1pm.

A thermoplastic composition according to claim 1, characterized in that the aluminum trihydroxide has an average particle size of less than 5 µm, less than 4 µm, less than 3 µm, less than 2 µm, about 1 µm or less than 1 µm.

The thermoplastic composition according to claim 1 or 2, characterized in that the added amount of the second synergistic flame retardant is less than 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5% , 2%, 1.5%, 1% or 0.5% by weight.

The thermoplastic composition according to any one of claims 1 to 3, characterized in that the second synergistic flame retardant is kaolin.

The thermoplastic composition according to any one of claims 1 to 4, characterized in that the thermoplastic composition is substantially non-antimony.

The thermoplastic composition according to any one of claims 1 to 5, characterized in that the second synergistic flame retardant is coated with an anti-caking agent such as stearic acid.

Thermoplastic composition according to any one of claims 4 to 6, characterized in that the added amount of kaolin is less than 5% by weight.

Use of a flame retardant system for the manufacture of a thermoplastic composite having an improved flame retardant consisting of a primary flame retardant and a second synergistic flame retardant, characterized in that the primary flame retardant is aluminum trihydroxide and the second, , and the added amount of aluminum trihydroxide is greater than 10% by weight and less than 25% by weight, and the added amount of the second synergistic flame retardant is less than 6% by weight, and the second synergistic flame retardant has an average particle size of less than 1pm.

Use according to Claim 8, characterized in that the thermoplastic composite is substantially antimony free.

Use according to claim 8 or 9, characterized in that the second synergistic flame retardant is kaolin.

A process for making a flame-retardant thermoplastic composite having a two-component flame retardant system, characterized in that the process comprises blending the thermoplastic components wherein the thermoplastic material is PVC with a primary flame retardant aluminum trihydroxide and a second synergistic flame retardant selected from the group consisting of with a homogeneous dispersion having an added amount of aluminum trihydroxide of greater than 10% by weight and less than 25% by weight, and an added amount of a second synergistic flame retardant of less than 6% by weight and an average synergistic flame retardant of less than 1 µm; and melting the homogeneous dispersion while mixing.

Process according to claim 11, characterized in that the primary flame retardant has an average particle size of less than 10 µm and the second, synergistic flame retardant is kaolin.