2.2. Preparation

Foaming BPRs with different B/P substitute rates (0, 10, 20, and 30 wt.%) were synthesized at the molar ratio of phenol (include bio-oil)/paraformaldehyde/NaOH, which was 1:2:0.4. Firstly, phenol and 75 wt.% NaOH solution (40 wt.%) were added into a 250 mL three-necked flask. Afterwards, the 75 wt.% paraformaldehyde was slowly added to the flask and kept at 65–75 °C for 20 min. The mixture was heated to 90 °C and held for 30 min. Secondly, the residual 25 wt.% paraformaldehyde, 25 wt.% NaOH solution (40 wt.%), and bio-oil were added and remained at 80 °C for 60 min. Thirdly, the pH value of the mixture was adjusted to between 6.8 to 7.2 using hydrochloric acid (37 wt.%), and the mixture was rapidly cooled down to 40 °C in 20 min to yield resin. The resultant resins were denoted as PR, 10%BPR, 20%BPR, and 30%BPR.

The viscosity of the resin was tested by an NDJ-5S rotating viscometer (CANY, Shanghai, China) at 25 °C. The solid content and curing time of resin were determined according to China National Standards (GB/T 14074-2013). In order to accurately determine the curing time of BPR in the foaming process, composite curing agents (p-toluenesulfonic acid/phosphoric acid) and the test temperature (75 °C) of the foaming process were used. In this test, 50 ± 0.1 g of resin and 6 ± 0.01 g of curing agent were stirred well at room temperature. The mixture (10 ± 0.1 g) was then placed in a test tube at 75 °C and the time until the stirring bar could not move was recorded. Each test above was repeated at least three times. The characterization of BFRs are summarized in Table 1.

Basic characteristics of phenolic resin (PR) and bio-oil phenolic resin (BPR).

Based on the weight of resin, 5 wt.% surfactants (Tween-80), 8 wt.% blowing agents (petroleum ether), and 12 wt.% composite curing agents (p-toluenesulfonic acid/phosphoric acid was 2:1) were added into the BPR and rapidly mixed well at room temperature. Then the mixture was poured into a mold and bubbled at 75 °C for 40 min. These prepared foams were denoted as PF, 10%BPF, 20%BPF, and 30%BPF.

The apparent density of foam was tested based on the China National Standard (GB/T 6343-2009). The compressive strength and flexural strength of the foam were measured with a universal testing machine (Insrton, Havisham, England) according to the China National Standard (GB/T 8813-2008 and GB/T 8812-2007). A LOI test of the foam was carried out using a JF-3 oxygen index meter (Jiangning Analysis Instrument Co., Jiangsu, China) according to the China National Standard (GB/T 2406-2008). The thermal conductivity of foam was investigated by the LFA467 thermal conductivity testing instrument (Netzsch, Selb, German). The pulverization ratio was obtained based on China National Standard (GB/T 12812-2006). In this test, a weight of 200 g was put on the sample (30 × 30 × 30 mm³). Then, the sample was pushed back and forth on a 300 mesh abrasive paper 30 times at a constant force and the distance of each single-pass friction was 250 mm. The pulverization ratio was measured by the weight loss of a sample after friction [7]. At least five replicates were used for these tests.