Smoke Suppressant Of Rigid Pvc For Wall Cladding Engineering Essay

PVC is the third most widely used commodity polymer. It is often used in construction and insulation applications. For construction applications, PVC can be used to make floors, wall claddings and ceilings whereas insulating cables and wires are the main fields in insulation applications.

PVC on its own (UPVC) is inherently self-distinguishing because of the high content of chlorine atoms. Rigid PVC has excellent fire resistance but it will also burn if a strong heat source is continuously applied. A large amount of smoke and toxic gases released by the combustion of PVC is one serious problem for PVC product. Smoke suppressants incorporated with flame retardants appropriately can be used to alleviate this problem.

Background of Altro Company

Altro Company founded in 1919 is one of the most excellent manufacturers and suppliers of interior surfaces in the world. It is based in United Kingdom and produces floors, wall claddings and ceilings. It aims to produce safe and hygienic products.

Objective of Project

To improve the fire performance especially smoke density rating of Altro wall cladding products in accordance with BS EN 13501-1:2007 +A1:2009 [1] without compromising appearance and mechanical properties

LITERATURE REVIEW

POLY (VINYL CHLORIDE)

Poly (vinyl chloride) always abbreviated to PVC, is the third most widely used polymer material in the world. PVC products can be split into rigid and plasticised.

1.1 Polymerisation

PVC resin is produced by the polymerisation of vinyl chloride monomers, which can be described as the following formula [1]: CH2=CHCl ↑ [CH2CHCl] n.

The mechanism of the PVC polymerisation is based on free radical polymerisation. Suspension, bulk and emulsion polymerisation all can be used to produce PVC resin. A great majority of PVC resin is polymerised by the suspension route while bulk and emulsion polymerisation are used to produce PVC resin for some special applications [1].

1.2 Structure and Properties

The glass transition temperature of PVC is about which is higher than room temperature whereas the glass transition temperatures of PE and PP are both much lower that room temperature. Thus, unplasticised PVC is more rigid and brittle than PE and PP. The volume of chlorine atom is much larger than that of hydrogen atom so it is more difficult for the PVC chain to closely pack and crystallise. Thus, PVC has a large amorphous region and typically has crystallinity less than 10% [1].

Even though PVC is largely amorphous, the polarity of chlorine atom leads to strong intermolecular attractions. Therefore, it has a higher tensile strength and modulus that that of PE and PP [1].

ADDITIVES

Additives for PVC can be split into two groups in terms of the function. The one is used to improve processibility including heat stabilizers, processing aids and lubricants, while the other is employed to enhance the properties of final products including plasticizers, impact modifiers, colorants, ultraviolet stabilizer. Flame retardants and smoke suppressants are fallen into the latter group and will be introduced in general in this section. More specific discussions on them will be carried on in section 5 and 6.

2.1 Heat Stabilisers

PVC resin is inherently very heat-sensitive. Heating PVC to temperature required for processing without heat stabiliser initially leads to yellowing, followed by gross discoloration, the production of hydrochloric acid, cross-linking and ultimately get to an infusible, unprocessable black mass.

PVC heat stabiliser can be grouped into primary stabilisers and secondary stabilisers. Primary stabiliser can be solely used in PVC to improve the heat stability. Lead stabilisers, organotin stabilisers and mixed metal stabilisers are the most common primary stabilisers (Lead stabilisers have been phasing out due to environmental issue). Secondary stabilisers such as hydrotalcite cannot be employed as the sole stabilisers in PVC but they can extend, complement and improve the heat stability when used with a primary stabiliser [2].

2.2 Processing Aids

Processing aids can be used in PVC formulations to accelerate and control the fusion process in PVC compounds due to they can increase the interactions between the PVC grains. In absence of processing aids, rigid PVC compounds can be poor in homogeneity, strength and elasticity during processing. The molecule of processing aids can bind together with the PVC particles and improve the melt strength. Thus, processing aids can strongly affect the rheological property of the PVC melt as well [2].

2.3 Lubricants

Lubricants can be used to reduce the adhesion between the polymer melt and the metal surface of processing machine. The interparticulate friction and rate of fusion can be affected by the lubricants as well. Lubricant additives are frequently added in rigid PVC because they are necessary to reduce the melt viscosity and make it possible for the machinery to process at high shear rate. Lubricants can significantly improve the stability of a compound due to they reduce the frictional heat build-up and often contain co-stability function groups [2].

2.4 Plasticisers

Plasticizers, most commonly phthalates, are always added to the formulation of plasticized PVC to make it softer and more flexible. They are often esters of polycarboxylic acids reacted with aliphatic alcohols of moderate chain length. Plasticizers could intervene between the polymer chains, separating them apart. Therefore, the glass transition temperature of PVC is lowered significantly by the plasticizers. However, its strength decreases with the addition of plasticizers [3].

2.5 Impact Modifiers

Impact modifiers are added to rigid PVC to improve the impact property of PVC. The impact resistance of rigid PVC compounds is the critical factor allowing their use in many applications such shatterproof bottles, crease-resistance clear packaging film, opaque pipe and window frame. Even though, rigid PVC has inherent toughness due to its polarity molecular structure and is ductile over a range of use conditions, it would be brittle under the condition of high deformation rate and concentrated stress without the incorporation of impact modifiers [3].

Impact modifiers for PVC could generally be divided into three types as follows:

Grafted particulate rubbery polymer such as methacrylate-butadiene-styrene (MBS), acrylate-butadiene-methacrylate, acylonitrile-butadiene-styrene (ABS)

Semicompatible plasticizing polymers such as chlorinated polyethylene (CPE) and ethylene-vinyl acetate (EVA)

Inorganics such as stearic acid coated calcium carbonate [2]

2.6 Colorants

Colorants are added to plastic formulations to produce colour in the polymer products. They are grouped into pigments and dyes. Dyes are soluble in the polymer, whereas pigments are insoluble in the polymer. Colorants are always in several forms including liquids and dry concentrates [3].

2.7 Ultraviolet Stabiliser

A variety of additives can be used to protect the plastics from degradation by ultraviolet (UV) light. The choice of UV stabiliser depends on the applications and the characteristics of polymers. There are two types of UV stabilizer. One works by screening out the light to restrict UV penetration into the polymer and another interrupts the chemical chain reactions causing by free radical. UV screeners are always pigment. They make the polymer translucent or opaque and absorb or reflect UV light, thus protecting the polymer. Titanium dioxide is available for the protection of PVC [3].

2.8 Flame retardants

It is well known unmodified PVC is a self-distinguishing material due to it high chlorine content. The limiting oxygen index (LOI) is as high as 45-48 while that of most other commodity polymers such as PE, PP and PS is lower than 20. However, the addition of plasticiser for plasticised PVC applications often largely reduces the fire resistance of PVC products. The LOI of plasticised PVC can be reduced to 24 [4]. Therefore, flame retardants are widely used in plasticised PVC applications such as insulation cables.

Even though rigid PVC is self-distinguishing, it will also burn as long as a continuous strong ignition source is applied. Thus, flame retardants are also used in some rigid PVC applications.

2.9 Smoke suppressants

One of the most serious problems of PVC combustion is that it produces large volume of smoke and toxic gases. The smoke and toxic gases is the main cause of fire related death [11]. Moreover, the formation of smoke and toxic gases reduces the visibility of the scene of a fire therefore increase the difficulty to save the trapped people for fire-fighters. Thus, smoke suppressants are employed to improve the smoke suppression property of PVC products.

PROCESSING

Processing of PVC is to apply sufficient heat and pressure to well blended PVC compounds to obtain well fused products in desired shape.

3.1 Dry Blending

Dry blending by high speed mixer is the first important step in the processing of PVC for achieving the required mechanical, physical and chemical properties, and the desired appearance of the product in the processing machines. The purpose of dry blending of rigid PVC is to obtain a homogeneous blend. The typical components of a rigid PVC power mixture are PVC resin, processing aids, impact modifier, fillers, stabilizers, lubricants and colorants [5].

3.2 Extrusion

Extrusion is a very efficient and widely used processing method for polymer materials. Extruders can be used to shape products directly after this mixing or an extruder can be used as the mixing and melting device combined with other shaping equipment. When used for direct shaping, shaping equipment with a desired cross-section is connected to the end of the extruder and this process is called extrusion [5].

Extruder can be classified into single crew extruders and twin screw extruders. Both the single screw extruders and the twin screw extruders can be used to process rigid PVC. Twin conical screw extruders have a more positive pumping action than single screw extruders and can provide more shear stress. Therefore, twin crew extruders are more effective in high output situation and are valuable with certain resins, especially those that are sensitive to heat such as PVC [5].

3.3 Gelation

It is well known that the properties of products not only depend on the composition of the PVC compound but also depend on the processing conditions. Different level of gelation as a function of processing temperature, leads to different physical and chemical properties. Thus, gelation is a very important consequence for the processing of PVC products.

Gilbert et al [6] investigated the gelation of rigid PVC compound by using differential scanning calorimetry. The DSC curves obtained by them is presented here. They got two melting endotherms, A and B as shown in Figure 1, by the DSC traces. They found that endotherm B shifted to higher temperatures and decreased in area with the increase of processing temperature, whereas endotherm A increased in area as the processing temperature increased.

Gilbert et al [6] explained that the endotherm A resulted from the melting of PVC crystallites which were formed from the cooling of the previous melted less perfect or smaller crystallites during the processing, and the temperature shift of endotherm B was due to the annealing of unmelted crystallites during previous processing. Relative less perfect and smaller crystallites would be melted at certain processing temperatures, so the endotherm A increased with the processing temperature. In addition, they concluded the area of endotherm A was related to the gelation level of PVC and the onset temperature of endotherm B was related directly to the maximum processing temperature of PVC.

Figure 1 DSC thermograms of compression moulded rigid PVC compounds at various temperatures [6]

The gelation level of rigid PVC product has a significant effect on the mechanical properties. The tensile strength of rigid PVC product increases as the gelation level increases. However, the impact strength of rigid PVC increases with the gelation level at the beginning then passes a maximum value and finally decreases with the gelation level [7].

FIRE PERFORMANCE TEST

4.1 UL 94 test

The UL 94 test is perhaps the most frequently used flammability test. It can be used to assess flammability of a wide range of thermoplastics which are intended to use in many applications. The UL 94 test actually contains several different test methods. The most commonly used method is the vertical burn method as shown in figure 2. In vertical burn method, a test specimen ( a bar of 13 mm by 125 mm by varying thickness) is suspended 10 mm above a calibrated methane burner and ignited. The flame is applied to five test specimens twice for 10 seconds. The fire performance is described by one of three ratings, V0, V1 or V2 depending on after-flame burn time for each specimen, the total after-flame burn time for all specimens, the afterglow time and the existence of flaming particles [8].

Figure 2 UL 94 vertical test apparatus [8]

4.2 Limiting Oxygen Index (LOI) Test

Another flammability test, LOI test is one of the oldest methods still used today. In this test, the minimum amount of oxygen in a mixture of oxygen and nitrogen which can support combustion is measured. An apparatus designed to imitate candle-like burning condition is used to evaluate three specimens (6.5 mm wide) as shown in figure xx. The result is actually a percentage which indicates the percentage of oxygen content in a mixture of oxygen and nitrogen required to support the combustion of the tested sample [8].

4.3 Smoke Density Tests

One of the most common smoke density tests is ASTM E662 Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials. The specific optical density of smoke generated by solid materials is measured in this test. The smoke density is determined by the attenuation of a light beam passing through the smoke within a closed chamber [8].

The smoke is caused by nonflaming pyrolytic decomposition and flaming combustion [8]. The apparatus of ASTM E662 is shown in figure 3.

Figure 3 ASTM E662 smoke chamber apparatus [8]

4.4 Cone Calorimetry

Cone calorimetry cited in ASTM E1354 which is standard test method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter. Cone calorimetry perhaps the most versatile fire test method can be used to test the ignitability, heat release rates, mass loss rates, effective heat of combustion and visible smoke development of materials and products [9].

Ignitability is determined by measurement of the time between initial exposure and sustained flaming and the rate of heat release is determined from the oxygen consumption. The effective heat of combustion is determined by the combination of mass loss rate and heat release rate. Smoke development is determined by the attenuation of light by the combustion product smoke [9].

FLAME RETARDANTS AND SMOKE SUPPRESSANTS FOR PVC

Hitt et al [10] overviewed the mechanisms of flame retardancy and smoke suppression for PVC. Following the review on mechanism, they gave a review on a variety of established flame retardants and smoke suppressants for PVC. The recent developments flame retardants and smoke suppressants for PVC were reviewed by them as well.

5.1 Mechanism of Flame Retardancy and Smoke Suppression

Innes et al [11] reviewed the mechanisms of flame retardancy and smoke suppression for polymers. Three mechanisms were mentioned in their review. Halogenated materials acting in the gas phase to disrupt the combustion is the first mechanism. This is why many kinds of flame retardants for polymer materials contain chlorine or bromine atoms. To form a protective layer at the surface (char formation) by phosphorous contained compounds can be considered the second mechanism. Phosphorous contained flame retardants is the third major flame retardant group of products. To release water that cools the materials and dilute the combustion gas by the decomposition of inorganic additive is the third mechanism. Inorganic additives like alumina trihydrate, magnesium hydroxide are widely used as flame retardants and smoke suppressants for polymer materials.

5.2 Established Flame Retardants and Smoke Suppressants

5.2.1 Inorganic Flame Retardants and Smoke Suppressants

Antimony Trioxide and related products

Antimony trioxide is a synergist in halogenated polymers such as PVC due to it has not effect in the absence of halogen. The typical loading level of antimony trioxide is in the range 3-7 phr. It reacts with HCl to form antimony trichloride [12]. The boiling point of trichloride is about 223 and is believed to act in the gas phase to poison the combustion reaction [13]. The clarity of antimony trioxide modified polymer is not good due to its large particle size. This pigmentation effect can be avoided by using more expensive antimony pentoxide [14].

Alumina trihydrate (ATH)

Some mineral additives such ATH and magnesium hydroxide, play dual role of flame retardants and mineral enhancement due to their high loading levels. The ATH is more widely used because of its more cost efficiency. It is believed to release water that cool the materials and dilute the combustion gas by decomposition at temperatures over 200 to achieve the flame retardancy and smoke suppression [13].

Zinc borate (ZB) is often used in combination with ATH as synergist. Ning et al [15] founded that the addition of a small amount of ATH and ZB mixture into PVC largely improves the flame retardancy and smoke suppression of PVC due to the mixture can greatly promote char formation and decrease the amount of hazardous gases releases.

Magnesium based compounds

The two most commonly used magnesium based flame retardants for PVC are Ultracarb and magnesium hydroxide [12]. Compared with ATH, the magnesium hydroxide is more expensive but it begins to release water at temperatures above 300 which make it an option in PVC applications requiring higher processing temperature [12]. Magnesium hydroxide is believed to be a more effective smoke suppressant than ATH due to its basicity [12].

Ultracarb, a natural mineral is the mixture of magnesium and hydroxycarbonate and huntite [14]. The price of Ultracarb is between ATH and magnesium hydroxide. It was found to be an effective flame retardant and smoke suppressant [14].

Molybdenum based compounds

The most common molybdenum based compound in flexible PVC formulation is Ammonium Octamolybdate (AOM). AOM is believed to promote the formation of trans-olefin structure from cis-olefin structures in the dehydrochlorinated polymer residue to decrease the smoke released during combustion. Relatively high price and negative effect on the stability of flexible PVC compounds are two main disadvantages of AOM [12].

Molybdenum trioxide is another smoke suppressant used in PVC formulation. It is slightly cheaper than AOM but people tend to use AOM due to AOM is more effective and molybdenum trioxide is blue in colour [12].

Zinc Based compounds

Zinc borate is the most commonly used zinc based compound in PVC. It acts both as flame retardant and smoke suppressant. It is often used to partially replace antimony trioxide. Zinc borate is believed to promote crosslinking and charring. In addition, it can form a barrier with other inorganic fillers. It also absorbs a small amount of combustion heat and has a little fuel dilution effect due to the dehydration process. FIREBRAKE ZB by Luzenac is the most widely used zinc borate product [14]. Care must be taken while using zinc borate in a PVC formulation due to it has negative effect on the heat stability [12].

Zinc stannate (ZS) and zinc hydroxystannate (ZHS) are both effective smoke suppressants and flame synergists because they can catalyze the dehydrochlorination of PVC and lead to rapid charring. Thomas [17] evaluated the fire performance of ZB and ZHS by LOI test and cone calorimetry. Results showed that ZHS is an excellent flame retardant and smoke suppressant for rigid PVC. She also found ZB is a good smoke suppressant as well as ZHS but it does not display an excellent flame retardant property of ZHS. A coarse grade of ZB decreases the impact strength of PVC. Finer grade of ZB can be used to overcome the impact strength deterioration problem but the heat stability of such PVC formulation will be reduced.

Like ZB, zinc sulphide can be used to partially replace the antimony trioxide in PVC without decrease of flame retardant properties [14]. It is believed to promote char formation whereas antimony trioxide catalyzes dehydrochlorination.

Copper compounds

The smoke suppression of copper oxalate in combination with a molybdate was studied by Starness et al [16]. They found such copper compound not only improves the smoke suppression but also increases the ignition time and reduces the mass loss rate under cone calorimetry condition. Reductive coupling reaction was believed to promote crosslinking during the combustion.

5.2.2 Flame Retarding and Smoke suppressing Plasticizer

In flexible PVC compound, the incorporation of phthalate plasticizers greatly reduces the flame retardancy of PVC. Thus, plasticized PVC has a less flame retardancy and tends to release a large amount of smoke if it is ignited. One most common approach to solve this problem is use phosphate ester to replace part of phthalate plasticizer. The phosphate ester plasticizers have been used in PVC since 1930s. They are widely used in flexible film, sheeting and other applications where relatively high flame retardancy and low smoke are required. The cresyl diphenyl phosphate is more commonly used in Europe and Asia due to its efficiency and price. However, it is less popular in US because of the volatility and toxicity issue. Trixylenyl phosphates are used in applications where high temperature performance or long term heat resistance is required [14].

Tetrabormo dioctyl phthalate is used as secondary plasticizer in flexible PVC applications. Halogen is released by it to enter the vapour phase to inhibit combustion. It is an effective flame retardant plasticizer due to the bromine is more active in the vapour phase than the chlorine evolved from PVC [12].

Chlorinated paraffins are occasionally used as plasticizers in flexible PVC application. It acts in the vapour phase to inhibit combustion by releasing halogen. A wide range of chlorine content chlorinated parafinns are available. Chlorinated paraffins with chlorine contents between 45% and 60% are the most commonly used in PVC. Due to the low cost of chlorinated paraffins, they are sometime used to partially replace of antimony trioxide [12].

RECENT DEVELOPMENTS

6.1 Coatings

Cuasck et al [18] reviewed on a new series of flame retardant smoke suppressants for PVC which are comprised of inorganic filler coated with ZHS or ZS. Compared with uncoated filler, it allows large reduction in additive loading level due to better dispersion of the ZHS/ZS within the polymer matrix.

The flame retardant and smoke suppressant properties of ZHS/ZS coated calcium carbonate for semi-rigid PVC were investigated by Xu et al [19] by means of LOI, anaerobic char yield and smoke density rating methods. The results show the addition of ZHS/ZS coated calcium carbonate improved the limiting oxygen index and anaerobic char yield, and decreased the smoke density rating and starting decomposition temperature. The tensile strength and elongation was reduced while the impact strength was enhanced.

Qu et al [20] evaluated the effect of ZHS coated alumina trihydrate or magnesium hydroxide on the flame retardant and smoke suppressant properties of flexible PVC. They found the ZHS coated metal hydroxides are more effective than the corresponding metal hydroxides in flame retardancy and smoke suppression. The PVC samples with ZHS coated metal hydroxides have lower decomposition temperature, weight loss and higher velocity of weight loss in the first stage than those with uncoated metal hydroxides.

6.2 Zinc/Tin compounds

Qu et al [21] studied metal hydroxystannates as flame retardants and smoke suppressants for semi-rigid PVC. LOI, smoke density rating, the solid yield test, scanning electron microscopy and differential thermal analysis were used to investigate the flame-retardant and smoke-suppressing mechanisms. The results showed that the addition of metal hydroxystannates increased LOI, and soli yield, and decreased smoke density rating and maximum smoke density. That is to say both the flame retardant and smoke suppressant properties were improved by the hydroxystannates compounds. They also found that the tin compounds might exert the action mainly in the condensed phase as Lewis acids.

Thermal behaviour and flame retardancy of flexible PVC modified with Al(OH)3 and ZnO were studied by Qu et al [22] suing thermal analytical techniques, limiting oxygen index and smoke density rating tests. Incorporation of a small amount of ZnO (1phr) improved flame retardancy and smoke suppression of flexible PVC indicating ZnO is an effective synergistic flame retardant with Al(OH)3. The addition of ZnO increased the activation energy of the char residue oxidation reaction thus enhance the flame retardant and smoke suppressant properties.

Wang el at [23] studied flame retardant property of Sb2O3/SnO2 and their synergism in flexible PVC. The results showed that Sb2O3, SnO2 and their mixture are all effective flame retardants for flexible PVC due to the LOI and char yield were greatly improved by the addition of these three flame retardants. SnO2 is a good synergistic flame retardant with Sb2O3 and can be used to replace Sb2O3, partly. Sb2O3 acts in condensed phase whereas SnO2 acts in vapour phase.

6.3 Copper Compounds

Ho et al [24] investigated the synergism of mixed-metal Cu(II) in the smoke suppression of plasticized PVC. They found various mixed-metal copper compounds exhibit to be effective smoke suppressants in flexible PVC formulations. Results also showed that a binary blend of CuMo and CuTi displayed better smoke suppressant activity than commercial additive, AOM indicating strong synergistic effect between them. CuMo was found to be a very effect smoke suppressant used alone.

6.4 Hydrotalcite

The effect of hydrotalcite and zinc oxide on smoke suppression of commercial rigid PVC was evaluated by Zhang et al [25]. The results showed that the addition of hydrotalcite and zinc oxide significantly improved the smoke suppression of rigid PVC formulation. The optimum addition level of hydrotalcite/zinc oxide is totally 5phr with the ratio of 3:2.

PVC/hydrotalcite nanocomposites prepared through vinyl chloride suspension polymerisation were modified with alkyl phosphate on the surface. The thermal stability, smoke emission and mechanical properties of PVC/hydrotalcite nanocomposites were investigated by Bao et al [26]. The results indicated that the well dispersed nano-scale hydrotalcite demonstrated an significant smoke suppression effect on PVC and the maximum smoke density was decreased about 1/3 and 1/2 when 2.5 wt% and 5.3% nano-scale hydrotalcite was added into PVC.

6.5 Organoclays

G Beyer [27] studied on PVC nanocomposites and new nanostructured flame retardants. The results showed that the addition of ATH, a modified montmorillonite and halloysite, an aluminosilicate (nanotubebased) filler improved the flame retardancy of a PVC cable formulation.

Yang et al [28] investigated the effect of Cu2+-organic montmorillonites on thermal decomposition and smoke emission of PVC by cone calorimetry. Results showed that Cu2+-organic montmorillonites promoted an early HCl elimination, crosslinking and char residue formation. The peak heat release rate, total heat release, peak smoke production rate, total smoke production and smoke extinction area were reduced by Cu2+-organic montmorillonites in the flaming process.

6.6 Molecular Heat Eater (MHE)

MHE is an innovative, non-toxic, environmental friendly and biodegradable flame retardant developed by Swedish company The Trulstech Group. It is derived from chemicals which can be found in foodstuff such as citric fruits and grapes [29]. Rather than a single chemical, MHE is a flexible flame retardant system consisting of a series of combinations of various carboxylic acids and alkali hydroxides. MHE is an effective flame retardant for more than 80 different materials.