Mechanical and physical properties of core–shell structured wood plastic composites: Effect of shells with hybrid mineral and wood fillers

Abstract

The effects of shells filled with silane treated precipitated calcium carbonate (TPCC) and wood fiber (WF) on properties of core–shell structured wood plastic composites (WPCs) were studied with two different core systems. In a weak core system made with recycled polypropylene (PP)/high density polyethylene (HDPE), coextruded WPCs with reinforced virgin HDPE shells showed significantly improved flexural strengths compared with their core-only controls. In a strong core system made of virgin HDPE, the composite flexural strengths were lowered in relation to their core-only controls. Impact strengths for both systems were noticeably improved with added shells. Specially, the impact strengths of the weak core coextruded WPCs showed up to 150% increases in relation to their core-only controls. Impact fracture types varied largely with core quality and filler composition in the shell. Water absorption of coextruded WPCs with high TPCC contents in the shell was lower than that of core-only controls and coextruded WPCs with high wood contents in the shell layer. The use of high percentage of plastic in the shell layer led to the increases of overall composite’s coefficient of thermal expansion (CTE). However, increased filler loadings in the shell helped reduce the CTE values, especially, for the weak core system.

Introduction

Coextrusion generally consists of two or more extruders combined with one die to produce multiple-layer products [1]. By combining molten multiple plastic layers with various properties into one profile, optimization of product performance is possible [2]. Water resistance, air entrapment, oxygen barrier, and increased toughness are some of the advantages of coextruded products [1], [2]. For example, coextrusion method has been applied to produce packaging films for maintaining content freshness, pipes requiring high mechanical properties, and other specialty products [3], [4].

Coextrusion in wood plastic composite (WPC) was first reported with a combination of a WPC core and a pure plastic shell layer. Stark and Matuana [5] investigated moisture uptake, flexural properties and weathering performance between non-coextruded and coextruded WPC with a pure high density polyethylene (HDPE) or a pure polypropylene (PP) shell. In their research, coextruded WPC demonstrated much reduced moisture uptake than non-coextruded WPC and there were almost no differences in flexural properties between them. They further studied the effects of a stabilized shell layer blended with HDPE and additives including a compatibilizer, a photostabilizer and a nanosized titanium dioxide (TiO₂) on the coextruded WPC after weathering tests [6]. The coextruded WPC with a stabilized shell layer showed better water resistance properties and the combination of a compatibilizer and a photostabilizer improved composite lightening by a synergistic effect. The individual use of TiO₂ or a photostabilizer showed a noticeably enhanced color stability for each capped WPC. However, a simultaneous use of both additives had a deleterious effect on color stability. Jin and Matuana [7] further showed improved water resistance through the application of a pure HDPE plastic shell onto an extruded WPC core. Water absorption (WA) and thickness swelling (TS) properties of coextruded WPC were better than those of non-coextruded WPC without a shell.

The addition of a pure plastic shell with a relatively low modulus over a WPC core negatively affected overall composite modulus. Jin and Matuana [8] reported a study using a PVC shell layer on a PVC-WF core. Increased flexural strength and decreased flexural modulus were observed for the coextruded composites. It was pointed out that high strength and low stiffness characteristics of PVC led to the decrease of composite flexural modulus. To acquire a simultaneous enhancement of both flexural modulus and flexural strength, they used carbon nano-tube (CNT) in the shell layer, in combination with two core compositions (40% versus 60% WF) and two processing conditions (low versus high temperature). The combination of a CNT-filled PVC shell and a high temperature processed WPC core showed better flexural strength and flexural modulus. However, the use of high-cost CNTs was not cost-effective for WPC products. Yao and Wu [9] investigated coextruded WPC using combinations of recycled PE core compositions (weak and strong) and a virgin PE shell with different WF contents and shell thicknesses. In the comparison of mechanical properties between weak and strong core systems, coextruded composites with a weak core showed much higher percentage increases in flexural and impact strength than those with a strong core. At the same shell thickness, less WF loading in the shell led to highly increased impact strength, but there were almost no changes in flexural modulus. On the other hand, the increase of impact strength and the decrease of flexural modulus were observed with increased shell thickness. Thus, they suggested that good flexural modulus of coextruded WPC resulted from the combination of a thinner shell and a higher modulus core.

The addition of mineral fillers into polymeric materials enhances their physical and mechanical properties [10], [11]. Among the mineral fillers, precipitated calcium carbonate (PCC) from chemical processes [11], [12], [13], [14], [15] has characteristics of very fine and regular particle size [16]. Thus, its uses in plastic composites have led to improved mechanical properties [17], [18]. However, the fine size of PCC conversely causes its aggregations in composites, leading to a deleterious effect on the properties of PCC filled composites [19]. Hence, to reduce this deleterious effect, surface treated PCC has been used. Lam et al. [19] investigated the effect of modified PCC in PP matrix through optical and thermal analysis. Nanosized PCC (n-PCC) and nanosized surface modified PCC (ns-PCC) filled PP composites were prepared. The initial decomposition temperature of ns-PCC/PP composites was higher than that of n-PCC/PP composites at the same filling ratio, indicating that the reinforcing effect of ns-PCC was increased by surface treatment. Cayer-Barrioz et al. [20] studied the interfacial adhesion between polyamide 66 (PA66) and PCC (untreated PCC; 3 wt% stearic acid treated PCC; amino acid treated PCC). Dynamic mechanical analysis (DMA) demonstrated the effects of the surface treated PCC through the storage modulus peak shifts by +13 K for the amino acid treatment and +6 K for the stearic acid treatment. Moreover, environmental scanning electron microscope (EnSEM) analysis confirmed the differences of adhesions among PCC filled composites. Kim et al. [21] investigated technical feasibility of using PCC of sugar origin as a reinforcing filler for bamboo fiber plastic composite. It was shown that air-dry PCC particles were in an agglomerated form made of individual cubic particles of about 1.2 μm in diameter. Compounding with plastic resin helped separate the PCC to smaller individual particles. The use of PCC led to a significant increase of flexural strength and flexural modulus of PCC filled composites after 10 wt% PCC loading level. For bamboo filled composites with PCC, tensile modulus and flexural modulus were improved with the increase of PCC content and with the use of surface treated bamboo filler. The use of raw and treated PCC in coextruded WPC (e.g., in the shell layer to modify overall composite properties) has not been reported.

The objectives of this study were to elucidate the effect of silane treated PCC (TPCC) and WF loadings in the shell layer on the mechanical, water absorption and thermal expansion properties of coextruded WPC with two different core systems.